Vortex breakdown simulation

NASA Technical Reports Server (NTRS)

Hafez, M.; Ahmad, J.; Kuruvila, G.; Salas, M. D.

1987-01-01

In this paper, steady, axisymmetric inviscid, and viscous (laminar) swirling flows representing vortex breakdown phenomena are simulated using a stream function-vorticity-circulation formulation and two numerical methods. The first is based on an inverse iteration, where a norm of the solution is prescribed and the swirling parameter is calculated as a part of the output. The second is based on direct Newton iterations, where the linearized equations, for all the unknowns, are solved simultaneously by an efficient banded Gaussian elimination procedure. Several numerical solutions for inviscid and viscous flows are demonstrated, followed by a discussion of the results. Some improvements on previous work have been achieved: first order upwind differences are replaced by second order schemes, line relaxation procedure (with linear convergence rate) is replaced by Newton's iterations (which converge quadratically), and Reynolds numbers are extended from 200 up to 1000.

Non-linear instability analysis of the two-dimensional Navier-Stokes equation: The Taylor-Green vortex problem

NASA Astrophysics Data System (ADS)

Sengupta, Tapan K.; Sharma, Nidhi; Sengupta, Aditi

2018-05-01

An enstrophy-based non-linear instability analysis of the Navier-Stokes equation for two-dimensional (2D) flows is presented here, using the Taylor-Green vortex (TGV) problem as an example. This problem admits a time-dependent analytical solution as the base flow, whose instability is traced here. The numerical study of the evolution of the Taylor-Green vortices shows that the flow becomes turbulent, but an explanation for this transition has not been advanced so far. The deviation of the numerical solution from the analytical solution is studied here using a high accuracy compact scheme on a non-uniform grid (NUC6), with the fourth-order Runge-Kutta method. The stream function-vorticity (ψ, ω) formulation of the governing equations is solved here in a periodic square domain with four vortices at t = 0. Simulations performed at different Reynolds numbers reveal that numerical errors in computations induce a breakdown of symmetry and simultaneous fragmentation of vortices. It is shown that the actual physical instability is triggered by the growth of disturbances and is explained by the evolution of disturbance mechanical energy and enstrophy. The disturbance evolution equations have been traced by looking at (a) disturbance mechanical energy of the Navier-Stokes equation, as described in the work of Sengupta et al., "Vortex-induced instability of an incompressible wall-bounded shear layer," J. Fluid Mech. 493, 277-286 (2003), and (b) the creation of rotationality via the enstrophy transport equation in the work of Sengupta et al., "Diffusion in inhomogeneous flows: Unique equilibrium state in an internal flow," Comput. Fluids 88, 440-451 (2013).

Velocity boundary conditions for vorticity formulations of the incompressible Navier-Stokes equations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kempka, S.N.; Strickland, J.H.; Glass, M.W.

1995-04-01

formulation to satisfy velocity boundary conditions for the vorticity form of the incompressible, viscous fluid momentum equations is presented. The tangential and normal components of the velocity boundary condition are satisfied simultaneously by creating vorticity adjacent to boundaries. The newly created vorticity is determined using a kinematical formulation which is a generalization of Helmholtz` decomposition of a vector field. Though it has not been generally recognized, these formulations resolve the over-specification issue associated with creating voracity to satisfy velocity boundary conditions. The generalized decomposition has not been widely used, apparently due to a lack of a useful physical interpretation. Anmore » analysis is presented which shows that the generalized decomposition has a relatively simple physical interpretation which facilitates its numerical implementation. The implementation of the generalized decomposition is discussed in detail. As an example the flow in a two-dimensional lid-driven cavity is simulated. The solution technique is based on a Lagrangian transport algorithm in the hydrocode ALEGRA. ALEGRA`s Lagrangian transport algorithm has been modified to solve the vorticity transport equation and the generalized decomposition, thus providing a new, accurate method to simulate incompressible flows. This numerical implementation and the new boundary condition formulation allow vorticity-based formulations to be used in a wider range of engineering problems.« less

A comparison of the Method of Lines to finite difference techniques in solving time-dependent partial differential equations. [with applications to Burger equation and stream function-vorticity problem

NASA Technical Reports Server (NTRS)

Kurtz, L. A.; Smith, R. E.; Parks, C. L.; Boney, L. R.

1978-01-01

Steady state solutions to two time dependent partial differential systems have been obtained by the Method of Lines (MOL) and compared to those obtained by efficient standard finite difference methods: (1) Burger's equation over a finite space domain by a forward time central space explicit method, and (2) the stream function - vorticity form of viscous incompressible fluid flow in a square cavity by an alternating direction implicit (ADI) method. The standard techniques were far more computationally efficient when applicable. In the second example, converged solutions at very high Reynolds numbers were obtained by MOL, whereas solution by ADI was either unattainable or impractical. With regard to 'set up' time, solution by MOL is an attractive alternative to techniques with complicated algorithms, as much of the programming difficulty is eliminated.

Accurate ω-ψ Spectral Solution of the Singular Driven Cavity Problem

NASA Astrophysics Data System (ADS)

Auteri, F.; Quartapelle, L.; Vigevano, L.

2002-08-01

This article provides accurate spectral solutions of the driven cavity problem, calculated in the vorticity-stream function representation without smoothing the corner singularities—a prima facie impossible task. As in a recent benchmark spectral calculation by primitive variables of Botella and Peyret, closed-form contributions of the singular solution for both zero and finite Reynolds numbers are subtracted from the unknown of the problem tackled here numerically in biharmonic form. The method employed is based on a split approach to the vorticity and stream function equations, a Galerkin-Legendre approximation of the problem for the perturbation, and an evaluation of the nonlinear terms by Gauss-Legendre numerical integration. Results computed for Re=0, 100, and 1000 compare well with the benchmark steady solutions provided by the aforementioned collocation-Chebyshev projection method. The validity of the proposed singularity subtraction scheme for computing time-dependent solutions is also established.

Boundary layer streaming in viscoelastic fluids

NASA Astrophysics Data System (ADS)

Bahrani, Seyed Amir; Costalanga, Maxime; Royon, Laurent; Brunet, Philippe; DSHE Team; Energy Team

2017-11-01

Oscillations of bodies immersed in fluids are known to generate secondary steady flows (streaming). These flows have strong similarities with acoustic streaming induced by sound and ultrasound waves. A typical situation, investigated here, is that of a cylinder oscillating perpendicular to its axis, generating two pairs of counter-rotating steady vortices due to the transfer of vorticity from an inner boundary layer. While most studies so far investigated the situation of newtonian fluids, here, we consider the situation of a viscoelastic fluid. By using Particle Image Velocimetry, we carry out an experimental study of the flow structure and magnitude over a range of amplitude (A up to 2.5 mm, nearly half the cylinder diameter) and frequency (f between 5 and 100 Hz). We observe unprecedented behaviors at higher frequency (f >50 Hz) : at high enough amplitude, the usual flow with 2 pairs of vortices is replaced by a more complex flow where 4 pairs of vortices are observed. At smaller frequency, we observe reversal large scale vortices that replace the usual inner and outer ones in Newtonian fluids. The main intention of this work is to understand the influence of the complex and nonlinear rheology on the mechanism of streaming flow. In this way, another source of purely rheological nonlinearity is expected, competing with hydrodynamic nonlinearity. We evidence the effect of elasticity in streaming.

Lidar investigation of wake vortices generated by a landing aircraft

NASA Astrophysics Data System (ADS)

Smalikho, Igor N.; Banakh, Viktor A.; Falits, Andrey V.

2017-11-01

The results of measurements of parameters of aircraft wake vortices by a Stream Line coherent Doppler lidar during the three-day experiment on the airfield of Tolmachevo Airport are presented. We have analyzed spatial dynamics and evolution of the wake vortices generated by aircrafts of various types: from the Airbus A319 passenger aircraft to the heavy Boeing B747-8 cargo aircraft entering the landing at Tolmachevo Airport. It is shown that the Stream Line lidar may well be used to obtain reliable information about the presence and intensity of aircraft wake vortices in the vicinity of the runway.

Environmental solid particle effects on compressor cascade performance

NASA Technical Reports Server (NTRS)

Tabakoff, W.; Balan, C.

1982-01-01

The effect of suspended solid particles on the performance of the compressor cascade was investigated experimentally in a specially built cascade tunnel, using quartz sand particles. The cascades were made of NACA 65(10)10 airfoils. Three cascades were tested, one accelerating cascade and two diffusing cascades. The theoretical analysis assumes inviscid and incompressible two dimensional flow. The momentum exchange between the fluid and the particle is accounted for by the interphase force terms in the fluid momentum equation. The modified fluid phase momentum equations and the continuity equation are reduced to the conventional stream function vorticity formulation. The method treats the fluid phase in the Eulerian system and the particle phase in Lagrangian system. The experimental results indicate a small increase in the blade surface static pressures, while the theoretical results indicate a small decrease. The theoretical analysis, also predicts the loss in total pressure associated with the particulate flow through the cascade.

Effects of Space Environment on Flow and Concentration During Directional Solidification

NASA Technical Reports Server (NTRS)

Benjapiyaporn, C.; Timchenko, V.; Leonardi, E.; deVahlDavis, G.; deGroh, H. C., III

2000-01-01

A study of directional solidification of a weak binary alloy (specifically, Bi - 1 at% Sn) based on the fixed grid single domain approach is being undertaken. The enthalpy method is used to solve for the temperature field over the computational domain including both the solid and liquid phases; latent heat evolution is treated with the aid of an effective specific heat coefficient. A source term accounting for the release of solute into the liquid during solidification has been incorporated into the solute transport equation. The vorticity-stream function formulation is used to describe thermosolutal convection in the liquid region. In this paper we numerically investigate the effects of g-jitter on directional solidification. A background gravity of 1 micro-g has been assumed, and new results for the effects of periodic disturbances over a range of amplitudes and frequencies on solute field and segregation have been presented.

Natural convection with evaporation in a vertical cylindrical cavity under the effect of temperature-dependent surface tension

NASA Astrophysics Data System (ADS)

Kozhevnikov, Danil A.; Sheremet, Mikhail A.

2018-01-01

The effect of surface tension on laminar natural convection in a vertical cylindrical cavity filled with a weak evaporating liquid has been analyzed numerically. The cylindrical enclosure is insulated at the bottom, heated by a constant heat flux from the side, and cooled by a non-uniform evaporative heat flux from the top free surface having temperature-dependent surface tension. Governing equations with corresponding boundary conditions formulated in dimensionless stream function, vorticity, and temperature have been solved by finite difference method of the second-order accuracy. The influence of Rayleigh number, Marangoni number, and aspect ratio on the liquid flow and heat transfer has been studied. Obtained results have revealed that the heat transfer rate at free surface decreases with Marangoni number and increases with Rayleigh number, while the average temperature inside the cavity has an opposite behavior; namely, it growths with Marangoni number and reduces with Rayleigh number.

Statistical properties of Charney-Hasegawa-Mima zonal flows

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anderson, Johan, E-mail: anderson.johan@gmail.com; Botha, G. J. J.

2015-05-15

A theoretical interpretation of numerically generated probability density functions (PDFs) of intermittent plasma transport events in unforced zonal flows is provided within the Charney-Hasegawa-Mima (CHM) model. The governing equation is solved numerically with various prescribed density gradients that are designed to produce different configurations of parallel and anti-parallel streams. Long-lasting vortices form whose flow is governed by the zonal streams. It is found that the numerically generated PDFs can be matched with analytical predictions of PDFs based on the instanton method by removing the autocorrelations from the time series. In many instances, the statistics generated by the CHM dynamics relaxesmore » to Gaussian distributions for both the electrostatic and vorticity perturbations, whereas in areas with strong nonlinear interactions it is found that the PDFs are exponentially distributed.« less

On the Numerical Solution of the Elliptic Monge—Ampère Equation in Dimension Two: A Least-Squares Approach

NASA Astrophysics Data System (ADS)

Dean, Edward J.; Glowinski, Roland

During his outstanding career, Olivier Pironneau has addressed the solution of a large variety of problems from the Natural Sciences, Engineering and Finance to name a few, an evidence of his activity being the many articles and books he has written. It is the opinion of these authors, and former collaborators of O. Pironneau (cf. [DGP91]), that this chapter is well-suited to a volume honoring him. Indeed, the two pillars of the solution methodology that we are going to describe are: (1) a nonlinear least squares formulation in an appropriate Hilbert space, and (2) a mixed finite element approximation, reminiscent of the one used in [DGP91] and [GP79] for solving the Stokes and Navier-Stokes equations in their stream function-vorticity formulation; the contributions of O. Pironneau on the two above topics are well-known world wide. Last but not least, we will show that the solution method discussed here can be viewed as a solution method for a non-standard variant of the incompressible Navier-Stokes equations, an area where O. Pironneau has many outstanding and celebrated contributions (cf. [Pir89], for example).

Quantification of topological changes of vorticity contours in two-dimensional Navier-Stokes flow.

PubMed

Ohkitani, Koji; Al Sulti, Fayeza

2010-06-01

A characterization of reconnection of vorticity contours is made by direct numerical simulations of the two-dimensional Navier-Stokes flow at a relatively low Reynolds number. We identify all the critical points of the vorticity field and classify them by solving an eigenvalue problem of its Hessian matrix on the basis of critical-point theory. The numbers of hyperbolic (saddles) and elliptic (minima and maxima) points are confirmed to satisfy Euler's index theorem numerically. Time evolution of these indices is studied for a simple initial condition. Generally speaking, we have found that the indices are found to decrease in number with time. This result is discussed in connection with related works on streamline topology, in particular, the relationship between stagnation points and the dissipation. Associated elementary procedures in physical space, the merging of vortices, are studied in detail for a number of snapshots. A similar analysis is also done using the stream function.

Characterizing the Severe Turbulence Environments Associated with Commercial Aviation Accidents. Part 2; Hydrostatic Mesobeta Scale Numerical Simulations of Supergradient Wind Flow and Streamwise Ageostrophic Frontogenesis

NASA Technical Reports Server (NTRS)

Kaplan, Michael L.; Huffman, Allan W.; Lux, Kevin M.; Cetola, Jeffrey D.; Charney, Joseph J.; Riordan, Allen J.; Lin, Yuh-Lang; Waight, Kenneth T., III; Proctor, Fred (Technical Monitor)

2003-01-01

Simulation experiments reveal key processes that organize a hydrostatic environment conducive to severe turbulence. The paradigm requires juxtaposition of the entrance region of a curved jet stream, which is highly subgeostrophic, with the entrance region of a straight jet stream, which is highly supergeostrophic. The wind and mass fields become misphased as the entrance regions converge resulting in the significant spatial variation of inertial forcing, centripetal forcing, and along- and cross-stream pressure gradient forcing over a mesobeta scale region. This results in frontogenesis and the along-stream divergence of cyclonic and convergence of cyclonic ageostrophic vertical vorticity. The centripetally forced mesoscale front becomes the locus of large gradients of ageostrophic vertical vorticity along an overturning isentrope. This region becomes favorable for streamwise vorticity gradient formation enhancing the environment for organization of horizontal vortex tubes in the presence of buoyant forcing.

On the size and distribution of rings and coherent vortices in the Sargasso Sea

NASA Astrophysics Data System (ADS)

Luce, David L.; Rossby, Tom

2008-05-01

The container motor vessel CMV Oleander, which operates between New Jersey and Bermuda, crosses the Gulf Stream and Sargasso Sea all year round on a semiweekly schedule. Using an acoustic Doppler current profiler, measurements of upper ocean currents have been made on a regular basis since the fall of 1992. In this paper we examine the database for evidence of axisymmetric coherent vortices including the distribution and intensity of cold core rings. To detect the existence of coherent vortices, the patterns of current vectors averaged between 40 and 80 m depth were fit to an axisymmetric Gaussian vortex model. The parameters of the model were axis location, maximum tangential, or swirl, speed, and radius at which the maximum swirl was measured. We were able to distinguish between the well-known cold core "rings" (CCRs) pinched from the Gulf Stream, and a population of cyclonic and anticyclonic "vortices" in the Sargasso Sea. Both the rings and the Sargasso Sea vortices showed radii of 64 ± 18 km, albeit with different swirl speeds. The rings, close to the Gulf Stream, exhibited a typical maximum swirl speed of 0.98 ± 0.40 m s-1 and a center relative vorticity of 0.64 ± 0.35 × 10-4 s-1, almost 80% of the planetary vorticity for the region. The more uniform population of Sargasso Sea vortices contained approximately equal numbers of cyclones and anticyclones, with mean speeds of +0.43 and -0.55 m s-1, and center relative vorticities of +0.24 × 10-4 s-1 and -0.29 × 10-4 s-1, respectively.

Network community-based model reduction for vortical flows

NASA Astrophysics Data System (ADS)

Gopalakrishnan Meena, Muralikrishnan; Nair, Aditya G.; Taira, Kunihiko

2018-06-01

A network community-based reduced-order model is developed to capture key interactions among coherent structures in high-dimensional unsteady vortical flows. The present approach is data-inspired and founded on network-theoretic techniques to identify important vortical communities that are comprised of vortical elements that share similar dynamical behavior. The overall interaction-based physics of the high-dimensional flow field is distilled into the vortical community centroids, considerably reducing the system dimension. Taking advantage of these vortical interactions, the proposed methodology is applied to formulate reduced-order models for the inter-community dynamics of vortical flows, and predict lift and drag forces on bodies in wake flows. We demonstrate the capabilities of these models by accurately capturing the macroscopic dynamics of a collection of discrete point vortices, and the complex unsteady aerodynamic forces on a circular cylinder and an airfoil with a Gurney flap. The present formulation is found to be robust against simulated experimental noise and turbulence due to its integrating nature of the system reduction.

Mechanism of tonal noise generation from circular cylinder with spiral fin

NASA Astrophysics Data System (ADS)

Yamashita, Ryo; Hayashi, Hidechito; Okumura, Tetsuya; Hamakawa, Hiromitsu

2014-12-01

The pitch of the spiral finned tube influences seriously to the acoustic resonance in the heat exchanger. In this research, the flow characteristics in relating to the aeolian tone from the finned cylinder are studied by the numerical simulation. It is observed that the tonal noise generated from the finned tube at two pitch spaces. The ratio of the fin pitch to the cylinder diameter is changed at 0.11 and 0.27. The tone level increases and the frequency decreases with the pitch shorter. The separation flow from the cylinder generates the span-wise vortices, Karman vortices, and the separation flow from the fin generates the stream-wise vortices. When the fin pitch ratio is small, the stream-wise vortices line up to span-wise and become weak rapidly. Only the Karman vortices are remained and integrate in span. So the Karman vortex became large. This causes the low frequency and the large aeolian tone.

Eulerian velocity reconstruction in ideal atmospheric dynamics using potential vorticity and potential temperature

NASA Astrophysics Data System (ADS)

Blender, R.

2009-04-01

An approach for the reconstruction of atmospheric flow is presented which uses space- and time-dependent fields of density ?, potential vorticity Q and potential temperature Î& cedil;[J. Phys. A, 38, 6419 (2005)]. The method is based on the fundamental equations without approximation. The basic idea is to consider the time-dependent continuity equation as a condition for zero divergence of momentum in four dimensions (time and space, with unit velocity in time). This continuity equation is solved by an ansatz for the four-dimensional momentum using three conserved stream functions, the potential vorticity, potential temperature and a third field, denoted as ?-potential. In zonal flows, the ?-potential identifies the initial longitude of particles, whereas potential vorticity and potential temperature identify mainly meridional and vertical positions. Since the Lagrangian tracers Q, Î&,cedil; and ? determine the Eulerian velocity field, the reconstruction combines the Eulerian and the Lagrangian view of hydrodynamics. In stationary flows, the ?-potential is related to the Bernoulli function. The approach requires that the gradients of the potential vorticity and potential temperature do not vanish when the velocity remains finite. This behavior indicates a possible interrelation with stability conditions. Examples with analytical solutions are presented for a Rossby wave and zonal and rotational shear flows.

Multi-dimensional upwinding-based implicit LES for the vorticity transport equations

NASA Astrophysics Data System (ADS)

Foti, Daniel; Duraisamy, Karthik

2017-11-01

Complex turbulent flows such as rotorcraft and wind turbine wakes are characterized by the presence of strong coherent structures that can be compactly described by vorticity variables. The vorticity-velocity formulation of the incompressible Navier-Stokes equations is employed to increase numerical efficiency. Compared to the traditional velocity-pressure formulation, high order numerical methods and sub-grid scale models for the vorticity transport equation (VTE) have not been fully investigated. Consistent treatment of the convection and stretching terms also needs to be addressed. Our belief is that, by carefully designing sharp gradient-capturing numerical schemes, coherent structures can be more efficiently captured using the vorticity-velocity formulation. In this work, a multidimensional upwind approach for the VTE is developed using the generalized Riemann problem-based scheme devised by Parish et al. (Computers & Fluids, 2016). The algorithm obtains high resolution by augmenting the upwind fluxes with transverse and normal direction corrections. The approach is investigated with several canonical vortex-dominated flows including isolated and interacting vortices and turbulent flows. The capability of the technique to represent sub-grid scale effects is also assessed. Navy contract titled ``Turbulence Modelling Across Disparate Length Scales for Naval Computational Fluid Dynamics Applications,'' through Continuum Dynamics, Inc.

Large-scale computation of incompressible viscous flow by least-squares finite element method

NASA Technical Reports Server (NTRS)

Jiang, Bo-Nan; Lin, T. L.; Povinelli, Louis A.

1993-01-01

The least-squares finite element method (LSFEM) based on the velocity-pressure-vorticity formulation is applied to large-scale/three-dimensional steady incompressible Navier-Stokes problems. This method can accommodate equal-order interpolations and results in symmetric, positive definite algebraic system which can be solved effectively by simple iterative methods. The first-order velocity-Bernoulli function-vorticity formulation for incompressible viscous flows is also tested. For three-dimensional cases, an additional compatibility equation, i.e., the divergence of the vorticity vector should be zero, is included to make the first-order system elliptic. The simple substitution of the Newton's method is employed to linearize the partial differential equations, the LSFEM is used to obtain discretized equations, and the system of algebraic equations is solved using the Jacobi preconditioned conjugate gradient method which avoids formation of either element or global matrices (matrix-free) to achieve high efficiency. To show the validity of this scheme for large-scale computation, we give numerical results for 2D driven cavity problem at Re = 10000 with 408 x 400 bilinear elements. The flow in a 3D cavity is calculated at Re = 100, 400, and 1,000 with 50 x 50 x 50 trilinear elements. The Taylor-Goertler-like vortices are observed for Re = 1,000.

Buoyancy and blockage effects on transient laminar opposing mixed convection heat transfer from two horizontal confined isothermal cylinder in tandem

NASA Astrophysics Data System (ADS)

Martínez-Suástegui, Lorenzo; Salcedo, Erick; Cajas, Juan; Treviño, César

2015-11-01

Transient mixed convection in a laminar cross-flow from two isothermal cylinders in tandem arrangement confined inside a vertical channel is studied numerically using the vorticity-stream function formulation of the unsteady two-dimensional Navier-Stokes and energy equations. Numerical experiments are performed for a Reynolds number based on cylinder diameter of Re = 200, Prandtl number of Pr = 7, blockage ratio of D/H = 0.2, a pitch-to-diameter ratio of L/D = 2, and several values of buoyancy strength or Richardson number Ri = Gr/Re2. The results reported herein demonstrate how the wall confinement, interference effects and opposing buoyancy affect the flow structure and heat transfer characteristics of the cylinder array. This research was supported by the Consejo Nacional de Ciencia y Tecnología (CONACYT), Grant number 167474 and by the Secretaría de Investigación y Posgrado del IPN, Grant number SIP 20141309.

MHD natural convection and entropy generation in an open cavity having different horizontal porous blocks saturated with a ferrofluid

NASA Astrophysics Data System (ADS)

Gibanov, Nikita S.; Sheremet, Mikhail A.; Oztop, Hakan F.; Al-Salem, Khaled

2018-04-01

In this study, natural convection combined with entropy generation of Fe3O4-water nanofluid within a square open cavity filled with two different porous blocks under the influence of uniform horizontal magnetic field is numerically studied. Porous blocks of different thermal properties, permeability and porosity are located on the bottom wall. The bottom wall of the cavity is kept at hot temperature Th, while upper open boundary is at constant cold temperature Tc and other walls of the cavity are supposed to be adiabatic. Governing equations with corresponding boundary conditions formulated in dimensionless stream function and vorticity using Brinkman-extended Darcy model for porous blocks have been solved numerically using finite difference method. Numerical analysis has been carried out for wide ranges of Hartmann number, nanoparticles volume fraction and length of the porous blocks. It has been found that an addition of spherical ferric oxide nanoparticles can order the flow structures inside the cavity.

Mathematical simulation of convective-radiative heat transfer in a ventilated rectangular cavity with consideration of internal mass transfer

NASA Astrophysics Data System (ADS)

Sheremet, M. A.; Shishkin, N. I.

2012-07-01

Mathematical simulation of the nonstationary regimes of heat-and-mass transfer in a ventilated rectangular cavity with heat-conducting walls of finite thickness in the presence of a heat-generating element of constant temperature has been carried out with account for the radiative heat transfer in the Rosseland approximation. As mechanisms of energy transfer in this cavity, the combined convection and the thermal radiation in the gas space of the cavity and the heat conduction in the elements of its fencing solid shell were considered. The mathematical model formulated in the dimensionless stream function-vorticity vector-temperature-concentration variables was realized numerically with the use of the finite-difference method. The streamline, temperature-field, and concentration distributions reflecting the influence of the Rayleigh number (Ra = 104, 105, 106), the nonstationarity (0 < τ ≤ 1000), and the optical thickness of the medium (τλ = 50, 100, 200) on the regimes of the gas flow and the heat-and-mass transfer in the cavity have been obtained.

Long-time asymptotics of the Navier-Stokes and vorticity equations on R(3).

PubMed

Gallay, Thierry; Wayne, C Eugene

2002-10-15

We use the vorticity formulation to study the long-time behaviour of solutions to the Navier-Stokes equation on R(3). We assume that the initial vorticity is small and decays algebraically at infinity. After introducing self-similar variables, we compute the long-time asymptotics of the rescaled vorticity equation up to second order. Each term in the asymptotics is a self-similar divergence-free vector field with Gaussian decay at infinity, and the coefficients in the expansion can be determined by solving a finite system of ordinary differential equations. As a consequence of our results, we are able to characterize the set of solutions for which the velocity field satisfies ||u(.,t)||(L(2)) = o(t(-5/4)) as t-->+ infinity. In particular, we show that these solutions lie on a smooth invariant submanifold of codimension 11 in our function space.

Growth and wall-transpiration control of nonlinear unsteady Görtler vortices forced by free-stream vortical disturbances

NASA Astrophysics Data System (ADS)

Marensi, Elena; Ricco, Pierre

2017-11-01

The generation, nonlinear evolution, and wall-transpiration control of unsteady Görtler vortices in an incompressible boundary layer over a concave plate is studied theoretically and numerically. Görtler rolls are initiated and driven by free-stream vortical perturbations of which only the low-frequency components are considered because they penetrate the most into the boundary layer. The formation and development of the disturbances are governed by the nonlinear unsteady boundary-region equations with the centrifugal force included. These equations are subject to appropriate initial and outer boundary conditions, which account for the influence of the upstream and free-stream forcing in a rigorous and mutually consistent manner. Numerical solutions show that the stabilizing effect on nonlinearity, which also occurs in flat-plate boundary layers, is significantly enhanced in the presence of centrifugal forces. Sufficiently downstream, the nonlinear vortices excited at different free-stream turbulence intensities Tu saturate at the same level, proving that the initial amplitude of the forcing becomes unimportant. At low Tu, the disturbance exhibits a quasi-exponential growth with the growth rate being intensified for more curved plates and for lower frequencies. At higher Tu, in the typical range of turbomachinery applications, the Görtler vortices do not undergo a modal stage as nonlinearity saturates rapidly, and the wall curvature does not affect the boundary-layer response. Good quantitative agreement with data from direct numerical simulations and experiments is obtained. Steady spanwise-uniform and spanwise-modulated zero-mass-flow-rate wall transpiration is shown to attenuate the growth of the Görtler vortices significantly. A novel modified version of the Fukagata-Iwamoto-Kasagi identity, used for the first time to study a transitional flow, reveals which terms in the streamwise momentum balance are mostly affected by the wall transpiration, thus offering insight into the increased nonlinear growth of the wall-shear stress.

First-Order System Least Squares for Velocity-Vorticity-Pressure Form of the Stokes Equations, with Application to Linear Elasticity

NASA Technical Reports Server (NTRS)

Cai, Zhiqiang; Manteuffel, Thomas A.; McCormick, Stephen F.

1996-01-01

In this paper, we study the least-squares method for the generalized Stokes equations (including linear elasticity) based on the velocity-vorticity-pressure formulation in d = 2 or 3 dimensions. The least squares functional is defined in terms of the sum of the L(exp 2)- and H(exp -1)-norms of the residual equations, which is weighted appropriately by by the Reynolds number. Our approach for establishing ellipticity of the functional does not use ADN theory, but is founded more on basic principles. We also analyze the case where the H(exp -1)-norm in the functional is replaced by a discrete functional to make the computation feasible. We show that the resulting algebraic equations can be uniformly preconditioned by well-known techniques.

Application of the method of lines for solutions of the Navier-Stokes equations using a nonuniform grid distribution

NASA Technical Reports Server (NTRS)

Abolhassani, J. S.; Tiwari, S. N.

1983-01-01

The feasibility of the method of lines for solutions of physical problems requiring nonuniform grid distributions is investigated. To attain this, it is also necessary to investigate the stiffness characteristics of the pertinent equations. For specific applications, the governing equations considered are those for viscous, incompressible, two dimensional and axisymmetric flows. These equations are transformed from the physical domain having a variable mesh to a computational domain with a uniform mesh. The two governing partial differential equations are the vorticity and stream function equations. The method of lines is used to solve the vorticity equation and the successive over relaxation technique is used to solve the stream function equation. The method is applied to three laminar flow problems: the flow in ducts, curved-wall diffusers, and a driven cavity. Results obtained for different flow conditions are in good agreement with available analytical and numerical solutions. The viability and validity of the method of lines are demonstrated by its application to Navier-Stokes equations in the physical domain having a variable mesh.

Numerical investigations of two-degree-of-freedom vortex-induced vibration in shear flow

NASA Astrophysics Data System (ADS)

Zhang, Hui; Liu, Mengke; Han, Yang; Li, Jian; Gui, Mingyue; Chen, Zhihua

2017-06-01

Exponential-polar coordinates attached to a moving cylinder are used to deduce the stream function-vorticity equations for two-degree-of-freedom vortex-induced vibration, the initial and boundary conditions, and the distribution of the hydrodynamic force, which consists of the vortex-induced force, inertial force, and viscous damping force. The fluid-structure interactions occurring from the motionless cylinder to the steady vibration are investigated numerically, and the variations of the flow field, pressure, lift/drag, and cylinder displacement are discussed. Both the dominant vortex and the cylinder shift, whose effects are opposite, affect the shear layer along the transverse direction and the secondary vortex along the streamwise direction. However, the effect of the cylinder shift is larger than that of the dominant vortices. Therefore, the former dominates the total effects of the flow field. Moreover, the symmetry of the flow field is broken with the increasing shear rate. With the effect of the background vortex, the upper vortices are strengthened, and the lower vortices are weakened; thus, the shear layer and the secondary vortices induced by the upper shedding vortices are strengthened, while the shear layer and the secondary vortices induced by the lower shedding vortices are weakened. Therefore, the amplitudes of the displacement and drag/lift dominated by the upper vortex are larger than those of the displacement and drag/lift dominated by the lower vortex.

Initial Circulation and Peak Vorticity Behavior of Vortices Shed from Airfoil Vortex Generators

NASA Technical Reports Server (NTRS)

Wendt, Bruce J.; Biesiadny, Tom (Technical Monitor)

2001-01-01

An extensive parametric study of vortices shed from airfoil vortex generators has been conducted to determine the dependence of initial vortex circulation and peak vorticity on elements of the airfoil geometry and impinging flow conditions. These elements include the airfoil angle of attack, chord length, span, aspect ratio, local boundary layer thickness, and free stream Mach number. In addition, the influence of airfoil-to-airfoil spacing on the circulation and peak vorticity has been examined for pairs of co-rotating and counter-rotating vortices. The vortex generators were symmetric airfoils having a NACA-0012 cross-sectional profile. These airfoils were mounted either in isolation, or in pairs, on the surface of a straight pipe. The turbulent boundary layer thickness to pipe radius ratio was about 17 percent. The circulation and peak vorticity data were derived from cross-plane velocity measurements acquired with a seven-hole probe at one chord-length downstream of the airfoil trailing edge location. The circulation is observed to be proportional to the free-stream Mach number, the angle-of-attack, and the span-to-boundary layer thickness ratio. With these parameters held constant, the circulation is observed to fall off in monotonic fashion with increasing airfoil aspect ratio. The peak vorticity is also observed to be proportional to the free-stream Mach number, the airfoil angle-of-attack, and the span-to-boundary layer thickness ratio. Unlike circulation, however, the peak vorticity is observed to increase with increasing aspect ratio, reaching a peak value at an aspect ratio of about 2.0 before falling off again at higher values of aspect ratio. Co-rotating vortices shed from closely spaced pairs of airfoils have values of circulation and peak vorticity under those values found for vortices shed from isolated airfoils of the same geometry. Conversely, counter-rotating vortices show enhanced values of circulation and peak vorticity when compared to values obtained in isolation. The circulation may be accurately modeled with an expression based on Prandtl's relationship between finite airfoil circulation and airfoil geometry. A correlation for the peak vorticity has been derived from a conservation relationship equating the moment at the airfoil tip to the rate of angular momentum production of the shed vortex, modeled as a Lamb (ideal viscous) vortex. This technique provides excellent qualitative agreement to the observed behavior of peak vorticity for low aspect ratio airfoils typically used as vortex generators.

Streamwise vortices destabilize swimming bluegill sunfish (Lepomis macrochirus).

PubMed

Maia, Anabela; Sheltzer, Alex P; Tytell, Eric D

2015-03-01

In their natural environment, fish must swim stably through unsteady flows and vortices, including vertical vortices, typically shed by posts in a flow, horizontal cross-flow vortices, often produced by a step or a waterfall in a stream, and streamwise vortices, where the axis of rotation is aligned with the direction of the flow. Streamwise vortices are commonly shed by bluff bodies in streams and by ships' propellers and axial turbines, but we know little about their effects on fish. Here, we describe how bluegill sunfish use more energy and are destabilized more often in flow with strong streamwise vorticity. The vortices were created inside a sealed flow tank by an array of four turbines with similar diameter to the experimental fish. We measured oxygen consumption for seven sunfish swimming at 1.5 body lengths (BL) s(-1) with the turbines rotating at 2 Hz and with the turbines off (control). Simultaneously, we filmed the fish ventrally and recorded the fraction of time spent maneuvering side-to-side and accelerating forward. Separately, we also recorded lateral and ventral video for a combination of swimming speeds (0.5, 1.5 and 2.5 BL s(-1)) and turbine speeds (0, 1, 2 and 3 Hz), immediately after turning the turbines on and 10 min later to test for accommodation. Bluegill sunfish are negatively affected by streamwise vorticity. Spills (loss of heading), maneuvers and accelerations were more frequent when the turbines were on than in the control treatment. These unsteady behaviors, particularly acceleration, correlated with an increase in oxygen consumption in the vortex flow. Bluegill sunfish are generally fast to recover from roll perturbations and do so by moving their pectoral fins. The frequency of spills decreased after the turbines had run for 10 min, but was still markedly higher than in the control, showing that fish partially adapt to streamwise vorticity, but not completely. Coping with streamwise vorticity may be an important energetic cost for stream fishes or migratory fishes. © 2015. Published by The Company of Biologists Ltd.

Experimental investigation of acoustic streaming in a cylindrical wave guide up to high streaming Reynolds numbers.

PubMed

Reyt, Ida; Bailliet, Hélène; Valière, Jean-Christophe

2014-01-01

Measurements of streaming velocity are performed by means of Laser Doppler Velocimetry and Particle Image Velociimetry in an experimental apparatus consisting of a cylindrical waveguide having one loudspeaker at each end for high intensity sound levels. The case of high nonlinear Reynolds number ReNL is particularly investigated. The variation of axial streaming velocity with respect to the axial and to the transverse coordinates are compared to available Rayleigh streaming theory. As expected, the measured streaming velocity agrees well with the Rayleigh streaming theory for small ReNL but deviates significantly from such predictions for high ReNL. When the nonlinear Reynolds number is increased, the outer centerline axial streaming velocity gets distorted towards the acoustic velocity nodes until counter-rotating additional vortices are generated near the acoustic velocity antinodes. This kind of behavior is followed by outer streaming cells only and measurements in the near wall region show that inner streaming vortices are less affected by this substantial evolution of fast streaming pattern. Measurements of the transient evolution of streaming velocity provide an additional insight into the evolution of fast streaming.

Some observed seasonal changes in extratropical general circulation: A study in terms of vorticity. [seasonal migrations of extra tropical frontal jet streams

NASA Technical Reports Server (NTRS)

Srivatsangam, S.; Reiter, E. R.

1973-01-01

Extratropical eddy distributions in four months typical of the four seasons are treated in terms of temporal mean and temporal r.m.s. values of the geostrophic relative vorticity. The geographical distributions of these parameters at the 300 mb level show that the arithmetic mean fields are highly biased representatives of the extratropical eddy distributions. The zonal arithmetic means of these parameters are also presented. These show that the zonal-and-time mean relative vorticity is but a small fraction of the zonal mean of the temporal r.m.s. relative vorticity, K. The reasons for considering the r.m.s. values as the temporal normal values of vorticity in the extratropics are given in considerable detail. The parameter K is shown to be of considerable importance in locating the extratropical frontal jet streams (EFJ) in time-and-zonal average distributions. The study leads to an understanding of the seasonal migrations of the EFJ which have not been explored until now.

Weak solutions of the three-dimensional vorticity equation with vortex singularities

NASA Technical Reports Server (NTRS)

Winckelmans, G.; Leonard, A.

1988-01-01

The extension of the concept of vortex singularities, developed by Saffman and Meiron (1986) for the case of two-dimensional point vortices in an incompressible vortical flow, to the three-dimensional case of vortex sticks (vortons) is investigated analytically. The derivation of the governing equations is explained, and it is demonstrated that the formulation obtained conserves total vorticity and is a weak solution of the vorticity equation, making it an appropriate means for representing three-dimensional vortical flows with limited numbers of vortex singularities.

Characterisation of columnar inertial modes in rapidly rotating spheres and spheroids

NASA Astrophysics Data System (ADS)

Maffei, S.; Jackson, A.; Livermore, P. W.

2017-12-01

We consider fluid-filled spheres and spheroidal containers of eccentricity ɛ in rapid rotation, as a proxy for the interior dynamics of stars and planets. The fluid motion is assumed to be quasi-geostrophic (QG): horizontal motions are invariant parallel to the rotation axis z, a characteristic which is handled by use of a stream function formulation which additionally enforces mass conservation and non-penetration at the boundary. By linearising about a quiescent background state, we investigate a variety of methods to study the QG inviscid inertial wave modes which are compared with fully 3-D calculations. We consider the recently-proposed weak formulation of the inviscid system valid in spheroids of arbitrary eccentricity, to which we present novel closed-form polynomial solutions. Our modal solutions accurately represent, in both spatial structure and frequency, the most z-invariant of the inertial wave modes in a spheroid, and constitute a simple basis set for the analysis of rotationally- dominated fluids. We further show that these new solutions are more accurate than those of the classical axial-vorticity equation, which is independent of ɛ and thus fails to properly encode the container geometry. We also consider the effects of viscosity for the cases of both no-slip and stress-free boundary conditions for a spherical container. Calculations performed under the columnar approximation are compared with 3-D solutions and excellent agreement has been found despite fundamental differences in the two formulations.

Measurements and modeling of flow structure in the wake of a low profile wishbone vortex generator

NASA Technical Reports Server (NTRS)

Wendt, B. J.; Hingst, W. R.

1994-01-01

The results of an experimental examination of the vortex structures shed from a low profile 'wishbone' generator are presented. The vortex generator height relative to the turbulent boundary layer was varied by testing two differently sized models. Measurements of the mean three-dimensional velocity field were conducted in cross-stream planes downstream of the vortex generators. In all cases, a counter-rotating vortex pair was observed. Individual vortices were characterized by three descriptors derived from the velocity data; circulation, peak vorticity, and cross-stream location of peak vorticity. Measurements in the cross plane at two axial locations behind the smaller wishbone characterize the downstream development of the vortex pairs. A single region of stream wise velocity deficit is shared by both vortex cores. This is in contrast to conventional generators, where each core coincides with a region of velocity deficit. The measured cross-stream velocities for each case are compared to an Oseen model with matching descriptors. The best comparison occurs with the data from the larger wishbone.

Nonlinear magnetic electron tripolar vortices in streaming plasmas.

PubMed

Vranjes, J; Marić, G; Shukla, P K

2000-06-01

Magnetic electron modes in nonuniform magnetized and unmagnetized streaming plasmas, with characteristic frequencies between the ion and electron plasma frequencies and at spatial scales of the order of the collisionless skin depth, are studied. Two coupled equations, for the perturbed (in the case of magnetized plasma) or self-generated (for the unmagnetized plasma case) magnetic field, and the temperature, are solved in the strongly nonlinear regime and stationary traveling solutions in the form of tripolar vortices are found.

A coarse-grid projection method for accelerating incompressible flow computations

NASA Astrophysics Data System (ADS)

San, Omer; Staples, Anne E.

2013-01-01

We present a coarse-grid projection (CGP) method for accelerating incompressible flow computations, which is applicable to methods involving Poisson equations as incompressibility constraints. The CGP methodology is a modular approach that facilitates data transfer with simple interpolations and uses black-box solvers for the Poisson and advection-diffusion equations in the flow solver. After solving the Poisson equation on a coarsened grid, an interpolation scheme is used to obtain the fine data for subsequent time stepping on the full grid. A particular version of the method is applied here to the vorticity-stream function, primitive variable, and vorticity-velocity formulations of incompressible Navier-Stokes equations. We compute several benchmark flow problems on two-dimensional Cartesian and non-Cartesian grids, as well as a three-dimensional flow problem. The method is found to accelerate these computations while retaining a level of accuracy close to that of the fine resolution field, which is significantly better than the accuracy obtained for a similar computation performed solely using a coarse grid. A linear acceleration rate is obtained for all the cases we consider due to the linear-cost elliptic Poisson solver used, with reduction factors in computational time between 2 and 42. The computational savings are larger when a suboptimal Poisson solver is used. We also find that the computational savings increase with increasing distortion ratio on non-Cartesian grids, making the CGP method a useful tool for accelerating generalized curvilinear incompressible flow solvers.

An experimental study of large-scale vortices over a blunt-faced flat plate in pulsating flow

NASA Astrophysics Data System (ADS)

Hwang, K. S.; Sung, H. J.; Hyun, J. M.

Laboratory measurements are made of flow over a blunt flat plate of finite thickness, which is placed in a pulsating free stream, U=Uo(1+Aocos 2πfpt). Low turbulence-intensity wind tunnel experiments are conducted in the ranges of Stp<=1.23 and Ao<=0.118 at ReH=560. Pulsation is generated by means of a woofer speaker. Variations of the time-mean reattachment length xR as functions of Stp and Ao are scrutinized by using the forward-time fraction and surface pressure distributions (Cp). The shedding frequency of large-scale vortices due to pulsation is measured. Flow visualizations depict the behavior of large-scale vortices. The results for non-pulsating flows (Ao=0) are consistent with the published data. In the lower range of Ao, as Stp increases, xR attains a minimum value at a particular pulsation frequency. For large Ao, the results show complicated behaviors of xR. For Stp>=0.80, changes in xR are insignificant as Ao increases. The shedding frequency of large-scale vortices is locked-in to the pulsation frequency. A vortex-pairing process takes place between two neighboring large-scale vortices in the separated shear layer.

Laser Doppler velocimeter system simulation for sensing aircraft wake vortices

NASA Technical Reports Server (NTRS)

Thomson, J. A. L.; Meng, J. C. S.

1974-01-01

A hydrodynamic model of aircraft vortex wakes in an irregular wind shear field near the ground is developed and used as a basis for modeling the characteristics of a laser Doppler detection and vortex location system. The trailing vortex sheet and the wind shear are represented by discrete free vortices distributed over a two-dimensional grid. The time dependent hydrodynamic equations are solved by direct numerical integration in the Boussinesq approximation. The ground boundary is simulated by images, and fast Fourier Transform techniques are used to evaluate the vorticity stream function. The atmospheric turbulence was simulated by constructing specific realizations at time equal to zero, assuming that Kolmogoroff's law applies, and that the dissipation rate is constant throughout the flow field. The response of a simulated laser Doppler velocimeter is analyzed by simulating the signal return from the flow field as sensed by a simulation of the optical/electronic system.

Receptivity of Flat-Plate Boundary Layer in a Non-Uniform Free Stream (Vorticity Normal to the Plate)

NASA Technical Reports Server (NTRS)

Kogan, M. N.; Shumilkin, V. G.; Ustinov, M. V.; Zhigulev, S. V.

1999-01-01

Experimental and theoretical studies of low speed leading edge boundary layer receptivity to free-stream vorticity produced by upstream wires normal to the leading edge are discussed. Data include parametric variations in leading edge configuration and details of the incident disturbance field including single and multiple wakes. The induced disturbance amplitude increases with increases in the leading edge diameter and wake interactions. Measurements agree with the theory of M. E. Goldstein.

Towards a theory of stochastic vorticity-augmentation. [tornado model

NASA Technical Reports Server (NTRS)

Liu, V. C.

1977-01-01

A new hypothesis to account for the formation of tornadoes is presented. An elementary one-dimensional theory is formulated for vorticity transfer between an ambient sheared wind and a transverse penetrating jet. The theory points out the relevant quantities to be determined in describing the present stochastic mode of vorticity augmentation.

Unsteady potential flow past a propeller blade section

NASA Technical Reports Server (NTRS)

Takallu, M. A.

1990-01-01

An analytical study was conducted to predict the effect of an oscillating stream on the time dependent sectional pressure and lift coefficients of a model propeller blade. The assumption is that as the blade sections encounter a wake, the actual angles of attack vary in a sinusoidal manner through the wake, thus each blade is exposed to an unsteady stream oscillating about a mean value at a certain reduced frequency. On the other hand, an isolated propeller at some angle of attack can experience periodic changes in the value of the flow angle causing unsteady loads on the blades. Such a flow condition requires the inclusion of new expressions in the formulation of the unsteady potential flow around the blade sections. These expressions account for time variation of angle of attack and total shed vortices in the wake of each airfoil section. It was found that the final expressions for the unsteady pressure distribution on each blade section are periodic and that the unsteady circulation and lift coefficients exhibit a hysteresis loop.

A viscous flow analysis for the tip vortex generation process

NASA Technical Reports Server (NTRS)

Shamroth, S. J.; Briley, W. R.

1979-01-01

A three dimensional, forward-marching, viscous flow analysis is applied to the tip vortex generation problem. The equations include a streamwise momentum equation, a streamwise vorticity equation, a continuity equation, and a secondary flow stream function equation. The numerical method used combines a consistently split linearized scheme for parabolic equations with a scalar iterative ADI scheme for elliptic equations. The analysis is used to identify the source of the tip vortex generation process, as well as to obtain detailed flow results for a rectangular planform wing immersed in a high Reynolds number free stream at 6 degree incidence.

"Submesoscale Soup" Vorticity and Tracer Statistics During the Lateral Mixing Experiment

NASA Astrophysics Data System (ADS)

Shcherbina, A.; D'Asaro, E. A.; Lee, C. M.; Molemaker, J.; McWilliams, J. C.

2012-12-01

A detailed view of upper-ocean velocity, vorticity, and tracer statistics was obtained by a unique synchronized two-vessel survey in the North Atlantic in winter 2012. In winter, North Atlantic Mode water region south of the Gulf Stream is filled with an energetic, homogeneous, and well-developed submesoscale turbulence field - the "submesoscale soup". Turbulence in the soup is produced by frontogenesis and the surface layer instability of mesoscale eddy flows in the vicinity of the Gulf Stream. This region is a convenient representation of the inertial range of the geophysical turbulence forward cascade spanning scales of o(1-100km). During the Lateral Mixing Experiment in February-March 2012, R/Vs Atlantis and Knorr were run on parallel tracks 1 km apart for 500 km in the submesoscale soup region. Synchronous ADCP sampling provided the first in-situ estimates of full 3-D vorticity and divergence without the usual mix of spatial and temporal aliasing. Tracer distributions were also simultaneously sampled by both vessels using the underway and towed instrumentation. Observed vorticity distribution in the mixed layer was markedly asymmetric, with sparse strands of strong anticyclonic vorticity embedded in a weak, predominantly cyclonic background. While the mean vorticity was close to zero, distribution skewness exceeded 2. These observations confirm theoretical and numerical model predictions for an active submesoscale turbulence field. Submesoscale vorticity spectra also agreed well with the model prediction.

Force Generation by Flapping Foils

NASA Astrophysics Data System (ADS)

Bandyopadhyay, P. R.; Donnelly, M.

1996-11-01

Aquatic animals like fish use flapping caudal fins to produce axial and cross-stream forces. During WW2, German scientists had built and tested an underwater vehicle powered by similar flapping foils. We have examined the forces produced by a pair of flapping foils. We have examined the forced produced by a pair of flapping foils attached to the tail end of a small axisymmetric cylinder. The foils operate in-phase (called waving), or in anti-phase (called clapping). In a low-speed water tunnel, we have undertaken time-dependent measurements of axial and cross-stream forces and moments that are exerted by the vortex shedding process over the entire body. Phase-matched LDV measurements of vorticity-velocity vectors, as well as limited flow visualization of the periodic vortex shedding process have also been carried out. The direction of the induced velocity within a pair of shed vortices determines the nature of the forces produced, viz., thrust or drag or cross-stream forces. The clapping mode produces a widely dispersed symmetric array of vortices which results in axial forces only (thrust and rag). On the other hand, the vortex array is staggered in the waving mode and cross-stream (maneuvering) forces are then generated.

Vortex Design Problem

NASA Astrophysics Data System (ADS)

Protas, Bartosz

2007-11-01

In this investigation we are concerned with a family of solutions of the 2D steady--state Euler equations, known as the Prandtl--Batchelor flows, which are characterized by the presence of finite--area vortex patches embedded in an irrotational flow. We are interested in flows in the exterior of a circular cylinder and with a uniform stream at infinity, since such flows are often employed as models of bluff body wakes in the high--Reynolds number limit. The ``vortex design'' problem we consider consists in determining a distribution of the wall--normal velocity on parts of the cylinder boundary such that the vortex patches modelling the wake vortices will have a prescribed shape and location. Such inverse problem have applications in various areas of flow control, such as mitigation of the wake hazard. We show how this problem can be solved computationally by formulating it as a free--boundary optimization problem. In particular, we demonstrate that derivation of the adjoint system, required to compute the cost functional gradient, is facilitated by application of the shape differential calculus. Finally, solutions of the vortex design problem are illustrated with computational examples.

Nonlinear development and secondary instability of Gortler vortices in hypersonic flows

NASA Technical Reports Server (NTRS)

Fu, Yibin B.; Hall, Philip

1991-01-01

In a hypersonic boundary layer over a wall of variable curvature, the region most susceptible to Goertler vortices is the temperature adjustment layer over which the basic state temperature decreases monotonically to its free stream value. Except for a special wall curvature distribution, the evolution of Goertler vortices trapped in the temperature adjustment layer will in general be strongly affected by the boundary layer growth through the O(M sup 3/2) curvature of the basic state, where M is the free stream Mach number. Only when the local wavenumber becomes as large as of order M sup 3/8, do nonparallel effects become negligible in the determination of stability properties. In the latter case, Goertler vortices will be trapped in a thin layer of O(epsilon sup 1/2) thickness which is embedded in the temperature adjustment layer; here epsilon is the inverse of the local wavenumber. A weakly nonlinear theory is presented in which the initial nonlinear development of Goertler vortices in the neighborhood of the neutral position is studied and two coupled evolution equations are derived. From these, it can be determined whether the vortices are decaying or growing depending on the sign of a constant which is related to wall curvature and the basic state temperature.

The inducement of planetary boundary layer mass convergence associated with varying vorticity beneath tropospheric wind maximum

NASA Technical Reports Server (NTRS)

Johnson, D. R.

1984-01-01

The effects of the vorticity distribution are applied to study planetary boundary layer mass convergence beneath free tropospheric wind maximum. For given forcing by viscous and pressure gradient forces beneath a wind maximum, boundary layer cross stream mass transport is increased by anticyclonic vorticity on the right flank and decreased by cyclonic vorticity on the left flank. Such frictionally forced mass transport induces boundary layer mass convergence beneath the relative wind maximum. This result is related to the empirical rule that the most intense convection and severe weather frequently develop beneath the 500 mb zero relative vorticity isopleth.

Leading-edge effects on boundary-layer receptivity

NASA Technical Reports Server (NTRS)

Gatski, Thomas B.; Kerschen, Edward J.

1990-01-01

Numerical calculations are presented for the incompressible flow over a parabolic cylinder. The computational domain extends from a region upstream of the body downstream to the region where the Blasius boundary-layer solution holds. A steady mean flow solution is computed and the results for the scaled surface vorticity, surface pressure and displacement thickness are compared to previous studies. The unsteady problem is then formulated as a perturbation solution starting with and evolving from the mean flow. The response to irrotational time harmonic pulsation of the free-stream is examined. Results for the initial development of the velocity profile and displacement thickness are presented. These calculations will be extended to later times to investigate the initiation of instability waves within the boundary-layer.

Receptivity of Flat-Plate Boundary Layer in a Non-Uniform Free Stream (Vorticity Normal to the Plate)

NASA Technical Reports Server (NTRS)

Kogan, M. N.; Ustinov, M. V.

1997-01-01

Work is devoted to study of free-stream vorticity normal to leading edge interaction with boundary layer over plate and resulting flow distortion influence on laminar-turbulent transition. In experiments made the wake behind the vertically stretched wire was used as a source of vortical disturbances and its effect on the boundary layer over the horizontally mounted plate with various leading edge shapes was investigated. The purpose of experiments was to check the predictions of theoretical works of M.E. Goldstein, et. al. This theory shows that small free-stream inhomogeneity interacting with leading edge produces considerable distortion of boundary layer flow. In general, results obtained confirms predictions of Goldstein's theory, i.e., the amplification of steady vortical disturbances in boundary layer caused by vortex lines stretching was observed. Experimental results fully coincide with predictions of theory for large Reynolds number, relatively sharp leading edge and small disturbances. For large enough disturbances the flow distortion caused by symmetric wake unexpectedly becomes antisymmetric in spanwise direction. If the leading edge is too blunt the maximal distortion takes place immediately at the nose and no further amplification was observed. All these conditions and results are beyond the scope of Goldstein's theory.

Monopolar vortices as relative equilibria and their dissipative decay

NASA Astrophysics Data System (ADS)

Vandefliert, B. W.; Vangroesen, E. W. C.

1991-11-01

Families of confined rotating monopolar vortices are characterized using a variational formulation with the angular momentum as the driving force for confinement. The characterization for positive monopolar vortices given, can be extended to negative vortices or to vortices within a rotating frame of reference. Besides the uniform Kirchhoff paths, new branches of vorticity solutions are found restricting the dynamics to levelsets of both the angular momentum and the quadratic anisotropy. The rotation rate of the smooth vorticity structures depends on the vorticity profile. This is made perceptible by considering both minimum energy vortices and minimizing vortices, rotating counterclockwise and clockwise respectively. An approximation for the decay of the vortices due to dissipation is given in terms of the dissipation of the integrals in the inviscid system. This description enables us to consider dissipation of vortices without loss of confinement. The elliptical Kirchhoff patches are found to symmetrize into circular patches. The minimum energy vortices gradually diminish while expending their support, while the maximum energy vortices are unstable for the dissipative evolution.

Wave theory of turbulence in compressible media

NASA Technical Reports Server (NTRS)

Kentzer, C. P.

1975-01-01

An acoustical theory of turbulence was developed to aid in the study of the generation of sound in turbulent flows. The statistical framework adopted is a quantum-like wave dynamical formulation in terms of complex distribution functions. This formulation results in nonlinear diffusion-type transport equations for the probability densities of the five modes of wave propagation: two vorticity modes, one entropy mode, and two acoustic modes. This system of nonlinear equations is closed and complete. The technique of analysis was chosen such that direct applications to practical problems can be obtained with relative ease.

A Global Interpolation Function (GIF) boundary element code for viscous flows

NASA Technical Reports Server (NTRS)

Reddy, D. R.; Lafe, O.; Cheng, A. H-D.

1995-01-01

Using global interpolation functions (GIF's), boundary element solutions are obtained for two- and three-dimensional viscous flows. The solution is obtained in the form of a boundary integral plus a series of global basis functions. The unknown coefficients of the GIF's are determined to ensure the satisfaction of the governing equations at selected collocation points. The values of the coefficients involved in the boundary integral equations are determined by enforcing the boundary conditions. Both primitive variable and vorticity-velocity formulations are examined.

Bacterial aggregation and biofilm formation in a vortical flow

PubMed Central

Yazdi, Shahrzad; Ardekani, Arezoo M.

2012-01-01

Bacterial aggregation and patchiness play an important role in a variety of ecological processes such as competition, adaptation, epidemics, and succession. Here, we demonstrate that hydrodynamics of their environment can lead to their aggregation. This is specially important since microbial habitats are rarely at rest (e.g., ocean, blood stream, flow in porous media, and flow through membrane filtration processes). In order to study the dynamics of bacterial collection in a vortical flow, we utilize a microfluidic system to mimic some of the important microbial conditions at ecologically relevant spatiotemporal scales. We experimentally demonstrate the formation of “ring”-shaped bacterial collection patterns and subsequently the formation of biofilm streamers in a microfluidic system. Acoustic streaming of a microbubble is used to generate a vortical flow in a microchannel. Due to bacteria's finite-size, the microorganisms are directed to closed streamlines and trapped in the vortical flow. The collection of bacteria in the vortices occurs in a matter of seconds, and unexpectedly, triggers the formation of biofilm streamers within minutes. Swimming bacteria have a competitive advantage to respond to their environmental conditions. In order to investigate the role of bacterial motility on the rate of collection, two strains of Escherichia coli bacteria with different motilities are used. We show that the bacterial collection in a vortical flow is strongly pronounced for high motile bacteria. PMID:24339847

Experimental examination of vorticity stripping from a wing-tip vortex in free-stream turbulence

NASA Astrophysics Data System (ADS)

Ghimire, Hari C.; Bailey, Sean C. C.

2018-03-01

Time-resolved stereoscopic particle image velocimetry measurements were conducted of a wing-tip vortex decaying in free-stream turbulence. The objective of the research was to experimentally investigate the mechanism causing the increased rate of decay of the vortex in the presence of turbulence. It was observed that the circulation of the vortex core experienced periods of rapid loss and recovery when immersed in free-stream turbulence. These events were not observed when the vortex was in a laminar free stream. A connection was made between these events and distortion of the vortex, coinciding with stripping of core fluid from the vortex core. Specifically, vortex stripping events were connected to asymmetry in the vortex core, and this asymmetry was associated with instances of rapid circulation loss. The increased rate of decay of the vortex in turbulence coincided with the formation of secondary vortical structures which wrapped azimuthally around the primary vortex.

Optimized open-flow mixing: insights from microbubble streaming

NASA Astrophysics Data System (ADS)

Rallabandi, Bhargav; Wang, Cheng; Guo, Lin; Hilgenfeldt, Sascha

2015-11-01

Microbubble streaming has been developed into a robust and powerful flow actuation technique in microfluidics. Here, we study it as a paradigmatic system for microfluidic mixing under a continuous throughput of fluid (open-flow mixing), providing a systematic optimization of the device parameters in this practically important situation. Focusing on two-dimensional advective stirring (neglecting diffusion), we show through numerical simulation and analytical theory that mixing in steady streaming vortices becomes ineffective beyond a characteristic time scale, necessitating the introduction of unsteadiness. By duty cycling the streaming, such unsteadiness is introduced in a controlled fashion, leading to exponential refinement of the advection structures. The rate of refinement is then optimized for particular parameters of the time modulation, i.e. a particular combination of times for which the streaming is turned ``on'' and ``off''. The optimized protocol can be understood theoretically using the properties of the streaming vortices and the throughput Poiseuille flow. We can thus infer simple design principles for practical open flow micromixing applications, consistent with experiments. Current Address: Mechanical and Aerospace Engineering, Princeton University.

A defect stream function, law of the wall/wake method for compressible turbulent boundary layers

NASA Technical Reports Server (NTRS)

Barnwell, Richard W.; Dejarnette, Fred R.; Wahls, Richard A.

1989-01-01

The application of the defect stream function to the solution of the two-dimensional, compressible boundary layer is examined. A law of the wall/law of the wake formulation for the inner part of the boundary layer is presented which greatly simplifies the computational task near the wall and eliminates the need for an eddy viscosity model in this region. The eddy viscosity model in the outer region is arbitrary. The modified Crocco temperature-velocity relationship is used as a simplification of the differential energy equation. Formulations for both equilibrium and nonequilibrium boundary layers are presented including a constrained zero-order form which significantly reduces the computational workload while retaining the significant physics of the flow. A formulation for primitive variables is also presented. Results are given for the constrained zero-order and second-order equilibrium formulations and are compared with experimental data. A compressible wake function valid near the wall has been developed from the present results.

Transient Boundary Layer Disturbance Growth and Bypass Transition Due to Realistic Roughness and Continued Study of Transition Over Riblets

DTIC Science & Technology

2011-03-19

producing negative streamwise vorticity). It is not clear, however, why these ωx pancakes take on this alternating layer form. Figuring out how new...streamwise vorticity. The stream ribbons are colored by the vorticity component along the direction of the ribbon. The upshot of such an image is...different colors . The right image of figure 21 is created from analyzing several photographs of each single collar vortex. Due to limitations in the dye

Steady flow instability in an annulus with deflectors at rotational vibration

NASA Astrophysics Data System (ADS)

Kozlov, Nikolai V.; Pareau, Dominique; Ivantsov, Andrey; Stambouli, Moncef

2016-12-01

Experimental study and direct numerical simulation of the dynamics of an isothermal low-viscosity fluid are done in a coaxial gap of a cylindrical container making rotational vibrations relative to its axis. On the inner surface of the outer wall of the container, semicircular deflectors are regularly situated, playing the role of flow activators. As a result of vibrations, the activators oscillate tangentially. In the simulation, a 2D configuration is considered, excluding the end-wall effects. In the experiment, a container with a large aspect ratio is used. Steady streaming is generated in the viscous boundary layers on the activators. On each of the latter, beyond the viscous domain, a symmetric vortices pair is formed. The steady streaming in the annulus has an azimuthal periodicity. With an increase in the vibration intensity, a competition between the vortices occurs, as a result of which one of the vortices (let us call it even) approaches the activator and the other one (odd) rolls away and couples with the vortices from the neighbouring pairs. Streamlines of the odd vortices close on each other, forming a cog-wheel shaped flow that encircles the inner wall. Comparison of the experiment and the simulation reveals an agreement at moderate vibration intensity.

A priori testing of subgrid-scale models for the velocity-pressure and vorticity-velocity formulations

NASA Technical Reports Server (NTRS)

Winckelmans, G. S.; Lund, T. S.; Carati, D.; Wray, A. A.

1996-01-01

Subgrid-scale models for Large Eddy Simulation (LES) in both the velocity-pressure and the vorticity-velocity formulations were evaluated and compared in a priori tests using spectral Direct Numerical Simulation (DNS) databases of isotropic turbulence: 128(exp 3) DNS of forced turbulence (Re(sub(lambda))=95.8) filtered, using the sharp cutoff filter, to both 32(exp 3) and 16(exp 3) synthetic LES fields; 512(exp 3) DNS of decaying turbulence (Re(sub(Lambda))=63.5) filtered to both 64(exp 3) and 32(exp 3) LES fields. Gaussian and top-hat filters were also used with the 128(exp 3) database. Different LES models were evaluated for each formulation: eddy-viscosity models, hyper eddy-viscosity models, mixed models, and scale-similarity models. Correlations between exact versus modeled subgrid-scale quantities were measured at three levels: tensor (traceless), vector (solenoidal 'force'), and scalar (dissipation) levels, and for both cases of uniform and variable coefficient(s). Different choices for the 1/T scaling appearing in the eddy-viscosity were also evaluated. It was found that the models for the vorticity-velocity formulation produce higher correlations with the filtered DNS data than their counterpart in the velocity-pressure formulation. It was also found that the hyper eddy-viscosity model performs better than the eddy viscosity model, in both formulations.

Planar incompressible Navier-Stokes and Euler equations: A geometric formulation

NASA Astrophysics Data System (ADS)

Dimitriou, Ioannis

2017-11-01

In this paper, a novel geometric approach for studying steady, two-dimensional, incompressible flows has been thoroughly developed. The continuity and momentum equations were expressed in the flow's intrinsic coordinate system in order to "accommodate" the geometric parameters characterizing it, namely, the local curvatures of the streamlines and their orthogonal trajectories. As a result, a new description of the governing equations was obtained, in which the concerned variables are the velocity magnitude v and a new quantity which was named geometric vorticity, Γ. The latter is defined by the curl of the global curvature vector KG and can be interpreted as the geometric signature of the known vorticity Ω. This approach leads to a new formulation of the Navier-Stokes and Euler equations, the so-called "velocity-curvature" formulation. In this framework, an expression for the flow velocity as a function of geometric parameters only was developed. This reveals that the physical information of a steady incompressible flow is imprinted in its geometry. It is this insight that makes the aforementioned formulation not only conceptually different to the existing classical descriptions, traditionally employed in both analytical and numerical applications, but also attractive, due to the advantages that it could provide at a theoretical and an experimental level. Finally, the derived results are briefly discussed, while emphasizing the implications that the identified geometry-physics interface might have in the future for planar flow analysis.

PIV and Hotwire Measurement and Analysis of Tip Vortices and Turbulent Wake Generated by a Model Horizontal Axis Wind Turbine

NASA Astrophysics Data System (ADS)

Green, D.; Tan, Y. M.; Chamorro, L. P.; Arndt, R.; Sotiropoulos, F.; Sheng, J.

2011-12-01

Understanding vortical flow structures and turbulence in the wake flow behind a Horizontal Axis Wind Turbine (HAWT) has widespread applications in efficient blade design. Moreover, the knowledge of wake-turbine interactions allows us to devise optimal operational parameters, such as the spatial allocation and control algorithms of wind turbines, for a densely populated wind farm. To understand the influence of tip vortices on energy containing mean flow and turbulence, characteristics of vortical structures and turbulence must be quantified thoroughly. In this study, we conduct phase-locked Particle Image Velocimetry (PIV) measurements of the flow before and after a model HAWT, which is located in a zero-pressure gradient wind tunnel with a cross section of 1.7 × 1.7 m and a test section of 16 m in length. A three-blade model HAWT with a diameter of 605 mm and tip-speed ratio of 5 is used. PIV images are recorded by a 2048 × 2048 CCD camera and streamed at 6 Hz continuously; and phased locked with the passage of the blade at its vertical position. Each PIV measurement covers a 0.13 × 0.13 m2 sample area with the spatial resolution of 63 μm and a vector spacing of 0.5 mm. All experiments are conducted at the free-stream wind speed of 10 m/s. Flow fields at thirty consecutive downstream locations up to six rotor diameters and 144 mid chord lengths are measured. At each location, we obtain at least 10,000 instantaneous PIV realizations or 20,000 images. Three different configurations: single, dual, and trio turbines located at 5 rotor diameter upstream to each other, are examined experimentally. The flow statistics include mean wake velocity distributions, characteristics of tip vortices evolving downstream, fluctuation velocity, turbulent kinetic energy, stresses, and energy spectra. We find that tip vortices decay much faster in the wake of the upstream turbines (multiple-turbine configurations), whereas they maintain the coherence and strength behind a single turbine. The tip vortices entrain the high speed free-stream fluids and subsequently replenish the loss of momentum into the wake. Such a mechanism is greatly mitigated in the multiple-turbine scenarios. On-going analysis is to elucidate the generation, evolution and dissipation of the tip vortices in the various configurations.

Motility versus fluctuations in mixtures of self-motile and passive agents.

PubMed

Hinz, Denis F; Panchenko, Alexander; Kim, Tae-Yeon; Fried, Eliot

2014-12-07

Many biological systems consist of self-motile and passive agents both of which contribute to overall functionality. However, little is known about the properties of such mixtures. Here we formulate a model for mixtures of self-motile and passive agents and show that the model gives rise to three different dynamical phases: a disordered mesoturbulent phase, a polar flocking phase, and a vortical phase characterized by large-scale counter rotating vortices. We use numerical simulations to construct a phase diagram and compare the statistical properties of the different phases with observed features of self-motile bacterial suspensions. Our findings afford specific insights regarding the interaction of microorganisms and passive particles and provide novel strategic guidance for efficient technological realizations of artificial active matter.

A Free Wake Numerical Simulation for Darrieus Vertical Axis Wind Turbine Performance Prediction

NASA Astrophysics Data System (ADS)

Belu, Radian

2010-11-01

In the last four decades, several aerodynamic prediction models have been formulated for the Darrieus wind turbine performances and characteristics. We can identified two families: stream-tube and vortex. The paper presents a simplified numerical techniques for simulating vertical axis wind turbine flow, based on the lifting line theory and a free vortex wake model, including dynamic stall effects for predicting the performances of a 3-D vertical axis wind turbine. A vortex model is used in which the wake is composed of trailing stream-wise and shedding span-wise vortices, whose strengths are equal to the change in the bound vortex strength as required by the Helmholz and Kelvin theorems. Performance parameters are computed by application of the Biot-Savart law along with the Kutta-Jukowski theorem and a semi-empirical stall model. We tested the developed model with an adaptation of the earlier multiple stream-tube performance prediction model for the Darrieus turbines. Predictions by using our method are shown to compare favorably with existing experimental data and the outputs of other numerical models. The method can predict accurately the local and global performances of a vertical axis wind turbine, and can be used in the design and optimization of wind turbines for built environment applications.

Factors which influence the behavior of turbofan forced mixer nozzles

NASA Technical Reports Server (NTRS)

Anderson, B. H.; Povinelli, L. A.

1981-01-01

A finite difference procedure was used to compute the mixing for three experimentally tested mixer geometries. Good agreement was obtained between analysis and experiment when the mechanisms responsible for secondary flow generation were properly modeled. Vorticity generation due to flow turning and vorticity generated within the centerbody lobe passage were found to be important. Results are presented for two different temperature ratios between fan and core streams and for two different free stream turbulence levels. It was concluded that the dominant mechanisms in turbofan mixers is associated with the secondary flows arising within the lobe region and their development within the mixing section.

Trapping and exclusion zones in complex streaming patterns around a large assembly of microfluidic bubbles under ultrasound

NASA Astrophysics Data System (ADS)

Combriat, Thomas; Mekki-Berrada, Flore; Thibault, Pierre; Marmottant, Philippe

2018-01-01

Pulsating bubbles have proved to be a versatile tool for trapping and sorting particles. In this article, we investigate the different streaming patterns that can be obtained with a group of bubbles in a confined geometry under ultrasound. In the presence of an external flow strong enough to oppose the streaming velocities but not drag the trapped bubbles, we observe either the appearance of exclusion zones near the bubbles or asymmetric streaming patterns that we interpret as the superposition of a two-dimensional (2D) streaming function and of a potential flow. When studying a lattice of several bubbles, we show that the streaming pattern can be accurately predicted by superimposing the contributions of every pair of bubbles present in the lattice, thus allowing one to predict the sizes and the shapes of exclusion zones created by a group of bubbles under acoustic excitation. We suggest that such systems could be used to enhance mixing at a small scale or to catch and release chemical species initially trapped in vortices created around bubble pairs.

Low frequency vibration induced streaming in a Hele-Shaw cell

DOE Office of Scientific and Technical Information (OSTI.GOV)

Costalonga, M., E-mail: maxime.costalonga@univ-paris-diderot.fr; Laboratoire Matière et Systèmes Complexes, UMR CNRS 7057, Université Paris Diderot, 10 rue Alice Domon et Léonie Duquet, 75205 Paris cedex 13; Brunet, P.

When an acoustic wave propagates in a fluid, it can generate a second order flow whose characteristic time is much longer than the period of the wave. Within a range of frequency between ten and several hundred Hz, a relatively simple and versatile way to generate streaming flow is to put a vibrating object in the fluid. The flow develops vortices in the viscous boundary layer located in the vicinity of the source of vibrations, leading in turn to an outer irrotational streaming called Rayleigh streaming. Because the flow originates from non-linear time-irreversible terms of the Navier-Stokes equation, this phenomenonmore » can be used to generate efficient mixing at low Reynolds number, for instance in confined geometries. Here, we report on an experimental study of such streaming flow induced by a vibrating beam in a Hele-Shaw cell of 2 mm span using long exposure flow visualization and particle-image velocimetry measurements. Our study focuses especially on the effects of forcing frequency and amplitude on flow dynamics. It is shown that some features of this flow can be predicted by simple scaling arguments and that this vibration-induced streaming facilitates the generation of vortices.« less

Acoustic streaming induced by two orthogonal ultrasound standing waves in a microfluidic channel.

PubMed

Doinikov, Alexander A; Thibault, Pierre; Marmottant, Philippe

2018-07-01

A mathematical model is derived for acoustic streaming in a microfluidic channel confined between a solid wall and a rigid reflector. Acoustic streaming is produced by two orthogonal ultrasound standing waves of the same frequency that are created by two pairs of counter-propagating leaky surface waves induced in the solid wall. The magnitudes and phases of the standing waves are assumed to be different. Full analytical solutions are found for the equations of acoustic streaming. The obtained solutions are used in numerical simulations to reveal the structure of the acoustic streaming. It is shown that the interaction of two standing waves leads to the appearance of a cross term in the equations of acoustic streaming. If the phase lag between the standing waves is nonzero, the cross term brings about circular vortices with rotation axes perpendicular to the solid wall of the channel. The vortices make fluid particles rotate and move alternately up and down between the solid wall and the reflector. The obtained results are of immediate interest for acoustomicrofluidic applications such as the ultrasonic micromixing of fluids and the manipulation of microparticles. Copyright © 2018 Elsevier B.V. All rights reserved.

DUST CAPTURE AND LONG-LIVED DENSITY ENHANCEMENTS TRIGGERED BY VORTICES IN 2D PROTOPLANETARY DISKS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Surville, Clément; Mayer, Lucio; Lin, Douglas N. C., E-mail: clement.surville@physik.uzh.ch

We study dust capture by vortices and its long-term consequences in global two-fluid inviscid disk simulations using a new polar grid code RoSSBi. We perform the longest integrations so far, several hundred disk orbits, at the highest resolution attainable in global disk simulations with dust, namely, 2048 × 4096 grid points. We vary a wide range of dust parameters, most notably the initial dust-to-gas ratio ϵ varies in the range of 10{sup −4}–10{sup −2}. Irrespective of the value of ϵ , we find rapid concentration of the dust inside vortices, reaching dust-to-gas ratios of the order of unity inside themore » vortex. We present an analytical model that describes this dust capture process very well, finding consistent results for all dust parameters. A vortex streaming instability develops, which invariably causes vortex destruction. After vortex dissipation large-scale dust rings encompassing a disk annulus form in most cases, which sustain very high dust concentration, approaching ratios of the order of unity; they persist as long as the duration of the simulations. They are sustained by a streaming instability, which manifests itself in high-density dust clumps at various scales. When vortices are particularly long-lived, rings do not form but dust clumps inside vortices can survive a long time and would likely undergo collapse by gravitational instability. Rings encompass almost an Earth mass of solid material, while even larger masses of dust do accumulate inside vortices in the earlier stage. We argue that rapid planetesimal formation would occur in the dust clumps inside the vortices as well as in the post-vortex rings.« less

Dust Capture and Long-lived Density Enhancements Triggered by Vortices in 2D Protoplanetary Disks

NASA Astrophysics Data System (ADS)

Surville, Clément; Mayer, Lucio; Lin, Douglas N. C.

2016-11-01

We study dust capture by vortices and its long-term consequences in global two-fluid inviscid disk simulations using a new polar grid code RoSSBi. We perform the longest integrations so far, several hundred disk orbits, at the highest resolution attainable in global disk simulations with dust, namely, 2048 × 4096 grid points. We vary a wide range of dust parameters, most notably the initial dust-to-gas ratio ɛ varies in the range of 10-4-10-2. Irrespective of the value of ɛ, we find rapid concentration of the dust inside vortices, reaching dust-to-gas ratios of the order of unity inside the vortex. We present an analytical model that describes this dust capture process very well, finding consistent results for all dust parameters. A vortex streaming instability develops, which invariably causes vortex destruction. After vortex dissipation large-scale dust rings encompassing a disk annulus form in most cases, which sustain very high dust concentration, approaching ratios of the order of unity they persist as long as the duration of the simulations. They are sustained by a streaming instability, which manifests itself in high-density dust clumps at various scales. When vortices are particularly long-lived, rings do not form but dust clumps inside vortices can survive a long time and would likely undergo collapse by gravitational instability. Rings encompass almost an Earth mass of solid material, while even larger masses of dust do accumulate inside vortices in the earlier stage. We argue that rapid planetesimal formation would occur in the dust clumps inside the vortices as well as in the post-vortex rings.

Preventing Jupiter's Great Red Spot from Turning Itself Inside-Out

NASA Astrophysics Data System (ADS)

Shetty, Sushil; Asay-Davis, Xylar; Marcus, Philip

2002-11-01

Previous simulations of Jupiter's Great Red Spot (GRS) have failed to reproduce its most prominent feature: its hollowness. Unlike most laboratory 2D vortices, where vorticity is peaked at the center, the GRS has nearly no fluid motion in its interior. The coherent fluid motion is confined to a narrow ring at the outer edge of the GRS that moves counter-clockwise around the vortex. Simulations show that isolated 2D vortices that are as hollow as the GRS are violently unstable, turning themselves inside-out within a few rotations of the vortex. How than can one explain the long-lived, stable GRS? The answer is that the GRS is not isolated but instead embedded in a system of east-west jet streams. The eastward jet streams correspond to strong (potential) vorticity gradients which act as guides for Rossby waves. We show that the interaction between the GRS and the Rossby waves stabilize the GRS. Furthermore, we show that the hollowness of the GRS is near its critical limit, so that if it were any more hollow it would become unstable. We suggest a plausible mechanism through which this critical hollowness is maintained.

Prescribed Velocity Gradients for Highly Viscous SPH Fluids with Vorticity Diffusion.

PubMed

Peer, Andreas; Teschner, Matthias

2017-12-01

Working with prescribed velocity gradients is a promising approach to efficiently and robustly simulate highly viscous SPH fluids. Such approaches allow to explicitly and independently process shear rate, spin, and expansion rate. This can be used to, e.g., avoid interferences between pressure and viscosity solvers. Another interesting aspect is the possibility to explicitly process the vorticity, e.g., to preserve the vorticity. In this context, this paper proposes a novel variant of the prescribed-gradient idea that handles vorticity in a physically motivated way. In contrast to a less appropriate vorticity preservation that has been used in a previous approach, vorticity is diffused. The paper illustrates the utility of the vorticity diffusion. Therefore, comparisons of the proposed vorticity diffusion with vorticity preservation and additionally with vorticity damping are presented. The paper further discusses the relation between prescribed velocity gradients and prescribed velocity Laplacians which improves the intuition behind the prescribed-gradient method for highly viscous SPH fluids. Finally, the paper discusses the relation of the proposed method to a physically correct implicit viscosity formulation.

Wave theory of turbulence in compressible media (acoustic theory of turbulence)

NASA Technical Reports Server (NTRS)

Kentzer, C. P.

1975-01-01

The generation and the transmission of sound in turbulent flows are treated as one of the several aspects of wave propagation in turbulence. Fluid fluctuations are decomposed into orthogonal Fourier components, with five interacting modes of wave propagation: two vorticity modes, one entropy mode, and two acoustic modes. Wave interactions, governed by the inhomogeneous and nonlinear terms of the perturbed Navier-Stokes equations, are modeled by random functions which give the rates of change of wave amplitudes equal to the averaged interaction terms. The statistical framework adopted is a quantum-like formulation in terms of complex distribution functions. The spatial probability distributions are given by the squares of the absolute values of the complex characteristic functions. This formulation results in nonlinear diffusion-type transport equations for the probability densities of the five modes of wave propagation.

Aerodynamic characteristics of NACA RM-10 missile in 8- by 6-foot supersonic wind tunnel at Mach numbers from 1.49 to 1.98 I : presentation and analysis of pressure measurements (stabilizing fins removed)

NASA Technical Reports Server (NTRS)

Luidens, Roger W; Simon, Paul C

1950-01-01

Experimental investigation of flow about a slender body of revolution (NACA RM-10 missile) aligned and inclined to a supersonic stream was conducted at Mach numbers from 1.49 to 1.98 at a Reynolds number of approximately 30,000,000. Boundary-layer measurements at zero angle of attack are correlated with subsonic formulations for predicting boundary-layer thickness and profile. Comparison of pressure coefficients predicted by theory with experimental values showed close agreement at zero angle of attack and angle of attack except over the aft leeward side of body. At angle of attack, pitot pressure measurements in plane of model base indicated a pair of symmetrically disposed vortices on leeward side of body.

Development of a Vorticity-Velocity Navier-Stokes Formulation for the Study of Compressibility Effects on Dynamic Stall

NASA Technical Reports Server (NTRS)

Ghia, K. N.; Ghia, U.

1996-01-01

The first major area of this study was to develop a vorticity-velocity formulation and numerical solution algorithms suitable for the analyses of incompressible as well as low-to- moderate-speed compressible flows. Research performed towards contributing to the determination of the appropriate vorticity and dilation creation boundary conditions suggested to temporarily set aside this approach and use a primitive-variable approach other than the pseudo-compressibility approach used. The second major area of study was initiated to comprehensively examine the INS-2D and INS-3D programs from the point of view of boundary conditions. The research carried out was documented in the form of two technical papers which are included in Appendices A and B; the boundary-condition related issues for INS-3D are briefly mentioned.

Interaction of disturbances with an oblique detonation wave attached to a wedge

NASA Technical Reports Server (NTRS)

Lasseigne, D. G.; Hussaini, M. Y.

1993-01-01

The linear response of an oblique overdriven detonation to impose free stream disturbances or to periodic movements of the wedge is examined. The free stream disturbances are assumed to be steady vorticity waves and the wedge motions are considered to be time periodic oscillations either about a fixed pivot point or along the plane of symmetry of the wedge aligned with the incoming stream. The detonation is considered to be a region of infinitesimal thickness in which a finite amount of heat is released. The response to the imposed disturbances is a function of the Mach number of the incoming flow, the wedge angle, and the exothermocity of the reaction within the detonation. It is shown that as the degree of overdrive increases, the amplitude of the response increases significantly; furthermore, a fundamental difference in the dependence of the response on the parameters of the problem is found between the response to a free stream disturbance and to a disturbance emanating from the wedge surface.

Structure measurements in a synthetic turbulent boundary layer

NASA Astrophysics Data System (ADS)

Arakeri, Jaywant H.

1987-09-01

Extensive hot-wire measurements have been made to determine the structure of the large eddy in a synthejc turbulent boundary layer on a flat-plate model. The experiments were carried out in a wind tunnel at a nominal free-stream velocity of 12 m/s. The synthetic turbulent boundary layer had a hexagonal pattern of eddies and a ratio of streamwise scale to spanwise scale of 3.2:1. The measured celerity of the large eddy was 84.2 percent of the free-stream velocity. There was some loss of coherence, but very little distortion, as the eddies moved downstream. Several mean properties of the synthetic boundary layer were found to agree quite well with the mean properties of a natural turbulent boundary layer at the same Reynolds number. The large eddy is composed of a pair of primary counter-rotating vortices about five [...] long in the streamwise direction and about one [...] apart in the spanwise direction, where [...] is the mean boundary-layer thickness. The sense of the primary pair is such as to pump fluid away from the wall in the region between the vortices. A secondary pair of counter-rotating streamwise vortices, having a sense opposite to that of the primary pair, is observed outside of and slightly downstream from the primary vortices. Both pairs of vortices extend across the full thickness of the boundary layer and are inclined at a shallow angle to the surface of the flat plate. The data show that the mean vorticity vectors are not tangential to the large-eddy vortices. In fact, the streamwise and normal vorticity components that signal the presence of the eddy are of the same order of magnitude. Definite signatures are obtained in terms of the mean skin-friction coefficient and the mean wake parameter averaged at constant phase. Velocities induced by the vortices are partly responsible for entrainment of irrotational fluid, for transport of momentum, for generation of Reynolds stresses, and for maintenance of streamwise and normal vorticity in the outer flow. A stretching mechanism is important in matching spanwise vorticity close to the wall to variations in turbulent shearing stress. Regions where the stretching term is large coincide with regions of large wall shearing stress and large turbulence production.

Unsteady Heat Transfer in Channel Flow using Small-Scale Vorticity Concentrations Effected by a Vibrating Reed

NASA Astrophysics Data System (ADS)

Hidalgo, Pablo; Glezer, Ari

2011-11-01

Heat transfer enhancement by small-scale vorticity concentrations that are induced within the core flow of a mm-scale heated channel are investigated experimentally. These small-scale motions are engendered by the cross stream vibrations of a streamwise cantilevered reed that spans most of the channel's width. The interactions between the reed the core flow over a range of flow rates lead to the formation, shedding, and advection of time-periodic vorticity concentrations that interact with the wall boundary layers, and increase cross stream mixing of the core flow. Heating of the channel walls is controlled using microfabricated serpentine resistive heaters embedded with streamwise arrays of temperature sensors. It is shown that the actuation disrupts the thermal boundary layers and result in significant enhancement of the local and global heat transfer along the channel compared to the baseline flow in the absence of the reed. The effect of the reed on the cross flow is measured using high resolution particle image velocimetry (PIV), and the reed motion is characterized using a laser-based position sensor. The blockage induced by the presence of the reed and its cross stream motion is characterized using detailed streamwise pressure distributions. Supported by DARPA and UTRC.

Interhemispheric comparison of atmospheric circulation features as evaluated from Nimbus satellite data

NASA Technical Reports Server (NTRS)

Reiter, E. R.; Vonderhaar, T. H.; Adler, R. F.; Srivatsangam, S.; Fields, A.

1973-01-01

A relationship is established between relative geostrophic vorticity on an isobaric surface and the Laplacian of the underlying layer-mean temperature. This relationship is used to investigate the distribution of vorticity and baroclinicity in a jet-stream model which is constantly recurrent in the winter troposphere. The investigation shows that the baroclinic and vorticity fields of the extratropical troposphere must be bifurcated with two extrema in the middle and subpolar latitudes. This pattern is present in daily tropospheric meridional cross-sections. The reasons for the disappearance of bifurcation in the time-and-longitude averaged distributions are discussed.

Finiteness of corner vortices

NASA Astrophysics Data System (ADS)

Kalita, Jiten C.; Biswas, Sougata; Panda, Swapnendu

2018-04-01

Till date, the sequence of vortices present in the solid corners of steady internal viscous incompressible flows was thought to be infinite. However, the already existing and most recent geometric theories on incompressible viscous flows that express vortical structures in terms of critical points in bounded domains indicate a strong opposition to this notion of infiniteness. In this study, we endeavor to bridge the gap between the two opposing stream of thoughts by diagnosing the assumptions of the existing theorems on such vortices. We provide our own set of proofs for establishing the finiteness of the sequence of corner vortices by making use of the continuum hypothesis and Kolmogorov scale, which guarantee a nonzero scale for the smallest vortex structure possible in incompressible viscous flows. We point out that the notion of infiniteness resulting from discrete self-similarity of the vortex structures is not physically feasible. Making use of some elementary concepts of mathematical analysis and our own construction of diametric disks, we conclude that the sequence of corner vortices is finite.

Calculation of flow about two-dimensional bodies by means of the velocity-vorticity formulation on a staggered grid

NASA Technical Reports Server (NTRS)

Stremel, Paul M.

1991-01-01

A method for calculating the incompressible viscous flow about two-dimensional bodies, utilizing the velocity-vorticity form of the Navier-Stokes equations using a staggered-grid formulation is presented. The solution is obtained by employing an alternative-direction implicit method for the solution of the block tridiagonal matrix resulting from the finite-difference representation of the governing equations. The boundary vorticity and the conservation of mass are calculated implicitly as a part of the solution. The mass conservation is calculated to machine zero for the duration of the computation. Calculations for the flow about a circular cylinder, a 2-pct thick flat plate at 90-deg incidence, an elliptic cylinder at 45-deg incidence, and a NACA 0012, with and without a deflected flap, at - 90-deg incidence are performed and compared with the results of other numerical investigations.

Numerical simulation of the vortical flow around a pitching airfoil

NASA Astrophysics Data System (ADS)

Fu, Xiang; Li, Gaohua; Wang, Fuxin

2017-04-01

In order to study the dynamic behaviors of the flapping wing, the vortical flow around a pitching NACA0012 airfoil is investigated. The unsteady flow field is obtained by a very efficient zonal procedure based on the velocity-vorticity formulation and the Reynolds number based on the chord length of the airfoil is set to 1 million. The zonal procedure divides up the whole computation domain in to three zones: potential flow zone, boundary layer zone and Navier-Stokes zone. Since the vorticity is absent in the potential flow zone, the vorticity transport equation needs only to be solved in the boundary layer zone and Navier-Stokes zone. Moreover, the boundary layer equations are solved in the boundary layer zone. This arrangement drastically reduces the computation time against the traditional numerical method. After the flow field computation, the evolution of the vortices around the airfoil is analyzed in detail.

Numerical simulation using vorticity-vector potential formulation

NASA Technical Reports Server (NTRS)

Tokunaga, Hiroshi

1993-01-01

An accurate and efficient computational method is needed for three-dimensional incompressible viscous flows in engineering applications. On solving the turbulent shear flows directly or using the subgrid scale model, it is indispensable to resolve the small scale fluid motions as well as the large scale motions. From this point of view, the pseudo-spectral method is used so far as the computational method. However, the finite difference or the finite element methods are widely applied for computing the flow with practical importance since these methods are easily applied to the flows with complex geometric configurations. However, there exist several problems in applying the finite difference method to direct and large eddy simulations. Accuracy is one of most important problems. This point was already addressed by the present author on the direct simulations on the instability of the plane Poiseuille flow and also on the transition to turbulence. In order to obtain high efficiency, the multi-grid Poisson solver is combined with the higher-order, accurate finite difference method. The formulation method is also one of the most important problems in applying the finite difference method to the incompressible turbulent flows. The three-dimensional Navier-Stokes equations have been solved so far in the primitive variables formulation. One of the major difficulties of this method is the rigorous satisfaction of the equation of continuity. In general, the staggered grid is used for the satisfaction of the solenoidal condition for the velocity field at the wall boundary. However, the velocity field satisfies the equation of continuity automatically in the vorticity-vector potential formulation. From this point of view, the vorticity-vector potential method was extended to the generalized coordinate system. In the present article, we adopt the vorticity-vector potential formulation, the generalized coordinate system, and the 4th-order accurate difference method as the computational method. We present the computational method and apply the present method to computations of flows in a square cavity at large Reynolds number in order to investigate its effectiveness.

Three dimensional steady subsonic Euler flows in bounded nozzles

NASA Astrophysics Data System (ADS)

Chen, Chao; Xie, Chunjing

The existence and uniqueness of three dimensional steady subsonic Euler flows in rectangular nozzles were obtained when prescribing normal component of momentum at both the entrance and exit. If, in addition, the normal component of the voriticity and the variation of Bernoulli's function at the entrance are both zero, then there exists a unique subsonic potential flow when the magnitude of the normal component of the momentum is less than a critical number. As the magnitude of the normal component of the momentum approaches the critical number, the associated flows converge to a subsonic-sonic flow. Furthermore, when the normal component of vorticity and the variation of Bernoulli function are both small, the existence and uniqueness of subsonic Euler flows with non-zero vorticity are established. The proof of these results is based on a new formulation for the Euler system, a priori estimate for nonlinear elliptic equations with nonlinear boundary conditions, detailed study for a linear div-curl system, and delicate estimate for the transport equations.

Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South Atlantic?

NASA Astrophysics Data System (ADS)

van Sebille, Erik; Johns, William E.; Beal, Lisa M.

2012-05-01

As part of the global thermohaline circulation, some North Atlantic Deep Water (NADW) exits the Atlantic basin to the south of Africa. Observations have shown that there is a quasi-zonal pathway centered at 25°S carrying NADW eastward, connecting the Deep Western Boundary Current to the Cape Basin. However, it has been unclear what sets this pathway. In particular, waters must move southward through the Cape Basin, thereby crossing isolines of planetary vorticity, in order to exit the basin. Here, we find that an eddy thickness flux induced by Agulhas rings moving northwestward forces a circulation of NADW through the Cape Basin. The pathway at 25°S feeds the southeastward flow of this circulation while conserving potential vorticity. Using Lagrangian floats advected for 300 years in a 1/10° resolution ocean model, we show that the most common pathway for NADW in our model lies directly below the Agulhas ring corridor. By analyzing the velocity and density fields in the model, we find that the decay of these rings, and their forward tilt with depth, results in a southward velocity, across isolines of planetary vorticity, of 1 to 2 cm/s in the deep waters. The associated stream function pattern yields a deep circulation transporting 4 Sv of NADW from the Deep Western Boundary Current at 25°S to the southern tip of Africa.

Aerodynamic Applications of Boundary Layer Control Using Embedded Streamwise Vortices

DTIC Science & Technology

2003-07-01

section, 0.02% free-stream turbulence level, free-stream velocity up to 18 m/s; the strain gauge can be used for aerodynamic force measurements. (2...section, free-stream velocity up to 28 m/s; equipped with the 3-component strain gauge (values of streamwise and normal forces measured up to 3N and 6...dimensional model: test section of 4m x 2.5m x 5.5m, free-stream velocities up to 42 m/s, multi-base 6-component strain gauge. Project Manager: Nina F

Goertler vortices in growing boundary layers: The leading edge receptivity problem, linear growth and the nonlinear breakdown stage

NASA Technical Reports Server (NTRS)

Hall, Philip

1989-01-01

Goertler vortices are thought to be the cause of transition in many fluid flows of practical importance. A review of the different stages of vortex growth is given. In the linear regime, nonparallel effects completely govern this growth, and parallel flow theories do not capture the essential features of the development of the vortices. A detailed comparison between the parallel and nonparallel theories is given and it is shown that at small vortex wavelengths, the parallel flow theories have some validity; otherwise nonparallel effects are dominant. New results for the receptivity problem for Goertler vortices are given; in particular vortices induced by free stream perturbations impinging on the leading edge of the walls are considered. It is found that the most dangerous mode of this type can be isolated and it's neutral curve is determined. This curve agrees very closely with the available experimental data. A discussion of the different regimes of growth of nonlinear vortices is also given. Again it is shown that, unless the vortex wavelength is small, nonparallel effects are dominant. Some new results for nonlinear vortices of 0(1) wavelengths are given and compared to experimental observations.

Simulation of separated flow past a bluff body using Navier-Stokes equations

NASA Technical Reports Server (NTRS)

Ghia, K. N.; Ghia, U.; Osswald, G. A.; Liu, C. A.

1987-01-01

Two-dimensional flow past a bluff body is presently simulated on the basis of an analysis that employs the incompressible, unsteady Navier-Stokes equations in terms of vorticity and stream function. The fully implicit, time-marching, alternating-direction, implicit-block Gaussian elimination used is a direct method with second-order spatial accuracy; this allows it to avoid the introduction of any artificial viscosity. Attention is given to the simulation of flow past a circular cylinder with and without symmetry, requiring the use of either the half or the full cylinder, respectively.

Reducing the Drag and Damage of a High-Speed Train by Analyzing and Optimizing its Boundary Layer Separation and Roll-up into Wake Vortices

NASA Astrophysics Data System (ADS)

Jiang, Chung-Hsiang; Marcus, Philip

2012-11-01

We present numerical calculations of the boundary layers and shed wake vortices behind several aerodynamic bodies and generic models of high-speed trains. Our calculations illustrate new visual diagnostics that we developed that clearly show where the separation of a boundary layer occurs and where, how, and with what angles (with respect to the stream-wise direction) the wake vortices form. The calculations also illustrate novel 3D morphing and mesh ``pushing and pulling'' techniques that allow us to change the shapes of aerodynamic bodies and models in a controlled and automated manner without spurious features appearing. Using these tools we have examined the patterns of the shed vortices behind generic bodies and trains and correlated them with the changes in the drag as well as with the effects of the shed vortices on the environment. In particular, we have applied these techniques to the end car of a next-generation, high-speed train in order to minimize the drag and to minimize the adverse effects of the shed vortices on the track ballast.

Plasma Relaxation and Topological Aspects in Electron Magnetohydrodynamics

NASA Astrophysics Data System (ADS)

Shivamoggi, Bhimsen

2016-10-01

Parker's formulation of isotopological plasma relaxation process toward minimum magnetics energy states in magnetohydrodynamics (MHD) is extended to electron MHD (EMHD). The lower bound on magnetic energy in EMHD is determined by both the magnetic field and the electron vorticity field topologies, and is shown to be reduced further in EMHD by an amount proportional to the sum of total electron-flow kinetic energy and total electron-flow enstrophy. The EMHD Beltrami condition becomes equivalent to the potential vorticity conservation equation in two-dimensional (2D) hydrodynamics, and the torsion coefficient and turns out to be proportional to potential vorticity. The winding pattern of the magnetic field lines appears to evolve therefore in the same way as potential vorticity lines in 2D hydrodynamics.

Plasma relaxation and topological aspects in electron magnetohydrodynamics

NASA Astrophysics Data System (ADS)

Shivamoggi, B. K.

2016-07-01

Parker's formulation of isotopological plasma relaxation process toward minimum magnetics energy states in magnetohydrodynamics (MHD) is extended to electron MHD (EMHD). The lower bound on magnetic energy in EMHD is determined by both the magnetic field and the electron vorticity field topologies, and is shown to be reduced further in EMHD by an amount proportional to the sum of total electron-flow kinetic energy and total electron-flow enstrophy. The EMHD Beltrami condition becomes equivalent to the potential vorticity conservation equation in two-dimensional (2D) hydrodynamics, and the torsion coefficient α turns out to be proportional to potential vorticity. The winding pattern of the magnetic field lines appears to evolve, therefore, in the same way as potential vorticity lines in 2D hydrodynamics.

On th meridional surface profile of the Gulf Stream at 55 deg W

NASA Technical Reports Server (NTRS)

Hallock, Zachariah R.; Teague, William J.

1995-01-01

Nine-month records from nine inverted echo sounders (IESs) are analyzed to describe the mean baroclinic Gulf Stream at 55 deg W. IES acoustic travel times are converted to thermocline depth which is optimally interpolated. Kinematic and dynamic parameters (Gulf Stream meridional position, velocity, and vorticity) are calculated. Primary Gulf Stream variabiltiy is attributed to meandering and and changes in direction. A mean, stream-coordinate (relative to Gulf Stream instantaneous position and direction) meridional profile is derived and compared with results presented by other investigators. The mean velocity is estimated at 0.84 m/s directed 14 deg to the right eastward, and the thermocline (12 c) drops 657 m (north to south), corresponding to a baroclinic rise of the surface of 0.87 m. The effect of Gulf Stream curvature on temporal mean profiles is found to be unimportant and of minimal importance overall. The derived, downstream current profile is well represented by a Gaussian function and is about 190 km wide where it crosses zero. Surface baroclinic transport is estimated to be 8.5 x 10(exp 4) sq m/s, and maximum shear (flanking the maximum) is 1.2 x 10(exp -5). Results compare well with other in situ observational results from the same time period. On the other hand, analyses (by others) of concurrent satellite altimetry (Geosat) suggest a considerable narrower, more intense mean Gulf Stream.

Second-order dissipative hydrodynamics for plasma with chiral asymmetry and vorticity

NASA Astrophysics Data System (ADS)

Gorbar, E. V.; Rybalka, D. O.; Shovkovy, I. A.

2017-05-01

By making use of the chiral kinetic theory in the relaxation-time approximation, we derive an Israel-Stewart type formulation of the hydrodynamic equations for a chiral relativistic plasma made of neutral particles (e.g., neutrinos). The effects of chiral asymmetry are captured by including an additional continuity equation for the axial charge, as well as the leading-order quantum corrections due to the spin of particles. In a formulation of the chiral kinetic theory used, we introduce a symmetric form of the energy-momentum tensor that is suitable for the description of a weakly nonuniform chiral plasma. By construction, the energy and momentum are conserved to the same leading order in the Planck constant as the kinetic equation itself. By making use of such a chiral kinetic theory and the Chapman-Enskog approach, we obtain a set of second-order dissipative hydrodynamic equations. The effects of the fluid vorticity and velocity fluctuations on the dispersion relations of chiral vortical waves are analyzed.

Stability of barotropic vortex strip on a rotating sphere

PubMed Central

Sohn, Sung-Ik; Kim, Sun-Chul

2018-01-01

We study the stability of a barotropic vortex strip on a rotating sphere, as a simple model of jet streams. The flow is approximated by a piecewise-continuous vorticity distribution by zonal bands of uniform vorticity. The linear stability analysis shows that the vortex strip becomes stable as the strip widens or the rotation speed increases. When the vorticity constants in the upper and the lower regions of the vortex strip have the same positive value, the inner flow region of the vortex strip becomes the most unstable. However, when the upper and the lower vorticity constants in the polar regions have different signs, a complex pattern of instability is found, depending on the wavenumber of perturbations, and interestingly, a boundary far away from the vortex strip can be unstable. We also compute the nonlinear evolution of the vortex strip on the rotating sphere and compare with the linear stability analysis. When the width of the vortex strip is small, we observe a good agreement in the growth rate of perturbation at an early time, and the eigenvector corresponding to the unstable eigenvalue coincides with the most unstable part of the flow. We demonstrate that a large structure of rolling-up vortex cores appears in the vortex strip after a long-time evolution. Furthermore, the geophysical relevance of the model to jet streams of Jupiter, Saturn and Earth is examined. PMID:29507524

Stability of barotropic vortex strip on a rotating sphere.

PubMed

Sohn, Sung-Ik; Sakajo, Takashi; Kim, Sun-Chul

2018-02-01

We study the stability of a barotropic vortex strip on a rotating sphere, as a simple model of jet streams. The flow is approximated by a piecewise-continuous vorticity distribution by zonal bands of uniform vorticity. The linear stability analysis shows that the vortex strip becomes stable as the strip widens or the rotation speed increases. When the vorticity constants in the upper and the lower regions of the vortex strip have the same positive value, the inner flow region of the vortex strip becomes the most unstable. However, when the upper and the lower vorticity constants in the polar regions have different signs, a complex pattern of instability is found, depending on the wavenumber of perturbations, and interestingly, a boundary far away from the vortex strip can be unstable. We also compute the nonlinear evolution of the vortex strip on the rotating sphere and compare with the linear stability analysis. When the width of the vortex strip is small, we observe a good agreement in the growth rate of perturbation at an early time, and the eigenvector corresponding to the unstable eigenvalue coincides with the most unstable part of the flow. We demonstrate that a large structure of rolling-up vortex cores appears in the vortex strip after a long-time evolution. Furthermore, the geophysical relevance of the model to jet streams of Jupiter, Saturn and Earth is examined.

Partial-depth lock-release flows

NASA Astrophysics Data System (ADS)

Khodkar, M. A.; Nasr-Azadani, M. M.; Meiburg, E.

2017-06-01

We extend the vorticity-based modeling concept for stratified flows introduced by Borden and Meiburg [Z. Borden and E. Meiburg, J. Fluid Mech. 726, R1 (2013), 10.1017/jfm.2013.239] to unsteady flow fields that cannot be rendered quasisteady by a change of reference frames. Towards this end, we formulate a differential control volume balance for the conservation of mass and vorticity in the fully unsteady parts of the flow, which we refer to as the differential vorticity model. We furthermore show that with the additional assumptions of locally uniform parallel flow within each layer, the unsteady vorticity modeling approach reproduces the familiar two-layer shallow-water equations. To evaluate its accuracy, we then apply the vorticity model approach to partial-depth lock-release flows. Consistent with the shallow water analysis of Rottman and Simpson [J. W. Rottman and J. E. Simpson, J. Fluid Mech. 135, 95 (1983), 10.1017/S0022112083002979], the vorticity model demonstrates the formation of a quasisteady gravity current front, a fully unsteady expansion wave, and a propagating bore that is present only if the lock depth exceeds half the channel height. When this bore forms, it travels with a velocity that does not depend on the lock height and the interface behind it is always at half the channel depth. We demonstrate that such a bore is energy conserving. The differential vorticity model gives predictions for the height and velocity of the gravity current and the bore, as well as for the propagation velocities of the edges of the expansion fan, as a function of the lock height. All of these predictions are seen to be in good agreement with the direct numerical simulation data and, where available, with experimental results. An energy analysis shows lock-release flows to be energy conserving only for the case of a full lock, whereas they are always dissipative for partial-depth locks.

Measurements of Supersonic Wing Tip Vortices

NASA Technical Reports Server (NTRS)

Smart, Michael K.; Kalkhoran, Iraj M.; Benston, James

1994-01-01

An experimental survey of supersonic wing tip vortices has been conducted at Mach 2.5 using small performed 2.25 chords down-stream of a semi-span rectangular wing at angle of attack of 5 and 10 degrees. The main objective of the experiments was to determine the Mach number, flow angularity and total pressure distribution in the core region of supersonic wing tip vortices. A secondary aim was to demonstrate the feasibility of using cone probes calibrated with a numerical flow solver to measure flow characteristics at supersonic speeds. Results showed that the numerically generated calibration curves can be used for 4-hole cone probes, but were not sufficiently accurate for conventional 5-hole probes due to nose bluntness effects. Combination of 4-hole cone probe measurements with independent pitot pressure measurements indicated a significant Mach number and total pressure deficit in the core regions of supersonic wing tip vortices, combined with an asymmetric 'Burger like' swirl distribution.

On the inlet vortex system. [preventing jet engine damage caused by debris pick-up

NASA Technical Reports Server (NTRS)

Bissinger, N. C.; Braun, G. W.

1974-01-01

The flow field of a jet engine with an inlet vortex, which can pick up heavy debris from the ground and damage the engine, was simulated in a small water tunnel by means of the hydrogen bubble technique. It was found that the known engine inlet vortex is accompained by a vortex system, consisting of two inlet vortices (the ground based and the trailing one), secondary vortices, and ground vortices. Simulation of the ground effect by an inlet image proved that the inlet vortex feeds on free stream vorticity and can exist without the presence of a ground boundary layer. The structural form of the inlet vortex system was explained by a simple potential flow model, which showed the number, location, and the importance of the stagnation points. A retractable horizontal screen or an up-tilt of the engine is suggested as countermeasure against debris ingestion.

An implicit time-marching method for studying unsteady flow with massive separation

NASA Technical Reports Server (NTRS)

Osswald, G. A.; Ghia, K. N.; Chia, U.

1985-01-01

A fully implicit time-marching method is developed such that all spatial derivatives are approximated using central differences, but no use is made of any artificial dissipation. The numerical method solves the discretized equations using Alternating Direction Implicit-Block Gaussian Elimination technique. The method is implemented in the unsteady analysis, which solves the incompressible Navier-Stokes equations in terms of vorticity and stream function in generalized orthogonal coordinates. A clustered conformal C-grid is employed, and every effort is made to resolve the various length scales in the flow problem. The metric discontinuity at the branch-cut is treated appropriately using analytic continuation. Introduction of the BGE reordering permits implicit treatment of the branch cut in the numerical method. The vorticity singularity at the cusped trailing edge is also appropriately treated. This accurate and efficient implicit method is used to study flow at Re = 1000, past a 12-percent thick symmetric Joukowski airfoil at high angle of attack 30 and 53 deg.

Water Tunnel Flow Visualization Study Through Poststall of 12 Novel Planform Shapes

NASA Technical Reports Server (NTRS)

Gatlin, Gregory M.; Neuhart, Dan H.

1996-01-01

To determine the flow field characteristics of 12 planform geometries, a flow visualization investigation was conducted in the Langley 16- by 24-Inch Water Tunnel. Concepts studied included flat plate representations of diamond wings, twin bodies, double wings, cutout wing configurations, and serrated forebodies. The off-surface flow patterns were identified by injecting colored dyes from the model surface into the free-stream flow. These dyes generally were injected so that the localized vortical flow patterns were visualized. Photographs were obtained for angles of attack ranging from 10' to 50', and all investigations were conducted at a test section speed of 0.25 ft per sec. Results from the investigation indicate that the formation of strong vortices on highly swept forebodies can improve poststall lift characteristics; however, the asymmetric bursting of these vortices could produce substantial control problems. A wing cutout was found to significantly alter the position of the forebody vortex on the wing by shifting the vortex inboard. Serrated forebodies were found to effectively generate multiple vortices over the configuration. Vortices from 65' swept forebody serrations tended to roll together, while vortices from 40' swept serrations were more effective in generating additional lift caused by their more independent nature.

Near-body vorticity dynamics of a square cylinder subjected to an inline pulsatile free stream flow

NASA Astrophysics Data System (ADS)

Krishnan, Hrisheekesh; Agrawal, Amit; Sharma, Atul; Sheridan, John

2016-09-01

In the present work, the effect of an inflow sinusoidal excitation that is superimposed over the mean flow on the vortex-shedding characteristics of a square cylinder is studied. The frequency of pulsation is varied around the natural vortex-shedding frequency, and the amplitude of pulsation is varied moderately in comparison to the cylinder diameter, at a fixed Reynolds number (=100). A flow regime map is prepared and compared with the experimental results, which are available for a circular cylinder that is subjected to inline excitation. We correlate the spectra to the corresponding flow regime. Visualization of the vorticity contours reveals that the significant interaction of the base-region vorticities with the main shear layer vorticities is important in the mechanism of formation of the several vortex-shedding modes. The strength and sign of base region vorticity with respect to the shear layers has a fundamental role to play in the mechanism of formation. It is hypothesized that the similarity in vortex-shedding modes across different excitation types, bluff body geometry, and for different parameters is due to the similarity in the underlying vorticity dynamics.

On the development of lift and drag in a rotating and translating cylinder

NASA Astrophysics Data System (ADS)

Martin-Alcantara, Antonio; Sanmiguel-Rojas, Enrique; Fernandez-Feria, Ramon

2014-11-01

The two-dimensional flow around a rotating cylinder is investigated numerically using a vorticity forces formulation with the aim of analyzing the flow structures, and their evolutions, that contribute to the lift and drag forces on the cylinder. The Reynolds number, based on the cylinder diameter and steady free-stream speed, considered is Re = 200 , while the non-dimensional rotation rate (ratio of the surface speed and free-stream speed) selected were α = 1 and 3. For α = 1 the wake behind the cylinder for the fully developed flow is oscillatory due to vortex shedding, and so are the lift and drag forces. For α = 3 the fully developed flow is steady with constant (high) lift and (low) drag. Each of these cases is considered in two different transient problems, one with angular acceleration of the cylinder and constant speed, and the other one with translating acceleration of the cylinder and constant rotation. Special attention is paid to explaining the mechanisms of vortex shedding suppression for high rotation (when α = 3) and its relation to the mechanisms by which the lift is enhanced and the drag is almost suppressed when the fully developed flow is reached. Supported by the Ministerio de Economia y Competitividad of Spain Grant No. DPI2013-40479-P.

The Flow Field Downstream of a Dynamic Low Aspect Ratio Circular Cylinder: A Parametric Study

NASA Astrophysics Data System (ADS)

Gildersleeve, Samantha; Dan, Clingman; Amitay, Michael

2015-11-01

Flow past a static, low aspect ratio cylinder (pin) has shown the formation of vortical structures, namely the horseshoe and arch-type vortex. These vortical structures may have substantial effects in controlling flow separation over airfoils. In the present experiments, the flow field associated with a low aspect ratio cylinder as it interacts with a laminar boundary layer under static and dynamic conditions was investigated through a parametric study over a flat plate. As a result of the pin being actuated in the wall-normal direction, the structures formed in the wake of the pin were seen to be a strong function of actuation amplitude, driving frequency, and aspect ratio of the cylinder. The study was conducted at a Reynolds number of 1875, based on the local boundary layer thickness, with a free stream velocity of 10 m/s. SPIV data were collected for two aspect ratios of 0.75 and 1.125, actuation amplitudes of 6.7% and 16.7%, and driving frequencies of 175 Hz and 350 Hz. Results indicate that the presence and interactions between vortical structures are altered in comparison to the static case and suggest increased large-scale mixing when the pin is driven at the shedding frequency (350 Hz). Supported by the Boeing Company.

A theoretical formulation of wave-vortex interactions

NASA Technical Reports Server (NTRS)

Wu, J. Z.; Wu, J. M.

1989-01-01

A unified theoretical formulation for wave-vortex interaction, designated the '(omega, Pi) framework,' is presented. Based on the orthogonal decomposition of fluid dynamic interactions, the formulation can be used to study a variety of problems, including the interaction of a longitudinal (acoustic) wave and/or transverse (vortical) wave with a main vortex flow. Moreover, the formulation permits a unified treatment of wave-vortex interaction at various approximate levels, where the normal 'piston' process and tangential 'rubbing' process can be approximated dfferently.

Vortex shedding from a blunt trailing edge with equal and unequal external mean velocities

NASA Technical Reports Server (NTRS)

Brinich, P. F.; Boldman, D. R.; Goldstein, M. E.

1975-01-01

A flow visualization study showed that strong Karman vortices are developed behind the blunt trailing edge of a plate when the free stream velocities over both surfaces are equal. These vortices tend to disappear when the surface velocities are unequal. This observation provided an explanation for the occurrence and disappearance of the lip noise often present in coaxial jets. Vortex formation and lip noise occurred at a Strouhal number of about 0.2 based on the lip thickness and the average of the external steady-state velocities. Results from theoretical calculations of the vortex formation, based on an inviscid, incompressible analysis of the motion of point vortices, were in good agreement with the experimental observations.

Tip Vortices of Isolated Wings and Helicopter Rotor Blades.

DTIC Science & Technology

1987-12-01

root to tip, as expected due to the induced downwash of the tip vor- tex and wake vortex sheet. Although the three different tip-caps produce very...the inherent limitation of not being able to model the vortex wake with these equations, although the Euler formulation has in it the necessary...physics to model vorticity transport correctly. These equations basically lack the physical mecha- nism needed to generate the vortex wake . However, in

Numerical and experimental study of a hydrodynamic cavitation tube

NASA Astrophysics Data System (ADS)

Hu, H.; Finch, J. A.; Zhou, Z.; Xu, Z.

1998-08-01

A numerical analysis of hydrodynamics in a cavitation tube used for activating fine particle flotation is described. Using numerical procedures developed for solving the turbulent k-ɛ model with boundary fitted coordinates, the stream function, vorticity, velocity, and pressure distributions in a cavitation tube were calculated. The calculated pressure distribution was found to be in excellent agreement with experimental results. The requirement of a pressure drop below approximately 10 m water for cavitation to occur was observed experimentally and confirmed by the model. The use of the numerical procedures for cavitation tube design is discussed briefly.

From catʼs eyes to multiple disjoint natural convection flow in tall tilted cavities: A direct primitive variables approach

NASA Astrophysics Data System (ADS)

Báez, Elsa; Nicolás, Alfredo

2013-11-01

Natural convection fluid flow in air-filled tall tilted cavities is studied numerically with a new direct projection method on the Boussinesq approximation in primitive variables. The study deals with “cat's eyes” instabilities and multiple disjoint cells as the aspect ratio A and the angle of inclination ϕ of the cavity vary. The flows are validated with those reported before using the stream function-vorticity variables. New cases, A=12 and 20 varying ϕ, lead to get more insight on the physical phenomenon.

Distribution of Circles on a Circle and Correlation Between Vortex Rings of Superfluids

NASA Astrophysics Data System (ADS)

Onur Fen, Mehmet; Erkoç, Šakír

2007-05-01

Superfluids are characterized by absence of viscosity. When superfluids are rotated, differently from normal fluids, they form more than one vortex in the containers where they are placed. The number of vortices change as the rotation velocity changes, but this change is not linear. M.W. Zwierlein et al. observed the vortices in experiments, observing up to a number of 80. Experiments also showed that the vortex distributions cannot include large spaces. By using experimental data, we noticed that when we think of vortices as vortex rings, their centers are at the same geometric location and these geometric locations are concentric circles. We generalized the distribution of these geometric places and formulized it. Our formula includes the magic circle numbers. When the number of vortices reach these magic numbers, the number of geometric locations increase by 1.

Formulation of Efficient Finite Element Prediction Models.

DTIC Science & Technology

1980-01-01

vorticity-divergence FEM formulation. This paper will compare these FEM formulations by considering the Vgeostrophic adjustment process with the linearized...by Fourier transforming the terms that are independent of t in (2.12)-(2.14) or (2.19)-(2.21). However, in this paper the final state will be...filtering in a baroclinic primitive equation model. 17 L . , 5. Conclusions The objective of this paper is to determine the response of various finite

Acoustic streaming related to minor loss phenomenon in differentially heated elements of thermoacoustic devices

NASA Astrophysics Data System (ADS)

Mironov, Mikhail; Gusev, Vitalyi; Auregan, Yves; Lotton, Pierrick; Bruneau, Michel; Piatakov, Pavel

2002-08-01

It is demonstrated that the differentially heated stack, the heart of all thermoacoustic devices, provides a source of streaming additional to those associated with Reynolds stresses in quasi-unidirectional gas flow. This source of streaming is related to temperature-induced asymmetry in the generation of vortices and turbulence near the stack ends. The asymmetry of the hydrodynamic effects in an otherwise geometrically symmetric stack is due to the temperature difference between stack ends. The proposed mechanism of streaming excitation in annular thermoacoustic devices operates even in the absence of thermo-viscous interaction of sound waves with resonator walls. copyright 2002 Acoustical Society of America.

Propagation and scattering of acoustic-vorticity waves in annular swirling flows

NASA Astrophysics Data System (ADS)

Golubev, Vladimir Viktorovich

1997-08-01

The dissertation presents a fundamental extension of unsteady aerodynamic theory developed to predict fluctuating forces on aircraft structural components. These excitations may result from a variety of upstream flow non-uniformities such as atmospheric turbulence, airframe tip vortices and wakes, engine inlet distortions and secondary flows. In the frame of reference of a downstream aircraft component, an upstream flow non- uniformity appears as a propagating vorticity wave (a gust). Classical treatment of gust interaction problems developed for uniform, potential upstream mean flows is based on the fact that it is possible to consider separately incident or scattered acoustic, entropic and vortical modes of unsteady flow motion. A purely vortical gust remains 'frozen' as it convects with the flow. The coupling between different unsteady components may occur only at the surface of a solid structure, or in the close vicinity of a lifting body. The classical approach, however, is not justified for an aircraft engine system where the internal turbomachinery flow is non-uniform and non-potential as it exhibits a strong swirling motion. In such a flow, acting centrifugal and Coriolis forces couple the various unsteady modes which thus can no longer be determined independently of each other. The new developed theory follows the decomposition of unsteady velocity field into vortical and potential components. In spite of the modal coupling, this decomposition elucidates the physical phenomena associated with unsteady swirling motion by indicating the degree of interaction between the various modes. It paves the way for generalizing the classical definition of a gust for vortical swirling flows. The concept of a generalized gust is developed based on the eigenmode pseudospectral analysis of the coupled equations of unsteady swirling motion. This analysis reveals two distinct regions of eigenvalues corresponding to pressure-dominated nearly-sonic and vorticity- dominated nearly-convected eigenmodes. A compact discrete spectrum of nearly-convected eigenvalues clusters with infinitely increasing density approaching an accumulation convected critical layer. The generalized gust is then identified with the nearly-convected eigenspectrum and formulated in terms of a non-amplifying nearly-convected wave and an instability wave growing in the critical layer. Based on the generalized gust model, a boundary-value problem of unsteady three-dimensional acoustic-vorticity waves propagating in a vortical swirling flow and impinging on a turbomachinery blading is formulated and solved numerically. A set of benchmark results reveals a significant effect of swirling flow motion on aerodynamic and acoustic response of the annular cascade.

Numerical investigation of MHD flow of blood and heat transfer in a stenosed arterial segment

NASA Astrophysics Data System (ADS)

Majee, Sreeparna; Shit, G. C.

2017-02-01

A numerical investigation of unsteady flow of blood and heat transfer has been performed with an aim to provide better understanding of blood flow through arteries under stenotic condition. The blood is treated as Newtonian fluid and the arterial wall is considered to be rigid having deposition of plaque in its lumen. The heat transfer characteristic has been analyzed by taking into consideration of the dissipation of energy due to applied magnetic field and the viscosity of blood. The vorticity-stream function formulation has been adopted to solve the problem using implicit finite difference method by developing well known Peaceman-Rachford Alternating Direction Implicit (ADI) scheme. The quantitative profile analysis of velocity, temperature and wall shear stress as well as Nusselt number is carried out over the entire arterial segment. The streamline and temperature contours have been plotted to understand the flow pattern in the diseased artery, which alters significantly in the downstream of the stenosis in the presence of magnetic field. Both the wall shear stress and Nusselt number increases with increasing magnetic field strength. However, wall shear stress decreases and Nusselt number enhances with Reynolds number. The results show that with an increase in the magnetic field strength upto 8 T, does not causes any damage to the arterial wall, but the study is significant for assessing temperature rise during hyperthermic treatment.

Three-Dimensional Structure of the Circulation Induced by a Shoaling Topographic Wave

NASA Astrophysics Data System (ADS)

Mizuta, G.; Hogg, N. G.

2003-12-01

Rectification of Rossby wave energy has been proposed as a mechanism for the maintenance of the recirculation cell of the Gulf Stream (Hogg 1988; Rizzoli et al. 1995). We investigated the three-dimensional structure of potential-vorticity flux and a mean flow induced by a topographic wave incident over a bottom slope analytically and numerically, focusing on the limit that bottom friction is the dominant dissipation process. In this limit it is shown that the topographic wave cannot be a steady source of the potential vorticity outside the bottom Ekman layer. Instead, the distribution of potential vorticity is determined from the initial transient of the topographic wave. This potential vorticity and the heat flux by the topographic wave at the bottom determine the mean flow, and give a relation between the horizontal and vertical scales of the mean flow. When the horizontal scale of the mean flow is larger than the internal deformation radius, the mean flow is almost constant with depth independent of whether or not the topographic wave is trapped near the bottom. Then the mean flow at the bottom is proportional to the divergence of vertically integrated Reynolds stress ∫ -D0 /line{u'v'} dz. This divergence, which is caused by bottom friction, is large when the group velocity, cg and the vertical scale, μ -1 of the wave motion are small. Thus the mean flow tends to be large where cg and μ -1 become small, and decreases as the topographic wave is dissipated by bottom friction. Since bottom friction also dissipates the mean flow, the mean flow asymptotes to a constant value as the friction becomes zero. These features of the potential-vorticity flux and the mean flow are reproduced in numerical experiments. It is also shown from the numerical experiment that the distribution of the mean flow depends on the amplitude of the wave because of the Doppler shift of the wave by the mean flow. These feature of the mean flow are preserved when we used stratification and bottom topography resembling to those over the continental slope near the Gulf Stream. The transport of the mean flow is about 20 Sv when the wave amplitude is about 2 cm/s. These numbers are similiar to those observed in the Gulf Stream region.

An experimental study of the effect of streamwise vorticity on supersonic mixing enhancement

NASA Technical Reports Server (NTRS)

Naughton, J. W.; Cattafesta, L. N.; Settles, G. S.

1989-01-01

An initial experimental study of the effect of streamwise vorticity on supersonic turbulent mixing has been carried out. A Mach 3 streamwise vortex is generated using a strutmounted swirl injector and is injected into a Mach 3.5 freestream. The resulting flowfield is investigated using both five-hole angularity probe and total temperature probe surveys. The results are compared to identical experiments with a baseline, swirl-free Mach 3 jet. Laser Light Sheet (LLS) images are used to observe the mixing phenomena. The entrainment of energy and mass is used to evaluate the degree of mixing between the two streams for both the vortex and jet cases. The results reveal that streamwise vorticity does lead to a modest mixing enhancement of about 34 percent for the conditions tested.

On the dynamics of stream piracy

NASA Astrophysics Data System (ADS)

Goren, L.; Willett, S. D.

2012-04-01

Drainage network reorganization by stream piracy is invoked repeatedly to explain the morphology of unique drainage patterns and as a possible mechanism inducing abrupt variations of sediment accumulation rates. However, direct evidence of stream piracy is usually rare, and is highly interpretation dependent. As a first step in assessing how probable capture events are and establishing the conditions that favor stream piracy versus the those that favor stable landscapes, we formulate analytically the physics of divide migration and capture events and study this formulation from a dynamical system point of view. The formulation is based on a one-dimensional topographic cross section between two channels that share a water divide. Two hillslope profiles diverge from the divide and drain into two fluvial bedrock tributaries, whose erosion rate is controlled by a stream power law. The rate of erosion at the bounding channels is thus a function of the upstream drainage area and local slope. A tectonically induced downward perturbation of the elevation of one of the bounding channels lowers the channel slope but at the same time increases the drainage area due to outward migration of the water divide. The changes in slope and area have opposing effect on the erosion rate at the bounding channels, so that the perturbation may either grow or be damped. We define the geomorphic and tectonic parameters that control the behavior of the system and find the regimes that lead to stable landscapes and to capture events.

Navy Tactical Applications Guide. Volume 2. Environmental Phenomena and Effects

DTIC Science & Technology

1979-01-01

usually distinguished: the polar-front jet stream, associated with extratropical frontal systems; and the subtropical jet stream, overlying the poleward...patterns have formed in the cold air behind a frontal cloud band which extends from North Africa into Southern Europe . Note that the cellular cloud field...but because of the future potential of such areas for rapid storm " , development. (See Case 3 for the further development of these vorticity centers

Time resolved flow-field measurements of a turbulent mixing layer over a rectangular cavity

NASA Astrophysics Data System (ADS)

Bian, Shiyao; Driscoll, James F.; Elbing, Brian R.; Ceccio, Steven L.

2011-07-01

High Reynolds number, low Mach number, turbulent shear flow past a rectangular, shallow cavity has been experimentally investigated with the use of dual-camera cinematographic particle image velocimetry (CPIV). The CPIV had a 3 kHz sampling rate, which was sufficient to monitor the time evolution of large-scale vortices as they formed, evolved downstream and impinged on the downstream cavity wall. The time-averaged flow properties (velocity and vorticity fields, streamwise velocity profiles and momentum and vorticity thickness) were in agreement with previous cavity flow studies under similar operating conditions. The time-resolved results show that the separated shear layer quickly rolled-up and formed eddies immediately downstream of the separation point. The vortices convect downstream at approximately half the free-stream speed. Vorticity strength intermittency as the structures approach the downstream edge suggests an increase in the three-dimensionality of the flow. Time-resolved correlations reveal that the in-plane coherence of the vortices decays within 2-3 structure diameters, and quasi-periodic flow features are present with a vortex passage frequency of ~1 kHz. The power spectra of the vertical velocity fluctuations within the shear layer revealed a peak at a non-dimensional frequency corresponding to that predicted using linear, inviscid instability theory.

Chaotic vortex filaments in a Bose–Einstein condensate and in superfluid helium

NASA Astrophysics Data System (ADS)

Nemirovskii, S. K.

2018-05-01

A statement of the quantum turbulence problem in both a Bose–Einstein condensate (BEC) and superfluid helium is formulated. In superfluid helium use is made of a so-called vortex filament method, in which quantum vortices are represented by stringlike objects, i.e. vortex lines. The dynamics of the vortex lines is determined by deterministic equations of motion, supplemented by random reconnections. Unlike He II, the laws of the dynamics of quantum vortices in BEC are based on the nonlinear Schrödinger equation. This makes it possible to obtain a microscopic description of the collision of vortices, the structure of a vortex filament, etc. A comparative analysis of these complementary approaches is carried out. It is shown that there are some features that do not automatically transfer the results obtained for BEC to vortices in He II and vice versa.

Crossflow effects on the growth rate of inviscid Goertler vortices in a hypersonic boundary layer

NASA Technical Reports Server (NTRS)

Fu, Yibin; Hall, Philip

1992-01-01

The effects of crossflow on the growth rate of inviscid Goertler vortices in a hypersonic boundary layer with pressure gradient are studied. Attention is focused on the inviscid mode trapped in the temperature adjustment layer; this mode has greater growth rate than any other mode. The eigenvalue problem which governs the relationship between the growth rate, the crossflow amplitude, and the wavenumber is solved numerically, and the results are then used to clarify the effects of crossflow on the growth rate of inviscid Goertler vortices. It is shown that crossflow effects on Goertler vortices are fundamentally different for incompressible and hypersonic flows. The neutral mode eigenvalue problem is found to have an exact solution, and as a by-product, we have also found the exact solution to a neutral mode eigenvalue problem which was formulated, but unsolved before, by Bassom and Hall (1991).

Coherent structures in bypass transition induced by a cylinder wake

NASA Astrophysics Data System (ADS)

Pan, Chong; Wang, Jin Jun; Zhang, Pan Feng; Feng, Li Hao

Flat-plate boundary layer transition induced by the wake vortex of a two-dimensional circular cylinder is experimentally investigated. Combined visualization and velocity measurements show a different transition route from the Klebanoff mode in free-stream turbulence-induced transition. This transition scenario is mainly characterized as: (i) generation of secondary transverse vortical structures near the flat plate surface in response to the von Kn vortex street of the cylinder; (ii) formation of hairpin vortices due to the secondary instability of secondary vortical structures; (iii) growth of hairpins which is accelerated by wake-vortex induction; (iv) formation of hairpin packets and the associated streaky structures. Detailed investigation shows that during transition the evolution dynamics and self-sustaining mechanisms of hairpins, hairpin packets and streaks are consistent with those in a turbulent boundary layer. The wake vortex mainly plays the role of generating and destabilizing secondary transverse vortices. After that, the internal mechanisms become dominant and lead to the setting up of a self-sustained turbulent boundary layer.

Secondary Instability of Stationary Crossflow Vortices in Mach 6 Boundary Layer Over a Circular Cone

NASA Technical Reports Server (NTRS)

Li, Fei; Choudhari, Meelan M.; Paredes-Gonzalez, Pedro; Duan, Lian

2015-01-01

Hypersonic boundary layer flows over a circular cone at moderate incidence can support strong crossflow instability. Due to more efficient excitation of stationary crossflow vortices by surface roughness, such boundary layer flows may transition to turbulence via rapid amplification of the high-frequency secondary instabilities of finite amplitude stationary crossflow vortices. The amplification characteristics of these secondary instabilities are investigated for crossflow vortices generated by an azimuthally periodic array of roughness elements over a 7-degree half-angle circular cone in a Mach 6 free stream. Depending on the local amplitude of the stationary crossflow mode, the most unstable secondary disturbances either originate from the second (i.e., Mack) mode instabilities of the unperturbed boundary layer or correspond to genuine secondary instabilities that reduce to stable disturbances at sufficiently small amplitudes of the stationary crossflow vortex. The predicted frequencies of dominant secondary disturbances are similar to those measured during wind tunnel experiments at Purdue University and the Technical University of Braunschweig, Germany.

A three-dimensional dual potential procedure with applications to wind tunnel inlets and interacting boundary layers

NASA Technical Reports Server (NTRS)

Rao, K. V.; Pletcher, R. H.; Steger, J. L.; Vandalsem, W. R.

1987-01-01

A dual potential decomposition of the velocity field into a scalar and a vector potential function is extended to three dimensions and used in the finite-difference simulation of steady three-dimensional inviscid rotational flows and viscous flow. The finite-difference procedure was used to simulate the flow through the 80 by 120 ft wind tunnel at NASA Ames Research Center. Rotational flow produced by the stagnation pressure drop across vanes and screens which are located at the entrance of the inlet is modeled using actuator disk theory. Results are presented for two different inlet vane and screen configurations. The numerical predictions are in good agreement with experimental data. The dual potential procedure was also applied to calculate the viscous flow along two and three dimensional troughs. Viscous effects are simulated by injecting vorticity which is computed from a boundary layer algorithm. For attached flow over a three dimensional trough, the present calculations are in good agreement with other numerical predictions. For separated flow, it is shown from a two dimensional analysis that the boundary layer approximation provides an accurate measure of the vorticity in regions close to the wall; whereas further away from the wall, caution has to be exercised in using the boundary-layer equations to supply vorticity to the dual potential formulation.

Development and application of a method for predicting rotor free wake positions and resulting rotor blade air loads. Volume 1: Model and results

NASA Technical Reports Server (NTRS)

Sadler, S. G.

1971-01-01

Rotor wake geometries are predicted by a process similar to the startup of a rotor in a free stream. An array of discrete trailing and shed vortices is generated with vortex strengths corresponding to stepwise radial and azimuthal blade circulations. The array of shed and trailing vortices is limited to an arbitrary number of azimuthal steps behind each blade. The remainder of the wake model of each blade is an arbitrary number of trailing vortices. Vortex element end points were allowed to be transported by the resultant velocity of the free stream and vortex-induced velocities. Wake geometry, wake flow, and wake-induced velocity influence coefficients are generated by this program for use in the blade loads portion of the calculations. Blade loads computations include the effects of nonuniform inflow due to a free wake, nonlinear airfoil characteristics, and response of flexible blades to the applied loads. Computed wake flows and blade loads are compared with experimentally measured data. Predicted blade loads, response and shears and moments are obtained for a model rotor system having two independent rotors. The effects of advance ratio, vertical separation of rotors, different blade radius ratios, and different azimuthal spacing of the blades of one rotor with respect to the other are investigated.

Investigation of Hypersonic Laminar Heating Augmentation in the Stagnation Region

NASA Technical Reports Server (NTRS)

Marineau, Eric C.; Lewis, Daniel R.; Smith, Michael S.; Lafferty, John F.; White, Molly E.; Amar, Adam J.

2012-01-01

Laminar stagnation region heating augmentation is investigated in the AEDC Tunnel 9 at Mach 10 by performing high frequency surface pressure and heat transfer measurements on the Orion CEV capsule at zero degree angle-of-attack for unit Reynolds numbers between 0.5 and 15 million per foot. Heating augmentation increases with Reynolds number, but is also model size dependent as it is absent on a 1.25-inch diameter model at Reynolds numbers where it reaches up to 15% on a 7-inch model. Heat transfer space-time correlations on the 7-inch model show that disturbances convect at the boundary layer edge velocity and that the streamwise integral scale increases with distance. Therefore, vorticity amplification due to stretching and piling-up in the stagnation region appears to be responsible for the stagnation point heating augmentation on the larger model. This assumption is reinforced by the f(exp -11/3) dependence of the surface pressure spectrum compared to the f(exp -1) dependence in the free stream. Vorticity amplification does not occur on the 1.25- inch model because the disturbances are too large. Improved free stream fluctuation measurements will be required to determine if significant vorticity is present upstream or mostly generated behind the bow shock.

Heat transfer with very high free-stream turbulence and streamwise vortices

NASA Technical Reports Server (NTRS)

Moffat, Robert J.; Maciejewski, Paul; Eaton, John K.; Pauley, Wayne

1986-01-01

Results are presented for two experimental programs related to augmentation of heat transfer by complex flow characteristics. In one program, high free stream turbulence (up to 63 percent) was shown to increase the Stanton number by more than a factor of 5, compared with the normally expected value based on x-Reynolds number. These experiments are being conducted in a free-jet facility, near the margins of the jet. To a limited extent, the mean velocity, turbulence intensity, and integral length scale can be separately varied. The results show that scale is a very important factor in determining the augmentation. Detailed studies of the turbulence structure are being carried out using an orthogonal triple hot-wire anemometer equipped with a fourth wire for measuring temperature. The v' component of turbulence appears to be distributed differently from u' or w'. In the second program, the velocity distributions and boundary layer thicknesses associated with a pair of counter-rotating, streamwise vortices were measured. There is a region of considerably thinned boundary layer between the two vortices when they are of approximately the same strength. If one vortex is much stronger than the other, the weaker vortex may be lifted off the surface and absorbed into the stronger.

Tomo-PIV measurements of the flow field in the wake of a sphere

NASA Astrophysics Data System (ADS)

Eshbal, Lior; David, Tom; Rinsky, Vladislav; van Hout, Rene; Greenblatt, David

2017-11-01

A sphere can be considered as a prototypical 3D bluff body. In order to improve our understanding of its 3D wake flow, a combination of time-resolved planar particle image velocimetry (PIV) and tomographic PIV (tomo-PIV) was implemented. Experiments were performed in a closed-loop water channel facility and sphere Reynolds numbers ReD = UD/ ν = 400, 800, 1200 and 2000, where U is the free-stream velocity, ν the kinematic viscosity and D the sphere diameter. The measurement volume (Height x Length x Width, 5 x 5 x 1.5 D3) comprised the sphere and the downstream wake. Tomo-PIV snap-shots were correlated with the time-resolved PIV such that the 3D temporal evolution of the shed vortices became clear. At ReD = 400, this procedure revealed shed hairpin vortices having a vertical plane of symmetry in agreement with many dye visualization studies. However, the measurements also revealed weaker induced hairpins resulting from the interaction of the near-wake flow and the surrounding free stream. These induced vortices were not visible in previous dye and smoke visualizations and have only been observed in simulations. Data processing of the data at higher ReD is currently ongoing. Israel Science Foundation Grant No. 1596/14.

An error analysis of least-squares finite element method of velocity-pressure-vorticity formulation for Stokes problem

NASA Technical Reports Server (NTRS)

Chang, Ching L.; Jiang, Bo-Nan

1990-01-01

A theoretical proof of the optimal rate of convergence for the least-squares method is developed for the Stokes problem based on the velocity-pressure-vorticity formula. The 2D Stokes problem is analyzed to define the product space and its inner product, and the a priori estimates are derived to give the finite-element approximation. The least-squares method is found to converge at the optimal rate for equal-order interpolation.

Vortices and quasiparticles near the superconductor-insulator transition in thin films.

PubMed

Galitski, Victor M; Refael, G; Fisher, Matthew P A; Senthil, T

2005-08-12

We study the low temperature behavior of an amorphous superconducting film driven normal by a perpendicular magnetic-field (B). For this purpose we introduce a new two-fluid formulation consisting of fermionized field-induced vortices and electrically neutralized Bogoliubov quasiparticles (spinons) interacting via a long-ranged statistical interaction. This approach allows us to access a novel non-Fermi-liquid phase, which naturally interpolates between the low B superconductor and the high B normal metal. We discuss the properties of the resulting "vortex metal" phase.

A dual potential formulation of the Navier-Stokes equations

NASA Technical Reports Server (NTRS)

Gegg, S. G.; Pletcher, R. H.; Steger, J. L.

1989-01-01

A dual potential formulation for numerically solving the Navier-Stokes equations is developed and presented. The velocity field is decomposed using a scalar and vector potential. Vorticity and dilatation are used as the dependent variables in the momentum equations. Test cases in two dimensions verify the capability to solve flows using approximations from potential flow to full Navier-Stokes simulations. A three-dimensional incompressible flow formulation is also described. An interesting feature of this approach to solving the Navier-Stokes equations is the decomposition of the velocity field into a rotational part (vector potential) and an irrotational part (scalar potential). The Helmholtz decomposition theorem allows this splitting of the velocity field. This approach has had only limited use since it increases the number of dependent variables in the solution. However, it has often been used for incompressible flows where the solution scheme is known to be fast and accurate. This research extends the usage of this method to fully compressible Navier-Stokes simulations by using the dilatation variable along with vorticity. A time-accurate, iterative algorithm is used for the uncoupled solution of the governing equations. Several levels of flow approximation are available within the framework of this method. Potential flow, Euler and full Navier-Stokes solutions are possible using the dual potential formulation. Solution efficiency can be enhanced in a straightforward way. For some flows, the vorticity and/or dilatation may be negligible in certain regions (e.g., far from a viscous boundary in an external flow). It is possible to drop the calculation of these variables then and optimize the solution speed. Also, efficient Poisson solvers are available for the potentials. The relative merits of non-primitive variables versus primitive variables for solution of the Navier-Stokes equations are also discussed.

Tip Vortex Cavitation

NASA Astrophysics Data System (ADS)

Maines, Brant H.; Arndt, Roger E. A.

2000-11-01

Cavitation in vortical flows is a problem of practical importance, that is relatively unexplored. Vortical structures of importance range from the eddies occurring randomly in space and time in turbulent flows to the developed vortices that occur at the tips of lifting surfaces and at the hubs of propellers and hydraulic turbines. A variety of secondary flow phenomena such as the horse shoe vortices that form around bridge piers, chute blocks and struts, and the secondary vortices found in the clearance passages of turbomachinery are also important cavitation sites. Tip vortex cavitation can be viewed as a canonical problem that captures many of the essential physics associated with vortex cavitation in general. This paper describes the inception process and focuses on the high levels of tension that can be sustained in the flow, which appears to scale with the blade loading. High speed video visualization indicates that the details of how free stream nuclei are ingested plays a major role in the nucleation and inception process. A new photographic technique was used to obtain high quality images of the bubble growth process at framing rates as high as 40,000 fps. Sponsored by the Office of Naval Research

Numerical modeling method on the movement of water flow and suspended solids in two-dimensional sedimentation tanks in the wastewater treatment plant.

PubMed

Zeng, Guang-Ming; Jiang, Yi-Min; Qin, Xiao-Sheng; Huang, Guo-He; Li, Jian-Bing

2003-01-01

Taking the distributing calculation of velocity and concentration as an example, the paper established a series of governing equations by the vorticity-stream function method, and dispersed the equations by the finite differencing method. After figuring out the distribution field of velocity, the paper also calculated the concentration distribution in sedimentation tank by using the two-dimensional concentration transport equation. The validity and feasibility of the numerical method was verified through comparing with experimental data. Furthermore, the paper carried out a tentative exploration into the application of numerical simulation of sedimentation tanks.

Free-stream disturbance, continuous Eigenfunctions, boundary-layer instability and transition

NASA Technical Reports Server (NTRS)

Grosch, C. E.

1980-01-01

A rational foundation is presented for the application of the linear shear flows to transition prediction, and an explicit method is given for carrying out the necessary calculations. The expansions used are shown to be complete. Sample calculations show that a typical boundary layer is very sensitive to vorticity disturbances in the inner boundary layer, near the critical layer. Vorticity disturbances three or four boundary layer thicknesses above the boundary are nearly uncoupled from the boundary layer in that the amplitudes of the discrete Tollmien-Schlicting waves are an extremely small fraction of the amplitude of the disturbance.

An analytical solution for Dean flow in curved ducts with rectangular cross section

NASA Astrophysics Data System (ADS)

Norouzi, M.; Biglari, N.

2013-05-01

In this paper, a full analytical solution for incompressible flow inside the curved ducts with rectangular cross-section is presented for the first time. The perturbation method is applied to solve the governing equations and curvature ratio is considered as the perturbation parameter. The previous perturbation solutions are usually restricted to the flow in curved circular or annular pipes related to the overly complex form of solutions or singularity situation for flow in curved ducts with non-circular shapes of cross section. This issue specifies the importance of analytical studies in the field of Dean flow inside the non-circular ducts. In this study, the main flow velocity, stream function of lateral velocities (secondary flows), and flow resistance ratio in rectangular curved ducts are obtained analytically. The effect of duct curvature and aspect ratio on flow field is investigated as well. Moreover, it is important to mention that the current analytical solution is able to simulate the Taylor-Görtler and Dean vortices (vortices in stable and unstable situations) in curved channels.

Acoustic microstreaming due to an ultrasound contrast microbubble near a wall

NASA Astrophysics Data System (ADS)

Mobadersany, Nima; Sarkar, Kausik

2017-11-01

In an ultrasound field, in addition to the sinusoidal motion of fluid particles, particles experience a steady streaming velocity due to nonlinear second order effects. Here, we have simulated the microstreaming flow near a plane rigid wall caused by the pulsations of contrast microbubbles. Although these microbubbles were initially developed as a contrast enhancing agents for ultrasound imaging, they generate additional therapeutic effects that can be harnessed for targeted drug delivery or blood brain barrier (BBB) opening. The microbubbles have a gas core coated with a stabilizing layer of lipids or proteins. We use analytical models as well as boundary element (BEM) simulation to simulate the flow around these bubbles implementing interfacial rheology models for the coating. The microstreaming flow is characterized by two wall bounded vortices. The size of the vortices decreases with the decrease of the separation from the wall. The vortex-induced shear stress is simulated and analyzed as a function of excitation parameters and geometry. These microstreaming shear stress plays a critical role in increasing the membrane permeability facilitating drug delivery or rupturing biological tissues.

Receptivity of flat-plate boundary layer in a non-uniform free stream (vorticity normal to the plate)

NASA Technical Reports Server (NTRS)

Kogan, M. N.

1994-01-01

Recent progress in both the linear and nonlinear aspects of stability theory has highlighted the importance of the receptivity problem. One of the most unclear aspects of receptivity study is the receptivity of boundary-layer flow normal to vortical disturbances. Some experimental and theoretical results permit the proposition that quasi-steady outer-flow vortical disturbances may trigger by-pass transition. In present work such interaction is investigated for vorticity normal to a leading edge. The interest in these types of vortical disturbances arise from theoretical work, where it was shown that small sinusoidal variations of upstream velocity along the spanwise direction can produce significant variations in the boundary-layer profile. In the experimental part of this work, such non-uniform flow was created and the laminar-turbulent transition in this flow was investigated. The experiment was carried out in a low-turbulence direct-flow wind tunnel T-361 at the Central Aerohydrodynamic Institute (TsAGI). The non-uniform flow was produced by laminar or turbulent wakes behind a wire placed normal to the plate upstream of the leading edge. The theoretical part of the work is devoted to studying the unstable disturbance evolution in a boundary layer with strongly non-uniform velocity profiles similar to that produced by outer-flow vorticity. Specifically, the Tollmien-Schlichting wave development in the boundary layer flow with spanwise variations of velocity is investigated.

On the effect of using the Shapiro filter to smooth winds on a sphere

NASA Technical Reports Server (NTRS)

Takacs, L. L.; Balgovind, R. C.

1984-01-01

Spatial differencing schemes which are not enstrophy conserving nor implicitly damping require global filtering of short waves to eliminate the build-up of energy in the shortest wavelengths due to aliasing. Takacs and Balgovind (1983) have shown that filtering on a sphere with a latitude dependent damping function will cause spurious vorticity and divergence source terms to occur if care is not taken to ensure the irrotationality of the gradients of the stream function and velocity potential. Using a shallow water model with fourth-order energy-conserving spatial differencing, it is found that using a 16th-order Shapiro (1979) filter on the winds and heights to control nonlinear instability also creates spurious source terms when the winds are filtered in the meridional direction.

Proceedings of the Gulf Stream Workshop Held at West Greenwich, Rhode Island on 23-26 April 1985,

DTIC Science & Technology

1985-04-01

complex. We now realize that a correct dytiamical description must intrinsically couple the mass-, momentum -, energy-, and vorticity-fluxes of a strong mean...path and structure, and the mass- and momentum transport. 2. Meander dynamics "intrinsic" to the Gulf Stream, such as growth and propagation of...contribute to the dissipation of momentum through wave and form drag. A general study of the influence of the seamounts seems more appropriate for the

A numerical study of the 2- and 3-dimensional unsteady Navier-Stokes equations in velocity-vorticity variables using compact difference schemes

NASA Technical Reports Server (NTRS)

Gatski, T. B.; Grosch, C. E.

1984-01-01

A compact finite-difference approximation to the unsteady Navier-Stokes equations in velocity-vorticity variables is used to numerically simulate a number of flows. These include two-dimensional laminar flow of a vortex evolving over a flat plate with an embedded cavity, the unsteady flow over an elliptic cylinder, and aspects of the transient dynamics of the flow over a rearward facing step. The methodology required to extend the two-dimensional formulation to three-dimensions is presented.

Baroclinic Vortices in Rotating Stratified Shearing Flows: Cyclones, Anticyclones, and Zombie Vortices

NASA Astrophysics Data System (ADS)

Hassanzadeh, Pedram

Large coherent vortices are abundant in geophysical and astrophysical flows. They play significant roles in the Earth's oceans and atmosphere, the atmosphere of gas giants, such as Jupiter, and the protoplanetary disks around forming stars. These vortices are essentially three-dimensional (3D) and baroclinic, and their dynamics are strongly influenced by the rotation and density stratification of their environments. This work focuses on improving our understanding of the physics of 3D baroclinic vortices in rotating and continuously stratified flows using 3D spectral simulations of the Boussinesq equations, as well as simplified mathematical models. The first chapter discusses the big picture and summarizes the results of this work. In Chapter 2, we derive a relationship for the aspect ratio (i.e., vertical half-thickness over horizontal length scale) of steady and slowly-evolving baroclinic vortices in rotating stratified fluids. We show that the aspect ratio is a function of the Brunt-Vaisala frequencies within the vortex and outside the vortex, the Coriolis parameter, and the Rossby number of the vortex. This equation is basically the gradient-wind equation integrated over the vortex, and is significantly different from the previously proposed scaling laws that find the aspect ratio to be only a function of the properties of the background flow, and independent of the dynamics of the vortex. Our relation is valid for cyclones and anticyclones in either the cyclostrophic or geostrophic regimes; it works with vortices in Boussinesq fluids or ideal gases, and non-uniform background density gradient. The relation for the aspect ratio has many consequences for quasi-equilibrium vortices in rotating stratified flows. For example, cyclones must have interiors more stratified than the background flow (i.e., super-stratified), and weak anticyclones must have interiors less stratified than the background (i.e., sub-stratified). In addition, this equation is useful to infer the height and internal stratification of some astrophysical and geophysical vortices because direct measurements of their vertical structures are difficult. In Chapter 3, we show numerically and experimentally that localized suction in rotating continuously stratified flows produces three-dimensional baroclinic cyclones. As expected from Chapter 2, the interiors of these cyclones are super-stratified. Suction, modeled as a small spherical sink in the simulations, creates an anisotropic flow toward the sink with directional dependence changing with the ratio of the Coriolis parameter to the Brunt-Vaisala frequency. Around the sink, this flow generates cyclonic vorticity and deflects isopycnals so that the interior of the cyclone becomes super-stratified. The super-stratified region is visualized in the companion experiments that we helped to design and analyze using the synthetic schlieren technique. Once the suction stops, the cyclones decay due to viscous dissipation in the simulations and experiments. The numerical results show that the vertical velocity of viscously decaying cyclones flows away from the cyclone's midplane, while the radial velocity flows toward the cyclone's center. This observation is explained based on the cyclo-geostrophic balance. This vertical velocity mixes the flow inside and outside of cyclone and reduces the super-stratification. We speculate that the predominance of anticyclones in geophysical and astrophysical flows is due to the fact that anticyclones require sub-stratification, which occurs naturally by mixing, while cyclones require super-stratification. In Chapter 4, we show that a previously unknown instability creates space-filling lattices of 3D turbulent baroclinic vortices in linearly-stable, rotating, stratified shear flows. The instability starts from a newly discovered family of easily-excited critical layers. This new family, named the baroclinic critical layer, has singular vertical velocities; the traditional family of (barotropic) critical layer has singular stream-wise velocities and is hard to excite. In our simulations, the baroclinic critical layers in rotating stably-stratified linear shear are excited by small-volume, small-amplitude vortices or waves. The excited baroclinic critical layers then intensify by drawing energy from the background shear and roll-up into large coherent 3D vortices that excite new critical layers and vortices. The vortices self-similarly replicate to create lattices of turbulent vortices. These vortices persist for all time and are called zombie vortices because they can occur in the dead zones of protoplanetary disks. The self-replication of zombie vortices can de-stabilize the otherwise linearly and finite-amplitude stable Keplerian shear and lead to the formation of stars and planets. (Abstract shortened by UMI.)

Navier-Stokes relaxation to sinh-Poisson states at finite Reynolds numbers

NASA Technical Reports Server (NTRS)

Montgomery, David; Shan, Xiaowen; Matthaeus, William H.

1993-01-01

A mathematical framework is proposed in which it seems possible to justify the computationally-observed relaxation of a two-dimensional Navier-Stokes fluid to a 'most probable', or maximum entropy, state. The relaxation occurs at large but finite Reynolds numbers, and involves substantial decay of higher-order ideal invariants such as enstrophy. A two-fluid formulation, involving interpenetrating positive and negative vorticity fluxes (continuous and square integrable) is developed, and is shown to be intimately related to the passive scalar decay problem. Increasing interpenetration of the two fluids corresponds to the decay of vorticity flux due to viscosity. It is demonstrated numerically that, in two dimensions, passive scalars decay rapidly, relative to mean-square vorticity (enstrophy). This observation provides a basis for assigning initial data to the two-fluid field variables.

Numerical calculations of velocity and pressure distribution around oscillating airfoils

NASA Technical Reports Server (NTRS)

Bratanow, T.; Ecer, A.; Kobiske, M.

1974-01-01

An analytical procedure based on the Navier-Stokes equations was developed for analyzing and representing properties of unsteady viscous flow around oscillating obstacles. A variational formulation of the vorticity transport equation was discretized in finite element form and integrated numerically. At each time step of the numerical integration, the velocity field around the obstacle was determined for the instantaneous vorticity distribution from the finite element solution of Poisson's equation. The time-dependent boundary conditions around the oscillating obstacle were introduced as external constraints, using the Lagrangian Multiplier Technique, at each time step of the numerical integration. The procedure was then applied for determining pressures around obstacles oscillating in unsteady flow. The obtained results for a cylinder and an airfoil were illustrated in the form of streamlines and vorticity and pressure distributions.

Generation and Maintenance of Recirculations by Gulf Stream Instabilities

DTIC Science & Technology

1999-02-01

Francois Primeau for endless discus- sions of various scientific problems, Kirill Pankratov for useful advice on the numerical methods in fluid...recirculation. J. Phys. Oceanogr., 18, 662-682. [7] Davis C. A. and K. A. Emanuel, 1991 : Potential vorticity diagnostics of cyclo- genesis. Mon. Weather. Rev

Midwestern streamflow, precipitation, and atmospheric vorticity influenced by Pacific sea-surface temperatures and total solar-irradiance variations

USGS Publications Warehouse

Perry, C.A.

2006-01-01

A solar effect on streamflow in the Midwestern United States is described and supported in a six-step physical connection between total solar irradiance (TSI), tropical sea-surface temperatures (SSTs), extratropical SSTs, jet-stream vorticity, surface-layer vorticity, precipitation, and streamflow. Variations in the correlations among the individual steps indicate that the solar/hydroclimatic mechanism is complex and has a time element (lag) that may not be constant. Correct phasing, supported by consistent spectral peaks between 0.092 and 0.096 cycles per year in all data sets within the mechanism is strong evidence for its existence. A significant correlation exists between total solar irradiance and the 3-year moving average of annual streamflow for Iowa (R = 0.67) and for the Mississippi River at St Louis, Missouri (R = 0.60), during the period 1950-2000. Published in 2005 by John Wiley & Sons, Ltd.

Cosmic-ray streaming perpendicular to the mean magnetic field. II - The gyrophase distribution function

NASA Technical Reports Server (NTRS)

Forman, M. A.; Jokipii, J. R.

1978-01-01

The distribution function of cosmic rays streaming perpendicular to the mean magnetic field in a turbulent medium is reexamined. Urch's (1977) discovery that in quasi-linear theory, the flux is due to particles at 90 deg pitch angle is discussed and shown to be consistent with previous formulations of the theory. It is pointed out that this flux of particles at 90 deg cannot be arbitrarily set equal to zero, and hence the alternative theory which proceeds from this premise is dismissed. A further, basic inconsistency in Urch's transport equation is demonstrated, and the connection between quasi-linear theory and compound diffusion is discussed.

Optimum performance and potential flow field of hovering rotors

NASA Technical Reports Server (NTRS)

Wu, J. C.; Sigman, R. K.

1975-01-01

Rotor and propeller performance and induced potential flowfields were studied on the basis of a rotating actuator disk concept, with special emphasis on rotors hovering out of ground effect. A new theory for the optimum performance of rotors hovering OGE is developed and presented. An extended theory for the optimum performance of rotors and propellers in axial motion is also presented. Numerical results are presented for the optimum distributions of blade-bound circulation together with axial inflow and ultimate wake velocities for the hovering rotor over the range of thrust coefficient of interest in rotorcraft applications. Shapes of the stream tubes and of the velocities in the slipstream are obtained, using available methods, for optimum and off-optimum circulation distributions for rotors hovering in and out of ground effect. A number of explicit formulae useful in computing rotor and propeller induced flows are presented for stream functions and velocities due to distributions of circular vortices over axi-symmetric surfaces.

Enabling CSPA Operations Through Pilot Involvement in Longitudinal Approach Spacing

NASA Technical Reports Server (NTRS)

Battiste, Vernol (Technical Monitor); Pritchett, Amy

2003-01-01

Several major airports around the United States have, or plan to have, closely-spaced parallel runways. This project complemented current and previous research by examining the pilots ability to control their position longitudinally within their approach stream.This project s results considered spacing for separation from potential positions of wake vortices from the parallel approach. This preventive function could enable CSPA operations to very closely spaced runways. This work also considered how pilot involvement in longitudinal spacing could allow for more efficient traffic flow, by allowing pilots to keep their aircraft within tighter arrival slots then air traffic control (ATC) might be able to establish, and by maintaining space within the arrival stream for corresponding departure slots. To this end, this project conducted several research studies providing an analytic and computational basis for calculating appropriate aircraft spacings, experimental results from a piloted flight simulator test, and an experimental testbed for future simulator tests. The following sections summarize the results of these three efforts.

Calculating forces on thin flat plates with incomplete vorticity-field data

NASA Astrophysics Data System (ADS)

Limacher, Eric; Morton, Chris; Wood, David

2016-11-01

Optical experimental techniques such as particle image velocimetry (PIV) permit detailed quantification of velocities in the wakes of bluff bodies. Patterns in the wake development are significant to force generation, but it is not trivial to quantitatively relate changes in the wake to changes in measured forces. Key difficulties in this regard include: (i) accurate quantification of velocities close to the body, and (ii) the effect of missing velocity or vorticity data in regions where optical access is obscured. In the present work, we consider force formulations based on the vorticity field, wherein mathematical manipulation eliminates the need for accurate near-body velocity information. Attention is restricted to nominally two dimensional problems, namely (i) a linearly accelerating flat plate, investigated using PIV in a water tunnel, and (ii) a pitching plate in a freestream flow, as investigated numerically by Wang & Eldredge (2013). The effect of missing vorticity data on the pressure side of the plate has a significant impact on the calculation of force for the pitching plate test case. Fortunately, if the vorticity on the pressure side remains confined to a thin boundary layer, simple corrections can be applied to recover a force estimate.

Multigrid method based on the transformation-free HOC scheme on nonuniform grids for 2D convection diffusion problems

NASA Astrophysics Data System (ADS)

Ge, Yongbin; Cao, Fujun

2011-05-01

In this paper, a multigrid method based on the high order compact (HOC) difference scheme on nonuniform grids, which has been proposed by Kalita et al. [J.C. Kalita, A.K. Dass, D.C. Dalal, A transformation-free HOC scheme for steady convection-diffusion on non-uniform grids, Int. J. Numer. Methods Fluids 44 (2004) 33-53], is proposed to solve the two-dimensional (2D) convection diffusion equation. The HOC scheme is not involved in any grid transformation to map the nonuniform grids to uniform grids, consequently, the multigrid method is brand-new for solving the discrete system arising from the difference equation on nonuniform grids. The corresponding multigrid projection and interpolation operators are constructed by the area ratio. Some boundary layer and local singularity problems are used to demonstrate the superiority of the present method. Numerical results show that the multigrid method with the HOC scheme on nonuniform grids almost gets as equally efficient convergence rate as on uniform grids and the computed solution on nonuniform grids retains fourth order accuracy while on uniform grids just gets very poor solution for very steep boundary layer or high local singularity problems. The present method is also applied to solve the 2D incompressible Navier-Stokes equations using the stream function-vorticity formulation and the numerical solutions of the lid-driven cavity flow problem are obtained and compared with solutions available in the literature.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cheviakov, Alexei F., E-mail: chevaikov@math.usask.ca

Partial differential equations of the form divN=0, N{sub t}+curl M=0 involving two vector functions in R{sup 3} depending on t, x, y, z appear in different physical contexts, including the vorticity formulation of fluid dynamics, magnetohydrodynamics (MHD) equations, and Maxwell's equations. It is shown that these equations possess an infinite family of local divergence-type conservation laws involving arbitrary functions of space and time. Moreover, it is demonstrated that the equations of interest have a rather special structure of a lower-degree (degree two) conservation law in R{sup 4}(t,x,y,z). The corresponding potential system has a clear physical meaning. For the Maxwell's equations,more » it gives rise to the scalar electric and the vector magnetic potentials; for the vorticity equations of fluid dynamics, the potentialization inverts the curl operator to yield the fluid dynamics equations in primitive variables; for MHD equations, the potential equations yield a generalization of the Galas-Bogoyavlenskij potential that describes magnetic surfaces of ideal MHD equilibria. The lower-degree conservation law is further shown to yield curl-type conservation laws and determined potential equations in certain lower-dimensional settings. Examples of new nonlocal conservation laws, including an infinite family of nonlocal material conservation laws of ideal time-dependent MHD equations in 2+1 dimensions, are presented.« less

Analogies between oscillation and rotation of bodies induced or influenced by vortex shedding

NASA Astrophysics Data System (ADS)

Lugt, H. J.

Vortex-induced or vortex-influenced rotation and oscillation of bodies in a parallel flow are discussed. A steady flow occurs if the body axis is parallel to the flow or if the axis of rotation is perpendicular to the flow. Flows around an oscillating body are quasi-steady only if the Strougal number is much smaller than unity. The connection between rotation and oscillation is demonstrated in terms of the autorotation of a Lanchester propeller, and conditions for stable autorotation are defined. The Riabouchinsky curve is shown to be typical of forces and torques on bodies with vortical wakes, including situations with fixed body axes perpendicular to the flow. A differential equation is formulated for rotational and oscillating bodies that shed vortices by extending the pendulum equation to include vortical effects expressed as a fifth-order polynomial.

Inflow/Outflow Conditions for Unsteady Aerodynamics and Aeroacoustics in Nonuniform Flow

NASA Technical Reports Server (NTRS)

Atassi, Oliver V.; Grady, Joseph E. (Technical Monitor)

2003-01-01

The effect of a nonuniform mean flow on the normal modes; the inflow/outflow nonreflecting boundary conditions; and the sound power are studied. The normal modes in an annular duct are computed using a spectral method in combination with a shooting method. The swirl causes force imbalance which couples the acoustic and vortical modes. The acoustic modes are distinguished from the vortical modes by their large pressure and small vorticity content. The mean swirl also produces a Doppler shift in frequency. This results in more counter-spinning modes cut-on at a given frequency than modes spinning with the swirl. Nonreflecting boundary conditions are formulated using the normal mode solutions. The inflow/outflow boundary conditions are implemented in a linearized Euler scheme and validated by computing the propagation of acoustic and vortical waves in a duct for a variety of swirling mean flows. Numerical results show that the evolution of the vortical disturbances is sensitive to the inflow conditions and the details of the wake excitations. All three components of the wake velocity must be considered to correctly compute the wake evolution and the blade upwash. For high frequencies, the acoustic-vortical mode coupling is weak and a conservation equation for the acoustic energy can be derived. Sound power calculations show significant mean flow swirl effects, but mode interference effects are small.

Enhancing critical current density of cuprate superconductors

DOEpatents

Chaudhari, Praveen

2015-06-16

The present invention concerns the enhancement of critical current densities in cuprate superconductors. Such enhancement of critical current densities include using wave function symmetry and restricting movement of Abrikosov (A) vortices, Josephson (J) vortices, or Abrikosov-Josephson (A-J) vortices by using the half integer vortices associated with d-wave symmetry present in the grain boundary.

Testing the effects of in-stream sediment sources and sinks on simulated watershed sediment yield using the coupled U.S. Army Corps of Engineers GSSHA Model and SEDLIB Sediment Transport Library

NASA Astrophysics Data System (ADS)

Floyd, I. E.; Downer, C. W.; Brown, G.; Pradhan, N. R.

2017-12-01

The Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model is the US Army Corps of Engineers' (USACE)'s only fully coupled overland/in-stream sediment transport model. While the overland sediment transport formulation in GSSHA is considered state of the art, the existing in-stream sediment transport formulation is less robust. A major omission in the formulation of the existing GSSHA in-stream model is the lack of in-stream sources of fine materials. In this effort, we enhanced the in-stream sediment transport capacity of GSSHA by linking GSSHA to the SEDLIB sediment transport library. SEDLIB was developed at the Coastal and Hydraulics Laboratory (CHL) under the System Wide Water Resources Program (SWWRP) and Flood and Coastal (F&C) research program. It is designed to provide a library of sediment flux formulations for hydraulic and hydrologic models, such as GSSHA. This new version of GSSHA, with the updated in-stream sediment transport simulation capability afforded by the linkage to SEDLIB, was tested in against observations in an experimental watershed that had previously been used as a test bed for GSSHA. The results show a significant improvement in the ability to model in-stream sources of fine sediment. This improved capability will broaden the applicability of GSSHA to larger watersheds and watersheds with complex sediment dynamics, such as those subjected to fire hydrology.

Nonlinear acoustic streaming in straight and tapered tubes

NASA Astrophysics Data System (ADS)

Tuttle, Brian C.

In thermoacoustic and Stirling devices such as the pulse-tube refrigerator, efficiency is diminished by the formation of a second-order mean velocity known as Rayleigh streaming. This flow emerges from the interaction of the working gas with the wall of the tube in a thin boundary layer. Recent studies have suggested that streaming velocity can be decreased in a tube by tapering it slightly. This research investigates that claim through the development of a numerical model of Rayleigh streaming in variously tapered tubes. It is found that the numerical simulation of streaming in a straight tube compares well with theory, and the application of different thermal boundary conditions at the tube wall shows that for pressurized helium, inner streaming vortices which appear near an adiabatic tube wall do not develop near an isothermal wall. An order analysis indicates that the temperature dependence of viscosity and thermal conductivity contributes appreciably to an accurate numerical model of streaming. Comparison of Rayleigh streaming in tapered tubes shows the effects of taper angle on the circulation and velocity of the mean flow.

Strong wave/mean-flow coupling in baroclinic acoustic streaming

NASA Astrophysics Data System (ADS)

Chini, Greg; Michel, Guillaume

2017-11-01

Recently, Chini et al. demonstrated the potential for large-amplitude acoustic streaming in compressible channel flows subjected to strong background cross-channel density variations. In contrast with classic Rayleigh streaming, standing acoustic waves of O (ɛ) amplitude acquire vorticity owing to baroclinic torques acting throughout the domain rather than via viscous torques acting in Stokes boundary layers. More significantly, these baroclinically-driven streaming flows have a magnitude that also is O (ɛ) , i.e. comparable to that of the sound waves. In the present study, the consequent potential for fully two-way coupling between the waves and streaming flows is investigated using a novel WKBJ analysis. The analysis confirms that the wave-driven streaming flows are sufficiently strong to modify the background density gradient, thereby modifying the leading-order acoustic wave structure. Simulations of the wave/mean-flow system enabled by the WKBJ analysis are performed to illustrate the nature of the two-way coupling, which contrasts sharply with classic Rayleigh streaming, for which the waves can first be determined and the streaming flows subsequently computed.

Electroosmotic flow in a microcavity with nonuniform surface charges.

PubMed

Halpern, David; Wei, Hsien-Hung

2007-08-28

In this work, we theoretically explore the characteristics of electroosmostic flow (EOF) in a microcavity with nonuniform surface charges. It is well known that a uniformly charged EOF does not give rise to flow separation because of its irrotational nature, as opposed to the classical problem of viscous flow past a cavity. However, if the cavity walls bear nonuniform surface charges, then the similitude between electric and flow fields breaks down, leading to the generation of vorticity in the cavity. Because this vorticity must necessarily diffuse into the exterior region that possesses a zero vorticity set by a uniform EOF, a new flow structure emerges. Assuming Stokes flow, we employ a boundary element method to explore how a nonuniform charge distribution along the cavity surface affects the flow structure. The results show that the stream can be susceptible to flow separation and exhibits a variety of flow structures, depending on the distributions of zeta potentials and the aspect ratio of the cavity. The interactions between patterned EOF vortices and Moffatt eddies are further demonstrated for deep cavities. This work not only has implications for electrokinetic flow induced by surface imperfections but also provides optimal strategies for achieving effective mixing in microgrooves.

Generation of multiple toroidal dust vortices by a non-monotonic density gradient in a direct current glow discharge plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kaur, Manjit, E-mail: manjit@ipr.res.in; Bose, Sayak; Chattopadhyay, P. K.

2015-09-15

Observation of two well-separated dust vortices in an unmagnetized parallel plate DC glow discharge plasma is reported in this paper. A non-monotonic radial density profile, achieved by an especially designed cathode structure using a concentric metallic disk and ring of different radii, is observed to produce double dust tori between cathode and anode. PIV analysis of the still images of the double tori shows oppositely rotating dust structures between the central disk and the ring. Langmuir probe measurements of background plasma shows a non-uniform plasma density profile between the disk and the ring. Location and sense of rotation of themore » dust vortices coincides with the location and direction of the radial gradient in the ion drag force caused by the radial density gradient. The experimentally observed dust vorticity matches well with the calculated one using hydrodynamic formulations with shear in ion drag dominating over the dust charge gradient. These results corroborate that a radial gradient in the ion drag force directed towards cathode is the principal cause of dust rotation.« less

Large Angle Unsteady Aerodynamic Theory of a Flat Plate

NASA Astrophysics Data System (ADS)

Manar, Field; Jones, Anya

2016-11-01

A purely analytical approach is taken for the evaluation of the unsteady loads on a flat plate. This allows for an extremely low cost theoretical prediction of the plate loads in the style of Wagner and Theodorsen, without making the assumption of small angle of attack or small disturbance flow. The forces and moments are evaluated using the time rate of change of fluid momentum, expressed as an integral of the vorticity field. The flow is taken as inviscid and incompressible with isolated vorticity bound to the plate and in the shed wake. The bound vorticity distribution on the plate is solved exactly using conformal mapping of the plate to a cylinder. In keeping with the original assumption of Wagner, the wake vorticity is assumed to remain stationary in an inertial reference frame and convection is disregarded. Formulation in this manner allows for a closed form solution of Wagner's problem valid at all angles of attack. Separation from the leading edge of the plate can also be included to further increase the fidelity of the model at high angles.

Field theoretical prediction of a property of the tropical cyclone

NASA Astrophysics Data System (ADS)

Spineanu, F.; Vlad, M.

2014-01-01

The large scale atmospheric vortices (tropical cyclones, tornadoes) are complex physical systems combining thermodynamics and fluid-mechanical processes. The late phase of the evolution towards stationarity consists of the vorticity concentration, a well known tendency to self-organization , an universal property of the two-dimensional fluids. It may then be expected that the stationary state of the tropical cyclone has the same nature as the vortices of many other systems in nature: ideal (Euler) fluids, superconductors, Bose-Einsetin condensate, cosmic strings, etc. Indeed it was found that there is a description of the atmospheric vortex in terms of a classical field theory. It is compatible with the more conventional treatment based on conservation laws, but the field theoretical model reveals properties that are almost inaccessible to the conventional formulation: it identifies the stationary states as being close to self-duality. This is of highest importance: the self-duality is known to be the origin of all coherent structures known in natural systems. Therefore the field theoretical (FT) formulation finds that the cuasi-coherent form of the atmospheric vortex (tropical cyclone) at stationarity is an expression of this particular property. In the present work we examine a strong property of the tropical cyclone, which arises in the FT formulation in a natural way: the equality of the masses of the particles associated to the matter field and respectively to the gauge field in the FT model is translated into the equality between the maximum radial extension of the tropical cyclone and the Rossby radius. For the cases where the FT model is a good approximation we calculate characteristic quantities of the tropical cyclone and find good comparison with observational data.

Advection within side-by-side liquid micro-cylinders in a cross-flow

NASA Astrophysics Data System (ADS)

Dong, Qingming; Sau, Amalendu

2017-11-01

The gaseous SO2 entrainment from outer air stream and dispersion in binary and ternary liquid micro-cylinders appearing side-by-side are examined hereby. The separation/attachment regulated non-uniform interfacial momentum exchange creates main stream driven "primary" and shear reversed "secondary" vortices in the liquid cylinders. At separation points, the sense of rotation of the generated "primary-secondary" vortex pair remains inward directed. We define such a vortex pair as the "inflow" type. However, at stagnation or attachment points, the sense of rotation of a "primary-primary" or "secondary-secondary" vortex pair remains outward directed, and such a vortex pair is defined as the "outflow" type. For the coupled water cylinders facing an oncoming stream contaminated by gaseous SO2, its absorption and internal transport are effectively controlled by dominant "inflow" and "outflow" natured dynamics of the said vortex pairs, besides by diffusion. The evolving "inflow" natured "primary-secondary" vortex pairs at separation points actively entrain the outer SO2, whereas the "outflow" natured vortex-pairs oppose SO2 entry through the stagnation regions. Moreover, the blockage induced steady-symmetric, steady-deflected, and flip-flopping air-jets through gaps, for varied gap-ratio (1 ≤ G/R ≤ 4) and Reynolds number (30 ≤ Re ≤ 160), create distinctive impact both on quantitative SO2 absorption (mso2 ') and convective nature of the SO2 transport in upper, lower, and middle cylinders, by virtue of modified strength and size of the inflow and outflow paired vortices. The present study shows that the tiny "secondary vortices" play important roles in SO2 entrainment and in effectively controlling the local absorption rate Rs o2. The sudden acceleration and upward/downward deflection of gap-flows enhanced near-neck advective SO2 entrainment by suitably strengthening the "inflow" natured local vortex dynamics. Conversely, for the reduced size of secondary vortices, the saturation becomes delayed. In addition, for decreased vertical spacing of micro-cylinders (R = 40 μm) falling below the diameter-length "2R," the SO2 absorption (mso2 ') only gets slower. We provide extensive analysis of two-phase transport phenomena in terms of interactive shear-stress, pressure, and characteristic time-ratio "Tr" of advection-diffusion processes, for varied G/R, Re, and liquid phase Peclet number "Pel" (96 ≤ Pel ≤ 1333), to present a better insight into the governing physics.

Scattering on two Aharonov-Bohm vortices

NASA Astrophysics Data System (ADS)

Bogomolny, E.

2016-12-01

The problem of two Aharonov-Bohm (AB) vortices for the Helmholtz equation is examined in detail. It is demonstrated that the method proposed by Myers (1963 J. Math. Phys. 6 1839) for slit diffraction can be generalised to obtain an explicit solution for AB vortices. Due to the singular nature of AB interaction the Green function and scattering amplitude for two AB vortices obey a series of partial differential equations. Coefficients entering these equations, fulfil ordinary non-linear differential equations whose solutions can be obtained by solving the Painlevé III equation. The asymptotics of necessary functions for very large and very small vortex separations are calculated explicitly. Taken together, this means that the problem of two AB vortices is exactly solvable.

Free-stream turbulence and concave curvature effects on heated, transitional boundary layers

NASA Technical Reports Server (NTRS)

Kim, J.; Simon, T. W.

1991-01-01

An experimental investigation of the transition process on flat-plate and concave curved-wall boundary layers for various free-stream turbulence levels was performed. Results show that for transition of a flat-plate, the two forms of boundary layer behavior, identified as laminar-like and turbulent-like, cannot be thought of as separate Blasius and fully-turbulent profiles, respectively. Thus, simple transition models in which the desired quantity is assumed to be an average, weighted on intermittency, of the theoretical laminar and fully turbulent values is not expected to be successful. Deviation of the flow identified as laminar-like from theoretical laminar behavior is shown to be due to recovery after the passage of a turbulent spot, while deviation of the flow identified as turbulent-like from the full-turbulent values is thought to be due to incomplete establishment of the fully-turbulent power spectral distribution. Turbulent Prandtl numbers for the transitional flow, computed from measured shear stress, turbulent heat flux and mean velocity and temperature profiles, were less than unity. For the curved-wall case with low free-stream turbulence intensity, the existence of Gortler vortices on the concave wall within both laminar and turbulent flows was established using liquid crystal visualization and spanwise velocity and temperature traverses. Transition was found to occur via a vortex breakdown mode. The vortex wavelength was quite irregular in both the laminar and turbulent flows, but the vortices were stable in time and space. The upwash was found to be more unstable, with higher levels of u' and u'v', and lower skin friction coefficients and shape factors. Turbulent Prandtl numbers, measured using a triple-wire probe, were found to be near unity for all post-transitional profiles, indicating no gross violation of Reynolds analogy. No evidence of streamwise vortices was seen in the high turbulence intensity case.

Vortex Particle-Mesh simulations of Vertical Axis Wind Turbine flows: from the blade aerodynamics to the very far wake

NASA Astrophysics Data System (ADS)

Chatelain, P.; Duponcheel, M.; Caprace, D.-G.; Marichal, Y.; Winckelmans, G.

2016-09-01

A Vortex Particle-Mesh (VPM) method with immersed lifting lines has been developed and validated. Based on the vorticity-velocity formulation of the Navier-Stokes equations, it combines the advantages of a particle method and of a mesh-based approach. The immersed lifting lines handle the creation of vorticity from the blade elements and its early development. LES of Vertical Axis Wind Turbine (VAWT) flows are performed. The complex wake development is captured in details and over very long distances: from the blades to the near wake coherent vortices, then through the transitional ones to the fully developed turbulent far wake (beyond 10 rotor diameters). The statistics and topology of the mean flow are studied. The computational sizes also allow insights into the detailed unsteady vortex dynamics, including some unexpected topological flow features.

A least-squares finite element method for 3D incompressible Navier-Stokes equations

NASA Technical Reports Server (NTRS)

Jiang, Bo-Nan; Lin, T. L.; Hou, Lin-Jun; Povinelli, Louis A.

1993-01-01

The least-squares finite element method (LSFEM) based on the velocity-pressure-vorticity formulation is applied to three-dimensional steady incompressible Navier-Stokes problems. This method can accommodate equal-order interpolations, and results in symmetric, positive definite algebraic system. An additional compatibility equation, i.e., the divergence of vorticity vector should be zero, is included to make the first-order system elliptic. The Newton's method is employed to linearize the partial differential equations, the LSFEM is used to obtain discretized equations, and the system of algebraic equations is solved using the Jacobi preconditioned conjugate gradient method which avoids formation of either element or global matrices (matrix-free) to achieve high efficiency. The flow in a half of 3D cubic cavity is calculated at Re = 100, 400, and 1,000 with 50 x 52 x 25 trilinear elements. The Taylor-Gortler-like vortices are observed at Re = 1,000.

Viscous theory of surface noise interaction phenomena

NASA Technical Reports Server (NTRS)

Yates, J. E.

1980-01-01

A viscous linear surface noise interaction problem is formulated that includes noise production by an oscillating surface, turbulent or vortical interaction with a surface, and scattering of sound by a surface. The importance of viscosity in establishing uniqueness of solution and partitioning of energy into acoustic and vortical modes is discussed. The results of inviscid two dimensional airfoil theory are used to examine the interactive noise problem in the limit of high reduced frequency and small Helmholtz number. It is shown that in the case of vortex interaction with a surface, the noise produced with the full Kutta condition is 3 dB less than the no Kutta condition result. The results of a study of an airfoil oscillating in a medium at rest are discussed. It is concluded that viscosity can be a controlling factor in analyses and experiments of surface noise interaction phenomena and that the effect of edge bluntness as well as viscosity must be included in the problem formulation to correctly calculate the interactive noise.

A numerical study of axisymmetric compressible non-isothermal and reactive swirling flow

NASA Astrophysics Data System (ADS)

Tavernetti, William E.; Hafez, Mohamed M.

2017-09-01

Non-linear dynamical phenomena in combustion processes is an active area of experimental and theoretical research. This is in large part due to increasingly strict environmental pressures to make gas turbine engines and industrial burners more efficient. Using numerical methods, for steady and unsteady confined and unconfined compressible flow, this study examines the modeling influence of compressibility for axisymmetric swirling flow. The compressible reactive Navier-Stokes equations in terms of stream function, vorticity, circulation are used. Results, details of the numerical algorithms, as well as numerical verification techniques and validation with sources from the literature will be presented. Understanding how vortex breakdown phenomena are affected by modeling reactant consumption with compressibility effect is the main goal of this study.

Numerical solutions of atmospheric flow over semielliptical simulated hills

NASA Technical Reports Server (NTRS)

Shieh, C. F.; Frost, W.

1981-01-01

Atmospheric motion over obstacles on plane surfaces to compute simulated wind fields over terrain features was studied. Semielliptical, two dimensional geometry and numerical simulation of flow over rectangular geometries is also discussed. The partial differential equations for the vorticity, stream function, turbulence kinetic energy, and turbulence length scale were solved by a finite difference technique. The mechanism of flow separation induced by a semiellipse is the same as flow over a gradually sloping surface for which the flow separation is caused by the interaction between the viscous force, the pressure force, and the turbulence level. For flow over bluff bodies, a downstream recirculation bubble is created which increases the aspect ratio and/or the turbulence level results in flow reattachment close behind the obstacle.

A cubic spline approximation for problems in fluid mechanics

NASA Technical Reports Server (NTRS)

Rubin, S. G.; Graves, R. A., Jr.

1975-01-01

A cubic spline approximation is presented which is suited for many fluid-mechanics problems. This procedure provides a high degree of accuracy, even with a nonuniform mesh, and leads to an accurate treatment of derivative boundary conditions. The truncation errors and stability limitations of several implicit and explicit integration schemes are presented. For two-dimensional flows, a spline-alternating-direction-implicit method is evaluated. The spline procedure is assessed, and results are presented for the one-dimensional nonlinear Burgers' equation, as well as the two-dimensional diffusion equation and the vorticity-stream function system describing the viscous flow in a driven cavity. Comparisons are made with analytic solutions for the first two problems and with finite-difference calculations for the cavity flow.

A vortex wake capturing method for potential flow calculations

NASA Technical Reports Server (NTRS)

Murman, E. M.; Stremel, P. M.

1982-01-01

A method is presented for modifying finite difference solutions of the potential equation to include the calculation of non-planar vortex wake features. The approach is an adaptation of Baker's 'cloud in cell' algorithm developed for the stream function-vorticity equations. The vortex wake is tracked in a Lagrangian frame of reference as a group of discrete vortex filaments. These are distributed to the Eulerian mesh system on which the velocity is calculated by a finite difference solution of the potential equation. An artificial viscosity introduced by the finite difference equations removes the singular nature of the vortex filaments. Computed examples are given for the two-dimensional time dependent roll-up of vortex wakes generated by wings with different spanwise loading distributions.

On cat's eyes and multiple disjoint cells natural convection flow in tall tilted cavities

NASA Astrophysics Data System (ADS)

Báez, Elsa; Nicolás, Alfredo

2014-10-01

Natural convection fluid flow in air-filled tall tilted cavities is studied numerically with a direct projection method applied on the unsteady Boussinesq approximation in primitive variables. The study is focused on the so called cat's eyes and multiple disjoint cells as the aspect ratio A and the angle of inclination ϕ of the cavity vary. Results have already been reported with primitive and stream function-vorticity variables. The former are validated with the latter ones, which in turn were validated through mesh size and time-step independence studies. The new results complemented with the previous ones lead to find out the fluid motion and heat transfer invariant properties of this thermal phenomenon, which is the novelty here.

Experimental parametric study of jet vortex generators for flow separation control

NASA Technical Reports Server (NTRS)

Selby, Gregory

1991-01-01

A parametric wind-tunnel study was performed with jet vortex generators to determine their effectiveness in controlling flow separation associated with low-speed turbulence flow over a two-dimensional rearward-facing ramp. Results indicate that flow-separation control can be accomplished, with the level of control achieved being a function of jet speed, jet orientation (with respect to the free-stream direction), and orifice pattern (double row of jets vs. single row). Compared to slot blowing, jet vortex generators can provide an equivalent level of flow control over a larger spanwise region (for constant jet flow area and speed). Dye flow visualization tests in a water tunnel indicated that the most effective jet vortex generator configurations produced streamwise co-rotating vortices.

Displacement and dissipation under the rotating tidal potential, in contrast to Love's geostationary potential

NASA Astrophysics Data System (ADS)

Bostrom, R. C.

The Earth rotates relative to the solunar gravity field. In consequence the M2, S2 tides are represented by permanent bulges, travelling westward around the Earth as distortion waves. The associated tidal stress ellipsoid progresses perpetually by rotation, without reversal. It is shown that under imperfect elasticity, in lieu of the body forces induced by Love's geostationary time-variant potential a rotating potential induces internal body couples, equally pervasive. Displacement is cumulative, and in the vortical mode formulated by Helmholtz (1858). Whereas in the geostationary formulation of Love cumulative distortion is nil, in actuality this motion is primary, and dimensionally capable of coupling with extant mantle convection. Unlike the marine tides, the bodily wave-tides proceed unhindered around the Earth unhindered by continental margins. Corrected for oceanic effects the complex Love numbers measure dissipation, as commonly supposed. However dissipation is the result of unmapped cumulative vortical displacement (a circulation component), rather than oscillatory forces having the form of a geographically stationary spheroidal eigenvibration. The characteristic period of the loss factor 1/Q is infinity rather than the period pertinent to seismicity or wobble, to which it is dimensionally unrelated. Although primary vorticity-induction is required by the existence of the rotating tidal potential, its tectonic consequences are a matter of speculation, treated elsewhere [1]. --- [1] Bostrom, R.C., 1998. Tectonic Consequences of the Earth's Rotation. Oxfo rd University Press.

Vortex generation and mixing in three-dimensional supersonic combustors

NASA Technical Reports Server (NTRS)

Riggins, D. W.; Vitt, P. H.

1993-01-01

The generation and evolution of the flow vorticity established by instream injector ramps in a high Mach number/high enthalpy scramjet combustor flow-field are described in detail for a number of computational cases. Classical fluid dynamic circulation is presented for these cases in order to clarify the spatial distribution and convection of the vorticity. The ability of the simulations to accurately represent Stokes Law of circulation is discussed and shown. In addition, the conservation of swirl (effectively the moment-of-momentum theorem) is presented for these flows. The impact of both turbulent diffusion and the vortex/ramp non-uniformity on the downstream mixing rate is clearly illustrated. A correlation over the length of the combustor between fuel-air mixing and a parameter called the vortex stirring length is demonstrated. Finally, computational results for a representative ramp injector are compared with experimental data. Influence of the stream vorticity on the effective turbulent Prandtl number used in the simulation is discussed.

Experimental characterization of wingtip vortices in the near field using smoke flow visualizations

NASA Astrophysics Data System (ADS)

Serrano-Aguilera, J. J.; García-Ortiz, J. Hermenegildo; Gallardo-Claros, A.; Parras, L.; del Pino, C.

2016-08-01

In order to predict the axial development of the wingtip vortices strength, an accurate theoretical model is required. Several experimental techniques have been used to that end, e.g. PIV or hot-wire anemometry, but they imply a significant cost and effort. For this reason, we have performed experiments using the smoke-wire technique to visualize smoke streaks in six planes perpendicular to the main stream flow direction. Using this visualization technique, we obtained quantitative information regarding the vortex velocity field by means of Batchelor's model for two chord-based Reynolds numbers, Re_c=3.33× 10^4 and 10^5. Therefore, this theoretical vortex model has been introduced in the integration of ordinary differential equations which describe the temporal evolution of streak lines as function of two parameters: the swirl number, S, and the virtual axial origin, overline{z_0}. We have applied two different procedures to minimize the distance between experimental and theoretical flow patterns: individual curve fitting at six different control planes in the streamwise direction and the global curve fitting which corresponds to all the control planes simultaneously. Both sets of results have been compared with those provided by del Pino et al. (Phys Fluids 23(013):602, 2011b. doi: 10.1063/1.3537791), finding good agreement. Finally, we have observed a weak influence of the Reynolds number on the values S and overline{z_0} at low-to-moderate Re_c. This experimental technique is proposed as a low cost alternative to characterize wingtip vortices based on flow visualizations.

Experimental and Theoretical Study of a Rectangular Wing in a Vortical Wake at Low Speed

NASA Technical Reports Server (NTRS)

Smith, Willard G.; Lazzeroni, Frank A.

1960-01-01

A systematic study has been made, experimentally and theoretically, of the effects of a vortical wake on the aerodynamic characteristics of a rectangular wing at subsonic speed. The vortex generator and wing were mounted on a reflection plane to avoid body-wing interference. Vortex position, relative to the wing, was varied both in the spanwise direction and normal to the wing. Angle of attack of the wing was varied from -40 to +60. Both chordwise and spanwise pressure distributions were obtained with the wing in uniform and vortical flow fields. Stream surveys were made to determine the flow characteristics in the vortical wake. The vortex-induced lift was calculated by several theoretical methods including strip theory, reverse-flow theory, and reverse-flow theory including a finite vortex core. In addition, the Prandtl lifting-line theory and the Weissinger theory were used to calculate the spanwise distribution of vortex-induced loads. With reverse-flow theory, predictions of the interference lift were generally good, and with Weissinger's theory the agreement between the theoretical spanwise variation of induced load and the experimental variation was good. Results of the stream survey show that the vortex generated by a lifting surface of rectangular plan form tends to trail back streamwise from the tip and does not approach the theoretical location, or centroid of circulation, given by theory. This discrepancy introduced errors in the prediction of vortex interference, especially when the vortex core passed immediately outboard of the wing tip. The wake produced by the vortex generator in these tests was not fully rolled up into a circular vortex, and so lacked symmetry in the vertical direction of the transverse plane. It was found that the direction of circulation affected the induced loads on the wing either when the wing was at angle of attack or when the vortex was some distance away from the plane of the wing.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Macek, Joseph H; Sternberg, James; Ovchinnikov, Serguei Yurevich

Deep minima in He(e,2e)He{sup +} triply differential cross sections are traced to vortices in atomic wave functions. Such vortices have been predicted earlier, but the present calculations show that they have also been observed experimentally, although not recognized as vortices. Their observation in (e,2e) measurements shows that vortices play an important role in electron correlations related to the transfer of angular momentum between incident and ejected electrons. The vortices significantly extend the list of known features that summarize the general picture of electron correlations in impact ionization.

The importance of dynamic stall in aerodynamic modeling of the Darrieus rotor

NASA Astrophysics Data System (ADS)

Fraunie, P.; Beguier, C.; Paraschivoiu, I.

The CAARDEX program is defined for analyzing the behavior of Darrieus wind turbines in terms of the Reynolds number, the geometrical characteristics of the wind turbine and the spreading of the stream tubes traversing the rotor volume. It is demonstrated that the maximum power conversion efficiency of the Darrieus rotor is 0.4, with the energy capture being divided at a 4:1 ratio upstream to downstream rotor. The model shows that the velocity induced on the rotor is a function of the specific velocity and solidity, and that previous stream tube theories are valid only at low values of these parameters. CARDAAX treats the rotor disk in terms of horizontal slices of stream tubes modeled separately for the upstream and downstream segments. Account is taken of the velocity profile in the atmospheric boundary layer, which can vary significantly in the case of large wind turbines, i.e., several hundred feet high. When applied to predicting the performance of a 1 kW, 2.6 m diam prototype Darrieus wind turbine in a 10 mps flow, fair agreement is obtained for power capture/wind velocity and cyclic aerodynamic forces. Additional flow visualization data is provided to illustrate the production of turbulence in the form of vortices shed between the blades.

Crystallized and amorphous vortices in rotating atomic-molecular Bose-Einstein condensates

PubMed Central

Liu, Chao-Fei; Fan, Heng; Gou, Shih-Chuan; Liu, Wu-Ming

2014-01-01

Vortex is a topological defect with a quantized winding number of the phase in superfluids and superconductors. Here, we investigate the crystallized (triangular, square, honeycomb) and amorphous vortices in rotating atomic-molecular Bose-Einstein condensates (BECs) by using the damped projected Gross-Pitaevskii equation. The amorphous vortices are the result of the considerable deviation induced by the interaction of atomic-molecular vortices. By changing the atom-molecule interaction from attractive to repulsive, the configuration of vortices can change from an overlapped atomic-molecular vortices to carbon-dioxide-type ones, then to atomic vortices with interstitial molecular vortices, and finally into independent separated ones. The Raman detuning can tune the ratio of the atomic vortex to the molecular vortex. We provide a phase diagram of vortices in rotating atomic-molecular BECs as a function of Raman detuning and the strength of atom-molecule interaction. PMID:24573303

Shedding of dual structures in the wake of a surface-mounted low aspect ratio cone

NASA Astrophysics Data System (ADS)

Chen, Zixiang; Martinuzzi, Robert J.

2018-04-01

The periodic shedding of vortex pairs in the turbulent wake of a surface-mounted right cone of aspect ratio 0.867 protruding a thin turbulent boundary layer is investigated experimentally. A phase-averaged volumetric velocity field is reconstructed from planar stereoscopic particle image velocimetry. During a typical (phase-averaged) shedding cycle, counter-rotating base vortices alternately form. These are tilted and stretched to merge with stream-wise tip vortices. The merged structure sheds and is convected downstream. A synthesis of earlier observations suggests that a similar shedding process exists for other low aspect ratio tapered geometries and is more complex than the shedding patterns observed for cantilevered cylinders, despite similarities of the mean flow field structure.

Forcing function modeling for flow induced vibration

NASA Technical Reports Server (NTRS)

Fleeter, Sanford

1993-01-01

The fundamental forcing function unsteady aerodynamics for application to turbomachine blade row forced response are considered, accomplished through a series of experiments performed in a rotating annular cascade and a research axial flow turbine. In particular, the unsteady periodic flowfields downstream of rotating rows of perforated plates, airfoils and turbine blade rows are measured with a cross hot-wire and an unsteady total pressure probe. The unsteady velocity and static pressure fields were then analyzed harmonically and split into vortical and potential gusts, accomplished by developing a gust splitting analysis which includes both gust unsteady static pressure and velocity data. The perforated plate gusts closely were found to be linear theory vortical gusts, satisfying the vortical gust constraints. The airfoil and turbine blade row generated velocity perturbations did not satisfy the vortical gust constraints. However, the decomposition of the unsteady flow field separated the data into a propagating vortical component which satisfied these vortical gust constraints and a decaying potential component.

Filtered Rayleigh scattering mixing measurements of merging and non-merging streamwise vortex interactions in supersonic flow

NASA Astrophysics Data System (ADS)

Ground, Cody R.; Gopal, Vijay; Maddalena, Luca

2018-04-01

By introducing large-scale streamwise vortices into a supersonic flow it is possible to enhance the rate of mixing between two fluid streams. However, increased vorticity content alone does not explicitly serve as a predictor of mixing enhancement. Additional factors, particularly the mutual interactions occurring between neighboring vortical structures, affect the underlying fundamental physics that influence the rate at which the fluids mix. As part of a larger systematic study on supersonic streamwise vortex interactions, this work experimentally quantifies the average rate of mixing of helium and air in the presence of two separate modes of vortex interaction, the merging and non-merging of a pair of co-rotating vortices. In these experiments vortex-generating expansion ramps are placed on a strut injector. The freestream Mach number is set at 2.5 and helium is injected as a passive scalar. Average injectant mole fractions at selected flow planes downstream of the injector are measured utilizing the filtered Rayleigh scattering technique. The filtered Rayleigh scattering measurements reveal that, in the domain surveyed, the merging vortex interaction strongly displaces the plume from its initial horizontal orientation while the non-merging vortex interaction more rapidly mixes the helium and air. The results of the current experiments are consistent with associated knowledge derived from previous analyses of the two studied configurations which have included the detailed experimental characterization of entrainment, turbulent kinetic energy, and vorticity of both modes of vortex interaction.

Coherent Structures and Evolution of Vorticity in Short-Crested Breaking Surface Waves

NASA Astrophysics Data System (ADS)

Kirby, James; Derakhti, Morteza

2017-11-01

We employ a multi-phase LES/VOF code to study turbulence and coherent structures generated during breaking of short-crested surface water waves. We examine the evolution of coherent vortex structures evolving at the scale of the width of the breaking event, and their long-time interaction with smaller vortex loops formed by the local instability of the breaking crest. Long-time results are often characterized by the detachment of the larger scale vortex loop from the surface and formation of a closed vortex ring. The evolution of circulation for the vortical flow field is examined. The initial concentration of forcing close to the free surface leads to spatial distributions of both span-wise and vertical vorticity distributions which are concentrated close to the surface. This result, which persists into shallow water, is at odds with the basic simplicity of the Peregrine mechanism, suggesting that even shallow flows such as the surf zone should be regarded as being forced (in dissipative situations) by a wave-induced surface stress rather than a uniform-over-depth body force. The localized forcing leads to the development of a complex pattern of stream-wise vorticity, comparable in strength to the vertical and span-wise components, and also persist into shallow water. NSF OCE-1435147.

Influence of a density increase on the evolution of the Kelvin-Helmholtz instability and vortices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Amerstorfer, U. V.; Erkaev, N. V.; Institute of Computational Modelling, 660036 Krasnoyarsk

2010-07-15

Results of two-dimensional nonlinear numerical simulations of the magnetohydrodynamic Kelvin-Helmholtz instability are presented. A boundary layer of a certain width is assumed, which separates the plasma in the upper layer from the plasma in the lower layer. A special focus is given on the influence of a density increase toward the lower layer. The evolution of the Kelvin-Helmholtz instability can be divided into three different phases, namely, a linear growth phase at the beginning, followed by a nonlinear phase with regular structures of the vortices, and finally, a turbulent phase with nonregular structures. The spatial scales of the vortices aremore » about five times the initial width of the boundary layer. The considered configuration is similar to the situation around unmagnetized planets, where the solar wind (upper plasma layer) streams past the ionosphere (lower plasma layer), and thus the plasma density increases toward the planet. The evolving vortices might detach around the terminator of the planet and eventually so-called plasma clouds might be formed, through which ionospheric material can be lost. For the special case of a Venus-like planet, loss rates are estimated, which are of the order of estimated loss rates from observations at Venus.« less

Nonylphenol ethoxylates and other additives in aircraft deicers, antiicers, and waters receiving airport runoff.

PubMed

Corsi, Steven R; Zitomer, Daniel H; Field, Jennifer A; Cancilla, Devon A

2003-09-15

Samples of nine different formulations of aircraft deicer and antiicer fluids (ADAF) were screened for the presence of selected surfactants. Nonylphenol ethoxylates (NPnEO) were identified in three ADAF formulations, octylphenol ethoxylates were identified in two formulations, and six formulations contained alcohol ethoxylates. A preliminary field study was conducted at General Mitchell International Airport, Milwaukee, WI, to quantify NPnEO (n = 1-15) and one of its byproducts, nonylphenol (NP), in airport runoff. Samples were collected from two airport outfalls, from the receiving stream, and from an upstream reference site during intensive ADAF application events. NPnEO was measured at concentrations up to 1190microg/L in airport outfall samples, up to 77 ug/L in samples from the receiving stream and less than 5.0 microg/L from the upstream reference. Concentrations of glycol and other ADAF-related constituents, including NPnEO, were reduced by approximately 1 order of magnitude between the outfall sites and the receiving stream site; however, concentrations of NP in the receiving stream remained similar to those from the outfalls (< 0.04 microg/L at the upstream reference, 0.98 and 7.67 microg/L at outfalls, and 3.89 microg/L in the receiving stream). The field data suggest that NP is generated through degradation of NPnEO from airport runoff.

Nonylphenol ethoxylates and other additives in aircraft deicers, antiicers, and waters receiving airport runoff

USGS Publications Warehouse

Corsi, Steven R.; Zitomer, Daniel H.; Field, Jennifer A.; Cancilla, Devon A.

2003-01-01

Samples of nine different formulations of aircraft deicer and antiicer fluids (ADAF) were screened for the presence of selected surfactants. Nonylphenol ethoxylates (NPnEO) were identified in three ADAF formulations, octylphenol ethoxylates were identified in two formulations, and six formulations contained alcohol ethoxylates. A preliminary field study was conducted at General Mitchell International Airport, Milwaukee, WI, to quantify NPnEO (n = 1-15) and one of its byproducts, nonylphenol (NP), in airport runoff. Samples were collected from two airport outfalls, from the receiving stream, and from an upstream reference site during intensive ADAF application events. NPnEO was measured at concentrations up to 1190microg/L in airport outfall samples, up to 77 ug/L in samples from the receiving stream and less than 5.0 microg/L from the upstream reference. Concentrations of glycol and other ADAF-related constituents, including NPnEO, were reduced by approximately 1 order of magnitude between the outfall sites and the receiving stream site; however, concentrations of NP in the receiving stream remained similar to those from the outfalls (< 0.04 microg/L at the upstream reference, 0.98 and 7.67 microg/L at outfalls, and 3.89 microg/L in the receiving stream). The field data suggest that NP is generated through degradation of NPnEO from airport runoff.

On computations of the integrated space shuttle flowfield using overset grids

NASA Technical Reports Server (NTRS)

Chiu, I-T.; Pletcher, R. H.; Steger, J. L.

1990-01-01

Numerical simulations using the thin-layer Navier-Stokes equations and chimera (overset) grid approach were carried out for flows around the integrated space shuttle vehicle over a range of Mach numbers. Body-conforming grids were used for all the component grids. Testcases include a three-component overset grid - the external tank (ET), the solid rocket booster (SRB) and the orbiter (ORB), and a five-component overset grid - the ET, SRB, ORB, forward and aft attach hardware, configurations. The results were compared with the wind tunnel and flight data. In addition, a Poisson solution procedure (a special case of the vorticity-velocity formulation) using primitive variables was developed to solve three-dimensional, irrotational, inviscid flows for single as well as overset grids. The solutions were validated by comparisons with other analytical or numerical solution, and/or experimental results for various geometries. The Poisson solution was also used as an initial guess for the thin-layer Navier-Stokes solution procedure to improve the efficiency of the numerical flow simulations. It was found that this approach resulted in roughly a 30 percent CPU time savings as compared with the procedure solving the thin-layer Navier-Stokes equations from a uniform free stream flowfield.

A Parametric Study of Jet Interactions with Rarefied Flow

NASA Technical Reports Server (NTRS)

Glass, C. E.

2004-01-01

Three-dimensional computational techniques, in particular the uncoupled CFD-DSMC of the present study, are available to be applied to problems such as jet interactions with variable density regions ranging from a continuum jet to a rarefied free stream. When the value of the jet to free stream momentum flux ratio approximately greater than 2000 for a sharp leading edge flat plate forward separation vortices induced by the jet interaction are present near the surface. Also as the free stream number density n (infinity) decreases, the extent and magnitude of normalized pressure increases and moves upstream of the nozzle exit. Thus for the flat plate model the effect of decreasing n (infinity) is to change the sign of the moment caused by the jet interaction on the flat plate surface.

Minimum-domain impulse theory for unsteady aerodynamic force

NASA Astrophysics Data System (ADS)

Kang, L. L.; Liu, L. Q.; Su, W. D.; Wu, J. Z.

2018-01-01

We extend the impulse theory for unsteady aerodynamics from its classic global form to finite-domain formulation then to minimum-domain form and from incompressible to compressible flows. For incompressible flow, the minimum-domain impulse theory raises the finding of Li and Lu ["Force and power of flapping plates in a fluid," J. Fluid Mech. 712, 598-613 (2012)] to a theorem: The entire force with discrete wake is completely determined by only the time rate of impulse of those vortical structures still connecting to the body, along with the Lamb-vector integral thereof that captures the contribution of all the rest disconnected vortical structures. For compressible flows, we find that the global form in terms of the curl of momentum ∇ × (ρu), obtained by Huang [Unsteady Vortical Aerodynamics (Shanghai Jiaotong University Press, 1994)], can be generalized to having an arbitrary finite domain, but the formula is cumbersome and in general ∇ × (ρu) no longer has discrete structures and hence no minimum-domain theory exists. Nevertheless, as the measure of transverse process only, the unsteady field of vorticity ω or ρω may still have a discrete wake. This leads to a minimum-domain compressible vorticity-moment theory in terms of ρω (but it is beyond the classic concept of impulse). These new findings and applications have been confirmed by our numerical experiments. The results not only open an avenue to combine the theory with computation-experiment in wide applications but also reveal a physical truth that it is no longer necessary to account for all wake vortical structures in computing the force and moment.

The Theory of a Free Jet of a Compressible Gas

NASA Technical Reports Server (NTRS)

Abramovich, G. N.

1944-01-01

In the present report the theory of free turbulence propagation and the boundary layer theory are developed for a plane-parallel free stream of a compressible fluid. In constructing the theory use was made of the turbulence hypothesis by Taylor (transport of vorticity) which gives best agreement with test results for problems involving heat transfer in free jets.

Effect of centrifugal forces on formation of secondary flow structures in a 180-degree curved artery model under pulsatile inflow conditions

NASA Astrophysics Data System (ADS)

Callahan, Shannon; Sajjad, Roshan; Bulusu, Kartik V.; Plesniak, Michael W.

2013-11-01

An experimental investigation of secondary flow structures within a 180-degree bent tube model of a curved artery was performed using phase-averaged, two-component, two-dimensional, particle image velocimetry (2C-2D PIV) under pulsatile inflow conditions. Pulsatile waveforms ranging from simple sinusoidal to physiological inflows were supplied. We developed a novel continuous wavelet transform algorithm (PIVlet 1.2) and applied it to vorticity fields for coherent secondary flow structure detection. Regime maps of secondary flow structures revealed new, deceleration-phase-dependent flow morphologies. The temporal instances where streamwise centrifugal forces dominated were associated with large-scale coherent structures, such as deformed Dean-, Lyne- and Wall-type (D-L-W) vortical structures. Magnitudes of streamwise and cross-stream centrifugal forces tend to balance during deceleration phases. Deceleration events were also associated with spatial reorganization and asymmetry in large-scale D-L-W secondary flow structures. Hence, the interaction between streamwise and cross-stream centrifugal forces that affects secondary flow morphologies is explained using a ``residual force'' parameter i.e., the difference in magnitudes of these forces. Supported by the NSF Grant No. CBET- 0828903 and GW Center for Biomimetics and Bioinspired Engineering.

The Boundary Layers in Fluids with Little Friction

NASA Technical Reports Server (NTRS)

Blasius, H.

1950-01-01

The vortices forming in flowing water behind solid bodies are not represented correctly by the solution of the potential theory nor by Helmholtz's jets. Potential theory is unable to satisfy the condition that the water adheres at the wetted bodies, and its solutions of the fundamental hydrodynamic equations are at variance with the observation that the flow separates from the body at a certain point and sends forth a highly turbulent boundary layer into the free flow. Helmholtz's theory attempts to imitate the latter effect in such a way that it joins two potential flows, jet and still water, nonanalytical along a stream curve. The admissibility of this method is based on the fact that, at zero pressure, which is to prevail at the cited stream curve, the connection of the fluid, and with it the effect of adjacent parts on each other, is canceled. In reality, however, the pressure at these boundaries is definitely not zero, but can even be varied arbitrarily. Besides, Helmholtz's theory with its potential flows does not satisfy the condition of adherence nor explain the origin of the vortices, for in all of these problems, the friction must be taken into account on principle, according to the vortex theorem.

Global structure transitions in an experimental induction furnace

NASA Astrophysics Data System (ADS)

Tasaka, Yuji; Galindo, Vladimir; Vogt, Tobias; Eckert, Sven

2017-11-01

Flows induced by alternating magnetic field (AMF) in a cylindrical vessel filled with liquid metal, alloy of GaInSn, were examined experimentally using ultrasonic Doppler velocimetry (UDV). Measurement lines of UDV arranged vertically set at different radial and azimuthal positions extracted flow structures and their time variations as spatio-temporal velocity maps in the opaque liquid metal layer. At low frequency of AMF, corresponding to shielding parameter S =μm σωR2 = O(1) (μm and σ are magnetic permeability and electric conductivity of the test fluid, ω angular frequency of AMF, and R the radius of cylindrical vessel), two toroidal vortices exist in the fluid layer as the large scale flow structure and have interactions each other. With increasing of S the structure has transition from toroidal vortex pair to four large scale circulations (S >= 100) via transient state, where strong interactions of two vortices are observed (30 < S < 100). Faster vertical stream is observed near the cylinder wall because of ski effect caused by AMF, and the time-averaged velocity of the stream takes maximum around S = 20 , which is little smaller value of S for the onset of the transient state. JSPS KAKENHI No. 15KK0219.

Effect of free-stream turbulence on boundary layer transition.

PubMed

Goldstein, M E

2014-07-28

This paper is concerned with the transition to turbulence in flat plate boundary layers due to moderately high levels of free-stream turbulence. The turbulence is assumed to be generated by an (idealized) grid and matched asymptotic expansions are used to analyse the resulting flow over a finite thickness flat plate located in the downstream region. The characteristic Reynolds number Rλ based on the mesh size λ and free-stream velocity is assumed to be large, and the turbulence intensity ε is assumed to be small. The asymptotic flow structure is discussed for the generic case where the turbulence Reynolds number εRλ and the plate thickness and are held fixed (at O(1) and O(λ), respectively) in the limit as [Formula: see text] and ε→0. But various limiting cases are considered in order to explain the relevant transition mechanisms. It is argued that there are two types of streak-like structures that can play a role in the transition process: (i) those that appear in the downstream region and are generated by streamwise vorticity in upstream flow and (ii) those that are concentrated near the leading edge and are generated by plate normal vorticity in upstream flow. The former are relatively unaffected by leading edge geometry and are usually referred to as Klebanoff modes while the latter are strongly affected by leading edge geometry and are more streamwise vortex-like in appearance. © 2014 The Author(s) Published by the Royal Society. All rights reserved.

Development of a linearized unsteady aerodynamic analysis for cascade gust response predictions

NASA Technical Reports Server (NTRS)

Verdon, Joseph M.; Hall, Kenneth C.

1990-01-01

A method for predicting the unsteady aerodynamic response of a cascade of airfoils to entropic, vortical, and acoustic gust excitations is being developed. Here, the unsteady flow is regarded as a small perturbation of a nonuniform isentropic and irrotational steady background flow. A splitting technique is used to decompose the linearized unsteady velocity into rotational and irrotational parts leading to equations for the complex amplitudes of the linearized unsteady entropy, rotational velocity, and velocity potential that are coupled only sequentially. The entropic and rotational velocity fluctuations are described by transport equations for which closed-form solutions in terms of the mean-flow drift and stream functions can be determined. The potential fluctuation is described by an inhomogeneous convected wave equation in which the source term depends on the rotational velocity field, and is determined using finite-difference procedures. The analytical and numerical techniques used to determine the linearized unsteady flow are outlined. Results are presented to indicate the status of the solution procedure and to demonstrate the impact of blade geometry and mean blade loading on the aerodynamic response of cascades to vortical gust excitations. The analysis described herein leads to very efficient predictions of cascade unsteady aerodynamic response phenomena making it useful for turbomachinery aeroelastic and aeroacoustic design applications.

Front propagation in a regular vortex lattice: Dependence on the vortex structure.

PubMed

Beauvier, E; Bodea, S; Pocheau, A

2017-11-01

We investigate the dependence on the vortex structure of the propagation of fronts in stirred flows. For this, we consider a regular set of vortices whose structure is changed by varying both their boundary conditions and their aspect ratios. These configurations are investigated experimentally in autocatalytic solutions stirred by electroconvective flows and numerically from kinematic simulations based on the determination of the dominant Fourier mode of the vortex stream function in each of them. For free lateral boundary conditions, i.e., in an extended vortex lattice, it is found that both the flow structure and the front propagation negligibly depend on vortex aspect ratios. For rigid lateral boundary conditions, i.e., in a vortex chain, vortices involve a slight dependence on their aspect ratios which surprisingly yields a noticeable decrease of the enhancement of front velocity by flow advection. These different behaviors reveal a sensitivity of the mean front velocity on the flow subscales. It emphasizes the intrinsic multiscale nature of front propagation in stirred flows and the need to take into account not only the intensity of vortex flows but also their inner structure to determine front propagation at a large scale. Differences between experiments and simulations suggest the occurrence of secondary flows in vortex chains at large velocity and large aspect ratios.

Vorticity and Λ polarization in baryon rich matter

NASA Astrophysics Data System (ADS)

Baznat, Mircea; Gudima, Konstantin; Prokhorov, George; Sorin, Alexander; Teryaev, Oleg; Zakharov, Valentin

2018-02-01

The polarization of Λ hyperons due to axial chiral vortical effect is discussed. The effect is proportional to (strange) chemical potential and is pronounced at lower energies in baryon-rich matter. The polarization of ¯ has the same sihn and larger magnitude. The emergence of vortical structures is observed in kinetic QGSM models. The hydrodynamical helicity separation receives the contribution of longitudinal velocity and vorticity implying the quadrupole structure of the latter. The transition from the quark vortical effects to baryons in confined phase may be achieved by exploring the axial charge. At the hadronic level the polarization corresponds to the cores of quantized vortices in pionic superfluid. The chiral vortical effects may be also studied in the frmework of Wigner function establishing the relation to the thermodynamical approach to polarization.

Polarization in heavy-ion collisions: magnetic field and vorticity

NASA Astrophysics Data System (ADS)

Baznat, M.; Gudima, K.; Prokhorov, G.; Sorin, A.; Teryaev, O.; Zakharov, V.

2017-12-01

The polarization of hyperons due to axial chiral vortical effect is discussed. The effect is proportional to (strange) chemical potential and is pronounced at lower energies, contrary to that of magnetic field. The polarization of antihyperons has the same sign and larger magnitude. The emergence of vortical structures is observed in kinetic QGSM models. The hydrodynamical helicity separation receives the contribution of longitudinal velocity and vorticity implying the quadrupole structure of the latter. The transition from the quark vortical effects to baryons in confined phase may be achieved by exploring the axial charge. At the hadronic level the polarization corresponds to the cores of quantized vortices in pionic superfluid. The chiral vortical effects may be also studied in the frmework of Wigner function establishing the relation to the thermodynamical approach to polarization.

Analysis of Influence of Heat Insulation on the Thermal Regime of Storage Tanks with Liquefied Natural Gas

NASA Astrophysics Data System (ADS)

Maksimov, Vyacheslav I.; Nagornova, Tatiana A.; Glazyrin, Viktor P.; Shestakov, Igor A.

2016-02-01

Is numerically investigated the process of convective heat transfer in the reservoirs of liquefied natural gas (LNG). The regimes of natural convection in a closed rectangular region with different intensity of heat exchange at the external borders are investigated. Is solved the time-dependent system of energy and Navier-Stokes equations in the dimensionless variables "vorticity - the stream function". Are obtained distributions of the hydrodynamic parameters and temperatures, that characterize basic regularities of the processes. The special features of the formation of circulation flows are isolated and the analysis of the temperature distribution in the solution region is carried out. Is shown the influence of geometric characteristics and intensity of heat exchange on the outer boundaries of reservoir on the temperature field in the LNG storage.

A rigorous solution of the Navier-Stokes equations for unsteady viscous flow at high Reynolds numbers around oscillating airfoils

NASA Technical Reports Server (NTRS)

Bratanow, T.; Aksu, H.; Spehert, T.

1975-01-01

A method based on the Navier-Stokes equations was developed for analyzing the unsteady incompressible viscous flow around oscillating airfoils at high Reynolds numbers. The Navier-Stokes equations have been integrated in their classical Helmholtz vorticity transport equation form, and the instantaneous velocity field at each time step was determined by the solution of Poisson's equation. A refined finite element was utilized to allow for a conformable solution of the stream function and its first space derivatives at the element interfaces. A corresponding set of accurate boundary conditions was applied; thus obtaining a rigorous solution for the velocity field. The details of the computational procedure and examples of computed results describing the unsteady flow characteristics around the airfoil are presented.

Critical Layers and Protoplanetary Disk Turbulence

NASA Astrophysics Data System (ADS)

Umurhan, Orkan M.; Shariff, Karim; Cuzzi, Jeffrey N.

2016-10-01

A linear analysis of the zombie vortex instability (ZVI) is performed in a stratified shearing sheet setting for three model barotropic shear flows. The linear analysis is done by utilizing a Green’s function formulation to resolve the critical layers of the associated normal-mode problem. The instability is the result of a resonant interaction between a Rossby wave and a gravity wave that we refer to as Z-modes. The associated critical layer is the location where the Doppler-shifted frequency of a distant Rossby wave equals the local Brunt-Väisälä frequency. The minimum required Rossby number for instability, {\\mathtt{Ro}}=0.2, is confirmed for parameter values reported in the literature. It is also found that the shear layer supports the instability in the limit where stratification vanishes. The ZVI is examined in a jet model, finding that the instability can occur for {\\mathtt{Ro}}=0.05. Nonlinear vorticity forcing due to unstable Z-modes is shown to result in the creation of a jet flow at the critical layer emerging as the result of the competition between the vertical lifting of perturbation radial vorticity and the radial transport of perturbation vertical vorticity. We find that the picture of this instability leading to a form of nonlinearly driven self-replicating pattern of creation and destruction is warranted: a parent jet spawns a growing child jet at associated critical layers. A mature child jet creates a next generation of child jets at associated critical layers of the former while simultaneously contributing to its own destruction via the Rossby wave instability.

Quasiconservation laws for compressible three-dimensional Navier-Stokes flow.

PubMed

Gibbon, J D; Holm, D D

2012-10-01

We formulate the quasi-Lagrangian fluid transport dynamics of mass density ρ and the projection q=ω·∇ρ of the vorticity ω onto the density gradient, as determined by the three-dimensional compressible Navier-Stokes equations for an ideal gas, although the results apply for an arbitrary equation of state. It turns out that the quasi-Lagrangian transport of q cannot cross a level set of ρ. That is, in this formulation, level sets of ρ (isopycnals) are impermeable to the transport of the projection q.

Large Eddy Simulation of Vertical Axis Wind Turbine wakes; Part I: from the airfoil performance to the very far wake

NASA Astrophysics Data System (ADS)

Chatelain, Philippe; Duponcheel, Matthieu; Caprace, Denis-Gabriel; Marichal, Yves; Winckelmans, Gregoire

2017-11-01

A vortex particle-mesh (VPM) method with immersed lifting lines has been developed and validated. Based on the vorticity-velocity formulation of the Navier-Stokes equations, it combines the advantages of a particle method and of a mesh-based approach. The immersed lifting lines handle the creation of vorticity from the blade elements and its early development. Large-eddy simulation (LES) of vertical axis wind turbine (VAWT) flows is performed. The complex wake development is captured in detail and over up to 15 diameters downstream: from the blades to the near-wake coherent vortices and then through the transitional ones to the fully developed turbulent far wake (beyond 10 rotor diameters). The statistics and topology of the mean flow are studied with respect to the VAWT geometry and its operating point. The computational sizes also allow insights into the detailed unsteady vortex dynamics and topological flow features, such as a recirculation region influenced by the tip speed ratio and the rotor geometry.

Optical diagnostics and computational modeling of reacting and non-reacting single and multiphase flows

NASA Astrophysics Data System (ADS)

Basu, Saptarshi

Three critical problem domains namely water transport in PEM fuel cell, interaction of vortices with diffusion flames and laminar diffusion layers and thermo-physical processes in droplets heated by a plasma or monochromatic radiation have been analyzed in this dissertation. The first part of the dissertation exhibits a unique, in situ, line-of-sight measurements of water vapor partial pressure and temperature in single and multiple gas channels on the cathode side of an operating PEM fuel cell. Tunable diode laser absorption spectroscopy was employed for these measurements for which water transitions sensitive to temperature and partial pressure were utilized. The technique was demonstrated in a PEM fuel cell operating under both steady state and time-varying load conditions. The second part of the dissertation is dedicated to the study of vortex interaction with laminar diffusion flame and non-reacting diffusion layers. For the non-reacting case, a detailed computational study of scalar mixing in a laminar vortex is presented for vortices generated between two gas streams. A detailed parametric study was conducted to determine the effects of vortex strength, convection time, and non-uniform temperature on scalar mixing characteristics. For the reacting case, an experimental study of the interaction of a planar diffusion flame with a line vortex is presented. The flame-vortex interactions are diagnosed by laser induced incandescence for soot yield and by particle image velocimetry for vortex flow characterization. The soot topography was studied as a function of the vortex strength, residence time, flame curvature and the reactant streams from which vortices are initiated. The third part of the dissertation is modeling of thermo-physical processes in liquid ceramic precursor droplets injected into plasma as used in the thermal spray industry to generate thermal barrier coatings on high value materials. Models include aerodynamic droplet break-up process, mixing of droplets in the high temperature plasma, heat and mass transfer within individual droplets as well as droplet precipitation and internal pressurization. The last part of the work is also concerned with the modeling of thermo-physical processes in liquid ceramic precursor droplets heated by monochromatic radiation. Purpose of this work was to evaluate the feasibility of studying precipitation kinetics and morphological changes in a droplet by mimicking similar heating rates as the plasma.

Roughness-induced generation of crossflow vortices in three-dimensional boundary layers

NASA Technical Reports Server (NTRS)

Choudhari, Meelan

1993-01-01

The receptivity theory of Goldstein and Ruban is extended within the nonasymptotic (quasi-parallel) framework of Zavol'skii et al to predict the roughness-induced generation of stationary and nonstationary instability waves in three-dimensional, incompressible boundary layers. The influence of acoustic-wave orientation, as well as that of different types of roughness geometries, including isolated roughness elements, periodic arrays, and two-dimensional lattices of compact roughness shapes, as well as random, but spatially homogeneous roughness distributions, is examined. The parametric study for the Falkner-Skan-Cooke family of boundary layers supports our earlier conjecture that the initial amplitudes of roughness-induced stationary vortices are likely to be significantly larger than the amplitudes of similarly induced nonstationary vortices in the presence of acoustic disturbances in the free stream. Maximum unsteady receptivity occurs when the acoustic velocity fluctuation is aligned with the wavenumber vector of the unsteady vortex mode. On the other hand, roughness arrays that are oriented somewhere close to the group velocity direction are likely to produce higher instability amplitudes. Limitations of the nonasymptotic theory are discussed, and future work is suggested.

Nonlinear effects in the bounded dust-vortex flow in plasma

NASA Astrophysics Data System (ADS)

Laishram, Modhuchandra; Sharma, Devendra; Chattopdhyay, Prabal K.; Kaw, Predhiman K.

2017-03-01

The vortex structures in a cloud of electrically suspended dust in a streaming plasma constitutes a driven system with a rich nonlinear flow regime. Experimentally recovered toroidal formations of this system have motivated study of its volumetrically driven-dissipative vortex flow dynamics using two-dimensional hydrodynamics in the incompressible Navier-Stokes regime. Nonlinear equilibrium solutions are obtained for this system where a nonuniformly driven two-dimensional dust flow exhibits distinct regions of localized accelerations and strong friction caused by stationary fluids at the confining boundaries resisting the dust flow. In agreement with observations in experiments, it is demonstrated that the nonlinear effects appear in the limit of small viscosity, where the primary vortices form scaling with the most dominant spatial scales of the domain topology and develop separated virtual boundaries along their periphery. This separation is triggered beyond a critical dust viscosity that signifies a structural bifurcation. Emergence of uniform vorticity core and secondary vortices with a newer level of identical dynamics highlights the applicability of the studied dynamics to gigantic vortex flows, such as the Jovian great red spot, to microscopic biophysical intracellular activity.

Laser Doppler technology applied to atmospheric environmental operating problems

NASA Technical Reports Server (NTRS)

Weaver, E. A.; Bilbro, J. W.; Dunkin, J. A.; Jeffreys, H. B.

1976-01-01

Carbon dioxide laser Doppler ground wind data were very favorably compared with data from standard anemometers. As a result of these measurements, two breadboard systems were developed for taking research data: a continuous wave velocimeter and a pulsed Doppler system. The scanning continuous wave laser Doppler velocimeter developed for detecting, tracking and measuring aircraft wake vortices was successfully tested at an airport where it located vortices to an accuracy of 3 meters at a range of 150 meters. The airborne pulsed laser Doppler system was developed to detect and measure clear air turbulence (CAT). This system was tested aboard an aircraft, but jet stream CAT was not encountered. However, low altitude turbulence in cumulus clouds near a mountain range was detected by the system and encountered by the aircraft at the predicted time.

InSTREAM: the individual-based stream trout research and environmental assessment model

Treesearch

Steven F. Railsback; Bret C. Harvey; Stephen K. Jackson; Roland H. Lamberson

2009-01-01

This report documents Version 4.2 of InSTREAM, including its formulation, software, and application to research and management problems. InSTREAM is a simulation model designed to understand how stream and river salmonid populations respond to habitat alteration, including altered flow, temperature, and turbidity regimes and changes in channel morphology. The model...

Influence of the least-squares phase on optical vortices in strongly scintillated beams

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen Mingzhou; Roux, Filippus S.; National Laser Centre, CSIR, P.O. Box 395, Pretoria 0001

2009-07-15

The optical vortices that exist in strongly scintillated beams make it difficult for conventional adaptive optics systems to remove the phase distortions. When the least-squares reconstructed phase is removed, the vortices still remain. However, we found that the removal of the least-squares phase induces a portion of the vortices to be annihilated during subsequent propagation, causing a reduction in the total number of vortices. This can be understood in terms of the restoration of equilibrium between explicit vortices, which are visible in the phase function, and vortex bound states, which are somehow encoded in the continuous phase fluctuations. Numerical simulationsmore » are provided to show that the total number of optical vortices in a strongly scintillated beam can be reduced significantly after a few steps of least-squares phase corrections.« less

Agradient velocity, vortical motion and gravity waves in a rotating shallow-water model

NASA Astrophysics Data System (ADS)

Sutyrin Georgi, G.

2004-07-01

A new approach to modelling slow vortical motion and fast inertia-gravity waves is suggested within the rotating shallow-water primitive equations with arbitrary topography. The velocity is exactly expressed as a sum of the gradient wind, described by the Bernoulli function,B, and the remaining agradient part, proportional to the velocity tendency. Then the equation for inverse potential vorticity,Q, as well as momentum equations for agradient velocity include the same source of intrinsic flow evolution expressed as a single term J (B, Q), where J is the Jacobian operator (for any steady state J (B, Q) = 0). Two components of agradient velocity are responsible for the fast inertia-gravity wave propagation similar to the traditionally used divergence and ageostrophic vorticity. This approach allows for the construction of balance relations for vortical dynamics and potential vorticity inversion schemes even for moderate Rossby and Froude numbers assuming the characteristic value of |J(B, Q)| = to be small. The components of agradient velocity are used as the fast variables slaved to potential vorticity that allows for diagnostic estimates of the velocity tendency, the direct potential vorticity inversion with the accuracy of 2 and the corresponding potential vorticity-conserving agradient velocity balance model (AVBM). The ultimate limitations of constructing the balance are revealed in the form of the ellipticity condition for balanced tendency of the Bernoulli function which incorporates both known criteria of the formal stability: the gradient wind modified by the characteristic vortical Rossby wave phase speed should be subcritical. The accuracy of the AVBM is illustrated by considering the linear normal modes and coastal Kelvin waves in the f-plane channel with topography.

Free stream capturing in fluid conservation law for moving coordinates in three dimensions

NASA Technical Reports Server (NTRS)

Obayashi, Shigeru

1991-01-01

The free-stream capturing technique for both the finite-volume (FV) and finite-difference (FD) framework is summarized. For an arbitrary motion of the grid, the FV analysis shows that volumes swept by all six surfaces of the cell have to be computed correctly. This means that the free-stream capturing time-metric terms should be calculated not only from a surface vector of a cell at a single time level, but also from a volume swept by the cell surface in space and time. The FV free-stream capturing formulation is applicable to the FD formulation by proper translation from an FV cell to an FD mesh.

Methodology for Scour Evaluation of US Army Installation Bridges: A Proposed Evaluation for Scour Risk and Channel Instability

DTIC Science & Technology

2013-01-01

rainfall runoff relations ....................................................... 9  Step 2: evaluate hydraulic conditions...earthquakes, landslides , or forest fires might result in a large sediment load. Major changes in the stream might be reflected in aggradation, degradation...removal of material from around piers, abutments, and embankments, caused by an acceleration of flow and by vortices induced by the flow around

A framework for estimating potential fluid flow from digital imagery

NASA Astrophysics Data System (ADS)

Luttman, Aaron; Bollt, Erik M.; Basnayake, Ranil; Kramer, Sean; Tufillaro, Nicholas B.

2013-09-01

Given image data of a fluid flow, the flow field, ⟨u,v⟩, governing the evolution of the system can be estimated using a variational approach to optical flow. Assuming that the flow field governing the advection is the symplectic gradient of a stream function or the gradient of a potential function—both falling under the category of a potential flow—it is natural to re-frame the optical flow problem to reconstruct the stream or potential function directly rather than the components of the flow individually. There are several advantages to this framework. Minimizing a functional based on the stream or potential function rather than based on the components of the flow will ensure that the computed flow is a potential flow. Next, this approach allows a more natural method for imposing scientific priors on the computed flow, via regularization of the optical flow functional. Also, this paradigm shift gives a framework—rather than an algorithm—and can be applied to nearly any existing variational optical flow technique. In this work, we develop the mathematical formulation of the potential optical flow framework and demonstrate the technique on synthetic flows that represent important dynamics for mass transport in fluid flows, as well as a flow generated by a satellite data-verified ocean model of temperature transport.

Neural Integration in Body Perception.

PubMed

Ramsey, Richard

2018-06-19

The perception of other people is instrumental in guiding social interactions. For example, the appearance of the human body cues a wide range of inferences regarding sex, age, health, and personality, as well as emotional state and intentions, which influence social behavior. To date, most neuroscience research on body perception has aimed to characterize the functional contribution of segregated patches of cortex in the ventral visual stream. In light of the growing prominence of network architectures in neuroscience, the current article reviews neuroimaging studies that measure functional integration between different brain regions during body perception. The review demonstrates that body perception is not restricted to processing in the ventral visual stream but instead reflects a functional alliance between the ventral visual stream and extended neural systems associated with action perception, executive functions, and theory of mind. Overall, these findings demonstrate how body percepts are constructed through interactions in distributed brain networks and underscore that functional segregation and integration should be considered together when formulating neurocognitive theories of body perception. Insight from such an updated model of body perception generalizes to inform the organizational structure of social perception and cognition more generally and also informs disorders of body image, such as anorexia nervosa, which may rely on atypical integration of body-related information.

Least-squares finite element solution of 3D incompressible Navier-Stokes problems

NASA Technical Reports Server (NTRS)

Jiang, Bo-Nan; Lin, Tsung-Liang; Povinelli, Louis A.

1992-01-01

Although significant progress has been made in the finite element solution of incompressible viscous flow problems. Development of more efficient methods is still needed before large-scale computation of 3D problems becomes feasible. This paper presents such a development. The most popular finite element method for the solution of incompressible Navier-Stokes equations is the classic Galerkin mixed method based on the velocity-pressure formulation. The mixed method requires the use of different elements to interpolate the velocity and the pressure in order to satisfy the Ladyzhenskaya-Babuska-Brezzi (LBB) condition for the existence of the solution. On the other hand, due to the lack of symmetry and positive definiteness of the linear equations arising from the mixed method, iterative methods for the solution of linear systems have been hard to come by. Therefore, direct Gaussian elimination has been considered the only viable method for solving the systems. But, for three-dimensional problems, the computer resources required by a direct method become prohibitively large. In order to overcome these difficulties, a least-squares finite element method (LSFEM) has been developed. This method is based on the first-order velocity-pressure-vorticity formulation. In this paper the LSFEM is extended for the solution of three-dimensional incompressible Navier-Stokes equations written in the following first-order quasi-linear velocity-pressure-vorticity formulation.

Fast acoustic streaming in standing waves: generation of an additional outer streaming cell.

PubMed

Reyt, Ida; Daru, Virginie; Bailliet, Hélène; Moreau, Solène; Valière, Jean-Christophe; Baltean-Carlès, Diana; Weisman, Catherine

2013-09-01

Rayleigh streaming in a cylindrical acoustic standing waveguide is studied both experimentally and numerically for nonlinear Reynolds numbers from 1 to 30 [Re(NL)=(U0/c0)(2)(R/δν)(2), with U0 the acoustic velocity amplitude at the velocity antinode, c0 the speed of sound, R the tube radius, and δν the acoustic boundary layer thickness]. Streaming velocity is measured by means of laser Doppler velocimetry in a cylindrical resonator filled with air at atmospheric pressure at high intensity sound levels. The compressible Navier-Stokes equations are solved numerically with high resolution finite difference schemes. The resonator is excited by shaking it along the axis at imposed frequency. Results of measurements and of numerical calculation are compared with results given in the literature and with each other. As expected, the axial streaming velocity measured and calculated agrees reasonably well with the slow streaming theory for small ReNL but deviates significantly from such predictions for fast streaming (ReNL>1). Both experimental and numerical results show that when ReNL is increased, the center of the outer streaming cells are pushed toward the acoustic velocity nodes until counter-rotating additional vortices are generated near the acoustic velocity antinodes.

Acoustic streaming: an arbitrary Lagrangian-Eulerian perspective.

PubMed

Nama, Nitesh; Huang, Tony Jun; Costanzo, Francesco

2017-08-25

We analyse acoustic streaming flows using an arbitrary Lagrangian Eulerian (ALE) perspective. The formulation stems from an explicit separation of time scales resulting in two subproblems: a first-order problem, formulated in terms of the fluid displacement at the fast scale, and a second-order problem, formulated in terms of the Lagrangian flow velocity at the slow time scale. Following a rigorous time-averaging procedure, the second-order problem is shown to be intrinsically steady, and with exact boundary conditions at the oscillating walls. Also, as the second-order problem is solved directly for the Lagrangian velocity, the formulation does not need to employ the notion of Stokes drift, or any associated post-processing, thus facilitating a direct comparison with experiments. Because the first-order problem is formulated in terms of the displacement field, our formulation is directly applicable to more complex fluid-structure interaction problems in microacoustofluidic devices. After the formulation's exposition, we present numerical results that illustrate the advantages of the formulation with respect to current approaches.

Introduction to Generalized Functions with Applications in Aerodynamics and Aeroacoustics

NASA Technical Reports Server (NTRS)

Farassat, F.

1994-01-01

Generalized functions have many applications in science and engineering. One useful aspect is that discontinuous functions can be handled as easily as continuous or differentiable functions and provide a powerful tool in formulating and solving many problems of aerodynamics and acoustics. Furthermore, generalized function theory elucidates and unifies many ad hoc mathematical approaches used by engineers and scientists. We define generalized functions as continuous linear functionals on the space of infinitely differentiable functions with compact support, then introduce the concept of generalized differentiation. Generalized differentiation is the most important concept in generalized function theory and the applications we present utilize mainly this concept. First, some results of classical analysis, are derived with the generalized function theory. Other applications of the generalized function theory in aerodynamics discussed here are the derivations of general transport theorems for deriving governing equations of fluid mechanics, the interpretation of the finite part of divergent integrals, the derivation of the Oswatitsch integral equation of transonic flow, and the analysis of velocity field discontinuities as sources of vorticity. Applications in aeroacoustics include the derivation of the Kirchhoff formula for moving surfaces, the noise from moving surfaces, and shock noise source strength based on the Ffowcs Williams-Hawkings equation.

Interaction of N-vortex structures in a continuum, including atmosphere, hydrosphere and plasma

NASA Astrophysics Data System (ADS)

Belashov, Vasily Yu.

2017-10-01

The results of analysis and numerical simulation of evolution and interaction of the N-vortex structures of various configuration and different vorticities in the continuum including atmosphere, hydrosphere and plasma are presented. It is found that in dependence on initial conditions the regimes of weak interaction with quasi-stationary evolution and active interaction with the "phase intermixing", when the evolution can lead to formation of complex forms of vorticity regions, are realized in the N-vortex systems. For the 2-vortex interaction the generalized critical parameter determining qualitative character of interaction of vortices is introduced. It is shown that for given initial conditions its value divides modes of active interaction and quasi-stationary evolution. The results of simulation of evolution and interaction of the two-dimensional and three-dimensional vortex structures, including such phenomena as dynamics of the atmospheric synoptic vortices of cyclonic types and tornado, hydrodynamic 4-vortex interaction and also interaction in the systems of a type of "hydrodynamic vortex - dust particles" are presented. The applications of undertaken approach to the problems of such plasma systems as streams of charged particles in a uniform magnetic field B and plasma clouds in the ionosphere are considered. It is shown that the results obtained have obvious applications in studies of the dynamics of the vortex structures dynamics in atmosphere, hydrosphere and plasma.

Three-Dimensional Navier-Stokes Simulations with Two-Equation Turbulence Models of Intersecting Shock-Waves/Turbulent Boundary Layer at Mach 8.3

NASA Technical Reports Server (NTRS)

Bardina, J. E.; Coakley, T. J.

1994-01-01

An investigation of the numerical simulation with two-equation turbulence models of a three-dimensional hypersonic intersecting (SWTBL) shock-wave/turbulent boundary layer interaction flow is presented. The flows are solved with an efficient implicit upwind flux-difference split Reynolds-averaged Navier-Stokes code. Numerical results are compared with experimental data for a flow at Mach 8.28 and Reynolds number 5.3x10(exp 6) with crossing shock-waves and expansion fans generated by two lateral 15 fins located on top of a cold-wall plate. This experiment belongs to the hypersonic database for modeling validation. Simulations show the development of two primary counter-rotating cross-flow vortices and secondary turbulent structures under the main vortices and in each corner singularity inside the turbulent boundary layer. A significant loss of total pressure is produced by the complex interaction between the main vortices and the uplifted jet stream of the boundary layer. The overall agreement between computational and experimental data is generally good. The turbulence modeling corrections show improvements in the predictions of surface heat transfer distribution and an increase in the strength of the cross-flow vortices. Accurate predictions of the outflow flowfield is found to require accurate modeling of the laminar/turbulent boundary layers on the fin walls.

Calculation of three-dimensional (3-D) internal flow by means of the velocity-vorticity formulation on a staggered grid

NASA Technical Reports Server (NTRS)

Stremel, Paul M.

1995-01-01

A method has been developed to accurately compute the viscous flow in three-dimensional (3-D) enclosures. This method is the 3-D extension of a two-dimensional (2-D) method developed for the calculation of flow over airfoils. The 2-D method has been tested extensively and has been shown to accurately reproduce experimental results. As in the 2-D method, the 3-D method provides for the non-iterative solution of the incompressible Navier-Stokes equations by means of a fully coupled implicit technique. The solution is calculated on a body fitted computational mesh incorporating a staggered grid methodology. In the staggered grid method, the three components of vorticity are defined at the centers of the computational cell sides, while the velocity components are defined as normal vectors at the centers of the computational cell faces. The staggered grid orientation provides for the accurate definition of the vorticity components at the vorticity locations, the divergence of vorticity at the mesh cell nodes and the conservation of mass at the mesh cell centers. The solution is obtained by utilizing a fractional step solution technique in the three coordinate directions. The boundary conditions for the vorticity and velocity are calculated implicitly as part of the solution. The method provides for the non-iterative solution of the flow field and satisfies the conservation of mass and divergence of vorticity to machine zero at each time step. To test the method, the calculation of simple driven cavity flows have been computed. The driven cavity flow is defined as the flow in an enclosure driven by a moving upper plate at the top of the enclosure. To demonstrate the ability of the method to predict the flow in arbitrary cavities, results will he shown for both cubic and curved cavities.

Origin of chaos near three-dimensional quantum vortices: A general Bohmian theory

NASA Astrophysics Data System (ADS)

Tzemos, Athanasios C.; Efthymiopoulos, Christos; Contopoulos, George

2018-04-01

We provide a general theory for the structure of the quantum flow near three-dimensional (3D) nodal lines, i.e., one-dimensional loci where the 3D wave function becomes equal to zero. In suitably defined coordinates (comoving with the nodal line) the generic structure of the flow implies the formation of 3D quantum vortices. We show that such vortices are accompanied by nearby invariant lines of the comoving quantum flow, called X lines, which are normally hyperbolic. Furthermore, the stable and unstable manifolds of the X lines produce chaotic scatterings of nearby quantum (Bohmian) trajectories, thus inducing an intricate form of the quantum current in the neighborhood of each 3D quantum vortex. Generic formulas describing the structure around 3D quantum vortices are provided, applicable to an arbitrary choice of 3D wave function. We also give specific numerical examples as well as a discussion of the physical consequences of chaos near 3D quantum vortices.

Origin of chaos near three-dimensional quantum vortices: A general Bohmian theory.

PubMed

Tzemos, Athanasios C; Efthymiopoulos, Christos; Contopoulos, George

2018-04-01

We provide a general theory for the structure of the quantum flow near three-dimensional (3D) nodal lines, i.e., one-dimensional loci where the 3D wave function becomes equal to zero. In suitably defined coordinates (comoving with the nodal line) the generic structure of the flow implies the formation of 3D quantum vortices. We show that such vortices are accompanied by nearby invariant lines of the comoving quantum flow, called X lines, which are normally hyperbolic. Furthermore, the stable and unstable manifolds of the X lines produce chaotic scatterings of nearby quantum (Bohmian) trajectories, thus inducing an intricate form of the quantum current in the neighborhood of each 3D quantum vortex. Generic formulas describing the structure around 3D quantum vortices are provided, applicable to an arbitrary choice of 3D wave function. We also give specific numerical examples as well as a discussion of the physical consequences of chaos near 3D quantum vortices.

Measurements in Transitional Boundary Layers Under High Free-Stream Turbulence and Strong Acceleration Conditions

NASA Technical Reports Server (NTRS)

Volino, Ralph J.; Simon, Terrence W.

1995-01-01

Measurements from transitional, heated boundary layers along a concave-curved test wall are presented and discussed. A boundary layer subject to low free-stream turbulence intensity (FSTI), which contains stationary streamwise (Gortler) vortices, is documented. The low FSTI measurements are followed by measurements in boundary layers subject to high (initially 8%) free-stream turbulence intensity and moderate to strong streamwise acceleration. Conditions were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. Mean flow characteristics as well as turbulence statistics, including the turbulent shear stress, turbulent heat flux, and turbulent Prandtl number, are documented. A technique called "octant analysis" is introduced and applied to several cases from the literature as well as to data from the present study. Spectral analysis was applied to describe the effects of turbulence scales of different sizes during transition. To the authors'knowledge, this is the first detailed documentation of boundary layer transition under such high free-stream turbulence conditions.

Size-sensitive particle trajectories in three-dimensional micro-bubble acoustic streaming flows

NASA Astrophysics Data System (ADS)

Volk, Andreas; Rossi, Massimiliano; Hilgenfeldt, Sascha; Rallabandi, Bhargav; Kähler, Christian; Marin, Alvaro

2015-11-01

Oscillating microbubbles generate steady streaming flows with interesting features and promising applications for microparticle manipulation. The flow around oscillating semi-cylindrical bubbles has been typically assumed to be independent of the axial coordinate. However, it has been recently revealed that particle motion is strongly three-dimensional: Small tracer particles follow vortical trajectories with pronounced axial displacements near the bubble, weaving a toroidal stream-surface. A well-known consequence of bubble streaming flows is size-dependent particle migration, which can be exploited for sorting and trapping of microparticles in microfluidic devices. In this talk, we will show how the three-dimensional toroidal topology found for small tracer particles is modified as the particle size increases up to 1/3 of the bubble radius. Our results show size-sensitive particle positioning along the axis of the semi-cylindrical bubble. In order to analyze the three-dimensional sorting and trapping capabilities of the system, experiments with an imposed flow and polydisperse particle solutions are also shown.

Interaction of a trailing vortex with an oscillating wing

NASA Astrophysics Data System (ADS)

McKenna, C.; Fishman, G.; Rockwell, D.

2018-01-01

A technique of particle image velocimetry is employed to characterize the flow structure of a trailing vortex incident upon the tip region of an oscillating wing (plate). The amplitude and velocity of the wing are nearly two orders of magnitude smaller than the wing chord and free stream velocity, respectively. Depending upon the outboard displacement of the incident vortex relative to the wing tip, distinctive patterns of upwash, downwash, and shed vorticity are observed. These patterns are a strong function of the phase of the wing motion during its oscillation cycle. At a given phase, the wing oscillation induces upwash that is reinforced by the upwash of the incident vortex, giving a maximum net upwash. Conversely, when these two origins of upwash counteract, rather than reinforce, one another during the oscillation cycle, the net upwash attains minimum value. Analogous interpretations hold for regions of maximum and minimum net downwash located outboard of the regions of upwash. The magnitude and scale of the vorticity shed from the tip of the wing are directly correlated with the net upwash, which takes different forms related to the outboard displacement of the incident vortex. As the location of the incident vortex is displaced towards the wing tip, both the maximum upwash and the maximum vorticity of the tip vortex initially increase and then decrease. For the limiting case where the incident vortex impinges directly upon the tip of the wing, there is no tip vortex or induced region of upwash. Furthermore, at small values of vortex displacement from the wing tip, the position of the incident vortex varies significantly from its nominal position during the oscillation cycle. All of the foregoing features are interpreted in conjunction with the flow topology in the form of streamlines and critical points, superposed on patterns of vorticity. It is shown that despite the small amplitude of the wing motion, the flow topology is fundamentally different at maximum positive and negative values of the velocity of the wing tip, that is, they are not symmetric.

Conditional sampling technique to test the applicability of the Taylor hypothesis for the large-scale coherent structures

NASA Technical Reports Server (NTRS)

Hussain, A. K. M. F.

1980-01-01

Comparisons of the distributions of large scale structures in turbulent flow with distributions based on time dependent signals from stationary probes and the Taylor hypothesis are presented. The study investigated an area in the near field of a 7.62 cm circular air jet at a Re of 32,000, specifically having coherent structures through small-amplitude controlled excitation and stable vortex pairing in the jet column mode. Hot-wire and X-wire anemometry were employed to establish phase averaged spatial distributions of longitudinal and lateral velocities, coherent Reynolds stress and vorticity, background turbulent intensities, streamlines and pseudo-stream functions. The Taylor hypothesis was used to calculate spatial distributions of the phase-averaged properties, with results indicating that the usage of the local time-average velocity or streamwise velocity produces large distortions.

Finite-analytic numerical solution of heat transfer in two-dimensional cavity flow

NASA Technical Reports Server (NTRS)

Chen, C.-J.; Naseri-Neshat, H.; Ho, K.-S.

1981-01-01

Heat transfer in cavity flow is numerically analyzed by a new numerical method called the finite-analytic method. The basic idea of the finite-analytic method is the incorporation of local analytic solutions in the numerical solutions of linear or nonlinear partial differential equations. In the present investigation, the local analytic solutions for temperature, stream function, and vorticity distributions are derived. When the local analytic solution is evaluated at a given nodal point, it gives an algebraic relationship between a nodal value in a subregion and its neighboring nodal points. A system of algebraic equations is solved to provide the numerical solution of the problem. The finite-analytic method is used to solve heat transfer in the cavity flow at high Reynolds number (1000) for Prandtl numbers of 0.1, 1, and 10.

Tropical cyclogenesis in a tropical wave critical layer: easterly waves

NASA Astrophysics Data System (ADS)

Dunkerton, T. J.; Montgomery, M. T.; Wang, Z.

2009-08-01

The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, is identified herein as the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. The entire sequence is likened to the development of a marsupial infant in its mother's pouch. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the "marsupial paradigm". A survey of 55 named tropical storms in 1998-2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cat's eyes, but are usually less regular, with one or more recirculation regions in the co-moving frame. It is shown that the kinematics of isolated proto-vortices carried by the wave also can be visualized in a frame of reference translating at or near the phase speed of the parent wave. The proper translation speeds for wave and vortex may vary with height owing to vertical shear and wave-vortex interaction. Some implications for entrainment/containment of vorticity and moisture in the cat's eye are discussed from this perspective, based on the observational survey.

A pseudo-dual-Doppler analysis of cyclic tornadogenesis

NASA Astrophysics Data System (ADS)

Dowell, David Collin

2000-06-01

Several tornadic storms formed in the Texas Panhandle on 8 June 1995, the date of the last mission of VORTEX (Verification of the Origins of Rotation in Tornadoes EXperiment). The southernmost storm in this severe weather outbreak produced a family of at least five tornadoes near the town of McLean. Airborne Doppler radar scans of this storm by the ELDORA (ELectra DOppler RAdar) offer the most detailed look to date at a storm producing a family of tornadoes. The goals of this study were twofold. The first was to determine a pseudo-dual-Doppler wind synthesis method in Cartesian coordinates appropriate for the analysis of the ELDORA data. Unique aspects of this part of the study include a comparison of wind synthesis methods based on variational formulations and the use of a non-uniform moving reference frame for the syntheses. A dual-Doppler formulation in which the radial velocity and continuity equations are all satisfied as weak constraints (Gamache 1997, Shapiro and Mewes 1999) yields a more accurate wind field than traditional (and variational) methods in which the radial velocity equations are satisfied exactly. The second goal of this study was to diagnose both the cyclic process and the formation of individual tornadoes. The McLean storm produced three large tornadoes at 18 min intervals. The last of these then lasted much longer (over one hour) and was stronger than the previous tornadoes. New pre-tornadic vortices formed on the east side of the updraft by tilting of strong environmental low-level horizontal vorticity into the vertical and then stretching of the vertical vorticity within the updraft. The vortices did not mature at low levels until they migrated to the west side of the updraft. Indirect evidence indicates that both baroclinic generation of horizontal vorticity and the rear downdraft may have played roles in tornado formation at this stage. The tornadic potential of a storm appears-to be related to the relative strength of low-level storm outflow and inflow beneath the west side of updraft. Cyclic tornadogenesis modes may be possible both when the inflow slightly dominates and when the outflow slightly dominates. The description of an inflow-dominated cyclic mode like that observed in the McLean storm is original. Internal cell interactions within the McLean storm appear to have helped the transition from the cyclic phase to a more steady phase.

Acoustic Streaming and Microparticle Enrichment within a Microliter Droplet Using a Lamb-Wave Resonator Array

NASA Astrophysics Data System (ADS)

Zhang, Hongxiang; Tang, Zifan; Wang, Zhan; Pan, Shuting; Han, Ziyu; Sun, Chongling; Zhang, Menglun; Duan, Xuexin; Pang, Wei

2018-06-01

We report the nonlinear acoustic streaming effect and the fast manipulation of microparticles by microelectromechanical Lamb-wave resonators in a microliter droplet. The device, consisting of four Lamb-wave resonators on a silicon die, generates cylindrical traveling waves in a liquid and efficiently drives nine horizontal vortices within a 1 -μ l droplet; the performance of the device coincides with the numerical model prediction. Experimentally, the particles are enriched at the stagnation center of the main vortex on the free surface of the droplet in open space without microfluidic channels. In addition, the trajectories of the particles in the droplet can be controlled by the excitation power.

Structure measurements in a synthetic turbulent boundary layer

NASA Astrophysics Data System (ADS)

Arakeri, Jaywant H.

Extensive hot-wire measurements were made to determine the structure of the large eddy in a synthetic turbulent boundary layer on a flat-plate model. The experiments were carried out in a wind tunnel at a nominal free-stream velocity of 12 m/s. The synthetic turbulent boundary layer had a hexagonal pattern of eddies and a ratio of streamwise scale to spanwise scale of 3.2:1. The measured celerity of the large eddy was 84.2 percent of the free-stream velocity. There was some loss of coherence, but very little distortion, as the eddies moved downstream. Several mean properties of the synthetic boundary layer were found to agree quite well with the mean properties of a natural turbulent boundary layer at the same Reynolds number. The large eddy is composed of a pair of primary counter-rotating vortices about five delta long in the steamwise direction and about one delta apart in the spanwise direction, where delta is the mean boundary-layer thickness. Definite signatures are obtained in terms of the mean skin-friction coefficient and the mean wake parameter averaged at constant phase. Velocities induced by the vortices are partly responsible for entrainment of irrotational fluid, for transport of momentum, for generation of Reynolds stresses, and for maintenance of streamwise and normal velocity in the outer flow.

Microalga propels along vorticity direction in a shear flow

NASA Astrophysics Data System (ADS)

Chengala, Anwar; Hondzo, Miki; Sheng, Jian

2013-05-01

Using high-speed digital holographic microscopy and microfluidics, we discover that, when encountering fluid flow shear above a threshold, unicellular green alga Dunaliella primolecta migrates unambiguously in the cross-stream direction that is normal to the plane of shear and coincides with the local fluid flow vorticity. The flow shear drives motile microalgae to collectively migrate in a thin two-dimensional horizontal plane and consequently alters the spatial distribution of microalgal cells within a given suspension. This shear-induced algal migration differs substantially from periodic rotational motion of passive ellipsoids, known as Jeffery orbits, as well as gyrotaxis by bottom-heavy swimming microalgae in a shear flow due to the subtle interplay between torques generated by gravity and viscous shear. Our findings could facilitate mechanistic solutions for modeling planktonic thin layers and sustainable cultivation of microalgae for human nutrition and bioenergy feedstock.

Acoustic streaming: an arbitrary Lagrangian–Eulerian perspective

PubMed Central

Nama, Nitesh; Huang, Tony Jun; Costanzo, Francesco

2017-01-01

We analyse acoustic streaming flows using an arbitrary Lagrangian Eulerian (ALE) perspective. The formulation stems from an explicit separation of time scales resulting in two subproblems: a first-order problem, formulated in terms of the fluid displacement at the fast scale, and a second-order problem, formulated in terms of the Lagrangian flow velocity at the slow time scale. Following a rigorous time-averaging procedure, the second-order problem is shown to be intrinsically steady, and with exact boundary conditions at the oscillating walls. Also, as the second-order problem is solved directly for the Lagrangian velocity, the formulation does not need to employ the notion of Stokes drift, or any associated post-processing, thus facilitating a direct comparison with experiments. Because the first-order problem is formulated in terms of the displacement field, our formulation is directly applicable to more complex fluid–structure interaction problems in microacoustofluidic devices. After the formulation’s exposition, we present numerical results that illustrate the advantages of the formulation with respect to current approaches. PMID:29051631

A complex analysis approach to the motion of uniform vortices

NASA Astrophysics Data System (ADS)

Riccardi, Giorgio

2018-02-01

A new mathematical approach to kinematics and dynamics of planar uniform vortices in an incompressible inviscid fluid is presented. It is based on an integral relation between Schwarz function of the vortex boundary and induced velocity. This relation is firstly used for investigating the kinematics of a vortex having its Schwarz function with two simple poles in a transformed plane. The vortex boundary is the image of the unit circle through the conformal map obtained by conjugating its Schwarz function. The resulting analysis is based on geometric and algebraic properties of that map. Moreover, it is shown that the steady configurations of a uniform vortex, possibly in presence of point vortices, can be also investigated by means of the integral relation. The vortex equilibria are divided in two classes, depending on the behavior of the velocity on the boundary, measured in a reference system rotating with this curve. If it vanishes, the analysis is rather simple. However, vortices having nonvanishing relative velocity are also investigated, in presence of a polygonal symmetry. In order to study the vortex dynamics, the definition of Schwarz function is then extended to a Lagrangian framework. This Lagrangian Schwarz function solves a nonlinear integrodifferential Cauchy problem, that is transformed in a singular integral equation. Its analytical solution is here approached in terms of successive approximations. The self-induced dynamics, as well as the interactions with a point vortex, or between two uniform vortices are analyzed.

Relaxation and self-organization in two-dimensional plasma and neutral fluid flow systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Das, Amita

Extensive numerical studies in the framework of a simplified two-dimensional model for neutral and plasma fluid for a variety of initial configurations and for both decaying and driven cases are carried out to illustrate relaxation toward a self-organized state. The dynamical model equation constitutes a simple choice for this purpose, e.g., the vorticity equation of the Navier-Stokes dynamics for the incompressible neutral fluids and the Hasegawa-Mima equation for plasma fluid flow system. Scatter plots are employed to observe a development of functional relationship, if any, amidst the generalized vorticity and its Laplacian. It is seen that they do not satisfymore » a linear relationship as the well known variational approach of enstrophy minimization subject to constancy of the energy integral for the two-dimensional (2D) system suggests. The observed nonlinear functional relationship is understood by separating the contribution to the scatter plot from spatial regions with intense vorticity patches and those of the background flow region where the background vorticity is weak or absent altogether. It is shown that such a separation has close connection with the known exact analytical solutions of the system. The analytical solutions are typically obtained by assuming a finite source of vorticity for the inner core of the localized structure, which is then matched with the solution in the outer region where vorticity is chosen to be zero. The work also demonstrates that the seemingly ad hoc choice of the linear vorticity source function for the inner region is in fact consistent with the self-organization paradigm of the 2D systems.« less

Quasi-steady vortical structures in vertically vibrating soap films

NASA Astrophysics Data System (ADS)

Vega, José M.; Higuera, F. J.; Weidman, P. D.

1998-10-01

An analysis of the quasi-steady streaming of the liquid in a vertically vibrated horizontal soap film is reported. The air around the soap film is seen to play a variety of roles: it transmits normal and tangential oscillatory stresses to the film, damps out Marangoni waves, and forces non-oscillatory deflection of the film and tangential motion of the liquid. Non-oscillatory volume forcing originating inside the liquid is also analysed. This forcing dominates the quasi-steady streaming when the excitation frequency is close to the eigenfrequency of a Marangoni mode of the soap film, while both volume forcing in the liquid and surface forcing of the gas on the liquid are important when no Marangoni mode resonates. Different manners by which the combined forcings can induce quasi-steady streaming motion are discussed and some numerical simulations of the quasi-steady liquid flow are presented.

Mach's principle: Exact frame-dragging via gravitomagnetism in perturbed Friedmann-Robertson-Walker universes with K=(±1,0)

NASA Astrophysics Data System (ADS)

Schmid, Christoph

2009-03-01

We show that there is exact dragging of the axis directions of local inertial frames by a weighted average of the cosmological energy currents via gravitomagnetism for all linear perturbations of all Friedmann-Robertson-Walker (FRW) universes and of Einstein’s static closed universe, and for all energy-momentum-stress tensors and in the presence of a cosmological constant. This includes FRW universes arbitrarily close to the Milne Universe and the de Sitter universe. Hence the postulate formulated by Ernst Mach about the physical cause for the time-evolution of inertial axes is shown to hold in general relativity for linear perturbations of FRW universes.—The time-evolution of local inertial axes (relative to given local fiducial axes) is given experimentally by the precession angular velocity Ω→gyro of local gyroscopes, which in turn gives the operational definition of the gravitomagnetic field: B→g≡-2Ω→gyro. The gravitomagnetic field is caused by energy currents J→ɛ via the momentum constraint, Einstein’s G0^i^ equation, (-Δ+μ2)A→g=-16πGNJ→ɛ with B→g=curlA→g. This equation is analogous to Ampère’s law, but it holds for all time-dependent situations. Δ is the de Rham-Hodge Laplacian, and Δ=-curlcurl for the vorticity sector in Riemannian 3-space.—In the solution for an open universe the 1/r2-force of Ampère is replaced by a Yukawa force Yμ(r)=(-d/dr)[(1/R)exp⁡(-μr)], form-identical for FRW backgrounds with K=(-1,0). Here r is the measured geodesic distance from the gyroscope to the cosmological source, and 2πR is the measured circumference of the sphere centered at the gyroscope and going through the source point. The scale of the exponential cutoff is the H-dot radius, where H is the Hubble rate, dot is the derivative with respect to cosmic time, and μ2=-4(dH/dt). Analogous results hold in closed FRW universes and in Einstein’s closed static universe.—We list six fundamental tests for the principle formulated by Mach: all of them are explicitly fulfilled by our solutions.—We show that only energy currents in the toroidal vorticity sector with ℓ=1 can affect the precession of gyroscopes. We show that the harmonic decomposition of toroidal vorticity fields in terms of vector spherical harmonics X→ℓm- has radial functions which are form-identical for the 3-sphere, the hyperbolic 3-space, and Euclidean 3-space, and are form-identical with the spherical Bessel-, Neumann-, and Hankel functions.—The Appendix gives the de Rham-Hodge Laplacian on vorticity fields in Riemannian 3-spaces by equations connecting the calculus of differential forms with the curl notation. We also give the derivation the Weitzenböck formula for the difference between the de Rham-Hodge Laplacian Δ and the “rough” Laplacian ∇2 on vector fields.

Mach's principle: Exact frame-dragging via gravitomagnetism in perturbed Friedmann-Robertson-Walker universes with K=({+-}1,0)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schmid, Christoph

We show that there is exact dragging of the axis directions of local inertial frames by a weighted average of the cosmological energy currents via gravitomagnetism for all linear perturbations of all Friedmann-Robertson-Walker (FRW) universes and of Einstein's static closed universe, and for all energy-momentum-stress tensors and in the presence of a cosmological constant. This includes FRW universes arbitrarily close to the Milne Universe and the de Sitter universe. Hence the postulate formulated by Ernst Mach about the physical cause for the time-evolution of inertial axes is shown to hold in general relativity for linear perturbations of FRW universes. -more » The time-evolution of local inertial axes (relative to given local fiducial axes) is given experimentally by the precession angular velocity {omega}-vector{sub gyro} of local gyroscopes, which in turn gives the operational definition of the gravitomagnetic field: B-vector{sub g}{identical_to}-2{omega}-vector{sub gyro}. The gravitomagnetic field is caused by energy currents J-vector{sub {epsilon}} via the momentum constraint, Einstein's G{sup 0-}circumflex{sub i-circumflex} equation, (-{delta}+{mu}{sup 2})A-vector{sub g}=-16{pi}G{sub N}J-vector{sub {epsilon}} with B-vector{sub g}=curl A-vector{sub g}. This equation is analogous to Ampere's law, but it holds for all time-dependent situations. {delta} is the de Rham-Hodge Laplacian, and {delta}=-curl curl for the vorticity sector in Riemannian 3-space. - In the solution for an open universe the 1/r{sup 2}-force of Ampere is replaced by a Yukawa force Y{sub {mu}}(r)=(-d/dr)[(1/R)exp(-{mu}r)], form-identical for FRW backgrounds with K=(-1,0). Here r is the measured geodesic distance from the gyroscope to the cosmological source, and 2{pi}R is the measured circumference of the sphere centered at the gyroscope and going through the source point. The scale of the exponential cutoff is the H-dot radius, where H is the Hubble rate, dot is the derivative with respect to cosmic time, and {mu}{sup 2}=-4(dH/dt). Analogous results hold in closed FRW universes and in Einstein's closed static universe.--We list six fundamental tests for the principle formulated by Mach: all of them are explicitly fulfilled by our solutions.--We show that only energy currents in the toroidal vorticity sector with l=1 can affect the precession of gyroscopes. We show that the harmonic decomposition of toroidal vorticity fields in terms of vector spherical harmonics X-vector{sub lm}{sup -} has radial functions which are form-identical for the 3-sphere, the hyperbolic 3-space, and Euclidean 3-space, and are form-identical with the spherical Bessel-, Neumann-, and Hankel functions. - The Appendix gives the de Rham-Hodge Laplacian on vorticity fields in Riemannian 3-spaces by equations connecting the calculus of differential forms with the curl notation. We also give the derivation the Weitzenboeck formula for the difference between the de Rham-Hodge Laplacian {delta} and the ''rough'' Laplacian {nabla}{sup 2} on vector fields.« less

Development of a defect stream function, law of the wall/wake method for compressible turbulent boundary layers. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Wahls, Richard A.

1990-01-01

The method presented is designed to improve the accuracy and computational efficiency of existing numerical methods for the solution of flows with compressible turbulent boundary layers. A compressible defect stream function formulation of the governing equations assuming an arbitrary turbulence model is derived. This formulation is advantageous because it has a constrained zero-order approximation with respect to the wall shear stress and the tangential momentum equation has a first integral. Previous problems with this type of formulation near the wall are eliminated by using empirically based analytic expressions to define the flow near the wall. The van Driest law of the wall for velocity and the modified Crocco temperature-velocity relationship are used. The associated compressible law of the wake is determined and it extends the valid range of the analytical expressions beyond the logarithmic region of the boundary layer. The need for an inner-region eddy viscosity model is completely avoided. The near-wall analytic expressions are patched to numerically computed outer region solutions at a point determined during the computation. A new boundary condition on the normal derivative of the tangential velocity at the surface is presented; this condition replaces the no-slip condition and enables numerical integration to the surface with a relatively coarse grid using only an outer region turbulence model. The method was evaluated for incompressible and compressible equilibrium flows and was implemented into an existing Navier-Stokes code using the assumption of local equilibrium flow with respect to the patching. The method has proven to be accurate and efficient.

Transitional and turbulent flat-plate boundary layers with heat transfer

NASA Astrophysics Data System (ADS)

Wu, Xiaohua; Moin, Parviz

2010-11-01

We report on our direct numerical simulation of two incompressible, nominally zero-pressure-gradient flat-plate boundary layers from momentum thickness Reynolds number 80 to 1950. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number=1. Throughout the entire flat-plate, the ratio of Stanton number and skin-friction St/Cfdeviates from the exact Reynolds analogy value of 0.5 by less than 1.5%. Turbulent Prandtl number t peaks at the wall. Preponderance of hairpin vortices is observed in both the transitional and turbulent regions of the boundary layers. In particular, the internal structure of merged turbulent spots is hairpin forest; the internal structure of infant turbulent spots is hairpin packet. Numerous hairpin vortices are readily detected in both the near-wall and outer regions of the boundary layers up to momentum thickness Reynolds number 1950. This suggests that the hairpin vortices in the turbulent region are not simply the aged hairpin forests convected from the upstream transitional region. Temperature iso-surfaces in the companion thermal boundary layers are found to be a useful tracer in identifying hairpin vortex structures.

Mixing enhancement strategies and their mechanisms in supersonic flows: A brief review

NASA Astrophysics Data System (ADS)

Huang, Wei

2018-04-01

Achieving efficient fuel-air mixing is a crucial issue in the design of the scramjet engine due to the compressibility effect on the mixing shear layer growth and the stringent flow residence time limitation induced by the high-speed crossflow, and the potential solution is to enhance mixing between air and fuel by introducing of streamwise vortices in the flow field. In this survey, some mixing enhancement strategies based on the traditional transverse injection technique proposed in recent years, as well as their mixing augmentation mechanisms, were reviewed in detail, namely the pulsed transverse injection scheme, the traditional transverse injection coupled with the vortex generator, and the dual transverse injection system with a front porthole and a rear air porthole arranged in tandem. The streamwise vortices, through the large-scale stirring motion that they introduce, are responsible for the extraction of large amounts of energy from the mean flow that can be converted into turbulence, ultimately leading to increased mixing effectiveness. The streamwise vortices may be obtained by taking advantage of the shear layer between a jet and the cross stream or by employing intrusive physical devices. Finally, a promising mixing enhancement strategy in supersonic flows was proposed, and some remarks were provided.

Characterizing cycle-to-cycle variations of the shedding cycle in the turbulent wake of a normal flat plate using generalized phase averages

NASA Astrophysics Data System (ADS)

Martinuzzi, Robert

2016-11-01

Quasi-periodic vortex shedding in the turbulent wake of a thin-flat plate placed normal to a uniform stream at Reynolds number of 6700 is investigated based on Particle Image Velocimetry experiments. The wake structure and vortex formation are characterized using a generalized phase average (GPA), a refinement of the triple decomposition of Reynolds and Hussain (1970) incorporating elements of mean-field theory (Stuart, 1958). The resulting analysis highlights the importance of cycle-to-cycle variations in characterizing vortex formation, wake topology and the residual turbulent Reynolds Stresses. For example, it is shown that during high-amplitude cycles vorticity is strongly concentrated within the well-organized shed vortices, whereas during low-amplitude cycles the shed vortices are highly distorted resulting in significant modulation of the shedding frequency. It is found that high-amplitude cycles contribute more to the coherent Reynolds stress field while the low-amplitude cycles contribute to the residual stress field. It is further shown that traditional phase-averaging techniques lead to an over-estimation of the residual stress field. Natural Sciences and Engineering Research Council of Canada.

Stability of streamwise vortices

NASA Technical Reports Server (NTRS)

Khorrami, M. K.; Grosch, C. E.; Ash, R. L.

1987-01-01

A brief overview of some theoretical and computational studies of the stability of streamwise vortices is given. The local induction model and classical hydrodynamic vortex stability theories are discussed in some detail. The importance of the three-dimensionality of the mean velocity profile to the results of stability calculations is discussed briefly. The mean velocity profile is provided by employing the similarity solution of Donaldson and Sullivan. The global method of Bridges and Morris was chosen for the spatial stability calculations for the nonlinear eigenvalue problem. In order to test the numerical method, a second order accurate central difference scheme was used to obtain the coefficient matrices. It was shown that a second order finite difference method lacks the required accuracy for global eigenvalue calculations. Finally the problem was formulated using spectral methods and a truncated Chebyshev series.

Right Ventricular Hemodynamics in Patients with Pulmonary Hypertension

NASA Astrophysics Data System (ADS)

Browning, James; Fenster, Brett; Hertzberg, Jean; Schroeder, Joyce

2012-11-01

Recent advances in cardiac magnetic resonance imaging (CMR) have allowed for characterization of blood flow in the right ventricle (RV), including calculation of vorticity and circulation, and qualitative visual assessment of coherent flow patterns. In this study, we investigate qualitative and quantitative differences in right ventricular hemodynamics between subjects with pulmonary hypertension (PH) and normal controls. Fifteen (15) PH subjects and 10 age-matched controls underwent same day 3D time resolved CMR and echocardiography. Echocardiography was used to determine right ventricular diastolic function as well as pulmonary artery systolic pressure (PASP). Velocity vectors, vorticity vectors, and streamlines in the RV were visualized in Paraview and total RV Early (E) and Atrial (A) wave diastolic vorticity was quantified. Visualizations of blood flow in the RV are presented for PH and normal subjects. The hypothesis that PH subjects exhibit different RV vorticity levels than normals during diastole is tested and the relationship between RV vorticity and PASP is explored. The mechanics of RV vortex formation are discussed within the context of pulmonary arterial pressure and right ventricular diastolic function coincident with PH.

Nanoscale assembly of superconducting vortices with scanning tunnelling microscope tip

PubMed Central

Ge, Jun-Yi; Gladilin, Vladimir N.; Tempere, Jacques; Xue, Cun; Devreese, Jozef T.; Van de Vondel, Joris; Zhou, Youhe; Moshchalkov, Victor V.

2016-01-01

Vortices play a crucial role in determining the properties of superconductors as well as their applications. Therefore, characterization and manipulation of vortices, especially at the single-vortex level, is of great importance. Among many techniques to study single vortices, scanning tunnelling microscopy (STM) stands out as a powerful tool, due to its ability to detect the local electronic states and high spatial resolution. However, local control of superconductivity as well as the manipulation of individual vortices with the STM tip is still lacking. Here we report a new function of the STM, namely to control the local pinning in a superconductor through the heating effect. Such effect allows us to quench the superconducting state at nanoscale, and leads to the growth of vortex clusters whose size can be controlled by the bias voltage. We also demonstrate the use of an STM tip to assemble single-quantum vortices into desired nanoscale configurations. PMID:27934960

Understanding the Fundamental Roles of Momentum and Vorticity Injections in Flow Control

DTIC Science & Technology

2016-09-02

production by pitched and skewed jets in a turbulent boundary layer . AIAA Journal 30, 640–647. DISTRIBUTION A: Distribution approved for public release...adverse pressure gradient along the suction surface, which ultimately results in a separated boundary layer . Such behavior of the boundary layer can... boundary layer either directly or by utilizing free stream momentum to energize the boundary layer (Gad-el-Hak, 2000a). Directly adding momentum to the

Three-dimensional relativistic field-electron interaction in a multicavity high-power klystron. 1: Basic theory

NASA Technical Reports Server (NTRS)

Kosmahl, H. G.

1982-01-01

A theoretical investigation of three dimensional relativistic klystron action is described. The relativistic axisymmetric equations of motion are derived from the time-dependent Lagrangian function for a charged particle in electromagnetic fields. An analytical expression of the fringing RF electric and magnetic fields within and in the vicinity of the interaction gap and the space-charge forces between axially and radially elastic deformable rings of charges are both included in the formulation. This makes an accurate computation of electron motion through the tunnel of the cavities and the drift tube spaces possible. Method of analysis is based on Lagrangian formulation. Bunching is computed using a disk model of electron stream in which the electron stream is divided into axisymmetric disks of equal charge and each disk is assumed to consist of a number of concentric rings of equal charges. The Individual representative groups of electrons are followed through the interaction gaps and drift tube spaces. Induced currents and voltages in interacting cavities are calculated by invoking the Shockley-Ramo theorem.

Disturbance functions of the Goertler instability on an airfoil

NASA Technical Reports Server (NTRS)

Dagenhart, J. R.; Mangalam, S. M.

1986-01-01

Goertler vortices arise in boundary layers along concave surfaces due to centrifugal effects. This paper presents some results of an experiment conducted to study the development of these vortices on an airfoil with a pressure gradient in the concave region where an attached laminar boundary layer was insured with suction through a perforated panel. A sublimating chemical technique was used to visualize Goertler vortices and the velocity field was measured by laser velocimetry. Experimental disturbance functions are compared with those predicted by the linear stability theory. The trend of vortex amplification in the concave zone and damping in the following convex region is shown to essentially follow the theoretical predictions.

Rotational flow in tapered slab rocket motors

NASA Astrophysics Data System (ADS)

Saad, Tony; Sams, Oliver C.; Majdalani, Joseph

2006-10-01

Internal flow modeling is a requisite for obtaining critical parameters in the design and fabrication of modern solid rocket motors. In this work, the analytical formulation of internal flows particular to motors with tapered sidewalls is pursued. The analysis employs the vorticity-streamfunction approach to treat this problem assuming steady, incompressible, inviscid, and nonreactive flow conditions. The resulting solution is rotational following the analyses presented by Culick for a cylindrical motor. In an extension to Culick's work, Clayton has recently managed to incorporate the effect of tapered walls. Here, an approach similar to that of Clayton is applied to a slab motor in which the chamber is modeled as a rectangular channel with tapered sidewalls. The solutions are shown to be reducible, at leading order, to Taylor's inviscid profile in a porous channel. The analysis also captures the generation of vorticity at the surface of the propellant and its transport along the streamlines. It is from the axial pressure gradient that the proper form of the vorticity is ascertained. Regular perturbations are then used to solve the vorticity equation that prescribes the mean flow motion. Subsequently, numerical simulations via a finite volume solver are carried out to gain further confidence in the analytical approximations. In illustrating the effects of the taper on flow conditions, comparisons of total pressure and velocity profiles in tapered and nontapered chambers are entertained. Finally, a comparison with the axisymmetric flow analog is presented.

Effects of 2 fungicide formulations on microbial and macroinvertebrate leaf decomposition under laboratory conditions

USGS Publications Warehouse

Elskus, Adria; Smalling, Kelly L.; Hladik, Michelle; Kuivila, Kathryn

2016-01-01

Aquatic fungi contribute significantly to the decomposition of leaves in streams, a key ecosystem service. However, little is known about the effects of fungicides on aquatic fungi and macroinvertebrates involved with leaf decomposition. Red maple (Acer rubrum) leaves were conditioned in a stream to acquire microbes (bacteria and fungi), or leached in tap water (unconditioned) to simulate potential reduction of microbial biomass by fungicides. Conditioned leaves were exposed to fungicide formulations QUILT (azoxystrobin + propiconazole) or PRISTINE (boscalid + pyraclostrobin), in the presence and absence of the leaf shredder, Hyalella azteca (amphipods; 7-d old at start of exposures) for 14 d at 23 °C. QUILT formulation (~ 0.3 μg/L, 1.8 μg/L, 8 μg/L) tended to increase leaf decomposition by amphipods (not significant) without a concomitant increase in amphipod biomass, indicating potential increased consumption of leaves with reduced nutritional value. PRISTINE formulation (~ 33 μg/L) significantly reduced amphipod growth and biomass (p<0.05), effects similar to those observed with unconditioned controls. The significant suppressive effects of PRISTINE on amphipod growth, and the trend towards increased leaf decomposition with increasing QUILT concentration, indicate the potential for altered leaf decay in streams exposed to fungicides. Further work is needed to evaluate fungicide effects on leaf decomposition under conditions relevant to stream ecosystems, including temperature shifts and pulsed exposures to pesticide mixtures.

Numerical Capture of Wing-tip Vortex Using Vorticity Confinement

NASA Astrophysics Data System (ADS)

Zhang, Baili; Lou, Jing; Kang, Chang Wei; Wilson, Alexander; Lundberg, Johan; Bensow, Rickard

2012-11-01

Tracking vortices accurately over large distances is very important in many areas of engineering, for instance flow over rotating helicopter blades, ship propeller blades and aircraft wings. However, due to the inherent numerical dissipation in the advection step of flow simulation, current Euler and RANS field solvers tend to damp these vortices too fast. One possible solution to reduce the unphysical decay of these vortices is the application of vorticity confinement methods. In this study, a vorticity confinement term is added to the momentum conservation equations which is a function of the local element size, the vorticity and the gradient of the absolute value of vorticity. The approach has been evaluated by a systematic numerical study on the tip vortex trailing from a rectangular NACA0012 half-wing. The simulated structure and development of the wing-tip vortex agree well with experiments both qualitatively and quantitatively without any adverse effects on the global flow field. It is shown that vorticity confinement can negate the effect of numerical dissipation, leading to a more or less constant vortex strength. This is an approximate method in that genuine viscous diffusion of the vortex is not modeled, but it can be appropriate for vortex dominant flows over short to medium length scales where viscous diffusion can be neglected.

Design and evaluation of a Dean vortex-based micromixer.

PubMed

Howell, Peter B; Mott, David R; Golden, Joel P; Ligler, Frances S

2004-12-01

A mixer, based on the Dean vortex, is fabricated and tested in an on-chip format. When fluid is directed around a curve under pressure driven flow, the high velocity streams in the center of the channel experience a greater centripetal force and so are deflected outward. This creates a pair of counter-rotating vortices moving fluid toward the inner wall at the top and bottom of the channel and toward the outer wall in the center. For the geometries studied, the vortices were first seen at Reynolds numbers between 1 and 10 and became stronger as the flow velocity is increased. Vortex formation was monitored in channels with depth/width ratios of 0.5, 1.0, and 2.0. The lowest aspect ratio strongly suppressed vortex formation. Increasing the aspect ratio above 1 appeared to provide improved mixing. This design has the advantages of easy fabrication and low surface area.

Roughness Induced Transition in a Supersonic Boundary Layer

NASA Technical Reports Server (NTRS)

Balakumar, Ponnampalam; Kergerise, Michael A.

2013-01-01

Direct numerical simulation is used to investigate the transition induced by threedimensional isolated roughness elements in a supersonic boundary layer at a free stream Mach number of 3.5. Simulations are performed for two different configurations: one is a square planform roughness and the other is a diamond planform roughness. The mean-flow calculations show that the roughness induces counter rotating streamwise vortices downstream of the roughness. These vortices persist for a long distance downstream and lift the low momentum fluid from the near wall region and place it near the outer part of the boundary layer. This forms highly inflectional boundary layer profiles. These observations agree with recent experimental observations. The receptivity calculations showed that the amplitudes of the mass-flux fluctuations near the neutral point for the diamond shape roughness are the same as the amplitude of the acoustic disturbances. They are three times smaller for the square shape roughness.

Motion of a curved vortex filament with decaying vortical core and axial velocity

NASA Technical Reports Server (NTRS)

Callegari, A. J.; Ting, L.

1978-01-01

The motion and decay of a curved vortex filament having large axial and circumferential velocity components in a three-dimensional stream are analyzed by using the method of matched asymptotic expansions of the incompressible Navier-Stokes equations. The small parameter is the square root of the ratio of the kinematic viscosity to the circulation. The outer region is analyzed by the classical Biot-Savart law, and its solution is matched to that of the inner region, where viscous effects are important. Equations describing the coupling between the inner vortex structure and the motion of the vortex filament as well as the time evolution of the inner vortex structure are obtained. Equations are derived for the motion of the vortex filament and for the change and decay in time and space of the leading-order circumferential and axial velocity and vorticity components. Solutions are constructed for these components in terms of initial data.

Nonlinear spatial evolution of inviscid instabilities on hypersonic boundary layers

NASA Technical Reports Server (NTRS)

Wundrow, David W.

1996-01-01

The spatial development of an initially linear vorticity-mode instability on a compressible flat-plate boundary layer is considered. The analysis is done in the framework of the hypersonic limit where the free-stream Mach number M approaches infinity. Nonlinearity is shown to become important locally, in a thin critical layer, when sigma, the deviation of the phase speed from unity, becomes o(M(exp -8/7)) and the magnitude of the pressure fluctuations becomes 0(sigma(exp 5/2)M(exp 2)). The unsteady flow outside the critical layer takes the form of a linear instability wave but with its amplitude completely determined by the nonlinear flow within the critical layer. The coupled set of equations which govern the critical-layer dynamics reflect a balance between spatial-evolution, (linear and nonlinear) convection and nonlinear vorticity-generation terms. The numerical solution to these equations shows that nonlinear effects produce a dramatic reduction in the instability-wave amplitude.

Local invariants in non-ideal flows of neutral fluids and two-fluid plasmas

NASA Astrophysics Data System (ADS)

Zhu, Jian-Zhou

2018-03-01

The main objective is the locally invariant geometric object of any (magneto-)fluid dynamics with forcing and damping (nonideal), while more attention is paid to the untouched dynamical properties of two-fluid fashion. Specifically, local structures, beyond the well-known "frozen-in" to the barotropic flows of the generalized vorticities, of the two-fluid model of plasma flows are presented. More general non-barotropic situations are also considered. A modified Euler equation [T. Tao, "Finite time blowup for Lagrangian modifications of the three-dimensional Euler equation," Ann. PDE 2, 9 (2016)] is also accordingly analyzed and remarked from the angle of view of the two-fluid model, with emphasis on the local structures. The local constraints of high-order differential forms such as helicity, among others, find simple formulation for possible practices in modeling the dynamics. Thus, the Cauchy invariants equation [N. Besse and U. Frisch, "Geometric formulation of the Cauchy invariants for incompressible Euler flow in flat and curved spaces," J. Fluid Mech. 825, 412 (2017)] may be enabled to find applications in non-ideal flows. Some formal examples are offered to demonstrate the calculations, and particularly interestingly the two-dimensional-three-component (2D3C) or the 2D passive scalar problem presents that a locally invariant Θ = 2θζ, with θ and ζ being, respectively, the scalar value of the "vertical velocity" (or the passive scalar) and the "vertical vorticity," may be used as if it were the spatial density of the globally invariant helicity, providing a Lagrangian prescription to control the latter in some situations of studying its physical effects in rapidly rotating flows (ubiquitous in atmosphere of astrophysical objects) with marked 2D3C vortical modes or in purely 2D passive scalars.

Slosh dynamics of a spin-stabilized spacecraft comprising off-axis tanks filled partially with liquid propellant

NASA Technical Reports Server (NTRS)

Fontenot, L. L.

1981-01-01

The fundamental nonlinear equations of motion were derived and the specialized to a steady-state rotation of the vehicle about a given axis of rotation. A thrust about the spin axis was introduced. A perturbation solution was derived which linearizes the problem. The effect of the centrifugal and coriolis accelerations together with vorticity are implicitly taken into consideration in the formulation. A variational formulation of the associated boundary conditions is presented. For practical cases it is shown that the simple classical pendulum representation for slosh is not very appealing for a spinning spacecraft unless severe restrictions are allowed.

Mixing enhancement of reacting parallel fuel jets in a supersonic combustor

NASA Technical Reports Server (NTRS)

Drummond, J. P.

1991-01-01

Pursuant to a NASA-Langley development program for a scramjet HST propulsion system entailing the optimization of the scramjet combustor's fuel-air mixing and reaction characteristics, a numerical study has been conducted of the candidate parallel fuel injectors. Attention is given to a method for flow mixing-process and combustion-efficiency enhancement in which a supersonic circular hydrogen jet coflows with a supersonic air stream. When enhanced by a planar oblique shock, the injector configuration exhibited a substantial degree of induced vorticity in the fuel stream which increased mixing and chemical reaction rates, relative to the unshocked configuration. The resulting heat release was effective in breaking down the stable hydrogen vortex pair that had inhibited more extensive fuel-air mixing.

Forces on particles in microstreaming flows

NASA Astrophysics Data System (ADS)

Hilgenfeldt, Sascha; Rallabandi, Bhargav; Thameem, Raqeeb

2015-11-01

In various microfluidic applications, vortical steady streaming from ultrasonically driven microbubbles is used in concert with a pressure-driven channel flow to manipulate objects. While a quantitative theory of this boundary-induced streaming is available, little work has been devoted to a fundamental understanding of the forces exerted on microparticles in boundary streaming flows, even though the differential action of such forces is central to applications like size-sensitive sorting. Contrary to other microfluidic sorting devices, the forces in bubble microstreaming act over millisecond times and micron length scales, without the need for accumulated deflections over long distances. Accordingly, we develop a theory of hydrodynamic forces on the fast time scale of bubble oscillation using the lubrication approximation, showing for the first time how particle displacements are rectified near moving boundaries over multiple oscillations in parallel with the generation of the steady streaming flow. The dependence of particle migration on particle size and the flow parameters is compared with experimental data. The theory is applicable to boundary streaming phenomena in general and demonstrates how particles can be sorted very quickly and without compromising device throughput. We acknowledge support by the National Science Foundation under grant number CBET-1236141.

From cat's eyes to disjoint multicellular natural convection flow in tall tilted cavities

NASA Astrophysics Data System (ADS)

Nicolás, Alfredo; Báez, Elsa; Bermúdez, Blanca

2011-07-01

Numerical results of two-dimensional natural convection problems, in air-filled tall cavities, are reported to study the change of the cat's eyes flow as some parameters vary, the aspect ratio A and the angle of inclination ϕ of the cavity, with the Rayleigh number Ra mostly fixed; explicitly, the range of the variation is given by 12⩽A⩽20 and 0°⩽ϕ⩽270°; about Ra=1.1×10. A novelty contribution of this work is the transition from the cat's eyes changes, as A varies, to a disjoint multicellular flow, as ϕ varies. These flows may be modeled by the unsteady Boussinesq approximation in stream function and vorticity variables which is solved with a fixed point iterative process applied to the nonlinear elliptic system that results after time discretization. The validation of the results relies on mesh size and time-step independence studies.

Argand-plane vorticity singularities in complex scalar optical fields: an experimental study using optical speckle.

PubMed

Rothschild, Freda; Bishop, Alexis I; Kitchen, Marcus J; Paganin, David M

2014-03-24

The Cornu spiral is, in essence, the image resulting from an Argand-plane map associated with monochromatic complex scalar plane waves diffracting from an infinite edge. Argand-plane maps can be useful in the analysis of more general optical fields. We experimentally study particular features of Argand-plane mappings known as "vorticity singularities" that are associated with mapping continuous single-valued complex scalar speckle fields to the Argand plane. Vorticity singularities possess a hierarchy of Argand-plane catastrophes including the fold, cusp and elliptic umbilic. We also confirm their connection to vortices in two-dimensional complex scalar waves. The study of vorticity singularities may also have implications for higher-dimensional fields such as coherence functions and multi-component fields such as vector and spinor fields.

A novel passive micromixer based on unbalanced splits and collisions of fluid streams

NASA Astrophysics Data System (ADS)

Ansari, Mubashshir Ahmad; Kim, Kwang-Yong; Anwar, Khalid; Kim, Sun Min

2010-05-01

A new passive micromixer based on the concept of unbalanced splits and cross-collisions of fluid streams is designed and fabricated. Experimental and numerical studies have been carried out on the micromixer at Reynolds numbers ranging from 10 to 80. The three-dimensional Navier-Stokes equations have been used to analyze the mixing and flow behavior of the micromixer, which is composed of two sub-channels of unequal widths which repeatedly undergo splitting and recombination. The difference between the mass flow rates in the two sub-channels creates an unbalanced collision of the two fluid streams. Mixing is mainly due to the combined effect of unbalanced collisions of the fluid streams and Dean vortices. The micromixer shows interesting mixing behavior for different ratios of the widths of the two split sub-channels. The sub-channels wherein the major sub-channel is twice as wide as the minor sub-channel exhibit the highest mixing performance at Reynolds numbers larger than 40. The results show the lowest mixing performance for the case of uniform width, where balanced collisions occur.

Influence of a non-uniform free stream velocity distribution on performance/acoustics of counterrotating propeller configurations

NASA Astrophysics Data System (ADS)

Allen, C. S.; Korkan, K. D.

1991-01-01

A methodology for predicting the performance and acoustics of counterrotating propeller configurations was modified to take into account the effects of a non-uniform free stream velocity distribution entering the disk plane. The method utilizes the analytical techniques of Lock and Theodorson as described by Davidson to determine the influence of the non-uniform free stream velocity distribution in the prediction of the steady aerodynamic loads. The unsteady load contribution is determined according to the procedure of Leseture with rigid helical tip vortices simulating the previous rotations of each propeller. The steady and unsteady loads are combined to obtain the total blade loading required for acoustic prediction employing the Ffowcs Williams-Hawking equation as simplified by Succi with the assumption of compact sources. The numerical method is used to redesign the previous commuter class counterrotating propeller configuration of Denner. The specifications, performance, and acoustics of the new design are compared with the results of Denner thereby determining the influence of the non-uniform free stream velocity distribution on these metrics.

Theory Can Help Structure Regression Models for Projecting Stream Conditions Under Alternative Land Use Scenarios

NASA Astrophysics Data System (ADS)

van Sickle, J.; Baker, J.; Herlihy, A.

2005-05-01

We built multiple regression models for Emphemeroptera/ Plecoptera/ Tricoptera (EPT) taxon richness and other indicators of biological condition in streams of the Willamette River Basin, Oregon, USA. The models were used to project the changes in condition that would be expected in all 2-4th order streams of the 30000 sq km basin under alternative scenarios of future land use. In formulating the models, we invoked the theory of limiting factors to express the interactive effects of stream power and watershed land use on EPT richness. The resulting models were parsimonious, and they fit the data in our wedge-shaped scatterplots slightly better than did a naive additive-effects model. Just as theory helped formulate our regression models, the models in turn helped us identify a new research need for the Basin's streams. Our future scenarios project that conversions of agricultural to urban uses may dominate landscape dynamics in the basin over the next 50 years. But our models could not detect any difference between the effects of agricultural and urban development in watersheds on stream biota. This result points to an increased need for understanding how agricultural and urban land uses in the Basin differentially influence stream ecosystems.

Flow past a rotating cylinder

NASA Astrophysics Data System (ADS)

Mittal, Sanjay; Kumar, Bhaskar

2003-02-01

Flow past a spinning circular cylinder placed in a uniform stream is investigated via two-dimensional computations. A stabilized finite element method is utilized to solve the incompressible Navier Stokes equations in the primitive variables formulation. The Reynolds number based on the cylinder diameter and free-stream speed of the flow is 200. The non-dimensional rotation rate, [alpha] (ratio of the surface speed and freestream speed), is varied between 0 and 5. The time integration of the flow equations is carried out for very large dimensionless time. Vortex shedding is observed for [alpha] < 1.91. For higher rotation rates the flow achieves a steady state except for 4.34 < [alpha] < 4:70 where the flow is unstable again. In the second region of instability, only one-sided vortex shedding takes place. To ascertain the instability of flow as a function of [alpha] a stabilized finite element formulation is proposed to carry out a global, non-parallel stability analysis of the two-dimensional steady-state flow for small disturbances. The formulation and its implementation are validated by predicting the Hopf bifurcation for flow past a non-rotating cylinder. The results from the stability analysis for the rotating cylinder are in very good agreement with those from direct numerical simulations. For large rotation rates, very large lift coefficients can be obtained via the Magnus effect. However, the power requirement for rotating the cylinder increases rapidly with rotation rate.

Structures and Evolutions of Explosive Cyclones over the Northwestern and Northeastern Pacific

NASA Astrophysics Data System (ADS)

Zhang, Shuqin; Fu, Gang

2018-06-01

In this study, the structures and evolutions of moderate (MO) explosive cyclones (ECs) over the Northwestern Pacific (NWP) and Northeastern Pacific (NEP) are investigated and compared using composite analysis with cyclone-relative coordinates. Final Operational Global Analysis data gathered during the cold seasons (October-April) of the 15 years from 2000 to 2015 are used. The results indicate that MO NWP ECs have strong baroclinicity and abundant latent heat release at low levels and strong upper-level forcing, which favors explosive cyclogenesis. The rapid development of MO NEP ECs results from their interaction with a northern cyclone and a large middle-level advection of cyclonic vorticity. The structural differences between MO NWP ECs and MO NEP ECs are significant. This results from their specific large-scale atmospheric and oceanic environments. MO NWP ECs usually develop rapidly in the east and southeast of the Japan Islands; the intrusion of cold dry air from the East Asian continent leads to strong baroclinicity, and the Kuroshio/Kuroshio Extension provides abundant latent heat release at low levels. The East Asian subtropical westerly jet stream supplies strong upper-level forcing. While MO NEP ECs mainly occur over the NEP, the low-level baroclinicity, upper-level jet stream, and warm ocean currents are relatively weaker. The merged cyclone associated with a strong middle-level trough transports large cyclonic vorticity to MO NEP ECs, which favors their rapid development.

Modification of inertial oscillations by the mesoscale eddy field

NASA Astrophysics Data System (ADS)

Elipot, Shane; Lumpkin, Rick; Prieto, GermáN.

2010-09-01

The modification of near-surface near-inertial oscillations (NIOs) by the geostrophic vorticity is studied globally from an observational standpoint. Surface drifter are used to estimate NIO characteristics. Despite its spatial resolution limits, altimetry is used to estimate the geostrophic vorticity. Three characteristics of NIOs are considered: the relative frequency shift with respect to the local inertial frequency; the near-inertial variance; and the inverse excess bandwidth, which is interpreted as a decay time scale. The geostrophic mesoscale flow shifts the frequency of NIOs by approximately half its vorticity. Equatorward of 30°N and S, this effect is added to a global pattern of blue shift of NIOs. While the global pattern of near-inertial variance is interpretable in terms of wind forcing, it is also observed that the geostrophic vorticity organizes the near-inertial variance; it is maximum for near zero values of the Laplacian of the vorticity and decreases for nonzero values, albeit not as much for positive as for negative values. Because the Laplacian of vorticity and vorticity are anticorrelated in the altimeter data set, overall, more near-inertial variance is found in anticyclonic vorticity regions than in cyclonic regions. While this is compatible with anticyclones trapping NIOs, the organization of near-inertial variance by the Laplacian of vorticity is also in very good agreement with previous theoretical and numerical predictions. The inverse bandwidth is a decreasing function of the gradient of vorticity, which acts like the gradient of planetary vorticity to increase the decay of NIOs from the ocean surface. Because the altimetry data set captures the largest vorticity gradients in energetic mesoscale regions, it is also observed that NIOs decay faster in large geostrophic eddy kinetic energy regions.

Extending high-order flux operators on spherical icosahedral grids and their application in a Shallow Water Model for transporting the Potential Vorticity

NASA Astrophysics Data System (ADS)

Zhang, Y.

2017-12-01

The unstructured formulation of the third/fourth-order flux operators used by the Advanced Research WRF is extended twofold on spherical icosahedral grids. First, the fifth- and sixth-order flux operators of WRF are further extended, and the nominally second- to sixth-order operators are then compared based on the solid body rotation and deformational flow tests. Results show that increasing the nominal order generally leads to smaller absolute errors. Overall, the fifth-order scheme generates the smallest errors in limited and unlimited tests, although it does not enhance the convergence rate. The fifth-order scheme also exhibits smaller sensitivity to the damping coefficient than the third-order scheme. Overall, the even-order schemes have higher limiter sensitivity than the odd-order schemes. Second, a triangular version of these high-order operators is repurposed for transporting the potential vorticity in a space-time-split shallow water framework. Results show that a class of nominally third-order upwind-biased operators generates better results than second- and fourth-order counterparts. The increase of the potential enstrophy over time is suppressed owing to the damping effect. The grid-scale noise in the vorticity is largely alleviated, and the total energy remains conserved. Moreover, models using high-order operators show smaller numerical errors in the vorticity field because of a more accurate representation of the nonlinear Coriolis term. This improvement is especially evident in the Rossby-Haurwitz wave test, in which the fluid is highly rotating. Overall, flux operators with higher damping coefficients, which essentially behaves like the Anticipated Potential Vorticity Method, present optimal results.

Non-equilibrium statistical mechanics theory for the large scales of geophysical flows

NASA Astrophysics Data System (ADS)

Eric, S.; Bouchet, F.

2010-12-01

The aim of any theory of turbulence is to understand the statistical properties of the velocity field. As a huge number of degrees of freedom is involved, statistical mechanics is a natural approach. The self-organization of two-dimensional and geophysical turbulent flows is addressed based on statistical mechanics methods. We discuss classical and recent works on this subject; from the statistical mechanics basis of the theory up to applications to Jupiter’s troposphere and ocean vortices and jets. The equilibrium microcanonical measure is built from the Liouville theorem. Important statistical mechanics concepts (large deviations, mean field approach) and thermodynamic concepts (ensemble inequivalence, negative heat capacity) are briefly explained and used to predict statistical equilibria for turbulent flows. This is applied to make quantitative models of two-dimensional turbulence, the Great Red Spot and other Jovian vortices, ocean jets like the Gulf-Stream, and ocean vortices. A detailed comparison between these statistical equilibria and real flow observations will be discussed. We also present recent results for non-equilibrium situations, for which forces and dissipation are in a statistical balance. As an example, the concept of phase transition allows us to describe drastic changes of the whole system when a few external parameters are changed. F. Bouchet and E. Simonnet, Random Changes of Flow Topology in Two-Dimensional and Geophysical Turbulence, Physical Review Letters 102 (2009), no. 9, 094504-+. F. Bouchet and J. Sommeria, Emergence of intense jets and Jupiter's Great Red Spot as maximum-entropy structures, Journal of Fluid Mechanics 464 (2002), 165-207. A. Venaille and F. Bouchet, Ocean rings and jets as statistical equilibrium states, submitted to JPO F. Bouchet and A. Venaille, Statistical mechanics of two-dimensional and geophysical flows, submitted to Physics Reports Non-equilibrium phase transitions for the 2D Navier-Stokes equations with stochastic forces (time series and probability density functions (PDFs) of the modulus of the largest scale Fourrier component, showing bistability between dipole and unidirectional flows). This bistability is predicted by statistical mechanics.

Observation of the Kelvin–Helmholtz Instability in a Solar Prominence

NASA Astrophysics Data System (ADS)

Yang, Heesu; Xu, Zhi; Lim, Eun-Kyung; Kim, Sujin; Cho, Kyung-Suk; Kim, Yeon-Han; Chae, Jongchul; Cho, Kyuhyoun; Ji, Kaifan

2018-04-01

Many solar prominences end their lives in eruptions or abrupt disappearances that are associated with dynamical or thermal instabilities. Such instabilities are important because they may be responsible for energy transport and conversion. We present a clear observation of a streaming kink-mode Kelvin–Helmholtz Instability (KHI) taking place in a solar prominence using the Hα Lyot filter installed at the New Vacuum Solar Telescope, Fuxian-lake Solar Observatory in Yunnan, China. On one side of the prominence, a series of plasma blobs floated up from the chromosphere and streamed parallel to the limb. The plasma stream was accelerated to about 20–60 km s‑1 and then undulated. We found that 2″- and 5″-size vortices formed, floated along the stream, and then broke up. After the 5″-size vortex, a plasma ejection out of the stream was detected in the Solar Dynamics Observatory/Atmospheric Imaging Assembly images. Just before the formation of the 5″-size vortex, the stream displayed an oscillatory transverse motion with a period of 255 s with the amplitude growing at the rate of 0.001 s‑1. We attribute this oscillation of the stream and the subsequent formation of the vortex to the KHI triggered by velocity shear between the stream, guided by the magnetic field and the surrounding media. The plasma ejection suggests the transport of prominence material into the upper layer by the KHI in its nonlinear stage.

Adaptation of a general circulation model to ocean dynamics

NASA Technical Reports Server (NTRS)

Turner, R. E.; Rees, T. H.; Woodbury, G. E.

1976-01-01

A primitive-variable general circulation model of the ocean was formulated in which fast external gravity waves are suppressed with rigid-lid surface constraint pressires which also provide a means for simulating the effects of large-scale free-surface topography. The surface pressure method is simpler to apply than the conventional stream function models, and the resulting model can be applied to both global ocean and limited region situations. Strengths and weaknesses of the model are also presented.

Formulation and preparation of Hanford Waste Treatment Plant direct feed low activity waste Effluent Management Facility core simulant

DOE Office of Scientific and Technical Information (OSTI.GOV)

McCabe, Daniel J.; Nash, Charles A.; Adamson, Duane J.

The Hanford Waste Treatment and Immobilization Plant (WTP) Low Activity Waste (LAW) vitrification facility will generate an aqueous condensate recycle stream (LAW Melter Off-Gas Condensate, LMOGC) from the off-gas system. The baseline plan for disposition of this stream during full WTP operations is to send it to the WTP Pretreatment Facility, where it will be blended with LAW, concentrated by evaporation and recycled to the LAW vitrification facility. However, during the Direct Feed LAW (DFLAW) scenario, planned disposition of this stream is to evaporate it in a new evaporator in the Effluent Management Facility (EMF) and then return it tomore » the LAW melter. It is important to understand the composition of the effluents from the melter and new evaporator so that the disposition of these streams can be accurately planned and accommodated. Furthermore, alternate disposition of the LMOGC stream would eliminate recycling of problematic components, and would enable less integrated operation of the LAW melter and the Pretreatment Facilities. Alternate disposition would also eliminate this stream from recycling within WTP when it begins operations and would decrease the LAW vitrification mission duration and quantity of glass waste, amongst the other problems such a recycle stream present. This LAW Melter Off-Gas Condensate stream will contain components that are volatile at melter temperatures and are problematic for the glass waste form, such as halides and sulfate. Because this stream will recycle within WTP, these components accumulate in the Melter Condensate stream, exacerbating their impact on the number of LAW glass containers that must be produced. Diverting the stream reduces the halides and sulfate in the recycled Condensate and is a key outcome of this work. This overall program examines the potential treatment and immobilization of this stream to enable alternative disposal. The objective of this task was to formulate and prepare a simulant of the LAW Melter Off-gas Condensate expected during DFLAW operations. That simulant can be used in evaporator testing to predict the composition of the effluents from the Effluent Management Facility (EMF) evaporator to aid in planning for their disposition. This document describes the method used to formulate a simulant of this LAW Melter Off-Gas Condensate stream, which, after pH adjustment, is the feed to the evaporator in the EMF.« less

Towards reduced order modelling for predicting the dynamics of coherent vorticity structures within wind turbine wakes

NASA Astrophysics Data System (ADS)

Debnath, M.; Santoni, C.; Leonardi, S.; Iungo, G. V.

2017-03-01

The dynamics of the velocity field resulting from the interaction between the atmospheric boundary layer and a wind turbine array can affect significantly the performance of a wind power plant and the durability of wind turbines. In this work, dynamics in wind turbine wakes and instabilities of helicoidal tip vortices are detected and characterized through modal decomposition techniques. The dataset under examination consists of snapshots of the velocity field obtained from large-eddy simulations (LES) of an isolated wind turbine, for which aerodynamic forcing exerted by the turbine blades on the atmospheric boundary layer is mimicked through the actuator line model. Particular attention is paid to the interaction between the downstream evolution of the helicoidal tip vortices and the alternate vortex shedding from the turbine tower. The LES dataset is interrogated through different modal decomposition techniques, such as proper orthogonal decomposition and dynamic mode decomposition. The dominant wake dynamics are selected for the formulation of a reduced order model, which consists in a linear time-marching algorithm where temporal evolution of flow dynamics is obtained from the previous temporal realization multiplied by a time-invariant operator. This article is part of the themed issue 'Wind energy in complex terrains'.

Multi-vortex crystal lattices in Bose-Einstein condensates with a rotating trap.

PubMed

Xie, Shuangquan; Kevrekidis, Panayotis G; Kolokolnikov, Theodore

2018-05-01

We consider vortex dynamics in the context of Bose-Einstein condensates (BECs) with a rotating trap, with or without anisotropy. Starting with the Gross-Pitaevskii (GP) partial differential equation (PDE), we derive a novel reduced system of ordinary differential equations (ODEs) that describes stable configurations of multiple co-rotating vortices (vortex crystals). This description is found to be quite accurate quantitatively especially in the case of multiple vortices. In the limit of many vortices, BECs are known to form vortex crystal structures, whereby vortices tend to arrange themselves in a hexagonal-like spatial configuration. Using our asymptotic reduction, we derive the effective vortex crystal density and its radius. We also obtain an asymptotic estimate for the maximum number of vortices as a function of rotation rate. We extend considerations to the anisotropic trap case, confirming that a pair of vortices lying on the long (short) axis is linearly stable (unstable), corroborating the ODE reduction results with full PDE simulations. We then further investigate the many-vortex limit in the case of strong anisotropic potential. In this limit, the vortices tend to align themselves along the long axis, and we compute the effective one-dimensional vortex density, as well as the maximum admissible number of vortices. Detailed numerical simulations of the GP equation are used to confirm our analytical predictions.

Numerical Simulation of Chemically Reacting Flows

DTIC Science & Technology

2015-09-03

62 (1986) 1-25. 6. O.L. Burchett, M.R. Birnbaum, and C.T. Oien, “ Compaction studies of palladium/aluminum powder ,” Sandia National Laboratories...interest to the Air Force. 15. SUBJECT TERMS Numerical methods, Diffusion Flames, Adaptive Gridding, Velocity-Vorticity, Compact Methods 16...discussed ab ot require th sure mass c mputational city formula e spectrum soot forma formulation lent agreem ing MC-Sm ork will lik ith compact iled

A boundary integral approach in primitive variables for free surface flows

NASA Astrophysics Data System (ADS)

Casciola, C.; Piva, R.

The boundary integral formulation, very efficient for free surface potential flows, was considered for its possible extension to rotational flows either inviscid or viscous. We first analyze a general formulation for unsteady Navier-Stokes equations in primitive variables, which reduces to a representation for the Euler equations in the limiting case of Reynolds infinity. A first simplified model for rotational flows, obtained by decoupling kinematics and dynamics, reduces the integral equations to a known kinematical form whose mathematical and numerical properties have been studied. The dynamics equations to complete the model are obtained for the free surface and the wake. A simple and efficient scheme for the study of the non linear evolution of the wave system and its interaction with the body wake is presented. A steady state version for the calculation of the wave resistance is also reported. A second model was proposed for the simulation of rotational separated regions, by coupling the integral equations in velocity with an integral equation for the vorticity at the body boundary. The same procedure may be extended to include the diffusion of the vorticity in the flowfield. The vortex shedding from a cylindrical body in unsteady motion is discussed, as a first application of the model.

Visualization of leading edge vortices on a series of flat plate delta wings

NASA Technical Reports Server (NTRS)

Payne, Francis M.; Ng, T. Terry; Nelson, Robert C.

1991-01-01

A summary of flow visualization data obtained as part of NASA Grant NAG2-258 is presented. During the course of this study, many still and high speed motion pictures were taken of the leading edge vortices on a series of flat plate delta wings at varying angles of attack. The purpose is to present a systematic collection of photographs showing the state of vortices as a function of the angle of attack for the four models tested.

Reynolds number effect on airfoil wake structures under pitching and heaving motion

NASA Astrophysics Data System (ADS)

Kim, Kyung Chun; Karbasian, Hamidreza; ExpTENsys Team

2017-11-01

Detached Eddy Simulation (DES) and particle image velocimetry (PIV) measurements were performed to investigate the wake flow characteristics of an airfoil under pitching and heaving motion. A NACA0012 airfoil was selected for the numerical simulation and experiments were carried out in a wind tunnel and a water tunnel at Reynolds number of 15,000 and 90,000, respectively. The airfoil oscillated around an axis located 1/4 distance from the leading edge chord. Two different angles of attack, 20° and 30°, were selected with +/-10° maximum amplitude of oscillation. In order to extract the coherent flow structures from time-resolved PIV data, proper orthogonal decomposition (POD) analysis was performed on 1,000 instantaneous realisations for each condition using the method of snapshots. Vorticity contour and velocity profiles for both PIV and DES results are in good agreement for pitching and heaving motion. At high Reynolds number, 3D stream-wise vortices appeared after generating span-wise vortices. The higher maximum angle of attack allows the leading edge vortex to grow stronger and that the angle of attack appears to be more important in influencing the growth of the leading edge vortex structure than the reduced frequency. National Research Foundation of Korea (No. 2011-0030013).

Vortex Interactions from a Finite Span Cylinder with a Laminar Boundary Layer for Varied Parameters

NASA Astrophysics Data System (ADS)

Gildersleeve, Samantha; Amitay, Michael

2017-11-01

Flow structures around a stationary, wall-mounted, finite-span cylindrical pin were investigated experimentally over a flat plate to explore the effects of varied aspect ratio and pin mean height with respect to the local boundary layer. Nine static pin configurations were tested where the pin's mean height to the local boundary layer thickness were 0.5, 1, and 1.5 for a range of aspect ratios between 0.125 and 1.125. The freestream velocity was fixed at 11 m/s, corresponding to ReD 2800, 5600, and 8400, respectively. Three-dimensional flowfields were reconstructed and analyzed from SPIV measurements where data were collected along cross-stream planes in the wake of the pin. This study focuses on three dominant vortical patterns associated with a finite span cylinder: the arch-type vortex horseshoe vortex, and the tip vortices Results indicate that both the aspect ratio and mean height play an important role in the behavior and interactions of these vortex structures which alter the wake characteristics significantly. Understanding the mechanisms by which the vortical structures may be strengthened while reducing adverse local pressure drag are key for developing more efficient means of passive and/or active flow control through finite span cylindrical pins and will be discussed in further detail. NDSEG Fellowship for Samantha Gildersleeve.

Characteristics of Plasma Synthetic Jet Actuators in Crossflow

NASA Astrophysics Data System (ADS)

Santhanakrishnan, Arvind; Jacob, Jamey

2006-11-01

The plasma synthetic jet actuator (PSJA) consists of two annular electrodes separated by dielectric material that results in a circular region of dielectric barrier discharge plasma. In quiescent conditions, this plasma ring produces a synthetic jet which can be used for active flow control applications. Unsteady pulsing of the actuator results in the formation of multiple primary and secondary vortex rings, the latter remaining fixed or trapped in space. The jet is observed to be formed by the advection and interaction of the primary vortices, resembling a conventional synthetic jet. This presentation examines the operation of the PSJA in a crossflow at three different jet to freestream velocity ratios. PIV measurements in the streamwise and cross-stream planes are used to illustrate the three dimensionality of the jet and associated vortical structures. The strength of the vortex ring is found to vary along its circumference due to interaction with the freestream. The boundary layer characteristics obtained from these experiments suggest that the mechanism of the PSJA in crossflow is similar to an active boundary layer trip. Both the penetration of the jet and effectiveness of the trip action are found to decrease with increase in freestream velocity. The effects of unsteady pulsing and increasing input power on actuator created crossflow vortices will also be presented.

Continuous Photo-Oxidation in a Vortex Reactor: Efficient Operations Using Air Drawn from the Laboratory

PubMed Central

2017-01-01

We report the construction and use of a vortex reactor which uses a rapidly rotating cylinder to generate Taylor vortices for continuous flow thermal and photochemical reactions. The reactor is designed to operate under conditions required for vortex generation. The flow pattern of the vortices has been represented using computational fluid dynamics, and the presence of the vortices can be easily visualized by observing streams of bubbles within the reactor. This approach presents certain advantages for reactions with added gases. For reactions with oxygen, the reactor offers an alternative to traditional setups as it efficiently draws in air from the lab without the need specifically to pressurize with oxygen. The rapid mixing generated by the vortices enables rapid mass transfer between the gas and the liquid phases allowing for a high efficiency dissolution of gases. The reactor has been applied to several photochemical reactions involving singlet oxygen (1O2) including the photo-oxidations of α-terpinene and furfuryl alcohol and the photodeborylation of phenyl boronic acid. The rotation speed of the cylinder proved to be key for reaction efficiency, and in the operation we found that the uptake of air was highest at 4000 rpm. The reactor has also been successfully applied to the synthesis of artemisinin, a potent antimalarial compound; and this three-step synthesis involving a Schenk-ene reaction with 1O2, Hock cleavage with H+, and an oxidative cyclization cascade with triplet oxygen (3O2), from dihydroartemisinic acid was carried out as a single process in the vortex reactor. PMID:28781513

An experimental and numerical investigation on the formation of stall-cells on airfoils

NASA Astrophysics Data System (ADS)

Manolesos, M.; Papadakis, G.; Voutsinas, S.

2014-12-01

Stall Cells (SCs) are large scale three-dimensional structures of separated flow that have been observed on the suction side of airfoils designed for or used on wind turbine blades. SCs are unstable in nature but can be stabilised by means of a localized disturbance; here in the form of a zigzag tape covering 10% of the wing span. Based on extensive tuft flow visualisations, the resulting flow was found macroscopically similar to the undisturbed flow. Next a combined investigation was carried out including pressure recordings, Stereo-PIV measurements and CFD simulations. The investigation parameters were the aspect ratio, the angle of attack and the Re number. Tuft and pressure data were found in good agreement. The 3D CFD simulations reproduced the structure of the SCs in qualitative agreement with the experimental data but had a delay of ~3deg in capturing the first appearance of a SC. The error in Cl max prediction was 7% compared to 19% for the 2D cases. Tests show that SCs grow with Re number and angle of attack. Also analysis of the time averaged computational results indicated the presence of three types of vortices: (a) the trailing edge line vortex (TELV) in the wake, (b) the separation line vortex (SLV) over the wing and (c) the SC vortices. The TELV and SLV run parallel to the trailing edge and are of opposite sign, while the SC vortices start normal to the wing suction surface, then bend towards the SC centre and later extend downstream, with their vorticity parallel to the free stream.

Quantized vortices in arbitrary dimensions and the normal-to-superfluid phase transition

NASA Astrophysics Data System (ADS)

Bora, Florin

The structure and energetics of superflow around quantized vortices, and the motion inherited by these vortices from this superflow, are explored in the general setting of a superfluid in arbitrary dimensions. The vortices may be idealized as objects of co-dimension two, such as one-dimensional loops and two-dimensional closed surfaces, respectively, in the cases of three- and four-dimensional superfluidity. By using the analogy between vortical superflow and Ampere-Maxwell magnetostatics, the equilibrium superflow containing any specified collection of vortices is constructed. The energy of the superflow is found to take on a simple form for vortices that are smooth and asymptotically large, compared with the vortex core size. The motion of vortices is analyzed in general, as well as for the special cases of hyper-spherical and weakly distorted hyper-planar vortices. In all dimensions, vortex motion reflects vortex geometry. In dimension four and higher, this includes not only extrinsic but also intrinsic aspects of the vortex shape, which enter via the first and second fundamental forms of classical geometry. For hyper-spherical vortices, which generalize the vortex rings of three dimensional superfluidity, the energy-momentum relation is determined. Simple scaling arguments recover the essential features of these results, up to numerical and logarithmic factors. Extending these results to systems containing multiple vortices is elementary due to the linearity of the theory. The energy for multiple vortices is thus a sum of self-energies and power-law interaction terms. The statistical mechanics of a system containing vortices is addressed via the grand canonical partition function. A renormalization-group analysis in which the low energy excitations are integrated approximately, is used to compute certain critical coefficients. The exponents obtained via this approximate procedure are compared with values obtained previously by other means. For dimensions higher than three the superfluid density is found to vanish as the critical temperature is approached from below.

Periodic solutions with prescribed minimal period of vortex type problems in domains

NASA Astrophysics Data System (ADS)

Bartsch, Thomas; Sacchet, Matteo

2018-05-01

We consider Hamiltonian systems with two degrees of freedom of point vortex type for in a domain . In the classical point vortex context the Hamiltonian is of the form where is the regular part of a hydrodynamic Green function in Ω, is the Robin function: , and , are the vortex strengths. We prove the existence of infinitely many periodic solutions with prescribed minimal period that are superpositions of a slow motion of the center of vorticity close to a star-shaped level line of h and of a fast rotation of the two vortices around their center of vorticity. The proofs are based on a recent higher dimensional version of the Poincaré–Birkhoff theorem due to Fonda and Ureña.

Static and Dynamic Flow Visualization Studies of Two Double-Delta Wing Models at High Angles of Attack

DTIC Science & Technology

1992-03-01

body, ft U.= free-stream velocity, ft/sec In the case of a wing pitching about its mid-chord location, it can be interpreted as the ratio of the...Over Moderately Swept Delta Wings," HTP -5 Workshop On Vortical Flow Breakdown and Structural Interactions, NASA Langley Research Center, August 15-16...January 6- 9,1992/Reno,Nevada. 18. User’s Manual , Flow Visualization Water Tunnel Operation for Model 1520, Eidelic International, Inc., Torrance

Nonlinear Dynamics of Vortices in Different Types of Grain Boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sheikhzada, Ahmad

As a major component of linear particle accelerators, superconducting radio-frequency (SRF) resonator cavities are required to operate with lowest energy dissipation and highest accelerating gradient. SRF cavities are made of polycrystalline materials in which grain boundaries can limit maximum RF currents and produce additional power dissipation sources due to local penetration of Josephson vortices. The essential physics of vortex penetration and mechanisms of dissipation of vortices driven by strong RF currents along networks of grain boundaries and their contribution to the residual surface resistance have not been well understood. To evaluate how GBs can limit the performance of SRF materials,more » particularly Nb and Nb3Sn, we performed extensive numerical simulations of nonlinear dynamics of Josephson vortices in grain boundaries under strong dc and RF fields. The RF power due to penetration of vortices both in weakly-coupled and strongly-coupled grain boundaries was calculated as functions of the RF field and frequency. The result of this calculation manifested a quadratic dependence of power to field amplitude at strong RF currents, an illustration of resistive behavior of grain boundaries. Our calculations also showed that the surface resistance is a complicated function of field controlled by penetration and annihilation of vortices and antivortices in strong RF fields which ultimately saturates to normal resistivity of grain boundary. We found that Cherenkov radiation of rapidly moving vortices in grain boundaries can produce a new instability causing generation of expanding vortex-antivortex pair which ultimately drives the entire GB in a resistive state. This effect is more pronounced in polycrystalline thin film and multilayer coating structures in which it can cause significant increase in power dissipation and results in hysteresis effects in I-V characteristics, particularly at low temperatures.« less

The Hilsch Tube, Rossby Vortices, and a Carnot Engine: Angular Momentum Transport in Astrophysics

NASA Astrophysics Data System (ADS)

Beckley, Howard F.; Klein, B.; Milburn, M.; Schindel, P.; Westpfahl, D. J.; Teare, S.; Li, H.; Colgate, S. A.

2008-05-01

We are attempting to demonstrate that the common laboratory vortex or Hilsch tube is a paradigm for the angular momentum transport by Rossby vortices in Keplerian accretion disks, either in super massive black hole formation or in star formation. Near supersonic rotating flow is induced in a cylinder by gas pressure injected through a tangential nozzle in a typical Ranque vortex or Hilsch tube. The gas exits through both an on-axis hole and a peripheral radially-aligned hole. The surprising result, demonstrated in hundreds of class rooms, is that one of the exit gas streams is hot and the other is cold. Depressing is that the typical explanation is given in terms of a "Maxwell daemon” that separates hot molecules from cold molecules, just as is the basis of any perpetual motion machine that violates the second law of thermodynamics. Instead we believe that the rotational flow is unstable to the formation of Rossby vortices that co-rotate with the azimuthal flow and act like semi-ridged turbine vanes. These quasi-vanes act like a Carnot turbine engine to the flow that escapes on axis and is therefore cooled by doing work. With the resulting free-energy, the vortices accelerate the peripheral flow which in turn becomes hot by friction with the cylinder wall. As a first step we expect to demonstrate that a free-running turbine, where metal vanes form the Carnot engine, will demonstrate the temperature effect. Such a suggestive result may lead to funding of time-dependent Schlerian photography of a vortex tube that can demonstrate the formation and pressure distribution of the Rossby vortices and coherent transport of angular momentum. This work is supported by a cooperative agreement between the New Mexico Institute of Mining and Technology, the University of California, Los Alamos National Laboratory, and the U.S. Dept. of Energy.

Combined action of transverse oscillations and uniform cross-flow on vortex formation and pattern of a circular cylinder

NASA Astrophysics Data System (ADS)

Lam, K. M.; Liu, P.; Hu, J. C.

2010-07-01

This paper attempts to study the roles of lateral cylinder oscillations and a uniform cross-flow in the vortex formation and wake modes of an oscillating circular cylinder. A circular cylinder is given lateral oscillations of varying amplitudes (between 0.28 and 1.42 cylinder-diameters) in a slow uniform flow stream (Reynolds number=284) to produce the 2S, 2P and P+S wake modes. Detailed flow information is obtained with time-resolved particle-image velocimetry and the phase-locked averaging techniques. In the 2S and 2P mode, the flow speeds relative to the cylinder movement are less than the uniform flow velocity and it is found that initial formation of a vortex is caused by shear-layer separation of the uniform flow on the cylinder. Subsequent development of the shear-layer vortices is affected by the lateral cylinder movement. At small cylinder oscillation amplitudes, vortices are shed in synchronization with the cylinder movement, resulting in the 2S mode. The 2P mode occurs at larger cylinder oscillation amplitudes at which each shear-layer vortex is found to undergo intense stretching and eventual bifurcation into two separate vortices. The P+S mode occurs when the cylinder moving speeds are, for most of the time, higher than the speed of the uniform flow. These situations are found at fast and large-amplitude cylinder oscillations in which the flow relative to the cylinder movement takes over the uniform flow in governing the initial vortex formation. The formation stages of vortices from the cylinder are found to bear close resemblance to those of a vortex street pattern of a cylinder oscillating in an otherwise quiescent fluid at Keulegan-Carpenter numbers around 16. Vortices in the inclined vortex street pattern so formed are then convected downstream by the uniform flow as the vortex pairs in the 2P mode.

Identifying Turbulent Structures through Topological Segmentation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bremer, Peer-Timo; Gruber, Andrea; Bennett, Janine C.

2016-01-01

A new method of extracting vortical structures from a turbulent flow is proposed whereby topological segmentation of an indicator function scalar field is used to identify the regions of influence of the individual vortices. This addresses a long-standing challenge in vector field topological analysis: indicator functions commonly used produce a scalar field based on the local velocity vector field; reconstructing regions of influence for a particular structure requires selecting a threshold to define vortex extent. In practice, the same threshold is rarely meaningful throughout a given flow. By also considering the topology of the indicator field function, the characteristics ofmore » vortex strength and extent can be separated and the ambiguity in the choice of the threshold reduced. The proposed approach is able to identify several types of vortices observed in a jet in cross-flow configuration simultaneously where no single threshold value for a selection of common indicator functions appears able to identify all of these vortex types.« less

Comparison of Four Mixed Layer Mesoscale Parameterizations and the Equation for an Arbitrary Tracer

NASA Technical Reports Server (NTRS)

Canuto, V. M.; Dubovikov, M. S.

2011-01-01

In this paper we discuss two issues, the inter-comparison of four mixed layer mesoscale parameterizations and the search for the eddy induced velocity for an arbitrary tracer. It must be stressed that our analysis is limited to mixed layer mesoscales since we do not treat sub-mesoscales and small turbulent mixing. As for the first item, since three of the four parameterizations are expressed in terms of a stream function and a residual flux of the RMT formalism (residual mean theory), while the fourth is expressed in terms of vertical and horizontal fluxes, we needed a formalism to connect the two formulations. The standard RMT representation developed for the deep ocean cannot be extended to the mixed layer since its stream function does not vanish at the ocean's surface. We develop a new RMT representation that satisfies the surface boundary condition. As for the general form of the eddy induced velocity for an arbitrary tracer, thus far, it has been assumed that there is only the one that originates from the curl of the stream function. This is because it was assumed that the tracer residual flux is purely diffusive. On the other hand, we show that in the case of an arbitrary tracer, the residual flux has also a skew component that gives rise to an additional bolus velocity. Therefore, instead of only one bolus velocity, there are now two, one coming from the curl of the stream function and other from the skew part of the residual flux. In the buoyancy case, only one bolus velocity contributes to the mean buoyancy equation since the residual flux is indeed only diffusive.

Continuous micro-vortex-based nanoparticle manipulation via focused surface acoustic waves.

PubMed

Collins, David J; Ma, Zhichao; Han, Jongyoon; Ai, Ye

2016-12-20

Despite increasing demand in the manipulation of nanoscale objects for next generation biological and industrial processes, there is a lack of methods for reliable separation, concentration and purification of nanoscale objects. Acoustic methods have proven their utility in contactless manipulation of microscale objects mainly relying on the acoustic radiation effect, though the influence of acoustic streaming has typically prevented manipulation at smaller length scales. In this work, however, we explicitly take advantage of the strong acoustic streaming in the vicinity of a highly focused, high frequency surface acoustic wave (SAW) beam emanating from a series of focused 6 μm substrate wavelength interdigital transducers patterned on a piezoelectric lithium niobate substrate and actuated with a 633 MHz sinusoidal signal. This streaming field serves to focus fluid streamlines such that incoming particles interact with the acoustic field similarly regardless of their initial starting positions, and results in particle displacements that would not be possible with a travelling acoustic wave force alone. This streaming-induced manipulation of nanoscale particles is maximized with the formation of micro-vortices that extend the width of the microfluidic channel even with the imposition of a lateral flow, occurring when the streaming-induced flow velocities are an order of magnitude larger than the lateral one. We make use of this acoustic streaming to demonstrate the continuous and differential focusing of 100 nm, 300 nm and 500 nm particles.

Effects of unsteady free stream velocity and free stream turbulence on stagnation point heat transfer

NASA Technical Reports Server (NTRS)

Gorla, R. S. R.

1984-01-01

The combined effects of transient free stream velocity and free stream turbulence on heat transfer at a stagnation point over a cylinder situated in a crossflow are studied. An eddy diffusivity model was formulated and the governing momentum and energy equations are integrated by means of the steepest descent method. The numerical results for the wall shear stress and heat transfer rate are correlated by a turbulence parameter. The wall friction and heat transfer rate increase with increasing free stream turbulence intensity.

Insights into the Streaming Instability in Protoplanetary Disks

NASA Astrophysics Data System (ADS)

Youdin, Andrew N.; Lin, Min-Kai; Li, Rixin

2017-10-01

The streaming instability is a leading mechanism to concentrate particles in protoplanetary disks, thereby triggering planetesimal formation. I will present recent analytical and numerical work on the origin of the streaming instability and its robustness. Our recent analytic work examines the origin of, and relationship between, a variety of drag-induced instabilities, including the streaming instability as well as secular gravitational instabilities, a drag instability driven by self-gravity. We show that drag instabilities are powered by a specific phase relationship between gas pressure and particle concentrations, which power the instability via pressure work. This mechanism is analogous to pulsating instabilities in stars. This mechanism differs qualitatively from other leading particle concentration mechanisms in pressure bumps and vortices. Our recent numerical work investigates the numerical robustness of non-linear particle clumping by the streaming instability, especially with regard to the location and boundary condition of vertical boundaries. We find that particle clumping is robust to these choices in boxes that are not too short. However, hydrodynamic activity away from the particle-dominated midplane is significantly affected by vertical boundary conditions. This activity affects the observationally significant lofting of small dust grains. We thus emphasize the need for larger scale simulations which connect disk surface layers, including outflowing winds, to the planet-forming midplane.

Compressible liquid flow in nano- or micro-sized circular tubes considering wall-liquid Lifshitz-van der Waals interaction

NASA Astrophysics Data System (ADS)

Zhang, Xueling; Zhu, Weiyao; Cai, Qiang; Shi, Yutao; Wu, Xuehong; Jin, Tingxiang; Yang, Lianzhi; Song, Hongqing

2018-06-01

Although nano- and micro-scale phenomena for fluid flows are ubiquitous in tight oil reservoirs or in nano- or micro-sized channels, the mechanisms behind them remain unclear. In this study, we consider the wall-liquid interaction to investigate the flow mechanisms behind a compressible liquid flow in nano- or micro-sized circular tubes. We assume that the liquid is attracted by the wall surface primarily by the Lifshitz-van der Waals (LW) force, whereas electrostatic forces are negligible. The long-range LW force is thus introduced into the Navier-Stokes equations. The nonlinear equations of motion are decoupled by using the hydrodynamic vorticity-stream functions, from which an approximate analytical perturbation solution is obtained. The proposed model considers the LW force and liquid compressibility to obtain the velocity and pressure fields, which are consistent with experimentally observed micro-size effects. A smaller tube radius implies smaller dimensionless velocity, and when the tube radius decreases to a certain radius Rm, a fluid no longer flows, where Rm is the lower limit of the movable-fluid radius. The radius Rm is calculated, and the results are consistent with previous experimental results. These results reveal that micro-size effects are caused by liquid compressibility and wall-liquid interactions, such as the LW force, for a liquid flowing in nano- or micro-sized channels or pores. The attractive LW force enhances the flow's radial resistance, and the liquid compressibility transmits the radial resistance to the streaming direction via volume deformation, thereby decreasing the streaming velocity.

Pressure Fluctuations Induced by a Hypersonic Turbulent Boundary Layer

NASA Technical Reports Server (NTRS)

Duan, Lian; Choudhari, Meelan M.; Zhang, Chao

2016-01-01

Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially-developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analyzed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean velocity pro les and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except in the immediate vicinity of the wall) while the propagation speed deviates from the Taylor's hypothesis in the free stream. Compared with the surface pressure fluctuations, which are primarily vortical, the acoustic pressure fluctuations in the free stream exhibit a significantly lower dominant frequency, a greater spatial extent, and a smaller bulk propagation speed. The freestream pressure structures are found to have similar Lagrangian time and spatial scales as the acoustic sources near the wall. As the Mach number increases, the freestream acoustic fluctuations exhibit increased radiation intensity, enhanced energy content at high frequencies, shallower orientation of wave fronts with respect to the flow direction, and larger propagation velocity.

Vorticity and Vertical Motions Diagnosed from Satellite Deep-Layer Temperatures. Revised

NASA Technical Reports Server (NTRS)

Spencer, Roy W.; Lapenta, William M.; Robertson, Franklin R.

1994-01-01

Spatial fields of satellite-measured deep-layer temperatures are examined in the context of quasigeostrophic theory. It is found that midtropospheric geostrophic vorticity and quasigeostrophic vertical motions can be diagnosed from microwave temperature measurements of only two deep layers. The lower- ( 1000-400 hPa) and upper- (400-50 hPa) layer temperatures are estimated from limb-corrected TIROS-N Microwave Sounding Units (MSU) channel 2 and 3 data, spatial fields of which can be used to estimate the midtropospheric thermal wind and geostrophic vorticity fields. Together with Trenberth's simplification of the quasigeostrophic omega equation, these two quantities can be then used to estimate the geostrophic vorticity advection by the thermal wind, which is related to the quasigeostrophic vertical velocity in the midtroposphere. Critical to the technique is the observation that geostrophic vorticity fields calculated from the channel 3 temperature features are very similar to those calculated from traditional, 'bottom-up' integrated height fields from radiosonde data. This suggests a lack of cyclone-scale height features near the top of the channel 3 weighting function, making the channel 3 cyclone-scale 'thickness' features approximately the same as height features near the bottom of the weighting function. Thus, the MSU data provide observational validation of the LID (level of insignificant dynamics) assumption of Hirshberg and Fritsch.

Hydrodynamical model of anisotropic, polarized turbulent superfluids. I: constraints for the fluxes

NASA Astrophysics Data System (ADS)

Mongiovì, Maria Stella; Restuccia, Liliana

2018-02-01

This work is the first of a series of papers devoted to the study of the influence of the anisotropy and polarization of the tangle of quantized vortex lines in superfluid turbulence. A thermodynamical model of inhomogeneous superfluid turbulence previously formulated is here extended, to take into consideration also these effects. The model chooses as thermodynamic state vector the density, the velocity, the energy density, the heat flux, and a complete vorticity tensor field, including its symmetric traceless part and its antisymmetric part. The relations which constrain the constitutive quantities are deduced from the second principle of thermodynamics using the Liu procedure. The results show that the presence of anisotropy and polarization in the vortex tangle affects in a substantial way the dynamics of the heat flux, and allow us to give a physical interpretation of the vorticity tensor here introduced, and to better describe the internal structure of a turbulent superfluid.

Accuracy of least-squares methods for the Navier-Stokes equations

NASA Technical Reports Server (NTRS)

Bochev, Pavel B.; Gunzburger, Max D.

1993-01-01

Recently there has been substantial interest in least-squares finite element methods for velocity-vorticity-pressure formulations of the incompressible Navier-Stokes equations. The main cause for this interest is the fact that algorithms for the resulting discrete equations can be devised which require the solution of only symmetric, positive definite systems of algebraic equations. On the other hand, it is well-documented that methods using the vorticity as a primary variable often yield very poor approximations. Thus, here we study the accuracy of these methods through a series of computational experiments, and also comment on theoretical error estimates. It is found, despite the failure of standard methods for deriving error estimates, that computational evidence suggests that these methods are, at the least, nearly optimally accurate. Thus, in addition to the desirable matrix properties yielded by least-squares methods, one also obtains accurate approximations.

Decaying two-dimensional turbulence in a circular container.

PubMed

Schneider, Kai; Farge, Marie

2005-12-09

We present direct numerical simulations of two-dimensional decaying turbulence at initial Reynolds number 5 x 10(4) in a circular container with no-slip boundary conditions. Starting with random initial conditions the flow rapidly exhibits self-organization into coherent vortices. We study their formation and the role of the viscous boundary layer on the production and decay of integral quantities. The no-slip wall produces vortices which are injected into the bulk flow and tend to compensate the enstrophy dissipation. The self-organization of the flow is reflected by the transition of the initially Gaussian vorticity probability density function (PDF) towards a distribution with exponential tails. Because of the presence of coherent vortices the pressure PDF become strongly skewed with exponential tails for negative values.

A semi-Lagrangian approach to the shallow water equation

NASA Technical Reports Server (NTRS)

Bates, J. R.; Mccormick, Stephen F.; Ruge, John; Sholl, David S.; Yavneh, Irad

1993-01-01

We present a formulation of the shallow water equations that emphasizes the conservation of potential vorticity. A locally conservative semi-Lagrangian time-stepping scheme is developed, which leads to a system of three coupled PDE's to be solved at each time level. We describe a smoothing analysis of these equations, on which an effective multigrid solver is constructed. Some results from applying this solver to the static version of these equations are presented.

Numerical study of droplet evaporation in an acoustic levitator

NASA Astrophysics Data System (ADS)

Bänsch, Eberhard; Götz, Michael

2018-03-01

We present a finite element method for the simulation of all relevant processes of the evaporation of a liquid droplet suspended in an acoustic levitation device. The mathematical model and the numerical implementation take into account heat and mass transfer across the interface between the liquid and gaseous phase and the influence of acoustic streaming on this process, as well as the displacement and deformation of the droplet due to acoustic radiation pressure. We apply this numerical method to several theoretical and experimental examples and compare our results with the well-known d2-law for the evaporation of spherical droplets and with theoretical predictions for the acoustic streaming velocity. We study the influence of acoustic streaming on the distribution of water vapor and temperature in the levitation device, with special attention to the vapor distribution in the emerging toroidal vortices. We also compare the evaporation rate of a droplet with and without acoustic streaming, as well as the evaporation rates in dependence of different temperatures and sound pressure levels. Finally, a simple model of protein inactivation due to heat damage is considered and studied for different evaporation settings and their respective influence on protein damage.

EDITORIAL: The FDR Prize The FDR Prize

NASA Astrophysics Data System (ADS)

Kida, Shigeo

2009-06-01

From the 45 papers published in the year 2008 in Fluid Dynamics Research the following paper has been selected for the second FDR prize: 'Propagation of very long water waves, with vorticity, over variable depth, with applications to tsunamis' by Adrian Constantin and Robin S Johnson, published in volume 40 (March 2008) pp 175-211. This paper takes, as its main theme, the analysis of the propagation of very long gravity waves in the ocean environment, with the possibility of applying the results to tsunamis. Both variable depth and some pre-existing vorticity are allowed in the model, but under the over-arching assumption of long waves; indeed, it is argued, the waves are so long that it is impossible for classical soliton theory to be the appropriate description of a developing tsunami. This aspect is supported by some simple scaling arguments, together with some observations associated with the tsunami of Boxing Day 2004. The formulation is based on two small scales: the slow scale on which the depth varies and the small amplitude of the wave (as initially generated in deep water). The technique adopted is that of matched asymptotic expansions. The solution, constructed for deep water, is not valid in suitably reduced depth of water; the solution in this shallow region (close inshore) is then matched to the deep-water solution. A novel feature of this work is the inclusion of a general distribution of vorticity in the absence of waves—intended to model the realistic ocean—which is based on the slow evolution scale for the bottom topography. Some general properties of such background flows are proved, and two specific examples have been obtained: constant vorticity everywhere (as far as the shoreline), and regions of isolated vorticity (for appropriate bottom profiles). The way in which the wave properties are modified in the presence of vorticity is described. The significant overall proposal in this theory, specifically applicable to tsunamis, is that it is the profile of the initial disturbance (generated by the seismic activity) that is the single most important ingredient in the formation of tsunami waves (provided, of course, the familiar requirement of a long, gently shelving beach is also present). This contention is described and developed, and supported by some graphical examples of the various types of solution that can be obtained; these include contributions from variable depth and suitable background vorticity.

Numerical study of large-eddy breakup and its effect on the drag characteristics of boundary layers

NASA Technical Reports Server (NTRS)

Kinney, R. B.; Taslim, M. E.; Hung, S. C.

1985-01-01

The break-up of a field of eddies by a flat-plate obstacle embedded in a boundary layer is studied using numerical solutions to the two-dimensional Navier-Stokes equations. The flow is taken to be incompressible and unsteady. The flow field is initiated from rest. A train of eddies of predetermined size and strength are swept into the computational domain upstream of the plate. The undisturbed velocity profile is given by the Blasius solution. The disturbance vorticity generated at the plate and wall, plus that introduced with the eddies, mix with the background vorticity and is transported throughout the entire flow. All quantities are scaled by the plate length, the unidsturbed free-stream velocity, and the fluid kinematic viscosity. The Reynolds number is 1000, the Blasius boundary layer thickness is 2.0, and the plate is positioned a distance of 1.0 above the wall. The computational domain is four units high and sixteen units long.

A Mixing Length Scale of Unlike Impinging Jets

NASA Astrophysics Data System (ADS)

Inoue, Chihiro; Fujii, Go; Daimon, Yu

2017-11-01

Bi-propellant thrusters in space propulsion systems often utilize unlike-doublet or triplet injectors. The impingement of hypergolic liquid jet streams of fuel and oxidizer involves the expanding sheet, droplet fragmentation, mixing, evaporation, and chemical reactions in liquid and gas phases, in which the rate controlling phenomenon is the mixing step. In this study, a defined length scale demonstrates the distribution of fuel and oxidizer, and therefore, represents their mixing states, allowing for providing a physical meaning of widely accepted practical indicator, so called Rupe factor, over half a century of injector design history. We concisely formulate the characteristic velocity in a consistent manner for doublet and triplet injectors as a function of propellant injection conditions. The validity of the present formulation is convinced by comparing with hot firing tests.

From genes to brain development to phenotypic behavior: "dorsal-stream vulnerability" in relation to spatial cognition, attention, and planning of actions in Williams syndrome (WS) and other developmental disorders.

PubMed

Atkinson, Janette; Braddick, Oliver

2011-01-01

Visual information is believed to be processed through two distinct, yet interacting cortical streams. The ventral stream performs the computations needed for recognition of objects and faces ("what" and "who"?) and the dorsal stream the computations for registering spatial relationships and for controlling visually guided actions ("where" and "how"?). We initially proposed a model of spatial deficits in Williams syndrome (WS) in which visual abilities subserved by the ventral stream, such as face recognition, are relatively well developed (although not necessarily in exactly the same way as in typical development), whereas dorsal-stream functions, such as visuospatial actions, are markedly impaired. Since these initial findings in WS, deficits of motion coherence sensitivity, a dorsal-stream function has been found in other genetic disorders such as Fragile X and autism, and as a consequence of perinatal events (in hemiplegia, perinatal brain anomalies following very premature birth), leading to the proposal of a general "dorsal-stream vulnerability" in many different conditions of abnormal human development. In addition, dorsal-stream systems provide information used in tasks of visuospatial memory and locomotor planning, and these systems are closely coupled to networks for attentional control. We and several other research groups have previously shown deficits of frontal and parietal lobe function in WS individuals for specific attention tasks [e.g., Atkinson, J., Braddick, O., Anker, S., Curran, W., & Andrew, R. (2003). Neurobiological models of visuospatial cognition in children with Williams Syndrome: Measures of dorsal-stream and frontal function. Developmental Neuropsychology, 23(1/2), 141-174.]. We have used the Test of Everyday Attention for Children (TEA-Ch) which aims to attempt to separate components of attention with distinct brain networks (selective attention, sustained attention, and attention control-executive function) testing a group of older children with WS, but this test battery is too demanding for many children and adults with WS. Consequently, we have devised a new set of tests of attention, the Early Childhood Attention Battery (ECAB). This uses similar principles to the TEA-Ch, but adapted for mental ages younger than 6 years. The ECAB shows a distinctive attention profile for WS individuals relative to their overall cognitive development, with relative strength in tasks of sustained attention and poorer performance on tasks of selective attention and executive control. These profiles, and the characteristic developmental courses, also show differences between children with Down's syndrome and WS. This chapter briefly reviews new research findings on WS in these areas, relating the development of brain systems in WS to evidence from neuroimaging in typically developing infants, children born very preterm, and normal adults. The hypothesis of "dorsal-stream(s) vulnerability" which will be discussed includes a number of interlinked brain networks, subserving not only global visual processing and formulation of visuomotor actions but interlinked networks of attention. Copyright © 2011 Elsevier B.V. All rights reserved.

Revisiting Horton's laws with considerations of the directly drained VS source area

NASA Astrophysics Data System (ADS)

Yang, Soohyun; Paik, Kyungrock

2015-04-01

River networks have been regarded as excellent examples of self-similar patterns in nature. Fractal characteristics of river networks have been quantified through scaling relations between several morphologic variables (e.g., Hack, 1957; Flint, 1974). In particular, Horton's legendary study on scaling properties between numbers and lengths of streams in different orders (Horton, 1945) has significantly influenced research studies in this subject. Today, Horton's laws are referred to the log-linear relationships of three variables across stream orders, i.e., number, length, and area which is later added by Schumm (1956). In a closer look, there is a conceptual inconsistency between their definitions though. While length is defined as the length of stream of a specific order only, area by its definition includes drainage area of lower order streams. To deal with this inconsistency, there was an attempt to distinguish the average area drained directly by the stream of a particular order in the Hortonian formulation (Marani et al., 1991; Beer and Borgas, 1993). Nevertheless, there remains an interesting problem in the definition of directly drained area for 1st order and for the rest orders in these studies. While the whole area of 1st order stream is regarded as the directly drained area in these studies, for a channel to form it needs the minimum drainage area named source area. In this study, we evaluate how significant considering this zero order area separately is in understanding overall river network organization. To this end, we define new expression for the directly drained area and revisit Horton's laws with a generalized formulation. To test the proposed ideas, several river networks extracted from digital elevation models (DEMs) are analyzed. References Beer, T., & Borgas, M. (1993). Horton's laws and the fractal nature of streams. Water Resources Research, 29(5), 1475-1487. Flint, J. J. (1974). Stream gradient as a function of order, magnitude, and discharge. Water Resources Research, 10(5), 969-973. Hack, J. T. (1957). Studies of longitudinal river profiles in Virginia and Maryland. US, Geological Survey Professional Paper, 294. Horton, R. E. (1945). Erosional development of streams and their drainage basins; hydrophysical approach to quantitative morphology. Geological Society of America Bulletin, 56(3), 275-370. Marani, A., Rigon, R., & Rinaldo, A. (1991). A note on fractal channel networks. Water Resources Research, 27(12), 3041-3049. Schumm, S. A. (1956). Evolution of drainage systems and slopes in badlands at Perth Amboy, New Jersey. Geological Society of America Bulletin, 67(5), 597-646.

Characteristics and controllability of vortices in ferromagnetics, ferroelectrics, and multiferroics.

PubMed

Zheng, Yue; Chen, W J

2017-08-01

Topological defects in condensed matter are attracting e significant attention due to their important role in phase transition and their fascinating characteristics. Among the various types of matter, ferroics which possess a switchable physical characteristic and form domain structure are ideal systems to form topological defects. In particular, a special class of topological defects-vortices-have been found to commonly exist in ferroics. They often manifest themselves as singular regions where domains merge in large systems, or stabilize as novel order states instead of forming domain structures in small enough systems. Understanding the characteristics and controllability of vortices in ferroics can provide us with deeper insight into the phase transition of condensed matter and also exciting opportunities in designing novel functional devices such as nano-memories, sensors, and transducers based on topological defects. In this review, we summarize the recent experimental and theoretical progress in ferroic vortices, with emphasis on those spin/dipole vortices formed in nanoscale ferromagnetics and ferroelectrics, and those structural domain vortices formed in multiferroic hexagonal manganites. We begin with an overview of this field. The fundamental concepts of ferroic vortices, followed by the theoretical simulation and experimental methods to explore ferroic vortices, are then introduced. The various characteristics of vortices (e.g. formation mechanisms, static/dynamic features, and electronic properties) and their controllability (e.g. by size, geometry, external thermal, electrical, magnetic, or mechanical fields) in ferromagnetics, ferroelectrics, and multiferroics are discussed in detail in individual sections. Finally, we conclude this review with an outlook on this rapidly developing field.

Structure, transport, and vertical coherence of the Gulf Stream from the Straits of Florida to the Southeast Newfoundland Ridge

NASA Astrophysics Data System (ADS)

Meinen, Christopher S.; Luther, Douglas S.

2016-06-01

Data from three independent and extensive field programs in the Straits of Florida, the Mid-Atlantic Bight, and near the Southeast Newfoundland Ridge are reanalyzed and compared with results from other historical studies to highlight the downstream evolution of several characteristics of the Gulf Stream's mean flow and variability. The three locations represent distinct dynamical regimes: a tightly confined jet in a channel; a freely meandering jet; and a topographically controlled jet on a boundary. Despite these differing dynamical regimes, the Gulf Stream in these areas exhibits many similarities. There are also anticipated and important differences, such as the loss of the warm core of the current by 42°N and the decrease in the cross-frontal gradient of potential vorticity as the current flows northward. As the Gulf Stream evolves it undergoes major changes in transport, both in magnitude and structure. The rate of inflow up to 60°W and outflow thereafter are generally uniform, but do exhibit some remarkable short-scale variations. As the Gulf Stream flows northward the vertical coherence of the flow changes, with the Florida Current and North Atlantic Current segments of the Gulf Stream exhibiting distinct upper and deep flows that are incoherent, while in the Mid-Atlantic Bight the Gulf Stream exhibits flows in three layers each of which tends to be incoherent with the other layers at most periods. These coherence characteristics are exhibited in both Eulerian and stream coordinates. The observed lack of vertical coherence indicates that great caution must be exercised in interpreting proxies for Gulf Stream structure and flow from vertically-limited or remote observations.

Structure, transport, and vertical coherence of the Gulf Stream from the Straits of Florida to the Southeast Newfoundland Ridge

NASA Astrophysics Data System (ADS)

Meinen, Christopher S.; Luther, Douglas S.

2016-05-01

Data from three independent and extensive field programs in the Straits of Florida, the Mid-Atlantic Bight, and near the Southeast Newfoundland Ridge are reanalyzed and compared with results from other historical studies to highlight the downstream evolution of several characteristics of the Gulf Stream's mean flow and variability. The three locations represent distinct dynamical regimes: a tightly confined jet in a channel; a freely meandering jet; and a topographically controlled jet on a boundary. Despite these differing dynamical regimes, the Gulf Stream in these areas exhibits many similarities. There are also anticipated and important differences, such as the loss of the warm core of the current by 42°N and the decrease in the cross-frontal gradient of potential vorticity as the current flows northward. As the Gulf Stream evolves it undergoes major changes in transport, both in magnitude and structure. The rate of inflow up to 60°W and outflow thereafter are generally uniform, but do exhibit some remarkable short-scale variations. As the Gulf Stream flows northward the vertical coherence of the flow changes, with the Florida Current and North Atlantic Current segments of the Gulf Stream exhibiting distinct upper and deep flows that are incoherent, while in the Mid-Atlantic Bight the Gulf Stream exhibits flows in three layers each of which tends to be incoherent with the other layers at most periods. These coherence characteristics are exhibited in both Eulerian and stream coordinates. The observed lack of vertical coherence indicates that great caution must be exercised in interpreting proxies for Gulf Stream structure and flow from vertically-limited or remote observations.

Deflection and trapping of a counter-rotating vortex pair by a flat plate

NASA Astrophysics Data System (ADS)

Nitsche, Monika

2017-12-01

The interaction of a counter-rotating vortex pair (dipole) with a flat plate in its path is studied numerically. The vortices are initially separated by a distance D (dipole size) and placed far upstream of a plate of length L . The plate is centered on the dipole path and inclined relative to it at an incident angle βi. At first, the plate is held fixed in place. The vortices approach the plate, travel around it, and then leave as a dipole with unchanged velocity but generally a different travel direction, measured by a transmitted angle βt. For certain plate angles the transmitted angle is highly sensitive to changes in the incident angle. The sensitivity increases as the dipole size decreases relative to the plate length. In fact, for sufficiently small values of D /L , singularities appear: near critical values of βi, the dipole trajectory undergoes a topological discontinuity under changes of βi or D /L . The discontinuity is characterized by a jump in the winding number of one vortex around the plate, and in the time that the vortices take to leave the plate. The jumps occur repeatedly in a self-similar, fractal fashion, within a region near the critical values of βi, showing the existence of incident angles that trap the vortices, which never leave the plate. The number of these trapping regions increases as the parameter D /L decreases, and the dependence of the motion on βi becomes increasingly complex. The simulations thus show that even in this apparently simple scenario, the inviscid dynamics of a two-point-vortex system interacting with a stationary wall is surprisingly rich. The results are then applied to separate an incoming stream of dipoles by an oscillating plate.

The effect of atmospheric diabatic heating on low-frequency oscillations

NASA Astrophysics Data System (ADS)

Yen, Ming-Cheng

A diagnostic scheme is devised to illustrate a chain relationship between diabatic heating and planetary-scale divergent and rotational circulations. The scheme consists of the velocity-potential maintenance equation, which relates diabatic heating and velocity potential, and the streamfunction budget equation, which depicts the streamfunction tendency caused by the imbalance between streamfunction tendencies induced by vorticity advection and source. The proposed scheme is employed to examine the effect of tropical diabatic heating on the annual variation of subtropical jet streams. It was found that annual variations of both tropical diabatic heating and planetary-scale divergent circulation exhibit an annual in-phase seesaw oscillation between the winter and summer hemispheres. The annual variation of subtropical jet streams is caused by the adjustment of atmospheric rotational flow through planetary-scale divergent circulation in response to the annual cycle of tropical diabatic heating.

An experimental trace gas investigation of fluid transport and mixing in a circular-to-rectangular transition duct

NASA Technical Reports Server (NTRS)

Reichert, B. A.; Hingst, W. R.; Okiishi, T. H.

1991-01-01

An ethylene trace gas technique was used to map out fluid transport and mixing within a circular to rectangular transition duct. Ethylene gas was injected at several points in a cross stream plane upstream of the transition duct. Ethylene concentration contours were determined at several cross stream measurement planes spaced axially within the duct. The flow involved a uniform inlet flow at a Mach number level of 0.5. Statistical analyses were used to quantitatively interpret the trace gas results. Also, trace gas data were considered along with aerodynamic and surface flow visualization results to ascertain transition duct flow phenomena. Convection of wall boundary layer fluid by vortices produced regions of high total pressure loss in the duct. The physical extent of these high loss regions is governed by turbulent diffusion.

MHD Turbulence, div B = 0 and Lattice Boltzmann Simulations

NASA Astrophysics Data System (ADS)

Phillips, Nate; Keating, Brian; Vahala, George; Vahala, Linda

2006-10-01

The question of div B = 0 in MHD simulations is a crucial issue. Here we consider lattice Boltzmann simulations for MHD (LB-MHD). One introduces a scalar distribution function for the velocity field and a vector distribution function for the magnetic field. This asymmetry is due to the different symmetries in the tensors arising in the time evolution of these fields. The simple algorithm of streaming and local collisional relaxation is ideally parallelized and vectorized -- leading to the best sustained performance/PE of any code run on the Earth Simulator. By reformulating the BGK collision term, a simple implicit algorithm can be immediately transformed into an explicit algorithm that permits simulations at quite low viscosity and resistivity. However the div B is not an imposed constraint. Currently we are examining a new formulations of LB-MHD that impose the div B constraint -- either through an entropic like formulation or by introducing forcing terms into the momentum equations and permitting simpler forms of relaxation distributions.

Investigation of compressible vortex flow characteristics

NASA Technical Reports Server (NTRS)

Muirhead, V. U.

1977-01-01

The nature of intense air vortices was studied and the factors which determine the intensity and rate of decay of both single and pairs of vortices were investigated. Vortex parameters of axial pressure differential, circulation, outflow rates, separation distance and directions of rotation were varied. Unconfined vortices, generated by a single rotating cage, were intensified by an increasing axial pressure gradient. Breakdown occurred when the axial gradient became negligible. The core radius was a function of the axial gradient. Dual vortices, generated by two counterrotating cages, rotated opposite to the attached cages. With minimum spacing only one vortex was formed which rotated in a direction opposite to the attached cage. When one cage rotated at half the speed of the other cage, one vortex formed at the higher speed cage rotating in the cage direction.

Towards reduced order modelling for predicting the dynamics of coherent vorticity structures within wind turbine wakes.

PubMed

Debnath, M; Santoni, C; Leonardi, S; Iungo, G V

2017-04-13

The dynamics of the velocity field resulting from the interaction between the atmospheric boundary layer and a wind turbine array can affect significantly the performance of a wind power plant and the durability of wind turbines. In this work, dynamics in wind turbine wakes and instabilities of helicoidal tip vortices are detected and characterized through modal decomposition techniques. The dataset under examination consists of snapshots of the velocity field obtained from large-eddy simulations (LES) of an isolated wind turbine, for which aerodynamic forcing exerted by the turbine blades on the atmospheric boundary layer is mimicked through the actuator line model. Particular attention is paid to the interaction between the downstream evolution of the helicoidal tip vortices and the alternate vortex shedding from the turbine tower. The LES dataset is interrogated through different modal decomposition techniques, such as proper orthogonal decomposition and dynamic mode decomposition. The dominant wake dynamics are selected for the formulation of a reduced order model, which consists in a linear time-marching algorithm where temporal evolution of flow dynamics is obtained from the previous temporal realization multiplied by a time-invariant operator.This article is part of the themed issue 'Wind energy in complex terrains'. © 2017 The Author(s).

Vorticity-divergence semi-Lagrangian global atmospheric model SL-AV20: dynamical core

NASA Astrophysics Data System (ADS)

Tolstykh, Mikhail; Shashkin, Vladimir; Fadeev, Rostislav; Goyman, Gordey

2017-05-01

SL-AV (semi-Lagrangian, based on the absolute vorticity equation) is a global hydrostatic atmospheric model. Its latest version, SL-AV20, provides global operational medium-range weather forecast with 20 km resolution over Russia. The lower-resolution configurations of SL-AV20 are being tested for seasonal prediction and climate modeling. The article presents the model dynamical core. Its main features are a vorticity-divergence formulation at the unstaggered grid, high-order finite-difference approximations, semi-Lagrangian semi-implicit discretization and the reduced latitude-longitude grid with variable resolution in latitude. The accuracy of SL-AV20 numerical solutions using a reduced lat-lon grid and the variable resolution in latitude is tested with two idealized test cases. Accuracy and stability of SL-AV20 in the presence of the orography forcing are tested using the mountain-induced Rossby wave test case. The results of all three tests are in good agreement with other published model solutions. It is shown that the use of the reduced grid does not significantly affect the accuracy up to the 25 % reduction in the number of grid points with respect to the regular grid. Variable resolution in latitude allows us to improve the accuracy of a solution in the region of interest.

Development of the wake behind a circular cylinder impulsively started into rotatory and rectilinear motion

NASA Technical Reports Server (NTRS)

Chen, Yen-Ming; Ou, Yuh-Roung; Pearlstein, Arne J.

1993-01-01

The temporal development of a 2D viscous incompressible flow generated by a circular cylinder started impulsively into steady rotatory and rectilinear motion is studied by integration of a velocity/vorticity formulation of the governing equations, using an explicit finite-difference/pseudo-spectral technique and an implementation of the Biot-Savart law. Results are presented for a Reynolds number of 200 (based on the cylinder diameter 2a and the magnitude U of the rectilinear velocity) for several values of the angular/rectilinear speed ratio alpha = omega(a)/U (where omega is the angular speed) up to 3.25. Several aspects of the kinematics and dynamics of the flow not considered earlier are discussed. For higher values of alpha, the results indicate that for Re = 200, vortex shedding does indeed occur for alpha = 3.25. However, consecutive vortices shed by the body can be shed from the same side and be of the same sense, in contrast to the nonrotating case, in which mirror-image vortices of opposite sense are shed alternately on opposite sides of the body. The implications of the results are discussed in relation to the possibility of suppressing vortex shedding by open or closed-loop control of the rotation rate.

SENSITIVITY OF INDICES OF BIOTIC INTEGRITY TO SIMULATED FISH ASSEMBLAGE CHANGES

EPA Science Inventory

Multi-metric indices of biotic integrity (IBI) are commonly used to assess condition of stream fish assemblages, but their ability to monitor trends within streams over time is largely unknown. We assessed the trend detection ability of two IBI formulations (one with traditional ...

Effects of energetic coherent motions on the power and wake of an axial-flow turbine

NASA Astrophysics Data System (ADS)

Chamorro, L. P.; Hill, C.; Neary, V. S.; Gunawan, B.; Arndt, R. E. A.; Sotiropoulos, F.

2015-05-01

A laboratory experiment examined the effects of energetic coherent motions on the structure of the wake and power fluctuations generated by a model axial-flow hydrokinetic turbine. The model turbine was placed in an open-channel flow and operated under subcritical conditions. The incoming flow was locally perturbed with vertically oriented cylinders of various diameters. An array of three acoustic Doppler velocimeters aligned in the cross-stream direction and a torque transducer were used to collect high-resolution and synchronous measurements of the three-velocity components of the incoming and wake flow as well as the turbine power. A strong scale-to-scale interaction between the large-scale and broadband turbulence shed by the cylinders and the turbine power revealed how the turbulence structure modulates the turbine behavior. In particular, the response of the turbine to the distinctive von Kármán-type vortices shed from the cylinders highlighted this phenomenon. The mean and fluctuating characteristics of the turbine wake are shown to be very sensitive to the energetic motions present in the flow. Tip vortices were substantially dampened and the near-field mean wake recovery accelerated in the presence of energetic motions in the flow. Strong coherent motions are shown to be more effective than turbulence levels for triggering the break-up of the spiral structure of the tip-vortices.

Tracking vortices in superconductors: Extracting singularities from a discretized complex scalar field evolving in time

DOE PAGES

Phillips, Carolyn L.; Guo, Hanqi; Peterka, Tom; ...

2016-02-19

In type-II superconductors, the dynamics of magnetic flux vortices determine their transport properties. In the Ginzburg-Landau theory, vortices correspond to topological defects in the complex order parameter field. Earlier, we introduced a method for extracting vortices from the discretized complex order parameter field generated by a large-scale simulation of vortex matter. With this method, at a fixed time step, each vortex [simplistically, a one-dimensional (1D) curve in 3D space] can be represented as a connected graph extracted from the discretized field. Here we extend this method as a function of time as well. A vortex now corresponds to a 2Dmore » space-time sheet embedded in 4D space time that can be represented as a connected graph extracted from the discretized field over both space and time. Vortices that interact by merging or splitting correspond to disappearance and appearance of holes in the connected graph in the time direction. This method of tracking vortices, which makes no assumptions about the scale or behavior of the vortices, can track the vortices with a resolution as good as the discretization of the temporally evolving complex scalar field. In addition, even details of the trajectory between time steps can be reconstructed from the connected graph. With this form of vortex tracking, the details of vortex dynamics in a model of a superconducting materials can be understood in greater detail than previously possible.« less

Extraction of Lateral-Directional Stability and Control Derivatives for the Basic F-18 Aircraft at High Angles of Attack

NASA Technical Reports Server (NTRS)

Iliff, Kenneth W.; Wang, Kon-Sheng Charles

1997-01-01

The results of parameter identification to determine the lateral-directional stability and control derivatives of an F-18 research aircraft in its basic hardware and software configuration are presented. The derivatives are estimated from dynamic flight data using a specialized identification program developed at NASA Dryden Flight Research Center. The formulation uses the linearized aircraft equations of motions in their continuous/discrete form and a maximum likelihood estimator that accounts for both state and measurement noise. State noise is used to model the uncommanded forcing function caused by unsteady aerodynamics, such as separated and vortical flows, over the aircraft. The derivatives are plotted as functions of angle of attack between 3 deg and 47 deg and compared with wind-tunnel predictions. The quality of the derivative estimates obtained by parameter identification is somewhat degraded because the maneuvers were flown with the aircraft's control augmentation system engaged, which introduced relatively high correlations between the control variables and response variables as a result of control motions from the feedback control system.

Convective Sedimentation of Colloidal Particles in a Bowl.

PubMed

Stiles; Kagan

1999-08-01

A physical model, which regards a colloidal dispersion as a single fluid continuum, is used to investigate cellular convection accompanying gravitational sedimentation in a hemispherical bowl with a thin cylindrical shaft along its vertical axis of symmetry. We have adapted the stream-function-vorticity form of the Navier-Stokes equations to describe momentum conservation in axially symmetric containers. These hydrodynamic equations have been coupled to the mass balance equation for binary hydrodynamic diffusion in the presence of a vertical gravitational field. Using finite-element software we have solved the equations governing coupled diffusive and hydrodynamic flow. A rapidly intensifying horizontal toroidal vortex develops around the axis of the bowl. This vortex is characterized by downward barycentric flow along the curved surface of the bowl and upward flow in the vicinity of its axis. We find that after a short period of time this large-scale cellular convection associated with the curved boundary of the bowl greatly enhances the rate of sedimentation. Copyright 1999 Academic Press.

Characterizing the Severe Turbulence Environments Associated With Commercial Aviation Accidents. Part 1; 44 Case Study Synoptic Observational Analyses

NASA Technical Reports Server (NTRS)

Kaplan, Michael L.; Huffman, Allan W.; Lux, Kevin M.; Charney, Joseph J.; Riordan, Allan J.; Lin, Yuh-Lang; Proctor, Fred H. (Technical Monitor)

2002-01-01

A 44 case study analysis of the large-scale atmospheric structure associated with development of accident-producing aircraft turbulence is described. Categorization is a function of the accident location, altitude, time of year, time of day, and the turbulence category, which classifies disturbances. National Centers for Environmental Prediction Reanalyses data sets and satellite imagery are employed to diagnose synoptic scale predictor fields associated with the large-scale environment preceding severe turbulence. These analyses indicate a predominance of severe accident-producing turbulence within the entrance region of a jet stream at the synoptic scale. Typically, a flow curvature region is just upstream within the jet entrance region, convection is within 100 km of the accident, vertical motion is upward, absolute vorticity is low, vertical wind shear is increasing, and horizontal cold advection is substantial. The most consistent predictor is upstream flow curvature and nearby convection is the second most frequent predictor.

Vortex developments over steady and accelerated airfoils incorporating a trailing edge jet

NASA Technical Reports Server (NTRS)

Finaish, F.; Okong'o, N.; Frigerio, J.

1993-01-01

Computational and experimental studies are conducted to investigate the influence of a trailing edge jet on flow separation and subsequent vortex formation over steady and accelerated airfoils at high angles of attack. A computer code, employing the stream function-vorticity approach, is developed and utilized to conduct numerical experiments on the flow problem. To verify and economize such efforts, an experimental system is developed and incorporated into a subsonic wind tunnel where streamline and vortex flow visualization experiments are conducted. The study demonstrates the role of the trailing edge jet in controlling flow separation and subsequent vortex development for steady and accelerating flow at angles past the static stall angle of attack. The results suggest that the concept of the trailing edge jet may be utilized to control the characteristics of unsteady separated flows over lifting surfaces. This control possibility seems to be quite effective and could have a significant role in controlling unsteady separated flows.

Correnti atmosferiche su Giove

NASA Astrophysics Data System (ADS)

Adamoli, Gianluigi

2006-06-01

UAI observations are presented concerning the surveillance of Jupiter's atmospheric currents by means of digital images. General statistics are derived about the latitude and speed of individual spots and currents in the period 2000-04, compared with the Voyager wind profile. Attention is drawn to the wind shear present at distinct latitudes, namely on the South edge of the SEB, across the NTB and across the NEB. Especially interesting were the 2003 remnants of the disappearing NTB, which showed a motion intermediate between the NTC and the NTBs jet streams. Vorticity was derived in all cases.

Horizontal and Vertical Structure of Velocity, Potential Vorticity and Energy in the Gulf Stream.

DTIC Science & Technology

1985-02-01

before. Finally, the equation for heat conservation, using standard . - notation, is: T u + w 3 RHS (2-15) at ax ay + where the RHS may include source and...may be rewritten: a o f 0 2 ah 30i .. .iaT + -R2 -+ w2! = RHS . at goz az Under an assumption of negligible mixing (i.e., RHS is small), vertical...Hk( + v.) Kk - 2i + 2 2 --k (k + N - P available potential energy EKE eddy kinetic energy MKE - mean kinetic energy RHS - right hand side LHS -left

Streaming flow from ultrasound contrast agents by acoustic waves in a blood vessel model.

PubMed

Cho, Eunjin; Chung, Sang Kug; Rhee, Kyehan

2015-09-01

To elucidate the effects of streaming flow on ultrasound contrast agent (UCA)-assisted drug delivery, streaming velocity fields from sonicated UCA microbubbles were measured using particle image velocimetry (PIV) in a blood vessel model. At the beginning of ultrasound sonication, the UCA bubbles formed clusters and translated in the direction of the ultrasound field. Bubble cluster formation and translation were faster with 2.25MHz sonication, a frequency close to the resonance frequency of the UCA. Translation of bubble clusters induced streaming jet flow that impinged on the vessel wall, forming symmetric vortices. The maximum streaming velocity was about 60mm/s at 2.25MHz and decreased to 15mm/s at 1.0MHz for the same acoustic pressure amplitude. The effect of the ultrasound frequency on wall shear stress was more noticeable. Maximum wall shear stress decreased from 0.84 to 0.1Pa as the ultrasound frequency decreased from 2.25 to 1.0MHz. The maximum spatial gradient of the wall shear stress also decreased from 1.0 to 0.1Pa/mm. This study showed that streaming flow was induced by bubble cluster formation and translation and was stronger upon sonication by an acoustic wave with a frequency near the UCA resonance frequency. Therefore, the secondary radiant force, which is much stronger at the resonance frequency, should play an important role in UCA-assisted drug delivery. Copyright © 2015 Elsevier B.V. All rights reserved.

Low-to-moderate Reynolds number swirling flow in an annular channel with a rotating end wall.

PubMed

Davoust, Laurent; Achard, Jean-Luc; Drazek, Laurent

2015-02-01

This paper presents a new method for solving analytically the axisymmetric swirling flow generated in a finite annular channel from a rotating end wall, with no-slip boundary conditions along stationary side walls and a slip condition along the free surface opposite the rotating floor. In this case, the end-driven swirling flow can be described from the coupling between an azimuthal shear flow and a two-dimensional meridional flow driven by the centrifugal force along the rotating floor. A regular asymptotic expansion based on a small but finite Reynolds number is used to calculate centrifugation-induced first-order correction to the azimuthal Stokes flow obtained as the solution at leading order. For solving the first-order problem, the use of an integral boundary condition for the vorticity is found to be a convenient way to attribute boundary conditions in excess for the stream function to the vorticity. The annular geometry is characterized by both vertical and horizontal aspect ratios, whose respective influences on flow patterns are investigated. The vertical aspect ratio is found to involve nontrivial changes in flow patterns essentially due to the role of corner eddies located on the left and right sides of the rotating floor. The present analytical method can be ultimately extended to cylindrical geometries, irrespective of the surface opposite the rotating floor: a wall or a free surface. It can also serve as an analytical tool for monitoring confined rotating flows in applications related to surface viscosimetry or crystal growth from the melt.

Development of an integrated BEM approach for hot fluid structure interaction

NASA Technical Reports Server (NTRS)

Dargush, Gary F.; Banerjee, Prasanta K.; Honkala, Keith A.

1991-01-01

The development of a boundary element formulation for the study of hot fluid-structure interaction in earth-to-orbit engine hot section components is described. The initial primary thrust of the program to date was directed quite naturally toward the examination of fluid flow, since boundary element methods for fluids are at a much less developed state. This required the development of integral formulations for both the solid and fluid, and some preliminary infrastructural enhancements to a boundary element code to permit coupling of the fluid-structure problem. Boundary element formulations are implemented in two dimensions for both the solid and the fluid. The solid is modeled as an uncoupled thermoelastic medium under plane strain conditions, while several formulations are investigated for the fluid. For example, both vorticity and primitive variable approaches are implemented for viscous, incompressible flow, and a compressible version is developed. All of the above boundary element implementations are incorporated in a general purpose two-dimensional code. Thus, problems involving intricate geometry, multiple generic modeling regions, and arbitrary boundary conditions are all supported.

Turbofan forced mixer lobe flow modeling. 2: Three-dimensional inviscid mixer analysis (FLOMIX)

NASA Technical Reports Server (NTRS)

Barber, T.

1988-01-01

A three-dimensional potential analysis (FLOMIX) was formulated and applied to the inviscid flow over a turbofan foced mixer. The method uses a small disturbance formulation to analytically uncouple the circumferential flow from the radial and axial flow problem, thereby reducing the analysis to the solution of a series of axisymmetric problems. These equations are discretized using a flux volume formulation along a Cartesian grid. The method extends earlier applications of the Cartesian method to complex cambered geometries. The effects of power addition are also included within the potential formulation. Good agreement is obtained with an alternate small disturbance analysis for a high penetration symmetric mixer in a planar duct. In addition, calculations showing pressure distributions and induced secondary vorticity fields are presented for practical trubofan mixer configurations, and where possible, comparison was made with available experimental data. A detailed description of the required data input and coordinate definition is presented along with a sample data set for a practical forced mixer configuration. A brief description of the program structure and subroutines is also provided.

Trapping of Embolic Particles in a Vessel Phantom by Cavitation-Enhanced Acoustic Streaming

PubMed Central

Maxwell, Adam D.; Park, Simone; Vaughan, Benjamin L.; Cain, Charles A.; Grotberg, James B.; Xu, Zhen

2014-01-01

Cavitation clouds generated by short, high-amplitude, focused ultrasound pulses were previously observed to attract, trap, and erode thrombus fragments in a vessel phantom. This phenomenon may offer a noninvasive method to capture and eliminate embolic fragments flowing through the bloodstream during a cardiovascular intervention. In this article, the mechanism of embolus trapping was explored by particle image velocimetry (PIV). PIV was used to examine the fluid streaming patterns generated by ultrasound in a vessel phantom with and without crossflow of blood-mimicking fluid. Cavitation enhanced streaming, which generated fluid vortices adjacent to the focus. The focal streaming velocity, uf, was as high as 120 cm/s, while mean crossflow velocities, uc, were imposed up to 14 cm/s. When a solid particle 3-4 mm diameter was introduced into crossflow, it was trapped near the focus. Increasing uf promoted particle trapping while increasing uc promoted particle escape. The maximum crossflow Reynolds number at which particles could be trapped, Rec, was approximately linear with focal streaming number, Ref, i.e. Rec = 0.25Ref + 67.44 (R2=0.76) corresponding to dimensional velocities uc=0.084uf + 3.122 for 20 < uf < 120 cm/s. The fluidic pressure map was estimated from PIV and indicated a negative pressure gradient towards the focus, trapping the embolus near this location. PMID:25109407

2D instabilities of surface gravity waves on a linear shear current

NASA Astrophysics Data System (ADS)

Francius, Marc; Kharif, Christian

2016-04-01

Periodic 2D surface water waves propagating steadily on a rotational current have been studied by many authors (see [1] and references therein). Although the recent important theoretical developments have confirmed that periodic waves can exist over flows with arbitrary vorticity, their stability and their nonlinear evolution have not been much studied extensively so far. In fact, even in the rather simple case of uniform vorticity (linear shear), few papers have been published on the effect of a vertical shear current on the side-band instability of a uniform wave train over finite depth. In most of these studies [2-5], asymptotic expansions and multiple scales method have been used to obtain envelope evolution equations, which allow eventually to formulate a condition of (linear) instability to long modulational perturbations. It is noted here that this instability is often referred in the literature as the Benjamin-Feir or modulational instability. In the present study, we consider the linear stability of finite amplitude two-dimensional, periodic water waves propagating steadily on the free surface of a fluid with constant vorticity and finite depth. First, the steadily propagating surface waves are computed with steepness up to very close to the highest, using a Fourier series expansions and a collocation method, which constitutes a simple extension of Fenton's method [6] to the cases with a linear shear current. Then, the linear stability of these permanent waves to infinitesimal 2D perturbations is developed from the fully nonlinear equations in the framework of normal modes analysis. This linear stability analysis is an extension of [7] to the case of waves in the presence of a linear shear current and permits the determination of the dominant instability as a function of depth and vorticity for a given steepness. The numerical results are used to assess the accuracy of the vor-NLS equation derived in [5] for the characteristics of modulational instabilities due to resonant four-wave interactions, as well as to study the influence of vorticity and nonlinearity on the characteristics of linear instabilities due to resonant five-wave and six-wave interactions. Depending on the dimensionless depth, superharmonic instabilities due to five-wave interactions can become dominant with increasing positive vorticiy. Acknowledgments: This work was supported by the Direction Générale de l'Armement and funded by the ANR project n°. ANR-13-ASTR-0007. References [1] A. Constantin, Two-dimensionality of gravity water flows of constant non-zero vorticity beneath a surface wave train, Eur. J. Mech. B/Fluids, 2011, 30, 12-16. [2] R. S. Johnson, On the modulation of water waves on shear flows, Proc. Royal Soc. Lond. A., 1976, 347, 537-546. [3] M. Oikawa, K. Chow, D. J. Benney, The propagation of nonlinear wave packets in a shear flow with a free surface, Stud. Appl. Math., 1987, 76, 69-92. [4] A. I Baumstein, Modulation of gravity waves with shear in water, Stud. Appl. Math., 1998, 100, 365-90. [5] R. Thomas, C. Kharif, M. Manna, A nonlinear Schrödinger equation for water waves on finite depth with constant vorticity, Phys. Fluids, 2012, 24, 127102. [6] M. M Rienecker, J. D Fenton, A Fourier approximation method for steady water waves , J. Fluid Mech., 1981, 104, 119-137 [7] M. Francius, C. Kharif, Three-dimensional instabilities of periodic gravity waves in shallow water, J. Fluid Mech., 2006, 561, 417-437

From flying wheel to square flow: Dynamics of a flow driven by acoustic forcing

NASA Astrophysics Data System (ADS)

Cambonie, Tristan; Moudjed, Brahim; Botton, Valéry; Henry, Daniel; Ben Hadid, Hamda

2017-12-01

Acoustic streaming designates the ability to drive quasisteady flows by acoustic propagation in dissipative fluids and results from an acoustohydrodynamics coupling. It is a noninvasive way of putting a fluid into motion using the volumetric acoustic force and can be used for different applications such as mixing purposes. We present an experimental investigation of a kind of square flow driven by acoustic streaming, with the use of beam reflections, in a water tank. Time-resolved experiments using particle image velocimetry have been performed to investigate the velocity field in the reference plane of the experiments for six powers: 0.5, 1, 2, 4, 6, and 8 W. The evolution of the flow regime from almost steady to strongly unsteady states is characterized using different tools: the plot of time-averaged and instantaneous velocity fields, the calculation of presence density maps for vortex positions and for the maximal velocity and vorticity crest lines, and the use of spatiotemporal maps of the waving observed on the jets created by acoustic streaming. A transition is observed between two regimes at moderate and high acoustic forcing.

Piecewise Potential Vorticity Inversion for Intense Extratropical Cyclones

NASA Astrophysics Data System (ADS)

Seiler, C.; Zwiers, F. W.

2017-12-01

Global climate models (GCMs) tend to simulate too few intense extratropical cyclones (ETCs) in the Northern Hemisphere (NH) under historic climate conditions. This bias may arise from the interactions of multiple drivers, including surface temperature gradients, latent heating in the lower troposphere, and the upper-level jet stream. Previous attempts to quantify the importance of these drivers include idealized model experiments or statistical approaches. The first method however cannot easily be implemented for a multi-GCM ensemble, and the second approach does not disentangle the interactions among drivers, nor does it prove causality. An alternative method that overcomes these limitations is piecewise potential vorticity inversion (PPVI). PPVI derives the wind and geopotential height fields by inverting potential vorticity (PV) for discrete atmospheric levels. Despite being a powerful diagnostic tool, PPVI has primarily been used to study the dynamics of individual events only. This study presents the first PPVI climatology for the 5% most intense NH ETCs that occurred from 1980 to 2016. Conducting PPVI to 3273 ETC tracks identified in ERA-Interim reanalysis, we quantified the contributions from 3 atmospheric layers to ETC intensity. The respective layers are the surface (1000 hPa), a lower atmospheric level (700-850 hPa) and an upper atmospheric level (100-500 hPa) that are associated with the contributions from surface temperature gradients, latent heating, and the jet stream, respectively. Results show that contributions are dominated by the lower level (40%), followed by the upper level (20%) and the surface (17%), while the remaining 23% are associated with the background flow. Contributions from the surface and the lower level are stronger in the western ocean basins owed to the presence of the warm ocean currents, while contributions from the upper level are stronger in the eastern basins. Vertical cross sections of ETC-centered composites show an undulation of the dynamic tropopause and the formation of a PV tower with values exceeding 1 PV unit during maximum ETC intensity. The dominant contribution from the lower level underlines the importance of latent heating for intense ETCs. The ability of GCMs to reproduce this mechanism remains to be assessed.

On the Goertler instability in hypersonic flows: Sutherland law fluids and real gas effects

NASA Technical Reports Server (NTRS)

Fu, Yibin B.; Hall, Philip; Blackaby, Nicholas D.

1990-01-01

The Goertler vortex instability mechanism in a hypersonic boundary layer on a curved wall is investigated. The precise roles of the effects of boundary layer growth, wall cooling, and gas dissociation is clarified in the determination of stability properties. It is first assumed that the fluid is an ideal gas with viscosity given by Sutherland's law. It is shown that when the free stream Mach number M is large, the boundary layer divides into two sublayers: a wall layer of O(M sup 3/2) thickness over which the basic state temperature is O(M squared) and a temperature adjustment layer of O(1) thickness over which the basic state temperature decreases monotonically to its free stream value. Goertler vortices which have wavelengths comparable with the boundary layer thickness are referred to as wall modes. It is shown that their downstream evolution is governed by a set of parabolic partial differential equations and that they have the usual features of Goertler vortices in incompressible boundary layers. As the local wavenumber increases, the neutral Goertler number decreases and the center of vortex activity moves towards the temperature adjustment layer. Goertler vortices with wavenumbers of order one or larger must necessarily be trapped in the temperature adjustment layer and it is this mode which is most dangerous. For this mode, it was found that the leading order term in the Goertler number expansion is independent of the wavenumber and is due to the curvature of the basic state. This term is also the asymptotic limit of the neutral Goertler numbers of the wall mode. To determine the higher order corrections terms in the Goertler number expansion, two wall curvature cases are distinguished. Real gas effects were investigated by assuming that the fluid is an ideal dissociating gas. It was found that both gas dissociation and wall cooling are destabilizing for the mode trapped in the temperature adjustment layer, but for the wall mode trapped near the wall the effect of gas dissociation can be either destabilizing or stabilizing.

Quantifying vorticity in magnetic particle suspensions driven by symmetric and asymmetric multiaxial fields.

DOE PAGES

Martin, James E.; Solis, Kyle Jameson

2015-08-07

We recently reported two methods of inducing vigorous fluid vorticity in magnetic particle suspensions. The first method employs symmetry-breaking rational fields. These fields are comprised of two orthogonal ac components whose frequencies form a rational number and an orthogonal dc field that breaks the symmetry of the biaxial ac field to create the parity required to induce deterministic vorticity. The second method is based on rational triads, which are fields comprised of three orthogonal ac components whose frequency ratios are rational (e.g., 1 : 2 : 3). For each method a symmetry theory has been developed that enables the predictionmore » of the direction and sign of vorticity as functions of the field frequencies and phases. However, this theory has its limitations. It only applies to those particular phase angles that give rise to fields whose Lissajous plots, or principal 2-d projections thereof, have a high degree of symmetry. Nor can symmetry theory provide a measure of the magnitude of the torque density induced by the field. In this paper a functional of the multiaxial magnetic field is proposed that not only is consistent with all of the predictions of the symmetry theories, but also quantifies the torque density. This functional can be applied to fields whose Lissajous plots lack symmetry and can thus be used to predict a variety of effects and trends that cannot be predicted from the symmetry theories. These trends include the dependence of the magnitude of the torque density on the various frequency ratios, the unexpected reversal of flow with increasing dc field amplitude for certain symmetry-breaking fields, and the existence of off-axis vorticity for rational triads, such as 1 : 3 : 5, that do not have the symmetry required to analyze by symmetry theory. As a result, experimental data are given that show the degree to which this functional is successful in predicting observed trends.« less

Characteristics and controllability of vortices in ferromagnetics, ferroelectrics, and multiferroics

NASA Astrophysics Data System (ADS)

Zheng, Yue; Chen, W. J.

2017-08-01

Topological defects in condensed matter are attracting e significant attention due to their important role in phase transition and their fascinating characteristics. Among the various types of matter, ferroics which possess a switchable physical characteristic and form domain structure are ideal systems to form topological defects. In particular, a special class of topological defects—vortices—have been found to commonly exist in ferroics. They often manifest themselves as singular regions where domains merge in large systems, or stabilize as novel order states instead of forming domain structures in small enough systems. Understanding the characteristics and controllability of vortices in ferroics can provide us with deeper insight into the phase transition of condensed matter and also exciting opportunities in designing novel functional devices such as nano-memories, sensors, and transducers based on topological defects. In this review, we summarize the recent experimental and theoretical progress in ferroic vortices, with emphasis on those spin/dipole vortices formed in nanoscale ferromagnetics and ferroelectrics, and those structural domain vortices formed in multiferroic hexagonal manganites. We begin with an overview of this field. The fundamental concepts of ferroic vortices, followed by the theoretical simulation and experimental methods to explore ferroic vortices, are then introduced. The various characteristics of vortices (e.g. formation mechanisms, static/dynamic features, and electronic properties) and their controllability (e.g. by size, geometry, external thermal, electrical, magnetic, or mechanical fields) in ferromagnetics, ferroelectrics, and multiferroics are discussed in detail in individual sections. Finally, we conclude this review with an outlook on this rapidly developing field.

Least-squares finite element solutions for three-dimensional backward-facing step flow

NASA Technical Reports Server (NTRS)

Jiang, Bo-Nan; Hou, Lin-Jun; Lin, Tsung-Liang

1993-01-01

Comprehensive numerical solutions of the steady state incompressible viscous flow over a three-dimensional backward-facing step up to Re equals 800 are presented. The results are obtained by the least-squares finite element method (LSFEM) which is based on the velocity-pressure-vorticity formulation. The computed model is of the same size as that of Armaly's experiment. Three-dimensional phenomena are observed even at low Reynolds number. The calculated values of the primary reattachment length are in good agreement with experimental results.

Flow structure of vortex-wing interaction

NASA Astrophysics Data System (ADS)

McKenna, Christopher K.

Impingement of a streamwise-oriented vortex upon a fin, tail, blade or wing represents a fundamental class of flow-structure interaction that extends across a range of applications. This interaction can give rise to time-averaged loading, as well as unsteady loading known as buffeting. The loading is sensitive to parameters of the incident vortex as well as the location of vortex impingement on the downstream aerodynamic surface, generically designated as a wing. Particle image velocimetry is employed to determine patterns of velocity, vorticity, swirl ratio, and streamlines on successive cross-flow planes upstream of and along the wing, which lead to volume representations and thereby characterization of the interaction. At locations upstream of the leading edge of the wing, the evolution of the incident vortex is affected by the presence of the wing, and is highly dependent on the spanwise location of vortex impingement. Even at spanwise locations of impingement well outboard of the wing tip, a substantial influence on the structure of the incident vortex at locations significantly upstream of the leading edge of the wing was observed. For spanwise locations close to or intersecting the vortex core, the effects of upstream influence of the wing on the vortex are to: decrease the swirl ratio; increase the streamwise velocity deficit; decrease the streamwise vorticity; increase the azimuthal vorticity; increase the upwash; decrease the downwash; and increase the root-mean-square fluctuations of both streamwise velocity and vorticity. The interrelationship between these effects is addressed, including the rapid attenuation of axial vorticity in presence of an enhanced defect of axial velocity in the central region of the vortex. Moreover, when the incident vortex is aligned with, or inboard of, the tip of the wing, the swirl ratio decreases to values associated with instability of the vortex, giving rise to enhanced values of azimuthal vorticity relative to the streamwise (axial) vorticity, as well as relatively large root-mean-square values of streamwise velocity and vorticity. Along the chord of the wing, the vortex interaction gives rise to distinct modes, which may involve either enhancement or suppression of the vortex generated at the tip of the wing. These modes are classified and interpreted in conjunction with computed modes at the Air Force Research Laboratory. Occurrence of a given mode of interaction is predominantly determined by the dimensionless location of the incident vortex relative to the tip of the wing and is generally insensitive to the Reynolds number and dimensionless circulation of the incident vortex. The genesis of the basic modes of interaction is clarified using streamline topology with associated critical points. Whereas formation of an enhanced tip vortex involves a region of large upwash in conjunction with localized flow separation, complete suppression of the tip vortex is associated with a small-scale separation-attachment bubble bounded by downwash at the wing tip. Oscillation of the wing at an amplitude and velocity nearly two orders of magnitude smaller than the wing chord and free stream velocity respectively can give rise to distinctive patterns of upwash, downwash, and shed vorticity, which are dependent on the outboard displacement of the incident vortex relative to the wing tip. Moreover, these patterns are a strong function of the phase of the wing motion during its oscillation cycle. At a given value of phase, the wing oscillation induces upwash that is reinforced by the upwash of the incident vortex, giving a maximum value of net upwash. Conversely, when these two origins of upwash counteract, rather than reinforce, one another during the oscillation cycle, the net upwash has its minimum value. Analogous interpretations hold for regions of maximum and minimum net downwash located outboard of the regions of upwash. During the oscillation cycle of the wing, the magnitude and scale of the vorticity shed from the tip of the wing are directly correlated with the net upwash, which takes different forms related to the outboard displacement of the incident vortex. As the location of the incident vortex is displaced towards the wing tip, both the maximum upwash and the maximum vorticity of the tip vortex initially increase, then decrease. For the limiting case where the incident vortex impinges directly upon the tip of the wing, there is no tip vortex or induced region of upwash. Furthermore, at small values of vortex displacement from the wing tip, the position of the incident vortex varies significantly from its nominal position during the oscillation cycle. For all locations of the incident vortex, it is shown that, despite the small amplitude of the wing motion, the flow topology is fundamentally different at maximum positive and negative values of the wing velocity, that is, they are not symmetric.

The Flow Field on Hydrofoils with Leading Edge Protuberances

NASA Astrophysics Data System (ADS)

Custodio, Derrick; Henoch, Charles; Johari, Hamid

2009-11-01

The exceptional mobility of the humpback whale has been linked to the use of its unique pectoral flippers. Biologists speculate that the flippers leading edge protuberances are a form of passive flow control. Force measurements on 2D hydrofoils with spanwise uniform leading edge protuberances, resembling those seen on the humpback whale flipper, were taken in a water tunnel and have revealed performance modifications when compared to a baseline NACA 63(4)-021 hydrofoil model. Qualitative flow visualization techniques and Particle Image Velocimetry (PIV) flow field measurements on the modified hydrofoils have shown that streamwise vortices originating from the shoulders of the protuberances are the likely cause of performance changes. Varying levels of interaction among adjacent streamwise vortices have been observed as a function of angle of attack and chord location. The circulation of these vortices as a function of angle of attack and spatial location was measured and an analysis of the vortex interactions will be presented.

Flour mill stream blending affects sugar snap cookie and Japanese sponge cake quality and oxidative cross-linking potential of soft white wheat.

PubMed

Ramseyer, Daniel D; Bettge, Arthur D; Morris, Craig F

2011-01-01

The purpose of this research was to study the functional differences between straight grade (75% extraction rate) and patent (60% extraction rate) flour blends from 28 genetically pure soft white and club wheat grain lots, as evidenced by variation in sugar snap cookie and Japanese sponge cake quality. Functional differences were examined relative to arabinoxylan content, protein content, and oxidative cross-linking potential of flour slurries. Oxidative cross-linking measurements were obtained on flour slurries with a low shear Bostwick consistometer and considered endogenous oxidative cross-linking potential (water alone) or enhanced oxidative cross-linking potential (with added hydrogen peroxide-peroxidase). A 2-way ANOVA indicated that flour blend was the greater source of variation compared to grain lot for all response variables except water-extractable arabinoxylan content. Patent flours produced larger sugar snap cookies and Japanese sponge cakes, and contained significantly less total and water-unextractable arabinoxylans, protein, and ash than did straight grade flours. Patent flours produced more viscous slurries for endogenous and enhanced cross-linking measurements compared to the straight grade flours. The functional differences between patent and straight grade flours appear to be related to the particular mill streams that were utilized in the formulation of the 2 flour blends and compositional differences among those streams. Journal of Food Science © 2011 Institute of Food Technologists® No claim to original US government works.

Baseline LAW Glass Formulation Testing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kruger, Albert A.; Mooers, Cavin; Bazemore, Gina

2013-06-13

The major objective of the baseline glass formulation work was to develop and select glass formulations that are compliant with contractual and processing requirements for each of the LAW waste streams. Other objectives of the work included preparation and characterization of glasses with respect to the properties of interest, optimization of sulfate loading in the glasses, evaluation of ability to achieve waste loading limits, testing to demonstrate compatibility of glass melts with melter materials of construction, development of glass formulations to support ILAW qualification activities, and identification of glass formulation issues with respect to contract specifications and processing requirements.

Technical Status Report: Preliminary Glass Formulation Report for INEEL HAW. Revision 1

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peeler, D.; Reamer, I.; Vienna, J.

1998-03-01

Preliminary glass formulation work has been initiated at Pacific Northwest National Laboratory (PNNL) and the Savannah River Technology Center (SRTC) to support immobilization efforts of Idaho National Engineering and Environmental Laboratory (INEEL) high activity waste (HAW). Based on current pretreatment flow sheet assumptions, several glasses were fabricated and tested using an average `All Blend` waste stream composition which is dominated by the presence of ZrO{sub 2} (i.e., approximately 80 wt percent). The results of this initial work show that immobilization via vitrification is a viable option for a specific INEEL HAW waste stream. Waste loadings of at least 19 wtmore » percent can be achieved for the `All Blend` stream while maintaining targeted processing and product performance criteria. This waste loading translates into a ZrO{sub 2} content in excess of 15 wt percent in the final glass waste form. Frits developed for this work are based in the alkali borosilicate system. Although the results indicate that vitrification can be used to immobilize the `All Blend` waste stream, the glass compositions are by no means optimized.« less

A three-dimensional model of co-rotating streams in the solar wind. 2: Hydrodynamic streams

NASA Technical Reports Server (NTRS)

Pizzo, V. J.

1979-01-01

Theoretical aspects of corotating solar wind dynamics on a global scale are explored by means of numerical simulations executed with a nonlinear, inviscid, adiabatic, single-fluid, three-dimensional (3-D) hydrodynamic formulation. A simple, hypothetical 3-D stream structure is defined on a source surface located at 35 solar radius and carefully documents its evolution to 1 AU under the influence of solar rotation. By manipulating the structure of this prototype configuration at the source surface, it is possible to elucidate the factors most strongly affecting stream evolution: (1) the intrinsic correlations among density, temperature, and velocity existing near the source; (2) the amplitude of the stream; (3) the longitudinal breadth of the stream; (4) the latitudinal breadth of the stream; and (5) the heliographic latitude of the centroid of the stream.

Drag reduction and thrust generation by tangential surface motion in flow past a cylinder

NASA Astrophysics Data System (ADS)

Mao, Xuerui; Pearson, Emily

2018-03-01

Sensitivity of drag to tangential surface motion is calculated in flow past a circular cylinder in both two- and three-dimensional conditions at Reynolds number Re ≤ 1000 . The magnitude of the sensitivity maximises in the region slightly upstream of the separation points where the contour lines of spanwise vorticity are normal to the cylinder surface. A control to reduce drag can be obtained by (negatively) scaling the sensitivity. The high correlation of sensitivities of controlled and uncontrolled flow indicates that the scaled sensitivity is a good approximation of the nonlinear optimal control. It is validated through direct numerical simulations that the linear range of the steady control is much higher than the unsteady control, which synchronises the vortex shedding and induces lock-in effects. The steady control injects angular momentum into the separating boundary layer, stabilises the flow and increases the base pressure significantly. At Re=100 , when the maximum tangential motion reaches 50% of the free-stream velocity, the vortex shedding, boundary-layer separation and recirculation bubbles are eliminated and 32% of the drag is reduced. When the maximum tangential motion reaches 2.5 times of the free-stream velocity, thrust is generated and the power savings ratio, defined as the ratio of the reduced drag power to the control input power, reaches 19.6. The mechanism of drag reduction is attributed to the change of the radial gradient of spanwise vorticity (partial r \\hat{ζ } ) and the subsequent accelerated pressure recovery from the uncontrolled separation points to the rear stagnation point.

Thermally driven mass flows in the convection zone of the sun

NASA Technical Reports Server (NTRS)

Dijkhuis, G. C.

1973-01-01

A formulation of the fluid dynamics of convective regions is developed which leads to an analytical description of the solar rotation, the Evershed flow, and the supergranulation. The starting point of the present formulation is the mixing length picture of convective equilibrium, but the earlier point mass model for convective molecules is replaced here by a model with both inertia and intrinsic moment of inertia. This extension introduces three rotational degrees of freedom into the dynamics of individual convective molecules, which enter into the dynamical equations for a mixing length fluid in the form of a separate vector field which we term the spin field. It is shown that for convective molecules having a spherically symmetric mass distribution, the spin field is proportional to the local vorticity.

Application of kinematic vorticity techniques for mylonitized Rocks in Al Amar suture, eastern Arabian Shield, Saudi Arabia

NASA Astrophysics Data System (ADS)

Hamimi, Z.; Kassem, O. M. K.; El-Sabrouty, M. N.

2015-09-01

The rotation of rigid objects within a flowing viscous medium is a function of several factors including the degree of non-coaxiality. The relationship between the orientation of such objects and their aspect ratio can be used in vorticity analyses in a variety of geological settings. Method for estimation of vorticity analysis to quantitative of kinematic vorticity number (Wm) has been applied using rotated rigid objects, such as quartz and feldspar objects. The kinematic vorticity number determined for high temperature mylonitic Abt schist in Al Amar area, extreme eastern Arabian Shield, ranges from ˜0.8 to 0.9. Obtained results from vorticity and strain analyses indicate that deformation in the area deviated from simple shear. It is concluded that nappe stacking occurred early during an earlier thrusting event, probably by brittle imbrications. Ductile strain was superimposed on the nappe structure at high-pressure as revealed by a penetrative subhorizontal foliation that is developed subparallel to tectonic contacts versus the underlying and overlying nappes. Accumulation of ductile strain during underplating was not by simple shear but involved a component of vertical shortening, which caused the subhorizontal foliation in the Al Amar area. In most cases, this foliation was formed concurrently with thrust sheets imbrications, indicating that nappe stacking was associated with vertical shortening.

Hydrodynamics of Fishlike Swimming: Effects of swimming kinematics and Reynolds number

NASA Astrophysics Data System (ADS)

Gilmanov, Anvar; Posada, Nicolas; Sotiropoulos, Fotis

2003-11-01

We carry out a series of numerical simulations to investigate the effects of swimming kinematics and Reynolds number on the flow past a three-dimensional fishlike body undergoing undulatory motion. The simulated body shape is that of a real mackerel fish. The mackerel was frozen and subsequently sliced in several thin fillets whose dimensions were carefully measured and used to construct the fishlike body shape used in the simulations. The flow induced by the undulating body is simulated by solving the 3D, unsteady, incompressible Navier-Stokes equations with the second-order accurate, hybrid Cartesian/Immersed Boundary formulation of Gilmanov and Sotiropoulos (J. Comp. Physics, under review, 2003). We consider in-line swimming at constant speed and carry out simulations for various types of swimming kinematics, varying the tailbeat amplitude, frequency, and Reynolds number (300

Three-Dimensional Simulations of Marangoni-Benard Convection in Small Containers by the Least-Squares Finite Element Method

NASA Technical Reports Server (NTRS)

Yu, Sheng-Tao; Jiang, Bo-Nan; Wu, Jie; Duh, J. C.

1996-01-01

This paper reports a numerical study of the Marangoni-Benard (MB) convection in a planar fluid layer. The least-squares finite element method (LSFEM) is employed to solve the three-dimensional Stokes equations and the energy equation. First, the governing equations are reduced to be first-order by introducing variables such as vorticity and heat fluxes. The resultant first-order system is then cast into a div-curl-grad formulation, and its ellipticity and permissible boundary conditions are readily proved. This numerical approach provides an equal-order discretization for velocity, pressure, vorticity, temperature, and heat conduction fluxes, and therefore can provide high fidelity solutions for the complex flow physics of the MB convection. Numerical results reported include the critical Marangoni numbers (M(sub ac)) for the onset of the convection in containers with various aspect ratios, and the planforms of supercritical MB flows. The numerical solutions compared favorably with the experimental results reported by Koschmieder et al..

p-Euler equations and p-Navier-Stokes equations

NASA Astrophysics Data System (ADS)

Li, Lei; Liu, Jian-Guo

2018-04-01

We propose in this work new systems of equations which we call p-Euler equations and p-Navier-Stokes equations. p-Euler equations are derived as the Euler-Lagrange equations for the action represented by the Benamou-Brenier characterization of Wasserstein-p distances, with incompressibility constraint. p-Euler equations have similar structures with the usual Euler equations but the 'momentum' is the signed (p - 1)-th power of the velocity. In the 2D case, the p-Euler equations have streamfunction-vorticity formulation, where the vorticity is given by the p-Laplacian of the streamfunction. By adding diffusion presented by γ-Laplacian of the velocity, we obtain what we call p-Navier-Stokes equations. If γ = p, the a priori energy estimates for the velocity and momentum have dual symmetries. Using these energy estimates and a time-shift estimate, we show the global existence of weak solutions for the p-Navier-Stokes equations in Rd for γ = p and p ≥ d ≥ 2 through a compactness criterion.

Residence-time framework for modeling multicomponent reactive transport in stream hyporheic zones

NASA Astrophysics Data System (ADS)

Painter, S. L.; Coon, E. T.; Brooks, S. C.

2017-12-01

Process-based models for transport and transformation of nutrients and contaminants in streams require tractable representations of solute exchange between the stream channel and biogeochemically active hyporheic zones. Residence-time based formulations provide an alternative to detailed three-dimensional simulations and have had good success in representing hyporheic exchange of non-reacting solutes. We extend the residence-time formulation for hyporheic transport to accommodate general multicomponent reactive transport. To that end, the integro-differential form of previous residence time models is replaced by an equivalent formulation based on a one-dimensional advection dispersion equation along the channel coupled at each channel location to a one-dimensional transport model in Lagrangian travel-time form. With the channel discretized for numerical solution, the associated Lagrangian model becomes a subgrid model representing an ensemble of streamlines that are diverted into the hyporheic zone before returning to the channel. In contrast to the previous integro-differential forms of the residence-time based models, the hyporheic flowpaths have semi-explicit spatial representation (parameterized by travel time), thus allowing coupling to general biogeochemical models. The approach has been implemented as a stream-corridor subgrid model in the open-source integrated surface/subsurface modeling software ATS. We use bedform-driven flow coupled to a biogeochemical model with explicit microbial biomass dynamics as an example to show that the subgrid representation is able to represent redox zonation in sediments and resulting effects on metal biogeochemical dynamics in a tractable manner that can be scaled to reach scales.

Vorticity scaling and intermittency in drift-interchange plasma turbulence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dura, P. D.; Hnat, B.; Robinson, J.

2012-09-15

The effects of spatially varying magnetic field strength on the scaling properties of plasma turbulence, modelled by an extended form of Hasegawa-Wakatani model, are investigated. We study changes in the intermittency of the velocity, density, and vorticity fields, as functions of the magnetic field inhomogeneity C=-{partial_derivative} ln B/{partial_derivative}x. While the velocity fluctuations are always self-similar and their scaling is unaffected by the value of C, the intermittency levels in density and vorticity change with parameter C, reflecting morphological changes in the coherent structures due to the interchange mechanism. Given the centrality of vorticity in conditioning plasma transport, this result ismore » of interest in scaling the results of transport measurements and simulations in tokamak edge plasmas, where drift-interchange turbulence in the presence of a magnetic field gradient is likely to occur.« less

A new reversal mode in exchange coupled antiferromagnetic/ferromagnetic disks: distorted viscous vortex

DOE PAGES

Gilbert, Dustin A.; Ye, Li; Varea, Aïda; ...

2015-04-28

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir 20Mn 80/Fe 20Ni 80 microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet,more » which leads to unexpected asymmetries in the annihilation and nucleation fields. Lastly, these results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Phillips, Carolyn L.; Guo, Hanqi; Peterka, Tom

In type-II superconductors, the dynamics of magnetic flux vortices determine their transport properties. In the Ginzburg-Landau theory, vortices correspond to topological defects in the complex order parameter field. Earlier, in Phillips et al. [Phys. Rev. E 91, 023311 (2015)], we introduced a method for extracting vortices from the discretized complex order parameter field generated by a large-scale simulation of vortex matter. With this method, at a fixed time step, each vortex [simplistically, a one-dimensional (1D) curve in 3D space] can be represented as a connected graph extracted from the discretized field. Here we extend this method as a function ofmore » time as well. A vortex now corresponds to a 2D space-time sheet embedded in 4D space time that can be represented as a connected graph extracted from the discretized field over both space and time. Vortices that interact by merging or splitting correspond to disappearance and appearance of holes in the connected graph in the time direction. This method of tracking vortices, which makes no assumptions about the scale or behavior of the vortices, can track the vortices with a resolution as good as the discretization of the temporally evolving complex scalar field. Additionally, even details of the trajectory between time steps can be reconstructed from the connected graph. With this form of vortex tracking, the details of vortex dynamics in a model of a superconducting materials can be understood in greater detail than previously possible.« less

VORTICAL MOTIONS OF BARYONIC GAS IN THE COSMIC WEB: GROWTH HISTORY AND SCALING RELATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Weishan; Feng, Long-long

The vortical motions of the baryonic gas residing in large-scale structures are investigated by cosmological hydrodynamic simulations. Proceeding in the formation of the cosmic web, the vortical motions of baryonic matter are pumped up by baroclinity in two stages, i.e., the formation of sheets and filaments. The mean curl velocities are about <1, 1–10, 10–150, and 5–50 km s{sup −1} in voids, sheets, filaments, and knots at z = 0, respectively. The scaling of the vortical velocity of gas can be well described by the She–Leveque hierarchical turbulence model in the range of l < 0.65(1.50) h{sup −1} Mpc inmore » a simulation with a box of size 25(100) h{sup −1} Mpc. The fractal Hausdorff dimension of vortical motions, d, revealed by velocity structure functions, is ∼2.1–2.3(∼1.8–2.1). It is slightly larger than the fractal dimension of mass distribution in filaments, D{sup f} ∼ 1.9–2.2, and smaller than the fractal dimension of sheets, D{sup s} ∼ 2.4–2.7. The vortical kinetic energy of baryonic gas is mainly transported by filaments. Both scalings of mass distribution and vortical velocity increments show distinctive transitions at the turning scale of ∼0.65(1.50) h{sup −1} Mpc, which may be closely related to the characteristic radius of density filaments.« less

Development of the wake behind a circular cylinder impulsively started into rotatory and rectilinear motion: Intermediate rotation rates

NASA Technical Reports Server (NTRS)

Chen, Yen-Ming; Ou, Yuh-Roung; Pearlstein, Arne J.

1991-01-01

The temporal development of two-dimensional viscous incompressible flow generated by a circular cylinder started impulsively into steady rotatory and rectilinear motion is studied by integration of a velocity/vorticity formulation of the governing equations, using an explicit finite-difference/pseudo-spectral technique and an implementation of the Biot-Savart law. Results are presented for a Reynolds number of 200 (based on the cylinder diameter 2a and the magnitude U of the rectilinear velocity) for several values of the angular/rectilinear speed ratio alpha = (omega x a)/U (where omega is the angular speed) up to 3.25. Several aspects of the kinematics and dynamics of the flow not considered earlier are discussed. For higher values of alpha, the results indicate that for Re = 200, vortex shedding does indeed occur for alpha = 3.25. The shedding process is; however, very different from that which gives rise to the usual Karman vortex street for alpha = 0. In particular, consecutive vortices shed by the body can be shed from the same side and be of the same sense, in contrast to the nonrotating case, in which mirror-image vortices of opposite sense are shed alternately on opposite sides of the body. The implications of the results are discussed in relation to the possibility of suppressing vortex shedding by open or closed-loop control of the rotation rate.

Prediction of asymmetric vortical flows around slender bodies using Navier-Stokes equations

NASA Technical Reports Server (NTRS)

Liu, C. H.; Wong, Tin-Chee; Kandil, Osama A.

1992-01-01

Steady and unsteady asymmetric vortical flows around slender bodies at high angles of attack are solved using the unsteady, compressible, thin-layer Navier-Stokes equations. An implicit, upwind-biased, flux-difference splitting, finite-volume scheme is used for the numerical computations. For supersonic flows past point cones, the locally conical flow assumption have been used for efficient computational studies of this phenomenon. Asymmetric flows past a 5-deg semiapex-angle circular cone at different angles of attack, free-stream Mach numbers, and Reynolds numbers have been studied in responses to different sources of disturbances. The effects of grid fineness and computational domain size have also been investigated. Next, the responses of three-dimensional supersonic asymmetric flow around a 5-deg circular cone at different angles of attack and Reynolds numbers to short-duration sideslip disturbances are presented. The results show that flow asymmetry becomes stronger as the Reynolds number and angles of attack are increased. One of the cases of flow over a cone-cylinder configuration is validated fairly well by experimental data.

Mixing Enhancement in a Lobed Injector

NASA Technical Reports Server (NTRS)

Smith, L. L.; Majamaki, A. J.; Lam, I. T.; Delabroy, O.; Karagozian, A. R.; Marble, F. E.; Smith, O. I.

1997-01-01

An experimental investigation of the non-reactive mixing processes associated with a lobed fuel injector in a coflowing air stream is presented. The lobed fuel injector is a device which generates streamwise vorticity, producing high strain rates which can enhance the mixing of reactants while delaying ignition in a controlled manner. The lobed injectors examined in the present study consist of two corrugated plates between which a fuel surrogate, CO2, is injected into coflowing air. Acetone is seeded in the CO2 supply as a fuel marker. Comparison of two alternative lobed injector geometries is made with a straight fuel injector to determine net differences in mixing and strain fields due to streamwise vorticity generation. Planar laser-induced fluorescence (PLIF) of the seeded acetone yields two-dimensional images of the scalar concentration field at various downstream locations, from which local mixing and scalar dissipation rates are computed. It is found that the lobed injector geometry can enhance molecular mixing and create a highly strained flowfield, and that the strain rates generated by scalar energy dissipation can potentially delay ignition in a reacting flowfield.

The velocity and vorticity fields of the turbulent near wake of a circular cylinder

NASA Technical Reports Server (NTRS)

Wallace, James; Ong, Lawrence; Moin, Parviz

1995-01-01

The purpose of this research is to provide a detailed experimental database of velocity and vorticity statistics in the very near wake (x/d less than 10) of a circular cylinder at Reynolds number of 3900. This study has determined that estimations of the streamwise velocity component in flow fields with large nonzero cross-stream components are not accurate. Similarly, X-wire measurements of the u and v velocity components in flows containing large w are also subject to the errors due to binormal cooling. Using the look-up table (LUT) technique, and by calibrating the X-wire probe used here to include the range of expected angles of attack (+/- 40 deg), accurate X-wire measurements of instantaneous u and v velocity components in the very near wake region of a circular cylinder has been accomplished. The approximate two-dimensionality of the present flow field was verified with four-wire probe measurements, and to some extent the spanwise correlation measurements with the multisensor rake. Hence, binormal cooling errors in the present X-wire measurements are small.

PIV Study on Flow around Leading-Edge Slat of 30P30N Airfoil

NASA Astrophysics Data System (ADS)

Ando, Ryosuke; Onishi, Yusaku; Sakakibara, Jun

2017-11-01

We measured flow velocity distribution around leading-edge slat using PIV. Simultaneously, noise measurement using microphone was also performed. A leading-edge slat and main wing model having a chord length of 160 mm was placed in the tunnel with free stream velocity of about 26m/s and chord Reynolds number of 2.8 x 105. Angle of attack was changed from 4 degrees to 10 degrees at two degree intervals. In this experiment, we investigated the relationship between the unsteady flow condition and the noise. At 4 degrees in the angle of attack, vortices shedding from the slat cusp were moved to the downstream. At 6 degrees or more, flow velocity distributions show that vortices were reattached on the slat lower surface and the flow in the slat cove recirculated. In FFT analysis of noise measurement, at 6 degrees in the angle of attack, there were some peaks on low frequency area and dominant peak on high frequency area was found. At 8 degrees or more, there were also some peaks on low frequency area. But dominant peak on high frequency area disappeared.

Aircraft High-Lift Aerodynamic Analysis Using a Surface-Vorticity Solver

NASA Technical Reports Server (NTRS)

Olson, Erik D.; Albertson, Cindy W.

2016-01-01

This study extends an existing semi-empirical approach to high-lift analysis by examining its effectiveness for use with a three-dimensional aerodynamic analysis method. The aircraft high-lift geometry is modeled in Vehicle Sketch Pad (OpenVSP) using a newly-developed set of techniques for building a three-dimensional model of the high-lift geometry, and for controlling flap deflections using scripted parameter linking. Analysis of the low-speed aerodynamics is performed in FlightStream, a novel surface-vorticity solver that is expected to be substantially more robust and stable compared to pressure-based potential-flow solvers and less sensitive to surface perturbations. The calculated lift curve and drag polar are modified by an empirical lift-effectiveness factor that takes into account the effects of viscosity that are not captured in the potential-flow solution. Analysis results are validated against wind-tunnel data for The Energy-Efficient Transport AR12 low-speed wind-tunnel model, a 12-foot, full-span aircraft configuration with a supercritical wing, full-span slats, and part-span double-slotted flaps.

Visualization Measurement of Streaming Flows Associated with a Single-Acoustic Levitator

NASA Astrophysics Data System (ADS)

Hasegawa, Koji; Abe, Yutaka; Kaneko, Akiko; Yamamoto, Yuji; Aoki, Kazuyoshi

2009-08-01

The purpose of the study is to experimentally investigate flow fields generated by an acoustic levitator. This flow field has been observed using flow visualization, PIV method. In the absent of a drop, the flow field was strongly influenced by sound pressure level (SPL). In light of the interfacial stability of a levitated drop, SPL was set at 161-163 [dB] in our experiments. In the case of any levitated drop at a pressure node of a standing wave, the toroidal vortices were appeared around a drop and clearly observed the flow fields around the drop by PIV measurement. It is found that the toroidal vortices around a levitated drop were strongly affected by the viscosity of a drop. For more detailed research, experiments in the reduced gravity were conducted with aircraft parabolic flights. By comparison with experimental results in the earth and reduced gravity, it is also indicated that the configuration of the external flow field around a drop is most likely to be affected by a position of a drop as well.

Vortical structures and development of laminar flow over convergent-divergent riblets

NASA Astrophysics Data System (ADS)

Xu, Fang; Zhong, Shan; Zhang, Shanying

2018-05-01

In this work, the development of a laminar boundary layer over a rectangular convergent-divergent riblet section with a finite streamwise length is studied experimentally using dye visualization and particle image velocimetry in a water flume. The flow topology over this highly directional spanwise roughness is established from this study. It is shown that convergent-divergent riblets generate a spanwise flow above the riblets from the diverging line toward the adjacent converging line. This consequently leads to the formation of a weak recirculating secondary flow in cross-stream planes across the boundary layer that creates a downwash motion over the diverging line and an upwash motion over the converging line. It is found that the fluid inside the riblet valley follows a helicoidal path and it also interacts with the crossflow boundary layer hence playing a key role in determining the structure of the secondary flow across the boundary layer. The impact of riblet wavelength on vortical structures is also revealed for the first time. A larger riblet wavelength is seen to produce a stronger upwash/downwash and hence a more intense secondary flow as well as a stronger deceleration effect on the crossflow. Furthermore, the streamwise development of the flow over the riblet section can be divided into a developing stage followed by a developed stage. In the developing stage, the magnitude of induced streamwise velocity and vorticity over the converging line continues to increase, whereas in the developed stage the values of these parameters remain essentially unchanged.

Nonlinear interaction of near-planar TS waves and longitudinal vortices in boundary-layer transition

NASA Technical Reports Server (NTRS)

Smith, F. T.

1988-01-01

The nonlinear interactions that evolve between a planar or nearly planar Tollmien-Schlichting (TS) wave and the associated longitudinal vortices are considered theoretically for a boundary layer at high Reynolds number. The vortex flow is either induced by the TS nonlinear forcing or is input upstream, and similarly for the nonlinear wave development. Three major kinds of nonlinear spatial evolution, Types 1-3, are found. Each can start from secondary instability and then become nonlinear, Type 1 proving to be relatively benign but able to act as a pre-cursor to the Types 2, 3 which turn out to be very powerful nonlinear interactions. Type 2 involves faster stream-wise dependence and leads to a finite-distance blow-up in the amplitudes, which then triggers the full nonlinear 3-D triple-deck response, thus entirely altering the mean-flow profile locally. In contrast, Type 3 involves slower streamwise dependence but a faster spanwise response, with a small TS amplitude thereby causing an enhanced vortex effect which, again, is substantial enough to entirely alter the meanflow profile, on a more global scale. Streak-like formations in which there is localized concentration of streamwise vorticity and/or wave amplitude can appear, and certain of the nonlinear features also suggest by-pass processes for transition and significant changes in the flow structure downstream. The powerful nonlinear 3-D interactions 2, 3 are potentially very relevant to experimental findings in transition.

The Dynamics of Controlled Flow Separation within a Diverter Duct Diffuser

NASA Astrophysics Data System (ADS)

Peterson, C. J.; Vukasinovic, B.; Glezer, A.

2016-11-01

The evolution and receptivity to fluidic actuation of the flow separation within a rectangular, constant-width, diffuser that is branched off of a primary channel is investigated experimentally at speeds up to M = 0.4. The coupling between the diffuser's adverse pressure gradient and the internal separation that constricts nearly half of the flow passage through the duct is controlled using a spanwise array of fluidic actuators on the surface upstream of the diffuser's inlet plane. The dynamics of the separating surface vorticity layer in the absence and presence of actuation are investigated using high-speed particle image velocimetry combined with surface pressure measurements and total pressure distributions at the primary channel's exit plane. It is shown that the actuation significantly alters the incipient dynamics of the separating vorticity layer as the characteristic cross stream scales of the boundary layer upstream of separation and of the ensuing vorticity concentrations within the separated flow increase progressively with actuation level. It is argued that the dissipative (high frequency) actuation alters the balance between large- and small-scale motions near separation by intensifying the large-scale motions and limiting the small-scale dynamics. Controlling separation within the diffuser duct also has a profound effect on the global flow. In the presence of actuation, the mass flow rate in the primary duct increases 10% while the fraction of the diverted mass flow rate in the diffuser increases by more than 45% at 0.7% actuation mass fraction. Supported by the Boeing Company.

Heat transfer characteristics within an array of impinging jets. Effects of crossflow temperature relative to jet temperature

NASA Technical Reports Server (NTRS)

Florschuetz, L. W.; Su, C. C.

1985-01-01

Spanwise average heat fluxes, resolved in the streamwise direction to one stream-wise hole spacing were measured for two-dimensional arrays of circular air jets impinging on a heat transfer surface parallel to the jet orifice plate. The jet flow, after impingement, was constrained to exit in a single direction along the channel formed by the jet orifice plate and heat transfer surface. The crossflow originated from the jets following impingement and an initial crossflow was present that approached the array through an upstream extension of the channel. The regional average heat fluxes are considered as a function of parameters associated with corresponding individual spanwise rows within the array. A linear superposition model was employed to formulate appropriate governing parameters for the individual row domain. The effects of flow history upstream of an individual row domain are also considered. The results are formulated in terms of individual spanwise row parameters. A corresponding set of streamwise resolved heat transfer characteristics formulated in terms of flow and geometric parameters characterizing the overall arrays is described.

Anomalous current from the covariant Wigner function

NASA Astrophysics Data System (ADS)

Prokhorov, George; Teryaev, Oleg

2018-04-01

We consider accelerated and rotating media of weakly interacting fermions in local thermodynamic equilibrium on the basis of kinetic approach. Kinetic properties of such media can be described by covariant Wigner function incorporating the relativistic distribution functions of particles with spin. We obtain the formulae for axial current by summation of the terms of all orders of thermal vorticity tensor, chemical potential, both for massive and massless particles. In the massless limit all the terms of fourth and higher orders of vorticity and third order of chemical potential and temperature equal zero. It is shown, that axial current gets a topological component along the 4-acceleration vector. The similarity between different approaches to baryon polarization is established.

Simulating Streamflow and Dissolved Organic Matter Export from small Forested Watersheds

NASA Astrophysics Data System (ADS)

Xu, N.; Wilson, H.; Saiers, J. E.

2010-12-01

Coupling the rainfall-runoff process and solute transport in catchment models is important for understanding the dynamics of water-quality-relevant constituents in a watershed. To simulate the hydrologic and biogeochemical processes in a parametrically parsimonious way remains challenging. The purpose of this study is to quantify the export of water and dissolved organic matter (DOM) from a forested catchment by developing and testing a coupled model for rainfall-runoff and soil-water flushing of DOM. Natural DOM plays an important role in terrestrial and aquatic systems by affecting nutrient cycling, contaminant mobility and toxicity, and drinking water quality. Stream-water discharge and DOM concentrations were measured in a first-order stream in Harvard Forest, Massachusetts. These measurements show that stream water DOM concentrations are greatest during hydrologic events induced by rainfall or snowmelt and decline to low, steady levels during periods of baseflow. Comparison of the stream-discharge data to calculations of a simple rainfall-runoff model reveals a hysteretic relationship between stream-flow rates and the storage of water within the catchment. A modified version of the rainfall-runoff model that accounts for hysteresis in the storage-discharge relationship in a parametrically simple way is capable of describing much, but not all, of the variation in the time-series data on stream discharge. Our ongoing research is aimed at linking the new rainfall-runoff formulation with coupled equations that predict soil-flushing and stream-water concentrations of DOM as functions of the temporal change in catchment water storage. This model will provide a predictive tool for examining how changes in climatic variables would affect the runoff generation and DOM fluxes from terrestrial landscape.

Notes on initial disturbance fields for the transition problem

NASA Technical Reports Server (NTRS)

Bushnell, Dennis

1990-01-01

Initial disturbance fields are categorized and discussed with respect to modes of disturbance and the variants which occur for each test technique. Attention is focused on four fluctuation modes included under the category of stream turbulence: vorticity fluctuations, entropy disturbances or temperature spottiness, noise, and concentration fluctuations. The mechanisms by which particulate matter can affect transition are discussed, along with electrostatic discharges with frequencies in the range from 200 KHz to 30 MHz which is within the critical disturbance-frequency range for many high-speed flows. Acoustic radiation from own vehicle, roughness, and Brownian motion are also covered.

Guide Vanes for Deflecting Fluid Currents with Small Loss of Energy

NASA Technical Reports Server (NTRS)

Krober, G

1933-01-01

The transverse momentum of the deflected air stream to be absorbed is divided between the intermediate and outside walls, so that the pressure increase on each wall is much smaller and the danger of separation is diminished. The formation of secondary vortices is also diminished. By taking as the basis profiles with high c(sub a), such as have proved practically favorable, it is not possible to find a satisfactory form of grid simply on the assumption that the flow is potential. The requirements called for the most uniform possible velocity distribution behind the bend and the smallest possible losses.

A design method for entrance sections of transonic wind tunnels with rectangular cross sections

NASA Technical Reports Server (NTRS)

Lionel, L.; Mcdevitt, J. B.

1975-01-01

A mathematical technique developed to design entrance sections for transonic or high-speed subsonic wind tunnels with rectangular cross sections is discribed. The transition from a circular cross-section setting chamber to a rectangular test section is accomplished smoothly so as not to introduce secondary flows (vortices or boundary-layer separation) into a uniform test stream. The results of static-pressure measurements in the transition region and of static and total-pressure surveys in the test section of a pilot model for a new facility at the Ames Research Center are presented.

Soap film flows: Statistics of two-dimensional turbulence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vorobieff, P.; Rivera, M.; Ecke, R.E.

1999-08-01

Soap film flows provide a very convenient laboratory model for studies of two-dimensional (2-D) hydrodynamics including turbulence. For a gravity-driven soap film channel with a grid of equally spaced cylinders inserted in the flow, we have measured the simultaneous velocity and thickness fields in the irregular flow downstream from the cylinders. The velocity field is determined by a modified digital particle image velocimetry method and the thickness from the light scattered by the particles in the film. From these measurements, we compute the decay of mean energy, enstrophy, and thickness fluctuations with downstream distance, and the structure functions of velocity,more » vorticity, thickness fluctuation, and vorticity flux. From these quantities we determine the microscale Reynolds number of the flow R{sub {lambda}}{approx}100 and the integral and dissipation scales of 2D turbulence. We also obtain quantitative measures of the degree to which our flow can be considered incompressible and isotropic as a function of downstream distance. We find coarsening of characteristic spatial scales, qualitative correspondence of the decay of energy and enstrophy with the Batchelor model, scaling of energy in {ital k} space consistent with the k{sup {minus}3} spectrum of the Kraichnan{endash}Batchelor enstrophy-scaling picture, and power-law scalings of the structure functions of velocity, vorticity, vorticity flux, and thickness. These results are compared with models of 2-D turbulence and with numerical simulations. {copyright} {ital 1999 American Institute of Physics.}« less

Final Report. Baseline LAW Glass Formulation Testing, VSL-03R3460-1, Rev. 0

DOE Office of Scientific and Technical Information (OSTI.GOV)

Muller, Isabelle S.; Pegg, Ian L.; Gan, Hao

2015-06-18

The major objective of the baseline glass formulation work was to develop and select glass formulations that are compliant with contractual and processing requirements for each of the LAW waste streams. Other objectives of the work included preparation and characterization of glasses with respect to the properties of interest, optimization of sulfate loading in the glasses, evaluation of ability to achieve waste loading limits, testing to demonstrate compatibility of glass melts with melter materials of construction, development of glass formulations to support ILAW qualification activities, and identification of glass formulation issues with respect to contract specifications and processing requirements.

The role of flow field structure in determining the aerodynamic response of a delta wing

NASA Astrophysics Data System (ADS)

Addington, Gregory Alan

Delta wings have long been known to exhibit nonlinear aerodynamic responses as a result of the presence of helical leading-edge vortices. This nonlinearity, found under both steady-state and unsteady conditions, is particularly profound in the presence of vortex burst. Modeling such aerodynamic responses with the Nonlinear Indicial Response (NIR) methodology provides a means of simulating these nonlinearities through its inclusion of motion history in addition to superposition. The NIR model also includes provisions for a finite number of discrete locations where the aerodynamic response is discontinuous with response to a state variable. These critical states also separate regions of states where the unsteady aerodynamic responses are potentially of highly-disparate characters. Although these critical states have been found in the past, their relationship with flow field bifurcation is uncertain. The purpose of this dissertation is to explore the relationship between nonlinear aerodynamic responses, critical states and flow field bifurcations from an experimental approach. This task has been accomplished by comparing a comprehensive database of skin-friction line topologies with static and unsteady aerodynamic responses. These data were collected using a 65sp° delta wing which rolled about an inclined longitudinal body axis. In this study, compelling, but not conclusive, evidence was found to suggest that a bifurcation in the skin-friction line topology was a necessary condition for the presence of a critical state. Although the presence of critical states was well predicted through careful observation and analysis of highly-resolved static loading data alone, their precise placement as a function of the independent variable was aided through the consideration of the locations of skin-friction line bifurcations. Furthermore, these static data were found to contain indications of the basic lagged or unlagged behavior of the unsteady aerodynamic response. This indication was found by comparing the relative rate of change seen in the estimated vortical- and potential-rolling-moment components. Through the review of these data in light of current theories on the mechanisms of leading-edge vortex breakdown, the formulation of a hypothesis regarding the relationship between both the static and unsteady aerodynamic response and vorticity dynamics was possible.

Understanding Hurricane Movement from a Potential Vorticity Perspective: a Numerical Model and AN Observational Study.

NASA Astrophysics Data System (ADS)

Wu, Chun-Chieh

In the first part of this thesis, we attempt to isolate the effect of background vertical shear. The hurricane is represented in a two-layer quasigeostrophic model as a point source of mass and zero potential vorticity air in the upper layer, collocated with a point cyclone in the lower layer. The model results show that Northern Hemisphere tropical cyclones should have a component of drift relative to the mean flow in a direction to the left of the background vertical shear. The effect of weak shear is found to be at least as strong as the beta effect, and the effect is maximized by a certain optimal ambient shear. The behavior of the model is sensitive to the thickness ratio of the two layers and is less sensitive to the ratio of the vortices' horizontal scale to the radius of deformation. Storms with stronger negative potential vorticity anomalies tend to exhibit more vortex drift. The validity of balance dynamics in the tropics also allows us to explore the dynamics of hurricanes using the potential vorticity (PV) framework. In the second part of this thesis, three observational case studies (Hurricane Bob and Tropical Storm Ana of 1991, and Hurricane Andrew of 1992) have been performed to demonstrate the use of PV diagnostics of hurricane movement from the twice-daily National Meteorological Center Northen Hemisphere final analyses gridded datasets. Using the seasonal climatology as the mean reference state, piecewise potential vorticity inversions are performed under the nonlinear balance condition. By examining the balanced flows at the central position of the hurricane, one can identify which PV perturbation has the most influence on hurricane movement. We also define the hurricane advection flow as the balanced flow (in the center of the storm) associated with the whole PV in the troposphere, except for the PV anomaly of the hurricane itself. The results from the observational study show that such a steering wind is a very good approximation to the real storm motion. This steering flow derived from the PV perspective is much more consistent and dynamically meaningful than the traditional steering stream, which is generally taken as the tropospheric annular mean flow. The results also show that hurricane movement is dominated by the balanced flows associated with the mean PV and perturbation PV in both the lower and upper troposphere. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.).

Influence of backflow on skin friction in turbulent pipe flow

NASA Astrophysics Data System (ADS)

Jalalabadi, Razieh; Sung, Hyung Jin

2018-06-01

A direct numerical simulation of a turbulent pipe flow (Reτ = 544) is used to investigate the influence of the backflow on the vortical structures that contribute to the local skin friction. The backflow is a rare event with a probability density function (PDF) of less than 10-3. The backflow is found to extend up to y+ ≈ 4 and is induced by the presence of a vortex in the buffer layer. The flow statistics are conditionally sampled under the condition of a negative streamwise velocity (u < 0) at y+ = 3. The conditionally averaged u <0 reaches its maximum at y+ ≈ 27. The intensified conditionally averaged velocity fluctuations contribute to vertical and spanwise momentum transport around the backflow. The ensemble averaged + and + reveal layered structures in the Q2 and Q4 events. A strong Q4 event appears above the backflow, flanked by two regions of Q2. The strong downwash of the flow along with the spanwise vortex induces the backflow. The upwash at upstream and downstream of the backflow enhances the movement of the low-speed flow in the streamwise and spanwise directions. The velocity-vorticity correlation reveals that the main contributions to Cf are the vorticity advection and vorticity stretching. The main contribution to the conditionally averaged Cf is the wall-normal gradient of the mean spanwise vorticity at the wall. The spanwise vorticity is positive above the backflow flanked by two regions of negative spanwise vorticity. The conditional PDF of the backflow under negative ul+ at y+ = 100 is more frequent than that under positive ul+.

Semidiscrete Galerkin modelling of compressible viscous flow past a circular cone at incidence. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Meade, Andrew James, Jr.

1989-01-01

A numerical study of the laminar and compressible boundary layer, about a circular cone in a supersonic free stream, is presented. It is thought that if accurate and efficient numerical schemes can be produced to solve the boundary layer equations, they can be joined to numerical codes that solve the inviscid outer flow. The combination of these numerical codes is competitive with the accurate, but computationally expensive, Navier-Stokes schemes. The primary goal is to develop a finite element method for the calculation of 3-D compressible laminar boundary layer about a yawed cone. The proposed method can, in principle, be extended to apply to the 3-D boundary layer of pointed bodies of arbitrary cross section. The 3-D boundary layer equations governing supersonic free stream flow about a cone are examined. The 3-D partial differential equations are reduced to 2-D integral equations by applying the Howarth, Mangler, Crocco transformations, a linear relation between viscosity, and a Blasius-type of similarity variable. This is equivalent to a Dorodnitsyn-type formulation. The reduced equations are independent of density and curvature effects, and resemble the weak form of the 2-D incompressible boundary layer equations in Cartesian coordinates. In addition the coordinate normal to the wall has been stretched, which reduces the gradients across the layer and provides high resolution near the surface. Utilizing the parabolic nature of the boundary layer equations, a finite element method is applied to the Dorodnitsyn formulation. The formulation is presented in a Petrov-Galerkin finite element form and discretized across the layer using linear interpolation functions. The finite element discretization yields a system of ordinary differential equations in the circumferential direction. The circumferential derivatives are solved by an implicit and noniterative finite difference marching scheme. Solutions are presented for a 15 deg half angle cone at angles of attack of 5 and 10 deg. The numerical solutions assume a laminar boundary layer with free stream Mach number of 7. Results include circumferential distribution of skin friction and surface heat transfer, and cross flow velocity distributions across the layer.

Nonreciprocal Transverse Photonic Spin and Magnetization-Induced Electromagnetic Spin-Orbit Coupling

PubMed Central

Levy, Miguel; Karki, Dolendra

2017-01-01

We present a formulation of electromagnetic spin-orbit coupling in magneto-optic media, and propose an alternative source of spin-orbit coupling to non-paraxial optics vortices. Our treatment puts forth a formulation of nonreciprocal transverse-spin angular-momentum-density shifts for evanescent waves in magneto-optic waveguide media. It shows that magnetization-induced electromagnetic spin-orbit coupling is possible, and that it leads to unequal spin to orbital angular momentum conversion in magneto-optic media evanescent waves in opposite propagation-directions. Generation of free-space helicoidal beams based on this conversion is shown to be spin-helicity- and magnetization-dependent. We show that transverse-spin to orbital angular momentum coupling into magneto-optic waveguide media engenders spin-helicity-dependent unidirectional propagation. This unidirectional effect produces different orbital angular momenta in opposite directions upon excitation-spin-helicity reversals. PMID:28059120

Motion of a cylinder adjacent to a free-surface: flow patterns and loading

NASA Astrophysics Data System (ADS)

Zhu, Q.; Lin, J.-C.; Unal, M. F.; Rockwell, D.

The flow structure and loading due to combined translatory and sinusoidal motion of a cylinder adjacent to a free-surface are characterized using a cinema technique of high-image-density particle image velocimetry and simultaneous force measurements. The instantaneous patterns of vorticity and streamline topology are interpreted as a function of degree of submergence beneath the free-surface. The relative magnitudes of the peak vorticity and the circulation of vortices formed from the upper and lower surfaces of the cylinder, as well as vortex formation from the free-surface, are remarkably affected by the nominal submergence. The corresponding streamline topology, interpreted in terms of foci, saddle points, and multiple separation and reattachment points also exhibit substantial changes with submergence. All of these features affect the instantaneous loading of the cylinder. Calculation of instantaneous moments of vorticity and the incremental changes in these moments during the cylinder motion allow identification of those vortices that contribute most substantially to the instantaneous lift and drag. Furthermore, the calculated moments are in general accord with the time integrals of the measured lift and drag acting on the cylinder for sufficiently large submergence.

A new diagnostic of stratospheric polar vortices

NASA Astrophysics Data System (ADS)

Gimeno, Luis; de La Torre, Laura; Nieto, Raquel; Gallego, David; Ribera, Pedro; García-Herrera, Ricardo

2007-11-01

We studied the main climatological features of the Arctic and Antarctic stratospheric vortices, using a new approach based on defining the vortex edge as the 50 hPa geostrophic streamline of maximum average velocity at each hemisphere. Given the use of NCAR-NCEP reanalysis data, it was thought advisable to limit the study to the periods 1958 2004 for the Northern Hemisphere (NH) and 1979 2004 for the Southern Hemisphere (SH). After describing the method and testing sample results with those from other approaches, we analysed the climatological means and trends of the four most distinctive characteristics of the vortices: average latitude, strength, area, and temperature. In general terms, our results confirm most of what is already known about the stratospheric vortices from previous studies that used different data and approaches. In addition, the new methodology provides some interesting new quantifications of the dominant wavenumber and its interannual variability, as well as the principal variability modes through an empirical orthogonal function analysis that was performed directly over the vortex trajectories. The main drawbacks of the methodology, such as noticeable problems characterising highly disturbed stratospheric structures as multiple or off-pole vortices, are also identified.

Streamwater Contamination After Aerial Application of a Pelletized Herbicide

Treesearch

James H. Miller; A.C. Bace

1980-01-01

Concentrations of hexazinone and its metabolites were monitored in a Piedmont stream for 8 months after aerial application of a pelletized formulation (Velpar® Gridball™). Downstream water-users and fish are probably safe from toxic exposure if treatment areas have only small streams (less than 50 cm average channel width) and if labeled rates are not exceeded....

Compressible flows with periodic vortical disturbances around lifting airfoils. Ph.D. Thesis - Notre Dame Univ.

NASA Technical Reports Server (NTRS)

Scott, James R.

1991-01-01

A numerical method is developed for solving periodic, three-dimensional, vortical flows around lifting airfoils in subsonic flow. The first-order method that is presented fully accounts for the distortion effects of the nonuniform mean flow on the convected upstream vortical disturbances. The unsteady velocity is split into a vortical component which is a known function of the upstream flow conditions and the Lagrangian coordinates of the mean flow, and an irrotational field whose potential satisfies a nonconstant-coefficient, inhomogeneous, convective wave equation. Using an elliptic coordinate transformation, the unsteady boundary value problem is solved in the frequency domain on grids which are determined as a function of the Mach number and reduced frequency. The numerical scheme is validated through extensive comparisons with known solutions to unsteady vortical flow problems. In general, it is seen that the agreement between the numerical and analytical results is very good for reduced frequencies ranging from 0 to 4, and for Mach numbers ranging from .1 to .8. Numerical results are also presented for a wide variety of flow configurations for the purpose of determining the effects of airfoil thickness, angle of attack, camber, and Mach number on the unsteady lift and moment of airfoils subjected to periodic vortical gusts. It is seen that each of these parameters can have a significant effect on the unsteady airfoil response to the incident disturbances, and that the effect depends strongly upon the reduced frequency and the dimensionality of the gust. For a one-dimensional (transverse) or two-dimensional (transverse and longitudinal) gust, the results indicate that airfoil thickness increases the unsteady lift and moment at the low reduced frequencies but decreases it at the high reduced frequencies. The results show that an increase in airfoil Mach number leads to a significant increase in the unsteady lift and moment for the low reduced frequencies, but a significant decrease for the high reduced frequencies.

Inlet Guide Vane Wakes Including Rotor Effects

NASA Astrophysics Data System (ADS)

Johnston, R. T.; Fleeter, S.

2001-02-01

Fundamental experiments are described directed at the investigation of forcing functions generated by an inlet guide vane (IGV) row, including interactions with the downstream rotor, for application to turbomachine forced response design systems. The experiments are performed in a high-speed research fan facility comprised of an IGV row upstream of a rotor. IGV-rotor axial spacing is variable, with the IGV row able to be indexed circumferentially, thereby allowing measurements to be made across several IGV wakes. With an IGV relative Mach number of 0.29, measurements include the IGV wake pressure and velocity fields for three IGV-rotor axial spacings. The decay characteristics of the IGV wakes are compared to the Majjigi and Gliebe empirical correlations. After Fourier decomposition, a vortical-potential gust splitting analysis is implemented to determine the vortical and potential harmonic wake gust forcing functions both upstream and downstream of the rotor. Higher harmonics of the vortical gust component of the IGV wakes are found to decay at a uniform rate due to viscous diffusion.

Vortical features for myocardial rotation assessment in hypertrophic cardiomyopathy using cardiac tagged magnetic resonance.

PubMed

Sanz-Estébanez, Santiago; Cordero-Grande, Lucilio; Sevilla, Teresa; Revilla-Orodea, Ana; de Luis-García, Rodrigo; Martín-Fernández, Marcos; Alberola-López, Carlos

2018-07-01

Left ventricular rotational motion is a feature of normal and diseased cardiac function. However, classical torsion and twist measures rely on the definition of a rotational axis which may not exist. This paper reviews global and local rotation descriptors of myocardial motion and introduces new curl-based (vortical) features built from tensorial magnitudes, intended to provide better comprehension about fibrotic tissue characteristics mechanical properties. Fifty-six cardiomyopathy patients and twenty-two healthy volunteers have been studied using tagged magnetic resonance by means of harmonic phase analysis. Rotation descriptors are built, with no assumption about a regular geometrical model, from different approaches. The extracted vortical features have been tested by means of a sequential cardiomyopathy classification procedure; they have proven useful for the regional characterization of the left ventricular function by showing great separability not only between pathologic and healthy patients but also, and specifically, between heterogeneous phenotypes within cardiomyopathies. Copyright © 2018 Elsevier B.V. All rights reserved.

A least-squares finite element method for incompressible Navier-Stokes problems

NASA Technical Reports Server (NTRS)

Jiang, Bo-Nan

1992-01-01

A least-squares finite element method, based on the velocity-pressure-vorticity formulation, is developed for solving steady incompressible Navier-Stokes problems. This method leads to a minimization problem rather than to a saddle-point problem by the classic mixed method and can thus accommodate equal-order interpolations. This method has no parameter to tune. The associated algebraic system is symmetric, and positive definite. Numerical results for the cavity flow at Reynolds number up to 10,000 and the backward-facing step flow at Reynolds number up to 900 are presented.

The Turbulent Structure of the Atmosphere: Vorticity, Winds and Temperature Emerge From Molecular Motion

NASA Astrophysics Data System (ADS)

Tuck, A. F.; Hovde, S. J.; Lovejoy, S.; Schertzer, D.

2007-12-01

Application of generalized scale invariance to horizontal airborne observations of winds, temperature, ozone and humidity reveals the atmosphere as a random, non-Gaussian Levy process, having mean scaling exponents H (conservation), C1 (intermittency) and alpha (Levy) of 0.56, 0.05 and 1.6 respectively in the cases of winds and temperature. A correlation between the intermittency of temperature and the ozone photodissociation rate in the Arctic lower stratosphere is interpreted in terms of the ring currents of non-equilibrium statistical mechanics in which vortices, fluid dynamical behavior, emerge from thermalized populations of Maxwellian molecules subjected to an anisotropy in the form of a flux. The emergence of jet streams and the definition of atmospheric temperature are examined in the light of these results. The vertical scaling of wind, temperature and humidity is examined through the depth of the troposphere using data observed by GPS dropsondes from the NOAA Gulfstream 4 aircraft over the eastern Pacific Ocean in boreal winter. The results exclude isotropic turbulence in the atmosphere, and reveal the structure of static, moist static and dynamic (Richardson number) stabilities to be sparse fractal sets. Each stable layer contains a set of smaller scale unstable sublayers, each of which in turn contains a set of stable sub-sublayers and so on. The moist static stability scales differently to the dry static stability in the lower troposphere. As with the 'horizontal' data, the 'vertical' data reveal a correlation between H for horizontal wind and measures of jet stream strength. It is pointed out that these results provide potentially a new way of testing numerical models of the atmosphere.

Vortex Chain in a Resonantly Pumped Polariton Superfluid

PubMed Central

Boulier, T.; Terças, H.; Solnyshkov, D. D.; Glorieux, Q.; Giacobino, E.; Malpuech, G.; Bramati, A.

2015-01-01

Exciton-polaritons are light-matter mixed states interacting via their exciton fraction. They can be excited, manipulated, and detected using all the versatile techniques of modern optics. An exciton-polariton gas is therefore a unique platform to study out-of-equilibrium interacting quantum fluids. In this work, we report the formation of a ring-shaped array of same sign vortices after injection of angular momentum in a polariton superfluid. The angular momentum is injected by a ℓ = 8 Laguerre-Gauss beam. In the linear regime, a spiral interference pattern containing phase defects is visible. In the nonlinear (superfluid) regime, the interference disappears and eight vortices appear, minimizing the energy while conserving the quantized angular momentum. The radial position of the vortices evolves in the region between the two pumps as a function of the density. Hydrodynamic instabilities resulting in the spontaneous nucleation of vortex-antivortex pairs when the system size is sufficiently large confirm that the vortices are not constrained by interference when nonlinearities dominate the system. PMID:25784592

The shallow water equation and the vorticity equation for a change in height of the topography.

PubMed

Da, ChaoJiu; Shen, BingLu; Yan, PengCheng; Ma, DeShan; Song, Jian

2017-01-01

We consider the shallow water equation and the vorticity equations for a variable height of topography. On the assumptions that the atmosphere is incompressible and a constant density, we simplify the coupled dynamic equations. The change in topographic height is handled as the sum of the inherent and changing topography using the perturbation method, together with appropriate boundary conditions of the atmosphere, to obtain the relationship between the relative height of the flow, the inherent topography and the changing topography. We generalize the conservation of the function of relative position, and quantify the relationship between the height of the topography and the relative position of a fluid element. If the height of the topography increases (decreases), the relative position of a fluid element descends (ascends). On this basis, we also study the relationship between the vorticity and the topography to find the vorticity decreasing (increasing) for an increasing (decreasing) height of the topography.

The shallow water equation and the vorticity equation for a change in height of the topography

PubMed Central

Shen, BingLu; Yan, PengCheng; Ma, DeShan; Song, Jian

2017-01-01

We consider the shallow water equation and the vorticity equations for a variable height of topography. On the assumptions that the atmosphere is incompressible and a constant density, we simplify the coupled dynamic equations. The change in topographic height is handled as the sum of the inherent and changing topography using the perturbation method, together with appropriate boundary conditions of the atmosphere, to obtain the relationship between the relative height of the flow, the inherent topography and the changing topography. We generalize the conservation of the function of relative position, and quantify the relationship between the height of the topography and the relative position of a fluid element. If the height of the topography increases (decreases), the relative position of a fluid element descends (ascends). On this basis, we also study the relationship between the vorticity and the topography to find the vorticity decreasing (increasing) for an increasing (decreasing) height of the topography. PMID:28591129

Regional statistics in confined two-dimensional decaying turbulence.

PubMed

Házi, Gábor; Tóth, Gábor

2011-06-28

Two-dimensional decaying turbulence in a square container has been simulated using the lattice Boltzmann method. The probability density function (PDF) of the vorticity and the particle distribution functions have been determined at various regions of the domain. It is shown that, after the initial stage of decay, the regional area averaged enstrophy fluctuates strongly around a mean value in time. The ratio of the regional mean and the overall enstrophies increases monotonously with increasing distance from the wall. This function shows a similar shape to the axial mean velocity profile of turbulent channel flows. The PDF of the vorticity peaks at zero and is nearly symmetric considering the statistics in the overall domain. Approaching the wall, the PDFs become skewed owing to the boundary layer.

The relationship between free-stream coherent structures and near-wall streaks at high Reynolds numbers

PubMed Central

Deguchi, K.; Hall, P.

2017-01-01

The present work is based on our recent discovery of a new class of exact coherent structures generated near the edge of quite general boundary layer flows. The structures are referred to as free-stream coherent structures and were found using a large Reynolds number asymptotic approach to describe equilibrium solutions of the Navier–Stokes equations. In this paper, first we present results for a new family of free-stream coherent structures existing at relatively large wavenumbers. The new results are consistent with our earlier theoretical result that such structures can generate larger amplitude wall streaks if and only if the local spanwise wavenumber is sufficiently small. In a Blasius boundary layer, the local wavenumber increases in the streamwise direction so the wall streaks can typically exist only over a finite interval. However, here it is shown that they can interact with wall curvature to produce exponentially growing Görtler vortices through the receptivity process by a novel nonparallel mechanism. The theoretical predictions found are confirmed by a hybrid numerical approach. In contrast with previous receptivity investigations, it is shown that the amplitude of the induced vortex is larger than the structures in the free-stream which generate it. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’. PMID:28167574

The relationship between free-stream coherent structures and near-wall streaks at high Reynolds numbers.

PubMed

Deguchi, K; Hall, P

2017-03-13

The present work is based on our recent discovery of a new class of exact coherent structures generated near the edge of quite general boundary layer flows. The structures are referred to as free-stream coherent structures and were found using a large Reynolds number asymptotic approach to describe equilibrium solutions of the Navier-Stokes equations. In this paper, first we present results for a new family of free-stream coherent structures existing at relatively large wavenumbers. The new results are consistent with our earlier theoretical result that such structures can generate larger amplitude wall streaks if and only if the local spanwise wavenumber is sufficiently small. In a Blasius boundary layer, the local wavenumber increases in the streamwise direction so the wall streaks can typically exist only over a finite interval. However, here it is shown that they can interact with wall curvature to produce exponentially growing Görtler vortices through the receptivity process by a novel nonparallel mechanism. The theoretical predictions found are confirmed by a hybrid numerical approach. In contrast with previous receptivity investigations, it is shown that the amplitude of the induced vortex is larger than the structures in the free-stream which generate it.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'. © 2017 The Author(s).

The Aerodynamic and Dynamic Loading of a Slender Structure by an Impacting Tornado-Like Vortex: The Influence of Relative Vortex-to-Structure Size on Structural Loading

NASA Astrophysics Data System (ADS)

Strasser, Matthew N.

Structural loading produced by an impacting vortex is a hazardous phenomenon that is encountered in numerous applications ranging from the destruction of residences by tornados to the chopping of tip vortices by rotors. Adequate design of structures to resist vortex-induced structural loading necessitates study of the phenomenon that control the structural loading produced by an impacting vortex. This body of work extends the current knowledge base of vortex-structure interaction by evaluating the influence of the relative vortex-to-structure size on the structural loading that the vortex produces. A computer model is utilized to directly simulate the two-dimensional impact of an impinging vortex with a slender, cylindrical structure. The vortex's tangential velocity profile (TVP) is defined by a normalization of the Vatistas analytical (TVP) which realistically replicates the documented spectrum of measured vortex TVPs. The impinging vortex's maximum tangential velocity is fixed, and the vortex's critical radius is incremented from one to one-hundred times the structure's diameter. When the impinging vortex is small, it interacts with vortices produced on the structure by the free stream, and maximum force coefficient amplitudes vary by more than 400% when the impinging vortex impacts the structure at different times. Maximum drag and lift force coefficient amplitudes reach asymptotic values as the impinging vortex's size increases that are respectively 94.77% and 10.66% less than maximum force coefficients produced by an equivalent maximum velocity free stream. The vortex produces maximum structural loading when its path is shifted above the structure's centerline, and maximum drag and lift force coefficients are respectively up to 4.80% and 34.07% greater than maximum force coefficients produced by an equivalent-velocity free stream. Finally, the dynamic load factor (DLF) concept is used to develop a generalized methodology to assess the dynamic amplification of a structure's response to vortex loading and to assess the dynamic loading threat that tornados pose. Typical civil and residential structures will not experience significant response amplification, but responses of very flexible structures may be amplified by up to 2.88 times.

A flying hot wire study of the turbulent near wake of a circular cylinder at Reynolds number of 140,000. Ph.D. Thesis. Progress Report

NASA Technical Reports Server (NTRS)

Cantwell, B. J.

1975-01-01

The phenomenology was studied of the processes of vortex formation and transport in the near wake, at a Reynolds number sufficiently high to insure a fully turbulent wake, but low enough to insure a laminar separation. The apparatus developed for measuring this flow consisted of X-array hot wire probes mounted on the ends of a pair of whirling arms. A computer controlled data acquisition system was slaved to the position of the rotating arm and managed, monitored, edited, and recorded the vast profusion of data which is continuously poured out by the device. Results are presented which show the instantaneous velocity, intermittency, vorticity, and stress fields as a function of phase for the first six diameters of the near wake. The stresses in the near wake emerge as a concatenation of peaks and valleys, some the result of strong induced motions in the outer flow which cause free stream fluid to move rapidly inward toward the center of the wake, others the result of the random motions of the background turbulence.

Numerical Studies of Boundary-Layer Receptivity

NASA Technical Reports Server (NTRS)

Reed, Helen L.

1995-01-01

Direct numerical simulations (DNS) of the acoustic receptivity process on a semi-infinite flat plate with a modified-super-elliptic (MSE) leading edge are performed. The incompressible Navier-Stokes equations are solved in stream-function/vorticity form in a general curvilinear coordinate system. The steady basic-state solution is found by solving the governing equations using an alternating direction implicit (ADI) procedure which takes advantage of the parallelism present in line-splitting techniques. Time-harmonic oscillations of the farfield velocity are applied as unsteady boundary conditions to the unsteady disturbance equations. An efficient time-harmonic scheme is used to produce the disturbance solutions. Buffer-zone techniques have been applied to eliminate wave reflection from the outflow boundary. The spatial evolution of Tollmien-Schlichting (T-S) waves is analyzed and compared with experiment and theory. The effects of nose-radius, frequency, Reynolds number, angle of attack, and amplitude of the acoustic wave are investigated. This work is being performed in conjunction with the experiments at the Arizona State University Unsteady Wind Tunnel under the direction of Professor William Saric. The simulations are of the same configuration and parameters used in the wind-tunnel experiments.

Nonlinear Stability and Structure of Compressible Reacting Mixing Layers

NASA Technical Reports Server (NTRS)

Day, M. J.; Mansour, N. N.; Reynolds, W. C.

2000-01-01

The parabolized stability equations (PSE) are used to investigate issues of nonlinear flow development and mixing in compressible reacting shear layers. Particular interest is placed on investigating the change in flow structure that occurs when compressibility and heat release are added to the flow. These conditions allow the 'outer' instability modes- one associated with each of the fast and slow streams-to dominate over the 'central', Kelvin-Helmholtz mode that unaccompanied in incompressible nonreacting mixing layers. Analysis of scalar probability density functions in flows with dominant outer modes demonstrates the ineffective, one-sided nature of mixing that accompany these flow structures. Colayer conditions, where two modes have equal growth rate and the mixing layer is formed by two sets of vortices, offer some opportunity for mixing enhancement. Their extent, however, is found to be limited in the mixing layer's parameter space. Extensive validation of the PSE technique also provides a unique perspective on central- mode vortex pairing, further supporting the view that pairing is primarily governed perspective sheds insight on how linear stability theory is able to provide such an accurate prediction of experimentally-observed, fully nonlinear flow phenomenon.

Digital image analysis of a turbulent flame

NASA Astrophysics Data System (ADS)

Zucherman, L.; Kawall, J. G.; Keffer, J. F.

1988-01-01

Digital image analysis of cine pictures of an unconfined rich premixed turbulent flame has been used to determine structural characteristics of the turbulent/non-turbulent interface of the flame. The results, comprising various moments of the interface position, probability density functions and correlation functions, establish that the instantaneous flame-interface position is essentially a Gaussian random variable with a superimposed quasi-periodical component. The latter is ascribable to a pulsation caused by the convection and the stretching of ring vortices present within the flame. To a first approximation, the flame can be considered similar to a three-dimensional axisymmetric turbulent jet, with superimposed ring vortices, in which combustion occurs.

Agenda Setting in Multilateral Contexts: The Example of the Decade of Education for Sustainable Development

ERIC Educational Resources Information Center

Sumida, Sugata

2017-01-01

A number of papers have focused on UN policy formulation processes, but little is known about how particular issues come to policy-makers' attention in the first place. This very early stage of policy formulation, called agenda setting, remains under-researched and more so in multilateral contexts. Applying Kingdon's multiple streams model to the…

THRSTER: A THRee-STream Ejector Ramjet Analysis and Design Tool

NASA Technical Reports Server (NTRS)

Chue, R. S.; Sabean, J.; Tyll, J.; Bakos, R. J.

2000-01-01

An engineering tool for analyzing ejectors in rocket based combined cycle (RBCC) engines has been developed. A key technology for multi-cycle RBCC propulsion systems is the ejector which functions as the compression stage of the ejector ramjet cycle. The THRee STream Ejector Ramjet analysis tool was developed to analyze the complex aerothermodynamic and combustion processes that occur in this device. The formulated model consists of three quasi-one-dimensional streams, one each for the ejector primary flow, the secondary flow, and the mixed region. The model space marches through the mixer, combustor, and nozzle to evaluate the solution along the engine. In its present form, the model is intended for an analysis mode in which the diffusion rates of the primary and secondary into the mixed stream are stipulated. The model offers the ability to analyze the highly two-dimensional ejector flowfield while still benefits from the simplicity and speed of an engineering tool. To validate the developed code, wall static pressure measurements from the Penn-State and NASA-ART RBCC experiments were used to compare with the results generated by the code. The calculated solutions were generally found to have satisfactory agreement with the pressure measurements along the engines, although further modeling effort may be required when a strong shock train is formed at the rocket exhaust. The range of parameters in which the code would generate valid results are presented and discussed.

THRSTER: A Three-Stream Ejector Ramjet Analysis and Design Tool

NASA Technical Reports Server (NTRS)

Chue, R. S.; Sabean, J.; Tyll, J.; Bakos, R. J.; Komar, D. R. (Technical Monitor)

2000-01-01

An engineering tool for analyzing ejectors in rocket based combined cycle (RBCC) engines has been developed. A key technology for multi-cycle RBCC propulsion systems is the ejector which functions as the compression stage of the ejector ramjet cycle. The THRee STream Ejector Ramjet analysis tool was developed to analyze the complex aerothermodynamic and combustion processes that occur in this device. The formulated model consists of three quasi-one-dimensional streams, one each for the ejector primary flow, the secondary flow, and the mixed region. The model space marches through the mixer, combustor, and nozzle to evaluate the solution along the engine. In its present form, the model is intended for an analysis mode in which the diffusion rates of the primary and secondary into the mixed stream are stipulated. The model offers the ability to analyze the highly two-dimensional ejector flowfield while still benefits from the simplicity and speed of an engineering tool. To validate the developed code, wall static pressure measurements from the Penn-State and NASA-ART RBCC experiments were used to compare with the results generated by the code. The calculated solutions were generally found to have satisfactory agreement with the pressure measurements along the engines, although further modeling effort may be required when a strong shock train is formed at the rocket exhaust. The range of parameters in which the code would generate valid results are presented and discussed.

Instability waves and transition in adverse-pressure-gradient boundary layers

NASA Astrophysics Data System (ADS)

Bose, Rikhi; Zaki, Tamer A.; Durbin, Paul A.

2018-05-01

Transition to turbulence in incompressible adverse-pressure-gradient (APG) boundary layers is investigated by direct numerical simulations. Purely two-dimensional instability waves develop on the inflectional base velocity profile. When the boundary layer is perturbed by isotropic turbulence from the free stream, streamwise elongated streaks form and may interact with the instability waves. Subsequent mechanisms that trigger transition depend on the intensity of the free-stream disturbances. All evidence from the present simulations suggest that the growth rate of instability waves is sufficiently high to couple with the streaks. Under very low levels of free-stream turbulence (˜0.1 % ), transition onset is highly sensitive to the inlet disturbance spectrum and is accelerated if the spectrum contains frequency-wave-number combinations that are commensurate with the instability waves. Transition onset and completion in this regime is characterized by formation and breakdown of Λ vortices, but they are more sporadic than in natural transition. Beneath free-stream turbulence with higher intensity (1-2 % ), bypass transition mechanisms are dominant, but instability waves are still the most dominant disturbances in wall-normal and spanwise perturbation spectra. Most of the breakdowns were by disturbances with critical layers close to the wall, corresponding to inner modes. On the other hand, the propensity of an outer mode to occur increases with the free-stream turbulence level. Higher intensity free-stream disturbances induce strong streaks that favorably distort the boundary layer and suppress the growth of instability waves. But the upward displacement of high amplitude streaks brings them to the outer edge of the boundary layer and exposes them to ambient turbulence. Consequently, high-amplitude streaks exhibit an outer-mode secondary instability.

Counter-rotating vortex pairs in the wake of a vertical axis wind turbine

NASA Astrophysics Data System (ADS)

Rolin, Vincent; Porté-Agel, Fernando

2017-04-01

Despite the rising popularity of vertical axis wind turbines, or VAWTs, the wakes behind these machines is much less well understood than those behind horizontal axis wind turbines, or HAWTs. A thorough understanding of wakes is important as they can cause turbines in wind farms to produce less power than anticipated and increase the fatigue loading on turbines due to vibrations. In order to gain a deeper understanding of the wake behind a vertical axis wind turbine in atmospheric flow stereo-PIV is implemented in a boundary-layer wind tunnel to produce snapshots of the 3-component velocity field in the wake at various downstream positions. The boundaries of the wake are readily observed due to the high velocity gradients and turbulence present here. Two pairs of counter-rotating vortices similar to those in the wake of yawed HAWTs are also observed. An examination of the momentum fluxes behind the turbine demonstrates that the mean flow induced by these vortices entrains a large quantity of momentum from the unperturbed boundary layer flow above the wake. This effect proves to play an even more significant role than turbulence in reintroducing momentum into the wake. In order to comprehend why the VAWT produces these vortices we modify the double-multiple stream-tube model typically used to predict VAWT performance to incorporate crosswind forces. The similarity between VAWT and yawed HAWT wakes is found not to be coincidental as both cases feature rotors which exert a lateral thrust on the incoming wind which leads to the creation of counter-rotating vortex pairs.

Linear stability analysis and nonlinear simulation of the channeling effect on viscous fingering instability in miscible displacement

NASA Astrophysics Data System (ADS)

Shahnazari, M. R.; Maleka Ashtiani, I.; Saberi, A.

2018-03-01

In this paper, the effect of channeling on viscous fingering instability of miscible displacement in porous media is studied. In fact, channeling is introduced as a solution to stabilize the viscous fingering instability. In this solution, narrow channels were placed next to the walls, and by considering an exponential function to model the channeling effect, a heterogeneous media is assumed. In linear stability analysis, the governing equations are transferred to Fourier space, and by introducing a novel numerical method, the transferred equations are analyzed. The growth rate based on the wave number diagram has been drawn up in three sections of the medium. It is found that the flow becomes more stable at the center and unstable along the walls when the permeability ratio is increased. Also when the permeability ratio is approximately equal to one, the channeling has no significant effect. In nonlinear simulations, by using stream function and vortices, new equations have been rewritten and it is shown that channeling has a profound effect on the growth of the fingers and mechanisms. In addition to the superposition of velocity vectors and concentration contours, the development of instability is investigated using the mixing length and sweep efficiency diagram. The results show that although channeling reduces instability, it increases the displacement process time.

Spatiotemporal dynamics and optical vortices in a photorefractive phase-conjugate resonator

NASA Technical Reports Server (NTRS)

Liu, Siuying Raymond; Indebetouw, Guy

1992-01-01

A truncated modal expansion approach is used to study the spatiotemporal dynamics of a phase-conjugate resonator as a function of Bragg detuning. The numerical results reveal a rich variety of behaviors. Emphasis is given to the spatial distribution of optical vortices, their trajectories and their relationship to the beam's spatial coherence. The limitations of the model are discussed and experimental results are presented for comparison with the model's predictions and assessment of its soundness.

On a viable first-order formulation of relativistic viscous fluids and its applications to cosmology

NASA Astrophysics Data System (ADS)

Disconzi, Marcelo M.; Kephart, Thomas W.; Scherrer, Robert J.

We consider a first-order formulation of relativistic fluids with bulk viscosity based on a stress-energy tensor introduced by Lichnerowicz. Choosing a barotropic equation-of-state, we show that this theory satisfies basic physical requirements and, under the further assumption of vanishing vorticity, that the equations of motion are causal, both in the case of a fixed background and when the equations are coupled to Einstein's equations. Furthermore, Lichnerowicz's proposal does not fit into the general framework of first-order theories studied by Hiscock and Lindblom, and hence their instability results do not apply. These conclusions apply to the full-fledged nonlinear theory, without any equilibrium or near equilibrium assumptions. Similarities and differences between the approach explored here and other theories of relativistic viscosity, including the Mueller-Israel-Stewart formulation, are addressed. Cosmological models based on the Lichnerowicz stress-energy tensor are studied. As the topic of (relativistic) viscous fluids is also of interest outside the general relativity and cosmology communities, such as, for instance, in applications involving heavy-ion collisions, we make our presentation largely self-contained.

A Dual-Plane PIV Study of Turbulent Heat Transfer Flows

NASA Technical Reports Server (NTRS)

Wernet, Mark P.; Wroblewski, Adam C.; Locke, Randy J.

2016-01-01

Thin film cooling is a widely used technique in turbomachinery and rocket propulsion applications, where cool injection air protects a surface from hot combustion gases. The injected air typically has a different velocity and temperature from the free stream combustion flow, yielding a flow field with high turbulence and large temperature differences. These thin film cooling flows provide a good test case for evaluating computational model prediction capabilities. The goal of this work is to provide a database of flow field measurements for validating computational flow prediction models applied to turbulent heat transfer flows. In this work we describe the application of a Dual-Plane Particle Image Velocimetry (PIV) technique in a thin film cooling wind tunnel facility where the injection air stream velocity and temperatures are varied in order to provide benchmark turbulent heat transfer flow field measurements. The Dual-Plane PIV data collected include all three components of velocity and all three components of vorticity, spanning the width of the tunnel at multiple axial measurement planes.

Computation of the inviscid supersonic flow about cones at large angles of attack by a floating discontinuity approach

NASA Technical Reports Server (NTRS)

Daywitt, J.; Kutler, P.; Anderson, D.

1977-01-01

The technique of floating shock fitting is adapted to the computation of the inviscid flowfield about circular cones in a supersonic free stream at angles of attack that exceed the cone half-angle. The resulting equations are applicable over the complete range of free-stream Mach numbers, angles of attack and cone half-angles for which the bow shock is attached. A finite difference algorithm is used to obtain the solution by an unsteady relaxation approach. The bow shock, embedded cross-flow shock, and vortical singularity in the leeward symmetry plane are treated as floating discontinuities in a fixed computational mesh. Where possible, the flowfield is partitioned into windward, shoulder, and leeward regions with each region computed separately to achieve maximum computational efficiency. An alternative shock fitting technique which treats the bow shock as a computational boundary is developed and compared with the floating-fitting approach. Several surface boundary condition schemes are also analyzed.

Stability analysis of confined V-shaped flames in high-velocity streams.

PubMed

El-Rabii, Hazem; Joulin, Guy; Kazakov, Kirill A

2010-06-01

The problem of linear stability of confined V-shaped flames with arbitrary gas expansion is addressed. Using the on-shell description of flame dynamics, a general equation governing propagation of disturbances of an anchored flame is obtained. This equation is solved analytically for V-flames anchored in high-velocity channel streams. It is demonstrated that dynamics of the flame disturbances in this case is controlled by the memory effects associated with vorticity generated by the perturbed flame. The perturbation growth rate spectrum is determined, and explicit analytical expressions for the eigenfunctions are given. It is found that the piecewise linear V structure is unstable for all values of the gas expansion coefficient. Despite the linearity of the basic pattern, however, evolutions of the V-flame disturbances are completely different from those found for freely propagating planar flames or open anchored flames. The obtained results reveal strong influence of the basic flow and the channel walls on the stability properties of confined V-flames.

A bulk viscosity approach for shock capturing on unstructured grids

NASA Astrophysics Data System (ADS)

Shoeybi, Mohammad; Larsson, Nils Johan; Ham, Frank; Moin, Parviz

2008-11-01

The bulk viscosity approach for shock capturing (Cook and Cabot, JCP, 2005) augments the bulk part of the viscous stress tensor. The intention is to capture shock waves without dissipating turbulent structures. The present work extends and modifies this method for unstructured grids. We propose a method that properly scales the bulk viscosity with the grid spacing normal to the shock for unstructured grid for which the shock is not necessarily aligned with the grid. The magnitude of the strain rate tensor used in the original formulation is replaced with the dilatation, which appears to be more appropriate in the vortical turbulent flow regions (Mani et al., 2008). The original form of the model is found to have an impact on dilatational motions away form the shock wave, which is eliminated by a proposed localization of the bulk viscosity. Finally, to allow for grid adaptation around shock waves, an explicit/implicit time advancement scheme has been developed that adaptively identifies the stiff regions. The full method has been verified with several test cases, including 2D shock-vorticity entropy interaction, homogenous isotropic turbulence, and turbulent flow over a cylinder.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cozzi, Alex D.; McCabe, Daniel J.

The Hanford Waste Treatment and Immobilization Plant (WTP) Low Activity Waste (LAW) vitrification facility will generate an aqueous condensate recycle stream (LAW Melter Off-Gas Condensate) from the off-gas system. The baseline plan for disposition of this stream during full WTP operations is to send it to the WTP Pretreatment Facility, where it will be blended with LAW, concentrated by evaporation and recycled to the LAW vitrification facility. However, during the Direct Feed LAW (DFLAW) scenario, planned disposition of this stream is to evaporate it in a new evaporator in the Effluent Management Facility (EMF) and then return it to themore » LAW melter. It is important to understand the composition of the effluents from the melter and new evaporator so that the disposition of these streams can be accurately planned and accommodated. Furthermore, alternate disposition of this stream would eliminate recycling of problematic components, and would enable less integrated operation of the LAW melter and the Pretreatment Facilities. Alternate disposition would also eliminate this stream from recycling within WTP when it begins operations and would decrease the LAW vitrification mission duration and quantity of glass waste. This LAW Melter Off-Gas Condensate stream will contain components that are volatile at melter temperatures and are problematic for the glass waste form, such as halides and sulfate, along with entrained, volatile, and semi-volatile metals, such as Hg, As, and Se. Because this stream will recycle within WTP, these components accumulate in the Melter Condensate stream, exacerbating their impact on the number of LAW glass containers that must be produced. Diverting the stream reduces the halides and sulfate that get recycled to the melter, and is a key objective of this work. This overall program examines the potential treatment and immobilization of this stream to enable alternative disposal. The objective of earlier tasks was to formulate and prepare a simulant of the LAW Melter Off-gas Condensate expected during DFLAW operations and use it in evaporator testing to predict the composition of the effluents from the Effluent Management Facility (EMF) evaporator to aid in planning for their disposition. The objective of this task was to test immobilization options for this evaporator bottoms aqueous stream. This document describes the method used to formulate a simulant of this EMF evaporator bottoms stream, immobilize it, and determine if the immobilized waste forms meet disposal criteria.« less

Review of the physics of enhancing vortex lift by unsteady excitation

NASA Technical Reports Server (NTRS)

Wu, J. Z.; Vakili, A. D.; Wu, J. M.

1991-01-01

A review aimed at providing a physical understanding of the crucial mechanisms for obtaining super lift by means of unsteady excitations is presented. Particular attention is given to physical problems, including rolled-up vortex layer instability and receptivity, wave-vortex interaction and resonance, nonlinear streaming, instability of vortices behind bluff bodies and their shedding, and vortex breakdown. A general theoretical framework suitable for handling the unsteady vortex flows is introduced. It is suggested that wings with swept and sharp leading edges, equipped with devices for unsteady excitations, could yield the first breakthrough of the unsteady separation barrier and provide super lift at post-stall angle of attack.

Theoretical aerodynamic characteristics of a family of slender wing-tail-body combinations

NASA Technical Reports Server (NTRS)

Lomax, Harvard; Byrd, Paul F

1951-01-01

The aerodynamic characteristics of an airplane configuration composed of a swept-back, nearly constant chord wing and a triangular tail mounted on a cylindrical body are presented. The analysis is based on the assumption that the free-stream Mach number is near unity or that the configuration is slender. The calculations for the tail are made on the assumption that the vortex system trailing back from the wing is either a sheet lying entirely in the plane of the flat tail surface or has completely "rolled up" into two point vortices that lie either in, above, or below the plane of the tail surface.

Effect of geometry variations on lee-surface vortex-induced heating for flat-bottom three-dimensional bodies at Mach 6

NASA Technical Reports Server (NTRS)

Hefner, J. N.

1973-01-01

Studies have shown that vortices can produce relatively severe heating on the leeward surfaces of conceptual hypersonic vehicles and that surface geometry can strongly influence this vortex-induced heating. Results which show the effects of systematic geometry variations on the vortex-induced lee-surface heating on simple flat-bottom three-dimensional bodies at angles of attack of 20 deg and 40 deg are presented. The tests were conducted at a free-stream Mach number of 6 and at a Reynolds number of 1.71 x 10 to the 7th power per meter.

Characteristics of mesoscale vortices over China in 2015

NASA Astrophysics Data System (ADS)

Shu, Yu; Sun, Jisong; Pan, Yinong

2017-12-01

Mesoscale vortices, which appear at middle and lower levels of rainstorms, are cyclonic circulations with a size ranging from tens of kilometers to several hundred kilometers. Mesoscale vortices often have close relationships with convective activities. The ERA-Interim dataset and an automatic vortex-searching method were used to identify the mesoscale vortices occurring over China in 2015 and their basic characteristics were analyzed. The mesoscale vortices are divided into three categories: mesoscale convective vortices, mesoscale stratiform vortices, and mesoscale dry vortices. The mesoscale convective vortices have the largest intensity, size, and duration, whereas the mesoscale dry vortices have the smallest. Mesoscale convective vortices are able to form in any direction of the parent mesoscale convective system, although the secondary convection tends to appear to the southeast of the parent vortices. The mesoscale vortices tend to generate in the transition area between high and low altitudes. The leeward side of the Tibetan Plateau is the main source region of mesoscale vortices in China. Most of vortices are generated at midday and midnight. The activities of mesoscale convective vortices and mesoscale stratiform vortices peak in summer, whereas those of the mesoscale dry vortices peak in winter.

Conformal invariance of the Lungren-Monin-Novikov equations for vorticity fields in 2D turbulence

NASA Astrophysics Data System (ADS)

Grebenev, V. N.; Wacławczyk, M.; Oberlack, M.

2017-10-01

We study the statistical properties of the vorticity field in two-dimensional turbulence. The field is described in terms of the infinite Lundgren-Monin-Novikov (LMN) chain of equations for multi-point probability density functions (pdf’s) of vorticity. We perform a Lie group analysis of the first equation in this chain using the direct method based on the canonical Lie-Bäcklund transformations devised for integro-differential equations. We analytically show that the conformal group is broken for the first LMN equation i.e. for the 1-point pdf at least for the inviscid case but the equation is still conformally invariant on the associated characteristic with zero-vorticity. Then, we demonstrate that this characteristic is conformally transformed. We find this outcome coincides with the numerical results about the conformal invariance of the statistics of zero-vorticity isolines, see e.g. Falkovich (2007 Russian Math. Surv. 63 497-510). The conformal symmetry can be understood as a ‘local scaling’ and its traces in two-dimensional turbulence were already discussed in the literature, i.e. it was conjectured more than twenty years ago in Polyakov (1993 Nucl. Phys. B 396 367-85) and clearly validated experimentally in Bernard et al (2006 Nat. Phys. 2 124-8).

Axisymmetric bluff-body flow: A vortex solver for thin shells

NASA Astrophysics Data System (ADS)

Strickland, J. H.

1992-05-01

A method which is capable of solving the axisymmetric flow field over bluff bodies consisting of thin shells such as disks, partial spheres, rings, and other such shapes is presented in this report. The body may be made up of several shells whose edges are separated by gaps. The body may be moved axially according to arbitrary velocity time histories. In addition, the surfaces may possess axial and radial degrees of flexibility such that points on the surfaces may be allowed to move relative to each other according to some specified function of time. The surfaces may be either porous or impervious. The present solution technique is based on the axisymmetric vorticity transport equation. Physically, this technique simulates the generation of vorticity at body surfaces in the form of discrete ring vortices which are subsequently diffused and convected into the boundary layers and wake of the body. Relatively large numbers of vortices (1000 or more) are required to obtain good simulations. Since the direct calculation of perturbations from large numbers of ring vortices is computationally intensive, a fast multipole method was used to greatly reduce computer processing time. Several example calculations are presented for disks, disks with holes, hemispheres, and vented hemispheres. These results are compared with steady and unsteady experimental data.

Optimal replenishment and credit policy in supply chain inventory model under two levels of trade credit with time- and credit-sensitive demand involving default risk

NASA Astrophysics Data System (ADS)

Mahata, Puspita; Mahata, Gour Chandra; Kumar De, Sujit

2018-03-01

Traditional supply chain inventory modes with trade credit usually only assumed that the up-stream suppliers offered the down-stream retailers a fixed credit period. However, in practice the retailers will also provide a credit period to customers to promote the market competition. In this paper, we formulate an optimal supply chain inventory model under two levels of trade credit policy with default risk consideration. Here, the demand is assumed to be credit-sensitive and increasing function of time. The major objective is to determine the retailer's optimal credit period and cycle time such that the total profit per unit time is maximized. The existence and uniqueness of the optimal solution to the presented model are examined, and an easy method is also shown to find the optimal inventory policies of the considered problem. Finally, numerical examples and sensitive analysis are presented to illustrate the developed model and to provide some managerial insights.

An Experiment on the Near Flow Field of the GE/ARL Mixer Ejector Nozzle

NASA Technical Reports Server (NTRS)

Zaman, K. B. M. Q.

2004-01-01

This report is a documentation of the results on flowfield surveys for the GE/ARL mixer-ejector nozzle carried out in an open jet facility at NASA Glenn Research Center. The results reported are for cold (unheated) flow without any surrounding co-flowing stream. Distributions of streamwise vorticity as well as turbulent stresses, obtained by hot-wire anemometry, are presented for a low subsonic condition. Pitot probe survey results are presented for nozzle pressure ratios up to 3.5. Flowfields both inside and outside of the ejector are considered. Inside the ejector, the mean velocity distribution exhibits a cellular pattern on the cross sectional plane, originating from the flow through the primary and secondary chutes. With increasing downstream distance an interchange of low velocity regions with adjacent high velocity regions takes place due to the action of the streamwise vortices. At the ejector exit, the velocity distribution is nonuniform at low and high pressure ratios but reasonably uniform at intermediate pressure ratios. The effects of two chevron configurations and a tab configuration on the evolution of the downstream jet are also studied. Compared to the baseline case, minor but noticeable effects are observed on the flowfield.

Particle-in-cell simulations of the lower-hybrid instability driven by an ion-ring distribution

NASA Astrophysics Data System (ADS)

Swanekamp, Stephen; Richardson, Steve; Mithaiwala, Manish; Crabtree, Chris

2013-10-01

Fully electromagnetic particle-in-cell simulations of the excitation of the lower-hybrid mode in a plasma driven by an ion-ring distribution using the Lsp code are presented. At early times the simulations agree with linear theory. The resulting wave evolution and non-linear plasma and ring-ion heating are compared with theoretical models [Mithaiwala et al. 2010; Crabtree et al., this meeting] and previous simulation results [Winske and Daughton, 2012]. 2D simulations show that when the magnetic field is perpendicular to the wave vector, k, the electrostatic potential fluctuations work in conjunction with the applied magnetic field causing a circular electron E ×B drift around a positively charged center. Similar phenomena are observed in 2D simulations of magnetic-field penetration into a spatially inhomogeneous unmagnetized plasma [Richardson et al., this meeting] where circular paramagnetic vortices are formed. These vortices are altered by the addition of a small, in-plane, component of magnetic field which allows electrons to stream along field lines effectively shorting out one component of the electric field. In this case, the vortex structures are no longer circular but elongated along the direction of the in-plane magnetic field component.

Initiation of Long-Wave Instability of Vortex Pairs at Cruise Altitudes

NASA Technical Reports Server (NTRS)

Rossow, Vernon J.

2011-01-01

Previous studies have usually attributed the initiation of the long-wave instability of a vortex pair to turbulence in the atmosphere or in the wake of the aircraft. The purpose here is to show by use of observations and photographs of condensation trails shed by aircraft at cruise altitudes that another initiating mechanism is not only possible but is usually the mechanism that initiates the long-wave instability at cruise altitudes. The alternate initiating mechanism comes about when engine thrust is robust enough to form an array of circumferential vortices around each jet-engine-exhaust stream. In those cases, initiation begins when the vortex sheet shed by the wing has rolled up into a vortex pair and descended to the vicinity of the inside bottom of the combined shear-layer vortex arrays. It is the in-and-out (up and down) velocity field between sequential circumferential vortices near the bottom of the array that then impresses disturbance waves on the lift-generated vortex pair that initiate the long-wave instability. A time adjustment to the Crow and Bate estimate for vortex linking is then derived for cases when thrust-based linking occurs.

Lift hysteresis at stall as an unsteady boundary-layer phenomenon

NASA Technical Reports Server (NTRS)

Moore, Franklin K

1956-01-01

Analysis of rotating stall of compressor blade rows requires specification of a dynamic lift curve for the airfoil section at or near stall, presumably including the effect of lift hysteresis. Consideration of the magnus lift of a rotating cylinder suggests performing an unsteady boundary-layer calculation to find the movement of the separation points of an airfoil fixed in a stream of variable incidence. The consideration of the shedding of vorticity into the wake should yield an estimate of lift increment proportional to time rate of change of angle of attack. This increment is the amplitude of the hysteresis loop. An approximate analysis is carried out according to the foregoing ideas for a 6:1 elliptic airfoil at the angle of attack for maximum lift. The assumptions of small perturbations from maximum lift are made, permitting neglect of distributed vorticity in the wake. The calculated hysteresis loop is counterclockwise. Finally, a discussion of the forms of hysteresis loops is presented; and, for small reduced frequency of oscillation, it is concluded that the concept of a viscous "time lag" is appropriate only for harmonic variations of angle of attack with time at mean conditions other than maximum lift.

Scaling laws and vortex profiles in two-dimensional decaying turbulence.

PubMed

Laval, J P; Chavanis, P H; Dubrulle, B; Sire, C

2001-06-01

We use high resolution numerical simulations over several hundred of turnover times to study the influence of small scale dissipation onto vortex statistics in 2D decaying turbulence. A scaling regime is detected when the scaling laws are expressed in units of mean vorticity and integral scale, like predicted in Carnevale et al., Phys. Rev. Lett. 66, 2735 (1991), and it is observed that viscous effects spoil this scaling regime. The exponent controlling the decay of the number of vortices shows some trends toward xi=1, in agreement with a recent theory based on the Kirchhoff model [C. Sire and P. H. Chavanis, Phys. Rev. E 61, 6644 (2000)]. In terms of scaled variables, the vortices have a similar profile with a functional form related to the Fermi-Dirac distribution.

Navier-Stokes, flight, and wind tunnel flow analysis for the F/A-18 aircraft

NASA Technical Reports Server (NTRS)

Ghaffari, Farhad

1994-01-01

Computational analysis of flow over the F/A-18 aircraft is presented along with complementary data from both flight and wind tunnel experiments. The computational results are based on the three-dimensional thin-layer Navier-Stokes formulation and are obtained from an accurate surface representation of the fuselage, leading-edge extension (LEX), and the wing geometry. However, the constraints imposed by either the flow solver and/or the complexity associated with the flow-field grid generation required certain geometrical approximations to be implemented in the present numerical model. In particular, such constraints inspired the removal of the empennage and the blocking (fairing) of the inlet face. The results are computed for three different free-stream flow conditions and compared with flight test data of surface pressure coefficients, surface tuft flow, and off-surface vortical flow characteristics that included breakdown phenomena. Excellent surface pressure coefficient correlations, both in terms of magnitude and overall trend, are obtained on the forebody throughout the range of flow conditions. Reasonable pressure agreement was obtained over the LEX; the general correlation tends to improve at higher angles of attack. The surface tuft flow and the off-surface vortex flow structures compared qualitatively well with the flight test results. To evaluate the computational results, a wind tunnel investigation was conducted to determine the effects of existing configurational differences between the flight vehicle and the numerical model on aerodynamic characteristics. In most cases, the geometrical approximations made to the numerical model had very little effect on overall aerodynamic characteristics.

Vortex identification from local properties of the vorticity field

NASA Astrophysics Data System (ADS)

Elsas, J. H.; Moriconi, L.

2017-01-01

A number of systematic procedures for the identification of vortices/coherent structures have been developed as a way to address their possible kinematical and dynamical roles in structural formulations of turbulence. It has been broadly acknowledged, however, that vortex detection algorithms, usually based on linear-algebraic properties of the velocity gradient tensor, can be plagued with severe shortcomings and may become, in practical terms, dependent on the choice of subjective threshold parameters in their implementations. In two-dimensions, a large class of standard vortex identification prescriptions turn out to be equivalent to the "swirling strength criterion" (λc i-criterion), which is critically revisited in this work. We classify the instances where the accuracy of the λc i-criterion is affected by nonlinear superposition effects and propose an alternative vortex detection scheme based on the local curvature properties of the vorticity graph (x ,y ,ω ) —the "vorticity curvature criterion" (λω-criterion)—which improves over the results obtained with the λc i-criterion in controlled Monte Carlo tests. A particularly problematic issue, given its importance in wall-bounded flows, is the eventual inadequacy of the λc i-criterion for many-vortex configurations in the presence of strong background shear. We show that the λω-criterion is able to cope with these cases as well, if a subtraction of the mean velocity field background is performed, in the spirit of the Reynolds decomposition procedure. A realistic comparative study for vortex identification is then carried out for a direct numerical simulation of a turbulent channel flow, including a three-dimensional extension of the λω-criterion. In contrast to the λc i-criterion, the λω-criterion indicates in a consistent way the existence of small scale isotropic turbulent fluctuations in the logarithmic layer, in consonance with long-standing assumptions commonly taken in turbulent boundary layer phenomenology.

Generalized derivation of the added-mass and circulatory forces for viscous flows

NASA Astrophysics Data System (ADS)

Limacher, Eric; Morton, Chris; Wood, David

2018-01-01

The concept of added mass arises from potential flow analysis and is associated with the acceleration of a body in an inviscid irrotational fluid. When shed vorticity is modeled as vortex singularities embedded in this irrotational flow, the associated force can be superimposed onto the added-mass force due to the linearity of the governing Laplace equation. This decomposition of force into added-mass and circulatory components remains common in modern aerodynamic models, but its applicability to viscous separated flows remains unclear. The present work addresses this knowledge gap by presenting a generalized derivation of the added-mass and circulatory force decomposition which is valid for a body of arbitrary shape in an unbounded, incompressible fluid domain, in both two and three dimensions, undergoing arbitrary motions amid continuous distributions of vorticity. From the general expression, the classical added-mass force is rederived for well-known canonical cases and is seen to be additive to the circulatory force for any flow. The formulation is shown to be equivalent to existing theoretical work under the specific conditions and assumptions of previous studies. It is also validated using a numerical simulation of a pitching plate in a steady freestream flow, conducted by Wang and Eldredge [Theor. Comput. Fluid Dyn. 27, 577 (2013), 10.1007/s00162-012-0279-5]. In response to persistent confusion in the literature, a discussion of the most appropriate physical interpretation of added mass is included, informed by inspection of the derived equations. The added-mass force is seen to account for the dynamic effect of near-body vorticity and is not (as is commonly claimed) associated with the acceleration of near-body fluid which "must" somehow move with the body. Various other consequences of the derivation are discussed, including a concept which has been labeled the conservation of image-vorticity impulse.

Quantum fluctuations of the superconducting cosmic string

NASA Technical Reports Server (NTRS)

Zhang, Shoucheng

1987-01-01

Quantum fluctuations of the proposed superconducting string with Bose charge carriers are studied in terms of the vortices on the string world sheet. In the thermodynamical limit, it is found that they appear in the form of free vortices rather than as bound pairs. This fluctuation mode violates the topological conservation law on which superconductivity is based. However, this limit may not be reached. The critical size of the superconducting string is estimated as a function of the coupling constants involved.

Vortex locking in direct numerical simulations of quantum turbulence.

PubMed

Morris, Karla; Koplik, Joel; Rouson, Damian W I

2008-07-04

Direct numerical simulations are used to examine the locking of quantized superfluid vortices and normal fluid vorticity in evolving turbulent flows. The superfluid is driven by the normal fluid, which undergoes either a decaying Taylor-Green flow or a linearly forced homogeneous isotropic turbulent flow, although the back reaction of the superfluid on the normal fluid flow is omitted. Using correlation functions and wavelet transforms, we present numerical and visual evidence for vortex locking on length scales above the intervortex spacing.

IUTAM Symposium on Vortex Dynamics: Formation, Structure and Function, 10-14 March 2013, Fukuoka, Japan

NASA Astrophysics Data System (ADS)

Fukumoto, Yasuhide

2014-06-01

This special issue of Fluid Dynamics Research contains the first of a two-part publication of the papers presented at the IUTAM Symposium on Vortex Dynamics: Formation, Structure and Function, held at the Centennial Hall, Kyushu University School of Medicine, Fukuoka, Japan, during the week of 10-14 March 2013. Vortices are ubiquitous structures in fluid mechanics spanning the range of scales from nanofluidics and microfluidics to geophysical and astrophysical flows. Vortices are the key to understanding many different phenomena. As a result, the subject of vortex dynamics continues to evolve and to constantly find new applications in biology, biotechnology, industrial and environmental problems. Vortices can be created by the separation of a flow from the surface of a body or at a density interface, and evolve into coherent structures. Once formed, a vortex acquires a function, depending on its individual structure. In this way, for example, insects gain lift and fish gain thrust. Surprisingly, despite the long history of vortex dynamics, only recently has knowledge about formation, structure and function of vortices been combined to yield new perspectives in the subject, thereby helping to solve outstanding problems brought about by modern advances in computer technology and improved experimental techniques. This symposium is a continuation, five years on, of the IUTAM Symposium '50 Years of Vortex Dynamics', Lyngby, Denmark that took place between 12-16 October 2008, organized by the late Professor Hassan Aref. Originally, Professor Aref was a member of the International Scientific Committee of this symposium and offered his enthusiasm and great expertise, to support its organization. To our shock, he suddenly passed away on 9 September 2011. Furthermore, Professor Slava Meleshko, a leading scientist of fluid and solid mechanics and an intimate friend of Professor Aref, was expected to make an eminent contribution to the symposium. Soon after this sad loss, Professor Meleshko unexpectedly passed away in a tragic traffic accident on 14 November 2011. This symposium was dedicated to the memory of Professors Aref and Meleshko, and started with a session commemorating the legacy of their work, organized by Professors P K Newton and G J F van Heijst. Professors Aref (1950-2011) and Meleshko (1951-2011) made fundamental contributions to fluid mechanics and vortex dynamics throughout their respective distinguished careers. Although mathematical in their fundamental approaches, both sought the connections between theory and experiment and searched for physical explanations in their work. With strong, warm, and embracing personalities, they each played key roles in developing and enriching international collaborations in the field of vortex dynamics through their considerable organizational and cooperative skills, and both made enormous contribution to the development of the IUTAM. Their scientific interests and personal lives overlapped considerably, and their impact in the field of vortex dynamics was honoured in the memorial session. The following sessions presented the development of new mathematical methods and theoretical concepts, bringing in novel techniques in vortex dynamics, stimulated by the continuous development of numerical method and new experimental results, in such aspects as vortex equilibria, spectra, instability and nonlinear dynamics of vortices in barotropic and baroclinic fluids, chaos, classical and quantum turbulence, and wall turbulence, flow separation and vortex-body interactions. Topical applications include biological locomotion, environmental problems, and Bose-Einstein condensates in condensed matter theory. Central fundamental issues in theoretical, numerical and experimental aspects of vortex dynamics were also covered during the symposium such as (1) The dynamics of point vortices in domains of non-trivial topology, its Hamiltonian formulation and new statistical approaches, (2) 3D instability of vortices, with effects of compressibility and stratification, (3) Stratified vortices in the atmosphere and oceans and MHD vortices in astrophysics, (4) Numerical methods for calculating vortex equilibria, (5) Numerical methods for calculating separation of vortices and vortex-vortex interactions with their application to fish and insect locomotion and wind turbines. The symposium was attended by 128 registered participants. The official scientific participants came from 16 nations: Algeria (1), Brazil (1), Canada (3), China (3), France (12), Germany (1), India (1), Italy (2), Japan (63), The Netherlands (1), Poland (3), Russia (7), Spain (2), UK (10), Ukraine (2), and USA (16). Just a hundred papers were presented. The technical program consisted of eight invited lectures, 48 contributed papers and 44 poster presentations. The International Scientific Committee (ISC) of the symposium consisted of Professors D G Crowdy, S Le Dizès, S G Llewellyn Smith, P K Newton, R L Ricca, G J F van Heijst and YF as the chair. The members of the ISC are gratefully acknowledged. Sincere thanks are extended to the Advisory Board and also to all the members of the Domestic Organizing Committee and Local Organizing Committee for their effort in making the symposium very successful. Financial support for the symposium was provided by the IUTAM, the Commemorative Organization for the Japan World Exposition '70, the CREST offered by the Japan Science and Technology Agency (JST), the Global COE Program of the Graduate School of Mathematics, Kyushu University offered by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and the Research Institute for Applied Mechanics and Institute of Mathematics for Industry, Kyushu University. All lecture presenters were strongly encouraged to submit papers for this IUTAM Symposium special issue of Fluid Dynamics Research. Poster presenters were also invited to do so. All the submitted papers were refereed, each by two reviewers, to the same standards applied for papers or review articles in regular issues of FDR. Some of the submitted papers do not appear in the special issue. The submitted papers were handled, for the reviewing procedure, by the three guest editors, Professors D G Crowdy, Y Hattori and S Le Dizès and YF, the associate editor of FDR. About 50 papers were accepted for publication in this special issue, which is published in two parts. This issue (vol 46, issue 3, June 2014) is part 1. Part 2 will be published as volume 46, issue 6, in December 2014. I hope that the special issue provides a sketch of the state of the art in the field of vortex dynamics and holds a key to open up future directions of study.

Discrete conservation properties for shallow water flows using mixed mimetic spectral elements

NASA Astrophysics Data System (ADS)

Lee, D.; Palha, A.; Gerritsma, M.

2018-03-01

A mixed mimetic spectral element method is applied to solve the rotating shallow water equations. The mixed method uses the recently developed spectral element histopolation functions, which exactly satisfy the fundamental theorem of calculus with respect to the standard Lagrange basis functions in one dimension. These are used to construct tensor product solution spaces which satisfy the generalized Stokes theorem, as well as the annihilation of the gradient operator by the curl and the curl by the divergence. This allows for the exact conservation of first order moments (mass, vorticity), as well as higher moments (energy, potential enstrophy), subject to the truncation error of the time stepping scheme. The continuity equation is solved in the strong form, such that mass conservation holds point wise, while the momentum equation is solved in the weak form such that vorticity is globally conserved. While mass, vorticity and energy conservation hold for any quadrature rule, potential enstrophy conservation is dependent on exact spatial integration. The method possesses a weak form statement of geostrophic balance due to the compatible nature of the solution spaces and arbitrarily high order spatial error convergence.

Model of random center vortex lines in continuous 2 +1 -dimensional spacetime

NASA Astrophysics Data System (ADS)

Altarawneh, Derar; Engelhardt, Michael; Höllwieser, Roman

2016-12-01

A picture of confinement in QCD based on a condensate of thick vortices with fluxes in the center of the gauge group (center vortices) is studied. Previous concrete model realizations of this picture utilized a hypercubic space-time scaffolding, which, together with many advantages, also has some disadvantages, e.g., in the treatment of vortex topological charge. In the present work, we explore a center vortex model which does not rely on such a scaffolding. Vortices are represented by closed random lines in continuous 2 +1 -dimensional space-time. These random lines are modeled as being piecewise linear, and an ensemble is generated by Monte Carlo methods. The physical space in which the vortex lines are defined is a torus with periodic boundary conditions. Besides moving, growing, and shrinking of the vortex configurations, also reconnections are allowed. Our ensemble therefore contains not a fixed but a variable number of closed vortex lines. This is expected to be important for realizing the deconfining phase transition. We study both vortex percolation and the potential V (R ) between the quark and antiquark as a function of distance R at different vortex densities, vortex segment lengths, reconnection conditions, and at different temperatures. We find three deconfinement phase transitions, as a function of density, as a function of vortex segment length, and as a function of temperature.

Computer program for prediction of the deposition of material released from fixed and rotary wing aircraft

NASA Technical Reports Server (NTRS)

Teske, M. E.

1984-01-01

This is a user manual for the computer code ""AGDISP'' (AGricultural DISPersal) which has been developed to predict the deposition of material released from fixed and rotary wing aircraft in a single-pass, computationally efficient manner. The formulation of the code is novel in that the mean particle trajectory and the variance about the mean resulting from turbulent fluid fluctuations are simultaneously predicted. The code presently includes the capability of assessing the influence of neutral atmospheric conditions, inviscid wake vortices, particle evaporation, plant canopy and terrain on the deposition pattern.

Vortex breakdown simulation - A circumspect study of the steady, laminar, axisymmetric model

NASA Technical Reports Server (NTRS)

Salas, M. D.; Kuruvila, G.

1989-01-01

The incompressible axisymmetric steady Navier-Stokes equations are written using the streamfunction-vorticity formulation. The resulting equations are discretized using a second-order central-difference scheme. The discretized equations are linearized and then solved using an exact LU decomposition, Gaussian elimination, and Newton iteration. Solutions are presented for Reynolds numbers (based on vortex core radius) 100-1800 and swirl parameter 0.9-1.1. The effects of inflow boundary conditions, the location of farfield and outflow boundaries, and mesh refinement are examined. Finally, the stability of the steady solutions is investigated by solving the time-dependent equations.

Method of measuring cross-flow vortices by use of an array of hot-film sensors

NASA Technical Reports Server (NTRS)

Agarwal, Aval K. (Inventor); Maddalon, Dal V. (Inventor); Mangalam, Siva M. (Inventor)

1993-01-01

The invention is a method for measuring the wavelength of cross-flow vortices of air flow having streamlines of flow traveling across a swept airfoil. The method comprises providing a plurality of hot-film sensors. Each hot-film sensor provides a signal which can be processed, and each hot-film sensor is spaced in a straight-line array such that the distance between successive hot-film sensors is less than the wavelength of the cross-flow vortices being measured. The method further comprises determining the direction of travel of the streamlines across the airfoil and positioning the straight-line array of hot film sensors perpendicular to the direction of travel of the streamlines, such that each sensor has a spanwise location. The method further comprises processing the signals provided by the sensors to provide root-mean-square values for each signal, plotting each root-mean-square value as a function of its spanwise location, and determining the wavelength of the cross-flow vortices by noting the distance between two maxima or two minima of root-mean-square values.

On the secondary instability of Taylor-Goertler vortices to Tollmien-Schlichting waves in fully developed flows

NASA Technical Reports Server (NTRS)

Bennett, James; Hall, Philip

1988-01-01

There are many flows of practical importance where both Tollmien-Schlichting waves and Taylor-Goertler vortices are possible causes of transition to turbulence. The effect of fully nonlinear Taylor-Goertler vortices on the growth of small amplitude Tollmien-Schlichting waves is investigated. The basic state considered is the fully developed flow between concentric cylinders driven by an azimuthal pressure gradient. It is hoped that an investigation of this problem will shed light on the more complicated external boundary layer problem where again both modes of instability exist in the presence of concave curvature. The type of Tollmien-Schlichting waves considered have the asymptotic structure of lower branch modes of plane Poiseuille flow. Whilst instabilities at lower Reynolds number are possible, the latter modes are simpler to analyze and more relevant to the boundary layer problem. The effect of fully nonlinear Taylor-Goertler vortices on both two-dimensional and three-dimensional waves is determined. It is shown that, whilst the maximum growth as a function of frequency is not greatly affected, there is a large destabilizing effect over a large range of frequencies.

On the secondary instability of Taylor-Goertler vortices to Tollmien-Schlichting waves in fully-developed flows

NASA Technical Reports Server (NTRS)

Bennett, James; Hall, Philip

1986-01-01

There are many flows of practical importance where both Tollmien-Schlichting waves and Taylor-Goertler vortices are possible causes of transition to turbulence. The effect of fully nonlinear Taylor-Goertler vortices on the growth of small amplitude Tollmien-Schlichting waves is investigated. The basic state considered is the fully developed flow between concentric cylinders driven by an azimuthal pressure gradient. It is hoped that an investigation of this problem will shed light on the more complicated external boundary layer problem where again both modes of instability exist in the presence of concave curvature. The type of Tollmein-Schlichting waves considered have the asymptotic structure of lower branch modes of plane Poisseulle flow. Whilst instabilities at lower Reynolds number are possible, the latter modes are simpler to analyze and more relevant to the boundary layer problem. The effect of fully nonlinear Taylor-Goertler vortices on both two-dimensional and three-dimensional waves is determined. It is shown that, whilst the maximum growth as a function of frequency is not greatly affected, there is a large destabilizing effect over a large range of frequencies.

The Flow Field on Hydrofoils with Leading Edge Protuberances

NASA Astrophysics Data System (ADS)

Custodio, Derrick; Henoch, Charles; Johari, Hamid

2008-11-01

The agility of the humpback whale has been attributed to the use of its pectoral flippers, on which protuberances are present along the leading edge. The forces and moments on hydrofoils with leading edge protuberances were measured in a water tunnel and were compared to a baseline NACA 63(4)-021 hydrofoil revealing significant performance differences. Three protuberance amplitudes and two spanwise wavelengths, closely resembling the morphology found in nature, were examined. Qualitative flow visualization techniques were used to examine flow patterns surrounding the hydrofoils, and Particle Image Velocimetry (PIV) was used to quantify these patterns. Flow visualizations have revealed counter-rotating vortices stemming from the shoulders of the protuberances. These streamwise vortices are a result of the spanwise pressure gradient brought about by the varying leading edge curvature. PIV was used to quantify the strength of these vortices as a function of angle of attack and leading edge geometry. At low angles of attack, these vortices are symmetric with respect to the protuberances; however, the symmetry is lost at high angles of attack. The loss of symmetry can be correlated with the separation point location on the hydrofoil.

Coherent vorticity extraction in resistive drift-wave turbulence: Comparison of orthogonal wavelets versus proper orthogonal decomposition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Futatani, S.; Bos, W.J.T.; Del-Castillo-Negrete, Diego B

2011-01-01

We assess two techniques for extracting coherent vortices out of turbulent flows: the wavelet based Coherent Vorticity Extraction (CVE) and the Proper Orthogonal Decomposition (POD). The former decomposes the flow field into an orthogonal wavelet representation and subsequent thresholding of the coefficients allows one to split the flow into organized coherent vortices with non-Gaussian statistics and an incoherent random part which is structureless. POD is based on the singular value decomposition and decomposes the flow into basis functions which are optimal with respect to the retained energy for the ensemble average. Both techniques are applied to direct numerical simulation datamore » of two-dimensional drift-wave turbulence governed by Hasegawa Wakatani equation, considering two limit cases: the quasi-hydrodynamic and the quasi-adiabatic regimes. The results are compared in terms of compression rate, retained energy, retained enstrophy and retained radial flux, together with the enstrophy spectrum and higher order statistics. (c) 2010 Published by Elsevier Masson SAS on behalf of Academie des sciences.« less

Intracardiac Vortex Dynamics by High-Frame-Rate Doppler Vortography-In Vivo Comparison With Vector Flow Mapping and 4-D Flow MRI.

PubMed

Faurie, Julia; Baudet, Mathilde; Assi, Kondo Claude; Auger, Dominique; Gilbert, Guillaume; Tournoux, Francois; Garcia, Damien

2017-02-01

Recent studies have suggested that intracardiac vortex flow imaging could be of clinical interest to early diagnose the diastolic heart function. Doppler vortography has been introduced as a simple color Doppler method to detect and quantify intraventricular vortices. This method is able to locate a vortex core based on the recognition of an antisymmetric pattern in the Doppler velocity field. Because the heart is a fast-moving organ, high frame rates are needed to decipher the whole blood vortex dynamics during diastole. In this paper, we adapted the vortography method to high-frame-rate echocardiography using circular waves. Time-resolved Doppler vortography was first validated in vitro in an ideal forced vortex. We observed a strong correlation between the core vorticity determined by high-frame-rate vortography and the ground-truth vorticity. Vortography was also tested in vivo in ten healthy volunteers using high-frame-rate duplex ultrasonography. The main vortex that forms during left ventricular filling was tracked during two-three successive cardiac cycles, and its core vorticity was determined at a sampling rate up to 80 duplex images per heartbeat. Three echocardiographic apical views were evaluated. Vortography-derived vorticities were compared with those returned by the 2-D vector flow mapping approach. Comparison with 4-D flow magnetic resonance imaging was also performed in four of the ten volunteers. Strong intermethod agreements were observed when determining the peak vorticity during early filling. It is concluded that high-frame-rate Doppler vortography can accurately investigate the diastolic vortex dynamics.

Advances in Glass Formulations for Hanford High-Aluminum, High-Iron and Enhanced Sulphate Management in HLW Streams - 13000

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kruger, Albert A.

2013-07-01

The current estimates and glass formulation efforts have been conservative in terms of achievable waste loadings. These formulations have been specified to ensure that the glasses are homogenous, contain essentially no crystalline phases, are processable in joule-heated, ceramic-lined melters and meet Hanford Tank Waste Treatment and Immobilization Plant (WTP) Contract terms. The WTP's overall mission will require the immobilization of tank waste compositions that are dominated by mixtures of aluminum (Al), chromium (Cr), bismuth (Bi), iron (Fe), phosphorous (P), zirconium (Zr), and sulphur (S) compounds as waste-limiting components. Glass compositions for these waste mixtures have been developed based upon previousmore » experience and current glass property models. Recently, DOE has initiated a testing program to develop and characterize HLW glasses with higher waste loadings and higher throughput efficiencies. Results of this work have demonstrated the feasibility of increases in waste loading from about 25 wt% to 33-50 wt% (based on oxide loading) in the glass depending on the waste stream. In view of the importance of aluminum limited waste streams at Hanford (and also Savannah River), the ability to achieve high waste loadings without adversely impacting melt rates has the potential for enormous cost savings from reductions in canister count and the potential for schedule acceleration. Consequently, the potential return on the investment made in the development of these enhancements is extremely favorable. Glass composition development for one of the latest Hanford HLW projected compositions with sulphate concentrations high enough to limit waste loading have been successfully tested and show tolerance for previously unreported tolerance for sulphate. Though a significant increase in waste loading for high-iron wastes has been achieved, the magnitude of the increase is not as substantial as those achieved for high-aluminum, high-chromium, high-bismuth or sulphur. Waste processing rate increases for high-iron streams as a combined effect of higher waste loadings and higher melt rates resulting from new formulations have been achieved. (author)« less

Advances in Glass Formulations for Hanford High-Alumimum, High-Iron and Enhanced Sulphate Management in HLW Streams - 13000

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kruger, Albert A.

2013-01-16

The current estimates and glass formulation efforts have been conservative in terms of achievable waste loadings. These formulations have been specified to ensure that the glasses are homogenous, contain essentially no crystalline phases, are processable in joule-heated, ceramic-lined melters and meet Hanford Tank Waste Treatment and Immobilization Plant (WTP) Contract terms. The WTP?s overall mission will require the immobilization of tank waste compositions that are dominated by mixtures of aluminum (Al), chromium (Cr), bismuth (Bi), iron (Fe), phosphorous (P), zirconium (Zr), and sulphur (S) compounds as waste-limiting components. Glass compositions for these waste mixtures have been developed based upon previousmore » experience and current glass property models. Recently, DOE has initiated a testing program to develop and characterize HLW glasses with higher waste loadings and higher throughput efficiencies. Results of this work have demonstrated the feasibility of increases in waste loading from about 25 wt% to 33-50 wt% (based on oxide loading) in the glass depending on the waste stream. In view of the importance of aluminum limited waste streams at Hanford (and also Savannah River), the ability to achieve high waste loadings without adversely impacting melt rates has the potential for enormous cost savings from reductions in canister count and the potential for schedule acceleration. Consequently, the potential return on the investment made in the development of these enhancements is extremely favorable. Glass composition development for one of the latest Hanford HLW projected compositions with sulphate concentrations high enough to limit waste loading have been successfully tested and show tolerance for previously unreported tolerance for sulphate. Though a significant increase in waste loading for high-iron wastes has been achieved, the magnitude of the increase is not as substantial as those achieved for high-aluminum, high-chromium, high-bismuth or sulphur. Waste processing rate increases for high-iron streams as a combined effect of higher waste loadings and higher melt rates resulting from new formulations have been achieved.« less

An analytical approach to fluid ratcheting in oscillatory boundary layer

NASA Astrophysics Data System (ADS)

Yu, Jie

2013-11-01

It is well known that oscillatory flows close to a rigid or flexible boundary induces a steady streaming due to viscosity. Under progressive motions, this becomes a unidirectional streaming near the boundary (e.g. mass transport or peristaltic pumping in water waves). This mechanism is shared by the phenomenon of ratcheting fluid in a narrow channel by vibrating the channel walls that are lined with asymmetric corrugations (shown by a recent experiment BAPS.2010.DFD.HC.3). A theory is presented here to describe the ratcheting effects in such a channel. A conformal transformation method, developed for waves over arbitrary periodic topographies (Yu & Howard, J. Fluid Mech. 2012), is adapted to deal with large corrugations of the channel walls. Under the assumption that the wall oscillations are of small amplitude, the vorticity dynamics can be analyzed in the mapped plane, obtaining the solution that describes the steady streaming field due to nonlinear convective inertia. The results are discussed, regarding the dependency of the pumping direction on the oscillation frequency of the walls and the effects of the end position relative to the phase of corrugations in the case of a finite length channel. Preliminary experimental data will be presented if time permits. Support by NFS (Grant CBET-0845957) during the period of this work is gratefully acknowledged.

High-resolution scanning Hall probe microscopy

NASA Astrophysics Data System (ADS)

Hallen, Hans D.; Hess, H. F.; Chang, A. M.; Pfeiffer, Loren N.; West, Kenneth W.; Mitzi, David B.

1993-06-01

A high resolution scanning Hall probe microscope is used to spatially resolve vortices in high temperature superconducting Bi2Sr2CaCu2O8+(delta) crystals. We observe a partially ordered vortex lattice at several different applied magnetic fields and temperatures. At higher temperatures, a limited amount of vortex re-arrangement is observed, but most vortices remain fixed for periods long compared to the imaging time of several hours even at temperatures as high as 75 degree(s)K (the superconducting transition temperature for these crystals is approximately 84 degree(s)K). A measure of these local magnetic penetration depth can be obtained from a fit to the surface field of several neighboring vortices, and has been measured as a function of temperature. In particular, we have measured the zero temperature penetration depth and found it to be 275 +/- 40 nm.

New scheme for braiding Majorana fermions.

PubMed

Wu, Long-Hua; Liang, Qi-Feng; Hu, Xiao

2014-12-01

Non-Abelian statistics can be achieved by exchanging two vortices in topological superconductors with each grabbing a Majorana fermion (MF) as zero-energy quasi-particle at the cores. However, in experiments it is difficult to manipulate vortices. In the present work, we propose a way to braid MFs without moving vortices. The only operation required in the present scheme is to turn on and off local gate voltages, which liberates a MF from its original host vortex and transports it along the prepared track. We solve the time-dependent Bogoliubov-de Gennes equation numerically, and confirm that the MFs are protected provided the switching of gate voltages for exchanging MFs are adiabatic, which takes only several nano seconds given reasonable material parameters. By monitoring the time evolution of MF wave-functions, we show that non-Abelian statistics is achieved.

Effect of the atmospheric quasi-biweekly oscillation on the vortices moving off the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Li, Lun; Zhang, Renhe; Wen, Min; Lü, Junmei

2018-02-01

In the present study, the relationship between the atmospheric quasi-biweekly oscillation (QBWO) and Tibetan Plateau vortices (TPVs) moving off the Tibetan Plateau was investigated based on the radiosonde and reanalysis data. It is found that the number of TPVs moving off the Tibetan Plateau (moving-off TPVs) has the distinct feature of the 10-20-day QBWO. 77% of the moving-off TPVs occur in the positive phases of the 10-20-day filtered 500 hPa vorticity over eastern Tibetan Plateau. Besides, distributions of the zonal and meridional components of E-vectors coincide well with the trajectories of TPVs, indicating the moving-off TPVs are well related with the propagation of the QBWO energy. The atmospheric circulations and related thermodynamic fields are discussed to reveal the mechanism of the effect of 10-20-day QBWO on the moving-off TPVs. It is found that the atmospheric circulations and heating fields of 10-20-day QBWO have major impact on the moving-off TPVs. In positive QBWO phases, at 500 hPa over eastern Tibetan Plateau, there appear negative geopotential height anomalies and anomalous cyclonic wind shear; the anomalous jet stream and positive geopotential heights at 200 hPa lying over the northeast of the Tibetan Plateau stretch eastward gradually, benefiting for the upper level divergence and ascending motion. The condensation latent heat is released and shifts eastward with the heating centers located at 400 hPa, which depresses the isobaric surface at 500 hPa. All these conditions are in favor of the maintenance and eastward movement of TPVs in the positive QBWO phases.

Spanwise effects on instabilities of compressible flow over a long rectangular cavity

NASA Astrophysics Data System (ADS)

Sun, Y.; Taira, K.; Cattafesta, L. N.; Ukeiley, L. S.

2017-12-01

The stability properties of two-dimensional (2D) and three-dimensional (3D) compressible flows over a rectangular cavity with length-to-depth ratio of L/D=6 are analyzed at a free-stream Mach number of M_∞ =0.6 and depth-based Reynolds number of Re_D=502. In this study, we closely examine the influence of three-dimensionality on the wake mode that has been reported to exhibit high-amplitude fluctuations from the formation and ejection of large-scale spanwise vortices. Direct numerical simulation (DNS) and bi-global stability analysis are utilized to study the stability characteristics of the wake mode. Using the bi-global stability analysis with the time-averaged flow as the base state, we capture the global stability properties of the wake mode at a spanwise wavenumber of β =0. To uncover spanwise effects on the 2D wake mode, 3D DNS are performed with cavity width-to-depth ratio of W/D=1 and 2. We find that the 2D wake mode is not present in the 3D cavity flow with W/D=2, in which spanwise structures are observed near the rear region of the cavity. These 3D instabilities are further investigated via bi-global stability analysis for spanwise wavelengths of λ /D=0.5{-}2.0 to reveal the eigenspectra of the 3D eigenmodes. Based on the findings of 2D and 3D global stability analysis, we conclude that the absence of the wake mode in 3D rectangular cavity flows is due to the release of kinetic energy from the spanwise vortices to the streamwise vortical structures that develops from the spanwise instabilities.

Dust-trapping Vortices and a Potentially Planet-triggered Spiral Wake in the Pre-transitional Disk of V1247 Orionis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kraus, Stefan; Kreplin, Alexander; Young, Alison K.

The radial drift problem constitutes one of the most fundamental problems in planet formation theory, as it predicts particles to drift into the star before they are able to grow to planetesimal size. Dust-trapping vortices have been proposed as a possible solution to this problem, as they might be able to trap particles over millions of years, allowing them to grow beyond the radial drift barrier. Here, we present ALMA 0.″04 resolution imaging of the pre-transitional disk of V1247 Orionis that reveals an asymmetric ring as well as a sharply confined crescent structure, resembling morphologies seen in theoretical models ofmore » vortex formation. The asymmetric ring (at 0.″17 = 54 au separation from the star) and the crescent (at 0.″38 = 120 au) seem smoothly connected through a one-armed spiral-arm structure that has been found previously in scattered light. We propose a physical scenario with a planet orbiting at ∼0.″3 ≈ 100 au, where the one-armed spiral arm detected in polarized light traces the accretion stream feeding the protoplanet. The dynamical influence of the planet clears the gap between the ring and the crescent and triggers two vortices that trap millimeter-sized particles, namely, the crescent and the bright asymmetry seen in the ring. We conducted dedicated hydrodynamics simulations of a disk with an embedded planet, which results in similar spiral-arm morphologies as seen in our scattered-light images. At the position of the spiral wake and the crescent we also observe {sup 12}CO(3-2) and H{sup 12}CO{sup +} (4-3) excess line emission, likely tracing the increased scale-height in these disk regions.« less

Dust-trapping Vortices and a Potentially Planet-triggered Spiral Wake in the Pre-transitional Disk of V1247 Orionis

NASA Astrophysics Data System (ADS)

Kraus, Stefan; Kreplin, Alexander; Fukugawa, Misato; Muto, Takayuki; Sitko, Michael L.; Young, Alison K.; Bate, Matthew R.; Grady, Carol; Harries, Tim T.; Monnier, John D.; Willson, Matthew; Wisniewski, John

2017-10-01

The radial drift problem constitutes one of the most fundamental problems in planet formation theory, as it predicts particles to drift into the star before they are able to grow to planetesimal size. Dust-trapping vortices have been proposed as a possible solution to this problem, as they might be able to trap particles over millions of years, allowing them to grow beyond the radial drift barrier. Here, we present ALMA 0.″04 resolution imaging of the pre-transitional disk of V1247 Orionis that reveals an asymmetric ring as well as a sharply confined crescent structure, resembling morphologies seen in theoretical models of vortex formation. The asymmetric ring (at 0.″17 = 54 au separation from the star) and the crescent (at 0.″38 = 120 au) seem smoothly connected through a one-armed spiral-arm structure that has been found previously in scattered light. We propose a physical scenario with a planet orbiting at ˜0.″3 ≈ 100 au, where the one-armed spiral arm detected in polarized light traces the accretion stream feeding the protoplanet. The dynamical influence of the planet clears the gap between the ring and the crescent and triggers two vortices that trap millimeter-sized particles, namely, the crescent and the bright asymmetry seen in the ring. We conducted dedicated hydrodynamics simulations of a disk with an embedded planet, which results in similar spiral-arm morphologies as seen in our scattered-light images. At the position of the spiral wake and the crescent we also observe 12CO(3-2) and H12CO+ (4-3) excess line emission, likely tracing the increased scale-height in these disk regions.

Scalar mixing and strain dynamics methodologies for PIV/LIF measurements of vortex ring flows

NASA Astrophysics Data System (ADS)

Bouremel, Yann; Ducci, Andrea

2017-01-01

Fluid mixing operations are central to possibly all chemical, petrochemical, and pharmaceutical industries either being related to biphasic blending in polymerisation processes, cell suspension for biopharmaceuticals production, and fractionation of complex oil mixtures. This work aims at providing a fundamental understanding of the mixing and stretching dynamics occurring in a reactor in the presence of a vortical structure, and the vortex ring was selected as a flow paradigm of vortices commonly encountered in stirred and shaken reactors in laminar flow conditions. High resolution laser induced fluorescence and particle imaging velocimetry measurements were carried out to fully resolve the flow dissipative scales and provide a complete data set to fully assess macro- and micro-mixing characteristics. The analysis builds upon the Lamb-Oseen vortex work of Meunier and Villermaux ["How vortices mix," J. Fluid Mech. 476, 213-222 (2003)] and the engulfment model of Baldyga and Bourne ["Simplification of micromixing calculations. I. Derivation and application of new model," Chem. Eng. J. 42, 83-92 (1989); "Simplification of micromixing calculations. II. New applications," ibid. 42, 93-101 (1989)] which are valid for diffusion-free conditions, and a comparison is made between three methodologies to assess mixing characteristics. The first method is commonly used in macro-mixing studies and is based on a control area analysis by estimating the variation in time of the concentration standard deviation, while the other two are formulated to provide an insight into local segregation dynamics, by either using an iso-concentration approach or an iso-concentration gradient approach to take into account diffusion.

Taylor-Goertler instabilities of Tollmien-Schlichting waves and other flows governed by the interactive boundary-layer equations

NASA Technical Reports Server (NTRS)

Hall, Philip; Bennett, James

1986-01-01

The Taylor-Goertler vortex instability equations are formulated for steady and unsteady interacting boundary-layer flows. The effective Goertler number is shown to be a function of the wall shape in the boundary layer and the possibility of both steady and unsteady Taylor-Goertler modes exists. As an example the steady flow in a symmetrically constricted channel is considered and it is shown that unstable Goertler vortices exist before the boundary layers at the wall develop the Goldstein singularity discussed by Smith and Daniels (1981). As an example of an unsteady spatially varying basic state, it is considered the instability of high-frequency large-amplitude two- and three-dimensional Tollmien-Schlichting waves in a curved channel. It is shown that they are unstable in the first 'Stokes-layer stage' of the hierarchy of nonlinear states discussed by Smith and Burggraf (1985). This instability of Tollmien-Schlichting waves in an internal flow can occur in the presence of either convex or concave curvature. Some discussion of this instability in external flows is given.

Role of edge superconducting states in trapping of multi-quanta vortices by microholes. Application of the bitter decoration technique

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bezryadin, A.; Pannetier, B.

1996-01-01

The Bitter decoration technique is used to study the trapping of single and multiple quanta vortices by a lattice of circular microholes. By keeping a thin superconducting layer (the bottom) inside each hole the authors are able to visualise the trapped vortices. From this they determine, for the first time, the filling factor FF, i.e. the number of vortices captured inside a hole. In all cases the sample is cooled at a constant field before making the decoration. Two qualitatively different states of the vortex crystal are observed: (1) In case when the interhole distance is much larger than themore » coherence length, the filling factor averaged over many identical holes () is a stepwise function of the magnetic flux (of the external field) through the hole, because each hole captures the same number of vortices. The density of fluxoids inside the openings is higher than in the uniform film, but much lower than it should be in the state of equilibrium. The authors claim that the number of trapped vortices is determined by the edge superconducting states which appear around each hole at the modified third critical field H{sub c3}* > H{sub c2}. Below H{sub c2} such states produce a surface barrier of a new type. This barrier for the vortex entrance and exit is due to the strong increase of the order parameter near the hole edge. It keeps constant the number of captured vortices during the cooling at a fixed field. (2) An increase of the hole density or of the hole radius initiates a sharp redistribution of fluxoids: all of them drop inside holes. This first order transition leads to a localization of all vortices and consequently to a qualitative change of the transport properties (TAFF in this case). In the resulting new state the filling factor is not any more the same for neighboring holes and its averaged value is equal to the frustration of the hole network.« less

Absolute/convective secondary instabilities and the role of confinement in free shear layers

NASA Astrophysics Data System (ADS)

Arratia, Cristóbal; Mowlavi, Saviz; Gallaire, François

2018-05-01

We study the linear spatiotemporal stability of an infinite row of equal point vortices under symmetric confinement between parallel walls. These rows of vortices serve to model the secondary instability leading to the merging of consecutive (Kelvin-Helmholtz) vortices in free shear layers, allowing us to study how confinement limits the growth of shear layers through vortex pairings. Using a geometric construction akin to a Legendre transform on the dispersion relation, we compute the growth rate of the instability in different reference frames as a function of the frame velocity with respect to the vortices. This approach is verified and complemented with numerical computations of the linear impulse response, fully characterizing the absolute/convective nature of the instability. Similar to results by Healey on the primary instability of parallel tanh profiles [J. Fluid Mech. 623, 241 (2009), 10.1017/S0022112008005284], we observe a range of confinement in which absolute instability is promoted. For a parallel shear layer with prescribed confinement and mixing length, the threshold for absolute/convective instability of the secondary pairing instability depends on the separation distance between consecutive vortices, which is physically determined by the wavelength selected by the previous (primary or pairing) instability. In the presence of counterflow and moderate to weak confinement, small (large) wavelength of the vortex row leads to absolute (convective) instability. While absolute secondary instabilities in spatially developing flows have been previously related to an abrupt transition to a complex behavior, this secondary pairing instability regenerates the flow with an increased wavelength, eventually leading to a convectively unstable row of vortices. We argue that since the primary instability remains active for large wavelengths, a spatially developing shear layer can directly saturate on the wavelength of such a convectively unstable row, by-passing the smaller wavelengths of absolute secondary instability. This provides a wavelength selection mechanism, according to which the distance between consecutive vortices should be sufficiently large in comparison with the channel width in order for the row of vortices to persist. We argue that the proposed wavelength selection criteria can serve as a guideline for experimentally obtaining plane shear layers with counterflow, which has remained an experimental challenge.

EDITORIAL: The FDR Prize The FDR Prize

NASA Astrophysics Data System (ADS)

Funakoshi, Mitsuaki

2011-08-01

From the 56 papers published in 2010 in Fluid Dynamics Research the following paper has been selected for the fourth FDR prize: 'Baroclinic multipole formation from heton interaction' by M A Sokolovskiy and X J Carton, published in volume 42 (August 2010) 045501. Coherent vortices are a universal feature of fluids at moderate and large Reynolds number, and have particular relevance to the quasi-two-dimensional flows used to model phenomena in the atmosphere and ocean. The structure and interaction of such vortices have proved a fascinating area for the researchers of fluid dynamics, including thoreticians, observers and experimentalists, together with related problems of how they mix fluids and how they transport scalars such as temperature and salinity. In this paper 'hetons' are considered; they are vortices of dipolar structures in a multilayer rotating fluid, carry thermal anomalies, and are relevant to transport in flows such as the Gulf Stream. The paper is a comprehensive study of the structure, invariants and interactions of two opposite-signed hetons in a two-layer fluid for several initial configurations and for several values of the Rossby radius of deformation, using models based on point vortex dynamics and contour dynamics of finite-area vortex regions. Different types of coupling and interactions are isolated and discussed. Depending on the initial configuration and the value of the radius of deformation, the time evolutions toward horizonal dipoles, vertically tilted dipoles, L-shaped dipoles, and Z-shaped tripoles are observed in the case of finite-area vortices. Using point vortex dynamics a rigorous analysis based on trilinear coordinates is performed, and the appearance of similar structures is shown analytically, except for the L-shaped dipoles. The contribution of this paper to the important problem of heton interaction is both profound and substantial. The study will be of great interest to a wide variety of readers and is likely to inspire further numerical and experimental work, as well being helpful in the interpretation and analysis of observations. Overall, the paper will undoubtedly have a large impact on the fluid dynamics community.

Conservative regularization of compressible dissipationless two-fluid plasmas

NASA Astrophysics Data System (ADS)

Krishnaswami, Govind S.; Sachdev, Sonakshi; Thyagaraja, A.

2018-02-01

This paper extends our earlier approach [cf. A. Thyaharaja, Phys. Plasmas 17, 032503 (2010) and Krishnaswami et al., Phys. Plasmas 23, 022308 (2016)] to obtaining à priori bounds on enstrophy in neutral fluids and ideal magnetohydrodynamics. This results in a far-reaching local, three-dimensional, non-linear, dispersive generalization of a KdV-type regularization to compressible/incompressible dissipationless 2-fluid plasmas and models derived therefrom (quasi-neutral, Hall, and ideal MHD). It involves the introduction of vortical and magnetic "twirl" terms λl 2 ( w l + ( q l / m l ) B ) × ( ∇ × w l ) in the ion/electron velocity equations ( l = i , e ) where w l are vorticities. The cut-off lengths λl and number densities nl must satisfy λl 2 n l = C l , where Cl are constants. A novel feature is that the "flow" current ∑ l q l n l v l in Ampère's law is augmented by a solenoidal "twirl" current ∑ l ∇ × ∇ × λl 2 j flow , l . The resulting equations imply conserved linear and angular momenta and a positive definite swirl energy density E * which includes an enstrophic contribution ∑ l ( 1 / 2 ) λl 2 ρ l wl 2 . It is shown that the equations admit a Hamiltonian-Poisson bracket formulation. Furthermore, singularities in ∇ × B are conservatively regularized by adding ( λB 2 / 2 μ 0 ) ( ∇ × B ) 2 to E * . Finally, it is proved that among regularizations that admit a Hamiltonian formulation and preserve the continuity equations along with the symmetries of the ideal model, the twirl term is unique and minimal in non-linearity and space derivatives of velocities.

Hybrid finite-difference/lattice Boltzmann simulations of microchannel and nanochannel acoustic streaming driven by surface acoustic waves

NASA Astrophysics Data System (ADS)

Tan, Ming K.; Yeo, Leslie Y.

2018-04-01

A two-dimensional hybrid numerical method that allows full coupling of the elastic motion in a piezoelectric solid (modeled using a finite-difference time-domain technique) with the resultant compressional flow in a fluid (simulated using a lattice Boltzmann scheme) is developed to study the acoustic streaming that arises in both microchannels and nanochannels under surface acoustic wave (SAW) excitation. In addition to verifying the model through a comparison of the simulations with results from experimental and numerical studies of microchannel and nanochannel flows driven by both standing and traveling SAWs in the literature, we highlight salient features of the flow field that arise and discuss the underlying mechanisms responsible for the flow. In microchannels, boundary layer streaming is the dominant mechanism when the channel height is below the sound wavelength in the liquid, whereas Eckart streaming—arising as a consequence of the attenuation of the sound wave in the liquid—dominates in the form of periodic vortices for larger channel heights. The absence of Eckart streaming and the overlapping of boundary layers in nanochannels with heights below the boundary layer thickness, on the other hand, give rise to a time-averaged dynamic acoustic pressure that results in an inertial-dominant flow, which paradoxically possesses a parabolic-like velocity profile resembling pressure-driven laminar flow. In contrast, if the nanochannel were to be filled instead with air, the significantly lower fluid density leads to a considerable reduction in the dynamic acoustic pressure and hence inertial forcing such that boundary layer streaming once again dominates, asymptotically imposing a slip condition along the channel surface that results in a negative pluglike velocity profile.

Optimal perturbations of a finite-width mixing layer near the trailing edge

NASA Astrophysics Data System (ADS)

Gumbart, James C.; Rabchuk, James

2002-03-01

The trailing edge of a surface separating two fluid flows can act as an efficient receptor for acoustic or other disturbances. The incident wave energy is converted by a linear mechanism into incipient flow instabilities which lead further downstream to the transition to turbulence. Understanding this process is essential for analyzing feedback loops and other resonances which can cause unwanted structural vibrations in the surface material or directed acoustic emissions from the mixing region. Previously, the modes of instability in a finite-width mixing layer near the trailing edge were studied as a function of frequency by assuming that vorticity was continually being introduced into the flow at the trailing edge by the forcing field. It was found that the initial amplitude of the growing instability mode was a sharply decreasing function of forcing frequency, and that the initial amplitude was a minimum for the frequency at which the rate of instability growth was a maximum^1. This result has led to a study of the adjoint equation for the perturbation stream function, whose eigensolutions are known to be associated with the optimal perturbation field for the frequency of forcing leading to the greatest instability growth downstream. We have obtained these solutions for a piecewise linear velocity profile near the trailing edge using group-theoretic techniques and have shown that they are indeed optimal. We have also analyzed the nature of the physical forcing field that might produce these optimal perturbations. ^1 Rabchuk, J.A., July 2000, Physics of Fluids.

Use of RORA for Complex Ground-Water Flow Conditions

USGS Publications Warehouse

Rutledge, A.T.

2004-01-01

The RORA computer program for estimating recharge is based on a condition in which ground water flows perpendicular to the nearest stream that receives ground-water discharge. The method, therefore, does not explicitly account for the ground-water-flow component that is parallel to the stream. Hypothetical finite-difference simulations are used to demonstrate effects of complex flow conditions that consist of two components: one that is perpendicular to the stream and one that is parallel to the stream. Results of the simulations indicate that the RORA program can be used if certain constraints are applied in the estimation of the recession index, an input variable to the program. These constraints apply to a mathematical formulation based on aquifer properties, recession of ground-water levels, and recession of streamflow.

Numerical study of comparison of vorticity and passive vectors in turbulence and inviscid flows

NASA Astrophysics Data System (ADS)

Ohkitani, Koji

2002-04-01

The nonlinear vortex stretching in incompressible Navier-Stokes turbulence is compared with a linear stretching process of passive vectors (PVs). In particular, we pay special attention to the difference of these processes under long and short time evolutions. For finite time evolution, we confirm our previous finding that the stretching effect of vorticity is weaker than that of general passive vectors for a majority of the initial conditions with the same energy spectra. The above difference can be explained qualitatively by examining the Biot-Savart formula. In order to see to what extent infinitesimal time development explains the above difference, we examine the probability density functions (PDFs) of the stretching rates of the passive vectors in the vicinity of a solution of Navier-Stokes equations. It is found that the PDFs are found to have a Gaussian distribution, suggesting that there are equally many PVs that stretched less and more than the vorticity. This suggests the importance of the vorticity-strain correlation built up over finite time in turbulence. We also discuss the case of Euler equations, where the dynamics of the Jacobian matrix relating the physical and material coordinates is examined numerically. A kind of alignment problem associated with the Cauchy-Green tensor is proposed and studied using the results of numerical simulations. It is found that vorticity tends to align itself with the most compressing eigenvector of the Cauchy-Green tensor. A two-dimensional counterpart of active-passive comparison is briefly studied. There is no essential difference between stretching of vorticity gradients and that of passive scalar gradients and a physical interpretation is given to it.

A Family of Vortices to Study Axisymmetric Vortex Breakdown and Reconnection

NASA Technical Reports Server (NTRS)

Young, Larry A.

2007-01-01

A new analytic model describing a family of vortices has been developed to study some of the axisymmetric vortex breakdown and reconnection fluid dynamic processes underlying body-vortex interactions that are frequently manifested in rotorcraft and propeller-driven fixed-wing aircraft wakes. The family of vortices incorporates a wide range of prescribed initial vorticity distributions -- including single or dual-core vorticity distributions. The result is analytical solutions for the vorticity and velocities for each member of the family of vortices. This model is of sufficient generality to further illustrate the dependence of vortex reconnection and breakdown on initial vorticity distribution as was suggested by earlier analytical work. This family of vortices, though laminar in nature, is anticipated to provide valuable insight into the vortical evolution of large-scale rotor and propeller wakes.

Relativistic thermodynamics, a Lagrangian field theory for general flows including rotation

NASA Astrophysics Data System (ADS)

Frønsdal, Christian

Any theory that is based on an action principle has a much greater predictive power than one that does not have such a formulation. The formulation of a dynamical theory of General Relativity, including matter, is here viewed as a problem of coupling Einstein’s theory of pure gravity to an independently chosen and well-defined field theory of matter. It is well known that this is accomplished in a most natural way when both theories are formulated as relativistic, Lagrangian field theories, as is the case with Einstein-Maxwell theory. Special matter models of this type have been available; here a more general thermodynamical model that allows for vortex flows is presented. In a wider context, the problem of subjecting hydrodynamics and thermodynamics to an action principle is one that has been pursued for at least 150 years. A solution to this problem has been known for some time, but only under the strong restriction to potential flows. A variational principle for general flows has become available. It represents a development of the Navier-Stokes-Fourier approach to fluid dynamics. The principal innovation is the recognition that two kinds of flow velocity fields are needed, one the gradient of a scalar field and the other the time derivative of a vector field, the latter closely associated with vorticity. In the relativistic theory that is presented here, the latter is the Hodge dual of an exact 3-form, well known as the notoph field of Ogievetskij and Palubarinov, the B-field of Kalb and Ramond and the vorticity field of Lund and Regge. The total number of degrees of freedom of a unary system, including the density and the two velocity fields is 4, as expected — as in classical hydrodynamics. In this paper, we do not reduce Einstein’s dynamical equation for the metric to phenomenology, which would have denied the relevance of any intrinsic dynamics for the matter sector, nor do we abandon the equation of continuity - the very soul of hydrodynamics.

An explicit canopy BRDF model and inversion. [Bidirectional Reflectance Distribution Function

NASA Technical Reports Server (NTRS)

Liang, Shunlin; Strahler, Alan H.

1992-01-01

Based on a rigorous canopy radiative transfer equation, the multiple scattering radiance is approximated by the asymptotic theory, and the single scattering radiance calculation, which requires an numerical intergration due to considering the hotspot effect, is simplified. A new formulation is presented to obtain more exact angular dependence of the sky radiance distribution. The unscattered solar radiance and single scattering radiance are calculated exactly, and the multiple scattering is approximated by the delta two-stream atmospheric radiative transfer model. The numerical algorithms prove that the parametric canopy model is very accurate, especially when the viewing angles are smaller than 55 deg. The Powell algorithm is used to retrieve biospheric parameters from the ground measured multiangle observations.

Methodology for Formulating Diesel Surrogate Fuels with Accurate Compositional, Ignition-Quality, and Volatility Characteristics

DOE PAGES

Mueller, Charles J.; Cannella, William J.; Bruno, Thomas J.; ...

2012-05-22

In this study, a novel approach was developed to formulate surrogate fuels having characteristics that are representative of diesel fuels produced from real-world refinery streams. Because diesel fuels typically consist of hundreds of compounds, it is difficult to conclusively determine the effects of fuel composition on combustion properties. Surrogate fuels, being simpler representations of these practical fuels, are of interest because they can provide a better understanding of fundamental fuel-composition and property effects on combustion and emissions-formation processes in internal-combustion engines. In addition, the application of surrogate fuels in numerical simulations with accurate vaporization, mixing, and combustion models could revolutionizemore » future engine designs by enabling computational optimization for evolving real fuels. Dependable computational design would not only improve engine function, it would do so at significant cost savings relative to current optimization strategies that rely on physical testing of hardware prototypes. The approach in this study utilized the state-of-the-art techniques of 13C and 1H nuclear magnetic resonance spectroscopy and the advanced distillation curve to characterize fuel composition and volatility, respectively. The ignition quality was quantified by the derived cetane number. Two well-characterized, ultra-low-sulfur #2 diesel reference fuels produced from refinery streams were used as target fuels: a 2007 emissions certification fuel and a Coordinating Research Council (CRC) Fuels for Advanced Combustion Engines (FACE) diesel fuel. A surrogate was created for each target fuel by blending eight pure compounds. The known carbon bond types within the pure compounds, as well as models for the ignition qualities and volatilities of their mixtures, were used in a multiproperty regression algorithm to determine optimal surrogate formulations. The predicted and measured surrogate-fuel properties were quantitatively compared to the measured target-fuel properties, and good agreement was found.« less

Simulation of spatially evolving turbulence and the applicability of Taylor's hypothesis in compressible flow

NASA Technical Reports Server (NTRS)

Lee, Sangsan; Lele, Sanjiva K.; Moin, Parviz

1992-01-01

For the numerical simulation of inhomogeneous turbulent flows, a method is developed for generating stochastic inflow boundary conditions with a prescribed power spectrum. Turbulence statistics from spatial simulations using this method with a low fluctuation Mach number are in excellent agreement with the experimental data, which validates the procedure. Turbulence statistics from spatial simulations are also compared to those from temporal simulations using Taylor's hypothesis. Statistics such as turbulence intensity, vorticity, and velocity derivative skewness compare favorably with the temporal simulation. However, the statistics of dilatation show a significant departure from those obtained in the temporal simulation. To directly check the applicability of Taylor's hypothesis, space-time correlations of fluctuations in velocity, vorticity, and dilatation are investigated. Convection velocities based on vorticity and velocity fluctuations are computed as functions of the spatial and temporal separations. The profile of the space-time correlation of dilatation fluctuations is explained via a wave propagation model.

Helical vortices: viscous dynamics and instability

NASA Astrophysics Data System (ADS)

Rossi, Maurice; Selcuk, Can; Delbende, Ivan; Ijlra-Upmc Team; Limsi-Cnrs Team

2014-11-01

Understanding the dynamical properties of helical vortices is of great importance for numerous applications such as wind turbines, helicopter rotors, ship propellers. Locally these flows often display a helical symmetry: fields are invariant through combined axial translation of distance Δz and rotation of angle θ = Δz / L around the same z-axis, where 2 πL denotes the helix pitch. A DNS code with built-in helical symmetry has been developed in order to compute viscous quasi-steady basic states with one or multiple vortices. These states will be characterized (core structure, ellipticity, ...) as a function of the pitch, without or with an axial flow component. The instability modes growing in the above base flows and their growth rates are investigated by a linearized version of the DNS code coupled to an Arnoldi procedure. This analysis is complemented by a helical thin-cored vortex filaments model. ANR HELIX.

Vortex variable range hopping in a conventional superconducting film

NASA Astrophysics Data System (ADS)

Percher, Ilana M.; Volotsenko, Irina; Frydman, Aviad; Shklovskii, Boris I.; Goldman, Allen M.

2017-12-01

The behavior of a disordered amorphous thin film of superconducting indium oxide has been studied as a function of temperature and magnetic field applied perpendicular to its plane. A superconductor-insulator transition has been observed, though the isotherms do not cross at a single point. The curves of resistance versus temperature on the putative superconducting side of this transition, where the resistance decreases with decreasing temperature, obey two-dimensional Mott variable-range hopping of vortices over wide ranges of temperature and resistance. To estimate the parameters of hopping, the film is modeled as a granular system and the hopping of vortices is treated in a manner analogous to hopping of charges. The reason the long-range interaction between vortices over the range of magnetic fields investigated does not lead to a stronger variation of resistance with temperature than that of two-dimensional Mott variable-range hopping remains unresolved.

Zero modes of the non-relativistic self-dual Chern-Simons vortices on the Toda backgrounds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoon, Yongsung

The two-dimensional self-dual equations are the governing equations of the static zero-energy vortex solutions for the non-relativistic, non-Abelian Chern-Simons models. The zero modes of the non-relativistic vortices are examined by index calculation for the self-dual equations. The index for the self-dual equations is zero for non-Abelian groups, but a non-zero index is obtained by the Toda Ansatz which reduces the self-dual equations to the Toda equations. The number of zero modes for the non-relativistic Toda vortices is 2 {Sigma}{sub {alpha},{beta}}{sup r}K{sub {alpha}{beta}}Q{sup {beta}} which is twice the total number of isolated zeros of the vortex functions. For the affine Todamore » system, there are additional adjoint zero modes which give a zero index for the SU(N) group.« less

Vortex/Body Interaction and Sound Generation in Low-Speed Flow

NASA Technical Reports Server (NTRS)

Kao, Hsiao C.

1998-01-01

The problem of sound generation by vortices interacting with an arbitrary body in a low-speed flow has been investigated by the method of matched asymptotic expansions. For the purpose of this report, it is convenient to divide the problem into three parts. In the first part the mechanism of the vortex/body interaction, which is essentially the inner solution in the inner region, is examined. The trajectories for a system of vortices rotating about their centroid are found to undergo enormous changes after interaction; from this, some interesting properties emerged. In the second part, the problem is formulated, the outer solution is found, matching is implemented, and solutions for acoustic pressure are obtained. In the third part, Fourier integrals are evaluated and predicated results presented. An examination of these results reveals the following: (a) the background noise can be either augmented or attenuated by a body after interaction, (b) sound generated by vortex/body interaction obeys a scaling factor, (C) sound intensity can be reduced substantially by positioning the vortex system in the "favorable" side of the body instead of the "unfavorable" side, and (d) acoustic radiation from vortex/bluff-body interaction is less than that from vortex/airfoil interaction under most circumstances.

Anelastic and Compressible Simulation of Moist Dynamics at Planetary Scales

NASA Astrophysics Data System (ADS)

Kurowski, M.; Smolarkiewicz, P. K.; Grabowski, W.

2015-12-01

Moist anelastic and compressible numerical solutions to the planetary baroclinic instability and climate benchmarks are compared. The solutions are obtained applying a consistent numerical framework for dis- crete integrations of the various nonhydrostatic flow equations. Moist extension of the baroclinic instability benchmark is formulated as an analog of the dry case. Flow patterns, surface vertical vorticity and pressure, total kinetic energy, power spectra, and total amount of condensed water are analyzed. The climate bench- mark extends the baroclinic instability study by addressing long-term statistics of an idealized planetary equilibrium and associated meridional transports. Short-term deterministic anelastic and compressible so- lutions differ significantly. In particular, anelastic baroclinic eddies propagate faster and develop slower owing to, respectively, modified dispersion relation and abbreviated baroclinic vorticity production. These eddies also carry less kinetic energy, and the onset of their rapid growth occurs later than for the compressible solutions. The observed differences between the two solutions are sensitive to initial conditions as they di- minish for large-amplitude excitations of the instability. In particular, on the climatic time scales, the anelastic and compressible solutions evince similar zonally averaged flow patterns with the matching meridional transports of entropy, momentum, and moisture.

Formulation of the relativistic moment implicit particle-in-cell method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Noguchi, Koichi; Tronci, Cesare; Zuccaro, Gianluca

2007-04-15

A new formulation is presented for the implicit moment method applied to the time-dependent relativistic Vlasov-Maxwell system. The new approach is based on a specific formulation of the implicit moment method that allows us to retain the same formalism that is valid in the classical case despite the formidable complication introduced by the nonlinear nature of the relativistic equations of motion. To demonstrate the validity of the new formulation, an implicit finite difference algorithm is developed to solve the Maxwell's equations and equations of motion. A number of benchmark problems are run: two stream instability, ion acoustic wave damping, Weibelmore » instability, and Poynting flux acceleration. The numerical results are all in agreement with analytical solutions.« less

Origins and demonstrations of electrons with orbital angular momentum

PubMed Central

Agrawal, Amit; Ercius, Peter A.; Grillo, Vincenzo; Herzing, Andrew A.; Harvey, Tyler R.; Linck, Martin; Pierce, Jordan S.

2017-01-01

The surprising message of Allen et al. (Allen et al. 1992 Phys. Rev. A 45, 8185 (doi:10.1103/PhysRevA.45.8185)) was that photons could possess orbital angular momentum in free space, which subsequently launched advancements in optical manipulation, microscopy, quantum optics, communications, many more fields. It has recently been shown that this result also applies to quantum mechanical wave functions describing massive particles (matter waves). This article discusses how electron wave functions can be imprinted with quantized phase vortices in analogous ways to twisted light, demonstrating that charged particles with non-zero rest mass can possess orbital angular momentum in free space. With Allen et al. as a bridge, connections are made between this recent work in electron vortex wave functions and much earlier works, extending a 175 year old tradition in matter wave vortices. This article is part of the themed issue ‘Optical orbital angular momentum’. PMID:28069765

Multipole Vortex Blobs (MVB): Symplectic Geometry and Dynamics.

PubMed

Holm, Darryl D; Jacobs, Henry O

2017-01-01

Vortex blob methods are typically characterized by a regularization length scale, below which the dynamics are trivial for isolated blobs. In this article, we observe that the dynamics need not be trivial if one is willing to consider distributional derivatives of Dirac delta functionals as valid vorticity distributions. More specifically, a new singular vortex theory is presented for regularized Euler fluid equations of ideal incompressible flow in the plane. We determine the conditions under which such regularized Euler fluid equations may admit vorticity singularities which are stronger than delta functions, e.g., derivatives of delta functions. We also describe the symplectic geometry associated with these augmented vortex structures, and we characterize the dynamics as Hamiltonian. Applications to the design of numerical methods similar to vortex blob methods are also discussed. Such findings illuminate the rich dynamics which occur below the regularization length scale and enlighten our perspective on the potential for regularized fluid models to capture multiscale phenomena.

Quantum corrections to the stress-energy tensor in thermodynamic equilibrium with acceleration

NASA Astrophysics Data System (ADS)

Becattini, F.; Grossi, E.

2015-08-01

We show that the stress-energy tensor has additional terms with respect to the ideal form in states of global thermodynamic equilibrium in flat spacetime with nonvanishing acceleration and vorticity. These corrections are of quantum origin and their leading terms are second order in the gradients of the thermodynamic fields. Their relevant coefficients can be expressed in terms of correlators of the stress-energy tensor operator and the generators of the Lorentz group. With respect to previous assessments, we find that there are more second-order coefficients and that all thermodynamic functions including energy density receive acceleration and vorticity dependent corrections. Notably, also the relation between ρ and p , that is, the equation of state, is affected by acceleration and vorticity. We have calculated the corrections for a free real scalar field—both massive and massless—and we have found that they increase, particularly for a massive field, at very high acceleration and vorticity and very low temperature. Finally, these nonideal terms depend on the explicit form of the stress-energy operator, implying that different stress-energy tensors of the scalar field—canonical or improved—are thermodynamically inequivalent.

Vortex nucleation in a dissipative variant of the nonlinear Schrödinger equation under rotation

DOE PAGES

Carretero-González, R.; Kevrekidis, P. G.; Kolokolnikov, T.

2016-03-01

In this work, we motivate and explore the dynamics of a dissipative variant of the nonlinear Schrödinger equation under the impact of external rotation. As in the well established Hamiltonian case, the rotation gives rise to the formation of vortices. We show, however, that the most unstable mode leading to this instability scales with an appropriate power of the chemical potential μ of the system, increasing proportionally to μ 2/3. The precise form of the relevant formula, obtained through our asymptotic analysis, provides the most unstable mode as a function of the atomic density and the trap strength. We showmore » how these unstable modes typically nucleate a large number of vortices in the periphery of the atomic cloud. However, through a pattern selection mechanism, prompted by symmetry-breaking, only few isolated vortices are pulled in sequentially from the periphery towards the bulk of the cloud resulting in highly symmetric stable vortex configurations with far fewer vortices than the original unstable mode. We conclude that these results may be of relevance to the experimentally tractable realm of finite temperature atomic condensates.« less

Direct numerical simulation of two-dimensional wall-bounded turbulent flows from receptivity stage.

PubMed

Sengupta, T K; Bhaumik, S; Bhumkar, Y G

2012-02-01

Deterministic route to turbulence creation in 2D wall boundary layer is shown here by solving full Navier-Stokes equation by dispersion relation preserving (DRP) numerical methods for flow over a flat plate excited by wall and free stream excitations. Present results show the transition caused by wall excitation is predominantly due to nonlinear growth of the spatiotemporal wave front, even in the presence of Tollmien-Schlichting (TS) waves. The existence and linear mechanism of creating the spatiotemporal wave front was established in Sengupta, Rao and Venkatasubbaiah [Phys. Rev. Lett. 96, 224504 (2006)] via the solution of Orr-Sommerfeld equation. Effects of spatiotemporal front(s) in the nonlinear phase of disturbance evolution have been documented by Sengupta and Bhaumik [Phys. Rev. Lett. 107, 154501 (2011)], where a flow is taken from the receptivity stage to the fully developed 2D turbulent state exhibiting a k(-3) energy spectrum by solving the Navier-Stokes equation without any artifice. The details of this mechanism are presented here for the first time, along with another problem of forced excitation of the boundary layer by convecting free stream vortices. Thus, the excitations considered here are for a zero pressure gradient (ZPG) boundary layer by (i) monochromatic time-harmonic wall excitation and (ii) free stream excitation by convecting train of vortices at a constant height. The latter case demonstrates neither monochromatic TS wave, nor the spatiotemporal wave front, yet both the cases eventually show the presence of k(-3) energy spectrum, which has been shown experimentally for atmospheric dynamics in Nastrom, Gage and Jasperson [Nature 310, 36 (1984)]. Transition by a nonlinear mechanism of the Navier-Stokes equation leading to k(-3) energy spectrum in the inertial subrange is the typical characteristic feature of all 2D turbulent flows. Reproduction of the spectrum noted in atmospheric data (showing dominance of the k(-3) spectrum over the k(-5/3) spectrum in Nastrom et al.) in laboratory scale indicates universality of this spectrum for all 2D turbulent flows. Creation of universal features of 2D turbulence by a deterministic route has been established here for the first time by solving the Navier-Stokes equation without any modeling, as has been reported earlier in the literature by other researchers.

Resonant Formation and Control of m-Fold Symmetric V-States

NASA Astrophysics Data System (ADS)

Friedland, Lazar; Shagalov, Arkadi

2000-10-01

Magnetized, pure electron plasmas trapped in a Malmberg-Penning trap can be modeled (in the drift approximation) by two-dimensional Euler equations of ideal fluids. The plasma density in this approximation is analogous to vorticity, while the radial electric field potential to the stream function of the fluid velocity field. For instance, electron plasma cylinder aligned with the magnetic field is analogous to a circular vortex patch solution of an ideal fluid. We shall show that by starting in such a circular equilibrium one can drive an m-fold symmetric interface (vortex) waves in two dimensions (V-states, discovered by Deem and Zabusky [1] nearly 20 years ago)into a highly nonlinear excitation by applying a weak external oscillating potential of appropriate symmetry and slowly varying the frequency of these oscillations. The phenomenon is due to autoresonance [2,3] in the system as the excited plasma (vortex) boundary preserves its functional form despite the drive, but self-adjusts the aspect ratio to synchronize with the driving potential oscillations. A similar approach can be used in controlling interface dynamics subject to global constraints in many other fields of physics. Work supported by Israel Science Foundation grant 607-97 and INTAS grant 99-1068. [1] G. Deem and N. Zabusky, Phys. Rev. Lett. 40, 859 (1978). [2] L. Friedland, Phys. Rev. E, 4106 (1999). [3] J. Fajans, E. Gilson, and L. Friedland, Phys. Rev. Lett. 82, 4444 (1999).

A comparison of Fick and Maxwell-Stefan diffusion formulations in PEMFC gas diffusion layers

NASA Astrophysics Data System (ADS)

Lindstrom, Michael; Wetton, Brian

2017-01-01

This paper explores the mathematical formulations of Fick and Maxwell-Stefan diffusion in the context of polymer electrolyte membrane fuel cell cathode gas diffusion layers. The simple Fick law with a diagonal diffusion matrix is an approximation of Maxwell-Stefan. Formulations of diffusion combined with mass-averaged Darcy flow are considered for three component gases. For this application, the formulations can be compared computationally in a simple, one dimensional setting. Despite the models' seemingly different structure, it is observed that the predictions of the formulations are very similar on the cathode when air is used as oxidant. The two formulations give quite different results when the Nitrogen in the air oxidant is replaced by helium (this is often done as a diagnostic for fuel cells designs). The two formulations also give quite different results for the anode with a dilute Hydrogen stream. These results give direction to when Maxwell-Stefan diffusion, which is more complicated to implement computationally in many codes, should be used in fuel cell simulations.

Flow Structures and Noise Produced by a Heated Rectangular Nozzle with a Third Stream and Aft Deck

NASA Astrophysics Data System (ADS)

Ruscher, Christopher; Gogineni, Sivaram; Kiel, Barry

2015-11-01

Jet noise is a huge issue that affects both civilian and military aviation and is a two-fold problem. Near-field noise causes hearing damage and is of great concern to the Navy. Far-field noise is also a concern for military and civilian aircraft. For military jets, the trend has shown that newer and more advanced planes are louder than their predecessors. Most of these planes are designed keeping the performance as the main driver in mind while the jet noise becomes an afterthought. To remedy this and to aid the design process, we propose to create a joint noise and performance prediction tool. To create this tool, one must understand how the near-field flow structures generate noise and how they are related to far-field noise. In the current work, we considered rectangular, three-stream nozzle with an aft deck and investigated the flow structures such as corner vortices, shocks and their impact on the noise generation mechanism. We have also used state-of-the-art data analytical tools such as wavelets, POD, and stochastic estimations.

An analytical design procedure for the determination of effective leading edge extensions on thick delta wings

NASA Technical Reports Server (NTRS)

Ghaffari, F.; Chaturvedi, S. K.

1984-01-01

An analytical design procedure for leading edge extensions (LEE) was developed for thick delta wings. This LEE device is designed to be mounted to a wing along the pseudo-stagnation stream surface associated with the attached flow design lift coefficient of greater than zero. The intended purpose of this device is to improve the aerodynamic performance of high subsonic and low supersonic aircraft at incidences above that of attached flow design lift coefficient, by using a vortex system emanating along the leading edges of the device. The low pressure associated with these vortices would act on the LEE upper surface and the forward facing area at the wing leading edges, providing an additional lift and effective leading edge thrust recovery. The first application of this technique was to a thick, round edged, twisted and cambered wing of approximately triangular planform having a sweep of 58 deg and aspect ratio of 2.30. The panel aerodynamics and vortex lattice method with suction analogy computer codes were employed to determine the pseudo-stagnation stream surface and an optimized LEE planform shape.

Influence of pressure driven secondary flows on the behavior of turbofan forced mixers

NASA Technical Reports Server (NTRS)

Anderson, B.; Povinelli, L.; Gerstenmaier, W.

1980-01-01

A finite difference procedure was developed to analyze the three dimensional subsonic turbulent flows in turbofan forced mixer nozzles. The method is based on a decomposition of the velocity field into primary and secondary flow components which are determined by solution of the equations governing primary momentum, secondary vorticity, thermal energy, and continuity. Experimentally, a strong secondary flow pattern was identified which is associated with the radial inflow and outflow characteristics of the core and fan streams and forms a very strong vortex system aligned with the radial interface between the core and fan regions. A procedure was developed to generate a similar generic secondary flow pattern in terms of two constants representing the average radial outflow or inflow in the core and fan streams as a percentage of the local streamwise velocity. This description of the initial secondary flow gave excellent agreement with experimental data. By identifying the nature of large scale secondary flow structure and associating it with characteristic mixer nozzle behavior, it is felt that the cause and effect relationship between lobe design and nozzle performance can be understood.

Mean-flow measurements of the flow field diffusing bend

NASA Technical Reports Server (NTRS)

Mcmillan, O. J.

1982-01-01

Time-average measurements of the low-speed turbulent flow in a diffusing bend are presented. The experimental geometry consists of parallel top and bottom walls and curved diverging side walls. The turning of the center line of this channel is 40 deg, the area ratio is 1.5 and the ratios of height and center-line length to throat width are 1.5 and 3, respectively. The diffusing bend is preceded and followed by straight constant area sections. The inlet boundary layers on the parallel walls are artificially thickened and occupy about 30% of the channel height; those on the side walls develop naturally and are about half as thick. The free-stream speed at the inlet was approximately 30 m/sec for all the measurements. Inlet boundary layer mean velocity and turbulence intensity profiles are presented, as are data for wall static pressures, and at six cross sections, surveys of the velocity-vector and static-pressure fields. The dominant feature of the flow field is a pair of counter-rotating streamwise vortices formed by the cross-stream pressure gradient in the bend on which an overall deceleration is superimposed.