Experimental study of premixed flames in intense isotropic turbulence
Bedat, B.; Cheng, R.K.
1994-04-01
A methodology for investigating premixed turbulent flames propagating in intense isotropic turbulence has been developed. The burner uses a turbulence generator developed by Videto and Santavicca and the flame is stabilized by weak-swirl generated by air injectors. This set-up produces stable premixed turbulent flames under a wide range of mixture conditions and turbulence intensities. The experiments are designed to investigate systematically the changes in flame structures for conditions which can be classified as wrinkled laminar flames, corrugated flames and flames with distributed reaction zones. Laser Doppler anemometry and Rayleigh scattering techniques are used to determine the turbulence and scalar statistics. In the intense turbulence, the flames are found to produce very little changes in the mean and rams velocities. Their flame speed increase linearly with turbulence intensity as for wrinkled laminar flames. The Rayleigh scattering pdfs for flames within the distributed reaction zone regime are distinctly bimodal. The probabilities of the reacting states (i.e. contributions from within the reaction zone) is not higher than those of wrinkled laminar flame. These results show that there is no drastic changes in flame structures at Karlovitz number close to unity. This suggest that the Klimov-Williams criterion under-predicts the resilience of wrinkled flamelets to intense turbulence.
The structure of intense vorticity in homogeneous isotropic turbulence
NASA Technical Reports Server (NTRS)
Jimenez, J.; Wray, A. A.; Saffman, P. G.; Rogallo, R. S.
1992-01-01
The structure of the intense vorticity regions is studied in numerically simulated homogeneous, isotropic, equilibrium turbulent flow fields at four different Reynolds numbers in the range Re(sub lambda) = 36-171. In accordance with previous investigators, this vorticity is found to be organized in coherent, cylindrical or ribbon-like, vortices ('worms'). A statistical study suggests that they are just especially intense features of the background, O(omega'), vorticity. Their radii scale with the Kolmogorov microscale and their lengths with the integral scale of the flow. An interesting observation is that the Reynolds number based on the circulation of the intense vortices, gamma/nu, increases monotonically with Re(sub lambda), raising the question of the stability of the structures in the limit of Re(sub lambda) approaching infinity. One and two-dimensional statistics of vorticity and strain are presented; they are non-gaussian, and the behavior of their tails depends strongly on the Reynolds number. There is no evidence of convergence to a limiting distribution in our range of Re(sub lambda), even though the energy spectra and the energy dissipation rate show good asymptotic properties in the higher Reynolds number cases. Evidence is presented to show that worms are natural features of the flow and that they do not depend on the particular forcing scheme.
Linearly Forced Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Lundgren, T. S.
2003-01-01
Stationary isotropic turbulence is often studied numerically by adding a forcing term to the Navier-Stokes equation. This is usually done for the purpose of achieving higher Reynolds number and longer statistics than is possible for isotropic decaying turbulence. It is generally accepted that forcing the Navier-Stokes equation at low wave number does not influence the small scale statistics of the flow provided that there is wide separation between the largest and smallest scales. It will be shown, however, that the spectral width of the forcing has a noticeable effect on inertial range statistics. A case will be made here for using a broader form of forcing in order to compare computed isotropic stationary turbulence with (decaying) grid turbulence. It is shown that using a forcing function which is directly proportional to the velocity has physical meaning and gives results which are closer to both homogeneous and non-homogeneous turbulence. Section 1 presents a four part series of motivations for linear forcing. Section 2 puts linear forcing to a numerical test with a pseudospectral computation.
DNS of Shock / Isotropic Turbulence Interaction
NASA Astrophysics Data System (ADS)
Grube, Nathan; Taylor, Ellen; Martín, Pino
2010-11-01
We discuss DNS of Shock / Isotropic Turbulence Interactions (SITI). We vary the incoming turbulence Mach number up to 0.8 and the convective Mach number up to 5 in order to determine their effects on the interaction. These cases are challenging due to the presence of shocklets in the incoming turbulence as well as significant motion of the main shock. Shock-capturing must be used at all points while still maintaining low enough numerical dissipation to preserve the turbulent fluctuations. We use the linearly- and nonlinearly-optimized Weighted Essentially Non-Oscillatory (WENO) method[1,2]. Particular attention is paid to the inflow boundary condition, where we find the use of snapshots of "frozen" turbulence from decaying isotropic box simulations to be unsatisfactory. We instead use time-varying inflow data generated by a separate forced isotropic turbulence simulation with a specified convection speed. This allows us to access flow conditions where the assumptions of Taylor's Hypothesis are not met. 1.) Mart'in, M.P., Taylor, E.M., Wu, M., and Weirs, V.G., JCP 220(1) 270-89, 2006. 2.) Taylor, E.M., Wu, M., and Mart'in, M.P., JCP 223(1) 384-97, 2007.
On the decay of homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
Skrbek, L.; Stalp, Steven R.
2000-08-01
Decaying homogeneous, isotropic turbulence is investigated using a phenomenological model based on the three-dimensional turbulent energy spectra. We generalize the approach first used by Comte-Bellot and Corrsin [J. Fluid Mech. 25, 657 (1966)] and revised by Saffman [J. Fluid Mech. 27, 581 (1967); Phys. Fluids 10, 1349 (1967)]. At small wave numbers we assume the spectral energy is proportional to the wave number to an arbitrary power. The specific case of power 2, which follows from the Saffman invariant, is discussed in detail and is later shown to best describe experimental data. For the spectral energy density in the inertial range we apply both the Kolmogorov -5/3 law, E(k)=Cɛ2/3k-5/3, and the refined Kolmogorov law by taking into account intermittency. We show that intermittency affects the energy decay mainly by shifting the position of the virtual origin rather than altering the power law of the energy decay. Additionally, the spectrum is naturally truncated due to the size of the wind tunnel test section, as eddies larger than the physical size of the system cannot exist. We discuss effects associated with the energy-containing length scale saturating at the size of the test section and predict a change in the power law decay of both energy and vorticity. To incorporate viscous corrections to the model, we truncate the spectrum at an effective Kolmogorov wave number kη=γ(ɛ/v3)1/4, where γ is a dimensionless parameter of order unity. We show that as the turbulence decays, viscous corrections gradually become more important and a simple power law can no longer describe the decay. We discuss the final period of decay within the framework of our model, and show that care must be taken to distinguish between the final period of decay and the change of the character of decay due to the saturation of the energy containing length scale. The model is applied to a number of experiments on decaying turbulence. These include the downstream decay of turbulence in
Small scale dynamics of isotropic viscoelastic turbulence
NASA Astrophysics Data System (ADS)
Nguyen, M. Quan; Delache, Alexandre; Simoëns, Serge; Bos, Wouter J. T.; El Hajem, Mamoud
2016-12-01
The comparison of the results of direct numerical simulations of isotropic turbulence of Newtonian and viscoelastic fluid provides evidence that viscoelasticity modifies qualitatively the behavior of the smallest scales: we observe a power law in the far dissipation range of the fluid kinetic energy spectrum and we show that it is a robust feature, roughly independent of the large scale dynamics. A detailed analysis of the energy transfer shows that at these scales energy is injected into the fluid flow through polymer relaxation. It is further shown that a part of the total energy is transferred among scales through an interaction of the velocity field with the polymer field.
Near isotropic behavior of turbulent thermal convection
NASA Astrophysics Data System (ADS)
Nath, Dinesh; Pandey, Ambrish; Kumar, Abhishek; Verma, Mahendra K.
2016-10-01
We investigate the anisotropy in turbulent convection in a three-dimensional (3D) box using direct numerical simulation. We compute the anisotropic parameter A =u⊥2/(2 u∥2) , where u⊥ and u∥ are the components of velocity perpendicular and parallel to the buoyancy direction, the shell and ring spectra, and shell-to-shell energy transfers. We observe that the flow is nearly isotropic for the Prandtl number Pr ≈1 , but the anisotropy increases with the Prandtl number. For Pr =∞ ,A ≈0.3 , anisotropy is not very significant even in extreme cases. We also observe that u∥ feeds energy to u⊥ via pressure. The computation of shell-to-shell energy transfers reveals that the energy transfer in turbulent convection is local and forward, similar to hydrodynamic turbulence. These results are consistent with the Kolmogorov's spectrum observed by Kumar et al. [Phys. Rev. E 90, 023016 (2014), 10.1103/PhysRevE.90.023016] for turbulent convection.
Spectra and statistics in compressible isotropic turbulence
NASA Astrophysics Data System (ADS)
Wang, Jianchun; Gotoh, Toshiyuki; Watanabe, Takeshi
2017-01-01
Spectra and one-point statistics of velocity and thermodynamic variables in isotropic turbulence of compressible fluid are examined by using numerical simulations with solenoidal forcing at the turbulent Mach number Mt from 0.05 to 1.0 and at the Taylor Reynolds number Reλ from 40 to 350. The velocity field is decomposed into a solenoidal component and a compressible component in terms of the Helmholtz decomposition, and the compressible velocity component is further decomposed into a pseudosound component, namely, the hydrodynamic component associated with the incompressible field and an acoustic component associated with sound waves. It is found that the acoustic mode dominates over the pseudosound mode at turbulent Mach numbers Mt≥0.4 in our numerical simulations. At turbulent Mach numbers Mt≤0.4 , there exists a critical wave number kc beyond which the pseudosound mode dominates while the acoustic mode dominates at small wave numbers k
Shocklet statistics in compressible isotropic turbulence
NASA Astrophysics Data System (ADS)
Wang, Jianchun; Gotoh, Toshiyuki; Watanabe, Takeshi
2017-02-01
Shocklet statistics in compressible isotropic turbulence are studied by using numerical simulations with solenoidal forcing, at the turbulent Mach number Mt ranging from 0.5 up to 1.0 and at the Taylor Reynolds number Reλ ranging from 110 to 250. A power-law region of the probability density function (PDF) of the shocklet strength Mn-1 (Mn is the normal shock Mach number) is observed. The magnitude of the power-law exponent is found to decrease with the increase of Mt. We show that the most probable shocklet strength is proportional to Mt3, and the shocklet thickness corresponding to the most probable shock Mach number is proportional to Mt-2 in our numerical simulations. The PDFs of the jumps of the velocity and thermodynamic variables across a shocklet exhibit a similar power-law scaling. The statistics of the jumps of the velocity and thermodynamic variables are further investigated by conditioned average. Nonlinear models for the conditional average of the jumps of the velocity and thermodynamic variables are developed and verified.
Helicity statistics in homogeneous and isotropic turbulence and turbulence models
NASA Astrophysics Data System (ADS)
Sahoo, Ganapati; De Pietro, Massimo; Biferale, Luca
2017-02-01
We study the statistical properties of helicity in direct numerical simulations of fully developed homogeneous and isotropic turbulence and in a class of turbulence shell models. We consider correlation functions based on combinations of vorticity and velocity increments that are not invariant under mirror symmetry. We also study the scaling properties of high-order structure functions based on the moments of the velocity increments projected on a subset of modes with either positive or negative helicity (chirality). We show that mirror symmetry is recovered at small scales, i.e., chiral terms are subleading and they are well captured by a dimensional argument plus anomalous corrections. These findings are also supported by a high Reynolds numbers study of helical shell models with the same chiral symmetry of Navier-Stokes equations.
Some Basic Laws of Isotropic Turbulent Flow
NASA Technical Reports Server (NTRS)
Loitsianskii, L. G.
1945-01-01
An Investigation is made of the diffusion of artificially produced turbulence behind screens or other turbulence producers. The method is based on the author's concept of disturbance moment as a certain theoretically well-founded measure of turbulent disturbances.
The signature of initial production mechanisms in isotropic turbulence decay
NASA Astrophysics Data System (ADS)
Meldi, M.
2016-03-01
In the present work the quantification of the time-lasting effects of production mechanisms in homogeneous isotropic turbulence decay is addressed. The analysis is developed through the use of theoretical tools as well as numerical calculations based on the eddy damped quasinormal Markovian (EDQNM) model. In both cases a modified Lin equation is used, which accounts for production mechanisms as proposed by Meldi, Lejemble, and Sagaut ["On the emergence of non-classical decay regimes in multiscale/fractal generated isotropic turbulence," J. Fluid Mech. 756, 816-843 (2014)]. The approaches used show that an exponential decay law can be observed if the intensity of the forcing is strong enough to drive the turbulence dynamics, before a power-law decay is eventually attained. The EDQNM numerical results indicate that the exponential regime can persist for long evolution times, longer than the observation time in grid turbulence experiments. A rigorous investigation of the self-similar behavior of the pressure spectrum has been performed by a comprehensive comparison of EDQNM data with direct numerical simulation (DNS)/experiments in the literature. While DNS and free decay EDQNM simulations suggest the need of a very high Reλ threshold in order to observe a clear -7/3 slope of the pressure inertial range, experimental data and forced EDQNM calculations indicate a significantly lower value. This observation suggests that the time-lasting effects of production mechanisms, which cannot be excluded in experiments, play a role in the lack of general agreement with classical numerical approaches. These results reinforce the urge to evolve the numerical simulation state of the art towards the prediction of realistic physical states.
The radiated noise from isotropic turbulence revisited
NASA Technical Reports Server (NTRS)
Lilley, Geoffrey M.
1993-01-01
The noise radiated from isotropic turbulence at low Mach numbers and high Reynolds numbers, as derived by Proudman (1952), was the first application of Lighthill's Theory of Aerodynamic Noise to a complete flow field. The theory presented by Proudman involves the assumption of the neglect of retarded time differences and so replaces the second-order retarded-time and space covariance of Lighthill's stress tensor, Tij, and in particular its second time derivative, by the equivalent simultaneous covariance. This assumption is a valid approximation in the derivation of the second partial derivative of Tij/derivative of t exp 2 covariance at low Mach numbers, but is not justified when that covariance is reduced to the sum of products of the time derivatives of equivalent second-order velocity covariances as required when Gaussian statistics are assumed. The present paper removes these assumptions and finds that although the changes in the analysis are substantial, the change in the numerical result for the total acoustic power is small. The present paper also considers an alternative analysis which does not neglect retarded times. It makes use of the Lighthill relationship, whereby the fourth-order Tij retarded-time covariance is evaluated from the square of similar second order covariance, which is assumed known. In this derivation, no statistical assumptions are involved. This result, using distributions for the second-order space-time velocity squared covariance based on the Direct Numerical Simulation (DNS) results of both Sarkar and Hussaini(1993) and Dubois(1993), is compared with the re-evaluation of Proudman's original model. These results are then compared with the sound power derived from a phenomenological model based on simple approximations to the retarded-time/space covariance of Txx. Finally, the recent numerical solutions of Sarkar and Hussaini(1993) for the acoustic power are compared with the results obtained from the analytic solutions.
Sudden relaminarisation and lifetimes in forced isotropic turbulence
NASA Astrophysics Data System (ADS)
Linkmann, Moritz; Morozov, Alexander
2015-11-01
We demonstrate an unexpected connection between isotropic turbulence and wall-bounded shear flows. We perform direct numerical simulations of isotropic turbulence forced at large scales at moderate Reynolds numbers and observe sudden transitions from chaotic dynamics to a spatially simple flow, analogous to the laminar state in wall bounded shear flows. We find that the survival probabilities of turbulence are exponential and the typical lifetimes increase super-exponentially with the Reynolds number, similar to results on relaminarisation of localised turbulence in pipe and plane Couette flow. Results from simulations subjecting the observed large-scale flow to random perturbations of variable amplitude demonstrate that it is a linearly stable simple exact solution that can be destabilised by a finite-amplitude perturbation, like the Hagen-Poiseuille profile in pipe flow. Our results suggest that both isotropic turbulence and wall-bounded shear flows qualitatively share the same phase-space dynamics.
The energy decay in self-preserving isotropic turbulence revisited
NASA Technical Reports Server (NTRS)
Speziale, Charles G.; Bernard, Peter S.
1992-01-01
The assumption of self-preservation allows for an analytical determination of the energy decay in isotropic turbulence. Here, the self-preserving isotropic decay problem is analyzed, yielding a more complete picture of self-serving isotropic turbulence. It is proven rigorously that complete self-serving isotropic turbulence admits two general types of asymptotic solutions: one where the turbulent kinetic energy K approximately t (exp -1) and one where K approximately t (sup alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one-point equations, it is demonstrated that the K approximately t (exp -1) and where K approximately t (sup -alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one-point equations, it is demonstrated that the K approximately t (exp -1) power law decay is the asymptotically consistent high Reynolds number solution; the K approximately 1 (sup -alpha) decay law is only achieved in the limit as t yields infinity and the turbulence Reynolds number vanishes. Arguments are provided which indicate that a K approximately t (exp -1) power law decay is the asymptotic state toward which a complete self-preserving isotropic turbulence is driven at high Reynolds numbers in order to resolve the imbalance between vortex stretching and viscous diffusion.
The energy decay in self-preserving isotropic turbulence revisited
NASA Technical Reports Server (NTRS)
Speziale, Charles G.; Bernard, Peter S.
1991-01-01
The assumption of self-preservation allows for an analytical determination of the energy decay in isotropic turbulence. Here, the self-preserving isotropic decay problem is analyzed, yielding a more complete picture of self-serving isotropic turbulence. It is proven rigorously that complete self-serving isotropic turbulence admits two general types of asymptotic solutions: one where the turbulent kinetic energy K approximately t (exp -1) and one where K approximately t (sup alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one-point equations, it is demonstrated that the K approximately t (exp -1) and where K approximately t (sup -alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one point equations, it is demonstrated that the K approximately t (exp -1) power law decay is the asymptotically consistent high Reynolds number solution; the K approximately 1 (sup - alpha) decay law is only achieved in the limit as t yields infinity and the turbulence Reynolds number vanishes. Arguments are provided which indicate that a K approximately t (exp -1) power law decay is the asymptotic state towards which a complete self-preseving isotropic turbulence is driven at high Reynolds numbers in order to resolve the imbalance between vortex stretching and viscous diffusion.
Particle dynamics during the transition from isotropic to anisotropic turbulence
NASA Astrophysics Data System (ADS)
Lee, Chung-Min; Gylfason, Armann; Toschi, Federico
2016-11-01
Turbulent fluctuations play an important role on the dynamics of particles in turbulence, enhancing their dispersion and mixing. In recent years the statistical properties of particles in several statistically stationary turbulent flows have been the subject of many numerical and experimental studies. In many natural and industrial environments, however, one deals with turbulence in a transient state. As a prototype system, we investigate the transition from an isotropic to an anisotropic flow, namely looking at the influence of a developing mean flow on the dynamics of particles. We simulate, via direct numerical simulation, stationary homogeneous and isotropic turbulence and then suddenly impose a mean shear or strain. This allows us to quantify the effects of the mean flow on particle dynamics in these transient periods. Preliminary results on single particle properties, such as velocities and accelerations will be reported.
Diffusion of Heat from a Line Source in Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Uberoi, Mahinder S; Corrsin, Stanley
1953-01-01
An experimental and analytical study has been made of some features of the turbulent heat diffusion behind a line heated wire stretched perpendicular to a flowing isotropic turbulence. The mean temperature distributions have been measured with systematic variations in wind speed, size of turbulence-producing grid, and downstream location of heat source. The nature of the temperature fluctuation field has been studied. A comparison of Lagrangian and Eulerian analyses for diffusion in a nondecaying turbulence yields an expression for turbulent-heat-transfer coefficient in terms of turbulence velocity and a Lagrangian "scale." the ratio of Eulerian to Lagrangian microscale has been determined theoretically by generalization of a result of Heisenberg and with arbitrary constants taken from independent sources, shows rough agreement with experimental results. A convenient form has been deduced for the criterion of interchangeability of instantaneous space and time derivatives in a flowing turbulence.
Studies of Shock Wave Interactions with Homogeneous and Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Briassulis, G.; Agui, J.; Watkins, C. B.; Andreopoulos, Y.
1998-01-01
A nearly homogeneous nearly isotropic compressible turbulent flow interacting with a normal shock wave has been studied experimentally in a large shock tube facility. Spatial resolution of the order of 8 Kolmogorov viscous length scales was achieved in the measurements of turbulence. A variety of turbulence generating grids provide a wide range of turbulence scales. Integral length scales were found to substantially decrease through the interaction with the shock wave in all investigated cases with flow Mach numbers ranging from 0.3 to 0.7 and shock Mach numbers from 1.2 to 1.6. The outcome of the interaction depends strongly on the state of compressibility of the incoming turbulence. The length scales in the lateral direction are amplified at small Mach numbers and attenuated at large Mach numbers. Even at large Mach numbers amplification of lateral length scales has been observed in the case of fine grids. In addition to the interaction with the shock the present work has documented substantial compressibility effects in the incoming homogeneous and isotropic turbulent flow. The decay of Mach number fluctuations was found to follow a power law similar to that describing the decay of incompressible isotropic turbulence. It was found that the decay coefficient and the decay exponent decrease with increasing Mach number while the virtual origin increases with increasing Mach number. A mechanism possibly responsible for these effects appears to be the inherently low growth rate of compressible shear layers emanating from the cylindrical rods of the grid.
Cosmic-ray pitch-angle scattering in isotropic turbulence
NASA Technical Reports Server (NTRS)
Bieber, John W.; Smith, Charles W.; Matthaeus, William H.
1988-01-01
A dissipation range is incorporated in the turbulence model to reconcile the divergent conclusions from studies of cosmic-ray pitch-angle scattering in isotropic magnetic turbulence. The Fokker-Planck coefficient for pitch-angle scattering is calculated. It is shown that the slab form of the Fokker-Plank coefficient (Jokipii, 1966) is valid at very low energies, while the nonslab form (Fisk, 1974) is valid at intermediate energies.
Assessing the Structure of Isotropic and Anisotropic Turbulent Magnetic Fields
NASA Astrophysics Data System (ADS)
Fatuzzo, Marco; Holden, Lisa; Grayson, Lindsay; Wallace, Kirk
2016-10-01
Turbulent magnetic fields permeate our universe, impacting a wide range of astronomical phenomena across all cosmic scales. A clear example is the magnetic field that threads the interstellar medium (ISM), which impacts the motion of cosmic rays through that medium. Understanding the structure of magnetic turbulence within the ISM and how it relates to the physical quantities that characterize it can thus inform our analysis of particle transport within these regions. Toward that end, we probe the structure of magentic turbulence through the use of Lyapunov exponents for a suite of isotropic and nonisotropic Alfvénic turbulence profiles. Our results provide a means of calculating a “turbulence lengthscale” that can then be connected to how cosmic rays propagate through magentically turbulent environments, and we perform such an analysis for molecular cloud environments.
Charge pariticle transport in the non-isotropic turbulences
NASA Astrophysics Data System (ADS)
Sun, P.; Jokipii, J. R.
2015-12-01
The scattering and diffusion of energetic charged particles is not only important for understanding phenomena such as diffusive shock acceleration but it also is a natural probe of the statistical characteristics of magnetohydrodynamic (MHD) turbulence. Although Parker's transport equation (Parker 1965) allows us to describe the propagation of charged particles, the transport coefficients needed in the equation must be determined. Using Quasi-Linear Theory (QLT, e.g. Jokipii (1966)), one finds that coefficients can be related to the correlation function or power spectrum of homogeneous magnetic turbulence. However, different turbulence models will generally have a different influence on particle's scattering and diffusion. Among those models developed in MHD Turbulence, such as isotropic, Slab plus 2D (Tu & Marsch 1993; Gray et al 1996; Bieber et al 1996), etc. Here, using test-particle orbit simulations to calculate the transport coefficients, we study particle transport in synthesized asymmetric turbulence using the form first proposed by Goldreich & Sridhar (1995). We developed and introduce a systematic method to synthesize scale-dependent non-isotropic magnetic turbulences. We also developed and introduce a method to synthesize the 3d turbulent magnetic field from the observed solar wind time series dataset. We present the comparison of their effects on charge particle transport with previous theories and models.
The decay of isotropic turbulence in a rapidly rotating frame
NASA Technical Reports Server (NTRS)
Speziale, C. G.; Mansour, N. N.; Rogallo, R. S.
1987-01-01
A direct numerical simulation of the decay of initially isotropic turbulence in a rapidly rotating frame was conducted. This 128 x 128 x 128 simulation was completed for a Reynolds number Re sub lambda = 15.3 and a Rossby number Ro sub lambda = 0.07 based on the initial turbulent kinetic energy and Taylor microscale. The numerical results indicate that the turbulence remains essentially isotropic during the major part of the decay (i.e., beyond the point where the turbulent kinetic energy has decayed to less than 10 percent of its initial value). The rapid rotation has the primary effect of shutting off the energy transfer so that the turbulence dissipation (and hence the rate of decay of the turbulent kinetic energy) is substantially reduced. Consequently, the anisotropy tensor remains essentially unchanged while the energy spectrum undergoes a nearly linear viscous decay (the same results that are predicted by Rapid Distortion Theory which is only formally valid for much shorter elapsed times. Surprisingly, no Taylor-Proudman reorganization of the flow to a two-dimensional state is observed. The implications that these results have on turbulence modeling are discussed briefly along with prospective future research.
Reynolds number scaling of velocity increments in isotropic turbulence
NASA Astrophysics Data System (ADS)
Iyer, Kartik P.; Sreenivasan, Katepalli R.; Yeung, P. K.
2017-02-01
Using the largest database of isotropic turbulence available to date, generated by the direct numerical simulation (DNS) of the Navier-Stokes equations on an 81923 periodic box, we show that the longitudinal and transverse velocity increments scale identically in the inertial range. By examining the DNS data at several Reynolds numbers, we infer that the contradictory results of the past on the inertial-range universality are artifacts of low Reynolds number and residual anisotropy. We further show that both longitudinal and transverse velocity increments scale on locally averaged dissipation rate, just as postulated by Kolmogorov's refined similarity hypothesis, and that, in isotropic turbulence, a single independent scaling adequately describes fluid turbulence in the inertial range.
Laboratory Study of Homogeneous and Isotropic Turbulence at High Reynolds Number
NASA Astrophysics Data System (ADS)
Pecenak, Zachary; Dou, Zhongwang; Yang, Fan; Cao, Lujie; Liang, Zach; Meng, Hui
2013-11-01
To study particle dynamics modified by isotropic turbulence at high Reynolds numbers and provide experimental data for DNS validation, we have developed a soccer-ball-shaped truncated icosahedron turbulence chamber with 20 adjoining hexagon surfaces, 12 pentagon surfaces and twenty symettrically displaced fans, which form an enclosed chamber of 1m diameter. We use Particle Image Velocimetry (PIV) technique to characterize the base turbulent flow, using different PIV set ups to capture various characteristic scales of turbulence. Results show that the stationary isotropic turbulence field is a spherical domain with diameter of 40 mm with quasi-zero mean velocities. The maximum rms velocity is ~1.5 m/s, corresponding to a Taylor microscale Re of 450. We extract from the PIV velocity field the whole set of turbulent flow parameters including: turbulent kinetic energy, turbulent intensity, kinetic energy dissipation rate, large eddy length and time scales, the Kolmogorov length, time and velocity scales, Taylor microscale and Re, which are critical to the study of inter-particle statistics modified by turbulence. This research is funded by an NSF grant CBET-0967407.
Computation of the sound generated by isotropic turbulence
NASA Technical Reports Server (NTRS)
Sarkar, S.; Hussaini, M. Y.
1993-01-01
The acoustic radiation from isotropic turbulence is computed numerically. A hybrid direct numerical simulation approach which combines direct numerical simulation (DNS) of the turbulent flow with the Lighthill acoustic analogy is utilized. It is demonstrated that the hybrid DNS method is a feasible approach to the computation of sound generated by turbulent flows. The acoustic efficiency in the simulation of isotropic turbulence appears to be substantially less than that in subsonic jet experiments. The dominant frequency of the computed acoustic pressure is found to be somewhat larger than the dominant frequency of the energy-containing scales of motion. The acoustic power in the simulations is proportional to epsilon (M(sub t))(exp 5) where epsilon is the turbulent dissipation rate and M(sub t) is the turbulent Mach number. This is in agreement with the analytical result of Proudman (1952), but the constant of proportionality is smaller than the analytical result. Two different methods of computing the acoustic power from the DNS data bases yielded consistent results.
Computation of large-scale statistics in decaying isotropic turbulence
NASA Technical Reports Server (NTRS)
Chasnov, Jeffrey R.
1993-01-01
We have performed large-eddy simulations of decaying isotropic turbulence to test the prediction of self-similar decay of the energy spectrum and to compute the decay exponents of the kinetic energy. In general, good agreement between the simulation results and the assumption of self-similarity were obtained. However, the statistics of the simulations were insufficient to compute the value of gamma which corrects the decay exponent when the spectrum follows a k(exp 4) wave number behavior near k = 0. To obtain good statistics, it was found necessary to average over a large ensemble of turbulent flows.
NASA Astrophysics Data System (ADS)
Abdelsamie, Abouelmagd H.; Lee, Changhoon
2013-03-01
The current paper examines the heavy particle statistics modification by two-way interaction in particle-laden isotropic turbulence in an attempt to interpret their statistics modification using the information of modulated turbulence. Moreover, we clarify the distinctions of this modification between decaying and stationary turbulence as an extension of our previous work [A. H. Abdelsamie and C. Lee, "Decaying versus stationary turbulence in particle-laden isotropic turbulence: Turbulence modulation mechanism," Phys. Fluids 24, 015106 (2012), 10.1063/1.3678332]. Direct Numerical Simulation (DNS) was carried out using 1283 grid points at a Taylor micro-scale Reynolds number of Rλ ˜ 70. The effect of O(10^6) solid particles with a different Stokes number (St) was implemented as a point-force approximation in the Navier-Stokes equation. Various statistics associated with particle dispersion are investigated, and the auto-correlations models which was provided by Jung et al. ["Behavior of heavy particles in isotropic turbulence," Phys. Rev. E 77, 016307 (2008), 10.1103/PhysRevE.77.016307] are extended in the current paper. DNS results reveal that the two-way coupling interaction enhances the fluid and heavy particle auto-correlation functions and the alignment between their velocity vectors for all Stokes numbers in decaying and stationary turbulence, but for different reasons. The modification mechanisms of particle dispersion statistics in stationary turbulence are different from those in decaying turbulence depending on the Stokes number, particularly for St <1.
A new approach to Lagrangian investigations of isotropic turbulence
NASA Astrophysics Data System (ADS)
Barjona, Manuel; B. da Silva, Carlos; Idmec Team
2016-11-01
A new numerical approach is used in conjunction with direct numerical simulations (DNS) of statistically stationary (forced) isotropic turbulence to investigate the high Reynolds number scaling properties of turbulence characteristics in a Lagrangian frame. The new method provides an alternative route to the determination of the classical Lagrangian turbulence quantities, such as the second order Lagrangian velocity structure function and two point particle separation, at a much higher Reynolds number than as obtained in previous numerical simulations, and displays excellent agreement with the classical theoretical predictions and existing numerical simulations and experimental data. The authors acknowledge the Laboratory for Advanced Computing at University of Coimbra for providing HPC, computing, consulting resources that have contributed to the research results reported within this paper. URL http://www.lca.uc.pt.
Modification of homogeneous and isotropic turbulence by solid particles
NASA Astrophysics Data System (ADS)
Hwang, Wontae
2005-12-01
Particle-laden flows are prevalent in natural and industrial environments. Dilute loadings of small, heavy particles have been observed to attenuate the turbulence levels of the carrier-phase flow, up to 80% in some cases. We attempt to increase the physical understanding of this complex phenomenon by studying the interaction of solid particles with the most fundamental type of turbulence, which is homogeneous and isotropic with no mean flow. A flow facility was developed that could create air turbulence in a nearly-spherical chamber by means of synthetic jet actuators mounted on the corners. Loudspeakers were used as the actuators. Stationary turbulence and natural decaying turbulence were investigated using two-dimensional particle image velocimetry for the base flow qualification. Results indicated that the turbulence was fairly homogeneous throughout the measurement domain and very isotropic, with small mean flow. The particle-laden flow experiments were conducted in two different environments, the lab and in micro-gravity, to examine the effects of particle wakes and flow structure distortion caused by settling particles. The laboratory experiments showed that glass particles with diameters on the order of the turbulence Kolmogorov length scale attenuated the fluid turbulent kinetic energy (TKE) and dissipation rate with increasing particle mass loadings. The main source of fluid TKE production in the chamber was the speakers, but the loss of potential energy of the settling particles also resulted in a significant amount of production of extra TKE. The sink of TKE in the chamber was due to the ordinary fluid viscous dissipation and extra dissipation caused by particles. This extra dissipation could be divided into "unresolved" dissipation caused by local velocity disturbances in the vicinity of the small particles and dissipation caused by large-scale flow distortions from particle wakes and particle clusters. The micro-gravity experiments in NASA's KC-135
Spark ignition of aviation fuel in isotropic turbulence
NASA Astrophysics Data System (ADS)
Krisman, Alex; Lu, Tianfeng; Borghesi, Giulio; Chen, Jacqueline
2016-11-01
Turbulent spark ignition occurs in combustion engines where the spark must establish a viable flame kernel that leads to stable combustion. A competition exists between kernel growth, due to flame propagation, and kernel attenuation, due to flame stretch and turbulence. This competition can be measured by the Karlovitz number, Ka, and kernel viability decreases rapidly for Ka >> 1 . In this study, the evolution of an initially spherical flame kernel in a turbulent field is investigated at two cases: Ka- (Ka = 25) and Ka+ (Ka = 125) using direct numerical simulation (DNS). A detailed chemical mechanism for jet fuel (Jet-A) is used, which is relevant for many practical conditions, and the mechanism includes a pyrolysis sub-model which is important for the ignition of large hydrocarbon fuels. An auxiliary non-reacting DNS generates the initial field of isotropic turbulence with a turbulent Reynolds number of 500 (Ka-) and 1,500 (Ka+). The kernel is then imposed at the center of the domain and the reacting DNS is performed. The Ka- case survives and the Ka+ case is extinguished. An analysis of the turbulence chemistry interactions is performed and the process of extinction is described. Department of Energy - Office of Basic Energy Science under Award No. DE-SC0001198.
Modeling inertial particle acceleration statistics in isotropic turbulence
NASA Astrophysics Data System (ADS)
Ayyalasomayajula, S.; Warhaft, Z.; Collins, L. R.
2008-09-01
Our objective is to explain recent Lagrangian acceleration measurements of inertial particles in decaying, nearly isotropic turbulence [Ayyalasomayajula et al., Phys. Rev. Lett. 97, 144507 (2006)]. These experiments showed that as particle inertial effects increased, the variance in the particle acceleration fluctuations was reduced, and the tails of the normalized particle acceleration probability density function (PDF) became systematically attenuated. We model this phenomenon using a base flow that consists of a two-dimensional array of evenly spaced vortices with signs and intensities that vary randomly in time. We simulate a large sample of inertial particles moving through the fluid without disturbing the flow (one-way coupling). Consistent with Bec et al. [J. Fluid Mech. 550, 349 (2006)], we find that our model exhibits preferential concentration or clustering of particles in regions located away from the vortex centers. That is, inertial particles selectively sample the flow field, oversampling regions with high strains and undersampling regions with high vorticities. At low Stokes numbers, this biased "sampling" of the flow is responsible for the reduction in the acceleration variance and partially explains the attenuation of the tails of the acceleration PDF. However, contrary to previous findings, we show that the tails of the PDF are also diminished by "filtering" induced by the attenuated response of the inertial particles to temporal variations in the fluid acceleration: Inertial particles do not respond to fluctuations with frequencies much higher than the inverse of the particle stopping time. We show that larger fluid acceleration events have higher frequencies and hence experience greater filtering by particle inertia. We contrast the vortex model with previous Lagrangian acceleration models by Sawford [Phys. Fluids A 3, 1577 (1991)] and Reynolds [Phys. Fluids 15, L1 (2003)] and show that although these models capture some aspects of the inertial
A spiral vortex model of homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
Higgins, Keith; Ooi, Andrew; Chong, Min
2002-11-01
The Lundgren-Townsend model of turbulent fine scales has been successful in predicting some of the properties of homogeneous isotropic turbulence. Lundgren obtained these results by averaging over an ensemble of nearly axisymmetric, unsteady, stretched spiral vortices. These vortical structures are represented in the model by a large-time asymptotic solution of the Navier-Stokes equations. Extending on the work of Pullin & Saffman [Phys. Fluids 8, 3072 (1996)], we calculate the energy spectrum and longitudinal velocity structure functions for a specific realisation of the Lundgren-Townsend model. Here the members of our ensemble are time-evolving spiral vortex structures resulting from the merging of stretched Burgers vortex tubes. The merging is computed numerically following the method of Buntine & Pullin [JFM 205, 263 (1989)]. We present results for a range of vortex Reynolds numbers.
Direct simulation of particle dispersion in a decaying isotropic turbulence
NASA Technical Reports Server (NTRS)
Elghobashi, S.; Truesdell, G. C.
1992-01-01
Results of a numerical investigation of the dispersion of solid particles in decaying isotropic turbulence are presented. The 3D time-dependent velocity field of a homogeneous nonstationary turbulence is computed using the method of direct numerical simulation (DNS). The dispersion characteristics of three different solid particles (corn, copper, and glass) injected in the flow are obtained by integrating the complete equation of particle motion along the instantaneous trajectories of 22-cubed particles for each particle type, and then by performing ensemble averaging. Good agreement was achieved between the present DNS results and the measured time development of the mean-square displacement of the particles. Questions of how and why the dispersion statistics of a solid particle differ from those of its corresponding fluid point and surrounding fluid and what influences inertia and gravity have on these statistics are also discussed.
The curvature of material surfaces in isotropic turbulence
NASA Astrophysics Data System (ADS)
Pope, S. B.; Yeung, P. K.; Girimaji, S. S.
1989-12-01
Direct numerical simulation is used to study the curvature of material surfaces in isotropic turbulence. The Navier-Stokes equation is solved by a 643 pseudospectral code for constant-density homogeneous isotropic turbulence, which is made statistically stationary by low-wavenumber forcing. The Taylor-scale Reynolds number is 39. An ensemble of 8192 infinitesimal material surface elements is tracked through the turbulence. For each element, a set of exact ordinary differential equations is integrated in time to determine, primarily, the two principal curvatures k1 and k2. Statistics are then deduced of the mean-square curvature M= (1)/(2) (k21+k22), and of the mean radius of curvature R=(k21+k22)-1/2. Curvature statistics attain an essentially stationary state after about 15 Kolmogorov time scales. Then the area-weighted expectation of R is found to be 12η, where η is the Kolmogorov length scale. For moderate and small radii (less than 10η) the probability density function (pdf) of R is approximately uniform, there being about 5% probability of R being less than η. The uniformity of the pdf of R, for small R, implies that the expectation of M is infinite. It is found that the surface elements with large curvatures are nearly cylindrical in shape (i.e., ‖k1‖≫‖k2‖ or ‖k2‖≫‖k1‖), consistent with the folding of the surface along nearly straight lines. Nevertheless the variance of the Gauss curvature K=k1k2 is infinite.
Fluctuations of thermodynamic variables in compressible isotropic turbulence
NASA Astrophysics Data System (ADS)
Donzis, Diego; Jagannathan, Shriram
2014-11-01
A distinguishing feature of compressible turbulence is the appearance of fluctuations of thermodynamic variables. While their importance is well-known in understanding these flows, some of their basic characteristics such as the Reynolds and Mach number dependence are not well understood. We use a large database of Direct Numerical Simulation of stationary compressible isotropic turbulence on up to 20483 grids at Taylor Reynolds numbers up to 450 and a range of Mach numbers (Mt ~ 0 . 1 - 0 . 6) to examine statistical properties of thermodynamic variables. Our focus is on the PDFs and moments of pressure, density and temperature. While results at low Mt are consistent with incompressible results, qualitative changes are observed at higher Mt with a transition around Mt ~ 0 . 3 . For example, the PDF of pressure changes from negatively to positively skewed as Mt increases. Similar changes are observed for temperature and density. We suggest that large fluctuations of thermodynamic variables will be log-normal at high Mt. We also find that, relative to incompressible turbulence, the correlation between enstrophy and low-pressure regions is weakened at high Mt which can be explained by the dominance of the so-called dilatational pressure.
Energy transfer and constrained simulations in isotropic turbulence
NASA Technical Reports Server (NTRS)
Jimenez, Javier
1993-01-01
The defining characteristic of turbulent flows is their ability to dissipate energy, even in the limit of zero viscosity. The Euler equations, if constrained in such a way that the velocity derivatives remain bounded, conserve energy. But when they arise as the limit of the Navier-Stokes (NS) equations, when the Reynolds number goes to infinity, there is persuasive empirical evidence that the gradients become singular as just the right function of Re for the dissipation to remain non-zero and to approach a well defined limit. It is generally believed that this limiting value of the dissipation is a property of the Euler equations themselves, independent of the particular dissipative mechanism involved, and that it can be normalized with the large scale properties of the turbulent flow (e.g. the kinetic energy per unit volume u'(exp 2)/2, and the integral scale L) without reference to the Reynolds number or to other dissipative quantities. This is usually taken to imply that the low wave number end of the energy spectrum, far from the dissipative range, is also independent of the particular mechanism chosen to dispose of the energy transfer. In the following sections, we present some numerical experiments on the effect of substituting different dissipation models into the truncated Euler equations. We will see that the effect is mainly felt in the 'near dissipation' range of the energy spectrum, but that this range can be quite wide in some cases, contaminating a substantial range of wave numbers. In the process, we will develop a 'practical' approximation to the subgrid energy transfer in isotropic turbulence, and we will gain insight into the structure of the nonlinear interactions among turbulent scales of comparable size, and into the nature of energy backscatter. Some considerations on future research directions are offered at the end.
The modified cumulant expansion for two-dimensional isotropic turbulence
NASA Astrophysics Data System (ADS)
Tatsumi, T.; Yanase, S.
1981-09-01
The two-dimensional isotropic turbulence in an incompressible fluid is investigated using the modified zero fourth-order cumulant approximation. The dynamical equation for the energy spectrum obtained under this approximation is solved numerically and the similarity laws governing the solution in the energy-containing and enstrophy-dissipation ranges are derived analytically. At large Reynolds numbers the numerical solutions yield the k to the -3rd power inertial subrange spectrum which was predicted by Kraichnan (1967), Leith (1968) and Batchelor (1969), assuming a finite enstrophy dissipation in the inviscid limit. The energy-containing range is found to satisfy an inviscid similarity while the enstrophy-dissipation range is governed by the quasi-equilibrium similarity with respect to the enstrophy dissipation as proposed by Batchelor (1969). There exists a critical time which separates the initial period and the similarity period in which the enstrophy dissipation vanishes and remains non-zero respectively in the inviscid limit.
Dynamics of Aerosol Particles in Stationary, Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Collins, Lance R.; Meng, Hui
2004-01-01
A detailed study of the dynamics of sub-Kolmogorov-size aerosol particles in stationary isotropic turbulence has been performed. The study combined direct numerical simulations (DNS; directed by Prof. Collins) and high-resolution experimental measurements (directed by Prof. Meng) under conditions of nearly perfect geometric and parametric overlap. The goal was to measure the accumulation of particles in low-vorticity regions of the flow that arises from the effect commonly referred to as preferential concentration. The grant technically was initiated on June 13, 2000; however, funding was not available until July 11, 2000. The grant was originally awarded to Penn State University (numerical simulations) and SUNY-Buffalo (experiments); however, Prof. Collins effort was moved to Cornell University on January 2002 when he joined that university. He completed the study there. A list of the specific tasks that were completed under this study is presented.
Random shearing direction models for isotropic turbulent diffusion
NASA Astrophysics Data System (ADS)
Majda, Andrew J.
1994-06-01
Recently, a rigorous renormalization theory for various scalar statistics has been developed for special modes of random advection diffusion involving random shear layer velocity fields with long-range spatiotemporal correlations. New random shearing direction models for isotropic turbulent diffusion are introduced here. In these models the velocity field has the spatial second-order statistics of an arbitrary prescribed stationary incompressible isotropic random field including long-range spatial correlations with infrared divergence, but the temporal correlations have finite range. The explicit theory of renormalization for the mean and second-order statistics is developed here. With ɛ the spectral parameter, for -∞<ɛ<4 and measuring the strength of the infrared divergence of the spatial spectrum, the scalar mean statistics rigorously exhibit a phase transition from mean-field behavior for ɛ<2 to anomalous behavior for ɛ with 2<ɛ<4 as conjectured earlier by Avellaneda and the author. The universal inertial range renormalization for the second-order scalar statistics exhibits a phase transition from a covariance with a Gaussian functional form for ɛ with ɛ<2 to an explicit family with a non-Gaussian covariance for ɛ with 2<ɛ<4. These non-Gaussian distributions have tails that are broader than Gaussian as ɛ varies with 2<ɛ<4 and behave for large values like exp(- C c | x|4-ɛ), with C c an explicit constant. Also, here the attractive general principle is formulated and proved that every steady, stationary, zero-mean, isotropic, incompressible Gaussian random velocity field is well approximated by a suitable superposition of random shear layers.
Nematic - isotropic phase transition in turbulent thermal convection
NASA Astrophysics Data System (ADS)
Ahlers, Guenter; Weiss, Stephan
2013-11-01
The nematic-isotropic transition of a liquid crystal (LC) at a temperature TNI is an example of a soft phase transition, where fluid properties, although discontinuous, change only very little and where the latent heat is small. Understanding thermal convection in the presence of such a phase change is relevant to convection in Earth's mantle where discontinuous changes of the crystalline structure occur. We report on turbulent Rayleigh-Bénard convection of a nematic LC while it undergoes a transition from the nematic to the isotropic phase in a cylindrical convection cell with aspect ratio Γ (height over diameter) of 0.50. The difference between the top- and the bottom-plate temperature ΔT =Tb -Tt was held constant, while the average temperature Tm = (Tb +Tt) / 2 was varied. There was a significant increase of heat transport when TNI was between Tb and Tt. Measurements of the temperatures along the side wall as a function of Tm showed several ranges with qualitatively different behavior of quantities such as the time-averaged side-wall temperature, temperature gradient, or temperature fluctuations. We interpret these different ranges in terms of processes in the thermal boundary layers close to the top and bottom plates. SW acknowledges support by the Deutsche Forschungsgemeinschaft. This work was supported by the U.S. National Science Foundation through Grant No. DMR11-58514.
Non-isotropic turbulence effects on spray combustion
NASA Technical Reports Server (NTRS)
Kim, Y. M.; Shang, H. M.; Chen, C. P.
1991-01-01
A numerical model for the prediction of local properties of statistically stationary spray-combusting flows is evaluated by comparison with experimental data. To appraise the relative performance of turbulence models, computations were carried out by the k-epsilon model and the algebraic stress model. The present numerical results show the qualitative agreement with experimental data. In terms of overall local flow properties, the algebraic stress model improves a degree of conformity to the experimental data due to its ability to introduce the nonisotropic turbulence effects. Two swirl numbers are considered to investigate the influence of swirl on the droplet evaporation and trajectories, and the effects of droplet/turbulence interactions in flow properties. It is found that the large swirl produces a higher evaporationn rate, and more intensive turbulent mixing and burning. The discrepancies observed in the results are attributed mainly to uncertainties in the initial spray size and velocity distributions, the droplet/wall impingement interaction, the combustion model with the fast chemistry and the turbulence models dealing with the strong streamline curvature and complex interactions between the dispersed droplets and the continuous gas-phase flows.
NASA Technical Reports Server (NTRS)
Eaton, John; Hwang, Wontae; Cabral, Patrick
2002-01-01
This research addresses turbulent gas flows laden with fine solid particles at sufficiently large mass loading that strong two-way coupling occurs. By two-way coupling we mean that the particle motion is governed largely by the flow, while the particles affect the gas-phase mean flow and the turbulence properties. Our main interest is in understanding how the particles affect the turbulence. Computational techniques have been developed which can accurately predict flows carrying particles that are much smaller than the smallest scales of turbulence. Also, advanced computational techniques and burgeoning computer resources make it feasible to fully resolve very large particles moving through turbulent flows. However, flows with particle diameters of the same order as the Kolmogorov scale of the turbulence are notoriously difficult to predict. Some simple flows show strong turbulence attenuation with reductions in the turbulent kinetic energy by up to a factor of five. On the other hand, some seemingly similar flows show almost no modification. No model has been proposed that allows prediction of when the strong attenuation will occur. Unfortunately, many technological and natural two-phase flows fall into this regime, so there is a strong need for new physical understanding and modeling capability. Our objective is to study the simplest possible turbulent particle-laden flow, namely homogeneous, isotropic turbulence with a uniform dispersion of monodisperse particles. We chose such a simple flow for two reasons. First, the simplicity allows us to probe the interaction in more detail and offers analytical simplicity in interpreting the results. Secondly, this flow can be addressed by numerical simulation, and many research groups are already working on calculating the flow. Our detailed data can help guide some of these efforts. By using microgravity, we can further simplify the flow to the case of no mean velocity for either the turbulence or the particles. In fact
Clustering of vertically constrained passive particles in homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
De Pietro, Massimo; van Hinsberg, Michel A. T.; Biferale, Luca; Clercx, Herman J. H.; Perlekar, Prasad; Toschi, Federico
2015-05-01
We analyze the dynamics of small particles vertically confined, by means of a linear restoring force, to move within a horizontal fluid slab in a three-dimensional (3D) homogeneous isotropic turbulent velocity field. The model that we introduce and study is possibly the simplest description for the dynamics of small aquatic organisms that, due to swimming, active regulation of their buoyancy, or any other mechanism, maintain themselves in a shallow horizontal layer below the free surface of oceans or lakes. By varying the strength of the restoring force, we are able to control the thickness of the fluid slab in which the particles can move. This allows us to analyze the statistical features of the system over a wide range of conditions going from a fully 3D incompressible flow (corresponding to the case of no confinement) to the extremely confined case corresponding to a two-dimensional slice. The background 3D turbulent velocity field is evolved by means of fully resolved direct numerical simulations. Whenever some level of vertical confinement is present, the particle trajectories deviate from that of fluid tracers and the particles experience an effectively compressible velocity field. Here, we have quantified the compressibility, the preferential concentration of the particles, and the correlation dimension by changing the strength of the restoring force. The main result is that there exists a particular value of the force constant, corresponding to a mean slab depth approximately equal to a few times the Kolmogorov length scale η , that maximizes the clustering of the particles.
Preferential Rotation of Chiral Dipoles in Isotropic Turbulence.
Kramel, Stefan; Voth, Greg A; Tympel, Saskia; Toschi, Federico
2016-10-07
We introduce a new particle shape which shows preferential rotation in three dimensional homogeneous isotropic turbulence. We call these particles chiral dipoles because they consist of a rod with two helices of opposite handedness, one at each end. 3D printing is used to fabricate these particles with a length in the inertial range and their rotations are tracked in a turbulent flow between oscillating grids. High aspect ratio chiral dipoles preferentially align with their long axis along the extensional eigenvectors of the strain rate tensor, and the helical ends respond to the extensional strain rate with a mean spinning rate that is nonzero. We use Stokesian dynamics simulations of chiral dipoles in pure strain flow to quantify the dependence of spinning on particle shape. Based on the known response to pure strain, we build a model that gives the spinning rate of small chiral dipoles using velocity gradients along Lagrangian trajectories from high resolution direct numerical simulations. The statistics of chiral dipole spinning determined with this model show surprisingly good agreement with the measured spinning of much larger chiral dipoles in the experiments.
Carroll, Jonathan J.; Frank, Adam; Blackman, Eric G.
2010-10-10
Feedback from protostellar outflows can influence the nature of turbulence in star-forming regions even if they are not the primary source of velocity dispersion for all scales of molecular clouds. For the rate and power expected in star-forming regions, we previously (Carroll et al.) demonstrated that outflows could drive supersonic turbulence at levels consistent with the scaling relations from Matzner although with a steeper velocity power spectrum than expected for an isotropically driven supersonic turbulent cascade. Here, we perform higher resolution simulations and combine simulations of outflow driven turbulence with those of isotropically forced turbulence. We find that the presence of outflows within an ambient isotropically driven turbulent environment produces a knee in the velocity power spectrum at the outflow scale and a steeper slope at sub-outflow scales than for a purely isotropically forced case. We also find that the presence of outflows flattens the density spectrum at large scales effectively reducing the formation of large-scale turbulent density structures. These effects are qualitatively independent of resolution. We have also carried out Principal Component Analysis (PCA) for synthetic data from our simulations. We find that PCA as a tool for identifying the driving scale of turbulence has a misleading bias toward low amplitude large-scale velocity structures even when they are not necessarily the dominant energy containing scales. This bias is absent for isotropically forced turbulence but manifests strongly for collimated outflow driven turbulence.
Large-deviation statistics of vorticity stretching in isotropic turbulence.
Johnson, Perry L; Meneveau, Charles
2016-03-01
A key feature of three-dimensional fluid turbulence is the stretching and realignment of vorticity by the action of the strain rate. It is shown in this paper, using the cumulant-generating function, that the cumulative vorticity stretching along a Lagrangian path in isotropic turbulence obeys a large deviation principle. As a result, the relevant statistics can be described by the vorticity stretching Cramér function. This function is computed from a direct numerical simulation data set at a Taylor-scale Reynolds number of Re(λ)=433 and compared to those of the finite-time Lyapunov exponents (FTLE) for material deformation. As expected, the mean cumulative vorticity stretching is slightly less than that of the most-stretched material line (largest FTLE), due to the vorticity's preferential alignment with the second-largest eigenvalue of strain rate and the material line's preferential alignment with the largest eigenvalue. However, the vorticity stretching tends to be significantly larger than the second-largest FTLE, and the Cramér functions reveal that the statistics of vorticity stretching fluctuations are more similar to those of the largest FTLE. In an attempt to relate the vorticity stretching statistics to the vorticity magnitude probability density function in statistically stationary conditions, a model Kramers-Moyal equation is constructed using the statistics encoded in the Cramér function. The model predicts a stretched-exponential tail for the vorticity magnitude probability density function, with good agreement for the exponent but significant difference (35%) in the prefactor.
Joint Statistics of Finite Time Lyapunov Exponents in Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Johnson, Perry; Meneveau, Charles
2014-11-01
Recently, the notion of Lagrangian Coherent Structures (LCS) has gained attention as a tool for qualitative visualization of flow features. LCS visualize repelling and attracting manifolds marked by local ridges in the field of maximal and minimal finite-time Lyapunov exponents (FTLE), respectively. To provide a quantitative characterization of FTLEs, the statistical theory of large deviations can be used based on the so-called Cramér function. To obtain the Cramér function from data, we use both the method based on measuring moments and measuring histograms (with finite-size correction). We generalize the formalism to characterize the joint distributions of the two independent FTLEs in 3D. The ``joint Cramér function of turbulence'' is measured from the Johns Hopkins Turbulence Databases (JHTDB) isotropic simulation at Reλ = 433 and results are compared with those computed using only the symmetric part of the velocity gradient tensor, as well as with those of instantaneous strain-rate eigenvalues. We also extend the large-deviation theory to study the statistics of the ratio of FTLEs. When using only the strain contribution of the velocity gradient, the maximal FTLE nearly doubles in magnitude and the most likely ratio of FTLEs changes from 4:1:-5 to 8:3:-11, highlighting the role of rotation in de-correlating the fluid deformations along particle paths. Supported by NSF Graduate Fellowship (DGE-1232825), a JHU graduate Fellowship, and NSF Grant CMMI-0941530. CM thanks Prof. Luca Biferale for useful discussions on the subject.
Large-deviation statistics of vorticity stretching in isotropic turbulence
NASA Astrophysics Data System (ADS)
Johnson, Perry L.; Meneveau, Charles
2016-03-01
A key feature of three-dimensional fluid turbulence is the stretching and realignment of vorticity by the action of the strain rate. It is shown in this paper, using the cumulant-generating function, that the cumulative vorticity stretching along a Lagrangian path in isotropic turbulence obeys a large deviation principle. As a result, the relevant statistics can be described by the vorticity stretching Cramér function. This function is computed from a direct numerical simulation data set at a Taylor-scale Reynolds number of Reλ=433 and compared to those of the finite-time Lyapunov exponents (FTLE) for material deformation. As expected, the mean cumulative vorticity stretching is slightly less than that of the most-stretched material line (largest FTLE), due to the vorticity's preferential alignment with the second-largest eigenvalue of strain rate and the material line's preferential alignment with the largest eigenvalue. However, the vorticity stretching tends to be significantly larger than the second-largest FTLE, and the Cramér functions reveal that the statistics of vorticity stretching fluctuations are more similar to those of the largest FTLE. In an attempt to relate the vorticity stretching statistics to the vorticity magnitude probability density function in statistically stationary conditions, a model Kramers-Moyal equation is constructed using the statistics encoded in the Cramér function. The model predicts a stretched-exponential tail for the vorticity magnitude probability density function, with good agreement for the exponent but significant difference (35%) in the prefactor.
Energy transfer and dissipation in forced isotropic turbulence
NASA Astrophysics Data System (ADS)
Linkmann, Moritz; McComb, W. David; Berera, Arjun; Yoffe, Samuel
2014-11-01
A model for the Reynolds number dependence of the dimensionless dissipation rate Cɛ is derived from the dimensionless Kármán-Howarth equation, resulting in Cɛ =Cɛ , ∞ + C /RL , where RL is the integral scale Reynolds number. The coefficients C and Cɛ , ∞ arise from asymptotic expansions of the dimensionless second- and third-order structure functions. The model equation is fitted to data from direct numerical simulations (DNS) of forced isotropic turbulence for integral scale Reynolds numbers up to RL = 5875 (Rλ = 435), which results in an asymptote for Cɛ in the infinite Reynolds number limit Cɛ , ∞ = 0 . 47 +/- 0 . 01 . Since the coefficients in the model equation are scale-dependent while the dimensionless dissipation rate is not, we modelled the scale dependences of the coefficients by an ad hoc profile function such that they cancel out, leaving the model equation scale-independent, as it must be. The profile function was compared to DNS data to very good agreement, provided we restrict the comparison to scales small enough to be well resolved in our simulations. This work has made use of the resources provided by the UK supercomputing service HECToR, made available through the Edinburgh Compute and Data Facility (ECDF). A.B. is supported by STFC, S.R.Y. and M.F.L. are funded by EPSRC.
Pressure and higher-order spectra for homogeneous isotropic turbulence
NASA Technical Reports Server (NTRS)
Pullin, D. I.; Rogallo, R. S.
1994-01-01
The spectra of the pressure, and other higher-order quantities including the dissipation, the enstrophy, and the square of the longitudinal velocity derivative are computed using data obtained from direct numerical simulation of homogeneous isotropic turbulence at Taylor-Reynolds numbers R(sub lambda) in the range 38 - 170. For the pressure spectra we find reasonable collapse in the dissipation range (of the velocity spectrum) when scaled in Kolmogorov variables and some evidence, which is not conclusive, for the existence of a k(exp -7/3) inertial range where k = absolute value of K, is the modulus of the wavenumber. The power spectra of the dissipation, the enstrophy, and the square of the longitudinal velocity derivative separate in the dissipation range, but appear to converge together in the short inertial range of the simulations. A least-squares curve-fit in the dissipation range for one value of R(sub lambda) = 96 gives a form for the spectrum of the dissipation as k(exp 0)exp(-Ck eta), for k(eta) greater than 0.2, where eta is the Kolmogorov length and C is approximately equal to 2.5.
Interacting scales and energy transfer in isotropic turbulence
NASA Technical Reports Server (NTRS)
Zhou, YE
1993-01-01
The dependence of the energy transfer process on the disparity of the interacting scales is investigated in the inertial and far-dissipation ranges of isotropic turbulence. The strategy for generating the simulated flow fields and the choice of a disparity parameter to characterize the scaling of the interactions is discussed. The inertial range is found to be dominated by relatively local interactions, in agreement with the Kolmogorov assumption. The far-dissipation is found to be dominated by relatively non-local interactions, supporting the classical notion that the far-dissipation range is slaved to the Kolmogorov scales. The measured energy transfer is compared with the classical models of Heisenberg, Obukhov, and the more detailed analysis of Tennekes and Lumley. The energy transfer statistics measured in the numerically simulated flows are found to be nearly self-similar for wave numbers in the inertial range. Using the self-similar form measured within the limited scale range of the simulation, an 'ideal' energy transfer function and the corresponding energy flux rate for an inertial range of infinite extent are constructed. From this flux rate, the Kolmogorov constant is calculated to be 1.5, in excellent agreement with experiments.
Analysis of the behavior of bubbles and droplets in isotropic turbulence
NASA Astrophysics Data System (ADS)
Snyder, Murray R.
The behavior and scale-dependent dispersion of small air bubbles, and the rise of slightly buoyant oil droplets in water under isotropic turbulence conditions, are analyzed computationally. The flow field is simulated using a pseudo-spectral code, while the bubble dynamics are analyzed by integration of a Lagrangian equation of motion with buoyancy, virtual mass, pressure, drag and lift forces. Consistent with experimental data, bubble rise velocities are increasingly suppressed with increasing turbulence intensity. The role of the lift force in moving the bubbles to the down-flow side of turbulent eddies, and consequently retarding their rise, is observed. Analysis also reveals that the vertical bubble velocities are characterized by asymmetric probability density functions that are positive or negative-skewed dependent upon the non-dimensional turbulence intensity and the Taylor length scale. Lagrangian bubble trajectories are used to determine dispersion characteristics, following the theoretical development of Cushman and Moroni (2001). The dispersion of 40 mum bubbles exhibits transition to Fickian behavior, and the process is weakly affected by the turbulence level for the entire range considered. Larger, 400 mum bubbles are shown to be more sensitive to turbulence level, with transition to Fickian behavior delayed in low turbulence fields. Computations are also performed to investigate the puzzling behavior observed by Friedman and Katz (2002), that the rise velocity of slightly buoyant droplets smaller than 800 mum in diameter is enhanced by turbulence whereas the rise of larger droplets is retarded. Using the quasi-steady, empirically-determined drag and lift coefficients, the observed experimental behavior could not be reproduced. Further, analysis of the effect of lift and history forces also indicates that, within a broad range of uncertainty, these forces do not account for the experimentally observed mean droplet rise. Guided by correlations obtained
NASA Astrophysics Data System (ADS)
Buaria, Dhawal; Yeung, P. K.; Sawford, B. L.
2016-11-01
An efficient massively parallel algorithm has allowed us to obtain the trajectories of 300 million fluid particles in an 81923 simulation of isotropic turbulence at Taylor-scale Reynolds number 1300. Conditional single-particle statistics are used to investigate the effect of extreme events in dissipation and enstrophy on turbulent dispersion. The statistics of pairs and tetrads, both forward and backward in time, are obtained via post-processing of single-particle trajectories. For tetrads, since memory of shape is known to be short, we focus, for convenience, on samples which are initially regular, with all sides of comparable length. The statistics of tetrad size show similar behavior as the two-particle relative dispersion, i.e., stronger backward dispersion at intermediate times with larger backward Richardson constant. In contrast, the statistics of tetrad shape show more robust inertial range scaling, in both forward and backward frames. However, the distortion of shape is stronger for backward dispersion. Our results suggest that the Reynolds number reached in this work is sufficient to settle some long-standing questions concerning Lagrangian scale similarity. Supported by NSF Grants CBET-1235906 and ACI-1036170.
On the effects of isotropic turbulence on the evaporation rate of a liquid droplet
NASA Astrophysics Data System (ADS)
Dodd, Michael; Ferrante, Antonino
2016-11-01
Our objective is to explain the effects of isotropic turbulence on the vaporization rate of a liquid droplet in conditions that are relevant to spray combustion applications. To this end, we have performed direct numerical simulation (DNS) of a single droplet in homogeneous isotropic turbulence using the volume-of-fluid method for resolving fully the process of momentum, heat, and mass transfer between the liquid droplet and the gas. The simulations were performed using 10243 grid points. The effect of turbulence on the droplet vaporization rate is investigated by varying the gas-phase Reynolds number based on the Taylor microscale, Reλ. Reλ is increased from 0 to 75 by increasing the r.m.s. velocity of the gas phase while keeping all other physical properties constant. We will present the droplet evaporation rate as a function of turbulence Reynolds number and investigate the physical mechanisms.
Analysis of the decay of temperature fluctuations in isotropic turbulence
NASA Technical Reports Server (NTRS)
Durbin, P. A.
1982-01-01
The Lagrangian dispersion theory of Durbin (1980) is used to analyze experiments by Warhaft and Lumley (1978) and by Sreenivasan et al. (1980) on temperature fluctuations in grid-generated turbulence. Both theory and experiment show that the decay exponent m depends on the ratio of the initial length scales of velocity and temperature, although when this ratio is greater than 2.5 such dependence is negligible. The theory shows that m is not truly constant, but within the range covered by the experiments it is nearly so. The agreement between theory and experiment lends credence to the idea that the decay of fluctuations is controlled largely by turbulent relative dispersion.
Direct numerical simulation of isotropic turbulence interacting with a weak shock wave
NASA Technical Reports Server (NTRS)
Lee, Sangsan; Lele, Sanjiva K.; Moin, Parviz
1993-01-01
Direct numerical simulations are used to investigate the interaction of isotropic quasi-incompressible turbulence with a weak shock wave. A linear analysis of the interaction is conducted for comparison with the simulations. Both the simulations and the analysis show that turbulence is enhanced during the interaction. Turbulent kinetic energy and transverse vorticity components are amplified, and turbulent lengthscales are decreased. It is suggested that the amplification mechanism is primarily linear. Simulations also showed a rapid evolution of turbulent kinetic energy just downstream of the shock, a behavior not reproduced by the linear analysis. Analysis of the budget of the turbulent kinetic energy transport equation shows that this behavior can be attributed to the pressure transport term. Multiple compression peaks were found along the mean streamlines at locations where the local shock thickness had increased significantly.
High-resolution simulations of forced compressible isotropic turbulence
NASA Astrophysics Data System (ADS)
Jagannathan, Shriram; Donzis, Diego
2011-11-01
Direct numerical simulations of compressible turbulent flows are several times more expensive than their incompressible counterparts. Therefore, using large computing resources efficiently is even more pressing when studying compressible turbulence. A highly scalable code is presented which is used to perform simulations aimed at understanding fundamental turbulent processes. The code, which is based on a 2D domain decomposition, is shown to scale well up to 128k cores. To attain a statistically stationary state a new scheme is developed which involves large-scale stochastic forcing (solenoidal or dilatational) and a procedure to keep mean internal energy constant. The resulting flows show characteristics consistent with results in the literature. The attainable Reynolds and turbulent Mach numbers for given computational resources depend on the number of grid points and the degree to which the smallest scales are resolved that are given by Kolmogorov scales. A systematic comparison of simulations at different resolutions suggests that the resolution needed depends on the particular statistic being considered. The resulting database is used to investigate small-scale universality, the scaling of spectra of velocity, density and temperature fields, structure functions and the trends towards high-Reynolds number asymptotes. Differences with incompressible results are highlighted.
Transmission of acoustic waves through mixing layers and 2D isotropic turbulence
NASA Astrophysics Data System (ADS)
Juve, D.; Blanc-Benon, P.; Comte-Bellot, G.
Ray tracing and parabolic equation methods have been used to study the properties of acoustic waves transmitted through turbulent velocity fields. A numerical simulation permits individual realizations of the turbulent field, which then allow, if desired, an ensemble averaging of the fields. Two flows have been considered, 2D isotropic turbulence and a 2D mixing layer. The following complementary aspects are developed: the occurrence of caustics, the reinforced or weakened zones of the acoustic field, the eigenrays between a source and a receiver, and the associated travel times, variances, and scintillation index.
A detailed look at turbulence intensity
Technology Transfer Automated Retrieval System (TEKTRAN)
The effect of turbulence intensity on energy capture by small wind turbines has been a point of debate in the last few years. Claims of 25% de-rating of the power curve for turbines installed at sites with high turbulence are not uncommon. Over the years, many attempts have been made to model the ef...
Mixing of a passive scalar in isotropic and sheared homogeneous turbulence
NASA Technical Reports Server (NTRS)
Shirani, E.; Ferziger, J. H.; Reynolds, W. C.
1981-01-01
In order to calculate the velocity and scalar fields, the three dimensional, time-dependent equations of motion and the diffusion equation were solved numerically. The following cases were treated: isotropic, homogeneous turbulence with decay of a passive scalar; and homogeneous turbulent shear flow with a passive scalar whose mean varies linearly in the spanwise direction. The solutions were obtained at relatively low Reynolds numbers so that all of the turbulent scales could be resolved without modeling. Turbulent statistics such as integral length scales, Taylor microscales, Kolmogorov length scale, one- and two-point correlations of velocity-velocity and velocity-scalar, turbulent Prandtl/Schmidt number, r.m.s. values of velocities, the scalar quantity and pressure, skewness, decay rates, and decay exponents were calculated. The results are compared with the available expermental results, and good agreement is obtained.
Observation of quantum decay of homogeneous, isotropic (grid) turbulence
NASA Astrophysics Data System (ADS)
Ihas, Gary; Munday, Lydia; Yang, Jihee; Thompson, Kyle; Guo, Wei; Chapurin, Roman; Fisher, Shaun; McClintock, Peter; Vinen, W. F.
2014-03-01
In classical grid turbulence fluid is forced through a stationary grid. In the quantum case a grid moves through an initially stationary superfluid driven by a linear motor. We have developed a motor using superconducting drive coils and bearings, moving a grid at constant speed (0 and 15 cm/s). Stalp et al[2] report the decay of vortex-line density L in the grid's wake measured by 2nd sound attenuation. L decayed at large times as t - 3 / 2, interpreted as a quasi-classical Richardson cascade of energy-containing eddies size limited by channel width, associated with a Kolmogorov energy spectrum. It is assumed eddies produced on a scale of the grid mesh grow through the classical fluids mechanism.[3] We can now test a semi-quantitative theory with different mesh grids or channel sizes, relating to the possible existence of inverse turbulent cascades. Our 2nd sound system is conventional, but with a novel phase and amplitude feedback loop making stringent constant temperature unnecessary. Both t - 3 / 2 and non-t - 3 / 2 decays have been observed with 2 mesh sizes. US NSF DMR#0602778 and #1007937 and EPSRC EP/H04762X/1.
The radiated noise from isotropic turbulence and heated jets
NASA Technical Reports Server (NTRS)
Lilley, G. M.
1995-01-01
Our understanding of aerodynamic noise has its foundations in the work of Sir James Lighthill (1952), which was the first major advance in acoustics since the pioneering work of Lord Rayleigh in the last century. The combination of Lighthill's theory of aerodynamic noise as applied to turbulent flows and the experimental growing database from the early 1950's was quickly exploited by various jet propulsion engine designers in reducing the noise of jet engines at takeoff and landing to levels marginally acceptable to communities living in the neighborhoods of airports. The success in this noise containment led to the rapid growth of fast economical subsonic civil transport aircraft worldwide throughout the 1960's and has continued to the present day. One important factor in this success story has been the improvements in the engine cycle that have led to both reductions in specific fuel consumption and noise. The second is the introduction of Noise Certification, which specifies the maximum noise levels at takeoff and landing that all aircraft must meet before they can be entered on the Civil Aircraft Register. The growing interest in the development of a new supersonic civil transport to replace 'Concorde' in the early years of the next century has led to a resurgence of interest in the more challenging problem of predicting the noise of hot supersonic jets and developing means of aircraft noise reduction at takeoff and landing to meet the standards now accepted for subsonic Noise Certification. The prediction of aircraft noise to the accuracy required to meet Noise Certification requirements has necessitated reliance upon experimental measurements and empirically derived laws based on the available experimental data bases. These laws have their foundation in the results from Lighthill's theory, but in the case of jet noise, where the noise is generated in the turbulent mixing region with the external ambient fluid, the complexity of the turbulent motion has
Choi, Yeontaek; Kim, Byung-Gu; Lee, Changhoon
2009-07-01
We provide an observation suggesting a strong correlation between helicity and enstrophy in fluid turbulence. Helicity statistics were obtained in a direct numerical simulation of forced isotropic turbulence. An investigation of coherent structures revealed that intermittently large local helicity was found in the core region of the coherent rotational structures, thus showing a strong correlation with local enstrophy, not dissipation. Statistics regarding the relative helicity and the correlation between velocity and vorticity conditioned on different levels of enstrophy clearly suggest that velocity and vorticity tend to be aligned in the core of the coherent structures.
NASA Technical Reports Server (NTRS)
Ling, S. C.; Saad, A.
1977-01-01
The energetic isotropic turbulence generated by a waterfall of low head was found to be developed in part through the unstable two-phase flow of entrained air bubbles. The resulting turbulent field had a turbulent Reynolds number in excess of 20,000 and maintained a self-similar structure throughout the decay period studied. The present study may provide some insight into the structure of turbulence produced by breaking waves over the ocean.
The role of bulk viscosity on the decay of compressible, homogeneous, isotropic turbulence
NASA Astrophysics Data System (ADS)
Johnsen, Eric; Pan, Shaowu
2016-11-01
The practice of neglecting bulk viscosity in studies of compressible turbulence is widespread. While exact for monatomic gases and unlikely to strongly affect the dynamics of fluids whose bulk-to-shear viscosity ratio is small and/or of weakly compressible turbulence, this assumption is not justifiable for compressible, turbulent flows of gases whose bulk viscosity is orders of magnitude larger than their shear viscosities (e.g., CO2). To understand the mechanisms by which bulk viscosity and the associated phenomena affect compressible turbulence, we conduct DNS of freely decaying compressible, homogeneous, isotropic turbulence for ratios of bulk-to-shear viscosity ranging from 0-1000. Our simulations demonstrate that bulk viscosity increases the decay rate of turbulent kinetic energy; while enstrophy exhibits little sensitivity to bulk viscosity, dilatation is reduced by an order of magnitude within the two eddy turnover time. Via a Helmholtz decomposition of the flow, we determined that bulk viscosity damps the dilatational velocity and reduces dilatational-solenoidal exchanges, as well as pressure-dilatation coupling. In short, bulk viscosity renders compressible turbulence incompressible by reducing energy transfer between translational and internal modes.
Dispersion of finite size droplets and solid particles in isotropic turbulence
NASA Astrophysics Data System (ADS)
Rosso, Michele
Turbulent disperse two-phase flows, of either fluid/fluid or fluid/solid type, are common in natural phenomena and engineering devices. Notable examples are atmospheric clouds, i.e. dispersed liquid water droplets and ice particles in a complex turbulent flow, and spray of fuel droplets in the combustion chamber of internal combustion engines. However, the physics of the interaction between a dispersed phase and turbulence is not yet fully understood. The objective of this study is to compare the dispersion of deformable finite size droplets with that of solid particles in a turbulent flow in the absence of gravity, by performing Direct Numerical Simulation (DNS). The droplets and the particles have the same diameter, of the order of the Taylor's microscale of turbulence, and the same density ratio to the carrier flow. The solid particle-laden turbulence is simulated by coupling a standard projection method with the Immersed Boundary Method (IBM). The solid particles are fully resolved in space and time without considering particle/particle collisions (two-way coupling). The liquid droplet-laden turbulence is simulated by coupling a variable-density projection method with the Accurate Conservative Level Set Method (ACLSM). The effect of the surface tension is accounted for by using the Ghost Fluid Method (GFM) in order to avoid any numerical smearing, while the discontinuities in the viscous term of the Navier-Stokes equation are smoothed out via the Continuum Surface Force approach. Droplet/droplet interactions are allowed (four-way coupling). The results presented here show that in isotropic turbulence the dispersion of liquid droplets in a given direction is larger than that of solid particles due to the reduced decay rate of turbulence kinetic energy via the four-way coupling effects of the droplets.
On the dynamics of small-scale vorticity in isotropic turbulence
NASA Technical Reports Server (NTRS)
Jimenez, Javier; Wray, A. A.
1994-01-01
It was previously shown that the strong vorticity in isotropic turbulence is organized into tubular vortices ('worms') whose properties were characterized through the use of full numerical simulations at several Reynolds numbers. At the time most of the observations were kinematic, and several scaling laws were discovered for which there was no theoretical explanation. In the meantime, further analysis of the same fields yielded new information on the generation of the vortices, and it was realized that even if they had to be formed by stretching, they were at any given moment actually compressed at many points of their axes. This apparent contradiction was partially explained by postulating axial inertial waves induced by the nonuniformity of the vortex cores, which helped to 'spread' the axial strain and allowed the vortices to remain compact even if not uniformly stretched. The existence of such solutions was recently proved numerically. The present report discusses a set of new numerical simulations of isotropic turbulence, and a reanalysis of the old ones, in an effort to prove or disprove the presence of these waves in actual turbulent flows and to understand the dynamics, as opposed to the kinematics, of the vortices.
Field Line Random Walk in Isotropic Magnetic Turbulence up to Infinite Kubo Number
NASA Astrophysics Data System (ADS)
Sonsrettee, W.; Wongpan, P.; Ruffolo, D. J.; Matthaeus, W. H.; Chuychai, P.; Rowlands, G.
2013-12-01
In astrophysical plasmas, the magnetic field line random walk (FLRW) plays a key role in the transport of energetic particles. In the present, we consider isotropic magnetic turbulence, which is a reasonable model for interstellar space. Theoretical conceptions of the FLRW have been strongly influenced by studies of the limit of weak fluctuations (or a strong mean field) (e.g, Isichenko 1991a, b). In this case, the behavior of FLRW can be characterized by the Kubo number R = (b/B0)(l_∥ /l_ \\bot ) , where l∥ and l_ \\bot are turbulence coherence scales parallel and perpendicular to the mean field, respectively, and b is the root mean squared fluctuation field. In the 2D limit (R ≫ 1), there has been an apparent conflict between concepts of Bohm diffusion, which is based on the Corrsin's independence hypothesis, and percolative diffusion. Here we have used three non-perturbative analytic techniques based on Corrsin's independence hypothesis for B0 = 0 (R = ∞ ): diffusive decorrelation (DD), random ballistic decorrelation (RBD) and a general ordinary differential equation (ODE), and compared them with direct computer simulations. All the analytical models and computer simulations agree that isotropic turbulence for R = ∞ has a field line diffusion coefficient that is consistent with Bohm diffusion. Partially supported by the Thailand Research Fund, NASA, and NSF.
Calculation of velocity structure functions for vortex models of isotropic turbulence
NASA Astrophysics Data System (ADS)
Saffman, P. G.; Pullin, D. I.
1996-11-01
Velocity structure functions (up'-up)m are calculated for vortex models of isotropic turbulence. An integral operator is introduced which defines an isotropic two-point field from a volume-orientation average for a specific solution of the Navier-Stokes equations. Applying this to positive integer powers of the longitudinal velocity difference then gives explicit formulas for (up'-up)m as a function of order m and of the scalar separation r. Special forms of the operator are then obtained for rectilinear stretched vortex models of the Townsend-Lundgren type. Numerical results are given for the Burgers vortex and also for a realization of the Lundgren-strained spiral vortex, and comparison with experimental measurement is made. In an Appendix, we calculate values of the velocity-derivative moments for the Townsend-Burgers model.
Optical intensity interferometry through atmospheric turbulence
NASA Astrophysics Data System (ADS)
Tan, P. K.; Chan, A. H.; Kurtsiefer, C.
2016-04-01
Conventional ground-based astronomical observations suffer from image distortion due to atmospheric turbulence. This can be minimized by choosing suitable geographic locations or adaptive optical techniques, and avoided altogether by using orbital platforms outside the atmosphere. One of the promises of optical intensity interferometry is its independence from atmospherically induced phase fluctuations. By performing narrow-band spectral filtering on sunlight and conducting temporal intensity interferometry using actively quenched avalanche photodiodes, the Solar g(2)(τ) signature was directly measured. We observe an averaged photon bunching signal of g(2)(τ) = 1.693 ± 0.003 from the Sun, consistently throughout the day despite fluctuating weather conditions, cloud cover and elevation angle. This demonstrates the robustness of the intensity interferometry technique against atmospheric turbulence and opto-mechanical instabilities, and the feasibility to implement measurement schemes with both large baselines and long integration times.
Large-eddy simulations of viscoelastic isotropic turbulence with the FENE-P fluid
NASA Astrophysics Data System (ADS)
Pinho, Fernando T.; Ferreira, Pedro O.; B. da Silva, Carlos; Idmec/Feup Collaboration
2016-11-01
A new subgrid-scale (SGS) model developed for large-eddy simulations (LES) of dilute polymer solutions described by the Finitely Extensible Nonlinear Elastic constitutive equation closed with the Peterlin approximation (FENE-P), is presented. The filtered conformation tensor evolution equation uses the self-similarity of the polymer stretching terms, and the global equilibrium of the trace of the conformation tensor, while the SGS stresses are modelled with the classical Smagorinsky model. The new closure is assessed in direct numerical simulations (DNS) of forced isotropic turbulence using classical a-priori tests, and in a-posteriori (LES) showing excellent agreement with all the exact (filtered DNS) results.
The upper atmosphere of Uranus - A critical test of isotropic turbulence models
NASA Technical Reports Server (NTRS)
French, R. G.; Elliot, J. L.; Sicardy, B.; Nicholson, P.; Matthews, K.
1982-01-01
Observations of the August 15, 1980, Uranus occultation of KM 12, obtained from Cerro Tololo InterAmerican Observatory, European Southern Observatory, and Cerro Las Campanas Observatory, are used to compare the atmospheric structure at points separated by approximately 140 km along the planetary limb. The results reveal striking, but by no means perfect correlation of the light curves, ruling out isotropic turbulence as the cause of the light curve spikes. The atmosphere is strongly layered, and any acceptable turbulence model must accommodate the axial ratios of greater than about 60 which are observed. The mean temperature of the atmosphere is 150 plus or minus 15 K for the region near number density 10 to the 14th per cu cm. Derived temperature variations of vertical scale approximately 130 km and amplitude plus or minus 5 K are in agreement for all stations, and correlated spikes correspond to low-amplitude temperature variations with a vertical scale of several kilometers.
NASA Astrophysics Data System (ADS)
Bateson, Colin; Aliseda, Alberto
2015-11-01
We present results from wind tunnel experiments on the evolution of small inertial (d ~ 10 - 200 μm) water droplets in homogeneous, isotropic, slowly decaying grid turbulence. High-speed imaging and a Particle Tracking algorithm are used to calculate relative velocity distributions. We analyze the preferential concentration, via the 2D Radial Distribution Function, and enhanced relative velocity of droplets resulting from their inertial interactions with the underlying turbulence. The two-dimensional particle velocities, measured from multi-image tracks along a streamwise plane, are conditionally analyzed with respect to the distance from the nearest particle. We focus on the non-normality of the statistics for the particle-particle separation velocity component to examine the influence of the inertial interaction with the turbulence on the dynamics of the droplets. We observe a negative bias (in the mean and mode) in the separation velocity of particles for short separations, signaling a tendency of particles to collide more frequently than a random agitation by turbulence would predict. The tails of the distribution are interpreted in terms of the collision/coalescence process and the probability of collisions that do not lead to coalescence.
On the effects of density ratio on droplet-laden isotropic turbulence
NASA Astrophysics Data System (ADS)
Ferrante, Antonino; Dodd, Michael
2016-11-01
Our objective is to determine the effects of varying the droplet- to carrier-fluid density ratio (ρd /ρc) on the interaction of droplets with turbulence. We performed DNS of 3130 finite-size, non-evaporating droplets of diameter approximately equal to the Taylor lengthscale and with 5% droplet volume fraction in decaying isotropic turbulence at initial Taylor-scale Reynolds number Reλ = 83 . We varied ρd /ρc from 1 to 100 while keeping the Weber number and dynamic viscosity ratio constant, Werms=1 and μd /μc =1. We derived the turbulence kinetic energy (TKE) equations for the two-fluid, carrier-fluid and droplet-fluid flow. These equations allow us to explain the pathways for TKE exchange between the carrier turbulent flow and the flow inside the droplet. We show that increasing ρd /ρc increases the decay rate of TKE in the two-fluid flow. The TKE budget shows that this increase is caused by an increase in the dissipation rate of TKE and a decrease in the power of the surface tension. The underlying physical mechanisms for these behaviors will be presented.
NASA Astrophysics Data System (ADS)
Imtiaz, Ahmad; Lu, Zhi-Ming; Liu, Yu-Lu
2014-01-01
Streamwise evolution of longitudinal and transverse velocity structure functions in a decaying homogeneous and nearly isotropic turbulence is reported for Reynolds numbers Reλ up to 720. First, two theoretical relations between longitudinal and transverse structure functions are examined in the light of recently derived relations and the results show that the low-order transverse structure functions can be well approximated by longitudinal ones within the sub-inertial range. Reconstruction of fourth-order transverse structure functions with a recently proposed relation by Grauer et al. is comparatively less valid than the relation already proposed by Antonia et al. Secondly, extended self-similarity methods are used to measure the scaling exponents up to order eight and the streamwise evolution of scaling exponents is explored. The scaling exponents of longitudinal structure functions are, at first location, close to Zybin's model, and at the fourth location, close to She—Leveque model. No obvious trend is found for the streamwise evolution of longitudinal scaling exponents, whereas, on the contrary, transverse scaling exponents become slightly smaller with the development of a steamwise direction. Finally, the stremwise variation of the order-dependent isotropy ratio indicates the turbulence at the last location is closer to isotropic than the other three locations.
Magnetic Field Line Random Walk in Isotropic Turbulence with Varying Mean Field
NASA Astrophysics Data System (ADS)
Sonsrettee, W.; Subedi, P.; Ruffolo, D.; Matthaeus, W. H.; Snodin, A. P.; Wongpan, P.; Chuychai, P.; Rowlands, G.; Vyas, S.
2016-08-01
In astrophysical plasmas, the magnetic field line random walk (FLRW) plays an important role in guiding particle transport. The FLRW behavior is scaled by the Kubo number R=(b/{B}0)({{\\ell }}\\parallel /{{\\ell }}\\perp ) for rms magnetic fluctuation b, large-scale mean field {{\\boldsymbol{B}}}0, and coherence scales parallel ({{\\ell }}\\parallel ) and perpendicular ({{\\ell }}\\perp ) to {{\\boldsymbol{B}}}0. Here we use a nonperturbative analytic framework based on Corrsin’s hypothesis, together with direct computer simulations, to examine the R-scaling of the FLRW for varying B 0 with finite b and isotropic fluctuations with {{\\ell }}\\parallel /{{\\ell }}\\perp =1, instead of the well-studied route of varying {{\\ell }}\\parallel /{{\\ell }}\\perp for b \\ll {B}0. The FLRW for isotropic magnetic fluctuations is also of astrophysical interest regarding transport processes in the interstellar medium. With a mean field, fluctuations may have variance anisotropy, so we consider limiting cases of isotropic variance and transverse variance (with b z = 0). We obtain analytic theories, and closed-form solutions for extreme cases. Padé approximants are provided to interpolate all versions of theory and simulations to any B 0. We demonstrate that, for isotropic turbulence, Corrsin-based theories generally work well, and with increasing R there is a transition from quasilinear to Bohm diffusion. This holds even with b z = 0, when different routes to R\\to ∞ are mathematically equivalent; in contrast with previous studies, we find that a Corrsin-based theory with random ballistic decorrelation works well even up to R = 400, where the effects of trapping are barely perceptible in simulation results.
Presumed PDF Modeling of Early Flame Propagation in Moderate to Intense Turbulence Environments
NASA Technical Reports Server (NTRS)
Carmen, Christina; Feikema, Douglas A.
2003-01-01
The present paper describes the results obtained from a one-dimensional time dependent numerical technique that simulates early flame propagation in a moderate to intense turbulent environment. Attention is focused on the development of a spark-ignited, premixed, lean methane/air mixture with the unsteady spherical flame propagating in homogeneous and isotropic turbulence. A Monte-Carlo particle tracking method, based upon the method of fractional steps, is utilized to simulate the phenomena represented by a probability density function (PDF) transport equation. Gaussian distributions of fluctuating velocity and fuel concentration are prescribed. Attention is focused on three primary parameters that influence the initial flame kernel growth: the detailed ignition system characteristics, the mixture composition, and the nature of the flow field. The computational results of moderate and intense isotropic turbulence suggests that flames within the distributed reaction zone are not as vulnerable, as traditionally believed, to the adverse effects of increased turbulence intensity. It is also shown that the magnitude of the flame front thickness significantly impacts the turbulent consumption flame speed. Flame conditions studied have fuel equivalence ratio s in the range phi = 0.6 to 0.9 at standard temperature and pressure.
NASA Astrophysics Data System (ADS)
Hackl, J. F.; Yeung, P. K.; Sawford, B. L.
2009-11-01
Numerical simulations at up to (4096^3) grid resolution have been conducted on machines with very large processor counts to obtain the statistics of Lagrangian particle pairs and tetrads in turbulent relative dispersion. Richardson-Obukhov scaling for mean-square pair separation adjusted for initial conditions is observed for intermediate initial separations, in support of prior estimates of about 0.6 for Richardson's constant. Simulations at (Rλ 650) have also been conducted for sufficient duration to obtain fully converged exit time statistics for independently moving particles at very large scales. The fact that all particle pairs reach such large scales of separation means the inertial subrange of exit times is also captured accurately. The results show Kolmogorov scaling for positive moments of exit time, but a strong dependence on initial separations for inverse moments. Inertial-range estimates of tetrad shape factors are reinforced by simulations at Taylor-scale Reynolds numbers up to about 1100. Tetrad shape parameters conditioned on cluster size are also examined in order to understand geometric features of turbulent dispersion in more detail.
Parametric Study of Decay of Homogeneous Isotropic Turbulence Using Large Eddy Simulation
NASA Technical Reports Server (NTRS)
Swanson, R. C.; Rumsey, Christopher L.; Rubinstein, Robert; Balakumar, Ponnampalam; Zang, Thomas A.
2012-01-01
Numerical simulations of decaying homogeneous isotropic turbulence are performed with both low-order and high-order spatial discretization schemes. The turbulent Mach and Reynolds numbers for the simulations are 0.2 and 250, respectively. For the low-order schemes we use either second-order central or third-order upwind biased differencing. For higher order approximations we apply weighted essentially non-oscillatory (WENO) schemes, both with linear and nonlinear weights. There are two objectives in this preliminary effort to investigate possible schemes for large eddy simulation (LES). One is to explore the capability of a widely used low-order computational fluid dynamics (CFD) code to perform LES computations. The other is to determine the effect of higher order accuracy (fifth, seventh, and ninth order) achieved with high-order upwind biased WENO-based schemes. Turbulence statistics, such as kinetic energy, dissipation, and skewness, along with the energy spectra from simulations of the decaying turbulence problem are used to assess and compare the various numerical schemes. In addition, results from the best performing schemes are compared with those from a spectral scheme. The effects of grid density, ranging from 32 cubed to 192 cubed, on the computations are also examined. The fifth-order WENO-based scheme is found to be too dissipative, especially on the coarser grids. However, with the seventh-order and ninth-order WENO-based schemes we observe a significant improvement in accuracy relative to the lower order LES schemes, as revealed by the computed peak in the energy dissipation and by the energy spectrum.
Isotropic boundary adapted wavelets for coherent vorticity extraction in turbulent channel flows
NASA Astrophysics Data System (ADS)
Farge, Marie; Sakurai, Teluo; Yoshimatsu, Katsunori; Schneider, Kai; Morishita, Koji; Ishihara, Takashi
2015-11-01
We present a construction of isotropic boundary adapted wavelets, which are orthogonal and yield a multi-resolution analysis. We analyze DNS data of turbulent channel flow computed at a friction-velocity based Reynolds number of 395 and investigate the role of coherent vorticity. Thresholding of the wavelet coefficients allows to split the flow into two parts, coherent and incoherent vorticity. The statistics of the former, i.e., energy and enstrophy spectra, are close to the ones of the total flow, and moreover the nonlinear energy budgets are well preserved. The remaining incoherent part, represented by the large majority of the weak wavelet coefficients, corresponds to a structureless, i.e., noise-like, background flow and exhibits an almost equi-distribution of energy.
Passive scalar convective-diffusive subrange for low Prandtl numbers in isotropic turbulence
NASA Astrophysics Data System (ADS)
Briard, A.; Gomez, T.
2015-01-01
In this Rapid Communication, we study the behavior of a strongly diffusive passive scalar field T submitted to a freely decaying, homogeneous and isotropic turbulence with eddy-damped quasinormal Markovian simulations. We present a new subrange located between the k-17 /3 inertial-diffusive subrange and the Kolmogorov wave number kη. This subrange is generated by small-scale convection linked to kη that balances diffusion effects. Thus, we build a typical length scale kCD -1 based on convection and diffusion and give an expression for the shape of the passive scalar spectrum in this subrange ET˜√{Pr}k-11 /3 using physical arguments. This result unifies two different theories coming from Batchelor [G. K. Batchelor, J. Fluid. Mech. 5, 113 (1959), 10.1017/S002211205900009X] and Chasnov [J. Chasnov et al., Phys. Fluids A 1, 1698 (1989), 10.1063/1.857535] and explains results previously obtained experimentally.
A Lagrangian study of scalar diffusion in isotropic turbulence with chemical reaction
NASA Astrophysics Data System (ADS)
Mitarai, S.; Riley, J. J.; Kosály, G.
2003-12-01
Direct numerical simulations are performed of a single-step, nonpremixed, Arrhenius-type reaction developing in isotropic, incompressible, decaying turbulence, for conditions where flame extinction and re-ignition occur. The Lagrangian characteristics of scalar diffusion, information necessary for modeling approaches such as some implementations of probability density function (PDF) methods, are investigated by tracking fluid particles. Focusing on the mixture fraction and temperature as the scalar variables of interest, fluid particles are characterized as continuously burning or noncontinuously burning based upon their recent time history, and noncontinuously burning particles are further characterized based upon their initial regions relative to the flame zone. The behavior of the mixture fraction and temperature fields is contrasted for the different types of particles characterized. Significant differences among these characterized particles are found, for example, in the unclosed conditional expectations of scalar diffusion appearing in the composition PDF equations.
Calculation of velocity structure functions for vortex models of isotropic turbulence
Saffman, P.G.; Pullin, D.I.
1996-11-01
Velocity structure functions ({ital u}{sub {ital p}}{sup {prime}}{minus}{ital u}{sub {ital p}}){sup {ital m}} are calculated for vortex models of isotropic turbulence. An integral operator is introduced which defines an isotropic two-point field from a volume-orientation average for a specific solution of the Navier{endash}Stokes equations. Applying this to positive integer powers of the longitudinal velocity difference then gives explicit formulas for ({ital u}{sub {ital p}}{sup {prime}}{minus}{ital u}{sub {ital p}}){sup {ital m}} as a function of order {ital m} and of the scalar separation {ital r}. Special forms of the operator are then obtained for rectilinear stretched vortex models of the Townsend{endash}Lundgren type. Numerical results are given for the Burgers vortex and also for a realization of the Lundgren-strained spiral vortex, and comparison with experimental measurement is made. In an Appendix, we calculate values of the velocity-derivative moments for the Townsend{endash}Burgers model. {copyright} {ital 1996 American Institute of Physics.}
NASA Astrophysics Data System (ADS)
Gotoh, Toshiyuki
2012-11-01
Spectrum of passive scalar variance at very high Schmidt number up to 1000 in isotropic steady turbulence has been studied by using very high resolution DNS. Gaussian random force and scalar source which are isotropic and white in time are applied at low wavenumber band. Since the Schmidt number is very large, the system was integrated for 72 large eddy turn over time for the system to forgot the initial state. It is found that the scalar spectrum attains the asymptotic k-1 spectrum in the viscous-convective range and the constant CB is found to be 5.7 which is larger than 4.9 obtained by DNS under the uniform mean scalar gradient. Reasons for the difference are inferred as the Reynolds number effect, anisotropy, difference in the scalar injection, duration of time average, and the universality of the constant is discussed. The constant CB is also compared with the prediction by the Lagrangian statistical theory for the passive scalar. The scalar spectrum in the far diffusive range is found to be exponential, which is consistent with the Kraichnan's spectrum. However, the Kraichnan spectrum was derived under the assumption that the velocity field is white in time, therefore theoretical explanation of the agreement needs to be explored. Grant-in-Aid for Scientific Research No. 21360082, Ministry of Education, Culture, Sports, Science and Technology of Japan.
NASA Astrophysics Data System (ADS)
Collins, Lance; Bragg, Andrew; Ireland, Peter
2014-11-01
In this talk, we consider the physical mechanism for the clustering of inertial particles in the inertial range of turbulence. By comparisons with DNS data we demonstrate that the mechanism in the theory of Zaichik et al. (Phys. Fluids 19, 113308, 2007) quantitatively describes the clustering of particles in the inertial range. We then analyze the theory for isotropic turbulence in the limit Reλ --> ∞ . For arbitrary St (Stokes number), there exists a separation in the inertial range beyond which Str << 1 , where Str is the Stokes number based on the eddy turnover timescale at separation r. The inertial-range clustering in this limit can be understood to be due to the preferential sampling of the coarse-grained velocity gradient tensor at that scale. At smaller separations, there may be transitions to Str ~ 1 , where a path history symmetry breaking effect dominates the clustering mechanism, and in some cases Str >> 1 , which implies ballistic behavior and a flat RDF. The scaling for each of these regimes is derived and compared to DNS, where applicable. Finally, we compare the results with the ``sweep-stick'' mechanism by Coleman and Vassilicos (Phys. Fluids 21, 113301, 2009) and discuss the similarities and differences between the two theories.
Direct numerical simulation of two-particle relative diffusion in isotropic turbulence
NASA Astrophysics Data System (ADS)
Yeung, P. K.
1994-10-01
The relative diffusion of fluid particle pairs in statistically stationary isotropic turbulence is studied by direct numerical simulation, at a Taylor-scale Reynolds number of about 90. The growth of two-particle separation exhibits asymptotic stages at small and large diffusion times. Through the two-particle separation, particle-pair velocity correlations are closely related to the Eulerian spatial structure of the turbulence. At large times, the square of the separation distance has a chi-square probability distribution. At the moderate Reynolds number of the simulations, for this asymptotic distribution to be reached before the particles begin to move independently of each other, the initial separation must be small compared to the Kolmogorov scale. In an inertial frame moving with the initial particle velocities, the velocity increments of two fluid particles become uncorrelated only if their initial velocities are uncorrelated, which requires their initial separation be large compared to the integral length scale. For sufficiently large initial separations, the relative velocity increments and mean-square dispersion in this moving frame display a resemblance to inertial range scaling, but with a proportionality constant that is much smaller than classical estimates. At large times, the degree of preferential alignment between the separation and relative velocity vectors is weak, but the product of the separation distance and the velocity component projected along the separation vector is sustained on average.
NASA Astrophysics Data System (ADS)
Yu, Huidan; Meneveau, Charles
2010-11-01
We study the Lagrangian time evolution of velocity gradient dynamics near the Vieillefosse tail. The data are obtained from fluid particle tracking through the 1024^4 space-time DNS of forced isotropic turbulence at Reλ=433, using a web-based public database (http://turbulence.pha.jhu.edu). Examination of individual time-series of velocity gradient invariants R and Q show that they are punctuated by strong peaks of negative Q and positive R. Most of these occur very close to the Viellefosse tail along Q = - (3/2^2/3) R^2/3. It is found there that the magnitude of pressure Hessian has positive Lagrangian time-derivative, meaning that it increases in order to resist the rapid growth. We also observe a "phase delay" of the pressure Hessian signals compared to those of R and Q, indicative of an "overshoot" of the controlling mechanism. We also examine the trajectories in the recently proposed 3-D extension of the R-Q plane (see Lüthi B, Holzner M, Tsinober A. 2009, J. Fluid Mech. 641, 497-507). Finally, Lagrangian models of the velocity gradient tensor are examined in the same light to identify similarities and differences with the observed dynamics. Such comparisons supply informative guidance to model improvements.
NASA Astrophysics Data System (ADS)
Marié, J. L.; Tronchin, T.; Grosjean, N.; Méès, L.; Öztürk, O. Can; Fournier, C.; Barbier, B.; Lance, M.
2017-02-01
The evaporation rate of diethyl ether droplets dispersing in a homogeneous, nearly isotropic turbulence is measured by following droplets along their trajectory. Measurements are performed at ambient temperature and pressure by using in-line digital holography. The holograms of droplets are recorded with a single high-speed camera (3 kHz), and droplets trajectories are reconstructed with an "inverse problem approach" (IPA) algorithm previously used in Chareyron et al. (New J Phys 14:043039, 2012) and Marié et al. (Exp Fluid 55(4):1708, 2014. doi: 10.1007/s00348-014-1708-6). The thermal/vapor concentration wakes developing around the droplets are visible behind each hologram. A standard reconstruction process is applied, showing that these wakes are aligned with the relative Lagrangian velocity seen by droplets at each instant. This relative velocity is that obtained from the dynamic equation of droplets motion and the positions and diameter of the droplets measured by holography and the IPA reconstruction. Sequences of time evolution of droplets 3D positions, diameter and 3D relative velocity are presented. In a number of cases, the evaporation rate of droplets changes along the trajectory and deviates from the value estimated with a standard film model of evaporation. This shows that turbulence may significantly influence the phase change process.
The rotation and translation of non-spherical particles in homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
Byron, Margaret
The motion of particles suspended in environmental turbulence is relevant to many scientific fields, from sediment transport to biological interactions to underwater robotics. At very small scales and simple shapes, we are able to completely mathematically describe the motion of inertial particles; however, the motion of large aspherical particles is significantly more complex, and current computational models are inadequate for large or highly-resolved domains. Therefore, we seek to experimentally investigate the coupling between freely suspended particles and ambient turbulence. A better understanding of this coupling will inform not only engineering and physics, but the interactions between small aquatic organisms and their environments. In the following pages, we explore the roles of shape and buoyancy on the motion of passive particles in turbulence, and allow these particles to serve as models for meso-scale aquatic organisms. We fabricate cylindrical and spheroidal particles and suspend them in homogeneous, isotropic turbulence that is generated via randomly-actuated jet arrays. The particles are fabricated with agarose hydrogel, which is refractive-index-matched to the surrounding fluid (water). Both the fluid and the particle are seeded with passive tracers, allowing us to perform Particle Image Velocimetry (PIV) simultaneously on the particle and fluid phase. To investigate the effects of shape, particles are fabricated at varying aspect ratios; to investigate the effects of buoyancy, particles are fabricated at varying specific gravities. Each particle type is freely suspended at a volume fraction of F=0.1%, for which four-way coupling interactions are negligible. The suspended particles are imaged together with the surrounding fluid and analyzed using stereoscopic PIV, which yields three velocity components in a two-dimensional measurement plane. Using image thresholding, the results are separated into simultaneous fluid-phase and solid-phase velocity
Large-deviation joint statistics of the finite-time Lyapunov spectrum in isotropic turbulence
Johnson, Perry L. Meneveau, Charles
2015-08-15
One of the hallmarks of turbulent flows is the chaotic behavior of fluid particle paths with exponentially growing separation among them while their distance does not exceed the viscous range. The maximal (positive) Lyapunov exponent represents the average strength of the exponential growth rate, while fluctuations in the rate of growth are characterized by the finite-time Lyapunov exponents (FTLEs). In the last decade or so, the notion of Lagrangian coherent structures (which are often computed using FTLEs) has gained attention as a tool for visualizing coherent trajectory patterns in a flow and distinguishing regions of the flow with different mixing properties. A quantitative statistical characterization of FTLEs can be accomplished using the statistical theory of large deviations, based on the so-called Cramér function. To obtain the Cramér function from data, we use both the method based on measuring moments and measuring histograms and introduce a finite-size correction to the histogram-based method. We generalize the existing univariate formalism to the joint distributions of the two FTLEs needed to fully specify the Lyapunov spectrum in 3D flows. The joint Cramér function of turbulence is measured from two direct numerical simulation datasets of isotropic turbulence. Results are compared with joint statistics of FTLEs computed using only the symmetric part of the velocity gradient tensor, as well as with joint statistics of instantaneous strain-rate eigenvalues. When using only the strain contribution of the velocity gradient, the maximal FTLE nearly doubles in magnitude, highlighting the role of rotation in de-correlating the fluid deformations along particle paths. We also extend the large-deviation theory to study the statistics of the ratio of FTLEs. The most likely ratio of the FTLEs λ{sub 1} : λ{sub 2} : λ{sub 3} is shown to be about 4:1:−5, compared to about 8:3:−11 when using only the strain-rate tensor for calculating fluid volume
Large-deviation joint statistics of the finite-time Lyapunov spectrum in isotropic turbulence
NASA Astrophysics Data System (ADS)
Johnson, Perry L.; Meneveau, Charles
2015-08-01
One of the hallmarks of turbulent flows is the chaotic behavior of fluid particle paths with exponentially growing separation among them while their distance does not exceed the viscous range. The maximal (positive) Lyapunov exponent represents the average strength of the exponential growth rate, while fluctuations in the rate of growth are characterized by the finite-time Lyapunov exponents (FTLEs). In the last decade or so, the notion of Lagrangian coherent structures (which are often computed using FTLEs) has gained attention as a tool for visualizing coherent trajectory patterns in a flow and distinguishing regions of the flow with different mixing properties. A quantitative statistical characterization of FTLEs can be accomplished using the statistical theory of large deviations, based on the so-called Cramér function. To obtain the Cramér function from data, we use both the method based on measuring moments and measuring histograms and introduce a finite-size correction to the histogram-based method. We generalize the existing univariate formalism to the joint distributions of the two FTLEs needed to fully specify the Lyapunov spectrum in 3D flows. The joint Cramér function of turbulence is measured from two direct numerical simulation datasets of isotropic turbulence. Results are compared with joint statistics of FTLEs computed using only the symmetric part of the velocity gradient tensor, as well as with joint statistics of instantaneous strain-rate eigenvalues. When using only the strain contribution of the velocity gradient, the maximal FTLE nearly doubles in magnitude, highlighting the role of rotation in de-correlating the fluid deformations along particle paths. We also extend the large-deviation theory to study the statistics of the ratio of FTLEs. The most likely ratio of the FTLEs λ1 : λ2 : λ3 is shown to be about 4:1:-5, compared to about 8:3:-11 when using only the strain-rate tensor for calculating fluid volume deformations. The results
The length distribution of streamline segments in homogeneous isotropic decaying turbulence
NASA Astrophysics Data System (ADS)
Schaefer, P.; Gampert, M.; Peters, N.
2012-04-01
by Schaefer et al. ["Fast and slow changes of the length of gradient trajectories in homogenous shear turbulence," in Advances in Turbulence XII, edited by B. Eckhardt (Springer-Verlag, Berlin, 2009), pp. 565-572] we will refer to the morphological part of the evolution of streamline segments as slow changes while the topological part of the evolution is referred to as fast changes. This separation yields a transport equation for the probability density function (pdf) P(l) of the arclength l of streamline segments in which the slow changes translate into a convection and a diffusion term when terms up to second order are included and the fast changes yield integral terms. The overall temporal evolution (morphological and topological) of the arclength l of streamline segments is analyzed and associated with the motion of the above isosurface. This motion is diffusion controlled for small segments, while large segments are mainly subject to strain and pressure fluctuations. The convection velocity corresponds to the first order jump moment, while the diffusion term includes the second order jump moment. It is concluded, both theoretically and from direct numerical simulations (DNS) data of homogeneous isotropic decaying turbulence at two different Reynolds numbers, that the normalized first order jump moment is quasi-universal, while the second order one is proportional to the inverse of the square root of the Taylor based Reynolds number Re_{λ }^{-1/2}. Its inclusion thus represents a small correction in the limit of large Reynolds numbers. Numerical solutions of the pdf equation yield a good agreement with the pdf obtained from the DNS data. The interplay of viscous drift acting on small segments and linear strain acting on large segments yield, as it has already been concluded for dissipation elements, that the mean length of streamline segments should scale with Taylor microscale.
MAGNETIC FIELD LINE RANDOM WALK IN ISOTROPIC TURBULENCE WITH ZERO MEAN FIELD
Sonsrettee, W.; Ruffolo, D.; Snodin, A. P.; Wongpan, P.; Subedi, P.; Matthaeus, W. H.; Chuychai, P. E-mail: david.ruf@mahidol.ac.th E-mail: pat.wongpan@postgrad.otago.ac.nz E-mail: prasub@udel.edu
2015-01-01
In astrophysical plasmas, magnetic field lines often guide the motions of thermal and non-thermal particles. The field line random walk (FLRW) is typically considered to depend on the Kubo number R = (b/B {sub 0})(ℓ{sub ∥}/ℓ ) for rms magnetic fluctuation b, large-scale mean field B {sub 0}, and parallel and perpendicular coherence scales ℓ{sub ∥} and ℓ , respectively. Here we examine the FLRW when R → ∞ by taking B {sub 0} → 0 for finite b{sub z} (fluctuation component along B {sub 0}), which differs from the well-studied route with b{sub z} = 0 or b{sub z} << B {sub 0} as the turbulence becomes quasi-two-dimensional (quasi-2D). Fluctuations with B {sub 0} = 0 are typically isotropic, which serves as a reasonable model of interstellar turbulence. We use a non-perturbative analytic framework based on Corrsin's hypothesis to determine closed-form solutions for the asymptotic field line diffusion coefficient for three versions of the theory, which are directly related to the k {sup –1} or k {sup –2} moment of the power spectrum. We test these theories by performing computer simulations of the FLRW, obtaining the ratio of diffusion coefficients for two different parameterizations of a field line. Comparing this with theoretical ratios, the random ballistic decorrelation version of the theory agrees well with the simulations. All results exhibit an analog to Bohm diffusion. In the quasi-2D limit, previous works have shown that Corrsin-based theories deviate substantially from simulation results, but here we find that as B {sub 0} → 0, they remain in reasonable agreement. We conclude that their applicability is limited not by large R, but rather by quasi-two-dimensionality.
The minimum energy decay rate in quasi-isotropic grid turbulence
NASA Astrophysics Data System (ADS)
Davidson, P. A.
2011-08-01
We consider high Reynolds number, freely-decaying, isotropic turbulence in which the large scales evolve in a self-similar manner when normalized by the integral scales, u and ℓ. As it is well known, a range of possible behaviors may be observed depending on the form of the longitudinal velocity correlation at large separation, uf∞=u 2f(r →∞). We consider the cases u2f∞=cmr-m,2≤m ≤6, whose spectral counterpart is E(k →0)~cmkm -1 for m <6, with or without a lnk correction, and E(k →0)~I k4 for m =6. (I is Loitsyansky's integral.) It has long been known that the cmm=constant during the decay. This, in turn, sets the energy decay rate as u2~t-(1-p)2m /(m+2), where p is the power-law exponent for the normalized dissipation rate, εℓ/εℓu3u3~t-p, observed empirically to be a small positive number in grid turbulence. We systematically explore the properties of these different classes of turbulence and arrive at the following conclusions. (i) The invariance of cm is a direct consequence of linear momentum conservation for m ≤4, and angular momentum conservation for m =5. (ii) The classical spectra of Saffman, E(k →0)~c3k2, and Batchelor, E(k →0)~Ik4, are robust in the sense that they emerge from a broad class of initial conditions. In particular, it is necessary only that <ωi ω'j >∞ ≤O(r-8) at t =0. The non-classical spectra (m =2,4,5), on the other hand, require very specific initial conditions in order to be realized, of the form <ωiω'j>∞=O(r-(m +2)). (Note the equality rather than the inequality here.) This makes the non-classical spectra less likely to be observed in practice. (iii) The case of m =2, which is usually associated with the u2~t-1 decay law, is pathological in a number of respects. For example, its spectral tensor diverges as k →0, and the long-range correlations
Miura, H.; Araki, K.
2014-07-15
Hall effects on local structures in homogeneous, isotropic, and incompressible magnetohydrodynamic turbulence are studied numerically. The transition of vortices from sheet-like to tubular structures induced by the Hall term is found, while the kinetic energy spectrum does not distinguish the two types of structures. It is shown by the use of the sharp low-pass filter that the transition occurs not only in the scales smaller than the ion skin depth but also in a larger scale. The transition is related with the forward energy transfer in the spectral space. Analyses by the use of the sharp low-pass filter show that the nonlinear energy transfer associated with the Hall term is dominated by the forward transfer and relatively local in the wave number space. A projection of the simulation data to a Smagorinsky-type sub-grid-scale model shows that the high wave number component of the Hall term may possibly be replaced by the model effectively.
A Stochastic Model for the Relative Motion of High Stokes Number Particles in Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Dhariwal, Rohit; Rani, Sarma; Koch, Donald
2014-11-01
In the current study, a novel analytical closure for the diffusion current in the PDF equation is presented that is applicable to high-inertia particle pairs with Stokes numbers Str >> 1 . Here Str is a Stokes number based on the time-scale τr of eddies whose size scales with pair separation r. Using this closure, Langevin equations were solved to evolve particle-pair relative velocities and separations in stationary isotropic turbulence. The Langevin equation approach enables the simulation of the full PDF of pair relative motion, instead of only the first few moments of the PDF as is the case in a moments-based approach. Accordingly, PDFs Ω (U | r) and Ω (Ur | r) are computed for various separations r, where the former is the PDF of relative velocity U and the latter is the PDF of the radial component of relative velocity Ur, both conditioned upon the separation r. Consistent with the DNS study of Sundaram & Collins, the Langevin simulations capture the transition of Ω (U | r) from being Gaussian at integral-scale separations to an exponential PDF at Kolmogorov-scale separations. The radial distribution functions (RDFs) computed from these simulations also show reasonable quantitative agreement with those from the DNS of Fevrier et al.
Intermittency in the isotropic component of helical and nonhelical turbulent flows
NASA Astrophysics Data System (ADS)
Martin, L. N.; Mininni, P. D.
2010-01-01
We analyze the isotropic component of turbulent flows spanning a broad range or Reynolds numbers. The aim is to identify scaling laws and their Reynolds number dependence in flows under different mechanical forcings. To this end, we applied an SO(3) decomposition to data stemming from direct numerical simulations with spatial resolutions ranging from 643 to 10243 grid points, and studied the scaling of high order moments of the velocity field. The study was carried out for two different flows obtained forcing the system with a Taylor-Green vortex or the Arn’old-Beltrami-Childress flow. Our results indicate that helicity has no significant impact on the scaling exponents as obtained from the generalized structure functions. Intermittency effects increase with the Reynolds number in the range of parameters studied, and in some cases are larger than what can be expected from several models of intermittency in the literature. The observed dependence of intermittency with the Reynolds number decreases if extended self-similarity is used to estimate the exponents.
Time resolved measurements of rigid fiber dispersion in near homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
Sabban, Lilach; Cohen, Asaf; van Hout, Rene; Empfl Environmental Multi-Phase Flow Laboratory Team
2013-11-01
Time resolved, planar particle image velocimetry (PIV, 3kHz) and two-orthogonal view, digital holographic cinematography (2kHz) was used to measure 3D fiber trajectories/orientation dynamics in near homogeneous isotropic air turbulence (HIT) with dilute suspended fibers. The PIV covered a field of view of 6 × 12 mm2 and the holography a volume of interest of 173 mm3, positioned at the center of the chamber. HIT (Reλ = 144) was generated in the center of a 403 cm3 cube by eight woofers mounted on each of its corners. Three different nylon fibers having a length of 0.5 mm and diameter of 10, 14 and 19 μm were released from the top of the chamber. Fibers had Stokes numbers of order one and are expected to accumulate in regions of low vorticity and settle along a path of local minimal drag. Fiber 3D trajectories/orientations have been obtained from the holography measurements and orientational/translational dispersion coefficients will be presented. In addition the flow field in the vicinity of tracked fibers has been resolved by the PIV, and results on fluid and fiber accelerations and position correlation with in-plane strain rate and out-of-plane vorticity will be presented.
NASA Astrophysics Data System (ADS)
Rani, Sarma; Koch, Donald
2012-11-01
In this study, we derived the Fokker-Planck equation governing the PDF of pair separation and relative velocity vectors of high St particles. The PDF equation contains a particle-pair diffusion coefficient in relative velocity space. We developed an analytical theory to predict this relative velocity-space pair diffusion coefficient in the limit of high St . Using the diffusion coefficient, Langevin-equation-based stochastic simulations were performed to evolve pair separation and velocity vectors in isotropic turbulence for particle Stokes numbers, St = 1 , 2 , 4 , 10 ,and, 20 and a Taylor micro-scale Reynolds number, Reλ = 75 . The most significant finding from the Langevin simulations is that our pair diffusivity theory successfully captures the transition of relative velocity PDF from a Gaussian PDF at separations of the order of integral length scale to a non-Gaussian PDF at smaller separations. The pair radial distribution functions (RDFs) computed using our theory show that as the Stokes number increased, particles preferentially accumulate even at integral length scale separations. Another significant finding of our approach is that the slope of RDF at Kolmogorov length scale separations for higher St particles is not zero.
NASA Astrophysics Data System (ADS)
Kaufmann, A.; Moreau, M.; Simonin, O.; Helie, J.
2008-06-01
The purpose of this paper is to evaluate the accuracy of the mesoscopic approach proposed by Février et al. [P. Février, O. Simonin, K.D. Squires, Partitioning of particle velocities in gas-solid turbulent flows into a continuous field and a spatially uncorrelated random distribution: theoretical formalism and numerical study, J. Fluid Mech. 533 (2005) 1-46] by comparison against the Lagrangian approach for the simulation of an ensemble of non-colliding particles suspended in a decaying homogeneous isotropic turbulence given by DNS. The mesoscopic Eulerian approach involves to solve equations for a few particle PDF moments: number density, mesoscopic velocity, and random uncorrelated kinetic energy (RUE), derived from particle flow ensemble averaging conditioned by the turbulent fluid flow realization. In addition, viscosity and diffusivity closure assumptions are used to compute the unknown higher order moments which represent the mesoscopic velocity and RUE transport by the uncorrelated velocity component. A detailed comparison between the two approaches is carried out for two different values of the Stokes number based on the initial fluid Kolmogorov time scale, St=0.17 and 2.2. In order to perform reliable comparisons for the RUE local instantaneous distribution and for the mesoscopic kinetic energy spectrum, the error due to the computation method of mesoscopic quantities from Lagrangian simulation results is evaluated and minimized. A very good agreement is found between the mesoscopic Eulerian and Lagrangian predictions for the small particle Stokes number case corresponding to the smallest particle inertia. For larger particle inertia, a bulk viscous term is included in the mesoscopic velocity governing equation to avoid spurious spatial oscillation that may arise due to the inability of the numerical scheme to resolve sharp number density gradients. As a consequence, for St=2.2, particle number density and RUE spatial distribution predicted by the
Absorption of intense microwaves and ion acoustic turbulence due to heat transport
De Groot, J.S.; Liu, J.M.; Matte, J.P.
1994-02-04
Measurements and calculations of the inverse bremsstrahlung absorption of intense microwaves are presented. The isotropic component of the electron distribution becomes flat-topped in agreement with detailed Fokker-Planck calculations. The plasma heating is reduced due to the flat-topped distributions in agreement with calculations. The calculations show that the heat flux at high microwave powers is very large, q{sub max} {approx} 0.3 n{sub e}v{sub e}T{sub e}. A new particle model to, calculate the heat transport inhibition due to ion acoustic turbulence in ICF plasmas is also presented. One-dimensional PIC calculations of ion acoustic turbulence excited due to heat transport are presented. The 2-D PIC code is presently being used to perform calculations of heat flux inhibition due to ion acoustic turbulence.
NASA Astrophysics Data System (ADS)
Dou, Zhongwang; Pecenak, Zachary K.; Cao, Lujie; Woodward, Scott H.; Liang, Zach; Meng, Hui
2016-03-01
Enclosed flow apparatuses with negligible mean flow are emerging as alternatives to wind tunnels for laboratory studies of homogeneous and isotropic turbulence (HIT) with or without aerosol particles, especially in experimental validation of Direct Numerical Simulation (DNS). It is desired that these flow apparatuses generate HIT at high Taylor-microscale Reynolds numbers ({{R}λ} ) and enable accurate measurement of turbulence parameters including kinetic energy dissipation rate and thereby {{R}λ} . We have designed an enclosed, fan-driven, highly symmetric truncated-icosahedron ‘soccer ball’ airflow apparatus that enables particle imaging velocimetry (PIV) and other whole-field flow measurement techniques. To minimize gravity effect on inertial particles and improve isotropy, we chose fans instead of synthetic jets as flow actuators. We developed explicit relations between {{R}λ} and physical as well as operational parameters of enclosed HIT chambers. To experimentally characterize turbulence in this near-zero-mean flow chamber, we devised a new two-scale PIV approach utilizing two independent PIV systems to obtain both high resolution and large field of view. Velocity measurement results show that turbulence in the apparatus achieved high homogeneity and isotropy in a large central region (48 mm diameter) of the chamber. From PIV-measured velocity fields, we obtained turbulence dissipation rates and thereby {{R}λ} by using the second-order velocity structure function. A maximum {{R}λ} of 384 was achieved. Furthermore, experiments confirmed that the root mean square (RMS) velocity increases linearly with fan speed, and {{R}λ} increases with the square root of fan speed. Characterizing turbulence in such apparatus paves the way for further investigation of particle dynamics in particle-laden homogeneous and isotropic turbulence.
Self-similar spiral flow structure in low Reynolds number isotropic and decaying turbulence
NASA Astrophysics Data System (ADS)
Vassilicos, J. C.; Brasseur, James G.
1996-07-01
It is rigorously proved for axisymmetric incompressible flows with bounded axial vorticity at infinity that if a spiral-helical streamline has a Kolmogorov capacity (box-counting dimension) DK>~1, then the velocity field must have a singularity at the axis of symmetry. Furthermore, certain types of singularity with DK=1 can be excluded. The Burgers and the Lundgren vortices are examples of strained vortices with different types of near-singular structure, and in both cases sections of streamlines have a well-defined DK>~1. However, the strain severely limits the region in space where DK is larger than 1. An algorithm is developed which detects streamlines with persistently strong curvature and calculates both the DK of the streamlines and the lower bound scale δmin of the range of self-similar scaling defined by DK. Error bounds on DK are also computed. The use of this algorithm partly relies on the fact that two to three turns of a spiral are enough to determine a spiral's DK. We detect well-defined self-similar scaling in the geometry of streamlines around vortex tubes in decaying isotropic direct numerical simulation turbulence with exceptionally fine small-scale resolution and Reλ around 20. The measured values of DK vary from DK=1 to DK~=1.60, and in general the self-similar range of length scales over which DK is well defined extends over one decade and ends at one of two well-defined inner scales, one just above and the other just below the Kolmogorov microscale η. We identify two different types of accumulation of length scales with DK>~1 on streamlines around the vortex tubes in the simulated turbulence: an accumulation of the streamline towards a central axis of the vortex tube in a spiral-helical fashion, and a helical and axial accumulation of the streamline towards a limit circle at the periphery of the vortex tube. In the latter case, the limit circle lies in a region along the axis of the vortex tube where there is a rapid drop in enstrophy. The
NASA Astrophysics Data System (ADS)
Aliseda, Alberto; Bourgoin, Mickael; Eswirp Collaboration
2014-11-01
We present preliminary results from a recent grid turbulence experiment conducted at the ONERA wind tunnel in Modane, France. The ESWIRP Collaboration was conceived to probe the smallest scales of a canonical turbulent flow with very high Reynolds numbers. To achieve this, the largest scales of the turbulence need to be extremely big so that, even with the large separation of scales, the smallest scales would be well above the spatial and temporal resolution of the instruments. The ONERA wind tunnel in Modane (8 m -diameter test section) was chosen as a limit of the biggest large scales achievable in a laboratory setting. A giant inflatable grid (M = 0.8 m) was conceived to induce slowly-decaying homogeneous isotropic turbulence in a large region of the test section, with minimal structural risk. An international team or researchers collected hot wire anemometry, ultrasound anemometry, resonant cantilever anemometry, fast pitot tube anemometry, cold wire thermometry and high-speed particle tracking data of this canonical turbulent flow. While analysis of this large database, which will become publicly available over the next 2 years, has only started, the Taylor-scale Reynolds number is estimated to be between 400 and 800, with Kolmogorov scales as large as a few mm . The ESWIRP Collaboration is formed by an international team of scientists to investigate experimentally the smallest scales of turbulence. It was funded by the European Union to take advantage of the largest wind tunnel in Europe for fundamental research.
Mechanisms for the clustering of inertial particles in the inertial range of isotropic turbulence
NASA Astrophysics Data System (ADS)
Bragg, Andrew D.; Ireland, Peter J.; Collins, Lance R.
2015-08-01
In this paper, we consider the physical mechanism for the clustering of inertial particles in the inertial range of isotropic turbulence. We analyze the exact, but unclosed, equation governing the radial distribution function (RDF) and compare the mechanisms it describes for clustering in the dissipation and inertial ranges. We demonstrate that in the limit Str≪1 , where Str is the Stokes number based on the eddy turnover time scale at separation r , the clustering in the inertial range can be understood to be due to the preferential sampling of the coarse-grained fluid velocity gradient tensor at that scale. When Str≳O (1 ) this mechanism gives way to a nonlocal clustering mechanism. These findings reveal that the clustering mechanisms in the inertial range are analogous to the mechanisms that we identified for the dissipation regime [see New J. Phys. 16, 055013 (2014), 10.1088/1367-2630/16/5/055013]. Further, we discuss the similarities and differences between the clustering mechanisms we identify in the inertial range and the "sweep-stick" mechanism developed by Coleman and Vassilicos [Phys. Fluids 21, 113301 (2009), 10.1063/1.3257638]. We show that the idea that initial particles are swept along with acceleration stagnation points is only approximately true because there always exists a finite difference between the velocity of the acceleration stagnation points and the local fluid velocity. This relative velocity is sufficient to allow particles to traverse the average distance between the stagnation points within the correlation time scale of the acceleration field. We also show that the stick part of the mechanism is only valid for Str≪1 in the inertial range. We emphasize that our clustering mechanism provides the more fundamental explanation since it, unlike the sweep-stick mechanism, is able to explain clustering in arbitrary spatially correlated velocity fields. We then consider the closed, model equation for the RDF given in Zaichik and
Mechanisms for the clustering of inertial particles in the inertial range of isotropic turbulence
Bragg, Andrew D.; Ireland, Peter J.; Collins, Lance R.
2015-08-27
In this study, we consider the physical mechanism for the clustering of inertial particles in the inertial range of isotropic turbulence. We analyze the exact, but unclosed, equation governing the radial distribution function (RDF) and compare the mechanisms it describes for clustering in the dissipation and inertial ranges. We demonstrate that in the limit St_{r} <<1, where St_{r} is the Stokes number based on the eddy turnover time scale at separation r, the clustering in the inertial range can be understood to be due to the preferential sampling of the coarse-grained fluid velocity gradient tensor at that scale. When St_{r}≳O(1) this mechanism gives way to a nonlocal clustering mechanism. These findings reveal that the clustering mechanisms in the inertial range are analogous to the mechanisms that we identified for the dissipation regime. Further, we discuss the similarities and differences between the clustering mechanisms we identify in the inertial range and the “sweep-stick” mechanism developed by Coleman and Vassilicos. We show that the idea that initial particles are swept along with acceleration stagnation points is only approximately true because there always exists a finite difference between the velocity of the acceleration stagnation points and the local fluid velocity. This relative velocity is sufficient to allow particles to traverse the average distance between the stagnation points within the correlation time scale of the acceleration field. We also show that the stick part of the mechanism is only valid for St_{r}<<1 in the inertial range. We emphasize that our clustering mechanism provides the more fundamental explanation since it, unlike the sweep-stick mechanism, is able to explain clustering in arbitrary spatially correlated velocity fields. We then consider the closed, model equation for the RDF given in Zaichik and Alipchenkov and use this, together with the results from our analysis, to predict the
Mechanisms for the clustering of inertial particles in the inertial range of isotropic turbulence
Bragg, Andrew D.; Ireland, Peter J.; Collins, Lance R.
2015-08-27
In this study, we consider the physical mechanism for the clustering of inertial particles in the inertial range of isotropic turbulence. We analyze the exact, but unclosed, equation governing the radial distribution function (RDF) and compare the mechanisms it describes for clustering in the dissipation and inertial ranges. We demonstrate that in the limit Str <<1, where Str is the Stokes number based on the eddy turnover time scale at separation r, the clustering in the inertial range can be understood to be due to the preferential sampling of the coarse-grained fluid velocity gradient tensor at that scale. When Str≳O(1)more » this mechanism gives way to a nonlocal clustering mechanism. These findings reveal that the clustering mechanisms in the inertial range are analogous to the mechanisms that we identified for the dissipation regime. Further, we discuss the similarities and differences between the clustering mechanisms we identify in the inertial range and the “sweep-stick” mechanism developed by Coleman and Vassilicos. We show that the idea that initial particles are swept along with acceleration stagnation points is only approximately true because there always exists a finite difference between the velocity of the acceleration stagnation points and the local fluid velocity. This relative velocity is sufficient to allow particles to traverse the average distance between the stagnation points within the correlation time scale of the acceleration field. We also show that the stick part of the mechanism is only valid for Str<<1 in the inertial range. We emphasize that our clustering mechanism provides the more fundamental explanation since it, unlike the sweep-stick mechanism, is able to explain clustering in arbitrary spatially correlated velocity fields. We then consider the closed, model equation for the RDF given in Zaichik and Alipchenkov and use this, together with the results from our analysis, to predict the analytic form of the RDF in the
Mechanisms for the clustering of inertial particles in the inertial range of isotropic turbulence.
Bragg, Andrew D; Ireland, Peter J; Collins, Lance R
2015-08-01
In this paper, we consider the physical mechanism for the clustering of inertial particles in the inertial range of isotropic turbulence. We analyze the exact, but unclosed, equation governing the radial distribution function (RDF) and compare the mechanisms it describes for clustering in the dissipation and inertial ranges. We demonstrate that in the limit Str≪1, where Str is the Stokes number based on the eddy turnover time scale at separation r, the clustering in the inertial range can be understood to be due to the preferential sampling of the coarse-grained fluid velocity gradient tensor at that scale. When Str≳O(1) this mechanism gives way to a nonlocal clustering mechanism. These findings reveal that the clustering mechanisms in the inertial range are analogous to the mechanisms that we identified for the dissipation regime [see New J. Phys. 16, 055013 (2014)]. Further, we discuss the similarities and differences between the clustering mechanisms we identify in the inertial range and the "sweep-stick" mechanism developed by Coleman and Vassilicos [Phys. Fluids 21, 113301 (2009)]. We show that the idea that initial particles are swept along with acceleration stagnation points is only approximately true because there always exists a finite difference between the velocity of the acceleration stagnation points and the local fluid velocity. This relative velocity is sufficient to allow particles to traverse the average distance between the stagnation points within the correlation time scale of the acceleration field. We also show that the stick part of the mechanism is only valid for Str≪1 in the inertial range. We emphasize that our clustering mechanism provides the more fundamental explanation since it, unlike the sweep-stick mechanism, is able to explain clustering in arbitrary spatially correlated velocity fields. We then consider the closed, model equation for the RDF given in Zaichik and Alipchenkov [Phys. Fluids 19, 113308 (2007)] and use this
NASA Astrophysics Data System (ADS)
Ferreira, Pedro O.; Pinho, Fernando T.; da Silva, Carlos B.
2016-12-01
A new subgrid-scale (SGS) model developed for large-eddy simulations (LES) of dilute polymer solutions, described by the finitely extensible nonlinear elastic constitutive equation closed with the Peterlin approximation, is presented. In this distortion similarity model (DSIM) the filtered conformation tensor evolution equation is based on the self-similarity of the polymer stretching terms, and on a global equilibrium of the trace of the conformation tensor, which is proportional to the elastic energy stored in the polymer molecules, while the SGS stresses are modelled with the classical Smagorinsky model. The DSIM closure is assessed in direct numerical simulations (DNS) of forced isotropic turbulence using classical a priori tests, and in a posteriori (LES) showing very good agreement with all the exact (filtered DNS) results. The DSIM model is simple to implement and computationally inexpensive and represents a major step forward in the numerical simulation of turbulent flows of Newtonian fluids with polymer additives.
NASA Astrophysics Data System (ADS)
Venkatachari, Balaji Shankar; Chang, Chau-Lyan
2016-11-01
The focus of this study is scale-resolving simulations of the canonical normal shock- isotropic turbulence interaction using unstructured tetrahedral meshes and the space-time conservation element solution element (CESE) method. Despite decades of development in unstructured mesh methods and its potential benefits of ease of mesh generation around complex geometries and mesh adaptation, direct numerical or large-eddy simulations of turbulent flows are predominantly carried out using structured hexahedral meshes. This is due to the lack of consistent multi-dimensional numerical formulations in conventional schemes for unstructured meshes that can resolve multiple physical scales and flow discontinuities simultaneously. The CESE method - due to its Riemann-solver-free shock capturing capabilities, non-dissipative baseline schemes, and flux conservation in time as well as space - has the potential to accurately simulate turbulent flows using tetrahedral meshes. As part of the study, various regimes of the shock-turbulence interaction (wrinkled and broken shock regimes) will be investigated along with a study on how adaptive refinement of tetrahedral meshes benefits this problem. The research funding for this paper has been provided by Revolutionary Computational Aerosciences (RCA) subproject under the NASA Transformative Aeronautics Concepts Program (TACP).
Statistics of intense turbulent vorticity events.
Moriconi, L
2004-08-01
We investigate statistical properties of vorticity fluctuations in fully developed turbulence, which are known to exhibit a strong intermittent behavior. Taking as the starting point the Navier-Stokes equations with a random force term correlated at large scales, we obtain in the high Reynolds number regime a closed analytical expression for the probability distribution function of an arbitrary component of the vorticity field. The central idea underlying the analysis consists in the restriction of the velocity configurational phase-space to a particular sector where the rate of strain and the rotation tensors can be locally regarded as slow and fast degrees of freedom, respectively. This prescription is implemented along the Martin-Siggia-Rose functional framework, whereby instantons and perturbations around them are taken into account within a steepest-descent approach.
Comparison of hot wire/laser velocimeter turbulence intensity measurements
NASA Technical Reports Server (NTRS)
Meyers, J. F.; Wilkinson, S. P.
1982-01-01
The question of whether a random measure of particle velocities yields a good statistical estimate of the stationary condition of the turbulence flow field was investigated by comparing hot-wire and laser velocimeter turbulence intensity measurements. Great care was taken to insure that the instrument precision of both the laser velocimeter and hot wire was maximized. In this attempt to reduce the measurement uncertainties in the hot wire, direct digitization of the analog output signal was performed with point-by-point conversion to velocity through a spline fit calibration curve and the turbulence intensity function was calculated statistically. Frequent calibrations of the hot wire were performed using the laser velocimeter as the velocity standard to account for the presence of the small seed particles in the air flow and signal drift in the hot wire.
Lateral Turbulence Intensity and Plume Meandering During Stable Conditions.
NASA Astrophysics Data System (ADS)
Hanna, Steven R.
1983-08-01
There is much evidence in the literature for the presence of mesoscale lateral meanders in the stable nighttime boundary layer. These meanders result in relatively high lateral turbulence intensities and diffusion rates when averaged over an hour. Anemometer data from 17 overnight experiments at Cinder Cone Butte in Idaho are analyzed to show that the dominant period of the mesoscale meanders is about two hours. Lidar cross-sections of tracer plumes from these same experiments show that the hourly average y is often dominated by meandering. Since meandering is not always observed for given meteorological conditions, it is suggested that nighttime diffusion cannot be accurately predicted without using onsite observations of wind fluctuations. In case no turbulence data are available, an empirical formula is suggested that predicts the hourly average lateral turbulence intensity as a function of wind speed and hour-to-hour variation in wind direction.
NASA Astrophysics Data System (ADS)
Bassenne, Maxime; Urzay, Javier; Park, George I.; Moin, Parviz
2016-03-01
This study investigates control-based forcing methods for incompressible homogeneous-isotropic turbulence forced linearly in physical space which result in constant turbulent kinetic energy, constant turbulent dissipation (also constant enstrophy), or a combination of the two based on a least-squares error minimization. The methods consist of proportional controllers embedded in the forcing coefficients. During the transient, the controllers adjust the forcing coefficients such that the controlled quantity achieves very early a minimal relative error with respect to its target stationary value. Comparisons of these forcing methods are made with the non-controlled approaches of Rosales and Meneveau ["Linear forcing in numerical simulations of isotropic turbulence: Physical space implementations and convergence properties," Phys. Fluids 17, 095106 (2005)] and Carroll and Blanquart ["A proposed modification to Lundgren's physical space velocity forcing method for isotropic turbulence," Phys. Fluids 25, 105114 (2013)], using direct numerical simulations (DNS) and large-eddy simulations (LES). The results indicate that the proposed constant-energetics forcing methods shorten the transient period from a user-defined artificial flow field to Navier-Stokes turbulence while maintaining steadier statistics. Additionally, the proposed method of constant kinetic-energy forcing behaves more robustly in coarse LES when initial conditions are employed that favor the occurrence of subgrid-scale backscatter, whereas the other approaches fail to provide physical turbulent flow fields. For illustration, the proposed forcing methods are applied to dilute particle-laden homogeneous-isotropic turbulent flows; the results serve to highlight the influences of the forcing strategies on the disperse-phase statistics.
Wouchuk, J G; Huete Ruiz de Lira, C; Velikovich, A L
2009-06-01
An exact analytical model for the interaction between an isolated shock wave and an isotropic turbulent vorticity field is presented. The interaction with a single-mode two-dimensional (2D) divergence-free vorticity field is analyzed in detail, giving the time and space evolutions of the perturbed quantities downstream. The results are generalized to study the interaction of a planar shock wave with an isotropic three-dimensional (3D) or 2D preshock vorticity field. This field is decomposed into Fourier modes, and each mode is assumed to interact independently with the shock front. Averages of the downstream quantities are made by integrating over the angles that define the orientation of the upstream velocity field. The ratio of downstream/upstream kinetic energies is in good agreement with existing numerical and experimental results for both 3D and 2D preshock vorticity fields. The generation of sound and the sonic energy flux radiated downstream from the shock front is also discussed in detail, as well as the amplification of transverse vorticity across the shock front. The anisotropy is calculated for the far downstream fields of both velocity and vorticity. All the quantities characteristic of the shock-turbulence interaction are reduced to closed-form exact analytical expressions. They are presented as explicit functions of the two parameters that govern the dynamics of the interaction: the adiabatic exponent gamma and the shock Mach number M1 . These formulas are further reduced to simpler exact asymptotic expressions in the limits of weak and strong shock waves (M_{1}-11, M_{1}1) and high shock compressibility of the gas (gamma-->1) .
The turbulent flame speed for low-to-moderate turbulence intensities
NASA Astrophysics Data System (ADS)
Matalon, Moshe; Fogla, Navin; Creta, Francesco
2014-11-01
Premixed flame propagation in two-dimensional turbulent flows is examined within the context of a hydrodynamic model. The flame is treated as a surface of density discontinuity and propagates against a turbulent flow of prescribed intensity and scale. A hybrid Navier-Stokes/interface capturing technique is used to describe the flow field throughout the entire domain and track the highly-fluctuating flame front which is allowed to attain folded conformations and form pockets of unburned gases that detach from the main flame surface and are rapidly consumed. A parametric study is conducted to examine the effects of the turbulence parameters: intensity and scale, and the combustion parameters: thermal expansion and mixture composition (or Markstein length). Markstein lengths are varied in order to span both, the Darrieus-Landau (DL) instability-free subcritical and the DL instability-affected supercritical regimes. Scaling laws for the turbulent flame speed, exhibiting explicit dependence on the system parameters, are proposed for low-to-moderate turbulence intensities.
NASA Astrophysics Data System (ADS)
Dou, Zhongwang; Bragg, Andrew; Hammond, Adam; Liang, Zach; Collins, Lance; Meng, Hui
2016-11-01
Effects of Reynolds number (Rλ) and Stokes number (St) on particle-pair relative velocity (RV) were studied using four-frame particle tracking in an enclosed turbulence chamber. Two tests were performed: varying Rλ between 246 and 357 at six St values, and varying St between 0.02 and 4.63 at five Rλ values. By comparing experimental and DNS results of mean inward particle-pair RV,
NASA Astrophysics Data System (ADS)
Tang, S. L.; Antonia, R. A.; Djenidi, L.; Danaila, L.; Zhou, Y.
2016-09-01
The transport equation for the mean scalar dissipation rate ɛ ¯ θ is derived by applying the limit at small separations to the generalized form of Yaglom's equation in two types of flows, those dominated mainly by a decay of energy in the streamwise direction and those which are forced, through a continuous injection of energy at large scales. In grid turbulence, the imbalance between the production of ɛ ¯ θ due to stretching of the temperature field and the destruction of ɛ ¯ θ by the thermal diffusivity is governed by the streamwise advection of ɛ ¯ θ by the mean velocity. This imbalance is intrinsically different from that in stationary forced periodic box turbulence (or SFPBT), which is virtually negligible. In essence, the different types of imbalance represent different constraints imposed by the large-scale motion on the relation between the so-called mixed velocity-temperature derivative skewness ST and the scalar enstrophy destruction coefficient Gθ in different flows, thus resulting in non-universal approaches of ST towards a constant value as Reλ increases. The data for ST collected in grid turbulence and in SFPBT indicate that the magnitude of ST is bounded, this limit being close to 0.5.
Turbulence generation through intense localized sources of energy
NASA Astrophysics Data System (ADS)
Maqui, Agustin; Donzis, Diego
2015-11-01
Mechanisms to generate turbulence in controlled conditions have been studied for nearly a century. Most common methods include passive and active grids with a focus on incompressible turbulence. However, little attention has been given to compressible flows, and even less to hypersonic flows, where phenomena such as thermal non-equilibrium can be present. Using intense energy from lasers, extreme molecule velocities can be generated from photo-dissociation. This creates strong localized changes in both the hydrodynamics and thermodynamics of the flow, which may perturb the flow in a way similar to an active grid to generate turbulence in hypersonic flows. A large database of direct numerical simulations (DNS) are used to study the feasibility of such an approach. An extensive analysis of single and two point statistics, as well as spectral dynamics is used to characterize the evolution of the flow towards realistic turbulence. Local measures of enstrophy and dissipation are studied to diagnose the main mechanisms for energy exchange. As commonly done in compressible flows, dilatational and solenoidal components are separated to understand the effect of acoustics on the development of turbulence. Further results for cases that assimilate laboratory conditions will be discussed. The authors gratefully acknowledge the support of AFOSR.
NASA Astrophysics Data System (ADS)
Lee, J. H.; Kevin; Monty, J. P.; Hutchins, N.
2016-08-01
The discrepancy between measured turbulence intensity obtained from experiments in wall-bounded turbulence and the fully resolved reference results (usually from DNS datasets) are often attributed to spatial resolution issues, especially in PIV measurements due to the presence of spatial averaging within the interrogation region/volume. In many cases, in particular at high Reynolds numbers (where there is a lack of DNS data), there is no attempt to verify that this is the case. There is a risk that attributing unexpected PIV statistics to spatial resolution, without careful checks, could mask wider problems with the experimental setup or test facility. Here, we propose a robust technique to validate the under-resolved PIV obtained turbulence intensity profiles for canonical wall-bounded turbulence. This validation scheme is independent of Reynolds number and does not rely on empirical functions. It is based on arguments that (1) the viscous-scaled small-scale turbulence energy is invariant with Reynolds number and that (2) the spatially under-resolved measurement is sufficient to capture the large-scale energy. This then suggests that we can estimate the missing energy from volume-filtered DNS data at much lower Reynolds numbers. Good agreement is found between the experimental results and estimation profiles for all three velocity components, demonstrating that the estimation tool successfully computes the missing energy for given spatial resolutions over a wide range of Reynolds numbers. A database for a canonical turbulent boundary layer and associated MATLAB function are provided that enable this missing energy to be calculated across a range of interrogation volume sizes, so that users do not require access to raw DNS data. This methodology and tool will provide PIV practitioners, investigating canonical wall-bounded turbulent flow with a convenient check of the effects of spatial resolution on a given experiment.
The influence of search strategies and homogeneous isotropic turbulence on planktonic contact rates
NASA Astrophysics Data System (ADS)
Rhodes, C. J.; Reynolds, A. M.
2007-12-01
Many species have been shown to adopt a Lévy-flight pattern of movement which are consistent with the most efficient way to locate sparsely distributed targets. Here, we consider a predator that conducts its search for prey in a turbulent environment. Such a situation is relevant to zooplankton-phytoplankton ecosystems. Kinematic simulation is used to represent the turbulent velocity field in the surface layers of the open ocean and contact with the prey is maximised for a predator swimming a Lévy flight with an exponent μsime1.2. The contact rate exceeds that recorded during straight-line swimming and passive advection. The observation that the contact rate is maximised for μsime1.2 appears to be not strongly dependent on predator swimming speed. The results are discussed in the context of recent work on planktonic search in laboratory conditions where Lévy-flight exponents of μsime2 were noted.
On the modification of particle dispersion in isotropic turbulence by free rotation of particle
NASA Astrophysics Data System (ADS)
Park, Yongnam; Lee, Changhoon
2008-11-01
Effect of a particle's spin is investigated numerically by considering the effect of lift occurring due to difference of rotations of a particle and of fluid such as the Saffman lift and Magnus force. These lift forces have been neglected in many previous works on particle-laden turbulence. The trajectory of particles can be changed by the lift forces, resulting in significant modification of the stochastic characteristics of heavy particles. Probability density functions and autocorrelations are examined of velocity, acceleration of solid particle and acceleration of fluid at the position of solid particle. Changes in velocity statistics are negligible but statistics related with acceleration are a little bit changed by particle's rotation. When a laden particle encounters with coherent structures during the motion, the particle's rotation might significantly affects the motion due to intermittently large fluid acceleration near coherent structures. The result can be used for development of stochastic model for particle dispersion. Detailed physical interpretation will be presented in the meeting.
An Analytical Model for the Three-Point Third-Order Velocity Correlation in Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Chang, Henry; Moser, Robert
2006-11-01
In turbulent flows, the three-point third-order velocity correlation Tijk(r,r') =
Pawar, Shashikant S; Arakeri, Jaywant H
2016-08-01
Frequency spectra obtained from the measurements of light intensity and angle of arrival (AOA) of parallel laser light propagating through the axially homogeneous, axisymmetric buoyancy-driven turbulent flow at high Rayleigh numbers in a long (length-to-diameter ratio of about 10) vertical tube are reported. The flow is driven by an unstable density difference created across the tube ends using brine and fresh water. The highest Rayleigh number is about 8×10^{9}. The aim of the present work is to find whether the conventional Obukhov-Corrsin scaling or Bolgiano-Obukhov (BO) scaling is obtained for the intensity and AOA spectra in the case of light propagation in a buoyancy-driven turbulent medium. Theoretical relations for the frequency spectra of log amplitude and AOA fluctuations developed for homogeneous isotropic turbulent media are modified for the buoyancy-driven flow in the present case to obtain the asymptotic scalings for the high and low frequency ranges. For low frequencies, the spectra of intensity and vertical AOA fluctuations obtained from measurements follow BO scaling, while scaling for the spectra of horizontal AOA fluctuations shows a small departure from BO scaling.
NASA Astrophysics Data System (ADS)
Ebert, M. A.
2002-07-01
An investigation was undertaken into possible dose conformity advantages and technical limitations of utilizing radially asymmetric internally applied radiation sources for intensity-modulated brachytherapy (IMBT). A feasible form of a source for IMBT would be a linear source with a high-intensity angular region, with some fractional transmission through the remainder of the source, which inhibits the resolution achievable in intensity modulation. Indexed rotation of the source about its axis would provide radial intensity modulation, which could compensate for variations in the spatial relationship between the source position and location of the target edge. Two treatment situations were simulated - one two-dimensional and one three-dimensional - both utilizing a single source (single catheter). The optimal intensity distribution of the source was determined by simulated annealing optimization using a conformality-based objective. The parameters in the optimization included the angular size of the source high-intensity region, and the fractional transmission through the low-intensity part of the source. Results indicate that limitations in source design suggest an optimal high-intensity resolution of approximately π/4 to π/8. The advantages of IMBT are rapidly reduced when fractional transmission through the low-intensity side of the source is increased.
Average intensity and spreading of a Lorentz-Gauss beam in turbulent atmosphere.
Zhou, Guoquan; Chu, Xiuxiang
2010-01-18
The propagation of a Lorentz-Gauss beam in turbulent atmosphere is investigated. Based on the extended Huygens-Fresnel integral and the Hermite-Gaussian expansion of a Lorentz function, analytical formulae for the average intensity and the effective beam size of a Lorentz-Gauss beam are derived in turbulent atmosphere. The average intensity distribution and the spreading properties of a Lorentz-Gauss beam in turbulent atmosphere are numerically demonstrated. The influences of the beam parameters and the structure constant of the atmospheric turbulence on the propagation of a Lorentz-Gauss beam in turbulent atmosphere are also discussed in detail.
NASA Astrophysics Data System (ADS)
Hosokawa, Iwao
2007-01-01
A decaying homogeneous isotropic turbulence is treated on the combined bases of the Kolmogorov hypothesis and the cross-independence hypothesis (for a closure of the Monin-Lundgren (ML) hierarchy of many-point velocity distributions) in turbulence. Similarity solutions for one- and two-point velocity distributions are obtained in the viscous, inertial and large-scale ranges of separation distance, from which we can give a reasonable picture of longitudinal and transverse velocity autocorrelation functions for any Reynolds number, even though they are distant from exact solutions of the infinite ML hierarchy. Possibility of non-similarity solutions with other reasonable and more realistic features is unveiled within the same theoretical framework. The cross-independence hypothesis is proved to be inconsistent with the Kolmogorov [1941b. Dissipation of energy in locally isotropic turbulence. Dokl. Akad. Nauk SSSR 32, 16-18.] theory in the inertial range. This is the main factor by which our special strategy (described in Introduction) is taken for solving this problem.
NASA Astrophysics Data System (ADS)
van Hinsberg, M. A. T.; Clercx, H. J. H.; Toschi, F.
2017-02-01
The Stokes drag force and the gravity force are usually sufficient to describe the behavior of sub-Kolmogorov-size (or pointlike) heavy particles in turbulence, in particular when the particle-to-fluid density ratio ρp/ρf≳103 (with ρp and ρf the particle and fluid density, respectively). This is, in general, not the case for smaller particle-to-fluid density ratios, in particular not for ρp/ρf≲102 . In that case the pressure gradient force, added mass effects, and the Basset history force also play important roles. In this study we focus on the understanding of the role of these additional forces, all of hydrodynamic origin, in the settling of particles in turbulence. In order to qualitatively elucidate the complex dynamics of such particles in homogeneous isotropic turbulence, we first focus on the case of settling of such particles in the flow field of a single vortex. After having explored this simplified case we extend our analysis to homogeneous isotropic turbulence. In general, we found that the pressure gradient force leads to a decrease in the settling velocity. This can be qualitatively understood by the fact that this force prevents the particles from sweeping out of vortices, a mechanism known as preferential sweeping which causes enhanced settling. Additionally, we found that the Basset history force can both increase and decrease the enhanced settling, depending on the particle Stokes number. Finally, the role of the nonlinear Stokes drag has been explored, confirming that it affects settling of inertial particles in turbulence, but only in a limited way for the parameter settings used in this investigation.
3D Wind: Quantifying wind speed and turbulence intensity
NASA Astrophysics Data System (ADS)
Barthelmie, R. J.; Pryor, S. C.; Wang, H.; Crippa, P.
2013-12-01
Integrating measurements and modeling of wind characteristics for wind resource assessment and wind farm control is increasingly challenging as the scales of wind farms increases. Even offshore or in relatively homogeneous landscapes, there are significant gradients of both wind speed and turbulence intensity on scales that typify large wind farms. Our project is, therefore, focused on (i) improving methods to integrate remote sensing and in situ measurements with model simulations to produce a 3-dimensional view of the flow field on wind farm scales and (ii) investigating important controls on the spatiotemporal variability of flow fields within the coastal zone. The instrument suite deployed during the field experiments includes; 3-D sonic and cup anemometers deployed on meteorological masts and buoys, anemometers deployed on tethersondes and an Unmanned Aerial Vehicle, multiple vertically-pointing continuous-wave lidars and scanning Doppler lidars. We also integrate data from satellite-borne instrumentation - specifically synthetic aperture radar and scatterometers and output from the Weather Research and Forecast (WRF) model. Spatial wind fields and vertical profiles of wind speed from WRF and from the full in situ observational suite exhibit excellent agreement in a proof-of-principle experiment conducted in north Indiana particularly during convective conditions, but showed some discrepancies during the breakdown of the nocturnal stable layer. Our second experiment in May 2013 focused on triangulating a volume above an area of coastal water extending from the port in Cleveland out to an offshore water intake crib (about 5 km) and back to the coast, and includes extremely high resolution WRF simulations designed to characterize the coastal zone. Vertically pointing continuous-wave lidars were operated at each apex of the triangle, while the scanning Doppler lidar scanned out across the water over 90 degrees azimuth angle. Preliminary results pertaining to
Turbulence at high resolution: intense events in dissipation, enstrophy and acceleration
NASA Astrophysics Data System (ADS)
Yeung, P. K.; Zhai, X. M.; Sreenivasan, K. R.
2014-11-01
Access to the Blue Waters supercomputer under the NSF Track 1 Petascale Resource Allocations program has allowed us to conduct an 81923 simulation of forced isotropic turbulence, with Taylor-scale Reynolds number close to 1300, and grid spacing at about 1.5 Kolmogorov scales. Extreme fluctuations in dissipation and enstrophy (over 10,000 times the mean) are observed, and found to scale similarly and occur together. Conditional sampling based on both dissipation and enstrophy shows that such extreme events in these variables are directly associated with strong intermittency in the fluid particle acceleration, which reaches values well beyond 100 standard deviations. An attempt is made to characterize in detail the formation of events of intense dissipation and enstrophy, including the transport, production and dissipation terms in the dissipation and enstrophy transport equations, as well as the nature of local flow conditions in principal strain-rate axes. Statistics of dissipation and enstrophy averaged over 3D sub-domains of linear size in the inertial range are also available. Both high Reynolds number and good small-scale resolution are important factors in these results. Supported by NSF Grant ACI-1036170.
Fichtl, G.H.
1983-09-01
When designing a wind energy converison system (WECS), it may be necessary to take into account the distribution of wind across the disc of rotation. The specific engineering applications include structural strength, fatigue, and control. This wind distribution consists of two parts, namely that associated with the mean wind profile and that associated with the turbulence velocity fluctuation field. The work reported herein is aimed at the latter, namely the distribution of turbulence velocity fluctuations across the WECS disk of rotation. A theory is developed for the two-time covariance matrix for turbulence velocity vector components for wind energy conversion system (WECS) design. The theory is developed for homogeneous and iotropic turbulance with the assumption that Taylor's hypothesis is valid. The Eulerian turbulence velocity vector field is expanded about the hub of the WECS. Formulae are developed for the turbulence velocity vector component covariance matrix following the WECS blade elements. It is shown that upon specification of the turbulence energy spectrum function and the WECS rotation rate, the two-point, two-time covariance matrix of the turbulent flow relative to the WECS bladed elements is determined. This covariance matrix is represented as the sum of nonstationary and stationary contributions. Generalized power spectral methods are used to obtain two-point, double frequency power spectral density functions for the turbulent flow following the blade elements. The Dryden turbulence model is used to demonstrate the theory. A discussion of linear system response analysis is provided to show how the double frequency turbulence spectra might be used to calculate response spectra of a WECS to turbulent flow. Finally the spectrum of the component of turbulence normal to the WECS disc of rotation, following the blade elements, is compared with experimental results.
NASA Technical Reports Server (NTRS)
Binienda, W. K.; Arnold, S. M.; Tan, H. Q.
1992-01-01
An essential part of describing the damage state and predicting the damage growth in a multicracked plate is the accurate calculation of stress intensity factors (SIF's). Here, a methodology and rigorous solution formulation for SIF's of a multicracked plate, with fully interacting cracks, subjected to a far-field arbitrary stress state is presented. The fundamental perturbation problem is derived, and the steps needed to formulate the system of singular integral equations whose solution gives rise to the evaluation of the SIF's are identified. This analytical derivation and numerical solution are obtained by using intelligent application of symbolic computations and automatic FORTRAN generation capabilities (described in the second part of this paper). As a result, a symbolic/FORTRAN package, named SYMFRAC, that is capable of providing accurate SIF's at each crack tip was developed and validated.
NASA Technical Reports Server (NTRS)
Arnold, S. M.; Binienda, W. K.; Tan, H. Q.; Xu, M. H.
1992-01-01
Analytical derivations of stress intensity factors (SIF's) of a multicracked plate can be complex and tedious. Recent advances, however, in intelligent application of symbolic computation can overcome these difficulties and provide the means to rigorously and efficiently analyze this class of problems. Here, the symbolic algorithm required to implement the methodology described in Part 1 is presented. The special problem-oriented symbolic functions to derive the fundamental kernels are described, and the associated automatically generated FORTRAN subroutines are given. As a result, a symbolic/FORTRAN package named SYMFRAC, capable of providing accurate SIF's at each crack tip, was developed and validated. Simple illustrative examples using SYMFRAC show the potential of the present approach for predicting the macrocrack propagation path due to existing microcracks in the vicinity of a macrocrack tip, when the influence of the microcrack's location, orientation, size, and interaction are taken into account.
Yi, S.; Kwon, J. M.; Rhee, T.; Diamond, P. H.
2012-11-15
This paper reports the results of gyrokinetic simulation studies of ion temperature gradient driven turbulence which investigate the role of non-resonant modes in turbulence spreading, turbulence regulation, and self-generated plasma rotation. Non-resonant modes, which are those without a rational surface within the simulation domain, are identified as nonlinearly driven, radially extended convective cells. Even though the amplitudes of such convective cells are much smaller than that of the resonant, localized turbulence eddies, we find from bicoherence analysis that the mode-mode interactions in the presence of such convective cells increase the efficiency of turbulence spreading associated with nonlocality phenomena. Artificial suppression of the convective cells shows that turbulence spreading is reduced, and that the turbulence intensity profile is more localized. The more localized turbulence intensity profile produces stronger Reynolds stress and E Multiplication-Sign B shear flows, which in turn results in more effective turbulence self-regulation. This suggests that models without non-resonant modes may significantly underestimate turbulent fluctuation levels and transport.
Intensity, Scale, and Spectra of Turbulence in Mixing Region of Free Subsonic Jet
NASA Technical Reports Server (NTRS)
Laurence, James C
1956-01-01
Report presents the results of the measurements of intensity of turbulence, the longitudinal and lateral correlation coefficients, and the spectra of turbulence in a 3.5-inch-diameter free jet measured with hot-wire anemometers at exit Mach numbers from 0.2 to 0.7 and Reynolds numbers from 192,000 to 725,000.
NASA Astrophysics Data System (ADS)
Yamaguchi, Atsushi; Ishihara, Takeshi
2016-09-01
In this study, a new motion compensation algorithm was proposed and verified by using numerical simulation. Compensated horizontal mean wind speed by using conventional method shows good agreement with reference wind speed regardless of the motion of the floater. However, turbulence intensity is always overestimated. The overestimation is more significant when the maximum pitch angle of the floater motion is larger. When proposed method is used, the overestimation of the turbulent intensity is improved and estimated turbulent intensity shows better agreement with reference value. There still remains underestimation of the turbulence intensity with the bias of -1.1%. This is probably caused by the low sampling frequency in LIDAR measurement and further research is needed to model the high frequency component of the wind speed for LIDAR measurement.
Directional Profiles of Wind Speed and Turbulence Intensity over Forest and Open Land
NASA Astrophysics Data System (ADS)
Beyer, Elisabeth; Dietz, Sebastian; Pinter, Anna
2014-05-01
More and more wind turbines are built onshore and reduce the available areas for wind energy. Forests are additional potential for wind energy priority areas. But the high roughness of wooden areas and the resulting turbulences make it difficult to assess sites in forests. In order to cope with this problem some measurements were done inside and outside wooden areas. Therefore met masts equipped with ultra sonic and cup anemometers and LIDAR were used. With the measured wind speed and its standard deviation the turbulence intensity was calculated. The results are direction dependent profiles of wind speed and turbulence intensity.
Flame Propagation in Low-Intensity Turbulence under Microgravity Conditions
NASA Technical Reports Server (NTRS)
Aldredge, R. C.
2001-01-01
The goal of the research is to understand the influences of the hydrodynamic instability on premixed-flame propagation. It is known that coupling between flame and flow-field dynamics in association with the hydrodynamic instability may lead to flame-generated turbulence, flame acceleration and enhancement of burning rates. As a result of such hydrodynamic coupling the transition from initially planar or wrinkled laminar flames to fast turbulent flames or detonations is possible, even when diffusive-thermal effects associated with non-unity reactant Lewis numbers are not destabilizing. It is important to identify methods of suppressing the hydrodynamic instability so as to insure fire safety, particularly in space.
Measuring anisotropy ellipse of atmospheric turbulence by intensity correlations of laser light.
Wang, Fei; Toselli, Italo; Li, Jia; Korotkova, Olga
2017-03-15
An experimental study has been performed of a laser beam propagating horizontally through the near-ground atmosphere above a grassy field at the University of Miami (UM) Coral Gables campus. The average intensity, scintillation index, and intensity correlation function are measured in the receiver plane for three channels with different turbulent conditions and at three different heights above the ground. Our results reveal that along short links (210 m) only the intensity correlation function captures the anisotropic information of turbulence, corresponding to the refractive index anisotropy ellipse of atmospheric fluctuations. In addition, we report an interesting phenomenon relating to turbulence eddy orientation near the ground. We confirmed that the experimental results are in agreement with the numerical simulations based on the multiple phase-screen method. Our findings provide an efficient method of determining the anisotropic parameters of atmospheric turbulence.
[Effects of turbulent fluctuation intensity on the growth of algae and water environment].
Lei, Yu; Long, Tian-yu; San, Lei; An, Qiang; Huang, Ning-qiu
2013-05-01
Through a self-designed vertically oscillating grid experiment device, the experiment was carried out to research the effects of the turbulence fluctuation intensity on the growth of algae and water environment in the water with adequate nutrients by changing the vibration frequency under a certain light and temperature conditions. The results showed that: the turbulent fluctuation has significant effects on algae growing that weak turbulence fluctuation can promote the growth of algae and strong turbulence fluctuation will inhibit the growth of algae in the range of experimental level. With the increase of the turbulent fluctuation intensity, the peak of algal biomass gradually delayed. Changes of nitrogen and phosphorus in the experimental process have significant differences, when the vibration frequency was up to 2.0 Hz, the maximum reduction of TN and TP were 55.2% and 69.0% lower compared with 0.5 Hz, which was closely associated with the growth of algae. With the intensity of turbulent fluctuation increases, nitrogen-phosphorus ratio first increases and then decreases corresponding to the peak of the algal biomass. Turbulent fluctuation can promote the pH and, dissolved oxygen quickly adjusted to the level of algae growing required, and the most appropriate value remains unchanged.
NASA Astrophysics Data System (ADS)
Jin, Guodong; Zhang, Jian; He, Guo-Wei; Wang, Lian-Ping
2010-12-01
Particle-laden turbulent flow is a typical non-equilibrium process characterized by particle relaxation time τp and the characteristic timescale of the flows τf, in which the turbulent mixing of heavy particles is related to different scales of fluid motions. The preferential concentration (PC) of heavy particles could be strongly affected by fluid motion at dissipation-range scales, which presents a major challenge to the large-eddy simulation (LES) approach. The errors in simulated PC by LES are due to both filtering and the subgrid scale (SGS) eddy viscosity model. The former leads to the removal of the SGS motion and the latter usually results in a more spatiotemporally correlated vorticity field. The dependence of these two factors on the flow Reynolds number is assessed using a priori and a posteriori tests, respectively. The results suggest that filtering is the dominant factor for the under-prediction of the PC for Stokes numbers less than 1, while the SGS eddy viscosity model is the dominant factor for the over-prediction of the PC for Stokes numbers between 1 and 10. The effects of the SGS eddy viscosity model on the PC decrease as the Reynolds number and Stokes number increase. LES can well predict the PC for particle Stokes numbers larger than 10. An SGS model for particles with small and intermediate Stokes numbers is needed to account for the effects of the removed SGS turbulent motion on the PC.
Measurements in film cooling flows: Hole L/D and turbulence intensity effects
Burd, S.W.; Kaszeta, R.W.; Simon, T.W.
1996-12-31
Hot-wire anemometry of simulated film cooling was used to study the influence of freestream turbulence intensity and film cooling hole length-to-diameter ratio on mean velocity and turbulence intensity. Measurements were made in the zone where the coolant and freestream flows mix. Flow from one row of film cooling holes with a streamwise injection of 35{degree} and no lateral injection and with a coolant- to-freestream flow velocity ratio of 1.0 was investigated under freestream turbulence levels of 0.5 and 12%. Coolant-to-freestream density ratio was unity. Two length-to-diameter ratios for the film cooling holes, 2.3 and 7.0, are tested. Results show that under low freestream turbulence conditions, pronounced differences exist in the flowfield between L/D=7.0 and 2.3; the differences are less prominent at high freestream turbulence intensities. Generally, short-L/D injection results in ``jetting`` of the coolant further into the freestream flow and enhanced mixing. Other changes in the flowfield attributable to a rise in freestream turbulence intensity to engine- representative conditions are documented. 15 figs, 2 tabs, refs.
Free-Stream Turbulence Intensity in the Langley 14- by 22-Foot Subsonic Tunnel
NASA Technical Reports Server (NTRS)
Neuhart, Dan H.; McGinley, Catherine B.
2004-01-01
An investigation was conducted using hot-wire anemometry to determine the turbulence intensity levels in the test section of the Langley 14- by 22-Foot Subsonic Tunnel in the closed or walls-down configuration. This study was one component of the three-dimensional High-Lift Flow Physics experiment designed to provide code validation data. Turbulence intensities were measured during two stages of the study. In the first stage, the free-stream turbulence levels were measured before and after a change was made to the floor suction surface of the wind tunnel s boundary layer removal system. The results indicated that the new suction surface at the entrance to the test section had little impact on the turbulence intensities. The second stage was an overall flow quality survey of the empty tunnel including measurements of the turbulence levels at several vertical and streamwise locations. Results indicated that the turbulence intensity is a function of tunnel dynamic pressure and the location in the test section. The general shape of the frequency spectrum is fairly consistent throughout the wind tunnel, changing mostly in amplitude (also slightly with frequency) with change in condition and location.
Probe systems for measuring static pressure and turbulence intensity in fluid streams
NASA Technical Reports Server (NTRS)
Rossow, Vernon J. (Inventor)
1993-01-01
A method and an apparatus for measuring time-averaged static or ambient pressure and turbulence intensity in a turbulent stream are discussed. The procedure involves placing a plurality of probes in the stream. Each probe responds in a different manner to characteristics of the fluid stream, preferably as a result of having varying cross sections. The responses from the probes are used to eliminate unwanted components in the measured quantities for accurate determination of selected characteristics.
Effect of turbulent atmosphere on the on-axis average intensity of Pearcey-Gaussian beam
NASA Astrophysics Data System (ADS)
F, Boufalah; L, Dalil-Essakali; H, Nebdi; A, Belafhal
2016-06-01
The propagation characteristics of the Pearcey-Gaussian (PG) beam in turbulent atmosphere are investigated in this paper. The Pearcey beam is a new kind of paraxial beam, based on the Pearcey function of catastrophe theory, which describes diffraction about a cusp caustic. By using the extended Huygens-Fresnel integral formula in the paraxial approximation and the Rytov theory, an analytical expression of axial intensity for the considered beam family is derived. Some numerical results for PG beam propagating in atmospheric turbulence are given by studying the influences of some factors, including incident beam parameters and turbulence strengths.
Lewis, D M
2003-05-07
It is a well-established fact that encounter rates between different species of planktonic microorganism, either swimming, or passively advected by the flow, are enhanced in the presence of turbulence. However, due to the complexity of the various calculations involved, current encounter rate theories are based on a number of simplifying approximations, which do not reflect reality. In particular, a typical planktonic predator is usually assumed to have perfect 'all round vision', i.e. it can perceive a prey particle at any relative orientation, provided it lies within some given contact radius R. Unfortunately, there is a wide body of experimental evidence that this is not the case. In this study the encounter problem for a predator with a limited field of sensory perception, swimming in a turbulent flow, is examined from first principles and a number of new modelling ideas proposed. A wide range of kinematic simulations are also undertaken to test these predictions. Particular attention is paid to the swimming strategy such a predator might undertake to enhance its encounter rate. It turns out that the predicted optimum swimming strategies differ radically from the results of previous work. Empirical evidence is also presented which appears to support these new findings.
NASA Technical Reports Server (NTRS)
Kerr, R. A.
1983-01-01
In a three dimensional simulation higher order derivative correlations, including skewness and flatness factors, are calculated for velocity and passive scalar fields and are compared with structures in the flow. The equations are forced to maintain steady state turbulence and collect statistics. It is found that the scalar derivative flatness increases much faster with Reynolds number than the velocity derivative flatness, and the velocity and mixed derivative skewness do not increase with Reynolds number. Separate exponents are found for the various fourth order velocity derivative correlations, with the vorticity flatness exponent the largest. Three dimensional graphics show strong alignment between the vorticity, rate of strain, and scalar-gradient fields. The vorticity is concentrated in tubes with the scalar gradient and the largest principal rate of strain aligned perpendicular to the tubes. Velocity spectra, in Kolmogorov variables, collapse to a single curve and a short minus 5/3 spectral regime is observed.
Bumblebee Flight in Heavy Turbulence
NASA Astrophysics Data System (ADS)
Engels, T.; Kolomenskiy, D.; Schneider, K.; Lehmann, F.-O.; Sesterhenn, J.
2016-01-01
High-resolution numerical simulations of a tethered model bumblebee in forward flight are performed superimposing homogeneous isotropic turbulent fluctuations to the uniform inflow. Despite tremendous variation in turbulence intensity, between 17% and 99% with respect to the mean flow, we do not find significant changes in cycle-averaged aerodynamic forces, moments, or flight power when averaged over realizations, compared to laminar inflow conditions. The variance of aerodynamic measures, however, significantly increases with increasing turbulence intensity, which may explain flight instabilities observed in freely flying bees.
Thorne, Meghan L; Poepping, Tamie L; Nikolov, Hristo N; Rankin, Richard N; Steinman, David A; Holdsworth, David W
2009-01-01
An in vitro investigation of turbulence intensity (TI) associated with a severe carotid stenosis in the presence of physiological cardiac variability is described. The objective of this investigation was to determine if fluctuations due to turbulence could be quantified with conventional Doppler ultrasound (DUS) in the presence of normal physiological cycle-to-cycle cardiac variability. An anthropomorphic model of a 70% stenosed carotid bifurcation was used in combination with a programmable flow pump to generate pulsatile flow with a mean flow rate of 6 mL/s. Utilizing the pump, we studied normal, nonrepetitive cycle-to-cycle cardiac variability (+/-3.9%) in flow, as well as waveform shapes with standard deviations equal to 0, 2 and 3 times the normal variation. Eighty cardiac cycles of Doppler data were acquired at two regions within the model, representing either laminar or turbulent flow; each measurement was repeated six times. Turbulence intensity values were found to be 11 times higher (p < 0.001), on average, in the turbulent region than in the laminar region, with a mean difference of 24 cm/s. Twenty cardiac cycles were required for confidence in TI values. In conclusion, these results indicate that it is possible to quantify TI in vitro, even in the presence of normal and exaggerated cycle-to-cycle cardiac variability.
Effects of q-profile structure on turbulence spreading: A fluctuation intensity transport analysis
Yi, S.; Kwon, J. M.; Diamond, P. H.; Hahm, T. S.
2014-09-15
This paper studies effects of q-profile structure on turbulence spreading. It reports results of numerical experiments using global gyrokinetic simulations. We examine propagation of turbulence, triggered by an identical linear instability in a source region, into an adjacent, linearly stable region with variable q-profile. The numerical experiments are designed so as to separate the physics of turbulence spreading from that of linear stability. The strength of turbulence spreading is measured by the penetration depth of turbulence. Dynamics of spreading are elucidated by fluctuation intensity balance analysis, using a model intensity evolution equation which retains nonlinear diffusion and damping, and linear growth. It is found that turbulence spreading is strongly affected by magnetic shear s, but is hardly altered by the safety factor q itself. There is an optimal range of modest magnetic shear which maximizes turbulence spreading. For high to modest shear values, the spreading is enhanced by the increase of the mode correlation length with decreasing magnetic shear. However, the efficiency of spreading drops for sufficiently low magnetic shear even though the mode correlation length is comparable to that for the case of optimal magnetic shear. The reduction of spreading is attributed to the increase in time required for the requisite nonlinear mode-mode interactions. The effect of increased interaction time dominates that of increased mode correlation length. Our findings of the reduction of spreading and the increase in interaction time at weak magnetic shear are consistent with the well-known benefit of weak or reversed magnetic shear for core confinement enhancement. Weak shear is shown to promote locality, as well as stability.
Yamaguchi, T; Kikkawa, S; Yoshikawa, T; Tanishita, K; Sugawara, M
1983-05-01
The blood flow velocity near the central axis of the canine ascending aorta was measured with a hot-film anemometer. The cardiac output and the heart rate were controlled at will by means of an extracorporeal circulation and by atrial pacing. The turbulent component of the blood flow velocity was calculated using an ensemble average technique. Ensemble average turbulent intensity was also calculated to show the time course of turbulence in the aorta. The ratio of the mean turbulence intensity to the time mean sectional average velocity in the aorta was constant in most animals regardless of the changes in fluid mechanical parameters. The correlation between the frequency parameter and the relative mean turbulence intensity was weakly positive. The power spectrum of the turbulence was also calculated.
Streamwise mean flow and turbulent intensity profiles in turbulent pipe flow
NASA Astrophysics Data System (ADS)
Vassilicos, John Christos; Laval, Jean-Philippe; Foucaut, Jean-Marc; Stanislas, Michel; Imperial-Lille Collaboration
2015-11-01
The Townsend-Perry attached eddy spectral model predicts that theintegral length-scale varies very slowly with distance to the wall inthe intermediate layer. The only way for the integral length scale'svariation to be more realistic while keeping with the Townsend-Perryattached eddy spectrum is to add a new wavenumber range to the modelat wavenumbers smaller than that spectrum. This necessary additionalso accounts for the high Reynolds number outer peak of the turbulentkinetic energy in the intermediate layer. An analytic expression isobtained for this outer peak in agreement with extremely high Reynoldsnumber data by Hultmark, Vallikivi, Bailey & Smits (2012,2013). Townsend's (1976) production-dissipation balance and thefinding of Dallas, Vassilicos & Hewitt (2009) that, in theintermediate layer, the eddy turnover time scales with skin frictionvelocity and distance to the wall implies that the mean flow gradienthas an outer peak at the same location as the turbulent kineticenergy. This is seen in the data of Hultmark, Vallikivi, Bailey Smits (2012, 2013). The same approach also predicts that the mean flowgradient has a logarithmic decay at distances to the wall larger thanthe position of the outer peak. This qualitative prediction is alsosupported by the aforementioned data.
Mao, Haidan; Zhao, Daomu
2010-01-18
Based on the intensity moments and Wigner distribution function, the second-order moments for broadband partially coherent flat-topped (BPCFT) beams in atmospheric turbulence are studied. The beam width of BPCFT beams in atmospheric turbulence is larger than that in free space. The beam width of BPCFT beams in atmospheric turbulence is larger than that of broadband fully coherent flat-topped (BFCFT) beams in atmospheric turbulence. The broader the bandwidth is, the larger the beam width of BPCFT beams becomes. Similar conclusion can be obtained by analyzing the divergence angle and beam propagation factor of BPCFT beams. The beam width of BPCFT beams in atmospheric turbulence is less affected by the broad spectral bandwidth than that in free space. The beam width of BFCFT beams in atmospheric turbulence is less affected by the broad spectral bandwidth than that of BPCFT beams in atmospheric turbulence.
NASA Astrophysics Data System (ADS)
Kamardin, A. P.; Nevzorova, I. V.; Odintsov, S. L.
2015-11-01
In the work, we consider estimates of the height of layer of intense turbulent heat exchange in stably stratified atmospheric boundary layer, made with the use of meteorological acoustic radar (sodar). Dependence of this height on temperature gradient is analyzed. Current temperature stratification of the atmosphere in the layer with height up to 1 000 m was determined with the help of MTP-5 meteorological temperature profiler.
Sankaran, Ramanan; Hawkes, Evatt R.; Yoo, Chun Sang; ...
2015-06-22
Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. We performed these simulations using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall onmore » the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Furthermore, statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.« less
Sankaran, Ramanan; Hawkes, Evatt R.; Yoo, Chun Sang; Chen, Jacqueline H.
2015-06-22
Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. We performed these simulations using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall on the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Furthermore, statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.
NASA Astrophysics Data System (ADS)
Wu, Yuqian; Zhang, Yixin; Wang, Qiu; Hu, Zhengda
2016-11-01
For Gaussian beams with three different partially coherent models, including Gaussian-Schell model (GSM), Laguerre-Gaussian Schell-model (LGSM) and Bessel-Gaussian Schell-model (BGSM) beams propagating through a biological turbulent tissue, the expression of the spatial coherence radius of a spherical wave propagating in a turbulent biological tissue, and the average intensity and beam spreading for GSM, LGSM and BGSM beams are derived based on the fractal model of power spectrum of refractive-index variations in biological tissue. Effects of partially coherent model and parameters of biological turbulence on such beams are studied in numerical simulations. Our results reveal that the spreading of GSM beams is smaller than LGSM and BGSM beams on the same conditions, and the beam with larger source coherence width has smaller beam spreading than that with smaller coherence width. The results are useful for any applications involved light beam propagation through tissues, especially the cases where the average intensity and spreading properties of the light should be taken into account to evaluate the system performance and investigations in the structures of biological tissue.
In vivo validation of numerical prediction for turbulence intensity in an aortic coarctation.
Arzani, Amirhossein; Dyverfeldt, Petter; Ebbers, Tino; Shadden, Shawn C
2012-04-01
This paper compares numerical predictions of turbulence intensity with in vivo measurement. Magnetic resonance imaging (MRI) was carried out on a 60-year-old female with a restenosed aortic coarctation. Time-resolved three-directional phase-contrast (PC) MRI data was acquired to enable turbulence intensity estimation. A contrast-enhanced MR angiography (MRA) and a time-resolved 2D PCMRI measurement were also performed to acquire data needed to perform subsequent image-based computational fluid dynamics (CFD) modeling. A 3D model of the aortic coarctation and surrounding vasculature was constructed from the MRA data, and physiologic boundary conditions were modeled to match 2D PCMRI and pressure pulse measurements. Blood flow velocity data was subsequently obtained by numerical simulation. Turbulent kinetic energy (TKE) was computed from the resulting CFD data. Results indicate relative agreement (error ≈10%) between the in vivo measurements and the CFD predictions of TKE. The discrepancies in modeled vs. measured TKE values were within expectations due to modeling and measurement errors.
NASA Astrophysics Data System (ADS)
Hari Prasad, K. B. R. R.; Srinivas, C. V.; Satyanarayana, A. N. V.; Naidu, C. V.; Baskaran, R.; Venkatraman, B.
2015-08-01
Season- and stability-dependent turbulence intensity ( σ u / u *, σ v / u *, σ w / u *) relationships are derived from experimental turbulence measurements following surface layer scaling and local stability at the tropical coastal site Kalpakkam, India for atmospheric dispersion parameterization. Turbulence wind components ( u', v', w') measured with fast response UltraSonic Anemometers during an intense observation campaign for wind field modeling called Round Robin Exercise are used to formulate the flux-profile relationships using surface layer similarity theory and Fast Fourier Transform technique. The new relationships (modified Hanna scheme) are incorporated in a Lagrangian Particle Dispersion model FLEXPART-WRF and tested by conducting simulations for a field tracer dispersion experiment at Kalpakkam. Plume dispersion analysis of a ground level hypothetical release indicated that the new turbulent intensity formulations provide slightly higher diffusivity across the plume relative to the original Hanna scheme. The new formulations for σ u , σ v , σ w are found to give better agreement with observed turbulent intensities during both stable and unstable conditions under various seasonal meteorological conditions. The simulated concentrations using the two methods are compared with those obtained from a classical Gaussian model and the observed SF6 concentration. It has been found that the new relationships provide comparatively higher diffusion across the plume relative to the model default Hanna scheme and provide downwind concentration results in better agreement with observations.
NASA Astrophysics Data System (ADS)
Pollak, D. A.; Nygaard, N. G.; Jimémez, M. M.; Wagner, R.; Sathe, A.
2014-12-01
To maximize offshore wind energy generation it is critical to have a comprehensive understanding of all atmospheric and oceanic parameters that could affect any part of the wind turbine system; prime of which is ambient turbulence intensity (TI), a measure of the degree of fluctuations about the mean wind speed within the three dimensional wind field (σU, standard deviation of wind speed). A wealth of data from nine meteorological towers located between 7 and 111 kilometers offshore in the Irish and North Seas were used to investigate the dependencies of TI and σU on wind speed, height, wind direction (θ) and fetch. The average turbulence intensity at 50m ranged from 6.32% to 7.44%, with TI decreasing with distance from the coast until 40-50 kilometers after which TI interestingly began to increase. In fact, when examining TI as a function of wind direction, TI was largest in sectors with fetch greater than 100-200 km and smallest in land sectors. Additional factors that influenced the TI dependency on θ, especially in the sites with reduced fetch, were coastal orientation, inland topography and atmospheric stability. Dissimilar to the relationships indicated in the IEC 61400-3 standards, σU is not linear, and TI not monotonically decreasing as a function of wind speed. Instead there is a height-dependent wind speed bin, between 8-14 m/s, where the transition from thermally- to mechanically-generated turbulence occurs. When comparing plots of σU and TI versus wind speed for all of the masts at various heights, it can be seen that there is near ubiquitous agreement between the masts. This implies that when averaging over all wind directions, these relationships are universal throughout Northern Europe. The vertical profiles of σU and TI are highly susceptible to different wind regimes. At low wind speeds σU and TI are nearly uniform with height, likely a result of the well-mixed nature of thermally-driven turbulence. At high wind speeds a strong, nearly
Wake Measurements at alpha ventus - Dependency on Stability and Turbulence Intensity
NASA Astrophysics Data System (ADS)
Westerhellweg, Annette; Cañadillas, Beatriz; Kinder, Friederike; Neumann, Tom
2014-12-01
Wind and power deficit in the wake are assessed for the offshore wind farm Alpha Ventus. Operational data are evaluated for the power deficit in the wake of a single wind turbine and in a row of wind turbines. The wake of a single wind turbine is described by the maximum power deficit and expansion width of the wake. The impact of atmospheric stability in respect to vertical wind shear and turbulence intensity is assessed showing that wake effects are more pronounced under stable conditions.
NASA Astrophysics Data System (ADS)
Ghassemlooy, Zabih; Popoola, Wasiu O.; Ahmadi, Vahid; Leitgeb, Erich
In this paper, we analyse the error performance of transmitter/receiver array free-space optical (FSO) communication system employing binary phase shift keying (BPSK) subcarrier intensity modulation (SIM) in clear but turbulent atmospheric channel. Subcarrier modulation is employed to eliminate the need for adaptive threshold detector. Direct detection is employed at the receiver and each subcarrier is subsequently demodulated coherently. The effect of irradiance fading is mitigated with an array of lasers and photodetectors. The received signals are linearly combined using the optimal maximum ratio combining (MRC), the equal gain combining (EGC) and the selection combining (SelC). The bit error rate (BER) equations are derived considering additive white Gaussian noise and log normal intensity fluctuations. This work is part of the EU COST actions and EU projects.
Wu, Yuqian; Zhang, Yixin; Zhu, Yun
2016-08-01
We studied Gaussian beams with three different partially coherent models, including the Gaussian-Schell model (GSM), Laguerre-Gaussian Schell model (LGSM), and Bessel-Gaussian Schell model (BGSM), propagating through oceanic turbulence. The expressions of average intensity, beam spreading, and beam wander for GSM, LGSM, and BGSM beams in the paraxial channel are derived. We make a contrast for the three models in numerical simulations and find that the GSM beam has smaller spreading than the others, and the LGSM beam needs longer propagation distance to transform into a well-like profile of average intensity than the BGSM beam in the same conditions. The salinity fluctuation has a greater contribution to the wander of LGSM and BGSM beams than that of the temperature fluctuation. Our results can be helpful in the design of an optical wireless communication link operating in oceanic environment.
NASA Astrophysics Data System (ADS)
Ji, Yong; She, Zhen-Su
2016-11-01
The streamwise turbulent intensity in wall turbulence (pipe and boundary layer) presents non-uniform distribution in both physical and wave number space. The well-known Townsend-Perry attached eddy hypothesis divides the energy spectrum into three distinct ranges: a constant range at small wavenumbers k
NASA Technical Reports Server (NTRS)
Mccolgan, C. J.; Larson, R. S.
1977-01-01
The effect of flight on the mean flow and turbulence properties of a 0.056m circular jet were determined in a free jet wind tunnel. The nozzle exit velocity was 122 m/sec, and the wind tunnel velocity was set at 0, 12, 37, and 61 m/sec. Measurements of flow properties including mean velocity, turbulence intensity and spectra, and eddy convection velocity were carried out using two linearized hot wire anemometers. This report contains the raw data and graphical presentations. The final technical report includes a description of the test facilities, test hardware, along with significant test results and conclusions.
NASA Astrophysics Data System (ADS)
Valente, Pedro C.; da Silva, Carlos B.; Pinho, Fernando T.
2013-11-01
We report a numerical study of statistically steady and decaying turbulence of FENE-P fluids for varying polymer relaxation times ranging from the Kolmogorov dissipation time-scale to the eddy turnover time. The total turbulent kinetic energy dissipation is shown to increase with the polymer relaxation time in both steady and decaying turbulence, implying a ``drag increase.'' If the total power input in the statistically steady case is kept equal in the Newtonian and the viscoelastic simulations the increase in the turbulence-polymer energy transfer naturally lead to the previously reported depletion of the Newtonian, but not the overall, kinetic energy dissipation. The modifications to the nonlinear energy cascade with varying Deborah/Weissenberg numbers are quantified and their origins investigated. The authors acknowledge the financial support from Fundação para a Ciência e a Tecnologia under grant PTDC/EME-MFE/113589/2009.
NASA Astrophysics Data System (ADS)
Herron, C. A.; Federrath, C.; Gaensler, B. M.; Lewis, G. F.; McClure-Griffiths, N. M.; Burkhart, Blakesley
2017-04-01
Previous studies have shown that star formation depends on the driving of molecular cloud turbulence, and differences in the driving can produce an order of magnitude difference in the star formation rate. The turbulent driving is characterized by the parameter ζ, with ζ = 0 for compressive, curl-free driving (e.g. accretion or supernova explosions), and ζ = 1 for solenoidal, divergence-free driving (e.g. Galactic shear). Here we develop a new method to measure ζ from observations of synchrotron emission from molecular clouds. We calculate statistics of mock synchrotron intensity images produced from magnetohydrodynamic simulations of molecular clouds, in which the driving was controlled to produce different values of ζ. We find that the mean and standard deviation of the log-normalized synchrotron intensity are sensitive to ζ, for values of ζ between 0 (curl-free driving) and 0.5 (naturally mixed driving). We quantify the dependence of zeta on the direction of the magnetic field relative to the line of sight. We provide best-fitting formulae for ζ in terms of the log-normalized mean and standard deviation of synchrotron intensity, with which ζ can be determined for molecular clouds that have similar Alfvénic Mach number to our simulations. These formulae are independent of the sonic Mach number. Signal-to-noise ratios larger than 5, and angular resolutions smaller than 5 per cent of the cloud diameter, are required to apply these formulae. Although there are no firm detections of synchrotron emission from molecular clouds, by combining Green Bank Telescope and Very Large Array observations it should be possible to detect synchrotron emission from molecular clouds, thereby constraining the value of ζ.
NASA Astrophysics Data System (ADS)
Stevens, Richard J. A. M.; Gayme, Dennice; Meneveau, Charles
2015-06-01
We use the recently introduced coupled wake boundary layer (CWBL) model to predict the effect of turbulence intensity on the performance of a wind farm. The CWBL model combines a standard wake model with a “top-down” approach to get improved predictions for the power output compared to a stand-alone wake model. Here we compare the CWBL model results for different turbulence intensities with the Horns Rev field measurements by Hansen et al., Wind Energy 15, 183196 (2012). We show that the main trends as function of the turbulence intensity are captured very well by the model and discuss differences between the field measurements and model results based on comparisons with LES results from Wu and Porté-Agel, Renewable Energy 75, 945-955 (2015).
NASA Astrophysics Data System (ADS)
Merino-Aceituno, Sara
2016-12-01
The isotropic 4-wave kinetic equation is considered in its weak formulation using model (simplified) homogeneous kernels. Existence and uniqueness of solutions is proven in a particular setting where the kernels have a rate of growth at most linear. We also consider finite stochastic particle systems undergoing instantaneous coagulation-fragmentation phenomena and give conditions in which this system approximates the solution of the equation (mean-field limit).
NASA Astrophysics Data System (ADS)
Goleneva, N. V.; Lavrinov, V.; Lavrinova, L. N.
2015-11-01
The wavefront sensor of Hartmann type consists of two parts: the optical and algorithmic. The parameters of the optical part of the sensor may vary. Since the time of "frozen" turbulence due to the Fried's length and to the cross wind transport turbulent distortion speed, the measurement Shack-Hartmann sensor depend on the intensity of turbulent distortions. In this paper are presented the results of the analysis of the measurements of the sensor according to the size of lens array and to the intensity of turbulent distortions. The analysis is performed on basis of a numerical model of the Shack-Hartmann wavefront sensor and on Kolmogorov's turbulence model.
NASA Astrophysics Data System (ADS)
Frisch, Uriel
1996-01-01
Written five centuries after the first studies of Leonardo da Vinci and half a century after A.N. Kolmogorov's first attempt to predict the properties of flow, this textbook presents a modern account of turbulence, one of the greatest challenges in physics. "Fully developed turbulence" is ubiquitous in both cosmic and natural environments, in engineering applications and in everyday life. Elementary presentations of dynamical systems ideas, probabilistic methods (including the theory of large deviations) and fractal geometry make this a self-contained textbook. This is the first book on turbulence to use modern ideas from chaos and symmetry breaking. The book will appeal to first-year graduate students in mathematics, physics, astrophysics, geosciences and engineering, as well as professional scientists and engineers.
NASA Astrophysics Data System (ADS)
Veiga Rodrigues, C.; Palma, J. M. L. M.
2014-06-01
Mesoscale results using the WRF model were downscaled from 3 km to 250 m resolution in a one-way coupling with VENTOS®/M. The results were compared against field measurements at one site comprising 4 meteorological masts, each with two sets of cup anemometers and wind vanes. The results showed that the addition of VENTOS®/M to the model chain improved the wind speed RMSE. Regarding the prediction of wind direction ambivalent results were obtained. Special attention was given to the prediction of turbulence intensity, particularly in reproducing its inverse proportionality with increasing wind speed (cf. IEC 61400-1 standard). The typical use of computational models in wind resource assessment, i.e., relying on decoupled methodologies and neutrally-stratified regimes, does not allow the representation of turbulence intensity for all wind speeds. The results obtained with VENTOS®/M were in agreement with the measured turbulence characteristics at both high and low wind speeds. Such was achieved without the coupling of any turbulence related field, relying solely on the turbulence model embedded in VENTOS®/M and its respective wall boundary conditions, based on Monin-Obukhov similarity theory. The behaviour under different stratification regimes was verified by analysing diurnal and nocturnal events separately.
Experimental assessment of helicopter rotor turbulence ingestion noise in hover
NASA Technical Reports Server (NTRS)
Simonich, J. C.; Schlinker, R. H.; Amiet, R. K.
1989-01-01
An experiment was conducted to assess the accuracy of a theory for non-isotropic turbulence ingestion. In order to generate non-isotropic turbulence in a controlled environment, a scale model rotor in a closed chamber was used so that the turbulence generated by the rotor was reingested by the recirculating flow. Simultaneous measurements of turbulence inflow properties and far field acoustics were acquired. Measurements confirmed that the inflow turbulence was highly non-isotropic. The measured aerodynamic properties were used as inputs for the noise prediction procedure. The general agreement between the non-isotropic noise prediction procedure and the experiment was good, although the procedure generally overpredicts the quasi-tonal low to mid range frequencies and underpredicts the higher broadband signals. The predicted sound power level as a function of polar angle was in close agreement with measurements, except near the rotor plane, which is not modeled by the present analysis. It is determined that the most sensitive parameter influencing the predicted noise was the turbulence intensity.
Leonardis, E.; Chapman, S. C.; Foullon, C.
2012-02-01
We focus on Hinode Solar Optical Telescope (SOT) calcium II H-line observations of a solar quiescent prominence (QP) that exhibits highly variable dynamics suggestive of turbulence. These images capture a sufficient range of scales spatially ({approx}0.1-100 arcsec) and temporally ({approx}16.8 s-4.5 hr) to allow the application of statistical methods used to quantify finite range fluid turbulence. We present the first such application of these techniques to the spatial intensity field of a long-lived solar prominence. Fully evolved inertial range turbulence in an infinite medium exhibits multifractal scale invariance in the statistics of its fluctuations, seen as power-law power spectra and as scaling of the higher order moments (structure functions) of fluctuations which have non-Gaussian statistics; fluctuations {delta}I(r, L) = I(r + L) - I(r) on length scale L along a given direction in observed spatial field I have moments that scale as ({delta}I(r, L){sup p}) {approx} L{sup {zeta}(p)}. For turbulence in a system that is of finite size, or that is not fully developed, one anticipates a generalized scale invariance or extended self-similarity (ESS) ({delta}I(r, L){sup p}) {approx} G(L){sup {zeta}(p)}. For these QP intensity measurements we find scaling in the power spectra and ESS. We find that the fluctuation statistics are non-Gaussian and we use ESS to obtain ratios of the scaling exponents {zeta}(p): these are consistent with a multifractal field and show distinct values for directions longitudinal and transverse to the bulk (driving) flow. Thus, the intensity fluctuations of the QP exhibit statistical properties consistent with an underlying turbulent flow.
A region of intense plasma wave turbulence on auroral field lines
NASA Technical Reports Server (NTRS)
Gurnett, D. A.; Frank, L. A.
1976-01-01
This report presents a detailed study of the plasma wave turbulence observed by HAWKEYE-1 and IMP-6 on high latitude auroral field lines and investigates the relationship of this turbulence to magnetic field and plasma measurements obtained in the same region.
Mixing and chemical reaction in sheared and nonsheared homogeneous turbulence
NASA Technical Reports Server (NTRS)
Leonard, Andy D.; Hill, James C.
1992-01-01
Direct numerical simulations were made to examine the local structure of the reaction zone for a moderately fast reaction between unmixed species in decaying, homogeneous turbulence and in a homogeneous turbulent shear flow. Pseudospectral techniques were used in domains of 64 exp 3 and higher wavenumbers. A finite-rate, single step reaction between non-premixed reactants was considered, and in one case temperature-dependent Arrhenius kinetics was assumed. Locally intense reaction rates that tend to persist throughout the simulations occur in locations where the reactant concentration gradients are large and are amplified by the local rate of strain. The reaction zones are more organized in the case of a uniform mean shear than in isotropic turbulence, and regions of intense reaction rate appear to be associated with vortex structures such as horseshoe vortices and fingers seen in mixing layers. Concentration gradients tend to align with the direction of the most compressive principal strain rate, more so in the isotropic case.
NASA Astrophysics Data System (ADS)
Pearson, Juli K.
The growing demand for increased efficiency in turbine engine designs has sparked a growing interest for research of air flow around curved surfaces. The turbine's operating conditions result in material property constraints, especially in the first stage turbine vanes and blades. These turbine vane components experience extreme loading conditions of both high temperature and high turbulence intensities exiting the combustor. The surface of the turbine blades has cylindrical leading edges that promote stabilizing flow accelerations. These convex surfaces can cause a reduced eddy diffusivity across the boundary layer. This thesis reviews measurements of velocity and turbulence intensities taken just shy of the thirty degrees offset from the stagnation line of a two-dimensional cylindrical leading edge under a wide range of turbulence and flow conditions flow conditions. Flow conditions and velocity measurements were gathered with respect to the distance to the surface. The length of the measurements extended from the surface to beyond the boundary layer's edge. The instrumentation used to collect data was a single wire driven by a constant temperature anemometer bridge. The hot wire is specially modified to measure data near the cylindrical leading edges curved surface. The traversing system allowed the acquisition of high-resolution boundary layer data. The traversing system was installed internally to the cylindrical leading edge to reduce probe blockage.
NASA Astrophysics Data System (ADS)
Majumdar, Arun K.; Luna, Carlos E.; Idell, Paul S.
2007-09-01
A new method of reconstructing and predicting an unknown probability density function (PDF) characterizing the statistics of intensity fluctuations of optical beams propagating through atmospheric turbulence is presented in this paper. The method is based on a series expansion of generalized Laguerre polynomials ; the expansion coefficients are expressed in terms of the higher-order intensity moments of intensity statistics. This method generates the PDF from the data moments without any prior knowledge of specific statistics and converges smoothly. The utility of reconstructed PDF relevant to free-space laser communication in terms of calculating the average bit error rate and probability of fading is pointed out. Simulated numerical results are compared with some known non-Gaussian test PDFs: Log-Normal, Rice-Nakagami and Gamma-Gamma distributions and show excellent agreement obtained by the method developed. The accuracy of the reconstructed PDF is also evaluated.
Probe shapes that measure time-averaged streamwise momentum and cross-stream turbulence intensity
NASA Technical Reports Server (NTRS)
Rossow, Vernon J. (Inventor)
1993-01-01
A method and apparatus for directly measuring the time-averaged streamwise momentum in a turbulent stream use a probe which has total head response which varies as the cosine-squared of the angle of incidence. The probe has a nose with a slight indentation on its front face for providing the desired response. The method of making the probe incorporates unique design features. Another probe may be positioned in a side-by-side relationship to the first probe to provide a direct measurement of the total pressure. The difference between the two pressures yields the sum of the squares of the cross-stream components of the turbulence level.
Tsai, Shen-Ta; Jhuo, Yue-Hua
2014-01-01
This study is concerned with a film cooling technique applicable to the protection of the endwalls of a gas turbine vane. In the experiments, cross-injection coolant flow from two-row, paired, inclined holes with nonintersecting centerlines was utilized. The test model is a scaled two-half vane. The levels of turbulence intensity used in the experiments are T.I. = 1.8%, 7%, and 12%. Other parameters considered in the film cooling experiments include three inlet Reynolds numbers (9.20 × 104, 1.24 × 105, and 1.50 × 105), three blowing ratios (0.5, 1.0, and 2.0), and three endwall conditions (smooth endwall and stepped endwall with forward-facing or backward-facing step). Thermochromic liquid crystal (TLC) technique with steady-state heat transfer experiments was used to obtain the whole-field film cooling effectiveness. Results show that, at low turbulence intensity, increasing Reynolds number decreases the effectiveness in most of the vane passage. There is no monotonic trend of influence by Reynolds number at high turbulence intensity. The effect of blowing ratio on the effectiveness has opposite trends at low and high turbulence levels. Increasing turbulent intensity decreases the effectiveness, especially near the inlet of the vane passage. With a stepped endwall, turbulence intensity has only mild effect on the film cooling effectiveness. PMID:24592153
Wu, Pey-Shey; Tsai, Shen-Ta; Jhuo, Yue-Hua
2014-01-01
This study is concerned with a film cooling technique applicable to the protection of the endwalls of a gas turbine vane. In the experiments, cross-injection coolant flow from two-row, paired, inclined holes with nonintersecting centerlines was utilized. The test model is a scaled two-half vane. The levels of turbulence intensity used in the experiments are T.I. = 1.8%, 7%, and 12%. Other parameters considered in the film cooling experiments include three inlet Reynolds numbers (9.20 × 10(4), 1.24 × 10(5), and 1.50 × 10(5)), three blowing ratios (0.5, 1.0, and 2.0), and three endwall conditions (smooth endwall and stepped endwall with forward-facing or backward-facing step). Thermochromic liquid crystal (TLC) technique with steady-state heat transfer experiments was used to obtain the whole-field film cooling effectiveness. Results show that, at low turbulence intensity, increasing Reynolds number decreases the effectiveness in most of the vane passage. There is no monotonic trend of influence by Reynolds number at high turbulence intensity. The effect of blowing ratio on the effectiveness has opposite trends at low and high turbulence levels. Increasing turbulent intensity decreases the effectiveness, especially near the inlet of the vane passage. With a stepped endwall, turbulence intensity has only mild effect on the film cooling effectiveness.
Effects of axisymmetric contractions on turbulence of various scales
NASA Technical Reports Server (NTRS)
Tan-Atichat, J.; Nagib, H. M.; Drubka, R. E.
1980-01-01
Digitally acquired and processed results from an experimental investigation of grid generated turbulence of various scales through and downstream of nine matched cubic contour contractions ranging in area ratio from 2 to 36, and in length to inlet diameter ratio from 0.25 to 1.50 are reported. An additional contraction with a fifth order contour was also utilized for studying the shape effect. Thirteen homogeneous and nearly isotropic test flow conditions with a range of turbulence intensities, length scales and Reynolds numbers were generated and used to examine the sensitivity of the contractions to upstream turbulence. The extent to which the turbulence is altered by the contraction depends on the incoming turbulence scales, the total strain experienced by the fluid, as well as the contraction ratio and the strain rate. Varying the turbulence integral scale influences the transverse turbulence components more than the streamwise component. In general, the larger the turbulence scale, the lesser the reduction in the turbulence intensity of the transverse components. Best agreement with rapid distortion theory was obtained for large scale turbulence, where viscous decay over the contraction length was negligible, or when a first order correction for viscous decay was applied to the results.
NASA Technical Reports Server (NTRS)
Wang, Chi R.; Yeh, Frederick C.
1987-01-01
A theoretical analysis and numerical calculations for the turbulent flow field and for the effect of free-stream turbulence on the surface heat transfer rate of a stagnation flow are presented. The emphasis is on the modeling of turbulence and its augmentation of surface heat transfer rate. The flow field considered is the region near the forward stagnation point of a circular cylinder in a uniform turbulent mean flow. The free stream is steady and incompressible with a Reynolds number of the order of 10 to the 5th power and turbulence intensity of less than 5 percent. For this analysis, the flow field is divided into three regions: (1) a uniform free-stream region where the turbulence is homogeneous and isotropic; (2) an external viscid flow region where the turbulence is distorted by the variation of the mean flow velocity; and, (3) an anisotropic turbulent boundary layer region over the cylinder surface. The turbulence modeling techniques used are the kappa-epsilon two-equation model in the external flow region and the time-averaged turbulence transport equation in the boundary layer region. The turbulence double correlations, the mean velocity, and the mean temperature within the boundary layer are solved numerically from the transport equations. The surface heat transfer rate is calculated as functions of the free-stream turbulence longitudinal microlength scale, the turbulence intensity, and the Reynolds number.
Noise of a model helicopter rotor due to ingestion of turbulence
NASA Technical Reports Server (NTRS)
Paterson, R. W.; Amiet, R. K.
1979-01-01
A theoretical and experimental investigation of the noise of a model helicoper rotor due to ingestion of turbulence was conducted. Experiments were performed with a 0.76 m dia, articulated model rotor for a range of inflow turbulence and rotor operating conditions. Inflow turbulence levels varied from approximately 2 to 19 percent and tip Mach number was varied from 0.3 to 0.52. Test conditions included ingestion of a atmospheric turbulence in outdoor hover as well as ingestion of grid generated isotropic turbulence in the wind tunnel airstream. In wind tunnel testing, both forward flight and vertical ascent (climb) were simulated. Far field noise spectra and directivity were measured in addition to incident turbulence intensities, length scales, and spectra. Results indicate that ingestion of atmospheric turbulence is the dominant helicopter rotor hover noise mechanism at the moderate to high frequencies which determine perceived noise level.
Measurements of Turbulence at Two Tidal Energy Sites in Puget Sound, WA
Thomson, Jim; Polagye, Brian; Durgesh, Vibhav; Richmond, Marshall C.
2012-06-05
Field measurements of turbulence are pre- sented from two sites in Puget Sound, WA (USA) that are proposed for electrical power generation using tidal current turbines. Rapidly sampled data from multiple acoustic Doppler instruments are analyzed to obtain statistical mea- sures of fluctuations in both the magnitude and direction of the tidal currents. The resulting turbulence intensities (i.e., the turbulent velocity fluctuations normalized by the harmonic tidal currents) are typically 10% at the hub- heights (i.e., the relevant depth bin) of the proposed turbines. Length and time scales of the turbulence are also analyzed. Large-scale, anisotropic eddies dominate the energy spectra, which may be the result of proximity to headlands at each site. At small scales, an isotropic turbulent cascade is observed and used to estimate the dissipation rate of turbulent kinetic energy. Data quality and sampling parameters are discussed, with an emphasis on the removal of Doppler noise from turbulence statistics.
NASA Technical Reports Server (NTRS)
Kiock, R.
1978-01-01
Turbulence intensity (Tu) measurements were made in two-dimensional and rotating cascades of blades in a low-speed cascade wind tunnel using hot-wire probes as sensors. The local Tu at Re = 1.6x100000 was determined in the wake zone behind a two-dimensional cascade. Then the values were recomputed for a rotating cascade, giving a mean turbulence intensity of 6.5% at 1/10 chord downstream and 2.9% at one chord. Fans were used for measurements on the rotating cascade. Re was equal to 7x100000. Frequency analysis was employed to separate the actual Tu of the entry flow from the effects caused by interaction with the rotor blades, showing that the true Tu increased from a few tenths of a percent to 6.6% in the 1st rotor, and from 7.2 to 9.3% in the 2d rotor. The Tu behind the 3d rotor was equal to 8.9%.
Scaling of Lyapunov Exponents in Homogeneous, Isotropic DNS
NASA Astrophysics Data System (ADS)
Fitzsimmons, Nicholas; Malaya, Nicholas; Moser, Robert
2013-11-01
Lyapunov exponents measure the rate of separation of initially infinitesimally close trajectories in a chaotic system. Using the exponents, we are able to probe the chaotic nature of homogeneous isotropic turbulence and study the instabilities of the chaotic field. The exponents are measured by calculating the instantaneous growth rate of a linear disturbance, evolved with the linearized Navier-Stokes equation, at each time step. In this talk, we examine these exponents in the context of homogeneous isotropic turbulence with two goals: 1) to investigate the scaling of the exponents with respect to the parameters of forced homogeneous isotropic turbulence, and 2) to characterize the instabilities that lead to chaos in turbulence. Specifically, we explore the scaling of the Lyapunov exponents with respect to the Reynolds number and with respect to the ratio of the integral length scale and the computational domain size.
Schwartz, M.W.
1981-04-30
A probabilistic safety criterion for isotropic flywheel rotors is established based on the tolerated noncontainment failure rates of commercial aircraft turbojet engine rotors. A technique is developed combining reliability with fracture mechanics, and a sample calculation provided, to show the energy-storage levels that isotropic flywheel rotors could achieve within the constraints of this safety criterion.
NASA Astrophysics Data System (ADS)
Zhang, J. A.; Marks, F. D.; Montgomery, M.; Lorsolo, S.
2010-12-01
Turbulent transport processes in the atmospheric boundary layer play an important role in the intensification and maintenance of a hurricane vortex. However, direct measurement of turbulence in the hurricane boundary layer has been scarce. This study analyzes the flight-level data collected by research aircraft that penetrated the eyewalls of Category 5 Hurricane Hugo (1989) and Category 4 Hurricane Allen (1980) between 1 km and the sea surface. Momentum flux, turbulent kinetic energy (TKE) and vertical eddy diffusivity are estimated before and during the eyewall penetrations. Spatial scales of turbulent eddies are determined through spectral analysis. The turbulence parameters estimated for the eyewall penetration leg are found to be nearly an order of magnitude larger than those for the leg outside the eyewall at similar altitudes. In the low-level intense eyewall region, the horizontal length scale of dominant turbulent eddies is found to be between 500 - 3000 m and the corresponding vertical length scale is approximately 100 - 200 m. The results suggest also that it is unwise to include the eyewall vorticity maximum (EVM) in the turbulence parameter estimation, since the EVMs are likely to be quasi two-dimensional vortex structures that are embedded within the three dimensional turbulence on the inside edge of the eyewall.
Turbulence Intensity at Inlet of 80- by 120-Foot Wind Tunnel Caused by Upwind Blockage
NASA Technical Reports Server (NTRS)
Salazar, Denise; Yuricich, Jillian
2014-01-01
In order to estimate the magnitude of turbulence in the National Full-Scale Aerodynamics Complex (NFAC) 80- by 120-Foot Wind Tunnel (80 x 120) caused by buildings located upwind from the 80 x 120 inlet, a 150th-scale study was performed that utilized a nominal two-dimensional blockage placed ahead of the inlet. The distance of the blockage ahead of the inlet was varied. This report describes velocity measurements made in the plane of the 80 x 120 model inlet for the case of zero ambient (atmospheric) wind.
NASA Technical Reports Server (NTRS)
Mielke, Amy F.; Seasholtz, Richard G.; Elam, Kristie A.; Panda, Jayanta
2004-01-01
A molecular Rayleigh scattering based flow diagnostic is developed to measure time average velocity, density, temperature, and turbulence intensity in a 25.4-mm diameter nozzle free jet facility. The spectrum of the Rayleigh scattered light is analyzed using a Fabry-Perot interferometer operated in the static imaging mode. The resulting fringe pattern containing spectral information of the scattered light is recorded using a low noise CCD camera. Nonlinear least squares analysis of the fringe pattern using a kinetic theory model of the Rayleigh scattered light provides estimates of density, velocity, temperature, and turbulence intensity of the gas flow. Resulting flow parameter estimates are presented for an axial scan of subsonic flow at Mach 0.95 for comparison with previously acquired pitot tube data, and axial scans of supersonic flow in an underexpanded screeching jet. The issues related to obtaining accurate turbulence intensity measurements using this technique are discussed.
Infinite Products of Random Isotropically Distributed Matrices
NASA Astrophysics Data System (ADS)
Il'yn, A. S.; Sirota, V. A.; Zybin, K. P.
2017-01-01
Statistical properties of infinite products of random isotropically distributed matrices are investigated. Both for continuous processes with finite correlation time and discrete sequences of independent matrices, a formalism that allows to calculate easily the Lyapunov spectrum and generalized Lyapunov exponents is developed. This problem is of interest to probability theory, statistical characteristics of matrix T-exponentials are also needed for turbulent transport problems, dynamical chaos and other parts of statistical physics.
NASA Astrophysics Data System (ADS)
Afarin, Yashar; Tabejamaat, Sadegh
2013-06-01
Large eddy simulations (LES) are employed to investigate the effect of the inlet turbulence intensity on the H2/CH4 flame structure in a hot and diluted co-flow stream which emulates the (Moderate or Intense Low-oxygen Dilution) MILD combustion regime. In this regard, three fuel inlet turbulence intensity profiles with the values of 4%, 7% and 10% are superimposed on the annular mixing layer. The effects of these changes on the flame structure under the MILD condition are studied for two oxygen concentrations of 3% and 9% (by mass) in the oxidiser stream and three hot co-flow temperatures 1300, 1500 and 1750 K. The turbulence-chemistry interaction of the numerically unresolved scales is modelled using the (Partially Stirred Reactor) PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influences of the turbulence intensity on the flame structure under the MILD condition are studied by using the profile of temperature, CO and OH mass fractions in both physical and mixture fraction spaces at two downstream locations. Also, the effects of this parameter are investigated by contours of OH, HCO and CH2O radicals in an area near the nozzle exit zone. Results show that increasing the fuel inlet turbulence intensity has a profound effect on the flame structure particularly at low oxygen mass fraction. This increment weakens the combustion zone and results in a decrease in the peak values of the flame temperature and OH and CO mass fractions. Furthermore, increasing the inlet turbulence intensity decreases the flame thickness, and increases the MILD flame instability and diffusion of un-burnt fuel through the flame front. These effects are reduced by increasing the hot co-flow temperature which reinforces the reaction zone.
Extreme events in computational turbulence
Yeung, P. K.; Zhai, X. M.; Sreenivasan, Katepalli R.
2015-01-01
We have performed direct numerical simulations of homogeneous and isotropic turbulence in a periodic box with 8,1923 grid points. These are the largest simulations performed, to date, aimed at improving our understanding of turbulence small-scale structure. We present some basic statistical results and focus on “extreme” events (whose magnitudes are several tens of thousands the mean value). The structure of these extreme events is quite different from that of moderately large events (of the order of 10 times the mean value). In particular, intense vorticity occurs primarily in the form of tubes for moderately large events whereas it is much more “chunky” for extreme events (though probably overlaid on the traditional vortex tubes). We track the temporal evolution of extreme events and find that they are generally short-lived. Extreme magnitudes of energy dissipation rate and enstrophy occur simultaneously in space and remain nearly colocated during their evolution. PMID:26424452
A hot-wire method for high-intensity turbulent flows
NASA Technical Reports Server (NTRS)
Mueller, U. R.
1983-01-01
A measuring technique for determing instantaneous, three-dimensional velocity vectors in highly turbulent flows by means of a 4-sensor hot-wire probe is described. As is well known, the hot-wire signal received in reversing flows cannot uniquely be interpreted. This difficulty is circumvented by tracking the thermal wake of a heated wire. Whenever the approximate flow direction is indicated by a temperature-sensitive wake detector, all components of the instantaneous velocity vector are evaluated by means of a digital data reduction method. Uniqueness of the solution derived from the triple-hot-wire response equations is examined. A first application of the proposed measuring technique in the recirculating flow downstream of a backward-facing step is described.
Faridzadeh, Monire; Gholami, Asghar; Ghassemlooy, Zabih; Rajbhandari, Sujan
2012-08-01
In this paper a hybrid modulation scheme based on pulse position modulation (PPM) and binary phase shift keying subcarrier intensity modulation (BPSK-SIM) schemes for free-space optical communications is proposed. The analytical bit error rate (BER) performance is investigated in weak and saturated turbulence channels and results are verified with the simulation data. Results show that performance of PPM-BPSK-SIM is superior to BPSK-SIM in all turbulence regimes; however, it outperforms 2-PPM for the turbulence variance σ(1)(2)>0.2. PPM-BPSK-SIM offers a signal-to-noise ratio (SNR) gain of 50 dB in the saturation regime compared to BPSK at a BER of 10(-6). The SNR gain in comparison to PPM improves as the strength of the turbulence level increases.
NASA Astrophysics Data System (ADS)
Chatterjee, Monish R.; Mohamed, Fathi H. A.
2014-10-01
In recent research, propagation of plane electromagnetic (EM) waves through a turbulent medium with modified von Karman phase characteristics was modeled and numerically simulated using transverse planar apertures representing narrow phase turbulence along the propagation path. The case for extended turbulence was also studied by repeating the planar phase screens multiple times over the propagation path and incorporating diffractive effects via a split-step algorithm. The goal of the research reported here is to examine two random phenomena: (a) atmospheric turbulence due to von Karman-type phase fluctuations, and (b) chaos generated in an acousto-optic (A-O) Bragg cell under hybrid feedback. The latter problem has been thoroughly examined for its nonlinear dynamics and applications in secure communications. However, the statistical characteristics (such as the power spectral density (PSD)) of the chaos have not been estimated in recent work. To that end, treating the chaos phenomena as a random process, the time waveforms of the chaos intensity and their spectra are numerically evaluated over a (large) number of time iterations. These spectra are then averaged to derive the equivalent PSD of the A-O chaos. For the turbulence problem, an optical beam passing through an input pinhole is propagated through a random phase screen (placed at different locations) to a desired distance (typically near-field) under different levels of turbulence strength. The resulting spatial intensity profile is then averaged and the process repeated over a (large) number of pre-specified time intervals. From this data, once again, the turbulence PSD is calculated via the Fourier spectra of the average intensity snapshots. The results for the two systems are compared.
Structure of wind-shear turbulence
NASA Technical Reports Server (NTRS)
Trevino, G.; Laituri, T. R.
1989-01-01
The statistical characteristics of wind shear turbulence are modelled. Isotropic turbulence serves as the basis of comparison for the anisotropic turbulence which exists in wind shear. The question of turbulence scales in wind shear is addressed from the perspective of power spectral density.
NASA Astrophysics Data System (ADS)
Korotkova, O.
2006-02-01
The intensity fluctuations of random electromagnetic beams propagating in the atmosphere are studied. For such beams it is shown that when the atmospheric fluctuations are weak then the scintillation index (the normalized variance of intensity fluctuations) of the beam at any distance from the source depends not only on the state of coherence but also on the degree of polarization of the beam in the source plane. In particular, we found that for initially unpolarized beams the scintillation index generally takes on smaller values than that for completely polarized beams. The presented analysis might be useful in the applications (e.g. communications, laser radars) where atmospheric effects can be mitigated by adjusting the coherence properties and the polarization properties of the source.
The effects of anisotropic free-stream turbulence on turbulent boundary layer behavior
NASA Technical Reports Server (NTRS)
Liang-Wei, F.; Hoffman, J. A.
1985-01-01
The effects of near-isotropic and highly anisotropic free-stream turbulence on mean flow properties of the turbulence structure of turbulent boundary layers in a near zero pressure gradient flow has been experimentally evaluated. Turbulence levels vary from 0.5% to 8.0% and the momentum thickness Reynolds number varies from 800 to 1100. The results indicate that the effects of free-stream turbulence on the classical boundary layer properties for near-isotropic turbulence which have been published by other investigators are similar to the case of highly anisotropic turbulence fields, while the effects of free-stream turbulence on the properties of the turbulent structure within the boundary layer for the case of near-isotropic turbulence are quite different compared to the highly anisotropic case.
Direct simulation of compressible turbulence in a shear flow
NASA Technical Reports Server (NTRS)
Sarkar, S.; Erlebacher, G.; Hussaini, M. Y.
1991-01-01
Compressibility effects on the turbulence in homogeneous shear flow are investigated. The growth of the turbulent kinetic energy was found to decrease with increasing Mach number: a phenomenon which is similar to the reduction of turbulent velocity intensities observed in experiments on supersonic free shear layers. An examination of the turbulent energy budget shows that both the compressible dissipation and the pressure-dilatation contribute to the decrease in the growth of kinetic energy. The pressure-dilatation is predominantly negative in homogeneous shear flow, in contrast to its predominantly positive behavior in isotropic turbulence. The different signs of the pressure-dilatation are explained by theoretical consideration of the equations for the pressure variance and density variance. Previously, the following results were obtained for isotropic turbulence: (1) the normalized compressible dissipation is of O(M(sub t)(exp 2)); and (2) there is approximate equipartition between the kinetic and potential energies associated with the fluctuating compressible mode. Both of these results were substantiated in the case of homogeneous shear. The dilatation field is significantly more skewed and intermittent than the vorticity field. Strong compressions seem to be more likely than strong expansions.
Accelerative propagation and explosion triggering by expanding turbulent premixed flames.
Akkerman, V'yacheslav; Chaudhuri, Swetaprovo; Law, Chung K
2013-02-01
The dynamics and morphology of outwardly propagating, accelerating turbulent premixed flames and the effect of flame acceleration on explosion triggering are analyzed. Guided by recent theoretical results and substantiated by experiments, we find that an expanding flame front in an externally forced, near-isotropic turbulent environment exhibits accelerative propagation given by a well-defined power law based on the average global flame radius. In this context the limits of the power-law exponent and the effective turbulence intensity experienced by the flame are derived. The power-law exponent is found to be substantially larger than that for the hydrodynamically unstable cellular laminar flames, hence facilitating the possibility of detonation triggering in turbulent environments. For large length scales, hydrodynamic instability is expected to provide additional acceleration, thus further favoring the attainment of detonation triggering.
NASA Technical Reports Server (NTRS)
Flegel, Ashlie B.; Giel, Paul W.; Welch, Gerard E.
2014-01-01
The effects of high inlet turbulence intensity on the aerodynamic performance of a variable speed power turbine blade are examined over large incidence and Reynolds number ranges. These results are compared to previous measurements made in a low turbulence environment. Both high and low turbulence studies were conducted in the NASA Glenn Research Center Transonic Turbine Blade Cascade Facility. The purpose of the low inlet turbulence study was to examine the transitional flow effects that are anticipated at cruise Reynolds numbers. The current study extends this to LPT-relevant turbulence levels while perhaps sacrificing transitional flow effects. Assessing the effects of turbulence at these large incidence and Reynolds number variations complements the existing database. Downstream total pressure and exit angle data were acquired for 10 incidence angles ranging from +15.8deg to -51.0deg. For each incidence angle, data were obtained at five flow conditions with the exit Reynolds number ranging from 2.12×10(exp 5) to 2.12×10(exp 6) and at a design exit Mach number of 0.72. In order to achieve the lowest Reynolds number, the exit Mach number was reduced to 0.35 due to facility constraints. The inlet turbulence intensity, Tu, was measured using a single-wire hotwire located 0.415 axial-chord upstream of the blade row. The inlet turbulence levels ranged from 8 to 15 percent for the current study. Tu measurements were also made farther upstream so that turbulence decay rates could be calculated as needed for computational inlet boundary conditions. Downstream flow field measurements were obtained using a pneumatic five-hole pitch/yaw probe located in a survey plane 7 percent axial chord aft of the blade trailing edge and covering three blade passages. Blade and endwall static pressures were acquired for each flow condition as well. The blade loading data show that the suction surface separation that was evident at many of the low Tu conditions has been eliminated. At
NASA Technical Reports Server (NTRS)
Flegel, Ashlie B.; Giel, Paul W.; Welch, Gerard E.
2014-01-01
The effects of high inlet turbulence intensity on the aerodynamic performance of a variable speed power turbine blade are examined over large incidence and Reynolds number ranges. These results are compared to previous measurements made in a low turbulence environment. Both high and low turbulence studies were conducted in the NASA Glenn Research Center Transonic Turbine Blade Cascade Facility. The purpose of the low inlet turbulence study was to examine the transitional flow effects that are anticipated at cruise Reynolds numbers. The current study extends this to LPT-relevant turbulence levels while perhaps sacrificing transitional flow effects. Assessing the effects of turbulence at these large incidence and Reynolds number variations complements the existing database. Downstream total pressure and exit angle data were acquired for 10 incidence angles ranging from +15.8deg to -51.0deg. For each incidence angle, data were obtained at five flow conditions with the exit Reynolds number ranging from 2.12×10(exp 5) to 2.12×10(exp 6) and at a design exit Mach number of 0.72. In order to achieve the lowest Reynolds number, the exit Mach number was reduced to 0.35 due to facility constraints. The inlet turbulence intensity, Tu, was measured using a single-wire hotwire located 0.415 axial-chord upstream of the blade row. The inlet turbulence levels ranged from 8 to 15 percent for the current study. Tu measurements were also made farther upstream so that turbulence decay rates could be calculated as needed for computational inlet boundary conditions. Downstream flow field measurements were obtained using a pneumatic five-hole pitch/yaw probe located in a survey plane 7 percent axial chord aft of the blade trailing edge and covering three blade passages. Blade and endwall static pressures were acquired for each flow condition as well. The blade loading data show that the suction surface separation that was evident at many of the low Tu conditions has been eliminated. At
NASA Astrophysics Data System (ADS)
Yousefi, M.; Kashani, F. D.; Mashal, A.
2017-02-01
In this research, an analytical expression for cross-spectral density matrix elements (and consequently, average intensity) of partially coherent flat-topped (PCFT) radial array laser beams in weak oceanic turbulence are derived based on the extended Huygens-Fresnel principle and the previously developed knowledge of the propagation of a partially coherent beam in atmosphere. Mean-squared beam width is calculated analytically using average intensity formula. The simulation is done by considering the effects of source parameters (such as the radius of the array setup’s circle and effective width of spectral degree of coherence) and turbulent ocean factors (such as the rate of dissipation of the turbulent kinetic energy per unit mass of fluid and relative strength of temperature-salinity fluctuations, Kolmogorov micro-scale, and the rate of dissipation of the mean squared temperature) in detail. It is found that when salinity fluctuations in the ocean dominate temperature fluctuations, the average intensity of the PCFT array beam becomes more broad and the array beam profile conversion process to a single wider Gaussian beam profile will occur at a faster rate. For the same turbulent conditions and the same initial beam width, the divergence of a flat-topped array beam is less than the Gaussian array beam. The simulation and calculation results are shown by graphs.
NASA Technical Reports Server (NTRS)
Flegel, Ashlie Brynn; Giel, Paul W.; Welch, Gerard E.
2014-01-01
The effects of inlet turbulence intensity on the aerodynamic performance of a variable speed power turbine blade are examined over large incidence and Reynolds number ranges. Both high and low turbulence studies were conducted in the NASA Glenn Research Center Transonic Turbine Blade Cascade Facility. The purpose of the low inlet turbulence study was to examine the transitional flow effects that are anticipated at cruise Reynolds numbers. The high turbulence study extends this to LPT-relevant turbulence levels while perhaps sacrificing transitional flow effects. Downstream total pressure and exit angle data were acquired for ten incidence angles ranging from +15.8 to 51.0. For each incidence angle, data were obtained at five flow conditions with the exit Reynolds number ranging from 2.12105 to 2.12106 and at a design exit Mach number of 0.72. In order to achieve the lowest Reynolds number, the exit Mach number was reduced to 0.35 due to facility constraints. The inlet turbulence intensity, Tu, was measured using a single-wire hotwire located 0.415 axial-chord upstream of the blade row. The inlet turbulence levels ranged from 0.25 - 0.4 for the low Tu tests and 8- 15 for the high Tu study. Tu measurements were also made farther upstream so that turbulence decay rates could be calculated as needed for computational inlet boundary conditions. Downstream flow field measurements were obtained using a pneumatic five-hole pitchyaw probe located in a survey plane 7 axial chord aft of the blade trailing edge and covering three blade passages. Blade and endwall static pressures were acquired for each flow condition as well. The blade loading data show that the suction surface separation that was evident at many of the low Tu conditions has been eliminated. At the extreme positive and negative incidence angles, the data show substantial differences in the exit flow field. These differences are attributable to both the higher inlet Tu directly and to the thinner inlet endwall
Effect of primary particle morphology on the structure of gels formed in intense turbulent shear.
Arosio, Paolo; Xie, Delong; Wu, Hua; Braun, Leonie; Morbidelli, Massimo
2010-05-04
We study the effect of primary particle morphology on intense shear-induced gelation without adding electrolytes. The primary particles are composed of a rubbery core grafted with a polystyrene shell. Depending on the shell-to-core mass ratio, the core can be partially covered by the shell, leading to strawberry-like morphology. It is found that at a fixed core mass the fractal dimension of the clusters constructing the gel decreases (i.e., more open cluster structure) as the shell mass increases, until reaching a plateau. The SEM pictures of the gels reveal that the structure variations are due to the occurrence of partial coalescence among particles, which decreases as the shell mass increases. In the region where the fractal dimension reaches a plateau, the coalescence is negligible. The conversion of the primary particles to gels is incomplete and increases as the extent of coalescence decreases. This is related to the fact that the smaller the extent of coalescence, the larger the cluster size. Thus, because of its cubic dependence on the cluster size, the aggregation rate increases as the extent of coalescence decreases, leading to increased conversion. It is therefore evident that the key parameter controlling the gel structure and the particle conversion is the core surface coverage by the shell. To further verify this conclusion, we have carried out the shear-induced gelation of another set of particles with varying core mass. It is found that the only parameter that can well correlate the values of the fractal dimension and particle conversion from the two sets of particles is the core surface coverage.
Ignition transition in turbulent premixed combustion
Shy, S.S.; Liu, C.C.; Shih, W.T.
2010-02-15
Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion in a centrally-ignited, fan-stirred cruciform burner capable of generating intense isotropic turbulence. Using the same methodology, this paper presents new complete MIE data sets for stoichiometric and rich cases at three different equivalence ratios {phi} = 1.0, 1.2 and 1.3, each covering a wide range of a turbulent Karlovitz number (Ka) indicating a time ratio between chemical reaction and turbulence. Thus, ignition transition in premixed turbulent combustion depending on both Ka and {phi} can be identified for the first time. It is found that there are two distinct modes on ignition in randomly stirred methane-air mixtures (ignition transition) separated by a critical Ka where values of Ka{sub c} {approx} 8-26 depending on {phi} with the minimum Ka{sub c} occurring near {phi} = 1. For Ka < Ka{sub c}, MIE increases gradually with Ka, flame kernel formation is similar to laminar ignition remaining a torus, and 2D laser tomography images of subsequent outwardly-propagating turbulent flames show sharp fronts. For Ka > Ka{sub c}, MIE increases abruptly with Ka, flame kernel is disrupted, and subsequent randomly-propagating turbulent flames reveal distributed-like fronts. Moreover, we introduce a reaction zone Peclet number (P{sub RZ}) indicating the diffusivity ratio between turbulence and chemical reaction, such that the aforementioned very scattering MIE data depending on Ka and {phi} can be collapsed into a single curve having two drastically different increasing slopes with P{sub RZ} which are separated by a critical P{sub RZ} {approx} 4.5 showing ignition transition. Finally, a physical model is proposed to explain these results. (author)
NASA Technical Reports Server (NTRS)
Haugstad, B. S.
1978-01-01
The nature and magnitude of turbulence-induced errors in atmospheric profiles derived from Doppler measurements made during radio occultations are investigated. It is found that turbulence in planetary atmospheres induces both fluctuating and systematic errors in derived profiles, but the errors of both types are very small. Consideration of the occultation of Mariner 10 by Venus and of the Pioneer occultations by Jupiter shows that the rms fractional errors in the atmospheric profiles derived from these observations were less than 0.01 in both temperature and pressure, while the fractional systematic errors were typically of the order of 1 millionth. The extent to which atmospheric profiles derived from radio and optical intensity measurements are affected by turbulence is also examined. The results indicate that turbulence in planetary atmospheres has only a marginal effect on derived profiles in the weak-scattering limit and that the turbulence-induced errors in this case are always much larger than the corresponding errors in profiles derived from radio Doppler measurements.
The isotropic Hamiltonian formalism
Vaisman, Izu
2011-02-10
A Hamiltonian formalism is a procedure that allows to associate a dynamical system to a function and that includes classical Hamiltonian mechanics as a particular case. The present, expository paper gives a survey of the Hamiltonian formalism defined by an isotropic subbundle of TM+T*M, in particular, by a Dirac structure. We discuss reduction and geometric quantization of the Hamiltonian dynamical systems provided by this formalism.
NASA Astrophysics Data System (ADS)
Madala, Srikanth; Satyanarayana, A. N. V.; Srinivas, C. V.
2015-12-01
Accurate representation of air pollutant dispersion is essential for environmental management and planning purposes. In this study, semi-empirical relationships of turbulence intensity (σu/u*, σv/u* and σw/u*) as a function of surface layer scaling and local stability are developed following boundary layer similarity concepts at Ranchi, a complex terrain in Jharkhand, Eastern India for various seasons. The impact of the new turbulence parameterization for air pollution dispersion simulation is studied by incorporating the same in the Hanna scheme of FLEXPART-WRF Lagrangian Particle dispersion model over study region. The model is used to estimate the ground level concentrations of nitrogen oxides (NOx) due to industrial and vehicular sources in study region. The meteorological parameters needed in air-quality simulation are simulated using the Advanced Research WRF (ARW) mesoscale model at high resolution (3 km). Three turbulence schemes (YSU, MYNN2 and ACM2) in ARW are alternatively tested in dispersion simulation and comparisons are made with available air quality data for eight days in different seasons (winter, pre-monsoon, monsoon and post-monsoon). Simulations with FLEXPART revealed distinct seasonal variation of dispersion patterns. It has been found that the new turbulence intensity relationships in FLEXPART improved the NOx concentration estimates by reducing the negative bias seen with default Hanna scheme. Further, the ARW simulated meteorological parameters using ACM2 and MYNN2 significantly reduced the bias in modeled pollutant concentrations. The study demonstrates the utility of high quality seasonal turbulence measurements in pollution dispersion model for better diffusion parameterization needed in air quality modeling.
Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations
NASA Astrophysics Data System (ADS)
Chaudhuri, Swetaprovo; Wu, Fujia; Law, Chung K.
2013-09-01
In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a ReT,f0.5 scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given ReT,f, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by ReT,M0.5 irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames.
Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations.
Chaudhuri, Swetaprovo; Wu, Fujia; Law, Chung K
2013-09-01
In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a Re_{T,f}^{0.5} scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given Re_{T,f}^{}, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by Re_{T,M}^{0.5} irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames.
Turbulence comes in bursts in stably stratified flows
NASA Astrophysics Data System (ADS)
Rorai, C.; Mininni, P. D.; Pouquet, A.
2014-04-01
There is a clear distinction between simple laminar and complex turbulent fluids; however, in some cases, as for the nocturnal planetary boundary layer, a stable and well-ordered flow can develop intense and sporadic bursts of turbulent activity that disappear slowly in time. This phenomenon is ill understood and poorly modeled and yet it is central to our understanding of weather and climate dynamics. We present here data from direct numerical simulations of stratified turbulence on grids of 20483 points that display the somewhat paradoxical result of measurably stronger events for more stable flows, not only in the temperature and vertical velocity derivatives as commonplace in turbulence, but also in the amplitude of the fields themselves, contrary to what happens for homogenous isotropic turbulent flows. A flow visualization suggests that the extreme values take place in Kelvin-Helmoltz overturning events and fronts that develop in the field variables. These results are confirmed by the analysis of a simple model that we present. The model takes into consideration only the vertical velocity and temperature fluctuations and their vertical derivatives. It indicates that in stably stratified turbulence, the stronger bursts can occur when the flow is expected to be more stable. The bursts are generated by a rapid nonlinear amplification of energy stored in waves and are associated with energetic interchanges between vertical velocity and temperature (or density) fluctuations in a range of parameters corresponding to the well-known saturation regime of stratified turbulence.
Dynamic multiscaling in magnetohydrodynamic turbulence.
Ray, Samriddhi Sankar; Sahoo, Ganapati; Pandit, Rahul
2016-11-01
We present a study of the multiscaling of time-dependent velocity and magnetic-field structure functions in homogeneous, isotropic magnetohydrodynamic (MHD) turbulence in three dimensions. We generalize the formalism that has been developed for analogous studies of time-dependent structure functions in fluid turbulence to MHD. By carrying out detailed numerical studies of such time-dependent structure functions in a shell model for three-dimensional MHD turbulence, we obtain both equal-time and dynamic scaling exponents.
NASA Astrophysics Data System (ADS)
Ostrovskii, Alexander G.; Zatsepin, Andrey G.
2016-10-01
This paper presents new observational data, which indicate that deep ventilation events in the aerobic zone extending across the upper part of the permanent pycnocline may occur sporadically in the Rim Current area, even during relatively warm seasons, when the seasonal thermocline is still notable. The strongest observed event of this type occurred on November 2014 off the continental shelf break near Gelendzhik Bay. Vertical profiles of dissolved oxygen were accurately measured using an SBE 52-MP Conductivity, Temperature, Depth (CTD) probe equipped with a fast-response SBE 43F oxygen sensor mounted on a moored Aqualog automatic mobile profiler. The analysis of the profiling data from October 6 through December 16, 2014, from depths between 35 m and 215 m revealed an anomaly on November 6-7. The dissolved oxygen exceeded the background levels by more than 0.2 ml/l (8.9 μM) at the 14.9-15.7 kg/m3 isopycnals in the pycnocline and reached approximately 1 ml/l (44.66 μM) for short periods. The peak absolute value of the dissolved oxygen reached an exceptionally high value of approximately 0.3 ml/l (13.4 μM) at the 15.9 kg/m3 isopycnal. The ventilation event increased the temperature by 0.2 °C at depths of 120-160 m. The simultaneous observations of both the thermohaline stratification and the ocean currents suggest that the ventilation event was associated with the sinking of pycnocline waters in the near-bottom Ekman layer along the continental slope and intense vertical turbulent exchange in the Rim Current area near the continental slope. The ventilation of the pycnocline when the overlaying upper ocean is stably stratified sharply differs from the convection reaching the Cold Intermediate Layer during extensive cooling of the sea surface. Indications of such ventilation events were also found in the Aqualog mooring data archive from 2012.
NASA Astrophysics Data System (ADS)
Zhang, Qiang
The effects of surface roughness, turbulence intensity, Mach number, and streamline curvature-airfoil shape on the aerodynamic performance of turbine airfoils are investigated in compressible, high speed flows. The University of Utah Transonic Wind Tunnel is employed for the experimental part of the study. Two different test sections are designed to produce Mach numbers, Reynolds numbers, passage mass flow rates, and physical dimensions, which match values along turbine blades in operating engines: (i) a nonturning test section with a symmetric airfoil, and (ii) a cascade test section with a cambered turbine vane. The nonuniform, irregular, three-dimensional surface roughness is characterized using the equivalent sand grain roughness size. Changing the airfoil surface roughness condition has a substantial effect on wake profiles of total pressure loss coefficients, normalized Mach number, normalized kinetic energy, and on the normalized and dimensional magnitudes of Integrated Aerodynamic Losses produced by the airfoils. Comparisons with results for a symmetric airfoil and a cambered vane show that roughness has more substantial effects on losses produced by the symmetric airfoil than the cambered vane. Data are also provided that illustrate the larger loss magnitudes are generally present with flow turning and cambered airfoils, than with symmetric airfoils. Wake turbulence structure of symmetric airfoils and cambered vanes are also studied experimentally. The effects of surface roughness and freestream turbulence levels on wake distributions of mean velocity, turbulence intensity, and power spectral density profiles and vortex shedding frequencies are quantified one axial chord length downstream of the test airfoils. As the level of surface roughness increases, all wake profile quantities broaden significantly and nondimensional vortex shedding frequencies decrease. Wake profiles produced by the symmetric airfoil are more sensitive to variations of surface
On the preferential sampling of helicity by isotropic helicoids
NASA Astrophysics Data System (ADS)
Biferale, Luca; Gustavsson, Kristian; Scatamacchia, Riccardo
2016-11-01
We present a theoretical and numerical study on the motion of isotropic helicoids in complex flows. These are particles whose motion is invariant under rotations but not under mirror reflections of the particle. This is the simplest, yet unexplored, extension of the much studied case of small spherical particles. We show that heavy isotropic helicoids, due to the coupling between translational and rotational degrees of freedom, preferentially sample different helical regions in laminar or chaotic advecting flows. This opens the way to control and engineer particles able to track complex flow structures with potential applications to microfluidics and turbulence. ERC AdG Grant NewTURB no. 339032.
NASA Technical Reports Server (NTRS)
Seasholtz, R. G.; Goldman, L. J.
1982-01-01
A technique for measuring a small optical axis velocity component in a flow with a large transverse velocity component is presented. Experimental results are given for a subsonic free jet operating in a laboratory environment, and for a 0.508 meter diameter turbine stator cascade. Satisfactory operation of the instrument was demonstrated in the stator cascade facility with an ambient acoustic noise level during operation of about 105 dB. In addition, the turbulence intensity measured with the interferometer was consistent with previous measurements taken with a fringe type laser anemometer.
NASA Astrophysics Data System (ADS)
Brunk, Brett Kenneth
1997-11-01
Pollutant and particle transport in estuaries is affected by a multitude of physical, chemical and biological processes. In this research the importance of equilibrium sorption and turbulent coagulation were studied. Sorption in estuaries was modeled using phenanthrene, bacterial extracellular polymer and kaolinite clay as surrogates for a hydrophobic organic pollutant, dissolved organic matter and inorganic suspended sediment, respectively. Experiments over a range of estuarine salinities showed that ionic strength had the largest effect on the extent of sorption, while the effect of extracellular polymer coatings on the mineral surfaces was insignificant. Further calculations using typical estuarine suspended sediment concentrations indicated that equilibrium sorption could not fully account for the solid/solution phase distribution of hydrophobic organic compounds in the estuarine water column. For particles that are small compared to the length scales of turbulence, the rate of coagulation is related to the dynamics of the smallest turbulent eddies since they have the highest shear rate. Experimental and theoretical effort focused on determining the coagulation rate of spherical particles in isotropic turbulence. A pair diffusion approximation valid for rapidly fluctuating flows was used to calculate the rate of coagulation in a randomly varying isotropic linear flow field. Dynamic simulations of particle coagulation in Gaussian turbulence were computed over a range of representative values of particle-particle interactions (i.e, hydrodynamic interactions and van der Waals attraction) and total strain (i.e., the product of the strain rate and its time scale). The computed coagulation rates for isotropic turbulence differed from analytical approximations valid at large and small total strain. As expected, particle interactions were found to be significant. Experimental measurements of coagulation in grid-generated turbulence were obtained by measuring the loss
Amato, Alberto; Fortini, Stefania; Watteaux, Romain; Diano, Marcello; Espa, Stefania; Esposito, Serena; Ferrante, Maria I; Peters, Francesc; Iudicone, Daniele; Ribera d'Alcalà, Maurizio
2016-03-01
In recent years, there has been a renewed interest in the impact of turbulence on aquatic organisms. In response to this interest, a novel instrument has been constructed, TURBOGEN, that generates turbulence in water volumes up to 13 l. TURBOGEN is fully computer controlled, thus, allowing for a high level of reproducibility and for variations of the intensity and characteristics of turbulence during the experiment. The calibration tests, carried out by particle image velocimetry, showed TURBOGEN to be successful in generating isotropic turbulence at the typical relatively low levels of the marine environment. TURBOGEN and its sizing have been devised with the long-term scope of analyzing in detail the molecular responses of plankton to different mixing regimes, which is of great importance in both environmental and biotechnological processes.
Effects of small-scale, high intensity inlet turbulence on flow in a two-dimensional diffuser
NASA Technical Reports Server (NTRS)
Hoffmann, J. A.; Gonzalez, G.
1984-01-01
The flow through a 2D experimental diffuser with channel width 2.60 cm and divergence angle (2 theta) 9 or 20 deg is investigated experimentally for inlet Reynolds number 78,300 and velocity 43.9 m/s, with and without vertical rods to generate inlet turbulence in excess of the limits defined by Hoffmann (1981) and Hoffmann and Gonzales (1983). Measurements are obtained using a thermal wall-flow-direction probe and a single hot-wire velocity probe, and the results are presented graphically. Significant increases in the pressure-recovery coefficient of the diffuser (10 percent at 9 deg and 22 percent at 20 deg) are attributed to the action of turbulence to reduce distortion and delay separation, thus creating an altered flow condition with symmetrical velocity profiles.
Zhu, Yingbin; Zhao, Daomu
2008-10-01
On the basis of the generalized diffraction integral formula for misaligned optical systems in the spatial domain, an analytical propagation expression for the elements of the cross-spectral density matrix of a random electromagnetic beam passing through a misaligned optical system in turbulent atmosphere is derived. Some analyses are illustrated by numerical examples relating to changes in the state of polarization of an electromagnetic Gaussian Schell-model beam propagating through such an optical system. It is shown that the misalignment has a significant influence on the intensity profile and the state of polarization of the beam, but the influence becomes smaller for the beam propagating in strong turbulent atmosphere. The method in this paper can be applied for sources that are either isotropic or anisotropic. It is shown that the isotropic sources and the anisotropic sources have different polarization properties on beam propagation.
Suppression of turbulent resistivity in turbulent Couette flow
Si, Jiahe Sonnenfeld, Richard G.; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe; Colgate, Stirling A.; Li, Hui; Nornberg, Mark D.
2015-07-15
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
Orientation statistics and settling velocity of ellipsoids in decaying turbulence
NASA Astrophysics Data System (ADS)
Siewert, C.; Kunnen, R. P. J.; Meinke, M.; Schröder, W.
2014-06-01
Motivated by applications in technology as well as in other disciplines where the motion of particles in a turbulent flow field is important, the orientation and settling velocity of ellipsoidal particles in a spatially decaying isotropic turbulent flow are numerically investigated. With respect to cloud microphysics ellipsoidal particles of various shapes are interpreted as archetypes of regular ice crystals, i.e., plates and columns approximated by oblate and prolate ellipsoids. The motion of 19 million small and heavy ellipsoidal particles is tracked by a Lagrangian point-particle model based on Stokes flow conditions. Five types of ellipsoids of revolution such as prolates, spheres, and oblates are considered. The orientation and settling velocity statistics are gathered at six turbulence intensities characterized by the turbulent kinetic energy dissipation rate ranging from 30 to 250 cm2s- 3. It is shown that the preferential orientation of ellipsoids is disturbed by the turbulent fluctuations of the fluid forces and moments. As the turbulence intensity increases the orientation probability distribution becomes more and more uniform. That is, the settling velocity of the ellipsoids is influenced by the turbulence level since the drag force is dependent on the orientation. The effect is more pronounced, the longer the prolate or the flatter the oblate is. The theoretical settling velocity based on the orientation probability of the non-spherical particles is smaller than that found in the simulation. The results show the existence of the preferential sweeping phenomenon also for non-spherical particles. These two effects of turbulence on the motion of ellipsoids change the settling velocity and as such the swept volume, that is expected to result in modified collision probabilities of ellipsoid-shaped particles.
Isotropic Monte Carlo Grain Growth
Mason, J.
2013-04-25
IMCGG performs Monte Carlo simulations of normal grain growth in metals on a hexagonal grid in two dimensions with periodic boundary conditions. This may be performed with either an isotropic or a misorientation - and incliantion-dependent grain boundary energy.
Inflow Turbulence Generation Methods
NASA Astrophysics Data System (ADS)
Wu, Xiaohua
2017-01-01
Research activities on inflow turbulence generation methods have been vigorous over the past quarter century, accompanying advances in eddy-resolving computations of spatially developing turbulent flows with direct numerical simulation, large-eddy simulation (LES), and hybrid Reynolds-averaged Navier-Stokes–LES. The weak recycling method, rooted in scaling arguments on the canonical incompressible boundary layer, has been applied to supersonic boundary layer, rough surface boundary layer, and microscale urban canopy LES coupled with mesoscale numerical weather forecasting. Synthetic methods, originating from analytical approximation to homogeneous isotropic turbulence, have branched out into several robust methods, including the synthetic random Fourier method, synthetic digital filtering method, synthetic coherent eddy method, and synthetic volume forcing method. This article reviews major progress in inflow turbulence generation methods with an emphasis on fundamental ideas, key milestones, representative applications, and critical issues. Directions for future research in the field are also highlighted.
Indentation of Transversely Isotropic Materials
NASA Astrophysics Data System (ADS)
Bhat, Talapady Srivatsa
Instrumented indentation, as a tool for characterization of mechanical properties, has well been established in the past decades. Studies have been conducted to understand the behavior of isotropic materials under indentation and techniques to accurately predict isotropic material properties have also been reported. Further, within the isotropic regime, work has been done to predict the indentation hardness without having to investigate the area of contact during indentation. Studies have also reported the prospect of utilizing indentation to predict the fatigue behavior of isotropic materials. This dissertation is made with the intent of extending the use of indentation, as a characterization tool, to the anisotropic regime. The effect of transverse isotropy on the indentation response of materials is systematically studied here. Extensive computational analysis is performed to elucidate the underlying deformation mechanics of indentation of transversely isotropic materials. Owing to the anisotropy, indentation may be performed parallel or perpendicular to the plane of isotropy of the specimen. It is observed that the indentation response varies significantly for each of these cases. The two cases are treated as unique and an identical systematic analysis is carried for both. The indentation orientations shall henceforth be referred to as transverse and longitudinal indentation for indentation parallel and perpendicular to the plane of isotropy respectively. A technique is developed capable of extracting the elastic-plastic properties of transversely isotropic materials from interpretation of indentation response in either direction. The technique is rigorously tested for its robustness, accuracy and uniqueness of results. A sensitivity analysis is performed to determine how sensitive the technique is to errors in experimental results. Rigorous studies are performed to understand the variation in pile-up or sink-in during indentation with varying anisotropy in the
NASA Technical Reports Server (NTRS)
Sewell, Jesse; Chew, Larry
1994-01-01
In recent years, the interest in developing a high-speed civil transport has increased. This has led to an increase in research activity on compressible supersonic flows, in particular the boundary layer. The structure of subsonic boundary layers has been extensively documented using conditional sampling techniques which exploit the knowledge of both u and v velocities. Researchers using these techniques have been able to explore some of the complex three-dimensional motions which are responsible for Reynolds stress production and transport in the boundary layer. As interest in turbulent structure has grown to include supersonic flows, a need for simultaneous multicomponent velocity measurements in these flows has developed. The success of conditional analysis in determining the characteristics of coherent motions and structures in the boundary layer relies on accurate, simultaneous measurement of two instantaneous velocity components.
NASA Technical Reports Server (NTRS)
Eggleston, John M; Diederich, Franklin W
1957-01-01
The correlation functions and power spectra of the rolling and yawing moments on an airplane wing due to the three components of continuous random turbulence are calculated. The rolling moments to the longitudinal (horizontal) and normal (vertical) components depend on the spanwise distributions of instantaneous gust intensity, which are taken into account by using the inherent properties of symmetry of isotropic turbulence. The results consist of expressions for correlation functions or spectra of the rolling moment in terms of the point correlation functions of the two components of turbulence. Specific numerical calculations are made for a pair of correlation functions given by simple analytic expressions which fit available experimental data quite well. Calculations are made for four lift distributions. Comparison is made with the results of previous analyses which assumed random turbulence along the flight path and linear variations of gust velocity across the span.
Charged Particle Diffusion in Isotropic Random Static Magnetic Fields
NASA Astrophysics Data System (ADS)
Subedi, P.; Sonsrettee, W.; Matthaeus, W. H.; Ruffolo, D. J.; Wan, M.; Montgomery, D.
2013-12-01
Study of the transport and diffusion of charged particles in a turbulent magnetic field remains a subject of considerable interest. Research has most frequently concentrated on determining the diffusion coefficient in the presence of a mean magnetic field. Here we consider Diffusion of charged particles in fully three dimensional statistically isotropic magnetic field turbulence with no mean field which is pertinent to many astrophysical situations. We classify different regions of particle energy depending upon the ratio of Larmor radius of the charged particle to the characteristic outer length scale of turbulence. We propose three different theoretical models to calculate the diffusion coefficient each applicable to a distinct range of particle energies. The theoretical results are compared with those from computer simulations, showing very good agreement.
Charged Particle Diffusion in Isotropic Random Magnetic Fields
NASA Astrophysics Data System (ADS)
Subedi, P.; Sonsrettee, W.; Blasi, P.; Ruffolo, D.; Matthaeus, W. H.; Montgomery, D.; Chuychai, P.; Dmitruk, P.; Wan, M.; Parashar, T. N.; Chhiber, R.
2017-03-01
The investigation of the diffusive transport of charged particles in a turbulent magnetic field remains a subject of considerable interest. Research has most frequently concentrated on determining the diffusion coefficient in the presence of a mean magnetic field. Here we consider the diffusion of charged particles in fully three-dimensional isotropic turbulent magnetic fields with no mean field, which may be pertinent to many astrophysical situations. We identify different ranges of particle energy depending upon the ratio of Larmor radius to the characteristic outer length scale of turbulence. Two different theoretical models are proposed to calculate the diffusion coefficient, each applicable to a distinct range of particle energies. The theoretical results are compared to those from computer simulations, showing good agreement.
Transversely isotropic poroelasticity arising from thin isotropic layers
Berryman, J.G.
1996-11-01
Percolation phenomena play central roles in the field of poroelasticity, where two distinct sets of percolating continua intertwine. A connected solid frame forms the basis of the elastic behavior of a poroelastic medium in the presence of confining forces, while connected pores permit a percolating fluid (if present) to influence the mechanical response of the system from within. The present paper discusses isotropic and anisotropic poroelastic media and establishes general formulas for the behavior of transversely isotropic poroelasticity arising from laminations of isotropic components. The Backus averaging method is shown to provide elementary means of constructing general formulas. The results for confined fluids are then compared with the more general Gassmann formulas that must be satisfied by any anisotropic poroelastic medium and found to be in complete agreement.
Transversely isotropic elasticity and poroelasticity arising from thin isotropic layers
Berryman, J.G.
1997-07-01
Since the classic work of Postma [1955] and Backus [1962], much has been learned about elastic constants in vertical transversely isotropic (VTI) media when the anisotropy is due to fine layering of isotropic elastic materials. However, new results are still being discovered. For example, the P-wave anisotropy parameter c{sub 11}/c{sub 33} lies in the range 1/4 {<=} c{sub 11}/c{sub 33} {<=} <{lambda}+2{mu}><1/({lambda}+2{mu})>, when the layers are themselves composed of isotropic elastic materials with Lame constants {lambda} and {mu} and the vertical average of the layers is symbolized by <{center_dot}>. The lower bound corrects a result of Postma. For porous layers, a connected solid frame forms the basis of the elastic behavior of a poroelastic medium in the presence of confining forces, while connected pores permit a percolating fluid (if present) to influence the mechanical response of the system from within. For isotropic and anisotropic poroelastic media, we establish general formulas for the behavior of transversely isotropic poroelasticity arising from laminations of isotropic components. The Backus averaging method is shown to provide elementary means of constructing general formulas. The results for confined fluids are then compared with the more general Gassmann [1951] formulas that must be satisfied by any anisotropic poroelastic medium and found to be in complete agreement. Such results are important for applications to oil exploration using AVO (amplitude versus offset) since the presence or absence of a fluid component, as well as the nature of the fluid, is the critical issue and the ways in which the fluid influences seismic reflection data still need to be better understood.
The Boundary Layer Late Afternoon and Sunset Turbulence Project
NASA Astrophysics Data System (ADS)
Lothon, Marie; Lohou, Fabienne; Darbieu, Clara; Couvreux, Fleur; Pino, David; Blay, Estel; Vila-Guerau de Arellano, Jordi; Pietersen, Henk; Hartogensis, Oscar; Pardyjak, Eric; Alexander, Daniel; Reuder, Joachim; Baaserud, Line; Nilsson, Erik; Jimenez, Maria Antonia; Faloona, Ian; Sastre-Marugan, Mariano; Angevine, Wayne M.; Canut, Guylaine; Bazile, Eric
2014-05-01
The BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) project aims at better understanding the turbulence processes which occur during the transition from a well-mixed convective boundary layer to a residual layer overlying a stabilized nocturnal layer. This phase of the diurnal cycle is challenging from both modeling and observational perspectives: it is transitory, most of the forcings are small or null during the transition and the turbulence regime changes from the fully convective regime of turbulence, close to homogeneous and isotropic, toward more heterogeneous and intermittent turbulence during its decay. Those issues motivated a field campaign that was conducted from 14 June to 8 July 2011 in southern France in complex terrain and consisted of a range of integrated instrument platforms including: full-size aircraft, Remotely Piloted Airplane Systems (RPAS), remote sensing instruments, radiosoundings, tethered balloons, surface flux stations, and various meteorological towers deployed over different surface covers. The boundary layer, from the earth's surface to free troposphere was densely probed during the entire day, with a focus and intense observations from midday until sunset. The field dataset now forms the base of a set of studies utilizing the observations and several types of models including: Large Eddy Simulation, Mesoscale models, forecast models. The presentation will expose an overview of this experiment and of the current observational and modeling studies, with the focus on: the turbulence decay process within the entire boundary layer from surface to the top, the mesoscale forcings of importance during BLLAST, the ability of the forecast models to represent the diurnal cycle, the relevance of the Monin Obukhov similarity theory, and shallow drainage flows. Reference: Lothon M. et al., 2012. The Boundary-Layer Late Afternoon and Sunset Turbulence field experiment, Proc. of the 20th Symposium on Boundary-Layers and Turbulence, 7
NASA Astrophysics Data System (ADS)
David, Gabrielle C. L.; Legleiter, Carl J.; Wohl, Ellen; Yochum, Steven E.
2013-02-01
roughness of the bed. The turbulence intensity is significant when considering hydraulics for predicting sediment transport and for habitat assessment. The plane-bed flow patterns were compared to results from a pool-riffle reach in the North Fork Cache La Poudre River, a step-pool reach in East St. Louis Creek, and a step-pool reach in Italy on the Rio Cordon. The comparison of the 3-D velocities among channel types showed that the plane-bed reach has higher streamwise velocities, but similar values of cross-stream and transverse velocities. Streamwise turbulence intensities were similar in both the plane-bed and step-pool reach in East St. Louis Creek. The analysis revealed that the connection between the flow characteristics and the channel was not only related to the gross morphology and location of larger clasts, but also to the development of shear layers from the convergence of flow. Therefore, it is essential to understand how water moving in one direction can alter the characteristics of another component of flow and how this interaction is connected to the bed morphology.
MULTI-SCALE COHERENT TURBULENCE AT TIDAL ENERGY SITES
Thomson, Jim; Kilcher, Levi; Harding, Samuel F.
2014-11-05
Turbulence is known to affect the performance and survivability of tidal turbines, yet characterization of turbulence in the field remains limited. Here, we refine and demonstrate a new approach to turbulence measurements, in which an array of multiple Acoustic Doppler Velocimeters (ADV) is suspended above the seabed at the hub height of a tidal turbine. These measurements provide information on the intensity, structure, and coherence of turbulence across the scale of a turbine rotor (< 10 m). Deployment of multiple moorings expands the analysis to array scales (> 10 m). Motion correction of the moored ADV data is essential to this approach and is verified using the turbulent kinetic energy spectra. Additional measurements include a bottommounted 5-beam Acoustic Doppler Current Profiler, from which scales can be assessed using the velocities a separation distances along a given beam. These methods are demonstrated with data collected at the site of the Snohomish PUD pilot project in Admiralty Inlet, Puget Sound, WA (USA). Coherent motion is found to be largely isotropic, such that coherence is high only at scales less than the advective length scale or the water depth, whichever is less.
Turbulence generation by waves
Kaftori, D.; Nan, X.S.; Banerjee, S.
1995-12-31
The interaction between two-dimensional mechanically generated waves, and a turbulent stream was investigated experimentally in a horizontal channel, using a 3-D LDA synchronized with a surface position measuring device and a micro-bubble tracers flow visualization with high speed video. Results show that although the wave induced orbital motion reached all the way to the wall, the characteristics of the turbulence wall structures and the turbulence intensity close to the wall were not altered. Nor was the streaky nature of the wall layer. On the other hand, the mean velocity profile became more uniform and the mean friction velocity was increased. Close to the free surface, the turbulence intensity was substantially increased as well. Even in predominantly laminar flows, the introduction of 2-D waves causes three dimensional turbulence. The turbulence enhancement is found to be proportional to the wave strength.
Effect of ambient turbulence on the evolution of a counter-rotating vortex pair.
NASA Astrophysics Data System (ADS)
Ahmed, Madiha; Hussain, Fazle
2007-11-01
In an attempt to explain and develop strategy for control of aircraft wake vortex in a turbulent atmosphere, the evolution of a vortex column dipole (a pair of counter-rotating vortices) in the presence of fine-scale (homogeneous and isotropic) freestream turbulence is studied via DNS of the Navier-Stokes equations. The freestream turbulence is found to significantly accelerate the vortex decay via a complex vortex-turbulence coupling scenario, which we study. External fine-scale turbulence is first stretched into azimuthal filaments (see also Melander & Hussain, PRE, vol 48 (1993)) which merge into threads through successive pairings and advect along the column dipole by self-induction. Oppositely-directed advection of opposite-signed threads forms thread dipoles which then move outward by mutual-induction and also eject column fluid (see also J. S. Marshall, JFM, vol 345 (1997)). This has the effect of enhancing both mixing with the ambient fluid and the nominally planar reconnection (cross-diffusion) between the column vortex pair. We then further explore the column vortex dipole-turbulence interaction scenario and vortex decay dependence on parameters like the column vortex Reynolds number, separation of the vortices, and the intensity and scale of freestream turbulence.
Fully-resolved DNS of finite-size particles exposed to a turbulent stream
NASA Astrophysics Data System (ADS)
Botto, Lorenzo; Prosperetti, Andrea
2008-11-01
A field of homogeneous isotropic turbulence is convected with a mean velocity past a group of fixed, finite-size particles and the structure and intensity of the resulting downstream turbulence are compared to the particle-free case. The diameter of the particles is larger than the Kolmogorov scale and is of the order of the Taylor micro-scale. The results illustrate the central role played by the particle wakes in destroying the isotropy and homogeneity of the incident turbulence. Furthermore, as a result of wake interactions, the time-dependent hydrodynamic forces on the downstream and upstream spheres are correlated. The numerical simulations are carried out on a uniform grid by employing the ``Physalis'' method which can be regarded as a combination of an immersed boundary and spectral method. Among other advantages, it does not require interpolation and its spectral convergence permits computations with relatively few grid nodes per particle.
Bi-isotropic constitutive relations
NASA Astrophysics Data System (ADS)
Sihvola, A. H.; Lindell, I. V.
1991-03-01
The constitutive relations of general bi-isotropic media, requiring four material parameters, can be written in different ways to describe their electromagnetic behavior. This communication contains a two-way 'dictionary' between a proposed formulation of the constitutive relations with three other sets of relations, generalized from relations used for chiral materials.
NASA Technical Reports Server (NTRS)
Han, Jongil; Lin, Yuh-Lang; Arya, S. Pal; Proctor, Fred H.
1999-01-01
The effects of ambient turbulence on decay and descent of aircraft wake vortices are studied using a validated, three-dimensional: large-eddy simulation model. Numerical simulations are performed in order to isolate the effect of ambient turbulence on the wake vortex decay rate within a neutrally-stratified atmosphere. Simulations are conducted for a range of turbulence intensities, by injecting wake vortex pairs into an approximately homogeneous and isotropic turbulence field. The decay rate of the vortex circulation increases clearly with increasing ambient turbulence level, which is consistent with field observations. Based on the results from the numerical simulations, simple decay models are proposed as functions of dimensionless ambient turbulence intensity (eta) and dimensionless time (T) for the circulation averaged over a range of radial distances. With good agreement with the numerical results, a Gaussian type of vortex decay model is proposed for weak turbulence: while an exponential type of Tortex decay model can be applied for strong turbulence. A relationship for the vortex descent based on above vortex decay model is also proposed. Although the proposed models are based on simulations assuming neutral stratification, the model predictions are compared to Lidar vortex measurements observed during stable, neutral, and unstable atmospheric conditions. In the neutral and unstable atmosphere, the model predictions appear to be in reasonable agreement with the observational data, while in the stably-stratified atmosphere, they largely underestimate the observed circulation decay with consistent overestimation of the observed vortex descent. The underestimation of vortex decay during stably-stratified conditions suggests that stratification has an important influence on vortex decay when ambient levels of turbulence are weak.
Stagnation point flow and heat transfer under free-steam turbulence
NASA Astrophysics Data System (ADS)
Xiong, Zhongmin
Stagnation point flow and heat transfer in the presence of free-stream turbulence is investigated through both numerical simulation and theoretical analysis. Large eddy simulations (LES), using fourth order finite differences in curvilinear coordinates in conjunction with an efficient linearized dual-time snub-iteration scheme, are performed to study free-stream turbulence impingement, upon an elliptical leading edge and the resulting heat transfer enhancement. A new blending procedure is developed through which independent, statistically identical realizations of homogeneous isotropic turbulence are combined to provide realistic representations of the free-stream turbulence. Results for different free-stream turbulence intensity, length scale, and Mach number are presented. Turbulence statistics and Reynolds stress budget at different streamwise locations are examined in detail. It is found that small scale, intense vortical structures generated by vortex stretching near the leading edge are directly responsible for the elevated heat transfer. The numerical results on the heat transfer enhancement show good agreement with the experimental measurements. In the theoretical study, the distortion of three dimensional unsteady disturbances in an incompressible Hiemenz boundary layer and its effects on heat, transfer are analyzed using linear vortex dynamics. An asymptotic expression for the vorticity evolution is obtained with explicit dependence on the length scale and frequency of the disturbance. It is shown that the vorticity amplification, and hence the heat transfer enhancement, increases with decreasing length scale, and the maximum value is found to be around five times the boundary layer thickness. Extending the analysis to free-stream turbulence, we derive a new scaling correlation for the relative heat transfer enhancement which incorporates turbulence intensity, integral length scale and mean flow Reynolds number. This correlation parameter is shown to
Space–time domain velocity distributions in isotropic radiative transfer in two dimensions
NASA Astrophysics Data System (ADS)
Rossetto, Vincent
2017-04-01
We compute the exact solutions of the radiative transfer equation in two dimensions for isotropic scattering. The intensity and the radiance are given in the space–time domain when the source is punctual and isotropic or unidirectional. These analytical results are compared to Monte-Carlo simulations in four particular situations.
The numerical analysis of a turbulent compressible jet
NASA Astrophysics Data System (ADS)
Debonis, James Raymond
2000-10-01
A numerical method to simulate high Reynolds number jet flows was formulated and applied to gain a better understanding of the flow physics. Large-eddy simulation was chosen as the most promising approach to model the turbulent structures due to its compromise between accuracy and computational expense. The filtered Navier-Stokes equations were developed including a total energy form of the energy equation. Sub-grid scale models for the momentum and energy equations were adapted from compressible forms of Smagorinsky's original model. The effect of using disparate temporal and spatial accuracy in a numerical scheme was discovered through one-dimensional model problems and a new uniformly fourth-order accurate numerical method was developed. Results from two and three dimensional validation exercises show that the code accurately reproduces both viscous and inviscid flows. Numerous axisymmetric jet simulations were performed to investigate the effect of grid resolution, numerical scheme, exit boundary conditions and sub-grid scale modeling on the solution and the results were used to guide the three-dimensional calculations. Three-dimensional calculations of a Mach 1.4 jet showed that this LES simulation accurately captures the physics of the turbulent flow. The agreement with experimental data relatively good and is much better than results in the current literature. Turbulent intensities indicate that the turbulent structures at this level of modeling are not isotropic and this information could lend itself to the development of improved sub-grid scale models for LES and turbulence models for RANS simulations. A two point correlation technique was used to quantify the turbulent structures. Two point space correlations were used to obtain a measure of the integral length scale, which proved to be approximately ½Dj. Two point space-time correlations were used to obtain the convection velocity for the turbulent structures. This velocity ranged from 0.57 to 0.71 Uj.
Turbulence Impact on Wind Turbines: Experimental Investigations on a Wind Turbine Model
NASA Astrophysics Data System (ADS)
Al-Abadi, A.; Kim, Y. J.; Ertunç, Ö.; Delgado, A.
2016-09-01
Experimental investigations have been conducted by exposing an efficient wind turbine model to different turbulence levels in a wind tunnel. Nearly isotropic turbulence is generated by using two static squared grids: fine and coarse one. In addition, the distance between the wind-turbine and the grid is adjusted. Hence, as the turbulence decays in the flow direction, the wind-turbine is exposed to turbulence with various energy and length scale content. The developments of turbulence scales in the flow direction at various Reynolds numbers and the grid mesh size are measured. Those measurements are conducted with hot-wire anemometry in the absence of the wind-turbine. Detailed measurements and analysis of the upstream and downstream velocities, turbulence intensity and spectrum distributions are done. Performance measurements are conducted with and without turbulence grids and the results are compared. Performance measurements are conducted with an experimental setup that allow measuring of torque, rotational speed from the electrical parameters. The study shows the higher the turbulence level, the higher the power coefficient. This is due to many reasons. First, is the interaction of turbulence scales with the blade surface boundary layer, which in turn delay the stall. Thus, suppressing the boundary layer and preventing it from separation and hence enhancing the aerodynamics characteristics of the blade. In addition, higher turbulence helps in damping the tip vortices. Thus, reduces the tip losses. Adding winglets to the blade tip will reduce the tip vortex. Further investigations of the near and far wake-surrounding intersection are performed to understand the energy exchange and the free stream entrainment that help in retrieving the velocity.
NASA Astrophysics Data System (ADS)
Pinsky, M.; Khain, A.; Rosenfeld, D.; Pokrovsky, A.
The motion of water drops and graupel particles within a turbulent medium is analyzed. The turbulence is assumed to be homogeneous and isotropic. It is demonstrated that the inertia of drops and graupel particles falling within a turbulent flow leads to the formation of significant velocity deviations from the surrounding air, as well as to the formation of substantial relative velocity between drops and graupel particles. The results of calculations of the continuous growth of raindrops and graupel particles moving within a cloud of small droplets are presented both in a non-turbulent medium and within turbulent flows of different turbulence intensity. Continuous growth of a drop-collector was calculated with the coalescence efficiency E ɛ=1, as well as using E ɛ values provided by Beard and Ochs [Beard, K.V., Ochs, H.T., 1984. Collection and coalescence efficiencies for accretion. J. Geophys. Res., 89: 7165-7169.] ranging from 0.5 to about 0.75 for different droplet sizes. In the case of graupel-droplet interaction E ɛ was assumed equal to 1. It is shown that in the case E ɛ=1 in a non-turbulent medium, the growth rates of graupel and raindrops are close. Under turbulent conditions typical of mature convective clouds, graupel grows much faster than a raindrop. In the case E ɛ<1 the growth rate of a water drop slows down significantly, so that graupel grows faster than raindrops even under non-turbulent conditions. Turbulence greatly increases the difference between the growth rates of graupel and drop-collectors. Possible consequences of the faster growth of graupel in terms of cloud microphysics are discussed.
Anisotropic Particles in Turbulence
NASA Astrophysics Data System (ADS)
Voth, Greg A.; Soldati, Alfredo
2017-01-01
Anisotropic particles are common in many industrial and natural turbulent flows. When these particles are small and neutrally buoyant, they follow Lagrangian trajectories while exhibiting rich orientational dynamics from the coupling of their rotation to the velocity gradients of the turbulence field. This system has proven to be a fascinating application of the fundamental properties of velocity gradients in turbulence. When particles are not neutrally buoyant, they experience preferential concentration and very different preferential alignment than neutrally buoyant tracer particles. A vast proportion of the parameter range of anisotropic particles in turbulence is still unexplored, with most existing research focusing on the simple foundational cases of axisymmetric ellipsoids at low concentrations in homogeneous isotropic turbulence and in turbulent channel flow. Numerical simulations and experiments have recently developed a fairly comprehensive picture of alignment and rotation in these cases, and they provide an essential foundation for addressing more complex problems of practical importance. Macroscopic effects of nonspherical particle dynamics include preferential concentration in coherent structures and drag reduction by fiber suspensions. We review the models used to describe nonspherical particle motion, along with numerical and experimental methods for measuring particle dynamics.
Constitutive modeling for isotropic materials
NASA Technical Reports Server (NTRS)
Ramaswamy, V. G.; Vanstone, R. H.; Dame, L. T.; Laflen, J. H.
1984-01-01
The unified constitutive theories for application to typical isotropic cast nickel base supperalloys used for air-cooled turbine blades were evaluated. The specific modeling aspects evaluated were: uniaxial, monotonic, cyclic, creep, relaxation, multiaxial, notch, and thermomechanical behavior. Further development of the constitutive theories to model thermal history effects, refinement of the material test procedures, evaluation of coating effects, and verification of the models in an alternate material will be accomplished in a follow-on for this base program.
Spherical 3D isotropic wavelets
NASA Astrophysics Data System (ADS)
Lanusse, F.; Rassat, A.; Starck, J.-L.
2012-04-01
Context. Future cosmological surveys will provide 3D large scale structure maps with large sky coverage, for which a 3D spherical Fourier-Bessel (SFB) analysis in spherical coordinates is natural. Wavelets are particularly well-suited to the analysis and denoising of cosmological data, but a spherical 3D isotropic wavelet transform does not currently exist to analyse spherical 3D data. Aims: The aim of this paper is to present a new formalism for a spherical 3D isotropic wavelet, i.e. one based on the SFB decomposition of a 3D field and accompany the formalism with a public code to perform wavelet transforms. Methods: We describe a new 3D isotropic spherical wavelet decomposition based on the undecimated wavelet transform (UWT) described in Starck et al. (2006). We also present a new fast discrete spherical Fourier-Bessel transform (DSFBT) based on both a discrete Bessel transform and the HEALPIX angular pixelisation scheme. We test the 3D wavelet transform and as a toy-application, apply a denoising algorithm in wavelet space to the Virgo large box cosmological simulations and find we can successfully remove noise without much loss to the large scale structure. Results: We have described a new spherical 3D isotropic wavelet transform, ideally suited to analyse and denoise future 3D spherical cosmological surveys, which uses a novel DSFBT. We illustrate its potential use for denoising using a toy model. All the algorithms presented in this paper are available for download as a public code called MRS3D at http://jstarck.free.fr/mrs3d.html
Kerstein, A.R.
1996-12-31
One-Dimensional Turbulence is a new turbulence modeling strategy involving an unsteady simulation implemented in one spatial dimension. In one dimension, fine scale viscous and molecular-diffusive processes can be resolved affordably in simulations at high turbulence intensity. The mechanistic distinction between advective and molecular processes is thereby preserved, in contrast to turbulence models presently employed. A stochastic process consisting of mapping {open_quote}events{close_quote} applied to a one-dimensional velocity profile represents turbulent advection. The local event rate for given eddy size is proportional to the velocity difference across the eddy. These properties cause an imposed shear to induce an eddy cascade analogous in many respects to the eddy cascade in turbulent flow. Many scaling and fluctuation properties of self-preserving flows, and of passive scalars introduced into these flows, are reproduced.
Interaction of Isotropic Turbulence with a Shock Wave
1992-03-01
5 1.3 Objectives and Overview....................................... 6 2. Linear Analysis...Length Scales .................................. 74 4.1.5 Thermodynamic Properties ................................ 75 5 4.1.6 M odeling Issues...78 4.2 Modification of a Shock Wave ...................................... 82 5 4.2.1 Statistics of a Shock Wave
Estimating three-demensional energy transfer in isotropic turbulence
NASA Technical Reports Server (NTRS)
Li, K. S.; Helland, K. N.; Rosenblatt, M.
1980-01-01
To obtain an estimate of the spectral transfer function that indicates the rate of decay of energy, an x-wire probe was set at a fixed position, and two single wire probes were set at a number of locations in the same plane perpendicular to the mean flow in the wind tunnel. The locations of the single wire probes are determined by pseudo-random numbers (Monte Carlo). Second order spectra and cross spectra are estimated. The assumption of isotropy relative to second order spectra is examined. Third order spectra are also estimated corresponding to the positions specified. A Monte Carlo Fourier transformation of the downstream bispectra corresponding to integration across the plane perpendicular to the flow is carried out assuming isotropy. Further integration is carried out over spherical energy shells.
Lagrangian analysis of premixed turbulent combustion in hydrogen-air flames
NASA Astrophysics Data System (ADS)
Darragh, Ryan; Poludnenko, Alexei; Hamlington, Peter
2016-11-01
Lagrangian analysis has long been a tool used to analyze non-reacting turbulent flows, and has recently gained attention in the reacting flow and combustion communities. The approach itself allows one to separate local molecular effects, such as those due to reactions or diffusion, from turbulent advective effects along fluid pathlines, or trajectories. Accurate calculation of these trajectories can, however, be rather difficult due to the chaotic nature of turbulent flows and the added complexity of reactions. In order to determine resolution requirements and verify the numerical algorithm, extensive tests are described in this talk for prescribed steady, unsteady, and chaotic flows, as well as for direct numerical simulations (DNS) of non-reacting homogeneous isotropic turbulence. The Lagrangian analysis is then applied to DNS of premixed hydrogen-air flames at two different turbulence intensities for both single- and multi-step chemical mechanisms. Non-monotonic temperature and fuel-mass fraction evolutions are found to exist along trajectories passing through the flame brush. Such non-monotonicity is shown to be due to molecular diffusion resulting from large spatial gradients created by turbulent advection. This work was supported by the Air Force Office of Scientific Research (AFOSR) under Award No. FA9550-14-1-0273, and the Department of Defense (DoD) High Performance Computing Modernization Program (HPCMP) under a Frontier project award.
Multifractality and the effect of turbulence on the chaotic dynamics of a HeNe laser
NASA Astrophysics Data System (ADS)
Gulich, Damián.; Zunino, Luciano; Pérez, Darío.; Garavaglia, Mario
2013-09-01
We propose the use of multifractal detrended fluctuation analysis (MF-DFA) to measure the influence of atmospheric turbulence on the chaotic dynamics of a HeNe laser. Fit ranges for MF-DFA are obtained with goodness of linear fit (GoLF) criterion. The chaotic behavior is generated by means of a simple interferometric setup with a feedback to the cavity of the gas laser. Such dynamics have been studied in the past and modeled as a function of the feedback level. Different intensities of isotropic turbulence have been generated with a turbulator device, allowing a structure constant for the index of refraction of air adjustable by means of a temperature difference parameter in the unit. Considering the recent interest in message encryption with this kind of setups, the study of atmospheric turbulence effects plays a key role in the field of secure laser communication through the atmosphere. In principle, different intensities of turbulence may be interpreted as different levels of white noise on the original chaotic series. These results can be of utility for performance optimization in chaotic free-space laser communication systems.
NASA Astrophysics Data System (ADS)
Nilsson, Erik; Lohou, Fabienne; Lothon, Marie; Pardyjak, Eric; Mahrt, Larry; Darbieu, Clara
2016-07-01
The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from midday until zero-buoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 intensive observation period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and mesoscale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of near-surface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near-surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near-surface production of TKE is compensated for by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of -0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are normalized with
The interaction of high-speed turbulence with flames: Turbulent flame speed
Poludnenko, A.Y.; Oran, E.S.
2011-02-15
Direct numerical simulations of the interaction of a premixed flame with driven, subsonic, homogeneous, isotropic, Kolmogorov-type turbulence in an unconfined system are used to study the mechanisms determining the turbulent flame speed, S{sub T}, in the thin reaction zone regime. High intensity turbulence is considered with the r.m.s. velocity 35 times the laminar flame speed, S{sub L}, resulting in the Damkoehler number Da=0.05. The simulations were performed with Athena-RFX, a massively parallel, fully compressible, high-order, dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion model, based on the one-step Arrhenius kinetics, represents a stoichiometric H{sub 2}-air mixture under the assumption of the Lewis number Le=1. Global properties and the internal structure of the flame were analyzed in an earlier paper, which showed that this system represents turbulent combustion in the thin reaction zone regime. This paper demonstrates that: (1) The flame brush has a complex internal structure, in which the isosurfaces of higher fuel mass fractions are folded on progressively smaller scales. (2) Global properties of the turbulent flame are best represented by the structure of the region of peak reaction rate, which defines the flame surface. (3) In the thin reaction zone regime, S{sub T} is predominantly determined by the increase of the flame surface area, A{sub T}, caused by turbulence. (4) The observed increase of S{sub T} relative to S{sub L} exceeds the corresponding increase of A{sub T} relative to the surface area of the planar laminar flame, on average, by {approx}14%, varying from only a few percent to as high as {approx}30%. (5) This exaggerated response is the result of tight flame packing by turbulence, which causes frequent flame collisions and formation of regions of high flame curvature >or similar 1/{delta}{sub L}, or ''cusps,'' where {delta}{sub L} is the thermal width of the laminar flame. (6) The local flame speed in the cusps
Phenomenology of turbulent convection
NASA Astrophysics Data System (ADS)
Verma, Mahendra; Chatterjee, Anando; Kumar, Abhishek; Samtaney, Ravi
2016-11-01
We simulate Rayleigh-Bénard convection (RBC) in which a fluid is confined between two thermally conducting plates. We report results from direct numerical simulation (DNS) of RBC turbulence on 40963 grid, the highest resolution hitherto reported, on 65536 cores of Cray XC40, Shaheen II, at KAUST. The non-dimensional parameters of our simulation are: the Rayleigh number Ra = 1 . 1 ×1011 (the highest ever for a pseudo-spectral simulation) and Prandtl number of unity. We present energy flux diagnostics of shell-to-shell (in wave number space) transfer. Furthermore, noting that convective flows are anisotropic due to buoyancy, we quantify anisotropy by subdividing each wavenumber shell into rings and quantify ring energy spectrum. An outstanding question in convective turbulence is the wavenumber scaling of the energy spectrum. Our pseudo-spectral simulations of turbulent thermal convection coupled with novel energy transfer diagnostics have provided a definitive answer to this question. We conclude that convective turbulence exhibits behavior similar to fluid turbulence, that is, Kolmogorov's k - 5 / 3 spectrum with forward and local energy transfers, along with a nearly isotropic energy distribution. The supercomputer Shaheen at KAUST was utilized for the simulations.
Are random fractal clusters isotropic\\?
NASA Astrophysics Data System (ADS)
Family, Fereydoon; Vicsek, Tamás; Meakin, Paul
1985-08-01
We have studied the shape of large clusters in the lattice-animal, percolation, and growing-percolation models. By calculating the radius of gyration tensor we find that in these models the clusters have an anisotropic shape. The results suggest that the critical droplets in related isotropic equilibrium models, such as the Ising model, may also be anisotropic. We have also determined the leading nonanalytic correction-to-scaling exponent by analyzing the anisotropy data and find that for percolation in two dimensions e~=0.47. .AE
A Transversely Isotropic Thermoelastic Theory
NASA Technical Reports Server (NTRS)
Arnold, S. M.
1989-01-01
A continuum theory is presented for representing the thermoelastic behavior of composites that can be idealized as transversely isotropic. This theory is consistent with anisotropic viscoplastic theories being developed presently at NASA Lewis Research Center. A multiaxial statement of the theory is presented, as well as plane stress and plane strain reductions. Experimental determination of the required material parameters and their theoretical constraints are discussed. Simple homogeneously stressed elements are examined to illustrate the effect of fiber orientation on the resulting strain distribution. Finally, the multiaxial stress-strain relations are expressed in matrix form to simplify and accelerate implementation of the theory into structural analysis codes.
Isotropic Negative Thermal Expansion Metamaterials.
Wu, Lingling; Li, Bo; Zhou, Ji
2016-07-13
Negative thermal expansion materials are important and desirable in science and engineering applications. However, natural materials with isotropic negative thermal expansion are rare and usually unsatisfied in performance. Here, we propose a novel method to achieve two- and three-dimensional negative thermal expansion metamaterials via antichiral structures. The two-dimensional metamaterial is constructed with unit cells that combine bimaterial strips and antichiral structures, while the three-dimensional metamaterial is fabricated by a multimaterial 3D printing process. Both experimental and simulation results display isotropic negative thermal expansion property of the samples. The effective coefficient of negative thermal expansion of the proposed models is demonstrated to be dependent on the difference between the thermal expansion coefficient of the component materials, as well as on the circular node radius and the ligament length in the antichiral structures. The measured value of the linear negative thermal expansion coefficient of the three-dimensional sample is among the largest achieved in experiments to date. Our findings provide an easy and practical approach to obtaining materials with tunable negative thermal expansion on any scale.
Premixed autoignition in compressible turbulence
NASA Astrophysics Data System (ADS)
Konduri, Aditya; Kolla, Hemanth; Krisman, Alexander; Chen, Jacqueline
2016-11-01
Prediction of chemical ignition delay in an autoignition process is critical in combustion systems like compression ignition engines and gas turbines. Often, ignition delay times measured in simple homogeneous experiments or homogeneous calculations are not representative of actual autoignition processes in complex turbulent flows. This is due the presence of turbulent mixing which results in fluctuations in thermodynamic properties as well as chemical composition. In the present study the effect of fluctuations of thermodynamic variables on the ignition delay is quantified with direct numerical simulations of compressible isotropic turbulence. A premixed syngas-air mixture is used to remove the effects of inhomogeneity in the chemical composition. Preliminary results show a significant spatial variation in the ignition delay time. We analyze the topology of autoignition kernels and identify the influence of extreme events resulting from compressibility and intermittency. The dependence of ignition delay time on Reynolds and turbulent Mach numbers is also quantified. Supported by Basic Energy Sciences, Dept of Energy, United States.
Anisotropic Turbulence and Protostellar Feedback in Molecular Clouds
NASA Astrophysics Data System (ADS)
Hansen, Charles Edward
I investigate the decay and regeneration of turbulence in molecular clouds and the resulting star formation in those clouds in the presence of protostellar feedback. Studies of turbulence generally only consider isotropic turbulence, while the turbulence in molecular clouds may be anisotropic. I perform a series of simulations of anisotropic turbulence and measure its decay rate. I find that anisotropic turbulence decays slower than isotropic turbulence. When I break the velocity dispersion into isotropic and anisotropic components, I find the decay time is the crossing time of the isotropic component, which can be much slower than the total velocity dispersion. As part of this study, I present a measure of anisotropy that can be calculated in observations of molecular clouds. I also investigate the effects of compression on turbulence. This is motivated by the need to replenish turbulent energy. Using a series of simulations of contracting turbulence, I find that turbulence behaves as a monatomic ideal gas under isotropic compression. I also find that compression in a single direction imparts energy to that direction, but does not transfer that energy to the other two directions. Finally, I perform a series of high resolution star formation simulations with adaptive mesh refinement (AMR) including hydrodynamics, gravity, radiation, protostellar outflows and protostellar luminosity. The simulations provide a self-consistent story of star formation, all while matching observations. The matched observations include the masses of both stars and prestellar cores, the clustering of cores and the luminosity function of protostars. In this story of star formation, cores form on the Jeans length of the host cloud. Each core forms a central star or binary, but also fragments repeatedly down 0.05 M⊙ stars. The stellar radiation prevents fragmentation below this mass scale, but is not important on larger scales. The protostellar outflows eject 2/3 of the incoming mass
On the Lundgren-Townsend model of turbulent fine scales
NASA Astrophysics Data System (ADS)
Pullin, D. I.; Saffman, P. G.
1993-01-01
Vorticity and velocity-derivative moments for homogeneous isotropic turbulence are calculated using the strained-spiral vortex model of turbulent fine scales given by Lundgren (1982). A specific form of the relaxing spiral vortex is proposed, modeled by a rolling-up vortex layer embedded in a background containing opposite signed vorticity and with zero total circulation at infinity.
Helicopter rotor noise due to ingestion of atmospheric turbulence
NASA Technical Reports Server (NTRS)
Simonich, J. C.; Amiet, R. K.; Schlinker, R. H.; Greitzer, E. M.
1986-01-01
A theoretical study was conducted to develop an analytical prediction method for helicopter main rotor noise due to the ingestion of atmospheric turbulence. This study incorporates an atmospheric turbulence model, a rotor mean flow contraction model and a rapid distortion turbulence model which together determine the statistics of the non-isotropic turbulence at the rotor plane. Inputs to the combined mean inflow and turbulence models are controlled by atmospheric wind characteristics and helicopter operating conditions. A generalized acoustic source model was used to predict the far field noise generated by the non-isotropic flow incident on the rotor. Absolute levels for acoustic spectra and directivity patterns were calculated for full scale helicopters, without the use of empirical or adjustable constants. Comparisons between isotropic and non-isotropic turbulence at the rotor face demonstrated pronounced differences in acoustic spectra. Turning and contraction of the flow for hover and low speed vertical ascent cases result in a 3 dB increase in the acoustic spectrum energy and a 10 dB increase in tone levels. Compared to trailing edge noise, turbulence ingestion noise is the dominant noise mechanism below approximately 30 rotor harmonics, while above 100 harmonics, trailing edge noise levels exceed turbulence ingestion noise by 25 dB.
ANISOTROPY LENGTHENS THE DECAY TIME OF TURBULENCE IN MOLECULAR CLOUDS
Hansen, Charles E.; McKee, Christopher F.; Klein, Richard I.
2011-09-01
The decay of isothermal turbulence with velocity anisotropy is investigated using computational simulations and synthetic observations. We decompose the turbulence into isotropic and anisotropic components with total velocity dispersions {sigma}{sub iso} and {sigma}{sub ani}, respectively. We find that the decay rate of the turbulence depends on the crossing time of the isotropic component only. A cloud of size L with significant anisotropy in its turbulence has a dissipation time, t{sub diss} = L/(2{sigma}{sub iso}). This translates into turbulent energy decay rates on the cloud scale that can be much lower for anisotropic turbulence than for isotropic turbulence. To help future observations determine whether observed molecular clouds have the level of anisotropy required to maintain the observed level of turbulence over their lifetimes, we performed a principal component analysis on our simulated clouds. Even with projection effects washing out the anisotropic signal, there is a measurable difference in the axis-constrained principal component analysis performed in directions parallel and perpendicular to the direction of maximum velocity dispersion. When this relative difference, {psi}, is 0.1, there is enough anisotropy for the dissipation time to triple the expected isotropic value. We provide a fit for converting {psi} into an estimate for the dissipation time, t{sub diss}.
Measuring isotropic subsurface light transport.
Happel, Kathrin; Dörsam, Edgar; Urban, Philipp
2014-04-21
Subsurface light transport can affect the visual appearance of materials significantly. Measuring and modeling this phenomenon is crucial for accurately reproducing colors in printing or for rendering translucent objects on displays. In this paper, we propose an apparatus to measure subsurface light transport employing a reference material to cancel out adverse signals that may bias the results. In contrast to other approaches, the setup enables improved focusing on rough surfaces (e.g. uncoated paper). We derive a measurement equation that may be used to deduce the point spread function (PSF) of subsurface light transport. Main contributions are the usage of spectrally-narrowband exchangeable LEDs allowing spectrally-resolved measurements and an approach based on quadratic programming for reconstructing PSFs in the case of isotropic light transport.
NASA Astrophysics Data System (ADS)
Gopalan, Balaji
In part I, high speed in-line digital holographic cinematography is used for studying turbulent diffusion of slightly buoyant 0.5-1.2 mm diameter diesel droplets (specific gravity of 0.85) and 50 mum diameter neutral density particles. Experiments are performed in a 50x50x70 mm3 sample volume in a controlled, nearly isotropic turbulence facility, which is characterized by 2-D PIV. An automated tracking program has been used for measuring velocity time history of more than 17000 droplets and 15000 particles. The PDF's of droplet velocity fluctuations are close to Gaussian for all turbulent intensities ( u'i ). The mean rise velocity of droplets is enhanced or suppressed, compared to quiescent rise velocity (Uq), depending on Stokes number at lower turbulence levels, but becomes unconditionally enhanced at higher turbulence levels. The horizontal droplet velocity rms exceeds the fluid velocity rms for most of the data, while the vertical ones are higher than the fluid only at the highest turbulence level. The scaled droplet horizontal diffusion coefficient is higher than the vertical one, for 1 < u'i /Uq < 5, consistent with trends of the droplet velocity fluctuations. Conversely, the scaled droplet horizontal diffusion timescale is smaller than the vertical one due to crossing trajectories effect. The droplet diffusion coefficients scaled by the product of turbulence intensity and an integral length scale is a monotonically increasing function of u'i /Uq. Part II of this work explains the formation of micron sized droplets in turbulent flows from crude oil droplets pre-mixed with dispersants. Experimental visualization shows that this breakup starts with the formation of very long and quite stable, single or multiple micro threads that trail behind millimeter sized droplets. These threads form in regions with localized increase in concentration of surfactant, which in turn depends on the flow around the droplet. The resulting reduction of local surface tension
Dielectrophoretic manipulation of nematic and isotropic droplets
NASA Astrophysics Data System (ADS)
Lee, Bomi; Song, Jang-Kun
2016-03-01
Dielectrophoresis can provide a delicate tool to control electrically neutral particles in colloid. The dielectrophoresis is usually applied to solid particles or heterogeneous liquid droplet in continuous liquid, but we devised and investigated the dielectrophoresis of isotropic droplets within nematic phase or vice versa. Using multi-components liquid crystal mixtures that exhibit relatively wide temperature range of nematic-isotropic coexistence, we achieved a field-induced phase separation between isotropic and nematic. We also fabricated the isotropic-nematic filaments that was achieved using a biased surface preference for either isotropic or nematic phase of the alignment layer [1]. The dielectrophoresis manipulations of isotropic and nematic droplets required much lower voltage compared to that for the electro wetting type devices. In addition, we observed the bi-directional actuation of isotropic droplets using anisotropic dielectric property of liquid crystal, which is not possible in usual dielectrophoresis. The bidirectional actuation was achieved by controlling the LC director within the cell so as to change the sign of the difference between the effective dielectric constant of nematic and isotropic liquid crystals. We simulated the bi-directional dielectrophoresis by performing the LC director calculation and the corresponding dielectrophoresis. The simulation results matched well with the experimental data. Thus, the bi-directional dielectrophoresis using isotropic and nematic droplets may open new possibility of electro- optical applications using liquid crystals.
Macroscopic simulation of isotropic permanent magnets
NASA Astrophysics Data System (ADS)
Bruckner, Florian; Abert, Claas; Vogler, Christoph; Heinrichs, Frank; Satz, Armin; Ausserlechner, Udo; Binder, Gernot; Koeck, Helmut; Suess, Dieter
2016-03-01
Accurate simulations of isotropic permanent magnets require to take the magnetization process into account and consider the anisotropic, nonlinear, and hysteretic material behaviour near the saturation configuration. An efficient method for the solution of the magnetostatic Maxwell equations including the description of isotropic permanent magnets is presented. The algorithm can easily be implemented on top of existing finite element methods and does not require a full characterization of the hysteresis of the magnetic material. Strayfield measurements of an isotropic permanent magnet and simulation results are in good agreement and highlight the importance of a proper description of the isotropic material.
Hairpin vortices in the outer and near wall regions of the canonical turbulent boundary layer
NASA Astrophysics Data System (ADS)
Wallace, James; Wu, Xiaohua; Moin, Parviz
2016-11-01
While the dominance of hairpin vortices and their significance for transport processes in the transitional and early turbulent regions of the canonical turbulent boundary layer has been widely accepted, opinion is divided about the developed flow downstream. Here we investigate the representative vortical structures in the outer and near wall regions for the momentum thickness Reynolds number, Reθ , of up to 3000 using the DNS database described in. This boundary layer grows spatially from a laminar state at Reθ = 80 beneath a freestream of continuous and nearly isotropic turbulence decaying from an intensity of 3 to 0.8%. The vortical structures are visualized with the swirling strength, λci. In the outer region hairpin vortices appear and are advected over distances corresponding to about 300 - 400 in Reθ within the fully turbulent region, demonstrating that they are not remnants of transitional structures. The near wall vortical structures are more difficult to visualize and require careful tuning of the swirling strength and making invisible the flow above the near wall region of the flow. The hairpins in this region occur in intermittent clusters that have features remarkably similar to transitional turbulent spots.
The relationship between strength of turbulence and backscattering radar power at HF and VHF
NASA Technical Reports Server (NTRS)
Hocking, W. K.
1983-01-01
The formulae relating turbulence and other atmospheric parameters to backscattered power for radar observations are reviewed. Emphasis is on the case of scatter from turbulent irregularities which have scales corresponding to the range of isotropic, inertial range turbulence. The applicability of this assumption is discussed. A formula is introduced for the mesosphere which relates ionospheric electron densities to backscattered power.
Kinematics of velocity and vorticity correlations in turbulent flow
NASA Astrophysics Data System (ADS)
Bernard, P. S.
1983-08-01
The kinematic problem of calculating second-order velocity moments from given values of the vorticity covariance is examined. Integral representation formulas for second-order velocity moments in terms of the two-point vorticity correlation tensor are derived. The special relationships existing between velocity moments in isotropic turbulence are expressed in terms of the integral formulas yielding several kinematic constraints on the two-point vorticity correlation tensor in isotropic turbulence. Numerical evaluation of these constraints suggests that a Gaussian curve may be the only form of the longitudinal velocity correlation coefficient which is consistent with the requirement of isotropy. It is shown that if this is the case, then a family of exact solutions to the decay of isotropic turbulence may be obtained which contains Batchelor's final period solution as a special case. In addition, the computed results suggest a method of approximating the integral representation formulas in general turbulent shear flows.
Kinematics of velocity and vorticity correlations in turbulent flow
NASA Technical Reports Server (NTRS)
Bernard, P. S.
1983-01-01
The kinematic problem of calculating second-order velocity moments from given values of the vorticity covariance is examined. Integral representation formulas for second-order velocity moments in terms of the two-point vorticity correlation tensor are derived. The special relationships existing between velocity moments in isotropic turbulence are expressed in terms of the integral formulas yielding several kinematic constraints on the two-point vorticity correlation tensor in isotropic turbulence. Numerical evaluation of these constraints suggests that a Gaussian curve may be the only form of the longitudinal velocity correlation coefficient which is consistent with the requirement of isotropy. It is shown that if this is the case, then a family of exact solutions to the decay of isotropic turbulence may be obtained which contains Batchelor's final period solution as a special case. In addition, the computed results suggest a method of approximating the integral representation formulas in general turbulent shear flows.
Dynamics of particle--turbulence interaction at the dissipative scales
NASA Astrophysics Data System (ADS)
Bocanegra Evans, Humberto; Dam, Nico; van de Water, Willem; JM Burgerscentrum Collaboration; COST Action, Particles in Turbulence Collaboration
2013-11-01
We present results of a novel phosphorescent tagging technique that is particularly suited to study particle-laden flows. Using phosphorescent droplets we probe the dynamics of particle-turbulence interaction at the dissipative length scales. We create a cloud of droplets within a chamber capable of generating homogeneous, isotropic turbulence with zero-mean flow. The droplets have Stokes number St ~ 1 , and the flow is intensely turbulent, with Reynolds number Reλ ~ 500 . Using a frequency-tripled Nd:YAG laser, we can tag a variety of volumes, such as thin slabs or thin, pencil-like cylinders. The droplets in these volumes glow during a few Kolmogorov times. By tracking the fate of pencil-shaped clouds using a fast (5 kHz) camera, we come to the surprising conclusion that they disperse faster than fluid elements, with a spreading rate reaching a maximum at St ~ 2 . Sheets of tagged droplets display preferential concentration at work; we discuss statistical quantities that can capture these events. This project is funded by Fundamenteel Onderzoek der Materie (FOM).
Light-Steered Isotropic Semiconductor Micromotors.
Chen, Chuanrui; Mou, Fangzhi; Xu, Leilei; Wang, Shaofei; Guan, Jianguo; Feng, Zunpeng; Wang, Quanwei; Kong, Lei; Li, Wei; Wang, Joseph; Zhang, Qingjie
2017-01-01
Intelligent photoresponsive isotropic semiconductor micromotors are developed by taking advantage of the limited penetration depth of light to induce asymmetrical surface chemical reactions. Independent of the Brownian motion of themselves, the as-proposed isotropic micromotors are able to continuously move with both motion direction and speed just controlled by light, as well as precisely manipulate particles for nanoengineering.
Rotor noise due to atmospheric turbulence ingestion. I - Fluid mechanics
NASA Technical Reports Server (NTRS)
Simonich, J. C.; Amiet, R. K.; Schlinker, R. H.; Greitzer, E. M.
1986-01-01
In the present analytical procedure for the prediction of helicopter rotor noise generation due to the ingestion of atmospheric turbulence, different models for turbulence fluid mechanics and the ingestion process are combined. The mean flow and turbulence statistics associated with the atmospheric boundary layer are modeled with attention to the effects of atmospheric stability length, windspeed, and altitude. The turbulence field can be modeled as isotropic, locally stationary, and homogeneous. For large mean flow contraction ratios, accurate predictions of turbulence vorticity components at the rotor face requires the incorporation of the differential drift of fluid particles on adjacent streamlines.
Constitutive modeling for isotropic materials
NASA Technical Reports Server (NTRS)
Lindholm, Ulric S.; Chan, Kwai S.
1986-01-01
The objective of the program is to evaluate and develop existing constitutive models for use in finite-element structural analysis of turbine engine hot section components. The class of constitutive equation studied is considered unified in that all inelastic deformation including plasticity, creep, and stress relaxation are treated in a single term rather than a classical separation of plasticity (time independent) and creep (time dependent) behavior. The unified theories employed also do not utilize the classical yield surface or plastic potential concept. The models are constructed from an appropriate flow law, a scalar kinetic relation between strain rate, temperature and stress, and evolutionary equations for internal variables describing strain or work hardening, both isotropic and directional (kinematic). This and other studies have shown that the unified approach is particularly suited for determining the cyclic behavior of superalloy type blade and vane materials and is entirely compatible with three-dimensional inelastic finite-element formulations. The behavior was examined of a second nickel-base alloy, MAR-M247, and compared it with the Bodner-Partom model, further examined procedures for determining the material-specific constants in the models, and exercised the MARC code for a turbine blade under simulated flight spectrum loading. Results are summarized.
Constitutive modeling for isotropic materials
NASA Technical Reports Server (NTRS)
Chan, K. S.; Lindholm, U. S.; Bodner, S. R.
1988-01-01
The third and fourth years of a 4-year research program, part of the NASA HOST Program, are described. The program goals were: (1) to develop and validate unified constitutive models for isotropic materials, and (2) to demonstrate their usefulness for structural analysis of hot section components of gas turbine engines. The unified models selected for development and evaluation were those of Bodner-Partom and of Walker. The unified approach for elastic-viscoplastic constitutive equations is a viable method for representing and predicting material response characteristics in the range where strain rate and temperature dependent inelastic deformations are experienced. This conclusion is reached by extensive comparison of model calculations against the experimental results of a test program of two high temperature Ni-base alloys, B1900+Hf and Mar-M247, over a wide temperature range for a variety of deformation and thermal histories including uniaxial, multiaxial, and thermomechanical loading paths. The applicability of the Bodner-Partom and the Walker models for structural applications has been demonstrated by implementing these models into the MARC finite element code and by performing a number of analyses including thermomechanical histories on components of hot sections of gas turbine engines and benchmark notch tensile specimens. The results of the 4-year program have been published in four annual reports. The results of the base program are summarized in this report. The tasks covered include: (1) development of material test procedures, (2) thermal history effects, and (3) verification of the constitutive model for an alternative material.
How Isotropic is the Universe?
NASA Astrophysics Data System (ADS)
Saadeh, Daniela; Feeney, Stephen M.; Pontzen, Andrew; Peiris, Hiranya V.; McEwen, Jason D.
2016-09-01
A fundamental assumption in the standard model of cosmology is that the Universe is isotropic on large scales. Breaking this assumption leads to a set of solutions to Einstein's field equations, known as Bianchi cosmologies, only a subset of which have ever been tested against data. For the first time, we consider all degrees of freedom in these solutions to conduct a general test of isotropy using cosmic microwave background temperature and polarization data from Planck. For the vector mode (associated with vorticity), we obtain a limit on the anisotropic expansion of (σV/H )0 <4.7 ×10-11 (95% C.L.), which is an order of magnitude tighter than previous Planck results that used cosmic microwave background temperature only. We also place upper limits on other modes of anisotropic expansion, with the weakest limit arising from the regular tensor mode, (σT ,reg/H )0 <1.0 ×10-6 (95% C.L.). Including all degrees of freedom simultaneously for the first time, anisotropic expansion of the Universe is strongly disfavored, with odds of 121 000:1 against.
How Isotropic is the Universe?
Saadeh, Daniela; Feeney, Stephen M; Pontzen, Andrew; Peiris, Hiranya V; McEwen, Jason D
2016-09-23
A fundamental assumption in the standard model of cosmology is that the Universe is isotropic on large scales. Breaking this assumption leads to a set of solutions to Einstein's field equations, known as Bianchi cosmologies, only a subset of which have ever been tested against data. For the first time, we consider all degrees of freedom in these solutions to conduct a general test of isotropy using cosmic microwave background temperature and polarization data from Planck. For the vector mode (associated with vorticity), we obtain a limit on the anisotropic expansion of (σ_{V}/H)_{0}<4.7×10^{-11} (95% C.L.), which is an order of magnitude tighter than previous Planck results that used cosmic microwave background temperature only. We also place upper limits on other modes of anisotropic expansion, with the weakest limit arising from the regular tensor mode, (σ_{T,reg}/H)_{0}<1.0×10^{-6} (95% C.L.). Including all degrees of freedom simultaneously for the first time, anisotropic expansion of the Universe is strongly disfavored, with odds of 121 000:1 against.
Non-linear dielectric effect in the isotropic phase above the isotropic-cholesteric phase transition
NASA Astrophysics Data System (ADS)
Mukherjee, Prabir K.; Chakraborty, Sumanta; Rzoska, Sylwester J.
2011-11-01
Using the Landau-de Gennes theory, the temperature, pressure and frequency dependence of the non-linear effect in the isotropic phase above the isotropic-cholesteric phase transition is calculated. The influence of pressure on the isotropic-cholesteric phase transition is discussed by varying the coupling between the orientational order parameter and the macroscopic polarization of polar cholesterics. Comparing the results of the calculations with existing data, we finally conclude that the model provides a description of the isotropic-cholesteric transition that takes all experimentally known features of the unusual negative and positive pretransitional effect in the isotropic phase of the system into account in a qualitatively correct way.
Broken symmetry in ideal magnetohydrodynamic turbulence
NASA Technical Reports Server (NTRS)
Shebalin, John V.
1993-01-01
A numerical study of the long-time evolution of a number of cases of inviscid, isotropic, incompressible, three-dimensional fluid, and magneto-fluid turbulence has been completed. The results confirm that ideal magnetohydrodynamic turbulence is non-ergodic if there is no external magnetic field present. This is due essentially to a canonical symmetry being broken in an arbitrary dynamical representation. The broken symmetry manifests itself as a coherent structure, i.e., a non-zero time-averaged part of the turbulent magnetic field. The coherent structure is observed, in one case, to contain about eighteen percent of the total energy.
Modeling turbulent flame propagation
Ashurst, W.T.
1994-08-01
Laser diagnostics and flow simulation techniques axe now providing information that if available fifty years ago, would have allowed Damkoehler to show how turbulence generates flame area. In the absence of this information, many turbulent flame speed models have been created, most based on Kolmogorov concepts which ignore the turbulence vortical structure, Over the last twenty years, the vorticity structure in mixing layers and jets has been shown to determine the entrainment and mixing behavior and these effects need to be duplicated by combustion models. Turbulence simulations reveal the intense vorticity structure as filaments and simulations of passive flamelet propagation show how this vorticity Creates flame area and defines the shape of the expected chemical reaction surface. Understanding how volume expansion interacts with flow structure should improve experimental methods for determining turbulent flame speed. Since the last decade has given us such powerful new tools to create and see turbulent combustion microscopic behavior, it seems that a solution of turbulent combustion within the next decade would not be surprising in the hindsight of 2004.
Inertial currents in isotropic plasma
NASA Technical Reports Server (NTRS)
Heinemann, M.; Erickson, G. M.; Pontius, D. H., Jr.
1994-01-01
The magnetospheric convection electric field contributes to Birkeland currents. The effects of the field are to polarize the plasma by displacing the bounce paths of the ions from those of electrons, to redistribute the pressure so that it is not constant along magnetic field lines, and to enhance the pressure gradient by the gradient of the bulk speed. Changes in the polarization charge during the convection of the plasma are neutralized by electrons in the form of field-aligned currents that close through the ionosphere. The pressure drives field-aligned currents through its gradient in the same manner as in quasi-static plasmas, but with modifications that are important if the bulk speed is of the order of the ion thermal speed; the variations in the pressure along field lines are maintained by a weak parallel potential drop. These effects are described in terms of the field-aligned currents in steady state, isotropic, MHD plasma. Solutions are developed by taking the MHD limit ot two-fluid solutions and illustrated in the special case of Maxwellian plasma for which the temperature is constant along magnetic field lines. The expression for the Birkeland current density is a generalization of Vasyliunas' expression for the field-aligned current density in quasi-static plasma and provides a unifying expression when both pressure gradients and ion inertia operate simultaneously as sources of field-aligned currents. It contains a full account of different aspects of the ion flow (parallel and perpendicular velocity and vorticity) that contribute to the currents. Contributions of ion inertia to field-aligned currents will occur in regions of strong velocity shear, electric field reversal, or large gradients in the parallel velocity or number density, and may be important in the low-latitude boundary layer, plasma sheet boundary layer, and the inner edge region of the plasma sheet.
The Numerical Analysis of a Turbulent Compressible Jet. Degree awarded by Ohio State Univ., 2000
NASA Technical Reports Server (NTRS)
DeBonis, James R.
2001-01-01
A numerical method to simulate high Reynolds number jet flows was formulated and applied to gain a better understanding of the flow physics. Large-eddy simulation was chosen as the most promising approach to model the turbulent structures due to its compromise between accuracy and computational expense. The filtered Navier-Stokes equations were developed including a total energy form of the energy equation. Subgrid scale models for the momentum and energy equations were adapted from compressible forms of Smagorinsky's original model. The effect of using disparate temporal and spatial accuracy in a numerical scheme was discovered through one-dimensional model problems and a new uniformly fourth-order accurate numerical method was developed. Results from two- and three-dimensional validation exercises show that the code accurately reproduces both viscous and inviscid flows. Numerous axisymmetric jet simulations were performed to investigate the effect of grid resolution, numerical scheme, exit boundary conditions and subgrid scale modeling on the solution and the results were used to guide the three-dimensional calculations. Three-dimensional calculations of a Mach 1.4 jet showed that this LES simulation accurately captures the physics of the turbulent flow. The agreement with experimental data was relatively good and is much better than results in the current literature. Turbulent intensities indicate that the turbulent structures at this level of modeling are not isotropic and this information could lend itself to the development of improved subgrid scale models for LES and turbulence models for RANS simulations. A two point correlation technique was used to quantify the turbulent structures. Two point space correlations were used to obtain a measure of the integral length scale, which proved to be approximately 1/2 D(sub j). Two point space-time correlations were used to obtain the convection velocity for the turbulent structures. This velocity ranged from 0.57 to
Two-dimensional convective turbulence
Gruzinov, A.V.; Kukharkin, N.; Sudan, R.N.
1996-02-01
We show that 2D {bold E{times}B} ionospheric turbulence of the electron density in the equatorial electrojet is isomorphic to the viscous convection of an ordinary fluid in a porous medium due to temperature gradients. Numerical simulations reveal the strong anisotropy in the turbulence, which consists of rising hot bubbles and falling cool bubbles. These bubbles break up into fingers leading to the formation of stable shear flows. After reaching a quasisteady state, the omnidirectional energy spectrum approaches a {ital k}{sup {minus}2} behavior, rather than {ital k}{sup {minus}5/3} as expected from isotropic turbulence. Physical mechanisms that lead to anisotropy are analyzed. {copyright} {ital 1996 The American Physical Society.}
Measurement of the Noise Resulting from the Interaction of Turbulence with a Lifting Surface
NASA Technical Reports Server (NTRS)
Hutcheson, Florence V.; Brooks, Thomas F.; Burley, Casey L.; Stead, Daniel J.
2011-01-01
An experimental study of the noise resulting from the interaction of an airfoil with incident turbulence is presented. The test models include NACA0015 airfoils of different chord lengths, a flat plate with a sharp leading edge, and an airfoil of same section as a reference Fowler flap. The airfoils are immersed in nearly isotropic turbulence. Two approaches for performing the noise measurements are used and compared. The effects that turbulence intensity and scales, airfoil geometry, velocity and angle of attack have on the incident turbulence interaction noise are examined. Detailed directivity measurements are presented. It is found that noise spectral levels beyond the peak frequency decrease more with decreasing airfoil leading edge sharpness, and that spectral peak level (at 0 deg. angle of attack) appears to be mostly controlled by the airfoil fs thickness and chord. Increase in turbulence scale and intensity are observed to lead to a uniform increase of the noise spectral levels with an LI(sup 2) dependence (where L is the turbulence longitudinal integral scale and I is the turbulence intensity). Noise levels are found to scale with the 6th power of velocity and the 2nd power of the airfoil chord. Sensitivity to changes in angle of attack appears to have a turbulence longitudinal integral scale to chord (C) ratio dependence, with large effects on noise for L/C greater than or equal to 1 and decreased effects as L/C becomes smaller than 1. For all L/C values, the directivity pattern of the noise resulting from the incident turbulence is seen to remain symmetric with respect to the direction of the mean flow until stall, at which point, the directivity becomes symmetric with respect to the airfoil chord. It is also observed that sensitivity to angle of attack changes is more pronounced on the model suction side than on the model pressure side, and in the higher frequency range of the spectra for the largest airfoils tested (L/C less than 0.24).
NASA Technical Reports Server (NTRS)
Chandler, C. L.
1987-01-01
In order to forecast turbulence, one needs to have an understanding of the cause of turbulence. Therefore, an attempt is made to show the atmospheric structure that often results when aircraft encounter moderate or greater turbulence. The analysis is based on thousands of hours of observations of flights over the past 39 years of aviation meteorology.
Turbulent Distortion of Condensate Accretion
NASA Technical Reports Server (NTRS)
Hazoume, R.; Orou Chabi, J.; Johnson, J. A., III
1997-01-01
When a simple model for the relationship between the density-temperature fluctuation correlation and mean values is used, we determine that the rate of change of turbulent intensity can influence directly the accretion rate of droplets. Considerable interest exists in the accretion rate for condensates in nonequilibrium flow with icing and the potential role which reactant accretion can play in nonequilibrium exothermic reactant processes. Turbulence is thought to play an important role in such flows. It has already been experimentally determined that turbulence influences the sizes of droplets in the heterogeneous nucleation of supersaturated vapors. This paper addresses the issue of the possible influence of turbulence on the accretion rate of droplets.
Shell models of magnetohydrodynamic turbulence
NASA Astrophysics Data System (ADS)
Plunian, Franck; Stepanov, Rodion; Frick, Peter
2013-02-01
Shell models of hydrodynamic turbulence originated in the seventies. Their main aim was to describe the statistics of homogeneous and isotropic turbulence in spectral space, using a simple set of ordinary differential equations. In the eighties, shell models of magnetohydrodynamic (MHD) turbulence emerged based on the same principles as their hydrodynamic counter-part but also incorporating interactions between magnetic and velocity fields. In recent years, significant improvements have been made such as the inclusion of non-local interactions and appropriate definitions for helicities. Though shell models cannot account for the spatial complexity of MHD turbulence, their dynamics are not over simplified and do reflect those of real MHD turbulence including intermittency or chaotic reversals of large-scale modes. Furthermore, these models use realistic values for dimensionless parameters (high kinetic and magnetic Reynolds numbers, low or high magnetic Prandtl number) allowing extended inertial range and accurate dissipation rate. Using modern computers it is difficult to attain an inertial range of three decades with direct numerical simulations, whereas eight are possible using shell models. In this review we set up a general mathematical framework allowing the description of any MHD shell model. The variety of the latter, with their advantages and weaknesses, is introduced. Finally we consider a number of applications, dealing with free-decaying MHD turbulence, dynamo action, Alfvén waves and the Hall effect.
Characterizing Wake Turbulence with Staring Lidar Measurements
NASA Astrophysics Data System (ADS)
Bastine, D.; Wächter, M.; Peinke, J.; Trabucchi, D.; Kühn, M.
2015-06-01
Lidar measurements in the German offshore wind farm Alpha Ventus were performed to investigate the turbulence characteristics of wind turbine wakes. In particular, we compare measurements of the free flow in the surroundings of the wind turbines with measurements in the inner region of a wake flow behind one turbine. Our results indicate that wind turbines modulate the turbulent structures of the flow on a wide range of scales. For the data of the wake flow, the power spectrum as well as the multifractal intermittency coefficient reveal features of homogeneous isotropic turbulence. Thus, we conjecture that on scales of the rotor a new turbulent cascade is initiated, which determines the features of the turbulent wake flow quite independently from the more complex wind flow in the surroundings of the turbine.
NASA Technical Reports Server (NTRS)
Goldstein, M. E.; Rosenbaum, B. M.
1972-01-01
A model, based on Lighthill's theory, for predicting aerodynamic noise from a turbulent shear flow is developed. This model is a generalization of the one developed by Ribner. Unlike Ribner's model, it does not require that the turbulent correlations factor into space and time-dependent parts. It replaces his assumption of isotropic. turbulence by the more realistic one of axisymmetric turbulence. The implications of the model for jet noise are discussed.
Observations of velocity shear driven plasma turbulence
NASA Technical Reports Server (NTRS)
Kintner, P. M., Jr.
1976-01-01
Electrostatic and magnetic turbulence observations from HAWKEYE-1 during the low altitude portion of its elliptical orbit over the Southern Hemisphere are presented. The magnetic turbulence is confined near the auroral zone and is similar to that seen at higher altitudes by HEOS-2 in the polar cusp. The electrostatic turbulence is composed of a background component with a power spectral index of 1.89 + or - .26 and an intense component with a power spectral index of 2.80 + or - .34. The intense electrostatic turbulence and the magnetic turbulence correlate with velocity shears in the convective plasma flow. Since velocity shear instabilities are most unstable to wave vectors perpendicular to the magnetic field, the shear correlated turbulence is anticipated to be two dimensional in character and to have a power spectral index of 3 which agrees with that observed in the intense electrostatic turbulence.
Application of a Reynolds stress turbulence model to the compressible shear layer
NASA Technical Reports Server (NTRS)
Sarkar, S.; Balakrishnan, L.
1990-01-01
Theoretically based turbulence models have had success in predicting many features of incompressible, free shear layers. However, attempts to extend these models to the high-speed, compressible shear layer have been less effective. In the present work, the compressible shear layer was studied with a second-order turbulence closure, which initially used only variable density extensions of incompressible models for the Reynolds stress transport equation and the dissipation rate transport equation. The quasi-incompressible closure was unsuccessful; the predicted effect of the convective Mach number on the shear layer growth rate was significantly smaller than that observed in experiments. Having thus confirmed that compressibility effects have to be explicitly considered, a new model for the compressible dissipation was introduced into the closure. This model is based on a low Mach number, asymptotic analysis of the Navier-Stokes equations, and on direct numerical simulation of compressible, isotropic turbulence. The use of the new model for the compressible dissipation led to good agreement of the computed growth rates with the experimental data. Both the computations and the experiments indicate a dramatic reduction in the growth rate when the convective Mach number is increased. Experimental data on the normalized maximum turbulence intensities and shear stress also show a reduction with increasing Mach number.
Characterizing glottal jet turbulence.
Alipour, Fariborz; Scherer, Ronald C
2006-02-01
Air pressure associated with airflow from the lungs drives the vocal folds into oscillation and allows the air to exit the glottis as a turbulent jet, even though laminar flow may enter the glottis from the trachea. The separation of the turbulence from the deterministic portion of the glottal jet was investigated in the excised canine larynx model. The present study is methodological in that the main goal was to examine three methods of obtaining reasonable representations of both the deterministic signal and the residual turbulence portion: (a) smoothing, (b) wavelet denoising, and (c) ensemble averaging. Ensemble averaging resulted in a deterministic signal that disregarded gross cyclic alterations while exaggerating the turbulence intensity. Wavelet denoising can perform an excellent analysis and synthesis of the glottal velocity, but was problematic in determining which levels of analysis to choose to represent both the deterministic and turbulence appropriately. Smoothing appeared to be the most appropriate for phonation velocities because it preserved gross cyclic variations important to perturbations and modulations, while extracting turbulence at what appears to be reasonable levels.
Propagation of multi-Gaussian Schell-model vortex beams in isotropic random media.
Tang, Miaomiao; Zhao, Daomu
2015-12-14
The effect of isotropic and homogeneous random media on propagation characteristics of recently introduced multi-Gaussian Schell-model (MGSM) vortex beams is investigated. The analytical formula for the cross-spectral density function of such a beam propagating in random turbulent media is derived and used to explore the evolution of the spectral density, the degree of coherence and the turbulence-induced spreading. An example illustrates the fact that, at sufficiently large distance from the source, the source correlations modulation of the spectral distribution in free space is shown to be suppressed by the uniformly correlated turbulence. The impacts, arising from the index M, the correlation width of the source and the properties of the medium on such characteristics are analyzed in depth.
Consistent Initial Conditions for the DNS of Compressible Turbulence
NASA Technical Reports Server (NTRS)
Ristorcelli, J. R.; Blaisdell, G. A.
1996-01-01
Relationships between diverse thermodynamic quantities appropriate to weakly compressible turbulence are derived. It is shown that for turbulence of a finite turbulent Mach number there is a finite element of compressibility. A methodology for generating initial conditions for the fluctuating pressure, density and dilatational velocity is given which is consistent with finite Mach number effects. Use of these initial conditions gives rise to a smooth development of the flow, in contrast to cases in which these fields are specified arbitrarily or set to zero. Comparisons of the effect of different types of initial conditions are made using direct numerical simulation of decaying isotropic turbulence.
Premixed Turbulent Flame Propagation in Microgravity
NASA Technical Reports Server (NTRS)
Menon, Suresh
1999-01-01
A combined numerical-experimental study has been carried out to investigate the structure and propagation characteristics of turbulent premixed flames with and without the influence of buoyancy. Experimentally, the premixed flame characteristics are studied in the wrinkled regime using a Couette flow facility and an isotropic flow facility in order to resolve the scale of flame wrinkling. Both facilities were chosen for their ability to achieve sustained turbulence at low Reynolds number. This implies that conventional diagnostics can be employed to resolve the smallest scales of wrinkling. The Couette facility was also built keeping in mind the constraints imposed by the drop tower requirements. Results showed that the flow in this Couette flow facility achieves full-developed turbulence at low Re and all turbulence statistics are in good agreement with past measurements on large-scale facilities. Premixed flame propagation studies were then carried out both using the isotropic box and the Couette facility. Flame imaging showed that fine scales of wrinkling occurs during flame propagation. Both cases in Ig showed significant buoyancy effect. To demonstrate that micro-g can remove this buoyancy effect, a small drop tower was built and drop experiments were conducted using the isotropic box. Results using the Couette facility confirmed the ability to carry out these unique reacting flow experiments at least in 1g. Drop experiments at NASA GRC were planned but were not completed due to termination of this project.
Studying Turbulence Using Numerical Simulation Databases, 2. Proceedings of the 1988 Summer Program
NASA Technical Reports Server (NTRS)
1988-01-01
The focus of the program was on the use of direct numerical simulations of turbulent flow for study of turbulence physics and modeling. A special interest was placed on turbulent mixing layers. The required data for these investigations were generated from four newly developed codes for simulation of time and spatially developing incompressible and compressible mixing layers. Also of interest were the structure of wall bounded turbulent and transitional flows, evaluation of diagnostic techniques for detection of organized motions, energy transfer in isotropic turbulence, optical propagation through turbulent media, and detailed analysis of the interaction of vortical structures.
Exact models for isotropic matter
NASA Astrophysics Data System (ADS)
Thirukkanesh, S.; Maharaj, S. D.
2006-04-01
We study the Einstein-Maxwell system of equations in spherically symmetric gravitational fields for static interior spacetimes. The condition for pressure isotropy is reduced to a recurrence equation with variable, rational coefficients. We demonstrate that this difference equation can be solved in general using mathematical induction. Consequently, we can find an explicit exact solution to the Einstein-Maxwell field equations. The metric functions, energy density, pressure and the electric field intensity can be found explicitly. Our result contains models found previously, including the neutron star model of Durgapal and Bannerji. By placing restrictions on parameters arising in the general series, we show that the series terminate and there exist two linearly independent solutions. Consequently, it is possible to find exact solutions in terms of elementary functions, namely polynomials and algebraic functions.
NASA Technical Reports Server (NTRS)
Liu, Xiao-Feng; Thomas, Flint O.; Nelson, Robert C.
2001-01-01
Turbulence kinetic energy (TKE) is a very important quantity for turbulence modeling and the budget of this quantity in its transport equation can provide insight into the flow physics. Turbulence kinetic energy budget measurements were conducted for a symmetric turbulent wake flow subjected to constant zero, favorable and adverse pressure gradients in year-three of research effort. The purpose of this study is to clarify the flow physics issues underlying the demonstrated influence of pressure gradient on wake development and provide experimental support for turbulence modeling. To ensure the reliability of these notoriously difficult measurements, the experimental procedure was carefully designed on the basis of an uncertainty analysis. Four different approaches, based on an isotropic turbulence assumption, a locally axisymmetric homogeneous turbulence assumption, a semi-isotropy assumption and a forced balance of the TKE equation, were applied for the estimate of the dissipation term. The pressure transport term is obtained from a forced balance of the turbulence kinetic energy equation. This report will present the results of the turbulence kinetic energy budget measurement and discuss their implication on the development of strained turbulent wakes.
PDF turbulence modeling and DNS
NASA Technical Reports Server (NTRS)
Hsu, A. T.
1992-01-01
The problem of time discontinuity (or jump condition) in the coalescence/dispersion (C/D) mixing model is addressed in probability density function (pdf). A C/D mixing model continuous in time is introduced. With the continuous mixing model, the process of chemical reaction can be fully coupled with mixing. In the case of homogeneous turbulence decay, the new model predicts a pdf very close to a Gaussian distribution, with finite higher moments also close to that of a Gaussian distribution. Results from the continuous mixing model are compared with both experimental data and numerical results from conventional C/D models. The effect of Coriolis forces on compressible homogeneous turbulence is studied using direct numerical simulation (DNS). The numerical method used in this study is an eight order compact difference scheme. Contrary to the conclusions reached by previous DNS studies on incompressible isotropic turbulence, the present results show that the Coriolis force increases the dissipation rate of turbulent kinetic energy, and that anisotropy develops as the Coriolis force increases. The Taylor-Proudman theory does apply since the derivatives in the direction of the rotation axis vanishes rapidly. A closer analysis reveals that the dissipation rate of the incompressible component of the turbulent kinetic energy indeed decreases with a higher rotation rate, consistent with incompressible flow simulations (Bardina), while the dissipation rate of the compressible part increases; the net gain is positive. Inertial waves are observed in the simulation results.
NASA Technical Reports Server (NTRS)
Wang, C. R.
1988-01-01
The present analyses of boundary layer flow and turbulence transport attempt to characterize the influence of freestream turbulence on the surface heat-transfer rate and stagnation point region skin friction of a circular cross-section cylinder in turbulent flow. The Reynolds stress-transport equations and k-epsilon two-equation turbulence modeling are used, yielding time-averaged turbulence double-correlations, mean-flow properties, surface heat-transfer rate, and skin-friction with freestream isotropic turbulence. A comparison of analytical results with experimental data indicates that large Reynolds normal stresses are induced at the boundary layer edge by the kinetic energy of the turbulence.
NASA Technical Reports Server (NTRS)
Wang, C. R.
1983-01-01
A turbulent boundary layer flow analysis of the momentum and thermal flow fields near the forward stagnation point due to a circular cylinder in turbulent cross flow is presented. Turbulence modeling length scale, anisotropic turbulence initial profiles and boundary conditions were identified as functions of the cross flow turbulence intensity and the boundary layer flow far field velocity. These parameters were used in a numerical computational procedure to calculate the mean velocity, mean temperature, and turbulence double correlation profiles within the flow field. The effects of the cross flow turbulence on the stagnation region momentum and thermal flow fields were investigated. This analysis predicted the existing measurements of the stagnation region mean velocity and surface heat transfer rate with cross flow Reynolds number and turbulence intensity less than 250,000 and 0.05, respectively.
Static spherically symmetric wormholes with isotropic pressure
NASA Astrophysics Data System (ADS)
Cataldo, Mauricio; Liempi, Luis; Rodríguez, Pablo
2016-06-01
In this paper we study static spherically symmetric wormhole solutions sustained by matter sources with isotropic pressure. We show that such spherical wormholes do not exist in the framework of zero-tidal-force wormholes. On the other hand, it is shown that for the often used power-law shape function there are no spherically symmetric traversable wormholes sustained by sources with a linear equation of state p = ωρ for the isotropic pressure, independently of the form of the redshift function ϕ (r). We consider a solution obtained by Tolman at 1939 for describing static spheres of isotropic fluids, and show that it also may describe wormhole spacetimes with a power-law redshift function, which leads to a polynomial shape function, generalizing a power-law shape function, and inducing a solid angle deficit.
Efficient modeling in transversely isotropic inhomogeneous media
Alkhalifah, T.
1993-11-01
An efficient modeling technique for transversely isotropic, inhomogeneous media, is developed using a mix of analytical equations and numerical calculations. The analytic equation for the raypath in a factorized transversely isotropic (FTI) media with linear velocity variation, derived by Shearer and Chapman, is used to trace between two points. In addition, I derive an analytical equation for geometrical spreading in FTI media that aids in preserving program efficiency; however, the traveltime is calculated numerically. I then generalize the method to treat general transversely isotropic (TI) media that are not factorized anisotropic inhomogeneous by perturbing the FTI traveltimes, following the perturbation ideas of Cerveny and Filho. A Kirchhoff-summation-based program relying on Trorey`s (1970) diffraction method is used to generate synthetic seismograms for such a medium. For the type of velocity models treated, the program is much more efficient than finite-difference and general ray-trace modeling techniques.
NASA Technical Reports Server (NTRS)
Goldstein, M.; Rosenbaum, B.
1973-01-01
A model based on Lighthill's theory for predicting aerodynamic noise from a turbulent shear flow is developed. This model is a generalization of the one developed by Ribner. It does not require that the turbulent correlations factor into space and time-dependent parts. It replaces his assumption of isotropic turbulence by the more realistic one of axisymmetric turbulence. In the course of the analysis, a hierarchy of equations is developed wherein each succeeding equation involves more assumptions than the preceding equation but requires less experimental information for its use. The implications of the model for jet noise are discussed. It is shown that for the particular turbulence data considered anisotropy causes the high-frequency self-noise to be beamed downstream.
Negative Poisson's ratio materials via isotropic interactions.
Rechtsman, Mikael C; Stillinger, Frank H; Torquato, Salvatore
2008-08-22
We show that under tension a classical many-body system with only isotropic pair interactions in a crystalline state can, counterintuitively, have a negative Poisson's ratio, or auxetic behavior. We derive the conditions under which the triangular lattice in two dimensions and lattices with cubic symmetry in three dimensions exhibit a negative Poisson's ratio. In the former case, the simple Lennard-Jones potential can give rise to auxetic behavior. In the latter case, a negative Poisson's ratio can be exhibited even when the material is constrained to be elastically isotropic.
The Two Isotropic Asymptotes of Fiber Composites,
1988-03-01
Voigt and Reuss models of summed stiffness and compliance. The compliance quasi-isotropic asymptote, which >’-:’ has evidently not been discussed in the...i,j,e)de o0 The resulting pseudo -isotropic compliance (series-model) Hooke’s law matrix is similar but not identical to Eq. (3): W(1) W(4) 0 a aIE...given by 1 W( ) 14) E : - Vc Wc 4W(5) 2[W(1) - W()] (8) c W() W(14 12 Direct formulas for the pseudo -isctrcpic moduli, in terms of the ply 1s natural
Turbulent flows near flat plates
NASA Astrophysics Data System (ADS)
Kambe, R.; Imamura, T.; Doi, M.
1980-08-01
The method to study the effect of the plate moving in the homogeneous or isotropic turbulence is presented. The crucial point of this method is to solve the Orr-Sommerfeld like equation, which is satisfied by the kernel of the Wiener-Hermite expansion of the velocity field, under the inhomogeneous boundary condition. In the special case of constant mean flow, our method gives the same result as that of Hunt and Graham and succeeds in explaining the experimental results of Thomas and Hancock. The method is also applied to the case of nonuniform mean flow, where the shear effect comes up.
Estimation of crystallinity in isotropic isotactic polypropylene with Raman spectroscopy.
Minogianni, Chrysa; Gatos, Konstantinos G; Galiotis, Costas
2005-09-01
The Raman spectrum of isotactic polypropylene (iPP) has been found to exhibit vibrational peaks in the region of 750 to 880 cm(-1) that are sensitive to the degree of crystallinity. These features are broadly assigned to various modes of methyl group rocking, rho(CH2), and there have been various attempts to assess crystallinity based on the integrated intensities of these bands. Various vibrational analyses performed in the past in combination with experimental studies have concluded that the presence of crystalline order with trans-gauche conformation gives rise to a peak at 809 cm(-1), which is assigned to a rho(CH2) mode coupled with the skeletal stretching mode. However, the presence of additional peaks at 830 cm(-1), 841 cm(-1), and 854 cm(-1), within the same envelope, have been the subject of controversy. In this work isotropic films of iPP derived from the same precursor of identical tacticity have been subjected to various degrees of annealing and the integrated intensities of the Raman bands were measured. The results showed that true 3d crystallinity in isotropic iPP can only be expressed by the 809 cm(-1) band whereas the band at 841 cm(-1) corresponds to an uncoupled rho(CH2) fundamental mode and thus is a measure of the amorphous content. The less intense satellite bands at 830 cm(-1) and 854 cm(-1) of solid iPP cannot be distinguished from the 841 cm(-1) band in the melt and are generally considered as intermediate phases possibly related to non-crystalline components with 3(1)-helical conformations. Independent differential scanning calorimetry (DSC) crystallinity measurements were in broad agreement with the Raman measurements based on the normalized intensity of the 809 cm(-1) Raman band. By comparing the Raman with the DSC data a new value for the theoretical heat of fusion for the 100% crystalline iPP has been proposed.
Small-scale universality in fluid turbulence
Schumacher, Jörg; Scheel, Janet D.; Krasnov, Dmitry; Donzis, Diego A.; Yakhot, Victor; Sreenivasan, Katepalli R.
2014-01-01
Turbulent flows in nature and technology possess a range of scales. The largest scales carry the memory of the physical system in which a flow is embedded. One challenge is to unravel the universal statistical properties that all turbulent flows share despite their different large-scale driving mechanisms or their particular flow geometries. In the present work, we study three turbulent flows of systematically increasing complexity. These are homogeneous and isotropic turbulence in a periodic box, turbulent shear flow between two parallel walls, and thermal convection in a closed cylindrical container. They are computed by highly resolved direct numerical simulations of the governing dynamical equations. We use these simulation data to establish two fundamental results: (i) at Reynolds numbers Re ∼ 102 the fluctuations of the velocity derivatives pass through a transition from nearly Gaussian (or slightly sub-Gaussian) to intermittent behavior that is characteristic of fully developed high Reynolds number turbulence, and (ii) beyond the transition point, the statistics of the rate of energy dissipation in all three flows obey the same Reynolds number power laws derived for homogeneous turbulence. These results allow us to claim universality of small scales even at low Reynolds numbers. Our results shed new light on the notion of when the turbulence is fully developed at the small scales without relying on the existence of an extended inertial range. PMID:25024175
Helicity fluctuations in incompressible turbulent flows
NASA Technical Reports Server (NTRS)
Rogers, Michael M.; Moin, Parviz
1987-01-01
Results from direct numerical simulations of several homogeneous flows and fully developed turbulent channel flow indicate that the probability distribution function (pdf) of relative helicity density exhibits at most a 20 percent deviation from a flat distribution. Isotropic flows exhibit a slight helical nature but the presence of mean strain in homogeneous turbulence suppresses helical behavior. All the homogeneous turbulent flows studied show no correlation between relative helicity density and the dissipation of turbulent kinetic energy. The channel flow simulations indicate that, except for low-dissipation regions near the outer edge of the buffer layer, there is no tendency for the flow to be helical. The strong peaks in the relative helicity density pdf and the association of these peaks with regions of low dissipation found in previous simulations by Pelz et al.(1985) are not observed.
Determination of angular distribution of radiation in an isotropically scattering slab
NASA Astrophysics Data System (ADS)
Cengel, Y. A.; Ozisik, M. N.; Yener, Y.
1984-02-01
Ozisik (1982) has employed the Galerkin method to arrive at a solution of the radiative transfer equation in an absorbing, emitting, isotropically scattering plane-parallel slab in order to predict radiation flux. This method is presently developed to accurately determine the angular distribution of radiation intensity anywhere in the medium, subject to general boundary conditions.
Magnetosheath electrostatic turbulence
NASA Technical Reports Server (NTRS)
Rodriquez, P.
1977-01-01
The spectrum of electrostatic plasma waves in the terrestrial magnetosheath was studied using the plasma wave experiment on the IMP-6 satellite. Electrostatic plasma wave turbulence is almost continuously present throughout the magnetosheath with broadband (20 Hz- 70 kHz) r.m.s. field intensities typically 0.01 - 1.0 millivolts/m. Peak intensities of about 1.0 millivolts/m near the electron plasma frequency (30 - 60 kHz) were detected occasionally. The components usually identified in the spectrum of magnetosheath electrostatic turbulence include a high frequency ( or = 30 kHz) component peaking at the electron plasma frequency f sub pe, a low frequency component with a broad intensity maximum below the nominal ion plasma frequency f sub pi (approximately f sub pe/43), and a less well defined intermediate component in the range f sub pi f f sub pe. The intensity distribution of magnetosheath electrostatic turbulence clearly shows that the low frequency component is associated with the bow shock, suggesting that the ion heating begun at the shock continues into the downstream magnetosheath.
A Controlled Laboratory Environment to Study EO Signal Degradation Due to Underwater Turbulence
2015-05-11
to examine and mitigate turbulence effects, we set up a laboratory turbulence environment allowing the variation of turbulence intensity. Convective ...profilers and fast thermistor probes. The turbulence measurements are complemented by computational fluid dynamics simulations of convective turbulence...outfitted with stainless steel plates at the bottom and top that can be temperature controlled (Fig. 1, left). In this tank, convective Rayleigh
Biomimetic Isotropic Nanostructures for Structural Coloration
Forster, Jason D.; Noh, Heeso; Liew, Seng Fatt; Saranathan, Vinodkumar; Schreck, Carl F.; Yang, Lin; Park, Jin-Gyu; Prum, Richard O.; Mochrie, Simon G.J.; O'Hern, Corey S.; Cao, Hui; Dufresne, Eric R.
2010-08-09
The self-assembly of films that mimic color-producing nanostructures in bird feathers is described. These structures are isotropic and have a characteristic length-scale comparable to the wavelength of visible light. Structural colors are produced when wavelength-independent scattering is suppressed by limiting the optical path length through geometry or absorption.
Interparticle collision mechanism in turbulence.
Choi, Jung-Il; Park, Yongnam; Kwon, Ohjoon; Lee, Changhoon
2016-01-01
Direct numerical simulations of particle-laden homogeneous isotropic turbulence are performed to investigate interparticle collisions in a wide range of Stokes numbers. Dynamics of the particles are described by Stokes drag including particle-particle interactions via hard-sphere collisions, while fluid turbulence is solved using a pseudospectral method. Particular emphasis is placed on interparticle-collision-based conditional statistics of rotation and dissipation rates of the fluid experienced by heavy particles, which provide essential information on the collision process. We also investigate the collision statistics of collision time interval and angle. Based on a Lamb vortex model for a vortex structure, we claim that collision events occur in the edge region for vortical structures in the intermediate-Stokes-number regime, suggesting that the sling effect enhances collision as well as clustering.
Numerical experiments in homogeneous turbulence
NASA Technical Reports Server (NTRS)
Rogallo, R. S.
1981-01-01
The direct simulation methods developed by Orszag and Patternson (1972) for isotropic turbulence were extended to homogeneous turbulence in an incompressible fluid subjected to uniform deformation or rotation. The results of simulations for irrotational strain (plane and axisymmetric), shear, rotation, and relaxation toward isotropy following axisymmetric strain are compared with linear theory and experimental data. Emphasis is placed on the shear flow because of its importance and because of the availability of accurate and detailed experimental data. The computed results are used to assess the accuracy of two popular models used in the closure of the Reynolds-stress equations. Data from a variety of the computed fields and the details of the numerical methods used in the simulation are also presented.
Leith diffusion model for homogeneous anisotropic turbulence
Rubinstein, Robert; Clark, Timothy T.; Kurien, Susan
2016-07-19
Here, a proposal for a spectral closure model for homogeneous anisotropic turbulence. The systematic development begins by closing the third-order correlation describing nonlinear interactions by an anisotropic generalization of the Leith diffusion model for isotropic turbulence. The correlation tensor is then decomposed into a tensorially isotropic part, or directional anisotropy, and a trace-free remainder, or polarization anisotropy. The directional and polarization components are then decomposed using irreducible representations of the SO(3) symmetry group. Under the ansatz that the decomposition is truncated at quadratic order, evolution equations are derived for the directional and polarization pieces of the correlation tensor. Here, numerical simulation of the model equations for a freely decaying anisotropic flow illustrate the non-trivial effects of spectral dependencies on the different return-to-isotropy rates of the directional and polarization contributions.
Leith diffusion model for homogeneous anisotropic turbulence
Rubinstein, Robert; Clark, Timothy T.; Kurien, Susan
2016-07-19
Here, a proposal for a spectral closure model for homogeneous anisotropic turbulence. The systematic development begins by closing the third-order correlation describing nonlinear interactions by an anisotropic generalization of the Leith diffusion model for isotropic turbulence. The correlation tensor is then decomposed into a tensorially isotropic part, or directional anisotropy, and a trace-free remainder, or polarization anisotropy. The directional and polarization components are then decomposed using irreducible representations of the SO(3) symmetry group. Under the ansatz that the decomposition is truncated at quadratic order, evolution equations are derived for the directional and polarization pieces of the correlation tensor. Here, numericalmore » simulation of the model equations for a freely decaying anisotropic flow illustrate the non-trivial effects of spectral dependencies on the different return-to-isotropy rates of the directional and polarization contributions.« less
Leith diffusion model for homogeneous anisotropic turbulence
NASA Astrophysics Data System (ADS)
Rubinstein, Robert; Clark, Timothy; Kurien, Susan
2016-11-01
A new spectral closure model for homogeneous anisotropic turbulence is proposed. The systematic development begins by closing the third-order correlation describing nonlinear interactions by an anisotropic generalization of the Leith diffusion model for isotropic turbulence. The correlation tensor is then decomposed into a tensorially isotropic part, or directional anisotropy, and a trace-free remainder, or polarization anisotropy. The directional and polarization components are then decomposed using irreducible representations of the SO(3) symmetry group. Under the ansatz that the decomposition is truncated at quadratic order, evolution equations are derived for the directional and polarization pieces of the correlation tensor. Numerical simulation of the model equations for a freely decaying anisotropic flow illustrate the non-trivial effects of spectral dependencies on the different return-to-isotropy rates of the directional and polarization contributions.
Transversely isotropic elasticity imaging of cancellous bone.
Shore, Spencer W; Barbone, Paul E; Oberai, Assad A; Morgan, Elise F
2011-06-01
To measure spatial variations in mechanical properties of biological materials, prior studies have typically performed mechanical tests on excised specimens of tissue. Less invasive measurements, however, are preferable in many applications, such as patient-specific modeling, disease diagnosis, and tracking of age- or damage-related degradation of mechanical properties. Elasticity imaging (elastography) is a nondestructive imaging method in which the distribution of elastic properties throughout a specimen can be reconstructed from measured strain or displacement fields. To date, most work in elasticity imaging has concerned incompressible, isotropic materials. This study presents an extension of elasticity imaging to three-dimensional, compressible, transversely isotropic materials. The formulation and solution of an inverse problem for an anisotropic tissue subjected to a combination of quasi-static loads is described, and an optimization and regularization strategy that indirectly obtains the solution to the inverse problem is presented. Several applications of transversely isotropic elasticity imaging to cancellous bone from the human vertebra are then considered. The feasibility of using isotropic elasticity imaging to obtain meaningful reconstructions of the distribution of material properties for vertebral cancellous bone from experiment is established. However, using simulation, it is shown that an isotropic reconstruction is not appropriate for anisotropic materials. It is further shown that the transversely isotropic method identifies a solution that predicts the measured displacements, reveals regions of low stiffness, and recovers all five elastic parameters with approximately 10% error. The recovery of a given elastic parameter is found to require the presence of its corresponding strain (e.g., a deformation that generates ɛ₁₂ is necessary to reconstruct C₁₂₁₂), and the application of regularization is shown to improve accuracy. Finally
Transversely Isotropic Elasticity Imaging of Cancellous Bone
Shore, Spencer W.; Barbone, Paul E.; Oberai, Assad A.; Morgan, Elise F.
2012-01-01
To measure spatial variations in mechanical properties of biological materials, prior studies have typically performed mechanical tests on excised specimens of tissue. Less invasive measurements, however, are preferable in many applications, such as patient-specific modeling, disease diagnosis, and tracking of age- or damage-related degradation of mechanical properties. Elasticity imaging (elastography) is a nondestructive imaging method in which the distribution of elastic properties throughout a specimen can be reconstructed from measured strain or displacement fields. To date, most work in elasticity imaging has concerned incompressible, isotropic materials. This study presents an extension of elasticity imaging to three-dimensional, compressible, transversely isotropic materials. The formulation and solution of an inverse problem for an anisotropic tissue subjected to a combination of quasi-static loads is described, and an optimization and regularization strategy that indirectly obtains the solution to the inverse problem is presented. Several applications of transversely isotropic elasticity imaging to cancellous bone from the human vertebra are then considered. The feasibility of using isotropic elasticity imaging to obtain meaningful reconstructions of the distribution of material properties for vertebral cancellous bone from experiment is established. However, using simulation, it is shown that an isotropic reconstruction is not appropriate for anisotropic materials. It is further shown that the transversely isotropic method identifies a solution that predicts the measured displacements, reveals regions of low stiffness, and recovers all five elastic parameters with approximately 10% error. The recovery of a given elastic parameter is found to require the presence of its corresponding strain (e.g., a deformation that generates ε12 is necessary to reconstruct C1212), and the application of regularization is shown to improve accuracy. Finally, the effects
Generation of Turbulent Inflow Conditions for Pipe Flow via an Annular Ribbed Turbulator
NASA Astrophysics Data System (ADS)
Moallemi, Nima; Brinkerhoff, Joshua
2016-11-01
The generation of turbulent inflow conditions adds significant computational expense to direct numerical simulations (DNS) of turbulent pipe flows. Typical approaches involve introducing boxes of isotropic turbulence to the velocity field at the inlet of the pipe. In the present study, an alternative method is proposed that incurs a lower computational cost and allows the anisotropy observed in pipe turbulence to be physically captured. The method is based on a periodic DNS of a ribbed turbulator upstream of the inlet boundary of the pipe. The Reynolds number based on the bulk velocity and pipe diameter is 5300 and the blockage ratio (BR) is 0.06 based on the rib height and pipe diameter. The pitch ratio is defined as the ratio of rib streamwise spacing to rib height and is varied between 1.7 and 5.0. The generation of turbulent flow structures downstream of the ribbed turbulator are identified and discussed. Suitability of this method for accurate representation of turbulent inflow conditions is assessed through comparison of the turbulent mean properties, fluctuations, Reynolds stress profiles, and spectra with published pipe flow DNS studies. The DNS results achieve excellent agreement with the numerical and experimental data available in the literature.
NASA Astrophysics Data System (ADS)
Tsubota, Makoto
2008-11-01
The present article reviews the recent developments in the physics of quantum turbulence. Quantum turbulence (QT) was discovered in superfluid 4He in the 1950s, and the research has tended toward a new direction since the mid 90s. The similarities and differences between quantum and classical turbulence have become an important area of research. QT is comprised of quantized vortices that are definite topological defects, being expected to yield a model of turbulence that is much simpler than the classical model. The general introduction of the issue and a brief review on classical turbulence are followed by a description of the dynamics of quantized vortices. Then, we discuss the energy spectrum of QT at very low temperatures. At low wavenumbers, the energy is transferred through the Richardson cascade of quantized vortices, and the spectrum obeys the Kolmogorov law, which is the most important statistical law in turbulence; this classical region shows the similarity to conventional turbulence. At higher wavenumbers, the energy is transferred by the Kelvin-wave cascade on each vortex. This quantum regime depends strongly on the nature of each quantized vortex. The possible dissipation mechanism is discussed. Finally, important new experimental studies, which include investigations into temperature-dependent transition to QT, dissipation at very low temperatures, QT created by vibrating structures, and visualization of QT, are reviewed. The present article concludes with a brief look at QT in atomic Bose-Einstein condensates.
Interactively variable isotropic resolution in computed tomography.
Lapp, Robert M; Kyriakou, Yiannis; Kachelriess, Marc; Wilharm, Sylvia; Kalender, Willi A
2008-05-21
An individual balancing between spatial resolution and image noise is necessary to fulfil the diagnostic requirements in medical CT imaging. In order to change influencing parameters, such as reconstruction kernel or effective slice thickness, additional raw-data-dependent image reconstructions have to be performed. Therefore, the noise versus resolution trade-off is time consuming and not interactively applicable. Furthermore, isotropic resolution, expressed by an equivalent point spread function (PSF) in every spatial direction, is important for the undistorted visualization and quantitative evaluation of small structures independent of the viewing plane. Theoretically, isotropic resolution can be obtained by matching the in-plane and through-plane resolution with the aforementioned parameters. Practically, however, the user is not assisted in doing so by current reconstruction systems and therefore isotropic resolution is not commonly achieved, in particular not at the desired resolution level. In this paper, an integrated approach is presented for equalizing the in-plane and through-plane spatial resolution by image filtering. The required filter kernels are calculated from previously measured PSFs in x/y- and z-direction. The concepts derived are combined with a variable resolution filtering technique. Both approaches are independent of CT raw data and operate only on reconstructed images which allows for their application in real time. Thereby, the aim of interactively variable, isotropic resolution is achieved. Results were evaluated quantitatively by measuring PSFs and image noise, and qualitatively by comparing the images to direct reconstructions regarded as the gold standard. Filtered images matched direct reconstructions with arbitrary reconstruction kernels with standard deviations in difference images of typically between 1 and 17 HU. Isotropic resolution was achieved within 5% of the selected resolution level. Processing times of 20-100 ms per frame
Quenching and anisotropy of hydromagnetic turbulent transport
Karak, Bidya Binay; Brandenburg, Axel; Rheinhardt, Matthias; Käpylä, Petri J.; Käpylä, Maarit J.
2014-11-01
Hydromagnetic turbulence affects the evolution of large-scale magnetic fields through mean-field effects like turbulent diffusion and the α effect. For stronger fields, these effects are usually suppressed or quenched, and additional anisotropies are introduced. Using different variants of the test-field method, we determine the quenching of the turbulent transport coefficients for the forced Roberts flow, isotropically forced non-helical turbulence, and rotating thermal convection. We see significant quenching only when the mean magnetic field is larger than the equipartition value of the turbulence. Expressing the magnetic field in terms of the equipartition value of the quenched flows, we obtain for the quenching exponents of the turbulent magnetic diffusivity about 1.3, 1.1, and 1.3 for Roberts flow, forced turbulence, and convection, respectively. However, when the magnetic field is expressed in terms of the equipartition value of the unquenched flows, these quenching exponents become about 4, 1.5, and 2.3, respectively. For the α effect, the exponent is about 1.3 for the Roberts flow and 2 for convection in the first case, but 4 and 3, respectively, in the second. In convection, the quenching of turbulent pumping follows the same power law as turbulent diffusion, while for the coefficient describing the Ω×J effect nearly the same quenching exponent is obtained as for α. For forced turbulence, turbulent diffusion proportional to the second derivative along the mean magnetic field is quenched much less, especially for larger values of the magnetic Reynolds number. However, we find that in corresponding axisymmetric mean-field dynamos with dominant toroidal field the quenched diffusion coefficients are the same for the poloidal and toroidal field constituents.
Interaction of turbulence with a detonation wave
NASA Technical Reports Server (NTRS)
Jackson, T. L.; Hussaini, M. Y.; Ribner, H. S.
1993-01-01
This paper addresses a specific reactive-flow configuration, namely, the interaction of a detonation wave with convected homogeneous isotropic weak turbulence (which can be constructed by a Fourier synthesis of small-amplitude vorticity waves). The effect of chemical heat release on the rms fluctuations downstream of the detonation is presented as a function of Mach number. In addition, for the particular case of the von Karman spectrum, the one-dimensional power spectra of these flow quantities are given.
Turbulence modeling and experiments
NASA Technical Reports Server (NTRS)
Shabbir, Aamir
1992-01-01
The best way of verifying turbulence is to do a direct comparison between the various terms and their models. The success of this approach depends upon the availability of the data for the exact correlations (both experimental and DNS). The other approach involves numerically solving the differential equations and then comparing the results with the data. The results of such a computation will depend upon the accuracy of all the modeled terms and constants. Because of this it is sometimes difficult to find the cause of a poor performance by a model. However, such a calculation is still meaningful in other ways as it shows how a complete Reynolds stress model performs. Thirteen homogeneous flows are numerically computed using the second order closure models. We concentrate only on those models which use a linear (or quasi-linear) model for the rapid term. This, therefore, includes the Launder, Reece and Rodi (LRR) model; the isotropization of production (IP) model; and the Speziale, Sarkar, and Gatski (SSG) model. Which of the three models performs better is examined along with what are their weaknesses, if any. The other work reported deal with the experimental balances of the second moment equations for a buoyant plume. Despite the tremendous amount of activity toward the second order closure modeling of turbulence, very little experimental information is available about the budgets of the second moment equations. Part of the problem stems from our inability to measure the pressure correlations. However, if everything else appearing in these equations is known from the experiment, pressure correlations can be obtained as the closing terms. This is the closest we can come to in obtaining these terms from experiment, and despite the measurement errors which might be present in such balances, the resulting information will be extremely useful for the turbulence modelers. The purpose of this part of the work was to provide such balances of the Reynolds stress and heat
Effect of Contraction on Turbulence and Temperature Fluctuations Generated by a Warm Grid
NASA Technical Reports Server (NTRS)
Mills, Robert R., Jr.; Corrsin, Stanley
1959-01-01
Hot-wire anemometer measurements were made of several statistical properties of approximately homogeneous and isotropic fields of turbulence and temperature fluctuations generated by a warm grid in a uniform airstream sent through a 4-to-1 contraction. These measurements were made both in the contraction and in the axisymmetric domain farther downstream. In addition to confirming the well-known turbulence anisotropy induced by strain, the data show effects on the skewnesses of both longitudinal velocity fluctuation (which has zero skewness in isotropic turbulence) and its derivative. The concomitant anisotropy in the temperature field accelerates the decay of temperature fluctuations.
Simulation and modeling of homogeneous, compressed turbulence
NASA Technical Reports Server (NTRS)
Wu, C. T.; Ferziger, J. H.; Chapman, D. R.
1985-01-01
Low Reynolds number homogeneous turbulence undergoing low Mach number isotropic and one-dimensional compression was simulated by numerically solving the Navier-Stokes equations. The numerical simulations were performed on a CYBER 205 computer using a 64 x 64 x 64 mesh. A spectral method was used for spatial differencing and the second-order Runge-Kutta method for time advancement. A variety of statistical information was extracted from the computed flow fields. These include three-dimensional energy and dissipation spectra, two-point velocity correlations, one-dimensional energy spectra, turbulent kinetic energy and its dissipation rate, integral length scales, Taylor microscales, and Kolmogorov length scale. Results from the simulated flow fields were used to test one-point closure, two-equation models. A new one-point-closure, three-equation turbulence model which accounts for the effect of compression is proposed. The new model accurately calculates four types of flows (isotropic decay, isotropic compression, one-dimensional compression, and axisymmetric expansion flows) for a wide range of strain rates.
Even harmonic generation in isotropic media of dissociating homonuclear molecules
Silva, R. E. F.; Rivière, P.; Morales, F.; Smirnova, O.; Ivanov, M.; Martín, F.
2016-01-01
Isotropic gases irradiated by long pulses of intense IR light can generate very high harmonics of the incident field. It is generally accepted that, due to the symmetry of the generating medium, be it an atomic or an isotropic molecular gas, only odd harmonics of the driving field can be produced. Here we show how the interplay of electronic and nuclear dynamics can lead to a marked breakdown of this standard picture: a substantial part of the harmonic spectrum can consist of even rather than odd harmonics. We demonstrate the effect using ab-initio solutions of the time-dependent Schrödinger equation for and its isotopes in full dimensionality. By means of a simple analytical model, we identify its physical origin, which is the appearance of a permanent dipole moment in dissociating homonuclear molecules, caused by light-induced localization of the electric charge during dissociation. The effect arises for sufficiently long laser pulses and the region of the spectrum where even harmonics are produced is controlled by pulse duration. Our results (i) show how the interplay of femtosecond nuclear and attosecond electronic dynamics, which affects the charge flow inside the dissociating molecule, is reflected in the nonlinear response, and (ii) force one to augment standard selection rules found in nonlinear optics textbooks by considering light-induced modifications of the medium during the generation process. PMID:27596609
Penetrative phototactic bioconvection in an isotropic scattering suspension
NASA Astrophysics Data System (ADS)
Panda, M. K.; Ghorai, S.
2013-07-01
Phototaxis is a directed swimming response dependent upon the light intensity sensed by micro-organisms. Positive (negative) phototaxis denotes the motion directed towards (away from) the source of light. Using the phototaxis model of Ghorai, Panda, and Hill ["Bioconvection in a suspension of isotropically scattering phototactic algae," Phys. Fluids 22, 071901 (2010)], 10.1063/1.3457163, we investigate two-dimensional phototactic bioconvection in an absorbing and isotropic scattering suspension in the nonlinear regime. The suspension is confined by a rigid bottom boundary, and stress-free top and lateral boundaries. The governing equations for phototactic bioconvection consist of Navier-Stokes equations for an incompressible fluid coupled with a conservation equation for micro-organisms and the radiative transfer equation for light transport. The governing system is solved efficiently using a semi-implicit second-order accurate conservative finite-difference method. The radiative transfer equation is solved by the finite volume method using a suitable step scheme. The resulting bioconvective patterns differ qualitatively from those found by Ghorai and Hill ["Penetrative phototactic bioconvection," Phys. Fluids 17, 074101 (2005)], 10.1063/1.1947807 at a higher critical wavelength due to the effects of scattering. The solutions show transition from steady state to periodic oscillations as the governing parameters are varied. Also, we notice the accumulation of micro-organisms in two horizontal layers at two different depths via their mean swimming orientation profile for some governing parameters at a higher scattering albedo.
ERIC Educational Resources Information Center
Hanratty, Thomas J.
1980-01-01
This paper gives an account of research on the structure of turbulence close to a solid boundary. Included is a method to study the flow close to the wall of a pipe without interferring with it. (Author/JN)
NASA Astrophysics Data System (ADS)
Nazarenko, Sergey
2015-07-01
Wave turbulence is the statistical mechanics of random waves with a broadband spectrum interacting via non-linearity. To understand its difference from non-random well-tuned coherent waves, one could compare the sound of thunder to a piece of classical music. Wave turbulence is surprisingly common and important in a great variety of physical settings, starting with the most familiar ocean waves to waves at quantum scales or to much longer waves in astrophysics. We will provide a basic overview of the wave turbulence ideas, approaches and main results emphasising the physics of the phenomena and using qualitative descriptions avoiding, whenever possible, involved mathematical derivations. In particular, dimensional analysis will be used for obtaining the key scaling solutions in wave turbulence - Kolmogorov-Zakharov (KZ) spectra.
Apparent anisotropy in inhomogeneous isotropic media
NASA Astrophysics Data System (ADS)
Lin, Fan-Chi; Ritzwoller, Michael H.
2011-09-01
Surface waves propagating through a laterally inhomogeneous medium undergo wavefield complications such as multiple scattering, wave front healing, and backward scattering. Unless accounted for accurately, these effects will introduce a systematic isotropic bias in estimates of azimuthal anisotropy. We demonstrate with synthetic experiments that backward scattering near an observing station will introduce an apparent 360° periodicity into the azimuthal distribution of anisotropy near strong lateral variations in seismic wave speeds that increases with period. Because it violates reciprocity, this apparent 1ψ anisotropy, where ψ is the azimuthal angle, is non-physical for surface waves and is, therefore, a useful indicator of isotropic bias. Isotropic bias of the 2ψ (180° periodicity) component of azimuthal anisotropy, in contrast, is caused mainly by wave front healing, which results from the broad forward scattering part of the surface wave sensitivity kernel. To test these predictions, we apply geometrical ray theoretic (eikonal) tomography to teleseismic Rayleigh wave measurements across the Transportable Array component of USArray to measure the directional dependence of phase velocities between 30 and 80 s period. Eikonal tomography accounts for multiple scattering (ray bending) but not finite frequency effects such as wave front healing or backward scattering. At long periods (>50 s), consistent with the predictions from the synthetic experiments, a significant 1ψ component of azimuthal anisotropy is observed near strong isotropic structural contrasts with fast directions that point in the direction of increasing phase speeds. The observed 2ψ component of azimuthal anisotropy is more weakly correlated with synthetic predictions of isotropic bias, probably because of the imprint of intrinsic structural anisotropy. The observation of a 1ψ component of azimuthal anisotropy is a clear indicator of isotropic bias in the inversion caused by unmodelled
Atmospheric turbulence not simply two-dimensional or three-dimensional
NASA Astrophysics Data System (ADS)
Schultz, Colin
2012-08-01
A complete mathematical description of turbulence is one of the most sought-after prizes in physics, and although the research of Pinel et al. does not provide a full account, it does aim to pin down the answer to one subset of that effort: Are two-dimensional (2-D) or 3-D the main options for atmospheric turbulence? In the earliest statistical descriptions, scientists assumed that turbulence was direction independent (isotropic) but in two separate regimes: at large scales being horizontally isotropic, while at small scales being isotropic in 3-D space. In this view, only large-scale turbulence behaves differently in the vertical and horizontal directions, that is, with horizontally stratified vortices.
Nonlinear polarization evolution of hybridly polarized beams by isotropic Kerr nonlinearity
NASA Astrophysics Data System (ADS)
Gu, Bing; Wen, Bo; Rui, Guanghao; Cui, Yiping
2016-11-01
Theoretically, we propose an investigation of the vectorial light field interacting with the isotropic Kerr medium. We obtain the analytical expression of the focal field of the hybrid polarized beam based on the vectorial Rayleigh-Sommerfeld formulas under the paraxial condition. Then we numerically simulate the far-field vectorial self-diffraction behavior and nonlinear ellipse rotation of a hybrid polarized beam by isotropic Kerr nonlinearity. Experimentally, we observe the vectorial self-diffraction behavior of the femtosecond-pulsed hybridly polarized beam in carbon disulfide at 800 nm, which is in agreement with the theoretical predictions. Our results demonstrate that the self-diffraction intensity pattern and the distribution of state of polarization (SoP) of a hybridly polarized beam could be manipulated by tuning the magnitude of the isotropic optical nonlinearity, which may find interesting applications in nonlinear mechanism analysis, nonlinear characterization technique, and spin angular momentum (SAM) manipulation.
Regimes of turbulence without an energy cascade
NASA Astrophysics Data System (ADS)
Barenghi, C. F.; Sergeev, Y. A.; Baggaley, A. W.
2016-10-01
Experiments and numerical simulations of turbulent 4He and 3He-B have established that, at hydrodynamic length scales larger than the average distance between quantum vortices, the energy spectrum obeys the same 5/3 Kolmogorov law which is observed in the homogeneous isotropic turbulence of ordinary fluids. The importance of the 5/3 law is that it points to the existence of a Richardson energy cascade from large eddies to small eddies. However, there is also evidence of quantum turbulent regimes without Kolmogorov scaling. This raises the important questions of why, in such regimes, the Kolmogorov spectrum fails to form, what is the physical nature of turbulence without energy cascade, and whether hydrodynamical models can account for the unusual behaviour of turbulent superfluid helium. In this work we describe simple physical mechanisms which prevent the formation of Kolmogorov scaling in the thermal counterflow, and analyze the conditions necessary for emergence of quasiclassical regime in quantum turbulence generated by injection of vortex rings at low temperatures. Our models justify the hydrodynamical description of quantum turbulence and shed light into an unexpected regime of vortex dynamics.
Regimes of turbulence without an energy cascade
Barenghi, C. F.; Sergeev, Y. A.; Baggaley, A. W.
2016-01-01
Experiments and numerical simulations of turbulent 4He and 3He-B have established that, at hydrodynamic length scales larger than the average distance between quantum vortices, the energy spectrum obeys the same 5/3 Kolmogorov law which is observed in the homogeneous isotropic turbulence of ordinary fluids. The importance of the 5/3 law is that it points to the existence of a Richardson energy cascade from large eddies to small eddies. However, there is also evidence of quantum turbulent regimes without Kolmogorov scaling. This raises the important questions of why, in such regimes, the Kolmogorov spectrum fails to form, what is the physical nature of turbulence without energy cascade, and whether hydrodynamical models can account for the unusual behaviour of turbulent superfluid helium. In this work we describe simple physical mechanisms which prevent the formation of Kolmogorov scaling in the thermal counterflow, and analyze the conditions necessary for emergence of quasiclassical regime in quantum turbulence generated by injection of vortex rings at low temperatures. Our models justify the hydrodynamical description of quantum turbulence and shed light into an unexpected regime of vortex dynamics. PMID:27761005
Monte-Carlo computation of turbulent premixed methane/air ignition
NASA Astrophysics Data System (ADS)
Carmen, Christina Lieselotte
The present work describes the results obtained by a time dependent numerical technique that simulates the early flame development of a spark-ignited premixed, lean, gaseous methane/air mixture with the unsteady spherical flame propagating in homogeneous and isotropic turbulence. The algorithm described is based upon a sub-model developed by an international automobile research and manufacturing corporation in order to analyze turbulence conditions within internal combustion engines. Several developments and modifications to the original algorithm have been implemented including a revised chemical reaction scheme and the evaluation and calculation of various turbulent flame properties. Solution of the complete set of Navier-Stokes governing equations for a turbulent reactive flow is avoided by reducing the equations to a single transport equation. The transport equation is derived from the Navier-Stokes equations for a joint probability density function, thus requiring no closure assumptions for the Reynolds stresses. A Monte-Carlo method is also utilized to simulate phenomena represented by the probability density function transport equation by use of the method of fractional steps. Gaussian distributions of fluctuating velocity and fuel concentration are prescribed. Attention is focused on the evaluation of the three primary parameters that influence the initial flame kernel growth-the ignition system characteristics, the mixture composition, and the nature of the flow field. Efforts are concentrated on the effects of moderate to intense turbulence on flames within the distributed reaction zone. Results are presented for lean conditions with the fuel equivalence ratio varying from 0.6 to 0.9. The present computational results, including flame regime analysis and the calculation of various flame speeds, provide excellent agreement with results obtained by other experimental and numerical researchers.
NASA Astrophysics Data System (ADS)
Movahed, Pooya
High-speed flows are prone to hydrodynamic interfacial instabilities that evolve to turbulence, thereby intensely mixing different fluids and dissipating energy. The lack of knowledge of these phenomena has impeded progress in a variety of disciplines. In science, a full understanding of mixing between heavy and light elements after the collapse of a supernova and between adjacent layers of different density in geophysical (atmospheric and oceanic) flows remains lacking. In engineering, the inability to achieve ignition in inertial fusion and efficient combustion constitute further examples of this lack of basic understanding of turbulent mixing. In this work, my goal is to develop accurate and efficient numerical schemes and employ them to study compressible turbulence and mixing generated by interactions between shocked (Richtmyer-Meshkov) and accelerated (Rayleigh-Taylor) interfaces, which play important roles in high-energy-density physics environments. To accomplish my goal, a hybrid high-order central/discontinuity-capturing finite difference scheme is first presented. The underlying principle is that, to accurately and efficiently represent both broadband motions and discontinuities, non-dissipative methods are used where the solution is smooth, while the more expensive and dissipative capturing schemes are applied near discontinuous regions. Thus, an accurate numerical sensor is developed to discriminate between smooth regions, shocks and material discontinuities, which all require a different treatment. The interface capturing approach is extended to central differences, such that smooth distributions of varying specific heats ratio can be simulated without generating spurious pressure oscillations. I verified and validated this approach against a stringent suite of problems including shocks, interfaces, turbulence and two-dimensional single-mode Richtmyer-Meshkov instability simulations. The three-dimensional code is shown to scale well up to 4000 cores
Particle Dispersion Within Zonal Jets in Two-Layer Beta-Plane Turbulence
2011-07-21
shows that the rate of diffusion due to turbulent motions is several orders of magnitude larger than the molecular diffusion rate. Given an isotropic...may be written as KT ( ’j) /(I VO1) and dimensional analysis yields KT , ’ (U0)/(0/L) = UL. The ratio of the eddy diffusivity to the molecular ...Reynolds number of the flow. Thus, the Reynolds number of a turbulent flow can be thought of as the ratio of molecular diffusion to turbulent diffusion
Takeuchi, Hiromitsu; Ishino, Shungo; Tsubota, Makoto
2010-11-12
We theoretically study the development of quantum turbulence from two counter-propagating superfluids of miscible Bose-Einstein condensates by numerically solving the coupled Gross-Pitaevskii equations. When the relative velocity exceeds a critical value, the countersuperflow becomes unstable and quantized vortices are nucleated, which leads to isotropic quantum turbulence consisting of two superflows. It is shown that the binary turbulence can be realized experimentally in a trapped system.
Takeuchi, Hiromitsu; Ishino, Shungo; Tsubota, Makoto
2010-11-12
We theoretically study the development of quantum turbulence from two counter-propagating superfluids of miscible Bose-Einstein condensates by numerically solving the coupled Gross-Pitaevskii equations. When the relative velocity exceeds a critical value, the countersuperflow becomes unstable and quantized vortices are nucleated, which leads to isotropic quantum turbulence consisting of two superflows. It is shown that the binary turbulence can be realized experimentally in a trapped system.
WYPIWYG hyperelasticity for isotropic, compressible materials
NASA Astrophysics Data System (ADS)
Crespo, José; Latorre, Marcos; Montáns, Francisco Javier
2016-10-01
Nowadays the most common approach to model elastic behavior at large strains is through hyperelasticity. Hyperelastic models usually specify the shape of the stored energy function. This shape is modulated by some material parameters that are computed so the predicted stresses best fit the experimental data. Many stored energy functions have been proposed in the literature for isotropic and anisotropic materials, either compressible or incompressible. What-You-Prescribe-Is-What-You-Get (WYPIWYG) formulations present a different approach which may be considered an extension of the infinitesimal framework. The shape of the stored energy is not given beforehand but computed numerically from experimental data solving the equilibrium equations. The models exactly fit the experimental data without any material parameter. WYPIWYG procedures have comparable efficiency in finite element procedures as classical hyperelasticity. In this work we present a WYPIWYG numerical procedure for compressible isotropic materials and we motivate the formulation through an equivalent infinitesimal model.
On curve and surface stretching in turbulent flow
NASA Technical Reports Server (NTRS)
Etemadi, Nassrollah
1989-01-01
Cocke (1969) proved that in incompressible, isotropic turbulence the average material line (material surface) elements increase in comparison with their initial values. Good estimates of how much they increase in terms of the eigenvalues of the Green deformation tensor were rigorously obtained.
Anisotropy in MHD turbulence due to a mean magnetic field
NASA Technical Reports Server (NTRS)
Shebalin, J. V.; Matthaeus, W. H.; Montgomery, D.
1982-01-01
The development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic turbulence. The anisotropy develops due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers. The effect is most pronounced at high mechanical and magnetic Reynolds numbers. The anisotropy is greater at the higher wave numbers.
Coherent structure extraction in turbulent channel flow using boundary adapted wavelets
NASA Astrophysics Data System (ADS)
Sakurai, Teluo; Yoshimatsu, Katsunori; Schneider, Kai; Farge, Marie; Morishita, Koji; Ishihara, Takashi
2017-04-01
We present a construction of isotropic boundary adapted wavelets, which are orthogonal and yield a multi-resolution analysis. We analyze direct numerical simulation data of turbulent channel flow computed at a friction Reynolds number of 395, and investigate the role of coherent vorticity. Thresholding of the vorticity wavelet coefficients allows to split the flow into two parts, coherent and incoherent vorticity. The coherent vorticity is reconstructed from their few intense wavelet coefficients. The statistics of the coherent part, i.e., energy and enstrophy spectra, are close to the statistics of the total flow, and moreover, the nonlinear energy budgets are very well preserved. The remaining incoherent part, represented by the large majority of the weak wavelet coefficients, corresponds to a structureless, i.e., noise-like, background flow whose energy is equidistributed.
Vorticity spectra in high Reynolds number anisotropic turbulence
NASA Astrophysics Data System (ADS)
Morris, Scott C.; Foss, John F.
2005-08-01
Assuming a turbulent flow to be homogeneous and isotropic allows for significant theoretical simplification in the description of its motions. The validity of these assumptions, first put forth by Kolmogorov [A. N. Kolmogorov, "The local structure of turbulence in incompressible viscous fluids for very large Reynolds numbers," C. R. Acad. Sci. URSS 30, 301 (1941)], has been the subject of considerable analytical development and extensive research as they are applied to actual flows. The present investigation describes the one-dimensional vorticity spectra of two flow fields: a single stream shear layer and the near surface region of an atmospheric boundary layer. Both flow fields exhibit a power-law region with a slope of -1 in the one-dimensional spectra of the spanwise component of vorticity in the same wave-number range for which the velocity spectra indicated the isotropic behavior. This is in strong disagreement with the isotropic prediction, which does not exhibit a power-law behavior.
Isotropic Contraction Of Mercury Due To Despinning
NASA Astrophysics Data System (ADS)
Matsuyama, Isamu; Bills, B. G.
2009-09-01
Mercury's slow rotation period of 59 days is presumably the result of solar tides driving its initial rotational state to the present 3:2 spin-orbit resonance. The observed large gravity coefficients can be explained as due to a remnant rotational bulge recording an initial rotation period of a few days (Matsuyama and Nimmo 2009). Despinning changes the shape of the rotational bulge, generating both compressional and extensional stresses (Melosh 1977). However, Mercury's surface is dominated by compressional tectonic features (Watters et al. 1998), and the inferred global contraction has been explained as due to thermal cooling (Solomon 1976). In addition to non-isotropic changes associated with the rotational flattening, despinning causes isotropic contraction of the entire planet. We consider the effect of the compressional stresses generated by this isotropic contraction on the predicted tectonic pattern. References Matsuyama and Nimmo. Gravity and tectonic patterns of Mercury: Effect of tidal deformation, spin-orbit resonance, nonzero eccentricity, despinning, and reorientation. J. Geophys. Res. (2009) vol. 114 pp. E01010 Melosh. Global tectonics of a despun planet. Icarus (1977) vol. 31 pp. 221-243 Solomon. Some aspects of core formation in Mercury. Icarus (1976) vol. 28 pp. 509-521 Watters et al. Topography of lobate scarps on Mercury: New constraints on the planet's contraction. Geology (1998) vol. 26 pp. 991-994
ODTLES : a model for 3D turbulent flow based on one-dimensional turbulence modeling concepts.
McDermott, Randy; Kerstein, Alan R.; Schmidt, Rodney Cannon
2005-01-01
This report describes an approach for extending the one-dimensional turbulence (ODT) model of Kerstein [6] to treat turbulent flow in three-dimensional (3D) domains. This model, here called ODTLES, can also be viewed as a new LES model. In ODTLES, 3D aspects of the flow are captured by embedding three, mutually orthogonal, one-dimensional ODT domain arrays within a coarser 3D mesh. The ODTLES model is obtained by developing a consistent approach for dynamically coupling the different ODT line sets to each other and to the large scale processes that are resolved on the 3D mesh. The model is implemented computationally and its performance is tested and evaluated by performing simulations of decaying isotropic turbulence, a standard turbulent flow benchmarking problem.
NASA Astrophysics Data System (ADS)
Lazarian, Alex
2003-07-01
HST and FUSE spectra of distant UV-bright sources reveal interstellar absorption lines of high stages of ionization {O VI, C IV, N V, Si IV} arising in many different astrophysical environments such as superbubbles, interstellar chimneys, high-velocity clouds, galaxy halos and cosmic filaments. Turbulence, always present in the magnetized ISM, must mix the hot { 10^6 K} gas with cooler gas within "turbulent mixing layers". Present theory, based on 1D steady-state flows, suggest the line ratios in these layers differ significantly from photoionized gas, radiative shocks, cooling zones, or conduction fronts. These models are use to infer mass and energy fluxes important to understanding the ISM. We propose to develop a suite of 3D time-dependent models that properly calculate turbulent mixing. We will produce synthetic UV absorption lines and optical emission lines directly relevant to HST observations that use GHRS, STIS, and eventually, COS. These models will allow us to explore the sensitivity of the spectral diagnostics to magnetic field strength, turbulence intensity, and relative velocity of the hot and cold gas. We will publish the resulting grid of spectral diagnostics and make them available through the Web.
On the subgrid-scale modeling of compressible turbulence
NASA Technical Reports Server (NTRS)
Speziale, C. G.; Erlebacher, G.; Zang, T. A.; Hussaini, M. Y.
1987-01-01
A subgrid-scale model recently derived for use in the large-eddy simulation of compressible turbulent flows is examined from a fundamental theoretical and computational standpoint. It is demonstrated that this model, which is applicable only to compressible turbulent flows in the limit of small density fluctuations, correlates somewhat poorly with the results of direct numerical simulations of compressible isotropic turbulence at low Mach numbers. An alternative model, based on Favre-filtered fields, is suggested which appears to reduce these limitations.
Large-eddy simulation of a turbulent mixing layer
NASA Technical Reports Server (NTRS)
Mansour, N. N.; Ferziger, J. H.; Reynolds, W. C.
1978-01-01
The three dimensional, time dependent (incompressible) vorticity equations were used to simulate numerically the decay of isotropic box turbulence and time developing mixing layers. The vorticity equations were spatially filtered to define the large scale turbulence field, and the subgrid scale turbulence was modeled. A general method was developed to show numerical conservation of momentum, vorticity, and energy. The terms that arise from filtering the equations were treated (for both periodic boundary conditions and no stress boundary conditions) in a fast and accurate way by using fast Fourier transforms. Use of vorticity as the principal variable is shown to produce results equivalent to those obtained by use of the primitive variable equations.
NASA Technical Reports Server (NTRS)
Tchen, C. M.
1986-01-01
Theoretical and numerical works in atmospheric turbulence have used the Navier-Stokes fluid equations exclusively for describing large-scale motions. Controversy over the existence of an average temperature gradient for the very large eddies in the atmosphere suggested that a new theoretical basis for describing large-scale turbulence was necessary. A new soliton formalism as a fluid analogue that generalizes the Schrodinger equation and the Zakharov equations has been developed. This formalism, processing all the nonlinearities including those from modulation provided by the density fluctuations and from convection due to the emission of finite sound waves by velocity fluctuations, treats large-scale turbulence as coalescing and colliding solitons. The new soliton system describes large-scale instabilities more explicitly than the Navier-Stokes system because it has a nonlinearity of the gradient type, while the Navier-Stokes has a nonlinearity of the non-gradient type. The forced Schrodinger equation for strong fluctuations describes the micro-hydrodynamical state of soliton turbulence and is valid for large-scale turbulence in fluids and plasmas where internal waves can interact with velocity fluctuations.
NASA Astrophysics Data System (ADS)
Brilouet, Pierre-Etienne; Canut, Guylaine; Durand, Pierre
2015-04-01
During winter, the North Western Mediterranean Sea is characterised by intense air-sea exchanges linked to regional strong winds (Mistral or Tramontana) which bring cold and dry continental air over a warmer sea. The HyMeX-ASICS-MED field campaign, devoted to intense sea-atmosphere exchange and deep oceanic convection analysis took place in the Gulf of Lion during winter 2013. The French ATR42 aircraft was operated to document the mean and turbulent structure of the atmospheric boundary layer (ABL) during strong wind conditions. The aircraft was equipped to measure turbulence fluctuations, thus allowing the computation of turbulence parameters. The flight strategy consisted of stacked horizontal legs oriented along and across the wind direction, in order to obtain information about the isotropy of the turbulent field and about coherent structures. Strong wind events were documented with 11 flights during which latent heat flux up to 600 W.m-2 were observed. The structure of the turbulent field is analysed through the integral length scale and the wavelength of the spectrum peak of the vertical velocity which represent the size of the large and the most energetic eddies, respectively. It reveals a stretching of turbulent eddies along the mean wind. This kind of organized structures plays a major role by modulating the transfers inside the ABL. In particular, this non-isotropic behaviour alters the flux estimates from along-wind samples. This last point is critical because surface and entrainment fluxes, deduced from extrapolation of the flux profiles, are essential parameters to characterise the coupling between air-sea exchanges and the ABL structure.
Rotating Rayleigh-Taylor turbulence
NASA Astrophysics Data System (ADS)
Boffetta, G.; Mazzino, A.; Musacchio, S.
2016-09-01
The turbulent Rayleigh-Taylor system in a rotating reference frame is investigated by direct numerical simulations within the Oberbeck-Boussinesq approximation. On the basis of theoretical arguments, supported by our simulations, we show that the Rossby number decreases in time, and therefore the Coriolis force becomes more important as the system evolves and produces many effects on Rayleigh-Taylor turbulence. We find that rotation reduces the intensity of turbulent velocity fluctuations and therefore the growth rate of the temperature mixing layer. Moreover, in the presence of rotation the conversion of potential energy into turbulent kinetic energy is found to be less effective, and the efficiency of the heat transfer is reduced. Finally, during the evolution of the mixing layer we observe the development of a cyclone-anticyclone asymmetry.
NASA Astrophysics Data System (ADS)
Skrbek, L.
2011-12-01
We review physical properties of quantum fluids He II and 3He-B, where quantum turbulence (QT) has been studied experimentally. Basic properties of QT in these working fluids are discussed within the phenomenological two-fluid model introduced by Landau. We consider counterflows in which the normal and superfluid components flow against each other, as well as co-flows in which the direction of the two fluids is the same. We pay special attention to the important case of zero temperature limit, where QT represents an interesting and probably the simplest prototype of three-dimensional turbulence in fluids. Experimental techniques to explore QT such as second sound attenuation, Andreev reflection, NMR, ion propagation are briefly introduced and results of various experiments on so-called Vinen QT and Kolmogorov QT both in He II and 3He are discussed, emphasizing similarities and differences between classical and quantum turbulence.
Turbulence in Compressible Flows
NASA Technical Reports Server (NTRS)
1997-01-01
Lecture notes for the AGARD Fluid Dynamics Panel (FDP) Special Course on 'Turbulence in Compressible Flows' have been assembled in this report. The following topics were covered: Compressible Turbulent Boundary Layers, Compressible Turbulent Free Shear Layers, Turbulent Combustion, DNS/LES and RANS Simulations of Compressible Turbulent Flows, and Case Studies of Applications of Turbulence Models in Aerospace.
2015-05-11
turbulence environment allowing the variation of turbulence intensity. Convective turbulence is generated in a large Rayleigh-Bénard type tank (5m by...energy and temperature variance dissipation rates in the tank, for different convective strengths. Optical image degradation in the tank is then...dynamics simulations of convective turbulence emulating the tank environment. These numerical simulations supplement the sparse laboratory measurements
Some Results Relevant to Statistical Closures for Compressible Turbulence
NASA Technical Reports Server (NTRS)
Ristorcelli, J. R.
1998-01-01
For weakly compressible turbulent fluctuations there exists a small parameter, the square of the fluctuating Mach number, that allows an investigation using a perturbative treatment. The consequences of such a perturbative analysis in three different subject areas are described: 1) initial conditions in direct numerical simulations, 2) an explanation for the oscillations seen in the compressible pressure in the direct numerical simulations of homogeneous shear, and 3) for turbulence closures accounting for the compressibility of velocity fluctuations. Initial conditions consistent with small turbulent Mach number asymptotics are constructed. The importance of consistent initial conditions in the direct numerical simulation of compressible turbulence is dramatically illustrated: spurious oscillations associated with inconsistent initial conditions are avoided, and the fluctuating dilatational field is some two orders of magnitude smaller for a compressible isotropic turbulence. For the isotropic decay it is shown that the choice of initial conditions can change the scaling law for the compressible dissipation. A two-time expansion of the Navier-Stokes equations is used to distinguish compressible acoustic and compressible advective modes. A simple conceptual model for weakly compressible turbulence - a forced linear oscillator is described. It is shown that the evolution equations for the compressible portions of turbulence can be understood as a forced wave equation with refraction. Acoustic modes of the flow can be amplified by refraction and are able to manifest themselves in large fluctuations of the compressible pressure.
Inverse energy cascade in rotational turbulence
NASA Astrophysics Data System (ADS)
Yu, Huidan (Whitney); Chen, Rou; Wang, Hengjie
2012-11-01
Rotation influences large-scale motions in the Earth's atmosphere and oceans and it is also important in many industrial applications such as turbo machinery, rotor-craft, and rotating channel etc. We study rotation effects on decaying isotropic turbulence through direct numerical simulation using lattice Boltzmann method. A Coriolis force characterized by the angular velocity of the frame of reference Ω is included in the lattice Boltzmann equations. The effects of rotation on fundamental turbulence features such as kinetic energy and enstrophy decay, energy spectrum, etc. are studied. The decay laws are quantitatively captured. Inverse energy cascade are observed in the 3D turbulence with and without rotation. The scaling of the inverse energy cascade and its relation to initial energy spectrum are explored. Indiana University-Purdue University Indianapolis (IUPUI).
NASA Technical Reports Server (NTRS)
Kim, J.; Simon, T. W.
1987-01-01
The effects of streamwise convex curvature, recovery, and freestream turbulence intensity on the turbulent transport of heat and momentum in a mature boundary layer are studied using a specially designed three-wire hot-wire probe. Increased freestream turbulence is found to increase the profiles throughout the boundary layer on the flat developing wall. Curvature effects were found to dominate turbulence intensity effects for the present cases considered. For the higher TI (turbulence intensity) case, negative values of the turbulent Prandtl number are found in the outer half of the boundary layer, indicating a breakdown in Reynolds analogy.
Scaling laws in magnetized plasma turbulence
Boldyrev, Stanislav
2015-06-28
Interactions of plasma motion with magnetic fields occur in nature and in the laboratory in an impressively broad range of scales, from megaparsecs in astrophysical systems to centimeters in fusion devices. The fact that such an enormous array of phenomena can be effectively studied lies in the existence of fundamental scaling laws in plasma turbulence, which allow one to scale the results of analytic and numerical modeling to the sized of galaxies, velocities of supernovae explosions, or magnetic fields in fusion devices. Magnetohydrodynamics (MHD) provides the simplest framework for describing magnetic plasma turbulence. Recently, a number of new features of MHD turbulence have been discovered and an impressive array of thought-provoking phenomenological theories have been put forward. However, these theories have conflicting predictions, and the currently available numerical simulations are not able to resolve the contradictions. MHD turbulence exhibits a variety of regimes unusual in regular hydrodynamic turbulence. Depending on the strength of the guide magnetic field it can be dominated by weakly interacting Alfv\\'en waves or strongly interacting wave packets. At small scales such turbulence is locally anisotropic and imbalanced (cross-helical). In a stark contrast with hydrodynamic turbulence, which tends to ``forget'' global constrains and become uniform and isotropic at small scales, MHD turbulence becomes progressively more anisotropic and unbalanced at small scales. Magnetic field plays a fundamental role in turbulent dynamics. Even when such a field is not imposed by external sources, it is self-consistently generated by the magnetic dynamo action. This project aims at a comprehensive study of universal regimes of magnetic plasma turbulence, combining the modern analytic approaches with the state of the art numerical simulations. The proposed study focuses on the three topics: weak MHD turbulence, which is relevant for laboratory devices, the solar
Physical Mechanisms of Two-Dimensional Turbulence
NASA Astrophysics Data System (ADS)
Ecke, Robert
2004-03-01
Turbulence has slowly yielded its mysteries through over 100 years of persistent effort. Recently experimental techniques and computation power have reached the stage where significant progress has been made on this very challenging problem. Two dimensional turbulence offers some real advantages in terms of reduced degrees of freedom such that the problem can now be thoroughly explored from many perspectives. Further, two-dimensional turbulence exhibits the basic phenomena of direct-enstrophy and inverse-energy cascades thought to apply to oceanic and atmospheric systems. We have investigated the properties of turbulence in two spatial dimensions using experimental measurements of the grid turbulence in a flowing soap film^1 and of the electromagnetically-forced turbulence in a thin salt layer floating on a dense immiscible fluid underlayer. We have also explored 2D turbulence using several different direct numerical simulations of homogeneous, isotropic turbulence in a periodic box^2. The data for both consist of high resolution fields of velocity; some are statistically independent sets and others are temporally resolved for dynamics. From this data we construct conventional Eulerian statistics, directly measure energy and enstrophy transfer^1, identify coherent structures in the flow, determine Lagrangian quantities, and calculate stretching fields. This comprehensive experimental and numerical characterization elucidates the physical mechanisms of two-dimensional turbulence. ^1 M.K. Rivera, W.B. Daniel and R.E. Ecke, Phys. Rev. Lett. 90, 104502 (2003). ^2 S. Chen, R.E. Ecke, G. Eyink, X. Wang, and Z. Xiao, Phys. Rev. Lett. 91, 214501 (2003).
Energy fluxes in turbulent separated flows
NASA Astrophysics Data System (ADS)
Mollicone, J.-P.; Battista, F.; Gualtieri, P.; Casciola, C. M.
2016-10-01
Turbulent separation in channel flow containing a curved wall is studied using a generalised form of Kolmogorov equation. The equation successfully accounts for inhomogeneous effects in both the physical and separation spaces. We investigate the scale-by-scale energy dynamics in turbulent separated flow induced by a curved wall. The scale and spatial fluxes are highly dependent on the shear layer dynamics and the recirculation bubble forming behind the lower curved wall. The intense energy produced in the shear layer is transferred to the recirculation region, sustaining the turbulent velocity fluctuations. The energy dynamics radically changes depending on the physical position inside the domain, resembling planar turbulent channel dynamics downstream.
Isotropic homogeneous universe with viscous fluid
Santos, N.O.; Dias, R.S.; Banerjee, A.
1985-04-01
Exact solutions are obtained for the isotropic homogeneous cosmological model with viscous fluid. The fluid has only bulk viscosity and the viscosity coefficient is taken to be a power function of the mass density. The equation of state assumed obeys a linear relation between mass density and pressure. The models satisfying Hawking's energy conditions are discussed. Murphy's model is only a special case of this general set of solutions and it is shown that Murphy's conclusion that the introduciton of bulk viscosity can avoid the occurrence of space-time singularity at finite past is not, in general, valid.
The dynamics of variable-density turbulence
Sandoval, D.L.
1995-11-01
The dynamics of variable-density turbulent fluids are studied by direct numerical simulation. The flow is incompressible so that acoustic waves are decoupled from the problem, and implying that density is not a thermodynamic variable. Changes in density occur due to molecular mixing. The velocity field, is in general, divergent. A pseudo-spectral numerical technique is used to solve the equations of motion. Three-dimensional simulations are performed using a grid size of 128{sup 3} grid points. Two types of problems are studied: (1) the decay of isotropic, variable-density turbulence, and (2) buoyancy-generated turbulence in a fluid with large density fluctuations. In the case of isotropic, variable-density turbulence, the overall statistical decay behavior, for the cases studied, is relatively unaffected by the presence of density variations when the initial density and velocity fields are statistically independent. The results for this case are in quantitative agreement with previous numerical and laboratory results. In this case, the initial density field has a bimodal probability density function (pdf) which evolves in time towards a Gaussian distribution. The pdf of the density field is symmetric about its mean value throughout its evolution. If the initial velocity and density fields are statistically dependent, however, the decay process is significantly affected by the density fluctuations. For the case of buoyancy-generated turbulence, variable-density departures from the Boussinesq approximation are studied. The results of the buoyancy-generated turbulence are compared with variable-density model predictions. Both a one-point (engineering) model and a two-point (spectral) model are tested against the numerical data. Some deficiencies in these variable-density models are discussed and modifications are suggested.
Lagrangian statistics of light particles in turbulence
NASA Astrophysics Data System (ADS)
Mercado, Julián Martínez; Prakash, Vivek N.; Tagawa, Yoshiyuki; Sun, Chao; Lohse, Detlef; (International CollaborationTurbulence Research)
2012-05-01
We study the Lagrangian velocity and acceleration statistics of light particles (micro-bubbles in water) in homogeneous isotropic turbulence. Micro-bubbles with a diameter db = 340 μm and Stokes number from 0.02 to 0.09 are dispersed in a turbulent water tunnel operated at Taylor-Reynolds numbers (Reλ) ranging from 160 to 265. We reconstruct the bubble trajectories by employing three-dimensional particle tracking velocimetry. It is found that the probability density functions (PDFs) of the micro-bubble acceleration show a highly non-Gaussian behavior with flatness values in the range 23 to 30. The acceleration flatness values show an increasing trend with Reλ, consistent with previous experiments [G. Voth, A. La Porta, A. M. Crawford, J. Alexander, and E. Bodenschatz, "Measurement of particle accelerations in fully developed turbulence," J. Fluid Mech. 469, 121 (2002)], 10.1017/S0022112002001842 and numerics [T. Ishihara, Y. Kaneda, M. Yokokawa, K. Itakura, and A. Uno, "Small-scale statistics in highresolution direct numerical simulation of turbulence: Reynolds number dependence of one-point velocity gradient statistics," J. Fluid Mech. 592, 335 (2007)], 10.1017/S0022112007008531. These acceleration PDFs show a higher intermittency compared to tracers [S. Ayyalasomayajula, Z. Warhaft, and L. R. Collins, "Modeling inertial particle acceleration statistics in isotropic turbulence," Phys. Fluids. 20, 095104 (2008)], 10.1063/1.2976174 and heavy particles [S. Ayyalasomayajula, A. Gylfason, L. R. Collins, E. Bodenschatz, and Z. Warhaft, "Lagrangian measurements of inertial particle accelerations in grid generated wind tunnel turbulence," Phys. Rev. Lett. 97, 144507 (2006)], 10.1103/PhysRevLett.97.144507 in wind tunnel experiments. In addition, the micro-bubble acceleration autocorrelation function decorrelates slower with increasing Reλ. We also compare our results with experiments in von Kármán flows and point-particle direct numerical simulations with periodic
Regularization of turbulence - a comprehensive modeling approach
NASA Astrophysics Data System (ADS)
Geurts, B. J.
2011-12-01
Turbulence readily arises in numerous flows in nature and technology. The large number of degrees of freedom of turbulence poses serious challenges to numerical approaches aimed at simulating and controlling such flows. While the Navier-Stokes equations are commonly accepted to precisely describe fluid turbulence, alternative coarsened descriptions need to be developed to cope with the wide range of length and time scales. These coarsened descriptions are known as large-eddy simulations in which one aims to capture only the primary features of a flow, at considerably reduced computational effort. Such coarsening introduces a closure problem that requires additional phenomenological modeling. A systematic approach to the closure problem, know as regularization modeling, will be reviewed. Its application to multiphase turbulent will be illustrated in which a basic regularization principle is enforced to physically consistently approximate momentum and scalar transport. Examples of Leray and LANS-alpha regularization are discussed in some detail, as are compatible numerical strategies. We illustrate regularization modeling to turbulence under the influence of rotation and buoyancy and investigate the accuracy with which particle-laden flow can be represented. A discussion of the numerical and modeling errors incurred will be given on the basis of homogeneous isotropic turbulence.
Evaporation of polydispersed droplets in a highly turbulent channel flow
NASA Astrophysics Data System (ADS)
Cochet, M.; Bazile, Rudy; Ferret, B.; Cazin, S.
2009-09-01
A model experiment for the study of evaporating turbulent two-phase flows is presented here. The study focuses on a situation where pre-atomized and dispersed droplets vaporize and mix in a heated turbulent flow. The test bench consists in a channel flow with characteristics of homogeneous and isotropic turbulence where fluctuations levels reach very high values (25% in the established zone). An ultrasonic atomizer allows the injection of a mist of small droplets of acetone in the carrier flow. The large range diameters ensure that every kind of droplet behavior with regards to turbulence is possible. Instantaneous concentration fields of the vaporized phase are extracted from fluorescent images (PLIF) of the two phase flow. The evolution of the mixing of the acetone vapor is analyzed for two different liquid mass loadings. Despite the high turbulence levels, concentration fluctuations remain significant, indicating that air and acetone vapor are not fully mixed far from the injector.
Talbot, L.; Cheng, R.K.
1993-12-01
Turbulent combustion is the dominant process in heat and power generating systems. Its most significant aspect is to enhance the burning rate and volumetric power density. Turbulent mixing, however, also influences the chemical rates and has a direct effect on the formation of pollutants, flame ignition and extinction. Therefore, research and development of modern combustion systems for power generation, waste incineration and material synthesis must rely on a fundamental understanding of the physical effect of turbulence on combustion to develop theoretical models that can be used as design tools. The overall objective of this program is to investigate, primarily experimentally, the interaction and coupling between turbulence and combustion. These processes are complex and are characterized by scalar and velocity fluctuations with time and length scales spanning several orders of magnitude. They are also influenced by the so-called {open_quotes}field{close_quotes} effects associated with the characteristics of the flow and burner geometries. The authors` approach is to gain a fundamental understanding by investigating idealized laboratory flames. Laboratory flames are amenable to detailed interrogation by laser diagnostics and their flow geometries are chosen to simplify numerical modeling and simulations and to facilitate comparison between experiments and theory.
1991-10-01
and complexity of thermochemistry . Accordingly a practical viewpoint is required to meet near-term work required for use in advanced CFD codes...teachers the opportunity to learn/explore/ teach turbulence issues. While such a product could be an invaluable eductaional tool (university), it also
NASA Technical Reports Server (NTRS)
Bardina, Jorge E.
1995-01-01
The objective of this work is to develop, verify, and incorporate the baseline two-equation turbulence models which account for the effects of compressibility into the three-dimensional Reynolds averaged Navier-Stokes (RANS) code and to provide documented descriptions of the models and their numerical procedures so that they can be implemented into 3-D CFD codes for engineering applications.
Cui, Linyan
2015-06-01
Analytic expressions for the temporal power spectra of irradiance fluctuations and angle of arrival (AOA) fluctuations are derived for optical waves propagating through weak anisotropic non-Kolmogorov atmospheric turbulence. In the derivation, the anisotropic non-Kolmogorov spectrum is adopted, which adopts the assumption of circular symmetry in the orthogonal plane throughout the path and the same degree of anisotropy along the propagation direction for all the turbulence cells. The final expressions consider simultaneously the anisotropic factor and general spectral power law values. When the anisotropic factor equals one (corresponding to the isotropic turbulence), the derived temporal power spectral models have good consistency with the known results for the isotropic turbulence. Numerical calculations show that the increased anisotropic factor alleviates the atmospheric turbulence's influence on the final expressions.
Structure in turbulent thermal convection
NASA Astrophysics Data System (ADS)
Balachandar, S.
1992-12-01
Small-scale features of vorticity, strain rate, and temperature gradients are considered in a Rayleigh-Bénard convection. The results reported are from a direct numerical simulation of turbulent convection performed in a rectangular box of aspect ratio 2√2 at a Rayleigh number of 6.5×106 and a Prandtl number of 0.72. In agreement with earlier results [Ashurst et al., Phys. Fluids 30, 2343 (1987) and Ruetsch and Maxey, Phys. Fluids A 3, 1587 (1991)], the intermediate strain rate is on an average positive, but the ratio of alpha, beta, and gamma strain rates are measured to be 5.3:1.0:-6.3. This result differs from the earlier result of 3:1:-4 obtained in homogeneous isotropic and shear turbulences. Buoyancy-induced vorticity production makes significant contribution to the overall enstrophy balance, especially close to the boundaries. Vorticity production by buoyancy is exclusively in the horizontal direction and is balanced by preferred production by stretching and tilting in the vertical direction, due to the preferred alignment of extensional alpha strain rate with the vertical direction. Such directional alignment of vorticity, strain rate, and scalar gradient is explained on the basis of preferred spatial orientation of coherent structures in thermal turbulence.
Isotropization of nematic liquid crystals by TMDSC
Chen, Wei; Dadmun, M.; Zhang, Ge; Boller, A.; Wunderlich, B. |
1997-12-01
Temperature-modulated differential scanning calorimetry (TMDSC) and traditional DSC are used to study the transition between the nematic liquid crystalline state and the isotropic liquid for two small molecules [4,4{prime}-azoxyanisole and N,N`-bis(4-n-octyloxybenzal)-1,4-phenylenediamine] and one macromolecule (4,4{prime}-dihydroxy-{alpha}-methylstilbene copolymerized with a 1:1 molar mixture of 1,7-dibromoheptane and 1,9-dibromononane). The DSC measurements with 4,4{prime}-azoxyanisole were used for temperature calibration with varying heating and cooling rates. Quasi-isothermal TMDSC with small temperature amplitude and standard TMDSC with underlying heating and cooling rates were utilized to analyze the breadth of the transitions. It could be verified that the isotropization transition of a nematic liquid crystal is, indeed, reversible for all three molecules. The nature of the transition changes, however, from relatively sharp, for small, rigid molecules, to about three kelvins wide for the small molecule with flexible ends, to as broad as 20 K for the macromolecule. It was also demonstrated that quantitative heats of fusion of sharp transitions can be extracted from TMDSC, but only from the time-domain heat-flow signal.
Quantifying Turbulence for Tidal Power Applications
Thomson, Jim; Richmond, Marshall C.; Polagye, Brian; Durgesh, Vibhav
2010-08-01
Using newly collected data from a tidal power site in Puget Sound, WA, metrics for turbulence quantification are assessed and discussed. The quality of raw ping Acoustic Doppler Current Profiler (ADCP) data for turbulence studies is evaluated against Acoustic Doppler Velocimeter (ADV) data at a point. Removal of Doppler noise from the raw ping data is shown to be a crucial step in turbulence quantification. Excluding periods of slack tide, the turbulent intensity estimates at a height of 4.6 m above the seabed are 8% and 11% from the ADCP and ADV, respectively. Estimates of the turbulent dissipation rate are more variable, from 10e-3 to 10e-1 W/m^3. An example analysis of coherent Turbulent Kinetic Energy (TKE) is presented.
NASA Technical Reports Server (NTRS)
Rued, Klaus
1987-01-01
The requirements for fundamental experimental studies of the influence of free stream turbulence, pressure gradients and wall cooling are discussed. Under turbine-like free stream conditions, comprehensive tests of transitional boundary layers with laminar, reversing and turbulent flow increments were performed to decouple the effects of the parameters and to determine the effects during mutual interaction.
Heat transfer and turbulence in a turbulated blade cooling circuit
NASA Astrophysics Data System (ADS)
Abuaf, N.; Kercher, D. M.
1992-06-01
The aero-thermal performance of a typical turbine blade three-pass turbulated cooling circuit geometry was investigated in a 10X plexiglas test model. The model closely duplicated the blade's leading edge, midchord and trailing edge cooling passage geometries. Steady state heat transfer coefficient distributions along the blade pressure side wall (convex surface) of the cooling circuit passages were measured with a thin-foil heater and a liquid crystal temperature sensor assembly. The heat transfer experiments were conducted on rib-roughened channels with staggered turbulators along the convex and concave surfaces of the cooling passages. Mid-channel axial velocity and turbulence intensity measurements were taken by hot wire anemometry at each passage end of the three-pass cooling circuit to characterize and relate the local thermal performance to the turbulence intensity levels. The near-atmospheric experimental data are compared with results of a Computational Fluid Dynamics (CFD) analysis at the operating internal environment for a 1X rotating model of the blade cooling circuit and other turbulator channel geometry heat transfer data investigations. The comparison between the measurements and analysis is encouraging. Differences with other heat transfer data appear reasonably understood and explainable.
Velocity and turbulence measurements in combustion systems
NASA Astrophysics Data System (ADS)
Goldstein, R. J.; Lau, K. Y.; Leung, C. C.
1983-06-01
A laser-Doppler velocimeter is used in the measurement of high-temperature gas flows. A two-stage fluidization particle generator provides magnesium oxide particles to serve as optical scattering centers. The one-dimensional dual-beam system is frequency shifted to permit measurements of velocities up to 300 meters per second and turbulence intensities greater than 100 percent. Exiting flows from can-type gas turbine combustors and burners with pre-mixed oxy-acetylene flames are described in terms of the velocity, turbulence intensity, and temperature profiles. The results indicate the influence of the combustion process on turbulence.
Bubble-induced turbulence study in homogeneous turbulent flow using DNS approach
NASA Astrophysics Data System (ADS)
Feng, Jinyong; Bolotnov, Igor
2015-11-01
The effect of a single bubble on the energy transfer to a homogeneous turbulent flow using DNS approach is investigated for various conditions. The single-phase turbulence is numerically generated by pressure-gradient driven uniform flow through a fully resolved turbulence generating grid. The turbulent intensity measured is uniform normal to the flow direction. The decay rate of the turbulent kinetic energy is validated against analytical power law. The collected instantaneous velocity is used as inflow condition for single-bubble simulations to study the bubble-induced turbulence (BIT). In interface-resolved two-phase simulation the bubble is kept at fixed positions by using a proportional-integral-derivative controller. This simulation set allows estimating the turbulent kinetic energy before and after the bubble, quantifying the BIT. Effects of bubble deformability, velocity and turbulent intensity are separately studied. We observe that for a nearly spherical bubble, the bubble-induced turbulence is positive, increasing the level of turbulent kinetic energy in the liquid phase. BIT is influenced by the other studied parameters and the presented work will contribute to the closure BIT model development in multiphase computational fluid dynamics modeling. The work is supported by NSF-CBET-Fluid Dynamics, Award #1333993.
Direct numerical simulation of turbulent reacting flows
Chen, J.H.
1993-12-01
The development of turbulent combustion models that reflect some of the most important characteristics of turbulent reacting flows requires knowledge about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between the turbulence and the chemistry is so strong in certain regimes that is is very difficult to isolate the role played by one individual phenomenon. Direct numerical simulation (DNS) is an extremely useful tool to study in detail the turbulence-chemistry interactions in certain well defined regimes. Globally, non-premixed flames are controlled by two limiting cases: the fast chemistry limit, where the turbulent fluctuations. In between these two limits, finite-rate chemical effects are important and the turbulence interacts strongly with the chemical processes. This regime is important because industrial burners operate in regimes in which, locally the flame undergoes extinction, or is at least in some nonequilibrium condition. Furthermore, these nonequilibrium conditions strongly influence the production of pollutants. To quantify the finite-rate chemistry effect, direct numerical simulations are performed to study the interaction between an initially laminar non-premixed flame and a three-dimensional field of homogeneous isotropic decaying turbulence. Emphasis is placed on the dynamics of extinction and on transient effects on the fine scale mixing process. Differential molecular diffusion among species is also examined with this approach, both for nonreacting and reacting situations. To address the problem of large-scale mixing and to examine the effects of mean shear, efforts are underway to perform large eddy simulations of round three-dimensional jets.
Evolution of the shock front and turbulence structures in the shock/turbulence interaction
NASA Technical Reports Server (NTRS)
Kevlahan, N.; Mahesh, K.; Lee, S.
1992-01-01
The interaction of a weak shock front with isotropic turbulence has been investigated using Direct Numerical Simulation (DNS). Two problems were considered: the ability of the field equation (the equation for a propagating surface) to model the shock; and a quantitative study of the evolution of turbulence structure using the database generated by Lee et al. Field equation model predictions for front shape have been compared with DNS results; good agreement is found for shock wave interaction with 2D turbulence and for a single steady vorticity wave. In the interaction of 3D isotropic turbulence with a normal shock, strong alignment of vorticity with the intermediate eigenvector of the rate of strain tensor (S(sup *)(sub ij) = S(sub ij) - (1/3)(delta(sub ij))(S(sub kk))) is seen to develop upstream of the shock and to be further amplified on passage through the shock. Vorticity tends to align at 90 deg to the largest eigenvector, but there is no preferred alignment with the smallest eigenvector. Upstream of the shock, the alignments continue to develop even after the velocity derivative skewness saturates. There is a significant tendency, which increases with time throughout the computational domain, for velocity to align with vorticity. The alignment between velocity and vorticity is strongest in eddy regions and weakest in convergence regions.
On the dynamics of homogeneous turbulence near a surface
NASA Astrophysics Data System (ADS)
Flores, Oscar; Riley, James J.
2011-11-01
It is becoming increasing clear that stably-stratified flows can support a stratified turbulence k - 5 / 3 inertial range, different from Kolmogorov's. Stratification inhibits vertical motions, but the large-scale quasi-horizontal motions produce strong vertical shearing and small-scale instabilities. The result is a k - 5 / 3 horizontal spectrum for the horizontal velocities at scales larger than the Ozmidov scale, the largest scale that can overturn. For smaller scales, the classical Kolmogorov k - 5 / 3 applies. Inspired by data taken near the water surface in a tidal river, we here explore to what extent the dynamics of the nonlinear spectral energy transfer of near-surface turbulence with no mean shear (i.e., horizontally isotropic turbulence bounded by free-slip and no-slip surfaces) is analogous to stably stratified turbulence. To that end, we perform DNS of decaying isotropic turbulence with Reλ ~ 100 , but bounded by a non-slip surface and a free slip surface. The behavior of the flow near the free-slip surface is similar to stratified turbulence, with a tentative k - 5 / 3 range, but the same is not true for the no-slip surface at the present Reynolds numbers. This research was supported by ARO and NSF. Chickadel et al. (2011) to appear in IEEE Geosci. Remote Sens. Lett.
Turbulence measurements in shock induced flow using hot wire anemometry
NASA Technical Reports Server (NTRS)
Hartung, Lin C.; Duffy, Robert E.; Trolier, James W.
1988-01-01
Heat transfer measurements over various geometric shapes have been made by immersing models in shock-induced flows. The heat transfer to a body is strongly dependent on the turbulence level of the stream. The interpretation of such heat transfer measurements requires a knowledge of the turbulence intensity. Turbulence intensity measurements, using hot-wire anemometry, have been successfully carried out in shock-induced flows. The experimental procedures for making such measurements and the techniques required are discussed.
NASA Astrophysics Data System (ADS)
Kühnen, Jakob; Hof, Björn
2015-11-01
We show that a simple modification of the velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarises. The annihilation of turbulence is achieved by a steady manipulation of the streamwise velocity component alone, greatly reducing control efforts. Several different control techniques are presented: one with a local modification of the flow profile by means of a stationary obstacle, one employing a nozzle injecting fluid through a small gap at the pipe wall and one with a moving wall, where a part of the pipe is shifted in the streamwise direction. All control techniques act on the flow such that the streamwise velocity profile becomes more flat and turbulence gradually grows faint and disappears. In a smooth straight pipe the flow remains laminar downstream of the control. Hence a reduction in skin friction by a factor of 8 and more can be accomplished. Stereoscopic PIV-measurements and movies of the development of the flow during relaminarisation are presented.
Kinematical uniqueness of homogeneous isotropic LQC
NASA Astrophysics Data System (ADS)
Engle, Jonathan; Hanusch, Maximilian
2017-01-01
In a paper by Ashtekar and Campiglia, invariance under volume preserving residual diffeomorphisms has been used to single out the standard representation of the reduced holonomy-flux algebra in homogeneous loop quantum cosmology (LQC). In this paper, we use invariance under all residual diffeomorphisms to single out the standard kinematical Hilbert space of homogeneous isotropic LQC for both the standard configuration space {{{R}}\\text{Bohr}} , as well as for the Fleischhack one {R}\\sqcup {{{R}}\\text{Bohr}} . We first determine the scale invariant Radon measures on these spaces, and then show that the Haar measure on {{{R}}\\text{Bohr}} is the only such measure for which the momentum operator is hermitian w.r.t. the corresponding inner product. In particular, the measure is forced to be identically zero on {R} in the Fleischhack case, so that for both approaches, the standard kinematical LQC-Hilbert space is singled out.
Isotropic MD simulations of dynamic brittle fracture
Espanol, P.; Rubio, M.A.; Zuniga, I.
1996-12-01
The authors present results obtained by molecular dynamics simulations on the propagation of fast cracks in triangular 2D lattices. Their aim is to simulate Mode 1 fracture of brittle isotropic materials. They propose a force law that respects the isotropy of the material. The code yields the correct imposed sound c{sub {parallel}}, shear c{sub {perpendicular}} and surface V{sub R} wave speeds. Different notch lengths are systematically studied. They observed that initially the cracks are linear and always branch at a particular critical velocity c* {approx} 0.8V{sub R} and that this occurs when the crack tip reaches the position of a front emitted from the initial crack tip and propagating at a speed c = 0.68V{sub R}.
Constitutive model development for isotropic materials
NASA Technical Reports Server (NTRS)
Kaufman, A.
1982-01-01
The objective is to develop a unified constitutive model for finite-element structural analysis of turbine engine hot section components. This effort constitutes a different approach for nonlinear finite-element computer codes which were heretofore based on classical inelastic methods. A unified constitutive theory will avoid the simplifying assumptions of classical theory and should more accurately represent the behavior of superalloy materials under cyclic loading conditions and high temperature environments. Model development will be directed toward isotropic, cast nickel-base alloys used for aircooled turbine blades and vanes. The contractor will select a base material for model development and an alternate material for verification purposes from a list of three alloys specified by NASA. The candidate alloys represent a cross-section of turbine blade and vane materials of interest to both large and small size engine manufacturers. Material stock for the base and alternate materials will be supplied to the Contractor by the government.
Isotropic cosmological singularities: other matter models
NASA Astrophysics Data System (ADS)
Tod, K. P.
2003-02-01
Isotropic cosmological singularities are singularities which can be removed by rescaling the metric. In some cases already studied, the existence and uniqueness of cosmological models with data at the singularity has been established (Anguige K and Tod K P 1999 Ann. Phys., NY 276 257-93, 294-320, Anguige K 2000 Ann. Phys., NY 285 395-419). These were cosmologies with, as source, either perfect fluids with linear equations of state or massless, collisionless particles. In this paper, we consider how to extend these results to a variety of other matter models. These are scalar fields, massive collisionless matter, the Yang-Mills plasma given by Choquet-Bruhat (Choquet-Bruhat Y 1996 Yang-Mills plasmas Global Structure and Evolution in General Relativity (Springer Lecture Notes in Physics vol 460) ed S Cotsakis and G W Gibbons (Berlin: Springer)) and matter satisfying the Einstein-Boltzmann equation.
Viscous propulsion in active transversely isotropic media
NASA Astrophysics Data System (ADS)
Cupples, G.; Dyson, R. J.; Smith, D. J.
2017-02-01
Taylor's swimming sheet is a classical model of microscale propulsion and pumping. Many biological fluids and substances are fibrous, having a preferred direction in their microstructure; for example cervical mucus is formed of polymer molecules which create an oriented fibrous network. Moreover, suspensions of elongated motile cells produce a form of active oriented matter. To understand how these effects modify viscous propulsion, we extend Taylor's classical model of small-amplitude zero-Reynolds-number propulsion of a 'swimming sheet' via the transversely-isotropic fluid model of Ericksen, which is linear in strain rate and possesses a distinguished direction. The energetic costs of swimming are significantly altered by all rheological parameters and the initial fibre angle. Propulsion in a passive transversely-isotropic fluid produces an enhanced mean rate of working, independent of the initial fibre orientation, with an approximately linear dependence of energetic cost on the extensional and shear enhancements to the viscosity caused by fibres. In this regime the mean swimming velocity is unchanged from the Newtonian case. The effect of the constant term in Ericksen's model for the stress, which can be identified as a fibre tension or alternatively a stresslet characterising an active fluid, is also considered. This stress introduces an angular dependence and dramatically changes the streamlines and flow field; fibres aligned with the swimming direction increase the energetic demands of the sheet. The constant fibre stress may result in a reversal of the mean swimming velocity and a negative mean rate of working if sufficiently large relative to the other rheological parameters.
Velocity analysis for transversely isotropic media
Alkhalifah, T.; Tsvankin, I.
1994-08-01
The main difficulty in extending seismic processing to anisotropic media is the recovery of anisotropic velocity fields from surface reflection data. Velocity analysis for transversely isotropic (TI) media can be done by inverting the dependence of P-wave moveout velocities on the ray parameter. P-wave NMO velocity in homogeneous TI media with a vertical symmetry axis depends just on the zero-dip value V{sub nmo} and a new effective parameter {eta} that reduces to the difference between Thomsen parameters {epsilon} and {delta} in the limit of weak anisotropy. It is possible to obtain {eta} and reconstruct the NMO velocity as a function of ray parameter using moveout velocities for two different dips. Moreover, V{sub nmo}(0) and {eta} determine not only the NMO velocity, but also also long-spread (nonhyperbollic) P-wave moveout for horizontal reflectors and time-migration impulse response. Inversion of dip-moveout information allows performance of all time-processing steps in TI media using only surface P-wave data. Isotropic time-processing methods remain entirely valid for elliptical anisotropy ({epsilon} = {delta}). Accurate time-to-depth conversion, however, requires the vertical velocity V{sub P0} be resolved independently. If I-P0 is known, then allisotropies {epsilon} and {delta} can be found by inverting two P-wave NMO velocities corresponding to a horizontal and a dipping reflector. If no information is available, all three parameters (V {sub P0}, {epsilon}, and {delta}) can be obtained by combining inversion results with shear-wave information. such as the P-SV or SV-SV wave NMO velocities for a horizontal reflector. Generalization of Tsvankin`s single-layer NMO equation for layered anisotropic media with a dipping reflector provides a basis for extending anisotropic velocity analysis to vertically inhomogeneous media. The influence of a stratified overburden on moveout velocity can be stripped through a Dix-type differentiation procedure.
Development and characterization of a variable turbulence generation system
NASA Astrophysics Data System (ADS)
Marshall, A.; Venkateswaran, P.; Noble, D.; Seitzman, J.; Lieuwen, T.
2011-09-01
Experimental turbulent combustion studies require systems that can simulate the turbulence intensities [ u'/ U 0 ~ 20-30% (Koutmos and McGuirk in Exp Fluids 7(5):344-354, 1989)] and operating conditions of real systems. Furthermore, it is important to have systems where turbulence intensity can be varied independently of mean flow velocity, as quantities such as turbulent flame speed and turbulent flame brush thickness exhibit complex and not yet fully understood dependencies upon both U 0 and u'. Finally, high pressure operation in a highly pre-heated environment requires systems that can be sealed, withstand high gas temperatures, and have remotely variable turbulence intensity that does not require system shut down and disassembly. This paper describes the development and characterization of a variable turbulence generation system for turbulent combustion studies. The system is capable of a wide range of turbulence intensities (10-30%) and turbulent Reynolds numbers (140-2,200) over a range of flow velocities. An important aspect of this system is the ability to vary the turbulence intensity remotely, without changing the mean flow velocity. This system is similar to the turbulence generators described by Videto and Santavicca (Combust Sci Technol 76(1):159-164, 1991) and Coppola and Gomez (Exp Therm Fluid Sci 33(7):1037-1048, 2009), where variable blockage ratio slots are located upstream of a contoured nozzle. Vortical structures from the slots impinge on the walls of the contoured nozzle to produce fine-scale turbulence. The flow field was characterized for two nozzle diameters using three-component Laser Doppler velocimetry (LDV) and hotwire anemometry for mean flow velocities from 4 to 50 m/s. This paper describes the key design features of the system, as well as the variation of mean and RMS velocity, integral length scales, and spectra with nozzle diameter, flow velocity, and turbulence generator blockage ratio.
Electron magnetohydrodynamics: Dynamics and turbulence
NASA Astrophysics Data System (ADS)
Lyutikov, Maxim
2013-11-01
We consider dynamics and turbulent interaction of whistler modes within the framework of inertialess electron magnetohydrodynamics (EMHD). We argue that there is no energy principle in EMHD: any stationary closed configuration is neutrally stable. On the other hand, the relaxation principle, the long term evolution of a weakly dissipative system towards Taylor-Beltrami state, remains valid in EMHD. We consider the turbulent cascade of whistler modes. We show that (i) harmonic whistlers are exact nonlinear solutions; (ii) collinear whistlers do not interact (including counterpropagating); (iii) waves with the same value of the wave vector k1=k2 do not interact; (iv) whistler modes have a dispersion that allows a three-wave decay, including into a zero frequency mode; (v) the three-wave interaction effectively couples modes with highly different wave numbers and propagation angles. In addition, linear interaction of a whistler with a single zero mode can lead to spatially divergent structures via parametric instability. All these properties are drastically different from MHD, so that the qualitative properties of the Alfvén turbulence can not be transferred to the EMHD turbulence. We derive the Hamiltonian formulation of EMHD, and using Bogoliubov transformation reduce it to the canonical form; we calculate the matrix elements for the three-wave interaction of whistlers. We solve numerically the kinetic equation and show that, generally, the EMHD cascade develops within a broad range of angles, while transiently it may show anisotropic, nearly two-dimensional structures. Development of a cascade depends on the forcing (nonuniversal) and often fails to reach a steady state. Analytical estimates predict the spectrum of magnetic fluctuations for the quasi-isotropic cascade ∝k-2. The cascade remains weak (not critically balanced). The cascade is UV local, while the infrared locality is weakly (logarithmically) violated.
A revisit of the equilibrium assumption for prediction of near-wall turbulence
NASA Astrophysics Data System (ADS)
Karimpour, Farid; Venayagamoorthy, Subhas
2014-11-01
Assuming equilibrium between the rates of production (P) and dissipation (ɛ) of the turbulent kinetic energy (k) is widely employed for prediction and modeling of turbulent flows. In this study, we revisit the consequence of using equilibrium assumption for prediction of near-wall turbulence. To this end, the relevant scales inherent in the turbulent viscosity (νt) formulation of the standard k- ɛ model is derived. We show that such turbulent viscosity formulations are not suitable for modeling near-wall turbulence. Furthermore, by using the turbulent viscosity (νt) formulation suggested by Durbin, we also show that the anisotropic Reynolds stress is correlated with the wall-normal, isotropic Reynolds stress. `A priori' tests are performed to assess the validity of the propositions using the direct numerical simulation (DNS) data of unstratified channel flow. The comparisons with the data are excellent and confirm our findings. Funded by the National Science Foundation.
Internal structure of a premixed turbulent flame
Rajan, S.; Smith, J.R.; Rambach, G.D.
1982-10-01
A pulsed laser and a multielement detector have been used to make instantaneous Rayleigh profiles along a line through a turbulent flame front thus eliminating the effects of flame front motion. The flame front in a premixed turbulent flame moves randomly about a mean position, giving rise to the visually observed flame brush or time-averaged flame thickness which is larger than the instantaneous thickness of the reaction zone. The physical characteristics and statistical properties of such turbulent flames reported previously were deduced from the time histories of Rayleigh scattered laser light at one or two points within the reaction zone. The study was conducted on a premixed propane-air flame stabilized on a rod at the exit plane of a square burner. Turbulence-producing screens below the burner exit controlled turbulent length scales while intensity was controlled with inlet mixture velocity. Turbulence properties of the cold reactants were determined by hot-wire anemometry. Mean and fluctuating velocity in the unburnt and burnt gases were measured using laser Doppler velocimetry. At the low level of turbulence studied, the instantaneous flame front thickness was found to be only slightly greater than the laminar flame thickness, and the magnitude of the density fluctuations only slightly greater than the cold flow turbulence intensity. Mean and rms values of density and velocity; density and velocity probability density functions; spatial density correlations; and comparison of data with the Bray-Moss-Libby model are presented.
Turbulent swirling layer with free surface
NASA Astrophysics Data System (ADS)
Bardet, Philippe; Peterson, Per; Savas, Omer
2007-11-01
A turbulent annular liquid wall jet, or vortex tube, generated by helical injection inside a tube was characterized experimentally. The resulting hollow confined swirling layer is proposed for use in a thick liquid first-wall chamber concept for inertial fusion power plants. The velocity fields were measured with a single camera split-screen stereoscopic particle image velocimetry scheme. The flow was studied at 5 stations between 1.5 and 4.5 ``vortex tube'' diameters downstream of the injection nozzle in a horizontal plane that coincides with the tube axis. Up to 1024 independent realizations were recorded and analyzed for Reynolds numbers ranging from 3,200 to 14,000 at each station. The turbulent structures are non-isotropic and non-homogeneous. Gradients in average velocity and Reynolds stress result in turbulent kinetic energy production. Between 1.5 and 3.5 diameters, the average azimuthal velocity profile alone is non uniform away from the wall. Persistent large vortical structures are observed. The turbulent kinetic energy decreases slowly with distance while the dissipation decreases rapidly. At 4.5 diameters, the wall effect influences strongly the average velocity profiles. The vortical structures disappear and the turbulent kinetic energy increases.
Turbulence topologies predicted using large eddy simulations
NASA Astrophysics Data System (ADS)
Wang, Bing-Chen; Bergstrom, Donald J.; Yin, Jing; Yee, Eugene
In this paper, turbulence topologies related to the invariants of the resolved velocity gradient and strain rate tensors are studied based on large eddy simulation. The numerical results presented in the paper were obtained using two dynamic models, namely, the conventional dynamic model of Lilly and a recently developed dynamic nonlinear subgrid scale (SGS) model. In contrast to most of the previous research investigations which have mainly focused on isotropic turbulence, the present study examines the influence of near-wall anisotropy on the flow topologies. The SGS effect on the so-called SGS dissipation of the discriminant is examined and it is shown that the SGS stress contributes to the deviation of the flow topology of real turbulence from that of the ideal restricted Euler flow. The turbulence kinetic energy (TKE) transfer between the resolved and subgrid scales of motion is studied, and the forward and backward scatters of TKE are quantified in the invariant phase plane. Some interesting phenomenological results have also been obtained, including a wing-shaped contour pattern for the density of the resolved enstrophy generation and the near-wall dissipation shift of the peak location (mode) in the joint probability density function of the invariants of the resolved strain rate tensor. The newly observed turbulence phenomenologies are believed to be important and an effort has been made to explain them on an analytical basis.
Magnetohydrodynamic Turbulence
NASA Astrophysics Data System (ADS)
Montgomery, David C.
2004-01-01
Magnetohydrodynamic (MHD) turbulence theory is modeled on neutral fluid (Navier-Stokes) turbulence theory, but with some important differences. There have been essentially no repeatable laboratory MHD experiments wherein the boundary conditions could be controlled or varied and a full set of diagnostics implemented. The equations of MHD are convincingly derivable only in the limit of small ratio of collision mean-free-paths to macroscopic length scales, an inequality that often goes the other way for magnetofluids of interest. Finally, accurate information on the MHD transport coefficients-and thus, the Reynolds-like numbers that order magnetofluid behavior-is largely lacking; indeed, the algebraic expressions used for such ingredients as the viscous stress tensor are often little more than wishful borrowing from fluid mechanics. The one accurate thing that has been done extensively and well is to solve the (strongly nonlinear) MHD equations numerically, usually in the presence of rectangular periodic boundary conditions, and then hope for the best when drawing inferences from the computations for those astrophysical and geophysical MHD systems for which some indisputably turbulent detailed data are available, such as the solar wind or solar prominences. This has led to what is perhaps the first field of physics for which computer simulations are regarded as more central to validating conclusions than is any kind of measurement. Things have evolved in this way due to a mixture of the inevitable and the bureaucratic, but that is the way it is, and those of us who want to work on the subject have to live with it. It is the only game in town, and theories that have promised more-often on the basis of some alleged ``instability''-have turned out to be illusory.
Recent developments in plasma turbulence and turbulent transport
Terry, P.W.
1997-09-22
This report contains viewgraphs of recent developments in plasma turbulence and turbulent transport. Localized nonlinear structures occur under a variety of circumstances in turbulent, magnetically confined plasmas, arising in both kinetic and fluid descriptions, i.e., in either wave-particle or three-wave coupling interactions. These structures are non wavelike. They cannot be incorporated in the collective wave response, but interact with collective modes through their shielding by the plasma dielectric. These structures are predicted to modify turbulence-driven transport in a way that in consistent with, or in some cases are confirmed by recent experimental observations. In kinetic theory, non wavelike structures are localized perturbations of phase space density. There are two types of structures. Holes are self-trapped, while clumps have a self-potential that is too weak to resist deformation and mixing by ambient potential fluctuations. Clumps remain correlated in turbulence if their spatial extent is smaller than the correlation length of the scattering fields. In magnetic turbulence, clumps travel along stochastic magnetic fields, shielded by the plasma dielectric. A drag on the clump macro-particle is exerted by the shielding, inducing emission into the collective response. The emission in turn damps back on the particle distribution via Landau dampling. The exchange of energy between clumps and particles, as mediated by the collective mode, imposes constraints on transport. For a turbulent spectrum whose mean wavenumber along the equilibrium magnetic field is nonzero, the electron thermal flux is proportional to the ion thermal velocity. Conventional predictions (which account only for collective modes) are larger by the square root of the ion to electron mass ratio. Recent measurements are consistent with the small flux. In fluid plasma,s localized coherent structures can occur as intense vortices.
Anisotropy, inhomogeneity and inertial-range scalings in turbulent convection
NASA Astrophysics Data System (ADS)
Rincon, François
2006-09-01
This paper provides a detailed study of turbulent statistics and scale-by-scale budgets in turbulent Rayleigh Bénard convection. It aims at testing the applicability of Kolmogorov and Bolgiano theories in the case of turbulent convection and at improving the understanding of the underlying inertial-range scalings, for which a general agreement is still lacking. Particular emphasis is laid on anisotropic and inhomogeneous effects, which are often observed in turbulent convection between two differentially heated plates. For this purpose, the SO(3) decomposition of structure functions and a method of description of inhomogeneities are used to derive inhomogeneous and anisotropic generalizations of Kolmogorov and Yaglom equations applying to Rayleigh Bénard convection, which can be extended easily to other types of anisotropic and/or inhomogeneous flows. The various contributions to these equations are computed in and off the central plane of a convection cell using data produced by a direct numerical simulation of turbulent Boussinesq convection at Ra {=} 10(6) and Pr {=} 1 with aspect ratio A {=} 5. The analysis of the isotropic part of the Kolmogorov equation demonstrates that the shape of the third-order velocity structure function is significantly influenced by buoyancy forcing and large-scale inhomogeneities, while the isotropic part of the mixed third-order structure function <(Deltatheta)(2Deltavec{u}>) appearing in the Yaglom equation exhibits a clear scaling exponent 1 in a small range of scales. The magnitudes of the various low ℓ degree anisotropic components of the equations are also estimated and are shown to be comparable to their isotropic counterparts at moderate to large scales. The analysis of anisotropies notably reveals that computing reduced structure functions (structure functions computed at fixed depth for correlation vectors boldsymbol{r} lying in specific planes only) in order to reveal scaling exponents predicted by isotropic theories
NASA Astrophysics Data System (ADS)
Nagata, Kouji; Sakai, Yasuhiko; Komori, Satoru
2011-06-01
Effects of weak, small-scale freestream turbulence on turbulent boundary layers with and without thermal convection are experimentally investigated using a wind tunnel. Two experiments are carried out: the first is isothermal boundary layers with and without grid turbulence, and the second is non-isothermal boundary layers with and without grid turbulence. Both boundary layers develop under a small favorable pressure gradient. For the latter case, the bottom wall of the test section is heated at a constant wall temperature to investigate the effects of thermal convection under the effects of freestream turbulence. For both cases, the turbulence intensity in the freestream is Tu = 1.3% ˜ 2.4%, and the integral length scale of freestream turbulence, L∞, is much smaller than the boundary layer thickness δ, i.e., L∞/δ ≪1. The Reynolds numbers Reθ based on the momentum thickness and freestream speed U∞ are Reθ = 560, 1100, 1310, and 2330 in isothermal boundary layers without grid turbulence. Instantaneous velocities, U and V, and instantaneous temperature T are simultaneously measured using a hot-wire anemometry and a constant-current resistance thermometer. The results show that the rms velocities and Reynolds shear stress normalized by the friction velocity are strongly suppressed by the freestream turbulence throughout the boundary layer in both isothermal and non-isothermal boundary layers. In the non-isothermal boundary layers, the normalized rms temperature and vertical turbulent heat flux are also strongly suppressed by the freestream turbulence. Turbulent momentum and heat transfer at the wall are enhanced by the freestream turbulence and the enhancement is notable in unstable stratification. The power spectra of u, v, and θ and their cospectra show that motions of almost all scales are suppressed by the freestream turbulence in both the isothermal and non-isothermal boundary layers.
Theoretical and experimental plastic strain ratios in planar isotropic textures
NASA Astrophysics Data System (ADS)
Kim, Insoo
1996-06-01
The plastic strain ratios of planar isotropic sheet specimens were studied by using unidirectionally solidified commercial Al. Sn and Al-Cu alloy sheets and Cu sheets electrodeposited under the various electrolysis conditions. The measured plastic strain ratios of [100] planar isotropic sheets by using unidirectionally solidified Al and Al-Cu alloy are about 0.17-0.52, that of [110] planar isotropic sheets using unidirectionally solidified Sn(BCT) are about 2.5, that of [110] planar isotropic sheets using electrodeposited Cu are 1.38-2.05 and that of [111] planar isotropic sheets using electrodeposited Cu are 2.61-2.85. There is a substantial discrepancy between the experimental plastic strain ratios which are measured from planar isotropic sheets and theoretical plastic strain ratios which are calculated by Backofen method and Bunge method, but the measured plastic strain ratio of [100] planar isotropic sheet using unidirectionally solidified Al and Al-Cu alloy is in good agreement with Bunge method and the measured plastic strain ratio of [110] and [111] planar isotropic sheets using electrodeposited Cu are in good agreement with Backofen method.
Effects of very high turbulence on convective heat transfer
NASA Technical Reports Server (NTRS)
Moffat, R. J.; Maciejewski, P. K.
1984-01-01
The effects of high-intensity, large-scale turbulence on turbulent boundary-layer heat transfer are studied. Flow fields were produced with turbulence intensities up to 40% and length scales up to several times the boundary layer thickness. In addition, three different types of turbulence will be compared to see whether they have the same effect on the boundary layer. The three are: the far field of a free jet, flow downstream of a grid, and flow downstream of a simulated gas turbine combustor. Each turbulence field will be characterized by several measures: intensity (by component), scale, and spectrum. Heat transfer will be measured on a 2.5 m long, 0.5 m wide flat plate using the energy-balance technique. The same plate will be used in each of the four flow fields; a low-turbulence tunnel for baseline data, and the three flow situations mentioned.
The structure of the extreme Lyapunov exponents in the inertial scales of turbulence
NASA Astrophysics Data System (ADS)
Vela-Martin, Alberto; Jimenez, Javier
2016-11-01
A fully reversible homogeneous isotropic turbulent system is constructed using inviscid LES to model energy fluxes in the far inertial range. Reversibility is exploited to efficiently calculate the highest/most unstable and lowest/most stable short-time Lyapunov exponents (STLE) of the system. When restricted to inertial modes, both extreme STLE have similar absolute value and inverse sign, suggesting the Hamiltonian nature of inertial dynamics. Their associated short-time Lyapunov vectors (STLV), which are complete flow fields that provide information on the perturbations to which the system is most/least sensitive, are found to be concentrated in small regions in physical space. The analysis of the structure of the STLV reveals that these small regions, where intense expansive and contractive events take place, are strongly dominated by the strain field of the flow. These regions are also characterized by a preferential alignment of the field of the STLV with the different eigenvectors of the strain tensor. However, no strong correlation of the STLV with the vorticity field is found. These results emphasize the active role of the strain in turbulence dynamics. Funded by the ERC COTURB project.
Turbulent transport phenomena in a channel with periodic rib turbulators
Liou, T.M.; Hwang, J.J.; Chen, S.H. )
1992-09-01
Periodic fully developed turbulent flow in a 2D channel with rib turbulators on two opposite walls has been studied numerically and experimentally. In numerical predictions, an algebraic Reynolds stress turbulence model is adopted, and a smoothed hybrid central/skew upstream difference scheme is developed. In experiments, the laser-Doppler velocimetry and laser holographic interferometry are employed to measure the local flow and heat transfer characteristics. The results are obtained with the ratio of pitch to rib height 5, 10, 15, and 20, for Reynolds number of 3.3 x 10 exp 4 and are presented in terms of the reattachment length, mean velocity and turbulent kinetic energy profiles, isotherm patterns, and distributions of local pressure recovery and Nusselt number. A detailed comparison with experimental data shows that the present calculations have an improvement over the previous work in the prediction of periodic ribbed-wall flow and heat transfer. In addition, regions susceptible to hot spots are identified by examining the distributions of the local Nusselt number. Furthermore, the enhancement of mean Nusselt number is documented in terms of relative contributions of the increased turbulence intensity and surface area provided by the ribs. 32 refs.
An investigation of turbulence structure in a low-Reynolds-number incompressible turbulent boundary
NASA Technical Reports Server (NTRS)
White, B. R.; Strataridakis, C. J.
1987-01-01
An existing high turbulence intensity level (5%) atmospheric boundary-layer wind tunnel has been successfully converted to a relatively low level turbulence (0.3%) wind tunnel through extensive modification, testing, and calibration. A splitter plate was designed, built, and installed into the wind-tunnel facility to create thick, mature, two-dimensional turbulent boundary layer flow at zero pressure gradient. Single and cross hot-wire measurements show turbulent boundary layer characteristics of good quality with unusually large physical size, i.e., viscous sublayer of the order of 1 mm high. These confirm the potential ability of the tunnel to be utilized for future high-quality near-wall turbulent boundary layer measurements. It compares very favorably with many low turbulence research tunnels.
Hierarchical structures in fully developed turbulence
NASA Astrophysics Data System (ADS)
Liu, Li
Analysis of the probability density functions (PDFs) of the velocity increment dvl and of their deformation is used to reveal the statistical structure of the intermittent energy cascade dynamics of turbulence. By analyzing a series of turbulent data sets including that of an experiment of fully developed low temperature helium turbulent gas flow (Belin, Tabeling, & Willaime, Physica D 93, 52, 1996), of a three-dimensional isotropic Navier-Stokes simulation with a resolution of 2563 (Cao, Chen, & She, Phys. Rev. Lett. 76, 3711, 1996) and of a GOY shell model simulation (Leveque & She, Phys. Rev. E 55, 1997) of a very big sample size (up to 5 billions), the validity of the Hierarchical Structure model (She & Leveque, Phys. Rev. Lett. 72, 366, 1994) for the inertial-range is firmly demonstrated. Furthermore, it is shown that parameters in the Hierarchical Structure model can be reliably measured and used to characterize the cascade process. The physical interpretations of the parameters then allow to describe differential changes in different turbulent systems so as to address non-universal features of turbulent systems. It is proposed that the above study provides a framework for the study of non-homogeneous turbulence. A convergence study of moments and scaling exponents is also carried out with detailed analysis of effects of finite statistical sample size. A quantity Pmin is introduced to characterize the resolution of a PDF, and hence the sample size. The fact that any reported scaling exponent depends on the PDF resolution suggests that the validation (or rejection) of a model of turbulence needs to carry out a resolution dependence analysis on its scaling prediction.
Component separation of a isotropic Gravitational Wave Background
NASA Astrophysics Data System (ADS)
Parida, Abhishek; Mitra, Sanjit; Jhingan, Sanjay
2016-04-01
A Gravitational Wave Background (GWB) is expected in the universe from the superposition of a large number of unresolved astrophysical sources and phenomena in the early universe. Each component of the background (e.g., from primordial metric perturbations, binary neutron stars, milli-second pulsars etc.) has its own spectral shape. Many ongoing experiments aim to probe GWB at a variety of frequency bands. In the last two decades, using data from ground-based laser interferometric gravitational wave (GW) observatories, upper limits on GWB were placed in the frequency range of 0~ 50-100 Hz, considering one spectral shape at a time. However, one strong component can significantly enhance the estimated strength of another component. Hence, estimation of the amplitudes of the components with different spectral shapes should be done jointly. Here we propose a method for "component separation" of a statistically isotropic background, that can, for the first time, jointly estimate the amplitudes of many components and place upper limits. The method is rather straightforward and needs negligible amount of computation. It utilises the linear relationship between the measurements and the amplitudes of the actual components, alleviating the need for a sampling based method, e.g., Markov Chain Monte Carlo (MCMC) or matched filtering, which are computationally intensive and cumbersome in a multi-dimensional parameter space. Using this formalism we could also study how many independent components can be separated using a given dataset from a network of current and upcoming ground based interferometric detectors.
Component separation of a isotropic Gravitational Wave Background
Parida, Abhishek; Jhingan, Sanjay; Mitra, Sanjit E-mail: sanjit@iucaa.in
2016-04-01
A Gravitational Wave Background (GWB) is expected in the universe from the superposition of a large number of unresolved astrophysical sources and phenomena in the early universe. Each component of the background (e.g., from primordial metric perturbations, binary neutron stars, milli-second pulsars etc.) has its own spectral shape. Many ongoing experiments aim to probe GWB at a variety of frequency bands. In the last two decades, using data from ground-based laser interferometric gravitational wave (GW) observatories, upper limits on GWB were placed in the frequency range of 0∼ 50−100 Hz, considering one spectral shape at a time. However, one strong component can significantly enhance the estimated strength of another component. Hence, estimation of the amplitudes of the components with different spectral shapes should be done jointly. Here we propose a method for 'component separation' of a statistically isotropic background, that can, for the first time, jointly estimate the amplitudes of many components and place upper limits. The method is rather straightforward and needs negligible amount of computation. It utilises the linear relationship between the measurements and the amplitudes of the actual components, alleviating the need for a sampling based method, e.g., Markov Chain Monte Carlo (MCMC) or matched filtering, which are computationally intensive and cumbersome in a multi-dimensional parameter space. Using this formalism we could also study how many independent components can be separated using a given dataset from a network of current and upcoming ground based interferometric detectors.
Methods of separation of variables in turbulence theory
NASA Technical Reports Server (NTRS)
Tsuge, S.
1978-01-01
Two schemes of closing turbulent moment equations are proposed both of which make double correlation equations separated into single-point equations. The first is based on neglected triple correlation, leading to an equation differing from small perturbed gasdynamic equations where the separation constant appears as the frequency. Grid-produced turbulence is described in this light as time-independent, cylindrically-isotropic turbulence. Application to wall turbulence guided by a new asymptotic method for the Orr-Sommerfeld equation reveals a neutrally stable mode of essentially three dimensional nature. The second closure scheme is based on an assumption of identity of the separated variables through which triple and quadruple correlations are formed. The resulting equation adds, to its equivalent of the first scheme, an integral of nonlinear convolution in the frequency describing a role due to triple correlation of direct energy-cascading.
Numerical simulation of turbulence in the presence of shear
NASA Technical Reports Server (NTRS)
Shaanan, S.; Ferziger, J. H.; Reynolds, W. C.
1975-01-01
The numerical calculations are presented of the large eddy structure of turbulent flows, by use of the averaged Navier-Stokes equations, where averages are taken over spatial regions small compared to the size of the computational grid. The subgrid components of motion are modeled by a local eddy-viscosity model. A new finite-difference scheme is proposed to represent the nonlinear average advective term which has fourth-order accuracy. This scheme exhibits several advantages over existing schemes with regard to the following: (1) the scheme is compact as it extends only one point away in each direction from the point to which it is applied; (2) it gives better resolution for high wave-number waves in the solution of Poisson equation, and (3) it reduces programming complexity and computation time. Examples worked out in detail are the decay of isotropic turbulence, homogeneous turbulent shear flow, and homogeneous turbulent shear flow with system rotation.
Spatiotemporal velocity-velocity correlation function in fully developed turbulence
NASA Astrophysics Data System (ADS)
Canet, Léonie; Rossetto, Vincent; Wschebor, Nicolás; Balarac, Guillaume
2017-02-01
Turbulence is a ubiquitous phenomenon in natural and industrial flows. Since the celebrated work of Kolmogorov in 1941, understanding the statistical properties of fully developed turbulence has remained a major quest. In particular, deriving the properties of turbulent flows from a mesoscopic description, that is, from the Navier-Stokes equation, has eluded most theoretical attempts. Here, we provide a theoretical prediction for the functional space and time dependence of the velocity-velocity correlation function of homogeneous and isotropic turbulence from the field theory associated to the Navier-Stokes equation with stochastic forcing. This prediction, which goes beyond Kolmogorov theory, is the analytical fixed point solution of nonperturbative renormalization group flow equations, which are exact in the limit of large wave numbers. This solution is compared to two-point two-times correlation functions computed in direct numerical simulations. We obtain a remarkable agreement both in the inertial and in the dissipative ranges.
Influence of initial mean helicity on homogeneous turbulent shear flow.
Jacobitz, Frank G; Schneider, Kai; Bos, Wouter J T; Farge, Marie
2011-11-01
Helicity statistics are studied in homogeneous turbulent shear flow. Initial mean helicity is imposed on an isotropic turbulence field using a decomposition of the flow into complex-valued helical waves. The initial decay of the turbulent kinetic energy is weakened in the presence of strong mean helicity, consistent with an analytic analysis of the spectral tensor of velocity correlations. While exponential growth of the mean turbulent kinetic energy is obtained, the mean helicity decays. Probability distribution functions (PDFs) of helicity are skewed and show that the imposed mean helicity prevails throughout the simulations. A wavelet-based scale-dependent analysis shows a trend to two dimensionalization for large scales of motion and a preference for helical motion at small scales. The magnitude of the skewness of the PDFs decreases for smaller scales. Joint PDFs indicate a strong correlation of the signs of both, helicity and superhelicity, for all cases. This correlation supports the conjecture that superhelicity dissipates helicity.
Anisotropic magnetohydrodynamic turbulence in a strong external magnetic field
NASA Technical Reports Server (NTRS)
Montgomery, D.; Turner, L.
1981-01-01
A strong external dc magnetic field introduces a basic anisotropy in incompressible MHD turbulence. The modifications that this is likely to produce in the properties of the turbulence are investigated for high Reynolds numbers. It is found that the turbulent spectrum splits into two parts: (1) an essentially two-dimensional spectrum with both the velocity field and the magnetic fluctuations perpendicular to the dc magnetic field, and (2) a generally weaker and more nearly isotropic spectrum of Alfven waves. These results are discussed in relation to measurements from the Culham-Harwell Zeta pinch device and the UCLA Macrorotor tokamak, as well as in relation to measurements of MHD turbulence in the solar wind.
Anisotropic magnetohydrodynamic turbulence in a strong external magnetic field
NASA Technical Reports Server (NTRS)
Montgomery, D.; Turner, L.
1981-01-01
A strong external dc magnetic field introduces a basic anisotropy into incompressible magnetohydrodynamic turbulence. The modifications that this is likely to produce in the properties of the turbulence are explored for the high Reynolds number case. The conclusion is reached that the turbulent spectrum splits into two parts: an essentially two dimensional spectrum with both the velocity field and magnetic fluctuations perpendicular to the dc magnetic field, and a generally weaker and more nearly isotropic spectrum of Alfven waves. A minimal characterization of the spectral density tensors is given. Similarities to measurements from the Culham-Harwell Zeta pinch device and the UCLA Macrotor Tokamak are remarked upon, as are certain implications for the Belcher and Davis measurements of magnetohydrodynamic turbulence in the solar wind.
The Spectral and Statistical Properties of Turbulence Generated by a Vortex/Blade-Tip Interaction
NASA Technical Reports Server (NTRS)
Devenport, William J.; Wittmer, Kenneth S.; Wenger, Christian W.
1997-01-01
The perpendicular interaction of a streamwise vortex with the tip of a lifting blade was studied in incompressible flow to provide information useful to the accurate prediction of helicopter rotor noise and the understanding of vortex dominated turbulent flows. The vortex passed 0.3 chord lengths to the suction side of the blade tip, providing a weak interaction. Single and two-point turbulence measurements were made using sub-miniature four sensor hot-wire probes 15 chord lengths downstream of the blade trailing edge; revealing the mean velocity and Reynolds stress tensor distributions of the turbulence, as well as its spanwise length scales as a function of frequency. The single point measurements show the flow downstream of the blade to be dominated by the interaction of the original tip vortex and the vortex shed by the blade. These vortices rotate about each other under their mutual induction, winding up the turbulent wakes of the blades. This interaction between the vortices appears to be the source of new turbulence in their cores and in the region between them. This turbulence appears to be responsible for some decay in the core of the original vortex, not seen when the blade is removed. The region between the vortices is not only a region of comparatively large stresses, but also one of intense turbulence production. Velocity autospectra measured near its center suggests the presence quasi-periodic large eddies with axes roughly parallel to a line joining the vortex cores. Detailed two-point measurements were made on a series of spanwise cuts through the flow so as to reveal the turbulence scales as they would be seen along the span of an intersecting airfoil. The measurements were made over a range of probe separations that enabled them to be analyzed not only in terms of coherence and phase spectra but also in terms of wave-number frequency (kappa-omega) spectra, computed by transforming the measured cross-spectra with respect to the spanwise separation
Microplane constitutive model for porous isotropic rocks
NASA Astrophysics Data System (ADS)
Baant, Zdenk P.; Zi, Goangseup
2003-01-01
The paper deals with constitutive modelling of contiguous rock located between rock joints. A fully explicit kinematically constrained microplane-type constitutive model for hardening and softening non-linear triaxial behaviour of isotropic porous rock is developed. The microplane framework, in which the constitutive relation is expressed in terms of stress and strain vectors rather than tensors, makes it possible to model various microstructural physical mechanisms associated with oriented internal surfaces, such as cracking, slip, friction and splitting of a particular orientation. Formulation of the constitutive relation is facilitated by the fact that it is decoupled from the tensorial invariance restrictions, which are satisfied automatically. In its basic features, the present model is similar to the recently developed microplane model M4 for concrete, but there are significant improvements and modifications. They include a realistic simulation of (1) the effects of pore collapse on the volume changes during triaxial loading and on the reduction of frictional strength, (2) recovery of frictional strength during shearing, and (3) the shear-enhanced compaction in triaxial tests, manifested by a deviation from the hydrostatic stress-strain curve. The model is calibrated by optimal fitting of extensive triaxial test data for Salem limestone, and good fits are demonstrated. Although these data do not cover the entire range of behaviour, credence in broad capabilities of the model is lend by its similarity to model M4 for concrete - an artificial rock. The model is intended for large explicit finite-element programs.
Constitutive modeling for isotropic materials (HOST)
NASA Technical Reports Server (NTRS)
Lindholm, U. S.; Chan, K. S.; Bodner, S. R.; Weber, R. M.; Walker, K. P.; Cassenti, B. N.
1985-01-01
This report presents the results of the second year of work on a problem which is part of the NASA HOST Program. Its goals are: (1) to develop and validate unified constitutive models for isotropic materials, and (2) to demonstrate their usefulness for structural analyses of hot section components of gas turbine engines. The unified models selected for development and evaluation are that of Bodner-Partom and Walker. For model evaluation purposes, a large constitutive data base is generated for a B1900 + Hf alloy by performing uniaxial tensile, creep, cyclic, stress relation, and thermomechanical fatigue (TMF) tests as well as biaxial (tension/torsion) tests under proportional and nonproportional loading over a wide range of strain rates and temperatures. Systematic approaches for evaluating material constants from a small subset of the data base are developed. Correlations of the uniaxial and biaxial tests data with the theories of Bodner-Partom and Walker are performed to establish the accuracy, range of applicability, and integability of the models. Both models are implemented in the MARC finite element computer code and used for TMF analyses. Benchmark notch round experiments are conducted and the results compared with finite-element analyses using the MARC code and the Walker model.
Nonlinear elastic inclusions in isotropic solids
Yavari, Arash; Goriely, Alain
2013-01-01
We introduce a geometric framework to calculate the residual stress fields and deformations of nonlinear solids with inclusions and eigenstrains. Inclusions are regions in a body with different reference configurations from the body itself and can be described by distributed eigenstrains. Geometrically, the eigenstrains define a Riemannian 3-manifold in which the body is stress-free by construction. The problem of residual stress calculation is then reduced to finding a mapping from the Riemannian material manifold to the ambient Euclidean space. Using this construction, we find the residual stress fields of three model systems with spherical and cylindrical symmetries in both incompressible and compressible isotropic elastic solids. In particular, we consider a finite spherical ball with a spherical inclusion with uniform pure dilatational eigenstrain and we show that the stress in the inclusion is uniform and hydrostatic. We also show how singularities in the stress distribution emerge as a consequence of a mismatch between radial and circumferential eigenstrains at the centre of a sphere or the axis of a cylinder. PMID:24353470
Isotropical conductive adhesives filled with silver nanowires
NASA Astrophysics Data System (ADS)
Tao, Y.; Xia, Y. P.; Zhang, G. Q.; Wu, H. P.; Tao, G. L.
2009-07-01
In this study, a solution-phase method was demonstrated to generate silver (Ag) nanowires with diameters in the range of 30~50nm and lengths of up to ~50μm, which was proceed by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). Fundamental material characterizations including X-ray diffraction transmission electro microscopy (TEM) and scanning electro microscopy (SEM) were conducted on these Ag nanowires. A novel kind of isotropical conductive adhesives (ICA) was prepared by using these Ag nanowires as conductive filler. Electrical property including bulk resistivity and mechanical property including shear strength were investigated and compared with that of conventional ICA filled with micrometer-sized Ag particles or nanometer-sized Ag particles. The average diameter of these Ag particles is about 1μm and 100 nm respectively. The results shown that ICA filled Ag nanowires exhibited higher conductivity, higher shear strength and low percolation threshold value than traditional ICA. Possible conductive mechanism was discussed based on theory calculation.
Constitutive modeling for isotropic materials (HOST)
NASA Technical Reports Server (NTRS)
Lindholm, Ulric S.; Chan, Kwai S.; Bodner, S. R.; Weber, R. M.; Walker, K. P.; Cassenti, B. N.
1984-01-01
The results of the first year of work on a program to validate unified constitutive models for isotropic materials utilized in high temperature regions of gas turbine engines and to demonstrate their usefulness in computing stress-strain-time-temperature histories in complex three-dimensional structural components. The unified theories combine all inelastic strain-rate components in a single term avoiding, for example, treating plasticity and creep as separate response phenomena. An extensive review of existing unified theories is given and numerical methods for integrating these stiff time-temperature-dependent constitutive equations are discussed. Two particular models, those developed by Bodner and Partom and by Walker, were selected for more detailed development and evaluation against experimental tensile, creep and cyclic strain tests on specimens of a cast nickel base alloy, B19000+Hf. Initial results comparing computed and test results for tensile and cyclic straining for temperature from ambient to 982 C and strain rates from 10(exp-7) 10(exp-3) s(exp-1) are given. Some preliminary date correlations are presented also for highly non-proportional biaxial loading which demonstrate an increase in biaxial cyclic hardening rate over uniaxial or proportional loading conditions. Initial work has begun on the implementation of both constitutive models in the MARC finite element computer code.
Analytical investigation of stratified isotropic media
NASA Astrophysics Data System (ADS)
Vytovtov, Konstantin A.
2005-04-01
A rigorous analytical approach for investigating a stratified medium with an arbitrary finite number of homogeneous isotropic layers in a period is developed. The approach is based on the translation matrix method. It is well known that the translation matrix for a period must be found as the product of the layer matrices. It is proved that this matrix can be represented as a finite sum of trigonometric matrices, and thus the dispersion relation of a stratified medium is written in an analytical form. All final expressions are obtained in terms of the constitutive parameters. To this author's knowledge, this is the first time that the new sign function that allows us to develop the presented analytical results has been described. The condition of the existence of a wave with an arbitrary period divisible by a structure period is found in analytical form. It is proved that changing the layer arrangement within the period does not affect the structure of the transmission and absorption bands.
Crossover from isotropic to directed percolation
NASA Astrophysics Data System (ADS)
Zhou, Zongzheng; Yang, Ji; Ziff, Robert M.; Deng, Youjin
2012-08-01
We generalize the directed percolation (DP) model by relaxing the strict directionality of DP such that propagation can occur in either direction but with anisotropic probabilities. We denote the probabilities as p↓=ppd and p↑=p(1-pd), with p representing the average occupation probability and pd controlling the anisotropy. The Leath-Alexandrowicz method is used to grow a cluster from an active seed site. We call this model with two main growth directions biased directed percolation (BDP). Standard isotropic percolation (IP) and DP are the two limiting cases of the BDP model, corresponding to pd=1/2 and pd=0,1 respectively. In this work, besides IP and DP, we also consider the 1/2
Isotropic microscale mechanical properties of coral skeletons
Pasquini, Luca; Molinari, Alan; Fantazzini, Paola; Dauphen, Yannicke; Cuif, Jean-Pierre; Levy, Oren; Dubinsky, Zvy; Caroselli, Erik; Prada, Fiorella; Goffredo, Stefano; Di Giosia, Matteo; Reggi, Michela; Falini, Giuseppe
2015-01-01
Scleractinian corals are a major source of biogenic calcium carbonate, yet the relationship between their skeletal microstructure and mechanical properties has been scarcely studied. In this work, the skeletons of two coral species: solitary Balanophyllia europaea and colonial Stylophora pistillata, were investigated by nanoindentation. The hardness HIT and Young's modulus EIT were determined from the analysis of several load–depth data on two perpendicular sections of the skeletons: longitudinal (parallel to the main growth axis) and transverse. Within the experimental and statistical uncertainty, the average values of the mechanical parameters are independent on the section's orientation. The hydration state of the skeletons did not affect the mechanical properties. The measured values, EIT in the 76–77 GPa range, and HIT in the 4.9–5.1 GPa range, are close to the ones expected for polycrystalline pure aragonite. Notably, a small difference in HIT is observed between the species. Different from corals, single-crystal aragonite and the nacreous layer of the seashell Atrina rigida exhibit clearly orientation-dependent mechanical properties. The homogeneous and isotropic mechanical behaviour of the coral skeletons at the microscale is correlated with the microstructure, observed by electron microscopy and atomic force microscopy, and with the X-ray diffraction patterns of the longitudinal and transverse sections. PMID:25977958
Aspects of Turbulent / Non-Turbulent Interfaces
NASA Technical Reports Server (NTRS)
Bisset, D. K.; Hunt, J. C. R.; Rogers, M. M.; Koen, Dennis (Technical Monitor)
1999-01-01
A distinct boundary between turbulent and non-turbulent regions in a fluid of otherwise constant properties is found in many laboratory and engineering turbulent flows, including jets, mixing layers, boundary layers and wakes. Generally, the flow has mean shear in at least one direction within t he turbulent zone, but the non-turbulent zones have no shear (adjacent laminar shear is a different case, e.g. transition in a boundary layer). There may be purely passive differences between the turbulent and non-turbulent zones, e.g. small variations in temperature or scalar concentration, for which turbulent mixing is an important issue. The boundary has several major characteristics of interest for the present study. Firstly, the boundary advances into the non-turbulent fluid, or in other words, nonturbulent fluid is entrained. Secondly, the change in turbulence properties across the boundary is remarkably abrupt; strong turbulent motions come close to the nonturbulent fluid, promoting entrainment. Thirdly, the boundary is irregular with a continually changing convoluted shape, which produces statistical intermittency. Its shape is contorted at all scales of the turbulent motion.
Turbulence and turbulent mixing in natural fluids
NASA Astrophysics Data System (ADS)
Gibson, Carl H.
2010-12-01
Turbulence and turbulent mixing in natural fluids begin with big bang turbulence powered by spinning combustible combinations of Planck particles and Planck antiparticles. Particle prograde accretions on a spinning pair release 42% of the particle rest mass energy to produce more fuel for turbulent combustion. Negative viscous stresses and negative turbulence stresses work against gravity, extracting mass-energy and space-time from the vacuum. Turbulence mixes cooling temperatures until strong-force viscous stresses freeze out turbulent mixing patterns as the first fossil turbulence. Cosmic microwave background temperature anisotropies show big bang turbulence fossils along with fossils of weak plasma turbulence triggered as plasma photon-viscous forces permitting gravitational fragmentation on supercluster to galaxy mass scales. Turbulent morphologies and viscous-turbulent lengths appear as linear gas-protogalaxy-clusters in the Hubble ultra-deep field at z~7. Protogalaxies fragment into Jeans mass clumps of primordial-gas planets at decoupling: the dark matter of galaxies. Shortly after the plasma-to-gas transition, planet mergers produce stars that explode on overfeeding to fertilize and distribute the first life.
Numerical Simulation of a Convective Turbulence Encounter
NASA Technical Reports Server (NTRS)
Proctor, Fred H.; Hamilton, David W.; Bowles, Roland L.
2002-01-01
A numerical simulation of a convective turbulence event is investigated and compared with observational data. The numerical results show severe turbulence of similar scale and intensity to that encountered during the test flight. This turbulence is associated with buoyant plumes that penetrate the upper-level thunderstorm outflow. The simulated radar reflectivity compares well with that obtained from the aircraft's onboard radar. Resolved scales of motion as small as 50 m are needed in order to accurately diagnose aircraft normal load accelerations. Given this requirement, realistic turbulence fields may be created by merging subgrid-scales of turbulence to a convective-cloud simulation. A hazard algorithm for use with model data sets is demonstrated. The algorithm diagnoses the RMS normal loads from second moments of the vertical velocity field and is independent of aircraft motion.
ULF turbulence in the Neptunian polar cusp
NASA Astrophysics Data System (ADS)
Farrell, W. M.; Lepping, R. P.; Smith, C. W.
1993-03-01
Results of a spectral analysis of the ULF wave turbulence in the Neptunian polar cusp are presented. The activity is characterized as broadbanded, extending up to a maximum frequency of about 0.5 Hz, and having maximum wave amplitudes as large as 6 percent of the dc magnetic field. Activity in the cusp region was particularly intense at its frontside and backside, associated with the magnetopause and cusp/magnetosphere boundaries, respectively. The turbulence, particularly that above f(ci), is tentatively identified as whistler mode. It is argued that such whistler mode turbulence should resonate with electrons having energies in the tens of kiloelectron volts. Observations indicate a very strong correlation of the ULF turbulence with the energetic electrons between 22 and 35 keV measured by Voyager's low-energy charged particle experiment. A vigorous interaction between the two is inferred. ULF wave turbulence in the cusp may represent a significant but not complete power source for the magnetosphere.
A Experimental Investigation of the Turbulent Structure in Two-Dimensional Momentumless Wakes
NASA Astrophysics Data System (ADS)
Park, Woonjean
By utilizing an airfoil-shaped model which permits air injection through the rear, two-dimensional momentumless wakes have been simulated in a wind tunnel. Mean velocity profiles have been measured (with four injection conditions) at Re = 5400, up to x/d = 75; these profiles are self-similar. In the momentumless wake (i.e. injection adjusted to provide a thrust which exactly cancels the model's drag), the decay rate of the centerline velocity difference, U_{d }, and the spreading rate of the wake were found to be much faster and slower, respectively, than that of the pure wake or pure jet. Smoke-wire flow visualizations are also presented; the characteristic turbulent vortical structures are easily identifiable for the wake and jet cases, while the structures for the momentumless wake are neither wake-like nor jet -like. Instead, a unique large-scale turbulence structure was observed near the boundary of the momentumless wake, which bursts out into the freestream quasi-periodically. For the momentumless wakes produced by three different jet-injection models, the three turbulence intensities (u^', v^' and w^') as well as the Reynolds stress were measured by hot-wire anemometry. Similarity of the turbulence intensities was observed; the overall shape of those profiles is Gaussian. The decay rate ( x^{-0.81}) of u ^' was observed to be universal for the three models. However, the rate of decay of U _{d} and the spreading rate are dependent on initial conditions, and ranged from x ^{-0.92} - x ^{-2.02} and x^ {0.3} - x^ {0.46}, respectively. U_ {d} decays much faster than does u ^'. Consequently, the balance between production and dissipation no longer exists and mean shear practically disappears some distance downstream; the flow becomes nearly isotropic beyond x/d ~eq 45. It has thus been verified that the unique property of this flowfield, which distinguishes it from jets and wakes, is the very rapid decay of both mean shear and turbulence.
Propagation of cosine-Gaussian-correlated Schell-model beams in atmospheric turbulence.
Mei, Zhangrong; Schchepakina, Elena; Korotkova, Olga
2013-07-29
A stochastic beam generated by a recently introduced source of Schell type with cosine-Gaussian spectral degree of coherence is shown to possess interesting novel features on propagation in isotropic and homogeneous atmospheric turbulence with general non-Kolmogorov power spectrum. It is shown that while at small distances from the source the beam's intensity exhibits annular profile with adjustable area of the dark region, the center disappears at sufficiently large distances and the beam's intensity tends to Gaussian form. Hence the 3D bottle beam is produced by the cumulative effect of the random source and the atmosphere. The distances at which the on-axis beam intensity has local minima and maxima are shown to have analytic dependence on the source and the atmospheric parameters. And the influence of the fractal constant of the atmospheric power spectrum and refractive-index structure constant on beam characteristics is analyzed in depth. The novel double-cycle qualitative change in the degree of coherence is shown to occur on atmospheric propagation which was not previously known for any other beams.
Compressibility effects on turbulent mixing
NASA Astrophysics Data System (ADS)
Panickacheril John, John; Donzis, Diego
2016-11-01
We investigate the effect of compressibility on passive scalar mixing in isotropic turbulence with a focus on the fundamental mechanisms that are responsible for such effects using a large Direct Numerical Simulation (DNS) database. The database includes simulations with Taylor Reynolds number (Rλ) up to 100, turbulent Mach number (Mt) between 0.1 and 0.6 and Schmidt number (Sc) from 0.5 to 1.0. We present several measures of mixing efficiency on different canonical flows to robustly identify compressibility effects. We found that, like shear layers, mixing is reduced as Mach number increases. However, data also reveal a non-monotonic trend with Mt. To assess directly the effect of dilatational motions we also present results with both dilatational and soleniodal forcing. Analysis suggests that a small fraction of dilatational forcing decreases mixing time at higher Mt. Scalar spectra collapse when normalized by Batchelor variables which suggests that a compressive mechanism similar to Batchelor mixing in incompressible flows might be responsible for better mixing at high Mt and with dilatational forcing compared to pure solenoidal mixing. We also present results on scalar budgets, in particular on production and dissipation. Support from NSF is gratefully acknowledged.
Multiscale equations for strongly stratified turbulent flows
NASA Astrophysics Data System (ADS)
Chini, Greg; Rocha, Cesar; Julien, Keith; Caulfield, Colm-Cille
2016-11-01
Strongly stratified turbulent shear flows are of fundamental importance owing to their widespread occurrence and their impact on diabatic mixing, yet direct numerical simulations of such flows remain challenging. Here, a reduced, multiscale description of turbulent shear flows in the presence of strong stable density stratification is derived via asymptotic analysis of the governing Boussinesq equations. The analysis explicitly recognizes the occurrence of dynamics on disparate spatiotemoporal scales, and yields simplified partial differential equations governing the coupled evolution of slowly-evolving small aspect-ratio ('pancake') modes and isotropic, strongly non-hydrostatic stratified-shear (e.g. Kelvin-Helmholtz) instability modes. The reduced model is formally valid in the physically-relevant regime in which the aspect-ratio of the pancake structures tends to zero in direct proportion to the horizontal Froude number. Relative to the full Boussinesq equations, the model offers both computational and conceptual advantages.
Vortex statistics in turbulent channel flows
NASA Astrophysics Data System (ADS)
Elsas, José Hugo; Augusto Moriconi, Luca Roberto
2016-11-01
In order to address the role of coherent structures in wall bounded turbulence, we study the statistics of morphological and kinematic properties of vortices, such as circulation, radius and height distributions. To accomplish that, we introduce a novel vortex identification method named as "vorticity curvature criterion" which is based on the local properties of the vorticity field. We furthermore employ a background subtraction procedure to remove shearing background effects expected to be present in the topology of the streamwise/wall-normal plane flow configurations. We discuss, through a comparative study of performance with the usual swirling strength criterion, and extending the previous analyses to the detection of coherent structures in the spanwise/wall normal planes, isotropization issues for the paradigmatic case of numerical turbulent channel flows. We acknowledge the funding from CNPq, CAPES and Faperj.
Energy spectra in elasto-inertial turbulence
NASA Astrophysics Data System (ADS)
Valente, P. C.; da Silva, C. B.; Pinho, F. T.
2016-07-01
Direct numerical simulations of statistically steady homogeneous isotropic turbulence in viscoelastic fluids described by the FENE-P model are presented. Emphasis is given to large polymer relaxation times compared to the eddy turnover time, which is a regime recently termed elasto-inertial turbulence. In this regime the polymers are ineffective in dissipating kinetic energy but they play a lead role in transferring kinetic energy to the small solvent scales which turns out to be concomitant with the depletion of the usual non-linear energy cascade. However, we show that the non-linear interactions are still highly active, but they lead to no net downscale energy transfer because the forward and reversed energy cascades are nearly balanced. Finally, we show that the tendency for a steeper elasto-inertial power-law spectra is reversed for large polymer relaxation times and the spectra tend towards the usual k-5/3 functional form.
Development of renormalization group analysis of turbulence
NASA Technical Reports Server (NTRS)
Smith, L. M.
1990-01-01
The renormalization group (RG) procedure for nonlinear, dissipative systems is now quite standard, and its applications to the problem of hydrodynamic turbulence are becoming well known. In summary, the RG method isolates self similar behavior and provides a systematic procedure to describe scale invariant dynamics in terms of large scale variables only. The parameterization of the small scales in a self consistent manner has important implications for sub-grid modeling. This paper develops the homogeneous, isotropic turbulence and addresses the meaning and consequence of epsilon-expansion. The theory is then extended to include a weak mean flow and application of the RG method to a sequence of models is shown to converge to the Navier-Stokes equations.
The analysis and modeling of dilatational terms in compressible turbulence
NASA Technical Reports Server (NTRS)
Sarkar, S.; Erlebacher, G.; Hussaini, M. Y.; Kreiss, H. O.
1989-01-01
It is shown that the dilatational terms that need to be modeled in compressible turbulence include not only the pressure-dilatation term but also another term - the compressible dissipation. The nature of these dilatational terms in homogeneous turbulence is explored by asymptotic analysis of the compressible Navier-Stokes equations. A non-dimensional parameter which characterizes some compressible effects in moderate Mach number, homogeneous turbulence is identified. Direct numerical simulations (DNS) of isotropic, compressible turbulence are performed, and their results are found to be in agreement with the theoretical analysis. A model for the compressible dissipation is proposed; the model is based on the asymptotic analysis and the direct numerical simulations. This model is calibrated with reference to the DNS results regarding the influence of compressibility on the decay rate of isotropic turbulence. An application of the proposed model to the compressible mixing layer has shown that the model is able to predict the dramatically reduced growth rate of the compressible mixing layer.
Modeling Rotating Turbulent Flows with the Body Force Potential Model.
NASA Astrophysics Data System (ADS)
Bhattacharya, Amitabh; Perot, Blair
2000-11-01
Like a Reynolds Stress Transport equation model, the turbulent potential model has an explicit Coriolis acceleration term that appears in the model that accounts for rotation effects. In this work the additional secondary effects that system rotation has on the dissipation rate, return-to-isotropy, and fast pressure strain terms are also included in the model. The resulting model is tested in the context of rotating isotropic turbulence, rotating homogeneous shear flow, rotating channel flow, and swirling pipe flow. Many of the model changes are applicable to Reynolds stress transport equation models. All model modifications are frame indifferent.
MEASUREMENTS AND COMPUTATIONS OF FUEL DROPLET TRANSPORT IN TURBULENT FLOWS
Joseph Katz and Omar Knio
2007-01-10
The objective of this project is to study the dynamics of fuel droplets in turbulent water flows. The results are essential for development of models capable of predicting the dispersion of slightly light/heavy droplets in isotropic turbulence. Since we presently do not have any experimental data on turbulent diffusion of droplets, existing mixing models have no physical foundations. Such fundamental knowledge is essential for understanding/modeling the environmental problems associated with water-fuel mixing, and/or industrial processes involving mixing of immiscible fluids. The project has had experimental and numerical components: 1. The experimental part of the project has had two components. The first involves measurements of the lift and drag forces acting on a droplet being entrained by a vortex. The experiments and data analysis associated with this phase are still in progress, and the facility, constructed specifically for this project is described in Section 3. In the second and main part, measurements of fuel droplet dispersion rates have been performed in a special facility with controlled isotropic turbulence. As discussed in detail in Section 2, quantifying and modeling the of droplet dispersion rate requires measurements of their three dimensional trajectories in turbulent flows. To obtain the required data, we have introduced a new technique - high-speed, digital Holographic Particle Image Velocimetry (HPIV). The technique, experimental setup and results are presented in Section 2. Further information is available in Gopalan et al. (2005, 2006). 2. The objectives of the numerical part are: (1) to develop a computational code that combines DNS of isotropic turbulence with Lagrangian tracking of particles based on integration of a dynamical equation of motion that accounts for pressure, added mass, lift and drag forces, (2) to perform extensive computations of both buoyant (bubbles) and slightly buoyant (droplets) particles in turbulence conditions
Three-dimensional time dependent computation of turbulent flow
NASA Technical Reports Server (NTRS)
Kwak, D.; Reynolds, W. C.; Ferziger, J. H.
1975-01-01
The three-dimensional, primitive equations of motion are solved numerically for the case of isotropic box turbulence and the distortion of homogeneous turbulence by irrotational plane strain at large Reynolds numbers. A Gaussian filter is applied to governing equations to define the large scale field. This gives rise to additional second order computed scale stresses (Leonard stresses). The residual stresses are simulated through an eddy viscosity. Uniform grids are used, with a fourth order differencing scheme in space and a second order Adams-Bashforth predictor for explicit time stepping. The results are compared to the experiments and statistical information extracted from the computer generated data.
Determining the alpha dynamo parameter in incompressible homogeneous magnetohydrodynamic turbulence
NASA Technical Reports Server (NTRS)
Matthaeus, W. H.; Goldstein, M. L.; Lantz, S. R.
1983-01-01
Alpha, an important parameter in dynamo theory, is proportional to either the kinetic, current, magnetic, or velocity helicity of the fluctuating magnetic field and fluctuating velocity field. The particular helicity to which alpha is proportional depends on the assumptions used in deriving the first order smoothed equations that describe the alpha effect. In two cases, when alpha is proportional to either the magnetic helicity or velocity helicity, alpha is determined experimentally from two point measurements of the fluctuating fields in incompressible, homogeneous turbulence having arbitrary symmetry. For the other two possibilities, alpha is determined if the turbulence is isotropic.
Application of a new K-tau model to near wall turbulent flows
NASA Technical Reports Server (NTRS)
Thangam, S.; Abid, R.; Speziale, Charles G.
1991-01-01
A recently developed K-tau model for near wall turbulent flows is applied to two severe test cases. The turbulent flows considered include the incompressible flat plate boundary layer with the adverse pressure gradients and incompressible flow past a backward facing step. Calculations are performed for this two-equation model using an anisotropic as well as isotropic eddy-viscosity. The model predictions are shown to compare quite favorably with experimental data.
Assessing Model Assumptions for Turbulent Premixed Combustion at High Karlovitz Number
2015-09-03
1309. [26] P. Carroll , G. Blanquart, A proposed modification to Lundgren’s physical space velocity forcing method for isotropic turbulence, Phys...Combust. Flame 159 (2012) 317–335. [30] P. Carroll , G. Blanquart, The effect of velocity field forcing techniques on the Karman-Howarth equation, J...331–368. [43] S. K. Lele, Compressibility effects on turbulence, Ann. Rev. Fluid Mech. 26 (1994) 211 – 254. [44] P. L. Carroll , G. Blanquart, A
Statistical turbulence theory and turbulence phenomenology
NASA Technical Reports Server (NTRS)
Herring, J. R.
1973-01-01
The application of deductive turbulence theory for validity determination of turbulence phenomenology at the level of second-order, single-point moments is considered. Particular emphasis is placed on the phenomenological formula relating the dissipation to the turbulence energy and the Rotta-type formula for the return to isotropy. Methods which deal directly with most or all the scales of motion explicitly are reviewed briefly. The statistical theory of turbulence is presented as an expansion about randomness. Two concepts are involved: (1) a modeling of the turbulence as nearly multipoint Gaussian, and (2) a simultaneous introduction of a generalized eddy viscosity operator.
Estimating Atmospheric Turbulence From Flight Records
NASA Technical Reports Server (NTRS)
Wingrove, R. C.; Bach, R. E., Jr.; Schultz, T. A.
1991-01-01
Method for estimation of atmospheric turbulence encountered by airplanes utilizes wealth of data captured by multichannel digital flight-data recorders and air-traffic-control radar. Developed as part of continuing effort to understand how airplanes respond to such potentially hazardous phenomena as: clear-air turbulence generated by destabilized wind-shear layers above mountains and thunderstorms, and microbursts (intense downdrafts striking ground), associated with thunderstorms. Reconstructed wind fields used to predict and avoid future hazards.
NASA Astrophysics Data System (ADS)
Venaille, Antoine; Nadeau, Louis-Philippe; Vallis, Geoffrey
2014-12-01
We investigate the non-linear equilibration of a two-layer quasi-geostrophic flow in a channel with an initial eastward baroclinically unstable jet in the upper layer, paying particular attention to the role of bottom friction. In the limit of low bottom friction, classical theory of geostrophic turbulence predicts an inverse cascade of kinetic energy in the horizontal with condensation at the domain scale and barotropization in the vertical. By contrast, in the limit of large bottom friction, the flow is dominated by ribbons of high kinetic energy in the upper layer. These ribbons correspond to meandering jets separating regions of homogenized potential vorticity. We interpret these results by taking advantage of the peculiar conservation laws satisfied by this system: the dynamics can be recast in such a way that the initial eastward jet in the upper layer appears as an initial source of potential vorticity levels in the upper layer. The initial baroclinic instability leads to a turbulent flow that stirs this potential vorticity field while conserving the global distribution of potential vorticity levels. Statistical mechanical theory of the 1 1/2 layer quasi-geostrophic model predicts the formation of two regions of homogenized potential vorticity separated by a minimal interface. We explain that cascade phenomenology leads to the same result. We then show that the dynamics of the ribbons results from a competition between a tendency to reach the equilibrium state and baroclinic instability that induces meanders of the interface. These meanders intermittently break and induce potential vorticity mixing, but the interface remains sharp throughout the flow evolution. We show that for some parameter regimes, the ribbons act as a mixing barrier which prevents relaxation toward equilibrium, favouring the emergence of multiple zonal (eastward) jets.
Coupling Turbulence in Hybrid LES-RANS Techniques
NASA Technical Reports Server (NTRS)
Woodruff, Stephen L.
2011-01-01
A formulation is proposed for hybrid LES-RANS computations that permits accurate computations during resolution changes, so that resolution may be changed at will in order to employ only as much resolution in each subdomain as is required by the physics. The two components of this formulation, establishing the accuracy of a hybrid model at constant resolutions throughout the RANS-to-LES range and maintaining that accuracy when resolution is varied, are demonstrated for decaying, homogeneous, isotropic turbulence.
Investigating source processes of isotropic events
NASA Astrophysics Data System (ADS)
Chiang, Andrea
explosion. In contrast, recovering the announced explosive yield using seismic moment estimates from moment tensor inversion remains challenging but we can begin to put error bounds on our moment estimates using the NSS technique. The estimation of seismic source parameters is dependent upon having a well-calibrated velocity model to compute the Green's functions for the inverse problem. Ideally, seismic velocity models are calibrated through broadband waveform modeling, however in regions of low seismicity velocity models derived from body or surface wave tomography may be employed. Whether a velocity model is 1D or 3D, or based on broadband seismic waveform modeling or the various tomographic techniques, the uncertainty in the velocity model can be the greatest source of error in moment tensor inversion. These errors have not been fully investigated for the nuclear discrimination problem. To study the effects of unmodeled structures on the moment tensor inversion, we set up a synthetic experiment where we produce synthetic seismograms for a 3D model (Moschetti et al., 2010) and invert these data using Green's functions computed with a 1D velocity mode (Song et al., 1996) to evaluate the recoverability of input solutions, paying particular attention to biases in the isotropic component. The synthetic experiment results indicate that the 1D model assumption is valid for moment tensor inversions at periods as short as 10 seconds for the 1D western U.S. model (Song et al., 1996). The correct earthquake mechanisms and source depth are recovered with statistically insignificant isotropic components as determined by the F-test. Shallow explosions are biased by the theoretical ISO-CLVD tradeoff but the tectonic release component remains low, and the tradeoff can be eliminated with constraints from P wave first motion. Path-calibration to the 1D model can reduce non-double-couple components in earthquakes, non-isotropic components in explosions and composite sources and improve
Computational investigation of the effects of turbulence, inertia, and gravity on particle dynamics
NASA Astrophysics Data System (ADS)
Ireland, Peter John
In this work, we examine the motion of particles which are subjected to varying levels of turbulence, inertia, and gravity, in both homogeneous and inhomogeneous turbulence. These investigations are performed through direct numerical simulation (DNS) of the Eulerian fluid velocity field combined with Lagrangian particle tracking. The primary motivation of these investigations is to better understand and model the dynamics and growth of water droplets in warm, cumulus clouds. In the first part of this work, we discuss the code we developed for these simulations, Highly Parallel Particle-laden flow Solver for Turbulence Research (HiPPSTR). HiPPSTR uses efficient parallelization strategies, time-integration techniques, and interpolation methods to enable massively parallel simulations of three-dimensional, particle-laden turbulence. In the second, third, and fourth sections of this work, we analyze simulations of particle-laden flows which are representative of those at the edges and cores of clouds. In the second section, we consider the mixing of droplets near interfaces with varying turbulence intensities and gravitational orientations, to provide insight into the dynamics near cloud edges. The simulations are parameterized to match wind-tunnel experiments of particle mixing which were conducted at Cornell, and the DNS and experimental results are compared and contrasted. Mixing is suppressed when turbulence intensities differ across the interface, and in all cases, the particle concentrations are subject to large fluctuations. In the third and fourth sections, we use HiPPSTR to analyze droplet motion in isotropic turbulence, which we take to be representative of adiabatic cloud cores. The third section examines the Reynolds-number scaling of single-particle and particle-pair statistics without gravity, while the fourth section shows results when gravity is included. While weakly inertial particles preferentially sample certain regions of the flow, gravity reduces
Energy Transfer in Rotating Turbulence
NASA Technical Reports Server (NTRS)
Cambon, Claude; Mansour, Nagi N.; Godeferd, Fabien S.; Rai, Man Mohan (Technical Monitor)
1995-01-01
The influence or rotation on the spectral energy transfer of homogeneous turbulence is investigated in this paper. Given the fact that linear dynamics, e.g. the inertial waves regime tackled in an RDT (Rapid Distortion Theory) fashion, cannot Affect st homogeneous isotropic turbulent flow, the study of nonlinear dynamics is of prime importance in the case of rotating flows. Previous theoretical (including both weakly nonlinear and EDQNM theories), experimental and DNS (Direct Numerical Simulation) results are gathered here and compared in order to give a self-consistent picture of the nonlinear effects of rotation on tile turbulence. The inhibition of the energy cascade, which is linked to a reduction of the dissipation rate, is shown to be related to a damping due to rotation of the energy transfer. A model for this effect is quantified by a model equation for the derivative-skewness factor, which only involves a micro-Rossby number Ro(sup omega) = omega'/(2(OMEGA))-ratio of rms vorticity and background vorticity as the relevant rotation parameter, in accordance with DNS and EDQNM results fit addition, anisotropy is shown also to develop through nonlinear interactions modified by rotation, in an intermediate range of Rossby numbers (Ro(omega) = (omega)' and Ro(omega)w greater than 1), which is characterized by a marco-Rossby number Ro(sup L) less than 1 and Ro(omega) greater than 1 which is characterized by a macro-Rossby number based on an integral lengthscale L and the micro-Rossby number previously defined. This anisotropy is mainly an angular drain of spectral energy which tends to concentrate energy in tile wave-plane normal to the rotation axis, which is exactly both the slow and the two-dimensional manifold. In Addition, a polarization of the energy distribution in this slow 2D manifold enhances horizontal (normal to the rotation axis) velocity components, and underlies the anisotropic structure of the integral lengthscales. Finally is demonstrated the
Development of turbulence in a dusty plasma
NASA Astrophysics Data System (ADS)
Schwabe, Mierk; Zhdanov, Sergey; Räth, Christoph
2016-10-01
Complex or dusty plasmas are low temperature plasmas which contain micrometer-sized particles (``dust''). The microparticles obtain high charges and interact with each other, effectively forming a solid, liquid or gas state in which the microparticles take over the role of molecules in conventional systems. Complex plasmas often are in a turbulent state, for instance when instabilities like the ``heartbeat'' instability or intense waves are present. The movement of the microparticles, the carriers of the turbulent interactions in complex plasmas, can be directly followed, unlike that of atoms and molecules in conventional experiments on turbulence. Here we present results of an experiment on the development of turbulence in a complex plasma in the PK-3 Plus laboratory on board the International Space Station. The microparticle cloud was first stabilized against an instability. Once the stabilization was turned off, the cloud became unstable, and the movement of the particles became turbulent. In the report, we show how the energy spectra evolve during the development of turbulence. In the case of fully developed turbulence, the spectra display multiple cascades explaining well the transport of turbulent energy and enstrophy.
CHARGED-PARTICLE TRANSPORT IN MAGNETIC TURBULENCE. I. A GLOBALLY ANISOTROPIC FIELD
Sun, P.; Jokipii, J. R.
2015-12-10
Collisionless magnetohydrodynamic Turbulence is common in large scale astrophysical environments. The determination of the transport of charged particles both parallel and perpendicular in such a system is of considerable interest. Quasi-linear analysis or direct numerical simulation can be used to find the effects of the turbulent magnetic field on the transport of charged particles. A number of different magnetic turbulence models have been proposed in the last several decades. We present here the results of studying particle transport in synthesized, anisotropic turbulence and compare the results with those obtained using the standard isotropic turbulence model in a series of papers. In this paper we consider the magnetic field turbulence model with global anisotropy.
NASA Astrophysics Data System (ADS)
Aryasova, Natalie; Reznikov, Yuri
2016-09-01
We study the effect of an isotropic-nematic (I -N ) phase transition on the liquid crystal alignment at untreated polymer surfaces. We demonstrate that the pattern at the untreated substrate in the planar cell where the other substrate is uniformly rubbed strongly depends on the temperature gradient across the cell during the I -N phase transition, being macroscopically isotropic if the untreated substrate is cooled faster, but becoming almost homogeneous along the rubbing direction in the opposite temperature gradient. We interpret the observed effect using complementary models of heat transfer and nematic elasticity. Based on the heat transfer model we show that the asymmetric temperature conditions in our experiments provide unidirectional propagation of the I -N interface during the phase transition and determine the initial director orientation pattern at the test's untreated surface. Using the Frank-Oseen model of nematic elasticity, we represent the three-dimensional director field in the nematic cell as a two-dimensional (2D) pattern at the untreated surface and perform 2D numeric simulations. The simulations explain the experimental results: Different initial director orientations at the untreated surface evolve into different stationary patterns.
A phenomenological treatment of rotating turbulence
NASA Technical Reports Server (NTRS)
Zhou, YE
1995-01-01
The strong similarity between the magnetohydrodynamic (MHD) turbulence and initially isotropic turbulence subject to rotation is noted. We then apply the MHD phenomenologies of Kraichnan and Matthaeus & Zhou to rotating turbulence. When the turbulence is subject to a strong rotation, the energy spectrum is found to scale as E(k) = C(sub Omega)(Omega(sub epsilon))(sup 1/2)k(sup -2), where Omega is the rotation rate, k is the wavenumber, and epsilon is the dissipation rate. This spectral form is consistent with a recent letter by Zeman. However, here the constant C(sub Omega) is found to be related to the Kolmogorov constant and is estimated in the range 1.22 - 1.87 for the typical values of the latter constant. A 'rule' that relates spectral transfer times to the eddy turnover time and the time scale for decay of the triple correlations is deduced. A hypothesis for the triple correlation decay rate leads to the spectral law which varies between the '-5/3' (without rotation) and '-2' laws (with strong rotation). For intermediate rotation rates, the spectrum varies according to the value of a dimensionless parameter that measures the strength of the rotation wavenumber k(sub Omega) = (Omega(sup 3)/epsiolon)(sup 1/2) relative to the wavenumber k. An eddy viscosity is derived with an explicit dependence on the rotation rate.
Universality and scaling in compressible turbulence
NASA Astrophysics Data System (ADS)
Donzis, Diego; Jagannathan, Shriram
2016-11-01
A large database of Direct Numerical Simulations (DNS) of stationary compressible isotropic turbulence at a range of Taylor Reynolds numbers (Rλ 38 - 450) and turbulent Mach numbers (Mt 0 . 1 - 0 . 6) is used to explore universality. While in incompressible turbulence self-similarity analysis leads to a single scaling parameter (Rλ), compressible turbulence expands the parameter space due to the coupling between hydrodynamics and thermodynamics, and the dependence on the mode of external forcing. While for the former it is common to use Mt as a scaling parameter, the effects of the latter are harder to quantify, and their consequences may have been attributed to a certain lack of universality. For instance, when the dilatational mode is forced, the variance and skewness of pressure shows significant scatter when plotted against Mt. Using a Helmholtz decomposition, we split the velocity field into solenoidal and dilatational modes, and propose scaling parameters that include the contribution from both modes. When expressed against these parameters, we observe a universal scaling regime regardless of the mode of excitation of forcing. Other quantities that follow this behavior are also discussed. Support from NSF and AFOSR is gratefully acknowledged.
Direct numerical simulation of chemically reacting turbulence
NASA Astrophysics Data System (ADS)
Miyauchi, Toshio; Tanahashi, Mamoru
In this paper, we present two results of direct numerical simulation of chemically reacting flows. One is direct numerical simulation of chemically reacting two-dimensional mixing layer and the other is direct numerical simulation of chemically reacting compressible isotropic turbulence. As for the mixing layer, a low Mach number approximation was used to take into account the variable density effects on the flow fields and to clarify the effects of heat release and density difference of a mean flow. In the case of density difference, expansion and baroclinic torque has a negative contribution to the local vorticity transport in the high density side and a positive contribution in the low density side which results in an asymmetric vortical structure structure. Thes density difference suppresses the growth of mixing layer and causes the overshoot of mean velocity only in the high density side which coincides with an experimental result. Coupling effects of heat release and desnity difference are also investigated. As for the homogeneous turbulence, fully compressible Navier-Stokes equations are solved to clarify the interaction between turbulence and chemical reaction in turbulent diffusion flame. The chemical reaction is suppressed by the increase of heat release because of the decrease of density and local Reynolds number. However, the decay of enstrophy with heat release is slower than that without heat release because of strong baroclinic torque which is generated near the reaction zone. Also, large amount of heat release causes increase in turbulent energy through the pressure dilatation term. The pressure dilatation term shows the periodic fluctuation which has an acoustic time scale. The fluctuation is enhanced by the heat release and travels in the turbulent field as pressure and dilatation waves.
NASA Astrophysics Data System (ADS)
Yi, Sumin; Jhang, Hogun; Kwon, J. M.
2016-10-01
We report the results of a gyrokinetic simulation study elucidating the characteristics of the current driven by electron temperature gradient (ETG) turbulence in toroidal geometry. We examined the amount of the ETG turbulence-driven current for different turbulence levels, which were obtained by varying the relative electron gyroradius ρ* = ρe/a. Simulations show that the amount of the ETG turbulence-driven current increases with ρ* due to the gyro-Bohm scaling of turbulence intensity. A perturbation of the equilibrium q-profile by the ETG turbulence-driven current becomes noticeable when ρ* > 1/4000. Even in a small ρ* case, the proportional relation between the ETG turbulence-driven current and turbulence intensity suggests that a considerable intrinsic current can be driven inside an edge pedestal where a steep gradient of the electron temperature profile can excite ETG turbulence in a narrow region.
Constitutive modeling for isotropic materials (HOST)
NASA Technical Reports Server (NTRS)
Chan, Kwai S.; Lindholm, Ulric S.; Bodner, S. R.; Hill, Jeff T.; Weber, R. M.; Meyer, T. G.
1986-01-01
The results of the third year of work on a program which is part of the NASA Hot Section Technology program (HOST) are presented. The goals of this program are: (1) the development of unified constitutive models for rate dependent isotropic materials; and (2) the demonstration of the use of unified models in structural analyses of hot section components of gas turbine engines. The unified models selected for development and evaluation are those of Bodner-Partom and of Walker. A test procedure was developed for assisting the generation of a data base for the Bodner-Partom model using a relatively small number of specimens. This test procedure involved performing a tensile test at a temperature of interest that involves a succession of strain-rate changes. The results for B1900+Hf indicate that material constants related to hardening and thermal recovery can be obtained on the basis of such a procedure. Strain aging, thermal recovery, and unexpected material variations, however, preluded an accurate determination of the strain-rate sensitivity parameter is this exercise. The effects of casting grain size on the constitutive behavior of B1900+Hf were studied and no particular grain size effect was observed. A systematic procedure was also developed for determining the material constants in the Bodner-Partom model. Both the new test procedure and the method for determining material constants were applied to the alternate material, Mar-M247 . Test data including tensile, creep, cyclic and nonproportional biaxial (tension/torsion) loading were collected. Good correlations were obtained between the Bodner-Partom model and experiments. A literature survey was conducted to assess the effects of thermal history on the constitutive behavior of metals. Thermal history effects are expected to be present at temperature regimes where strain aging and change of microstructure are important. Possible modifications to the Bodner-Partom model to account for these effects are outlined
Space-Time Correlations and Dynamic Coupling in Turbulent Flows
NASA Astrophysics Data System (ADS)
He, Guowei; Jin, Guodong; Yang, Yue
2017-01-01
Space-time correlation is a staple method for investigating the dynamic coupling of spatial and temporal scales of motion in turbulent flows. In this article, we review the space-time correlation models in both the Eulerian and Lagrangian frames of reference, which include the random sweeping and local straining models for isotropic and homogeneous turbulence, Taylor's frozen-flow model and the elliptic approximation model for turbulent shear flows, and the linear-wave propagation model and swept-wave model for compressible turbulence. We then focus on how space-time correlations are used to develop time-accurate turbulence models for the large-eddy simulation of turbulence-generated noise and particle-laden turbulence. We briefly discuss their applications to two-point closures for Kolmogorov's universal scaling of energy spectra and to the reconstruction of space-time energy spectra from a subset of spatial and temporal signals in experimental measurements. Finally, we summarize the current understanding of space-time correlations and conclude with future issues for the field.
Inverse Energy Cascades in Rotating Turbulence
NASA Astrophysics Data System (ADS)
Rosenberg, D. L.; Marino, R.; Mininni, P.; Pouquet, A.
2013-12-01
We present the results of direct numerical simulations (DNS) of rapidly rotating turbulent flows on grids of 20483 grid points that are forced at intermediate scales. Injection of energy at such scales at small Rossby numbers (~0.04) leads to a direct cascade toward small scales and an inverse cascade toward large scales. These results essentially validate those obtained using large eddy simulation (LES) (Sen et al., PRE 86:036319 (2012)): for a (helical) forcing that injects energy largely in 2D modes, the large scale energy spectrum scales as kperp-5/3, consistent with Kolmogorov-Kraichnan-Batchelor-Leith phenomenology; for a nonhelical isotropic forcing, the large scale energy spectrum scales as kperp-3. The (helical) anisotropic forcing DNS solution, like that of the LES models, shows a k-1 isotropic energy spectrum, which Sen et al. attribute to a large scale shear. The higher resolution of the DNS runs allows us to carry out probability distribution and conditional analyses that show that this interpretation may, in fact, be consistent with wall-bounded turbulent shear flow.
Doppler lidar signal and turbulence study
NASA Technical Reports Server (NTRS)
Frost, W.; Huang, K. H.
1983-01-01
Wind fields were measured with the ground-based NASA/MSFC lidar are compared with the in situ NASA RB-57 aircraft measurements. The mean wind fields, the turbulence intensities, and the turbulence spectra determined from measurements by both systems are in very good agreement. Turbulence intensities and spectra were calculated from the fluctuations with time in the radial wind speed component. The second moment or Doppler frequency spectral width of the lidar measurements was also compared with turbulence intensities measured by the aircraft. These second moments could only be resolved at the very low altitudes (in three range bins). Turbulence intensities estimated from the spectral width data were an order of magnitude higher than those measured by the aircraft. An interesting boundary layer evolved during the progress of the experiment. The breakup of a stable boundary layer resulted in winds blowing in one direction above 600 m msl and in the opposite direction below that level. Both the aircraft and the lidar systems clearly identified this unusual boundary layer flow and showed the identical trends.
Characterizing error propagation in quantum circuits: the Isotropic Index
NASA Astrophysics Data System (ADS)
Fonseca de Oliveira, André L.; Buksman, Efrain; Cohn, Ilan; García López de Lacalle, Jesús
2017-02-01
This paper presents a novel index in order to characterize error propagation in quantum circuits by separating the resultant mixed error state in two components: an isotropic component that quantifies the lack of information, and a disalignment component that represents the shift between the current state and the original pure quantum state. The Isotropic Triangle, a graphical representation that fits naturally with the proposed index, is also introduced. Finally, some examples with the analysis of well-known quantum algorithms degradation are given.
Geostrophic Turbulence in the Frequency-Wavenumber Domain: Eddy-Driven Low-Frequency Variability
2014-01-01
oceanic geostrophic turbulence. Our main diagnostics are spectral fluxes and spectral transfers—long utilized in wavenumber space analyses of...isotropic wavenumber–frequency spectral diagnostics . In a companion paper, we analyze spectral diagnostics in the anisotropic wavenumber–frequency domain to...2013), we will also compute spectral diagnostics from a satellite altimeter product. The Archiving, Validation, and Interpretation of Satellite
Shielded Superconducting Linear Motor for Towed-Grid Studies of Quantum Turbulence
Liu Shuchen; Zhou Yihui; Ihas, Gary G.
2006-09-07
A motor is described which can pull a grid through a channel of pure superfluid 4He to produce homogeneous isotropic turbulence. The motor is composed of a superconducting solenoid inside a superconducting shield to minimize Joule and eddy current heating of the liquid helium. Computer simulations show the design to be feasible.
Heat transfer and turbulence in a turbulated blade cooling circuit
Abuaf, N. ); Kercher, D.M. )
1994-01-01
The aerothermal performance of a typical turbine blade three-pass turbulated cooling circuit geometry was investigated in a 10X plexiglass test model. The model closely duplicated the blade's leading edge, midchord, and trailing edge cooling passage geometries. Steady-state heat transfer coefficient distributions along the blade pressure side wall (convex surface) of the cooling circuit passages were measured with a thin-foil heater and a liquid crystal temperature sensor assembly. The heat transfer experiments were conducted on rib-roughened channels with staggered turbulators along the convex and concave surfaces of the cooling passages. Midchannel axial velocity and turbulence intensity measurements were taken by hot-wire anemometry at each passage end of the three-pass cooling circuit to characterize and relate the experimental data are compared with results of a Computational Fluid Dynamics (CFD) analysis at the operating internal environment for a 1X rotating model of the blade cooling circuit and other turbulator channel geometry heat transfer data investigations. The comparison between the measurements and analysis is encouraging. Differences with other heat transfer data appear reasonably understood and explainable.
Aerodynamic Performance and Turbulence Measurements in a Turbine Vane Cascade
NASA Technical Reports Server (NTRS)
Boyle, Robert J.; Lucci, Barbara L.; Senyitko, Richard G.
2002-01-01
Turbine vane aerodynamics were measured in a three vane linear cascade. Surface pressures and blade row losses were obtained over a range of Reynolds and Mach number for three levels of turbulence. Comparisons are made with predictions using a quasi-3D Navier-Stokes analysis. Turbulence intensity measurement were made upstream and downstream of the vane. The purpose of the downstream measurements was to determine how the turbulence was affected by the strong contraction through 75 deg turning.
NASA Astrophysics Data System (ADS)
Gao, Zhiwen; Zhou, Youhe
2015-04-01
Real fundamental solution for fracture problem of transversely isotropic high temperature superconductor (HTS) strip is obtained. The superconductor E-J constitutive law is characterized by the Bean model where the critical current density is independent of the flux density. Fracture analysis is performed by the methods of singular integral equations which are solved numerically by Gauss-Lobatto-Chybeshev (GSL) collocation method. To guarantee a satisfactory accuracy, the convergence behavior of the kernel function is investigated. Numerical results of fracture parameters are obtained and the effects of the geometric characteristics, applied magnetic field and critical current density on the stress intensity factors (SIF) are discussed.
Incompressible Turbulent Wing-Body Junction Flow
NASA Technical Reports Server (NTRS)
Krishnamurthy, R.; Cagle, Corey D.; Chandra, S.
1998-01-01
-stream flow. The lateral curvature of the wing/strat causes the oncoming turbulent layer to skew about am axis (x-axis) parallel to the plane (xz-plane) of the mean shear. This is the principle mechanism for the generation of secondary flow. Such skew-induced secondary flows are slow to be attenuated by Reynolds stresses. Additional contribution to the generation of secondary flow comes from anisotropies in Reynolds stresses. Upstream of the strut, the mean-vorticity is directed span wise (along the y-direction). The presence of secondary flow in the vicinity of the strut causes the vorticity to stretch around the obstacle in a horse-shoe shape, with each leg having a vorticity of the opposite sense. The blockage effect of the strut imposes a severe adverse pressure gradient on the oncoming turbulent shear layer, causing boundary layer separation ahead of the leading edge, resulting in a vortex that rolls up and flows downstream into the juncture region. The separation vortices trailing in the wake of the wing can alter the lift or drag characteristics of the surfaces downstream of the wing-body juncture. Likewise, on submarines, the wake flow behind the appendage can degrade the performance of the propeller located downstream. The complex nature of this flow is caused by the presence of all six components of Reynolds stresses. Devenport and Simpson report that in the vicinity of the horse-shoe vortex there is intense recirculation with turbulent stresses being much larger than those normally observed in turbulent flows. These features contribute to making this flow a challenge to predict numerically. Some of the past studies provide useful insights into this flow that would guide our numerical efforts. In measurements reported by Shabaka and Bradshaw, the eddy viscosity tensor is seen to be non-isotropic and has negative components in certain regions. In an effort to evaluate the closure assumptions of various turbulence models, Devenport and Simpson used their own extensive
Turbulent Premixed Flames in Microgravity
NASA Technical Reports Server (NTRS)
Menon, Suresh
1996-01-01
The experimental cold-flow facility is now full operational and is currently being used to obtain baseline turbulence data in a Couette flow. The baseline turbulence data is necessary to confirm the capability of the chosen device to generate and maintain the required turbulence intensity. Subsequent reacting flow studies will assume that a similar turbulent flow field exists ahead of the premixed flame. Some modifications and refinements had to be made to enable accurate measurements. It consists of two rollers, one (driven by a motor) which drives a continuous belt and four smaller rollers used to set the belt spacing and tension to minimize belt flutter. The entire assemble is enclosed in a structure that has the dimensions to enable future drop tower experiments of the hot facility. All critical dimensions are the same as the original plans except for the pulley ratio which has been changed to enable a wider operating regime in terms of the Reynolds number. With the current setup, Reynolds numbers as low as 100 and as high as 14,000 can be achieved. This is because the in-between belt spacing can be varied from 1 cm to 7.6 cm, and the belt speed can be accurately varied from .15 m/sec to 3.1 m/sec.
NASA Astrophysics Data System (ADS)
Liu, Yangwei; Lu, Lipeng; Fang, Le; Gao, Feng
2011-06-01
The correlation between the velocity helicity and the energy backscatter is proved in a DNS case of 256 3-grid homogeneous isotropic decaying turbulence. The helicity is then proposed to be employed to improve turbulence models and SGS models. Then Spalart-Allmaras turbulence model (SA) is modified with the helicity to take account of the energy backscatter, which is significant in the region of corner separation in compressors. By comparing the numerical results with experiments, it can be concluded that the modification for SA model with helicity can appropriately represent the energy backscatter, and greatly improves the predictive accuracy for simulating the corner separation flow in compressors.
Laser beam propagation in atmospheric turbulence
NASA Technical Reports Server (NTRS)
Murty, S. S. R.
1979-01-01
The optical effects of atmospheric turbulence on the propagation of low power laser beams are reviewed in this paper. The optical effects are produced by the temperature fluctuations which result in fluctuations of the refractive index of air. The commonly-used models of index-of-refraction fluctuations are presented. Laser beams experience fluctuations of beam size, beam position, and intensity distribution within the beam due to refractive turbulence. Some of the observed effects are qualitatively explained by treating the turbulent atmosphere as a collection of moving gaseous lenses of various sizes. Analytical results and experimental verifications of the variance, covariance and probability distribution of intensity fluctuations in weak turbulence are presented. For stronger turbulence, a saturation of the optical scintillations is observed. The saturation of scintillations involves a progressive break-up of the beam into multiple patches; the beam loses some of its lateral coherence. Heterodyne systems operating in a turbulent atmosphere experience a loss of heterodyne signal due to the destruction of coherence.
Geometrical Monte Carlo simulation of atmospheric turbulence
NASA Astrophysics Data System (ADS)
Yuksel, Demet; Yuksel, Heba
2013-09-01
Atmospheric turbulence has a significant impact on the quality of a laser beam propagating through the atmosphere over long distances. Turbulence causes intensity scintillation and beam wander from propagation through turbulent eddies of varying sizes and refractive index. This can severely impair the operation of target designation and Free-Space Optical (FSO) communications systems. In addition, experimenting on an FSO communication system is rather tedious and difficult. The interferences of plentiful elements affect the result and cause the experimental outcomes to have bigger error variance margins than they are supposed to have. Especially when we go into the stronger turbulence regimes the simulation and analysis of the turbulence induced beams require delicate attention. We propose a new geometrical model to assess the phase shift of a laser beam propagating through turbulence. The atmosphere along the laser beam propagation path will be modeled as a spatial distribution of spherical bubbles with refractive index discontinuity calculated from a Gaussian distribution with the mean value being the index of air. For each statistical representation of the atmosphere, the path of rays will be analyzed using geometrical optics. These Monte Carlo techniques will assess the phase shift as a summation of the phases that arrive at the same point at the receiver. Accordingly, there would be dark and bright spots at the receiver that give an idea regarding the intensity pattern without having to solve the wave equation. The Monte Carlo analysis will be compared with the predictions of wave theory.
NASA Technical Reports Server (NTRS)
Cuzzi, J. N.; Hogan, R.; Dobrovolskis, A.; Paque, J.
2006-01-01
It is generally agreed that individual chondrules formed as entities in a gaseous nebula prior to being accumulated into a meteorite parent body, within which they incur various forms of modification before arriving in our labs. While there are major unanswered questions about the properties of the nebula environment in which chondrules formed, the process by which the most primitive meteorites are formed overwhelmingly from chondrules must then be an aspect of "nebula processing". Textures in certain fragments of primitive meteorites might be summarized as being primarily chondrules and clastic, chondrule-sized, fragments of other minerals, each covered with a rim of fine dust with physical and chemical properties which are essentially independent of the composition and mineralogy of the underlying chondrule. This (unfortunately rather rare) texture was called "primary accretionary texture" to reflect their belief that it precedes subsequent stages in which fragmentation, comminution, mixing, heating, and other forms of alteration occur on the parent body(-ies). The size distribution of these chondrules and fragments, and the properties of their dusty rims, are key clues regarding the primary nebula accretion process. Even in the much more abundant meteorites which have clearly suffered internal mixing, abrasion, grinding, and even mineralogical alteration or replacement (due presumably to the collisional growth and heating process itself), key chondrule properties such as mean size and density remain relatively well defined, and well defined rims persist in many cases. It has been our goal to infer the key nebula processes indirectly from the properties of these very earliest primitive meteorites by making use of a theoretical framework in which the nebula possesses a plausible level of isotropic turbulence. We have shown that turbulence has the property of concentrating one particular particle size by orders of magnitude, where the preferentially concentrated
Effect of free-stream turbulence on film cooling
NASA Technical Reports Server (NTRS)
Marek, C. J.; Tacina, R. R.
1975-01-01
Film-cooling experiments were conducted at four levels of free-stream turbulence to test the hypothesis that the film-cooling effectiveness is inversely related to the free-stream turbulence level. The hot-gas operating conditions were held constant at a temperature of 590 K, a pressure of 1 atmosphere, and a velocity of 62 m/sec. The film-cooling air was at ambient inlet temperature, and the film-cooling flow rates were 2.5, 5.0, 7.5, and 10.0 percent of the total airflow. Blockage plates with blockage areas of 0, 52, 72, and 90 percent were placed upstream of the film-cooling slot and produced axial turbulence intensities of 7, 14, 23, and 35 percent, respectively. The film-cooling effectiveness decreased as much as 50 percent as the freestream turbulence intensity was increased from 7 to 35 percent. The value of the turbulent mixing coefficient used in previous work was compared with the axial turbulence intensity. The turbulent mixing coefficient was found to be 10 to 40 percent of the axial turbulence intensity.
Investigations of turbulent motions and particle acceleration in solar flares
NASA Technical Reports Server (NTRS)
Jakimiec, J.; Fludra, A.; Lemen, J. R.; Dennis, B. R.; Sylwester, J.
1986-01-01
Investigations of X-raya spectra of solar flares show that intense random (turbulent) motions are present in hot flare plasma. Here it is argued that the turbulent motions are of great importance for flare development. They can efficiently enhance flare energy release and accelerate particles to high energies.
Wind turbulence characterization for wind energy development
NASA Astrophysics Data System (ADS)
Wendell, L. L.; Gower, G. L.; Morris, V. R.; Tomich, S. D.
1991-09-01
As part of its support of the U.S. Department of Energy's (DOE's) Federal Wind Energy Program, the Pacific Northwest Laboratory (PNL) has initiated an effort to work jointly with the wind energy community to characterize wind turbulence in a variety of complex terrains at existing or potential sites of wind turbine installation. Five turbulence characterization systems were assembled and installed at four sites in the Tehachapi Pass in California, and one in the Green Mountains near Manchester, Vermont. Data processing and analyses techniques were developed to allow observational analyses of the turbulent structure; this analysis complements the more traditional statistical and spectral analyses. Preliminary results of the observational analyses, in the rotating framework or a wind turbine blade, show that the turbulence at a site can have two major components: (1) engulfing eddies larger than the rotor, and (2) fluctuating shear due to eddies smaller than the rotor disk. Comparison of the time series depicting these quantities at two sites showed that the turbulence intensity (the commonly used descriptor of turbulence) did not adequately characterize the turbulence at these sites.
Wind turbulence characterization for wind energy development
Wendell, L.L.; Gower, G.L.; Morris, V.R.; Tomich, S.D.
1991-09-01
As part of its support of the US Department of Energy's (DOE's) Federal Wind Energy Program, the Pacific Northwest Laboratory (PNL) has initiated an effort to work jointly with the wind energy community to characterize wind turbulence in a variety of complex terrains at existing or potential sites of wind turbine installation. Five turbulence characterization systems were assembled and installed at four sites in the Tehachapi Pass in California, and one in the Green Mountains near Manchester, Vermont. Data processing and analyses techniques were developed to allow observational analyses of the turbulent structure; this analysis complements the more traditional statistical and spectral analyses. Preliminary results of the observational analyses, in the rotating framework or a wind turbine blade, show that the turbulence at a site can have two major components: (1) engulfing eddies larger than the rotor, and (2) fluctuating shear due to eddies smaller than the rotor disk. Comparison of the time series depicting these quantities at two sites showed that the turbulence intensity (the commonly used descriptor of turbulence) did not adequately characterize the turbulence at these sites. 9 refs., 10 figs.,
Numerical simulation of premixed turbulent methane combustion
Bell, John B.; Day, Marcus S.; Grcar, Joseph F.
2001-12-14
In this paper we study the behavior of a premixed turbulent methane flame in three dimensions using numerical simulation. The simulations are performed using an adaptive time-dependent low Mach number combustion algorithm based on a second-order projection formulation that conserves both species mass and total enthalpy. The species and enthalpy equations are treated using an operator-split approach that incorporates stiff integration techniques for modeling detailed chemical kinetics. The methodology also incorporates a mixture model for differential diffusion. For the simulations presented here, methane chemistry and transport are modeled using the DRM-19 (19-species, 84-reaction) mechanism derived from the GRIMech-1.2 mechanism along with its associated thermodynamics and transport databases. We consider a lean flame with equivalence ratio 0.8 for two different levels of turbulent intensity. For each case we examine the basic structure of the flame including turbulent flame speed and flame surface area. The results indicate that flame wrinkling is the dominant factor leading to the increased turbulent flame speed. Joint probability distributions are computed to establish a correlation between heat release and curvature. We also investigate the effect of turbulent flame interaction on the flame chemistry. We identify specific flame intermediates that are sensitive to turbulence and explore various correlations between these species and local flame curvature. We identify different mechanisms by which turbulence modulates the chemistry of the flame.
Statistics of Quantum Turbulence in Superfluid He
NASA Astrophysics Data System (ADS)
L'vov, V. S.; Pomyalov, A.
2016-11-01
Based on our current understanding of statistics of quantum turbulence as well as on results of intensive ongoing analytical, numerical and experimental studies, we overview here the following problems in the large-scale, space-homogeneous, steady-state turbulence of superfluid ^4 He and ^3 He: (1) energy spectra of normal and superfluid velocity components; (2) cross-correlation function of normal and superfluid velocities; (3) energy dissipation by mutual friction and viscosity; (4) energy exchange between normal and superfluid components; (5) high-order statistics and intermittency effects. The statistical properties are discussed for turbulence in different types of flows: coflow of ^4 He; turbulent ^3 He with the laminar normal fluid; pure superflow and counterflow in ^4 He.
Turbulence closure modeling near rigid boundaries
NASA Technical Reports Server (NTRS)
Durbin, Paul A.
1991-01-01
The near-wall region plays an essential role in turbulent boundary layers: it is a region of high shear; the peak rate of production and peak intensity of turbulence occurs there; and the peak rate of dissipation occurs right at the wall. Nevertheless, this region has received less attention from modelers than have more nearly homogeneous flows. One reason for this is that when the boundary layer is near equilibrium, experimental data can be used to prescribe the flow in the wall layer. Another reason is that most turbulence models are developed under assumptions of near homogeneity. This is a poor approximation in the wall region. A single-point moment closure model for the strongly non-homogeneous A turbulent flow near a rigid boundary is developed.
NASA Technical Reports Server (NTRS)
Potter, J. Leith; Barnett, R. Joel; Fisher, Carl E.; Koukousakis, Costas E.
1986-01-01
Experiments were conducted to determine if free-stream turbulence scale affects separation of turbulent boundary layers. In consideration of possible interrelation between scale and intensity of turbulence, the latter characteristic also was varied and its role was evaluated. Flow over a 2-dimensional airfoil in a subsonic wind tunnel was studied with the aid of hot-wire anemometry, liquid-film flow visualization, a Preston tube, and static pressure measurements. Profiles of velocity, relative turbulence intensity, and integral scale in the boundary layer were measured. Detachment boundary was determined for various angles of attack and free-stream turbulence. The free-stream turbulence intensity and scale were found to spread into the entire turbulent boundary layer, but the effect decreased as the airfoil surface was approached. When the changes in stream turbulence were such that the boundary layer velocity profiles were unchanged, detachment location was not significantly affected by the variations of intensity and scale. Pressure distribution remained the key factor in determining detachment location.
Interaction of unsteady, turbulent vortical structures with a turbulent boundary layer
NASA Astrophysics Data System (ADS)
Kothmann, Bruce D.; Pauley, Wayne R.
1992-01-01
The interaction of unsteady, turbulent, spanwise vortices with a turbulent boundary layer has been studied using smoke-wire photographs and time-resolved hot-wire measurements. Inner-variable scaling of the ensemble-averaged instantaneous velocity and turbulence intensity profiles was found to be meaningful at all times during the interaction. However, predictions of changes in wall shear stress using the logarithmic law of the wall, with the usual constants, were not reliable. A detailed account of the experimental procedure and data reduction schemes is given, in addition to comments on the use of ensemble-averaging for the analysis of low-speed unsteady flows.
Abdo, A. A.
2011-08-19
We report on the first Fermi Large Area Telescope (LAT) measurements of the so-called 'extra-galactic' diffuse {gamma}-ray emission (EGB). This component of the diffuse {gamma}-ray emission is generally considered to have an isotropic or nearly isotropic distribution on the sky with diverse contributions discussed in the literature. The derivation of the EGB is based on detailed modelling of the bright foreground diffuse Galactic {gamma}-ray emission (DGE), the detected LAT sources and the solar {gamma}-ray emission. We find the spectrum of the EGB is consistent with a power law with differential spectral index {gamma} = 2.41 {+-} 0.05 and intensity, I(> 100 MeV) = (1.03 {+-} 0.17) x 10{sup -5} cm{sup -2} s{sup -1} sr{sup -1}, where the error is systematics dominated. Our EGB spectrum is featureless, less intense, and softer than that derived from EGRET data.
Beyond the Maltese Cross: Geometry of Turbulence Between 0.2 and 1 au
NASA Astrophysics Data System (ADS)
Verdini, Andrea; Grappin, Roland
2016-11-01
The spectral anisotropy of turbulent structures has been measured in the solar wind since 1990, relying on the assumption of axisymmetry about the mean magnetic field, B 0. However, several works indicate that this hypothesis might be partially wrong, thus raising two questions: (i) is it correct to interpret measurements at 1 au (the so-called Maltese cross) in term of a sum of slab and two-dimensional (2D) turbulence; and (ii) what information is really contained in the Maltese cross? We solve direct numerical simulations of the magnetohydrodynamic equations including the transverse stretching exerted by the solar wind flow and study the genuine 3D anisotropy of turbulence as well as that one resulting from the assumption of axisymmetry about B 0. We show that the evolution of the turbulent spectrum from 0.2 to 1 au depends strongly on its initial anisotropy. An axisymmetric spectrum with respect to B 0 keeps its axisymmetry, i.e., resists stretching perpendicular to radial, while an isotropic spectrum becomes essentially axisymmetric with respect to the radial direction. We conclude that close to the Sun, slow-wind turbulence has a spectrum that is axisymmetric around B 0 and the measured 2D component at 1 au describes the real shape of turbulent structures. In contrast, fast-wind turbulence has a more isotropic spectrum at the source and becomes radially symmetric at 1 au. Such structure is hidden by the symmetrization applied to the data that instead returns a slab geometry.
Investigation of subgrid models in homogeneous incompressible turbulence
NASA Astrophysics Data System (ADS)
Teissedre, C.
1987-08-01
A data base of simulated homogeneous, incompressible turbulence in an anisotropic regime was derived using a direct simulation code on a parallel processing computer. The simulated distributions were used to validate subgrid models of the turbulent viscosity and similitude type (analogy between the near field of the cut-off and the subgrid field). The first type of model accounts for the evolution of turbulent kinetic energy well, while the second type, although it better represents the exact value of stress in the subgrid, seems to present a defect of nondissipation. Tests of a model of perturbation of nonlinear terms were performed in an isotropic situation with large structures. The results show the same kind of nondissipative behavior as for the similitude model.
Decay of Finite Temperature Superfluid Helium-4 Turbulence
NASA Astrophysics Data System (ADS)
Kivotides, Demosthenes
2015-10-01
A mesoscopic model of superfluid helium-4, that describes the dynamics of individual topological defects of the ground state (superfluid vortices) and their (self-consistent) interactions with its quasi-particle excitations (normal-fluid), is solved numerically in order to analyse the physics of decaying homogeneous, isotropic turbulence. The calculations predict several temporal decay regimes not present in classical turbulence decay, the corresponding superfluid and normal-fluid energy spectra, and the experimentally observed scaling for the superfluid vortex line density at large times. The results demonstrate that the origin of this scaling is the energy spent by the superfluid in order to sustain a fluctuating low Reynolds number flow in the normal-fluid, and not the locking of turbulent superfluid and normal-fluid vorticities.
The structure of the vorticity field in homogeneous turbulent flows
NASA Technical Reports Server (NTRS)
Rogers, Michael M.; Moin, Parviz
1987-01-01
The structures of the vorticity fields in several homogeneous irrotational straining flows and a homogeneous turbulent shear flow were examined using a database generated by direct numerical simulation of the unsteady Navier-Stokes equations. In all cases, strong evidence was found for the presence of coherent vortical structures. The initially isotropic vorticity fields were rapidly affected by imposed mean strain and the rotational component of mean shear and developed accordingly. In the homogeneous turbulent shear-flow cases, the roll-up of mean vorticity into characteristic hairpin vortices was clearly observed, supporting the view that hairpin vortices are an important vortical structure in all turbulent shear flows; the absence of mean shear in the homogeneous irrotational straining flows precludes the presence of hairpin-like vortices.
Direct numerical simulation and analysis of shock turbulence interaction
NASA Technical Reports Server (NTRS)
Lee, Sangsan; Lele, Sanjiva K.; Moin, Parviz
1991-01-01
Two kinds of linear analysis, rapid distortion theory (RDT) and linear interaction analysis (LIA), were used to investigate the effects of a shock wave on turbulence. Direct numerical simulations of two-dimensional isotropic turbulence interaction with a normal shock were also performed. The results from RDT and LIA are in good agreement for weak shock waves, where the effects of shock front curvature and shock front unsteadiness are not significant in producing vorticity. The linear analyses predict wavenumber-dependent amplification of the upstream one-dimensional energy spectrum, leading to turbulence scale length scale decrease through the interaction. Instantaneous vorticity fields show that vortical structures are enhanced while they are compressed in the shock normal direction. Entrophy amplfication through the shock wave compares favorably with the results of linear analyses.
An alternative to Reynolds stresses in turbulent channels
NASA Astrophysics Data System (ADS)
Jimenez, Javier
2016-11-01
It is remarked that fluxes in conservation laws, such as the Reynolds stresses in the momentum equation of turbulent shear flows, or the spectral energy flux in isotropic turbulence, are only defined up to an arbitrary solenoidal field. While this is not usually significant for long-time averages, it becomes important when fluxes are modelled locally in large-eddy simulations, or in the analysis of intermittency and cascades. As an example, a numerical procedure is introduced to compute fluxes in scalar conservation equations in such a way that their total integrated magnitude is minimised. The result is an irrotational vector field that derives from a potential, thus minimising sterile flux 'circuits'. The algorithm is generalised to tensor fluxes and applied to the transfer of momentum in a turbulent channel. The resulting instantaneous Reynolds stresses are compared with their traditional expressions, and found to be substantially different. Funded by the Coturb project of the ERC.
Diffusion of Sound Waves in a Turbulent Atmosphere
NASA Technical Reports Server (NTRS)
Lyon, Richard H.
1960-01-01
The directional and frequency diffusion of a plane monochromatic 2 sound wave in statistically homogeneous, isotropic, and stationary turbulence is analyzed theoretically. The treatment is based on the diffusion equation for the energy density of sound waves, using the scattering cross section derived by Kraichnan for the type of turbulence assumed here. A form for the frequency-wave number spectrum of the turbulence is adopted which contains the pertinent parameters of the flow and is adapted to ease of calculation. A new approach to the evaluation of the characteristic period of the flow is suggested. This spectrum is then related to the scattering cross section. Finally, a diffusion equation is derived as a small-angle scattering approximation to the rigorous transport equation. The rate of spread of the incident wave in frequency and direction is calculated, as well as the power spectrum and autocorrelation for the wave.
An integral turbulent kinetic energy analysis of free shear flows
NASA Technical Reports Server (NTRS)
Peters, C. E.; Phares, W. J.
1973-01-01
Mixing of coaxial streams is analyzed by application of integral techniques. An integrated turbulent kinetic energy (TKE) equation is solved simultaneously with the integral equations for the mean flow. Normalized TKE profile shapes are obtained from incompressible jet and shear layer experiments and are assumed to be applicable to all free turbulent flows. The shear stress at the midpoint of the mixing zone is assumed to be directly proportional to the local TKE, and dissipation is treated with a generalization of the model developed for isotropic turbulence. Although the analysis was developed for ducted flows, constant-pressure flows were approximated with the duct much larger than the jet. The axisymmetric flows under consideration were predicted with reasonable accuracy. Fairly good results were also obtained for the fully developed two-dimensional shear layers, which were computed as thin layers at the boundary of a large circular jet.
Wavefront sensing for anisotropic turbulence using digital holography
NASA Astrophysics Data System (ADS)
Thurman, Samuel T.; Gatt, Philip; Alley, Thomas
2016-09-01
We report on digital holographic imaging through atmospheric turbulence. Data recorded with aberrations is corrected during post processing using an iterative sharpness-metric maximization algorithm. Assuming the correction cancels the actual wavefront error, this process is equivalent to wavefront sensing. Much of our past work focused on imaging through isotropic turbulence with phase corrections using a Zernike-polynomial expansion. Here, we describe algorithm modifications for imaging through anisotropic turbulence, similar to what is seen when looking through the aero-optic boundary layer surrounding a moving aircraft. Specifically, we explore tradeoffs associated with switching from a Zernike representation to Karhunen-Loève basis functions. In some cases, the dimensionality of the phase correction estimation algorithm can be reduced significantly by this change. This reduces the computational burden
Large-scale Cosmic-Ray Anisotropy as a Probe of Interstellar Turbulence
NASA Astrophysics Data System (ADS)
Giacinti, Gwenael; Kirk, John G.
2017-02-01
We calculate the large-scale cosmic-ray (CR) anisotropies predicted for a range of Goldreich–Sridhar (GS) and isotropic models of interstellar turbulence, and compare them with IceTop data. In general, the predicted CR anisotropy is not a pure dipole; the cold spots reported at 400 TeV and 2 PeV are consistent with a GS model that contains a smooth deficit of parallel-propagating waves and a broad resonance function, though some other possibilities cannot, as yet, be ruled out. In particular, isotropic fast magnetosonic wave turbulence can match the observations at high energy, but cannot accommodate an energy dependence in the shape of the CR anisotropy. Our findings suggest that improved data on the large-scale CR anisotropy could provide a valuable probe of the properties—notably the power-spectrum—of the interstellar turbulence within a few tens of parsecs from Earth.
Local isotropy in buoyancy-generated turbulence
NASA Astrophysics Data System (ADS)
Chasnov, Jeffrey R.
1991-12-01
Batchelor et al. (1992) recently considered the turbulent motion generated by buoyancy forces acting on random fluctuations in the density of an infinite fluid. This homogeneous buoyancy-generated flow field with zero mean density gradient was conceived as an idealized system which, like isotropic turbulence, may be useful as a vehicle for the general study of turbulence. The Batchelor et al. study relied partly on theoretical analysis and partly on direct and large-eddy numerical simulations of the flow field. To this mix, we add here a two-point closure study based on the eddy-damped quasi-normal Markovian (EDQNM) closure model applied to axisymmetric turbulence. The EDQNM model has been shown to yield reasonably accurate quantitative results for a variety of problems in homogeneous turbulence (Lesieur 1987). The main advantage here in applying EDQNM to the buoyancy-driven flow field is the wide range of wavenumbers over which a solution of the EDQNM equations may be solved. Whereas a typical large-eddy simulation using 128(exp 3) grid points has a wavenumber range of only 60, the EDQNM calculation can be easily run with a wavenumber range of several decades. Because of the growth in length scales in the buoyancy-driven flow field, this large wavenumber range allows for a solution of the flow field well into its asymptotic regime. Recent comparisons between large-eddy simulations and closure theory (Herring 1990) indicate that a time longer than that attainable by current large-eddy simulations is required to reach flow asymptotics and that conclusions based on large-eddy simulation results may be based only on an intermediate transient state. We briefly introduce the EDQNM equations for the buoyancy-generated flow field. We then present a Kolmogorov-like theoretical argument on the scaling of the small-scale spectra. This scaling is then confirmed by numerical solution of the EDQNM equations. We briefly conclude with possible future research directions.
Line segments in homogeneous scalar turbulence
NASA Astrophysics Data System (ADS)
Gauding, Michael; Goebbert, Jens Henrik; Hasse, Christian; Peters, Norbert
2015-09-01
The local structure of a turbulent scalar field in homogeneous isotropic turbulence is analyzed by direct numerical simulations (DNS) with different Taylor micro-scale based Reynolds numbers between 119 and 529. A novel signal decomposition approach is introduced where the signal of the scalar along a straight line is partitioned into segments based on the local extremal points of the scalar field. These segments are then parameterized by the distance ℓ between adjacent extremal points and the scalar difference Δϕ at the extrema. Both variables are statistical quantities and a joint distribution function of these quantities contains most information to statistically describe the scalar field. It is highlighted that the marginal distribution function of the length becomes independent of Reynolds number when normalized by the mean length ℓm. From a statistical approach, it is further shown that the mean length scales with the Kolmogorov length, which is also confirmed by DNS. For turbulent mixing, the scalar gradient plays a paramount role. Turbulent scalar fields are characterized by cliff-ramp-like structures manifesting the occurrence of localized large scalar gradients. To study turbulent mixing, a segment-based gradient is defined as Δϕ/ℓ. Joint statistics of the length and the segment-based gradient provide novel understanding of cliff-ramp-like structures. Ramp-like structures are unveiled by the asymmetry of the joint distribution function of the segment-based gradient and the length. Cliff-like structures are further analyzed by conditional statistics and it is shown from DNS that the width of cliffs scales with the Kolmogorov length scale.