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1

Boundary-layer friction in midlatitude cyclones  

Microsoft Academic Search

SUMMARY Results from an idealized three-dimensional baroclinic life-cycle model are interpreted ina potential vorticity (PV) framework to identify the physical mechanisms by which frictional processes acting in the atmospheric boundary layer modify and reduce the baroclinic development of a midlatitude storm. Considering a life cycle where theonly non-conservative process acting is boundary-layer friction, therate of change of depth-averaged PV within

D. S. Adamson; S. E. Belcher; B. J. Hoskins; R. S. Plant

2006-01-01

2

Skin friction in zero-pressure-gradient boundary layers.  

PubMed

A global approach leading to a self-consistent solution to the Navier-Stokes-Prandtl equations for zero-pressure-gradient boundary layers is presented. It is shown that as Re(?)? ?, the dynamically defined boundary layer thickness ?(x) ? x/ln2 ?Rex and the skin friction ? = 2?(w)/?U(0)(2) ? 1/ln2 ??(x). Here ?(w) and U0 are the wall shear stress and free stream velocity, respectively. The theory is formulated as an expansion in powers of a small dimensionless parameter d?(x)/dx ? 0 in the limit x ? ?. PMID:21230338

Yakhot, Victor

2010-10-04

3

Skin friction in zero-pressure-gradient boundary layers  

NASA Astrophysics Data System (ADS)

A global approach leading to a self-consistent solution to the Navier-Stokes-Prandtl equations for zero-pressure-gradient boundary layers is presented. It is shown that as Re??? , the dynamically defined boundary layer thickness ?(x)?x/ln2Rex and the skin friction ?=(2?w)/(?U02)?1/ln2?(x) . Here ?w and U0 are the wall shear stress and free stream velocity, respectively. The theory is formulated as an expansion in powers of a small dimensionless parameter (d?(x))/(dx)?0 in the limit x?? .

Yakhot, Victor

2010-10-01

4

Skin friction measurements following manipulation of a turbulent boundary layer  

Microsoft Academic Search

Results are presented from three experiments in which direct, local measurements of the skin friction reduction due to flat-plate turbulence manipulators for overall viscous drag reduction were obtained. The results suggest that only a very small overall net drag reduction will be possible for such devices at moderate momentum thickness-derived Reynolds number values, since maximum skin friction drag is neither

V. D. Nguyen; A. M. Savill; R. V. Westphal

1987-01-01

5

A new method of calculating the boundary layer characteristics downstream of manipulators. I - Boundary layer integral parameters. II - Skin friction and net drag reduction  

Microsoft Academic Search

In the present method for the calculation of boundary layer characteristics downstream of flow manipulators, the wake-superposition model employed furnishes an effective and simple technique for the derivation of such boundary layer integral parameters as the local skin friction coefficient, the momentum thickness, and the form factor. Mean velocity profiles in excellent agreement with experiment are thereby obtained. The second

P. E. Roach

1987-01-01

6

Friction law and turbulent properties in a laboratory Ekman boundary layer  

NASA Astrophysics Data System (ADS)

We use spin-up/spin-down laboratory experiments to study the neutrally stratified Ekman boundary layer. The experiments are performed in the 13 m diameter, 1 m deep Coriolis rotating tank of the LEGI in Grenoble, France. A global flow rotation is produced by an initial change in the tank rotation speed. It then slowly decays under the effect of Ekman friction, evolving from the turbulent state to the laminar state. It is checked that the Ekman layer itself remains in a quasi-steady state during this decay. The velocity is measured by Particle Imaging Velocimetry (PIV) at two scales: the global rotation in a horizontal plane, and the vertical profile inside the boundary layer, where the three velocity components are obtained by stereoscopic PIV. The friction law is obtained by relating the decay rate of the bulk velocity to the velocity itself. This method is justified by the fact that this bulk velocity is independent of height beyond the top of the boundary layer (a few cm), as expected from the Taylor-Proudman theorem for rotating fluids. The local measurements inside the boundary layer provide profiles of the mean velocity and Reynolds stress components, in particular the cross-isobar angle between the interior and near surface velocities. In the laminar regime, good agreement is obtained with the classical Ekman's theory, which validates the method. In the turbulent regime, the results are found consistent with the classical Atmospheric Boundary Layer (ABL) model based on the von Karman logarithmic layer. Our experiments therefore indicate that this theory, in principle valid for very large Reynolds numbers, is already relevant close to the transitional regimes. A fit of the empirical coefficients A and B appearing in this theory yields A = 3.3 and B = 3.0. Extrapolating the results to the atmospheric case gives a friction velocity u* about 12% higher than the traditional fit for the ABL. We may safely deduce that for the oceanic bottom boundary layer, corresponding to lower Reynolds numbers than the atmosphere, our result provides a correct estimate within 10%. The previous laboratory results of Caldwell et al. [``A laboratory study of the turbulent Ekman layer,'' Geophys. Fluid Dyn. 3, 125-160 (1972)] provided frictions velocities about 20% higher than in our experiments, and slightly higher cross-isobar angles. We attribute this difference to the higher vortical Rossby number Rot in those experiments, and maybe also to roughness effects. We take into account the effect of this vortical Rossby number within the framework of the Ekman layer (Rot --> 0) by replacing the tank rotation rate by the fluid rotation rate.

Sous, D.; Sommeria, J.; Boyer, D.

2013-04-01

7

Mean flow in turbulent boundary layers disturbed to alter skin friction  

Microsoft Academic Search

A comparative evaluation is made of recent developments in methods for the reduction of boundary layer drag, encompassing longitudinal surface riblets, 'outer layer' devices, (OLDs) and longitudinal convex surface curvature. The boundary layer of a surface with a longitudinal concave curvature is also studied, to complement the convex case results. The net drag reductions achievable by both riblets and OLDs

P. R. Bandyopadhyay

1986-01-01

8

Effect of local heat supply to a turbulent boundary layer on the friction  

Microsoft Academic Search

The results of calculating a supersonic turbulent boundary layer on a flat plate in the presence of thermal energy supply\\u000a to the boundary layer are presented. Two methods of energy supply are considered: heating a local interval of the surface,\\u000a which is otherwise thermally insulated and using a local volume heat source. It is shown that for the same amount

A. V. Kazakov; M. N. Kogan; A. P. Kuryachii

1997-01-01

9

Skin friction Reduction by Introduction of Micro-bubbles into Turbulent Boundary Layer  

Microsoft Academic Search

The phenomenon of drag reduction by the injection of micro- bubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Two variants namely the Inhomogeneous and MUSIG (MUltiple SIze Group) based on Population balance models are investigated. The simulated results are compared against the experimental findings of Madavan et al (1). The model employed in the investigation

K. Mohanarangam; C. P. Cheung

10

Skin-friction drag reduction in laminar and turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The skin-friction drag in a constant, mass-flux plane channel flow is sustained below that corresponding to laminar flow when wall-normal blowing and suction at the upper and lower walls are applied as upstream traveling waves. The control is distributed such that the net mass-flux of the channel is not modified. Direct numerical simulations demonstrate that these upstream traveling waves can induce sublaminar viscous drag in fully developed laminar and turbulent flows. Furthermore, it is found that the observed phenomena can be characterized by the linearized governing equations. The Navier-Stokes equations can be linearized by describing the flow velocities as perturbations about a mean, resulting in equations that describe the dynamics of the perturbations. A spectral decomposition, involving a two-dimensional Fourier expansion in the streamwise and spanwise directions and a Galerkin projection of Chebyshev polynomials in the wall-normal direction, is used to spatially discretize these equations, leading to a state space model. The traveling wave control is then introduced as a temporally, phase-shifting, wall-normal velocity wall boundary condition at a particular Fourier wavenumber. This linear model was used to develop reduced-order linear-quadratic-Gaussian (LQG) controllers. While these controls could achieve significant drag reduction, they appeared to be a performance limit. However, they could induce transient sublaminar drag under certain conditions. A nonlinear minimization using full-order nonlinear simulations was attempted to capture this transient behavior on a periodic basis. The upstream traveling wave was discovered in the course of that study. The same linear models can represent the sustained sublaminar drag. Using a recent formulation which expresses the viscous drag of a fully developed channel flow as a sum of laminar drag and the Reynolds shear stress, it is shown that the Reynolds shear stress calculated from the controlled linear model state provides good predictions of how the nonlinear flow will respond. This provides a computationally efficient environment in which to find the optimal amplitude and speed to set a traveling wave to gain the most predicted drag reduction for a specific level of input power.

Kang, Sung Moon

11

Effect of Free-Stream Turbulence on the Reduction of Surface Friction in a Turbulent Boundary Layer Behind LEBU-Devices  

Microsoft Academic Search

The effect of increased free-stream turbulence on the reduction of the surface friction coefficient c\\u000a \\u000a f\\u000a in a turbulent boundary layer behind large-eddy break-up (LEBU) devices is investigated using a gravimetric method. The turbulence\\u000a level was ? ? 1.9–4.9 % and the turbulence scale L\\u000a \\u000a e\\u000a ? 40–110 mm. The boundary layer Reynolds number Re** was varied from 2300 to

N. V. Samoilova; V. G. Shumilkin

2005-01-01

12

Effect of Boundary Layer Thickness and Entropy Layer on Boundary Layer Combustion  

Microsoft Academic Search

This project investigates the possibilities of scramjet combustor performance enhancement by reducing the skin friction through boundary layer combustion. Experiments were conducted in the T4 Stalker tube to investigate the influence of boundary layer thickness and entropy layers on the ignition of a hydrogen air mixture near the wall of a constant area duct. The hydrogen was injected tangentially from

R. M. Kirchhartz; D. J. Mee; R. J. Stalker

13

Semimetallic Brake Friction Materials Containing ZrSiO4: Friction Performance and Friction Layers Evaluation  

Microsoft Academic Search

The effect of ZrSiO4 (zircon) content on friction performance and friction surfaces of semimetallic brake friction materials is discussed. The experimental results indicate that the varying content of zircon affects the friction performance as well as plays crucial role in the iron film formation on the friction surfaces. The friction layers, formed during friction process, were carefully characterized using scanning

Vlastimil Mat?jka; Gražyna Simha Martynková; Yuning Ma; Yafei Lu

2009-01-01

14

Boundary Layer Simulator Improvement.  

National Technical Information Service (NTIS)

Boundary Layer Integral Matrix Procedure (BLIMPJ) has been identified by the propulsion community as the rigorous boundary layer program in connection with the existing JANNAF reference programs. The improvements made to BLIMPJ and described herein have p...

S. C. Praharaj C. P. Schmitz J. A. Nouri

1989-01-01

15

Statistical theory of the boundary friction of atomically flat solid surfaces in the presence of a lubricant layer  

NASA Astrophysics Data System (ADS)

A rheological model and a thermodynamic model are proposed for describing the melting of an ultrathin lubricant film between atomically flat solid surfaces. Hysteresis phenomena are considered, allowing for the stress and strain dependence of the lubricant shear modulus. The self-similar regime of lubricant melting is studied taking the additive noncorrelated noise of basic parameters into account. The regions of dry, sliding, and stick-slip friction are determined in the phase diagram. Shear stress time series are obtained by numerically analyzing the Langevin equation and are then subjected to multifractal fluctuation analysis. The dependence of the stationary friction force on the lubricant temperature and on the shear velocity of rubbing surfaces is investigated.

Khomenko, Alexei V.; Lyashenko, I. A.

2012-10-01

16

Boundary layer transition studies  

Microsoft Academic Search

A small-scale wind tunnel previously used for turbulent boundary layer experiments was modified for two sets of boundary layer transition studies. The first study concerns a laminar separation\\/turbulent reattachment. The pressure gradient and unit Reynolds number are the same as the fully turbulent flow of Spalart and Watmuff. Without the trip wire, a laminar layer asymptotes to a Falkner &

Jonathan H. Watmuff

1995-01-01

17

The atmospheric boundary layer  

Microsoft Academic Search

In this book, the author successfully reviews the current state of affairs in boundary-layer meteorology research. The book is organized into nine chapters. The first chapter is an introduction to the topic of the atmospheric boundary layer. The second chapter is a survey of turbulence theory. The third chapter reviews the similarity relationships that have been formulated for the various

J. R. Garratt

1992-01-01

18

The Atmospheric Boundary Layer  

ERIC Educational Resources Information Center

|Discusses some important parameters of the boundary layer and effects of turbulence on the circulation and energy dissipation of the atmosphere. Indicates that boundary-layer research plays an important role in long-term forecasting and the study of air-pollution meteorology. (CC)|

Tennekes, Hendrik

1974-01-01

19

Boundary layer transition  

Microsoft Academic Search

The boundary layer stability, its active control by sound and surface heating and the effect of curvature are studied numerically and experimentally for subsonic flow. In addition, the experimental and flight test data are correlated using the stability theory for supersonic Mach numbers. Active transition fixing and feedback control of boundary layer by sound interactions are experimentally investigated at low

L. Maestrello; A. Bayliss; S. M. Mangalam; M. R. Malik

1986-01-01

20

The Atmospheric Boundary Layer  

ERIC Educational Resources Information Center

Discusses some important parameters of the boundary layer and effects of turbulence on the circulation and energy dissipation of the atmosphere. Indicates that boundary-layer research plays an important role in long-term forecasting and the study of air-pollution meteorology. (CC)

Tennekes, Hendrik

1974-01-01

21

Stability and drag reduction in a boundary layer with microbubbles  

Microsoft Academic Search

When microbubbles are injected in a turbulent boundary layer, they promote a reduction of the local skin friction and a sharp mean velocity gradient between the two microbubble free regions in the boundary layer, i.e., the outer edge and the near wall region. In connection with the apparent stability of the bubbles in the boundary layer, the stability of a

Joao M. Neves

1988-01-01

22

Friction microprobe investigation of particle layer effects on sliding friction  

SciTech Connect

Interfacial particles (third-bodies), resulting from wear or external contamination, can alter and even dominate the frictional behavior of solid-solid sliding in the absence of effective particle removal processes (e.g., lubricant flow). A unique friction microprobe, developed at Oak Ridge National Laboratory, was used to conduct fine- scale friction studies using 1.0 mm diameter stainless steel spheres sliding on several sizes of loose layers of fine aluminum oxide powders on both aluminum and alumina surfaces. Conventional, pin-on-disk experiments were conducted to compare behavior with the friction microprobe results. The behavior of the relatively thick particle layers was found to be independent of the nature of underlying substrate, substantiating previous work by other investigators. The time-dependent behavior of friction, for a spherical macrocontact starting from rest, could generally be represented by a series of five rather distinct phases involving static compression, slider breakaway, transition to steady state, and dynamic layer instability. A friction model for the steady state condition, which incorporates lamellar powder layer behavior, is described.

Blau, P.J.

1993-01-01

23

Boundary layer relaxation from convex curvature  

Microsoft Academic Search

Convex curvature has promise as a means of viscous drag reduction because it has been shown to result in a decrease in skin friction. In order to improve the understanding of curvature effects and their potential for drag reduction, an examination was made of the flat plate relaxation behavior of a turbulent boundary layer recovering from 90 deg of strong

Amy Elsa Alving

1988-01-01

24

Boundary layer receptivity - Theory  

NASA Astrophysics Data System (ADS)

The receptivity mechanisms by which free-stream disturbances generate instability waves in laminar boundary layers are discussed. Free-stream disturbances have wavelengths which are generally much longer than those of instability waves. Hence, the transfer of energy from the free-stream disturbance to the instability wave requires a wavelength conversion mechanism. Recent analyses using asymptotic methods have shown that the wavelength conversion takes place in regions of the boundary layer where the mean flow adjusts on a short streamwise length scale. This paper reviews recent progress in the theoretical understanding of these phenomena.

Kerschen, E. J.

25

The effects of spatially complex inner shelf roughness on boundary layer turbulence and current and wave friction: Tairua embayment, New Zealand  

NASA Astrophysics Data System (ADS)

A field study of bedforms, associated hydraulic roughness, and turbulence was conducted on the inner shelf off the east coast of New Zealand's North Island under conditions that included two significant storm events. Sharply contrasting rough and smooth beds were characterized via field mapping and deployment of instrumented benthic tripods. Rough areas of coarse sand exhibited ripples with heights and lengths of ˜25 and ˜100 cm, while smooth areas supported smaller ripples with heights and lengths of ˜5 cm and ˜20 cm. Contacts between the two surfaces were sharp and maintained their position. Roughness contrasts were enhanced significantly during storms, which simultaneously accentuated migrating orbital ripples over the coarse bed and replaced ripples on the fine sediment bed with smoother hummocky features. Spectra of the fluctuating vertical velocity components, w', from both smooth and rough sites showed good fits to -5/3 slopes within the inertial sub range enabling independent estimates of wave-averaged bed stress to be made via the inertial dissipation method (IDM). We also utilized the vertical fluctuation data to obtain alternative estimates of the wave friction factor, fw, following Smyth and Hay (J. Phys. Oceanography 32 (2002) 3490); SH. These two methods yielded generally similar results. Under high wave conditions, fw estimated via IDM averaged 0.027 at the rough site and 0.0045 at the smooth site while the SH method gave respective values of 0.027 and 0.013. Under low-energy conditions, fw from IDM averaged 0.0082 at the rough site and 0.012 at the smooth site, while the SH method yielded mean values of 0.0080 and 0.016. Thus, fw was much larger at the rough site than at the smooth site during storms but smaller at the rough site during fair weather. During storms, structured vortices with frequencies at the first harmonic of the swell waves formed over the rough surface and penetrated above the wave current boundary layer causing retardation of mean currents. Such storm-induced vortices were only intermittently present over the smooth surface. The application of Nielsen's (J. Geophys. Res. 86 (1981) 6467) roughness model produced some qualitatively similar trends in fw, although predicted fw was larger than observed values at the rough site. Additionally, low modelled fw over the smooth bed during high energy was based on plane bed theory rather than the inferred hummocky bed.

Trembanis, A. C.; Wright, L. D.; Friedrichs, C. T.; Green, M. O.; Hume, T.

2004-08-01

26

Plasma sheet boundary layer  

Microsoft Academic Search

The plasma sheet boundary layer is a temporally variable transition region located between the magnetotail lobes and the central plasma sheet. We have made a survey of these regions by using particle spectra and three-dimensional velocity-space distributions sampled by the ISEE 1 LEPEDEA. Ion composition measurements obtained by the Lockhead ion mass spectrometers indicate that ionospheric ions play a crucial

T. E. Eastman; L. A. Frank; W.K. Peterson; W. Lennartsson

1984-01-01

27

Viscous drag reduction in boundary layers  

Microsoft Academic Search

The present volume discusses the development status of stability theory for laminar flow control design, applied aspects of laminar-flow technology, transition delays using compliant walls, the application of CFD to skin friction drag-reduction, active-wave control of boundary-layer transitions, and such passive turbulent-drag reduction methods as outer-layer manipulators and complex-curvature concepts. Also treated are such active turbulent drag-reduction technique applications as

Dennis M. Bushnell; Jerry N. Hefner

1990-01-01

28

Laminar vortex boundary layers  

NASA Astrophysics Data System (ADS)

The interaction of a Burgers--Rott vortex with a rigid no-slip normal wall is investigated via direct numerical simulations of the unsteady axisymmetric Navier--Stokes equations. The flows in the boundary layer and away from the vortex core have a self-similar structure, i.e. the solutions at time t, radius r, height z, and Reynolds number Re can be reduced to single profiles for the angular momentum and the azimuthal vorticity dependent on a single similarity variable. The similarity variable is the direction normal to the wall scaled by Re^1/2 and a function of r and t. The boundary layer flow near the axis for low-Re consists of a matching between a Bödewadt-like flow near r=0, where the vortex flow is near solid-body rotation, and a potential vortex boundary layer flow. For medium Re, waves form within the core radius resulting from the inflection points in the Bödewadt-like profiles. At large Re, there are also waves that travel vertically along the interface between the rotational core and the irrotational flow outside the core at r? 1.

Arrese, Juan C.; Lopez, John M.

1996-11-01

29

Microbubble Drag Reduction in Liquid Turbulent Boundary Layers  

Microsoft Academic Search

The interactions between a dense cloud of small bubbles and a liquid turbulent boundary layer are reviewed on the basis of available experimental observations to understand and quantify their capability for reducing skin friction. Gas bubbles are generally introduced into the boundary layer by injection through a porous surface or by electrolysis. After injection, the bubbles stay near the wall

Charles L. Merkle; Steven Deutsch

1992-01-01

30

Boundary layer transition studies  

NASA Astrophysics Data System (ADS)

A small-scale wind tunnel previously used for turbulent boundary layer experiments was modified for two sets of boundary layer transition studies. The first study concerns a laminar separation/turbulent reattachment. The pressure gradient and unit Reynolds number are the same as the fully turbulent flow of Spalart and Watmuff. Without the trip wire, a laminar layer asymptotes to a Falkner & Skan similarity solution in the FPG. Application of the APG causes the layer to separate and a highly turbulent and approximately 2D mean flow reattachment occurs downstream. In an effort to gain some physical insight into the flow processes a small impulsive disturbance was introduced at the C(sub p) minimum. The facility is totally automated and phase-averaged data are measured on a point-by-point basis using unprecedently large grids. The evolution of the disturbance has been tracked all the way into the reattachment region and beyond into the fully turbulent boundary layer. At first, the amplitude decays exponentially with streamwise distance in the APG region, where the layer remains attached, i.e. the layer is viscously stable. After separation, the rate of decay slows, and a point of minimum amplitude is reached where the contours of the wave packet exhibit dispersive characteristics. From this point, exponential growth of the amplitude of the disturbance is observed in the detached shear layer, i.e. the dominant instability mechanism is inviscid. A group of large-scale 3D vortex loops emerges in the vicinity of the reattachment. Remarkably, the second loop retains its identify far downstream in the turbulent boundary layer. The results provide a level of detail usually associated with CFD. Substantial modifications were made to the facility for the second study concerning disturbances generated by Suction Holes for laminar flow Control (LFC). The test section incorporates suction through interchangeable porous test surfaces. Detailed studies have been made using isolated holes in the impervious test plate that used to establish the Blasius base flow. The suction is perturbed harmonically and data are averaged on the basis of the phase of the disturbance, for conditions corresponding to strong suction and without suction.

Watmuff, Jonathan H.

1995-02-01

31

Magnetohydrodynamic boundary layer control system  

NASA Astrophysics Data System (ADS)

An active boundary layer control system for marine vehicles is disclosed. The boundary layer control system comprises a plurality of magnets and seawater electrodes placed in circumferential rows around the beam of the hull. The magnets and electrodes are positioned so that a Lorentz force generated by the interacting magnetic and electric fields will drive the boundary layer flow in an axial direction toward the aft end of the bull. The boundary control system reduces turbulence and may relaminarize boundary layer flow, thereby reducing radiated noise.

Meng, James C.

1993-12-01

32

Thermal activation in boundary lubricated friction  

Microsoft Academic Search

The friction coefficients for copper pairs lubricated with fatty acids and fluorinated fatty acids have been measured over a wide range of sliding speeds and temperatures. Sliding speeds in the range 10?7?10?2 m s?1 and temperatures in the range 4.2–300 K were used. The friction coefficients near 300 K are generally low and increase with sliding speed, while the friction

P. C. Michael; E. Rabinowicz; Y. Iwasa

1996-01-01

33

An Annular Contact Friction and Boundary Lubrication Tester  

Microsoft Academic Search

A simple, inexpensive, easy to use, and very accurate annular contact friction and boundary lubrication tester is described. Data are presented to show excellent reproducibility and extremely low experimental scatter. The system allows the accurate simultaneous measurement of normal load, friction force, ambient fluid temperature, and mean surface contact temperature. Analyses are presented, which are applicable to other testers in

Eugene F. Finkin

1968-01-01

34

Minimalist turbulent boundary layer model  

Microsoft Academic Search

We discuss an elementary model of a turbulent boundary layer over a flat surface given as a vertical random distribution of spanwise Lamb-Oseen vortex configurations placed over a nonslip boundary-condition line. We are able to reproduce several important features of realistic flows, such as the viscous and logarithmic boundary sublayers, and the general behavior of the first statistical moments (turbulent

L. Moriconi

2009-01-01

35

Momentum transfer in boundary layers  

Microsoft Academic Search

The continuity and momentum equations of fluid flow are considered along with thin-shear-layer equations, the analysis of laminar shear layers, the analysis of turbulent shear layers, numerical methods for thin shear layers, numerical solutions of laminar and turbulent boundary layers, aspects of stability and transition, and complex shear layers and viscous\\/inviscid interactions. Three-dimensional and unsteady flows are discussed, taking into

T. Cebeci; P. Bradshaw

1977-01-01

36

Minimalist turbulent boundary layer model.  

PubMed

We discuss an elementary model of a turbulent boundary layer over a flat surface given as a vertical random distribution of spanwise Lamb-Oseen vortex configurations placed over a nonslip boundary-condition line. We are able to reproduce several important features of realistic flows, such as the viscous and logarithmic boundary sublayers, and the general behavior of the first statistical moments (turbulent intensity, skewness, and flatness) of the streamwise velocity fluctuations. As an application, we advance some heuristic considerations on the boundary layer underlying kinematics that could be associated with the phenomenon of drag reduction by polymers, finding a suggestive support from its experimental signatures. PMID:19518332

Moriconi, L

2009-04-06

37

Minimalist turbulent boundary layer model  

NASA Astrophysics Data System (ADS)

We discuss an elementary model of a turbulent boundary layer over a flat surface given as a vertical random distribution of spanwise Lamb-Oseen vortex configurations placed over a nonslip boundary-condition line. We are able to reproduce several important features of realistic flows, such as the viscous and logarithmic boundary sublayers, and the general behavior of the first statistical moments (turbulent intensity, skewness, and flatness) of the streamwise velocity fluctuations. As an application, we advance some heuristic considerations on the boundary layer underlying kinematics that could be associated with the phenomenon of drag reduction by polymers, finding a suggestive support from its experimental signatures.

Moriconi, L.

2009-04-01

38

Turbulent boundary layers developing over compliant surfaces  

NASA Astrophysics Data System (ADS)

The problem of reducing drag due to skin friction remains of interest. This is the case because of the significant benefits that would result from an application of a drag reducing scheme on airplanes, ships or underwater vehicles. One of the techniques that have been proposed for such a scheme is wall compliance. Wall compliance could, in principle, work in two ways: either it could delay transition, or it could modify the inner part of a turbulent boundary layer so that reduced skin friction would result. The objective of this research program was to develop prediction techniques for high Reynolds number turbulent flows over compliant surfaces. This objective was pursued by evaluating the wall induced Reynolds stresses using solutions of the liner momentum equations.

Lekoudis, S. G.; Sengupta, T.

1983-05-01

39

On the stability of the swept leading-edge boundary layer  

Microsoft Academic Search

It has been acknowledged for a long time that the leading-edge boundary layer of a swept wing plays a critical role in the generation of skin friction. Instabilities in the swept leading-edge boundary layer can trigger the transition to a turbulent boundary layer already in the attachment-line region thereby increasing the overall skin friction substantially. Research has identified conditions for

Dominik Obrist

2000-01-01

40

Boundary Layer Control on Airfoils.  

ERIC Educational Resources Information Center

|A phenomena, boundary layer control (BLC), produced when visualizing the fluidlike flow of air is described. The use of BLC in modifying aerodynamic characteristics of airfoils, race cars, and boats is discussed. (KR)|

Gerhab, George; Eastlake, Charles

1991-01-01

41

Frictional anisotropy under boundary lubrication: effect of surface texture.  

SciTech Connect

The friction coefficient was measured under boundary lubrication with a ball-on-flat contact configuration in unidirectional sliding. The ball was smooth and hardened 52100 steel. Discs were made from case-carburized and hardened 4620, annealed 1080, and 1018 steels with directionally ground surfaces. A synthetic lubricant of stock polyalphaolefin was used for testing. During testing with each material, a frictional spike was observed whenever the ball slid parallel to the grinding ridge on the disc surface. The average friction coefficient for all tests was about 0.1, which is typical for the boundary lubrication regime. The magnitude of the frictional spikes, which reached as high as a friction coefficient of 0.25, and their persistence depended on the hardness of the disc surface. On the basis of elastohydrodynamic theory, coupled with the observation of severe plastic deformation on the ridges parallel to the sliding direction, the frictional spike could be due to localized plastic deformation on the disc surface at locations of minimal thickness for the lubricant fluid film. This hypothesis was further supported by lack of frictional spikes in tests using discs coated with a thin film of diamond-like carbon, in which plastic deformation is minimal.

Ajayi, O. O.; Erck, R. A.; Lorenzo-Martin, C.; Fenske, G. R.; Energy Systems

2009-06-15

42

Asymptotic similarity in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The turbulent boundary layer is one of the most fundamental and important applications of fluid mechanics. Despite great practical interest and its direct impact on frictional drag among its many important consequences, no theory absent of significant inference or assumption exists. Numerical simulations and empirical guidance are used to produce models and adequate predictions, but even minor improvements in modeling parameters or physical understanding could translate into significant improvements in the efficiency of aerodynamic and hydrodynamic vehicles. Classically, turbulent boundary layers and fully-developed turbulent channels and pipes are considered members of the same "family," with similar "inner" versus "outer" descriptions. However, recent advances in experiments, simulations, and data processing have questioned this, and, as a result, their fundamental physics. To address a full range of pressure gradient boundary layers, a new approach to the governing equations and physical description of wall-bounded flows is formulated, using a two variable similarity approach and many of the tools of the classical method with slight but significant variations. A new set of similarity requirements for the characteristic scales of the problem is found, and when these requirements are applied to the classical "inner" and "outer" scales, a "similarity map" is developed providing a clear prediction of what flow conditions should result in self-similar forms. An empirical model with a small number of parameters and a form reminiscent of Coles' "wall plus wake" is developed for the streamwise Reynolds stress, and shown to fit experimental and numerical data from a number of turbulent boundary layers as well as other wall-bounded flows. It appears from this model and its scaling using the free-stream velocity that the true asymptotic form of u'2 may not become self-evident until Retheta ? 275,000 or delta+ ? 105, if not higher. A perturbation expansion made possible by the novel inclusion of the scaled streamwise coordinate is used to make an excellent prediction of the shear Reynolds stress in zero pressure gradient boundary layers and channel flows, requiring only a streamwise mean velocity profile and the new similarity map. Extension to other flows is promising, though more information about the normal Reynolds stresses is needed. This expansion is further used to infer a three layer structure in the turbulent boundary layer, and modified two layer structure in fully-developed flows, by using the classical inner and logarithmic profiles to determine which portions of the boundary layer are dominated by viscosity, inertia, or turbulence. A new inner function for U+ is developed, based on the three layer description, providing a much more simplified representative form of the streamwise mean velocity nearest the wall.

Duncan, Richard D.

43

Experimental study of the wall pressure fluctuations under a turbulent boundary layer downstream of tandem aerofoil external manipulators  

Microsoft Academic Search

The boundary layer alteration downstream of a tandem of aerofoil external manipulators was evaluated by mean and turbulent characteristic measurements. The results show that the manipulated boundary layer structure remains identical to the natural one's. The local skin friction reaches a maximum of 20 percent at 36 boundary layer thicknesses downstream the device and then persists up to 100 boundary

Philippe Olivero

1990-01-01

44

Stability of Compressible Boundary Layers.  

National Technical Information Service (NTIS)

The stability of compressible 2-D and 3-D boundary layers is reviewed. The stability of 2-D compressible flows differs from that of incompressible flows in two important features: There is more than one mode of instability contributing to the growth of di...

A. H. Nayfeh

1989-01-01

45

Boundary Layer Over Uniform Roughness.  

National Technical Information Service (NTIS)

The boundary-layer survey is conducted of a rough plate consisting of uniform spherical particles arranged in a random and in highly compact fashion. Strong turbulent mixing is observed in a region within a quarter of the particle size from the top of the...

J. Wu

1970-01-01

46

Laminar boundary layer over riblets  

Microsoft Academic Search

Laser doppler anemometry (LDA) measurements and numerical calculations have been made for a laminar boundary layer on triangular riblets. Calculated mean velocity distributions along the riblet contour are in good agreement with the measured ones. The results show that no transversal motion exists above and within the riblet valleys (e.g., no secondary motion). It is found that despite the large

L. Djenidi; F. Anselmet; J. Liandrat; L. Fulachier

1994-01-01

47

A Theory of Gravity Wave Absorption by a Boundary Layer  

Microsoft Academic Search

A one-layer model of the atmospheric boundary layer (BL) is proposed to explain the nature of lee-wave attenuation and gravity wave absorption seen in numerical simulations. Two complex coefficients are defined: the compliance coefficient and the wave reflection coefficient. A real-valued ratio of reflected to incident wave energy is also useful. The key result is that, due to horizontal friction,

Ronald B. Smith; Qingfang Jiang; James D. Doyle

2006-01-01

48

Boundary lubrication under pressure: could the friction jump down, instead of up?  

NASA Astrophysics Data System (ADS)

The sliding friction during pressure squeezout of a boundary lubricated contact has been shown [1,2] to undergo upward jumps every time a lubricant atomic layer is expelled. Here we ask the question whether the jump could not be downward. Whereas most studies focus on the layered structure which the confined lubricant takes in the normal direction, the element we wish to consider is a possible change of parallel periodicity occurring at the squeezout transition. Such changes have been reported in simulations [3], but their effect has not been discussed so far. One possible effect could be a transition of the slider-lubricant interface commensurability, producing a switch of the frictional mechanism, from lubricant melting-freezing in a commensurate state, to superlubric in an incommensurate one -- in this case with a drop of friction for increasing load. We exemplify this effect by MD simulations, where we replace for convenience the open squeezout system with a closed system, where the lubricant is sealed between the sliders. As the number of layers drops under pressure, the planar lubricant structural lattice parameter also drops. This change reflects in a sliding friction jump, which is easily observed to be downwards. The potential observability of load-induced friction drops will be discussed. [4pt] [1] J.N. Israelachvili et al., Science 240, 189 (1988). [0pt] [2] J. Gao et al., J. Phys. Chem. B 102, 5033 (1998). [0pt] [3] U. Tartaglino et al., J. Chem. Phys. 125, 014704 (2006).

Vanossi, Andrea; Benassi, Andrea; Varini, Nicola; Tosatti, Erio

2012-02-01

49

Internal friction and boundary conditions in lossy fluid seabeds  

SciTech Connect

There are two distinct mechanisms associated with compressional wave absorption in lossy media, internal relaxation and internal friction. For the special case of propagation in an homogeneous, unbounded medium, both mechanisms can be modeled by adopting the convention of a complex sound speed and are, in this sense, equivalent. For the more realistic case of propagation in a stratified medium, the convention of complex sound speed does not give a correct description for losses which modify the linearized equation of motion, such as internal friction. In the presence of boundaries, internal friction can be modeled by the introduction of a complex quiescent density in addition to complex sound speed. Propagation models which use complex sound speed only in the presence of boundaries make the tacit assumption that seafloor losses are caused by internal relaxations only. A solution is developed for propagation in a lossy Pekeris channel where absorption in the lower fluid is caused by internal friction. The example that has been considered yields a sound level 3 dB less than the standard description over a 50-km path. {copyright} {ital 1997 Acoustical Society of America.}

Deane, G.B. [Marine Physical Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0238 (United States)

1997-01-01

50

An analytical model of capped turbulent oscillatory bottom boundary layers  

NASA Astrophysics Data System (ADS)

An analytical model of capped turbulent oscillatory bottom boundary layers (BBLs) is proposed using eddy viscosity of a quadratic form. The common definition of friction velocity based on maximum bottom shear stress is found unsatisfactory for BBLs under rotating flows, and a possible extension based on turbulent kinetic energy balance is proposed. The model solutions show that the flow may slip at the top of the boundary layer due to capping by the water surface or stratification, reducing the bottom shear stress, and that the Earth's rotation induces current and bottom shear stress components perpendicular to the interior flow with a phase lag (or lead). Comparisons with field and numerical experiments indicate that the model predicts the essential characteristics of the velocity profiles, although the agreement is rather qualitative due to assumptions of quadratic eddy viscosity with time-independent friction velocity and a well-mixed boundary layer. On the other hand, the predicted linear friction coefficients, phase lead, and veering angle at the bottom agreed with available data with an error of 3%-10%, 5°-10°, and 5°-10°, respectively. As an application of the model, the friction coefficients are used to calculate e-folding decay distances of progressive internal waves with a semidiurnal frequency.

Shimizu, Kenji

2010-03-01

51

Boundary layer sensitivity and receptivity  

NASA Astrophysics Data System (ADS)

The relation between the receptivity and the sensitivity of the incompressible flow in the boundary layer over a flat plate to harmonic perturbations is determined. Receptivity describes the birth of a disturbance, whereas sensitivity is a concept of larger breath, describing the modification incurred by the state of a system as a response to parametric variations. The governing equations ruling the system's state are the non-local stability equations. Receptivity and sensitivity functions can be obtained from the solution of the adjoint system of equations. An application to the case of Tollmien-Schlichting waves spatially developing in a flat plate boundary layer is studied. To cite this article: C. Airiau et al., C. R. Mecanique 330 (2002) 259-265.

Airiau, Christophe; Walther, Steeve; Bottaro, Alessandro

52

Drag reduction in a flat-plate boundary layer flow by polymer additives  

NASA Astrophysics Data System (ADS)

This paper presents a theoretical study on the velocity distribution and the friction factor of boundary layer flows with polymer additives starting from the concept of ``stress deficit.'' A novel method of order of magnitude analysis is developed, which converts the governing equations of boundary layer flow into a solvable ordinary differential equation, thus the total shear stress distribution is obtained, then the formulas for the mean velocity profiles and the friction factor for a boundary layer flow are derived after introducing appropriate expressions for the ``effective viscosity'' and the thickness of viscous sublayer. The derived velocity equation is able to depict the velocity from a solid wall to the outer edge of boundary layer with or without polymer additives using only one fitted parameter D* that is a function of polymer species, its concentration, and Reynolds number. By integrating the velocity profiles, the friction factor and the thickness of boundary layer development are obtained. Experimental data agree well with the theoretical results.

Yang, Shu-Qing; Dou, G.

2005-06-01

53

Boundary layer receptivity and control  

NASA Astrophysics Data System (ADS)

Receptivity processes initiate natural instabilities in a boundary layer. The instabilities grow and eventually break down to turbulence. Consequently, receptivity questions are a critical element of the analysis of the transition process. Success in modeling the physics of receptivity processes thus has a direct bearing on technological issues of drag reduction. The means by which transitional flows can be controlled is also a major concern: questions of control are tied inevitably to those of receptivity. Adjoint systems provide a highly effective mathematical method for approaching many of the questions associated with both receptivity and control. The long term objective is to develop adjoint methods to handle increasingly complex receptivity questions, and to find systematic procedures for deducing effective control strategies. The most elementary receptivity problem is that in which a parallel boundary layer is forced by time-harmonic sources of various types. The characteristics of the response to such forcing form the building blocks for more complex receptivity mechanisms. The first objective of this year's research effort was to investigate how a parallel Blasius boundary layer responds to general direct forcing. Acoustic disturbances in the freestream can be scattered by flow non-uniformities to produce Tollmien-Schlichting waves. For example, scattering by surface roughness is known to provide an efficient receptivity path. The present effort is directed towards finding a solution by a simple adjoint analysis, because adjoint methods can be extended to more complex problems. In practice, flows are non-parallel and often three-dimensional. Compressibility may also be significant in some cases. Recent developments in the use of Parabolized Stability Equations (PSE) offer a promising possibility. By formulating and solving a set of adjoint parabolized equations, a method for mapping the efficiency with which external forcing excites the three-dimensional motions of a non-parallel boundary layer was developed. The method makes use of the same computationally efficient formulation that makes the PSE currently so appealing. boundary layers involves the application of localized mean wall suction.

Hill, D. C.

1993-12-01

54

Boundary Layer Transport of Small Particles.  

National Technical Information Service (NTIS)

The transport of small particles across the aerodynamic boundary layer that developed over a smooth, flat, acrylic plate and their subsequent deposition was investigated. The velocity boundary layer over the flat plate was characterized for a wind tunnel ...

D. McCready

1984-01-01

55

Nonequilibrium Chemistry Boundary Layer Integral Matrix Procedure.  

National Technical Information Service (NTIS)

The development of an analytic procedure for the calculation of nonequilibrium boundary layer flows over surfaces of arbitrary catalycities is described. An existing equilibrium boundary layer integral matrix code was extended to include nonequilibrium ch...

A. C. Buckingham H. Tong H. L. Morse

1973-01-01

56

Turbulent friction drag reduction: Boundary layer manipulators  

Microsoft Academic Search

Internal manipulation by slant of thin longitudinal riblets is addressed experimentally in three dimensional flow. An ONERA D profile mounted between the lateral walls of a subsonic wind tunnel is maintained at an angle sweep of 22.5 degrees and at incidence 0 degrees. When the profile is recovered by different models of ribleted walls the variations on the drag coefficient

E. Coustols

1990-01-01

57

Turbulent friction drag reduction: Boundary layer manipulators  

NASA Astrophysics Data System (ADS)

Internal manipulation by slant of thin longitudinal riblets is addressed experimentally in three dimensional flow. An ONERA D profile mounted between the lateral walls of a subsonic wind tunnel is maintained at an angle sweep of 22.5 degrees and at incidence 0 degrees. When the profile is recovered by different models of ribleted walls the variations on the drag coefficient are estimated by wake explorations and compared to the drag of the profile encased in a smooth sheet of equivalent thickness.

Coustols, E.

1990-03-01

58

Synoptic Controls on Boundary-Layer Characteristics  

Microsoft Academic Search

We report the characteristics of the three-dimensional, time evolving, atmospheric boundary layer that develops beneath an idealised, dry, baroclinic weather system. The boundary-layer structure is forced by thermal advection associated with the weather system. Large positive heat fluxes behind the cold front drive a vigorous convective boundary layer, whereas moderate negative heat fluxes in the warm sector between the cold

Victoria A. Sinclair; Stephen E. Belcher; Suzanne L. Gray

2010-01-01

59

Boundary condition effects on turbulent boundary layer wall pressure fluctuations  

Microsoft Academic Search

To better understand the flow and pressure field associated with automobile underbodies, laboratory experiments were conducted to map the velocity and pressure fluctuation statistics on two simulated underbodies operating next to a moving ground plane. The effect of boundary condition changes on the statistics for underbodies subject to equilibrium turbulent boundary layer flow and turbulent boundary layer flow perturbed by

Timothy Allen Brungart

1997-01-01

60

Turbulent boundary layer at the initial portion of a pipe with rough walls  

Microsoft Academic Search

The development of a turbulent boundary layer at the initial portion of a pipe with rough walls is considered in the framework of the boundary-layer theory. It is shown that the consideration of roughness can be carried out by introducing into the “standard” law of friction a function which takes into account this factor. An experimental investigation is carried out

V. V. Kuz'min; A. A. Tupichenkov; A. V. Fafurin

1971-01-01

61

Drag reduction in a flat-plate boundary layer flow by polymer additives  

Microsoft Academic Search

This paper presents a theoretical study on the velocity distribution and the friction factor of boundary layer flows with polymer additives starting from the concept of ``stress deficit.'' A novel method of order of magnitude analysis is developed, which converts the governing equations of boundary layer flow into a solvable ordinary differential equation, thus the total shear stress distribution is

Shu-Qing Yang; G. Dou

2005-01-01

62

Investigation of the turbulent boundary layer on a permeable rough surface with heat transfer  

Microsoft Academic Search

The present paper reports on the integral method of the skin friction and heat transfer calculation for nonadiabatic compressible turbulent boundary layer (TBL) on a permeable rough surface in the presence of the heat transfer and pressure gradient. The influence of the roughness is presented as relative variations of the skin friction coefficient and Stanton number. The roughness geometry is

G. F. Sivykh; E. G. Zaulichnii

1978-01-01

63

Velocities, turbulence, and skin friction in a deep-sea logarithmic layer  

Microsoft Academic Search

Speed, turbulence, skin friction, and drag measurements made with metal-clad hot wires, epoxy-coated hot films, and Savonius rotors are reported for a deep-sea boundary layer at a water depth of ~5000 m. They include data from heights z<30 cm, a region hitherto only investigated in detail by Chriss and Caldwell (1982) for a shelf site. A mean speed logarithmic layer

Giselher Gust; Georges L. Weatherly

1985-01-01

64

Investigation and modeling of frictional boundary conditions in oblique cutting of aluminum alloys  

NASA Astrophysics Data System (ADS)

Friction at the cutting tool interface has been studied for 60 years, yet an accurate model of friction is largely unavailable, especially in operations such as turning, where the interface is inaccessible due the continuous contact between chip and tool. A historical perspective of friction in turning is provided to better understand the purpose of this thesis. The contradictions arising from different frictional boundary condition assumptions in machining were analyzed. Experimental observations were substantiated in the light of the literature review. Friction conditions at the tool chip interface were found to be more complex than the simple models of seizure followed by sliding, which is accepted in most machining models. This thesis investigated the surface topology of cutting tools in conventional turning operation, which is one of the oldest and common machining processes. Two different aluminum alloys Al-2024 and Al-6061 were used in turning experiments with carbide tools to define the frictional conditions as these alloys exhibited a wide range of frictional contacts at different machining conditions. Experiments were conducted using carbide cutting tools at a range of speeds, feed rates, and depths of cut, which are commonly utilized in industrial applications. The analysis of tool chip interface at microscopic levels revealed further details of seizure and sliding zone formation. Newer techniques developed in microscopy and surface characterization were used to characterize the interface in a non-destructive manner. Scanning electron microscopy (SEM), surface profilometer and laser scanning confocal microscopy (LSCM) techniques helped us in the understanding of the frictional boundaries. Analysis of SEM images obtained by turning experiments revealed three distinct regions whose topology is closely related to turning parameters. These different zones were named as primary sticking zone, sliding zone and secondary sticking zone. Furthermore, with the assistance of a developed computer code, the real area of contact and each different contact area were determined numerically. Therefore, this study is the first attempt in literature both identifies the frictional contact areas and computes their exact numerical values. The SEM backscattering technique showed that the workpiece material behavior is different in the built up edge and sticking areas. This finding was especially used to identify the preliminary and secondary sticking areas. Thus, it has been showed first time that the deposited layers on frictional areas show different material characteristics. With the help of tool surface image analysis, area calculation algorithm, chemical composition identification, and earlier efforts cited in the literature, we proposed a stress-model which accurately predicted experimental normal and shear forces in oblique cutting of aluminum alloys for most tested conditions.

Kilic, Dursun Sedat

65

Kinetics of stick-slip friction in boundary lubrication  

NASA Astrophysics Data System (ADS)

We show that new important features are brought to the kinetics and dynamics of frictional stick-slip motion in an earthquakelike model of boundary lubrication by introducing a distribution of static breaking thresholds of individual contacts. In particular the condition for elastic instability and details of the slip motion are heavily affected. Among the novel emerging properties is the role of other parameters such as the delay time of contact reforming, the strength of elastic interaction between the contacts, and the elasticity of the contacts and of the slider. We simulate the model dynamics, choosing parameters appropriate to describe a recent surface force apparatus experiment (Klein J., Phys. Rev. Lett., 98 (2007) 056101) whose results are now explained with a totally normal boundary lubricant film viscosity.

Braun, O. M.; Tosatti, E.

2009-11-01

66

Flat plate turbulent boundary-layer control using vertical LEBUs  

NASA Astrophysics Data System (ADS)

Necessity of aerodynamic drag reduction of aircrafts and other moving objects stimulates researchers for finding out new means of the near-wall turbulence control. In [1] it has been found that the vertical positioning of the LEBUs in boundary layer can be much more efficient compared to the conventional horizontal one, although, according to the same authors, the devices were far from being optimized. Present work is focused upon the study of possibility of turbulent skin-friction reduction using flow-aligned vertical LEBUs, the LEBUs being mounted perpendicular to the flat plate surface in nominally gradient-free incompressible turbulent boundary layer. The Reynolds number based on the momentum thickness of the boundary layer at the LEBUs' position was 1099. All measurements were performed using a computer-controlled automated system of space/time hot wire visualization of mean and fluctuating components of the velocity field. The system provided accuracy not worse than approximately ±2 µm along x, y, and z coordinates. Local skin friction C f in the regular (unmodified) shear flow was determined from the condition of the best correspondence between measured and and classic velocity coefficient profiles in the region of the law of the wall functionality U^+ = A log y+ + B with known coefficients A and B. In the modified boundary layer C f was determined by the mean velocity gradient at the wall (partial U/partial y)_{y=0}. The measurement technique is given in more detail in [2].

Kornilov, V. I.; Boiko, A. V.

67

Molecular origins of friction. The force on adsorbed layers  

NASA Astrophysics Data System (ADS)

Simulations and perturbation theory are used to study the molecular origins of an ideal model system, a layer of adsorbed molecules sliding over a substrate. These calculations reproduce several surprising features of experimental results. In most cases, the frictional force on a solid monolayer has a different form from that observed between macroscopic solids. No threshold force or static friction is needed to initiate sliding; instead, the velocity is proportional to the force. As in experiments, incommensurate solid layers actually slide more readily than fluid layers. A comparison of experiment, simulation, and analytic results shows that dissipation arises from anharmonic coupling between phonon modes and substrate-induced deformations in the adsorbate.

Cieplak, Marek; Smith, Elizabeth D.; Robbins, Mark O.

1994-08-01

68

Kahuku kite wind study. I. Kahuka beach boundary layer  

SciTech Connect

In the coastal plain of Kahuku, Oahu, during August 1980 and February to April 1981, the boundary layer and the mechanism that creates it were investigated. Four sets of two automatically-recording tethered aerodynamically lifting anemometer (TALA) kites flying continuously at 100 and 300 ft, and conventional 30 ft instruments were used concurrently at four sites along a transect parallel to the prevailing trade winds. Hand-held short-term kite measurements were used to verify the data from the prototype automatic kites during the first survey. Because of surface heating and vertical mixing, a rapidly expanding boundary develops soon after sunrise. Other forces that modify the daytime air flow in the lower layers are: surface friction, local scale thermal wind, a sea breeze and mechanical forcing. The nighttime boundary is established through heat conduction from the surface air to the ground. This layer grows slowly and reaches only a few hundred feet in depth. Other mechanisms that modify the winds in and around the nighttime boundary layer include: confluence into the boundary layer, local scale thermal wind, land breeze-drainage winds, and friction. In the second survey, the boundary layer was neither as high nor as well-developed as in the first because the ground was saturated after the winter rains. The consequent latent heat exchange prevented extreme surface temperature fluctuation. The commonly-used wind profile law exponent was found to depend on speed in the lower 100 ft of the atmosphere above which the exponent is constant. Estimates of long-term speeds at some sites differed by up to four mph between the two surveys. This underlines the importance of surveying not only all major wind regimes but also under different surface conditions. Long term speed estimates for the sties are high enough to make the area probably profitable for wind power development.

Daniels, P.A.; Oshiro, N.E.

1982-09-01

69

Turbulent oceanic western-boundary layers at low latitude  

NASA Astrophysics Data System (ADS)

Low latitude oceanic western-boundary layers range within the most turbulent regions in the worlds ocean. The Somali current system with the Great Whirl and the Brazilian current system with its eddy shedding are the most prominent examples. Results from analytical calculations and integration of a one layer reduced-gravity fine resolution shallow water model is used to entangle this turbulent dynamics. Two types of wind-forcing are applied: a remote Trade wind forcing with maximum shear along the equator and a local Monsoon wind forcing with maximum shear in the vicinity of the boundary. For high values of the viscosity (> 1000m2s-1) the stationary solutions compare well to analytical predictions using Munk and inertial layer theory. When lowering the friction parameter time dependence results. The onset of instability is strongly influenced by inertial effects. The unstable boundary current proceeds as a succession of anti-cyclonic coherent eddies performing a chaotic dynamics in a turbulent flow. The dynamics is governed by the turbulent fluxes of mass and momentum. We determine these fluxes by analyzing the (potential) vorticity dynamics. We demonstrate that the boundary-layer can be separated in four sub-layers, which are (starting from the boundary): (1) the viscous sub-layer (2) the turbulent buffer-layer (3) the layer containing the coherent structures and (4) the extended boundary layer. The characteristics of each sub-layer and the corresponding turbulent fluxes are determined, as are the dependence on latitude and the type of forcing. A new pragmatic method of determining the eddy viscosity, based on Munk-layer theory, is proposed. Results are compared to observations and solutions of the multi-level primitive equation model (DRAKKAR).

Quam Cyrille Akuetevi, Cataria; Wirth, Achim

2013-04-01

70

Microgravity Effects on Plant Boundary Layers  

NASA Astrophysics Data System (ADS)

The goal of these series of experiment was to determine the effects of microgravity conditions on the developmental boundary layers in roots and leaves and to determine the effects of air flow on boundary layer development. It is hypothesized that microgravity induces larger boundary layers around plant organs because of the absence of buoyancy-driven convection. These larger boundary layers may affect normal metabolic function because they may reduce the fluxes of heat and metabolically active gases (e.g., oxygen, water vapor, and carbon dioxide. These experiments are to test whether there is a change in boundary layer associated with microgravity, quantify the change if it exists, and determine influence of air velocity on boundary layer thickness under different gravity conditions.

Stutte, Gary; Monje, Oscar

2005-08-01

71

Synoptic Controls on Boundary-Layer Characteristics  

Microsoft Academic Search

We report the characteristics of the three-dimensional, time evolving, atmospheric boundary layer that develops beneath an\\u000a idealised, dry, baroclinic weather system. The boundary-layer structure is forced by thermal advection associated with the\\u000a weather system. Large positive heat fluxes behind the cold front drive a vigorous convective boundary layer, whereas moderate\\u000a negative heat fluxes in the warm sector between the cold

Victoria A. Sinclair; Stephen E. Belcher; Suzanne L. Gray

2010-01-01

72

Synoptic Controls on Boundary-Layer Characteristics  

NASA Astrophysics Data System (ADS)

We report the characteristics of the three-dimensional, time evolving, atmospheric boundary layer that develops beneath an idealised, dry, baroclinic weather system. The boundary-layer structure is forced by thermal advection associated with the weather system. Large positive heat fluxes behind the cold front drive a vigorous convective boundary layer, whereas moderate negative heat fluxes in the warm sector between the cold and warm fronts generate shallow, stably stratified or neutral boundary layers. The forcing of the boundary-layer structure is quantified by forming an Eulerian mass budget integrated over the depth of the boundary layer. The mass budget indicates that tropospheric air is entrained into the boundary layer both in the vicinity of the high-pressure centre, and behind the cold front. It is then transported horizontally within the boundary layer and converges towards the cyclone's warm sector, whence it is ventilated out into the troposphere. This cycling of air is likely to be important for the ventilation of pollution out of the boundary layer, and for the transformation of the properties of large-scale air masses.

Sinclair, Victoria A.; Belcher, Stephen E.; Gray, Suzanne L.

2010-03-01

73

Transitionally rough zero pressure gradient turbulent boundary layers  

NASA Astrophysics Data System (ADS)

Near-wall measurements are performed to study the effects of surface roughness and viscous shear stresses on the transitionally rough regime (5 < k + < 70) of a zero pressure gradient turbulent boundary layer. The x-dependence is known from the eleven consecutive measurements in the streamwise direction, which allows for the computation of the streamwise gradients in the boundary layer equations. Thus, the skin friction is computed from the integrated boundary layer equation with errors of 3 and 5% for smooth and rough, respectively. It is found that roughness destroys the viscous layer near the wall, thus, reducing the contribution of the viscous stress in the wall region. As a result, the contribution in the wall shear stress due to form drag increases, while the viscous stress decreases. This yields Reynolds number invariance in the skin friction as k + increases into the fully rough regime. Furthermore, the roughness at the wall reduces the high peak of the streamwise component of the Reynolds stress in the near-wall region. However, for the Reynolds wall-normal and shear stress components, its contribution is not significantly altered for sand grain roughness.

Brzek, Brian G.; Cal, Raúl Bayoán; Johansson, Gunnar; Castillo, Luciano

2008-01-01

74

Incompressible Fluids in Thin Domains with Navier Friction Boundary Conditions (II)  

NASA Astrophysics Data System (ADS)

We study the Navier-Stokes equations for incompressible fluids in a three-dimensional thin two-layer domain whose top, bottom and interface boundaries are not flat. In addition to the Navier friction boundary conditions on the top and bottom boundaries of the domain, and the periodicity condition on the sides, the fluid velocities are subject to an interface boundary condition which relates the normal stress of each fluid to the relative velocity between them on the common boundary. We prove that the strong solutions exist for all time if the initial data and body force, measured in relevant norms, are appropriately large as the domain becomes very thin. In our analysis, the interface boundary condition is interpreted as a variation of the Navier boundary conditions containing an interaction part. The effect of that interaction on the Stokes operator and the nonlinear term of the Navier-Stokes equations is expressed and carefully estimated in different ways in order to obtain suitable inequalities.

Hoang, Luan Thach

2013-06-01

75

Transitional and turbulent boundary layer with heat transfer  

NASA Astrophysics Data System (ADS)

We report on our direct numerical simulation of an incompressible, nominally zero-pressure-gradient flat-plate boundary layer from momentum thickness Reynolds number 80-1950. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number Pr=1. Skin-friction coefficient and other boundary layer parameters follow the Blasius solutions prior to the onset of turbulent spots. Throughout the entire flat-plate, the ratio of Stanton number and skin-friction St/Cf deviates from the exact Reynolds analogy value of 0.5 by less than 1.5%. Mean velocity and Reynolds stresses agree with experimental data over an extended turbulent region downstream of transition. Normalized rms wall-pressure fluctuation increases gradually with the streamwise growth of the turbulent boundary layer. Wall shear stress fluctuation, ?w,rms'+, on the other hand, remains constant at approximately 0.44 over the range, 800boundary layer edge with no near-wall secondary peak, in good agreement with previous boundary layer heat transfer experiments. In the transitional region, turbulent spots are tightly packed with numerous hairpin vortices. With the advection and merging of turbulent spots, these young isolated hairpin forests develop into the downstream turbulent region. Isosurfaces of temperature up to Re?=1900 are found to display well-resolved signatures of hairpin vortices, which indicates the persistence of the hairpin forests.

Wu, Xiaohua; Moin, Parviz

2010-08-01

76

Compressible Ekman–Hartmann boundary layers  

Microsoft Academic Search

We consider the effect of compressibility on mixed Ekman–Hartmann boundary layers on an infinite plane (z = 0), in the presence of an external magnetic field oblique to the boundary. The aim is to investigate the influence of the magnetic pressure on the fluid density, and hence, via mass conservation, on the mass flow into or out of the boundary

Krzysztof A. Mizerski; David W. Hughes

2010-01-01

77

Turbulent boundary layer on a moving surface  

Microsoft Academic Search

An integral and a numerical method are proposed for calculating the turbulent boundary layer on a moving surface (flap) in the presence of a longitudinal pressure gradient under conditions of monotonic velocity profiles. The integral method is a modification of Fediaevskii's et al. (1973) integral method (for calculating turbulent boundary layers in an incompressible fluid) to include an airfoil moving

A. S. Ginevskii; G. N. Emalianova; A. V. Kolesnikov

1976-01-01

78

Boundary layers on a rotating disk  

Microsoft Academic Search

A numerical method is proposed for predicting the three-dimensional boundary layer that develops on a rotating disk in the presence of a steady incompressible axisymmetric flow. The method employs the eddy viscosity concept to model the Reynolds shear stress terms in the boundary layer equations. The governing nonlinear difference equations are solved by Newton's method using an efficient block-tridiagonal factorization

Tuncer Cebeci; D. E. Abbott

1975-01-01

79

Cyclone separator having boundary layer turbulence control  

DOEpatents

A cyclone separator including boundary layer turbulence control that is operable to prevent undue build-up of particulate material at selected critical areas on the separator walls, by selectively varying the fluid pressure at those areas to maintain the momentum of the vortex, thereby preventing particulate material from inducing turbulence in the boundary layer of the vortical fluid flow through the separator.

Krishna, Coimbatore R. (Mt. Sinai, NY); Milau, Julius S. (Port Jefferson, NY)

1985-01-01

80

Boundary Layers of Air Adjacent to Cylinders  

PubMed Central

Using existing heat transfer data, a relatively simple expression was developed for estimating the effective thickness of the boundary layer of air surrounding cylinders. For wind velocities from 10 to 1000 cm/second, the calculated boundary-layer thickness agreed with that determined for water vapor diffusion from a moistened cylindrical surface 2 cm in diameter. It correctly predicted the resistance for water vapor movement across the boundary layers adjacent to the (cylindrical) inflorescence stems of Xanthorrhoea australis R. Br. and Scirpus validus Vahl and the leaves of Allium cepa L. The boundary-layer thickness decreased as the turbulence intensity increased. For a turbulence intensity representative of field conditions (0.5) and for ?windd between 200 and 30,000 cm2/second (where ?wind is the mean wind velocity and d is the cylinder diameter), the effective boundary-layer thickness in centimeters was equal to [Formula: see text].

Nobel, Park S.

1974-01-01

81

Receptivity of boundary layers to convected gusts  

NASA Astrophysics Data System (ADS)

The primary objective of this study is to determine experimentally the receptivity of a laminar boundary layer to convected gusts. Receptivity is the process by which external disturbances transfer energy to instabilities in the boundary layer. The term convected gust refers to a transient or periodic vortical disturbance convected by the freestream. The experimental approach consisted of hot-wire studies of the boundary layer response to vortical disturbances produced by an array of oscillating ribbons. The boundary layer is remarkably insensitive to large-scale vortical perturbations. No significant Tollmien-Schlichting waves were observed. The vortical disturbances are rapidly damped in the boundary layer and cause only weak oscillations that are primarily of the same frequency and wavelength as the external disturbance.

Parekh, D. E.

1993-03-01

82

Computation and simulation of wake-generated unsteady pressure and boundary layers in cascades. Part 2: Simulation of unsteady boundary layer flow physics  

SciTech Connect

The unsteady pressure and boundary layers on a turbomachinery blade row arising from periodic wakes due to upstream blade rows are investigated in this paper. Numerical simulations are carried out to understand the effects of the wake velocity defect and the wake turbulence intensity on the development of unsteady blade boundary layers. The boundary layer transition on the blade is found to be strongly influenced by the unsteady wake passing. Periodic transitional patches are generated by the high turbulence intensity in the passing wakes and transported downstream. The time-dependent transition results in large unsteadiness in the instantaneous local skin friction coefficient and a smoother time-averaged transition curve than the one observed in the steady boundary layer. A parametric study is then carried out to determine the influence of wake parameters on the development of the unsteady blade boundary layers. It is shown that the unsteadiness in the blade boundary layer increases with a decrease in the axial gap, an increase in wake/blade count ratio, or an increase in the wake traverse speed. The time-averaged boundary layer momentum thickness at the trailing edge of the blade is found to increase significantly for higher wake/blade count ratio and larger wake traverse speed. Increase of the wake/blade count ratio also results in higher frictional drag of the blade.

Fan, S.; Lakshminarayana, B. [Pennsylvania State Univ., University Park, PA (United States). Center for Gas Turbines and Power

1996-01-01

83

Problems of matter-antimatter boundary layers  

Microsoft Academic Search

This paper outlines the problems of the quasi-steady matter-antimatter boundary layers discussed in Klein-Alfvén's cosmological theory, and a crude model of the corresponding ambiplasma balance is presented:(i)At interstellar particle densities, no well-defined boundary layer can exist in presence of neutral gas, nor can such a layer be sustained in an unmagnetized fully ionized ambiplasma.(ii)Within the limits of applicability of the

B. Lehnert

1977-01-01

84

Boundary layer processes in the Martian magnetosphere  

NASA Astrophysics Data System (ADS)

At Mars, in the absence of a global magnetic field, the flowing solar wind interacts directly with ionized atmospheric constituents, forming an induced magnetosphere. Turbulent boundary layer processes provide one means by which solar wind and atmospheric plasma can mix and exchange momentum, potentially leading to atmospheric escape. In this presentation, we describe MGS MAG/ER observations of Martian boundary layer oscillations, identified using a combination of magnetic field and electron data. We analyze electron distributions to identify interaction regions, and utilize magnetic field data to determine wave properties and boundary position and morphology. Our observations suggest several modes of interaction, including large-scale pulsations of a relatively smooth boundary, and corrugated boundary layer structures possibly indicative of the presence of boundary instabilities. We discuss each of these modes, and the implications for atmospheric escape process.

Halekas, J. S.; Brain, D. A.; Eastwood, J. P.

2010-12-01

85

Friction of Greases and Grease Components during Boundary Lubrication  

Microsoft Academic Search

Measurements have been made of the coefficient of friction of a steel pin sliding on a steel ring lubricated with three commercial greases, various experimental greases or pastes, and other components of greases. After break-in at room temperature, heat was applied which reduced the friction of the greases to about half their room temperature value. The friction increased on cooling.

Douglas Godfrey

1964-01-01

86

A comparison of boundary layer receptivity mechanisms  

NASA Astrophysics Data System (ADS)

Boundary layer receptivity mechanisms are investigated, and the implications of receptivity for the prediction of boundary layer transition are discussed. The receptivity of the Blasius boundary layer to a variety of freestream disturbances is first studied. Due to diffraction by the leading edge, oblique acoustic waves at low Mach numbers are found to produce the strongest receptivity. The case of the receptivity produced by acoustic waves interacting with porous suction surfaces in subsonic flow is then considered, and receptivity is shown to occur due to the rapid adjustment in the mean flow and to local scattering of the acoustic wave by the nonzero admittance of the porous surface.

Heinrich, R. A.; Choudhari, M.; Kerschen, E. J.

87

Boundary layers on longitudinally grooved walls (riblets)  

NASA Astrophysics Data System (ADS)

The boundary layer of riblets has been investigated in a hydrodynamic wind tunnel. For the case of triangularly grooved riblets, laser velocimetry visualizations show flow stabilization to occur for a turbulent boundary layer, and a decreased longitudinal velocity profile slope and a rapid relaxation downstream to occur for a laminar boundary layer. U-shaped grooves are found to have no effect. Visualizations of triangularly gooved riblets of several dimensions indicate that no counterrotating vortices exist in the grooves. This result is confirmed by profiles of the longitudinal velocity component, which show an increase in the velocity gradient near the crest and a significant decrease in the groove.

Fulachier, L.; Djenidi, L.; Anselmet, F.

1987-10-01

88

Growth Of The Summer Daytime Convective Boundary Layer At Anand  

NASA Astrophysics Data System (ADS)

The heights of the daytime convective boundary layer (CBL), computed by a one-dimensional model for a bare soil surface at a semi-arid station,Anand, during the dry and hot summer month of May 1997, are presented. As input, the model requires surface heat flux, friction velocity and air temperature as functions of time. Temperature data at the one-metre level from a tower and sonic anemometer data at 9.5 m collected during the period 13-17 May 1997 in the Land Surface Processes Experiment (LASPEX-97) are used to compute hourly values of surface heat flux, friction velocity and Obukhov length following the operational method suggested by Holtslag and Van Ulden [J. Climate Appl. Meteorol. 22,517-529 (1983)]. The model has been tested with different values for the potential temperature gradient ( ) above the inversion. The model-estimated CBL heights comparefavourably with observed heights obtained from radiosonde ascents.

Nagar, S. G.; Tyagi, Ajit; Seetaramayya, P.; et al.

89

Turbulent boundary layer manipulation by outer-layer devices  

Microsoft Academic Search

A turbulent boundary layer manipulated by outer-layer devices has been studied. Experiments have been conducted in the 0.70 by 0.50 m2 low speed wind tunnel of the ‘Modesto Panetti’ Aeronautical Laboratory of the Politecnico di Torino. Mean values and turbulent quantities measured in the natural and manipulated boundary layers are shown for comparison. The mechanisms to explain the observed skin

G. Iuso; M. Onorato

1995-01-01

90

Porous Material Development for Boundary Layer Control.  

National Technical Information Service (NTIS)

A description is presented of the development of a new porous material that was undertaken specifically for the application of area suction boundary layer control to underwater vehicles. This material was subsequently used in the manufacture of several hy...

G. May W. B. Giles

1964-01-01

91

Analysis of Nosetip Boundary Layer Transition Data.  

National Technical Information Service (NTIS)

A critical analysis is presented of the available wind tunnel data simulating nosetip boundary layer transition on reentry vehicles. It is agreed that transition should depend on surface roughness, surface temperature, and surface curvature. The Reynolds ...

M. L. Finson

1976-01-01

92

Radiation from Air-Teflon Boundary Layers.  

National Technical Information Service (NTIS)

Radiation profiles in an ablating flat plate air-teflon laminar boundary layer were studied both experimentally and theoretically. The experiments were conducted in a one atmosphere, 3000 - 6000K, subsonic free stream produced by an arc jet. Spatially res...

R. A. Greenberg K. L. Wray L. A. Young N. H. Kemp

1970-01-01

93

Sound from Turbulent Boundary Layer Excited Panels.  

National Technical Information Service (NTIS)

The acoustic power radiated by thin flexible panels excited by turbulent boundary layer pressure fluctuations is estimated using a modal analysis, light fluid loading effects being included. Previous estimates of the modal radiation coefficients are impro...

H. G. Davies

1969-01-01

94

Control of friction coefficient by applying electric fields across liquid crystal boundary films  

NASA Astrophysics Data System (ADS)

Experiments are described in which friction coefficients under boundary lubrication regime are measured with a nematic liquid crystal as the lubricant. Using steel-to-steel contacts, with one of the mating surfaces coated with a thin insulating film, electric fields are applied across the liquid crystal film. A substantial decrease in the friction coefficient is observed with both dc and low-frequency ac voltages, suggesting the possibility of controlling friction by external fields.

Kimura, Yoshitsugu; Nakano, Ken; Kato, Takashi; Morishita, Shin

1994-06-01

95

A computational study of turbulent flow separation for a circular cylinder using skin friction boundary conditions  

Microsoft Academic Search

\\u000a In this paper we present a computational study of turbulent flow separation for a circular cylinder at high Reynolds numbers.\\u000a We use a stabilized finite element method together with skin friction boundary conditions, where we study flow separation\\u000a with respect to the decrease of a friction parameter. In particular, we consider the case of zero friction corresponding to\\u000a pure slip

Johan Hoffman; Niclas Jansson

96

An experimental study of polymer drag reduction and boundary layer diffusion characteristics for incompressible flow over a flat plate  

Microsoft Academic Search

The effect of injecting polymer into a developing boundary layer was determined through the measurement of polymer boundary layer concentration distributions and velocity profiles. To reach this objective, methods to predict local skin friction coefficients were developed as a function of local polymer concentrations and experimentally verified. Tests were also performed using the injection of water and solutions of a

J. Miguel

1978-01-01

97

Reflection of sound by a boundary layer  

Microsoft Academic Search

A method is given for the calculation of the reflection coefficient for plane waves incident upon a plane boundary when the acoustic medium flows parallel to the plane and the resulting boundary layer is of finite thickness. Results are given for limited ranges of the important parameters, which are the Mach number of the flow, the angle of incidence, the

R. S. Brand; R. T. Nagel

1982-01-01

98

Turbulent Boundary Layers with Assigned Wall Shear.  

National Technical Information Service (NTIS)

This paper considers the problem of computing the external velocity distribution on a two-dimensional body in an incompressible boundary-layer flow for a specified wall shear. This leads to an 'overdetermined' boundary-value problem for a partial differen...

T. Cebeci N. Berkant I. Silivri H. B. Keller

1974-01-01

99

BUBBLE – an Urban Boundary Layer Meteorology Project  

Microsoft Academic Search

Summary The Basel UrBan Boundary Layer Experiment (BUBBLE) was a year-long experimental effort to investigate in detail the boundary layer structure in the City of Basel, Switzerland. At several sites over different surface types (urban, sub-urban and rural reference) towers up to at least twice the main obstacle height provided turbulence observations at many levels. In addition, a Wind Profiler

M. W. Rotach; R. Vogt; C. Bernhofer; E. Batchvarova; A. Christen; A. Clappier; B. Feddersen; S.-E. Gryning; G. Martucci; H. Mayer; V. Mitev; T. R. Oke; E. Parlow; H. Richner; M. Roth; Y.-A. Roulet; D. Ruffieux; J. A. Salmond; M. Schatzmann; J. A. Voogt

2005-01-01

100

Direct numerical simulation of polymer-induced drag reduction in turbulent boundary layer flow of inhomogeneous polymer solutions  

Microsoft Academic Search

Skin-friction drag reduction in turbulent boundary layer flow of inhomogeneous polymer solutions is investigated using direct numerical simulations. A continuum constitutive model (FENE-P) accounting for the effects of polymer microstructure and concentration is used to describe the effect of viscoelasticity. The evolution of wall friction along the streamwise direction is a function of the dynamics of the polymer distribution in

Costas D. Dimitropoulos; Yves Dubief; Eric S. G. Shaqfeh; Parviz Moin

2006-01-01

101

Numerical Simulation of Active Control of Boundary Layer Transition,  

National Technical Information Service (NTIS)

A numerical model has been developed for investigating boundary layer transition control for a three-dimensional flat plate boundary layer. Control of a periodically forced boundary layer in an incompressible fluid is studied using surface heating techniq...

H. F. Fasel L. D. Kral

1988-01-01

102

Klebanoff modes in 3D boundary layers  

NASA Astrophysics Data System (ADS)

We present an investigation based on simplified direct numerical simulations of the generation of Klebanoff modes by free-stream vorticity in Falkner-Skan-Cooke boundary layers. The free-stream vorticity is generated by a source taking the form d(x)d(z-zf)exp(iby) derived from a body force, where (x,y,z) are the streamwise, spanwise and wall-normal co-ordinates, zf is located above the boundary layer and near its edge, and b is the spanwise wave-number. This vorticity source creates a diffused and corrugated sheet of streamwise vorticity that drives the boundary layer with a wall-normal velocity, thereby generating streak-like structures within the boundary layer. Simulations of streak evolution have been carried out for a range of 2D and 3D boundary layers with favourable and adverse streamwise pressure gradients. In general, increasing the degree of adverse pressure gradient and sweep makes the boundary layer more receptive to streak formation. The streaks also change form as the sweep angle is increased and there is also evidence of secondary instability.

Carpenter, Peter W.; Kudar, Karen; Davies, Chris

2004-11-01

103

Designing LES of the High Reynolds Surface Layer to Account for Numerical Friction in the Algorithm.  

NASA Astrophysics Data System (ADS)

Numerical friction stabilizes large-eddy simulation (LES), but also impacts accuracy. We explore this issue using a theory (Brasseur & Wei 2010) where the LES is designed in a 3-parameter space that quantifies the level of friction in the SFS stress model (ReLES), the relative content of resolved to SFS stress (), and surface layer resolution. To achieve law-of-the-wall in the mean, the LES must be in the ``high-accuracy zone'' (HAZ) of the -ReLES parameter space. Using rough-wall channel flow and atmospheric boundary layer LES, we analyze simulations that are identical except for spectral vs. finite volume (FV) algorithms. Numerical friction shifts the LES away from the HAZ in the -ReLES parameter space consistent with changes in mean shear-rate. The effective low pass filter from numerical friction shifts the total stress from resolved to subfilter-scale contributions, and effect that is more apparent when the spectral version of the LES is in the HAZ. A consequence is the enhancement of streamwise coherence in turbulence structure, particularly apparent in the integral scales. We shall discuss the requirements to adjust the FV LES to match a corresponding spectral LES in the HAZ, and differences in efficiency and accuracy. Support: NSF, DOE.

Brasseur, James; Vijayakumar, Ganesh; Churchfield, Matthew; Lavely, Adam; Paterson, Eric; Moriarty, Patrick

2011-11-01

104

Modification of Boundary Lubrication by Oil-Soluble Friction Modifier Additives  

Microsoft Academic Search

The molecular-level function of model and commercial friction modifier additives in lubricants of the type used at the wet clutch interface in automatic transmissions has been studied using a surface forces apparatus (SFA) modified for oscillatory shear. The nanorheological properties of tetradecane with and without a model friction modifier additive (1-hexadecylamine) were examined in the boundary lubrication regime and compared

Yingxi Zhu; Hiroko Ohtani; Michael L. Greenfield; Marina Ruths; Steve Granicka

2003-01-01

105

Historical review (pre-1980) of magnetospheric boundary layers and the low-latitude boundary layer  

NASA Astrophysics Data System (ADS)

This historical survey covers research on the magnetospheric boundary layers through 1979 with a focus on observations and on the low-latitude boundary layer (LLBL) of Earth's magnetosphere. An effort was made to identify and read every paper published prior to 1980 that made a significant contribution towards identifying and characterizing boundary regions of the outer magnetosphere. The total list of over 80 papers is available at http://www.plasmas.org/BL. A sketch of Earth's magnetosphere is shown in Figure 1, which illustrates the major regimes and boundary layers. The magnetospheric boundary layer denotes all exterior boundary layers adjoining the magnetopause, including the dayside and tail flank portions of the LLBL, the exterior cusp region and entry layer, and the plasma mantle. There is a plasma sheet boundary layer separating the lobe and plasma sheet regions, but that boundary layer is not reviewed here [see Eastman et al., 1984]. The magnetotail boundary layer refers to both the plasma mantle and the tail flank portion of the LLBL.

Eastman, Timothy E.

106

Calculation of three-dimensional boundary layers on rotor blades using integral methods  

SciTech Connect

The important effects of rotation and compressibility on rotor blade boundary layers are theoretically investigated. The calculations are based on the momentum integral method and results from calculations of a transonic compressor rotor are presented. Influence of rotation is shown by comparing the incompressible rotating flow with the stationary one. Influence of compressibility is shown by comparing the compressible rotating flow with the incompressible rotating one. Two computer codes for three-dimensional laminar and turbulent boundary layers, originally developed by SSPA Maritime Consulting AB, have been further developed by introducing rotation and compressibility terms into the boundary layer equations. The effect of rotation and compressibility on the transition have been studied. The Coriolis and centrifugal forces that contribute to the development of the boundary layers and influence its behavior generate crosswise flow inside the blade boundary layers, the magnitude of which depends upon the angular velocity of the rotor and the rotor geometry. The calculations show the influence of rotation and compressibility on the boundary layer parameters. Momentum thickness and shape factor increase with increasing rotation and decrease when compressible flow is taken into account. For skin friction such effects have inverse influences. The different boundary layer parameters behave similarly on the suction and pressure sides with the exception of the crossflow angle, the crosswise momentum thickness, and the skin friction factor. The codes use a nearly orthogonal streamline coordinate system, which is fixed to the blade surface and rotates with the blade.

Karimipanah, M.T.; Olsson, E. (Chalmers Univ. of Technology, Goeteborg (Sweden))

1993-04-01

107

Boundary Layer Cloudiness Parameterizations Using ARM Observations  

SciTech Connect

This study used DOE ARM data and facilities to: (1) study macroscopic properties of continental stratus clouds at SGP and the factors controlling these properties, (2) develop a scientific basis for understanding the processes responsible for the formation of boundary layer clouds using ARM observations in conjunction with simple parametric models and LES, and (3) evaluate cumulus cloud characteristics retrieved from the MMCR operating at TWP-Nauru. In addition we have used high resolution 94 GHz observations of boundary layer clouds and precipitation to: (1) develop techniques for using high temporal resolution Doppler velocities to study large-eddy circulations and turbulence in boundary layer clouds and estimate the limitations of using current and past MMCR data for boundary layer cloud studies, (2) evaluate the capability and limitations of the current MMCR data for estimating reflectivity, vertical velocities, and spectral under low- signal-to-noise conditions associated with weak no n-precipitating clouds, (3) develop possible sampling modes for the new MMCR processors to allow for adequate sampling of boundary layer clouds, and (4) retrieve updraft and downdraft structures under precipitating conditions.

Bruce Albrecht

2004-09-15

108

Calculation of the turbulent boundary layer in the initial section of pipe  

SciTech Connect

This article constructs a flow model for the turbulent boundary layer in a pipe operating under conditions of pressure and gravitation encountered in a hydroelectric power plant. Pipe roughness and friction factors are taken into account as are hydraulic conductivity and pipe dimension considerations. Continuity equations are given and the accuracy of the model is compared against experimental data.

Temirkhanov, A.M.; Spivak, V.M.

1987-11-01

109

Calculation of the turbulent boundary layer in the initial section of pipe  

Microsoft Academic Search

This article constructs a flow model for the turbulent boundary layer in a pipe operating under conditions of pressure and gravitation encountered in a hydroelectric power plant. Pipe roughness and friction factors are taken into account as are hydraulic conductivity and pipe dimension considerations. Continuity equations are given and the accuracy of the model is compared against experimental data.

A. M. Temirkhanov; V. M. Spivak

1987-01-01

110

Drag reduction of turbulent boundary layers by means of grooved surfaces  

Microsoft Academic Search

The phenomenon that the friction factor of a flat plate can be decreased by coating the surface with small longitudinal grooves was studied. This phenomenon occurs in a turbulent boundary layer flow and apparently the grooves influence the eddies in the flow in such a way that the stress transport shows a 7 percent decrease. Water channel and wind tunnel

Cornelis Johannes Adrianus Pulles

1988-01-01

111

Injection into a turbulent boundary layer through porous surfaces with different surface geometries  

Microsoft Academic Search

The turbulent boundary layer over a perforated titanium wall is analyzed with and without injection, and the results are compared to previous work on a smooth, solid wall and a porous, sintered-metal wall at the same conditions. Results for the case of no injection show that the perforated titanium and porous, sintered metal walls have local skin friction coefficients that

F. S. Collier Jr.; J. A. Schetz

1983-01-01

112

Turbulence structure and polymer drag reduction in adverse pressure gradient boundary layers  

Microsoft Academic Search

The ability of some solutions of long chain polymers to greatly reduce the pressure drop in pipes is well documented and commercially applied. In addition, a number of experiments indicate that large reductions of wall friction are possible by injecting polymer into a boundary layer. Therefore, it is reasonable to expect that drag reducing polymers could significantly improve the performance

John E. Koskie; William G. Tiederman

1991-01-01

113

An appraisal of 'flat plate' closure for the approximate solution of boundary layer problems  

Microsoft Academic Search

The usefulness of zero pressure gradient, flat plate closure relations in providing approximate solutions for the boundary layer momentum and energy integral equations is examined. Expressions are obtained for skin friction, surface heat transfer rate and local Reynolds analogy factor under general compressible flow conditions. For laminar flows the predictions are compared with well known similarity solutions, with some exact

D. I. A. Poll; C. M. Hellon

1987-01-01

114

Interfacial slip friction at a fluid-solid cylindrical boundary  

NASA Astrophysics Data System (ADS)

Recently we proposed a method to calculate the interfacial friction coefficient between fluid and solid at a planar interface. In this work we extend the method to cylindrical systems where the friction coefficient is curvature dependent. We apply the method to methane flow in carbon nanotubes, and find good agreement with non-equilibrium molecular dynamics simulations. The proposed method is robust, general, and can be used to predict the slip for cylindrical nanofluidic systems.

Kannam, Sridhar Kumar; Todd, B. D.; Hansen, J. S.; Daivis, Peter J.

2012-06-01

115

Hypersonic Boundary Layer\\/Shockwave Interaction Problems  

Microsoft Academic Search

The hypersonic shockwave boundary layer-interaction problem was defined with the use of the full Navier- Stokes (NS) equations and a FORTRAN code was developed to provide numerical solutions to this problem. Further, this problem was studied under two specified sets of boundary conditions: adiabatic wall and constant wall conditions. The MacCormack Predictor-Corrector technique was used in developing this NS code.

Haile Lindsay; Frederick Ferguson

116

Hypersonic Boundary Layer\\/Oblique Shockwave Interaction  

Microsoft Academic Search

The hypersonic boundary layer\\/oblique shockwave interaction problem was defined with the use of the full Navier-Stokes (NS) equations and a FORTRAN code was developed to provide numerical solutions to this problem. Further, this problem was studied under two specified sets of boundary conditions: adiabatic wall and constant wall conditions. The MacCormack Technique was used in developing this NS code. To

Haile Lindsay

2005-01-01

117

Boundary layer energy transport in plasma devices  

NASA Astrophysics Data System (ADS)

The purpose of this research was to develop a model of boundary-layer energy transport in electric launchers, and perform a numerical simulation to investigate the influence of turbulence, thermal radiation and ablation on energy flux to the surface. The model developed combines boundary-layer conservation equations with a k-o turbulence model and multi-group radiation transport, and uses plasma models for fluid properties such as viscosity, thermal conductivity and specific heat capacity. The resulting TURBFIRE computer code is the first code to model turbulence and radiation transport in a self-consistent manner for electric launchers. Although approximations have been made to simplify the physics enough to permit a numerical solution, this is the most comprehensive boundary-layer simulation of turbulence and radiation transport to date.

Orton, Nigel Paul

2000-11-01

118

Aerosol buffering of marine boundary layer cloudiness  

NASA Astrophysics Data System (ADS)

The role of aerosol particles in maintaining a cloudy boundary layer in the remote marine environment is explored. It has previously been shown that precipitation can result in the transition from a closed- to open-cellular state but that the boundary layer cannot maintain this open-cell state without a resupply of particles. Potential sources include wind-driven production of sea salt particles from the ocean, nucleation from the gas phase, and entrainment from the free troposphere. Here we investigate with model simulations how the interplay of cloud properties, aerosol production, and boundary layer dynamics results in aerosol sources acting as a buffer against processes that destabilize cloudiness and the dynamic state of the marine boundary layer. For example, at nighttime, cloud liquid water increases in the absence of solar heating, resulting in increased precipitation, stronger cloud top cooling, accelerated boundary layer turbulence, and faster surface wind speeds. Faster surface wind speeds drive an enhanced flux of sea salt aerosol, at a time when aerosol particles are scavenged more readily by enhanced precipitation. In contrast, absorption of solar radiation during daytime reduces cloud water, decelerates boundary layer turbulence, reduces surface wind speeds, and therefore slows surface emissions. This is compensated by nucleation of small aerosol particles from the gas phase in response to the nigh complete removal of cloud condensation nuclei in precipitating open cell walls. These newly formed particles need to grow to larger sizes before they can serve as cloud condensation nuclei (CCN), but will likely contribute to the CCN population during the nighttime and, together with ocean emissions, buffer the system against precipitation removal.

Kazil, J.; Feingold, G.; Wang, H.

2010-12-01

119

Waves which travel upstream in boundary layers  

NASA Astrophysics Data System (ADS)

Upstream propagation and diffusion of vorticity in a boundary layer is described by a numerical solution of the Orr-Sommerfeld equation. This traveling wave grows very rapidly in the downstream direction. The growth rate is approximately exp(+ R (sub delta)x) where R sub delta is the Reynolds number based on the characteristic boundary layer thickness, and x is the streamwise coordinate nondimensionalized against delta. Far from the boundary layer, the solution oscillates neutrally in the Y-direction. Analyses reveal high frequency wave which oscillates and decays in the y-direction approximately as exp(-i R(sub delta) y - omega Y) where omega is the frequency. This high frequency wave can survive into the freestream. Numerical solutions of the Orr-Sommerfeld equation with a Blasius layer are obtained by a series expansion of Chebyshev polynomials. Since the y-wavenumber of the oscillations increases with increasing Reynolds number, the calculations have been restricted to low Reynolds numbers. In the boundary-value problem, this solution appears as a branch line in Laplace space. It is one of the possible solutions in a mathematically complete description of the spatial evolution of fluctuations. This traveling wave represents one of the upstream influences of a boundary in a calculational domain. Another mechanism of upstream influence is the growing standing wave.

Rogler, H. L.

1985-07-01

120

Frictional figures of merit for single layered nanostructures.  

PubMed

We determine the frictional figures of merit for a pair of layered honeycomb nanostructures, such as graphane, fluorographene, MoS2 and WO2 moving over each other, by carrying out ab initio calculations of interlayer interaction under constant loading force. Using the Prandtl-Tomlinson model we derive the critical stiffness required to avoid stick-slip behavior. We show that these layered structures have low critical stiffness even under high loading forces due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby the materials avoid the stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, tungsten dioxide displays a much better performance relative to others and heralds a potential superlubricant. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers. PMID:22540600

Cahangirov, S; Ataca, C; Topsakal, M; Sahin, H; Ciraci, S

2012-03-21

121

Frictional Figures of Merit for Single Layered Nanostructures  

NASA Astrophysics Data System (ADS)

We determine the frictional figures of merit for a pair of layered honeycomb nanostructures, such as graphane, fluorographene, MoS2 and WO2 moving over each other, by carrying out ab initio calculations of interlayer interaction under constant loading force. Using the Prandtl-Tomlinson model we derive the critical stiffness required to avoid stick-slip behavior. We show that these layered structures have low critical stiffness even under high loading forces due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby the materials avoid the stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, tungsten dioxide displays a much better performance relative to others and heralds a potential superlubricant. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers.

Cahangirov, S.; Ataca, C.; Topsakal, M.; Sahin, H.; Ciraci, S.

2012-03-01

122

Electric arc behavior in a boundary layer  

NASA Astrophysics Data System (ADS)

The objective of this work is to understand how the size of an arc on the electrode of a magneto-hydrodynamic (MHD) generator or accelerator depends on operating parameters such as boundary-layer shape and current density. Arc size has an important bearing on device lifetime and performance. A boundary layer in an MHD device is a region of exceedingly steep gradients including, in particular, the gradient of electrical conductivity. A theory relating arc size to the characteristics of these gradients is developed and compared with behavior observed in various MHD devices, most recently that in the Component Development and Integration Facility in Butte, Montana.

Rosa, Richard; Farrar, Larry; Trudnowski, Dan

1988-10-01

123

Boundary layer halogens in coastal Antarctica.  

PubMed

Halogens influence the oxidizing capacity of Earth's troposphere, and iodine oxides form ultrafine aerosols, which may have an impact on climate. We report year-round measurements of boundary layer iodine oxide and bromine oxide at the near-coastal site of Halley Station, Antarctica. Surprisingly, both species are present throughout the sunlit period and exhibit similar seasonal cycles and concentrations. The springtime peak of iodine oxide (20 parts per trillion) is the highest concentration recorded anywhere in the atmosphere. These levels of halogens cause substantial ozone depletion, as well as the rapid oxidation of dimethyl sulfide and mercury in the Antarctic boundary layer. PMID:17641195

Saiz-Lopez, Alfonso; Mahajan, Anoop S; Salmon, Rhian A; Bauguitte, Stephane J-B; Jones, Anna E; Roscoe, Howard K; Plane, John M C

2007-07-20

124

Runup and boundary layers on sloping beaches  

NASA Astrophysics Data System (ADS)

The present study is devoted to discrepancies between experimental and theoretical runup heights on an inclined plane, which have occasionally been reported in the literature. In a new study on solitary wave-runup on moderately steep slopes, in a wave tank with 20 cm water depth, detailed observations are made for the shoreline motion and velocity profiles during runup. The waves are not breaking during runup, but they do break during the subsequent draw-down. Both capillary effects and viscous boundary layers are detected. In the investigated cases the onshore flow is close to the transitional regime between laminar and turbulent boundary layers. The flow behaviour depends on the amplitude of the incident wave and the location on the beach. Stable laminar flow, fluctuations (Tollmien-Schlichting waves), and formation of vortices are all observed. Comparison with numerical simulations showed that the experimental runup heights were markedly smaller than predictions from inviscid theory. The observed and computed runup heights are discussed in the context of preexisting theory and experiments. Similar deviations are apparent there, but have often been overlooked or given improper physical explanations. Guided by the absence of turbulence and irregular flow features in parts of the experiments we apply laminar boundary layer theory to the inundation flow. Outer flows from potential flow models are inserted in a nonlinear, numerical boundary layer model. Even though the boundary layer model is invalid near the moving the shoreline, the computed velocity profiles are found to compare well with experiments elsewhere, until instabilities are observed in the measurements. Analytical, linear boundary layer solutions are also derived both for an idealized swash zone motion and a polynomial representation of the time dependence of the outer flow. Due to lacking experimental or theoretical descriptions of the contact point dynamics no two-way coupling of the boundary layer model and the inviscid runup models is attempted. Instead, the effect of the boundary layer on the maximum runup is estimated through integrated losses of onshore volume transport and found to be consistent with the differences between inviscid theory and experiments.

Pedersen, G. K.; Lindstrøm, E.; Bertelsen, A. F.; Jensen, A.; Laskovski, D.; Sælevik, G.

2013-01-01

125

Boundary-layer flows in rotating cavities  

NASA Astrophysics Data System (ADS)

Differential boundary layer equations modelling the flow between two corotating air-cooled gas-turbine disks are solved to study the velocity distribution inside the entraining and nonentraining boundary layers and in the inviscid core. The equations are discretized using the box scheme of Keller and Cebeci (1972), and the Cebeci-Smith (1974) eddy-viscosity model is used to treat the turbulent-flow case. Good agreement between the present computations and previous experimental results is obtained for a wide range of flow rates and rotational speeds.

Ong, C. L.; Owen, J. M.

1988-06-01

126

Orientation dependent molecular friction on organic layer compound crystals  

NASA Astrophysics Data System (ADS)

High resolution friction force maps of the benzylammonium terminated crystalline surface of a layer compound are presented. The lateral force map acquired with an atomic force microscope, reveals a significant contrast between different molecular orientations yielding molecular rows which differ from their neighboring ones. The single crystals are formed by stacks of copper oxalate sheets sandwiched between stereoregular organic cations, resulting in highly organized surface structures. Single molecular defects are observed at small loads. The experimental results are compared with numerical calculations which indicate a transition from an unperturbed state at small loads to a distorted state at higher loads.

Fessler, Gregor; Zimmermann, Iwan; Glatzel, Thilo; Gnecco, Enrico; Steiner, Pascal; Roth, Raphael; Keene, Tony D.; Liu, Shi-Xia; Decurtins, Silvio; Meyer, Ernst

2011-02-01

127

Numerical investigation of influence of rotor/stator interaction on blade boundary layer flow in a low speed compressor  

NASA Astrophysics Data System (ADS)

Numerical method was applied to the unsteady flow simulation at the mid span of a two-stage low speed compressor, and the blade boundary layer flow under rotor/stator interaction was investigated. By the model of wake/boundary layer interaction provided in this paper, the simulated blade frictional force and the boundary layer turbulent kinetic energy, the influence of wake/potential flow interaction on the blade boundary layer flow was analyzed in detail. The results show that under the condition of rotor/stator interaction, the wake is able to induce the stator laminar boundary layer flow to develop into turbulent flow within a certain range of wake interaction. In the stator suction boundary layer, an undisturbed region occurs behind the rotor wake, which extends the laminar flow range, and the wake with high turbulent intensity has the capability to control the boundary layer separation under adverse pressure gradient.

Yao, Hongwei; Yan, Peigang; Han, Wanjin

2011-02-01

128

Coastal boundary layer transition within tropical cyclones at landfall  

NASA Astrophysics Data System (ADS)

Hurricanes pose a great risk to life and property with their high winds, excessive rainfall, wave action, and storm surge. Predicting changes within hurricanes at and near the time of landfall requires an understanding of the dynamics that drive the boundary layer flow. Forecasters predict the timing, duration, and effects of the intense winds associated with a hurricane when it comes ashore, while emergency management officials call for public evacuations based upon these forecasts. One region where understanding the magnitude and structure of the wind is critical is within the surface layer just downstream of the coastline in the onshore flow. Within this region the flow begins to adjust to changes in surface triggered by its passage from the shallow coastal waters to the less homogeneous and rougher land. This adjustment may include a slowing of the mean wind with an increase in turbulence, both resulting from the increased friction of the man-made and natural terrain. Hurricane observing programs consisting of portable and mobile equipment and regional coastal mesoscale observing networks are leading to a better understanding of the processes involved with these flow modifications. The Texas Tech University Wind Engineering Mobile Instrumented Tower Experiment (WEMITE) continues to play a leading role in the observation and analysis of the boundary layer of tropical cyclones at landfall. In order to gain further insight into the characteristics of this coastal transition zone, experiments were planned utilizing portable in-situ and remote measuring devices to be placed within the onshore flow at landfall. Experiment plan designs along with results from these experiments are discussed, including the analysis of a dataset collected by multiple institutions during the landfall of Hurricane Lili (2002) along the south-central Louisiana coast. Investigation reveals the existence of frictionally-induced changes in the boundary layer downwind of the coastline within the right semicircle with respect to Lili's forward motion. In the outer reaches of Lili, these transitions appear similar to internal boundary layers produced by flow moving over an abrupt change in surface. The impact on the magnitude of the wind within this near-shore region is a reduction of 4--10% per 10 km distance from the coast up to 50 km inland for open terrain. Results of the study show this reduction to be an exponential function of distance from the coast, which is dependent upon surface roughness. This rate of wind decay slows with farther progression inland and appears to be much faster than the rate found in some modeling studies. In contrast, near Lili's circulation center, little or no decrease in the magnitude of the mean wind was found for distances of up to 20 km inland.

Howard, James Robert

129

Boundary layer control device for duct silencers  

NASA Astrophysics Data System (ADS)

A boundary layer control device includes a porous cover plate, an acoustic absorber disposed under the porous cover plate, and a porous flow resistive membrane interposed between the porous cover plate and the acoustic absorber. The porous flow resistive membrane has a flow resistance low enough to permit sound to enter the acoustic absorber and high enough to damp unsteady flow oscillations.

Schmitz, Fredric H.; Soderman, Paul T.

1993-11-01

130

Wall functions for unsteady turbulent boundary layers  

Microsoft Academic Search

A method for obtaining wall functions for a turbulent boundary layer flow, in which the free-stream velocity oscillates periodically about a nonzero mean, is presented. Particular expressions of these wall functions are proposed for a k-epsilon turbulence model.

A. N. Menendez; B. R. Ramaprian

1985-01-01

131

Simulating observed boundary layer clouds on Mars  

Microsoft Academic Search

A microphysical model for Mars dust and ice clouds has been applied in combination with a model of the planetary boundary layer (PBL) for the interpretation of measurements by the LIDAR instrument on the Phoenix Mars mission. The model simulates nighttime clouds and fall streaks within the PBL that are similar in structure to the LIDAR observations. The observed regular

F. Daerden; J. A. Whiteway; R. Davy; C. Verhoeven; L. Komguem; C. Dickinson; P. A. Taylor; N. Larsen

2010-01-01

132

Interaction between Trapped Waves and Boundary Layers  

Microsoft Academic Search

The absorption of trapped lee waves by the atmospheric boundary layer (BL) is investigated based on numerical simulations and theoretical formulations. It is demonstrated that the amplitude of trapped waves decays exponentially with downstream distance due to BL absorption. The decay coefficient, , defined as the inverse of the e-folding decay distance, is found to be sensitive to both surface

Qingfang Jiang; James D. Doyle; Ronald B. Smith

2006-01-01

133

Diagnosis of boundary-layer circulations.  

PubMed

Diagnoses of circulations in the vertical plane provide valuable insights into aspects of the dynamics of the climate system. Dynamical theories based on geostrophic balance have proved useful in deriving diagnostic equations for these circulations. For example, semi-geostrophic theory gives rise to the Sawyer-Eliassen equation (SEE) that predicts, among other things, circulations around mid-latitude fronts. A limitation of the SEE is the absence of a realistic boundary layer. However, the coupling provided by the boundary layer between the atmosphere and the surface is fundamental to the climate system. Here, we use a theory based on Ekman momentum balance to derive an SEE that includes a boundary layer (SEEBL). We consider a case study of a baroclinic low-level jet. The SEEBL solution shows significant benefits over Ekman pumping, including accommodating a boundary-layer depth that varies in space and structure, which accounts for buoyancy and momentum advection. The diagnosed low-level jet is stronger than that determined by Ekman balance. This is due to the inclusion of momentum advection. Momentum advection provides an additional mechanism for enhancement of the low-level jet that is distinct from inertial oscillations. PMID:23588045

Beare, Robert J; Cullen, Michael J P

2013-04-15

134

Reduction of turbulent drag: Boundary layer manipulators  

Microsoft Academic Search

The drag reduction on airbus profiles is investigated. External and internal boundary layer manipulators are applied. The wind tunnel wall geometry and the model surface geometry are modified, carving riblets in the sense of the main flow. The change induced in the flow are studied using hotwire anemometry and spectral analysis. Direct drag measurements on Airbus profiles indicate a drag

E. Coustols

1989-01-01

135

Turbulent boundary layer structure, drag reduction  

Microsoft Academic Search

The research work has progressed along the following lines: (1) continuation of theoretical work on the effects of large-eddy breakup devices on turbulent eddies; (2) theoretical study of the role of pressures in the wall turbulence generation process; (3) application of a theoretical model for designing smart wall boundary layer control; (4) use of a new bursting model for predicting

Marten T. Landahl; Joseph H. Haritonidis

1986-01-01

136

Particle-laden boundary layers and singularities  

NASA Astrophysics Data System (ADS)

The dusty-gas model for flow in dilute particle suspensions generates a singularity in particle volume fraction in a variety of viscous boundary layer problems. Such a singularity, at say x=xs along the wall, makes it impossible to continue the solution to the equations. Previously, we have found that computation of the Blasius boundary layer, with a modified equation set that permits fluid volume fraction significantly different from 1, still leads to a velocity singularity at a slightly modified location.ootnotetextFoster, Duck & Hewitt, Bull. Amer. Phys. Soc., November, 2006 Contrary to some published work, the Saffman force has not been found to mitigate the singularity for the conventional equation set, and again here, though the Saffman force does become comparable to the Stokes drag near the singularity, it alters the structure only slightly, and does not remove it. If ?o is the particle volume fraction of the fluid in which the boundary layer is embedded, then in certain re-scaled coordinates, the singularity occurs in a region ?ox?o/|?o| about xs, where a reduced set of equations applies. Within this region, there is a downstream-running ray from the origin on which ??1. However, the vertical fluid and particle velocity components are unbounded on that line. On replacing the line with a solid surface of particle material, a narrow boundary layer may be inserted, in which velocity singularities are removed.

Foster, M. R.

2007-11-01

137

Linear Optimal Controllers for Turbulent Boundary Layers  

Microsoft Academic Search

Several investigators have shown recently that controller design based on a system control-theoretic approach has led to significant viscous-drag reduction in turbulent boundary layers. Although these new approaches rely less on the investigator's physical insight into the flow (in contrast to most existing approaches), there still remain many design issues that need to be addressed. These are, but not limited

Junwoo Lim; John Kim; Sung Moon Kang; Jason Speyer

2001-01-01

138

Turbulent boundary layer characteristics over streamwise grooves  

Microsoft Academic Search

Turbulent boundary layer characteristics over streamwise grooves (on the order of 15 wall units in height and spacing) have been quantified in a water channel using hot film anemometry. The turbulent structure over both smooth and riblet surfaces has been investigated using traditional cross correlations and triple correlations. The VITA technique has been used to detect and count turbulent bursts

J. A. Gallagher; A. S. W. Thomas

1984-01-01

139

A Wake Boundary Layer Mixing Experiment.  

National Technical Information Service (NTIS)

The mixing of a boundary layer was investigated under zero pressure gradient with a two dimensional wake shed from a plate. Measurements of mean and fluctuating quantities were carried out for this flow in particular, as well as for one of the component f...

P. J. Pot

1979-01-01

140

INDIVIDUAL TURBULENT CELL INTERACTION: BASIS FOR BOUNDARY LAYER ESTABLISHMENT  

EPA Science Inventory

Boundary layers are important in determining the forces on objects in flowing fluids, mixing characteristics, and other phenomena. For example, benthic boundary layers are frequently active resuspension layers that determine bottom turbidity and transniissivity. Traditionally, bo...

141

Turbulent boundary layer pressure fluctuations at large scales  

NASA Astrophysics Data System (ADS)

The pressure fluctuations underneath a turbulent boundary are an important excitation source for noise and vibration for both aircraft and ships. This presentation describes experimental results from a new study of flat-plate turbulent boundary layer pressure fluctuations in water at large scales and high Reynolds number. The experiments were performed at the U.S. Navy's William B. Morgan Large Cavitation Channel in Memphis, TN on a polished flat plate 3.05 m wide, 12.8 m long, and 0.18 m thick. Flow velocity, skin friction, surface pressure, and plate acceleration measurements were made at multiple downstream locations at flow speeds ranging from 0.5 m/s to 19 m/s for a Reynolds number (based on downstream distance) range of several million to 200 million. Dynamic surface pressures were recorded with 16 flush mounted pressure transducers forming an L-array with streamwise dimension of 0.264 m and cross-stream dimension of 0.391 m. Measured 99% boundary-layer thicknesses were typically of order 0.10 m. Results for spatial and temporal correlation functions, as well as auto- and cross-spectra are presented and compared with prior lower-Reynolds-number results using either inner or outer variable scaling. [Work sponsored by the Defense Advanced Research Projects Agency and ONR Code 333.

Sanders, Wendy; Judge, Carolyn; Winkel, Eric; Ceccio, Steven; Dowling, David; Perlin, Marc

2002-05-01

142

Singularity Analysis and Boundary Integral Equation Method for Frictional Crack Problems in Two-Dimensional Elasticity  

Microsoft Academic Search

This study investigates the stress singularities in the neighborhood of the tip of a sliding crack with Coulomb-type frictional\\u000a contact surfaces, and applies the boundary integral equation method to solve some frictional crack problems in plane elasticity.\\u000a A universal approach to the determination of the complex order of stress singularity is established analytically by using\\u000a the series expansion of the

K. T. Chau; Y. B. Wang

1998-01-01

143

Compressible Ekman layers on curved boundaries  

NASA Astrophysics Data System (ADS)

The effects of the compressible Ekman layer on a rotating nonsymmetric surface are described. Equivalent boundary conditions on the 'interior' flow are obtained in terms of the primitive variables of velocity and temperature. Expressions are derived for the local flux vector of mass in the Ekman layer and for the mass transport induced into the Ekman layer from the interior. The interior temperature at the boundary is given as an explicit function of the interior velocity there and the imposed wall conditions. The interior velocity is itself determined from the suction formula by a direct and fairly routine procedure. Examples are used to show that these formulas markedly simplify the problem of motion in a centrifuge of rather arbitrary shape.

Greenspan, H. P.

1984-04-01

144

Nonuniqueness in wakes and boundary layers  

NASA Astrophysics Data System (ADS)

Instreamlined flow past a flat plate aligned with a uniform stream, it is shown that the Goldstein near wake and the Blasius boundary layer are nonunique solutions locally for the classical boundary layer equations, whereas the Rott-Hakkien very near wake appears to be unique. Concerning non-streamlined flow, new similarity forms are described for the pressure free vicous symmetric closure of a predominantly slender long wake beyond a large-scale separation. Features arising include nonuniqueness, singularities and algebraic behavior, consistent with non-entraining shear layers with algebraic decay. Nonuniqueness also seems possible in reattachment onto a solid surface and for nonsymmetric or pressure controlled flows including the wake of a symmetric cascade.

Smith, F. T.

1983-05-01

145

Boundary layer effects on sound in a circular duct  

Microsoft Academic Search

Boundary layer effects on an acoustic field in a unidirectional flow with transverse shear are studied. The acoustic pressure variation in the direction normal to that of the flow is governed in the boundary layer by a second order differential equation. The problem in the boundary layer is reduced from a two point boundary value problem to a one point

R. T. Nagel; R. S. Brand

1982-01-01

146

Bursting frequency prediction in turbulent boundary layers  

SciTech Connect

The frequencies of the bursting events associated with the streamwise coherent structures of spatially developing incompressible turbulent boundary layers were predicted using global numerical solution of the Orr-Sommerfeld and the vertical vorticity equations of hydrodynamic stability problems. The structures were modeled as wavelike disturbances associated with the turbulent mean flow. The global method developed here involves the use of second and fourth order accurate finite difference formula for the differential equations as well as the boundary conditions. An automated prediction tool, BURFIT, was developed. The predicted resonance frequencies were found to agree very well with previous results using a local shooting technique and measured data.

LIOU,WILLIAM W.; FANG,YICHUNG

2000-02-01

147

Transition of MHD boundary layer flow past a stretching sheet  

NASA Astrophysics Data System (ADS)

In this paper a study is carried out to understand the transition effect of boundary layer flow: (1) due to a suddenly imposed magnetic field over a viscous flow past a stretching sheet and (2) due to sudden withdrawal of magnetic field over a viscous flow past a stretching sheet under a magnetic field. In both the cases the sheet stretches linearly along the direction of the fluid flow. Governing equations have been non-dimensionalised and the non-dimensionalised equations have been solved using the implicit finite difference method of Crank-Nicholson type. Comparison between the steady state exact solutions and the steady state computed solutions has been carried out. Graphical representation of the dimensionless horizontal velocity, vertical velocity and local skin friction profiles of the steady state and unsteady state has been presented. Computation has been carried out for various values of the magnetic parameter M. The obtained results has been interpreted and discussed.

Kumaran, V.; Vanav Kumar, A.; Pop, I.

2010-02-01

148

Experimental Study of the Boundary Layer Formation over Three Dimensional Arrays of Embedded Hexagonal Cavities  

NASA Astrophysics Data System (ADS)

With increasing fuel costs, research into reducing drag over solid surfaces in high Reynolds number flows is still an area of interest. There have been many studies examining the boundary layer flow over two-dimensional microgeometries (e.g. riblets), but very few studies involving three dimensional microgeometries. The main objective of this study was to examine how embedded vortices, forming in hexagonal cavities, affect the boundary layer flow over a solid surface. It is believed that stable embedded vortices produce a partial slip condition, which could result in decreasing the skin friction and delaying the transition to turbulence while also acting as a means of separation control. To study the boundary layer flow, a model was constructed using a hexagonal array of cavities embedded into a flat plate. Using a water tunnel, dye visualization and DPIV measurements, the boundary layer flow forming above the cavities was examined. Measurements were also compared when changing the orientation of the hexagonal cavities.

Melnick, Blake; Lang, Amy

2008-11-01

149

The effects of external conditions in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The effects of multiple external conditions on turbulent boundary layers were studied in detail. These external conditions include: surface roughness, upstream turbulence intensity, and pressure gradient. Furthermore, the combined effects of these conditions show the complicated nature of many realistic flow conditions. It was found that the effects of surface roughness are difficult to generalize, given the importance of so many parameters. These parameters include: roughness geometry, roughness regime, roughness height to boundary layer thickness, (k/delta), roughness parameter, ( k+), Reynolds number, and roughness function (Delta B+). A further complication, is the difficulty in computing the wall shear stress, tauw/rho. For the sand grain type roughness, the mean velocity and Reynolds stresses were studied in inner and outer variables, as well as, boundary layer parameters, anisotropy tensor, production term, and viscous stress and form drag contributions. To explore the effects of roughness and Reynolds number dependence in the boundary layer, a new experiment was carefully designed to properly capture the x-dependence of the single-point statistics. It was found that roughness destroys the viscous layer near the wall, thus, reducing the contribution of the viscous stress in the wall region. As a result, the contribution in the skin friction due to form drag increases, while the viscous stress decreases. This yields Reynolds number invariance in the skin friction, near-wall roughness parameters, and inner velocity profiles as k + increases into the fully rough regime. However, in the transitionally rough regime, (i.e., 5 < k+ < 70), it was found that these parameters are functions of both Reynolds number and roughness. For the sand grain type roughnesses, only the Zagarola and Smits scaling, Uinfinitydelta*/delta, is able to remove the effects of roughness and Reynolds number from the velocity profiles in outer variables, provided there is no freestream turbulence. However, each scaling for the velocity deficit profiles results in self-similar solutions for fixed experimental conditions. When examining the Reynolds stresses in the inner region, (i.e., 0 < (y + epsilon)+ < 0:1delta +), the < u2 > component shows the largest influence of roughness, where the high peak near the wall was decreased and became nearly flat for the fully rough regime profiles. In addition, the Reynolds stresses in outer variables show self-similarity for fixed experimental conditions. However, as the roughness parameter, k +, increases, all Reynolds stress profiles become similar in shape indicating increased isotropy near the wall. Furthermore, the boundary layer parameters and production terms also show a considerable increase due to roughness. This study of rough wall turbulence was also combined with high freestream turbulence. The freestream turbulence was generated with the use of an active grid, which resulted in freestream turbulence levels of 6.2% and 5.2% at the two downstream measuring locations. The effect of the freestream turbulence on this rough surface significantly alters the mean velocity deficit profiles. In inner variables, the velocity profiles show a significantly reduced wake region, while in outer variables, a more full profile indicates increased momentum transport towards the wall. Furthermore, the effects of freestream turbulence are clearly identifiable in the Reynolds stress profiles. Furthermore, pressure gradient flows are also difficult to generalize, given that a significant difference in the boundary layer structure exists between different external pressure gradients, (i.e., FPG, ZPG, and APG). This was examined through the scaling of the velocity and Reynolds stresses from multiple data sets. (Abstract shortened by UMI.)

Brzek, Brian G.

150

Boundary layer control of rotating convection systems.  

PubMed

Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems. PMID:19148097

King, Eric M; Stellmach, Stephan; Noir, Jerome; Hansen, Ulrich; Aurnou, Jonathan M

2009-01-15

151

Turbulent Boundary Layers on Rough Surfaces  

Microsoft Academic Search

The incompressible zero-pressure-gradient turbulent boundary layers on rough surface are investigated in light of the effects of the Reynolds number and roughness. The experimental data from various researchers were collected and analyzed. The true asymptotic profile (self-preserving solution) for the outer flow is found when the profiles are normalized by the Zagarola\\/Smits (1998) scaling, U_? delta\\/delta. This scaling successfully remove

Junghwa Seo; Luciano Castillo

2003-01-01

152

Active Control of Laminar Boundary Layer Disturbances  

Microsoft Academic Search

\\u000a Active suppression of the naturally occurring travelling wave disturbances that amplify in laminar boundary layers and cause\\u000a the transition from laminar to turbulent flow is considered. Both open-loop and closed-loop schemes are discussed. Numerical\\u000a predictions, based on linear stability theory, have been used to model the behaviour of the flow disturbances and the controlled\\u000a waves. Predictions based on these models

M. Gaster

153

Turbulent boundary layer drag reduction using riblets  

Microsoft Academic Search

An experimental study of low-speed turbulent boundary layer flow over longitudinally grooved surfaces (i.e., riblets) is discussed. Results obtained with a highly accurate drag balance indicate that v-groove riblet surfaces can produce consistent net drag reductions as large as 8 percent provided the height and spacing of the grooves in terms of law of the wall variables are less than

M. J. Walsh

1982-01-01

154

Reduction of turbulent drag: Boundary layer manipulators  

NASA Astrophysics Data System (ADS)

The drag reduction on airbus profiles is investigated. External and internal boundary layer manipulators are applied. The wind tunnel wall geometry and the model surface geometry are modified, carving riblets in the sense of the main flow. The change induced in the flow are studied using hotwire anemometry and spectral analysis. Direct drag measurements on Airbus profiles indicate a drag reduction of 3.5 percent. Experiments using cylindrical bodies in transonic flow show a drag reduction of 8 percent.

Coustols, E.

1989-03-01

155

Linearization of turbulent boundary-layer equations  

NASA Astrophysics Data System (ADS)

In previous solutions of the turbulent boundary layer equations that use the eddy viscosity concept in algebraic form, Newton's method has been applied to all terms except the viscous term, where the values of the eddy viscosity were assumed from a previous iteration. The method presently set out and evaluated was devised to allow the application of Newton's method to all terms of the momentum equation, using the eddy viscosity formulation of Cebeci and Smith (1974).

Cebeci, T.; Chang, K. C.; Mack, D. P.

1984-12-01

156

Effect of variability on boundary layer separation  

NASA Astrophysics Data System (ADS)

The behavior of a boundary layer during the accelerated motion of a cylinder in flow of a liquid was investigated experimentally using a hydrodynamic testing apparatus. Test results are presented for cylinders of 0.5-1 cm in diameter over the Reynolds number range 0.1-50,000 along the cylinder diameter for both laminar and turbulent external flow. The results can be used in the design of different kinds of process equipment involving nonstationary flow past various bodies.

Gudkov, V. A.

1991-10-01

157

Nearly Free Convection in Thermally Stratified Boundary Layers  

NASA Astrophysics Data System (ADS)

Thermally stratified boundary layers have generally been handled through Monin-Obukhov theory, although there has been considerable discussion about the free convection limit of this theory. Based on two atmospheric boundary layer experiments, one at Jodhpur in India and the other in Oklahoma, US, it is shown here that if the mean wind is sufficiently low, the drag varies linearly with wind while the heat flux continues to be governed by the free convection law. These characteristics define what may be called the `weakly forced convection' sub-regime within the broader regime of mixed convection. To make scaling arguments in this sub-regime, it is shown that it is useful to adopt the heat flux, rather than the wall stress (equivalently friction velocity) as Monin-Obukhov theory does. Several candidates for a heat-flux velocity scale are considered, and their relative merits assessed. These arguments lead to novel definitions of drag and heat transfer coefficients that are independent of wind speed.

Narasimha, Roddam

2005-11-01

158

Boundary-layer turbulence characteristics during aeolian saltation  

NASA Astrophysics Data System (ADS)

A great deal of effort has been expended in measuring turbulence phenomena in clean air flows. However, no previous measurements have been successfully made of the vertical distributions of turbulence intensity and Reynolds stress in a fully adjusted boundary-layer flow saturated with saltating particles. The present wind tunnel study addresses this knowledge gap using a custom designed laser-Doppler anemometer (LDA). The amount of turbulence is found to increase with the introduction of saltating particles to the airflow. Over the lowest 15% of boundary layer, vertical profiles of the streamwise wind speed provide friction velocities that lie well within the narrow range of those derived from direct measurement of the Reynolds stress. Relative to clean air, aeolian saltation is demonstrated to increase the magnitude but not the frequency of burst-sweep events that primarily contribute to the total fluid stress. Within several millimeters above the bed surface, all vertical profiles of wind speed converge upon a focal point, as the local fluid stress declines toward the mobile bed.

Li, Bailiang; McKenna Neuman, Cheryl

2012-06-01

159

The interaction of synthetic jets with turbulent boundary layers  

NASA Astrophysics Data System (ADS)

In recent years, a promising approach to the control of wall bounded as well as free shear flows, using synthetic jet (oscillatory jet with zero-net-mass-flux) actuators, has received a great deal of attention. A variety of impressive flow control results have been achieved experimentally by many researchers including the vectoring of conventional propulsive jets, modification of aerodynamic characteristics of bluff bodies, control of lift and drag of airfoils, reduction of skin-friction of a flat plate boundary layer, enhanced mixing in circular jets, and control of external as well as internal flow separation and of cavity oscillations. More recently, attempts have been made to numerically simulate some of these flowfields. Numerically several of the above mentioned flow fields have been simulated primarily by employing the Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with a turbulence model and a limited few by Direct Numerical Simulation (DNS). In simulations, both the simplified boundary conditions at the exit of the jet as well as the details of the cavity and lip have been included. In this dissertation, I describe the results of simulations for several two- and three-dimensional flowfields dealing with the interaction of a synthetic jet with a turbulent boundary layer and control of separation. These simulations have been performed using the URANS equations in conjunction with either one- or a two-equation turbulence model. 2D simulations correspond to the experiments performed by Honohan at Georgia Tech. and 3D simulations correspond to the CFD validation test cases proposed in the NASA Langley Research Center Workshop---"CFD Validation of Synthetic Jets and Turbulent Separation Control" held at Williamsburg VA in March 2004. The sources of uncertainty due to grid resolution, time step, boundary conditions, turbulence modeling etc. have been examined during the computations. Extensive comparisons for various flow variables are made with the experimental data; fair agreement is obtained.

Cui, Jing

160

Control of Nanoscale Friction on Gold in an Ionic Liquid by a Potential-Dependent Ionic Lubricant Layer  

NASA Astrophysics Data System (ADS)

The lubricating properties of an ionic liquid on gold surfaces can be controlled through application of an electric potential to the sliding contact. A nanotribology approach has been used to study the frictional behavior of 1-butyl-1-methylpyrrolidinium tris(penta?uoroethyl) tri?uorophosphate ([Py1,4]FAP) confined between silica colloid probes or sharp silica tips and a Au(111) substrate using atomic force microscopy. Friction forces vary with potential because the composition of a confined ion layer between the two surfaces changes from cation-enriched (at negative potentials) to anion-enriched (at positive potentials). This offers a new approach to tuning frictional forces reversibly at the molecular level without changing the substrates, employing a self-replenishing boundary lubricant of low vapor pressure.

Sweeney, James; Hausen, Florian; Hayes, Robert; Webber, Grant B.; Endres, Frank; Rutland, Mark W.; Bennewitz, Roland; Atkin, Rob

2012-10-01

161

Calculation and Measurement of Separated Turbulent Boundary Layers.  

National Technical Information Service (NTIS)

An inverse integral prediction method for the development of separated turbulent boundary layers developed from the lag-entrainment method is described. The inverse method uses the concept of equilibrium separated boundary layer flows and the predicted ch...

P. D. Smith R. C. Hastings B. R. Williams

1982-01-01

162

Linear Stability Theory and Three-Dimensional Boundary Layer Transition.  

National Technical Information Service (NTIS)

The viewgraphs and discussion of linear stability theory and three dimensional boundary layer transition are provided. The ability to predict, using analytical tools, the location of boundary layer transition over aircraft-type configurations is of great ...

R. E. Spall M. R. Malik

1992-01-01

163

Physics of ice friction  

NASA Astrophysics Data System (ADS)

Although the study of friction has a long history, ice friction has only been investigated during the last century. The basic physical concepts underlying the different friction regimes, such as boundary, mixed, and hydrodynamic friction are also relevant to ice friction. However, these friction regimes must be described with respect to the thickness of the lubricating liquidlike layer on ice. In this review the state of knowledge on the physics of ice friction is discussed. Surface melting theories are introduced. These theories attempt to explain the existence and nature of the liquidlike surface layer on ice at any temperature and without any load applied. Pressure melting, as the long-time explanation for the ease of ice friction, is discussed, together with the prevailing theory of frictional heating. The various laboratory setups for ice friction measurements are presented as well as their advantages and disadvantages. The individual influence of the different parameters on the coefficient of ice friction is discussed; these include the effects of temperature, sliding velocity, normal force exerted by the sliding object, the contact area between ice and slider, relative humidity, and also properties of the slider material such as surface roughness, surface structure, wettability, and thermal conductivity. Finally, the most important ice friction models based on the frictional heating theory are briefly introduced and research directions on the subject of ice friction are discussed.

Kietzig, Anne-Marie; Hatzikiriakos, Savvas G.; Englezos, Peter

2010-04-01

164

ADSORPTION MODELING OF FATTY ESTERS AND OLEIC ESTOLIDES VIA BOUNDARY LUBRICATION COEFFICIENT OF FRICTION MEASUREMENTS  

Technology Transfer Automated Retrieval System (TEKTRAN)

The frictional behaviors of a variety of fatty esters (methyl palmitate (MP), methyl laurate (ML), and 2-ethylhexyl oleate (EHO)) and oleic estolide esters (methyl oleic estolide ester (ME) and 2-ethylhexyl oleic estolide ester (EHE)) as additives in hexadecane have been examined in a boundary lubri...

165

Thermomechanical model with adaptive boundary conditions for friction stir welding of Al 6061  

Microsoft Academic Search

Thermo-mechanical simulation of friction stir welding can predict the transient temperature field, active stresses developed, forces in all the three dimensions and may be extended to determine the residual stress. The thermal stresses constitute a major portion of the total stress developed during the process. Boundary conditions in the thermal modeling of process play a vital role in the final

Vijay Soundararajan; Srdja Zekovic; Radovan Kovacevic

2005-01-01

166

Daytime Evolution of Relative Humidity at the Boundary Layer Top  

Microsoft Academic Search

Data from the HAPEX-MOBILHY field program and results from a one-dimensionalmodel of the soil and atmospheric boundary layer are analyzed to study the daytimeevolution of the relative humidity at the boundary-layer top. This evolution is thought tocontrol the development of boundary-layer clouds. This study examines thedependence of boundary-layer relative humidity on soil moisture, large-scale verticalmotion, and the moisture content and

M. Ek; L. Mahrt

1994-01-01

167

Remote Sensing of Boundary layer Trace Gases in the Presence of dynamic boundary layer events  

NASA Astrophysics Data System (ADS)

With the suite of instruments located at UMBC we are able to analyze how low level jets and convective rolls affect boundary layer carbon monoxide and ozone. Low level jets are an excellent transport mechanism for boundary layer air. These jets alter the nominal nocturnal ozone cycle and inhibit the total depletion of ozone during night time. Using BBAERI (Baltimore Bomem Atmospheric Emitted Radiance Interferometer) we retrieve boundary layer CO and total column ozone during both day and night. Comparing inactive to active nights we can better understand the effect jets have on the ozone cycle. The WRF regional model is used to simulate nocturnal jet events to elucidate source regions for the BBAERI retrievals of trace gas abundances. Previous studies using the AERI at the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site near Lamont, Oklahoma have demonstrated the impact of horizontal convective rolls on boundary layer water vapor profiles. We will present results of CO retrievals from BBAERI and AERI in the presence of convective rolls to better understand how they mix CO. These observations demonstrate the utility of BBAERI and other AERIs for remotely monitoring boundary layer composition and dynamics.

Wilson, R. C.; McMillan, W. W.; Delgado, R.; Hoff, R.; Weldegabar, M.

2008-12-01

168

A simple model of the hurricane boundary layer  

Microsoft Academic Search

A simple, steady, moist, axisymmetric, constant depth, slab model for the hurricane boundary layer is investigated. High-resolution solutions of the boundary layer equations are obtained by integrating inwards from some large radius, at which it is assumed that geostrophic balance and convective-radiative balance exists. In all the solutions obtained, the tangential wind speed in the boundary layer approaches that above

Roger K. Smith

2002-01-01

169

Mixing length in low Reynolds number compressible turbulent boundary layers  

Microsoft Academic Search

The paper studies the effect of low Reynolds number in high-speed turbulent boundary layers on variations of mixing length. Boundary layers downstream of natural transition on plates, cones and cylinders, and boundary layers on nozzle walls without laminarization-retransition are considered. The problem of whether low Reynolds number amplification of shear stress is a result of transitional flow structure is considered.

D. M. Bushnell; A. M. Cary Jr.; B. B. Holley

1975-01-01

170

THE EFFECTS OF PERIODIC WAKE STRUCTURES ON TURBULENT BOUNDARY LAYERS  

Microsoft Academic Search

Compressor and turbine blade boundary layers in axial-flow turbomachines are subject to periodically disturbed flow. This study modelled these conditions in a wind tunnel with circular cylinders traversing in front of a flat plate. Turbulent boundary layer velocity profiles on the flat plate were measured with a hot-wire anemometer. The turbulence intensity in the boundary layer was found to be

R. M. Holland; R. L. Evans

1996-01-01

171

Validation of High-Speed Turbulent Boundary Layer and Shock-Boundary Layer Interaction Computations with the OVERFLOW Code.  

National Technical Information Service (NTIS)

The capability of the OVERFLOW code to accurately compute high-speed turbulent boundary layers and turbulent shock-boundary layer interactions is being evaluated. Configurations being investigated include a Mach 2.87 flat plate to compare experimental vel...

A. B. Oliver A. S. Lyrintizis G. A. Blaisdell R. P. Lillard

2006-01-01

172

The efficient calculation of turbulent boundary layers  

NASA Astrophysics Data System (ADS)

A simplified approach to the computation of turbulent boundary layers (TBL) by implicit-difference methods is developed and demonstrated. It is shown that the viscous sublayer can be neglected when integrating the TBL equations if the boundary conditions near the wall are appropriately modified. Logarithmic coordinates are introduced to improve the accuracy of the difference formulas. A number of numerical examples are calculated, and the results and computation times are compared in graphs and tables with those of the transformation of Cebeci and Smith (1968), which integrates the sublayer. In the case of the TBL on a plane surface, time savings of 56 and 65 percent are achieved for Reynolds numbers of 22,000 and 240,000, respectively.

Rotta, J. C.

1983-12-01

173

Stationary Flow in the Planetary Boundary Layer with an Inversion Layer and a Sea Breeze  

Microsoft Academic Search

An analysis is made of the effects of an inversion layer and a sea breeze on the stationary flow in a planetary boundary layer. It is shown that an intense temperature inver- sion virtually acts as a boundary surface which separates a planetry boundary layer into two layers. In the lower layer the flow is primarily thermally driven whereas in

S.-K. Kao

1960-01-01

174

Bottom boundary layer flow profiling system  

SciTech Connect

An autonomous profiling system is being developed to measure physical and optical properties in ocean-bottom boundary layers. System sensors will include electromagnetic current meters, temperature sensors, transmissometers, and water sample bottles affixed in a vertical array to bottom-supported instrument frame at heights ranging from 0.25-5.00 m (1-16 ft) above its base. The instrumentation will measure high-frequency property fluctuations (5 Hz) as well as mean values. High-capacity tape recorders will permit unattended deployments for up to 3 months. Field tests will be conducted in the Gulf of Mexico.

Adams, C.E. Jr.; Fredericks, R.D.

1984-04-01

175

The minisodar and planetary boundary layer studies  

SciTech Connect

The minisodar, in addition to being smaller than conventional sodar, operates at higher frequencies, obtains usable signal returns closer to the surface, and can use smaller range gates. Because the max range is generally limited to the lower 200 m above the surface, the minisodar is not able to interrogate the entire daytime atmospheric Planetary Boundary Layer (PBL); however it can be a very useful tool for understanding the PBL. In concert with other instruments, the minisodar can add significant new insights to our understanding of the PBL. This paper gives examples of past and potential uses of minisodars in such situations.

Coulter, R.L.

1996-06-01

176

Consideration on boundary-layer receptivity  

NASA Astrophysics Data System (ADS)

On the boundary layer receptivity, the present paper describes the results of numerical simulations and laboratory experiments to clarify the condition under which external disturbances such as sound and free stream turbulence can excite Tollmien Schlichting (T-S) waves. The most important finding is that the unsteady pressure gradient on the wall imposed by the external disturbance need to have such proper spatial scales as to match lambda(sub TS), the wavelength of T-S wave to be excited. In this case, the unsteady pressure gradient induces the vortical wave of the proper scale and then T-S wave.

Nishioka, Michio

1991-09-01

177

Chaotic motion in an oscillatory boundary layer.  

PubMed

The chaotic time oscillations in an incompressible fluid driven into motion by a harmonic time-varying pressure gradient is examined. Special attention is given to centrifugal destabilization of the viscous boundary layer. The basic flow is shown to be linearly unstable. For increasing modulation amplitude, the flow exhibits chaotic oscillations. The energy exchange between subharmonics and superharmonics of the least-stable spanwise wave number is considered. The presence of subharmonic Fourier modes are shown to accelerate the transition to temporally chaotic motion. (c) 1996 American Institute of Physics. PMID:12780290

Mehta, V.; Thompson, C.; Mulpur, A.; Chandra, K.

1996-12-01

178

Friction  

Microsoft Academic Search

\\u000a Friction materials such as disk pads, brake linings, and clutch facings are widely used for automotive applications. Friction\\u000a materials function during braking due to frictional resistance that transforms kinetic energy into thermal energy. There has\\u000a been a rudimentary evolution, from materials like leather or wood to asbestos fabric or asbestos fabric saturated with various\\u000a resins such as asphalt or resin

Yoshihiro Matsuo; Daryl D. Clarke; Shinichi Ozeki

2010-01-01

179

Friction-term response to boundary-condition type in flow models  

USGS Publications Warehouse

The friction-slope term in the unsteady open-channel flow equations is examined using two numerical models based on different formulations of the governing equations and employing different solution methods. The purposes of the study are to analyze, evaluate, and demonstrate the behavior of the term in a set of controlled numerical experiments using varied types and combinations of boundary conditions. Results of numerical experiments illustrate that a given model can respond inconsistently for the identical resistance-coefficient value under different types and combinations of boundary conditions. Findings also demonstrate that two models employing different dependent variables and solution methods can respond similarly for the identical resistance-coefficient value under similar types and combinations of boundary conditions. Discussion of qualitative considerations and quantitative experimental results provides insight into the proper treatment, evaluation, and significance of the friction-slope term, thereby offering practical guidelines for model implementation and calibration.

Schaffranek, R. W.; Lai, C.

1996-01-01

180

Friction behaviour of Si-DLC\\/DLC multi layer films on steel substrate in water environment  

Microsoft Academic Search

The tribological properties of diamond-like carbon (DLC) films over-coated by various thicknesses of Si-doped layers were investigated in a water environment. The multi-layer DLC films were deposited on a stainless steel substrate by pulsed-bias CVD technique. A dependence of friction coefficients on thickness of the over-coated layer was not observed and the friction coefficients of the films showed a stable

T. Ohana; M. Suzuki; T. Nakamura; A. Tanaka; Y. Koga

2005-01-01

181

Drag reduction for external and internal boundary layers using riblets and polymers  

Microsoft Academic Search

The efficiency of riblets and a drag-reducing polymer solution (a polyacrylamide slurry) in high-speed water tunnels for reducing drag in turbulent boundary layers was investigated in two experiments. One was an external flow experiment, in which riblets were applied to a flat plate in a high-speed water tunnel and the skin friction drag was calculated from velocity profile data. The

Laurel W. Reidy; Greg W. Anderson

1988-01-01

182

Experimental investigation of turbulent boundary layers manipulated with internal devices - Riblets  

NASA Astrophysics Data System (ADS)

The purpose of this paper is to summarize the current status of the experimental research, as regards internal devices, carried out in low-speed wind tunnel when manipulating two-dimensional boundary layers. The main objective is to provide an estimate of attainable drag reduction performances in zero as well as adverse pressure gradient flows. Moreover, the effect of combining internal and external devices has been checked in order to control whether or not the overall skin-friction reductions are additive.

Coustols, E.; Cousteix, J.

183

Microbubble drag reduction in rough walled turbulent boundary layers with comparison against polymer drag reduction  

Microsoft Academic Search

Experiments were conducted in the 12-inch diameter tunnel at the Applied Research Laboratory, Pennsylvania State University using the tunnel wall boundary layer to determine the influence of surface roughness on microbubble drag reduction. To accomplish this, carbon dioxide was injected through a slot at rates of 0.001 m 3\\/s to 0.011 m 3\\/s, and the resulting skin friction drag measured on a

S. Deutsch; M. Moeny; A. A. Fontaine; H. Petrie

2004-01-01

184

Streamwise development of turbulent boundary-layer drag reduction with polymer injection  

Microsoft Academic Search

Zero-pressure-gradient turbulent boundary-layer drag reduction by polymer injection has been studied with particle image velocimetry. Flow fields ranging from low to maximum drag reduction have been investigated. A previously developed technique y\\/ has been used to evaluate the skin friction, drag reduction and polymer stress. Current results agree well with the semi-log plot of drag reduction vs. normalized polymer flux

Y. X. Hou; V. S. R. Somandepalli; M. G. Mungal

2008-01-01

185

Acoustics of friction  

NASA Astrophysics Data System (ADS)

This article presents an overview of the acoustics of friction by covering friction sounds, friction-induced vibrations and waves in solids, and descriptions of other frictional phenomena related to acoustics. Friction, resulting from the sliding contact of solids, often gives rise to diverse forms of waves and oscillations within solids which frequently lead to radiation of sound to the surrounding media. Among the many everyday examples of friction sounds, violin music and brake noise in automobiles represent the two extremes in terms of the sounds they produce and the mechanisms by which they are generated. Of the multiple examples of friction sounds in nature, insect sounds are prominent. Friction also provides a means by which energy dissipation takes place at the interface of solids. Friction damping that develops between surfaces, such as joints and connections, in some cases requires only microscopic motion to dissipate energy. Modeling of friction-induced vibrations and friction damping in mechanical systems requires an accurate description of friction for which only approximations exist. While many of the components that contribute to friction can be modeled, computational requirements become prohibitive for their contemporaneous calculation. Furthermore, quantification of friction at the atomic scale still remains elusive. At the atomic scale, friction becomes a mechanism that converts the kinetic energy associated with the relative motion of surfaces to thermal energy. However, the description of the conversion to thermal energy represented by a disordered state of oscillations of atoms in a solid is still not well understood. At the macroscopic level, friction interacts with the vibrations and waves that it causes. Such interaction sets up a feedback between the friction force and waves at the surfaces, thereby making friction and surface motion interdependent. Such interdependence forms the basis for friction-induced motion as in the case of ultrasonic motors and other examples. Last, when considered phenomenologically, friction and boundary layer turbulence exhibit analogous properties and, when compared, each may provide clues to a better understanding of the other.

Akay, Adnan

2002-04-01

186

Near Critical Phenomena in Laminar Boundary Layers  

NASA Astrophysics Data System (ADS)

Recent developments in the construction of airfoils and rotorblades are characterized by an increasing interest in the application of so-called smart structures for active flow control. These are characterized by an interplay of sensors, actuators, real-time controlling data processing systems and the use of new materials e.g. shape alloys with the aim to increase manoeuvrability, reduce drag and radiated sound. The optimal use of such devices obviously requires a detailed insight into the flow phenomena to be controlled and in particular their sensitivity to external disturbances. In this connection locally separated boundary layer flows are of special interest. Asymptotic analysis of boundary layer separation in the limit of large Reynolds number Re? ? has shown that in a number of cases which are of importance from a practical point of view solutions of the resulting interaction equations describing two-dimensional steady flows exist up to a limiting value ? c of the relevant controlling parameter ? only while two branches of solutions exist in a regime ? < ? c . The present study aims at a better understanding of near critical flows ? ? — ? c ? ? 0 and in particular the changes of the flow behaviour associated with the passage of ? through ? c .

Kluwick, A.; Braun, S.; Cox, E. A.

187

Effect of friction on subsurface layer microstructure in austenitic and martensitic steels  

Microsoft Academic Search

By means of transmission and scanning electron microscopy, the microstructure of subsurface layer on specimens after friction tests has been examined. The load and sliding speed dependencies of the coefficient of friction and temperature have been obtained on specimens from austenitic and martensitic steels. It has been shown that severe adhesive wear (seizure) conditions result in formation of a 20-

S. Yu. Tarassov; A. V. Kolubaev

1999-01-01

188

Advective heat transport and boundary layer decoupling controlling the melt dynamics of a patchy snow cover  

NASA Astrophysics Data System (ADS)

Once the mountain snow-cover gets patchy in the course of the ablation season two processes are expected to increase in magnitude: the advective heat ransport and the near-surface boundary layer decoupling. These two processes, which have an opposite effect on sensible heat transport onto the snow surface, are, however, not well understood. The aim of this study is to investigate the effects of locally developing atmospheric stratification over snow patches. Especially stable internal boundary layers over cold surfaces can result in a decoupling of the near-surface air from the warmer atmosphere. In this investigation we are particularly interested in the effect of boundary layer decoupling on the net sensible heat flux towards the snow surface. At two experimental sites we applied local eddy flux measurements over snow patches at three different heights above the snow surface. The measurement results suggested wind velocity, turbulence intensity, wind fetch distance and topographical curvature to be driving factors for boundary layer growth above patchy snow covers. These factors also control the efficiency of advective heat transport to contribute to snow ablation. The turbulence data clearly show that boundary layer decoupling inhibits the transfer of additional energy to the snow cover potentially gained from advective heat transport, leading to an upward flux of sensible heat above the stable internal layer. The atmospheric decoupling primarily occurs for shallow stable internal boundary layers, calm winds and low friction velocities. Contrary, the transfer of sensible heat towards the snow cover is promoted by high mechanical turbulence initiated by strong winds. Advective heat transport is shown to be especially effective under these conditions. Thus, strong winds additionally increase the role of advective heat transport by decreasing boundary layer decoupling. Furthermore, concave topographies reduce snow ablation by enhancing the potential of boundary layer decoupling. The atmospheric decoupling is thus shown to be a key mechanism in snow patch survival.

Mott, R.; Gromke, C.; Grünewald, T.; Lehning, M.

2012-04-01

189

Lidar probing the urban nocturnal boundary layer  

NASA Astrophysics Data System (ADS)

Lidar observations to study the nocturnal boundary layer in the atmosphere were made on selected evenings during December 1997 - March 1998 at the City University of Hong Kong (lat. 20 degree(s)20'6', long. 114 degree(s)10'18', at 57 m AMSL), Hong Kong. The ground-based Nd:YAG lidar has been operated to detect the vertical distribution of aerosols in the NBL at a subtropical urban site. It is shown that the vertical relative signal profiles can be employed to determine the heights of the single or multiple nocturnal inversions. In a clear sky and light wind evening transition period, the strong radiative cooling caused the air near the ground becomes stably stratified. The nocturnal inversion starts to emerge soon before sunset and grows vertically as the night progresses. The study also showed that the temporal evolution of the nocturnal inversion depth was rapidly increased soon after sunset and a slower rate in the midnight hours. The results of the study indicate that the vertical aerosol distribution in the multiple-layer is more complicated than that in the single-layer, of NBL. The early morning transition of the NBL is also discussed. A comparison of the lidar aerosol signals and radiosonde measurements was performed to evaluate the consistency of observations between the different systems.

Mok, T. M.; Leung, Kang M.; Ho, A. H.; Chan, J. C.; Ng, C. N.

1998-08-01

190

High Energy Benthic Boundary Layer Experiment: HEBBLE  

NASA Astrophysics Data System (ADS)

HEBBLE's precise aim is to develop and to test explicit predictions about the response of adhesive/cohesive marine sediments to imposed and controlled stresses [Hollister et al., 1980; Kerr, 1980]. Pursuit of this goal has necessitated a co-ordinated, interdisciplinary effort, to date including physical oceanographers, sedimentologists, radiochemists and biochemists, and biological oceanographers.Current produced bed features reflect significant momentum exchange between the fluid boundary layer and the sediment surface. From photographs, and the few current meter records available, it appears that vast areas of the deep sea are presently being modified by energetic flows. The bed forms range in scale from kilometers to millimeters and are found where near bottom currents have been delineated by maxima in near bottom potential temperature. Moreover, on the Scotian Rise for example, many of these bedforms are being produced by present day currents because rapid destruction of the features by benthic organisms is evidenced in stereo-photographs.

Nowell, Arthur R. M.; Hollister, Charles D.; Jumars, Peter A.

191

Linear Optimal Controllers for Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

Several investigators have shown recently that controller design based on a system control-theoretic approach has led to significant viscous-drag reduction in turbulent boundary layers. Although these new approaches rely less on the investigator's physical insight into the flow (in contrast to most existing approaches), there still remain many design issues that need to be addressed. These are, but not limited to, the choice of cost functions for control and estimation, system model reduction, system dynamics estimation, robustness of the controlled system, and centralized vs. de-centralized control. We have performed a comparative study with different reduced-order models and cost functions in our numerical experiments of a low-Reynolds number turbulent channel flow in order to address some of the issues mentioned above. Results from these controllers and those from existing controllers and their implications will be presented.

Lim, Junwoo; Kim, John; Kang, Sung Moon; Speyer, Jason

2001-11-01

192

Boundary-layer instability noise on aerofoils  

NASA Astrophysics Data System (ADS)

An experimental and theoretical investigation has been carried out to understand the tonal noise generation mechanism on aerofoils at moderate Reynolds number. Experiments were conducted on a NACA0012 aerofoil section in a low-turbulence closed working section wind tunnel. Narrow band acoustic tones were observed up to 40 dB above background noise. The ladder structure of these tones was eliminated by modifying the tunnel to approximate to anechoic conditions. High-resolution flow velocity measurements have been made with a three-component laser-Doppler anemometer (LDA) which have revealed the presence of strongly amplified boundary-layer instabilities in a region of separated shear flow just upstream of the pressure surface trailing edge, which match the frequency of the acoustic tones. Flow visualization experiments have shown these instabilities to roll up to form a regular Kármán-type vortex street.

Nash, Emma C.; Lowson, Martin V.; McAlpine, Alan

1999-03-01

193

Boundary layer receptivity to freestream turbulence  

NASA Astrophysics Data System (ADS)

Freestream turbulence of weak but adjustable intensity incident upon a flat plate test model induced Tollmien-Schichting (TS) waves and wave packets. These were studied using a newly-developed sensor by which the packets could be followed from a station nearly as far forward as the minimum critical Reynolds number to the onset of transition. Conclusions reached from these studies were: (1) weak freestream turbulence induced TS wave packets in a Blasius boundary-layer, (2) packets gained strength during propagation through expansion of lateral and longitudinal scales, and through an increase in peak amplitude, (3) the average wave strength at stations of observation was not related to the freestream velocity fluctuation in a linear manner, and (4) packets typically evolved into turbulent spots.

Kendall, James M.

1990-06-01

194

Turbulent boundary layer drag reduction using riblets  

NASA Astrophysics Data System (ADS)

An experimental study of low-speed turbulent boundary layer flow over longitudinally grooved surfaces (i.e., riblets) is discussed. Results obtained with a highly accurate drag balance indicate that v-groove riblet surfaces can produce consistent net drag reductions as large as 8 percent provided the height and spacing of the grooves in terms of law of the wall variables are less than 25 wall units. Momentum balances confirmed these direct drag measurements. Conditionally sampled data indicate that the burst frequency for riblets is approximately the same as that for a flat plate but turbulence intensity is reduced. Attempts to optimize the net drag reduction by varying riblet cross-sectional geometry and alignment are also discussed.

Walsh, M. J.

1982-01-01

195

An experimental and computational investigation of the transport of Reynolds stress in an axisymmetric swirling boundary layer  

Microsoft Academic Search

A coordinated experimental and computational investigation of the transport and interaction of the individual Reynolds stresses within a turbulent axisymmetric swirling boundary layer flowing over a stationary cylinder is described. The cylinder is instrumented with new directional surface fence skin-friction gages developed for the experiment. The longitudinal and transverse mean velocity profiles are measured with a miniature directional pressure probe.

H. Higuchi; M. W. Rubesin

1981-01-01

196

Laminar turbulent boundary layers; Proceedings of the Energy Sources Technology Conference, New Orleans, LA, February 12-16, 1984  

Microsoft Academic Search

Among the topics discussed are drag and aeroacoustic noise characteristics due to the coupled roughness and blowing of surfaces, skin friction and heat transfer for combined roughness and mass addition, the effect of drag-reducing additives on the development and separation of a turbulent boundary layer with adverse pressure gradient, numerical investigations of the microbubble drag reduction mechanism, the effects of

E. M. Uram; H. E. Weber

1984-01-01

197

Friction  

NSDL National Science Digital Library

This website from Kathleen Cummings Dominguez at the Illinois Institute of Technology provides a lesson plan on the concepts of friction. It describes a lesson plan which will engage students in active classroom learning.

Dominguez, Kathleen C.

2010-03-17

198

Slip Effects on Boundary Layer Flow and Heat Transfer Along a Stretching Cylinder  

NASA Astrophysics Data System (ADS)

An axi-symmetric laminar boundary layer flow of a viscous incompressible fluid and heat transfer towards a stretching cylinder is presented. Velocity slip is considered instead of the no-slip condition at the boundary. Similarity transformations are used to convert the partial differential equations corresponding to the momentum and heat equations into non-linear ordinary differential equations. Numerical solutions of these equations are obtained by the shooting method. It is found that the velocity decreases with increasing the slip parameter. The skin friction as well as the heat transfer rate at the surface is larger for a cylinder compared to those for a flat plate.

Mukhopadhyay, S.; Gorla, R. S. R.

2013-06-01

199

Halogen chemistry in the marine boundary layer  

NASA Astrophysics Data System (ADS)

Important atmospheric sources of iodine include the air-sea exchange of biogenic iodocarbons, and the emission of I2 from macro-algae. The major source of bromine is the release of bromide ions from sea-salt aerosol. The subsequent atmospheric chemistry of these halogens (1), changes the oxidizing capacity of the marine boundary layer by destroying ozone and changing the hydroxyl radical concentration; (2), reacts efficiently with dimethyl sulphide and mercury (in the polar regions); and (3), leads to the formation of ultra-fine particles which may contribute to cloud condensation nuclei (CCN) and hence affect climate. This paper will report observations of IO, BrO, OIO and I2 made by the technique of differential optical absorption spectroscopy, in several contrasting marine environments: the equatorial mid-Atlantic (Cape Verde); mid-latitude clean coastal (Mace Head, Ireland); polluted coastal (Roscoff, France); and the polar marine boundary layer (Hudson Bay, Canada). Both IO and BrO are observed in all these locations at significant concentrations (> 1 pptv), and so have a major impact on (1) and (2) above. To complement the field campaigns we have also carried out wide-ranging laboratory investigation. A new study of OIO photochemistry shows that absorption in the visible bands between 490 and 630 nm leads to I atom production with a quantum yield of unity, which now means that iodine is a particularly powerful ozone-depleting agent. We have also studied the formation and growth kinetics of iodine oxide nano-particles, and their uptake of water, sulphuric acid and di-carboxylic organic acids, in order to model their growth to a size where they can act as CCN. Their ice-nucleating properties will also be reported.

Plane, J. M. C.; Gomez Martin, J. C.; Kumar, R.; Mahajan, A. S.; Oetjen, H.; Saunders, R. W.

2009-04-01

200

Boundary layer (shear-band) in frustrated viscoplastic flows  

NASA Astrophysics Data System (ADS)

We show that frustrated creep flows of yield stress fluids give rise to a boundary layer, which takes the form of a liquid region of uniform significant thickness separating two solid regions. In this boundary layer the shear rate is approximately constant for a given flow rate and the layer thickness varies extremely slowly with the flow rate.

Chevalier, T.; Rodts, S.; Chateau, X.; Boujlel, J.; Maillard, M.; Coussot, P.

2013-05-01

201

Saturn's low-latitude boundary layer: 2. Electron structure  

Microsoft Academic Search

The boundary of a planetary magnetosphere is the site of mass, momentum, and energy transport. This transport produces a layer of mixed solar wind and magnetospheric plasma inside and adjacent to the boundary. In the case of Earth, the electron structure of this layer is distinctive, and has been explained by models of the layer on open magnetic field lines.

A. Masters; A. P. Walsh; A. N. Fazakerley; A. J. Coates; M. K. Dougherty

2011-01-01

202

Direct numerical simulation of a high-entrainment turbulent boundary layer  

NASA Astrophysics Data System (ADS)

It has been reported that certain rough surfaces modify the outer region of turbulent boundary layers, but not those of channels or pipes. Besides their surface geometries, all those experiments share relatively large spreading and entrainment rates, which is known to modify the outer intermittent layers of external turbulent flows, but is absent from channels. To separate the effect of surface geometry from that of entrainment, we present a direct simulation of a zero-pressure-gradient turbulent boundary layer, at Re?= 1400-4500, in which the friction coefficient is augmented by a smooth volumetric force, restricted to the viscous layer below y^+=25, and proportional to the streamwise component of the velocity. The spreading rate increases by 70%, equivalent to a sand roughnes ks^+ 60. The resulting changes in the velocity and pressure fluctuations, and in the velocity correlation lengths, are compared with those of rough-wall experiments.

Borrell, Guillem; Gungor, Ayse G.; Jimenez, Javier

2011-11-01

203

Improved boundary layer depth retrievals from MPLNET  

NASA Astrophysics Data System (ADS)

Continuous lidar observations of the planetary boundary layer (PBL) depth have been made at the Micropulse Lidar Network (MPLNET) site in Greenbelt, Maryland, since April 2001. However, because of issues with the operational PBL depth algorithm, the data are not reliable for determining seasonal and diurnal trends. Therefore, an improved PBL depth algorithm has been developed which uses a combination of the wavelet technique and image processing. The new algorithm is less susceptible to contamination by clouds and residual layers and, in general, produces lower PBL depths. A 2010 comparison shows the operational algorithm overestimates the daily mean PBL depth when compared to the improved algorithm (1.85 and 1.07 km, respectively). The improved MPLNET PBL depths are validated using radiosonde comparisons, which suggests the algorithm performs well to determine the depth of a fully developed PBL. A comparison with the Goddard Earth Observing System version 5 (GEOS-5) model suggests that the model may underestimate the maximum daytime PBL depth by ˜410 m during the spring and summer. The best agreement between MPLNET and GEOS-5 occurred during the fall and they differed the most in the winter.

Lewis, Jasper R.; Welton, Ellsworth J.; Molod, Andrea M.; Joseph, Everette

2013-09-01

204

Turbulent flow in converging nozzles, part one: boundary layer solution  

Microsoft Academic Search

The boundary layer integral method is used to investigate the development of the turbulent swirling flow at the entrance region\\u000a of a conical nozzle. The governing equations in the spherical coordinate system are simplified with the boundary layer assumptions\\u000a and integrated through the boundary layer. The resulting sets of differential equations are then solved by the fourth-order\\u000a Adams predictor-corrector method.

R. Maddahian; B. Farhanieh; B. Firoozabadi

2011-01-01

205

Quadrature formulas for functions with a boundary-layer component  

NASA Astrophysics Data System (ADS)

Quadrature formulas for one-variable functions with a boundary-layer component are constructed and studied. It is assumed that the integrand can be represented as the sum of a regular and a boundary-layer component, the latter having high gradients that reduce the accuracy of classical quadrature formulas, such as the trapezoidal and Simpson rules. The formulas are modified so that their error is independent of the gradients of the boundary-layer component. Results of numerical experiments are presented.

Zadorin, A. I.; Zadorin, N. A.

2011-11-01

206

Self-preservation of rough-wall turbulent boundary layers  

Microsoft Academic Search

It is proposed that all fully rough-wall boundary layers should satisfy self-preservation more closely than a smooth-wall boundary layer. Previous work has shown that the self-preserving forms of the momentum and turbulent kinetic energy equations for a zero pressure gradient turbulent boundary layer, at sufficiently high Reynolds number, require that the wall shear stress is constant with x, and the

R. J. Smalley; R. A. Antonia; L. Djenidi

2001-01-01

207

On the extension of the wind profile over homogeneous terrain beyond the surface boundary layer  

NASA Astrophysics Data System (ADS)

Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Høvsøre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50 80 m. At higher levels deviations from the measurements progressively occur. For applied use an extension to the wind profile in the surface layer is formulated for the entire boundary layer, with emphasis on the lowest 200 300 m and considering only wind speeds above 3 m s-1 at 10 m height. The friction velocity is taken to decrease linearly through the boundary layer. The wind profile length scale is composed of three component length scales. In the surface layer the first length scale is taken to increase linearly with height with a stability correction following Monin-Obukhov similarity. Above the surface layer the second length scale ( L MBL ) becomes independent of height but not of stability, and at the top of the boundary layer the third length scale is assumed to be negligible. A simple model for the combined length scale that controls the wind profile and its stability dependence is formulated by inverse summation. Based on these assumptions the wind profile for the entire boundary layer is derived. A parameterization of L MBL is formulated using the geostrophic drag law, which relates friction velocity and geostrophic wind. The empirical parameterization of the resistance law functions A and B in the geostrophic drag law is uncertain, making it impractical. Therefore an expression for the length scale, L MBL , for applied use is suggested, based on measurements from the two sites.

Gryning, Sven-Erik; Batchvarova, Ekaterina; Brümmer, Burghard; Jørgensen, Hans; Larsen, Søren

2007-08-01

208

Seasonal Simulations of the Planetary Boundary Layer and Boundary-Layer Stratocumulus Clouds with a General Circulation Model  

Microsoft Academic Search

The UCLA general circulation model (GCM) has been used to simulate the seasonally varying planetary boundary layer (PBL), as well as boundary-layer stratus and stratocumulus clouds. The PBL depth is a prognostic variable of the GCM, incorporated through the use of a vertical coordinate system in which the PBL is identified with the lowest model layer.Stratocumulus clouds are assumed to

David A. Randall; James A. Abeles; Thomas G. Corsetti

1985-01-01

209

Observations of mesoscale and boundary-layer circulations affecting dust uplift and transport in the Saharan boundary layer  

NASA Astrophysics Data System (ADS)

Observations of the Saharan boundary layer, made during the GERBILS field campaign, show that mesoscale land surface temperature variations (which were related to albedo variations) induced mesoscale circulations, and that mesoscale and boundary-layer circulations affected dust uplift and transport. These processes are unrepresented in many climate models, but may have significant impacts on the vertical transport and uplift of desert dust. Mesoscale effects in particular tend to be difficult to parameterise. With weak winds along the aircraft track, land surface temperature anomalies with scales of greater than 10 km are shown to significantly affect boundary-layer temperatures and winds. Such anomalies are expected to affect the vertical mixing of the dusty and weakly stratified Saharan Air Layer (SAL). Mesoscale variations in winds are also shown to affect dust loadings in the boundary-layer. In a region of local uplift, with strong along-track winds, boundary-layer rolls are shown to lead to warm moist dusty updraughts in the boundary layer. Large eddy model (LEM) simulations suggest that these rolls increased uplift by approximately 30%. The modelled effects of boundary-layer convection on uplift is shown to be larger when the boundary-layer wind is decreased, and most significant when the mean wind is below the threshold for dust uplift and the boundary-layer convection leads to uplift which would not otherwise occur.

Marsham, J. H.; Parker, D. J.; Grams, C. M.; Grey, W. M. F.; Johnson, B. T.

2008-05-01

210

Application of the Dorodnitsyn finite element method to swirling boundary layer flow  

NASA Astrophysics Data System (ADS)

The Dorodnitsyn finite element method for turbulent boundary layer flow with surface mass transfer is extended to include axisymmetric swirling internal boundary layer flow. Turbulence effects are represented by the two-layer eddy viscosity model of Cebeci and Smith (1974) with extensions to allow for the effect of swirl. The method is applied to duct entry flow and a 10 degree included-angle conical diffuser, and produces results in close agreement with experimental measurements with only 11 grid points across the boundary layer. The introduction of swirl (we/ue = 0.4) is found to have little effect on the axial skin friction in either a slightly favorable or adverse pressure gradient, but does cause an increase in the displacement area for an adverse pressure gradient. Surface mass transfer (blowing or suction) causes a substantial reduction (blowing) in axial skin friction and an increase in the displacement area. Both suction and the adverse pressure gradient have little influence on the circumferential velocity and shear stress components. Consequently in an adverse pressure gradient the flow direction adjacent to the wall is expected to approach the circumferential direction at some downstream location.

Fletcher, C. A. J.

1985-05-01

211

Scaling of high-Reynolds number turbulent boundary layers in the National Diagnostic Facility  

NASA Astrophysics Data System (ADS)

Velocity, shear stress and Reynolds stress were measured using hot-wire anemometry in three distinct turbulent boundary layers in the National Diagnostic Facility and have demonstrated some new and unique features of these layers up to a momentum thickness Reynolds number of 50,000. In agreement with the literature, mean velocity profiles showed that the so-called logarithmic region grew continuously for increasing Reynolds number for all of the boundary layers confirming an extended disparity between small and large scale motions. It was also observed from the mean velocity profiles that minor surface roughness elements of less than 3 viscous lengths caused a noticeable overshoot of the log-law line as the Reynolds number increased. Detailed spectra computed from long time-series of the streamwise velocity revealed a bi-modal distribution of energy in the boundary layer close to the wall, within the viscous and buffer layers, asserting that the large-scale, low-frequency motions of the boundary layer are important in the near-wall region. Direct shear stress measurements using wire-on-wall and MEMS sensors have been compared to the Clauser prediction of friction velocity and demonstrated scaling consistent with or better then Clauser friction velocity. The measured wall shear stress was found to be higher than the values inferred from the Clauser approach with a diminishing discrepancy as the boundary layer developed far downstream towards very high Reynolds numbers. Also, it was demonstrated that the Clauser scaling tends to mask the streamwise (Resb{x}) dependencies of the boundary layer quantities that were observed repeatedly when the mean and fluctuating quantities were scaled with the measured friction velocity. Instantaneous velocity and Reynolds stress time-series were filtered based on the collapse and the bi-modal structure of the streamwise velocity spectra to extract details concerning the inner and outer scaling of the turbulence quantities. It was observed for both the streamwise turbulence and the Reynolds stress that the overlap region grows continuously with Reynolds number. This overlap region scaled with inner variables at the lowest Reynolds numbers, but at the higher Reynolds numbers, the growth of the overlap region was attributed to an increase in the amount of large scale motion present. Further, the near-wall turbulent bursting frequency was investigated using the same filtering techniques and showed that inner-variable scaling collapsed the bursting frequency very well when the largest scales were filtered from the time-series. The scaled bursting frequency was shown to be independent of the detection threshold level and the high-pass filtering cut-off frequency over the range 10

Hites, Michael Hubert

212

On Internal Gravity Waves Associated with the Stable Boundary Layer  

NASA Astrophysics Data System (ADS)

The stable planetary boundary layer at the baseof the residual layer supports internalwaves that are unambiguously boundary layer incharacter. Some of these wavesare instabilities and some are neutrally stable modes, but they all have critical levelsin the residual layer. These waves exist for a broad range of conditions and should bea major component of any ducted disturbance that propagates within ninety degreesof the wind direction. The wave properties can be computed without the numericaldifficulties usually associated with critical-level systems.

Chimonas, George

213

Helical circulations in the typhoon boundary layer  

NASA Astrophysics Data System (ADS)

Low-level wind data from the WSR-88D in Guam obtained in Typhoon Dale (1996) and Typhoon Keith (1997) are analyzed for coherent structures. Consistent with the results of previous studies of Atlantic hurricanes, velocity anomalies associated with coherent structures were found in the boundary layer of both storms. A total of 99 cases of coherent structures, also known as roll vortices, were documented during a 6 h evaluation period for each storm. Storm-relative roll location, roll vorticity, asymmetries in the upward and downward momentum fluxes, and signatures of circulations transverse to the mean flow associated with roll circulations were explored. The effects of terrain and convective precipitation systems, such as rainbands, on the occurrence of rolls were investigated. The results support and extend prior findings of roll observations, and can be used to help validate theoretical and numerical models of coherent structures within tropical cyclones. Moreover, the wind variations documented in this study may have application for wave runup and wind damage potential in tropical cyclones.

Ellis, Ryan; Businger, Steven

2010-03-01

214

Pattern Formation in Cathode Boundary Layer Microdischarges  

NASA Astrophysics Data System (ADS)

Direct current glow discharges in xenon between a planar, 100 ?m thick cathode and a ring shaped anode, separated by 250 ?m, were found to be stable up to atmospheric pressure. Photographs in the visible and VUV (172 nm) range of the spectrum show the transition from a homogeneous to a structured plasma. The plasma patterns, regularly arranged filaments that are most pronounced at lower pressures (100 Torr), show discrete changes when the current is decreased by fractions of mA. This selforganization of the plasma requires the presence of a second stable branch in addition to the abnormal cathode fall in the voltage-current density characteristic of the "cathode boundary layer" (CBL) discharges. A model of the cathode fall by von Engel and Steenbeck [1], which was modified to take thermal conduction as a loss process into account, in addition to radiation, indicates the presence of stable plasma filaments at current densities in the range from 10 to 100 A/cm^2, before transition into an arc. [1] A. von Engel and M. Steenbeck, "Elektrische Gasentladungen, ihre Physik und Technik," Vol. 2, p. 121. Work supported by NSF (CTS-0078618 and INT-0001438).

Schoenbach, K. H.; Moselhy, M. M.

2003-10-01

215

The Near-Calm Stable Boundary Layer  

NASA Astrophysics Data System (ADS)

For the near-calm stable boundary layer, nominally 2-m mean wind speed <0.5 ms-1, the time-average turbulent flux is dominated by infrequent mixing events. These events are related to accelerations associated with wave-like motions and other more complex small-scale motions. In this regime, the relationship between the fluxes and the weak mean flow breaks down. Such near-calm conditions are common at some sites. For very weak winds and strong stratification, the characteristics of the fluctuating quantities change slowly with increasing scale and the separation between the turbulence and non-turbulent motions can become ambiguous. Therefore, a new analysis strategy is developed based on the scale dependence of selected flow characteristics, such as the ratio of the fluctuating potential energy to the kinetic energy. In contrast to more developed turbulence, correlations between fluctuating quantities are small, and a significant heat flux is sometimes carried by very weak vertical motions with large temperature fluctuations. The relation of the flux events to small-scale increases of wind speed is examined. Large remaining uncertainties are noted.

Mahrt, Larry

2011-09-01

216

DNS of Hypersonic Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

We present a direct numerical simulation database of supersonic and hypersonic turbulent boundary layers. The systematic procedure for initializing the turbulent flow fields at controlled Mach number and Reynolds number conditions is described. It is shown that simulation transients are less than 10% of the time required for gathering statistical data of the turbulent flows. The experimental conditions of Debiève ( Debieve, Gouin, and Gaviglio, Proceedings ICHMT/IUTAM Symposium on the Structure of Turbulence and Heat and Mass Transfer, Dubrovnik, 1981. ) ( Debieve, Thèse Université d'Aix Marseille II, 1983, Marseille, France. ) and Elena ( Eléna, Lacharme, and Gaviglio, In: Dybb, A. & Pfund, P.A. (eds), International Symposium on Laser Anemometry. ASME, 1985.) ( Eléna and Lacharme, ) are simulated and the simulation and experimental data are in excellent agreement. Using the direct numerical simulation database we perform parametric studies varying freestream Mach number in the range of 3 to 8 and wall-temperature condition for wall-to-freestream-temperature ratio of 2 to 5.5.

Pino Martin, M.

2004-11-01

217

Excitation of secondary instabilities in boundary layers  

NASA Astrophysics Data System (ADS)

The receptivity to fundamental and subharmonic secondary instabilities is analysed for two-dimensional boundary layers. Fundamental modes are excited by the direct scattering of Tollmien Schlichting (TS) waves over surface variations. The excitation of subharmonic modes stems from the combined scattering of acoustic free-stream disturbances and TS waves over surface variations. The surface variations are localized in their streamwise extent and are the result of roughness or suction. The velocity field is expanded in terms of small parameters characterizing the acoustic disturbance and the surface variation. The TS wave is included as part of the base flow leading to a non-homogeneous system with periodic coefficients governing the receptivity. The receptivity amplitudes show a strong dependence on the TS-wave amplitude, and for subharmonic modes a strong dependence on the TS-wave phase at the location of the surface variation. The receptivity analysis shows a significant bias toward fundamental modes of secondary instability for larger TS-wave amplitudes except for conditions of extremely high free-stream sound level. A combination of receptivity results and stability results suggests a bias toward subharmonic modes for TS-wave amplitudes below 0.5% and toward fundamental modes for TS-wave amplitudes above 0.5% (normalized by the local edge velocity).

Crouch, J. D.

1997-04-01

218

Friction  

NSDL National Science Digital Library

The representation demonstrates, through an animated, narrated slide-show, how frictional forces, including air resistance, can affect the motion of an object. This resource also includes an interactive test and review of the material. One is also able to download "myskoool" which allows allows one to download lessons to run offline and use anytime.

219

Friction welding of dissimilar metal joints with intermediate layers  

Microsoft Academic Search

Purpose: Metals such as titanium, vanadium, zirconium, niobium, molybdenum and also tantalum and tungsten must be protected at elevated temperatures from the effects of oxygen, nitrogen and hydrogen. For this reason, it is of interest, both from the innovative and practical points of view, to investigate the possibility of using the process of friction welding to produce joints in these

A. Ambroziak; M. Korzeniowski; P. Kustro?

220

Friction factor of two-dimensional rough-boundary turbulent soap film flows.  

PubMed

We use momentum-transfer arguments to predict the friction factor f in two-dimensional turbulent soap film flows with rough boundaries (an analog of three-dimensional pipe flow) as a function of Reynolds number Re and roughness r , considering separately the inverse energy cascade and the forward enstrophy cascade. At intermediate Re, we predict a Blasius-like friction factor scaling of f proportional, variant Re{-1/2} in flows dominated by the enstrophy cascade, distinct from the energy cascade scaling of Re{-1/4} . For large Re, f approximately r in the enstrophy-dominated case. We use conformal map techniques to perform direct numerical simulations that are in satisfactory agreement with theory and exhibit data collapse scaling of roughness-induced criticality, previously shown to arise in the three-dimensional pipe data of Nikuradse. PMID:19658555

Guttenberg, Nicholas; Goldenfeld, Nigel

2009-06-25

221

Tne transfer of heat in turbulent boundary layers with injection or suction - Universal laws and Stanton number equations  

NASA Astrophysics Data System (ADS)

Two new expressions for the law of the wall associated with turbulent boundary layer heat transfer in the presence of injection or suction are derived; attention is given to the extension of these expressions to the defect layer through the use of Coles' function. The resulting temperature-defect expressions yield bilogarithmic expressions for Stanton number that are similar to the skin friction equation. These differ significantly from previously proposed expressions.

Faraco-Medeiros, Marcello A.; Silva-Freire, Atila P.

1992-04-01

222

The Coriolis effect on coherent structures in planetary boundary layers  

Microsoft Academic Search

Coherent structures are often visible in atmospheric boundary layers as convective clouds and irregular fog. Large eddy simulations (LES) provide data to study the coherent structures by means of multivariate methods of statistical analysis. One of such methods is a proper orthogonal decomposition (POD). A POD can isolate most energetic three-dimensional structures in turbulent boundary layers. Coherent structures in planetary

Igor N Esau

2003-01-01

223

The Boundary Layer on a Flat Plate in Anisotropic Magnetohydrodynamics.  

National Technical Information Service (NTIS)

It is demonstrated that when calculating the boundary layer, despite the presence of thermal flow due to Larmor precession of electrons, it is possible to consider the temperature at the limit of the boundary layer as fixed. For this purpose the problem o...

V. B. Varanov A. G. Kukilovskii G. A. Lyubimov

1965-01-01

224

The mesoscale responses of a locally heated planetary boundary layer  

Microsoft Academic Search

The steady boundary layer responses that occur over the Great Lakes region during wintertime cold air outbreaks are examined using a 2-D, linear, analytic model. The boundary layer is modeled as an idealized, constantly stratified, viscous, rotating Boussinesq fluid that move uniformly between two horizontally infinite, rigid, stress free plates. The heat from the lakes is parameterized in terms of

Peter John Sousounis

1990-01-01

225

Impact of Injection Schemes on Transition in Hypersonic Boundary Layers.  

National Technical Information Service (NTIS)

Three geometries are explored for injecting CO2 into the boundary layer of a sharp five degree half-angle cone. The impact of the injection geometry, namely discrete injection holes or a porous conical section, on tripping the boundary layer is examined, ...

H. G. Hornung I. A. Leyva J. Shepherd J. S. Jewell S. Laurence

2009-01-01

226

Surface Roughness Effects on the Hypersonic Turbulent Boundary Layer.  

National Technical Information Service (NTIS)

An experimental investigation of the response of a hypersonic turbulent boundary layer to a step change in surface roughness has been performed. The boundary layer on a flat nozzle wall of a Mach 6 wind tunnel was subjected to abrupt changes in surface ro...

T. Kubota D. E. Berg

1977-01-01

227

Vertical Transport of Water in the Martian Boundary Layer.  

National Technical Information Service (NTIS)

We are continuing our examination of the transport of H2O through the martian boundary layer, and we have written a one-dimensional numerical model of the exchange of H2O between the atmosphere and subsurface of Mars through the planetary boundary layer (...

A. P. Zent R. M. Haberle H. C. Houben

1993-01-01

228

Entrainment process of carbon dioxide in the atmospheric boundary layer  

Microsoft Academic Search

Aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide (CO2) were taken above a grassland in a convective atmospheric boundary layer. The observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dioxide in the boundary layer. From the observations we were able to estimate the vertical profiles of the

Jordi Vilà-Guerau de Arellano; Beniamino Gioli; Franco Miglietta; Harm J. J. Jonker; Henk Klein Baltink; Ronald W. A. Hutjes; Albert A. M. Holtslag

2004-01-01

229

Bristled shark skin: a microgeometry for boundary layer control?  

Microsoft Academic Search

There exists evidence that some fast-swimming shark species may have the ability to bristle their scales during fast swimming. Experimental work using a water tunnel facility has been performed to investigate the flow field over and within a bristled shark skin model submerged within a boundary layer to deduce the possible boundary layer control mechanisms being used by these fast-swimming

230

Boundary layer phenomena in combustion-driven MHD power generators  

Microsoft Academic Search

An analytic and experimental study of boundary layer phenomena in combustion-driven MHD power generators has been conducted. Measurements of heat transfer rates and velocity, temperature, and electron number density profiles were compared with numerical calculations. The analysis is based on the two-dimensional boundary layer equations. The velocity, temperature, and electron number density profiles were measured at the downstream end of

J. W. Daily

1975-01-01

231

Development of instrumentation for boundary layer transition detection  

Microsoft Academic Search

A steady state heat transfer technique is developed and evaluated for detecting boundary layer transition on a flat plate in incompressible flow. The method involves adhering encapsulated temperature sensitive liquid crystals to a constant heat flux surface. A heater composed of unidirectional carbon fibers is developed and tested with the aim of in-flight boundary layer transition detection on a natural

Steven B. Harrison

1991-01-01

232

Modelling the low-latitude boundary layer with reconnection entry  

SciTech Connect

The authors develop a one-dimensional Low-Latitude Boundary Layer (LLBL) model for northward IMF. The boundary layer in this model is uniform in the direction normal to the magnetopause, a `plateau-type` boundary layer. The boundary layer motion is decoupled from the magnetosheath motion and driven by the plasma pressure associated with the incoming solar wind plasma near local noon, which has become entrained on closed field lines as a result of reconnection in the cusp region. Dissipation in the ionosphere at the feet of the boundary layer field lines opposes this motion. There are two physical solutions for the model. In one, the boundary layer reaches a terminal velocity in the tail as the boundary layer plasma effectively joins the solar wind flow. In the other solution, the flow is nearly stopped in the far tail. In combination with other mechanisms, this latter solution may correspond to the case in which the boundary layer plasma participates in magnetospheric convection and returns sunward. The density, velocity, and thickness as functions of distance from local noon are studied, assuming that the magnetopause has elliptical shape and the magnetospheric field is dipolar.

Song, P.; Holzer, T.E. [National Center for Atmospheric Research, Boulder, CO (United States); Russell, C.T.; Wang, Z. [Univ. of California, Los Angeles, CA (United States)

1994-04-01

233

Beta limitation of matter-antimatter boundary layers  

Microsoft Academic Search

A model for a boundary layer which separates a cloud of matter from one of antimatter in a magnetized ambiplasma, in which steady pressure equilibrium ceases to exist when a certain beta limit is exceeded is discussed. The latter is defined as the ratio between the ambiplasma and magnetic field pressures which balance each other in the boundary layer. Thus,

B. Lehnert

1987-01-01

234

On fluid dynamic drag reduction in some boundary layer flows  

Microsoft Academic Search

Summary The problem of boundary layer flow on a flat plate with injection and a constant velocity opposite in direction to that of the uniform mainstream is analyzed. It is shown that the solution of this boundary layer problem not only depends on the ratio of the velocity of the plate to the velocity of the free stream (?), but

K. Vajravelu; R. N. Mohapatra

1990-01-01

235

Effects of coastal forcing on turbulence and boundary- layer structure  

Microsoft Academic Search

Coastal mountains of significant elevation impose constraints for the surrounding flow. The aim of this study is to describe the modifications of the marine atmospheric boundary layer that occur offshore of the west coast of the United States. Aircraft measurements, up to 1000 km off the coast from two experiments, are used. This boundary layer is capped by a subsidence

Linda Maria Viktoria Strom

1999-01-01

236

Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration  

Microsoft Academic Search

The influence of concentration polarization on the permeate flux in the ultrafiltration of aqueous Dextran T70 solutions can be described by (i) the osmotic pressure model and (ii) the boundary layer resistance model. In the latter model the hydrodynamic resistance of the non-gelled boundary layer is computed using permeability data of the Dextran molecules obtained by sedimentation experiments. It is

J. G. Wijmans; S. Nakao; Berg van den J. W. A; F. R. Troelstra; C. A. Smolders

1985-01-01

237

Drag reduction in liquid boundary layers by gas injection  

Microsoft Academic Search

The present evaluation of the development status of the introduction of gases into fluid boundary layers for viscous drag reduction emphasizes microbubble injection processes in which the bubbles form an emulsion in the background liquid's turbulent boundary layer. A considerable body of continuous-film surface lubrication research exists in the Soviet literature. The injection of a gas into a liquid turbulent

Charles L. Merkle; Steven Deutsch

1990-01-01

238

Modelling the low-latitude boundary layer with reconnection entry  

NASA Astrophysics Data System (ADS)

We develop a one-dimensional Low Latitude Boundary Layer (LLBL) model for northward interplanetary magnetic field (IMF). The boundary layer in this model is uniform in the direction normal to the magnetopause, a 'plateau-type' boundary layer. The boundary layer motion is decoupled from the magnetosheath motion and driven by the plasma pressure associated with the incoming solar wind plasma near local noon, which has become entrained on closed field lines as a result of reconnection in the cusp region. Dissipation in the ionosphere at the feet of the boundary layer field lines opposes this motion. There are two physical solutions for the model. In one, the boundary layer reaches a terminal velocity in the tail as the boundary layer plasma effectively joins the solar wind flow. In the other solution, the flow is nearly stopped in the far tail. In combination with other mechanisms, this latter solution may correspond to the case in which the boundary layer plasma participates in magnetospheric convection and returns sunward. The density, velocity, and thickness as functions of distance from local noon are studied, assuming that the magnetopause hasa elliptical shape and the magnetospheric field is dipolar.

Song, P.; Holzer, T. E.; Russell, C. T.; Wang, Z.

1994-04-01

239

Dusty boundary layer in a surface-burst explosion  

SciTech Connect

Dusty boundary layers are an inherent feature of explosions over ground surfaces. Detailed knowledge of dusty boundary layer characteristics is needed in explosion safety analysis (e.g., to calculate the drag loads on structures). Also, to predicct the amount of dust in the rising fireball of an explsion, one must know the dusty boundary layer swept up during the positive and negative phases of the blast wave and how much of this boundary layer dust is entrained into the stem of the dust cloud. This paper describes the results of numerical simulations of the dusty boundary layer created by a surface burst explosion. The evolution of the flow was calculated by a high-order Godunov code that solves the nonsteady conservation laws.

Kuhl, A.L. [Lawrence Livermore National Lab., El Segundo, CA (United States); Ferguson, R.E.; Chien, K.Y.; Collins, J.P. [Naval Surface Warfare Center, Silver Spring, MD (United States)

1993-08-01

240

Cavitation erosion of NiAl-bronze layers generated by friction surfacing  

Microsoft Academic Search

Friction surfacing is a solid-state process, which allows deposition welding at temperatures below the melting range. For this investigation coating layers of NiAl-bronze were deposited by friction surfacing on self-mating substrates, followed by microstructural characterisation. Further, cavitation tests were performed in order to investigate wear resistance. Cavitation erosion mechanisms were analysed by means of optical and electron microscopy. All coatings

Stefanie Hanke; Alfons Fischer; Matthias Beyer; Jorge dos Santos

2011-01-01

241

Structure and friction-reducing property of the sulfide layer produced by ion sulfuration  

SciTech Connect

Sulfide layers with a certain thickness were made on the surface of 1045 and 52100 steels by means of the low-temperature ion sulfuration technique. Metallography, scanning electron microscope (SEM) + energy-dispersive x-ray analysis (EDX), and x-ray diffraction (XRD) were adopted to analyze the structure of sulfide layers; the tribological properties of the layers lubricated by paraffin oil were also investigated on a reciprocating tester. The results showed that sulfide layer is porous, and its structure is mainly composed of FeS, FeS{sub 2}, and substrate phases. The sulfide layer possessed a remarkable friction-reducing effect; its friction coefficient was lower on average, by about 50%, than that of the surface without layer. With the increase of layer thickness, its friction coefficient was unchanged, and under low load conditions, its operational period was prolonged. Under the same experimental conditions, the operational period of sulfide layer on 52100 steel was longer than that on 1045 steel, and its friction coefficient was lower as well.

Ning, Z.; Da-Ming, Z.; Yan-Hua, W.; Jia-Jun, L.; Xiao-Dong, F.; Ming-Xi, G.

2000-04-01

242

Effects of Normal Swirling Jet on a Boundary Layer Flow  

NASA Astrophysics Data System (ADS)

To investigate the effects of hurricane flow on its surrounding boundary layer, a normal counter clockwise swirling jet at 14.4 m/s average velocity is created in a flat-plate boundary layer flow at 8.1 m/s. The boundary layer parameters and, mean and root mean-squared velocity data are measured at the four quadrants (port, windward, starboard and leeward) of the swirling jet using a Pitot tube and a hot wire anemometer. The boundary layer flow is decelerated due to the opposing flow created by the swirl on the port side. The starboard side flow is accelerated due to the swirl direction being parallel to the freestream. On the windward side, the boundary layer flow is slightly decelerated. At this location the swirl direction is normal to the free stream and acts as wall with slippage. For the leeward location boundary layer flow is decelerated due to the wake and blockage effects created by the normal swirling jet. The boundary layer turbulence intensity on the port side increases to its peak at about half boundary layer thickness and drops as it approaches the free stream. Whereas the boundary layer intensity on the starboard side is small near the wall but increases linearly up until it reaches two times the boundary layer thickness. From that point on it decreases linearly. There is a greatest loss of momentum or momentum flux on the leeward location, primarily due to the wake and flow blockage. The Log-law appears to be completely washed-out in the port side velocity profile

Caballero, Irving I.; Subramanian, Chelakara S.

2002-11-01

243

Entrainment and detrainment from a model boundary layer  

NASA Astrophysics Data System (ADS)

A two-dimensional inviscid flow with piecewise-uniform regions of vorticity is studied as a model of the high-Reynolds-number mixing between a boundary layer and an outer layer. It is found that an initial disturbance to the boundary-layer thickness breaks down into a wave field plus, if the initial disturbance is steep enough, a volume of entrained fluid. The entrained fluid is drawn from the outer layer and then folded into a crevice. The crevice stretches, and eventually pinches off, becoming completely enveloped within the boundary layer. Though the entrained fluid is slender in shape, its volume is significant. Very steep disturbances result in detrainment, in which a small parcel of fluid detaches from the boundary layer and curls into the outer layer. The v-velocity field agrees with many features of Kovasznay et al.'s (1970) measurements in the turbulent boundary layer. This correspondence with fully turbulent flow, plus the characteristics of folding and stretching large volumes of fluid, make the process presented here a candidate for a mechanism by which high-Reynolds-number boundary layers mix with outer-layer fluid.

Sun, Meihong; Lichter, Seth

2003-06-01

244

Frictional sliding in layered rock: laboratory-scale experiments  

SciTech Connect

The work is part of the rock mechanics effort for the Yucca Mountain Site Characterization Program. The laboratory-scale experiments are intended to provide high quality data on the mechanical behavior of jointed structures that can be used to validate complex numerical models for rock-mass behavior. Frictional sliding between simulated rock joints was studied using phase shifting moire interferometry. A model, constructed from stacks of machined and sandblasted granite plates, contained a central hole bore normal to the place so that frictional slip would be induced between the plates near the hole under compressive loading. Results show a clear evolution of slip with increasing load. Since the rock was not cycled through loading- unloading, the quantitative differences between the three data sets are probably due to a ``wearing-in`` effect. The highly variable spatial frequency of the data is probably due to the large grain size of the granite and the stochastic frictional processes. An unusual feature of the evolution of slip with increasing load is that as the load gets larger, some plates seem to return to a null position. Figs, 6 refs.

Buescher, B.J.; Perry, K.E. Jr.; Epstein, J.S.

1996-09-01

245

High Reynolds number rough wall turbulent boundary layer experiments using Braille surfaces  

NASA Astrophysics Data System (ADS)

This paper details smooth, transitional and fully rough turbulent boundary layer experiments in the New Mexico State high Reynolds number rough wall wind tunnel. The initial surface tested was generated with a Braille printer and consisted of an uniform array of Braille points. The average point height being 0.5mm, the spacing between the points in the span was 0.5mm and the surface consisted of span wise rows separated by 4mm. The wavelength to peak ratio was 8:1. The boundary layer thickness at the measurement location was 190mm giving a large separation of roughness height to layer thickness. The maximum friction velocity was u?=1.5m/s at Rex=3.8 x10^7. Results for the skin friction co-efficient show that this surface follows a Nikuradse type inflectional curve and that Townsends outer layer similarity hypothesis is valid for rough wall flows with a large separation of scales. Mean flow and turbulence statistics will be presented.

Harris, Michael; Monty, Jason; Nova, Todd; Allen, James; Chong, Min

2007-11-01

246

Theoretical and modeling studies of the marine planetary boundary layer  

NASA Astrophysics Data System (ADS)

The research goals were: (1) to gain an improved theoretical understanding of and a predictive capability for partly cloudy boundary layers, and to test these ideas against data acquired in the field and (2) to investigate the role of air-sea interactions in regulating cloud amount in the marine boundary layer. The objectives were to produce an extensive set of theoretical and numerical results, leading to better physical understanding of the cloudy marine boundary layer and provide a theoretical basis for the planning and execution of the Atlantic Stratocumulus Transition Experiment (ASTEX). A new type of boundary-layer model was developed that combines second order closure with a bulk representation of the vertical structure. The boundary-layer depth and turbulence kinetic energy (TKE) are prognostically determined. The large turbulent eddies that are primarily responsible for the fluxes are modeled as convective circulations, with ascending and descending branches. The interior of the boundary layer is bounded above by a thin entrainment layer and below by a thin ventilation layer. Conservative variables such as the the equivalent potential temperature have quadratic profiles in the interior. Convective circulations occur, with rising branches occupying fractional area sigma, which is predicted by the model. The upper ocean is represented by a mixed layer whose depth can be either fixed or variable, depending on the objectives of the numerical experiment being conducted.

Randall, David A.

1990-12-01

247

Planetary boundary layer feedbacks in climate system  

NASA Astrophysics Data System (ADS)

A remarkable feature of the ongoing global warming is the asymmetry in trends of the daily minima, ?min, and maxima, ?max, of the surface air temperature (SAT): ?min increases faster than ?max, so that the daily temperature range (DTR), ?max-?min, basically decreases. The state of the art general circulation and climate models (GCMs) do not reproduce it and predict approximately the same change rates for ?min and ?max. We propose that the difference in trends of ?min and ?max is caused by the strong stability dependence of the height, h, of the planetary boundary layer (PBL). Indeed, the daytime warming is associated with deep convective (C) PBLs (with the heights hC ~ 103m), in contrast to the nocturnal and/or wintertime cooling associated with shallower mid-latitudinal nocturnal stable (NS) PBLs (with hNS ~ 200m) and even shallower high-latitudinal long-lived stable (LS) PBLs (with hLS ~ 30-50m) developing during longer than night periods of the persistent surface cooling. As a result, one and the same increment, ?Q0, in the surface heat flux leads to only minor increment in ?max in deep C PBLs, but essential increments in ?min in shallow NS and especially NS PBLs. The latter type of the PBL has been discovered only recently and is not yet accounted for in modern GCMs. In the present paper, we derive theoretical estimates of the variations, ??min and ??max, in the SAT minima and maxima associated with the stable and convective PBLs, respectively, and by this means explain the observed asymmetry in the growth rates of ?min and ?max. To characterise the role of PBLs in the climate system, we introduce the concepts of local and general PBL feedbacks. Besides the strengths of feedbacks, we propose to take into account the reaction times of different mechanisms. The proposed concepts could be applied to different climate-change problems from global (as in this paper) to local, in particular, to those caused by the land-use modification.

Zilitinkevich, S.; Esau, I.

2009-09-01

248

The Role of Adsorbed Water on the Friction of a Layer of Submicron Particles  

NASA Astrophysics Data System (ADS)

Anomalously low values of friction observed in layers of submicron particles deformed in simple shear at high slip velocities are explained as the consequence of a one nanometer thick layer of water adsorbed on the particles. The observed transition from normal friction with an apparent coefficient near ?=0.6 at low slip speeds to a coefficient near ?=0.3 at higher slip speeds is attributed to a competition between the time required to extrude the water layer from between neighboring particles in a force chain and the average lifetime of the chain. At low slip speeds the time required for extrusion is less than the average lifetime of a chain so the particles make contact and lock. As slip speed increases, the average lifetime of a chain decreases until it is less than the extrusion time and the particles in a force chain never come into direct contact. If the adsorbed water layer allows the otherwise rough particles to rotate, then the coefficient of friction will drop to ?=0.3 appropriate for rotating spheres. At the highest slip speeds particle temperatures rise above 100 C, the water layer vaporizes, the particles contact and lock, and the coefficient of friction rises to ?=0.6. The observed onset of weakening at slip speeds near 0.001 m/s is consistent with the measured viscosity of a 1nm thick layer of adsorbed water, with a minimum particle radius of about 20 nm, and with reasonable assumptions about the distribution of force chains guided by experimental observation. The reduction of friction and range of velocities over which it occurs decreases with increasing normal stress as predicted by the model. Moreover, the analysis predicts that this high speed weakening mechanism should operate only for particles with radii smaller than about 1 ?m. For larger particles the slip speed required for weakening is so large that frictional heating will evaporate the adsorbed water and weakening will not occur.

Sammis, C. G.; Lockner, D. A.; Reches, Z.

2011-12-01

249

The role of adsorbed water on the friction of a layer of submicron particles  

USGS Publications Warehouse

Anomalously low values of friction observed in layers of submicron particles deformed in simple shear at high slip velocities are explained as the consequence of a one nanometer thick layer of water adsorbed on the particles. The observed transition from normal friction with an apparent coefficient near ? = 0.6 at low slip speeds to a coefficient near ? = 0.3 at higher slip speeds is attributed to competition between the time required to extrude the water layer from between neighboring particles in a force chain and the average lifetime of the chain. At low slip speeds the time required for extrusion is less than the average lifetime of a chain so the particles make contact and lock. As slip speed increases, the average lifetime of a chain decreases until it is less than the extrusion time and the particles in a force chain never come into direct contact. If the adsorbed water layer enables the otherwise rough particles to rotate, the coefficient of friction will drop to ? = 0.3, appropriate for rotating spheres. At the highest slip speeds particle temperatures rise above 100°C, the water layer vaporizes, the particles contact and lock, and the coefficient of friction rises to ? = 0.6. The observed onset of weakening at slip speeds near 0.001 m/s is consistent with the measured viscosity of a 1 nm thick layer of adsorbed water, with a minimum particle radius of approximately 20 nm, and with reasonable assumptions about the distribution of force chains guided by experimental observation. The reduction of friction and the range of velocities over which it occurs decrease with increasing normal stress, as predicted by the model. Moreover, the analysis predicts that this high-speed weakening mechanism should operate only for particles with radii smaller than approximately 1 ?m. For larger particles the slip speed required for weakening is so large that frictional heating will evaporate the adsorbed water and weakening will not occur.

Sammis, Charles G.; Lockner, David A.; Reches, Ze’ev

2011-01-01

250

Matter-antimatter boundary layers with a magnetic neutral sheet  

Microsoft Academic Search

An earlier model of matter-antimatter boundary layers has been extended to include a sheet with a reversed magnetic field. The derived layer thickness is largely unaffected by a magnetic field-reversal, provided that the width of the corresponding magnetic neutral sheet becomes substantially smaller than the layer thickness. This condition is likely to be satisfied within parameter ranges of cosmical interest.

B. Lehnert

1978-01-01

251

Mixed convection boundary layer flow over a horizontal circular cylinder with Newtonian heating  

NASA Astrophysics Data System (ADS)

The steady mixed convection boundary layer flow over a horizontal circular cylinder, generated by Newtonian heating in which the heat transfer from the surface is proportional to the local surface temperature, is considered in this study. The governing boundary layer equations are first transformed into a system of non-dimensional equations via the non-dimensional variables, and then into non-similar equations before they are solved numerically using a numerical scheme known as the Keller-box method. Numerical solutions are obtained for the skin friction coefficient Re 1/2 C f and the local wall temperature ? w ( x) as well as the velocity and temperature profiles with two parameters, namely the mixed convection parameter ? and the Prandtl number Pr.

Salleh, Mohd Zuki; Nazar, Roslinda; Pop, Ioan

2010-12-01

252

Measurements of Boundary Layer Structure at Fort Cobb During CLASIC, June 2007  

NASA Astrophysics Data System (ADS)

A tethersonde system was deployed at Fort Cobb, Oklahoma during the Cloud and Land Surface Interaction Campaign (CLASIC) June 8-24 2007 with the objective of characterizing the diurnal cycle of lower boundary layer structure up to 500 m including wind, pressure, temperature, humidity as well as CO2 profiles over harvested wheat. One unique feature of this data set is that includes fair weather, pre-storm and post-storm conditions for a record monthly rainfall in Oklahoma, in excess of 300 mm at the site. Here, we discuss specifically the diurnal cycle of (potential temperature) and q (specific humidity) and overall boundary layer structure during the duration of the field campaign with an emphasis on conditions before and after one major rain event. Preliminary regional estimates of surface roughness and friction velocity, and sensible heat flux and latent heat flux are also presented.

Li, W.; Barros, A. P.; Kang, D. H.; Prat, O. P.; Shrestha, P.; Tao, K.; Giovannettone, J.; Munoz, F.; Patrick, W.; Peters-Lidard, C.; Jackson, T.

2007-12-01

253

Marine Boundary-layer Height Estimated From The Hirlam Model  

NASA Astrophysics Data System (ADS)

Two weeks of radiosonde measurements of the boundary-layer height over a small island (Christiansoe) in the Baltic Sea is discussed. During the experiment the water was generally warmer than the air which is a typical feature of the Baltic Sea during the late summer, autumn and early winter. This results in a positive heat flux over the sea and the generation of convectively driven marine boundary layer. The boundary-layer heights that could be inferred from the measurements were compared to estimates derived from the operational numerical weather prediction High Resolution Limited Area Model HIRLAM (a version of the Swedish Meteorological and Hydrological Institute with grid resolution of 22.5 km times 22.5 km). The hight of the boundary layer does not form a part from the HIRLAM model, but has to be estimated from them. In this study we applied and compared two methods to extract the boundary- layer height from the Hirlam output data; both are based on a bulk Richardson-number approach. For both methods the boundary-layer height is defined as the height where the bulk Richardson number reaches a critical value, typically 0.25. For southwesterly winds it was found that a relatively large island (Bornholm) lying 20 km upwind of the measuring site influences the boundary-layer height. In this situation Richardson- number based methods with the HIRLAM data fail most likely because the island of Bornholm and the water fatch to the measuring site are about the size of the grid resolution of HIRLAM model and therefore poorly resolved. The grid resolution is too coarse to reflect the mesoscale features that control the boundary-layer height over Christiansoe. For northerly wind the water fatch to the measuring site is about 100 km and the Richardson-number methods reproduce the measured height of the boundary layer. This suggests that the HIRLAM model adequately resolves a water fatch of 100 km with respect predictions of the height of the marine boundary layer. Originally the critical Richarson numbers for both methods are determined from measurements of the height of the boundary layer over land. In this study the boundary-layer height predicted by one of the Richardson-number methods is systematically higher than for the other. This suggests, considering the low roughness of the sea surface, that there is dependence between the surface roughness and the critical Richardon numbers and that the dependence is not the same for the numbers used.

Batchvarova, E.; Gryning, S.-E.; Jensen, N.-O.

254

Boundary layer receptivity to convected gusts and sound  

NASA Astrophysics Data System (ADS)

The receptivity of a laminar boundary layer to sound and convected gusts is examined experimentally, considering the coupling between these external disturbances and the boundary layer in the vicinity of a 24:1 elliptic leading edge, a porous strip, and a forward-facing step. A conventional loudspeaker generates the acoustic disturbance, and an array of oscillating ribbons produces a vortical disturbance in the form of a periodic convected gust. Techniques for decoupling the excitation from the boundary layer response and comparisons of receptivity mechanisms are discussed.

Parekh, D. E.; Pulvin, P.; Wlezien, R. W.

255

Transitional and turbulent flat-plate boundary layers with heat transfer  

NASA Astrophysics Data System (ADS)

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

Wu, Xiaohua; Moin, Parviz

2010-11-01

256

Buoyancy effects on the laminar boundary layer heat transfer along vertically moving cylinders  

NASA Astrophysics Data System (ADS)

The local similarity method (Lloyd, Sparrow, 1970) is used to study the effects of buoyancy force on the laminar boundary layer heat transfer along vertically moving cylinders. Cases of prescribed surface temperature and wall heat flux in power of streamwise distance are analyzed. Local similarity solutions are obtained to show the effects of the transverse curvature of the cylinder surface and buoyancy parameters on the surface friction and heat transfer rate. It is known, however, that the local non-similarity method (Sparrow, Quack, Boerner, 1970) and the finite difference method (Mucoglu, Chen, 1979) would give more accurate results.

Lin, H.-T.; Shih, Y.-P.

1981-01-01

257

Turbulent boundary layer measurements over flat surfaces coated by nanostructured marine antifoulings  

NASA Astrophysics Data System (ADS)

Whilst recent developments of nanotechnology are being exploited by chemists and marine biologists to understand how the completely environmentally friendly foul release coatings can control marine biofouling and how they can be developed further, the understanding of the hydrodynamic performances of these new generation coatings is being overlooked. This paper aims to investigate the relative boundary layer, roughness and drag characteristics of some novel nanostructured coatings, which were developed through a multi-European and multi-disciplined collaborative research project AMBIO (2010), within the framework of turbulent flows over rough surfaces. Zero-pressure-gradient, turbulent boundary layer flow measurements were conducted over flat surfaces coated with several newly developed nanostructured antifouling paints, along with some classic reference surfaces and a state-of-the-art commercial coating, in the Emerson Cavitation Tunnel (ECT) of Newcastle University. A large flat plane test bed that included interchangeable flat test sections was used for the experiments. The boundary layer data were collected with the aid of a two-dimensional DANTEC Laser Doppler Velocimetry (LDV) system. These measurements provided the main hydrodynamic properties of the newly developed nanostructured coatings including local skin friction coefficients, roughness functions and Reynolds stresses. The tests and subsequent analysis indicated the exceptionally good frictional properties of all coatings tested, in particular, the drag benefit of some new nanostructured coatings in the Reynolds number range investigated. The rapidly decreasing roughness function trends of AKZO19 and AKZO20 as the ks^{ + } increases were remarkable along with the dissimilar roughness function character of all tested coatings to the well-known correlation curves warranting further research at higher Reynolds numbers. The wall similarity concept for the Reynolds stresses was only validated for the transitionally rough surfaces from (y + \\varepsilon)^{ + } ? 100 up to the end of the boundary layer.

Ünal, U?ur Oral; Ünal, Burcu; Atlar, Mehmet

2012-06-01

258

Turbulent boundary layer flow subject to streamwise oscillation of spanwise wall-velocity  

NASA Astrophysics Data System (ADS)

Direct numerical simulations have been performed to study the effect of a stationary distribution of spanwise wall-velocity that oscillates in the streamwise direction on a turbulent boundary layer. For the first time, a spatially developing flow with this type of forcing is studied. The part of the boundary layer which flows over the alternating wall-velocity section is greatly affected with a drag reduction close to 50% which exhibits an oscillatory distribution with a wavenumber which is twice that of the imposed wall-velocity. The maximum in drag reduction occurs where the wall velocity is at its maximum (or minimum) and the minimum occurs where the wall velocity is zero. Comparisons of the mean spanwise velocity profiles with the analytical solution to the laminar Navier-Stokes equations show very good agreement. The streamwise velocity profile indicates a thickening of the viscous sub-layer when scaled with the local friction velocity and an upward shifting of the logarithmic region when scaled with the reference (unmanipulated) friction velocity. An estimation of the idealized power consumption shows that--with the present wall forcing magnitude--more energy is required for the spatial oscillation than what is saved by drag reduction.

Skote, M.

2011-08-01

259

Energy dissipating structures in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

We present numerical experiments of a dipole crashing into a wall, a generic event in two-dimensional incompressible flows with solid boundaries. The Reynolds number Re is varied from 985 to 7880, and no-slip boundary conditions are approximated by Navier boundary conditions with a slip length proportional to Re-1. Energy dissipation is shown to first set up within a vorticity sheet of thickness proportional to Re-1 in the neighborhood of the wall, and to continue as this sheet rolls up into a spiral and detaches from the wall. The energy dissipation rate integrated over these regions appears to converge towards Rey-independent values, indicating the existence of energy dissipating structures that persist in the vanishing viscosity limit. Details can be found in Nguyen van yen, Farge and Schneider, PRL, 106, 184502 (2011).

Farge, Marie; Nguyen van Yen, Romain; Schneider, Kai

2011-11-01

260

Boundary layer aerosols at Trivandrum tropical coast  

NASA Astrophysics Data System (ADS)

Most of the atmospheric particulate matter produced from the earth's surface and injected into the Atmospheric Boundary Layer (ABL) through various natural and anthropogenic processes subsequently intrude into the free troposphere and above depending on prevailing turbulent and meteorological features. Properties of ABL aerosols at Trivandrum (8.5°N, 77 °E), a typical coastal station situated near the Arabian Sea uncontaminated by any major industrial sources, are studied using a High Volume Sampler (HVS), a Low Pressure Impactor (LPI), a CW lidar at 0.5?m wavelength (CWL) and a multi-wavelength (0.4-1.02 ?m) solar radiometer (MWR). While HVS provides hourly variation of surface TSP (Total Suspended Particles) concentration, LPI measures the size resolved aerosol mass concentration (in the aerodynamic radius range 0.05 to 14 ?m) in 12 size-bins averaged for a few days in a month, CWL gives altitude profile of aerosol extinction and number density (Na) up to ~2km and MWR gives the columnar aerosol optical depth (AOD). While the diurnal variation of TSP concentration strongly depends on features of mesoscale circulation, the seasonal TSP pattern depends on local meteorology and synoptic circulation. In winter, TSP concentration is low during sea breeze (SB) and high during land breeze (LB). This TSP contrast decreases in summer and reverses during the monsoon period. On an average, TSP concentration varies in the range 30 to 160 ?g/m 3 with two maxima, during winter and monsoon periods (attributed respectively to increased production of continental aerosols and that of sea-spray aerosols by surface winds). Relative concentration of small particles (radius r < 0.15?m) is high in winter and low during the monsoon period, while that for particles in the intermediate size range (0.151.5?m) remains almost steady through out the year. Though on an average the number -size distribution approximates to a power law type decrease with increase in size (size index 4.05 +/-0.15), three modes are discernable around 0.05, 0.5 and 5 ?m. Altitude profile of Na is characterised by well-mixed region, entrainment region and upper mixing region. Aerosol -mixing height generally lies in the range 150 to 400m depending on the strength of vertical eddy mixing in ABL. Above this, Na decreases almost exponentially with a scale height of 0.5-1.5km. Sea-spray contribution to Na shows a significant non-linear dependence on surface wind speed. About 10-30% of the columnar AOD is contributed (high in winter and monsoon periods) by the mixing region. An increasing trend in mixing region AOD also is observed at this site from 1989.

Parameswaran, K.; Kumar, S. Sunil; Rajeev, K.; Nair, P.; Krishna Murthy, K.

261

Turbulent Transfer in the Marine Planetary Boundary Layer.  

National Technical Information Service (NTIS)

Work under this contract focused on turbulent processes in the marine atmospheric boundary layer (MABL) and how they effect mixing between ocean and the free atmosphere. In particular, we investigated specific mechanisms of this turbulence within the MABL...

S. Jodha S. Khalsa H. P. Hanson

1994-01-01

262

3D (3 Dimensional) Boundary Layer Computations on Swept Wings.  

National Technical Information Service (NTIS)

A three-dimensional turbulent boundary layer code based on an integral method developed by Cousteix and Aupoix has been made operational on the NAL computer. The turbulent modeling is based on an improved mixing length hypothesis and similarity solutions....

N. R. Subramanian M. I. James

1987-01-01

263

Structure of the Laminar Ablating Air-Teflon Boundary Layer.  

National Technical Information Service (NTIS)

Radiation profiles in an ablating flat plate air-teflon laminar boundary layer were studied both experimentally and theoretically. The experiments were conducted in a one atmosphere, 3000 - 6000K, subsonic free stream produced by an arc jet. Spatially res...

R. A. Greenberg N. H. Kemp K. L. Wray

1968-01-01

264

Computational Evaluation of Quiet Tunnel Hypersonic Boundary Layer Stability Experiments.  

National Technical Information Service (NTIS)

A computational evaluation of two stability experiments conducted in the NASA Langley Mach 6 axisymmetric quiet nozzle test chamber facility is conducted. Navier- Stokes analysis of the mean flow and linear stability theory analysis of boundary layer dist...

M. L. Manning

2000-01-01

265

Stability of Boundary Layers at High Supersonic and Hypersonic Speeds.  

National Technical Information Service (NTIS)

The thrust of this research program has been the improvement of our capabilities for analyzing stability and transition of boundary layers at supersonic speeds. During the first phase, our efforts were primarily directed toward analytical studies, establi...

T. Herbert

1992-01-01

266

Cross Stream Differencing for Integral Boundary Layer Equations.  

National Technical Information Service (NTIS)

An efficient, stable, explicit, first-order cross-stream differencing scheme having low truncation error is derived and applied to three dimensional integral boundary layer equations. The analysis is considered in detail for the particular case of the mom...

D. Hally

1987-01-01

267

ATMOSPHERIC DISPERSION MODELING BASED UPON BOUNDARY LAYER PARAMETERIZATION  

EPA Science Inventory

Characteristic scaling parameters in the planetary boundary layer have been applied to estimate the dispersion of nonbuoyant gaseous pollutants. Vertical and lateral spread are treated separately, and the choice of parameters for the dispersion models depends upon the actual stat...

268

Hybrid spectral element/asymptotic method for boundary layers problems  

SciTech Connect

This report presents an efficient higher-order numerical approach to many-dimensional problems the boundary layers. This method uses a coarse mesh penalty-spectral element method with a one-dimensional asymptotic approximation.

Zrahia, U.; Orszag, S.A. [Princeton Univ., NJ (United States); Israeli, M. [Israel Institute of Technology, Haifa (Israel)

1997-12-01

269

Differences in the structure of a planetary magnetopause boundary layer  

NASA Astrophysics Data System (ADS)

The boundary of a planetary magnetosphere is known as a magnetopause, and is the site of energy, mass, and momentum transfer. The structure of internal boundary layers adjacent to these boundaries is intimately related to the processes responsible for this transport. We use thermal electron observations made by the Cassini spacecraft to examine the structure of Saturn’s low-latitude internal boundary layer. By analyzing the relationship between the electron density and temperature during the crossings we demonstrate that the structure of the layer is variable. At some of the crossings the major changes in electron density and temperature occur in distinct regions of the layer (as for previously reported examples at Earth), whereas at others the two quantities change over a similar region. We discuss the possible explanations for this phenomenon, and what this could tell us about how the solar wind interacts with a planetary magnetosphere.

Coates, A. J.; Masters, A.; Walsh, A. P.; Fazakerley, A. N.; Dougherty, M. K.

2010-12-01

270

Investigation of the Stable Atmospheric Boundary Layer at Halley Antarctica  

NASA Astrophysics Data System (ADS)

Boundary-layer measurements from the Brunt Ice Shelf, Antarctica are analyzed to determine flux-profile relationships. Dimensionless quantities are derived in the standard approach from estimates of wind shear, potential temperature gradient, Richardson number, eddy diffusivities for momentum and heat, Prandtl number, mixing length and turbulent kinetic energy. Nieuwstadt local scaling theory for the stable atmospheric boundary-layer appears to work well departing only slightly from expressions found in mid-latitudes. An - single-column model of the stable boundary layer is implemented based on local scaling arguments. Simulations based on the first GEWEX Atmospheric Boundary-Layer Study case study are validated against ensemble-averaged profiles for various stability classes. A stability-dependent function of the dimensionless turbulent kinetic energy allows a better fit to the ensemble profiles.

Rodrigo, Javier Sanz; Anderson, Philip S.

2013-09-01

271

Boundary-Layer-Tripping Studies of Compressible Dynamic Stall Flow.  

National Technical Information Service (NTIS)

The challenging task of 'properly' tripping the boundary layer near the leading edge of an airfoil experiencing compressible dynamic stall has been addressed. Real-time interferometry studies have been conducted on an oscillating airfoil undergoing compre...

M. S. Chandrasekhara M. C. Wilder L. W. Carr

1996-01-01

272

Some Topics Relating to Modelling of Dispersion in Boundary Layer.  

National Technical Information Service (NTIS)

This special report discusses six topics all of the major current interest in modelling of dispersion in the atmospheric boundary layer. These are the second-order closure modelling of turbulence, crosswind dispersion and the properties of turbulence, win...

F. Pasquill

1975-01-01

273

The Behaviour of Similar Solutions in a Compressible Boundary Layer.  

National Technical Information Service (NTIS)

The paper discusses the mathematical properties of similar solutions of the boundary-layer equations in a compressible model fluid. Assuming a favourable pressure gradient and that backflow is not present, the results include (among other things) a rigoro...

J. B. McLeod J. Serrin

1968-01-01

274

A multidisciplinary optimization method for designing boundary layer ingesting inlets  

Microsoft Academic Search

The Blended-Wing-Body is a conceptual aircraft design with rear-mounted, over-wing engines. Two types of engine installations have been considered for this aircraft. One installation is quite conventional with podded engines mounted on pylons. The other installation has partially buried engines with boundary layer ingesting inlets. Although ingesting the low-momentum flow in a boundary layer can improve propulsive efficiency, poor inlet

David Leonard Rodriguez

2001-01-01

275

Benthic boundary layer processes in coastal environments: An introduction  

Microsoft Academic Search

This special issue ofGeo-Marine Letters “Benthic Boundary Layer Processes in Coastal Environments” includes 20 papers devoted to results of recent near-shore experiments supported by the Coastal Benthic Boundary Layer (CBBL) program. Experiments were conducted in gas-rich muddy sediments of Eckernförde Bay of the Baltic Sea and on relict sandy sediments of the West Florida Sand Sheet. In this introductory paper

M. D. Richardson; W. R. Bryant

1996-01-01

276

The behavior of vertically integrated boundary-layer winds  

Microsoft Academic Search

This research provides a study of the behavior of vertically integrated boundary-layer winds (IBLWs). This information should be helpful for both theoretical and practical applications, e.g., boundary-layer parameterization in general circulation models, air pollution models, and low-level parachuting operations. The study concerned itself with winds integrated up to a height of 300 m in the United States. The only data

René V. Cormier

1975-01-01

277

Boundary Layer Structure and Processes in Mid - Ocean Storms  

Microsoft Academic Search

Measurements taken during the Storm Transfer and Response Experiment (STREX) are used to analyze boundary layer structures and processes in the vicinity of North Pacific storms. Case studies are carried out for the pre -frontal, post-frontal, and frontal sectors of storms. The effects of sub-grid scale processes on the boundary layer and the overlying atmosphere receive special emphasis. The pre-frontal

Nicholas A. Bond

1986-01-01

278

Drag and heat transfer relations for the planetary boundary layer  

Microsoft Academic Search

A theory is offered for the drag and heat transfer relations in the statistically steady, horizontally homogeneous, diabatic, barotropic planetary boundary layer. The boundary layer is divided into three regionsR1,R2, andR3, in which the heights are of the order of magnitude ofz0,L, andh, respectively, wherez0 is the roughness length for either momentum or temperature,L is the Obukhov length, andh is

Robert R. Long; Larry J. Guffey

1977-01-01

279

Boundary-Layer Turbulence Over The Nebraska Sandhills  

Microsoft Academic Search

Data from National Centre for Atmospheric Research (NCAR) Queen Air boundary-layer flights over the Nebraska Sandhills are analyzed to investigate the effects of these low hills on boundary-layer turbulence. The Sandhills are an area of anisotropic rolling terrain with characteristic wavelengths of order 2km and rms height variations of order 25m. The biggest impact is found in early morning flight

Yoseph G. Mengesha; Peter A. Taylor; Donald H. Lenschow

2001-01-01

280

Study on ground clutter prevention fences for boundary layer radars  

NASA Astrophysics Data System (ADS)

A low elevation sidelobe suppression algorithm based on the uniform physical theory of diffraction (PTD) is developed to simulate ground clutter prevention fences for boundary layer radars (BLRs). As applications to the algorithm, the most suitable fence is achieved for the lower troposphere radar (LTR) and the L-28 boundary layer radar, respectively. The developed algorithm can also be applied to other radar systems where reducing low elevation sidelobes is desired.

Rao, Qinjiang; Hashiguchi, Hiroyuki; Fukao, Shoichiro

2003-04-01

281

An investigation of boundary-layer behavior on wavy walls  

Microsoft Academic Search

A numerical approach has been developed to study the viscous flow over wavy walls. It is based on two-dimensional compressible turbulent boundary-layer equations and involves functions for pressure gradient and pressure phase lag as well as for the effects of surface curvature on boundary-layer behavior. Results are presented for three kinds of wavy wall and provide further evidence of reduction

Suxun Li; Guang Li Chen

1989-01-01

282

High Reynolds number thick axisymmetric turbulent boundary layer measurements  

Microsoft Academic Search

Experimental measurements of the wall shear stress and momentum thickness for thick axisymmetric turbulent boundary layers are presented. The use of a full-scale towing tank allowed zero pressure gradient turbulent boundary layers to be developed on cylinders with diameters of 0.61, 0.89, and 2.5 mm and lengths ranging from 30 m to 150 m. Moderate to high Reynolds numbers (10 4Re ?5, 10

K. M. Cipolla; W. L. Keith

2003-01-01

283

Drag Reduction for External and Internal Boundary Layers Using Riblets and Polymers.  

National Technical Information Service (NTIS)

Two hydrodynamic experiments were conducted to measure drag reduction using riblets in turbulent boundary layers. The first was an external boundary layer experiment using a flat plate in a water tunnel, and the second was an internal boundary layer exper...

L. W. Reidy G. W. Anderson

1988-01-01

284

Influences on the Height of the Stable Boundary Layer as Seen in LES.  

National Technical Information Service (NTIS)

Climate models, numerical weather prediction (NWP) models, and atmospheric dispersion models often rely on parameterizations of planetary boundary layer height. In the case of a stable boundary layer, errors in boundary layer height estimation can result ...

B. Kosovic J. K. Lundquist

2004-01-01

285

Velocity spectra in the marine atmospheric boundary layer  

NASA Astrophysics Data System (ADS)

Velocity spectra in the marine atmospheric boundary layerAnn-Sofi Smedman and Ulf Högström Department of Earth Sciences, Meteorology, Uppsala University Spectra of longitudinal and vertical velocity have been studied at a marine site, Östergarnsholm, in the Baltic Sea during a period of six days with near neutral or slightly unstable conditions when the wave state gradually changed from pure wind sea to strong swell having approximately the same direction as the wind. During the period with pure wind sea, u- and w-spectra are shown to be in complete agreement with a new theory for the neutral atmospheric surface layer, which was earlier shown to agree very well with measurements over flat land surfaces. These spectra are used as references for the analysis of the corresponding spectra during the subsequent time period. The sensible heat flux was upward but small during the entire sequence of events. The analysis shows that the shape of both the longitudinal wind spectrum and the vertical wind spectrum start to deviate from their ‘ideal' forms as soon as the wave age parameter co/U exceeds about 0.8. The spectral modification appears to start at a frequency around 0.2 Hz, resulting in two processes that evolve gradually as co/U increases: 1) A down-scale cascade effect, which causes the lower frequency limit of the f—2/3-range to gradually move to higher values of normalized frequency, f = nz/U. This effect proceeds at a greater pace in the vertical velocity component and more slowly in the longitudinal. 2) A low-frequency modification, which lifts the entire spectral level in the normalized w-spectra below 0.2 and creates a maximum in the corresponding normalized u-spectra which gradually moves towards lower frequencies. It is argued that this low-frequency spectral behaviour is a secondary effect caused by strong reduction of the friction velocity u*, which in turn, is a result of an upward directed uw co-spectral component accomplished by swell.

Smedman, A.-S.

2009-09-01

286

A novel polyvinyl alcohol hydrogel functionalized with organic boundary lubricant for use as low-friction cartilage substitute: synthesis, physical/chemical, mechanical, and friction characterization.  

PubMed

A novel material design was developed by functionalizing polyvinyl alcohol hydrogel with an organic low-friction boundary lubricant (molar ratios of 0.2, 0.5, and 1.0 moles of lauroyl chloride). The hydrogels were fabricated using two different techniques. First, the boundary lubricant was initially functionalized to the polymer, then the hydrogels were created by physically crosslinking the reacted polymer. Second, hydrogels were initially created by crosslinking pure polyvinyl alcohol, with the functionalization reaction performed on the fully formed gel. After the reaction, Fourier transform infrared spectroscopy and attenuated total reflectance spectra revealed a clear ester peak, the diminishment of the alcohol peak, and the amplification of the alkyl peaks, which confirmed attachment of the hydrocarbon chains to the polymer. Additional chemical characterization occurred through elemental analysis where an average increase of 22% carbon and 40% hydrogen provided further confirmation of attachment. Physical characterization of the boundary lubricant functionalized hydrogels was performed by water content and contact angle measurements. Water content dependency showed that method 1 had a direct relationship with boundary lubricant concentration, and method 2 displayed an inverse relationship. The contact angle increased as boundary lubricant concentration increased for the pure matrix material for both processing methods, suggesting that the hydrocarbons produced surface properties that mimic natural cartilage, and contact behavior of the biphasic system was dependent on processing method. Friction tests demonstrated a significant decrease in friction coefficient, with a maximum decrease of 70% and a minimum decrease of 24% for boundary lubricant functionalized hydrogels compared with nonfunctionalized polyvinyl alcohol hydrogels. PMID:22807285

Blum, Michelle M; Ovaert, Timothy C

2012-07-18

287

Disturbances to Air-Layer Skin-Friction Drag Reduction at High Reynolds Numbers  

Microsoft Academic Search

Skin friction drag on a flat surface may be reduced by more than 80% when a layer of air separates the surface from a flowing liquid compared to when such an air layer is absent. Past large-scale experiments utilizing the US Navy's Large Cavitation Channel and a flat-plate test model 3 m wide and 12.9 m long have demonstrated air

David Dowling; Brian Elbing; Simo Makiharju; Andrew Wiggins; Marc Perlin; Steven Ceccio

2009-01-01

288

Charnock dynamics: a model for the velocity structure in the wave boundary layer of the air–sea interface  

Microsoft Academic Search

We present a unified model of the air–sea boundary layer, which takes account of the air–sea momentum exchange across the\\u000a sea surface. The recognition of the importance of the velocity shears in the water (which comprise a frictional shear and\\u000a the Stokes shear due to the wave motion) in determining the sea surface roughness is a distinctive feature of the

John A. T. Bye; Jörg-Olaf Wolff

2008-01-01

289

Diffusion of Drag-Reducing Polymers within a High-Reynolds-Number, Rough-Wall Turbulent Boundary Layer  

Microsoft Academic Search

Two experiments were conducted to investigate polymer drag reduction (PDR) within high Reynolds number (to 200 million based on downstream distance), rough-wall turbulent boundary layers. The first experiment was conducted at the U.S. Navy's Large Cavitation Channel on a 12.9 m long flat-plate at speeds to 20 m\\/s with the surface hydraulically smooth and fully rough. Local skin-friction measurements on

Brian Elbing; Marc Perlin; David Dowling; Michael Solomon; Steven Ceccio

2008-01-01

290

Incompressible Boundary Layer Transition Flight Experiments Over a Nonaxisymmetric Fuselage Forebody and Comparisons with Laminar Boundary Layer Stability Theory  

Microsoft Academic Search

Analyses of previous boundary-layer transition experiments over axisymmetric bodies indicates a potential for achieving substantial amounts of laminar flow over such shapes. Achievement of natural laminar flow over portions of nonlifting aircraft geometries, such as fuselage forebodies, tip tanks or engine nacelles, could significantly contribute to the reduction of total aircraft viscous drag. A modern surface-panel method, a streamwise boundary-layer

Paul M. H. W. Vijgen

1990-01-01

291

ON AERODYNAMIC AND BOUNDARY LAYER RESISTANCES WITHIN DRY DEPOSITION MODELS  

EPA Science Inventory

There have been many empirical parameterizations for the aerodynamic and boundary layer resistances proposed in the literature, e.g. those of the Meyers Multi-Layer Deposition Model (MLM) used with the nation-wide dry deposition network. Many include arbitrary constants or par...

292

Wet but not slippery: boundary friction in tree frog adhesive toe pads  

PubMed Central

Tree frogs are remarkable for their capacity to cling to smooth surfaces using large toe pads. The adhesive skin of tree frog toe pads is characterized by peg-studded hexagonal cells separated by deep channels into which mucus glands open. The pads are completely wetted with watery mucus, which led previous authors to suggest that attachment is solely due to capillary and viscous forces generated by the fluid-filled joint between the pad and the substrate. Here, we present evidence from single-toe force measurements, laser tweezer microrheometry of pad mucus and interference reflection microscopy of the contact zone in Litoria caerulea, that tree frog attachment forces are significantly enhanced by close contacts and boundary friction between the pad epidermis and the substrate, facilitated by the highly regular pad microstructure.

Federle, W; Barnes, W.J.P; Baumgartner, W; Drechsler, P; Smith, J.M

2006-01-01

293

Development of a micro-PIV/ LIF System for the Study of High Reynolds Number Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

Skin friction constitutes a considerable proportion of the total drag on a ship. The flow mechanisms giving rise to friction drag occur mostly in a thin region near the solid surface. A HIgh Reynolds number, smooth, flat PLATE (HIPLATE) model is used to conduct equilibrium boundary layer experiments at scales approaching prototype applications. The plate measures 3 m wide by 12.9 m long, and Reynolds numbers (based on downstream distance) of 200 million have been achieved. This talk reports on a friction-drag-reduced flow study by means of injecting long-chain, water-soluble polymer into the boundary layer. We describe the details of a particle imaging velocimetry (PIV) system to measure the mean and fluctuating velocity profiles in the inner region of the boundary layer (y < 2 mm, or y+ < 400), with a vector spacing of approximately 40 microns. A laser induced fluorescence (LIF) system is also used to measure the mean and fluctuating concentration profiles of the injected polymer. Preliminary results are discussed. [Sponsored by DARAPA

Oweis, Ghanem; Winkel, Eric; Dowling, David; Ceccio, Steven

2004-11-01

294

A model for the height of the internal boundary layer over an area with an irregular coastline  

NASA Astrophysics Data System (ADS)

A model for the time and space variation of the internal boundary-layer height over a land area with an irregular coastline is presented. It is based on the analytical model of the boundary-layer height proposed by Gryning and Batchvarova (1990) and Batchvarova and Gryning (1991), The model accounts for the temperature jump and the mean vertical air motion at the top of the internal boundary-layer. Four cases from experiments in Nanticoke and Vancouver are used for model validation. The agreement between the calculated and measured internal boundary layer height at the observational sites is fairly good. The input information for the model consist of wind speed and direction, friction velocity and kinematic heat flux in time and space for the area, and the potential temperature gradient and the mean vertical air motion above the internal boundary layer. For the experiments used in the validation the effect of subsidence is relatively important in the afternoon under low wind speed high pressure conditions, lowering the height of the internal boundary layer by up to 10%, and it is negligible in the morning hours. The effect of the mixing height over the sea is found to be negligible.

Gryning, Sven-Erik; Batchvarova, Ekaterina

1996-03-01

295

Large Eddy Simulation study of scalar transport in fully developed wind-turbine array boundary layers  

NASA Astrophysics Data System (ADS)

Large wind farms are attaining scales at which two-way interactions with the atmospheric boundary layer must be taken into account. A recent study by Baidya et al. (PNAS 2010) has shown that wind farms increase scalar fluxes at the surface. Numerical simulations from Calaf et al. (Pof 2010) together with laboratory experiments from Cal et al. (JSRE 2010) showed that the friction velocity underneath the wind turbines is decreased. Conversely, above the turbine, friction velocity is increased. To shed light onto the relevant phenomena, a suite of Large Eddy Simulations of an infinite (fully developed) wind turbine array boundary layer, including passive scalar transport, is performed. Results clearly show an overall increase of scalar fluxes in the presence of wind turbines, of about 10-15%. And this increase is not highly dependent on wind turbine loading or spacing. This resultant increase in the scalar fluxes can be explained through a balance between two competing effects. Further, following the approach of Calaf et al. (PoF 2010), a single-column model has been developed which confirms the observed trends.

Calaf, Marc; Parlange, Marc B.; Meneveau, Charles

2011-11-01

296

Aeroelastically deflecting flaps for shock/boundary-layer interaction control  

NASA Astrophysics Data System (ADS)

An aeroelastic mesoflap system has been developed to improve the downstream flow properties of an oblique shock/boundary-layer interaction. The mesoflap system employs a set of small flaps over a cavity, whereby the flaps downstream of the interaction bend downward aeroelastically to bleed the flow and the upstream flaps bend upward to re-inject this same mass flow upstream. This recirculating system requires no net mass bleed and therefore has advantages for boundary layer control in external or mixed-compression supersonic aircraft inlets. In addition, the system may be applicable in other aerospace applications where boundary-layer control can help remedy the adverse effects of shock interactions. Several mesoflap systems have been fabricated and examined experimentally to investigate their aerodynamic and structural performance. Each mesoflap is rigidly attached to a spar on its upstream end while the remainder of the flap is free to deflect aeroelastically. The flap length is nominally a few boundary-layer thicknesses in dimension, while the flap thickness is small enough to allow tip deflections that are of the order of the boundary-layer momentum thickness. Experiments were conducted for a Mach 2.41 impinging oblique shock wave interaction with a turbulent boundary layer. Spanwise-centered laser Doppler velocimeter measurements indicate that certain mesoflap designs can show significant flow improvement as compared to the solid-wall case, including increased stagnation pressure recovery and a 7% reduction in boundary layer thickness and sonic thickness. However, one drawback of the mesoflap system is the potential for fatigue, which in some cases led to microcracking followed by flap failure. Structural design improvements to alleviate and avoid this problem included a lower profile spar design, substitution of Nitinol for aluminum as the flap material, and use of stress-relieving holes at the ends of the flap cut-outs.

Gefroh, D.; Loth, E.; Dutton, C.; Hafenrichter, E.

2003-06-01

297

Direct numerical simulations of turbulent thermal boundary layers subjected to adverse streamwise pressure gradients  

NASA Astrophysics Data System (ADS)

An innovative method for prescribing turbulent thermal inflow information in spatially developing boundary layers under streamwise pressure gradients is introduced for attached flows. The approach is tested and validated in a suite of Direct Numerical Simulations (DNS) of thermal boundary layers for zero (ZPG) and adverse (APG) pressure gradients with momentum thickness Reynolds numbers (Re?) up to 3000. The turbulent thermal data are generated based on the dynamic multi-scale approach proposed by Araya et al. [``A dynamic multi-scale approach for turbulent inflow boundary conditions in spatially evolving flows,'' J. Fluid Mech. 670, 581-605 (2011)], which is extended to include thermal field simulations in the present article. The approach is based on the original rescaling-recycling method developed by Lund, Wu, and Squires [``Generation of turbulent inflow data for spatially developing boundary layer simulations,'' J. Comput. Phys. 140, 233-258 (1998)] for ZPG flows. Isothermal walls are considered for the thermal field and the molecular Prandtl number is 0.71. In addition, only inlet momentum/thermal boundary layer thicknesses must be prescribed while other flow parameters such as the inlet friction velocity, u?, and friction temperature, ??, are computed dynamically based on the flow solution obtained downstream by means of a test plane. This plane is located between the inlet and recycle stations. Based on the unique and extensive DNS results of heat transfer obtained in this investigation, the effects of Reynolds numbers and adverse pressure gradients on the flow and thermal parameters are also explored and visualized. The principal outcome of adverse pressure gradient on the flow parameters has been determined as a secondary peak, particularly on the streamwise velocity fluctuations in the outer region, which shows clear evidence of energy production in the outer flow and not only in the buffer layer as traditionally known. Nevertheless, this peak is not so obvious on the thermal fluctuations but it is hypothesized that the reason is mainly attributed to the absence of a freestream thermal gradient, as imposed in the velocity field. Furthermore, the high-speed streaks in the buffer layer are observed to be notably shorter and wider in a Strong APG than in the ZPG case. Finally, a significant decrease of the turbulent Prandtl number is attributed to the presence of a Strong APG.

Araya, Guillermo; Castillo, Luciano

2013-09-01

298

Numerical studies of boundary-layer receptivity  

NASA Astrophysics Data System (ADS)

Direct numerical simulations (DNS) of the acoustic receptivity process on a semi-infinite flat plate with a modified-super-elliptic (MSE) leading edge are performed. The incompressible Navier-Stokes equations are solved in stream-function/vorticity form in a general curvilinear coordinate system. The steady basic-state solution is found by solving the governing equations using an alternating direction implicit (ADI) procedure which takes advantage of the parallelism present in line-splitting techniques. Time-harmonic oscillations of the farfield velocity are applied as unsteady boundary conditions to the unsteady disturbance equations. An efficient time-harmonic scheme is used to produce the disturbance solutions. Buffer-zone techniques have been applied to eliminate wave reflection from the outflow boundary. The spatial evolution of Tollmien-Schlichting (T-S) waves is analyzed and compared with experiment and theory. The effects of nose-radius, frequency, Reynolds number, angle of attack, and amplitude of the acoustic wave are investigated. This work is being performed in conjunction with the experiments at the Arizona State University Unsteady Wind Tunnel under the direction of Professor William Saric. The simulations are of the same configuration and parameters used in the wind-tunnel experiments.

Reed, Helen L.

1995-09-01

299

Micro\\/nanoscaled irreversible Otto engine cycle with friction loss and boundary effects and its performance characteristics  

Microsoft Academic Search

An irreversible cycle model of the micro-\\/nanoscaled Otto engine cycle with internal friction loss is established. The general expressions of the work output and efficiency of the cycle are calculated based on the finite system thermodynamic theory, in which the quantum boundary effect of gas particles as working substance and the mechanical Casimir effect of gas system are considered. It

Wenjie Nie; Qinghong Liao; ChunQiang Zhang; Jizhou He

2010-01-01

300

Continuum emission and broadband electrostatic noise at the low latitude boundary layer: A diagnostic of boundary layer dynamics  

NASA Astrophysics Data System (ADS)

We examine the plasma and radio waves near the inner edge of the low latitude boundary layer (LLBL) during a period of time when the layer is in a ``pulsed'' oscillating configuration. Previous work suggests there are numerous possible explanations for such oscillations including a Kelvin-Helmholtz instability, flux transfer events, and a boundary response to solar wind pressure oscillations. We demonstrate that the inner edge of the LLBL is indeed in motion, based upon the influence this motion has on the incident freely-propagating continuum emission. We also demonstrate that the inner boundary contains impulsive broadband events that appear, in high resolution, as a series of bipolar solitary pulses. These are the result of a kinetic electron beam instability occurring in association with the boundary fluid motion. We suggest that the large-scale fluid motion drives the kinetic instability via particle evacuation near the oscillating boundary.

Farrell, W. M.; Fitzenreiter, R. J.; Kaiser, M. L.; Goetz, K.; Maksimovic, M.; Reiner, M. J.

2002-06-01

301

Effects of silicon carbide in semi-metallic brake materials on friction performance and friction layer formation  

Microsoft Academic Search

The research presented in this paper is focused on the effect of silicon carbide (SiC) on friction–wear properties of semi-metallic friction composites (FC). Semi-metallic FC with increasing content of silicon carbide (SiC: 0, 3.4, 5.6, 9 and 14.6vol.%) were prepared and slid against cast iron disc and their friction–wear properties were evaluated. The friction coefficient (?) was observed to increase

Vlastimil Mat?jka; Yafei Lu; Yanli Fan; Gabriela Kratošová; Jana Lešková

2008-01-01

302

The Asymmetric Boundary layer Flow Under a Translating Hurricane.  

NASA Astrophysics Data System (ADS)

An investigation is made of the role of the translation of a hurricane in determining the distribution of boundary layer winds and in the organization of convection. A slab boundary layer model of constant depth is used to analyze the steady flow under a specified translating symmetric vortex in gradient balance. A truncated spectral formulation is used, including asymmetries through wavenumber 2. The role of linear and nonlinear asymmetric effects in the determination of the boundary layer response is diagnosed. These effects am relevant to relatively slowly and rapidly translating hurricanes, respectively.The analysis is compared to observations of Hurricanes Frederic of 1979 and Allen of 1980, as well as to other observational and theoretical cures. Allen's translation speed was approximately twice that of Frederic. It is found that the simple boundary layer formulation simulates the qualitative features of the wind field observed in Frederic. The distribution of convection in Frederic and Allen compares favorably with boundary layer convergence diagnosed from the model.

Shapiro, Lloyd J.

1983-08-01

303

A numerical study of compressible turbulent boundary layers  

NASA Astrophysics Data System (ADS)

Compressible turbulent boundary layers with free-stream Mach number ranging from 2.5 up to 20 are analyzed by means of direct numerical simulation of the Navier-Stokes equations. The fluid is assumed to be an ideal gas with constant specific heats. The simulation generates its inflow condition using the rescaling-recycling method. The main objective is to study the effect of Mach number on turbulence statistics and near-wall turbulence structures. The present study shows that supersonic/hypersonic boundary layers at zero pressure gradient exhibit close similarities to incompressible boundary layers and that the main turbulence statistics can be correctly described as variable-density extensions of incompressible results. The study also shows that the spanwise streak's spacing of 100 wall units in the inner region (y+~15) still holds for the considered high Mach numbers. The probability density function of the velocity dilatation shows significant variations as the Mach number is increased, but it can also be normalized by accounting for the variable-density effect. The compressible boundary layer also shows an additional similarity to the incompressible boundary layer in the sense that without the linear coupling term, near-wall turbulence cannot be sustained.

Lagha, M.; Kim, J.; Eldredge, J. D.; Zhong, X.

2011-01-01

304

Measurement and calculation of fluid dynamic characteristics of rough-wall turbulent boundary-layer flows  

SciTech Connect

Experimental measurements of profiles of mean velocity and distributions of boundary-layer thickness and skin friction coefficient from aerodynamically smooth, transitionally rough, and fully rough turbulent boundary-layer flows are presented for four surfaces-three rough and one smooth. The rough surfaces are composed of 1.27 mm diameter hemispheres spaced in staggered arrays 2, 4, and 10 base diameters apart, respectively, on otherwise smooth walls. The current incompressible turbulent boundary-layer rough-wall air flow data are compared with previously published results on another, similar rough surface. It is shown that fully rough mean velocity profiles collapse together when scaled as a function of momentum thickness, as was reported previously. However, this similarity cannot be used to distinguish roughness flow regimes, since a similar degree of collapse is observed in the transitionally rough regimes, since a similar degree of collapse is observed in the transitionally rough data. Observation of the new data shows that scaling on the momentum thickness alone is not sufficient to produce similar velocity profiles for flows over surfaces of different roughness character. The skin friction coefficient data versus the ratio of the momentum thickness to roughness height collapse within the data uncertainty, irrespective of roughness flow regime, with the data for each rough surface collapsing to a different curve. Calculations made using the previously published discrete element prediction method are compared with data from the rough surfaces with well-defined roughness elements, and it is shown that the calculations are in good agreement with the data.

Hosni, M.H. (Kansas State Univ., Manhattan, KS (United States). Dept. of Mechanical Engineering); Coleman, H.W. (Univ. of Alabama, Huntsville, AL (United States). Mechanical Engineering Dept.); Taylor, R.P. (Mississippi State Univ., Mississippi State, MS (United States). Mechanical and Nuclear Engineering Dept.)

1993-09-01

305

Boundary layer flow and heat transfer over a stretching sheet with convective boundary conditions  

NASA Astrophysics Data System (ADS)

In this paper, the steady boundary layer flow and heat transfer over a stretching sheet with convective boundary conditions are studied, where the heat is supplied to the convecting fluid through a bounding surface with a finite heat capacity. The nonlinear boundary layer equations are transformed into ordinary differential equations which are then solved numerically via the Keller-box method. Numerical solutions are obtained for the wall temperature, the local heat transfer coefficient for various values of the Prandtl number and the conjugate parameter. The effect of these parameters is discussed and it was found that the boundary layer thickness increases as conjugate parameter increases but opposite trend is observed for the increasing value of Pr.

Sarif, N. M.; Salleh, M. Z.; Nazar, R.

2013-04-01

306

Feasibility of generating an artificial burst in a turbulent boundary layer, phase 2  

NASA Astrophysics Data System (ADS)

Various drag accounts for about half of the total drag on commercial aircraft at subsonic cruise conditions. Two avenues are available to achieve drag reduction: either laminar flow control or turbulence manipulation. The present research deals with the latter approach. The primary objective of Phase 2 research was to investigate experimentally the feasibility of substantially reducing the skin-friction drag in a turbulent boundary layer. The method combines the beneficial effects of suction and a longitudinally ribbed surface. At a sufficiently large spanwise separation, the streamwise grooves act as a nucleation site causing a focusing of low-speed streaks over the peaks. Suction is then applied intermittently through longitudinal slots located at selected locations along those peaks to obliterate the low-speed regions and to prevent bursting. Phase 2 research was divided into two tasks. In the first, selective suction from a single streamwise slot was used to eliminate either a single burst-like event or a periodic train of artificially generated bursts in laminar and turbulent boundary layers that develop on a flat plate towed in a water channel. The results indicate that equivalent values of the suction coefficient as low as 0.0006 were sufficient to eliminate the artificially generated bursts in a laminar boundary layer.

Gad-El-Hak, Mohamed

1989-03-01

307

Diffusion of drag-reducing polymer solutions within a rough-walled turbulent boundary layer  

NASA Astrophysics Data System (ADS)

The influence of surface roughness on diffusion of wall-injected, drag-reducing polymer solutions within a turbulent boundary layer was studied with a 0.94 m long flat-plate test model at speeds of up to 10.6 m s-1 and Reynolds numbers of up to 9×106. The surface was hydraulically smooth, transitionally rough, or fully rough. Mean concentration profiles were acquired with planar laser induced fluorescence, which was the primary flow diagnostic. Polymer concentration profiles with high injection concentrations (>=1000 wppm) had the peak concentration shifted away from the wall, which was partially attributed to a lifting phenomenon. The diffusion process was divided into three zones-initial, intermediate, and final. Studies of polymer injection into a polymer ocean at concentrations sufficient for maximum drag reduction indicated that the maximum initial zone length is of the order of 100 boundary layer thicknesses. The intermediate zone results indicate that friction velocity and roughness height are important scaling parameters in addition to flow and injection conditions. Lastly, the current results were combined with those in Petrie et al. [``Polymer drag reduction with surface roughness in flat-plate turbulent boundary layer flow,'' Exp. Fluids 35, 8 (2003)] to demonstrate that the influence of polymer degradation increases with increased surface roughness.

Elbing, Brian R.; Dowling, David R.; Perlin, Marc; Ceccio, Steven L.

2010-04-01

308

Large-eddy simulation of the zero pressure gradient, turbulent boundary layer  

NASA Astrophysics Data System (ADS)

Large-eddy simulations (LES) of the zero-pressure gradient, smooth-wall, flat-plate turbulent boundary layer are presented. The LES combines the stretched-vortex, subgrid-scale (SGS) model with a tailored, near-wall model designed to incorporate anisotropic vorticity scales in the presence of the wall. Specifically, an approximate analytic integration of the stream-wise momentum equation across the near-wall layer, with inner-scaling used to reduce inertial terms, leads to a hyperbolic partial differential equation for the wall shear stress. This is coupled to an SGS model of streamwise, attached vortices in the presence of the wall, constructed to capture the principal dynamical behavior of longitudinal vortices in wall-normal transport of streamwise momentum. The result is an effective slip-velocity boundary condition for the LES at a raised "virtual wall" together with a dynamical calculation of the K'arm'an constant. Presently we demonstrate LES of the spatially developing, turbulent boundary layer at Reynolds numbers Re? based on the free-stream velocity and the momentum thickness in the range Re?= 10^3 -10^12. At large Re?, the calculated skin-friction coefficient agrees well with the Coles-Fernholz relation.

Inoue, Michio; Pullin, D. I.

2010-11-01

309

Hypersonic turbulent wall boundary layer computations  

NASA Astrophysics Data System (ADS)

The Baldwin-Lomax algebraic turbulence model was modified for hypersonic flow conditions. Two coefficients in the outer layer eddy viscosity model were determined as functions of Mach number and temperature ratio. By matching the solutions from the Baldwin-Lomax model to those from the Cebeci-Smith model for a flat plate at hypersonic speed, the new values of the coefficient were obtained. The results show that the values of C sub cp and C sub kleb are functions of both Mach number and wall temperature ratio. The C sub cp and C sub kleb variations with Mach number and wall temperature were used for the calculations of both a 4 deg wedge flow at Mach 18 and an axisymmetric Mach 20 nozzle flow. The Navier-Stokes equations with thin layer approximation were solved for the above hypersonic flow conditions and the results were compared with existing experimental data. The agreement between the numerical solutions and the existing experimental data were good. The modified Baldwin-Lomax model thus is useful in the computations of hypersonic flows.

Kim, S. C.; Harloff, G. J.

1988-05-01

310

Drag reduction for external and internal boundary layers using riblets and polymers  

NASA Astrophysics Data System (ADS)

The efficiency of riblets and a drag-reducing polymer solution (a polyacrylamide slurry) in high-speed water tunnels for reducing drag in turbulent boundary layers was investigated in two experiments. One was an external flow experiment, in which riblets were applied to a flat plate in a high-speed water tunnel and the skin friction drag was calculated from velocity profile data. The second was an internal flow experiment, in which riblets were applied to the inside of a 6-in diameter pipe and the friction factor was calculated from mass flow rate and pressure drop measurements. Both experiments used adhesive-backed vinyl riblet film with 0.003-in height and spacing of the symmetric V-grooves. For the flat plate test, free stream velocity and Re data indicated a maximum drag reduction of about 8.1 percent. With riblets in the pipe, however, there was about three times as much friction reduction. When the polymer slurry was used in conjunction with riblets in the pipe flow, the total drag reduction was approximately equal to the sum of the drag reductions of the two techniques used separately, with some dependence on Reynolds number.

Reidy, Laurel W.; Anderson, Greg W.

1988-01-01

311

Correlations study in shock wave\\/turbulent boundary layer interaction  

Microsoft Academic Search

The shock waves\\/turbulent boundary layer interaction is a problem of critical importance that is frequently encountered in\\u000a designing flying vehicles. Presently, the most topical issue is the investigation of nonstationary phenomena (in particular,\\u000a low-frequency effects) involved in this interaction. We have experimentally studied separated flows in the zone of interaction\\u000a between an obliquely incident shock wave and a turbulent boundary

P. A. Polivanov; A. A. Sidorenko; A. A. Maslov

2010-01-01

312

Saltating Particles in a Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

Experiments on aeolian sand transport were carried out in a wind tunnel at the University of Aarhus in Denmark for a wide range of wind speeds. The saltating particles were analyzed using imaging techniques (PIV and PTV). Vertical profiles of particle concentration and velocity were extracted. The particle concentration was found to decrease exponentially with the height above the bed and the characteristic decay height was independent of the wind speed [1]. In contrast with the logarithmic profile of the wind speed, the particle velocity was found to vary linearly with the height. In addition, the particle slip velocity is finite and invariant with the wind speed. These results are shown to be closely related to the features of the splash function that characterizes the impact of the saltating particles onto a sand bed. A numerical simulation was developed that explicitly incorporates low velocity moments of the splash function in a calculation of the boundary conditions that apply at the bed [1]. The overall features of the experimental measurements are well reproduced by the simulation.

Valance, A.; El Moctar, A. Ould; Dupont, P.; Cantat, I.; Jenkins, J. T.

2010-05-01

313

Spatial Optimal Disturbances in Three-Dimensional Boundary Layers  

NASA Astrophysics Data System (ADS)

A parabolised set of equations is used to compute spatial optimal disturbances in Falkner-Skan-Cooke boundary layers. These disturbances associated with maximum energy growth initially take the form of vortices which are tilted against the direction of the mean crossflow shear. They evolve into bended streaks while traveling downstream and finally into crossflow disturbances when entering the supercritical domain of the boundary layer. Two physical mechanisms, namely the lift-up and the Orr-mechanism, can be identified as being responsible for nonmodal growth in three-dimensional boundary layers. A parametric study is presented where, amongst others, the influences of pressure gradient and sweep angle on optimal growth are investigated. It turns out that substantial disturbance growth is already found in regions of the flow where modal disturbances are damped.

Tempelmann, David; Hanifi, Ardeshir; Henningson, Dan S.

314

Lag model for turbulent boundary layers over rough bleed surfaces  

NASA Astrophysics Data System (ADS)

Boundary-layer mass removal (bleed) through spanwise bands of holes on a surface is used to prevent or control separation and to stabilize the normal shock in supersonic inlets. The addition of a transport equation lag relationship for eddy viscosity to the rough wall algebraic turbulence model of Cebeci and Chang was found to improve agreement between predicted and measured mean velocity distributions downstream of a bleed band. The model was demonstrated for a range of bleed configurations, bleed rates, and local freestream Mach numbers. In addition, the model was applied to the boundary-layer development over acoustic lining materials for the inlets and nozzles of commercial aircraft. The model was found to yield accurate results for integral boundary-layer properties unless there was a strong adverse pressure gradient.

Lee, J.; Sloan, M. L.; Paynter, G. C.

1994-07-01

315

Nonlocalized receptivity of boundary layers to three-dimensional disturbances  

NASA Astrophysics Data System (ADS)

The nonlocalized receptivity of the Blasius boundary layer over a wavy surface is analyzed using two different approaches. First, a mode-interaction theory is employed to unveil basic mechanisms and to explore the interplay between different components of the disturbance field. The second approach is derived from the parabolized stability equations. These nonlinear equations incorporate the effects of the stream-wise divergence of the boundary layer. The analysis provides results for three-dimensional disturbances and also considers nonparallel effects. Results for two-dimensional disturbances demonstrate that nonparallel effects are negligible and substantiates the mechanism described by the mode-interaction theory. Nonparallel effects become significant with increasing three-dimensionality. Receptivity amplitudes are shown to be large over a broad range of surface wave numbers. When operative, this mechanism is likely to dominate the boundary-layer receptivity.

Crouch, J. D.; Bertolotti, F. P.

1992-01-01

316

Measurement of boundary-layer receptivity at suction surfaces  

NASA Astrophysics Data System (ADS)

The receptivity of a boundary layer to external acoustic disturbances in the vicinity of a narrow suction slot is experimentally investigated. The relative importance of the leading-edge and suction-slot receptivity mechanisms is explored. A flat-plate, zero-pressure-gradient boundary layer with and without passive suction surfaces is irradiated by acoustic plane waves, and the coupling between the incident sound field and Tollmien-Schlichting waves is characterized. Suction slot locations and forcing frequencies are chosen so that the slot is positioned near the streamwise location corresponding to the lower branch of the neutral stability curve. Both amplified and damped modes are considered. The slot receptivity mechanism is validated and shown to produce a boundary-layer response at the slot of the same order as waves convected from the leading edge.

Wlezien, R. W.

1989-03-01

317

Effect of frictional heating on the surface-layer structure and tribological properties of titanium nickelide  

NASA Astrophysics Data System (ADS)

The effect of frictional heating (whose intensity was varied at the expense of changes in the sliding velocity from 0.35 to 9.00 m/s) on the rate of wear, friction coefficient, friction thermopower, structure, and microhardness of the Ti49.4Ni50.6 alloy in a microcrystalline (MC) state with grains 20-30 ?m in size and in a submicrocrystalline (SMC) state with grains 300 nm in size has been investigated. The tribological tests were conducted under the conditions of dry sliding friction in air using the finger-disk (made of steel Kh12M, hardness HRC = 63) scheme at a normal load of 98 N. Due to the frictional heating, the temperature in the surface layer 0.5 mm thick of the samples changed from 150-200 (at a sliding velocity of 0.35 m/s) to 1100°C (at a velocity of 9 m/s). The alloy structure has been studied with the help of metallographic and electronmicroscopic (scanning and transmission microscopy) methods. It has been shown that the rate of wear of the titanium nickelide in the MC and SMC structural states is more than an order of magnitude lower than in the 12Kh18N9 steel and several times less than in the 40Kh13 steel. The fracture of the friction surface of the titanium nickelide occurs predominantly by the fatigue or oxidation-fatigue mechanisms, which are characterized by a relatively low wear rate, whereas the 40Kh13 and 12Kh18N9 steels show a tendency to intense thermal adhesive wear (seizure) at velocities higher than 0.35 m/s. It has been shown by the electron-microscopic investigation that nanocrystalline structures consisting of crystals of the B2 phase, oxides of the TiO2 type, and some amount of martensite B19' are formed in the process of friction in the surface layer of the titanium nickelide. It has been concluded that an enhanced wear resistance of the titanium nickelide is caused by the high heat resistance (strength) and high fracture toughness of the nanocrystalline B2 phase and by the presence of high-strength thermostable oxides of the TiO2 type formed upon friction.

Korshunov, L. G.; Pushin, V. G.; Chernenko, N. L.

2011-09-01

318

Field observations of the wave bottom boundary layer  

NASA Astrophysics Data System (ADS)

This paper presents a comprehensive set of velocity and suspended sediment observations in the nearshore wave bottom boundary layer, collected during the Duck94 field experiment on the Outer Banks of the North Carolina coast. Cross-shore velocity measurements in the wave bottom boundary layer were made using five hot film anemometers, nominally spaced from 1 to 5 cm above the bed in 2 m of water depth. The time-varying location of the seabed was estimated to roughly 1 cm with a stacked set of bed-penetrating fiber-optic backscatter sensors. The instrument array was intermittently located in the surf zone on the crest of a bar. The location of the bottom varied several centimeters over a 34 min data run. Even over 4 min segments of quasi-steady statistics, occasional large waves caused short erosion and redeposition events, complicating the definition of bottom location and causing the root-mean-square velocity statistics to be nonzero below the mean bed location. This leads to obvious difficulties in comparisons with two, one-dimensional time-dependent, eddy viscosity wave bottom boundary layer models. For example, bed shears based on rms amplitude decay were lower than predicted. The observations show some evidence for a velocity overshoot region within the wave bottom boundary layer. The observations were compared with two linear eddy viscosity models. Larger estimates of a constant eddy viscosity and smaller than predicted phase leads are indicative of more rapid mixing of momentum than predicted by the models. The phase and amplitude frequency response estimated with frequency domain empirical orthogonal functions shows a nonlinear response of the wave bottom boundary layer over the incident band. These observations are among the first coherent looks at the wave bottom boundary layer under conditions of significant sediment response. They highlight the added complexity of the dynamics in natural environments.

Foster, D. L.; Beach, R. A.; Holman, R. A.

2000-08-01

319

Boundary-layer dynamics of a developing vortex  

NASA Astrophysics Data System (ADS)

The axisymmetric vortices in solid and nonsolid rotation are analyzed to investigate the dynamics of the boundary layer. The governing expressions are similarity transformed to nonlinear parabolic partial differential equations solved by a Galerkin method for simplified initial conditions; these conditions allow non-zero tangential and vertical velocities at the lower and upper boundaries. A time-dependent Taylor boundary condition at the lower extremity of the vortex induces gradual fluid spin-down, so that the non-slip steady-state solutions are progressively approached.

Raymond, W. H.; Rao, G. V.

1981-02-01

320

Velocity measurements in a turbulent natural convection boundary layer  

NASA Astrophysics Data System (ADS)

The initial results from an investigation of the velocity and temperature profiles for a turbulent natural convective boundary layer using laser Doppler anemometry (LDA) are reported. A DISA type seeding generator spewed corn oil droplets over a flat plate immersed in a flow in order to obtain velocity fluctuations, and thermocouples provided data for the temperature profiles. The plate was maintained at a surface temperature of 80 C. Sufficient measurements were made to characterize the laminar, transition, and the turbulent regions. The measurements taken in the boundary layer satisfied an integral energy balance, with residence time weighting of the values yielding corrections that amounted to only 2-3 percent.

Cheesewright, R.; Ierokiopitis, E.

321

Extended self-similarity in boundary layer turbulence  

NASA Astrophysics Data System (ADS)

It is shown that a lack of isotropy narrows the range of spatial scales where turbulent flows exhibit extended self-similarity (ESS), namely, self-scaling of velocity structure functions. This effect holds irrespectively of the order of the structure functions and explains why early experiments on turbulent boundary layers failed to observe ESS. The shrinking of the ESS range of scales is well captured by the approximate analytical scaling functions developed by Sreenivasan and co-workers [Phys. Rev. E 48, R33 (1993); 48, 5 (1993); 48, R3217 (1993)] to fit atmospheric boundary layer data.

Amati, G.; Benzi, R.; Succi, S.

1997-06-01

322

Grain-boundary layering transitions in a model bicrystal  

NASA Astrophysics Data System (ADS)

We investigate numerically a series of layering transitions associated with grain-boundary segregation in a lattice-gas model of a binary alloy. By examining the dependence of the excess composition on temperature and relative chemical potential, we find a series of first-order layering transitions that depend on the nature of the grain boundary. Diagrams are constructed to illustrate the richness of the phase-like behavior (complexion transitions) and its dependence on grain misorientation and stress. Finally, the connection between the thermodynamics of this prototypical model and recent observations of complexion transitions associated with interfaces in metals and ceramics is explored.

Rickman, J. M.; Chan, H. M.; Harmer, M. P.; Luo, J.

2013-12-01

323

Bias-controlled friction of InAs nanowires on a silicon nitride layer studied by atomic force microscopy  

Microsoft Academic Search

By studying how nanowires lying on a surface bend when pushed by an atomic force microscopy tip we are able to measure the friction between them and the substrate. Here, we show how the friction between InAs nanowires and an insulating silicon nitride layer varies when a dc voltage is applied to the tip during manipulation. The bias charges the

G. Conache; A. Ribayrol; L. E. Fröberg; M. T. Borgström; L. Samuelson; L. Montelius; H. Pettersson; S. M. Gray

2010-01-01

324

Fabrication of Nano-Composite Surface Layers on Aluminium Employing Friction Stir Processing Technique  

SciTech Connect

Al/Al{sub 2}O{sub 3} nano-composite surface layer was fabricated via friction stir processing technique. Commercial AA6082 aluminium alloy extruded bar and nanometric Al{sub 2}O{sub 3} powder were subjected to friction stir processing at a substrate travel speed of 80 mm/min and a tool rotation speed of 1000 rpm using a hardened H-13 tool steel. The grain structure and reinforcement particles were investigated by using optical and scanning electron microscopy. Results show that Al{sub 2}O{sub 3} particles can be more uniformly dispread in aluminium substrate by increasing the number of processing passes. Also, hardness enhancement of the nano-composite surface layer was found. This is attributed to uniform dispersion of Al{sub 2}O{sub 3} particles.

Bozorg, S. F. K.; Zarghani, A. S.; Zarei-Hanzaki, A. [School of Metallurgy and Materials Engineering, Faculty of Engineering, University of Tehran, Tehran, P.O. Box: 14395-553 (Iran, Islamic Republic of)

2010-03-11

325

Laminar boundary layer flow of saturated vapor and its condensate over a horizontal tube  

NASA Astrophysics Data System (ADS)

The steady two-dimensional laminar flow of a stream of saturated vapor flowing over a tube that is kept at a uniform temperature, below the saturation temperature, is examined. Owing to the temperature difference between the vapor stream and the solid surface a film of condensate is generated that flows along the surface due to shear, pressure-drop, and gravity. In the limit as the boundary layer and film thickness remain smaller than the radius of curvature of the surface a simplified lubrication-type formulation describes the temperature and flow fields in the film, whereas the usual boundary layer formulation is applied in the vapor boundary layer. The case of flow past a horizontal cylinder of radius R is investigated numerically with the oncoming stream aligned with gravity. The parameters that control momentum and heat transfer in this problem are the viscosity ratio, ?cs/?ss, the density ratio, ?cs/?ss, the Prandtl number, Pr=cpcs?cs/kcs, the Froude number, Fr=U?2/(g?R), and finally the thickness ratio between the condensate and the vapor boundary layer, ?, which is also a measure of the temperature difference between the vapor stream and the tube wall. Then, the Nusselt number and the skin friction coefficient, averaged over the upper half of the cylinder, are calculated for a wide parameter range. When Fr is very small and V relatively large the flow remains attached until the trailing stagnation point of the cylinder. As the effect of adverse pressure drop becomes more pronounced (Fr increases or V decreases) it is shown that the solution exhibits two different types of singularity in the rear part of the cylinder. The first one is a typical Goldstein singularity because it appears at the tube wall and it is associated with vanishing skin friction (wall shear) and rapidly increasing film thickness. The second one takes place near the interface between the vapor stream and the film of condensate in a region where very small velocities prevail in conjunction with vanishing shear rate. The latter has not been reported so far and it is expected to affect the flow locally, as opposed to the Goldstein singularity which is known to lead to massive separation in the case of a cylindrical surface. Upon proper rescaling of Fr and V, Fr'=Fr ?ss/?cs, V'=V6(?ss/?cs)7/2(?cs/?ss)7/2, a critical curve is produced in the (Fr',V') plane that marks the boundary curve separating the two types of singular behavior for most of the numerical data obtained herein for the steam-water system, irrespective of the saturation temperature.

Smyrnaios, D. N.; Pelekasis, N. A.; Tsamopoulos, J. A.

2002-06-01

326

Surface heating due to turbulent boundary-layer flows  

SciTech Connect

Numerical analysis is made of surface-heating history when a surface is exposed to transient thermal fluxes from a turbulent compressible boundary layer. The conservation equations are solved by means of a factored ADI method. Results display high heat fluxes at the surface, causing sufficient rise in surface temperatures to quickly reach melting in some substances. The melting liquid-layer case is also briefly discussed.

Kang, S.W.; Levatin, J.L.

1981-01-20

327

Disturbances to Air-Layer Skin-Friction Drag Reduction at High Reynolds Numbers  

NASA Astrophysics Data System (ADS)

Skin friction drag on a flat surface may be reduced by more than 80% when a layer of air separates the surface from a flowing liquid compared to when such an air layer is absent. Past large-scale experiments utilizing the US Navy's Large Cavitation Channel and a flat-plate test model 3 m wide and 12.9 m long have demonstrated air layer drag reduction (ALDR) on both smooth and rough surfaces at water flow speeds sufficient to reach downstream-distance-based Reynolds numbers exceeding 100 million. For these experiments, the incoming flow conditions, surface orientation, air injection geometry, and buoyancy forces all favored air layer formation. The results presented here extend this prior work to include the effects that vortex generators and free stream flow unsteadiness have on ALDR to assess its robustness for application to ocean-going ships. Measurements include skin friction, static pressure, airflow rate, video of the flow field downstream of the injector, and profiles of the flowing air-water mixture when the injected air forms bubbles, when it is in transition to an air layer, and when the air layer is fully formed. From these, and the prior measurements, ALDR's viability for full-scale applications is assessed.

Dowling, David; Elbing, Brian; Makiharju, Simo; Wiggins, Andrew; Perlin, Marc; Ceccio, Steven

2009-11-01

328

Multiple pass and multiple layer friction stir welding and material enhancement processes  

DOEpatents

Processes for friction stir welding, typically for comparatively thick plate materials using multiple passes and multiple layers of a friction stir welding tool. In some embodiments a first portion of a fabrication preform and a second portion of the fabrication preform are placed adjacent to each other to form a joint, and there may be a groove adjacent the joint. The joint is welded and then, where a groove exists, a filler may be disposed in the groove, and the seams between the filler and the first and second portions of the fabrication preform may be friction stir welded. In some embodiments two portions of a fabrication preform are abutted to form a joint, where the joint may, for example, be a lap joint, a bevel joint or a butt joint. In some embodiments a plurality of passes of a friction stir welding tool may be used, with some passes welding from one side of a fabrication preform and other passes welding from the other side of the fabrication preform.

Feng, Zhili (Knoxville, TN); David, Stan A. (Knoxville, TN); Frederick, David Alan (Harriman, TN)

2010-07-27

329

The collapse of turbulence in the atmospheric boundary layer  

NASA Astrophysics Data System (ADS)

A well-known phenomenon in the atmospheric boundary layer is the fact that winds may become very weak in the evening after a clear sunny day. In these quiet conditions usually hardly any turbulence is present. Consequently this type of boundary layer is referred to as the quasi-laminar boundary layer. In spite of its relevance, the appearance of laminar boundary layers is poorly understood and forms a long standing problem in meteorological research. Here we investigate an analogue problem in the form of a stably stratified channel flow. The flow is studied with a simplified atmospheric model as well as with Direct Numerical Simulations. Both models show remarkably similar behaviour with respect to the mean variables such as temperature and wind speed. The similarity between both models opens new way for understanding and predicting the laminarization process. Mathematical analysis on the simplified model shows that relaminarization can be understood from the existence of a definite limit in the maximum sustainable heat flux under stably stratified conditions. This fascinating aspect will be elaborated in future work.

van de Wiel, B. J. H.; Moene, A. F.; Jonker, H. J. J.; Clercx, H. J. H.

2011-12-01

330

Hair receptor sensitivity to changes in laminar boundary layer shape.  

PubMed

Biologists have shown that bat wings contain distributed arrays of flow-sensitive hair receptors. The hair receptors are hypothesized to feedback information on airflows over the bat wing for enhanced stability or maneuverability during flight. Here, we study the geometric specialization of hair-like structures for the detection of changes in boundary layer velocity profiles (shapes). A quasi-steady model that relates the flow velocity profile incident on the longitudinal axis of a hair to the resultant moment and shear force at the hair base is developed. The hair length relative to the boundary layer momentum thickness that maximizes the resultant moment and shear-force sensitivity to changes in boundary layer shape is determined. The sensitivity of the resultant moment and shear force is shown to be highly dependent on hair length. Hairs that linearly taper to a point are shown to provide greater output sensitivity than hairs of uniform cross-section. On an order of magnitude basis, the computed optimal hair lengths are in agreement with the range of hair receptor lengths measured on individual bat species. These results support the hypothesis that bats use hair receptors for detecting changes in boundary layer shape and provide geometric guidelines for artificial hair sensor design and application. PMID:20157224

Dickinson, B T

2010-02-16

331

On the origin of the high-latitude boundary layer  

Microsoft Academic Search

This paper presents a statistical study of the high-latitude boundary layer (HLBL) performed on 53 Interball-1 magnetopause crossings. In the study we verify if antiparallel merging is the main source of HLBL formation when the IMF is nearly horizontal. To provide such a study we designed a new coordinate system which allowed us to analyze HLBL under varied interplanetary conditions.

A. Fedorov; E. Budnik; J.-A. Sauvaud

2002-01-01

332

CFD simulation of the atmospheric boundary layer: wall function problems  

Microsoft Academic Search

Accurate Computational Fluid Dynamics (CFD) simulations of atmospheric boundary layer (ABL) flow are essential for a wide variety of atmospheric studies including pollutant dispersion and deposition. The accuracy of such simulations can be seriously compromised when wall-function roughness modifications based on experimental data for sand-grain roughened pipes and channels are applied at the bottom of the computational domain. This type

Bert Blocken; Ted Stathopoulos; Jan Carmeliet

333

Aircraft measurements within the planetary boundary layer over water  

Microsoft Academic Search

The basic overall objective of the program has been to develop a relatively inexpensive airborne sensing system for study of the marine boundary layer in support of the NAVAIR Marine Fog Investigation. This extends into the third dimension measurement of most of the significant parameters which have been studied from ships and land stations. The operational flexibility of the aircraft

R. Markson

1977-01-01

334

A Boundary-Layer Parametrisation for Martian General Circulation Model  

Microsoft Academic Search

The representation of the Planetary Boundary-Layer (PBL) is essential for use of General Circulation Models (GCM) results for planetary exploration missions design (prediction of near surface profiles of wind and temperature means, as well as turbulent fluctuations). However, the representation of the PBL was considered of secondary importances in early developments of General Circulation Models (GCM). Now, long duration simulations

R. Fournier; E. Deleersnijder; F. Hourdin; A. Lahellec; F. Forget; O. Talagrand

1996-01-01

335

A Qualitative Description of Boundary Layer Wind Speed Records  

Microsoft Academic Search

The complexity of the atmosphere endows it with the property of turbulence by virtue of which, wind speed variations in the atmospheric boundary layer (ABL) exhibit highly irregular fluctuations that persist over a wide range of temporal and spatial scales. Despite the large and significant body of work on microscale turbulence, understanding the statistics of atmospheric wind speed variations has

Radhakrishnan Nagarajan

2006-01-01

336

Iodine monoxide in the Western Pacific marine boundary layer  

NASA Astrophysics Data System (ADS)

A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved, if an additional sea-air flux of inorganic iodine (e.g. I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.

Großmann, K.; Frieß, U.; Peters, E.; Wittrock, F.; Lampel, J.; Yilmaz, S.; Tschritter, J.; Sommariva, R.; von Glasow, R.; Quack, B.; Krüger, K.; Pfeilsticker, K.; Platt, U.

2012-10-01

337

Iodine monoxide in the Western Pacific marine boundary layer  

NASA Astrophysics Data System (ADS)

A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved if an additional sea-air flux of inorganic iodine (e.g., I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.

Großmann, K.; Frieß, U.; Peters, E.; Wittrock, F.; Lampel, J.; Yilmaz, S.; Tschritter, J.; Sommariva, R.; von Glasow, R.; Quack, B.; Krüger, K.; Pfeilsticker, K.; Platt, U.

2013-03-01

338

ATMOSPHERIC DISPERSION IN THE ARCTIC: WINTERTIME BOUNDARY-LAYER MEASUREMENTS  

EPA Science Inventory

The wintertime arctic atmospheric boundary layer was investigated with micro-meteorological and SF6 tracer measurements collected in Prudhoe Bay, AK. he flat, snow-covered tundra surface at this site generates a very small (0.03 cm) surface roughness. he relatively warm maritime ...

339

Sources of iodine in the tropical marine boundary layer  

NASA Astrophysics Data System (ADS)

The importance of halogens in the chemical processes of the marine boundary layer has long been recognized, although many uncertainties remain. The role of iodine is particularly uncertain, especially with regard to its atmospheric sources. In this work, iodine chemistry in the tropical marine boundary layer was studied using a one dimensional model constrained to measured fluxes of iodinated hydrocarbons. The concentrations of iodocarbons were simultaneously measured in marine air and water during the RHaMBLe cruise, which took place in May-June 2007 around the Cape Verde archipelago. The sea-air fluxes of selected iodocarbons calculated from these measurements were used in a 1-D model to calculate the atmospheric concentrations of inorganic iodine species in the marine boundary layer at the latitude of Cape Verde (17 degrees N). The model was a revised version of MISTRA, with an updated module of gas-phase inorganic and DMS chemistry, and included the new iodine aqueous-phase chemical mechanism developed by Pechtl et al. (2007). The model results were qualitatively compared with the observations of IO taken at the Cape Verde Atmospheric Observatory during a previous campaign. The model was then used to determine the role of the measured iodocarbons as sources of iodine in the marine boundary layer and to study their impact on the concentration of tropospheric ozone.

Sommariva, R.; von Glasow, R.; Jones, C. E.; Hornsby, K. E.; Carpenter, L. J.; McFiggans, G.

2009-04-01

340

The core-mantle boundary layer and deep Earth dynamics  

Microsoft Academic Search

Recent seismological work has revealed new structures in the boundary layer between the Earth's core and mantle that are altering and expanding perspectives of the role this region plays in both core and mantle dynamics. Clear challenges for future research in seismological, experimental, theoretical and computational geophysics have emerged, holding the key to understanding both this dynamic system and geological

Thorne Lay; Quentin Williams; Edward J. Garnero

1998-01-01

341

Acoustic explorations of the upper ocean boundary layer  

Microsoft Academic Search

The upper ocean boundary layer is an important but difficult to probe part of the ocean. A better understanding of small scale processes at the air-sea interface, including the vertical transfer of gases, heat, mass and momentum, are crucial to improving our understanding of the coupling between atmosphere and ocean. Also, this part of the ocean contains a significant part

Svein Vagle

2005-01-01

342

On the Stability of 3D Boundary Layers  

Microsoft Academic Search

We have extended the parabolized stability equations (PSE) to fully 3D boundary layers that vary with all spatial variables. The extended PSE permit the calculation of streamwise and spanwise growth rates of disturbances based on first principles. The method provides improved N factors for the design of turbine vanes and blades, realistic wings with taper, twist, root, tip, and engines,

Th. Herbert

1996-01-01

343

Thickness and concentration profile of the boundary layer in electrodialysis  

Microsoft Academic Search

Back electrical motive force (emf) measurements with spiral electrodialysis (SpED) modules showed that obtaining the profile of the back emf transient curves during depolarization is difficult from the Nernst model, and the assumption of a linear concentration profile in a stirred polarized boundary layer is oversimplified. A non-linear concentration distribution model derived from the error function is introduced.

M. Law; T. Wen; G. S. Solt

1997-01-01

344

Torpedo Drag Reduction Using Magnetohydrodynamic Boundary-Layer Control.  

National Technical Information Service (NTIS)

Study for the drag reduction effect by use of MHD boundary-layer control had been carried out by the present investigators, with the assumption of non-zero electric field. The momentum integral method was used to obtain an optimum magnetic field distribut...

Y. K. Wu G. F. Anderson

1972-01-01

345

ANALYTICAL PARAMETERIZATIONS OF DIFFUSION: THE CONVECTIVE BOUNDARY LAYER  

EPA Science Inventory

A brief review is made of data bases which have been used for developing diffusion parameterizations for the convective boundary layer (CBL). A variety of parameterizations for lateral and vertical dispersion, (sigma sub) and (sigma sub z), are surveyed; some of these include mec...

346

Stable Boundary-Layer Scaling Regimes: The Sheba Data  

Microsoft Academic Search

Turbulent and mean meteorological data collected at five levels on a 20-m tower over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are analyzed to examine different regimes of the stable boundary layer (SBL). Eleven months of measurements during SHEBA cover a wide range of stability conditions, from the weakly unstable regime to

Andrey A. Grachev; Christopher W. Fairall; P. Ola G. Persson; Edgar L. Andreas; Peter S. Guest

2005-01-01

347

Propagation of propeller tone noise through a fuselage boundary layer  

NASA Astrophysics Data System (ADS)

In earlier experimental and analytical studies, it was found that the boundary layer on an aircraft could provide significant shielding from propeller noise at typical transport airplane cruise Mach numbers. In this paper a new three-dimensional theory is described that treats the combined effects of refraction and scattering by the fuselage and boundary layer. The complete wave field is solved by matching analytical expressions for the incident and scattered waves in the outer flow to a numerical solution in the boundary layer flow. The model for the incident waves is a near-field frequency-domain propeller source theory developed previously for free field studies. Calculations for an advanced turboprop (Prop-Fan) model flight test at 0.8 Mach number show a much smaller than expected pressure amplification at the noise directivity peak, strong boundary layer shielding in the forward quadrant, and shadowing around the fuselage. Results are presented showing the difference between fuselage surface and free-space noise predictions as a function of frequency and Mach number. Comparison of calculated and measured effects obtained in a Prop-Fan model flight test show good agreement, particularly near and aft of the plane of rotation at high cruise Mach number.

Hanson, D. B.; Magliozzi, B.

1984-01-01

348

Basic study on blockage effects in turbulent boundary layer flows  

Microsoft Academic Search

The blockage effects on aerostatic forces such as the drag coefficient, CD, and the base pressure coefficient, Cpb, were experimentally examined by using two-dimensional rectangular cylinders, Blh = 1.0, placed on the floor normal to the flow direction in turbulent boundary layer flows (TBLF). At the same time, the effects of the wind characteristics of the TBLF on the aerostatic

M. Noda; H. Utsunomiya; F. Nagao

1995-01-01

349

On boundary-layer transition in transonic flow  

Microsoft Academic Search

Boundary-layer transition in transonic external flow is addressed theoretically. The transonic area is rich in different flow structures, and transition paths, and the work has wide potential application in transonic aerodynamics, including special reference to the example of flow transition over an engine nacelle. The investigation is intended partly to aid, compare with, and detect any limitations of, a quasi-parallel

R. I. Bowles; F. T. Smith

1993-01-01

350

Irregularity excitation associated with charged dust cloud boundary layers  

NASA Astrophysics Data System (ADS)

Irregularity generation associated with dust cloud expansion through a background plasma along a magnetic field is investigated. Because of the dust charging process, a boundary layer is produced, separating the dusty plasma generated and the background plasma. It is observed that under appropriate conditions, localized plasma irregularities may be generated in this boundary layer. Theoretical and computational models are used to study the evolution of relevant plasma instabilities thought to play a dominant role in irregularity production. An electron flow develops along the boundary layer of the dust cloud, and plasma irregularities are generated in response to this flow. Several aspects of the cloud's structure (thickness of the boundary layer, average particle size and density, collisional processes, and cloud expansion speed) and the ambient plasma are varied to determine the effect of these quantities on the resulting irregularities. The relevance of these results to past experimental observations in space and the laboratory for applications to the expansion of naturally or artificially created dust clouds is discussed.

Mahmoudian, A.; Scales, W. A.

2012-02-01

351

The role of acoustic feedback in boundary-layer instability  

NASA Astrophysics Data System (ADS)

In this paper, the classical triple-deck formalism is employed to investigate two instability problems in which acoustic feedback loop plays an essential role. The first concerns a boundary layer over a flat plate, on which two well separated roughness elements are present. A spatially amplifying Tollmien-Schlichting (T-S) wave between the roughness elements is scattered by the downstream roughness to emit a sound wave, which propagates upstream and impinges on the upstream roughness to regenerate the T-S wave thereby forming a closed feedback loop in the streamwise direction. Numerical calculations suggest that at high Reynolds numbers and for moderate roughness heights the long-range acoustic coupling may lead to global instability, which is characterized by self-sustained oscillations at discrete frequencies. The dominant peak frequency may jump from one value to another as the Reynolds number, or the distance between the roughness elements, is varied gradually. The second problem concerns supersonic 'twin boundary layers', which develop along the two well-separated parallel flat plates. The two boundary layers are in mutual interaction through the impinging and reflected acoustic waves. It is found that the interaction leads to a new instability that is absent in the usual unconfined boundary layer.

Wu, Xuesong

2013-10-01

352

Beta Limitation of Matter-Antimatter Boundary Layers.  

National Technical Information Service (NTIS)

A model for a boundary layer which separates a cloud of matter from one of antimatter in a magnetized ambiplasma, in which steady pressure equilibrium ceases to exist when a certain beta limit is exceeded, is discussed. The latter is defined as the ratio ...

B. Lehnert

1987-01-01

353

Beta limitation of matter-antimatter boundary layers  

Microsoft Academic Search

A model has earlier been proposed for a boundary layer which separates a cloud of matter from one of antimatter in a magnetized ambiplasma. In this model steady pressure equilibrium ceases to exist when a certain beta limit is exceeded. The latter is defined as the ratio between the ambiplasma and magnetic field pressures which balance each other in the

B. Lehnert

1988-01-01

354

Vertical velocity structure of nonprecipitating continental boundary layer stratocumulus clouds  

Microsoft Academic Search

Continental boundary layer (BL) stratocumulus clouds affect the local weather by modulating the surface energy and moisture budgets and are also intimately tied to the diurnal cycle of the turbulence in the BL. Vertical velocity structure of these clouds is studied using data from the Atmospheric Radiation Measurement Program's Southern Great Plains observing facility located near Lamont, Oklahoma. Data from

Virendra P. Ghate; Bruce A. Albrecht; Pavlos Kollias

2010-01-01

355

Nighttime Chemistry in the Polluted Boundary Layer (Invited)  

Microsoft Academic Search

Chemistry in the urban nocturnal boundary layer (NBL) has received surprisingly little attention in the past. Surface observations often see low ozone and high NO levels, which lead to low nocturnal radical levels and consequently slow chemistry near the ground. Above the surface, however, ozone and radical levels, for example of NO3, are considerably higher, and more efficient chemical pathways

J. Stutz; K. Wong; C. Tsai; O. Pikelnaya

2009-01-01

356

Shock Wave / Boundary Layer Interaction Experiment on Control Surface.  

National Technical Information Service (NTIS)

The shock wave / boundary layer interaction (SWBLI) experiment is part of the EXPERT mission. SWBLI is studied on two identical fixed compression ramps made of C/SiC, which are models for control surfaces. The flow separates on the flat surfaces upstream ...

L. Prochazka S. Schlamp T. Rosgen

2007-01-01

357

COMPRESSIBLE TURBULENT BOUNDARY-LAYER FLOW CONTROL OVER A WEDGE  

Microsoft Academic Search

The effects of blowing and suction on the steady compressible boundary-layer flow with adverse pressure gradient and heat transfer over a wedge are numerically examined. The fluid is considered to be a compressible, viscous and Newtonian ideal gas (air) and it is subjected to a constant velocity of suction\\/injection applied globally to the wedge or locally to specific slots on

M. Xenos; E. Tzirtzilakis; N. Kafoussias

358

The effects of cylindrical surface modifications on turbulent boundary layers  

Microsoft Academic Search

A study employing a combination of hydrogen bubble-wire flow visualization and hot-film anemometry measurements has been conducted to determine the effects sublayer scale streamwise surface modifications on the structure and flow characteristics of turbulent boundary layers. The surface modifications were created using very fine monofilament fishing line of an approximate non-dimensional height of h + = 4. Spanwise line spacings

J. B. Johansen; C. R. Smith

1983-01-01

359

Power laws for rough wall turbulent boundary layers  

Microsoft Academic Search

An assessment of the ability of power laws to describe the mean velocity profile in the overlap region of a zero pressure gradient turbulent boundary layer is reported. The experiments were performed in a wind tunnel on smooth and four different types of rough surfaces at moderate Reynolds numbers. A novel modification to the power law velocity profile is proposed

N. A. Kotey; D. J. Bergstrom; M. F. Tachie

2003-01-01

360

Theoretical investigation of turbulent boundary layer over a wavy surface  

Microsoft Academic Search

The important features of the two-dimensional incompressible turbulent flow over a wavy surface of wavelength comparable with the boundary layer thickness are analyzed. A turbulent field method using a model equation for turbulent shear stress was employed with suitable modification to cover the viscous sublayer. The governing differential equations are linearized based on the small but finite amplitude to wavelength

L. C. Chang

1975-01-01

361

Atmospheric boundary layer modification in the marginal ice zone  

Microsoft Academic Search

A case study of the Andreas et al. (1984) data on atmospheric boundary layer modification in the marginal ice zone is made. Our model is a two-dimensional, multilevel, linear model with turbulence, lateral and vertical advection, and radiation. Good agreement between observed and modeled temperature cross sections is obtained. In contrast to the hypothesis of Andreas et al., we find

Theodore J. Bennett; Kenneth Hunkins

1986-01-01

362

Why Rolls are Prevalent in the Hurricane Boundary Layer  

Microsoft Academic Search

Recent remote sensing observations show that the hurricane boundary layer flow, although energetic, is not a region of homogeneous turbulence. In fact, the observations convincingly demonstrate that a large fraction of the turbulent flow in the regions away from the deep convective rainbands is highly organized into intense horizontal roll vortices that are approximately aligned with the mean wind and

Ralph C. Foster

2005-01-01

363

Numerical Solution of the Turbulent-Boundary-Layer Equations.  

National Technical Information Service (NTIS)

The report presents a numerical solution of turbulent boundary-layer equations for both compressible and incompressible flows. An eddy viscosity concept is used to eliminate the Reynolds shear-stress term, and an eddy-conductivity concept is used to elimi...

A. M. O. Smith T. Cebeci

1967-01-01

364

Unsteady boundary layers with an intelligent numerical scheme  

Microsoft Academic Search

A numerical method has been developed to represent unsteady boundary layers with large flow reversal. It makes use of the characteristic box scheme which examines the finite-difference grid in relation to the magnitude and direction of local velocity and reaches and implements a decision to ensure that the Courant, Friedricks and Lewey stability criterion is not violated. The method has

Tuncer Cebeci

1986-01-01

365

A boundary-layer collision in a curved duct  

Microsoft Academic Search

It is shown that the boundary layer, generated at the entrance to a curved duct by an incoming flow at large values of the Dean number, develops a collision structure in the neighborhood of the inner wall, after evolving for a finite distance down the duct. This result is not to be anticipated from the results of previous studies of

KEITH STEWARTSON; TUNCER CEBECI; K. C. Chang

1980-01-01

366

Calculation of incompressible rough-wall boundary-layer flows  

Microsoft Academic Search

The algebraic eddy viscosity model of Cebeci and Smith has been modified to account for wall roughness by incorporating a suggestion of Rotta. The boundary-layer equations are solved, with this model, by the accurate and efficient Keller Box scheme for a wide variety of experimental configurations. These include adverse, zero, and favorable pressure gradients, and roughness elements that approach the

T. Cebeci; K. C. Chang

1978-01-01

367

Drag reduction by microbubbles in a turbulent boundary layer  

Microsoft Academic Search

An experimental characterization of the turbulent boundary layer over a flat plate in the presence of small amounts of microbubbles is performed. The average diameter of the injected bubbles is comparable with the local Kolmogorov lengthscale, and the bulk void fraction C is approximately 0.1%. The velocity field of the liquid phase, as well as the bubble characteristics, is acquired

Boris Jacob; Angelo Olivieri; Massimo Miozzi; Emilio F. Campana; Renzo Piva

2010-01-01

368

Polymer drag reduction of a zero pressure gradient boundary layer  

Microsoft Academic Search

The objective was to determine the response of a zero pressure gradient boundary layer to slot injection of drag reducing polymer solution. Attention was focused on the region far downstream of the injector. Two-component velocity data were acquired with a laser Doppler velocimeter. Mean concentration data were measured using a laser induced fluorescence technique. A 1000 ppm solution of Separan

John E. Koskie; William G. Tiederman

1991-01-01

369

Boundary layer effects above a Himalayan valley near Mount Everest  

Microsoft Academic Search

Periodical Wind Profiler and Radio Acoustic Sounding System observations have been commenced at the Himalayas' northern slope nearby Mount Everest in September 2005. Primarily data sets obtained 25 km remote from the glacier edge are utilized for a preliminary discussion of planetary boundary layer circulation resembling high alpine mountainous regions. Substantial findings include the detection of two wind shears and

Fanglin Sun; Yaoming Ma; Maoshan Li; Weiqiang Ma; Hui Tian; Stefan Metzger

2007-01-01

370

Turbulent dispersion in the Atmospheric Convective Boundary Layer  

Microsoft Academic Search

The dispersion of a plume in the Atmospheric Boundary Layer is a very complex phenomenon that includes the transport, the mixing and the chemical transformations of the plume material. When a plume is dispersed in the ABL, its shape, evolution, and internal structure are determined by the interaction between the plume and the turbulent eddies that characterize the atmospheric motion.

A. Dosio

2005-01-01

371

The bottom boundary layer of the deep ocean  

Microsoft Academic Search

Some aspects of the bottom boundary layer of the deep ocean are exhibited in profiles of salinity and temperature made with a Woods Hole Oceanographic Institution\\/Brown CTD microprofiler. Profiles from the center of the Hatteras Abyssal Plain have a signature that is characteristic of mixing up a uniformly stratified region. Over rough or sloping topography, to the east and west

Laurence Armi; Robert C. Millard

1976-01-01

372

Retinal layer segmentation of macular OCT images using boundary classification  

PubMed Central

Optical coherence tomography (OCT) has proven to be an essential imaging modality for ophthalmology and is proving to be very important in neurology. OCT enables high resolution imaging of the retina, both at the optic nerve head and the macula. Macular retinal layer thicknesses provide useful diagnostic information and have been shown to correlate well with measures of disease severity in several diseases. Since manual segmentation of these layers is time consuming and prone to bias, automatic segmentation methods are critical for full utilization of this technology. In this work, we build a random forest classifier to segment eight retinal layers in macular cube images acquired by OCT. The random forest classifier learns the boundary pixels between layers, producing an accurate probability map for each boundary, which is then processed to finalize the boundaries. Using this algorithm, we can accurately segment the entire retina contained in the macular cube to an accuracy of at least 4.3 microns for any of the nine boundaries. Experiments were carried out on both healthy and multiple sclerosis subjects, with no difference in the accuracy of our algorithm found between the groups.

Lang, Andrew; Carass, Aaron; Hauser, Matthew; Sotirchos, Elias S.; Calabresi, Peter A.; Ying, Howard S.; Prince, Jerry L.

2013-01-01

373

Simultaneous profiling of the Arctic Atmospheric Boundary Layer  

Microsoft Academic Search

The structure of the Arctic atmospheric boundary layer (AABL) and the heat and moisture fluxes between relatively warm water and cold air above non-sea-ice-covered water (such as fjords, leads and polynyas) are of great importance for the sensitive Arctic climate system. So far, such processes are not sufficiently resolved in numerical weather prediction (NWP) and climate models. Especially for regions

S. Mayer; M. Jonassen; J. Reuder

2009-01-01

374

Impulse response of laminar boundary layer and receptivity  

NASA Astrophysics Data System (ADS)

Impulse response of a laminar boundary layer of a flat plate to periodic surface suction and blowing is analyzed through a numerical computation based on parallel flow approximation. Bilateral Laplace transform is used to eliminate the problem of individually identifying the various eigen modes and contributions of other singularities.

Sengupta, Tapan K.

375

On the growth of turbulent regions in laminar boundary layers  

NASA Astrophysics Data System (ADS)

Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a turbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.

Gad-El-Hak, M.; Riley, J. J.; Blackwelder, R. F.

1981-09-01

376

TURBULENCE PARAMETERS IMPACTING DISPERSION IN AN URBAN CONVECTIVE BOUNDARY LAYER  

EPA Science Inventory

Turbulence measurements of the three dimensional wind components were collected by an instrumented research aircraft on 7 days in August 1976. These aircraft flights were conducted as part of the Regional Air Pollution Study (RAPS) urban boundary layer field program in St. Louis,...

377

Navier-Stokes simulation of boundary-layer transition  

NASA Astrophysics Data System (ADS)

This report describes the successful efforts to computationally model the receptivity of the laminar boundary layer on a semi-infinite flat plat with an elliptic leading edge by a spatial simulation. The compressible flow is simulated by solving the governing full Navier-Stokes equations in general curvilinear coordinates by a finite difference method. First, the steady basic-state solution is obtained in a transient approach using spatially varying time steps. Then, a small-amplitude acoustic disturbances of the freestream velocity are applied as unsteady boundary conditions, and the governing equations are solved time-accurately to evaluate the spatial and temporal developments of the perturbation leading to instability waves (Tollmien-Schlichting waves) in the boundary layer. The effect of leading type radius on receptivity is determined.

Reed, Helen L.

1990-05-01

378

Numerical investigation of the stable nocturnal boundary layer  

NASA Astrophysics Data System (ADS)

The governing equations for the wind field and temperature field within the flat nocturnal atmospheric boundary layer (FNABL, (30)) are a highly nonlinear system of parabolic PDEs. This system is discretized into a crude two-layer numerical model via the finite difference approximation and the Monin-Obukhov similarity theory ( (22)), and analyzed as a set of ODEs. The steady state problem is also transformed into an equivalent system of first order ODEs and then discretized into a very accurate 'multi-layer' model using the orthogonal collocation method ( (12)). Some numerical techniques for nonlinear problems such as numerical continuation and bifurcation analysis are used to study the steady state solutions as some physical parameters vary. The resulting bifurcation diagrams from the two layer and multilayer models have qualitatively similar behavior. This implies that the two-layer model, though mathematically crude, does capture some essential features of the original system. Time dependent solutions of the two layer model are computed via the fourth-order Runge-Kutta technique, for various combinations of parameters, and they match and support related bifurcation diagrams. Physically realistic wind and temperature profiles over the boundary layer are computed from the 'multi-layer' model. Our results imply that operational application of this type of model of frost or pollution dispersion may not be made with confidence for certain parameter regimes, and they have important implications for the predictability of the nocturnal boundary layer for frost prediction or pollution dispersion. Space discretization for simple parabolic PDEs from an AUTO demo via pseudospectral method with Chebyshev basis functions is very accurate, and seems promising for future application to our problem.

Shi, Xingzhong

1997-10-01

379

Characteristics of nonlinear evolution of wavepackets in boundary layers  

NASA Astrophysics Data System (ADS)

The nonlinear evolution of a finite-amplitude disturbance in a 3-D supersonic boundary layer over a cone was investigated recently by Liu et al. using direct numerical simulation (DNS). It was found that certain small-scale 3-D disturbances amplified rapidly. These disturbances exhibit the characteristics of second modes, and the most amplified components have a well-defined spanwise wavelength, indicating a clear selectivity of the amplification. In the case of a cone, the three-dimensionality of the base flow and the disturbances themselves may be responsible for the rapid amplification. In order to ascertain which of these two effects are essential, in this study we carried out DNS of the nonlinear evolution of a spanwise localized disturbance (wavepacket) in a flat-plate boundary layer. A similar amplification of small-scale disturbances was observed, suggesting that the direct reason for the rapid amplification is the three-dimensionality of the disturbances rather than the three-dimensional nature of the base flow, even though the latter does alter the spanwise distribution of the disturbance. The rapid growth of 3-D waves may be attributed to the secondary instability mechanism. Further simulations were performed for a wavepacket of first modes in a supersonic boundary layer and of Tollmien-Schlichting (T-S) waves in an incompressible boundary layer. The results show that the amplifying components are in the band centered at zero spanwise wavenumber rather than at a finite spanwise wavenumber. It is therefore concluded that the rapid growth of 3-D disturbances in a band centered at a preferred large spanwise wavenumber is the main characteristic of nonlinear evolution of second mode disturbances in supersonic boundary layers.

Yu, Min; Luo, JiSheng; Li, Jia

2013-02-01

380

The influence of bulges on boundary-layer instability  

NASA Astrophysics Data System (ADS)

Local disturbances caused by a spanwise surface corrugation affect the position of the boundary-layer transition, and so the drag, of an object. This premature transition from laminar to turbulent flow is often associated with a separation of the laminar boundary-layer from its surface. Also the roughness-induced separation bubble provides an important link between the pressure and velocity fluctuations in the environment and the development of the disturbance in the laminar boundary-layer, i.e., the receptivity problem. To investigate the influence of a laminar separation bubble on boundary-layer instability, a separated flow generated by a velocity gradient over a flat plate was analyzed by direct numerical simulation using finite-difference solutions of the Navier-Stokes equations. The bubble acts as a strong amplifier of the instability waves and a highly nonlinear flow field is shown to develop downstream of the bubble. Consequently, the results of the direct numerical simulation differ noticeably from those of the classical linear stability theory proving the fact that the nonparallel effects together with the nonlinear interactions are crucial to this flow development. In the present paper, the effect of physical perturbations such as humps and hollows on boundary-layer instability is analyzed. This problem has been considered theoretically by several researchers (e.g., Nayfeh et al., 1987 and 1990; Cebeci et al., 1988). They used linear stability theory in their approach which does not include the nonparallel nor the nonlinear effects. Therefore, to account for these important effects in studying flow over humps and hollows the direct simulation technique is being implemented in generalized coordinates.

Elli, S.; Vandam, C. P.

381

Saturn's low-latitude boundary layer: 2. Electron structure  

NASA Astrophysics Data System (ADS)

The boundary of a planetary magnetosphere is the site of mass, momentum, and energy transport. This transport produces a layer of mixed solar wind and magnetospheric plasma inside and adjacent to the boundary. In the case of Earth, the electron structure of this layer is distinctive, and has been explained by models of the layer on open magnetic field lines. In this paper we examine the electron structure of Saturn's low-latitude boundary layer (LLBL) using observations made by the Cassini spacecraft; the typical properties and variability of Saturn's LLBL are examined in a companion paper. By analyzing the relationship between the electron density and temperature measured during Cassini magnetopause crossings we demonstrate that the electron structure of Saturn's LLBL is highly variable. At some of the crossings the structure of Saturn's LLBL is similar to previously reported examples of the structure of Earth's LLBL, where the major changes in electron density and temperature clearly occur in different regions of the layer, producing a distinctive shape to the temperature-density distribution. However, at many crossings the structure of Saturn's LLBL is unlike the previously reported examples of the structure of Earth's LLBL, since they lack the same distinctive shape to the distribution. We discuss the possible explanations for these differences in the electron structure of Saturn's LLBL, and what these differences could tell us about how the solar wind interacts with a planetary magnetosphere.

Masters, A.; Walsh, A. P.; Fazakerley, A. N.; Coates, A. J.; Dougherty, M. K.

2011-06-01

382

On a numerical solution of incompressible turbulent boundary layer flow  

NASA Astrophysics Data System (ADS)

The application of the finite element-differential method for turbulent flow problems is considered for a steady two-dimensional incompressible boundary layer flow past a semiinfinite flat plate, in which the boundary layer region is composed of laminar, transitional, and turbulent regimes. The two-layer Cebeci-Smith (1974) closure model was chosen for the eddy viscosity. For the first numerical experiment, a fixed 16-element discretization model is selected with H chosen as 7.0 to cover a large range of local Reynolds numbers. The computed effective eddy profile exhibits oscillatory behavior in the inner region. The next experiment uses an adjustable 15-element model. Finally, a simple adaptive element generation technique for the turbulent flow problem is developed and successfully tested.

Hsu, C.-C.; Chang, T.-H.

383

Characterization and friction performance of Zn/Mg/Al-CO3 layered double hydroxides  

NASA Astrophysics Data System (ADS)

Zn/Mg/Al-CO3 layered double hydroxides (LDHs) were synthesized by coprecipitation method and the products were surface modified by oleic acid. The materials were characterized by X-ray diffraction, scanning electron microscopy, wavelength dispersive X-ray fluorescence, Fourier transform infrared spectroscopy, thermogravimetry and differential scanning calorimetry. The tribological property of LDHs in base oil was studied by using a four-ball friction test machine. The results showed that Zn/Mg/Al-CO32--LDHs had high crystallinity and hexagonal lamellar structure with average disk diameter of about 150 nm and the chemical formula was Zn0.40Mg0.28Al0.32(OH)2(CO3)0.16·0.28H2O. The interaction between LDHs and oleic acid molecules was based on chemisorption with a monomolecular layer on the surface of laminate. The friction test results indicated that base oil with 0.5 wt% LDHs performed optimal antifriction property and the friction coefficient and wear scar diameter reduced by 68.6% and 24.6% respectively.

Li, Shuo; Bai, Zhimin; Zhao, Dong

2013-11-01

384

Explicit Solvent Simulations of Friction between Brush Layers of Charged and Neutral Bottle-Brush Macromolecules  

SciTech Connect

We study friction between charged and neutral brush layers of bottle-brush macromolecules using molecular dynamics simulations. In our simulations the solvent molecules were treated explicitly. The deformation of the bottle-brush macromolecules under the shear were studied as a function of the substrate separation and shear stress. For charged bottle-brush layers we study effect of the added salt on the brush lubricating properties to elucidate factors responsible for energy dissipation in charged and neutral brush systems. Our simulations have shown that for both charged and neutral brush systems the main deformation mode of the bottle-brush macromolecule is associated with the backbone deformation. This deformation mode manifests itself in the backbone deformation ratio, , and shear viscosity, , to be universal functions of the Weissenberg number W. The value of the friction coefficient, , and viscosity, , are larger for the charged bottle-brush coatings in comparison with those for neutral brushes at the same separation distance, D, between substrates. The additional energy dissipation generated by brush sliding in charged bottle-brush systems is due to electrostatic coupling between bottle-brush and counterion motion. This coupling weakens as salt concentration, cs, increases resulting in values of the viscosity, , and friction coefficient, , approaching corresponding values obtained for neutral brush systems.

Carrillo, Jan-Michael [University of Connecticut; Brown, W Michael [ORNL; Dobrynin, Andrey [University of Connecticut

2012-01-01

385

Effects of oxide layer on the friction characteristics between TiN coated ball and steel disk in dry sliding  

Microsoft Academic Search

In this study, the effect of oxide layer on friction characteristic of titanium nitride (TiN) coated ball and steel disk was investigated through sliding test. Sliding tests were conducted in both air and nitrogen environments to study the role of oxide layer in sliding motion as well as to investigate the effect of oxide layer on TiN coating. For the

Chung-Woo Cho; Young-Ze Lee

2003-01-01

386

Behaviour of Atmospheric Boundary Layer Height at Dome C, Antarctica  

NASA Astrophysics Data System (ADS)

The Antarctic Atmospheric Boundary Layer presents characteristics which are substantially different from the mid-latitudes ABLs. On the Antarctic plateau two different extreme situations are observed. During the summer a mixing height develops during the warmer hours of the day although the sensible heat flux is reduced compared to that at mid-latitudes. During the winter a long lived stable boundary layer is continuously present, the residual layer is never observed, consequently the inversion layer is connected at the free atmosphere. To understand the stable ABL process the STABLEDC (Study of the STAble Boundary Layer Environmental at Dome C) experimental field was held at Concordia, the French Italian plateau station at Dome C, during 2005. In the same period the RMO (Routine Measurements Observations) started. The data included turbulence data at the surface, temperature profiles by a microwave profiler (MTP-5P), a mini-sodar and radio-soundings. In this work we will show the results of a comparison of the ABL height at Concordia (3233 m a.s.l) during the summer and the winter using direct measurements and parameterization. The winter ABL height was estimated directly using experimental data (radio-soundings and radiometer temperature and wind velocity profiles) and different methods proposed in literature. The stable ABL height was also estimated using the formulation proposed by Zilitinkevich et al. (2007) for the long-lived stable boundary layer. The correlation of ABL height with the temperature and wind speed is also shown. The summer mixing height was instead estimated by mini-sodar data and compared with the height given by the model suggested by Batchvarova and Gryning (1991) which use as input the turbulence data.

Pietroni, I.; Argentini, S.

2009-09-01

387

Integral Scales for the Nocturnal Boundary Layer. Part 1: Empirical Depth Relationships.  

NASA Astrophysics Data System (ADS)

The stable-layer thickness h and near-surface potential temperature strength s, of the nocturnal boundary layer (NBL) are shown to have a `background' square-root of time dependence. Superimposed upon this background are other time variations caused by changes in bulk turbulence parameter B and average surface heat flux H: h = 5(HtB) 1/2 and s = (HtB1) 1/2 ). As an intentionally different approach to the NBL problem B is modeled in terms of forcings external to the NBL rather than in terms of internal variables such as friction velocity or Obukhov length. Nocturnal boundary layer observations from the Wangara and Koorin field experiments in Australia are used to guide some dimensional arguments to yield B (GUG1)(|fUG|Zs)3/2/(QHg), where UG is the geostrophic wind vector, f the Coriolis parameter, g the acceleration due to gravity, Zs is a site and wind-direction-dependent empirical parameter and the overbear indicates time-average since transition (near sunset). Apparently, Zs is a measure of the influence of terrain features such as roughness and slope on NBL development. The resulting model is shown to be adaptable to frost-warning and air-quality applications.

Stull, Roland B.

1983-04-01

388

The Role of Lateral Boundary Conditions and Boundary Layer in air Quality Modelling System  

NASA Astrophysics Data System (ADS)

Often in Europe, high concentrations of O3 and NOx exceed the allowed maximum levels defined by the European Commission. The model system WRF-CMAQ is a regional air quality modelling system, which we applied to the European continent with a horizontal resolution of 12km × 12km and 8 vertical layers for CMAQ. The EMEP emission inventory was compiled and applied to the model domain. Due to uncertain external influence, the definition of adequate lateral boundary conditions for gas phase chemistry in a regional model is a complex issue and an important source of errors. Sensitivity studies were performed for a tested month (May 2004) to assess the impact of boundary conditions and boundary layer on the quality of the simulations. In order to evaluate the performances of the model, model simulations were compared to 70 and 21 stations from the EMEP network for O3 and NO2, respectively, throughout Europe. Basically, synthetic boundary conditions over 6 vertical layers were used in preliminary simulations. Then, climatological data provided by the global climate-chemistry model LMDz-INCA2 were used to define consistent lateral conditions and simulations were also performed using the May-2004 data from this model (both over 6 or 15 layers). Climatological data provided reliable conditions for the model boundaries but did not improve the quality of simulated O3 in the model domain (mean normalized gross error (MNGE) of 23% compared to 20% for synthetic profiles). Besides, the use of higher vertical resolution notably improved the trend and daily variations of O3 and hindered unrealistic subsidence of O3-enriched air from aloft, reducing the MNGE from 23% to 18%. The chemistry of NO2 was found to be mostly governed by local emissions, with little influence of the boundary conditions. A best-fitting configuration of boundary conditions will be discussed. Also, this work analyzes the influence of two different planetary boundary layer (PBL) parameterization schemes: the YSU PBL scheme based on Hong. et al (1996) and a new stable boundary layer scheme allowing the computation of vertical diffusion coefficients at all timesteps. The two schemes do not differ sufficiently from one another to lead to a significant improvement in the simulation of the chemistry. In the view of excessive NO2 simulated at night, other PBL parameterizations will be discussed.

Piot, M.; Jorba, O.; Jimenez, P.; Baldasano, J.

2008-12-01

389

Hydrodynamical calculations towards steady state structures in boundary layers in accretion disks. 1: 1-D polytropic boundary layers  

NASA Astrophysics Data System (ADS)

We apply the one dimensional version of the multi-dimensional robust solver developed by Hujeirat & Rannacher (1994) to follow the viscous evolution of various models of polytropic boundary layers in accretion disks around a non-magnetic white dwarf. In this paper, we present the results of 13 different time-dependent hydrodynamical calculations. The results indicate that (1) no steady structure of the flow in the boundary layer is encountered. The solutions show that a quasi-standing shock is always present for very small alpha and high stellar rotation, (2) the rate of accreted angular momentum into the central object is much less than the corresponding Keplerian one, (3) the radial extent of the boundary layer (delta RBL) is much smaller than the vertical one as well as the radial extent of classical viscous boundary layers, (4) delta RBL increases with the rotational speed of the star, (5) an instability of the shock position is detected as well as quasi-periodic oscillations which are viscosity-dependent, (6) the polytropic equation of state with gamma = 2 yields unstable and chaotic behavior of the flow in the disk region.

Hujeirat, A.

1995-03-01

390

Incompressible boundary layer transition flight experiments over a nonaxisymmetric fuselage forebody and comparisons with laminar boundary layer stability theory  

Microsoft Academic Search

Analyses of previous boundary-layer transition experiments over axisymmetric bodies indicates a potential for achieving substantial amounts of laminar flow over such shapes. Achievement of natural laminar flow over portions of nonlifting aircraft geometries, such as fuselage to forebodies, tip tanks or engine nacelles, could significantly contribute to the reduction of total aircraft viscous drag. A modern surface-panel method, a streamwise

Paul M. H. W. Vijgen

1990-01-01

391

Asymptotic analysis of the k-epsilon turbulent boundary layer  

NASA Astrophysics Data System (ADS)

Matched expansions in the limit of infinite Reynolds number are presently used to analyze the representation of attached turbulent boundary-layer flows yielded by the standard form of the k-epsilon model, whose structure is made up of a thin viscous wall-layer, a thick outer 'defect layer' region, and a thin region at the outer edge of the defect layer. Similarity equations governing the outer-layer flow are obtained for equilibrium flow situations, yielding profiles that are not analytic at the outer edge; the asymptotic behavior at the outer edge has a strong influence on the shape of the profiles throughout a significant portion of the entire outer region. The asymptotic results may be the basis of zonal modeling for complex turbulent flow fields.

Bogucz, E. A.

392

The possibility of drag reduction by outer layer manipulators in turbulent boundary layers  

Microsoft Academic Search

Experiments were carried out with the aim of investigating the possibility of obtaining a net drag reduction on a finite body by manipulating the outer layer structure of the turbulent boundary layer. The experiments were carried out in a 260 m long towing tank, where large eddy breakup devices (LEBU’s) were used in single and tandem configurations on a large

Alexander Sahlin; Arne V. Johansson; P. Henrik Alfredsson

1988-01-01

393

Pressure field due to drag reducing outer layer devices in turbulent boundary layers  

Microsoft Academic Search

The wall static pressure in the vicinity of drag reducing outer layer devices in flat wall turbulent boundary layers has been measured and compared with an inviscid theory. Symmetric and cambered airfoil devices have been examined at small angles of attack and very low chord Reynolds numbers. Airfoil devices impose a sequence of strong favorable and adverse pressure gradients on

P. R. Bandyopadhyay; R. D. Watson

1987-01-01

394

The location of thermal shelf fronts and the variability of the heights of tidal benthic boundary layers  

NASA Astrophysics Data System (ADS)

It is often hypothesized that the locus of a thermal shelf fronts is where the water depth (D) is equal to the thickness of the tidal frictional bottom boundary layer h. To determine the proper scales for tidal benthic boundary layers, we present simple, but rather general arguments which demonstrate that benthic boundary layers (BBL) in neutrally stratified environment must be defined by Ekman scale Le=u*/Ù, where u* is friction velocity, based on the bottom stress ôb=pu2*, p-water density, and Ù-Coriolis parameter. This result differs from those suggested by the numerical simulation of the formation of BBL in initially continuously stratified fluid, subject to a suddenly imposed barotropic pressure gradient as well as by direct observations of the intensity of turbulence close to the sea bottom, which indicated that the thickness of the well-mixed turbulent region close to the bottom of the sea is very often significantly less than Le. Recently, Stigebrandt has argued that it can be explained by introducing the numerical constant ë in the expression h = ë Le (ë [[SYMBOL MISSING

Kitaigorodskii, S. A.

1992-10-01

395

Boundary Layer Depth, Entrainment, and Decoupling in the Cloud-Capped Subtropical and Tropical Marine Boundary Layer.  

NASA Astrophysics Data System (ADS)

Estimates of marine boundary layer (MBL) depth and degree of decoupling for two regions of the subtropical and tropical east Pacific are presented using satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). These data are combined in a novel way with NCEP reanalysis data and a mixing line parameterization to estimate the mean entrainment rate we over the two regions. Mean entrainment rates vary geographically and have maxima just downwind of the Californian coast (e 4 5 mm s-1), and also in the core of the equatorial east Pacific cold tongue where mean we exceeds 6 mm s-1. Entrainment exceeds subsidence by 30% or less in the subtropical stratocumulus regions. North of the equatorial cold tongue entrainment greatly exceeds subsidence, producing a rapid deepening of the MBL as air flows over a marked SST gradient.Shallow MBLs (zi < 500 700 m) are found to be well mixed in general. The decoupling increases markedly for deeper boundary layers and is well parameterized as a function of the thickness of the layer extending from the top of the surface mixed layer to the MBL inversion. This study demonstrates new ways in which large-scale observational and reanalysis datasets may be used to aid understanding of MBL boundary layer and cloud systems.


Wood, Robert; Bretherton, Christopher S.

2004-09-01

396

Boundary Layer Structure and Dynamics in Outer Hurricane Rainbands.  

NASA Astrophysics Data System (ADS)

Results of hurricane boundary layer experiments conducted in outer rainbands of Hurricanes Josephine (1984) and Earl (1986) are presented. Comparisons of precipitation, kinematic, and thermodynamic structures in these storms and in Hurricane Floyd (1981) indicate that principal rainbands have common characteristic mesoscale and convective scale features in the boundary layer. The two-dimensional mesoscale structure suggests that rainbands are made up of a linear aggregate of cellular reflectivity elements (on the inner, upshear side of the band) and stratiform rain (on the outer downshear side). The band is oriented perpendicular to the shear above the boundary layer and cells move downband at about 80% of the maximum wind. Alongband and crossband wind maxima, and maximum equivalent potential temperatures are located on the outer side of the band axis, with minima 4-8 km to the inner side. Updrafts and downdrafts are preferentially located on the inner side of the band axis, along with maximum crossband convergence, cyclonic shear vorticity, and minimum equivalent potential temperatures. Downdraft transport of cool and dry air from middle levels on the inner side of the rainband was responsible for modifying mixed layer structure adjacent to the band on alongband scales of 100 km. An undisturbed mixed layer of 500 m was present on the outer side of the band while a variety of structures were observed on the inner side indicative of both disturbed and recovering mixed layers. Application of a mixed layer model to low level flow trajectories from the outer rainband to the eyewall indicates that under some conditions, the mixed layer may not recover sufficiently and low surface equivalent potential temperature air may reach the eyewall. These conditions are associated with suppressed flow in a region of positive divergence with moderate rainfall from a middle level anvil cloud. Incomplete recovery was most evident when a recovering mixed layer exhibited a negative jump in water vapor mixing ratio. Differential evaporation cooling over the transition layer drives entrainment of dry air from above which overcomes any evaporation moistening, resulting in a drier mixed layer (with lower surface equivalent potential temperature). Depending on the humidity profile and spatial scale of the initial disturbed mixed layer, the model results suggest that incomplete recovery may be responsible for transitional changes in hurricane intensity.

Powell, Mark Dillon

397

A numerical study of compressible turbulent boundary layers  

NASA Astrophysics Data System (ADS)

Compressible turbulent boundary layers with free-stream Mach number ranging from 2.5 up to 20 are analyzed by means of direct numerical simulation of the Navier--Stokes equations. The simulation generates its inflow condition using the rescaling-recycling method. The main objective is to study the effect of Mach number on turbulence statistics and near-wall turbulence structures. The present study shows that the main turbulence statistics can be correctly described as variable-density extensions of incompressible results. We show that the apparent increase in the magnitude of the fluctuating Mach number with increasing free-stream Mach number is a variable-property effect. Using the mean density to scale the fluctuating Mach number collapses results for different freestream Mach number. The increase in the pdf tails of the dilatation is also shown to be a variable-property effect. Compressible boundary layers are shown to be similar to incompressible boundary layers in that, without the linear coupling term, the turbulence cannot be sustained. The linear coupling term is necessary to generate the wall-layer streaks. For an adiabatic wall, the near-wall structure exhibits the same characteristics as in incompressible turbulent flow in terms of the spanwise spacing of the streaks ( 100^+). For isothermal walls, near-wall turbulence structures show their dependence on the surface heat flux.

Lagha, Maher; Kim, John; Eldredge, Jeff; Zhong, Xiaolin

2010-11-01

398

Mixing of Ozone at Boundary Layer Top - A Lidar Study  

NASA Astrophysics Data System (ADS)

Studies of boundary layer dynamics and entrainment were carried out using field campaign data from the mobile aerosol and ozone profiler operated by the Facility for Ground based Atmospheric Measurements (FGAM) in the United Kingdom (UK). The UK-based field campaigns investigated include the Tropospheric ORganic CHemistry experiment (TORCH) in 2003, the Convective Storm Initiation Project (CSIP) in 2005 and the Leicester Air quality Measurement Project (LAMP) in 2007. The profiler is a DIfferential Absorption Lidar (DIAL) system that operates at five wavelengths simultaneously in the near ultraviolet (266nm, 289nm, 299nm, 316nm and 355nm) and has a range between 100m and 5km, depending on the meteorological conditions. Vertical aerosol backscatter profiles were calculated and ozone profiles of the boundary layer were deduced. The error in ozone mixing ratio was +/- 3ppbv. Unlike other ozone lidars, the UFAM profiler can be run at high temporal resolutions of down to 1 minute. From these profiles it was possible to follow entrainment events and the mixing of aerosol and ozone at the top of the convective boundary layer. Case studies are presented including re-entrainment of previously detrained polluted air and development of residual layers from preceding days. The chemical and physical properties of the air parcels were looked at in greater detail using accompanying instruments at each measurement site.

Ricketts, H. M. A.; Vaughan, G.; Norton, E. G.; Wareing, D. P.

2009-09-01

399

Studies of the Marine Boundary Layer at Tarapur  

NASA Astrophysics Data System (ADS)

Simultaneous observations were made of the Marine Boundary Layer at Tarapur, a site near Bombay on the sea coast, by acoustic sounder and instrumented tower. The meteorological tower was used to sense wind and temperature at various levels up to a height of 120 m while the acoustic sounder was used to examine the thermal structure of the boundary layer up to a height of 700 m. Data recorded for the year 1982 have been analysed. Analysis of the data shows that while the normal structures of thermal echoes and shear echoes represent the mixing depth of the atmospheric boundary layer, the often observed elevated layers are due to sea breeze reversals with their base giving a measure of the depth of the sea-breeze circulation during the day. A sea breeze has been detected during both spring (March to May) and autumn (October to December) months. The onset times are around 1000 hr during spring months and around noon during the autumn period, the height of development being respectively up to 500 and 350 m. The capability of the sodar to detect the base and thickness of the sea breeze, is clearly revealed.

Singal, S. P.; Aggarwal, S. K.; Pahwa, D. R.; Adiga, B. B.

1986-12-01

400

Inner-Layer Structure of a Shear-Driven 3-D Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

An investigation of a planar, shear-driven 3-D turbulent boundary layer (3DTBL) was performed to examine the effects of variable skewing on the turbulence structure and flow physics of the non-equilibrium flow field. Particle image velocimetry (PIV) measurements were acquired in both the xy-plane (perpendicular to the wall) and the xz-plane (parallel to the wall) to examine modifications to the near-wall turbulence when subject to varying strengths of crossflow. These measurements reveal significant changes to the inner region of the boundary layer, particularly at higher shear rates. Increased spanwise shear leads to the breakup of larger organized flow structures into smaller structures that are displaced out from the near-wall region of the boundary layer, leading to increased transport and a thickening of the inner region of the boundary layer. The crossflow is also associated with increases in the (normal and shear) Reynolds stresses, particularly over the translating wall section. The discontinuity at the trailing edge of the translating wall results in an initial decrease of the streamwise normal stress, which subsequently recovers and increases above 2-D levels. Another effect of the crossflow is the disruption of the initial 2-D boundary layer spanwise vorticity layer, contributing to the increased momentum transfer in this region.

Kiesow, Robert; Plesniak, Michael

1999-11-01

401

Boundary layer analysis for two-dimensional slot jet impingement on inclined plates  

SciTech Connect

The laminar boundary layer flow when a two-dimensional slot jet impinges on a flat plate at some angle is analyzed theoretically. The conservation equations in primitive variables are solved using a finite-difference technique. The computed results at 0 and 90 deg angle of impingement are in perfect agreement with standard solutions available in the literature. The influence of the angle of impingement on the velocity and temperature profiles is studied. The presence of a stagnation point when the plate is not parallel to the oncoming jet is found to affect considerably the local Nusselt number and skin friction coefficient. These parameters attain very large values close to the stagnation point at small angles of impingement. However, far from the stagnation point, they approach values corresponding to a flat plate at zero incidence, irrespective of the angle of jet impingement.

Garg, V.K.; Jayaraj, S. (Indian Institute Technology, Kanpur (India))

1988-08-01

402

Dynamical Simulation of Cloudy Boundary Layer Flow during Cold Air Outbreaks  

Microsoft Academic Search

A two-dimensional primitive equation planetary boundary layer model has been constructed and applied to simulate downwind evolution of coupled dynamical, thermodynamical and cloud properties in the planetary boundary layer (PBL) developed during cold air outbreaks over warm ocean. A layered parametric approach is adopted to model the inversion -capped convective boundary layer filled with shallow cumuli, or topped by stratocumulus

Chiu-Wai Yuen

1983-01-01

403

Boundary layer ozone - An airborne survey above the Amazon Basin  

NASA Astrophysics Data System (ADS)

Ozone data obtained over the forest canopy of the Amazon Basin during July and August 1985 in the course of NASA's Amazon Boundary Layer Experiment 2A are discussed, and ozone profiles obtained during flights from Belem to Tabatinga, Brazil, are analyzed to determine any cross-basin effects. The analyses of ozone data indicate that the mixed layer of the Amazon Basin, for the conditions of undisturbed meteorology and in the absence of biomass burning, is a significant sink for tropospheric ozone. As the coast is approached, marine influences are noted at about 300 km inland, and a transition from a forest-controlled mixed layer to a marine-controlled mixed layer is noted.

Gregory, Gerald L.; Browell, Edward V.; Warren, Linda S.

1988-02-01

404

The Boundary Layer Flows of a Rivlin-Ericksen Fluid  

NASA Astrophysics Data System (ADS)

The present work deals with the two-dimensional incompressible, laminar, steady-state boundary layer equations. First, we determine a family of velocity distributions outside the boundary layer such that these problems may have similarity solutions. We study the Falkner-Skan flow of a viscoelastic fluid governed by second order model, as the Reynolds number Re? ?. We obtain an ordinary forth order differential equation to obtain the stream function, velocity profile and the stress. The stream function is then governed by a generalized Falkner-Skan equation. In comparison with Newtonian Falkner-Skan equation that has two coefficients this new one has four coefficients that two of them represent elastic properties of the fluid. The effects of the elastic parameter on the velocity filed have been discussed. As it is shown in the figure there is a good agreement between numerical results and previous special cases confirm the validity of the presented algorithm.

Sadeghy, K.; Khabazi, N.; Taghavi, S. M.

405

Clues and modelling for missing boundary layer in cataclysmic variables  

SciTech Connect

Recently, it has become observationally evident that during a dwarf nova (DN) outburst, a significant portion of the gravitational energy accreted onto the white dwarf (WD) may not be radiated away instantly from a narrow boundary layer as predicted by the standard disk theory. Instead, it may be stored in the WD through various mechanisms and the radiative area may be much larger; thus the long-puzzling {ital missing boundary layer} may be accounted for when the response of the WD to the accretion is considered. The results from our group and collaborators on this aspect are outlined in the first part. A progress report on the development, of a new numerical model forms the second.

Huang, M.; Sion, E.M. [Villanova Univ., PA (United States); Sparks, W.M. [Los Alamos National Lab., NM (United States)

1996-12-31

406

Aerosol Observations by Lidar in the Nocturnal Boundary Layer  

NASA Astrophysics Data System (ADS)

Aerosol observations by lidar in the nocturnal boundary layer (NBL) were performed in Potenza, Southern Italy, from 20 January to 20 February 1997. Measurements during nine winter nights were considered, covering a variety of boundary-layer conditions. The vertical profiles of the aerosol backscattering coefficient at 355 and 723.37 nm were determined through a Klett-modified iterative procedure, assuming the extinction-to-backscattering ratio within the NBL has a constant value. Aerosol average size characteristics were retrieved from almost simultaneous profiles of the aerosol backscattering coefficient at 355 and 723.37 nm, the measurements being consistent with an accumulation mode radius not exceeding 0.4 m. Similar results in terms of aerosol sizes were obtained from measurements of the extinction-to-backscattering ratio profile at 355 nm performed on six nights during the measurement campaign. Backscattering profiles at 723.37 nm were also converted into profiles of aerosol liquid water content.

di Girolamo, Paolo; Ambrico, Paolo Francesco; Amodeo, Aldo; Boselli, Antonella; Pappalardo, Gelsomina; Spinelli, Nicola

1999-07-01

407

On multiscale homogenization problems in boundary layer theory  

NASA Astrophysics Data System (ADS)

This paper is concerned with the homogenization of the equations describing a magnetohydrodynamic boundary layer flow past a flat plate, the flow being subjected to velocities caused by injection and suction. The fluid is assumed incompressible, viscous and electrically conducting with a magnetic field applied transversally to the direction of the flow. The velocities of injection and suction and the applied magnetic field are represented by rapidly oscillating functions according to several scales. We derive the homogenized equations, prove convergence results and establish error estimates in a weighted Sobolev norm and in C 0-norm. We also examine the asymptotic behavior of the solutions of the equations governing a boundary layer flow past a rough plate with a locally periodic oscillating structure.

Amirat, Youcef; Chechkin, Gregory A.; Romanov, Maxim

2012-06-01

408

Boundary layer study. Phase 4: Experimental validation test plan  

NASA Astrophysics Data System (ADS)

The trend towards very large area ratio nozzles, which result in performance gains for space propulsion applications, has increased the need for detailed knowledge of the momentum losses due to nozzle viscous effects (i.e., boundary layer). These losses degrade overall system performance, such as increasing system weight, decreasing useful payload weight, and/or decreasing effective system range. Another important factor in designing propulsive nozzles is the detailed knowledge of heat transfer at the wall for regeneratively cooled walls and/or material performance. Phase 4 (described in this report) consists of preparing an experimental test plan, which, if executed, would validate computational techniques used in evaluating propulsion performance losses due to boundary layers in rocket nozzles. The importance of the loss and the basis for the experimental work are established. Experimental techniques are also reviewed. Visits to numerous experimental facilities are described along with recommendations from these facilities. Finally, recommendations as to the diagnostic techniques of choice are made.

Kehtarnavaz, H.; Coats, D. E.

1990-11-01

409

Receptivity of boundary layers - Asymptotic theory and experiment  

NASA Astrophysics Data System (ADS)

The sources of disturbance (vibrators, small jets, vortices, sound waves) in a boundary layer are considered, emphasizing their ability to provoke the onset of eigenoscillations with exponentially growing amplitude. Harmonic sources give rise to the Tollmien-Schlichting waves, whereas impulsive sources excite wave packets. General requirements are stated for the temporal and spatial characteristics of the signals emitted by the devices causing disturbance, as well as for obstacles met by signals when propagating. To scale the frequencies and wavenumbers in terms of the Reynolds number taking on indefinitely large values, the asymptotic theory of an interacting boundary layer with the triple-deck structure is used. The conclusions from the asymptotic analysis are in line with the results of measurements in wind tunnels when the Reynolds numbers were moderate.

Kozlov, V. V.; Ryzhov, O. S.

1990-06-01

410

Leading-edge effects on boundary-layer receptivity  

NASA Astrophysics Data System (ADS)

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

Gatski, Thomas B.; Kerschen, Edward J.

1990-07-01

411

Calculation of hypersonic laminar boundary layers with chemical kinetics  

NASA Astrophysics Data System (ADS)

CLIM, a computer program developed to calculate the laminar boundary layer in axisymmetric steady hypersonic flows such as those on reentry vehicles, is described and demonstrated. CLIM takes into account the effects of transverse or longitudinal curvature, wall injection, and chemical reactions of equilibrium or nonequilibrium gases, permitting the treatment of ablation phenomena. The basic hypotheses are outlined; the program equations are derived; the finite-difference discretization is explained; and results are presented graphically for test problems involving a 10-deg-half-angle cone, a Shuttle-like hyperboloid, and the ablation of a carbon wall. When programmed on an IBM 3081, CLIM comprises about 3000 lines and can compute the wall stresses and the boundary-layer thickness, velocity, temperature, density, and concentration profiles for a 50-section body and 10 gas species in less than 1 min for nondissociated air, about 1 min for equilibrium air, and less than 10 min for nonequilibrium air.

Noel, F.; Boukhobza, P.

1986-11-01

412

Entropy generation in the viscous parts of turbulent boundary layers  

SciTech Connect

The local (pointwise) entropy generation rate per unit volume S is a key to improving many energy processes and applications. Consequently, in the present study, the objectives are to examine the effects of Reynolds number and favorable streamwise pressure gradients on entropy generation rates across turbulent boundary layers on flat plates and—secondarily—to assess a popular approximate technique for their evaluation. About two-thirds or more of the entropy generation occurs in the viscous part, known as the viscous layer. Fundamental new results for entropy generation in turbulent boundary layers are provided by extending available direct numerical simulations. It was found that, with negligible pressure gradients, results presented in wall coordinates are predicted to be near “universal” in the viscous layer. This apparent universality disappears when a significant pressure gradient is applied; increasing the pressure gradient decreases the entropy generation rate. Within the viscous layer, the approximate evaluation of S differs significantly from the “proper” value but its integral, the entropy generation rate per unit surface area S, agrees within 5% at its edge.

Donald M. McEligot; Edmund J. Walsh; Eckart Laurien; Philippe R. Spalart

2008-06-01

413

Boundary-layer equations in generalized curvilinear coordinates  

Microsoft Academic Search

A set of higher-order boundary-layer equations is derived valid for three-dimensional compressible flows. The equations are written in a generalized curvilinear coordinate system, in which the surface coordinates are nonorthogonal; the third axis is restricted to be normal to the surface. Also, higher-order viscous terms which are retained depend on the surface curvature of the body. Thus, the equations are

Argyris G. Panaras

1987-01-01

414

A zonal grid algorithm for DNS of turbulent boundary layers  

Microsoft Academic Search

A zonal grid algorithm for direct numerical simulation (DNS) of incompressible turbulent flows within a Finite-Volume framework is presented. The algorithm uses fully coupled embedded grids and a conservative treatment of the grid-interface variables. A family of conservative prolongation operators is tested in a 2D vortex dipole and a 3D turbulent boundary layer flow. These tests show that both, first-

Michael Manhart

2004-01-01

415

Weak Boundary Layers in Styrene-Butadiene Rubber  

Microsoft Academic Search

In this paper two kinds of weak boundary layers (WBL) in synthetic vulcanized styrene-butadiene rubber are described.i) WBL produced by the presence of antiadhesion compounds of the rubber formulation (zinc stearate, microcrystalline paraffin wax). These WBL cannot be effectively removed by solvent wiping, whether followed by washing with an ethanol\\/water mix or not. Although this treatment allowed a significant removal

M. M. Pastor-Blas; M. S. Sánchez-Adsuar; J. M. Martín-Martínez

1995-01-01

416

The Amazon Boundary Layer Experiment: Wet season 1987  

Microsoft Academic Search

The Amazon Boundary Layer Experiment (ABLE 2B) used data from aircraft, ground-based, and satellite platforms to characterize the chemistry and dynamics of the lower atmosphere over the Amazon Basin during wet season (April-May 1987) conditions. This paper reports the experimental design for ABLE 2B and a brief summary of the results for the combined ABLE 2A and ABLE 2B. The

R. C. Harriss; M. Garstang; S. C. Wofsy; S. M. Beck; R. J. Bendura; J. R. B. Coelho; J. W. Drewry; J. M. Hoell; P. A. Matson; R. J. McNeal; L. C. B. Molion; R. L. Navarro; V. Rabine; R. L. Snell

1990-01-01

417

The Turbulent Structure of Drag Reducing Boundary Layer Flows  

Microsoft Academic Search

The turbulent structure of wall-bounded drag reduced flow has been studied with particle image velocimetry (PIV) in a zero-pressure-gradient boundary layer. Drag reduction was achieved by injection of a concentrated polymer solution through a spanwise slot along the test wall at a distance approximately 2 m upstream of the PIV measurement station. For comparison, water was injected at the same

C. M. White; V. Somandepalli; M. G. Mungal

418

Nanodiamonds in the Younger Dryas Boundary Sediment Layer  

Microsoft Academic Search

We report abundant nanodiamonds in sediments dating to 12.9 ± 0.1 thousand calendar years before the present at multiple locations across North America. Selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval. Cubic diamonds form under high temperature-pressure regimes, and n-diamonds also require extraordinary conditions, well outside the range

D. J. Kennett; J. P. Kennett; C. Mercer; S. S. Que Hee; L. Bement; T. E. Bunch; M. Sellers; W. S. Wolbach

2009-01-01

419

Correlation study in shock wave–turbulent boundary layer interaction  

Microsoft Academic Search

Shock wave–turbulent boundary layer interaction is a critical problem in aircraft design. Therefore, a thorough understanding\\u000a of the processes occurring in such flows is necessary. The most important task is to study the unsteady phenomena, in particular,\\u000a the low-frequency ones, for this interaction. An experimental study of separated flow has been performed in the zone of interaction\\u000a of the incident

P. A. Polivanov; A. A. Sidorenko; A. A. Maslov

2011-01-01

420

Automated large-eddy simulations of realistic atmospheric boundary layers  

NASA Astrophysics Data System (ADS)

Large-eddy simulations (LES) are the most realistic numerical model available for studies of flow in the Atmospheric Boundary Layer (ABL). However, since large scale processes cannot be solved within the LES domain of typically 10x10x5km, LES is nearly always limited to idealized circumstances. In this study, we use the mean state of a regional model to drive and relaxing LES with the analysis of a regional model, in analogy to the way single column models (SCMs) are driven. That way, many different days can be studied, each with their own meteorological characterization. Specifically, diurnal cycles of clear and cloud-topped boundary layers are investigated, including effects of precipitation and soil feed backs. The focus of this study lies on the meteorology around the Cabauw tower in the Netherlands during May 2008. During this month, Cabauw was the focus of the Intensive Observation Period of the EUCAARI-IMPACT campaign, which gives a very rich set of observations to compare with. Given a sufficiently large relaxation time, boundary layer processes are given the room in LES to fully develop, and characteristics such as cloud height and boundary layer thermodynamics compare well with the environment. In order to robustly run LES for the various days without strong assumptions, it turns out to be important to have a reliable radiation and surface model. The LES results are embedded in an intercomparison test bed, where LES, several SCMs, and observations can be compared with each other on a daily basis. In this way, a wide range of studies can be done. For instance, the role of individual processes, like radiation, cloud microphysics, or soil and vegetation, can be directly assessed in the controlled environment of LES. Furthermore, the availability in LES of all relevant variables in three dimensions and with a high time resolution in LES allows us to diagnose relations that form the basic assumptions of large-scale model parametrizations.

Heus, T.; Siebesma, A. P.; Neggers, R. A. J.

2009-09-01

421

Transient boundary-layer flows in combustion environments  

SciTech Connect

Unsteady boundary-layer flow equations characterizing hot, burning environments are solved numerically by means of a factored ADI method under transient and/or streamwise varying core-flow conditions. Calculated results for compressible, turbulent flow cases show that high heat fluxes at the wall due to turbulence and changing edge conditions may bring about severe temperature increase at the wall, causing melting and hence erosion of the surface itself.

Kang, S.W.; Levatin, J.L.

1980-11-01

422

The effects of cylindrical surface modifications on turbulent boundary layers  

Microsoft Academic Search

A study employing hydrogen bubble-wire flow visualization and hot-film anemometry measurements has been conducted to determine the effects of sublayer-scale streamwise surface modifications of approximate nondimensional height of h(+) = 4 on the structure and flow characteristics of turbulent boundary layers. The visualization results indicate that the surface modifications did affect the streak spacing characteristics, with the greatest effect occurring

J. B. Johansen; C. R. Smith

1985-01-01

423

Spatially developing secondary instabilities in compressible swept airfoil boundary layers  

Microsoft Academic Search

Two-dimensional eigenvalue analysis is used on a massive scale to study the spatial instabilities of compressible shear flows\\u000a with two inhomogeneous directions. The main focus of the study is crossflow dominated swept-wing boundary layers although\\u000a the methodology can also be applied to study other types of flows, such as the attachment-line flow. Certain unique aspects\\u000a of formulating a spatial, two-dimensional

Fei Li; Meelan M. Choudhari

2011-01-01

424

Some physical aspects of shock wave/boundary layer interactions  

NASA Astrophysics Data System (ADS)

When the flow past a vehicle flying at high velocity becomes supersonic, shock waves form, caused either by a change in the slope of a surface, a downstream obstacle or a back pressure constraining the flow to become subsonic. In modern aerodynamics, one can cite a large number of circumstances where shock waves are present. The encounter of a shock wave with a boundary layer results in complex phenomena because of the rapid retardation of the boundary layer flow and the propagation of the shock in a multilayered structure. The consequence of shock wave/boundary layer interaction (SWBLI) are multiple and often critical for the vehicle or machine performance. The shock submits the boundary layer to an adverse pressure gradient which may strongly distort its velocity profile. At the same time, in turbulent flows, turbulence production is enhanced which amplifies the viscous dissipation leading to aggravated performance losses. In addition, shock-induced separation most often results in large unsteadiness which can damage the vehicle structure or, at least, severely limit its performance. The article first presents basic and well-established results on the physics of SWBLI corresponding to a description in terms of an average two-dimensional steady flow. Such a description allows apprehending the essential properties of SWBLIs and drawing the main features of the overall flow structure associated with SWBLI. Then, some emphasis is placed on unsteadiness in SWBLI which constitutes a salient feature of this phenomenon. In spite of their importance, fluctuations in SWBLI have been considered since a relatively recent date although they represent a domain which deserves a special attention because of its importance for a clear physical understanding of interactions and of its practical consequences as in aeroelasticity.

Délery, Jean; Dussauge, Jean-Paul

2009-12-01

425

Perturbed boundary layer diffusion flames. Ph. D. thesis  

SciTech Connect

Thermal cracking is shown to have a significant effect on the temperature profiles in a boundary layer diffusion flame. Measurements of temperature profiles in a sooting free flow flame are compared with classic flame sheet model results. This comparison reveals a large overprediction of temperatures in the region between the fuel surface and the flame. The principle cause of this overprediction is the neglect of thermal cracking in the flame sheet model. The endothermicity of the cracking phenomena is analytically treated in a forced flow flame through the introduction of new Shvab-Zeldovich variables and a simple cracking sheet model, analogous to the flame sheet model. Two new parameters D sub cr and Q' are found which control the temperature at which cracking occurs and the amount of energy absorbed in the process. A second possible cause for the temperature overprediction is the sink effect of radiative heat transfer from soot in the boundary layer. This effect and the effects of wall emission and normal buoyancy on a horizontal boundary layer diffusion flame are studied by perturbing the cracking sheet solution. The principle effect of soot radiation is an increased blowing rate at the wall. This blowing is offset by the radiative wall emission. The postulated sink effect of soot emission is negligible. The major buoyancy effect is primarily the inducement of a velocity overshoot.

Ang, J.A.

1987-03-01

426

Acoustic explorations of the upper ocean boundary layer  

NASA Astrophysics Data System (ADS)

The upper ocean boundary layer is an important but difficult to probe part of the ocean. A better understanding of small scale processes at the air-sea interface, including the vertical transfer of gases, heat, mass and momentum, are crucial to improving our understanding of the coupling between atmosphere and ocean. Also, this part of the ocean contains a significant part of the total biomass at all trophic levels and is therefore of great interest to researchers in a range of different fields. Innovative measurement plays a critical role in developing our understanding of the processes involved in the boundary layer, and the availability of low-cost, compact, digital signal processors and sonar technology in self-contained and cabled configurations has led to a number of exciting developments. This talk summarizes some recent explorations of this dynamic boundary layer using both active and passive acoustics. The resonant behavior of upper ocean bubbles combined with single and multi-frequency broad band active and passive devices are now giving us invaluable information on air-sea gas transfer, estimation of biological production, marine mammal behavior, wind speed and precipitation, surface and internal waves, turbulence, and acoustic communication in the surf zone.

Vagle, Svein

2005-04-01

427

A planetary boundary layer observational capability in Kansas  

SciTech Connect

An initiative is underway to establish the Argonne Boundary Layer Experiments (ABLE) facility to provide continuous, long-term observations of the planetary boundary layer (PBL) with state-of-the-art instruments. Planning for ABLE began during 1995, and implementation is expected to be mostly complete by 1998. ABLE will be located within the area now occupied by the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site of DOE`s Atmospheric Radiation Measurement (ARM) program. The Argonne facility will concentrate on measuring at spatial scales considerably smaller than addressed with CART. When it is fully functional, ABLE will offer atmospheric scientists the opportunity to remotely {open_quote}collect{close_quote} data in real time without necessarily leaving their home offices. Specialized computer analysis and visualization software will be developed and provided by ABLE to facilitate analysis by remote users. ABLE will host specialized field campaigns for which it can provide supplementary measurements and the required facilities for shorter-term instrument deployments. In addition, ABLE will function as the proving ground for new technologies for atmospheric boundary layer research. 1 ref., 1 fig.

Wesely, M.L.; Coulter, R.L.; Klazura, G.E. [and others

1997-03-01

428

Linear stability of three-dimensional boundary layers  

NASA Astrophysics Data System (ADS)

Stability of compressible three-dimensional boundary layers on a swept wing model is studied within the framework of the linear theory. The analysis based on the approximation of local self-similarity of the mean flow was performed within the Falkner-Skan-Cooke solution extended to compressible flows. The calculated characteristics of stability for a subsonic boundary layer are found to agree well with the measured results. In the case of a supersonic boundary layer, the results calculated for a Mach number M = 2 are also in good agreement with the measured spanwise scales of nonstationary vortices of the secondary flow. The calculated growth rates of disturbances, however, are substantially different from the measured values. This difference can be attributed to a high initial amplitude of disturbances generated in the experiment, which does not allow the linear stability theory to be applied. The evolution of natural disturbances with moderate amplitudes is fairly well predicted by the theory. The effect of compressibility on crossflow instability modes is demonstrated to be insignificant.

Gaponov, S. A.; Smorodskii, B. V.

2008-03-01

429

Concentration boundary layers in osmotic membrane transport processes  

NASA Astrophysics Data System (ADS)

It has long been recognized, that the osmotic transport characteristics of membranes may be strongly influenced by the presence of unstirred concentration boundary layers adjacent to the membrane [1,2]. Previous experimental as well as theoretical works have focused on the case where the solution on both sides of the membrane remain well-mixed due to an external stirring mechanism. We present a theoretical investigation the effects of concentration boundary layers on the efficiency of osmotic pumping processes in the absence of external stirring i.e. when the stirring is provided by the osmotically generated flow itself. For such systems, we show that no well defined boundary layer thickness exist and that the reduction in concentration can be estimated by a surprisingly simple mathematical relation valid across a wide range of geometries and P'eclet numbers. [4pt] [1] T.J.Pedley, Q. Rev. Biophys., 1983, 16, 115[0pt] [2] K.H.Jensen et al., Lab Chip, 2009, 9, 2093

Jensen, Kaare; Bohr, Tomas; Bruus, Henrik

2009-11-01

430

Boundary layer structure during sea breeze conditions at Ahtopol, Bulgaria  

NASA Astrophysics Data System (ADS)

Continuous sodar (Scintec MFAS) and ultrasonic anemometer (Typhoon - Obninsk make) measurements were initiated in summer 2008 at the meteorological observatory of Ahtopol at the Black Sea coast (south-east Bulgaria) under a Bulgarian-Russian collaborative programme. These observations of high resolution form the basis for studies of the atmospheric boundary layer turbulence and vertical structure at a coastal site. This sodar is unique in Bulgaria and provides the first continuous high resolution data on the wind profile up to 400 - 500 m above the ground. In addition, the continuous turbulence parameters monitoring allows atmospheric boundary studies needed for different applications. The meteorological observatory at Ahtopol is under development as a background atmospheric composition station in coastal area and the wind data are essential for the studies of gases exchange under breeze conditions. The measurements revealed quite different sea breeze seasons during the years 2008 to 2011 and within the individual seasons, a number of different sea breeze types were identified depending on the interaction of local and larger-scale forcing. In this study we investigate the turbulence parameters and the vertical structure of the boundary layer related to only to sea breeze conditions. We also study the wind profile within the first 400 - 500 m above the ground. For the surface layer, we test the free convection theory against the sodar observations.

Barantiev, D.; Batchvarova, E.; Novitzky, M. A.

2012-04-01

431

Measurements of Instability and Transition in Hypersonic Boundary Layers  

NASA Astrophysics Data System (ADS)

Several studies on boundary-layer instability and transition have been conducted in the Boeing/AFOSR-Mach 6 Quiet Tunnel (BAM6QT) and the Sandia Hypersonic Wind Tunnels (HWT) at Mach 5 and 8. The first study looked at the effect of freestream noise on roughness- induced transition on a blunt cone. Temperature-sensitive paints were used to visualize the wake of an isolated roughness element at zero deg angle of attack in the BAM6QT. Transition was always delayed under quiet flow compared to noisy flow, even for an effective trip height. The second study measured transitional surface pressure fluctuations on a seven degree half-angle sharp cone in the HWT under noisy flow and in the BAM6QT under noisy and quiet flow. Fluctuations under laminar boundary layers reflected tunnel noise levels. Transition on the model only occurred under noisy flow, and fluctuations peaked during transition. Measurements of second- mode waves showed the waves started to grow under a laminar boundary layer, saturated, and then broke down near the peak in transitional pressure fluctuations. The third study looked at the development of wave packets and turbulent spots on the BAM6QT nozzle wall. A spark perturber was used to generate controlled disturbances. Measurements of the internal structure of the pressure field of the disturbances were made.

Casper, K. M.; Schneider, S. P.; Beresh, S. J.

2011-08-01

432

Transient deployment of flat winglets inside a turbulent boundary layer  

NASA Astrophysics Data System (ADS)

An experiment has been designed to investigate the flow mechanisms responsible for the augmented force generation during the transient flapping of winglets. Square and triangular flaps hinged at the wall beneath the flow have been used which were rotated with angular velocities between 10 and 100 rad/s. Strouhal numbers between 0.05 and 0.4 and Stokes numbers between 3800 and 38,000 were achieved. Experiments with two different boundary layers were also carried out. In the first one, the boundary layer thickness to the winglet's height ratio was 1.3 and in the second 0.6. Particle Image Velocimetry was used to provide qualitative and quantitative information of the flow field. The dynamic lift and drag force coefficients during the transient deployment are different than the corresponding coefficients under stationary conditions at the same deployment angle after adjusting for inertial effects. These effects are enhanced with increasing Strouhal number and decrease with increasing boundary layer thickness.

Pierides, Alexis; Andreopoulos, Yiannis

2008-11-01

433

Effect of thermally induced perturbation in supersonic boundary layers  

NASA Astrophysics Data System (ADS)

This paper investigates the mechanism of steady and unsteady thermal perturbation (also denoted as thermal bump) in a Mach 1.5 flat plate boundary layer. A high-fidelity upwind-biased third-order Roe scheme is used with the compressive van Leer harmonic limiter on a suitably refined mesh. The study consists of two parts. In the first part, the effects of the steady and pulsed thermal bumps are explored. It is shown that the finite-span thermal bumps generate streamwise vortices. With steady heating, the disturbance decays downstream. However, when the thermal bump is pulsed, vortex shedding is observed and the streamwise vortical disturbance grows with downstream distance, consistent with linear stability analysis. The integrated disturbance energy indicates that streamwise kinetic disturbance energy growth dominates over those associated with other two velocity and thermodynamic components. The second part of this paper explores the physical consequences of the nonlinear dynamics between the vortices produced by the pulsed bump and the compressible boundary layer. The resulting three-dimensional flow distortion generates hairpin structures which are aligned in the streamwise direction, suggesting that the transition process bears some similarity to K-type breakdown. The arrangement of these vortices is connected to the low-speed streaks observed in the evolving boundary layer. The shape factor, velocity, and Reynolds stress profiles suggest that the perturbed flow shows initiation of transition to turbulence, but remains transitional at the end of the plate.

Yan, Hong; Gaitonde, Datta

2010-06-01

434

Large-eddy simulation of shock-wave/turbulent-boundary-layer interaction  

NASA Astrophysics Data System (ADS)

Well-resolved large-eddy simulations (LES) are performed in order to investigate flow phenomena and turbulence structure of the boundary layer along a supersonic compression ramp. The numerical simulations directly reproduce an available experimental result. The compression ramp has a deflection angle of beta {=} 25(°) . The mean free-stream Mach number is M_infty {=} 2.95. The Reynolds number based on the incoming boundary-layer thickness is Re_{delta_0} {=} 63 560 in accordance with the reference experiment. These simulations overcome deficiencies of earlier direct numerical simulations (DNS) and LES in terms of ramp-deflection angle, Reynolds number and spanwise size of the computational domain which is required for capturing the essential flow phenomena. The filtered conservation equations for mass, momentum and energy are solved with a high-order finite-difference scheme. The effect of subgrid scales is modelled by the approximate deconvolution model. About 18.5 {×} 10(6) grid points are used for discretizing the computational domain. To obtain mean flow and turbulence structure the flow is sampled 1272 times over 703 characteristic time scales of the incoming boundary layer. Statistical data are computed from these samples. An analysis of the data shows good agreement with the experiment in terms of mean quantities such as shock position, separation and reattachment location, skin-friction and surface-pressure distributions, and turbulence structure. The computational data confirm theoretical and experimental results on fluctuation amplification across the interaction region. In the wake of the main shock a shedding of shocklets is observed. The temporal behaviour of the coupled shock separation system agrees well with experimental data. Unlike previous DNS the present simulation data provide indications of a large-scale shock motion. Also, evidence for the existence of three-dimensional large-scale streamwise structures, commonly referred to as Görtler-like vortices, is found.

Loginov, Maxim S.; Adams, Nikolaus A.; Zheltovodov, Alexander A.

2006-10-01

435

A model of a turbulent boundary layer with a nonzero pressure gradient  

PubMed Central

According to a model of the turbulent boundary layer that we propose, in the absence of external turbulence the intermediate region between the viscous sublayer and the external flow consists of two sharply separated self-similar structures. The velocity distribution in these structures is described by two different scaling laws. The mean velocity u in the region adjacent to the viscous sublayer is described by the previously obtained Reynolds-number-dependent scaling law ? = u/u* = A??, A = 1/\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\setlength{\\oddsidemargin}{-69pt} \\begin{document} \\begin{equation*}\\sqrt{3}\\end{equation*}\\end{document} ln Re? + 5/2, ? = 3/2 In Re?,?? = u*y/?. (Here u* is the dynamic or friction velocity, y is the distance from the wall, ? the kinematic viscosity of the fluid, and the Reynolds number Re? is well defined by the data.) In the region adjacent to the external flow, the scaling law is different: ? = B??. The power ? for zero-pressure-gradient boundary layers was found by processing various experimental data and is close (with some scatter) to 0.2. We show here that for nonzero-pressure-gradient boundary layers, the power ? is larger than 0.2 in the case of an adverse pressure gradient and less than 0.2 for a favorable pressure gradient. Similarity analysis suggests that both the coefficient B and the power ? depend on Re? and on a new dimensionless parameter P proportional to the pressure gradient. Recent experimental data of Perry, Maruši?, and Jones were analyzed, and the results are in agreement with the model we propose.

Barenblatt, G. I.; Chorin, A. J.; Prostokishin, V. M.

2002-01-01

436

Probing high-Reynolds-number effects in numerical boundary layers  

NASA Astrophysics Data System (ADS)

We study the high-Reynolds-number behavior of a turbulent boundary layer in the low supersonic regime through very-large-scale direct numerical simulation (DNS). For the first time a Reynolds number is attained in DNS (Re?=?/?v~4000, where ? is the boundary layer thickness and ?v is the viscous length scale) at which theoretical predictions and experiments suggest the occurrence of phenomena pertaining to the asymptotic Reynolds number regime. From comparison with previous DNS data at lower Reynolds number we find evidence of a continuing trend toward a stronger imprint of the outer-layer structures onto the near-wall region. This effect is clearly manifested both in flow visualizations, and in energy spectra. More than a decade of nearly-logarithmic variation is observed in the mean velocity profiles, with log-law constants k ~ 0.394, C ~ 4.84, and a trend similar to experiments. We find some supporting evidence for the debated existence of a k-1 region in the power spectrum of streamwise velocity fluctuations, which extends up to y+ ~ 150, and of a k-5/3 spectral range in the outer layer.

Pirozzoli, Sergio; Bernardini, Matteo

2013-02-01

437

The Benthic Boundary Layer: Transport Processes and Biogeochemistry  

NASA Astrophysics Data System (ADS)

Interdisciplinary research is certainly one of the current buzzwords that needs to be incorporated in virtually every grant proposal. The idea that integration of different scientific fields is a prerequisite for progress in Earth sciences is now well recognized. The benthic boundary layer (BBL) is one area of research in which physicists, chemists, biologists, geologists, and engineers have worked in close and fruitful cooperation for several decades. The BBL comprises the near-bottom layer of water, the sediment-water interface, and the top layer of sediment that is directly influenced by the overlying water. In 1974, a BBL conference in France resulted in a book titled The Benthic Boundary Layer edited by I.N. McCave. This publication contained contributions from scientists from a wide range of disciplines and gave an overview of the state-of-the-art of BBL research. However, science has moved on in the past 25 years. Significant conceptual and technological progress has been made, and it is definitely time for an update.

van Duren, Luca A.; Middelburg, Jack J.

438

Transpiration and Film Cooling Boundary Layer Computer Program. Volume 2 Computer Program and Users Manual.  

National Technical Information Service (NTIS)

A finite difference turbulent boundary layer computer program which allows for mass transfer wall cooling and equilibrium chemistry effects is presented. The program is capable of calculating laminar or turbulent boundary layer solutions for an arbitrary ...

R. J. Gloss

1971-01-01

439

Calculation of Hypersonic Non-Equilibrium Viscous Flow Using Second Order Boundary Layer Theory.  

National Technical Information Service (NTIS)

A method is presented for the calculation of hypersonic boundary layers in chemical nonequilibrium. The second order boundary layer equations are solved using a finite difference space marching method. The flow and the chemistry are solved simultaneously ...

C. Mundt F. Monnoyer

1990-01-01

440

An Experimental and Analytical Study of Boundary Layers in Highly Turbulent Freestreams.  

National Technical Information Service (NTIS)

Experimental research has been conducted to determine the influence of free-stream turbulence on zero pressure gradient, incompressible, fully turbulent boundary layer flow. During this period convective heat transfer coefficients, boundary layer mean vel...

M. F. Blair

1979-01-01

441

The Influence of Free-Stream Turbulence on the Zero Pressure Gradient Fully Turbulent Boundary Layer.  

National Technical Information Service (NTIS)

Experimental research has been conducted to determine the influence of free-stream turbulence on zero pressure gradient fully turbulent boundary layer flow. During this period convective heat transfer coefficients, boundary layer mean velocity and tempera...

M. F. Blair M. J. Werle

1980-01-01

442

DYNAMIC PLANE-STRAIN SHEAR RUPTURE WITH A SLIP-WEAKENING FRICTION LAW CALCULATED BY A BOUNDARY INTEGRAL METHOD.  

USGS Publications Warehouse

A numerical boundary integral method, relating slip and traction on a plane in an elastic medium by convolution with a discretized Green function, can be linked to a slip-dependent friction law on the fault plane. Such a method is developed here in two-dimensional plane-strain geometry. Spontaneous plane-strain shear ruptures can make a transition from sub-Rayleigh to near-P propagation velocity. Results from the boundary integral method agree with earlier results from a finite difference method on the location of this transition in parameter space. The methods differ in their prediction of rupture velocity following the transition. The trailing edge of the cohesive zone propagates at the P-wave velocity after the transition in the boundary integral calculations. Refs.

Andrews, D. J.

1985-01-01

443

Non-Darcy effects on convective boundary layer flow past a semi-infinite vertical plate in saturated porous media  

Microsoft Academic Search

The composite effects of viscosity, porosity, buoyancy parameter, thermal conductivity ratio and non-Darcy effects of Brinkman\\u000a friction and Forscheimmer quadratic drag on the mixed convection boundary layer flow past a semi-infinite plate in a fully-saturated\\u000a porous regime are theoretically and numerically investigated using Keller’s implicit finite-difference technique and a double-shooting\\u000a Runge-Kutta method. The Brinkman Forcheimer-extended Darcy model is implemented in

H. S. Takhar; O. A. Beg

1996-01-01

444

Slip Effects on an Unsteady Boundary Layer Stagnation-Point Flow and Heat Transfer towards a Stretching Sheet  

NASA Astrophysics Data System (ADS)

An analysis is presented for an unsteady boundary layer stagnation-point flow of a Newtonian fluid and the heat transfer towards a stretching sheet taking non-conventional partial slip conditions at the sheet. The self-similar equations are obtained using similarity transformations and solved numerically by the shooting method. Effects of the parameters involved in the equations, especially velocity slip and thermal slip parameters on the velocity and temperature profiles, are analyzed extensively. It is revealed that due to the velocity and thermal slip parameters, the rate of heat transfer from the sheet and the wall skin friction change significantly.

Krishnendu, Bhattacharyya; Swati, Mukhopadhyay; C. Layek, G.

2011-09-01

445

Boundary layer effects on lee wave resonance in the semi-T-REX environment  

NASA Astrophysics Data System (ADS)

At main focus of the Terrain-induced Rotor Experiment (T-REX) is a tight coupling of mountain-waves and boundary layer displayed in the formation of atmospheric rotors. In our earlier study of lee wave resonances over double bell-shaped obstacles, in which the profiles of wind speed and stability as well as terrain were derived from the T-REX observations of lee wave events over the Sierra Nevada and Inyo Mountains, we focused on the mountain-wave part of the problem, examining the interaction of trapped lee waves with downstream orography in absence of friction. It was shown that the displayed variation of wave amplitude and drag with the ridge separation distance can be attributed to nonlinear wave resonance. In this study, effects of the frictional boundary layer on lee wave resonance are investigated by means of high-resolution 2D numerical simulations with the NRL COAMPS model, using the same idealized representation of the T-REX environment from our earlier study. Due to complex nonlinear interactions between the lee waves and boundary layer, new results show a different resonance pattern, which no longer displays itself in wave amplitudes or drag but rather in the amplitude ratio and steady-state averaged reversed flow strength underneath the primary lee-wave crest. The total destructive interference, in which waves completely cancel out in the lee of the downstream peak, is also observed for certain ridge separation distances when the height of the downstream obstacle is 2/3 the height of the upstream one. The flow within the valley exhibits a highly complex structure. Regions of reversed flow form at the surface underneath the lee wave crests and are lifted above the ground, extending up to 2 km AGL for 3 km high obstacles. The flow within a rotor is unsteady with multiple sub-vortices. The location of the inversion relative to the mountaintop is shown to significantly affect the resonance pattern, reversed flow strength, and the wave steadiness.

Stiperski, I.; Grubiši?, V.

2009-04-01

446

Integrating complementarity into the 2D displacement discontinuity boundary element method to model faults and fractures with frictional contact properties  

NASA Astrophysics Data System (ADS)

We present a two-dimensional displacement discontinuity method (DDM) in combination with a complementarity solver to simulate quasi-static slip on cracks as models for faults and fractures in an otherwise homogeneous, isotropic, linear elastic material. A complementarity algorithm enforces appropriate contact boundary conditions along the cracks so that variable friction and frictional strength can be included. This method accurately computes slip and opening distributions along the cracks, displacement and stress fields within the surrounding material, and stress intensity factors at the crack tips. The DDM with complementarity is a simple yet powerful tool to investigate many aspects of the mechanical behavior of faults and fractures in Earth's brittle crust. Implementation in Excel and Matlab enables easy saving, organization, and sharing.

Ritz, Elizabeth; Mutlu, Ovunc; Pollard, David D.

2012-08-01

447

Bifurcation of the cusp: Implications for understanding boundary layers  

NASA Astrophysics Data System (ADS)

Event analyses and magnetohydrodynamic (MHD) modeling provide complementary insights into solar-wind/magnetosphere-ionosphere coupling when the interplanetary magnetic field (IMF) has a stronger Y than Z component. The sources for convection and particle precipitation within the cusp become spatially bifurcated. Incoming surfaces of constant phase in the interplanetary electric field (IEF) can be tilted with respect to the Sun-Earth line. This forces the two hemispheres to respond to the same elements of the solar wind stream at significantly different times. We consider a case in which ground and rocket measurements indicate that IEF phase planes interacted first with the magnetopause in the Southern Hemisphere at lag times significantly less than the simple adjection time between an L1 monitor and Earth. Magnetic merging on the Northern Hemisphere magnetopause occurred later. The timing differences are related to the phase-plane tilts and the strong IMF BX. Auroral emissions created by electrons injected from the Southern Hemisphere merging line can appear in close proximity to those from Northern Hemisphere sites, within an all-sky imager's field-of-view. Bifurcation is driven by IMF BY, while BX controls differences in the timing of interactions with the two hemispheres. Detailed harmonization of auroral features with interplanetary drivers strongly supports the utility of the antiparallel merging criterion for estimating when and where the IMF-magnetosphere interactions occur. We compare empirical results with MHD simulations to help constrain interpretations of magnetospheric boundary layers. Merging at high latitudes creates layers of open field lines that drape over the dayside magnetosphere to form an open boundary layer. MHD modeling suggests that open boundary layers may become quite thick along the magnetospheric flank equatorward of the sash. Simulations and the empirical results indicate that merging in the conjugate hemisphere drives the smaller ionospheric convection cell.

Maynard, N. C.; Burke, W. J.; Moen, J.; Sandholt, P. E.; Lester, M.; Ober, D. M.; Weimer, D. R.; White, W. E.

448

An Experimental Study on the Effect of Machining-Induced White Layer on Frictional and Wear Performance at Dry and Lubricated Sliding Contact  

Microsoft Academic Search

The frictional and wear performance of a machined component depends strongly on the surface properties. A white layer on the machining surface is often produced at abusive machining conditions. However, the effect of white layer on frictional and wear performance has received little attention. Dry and lubricated sliding contact tests for white layer surfaces by turning and grinding were carried

Y. B. Guo; R. A. Waikar

2009-01-01

449

Coupling the dynamics of boundary layers and evolutionary dunes.  

PubMed

A theoretical formulation and corresponding numerical solutions are presented for fluid flow and sediment transport past evolutionary sand dunes. Time-dependent curvilinear coordinates are employed to fully couple flow aloft with the developing landform. The differential conservation law that defines shape of the lower boundary depends on details of local surface stress, thereby favoring the large eddy simulation of the boundary layer. To shrink the gap between the time scales characteristic of planetary boundary layer flows O(10(3)) s and sand dune evolution O(10(6)) s, a hypothetical "severe-wind scenario" is adopted with the saltation flux amplified up to 3 orders of magnitude. While the results are largely insensitive to the rescaling, the efficacy of computations is greatly improved. The flux-form partial differential equation for the interface profile--via saltation and sand avalanches--is formulated as an advection-diffusion equation, to facilitate discrete integrations. Numerical experiments verify the adopted theoretical framework by reproducing scaling results reported in the literature. The versatility of the approach is illustrated with evolution of a sandhole--an example of application likely never addressed in the literature, yet realizable in nature. PMID:19518224

Ortiz, Pablo; Smolarkiewicz, Piotr K

2009-04-20

450

A numerical calculation of three dimensional incompressible laminar, transition and turbulent boundary layers  

Microsoft Academic Search

A method is presented for calculating the 3D boundary layers in incompressible viscous flow, including a differential method for laminar boundary layers, an integral method for turbulent boundary layers, and a numerical solution method for the Orr-Sommerfeld equation in which transition occurs only if the amplification of the disturbance reaches a certain level. These methods are used to calculate a

Jiaxiang Yan; Bin Yu

1991-01-01