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1

Turbulence and skin friction evolutions in an oscillating boundary layer  

Microsoft Academic Search

The paper presents and analyzes experimental results on an oscillating flat plate turbulent boundary layer. Emphasis is placed on quantities, which characterize the evolution of turbulence and of the skin friction. These results have been obtained in a rather large range of reduced frequencies with (omega nu)\\/(U-tau\\/bar\\/)-squared in the interval from 0.0035 to 0.013 and (omega XO)\\/(U-e\\/bar\\/) in the interval

J. Cousteix; R. Houdeville

1985-01-01

2

Measurements of local skin friction in a microbubble-modified turbulent boundary layer  

Microsoft Academic Search

The present local skin friction reduction measurements by means of an array of flush-mounted hot film probes in a microbubble-modified, zero-pressure gradient turbulent boundary layer have both qualitatively and quantitatively confirmed earlier integrated skin friction measurements that indicated the reduction to be a function of plate orientation, gas flow rate, and freestream velocity. While skin friction is reduced monotonically for

N. K. Madavan; S. Deutsch; C. L. Merkle

1985-01-01

3

Friction measurement in zero and adverse pressure gradient boundary layer using oil droplet interferometric method  

NASA Astrophysics Data System (ADS)

The oil droplet interferometric technique has been used to investigate the skin friction distribution along a zero and adverse pressure gradient boundary layer developing in the Laboratoire de Mécanique de Lille wind tunnel. This experimental task was a part of the WALLTURB project, funded by the European Community, in order to bring significant progress in the understanding of near wall turbulence in boundary layers. Skin friction values close to 0.01 Pa have been measured with this optical method. A comparison with the results obtained with hot-wire anemometry and macro-PIV demonstrates the great potential of the oil droplet technique.

Pailhas, Guy; Barricau, P.; Touvet, Y.; Perret, L.

2009-08-01

4

Bubble friction drag reduction in a high-Reynolds-number flat-plate turbulent boundary layer  

Microsoft Academic Search

Turbulent boundary layer skin friction in liquid flows may be reduced when bubbles are present near the surface on which the boundary layer forms. Prior experimental studies of this phenomenon reached downstream-distance-based Reynolds numbers (Re_{x}) of several million, but potential applications may occur at Re_{x} orders of magnitude higher. This paper presents results for Re_{x} as high as 210 million

Wendy C. Sanders; Eric S. Winkel; David R. Dowling; Marc Perlin; Steven L. Ceccio

2006-01-01

5

Effects of Riblets on Skin Friction in High-Speed Turbulent Boundary Layers  

NASA Technical Reports Server (NTRS)

Direct numerical simulations of spatially developing turbulent boundary layers over riblets are conducted to examine the effects of riblets on skin friction at supersonic speeds. Zero-pressure gradient boundary layers with an adiabatic wall, a Mach number of M1 = 2.5, and a Reynolds number based on momentum thickness of Re = 1720 are considered. Simulations are conducted for boundary-layer flows over a clean surface and symmetric V- groove riblets with nominal spacings of 20 and 40 wall units. The DNS results confirm the few existing experimental observations and show that a drag reduction of approximately 7% is achieved for riblets with proper spacing. The influence of riblets on turbulence statistics is analyzed in detail with an emphasis on identifying the differences, if any, between the drag reduction mechanisms for incompressible and high-speed boundary layers.

Duan, Lian; Choudhari, Meelan M.

2012-01-01

6

Introduction to boundary-layer theory. [viscous friction loss calculation for turbine blade design  

NASA Technical Reports Server (NTRS)

The pressure ratio across a turbine provides a certain amount of ideal energy that is available to the turbine for producing work. The portion of the ideal energy that is not converted to work is considered to be a loss. One of the more important and difficult aspects of turbine design is the prediction of the losses. The primary cause of losses is the boundary layer that develops on the blade and end wall surfaces. Boundary-layer theory is used to calculate the parameters needed to estimate viscous (friction) losses.

Mcnally, W. D.

1973-01-01

7

Pressure-gradient effects on hypersonic turbulent skin friction and boundary-layer profiles.  

NASA Technical Reports Server (NTRS)

Local skin friction, total-temperature profiles, and pitot-pressure profiles were measured on the wall of a Mach-7.4 wind tunnel. The wall to adiabatic wall temperature ratio was varied from 0.3 to 0.5. Boundary-layer characteristics were compared with those predicted by a finite-difference method. Local skin friction was predicted to within 15%. Pressure-gradient effects on the temperature and Mach number distributions and the shape factor (displacement thickness/momentum thickness) were underpredicted, but the velocity distributions were closely predicted.

Hopkins, E. J.; Keener, E. R.

1972-01-01

8

Skin-friction measurements in high-enthalpy hypersonic boundary layers  

NASA Astrophysics Data System (ADS)

Skin-friction measurements are reported for high-enthalpy and high-Mach-number laminar, transitional and turbulent boundary layers. The measurements were performed in a free-piston shock tunnel with air-flow Mach number, stagnation enthalpy and Reynolds numbers in the ranges of 4.4 6.7, 3 13 MJ kg(-1) and 0.16× 10(6) 21× 10(6) , respectively. Wall temperatures were near 300 K and this resulted in ratios of wall enthalpy to flow-stagnation enthalpy in the range of 0.1 0.02. The experiments were performed using rectangular ducts. The measurements were accomplished using a new skin-friction gauge that was developed for impulse facility testing. The gauge was an acceleration compensated piezoelectric transducer and had a lowest natural frequency near 40 kHz. Turbulent skin-friction levels were measured to within a typical uncertainty of ± 7%. The systematic uncertainty in measured skin-friction coefficient was high for the tested laminar conditions; however, to within experimental uncertainty, the skin-friction and heat-transfer measurements were in agreement with the laminar theory of van Driest (1952). For predicting turbulent skin-friction coefficient, it was established that, for the range of Mach numbers and Reynolds numbers of the experiments, with cold walls and boundary layers approaching the turbulent equilibrium state, the Spalding & Chi (1964) method was the most suitable of the theories tested. It was also established that if the heat transfer rate to the wall is to be predicted, then the Spalding & Chi (1964) method should be used in conjunction with a Reynolds analogy factor near unity. If more accurate results are required, then an experimentally observed relationship between the Reynolds analogy factor and the skin-friction coefficient may be applied.

Goyne, C. P.; Stalker, R. J.; Paull, A.

2003-06-01

9

On local perturbations of a boundary layer with small skin friction  

Microsoft Academic Search

This paper studies the effect of two-dimensional surface irregularities on the flow in a plane steady boundary layer with\\u000a small skin friction in an incompressible fluid. A detailed analysis is carried out for the flow regime with a given pressure\\u000a gradient determined on the scale of the small irregularity by its shape. It is shown that there is a critical

Vik V. Sychev

1998-01-01

10

Manipulation of turbulent boundary layers by outer-layer devices - Skin-friction and flow-visualization results  

Microsoft Academic Search

The ability of devices introduced into the outer region of a low-Reynolds-number turbulent boundary layer to reduce surface drag is investigated. Direct measurements of local skin friction (using a floating-plate drag balance) and laser-sheet smoke flow visualization results were obtained for the cases of single flat plates, a cylinder with the same drag as one of these, and two plates

A. M. Savill; J. C. Mumford

1988-01-01

11

Skin friction and velocity profile family for compressible turbulent boundary layers  

NASA Technical Reports Server (NTRS)

The paper presents a general approach to constructing mean velocity profiles for compressible turbulent boundary layers with isothermal or adiabatic walls. The theory is based on a density-weighted transformation that allows the extension of the incompressible similarity laws of the wall to the compressible regions. The velocity profile family is compared to a range of experimental data, and excellent agreement is obtained. A self-consistent skin friction law, which satisfies the proposed velocity profile family, is derived and compared with the well-known Van Driest II theory for boundary layers in zero pressure gradient. The results are found to be at least as good as those obtained by using the Van Driest II transformation.

Huang, P. G.; Bradshaw, P.; Coakley, T. J.

1993-01-01

12

Some effects of finite spatial resolution on skin friction measurements in turbulent boundary layers  

NASA Technical Reports Server (NTRS)

The effects of finite spatial resolution often cause serious errors in measurements in turbulent boundary layers, with particularly large effects for measurements of fluctuating skin friction and velocities within the sublayer. However, classical analyses of finite spatial resolution effects have generally not accounted for the substantial inhomogeneity and anisotropy of near-wall turbulence. The present study has made use of results from recent computational simulations of wall-bounded turbulent flows to examine spatial resolution effects for measurements made at a wall using both single-sensor probes and those employing two sensing volumes in a V shape. Results are presented to show the effects of finite spatial resolution on a variety of quantitites deduced from the skin friction field.

Westphal, Russell V.

1988-01-01

13

Friction and Wear Modifiers Using Solvent Partitioning of Hydrophilic Surface-interactive Chemicals Contained in Boundary Layer-targeted Emulsions  

NASA Technical Reports Server (NTRS)

A wear and/or friction reducing additive for a lubricating fluid in which the additive is a combination of a moderately hydrophilic single-phase compound and an anti-wear and/or anti-friction aqueous salt solution. The aqueous salt solution produces a coating on boundary layer surfaces. The lubricating fluid can be an emulsion-free hydrophobic oil, hydraulic fluid, antifreeze, or water. Preferably, the moderately hydrophilic single-phase compound is sulfonated castor oil and the aqueous salt solution additionally contains boric acid and zinc oxide. The emulsions produced by the aqueous salt solutions, the moderately hydrophilic single-phase compounds, or the combination thereof provide targeted boundary layer organizers that significantly enhance the anti-wear and/or anti-friction properties of the base lubricant by decreasing wear and/or friction of sliding and/or rolling surfaces at boundary layers.

Richmond, Robert Chafee (Inventor); Schramm, Jr., Harry F. (Inventor); Defalco, Francis G. (Inventor)

2013-01-01

14

On the manner in which outer layer disturbances affect turbulent boundary layer skin friction  

Microsoft Academic Search

This paper presents results, from bth experimental studies and turbulence modelling, of the manipulated flow behind flat plate devices introduced into the outer region of turbulent boundary layers. An analysis of mean velocity profiles, in conjunction with drag balance and other measurements for u sub tau, reveals only small changes in the log law and little influence of plate thickness

A. M. Savill

1987-01-01

15

Laser interferometer skin-friction measurements of crossing-shock-wave/turbulent-boundary-layer interactions  

NASA Technical Reports Server (NTRS)

Wall shear stress measurements beneath crossing-shock-wave/turbulent boundary-layer interactions have been made for three interactions of different strengths. The interactions are generated by two sharp fins at symetric angles of attack mounted on a flat plate. The shear stress measurements were made for fin angles of 7 and 11 deg at Mach 3 and 15 deg at Mach 3.85. The measurements were made using a laser interferometer skin-friction meter, a device that determines the wall shear by optically measuring the time rate of thinning of an oil film placed on the test model surface. Results of the measurements reveal high skin-friction coefficients in the vicinity of the fin/plate junction and the presence of quasi-two-dimensional flow separation on the interaction center line. Additionally, two Navier-Stokes computations, one using a Baldwin-Lomax turbulence model and one using a k-epsilon model, are compared with the experimental results for the Mach 3.85, 15-deg interaction case. Although the k-epsilon model did a reasonable job of predicting the overall trend in portions of the skin-friction distribution, neither computation fully captured the physics of the near-surface flow in this complex interaction.

Garrison, T. J.; Settles, G. S.; Narayanswami, N.; Knight, D. D.

1994-01-01

16

Reduction of skin-friction in a microbubble-laden spatially developing turbulent boundary layer over a flat plate  

Microsoft Academic Search

Experimental evidence during the past three decades indicates that injection of gaseous microbubbles into a liquid turbulent boundary layer over a flat plate or over axisymmetrical bodies can reduce the skin-friction by as much as 80% from its value without bubble injection. However, the basic physical mechanisms responsible for that reduction are not yet fully understood. The present study is

Antonino Ferrante

2004-01-01

17

Evaluation of the effect of discrete fluid injection on turbulent boundary layer skin friction in the presence of wall roughness  

Microsoft Academic Search

The purpose of the present experimental study was to determine if discrete mass injection into a turbulent boundary layer over a rough wall would effectively smooth the surface and reduce the skin friction drag. Accordingly, a test program was planned and a series of tests were conducted in the Free Surface Water Tunnel at the California Institute of Technology. In

W. W. Haigh; D. D. Mantrom; J. E. Lewis

1976-01-01

18

Reducing skin friction by boundary layer combustion on a generic scramjet model  

Microsoft Academic Search

Combustion of hydrogen in the boundary layer of a simplified scramjet model is investigated. The analytical model for combustion is introduced for a flat plate in a turbulent boundary layer. Investigations are carried out for a fixed scramjet geometry and Mach 10 at 36 km altitude. Fuel is injected in the combustor as well as into the boundary layers of

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

2005-01-01

19

An extension of the transpired skin-friction equation to compressible turbulent boundary layers  

Microsoft Academic Search

W INJECTION or suction of fluid into the turbulent boundary layer has proved over the years to be an effective means of controlling the properties of fluid flowing over a surface. The result of this injection or suction of fluid is to modify the velocity and temperature distributions through the boundary layer so that the drag and the heat transfer

ATILA P. SILVA-FREIRE

20

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

21

Direct measurements and analysis of skin friction and cooling downstream of multiple flush-slot injection into a turbulent Mach 6 boundary layer  

NASA Technical Reports Server (NTRS)

Experiments were conducted to determine the reduction in surface skin friction and the effectiveness of surface cooling downstream of one to four successive flush slots injecting cold air at an angle of 10 deg into a turbulent Mach 6 boundary layer. Data were obtained by direct measurement of surface shear and equilibrium temperatures, respectively. Increasing the number of slots decreased the skin friction, but the incremental improvement in skin-friction reduction decreased as the number of slots was increased. Cooling effectiveness was found to improve, for a given total mass injection, as the number of slots was increased from one to four. Comparison with previously reported step-slot data, however, indicated that step slots with tangential injection are more effective for both reducing skin friction and cooling than the present flush-slot configuration. Finite-difference predictions are in reasonable agreement with skin-friction data and with boundary-layer profile data.

Howard, F. G.; Strokowski, A. J.

1978-01-01

22

Scaling properties of velocity and temperature spectra above the surface friction layer in a convective atmospheric boundary layer  

NASA Astrophysics Data System (ADS)

We report velocity and temperature spectra measured at nine levels from 1.42 meters up to 25.7 m over a smooth playa in Western Utah. Data are from highly convective conditions when the magnitude of the Obukhov length (our proxy for the depth of the surface friction layer) was less than 2 m. Our results are somewhat similar to the results reported from the Minnesota experiment of Kaimal et al. (1976), but show significant differences in detail. Our velocity spectra show no evidence of buoyant production of kinetic energy at at the scale of the thermal structures. We interpret our velocity spectra to be the result of outer eddies interacting with the ground, not "local free convection". We observe that velocity spectra represent the spectral distribution of the kinetic energy of the turbulence, so we use energy scales based on total turbulence energy in the convective boundary layer (CBL) to collapse our spectra. For the horizontal velocity spectra this scale is (zi ?o)2/3, where zi is inversion height and ?o is the dissipation rate in the bulk CBL. This scale functionally replaces the Deardorff convective velocity scale. Vertical motions are blocked by the ground, so the outer eddies most effective in creating vertical motions come from the inertial subrange of the outer turbulence. We deduce that the appropriate scale for the peak region of the vertical velocity spectra is (z ?o)2/3 where z is height above ground. Deviations from perfect spectral collapse under these scalings at large and small wavenumbers are explained in terms of the energy transport and the eddy structures of the flow. We find that the peaks of the temperature spectra collapse when wavenumbers are scaled using (z1/2 zi1/2). That is, the lengths of the thermal structures depend on both the lengths of the transporting eddies, ~9z, and the progressive aggregation of the plumes with height into the larger-scale structures of the CBL. This aggregation depends, in top-down fashion, on zi. The whole system is therefore highly organized, with even the smallest structures conforming to the overall requirements of the whole flow.

McNaughton, K. G.; Clement, R. J.; Moncrieff, J. B.

2007-06-01

23

Boundary-layer transition and global skin friction measurement with an oil-fringe imaging technique  

NASA Technical Reports Server (NTRS)

A new oil-fringe imaging system skin friction (FISF) technique to measure skin friction on wind tunnel models is presented. In the method used to demonstrate the technique, lines of oil are applied on surfaces that connect the intended sets of measurement points, and then a wind tunnel is run so that the oil thins and forms interference fringes that are spaced in proportion to local skin friction. After a run the fringe spacings are imaged with a CCD-array digital camera and measured on a computer. Skin friction and transition measurements on a two-dimensional wing are presented and compared with computational predictions.

Monson, Daryl J.; Mateer, George G.; Menter, Florian R.

1993-01-01

24

Experimental study of skin friction drag reduction on superhydrophobic flat plates in high Reynolds number boundary layer flow  

NASA Astrophysics Data System (ADS)

In this paper, we report the measurement of skin friction drag on superhydrophobic-coated flat plates in high Reynolds (Re) number boundary layer flows, using a high-speed towing tank system. Aluminum flat plates with a large area (4 feet × 2 feet, 3/8 in. thick) and sharpened leading/trailing edges (1 in. long) were prepared as a boundary layer flow model. Spray coating of hydrophobic nanoparticles was applied to make two different types of superhydrophobic coatings: one with low contact angle and high contact angle hysteresis, and the other with high contact angle and low contact angle hysteresis. Skin friction drag of the superhydrophobic plates was measured in the flow speed up to 30 ft/s to cover transition and turbulent flow regimes (105 < ReL < 107), and was compared to that of an uncoated bare aluminum plate. A significant drag reduction was observed on the superhydrophobic plate with high contact angle and low contact angle hysteresis up to ~30% in transition regime (105 < ReL < 106), which is attributed to the shear-reducing air layer entrapped on the superhydrophobic surface. However, in fully turbulence regime (106 < ReL < 107), an increase of drag was observed, which is ascribed to the morphology of the surface air layer and its depletion by high shear flow. The texture of superhydrophobic coatings led to form a rugged morphology of the entrapped air layer, which would behave like microscale roughness to the liquid flow and offset the drag-reducing effects in the turbulent flow. Moreover, when the superhydrophobic coating became wet due to the removal of air by high shear at the boundary, it would amplify the surface roughness of solid wall and increase the drag in the turbulent flow. The results illustrate that drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the effective drag reduction using superhydrophobic surfaces, especially in high Re number turbulent flow regimes.

Aljallis, Elias; Sarshar, Mohammad Amin; Datla, Raju; Sikka, Vinod; Jones, Andrew; Choi, Chang-Hwan

2013-02-01

25

Multi-scale friction modeling for manufacturing processes: The boundary layer regime  

Microsoft Academic Search

This paper presents a multi-scale friction model for largescale forming simulations. A friction framework has been developed including the effect of surface changes due to normal loading and straining the underlying bulk material. A fast and\\u000aefficient translation from micro to macro modeling, based on stochastic methods, is incorporated to reduce the computational effort. Adhesion and ploughing effects have been

J. Hol; D. K. Karupannasamy; V. T. Meinders; Hitomi Yamaguchi; Laine Mears; Miguel Angel Selles Canto; Masahiko Yoshino; Shreyes Melkote

2012-01-01

26

High-Reynolds-number turbulent-boundary-layer wall pressure fluctuations with skin-friction reduction by air injection.  

PubMed

The hydrodynamic pressure fluctuations that occur on the solid surface beneath a turbulent boundary layer are a common source of flow noise. This paper reports multipoint surface pressure fluctuation measurements in water beneath a high-Reynolds-number turbulent boundary layer with wall injection of air to reduce skin-friction drag. The experiments were conducted in the U.S. Navy's Large Cavitation Channel on a 12.9-m-long, 3.05-m-wide hydrodynamically smooth flat plate at freestream speeds up to 20 ms and downstream-distance-based Reynolds numbers exceeding 200 x 10(6). Air was injected from one of two spanwise slots through flush-mounted porous stainless steel frits (approximately 40 microm mean pore diameter) at volume flow rates from 17.8 to 142.5 l/s per meter span. The two injectors were located 1.32 and 9.78 m from the model's leading edge and spanned the center 87% of the test model. Surface pressure measurements were made with 16 flush-mounted transducers in an "L-shaped" array located 10.7 m from the plate's leading edge. When compared to no-injection conditions, the observed wall-pressure variance was reduced by as much as 87% with air injection. In addition, air injection altered the inferred convection speed of pressure fluctuation sources and the streamwise coherence of pressure fluctuations. PMID:18529171

Winkel, Eric S; Elbing, Brian R; Ceccio, Steven L; Perlin, Marc; Dowling, David R

2008-05-01

27

Direct measurements and analysis of skin friction and cooling downstream of multiple flush-slot injection into a turbulent Mach 6 boundary layer  

Microsoft Academic Search

Experiments were conducted to determine the reduction in surface skin friction and the effectiveness of surface cooling downstream of one to four successive flush slots injecting cold air at an angle of 10 deg into a turbulent Mach 6 boundary layer. Data were obtained by direct measurement of surface shear and equilibrium temperatures, respectively. Increasing the number of slots decreased

F. G. Howard; A. J. Strokowski

1978-01-01

28

Skin-friction drag reduction in laminar and turbulent boundary layers  

Microsoft Academic Search

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

Sung Moon Kang

2006-01-01

29

A new method of calculating the boundary layer characteristics downstream of manipulators. Part 2: Skin friction and net drag reduction  

Microsoft Academic Search

A simple wake model which described the qualitative and quantitative skin friction reducing potential of manipulators was developed. The device drag may be ascertained with a reasonable degree of certainty for flat plate manipulators. Consideration of the average friction drag shows that the optimum device height is 0.6 delta. From net drag considerations, this optimum height is found to be

P. E. Roach

1987-01-01

30

Heat Transfer Through Turbulent Friction Layers  

NASA Technical Reports Server (NTRS)

The "general Prandtl number" Pr(exp 1) - A(sub q)/A Pr, aside from the Reynolds number determines the ratio of turbulent to molecular heat transfer, and the temperature distribution in turbulent friction layers. A(sub q) = exchange coefficient for heat; A = exchange coefficient for momentum transfer. A formula is derived from the equation defining the general Prandtl number which describes the temperature as a function of the velocity. For fully developed thermal boundary layers all questions relating to heat transfer to and from incompressible fluids can be treated in a simple manner if the ratio of the turbulent shear stress to the total stress T(sub t)/T in the layers near the wall is known, and if the A(sub q)/A can be regarded as independent of the distance from the wall. The velocity distribution across a flat smooth channel and deep into the laminar sublayer was measured for isothermal flow to establish the shear stress ratio T(sub t)/T and to extend the universal wall friction law. The values of T(sub t)/T which resulted from these measurements can be approximately represented by a linear function of the velocity in the laminar-turbulent transition zone. The effect of the temperature relationship of the material values on the flow near the wall is briefly analyzed. It was found that the velocity at the laminar boundary (in contrast to the thickness of the laminar layer) is approximately independent of the temperature distribution. The temperature gradient at the wall and the distribution of temperature and heat flow in the turbulent friction layers were calculated on the basis of the data under two equations. The derived formulas and the figures reveal the effects of the Prandtl number, the Reynolds number, the exchange quantities and the temperature relationship of the material values.

Reichardt, H.

1943-01-01

31

The Impact of Surface Friction on Boundary Layer Separation for Different Mountain Flow Regimes: An Analysis Based on Large-Eddy-Simulations  

NASA Astrophysics Data System (ADS)

Boundary layer separation (BLS) may occur when a strong external adverse pressure gradient force is imposed on the boundary layer flow, leading to detachment of streamlines from the surface due to a strong deceleration of the flow within the boundary layer. This process commonly occurs at the salient edge of very sharp obstacles. In stably stratified flows pressure perturbations strong enough to cause BLS can also be induced by internal gravity waves. A well-known phenomenon related to wave-induced BLS is that of atmospheric rotors that form on the lee side of mountain ranges. Rotors are boundary-layer zones characterized by strong turbulence, surface wind reversals, large values of spanwise vorticity and neutral stability. Due to the high intensity of turbulence, atmospheric rotors are known to pose a hazard for general aviation and road traffic and can significantly impact the energy yield of wind parks in mountainous terrain. Hence, the onset of BLS and formation of rotors have been extensively investigated in recent years. However, only a few systematic studies of the processes involved in the formation of rotors are available in the literature. In this study, the CM1 model is used to explore the impact of different mountain flow regimes (from hydrostatic to non-hydrostatic and from weakly to strongly non-linear) and the surface exchange coefficient for momentum on the size and strength of rotors. In addition to that, a feedback mechanism of BLS onto the larger-scale flow is investigated. The results show that the governing flow regime has a strong impact on the strength and size of atmospheric rotors, whereas friction mainly influences the rotor interior structure. The most intense rotors, as measured by the strength of surface reversed flow, are found to occur in strongly non-linear and non-hydrostatic flows. The largest rotors instead do not necessarily occur in the strongest non-linear flow regime, in which the largest amplitudes of mountain waves are to be expected; rather we find those in moderately non-linear flows. A possible explanation for that is that for flows in the strongest non-linear regime, the BLS is associated with low-level mountain-wave breaking which leads to the formation of a lee wave train underneath a self-induced critical level and an attendant train of smaller rotors underneath the lee wave crests. It appears also that the trapped flow over this train of rotors excites an upward propagating gravity wave (similar to a terrain generated internal gravity wave) which partially passes through the critical level and changes the structure of the larger-scale flow above. The sensitivity tests for the strength of surface friction show that with increasing surface drag the rotor interior structure changes from a single coherent horizontal vortex to a more complex interior filled with sub-vortices that impact the overall shape of the rotor.

Sachsperger, Johannes; Serafin, Stefano; Grubiši?, Vanda

2014-05-01

32

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

33

Simple Lagrangian formulation of bubbly flow in a turbulent boundary layer (bubbly boundary layer flow)  

Microsoft Academic Search

For the theoretical consideration of a system for reducing skin friction, a mathematical model was derived to represent, in a two-phase field, the effect on skin friction of the injection of micro air bubbles into the turbulent boundary layer of a liquid stream. Based on the Lagrangian method, the equation of motion governing a single bubble was derived. The random

Yuki Yoshida; Yoshiaki Takahashi; Hiroharu Kato; Akira Masuko; Osamu Watanabe

1997-01-01

34

Boundary Layer Theory. Part 1; Laminar Flows  

NASA Technical Reports Server (NTRS)

The purpose of this presentation is to give you a survey of a field of aerodynamics which has for a number of years been attracting an ever growing interest. The subject is the theory of flows with friction, and, within that field, particularly the theory of friction layers, or boundary layers. As you know, a great many considerations of aerodynamics are based on the so-called ideal fluid, that is, the frictionless incompressible fluid. By neglect of compressibility and friction the extensive mathematical theory of the ideal fluid (potential theory) has been made possible.

Schlichting, H.

1949-01-01

35

Smooth attractors of finite dimension for von Karman evolutions with nonlinear frictional damping localized in a boundary layer  

NASA Astrophysics Data System (ADS)

In this paper dynamic von Karman equations with localized interior damping supported in a boundary collar are considered. Hadamard well-posedness for von Karman plates with various types of nonlinear damping are well known, and the long-time behavior of nonlinear plates has been a topic of recent interest. Since the von Karman plate system is of "hyperbolic type" with critical nonlinearity (noncompact with respect to the phase space), this latter topic is particularly challenging in the case of geometrically constrained, nonlinear damping. In this paper we first show the existence of a compact global attractor for finite energy solutions, and we then prove that the attractor is both smooth and finite dimensional. Thus, the hyperbolic-like flow is stabilized asymptotically to a smooth and finite dimensional set.

Geredeli, Pelin G.; Lasiecka, Irena; Webster, Justin T.

36

Separated laminar boundary layers  

NASA Technical Reports Server (NTRS)

Classical boundary layer theory is inadequate to deal with the problem of flow separation owing to its underlying assumption that the boundary layer has an insignificant effect on the external stream. This difficulty is resolved by a theory which includes interaction with the external flow. This theory is described from the viewpoint of the asymptotic triple deck structure. Several triple deck studies are reviewed with emphasis on results of interest in aeronautical applications.

Burggraf, O. R.

1976-01-01

37

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

38

The Martian Surface Boundary Layer  

NASA Technical Reports Server (NTRS)

The acquisition of meteorological data from the surface of Mars by the two Viking Landers and Mars Pathfinder make it possible to estimate atmospheric boundary layer parameters and surface properties at three different locations on the planet. Because the Martian atmosphere is so thin the majority of the solar radiance is converted to heat at the surface. The difference between surface and atmospheric temperature can also constraint surface albedo, thermal inertia, and infrared emissivity. The Mars Pathfinder Atmospheric Structure Instrument/Meteorological package (ASI/MET) was the most capable weather monitoring system ever sent to the surface of another planet to date. One of the prime objectives of the ASI/MET package is to characterize the surface boundary layer parameters, particularly the heat and momentum fluxes, scaling temperature and friction velocity, and estimate surface roughness. Other important boundary layer parameters, such as Richardson Number, Monin-Obukhov length, analysis of turbulence characteristics of wind and temperature, and atmospheric stability class can also be determined from these measurements.

Wilson, G. R.; Joshi, M.

1999-01-01

39

Boundary layer simulator improvement  

NASA Technical Reports Server (NTRS)

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 potential applications in the design of the future Orbit Transfer Vehicle engines. The turbulence model is validated to include the effects of wall roughness and a way is devised to treat multiple smooth-rough surfaces. A prediction of relaminarization regions is examined as is the combined effects of wall cooling and surface roughness on relaminarization. A turbulence model to represent the effects of constant condensed phase loading is given. A procedure is described for thrust decrement calculation in thick boundary layers by coupling the T-D Kinetics Program and BLIMPJ and a way is provided for thrust loss optimization. Potential experimental studies in rocket nozzles are identified along with the required instrumentation to provide accurate measurements in support of the presented new analytical models.

Praharaj, Sarat C.; Schmitz, Craig P.; Nouri, Joseph A.

1989-01-01

40

Friction layers and friction films on PMC brake pads  

Microsoft Academic Search

A rather new tool, the focused ion beam (FIB) technique, was used to characterise superficial layers at micro-contact areas of a commercial brake pad. The friction material was a polymer matrix composite (PMC) with approximately 50% metal content (semi-metallic) and the counter part was a cast iron rotor. Though the contact areas were not visible as topographic features, they could

W. Österle; I. Urban

2004-01-01

41

Boundary layer simulator improvement  

NASA Technical Reports Server (NTRS)

High chamber pressure expander cycles proposed for orbit transfer vehicles depend primarily on the heat energy transmitted from the combustion products through the thrust wall chamber wall. The heat transfer to the nozzle wall is affected by such variables as wall roughness, relamarization, and the presence of particles in the flow. Motor performance loss for these nozzles with thick boundary layers is inaccurate using the existing procedure coded BLIMPJ. Modifications and innovations to the code are examined. Updated routines are listed.

Praharaj, S. C.; Schmitz, C.; Frost, C.; Engel, C. D.; Fuller, C. E.; Bender, R. L.; Pond, J.

1984-01-01

42

The Atmospheric Boundary Layer  

NASA Astrophysics Data System (ADS)

A comprehensive and lucid account of the physics and dynamics of the lowest one to two kilometers of the Earth's atmosphere in direct contact with the Earth's surface, known as the atmospheric boundary layer (ABL). Dr. Garratt emphasizes the application of the ABL problems to numerical modeling of the climate, which makes this book unique among recent texts on the subject. He begins with a brief introduction to the ABL before leading to the development of mean and turbulence equations and the many scaling laws and theories that are the cornerstone of any serious ABL treatment. Modeling of the ABL is crucially dependent for its realism on the surface boundary conditions, so chapters four and five deal with aerodynamic and energy considerations, with attention given to both dry and wet land surfaces and the sea. The author next treats the structure of the clear-sky, thermally stratified ABL, including the convective and stable cases over homogeneous land, the marine ABL, and the internal boundary layer at the coastline. Chapter seven then extends this discussion to the cloudy ABL. This is particularly relevant to current research because the extensive stratocumulus regions over the subtropical oceans and stratus regions over the Arctic have been identified as key players in the climate system. In the final chapters, Dr. Garratt summarizes the book's material by discussing appropriate ABL and surface parameterization schemes in general circulation models of the atmosphere that are being used for climate stimulation.

Garratt, J. R.

1994-05-01

43

Boundary layer stability calculations  

NASA Technical Reports Server (NTRS)

In this paper numerical calculation of the spatial stability of disturbances in the parallel and nonparallel Blasius boundary layers is considered. Chebyshev polynomials are used for discretization. The problem with the boundary condition at infinity is overcome, and the resulting nonlinear matrix eigenvalue problem is attacked directly. The secondary eigenvalue problem for three-dimensional disturbances is shown to be uniformly stable, and particular solutions of this problem generated by the Orr-Sommerfeld equation are shown. A numerical solution of the nonparallel problem is considered using Chebyshev polynomials. The matrix equations are analyzed directly and the problem of uniqueness of the nonparallel correction is settled by careful application of the Fredholm alternative. Nonparallel corrections to the streamwise eigenfunction are shown.

Bridges, Thomas J.; Morris, Philip J.

1987-01-01

44

Boundary layer transition studies  

NASA Technical Reports Server (NTRS)

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. The technique was enhanced by using up to nine multiple probes to reduce the experimental run-time. In both cases, 3D contour surfaces in the vicinity of the hole show highly 3D TS waves which fan out in the spanwise direction forming bow-shaped waves downstream. The case without suction has proved useful for evaluating calculation methods. With suction, the perturbations on the centerline are much stronger and decay less rapidly, while the TS waves in the far field are similar to the case without suction. Downstream, the contour surfaces of the TS waves develop spanwise irregularities which eventually form into clumps. The spanwise clumping is evidence of a secondary instability that could be associated with suction vortices. Designers of porous surfaces use Goldsmith's Criterion to minimize cross-stream interactions. It is shown that partial TS wave cancellation is possible, depending on the hole spacing, disturbance frequency and free-stream velocity. New high-performance Constant Temperature Hot-Wire Anemometers were designed and built, based on a linear system theory analysis that can be extended to arbitrary order. The motivation was to achieve the highest possible frequency reponse while ensuring overall system stability. The performance is equal to or superior to commercially available instruments at about 10% of the cost. Details, such as fabrication drawings and a parts list, have been published to enable the instrument to be construced by others.

Watmuff, Jonathan H.

1995-01-01

45

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

46

The Boundary Layer Radiometer  

NASA Astrophysics Data System (ADS)

The Boundary Layer Radiometer is a small, low mass (<1kg) radiometer with only a single moving part - a scan/calibration mirror. The instrument consists of a three mirror telescope system incorporating an intermediate focus for use with miniature infrared and visible filters. It also has an integrated low power blackbody calibration target to provide long-term calibration stability The instrument may be used as an upward looking boundary layer radiometer for both the terrestrial and Martian atmospheres with appropriate filters for the mid-infrared carbon dioxide band, as well as a visible channel for the detection of aerosol components such as dust. The scan mirror may be used to step through different positions from the local horizon to the zenith, allowing the vertical temperature profile of the atmosphere to be retrieved. The radiometer uses miniature infrared filter assemblies developed for previous space-based instruments by Oxford, Cardiff and Reading Universities. The intermediate focus allows for the use of upstream blocking filters and baffles, which not only simplifies the design of the filters and focal plane assembly, but also reduces the risk of problems due to stray light. Combined with the calibration target this means it has significant advantages over previous generations of small radiometers.

Irshad, Ranah; Bowles, N. E.; Calcutt, S. B.; Hurley, J.

2010-10-01

47

Boundary-Layer & health  

NASA Astrophysics Data System (ADS)

It has long been known that specific atmospheric processes, such as weather and longer-term climatic fluctuations, affect human health. The biometeorological literature refers to this relationship as meteorotropism, defined as a change in an organism that is correlated with a change in atmospheric conditions. Plenty of (patho)physiological functions are affected by those conditions - like the respiratory diseases - and currently it is difficult to put any limits for pathologies developed in reply. Nowadays the importance of atmospheric boundary layer and health is increasingly recognised. A number of epidemiologic studies have reported associations between ambient concentrations of air pollution, specifically particulate pollution, and adverse health effects, even at the relatively low concentrations of pollution found. Since 1995 there have been over twenty-one studies from four continents that have explicitly examined the association between ambient air pollutant mixes and daily mortality. Statistically significant and positive associations have been reported in data from various locations around the world, all with varying air pollutant concentrations, weather conditions, population characteristics and public health policies. Particular role has been given to atmospheric boundary layer processes, the impact of which for specific patient-cohort is, however, not well understood till now. Assessing and monitoring air quality are thus fundamental to improve Europe's welfare. One of current projects run by the "European Medical Association" - PASODOBLE will develop and demonstrate user-driven downstream information services for the regional and local air quality sectors by combining space-based and in-situ data with models in 4 thematic service lines: - Health community support for hospitals, pharmacies, doctors and people at risk - Public information for regions, cities, tourist industry and sporting event organizers - Compliance monitoring support on particulate matter for regional environmental agencies - Local forecast model evaluation support for local authorities and city bodies. Giving value to the above listed aspects, PASODOBLE objectives are following: - Evolution of existing and development of new sustainable air quality services for Europe on regional and local scales - Development and testing of a generic service framework for coordinated input data acquisition and customizable user-friendly access to services - Utilization of multiple cycles of delivery, use and assessment versus requirements and market planning in cooperation with users - Promotion and harmonisation of best practise tools for air quality communities. Further European multidisciplinary projects should be created to better understand the most prevalent atmospheric factors to be impacted in predictive, preventive and personalised medicine considered as the central concept for future medicine.

Costigliola, V.

2010-09-01

48

Boundary and mixed lubrication friction modeling under forming process conditions  

NASA Astrophysics Data System (ADS)

A multi-scale friction model for large-scale forming simulations is presented. A framework has been developed for the boundary and mixed lubrication regime, including the effect of surface changes due to normal loading, sliding and straining the underlying bulk material. Adhesion and ploughing effects have been accounted for to characterize friction conditions on the micro scale. To account for the lubricant effects special hydrodynamic contact elements have been developed. Pressure degrees of freedom are introduced to capture the pressure values which are computed by a finite element discretization of the 2D averaged Reynolds equations. The boundary friction model and the hydrodynamic friction model have been coupled to cover the boundary and mixed lubrication regime. To prove the numerical efficiency of the multi-scale friction model, finite element simulations have been carried out on a top hat section. The computed local friction coefficients show to be dependent on the punch stroke, punch speed and location in the product, and are far from constant. The location and range of friction coefficient values are in the order of what to expect from practice. The agreement between the numerical results and the experiments for different lubrication types and amount of lubrication is good. The multi-scale friction model proves to be stable, and compared to a Coulomb-based FE simulation, with only a modest increase in computation time.

Meinders, V. T.; Hol, J.; van den Boogaard, A. H.

2013-12-01

49

DNS of Decelerating Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

We conduct Direct Numerical Simulation (DNS) of turbulent plane-channel flow subjected to bulk deceleration and to a uniform strain field consisting of streamwise compression (partial U/partial x < 0) and wall-normal stretching (partial V/partial y > 0). This creates a time-developing flow containing most of the essential physics of spatially developing APG boundary layers, particularly in the outer layer. The logistics are much more favorable than those of a true spatial case, both for the DNS and for the testing of turbulence models. The computations are performed at two Reynolds numbers, with initial Re_? = 180 and 395, and advanced past the point when the deceleration causes the mean skin friction to change sign. This parallel-flow analog of ``separation'' isolates outer-layer features of separated boundary layers that are due to mean strain and vanishing surface stress from those caused by streamline curvature. Files are available to allow comparisons at the level of skin friction, velocity and stress profiles, and Reynolds-stress budgets. Changes in the velocity--pressure-gradient term ?_ij are found to dominate the initial evolution of the flow.

Coleman, G. N.; Kim, J.; Spalart, P. R.

1998-11-01

50

Some characteristics of turbulent boundary layers in rapidly accelerated flows  

NASA Technical Reports Server (NTRS)

An analysis of time-mean-turbulent boundary layer velocity profiles measured in a rapidly accelerating flow suggests that the outer region of the velocity profiles consists of essentially inviscid, rotational flow. The extent of this inviscid outer region was observed in some cases to exceed 90 percent of what is ordinarily thought of as the turbulent boundary layer thickness. On the other hand, the inner frictional region of these velocity profiles appears to have turbulent characteristics similar to those of more conventional turbulent boundary layers. Hence, the outer edge boundary condition for this inner region is more properly the external rotational flow region than the free stream.

Brinich, P. F.; Neumann, H. E.

1971-01-01

51

Vortex boundary-layer interactions  

NASA Technical Reports Server (NTRS)

The interaction of a turbulent boundary layer on a flat plate with a strong artificially generated longitudinal vortex which may or may not actually enter the boundary layer is studied. The vortices are generated by a delta wing suspended ahead of the test plate, so that the configuration is approximately that of a close coupled carnard with zero main-wing sweep and an invisible body. All necessary configuration and parametric checks are completed, and data acquisition and analysis on the first configuration chosen for detailed study, in which the vortex starts to merge with the boundary layer a short distance downstream of the leading edge of the test plate, are nearly complete.

Bradshaw, P.

1985-01-01

52

Flow unsteadiness effects on boundary layers  

NASA Technical Reports Server (NTRS)

The development of boundary layers at high subsonic speeds in the presence of either mass flux fluctuations or acoustic disturbances (the two most important parameters in the unsteadiness environment affecting the aerodynamics of a flight vehicle) was investigated. A high quality database for generating detailed information concerning free-stream flow unsteadiness effects on boundary layer growth and transition in high subsonic and transonic speeds is described. The database will be generated with a two-pronged approach: (1) from a detailed review of existing literature on research and wind tunnel calibration database, and (2) from detailed tests in the Boundary Layer Apparatus for Subsonic and Transonic flow Affected by Noise Environment (BLASTANE). Special instrumentation, including hot wire anemometry, the buried wire gage technique, and laser velocimetry were used to obtain skin friction and turbulent shear stress data along the entire boundary layer for various free stream noise levels, turbulence content, and pressure gradients. This database will be useful for improving the correction methodology of applying wind tunnel test data to flight predictions and will be helpful for making improvements in turbulence modeling laws.

Murthy, Sreedhara V.

1989-01-01

53

Power Law for Rough Favorable Pressure Gradient Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

Current studies on rough favorable pressure gradient (FPG) boundary layers are very challenging particularly since it is difficult to obtain values of skin friction as a function of roughness, pressure gradient strength and Re?. This study presents a new modified form of the power law from George and Castillo (1997) developed for smooth zero pressure gradient boundary layers. The new form accounts for mild pressure gradients, and aids in the account for rough surface boundary layers. Emphasis will be given to its application to rough FPG flows. The values of the skin friction for smooth FPG boundary layers are obtained within 3%. Moreover, the composite profile for the mean velocity accurately describes both FPG and APG flows. The modified power law solution has the advantage of being a continuous solution for smooth and rough profiles subject to external pressure gradients. In addition, by using the Navier Stokes equation the Reynolds shear stress is accurately calculated from the composite description of the mean velocity.

Newhall, Katherine; Castillo, Luciano

2006-11-01

54

Approximation theory for boundary layer suction through individual slits  

NASA Technical Reports Server (NTRS)

The basic concepts of influencing boundary layers are summarized, especially the prevention of flow detachment and the reduction of frictional resistance. A mathematical analysis of suction through a slit is presented with two parameters, for thickness and for shape of the boundary layer, being introduced to specify the flow's velocity profile behind the slit. An approximation of the shape parameter produces a useful formula, which can be used to determine the most favorable position of the slit. An aerodynamic example is given.

Walz, A.

1979-01-01

55

Recent advances in active control of turbulent boundary layers  

NASA Astrophysics Data System (ADS)

In this article, we review the recent progress in active control of a turbulent boundary layer for skin-friction drag reduction. Near-wall coherent structures, which are closely associated with large skin-friction drag and are thus often the target to be manipulated, are discussed briefly, providing a rationale of various control strategies. Open- and closed-loop controls are extensively reviewed, largely focusing on techniques and drag-reduction mechanisms. Finally, some concluding remarks are given.

Zhou, Yu; Bai, HongLei

2011-07-01

56

Measurement of Heat Transfer in the Turbulent Boundary Layer on a Flat Plate in a Supersonic Flow and Comparison with Skin Friction Results  

NASA Technical Reports Server (NTRS)

Local heat-transfer rates on the surface of a heated flat plate at zero incidence to an air stream flowing at Mach numbers of 1.69 and 2.27 are presented. The Reynolds number range for both Mach numbers was 1 million to 10 million. Surface temperatures were maintained near recovery temperature. It was found that the variation of heat transfer with Mach number was in agreement with previously reported variations of directly measured skin friction with Mach number on unheated bodies. The variation with Mach number of the average skin coefficient, as determined from impact-pressure surveys, was in agreement with that from other momentum loss measurements but differed from the variation obtained from directly measured skin friction as reported by others. (author)

Pappas, C C

1954-01-01

57

Modified diffusion model of pseudo-shock waves considering upstream boundary layers  

Microsoft Academic Search

Experiments were performed to investigate the effects of the upstream boundary layer on pseudo-shock waves, by varying the Mach number and boundary layer thickness upstream of the pseudo-shock. A new flow model is presented in which both the effects of the upstream boundary layer and wall friction are taken into account. The configuration and axial static pressure distribution of the

T. Ikui; K. Matsuo; K. Sasaguchi

1981-01-01

58

Frictional anisotropy under boundary lubrication: Effect of surface texture  

Microsoft Academic Search

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

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

2009-01-01

59

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

60

Velocity profiles for turbulent boundary layers under freestream turbulence  

NASA Astrophysics Data System (ADS)

Correlations for changes of skin friction coefficients and wake parameters, relative to the low freestream turbulence condition, are presented for the case of turbulent boundary layers under freestream turbulence with zero and adverse pressure gradients. The turbulent boundary layers were evaluated on a plate in a wind tunnel using a monoplane rod set turbulence generator; comparisons were also made using the data of several other investigators. The results, which define the velocity profiles within the boundary layers, were found to collapse for a large range of the pressure gradient parameter.

Hoffmann, J. A.; Mohammadi, K.

1991-09-01

61

Steady-state wear and friction in boundary lubrication studies  

NASA Technical Reports Server (NTRS)

A friction and wear study was made at 20 C to obtain improved reproducibility and reliability in boundary lubrication testing. Ester-base and C-ether-base fluids were used to lubricate a pure iron rider in sliding contact with a rotating M-50 steel disk in a friction and wear apparatus. Conditions included loads of 1/2 and 1 kg and sliding velocities of 3.6 to 18.2 m/min in a dry air atmosphere and stepwise time intervals from 1 to 250 min for wear measurements. The wear rate results were compared with those from previous studies where a single 25 min test period was used. Satisfactory test conditions for studying friction and wear in boundary lubrication for this apparatus were found to be 1 kg load; sliding velocities of 7.1 to 9.1 m/min (50 rpm disk speed); and use of a time stepwise test procedure. Highly reproducible steady-state wear rates and steady-state friction coefficients were determined under boundary conditions. Wear rates and coefficients of friction were constant following initially high values during run-in periods.

Loomis, W. R.; Jones, W. R., Jr.

1980-01-01

62

Unsteady turbulent boundary layer analysis  

NASA Technical Reports Server (NTRS)

The governing equations for an unsteady turbulent boundary layer on a swept infinite cylinder, composed of a continuity equation, a pair of momentum equations and a pair of turbulent energy equations which include upstream history efforts, are solved numerically. An explicit finite difference analog to the partial differential equations is formulated and developed into a computer program. Calculations were made for a variety of unsteady flows in both two and three dimensions but primarily for two dimensional flow fields in order to first understand some of the fundamental physical aspects of unsteady turbulent boundary layers. Oscillating free stream flows without pressure gradient, oscillating retarded free stream flows and monotonically time-varying flows have all been studied for a wide frequency range. It was found that to the lowest frequency considered, the lower frequency bound being determined by economic considerations (machine time), there were significant unsteady effects on the turbulent boundary layer.

Singleton, R. E.; Nash, J. F.; Carl, L. W.; Patel, V. C.

1973-01-01

63

Physics of magnetospheric boundary layers  

NASA Technical Reports Server (NTRS)

This final report was concerned with the ideas that: (1) magnetospheric boundary layers link disparate regions of the magnetosphere-solar wind system together; and (2) global behavior of the magnetosphere can be understood only by understanding its internal linking mechanisms and those with the solar wind. The research project involved simultaneous research on the global-, meso-, and micro-scale physics of the magnetosphere and its boundary layers, which included the bow shock, the magnetosheath, the plasma sheet boundary layer, and the ionosphere. Analytic, numerical, and simulation projects were performed on these subjects, as well as comparisons of theoretical results with observational data. Other related activity included in the research included: (1) prediction of geomagnetic activity; (2) global MHD (magnetohydrodynamic) simulations; (3) Alfven resonance heating; and (4) Critical Ionization Velocity (CIV) effect. In the appendixes are list of personnel involved, list of papers published; and reprints or photocopies of papers produced for this report.

Cairns, Iver H.

1995-01-01

64

Internal Layers in The Turbulent Boundary Layer*  

NASA Astrophysics Data System (ADS)

Coherent structures in zero-pressure gradient turbulent boundary layers are investigated using particle image velocimetry (PIV). Instantaneous streamwise and normal velocity components were measured in a two-dimensional plane at Req = 1000 and 7000. The images have a wide field of view to capture over three d in the streamwise direction. Recent evidence1 suggests that growing, internal boundary layers exist close to the wall, and may be modulated by the large-scale bulges of the outer region. The investigation focuses on the existance and nature of these layer, the properties of outer region turbulent bulges, and the possible interaction of these two structures. 1 C. D. Meinhart and R. J. Adrian, Phys., Fluids 7 1995. * Work supported by ONR Grant No. N00014-93-1-0552

Tomkins, C. D.; Adrian, Ronald J.

1996-11-01

65

Steady-State Wear and Friction in Boundary Lubrication Studies.  

National Technical Information Service (NTIS)

A friction and wear study was made at 20 C to obtain improved reproducibility and reliability in boundary lubrication testing. Ester-base and C-ether-base fluids were used to lubricate a pure iron rider in sliding contact with a rotating M-50 steel disk i...

W. R. Jones W. R. Loomis

1980-01-01

66

``Broadband'' plasma waves in the boundary layers  

Microsoft Academic Search

Boundary layers are commonly encountered in space and astrophysical plasmas. For example, interaction of solar wind plasma with the planets and comets produces magnetopause and cometopause boundary layers, respectively. Generally, the boundary layers are formed when plasmas with different characteristics interact with each other. The plasma sheet boundary layer in the Earth's magnetotail is formed owing to the interaction of

G. S. Lakhina; B. T. Tsurutani; H. Kojima; H. Matsumoto

2000-01-01

67

Studies of hypersonic boundary layer behavior  

Microsoft Academic Search

Here, we present the final technical report on AFOSR Grant F49620-93-0064, Studies Of Hypersonic Boundary Layer Behavior. The grant covered three interrelated research efforts: a study of the structure of hypersonic turbulent boundary layers and shock wave boundary layer interactions, a study of boundary layer transition at supersonic and hypersonic speeds, and the development and application of new optical techniques

A. J. Smits; R. B. Miles; G. L. Brown

1995-01-01

68

On friction layer formation in polymer matrix composite materials for brake applications  

Microsoft Academic Search

Due to the complexity of friction phenomena in polymer matrix composites, the friction mechanisms have not been fully understood. This paper concentrates on the characterization of friction layer formation and correlation of friction layer properties to the performance of a recently developed family of polymer matrix composites. It was demonstrated that character of the friction layer determines the friction performance

Peter Filip; Zdenek Weiss; David Rafaja

2002-01-01

69

Deposition of hard surfacing layer by friction surfacing  

Microsoft Academic Search

Although several surfacing processes have been proposed and applied, none has been able to apply a 1 mm thick hard layer. This paper describes fundamental research to establish friction surfacing as a technique for applying a 1 mm thick hard layer.Experiments were run to clarify the effects of operating parameters on surface quality using a conventional friction welder fitted with

T. Shinoda; S. Okamoto; S. Takemoto; Y. Kato; T. Shimizu

1996-01-01

70

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

71

Application of Vortex Generators to Bubbly Boundary Layers  

Microsoft Academic Search

The use of micro-bubbles for skin friction drag reduction has been known at least since the 1970's. However, a limitation on the effectiveness of micro-bubbles has been bubble migration, whereby bubbles move to the outer parts of the boundary layer. Since these bubbles need to be fairly close to the wall to be useful (typically in the buffer layer or

David Jeon; Mory Gharib

2004-01-01

72

Manipulation of the structure of a turbulent boundary layer  

NASA Astrophysics Data System (ADS)

The manipulation of a turbulent boundary layer for the purpose of net drag reduction is an attractive topic for research, because even modest success will result in large energy savings. The focus is on passive manipulation, one of the simplest manipulation techniques. The most promising manipulator is the so-called BLADE device, consisting of two thin ribbons or foils suspended in the outer portion of the boundary layer. When the research was begun, there was significant controversy over the magnitude of the net drag reduction possible (20 percent) and the maximum skin friction reduction obtainable (50 percent). Accurate local skin-friction measurements were made using sublayer fences in a perturbed boundary layer. By comparing the direct measurements with those obtained by indirect methods, it was determined that the degree of drag reduction obtained depends on the method used to calculate the combined device drag and skin friction drag. Using auto and two-point correlation measurements as well as space-time correlations, the effects of BLADE were investigated on the turbulent structures in the boundary layer, comparing them with wire devices, which are not known to produce a net reduction in drag. The space-time correlation revealed that the most significant effect of the BLADE device was on the large structures (the dominant structures in the outer region of the boundary layer). The inner layer devices consisting of sublayer wires were also investigated. The results from both the inner and outer layer manipulations suggest that the effective alteration of a turbulent boundary layer depends on the scaling of the device.

Lynn, Theodore Brooks

73

Stability of compressible boundary layers  

NASA Technical Reports Server (NTRS)

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 disturbances in supersonic laminar boundary layers and the most unstable first mode wave is 3-D. Whereas viscosity has a destabilizing effect on incompressible flows, it is stabilizing for high supersonic Mach numbers. Whereas cooling stabilizes first mode waves, it destabilizes second mode waves. However, second order waves can be stabilized by suction and favorable pressure gradients. The influence of the nonparallelism on the spatial growth rate of disturbances is evaluated. The growth rate depends on the flow variable as well as the distance from the body. Floquet theory is used to investigate the subharmonic secondary instability.

Nayfeh, Ali H.

1989-01-01

74

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

75

Mechanisms leading to net drag reduction in manipulated turbulent boundary layers  

NASA Technical Reports Server (NTRS)

Turbulent boundary layers have been manipulated successfully using passive devices called BLADES (boundary-layer alteration devices) leading to net drag reduction. Measurements of various turbulence quantities including intermittency have led to the identification of some of the mechanisms involved in skin-friction reduction. Velocity and vorticity fluctuations associated with oncoming large scales are inhibited significantly by the manipulator blades and the vorticity shed in their wake. The manipulated boundary layers exhibit a reduced intermittency in the outer part of the layer. The effects of manipulation relax with downstream distance and the boundary layer returns toward normal conditions after 100 or 150 boundary-layer thicknesses.

Guezennec, Y. G.; Nagib, H. M.

1990-01-01

76

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

77

Turbulent Boundary Layer Inner-Outer Interactions.  

National Technical Information Service (NTIS)

A summary of work performed to study the interactions between the inner and outer regions of a turbulent boundary layer is presented. The interactions were studied by observing the response of the boundary layer to different perturbations. The inner regio...

D. G. Bogard C. Lim A. Kohli

1993-01-01

78

Turbulent boundary-layer control with plasma actuators.  

PubMed

This paper reviews turbulent boundary-layer control strategies for skin-friction reduction of aerodynamic bodies. The focus is placed on the drag-reduction mechanisms by two flow control techniques-spanwise oscillation and spanwise travelling wave, which were demonstrated to give up to 45 per cent skin-friction reductions. We show that these techniques can be implemented by dielectric-barrier discharge plasma actuators, which are electric devices that do not require any moving parts or complicated ducting. The experimental results show different modifications to the near-wall structures depending on the control technique. PMID:21382824

Choi, Kwing-So; Jukes, Timothy; Whalley, Richard

2011-04-13

79

Study of boundary-layer transition using transonic-cone preston tube data  

NASA Technical Reports Server (NTRS)

The laminar boundary layer on a 10 degree cone in a transonic wind tunnel was studied. The inviscid flow and boundary layer development were simulated by computer programs. The effects of pitch and yaw angles on the boundary layer were examined. Preston-tube data, taken on the boundary-layer-transition cone in the NASA Ames 11 ft transonic wind tunnel, were used to develope a correlation which relates the measurements to theoretical values of laminar skin friction. The recommended correlation is based on a compressible form of the classical law-of-the-wall. The computer codes successfully simulates the laminar boundary layer for near-zero pitch and yaw angles. However, in cases of significant pitch and/or yaw angles, the flow is three dimensional and the boundary layer computer code used here cannot provide a satisfactory model. The skin-friction correlation is thought to be valid for body geometries other than cones.

Reed, T. D.; Moretti, P. M.

1980-01-01

80

A brief review of some mechanisms causing boundary layer transition at high speeds  

NASA Technical Reports Server (NTRS)

In high speed flight, the state of the boundary layer can strongly influence the design of vehicles through its effect on skin friction drag and aerodynamic heating. The major mechanisms causing boundary layer transition on high speed vehicles are briefly reviewed and some empirical relations from the unclassified literature are given for the transition Reynolds numbers.

Tauber, M. E.

1990-01-01

81

Experimental Results from a Flat Plate, Turbulent Boundary Layer Modified for the Purpose of Drag Reduction  

Microsoft Academic Search

Recent experiments on a flat plate, turbulent boundary layer at high Reynolds numbers (>10^7) were performed to investigate various methods of reducing skin friction drag. The methods used involved injecting either air or a polymer solution into the boundary layer through a slot injector. Two slot injectors were mounted on the model with one located 1.4 meters downstream of the

Brian R. Elbing

2006-01-01

82

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

83

Control of Boundary-Layer Separation for Lifting Surfaces  

Microsoft Academic Search

The lack of understanding of most of the relevant physical mechanisms when applying flow control limits the prospects of successfully transitioning flow-control technologies into real flight vehicles. Successful control of boundary-layer separation for lifting surfaces promises major performance gains especially when large laminar runs are desired in order to minimize the skin-friction drag. We systematically explore the fundamental mechanisms of

W. Balzer; A. Gross; H. F. Fasel

2009-01-01

84

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

85

Nonequilibrium chemistry boundary layer integral matrix procedure  

NASA Technical Reports Server (NTRS)

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 chemistry while retaining all of the general boundary condition features built into the original code. For particular application to the pitch-plane of shuttle type vehicles, an approximate procedure was developed to estimate the nonequilibrium and nonisentropic state at the edge of the boundary layer.

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

1973-01-01

86

Vortex boundary-layer interactions  

NASA Technical Reports Server (NTRS)

Parametric studies to identify a vortex generator were completed. Data acquisition in the first chosen configuration, in which a longitudinal vortex pair generated by an isolated delta wing starts to merge with a turbulent boundary layer on a flat plate fairly close to the leading edge is nearly completed. Work on a delta-wing/flat-plate combination, consisting of a flow visualization and hot wire measurements taken with a computer controlled traverse gear and data logging system were completed. Data taking and analysis have continued, and sample results for another cross stream plane are presented. Available data include all mean velocity components, second order mean products of turbulent fluctuations, and third order mean products. Implementation of a faster data logging system was accomplished.

Bradshaw, P.

1986-01-01

87

Boundary layer theory and subduction  

NASA Astrophysics Data System (ADS)

Numerical models of thermally activated convective flow in Earth's mantle do not resemble active plate tectonics because of their inability to model successfully the process of subduction, other than by the inclusion of artificial weak zones. Here we show, using a boundary layer argument, how the 'rigid lid' style of convection favored by thermoviscous fluids leads to lithospheric stresses which may realistically exceed the yield stress and thus cause subduction ot occur through the visoc-plastic failure of lithospheric rock. An explicit criterion for the failure of the lid is given, which is sensitive to the internal viscosity eta(sub a) below the lid. For numbers appropriate to Earth's mantle, this criterion is approximately eta(sub a) greater than 10(exp 21) Pa s.

Fowler, A. C.

1993-12-01

88

Boundary layer theory and subduction  

SciTech Connect

Numerical models of thermally activated convective flow in Earth`s mantle do not resemble active plate tectonics because of their inability to model successfully the process of subduction, other than by the inclusion of artificial weak zones. Here we show, using a boundary layer argument, how the `rigid lid` style of convection favored by thermoviscous fluids leads to lithospheric stresses which may realistically exceed the yield stress and thus cause subduction ot occur through the visoc-plastic failure of lithospheric rock. An explicit criterion for the failure of the lid is given, which is sensitive to the internal viscosity eta(sub a) below the lid. For numbers appropriate to Earth`s mantle, this criterion is approximately eta(sub a) greater than 10(exp 21) Pa s.

Fowler, A.C. [Oxford Univ., Oxford (United Kingdom)

1993-12-01

89

Tribological Properties of Surface Layer with Boron in Friction Pairs  

NASA Astrophysics Data System (ADS)

The aim of the present work is to determine the influence of technologically produced boron surface layers on the friction parameters in the sliding pairs under the conditions of mixed friction. The tribological evaluation included ion nitrided, pack borided, laser borided, quenched and tempered surface layers and TiB2 coating deposited on 38CrAlMo5-10, 46Cr2 and 30MnB4 steels. Modified surface layers of annular samples were matched under test conditions with counter-sample made from AlSn20 bearing alloy. Tested sliding pairs were lubricated with 15 W/40 Lotos mineral engine oil. The tribological tests were conducted on a T-05 block on ring tester. The applied steel surface layer modification with boron allows surface layers to be created with pre-determined tribological characteristics required for the elements of kinematic pairs operating in the conditions of sliding friction. Pack boronizing reduces the friction coefficient during the start-up of the frictional pair and the maximum start-up resistance level is similar to the levels of pairs with nitrided surface layers.

Lubas, Janusz

90

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

91

An experimental investigation of turbulent boundary layers along curved surfaces  

NASA Technical Reports Server (NTRS)

A curved wall tunnel was designed, and an equilibrium turbulent boundary layer was set up on the straight section preceding the curved test section. Turbulent boundary layer flows with uniform and adverse pressure distributions along convex and concave walls were investigated. Hot-wire measurements along the convex surface indicated that turbulent mixing between fluid layers was very much reduced. However, the law of the wall held and the skin friction, thus determined, correlated well with other measurements. Hot-wire measurements along the concave test wall revealed a system of longitudinal vortices inside the boundary layer and confirmed that concave curvature enhances mixing. A self-consistent set of turbulent boundary layer equations for flows along curved surfaces was derived together with a modified eddy viscosity. Solution of these equations together with the modified eddy viscosity gave results that correlated well with the present data on flows along the convex surface with arbitrary pressure distribution. However, it could only be used to predict the mean characteristics of the flow along concave walls because of the existence of the system of longitudinal vortices inside the boundary layer.

So, R. M. C.; Mellor, G. L.

1972-01-01

92

Three-dimensional boundary layers approaching separation  

NASA Technical Reports Server (NTRS)

The theory of semi-similar solutions of the laminar boundary layer equations is applied to several flows in which the boundary layer approaches a three-dimensional separation line. The solutions obtained are used to deduce the nature of three-dimensional separation. It is shown that in these cases separation is of the "ordinary" type. A solution is also presented for a case in which a vortex is embedded within the three-dimensional boundary layer.

Williams, J. C., III

1976-01-01

93

Shear instability in the subsurface layer of a material in friction  

NASA Astrophysics Data System (ADS)

The shear instability in subsurface layers of a material in friction has been investigated. As the friction surface is approached, several characteristic regions can be distinguished in the bulk of the metal: the region of plastic deformation and texturing (I), the region of severe fragmentation (II), the region of turbulent flow (III), and the region of laminar flow (IV). Regions I and II can be referred to as regions of conventional plastic deformation, whereas regions III and IV correspond to regions of the development of shear instability of the Kelvin-Helmholtz type at the shear boundary. The possibility of implementing this phenomenon within the hydrodynamic approach has been evaluated.

Tarasov, S. Yu.; Rubtsov, V. E.

2011-02-01

94

Structure of the low latitude boundary layer  

NASA Technical Reports Server (NTRS)

Observations at high temporal resolution of the frontside magnetopause and plasma boundary layer, made with the LASL/MPE fast plasma analyzer onboard the ISEE 1 and 2 spacecraft, revealed a complex quasiperiodic structure of some of the observed boundary layers. A cool tailward streaming boundary layer plasma was seen intermittently, with intervening periods of hot tenuous plasma which has properties similar to the magnetospheric population. While individual encounters with the boundary layer plasma last only a few minutes, the total observation time may extend over one hour or more.

Sckopke, N.; Paschmann, G.; Haerendel, G.; Sonnerup, B. U. O.; Bame, S. J.; Forbes, T. G.; Hones, E. W., Jr.; Russell, C. T.

1980-01-01

95

Particle motion in atmospheric boundary layers of Mars and Earth  

NASA Technical Reports Server (NTRS)

To study the eolian mechanics of saltating particles, both an experimental investigation of the flow field around a model crater in an atmospheric boundary layer wind tunnel and numerical solutions of the two- and three-dimensional equations of motion of a single particle under the influence of a turbulent boundary layer were conducted. Two-dimensional particle motion was calculated for flow near the surfaces of both Earth and Mars. For the case of Earth both a turbulent boundary layer with a viscous sublayer and one without were calculated. For the case of Mars it was only necessary to calculate turbulent boundary layer flow with a laminar sublayer because of the low values of friction Reynolds number; however, it was necessary to include the effects of slip flow on a particle caused by the rarefied Martian atmosphere. In the equations of motion the lift force functions were developed to act on a single particle only in the laminar sublayer or a corresponding small region of high shear near the surface for a fully turbulent boundary layer. The lift force functions were developed from the analytical work by Saffman concerning the lift force acting on a particle in simple shear flow.

White, B. R.; Iversen, J. D.; Greeley, R.; Pollack, J. B.

1975-01-01

96

Modeling cathode boundary layer discharges  

NASA Astrophysics Data System (ADS)

A Cathode Boundary Layer Discharge or CBL (Schoenbach, et al Plasma Sources Sci. Technol. 13, 177,2004) is an electrode/dielectric/electrode sandwich with a central hole pierced through the dielectric and one of the electrodes (the anode). Thus, the cathode surface area available to the discharge is limited by the annular dielectric, and the discharge operates in an abnormal glow mode with a positive V-I characteristic at higher current. Using a two-dimensional fluid model, we have studied the electrical properties of CBLs in argon at 100 and 400 torr pressure. The spatial profiles of charged particle and metastable densities, potential, and gas temperature, as well as calculated V-I characteristics will be shown for a range of conditions for a 800 micron hole diameter. One interesting result (anticipated in the work of Belostotskiy, et al, Plasma Sources Sci. Technol 17, 045018, 2008) is that there is a sharp increase in the slope of the V-I characteristic when gas heating is taken into account. This current limiting effect is not observed when the discharge is able to expand on the outer surface of the cathode as in the case of the MicroHollow Cathode Discharge (MHCD) configuration, for example.

Munoz-Serrano, E.; Boeuf, J. P.; Pitchford, L. C.

2009-10-01

97

Calculation of a Turbulent Boundary Layer for Substantial Positive Pressure Gradients.  

National Technical Information Service (NTIS)

In this article is exponded a new method of calculation ofa turbulent boundary layer, which is based on the boundary laws of friction and heat transfer in the diffusion region of a gas flow. There are obtained values of critical parameters at point of abr...

S. S. Kytateladze A. I. Leont'ev

1964-01-01

98

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

99

Transient response of a turbulent boundary layer  

NASA Technical Reports Server (NTRS)

A unique feature of the present ensemble-averaged measurements of a turbulent boundary layer's transient response to a spontaneous change in the free stream velocity distribution, is that the test boundary layer is a standard, steady, flat plate turbulent boundary layer at the entrance to the unsteady region, and is then subjected to sudden changes in free stream velocity distribution in the test section. These water tunnel tests were controlled by minicomputer. It is noted that the boundary layer development was relatively slow, with a characteristic time that was greater than the free stream time-of-flight by a factor of as much as 3. Response varied dramatically across the boundary layer, and the evolution of the turbulent stress field occurred on the same time scale as that of the ensemble-averaged velocity field.

Parikh, P. G.; Jayaraman, R.; Reynolds, W. C.; Carr, L. W.

1983-01-01

100

Effect of sulphide layers on the tribological behavior of steels under boundary lubrication conditions  

NASA Astrophysics Data System (ADS)

Sulphide layers were prepared on the surface of AISI 1045 steel by ion sulphurization. The anti-scuffing, friction-reducing, and wear-resistant behavior of these sulphurized surfaces were investigated systematically using a ball-on-disc wear tester with engine oil as a lubricant. SEM, EDX, XPS, and AES were used to examine the morphologies and compositions of wear scars and boundary films. Sulphide layers improved anti-scuffing properties remarkably at low sliding velocities, and exhibited good friction-reducing and wear-resistant effects. During friction the sulphide layer promoted the growth of oxide, and iron sulphide could be decomposed and regenerated to form FeS again on the rubbing surface. An appropriately thick sulphide layer optimizes the sulphur-to-oxygen concentration in the boundary film resulting in the highest load-bearing capacity, and shows that thicker sulphide layers are unnecessary.

Ning, Zhang; Da-Ming, Zhuang; Jia-Jun, Liu; Xiao-Dong, Fang; Ming-Xi, Guan

2001-09-01

101

Longitudinal vortices imbedded in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The attenuation of skew-induced longitudinal vortices by turbulent or viscous stresses is studied for the case of pure, artificially-generated longitudinal vortices entrained into initially two-dimensional boundary layers in nominally zero pressure gradients. Three types of vortex-boundary interactions are studied in detail: (1) an isolated vortex in a two-dimensional boundary layer; (2) a vortex pair in a turbulent boundary layer with the common flow between the vortices moving away from the surface; (3) a vortex pair in a boundary layer with the common flow moving towards the surface. Detailed mean flow and turbulence measurements are made, showing that the eddy viscosities defined for the different shear-stress components behave in different and complicated ways. Terms in the Reynolds stress transport equations, notably the triple products that effect turbulent diffusion of Reynolds stress, also fail to obey simple rules.

Mehta, R. D.; Shabaka, I. M. M.; Shibl, A.; Bradshaw, P.

1983-01-01

102

Computation of the shock-wave boundary layer interaction with flow separation  

NASA Technical Reports Server (NTRS)

The boundary layer concept is used to describe the flow near the wall. The external flow is approximated by a pressure displacement relationship (tangent wedge in linearized supersonic flow). The boundary layer equations are solved in finite difference form and the question of the presence and unicity of the solution is considered for the direct problem (assumed pressure) or converse problem (assumed displacement thickness, friction ratio). The coupling algorithm presented implicitly processes the downstream boundary condition necessary to correctly define the interacting boundary layer problem. The algorithm uses a Newton linearization technique to provide a fast convergence.

Ardonceau, P.; Alziary, T.; Aymer, D.

1980-01-01

103

Planetary Boundary Layer Simulation Using TASS  

NASA Technical Reports Server (NTRS)

Boundary conditions to an existing large-eddy simulation model have been changed in order to simulate turbulence in the atmospheric boundary layer. Several options are now available, including the use of a surface energy balance. In addition, we compare convective boundary layer simulations with the Wangara and Minnesota field experiments as well as with other model results. We find excellent agreement of modelled mean profiles of wind and temperature with observations and good agreement for velocity variances. Neutral boundary simulation results are compared with theory and with previously used models. Agreement with theory is reasonable, while agreement with previous models is excellent.

Schowalter, David G.; DeCroix, David S.; Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael

1996-01-01

104

A drag reduction method for turbulent boundary layers  

NASA Technical Reports Server (NTRS)

A novel method to reduce skin friction drag in a turbulent boundary layer is presented. The technique combines the beneficial effects of a longitudinally ribbed surface and suction. 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. During the first phase of the present research, 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. The experiments were conducted using a flat plate towed in an 18-m water channel. Flow visualization and hot-film probe measurements were used together with pattern recognition algorithms to demonstrate the feasibility of the drag-reducing method.

Gad-El-hak, Mohamed; Blackwelder, Ron F.

1987-01-01

105

Boundary-layer linear stability theory  

NASA Technical Reports Server (NTRS)

Most fluid flows are turbulent rather than laminar and the reason for this was studied. One of the earliest explanations was that laminar flow is unstable, and the linear instability theory was first developed to explore this possibility. A series of early papers by Rayleigh produced many notable results concerning the instability of inviscid flows, such as the discovery of inflectional instability. Viscosity was commonly thought to act only to stabilize the flow, and flows with convex velocity profiles appeared to be stable. The investigations that led to a viscous theory of boundary layer instability was reported. The earliest application of linear stability theory to transition prediction calculated the amplitude ratio of the most amplified frequency as a function of Reynolds number for a Blasius boundary layer, and found that this quantity had values between five and nine at the observed Ret. The experiment of Schubauer and Skramstad (1947) completely reversed the prevailing option and fully vindicated the Gottingen proponents of the theory. This experiment demonstrated the existence of instability waves in a boundary layer, their connection with transition, and the quantitative description of their behavior by the theory of Tollmien and Schlichting. It is generally accepted that flow parameters such as pressure gradient, suction and heat transfer qualitatively affect transition in the manner predicted by the linear theory, and in particular that a flow predicted to be stable by the theory should remain laminar. The linear theory, in the form of the e9, or N-factor is today in routine use in engineering studies of laminar flow. The stability theory to boundary layers with pressure gradients and suction was applied. The only large body of numerical results for exact boundary layer solutions before the advent of the computer age by calculating the stability characteristics of the Falkner-Skan family of velocity profiles are given. When the digital computer reached a stage of development which permit the direct solution of the primary differential equations, numerical results were obtained from the linear theory during the next 10 years for many different boundary layer flows: three dimensional boundary layers; free convention boundary layers; compressible boundary layers; boundary layers on compliant walls; a recomputation of Falkner-Skan flows; unsteady boundary layers; and heated wall boundary layers.

Mack, L. M.

1984-01-01

106

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.

107

An Experimental Investigation Of A Highly Accelerated Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

An investigation of a highly accelerated turbulent boundary layer (Re_? = 5000) is in progress at the Hessert Center for Aerospace Research at the University of Notre Dame. Unique to this experiment is the ability to expose the boundary layer to a range of favorable pressure gradients in which the acceleration parameter K = frac?u^2 fracdudx is maintained constant over an appreciable streamwise distance. Values of 0.5 x 10-6 < K < 4.5 x 10-6 have been investigated. The objective of the research is to elucidate the fundamental mechanisms responsible for turbulent boundary layer relaminarization, or reverse transition, by examining the underlying flow structure of the turbulent boundary layer and exploring how these structures change during rapid flow acceleration. Results to be discussed include profiles of the mean velocity, fluctuating streamwise and wall normal velocity, and the Reynolds stress. In addition, skin friction values measured using oil film interferometry, flow visualization results, and preliminary 2D PIV velocity contours will be presented.

Bourassa, Corey; Thomas, Flint; Nelson, Robert

2001-11-01

108

Exact Calculation of Laminar Boundary Layer in Longitudinal Flow over a Flat Plate with Homogeneous Suction  

NASA Technical Reports Server (NTRS)

Lately it has been proposed to reduce the friction drag of a body in a flow for the technically important large Reynolds numbers by the following expedient: the boundary layer, normally turbulent, is artificially kept laminar up to high Reynolds numbers by suction. The reduction in friction drag thus obtained is of the order of magnitude of 60 to 80 percent of the turbulent friction drag, since the latter, for large Reynolds numbers, is several times the laminar friction drag. In considering the idea mentioned one has first to consider whether suction is a possible means of keeping the boundary layer laminar. This question can be answered by a theoretical investigation of the stability of the laminar boundary layer with suction. A knowledge, as accurate as possible, of the velocity distribution in the laminar boundary layer with suction forms the starting point for the stability investigation. E. Schlichting recently gave a survey of the present state of calculation of the laminar boundary layer with suction.

Iglisch, Rudolf

1949-01-01

109

Boundary layer flow visualization for flight testing  

NASA Technical Reports Server (NTRS)

Flow visualization is used extensively in flight testing to determine aerodynamic characteristics such as surface flow direction and boundary layer state. Several visualization techniques are available to the aerodynamicist. Two of the most popular are oil flows and sublimating chemicals. Oil is used to visualize boundary layer transition, shock wave location, regions of separated flow, and surface flow direction. Boundary layer transition can also be visualized with sublimating chemicals. A summary of these two techniques is discussed, and the use of sublimating chemicals is examined in some detail. The different modes of boundary layer transition are characterized by different patterns in the sublimating chemical coating. The discussion includes interpretation of these chemical patterns and the temperature and velocity operating limitations of the chemical substances. Information for selection of appropriate chemicals for a desired set of flight conditions is provided.

Obara, Clifford J.

1986-01-01

110

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

111

Swept shock/boundary layer interaction experiments in support of CFD code validation  

NASA Technical Reports Server (NTRS)

Research on the topic of shock wave/turbulent boundary layer interaction was carried out. Skin friction and surface pressure measurements in fin-induced, swept interactions were conducted, and heat transfer measurements in the same flows are planned. The skin friction data for a strong interaction case (Mach 4, fin-angles equal 16 and 20 degrees) were obtained, and their comparison with computational results was published. Surface pressure data for weak-to-strong fin interactions were also obtained.

Settles, G. S.; Lee, Y.

1990-01-01

112

Boundary layer control on magnetohydrodynamic numerical simulations  

NASA Astrophysics Data System (ADS)

In the context of planetary interiors the geometry and magnitude of magnetic fields has been found to be controlled by the heat flux through the outer boundary (e.g., Olson & Christensen 2006). However, there is indication, based on experimental work, that the balance between thermal, viscous, and Coriolis effects defines the flow behaviour in non-magnetic fluids by means of boundary layer control (King et al. 2009). Using numerical models of magnetohydrodynamic fluids in spherical shells, we find that the force balances at the top boundary may be determining the overall behaviour of the flow. Similar to flow control by boundary layers in non-magnetic convective systems, we find that boundary layers associated with temperature, viscosity, Coriolis forces, and magnetic fields play an important role in determining the large scale flow. Boundary layer relative thicknesses are found to correlate with the internal force balances. For planetary dynamos, this implies that the boundary conditions at the top of the dynamo region - not only the heat flux through the boundary - control the mode of convection as well as magnetic field magnitude, geometry and secular variation.

Gomez Perez, N.; Heimpel, M. H.

2012-12-01

113

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

114

Varicose instabilities in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

An investigation of a model of turbulence generation in the wall region of a turbulent boundary layer is made through direct numerical simulations. The model is based on the varicose instability of a streak. First, a laminar boundary layer disturbed by a continuous blowing through a slot is simulated in order to reproduce and further investigate the results reported from the experiments of Acarlar and Smith [J. Fluid Mech. 175, 43 (1987)]. An isolated streak with an inflectional profile is generated that becomes unstable, resulting in a train of horseshoe vortices. The frequency of the vortex generation is equal to the experimental results. Comparison of the instability characteristics to those predicted through an Orr-Sommerfeld analysis are in good agreement. Second, a direct numerical simulation of a turbulent boundary layer is performed to point out the similarities between the horseshoe vortices in a turbulent and a laminar boundary layer. The characteristics of streaks and the vortical structures surrounding them in a turbulent boundary layer compare well with the model streak. The results of the present study show that one mechanism for the generation of horseshoe vortices in turbulent boundary layers is related to a normal inflectional instability of the streaks.

Skote, M.; Haritonidis, J. H.; Henningson, D. S.

2002-07-01

115

Anomalous Low Friction Under Boundary Lubrication of Steel Surfaces by Polyols  

Microsoft Academic Search

We present here anomalous low friction obtained with highly polished steel on steel hard contact lubricated by glycerol under\\u000a severe mixed and boundary regimes (? ratio below 1). We investigated the effects of contact pressure, sliding speed, and temperature\\u000a on friction coefficient and electrical contact resistance. The mechanism of low friction (typically below 0.02) is thought\\u000a to have two origins:

L. Joly-Pottuz; J. M. Martin; M. I. De Barros Bouchet; M. Belin

2009-01-01

116

Assessment of inflow boundary conditions for compressible turbulent boundary layers  

NASA Astrophysics Data System (ADS)

A description of different inflow methodologies for turbulent boundary layers, including validity and limitations, is presented. We show that the use of genuine periodic boundary conditions, in which no alteration of the governing equations is made, results in growing mean flow and decaying turbulence. Premises under which the usage is valid are presented and explained, and comparisons with the extended temporal approach [T. Maeder, N. A. Adams, and L. Kleiser, ``Direct simulation of turbulent supersonic boundary layers by an extended temporal approach,'' J. Fluid Mech. 429, 187 (2001)] are used to assess the validity. Extending the work by Lund et al. [J. Comput. Phys. 140, 233 (1998)], we propose an inflow generation method for spatial simulations of compressible turbulent boundary layers. The method generates inflow by reintroducing a rescaled downstream flow field to the inlet of a computational domain. The rescaling is based on Morkovin's hypothesis [P. Bradshaw, ``Compressible turbulent shear layers,'' Annu. Rev. Fluid Mech. 9, 33 (1977)] and generalized temperature-velocity relationships. This method is different from other existing rescaling techniques [S. Stolz and N. A. Adams, ``Large-eddy simulation of high-Reynolds-number supersonic boundary layers using the approximate deconvolution model and a rescaling and recycling technique,'' Phys. Fluids 15, 2398 (2003); G. Urbin and D. Knight, ``Large-eddy simulation of a supersonic boundary layer using an unstructured grid,'' AIAA J. 39, 1288 (2001)], in that a more consistent rescaling is employed for the mean and fluctuating thermodynamic variables. The results are compared against the well established van Driest II theory and indicate that the method is efficient and accurate.

Xu, Sheng; Martin, M. Pino

2004-07-01

117

Entrainment Parameterization in Convective Boundary Layers.  

NASA Astrophysics Data System (ADS)

Various runs were performed with a large eddy simulation (LES) model to evaluate different types of entrainment parametrizations. For this evaluation, three types of boundary layers were simulated: a clear convective boundary layer (CBL), a boundary layer containing a smoke concentration, and a cloud-topped boundary layer. It is shown that the assumption that the entrainment flux equals the product of the entrainment rate and the jump over a discontinuous inversion is not valid in CBLs simulated by an LES model. A finite inversion thickness (i.e., a first-order jump model) is needed to define an entrainment flux for which this approximation of the flux is valid. This entrainment flux includes not only the buoyancy flux at the inversion, but also the surface heat flux. The parameterization of the buoyancy flux at the inversion is evaluated for different closures, as suggested in the literature (i.e., Eulerian partitioning, process partitioning, and a closure developed by Deardorff), and where needed is extended for use in a first-order jump model. The closure based on process partitioning is found to yield consistent results in all types of convective boundary layers and shows the best agreement with the limit found in LES results if the longwave radiative flux divergence takes place in a much shallower layer than the mixed layer.

Vanzanten, Margreet C.; Duynkerke, Peter G.; Cuijpers, Joannes W. M.

1999-03-01

118

Forming Nanocrystalline Surface Layers in Copper Using Friction Stir Processing  

Microsoft Academic Search

In the present work, a nanostructured layer was formed on the surface of pure copper via friction stir processing (FSP). The surface nanostructuring can be of significant importance in terms of surface-dependent properties such as fatigue, wear, coating, and even corrosion. During FSP, the applied rotating and traverse speeds were 1600 rpm and 50 mm\\/min, respectively. The results show that a nanograin

K. Dehghani; M. Mazinani

2011-01-01

119

Vortex filament stability and boundary layer dynamics  

SciTech Connect

Coherent structures in fluid boundary layers at high Reynolds numbers are a prominent feature of these flows. The structures appear as concentrations of vorticity into hairpin'' and other shapes. We explore the inviscid interaction and stability of vortex filaments initially situated spanwise to the mean flow in a model of a boundary layer. Both for a single vortex filament and its image through the boundary and for an infinite line of such filaments with their images we find a linear instability associated with deformations of the filament along its length with maximum instability having a wavelength on the order of the height of the filament above the boundary. The linear unstable manifold for this instability points at approximately 45[degree] from the plane of the boundary in accord with experimental observations and numerical modeling of these coherent structures. This provides a dynamical origin to the observations of the orientation of these coherent structures.

Abarbanel, H.D.I. (Department of Physics, Institute for Nonlinear Science and Marine Physical Laboratory, Scripps Institution of Oceanography, University of California, San Diego, Mail Code 0402, La Jolla, California 92093-0402 (United States)); Lisi, A.G. (Department of Physics and Institute for Nonlinear Science, University of California, San Diego, Mail Code 0402, La Jolla, California 92093-0402 (United States)); Rouhi, A.; Wright, J.A. (Institute for Nonlinear Science, University of California, San Diego, Mail Code 0402, La Jolla, California 92093-0402 (United States))

1994-08-01

120

Ground observations of magnetospheric boundary layer phenomena  

NASA Technical Reports Server (NTRS)

Several classes of traveling vortices in the dayside ionosphere convection have been detected and tracked using the Greenland magnetometer chain (Friis-Christensen et al., 1988, McHenry et al., 1989). One class observed during quiet times consists of a continuous series of vortices moving generally antisunward for several hours at a time. The vortices' strength is seen to be approximately steady and neighboring vortices rotate in opposite directions. Sondrestrom radar observations show that the vortices are located at the ionospheric convection reversal boundary. Low altitude DMSP observations indicate the vortices are on field lines which map to the inner edge of the low latitude boundary layer. Because the vortices are conjugate to the boundary layer, repeat in a regular fashion and travel antisunward, it is argued that this class of vortices is caused by the Kelvin-Helmholtz instability of the inner edge of the magnetospheric boundary layer.

Mchenry, Mark A.; Clauer, C. Robert; Friis-Christensen, Eigil; Newell, Patrick T.; Kelly, J. D.

1990-01-01

121

Experiments in Boundary-Layer Turbulence  

NASA Technical Reports Server (NTRS)

Motion in layer highly three-dimensional. Report describes experimental studies of disturbances induced by weak free-stream turbulence in pre-transitional Blasius boundary layer. Asks and partially answers some fundamental questions concerning large-amplitude, low-frequency disturbances.

Kendall, James M., Jr.

1987-01-01

122

Some Basic Aspects of Magnetohydrodynamic Boundary-Layer Flows  

NASA Technical Reports Server (NTRS)

An appraisal is made of existing solutions of magnetohydrodynamic boundary-layer equations for stagnation flow and flat-plate flow, and some new solutions are given. Since an exact solution of the equations of magnetohydrodynamics requires complicated simultaneous treatment of the equations of fluid flow and of electromagnetism, certain simplifying assumptions are generally introduced. The full implications of these assumptions have not been brought out properly in several recent papers. It is shown in the present report that for the particular law of deformation which the magnetic lines are assumed to follow in these papers a magnet situated inside the missile nose would not be able to take up any drag forces; to do so it would have to be placed in the flow away from the nose. It is also shown that for the assumption that potential flow is maintained outside the boundary layer, the deformation of the magnetic lines is restricted to small values. The literature contains serious disagreements with regard to reductions in heat-transfer rates due to magnetic action at the nose of a missile, and these disagreements are shown to be mainly due to different interpretations of reentry conditions rather than more complicated effects. In the present paper the magnetohydrodynamic boundary-layer equation is also expressed in a simple form that is especially convenient for physical interpretation. This is done by adapting methods to magnetic forces which in the past have been used for forces due to gravitational or centrifugal action. The simplified approach is used to develop some new solutions of boundary-layer flow and to reinterpret certain solutions existing in the literature. An asymptotic boundary-layer solution representing a fixed velocity profile and shear is found. Special emphasis is put on estimating skin friction and heat-transfer rates.

Hess, Robert V.

1959-01-01

123

Mechanical response of an inclined frictional granular layer approaching unjamming  

NASA Astrophysics Data System (ADS)

We present an orthotropic elastic analysis of frictional granular layers under gravity by studying their stress response to a localized overload at the layer surface for several substrate tilt angles. The distance to the unjamming transition is controlled by the tilt angle ? with respect to the critical angle ?c. We find that the shear modulus of the system decreases with ?, but reaches a finite value as ? ? ?c. We also analyze the vibration modes of the system and show that the soft modes play an increasing, though not crucial, role approaching the transition.

Atman, A. P. F.; Claudin, P.; Combe, G.; Mari, R.

2013-02-01

124

Lear jet boundary layer/shear layer laser propagation experiments  

NASA Technical Reports Server (NTRS)

Optical degradations of aircraft turbulent boundary layers with shear layers generated by aerodynamic fences are analyzed. A collimated 2.5 cm diameter helium-neon laser (0.63 microns) traversed the approximate 5 cm thick natural aircraft boundary layer in double pass via a reflective airfoil. In addition, several flights examined shear layer-induced optical degradation. Flight altitudes ranged from 1.5 to 12 km, while Mach numbers were varied from 0.3 to 0.8. Average line spread function (LSF) and Modulation Transfer Function (MTF) data were obtained by averaging a large number of tilt-removed curves. Fourier transforming the resulting average MTF yields an LSF, thus affording a direct comparison of the two optical measurements. Agreement was good for the aerodynamic fence arrangement, but only fair in the case of a turbulent boundary layer. Values of phase variance inferred from the LSF instrument for a single pass through the random flow and corrected for a large aperture ranged from 0.08 to 0.11 waves (lambda = .63 microns) for the boundary layer. Corresponding values for the fence vary from 0.08 to 0.16 waves. Extrapolation of these values to 10.6 microns suggests negligible degradation for a CO2 laser transmitted through a 5 cm thick, subsonic turbulent boundary layer.

Gilbert, K.

1980-01-01

125

The effects of a favourable pressure gradient and of the Reynolds number on an incompressible axisymmetric turbulent boundary layer. Part 2. The boundary layer with relaminarization  

NASA Astrophysics Data System (ADS)

This is an experimental investigation of two turbulent boundary layers (cases 2 and 4) where the streamwise negative pressure gradient changes mean properties of the flow, e.g. mean velocity profiles and skin friction, so that they display laminar-like behaviour. The maximum acceleration parameter K[less-than-or-eq, slant]4×10[minus sign]6 and the starting value of the Reynolds number is 862 or 2564. Relaminarization occurs in both boundary layers as a gradual change of the turbulence properties and is not catastrophic. Retransition, however, is a fast process due to the remaining turbulence structure and may be compared with bypass transition. Together with an extensive investigation of the turbulence structure as in the companion paper, Part 1, which describes two cases (1 and 3) of boundary layers which remain turbulent, spectra and integral length scales for all four boundary layers are discussed.

Warnack, D.; Fernholz, H. H.

1998-03-01

126

High enthalpy hypersonic boundary layer flow  

NASA Technical Reports Server (NTRS)

A theoretical and experimental study of an ionizing laminar boundary layer formed by a very high enthalpy flow (in excess of 12 eV per atom or 7000 cal/gm) with allowance for the presence of helium driver gas is described. The theoretical investigation has shown that the use of variable transport properties and their respective derivatives is very important in the solution of equilibrium boundary layer equations of high enthalpy flow. The effect of low level helium contamination on the surface heat transfer rate is minimal. The variation of ionization is much smaller in a chemically frozen boundary layer solution than in an equilibrium boundary layer calculation and consequently, the variation of the transport properties in the case of the former was not essential in the integration. The experiments have been conducted in a free piston shock tunnel, and a detailed study of its nozzle operation, including the effects of low levels of helium driver gas contamination has been made. Neither the extreme solutions of an equilibrium nor of a frozen boundary layer will adequately predict surface heat transfer rate in very high enthalpy flows.

Yanow, G.

1972-01-01

127

Pressure Gradient Boundary Layers With Eventual Separation  

NASA Astrophysics Data System (ADS)

Using the similarity analysis for turbulent boundary layer with pressure gradient by Castillo and George(Castillo, L. and George, W.K.,``Similarity Analysis for Turbulent Boundary Layer with Pressure Gradient: out flow,'' AIAA Journal, Vol.39,2001) it will be shown that the outer part of adverse pressure gradient turbulent boundary layers tends to remain in equilibrium similarity, even near (and sometimes past) separation. Thus such boundary layers are characterized by a single pressure parameter, ?_? =frac? ? U_? ^2d? /dxfracdP_? dx, and its value appears to be the same for all adverse pressure gradient flows; i.e., ?_? ? 0.22. Using this pressure parameter and the momentum integral boundary layer equation, it is possible to show that the shape factor at separation must have a single value, H_sep ? 2.5. Both the conditions for equilibrium similarity and the value of H_sep are shown to be in reasonable agreement with a variety of experimental estimates.

Wang, Xia; Castillo, Luciano; George, William K.

2001-11-01

128

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

129

Boundary-Layer-Ingesting Inlet Flow Control  

NASA Technical Reports Server (NTRS)

An experimental study was conducted to provide the first demonstration of an active flow control system for a flush-mounted inlet with significant boundary-layer-ingestion in transonic flow conditions. The effectiveness of the flow control in reducing the circumferential distortion at the engine fan-face location was assessed using a 2.5%-scale model of a boundary-layer-ingesting offset diffusing inlet. The inlet was flush mounted to the tunnel wall and ingested a large boundary layer with a boundary-layer-to-inlet height ratio of 35%. Different jet distribution patterns and jet mass flow rates were used in the inlet to control distortion. A vane configuration was also tested. Finally a hybrid vane/jet configuration was tested leveraging strengths of both types of devices. Measurements were made of the onset boundary layer, the duct surface static pressures, and the mass flow rates through the duct and the flow control actuators. The distortion and pressure recovery were measured at the aerodynamic interface plane. The data show that control jets and vanes reduce circumferential distortion to acceptable levels. The point-design vane configuration produced higher distortion levels at off-design settings. The hybrid vane/jet flow control configuration reduced the off-design distortion levels to acceptable ones and used less than 0.5% of the inlet mass flow to supply the jets.

Owens, Lewis R.; Allan, Brian G.; Gorton, Susan A.

2008-01-01

130

Modification in drag of turbulent boundary layers resulting from manipulation of large-scale structures  

NASA Technical Reports Server (NTRS)

The effects of placing a parallel-plate turbulence manipulator in a boundary layer are documented through flow visualization and hot wire measurements. The boundary layer manipulator was designed to manage the large scale structures of turbulence leading to a reduction in surface drag. The differences in the turbulent structure of the boundary layer are summarized to demonstrate differences in various flow properties. The manipulator inhibited the intermittent large scale structure of the turbulent boundary layer for at least 70 boundary layer thicknesses downstream. With the removal of the large scale, the streamwise turbulence intensity levels near the wall were reduced. The downstream distribution of the skin friction was also altered by the introduction of the manipulator.

Corke, T. C.; Guezennec, Y.; Nagib, H. M.

1981-01-01

131

Boundary layers in dilute particle suspensions  

NASA Astrophysics Data System (ADS)

Boundary layers in dilute particle suspensions have been found to have a number of interesting features. The development of a singularity at the wall has recently been found to be common to many of these flows, ootnotetextSee Foster, Duck & Hewitt, J. Fluid Mech. 474 (2003) and Duck, Hewitt & Foster, J. Fluid Mech. 514, (2004) and we note here that Falkner-Skan-type boundary layers (layers with `edge' velocity proportional to x^m) and the boundary layer under a linearly decelerating flow ootnotetextHowarth (1934) also break down at the wall in the absence of gravity, but can be singularity-free for heavy particles. In addition, we find that matching of the Falkner-Skan profile to an outer flow is problematic for some values of m, though the case most studied heretofore---the Blasius case (for m=0)---does not feature this difficulty. Finally, a boundary layer that does not develop a singularity takes on a the typical Falkner-Skan self-similarity far downstream, in the absence of gravity. For heavy particles, however, gravity causes a constant drift of particles toward the wall, and a constant-thickness far-downstream layer. The far-downstream behavior in a light-particle suspension is different, with a particle-free zone between the wall and a particle `shock' that grows like x^(1-m).

Foster, M. R.; Duck, P. W.; Hewitt, R. E.

2005-11-01

132

Simulation of bubble migration in a turbulent boundary layer  

NASA Astrophysics Data System (ADS)

This paper presents the results from a one-way coupled, Euler-Lagrangian, direct numerical simulation of bubbles injected into a turbulent boundary layer. The Reynolds number of the turbulent boundary layer varies from 420friction drag reduction in a turbulent boundary layer, although the Reynolds number of the simulation is lower than the experiment. After injection, bubbles move away from the wall as they travel downstream with the flow. Mean bubble diffusion is compared to Sanders et al. and the passive scalar diffusion results given by Poreh and Cermak [Int. J. Heat Mass Transfer 7, 1083 (1964)]. The mean diffusion profiles in the Sanders experiment and the simulation are comparable to the passive scalar results. Except very near the wall, the profiles of bubble concentration are also found to be similar to passive scalar results. The forces on a bubble were analyzed through budgets and the carrier-fluid acceleration was found to be the reason for moving the bubbles away from the wall.

Mattson, M.; Mahesh, K.

2011-04-01

133

Mechanics of Boundary Layer Transition. Part 5: Boundary Layer Stability theory in incompressible and compressible flow  

NASA Technical Reports Server (NTRS)

The fundamentals of stability theory, its chief results, and the physical mechanisms at work are presented. The stability theory of the laminar boundary determines whether a small disturbance introduced into the boundary layer will amplify or damp. If the disturbance damps, the boundary layer remains laminar. If the disturbance amplifies, and by a sufficient amount, then transition to turbulence eventually takes place. The stability theory establishes those states of the boundary layer which are most likely to lead to transition, identifys those frequencies which are the most dangerous, and indicates how the external parameters can best be changed to avoid transition.

Mack, L. M.

1967-01-01

134

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

135

Stability of separating subsonic boundary layers  

NASA Technical Reports Server (NTRS)

The primary and subharmonic instabilities of separating compressible subsonic two-dimensional boundary layers in the presence of a two-dimensional roughness element on a flat plate are investigated. The roughness elements considered are humps and forward- and backward-facing steps. The use of cooling and suction to control these instabilities is studied. The similarities and differences between the instability characteristics of separating boundary layers and those of the boundary layer over a flat plate with a zero pressure gradient are pointed out and discussed. The theoretical results agree qualitatively and quantitatively with the experimental data of Dovgal and Kozlov. Cooling and suction decrease the growth rates of primary and subharmonic waves in the attached-flow regions but increase them in the separated-flow regions.

Masad, Jamal A.; Nayfeh, Ali H.

1994-01-01

136

Diffusion processes in the magnetopause boundary layer  

NASA Technical Reports Server (NTRS)

A quantitative estimate is calculated for the effect of wave-particle scattering on the structure of the magnetopause boundary layer. It is assumed that large cross-B electric fields are absent in the observed penetration of magnetosheath plasma into the magnetopause boundary layer, thus allowing for cross-field transport comparable to 10% of the Bohm diffusion. It is shown that magnetosheath ions, resonant with low frequency electrostatic waves, can account for the typical boundary layer thickness when transported at 10% of the diffusion rate 1000 sq km/sec. The conditions are required to occur at all local times and under all interplanetary conditions. Significant mass and momentum transfer are then possible across the magnetopause when field merging is not occurring.

Tsurutani, B. T.; Thorne, R. M.

1982-01-01

137

Effects of uniform blowing or suction from a spanwise slot on a turbulent boundary layer flow  

Microsoft Academic Search

Effects of uniform blowing or suction from a spanwise slot on a turbulent boundary layer flow are investigated using the direct numerical simulation technique. The magnitudes of blowing or suction are less than 10% of the free-stream velocity. The skin friction and near-wall turbulence intensities are significantly changed by blowing and suction. In the case of uniform blowing, the skin

Haecheon Choi

1999-01-01

138

Boundary layer blockage in expansion tube nozzles  

NASA Technical Reports Server (NTRS)

The results of a first order perfect gas correction for the effects of the boundary layer formation within expansion tubes with nozzles are presented. The analytical model developed to describe the boundary layer formation within the expansion tube and an expansion nozzle located at the end of the acceleration tube is based on the Karman integral equations. The results of this analytical model are compared with experimental data from an expansion diffuser. The model provides a useful tool for the preliminary design of nozzles for such facilities.

Sudnitsin, Olga; Morgan, Richard G.

1995-01-01

139

Boundary-layer theory for blast waves  

NASA Technical Reports Server (NTRS)

It is profitable to consider the blast wave as a flow field consisting of two regions: the outer, which retains the properties of the inviscid solution, and the inner, which is governed by flow equations including terms expressing the effects of heat transfer and, concomitantly, viscosity. The latter region thus plays the role of a boundary layer. Reported here is an analytical method developed for the study of such layers, based on the matched asymptotic expansion technique combined with patched solutions.

Kim, K. B.; Berger, S. A.; Kamel, M. M.; Korobeinikov, V. P.; Oppenheim, A. K.

1975-01-01

140

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

2012-03-23

141

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

142

Turbulent Boundary Layer Flow over Superhydrophobic Surfaces.  

National Technical Information Service (NTIS)

The objective of this project was to determine whether drag caused by turbulence in boundary layer flow can be reduced through the use of modified surfaces. This study encompassed the testing of four different surfaces: 1) Teflon SLIP, 2) Aluminum SLIP, 3...

A. J. Rydalch

2013-01-01

143

Convection in the atmospheric boundary layer  

Microsoft Academic Search

Recent advances in the study of clear and partly cloudy convective boundary layers are reviewed. The techniques and results of observational studies of the structure and dynamics of convective elements are discussed. These findings have important consequences for pollution dispersion. They also form the basis for a test of LES dispersion models which is more discriminating than those based on

George S. Young

1988-01-01

144

Boundary layer control device for duct silencers  

NASA Technical Reports Server (NTRS)

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. (inventor); Soderman, Paul T. (inventor)

1993-01-01

145

Note on the calculation of boundary layers  

NASA Technical Reports Server (NTRS)

The properties of the solutions of the hydrodynamic equations of viscous fluid by "boundary-layer omission" are discussed. A method is indicated for the numerical determination of the solution for a known initial profile u(x(sub o),y) and pressure distribution p(x) within the region.

Prandtl, L

1940-01-01

146

Turbulences in Boundary Layer of Flat Plates  

NASA Astrophysics Data System (ADS)

The aeroelastic assessment of turbulences appearing in boundary layer of flat plates tested in the wind tunnel is treated in present paper. The approach suggested takes into account multiple functions in the analysis of flat plates subjected to laminar and turbulent wind forcing. Analysis and experimental assessments in the aerodynamic tunnel are presented. Some results obtained are discussed

Tesar, Alexander

2014-06-01

147

Soot and radiation in combusting boundary layers  

Microsoft Academic Search

In most fires thermal radiation is the dominant mode of heat transfer. Carbon particles within the fire are responsible for most of this emitted radiation and hence warrant quantification. As a first step toward understanding thermal radiation in full scale fires, an experimental and theoretical study is presented for a laminar combusting boundary layer. Carbon particulate volume fraction profiles and

1982-01-01

148

Soot and radiation in combusting boundary layers  

Microsoft Academic Search

In most fires thermal radiation is the dominant mode of heat transfer. Carbon particles within the fire are responsible for most of this emitted radiation and hence warrant quantification. As a first step toward understanding thermal radiation in full scale fires, an experimental and theoretical study is presented for a laminar combusting boundary layer. Carbon particulate volume fraction profiles and

1981-01-01

149

Boundary Layer Transition on X-43A  

NASA Technical Reports Server (NTRS)

The successful Mach 7 and 10 flights of the first fully integrated scramjet propulsion systems by the Hyper-X (X-43A) program have provided the means with which to verify the original design methodologies and assumptions. As part of Hyper-X s propulsion-airframe integration, the forebody was designed to include a spanwise array of vortex generators to promote boundary layer transition ahead of the engine. Turbulence at the inlet is thought to provide the most reliable engine design and allows direct scaling of flight results to groundbased data. Pre-flight estimations of boundary layer transition, for both Mach 7 and 10 flight conditions, suggested that forebody boundary layer trips were required to ensure fully turbulent conditions upstream of the inlet. This paper presents the results of an analysis of the thermocouple measurements used to infer the dynamics of the transition process during the trajectories for both flights, on both the lower surface (to assess trip performance) and the upper surface (to assess natural transition). The approach used in the analysis of the thermocouple data is outlined, along with a discussion of the calculated local flow properties that correspond to the transition events as identified in the flight data. The present analysis has confirmed that the boundary layer trips performed as expected for both flights, providing turbulent flow ahead of the inlet during critical portions of the trajectory, while the upper surface was laminar as predicted by the pre-flight analysis.

Berry, Scott; Daryabeigi, Kamran; Wurster, Kathryn; Bittner, Robert

2008-01-01

150

Turbulence in a separated boundary layer  

NASA Astrophysics Data System (ADS)

Results are reported from an extensive experimental investigation of a flat-plate turbulent boundary subjected to an adverse pressure gradient sufficiently strong to lead to the formation of a large separation region. The gradient was produced by applying strong suction through a porous cylinder fitted with a rear flap and mounted above the boundary layer with its axis in the spanwise direction. Data on the structure of the turbulent flow within the separation region are presented in extensive graphs, and many features are shown to be similar to those that occur in separated regions produced in a very dissimilar manner. Similarities with boundary layers separating under the action of much weaker pressure gradients are found as well. These similarities and also some noticeable differences are discussed, and several inferences concerning the application of turbulence models to separated flows are presented.

Dianat, M.; Castro, Ian P.

1991-05-01

151

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...

152

Experimental study of the boundary layer over an airfoil in plunging motion  

NASA Astrophysics Data System (ADS)

This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions. It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer. The wind tunnel measurements involved surfacemounted hot-film sensors and boundary-layer rake. The experiments were conducted at Reynolds numbers of 0.42×106 to 0.84 × 106 and the reduced frequency was varied from 0.01 to 0.11. The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases. For the static tests, boundary layer transition occurred through a laminar separation bubble. By increasing the angle of attack, disturbances and the transition location moved toward the leading edge. For the dynamic tests, earlier transition occurred with increasing rather than decreasing effective angle of attack. The mean angle of attack and the oscillating parameters significantly affected the state of the boundary layer. By increasing the reduced frequency, the boundary layer transition was promoted to the upstroke portion of the equivalent angle of attack, but the quasi skin friction coefficient was decreased.

Marzabadi, F. Rasi; Soltani, M. R.

2012-04-01

153

Numerical experiments on dense water descending with bottom boundary layer model in high latitude  

NASA Astrophysics Data System (ADS)

Importance of dense water formation in the Arctic shelf and slope has been suggested from observations in the Arctic coastal region, and several numerical experiments also showed dense water formation in the Arctic coast. The dense water is believed as one of the source of cold halocline, and is thought to transported from coastal region toward basin through mainly bottom boundary layer. Bottom boundary plays an important role in ocean circulation. Bottom boundary layer transports dense water to one's neutral depth on formation process of intermediate and deep water. Fine grid interval is needed to simulate bottom boundary layer, but now computational resources are not sufficient to simulate bottom boundary layer exactly, especially in global ocean circulation experiment with coarse grid. Several bottom boundary layer models therefore have been developed. But transport by bottom boundary layer is not simulated well as observed, and tends to underestimate the transport. It is important to improve bottom boundary layer model with coarse grid. So we carry out sensitivity experiments with bottom boundary layer model on its parameterization, especially on bottom friction and height of bottom boundary layer. We carry out numerical experiments of descending of dense water with and without bottom boundary layer. The primitive equation model with z-coordinate (MOM) is used for rectangular idealized model ocean. The model ocean assumes the shelf region in the Arctic Ocean, with zonal lateral boundary condition being cyclic. Bottom is inclined meridionally as southern boundary has 100 meters depth, and northern boundary depth has 470 meters depth. As forcing, surface salt flux is added from southern boundary to y=30 km. The forcing assumes brine rejection in coastal polynya in the Arctic. The flux gradually decreases from y =30 km, and becomes zero at 50 km, and is homogeneous zonally. Only salt flux is used, and surface wind and heat flux are not used. Bottom boundary layer following Gnanadesikan's model is used. The layer depth is constant. Numerical experiments with coarse grid are carried out to simulate descending of the dense water. The surface salt flux makes surface water denser at southern boundary. The water descends to bottom, and starts to flow along bottom slope. In case of experiments without bottom boundary layer model, dense water flows almost along coast. On the other hand, in case of experiments with bottom boundary layer model, dense water start to descends toward ocean basin. Even in case of experiments with coarse grid, dense water descending is reproduced. These transport are compared with results of fine grid experiments.

Takahashi, J.; Wang, J.; Ikeda, M.

2002-12-01

154

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

Microsoft Academic Search

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

Yoshitsugu Kimura; Ken Nakano; Takashi Kato; Shin Morishita

1994-01-01

155

Finite element\\/boundary element coupling for two-body elastoplastic contact problems with friction  

Microsoft Academic Search

We consider two-body contact problems in elastoplasticity (plasticity with isotropic hardening) with and without friction\\u000a and present solution procedures based on the coupling of finite elements and boundary elements. One solution method consists\\u000a in rewriting the problem with penalty terms taking care of the frictional contact conditions [4], see also [8]. Then, its\\u000a discretized version is solved by applying the

A. Chernov; S. Geyn; M. Maischak; E. P. Stephan

156

Force microscopy of layering and friction in an ionic liquid.  

PubMed

The mechanical properties of the ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]) in confinement between a SiOx and a Au(1?1?1) surface are investigated by means of atomic force microscopy (AFM) under electrochemical control. Up to 12 layers of ion pairs can be detected through force measurements while approaching the tip of the AFM to the surface. The particular shape of the force versus distance curve is explained by a model for the interaction between tip, gold surface and ionic liquid, which assumes an exponentially decaying oscillatory force originating from bulk liquid density correlations. Jumps in the tip-sample distance upon approach correspond to jumps of the compliant force sensor between branches of the oscillatory force curve. Frictional force between the laterally moving tip and the surface is detected only after partial penetration of the last double layer between tip and surface. PMID:24919549

Hoth, Judith; Hausen, Florian; Müser, Martin H; Bennewitz, Roland

2014-07-16

157

Force microscopy of layering and friction in an ionic liquid  

NASA Astrophysics Data System (ADS)

The mechanical properties of the ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]) in confinement between a SiOx and a Au(1?1?1) surface are investigated by means of atomic force microscopy (AFM) under electrochemical control. Up to 12 layers of ion pairs can be detected through force measurements while approaching the tip of the AFM to the surface. The particular shape of the force versus distance curve is explained by a model for the interaction between tip, gold surface and ionic liquid, which assumes an exponentially decaying oscillatory force originating from bulk liquid density correlations. Jumps in the tip–sample distance upon approach correspond to jumps of the compliant force sensor between branches of the oscillatory force curve. Frictional force between the laterally moving tip and the surface is detected only after partial penetration of the last double layer between tip and surface.

Hoth, Judith; Hausen, Florian; Müser, Martin H.; Bennewitz, Roland

2014-07-01

158

The effects of a favourable pressure gradient and of the Reynolds number on an incompressible axisymmetric turbulent boundary layer. Part 2. The boundary layer with relaminarization  

Microsoft Academic Search

This is an experimental investigation of two turbulent boundary layers (cases 2 and 4) where the streamwise negative pressure gradient changes mean properties of the flow, e.g. mean velocity profiles and skin friction, so that they display laminar-like behaviour. The maximum acceleration parameter K[less-than-or-eq, slant]4×10[minus sign]6 and the starting value of the Reynolds number is 862 or 2564. Relaminarization occurs

D. Warnack; H. H. Fernholz

1998-01-01

159

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

160

Particulate plumes in boundary layers with obstacles  

NASA Astrophysics Data System (ADS)

This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by non-slip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of big wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations.We deal with describing big field gradients with the aid of scheme viscosity of numerical algorithm used to model near-surface phenomena.

Petrosyan, Arakel; Karelsky, Kirill

2013-04-01

161

Hairpin vortices in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The present work addresses the question whether hairpin vortices are a dominant feature of near-wall turbulence and which role they play during transition. First, the parent-offspring mechanism is investigated in temporal simulations of a single hairpin vortex introduced in a mean shear flow corresponding to turbulent channels and boundary layers up to Re? = 590. Using an eddy viscosity computed from resolved simulations, the effect of a turbulent background is also considered. Tracking the vortical structure downstream, it is found that secondary hairpins are created shortly after initialization. Thereafter, all rotational structures decay, whereas this effect is enforced in the presence of an eddy viscosity. In a second approach, a laminar boundary layer is tripped to transition by insertion of a regular pattern of hairpins by means of defined volumetric forces representing an ejection event. The idea is to create a synthetic turbulent boundary layer dominated by hairpin-like vortices. The flow for Re? < 250 is analysed with respect to the lifetime of individual hairpin-like vortices. Both the temporal and spatial simulations demonstrate that the regeneration process is rather short-lived and may not sustain once a turbulent background has formed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former DNS studies is an outer layer phenomenon not being connected to the onset of near-wall turbulence.

Eitel-Amor, G.; Flores, O.; Schlatter, P.

2014-04-01

162

Friction  

NSDL National Science Digital Library

This web resource was created as a study in friction by students at an Advanced Mechanics class at Davidson College in Davidson. The resource includes a historical background as well as the various factors that affect friction. The analysis includes sections about Rolling Friction, Velocity Dependence, Stick-Slip mechanism, Adhesion, Wear, Lubrication, Viscosity and Wet vs. Dry Friction.

Conatser, Ryan; Kinnaman, Ben; Lassiter, Mike

2011-04-13

163

An Approximate Method for Calculation of the Laminar Boundary Layer with Suction for Bodies of Arbitrary Shape  

NASA Technical Reports Server (NTRS)

Various ways were tried recently to decrease the friction drag of a body in a flow; they all employ influencing the boundary layer. One of them consists in keeping the boundary layer Laminar by suction; promising tests have been carried out. Since for large Reynolds numbers the friction drag of the laminar boundary layer is much lower than that of the turbulent boundary layer, a considerable saving in drag results from keeping the boundary layer laminar, even with the blower power required for suction taken into account. The boundary layer is kept laminar by suction in two ways: first, by reduction of the thickness of the boundary layer and second, by the fact that the suction changes the form of the velocity distribution so that it becomes more stable, in a manner similar to the change by a pressure drop. There by the critical Reynolds number of the boundary layer (USigma*/V) (sub crit) becomes considerably higher than for the case without suction. This latter circumstance takes full effect only if continuous suction is applied which one might visualize realized through a porous wall. Thus the suction quantities required for keeping the boundary layer laminar become so small that the suction must be regarded as a very promising auxiliary means for drag reduction.

Schlichting, H.

1949-01-01

164

The Effect of Nonlinear Critical Layers on Boundary Layer Transition  

NASA Technical Reports Server (NTRS)

Asymptotic methods are used to describe the nonlinear self-interaction between pairs of oblique instability modes that eventually develops when initially linear and spatially growing instability waves evolve downstream in nominally two-dimensional and spanwise periodic laminar boundary layers. The first nonlinear reaction takes place locally within a so-called 'critical layer' with the flow outside this layer consisting of a locally parallel mean flow plus an appropriate superposition of linear instability waves. The amplitudes of these waves are determined by either a single integro-differential equation or by a pair of integro-differential equations with quadratic to quartic-type nonlinearities.

Goldstein, Marvin E.

1995-01-01

165

Changes in the turbulent boundary layer structure associated with net drag reduction by outer layer manipulators  

NASA Technical Reports Server (NTRS)

A specially designed wind tunnel was used to examine the effects of tandemly arranged parallel plate manipulators (TAPPMs) on a turbulent boundary-layer structure and the associated drag. Momentum balances, as well as measurements of the local shear stress from the velocity gradient near the wall, were used to obtain the net drag and local skin friction changes. Two TAPPMs, identical except for the thickness of their plates, were used in the study. Results with .003 inch plates were a maximum net drag reduction of 10 percent at 58 beta sub o (using a momentum balance). At 20 beta sub o, simultaneous laser sheet flow visualization and hot-wire anemometry data showed that the Reynolds stress in the large eddies was significantly reduced, as were the streamwise and normal velocity components. Using space-time correlations the reductions were again identified. Furthermore, quantitative flow visualization showed that the outward normal velocity of the inner region was also significantly decreased in the region around 20 beta sub o. However, throughout the first 130 beta sub o, the measured sublayer thickness with the TAPPMs in place was 15 to 20 percent greater. The data showed that the skin friction, as well as the structure of the turbulence, was strongly modified in the first 35 beta sub o, but that they both significantly relaxed toward unmanipulated boundary layer values by 50 beta sub o.

Rashidnia, N.; Falco, R. E.

1987-01-01

166

Towards Petascale DNS of High Reynolds-Number Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

In flight vehicles, a large portion of fuel consumption is due to skin-friction drag. Reduction of this drag will significantly reduce the fuel consumption of flight vehicles and help our nation to reduce CO 2 emissions. In order to reduce skin-friction drag, an increased understanding of wall-turbulence is needed. Direct numerical simulation (DNS) of spatially developing turbulent boundary layers (SDTBL) can provide the fundamental understanding of wall-turbulence in order to produce models for Reynolds averaged Navier-Stokes (RANS) and large-eddy simulations (LES). DNS of SDTBL over a flat plate at Retheta = 1430 - 2900 were performed. Improvements were made to the DNS code allowing for higher Reynolds number simulations towards petascale DNS of turbulent boundary layers. Mesh refinement and improvements to the inflow and outflow boundary conditions have resulted in turbulence statistics that match more closely to experimental results. The Reynolds stresses and the terms of their evolution equations are reported.

Webster, Keegan R.

167

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.

168

Boundary layer transition detection by luminescence imaging  

NASA Astrophysics Data System (ADS)

In recent experiments we have demonstrated the feasibility of a new approach to boundary layer transition detection. This new approach employs the temperature dependence of certain photoluminescent materials in the form of a surface coating or 'paint' to detect the change in heat transfer characteristics that accompany boundary layer transition. The feasibility experiments were conducted for low subsonic to transonic Mach numbers on two-dimensional airfoil and flat plate configurations. Paint derived transition locations were determined and compared to those obtained from Preston pressure probe measurements. Artificial heating of the models was used to obtain transition temperature signatures suitable for the instrumentation available to us. Initial estimates show, however, that passive kinetic heating at high Mach numbers is a promising alternative.

McLachlan, B. G.; Bell, J. H.; Gallery, J.; Gouterman, M.; Callis, J.

1993-01-01

169

BOREAS AFM-6 Boundary Layer Height Data  

NASA Technical Reports Server (NTRS)

The Boreal Ecosystem-Atmosphere Study (BOREAS) Airborne Fluxes and Meteorology (AFM)-6 team from National Oceanic and Atmospheric Adminsitration/Environment Technology Laboratory (NOAA/ETL) operated a 915-MHz wind/Radio Acoustic Sounding System (RASS) profiler system in the Southern Study Area (SSA) near the Old Jack Pine (OJP) site. This data set provides boundary layer height information over the site. The data were collected from 21 May 1994 to 20 Sep 1994 and are stored in tabular ASCII files. The boundary layer height data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).

Wilczak, James; Hall, Forrest G. (Editor); Newcomer, Jeffrey A. (Editor); Smith, David E. (Technical Monitor)

2000-01-01

170

Interaction between soil hydrology and boundary-layer development  

Microsoft Academic Search

A two-layer model of soil hydrology and thermodynamics is combined with a one-dimensional model of the planetary boundary layer to study various interactions between evolution of the boundary layer and soil moisture transport. Boundary-layer moistening through surface evaporation reduces the potential and actual surface evaporation as well as the boundary-layer growth. With more advanced stages of soil drying, the restricted

H.-L. Pan; L. Mahrt

1987-01-01

171

Dynamical modeling of marine boundary layer convection  

Microsoft Academic Search

The interaction between dynamics and infrared radiation is investigated, as well as the problem of entrainment instability in the cloud-topped marine boundary layer. A two-dimensional Boussinesq moist model with a numerical technique (Fourier-Chebysheve tau method) and resolution sufficient to simulate cloud top processes was developed. Previous measurements suggest that the cloud-top radiative cooling is likely to undergo significant horizontal and

Hung-Chi Kuo

1987-01-01

172

Dynamical Modeling of Marine Boundary Layer Convection  

Microsoft Academic Search

This dissertation investigates the interaction between dynamics and infrared radiation as well as the problem of entrainment instability in the cloud-topped marine boundary layer. A two-dimensional Boussinesq moist model with a numerical technique (Fourier-Chebysheve tau method) and resolution sufficient to simulate cloud top processes has been developed. Previous measurements suggest that the cloud-top radiative cooling is likely to undergo significant

Hung-Chi Kuo

1987-01-01

173

Lidar probing the urban nocturnal boundary layer  

Microsoft Academic Search

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

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

1998-01-01

174

Turbulent boundary layer studies using polynomial interpolation  

Microsoft Academic Search

For laminar flows, higher-order differential methods derived from polynomial spline interpolation and Hermitian collocation have significantly reduced the numbers of mesh points required to achieve accuracy equal to that of conventional second-order finite-difference methods. This study is intended to evaluate the applicability of two of these techniques for simulating the thicker, higher-shear turbulent boundary layer on a flat plate, with

S. G. Rubin; P. K. Khosla; S. Rivera

1977-01-01

175

Boundary Layer Control for Hypersonic Airbreathing Vehicles  

NASA Technical Reports Server (NTRS)

Active and passive methods for tripping hypersonic boundary layers have been examined in NASA Langley Research Center wind tunnels using a Hyper-X model. This investigation assessed several concepts for forcing transition, including passive discrete roughness elements and active mass addition (or blowing), in the 20-Inch Mach 6 Air and the 31-Inch Mach 10 Air Tunnels. Heat transfer distributions obtained via phosphor thermography, shock system details, and surface streamline patterns were measured on a 0.333-scale model of the Hyper-X forebody. The comparisons between the active and passive methods for boundary layer control were conducted at test conditions that nearly match the Hyper-X nominal Mach 7 flight test-point of an angle-of-attack of 2-deg and length Reynolds number of 5.6 million. For passive roughness, the primary parametric variation was a range of trip heights within the calculated boundary layer thickness for several trip concepts. The passive roughness study resulted in a swept ramp configuration, scaled to be roughly 0.6 of the calculated boundary layer thickness, being selected for the Mach 7 flight vehicle. For the active blowing study, the manifold pressure was systematically varied (while monitoring the mass flow) for each configuration to determine the jet penetration height, with schlieren, and transition movement, with the phosphor system, for comparison to the passive results. All the blowing concepts tested, which included various rows of sonic orifices (holes), two- and three-dimensional slots, and random porosity, provided transition onset near the trip location with manifold stagnation pressures on the order of 40 times the model surface static pressure, which is adequate to ensure sonic jets. The present results indicate that the jet penetration height for blowing was roughly half the height required with passive roughness elements for an equivalent amount of transition movement.

Berry, Scott A.; Nowak, Robert J.; Horvath, Thomas J.

2004-01-01

176

Shock-wave boundary layer interactions  

NASA Technical Reports Server (NTRS)

Presented is a comprehensive, up-to-date review of the shock-wave boundary-layer interaction problem. A detailed physical description of the phenomena for transonic and supersonic speed regimes is given based on experimental observations, correlations, and theoretical concepts. Approaches for solving the problem are then reviewed in depth. Specifically, these include: global methods developed to predict sudden changes in boundary-layer properties; integral or finite-difference methods developed to predict the continuous evolution of a boundary-layer encountering a pressure field induced by a shock wave; coupling methods to predict entire flow fields; analytical methods such as multi-deck techniques; and finite-difference methods for solving the time-dependent Reynolds-averaged Navier-Stokes equations used to predict the development of entire flow fields. Examples are presented to illustrate the status of the various methods and some discussion is devoted to delineating their advantages and shortcomings. Reference citations for the wide variety of subject material are provided for readers interested in further study.

Delery, J.; Marvin, J. G.; Reshotko, E.

1986-01-01

177

Longitudinal vortices in boundary layer transition  

NASA Astrophysics Data System (ADS)

It is recognized that longitudinal vortices in boundary layer can attain significant amplitudes under the influence of moderate to high freestream turbulence owing to transient growth. The present theoretical-numerical study is carried out in order to understand the origin and effect of longitudinal vortices in boundary layer transition. The origin of longitudinal vortices is connected to freestream turbulence through the continuum modes of the Orr-Sommerfeld and Squire equations. With the inclusion of transverse flow velocity, the theoretical results match several inherent characteristics of longitudinal vortices observed experimentally. The linear instability of the flow with longitudinal vortices superimposed upon an otherwise unperturbed boundary layer is examined using the Floquet theory. Under mild modulations, the three-dimensional unstable modes have the same spanwise wavelength as longitudinal vortices and maintain the characteristics of two-dimensional Tollmien-Schlichting (TS) waves. If the spanwise modulation is sufficiently strong, a new type of instability, very different from TS waves, appears. The unstable modes of the new instability intensify as a result of subharmonic resonance at rates and frequencies much higher than TS waves. The subharmonic modes have a modal shape different from that of TS waves and they propagate faster than the TS waves. Longitudinal vortices with narrower spanwise length scales will cause subharmonic secondary modes to grow at higher frequencies. It is likely that the fast amplification of the secondary modes, not longitudinal vortices themselves, will lead to imminent transition.

Su, Yi-Chung

1999-11-01

178

Evaluating Langmuir turbulence parameterizations in the ocean surface boundary layer  

NASA Astrophysics Data System (ADS)

is expected that surface gravity waves play an important role in the dynamics of the ocean surface boundary layer (OSBL), quantified with the turbulent Langmuir number (La=?{u&/us0}, where u* and us0 are the friction velocity and surface Stokes drift, respectively). However, simultaneous measurements of the OSBL dynamics along with accurate measurements of the wave and atmospheric forcing are lacking. Measurements of the turbulent dissipation rate ? were collected using the Air-Sea Interaction Profiler (ASIP), a freely rising microstructure profiler. Two definitions for the OSBL depth are used: the mixed layer derived from measurements of density >(h?>), and the mixing layer >(h?>) determined from direct measurements of ?. When surface buoyancy forces are relatively small, ??La-2 only near the surface with no dependency on La at mid-depths of the OSBL when using h? as the turbulent length scale. However, if h? is used then the dependence of ? with La-2 is more uniform throughout the OSBL. For relatively high destabilizing surface buoyancy forces, ? is proportional to the ratio of the OSBL depth against the Langmuir stability length LL. During destabilizing conditions, the mixed and mixing layer depths are nearly identical, but we have relatively few measurements under these conditions, rather than any physical implications. Observations of epsilon are compared with the OSBL regime diagram of Belcher et al. (2012) and are generally within an order of magnitude, but there is an improved agreement if h? is used as the turbulent length scale rather than h?.

Sutherland, G.; Christensen, K. H.; Ward, B.

2014-03-01

179

A model of the planetary boundary layer over a snow surface  

NASA Technical Reports Server (NTRS)

A model of the planetary boundary layer over a snow surface has been developed. It contains the vertical heat exchange processes due to radiation, conduction, and atmospheric turbulence. Parametrization of the boundary layer is based on similarity functions developed by Hoffert and Sud (1976), which involve a dimensionless variable, dependent on boundary-layer height and a localized Monin-Obukhov length. The model also contains the atmospheric surface layer and the snowpack itself, where snowmelt and snow evaporation are calculated. The results indicate a strong dependence of surface temperatures, especially at night, on the bursts of turbulence which result from the frictional damping of surface-layer winds during periods of high stability, as described by Businger (1973). The model also shows the cooling and drying effect of the snow on the atmosphere, which may be the mechanism for air mass transformation in sub-Arctic regions.

Halberstam, I.; Melendez, R.

1979-01-01

180

The role of nonlinear critical layers in boundary layer transition  

NASA Technical Reports Server (NTRS)

Asymptotic methods are used to describe the nonlinear self-interaction between pairs of oblique instability modes that eventually develops when initially linear spatially growing instability waves evolve downstream in nominally two-dimensional laminar boundary layers. The first nonlinear reaction takes place locally within a so-called 'critical layer', with the flow outside this layer consisting of a locally parallel mean flow plus a pair of oblique instability waves - which may or may not be accompanied by an associated plane wave. The amplitudes of these waves, which are completely determined by nonlinear effects within the critical layer, satisfy either a single integro-differential equation or a pair of integro-differential equations with quadratic to quartic-type nonlinearities. The physical implications of these equations are discussed.

Goldstein, M.E.

1995-01-01

181

Relaxation of an unsteady turbulent boundary layer on a flat plate in an expansion tube  

NASA Technical Reports Server (NTRS)

An analysis is presented for the relaxation of a turbulent boundary layer on a semi-infinite flat plate after passage of a shock wave and a trailing driver gas-driven gas interface. The problem has special application to expansion-tube flows. The flow-governing equations have been transformed into the Crocco variables, and a time-similar solution is presented in terms of the dimensionless distance-time variable alpha and the dimensionless velocity variable beta. An eddy-viscosity model, similar to that of time-steady boundary layers, is applied to the inner and outer regions of the boundary layer. A turbulent Prandtl number equal to the molecular Prandtl number is used to relate the turbulent heat flux to the eddy viscosity. The numerical results, obtained by using the Gauss-Seidel line-relaxation method, indicate that a fully turbulent boundary layer relaxes faster to the final steady-state values of heat transfer and skin friction than a laminar boundary layer. The results also give a fairly good estimate of the local skin friction and heat transfer for near steady-flow conditions.

Gurta, R. N.; Trimpi, R. L.

1974-01-01

182

Boundary Layer Experiment 1996 (BLX96).  

NASA Astrophysics Data System (ADS)

The University of Wyoming King Air aircraft was the primary instrument platform for turbulence measurements in the bottom half of the convective boundary layer during 15 July-13 August 1996. A total of 12 successful research flights were made, each of about 4.5-h duration. Crosswind (east-west) flight patterns were flown in Oklahoma and Kansas over three sites of different land use: forest, pasture, and crops.Measurements of mean values, turbulent deviations, and turbulent fluxes of temperature, moisture, and momentum were made to test theories of convective transport, the radix layer, and cumulus potential. Additional portions of each flight included slant soundings and near-surface horizontal flights in order to determine mixed layer (ML) scaling variables such as ML depth zi, Deardorff velocity w and buoyancy velocity wB. While the ML was shallower and the ground wetter than anticipated based on climatology, a high-quality dataset was obtained.

Stull, Roland; Santoso, Edi; Berg, Larry; Hacker, Joshua

1997-06-01

183

Calculation of Turbulent Boundary Layer Wall Pressure Spectra.  

National Technical Information Service (NTIS)

This study is an investigation into the suitability of various wavevector-frequency models of turbulent boundary layer wall pressure fluctuations for the prediction of experimental measurements of turbulent boundary layer wall pressure spectra. Three sepa...

D. E. Capone G. C. Lauchle

1993-01-01

184

Boundary layer transition and separation on a compressor rotor airfoil  

SciTech Connect

This paper presents boundary layer calculations based on these potential flow results and comparison with surface flow visualization results indicating the locations of boundary layer transition and separation. 4 refs.

Dring, R.P.

1982-01-01

185

Numerical solution of the free convection boundary layer flow over a horizontal circular cylinder with convective boundary conditions  

NASA Astrophysics Data System (ADS)

Numerical solution for the steady free convection boundary layer flow near the lower stagnation point of a horizontal circular cylinder subjected to a convective boundary condition, where the heat is supplied to the fluid through a bounding surface with a finite heat capacity are presented in this paper. The governing boundary layer equations are transformed using non-similar variables into non-similar equations and were solved numerically using an implicit finite difference scheme known as the Keller-box method. The solutions are obtained for the skin friction coefficient, the local wall temperature, as well as the velocity and temperature profiles with two the variations of two parameters, namely the conjugate parameter ? and the Prandtl number Pr.

Sarif, Norhafizah Md; Salleh, Mohd Zuki; Tahar, Razman Mat; Nazar, Roslinda

2014-06-01

186

Leaky waves in boundary layer flow  

NASA Astrophysics Data System (ADS)

Linear stability analysis of boundary layer flow is traditionally performed by solving the Orr-Sommerfeld equation (OSE), either in a temporal or a spatial framework. The mode structure of the OSE is in both cases composed of a finite number of discrete modes which decay at infinity in the wall- normal direction y, and a continuous spectrum of propagating modes behaving as (±ik y) when y->?, with real k. A peculiarity of this structure is that the number of discrete modes changes with the Reynolds number, Re. They indeed seem to disappear behind the continuous spectrum at certain Re. This phenomenon is here investigated by studying the response of the Blasius boundary layer forced instantaneously in space and time. Since the solution of the forced and homogeneous Laplace-transformed problem both depend on the free-stream boundary conditions, it is shown here that a suitable change of variables can remove the branch cut in the Laplace plane. As a result, integration of the inverse Laplace transform along the two sides of the branch cut, which gives rise to the continuous spectrum, can be replaced by a sum of residues corresponding to an additional set of discrete eigenvalues. These new modes grow at infinity in the y direction, and are analogous to the leaky waves found in the theory of optical waveguides, i.e. optical fibers, which are attenuated in the direction of the waveguide but grow unbounded in the direction perpendicular to it.

Pralits, Jan

2005-11-01

187

Some further thoughts on the zero-pressure-gradient turbulent boundary layer  

NASA Astrophysics Data System (ADS)

In spite of its remarkable ability to describe recent friction and integral boundary layer thickness data, there have been consistent claims that power-law based theories of turbulent boundary layers are not viable. In particular it is argued that they do not properly describe the velocity data in the overlap region near the wall. These claims are examined in detail using recent experiments for which the data is publicly available. (Smith and Osterlund). The mean velocity data from both experiments are seen to be described by the George/Castillo/Wosnik theoretical profile to within fractions of a percent for y^+ > 30.

George, W. K.

2005-11-01

188

Analysis of a turbulent boundary layer over a moving ground plane  

NASA Technical Reports Server (NTRS)

Four methods of predicting the integral and friction parameters for a turbulent boundary layer over a moving ground plane were evaluated by using test information obtained in 76.2- by 50.8-centimeter tunnel. The tunnel was operated in the open sidewall configuration. These methods are (1) relative integral parameter method, (2) modified power law method, (3) relative power law method, and (4) modified law of the wall method. The modified law of the wall method predicts a more rapid decrease in skin friction with an increase in the ratio of belt velocity to free steam velocity than do methods (1) and (3).

Roper, A. T.; Gentry, G. L., Jr.

1972-01-01

189

Turbulence modeling for sharp-fin-induced shock wave/turbulent boundary-layer interactions  

NASA Technical Reports Server (NTRS)

Solutions of the Reynolds averaged Navier-Stokes equations are presented and compared with a family of experimental results for the 3-D interaction of a sharp fin induced shock wave with a turbulent boundary layer. Several algebraic and two equation eddy viscosity turbulence models are employed. The computed results are compared with experimental surface pressure, skin friction, and yaw angle data as well as the overall size of the interaction. Although the major feature of the flow fields are correctly predicted, several discrepancies are noted. Namely, the maximum skin friction values are significantly underpredicted for the strongest interaction cases. These and other deficiencies are discussed.

Horstman, C. C.

1990-01-01

190

Spatial Linear Instability of Confluent Wake/Boundary Layers.  

National Technical Information Service (NTIS)

The spatial linear instability of incompressible confluent wake/boundary layers is analyzed. The flow model adopted is a superposition of the Blasius boundary layer and a wake located above the boundary layer. The Orr-Sommerfeld equation is solved using a...

W. W. Liou F. J. Liu

2001-01-01

191

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

192

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

193

Acoustic radar investigations of boundary layer phenomena  

NASA Technical Reports Server (NTRS)

A comparison is made between acoustic radar echoes and conventional meteorological data obtained from the WKY tower, for the purpose of better understanding the relationships between acoustic radar echoes and boundary layer processes. Two thunderstorm outflow cases are presented and compared to both acoustic radar data and Charba's gust front model. The acoustic radar echoes reveal the boundary between warm and cold air and other areas of mixing and strong thermal gradient quite well. The thunderstorm outflow of 27 June 1972 is found to compare with in most respects to Charba's gust front model. The major difference is the complete separation of the head from the main body of cold air, probably caused by erosion of the area behind the head by mixing with the ambient air. Two cases of nocturnal inversions caused by advection of warmer air aloft are presented. It is found that areas of turbulent mixing or strong thermal gradient can be identified quite easily in the acoustic radar record.

Marks, J. R.

1974-01-01

194

Flow Quality and Boundary Layer Transition  

NASA Technical Reports Server (NTRS)

The widely held view is that transition to turbulence in the Blasius boundary layer occurs via amplification and eventual nonlinear breakdown of initially small amplitude instabilities i.e. Tollmien-Schlichting (TS) waves. However this scenario is only observed for low amplitude free-stream turbulence levels, i.e. u/U < 0.1%. Bypass of linear TS instability mechanism occurs for higher EST levels, yet considerable differences exist between the few experiments carefully designed to assess the effect of EST on transition. The consensus is that EST leads to longitudinal streaks that form near the leading edge in the boundary layer . These streaks appeal to be regions of concentrated streamwise vorticity and they are often referred to as Klebanoff modes. The importance of mean flow free-stream nonuniformity (FSN) is not as widely appreciated as EST for characterizing wind tunnel flow quality. Here it is shown that, although the v like generated by a d=50micron wire located upstream of the contraction (Re(sub d)=6.6, x/d=45,000) is immeasurably small by the time it interacts with the leading edge in the test section, it is responsible for generation of a pair of weak streamwise vortices in the boundary layer downstream. The characteristics of these wake-induced vortices and their effect on TS waves are demonstrated. Small remnant FSN variations are also shown to exist downstream of a turbulence grid. The question arises Are the adverse effects introduced by the turbulence grid caused by FST or by small remnant FSN variations?

Watmuff, Jonathan H; Tobak, M.; Davis, Sanford S. (Technical Monitor)

1997-01-01

195

Boundary-Layer-Ingesting Inlet Flow Control  

NASA Technical Reports Server (NTRS)

This paper gives an overview of a research study conducted in support of the small-scale demonstration of an active flow control system for a boundary-layer-ingesting (BLI) inlet. The effectiveness of active flow control in reducing engine inlet circumferential distortion was assessed using a 2.5% scale model of a 35% boundary-layer-ingesting flush-mounted, offset, diffusing inlet. This experiment was conducted in the NASA Langley 0.3-meter Transonic Cryogenic Tunnel at flight Mach numbers with a model inlet specifically designed for this type of testing. High mass flow actuators controlled the flow through distributed control jets providing the active flow control. A vortex generator point design configuration was also tested for comparison purposes and to provide a means to examine a hybrid vortex generator and control jets configuration. Measurements were made of the onset boundary layer, the duct surface static pressures, and the mass flow through the duct and the actuators. The distortion and pressure recovery were determined by 40 total pressure measurements on 8 rake arms each separated by 45 degrees and were located at the aerodynamic interface plane. The test matrix was limited to a maximum free-stream Mach number of 0.85 with scaled mass flows through the inlet for that condition. The data show that the flow control jets alone can reduce circumferential distortion (DPCP(sub avg)) from 0.055 to about 0.015 using about 2.5% of inlet mass flow. The vortex generators also reduced the circumferential distortion from 0.055 to 0.010 near the inlet mass flow design point. Lower inlet mass flow settings with the vortex generator configuration produced higher distortion levels that were reduced to acceptable levels using a hybrid vortex generator/control jets configuration that required less than 1% of the inlet mass flow.

Owens, Lewis R.; Allan, Brian G.; Gorton, Susan A.

2006-01-01

196

Boundary-Layer-Ingesting Inlet Flow Control  

NASA Technical Reports Server (NTRS)

This paper gives an overview of a research study conducted in support of the small-scale demonstration of an active flow control system for a boundary-layer-ingesting (BLI) inlet. The effectiveness of active flow control in reducing engine inlet circumferential distortion was assessed using a 2.5% scale model of a 35% boundary-layer-ingesting flush-mounted, offset, diffusing inlet. This experiment was conducted in the NASA Langley 0.3-meter Transonic Cryogenic Tunnel at flight Mach numbers with a model inlet specifically designed for this type of testing. High mass flow actuators controlled the flow through distributed control jets providing the active flow control. A vortex generator point design configuration was also tested for comparison purposes and to provide a means to examine a hybrid vortex generator and control jets configuration. Measurements were made of the onset boundary layer, the duct surface static pressures, and the mass flow through the duct and the actuators. The distortion and pressure recovery were determined by 40 total pressure measurements on 8 rake arms each separated by 45 degrees and were located at the aerodynamic interface plane. The test matrix was limited to a maximum free-stream Mach number of 0.85 with scaled mass flows through the inlet for that condition. The data show that the flow control jets alone can reduce circumferential distortion (DPCPavg) from 0.055 to about 0.015 using about 2.5% of inlet mass flow. The vortex generators also reduced the circumferential distortion from 0.055 to 0.010 near the inlet mass flow design point. Lower inlet mass flow settings with the vortex generator configuration produced higher distortion levels that were reduced to acceptable levels using a hybrid vortex generator/control jets configuration that required less than 1% of the inlet mass flow.

Owens, Lewis R.; Allan, Brian G.; Gorton, Susan A.

2006-01-01

197

SAID: A turbulent plasmaspheric boundary layer (Invited)  

NASA Astrophysics Data System (ADS)

The new features of subauroral ion drifts (SAID) observed from the Cluster, DMSP, and Polar satellites are reviewed, including the discovery of SAID-related plasma waves. These observations confirm and expand on the Mishin and Puhl-Quinn [GRL, 34, L24101, 2007] concept of the SAID channel being a turbulent boundary layer, formed via a short circuit of the substorm-injected plasmoid by the plasmasphere. These observations show that SAID formation is related to enhanced lower hybrid/fast magnetosonic waves. Their excitation leads to anomalous circuit resistivity and magnetic diffusion, similar to the well-documented plasmoid-magnetic barrier problem, including impulsive penetration at the magnetopause.

Mishin, E. V.

2010-12-01

198

SAID: A turbulent plasmaspheric boundary layer  

NASA Astrophysics Data System (ADS)

This paper presents novel features of subauroral ion drifts (SAID) observed from a unique conjunction of the Cluster, DMSP, and Polar satellites, including the discovery of SAID-related plasma waves. These observations confirm and expand on our proposed concept of the SAID channel being a turbulent boundary layer, formed via a short circuit of the substorm-injected plasmoid by the plasmasphere. We show that SAID formation is related to enhanced lower hybrid/fast magnetosonic waves. Their excitation leads to anomalous circuit resistivity and magnetic diffusion, similar to the well-documented plasmoid-magnetic barrier problem, including impulsive penetration at the magnetopause.

Mishin, E. V.; Puhl-Quinn, P. A.; Santolik, O.

2010-04-01

199

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

200

Boundary-layer Transition at Supersonic Speeds  

NASA Technical Reports Server (NTRS)

Recent results of the effects of Mach number, stream turbulence, leading-edge geometry, leading-edge sweep, surface temperature, surface finish, pressure gradient, and angle of attack on boundary-layer transition are summarized. Factors that delay transition are nose blunting, surface cooling, and favorable pressure gradient. Leading-edge sweep and excessive surface roughness tend to promote early transition. The effects of leading-edge blunting on two-dimensional surfaces and surface cooling can be predicted adequately by existing theories, at least in the moderate Mach number range.

Low, George M

1956-01-01

201

Performance and boundary-layer evaluation of a sonic inlet  

NASA Technical Reports Server (NTRS)

Tests were conducted to determine the boundary layer characteristics and aerodynamic performance of a radial vane sonic inlet with a length/diameter ratio of 1 for several vane configurations. The sonic inlet was designed with a slight wavy wall type of diffuser geometry, which permits operation at high inlet Mach numbers (sufficiently high for good noise suppression) without boundary layer flow separation and with good total pressure recovery. A new method for evaluating the turbulent boundary layer was developed to separate the boundary layer from the inviscid core flow, which is characterized by a total pressure variation from hub to tip, and to determine the experimental boundary layer parameters.

Schmidt, J. F.; Ruggeri, R. S.

1976-01-01

202

Soot profiles in boundary-layer flames  

SciTech Connect

Carbon particulate volume fractions and approximate particle size distributions are measured in a free laminar combusting boundary layer for liquid hydrocarbon fuels (n-heptane, iso-octane, cyclohexane, cyclohexene, toluene) and polymethylmethacrylate (PMMA). A multiwavelength laser transmission technique determines a most probable radius and the total particle concentration, which are two parameters in an assumed form for the size distribution. In the combusting boundary layer, a sooting region exists between the pyrolyzing fuel surface and the flame zone. The liquid fuel soot volume fractions, f/sub v/, range from f/sub v/ approx. 10/sup -7/ for n-heptane, a paraffin, to f/sub v approx. 10/sup -5/ for toluene, an aromatic. The PMMA volume fractions, f/sub v/ approx. 5 X 10/sup -7/, are approximately the same as the values previously reported for pool fires. The soot volume fractions increase with height; convection of carbon particles downstream widens the soot region with height. For all fuels tested, the most probable radius is between 20 nm and 50 nm, and it changes only slightly with height and distance from the fuel surface.

Beier, R.A.; Pagni, P.J.

1981-12-01

203

Boundary Layer Transition Flight Experiment Overview  

NASA Technical Reports Server (NTRS)

In support of the Boundary Layer Transition Flight Experiment (BLT FE) Project, a manufactured protuberance tile was installed on the port wing of Space Shuttle Orbiter Discovery for STS-119, STS-128, STS-131 and STS-133 as well as Space Shuttle Endeavour for STS-134. Additional instrumentation was installed in order to obtain more spatially resolved measurements downstream of the protuberance. This paper provides an overview of the BLT FE Project with emphasis on the STS-131 and STS-133 results. A high-level overview of the in-situ flight data is presented, along with a summary of the comparisons between pre- and post-flight analysis predictions and flight data. Comparisons show that empirically correlated predictions for boundary layer transition onset time closely match the flight data, while predicted surface temperatures were significantly higher than observed flight temperatures. A thermocouple anomaly observed on a number of the missions is discussed as are a number of the mitigation actions that will be taken on the final flight, STS-134, including potential alterations of the flight trajectory and changes to the flight instrumentation.

Berger, Karen T.; Anderson, Brian P.; Campbell, Charles H.; Garske, Michael T.; Saucedo, Luis A.; Kinder, Gerald R.; Micklos, Ann M.

2011-01-01

204

BUBBLE an Urban Boundary Layer Meteorology Project  

NASA Astrophysics Data System (ADS)

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 and a Lidar near the city center were profiling the entire lower troposphere. During an intensive observation period (IOP) of one month duration, several sub-studies on street canyon energetics and satellite ground truth, as well as on urban turbulence and profiling (sodar, RASS, tethered balloon) were performed. Also tracer experiments with near-roof-level release and sampling were performed. In parallel to the experimental activities within BUBBLE, a meso-scale numerical atmospheric model, which contains a surface exchange parameterization, especially designed for urban areas was evaluated and further developed. Finally, the area of the full-scale tracer experiment which also contains several sites of other special projects during the IOP (street canyon energetics, satellite ground truth) is modeled using a very detailed physical scale-model in a wind tunnel. In the present paper details of all these activities are presented together with first results.

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

2005-07-01

205

Investigations of Compression Shocks and Boundary Layers in Gases Moving at High Speed  

NASA Technical Reports Server (NTRS)

The mutual influences of compression shocks and friction boundary layers were investigated by means of high speed wind tunnels.Schlieren optics provided a clear picture of the flow phenomena and were used for determining the location of the compression shocks, measurement of shock angles, and also for Mach angles. Pressure measurement and humidity measurements were also taken into consideration.Results along with a mathematical model are described.

Ackeret, J.; Feldmann, F.; Rott, N.

1947-01-01

206

Boundary layer parametrization for atmospheric diffusion models by meteorological measurements at ground level  

Microsoft Academic Search

Summary  Over recent years several theoretical and experimental works have made it possible to evaluate the basic parameters describing\\u000a the atmospheric boundary layer by means of elementary meteorological measurements in proximity to the ground. This has opened\\u000a up the possibility of utilizing atmospheric pollutant dispersion models able to use direct evaluations of atmospheric turbulence\\u000a through friction velocity and the Monin-Obukhov length.

R. Bellasio; G. Lanzani; M. Tamponi; T. Tirabassi

1994-01-01

207

Active and inactive motions in a turbulent boundary layer - Interactions with free-stream turbulence  

Microsoft Academic Search

The ideas of active and inactive motions are used to study the effects of grid-generated turbulence on flat plate turbulent boundary layers. Three-component laser doppler velocimetry measurements were made in flows with 5 percent and 7.5 percent freestream turbulence intensity. Skin friction measurements indicate that increases in shear stress production may be limited even as freestream turbulence levels are increased.

P. L. Johnson; J. P. Johnston

1989-01-01

208

Bypass transition in compressible boundary layers  

NASA Technical Reports Server (NTRS)

Transition to turbulence in aerospace applications usually occurs in a strongly disturbed environment. For instance, the effects of free-stream turbulence, roughness and obstacles in the boundary layer strongly influence transition. Proper understanding of the mechanisms leading to transition is crucial in the design of aircraft wings and gas turbine blades, because lift, drag and heat transfer strongly depend on the state of the boundary layer, laminar or turbulent. Unfortunately, most of the transition research, both theoretical and experimental, has focused on natural transition. Many practical flows, however, defy any theoretical analysis and are extremely difficult to measure. Morkovin introduced in his review paper the concept of bypass transition as those forms of transition which bypass the known mechanisms of linear and non-linear transition theories and are currently not understood by experiments. In an effort to better understand the mechanisms leading to transition in a disturbed environment, experiments are conducted studying simpler cases, viz. the effects of free stream turbulence on transition on a flat plate. It turns out that these experiments are very difficult to conduct, because generation of free stream turbulence with sufficiently high fluctuation levels and reasonable homogeneity is non trivial. For a discussion see Morkovin. Serious problems also appear due to the fact that at high Reynolds numbers the boundary layers are very thin, especially in the nose region of the plate where the transition occurs, which makes the use of very small probes necessary. The effects of free-stream turbulence on transition are the subject of this research and are especially important in a gas turbine environment, where turbulence intensities are measured between 5 and 20 percent, Wang et al. Due to the fact that the Reynolds number for turbine blades is considerably lower than for aircraft wings, generally a larger portion of the blade will be in a laminar transitional state. The effects of large free stream turbulence in compressible boundary layers at Mach numbers are examined both in the subsonic and transonic regime using direct numerical simulations. The flow is computed over a flat plate and curved surface. while many applications operate in the transonic regime. Due the nature of their numerical scheme, a non-conservation formulation of the Navier-Stokes equations, it is a non-trivial extension to compute flow fields in the transonic regime. This project aims at better understanding the effects of large free-stream turbulence in compressible boundary layers at mach number both in the subsonic and transonic regime using direct numerical simulations. The present project aims at computing the flow over a flat plate and curved surface. This research will provide data which can be used to clarify mechanisms leading to transition in an environment with high free stream turbulence. This information is useful for the development of turbulence models, which are of great importance for CFD applications, and are currently unreliable for more complex flows, such as transitional flows.

Vandervegt, J. J.

1992-01-01

209

Reduction of friction and wear by grooves applied on the nanoscale polished surface in boundary lubrication conditions  

PubMed Central

The evolution of a friction surface geometry with initially directed microscale grooves on a nanoscale polished surface in ring-on-block sliding contact is studied experimentally. Reduced wear and friction is observed when the orientation of grooves coincides with the direction of sliding. A new compressive-vacuum hypothesis of friction force nature under a condition of boundary lubrication is proposed, which successfully explains the observed phenomena. Grooves supply lubricant into the contact zone and facilitate its devacuumization, which lead to substantial reduction of surface wear. The obtained results enable developing optimized roughness profiles of friction surfaces to create high-performance durable friction units.

2014-01-01

210

A study of the effects of Lebu devices on turbulent boundary layer drag  

NASA Technical Reports Server (NTRS)

Initial measurements of the changes in local skin friction, velocity profile shape, and turbulence structure which result from the placement of tandem plates parallel to the wall in the outer region of thick turbulent boundary layers were made. Using a tunnel with a .75 m x 1.2 m x 7.3 m test section, which diverged so as to keep the pressure gradient less than 2x1000/ft, on the test wall, a skin friction reduction of approximately 30% was measured at xi/h = 62. This relaxed to a reduction of approximately 16% at xi/h = 124 for h/delta M = .6. The c sub f measurements for both the normal and modified boundary layers were obtained by measuring the slope of the velocity profile within the linear sublayer. Visual results indicated a continued presence of strong large eddy structure downstream of the devices. Local skin friction reduction of 12% at xi/h = 62 was also obtained with the manipulators above the boundary layer at y/delta m = 1.1.

Falco, R. E.

1983-01-01

211

An eddy-viscosity treatment of the unsteady turbulent boundary layer on a flat plate in an expansion tube  

NASA Technical Reports Server (NTRS)

An analysis is presented for the relaxation of a turbulent boundary layer on a semiinfinite flat plate after passage of a shock wave and a trailing driver gas-driven gas interface. The problem has special application to expansion tube flows. The flow-governing equations have been transformed into the Lamcrocco variables. The numerical results indicate that a fully turbulent boundary layer relaxes faster to the final steady-state values of heat transfer and skin-friction than a fully laminar boundary layer.

Gupta, R. N.; Trimpi, R. L.

1974-01-01

212

Application of Vortex Generators to Bubbly Boundary Layers  

NASA Astrophysics Data System (ADS)

The use of micro-bubbles for skin friction drag reduction has been known at least since the 1970's. However, a limitation on the effectiveness of micro-bubbles has been bubble migration, whereby bubbles move to the outer parts of the boundary layer. Since these bubbles need to be fairly close to the wall to be useful (typically in the buffer layer or closer), this reduces the usefulness of bubbles for drag reduction. In this project, we have investigated the use of wall-mounted vortex generators as a method for holding bubbles in the near wall region. We have tested a variety of different vortex generator geometries and will show what kind of effect they have on bubble migration. For these tests, bubbles were generated by electrolysis directly from the vortex generators, in order to maximize the injection of bubbles into the vortices. The authors would like to thank the Office of Naval Research for their generous support under Grant No. N00014-00-1-0110.

Jeon, David; Gharib, Mory

2004-11-01

213

Investigation of turbulent boundary layer structures using Tomographic PIV  

NASA Astrophysics Data System (ADS)

Tomographic particle image velocimetry (TPIV) data were acquired in the logarithmic region of a zero pressure gradient turbulent boundary layer flow at friction Reynolds number Re? = 1160. Experiments were conducted in a suction type wind tunnel seeded with olive oil particles of diameter ˜ 1?m. The volume of interest was illuminated by two Nd:YAG laser beams expanded with appropriate optics into sheets of 8mm thickness in the wall-normal direction (z). Images were acquired by four 2k x 2k pixel cameras, and correlation of reconstructed fields provided the full velocity gradient tensor in a volume of 0.7? x 0.7? x 0.07?, which resolved the region z^+ = 70-150 in the log layer. Various vortex identification techniques, such as Galilean decomposition and iso-surfaces of two- and three-dimensional swirl, were utilized to visualize and analyze the eddy structures present in instantaneous fields. The results of the present study will be compared to results from earlier experimental studies that relied on planar PIV data only to identify vortices and vortex packets as well as from a direct numerical simulation of fully developed channel flow at comparable Re?.

Saikrishnan, Neelakantan; Longmire, Ellen; Wieneke, Bernd

2008-11-01

214

Acoustics of friction.  

PubMed

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. PMID:12002837

Akay, Adnan

2002-04-01

215

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

216

Modification in drag of turbulent boundary layers resulting from manipulation of large-scale structures  

NASA Technical Reports Server (NTRS)

The effects of placing a parallel-plate turbulence manipulator in a boundary layer are investigated by means of flow visualization and hot-wire measurements. The manipulator was found to be effective in inhibiting the intermittent large scale structure of the boundary layer, and the effect persists for at least 70 boundary-layer thicknesses downstream of the manipulator. With the removal of the large scale, there was an observed reduction in the streamwise turbulence intensity levels near the wall. The downstream distribution of the skin friction coefficient, determined from the stream wise change in fluid momentum, was also altered by the introduction of the manipulator. The data from both the visualization and hot wire studies are provided.

Corke, T. C.; Guezennec, Y.; Nagib, H. M.

1980-01-01

217

Turbulent boundary layer over solid and porous surfaces with small roughness  

NASA Technical Reports Server (NTRS)

Skin friction and profiles of mean velocity, axial and normal turbulence intensity, and Reynolds stress in the untripped boundary layer were measured directly on a large diameter, axisymmetric body with: (1) a smooth, solid surface; (2) a sandpaper-roughened, solid surface; (3) a sintered metal, porous surface; (4) a smooth, perforated titanium surface; (5) a rough solid surface made of fine, diffusion bonded screening, and (6) a rough, porous surface of the same screening. Results obtained for each of these surfaces are discussed. It is shown that a rough, porous wall simply does not influence the boundary layer in the same way as a rough solid wall. Therefore, turbulent transport models for boundary layers over porous surfaces either with or without injection or suction, must include both surface roughness and porosity effects.

Kong, F. Y.; Schetz, J. A.; Collier, F.

1982-01-01

218

Boundary layer flow and heat transfer past a moving plate with suction and injection  

NASA Astrophysics Data System (ADS)

The behavior of an incompressible steady boundary layer flow past a permeable semi-infinite flat plate moving in a free stream is discussed in this paper. In addition to the mass transfer from the plate (suction or injection), the viscous dissipation term is also included into the energy equation. The solutions of the transformed ordinary differential equations are obtained numerically using an implicit finite-difference method. The numerical results are given for the velocity and temperature profiles as well as for the skin friction coefficient and the local Nusselt number for various values of the suction/injection parameter ?, ratio of the wall velocity to the free stream velocity parameter ?, Prandtl number Pr and Eckert number Ec. It is found that suction increases the heat transfer by decreasing the thermal boundary layer thickness and the reverse happens for injection. Furthermore, it is also found that the boundary layer equations have non-unique (dual) solutions in some cases.

Ishak, Anuar; Nazar, Roslinda; Pop, Ioan

2014-06-01

219

Boundary Layer Resolving Pseudospectral Methods For Singular Perturbation Problems  

Microsoft Academic Search

. Pseudospectral methods are investigated for singularly perturbed boundary valueproblems for ordinary differential equations which possess boundary layers. It is well known that ifthe boundary layer is very small then a very large number of spectral collocation points is requiredto obtain accurate solutions. We introduce here a new effective procedure, based on coordinatestretching and the Chebyshev pseudospectral method to resolve

Tao Tang Manfred R. Trummer

1993-01-01

220

Modelling of the Evolving Stable Boundary Layer  

NASA Astrophysics Data System (ADS)

A single-column model of the evolving stable boundary layer (SBL) is tested for self-similar properties of the flow and effects of ambient forcing. The turbulence closure of the model is diagnostic, based on the K-theory approach, with a semi-empirical form of the mixing length, and empirical stability functions of the Richardson number. The model results, expressed in terms of local similarity scales, are universal functions, satisfied in the entire SBL. Based on similarity expression, a realizability condition is derived for the minimum allowable turbulent heat flux in the SBL. Numerical experiments show that the development of "horse-shoe" shaped, fixed-elevation hodographs in the interior of the SBL around sunrise is controlled by effects imposed by surface thermal forcing.

Sorbjan, Zbigniew

2014-06-01

221

Chemistry of a polluted cloudy boundary layer  

NASA Astrophysics Data System (ADS)

A one-dimensional photochemical model for cloud-topped boundary layers has been developed to include descriptions of gas- and aqueous-phase chemistry and the radiation field in and below the cloud. The model is applied to the accumulation of pollutants during a wintertime episode with low stratus over Bakersfield, CA. The mechanisms of sulfate production and the balance between the concentrations of acids and bases are examined. It is shown that most of the sulfate production may be explained by the Fe(III)-catalyzed autoxidation of S(IV). Another source of sulfate is the oxidation of SO2 by OH in both the gas and the aqueous phase. It is shown that the sulfate production in the model is controlled by the availability of NH3. It is suggested that this explains the balance observed between total concentration of acids and bases.

Jacob, Daniel J.; Gottlieb, Elaine W.; Prather, Michael J.

1989-09-01

222

Streamwise vortices in heated boundary layers  

NASA Technical Reports Server (NTRS)

The nonlinear instability of the boundary layer on a heated flat plate placed in an oncoming flow is investigated. Such flows are unstable to stationary vortex instabilities and inviscid traveling wave disturbances governed by the Taylor-Goldstein equation. For small temperature differences the Taylor-Goldstein equation reduces to Rayleigh's equation. When the temperature difference between the wall and free stream is small the preferred mode of instability is a streamwise vortex. It is shown in this case that the vortex, assumed to be of small wavelength, restructures the underlying mean flow to produce a profile which can be massively unstable to inviscid traveling waves. The mean state is shown to be destabilized or stabilized to inviscid waves depending on whether the Prandtl number is less or greater than unity.

Hall, Philip

1992-01-01

223

Boundary layer receptivity to freestream turbulence  

NASA Technical Reports Server (NTRS)

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.

1990-01-01

224

DYNAMIC BOUNDARY CONTROL OF BEAMS USING ACTIVE CONSTRAINED LAYER DAMPING  

Microsoft Academic Search

A globally stable boundary control strategy is developed to damp the vibration of beams fully treated with active constrained layer damping (ACLD) treatments. The devised boundary controller is compatible with the operating nature of the ACLD treatments where the strain induced generates a control force and moment acting at the boundary of the treated beam. The development of the boundary

A. Baz

1997-01-01

225

Some measurements in synthetic turbulent boundary layers  

NASA Astrophysics Data System (ADS)

Synthetic turbulent boundary layers are examined which were constructed on a flat plate by generating systematic moving patterns of turbulent spots in a laminar flow. The experiments were carried out in a wind tunnel at a Reynolds number based on plate length of 1,700,000. Spots were generated periodically in space and time near the leading edge to form a regular hexagonal pattern. The disturbance mechanism was a camshaft which displaced small pins momentarily into the laminar flow at frequencies up to 80 Hz. The main instrumentation was a rake of 24 hot wires placed across the flow in a line parallel to the surface. The main measured variable was local intermittency; i.e., the probability of observing turbulent flow at a particular point in space and time. The results are reported in x-t diagrams showing the evolution of various synthetic flows along the plate. The dimensionless celerity or phase velocity of the large eddies is found to be 0.88, independent of eddy scale. All patterns with sufficiently small scales eventually showed loss of coherence as they moved downstream. A novel phenomenon called eddy transposition was observed in several flows which contained appreciable laminar regions. The large eddies shifted in formation to new positions, intermediate to their original ones, while preserving their hexagonal pattern. The present results, together with some empirical properties of a turbulent spot, are used to estimate the best choice of scales for constructing a synthetic boundary layer suitable for detailed study. The values recommended are: spanwise scale/thickness = 2.5, streamwise scale/thickness = 8.

Savas, O.

1980-01-01

226

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

227

Acoustics of laminar boundary layers breakdown  

NASA Astrophysics Data System (ADS)

Boundary layer flow transition has long been suggested as a potential noise source in both marine (sonar-dome self noise) and aeronautical (aircraft cabin noise) applications, owing to the highly transient nature of process. The design of effective noise control strategies relies upon a clear understanding of the source mechanisms associated with the unsteady flow dynamics during transition. Due to formidable mathematical difficulties, theoretical predictions either are limited to early linear and weakly nonlinear stages of transition, or employ acoustic analogy theories based on approximate source field data, often in the form of empirical correlation. In the present work, an approach which combines direct numerical simulation of the source field with the Lighthill acoustic analogy is utilized. This approach takes advantage of the recent advancement in computational capabilities to obtain detailed information about the flow-induced acoustic sources. The transitional boundary layer flow is computed by solving the incompressible Navier-Stokes equations without model assumptions, thus allowing a direct evaluation of the pseudosound as well as source functions, including the Lighthill stress tensor and the wall shear stress. The latter are used for calculating the radiated pressure field based on the Curle-Powell solution of the Lighthill equation. This procedure allows a quantitative assessment of noise source mechanisms and the associated radiation characteristics during transition from primary instability up to the laminar breakdown stage. In particular, one is interested in comparing the roles played by the fluctuating volume Reynolds stress and the wall-shear-stresses, and in identifying specific flow processes and structures that are effective noise generators.

Wang, Meng

1994-12-01

228

Acoustics of laminar boundary layers breakdown  

NASA Technical Reports Server (NTRS)

Boundary layer flow transition has long been suggested as a potential noise source in both marine (sonar-dome self noise) and aeronautical (aircraft cabin noise) applications, owing to the highly transient nature of process. The design of effective noise control strategies relies upon a clear understanding of the source mechanisms associated with the unsteady flow dynamics during transition. Due to formidable mathematical difficulties, theoretical predictions either are limited to early linear and weakly nonlinear stages of transition, or employ acoustic analogy theories based on approximate source field data, often in the form of empirical correlation. In the present work, an approach which combines direct numerical simulation of the source field with the Lighthill acoustic analogy is utilized. This approach takes advantage of the recent advancement in computational capabilities to obtain detailed information about the flow-induced acoustic sources. The transitional boundary layer flow is computed by solving the incompressible Navier-Stokes equations without model assumptions, thus allowing a direct evaluation of the pseudosound as well as source functions, including the Lighthill stress tensor and the wall shear stress. The latter are used for calculating the radiated pressure field based on the Curle-Powell solution of the Lighthill equation. This procedure allows a quantitative assessment of noise source mechanisms and the associated radiation characteristics during transition from primary instability up to the laminar breakdown stage. In particular, one is interested in comparing the roles played by the fluctuating volume Reynolds stress and the wall-shear-stresses, and in identifying specific flow processes and structures that are effective noise generators.

Wang, Meng

1994-01-01

229

Skin-friction reduction by multiple cavities  

Microsoft Academic Search

A possibility of reducing the skin-friction drag in a turbulent boundary layer with a two-dimensional cavity (elongated in the spanwise direction) has been investigated in the literature. In our previous numerical study (Hahn et al. 1999), we have shown that a single cavity indeed reduces the skin friction by about 10% in a turbulent boundary layer. Now, the question is

Chulkyu Kim

2005-01-01

230

Boundary-layer and shock-layer solutions to singularly perturbed boundary-value problems  

SciTech Connect

This dissertation concerns the study of certain singularly perturbed boundary value problems. In the first part of this dissertation (Chapters 2 and 3), a singularly perturbed nonlinear system of differential equations are considered over a compact interval, subject to general boundary conditions that allow the coupling of the boundary values at the different endpoints. It is shown, subject to suitable conditions, that there exists solutions of boundary-layer type, i.e., solutions that experience a rapid variation at one or both endpoints. In the second part (Chapter 4), a singularly perturbed second-order scalar differential equation is considered over a compact interval subject to Dirichlet boundary conditions. Subject to suitable conditions, there exist solutions of shock-layer type, i.e., solutions that experience a rapid transition at an interior point. For both the singularly perturbed system and the second-order scalar equation, a proposed approximate solution is constructed using the O'Malley construction, and a Riccati transformation is then used in a direct construction of the Green function for linearization of the problem about the proposed approximate solution.

Jeffries, J.S.

1987-01-01

231

Turbulent eddy viscosity modeling in transonic shock/boundary-layer interactions  

NASA Technical Reports Server (NTRS)

The treatment of turbulence effects on transonic shock/turbulent boundary layer interaction is addressed within the context of a triple deck approach valid for arbitrary practical Reynolds numbers between 1000 and 10 billion. The modeling of the eddy viscosity and basic turbulent boundary profile effects in each deck is examined in detail using Law-of-the-Wall/Law-of-the-Wake concepts as the foundation. Results of parametric studies showing how each of these turbulence model aspects influences typical interaction zone property distributions (wall pressure, displacement thickness and local skin friction) are presented and discussed.

Inger, G. R.

1989-01-01

232

On the nonlinear dynamics of the boundary layer of intense atmospheric vortex  

NASA Astrophysics Data System (ADS)

The paper describes a stationary model of the boundary layer of a large-scale vortex. The model takes into account two types of nonlinearities: (1) advection of the momentum and centrifugal accelerations, that is related to large values of the Rossby number; (2) a nonlinear turbulent friction on the lower boundary (generally speaking, not only quadratic, but also more complicated). Nevertheless, the model may be studied analytically. This approach allows discovery and analysis of several universal regularities, which are not revealed by numerical modeling. The model provides the actual values for the speed of the surface wind and the angle of cross-isobar surface flow in the tropical cyclones.

Ingel, Lev Kh.

2005-11-01

233

Large-Eddy Simulation of the Flat-plate Turbulent Boundary Layer at High Reynolds numbers  

NASA Astrophysics Data System (ADS)

The near-wall, subgrid-scale (SGS) model [Chung and Pullin, "Large-eddy simulation and wall-modeling of turbulent channel flow'', J. Fluid Mech. 631, 281--309 (2009)] is used to perform large-eddy simulations (LES) of the incompressible developing, smooth-wall, flat-plate turbulent boundary layer. In this model, the stretched-vortex, SGS closure is utilized in conjunction with a tailored, near-wall model designed to incorporate anisotropic vorticity scales in the presence of the wall. The composite SGS-wall model is presently incorporated into a computer code suitable for the LES of developing flat-plate boundary layers. This is then used to study several aspects of zero- and adverse-pressure gradient turbulent boundary layers. First, LES of the zero-pressure gradient turbulent boundary layer are performed at Reynolds numbers Retheta based on the free-stream velocity and the momentum thickness in the range Retheta = 103-1012. Results include the inverse skin friction coefficient, 2/Cf , velocity profiles, the shape factor H, the Karman "constant", and the Coles wake factor as functions of Re theta. Comparisons with some direct numerical simulation (DNS) and experiment are made, including turbulent intensity data from atmospheric-layer measurements at Retheta = O (106). At extremely large Retheta , the empirical Coles-Fernholz relation for skin-friction coefficient provides a reasonable representation of the LES predictions. While the present LES methodology cannot of itself probe the structure of the near-wall region, the present results show turbulence intensities that scale on the wall-friction velocity and on the Clauser length scale over almost all of the outer boundary layer. It is argued that the LES is suggestive of the asymptotic, infinite Reynolds-number limit for the smooth-wall turbulent boundary layer and different ways in which this limit can be approached are discussed. The maximum Retheta of the present simulations appears to be limited by machine precision and it is speculated, but not demonstrated, that even larger Retheta could be achieved with quad- or higher-precision arithmetic. Second, the time series velocity signals obtained from LES within the logarithmic region of the zero-pressure gradient turbulent boundary layer are used in combination with an empirical, predictive inner--outer wall model [Marusic et al., "Predictive model for wall-bounded turbulent flow'', Science 329, 193 (2010)] to calculate the statistics of the fluctuating streamwise velocity in the inner region of the zero-pressure gradient turbulent boundary layer. Results, including spectra and moments up to fourth order, are compared with equivalent predictions using experimental time series, as well as with direct experimental measurements at Reynolds numbers Retau based on the friction velocity and the boundary layer thickness, Retau = 7,300, 13,600 and 19,000. LES combined with the wall model are then used to extend the inner-layer predictions to Reynolds numbers Retau = 62,000, 100,000 and 200,000 that lie within a gap in log(Retau) space between laboratory measurements and surface-layer, atmospheric experiments. The present results support a log-like increase in the near-wall peak of the streamwise turbulence intensities with Retau and also provide a means of extending LES results at large Reynolds numbers to the near-wall region of wall-bounded turbulent flows. Finally, we apply the wall model to LES of a turbulent boundary layer subject to an adverse pressure gradient. Computed statistics are found to be consistent with recent experiments and some Reynolds number similarity is observed over a range of two orders of magnitude.

Inoue, Michio

234

Acoustic sounding in the planetary boundary layer  

NASA Technical Reports Server (NTRS)

Three case studies are presented involving data from an acoustic radar. The first two cases examine data collected during the passage of a mesoscale cold-air intrusion, probably thunderstorm outflow, and a synoptic-scale cold front. In these studies the radar data are compared to conventional meteorological data obtained from the WKY tower facility for the purpose of radar data interpretation. It is shown that the acoustic radar echoes reveal the boundary between warm and cold air and other areas of turbulent mixing, regions of strong vertical temperature gradients, and areas of weak or no wind shear. The third case study examines the relationship between the nocturnal radiation inversion and the low-level wind maximum or jet in the light of conclusions presented by Blackadar (1957). The low-level jet is seen forming well above the top of the inversion. Sudden rapid growth of the inversion occurs which brings the top of the inversion to a height equal that of the jet. Coincident with the rapid growth of the inversion is a sudden decrease in the intensity of the acoustic radar echoes in the inversion layer. It is suggested that the decrease in echo intensity reveals a decrease in turbulent mixing in the inversion layer as predicted by Blackadar. It is concluded that the acoustic radar can be a valuable tool for study in the lower atmosphere.

Kelly, E. H.

1974-01-01

235

Low-friction behaviour of boundary-lubricated diamond-like carbon coatings containing tungsten  

Microsoft Academic Search

It is generally accepted and well described that the mechanism by which extreme-pressure (EP) and antiwear (AW) additives reduce the friction and wear of metallic surfaces under boundary lubrication is the formation of tribochemical films. Although investigations of diamond-like carbon (DLC)-coated surfaces showed improved tribological properties when lubricated by additivated oil, the mechanism responsible is not fully understood. Therefore, the

B. Podgornik; D. Hren; J. Vizintin

2005-01-01

236

Winds in the Marine Boundary Layer: A Forecaster's Guide  

NSDL National Science Digital Library

This module is intended for experienced forecasters moving from a land-based area to a coastal or Great Lakes region where both over-land and over-water forecast areas exist. This module highlights the differences between marine boundary layer and terrestrial boundary layer winds. The experienced forecaster is relatively familiar with the boundary layer over land and the associated implications for the wind field. Using this as a base, the module compares this known quantity with the lesser-known processes that occur in the marine boundary layer. Three major topics that influence marine boundary layer winds are discussed: stability within the boundary layer, isallobaric influence, and the effects of convection and tropical cyclones.

Spangler, Tim

2006-12-01

237

Turbulent Boundary Layer in High Rayleigh Number Convection in Air  

NASA Astrophysics Data System (ADS)

Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bénard experiment are presented for the Rayleigh number Ra =1.4×1010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re ?200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.

du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, André; Willert, Christian

2014-03-01

238

New evolution equations for turbulent boundary layers  

NASA Astrophysics Data System (ADS)

Perry, Marusic & Li (1994) (Phys. Fluids, vol. 6(2) part 2) initially developed a mathematical framework for computing the evolution of boundary layers using classical similarity laws such as Prandtl's law of the wall and Coles' law of the wake together with the momentum integral and differential equations. It was found that these equations show that there are 4 parameters which control the streamwise evolution of the layer and the Reynolds shear stress distribution and these are S, ?, ? and ?. S = U_1/U_?, ? is Coles wake factor, ? is the Clauser pressure gradient parameter and ?=S?_cd?/dx. In this early work the evolution equations were incomplete and the only problems which could be solved were the so called quasi-equilibrium flow cases where it could be assumed that ? was sufficiently small to neglect its effect. Here we present the full set of evolution equations for finite ? so that the more general problem of non-equilibrium layers can be tackled. In this initial study here, closure is obtained assuming that \\calF[S, ?, ?, ?] = 0 and this function is mapped out semi-empirically. The formulation is consistent with the recently extended attached eddy hypothesis of Perry & Marusic (1995) (JFM vol. 298) from which once the mean flow evolution has been calculated, the broadband turbulence intensities and spectra can be calculated. The use of topology as a diagnostic tool to interpret DNS data tends to support this recently developed hypothesis (Chong et al. 1998) (JFM vol. 357) and preliminary modeling is carried out in conjunction with these evolution equations so as to obtain closure based on physical arguments. Some nonequilibrium flow data is compared with computations using these new evolution equations.

Perry, A. E.

1998-11-01

239

Boundary-layer wind structure in a landfalling tropical cyclone  

Microsoft Academic Search

In this study, a slab boundary layer model with a constant depth is used to analyze the boundary-layer wind structure in a\\u000a landfalling tropical cyclone. Asymmetry is found in both the tangential and radial components of horizontal wind in the tropical\\u000a cyclone boundary layer at landfall. For a steady tropical cyclone on a straight coastline at landfall, the magnitude of

Xiaodong Tang; Zhemin Tan

2006-01-01

240

Heat transfer in a viscoelastic boundary layer flow through a porous medium  

NASA Astrophysics Data System (ADS)

This paper examines the viscoelastic fluid flow and heat transfer characteristics in a saturated porous medium over an impermeable stretching surface with frictional heating and internal heat generation or absorption. The heat transfer analysis has been carried out for two different heating processes, namely (i) with prescribed surface temperature (PST-case) and (ii) prescribed surface heat flux (PHF-case). The governing equations for the boundary layer flow problem result similar solutions. For the specified five boundary conditions, it is not possible to solve directly the resulting sixth-order nonlinear ordinary differential equation. For the present incompressible boundary layer flow problem with constant physical parameters, the momentum equation is decoupled from the energy equation. Two closed-form solutions for the momentum equation are obtained and identified the realistic solution of the physical problem. Exact solution for the velocity field and the skin-friction are obtained. Also, the solution for the temperature and the heat transfer characteristics are obtained in terms of Kummer's function. Asymptotic results for the temperature function for large Prandtl numbers are presented. The work due to deformation in the energy equation, which is essential and escaped from the attention of researchers while formulating the visco-elastic boundary layer flow problems, is considered. Drastic variation in the values of heat transfer coefficient is observed when the work due to deformation is ignored.

Pillai, K. M. C.; Sai, K. S.; Swamy, N. S.; Nataraja, H. R.; Tiwari, S. B.; Rao, B. N.

241

Nonparallel stability of boundary layers with pressure gradients and suction  

NASA Technical Reports Server (NTRS)

An analysis is presented for the linear nonparallel stability of boundary layer flows with pressure gradients and suction. The effect of the boundary layer growth is included by using the method of multiple scales. The present analysis is compared with those of Bouthier and Gaster and the roles of the different definitions of the amplification rates are discussed. The results of these theories are compared with experimental data for the Blasius boundary layer. Calculations are presented for stability characteristics of boundary layers with pressure gradients and nonsimilar suction distributions.

Saric, W. S.; Nayfeh, A. H.

1977-01-01

242

Effect of the noise on boundary layer transition  

NASA Astrophysics Data System (ADS)

In this paper, an experimental study of stream noise affecting the boundary layer transition in the test section of a wind tunnel is conducted. The effect of sortie input on the boundary layer transition is examined and stream noise in test section is measured. The results show that there are sensitive regions of sonic frequency and pressure affecting the boundary layer transition. The frequency band of the stream noise in the test section is wide and the low frequency is dominant and important in its effect on the boundary layer transition. Stream noise increases with the wind velocity and turbulence rises with added grid in the settling chamber.

Zheng, Guofeng

1993-04-01

243

Three-dimensional boundary layer stability and transition  

NASA Technical Reports Server (NTRS)

Nonparallel and nonlinear stability of a three-dimensional boundary layer, subject to crossflow instability, is investigated using parabolized stability equations (PSEs). Both traveling and stationary disturbances are considered and nonparallel effect on crossflow instability is found to be destabilizing. Our linear PSE results for stationary disturbances agree well with the results from direct solution of Navier-Stokes equations obtained by Spalart (1989). Nonlinear calculations have been carried out for stationary vortices and the computed wall vorticity pattern results in streamwise streaks which resemble remarkably well with the surface oil-flow visualizations in swept-wing experiments. Other features of the stationary vortex development (half-mushroom structure, inflected velocity profiles, vortex doubling, etc.) are also captured in our nonlinear calculations. Nonlinear interaction of the stationary amplitude of the stationary vortex is large as compared to the traveling mode, and the stationary vortex dominates most of the downstream development. When the two modes have the same initial amplitude, the traveling mode dominates the downstream development owing to its higher growth rate, and there is a tendency for the stationary mode to be suppressed. The effect of nonlinear wave development on the skin-friction coefficient is also computed.

Malik, M. R.; Li, F.

1992-01-01

244

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

245

Boundary layer features observed during NAME 2004  

NASA Astrophysics Data System (ADS)

S-Pol radar data from the North American Monsoon Experiment (NAME) are examined to investigate the characteristics of sea breezes that occurred during the North American Monsoon in the late summer of 2004, as well as their role in modulating monsoon convection. Zero degree plan position indicated (PPI) scans were examined to determine the presence of a sea breeze fine line in the S-Pol radar data. Sea breeze fine lines were typically observed over land very near the coast of the Gulf of California (GoC), and usually moved onshore around 1700--1800 UTC (11:00 AM--12:00 PM local time), and then continued to move slowly inland on the coastal plain. The sea breezes typically moved on land and dissipated before any significant interactions with Sierra Madre Occidental (SMO) convection could occur. Fine lines varied in reflectivity strength, but were typically around 10 to 20 dBZ. Surface winds from the Estacion Obispo (ETO) supersite were analyzed to confirm the presence of a shift in wind direction on days in which a fine line had been identified. Typically winds changed from light and variable to consistently out of the west or southwest. Vertical plots of S-Pol reflectivity were created to examine sea breeze structure in the vertical, but these were not found to be useful as the sea breeze signature was nearly impossible to distinguish from other boundary layer features. Horizontal structure was further investigated using wind profiler relative reflectivity, vertical velocity, and horizontal winds from the profiler located at ETO. Relative reflectivity and vertical velocity fields revealed a complex boundary layer structure on some days of repeating updrafts and downdrafts. Further examination of S-Pol PPI data revealed that these vertical motions are likely due to the presence of horizontal convective rolls. Profiler horizontal winds revealed that the depth and vertical structure of the sea breezes varied significantly from day to day, but that the height of the sea breeze is around 1 km above the ground. Sea breezes observed during NAME almost never initiated convection on their own. It is hypothesized that a weak thermal contrast between the GoC and the land leads to comparatively weak sea breezes, which don't have enough lift to trigger convection.

Stuckmeyer, Elizabeth A.

246

Control of the Transitional Boundary Layer  

NASA Astrophysics Data System (ADS)

This work makes advances in the delay of boundary layer transition from laminar to turbulent flow via feedback control. The applications include the reduction of drag over streamline bodies (e.g., airplane wings) and the decrease of mixing and heat transfer (e.g., over turbine blades in jet engines). A difficulty in many fields is designing feedback controllers for high-dimensional systems, be they experiments or high-fidelity simulations, because the required time and resources are too large. A cheaper alternative is to approximate the high-dimensional system with a reduced-order model and design a controller for the model. We implement several model reduction algorithms in "modred", an open source and publicly available library that is applicable to a wide range of problems. We use this library to study the role of sensors and actuators in feedback control of transition in the 2D boundary layer. Previous work uses a feedforward configuration in which the sensor is upstream of the actuator, but we show that the actuator-sensor pair is unsuitable for feedback control due to an inability to sense the exponentially-growing Tollmien-Schlichting waves. A new actuator-sensor pair is chosen that more directly affects and measures the TS waves, and as a result it is effective in a feedback configuration. Lastly, the feedback controller is shown to outperform feedforward controllers in the presence of unmodeled disturbances. Next, we focus on a specific type of actuator, the single dielectric barrier discharge (SDBD) plasma actuator. An array of these plasma actuators is oriented to produce stream-wise vorticity and thus directly cancel the structures with the largest transient growth (so-called stream-wise streaks). We design a feedback controller using only experimental data by first developing an empirical input-output quasi-steady model. Then, we design feedback controllers for the model such that the controllers perform well when applied to the experiment. Lastly, we also simulate the plasma actuators and determine a suitable numerical model for the forces they create by comparing with experimental results. This physical force model is essential to future numerical studies on delaying bypass transition via feedback control and plasma actuation.

Belson, Brandt A.

247

Friction  

NASA Astrophysics Data System (ADS)

Friction materials such as disk pads, brake linings, and clutch facings are widely used for automotive applications. Friction materials function during braking due to frictional resistance that transforms kinetic energy into thermal energy. There has been a rudimentary evolution, from materials like leather or wood to asbestos fabric or asbestos fabric saturated with various resins such as asphalt or resin combined with pitch. These efforts were further developed by the use of woven asbestos material saturated by either rubber solution or liquid resin binder and functioned as an internal expanding brake, similar to brake lining system. The role of asbestos continued through the use of chopped asbestos saturated by rubber, but none was entirely successful due to the poor rubber heat resistance required for increased speeds and heavy gearing demands of the automobile industry. The use of phenolic resins as binder for asbestos friction materials provided the necessary thermal resistance and performance characteristics. Thus, the utility of asbestos as the main friction component, for over 100 years, has been significantly reduced in friction materials due to asbestos identity as a carcinogen. Steel and other fibrous components have displaced asbestos in disk pads. Currently, non-asbestos organics are the predominate friction material. Phenolic resins continue to be the preferred binder, and increased amounts are necessary to meet the requirements of highly functional asbestos-free disk pads for the automotive industry. With annual automobile production exceeding 70 million vehicles and additional automobile production occurring in developing countries worldwide and increasing yearly, the amount of phenolic resin for friction material is also increasing (Fig. 14.1). Fig. 14.1 Worldwide commercial vehicle production In recent years, increased fuel efficiency of passenger car is required due to the CO2 emission issue. One of the solutions to improve fuel efficiency is to lower the car body weight. It means that the weight of car components must be decreased. In the case of reduced weight for friction parts, the load applied to the friction parts would be higher (more heat also) and trend would lead to phenolic resins with improved heat resistance.

Matsuo, Yoshihiro; Clarke, Daryl D.; Ozeki, Shinichi

248

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

249

Non-equilibrium Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

Clauser (1954) in his now classical paper on pressure gradients concluded that equilibrium flows are very special flows difficult to achieve experimentally and that few flows were in equilibrium. However, using a similarity analysis of the Navier-Stokes equations Castillo and George(Castillo, L. and George, W.K.,``Similarity Analysis for Turbulent Boundary Layer with Pressure Gradient: out flow,'' AIAA Journal, Vol.39,2001) concluded that an equilibrium flow is one where the pressure parameter, ? =frac? ? U_? ^2d? /dxfracdP_? dx, is a constant. They further concluded that most flows are in equilibrium and the exception are non-equilibrium flows; those where the ? neq constant. Using the equations of motion and similarity analysis, it will be shown that even non-equilibrium flows-mainly those over airfoils or with sudden changes on external pressure gradient, and separated flows-are in an equilibrium state but locally. Moreover, in the case of airfoils where the external pressure gradient changes from favorable to zero pressure gradient and to adverse pressure gradient, three distinctive regions are identified each given by a constant value of ?. The most interesting result is that there seems to be a universal value for the pressure parameter, ? ~= 0.22, for all adverse pressure gradients including relaxed flows and separated flows.

Castillo, Luciano; Wang, Xia

2001-11-01

250

Effect of sound on boundary layer stability  

NASA Technical Reports Server (NTRS)

Experiments are conducted in the Arizona State University Unsteady Wind Tunnel with a zero-pressure-gradient flat-plate model that has a 67:1 elliptical leading edge. Boundary-layer measurements are made of the streamwise fluctuating-velocity component in order to identify the amplified T-S waves that are forced by downstream-traveling sound waves. Measurements are taken with circular 3-D roughness elements placed at the Branch 1 neutral stability point for the frequency under consideration, and then with the roughness element downstream of Branch 1. These roughness elements have a principal chord dimension equal to 2 lambda(sub TS)/pi of the T-S waves under study and are 'stacked' in order to resemble a Gaussian height distribution. Measurements taken just downstream of the roughness (with leading-edge T-S waves, surface roughness T-S waves, instrumentation sting vibrations, and the Stokes wave subtracted) show the generation of 3-D T-S waves, but not in the characteristic heart-shaped disturbance field predicted by 3-D asymptotic theory. Maximum disturbance amplitudes are found on the roughness centerline. However, some near-field characteristics predicted by numerical modeling are observed.

Saric, William S.; Spencer, Shelly Anne

1993-01-01

251

Effect of sound on boundary layer stability  

NASA Technical Reports Server (NTRS)

Experiments are conducted in the Arizona State University Unsteady Wind Tunnel with a zero-pressure-gradient flat-plate model that has a 67:1 elliptical leading edge. Boundary-layer measurements are made of the streamwise fluctuating-velocity component in order to identify the amplified T-S waves that are forced by downstream-travelling, sound waves. Measurements are taken with circular 3-D roughness elements placed at the Branch 1 neutral stability point for the frequency under consideration, and then with the roughness element downstream of Branch 1. These roughness elements have a principal chord dimension equal to 2(lambda)(sub TS)/pi, of the T-S waves under study and are 'stacked' in order to resemble a Gaussian height distribution. Measurements taken just downstream of the roughness (with leading-edge T-S waves, surface roughness T-S waves, instrumentation sting vibrations and the Stokes wave subtracted) show the generation of 3-D-T-S waves, but not in the characteristic heart-shaped disturbance field predicted by 3-D asymptotic theory. Maximum disturbance amplitudes are found on the roughness centerline. However, some near-field characteristics predicted by numerical modelling are observed.

Saric, William S. (Principal Investigator); Spencer, Shelly Anne

1993-01-01

252

Interaction between surface and atmosphere in a convective boundary layer  

NASA Astrophysics Data System (ADS)

Solar heating of the surface causes the near surface air to warm up and with sufficient buoyancy it ascends through the atmosphere as surface-layer plumes and thermals. The cold fluid from the upper part of the boundary layer descends as downdrafts. The downdrafts and thermals form streamwise roll vortices. All these turbulent coherent structures are important because they contribute most of the momentum and heat transport. While these structures have been studied in depth, their imprint on the surface through energy budget in a convective atmospheric boundary layer has received little attention. The main objective of the present study is to examine the turbulence-induced surface temperature fluctuations for different surface properties and stratification. Experiments were performed to measure atmospheric turbulence using sonic anemometers, fine wire thermocouples and LIDAR; and surface temperature using an infra-red camera over grass and artificial turf fields. The surface temperature fluctuations were found to be highly correlated to the turbulent coherent structures and follow the processes postulated in the surface renewal theory. The spatio-temporal scales and advection speed of the surface temperature fluctuation were found to match with those of turbulent coherent structures. A parametric direct numerical simulation (DNS) study was then performed by solving the solid-fluid heat transport mechanism numerically for varying solid thermal properties, solid thickness and strength of stratification. Even though there were large differences in the friction Reynolds and Richardson numbers between the experiments and numerical simulations, similar turbulent characteristics were observed. The ejection (sweep) events tend to be aligned with the streamwise direction to form roll vortices with unstable stratification. The solid-fluid interfacial temperature fluctuations increase with the decreases in solid thermal inertia; and with the increase in solid thickness to attain a constant value for a sufficiently thick solid. The temperature fluctuation changes from a Gaussian distribution near the wall to a positively skewed distribution away from the wall. The turbulent temperature fluctuations influence the solid interfacial temperature by thermal conduction only. These studies provided unique insights into the solid-fluid coupled heat transport in low and high Reynolds number flows. This turbulence induced surface temperature fluctuation can influence the performances of several satellite remote sensing models.

Garai, Anirban

253

Modelling the low-latitude boundary layer with reconnection entry  

NASA Technical Reports Server (NTRS)

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.

1994-01-01

254

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

255

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

256

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

257

Boundary Layer Integral Matrix Procedure: Verification of Models.  

National Technical Information Service (NTIS)

The three turbulent models currently available in the JANNAF version of the Aerotherm Boundary Layer Integral Matrix Procedure (BLIMP-J) code were studied. The BLIMP-J program is the standard prediction method for boundary layer effects in liquid rocket e...

W. S. Bonnett R. M. Evans

1977-01-01

258

Numerical simulation of shock wave-turbulent boundary layer interaction  

Microsoft Academic Search

Numerical solutions of the Navier-Stokes equations are presented for the interactions of a shock wave and a turbulent boundary layer. The turbulent closure is provided by a relaxation eddy viscosity model which approximates the response of turbulence to a severe pressure gradient. The eddy viscosity model is verified by investigating shock impingement on a turbulent boundary layer. Computations were performed

J. S. Shang; W. L. Hankey Jr.; C. H. Law

1976-01-01

259

The Application of Optimal Control to Boundary Layer Flow  

Microsoft Academic Search

Modern optimal control theory can be used to calculate the optimal steady suction needed to e.g. relaminarize the flow or to delay transition. This has been used to devise the best possible suction distributions for keeping the flow laminar, and applied for flat plate boundary layers as well as boundary layers on swept wings of airplanes. Optimal control theory can

D. Henningson; A. Hanifi

260

On optical imaging through aircraft turbulent boundary layers  

NASA Technical Reports Server (NTRS)

Optical resolution quality as affected by aircraft turbulent boundary layers is analyzed. Wind-tunnel data was analyzed to obtained the variation of boundary layer turbulence scale length and mass density rms fluctuations with Mach number. The data gave good agreement with a mass density fluctuation turbulence spectrum that is either isotropic of orthogonally anisotropic. The data did not match an isotropic turbulence velocity spectrum which causes an anisotropic non-orthogonal mass density fluctuation spectrum. The results indicate that the average mass density rms fluctuation is about 10% of the maximum mass density across the boundary layer and that the transverse turbulence scale size is about 10% of the boundary layer thickness. The results indicate that the effect of the turbulent boundary layer is large angle scattering which decreases contrast but not resolution. Using extinction as a criteria the range of acceptable aircraft operating conditions are given.

Sutton, G. W.

1980-01-01

261

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

262

On optical imaging through aircraft turbulent boundary layers  

NASA Astrophysics Data System (ADS)

Optical resolution quality as affected by aircraft turbulent boundary layers is analyzed. Wind-tunnel data was analyzed to obtained the variation of boundary layer turbulence scale length and mass density rms fluctuations with Mach number. The data gave good agreement with a mass density fluctuation turbulence spectrum that is either isotropic of orthogonally anisotropic. The data did not match an isotropic turbulence velocity spectrum which causes an anisotropic non-orthogonal mass density fluctuation spectrum. The results indicate that the average mass density rms fluctuation is about 10% of the maximum mass density across the boundary layer and that the transverse turbulence scale size is about 10% of the boundary layer thickness. The results indicate that the effect of the turbulent boundary layer is large angle scattering which decreases contrast but not resolution. Using extinction as a criteria the range of acceptable aircraft operating conditions are given.

Sutton, G. W.

1980-04-01

263

Dynamic behavior of an unsteady trubulent boundary layer  

NASA Technical Reports Server (NTRS)

Experiments on an unsteady turbulent boundary layer are reported in which the upstream portion of the flow is steady (in the mean) and in the downstream region, the boundary layer sees a linearly decreasing free stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70% greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at higher amplitude is quite similar. At sufficiently high frequencies, the boundary layer thickness remains frozen at the mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle.

Parikh, P. G.; Reynolds, W. C.; Jayaramen, R.; Carr, L. W.

1981-01-01

264

Perturbing Spanwise Modes in Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

The objective of the current study was to manipulate the coherent vortex packets in a turbulent boundary layer at Re ?=2480 by inserting a small scale cylinder array and to improve the understanding of the downstream flow stability issues. The height of the cylinders was H/?=0.2 (H+=500) with aspect ratio (AR=cylinder height/base diameter) of 4, and three cases were studied using single array of 0.2?, 0.4? and 0.6? spaced cylinders. Both fixed location data and flying data were acquired at z+=296 using PIV, and the spanwise scales of the packets and the wake-packet interactions downstream of the cylinder array were also discussed. The non-perturbed flow was studied first and the dominant spanwise scale of the vortex packets was found to be ˜0.6?. From the flying data, the organization of vortex packets was found to persist over a streamwise distance of ˜8?. The averaged results of the perturbed cases showed a spanwise variation of the streamwise velocity downstream of the cylinder array, and the spanwise scales of the low speed regions were most stable for the 0.6? spacing case. Also, distinct downwash behavior was observed directly behind each cylinder. The flying data showed frequent spanwise interactions of cylinder wakes in the 0.2? case and the downstream structures were affected greatly by the incoming flow condition. The 0.4? and 0.6? cases were discussed based on the relative spanwise location of the upstream vortex packets and the cylinders and it was concluded that the organization of flow structures was most stable when the perturbation scale was the same as the dominant spanwise mode of the non-perturbed flow.

Zheng, Shaokai

265

Boundary Layer Control of Rotating Convection Systems  

NASA Astrophysics Data System (ADS)

Rotating convection is ubiquitous in the natural universe, and is likely responsible for planetary processes such magnetic field generation. Rapidly rotating convection is typically organized by the Coriolis force into tall, thin, coherent convection columns which are aligned with the axis of rotation. This organizational effect of rotation is thought to be responsible for the strength and structure of magnetic fields generated by convecting planetary interiors. As thermal forcing is increased, the relative influence of rotation weakens, and fully three-dimensional convection can exist. It has long been assumed that rotational effects will dominate convection dynamics when the ratio of buoyancy to the Coriolis force, the convective Rossby number, Roc, is less than unity. We investigate the influence of rotation on turbulent Rayleigh-Benard convection via a suite of coupled laboratory and numerical experiments over a broad parameter range: Rayleigh number, 10310; Ekman number, 10-6? E ? ?; and Prandtl number, 1? Pr ? 100. In particular, we measure heat transfer (as characterized by the Nusselt number, Nu) as a function of the Rayleigh number for several different Ekman and Prandtl numbers. Two distinct heat transfer scaling regimes are identified: non-rotating style heat transfer, Nu ~ Ra2/7, and quasigeostrophic style heat transfer, Nu~ Ra6/5. The transition between the non-rotating regime and the rotationally dominant regime is described as a function of the Ekman number, E. We show that the regime transition depends not on the global force balance Roc, but on the relative thicknesses of the thermal and Ekman boundary layers. The transition scaling provides a predictive criterion for the applicability of convection models to natural systems such as Earth's core.

King, E. M.; Stellmach, S.; Noir, J.; Hansen, U.; Aurnou, J. M.

2008-12-01

266

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.

267

First results of a study on turbulent boundary layers in oscillating flow with a mean adverse pressure gradient  

NASA Technical Reports Server (NTRS)

The development of a turbulent unsteady boundary layer with a mean pressure gradient strong enough to induce separation, in order to complete the extend results obtained for the flat plate configuration is presented. The longitudinal component of the velocity is measured using constant temperature hot wire anemometer. The region where negative velocities exist is investigated with a laser Doppler velocimeter system with BRAGG cells. The boundary layer responds by forced pulsation to the perturbation of potential flow. The unsteady effects observed are very important. The average location of the zero skin friction point moves periodically at the perturbation frequency. Average velocity profiles from different instants in the cycle are compared. The existence of a logarithmic region enables a simple calculation of the maximum phase shift of the velocity in the boundary layer. An attempt of calculation by an integral method of boundary layer development is presented, up to the point where reverse flow starts appearing.

Houdeville, R.; Cousteix, J.

1979-01-01

268

Experimental Investigation of the Effects of Acceleration on Heat Transfer in the Turbulent Boundary Layer  

NASA Technical Reports Server (NTRS)

The objective of this research was to experimentally investigate the combined effects of freestream acceleration and surface roughness on heat transfer and fluid flow in the turbulent boundary layer. The experiments included a variety of flow conditions ranging from aerodynamically smooth to transitionally rough to fully rough boundary layers with accelerations ranging from moderate to moderately strong. The test surfaces used were a smooth-wall test surface and two rough-wall surfaces which were roughened with 1.27 mm diameter hemispheres spaced 2 and 4 base diameters apart in a staggered array. The measurements consisted of Stanton number distributions, mean temperature profiles, skin friction distributions, mean velocity profiles, turbulence intensity profiles, and Reynolds stress profiles. The Stanton numbers for the rough-wall experiments increased with acceleration. For aerodynamically smooth and transitionally rough boundary layers, the effect of roughness is not seen immediately at the beginning of the accelerated region as it is for fully rough boundary layers; however, as the boundery layer thins under acceleration, the surface becomes relatively rougher resulting in a sharp increase in Stanton number.

Chakroun, Walid M.; Taylor, Robert P.

1996-01-01

269

Low-friction diamond-like carbon (DLC)-layers for humid environment  

Microsoft Academic Search

Even though amorphous diamond-like carbon (DLC) coated tools with their low friction and high wear resistance are well established in a wide field of industrial applications, their employment into humid environments is limited. In humid environments hydrogenated DLC-layers show high friction coefficients and low wear resistance.This work is focused on the development of DLC-coating systems with high wear resistance and

Wolfgang Tillmann; Evelina Vogli; Fabian Hoffmann

2007-01-01

270

Molecular Simulations of Kinetic-Friction Modification in Nanoscale Fluid Layers  

Microsoft Academic Search

Molecular simulations are used to explore kinetic-friction modification in nanoscale fluid layers of oil and additive confined\\u000a between sheared parallel walls. The molecules are represented by coarse-grained bead-spring models that reflect the essential\\u000a solvophilic and solvophobic natures of the chemical groups. The degree of friction modification is surveyed as a function\\u000a of wall separation, sliding velocity, additive molecular weight and

Matthew R. Farrow; Alexandros Chremos; Philip J. Camp; Steven G. Harris; Raymond F. Watts

2011-01-01

271

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

272

On the stability of the decelerating laminar boundary layer  

NASA Technical Reports Server (NTRS)

The stability of a decelerating boundary-layer flow is investigated experimentally and numerically. Experimentally, a flat plate having a Blasius boundary layer is decelerated in an 18 m towing tank. The boundary layer becomes unstable to two-dimensional waves, which break down into three-dimensional patterns, hairpin vortices, and finally turbulent bursts when the vortices lift off the wall. The unsteady boundary-layer equations are solved numerically to generate instantaneous velocity profiles for a range of boundary and initial conditions. A quasi-steady approximation is invoked and the stability of local velocity profiles is determined by solving the Orr-Sommerfeld equation using Chebyshev matrix methods. Comparisons are made between the numerical predictions and the experimentally observed instabilities.

Gad-El-hak, M.; Mcmurray, J. T.; Davis, S. H.; Orszag, S. A.

1984-01-01

273

Interaction of the planetary boundary layer depth with aerosol and boundary-layer clouds  

NASA Astrophysics Data System (ADS)

The depth of the planetary boundary layer (PBL) is driven by surface heating, with strong diurnal and seasonal cycles. Methods to detect the PBL depth from remote sensing instruments such as lidar and infrared spectrometer can take advantage of their high temporal resolution to produce detailed information about PBL development, which in turn has implications for weather, air quality and climate. An algorithm combining two common methods for PBL depth detection (wavelet covariance and iterative curve-fitting) has been evaluated by intercomparison among multiple instruments at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. Radiosonde-derived PBL depths at SGP accounted for over two-thirds the variation in PBL depths from atmospheric emitted radiance interferometer (AERI), and over half the variation in PBL depths from micropulse lidar (MPL). The results are sufficiently robust that the algorithm can be used at other locations that have only one source of atmospheric profiles. The new continuous PBL data set can be used to improve model parameterizations of PBL and our understanding of atmospheric transport of pollutants. Using ground-based MPL profiles from China and the U.S., this study investigates the behavior of the PBL in the presence of aerosol loading, in which the aerosol direct effect would have altitude-dependent consequences, and the interaction of PBL, aerosol and boundary-layer clouds. PBL depths detected by MPL, AERI and radiosonde, overlaid on MPL backscatter during a nine-day period of typical conditions.

Sawyer, V. R.; Li, Z.

2013-12-01

274

Chemistry of a polluted cloudy boundary layer  

SciTech Connect

A one-dimensional photochemical model for cloud-topped boundary layers is developed which includes detailed descriptions of gas-phase and aqueous-phase chemistry, and of the radiation field in and below cloud. The model is used to interpret the accumulation of pollutants observed over Bakersfield, California, during a wintertime stagnation episode with low stratus. The main features of the observations are well simulated; in particular, sulfate accumulates progressively over the course of the episode due to sustained aqueous-phase oxidation of SO{sub 2} in the stratus cloud. The major source of sulfate is the reaction S(IV)+Fe(III), provided that this reaction proceeds by a non radical mechanism in which Fe(III) is not reduced. A radical mechanism with SO{sup {minus}}{sub 3} and Fe(II) as immediate products would quench sulfate production because of depletion of Fe(III). The model results suggest that the non radical mechanism is more consistent with observations, although this result follows from the absence of a rapid Fe(II) oxidation pathway in the model. Even with the non-radical mechanis most of the soluble iron is present as FE(II) because Fe(III) is rapidly reduced by O{sup {minus}}{sub 2}. The S(IV)+Fe(III) reacton provides the principal source of H{sub 2}O{sub 2} in the model; photochemical production of H{sub 2}O{sub 2} from HO{sub 2} or O{sub 2}({minus}I) is slow because HO{sub 2} is depleted by high levels of NO{sub {ital x}}. The aqueous-phase reaction S(IV)+OH initiates a radical-assisted S(IV) oxidation chain but we find that the chain is not propagated due to efficient termination by SO{sup {minus}}{sub 4}+Cl{sup {minus}} followed by Cl+H{sub 2}O. A major uncertainty attached to that result is that the reactivities of S(IV)-carbonyl adducts with radical oxidants are unknown.

Jacob, D.J.; Gottlieb, E.W. (Department of Earth and Planetary Sciences and Division of Applied Sciences, Harvard University, Cambridge, Massachusetts (US)); Prather, M.J. (NASA Goddard Institute of Space Studies, New York, New York)

1989-09-20

275

Convective Surface Layers: Influence of the Boundary Layer Depth  

NASA Astrophysics Data System (ADS)

Two methods, one semi-empirical and the other theoretical, are employed to develop a new theory for predicting the behavior of turbulence characteristics in convective surface layers. This theory is based on three length scales; the height above the ground, the Monin-Obukhov length, and the boundary layer depth (h). The chief advantages are its relative simplicity, its ability to predict the height dependence of the mean gradients and second moments in a manner consistent with their observed behavior in the atmosphere and in large-eddy simulations, and it is consistent with the theoretical requirements of free convection similarity. The semi-empirical method is based on nonlinear least squares fits to a generalized Businger-Dyer formulation with up to four unknown parameters. Using data obtained primarily from the Kansas and Minnesota field experiments, this method is applied to the universal functions for the wind and temperature gradients, the standard deviation of the horizontal wind velocity (v(,*)), the variances of vertical velocity and temperature, the streamwise heat flux, and the turbulence dissipation. Analyses of the Kansas data are performed using both drag-plate and eddy -correlation measurements of u(,*). To indirectly include the influence of h and also obtain the best statisti- cal results, the dimensionless shear, (xi) and Ri are modified by the ratio (kappa)v(,*)/u(,*). Unfortunately, this procedure often results in solutions which do not satisfy the requirements of free convection theory. An apparent reason for this is found when the differences between the drag-plate and eddy-correlation values of n(,*) are analyzed. Using the flow distortion equations developed by Wyngaard (1982), results in close agreement with Wieringa (1980) are obtained. Despite this, it is found that the ratio (kappa)v(,*)/u(,*) is apparently equal to unity in neutral conditions and that the height dependence of some quantities may not be the same as is currently accepted. A theoretical approach which imposes free convection require- ments is developed. Vector-like quantities are found to be propor- tional to the shear stress and all quantities are found to depend on both w(,*) and w(theta)(' )in an identical manner. New flux-gradient relationships are proposed.

Loveland, Kurt Tyler

276

Spatial Linear Instability of Confluent Wake/Boundary Layers  

NASA Technical Reports Server (NTRS)

The spatial linear instability of incompressible confluent wake/boundary layers is analyzed. The flow model adopted is a superposition of the Blasius boundary layer and a wake located above the boundary layer. The Orr-Sommerfeld equation is solved using a global numerical method for the resulting eigenvalue problem. The numerical procedure is validated by comparing the present solutions for the instability of the Blasius boundary layer and for the instability of a wake with published results. For the confluent wake/boundary layers, modes associated with the boundary layer and the wake, respectively, are identified. The boundary layer mode is found amplified as the wake approaches the wall. On the other hand, the modes associated with the wake, including a symmetric mode and an antisymmetric mode, are stabilized by the reduced distance between the wall and the wake. An unstable mode switching at low frequency is observed where the antisymmetric mode becomes more unstable than the symmetric mode when the wake velocity defect is high.

Liou, William W.; Liu, Feng-Jun; Rumsey, C. L. (Technical Monitor)

2001-01-01

277

A Boundary Layer Interacting with a Point Vortex  

NASA Astrophysics Data System (ADS)

A model of the interaction of a boundary layer with a point vortex is presented. By varying the vortex strength and distance from the wall, a wide range of phenomena--localized disturbances, wave trains, entrainment of irrotational fluid, ejection of vortical fluid--is seen, which mimic the diversity of behavior found in actual boundary layers. The model equations, which are two-dimensional and inviscid, are expected to be valid at large Reynolds number. Both the evolution of the boundary layer and the trajectory of the vortex, which may be above or within the boundary layer, are determined. Analytical results are obtained where possible; otherwise, contour dynamics is used. It is found that when the vortex is within the boundary layer, a resonance condition yields a lengthening interval of growing waves. For a vortex extremely close to the wall, the disturbance amplitude is independent of the vortex strength. When the vortex is above the boundary layer, it may produce a second vortex-like structure within the boundary layer undercut by a narrow crevice of entrained irrotational fluid. Under other conditions, a spire of irrotational fluid is ejected.

Lichter, Seth

1997-11-01

278

On the theory of laminar boundary layers involving separation  

NASA Technical Reports Server (NTRS)

This paper presents a mathematical discussion of the laminar boundary layer, which was developed with a view of facilitating the investigation of those boundary layers in particular for which the phenomenon of separation occurs. The treatment starts with a slight modification of the form of the boundary layer equation first published by Von Mises. Two approximate solutions of this equation are found, one of which is exact at the outer edge of the boundary layer while the other is exact at the wall. The final solution is obtained by joining these two solutions at the inflection points of the velocity profiles. The final solution is given in terms of a series of universal functions for a fairly broad class of potential velocity distributions outside of the boundary layer. Detailed calculations of the boundary layer characteristics are worked out for the case in which the potential velocity is a linear function of the distance from the upstream stagnation point. Finally, the complete separation point characteristics are determined for the boundary layer associated with a potential velocity distribution made up of two linear functions of the distance from the stagnation point. It appears that extensions of the detailed calculations to more complex potential flows can be fairly easily carried out by using the explicit formulae given in the paper. (author)

Von Karman, TH; Millikan, C

1934-01-01

279

Titan's planetary boundary layer structure at the Huygens landing site  

Microsoft Academic Search

Huygens Atmospheric Structure Instrument (HASI) for the first time performed an in situ measurement of the thermal structure in Titan's atmosphere with a vertical resolution sufficient to analyze the planetary boundary layer (PBL). The vertical potential temperature profile reveals the presence of a weakly convective PBL, with a surface layer thickness of 10 m and an outer layer with a

Tetsuya Tokano; Francesca Ferri; Giacomo Colombatti; Teemu Mäkinen; Marcello Fulchignoni

2006-01-01

280

Slip with friction and penetration with resistance boundary conditions for the Navier-Stokes equations--numerical tests and aspects of the implementation  

NASA Astrophysics Data System (ADS)

We consider slip with friction and penetration with resistance boundary conditions in the steady state Navier-Stokes equations. This paper describes some aspects of the implementation of these boundary conditions for finite element discretizations. Numerical tests on two- and three-dimensional channel flows across a step using the slip with friction boundary condition study the influence of the friction parameter on the position of the reattachment point and the reattachment line of the recirculating vortex, respectively.

John, Volker

2002-10-01

281

Friction laws for lubricated nanocontacts  

NASA Astrophysics Data System (ADS)

We have used friction force microscopy to probe friction laws for nanoasperities sliding on atomically flat substrates under controlled atmosphere and liquid environment, respectively. A power law relates friction force and normal load in dry air, whereas a linear relationship, i.e., Amontons' law, is observed for junctions fully immersed in model lubricants, namely, octamethylciclotetrasiloxane and squalane. Lubricated contacts display a remarkable friction reduction, with liquid and substrate specific friction coefficients. Comparison with molecular dynamics simulations suggests that load-bearing boundary layers at junction entrance cause the appearance of Amontons' law and impart atomic-scale character to the sliding process; continuum friction models are on the contrary of limited predictive power when applied to lubrication effects. An attempt is done to define general working conditions leading to the manifestation of nanoscale lubricity due to adsorbed boundary layers.

Buzio, R.; Boragno, C.; Valbusa, U.

2006-09-01

282

Elevated Residual Layers and Their Influence on Surface Boundary-Layer Evolution  

Microsoft Academic Search

Elevated mixed layers (EMLs) are an important factor in the development of springtime thunderstorms over the United States. EMLs can be considered a subset of a larger class, called residual layers, since the mean state variables are the same, at least initially, as those of the boundary layers in which EMLs a formed. It is possible, however, for boundary or

David J. Stensrud

1993-01-01

283

Experimental study of boundary layer transition on a heated flat plate  

NASA Technical Reports Server (NTRS)

A detailed investigation to the document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional, and low Reynolds number turbulent boundary layers are presented. Measurements were done in a low-speed, closed-loop wind tunnel with a freestream velocity of 100 ft/s and zero pressure gradient over a range of freestream turbulence intensities from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate, and Reynolds shear stress were all consistent with previously published data. Reynolds analogy factors for momentum thickness Reynolds number, Re(sub theta) less than 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length and uniform heat flux. A small dependence of turbulence results on the freestream turbulence intensity was observed.

Sohn, K. H.; Reshotko, E.; Zaman, K. B. M. Q.

1991-01-01

284

Investigation of blown boundary layers with an improved wall jet system  

NASA Technical Reports Server (NTRS)

Measurements were made in a two dimensional incompressible wall jet submerged under a thick upstream boundary layer with a zero pressure gradient and an adverse pressure gradient. The measurements included mean velocity and Reynolds stresses profiles, skin friction, and turbulence spectra. The measurements were confined to practical ratios (less than 2) of the jet velocity to the free stream velocity. The wall jet used in the experiments had an asymmetric velocity profile with a relatively higher concentration of momentum away from the wall. An asymmetric jet velocity profile has distinct advantages over a uniform jet velocity profile, especially in the control of separation. Predictions were made using Irwin's (1974) method for blown boundary layers. The predictions clearly show the difference in flow development between an asymmetric jet velocity profile and a uniform jet velocity profile.

Saripalli, K. R.; Simpson, R. L.

1980-01-01

285

A numerical investigation of supersonic nozzle boundary layer transition  

NASA Technical Reports Server (NTRS)

The boundary layer stability within the high-area-ratio nozzle at NASA Lewis was tested at five chamber pressure conditions. For all nozzle conditions examined, Taylor-Goertler vortices grew more rapidly than Tollmien-Schlichting waves. The experimental heat flux was accurately predicted by a laminar boundary layer computation, thereby confirming the laminar nature of the nozzle boundary layer flow. When the chamber pressure was increased in a series of cases, the transition point occurred farther upstream. The number of vortices contained in the dominant instability increased with chamber pressure.

Pauley, Laura L.; Dagher, Samir N.

1991-01-01

286

Formation of pre-sheath boundary layers in electronegative plasmas  

SciTech Connect

In electronegative plasmas Coulomb scattering between positive and negative ions can lead to the formation of a pre-sheath boundary layer containing the bulk of the negative ions. The negative ion boundary layer forms when momentum transfer from positive to negative ions dominates the negative ion acceleration from the electric field. This condition is met in Inductively Coupled Plasma reactors that operate at low pressure and high plasma density. Simulations of the GEC reactor for Chlorine and Oxygen chemistries using the INDUCT95 2D model are presented showing the pre-sheath boundary layer structure as a function of applied power and neutral pressure.

Vitello, P., LLNL

1998-05-01

287

Boundary layer flow and heat transfer past a shrinking sheet in a copper-water nanofluid  

NASA Astrophysics Data System (ADS)

The problem of laminar fluid flow which results from the shrinking of a flat surface in a water-based copper (Cu) nanofluid is considered in this study. The model used for the nanofluid incorporates the effect of the nanoparticles volume fraction. The governing partial differential equations are transformed into ordinary differential equations by similarity transformations. The transformed equations are solved numerically by using a shooting method. Results for the skin friction coefficient, local Nusselt number, velocity profiles and temperature profiles are presented for different values of the governing parameters. The analysis reveals the conditions for the existence of the steady boundary layer flow due to shrinking of the sheet and it is found that when the mass suction parameter exceeds a certain critical value, steady flow is possible. Dual solutions for the velocity and temperature distributions are obtained. With increasing values of the nanoparticles volume fraction, the skin friction and the heat transfer coefficient increase.

Aleng, Nur Liyana; Bachok, Norfifah; Arifin, Norihan Md.; Ishak, Anuar

2014-06-01

288

Particle size and boundary geometry effects on the bulk friction coefficient of sheared granular materials in the inertial regime  

NASA Astrophysics Data System (ADS)

Glass beads of varying diameters (d=2,3,4, and 5 mm) are used to measure the ratio of shear-to-normal stress, or bulk friction coefficient, generated inside an annular shear cell at high shearing rates. The effects of the particle size, the solids concentration, and the shear rate are explored. It is found that (1) for a given particle size, the magnitude of the bulk friction coefficient decreases with increasing solids concentration, (2) for a given solids concentration, the bulk friction coefficient decreases with increasing particle size, and (3) the bulk friction coefficient is independent of the shear rate except for cases with low solids concentration, where it decreases with increasing shear rate. The boundary geometry is found to affect bulk friction only for dilute (low solids concentration) flows involving small particles.

Orlando, Andrés D.; Shen, Hayley H.

2014-03-01

289

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

290

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

291

Self-similar analysis of fluid flow and heat-mass transfer of nanofluids in boundary layer  

NASA Astrophysics Data System (ADS)

Processes of heat, momentum, and concentration transport in a boundary layer of a nanofluid near a flat wall were studied. The study was performed by means of numerical analysis of boundary layer equations in a self-similar form. Self-similar forms of these equations were obtained based on symmetry properties (Lie groups). In doing so, dependence of physical properties (viscosity, thermal conductivity, and diffusion coefficient) on concentration of nanofluids and temperature were taken into account. Effects of concentration of the nano-particles on velocity and temperature profiles, as well as on the relative Nusselt numbers and skin-friction coefficients, were elucidated.

Avramenko, A. A.; Blinov, D. G.; Shevchuk, I. V.

2011-08-01

292

Evaluation of boundary lubricants using steady-state wear and friction  

NASA Technical Reports Server (NTRS)

A friction and wear study was made at 20 C to establish operating limits and procedures for obtaining improved reproducibility and reliability in boundary lubrication testing. Ester base and C-other base fluids were used to lubricate a pure iron rider in sliding contact with a rotating M-50 steel disk in a pin-on-disk apparatus. Results of a parametric study with varying loads and speeds slowed that satisfactory test conditions for studying the direction and wear characteristics in the boundary lubrication regime with this test device were found to be 1 kilogram load; 7 to 9 meters-per-minute (50 rpm) surface speed; dry air test atmosphere (less than 100 ppm H2O); and use of a time stepwise procedure for measuring wear. Highly reproducible steady-state wear rates resulted from the two fluid studies which had a linearity of about 99 percent after initially higher wear rates and friction coefficients during run-in periods of 20 to 40 minutes.

Loomis, W. R.; Jones, W. R., Jr.

1981-01-01

293

Coherent Motions of the Turbulent Boundary Layer (Invited)  

NASA Astrophysics Data System (ADS)

Over the last decade a model has been developed in which the structure of the turbulent boundary consists of quasi-streamwise vortices near the wall, a hierarchy of hairpin vortex packets that extends through the logarithmic layer, large-scale motions having streamwise extent of the order of the thickness of the boundary layer, and very-large-scale motions that are much longer than the boundary layer thickness. Figure 1 shows a cartoon sketch of the hairpin packet hierarchy. The evidence indicates that the large and very-large-scale motions become increasingly important as the Reynolds number increases, implying that geophysical boundary layer have considerably different character than low Reynolds number laboratory experiments and simulations. Work is in progress to discern the form of the large motions and incorporate them into a more complete model. Fig. 1. Hierarchy of hairpin packets begins at the surface.

Adrian, R. J.

2009-12-01

294

Development of Instrumentation for Boundary Layer Transition Detection.  

National Technical Information Service (NTIS)

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 surfa...

S. B. Harrison

1991-01-01

295

Boundary Layers in Strain-Gradient Theory of Linear Elasticity.  

National Technical Information Service (NTIS)

In a previous investigation based on couple-stress theory the presence of an elastic boundary layer of minute thickness was established in which the states of stress and displacement differ essentially and significantly from classical elasticity solutions...

M. A. Sadowsky S. L. Pu

1970-01-01

296

Analytical Parameterizations of Diffusion: The Convective Boundary Layer.  

National Technical Information Service (NTIS)

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; ...

G. A. Briggs

1985-01-01

297

Boundary Layer Integral Matrix Procedure Code Modifications and Verifications.  

National Technical Information Service (NTIS)

A summary of modifications to Aerotherm's Boundary Layer Integral Matrix Procedure (BLIMP) code is presented. These modifications represent a preliminary effort to make BLIMP compatible with other JANNAF codes and to adjust the code for specific applicati...

R. M. Evans H. L. Morse

1974-01-01

298

On Strong Slot Injection Into a Subsonic Laminar Boundary Layer.  

National Technical Information Service (NTIS)

This paper is concerned with the problem of strong slot injection into a subsonic laminar boundary layer at asymptotically high Reynolds number. The problem is formulated and the governing equations are presented within the context of triple deck theory. ...

M. Napolitano R. E. Messick

1978-01-01

299

Asymptotic analysis: Working note (number sign)3, boundary layers.  

National Technical Information Service (NTIS)

In this chapter the authors discuss the asymptotic approximation of functions that display boundary-layer behavior. The purpose here is to introduce the basic concepts underlying the phenomenon, to illustrate its importance, and to describe some of the fu...

M. Garbey H. G. Kaper

1993-01-01

300

Boundary-layer transition effects on airplane stability and control  

NASA Technical Reports Server (NTRS)

Surface contamination of laminar-flow airfoils can significantly modify the location of transition from laminar-to-turbulent boundary-layer flow. The contamination can be the result of insect debris, environmental effects such as ice crystals and moisture due to mist or rain, surface damage, or other contamination adhering to the surface. Location and mode of transition have a dominant effect on the lift-and-drag characteristics of a lifting surface. The influences of laminar boundary-layer flow behavior on airplane stability and control are examined through theoretical results and experimental (wind-tunnel and free-flight) data. For certain airfoils with a relatively steep pressure recovery it is shown that loss of laminar flow near the leading edge can result in premature separation of the turbulent boundary layer and, consequently, in loss of lift and control effectiveness. Aerodynamic modifications which minimize boundary-layer transition effects on airplane stability and control are also discussed.

Van Dam, C. P.; Holmes, B. J.

1986-01-01

301

Microphone System for the Measurement of Boundary Layer Pressure Fluctuations.  

National Technical Information Service (NTIS)

This report presents a description of a condenser microphone system developed for use in the measurement of boundary layer pressure fluctuations. Prime consideration in the design of the transducer of the system was the requirement for operation in abnorm...

J. C. Ortega

1967-01-01

302

Theoretical and Modeling Studies of the Marine Planetary Boundary Layer.  

National Technical Information Service (NTIS)

We do not currently understand what determines the fractional cloudiness in partly cloudy boundary layers, how it is influenced by cloud-top entrainment instability, or what controls the transition from stratocumulus to cumulus conditions. These questions...

D. A. Randall

1991-01-01

303

Streamwise velocity statistics in turbulent boundary layers that spatially develop to high Reynolds number  

NASA Astrophysics Data System (ADS)

Well-resolved measurements of the streamwise velocity in zero pressure gradient turbulent boundary layers are presented for friction Reynolds numbers up to 19,670. Distinct from most studies, the present boundary layers undergo nearly a decade increase in Reynolds number solely owing to streamwise development. The profiles of the mean and variance of the streamwise velocity exhibit logarithmic behavior in accord with other recently reported findings at high Reynolds number. The inner and mid-layer peaks of the variance profile are evidenced to increase at different rates with increasing Reynolds number. A number of statistical features are shown to correlate with the position where the viscous force in the mean momentum equation loses leading order importance, or similarly, where the mean effect of turbulent inertia changes sign from positive to negative. The near-wall peak region in the 2-D spectrogram of the fluctuations is captured down to wall-normal positions near the edge of the viscous sublayer at all Reynolds numbers. The spatial extent of this near-wall peak region is approximately invariant under inner normalization, while its large wavelength portion is seen to increase in scale in accord with the position of the mid-layer peak, which resides at a streamwise wavelength that scales with the boundary layer thickness.

Vincenti, P.; Klewicki, J.; Morrill-Winter, C.; White, C. M.; Wosnik, M.

2013-12-01

304

The turbulent boundary layer on a porous plate: An experimental study of the fluid mechanics for adverse free stream pressure gradients  

NASA Technical Reports Server (NTRS)

An experimental investigation of transpired turbulent boundary layers in zero and adverse pressure gradients has been carried out. Profiles of: (1) the mean velocity, (2) the three intensities of the turbulent fluctuations, and (3) the Reynolds stress were obtained by hot-wire anemometry. The friction coefficients were measured by using an integrated form of the boundary layer equation to extrapolate the measured shear stress profiles to the wall.

Anderson, P. S.; Kays, W. M.; Moffat, R. J.

1972-01-01

305

On stability of free laminar boundary layer between parallel streams  

NASA Technical Reports Server (NTRS)

An analysis and calculations on the stability of the free laminar boundary layer between parallel streams were made for an incompressible fluid using the Tollmien-Schlichting theory of small disturbances. Because the boundary conditions are at infinity, two solutions of the Orr-Sommerfeld stability equations need not be considered, and the remaining two solutions are exponential in character at the infinite boundaries. The calculations show that the flow is unstable except for very low Reynolds numbers.

Lessen, Martin

1950-01-01

306

Swirling Flow Problem in Boundary Layer Theory.  

National Technical Information Service (NTIS)

The paper deals with the boundary value problem on t > or = 0 given by f triple primed + f(f double primed) + beta(g sup 2 - omega sup 2 - f primed sup 2) = 0, g double primed + f(g primed) - 2b(f primed)g = 0, f(0), f primed(0), g(0) given and f primed (...

P. Hartman

1971-01-01

307

Chebyshev solution of laminar boundary layer flow  

Microsoft Academic Search

An expansion procedure using the Chebyshev polynomials is proposed by using El-Gendi method [1], which yields more accurate results than those computed by P. M. Beckett [2] and A. R. Wadia and F. R. Payne [6] as indicated from solving the Falkner-Skan equation, which uses a boundary value technique. This method is accomplished by starting with Chebyshev approximation for the

H. Nasr; I. A. Hassanien; H. M. El-Hawary

1990-01-01

308

A Distributional Approach to the Boundary Layer Theory  

Microsoft Academic Search

We present a distributional approach for solving problems in the boundary layer and singular perturbation theories. We discuss the asymptotic expansions in the inner and outer regions and their matching in the overlapping domain by our theory. This technique is illustrated with various examples from the initial value and boundary value problems. From this discussion we find that the distributional

R. Estrada; R. P. Kanwal

1995-01-01

309

Calculation of turbulent boundary layer wall pressure spectra  

Microsoft Academic Search

This study is an investigation into the suitability of various wavevector-frequency models of turbulent boundary layer wall pressure fluctuations for the prediction of experimental measurements of turbulent boundary layer wall pressure spectra. Three separate models of the wavevector-frequency spectrum proposed by D. M. Chase in 1980 and 1987 are evaluated. The wavevector-frequency spectral models are integrated numerically using a formulation

D. E. Capone; G. C. Lauchle

1993-01-01

310

Tropical boundary layer equilibrium in the last ice age  

NASA Technical Reports Server (NTRS)

A radiative-convective boundary layer model is used to assess the effect of changing sea surface temperature, pressure, wind speed, and the energy export from the tropics on the boundary layer equilibrium equivalent potential temperature. It remains difficult to reconcile the observations that during the last glacial maximum (18,000 yr BP) the snowline on the tropical mountains fell 950 m, while the tropical sea surface temperatures fell only 1-2 K.

Betts, Alan K.; Ridgway, W.

1992-01-01

311

Boundary Layers in Self-Gravitating, Collisional Rings  

Microsoft Academic Search

Recently, it has been proposed that boundary layers on the edges of planetary rings may play a key role in the behavior of the ring as a whole. In particular, Chiang and Goldreich (2002) proposed that that narrow eccentric rings have high-velocity-dispersion boundary layers that work against apse alignment. Self-gravity should be a key player in the formation of these

J. W. Weiss; G. R. Stewart

2003-01-01

312

Vortex Shedding from a Hemisphere in a Turbulent Boundary Layer  

Microsoft Academic Search

:   Supercritical turbulent boundary layer flow over a hemisphere with a rough surface (Re= 150000) has been simulated using Large Eddy Simulation (LES) and analyzed using the Karhunen--Loève expansion (“Proper Orthogonal\\u000a Decomposition,” POD). The time-dependent inflow condition is provided from a separate LES of a boundary layer developing behind\\u000a a barrier fence and a set of vorticity generators. LES results

Michael Manhart

1998-01-01

313

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

314

Flow visualization of shock-boundary layer interaction  

NASA Technical Reports Server (NTRS)

Two and three-dimensional shock-boundary layer interaction data were obtained from supersonic wind tunnel tests. These interactions are studied both with and without boundary layer bleed. The data verify computational fluid dynamic codes. Surface static pressure, pitot pressure, flow angularity, and bleed rates, are studied by flow visualization techniques. Surface oil flow using fluorescent dye and laser sheet using water droplets as the scattering material are used for flow visualization.

Hingst, W. R.; Jurkovich, M.

1982-01-01

315

Separating and turbulent boundary layer calculations using polynomial interpretation  

NASA Technical Reports Server (NTRS)

Higher order numerical methods derived from polynomial spline interpolation or Hermitian differencing are applied to a separating laminar boundary layer, i.e., the Howarth problem, and the turbulent flat plate boundary layer flow. Preliminary results are presented. It is found that accuracy equal to that of conventional second order accurate finite difference methods is achieved with many fewer mesh points and with reduced computer storage and time requirements.

Rubin, S. G.; Rivera, S.

1977-01-01

316

Turbulent boundary layer in high rayleigh number convection in air.  

PubMed

Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bénard experiment are presented for the Rayleigh number Ra=1.4×1010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re?200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal. PMID:24724653

du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, André; Willert, Christian

2014-03-28

317

Influence of Bowen Ratio on Boundary-Layer Cloud Structure.  

NASA Astrophysics Data System (ADS)

An investigation of the influence of the ratio of surface sensible heat flux to latent heat flux, the Bowen ratio. on the structure of boundary-layer clouds is carried out utilizing numerical large eddy simulations (LES). The role of cloud-top radiational cooling, cloud-top temperature and moisture jump conditions, and wind shear are included in a secondary way. Although no detailed comparisons have been made, the LES results appear to be qualitatively consistent with the Atlantic Stratocumulus Transition Experiment, the recent field study on marine boundary-layer cloud structure. Some conclusions that follow from an examination of these LES results are the following: First, there is a highly bimodal character to the cloud ceiling frequency within a very low Bowen ratio boundary layer. The updrafts tend to produce a lower cloud ceiling than the surrounding environment with its weak downdrafts. Second, a very low Bowen ratio with the aid of some boundary-layer shear makes the development of persistent microcell cloud circulations possible within the boundary layer. Third, when the surface latent heat flux is the dominant factor in the dynamics of the boundary layer, the approach to a conditionally stable lapse rate results in the potential for subsequent decoupling. Last, the maximum partial cloud fraction is very well represented by the relation suggested by Sommeria and Deardorff for a Gaussian probability distribution for the range of conditions studied.

Lewellen, D. C.; Lewellen, W. S.; Yoh, S.

1996-01-01

318

Hypersonic Turbulent Wall Boundary Layer Computations.  

National Technical Information Service (NTIS)

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 Baldwi...

S. C. Kim G. J. Harloff

1988-01-01

319

FEM simulation on rotating piercing process of double-layer clad sheet with Coulomb friction  

NASA Astrophysics Data System (ADS)

This study proposes a new piercing technology with rotating punch on the double-layer clad sheet; it carries out an FEM simulation on rotating piercing process using DEFORM-3D commercial software. Frictions among the punch, the blank holder, the dies and the double-layer clad sheet material are assumed as Coulomb friction, but can be different. The surface of the inner diameter, the effective stress, the effective strain, velocity field, damage, burr and the shearing force can be determined form the FEM simulation. In this study, effects of various piercing conditions such as the clearance, the punch nose angle, the frictional factor, the rotating angular velocity, the shearing force, and burr on shearing characteristics are explored effectively to realize the feasibility of FEM model.

Tzou, Gow-Yi; Hwang, Yeong-Maw; Teng, Hsiang-Yu

2013-12-01

320

Dense gas boundary layer experiments: Visualization, pressure measurements, concentration evaluation  

SciTech Connect

This technical report describes methods that were applied to investigate turbulent boundary layers generated by inviscid, baroclinic effects. The Cranz-Schardin 24-sparks camera was used to visualize the interactions of a planar shock wave with a Freon R12-layer. The shock propagates more slowly in the Freon layer than in air because of its smaller sound speed. This causes the shock front to be curved and to be reflected between the wall and the layer interface. As a consequence of the reflection process, a series of compression and expansion waves radiate from the layer. Large fluctuations in the streamwise velocity and in pressure develop for about 1 ms. These waves strongly perturb the interface shear layer, which rapidly transitions to a turbulent boundary flow. Pressure measurements showed that the fluctuations in the Freon layer reach a peak pressure 4 times higher than in the turbulent boundary flow. To characterize the preshock Freon boundary layer, concentration measurements were performed with a differential interferometry technique. The refraction index of Freon R12 is so high that Mach-Zehnder interferometry was not successful in these experiments. The evaluation of the concentration profile is described here in detail. Method and results of corresponding LDV measurements under the same conditions are presented in a different report, EMI Report T 9/92. The authors plan to continue the dense gas layer investigations with the gas combination helium/Freon.

Reichenbach, H.; Neuwald, P. [Ernst-Mach-Institut, Freiburg (DE); Kuhl, A.L. [R and D Associates, Los Angeles, CA (United States)

1992-11-01

321

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

322

Magnetic domination of recollimation boundary layers in relativistic jets  

NASA Astrophysics Data System (ADS)

We study the collimation of relativistic magnetohydrodynamic jets by the pressure of an ambient medium, in the limit where the jet interior loses causal contact with its surroundings. This follows up a hydrodynamic study in a previous paper, adding the effects of a toroidal magnetic field threading the jet. As the ultrarelativistic jet encounters an ambient medium with a pressure profile with a radial scaling of p ? r-? where 2 < ? < 4, it loses causal contact with its surroundings and forms a boundary layer with a large pressure gradient. By constructing self-similar solutions to the fluid equations within this boundary layer, we examine the structure of this layer as a function of the external pressure profile. We show that the boundary layer always becomes magnetically dominated far from the source, and that in the magnetic limit, physical self-similar solutions are admitted in which the total pressure within the layer decreases linearly with distance from the contact discontinuity inwards. These solutions suggest a 'hollow-cone' behaviour of the jet, with the boundary-layer thickness prescribed by the value of ?. In contrast to the hydrodynamical case, however, the boundary layer contains an asymptotically vanishing fraction of the jet energy flux.

Kohler, Susanna; Begelman, Mitchell C.

2012-10-01

323

The separated turbulent boundary layer over a wavy wall  

NASA Technical Reports Server (NTRS)

A study and application of the fourth order spline collocation procedure, numerical solution of boundary layer like differential equations, is presented. A simple inversion algorithm for the simultaneous solution of the resulting difference equations is given. Particular attention is focused on the boundary condition representation for the spline second derivative approximations. Solutions using the spline procedure, as well as the three point finite difference method, are presented for several model problems in order to assess and improve the spline numerical scheme. Application of the resulting algorithm to the incompressible laminar self similar boundary layer equations is presented.

Polak, A.; Werle, M. J.

1977-01-01

324

Boundary Layer Clouds and Vegetation–Atmosphere Feedbacks  

Microsoft Academic Search

An analysis of boundary layer cumulus clouds and their impact on land surface-atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (;1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls

Jeffrey M. Freedman; David R. Fitzjarrald; Kathleen E. Moore; Ricardo K. Sakai

2001-01-01

325

ILLIAC 4 and lifting surface theory with boundary layer  

NASA Technical Reports Server (NTRS)

Aerodynamic flutter and a re-written computer program for its study are discussed. Data cover: (1) lifting surface theory with boundary layer, (2) incompressible, two dimensional, unsteady flow with control surfaces, (3) improved unsteady theory, (4) combined transonic airfoil thickness and shear layer thickness effects, and (5) bending-torsion flutter calculations.

Dowell, E. H.

1976-01-01

326

BULK MODELS OF THE ATMOSPHERIC CONVECTIVE BOUNDARY LAYER  

Microsoft Academic Search

The paper presents an overview of modeling the atmospheric convective boundary layer (CBL) using bulk parameterizations for the vertical structure of the layer. Such parameterizations are constructed based on empirical knowledge about vertical distributions of meteorological variables in the CBL. Two main types of CBL bulk models are presented and discussed. The first model considered is the so-called zero-order jump

E. FEDOROVICH

1998-01-01

327

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...

328

Unsteady gun-barrel boundary-layer calculations  

Microsoft Academic Search

The paper describes current progress in the calculation of nonsteady boundary layers involved in internal flow situations to determine the dominant mechanisms of erosion. A time-dependent description of the viscous (laminar and turbulent) layer is formulated, and numerical solutions are obtained by application of a factored ADI scheme. The conservation equations in the alternate-sweep form are then solved under prescribed

S. W. Kang; J. L. Levatin

1980-01-01

329

Compressibility Considerations for kappa-omega Turbulence Models in Hypersonic Boundary Layer Applications  

NASA Technical Reports Server (NTRS)

The ability of kappa-omega models to predict compressible turbulent skin friction in hypersonic boundary layers is investigated. Although uncorrected two-equation models can agree well with correlations for hot-wall cases, they tend to perform progressively worse - particularly for cold walls - as the Mach number is increased in the hypersonic regime. Simple algebraic models such as Baldwin-Lomax perform better compared to experiments and correlations in these circumstances. Many of the compressibility corrections described in the literature are summarized here. These include corrections that have only a small influence for kappa-omega models, or that apply only in specific circumstances. The most widely-used general corrections were designed for use with jet or mixing-layer free shear flows. A less well-known dilatation-dissipation correction intended for boundary layer flows is also tested, and is shown to agree reasonably well with the Baldwin-Lomax model at cold-wall conditions. It exhibits a less dramatic influence than the free shear type of correction. There is clearly a need for improved understanding and better overall physical modeling for turbulence models applied to hypersonic boundary layer flows.

Rumsey, C. L.

2009-01-01

330

Turbulent boundary layers: Inflow effects and cross-validation of simulation and experiment  

NASA Astrophysics Data System (ADS)

A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows [Schlatter & "Orl"u, J. Fluid Mech. 659, 116 (2010)] showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. Several DNS of a zero pressure-gradient (ZPG) turbulent boundary layer (TBL) àla Schlatter et al. [Phys. Fluids 21, 051702 (2009)] with physically different inflow conditions and tripping effects were performed. Most of the differences observed when comparing available DNS could thereby be traced back to different initial conditions. It was also found, that if transition is initiated at a low enough Reynolds number (based on the momentum-loss thickness) Re?< 300, all data agree well for both inner and outer layer for Re?>2000; a result that gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments. Based on these results a detailed comparison between DNS and experiment of a ZPG TBL flow at Re?=2500 and 4000 is presented. Good agreement is obtained for integral quantities, mean and fluctuating streamwise velocity profiles, but also for the probability distribution and spectral map throughout the boundary layer.

Oerlue, Ramis; Schlatter, Philipp

2011-11-01

331

Free convection boundary layer flow near the lower stagnation point of a solid sphere with convective boundary conditions in a micropolar fluid  

NASA Astrophysics Data System (ADS)

In this paper, the mathematical model for free convection boundary layer flow in a micropolar fluid near the lower stagnation point of a solid sphere with convective boundary conditions, in which the heat is supplied through a bounding surface of finite thickness and finite heat capacity, is considered. The transformed and reduced boundary layer equations in the form of ordinary differential equations are solved numerically using an implicit finite difference scheme known as the Keller-box method. Numerical solutions are obtained for the local wall temperature and the local skin friction coefficient, as well as the velocity, angular velocity and temperature profiles. The features of the flow and heat transfer characteristics for different values of the material or micropolar parameter K, the Prandtl number Prand the conjugate parameter ?are analyzed and discussed.

Alkasasbeh, Hamzeh Taha; Salleh, Mohd Zuki; Tahar, Razman Mat; Nazar, Roslinda; Pop, Ioan

2014-06-01

332

Stability of a Time Dependent Boundary Layer.  

National Technical Information Service (NTIS)

The aim of this article is to determine the stability characteristics of a Rayleigh layer, which is known to occur when the fluid above a flat plate has a velocity imparted to it (parallel to the plate). This situation is intrinsically unsteady, however, ...

S. R. Otto

1993-01-01

333

Experimental study of boundary layer transition with elevated freestream turbulence on a heated flat plate  

NASA Technical Reports Server (NTRS)

A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional, and low Reynolds number turbulent boundary layers are presented. Measurements were acquired in a low-speed, closed-loop wind tunnel with a freestream velocity of 100 ft/s and zero pressure gradient over a range of freestream turbulence intensities (TI) from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate and Reynolds shear stress were all consistent with previously published data. Reynolds analogy factors for R(sub theta) is less than 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length. The measured laminar value of Reynolds analogy factor was as much as 53 percent higher than the Pr(sup -2/3). A small dependence of turbulent results on TI was observed. Conditional sampling performed in the transitional boundary layer indicated the existence of a near-wall drop in intermittency, pronounced at certain low intermittencies, which is consistent with the cross-sectional shape of turbulent spots observed by others. Non-turbulent intervals were observed to possess large magnitudes of near-wall unsteadiness and turbulent intervals had peak values as much as 50 percent higher than were measured at fully turbulent stations. Non-turbulent and turbulent profiles in transitional boundary layers cannot be simply treated as Blasius and fully turbulent profiles, respectively. The boundary layer spectra indicate predicted selective amplification of T-S waves for TI is approximately 0.4 percent. However, for TI is approximately 0.8 and 1.1 percent, T-S waves are localized very near the wall and do not play a dominant role in transition process.

Sohn, Ki-Hyeon; Reshotko, Eli

1991-01-01

334

Sheared boundary layers in turbulent Rayleigh-Benard convection  

NASA Astrophysics Data System (ADS)

Thermal boundary layers in turbulent Rayleigh-Benard convection are studied experimentally using a novel system in which the convecting fluid is sheared from below with a flowing layer of mercury. Oscillatory shear substantially alters the spatial structure and frequency of the eruptions, with minimal effect on the heat flux (less than 5 percent). The temperature probability distribution function (PDF) just above the lower boundary layer changes from Gaussian to exponential without significant changes in the interior PDF. Implications for theories of 'hard' turbulence are discussed.

Solomon, T. H.; Gollub, J. P.

1990-05-01

335

Numerical Studies of Boundary-Layer Receptivity  

NASA Technical Reports Server (NTRS)

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.

1995-01-01

336

Design and Evaluation of a New Boundary-Layer Rake for Flight Testing  

NASA Technical Reports Server (NTRS)

A new boundary-layer rake has been designed and built for flight testing on the NASA Dryden Flight Research Center F-15B/Flight Test Fixture. A feature unique to this rake is its curved body, which allows pitot tubes to be more densely clustered in the near-wall region than conventional rakes allow. This curved rake design has a complex three-dimensional shape that requires innovative solid-modeling and machining techniques. Finite-element stress analysis of the new design shows high factors of safety. The rake has passed a ground test in which random vibration measuring 12 g rms was applied for 20 min in each of the three normal directions. Aerodynamic evaluation of the rake has been conducted in the NASA Glenn Research Center 8 x 6 Supersonic Wind Tunnel at Mach 0-2. The pitot pressures from the new rake agree with conventional rake data over the range of Mach numbers tested. The boundary-layer profiles computed from the rake data have been shown to have the standard logarithmic-law profile. Skin friction values computed from the rake data using the Clauser plot method agree with the Preston tube results and the van Driest II compressible skin friction correlation to approximately +/-5 percent.

Bui, Trong T.; Oates, David L.; Gonsalez, Jose C.

2000-01-01

337

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

338

Feasibility study of optical boundary layer transition detection method  

NASA Technical Reports Server (NTRS)

A high sensitivity differential interferometer was developed to locate the region where the boundary layer flow undergoes transition from laminar to turbulent. Two laboratory experimental configurations were used to evaluate the performance of the interferometer: open shear layer, and low speed wind tunnel turbulent spot configuration. In each experiment, small temperature fluctuations were introduced as the signal source. Simultaneous cold wire measurements were compared with the interferometer data. The comparison shows that the interferometer is sensitive to very weak phase variations in the order of 0.001 the laser wavelength. An attempt to detect boundary layer transition over a flat plate at NASA-Langley Unitary Supersonic Wind Tunnel using the interferometer system was performed. The phase variations during boundary layer transition in the supersonic wind tunnel were beyond the minimum signal-to-noise level of the instrument.

Azzazy, M.; Modarress, D.; Trolinger, J. D.

1986-01-01

339

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

340

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

341

The Boundary Layer Late Afternoon and Sunset Turbulence Project  

NASA Astrophysics Data System (ADS)

The BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) project aims at better understanding the turbulence processes which occur during the transition from a well-mixed convective boundary layer to a residual layer overlying a stabilized nocturnal layer. This phase of the diurnal cycle is challenging from both modeling and observational perspectives: it is transitory, most of the forcings are small or null during the transition and the turbulence regime changes from the fully convective regime of turbulence, close to homogeneous and isotropic, toward more heterogeneous and intermittent turbulence during its decay. Those issues motivated a field campaign that was conducted from 14 June to 8 July 2011 in southern France in complex terrain and consisted of a range of integrated instrument platforms including: full-size aircraft, Remotely Piloted Airplane Systems (RPAS), remote sensing instruments, radiosoundings, tethered balloons, surface flux stations, and various meteorological towers deployed over different surface covers. The boundary layer, from the earth's surface to free troposphere was densely probed during the entire day, with a focus and intense observations from midday until sunset. The field dataset now forms the base of a set of studies utilizing the observations and several types of models including: Large Eddy Simulation, Mesoscale models, forecast models. The presentation will expose an overview of this experiment and of the current observational and modeling studies, with the focus on: the turbulence decay process within the entire boundary layer from surface to the top, the mesoscale forcings of importance during BLLAST, the ability of the forecast models to represent the diurnal cycle, the relevance of the Monin Obukhov similarity theory, and shallow drainage flows. Reference: Lothon M. et al., 2012. The Boundary-Layer Late Afternoon and Sunset Turbulence field experiment, Proc. of the 20th Symposium on Boundary-Layers and Turbulence, 7-13 July, Boston, MA, USA.

Lothon, Marie; Lohou, Fabienne; Darbieu, Clara; Couvreux, Fleur; Pino, David; Blay, Estel; Vila-Guerau de Arellano, Jordi; Pietersen, Henk; Hartogensis, Oscar; Pardyjak, Eric; Alexander, Daniel; Reuder, Joachim; Baaserud, Line; Nilsson, Erik; Jimenez, Maria Antonia; Faloona, Ian; Sastre-Marugan, Mariano; Angevine, Wayne M.; Canut, Guylaine; Bazile, Eric

2014-05-01

342

Surface modes in sheared boundary layers over impedance linings  

NASA Astrophysics Data System (ADS)

Surface modes, being duct modes localized close to the duct wall, are analysed within a lined cylindrical duct with uniform flow apart from a thin boundary layer. As well as full numerical solutions of the Pridmore-Brown equation, simplified mathematical models are given where the duct lining and boundary layer are lumped together and modelled using a single boundary condition (a modification of the Myers boundary condition previously proposed by the author), from which a surface mode dispersion relation is derived. For a given frequency, up to six surface modes are shown to exist, rather than the maximum of four for uniform slipping flow. Not only is the different number and behaviour of surface modes important for frequency-domain mode-matching techniques, which depend on having found all relevant modes during matching, but the thin boundary layer is also shown to lead to different convective and absolute stability than for uniform slipping flow. Numerical examples are given comparing the predictions of the surface mode dispersion relation to full solutions of the Pridmore-Brown equation, and the accuracy with which surface modes are predicted is shown to be significantly increased compared with the uniform slipping flow assumption. The importance of not only the boundary layer thickness but also its profile (tanh or linear) is demonstrated. A Briggs–Bers stability analysis is also performed under the assumption of a mass–spring–damper or Helmholtz resonator impedance model.

Brambley, E. J.

2013-08-01

343

The Atmospheric Boundary Layer Growth In An Urban Area  

NASA Astrophysics Data System (ADS)

The development and maintenance of the atmospheric boundary layer (ABL) plays a key role on the distribution of atmospheric constituents, specially in a polluted urban area. In particular, the atmospheric boundary layer has a direct impact on the con- centration and transformation of pollutants. It is therefore very important to obtain an accurate estimation of the boundary layer growth. The ABL growth is driven primarily by the surface fluxes (sensible and latent) and entrainment of warmer air from the free troposphere. It can also be influenced by the presence of mesoscale phenomena such as the sea-breeze or the mountain valley circulation. In order to analyse the different mechanisms which control the boundary layer growth, we have simulated by means of the non-hydrostatic model MM5 several boundary layer observed in the city of Barcelona (Spain). Sensitivity analysis of the modelled ABL are carried out by using various descriptions of the planetary boundary layer. Direct and continuous measure- ments of the boundary layer height taken by a LIDAR are used to evaluate the results obtained by the model. Depending on the LIDAR data availability, we have repeated our study under different meteorological situations. The intercomparison shows that the modelled boundary layer strongly depends on the selected parameterisation. In our presentation, we will show that in general the obser- vations provide the highest value of the maximum of ABL height. Moreover, the more simple parameterisation (Medium Range Forecast) simulates an ABL with values of the inversion height similar to those found with the LIDAR. On the contrary, the pa- rameterisation that solves the TKE prognostic equation (ETA) yields lowest values of the ABL height and does not fit with the observations. One possible source of differ- ences may be the heat and specific humidity surface fluxes calculated by the model. It is therefore advisable to measure surface fluxes in combination with LIDAR obser- vations to fully understand the boundary layer growth in an urban area. In addition, it would be also interesting to make this campaign at different places in the Barcelona area, in order to study the possible breeze effects and the influence of soil conditions in the ABL growth.

Pino, D.; Vilà-Guerau de Arellano, J.; Comerón, A.; Rocadenbosch, F.

344

Dynamic behavior of an unsteady turbulent boundary layer  

NASA Technical Reports Server (NTRS)

Experiments on an unsteady turbulent boundary layer are reported in which the upstream portion of the flow is steady (in the mean) and in the downstream region, the boundary layer sees a linearly decreasing free stream velocity. This velocity gradient oscillates in time, at frequencies ranging from zero to approximately the bursting frequency. For the small amplitude, the mean velocity and mean turbulence intensity profiles are unaffected by the oscillations. The amplitude of the periodic velocity component, although as much as 70 percent greater than that in the free stream for very low frequencies, becomes equal to that in the free stream at higher frequencies. At high frequencies, both the boundary layer thickness and the Reynolds stress distribution across the boundary layer become frozen. The behavior at higher amplitude is quite similar. At sufficiently high frequencies, the boundary layer thickness remains frozen at the mean value over the oscillation cycle, even though flow reverses near the wall during a part of the cycle. Previously announced in STAR as N81-28392

Parikh, P. G.; Reynolds, W. C.; Jayaraman, R.; Carr, L. W.

1981-01-01

345

Bending Boundary Layers in Laminated-Composite Circular Cylindrical Shells  

NASA Technical Reports Server (NTRS)

An analytical, parametric study of the attenuation of bending boundary layers or edge effects in balanced and unbalanced, symmetrically and unsymmetrically laminated thin cylindrical shells is presented for nine contemporary material systems. The analysis is based on the linear Sanders-Koiter shell equations and specializations to the Love-Kirchhoff shell equations and Donnell's equations are included. Two nondimensional parameters are identified that characterize and quantify the effects of laminate orthotropy and laminate anisotropy on the bending boundary-layer decay length in a very general and encompassing manner. A substantial number of structural design technology results are presented for a wide range of laminated-composite cylinders. For all the laminate constructions considered, the results show that the differences between results that were obtained with the Sanders-Koiter shell equations, the Love-Kirchhoff shell equations, and Donnell's equations are negligible. The results also show that the effect of anisotropy in the form of coupling between pure bending and twisting has a negligible effect on the size of the bending boundary-layer decay length of the balanced, symmetrically laminated cylinders considered. Moreover, the results show that coupling between the various types of shell anisotropies has a negligible effect on the calculation of the bending boundary-layer decay length in most cases. The results also show that in some cases neglecting the shell anisotropy results in underestimating the bending boundary-layer decay length and in other cases it results in an overestimation.

Nemeth, Michael P.; Smeltzer, Stanley S., III

2000-01-01

346

Stability and separation of freely interacting boundary layers  

NASA Astrophysics Data System (ADS)

The triple-deck theory to describe boundary-layer free interaction and separation is carefully investigated from the viewpoint of how it predicts stability properties of a viscous flow. The linearized version of this theory gives the same results as the linear stability theory based on the Orr-Sommerfeld equation if the flow is incompressible and the wave number of small disturbances goes to zero. Hence, a rather general criterion results to fix limits for a subsonic boundary layer to be stable. On the contrary, the linear approximation to the triple-deck theory leads to discouraging conclusions when the velocity of the oncoming stream exceeds the speed of sound, since it fails to reveal the boundary-layer instability. The Prandtl equations with a self-induced pressure gradient included are used to formulate a nonlinear approach for elucidating stability properties of freely inter acting boundary layers both for subsonic and supersonic cases. The shock-wave boundary-layer interaction and separation on a moving wall is numerically studied, the formation of two recirculation bubbles being the most striking feature. With the shock strength increasing, both bubbles tend to divide into smaller vortex cells whence the nonsteady process of velocity field "breathing" stems.

Ryzhov, Oleg S.; Zhuk, Vladimir I.

347

Bending Boundary Layers in Laminated-Composite Circular Cylindrical Shells  

NASA Technical Reports Server (NTRS)

A study of the attenuation of bending boundary layers in balanced and unbalanced, symmetrically and unsymmetrically laminated cylindrical shells is presented for nine contemporary material systems. The analysis is based on the linear Sanders-Koiter shell equations and specializations to the Love-Kirchhoff shell equations and Donnell's equations are included. Two nondimensional parameters are identified that characterize the effects of laminate orthotropy and anisotropy on the bending boundary-layer decay length in a very general manner. A substantial number of structural design technology results are presented for a wide range of laminated-composite cylinders. For all laminates considered, the results show that the differences between results obtained with the Sanders-Koiter shell equations, the Love-Kirchhoff shell equations, and Donnell's equations are negligible. The results also show that the effect of anisotropy in the form of coupling between pure bending and twisting has a negligible effect on the size of the bending boundary-layer decay length of the balanced, symmetrically laminated cylinders considered. Moreover, the results show that coupling between the various types of shell anisotropies has a negligible effect on the calculation of the bending boundary-layer decay length in most cases. The results also show that, in some cases, neglecting the shell anisotropy results in underestimating the bending boundary-layer decay length and, in other cases, results in an overestimation.

Nemeth, Michael P.; Smeltzer, Stanley S., III

2000-01-01

348

Summary of experimentally determined facts concerning the behavior of the boundary layer and performance of boundary layer measurements. [considering sailing flight  

NASA Technical Reports Server (NTRS)

A summary report of boundary layer studies is presented. Preliminary results of experimental measurements show that: (1) A very thin layer (approximately 0.4 mm) of the boundary layer seems to be accelerated; (2) the static pressure of the outer flow does not remain exactly constant through the boundary layer; and (3) an oncoming boundary layer which is already turbulent at the suction point can again become laminar behind this point without being completely sucked off.

Vanness, W.

1978-01-01

349

Interferometric data for a shock-wave/boundary-layer interaction  

NASA Technical Reports Server (NTRS)

An experimental study of the axisymmetric shock-wave / boundary-layer strong interaction flow generated in the vicinity of a cylinder-cone intersection was conducted. The study data are useful in the documentation and understanding of compressible turbulent strong interaction flows, and are part of a more general effort to improve turbulence modeling for compressible two- and three-dimensional strong viscous/inviscid interactions. The nominal free stream Mach number was 2.85. Tunnel total pressures of 1.7 and 3.4 atm provided Reynolds number values of 18 x 10(6) and 36 x 10(6) based on model length. Three cone angles were studied giving negligible, incipient, and large scale flow separation. The initial cylinder boundary layer upstream of the interaction had a thickness of 1.0 cm. The subsonic layer of the cylinder boundary layer was quite thin, and in all cases, the shock wave penetrated a significant portion of the boundary layer. Owing to the thickness of the cylinder boundary layer, considerable structural detail was resolved for the three shock-wave / boundary-layer interaction cases considered. The primary emphasis was on the application of the holographic interferometry technique. The density field was deduced from an interferometric analysis based on the Able transform. Supporting data were obtained using a 2-D laser velocimeter, as well as mean wall pressure and oil flow measurements. The attached flow case was observed to be steady, while the separated cases exhibited shock unsteadiness. Comparisons with Navier-Stokes computations using a two-equation turbulence model are presented.

Dunagan, Stephen E.; Brown, James L.; Miles, John B.

1986-01-01

350

Hypersonic turbulent wall boundary layer computations  

NASA Technical Reports Server (NTRS)

The Baldwin-Lomax (1978) 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 (1974) model for a flat plate at hypersonic speed, the new values of the coefficients were obtained. The results show that the values of C(cp) and C(kleb) are functions of both Mach number and wall temperature ratio. The C(cp) and C(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.

1988-01-01

351

Hypersonic turbulent wall boundary layer computations  

NASA Technical Reports Server (NTRS)

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.

1988-01-01

352

Turbulent boundary layer to single-stream shear layer: the transition region  

NASA Astrophysics Data System (ADS)

This communication presents the results and conclusions of an experimental study of the near-separation region of a single-stream shear layer. The momentum thickness at separation (x {=} 0) was theta_0 {=} 9.6 mm, with Reynolds number shape Re_theta {=} 4650. Boundary layer separation was caused by a sharp 90(°) edge. Detailed single- and multi-point measurements of the velocity field were acquired at the streamwise locations 0 {<} x/theta_0 {<} 100. This represents the transition region between two of the canonical turbulent shear flows: the zero-pressure-gradient turbulent boundary layer and the single-stream shear layer. From the viewpoint of a separating boundary layer, the results describe how the turbulent flow reacts to a sudden change in wall boundary conditions. From the viewpoint of the developed shear layer, the results describe the transition to the self-similar region. The data acquired suggest that the initial shear layer instability occurs in the region very near separation (x {?} theta_0), and that it involves only the vorticity filaments which originate in the near-wall region of the upstream boundary layer. This ‘near-wall region’ roughly defines the origin of a narrow wedge-shaped domain that was identified from the velocity statistics. This domain is termed the ‘sub-shear layer’. The statistics of the velocity field in the region bounded by the sub-shear layer and the free-stream flow were found to represent the normative continuation of the upstream boundary layer. The sub-shear layer has been found to exhibit many of the standard features observed in fully developed shear layers. For example, velocity measurements on the entrainment side of the shear layer indicate that large-scale motions with spanwise coherence were observed. The streamwise dependence of the dominant frequency, convection velocity, and spanwise velocity correlation have been documented in order to characterize the sub-shear layer phenomenon.

Morris, Scott C.; Foss, John F.

2003-11-01

353

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

NASA Technical Reports Server (NTRS)

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

1989-01-01

354

Modified diffusion model of pseudo-shock waves considering upstream boundary layers  

NASA Astrophysics Data System (ADS)

Experiments were performed to investigate the effects of the upstream boundary layer on pseudo-shock waves, by varying the Mach number and boundary layer thickness upstream of the pseudo-shock. A new flow model is presented in which both the effects of the upstream boundary layer and wall friction are taken into account. The configuration and axial static pressure distribution of the pseudo-shock were found to depend on both the upstream Mach number at the center of the duct and the ratio of the displacement thickness to the equivalent diameter. An experimental formula of the dimensionless length of the pseudo-shock L/D was obtained in terms of the mean Mach number in the supersonic region just upstream of the pseudo-shock and the ratio of the mass flow rate in the subsonic region to the total mass flow rate. The calculated values of axial static pressure distributions within the pseudo-shock and the static and total pressure ratios across the pseudo-shock were found to agree better with experimental values than did previous flow models.

Ikui, T.; Matsuo, K.; Sasaguchi, K.

1981-11-01

355

Hot-Film and Hot-Wire Anemometry for a Boundary Layer Active Flow Control Test  

NASA Technical Reports Server (NTRS)

Unsteady active flow control (AFC) has been used experimentally for many years to minimize bluff-body drag. This technology could significantly improve performance of rotorcraft by cleaning up flow separation. It is important, then, that new actuator technologies be studied for application to future vehicles. A boundary layer wind tunnel was constructed with a 1ft-x-3ft test section and unsteady measurement instrumentation to study how AFC manipulates the boundary layer to overcome adverse pressure gradients and flow separation. This unsteady flow control research requires unsteady measurement methods. In order to measure the boundary layer characteristics, both hot-wire and hot-film Constant Temperature Anemometry is used. A hot-wire probe is mounted in the flow to measure velocity while a hot-film array lays on the test surface to measure skin friction. Hot-film sensors are connected to an anemometer, a Wheatstone bridge circuit with an output that corresponds to the dynamic flow response. From this output, the time varying flow field, turbulence, and flow reversal can be characterized. Tuning the anemometers requires a fan test on the hot-film sensors to adjust each output. This is a delicate process as several variables drastically affect the data, including control resistance, signal input, trim, and gain settings.

Lenahan, Keven C.; Schatzman, David M.; Wilson, Jacob Samuel

2013-01-01

356

Secondary instability in boundary-layer flows  

NASA Technical Reports Server (NTRS)

The stability of a secondary Tollmien-Schlichting wave, whose wavenumber and frequency are nearly one half those of a fundamental Tollmien-Schlichting instability wave is analyzed using the method of multiple scales. Under these conditions, the fundamental wave acts as a parametric exciter for the secondary wave. The results show that the amplitude of the fundamental wave must exceed a critical value to trigger this parametric instability. This value is proportional to a detuning parameter which is the real part of k - 2K, where k and K are the wavenumbers of the fundamental and its subharmonic, respectively. For Blasius flow, the critical amplitude is approximately 29% of the mean flow, and hence many other secondary instabilities take place before this parametric instability becomes significant. For other flows where the detuning parameter is small, such as free-shear layer flows, the critical amplitude can be small, thus the parametric instability might play a greater role.

Nayfeh, A. H.; Bozatli, A. N.

1979-01-01

357

Unsteady laminar compressible boundary layers with vectored mass transfer and an applied magnetic field  

NASA Astrophysics Data System (ADS)

This paper deals with the laminar unsteady compressible boundary layer flow of an electrically conducting viscous fluid at the stagnation point of an axisymmetric blunt-nosed body with vectored mass transfer, nonzero dissipation parameter m, and an applied magnetic field. The semisimilar solutions have been obtained numerically using an implicit finite difference scheme in combination with the quasi-linearization technique. The results have been obtained for an accelerating/decelerating stream and a fluctuating stream. The skin friction responds to the fluctuation in the free-stream more compared to the heat transfer. In presence of the magnetic field, a velocity overshoot occurs which gives rise to the point of inflection in velocity profile and for small wall values of the total enthalpy. Skin friction can be reduced by applying a greater vectored mass transfer compared to the mass transfer applied normal to the surface. The results also show that the skin friction coefficient increases but Nusselt number decreases as the magnetic parameter M increases.

Roy, S.; Nath, G.

1992-01-01

358

Asymptotic analysis: Working note {number_sign}3, boundary layers  

SciTech Connect

In this chapter the authors discuss the asymptotic approximation of functions that display boundary-layer behavior. The purpose here is to introduce the basic concepts underlying the phenomenon, to illustrate its importance, and to describe some of the fundamental tools available for its analysis. To achieve their purpose in the clearest way possible, the authors will work with functions that are assumed to be given explicitly -- that is, functions f : (0,{epsilon}{sub 0}) {yields} X whose expressions are known, at least in principle. Only in the following chapter will they begin the study of functions that are given implicitly as solutions of boundary value problems -- the real stuff of which singular perturbation theory is made. Boundary-layer behavior is associated with asymptotic expansions that are regular {open_quotes}almost everywhere{close_quotes} -- that is, expansions that are regular on every compact subset of the domain of definition, but not near the boundary. These regular asymptotic expansions can be continued in a certain sense all the way up to the boundary, but a separate analysis is still necessary in the boundary layer. The boundary-layer analysis is purely local and aims at constructing local approximations in the neighborhood of each point of the singular part of the boundary. The problem of finding an asymptotic approximation is thus reduced to matching the various local approximations to the existing regular expansion valid in the interior of the domain. The authors are thinking, for example, of fluid flow (viscosity), combustion (Lewis number), and superconductivity (Ginzburg-Landau parameter) problems. Their solution may remain smooth over a wide range of parameter values, but as the parameters approach critical values, complicated patterns may emerge.

Garbey, M. [Universite Claude Bernard Lyon 1, Villeurbanne (France). Laboratoire d`Analyse Numerique; Kaper, H.G. [Argonne National Lab., IL (United States)

1993-09-01

359

Hypersonic flow separation in shock wave boundary layer interactions  

NASA Technical Reports Server (NTRS)

An assessment is presented for the experimental data on separated flow in shock wave turbulent boundary layer interactions at hypersonic and supersonic speeds. The data base consists mainly of two dimensional and axisymmetric interactions in compression corners or cylinder-flares, and externally generated oblique shock interactions with boundary layers over flat plates or cylindrical surfaces. The conditions leading to flow separation and the subsequent changes in the flow empirical correlations for incipient separation are reviewed. The effects of the Mach number, Reynolds number, surface cooling and the methods of detecting separation are discussed. The pertinent experimental data for the separated flow characteristics in separated turbulent boundary layer shock interaction are also presented and discussed.

Hamed, A.; Kumar, Ajay

1992-01-01

360

Effect of Blowing on Boundary Layer of Scarf Inlet  

NASA Technical Reports Server (NTRS)

When aircraft operate in stationary or low speed conditions, airflow into the engine accelerates around the inlet lip and pockets of turbulence that cause noise and vibration can be ingested. This problem has been encountered with engines equipped with the scarf inlet, both in full scale and in model tests, where the noise produced during the static test makes it difficult to assess the noise reduction performance of the scarf inlet. NASA Langley researchers have implemented boundary layer control in an attempt to reduce the influence of the flow nonuniformity in a 12-in. diameter model of a high bypass fan engine mounted in an anechoic chamber. Static pressures and boundary layer profiles were measured in the inlet and far field acoustic measurements were made to assess the effectiveness of the blowing treatment. The blowing system was found to lack the authority to overcome the inlet distortions. Methods to improve the implementation of boundary layer control to reduce inlet distortion are discussed.

Gerhold, Carl H.; Clark, Lorenzo R.

2004-01-01

361

Turbulence in the convective boundary layer observed by microwave interferometry  

SciTech Connect

A 9-antenna, 400 meter microwave interferometer was utilized in SALSA MEX on the San Pedro River area in July and August, 1997, to measure the turbulence in the Convective Boundary Layer. Water vapor has an appreciable index of refraction at radio frequencies around 10 GHz, and acts as a passive tracer of the magnitude and motion of turbulence. The relative phase changes of a signal from a satellite were tracked by an array of 9 antennas, and the phase differences between antennas were then used to derive the turbulence properties of the boundary layer. Preliminary analysis shows clearly different characteristics for the convection activity of the boundary layer between day and night. From the structure function analysis they can see that the turbulence structure starts to decorrelate at scale sizes of 200 meters for a temporal passband around 100 seconds. Derivation of average wind fields is currently in process.

Shao, X.M.; Carlos, R.C.; Kirkland, M.W.

1997-12-01

362

The Turbulent Boundary Layer on a Rough Curvilinear Surface  

NASA Technical Reports Server (NTRS)

A number of semiempirical approximate methods exist for determining the characteristics of the turbulent boundary layer on a curvilinear surface. At present, among these methods, the one proposed by L. G. Loitsianskii is given frequent practical application. This method is sufficiently effective and permits, in the case of wing profiles with technically smooth surfaces, calculating the basic characteristics of the boundary layer and the values of the overall drag with an accuracy which suffices for practical purposes. The idea of making use of the basic integral momentum equation ((d delta(sup xx))/dx) + ((V' delta(sup xx))/V) (2 + H) = (tau(sub 0))/(rho V(exp 2)) proves to be fruitful also for the solution of the problems in the determination of the characteristics of the turbulent boundary layer on a rough surface.

Droblenkov, V. F.

1958-01-01

363

The phase-dependence of a swirling, turbulent boundary layer  

NASA Astrophysics Data System (ADS)

Measurements have been made in a swirling turbulent boundary layer affected by the angular momentum instability. The flow field seen by a rotating observer was obtained from a fixed hot-wire probe using the technique of phase-locked averaging. The instability is localized to the wall region of the boundary layer and there is some evidence that it has produced 'streamwise' vortices analogous to the Taylor-Goertler vortices in a boundary layer on a concave wall. The mean velocities and Reynolds stresses show a significant phase dependence which appears to originate in the swirl generator. The conventional Reynolds stresses are not equally affected by the phase dependence and this has implications for the turbulence modeling of swirling flows.

Koh, S. G.; Clausen, P. D.; Wood, D. H.

364

Boundary layer effects on particle impaction and capture  

NASA Technical Reports Server (NTRS)

The inertial impaction and deposition of small particles on larger bodies with viscous boundary layers are considered theoretically, in a detailed comment on a paper by Menguturk et al. (1983). Topics addressed include cushion effects, the dimensionless groups corresponding to the diameter range (3-6 microns) examined by Menguturk et al. in a numerical example, analogous effects of particle-gas energy and mass exchange in boundary layers, and the combined effects of particle inertia and diffusion. It is argued that the inertial effects can be characterized in terms of a body, boundary-layer, or sublayer Stokes number. In a reply by Menguturk et al., the focus is on the application of the theoretical model to the erosion of blade surfaces in large gas turbines; the Stokes number is found to be of limited practical value in these cases, because the particle motion is not primarily normal to the blade surfaces.

Rosner, D. E.; Fernandez De La Mora, J.

1984-01-01

365

Method for laminar boundary layer transition visualization in flight  

NASA Technical Reports Server (NTRS)

Disclosed is a method of visualizing laminar to turbulent boundary layer transition, shock location, and laminar separation bubbles around a test surface. A liquid crystal coating is formulated using an unencapsulated liquid crystal operable in a temperature bandwidth compatible with the temperature environment around the test surface. The liquid crystal coating is applied to the test surface, which is preferably pretreated by painting with a flat, black paint to achieve a deep matte coating, after which the surface is subjected to a liquid or gas flow. Color change in the liquid crystal coating is produced in response to differences in relative shear stress within the boundary layer around the test surface. The novelty of this invention resides in the use of liquid crystals which are sensitive to shear stress to show aerodynamic phenomena such as a boundary layer transition, shock location, and laminar separation bubbles around a test surface.

Holmes, Bruce J. (inventor); Gall, Peter D. (inventor)

1988-01-01

366

Numerical Simulations of Wake/Boundary Layer Interactions  

NASA Technical Reports Server (NTRS)

Direct and large-eddy simulations of the interaction between the wake of a circular cylinder and a flat-plate boundary layer are conducted. Two Reynolds numbers are examined. The simulations indicate that at the lower Reynolds number the boundary layer is buffeted by the unsteady Karman vortex street shed by the cylinder. The fluctuations, however, cannot be self-sustained due to the low Reynolds-number, and the flow does not reach a turbulent state within the computational domain. In contrast, in the higher Reynolds-number case, boundary-layer fluctuations persist after the wake has decayed (due, in part, to the higher values of the local Reynolds number Re(sub theta) achieved in this case); some evidence could be observed that a self-sustaining turbulence generation cycle was beginning to be established.

Piomelli, Ugo; Choudhari, Meelan M.; Ovchinnikov, Victor; Balaras, Elias

2003-01-01

367

DNS of Turbulent Boundary Layers under Highenthalpy Conditions  

NASA Astrophysics Data System (ADS)

To study real-gas effects and turbulence-chemistry interaction, direct numerical simulations (DNS) of hypersonic boundary layers are conducted under typical hypersonic conditions. We consider the boundary layer on a lifting-body consisting of a flat plate at an angle of attack, which flies at altitude 30km with a Mach number 21. Two different inclined angles, 35^o and 8^o, are considered,representing blunt and slender bodies. Both noncatalytic and supercatalytic wall conditions are considered. The DNS data are studied to assess the validity of Morkovin's hypothesis, the strong Reynolds analogy, as well as the behaviors of turbulence structures under high-enthalpy conditions.Relative to low-enthalpy conditions [1], significant differences in typical scalings are observed. [4pt] [1] L. Duan and I. Beekman and M. P. Mart'in, Direct numerical simulation of hypersonic turbulent boundary layers. Part 2: Effect of temperature, J. Fluid Mech. 655 (2010), 419-445.

Duan, Lian; Martín, Pino

2010-11-01

368

Blow-up and control of marginally separated boundary layers.  

PubMed

Interactive solutions for steady two-dimensional laminar marginally separated boundary layers are known to exist up to a critical value Gamma(c) of the controlling parameter (e.g. the angle of attack of a slender airfoil) Gamma only. Here, we investigate three-dimensional unsteady perturbations of such boundary layers, assuming that the basic flow is almost critical, i.e. in the limit Gamma(c)-Gamma-->0. It is then shown that the interactive equations governing such perturbations simplify significantly, allowing, among others, a systematic study of the blow-up phenomenon observed in earlier investigations and the optimization of devices used in boundary-layer control. PMID:16105768

Braun, Stefan; Kluwick, Alfred

2005-05-15

369

Simple turbulence models and their application to boundary layer separation  

NASA Technical Reports Server (NTRS)

Measurements in the boundary layer and wake of a stalled airfoil are presented in two coordinate systems, one aligned with the airfoil chord, the other being conventional boundary layer coordinates. The NACA 4412 airfoil is studied at a single angle of attack corresponding to maximum lift, the Reynolds number based on chord being 1.5 x 10 to the 6th power. Turbulent boundary layer separation occurred at the 85 percent chord position. The two-dimensionality of the flow was documented and the momentum integral equation studied to illustrate the importance of turbulence contributions as separation is approached. The assumptions of simple eddy-viscosity and mixing-length turbulence models are checked directly against experiment. Curvature effects are found to be important as separation is approached.

Wadcock, A. J.

1980-01-01

370

Additive thermochemical effects in turbulent erosive boundary layers  

SciTech Connect

Previously obtained interior ballistics and wall boundary layer modeling results indicate that significant reduction in erosive heating can be expected when finely divided particles are dispersed through the propellant combustion flow field. Attention was first placed on the particle size influences, together with particle dispersal dynamics in both turbulent combustion core flow and the erosive wall boundary layer region. Submicron thermochemically inert particles were predicted to disperse readily to the near wall region where they were then entrained in the boundary layer. This was estimated to substantially reduce the predicted erosive heat and mass transfer and experimentally confirmed. Examination of the time-averaged turbulent boundary layer macrostructure changes indicated that inertial influences were primarily responsible for this reduction in erosive heating to gun barrel walls. The boundary layers were thickened by the additives and erosive diffusion gradients were correspondingly reduced. The isolated inertial mechanisms are now understood but are difficult to apply in general dimensional analysis scaling or in analytical heat transfer correlation predictions. Three major factors which contribute to these difficulties are: time dependence of both developing mean flow and particle field; turbulence-particle interactions; and thermochemical heat release and exchange between reactive gas components and particles, particles and wall surface, and reactive gas and wall surface. To help illustrate the influence of thes mechanisms and provide a basis for prediction, the influence of submicron additives in unsteady turbulent boundary layer growth and interaction regions adjacent to a model of a chemically active metallic (steel) surface are examined. Equilibrium chemistry is assumed for all phases.

Buckingham, A.C.; Levatin, J.L.

1983-01-18

371

Similarity Equations for Wind and Temperature Profiles in the Radix Layer, at the Bottom of the Convective Boundary Layer.  

NASA Astrophysics Data System (ADS)

In the middle of the convective boundary layer, also known as the mixed layer, is a relatively thick region where wind speed and potential temperature are nearly uniform with height. Below this uniform layer (UL), wind speed decreases to zero at the ground, and potential temperature increases to the surface skin value. This whole region below the UL is called the radix layer (RxL), and is of order hundreds of meters thick. Within the bottom of the RxL lies the classical surface layer (order of tens of meters thick) that obeys traditional Monin-Obukhov similarity theory.The RxL depth is shown to depend on friction velocity, Deardorff velocity, and boundary layer depth. The wind RxL is usually thicker than the temperature RxL. Using RxL depth, UL wind speed, and UL potential temperature as length, velocity, and temperature scales, respectively, one can form dimensionless heights, velocities, and temperatures. When observations obtained within the RxL are plotted in this dimensionless framework, the data collapse into similarity curves. This data collapse is tightly packed for data collected over single-location homogeneous surfaces, and shows more scatter for data collected along 72-km flight tracks over heterogeneous surfaces. Empirical profile equations are proposed to describe this RxL similarity. When these profile equations are combined with the flux equations from convective transport theory, the results are new flux-profile equations for a deep region within the bottom of the convective boundary layer.These RxL profile similarity equations are calibrated using data from four sites with different roughnesses: Minnesota, BLX96-Lamont, BLX96-Meeker, and BLX96-Winfield. The empirical parameters are found to be invariant from site to site, except for the profile shape parameter for wind speed. This parameter is found to depend on standard deviation of terrain elevation, rather than on the aerodynamic roughness length. The resulting parameter values are compared with independent data from a forested fifth site, Koorin, and it is found that displacement height must be subtracted from all the heights in the RxL profile equations. The resulting profile equations could be useful for calculating wind loading on bridges, wind turbine power estimation, air pollutant transport, or other applications where wind speeds or temperatures are needed over the bottom hundreds of meters of the convective boundary layer.

Santoso, Edi; Stull, Roland

2001-06-01

372

Receptivity of Hypersonic Boundary Layers over Straight and Flared Cones  

NASA Technical Reports Server (NTRS)

The effects of adverse pressure gradients on the receptivity and stability of hypersonic boundary layers were numerically investigated. Simulations were performed for boundary layer flows over a straight cone and two flared cones. The steady and the unsteady flow fields were obtained by solving the two-dimensional Navier-Stokes equations in axi-symmetric coordinates using the 5th order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. The mean boundary layer profiles were analyzed using local stability and non-local parabolized stability equations (PSE) methods. After the most amplified disturbances were identified, two-dimensional plane acoustic waves were introduced at the outer boundary of the computational domain and time accurate simulations were performed. The adverse pressure gradient was found to affect the boundary layer stability in two important ways. Firstly, the frequency of the most amplified second-mode disturbance was increased relative to the zero pressure gradient case. Secondly, the amplification of first- and second-mode disturbances was increased. Although an adverse pressure gradient enhances instability wave growth rates, small nose-tip bluntness was found to delay transition due to the low receptivity coefficient and the resulting weak initial amplitude of the instability waves. The computed and measured amplitude-frequency spectrums in all three cases agree very well in terms of frequency and the shape except for the amplitude.

Balakumar, Ponnampalam; Kegerise, Michael A.

2010-01-01

373

Numerical Modeling of the Evolving Stable Boundary Layer  

NASA Astrophysics Data System (ADS)

A single-column model of the evolving stable boundary layer is tested for the consistency of turbulence parameterization, self-similar properties of the flow, and effects of ambient forcing. The turbulence closure of the model is based on the K-theory approach, with stability functions based on empirical data, and a semi-empirical form of the mixing length. The model has one internal, governing stability parameter, the Richardson number Ri, which dynamically adjusts to the boundary conditions and to external forcing. Model results, expressed in terms of local similarity scales, are universal functions of the Richardson number, i.e. they are satisfied in the entire stable boundary layer, for all instants of time, and all kinds of external forcing. Based on similarity expression, a realizability condition is derived for the minimum turbulent heat flux in the stable boundary layer. Numerical experiments show that the development of 'horse-shoe' shaped, 'fixed-elevation' wind hodographs in the interior of the stable boundary layer are solely caused by effects imposed by surface thermal forcing, and are not related to the inertial oscillation mechanism.

Sorbjan, Z.

2013-12-01

374

Dual characterization of boundary friction thanks to the harmonic tribometer: Identification of viscous and solid friction contributions  

NASA Astrophysics Data System (ADS)

All the usual characterization of an interface submitted to shear is generally achieved thanks to an experiment using a pair of two loaded solids in continuous sliding. So, the friction coefficient value can be determined. But, only this friction value is quite poor information and is not able to reveal the complexity of the interfacial processes occurring in the sliding contact. A novel method has been developed at LTDS, allowing us to identify both the velocity-dependent and the solid friction contributions. It is based on a contact submitted to constant normal load, one solid being supported by a 1D mechanical oscillator, under the form of an elastic bi-blade, able to return to its equilibrium position through damped oscillations. The way the amplitude is decaying during this elastic recovery is recorded. A mechanical model has been developed, in order to fit experimental data, allowing us to identify two different contributions: the velocity-dependent contribution (typically viscous damping), and the solid-like contribution (typically solid friction). This technique has been applied here to a lubricated sphere-on-plane contact. The contacting surfaces are made of AISI 52100 polished steel. Two model products are tested: pure glycerol and 1,3-buthylene glycol. First results are presented, showing the capabilities of this technique for giving a new insight on the mixed lubrication regime description.

Belin, M.; Kakizawa, M.; Rigaud, E.; Martin, J. M.

2010-11-01

375

Existence results for nano boundary layer flows with nonlinear Navier boundary condition  

NASA Astrophysics Data System (ADS)

The standard no slip boundary condition of classical fluid mechanics is no longer valid at the micro- and nano-scale and should be replaced by a boundary condition that allows some degree of tangential slip. In the present work, the classical laminar boundary layer equation of the flow away from the origin past a wedge with the no-slip boundary condition replaced by a nonlinear Navier boundary condition is revisited. This boundary condition includes an arbitrary index parameter, denoted by n>0, which appears in the coefficients of the differential equation to be solved. It is proved corresponding to the value n=1/3, there are exactly three situations for the problem: (i) there is no solution; (ii) there exist two solutions; (iii) there exist four solutions. Furthermore, the exact analytical solution of the problem is given in terms of parabolic cylinder functions for further physical interpretations.

Shivanian, Elyas

2013-12-01

376

Oscillations of the Boundary Layer and High-frequency QPOs  

NASA Astrophysics Data System (ADS)

We observed persistent high-frequency oscillations of the boundary layer near an accreting, weakly-magnetized star in global 3D MHD simulations. The tilted dipole magnetic field is not strong enough to open a gap between the star and the disk. Instead, it forms a highly-wrapped azimuthal field near the surface of the star which slows down rotation of the disk matter, while a small tilt of the field excites oscillations of the boundary layer with a frequency below the Keplerian frequency. This mechanism may be responsible for the high-frequency oscillations in accreting neutron stars, white dwarfs and classical T Tauri stars.

Blinova, A. A.; Bachetti, M.; Romanova, M. M.

2014-01-01

377

Bubbly turbulent drag reduction is a boundary layer effect.  

PubMed

In turbulent Taylor-Couette flow, the injection of bubbles reduces the overall drag. On the other hand, rough walls enhance the overall drag. In this work, we inject bubbles into turbulent Taylor-Couette flow with rough walls (with a Reynolds number up to 4 x 10(5), finding an enhancement of the dimensionless drag as compared to the case without bubbles. The dimensional drag is unchanged. As in the rough-wall case no smooth boundary layers can develop, the results demonstrate that bubbly drag reduction is a pure boundary layer effect. PMID:17359101

van den Berg, Thomas H; van Gils, Dennis P M; Lathrop, Daniel P; Lohse, Detlef

2007-02-23

378

A New Scaling for Adverse Pressure Gradient Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

A new scaling for strong adverse pressure gradient (APG) turbulent boundary layers (TBL) is presented. The new scaling is applied to data from the author's APG TBL experiment as well as several previous experimental studies. Both steady and unsteady flows are considered. The new scaling is shown to provide an excellent collapse of not only the mean velocity but also the turbulent stress profiles. The physical motivation for the scaling is presented in terms of underlying stability mechanisms as evidenced by a series of conditional boundary layer measurements. The implications of the scaling on the physics of strong APG TBL flows is also discussed.

Thomas, Flint; Schatzman, David

2011-11-01

379

Effects of shock on hypersonic boundary layer stability  

NASA Astrophysics Data System (ADS)

The design of hypersonic vehicles requires the estimate of the laminar to turbulent transition location for an accurate sizing of the thermal protection system. Linear stability theory is a fast scientific way to study the problem. Recent improvements in computational capabilities allow computing the flow around a full vehicle instead of using only simplified boundary layer equations. In this paper, the effect of the shock is studied on a mean flow provided by steady Computational Fluid Dynamics (CFD) computations and simplified boundary layer calculations.

Pinna, F.; Rambaud, P.

2013-06-01

380

Effects of variable properties in film cooled turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The effects of variable properties on heat transfer in a film-cooled turbulent boundary layer were investigated. A new procedure was developed to deduce adiabatic effectiveness from heat transfer coefficients based on the wall to freestream temperature difference, where both are representative of variable property flow conditions. The new technique was shown to be valid using data from the literature for injection into a turbulent boundary layer from one and two rows of injection holes. From these results, the variation of the coolant to mainstream density ratio was shown to have a significantly greater effect on heat transfer than variations of viscosity and thermal conductivity.

Walz, A. F., Jr.

1986-03-01

381

Boundary Layer Dynamics as seen from MER Mini-TES  

NASA Astrophysics Data System (ADS)

We will present results from the high temporal frequency observations of the martian atmospheric boundary layer from MER Mini-TES. This work will address not only the detailed retrievals from these observations, but also their diurnal trends, seasonal differences and differences between the rovers. We will also discuss analysis of the information content of the Mini-TES retrievals. The extent of convection, the strength of turbulent overturnings and the dominant frequencies for each of these processes will be reported. Inferences about the convective or shear instability of the atmospheric boundary layer will be drawn from the results.

Banfield, D.; Smith, M.; Wolff, M.; Christensen, P.

2005-12-01

382

Laminar Boundary Layer Behind a Strong Shock Moving into Air  

NASA Technical Reports Server (NTRS)

The laminar wall boundary layer behind a strong shock advancing into stationary air has been determined. Numerical results have been obtained for shock Mach numbers up to 14 using real gas values for density and viscosity and assuming Prandtl and Lewis numbers of 0.72 and 1, respectively. The numerical results for shear and heat transfer agree, within 4 percent, with a previously presented approximate analytical expression for these quantities. A slight modification of this expression results in agreement with the numerical data to within 2.5 percent. Analytical expressions for boundary-layer thickness and displacement thickness, correct to within 4 percent for the present data, have also been obtained.

Mirels, Harold

1961-01-01

383

An Innovative Flow-Measuring Device: Thermocouple Boundary Layer Rake  

NASA Technical Reports Server (NTRS)

An innovative flow-measuring device, a thermocouple boundary layer rake, was developed. The sensor detects the flow by using a thin-film thermocouple (TC) array to measure the temperature difference across a heater strip. The heater and TC arrays are microfabricated on a constant-thickness quartz strut with low heat conductivity. The device can measure the velocity profile well into the boundary layer, about 65 gm from the surface, which is almost four times closer to the surface than has been possible with the previously used total pressure tube.

Hwang, Danny P.; Fralick, Gustave C.; Martin, Lisa C.; Wrbanek, John D.; Blaha, Charles A.

2001-01-01

384

Wall shear stress in Görtler vortex boundary layer flow  

NASA Astrophysics Data System (ADS)

The development of wall shear stress in concave surface boundary layer flows in the presence of Görtler vortices was experimentally studied by means of hot-wire measurements. The wavelengths of the vortices were preset by thin vertical perturbation wires so to produce the most amplified wavelengths. Three different vortex wavelengths of 12, 15, and 20 mm were considered, and near-wall velocity measurements were carried out to obtain the ``linear'' layers of velocity profiles in the boundary layers. The wall shear stress coefficient Cf was estimated from the velocity gradient of the ``linear'' layer. The streamwise developments of boundary layer displacement and momentum thickness at both upwash and downwash initially follow the Blasius (laminar boundary layer) curve up to a certain streamwise location. Further downstream, they depart from the Blasius curve such that they increase at upwash and decrease at downwash before finally converge to the same value due to the increased mixing as a consequence of transition to turbulence. The spanwise-averaged wall shear stress coefficient Cf, which initially follows the Blasius curve, increases well above the local turbulent boundary layer value further downstream due to the nonlinear effect of Görtler instability and the secondary instability modes. Three different regions are identified based on the streamwise development of Cf, namely linear, nonlinear, and transition to turbulence regions. The onset of nonlinear region is defined as the streamwise location where the Cf begins to depart from the Blasius curve. In the nonlinear region, the spanwise distribution of Cf at the downwash becomes narrower, and there is no inflection point found further downstream.

Tandiono; Winoto, S. H.; Shah, D. A.

2009-08-01

385

Flow characteristics around a circular cylinder with a slit. II Effect of boundary layer suction  

Microsoft Academic Search

The effects of boundary layer suction are investigated for a circular cylinder with a two-dimensional slit placed along a diameter. Flow characteristics around the cylinder are examined for cases of intermittent boundary layer suction (Pattern I), alternate boundary layer suction and blowing (Pattern II), and the transition regions. Among other results, it is found that the boundary layer suction is

T. Igarashi

1982-01-01

386

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

387

High-speed boundary layer transition induced by a discrete roughness element  

NASA Astrophysics Data System (ADS)

The effect of a hemispherical bump on a Mach 3.37 laminar boundary layer is studied using DNS for three conditions with k/?= 2.54, 0.25 and 0.125, where k is the roughness height. The simulation parameters are based on the experiment by Danehy et. al. (AIAA-2009-394). The flow downstream of the roughness is transitional for all the three conditions accompanied by a rise in skin friction and heat transfer. Upon interaction with the roughness element, the boundary layer separates to form a series of spanwise vortices upstream and a shear layer. These vortices wrap around the roughness to yield a system of streamwise vortices downstream. Perturbation of the shear layer due to the vortices results in the formation of hairpin-shaped vortices further downstream of the roughness. While hairpin vortices were observed in both the center plane and off-symmetry planes on either side for the smallest ? case, they were observed only in the center plane for the other cases.

Iyer, Prahladh; Mahesh, Krishnan

2011-11-01

388

A model of wind shear and turbulence in the surface boundary layer  

NASA Technical Reports Server (NTRS)

A model of wind and turbulence has been described for the surface boundary layer. The wind structure in the surface layer is considered to be a function of the surface parameters, stability, and height. The surface parameters considered are: (1) the surface roughness length; (2) the surface friction velocity; and (3) the zero plane displacement height. The stability parameter, Z/L, where L is the Monin-Obukov stability length, describes the thermal effect on the wind profile. The logarithmic wind profile is used to describe the mean wind field in the neutral boundary layer, and a logarithmic profile with a stability defect is used to describe the stable and unstable atmospheric conditions. For the very stable conditions, the logarithmic wind law does not hold. Under this condition, the layers of the atmosphere become disconnected and large scale frontal motions are the predominate factor in defining the wind profile. Figures are presented which represent some typical wind profiles in the very stable condition.

Luers, J. K.

1973-01-01

389

The Saharan atmospheric boundary layer: Turbulence, stratification and mixing  

NASA Astrophysics Data System (ADS)

High-resolution large-eddy model simulations, combined with aircraft and radiosonde observations from the Fennec observational campaign are used to describe the vertical structure of the Saharan atmospheric boundary layer (SABL). The SABL, probably the deepest dry convective boundary layer on Earth, is crucial in controlling the vertical redistribution and long-range transport of dust, heat, water and momentum in the Sahara, with significant implications for the large-scale Saharan heat low and West African monsoon systems. The daytime SABL has a unique structure, with an actively growing convective region driven by high sensible heating at the surface, capped by a weak (?1K) temperature inversion and a deep, near-neutrally stratified Saharan residual layer (SRL) above it, which is mostly well mixed in humidity and temperature and reaches a height of ~500hPa. Large-eddy model (LEM) simulations were initialized with radiosonde data and driven by surface heat flux observations from Fennec supersite-1 at Bordj Bardji Mokhtar (BBM), southern Algeria. Aircraft observations are used to validate the processes of interest identified in the model, as well as providing unprecedented detail of the turbulent characteristics of the SABL. Regular radiosondes from BBM during June 2011 are used to generate a climatology of the day-time SABL structure, providing further evidence that the processes identified with the LEM are recurrent features of the real SABL. The model is shown to reproduce the typical SABL structure from observations, and different tracers are used to illustrate the penetration of the convective boundary layer into the residual layer above as well as mixing processes internal to the residual layer. Despite the homogeneous surface fluxes and tracer initialization, the large characteristic length-scale of the turbulent eddies leads to large horizontal changes in boundary layer depth (which control the formation of clouds) and significant heterogeneity in tracer concentrations, demonstrating the potential for variability in, for example, dust concentrations independent of external forcings. The residual layer, where long-range transport can take place, is analyzed in particular detail. Various processes which can lead to transport into and mixing within the residual layer are explored, including shear-driven turbulence at the residual layer top and the potential for detrainment from the convective boundary layer top due to the combination of a weak lid and a neutral layer above.

Garcia-Carreras, Luis; Parker, Douglas J.; Marsham, John H.; Rosenberg, Philip D.; Marenco, Franco; Mcquaid, James B.

2013-04-01

390

A new absorbing layer boundary condition for the wave equation  

SciTech Connect

A new absorbing boundary condition using an absorbing layer is presented for application to finite-difference time-domain (FDTD) calculation of the wave equation. This algorithm is by construction a hybrid between the Berenger perfectly matched layer (PML) algorithm and the one-way Sommerfeld algorithm. The new prescription contains both of these earlier ones as particular cases, and retains benefits from both. Numerical results indicate that the new algorithm provides absorbing rates superior to those of the PML algorithm.

Vay, J.L.

2000-09-11

391

Modeling the planetary boundary layer — Extension to the stable case  

Microsoft Academic Search

A higher-order-closure model, which contains equations for turbulence covariances as well as the mean field, was developed and used to investigate the structure of the stably-stratified planetary boundary layer. The calculated surface-layer profiles of wind shear, temperature gradient, and dissipation rate agree well with the 1968 Kansas data. A simulation of the evolution of the nocturnal PBL reproduces fairly accurately

J. C. Wyngaard; Hanscom AFB

1975-01-01

392

Investigation of the Stability of Austenite Formed in Surface Layers During the Friction of High-Strength Cast Irons.  

National Technical Information Service (NTIS)

To determine the stability of austenite formed during friction in surface layers of high strength cast irons the amount of gamma-phase was monitored over a span of 4 years during friction. The amount of gamma-phase was independent of the pressure and of t...

L. I. Markovskaya

1970-01-01

393

Dynamic Boundary Control of Beams Using Active Constrained Layer Damping  

NASA Astrophysics Data System (ADS)

A globally stable boundary control strategy is developed to damp the vibration of beams fully treated with active constrained layer damping (ACLD) treatments. The devised boundary controller is compatible with the operating nature of the ACLD treatments where the strain induced generates a control force and moment acting at the boundary of the treated beam. The development of the boundary control strategy is based on a distributed-parameter model of the beam/ACLD system in order to avoid the classical spillover problems resulting from using 'truncated' finite element models. Such an approach makes the boundary controller capable of controlling all the modes of vibration of the ACLD-treated beams and guarantees that the total energy norm of the system is decreasing continuously with time. The control strategy is provided also with a dynamic compensator to shape the vibration damping characteristics of the ACLD in the frequency domain. The effectiveness of the ACLD in damping out the vibration of cantilevered beams is determined for different control gains and compared with the performance of conventional passive constrained layer damping (PCLD). The results obtained demonstrate the high damping characteristics of the boundary controller particularly over broad frequency bands.

Baz, A.

1997-11-01

394

An Adaptive Finite Difference Solver for Nonlinear Two-Point Boundary Problems with Mild Boundary Layers  

Microsoft Academic Search

A variable-order finite difference solver for first-order nonlinear system subject to two-point boundary conditions is described. The method uses deferred corrections, and adaptive meshes are automatically produced in order to detect and resolve mild boundary layers and other sharp-gradient situations. A set of numerical examples solved with an implementation of the algorithm is presented, together with comparisons with several other

M. Lentini; V. Pereyra

1977-01-01

395

Transportation of particulate plumes in boundary layer with obstacles  

NASA Astrophysics Data System (ADS)

This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high field gradients with the aid of scheme viscosity of numerical algorithm used to model near-surface phenomena. This idea is implemented in the model of ideal gas equations with variable equation of state describing particulates transportation within boundary layer with obstacles. 1

Petrosyan, A.; Karelsky, K.; Smirnov, I.

2010-05-01

396

Modeling of particulate plumes transportation in boundary layers with obstacles  

NASA Astrophysics Data System (ADS)

This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high field gradients with the aid of scheme viscosity of numerical algorithm used to model near-surface phenomena. This idea is implemented in the model of ideal gas equations with variable equation of state describing particulates transportation within boundary layer with obstacles.

Karelsky, K. V.; Petrosyan, A. S.

2012-04-01

397

Transportation of particulate plumes in boundary layer with obstacles  

NASA Astrophysics Data System (ADS)

This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high field gradients with the aid of scheme viscosity of numerical algorithm used to model near-surface phenomena. This idea is implemented in the model of ideal gas equations with variable equation of state describing particulates transportation within boundary layer with obstacles. 1

Karelsky, K.; Petrosyan, A.; Smirnov, I.

2009-09-01

398

Modeling of particulate plumes transportation in boundary layers with obstacles  

NASA Astrophysics Data System (ADS)

This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high field gradients with the aid of scheme viscosity of numerical algorithm used to model near-surface phenomena. This idea is implemented in the model of ideal gas equations with variable equation of state describing particulates transportation within boundary layer with obstacles.

Karelsky, K.; Petrosyan, A.; Smirnov, I.

2010-09-01

399

Transport of Particulates in Boundary Layer with Obstacles  

NASA Astrophysics Data System (ADS)

This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high field gradients with the aid of scheme viscosity of numerical algorithm used to model near-surface phenomena. This idea is implemented in the model of ideal gas equations with variable equation of state describing particulates transportation within boundary layer with obstacles.

Karelsky, Kirill; Petrosyan, Arakel

2014-05-01

400

Radon daughter disequilibria in the lower marine boundary layer  

Microsoft Academic Search

Radon daughters are produced as free ions, but they become attached to aerosol particles at a rate depending on the particle concentration. In the lower marine boundary layer, most of those which do not become attached plate out on the ocean surface. In this paper a simple model is used to examine the influence of several parameters on radon\\/radon daughter

S. Whittlestone; Private MailBag

1990-01-01

401

Stability of hypersonic boundary-layer flows with chemistry  

NASA Technical Reports Server (NTRS)

The effects of nonequilibrium chemistry and three dimensionality on the stability characteristics of hypersonic flows are discussed. In two-dimensional (2-D) and axisymmetric flows, the inclusion of chemistry causes a shift of the second mode of Mack to lower frequencies. This is found to be due to the increase in size of the region of relative supersonic flow because of the lower speeds of sound in the relatively cooler boundary layers. Although this shift in frequency is present in both the equilibrium and nonequilibrium air results, the equilibrium approximation predicts modes which are not observed in the nonequilibrium calculations (for the flight conditions considered). These modes are superpositions of incoming and outgoing unstable disturbances which travel supersonically relative to the boundary-layer edge velocity. Such solutions are possible because of the finite shock stand-off distance. Their corresponding wall-normal profiles exhibit an oscillatory behavior in the inviscid region between the boundary-layer edge and the bow shock. For the examination of three-dimensional (3-D) effects, a rotating cone is used as a model of a swept wing. An increase of stagnation temperature is found to be only slightly stabilizing. The correlation of transition location (N = 9) with parameters describing the crossflow profile is discussed. Transition location does not correlate with the traditional crossflow Reynolds number. A new parameter that appears to correlate for boundary-layer flow was found. A verification with experiments on a yawed cone is provided.

Reed, Helen L.; Stuckert, Gregory K.; Haynes, Timothy S.

1993-01-01

402

Surface energy balance and boundary layer development during snowmelt  

NASA Astrophysics Data System (ADS)

Improved prediction of snowmelt requires comprehensive data collection, including surface, subsurface, and atmospheric processes, during the snowmelt period. We report results of field research in which all components of the surface energy balance were measured during two different snowmelt periods, along with boundary layer soundings. The two periods were quite different, the first being overcast and the second occurring under clear skies. However, snowmelt was estimated relatively well from the cumulative residual of the energy balance in both cases. Downward infrared radiation and sensible heat flux were important contributors to the melt during overcast conditions, with net radiation providing about two thirds of the energy for melt and sensible heat providing the remainder. The sunny melt was dominated by direct solar heating of the surface. In both cases, estimation of melt as a residual of the energy balance agreed well with visual and gravimetric observations. The boundary layer soundings revealed the importance of advection, which was generally consistent with synoptic patterns during the period of the study. The data also showed a transition from advection-dominated to turbulence-dominated boundary layer budgets as the snowpack disappeared. The potential for convective cloud formation was also examined. Surface heating and entrainment outweighed adiabatic cooling and evaporation, resulting in the boundary layer top relative humidity decreasing as the snow melted and turbulent mixing increased.

Baker, J. M.; Davis, K. J.; Liknes, G. C.

1999-08-01

403

Secondary eyewall formation as a progressive boundary layer response  

NASA Astrophysics Data System (ADS)

The robust observational (satellite based) evidence that secondary eyewalls are common features in major hurricanes contrasts with the scarce in situ observations of the phenomena and its life cycle. This lack of observations has resulted in an incomplete understanding of the dynamics of secondary eyewall formation (SEF). A wide variety of physical processes have been invoked to explain SEF, but only the recently proposed theory of a progressive boundary layer control in SEF has been supported by a variety of full physics mesoscale numerical integrations. The RAINEX field project provided unique observations of the secondary eyewall of Hurricane Rita (2005) both before and during the time Rita exhibited a clear secondary eyewall structure. These observations have contributed to the advancement of the understanding of the secondary eyewall phenomenon. However, in the RAINEX experiment, there was limited data sampling during the development of the secondary wind maxima, thereby precluding a complete observational investigation of the dynamics of SEF. In this presentation we adopt an azimuthally-averaged perspective of the flow dynamics and we test the newly proposed theory of a progressive boundary layer control on SEF. Specifically, we use both RAINEX data as well as data from high resolution, full physics mesoscale numerical simulations to initialize and force an axisymmetric slab boundary layer model with radial diffusion included. The objective is to investigate whether such a reduced boundary layer model can generate secondary wind maxima as a response to environments like those that result in SEF in nature and in full physics simulations.

Abarca, S. F.; Montgomery, M. T.; Bell, M. M.

2012-12-01

404

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

405

Boundary-Layer Receptivity and Integrated Transition Prediction  

NASA Technical Reports Server (NTRS)

The adjoint parabold stability equations (PSE) formulation is used to calculate the boundary layer receptivity to localized surface roughness and suction for compressible boundary layers. Receptivity efficiency functions predicted by the adjoint PSE approach agree well with results based on other nonparallel methods including linearized Navier-Stokes equations for both Tollmien-Schlichting waves and crossflow instability in swept wing boundary layers. The receptivity efficiency function can be regarded as the Green's function to the disturbance amplitude evolution in a nonparallel (growing) boundary layer. Given the Fourier transformed geometry factor distribution along the chordwise direction, the linear disturbance amplitude evolution for a finite size, distributed nonuniformity can be computed by evaluating the integral effects of both disturbance generation and linear amplification. The synergistic approach via the linear adjoint PSE for receptivity and nonlinear PSE for disturbance evolution downstream of the leading edge forms the basis for an integrated transition prediction tool. Eventually, such physics-based, high fidelity prediction methods could simulate the transition process from the disturbance generation through the nonlinear breakdown in a holistic manner.

Chang, Chau-Lyan; Choudhari, Meelan

2005-01-01

406

On the Effects of Surface Roughness on Boundary Layer Transition  

NASA Technical Reports Server (NTRS)

Surface roughness can influence laminar-turbulent transition in many different ways. This paper outlines selected analyses performed at the NASA Langley Research Center, ranging in speed from subsonic to hypersonic Mach numbers and highlighting the beneficial as well as adverse roles of the surface roughness in technological applications. The first theme pertains to boundary-layer tripping on the forebody of a hypersonic airbreathing configuration via a spanwise periodic array of trip elements, with the goal of understanding the physical mechanisms underlying roughness-induced transition in a high-speed boundary layer. The effect of an isolated, finite amplitude roughness element on a supersonic boundary layer is considered next. The other set of flow configurations examined herein corresponds to roughness based laminar flow control in subsonic and supersonic swept wing boundary layers. A common theme to all of the above configurations is the need to apply higher fidelity, physics based techniques to develop reliable predictions of roughness effects on laminar-turbulent transition.

Choudhari, Meelan M.; Li, Fei; Chang, Chau-Lyan; Edwards, Jack

2009-01-01

407

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 ...

408

Response of the Tropical Boundary Layer to Weak Surface Forcing  

NASA Technical Reports Server (NTRS)

During the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE), a series of airborne thermal infrared observations and in situ atmospheric measurements were made near the sea surface through heights exceeding 4 km. Air movements associated with the sea surface temperature and the marine atmospheric boundary layer were studied.

Hagan, D.; Rogers, D.

1995-01-01

409

A wave equation for the SGEMP boundary layer  

NASA Astrophysics Data System (ADS)

Time-independent solutions of the fluid equations pertaining to the SGEMP boundary layer are found and oscillations about these static solutions are considered. Unstable waves are found by linearizing the fluid equations. An explanation is proposed for the absence of significant oscillations when electrons are emitted with a realistic energy spectrum. Numerical simulations are used to substantiate the validity of the analytic solutions.

Perez, J. D.

1984-12-01

410

Adiabatic Oscillations of the One-Dimensional SGEMP Boundary Layer  

Microsoft Academic Search

Adiabatic oscillations of the one-dimensional SGEMP boundary layer about equilibrium are described in a fluid approximation. The solution for monoenergetic emission normal to the surface is found and compared with a computer solution. Solutions for photoelectron emission with a cosine distribution for a monoenergetic and a linear times exponential energy distribution are also found. It appears that these latter solutions

Roger Stettner

1977-01-01

411

Scaling Behavior of the Time-Dependent SGEMP Boundary Layer  

Microsoft Academic Search

The analysis and results given here show that boundary layer dynamics obeys very useful scaling laws which permit one solution of the basic equations to hold for many cases. In particular, during the time that the X-ray pulse is linearly rising, or when the pulse time history changes slowly after a rapid rise, (or when the pulse behaves as any

N. J. CarronandC; C. L. Longmire

1978-01-01

412

A wave equation for the SGEMP boundary layer  

SciTech Connect

A static solution and a linear dispersion relationship are obtained from fluid equations to address the frequency content of the radiation from the SGEMP boundary layer. Numerical simulations confirm the results and indicate nonlinear behavior. No new SGEMP threat to satellites is predicted.

Perez, J.D.

1984-12-01

413

Microscale heat transfer enhancement using thermal boundary layer redeveloping concept  

Microsoft Academic Search

We demonstrated a new silicon microchannel heat sink, composing of parallel longitudinal microchannels and several transverse microchannels, which separate the whole flow length into several independent zones, in which the thermal boundary layer is in developing. The redeveloping flow is repeated for all of the independent zones thus the overall heat transfer is greatly enhanced. Meanwhile, the pressure drops are

J. L. Xu; Y. H. Gan; D. C. Zhang; X. H. Li

2005-01-01

414

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

415

Measurements Of Turbulence In Boundary-Layer Flows  

NASA Technical Reports Server (NTRS)

Report describes experimental study of turbulence in two boundary-layer flows with adverse gradients of pressure. Flows produced about cylinder oriented with axis along that of low-speed wind tunnel of rectangular cross section. Fluctuations of velocities analyzed with respect to various mathematical models of turbulence.

Driver, David M.

1993-01-01

416

Higher eigenmodes in the Blasius boundary-layer stability problem  

NASA Technical Reports Server (NTRS)

The higher spatial-stability eigenmodes for the Blasius boundary layer are examined by using asymptotic theory, and an infinite number of modes are found. The asymptotic results are shown to be in close agreement with results from a direct numerical solution of the Orr-Sommerfeld problem. The asymptotic theory would therefore provide an efficient tool in exploratory searches for the eigenvalues.

Hultgren, Lennart S.

1987-01-01

417

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

2010-03-01

418

Flow physics of discrete boundary layer suction measurements and predictions  

Microsoft Academic Search

The primary objective of this work is to determine the detailed characteristics of the flow features induced in a boundary layer by suction through laminar flow control (LFC) perforations. An additional goal is to validate a predictive method for generic LFC suction surfaces and to apply this technique to typical flight condition configurations. Fundamental insights into the flow physics of

D. G. MacManus; J. A. Eaton

2000-01-01