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1

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

2

Boundary layer response to wind gusts  

E-print Network

recording and reduction. Velocity fluctuations in the test nocti. on and in tho boundary layer wore naasured using hot sire ~ters of conventional oon figuration. Two hot wires, 0. 0002 inches in dianaxor aad 0. 060 inches long ware fahricat?I at Texas k... the valves or the tunnel. In general, mechanically induced noise ves loss than xha hot wire amplifier noise level and vss& therefore, ignored. Tha boundary layer traverse mechanism, figure 4, consisted of a depth microeetex' mounted under tbo wind tunnel...

Morland, Bruce Thomas

2012-06-07

3

Laboratory synthesis of turbulent boundary layer wall-pressures and the induced vibro-acoustic response  

E-print Network

Laboratory synthesis of turbulent boundary layer wall-pressures and the induced vibro is considered of synthesizing spatially correlated random pressure fields whose statistical properties Introduction Synthesizing the response of structures to Turbulent Boundary Layer (TBL) wall-pressure

Boyer, Edmond

4

Total Solar Eclipses and Atmospheric Boundary Layer Response  

NASA Astrophysics Data System (ADS)

The effect of three total solar eclipses on meteorological parameters is discussed in the paper. Measurements were conducted at the village of Ravnets,General Toshevo municipality, Bulgaria, 1999,in Manavgat, near Antalya, Turkey, 2006 and in Tian Huang Ping, China, 2009. The observed decrease of the sky illumination (incoming solar radiation) during the eclipses was proportional to the percentage of solar coverage. The after eclipse sky illumination level is due to the effect of the natural change of the solar elevation angle. For the 1999 TSE it did not regain its pre eclipse value, it has exactly the same value for the 2006 TSE, and, It is three times larger than the pre eclipse value for the 2009 TSE. This fact can be easily explained by the Local Time of the maximum of the eclipses: LT 13:12, LT 12:58, and LT 09:34, respectively. Measurements showed significant changes in the surface air temperature. The minimum of the air temperature during the 2009 TSE (Tmin=4.5°C) was measured 6 min after the end of the total phase. This minimal temperature drop and larger time lag can be explained with the huge artificial lake near the place of observation, which minimizes the temperature response due to its larger heat capacity. During the 1999 TSE, minimal temperature (Tmin=6.4°C) is measured 7 min 30 s after the total phase, and for the 2006 TSE (Tmin=5°C) - 5 min. It is in accordance with the fact that the temperature minima at residential/commercial stations occurred in general, before the minima at stations in agricultural terrains. In 2006 we were at the yard of the hotel, and in 1999 in the countryside. The wind velocity drops during the total phase as a result of the cooling and stabilization of the atmospheric boundary layer. The wind direction during the total phase changes and the wind begins to blow in the same direction as the direction of motion of the lunar shadow on the earth. Cirrus and cirrostratus clouds were observed during the 2006 total solar eclipse. Cloud structures in the form of narrow concentric arcs, equally detached from one another were observed for 20 minutes, after the beginning of the maximum phase of the 1999 TSE.

Stoev, A.; Stoeva, P.; Kuzin, S.

2012-11-01

5

Response of a hypersonic boundary layer to freestream pulse acoustic disturbance.  

PubMed

The response of hypersonic boundary layer over a blunt wedge to freestream pulse acoustic disturbance was investigated. The stability characteristics of boundary layer for freestream pulse wave and continuous wave were analyzed comparatively. Results show that freestream pulse disturbance changes the thermal conductivity characteristics of boundary layer. For pulse wave, the number of main disturbance clusters decreases and the frequency band narrows along streamwise. There are competition and disturbance energy transfer among different modes in boundary layer. The dominant mode of boundary layer has an inhibitory action on other modes. Under continuous wave, the disturbance modes are mainly distributed near fundamental and harmonic frequencies, while under pulse wave, the disturbance modes are widely distributed in different modes. For both pulse and continuous waves, most of disturbance modes slide into a lower-growth or decay state in downstream, which is tending towards stability. The amplitude of disturbance modes in boundary layer under continuous wave is considerably larger than pulse wave. The growth rate for the former is also considerably larger than the later the disturbance modes with higher growth are mainly distributed near fundamental and harmonic frequencies for the former, while the disturbance modes are widely distributed in different frequencies for the latter. PMID:24737993

Wang, Zhenqing; Tang, Xiaojun; Lv, Hongqing

2014-01-01

6

Response of a Hypersonic Boundary Layer to Freestream Pulse Acoustic Disturbance  

PubMed Central

The response of hypersonic boundary layer over a blunt wedge to freestream pulse acoustic disturbance was investigated. The stability characteristics of boundary layer for freestream pulse wave and continuous wave were analyzed comparatively. Results show that freestream pulse disturbance changes the thermal conductivity characteristics of boundary layer. For pulse wave, the number of main disturbance clusters decreases and the frequency band narrows along streamwise. There are competition and disturbance energy transfer among different modes in boundary layer. The dominant mode of boundary layer has an inhibitory action on other modes. Under continuous wave, the disturbance modes are mainly distributed near fundamental and harmonic frequencies, while under pulse wave, the disturbance modes are widely distributed in different modes. For both pulse and continuous waves, most of disturbance modes slide into a lower-growth or decay state in downstream, which is tending towards stability. The amplitude of disturbance modes in boundary layer under continuous wave is considerably larger than pulse wave. The growth rate for the former is also considerably larger than the later the disturbance modes with higher growth are mainly distributed near fundamental and harmonic frequencies for the former, while the disturbance modes are widely distributed in different frequencies for the latter. PMID:24737993

Wang, Zhenqing; Tang, Xiaojun; Lv, Hongqing

2014-01-01

7

Fuselage Structure Response to Boundary Layer, Tonal Sound, and Jet Noise  

NASA Technical Reports Server (NTRS)

Experiments have been conducted to study the response of curved aluminum and graphite-epoxy fuselage structures to flow and sound loads from turbulent boundary layer, tonal sound, and jet noise. Both structures were the same size. The aluminum structure was reinforced with tear stoppers, while the graphite-epoxy structure was not. The graphite-epoxy structure weighed half as much as the aluminum structure. Spatiotemporal intermittence and chaotic behavior of the structural response was observed, as jet noise and tonal sound interacted with the turbulent boundary layer. The fundamental tone distributed energy to other components via wave interaction with the turbulent boundary layer. The added broadband sound from the jet, with or without a shock, influenced the responses over a wider range of frequencies. Instantaneous spatial correlation indicates small localized spatiotemporal regions of convected waves, while uncorrelated patterns dominate the larger portion of the space. By modifying the geometry of the tear stoppers between panels and frame, the transmitted and reflected waves of the aluminum panels were significantly reduced. The response level of the graphite-epoxy structure was higher, but the noise transmitted was nearly equal to that of the aluminum structure. The fundamental shock mode is between 80 deg and 150 deg and the first harmonic is between 20 deg and 80 deg for the underexpanded supersonic jet impinging on the turbulent boundary layer influencing the structural response. The response of the graphite-epoxy structure due to the fundamental mode of the shock impingement was stabilized by an externally fixed oscillator.

Maestrello, L.

2004-01-01

8

The Response Of, and the Acoustic Radiation from Panels Excited by Turbulent Boundary Layers.  

National Technical Information Service (NTIS)

The principal aim of the work is to develop approximate solutions for both the response of skin-stringer panels to turbulent boundary layer excitation and the subsequently acoustic radiation from these structures. As a first step, the solution for the res...

E. Szechenyi

1970-01-01

9

The atmospheric boundary layer  

SciTech Connect

In this book, the author successfully reviews the current state of affairs in boundary-layer meteorology research. The book is organized into nine chapters. The first chapter is an introduction to the topic of the atmospheric boundary layer. The second chapter is a survey of turbulence theory. The third chapter reviews the similarity relationships that have been formulated for the various scaling regimes in the boundary layer. Chapter 4 deals with the deviations of the homogeneous boundary layer. In chapter 5 the boundary conditions for the atmospheric boundary layers are considered, that is, the energy fluxes at the earth's surface. In chapter 6 the characteristics and dynamics are discussed for the various prototypes of the atmospheric boundary layer, such as the convective and the stable boundary layer. Boundary-layer clouds are the subject of chapter 7. The final chapters, 8 and 9, discuss the use of boundary-layer meteorology in formulating parameterization schemes. In the preface of the book, the author states that his goal is to provide a book for researchers in atmospheric and associated sciences. The book will be an asset to any scientist active in boundary-layer meteorology or a related field.

Garratt, J.R.

1992-01-01

10

The response of a flat plate boundary layer to an orthogonally arranged dielectric barrier discharge actuator  

NASA Astrophysics Data System (ADS)

The jetting characteristics of dielectric barrier discharge (DBD) actuators make these devices suitable for augmenting boundary layer flows. The associated change to the hydrodynamic stability of the fluid arising from the actuator provides a mechanism through which a DBD-based laminar flow control (LFC) system can be developed. Historically, DBD actuators with electrodes arranged parallel to each other have been used for LFC with mixed results. An alternative is to use an actuator with electrodes placed orthogonally to each other. Orthogonally arranged actuators exhibit different jetting characteristics to conventional ones, and as such understanding the effect that these actuators have on the mean velocity profile within a flat plate boundary layer is of significant interest to the development of DBD-based LFC technology. In this investigation, the velocity distribution within a flat plate boundary layer in a zero pressure gradient is measured in response to the operation of an orthogonally arranged actuator. The results suggest that significant thinning of the boundary layer can be realized with an orthogonally arranged actuator, over a short distance downstream of the device, and used in conjunction with a subtle suction effect, this thinning can be exacerbated. However, further downstream, rapid thickening of the layer, supported by a decrease in the shape factor of the flow suggests that the layer becomes unstable, in an accelerated fashion, to the presence of the actuator. Hence the stability of the layer is found to be significantly altered by the presence of the orthogonally arranged actuator, a requisite for a LFC system. However, since the actuator produces a destabilizing effect, the development of a successful LFC system based on orthogonal actuators will require further work.

Gibson, B. A.; Arjomandi, M.; Kelso, R. M.

2012-01-01

11

The atmospheric boundary layer  

Microsoft Academic Search

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

J. R. Garratt

1992-01-01

12

Response of the Blasius boundary layer to free-stream vorticity  

NASA Astrophysics Data System (ADS)

Two- and three-dimensional vortical modes that solve the linearized Navier-Stokes equations in the free stream are used in the present theory to represent some of the key features of low-level turbulence. Excluding the leading edge, the effect of these modes on the Blasius boundary layer is investigated using the parabolized stability equations (PSE). When the vortical modes are steady, or have low frequencies, the PSE analysis is started at a location x0 from the solution to a new set of ordinary differential equations. This solution is able to satisfy the linearized Navier-Stokes equations in a rather large neighborhood of x0. When the vortical modes have frequencies equal to those of unstable Tollmien-Schlichting waves, the scattering of the vortical modes by surface undulation produces only a weak response in the boundary layer, in agreement with other investigations. However, when steady and low-frequency vortical modes are considered, the analysis yields results that successfully reproduce a number of the experimental measurements of Kendall [AIAA Paper 90-1504 (1990)] on streaky structures, known as Klebanoff modes, that cause a periodic spanwise modulation of the streamwise velocity.

Bertolotti, F. P.

1997-08-01

13

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

14

A synthesis approach for reproducing the response of aircraft panels to a turbulent boundary layer excitation.  

PubMed

Random wall-pressure fluctuations due to the turbulent boundary layer (TBL) are a feature of the air flow over an aircraft fuselage under cruise conditions, creating undesirable effects such as cabin noise annoyance. In order to test potential solutions to reduce the TBL-induced noise, a cost-efficient alternative to in-flight or wind-tunnel measurements involves the laboratory simulation of the response of aircraft sidewalls to high-speed subsonic TBL excitation. Previously published work has shown that TBL simulation using a near-field array of loudspeakers is only feasible in the low frequency range due to the rapid decay of the spanwise correlation length with frequency. This paper demonstrates through theoretical criteria how the wavenumber filtering capabilities of the radiating panel reduces the number of sources required, thus dramatically enlarging the frequency range over which the response of the TBL-excited panel is accurately reproduced. Experimental synthesis of the panel response to high-speed TBL excitation is found to be feasible over the hydrodynamic coincidence frequency range using a reduced set of near-field loudspeakers driven by optimal signals. Effective methodologies are proposed for an accurate reproduction of the TBL-induced sound power radiated by the panel into a free-field and when coupled to a cavity. PMID:21302997

Bravo, Teresa; Maury, Cédric

2011-01-01

15

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

16

Remote Sensing And Surface Observations Of The Response Of The Atmospheric Boundary Layer To A Solar Eclipse  

NASA Astrophysics Data System (ADS)

On 11 August 1999, a near-total solar eclipse (80%) was observed in Campistrous, France. The influence of this particular event on the atmospheric boundary layer was observed with a UHF-RASS radar, a sodar and an instrumented mast. The changes in turbulence intensity, radar reflectivity, and temperature on the radiative budget are described in relation to collocated ground meteorological data. The impact of the eclipse induces a clear response of the atmosphere, with a time lag of 15 to 30 min, perceptible in several mean and turbulent meteorological variables up to the top of the atmospheric boundary layer.

Girard-Ardhuin, Fanny; Bénech, B.; Campistron, B.; et al.

17

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

18

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

19

Boundary layer transition studies  

NASA Astrophysics Data System (ADS)

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

Watmuff, Jonathan H.

1995-02-01

20

Modeling the urban boundary layer  

NASA Technical Reports Server (NTRS)

A summary and evaluation is given of the Workshop on Modeling the Urban Boundary Layer; held in Las Vegas on May 5, 1975. Edited summaries from each of the session chairpersons are also given. The sessions were: (1) formulation and solution techniques, (2) K-theory versus higher order closure, (3) surface heat and moisture balance, (4) initialization and boundary problems, (5) nocturnal boundary layer, and (6) verification of models.

Bergstrom, R. W., Jr.

1976-01-01

21

Orthogonal Boundary-Layer Flows  

NASA Astrophysics Data System (ADS)

A theoretical study is made for boundary-layer flows of different strengths intersecting each other at right angles. Analytic solutions are found for orthogonally interesecting Bickley jets, wall jets, wakes, and uniform shear flows. The equations for intersecting Blasius boundary layers and mixing layers are found and solved numerically. In all cases the development of the boundary-layer thickness for flow in the x-z plane is proportional to a fractional power of (x + z). Extensions of the work are envisioned to include plate transpiration and stretching for the wall-bounded flows.

Weidman, Patrick

2011-11-01

22

A study of the effect of a boundary layer profile on the dynamic response and acoustic radiation of flat panels. Ph.D. Thesis - Virginia Univ.  

NASA Technical Reports Server (NTRS)

The response of a thin, elastic plate to a harmonic force which drives the plate from below and a compressible air stream with a viscous boundary layer flowing parallel to the upper surface along the length was investigated. Equations governing the forced response of the coupled plate-aerodynamic system are derived along with appropriate boundary conditions. Calculations of basic solution parameters for a linear velocity profile and for a Blasius profile showed that the same system response could be obtained from each profile if appropriate values of boundary layer thickness were chosen for each profile.

Mixson, J. S.

1973-01-01

23

Boundary Layer Control on Airfoils.  

ERIC Educational Resources Information Center

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

Gerhab, George; Eastlake, Charles

1991-01-01

24

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

25

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

26

Experimental Measurement of Transonic Fan Wake Response to Uniform and Simulated Boundary Layer Ingesting Inlet Flows  

NASA Technical Reports Server (NTRS)

BWB Aircraft with embedded engines and BLI inlets offer attractive advantages in terms of reduced noise from engines and increased range and fuel economy. The BLI inlet produces inlet distortion patterns that can reduce fan performance and stall margin, and can produce undesirable forced responses. Knowledge of the dynamic response of fan flow when subjected to flow distortions of the type produced by BLI inlets is important for the design of distortion tolerant fans. This project is investigating fan response to flow distortion by measuring the response of the fan of a JT15D engine to a flow pattern following the results of the NASA Inlet A BLI wind tunnel tests.

O'Brien, Walter F.; Ferrar, Anthony M.; Arend, David

2011-01-01

27

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

28

The kinematics of turbulent boundary layer structure  

NASA Technical Reports Server (NTRS)

The long history of research into the internal structure of turbulent boundary layers has not provided a unified picture of the physics responsible for turbulence production and dissipation. The goals of the present research are to: (1) define the current state of boundary layer structure knowledge; and (2) utilize direct numerical simulation results to help close the unresolved issues identified in part A and to unify the fragmented knowledge of various coherent motions into a consistent kinematic model of boundary layer structure. The results of the current study show that all classes of coherent motion in the low Reynolds number turbulent boundary layer may be related to vortical structures, but that no single form of vortex is representative of the wide variety of vortical structures observed. In particular, ejection and sweep motions, as well as entrainment from the free-streem are shown to have strong spatial and temporal relationships with vortical structures. Disturbances of vortex size, location, and intensity show that quasi-streamwise vortices dominate the buffer region, while transverse vortices and vortical arches dominate the wake region. Both types of vortical structure are common in the log region. The interrelationships between the various structures and the population distributions of vortices are combined into a conceptual kinematic model for the boundary layer. Aspects of vortical structure dynamics are also postulated, based on time-sequence animations of the numerically simulated flow.

Robinson, Stephen Kern

1991-01-01

29

Tropical cyclone boundary layer shocks  

E-print Network

This paper presents numerical solutions and idealized analytical solutions of axisymmetric, $f$-plane models of the tropical cyclone boundary layer. In the numerical model, the boundary layer radial and tangential flow is forced by a specified pressure field, which can also be interpreted as a specified gradient balanced tangential wind field $v_{\\rm gr}(r)$ or vorticity field $\\zeta_{\\rm gr}(r)$. When the specified $\\zeta_{\\rm gr}(r)$ field is changed from one that is radially concentrated in the inner core to one that is radially spread, the quasi-steady-state boundary layer flow transitions from a single eyewall shock-like structure to a double eyewall shock-like structure. To better understand these structures, analytical solutions are presented for two simplified versions of the model. In the simplified analytical models, which do not include horizontal diffusion, the $u(\\partial u/\\partial r)$ term in the radial equation of motion and the $u[f+(\\partial v/\\partial r)+(v/r)]$ term in the tangential equat...

Slocum, Christopher J; Taft, Richard K; Schubert, Wayne H

2014-01-01

30

Response of the topography and gravity field on Venus to mantle upwelling beneath a chemical boundary layer  

NASA Technical Reports Server (NTRS)

The long wavelength correlation of the gravity and topography and the large apparent depths of compensation (approximately 150-300 km) for large highland regions on Venus require significant differences between the interior structure of Earth and Venus. The morphology, geologic history, and large apparent depths of compensation for many highlands have been interpreted to indicate areas of mantle upwelling. A large apparent depth of compensation at a mantle upwelling is generally interpreted to indicate the base of the thermal boundary layer of convection. A boundary layer thickness of 150-300 km implies that the interior of Venus is presently much colder than Earth and thus tectonically less active. The recent Magellan mission has provided contradictory evidence regarding the present level of tectonic activity on Venus, prompting considerable debate. In this study, we investigate the possibility that a chemical boundary layer acts together with a thermal boundary layer to produce large apparent depths of compensation, or equivalently, large geoid-to-topography-ratios (GTR's). The crust of a planet forms through partial melting of mantle materials. Both the melt and the residuum are lower in density than unmelted (or undepleted) mantle. In the absence of vigorous plate tectonics, a thick layer of buoyant residuum, or depleted mantle, may collect beneath the lithosphere. In this scenario, the thermal lithosphere does not need to be thick and cold to match the GTR's. Cooling of the depleted layer may lead to overturn of the upper mantle and episodic resurfacing with time scales on the order of 300-500 MY, consistent with the resurfacing age of Venus.

Smrekar, Suzanne E.; Parmentier, E. Marc

1993-01-01

31

Compressible turbulent boundary layer interaction experiments  

NASA Technical Reports Server (NTRS)

Four phases of research results are reported: (1) experiments on the compressible turbulent boundary layer flow in a streamwise corner; (2) the two dimensional (2D) interaction of incident shock waves with a compressible turbulent boundary layer; (3) three dimensional (3D) shock/boundary layer interactions; and (4) cooperative experiments at Princeton and numerical computations at NASA-Ames.

Settles, G. S.; Bogdonoff, S. M.

1981-01-01

32

Modelling the transitional boundary layer  

NASA Technical Reports Server (NTRS)

Recent developments in the modelling of the transition zone in the boundary layer are reviewed (the zone being defined as extending from the station where intermittency begins to depart from zero to that where it is nearly unity). The value of using a new non-dimensional spot formation rate parameter, and the importance of allowing for so-called subtransitions within the transition zone, are both stressed. Models do reasonably well in constant pressure 2-dimensional flows, but in the presence of strong pressure gradients further improvements are needed. The linear combination approach works surprisingly well in most cases, but would not be so successful in situations where a purely laminar boundary layer would separate but a transitional one would not. Intermittency-weighted eddy viscosity methods do not predict peak surface parameters well without the introduction of an overshooting transition function whose connection with the spot theory of transition is obscure. Suggestions are made for further work that now appears necessary for developing improved models of the transition zone.

Narasimha, R.

1990-01-01

33

Boundary Layer Heights from CALIOP  

NASA Astrophysics Data System (ADS)

This work is focused on the development of a planetary boundary layer (PBL) height retrieval algorithm for CALIOP and validation studies. Our current approach uses a wavelet covariance transform analysis technique to find the top of the boundary layer. We use the methodology similar to that found in Davis et. al. 2000, ours has been developed to work with the lower SNR data provided by CALIOP, and is intended to work autonomously. Concurrently developed with the CALIOP algorithm we will show results from a PBL height retrieval algorithm from profiles of potential temperature, these are derived from Aircraft Meteorological DAta Relay (AMDAR) observations. Results from 5 years of collocated AMDAR - CALIOP retrievals near O'Hare airport demonstrate good agreement between the CALIOP - AMDAR retrievals. In addition, because we are able to make daily retrievals from the AMDAR measurements, we are able to observe the seasonal and annual variation in the PBL height at airports that have sufficient instrumented-aircraft traffic. Also, a comparison has been done between the CALIOP retrievals and the NASA Langley airborne High Spectral Resolution Lidar (HSRL) PBL height retrievals acquired during the GoMACCS experiment. Results of this comparison, like the AMDAR comparison are favorable. Our current work also involves the analysis and verification of the CALIOP PBL height retrieval from the 6 year CALIOP global data set. Results from this analysis will also be presented.

Kuehn, R.; Ackerman, S. A.; Holz, R.; Roubert, L.

2012-12-01

34

Observational and numerical analysis of atmospheric boundary-layer response to summertime thermal and dynamic forcings in Miers Valley, Antarctica  

NASA Astrophysics Data System (ADS)

In response to solar insolation the McMurdo Dry Valleys (MDVs) summertime airflow patterns show bimodal oscillations, either up-valley or down-valley. Miers Valley (78°6'S, 164°0'E) is one of the four, east-west oriented ice-free valleys in the Denton Hill area of southern Victoria Land and one of the biodiversity 'hotspots'. This glacially excavated valley is considerably smaller than the larger valley systems such as Wright or Victoria. While most atmospheric research has been carried out in the larger valley systems, little work has been done on this part of the MDVs; this research presents the first investigation of Miers valley's microclimate. We aim to provide insight into diurnal evolution of boundary layer and physical mechanism(s) responsible for local circulations, and determine their spatial extent during summer 2012 (13-25 January). This was achieved through high-resolution numerical modeling using a polar optimized weather prediction model (PWRF) alongside data obtained from an eddy-covariance system and a wind profiler (SODAR) to elucidate the role of local thermal forcing during synoptic quiescence. Measurements showed that persistent up-valley winds were present within and above a mixed layer (120 ± 30 mAGL), peaking around 75 mAGL upto 6 m/s between 1800 to 2000 LST. Several mechanisms that can produce up-valley flows in MDVs have been proposed. A regional intrusion of sea breeze currents originating from McMurdo Sound and the Ross Sea area is one of the possible forcings for such valley winds. Another suggested forcing is flow deflection due to the blocking effect of Ross Island (RI). Numerical simulations confirm that up-valley easterlies have both local and regional thermal and dynamic forcings. The thermal forcing is controlled by the variation in solar elevation, topographic shadowing, and clouds, which cause a heterogeneous thermal pattern and produce a cross-valley thermal gradient, and spatially larger gradient between the valley atmosphere and the Ross Ice Shelf (RIS) forcing the local wind system. PWRF simulates different spatial wind pattern for each half of the valley, mainly because of the shape of topography and the distance from RIS. The eastern half of the valley constantly showed up-valley wind whereas the valley interior, (which includes the measurement site) experienced a weak and shallow down-valley flow when the surface is topographically shadowed. The dynamic forcing results from the build-up of a meso-high pressure zone south of RI due to katabatic interaction with topography and the channeling effect of RI and the adjacent coastal topography, which cause intrusion of an easterly flow against the pressure gradient pattern lying over the nearby Koettlitz Glacier.

Soltanzadeh, Iman; Katurji, Marwan; Zawar-Reza, Peyman; Storey, Bryan

2013-04-01

35

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

36

Leading edge effects on boundary layer receptivity  

NASA Astrophysics Data System (ADS)

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

Gatski, Thomas B.; Kerschen, Edward J.

37

Stochastic rocket dynamics under random nozzle side loads: Ornstein-Uhlenbeck boundary layer separation and its coarse grained connection to side loading and rocket response  

E-print Network

A long-standing, though ill-understood problem in rocket dynamics, rocket response to random, altitude-dependent nozzle side-loads, is investigated. Side loads arise during low altitude flight due to random, asymmetric, shock-induced separation of in-nozzle boundary layers. In this paper, stochastic evolution of the in-nozzle boundary layer separation line, an essential feature underlying side load generation, is connected to random, altitude-dependent rotational and translational rocket response via a set of simple analytical models. Separation line motion, extant on a fast boundary layer time scale, is modeled as an Ornstein-Uhlenbeck process. Pitch and yaw responses, taking place on a long, rocket dynamics time scale, are shown to likewise evolve as OU processes. Stochastic, altitude-dependent rocket translational motion follows from linear, asymptotic versions of the full nonlinear equations of motion; the model is valid in the practical limit where random pitch, yaw, and roll rates all remain small. Comp...

Keanini, R G; Tkacik, Peter T; Weggel, David C; Knight, P Douglas

2011-01-01

38

Diamagnetic boundary layers: A kinetic theory  

Microsoft Academic Search

We present a kinetic theory for boundary layers associated with MHD tangential ‘discontinuities’ in a collisionless magnetized plasma such as those observed in the solar wind. The theory consists of finding self-consistent solutions of Vlasov's equation and Maxwell's equation for stationary, one-dimensional boundary layers separating two Maxwellian plasma states. Layers in which the current is carried by electrons are found

J. Lemaire; L. F. Burlaga

1976-01-01

39

Microgravity Effects on Plant Boundary Layers  

NASA Technical Reports Server (NTRS)

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

Stutte, Gary; Monje, Oscar

2005-01-01

40

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

41

Turbulent boundary layers with secondary flow  

NASA Technical Reports Server (NTRS)

An experimental analysis of the boundary layer on a plane wall, along which the flow occurs, whose potential flow lines are curved in plane parallel to the wall is discussed. According to the equation frequently applied to boundary layers in a plane flow, which is usually obtained by using the pulse law, a generalization is derived which is valid for boundary layers with spatial flow. The wall shear stresses were calculated with this equation.

Grushwitz, E.

1984-01-01

42

Numerical studies in boundary-layer stability  

NASA Astrophysics Data System (ADS)

The applicability of interactive-boundary-layer theory and thin-layer Navier-Stokes methods to problems in Blasius boundary-layer stability is investigated. First, the classical linear stability problem, based on the parallel flow approximation, is solved and results are compared to those obtained from a solution to the Orr-Sommerfeld equation. Then studies on the nonlinear evolution of localized disturbances in the Blasius boundary layer are carried out using two different, three-dimensional, unsteady, interactive-boundary-layer models and a thin-layer Navier-Stokes model. Results are, whenever possible, compared to those obtained from the full Navier-Stokes equations. A simple, yet very efficient, method of solving the unsteady incompressible thin-layer Navier-Stokes equations is introduced and it drastically reduced the computational time, relative to comparable studies based on the full Navier-Stokes equations.

Yahiaoui, Mondher

43

AIAA 20042128 Boundary Layer Flow Control Using  

E-print Network

AIAA 2004­2128 Boundary Layer Flow Control Using AC Discharge Plasma Actuators Jamey Jacob, Propulsion Directorate Wright-Patterson Air Force Base, OH J. Estevadeordal ISSI, Dayton, OH AIAA 2nd Flow­4344 #12;Boundary Layer Flow Control Using AC Discharge Plasma Actuators Jamey Jacob Mechanical Engineering

Jacob, Jamey

44

8, 1074910790, 2008 Boundary-layer top  

E-print Network

lidar, a vertical-wind Doppler lidar, and ac- companying radiosonde profiling of temperatureACPD 8, 10749­10790, 2008 Boundary-layer top from lidar H. Baars et al. Title Page Abstract Chemistry and Physics Discussions Continuous monitoring of the boundary-layer top with lidar H. Baars, A

Paris-Sud XI, Université de

45

Boundary Layers of Air Adjacent to Cylinders  

PubMed Central

Using existing heat transfer data, a relatively simple expression was developed for estimating the effective thickness of the boundary layer of air surrounding cylinders. For wind velocities from 10 to 1000 cm/second, the calculated boundary-layer thickness agreed with that determined for water vapor diffusion from a moistened cylindrical surface 2 cm in diameter. It correctly predicted the resistance for water vapor movement across the boundary layers adjacent to the (cylindrical) inflorescence stems of Xanthorrhoea australis R. Br. and Scirpus validus Vahl and the leaves of Allium cepa L. The boundary-layer thickness decreased as the turbulence intensity increased. For a turbulence intensity representative of field conditions (0.5) and for ?windd between 200 and 30,000 cm2/second (where ?wind is the mean wind velocity and d is the cylinder diameter), the effective boundary-layer thickness in centimeters was equal to [Formula: see text]. PMID:16658855

Nobel, Park S.

1974-01-01

46

Stable Layers in the Atmospheric Boundary Layer  

NASA Astrophysics Data System (ADS)

Field experimental studies on the establishment and growth of the nocturnal stable layer near the ground were made in January, 1998 using a tethered balloon at a site in Phoenix, Arizona. Days and nights with clear skies and light surface winds were of particular interest because small particle and carbon monoxide concentrations can be high during such times. Closest to the ground a shallow stable layer 20 meters deep with a buoyancy frequency (N) of 0.05 1/s rapidly developed before sundown. The height of this layer and N remained constant throughout the night. Above the 20-meter level, there was a transition layer which was also stable with N = 0.025 1/s. This transition layer grew throughout the night and reached 120 meters by dawn. Above the transition layer was a neutrally stable (residual) layer left over from the previous day. An unsteady layer 10 to 100 m thick with N = 0.025 1/s was also found at the top of the troposphere with the neutrally stable troposphere below and the stable stratosphere above. The growth and/or decay of turbulence in such stable layers will be discussed in light of recent theoretical developments.

Mahalov, A.; Berman, N. S.; Fernando, H. J. S.; Yu, F.; Pardyjak, E.

1998-11-01

47

The surface roughness and planetary boundary layer  

Microsoft Academic Search

Applications of the entrainment process to layers at the boundary, which meet the self similarity requirements of the logarithmic profile, have been studied. By accepting that turbulence has dominating scales related in scale length to the height above the surface, a layer structure is postulated wherein exchange is rapid enough to keep the layers internally uniform. The diffusion rate is

James W. Telford

1980-01-01

48

Resonant properties of a nonlinear dissipative layer excited by a vibrating boundary: Q-factor and frequency response.  

PubMed

Simplified nonlinear evolution equations describing non-steady-state forced vibrations in an acoustic resonator having one closed end and the other end periodically oscillating are derived. An approach based on a nonlinear functional equation is used. The nonlinear Q-factor and the nonlinear frequency response of the resonator are calculated for steady-state oscillations of both inviscid and dissipative media. The general expression for the mean intensity of the acoustic wave in terms of the characteristic value of a Mathieu function is derived. The process of development of a standing wave is described analytically on the base of exact nonlinear solutions for different laws of periodic motion of the wall. For harmonic excitation the wave profiles are described by Mathieu functions, and their mean energy characteristics by the corresponding eigenvalues. The sawtooth-shaped motion of the boundary leads to a similar process of evolution of the profile, but the solution has a very simple form. Some possibilities to enhance the Q-factor of a nonlinear system by suppression of nonlinear energy losses are discussed. PMID:15759681

Enflo, B O; Hedberg, C M; Rudenko, O V

2005-02-01

49

Multiple solutions of a boundary layer problem  

NASA Astrophysics Data System (ADS)

The laminar boundary layer flow on a continuous moving porous flat plate with suction or injection is governed by the nonlinear differential equation f?(?)+f(?)f?(?)=0, with boundary conditions f(0)=-C,f'(0)=?,f'(+?)=1, where ? is the similarity variable, f( ?) is related to the stream function, and C and ? are constants. This paper presents a rigorous proof of the existence of multiple solutions to the boundary value problem by a shooting method on [0, ?).

Lu, Chunqing

2007-08-01

50

Thunderstorm influence on boundary layer winds  

E-print Network

THUNDERSTORM INFLUENCE ON BOUNDARY LAYER WINDS A Thesis by JILL MARIE SCHMIDT Submitted to the Graduate College of Texas ASM University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE December 1986 Major... Subject: Meteorology THUNDERSTORM INFLUENCE ON BOUNDARY LAYER WINDS A Thesis by JILL MARIE SCHMIDT Approved as to style and content by: James R. Scog s (Chairman of Committee) Kenneth C. Brundidge (Member) Qmer . Jenklns (Member) James R...

Schmidt, Jill Marie

2012-06-07

51

SH Wave Number Green's Function for a Layered, Elastic Half-Space. Part I: Theory and Dynamic Canyon Response by the Discrete Wave Number Boundary Element Method  

NASA Astrophysics Data System (ADS)

We present a closed-form frequency-wave number ( ? - k) Green's function for a layered, elastic half-space under SH wave propagation. It is shown that for every ( ? - k) pair, the fundamental solution exhibits two distinctive features: (1) the original layered system can be reduced to a system composed by the uppermost superficial layer over an equivalent half-space; (2) the fundamental solution can be partitioned into three different fundamental solutions, each one carrying out a different physical interpretation, i.e., an equivalent half-space, source image impact, and dispersive wave effect, respectively. Such an interpretation allows the proper use of analytical and numerical integration schemes, and ensures the correct assessment of Cauchy principal value integrals. Our method is based upon a stiffness-matrix scheme, and as a first approach we assume that observation points and the impulsive SH line-source are spatially located within the uppermost superficial layer. We use a discrete wave number boundary element strategy to test the benefits of our fundamental solution. We benchmark our results against reported solutions for an infinitely long circular canyon subjected to oblique incident SH waves within a homogeneous half-space. Our results show an almost exact agreement with previous studies. We further shed light on the impact of horizontal strata by examining the dynamic response of the circular canyon to oblique incident SH waves under different layered half-space configurations and incident angles. Our results show that modifications in the layering structure manifest by larger peak ground responses, and stronger spatial variability due to interactions of the canyon geometry with trapped Love waves in combination with impedance contrast effects.

Restrepo, Doriam; Gómez, Juan David; Jaramillo, Juan Diego

2014-09-01

52

Large eddy simulation of atmospheric boundary layer over wind farms using a prescribed boundary layer approach  

NASA Astrophysics Data System (ADS)

Large eddy simulation (LES) of flow in a wind farm is studied in neutral as well as thermally stratified atmospheric boundary layer (ABL). An approach has been practiced to simulate the flow in a fully developed wind farm boundary layer. The approach is based on the Immersed Boundary Method (IBM) and involves implementation of an arbitrary prescribed initial boundary layer (See [1]). A prescribed initial boundary layer profile is enforced through the computational domain using body forces to maintain a desired flow field. The body forces are then stored and applied on the domain through the simulation and the boundary layer shape will be modified due to the interaction of the turbine wakes and buoyancy contributions. The implemented method is capable of capturing the most important features of wakes of wind farms [1] while having the advantage of resolving the wall layer with a coarser grid than typically required for such problems.

Sarlak, H.; Sørensen, J. N.; Mikkelsen, R.

2012-09-01

53

Characterization of internal boundary layer capacitors  

SciTech Connect

Internal boundary layer capacitors were characterized by scanning transmission electron microscopy and by microscale electrical measurements. Data are given for the chemical and physical characteristics of the individual grains and boundaries, and their associated electric and dielectric properties. Segregated internal boundary layers were identified with resistivities of 10/sup 12/-10/sup 13/ ..cap omega..-cm. Bulk apparent dielectric constants were 10,000-60,000. A model is proposed to explain the dielectric behavior in terms of an equivalent n-c-i-c-n representation of ceramic microstructure, which is substantiated by capacitance-voltage analysis.

Park, H. D.; Payne, D. A.

1980-05-01

54

Alpha models and boundary-layer turbulence  

NASA Astrophysics Data System (ADS)

We study boundary-layer turbulence using the Navier-Stokes-alpha model obtaining an extension of the Prandtl equations for the averaged flow in a turbulent boundary layer. In the case of a zero pressure gradient flow along a flat plate, we derive a nonlinear fifth-order ordinary differential equation, an extension of the Blasius equation. We study it analytically and prove the existence of a two-parameter family of solutions satisfying physical boundary conditions. From this equation we obtain a theoretical prediction of the skin-friction coefficient in a wide range of Reynolds numbers based on momentum thickness, and deduce the maximal value of the skin-friction coefficient in the turbulent boundary layer. The two-parameter family of solutions to the equation matches experimental data in the transitional boundary layers with different free stream turbulence intensity. A one-parameter sub-family of solutions, obtained using our skin-friction coefficient law, matches experimental data in the turbulent boundary layer for moderately large Reynolds numbers.

Cheskidov, Alexey

55

Stability of spatially developing boundary layers  

NASA Astrophysics Data System (ADS)

A new formulation of the stability of boundary-layer flows in pressure gradients is presented, taking into account the spatial development of the flow. The formulation assumes that disturbance wavelength and eigenfunction vary downstream no more rapidly than the boundary-layer thickness, and includes all terms of O(1) and O(R(exp -1)) in the boundary-layer Reynolds number R. Although containing the Orr-Sommerfeld operator, the present approach does not yield the Orr-Sommerfeld equation in any rational limit. In Blasius flow, the present stability equation is consistent with that of Bertolotti et al. (1992) to terms of O(R(exp -1)). For the Falkner-Skan similarity solutions neutral boundaries are computed without the necessity of having to march in space. Results show that the effects of spatial growth are striking in flows subjected to adverse pressure gradients.

Govindarajan, Rama

1993-07-01

56

Thermosolutal Marangoni Forced Convection Boundary Layers  

Microsoft Academic Search

An analysis is made for the forced convection thermal and solute concentration Marangoni boundary layers (thin dissipative layers) that can be formed along the surface, which separates two immiscible fluids in surface driven flows when the appropriately defined Reynolds number is large enough. Similarity equations for the case in which an external pressure gradient is imposed are derived. These equations

I. Pop; A. Postelnicu; T. Gro?an

2001-01-01

57

Mathematical model of a kinetic boundary layer  

NASA Astrophysics Data System (ADS)

The two-dimensional (plane) problem of a hypersonic kinetic boundary layer developing on a thin body in the case of a homogeneous polyatomic gas flow with no dissociation or electron excitation is considered assuming that energy exchange between translational and internal molecular degrees of freedom is easy. (The approximation of a hypersonic kinetic boundary layer arises from the kinetic theory of gases and, within the thin-layer model, takes into account the strong nonequilibrium of the hypersonic flow with respect to translational and internal degrees of freedom of the gas particles.) A method is proposed for constructing the solution of the given kinetic problem in terms of a given solution of an equivalent well-studied classical Navier-Stokes hypersonic boundary layer problem (which is traditionally formulated on the basis of the Navier-Stokes equations).

Ankudinov, A. L.

2014-06-01

58

Dynamics of Thin Astrophysical Boundary Layers  

NASA Astrophysics Data System (ADS)

We study the modal structure and angular momentum transport mechanisms of astrophysical boundary layers. We focus on the case where the accretion disk extends all the way to surface of the star and the boundary layer is thin in comparison with the stellar radius. Such a scenario is applicable, for example, to weakly magnetized neutron stars and white dwarfs, for which the strength of the magnetic field outside the star is too small to disrupt the disk and funnel matter to the poles. Within the boundary layer, material rotating at the Keplerian velocity within the disk slows down to the rotational velocity inside the star. This generates intense velocity gradients and makes the boundary layer susceptible to shear instabilities. By performing a linear stability analysis for the simplified case of a plane-parallel, compressible shear layer, we argue that astrophysical boundary layers are unstable to the sonic instability. This instability is part of a more general class of acoustic instabilities that includes the Papaloizou-Pringle instability. We confirm the predictions of our linear stability analysis by running a suite of simulations in 2D and 3D, with and without stratification, and with and without magnetic field. In our numerical experiments, we find that acoustic modes excited by the sonic instability persist even in the nonlinear regime. We explain the morphological properties and derive analytic formulas for the pattern speed of these acoustic modes. Our work has significant implications for semianalytic models describing the structure and spectral emission from boundary layers. Typically, these models adopt a local, effective viscosity prescription for the angular momentum transport. However, in our simulations we find that angular momentum transport in the boundary layer is facilitated by acoustic modes. In this scenario, accreting material inside the boundary layer loses angular momentum to sound waves that propagate into both the star and the disk. Since transport of angular momentum by waves is inherently nonlocal, our work invites the construction of new phenomenological models of the boundary layer in which angular momentum is transported by waves rather than by an anomalous viscosity.

Belyaev, Mikhail Alexander

59

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

60

Numerical simulation of supersonic boundary layer transition  

NASA Technical Reports Server (NTRS)

The present contribution reviews some of the recent progress obtained at our group in the direct numerical simulation (DNS) of compressible boundary layer transition. Elements of the different simulation approaches and numerical techniques employed are surveyed. Temporal and spatial simulations, as well as comparisons with results obtained from Parabolized Stability Equations, are discussed. DNS results are given for flat plate boundary layers in the Mach number range 1.6 to 4.5. A temporal DNS at Mach 4.5 has been continued through breakdown all the way to the turbulent stage. In addition results obtained with a recently developed extended temporal DNS approach are presented, which takes into account some nonparallel effects of a growing boundary layer. Results from this approach are quite close to those of spatial DNS, while preserving the efficiency of the temporal DNS.

Guo, Y.; Adams, N. A.; Sandham, N. D.; Kleiser, L.

1994-01-01

61

Asymptotic similarity in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

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

Duncan, Richard D.

62

Effect of boundary layer on thrust deduction  

NASA Astrophysics Data System (ADS)

It is noted that methods of computing thrust deduction usually ignore viscous effects and assume that the flow field of the ship and propeller is irrotational. The computed values of the thrust deduction with and without the boundary layer and wake were compared. A streamlined body of revolution was selected, and a sink on the axis behind the body was used as a simple mechanism to simulate the suction at the stern induced by a propeller. When the boundary layer and wake are present, the thick boundary near the tail of the body is first calculated by a previously developed method in which the equation of a thick boundary layer and wake are solved numerically by finite differences, and the outer irrotational flow is obtained as the solution of an integral equation. An iteration procedure in which the inner and outer flows are successively adjusted converges to the desired solution. It was found that results obtained in the wake were not sufficiently accurate, so that a momentum analysis using a special control volume was used to calculate the viscous drag with and without the sink. The calculated values of the thrust deuction are C sub D = 0.00021 from irrotational flow and 0.00043 from the boundary layer potential-flow interaction method.

Dinavahi, S. P. G.; Landweber, L.

1981-11-01

63

Calculation methods for compressible turbulent boundary layers, 1976  

NASA Technical Reports Server (NTRS)

Equations and closure methods for compressible turbulent boundary layers are discussed. Flow phenomena peculiar to calculation of these boundary layers were considered, along with calculations of three dimensional compressible turbulent boundary layers. Procedures for ascertaining nonsimilar two and three dimensional compressible turbulent boundary layers were appended, including finite difference, finite element, and mass-weighted residual methods.

Bushnell, D. M.; Cary, A. M., Jr.; Harris, J. E.

1977-01-01

64

Rethinking the Boundaries: Response  

ERIC Educational Resources Information Center

In his Keynote address, Dr. Kretchmar suggests that children who demonstrate an inability or unwillingness to play may have a "play disability," and thus offers interventions for remediation. In response, Oslin and Collier argue that due to cultural influences, it is more likely that physical educators are play disabled. Oslin and Collier share…

Oslin, Judy; Collier, Connie

2012-01-01

65

TURBULENCE IN SUPERSONIC AND HYPERSONIC BOUNDARY LAYERS  

E-print Network

TURBULENCE IN SUPERSONIC AND HYPERSONIC BOUNDARY LAYERS Alexander J. Smits and M. Pino Martin in supersonic and hypersonic flow where the effects of compressibility have a direct influence on the turbulence. Experimental and DNS results are presented and compared. Key words: Turbulence, supersonic, hypersonic, shocks

Martín, Pino

66

Acoustics of laminar boundary layers breakdown  

Microsoft Academic Search

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

Meng Wang

1994-01-01

67

Goertler instability of a hypersonic boundary layer  

Microsoft Academic Search

The Goertler instability of a hypersonic boundary layer and its influence on the wall heat transfer are experimentally analyzed. Measurements, made in a wind tunnel by means of a computerized infrared (IR) imaging system, refer to the flow over two-dimensional concave walls. Wall temperature maps (that are interpreted as surface flow visualizations) and spanwise heat transfer fluctuations are presented. Measured

L. de Luca; G. Cardone; D. Aymer de la Chevalerie; A. Fonteneau

1993-01-01

68

Thick diffusion limit boundary layer test problems  

SciTech Connect

We develop two simple test problems that quantify the behavior of computational transport solutions in the presence of boundary layers that are not resolved by the spatial grid. In particular we study the quantitative effects of 'contamination' terms that, according to previous asymptotic analyses, may have a detrimental effect on the solutions obtained by both discontinuous finite element (DFEM) and characteristic-method (CM) spatial discretizations, at least for boundary layers caused by azimuthally asymmetric incident intensities. Few numerical results have illustrated the effects of this contamination, and none have quantified it to our knowledge. Our test problems use leading-order analytic solutions that should be equal to zero in the problem interior, which means the observed interior solution is the error introduced by the contamination terms. Results from DFEM solutions demonstrate that the contamination terms can cause error propagation into the problem interior for both orthogonal and non-orthogonal grids, and that this error is much worse for non-orthogonal grids. This behavior is consistent with the predictions of previous analyses. We conclude that these boundary layer test problems and their variants are useful tools for the study of errors that are introduced by unresolved boundary layers in diffusive transport problems. (authors)

Bailey, T. S. [Lawrence Livermore National Laboratory, 7000 East Avenue, L-095, Livermore, CA 94551 (United States); Warsa, J. S.; Chang, J. H. [Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87544 (United States); Adams, M. L. [Texas A and M University, Department of Nuclear Engineering, College Station, TX 77843-3133 (United States)

2013-07-01

69

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

70

A Vertically Resolved Planetary Boundary Layer  

NASA Technical Reports Server (NTRS)

Increase of the vertical resolution of the GLAS Fourth Order General Circulation Model (GCM) near the Earth's surface and installation of a new package of parameterization schemes for subgrid-scale physical processes were sought so that the GLAS Model GCM will predict the resolved vertical structure of the planetary boundary layer (PBL) for all grid points.

Helfand, H. M.

1984-01-01

71

Effect of boundary layer on thrust deduction  

Microsoft Academic Search

It is noted that methods of computing thrust deduction usually ignore viscous effects and assume that the flow field of the ship and propeller is irrotational. The computed values of the thrust deduction with and without the boundary layer and wake were compared. A streamlined body of revolution was selected, and a sink on the axis behind the body was

S. P. G. Dinavahi; L. Landweber

1981-01-01

72

Advection in Accretion Disk Boundary Layers  

NASA Technical Reports Server (NTRS)

Recent numerical simulations of accretion disk boundary layers have shown qualitatively the importance tance of advected energy in the inner region of the disk. In this short paper we present quantitative results of advective boundary layers in the optically thick regime. Numerical results are obtained for various systems, by means of a one-dimensional time-dependent numerical code. At high accretion mass rates, dot-M approx. = 10(exp -4) solar mass/yr, or low values of the viscosity parameter, alpha approx. = 0.001-0.01 (characteristic of FU Orionis systems and some symbiotic stars), the optical thickness in the inner part of the disk becomes very large (tau much greater than 1). The disk, unable to cool efficiently, becomes geometrically thick (H/r approx. = 0.5). The energy dissipated in the dynamical boundary layer is radiated outward to larger radii and advected into the star. The boundary-layer luminosity is only a fraction of its expected value; the rest of the energy is advected into the star. The fraction of the advected energy is zeta = L(sub adv)/L(sub acc) approx. = 0.1 in symbiotic stars (accretion onto a low-mass main-sequence star) and zeta approx. = 0.2 in FU Ori systems (accretion onto a pre-main sequence star).

Godon, Patrick

1997-01-01

73

Accretion disk boundary layers in cataclysmic variables. 1: Optically thick boundary layers  

NASA Technical Reports Server (NTRS)

We develop numerical models of accretions disks in cataclysmic variables (CVs), including and emphasizing the boundary layer region where the accretion disk meets the accreting white dwarf. We confine ourselves to solutions where the boundary layer region is vertically optically thick, and find that these solutions share several common features. The angular and radial velocities of the accreting material drop rapidly in a dynamical boundary layer, which has a radial width approximately 1%-3% of the white dwarf radius. The energy dissipated in this region diffuses through the inner part of the disk and is radiated from the disk surface in a thermal boundary layer, which has a radial width comparable to the disk thickness, approximately 5%-15% of the white dwarf radius. We examine the dependence of the boundary layer structure on the mass accretion rate, the white dwarf mass and rotation rate, and the viscosity parameter alpha. We delineate the boundary between optically thick and optically thin boundary layer solutions as a function of these parameters and suggest that by means of a careful comparison with observations it may be possible to estimate alpha in CVs. We derive an expression for the total boundary layer luminosities as a function of the parameters and show that it agrees well with the luminosites of our numerical solutions. Finally, we calcuate simple blackbody continuum spectra of the boundary layer and disk emission for our solutions and compare these to soft X-ray, EUV, and He II emission-line observations of CVs. We show that, through such comparisons, it may be possible to determine the rotation rates of the accreting stars in CVs, and perhaps also the white dwarf masses and the accretion rates. The spectra are quite insensitive to alpha, so the uncertainty in this parameter does not affect such comparisons.

Popham, Robert; Narayan, Ramesh

1995-01-01

74

Accommodation between transpiring vegetation and the convective boundary layer  

Microsoft Academic Search

A simple scheme is developed to describe how vegetation and the convective boundary layer (CBL) interact during daylight in terms of water and sensible heat exchange. The response of vegetation to a prescribed atmospheric state is defined by a quadratic equation obtained by combining the Penman-Monteith equation with a new relation between surface conductance and transpiration rate based on laboratory

J. L. Monteith

1995-01-01

75

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

76

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

77

Sea-salt injections into the low-latitude marine boundary layer: The transient response in three Earth system models  

NASA Astrophysics Data System (ADS)

proposed mechanisms for counteracting global warming through solar radiation management is the deliberate injection of sea salt acting via marine cloud brightening and the direct effect of sea-salt aerosols. In this study, we show results from multidecadal simulations of such sea-salt climate engineering (SSCE) on top of the RCP4.5 emission scenario using three Earth system models. As in the proposed "G3" experiment of the Geoengineering Model Intercomparison Project, SSCE is designed to keep the top-of-atmosphere radiative forcing at the 2020 level for 50 years. SSCE is then turned off and the models run for another 20 years, enabling an investigation of the abrupt warming associated with a termination of climate engineering ("termination effect"). As in former idealized studies, the climate engineering in all three models leads to a significant suppression of evaporation from low-latitude oceans and reduced precipitation over low-latitude oceans as well as in the storm-track regions. Unlike those studies, however, we find in all models enhanced evaporation, cloud formation, and precipitation over low-latitude land regions. This is a response to the localized cooling over the low-latitude oceans imposed by the SSCE design. As a result, the models obtain reduced aridity in many low-latitude land regions as well as in southern Europe. Terminating the SSCE leads to a rapid near-surface temperature increase, which, in the Arctic, exceeds 2 K in all three models within 20 years after SSCE has ceased. In the same period September Arctic sea ice cover shrinks by over 25%.

Alterskjær, Kari; Kristjánsson, Jón Egill; Boucher, Olivier; Muri, Helene; Niemeier, Ulrike; Schmidt, Hauke; Schulz, Michael; Timmreck, Claudia

2013-11-01

78

Stability of boundary layers along curved surfaces  

NASA Astrophysics Data System (ADS)

The stability of a boundary layer along a curved wall was first studied by Gortler (27) in 1940. Further investigations have been conducted since 1980. An important contribution was made by Hall (29), when he first proposed a more rigorous approach to this problem. Gortler had used a simple approximation which allowed him to reduce the problem to a set of ordinary differential equations. However such an assumption ignored the growth of the boundary layer, which Hall showed could not be neglected. Hall then rederived the formulation to obtain a set of partial differential equations. We solved the system of equations, including the nonlinear terms, with a method proposed by Herbert (3), called the Parabolic Stability Equations (PSE). The flow is divided into a basic profile, which satisfies the Prandtl boundary layer equations, and a perturbation. We compared our results for the Blasius profile with those of Bottaro, Klinnmann, and Zebib (8) and found excellent agreement between our calculations and their finite- volume simulations. We then applied our code to the wall jet profile. We were able to capture the growth of steady vortices, located in the inner region of the jet for a concave wall, and the outer region for a convex wall, as predicted by Florian's (20) linear inviscid argument. Our calculations were in good agreement with Matsson's (49) experimental results. We also studied the influence of crossflow on a boundary layer. Crossflow might lead to streamwise vortices along a flat plate, and contrary to Gortler vortices, crossflow vortices are co-rotating instead of counter-rotating. We investigated the interactions between these two types of vortices and compared the results with experimental measurements obtained by Bippes (4). The final part of the thesis is the simulation of the secondary time-dependent instability originating from the shear profiles created by the primary streamwise vortices. The onset of the instability is studied by marching both in space and time. If no forcing is prescribed the time-dependent code predicts a steady solution. Time-dependent boundary conditions are then applied by solving the linear stability problem at some streamwise location to obtain the most dangerous streamwise perturbation velocity and the corresponding frequency. We found that the varicose mode is more amplified in the streamwise direction than the sinuous mode. Similarly, if both modes are included in the initial conditions, the varicose mode is still dominant. Furthermore, as the flow evolves downstream, the unsteady behavior exhibits a more complex time-dependence, which was also observed in the experiments of Swearingen and Blackwealder (66). In our computations, higher harmonics are observed near the wall and propagate into the boundary layer.

Le Cunff, Cedric

79

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

80

Boundary layer flow in Trombe wall ducts  

NASA Astrophysics Data System (ADS)

A finite difference material solution technique for the boundary layer equations with an eddy viscosity formulation for turbulence is developed for two-dimensional free convection duct flows. Heat transfer and mass flow rates for air are determined for ducts of uniform but unequal surface temperatures characteristic of Trombe wall ducts. The effect of vents through the wall are modeled as an external pressure drop. Correlations of Nusselt versus Grashof numbers using the duct height as the reference length reduce the heat transfer results for a given vent restriction to a single curve for duct aspect ratios from 10 to 100. Limits to the solution due to the onset of downward flow of air near the cooler surface and the consequent breakdown of the boundary layer assumption are presented.

Pratt, R.; Karaki, S.

1980-07-01

81

X-33 HYPERSONIC BOUNDARY LAYER TRANSITION  

Microsoft Academic Search

Boundary layer and aeroheating characteristics of several X-33 configurations have been experimentally examinedin the Langley 20-Inch Mach 6 Air Tunnel. Global surface heat transfer distributions, surface streamlinepatterns, and shock shapes were measured on 0.013-scale models at Mach 6 in air. Parametric variations includeangles-of-attack of 20-deg, 30-deg, and 40-deg; Reynolds numbers based on model length of 0.9 to 6.6 million; andbody-flap

Scott A. Berry; Thomas J. Horvath; Brian R. Hollis; Richard A. Thompson; H. Harris Hamilton

1999-01-01

82

The boundary layer on compressor cascade blades  

NASA Technical Reports Server (NTRS)

Some redesign of the cascade facility was necessary in order to incoporate the requirements of the LDA system into the design. Of particular importance was the intended use of a combination of suction upstream of the blade pack with diverging pack walls, as opposed to blade pack suction alone, for spanwise dimensionality control. An ARL blade was used to redo some tests using this arrangement. Preliminary testing and boundary layer measurements began on the double circular arc blades.

Deutsch, S.

1981-01-01

83

Entropy Production in Relativistic Jet Boundary Layers  

E-print Network

Hot relativistic jets, passing through a background medium with a pressure gradient p \\propto r^{-\\eta} where 2 power. In previous work, we developed a self-similar description of the boundary layer assuming isentropic flow, but we found that such models respect global energy conservation only for the special case \\eta = 8/3. Here we demonstrate that models with \\eta tidal disruption events and gamma-ray bursts from ...

Kohler, Susanna

2014-01-01

84

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

85

Oblique-shock\\/turbulent-boundary-layer interaction  

Microsoft Academic Search

The present numerical investigation uses well-resolved large-eddy simulations to study the low-frequency unsteady motions observed in shock-wave\\/turbulent-boundary-layer interactions. Details about the numerical aspects of the simulations and the subsequent data analysis can be found in three papers by the authors (Theo. Comput. Fluid Dyn., 23:79--107 (2009); Shock Waves, 19(6):469--478 (2009) and J. of Fluid Mech. (2011)). The fluid dynamics video

Emile Touber; Neil D. Sandham

2010-01-01

86

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

87

Turbulent boundary layers in long computational domains  

NASA Astrophysics Data System (ADS)

A new series of numerical simulations of spatially evolving turbulent boundary layers is discussed. The very long computational domain starts at a low Re?=180, where laminar-turbulent transition is initiated, reaching up to the (computationally very) high Re?=8500. In the domain, the boundary layer develops naturally from the tripping location to the higher Reynolds numbers without any re-injection or recycling procedures. In consequence, this computational setup allows us to study, e.g., the mean flow development and the scaling behavior of the fluctuating energy free from pseudo-periodic effects. However, such domains require a large number of grid points; in the present case up to 10 billion for running well-resolved large-eddy simulation. The present results show excellent agreement with wind-tunnel experiments at similar Re and previous (lower-Re) simulations (both direct and large- eddy simulations). The mean velocity profiles closely follow the correlation proposed by Monkewitz et al. (2007), just about reaching the plateau in the log-law diagnostic function. In a second part, three-dimensional visualizations of the evolving turbulent boundary layer are discussed with special focus on the persistence of transitional flow structures towards higher Reynolds numbers, having a highly unordered appearance.

Schlatter, Philipp; Li, Qiang; Oerlue, Ramis; Brethouwer, Geert; Johansson, Arne V.; Alfredsson, P. Henrik; Henningson, Dan S.

2011-11-01

88

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

89

The nonlinear boundary layer to the Boltzmann equation for cutoff soft potential with physical boundary condition  

Microsoft Academic Search

We consider the nonlinear boundary layer to the Boltzmann equation for cutoff soft potential with physical boundary condition, i.e., the Dirichlet boundary condition with weak diffuse effect. Under the assumption that the distribution function of gas particles tends to a global Maxwellian in the far field, we will show the boundary layer exist if the boundary data satisfy the solvability

Jie Sun; Qianzhu Tian

2011-01-01

90

Numerical Simulations of Hypersonic Boundary Layer Transition  

NASA Astrophysics Data System (ADS)

Numerical schemes for supersonic flows tend to use large amounts of artificial viscosity for stability. This tends to damp out the small scale structures in the flow. Recently some low-dissipation methods have been proposed which selectively eliminate the artificial viscosity in regions which do not require it. This work builds upon the low-dissipation method of Subbareddy and Candler which uses the flux vector splitting method of Steger and Warming but identifies the dissipation portion to eliminate it. Computing accurate fluxes typically relies on large grid stencils or coupled linear systems that become computationally expensive to solve. Unstructured grids allow for CFD solutions to be obtained on complex geometries, unfortunately, it then becomes difficult to create a large stencil or the coupled linear system. Accurate solutions require grids that quickly become too large to be feasible. In this thesis a method is proposed to obtain more accurate solutions using relatively local data, making it suitable for unstructured grids composed of hexahedral elements. Fluxes are reconstructed using local gradients to extend the range of data used. The method is then validated on several test problems. Simulations of boundary layer transition are then performed. An elliptic cone at Mach 8 is simulated based on an experiment at the Princeton Gasdynamics Laboratory. A simulated acoustic noise boundary condition is imposed to model the noisy conditions of the wind tunnel and the transitioning boundary layer observed. A computation of an isolated roughness element is done based on an experiment in Purdue's Mach 6 quiet wind tunnel. The mechanism for transition is identified as an instability in the upstream separation region and a comparison is made to experimental data. In the CFD a fully turbulent boundary layer is observed downstream.

Bartkowicz, Matthew David

91

Turbulent boundary layers subjected to multiple curvatures and pressure gradients  

NASA Technical Reports Server (NTRS)

The effects of abruptly applied cycles of curvatures and pressure gradients on turbulent boundary layers are examined experimentally. Two two-dimensional curved test surfaces are considered: one has a sequence of concave and convex longitudinal surface curvatures and the other has a sequence of convex and concave curvatures. The choice of the curvature sequences were motivated by a desire to study the asymmetric response of turbulent boundary layers to convex and concave curvatures. The relaxation of a boundary layer from the effects of these two opposite sequences has been compared. The effect of the accompaying sequences of pressure gradient has also been examined but the effect of curvature dominates. The growth of internal layers at the curvature junctions have been studied. Measurements of the Gortler and corner vortex systems have been made. The boundary layer recovering from the sequence of concave to convex curvature has a sustained lower skin friction level than in that recovering from the sequence of convex to concave curvature. The amplification and suppression of turbulence due to the curvature sequences have also been studied.

Bandyopadhyay, Promode R.; Ahmed, Anwar

1993-01-01

92

System Identification and Active Control of a Turbulent Boundary Layer  

E-print Network

An experimental investigation is made into the active control of the near-wall region of a turbulent boundary layer using a linear control scheme. System identification in the boundary layer provides optimal transfer ...

Rathnasingham, Ruben

93

The surface roughness and planetary boundary layer  

NASA Astrophysics Data System (ADS)

Applications of the entrainment process to layers at the boundary, which meet the self similarity requirements of the logarithmic profile, have been studied. By accepting that turbulence has dominating scales related in scale length to the height above the surface, a layer structure is postulated wherein exchange is rapid enough to keep the layers internally uniform. The diffusion rate is then controlled by entrainment between layers. It has been shown that theoretical relationships derived on the basis of using a single layer of this type give quantitatively correct factors relating the turbulence, wind and shear stress for very rough surface conditions. For less rough surfaces, the surface boundary layer can be divided into several layers interacting by entrainment across each interface. This analysis leads to the following quantitatively correct formula compared to published measurements. 1 24_2004_Article_BF00877766_TeX2GIFE1.gif {? _w }/{u^* } = ( {2/{9Aa}} )^{{1/4}} ( {1 - 3^{{1/2}{ a/k{d_n }/z{? _w }/{u^* }z/L} )^{{1/4}} = 1.28(1 - 0.945({{? _w }/{u^* }}}) {{z/L}})^{{1/4 where u^* = ( {{tau/?}}^{{1/2}}, ? w is the standard deviation of the vertical velocity, z is the height and L is the Obukhov scale lenght. The constants a, A, k and d n are the entrainment constant, the turbulence decay constant, Von Karman's constant, and the layer depth derived from the theory. Of these, a and A, are universal constants and not empirically determined for the boundary layer. Thus the turbulence needed for the plume model of convection, which resides above these layers and reaches to the inversion, is determined by the shear stress and the heat flux in the surface layers. This model applies to convection in cool air over a warm sea. The whole field is now determined except for the temperature of the air relative to the water, and the wind, which need a further parameter describing sea surface roughness. As a first stop to describing a surface where roughness elements of widely varying sizes are combined this paper shows how the surface roughness parameter, z 0, can be calculated for an ideal case of a random distribution of vertical cylinders of the same height. To treat a water surface, with various sized waves, such an approach modified to treat the surface by the superposition of various sized roughness elements, is likely to be helpful. Such a theory is particularly desirable when such a surface is changing, as the ocean does when the wind varies. The formula, 2 24_2004_Article_BF00877766_TeX2GIFE2.gif {0.118}/{a_s C_D }< z_0< {0.463}/{a_s C_D (u^* )} is the result derived here. It applies to cylinders of radius, r, and number, m, per unit boundary area, where a s = 2rm, is the area of the roughness elements, per unit area perpendicular to the wind, per unit distance downwind. The drag coefficient of the cylinders is C D . The smaller value of z o is for large Reynolds numbers where the larger scale turbulence at the surface dominates, and the drag coefficient is about constant. Here the flow between the cylinders is intermittent. When the Reynolds number is small enough then the intermittent nature of the turbulence is reduced and this results in the average velocity at each level determining the drag. In this second case the larger limit for z 0 is more appropriate.

Telford, James W.

1980-03-01

94

Boundary layer manipulators at high Reynolds numbers  

NASA Technical Reports Server (NTRS)

Airfoil large-eddy breakup (LEBU) devices were tested on an axisymmetric body in the Langley Two Tank up to speeds of 50 ft/sec. NACA-0009, NACA-2412, E-193, and Clark Y contours were tested in single and tandem configurations. At the higher Reynolds numbers local skin friction downstream of the devices showed minimal reductions O (10 percent) and total body drag was increased 1 to 3 percent. At lower Reynolds numbers skin-friction reductions as large as 25 percent were measured and total body drag tended toward net reductions. The loss of effectiveness with increasing Reynolds number of conventional, outer layer devices suggests a decoupling of the outer and inner scales in high Reynolds number turbulent boundary layers.

Anders, J. B.

1990-01-01

95

Sound radiation due to boundary layer transition  

NASA Technical Reports Server (NTRS)

This report describes progress made to date towards calculations of noise produced by the laminar-turbulence transition process in a low Mach number boundary layer formed on a rigid wall. The primary objectives of the study are to elucidate the physical mechanisms by which acoustic waves are generated, to clarify the roles of the fluctuating Reynolds stress and the viscous stress in the presence of a solid surface, and to determine the relative efficiency as a noise source of the various transition stages. In particular, we will examine the acoustic characteristics and directivity associated with three-dimensional instability waves, the detached high-shear layer, and turbulent spots following a laminar breakdown. Additionally, attention will be paid to the unsteady surface pressures during the transition, which provide a source of flow noise as well as a forcing function for wall vibration in both aeronautical and marine applications.

Wang, Meng

1993-01-01

96

Laminar Boundary Layers in Convective Heat Transport  

NASA Astrophysics Data System (ADS)

We study Rayleigh-Bénard convection in the high-Rayleigh-number regime and infinite-Prandtl-number limit, i.e., we consider a fluid in a container that is exposed to strong heating of the bottom and cooling of the top plate in the absence of inertia effects. While the dynamics in the bulk are characterized by a chaotic heat flow, close to the horizontal walls, the fluid is essentially motionless. We derive local bounds on the temperature field in the boundary layers and prove that the temperature profile is essentially linear. The results depend only logarithmically on the system parameters. An important tool in our analysis is a new Hardy-type estimate for the convecting velocity field, which yields control of the fluid motion in the layer. The bounds on the temperature field are derived via local maximal regularity estimates for convection-diffusion equations.

Seis, Christian

2013-12-01

97

Boundary Layer Turbulence Index: Progress and Recent Developments  

E-print Network

A boundary layer turbulence index (TIBL) product has been developed to assess the potential for turbulence in the lower troposphere, generated using RUC-2 numerical model data. The index algorithm approximates boundary layer turbulent kinetic energy by parameterizing vertical wind shear, responsible for mechanical production of TKE, and kinematic heat flux, parameterized by the vertical temperature lapse rate and responsible for buoyant production of TKE. Validation for the TIBL product has been conducted for selected nonconvective wind events during the 2008 winter season over the Idaho National Laboratory mesonet domain. This paper presents studies of four significant wind events between December 2007 and February 2008 over southeastern Idaho. Based on the favorable results highlighted from validation statistics and in the case studies, the RUC TIBL product has demonstrated operational utility in assessing turbulence hazards to low-flying aircraft and ground transportation, and in the assessment of wildfire...

Pryor, Kenneth L

2008-01-01

98

Boundary layer elasto-optic switching in ferroelectric liquid crystals  

NASA Technical Reports Server (NTRS)

The first experimental observation of a change in the director azimuthal angle due to applied shear stress is reported in a sample configuration involving a liquid-crystal-coated top surface exposed directly to gas flow. The electrooptic response caused by the shear stress is large, fast, and reversible. These findings are relevant to the use of liquid crystals in boundary layer investigations on wind tunnel models.

Parmar, D. S.

1992-01-01

99

Sound Radiation from a Turbulent Boundary Layer  

NASA Technical Reports Server (NTRS)

If the restriction of incompressibility in the turbulence problem is relaxed, the phenomenon of energy radiation in the form of sound from the turbulent zone arises. In order to calculate this radiated energy, it is shown that new statistical quantities, such as time-space correlation tensors, have to be known within the turbulent zone in addition to the conventional quantities. For the particular case of the turbulent boundary layer, indications are that the intensity of radiation becomes significant only in supersonic flows. Under these conditions, the recent work of Phillips is examined together with some experimental findings of the author. It is shown that the qualitative features of the radiation field (intensity, directionality) as predicted by the theory are consistent with the measurements; however, even for the highest Mach number flow, some of the assumptions of the asymptotic theory are not yet satisfied in the experiments. Finally, the question of turbulence damping due to radiation is discussed, with the result that in the Mach number range covered by the experiments, the energy lost from the boundary layer due to radiation is a small percentage of the work done by the wall shearing stresses.

Laufer, J.

1961-01-01

100

X-33 Hypersonic Boundary Layer Transition  

NASA Technical Reports Server (NTRS)

Boundary layer and aeroheating characteristics of several X-33 configurations have been experimentally examined in the Langley 20-Inch Mach 6 Air Tunnel. Global surface heat transfer distributions, surface streamline patterns, and shock shapes were measured on 0.013-scale models at Mach 6 in air. Parametric variations include angles-of-attack of 20-deg, 30-deg, and 40-deg; Reynolds numbers based on model length of 0.9 to 6.6 million; and body-flap deflections of 0, 10 and 20-deg. The effects of discrete and distributed roughness elements on boundary layer transition, which included trip height, size, location, and distribution, both on and off the windward centerline, were investigated. The discrete roughness results on centerline were used to provide a transition correlation for the X-33 flight vehicle that was applicable across the range of reentry angles of attack. The attachment line discrete roughness results were shown to be consistent with the centerline results, as no increased sensitivity to roughness along the attachment line was identified. The effect of bowed panels was qualitatively shown to be less effective than the discrete trips; however, the distributed nature of the bowed panels affected a larger percent of the aft-body windward surface than a single discrete trip.

Berry, Scott A.; Horvath, Thomas J.; Hollis, Brian R.; Thompson, Richard A.; Hamilton, H. Harris, II

1999-01-01

101

Halogen chemistry in the trosopheric boundary layer  

NASA Astrophysics Data System (ADS)

Iodine and bromine chemistry can affect the lower troposphere in several important ways: (1), change the oxidizing capacity by destroying ozone and affecting the hydroxyl radical concentration; (2), react efficiently with dimethyl sulphide (in the marine boundary layer) and mercury (in the polar regions); and (3), form ultra-fine particles (iodine oxides are highly condensable), which may contribute to cloud condensation nuclei and hence affect climate. This paper will report measurements of IO, BrO, OIO and I2 , made by the technique of differential optical absorption spectroscopy (DOAS), in several contrasting environments: equatorial clean mid-ocean (Cape Verde); mid-latitude clean coastal (Mace Head, Ireland); polluted coastal (Roscoff, France); and the polar boundary layer (Halley Bay, Antarctica and Hudson Bay, Canada). Both IO and BrO are observed in all these locations at concentrations (> 1 pptv), and so have a major impact on (1) and (2) above. The concentrations of IO in coastal Antarctica, and coastlines rich in certain species of macro-algae, are large enough (> 10 pptv) to promote ultra-fine particle formation. Recently, the first satellite measurements of IO, using the SCIAMACHY instrument on ENVISAT, have been reported by two groups; their results will be compared with the ground-based measurements.

Plane, John M. C.; Mahajan, Anoop; Oetjen, Hilke

102

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

103

SUPERSONIC SHEAR INSTABILITIES IN ASTROPHYSICAL BOUNDARY LAYERS  

SciTech Connect

Disk accretion onto weakly magnetized astrophysical objects often proceeds via a boundary layer (BL) that forms near the object's surface, in which the rotation speed of the accreted gas changes rapidly. Here, we study the initial stages of formation for such a BL around a white dwarf or a young star by examining the hydrodynamical shear instabilities that may initiate mixing and momentum transport between the two fluids of different densities moving supersonically with respect to each other. We find that an initially laminar BL is unstable to two different kinds of instabilities. One is an instability of a supersonic vortex sheet (implying a discontinuous initial profile of the angular speed of the gas) in the presence of gravity, which we find to have a growth rate of order (but less than) the orbital frequency. The other is a sonic instability of a finite width, supersonic shear layer, which is similar to the Papaloizou-Pringle instability. It has a growth rate proportional to the shear inside the transition layer, which is of order the orbital frequency times the ratio of stellar radius to the BL thickness. For a BL that is thin compared to the radius of the star, the shear rate is much larger than the orbital frequency. Thus, we conclude that sonic instabilities play a dominant role in the initial stages of nonmagnetic BL formation and give rise to very fast mixing between disk gas and stellar fluid in the supersonic regime.

Belyaev, Mikhail A.; Rafikov, Roman R., E-mail: rrr@astro.princeton.edu [Department of Astrophysical Sciences, Princeton University, Ivy Lane, Princeton, NJ 08540 (United States)

2012-06-20

104

Supersonic Shear Instabilities in Astrophysical Boundary Layers  

NASA Astrophysics Data System (ADS)

Disk accretion onto weakly magnetized astrophysical objects often proceeds via a boundary layer (BL) that forms near the object's surface, in which the rotation speed of the accreted gas changes rapidly. Here, we study the initial stages of formation for such a BL around a white dwarf or a young star by examining the hydrodynamical shear instabilities that may initiate mixing and momentum transport between the two fluids of different densities moving supersonically with respect to each other. We find that an initially laminar BL is unstable to two different kinds of instabilities. One is an instability of a supersonic vortex sheet (implying a discontinuous initial profile of the angular speed of the gas) in the presence of gravity, which we find to have a growth rate of order (but less than) the orbital frequency. The other is a sonic instability of a finite width, supersonic shear layer, which is similar to the Papaloizou-Pringle instability. It has a growth rate proportional to the shear inside the transition layer, which is of order the orbital frequency times the ratio of stellar radius to the BL thickness. For a BL that is thin compared to the radius of the star, the shear rate is much larger than the orbital frequency. Thus, we conclude that sonic instabilities play a dominant role in the initial stages of nonmagnetic BL formation and give rise to very fast mixing between disk gas and stellar fluid in the supersonic regime.

Belyaev, Mikhail A.; Rafikov, Roman R.

2012-06-01

105

Digital image filtering in visualized boundary layers  

NASA Technical Reports Server (NTRS)

The application of two-dimensional low-pass matched filtering is presented for use in objective processing of digitized flow visualization images in order to identify instantaneous large-scale organized structures in turbulent boundary layers. The images were digitally acquired simultaneously with the outputs of a two-dimensional rake of hot-wire sensors in the field of view of the digital camera. Two-dimensional low wavenumber analysis brought out patterns in the visualization images which consisted of slender inclined structures having an average streamwise scale of 100-200 v/u (tau) and a length on the order of 1-2 delta. The similarly processed two-dimensional streamwise velocity reconstructions reveal similar features. The ensemble statistics indicate that these inclined features brought out by this processing may be a basic flow module in higher Reynolds number flows which links the so-called wall 'bursting' process and the larger outer scale motions.

Corke, T. C.

1984-01-01

106

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

107

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

108

Prediction and control of transition in hypersonic boundary layers  

NASA Technical Reports Server (NTRS)

In this paper, the role of compressible linear stability theory in prediction of boundary layer transition at supersonic and hypersonic speeds is investigated. Computations for sharp cones, using the e exp N method with N = 10, show that the first oblique Tollmien-Schlichting mode is responsible for transition at adiabatic wall conditions for freestream Mach numbers up to 7. For cold walls, the two-dimensional second mode dominates the transition process at lower hypersonic Mach numbers due to the well-known destabilizing effect of cooling on the second mode. It is shown that pressure gradient and suction may be used to stabilize this mode. Some results on the real gas effects on hypersonic boundary-layer stability are presented.

Malik, Mujeeb R.

1987-01-01

109

Leading-edge effects on boundary-layer receptivity  

NASA Astrophysics Data System (ADS)

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

Gatski, Thomas B.; Kerschen, Edward J.

1990-07-01

110

Active Boundary Layer Trip for Supersonic Flows  

NASA Astrophysics Data System (ADS)

The last decade has been full of excitement and success for the hypersonic community thanks to various Scramjet ground tests and launches. These studies have shown promising potentials but the viability to perform commercial flights at Mach 8 is still to be demonstrated. An ideal Scramjet is one which is capable of self- starting over a wide range of angles of attack and Mach number. The Scramjet designer has to ensure that the boundary layer over the inlet ramp is fully turbulent where shocks impact, hence reducing the risks of chocked flow conditions. Most studies have issued the efficiency of roughness trip to trigger the boundary layer transition. At hypersonic speed, heat transfer and drag dramatically increase resulting in skin friction averaging at 40% of the overall drag. This study investigates the possibility of triggering transition using perpendicular air jets on a flat plate place in a hypersonic cross-flow. Experiments were conducted in the von Karman Institute hypersonic blow down wind tunnel H3. This facility is mounted with a Mach 6 contoured nozzles and provides flows with Reynolds number in the range of 10x106/m to 30x106/m. The model consist of a flat plate manufactured with a built -in settling chamber, equipped with a pressure tap and a thermocouple to monitor the jet conditions. A first flat plate was manufactured with a black-coated Plexiglas top, for surface heat transfer measurement using an infrared camera. On the second model, a Upilex sheet equipped with 32 thin film gages was glued, time dependent heat transfer measurements up to 60kHz. The jet injection conditions have been varied and a Mach number of 5.5 kept constant. The flow topology was investigated using fast schlieren techniques and oil flow, in order to gain a better understanding.

Schloegel, F.; Panigua, G.; Tirtey, S.

2009-01-01

111

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

112

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

113

Turbulence measurements in hypersonic boundary layers using constant-temperature anemometry and Reynolds stress measurements in hypersonic boundary layers  

NASA Technical Reports Server (NTRS)

The primary objective in the two research investigations performed under NASA Langley sponsorship (Turbulence measurements in hypersonic boundary layers using constant temperature anemometry and Reynolds stress measurements in hypersonic boundary layers) has been to increase the understanding of the physics of hypersonic turbulent boundary layers. The study began with an extension of constant-temperature thermal anemometry techniques to a Mach 11 helium flow, including careful examinations of hot-wire construction techniques, system response, and system calibration. This was followed by the application of these techniques to the exploration of a Mach 11 helium turbulent boundary layer (To approximately 290 K). The data that was acquired over the course of more than two years consists of instantaneous streamwise mass flux measurements at a frequency response of about 500 kHz. The data are of exceptional quality in both the time and frequency domain and possess a high degree of repeatability. The data analysis that has been performed to date has added significantly to the body of knowledge on hypersonic turbulence, and the data reduction is continuing. An attempt was then made to extend these thermal anemometry techniques to higher enthalpy flows, starting with a Mach 6 air flow with a stagnation temperature just above that needed to prevent liquefaction (To approximately 475 F). Conventional hot-wire anemometry proved to be inadequate for the selected high-temperature, high dynamic pressure flow, with frequent wire breakage and poor system frequency response. The use of hot-film anemometry has since been investigated for these higher-enthalpy, severe environment flows. The difficulty with using hot-film probes for dynamic (turbulence) measurements is associated with construction limitations and conduction of heat into the film substrate. Work continues under a NASA GSRP grant on the development of a hot film probe that overcomes these shortcomings for hypersonic flows. Each of the research tasks performed during the NASA Langley research grants is discussed separately below.

Spina, Eric F.

1995-01-01

114

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

115

PIV Measurements of a Shock Wave\\/Turbulent Boundary Layer Interaction  

Microsoft Academic Search

Particle Image Velocimetry is used to investigate the interaction between an incident planar shock wave and turbulent boundary layer developing on a flat plate at Mach 2.1. The mean velocity profile and deduced skin friction coefficient of the undisturbed boundary layer show good agreement with theory. A particle response assessment establishes the fidelity of the tracer particles. The interaction region

R. A. Humble; F. Scarano; B. W. van Oudheusden; M. Tuinstra

2006-01-01

116

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

117

Simulating supercell thunderstorms in a convective boundary layer: Effects on storm and boundary layer properties  

NASA Astrophysics Data System (ADS)

Nearly all previous numerical simulations of supercell thunderstorms have neglected surface uxes of heat, moisture, and momentum as well as horizontal inhomogeneities in the near-storm environment from resulting dry boundary layer convection. This investigation uses coupled radiation and land-surface schemes within an idealized cloud model to identify the effects of organized boundary layer convection in the form of horizontal convective rolls (HCRs) on the strength, structure, and evolution of simulated supercell thunderstorms. The in uence of HCRs and the importance of their orientation relative to storm motion is tested by comparing simulations with a convective boundary layer (CBL) against those with a horizontally homogeneous base state having the same mean environment. The impact of anvil shading on the CBL is tested by comparing simulations with and without the effects of clouds in the radiative transfer scheme. The results of these simulations indicate that HCRs provide a potentially important source of environmental vertical vorticity in the sheared, near-storm boundary layer. These vorticity perturbations are amplified both beneath the main supercell updraft and along the trailing out ow boundary, leading to the formation of occasionally intense misovortices. HCRs perpendicular to storm motion are found to have a detrimental effect on the strength and persistence of the lowlevel mesocyclone, particularly during its initial development. Though the mean environment is less supportive of low-level rotation with a wind profile conducive to HCRs oriented parallel to storm motion, such HCRs are found to often enhance the low-level mesocyclone circulation. When anvil shading is included, stabilization results in generally weaker low-level mesocyclone circulation, regardless of HCR orientation. Moreover, HCRs diminish in the near-storm environment such that the effects of HCRs on the supercell are mitigated. HCRs are also shown to be a necessary condition for the formation of so-called "feeder clouds" and anking line convection in these simulations. These findings suggest potentially important rami fications regarding both non-mesocyclone and mesocyclone tornadoes in supercell thunderstorms in an environment with active boundary layer convection.

Nowotarski, Christopher J.

118

Electromagnetic precipitation and ducting of particles in turbulent boundary layers  

NASA Technical Reports Server (NTRS)

A method for analyzing magnetic migration of particles in turbulent flows is applied to the prediction of particle trajectories and densities in turbulent aerodynamic boundary layers. Results for conditions typical of aircraft with 30-40 micron particles indicate a large upstream collection and a 5% loss of particles during one pass through the boundary layer. The capacity of the magnetic field to achieve a balance with turbulent diffusion in confining the particles to the boundary layer is discussed.

Davey, K. R.; Melcher, J. R.

1980-01-01

119

Fuselage boundary layer effects on sound propagation and scattering  

NASA Astrophysics Data System (ADS)

The effects of a fuselage and its boundary layer on sound propagation to the fuselage surface and on sound scattering in the farfield were analyzed. A hard-wall infinite cylinder with a boundary layer of both velocity and temperature variations was modeled to simulate the fuselage of an aircraft in flight. Examples for a monopole noise source outside the boundary layer showed considerable noise attenuation on the cylindrical surface forward of the source and much less effect on the downstream side. Data from a transonic wind tunnel test showed the same trends. For enroute and airport community noise, the boundary layer alters the interference pattern caused by the fuselage.

Lu, H. Y.

1989-04-01

120

Incorporation of the planetary boundary layer in atmospheric models  

NASA Technical Reports Server (NTRS)

The topics discussed include the following: perspectives on planetary boundary layer (PBL) measurements; current problems of PBL parameterization in mesoscale models; and convective cloud-PBL interactions.

Moeng, Chin-Hoh; Wyngaard, John; Pielke, Roger; Krueger, Steve

1993-01-01

121

Characterization of Surface and Grain Boundary Layer of Barium Titanate.  

National Technical Information Service (NTIS)

The object of this investigation was to characterize the surface layer of ceramic particles and the grain boundary layer of ceramic materials by a few selected electrical and optical experiments, observe their unusual changes, if any, and infer from them ...

A. K. Goswami

1970-01-01

122

Secondary instabilities in compressible boundary layers  

NASA Technical Reports Server (NTRS)

Secondary instabilities are examined in compressible boundary layers at Mach numbers M(sub infinity) = 0, 0.8, 1.6, and 4.5. It is found that there is a broad-band of highly unstable 3-d secondary disturbances whose growth rates increase with increasing primary wave amplitude. At M(sub infinity) is less than or equal to 1.6, fundamental resonance dominates at relatively high (2-d) primary disturbance amplitude, while subharmonic resonance is characterized by a low (2-d) primary amplitude. At M(sub infinity) = 4.5, the subharmonic instability which arises from the second mode disturbance is the strongest type of secondary instability. The influence of the inclination, theta, of the primary wave with respect to the mean flow direction on secondary instability is investigated at M(sub infinity) = 1.6 for small to moderate values of theta. It is found that the strongest fundamental instability occurs when the primary wave is inclined at 10 deg to the mean flow direction, although a 2-d primary mode yields the most amplified subharmonic. The subharmonic instability at a high value of theta (namely, theta = 45 deg) is also discussed. Finally, a subset of the secondary instability results are compared against direct numerical simulations.

Ng, Lian; Erlebacher, Gordon

1990-01-01

123

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

124

D": The Enigmatic Magmatic Boundary Layer  

NASA Astrophysics Data System (ADS)

The structures and dynamics of the basal region of Earth's mantle lie at the crux of a broad suite of geophysical questions. These include: how Earth's silicate mantle and iron-rich core interact; how volcanic hot spots are generated; how (and at what rate) heat is transported from Earth's deepest interior; and how the Earth's silicate mantle has differentiated through time. The seismically anomalous lowermost 200-300 km of Earth's mantle are characterized by both small or negative changes in velocity with depth, and seismic discontinuities about 250 km above the core-mantle boundary (CMB), and 5-40 km directly above the core. The origin of the former discontinuity remains unclear, but it is likely that it marks the top of the anomalous region at the base of the mantle. The latter discontinuity, characterized by decreases in compressional wave velocity of about -10 percent and changes in shear velocity of about -30 percent (the Ultra-Low Velocity Zone, or ULVZ), is most readily explained by the presence of 5-30 percent partial melt in this zone. The key issue associated with the presence of partial melt at the absolute base of Earth's mantle is not why there is melt present at this depth--this is simply a natural consequence of intercepting the solidus of the basal material--but rather why the melt has not segregated from the coexisting solids into a pure melt layer. The apparent maintenance of a mixture of solid and liquid at the base of Earth's mantle strongly implies that the density difference between melt and solid under these conditions is vanishingly small, and that the solid and liquid compositions have evolved to the point where they coexist at the same depth. Moreover, the partially molten slurry is unlikely to be completely confined to the base of the mantle. The negative shear velocity and small compressional velocity gradients throughout the bulk of D", when coupled with the probable small values of the temperature derivatives of the elastic properties of minerals at the pressures of the core-mantle boundary, are most readily explained by the ubiquitous presence of a small fraction of partial melt (less than 1 percent) distributed throughout the lowermost 200-300 km of the mantle. Seismic evidence indicates that some of this melt may be confined within lamellae distributed at variable depths within D".This ubiquity of melt within the bottom boundary layer of Earth's mantle implies that the geodynamics of the bottom of the mantle is unlikely to be dominated by the solid-state convective circulation present within the bulk of Earth's mantle, but instead by classical magmatic processes: magma ascent and re-equilibration, fractional crystallization, and ubiquitous dissolution and reprecipitation.

Williams, Q.

2003-12-01

125

Green House Gases Flux Model in Boundary Layer  

NASA Astrophysics Data System (ADS)

Analytical dynamic model of the turbulent flux in the three-layer boundary system is presented. Turbulence is described as a presence of the non-zero vorticity. The generalized advection-diffusion-reaction equation is derived for an arbitrary number of components in the flux. The fluxes in the layers are objects for matching requirements on the boundaries between the layers. Different types of transport mechanisms are dominant on the different levels of the layers.

Nurgaliev, Ildus

126

Aeromechanics Analysis of a Boundary Layer Ingesting Fan  

NASA Technical Reports Server (NTRS)

Boundary layer ingesting propulsion systems have the potential to significantly reduce fuel burn but these systems must overcome the challe nges related to aeromechanics-fan flutter stability and forced response dynamic stresses. High-fidelity computational analysis of the fan a eromechanics is integral to the ongoing effort to design a boundary layer ingesting inlet and fan for fabrication and wind-tunnel test. A t hree-dimensional, time-accurate, Reynolds-averaged Navier Stokes computational fluid dynamics code is used to study aerothermodynamic and a eromechanical behavior of the fan in response to both clean and distorted inflows. The computational aeromechanics analyses performed in th is study show an intermediate design iteration of the fan to be flutter-free at the design conditions analyzed with both clean and distorte d in-flows. Dynamic stresses from forced response have been calculated for the design rotational speed. Additional work is ongoing to expan d the analyses to off-design conditions, and for on-resonance conditions.

Bakhle, Milind A.; Reddy, T. S. R.; Herrick, Gregory P.; Shabbir, Aamir; Florea, Razvan V.

2013-01-01

127

Computation of Hypersonic Double Wedge Shock / Boundary Layer Interaction  

E-print Network

Computation of Hypersonic Double Wedge Shock / Boundary Layer Interaction B. Reinartz and J analysis of hypersonic inlet flows has been initiated. For an air breathing hypersonic propulsion system with thick hypersonic boundary layers causes large separation zones that reduce the captured mass flow

128

Turbulence Structure and Wall Signature in Hypersonic Turbulent Boundary Layer  

E-print Network

Turbulence Structure and Wall Signature in Hypersonic Turbulent Boundary Layer Yin-Chiu Kan , Clara and hypersonic turbulent boundary layer datasets from direct numerical simulation (DNS). Contour plots and Marusic5 and Mathis, Hutchins and Marusic16 ). In contrast to supersonic and hypersonic flow regimes

Martín, Pino

129

2-D airfoil tests including side wall boundary layer measurements  

Microsoft Academic Search

The data presented in this contribution were obtained in the DLR Transonic Wind Tunnel Braunschweig. The intent of the experiment was to provide data giving information on the development of the TWB-side wall boundary layer in the presence of a typical transonic airfoil model for further investigation of the influence of the side wall boundary layer on 2-D airfoil measurements.

W. Bartelheimer; K. H. Horstmann; W. Puffert-Meissner

1994-01-01

130

Fluctuating boundary layer on a heated horizontal plate  

Microsoft Academic Search

Summary The paper deals with the boundary layer flow and heat transfer on a horizontal plate whose temperature differs from that of ambient fluid. The basic flow is purely induced by buoyancy which is caused by the difference of temperature in the plate and the fluid. The oscillation in the plate temperature causes a time dependent boundary layer flow and

P. Singh; V. P. Sharma; U. N. Misra

1978-01-01

131

Turbulent boundary layer heat transfer on curved surfaces  

Microsoft Academic Search

Heat transfer measurements for a turbulent boundary layer on a convex and concave, constant-temperature surface are presented. The heat transferred on the convex surface was found to be less than that for a flat surface, while the heat transferred to the boundary layer on the concave surface was greater. It was also found that the heat transferred on the convex

R. E. Mayle; M. F. Blair; F. C. Kopper

1979-01-01

132

Boundary layer receptivity mechanisms relevant to laminar flow control  

Microsoft Academic Search

Receptivity processes by which free-stream acoustic waves generate instability waves in boundary layers are investigated. Concentration is placed on mechanisms associated with local regions of short scale variation in wall suction or admittance distribution. These mechanisms are relevant to laminar flow control technology, in which suction is utilized to control the growth of boundary layer instabilities. The receptivity process requires

Meelan Choudhari

1990-01-01

133

Numerical investigation of condensing steam flow in boundary layers  

Microsoft Academic Search

The paper describes a numerical method for the prediction of condensing steam flow within compressible boundary layers. The method is based on a simple stream function technique, which enables straightforward integration of the nucleation and droplet growth equations in a Lagrangian frame of reference. Calculations show how viscous dissipation and reduced expansion rate within a typical boundary layer influence nucleation

A. J White

2000-01-01

134

Symmetries in Turbulent Boundary Layer Flows  

NASA Technical Reports Server (NTRS)

The objective is the development of a new theory which enables the algorithmic computation of all self-similar mean velocity profiles. The theory is based on Liegroup analysis and unifies a large set of self-similar solutions for the mean velocity of stationary parallel turbulent shear flows. The results include the logarithmic law of the wall, an algebraic law, the viscous sublayer, the linear region in the middle of a Couette flow and in the middle of a rotating channel flow, and a new exponential mean velocity profile not previously reported. Experimental results taken in the outer parts of a high Reynolds number flat-plate boundary layer, strongly support the exponential profile. From experimental as well as from DNS data of a turbulent channel flow the algebraic scaling law could be confirmed in both the center region and in the near wall region. In the case of the logarithmic law of the wall, the scaling with the wall distance arises as a result of the analysis and has not been assumed in the derivation. The crucial part of the derivation of all the different mean velocity profiles is to consider the invariance of the equation for the velocity fluctuations at the same time as the invariance of the equation for the velocity product equations. The latter is the dyad product of the velocity fluctuations with the equation for the velocity fluctuations. It has been proven that all the invariant solutions are also consistent with similarity of all velocity moment equations up to any arbitrary order.

Oberlack, M.

1996-01-01

135

Local boundary layer scales in turbulent Rayleigh-Benard convection  

E-print Network

We compute fully local boundary layer scales in three-dimensional turbulent Rayleigh-Benard convection. These scales are directly connected to the highly intermittent fluctuations of the fluxes of momentum and heat at the isothermal top and bottom walls and are statistically distributed around the corresponding mean thickness scales. The local boundary layer scales also reflect the strong spatial inhomogeneities of both boundary layers due to the large-scale, but complex and intermittent, circulation that builds up in closed convection cells. Similar to turbulent boundary layers, we define inner scales based on local shear stress which can be consistently extended to the classical viscous scales in bulk turbulence, e.g. the Kolmogorov scale, and outer scales based on slopes at the wall. We discuss the consequences of our generalization, in particular the scaling of our inner and outer boundary layer thicknesses and the resulting shear Reynolds number with respect to Rayleigh number. The mean outer thickness s...

Scheel, Janet D

2014-01-01

136

Differential analysis for the turbulent boundary layer on a compressor blade element (including boundary-layer separation)  

NASA Technical Reports Server (NTRS)

A two-dimensional differential analysis is developed to approximate the turbulent boundary layer on a compressor blade element with strong adverse pressure gradients, including the separated region with reverse flow. The predicted turbulent boundary layer thicknesses and velocity profiles are in good agreement with experimental data for a cascade blade, even in the separated region.

Schmidt, J. F.; Todd, C. A.

1974-01-01

137

Boundary Layer Rolls Observed Above and Below a Jet in a Marine Boundary Layer  

NASA Astrophysics Data System (ADS)

We have flown a coherent Doppler wind lidar (DWL) on the Cirpas Twin Otter off the California coast near Monterey since 2003. One scientific purpose of these flights is to understand the relationship between the turbulent fluxes measured on the aircraft or on other platforms and the observed structure of the marine boundary layer (MBL). Two common features are found in the MBL flow: (1) a strong jet at approximately 200 m above the sea surface; and (2) organized large eddies (OLE) in the form of roll vortices that are approximately aligned along the mean wind direction. On two flights (April 13, 2007 and September 30, 2012), the DWL data indicated that roll OLE existed simultaneously both above and below the jet. The DWL winds suggest that the OLE in these layers are sometimes independent and sometimes connected. Standard flux data are obtained on the Twin Otter at flight level, which is nominally 300 m. The 10 Hz wind and temperature data exhibit variability at spatial scales corresponding to the OLE wavelength. We have constructed a nonlinear theoretical model that includes triad wave-wave interactions to test the hypothesis that rolls could form both above and below the jet. This model shows that this is possible and that the rolls in the two layers could have unique characteristics compared to standard boundary layer rolls. The model further shows that the rolls above and below the jet are due to separate instabilities that interact. This is consistent with the observations of both connected and independent OLE above and below the jet. Contrast-enhanced DWL line-of-sight winds. Jet maximum 200 m below aircraft. Typical resonant triad solution for rolls above and below a PBL jet.

Foster, R. C.; Emmitt, G. D.; Godwin, K.; Greco, S.

2013-12-01

138

Further Improvements to Nozzle Boundary Layer Calculations in BLIMPJ  

NASA Technical Reports Server (NTRS)

Further improvements made to advance the current Boundary Layer Integral Matrix Procedure - Version J (BLIMPJ) containing previously modeled simplified calculation methods by accounting for condensed phase, thick boundary layer and free stream turbulence effects are discussed. The condensed phase effects were included through species composition effect considered via input to the code and through particle damping effect considered via a turbulence model. The thrust loss calculation procedure for thick boundary layer effects was improved and the optimization of net thrust with respect to nozzle length was performed. The effects of free stream turbulence were approximately modeled in the turbulence model.

Praharaj, S. C.; Gross, Klaus W.

1989-01-01

139

Shock wave-boundary layer interactions in rarefied gas flows  

NASA Technical Reports Server (NTRS)

A numerical study is presented, using the direct simulation Monte Carlo (DSMC) method, of shock wave-boundary layer interactions in low density supersonic flows. Test cases include two-dimensional, axially-symmetric and three-dimensional flows. The effective displacement angle of the boundary layer is calculated for representative flat plate, wedge, and cone flows. The maximum pressure, shear stress, and heat transfer in the shock formation region is determined in each case. The two-dimensional reflection of an oblique shock wave from a flat plate is studied, as is the three-dimensional interaction of such a wave with a sidewall boundary layer.

Bird, G. A.

1991-01-01

140

Influences on the Height of the Stable Boundary Layer as seen in LES  

SciTech Connect

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 in gross errors in boundary-layer evolution and in prediction of turbulent mixing within the boundary layer.

Kosovic, B; Lundquist, J

2004-06-15

141

Boundary layer structure over areas of heterogeneous heat fluxes  

SciTech Connect

In general circulation models (GCMs), some properties of a grid element are necessarily considered homogeneous. That is, for each grid volume there is associated a particular combination of boundary layer depth, vertical profiles of wind and temperature, surface fluxes of sensible and latent heat, etc. In reality, all of these quantities may exhibit significant spatial variations the grid area, and the larger the area the greater the likely variations. In balancing the benefits of higher resolution against increased computational time and expense, it is useful to consider what the consequences of such subgrid-scale variability may be. Moreover, in interpreting the results of a simulation, one must be able to define an appropriate average value over a grid. There are two aspects of this latter problem: (1) in observations, how does one take a set of discrete or volume-averaged measurements and relate these to properties of the entire domain, and (2) in computations, how can subgrid-scale features be accounted for in the model parameterizations To address these and related issues, two field campaigns were carried out near Boardman, Oregon, in June 1991 and 1992. These campaigns were designed to measure the surface fluxes of latent and sensible heat over adjacent areas with strongly contrasting surface types and to measure the response of the boundary layer to those fluxes. This paper discusses some initial findings from those campaigns.

Doran, J.C. (Pacific Northwest Lab., Richland, WA (United States)); Barnes, F.J. (Los Alamos National Lab., NM (United States)); Coulter, R.L. (Argonne National Lab., IL (United States)); Crawford, T.L. (National Oceanic and Atmospheric Administration, Oak Ridge, TN (United States). Air Resources Lab. Atmospheric Turbulence and Diffusion Div.)

1993-01-01

142

Boundary layer structure over areas of heterogeneous heat fluxes  

SciTech Connect

In general circulation models (GCMs), some properties of a grid element are necessarily considered homogeneous. That is, for each grid volume there is associated a particular combination of boundary layer depth, vertical profiles of wind and temperature, surface fluxes of sensible and latent heat, etc. In reality, all of these quantities may exhibit significant spatial variations the grid area, and the larger the area the greater the likely variations. In balancing the benefits of higher resolution against increased computational time and expense, it is useful to consider what the consequences of such subgrid-scale variability may be. Moreover, in interpreting the results of a simulation, one must be able to define an appropriate average value over a grid. There are two aspects of this latter problem: (1) in observations, how does one take a set of discrete or volume-averaged measurements and relate these to properties of the entire domain, and (2) in computations, how can subgrid-scale features be accounted for in the model parameterizations? To address these and related issues, two field campaigns were carried out near Boardman, Oregon, in June 1991 and 1992. These campaigns were designed to measure the surface fluxes of latent and sensible heat over adjacent areas with strongly contrasting surface types and to measure the response of the boundary layer to those fluxes. This paper discusses some initial findings from those campaigns.

Doran, J.C. [Pacific Northwest Lab., Richland, WA (United States); Barnes, F.J. [Los Alamos National Lab., NM (United States); Coulter, R.L. [Argonne National Lab., IL (United States); Crawford, T.L. [National Oceanic and Atmospheric Administration, Oak Ridge, TN (United States). Air Resources Lab. Atmospheric Turbulence and Diffusion Div.

1993-01-01

143

Particle image velocimetry measurements of a shock wave\\/turbulent boundary layer interaction  

Microsoft Academic Search

Particle image velocimetry is used to investigate the interaction between an incident shock wave and a turbulent boundary\\u000a layer at Mach 2.1. A particle response assessment establishes the fidelity of the tracer particles. The undisturbed boundary\\u000a layer is characterized in detail. The mean velocity field of the interaction shows the incident and reflected shock wave pattern,\\u000a as well as the

R. A. Humble; F. Scarano; B. W. van Oudheusden

2007-01-01

144

Further studies of unsteady boundary layers with flow reversal  

NASA Technical Reports Server (NTRS)

One set of calculations was performed using the first order, time dependent turbulent boundary layer equations, and extended earlier work by Nash and Patel to a wider range of flows. Another set of calculations was performed for laminar flow using the time dependent Navier-Stokes equations. The results of the calculations confirm previous conclusions concerning the existence of a regime of unseparated flow, containing an embedded region of reversal, which is accessible to first order boundary layer theory. However, certain doubts are cast on the precise nature of the events which accompany the eventual breakdown of the theory due to singularity onset. The earlier view that the singularity appears as the final event in a sequence involving rapid thickening of the boundary layer and the formation of a localized region of steep gradients is called into question by the present results. It appears that singularity onset is not necessarily preceded by rapid boundary layer thickening, or even necessarily produces immediate thickening.

Nash, J. F.

1976-01-01

145

Interacting turbulent boundary layer over a wavy wall  

NASA Technical Reports Server (NTRS)

The two dimensional supersonic flow of a thick turbulent boundary layer over a train of relatively small wave-like protuberances is considered. The flow conditions and the geometry are such that there exists a strong interaction between the viscous and inviscid flow. The problem cannot be solved without inclusion of interaction effects due to the occurrence of the separation singularity in classical boundary layer methods. The interacting boundary layer equations are solved numerically using a time-like relaxation method with turbulence effects represented by the inclusion of the eddy viscosity model. Results are presented for flow over a train of up to six waves for Mach numbers of 10 and 32 million/meter, and wall temperature rations (T sub w/T sub 0) of 0.4 and 0.8. Limited comparisons with independent experimental and analytical results are also given. Detailed results on the influence of small protuberances on surface heating by boundary layers are presented.

Polak, A.; Werle, M. J.

1977-01-01

146

Performance of a boundary layer ingesting propulsion system  

E-print Network

This thesis presents an assessment of the aerodynamic performance of an aircraft propulsion system, with embedded engines, in the presence of aircraft fuselage boundary layer ingestion (BLI). The emphasis is on defining ...

Plas, Angélique (Angélique Pascale)

2006-01-01

147

ATMOSPHERIC DISPERSION MODELING BASED UPON BOUNDARY LAYER PARAMETERIZATION  

EPA Science Inventory

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

148

Examining A Hypersonic Turbulent Boundary Layer at Low Reynolds Number  

E-print Network

The purpose of the current study was to answer several questions related to hypersonic, low Reynolds number, turbulent boundary layers, of which available data related to turbulence quantities is scarce. To that end, a unique research facility...

Semper, Michael Thomas

2013-05-15

149

Prehistory of Instability in a Hypersonic Boundary Layer  

Microsoft Academic Search

.   The initial phase of hypersonic boundary-layer transition comprising excitation of boundary-layer modes and their downstream\\u000a evolution from receptivity regions to the unstable region (instability prehistory problem) is considered. The disturbance\\u000a spectrum reveals the following features: (1) the first and second modes are synchronized with acoustic waves near the leading\\u000a edge; (2) further downstream, the first mode is synchronized with

Alexander V. Fedorov; Andrew P. Khokhlov

2001-01-01

150

Acoustic sources in the low Mach number turbulent boundary layer  

NASA Technical Reports Server (NTRS)

The sources of sound production in a low Mach number turbulent boundary layer are examined. The sources are shown to be quadrupole in nature and to result from supersonically convecting wave-number components of the fluctuating Reynolds' normal stresses. The primary Tollmien-Schlichting instability of the boundary layer is found to radiate no sound. Analysis of various vortical phenomena suggests that the primary source is the process of formation of horseshoe vortices, with viscous sublayer bursts a possible secondary source.

Hardin, Jay C.

1991-01-01

151

Observations on streamwise vortices in laminar and turbulent boundary layers  

NASA Technical Reports Server (NTRS)

The frequent but often unsuspected presence of streamwise vortices in nominally two dimensional laminar and turbulent boundary layers and some of their consequences are described. Since there is no body of systematic information on streamwise vortices imbedded in boundary layers, a number of issues concerning their occurrence and behavior are discussed in the form of a set of succinct observations. Desirable experimental and numerical research to remedy the current lack of knowledge is recommended.

Morkovin, M. V.

1979-01-01

152

Shock wave oscillation driven by turbulent boundary layer fluctuations  

NASA Technical Reports Server (NTRS)

Pressure fluctuations due to the interaction of a shock wave with a turbulent boundary layer were investigated. A simple model is proposed in which the shock wave is convected from its mean position by velocity fluctuations in the turbulent boundary layer. Displacement of the shock is assumed limited by a linear restoring mechanism. Predictions of peak root mean square pressure fluctuation and spectral density are in excellent agreement with available experimental data.

Plotkin, K. J.

1972-01-01

153

New algebraic approaches to classical boundary layer problems  

Microsoft Academic Search

Classical non-steady boundary layer equations are fundamental nonlinear partial differential equations in the boundary layer\\u000a theory of fluid dynamics. In this paper, we introduce various schemes with multiple parameter functions to solve these equations\\u000a and obtain many families of new explicit exact solutions with multiple parameter functions. Moreover, symmetry transformations\\u000a are used to simplify our arguments. The technique of moving

Xiao Ping Xu

2011-01-01

154

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

155

Study on ground clutter prevention fences for boundary layer radars  

NASA Astrophysics Data System (ADS)

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

Rao, Qinjiang; Hashiguchi, Hiroyuki; Fukao, Shoichiro

2003-04-01

156

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

157

Dayside boundary layer under northward IMF: A Cluster perspective  

NASA Astrophysics Data System (ADS)

It has been proposed that the Low Latitude Boundary Layer (LLBL) was formed by high-latitude reconnection when the IMF is northward. To study the relationship between the low-latitude boundary layer and high-latitude boundary layer under northward IMF condition, we present statistical results based on 3 years of data obtained by Cluster when these spacecraft were in the vicinity of the dayside magnetopause during northward IMF. In total 341 cases of Cluster crossing of Low Latitude Boundary Layer (LLBL) and High Latitude Boundary Layer (HLBL) (according to the definition by Phan et al [1996a,b]) have been analyzed in detail in order to study the relation between the LLBL and the HLBL. The plasma density, temperature, velocity, energetic particle flux and magnetic field geometry change across the magnetopause under northward IMF were analyzed by a superposed epoch analysis. It has been suggested [Zong et al, 2004] that the solar wind plasma density decreases in the magnetospheric boundary region in an exponential mannerwith an e-folding distance of 1000 km during northward IMF in a case study. In this statistical study, we explore further the relation between the distance to magnetopause and the penetration of solar wind plasma inside the magnetopause. Phan, T. D., and G. Paschmann, Low-latitude dayside magnetopause and boundary layer for high magnetic sheath: 1. Structure and motion, J. Geophys. Res.,101, 7801-7815, 1996 Phan, T. D., G. Paschmann, and B. U. O. Sonnerup, Low-latitude dayside magnetopause and boundary layer for high magnetic sheath: 2. Occurrence of magnetic reconnection, J. Geophys. Res.,101, 7817-7828, 1996 Zong, Q.-G., T. A. Fritz, H. Spence, K. Oksavik, Z.-Y. Pu, A. Korth, and P. W. Daly, Energetic particle sounding of the magnetopause: A contribution by Cluster/RAPID, J. Geophys. Res.,109, A04207, 2004

Zhang, H.; Fritz, T.; Zong, Q.; Daly, P.

2004-12-01

158

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

NASA Technical Reports Server (NTRS)

The capability of the OVERFLOW code to accurately compute high-speed turbulent boundary layers and turbulent shock-boundary layer interactions is being evaluated. Configurations being investigated include a Mach 2.87 flat plate to compare experimental velocity profiles and boundary layer growth, a Mach 6 flat plate to compare experimental surface heat transfer,a direct numerical simulation (DNS) at Mach 2.25 for turbulent quantities, and several Mach 3 compression ramps to compare computations of shock-boundary layer interactions to experimental laser doppler velocimetry (LDV) data and hot-wire data. The present paper describes outlines the study and presents preliminary results for two of the flat plate cases and two small-angle compression corner test cases.

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

2006-01-01

159

A preliminary assessment of the Titan planetary boundary layer  

NASA Technical Reports Server (NTRS)

Results of a preliminary assessment of the characteristic features of the Titan planetary boundary are addressed. These were derived from the combined application of a patched Ekman surface layer model and Rossby number similarity theory. Both these models together with Obukhov scaling, surface speed limits and saltation are discussed. A characteristic Akman depth of approximately 0.7 km is anticipated, with an eddy viscosity approximately equal to 1000 sq cm/s, an associated friction velocity approximately 0.01 m/s, and a surface wind typically smaller than 0.6 m/s. Actual values of these parameters probably vary by as much as a factor of two or three, in response to local temporal variations in surface roughness and stability. The saltation threshold for the windblown injection of approximately 50 micrometer particulates into the atmosphere is less than twice the nominal friction velocity, suggesting that dusty breezes might be an occassional feature of the Titan meteorology.

Allison, Michael

1992-01-01

160

Boundary layer receptivity to unsteady free-stream pressure gradients  

NASA Technical Reports Server (NTRS)

A linear triple-deck theory is applied to an examination of the response of a boundary layer to short-scale variations in an unsteady free-stream disturbance field. Two-dimensional incompressible flows are considered, and a locally-parallel Blasius mean flow is assumed. A simple pulsating pressure source and a traveling pressure field in the free-stream are modeled by introducing appropriate pressure sources in the upper deck of the triple-deck structure. The modification in unsteady thickness is obtained for these cases, the results are related to the Tollmien-Schlichting instability wave, and the generation of unstable Tollmien-Schlichting waves for both experiments modeled is confirmed.

Heinrich, Roland A. E.; Kerschen, Edward J.; Gatski, Thomas B.

1990-01-01

161

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...

162

Stabilization of Hypersonic Boundary Layers by Porous Coatings  

Microsoft Academic Search

A second-mode stability analysis has been performed for a hypersonic boundary layer on a wall covered by a porous coating with equally spaced cylindrical blind microholes. Massive reduction of the second mode amplié - cation is found to be due to the disturbance energy absorption by the porous layer. This stabilization effect was demonstrated by experiments recently conducted on a

Alexander V. Fedorov; Norman D. Malmuth; Adam Rasheed; Hans G. Hornung

2001-01-01

163

Methodology for automatic boundary layer detection using ground penetrating radar  

Microsoft Academic Search

Ground penetrating radars are u of application. A challenging task is t interpretation of measurement data wherefore experts' help is necessary. In this paper an au layer detection based on selective time-freq proposed. Developed algorithms are tested on acquired with an integrated GPR system o excavator in open cast mining. Keywords—Automatic boundary layer frequency analysis; GPR; open cast mining I.

Tobias Mathiak; Martin Kesting; Ludger Overmeyer; Veronika Gau

2011-01-01

164

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

165

Optimal Control of Shock Wave Turbulent Boundary Layer Interactions Using Micro-Array Actuation  

NASA Technical Reports Server (NTRS)

The intent of this study on micro-array flow control is to demonstrate the viability and economy of Response Surface Methodology (RSM) to determine optimal designs of micro-array actuation for controlling the shock wave turbulent boundary layer interactions within supersonic inlets and compare these concepts to conventional bleed performance. The term micro-array refers to micro-actuator arrays which have heights of 25 to 40 percent of the undisturbed supersonic boundary layer thickness. This study covers optimal control of shock wave turbulent boundary layer interactions using standard micro-vane, tapered micro-vane, and standard micro-ramp arrays at a free stream Mach number of 2.0. The effectiveness of the three micro-array devices was tested using a shock pressure rise induced by the 10 shock generator, which was sufficiently strong as to separate the turbulent supersonic boundary layer. The overall design purpose of the micro-arrays was to alter the properties of the supersonic boundary layer by introducing a cascade of counter-rotating micro-vortices in the near wall region. In this manner, the impact of the shock wave boundary layer (SWBL) interaction on the main flow field was minimized without boundary bleed.

Anderson, Bernhard H.; Tinapple, Jon; Surber, Lewis

2006-01-01

166

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

167

Shock-like structures in the tropical cyclone boundary layer  

NASA Astrophysics Data System (ADS)

This paper presents high horizontal resolution solutions of an axisymmetric, constant depth, slab boundary layer model designed to simulate the radial inflow and boundary layer pumping of a hurricane. Shock-like structures of increasing intensity appear for category 1-5 hurricanes. For example, in the category 3 case, the u>(?u/?r>) term in the radial equation of motion produces a shock-like structure in the radial wind, i.e., near the radius of maximum tangential wind the boundary layer radial inflow decreases from approximately 22 m s-1 to zero over a radial distance of a few kilometers. Associated with this large convergence is a spike in the radial distribution of boundary layer pumping, with updrafts larger than 22 m s-1 at a height of 1000 m. Based on these model results, it is argued that observed hurricane updrafts of this magnitude so close to the ocean surface are attributable to the dry dynamics of the frictional boundary layer rather than moist convective dynamics. The shock-like structure in the boundary layer radial wind also has important consequences for the evolution of the tangential wind and the vertical component of vorticity. On the inner side of the shock the tangential wind tendency is essentially zero, while on the outer side of the shock the tangential wind tendency is large due to the large radial inflow there. The result is the development of a U-shaped tangential wind profile and the development of a thin region of large vorticity. In many respects, the model solutions resemble the remarkable structures observed in the boundary layer of Hurricane Hugo (1989).

Williams, Gabriel J.; Taft, Richard K.; McNoldy, Brian D.; Schubert, Wayne H.

2013-06-01

168

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

169

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

Microsoft Academic Search

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

H. Reichenbach; P. Neuwald; A. L. Kuhl

1992-01-01

170

Screening of selective radiation in a boundary layer  

Microsoft Academic Search

An analysis of numerous calculations of the flow and radiative-convective heat exchange in a hypersonic shock layer near a blunt body, both at an impermeable surface [1] and in the presence of ablation [1–4], made it possible to establish some relationships connected with the screening in the boundary layer of radiation from the high-temperature part of the shock layer. It

T. V. Kondranin; I. N. Kuz'minskii

1978-01-01

171

Diamagnetic boundary layers - A kinetic theory. [for collisionless magnetized plasmas  

NASA Technical Reports Server (NTRS)

A kinetic theory is presented for boundary layers associated with MHD tangential 'discontinuities' in a collisionless magnetized plasma, such as those observed in the solar wind. The theory consists of finding self-consistent solutions of Vlasov's equation and Maxwell's equation for stationary one-dimensional boundary layers separating two Maxwellian plasma states. Layers in which the current is carried by electrons are found to have a thickness of the order of a few electron gyroradii, but the drift speed of the current-carrying electrons is found to exceed the Alfven speed, and accordingly such layers are not stable. Several types of layers in which the current is carried by protons are discussed; in particular, cases are considered in which the magnetic-field intensity, direction, or both, changed across the layer. In every case, the thickness was of the order of a few proton gyroradii, and the field changed smoothly, although the characteristics depended somewhat on the boundary conditions. The drift speed was always less than the Alfven speed, consistent with stability of such structures. These results are consistent with observations of boundary layers in the solar wind near 1 AU.

Lemaire, J.; Burlaga, L. F.

1976-01-01

172

Modeling the Urban Boundary and Canopy Layers  

EPA Science Inventory

Today, we are confronted with increasingly more sophisticated application requirements for urban modeling. These include those that address emergency response to acute exposures from toxic releases, health exposure assessments from adverse air quality, energy usage, and character...

173

Optimum forebody shaping for axisymmetric submersibles with turbulent boundary layers and BLO (Boundary Layer Control) afterbodies  

NASA Astrophysics Data System (ADS)

One objective of the Advanced Undersea Vehicle (AUV) program is to design a low drag vehicle. The approach in this investigation is boundary layer control by means of an annular suction slot located on the afterbody. Although wind tunnel data showed significant reduction in propulsive power over conventional shapes, an attempt was made to achieve further reduction by means of forebody shaping. Two methods were used to vary the geometric parameters for this analysis. The direct method, based on the mathematical development of the Series 58 bodies, allows the definition of a shape by a fifth-order polynomial based on the four fundamental parameters of fineness ratio, nose radius of curvature, location of maximum thickness, and prismatic coefficient. The inverse method allows various velocity distributions to define the body shape. The shapes derived by this method have flat velocity distributions and show similar trends to the polynomial shapes (about 3-percent reduction in propulsive power). The range of fineness ratios analyzed was from 1 to 10 at a volume-based Reynolds number of 3.2 million. In the range of 2.5 to 8, fineness ratio did not affect propulsive power more than 6 percent. A maximum improvement of 3 percent as shown by varying the meridian section.

Neumann, B. J.

1983-07-01

174

Turbulent boundary-layer structure of flows over freshwater biofilms  

NASA Astrophysics Data System (ADS)

The structure of the turbulent boundary-layer for flows over freshwater biofilms dominated by the diatom Tabellaria flocculosa was investigated. Biofilms were grown on large test plates under flow conditions in an Australian hydropower canal for periods up to 12 months. Velocity-profile measurements were obtained using LDV in a recirculating water tunnel for biofouled, smooth and artificially sandgrain roughened surfaces over a momentum thickness Reynolds number range of 3,000-8,000. Significant increases in skin friction coefficient of up to 160 % were measured over smooth-wall values. The effective roughnesses of the biofilms, k s, were significantly higher than their physical roughness measured using novel photogrammetry techniques and consisted of the physical roughness and a component due to the vibration of the biofilm mat. The biofilms displayed a k-type roughness function, and a logarithmic relationship was found between the roughness function and roughness Reynolds number based on the maximum peak-to-valley height of the biofilm, R t. The structure of the boundary layer adhered to Townsend's wall-similarity hypothesis even though the scale separation between the effective roughness height and the boundary-layer thickness was small. The biofouled velocity-defect profiles collapsed with smooth and sandgrain profiles in the outer region of the boundary layer. The Reynolds stresses and quadrant analysis also collapsed in the outer region of the boundary layer.

Walker, J. M.; Sargison, J. E.; Henderson, A. D.

2013-12-01

175

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

176

Characterizing Boundary Layer Properties for Estimating Urban Greenhouse Gas Emissions  

NASA Astrophysics Data System (ADS)

The Indianapolis Flux Experiment (INFLUX) aims to develop, evaluate and improve methodologies for quantification of greenhouse gas fluxes from urban areas through a multi-year modeling and observational study. The study incorporates measurements of greenhouse gases from periodic aircraft observations as well as from a surface-based network of towers in the area. Recently, we installed a scanning Doppler lidar east of downtown Indianapolis to characterize boundary layer properties important for the aircraft and modeling studies. A scan sequence, including conical scans, vertical scans along two orthogonal directions, and zenith staring is repeated every 20 minutes. The lidar measurements of the radial velocity and backscatter intensity are processed to estimate boundary layer depth, turbulent mixing, aerosol distribution, and wind speed and direction. These lidar-derived boundary layer parameters are used in conjunction with the aircraft greenhouse gas concentration measurements in mass-balance studies and for investigating model performance. The lidar wind profile measurements can also be ingested into models to improve inverse flux estimates. We present here an overview of the first several months of lidar observations from Indianapolis, including performance evaluation, comparison with model estimates, diurnal and seasonal variability of the measurements, and use of the data for model ingest. We also discuss different techniques for estimating boundary layer depth from the observations and the application for mass-balance studies, and introduce plans for deploying a second instrument to study horizontal variability of the measured boundary layer properties.

Hardesty, R. M.; Brewer, A.; Sandberg, S.; Weickmann, A.; Sweeney, C.; Karion, A.; Davis, K. J.; Shepson, P. B.; Lauvaux, T.; Cambaliza, M. L.; Miles, N. L.; Whetstone, J. R.

2013-12-01

177

Linear and nonlinear PSE for compressible boundary layers  

NASA Technical Reports Server (NTRS)

Compressible stability of growing boundary layers is studied by numerically solving the partial differential equations under a parabolizing approximation. The resulting parabolized stability equations (PSE) account for nonparallel as well as nonlinear effects. Evolution of disturbances in compressible flat-plate boundary layers are studied for freestream Mach numbers ranging from 0 to 4.5. Results indicate that the effect of boundary-layer growth is important for linear disturbances. Nonlinear calculations are performed for various Mach numbers. Two-dimensional nonlinear results using the PSE approach agree well with those from direct numerical simulations using the full Navier-Stokes equations while the required computational time is less by an order of magnitude. Spatial simulation using PSE were carried out for both the fundamental and subharmonic type breakdown for a Mach 1.6 boundary layer. The promising results obtained show that the PSE method is a powerful tool for studying boundary-layer instabilities and for predicting transition over a wide range of Mach numbers.

Chang, Chau-Lyan; Malik, Mujeeb R.; Erlebacher, Gordon; Hussaini, M. Yousuff

1993-01-01

178

Strong vortex/boundary layer interactions. I - Vortices high  

NASA Technical Reports Server (NTRS)

Detailed measurements with hot-wires and pressure probes are presented for the interaction between a turbulent longitudinal vortex pair with 'common flow' down, and a turbulent boundary layer. The interaction has a larger value of the vortex circulation parameter, and therefore better represents many aircraft/vortex interactions, than those studied previously. The vortices move down towards the boundary layer, but only the outer parts of the vortices actually enter it. Beneath the vortices the boundary layer is thinned by lateral divergence to the extent that it almost ceases to grow. Outboard of the vortices the boundary layer is thickened by lateral convergence. The changes in turbulence structure parameters in the boundary layer appear to be due to the effects of 'extra-rate-of-strain' produced by lateral divergence (or convergence) and by free-stream turbulence. The effect of the interaction on the vortices (other than the inviscid effect of the image vortices below the surface) is small. The flow constitutes a searching test case for prediction methods for three-dimensional turbulent flows.

Cutler, A. D.; Bradshaw, P.

1993-01-01

179

Anisotropic Mesh Adaptivity for Turbulent Flows with Boundary Layers  

NASA Astrophysics Data System (ADS)

Turbulent flows are found everywhere in nature and are studied, analyzed and simulated using various experimental and numerical tools. For computational analysis, a variety of turbulence models are available and the accuracy of these models in capturing the phenomenon depends largely on the mesh spacings, especially near the walls, in the boundary layer region. Special semi-structured meshes called "mesh boundary layers" are widely used in the CFD community in simulations of turbulent flows, because of their graded and orthogonal layered structure. They provide an efficient way to achieve very fine and highly anisotropic mesh spacings without introducing poorly shaped elements. Since usually the required mesh spacings to accurately resolve the flow are not known a priori to the simulations, an adaptive approach based on a posteriori error indicators is used to achieve an appropriate mesh. In this study, we apply the adaptive meshing techniques to turbulent flows with a focus on boundary layers. We construct a framework to calculate the critical wall normal mesh spacings inside the boundary layers based on the flow physics and the knowledge of the turbulence model. This approach is combined with numerical error indicators to adapt the entire flow region. We illustrate the effectiveness of this hybrid approach by applying it to three aerodynamic flows and studying their superior performance in capturing the flow structures in detail. We also demonstrate the capabilities of the current developments in parallel boundary layer mesh adaptation by applying them to two internal flow problems. We also study the application of adaptive boundary layer meshes to complex geometries like multi element wings. We highlight the advantage of using such techniques for superior wake and tip region resolution by showcasing flow results. We also outline the future direction for the adaptive meshing techniques to be useful to the large scale flow computations.

Chitale, Kedar C.

180

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

181

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

182

Three dimensional boundary layer separation in supersonic flow  

NASA Technical Reports Server (NTRS)

An account is given of a detailed experimental investigation of three dimensional boundary layer separation in supersonic flow. In investigating three dimensional effects on supersonic separation, models were chosen which exhibited departures from two dimensional flow in the simplest way. The plane compression corner was replaced by a plate attached to a swept back wedge formed by two obliquely intersecting planes. Maintaining a constant tunnel Mach number of 2.5, surface pressure measurements were made on these models at static orifices spaced along the centerline and along three parallel lines. The flow parameters in the boundary layer and separated regions adjacent to the model surface were measured by traversing hot wire and pitot probes. The traverses were taken across the boundary layer and reversed flow regions in a direction normal to the body surface; they were made in several vertical planes, including the plane of symmetry.

Bachalo, W. D.; Holt, M.

1976-01-01

183

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

184

A compilation of unsteady turbulent boundary-layer experimental data  

NASA Technical Reports Server (NTRS)

An extensive literature search was conducted and those experiments related to unsteady boundary layer behavior were cataloged. In addition, an international survey of industrial, university, and governmental research laboratories was made in which new and ongoing experimental programs associated with unsteady turbulent boundary layer research were identified. Pertinent references were reviewed and classified based on the technical emphasis of the various experiments. Experiments that include instantaneous or ensemble averaged profiles of boundary layer variables are stressed. The experimental apparatus and flow conditions are described and summaries of acquired data and significant conclusions are summarized. Measurements obtained from the experiments which exist in digital form were stored on magnetic tape. Instructions are given for accessing these data sets for further analysis.

Carr, L. W.

1981-01-01

185

On subcritical instability of the attachment line boundary layer  

NASA Astrophysics Data System (ADS)

Subcritical instability in the two-dimensional incompressible attachment-line boundary layer remains a topic of debate, after the apparently contradictory results of Hall and Malik (1986) on one hand and Spalart (1988) and Jimenez et al. (1990) on the other. Direct Numerical Simulation (DNS) results are presented, aiming at addressing this question. Extensive numerical experimentation has been performed and all results obtained suggest that the two-dimensional model equations describing leading edge boundary layer (LEBL) flow doe not support solutions growing subcritically in Reynolds number, although the nonlinear neutral loop is seen to bifurcate from its linear counterpart in a manner consistent with the predictions of the theory of Hall and Malik (1986). Nonlinear neutral loops have been obtained suggesting that the two-dimensional model LEBL flow is similar to the classical Blasius boundary layer in terms of the location, in parameter space, of the experimentally observed naturally occurring instability waves.

Theofilis, Vassilios

1994-12-01

186

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

187

Supersonic separated turbulent boundary - layer over a wavy wall  

NASA Technical Reports Server (NTRS)

A prediction method is developed for calculating distributions of surface heating rates, pressure and skin friction over a wavy wall in a two-dimensional supersonic flow. Of particular interest is the flow of thick turbulent boundary layers. The surface geometry and the flow conditions considered are such that there exists a strong interaction between the viscous and inviscid flow. First, using the interacting turbulent boundary layer equations, the problem is formulated in physical coordinates and then a reformulation of the governing equations in terms of Levy-Lees variables is given. Next, a numerical scheme for solving interacting boundary layer equations is adapted. A number of modifications which led to the improvement of the numerical algorithm are discussed. Finally, results are presented for flow over a train of up to six waves at various flow conditions.

Polak, A.; Werle, M. J.

1977-01-01

188

Effects of forebody geometry on subsonic boundary-layer stability  

NASA Technical Reports Server (NTRS)

As part of an effort to develop computational techniques for design of natural laminar flow fuselages, a computational study was made of the effect of forebody geometry on laminar boundary layer stability on axisymmetric body shapes. The effects of nose radius on the stability of the incompressible laminar boundary layer was computationally investigated using linear stability theory for body length Reynolds numbers representative of small and medium-sized airplanes. The steepness of the pressure gradient and the value of the minimum pressure (both functions of fineness ratio) govern the stability of laminar flow possible on an axisymmetric body at a given Reynolds number. It was found that to keep the laminar boundary layer stable for extended lengths, it is important to have a small nose radius. However, nose shapes with extremely small nose radii produce large pressure peaks at off-design angles of attack and can produce vortices which would adversely affect transition.

Dodbele, Simha S.

1990-01-01

189

Three dimensional shock wave/boundary layer interactions  

NASA Astrophysics Data System (ADS)

An investigation into a three-dimensional, curved shock wave interacting with a three-dimensional, curved boundary layer on a slender body is presented. Three different nose profiles mounted on a cylindrical body were tested in a supersonic wind tunnel and numerically simulated by solving the Navier-Stokes equations. The conical and hemispherical nose profiles tested were found to generate shock waves of sufficient strength to separate the boundary layer on the cylinder, while the shock wave generated by the ogival profile did not separate the boundary layer. For the separated flow, separation was found to occur predominantly on the windward side of the cylinder with the lee-side remaining shielded from the direct impact of the incident shock wave. A thickening of the boundary layer on the lee-side of all the profiles was observed, and in the conical and hemispherical cases this leads to the re-formation of the incident shock wave some distance away from the surface of the cylinder. A complex reflection pattern off the shock wave/boundary layer interaction (SWBLI) was also identified for the separated flow cases. For comparative purposes, an inviscid simulation was performed using the hemispherical profile. Significant differences between the viscous and inviscid results were noted including the absence of a boundary layer leading to a simplified shock wave reflection pattern forming. The behaviour of the incident shock wave on the lee-side of the cylinder was also affected with the shock wave amalgamating on the surface of the cylinder instead of away from the surface as per the viscous case. Test data from the wind tunnel identified two separation lines present on the cylindrical surface of the hemispherical SWBLI generator. The pair of lines were not explicitly evident in the original CFD simulations run, but were later identified in a high-resolution simulation.

Mowatt, S.; Skews, B.

2011-09-01

190

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

191

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

192

Carbon vaporization into a nonequilibrium, stagnation-point boundary layer  

NASA Technical Reports Server (NTRS)

The heat transfer to the stagnation point of an ablating carbonaceous heat shield, where both the gas-phase boundary layer and the heterogeneous surface reactions are not in chemical equilibrium, is examined. Specifically, the nonequilibrium changes in the mass fraction profiles of carbon species calculated for frozen flow are studied. A set of equations describing the steady-state, nonequilibrium laminar boundary layer in the axisymmetric stagnation region, over an ablating graphite surface, is solved, with allowance for the effects of finite rate of carbon vaporization.

Suzuki, T.

1978-01-01

193

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

194

Boundary layer on a circular cylinder in axial flow  

NASA Astrophysics Data System (ADS)

Quasi-similar solutions are presented for the boundary layer on a circular cylinder in axial flow, using a Keller-Box numerical scheme to solve for velocity components rather than a stream function. The solutions extend earlier results considerably and cover a wide range of cylinder radii from very small (needle case) to very large (Blasius case). Velocity profiles, skin friction, and boundary-layer thickness parameters are presented and compared with earlier results. The results are given in sufficient detail to provide useful guidelines for engineering applications.

Sawchuk, S. P.; Zamir, M.

1992-06-01

195

Characteristics of turbulence in boundary layer with zero pressure gradient  

NASA Technical Reports Server (NTRS)

The results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented. Measurements with the hot-wire anemometer were made of turbulent energy and turbulent shear stress, probability density and flattening factor of u-fluctuation (fluctuation in x-direction), spectra of turbulent energy and shear stress, and turbulent dissipation. The importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated. Attention is given to the energy balance and the intermittent character of the outer region of the boundary layer. Also several interesting features of the spectral distribution of the turbulent motions are discussed.

Klebanoff, P S

1955-01-01

196

Hypersonic crossing shock-wave/turbulent-boundary-layer interactions  

NASA Technical Reports Server (NTRS)

Experimental data for two three-dimensional intersecting shock-wave/turbulent boundary-layer interaction flows at Mach 8.3 are presented. The test bodies, composed of two sharp fins fastened to a flat plate test bed, were designed to generate flows with varying degrees of pressure gradient, boundary-layer separation, and turning angle. The data include surface pressure and heat transfer distributions as well as mean flow field surveys both in the undisturbed and interaction regimes. The data are presented in a convenient form to be used to validate existing or future computational models of these hypersonic flows.

Kussoy, M. I.; Horstman, K. C.; Horstman, C. C.

1993-01-01

197

Stratospheric ozone in the planetary boundary layer  

SciTech Connect

The impact of stratospheric ozone associated with rapid transport processes at remote Whiteface Mountain, New York was studied using /sup 7/Be as a stratospheric tracer. The seasonal variation of /sup 7/Be concentration suggests that the greatest impact of stratospheric ozone at Whiteface Mountain occurs during late spring and early summer consistent with the time when the impact of tropopause folding should be maximum. This is also the time when tropospheric photochemical ozone production is likely to be maximum. The analysis of 33 cases with peak /sup 7/Be > 175 fCi/m/sup 3/ shows strong correlation between averaged O/sub 3/ and /sup 7/Be concentration. Assuming peak /sup 7/Be is associated with rapid transport we estimate 12 ppbv as the 24th average increased O/sub 3/ due to tropopause folding events. Also the average SO/sub 4/2/sup -/ concentration peaked a day after /sup 7/Be suggesting the backside of the weather system responsible for rapid subsidence also favors transport to this site from distant urban-industrial centers. Thus, photochemical pollution episodes may be intensified by stratospheric subsidence.

Dutkiewicz, V.A.; Husain, L.; Rusheed, A.

1980-01-01

198

Physical description of boundary-layer transition: Experimental evidence  

NASA Technical Reports Server (NTRS)

The problems of understanding the origins of turbulent flow and transition to turbulent flow are the most important unsolved problems of fluid mechanics and aerodynamics. It is well known that the stability, transition, and turbulent characteristics of bounded shear layers are fundamentally different from those of free shear layers. Likewise, the stability, transition, and turbulent characteristics of open systems are fundamentally different from those of closed systems. Because of the influence of indigenous disturbances, surface geometry and roughness, sound, heat transfer, and ablation, it is not possible to develop general prediction schemes for transition location and the nature of turbulent structures in boundary-layer flows. At the present time no mathematical model exists that can predict the transition Reynolds number on a flat plate. The recent progress in this area is encouraging, in that a number of distinct transition mechanisms have been found experimentally. The theoretical work finds them to be amplitude and Reynolds-number dependent. The theory remains rather incomplete with regard to predicting transition. Amplitude and spectral characteristics of the disturbances inside the laminar viscous layer strongly influence which type of transition occurs. The major need in this area is to understand how freestream disturbances are entrained into the boundary layer, i.e., to answer the question of receptivity. We refer receptivity to the mechanism(s) that cause freestream disturbances to enter the boundary layer and create the initial amplitudes for unstable waves.

Saric, William S.

1994-01-01

199

Receptivity of Supersonic Boundary Layers to Acoustic Disturbances  

NASA Technical Reports Server (NTRS)

Boundary layer receptivity to two-dimensional slow and fast acoustic waves is investigated by solving Navier-Stokes equations for Mach 4.5 flow over a flat plate with a finite-thickness leading edge. Higher order spatial and temporal schemes are employed to obtain the solution whereby the flat-plate leading edge region is resolved by providing a sufficiently refined grid. The results show that the instability waves are generated in the leading edge region and that the boundary-layer is much more receptive to slow acoustic waves (by almost a factor of 20) as compared to the fast waves. Hence, this leading-edge receptivity mechanism is expected to be more relevant in the transition process for high Mach number flows. The effect of acoustic wave incidence angle is also studied and it is found that the receptivity of the boundary layer on the windward side (with respect to the acoustic forcing) decreases by more than a factor of 4 when the incidence angle is increased from 0 to 45 deg. However, the receptivity coefficient for the leeward side is found to vary relatively weakly with the incidence angle. The effect of leading-edge thickness is also studied and bluntness is found to stabilize the boundary layer. The relative significance of fast acoustic waves is enhanced in the presence of bluntness.

Malik, Mujeeb R.; Balakumar, P.

2005-01-01

200

CFD simulation of the atmospheric boundary layer: wall function problems  

Microsoft Academic Search

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

Bert Blocken; Ted Stathopoulos; Jan Carmeliet

2007-01-01

201

Large-Scale Streamwise Turbulent Structures in Hypersonic Boundary Layers  

E-print Network

Prior research in the field of boundary layer turbulence has identified streamwise-elongated large-scale turbulence structures in both low speed compressible and high speed (M=2.0) flow. No experimental work has been done in any flow of M> or =3...

English, Benjamin L.

2013-04-22

202

Numerical calculations of shock-wave/boundary-layer flow interactions  

NASA Technical Reports Server (NTRS)

The paper presents results of calculations for 2-D supersonic turbulent compression corner flows. The results seem to indicate that the newer, improved kappa-epsilon models offer limited advantages over the standard kappa-epsilon model in predicting the shock-wave/boundary-layer flows in the 2-D compression corner over a wide range of corner angles and flow conditions.

Huang, P. G.; Liou, W. W.

1994-01-01

203

Modelling hyporheic exchange: From the boundary layer to the basin  

E-print Network

Modelling hyporheic exchange: From the boundary layer to the basin M.J. Stewardson a , S.B. Grant a processes at the basin-scale including nutrient cycling and retention; movements of organisms to complete hydrological connectivity at the basin-scale but this is not true for the vertical dimension. Understanding

Marusic, Ivan

204

Plasma characteristics of the boundary layer in the Martian magnetosphere  

Microsoft Academic Search

Plasma and magnetic field data from circular orbits of the Phobos 2 spacecraft near Mars are examined to provide a description of the plasma properties of inner regions of the Mars magnetosheath and the boundary layer\\/plasma mantle. The data are analyzed in the VB coordinate system which is reasonable for draping magnetospheres of nonmagnetized planets and comets. It is shown

E. Dubinin; K. Sauer; R. Lundin; O. Norberg; J.-G. Trotignon; K. Schwingenschuh; M. Delva; W. Riedler

1996-01-01

205

Fluid-dynamic boundary layers in CFB boilers  

Microsoft Academic Search

The characteristics of the fluid-dynamic boundary layer (FBL) were studied in the 12 MW circulating fluidized bed boiler at Chalmers University of Technology. For a comparison, some additional measurements were made in a 165 MW CFB boiler. The FBL consists mainly of the falling film of particles at the furnace wall. The distance from the wall to the zero net

Wennan Zhang; Filip Johnsson; Bo Leckner

1995-01-01

206

Aircraft measurements within the planetary boundary layer over water  

Microsoft Academic Search

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

R. Markson

1977-01-01

207

EVOLUTION CALCULATIONS FOR TURBULENT BOUNDARY LAYERS APPROACHING EQUILIBRIUM SINK FLOW  

E-print Network

EVOLUTION CALCULATIONS FOR TURBULENT BOUNDARY LAYERS APPROACHING EQUILIBRIUM SINK FLOW I. MARUSIC ap- proaching a smooth wall equilibrium sink ow is con- sidered. The closure problem is described gradient parameter where is the displacement thickness, p is the freestream static pressure, 0 is the wall

Marusic, Ivan

208

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

209

Propagation of propeller tone noise through a fuselage boundary layer  

NASA Technical Reports Server (NTRS)

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

Hanson, D. B.; Magliozzi, B.

1984-01-01

210

Boundary layer effects above a Himalayan valley near Mount Everest  

Microsoft Academic Search

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

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

2007-01-01

211

SUPPRESSION DYNAMICS OF A BOUNDARY-LAYER DIFFUSION FLAME  

Microsoft Academic Search

Flame stability, suppression, and extinction phenomena are intrinsically time dependent. Solutions are obtained for unsteady, full Navier-Stokes equations using Barely Implicit Correction to Flux Corrected Transport (BIC- FCT) algorithms for a boundary-layer diffusion flame formed over a flat porous plate, through which a fuel gas is injected uniformly. The solutions include calculation of surface temperature and composition as functions of

Ramagopal Ananth; Chuka C. Ndubizu; P. A. Tatem; Gopal Patnaik; K. Kailasanath

2001-01-01

212

TURBULENCE PARAMETERS IMPACTING DISPERSION IN AN URBAN CONVECTIVE BOUNDARY LAYER  

EPA Science Inventory

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

213

Turbulent dispersion in the Atmospheric Convective Boundary Layer  

Microsoft Academic Search

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

A. Dosio

2005-01-01

214

Active methods to measure multilayer Planetary Boundary Layer Dynamics  

Microsoft Academic Search

The need to characterize in a robust way Planetary Boundary Layer (PBL) Heights is crucial as in air quality forcast and transport models. Incorrect determination of PBL heights can severely distort the surface level PM2.5 predictions crucial in determining whether New York City is in compliance. It has been amply demonstrated that lidar systems have repeatedly proven to be valuable

Y. Wu; B. Gross

2009-01-01

215

Inorganic bromine in the marine boundary layer: a critical review  

Microsoft Academic Search

The cycling of inorganic bromine in the marine boundary layer (mbl) has received increased attention in recent years. Bromide, a constituent of sea water, is injected into the atmosphere in association with sea-salt aerosol by breaking waves on the ocean surface. Measurements reveal that supermicrometer sea-salt aerosol is substantially depleted in bromine (often exceeding 50%) relative to conservative tracers, whereas

R. Sander; W. C. Keene; A. A. P. Pszenny; R. Arimoto; G. P. Ayers; E. Baboukas; J. M. Cainey; P. J. Crutzen; R. A. Duce; G. Hönninger; B. J. Huebert; W. Maenhaut; N. Mihalopoulos; V. C. Turekian; R. van Dingenen

2003-01-01

216

ACTIVE FLOW CONTROL ON A BOUNDARY-LAYER-INGESTING INLET  

Microsoft Academic Search

Boundary layer ingestion (BLI) is explored as a means to improve overall system performance for a Blended Wing Body configuration. The benefits of BLI for vehicle system performance benefit are assessed with a process derived from first principles suitable for highly-integrated propulsion systems. This performance evaluation process provides a framework within which to assess the benefits of an integrated BLI

Susan Althoff Gorton; Lewis R. Owens; Luther N. Jenkins; Brian G. Allan; Ernest P. Schuster

217

Simulations of Serpentine Plasma Actuators in a Laminar Boundary Layer  

E-print Network

as components of active and passive flow control systems. One concern with in applying these actuators of 11 American Institute of Aeronautics and Astronautics #12; Velocity ratio I Integrated value. Introduction Boundary layer control is one aspect of fluid dynamics that is becoming increasingly more

Roy, Subrata

218

ON HYDROMAGNETIC STRESSES IN ACCRETION DISK BOUNDARY LAYERS  

SciTech Connect

Detailed calculations of the physical structure of accretion disk boundary layers, and thus their inferred observational properties, rely on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. In order to shed light on physically viable mechanisms for angular momentum transport in this inner disk region, we examine the generation of hydromagnetic stresses and energy density in differentially rotating backgrounds with angular frequencies that increase outward in the shearing-sheet framework. We isolate the modes that are unrelated to the standard MRI and provide analytic solutions for the long-term evolution of the resulting shearing MHD waves. We show that, although the energy density of these waves can be amplified significantly, their associated stresses oscillate around zero, rendering them an inefficient mechanism to transport significant angular momentum (inward). These findings are consistent with the results obtained in numerical simulations of MHD accretion disk boundary layers and challenge the standard assumption of efficient angular momentum transport in the inner disk regions. This suggests that the detailed structure of turbulent MHD accretion disk boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity.

Pessah, Martin E. [Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen O (Denmark); Chan, Chi-kwan, E-mail: mpessah@nbi.dk, E-mail: ckch@nordita.org [NORDITA, Roslagstullsbacken 23, 106 91 Stockholm (Sweden)

2012-05-20

219

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

220

Numerical simulation of boundary-layer disturbance evolution  

E-print Network

, or interact with, flow- control devices in the form of compliant panels, suction slots evolution in incompressible boundary layers frequently require an assumption of laminar flow experiments, it is usually only possible to maintain laminar flow over a very limited range of Reynolds

Davies, Christopher

221

Calculation of turbulent boundary layer wall pressure spectra  

NASA Astrophysics Data System (ADS)

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 for the point wall pressure spectrum (based on the work of Uberoi and Kovasznay). The representation of the wall pressure spectrum used accounts for the effect of a finite sized transducer on the measured wall pressure spectrum. By accounting for the area averaging effect of finite sized transducers on the measured turbulent boundary layer wall pressure spectra, it was possible to use Chase's rigorous formulations for the wavevector-frequency spectrum instead of the point pressure spectrum representations which assume an infinitely small measurement sensor. Results from the numerical integrations are compared to recent experimental data to determine which model of the wavevector-frequency spectrum most accurately predicts measured turbulent boundary layer wall pressure spectra. Data from experiments using fluids with a wide range of physical properties (air, water, and glycerine) are used for comparison purposes. Using the selected model, new empirical constants are established for use in the model for each fluid under consideration. Justification for use of the new empirical constants is given, and current limitations of the wavevector-frequency models are discussed.

Capone, D. E.; Lauchle, G. C.

1993-08-01

222

Numerical Calculations of Shock-Wave/Boundary-Layer Flow Interactions  

NASA Technical Reports Server (NTRS)

The paper presents results of calculations for 2-D supersonic turbulent compression corner flows. The results seem to indicate that the newer, improved kappa-epsilon models offer limited advantages over the standard kappa-epsilon model in predicting the shock-wave/boundary-layer flows in the 2-D compression corner over a wide range of corner angles and flow conditions.

Huang, P. G.; Liou, W. W.

1994-01-01

223

Flow phenomena peculiar to calculation of compressible turbulent boundary layers  

NASA Technical Reports Server (NTRS)

Calculation procedures for compressible turbulent boundary layers were based upon techniques, modeling constants, etc., developed originally for the low speed case. Significant differences and new or altered physics which occur in the compressible case were considered, as compared with the low speed situation. Possible pitfalls and sources of inaccuracy in the calculations were indicated.

1977-01-01

224

Measuring techniques for wall shearing stress in turbulent boundary layer  

NASA Astrophysics Data System (ADS)

This paper presents the calibration results and the comparison of various measuring techniques for wall shearing stress measurement, both in compressible and incompressible turbulent boundary layers. Techniques including Preston tube, Stanton tube, sublayer fence, surface hot-film and computational Preston tube method are discussed.

Dai, Changhui; Liu, Tianshu; Teng, Yongguang; Ming, Xiao

1988-05-01

225

ANALYTICAL PARAMETERIZATIONS OF DIFFUSION: THE CONVECTIVE BOUNDARY LAYER  

EPA Science Inventory

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

226

Retinal layer segmentation of macular OCT images using boundary classification.  

PubMed

Optical coherence tomography (OCT) has proven to be an essential imaging modality for ophthalmology and is proving to be very important in neurology. OCT enables high resolution imaging of the retina, both at the optic nerve head and the macula. Macular retinal layer thicknesses provide useful diagnostic information and have been shown to correlate well with measures of disease severity in several diseases. Since manual segmentation of these layers is time consuming and prone to bias, automatic segmentation methods are critical for full utilization of this technology. In this work, we build a random forest classifier to segment eight retinal layers in macular cube images acquired by OCT. The random forest classifier learns the boundary pixels between layers, producing an accurate probability map for each boundary, which is then processed to finalize the boundaries. Using this algorithm, we can accurately segment the entire retina contained in the macular cube to an accuracy of at least 4.3 microns for any of the nine boundaries. Experiments were carried out on both healthy and multiple sclerosis subjects, with no difference in the accuracy of our algorithm found between the groups. PMID:23847738

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

2013-07-01

227

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

228

The boundary layer on a finite flat plate  

NASA Astrophysics Data System (ADS)

The problem of finding the flow over a finite flat plate aligned with a uniform free stream is revisited. Multigrid is used to obtain accurate numerical solutions up to a Reynolds number of 4000. Fourier boundary conditions keep the computational domain small, with no loss of accuracy. Near the trailing edge, excellent agreement with first-order triple-deck theory is found. However, previous comparisons between computations, experiments, and triple-deck theory are shown to be misleading: In fact, triple-deck theory only accounts for half the drag excess (that part not due to the first-order Blasius boundary layer) even at R=4000. The remainder is shown to be due to, among other things, a large displacementlike effect in the boundary layer, i.e., an O(R-1) increase in skin friction extending over the whole plate.

McLachlan, Robert I.

1991-02-01

229

Modeling Disturbance Dynamics in Transitional and Turbulent Boundary Layers  

NASA Technical Reports Server (NTRS)

The dynamics of an ensemble of linear disturbances in boundary-layer flows at various Reynolds numbers is studied through an analysis of the transport equations for the mean disturbance kinetic energy and energy dissipation rate. Effects of adverse and favorable pressure-gradients on the disturbance dynamics are also included in the analysis. Unlike the fully turbulent regime where nonlinear phase scrambling of the fluctuations affects the flow field even in proximity to the wall, the early stage transition regime fluctuations studied here are influenced across the boundary layer by the solid boundary. In addition, the dominating dynamics in the disturbance kinetic energy equation is governed by the energy production, pressure-transport and viscous diffusion - also in contrast to the fully turbulent regime. For the disturbance dissipation rate, a dynamic balance exists between the destruction and diffusion of dissipation.

Grosch, C. E.; Gatski, T. B. (Technical Monitor)

2002-01-01

230

Transitional and turbulent boundary layer with heat transfer  

NASA Astrophysics Data System (ADS)

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

Wu, Xiaohua; Moin, Parviz

2010-08-01

231

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

232

Linear segmentation algorithm for detecting layer boundary with lidar.  

PubMed

The automatic detection of aerosol- and cloud-layer boundary (base and top) is important in atmospheric lidar data processing, because the boundary information is not only useful for environment and climate studies, but can also be used as input for further data processing. Previous methods have demonstrated limitations in defining the base and top, window-size setting, and have neglected the in-layer attenuation. To overcome these limitations, we present a new layer detection scheme for up-looking lidars based on linear segmentation with a reasonable threshold setting, boundary selecting, and false positive removing strategies. Preliminary results from both real and simulated data show that this algorithm cannot only detect the layer-base as accurate as the simple multi-scale method, but can also detect the layer-top more accurately than that of the simple multi-scale method. Our algorithm can be directly applied to uncalibrated data without requiring any additional measurements or window size selections. PMID:24216909

Mao, Feiyue; Gong, Wei; Logan, Timothy

2013-11-01

233

Turbulent boundary layer on a convex, curved surface  

NASA Technical Reports Server (NTRS)

The effects of strong convex curvature on boundary layer turbulence were investigated. The data gathered on the behavior of Reynolds stress suggested the formulation of a simple turbulence model. Three sets of data were taken on two separate facilities. Both rigs had flow from a flat surface, over a convex surface with 90 deg of turning, and then onto a flat recovery surface. The geometry was adjusted so that, for both rigs, the pressure gradient along the test surface was zero - thus avoiding any effects of streamwise acceleration on the wall layers. Results show that after a sudden introduction of curvature, the shear stress in the outer part of the boundary layer is sharply diminished and is even slightly negative near the edge. The wall shear also drops off quickly downstream. In contrast, when the surface suddenly becomes flat again, the wall shear and shear stress profiles recover very slowly towards flat wall conditions.

Gillis, J. C.; Johnston, J. P.; Kays, W. M.; Moffat, R. J.

1980-01-01

234

Hypersonic Turbulent Boundary-Layer and Free Sheer Database Datasets  

NASA Technical Reports Server (NTRS)

A critical assessment and compilation of data are presented on attached hypersonic turbulent boundary layers in pressure gradients and compressible turbulent mixing layers. Extensive searches were conducted to identify candidate experiments, which were subjected to a rigorous set of acceptance criteria. Accepted datasets are both tabulated and provided in machine-readable form. The purpose of this database effort is to make existing high quality data available in detailed form for the turbulence-modeling and computational fluid dynamics communities. While significant recent data were found on the subject of compressible turbulent mixing, the available boundary-layer/pressure-gradient experiments are all older ones of which no acceptable data were found at hypersonic Mach numbers.

Settles, Gary S.; Dodson, Lori J.

1993-01-01

235

Provenance of the K/T boundary layers  

NASA Technical Reports Server (NTRS)

An array of chemical, physical and isotopic evidence indicates that an impact into oceanic crust terminated the Cretaceous Period. Approximately 1500 cu km of debris, dispersed by the impact fireball, fell out globally in marine and nonmarine environments producing a 2 to 4 mm thick layer (fireball layer). In North American locales, the fireball layer overlies a 15 to 25 mm thick layer of similar but distinct composition. This 15 to 25 mm layer (ejecta layer) may represent approximately 1000 cu km of lower energy ejecta from a nearby impact site. Isotopic and chemical evidence supports a mantle provenance for the bulk of the layers. The extraordinary REE pattern of the boundary clays was modelled as a mixture of oceanic crust, mantle, and approximately 10 percent continental material. The results are presented. If the siderophiles of the ejecta layer were derived solely from the mantle, a test may be available to see if the siderophile element anomaly of the fireball layer had an extraterrestrial origin. Radiogenic Os-187 is depleted in the mantle relative to an undifferentiated chondritic source. Os-187/Os-186 ratios of 1.049 and 1.108 were calculated for the ejecta and fireball layers, respectively.

Hildebrand, A. R.; Boynton, W. V.

1988-01-01

236

Investigations on entropy layer along hypersonic hyperboloids using a defect boundary layer  

NASA Technical Reports Server (NTRS)

A defect approach coupled with matched asymptotic expansions is used to derive a new set of boundary layer equations. This method ensures a smooth matching of the boundary layer with the inviscid solution. These equations are solved to calculate boundary layers over hypersonic blunt bodies involving the entropy gradient effect. Systematic comparisons are made for both axisymmetric and plane flows in several cases with different Mach and Reynolds numbers. After a brief survey of the entropy layer characteristics, the defect boundary layer results are compared with standard boundary layer and full Navier-Stokes solutions. The entropy gradient effects are found to be more important in the axisymmetric case than in the plane one. The wall temperature has a great influence on the results through the displacement effect. Good predictions can be obtained with the defect approach over a cold wall in the nose region, with a first order solution. However, the defect approach gives less accurate results far from the nose on axisymmetric bodies because of the thinning of the entropy layer.

Brazier, J. P.; Aupoix, B.; Cousteix, J.

1992-01-01

237

Footprint characteristics of scalar concentration in the convective boundary layer  

NASA Astrophysics Data System (ADS)

Footprint characteristics for passive scalar concentration in the convective boundary layer (CBL) are investigated. A backward Lagrangian stochastic (LS) dispersion model and a large eddy simulation (LES) model are used in the investigation. Typical characteristics of the CBL and their responses to the surface heterogeneity are resolved from the LES. Then the turbulence fields are used to drive the backward LS dispersion. To remedy the spoiled description of the turbulence near the surface, Monin-Obukhov similarity is applied to the lowest LES level and the surface for the modeling of the backward LS dispersion. Simulation results show that the footprint within approximately 1 km upwind predominates in the total contribution. But influence from farther distances also exists and is even slightly greater than that from closer locations. Surface heterogeneity may change the footprint pattern to a certain degree. A comparison to three analytical models provides a validation of the footprint simulations, which shows the possible influence of along-wind turbulence and the large eddies in the CBL, as well as the surface heterogeneity.

Guo, Xiaofeng; Cai, Xuhui

2005-11-01

238

On buffer layers as non-reflecting computational boundaries  

NASA Technical Reports Server (NTRS)

We examine an absorbing buffer layer technique for use as a non-reflecting boundary condition in the numerical simulation of flows. One such formulation was by Ta'asan and Nark for the linearized Euler equations. They modified the flow inside the buffer zone to artificially make it supersonic in the layer. We examine how this approach can be extended to the nonlinear Euler equations. We consider both a conservative and a non-conservative form modifying the governing equations in the buffer layer. We compare this with the case that the governing equations in the layer are the same as in the interior domain. We test the effectiveness of these buffer layers by a simulation of an excited axisymmetric jet based on a nonlinear compressible Navier-Stokes equations.

Hayder, M. Ehtesham; Turkel, Eli L.

1996-01-01

239

Scaling of heat transfer augmentation due to mechanical distortions in hypervelocity boundary layers  

NASA Astrophysics Data System (ADS)

We examine the response of hypervelocity boundary layers to global mechanical distortions due to concave surface curvature. Surface heat transfer and visual boundary layer thickness data are obtained for a suite of models with different concave surface geometries. Results are compared to predictions using existing approximate methods. Near the leading edge, good agreement is observed, but at larger pressure gradients, predictions diverge significantly from the experimental data. Up to a factor of five underprediction is reported in regions with greatest distortion. Curve fits to the experimental data are compared with surface equations. We demonstrate that reasonable estimates of the laminar heat flux augmentation may be obtained as a function of the local turning angle for all model geometries, even at the conditions of greatest distortion. This scaling may be explained by the application of Lees similarity. As a means of introducing additional local distortions, vortex generators are used to impose streamwise structures into the boundary layer. The response of the large scale vortices to an adverse pressure gradient is investigated. Surface streak evolution is visualized over the different surface geometries using fast response pressure sensitive paint. For a flat plate baseline case, heat transfer augmentation at similar levels to turbulent flow is measured. For the concave geometries, increases in heat transfer by factors up to 2.6 are measured over the laminar values. The scaling of heat transfer with turning angle that is identified for the laminar boundary layer response is found to be robust even in the presence of the imposed vortex structures.

Flaherty, W.; Austin, J. M.

2013-10-01

240

Behavior of Turbulent Structures within a Mach 5 Mechanically Distorted Boundary Layer  

E-print Network

High-resolution particle image velocimetry (PIV) is employed to resolve the velocity fields within a Mach 4.9 mechanically distorted turbulent boundary layer (Re? ? 40,000). The goal of this study is to directly observe the mechanisms responsible...

Peltier, Scott Jacob

2013-08-05

241

A general integral form of the boundary-layer equation for incompressible flow with an application to the calculation of the separation point of turbulent boundary layers  

NASA Technical Reports Server (NTRS)

A general integral form of the boundary-layer equation, valid for either laminar or turbulent incompressible boundary-layer flow, is derived. By using the experimental finding that all velocity profiles of the turbulent boundary layer form essentially a single-parameter family, the general equation is changed to an equation for the space rate of change of the velocity-profile shape parameter. The lack of precise knowledge concerning the surface shear and the distribution of the shearing stress across turbulent boundary layers prevented the attainment of a reliable method for calculating the behavior of turbulent boundary layers.

Tetervin, Neal; Lin, Chia Chiao

1951-01-01

242

Observed bottom boundary layer transport and uplift on the continental shelf adjacent to a western boundary current  

NASA Astrophysics Data System (ADS)

boundary currents strongly influence the dynamics on the adjacent continental shelf and in particular the cross-shelf transport and uplift through the bottom boundary layer. Four years of moored in situ observations on the narrow southeastern Australian shelf (in water depths of between 65 and 140 m) were used to investigate bottom cross-shelf transport, both upstream (30°S) and downstream (34°S) of the separation zone of the East Australian Current (EAC). Bottom transport was estimated and assessed against Ekman theory, showing consistent results for a number of different formulations of the boundary layer thickness. Net bottom cross-shelf transport was onshore at all locations. Ekman theory indicates that up to 64% of the transport variability is driven by the along-shelf bottom stress. Onshore transport in the bottom boundary layer was more intense and frequent upstream than downstream, occurring 64% of the time at 30°S. Wind-driven surface Ekman transport estimates did not balance the bottom cross-shelf flow. At both locations, strong variability was found in bottom water transport at periods of approximately 90-100 days. This corresponds with periodicity in EAC fluctuations and eddy shedding as evidenced from altimeter observations, highlighting the EAC as a driver of variability in the continental shelf waters. Ocean glider and HF radar observations were used to identify the bio-physical response to an EAC encroachment event, resulting in a strong onshore bottom flow, the uplift of cold slope water, and elevated coastal chlorophyll concentrations.

Schaeffer, A.; Roughan, M.; Wood, J. E.

2014-08-01

243

Grain boundary layers in nanocrystalline ferromagnetic zinc oxide  

NASA Astrophysics Data System (ADS)

The complete solubility of an impurity in a polycrystal increases with decreasing grain size, because the impurity dissolves not only in the crystallite bulk but also on the grain boundaries. This effect is especially strong when the adsorption layers (or the grain boundary phases) are multilayer. For example, the Mn solubility in the nanocrystalline films (where the size of grains is ˜20 nm) is more than three times greater than that in the ZnO single crystals. The thin nanocrystalline Mn-doped ZnO films in the Mn concentration range 0.1-47 at % have been obtained from organic precursors (butanoates) by the "liquid ceramic" method. They have ferromagnetic properties, because the specific area of the grain boundaries in them is greater than the critical value [B.B. Straumal et al., Phys. Rev. B 79, 205206 (2009)]. The high-resolution electron transmission microscopy studies show that the ZnO nanocrystalline grains with the wurtzite lattice are separated by amorphous layers whose thickness increases with the Mn concentration. The morphology of these layers differs greatly from the structure of the amorphous prewetting films on the grain boundaries in the ZnO:Bi2O3 system.

Straumal, B. B.; Myatiev, A. A.; Straumal, P. B.; Mazilkin, A. A.; Protasova, S. G.; Goering, E.; Baretzky, B.

2010-09-01

244

BLSTA: A boundary layer code for stability analysis  

NASA Technical Reports Server (NTRS)

A computer program is developed to solve the compressible, laminar boundary-layer equations for two-dimensional flow, axisymmetric flow, and quasi-three-dimensional flows including the flow along the plane of symmetry, flow along the leading-edge attachment line, and swept-wing flows with a conical flow approximation. The finite-difference numerical procedure used to solve the governing equations is second-order accurate. The flow over a wide range of speed, from subsonic to hypersonic speed with perfect gas assumption, can be calculated. Various wall boundary conditions, such as wall suction or blowing and hot or cold walls, can be applied. The results indicate that this boundary-layer code gives velocity and temperature profiles which are accurate, smooth, and continuous through the first and second normal derivatives. The code presented herein can be coupled with a stability analysis code and used to predict the onset of the boundary-layer transition which enables the assessment of the laminar flow control techniques. A user's manual is also included.

Wie, Yong-Sun

1992-01-01

245

Edge Plasma Boundary Layer Generated By Kink Modes in Tokamaks  

SciTech Connect

This paper describes the structure of the electric current generated by external kink modes at the plasma edge using the ideally conducting plasma model. It is found that the edge current layer is created by both wall touching and free boundary kink modes. Near marginal stability, the total edge current has a universal expression as a result of partial compensation of the ?-functional surface current by the bulk current at the edge. The resolution of an apparent paradox with the pressure balance across the plasma boundary in the presence of the surface currents is provided.

L.E. Zakharov

2010-11-22

246

Boundary layer studies related to fusion theory. Final report  

SciTech Connect

The described work studied the boundary between closed and open field lines in EBT geometry, with emphasis on the microstability properties. These properties were established primarily for drift waves in the lower hybrid range of frequencies. The transport due to these modes was evaluated by a self-consistent treatment, using quasilinear models in a plasma diffusion code. The model was benchmarked against the EDT experimental results from ORNL and the sensitivity to transport model established. Viscosity was estimated to be negligible compared with anomalous transport. Drift wave turbulence gave a boundary layer size much more consistent with experiment than either collisional transport or Bohm diffusion.

None

1981-09-29

247

LES of a Stratified Boundary Layer under an Oscillating Current  

NASA Astrophysics Data System (ADS)

A numerical study based on large-eddy simulation (LES) is performed in the case of an oscillating tidal flow with a uniform ambient stratification. Here, the Reynolds number Re?=U0?s/?=1790 (U0= maximum amplitude of the outer flow, ?s= ?2 ?/? is the Stokes layer thickness, ? is the kinematic viscosity of the fluid and ? the angular frequency of the oscillatory current), and N?^2/2?= 500 where N? is the buoyancy frequency of the overlying stratified layer. Turbulence appears at a tidal phase of approximately ?/4 and is sustained throughout the deceleration phase (?/2layer between the wall and the thermocline. For both the stratified and unstratified cases, there is a log layer over a significant extent of the tidal cycle. Our unstratified flow results are verified against the numerical simulations of Salon et ; al (2007) %. JFM, 2007, vol 570, 253-296 and experimental data of Jensen et ; al. (1987). %JFM, 1987, vol 206, 256-297. In the presence of stratification, the boundary layer height decreases substantially and the wall shear stress increases slightly with respect to the unstratified case. Stratification effects on boundary layer turbulence and on the thermal field including the formation and collapse of the thermocline will be discussed.

Gayen, Bishakhdatta; Sarkar, Sutanu; Taylor, John

2008-11-01

248

Urban air pollution modelling and measurements of boundary layer height  

NASA Astrophysics Data System (ADS)

An urban field trial has been undertaken with the aim of assessing the performance of the boundary layer height (BLH) determination of two models: the Met Office Unified Model (UM) and a Gaussian-type plume model, ADMS. Pulsed Doppler lidar data were used to measure mixing layer height and cloud base heights for a variety of meteorological conditions over a 3 week period in July 2003. In this work, the daily growth and decay of the BLH from the lidar data and model simulations for 5 days are compared. The results show that although the UM can do a good job of reproducing the boundary layer growth, there are occasions where the BLH is overestimated by 30-100%. Within dispersion models it is the BLH that effectively limits the height to which pollution disperses, so these results have very important implications for pollution dispersion modelling. The results show that correct development of the boundary layer in the UM is critically dependant on morning cloud cover. The ADMS model is used routinely by local authorities in the UK for local air-quality forecasting. The ADMS model was run under three settings; an 'urban' roughness, a 'rural' roughness and a 'transition' roughness. In all cases, the 'urban' setting over estimated the BLH and is clearly a poor predictor of urban BLH. The 'transition' setting, which distinguishes between the meteorological data input site and the dispersion modelling site, gave the best results under the well mixed conditions of the trial.

Davies, F.; Middleton, D. R.; Bozier, K. E.

249

A modeling study of marine boundary layer clouds  

NASA Technical Reports Server (NTRS)

Marine boundary layer (MBL) clouds are important components of the earth's climate system. These clouds drastically reduce the amount of solar radiation absorbed by the earth, but have little effect on the emitted infrared radiation on top of the atmosphere. In addition, these clouds are intimately involved in regulating boundary layer turbulent fluxes. For these reasons, it is important that general circulation models used for climate studies must realistically simulate the global distribution of the MBL. While the importance of these cloud systems is well recognized, many physical processes involved in these clouds are poorly understood and their representation in large-scale models remains an unresolved problem. The present research aims at the development and improvement of the parameterization of these cloud systems and an understanding of physical processes involved. This goal is addressed in two ways. One is to use regional modeling approach to validate and evaluate two-layer marine boundary layer models using satellite and ground-truth observations; the other is to combine this simple model with a high-order turbulence closure model to study the transition processes from stratocumulus to shallow cumulus clouds. Progress made in this effort is presented.

Wang, Shouping; Fitzjarrald, Daniel E.

1993-01-01

250

Hypersonic Boundary Layer Stability over a Flared Cone in a Quiet Tunnel  

NASA Technical Reports Server (NTRS)

Hypersonic boundary layer measurements were conducted over a flared cone in a quiet wind tunnel. The flared cone was tested at a freestream unit Reynolds number of 2.82x106/ft in a Mach 6 flow. This Reynolds number provided laminar-to-transitional flow over the model in a low-disturbance environment. Point measurements with a single hot wire using a novel constant voltage anemometry system were used to measure the boundary layer disturbances. Surface temperature and schlieren measurements were also conducted to characterize the laminar-to-transitional state of the boundary layer and to identify instability modes. Results suggest that the second mode disturbances were the most unstable and scaled with the boundary layer thickness. The integrated growth rates of the second mode compared well with linear stability theory in the linear stability regime. The second mode is responsible for transition onset despite the existence of a second mode sub-harmonic. The sub-harmonic wavelength also scales with the boundary layer thickness. Furthermore, the existence of higher harmonics of the fundamental suggests that non-linear disturbances are not associated with high free stream disturbance levels.

Lachowicz, Jason T.; Chokani, Ndaona; Wilkinson, Stephen P.

1996-01-01

251

Hypersonic Boundary Layer Stability Experiments in a Quiet Wind Tunnel with Bluntness Effects  

NASA Technical Reports Server (NTRS)

Hypersonic boundary layer measurements over a flared cone were conducted in a Mach 6 quiet wind tunnel at a freestream unit Reynolds number of 2.82 million/ft. This Reynolds number provided laminar-to-transitional flow over the cone model in a low-disturbance environment. Four interchangeable nose-tips, including a sharp-tip, were tested. Point measurements with a single hot-wire using a novel constant voltage anemometer were used to measure the boundary layer disturbances. Surface temperature and schlieren measurements were also conducted to characterize the transitional state of the boundary layer and to identify instability modes. Results suggest that second mode disturbances were the most unstable and scaled with the boundary layer thickness. The second mode integrated growth rates compared well with linear stability theory in the linear stability regime. The second mode is responsible for transition onset despite the existence of a second mode subharmonic. The subharmonic disturbance wavelength also scales with the boundary layer thickness. Furthermore, the existence of higher harmonics of the fundamental suggests that nonlinear disturbances are not associated with 'high' free stream disturbance levels. Nose-tip radii greater than 2.7% of the base radius completely stabilized the second mode.

Lachowicz, Jason T.; Chokani, Ndaona

1996-01-01

252

Model simulations of the Arctic atmospheric boundary-layer from the SHEBA year.  

PubMed

We present Arctic atmospheric boundary-layer modeling with a regional model COAMPS, for the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment. Model results are compared to soundings, near-surface measurements and forecasts from the ECMWF model. The near-surface temperature is often too high in winter, except in shorter periods when the boundary layer was cloud-capped and well-mixed due to cloud-top cooling. Temperatures are slightly too high also during the summer melt season. Effects are too high boundary-layer moisture and formation of too dense stratocumulus, generating a too deep well-mixed boundary layer with a cold bias at the simulated boundary-layer top. Errors in temperature and therefore moisture are responsible for large errors in heat flux, in particular in solar radiation, by forming these clouds. We conclude that the main problems lie in the surface energy balance and the treatment of the heat conduction through the ice and snow and in how low-level clouds are treated. PMID:15264600

Tjernström, Michael; Zagar, Mark; Svensson, Gunilla

2004-06-01

253

Numerical Solutions of Free Convection Boundary Layer Flow on a Solid Sphere with Convective Boundary Conditions  

NASA Astrophysics Data System (ADS)

The free convection boundary layer flow on a solid sphere with convective boundary conditions has been investigated. The basic equations of boundary layer are transformed into a non-dimensional form and reduced to nonlinear systems of partial differential equations are solved numerically using an implicit finite difference scheme known as the Keller-box method. Numerical results are obtained for the wall temperature, the local heat transfer coefficient and the local skin friction coefficient, as well as the velocity and temperature profiles of the fluid. The features of the flow and heat transfer characteristics for Prandtl number, Pr = 0.7 7 and 100, the conjugate parameter y = 0.05, 0.1, 0.2 and the coordinate running along the surface of the sphere, 0° <= x <= 120° are analyzed and discussed.

Alkasasbeh, H. T.; Salleh, M. Z.; Tahar, R. M.; Nazar, R.

2014-04-01

254

Influences on the Height of the Stable Boundary Layer as seen in Large-Eddy Simulations  

SciTech Connect

Numerical weather prediction (NWP) models and atmospheric dispersion models rely on parameterizations of planetary boundary layer height. In the case of a stable boundary layer, errors in boundary layer height estimation can result in gross errors in boundary-layer evolution and in prediction of turbulent mixing within the boundary layer. We use large-eddy simulations (LES) of moderately stable boundary layers to characterize the effects of various physical processes on stable boundary layers. The stable boundary layer height is assumed to be a function of surface friction velocity, geostrophic wind, Monin-Obukhov length, and the strength of the temperature inversion atop the stable boundary layer. This temperature inversion induces gravity waves with a frequency determined by the strength of the temperature inversion.

Kosovic, B; Lundquist, J K

2004-03-29

255

FOREWORD: International Conference on Planetary Boundary Layer and Climate Change  

NASA Astrophysics Data System (ADS)

One of the greatest achievements of climate science has been the establisment of the concept of climate change on a multitude of time scales. The Earth's complex climate system does not allow a straightforward interpretation of dependences between the external parameter perturbation, internal stochastic system dynamics and the long-term system response. The latter is usually referred to as climate change in a narrow sense (IPCC, 2007). The focused international conference "Planetary Boundary Layers and Climate Change" has addressed only time scales and dynamical aspects of climate change with possible links to the turbulent processes in the Planetary Boundary Layer (PBL). Although limited, the conference topic is by no means singular. One should clearly understand that the PBL is the layer where 99% of biosphere and human activity are concentrated. The PBL is the layer where the energy fluxes, which are followed by changes in cryosphere and other known feedbacks, are maximized. At the same time, the PBL processes are of a naturally small scale. What is the averaged long-term effect of the small-scale processes on the long-term climate dynamics? Can this effect be recognized in existing long-term paleo-climate data records? Can it be modeled? What is the current status of our theoretical understanding of this effect? What is the sensitivity of the climate model projections to the representation of small-scale processes? Are there significant indirect effects, e.g. through transport of chemical components, of the PBL processes on climate? These and other linked questions have been addressed during the conference. The Earth's climate has changed many times during the planet's history, with events ranging from ice ages to long periods of warmth. Historically, natural factors such as the amount of energy released from the Sun, volcanic eruptions and changes in the Earth's orbit have affected the Earth's climate. Beginning late in the 18th century, human activities associated with the Industrial Revolution such as the addition of greenhouse gases and aerosols has changed the composition of the atmosphere. These changes are likely to have influenced temperature, precipitation, storms and sea level (IPCC, 2007). However, these features of the climate also vary naturally, so determining what fraction of climate changes are due to natural variability versus human activities is challenging and not yet a solved problem. Africa is vulnerable to climate change as its ability to adaptat and mitigate is considerably dampened (IPCC, 2007). Climate change may impede a nations ability to achieve sustainable development and the Millennium Development Goals, and because of that Africa (particularly sub-tropical Africa) will experience increased levels of water stress and reduced agricultural yields of up to 50% by 2020. An example of the scale of the region's vulnerability was demonstrated during the last very dry year (1991/92) when 30% of the southern African population was put on food aid and more than one million people were displaced. Climate change in Africa is essentially dependent on our understanding of the PBL processes both due to the indispensible role of the atmospheric convection in the African climate and due to its tele-connections to other regions, e.g. the tropical Pacific and Indian monsoon regions. Although numerous publications attribute the observed changes to one or another modification of the convective patterns, further progress is impeded by imperfections of the small-scale process parameterizations in the models. The uncertainties include parameter uncertainties of known physical processes, which could be reduced through better observations/modelling, as well as uncertainties in our knowledge of physical processes themselves (or structural uncertainties), which could be reduced only through theoretical development and design of new, original observations/experiments (Oppenheimer et al., Science, 2007). Arguably, the structural uncertainties is hard to reduce and this could be one of the reasons determinin

Djolov, G.; Esau, I.

2010-05-01

256

Boundary layer ozone - An airborne survey above the Amazon Basin  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

257

Direct numerical simulation of supersonic turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The objectives of this research were to develop a method by which the spatially developing compressible turbulent boundary layer could be simulated using a temporally developing numerical simulation and to study the physics of the compressible turbulent boundary layer. We take advantage of the technique developed by Spalart (1987, 1988) for the incompressible case. In this technique, it is recognized that the boundary layer exhibits slow growth in the streamwise direction, so the turbulence can be treated as approximately homogeneous in this direction. The slow growth is accounted for with a coordinate transformation and a multiple scale analysis. The result is a modified system of equations (Navier-Stokes plus some extra terms, which we call "slow growth terms") that are homogeneous in both the streamwise and spanwise directions and represent the state of the boundary layer at a given streamwise location (or, equivalently, a given thickness). The compressible Navier-Stokes equations are solved using a mixed Fourier and B-spline "spectral" method. The dependent variables are expanded in terms of a Fourier representation in the horizontal directions and a B-spline representation in the wall-normal direction. In the wall-normal direction non-reflecting boundary conditions are used at the freestream boundary, and zero-heat-flux no-slip boundary conditions are used at the wall. This combination of splines and Fourier methods produces a very accurate numerical method. Mixed implicit/explicit time discretization is used. Results are presented for a case with a Mach number of 2.5, and a Reynolds number, based on momentum integral thickness and wall viscosity, of Rsb{thetasp'} = 840. The results show that the van Driest transformed velocity satisfies the incompressible scalings and a narrow logarithmic region is obtained. The results for the turbulence intensities compare well with the incompressible simulations of Spalart. Pressure fluctuations are found to be higher than in incompressible flow. Morkovin's strong Reynolds analogy does not agree with the results of the simulation, however, an analogy is found between the rate of turbulent heat transfer and the rate of turbulent momentum transfer. Reynolds stress and turbulent kinetic energy budgets are computed and compared with the budgets from Spalart's incompressible simulations.

Guarini, Stephen

258

Fluctuating pressure loads under high speed boundary layers  

NASA Technical Reports Server (NTRS)

Aeroacoustic fatigue is anticipated to control the design of significant portions of the structures of high-speed vehicles. This is due to contemplated long-duration flights at high dynamic pressures and Mach numbers with related high skin temperatures. Fluctuating pressure loads are comparatively small beneath attached turbulent boundary layers, but become important in regions of flow separation such as compression and expansion corners on elevons and rudders. The most intense loads are due to shock/boundary-layer interaction. These flows may occur in the engine-exhaust wall jet and in flows over control surfaces. A brief review is given of available research in these areas with a description of work under way at Langley Research Center.

Zorumski, William E.

1987-01-01

259

Possibilities for drag reduction by boundary layer control  

NASA Technical Reports Server (NTRS)

The mechanics of laminar boundary layer transition are reviewed. Drag possibilities for boundary layer control are analyzed using assumed conditions of transition Reynolds number, inlet loss, number of slots, blower efficiency, and duct losses. Although the results of such analysis are highly favorable, those obtained by experimental investigations yield conflicting results, showing only small gains, and sometimes losses. Reduction of this data indicates that there is a lower limit to the quantity of air which must be removed at the slot in order to stabilize the laminar flow. The removal of insufficient air permits transition to occur while the removal of excessive amounts of air results in high power costs, with a net drag increases. With the estimated value of flow coefficient and duct losses equal to half the dynamic pressure, drag reductions of 50% may be obtained; with twice this flow coefficient, the drag saving is reduced to 25%.

Naiman, I.

1946-01-01

260

Nonparallel instability of supersonic and hypersonic boundary layers  

NASA Technical Reports Server (NTRS)

Multiple scaling technique is used to examine the nonparallel instability of supersonic and hypersonic boundary-layer flows to three dimensional (first mode) and two dimensional (second mode) disturbances. The method is applied to the flat plate boundary layer for a range of Mach numbers from 0 to 10. Growth rates of disturbances are calculated based on three different criteria: following the maximum of the mass-flow disturbance, using an integral of the disturbance kinetic energy, and using the integral of the square of the mass-flow amplitude. By following the maximum of the mass-flow disturbance, the calculated nonparallel growth rates are in good quantitative agreement with the experimental results at Mach number 4.5.

El-Hady, Nabil M.

1991-01-01

261

3D LDV Measurements in Oscillatory Boundary Layers  

NASA Astrophysics Data System (ADS)

The oscillatory boundary layer represents a particular case of unsteady wall-bounded flows in which fluid particles follow a periodic sinusoidal motion. Unlike steady boundary layer flows, the oscillatory flow regime and bed roughness character change in time along the period for every cycle, a characteristic that introduces a high degree of complexity in the analysis of these flows. Governing equations can be derived from the general Navier-Stokes equations for the motion of fluids, from which the exact solution for the laminar oscillatory boundary layer is obtained (also known as the 2nd Stokes problem). No exact solution exists for the turbulent case, thus, understanding of the main flow characteristics comes from experimental work. Several researchers have reported experimental work in oscillatory boundary layers since the 1960's; however, larger scale facilities and the development of newer measurement techniques with improved temporal and spatial resolution in recent years provides a unique opportunity to achieve a better understanding about this type of flows. Several experiments were performed in the Large Oscillatory Water and Sediment Tunnel (LOWST) facility at the Ven Te Chow Hydrosystems Laboratory, for a range of Reynolds wave numbers between 6x10^4 < Rew < 6x10^6 over a flat and smooth bottom. A 3D Laser Doppler Velocimetry (LDV) system was used to measure instantaneous flow velocities with a temporal resolution up to ~ 1,000 Hz. It was mounted on a 3-axis traverse with a spatial resolution of 0.01 mm in all three directions. The closest point to the bottom was measured at z = 0.2 mm (z+ ? 4), which allowed to capture boundary layer features with great detail. In order to achieve true 3D measurements, 2 probes were used on a perpendicular configuration, such that u and w components were measured from a probe on the side of the flume and v component was measured from a probe pointing down through and access window on top of the flume. The top probe was submerged in a water container, such that the focal length remained constant and coincidence in the measurement volume for all 3 components was maintained when traversing the probes along the measurement profiles. Results show the existence of high turbulence levels inside the boundary layer up to about 30 mm away from the bottom. The streamwise component u shows greater intensities closer to the bottom and ahead of the freestream velocity maximum. On the contrary, the vertical component w shows smaller values of turbulent intensity, located higher up in the profile and lagging with respect to the freestream velocity maximum. Meanwhile, the spanwise component v shows similar intensities than w, happening in phase with it, but distributed all along the boundary layer, overlapping the areas of greater intensity of u and w. In addition, wall shear stress and other turbulent magnitudes related to the boundary layer were analyzed from the experimental results obtained through this research.

Mier, J. M.; Garcia, M. H.

2012-12-01

262

Crossing shock wave-turbulent boundary layer interactions  

NASA Technical Reports Server (NTRS)

Three-dimensional interactions between crossing shock waves generated by symmetric sharp fins and a turbulent boundary layer on a flat plate are investigated experimentally and theoretically at Mach number 2.95 and freestream unit Reynolds number 1.96 x 10 to the 7th/ft. The incoming boundary layer has a thickness of 4 mm at the location of the fin leading edges. A comparison of experimental and computational results for two sets of fin angles (11 x 11 and 9 x 9 deg) shows general agreement with regard to surface pressure measurements and surface streamline patterns. The principal feature of the streamline structure is a collision of counterrotating vortical structures emanating from near the fin leading edges and meeting at the geometric centerline of the interaction.

Narayanswami, N.; Knight, D. D.; Bogdonoff, S. M.; Horstman, C. C.

1991-01-01

263

VALIDITY OF GENERATION SYSTEM FOR SOLITARY WAVE BOUNDARY LAYER  

NASA Astrophysics Data System (ADS)

The present study is concerning bottom boundary layer beneath solitary wave over smooth beds condition. A new generation system was proposed to generate an oscillatory motion similar to solitary wave in a closed conduit water tunnel using a mechanical system. This generation system facilitates easy measurement of periodical oscillatory motion to replace solitary wave motion with a sufficient tranquil period. The velocities were measured by using a Laser Doppler Veloci-meter (LDV) at 17 to 22 points in the vertical direction. The experiments were accomplished with different velocities under single and periodical oscillatory motion conditions and validations have been done in some various terms of experiment relevant to solitary wave boundary layer. A good agreement is achieved in validation of free stream velocity and also both single and periodical oscillatory motion measurements methods. Furthermore, validation of velocity distribution in time variation obtains a critical Reynolds number which has a good agreement with the finding of previous researchers.

Winarta, Bambang; Tanaka, Hitoshi

264

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

265

Characteristics of Mach 10 transitional and turbulent boundary layers  

NASA Technical Reports Server (NTRS)

Measurements of the mean flow properties of transitional and turbulent boundary layers in helium on 4 deg and 5 deg wedges were made for flows with edge Mach numbers from 9.5 to 11.3, ratios of wall temperature to total temperature of 0.4 to 0.95, and maximum length Reynolds numbers of one hundred million. The data include pitot and total temperature surveys and measurements of heat transfer and surface shear. In addition, with the assumption of local similarity, turbulence quantities such as the mixing length were derived from the mean flow profiles. Low Reynolds number and precursor transition effects were significant factors at these test conditions and were included in finite difference boundary layer predictions.

Watson, R. D.

1978-01-01

266

Benthic boundary layer processes in the Lower Florida Keys  

USGS Publications Warehouse

This special issue of Geo-Marine Letters, "Benthic Boundary Layer Processes in the Lower Florida Keys," includes 12 papers that present preliminary results from the Key West Campaign. The Dry Tortugas and Marquesas Keys test sites were selected by a group of 115 scientists and technicians to study benthic boundary layer processes in a carbonate environment controlled by bioturbation and biogeochemical processes. Major activities included remote sediment classification; high-frequency acoustic scattering experiments; sediment sampling for radiological, geotechnical, biological, biogeochemical, physical, and geoacoustic studies; and hydrodynamic studies using an instrumented tetrapod. All these data are being used to improve our understanding of the effects of environmental processes on sediment structure and behavior.

Lavoie, D.L.; Richardson, M.D.; Holmes, C.

1997-01-01

267

A review of unsteady turbulent boundary-layer experiments  

NASA Technical Reports Server (NTRS)

The essential results of a comprehensive review of existing unsteady turbulent boundary-layer experiments are presented. Different types of unsteady flow facilities are described, and the related unsteady turbulent boundary-layer experiments are cataloged and discussed. The measurements that were obtained in the various experiments are described, and a complete list of experimental results is presented. All the experiments that measured instantaneous values of velocity, turbulence intensity, or turbulent shear stress are identified, and the availability of digital data is indicated. The results of the experiments are analyzed, and several significant trends are identified. An assessment of the available data is presented, delineating gaps in the existing data, and indicating where new or extended information is needed. Guidelines for future experiments are included.

Carr, L. W.

1981-01-01

268

Boundary layer flow on a long thin cylinder  

NASA Astrophysics Data System (ADS)

The development of the boundary layer along a long thin cylinder aligned with the flow is considered. Numerical solutions are presented and compared with previous asymptotic results. Very near the leading edge the flow is given by the Blasius solution for a flat plate. However, there is soon a significant deviation from Blasius flow, with a thinner boundary layer and higher wall shear stress. Linear normal mode stability of the flow is investigated. It is found that for Reynolds numbers less than a critical value of 1060 the flow is unconditionally stable. Also, axisymmetric modes are only the fourth least stable modes for this problem, with the first three three-dimensional modes all having a lower critical Reynolds number. Further, for Reynolds numbers above the critical value, the flow is unstable only for a finite distance, and returns to stability sufficiently far downstream.

Tutty, O. R.; Price, W. G.; Parsons, A. T.

2002-02-01

269

Nonlinear interaction of two waves in boundary-layer flows  

NASA Technical Reports Server (NTRS)

First-order nonlinear interactions of Tollmien-Schlichting waves of different frequencies and initial amplitudes in boundary-layer flows are analyzed using the method of multiple scales. Numerical results for flow past a flat plate show that the spatial detuning wipes out resonant interactions unless the initial amplitudes are very large. Thus, a wave having a moderate amplitude has little influence on its subharmonic although it has a strong influence on its second harmonic. Moreover, two waves having moderate amplitudes have a strong influence on their difference frequency. The results show that the difference frequency can be very unstable when generated by the nonlinear interaction, even though it may be stable when introduced by itself in the boundary layer.

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

1980-01-01

270

Optimal disturbances in boundary layers subject to streamwise pressure gradient  

NASA Technical Reports Server (NTRS)

An analysis of the optimal non-modal growth of perturbations in a boundary layer in the presence of a streamwise pressure gradient is presented. The analysis is based on PSE equations for an incompressible fluid. Examples with Falkner-Scan profiles indicate that a favorable pressure gradient decreases the non-modal growth, while an unfavorable pressure gradient leads to an increase of the amplification. It is suggested that the transient growth mechanism be utilized to choose optimal parameters of tripping elements on a low-pressure turbine (LPT) airfoil. As an example, a boundary layer flow with a streamwise pressure gradient corresponding to the pressure distribution over a LPT airfoil is considered. It is shown that there is an optimal spacing of the tripping elements and that the transient growth effect depends on the starting point.

Ashpis, David E.; Tumin, Anatoli

2003-01-01

271

Atmospheric surface and boundary layers of the Amazon Basin  

NASA Technical Reports Server (NTRS)

Three phases of work were performed: design of and preparation for the Amazon Boundary Layer Experiment (ABLE 2-A); execution of the ABLE 2-A field program; and analysis of the ABLE 2-A data. Three areas of experiment design were dealt with: surface based meteorological measurements; aircraft missions; and project meteorological support. The primary goal was to obtain a good description of the structure of the atmosphere immediately above the rain forest canopy (top of canopy to a few thousand meters), to describe this region during the growing daytime phase of the boundary layer; and to examine the nighttime stratified state. A secondary objective was to examine the role that deep convective storms play in the vertical transport of heat, water vapor, and other trace gases. While significant progress was made, much of the analysis remains to be done.

Garstang, Michael

1987-01-01

272

Atmospheric tides on Venus. III - The planetary boundary layer  

NASA Astrophysics Data System (ADS)

Diurnal solar heating of Venus' surface produces variable temperatures, winds, and pressure gradients within a shallow layer at the bottom of the atmosphere. The corresponding asymmetric mass distribution experiences a tidal torque tending to maintain Venus' slow retrograde rotation. It is shown that including viscosity in the boundary layer does not materially affect the balance of torques. On the other hand, friction between the air and ground can reduce the predicted wind speeds from about 5 to about 1 m/sec in the lower atmosphere, more consistent with the observations from Venus landers and descent probes. Implications for aeolian activity on Venus' surface and for future missions are discussed.

Dobrovolskis, A. R.

1983-10-01

273

Nonlinear Interaction of Frequency-Detuned Modes in Boundary Layers  

NASA Technical Reports Server (NTRS)

The present critical-layer asymptotic analysis for the nonlinear interaction of frequency-detuned modes in boundary-layer transition indicates that the interaction between a plane mode at the fundamental frequency and a pair of symmetrical oblique waves at the near-subharmonic frequency amplifies another pair of symmetrical oblique waves at the 'mirror frequency'. This type of interaction is stronger in the frequency-detuned case than the resonant triad case, and leads to a sharp drop in the oblique waves' peak with small detuning.

Mankbadi, Reda R.

1993-01-01

274

Air Flow in a Separating Laminar Boundary Layer  

NASA Technical Reports Server (NTRS)

The speed distribution in a laminar boundary layer on the surface of an elliptic cylinder, of major and minor axes 11.78 and 3.98 inches, respectively, has been determined by means of a hot-wire anemometer. The direction of the impinging air stream was parallel to the major axis. Special attention was given to the region of separation and to the exact location of the point of separation. An approximate method, developed by K. Pohlhausen for computing the speed distribution, the thickness of the layer, and the point of separation, is described in detail; and speed-distribution curves calculated by this method are presented for comparison with experiment.

Schubauer, G B

1936-01-01

275

A kappa-epsilon calculation of transitional boundary layers  

NASA Technical Reports Server (NTRS)

A recently proposed kappa-epsilon model for low Reynolds number turbulent flows was modified by introducing a new damping function f(sub mu). The modified model is used to calculate the transitional boundary layer over a flat plate with different freestream turbulence levels. It is found that the model could mimic the transitional flow. However, the predicted transition is found to be sensitive to the initial conditions.

Yang, Z.; Shih, T. H.

1992-01-01

276

Spherical bubble motion in a turbulent boundary layer  

Microsoft Academic Search

Monodisperse dilute suspensions of spherical air bubbles in a tap-water turbulent vertical boundary layer were experimentally studied to note their motion and distribution. Bubbles with diameters of 0.37-1.2 mm were injected at various transverse wall-positions for free-stream velocities between 0.4 and 0.9 m\\/s. The bubbles were released from a single injector at very low frequencies such that two-way coupling and

Keith Felton; Eric Loth

2001-01-01

277

Turbulent friction in a boundary layer of compressible gas  

NASA Astrophysics Data System (ADS)

The main processes of turbulent momentum and heat transfer in the developed flow of a compressible gas are considered. Results are presented on the dependence of the relative friction coefficient on the temperature factor and on the Mach number. An analysis is made of the theoretical and experimental distributions of air velocity in the turbulent boundary layer on a cylinder. Calculations using the proposed turbulent-viscosity model agree well with experimental data.

Dobrocheev, O. V.; Motulevich, V. P.

1987-10-01

278

On the Secondary Instability of Three-Dimensional Boundary Layers  

Microsoft Academic Search

.   One of the possible transition scenarios in three-dimensional boundary layers, the saturation of stationary crossflow vortices\\u000a and their secondary instability to high-frequency disturbances, is studied using the Parabolized Stability Equations (PSE)\\u000a and Floquet theory. Starting from nonlinear PSE solutions, we investigate the region where a purely stationary crossflow disturbance\\u000a saturates for its secondary instability characteristics utilizing global and local

Erik Janke; Ponnampalam Balakumar

2000-01-01

279

Sheet flow and suspension of sand in oscillatory boundary layers  

Microsoft Academic Search

after revisionTime-dependent measurements of flow velocities and sediment concentrations were conducted in a large oscillating water tunnel. The measurements were aimed at the flow and sediment dynamics in and above an oscillatory boundary layer in plane bed and sheet-flow conditions. Two asymmetric waves and one sinusoidal wave were imposed using quartz sand with D50 = 0.21 mm. A new electro-resistance

Jan S. Ribberink; Abdullah A. Al-Salem

1995-01-01

280

LASTRAC.3d: Transition Prediction in 3D Boundary Layers  

NASA Technical Reports Server (NTRS)

Langley Stability and Transition Analysis Code (LASTRAC) is a general-purpose, physics-based transition prediction code released by NASA for laminar flow control studies and transition research. This paper describes the LASTRAC extension to general three-dimensional (3D) boundary layers such as finite swept wings, cones, or bodies at an angle of attack. The stability problem is formulated by using a body-fitted nonorthogonal curvilinear coordinate system constructed on the body surface. The nonorthogonal coordinate system offers a variety of marching paths and spanwise waveforms. In the extreme case of an infinite swept wing boundary layer, marching with a nonorthogonal coordinate produces identical solutions to those obtained with an orthogonal coordinate system using the earlier release of LASTRAC. Several methods to formulate the 3D parabolized stability equations (PSE) are discussed. A surface-marching procedure akin to that for 3D boundary layer equations may be used to solve the 3D parabolized disturbance equations. On the other hand, the local line-marching PSE method, formulated as an easy extension from its 2D counterpart and capable of handling the spanwise mean flow and disturbance variation, offers an alternative. A linear stability theory or parabolized stability equations based N-factor analysis carried out along the streamline direction with a fixed wavelength and downstream-varying spanwise direction constitutes an efficient engineering approach to study instability wave evolution in a 3D boundary layer. The surface-marching PSE method enables a consistent treatment of the disturbance evolution along both streamwise and spanwise directions but requires more stringent initial conditions. Both PSE methods and the traditional LST approach are implemented in the LASTRAC.3d code. Several test cases for tapered or finite swept wings and cones at an angle of attack are discussed.

Chang, Chau-Lyan

2004-01-01

281

A numerical simulation of boundary-layer flows near shelterbelts  

Microsoft Academic Search

We have developed a shelterbelt boundary-layer numerical model to study the patterns and dynamic processes relating to flow interaction with shelterbelts. The model simulates characteristics of all three zones of airflow passing over and through shelterbelts: the windward windspeed-reduction zone, the overspeeding zone above the shelterbelt, and the leeward windspeed-reduction zone. Locations of the maximum windspeed reduction and recirculation zone,

Hao Wang; Eugene S. Takle

1995-01-01

282

Vortex/boundary-layer interactions: Data report, volume 2  

NASA Technical Reports Server (NTRS)

This report summarizes the work done under NASA grant NAGw-581, Vortex/Boundary-Layer Interactions, to date. The experimental methods are discussed in detail and the results presented as a large number of figures, but are not fully interpreted. This report should be useful to anyone who wishes to make further use of the data (available on floppy disc or magnetic tape) for the development of turbulence models or the validation of predictive methods. Journal papers are in preparation.

Cutler, A. D.; Bradshaw, P.

1987-01-01

283

Streamline-curvature effect in three-dimensional boundary layers  

NASA Technical Reports Server (NTRS)

The effect of including wall and streamline curvature terms in swept-wing boundary-layer stability calculations is studied. The linear disturbance equations are cast on a fixed, body-intrinsic, curvilinear coordinate system. Those nonparallel terms which contribute mainly to the streamline-curvature effect are retained in this formulation and approximated by their local finite-difference values. Convex-wall curvature has a stabilizing effect, while streamline curvature is destabilizing if the curvature exceeds a critical value.

Reed, Helen L.; Lin, Ray-Sing; Petraglia, Media M.

1992-01-01

284

SCALING OF THE ANOMALOUS BOOST IN RELATIVISTIC JET BOUNDARY LAYER  

SciTech Connect

We investigate the one-dimensional interaction of a relativistic jet and an external medium. Relativistic magnetohydrodynamic simulations show an anomalous boost of the jet fluid in the boundary layer, as previously reported. We describe the boost mechanism using an ideal relativistic fluid and magnetohydrodynamic theory. The kinetic model is also examined for further understanding. Simple scaling laws for the maximum Lorentz factor are derived, and verified by the simulations.

Zenitani, Seiji; Hesse, Michael; Klimas, Alex, E-mail: Seiji.Zenitani-1@nasa.go [NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)

2010-04-01

285

Vortex/boundary-layer interactions: Data report, volume 1  

NASA Technical Reports Server (NTRS)

This report summarizes the work done under NASA Grant NAGw-581, Vortex/Boundary Layer Interactions. The experimental methods are discussed in detail and numerical results are presented, but are not fully interpreted. This report should be useful to anyone who wishes to make further use of the data (available on floppy disc or magnetic tape) for the development of turbulence models or the validation of predictive methods. Journal papers are in course of preparation.

Cutler, A. D.; Bradshaw, P.

1987-01-01

286

Linear stability theory and three-dimensional boundary layer transition  

NASA Technical Reports Server (NTRS)

The viewgraphs and discussion of linear stability theory and three dimensional boundary layer transition are provided. The ability to predict, using analytical tools, the location of boundary layer transition over aircraft-type configurations is of great importance to designers interested in laminar flow control (LFC). The e(sup N) method has proven to be fairly effective in predicting, in a consistent manner, the location of the onset of transition for simple geometries in low disturbance environments. This method provides a correlation between the most amplified single normal mode and the experimental location of the onset of transition. Studies indicate that values of N between 8 and 10 correlate well with the onset of transition. For most previous calculations, the mean flows were restricted to two-dimensional or axisymmetric cases, or have employed simple three-dimensional mean flows (e.g., rotating disk, infinite swept wing, or tapered swept wing with straight isobars). Unfortunately, for flows over general wing configurations, and for nearly all flows over fuselage-type bodies at incidence, the analysis of fully three-dimensional flow fields is required. Results obtained for the linear stability of fully three-dimensional boundary layers formed over both wing and fuselage-type geometries, and for both high and low speed flows are discussed. When possible, transition estimates form the e(sup N) method are compared to experimentally determined locations. The stability calculations are made using a modified version of the linear stability code COSAL. Mean flows were computed using both Navier Stokes and boundary-layer codes.

Spall, Robert E.; Malik, Mujeeb R.

1992-01-01

287

Effect of compliant wall motion on turbulent boundary layers  

NASA Technical Reports Server (NTRS)

A critical analysis of available wall data which indicated drag reduction under turbulent boundary layers. Detailed structural dynamic calculations suggest the surfaces responded in a resonant, rather than compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: flexible pipe flown, and waterbacked membranes in air. Analysis indicates the wall motion for the remaining data is typified by short wave lengths in agreement with the requirement of a possible compliant wall drag reduction mechanism recently suggested by Langley.

Bushnell, D. M.; Hefner, J. N.; Ash, R. L.

1977-01-01

288

Interaction of Pulsed Vortex Generator Jets with Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

Vortex Generator Jets (VGJ) have been proposed as a means for active control of turbulent boundary layer separation by Johnston footnote AIAA J. 28, 989 (1990). It has been shown that a vortex generator jet can create weak longitudinal vorticity of a single sign when the surface-mounted jets are pitched and skewed with respect to the solid surface. The primary advantages of VGJs when compared to solid vortex generators are their lack of parasitic drag when the jets are off and the ability to rapidly activate and deactivate the jets for dynamic control. Pulsing of the jets is proposed as a way of increasing the turbulent mixing and therefore, improving the performance of vortex generator jets. Initial experiments with jets pitched at 45 deg and skewed at 90 deg degrees in air have indicated that large-scale turbulent structures are formed by the pulsed VGJs. Subsequent flow visualization experiments in a water tunnel suggest that fully-modulated jets embedded in a flat plate boundary layer result in a series of puffs which penetrate through the boundary layer. The influence of jet velocity, diameter, pulsing frequency and duty-cycle will be discussed. * Supported by NSF and PSI.

McManus, K. R.; Johari, H.

1996-11-01

289

Surface-cooling effects on compressible boundary-layer instability  

NASA Technical Reports Server (NTRS)

The influence of surface cooling on compressible boundary layer instability is discussed theoretically for both viscous and inviscid modes, at high Reynolds numbers. The cooling enhances the surface heat transfer and shear stress, creating a high heat transfer sublayer. This has the effect of distorting and accentuating the viscous Tollmien-Schlichting modes to such an extent that their spatial growth rates become comparable with, and can even exceed, the growth rates of inviscid modes, including those found previously. This is for moderate cooling, and it applies at any Mach number. In addition, the moderate cooling destabilizes otherwise stable viscous or inviscid modes, in particular triggering outward-traveling waves at the edge of the boundary layer in the supersonic regime. Severe cooling is also discussed as it brings compressible dynamics directly into play within the viscous sublayer. All the new cooled modes found involve the heat transfer sublayer quite actively, and they are often multi-structured in form and may be distinct from those observed in previous computational and experimental investigations. The corresponding nonlinear processes are also pointed out with regard to transition in the cooled compressible boundary layer. Finally, comparisons with Lysenko and Maslov's (1984) experiments on surface cooling are presented.

Seddougui, Sharon O.; Bowles, R. I.; Smith, F. T.

1990-01-01

290

Measurements of Instability and Transition in Hypersonic Boundary Layers  

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

291

Görtler instability of the axisymmetric boundary layer along a cone  

NASA Astrophysics Data System (ADS)

Exact partial differential equations are derived to describe Görtler instability, caused by a weakly concave wall, of axisymmetric boundary layers with similar velocity profiles that are decomposed into a sequence of ordinary differential systems on the assumption that the solution can be expanded into inverse powers of local Reynolds number. The leading terms of the series solution are determined by solving a non-parallel version of Görtler’s eigenvalue problem and lead to a neutral stability curve and finite values of critical Görtler number and wave number for stationary and longitudinal vortices. Higher-order terms of the series solution indicate Reynolds-number dependence of Görtler instability and a limited validity of Görtler’s approximation based on the leading terms only. The present formulation is simply applicable to two-dimensional boundary layers of similar profiles, and critical Görtler number and wave number of the Blasius boundary layer on a flat plate are given by G2c = 1.23 and ?2c = 0.288, respectively, if the momentum thickness is chosen as the reference length.

ITOH, Nobutake

2014-10-01

292

Effect of Surface Thermal Perturbations on Compressible Boundary Layer Stability  

NASA Astrophysics Data System (ADS)

High-speed laminar-turbulent boundary layer transition is a critical issue for re-entry and sustained hypersonic cruise vehicles. Turbulent wall heating rates can increase several orders of magnitude compared to laminar rates and skin friction drag can become a major component of the overall drag. We analyze approaches to modulate transition by altering the stability features of the boundary layer through the use of thermal perturbations. To this end, high-fidelity numerical simulations to generate basic states for Mach 1.5 and Mach 5.6 flat plate boundary layers with and without thermal bumps. Linear Parabolized Stability Equations (PSE) are solved using the STABL software suite to establish the flow stability characteristics under baseline (no excitation), constant and pulsed bump cases for each freestream Mach number. The effects are described in terms of neutral curves showing amplification for various frequencies versus Reynolds number. The three-dimensional flow structure is also examined near the breakdown to turbulence flow region to gain insight into the final stages of transition.

Alba, Christopher; Gaitonde, Datta

2009-11-01

293

Some characteristics of bypass transition in a heated boundary layer  

NASA Technical Reports Server (NTRS)

Experimental measurements of both mean and conditionally sampled characteristics of laminar, transitional and low Reynolds number turbulent boundary layers on a heated flat plate are presented. Measurements were obtained in air over a range of freestream turbulence intensities from 0.3 percent to 6 percent with a freestream velocity of 30.5 m/s and zero pressure gradient. Conditional sampling performed in the transitional boundary layers indicate the existence of a near-wall drop in intermittency, especially pronounced at low intermittencies. Nonturbulent intervals were observed to possess large levels of low-frequency unsteadiness, and turbulent intervals had peak intensities as much as 50 percent higher than were measured at fully turbulent stations. Heat transfer results were consistent with results of previous researches and Reynolds analogy factors were found to be well predicted by laminar and turbulent correlations which accounted for unheated starting length. A small dependence of the turbulent Reynolds analogy factors on freestream turbulence level was observed. Laminar boundary layer spectra indicated selective amplification of unstable frequencies. These instabilities appear to play a dominant role in the transition process only for the lowest freestream turbulence level studied, however.

Sohn, K. H.; Reshotko, E.; O'Brien, J. E.

1989-01-01

294

Some characteristics of bypass transition in a heated boundary layer  

NASA Technical Reports Server (NTRS)

Experimental measurements of both mean and conditionally sampled characteristics of laminar, transitional and low Reynolds number turbulent boundary layers on a heated flat plate are presented. Measurements were obtained in air over a range of freestream turbulence intensities from 0.3 percent to 6 percent with a freestream velocity of 30.5 m/s and zero pressure gradient. Conditional sampling performed in the transitional boundary layers indicate the existence of a near-wall drop in intermittency, especially pronounced at low intermittencies. Nonturbulent intervals were observed to possess large levels of low-frequency unsteadiness, and turbulent intervals had peak intensities as much as 50 percent higher than were measured at fully turbulent stations. Heat transfer results were consistent with results of previous researchers and Reynolds analogy factors were found to be well predicted by laminar and turbulent correlations which accounted for unheated starting length. A small dependence of the turbulent Reynolds analogy factors on freestream turbulence level was observed. Laminar boundary layer spectra indicated selective amplification of unstable frequencies. These instabilities appear to play a dominant role in the transition process only for the lowest freestream turbulence level studied, however.

Sohn, K. H.; Obrien, J. E.; Reshotko, E.

1989-01-01

295

Improving subtropical boundary layer cloudiness in the 2011 NCEP GFS  

NASA Astrophysics Data System (ADS)

The current operational version of National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) shows significant low cloud bias. These biases also appear in the Coupled Forecast System (CFS), which is developed from the GFS. These low cloud biases degrade seasonal and longer climate forecasts, particularly of short-wave cloud radiative forcing, and affect predicted sea surface temperature. Reducing this bias in the GFS will aid the development of future CFS versions and contributes to NCEP's goal of unified weather and climate modelling. Changes are made to the shallow convection and planetary boundary layer parameterisations to make them more consistent with current knowledge of these processes and to reduce the low cloud bias. These changes are tested in a single-column version of GFS and in global simulations with GFS coupled to a dynamical ocean model. In the single-column model, we focus on changing parameters that set the following: the strength of shallow cumulus lateral entrainment, the conversion of updraught liquid water to precipitation and grid-scale condensate, shallow cumulus cloud top, and the effect of shallow convection in stratocumulus environments. Results show that these changes improve the single-column simulations when compared to large eddy simulations, in particular through decreasing the precipitation efficiency of boundary layer clouds. These changes, combined with a few other model improvements, also reduce boundary layer cloud and albedo biases in global coupled simulations.

Fletcher, J. K.; Bretherton, C. S.; Xiao, H.; Sun, R.; Han, J.

2014-09-01

296

Thermocapillary Bubble Migration: Thermal Boundary Layers for Large Marangoni Numbers  

NASA Technical Reports Server (NTRS)

The migration of an isolated gas bubble in an immiscible liquid possessing a temperature gradient is analyzed in the absence of gravity. The driving force for the bubble motion is the shear stress at the interface which is a consequence of the temperature dependence of the surface tension. The analysis is performed under conditions for which the Marangoni number is large, i.e. energy is transferred predominantly by convection. Velocity fields in the limit of both small and large Reynolds numbers are used. The thermal problem is treated by standard boundary layer theory. The outer temperature field is obtained in the vicinity of the bubble. A similarity solution is obtained for the inner temperature field. For both small and large Reynolds numbers, the asymptotic values of the scaled migration velocity of the bubble in the limit of large Marangoni numbers are calculated. The results show that the migration velocity has the same scaling for both low and large Reynolds numbers, but with a different coefficient. Higher order thermal boundary layers are analyzed for the large Reynolds number flow field and the higher order corrections to the migration velocity are obtained. Results are also presented for the momentum boundary layer and the thermal wake behind the bubble, for large Reynolds number conditions.

Balasubramaniam, R.; Subramanian, R. S.

1996-01-01

297

Improving subtropical boundary layer cloudiness in the 2011 NCEP GFS  

NASA Astrophysics Data System (ADS)

The current operational version of National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) shows significant low cloud bias. These biases also appear in the Coupled Forecast System (CFS), which is developed from the GFS. These low cloud biases degrade seasonal and longer climate forecasts, particularly of shortwave cloud radiative forcing, and affect predicted sea-surface temperature. Reducing this bias in the GFS will aid the development of future CFS versions and contributes to NCEP's goal of unified weather and climate modelling. Changes are made to the shallow convection and planetary boundary layer parametrisations to make them more consistent with current knowledge of these processes and to reduce the low cloud bias. These changes are tested in a single-column version of GFS and in global simulations with GFS coupled to a dynamical ocean model. In the single column model, we focus on changing parameters that set the following: the strength of shallow cumulus lateral entrainment, the conversion of updraught liquid water to precipitation and grid-scale condensate, shallow cumulus cloud top, and the effect of shallow convection in stratocumulus environments. Results show that these changes improve the single-column simulations when compared to large eddy simulations, in particular through decreasing the precipitation efficiency of boundary layer clouds. These changes, combined with a few other model improvements, also reduce boundary layer cloud and albedo biases in global coupled simulations.

Fletcher, J. K.; Bretherton, C. S.; Xiao, H.; Sun, R.; Han, J.

2014-04-01

298

Coupled wake boundary layer model of wind-farms  

E-print Network

We present and test a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a wind-farm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall wind-farm boundary layer structure. The wake expansion/superposition model captures the effect of turbine positioning, while the top-down portion adds the interaction between the wind-turbine wakes and the atmospheric boundary layer. Each portion of the model requires specification of a parameter that is not known a-priori. For the wake model the wake expansion coefficient is required, while the top-down model requires an effective span-wise turbine spacing within which the model's momentum balance is relevant. The wake expansion coefficient is obtained by matching the predicted mean velocity at the turbine from both approaches, while the effective span-wise turbine spacing depends on turbine positioning and thus can be determined from the wake expansion...

Stevens, Richard J A M; Meneveau, Charles

2014-01-01

299

Boundary Layer Transition in the NTF: HSR Experience and Plans  

NASA Technical Reports Server (NTRS)

Efforts towards understanding boundary layer transition characteristics on a High Speed Civil Transport (HSCT)-class configuration in the National Transonic Facility (NTF) are ongoing. The majority of the High Speed Research (HSR) data base in the NTF has free transition on the wing, even at low Reynolds numbers (Rn) attainable in conventional facilities. Limited data has been obtained and is described herein showing the effects of a conventional, Braslow method based wing boundary-layer trip on drag. Comparisons are made using force data polars and surface flow visualization at selected angles-of-attack and Mach number. Minimum drag data obtained in this study suggest that boundary layer transition occurred very near the wing leading edge by a chord Rn of 30 million. Sublimating chemicals were used in the air mode of operation only at low Rn and low angles-of-attack with no flap deflections; sublimation results suggest that the forebody and outboard wing panel are the only regions with significant laminar flow. The process and issues related to the sublimating chemical technique as applied in the NTF are discussed. Beyond the existing experience, status of efforts to develop a production transition detection system applicable to both air and cryogenic nitrogen environments is presented.

Owens, Lewis R., Jr.; Wahls, Richard A.; Hamner, Marvine P.

1999-01-01

300

The nonlinear development of Gortler vortices in growing boundary layers  

NASA Technical Reports Server (NTRS)

The development of Gortler vortices in boundary layers over curved walls in the nonlinear regime is investigated. The growth of the boundary layer makes a parallel flow analysis impossible except in the high wavenumber regime so in general the instability equations must be integrated numerically. Here the spanwise dependence of the basic flow is described using Fourier series expansion whilst the normal and streamwise variations are taken into account using finite differences. The calculations suggest that a given disturbance imposed at some position along the wall will eventually reach a local equilibrium state essentially independent of the initial conditions. In fact, the equilibrium state reached is qualitatively similar to the large amplitude high wave-number solution described asymptotically by Hall (1982). In general, it is found that the nonlinear interactions are dominated by a mean field type of interaction between the mean flow and the fundamental. Thus, even though higher harmonics of the fundamental are necessarily generated, most of the disturbance energy is confined to the mean flow correction and the fundamental. A major result of the calculations is finding that the downstream velocity field develops a strongly inflection character as the flow moves downstream. The latter result suggests that the major effect of Gortler vortices on boundary layers of practical importance might be to make them highly receptive to rapidly growing Rayleigh modes of instability.

Hall, Philip

1986-01-01

301

The nonlinear development of Goertler vortices in growing boundary layers  

NASA Technical Reports Server (NTRS)

The development of Goertler vortices in boundary layers over curved walls in the nonlinear regime is investigated. The growth of the boundary layer makes a parallel flow analysis impossible except in the high wavenumber regime so in general the instability equations must be integrated numerically. Here the spanwise dependence of the basic flow is described using Fourier series expansion while the normal and streamwise variations are taken into account using finite differences. The calculations suggest that a given disturbance imposed at some position along the wall will eventually reach a local equilibrium state essentially independent of the initial conditions. In fact, the equilibrium state reached is qualitatively similar to the large amplitude high wave-number solution described asymptotically by Hall (1982). In general, it is found that the nonlinear interactions are dominated by a mean field type of interaction between the mean flow and the fundamental. Thus, even though higher harmonics of the fundamental are necessarily generated, most of the disturbance energy is confined to the mean flow correction and the fundamental. A major result of the calculations is finding that the downstream velocity field develops a strongly inflectional character as the flow moves downstream. The latter result suggests that the major effect of Goertler vortices on boundary layers of practical importance might be to make them highly receptive to rapidly growing Rayleigh modes of instability.

Hall, Philip

1988-01-01

302

PSE approach to optimal disturbances in compressible boundary layers  

NASA Astrophysics Data System (ADS)

The problem of transient growth in compressible boundary layers is considered within the scope of partial differential equations. As it follows from previous investigations, the optimal disturbances correspond to steady counter rotating streamwise vortices. The corresponding scaling of the perturbations leads to the governing equations as for Gortler type of instability with the Gortler number equal to zero. The iteration procedure employs back and forth marching solutions of the adjoint and original systems of equations. At low Mach numbers, the results agree with results by Andersson, Berggren and Henningson (1999) and Luchini (2000) for Blasius boundary layer flow. In the case of a parallel flow, the method leads to the same results obtained for compressible flow within the scope of linearized Navier-Stokes equations (Tumin and Reshotko, 2001). The new method is applied to analysis of optimal disturbances in compressible boundary layers at various Mach numbers and temperature factors. Experimental data by E. White (2002) on transient growth are discussed in the light of the new results.

Tumin, Anatoli; Reshotko, Eli

2002-11-01

303

Cloud-Scale Numerical Modeling of the Arctic Boundary Layer  

NASA Technical Reports Server (NTRS)

The interactions between sea ice, open ocean, atmospheric radiation, and clouds over the Arctic Ocean exert a strong influence on global climate. Uncertainties in the formulation of interactive air-sea-ice processes in global climate models (GCMs) result in large differences between the Arctic, and global, climates simulated by different models. Arctic stratus clouds are not well-simulated by GCMs, yet exert a strong influence on the surface energy budget of the Arctic. Leads (channels of open water in sea ice) have significant impacts on the large-scale budgets during the Arctic winter, when they contribute about 50 percent of the surface fluxes over the Arctic Ocean, but cover only 1 to 2 percent of its area. Convective plumes generated by wide leads may penetrate the surface inversion and produce condensate that spreads up to 250 km downwind of the lead, and may significantly affect the longwave radiative fluxes at the surface and thereby the sea ice thickness. The effects of leads and boundary layer clouds must be accurately represented in climate models to allow possible feedbacks between them and the sea ice thickness. The FIRE III Arctic boundary layer clouds field program, in conjunction with the SHEBA ice camp and the ARM North Slope of Alaska and Adjacent Arctic Ocean site, will offer an unprecedented opportunity to greatly improve our ability to parameterize the important effects of leads and boundary layer clouds in GCMs.

Krueger, Steven K.

1998-01-01

304

Computational Study of Hypersonic Boundary Layer Stability on Cones  

NASA Astrophysics Data System (ADS)

Due to the complex nature of boundary layer laminar-turbulent transition in hypersonic flows and the resultant effect on the design of re-entry vehicles, there remains considerable interest in developing a deeper understanding of the underlying physics. To that end, the use of experimental observations and computational analysis in a complementary manner will provide the greatest insights. It is the intent of this work to provide such an analysis for two ongoing experimental investigations. The first focuses on the hypersonic boundary layer transition experiments for a slender cone that are being conducted at JAXA's free-piston shock tunnel HIEST facility. Of particular interest are the measurements of disturbance frequencies associated with transition at high enthalpies. The computational analysis provided for these cases included two-dimensional CFD mean flow solutions for use in boundary layer stability analyses. The disturbances in the boundary layer were calculated using the linear parabolized stability equations. Estimates for transition locations, comparisons of measured disturbance frequencies and computed frequencies, and a determination of the type of disturbances present were made. It was found that for the cases where the disturbances were measured at locations where the flow was still laminar but nearly transitional, that the highly amplified disturbances showed reasonable agreement with the computations. Additionally, an investigation of the effects of finite-rate chemistry and vibrational excitation on flows over cones was conducted for a set of theoretical operational conditions at the HIEST facility. The second study focuses on transition in three-dimensional hypersonic boundary layers, and for this the cone at angle of attack experiments being conducted at the Boeing/AFOSR Mach-6 quiet tunnel at Purdue University were examined. Specifically, the effect of surface roughness on the development of the stationary crossflow instability are investigated in this work. One standard mean flow solution and two direct numerical simulations of a slender cone at an angle of attack were computed. The direct numerical simulations included a digitally-filtered, randomly distributed surface roughness and were performed using a high-order, low-dissipation numerical scheme on appropriately resolved grids. Comparisons with experimental observations showed excellent qualitative agreement. Comparisons with similar previous computational work were also made and showed agreement in the wavenumber range of the most unstable crossflow modes.

Gronvall, Joel Edwin

305

Tropospheric ozone: Effects of cloud chemistry and boundary layer processes  

NASA Astrophysics Data System (ADS)

The sensitivity of tropospheric O3 to aqueous-phase radical chemistry in clouds is examined using photochemical model calculations of the O3 production efficiency per unit NO x and the chemical lifetime of O3. Based on typical cloud volume fractions and frequencies of air processing by clouds, we estimate that the maximum perturbation to O3 from cloud chemistry in the tropics and mid- latitudes summer is less than 3%. This result is supported by calculations using a three-dimensional, continental-scale model for North America. We argue that previous model analyses which reported large perturbations to O3 from cloud chemistry either did not use suitably quantitative diagnostics or assumed high liquid water abundances and high solubility for CH3O2. A 3-dimensional, continental-scale photochemical model is used to investigate seasonal variations in the budgets of O3 and NO y species (including NO x and its oxidation products) in the boundary layer over the United States, and to estimate the export of these species from the U.S. boundary layer to the global atmosphere. Model results are evaluated with year-round observations for O 3, CO, and NO y species. The model reproduces the observed seasonal variations of O3, NO x, NO x/NO y, and NO y deposition flux. It overestimates O3 over the south-central United States by 10-20 ppbv in summer and fall. The mean NO x/NO y ratio in the U.S. boundary layer in the model ranges from 0.2 in summer to 0.6 in winter, reflecting the seasonal change in the chemical lifetime of NO x. Formation of hydroxy organic nitrates during oxidation of isoprene, followed by decomposition of these nitrates to HNO3, accounts for 30% of the chemical sink of NO x in the U.S. boundary layer in summer. Peroxyacylnitrates (PANs) are most abundant in the U.S. boundary layer in spring (25% of total NO y), reflecting a combination of active photochemistry and low temperatures, and least abundant in winter (10% of NO y). The fraction of NO x emitted in the United States exported to the global atmosphere as NO x or PANs ranges from 15% in summer to 25% in winter. This export responds less-than-linearly to changes in NO x emissions in summer, but more-than-linearly in winter. The annual mean export of NO x + PANs from the U.S. boundary layer is estimated to be 1.2 Tg N yr-1, comparable to current estimates for the lightning source of NO x in the Northern Hemisphere; the resulting source of O3 production in the global troposphere is estimated to be 2 times more important than the direct export of O3 pollution from the U.S. boundary layer.

Liang, Jinyou

1997-11-01

306

Numerical simulations of spatially developing, accelerating boundary layers  

NASA Astrophysics Data System (ADS)

We present the results of direct and large-eddy simulations of spatially developing boundary layers subjected to favorable pressure gradient, strong enough to cause reversion of the flow towards a quasi-laminar state. The numerical results compare well with experimental data. Visualization of the flow structures shows the well-known stabilization of the streaks, the re-orientation of outer layer vortices in the streamwise direction, and the appearance of turbulent spots in the re-transition region. Both instantaneous visualizations and turbulent statistics highlight the significant damping of wall-normal and spanwise fluctuations. The fast component of the pressure fluctuations appears to be the main driver of this process, contributing to reduce pressure fluctuations and, as a consequence, the energy redistribution term in the Reynolds stress budgets. The streamwise stresses, in whose budget a separate production term plays a role, do not decay but remain frozen at their upstream value. The decrease of wall-normal and spanwise fluctuations appears to be the main cause of the inner-layer stabilization, by disrupting the generation and subsequent growth of streaks, consistent with various models of the turbulence-generation cycle proposed in the literature. The outer layer seems to play a passive role in this process. The stretching and reorientation of the outer-layer vortices results in a more orderly and organized structure; since fewer ejections occur, the inner layer does not break this re-organization, which is maintained until re-transition begins.

Piomelli, Ugo; Yuan, Junlin

2013-10-01

307

Coupling the dynamics of boundary layers and evolutionary dunes.  

PubMed

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

Ortiz, Pablo; Smolarkiewicz, Piotr K

2009-04-01

308

On the partially reacted boundary layer in rate sticks  

NASA Astrophysics Data System (ADS)

Using our temperature dependent reactive flow model (TDRR) to simulate detonation in a rate stick, we observe that a partially reacted layer (PRL) is formed near the boundary. We are not aware that such a PRL has been observed in tests, and this is why we regarded it in the past as a numerical artifact. Assuming that such an artefact may be caused by the finite rise time of the detonation shock, we showed in [1] how it can be eliminated by delaying the outward boundary motion for a length of time comparable with the shock rise time. Here we revisit the PRL problem. We first show that it is not a numerical artifact but a real phenomenon. We do this by repeating the reactive flow run with a finer mesh. By looking at the PRL structure, we see that doubling the resolution affects the PRL only slightly. We then conjecture that the PRL formation has to do with the finite duration of the reaction process (or the finite extent of the reaction zone). By the time the boundary rarefaction reaches a cell near the boundary, it may be only partially reacted, and its reaction may therefore be cut off. To establish our conjecture we show how the PRL structure changes with the reaction duration.

Partom, Y.

2014-05-01

309

High-Order Finite-Difference Schemes for Numerical Simulation of Hypersonic Boundary-Layer Transition  

Microsoft Academic Search

Direct numerical simulation (DNS) has become a powerful tool in studying fundamental phenomena of laminar-turbulent transition of high-speed boundary layers. Previous DNS studies of supersonic and hypersonic boundary layer transition have been limited to perfect-gas flow over flat-plate boundary layers without shock waves. For hypersonic boundary layers over realistic blunt bodies, DNS studies of transition need to consider the effects

Xiaolin Zhong

1998-01-01

310

Determining Boundary-Layer Height from Aircraft Measurements  

NASA Astrophysics Data System (ADS)

The height of the atmospheric boundary layer (ABL) is an important variable in both observational studies and model simulations. The most commonly used measurement for obtaining ABL height is a rawinsonde profile. Mesoscale or regional scale models use a bulk Richardson number based on profiles of the forecast variables. Here we evaluate the limitations of several frequently-used approaches for defining ABL height from a single profile, and identify the optimal threshold value for each method if profiles are the only available measurements. Aircraft measurements from five field projects are used, representing a variety of ABL conditions including stable, convective, and cloud-topped boundary layers over different underlying surfaces. ABL heights detected from these methods were validated against the `true' value determined from aircraft soundings, where ABL height is defined as the top of the layer with significant turbulence. A detection rate was defined to denote how often the ABL height was correctly diagnosed with a particular method. The results suggest that the temperature gradient method provides the most reasonable estimates, although the detection rate and suitable detection criteria vary for different types of ABL. The Richardson number method, on the other hand, is in most cases inadequate or inferior to the other methods that were tried. The optimal range of the detection criteria is given for all ABL types examined in this study.

Dai, C.; Wang, Q.; Kalogiros, J. A.; Lenschow, D. H.; Gao, Z.; Zhou, M.

2014-09-01

311

Cloud-Scale Numerical Modeling of the Arctic Boundary Layer  

NASA Technical Reports Server (NTRS)

The research objective of this NASA grant-funded project was to determine in detail how large-scale processes. in combination with cloud-scale radiative, microphysical, and dynamical processes, govern the formation and multi-layered structure of Arctic stratus clouds. This information will be useful for developing and improving 1D (one dimensional) boundary layer models for the Arctic. Also, to quantitatively determine the effects of leads on the large-scale budgets of sensible heat, water vapor, and condensate in a variety of Arctic winter conditions. This information will be used to identify the most important lead-flux processes that require parameterization in climate models. Our approach was to use a high-resolution numerical model, the 2D (two dimensional) University of Utah Cloud Resolving Model (UU CRM), and its 1D version, the University of Utah Turbulence Closure Model (UU TCM), a boundary layer model based on third-moment turbulence closure, as well as a large-eddy simulation (LES) model originally developed by C.H. Moeng.

Kruegen, Steven K.; Delnore, Victor E. (Technical Monitor)

2002-01-01

312

Universality of the quantum boundary layer for a Maxwellian gas  

NASA Astrophysics Data System (ADS)

For an ideal gas confined in a rectangular domain, it has been shown that the density is not homogenous even in thermodynamic equilibrium and it goes to zero within a layer near to the boundaries due to the wave character of particles. This layer has been called the quantum boundary layer (QBL). In literature, an analytical expression for the thickness of QBL has been given for only a rectangular domain since both energy eigenvalues and eigenfunctions of the Schrödinger equation can analytically be obtained for only a rectangular domain. In this study, ideal Maxwellian gases confined in spherical and cylindrical domains are considered to investigate whether the thickness of QBL is independent of the domain shape. Although the energy eigenvalues are the roots of Bessel functions and there is no analytical expression giving the roots, the thickness of QBL is expressed analytically by considering the density distributions and using some simplifications based on the numerical calculations. It is found that QBL has the same thickness for the domains of different shapes. Therefore, QBL seems to have a universal thickness independent of the domain shape for an ideal Maxwellian gas.

Firat, C.; Sisman, A.

2009-06-01

313

Vertical ozone characteristics in urban boundary layer in Beijing.  

PubMed

Vertical ozone and meteorological parameters were measured by tethered balloon in the boundary layer in the summer of 2009 in Beijing, China. A total of 77 tethersonde soundings were taken during the 27-day campaign. The surface ozone concentrations measured by ozonesondes and TEI 49C showed good agreement, albeit with temporal difference between the two instruments. Two case studies of nocturnal secondary ozone maxima are discussed in detail. The development of the low-level jet played a critical role leading to the observed ozone peak concentrations in nocturnal boundary layer (NBL). The maximum of surface ozone was 161.7 ppbv during the campaign, which could be attributed to abundant precursors storage near surface layer at nighttime. Vertical distribution of ozone was also measured utilizing conventional continuous analyzers on 325-m meteorological observation tower. The results showed the NBL height was between 47 and 280 m, which were consistent with the balloon data. Southerly air flow could bring ozone-rich air to Beijing, and the ozone concentrations exceeded the China's hourly ozone standard (approximately 100 ppb) above 600 m for more than 12 h. PMID:23129408

Ma, Zhiqiang; Xu, Honghui; Meng, Wei; Zhang, Xiaoling; Xu, Jing; Liu, Quan; Wang, Yuesi

2013-07-01

314

Fullerenes in the cretaceous-tertiary boundary layer  

SciTech Connect

High-pressure liquid chromatography with ultraviolet-visible spectral analysis of toluene extracts of samples from two Cretaceous-Tertiary (K-T) boundary sites in New Zealand has revealed the presence of C[sub 60] at concentrations of 0.1 to 0.2 parts per million of the associated soot. This technique verified also that fullerenes are produced in similar amounts in the soots of common flames under ambient atmospheric conditions. Therefore, the C[sub 60] in the K-T boundary layer may have originated in the extensive wildfires that were associated with the cataclysmic impact event that terminated the Mezozoic era about 65 million years ago.

Heymann, D.; Chibante, L.P.F.; Smalley, R.E. (Rice Univ., Houston, TX (United States)); Brooks, R.R. (Massey Univ., Palmerston North (New Zealand)); Wolbach, W.S. (Illinois Wesleyan Univ., Bloomington, IL (United States))

1994-07-29

315

A visual study of vortex-induced subcritical instability on a flat plate laminar boundary layer  

Microsoft Academic Search

This paper reports results of our experimental investigation on flow instability on a flat plate laminar boundary layer caused by a “captive” vortex migrating far outside the boundary layer. Results show that the sign of the circulation associated with the vortex is the main determinant for the severity of the boundary layer instability. A captive vortex with an opposite sign

T. T. Lim; T. K. Sengupta; M. Chattopadhyay

2004-01-01

316

A COMPARISON OF MODELS FOR THE WAVENUMBER–FREQUENCY SPECTRUM OF TURBULENT BOUNDARY LAYER PRESSURES  

Microsoft Academic Search

Aircraft cabin noise due to the fuselage boundary layer is determined by, among other factors, the wavenumber–frequency spectrum of the fluctuating boundary layer pressures, a quantity for which a number of models have been proposed. In this work predictions for the sound radiated by a boundary layer driven plate are investigated, with a view to determining which model is most

W. R. Graham

1997-01-01

317

Bound vortex boundary layer control with application to V\\/STOL airplanes  

Microsoft Academic Search

Effectiveness of the bound vortex boundary layer control is assessed with reference to airfoils modified with a leading edge rotating cylinder. Results of the test program and the numerical models suggest the following: • The surface singularity method in conjunction with the boundary layer correction scheme is capable of predicting useful information concerning bound vortex boundary layer control. The predicted

V. J. Modi; F. Mokhtarian; T. Yokomizo; G. Ohta; T. Oinuma

1988-01-01

318

A revised slug model boundary layer correction for starting jet vorticity flux  

Microsoft Academic Search

The flux of vorticity from a piston-cylinder vortex generator is commonly approximated using a model in which the fluid efflux is treated as a uniform slug of fluid with negligible boundary layer thickness. Shusser et al. (2002) introduced a correction to the slug model that accounts for boundary layer growth within the cylinder. We show that their implemented boundary layer

John O. Dabiri; Morteza Gharib

2004-01-01

319

A simple model of the atmospheric boundary layer; sensitivity to surface evaporation  

Microsoft Academic Search

A simple formulation of the boundary layer is developed for use in large-scale models and other situations where simplicity is required. The formulation is suited for use in models where some resolution is possible within the boundary layer, but where the resolution is insufficient for resolving the detailed boundary-layer structure and overlying capping inversion. Surface fluxes are represented in terms

I B Troen; L. Mahrt

1986-01-01

320

Nonlinear spatial evolution of inviscid instabilities on hypersonic boundary layers  

NASA Technical Reports Server (NTRS)

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

Wundrow, David W.

1996-01-01

321

Adaptive nonlinear polynomial neural networks for control of boundary layer/structural interaction  

NASA Astrophysics Data System (ADS)

The acoustic pressures developed in a boundary layer can interact with an aircraft panel to induce significant vibration in the panel. Such vibration is undesirable due to the aerodynamic drag and structure-borne cabin noises that result. The overall objective of this work is to develop effective and practical feedback control strategies for actively reducing this flow-induced structural vibration. This report describes the results of initial evaluations using polynomial, neural network-based, feedback control to reduce flow induced vibration in aircraft panels due to turbulent boundary layer/structural interaction. Computer simulations are used to develop and analyze feedback control strategies to reduce vibration in a beam as a first step. The key differences between this work and that going on elsewhere are as follows: that turbulent and transitional boundary layers represent broadband excitation and thus present a more complex stochastic control scenario than that of narrow band (e.g., laminar boundary layer) excitation; and secondly, that the proposed controller structures are adaptive nonlinear infinite impulse response (IIR) polynomial neural network, as opposed to the traditional adaptive linear finite impulse response (FIR) filters used in most studies to date. The controllers implemented in this study achieved vibration attenuation of 27 to 60 dB depending on the type of boundary layer established by laminar, turbulent, and intermittent laminar-to-turbulent transitional flows. Application of multi-input, multi-output, adaptive, nonlinear feedback control of vibration in aircraft panels based on polynomial neural networks appears to be feasible today. Plans are outlined for Phase 2 of this study, which will include extending the theoretical investigation conducted in Phase 2 and verifying the results in a series of laboratory experiments involving both bum and plate models.

Parker, B. Eugene, Jr.; Cellucci, Richard L.; Abbott, Dean W.; Barron, Roger L.; Jordan, Paul R., III; Poor, H. Vincent

1993-12-01

322

Observations of the atmospheric boundary layer height over Abu Dhabi, United Arab Emirates: Investigating boundary layer climatology in arid regions  

NASA Astrophysics Data System (ADS)

Strong sensible heat fluxes and deep turbulent mixing - together with marked dustiness and a low substrate water content - represent a characteristic signature in the boundary layer over hot deserts, resulting in "thicker" mixing layers and peculiar optical properties. Beside these main features however, desert ABLs present extremely complex local structures that have been scarcely addressed in the literature, and whose understanding is essential in modeling processes such as the transport of dust and pollutants, and turbulent fluxes of momentum, heat and water vapor in hyper-arid regions. In this study, we analyze a continuous record of observations of the atmospheric boundary layer (ABL) height from a single lens LiDAR ceilometer operated at Masdar Institute Field Station (24.4oN, 54.6o E, Abu Dhabi, United Arab Emirates), starting March 2013. We compare different methods for the estimation of the ABL height from Ceilometer data such as, classic variance-, gradient-, log gradient- and second derivation-methods as well as recently developed techniques such as the Bayesian Method and Wavelet covariance transform. Our goal is to select the most suited technique for describing the climatology of the ABL in desert environments. Comparison of our results with radiosonde observations collected at the nearby airport of Abu Dhabi indicate that the WCT and the Bayesian method are the most suitable tools to accurately identify the ABL height in all weather conditions. These two methods are used for the definition of diurnal and seasonal climatologies of the boundary layer conditional to different atmospheric stability classes.

Marzooqi, Mohamed Al; Basha, Ghouse; Ouarda, Taha B. M. J.; Armstrong, Peter; Molini, Annalisa

2014-05-01

323

Role of Helicity In Stability of The Atmospheric Boundary Layer  

NASA Astrophysics Data System (ADS)

The concept of helicity was introduced in magnetic hydrodynamics in the 1960s. Later, it was recognized that helicity plays a notable role in the usual "terrestrialT hydrodynamics, supporting the stability of structures and determining the develop- ment and destruction of intense vortices similar to tornadoes, tropical cyclones, cloud streets, etc. The Earth's rotation and surface friction generate a large-scale geostrophic wind rotating with height, referred to as the Ekman flow or the Ekman spiral. The Ek- man flow is helical and obviously produces helicity of the turbulent flow component. In turn, the helical properties of turbulence may change the structure of the Reynolds stress tensor, which affects steady-state regimes, including the Ekman flow itself. The self-consistent, semi-empirical model of the Ekman boundary layer with allowance for the helicity of the turbulent velocity field has been constructed. The model is de- velopment of the Monin model. Helicity reduces the mean turbulent energy, modifies the Ekman flow, diminishes the turn angle of the Ekman spiral and increases the effec- tive depth of the boundary layer. These effects are direct reflection of reduction of the energy flow to the small scales in helical turbulence. The turbulent helicity account- ing raises a dynamic inflection point instability threshold. The threshold of parallel instability on the contrary is slightly lowered. Thus essentially grow unstable modes increments. There are changes in scales and orientation of the unstable modes. The comparison with classical and modern boundary layer models and observation data on secondary roll circulation is discussed.

Chkhetiani, O. G.; Ponomarev, V. M.; Khapaev, A. A.

324

Investigation of Separation of the Turbulent Boundary Layer  

NASA Technical Reports Server (NTRS)

An investigation was conducted on a turbulent boundary layer near a smooth surface with pressure gradients sufficient to cause flow separation. The reynolds number was high, but the speeds were entirely within the incompressible flow range. The investigation consisted of measurements of mean flow, three components of turbulence intensity, turbulent shearing stress, and correlations between two fluctuation components at a point and between the same component of different points. The results are given in the form of tables and graphs. The discussion deals first with separation and then with the more fundamental question of basic concepts of turbulent flow.

Schubauer, G B; Klebanoff, P S

1951-01-01

325

Numerical simulation of convective boundary layer above polynyas and leads.  

NASA Astrophysics Data System (ADS)

Arctic region is very important as one of drivers for global atmosphere circulation. Meanwhile, results of modern global atmospheric models, both climatic and weather forecasting differs significantly from each other and observations in this region. One of the reasons for these uncertainties can be inaccurate simulation of ice and snow cover distribution, which accuracy depends in turn on variety of factors. Among others, appropriate parameterizations of atmospheric boundary layer over inhomogeneous surface, not explicitly resolved at the atmospheric model grid, can decrease these inaccuracies. The main objective of these parameterizations is to calculate surface heat and water vapor fluxes, averaged over the whole model cell. However, due to great differences in structure of boundary layers formed over cold ice and relatively warm open water, which cause nonlinear dependencies,the parameterizations suggested to the moment can hardly be regarded as applicable for "complete" set of synoptic scenarios . The present paper attempts to improve standard mosaic method of flux aggregation, which is still common in climate models [1]. The main idea is to derive heat fluxes using data from numerical experiments, explicitly reproducing most of sub grid (for global models) turbulence motions spectra, and compare with fluxes calculated using mosaic method implying the part of model domain to be a global model cell. The study is based on idealized high resolution (~10 m) experiments with typically observed surface parameters (temperature and roughness), ice-open water distribution, initial temperature and wind profiles distribution included in Large Eddy Simulation model of Insitute of Numerical Mathematics RAS [2],[3]. Analysis of other boundary layer characteristics such as its height, eddy diffusivity profiles, kinetic energy is presented. The modeling results are compared with field experiments' data gathered at White Sea. References: 1. V.M. Stepanenko, P.M. Miranda, V.N. Lykosov. Numerical simulation of mesoscale iteration of atmosphere and hydrological inhomogeneous surface (in Russian). Computational technologies,2006, vol. 11 No.7: p.118-127 2. A.V. Glazunov, V.N. Lykossov. Large eddy simulation of interaction of ocean and atmospheric boundary layers. Russian Journal of Numerical Analysis and Mathematical Modeling. 2003 Vol.18, No. 4: p.279-295 3. Glazunov A.V. Modeling of neutral-stratified turbulent flow over horizontal rough surface(in Russian) Izvestiya. Atmospheric and Oceanic Physics vol.42, No3: p.307-325

Debolskiy, Andrey; Stepanenko, Victor

2013-04-01

326

Regional scale evaporation and the atmospheric boundary layer  

NASA Technical Reports Server (NTRS)

In this review we briefly summarize some current models of evaporation and the atmospheric boundary layer (ABL) and discuss new experimental and computational oppurtunities that may aid our understanding of evaporation at these larger scales. In particular, consideration is given to remote sensing of the atmosphere, computational fluid dynamics and the role numerical models can play in understanding land-atmosphere interactions. These powerful modeling and measurement tools are allowing us to visualize and study spatial and temporal scales previously untouched, thereby increasing the oppurtunities to improve our understanding of land-atmosphere interaction.

Parlange, Marc B.; Eichinger, William E.; Albertson, John D.

1995-01-01

327

Boundary layer effects above a Himalayan valley near Mount Everest  

NASA Astrophysics Data System (ADS)

Periodical Wind Profiler and Radio Acoustic Sounding System observations have been commenced at the Himalayas' northern slope nearby Mount Everest in September 2005. Primarily data sets obtained 25 km remote from the glacier edge are utilized for a preliminary discussion of planetary boundary layer circulation resembling high alpine mountainous regions. Substantial findings include the detection of two wind shears and the phenomenon of glacier wind at a distance of 25 km from the glaciers. The latter lead to a reversed compensatory flow in a vertical scale of up to 2000 m above ground level, pointing at supra regional impact.

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

2007-04-01

328

Active wave control of boundary-layer transition  

NASA Technical Reports Server (NTRS)

The physical processes involved in the boundary layer instability-generation and transition process seem to pose basic restrictions on the implementation of active, wave-based transition-control methods. While suppression short of elimination is possible by these means for primary disturbances, wave-interaction instabilities demand that the control be implemented almost immediately after the appearance of primary disturbances; even slight delays can negate the intended beneficial effects, and this basic problem is exacerbated at the higher Reynolds numbers typical of aircraft in cruising flight. Three-dimensional disturbances are noted to be important in this context, together with the continuous regeneration of TS waves.

Thomas, Andrew S. W.

1990-01-01

329

Blasius boundary layer solution with slip flow conditions  

NASA Astrophysics Data System (ADS)

As the number of applications of micro electro mechanical systems, or MEMS, increase, the variety of flow geometries that must be analyzed at the micro-scale is also increasing. To date, most of the work on MEMS scale fluid mechanics has focused on internal flow geometries, such as microchannels. As applications such as micro-scale flyers are considered, it is becoming necessary to consider external flow geometries. Adding a slip-flow condition to the Blasius boundary layer allows these flows to be studied without extensive computation.

Martin, Michael J.; Boyd, Iain D.

2001-08-01

330

Numerical analysis of Weyl's method for integrating boundary layer equations  

NASA Technical Reports Server (NTRS)

A fast method for accurate numerical integration of Blasius equation is proposed. It is based on the limit interchange in Weyl's fixed point method formulated as an iterated limit process. Each inner limit represents convergence to a discrete solution. It is shown that the error in a discrete solution admits asymptotic expansion in even powers of step size. An extrapolation process is set up to operate on a sequence of discrete solutions to reach the outer limit. Finally, this method is extended to related boundary layer equations.

Najfeld, I.

1982-01-01

331

Turbulence modeling in shock wave/turbulent boundary layer interactions  

NASA Technical Reports Server (NTRS)

The research performed was an experimental program to help develop turbulence models for shock wave boundary layer interactions. The measurements were taken in a Mach 3, 16 deg compression corner interaction, at a unit Reynolds number of 63 x 10(exp 6)/m. The data consisted of heat transfer data taken upstream and downstream of the interaction, hot wire measurements of the instantaneous temperature and velocity fluctuations to verify the Strong Reynolds Analogy, and single- and double-pulsed Rayleigh scattering images to study the development of the instantaneous shock/turbulence interaction.

Smits, A. J.

1992-01-01

332

Distributed boundary layer suction utilizing wing tip effects  

E-print Network

AND RECOMMENDATIONS 31 Recommendations for Other Applications Continuation of Investigation 32 33 REFERENCES PRESENTATION OF DATA . APPENDIX 34 53 iv LIST OF FIGURES Figure Page Mounting Panel for Changing Angles of Attack. Cut-away of Wing Model Shape... of Hoerner Tip Used for Suction. Wake Rake 10 Boundary Layer Probe. 10 10 Diagram of Normal Wing Force. Pro]ection of Pressure Force on the Chord Spanwise Tip Flow Diagram of Angles of Attack Wing Under Construction . 13 15 18 26 37 Tunnel...

Edwards, Jay Thomas

2012-06-07

333

Orbiter Entry Aeroheating Working Group Viscous CFD Boundary Layer Transition Trailblazer Solutions  

NASA Technical Reports Server (NTRS)

Boundary layer transition correlations for the Shuttle Orbiter have been previously developed utilizing a two-layer boundary layer prediction technique. The particular two-layer technique that was used is limited to Mach numbers less than 20. To allow assessments at Mach numbers greater than 20, it is proposed to use viscous CFD to the predict boundary layer properties. This report addresses if the existing Orbiter entry aeroheating viscous CFD solutions, which were originally intended to be used for heat transfer rate predictions, adequately resolve boundary layer edge properties and if the existing two-layer results could be leveraged to reduce the number of needed CFD solutions. The boundary layer edge parameters from viscous CFD solutions are extracted along the wind side centerline of the Space Shuttle Orbiter at reentry conditions, and are compared with results from the two-layer boundary layer prediction technique. The differences between the viscous CFD and two-layer prediction techniques vary between Mach 6 and 18 flight conditions and Mach 6 wind tunnel conditions, and there is not a straightforward scaling between the viscous CFD and two-layer values. Therefore: it is not possible to leverage the existing two-layer Orbiter flight boundary layer data set as a substitute for a viscous CFD data set; but viscous CFD solutions at the current grid resolution are sufficient to produce a boundary layer data set suitable for applying edge-based boundary layer transition correlations.

Wood, William A.; Erickson, David W.; Greene, Francis A.

2007-01-01

334

The Denver Cyclone. Part II: Interaction with the Convective Boundary Layer.  

NASA Astrophysics Data System (ADS)

The effect of surface heating on the flow past an isolated obstacle is examined with the aid of a nonlinear numerical model. These simulations extend the results of Part I, which considered the adiabatic, stratified flow around the obstacle. When the obstacle is heated, substantial low-level shear develops in the lee as the flow converges at low levels and diverges above. A linear model is developed to explain some of the details of this shear pattern. In this model, vertical shear is produced by differential heating and removed by mixing.Some of the small-scale circulations that develop in the convective boundary layer are then discussed. A thermal instability predominates in the lowest levels of the boundary layer with its axis aligned along the low-level shear vector. Higher in the boundary layer, a transverse mode appears and breaks the thermal instability into three-dimensional maxima. The transverse nature of this mode, the existence of an inflection point, and the low Richardson number suggest that this mode is a shearing instability.The convergence/vorticity zone in the lee of the obstacle (described in Part I) is then examined in detail. Several small-scale vortices develop along this zone at points where the thermal instabilities intersect. Observational studies have indicated that these boundary layer vortices often spawn tornadoes. It is shown that the vertical vorticity in these circulations is due to stretching of the preexisting vorticity along the convergence zone.The small-scale circulations in the boundary layer force a gravity wave response (with 10 km) in the stratified atmosphere above. The vertical velocity in these waves exceeds 1 m s1 in certain regions of the flow. A model is developed to explain how the boundary layer eddies with horizontal scales of 2-4 km can force a 10 km wave response above. This model depends on the fact that the vertical group velocity is inversely proportional to the horizontal wavelength as well as on a feedback process in which the gravity waves modulate the boundary layer eddies.

Crook, N. Andrew; Clark, Terry L.; Moncrieff, Mitchell W.

1991-10-01

335

Aerodynamic Models for Hurricanes III. Modeling hurricane boundary layer  

E-print Network

The third paper of the series (see previous ones in Refs.[1-2]) discusses basic physicalprocesses in the (quasi-) steady hurricane boundary layer (HBL), develops an approximate airflow model, establishes the HBL structure, and presents integral balance relations for dynamic and thermodynamic variables in HBL. Models of evaporation and condensation are developed, where the condensation is treated similarly to the slow combustion theory. A turbulent approximation for the lower sub-layer of HBL is applied to the sea-air interaction to establish the observed increase in angular momentum in the outer region of HBL.A closed set of balance relations has been obtained. Simple analytical solution of the set yields expressions for the basic dynamic variables - maximal tangential and radial velocities in hurricane, maximal vertical speed in eye wall, the affinity speed of hurricane travel, and the maximal temperature increase after condensation. Estimated values of the variables seem to be realistic. An attempt is also ...

Leonov, Arkady I

2008-01-01

336

Nanodiamonds in the Younger Dryas boundary sediment layer.  

PubMed

We report abundant nanodiamonds in sediments dating to 12.9 +/- 0.1 thousand calendar years before the present at multiple locations across North America. Selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval. Cubic diamonds form under high temperature-pressure regimes, and n-diamonds also require extraordinary conditions, well outside the range of Earth's typical surficial processes but common to cosmic impacts. N-diamond concentrations range from approximately 10 to 3700 parts per billion by weight, comparable to amounts found in known impact layers. These diamonds provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondrites or comets at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America. PMID:19119227

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

2009-01-01

337

Data Assimilation Strategies in the Planetary Boundary Layer  

NASA Astrophysics Data System (ADS)

We investigate the effect of the assimilation of surface and boundary-layer mass-field observations on the planetary boundary layer (PBL) within a one-dimensional (1D) version of the non-hydrostatic Fifth-Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5). We focus on the vertical extent and effects of mass-field nudging within the PBL based on surface observations, and the added value of assimilating column mass observations within the PBL. Model experiments for dynamic initialization and dynamic analysis are conducted and composited for 29 May, 6 June, and 7 June 2002 during the International H2O Project (IHOP) over the Southern Great Plains, U.S.A. Advantages are found when the data assimilation uses the innovation (the difference between the modelled value and the observed value) calculated by comparing the surface mass-field observation to the model value at the 2-m observation height rather than at the lowest model level. It is shown that this innovation can be applied throughout the model-diagnosed PBL via nudging during free-convective conditions because of the well-mixed nature of the PBL. However, in stable conditions, due to decreased vertical mixing the surface innovation may be best applied only in a shallow layer adjacent to the surface. Surface air-temperature innovations were also applied to the top soil-layer temperature to minimize disruption to the surface energy balance. In combination with the surface observations, the use of within-PBL mass-field data assimilation improves the simulated PBL structure.

Reen, Brian P.; Stauffer, David R.

2010-11-01

338

Onset of new particle formation in boundary layer  

NASA Astrophysics Data System (ADS)

At this moment, the mechanisms of atmospheric new particle formation (NPF), and the vapors participating in this process are not truly understood. Especially, in which part of the atmosphere the NPF takes place, is still an open question. To detect directly the very first steps of NPF in the atmosphere, we measured these chemical and physical processes within the Planetary Boundary Layer (PBL). We used airborne Zeppelin and Cessna measurements, and ground based in-situ measurements. Using Zeppelin, we focused on the time of the development of the PBL (altitudes up to 1 km) from sunrise until noon to measure vertical profiles of aerosol particles and chemical compounds. This is also the time when NPF typically occurs at ground level. On summer 2012, Zeppelin was measuring nucleation occurring in the polluted Po Valley area, Northern Italy, especially over the San Pietro Capofiume field site. A year later, Zeppelin had a spring campaign in boreal forest area, close to Hyytiälä field site in Southern Finland. During both campaigns, we aimed on measuring the vertical and the horizontal extension for NPF events using an instrumented Zeppelin. The vertical profile measurements represent the particle and gas concentrations in the lower parts of the atmosphere: the residual layer, the nocturnal boundary layer, and the PBL. At the same time, the ground based measurements records present conditions in the surface layer. The key instruments to measure the onset of NPF were an Atmospheric Pressure interface Time-Of-Flight mass spectrometer (APi-TOF), a Particle Size Magnifier (PSM), and a Neutral cluster and Air Ion Spectrometer (NAIS). These instruments are able to measure particles at the size range ~1-2 nm where atmospheric nucleation and cluster activation takes place. The onset of NPF was usually observed onboard Zeppelin when it was measuring inside the rising mixed layer which is connected to the surface layer by effective vertical mixing. The newly formed, subsequently growing, particles were observed to be homogeneously distributed inside the mixed layer. These measurements are part of the PEGASOS project which aims to quantify the magnitude of regional to global feedbacks between the atmospheric chemistry and physics, and thus quantify the changing climate.

Manninen, Hanna E.; Lampilahti, Janne; Mirme, Sander; Nieminen, Tuomo; Ehn, Mikael; Pullinen, Iida

2014-05-01

339

Climate factors of the high-latitude planetary boundary layer  

NASA Astrophysics Data System (ADS)

The global earth system models (ESMs) are widely used to study the earth's system climate and its future change in climate forcing scenarios. However, the coarse spatial resolution of the state-of-the-art ESMs requires parameterization of the small-scale climate dynamics and the essential physical processes in the climate system. The vertical turbulent exchange in the planetary boundary layer (PBL) is one of the key small-scale dynamical processes with large and non-trivial effect on the earth's climate. The PBL effects are particularly significant in high latitudes where the PBL is shallow and sensitive to the changes in the surface and cloud properties. This study highlights four climate factors linked to the high-latitude PBL: 1. Amplification of the climate forcing signal and the natural variability in the shallow PBL 2. Asymmetric response of the PBL on warm and cold advection events 3. Suppression/enhancement of the vertical mixing by PBL structures over heterogenic surface 4. Indirect effect of the PBL variability on larger climatologic spatial and time scales Recent research efforts revealed that the direct turbulent coupling between the atmosphere and the ocean or land surface as expressed through the flux-gradient approximation is not adequate on both the large climatologic scales and the small turbulence scales. On the large scales, the aggregated PBL effects depend on the degree of adjustment between the component of the climate system, e.g. between the atmospheric circulation and the ocean heat content anomalies. On the small scales, the flux-gradient approximation is not appropriate to describe the non-local turbulent mixing dynamics and the aggregated effects of the turbulence self-organization. Any state-of-the-art ESM includes a number of physical and bio-geochemical processes with the aggregated climate impact determined by the details of the turbulence dynamics. It justifies the efforts towards systematic studies of the climate factors sensitive to the asymmetric and indirect aggregated effects of the PBL.

Esau, Igor

2014-05-01

340

Free-stream disturbances, continuous eigenfunctions, boundary-layer instability and transition  

NASA Technical Reports Server (NTRS)

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

Salwen, H.

1980-01-01

341

About possible mechanisms of influence of gas bubbles on characteristics of turbulent boundary layer  

Microsoft Academic Search

Two different mechanisms responsible for the were revealed impact of gas bubbl injected into a boundary layer on the shear\\u000a stress on the wetted surfaces. Both mechanisms exist due to extremely high sensitivity of bubbles even to very low pressure\\u000a gradients and due to a high value of the virtual mass and coefficient of viscous drag for bubbles. The first

L. I. Maltsev; A. G. Malyuga; B. G. Novikov

2006-01-01

342

Transition in a Supersonic Boundary Layer Due to Acoustic Disturbances  

NASA Technical Reports Server (NTRS)

The boundary layer receptivity process due to the interaction of three-dimensional slow and fast acoustic disturbances with a blunted flat plate is numerically investigated at a free stream Mach number of 3.5 and at a high Reynolds number of 10(exp 6)/inch. The computations are performed with and without two-dimensional isolated roughness element located near the leading edge. Both the steady and unsteady solutions are obtained by solving the full Navier-Stokes equations using the fifth-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 simulations showed that the linear instability waves are generated very close to the leading edge. The wavelength of the disturbances inside the boundary layer first increases gradually and becomes longer than the wavelength for the instability waves within a short distance from the leading edge. The wavelength then decreases gradually and merges with the wavelength for the Tollmien-Schlichting wave. The initial amplitudes of the instability waves near the neutral points, the receptivity coefficients, are about 1.20 and 0.07 times the amplitude of the free-stream disturbances for the slow and the fast waves respectively. It was also revealed that small isolated roughness element does not enhance the receptivity process for the given nose bluntness.

Balakumar, P.

2005-01-01

343

Transition in a Supersonic Boundary Layer due to Acoustic Disturbances  

NASA Technical Reports Server (NTRS)

The boundary layer receptivity process due to the interaction of three-dimensional slow and fast acoustic disturbances with a blunted flat plate is numerically investigated at a free stream Mach number of 3.5 and at a high Reynolds number of 106/inch. The computations are performed with and without two-dimensional isolated roughness element located near the leading edge. Both the steady and unsteady solutions are obtained by solving the full Navier-Stokes equations 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 simulations showed that the linear instability waves are generated very close to the leading edge. The wavelength of the disturbances inside the boundary layer first increases gradually and becomes longer than the wavelength for the instability waves within a short distance from the leading edge. The wavelength then decreases gradually and merges with the wavelength for the Tollmien_Schlichting wave. The initial amplitudes of the instability waves near the neutral points, the receptivity coefficients, are about 1.20 and 0.07 times the amplitude of the free-stream disturbances for the slow and the fast waves respectively. It was also revealed that small isolated roughness element does not enhance the receptivity process for the given nose bluntness.

Balakumar, Ponnampalam

2004-01-01

344

Characteristic Lifelength of Coherent Structure in the Turbulent Boundary Layer  

NASA Technical Reports Server (NTRS)

A characteristic lifelength is defined by which a Gaussian distribution is fit to data correlated over a 3 sensor array sampling streamwise sidewall pressure. The data were acquired at subsonic, transonic and supersonic speeds aboard a Tu-144. Lifelengths are estimated using the cross spectrum and are shown to compare favorably with Efimtsov's prediction of correlation space scales. Lifelength distributions are computed in the time/frequency domain using an interval correlation technique on the continuous wavelet transform of the original time data. The median values of the lifelength distributions are found to be very close to the frequency averaged result. The interval correlation technique is shown to allow the retrieval and inspection of the original time data of each event in the lifelength distribution, thus providing a means to locate and study the nature of the coherent structure in the turbulent boundary layer. The lifelength data can be converted to lifetimes using the convection velocity. The lifetime of events in the time/frequency domain are displayed in Lifetime Maps. The primary purpose of the paper is to validate these new analysis techniques so that they can be used with confidence to further characterize coherent structure in the turbulent boundary layer.

Palumbo, Daniel L.

2006-01-01

345

Flow Coefficient Behavior for Boundary Layer Bleed Holes and Slots  

NASA Technical Reports Server (NTRS)

An experimental investigation into the flow coefficient behavior for nine boundary layer bleed orifice configurations is reported. This test was conducted for the purposes of exploring boundary layer control through mass flow removal and does not address issues of stability bleed. Parametric data consist of bleed region flow coefficient as a function of Mach number, bleed plenum pressure, and bleed orifice geometry. Seven multiple hole configurations and two single slot configurations were tested over a supersonic Mach number range of 1.3 to 2.5 (nominal). Advantages gained by using multiple holes in a bleed region instead of a single spanwise slot are discussed and the issue of modeling an entire array of bleed orifices based on the performance of a single orifice is addressed. Preconditioning the flow approaching a 90 degree inclined (normal) hole configuration resulted in a significant improvement in the performance of the configuration. The same preconditioning caused only subtle changes in performance for a 20 degree inclined (slanted) configuration.

Willis, B. P.; Davis, D. O.; Hingst, W. R.

1995-01-01

346

Sound generation by boundary-layer flow over steps  

NASA Astrophysics Data System (ADS)

Large-eddy simulations of turbulent boundary layer flows over backward and forward facing steps are performed to study flow-induced noise at low Mach number. The Reynolds number is 21000 based on the step height and free-stream velocity. The boundary layer thickness is approximately twice the step height near the step. Statistics of wall pressure fluctuations such as the root-mean-square values and frequency spectra yield favorable comparisons with previous experimental measurements. Sound generated by flow over backward and forward steps is examined in the framework of Lighthill's acoustic analogy. Green's function for the step geometry that is valid for an acoustically compact step height is employed to evaluate the volume integral in the solution to Lighthill's equation. For a far-field observer, the steps act primarily as a dipole source aligned in the streamwise direction. In line with experimental results, the flow over a forward step emits sound that is significantly stronger than that from a backward step. The underlying reason is analyzed in terms of source strength and distribution relative to the Green's function distribution. It is found that the forward step generates stronger source in regions closer to the upper step corner, which is acoustically most important.

Ji, Minsuk; Wang, Meng

2008-11-01

347

Supersonic Boundary-Layer Control: Bleed-Induced Shock Structure  

NASA Astrophysics Data System (ADS)

Bleed of a supersonic boundary layer through rows of normal and inclined circular holes has been found to be effective in controlling flow separation and in minimizing flow distortions caused by adverse pressure gradients from incident and reflected shock waves and from curvatures in geometry. This is accomplished by not just removing low momentum fluid next to walls but also through the formation of what are referred to as barrier shocks, one about each bleed hole, which collectively can block downstream adverse pressure gradients from propagating upstream. Unfortunately, these shocks also introduce considerable disturbances into the flow by curving and bending three-dimensionally incident and reflected shock waves. The structure of these barrier shocks is quite complicated because of the spanwise convex geometry of the holes and the interactions with the flow in neighboring bleed holes. Computations based on the low-Reynolds-number shear-stress-transport k-omega turbulence model were used to study the structure of the barrier shocks. Parameters investigated include rows of aligned and misaligned normal and inclined circular holes in which the diameter of the holes is comparable to the displacement thickness of the approaching boundary-layer flow.

Shih, Tom; Flores, Andrew

1999-11-01

348

The turning of the wind in the atmospheric boundary layer  

NASA Astrophysics Data System (ADS)

Here we use accurate observations of the wind speed vector to analyze the behavior with height of the wind direction. The observations are a combination of tall meteorological mast and long-range wind lidar measurements covering the entire atmospheric boundary layer. The observations were performed at the Høvsøre site in Denmark, which is a flat farmland area with a nearly homogeneous easterly upstream sector. Therefore, within that sector, the turning of the wind is caused by a combination of atmospheric stability, Coriolis, roughness, horizontal pressure gradient and baroclinity effects. Atmospheric stability was measured using sonic anemometers placed at different heights on the mast. Horizontal pressure gradients and baroclinity are derived from outputs of a numerical weather prediction model and are used to estimate the geostrophic wind. It is found, for these specific and relatively short periods of analysis, that under both barotropic and baroclinic conditions, the model predicts the gradient and geostrophic wind well, explaining for a particular case an 'unusual' backing of the wind. The observed conditions at the surface, on the other hand, explain the differences in wind veering. The simulated winds underpredict the turning of the wind and the boundary-layer winds in general.

Peña, Alfredo; Gryning, Sven-Erik; Floors, Rogier

2014-06-01

349

A Boundary-Layer Scaling for Turbulent Katabatic Flow  

NASA Astrophysics Data System (ADS)

Scaling relationships are proposed for the turbulent katabatic flow of a stably stratified fluid down a homogeneously cooled planar slope—the turbulent analogue of a Prandtl-type slope flow. The Theorem predicts that such flows are controlled by three non-dimensional parameters: the slope angle, the Prandtl number, and a Reynolds number defined in terms of the surface thermal forcing (surface buoyancy or surface buoyancy flux), Brunt-Väisälä frequency, slope angle, and molecular viscosity and diffusivity coefficients. However, by exploiting the structure of the governing differential equations in a boundary-layer form, scaled equations are deduced that involve only two non-dimensional parameters: the Prandtl number and a modified Reynolds number. In the proposed scaling framework, the slope angle does not appear as an independent governing parameter, but merely acts as a stretching factor in the scales for the dependent and independent variables, and appears in the Reynolds number. Based on the boundary-layer analysis, we hypothesize that the full katabatic-flow problem is largely controlled by two rather than three parameters. Preliminary tests of the scaling hypothesis using data from direct numerical simulations provide encouraging results.

Shapiro, Alan; Fedorovich, Evgeni

2014-10-01

350

Hypersonic boundary-layer transition on a flared cone  

NASA Astrophysics Data System (ADS)

Transition on a flared cone with zero angle of attack was studied in our newly established Mach 6 quiet wind tunnel (M6QT) via wall pressure measurement and flow visualization. High-frequency pressure transducers were used to measure the second-mode waves' amplitudes and frequencies. Using pulsed schlieren diagnostic and Rayleigh scattering technique, we got a clear evolution of the second-mode disturbances. The second-mode waves exist for a long distance, which means that the second-mode waves grow linearly in a large region. Strong Mach waves are radiated from the edge of the boundary layer. With further development, the second-mode waves reach their maximum magnitude and harmonics of the second-mode instability appear. Then the disturbances grow nonlinearly. The second modes become weak and merge with each other. Finally, the nonlinear interaction of disturbance leads to a relatively quiet zone, which further breaks down, resulting in the transition of the boundary layer. Our results show that transition is determined by the second mode. The quiet zone before the final breakdown is observed in flow visualization for the first time. Eventual transition requires the presence of a quiet zone generated by nonlinear interactions.

Zhang, Chuan-Hong; Tang, Qing; Lee, Cun-Biao

2013-02-01

351

Generation of 'artificial' bursts in a turbulent boundary layer  

NASA Technical Reports Server (NTRS)

In an effort to better understand the physics and structure of coherent events in a turbulent boundary layer, an attempt is made to produce 'artificial' bursts. These are generated in a unique turbulent boundary layer, developed on a flat plate towed in an 18-m water channel, and thus with negligible freestream turbulence. The burst-like events are produced by either withdrawing near-water fluid from two minute holes separated in the spanwise direction, or by pitching a miniature delta wing that is flush-mounted to the wall. Either of these two actions generates a hairpin-like vortex and low-speed streak that resemble naturally occurring structures. The resulting sequence of events that occur at a given location can be controlled at will, thus allowing detailed examination via phase-locked measurements and flow visualization. In this paper, the artificial bursts are compared with natural, random bursts, using flow visualization and hot-film signals. Detailed quantitative data on topographical details and dynamical significance of the bursting structure will be addressed in a forthcoming article.

Gad-El-hak, M.; Hussain, A. K. M. F.

1986-01-01

352

Clouds, Precipitation and Marine Boundary Layer Structure during MAGIC (Invited)  

NASA Astrophysics Data System (ADS)

Marine boundary layer clouds in the subtropics play a key role in cloud-climate feedbacks that are poorly understood and are key elements in biases in seasonally coupled model forecasts and simulated mean climate. In particular, the representation of the transition from the stratocumulus (Sc) regime, to shallow cumulus (Cu) underlines one of the most challenging problems to the modeling community In MAGIC, the Marine ARM (Atmospheric Radiation Measurement) GPCI (Global Energy and Water Cycle Experiment [GEWEX] Cloud System Studies [GCSS] Pacific Cross-section Intercomparison) Investigation of Clouds study the second Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF2) during the Marine ARM GPCI Investigation of Clouds (MAGIC) provided an unparalleled dataset to study the statistical properties of MBL clouds and the transitions between Sc and Cu. Utilizing AMF2, we develop an objective scheme to identify MBL cloud occurrence across each leg and to recognize some important properties of different MBL cloud (e.g. Sc and Cu) and precipitation types. The variability and frequency of occurrence of the different cloud and precipitation events is presented with emphasis on the various MBL cloud structures. A statistical analysis of macroscopic properties (e.g. Inversion and transition layer) and cloud structure (e.g. cloud boundaries) is preformed relating to the thermodynamic profiles. Further emphasis is placed on the differentiation between Cu and Sc regimes as well as the presence of decoupling.

Kollias, P.; Zhou, X.; Lewis, E. R.

2013-12-01

353

Optimal Disturbances in Boundary Layers Subject to Streamwise Pressure Gradient  

NASA Technical Reports Server (NTRS)

An analysis of the non-modal growth of perturbations in a boundary layer in the presence of a streamwise pressure gradient is presented. The analysis is based on PSE equations for an incompressible fluid. Examples with Falkner- Skan profiles indicate that a favorable pressure gradient decreases the non-modal growth while an unfavorable pressure gradient leads to an increase of the amplification. It is suggested that the transient growth mechanism be utilized to choose optimal parameters of tripping elements on a low-pressure turbine (LPT) airfoil. As an example, a boundary-layer flow with a streamwise pressure gradient corresponding to the pressure distribution over a LPT airfoil is considered. It is shown that there is an optimal spacing of the tripping elements and that the transient growth effect depends on the starting point. The amplification is found to be small at the LPT s very low Reynolds numbers, but there is a possibility to enhance the transient energy growth by means of wall cooling.

Ashpis, David E.; Tumin, Anatoli

2003-01-01

354

Application of a general boundary layer analysis to turbulent boundary layers subjected to strong favorable pressure gradients  

NASA Technical Reports Server (NTRS)

Theoretical predictions of turbulent boundary layer development under the influence of strong favorable pressure gradients made using a finite-difference calculation procedure are compared to experimental data. Comparisons are presented for low speed flows with and without wall heat transfer as well as for supersonic flows with adiabatic walls. The turbulence model used is governed by an integral form of the turbulence kinetic energy equation and the results are compared with predictions made using a conventional equilibrium turbulence model based upon Prandtl's mixing length, a Clauser-type eddy viscosity model used by Cebecci and Mosinskis, and a two-equation turbulence energy model of Launder and Jones.

Kreskovsky, J. P.; Shamroth, S. J.; Mcdonald, H.

1975-01-01

355

Numerical Modeling Studies of Wake Vortex Transport and Evolution Within the Planetary Boundary Layer  

NASA Technical Reports Server (NTRS)

In support of the wake vortex effect of the Terminal Area Productivity program, we have put forward four tasks to be accomplished in our proposal. The first task is validation of two-dimensional wake vortex-turbulence interaction. The second task is investigation of three-dimensional interaction between wake vortices and atmospheric boundary layer (ABL) turbulence. The third task is ABL studies. The, fourth task is addition of a Klemp-Durran condition at the top boundary for TASS model. The accomplishment of these tasks will increase our understanding of the dynamics of wake vortex and improve forecasting systems responsible for air safety and efficiency. The first two tasks include following three parts: (a) Determine significant length scale for vortex decay and transport, especially the length scales associated with the onset of Crow instability (Crow, 1970); (b) Study the effects of atmospheric turbulence on the decay of the wake vortices; and (c) Determine the relationships between decay rate, transport properties and atmospheric parameters based on large eddy simulation (LES) results and the observational data. These parameters may include turbulence kinetic energy, dissipation rate, wind shear and atmospheric stratification. The ABL studies cover LES modeling of turbulence structure within planetary boundary layer under transition and stable stratification conditions. Evidences have shown that the turbulence in the stable boundary layer can be highly intermittent and the length scales of eddies are very small compared to those in convective case. We proposed to develop a nesting grid mesh scheme and a modified Klemp-Durran conditions (Klemp and Wilhelmson, 1978) at the top boundary for TASS model to simulate planetary boundary layer under stable stratification conditions. During the past year, our group has made great efforts to carry out the above mentioned four tasks simultaneously. The work accomplished in the last year will be described in the next section.

Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael L.; Shen, Shaohua

1998-01-01

356

Atmospheric boundary layer rolls: Quantification of their effect on the hydrodynamics in the German Bight  

NASA Astrophysics Data System (ADS)

Atmospheric boundary layer rolls and their impact on upper ocean circulation were investigated using a combination of two high-resolution data sources: (1) data from the Advanced Synthetic Aperture Radar (ASAR) onboard ENVISAT with a spatial sampling of approximately 500 m × 500 m and (2) continuous observations taken at the research platform FINO 1 with 2 min temporal sampling at eight heights between 33 and 100 m. The parallel analysis of instantaneous image data in combination with the FINO 1 time series enabled us to quantify both the spatial and temporal dynamics of mesoscale and submesoscale wind variations. The influence of these variations with different temporal and spatial scales on the hydrodynamics of the German Bight was addressed using outputs from a three-dimensional circulation model. It was demonstrated that while the coupling between wind and tidal forcing triggered substantial responses at mesoscales, the response of surface currents and sea surface temperature to the atmospheric boundary layer rolls appeared relatively weak. However, these ocean surface responses closely follow the surface footprint of the atmospheric boundary layer rolls, the signatures of which become more pronounced in the absence of strong tidal flows.

Müller, Sabine; Stanev, Emil V.; Schulz-Stellenfleth, Johannes; Staneva, Joanna; Koch, Wolfgang

2013-10-01

357

Prandtl boundary layers for the Phan-Thien Tanner and Giesekus fluid  

NASA Astrophysics Data System (ADS)

The Prandtl equations, arising naturally in the description of high Reynolds number boundary layers, have turned out to be quite difficult from the point of view of mathematical analysis. Recent work by the author has shown that the analogous problem for the upper-convected Maxwell fluid is actually better behaved, and the well-posedness of the boundary layer equations has been established. In this paper, boundary layers for the Phan-Thien-Tanner and Giesekus fluid are considered. It turns out that there are two fundamentally different types of boundary layers, which we shall call elastic and viscometric boundary layers. The elastic boundary layers will be analyzed in an analogous fashion as those for the upper-convected Maxwell fluid. On the other hand, for viscometric boundary layers, which occur only for the PTT fluid, the equations are equivalent to those for a power law fluid.

Renardy, Michael

2014-05-01

358

Interface Modes and Their Instabilities in Accretion Disc Boundary Layers  

E-print Network

We study global non-axisymmetric oscillation modes trapped near the inner boundary of an accretion disc. Observations indicate that some of the quasi-periodic oscillations (QPOs) observed in the luminosities of accreting compact objects (neutron stars, black holes and white dwarfs) are produced in the inner-most regions of accretion discs or boundary layers. Two simple models are considered in this paper: The magnetosphere-disc model consists of a thin Keplerian disc in contact with a uniformly rotating magnetosphere with and low plasma density, while the star-disc model involves a Keplerian disc terminated at the stellar atomosphere with high density and small density scale height. We find that the interface modes at the magnetosphere-disc boundary are generally unstable due to Rayleigh-Taylor and/or Kelvin-Helmholtz instabilities. However, differential rotation of the disc tends to suppress Rayleigh-Taylor instability and a sufficiently high disc sound speed (or temperature) is needed to overcome this suppression and to attain net mode growth. On the other hand, Kelvin-Helmholtz instability may be active at low disc sound speeds. We also find that the interface modes trapped at the boundary between a thin disc and an unmagnetized star do not suffer Rayleigh-Taylor or Kelvin-Helmholtz instability, but can become unstable due to wave leakage to large disc radii and, for sufficiently steep disc density distributions, due to wave absorption at the corotation resonance in the disc. The non-axisymmetric interface modes studied in this paper may be relevant to the high-frequency QPOs observed in some X-ray binaries and in cataclysmic variables.

David Tsang; Dong Lai

2008-12-20

359

Laser Doppler velocimeter measurements of boundary layer velocity and turbulent intensities in Mach 2.5 flow  

NASA Technical Reports Server (NTRS)

In recent years, the interest in developing a high-speed civil transport has increased. This has led to an increase in research activity on compressible supersonic flows, in particular the boundary layer. The structure of subsonic boundary layers has been extensively documented using conditional sampling techniques which exploit the knowledge of both u and v velocities. Researchers using these techniques have been able to explore some of the complex three-dimensional motions which are responsible for Reynolds stress production and transport in the boundary layer. As interest in turbulent structure has grown to include supersonic flows, a need for simultaneous multicomponent velocity measurements in these flows has developed. The success of conditional analysis in determining the characteristics of coherent motions and structures in the boundary layer relies on accurate, simultaneous measurement of two instantaneous velocity components.

Sewell, Jesse; Chew, Larry

1994-01-01

360

Seasonal simulations of the planetary boundary layer and boundary-layer stratocumulus clouds with a general circulation model  

NASA Technical Reports Server (NTRS)

The formulation of the planetary boundary layer (PBL) and stratocumulus parametrizations in the UCLA general circulation model (GCM) are briefly summarized, and extensive new results are presented illustrating some aspects of the simulated seasonal changes of the global distributions of PBL depth, stratocumulus cloudiness, cloud-top entrainment instability, the cumulus mass flux, and related fields. Results from three experiments designed to reveal the sensitivity of the GCM results to aspects of the PBL and stratocumulus parametrizations are presented. The GCM results show that the layer cloud instability appears to limit the extent of the marine subtropical stratocumulus regimes, and that instability frequently occurs in association with cumulus convection over land. Cumulus convection acts as a very significant sink of PBL mass throughout the tropics and over the midlatitude continents in winter.

Randall, D. A.; Abeles, J. A.; Corsetti, T. G.

1985-01-01

361

Long-term Observations of the Convective Boundary Layer Using Insect Radar Returns at the SGP ARM Climate Research Facility  

SciTech Connect

A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004-08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes-thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.

Chandra, A S; Kollias, P; Giangrande, S E; Klein, S A

2009-08-20

362

Effects of boundary-layer stability on urban heat island-induced circulation  

Microsoft Academic Search

Summary  The effects of atmospheric boundary-layer stability on urban heat island-induced circulation are numerically and theoretically\\u000a investigated using a nonlinear numerical model (ARPS) and a two-layer linear analytical model. Numerical model simulations\\u000a show that as the boundary layer becomes less stable, a downwind updraft cell induced by the urban heat island strengthens.\\u000a It is also shown that as the boundary layer

J.-J. Baik; Y.-H. Kim; J.-J. Kim; J.-Y. Han

2007-01-01

363

Effects of boundary-layer stability on urban heat island-induced circulation  

Microsoft Academic Search

The effects of atmospheric boundary-layer stability on urban heat island-induced circulation are numerically and theoretically investigated using a nonlinear numerical model (ARPS) and a two-layer linear analytical model. Numerical model simulations show that as the boundary layer becomes less stable, a downwind updraft cell induced by the urban heat island strengthens. It is also shown that as the boundary layer

J.-J. Baik; Y.-H. Kim; J.-J. Kim; J.-Y. Han

2007-01-01

364

Sensing the Stable Boundary Layer in a Towing Tank  

NASA Astrophysics Data System (ADS)

Understanding and forecasting the stable atmospheric boundary layer (SBL) over land is a challenge for already several decades. Generally, the SBL covers two different regimes. The first regime is the weakly SBL, characterised by well defined wind driven turbulence. The second regime covers the very SBL with weak turbulence, and then additional processes become relevant, such as meandering motions, gravity waves, drainage flows, intermittent turbulence and radiation divergence. Especially in this regime this complexity limits the understanding of the SBL and its representation in numerical weather prediction, climate models and air pollution models. For calm conditions, these models typically overestimate near surface temperature and wind speed, with adverse effects for understanding polar climate and end users in agriculture, transportation, and air quality assessment. To improve our understanding of the SBL, we study SBL turbulence in the CNRM-GAME stratified water flume in Toulouse. This unique facility, particularly well suited for stratified flow and BL studies, provides novel laboratory observations that extend earlier efforts of field observations and wind tunnel studies. Among other things, laboratory observations have the advantage of statistical robustness due to repeatability of the experiment and provide access to an extensive set of data. Hence, a 3x3 m2 plate covered with LEGO of Lx=1.57 cm and Ly=3.57 cm, (roughness length = 0.0014 m, and roughness density =0.250, index of frontal area = 0.125) was towed at different velocities through the tank of 22 x 3 x 1.6 m. In this way we were able to achieve an SBL of ~10 cm with bulk Richardson numbers in the range between 0.05 and 0.25, and turbulence with a well-behaved inertial subrange. We focus on the estimation of the non-dimensional velocity and density profiles, on higher order turbulent statistics (important for plume dispersion), as well as on the turbulence spectral behaviour. Finally, we aim to quantify the transition from weakly to very stable boundary layer, and the transition from a turbulent to laminar boundary layer in terms of non-dimensional quantities.

Steeneveld, G. J.; Dobrovolschi, D.; Paci, A.; Eiff, O.; Lacaze, L.; Holtslag, A. A. M.

2010-09-01

365

Boundary-layer processes cause GCM biases in Arctic winter  

NASA Astrophysics Data System (ADS)

Temperature inversions are a common feature of the Arctic wintertime boundary layer. They have important impacts on both radiative and turbulent heat fluxes and partly determine local climate change feedbacks. Inversions and the associated surface fluxes are poorly represented in current climate models, with many models overestimating the typical strength of temperature inversions. Understanding the spread and biases in inversion strength modelled by global climate models is therefore an important step in better understanding Arctic climate and its present and future changes. Here, we show how the cooling of relatively warm and moist are masses advected from lower latitudes leads to the emergence of a clear and a cloudy state of the Arctic winter boundary layer. During this process of formation of Arctic air, radiative cooling leads to saturation and thus triggers the formation of a high-emissivity liquid-containing cloud which limits surface radiative cooling in the cloudy state. Further radiative cooling drives the transition to a low-emissivity ice cloud which allows the surface to cool radiatively and is therefore associated with the clear state of the boundary layer. Temperature inversions are initially created by warm air advection, then eroded by radiative cooling aloft in the cloudy state and created again by surface cooling in the clear state. This results in stronger typical inversions in the clear than in the cloudy state. Comparing model output to observations, we find that many CMIP5 models do not realistically represent the cloudy state. This results in excessive surface radiative cooling, which leads to an overestimation of inversion strength in one group of models, whereas other models produce weak inversions despite strong surface cooling. An idealised single-column model experiment of the formation of Arctic air reveals that the lack of a cloudy state is linked to inadequate mixed-phase cloud microphysics. In models lacking a cloudy state, freezing of cloud liquid water occurs at too warm temperatures. Excessive turbulent and conductive heat fluxes can weaken temperature inversions despite surface radiative cooling, which can explain why some models produce weak inversions despite lacking a cloudy state. A redistribution from the clear to the cloudy state in a warming climate would act to amplify Arctic surface warming. Results from the MPI-ESM-LR suggest that such a feedback does indeed exist. A better understanding and model representation of Arctic mixed-phase clouds is required to verify and quantify the effect of this mechanism on Arctic climate change.

Pithan, Felix; Medeiros, Brian; Mauritsen, Thorsten

2013-04-01

366

Surface Temperature and Surface-Layer Turbulence in a Convective Boundary Layer  

NASA Astrophysics Data System (ADS)

Previous laboratory and atmospheric experiments have shown that turbulence influences the surface temperature in a convective boundary layer. The main objective of this study is to examine land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency infrared imagery and sonic anemometer measurements were obtained during the boundary layer late afternoon and sunset turbulence (BLLAST) experimental campaign. Temporal turbulence data in the surface-layer are then analyzed jointly with spatial surface-temperature imagery. The surface-temperature structures (identified using surface-temperature fluctuations) are strongly linked to atmospheric turbulence as manifested in several findings. The surface-temperature coherent structures move at an advection speed similar to the upper surface-layer or mixed-layer wind speed, with a decreasing trend with increase in stability. Also, with increasing instability the streamwise surface-temperature structure size decreases and the structures become more circular. The sequencing of surface- and air-temperature patterns is further examined through conditional averaging. Surface heating causes the initiation of warm ejection events followed by cold sweep events that result in surface cooling. The ejection events occur about 25 % of the time, but account for 60-70 % of the total sensible heat flux and cause fluctuations of up to 30 % in the ground heat flux. Cross-correlation analysis between air and surface temperature confirms the validity of a scalar footprint model.

Garai, Anirban; Pardyjak, Eric; Steeneveld, Gert-Jan; Kleissl, Jan

2013-07-01

367

Experimental study of laminar boundary layer receptivity to a traveling pressure field  

NASA Technical Reports Server (NTRS)

Study is made of the production of Tollmien-Schlichting (T-S) waves within a Blasius boundary layer by an externally-imposed, two-dimensional, pressure field of a particular type. The field is that due to flow past a relatively small cylinder aligned parallel to the test plate and normal to the flow. The cylinder was carried on a circular path about an axis located well above the plate and such that the closest approach of the cylinder to the layer was several times the layer thickness or cylinder diameter. The turbulent wake did not, therefore, directly perturb the layer. Through adjustment of the circumferential velocity, the pressure field was swept along a certain region of the plate with controllable speed. Both the pressure field and the layer response are described in detail. It is shown that the production of T-S waves was sharply maximal for the case of circumferential speed near that of freely-propagating T-S waves, and that the coupling efficiency was extremely high for this case of resonance. The relevance of this energy transfer process to T-S wave production by the unsteady pressure fields resulting from wind tunnel wall turbulent boundary layers for both subsonic and supersonic flow is mentioned.

Kendall, James M.

1987-01-01

368

Predictive Inner-Outer Model for Turbulent Boundary Layers Applied to Hypersonic DNS Data  

E-print Network

Predictive Inner-Outer Model for Turbulent Boundary Layers Applied to Hypersonic DNS Data Clara numerical simulation (DNS) data of supersonic and hypersonic turbulent boundaries with Mach 3 and Mach 7, and Martin12­14 on DNS of hypersonic turbulent boundary layers demonstrates the existence of large scale

Martín, Pino

369

Effects of surface wave breaking on the oceanic boundary layer Hailun He1,2  

E-print Network

Effects of surface wave breaking on the oceanic boundary layer Hailun He1,2 and Dake Chen1 (General Ocean Turbulence Model, GOTM) to investigate the effects of wave breaking on the oceanic boundary wave breaking on the oceanic boundary layer, Geophys. Res. Lett., 38, L07604, doi:10.1029/2011GL046665

Chen, .Dake

370

Thickness of the Atmospheric Boundary Layer Above Dome A, Antarctica, during 2009  

E-print Network

Thickness of the Atmospheric Boundary Layer Above Dome A, Antarctica, during 2009 C. S. BONNER,1 M s at Dome A, Antarctica between 2009 February 4 and 2009 August 18. The median thickness of the boundary increase. Winds within the boundary layer over Antarctica are usually katabatic in na- ture. This suggests

Ashley, Michael C. B.

371

Unsteady Phenomena in Shock Wave/Boundary Layer Interaction  

NASA Technical Reports Server (NTRS)

A brief review is given of the unsteadiness of shock wave/turbulent boundary layer interaction. The focus is on interactions generated by swept and unswept compression ramps, by flares, steps and incident shock waves, by cylinders and blunt fins, and by glancing shock waves. The effects of Mach number, Reynolds number, and separated flow scale are discussed as are the physical causes of the unsteadiness. The implications that the unsteadiness has for interpreting time-average surface and flowfield data, and for comparisons of such experimental data with computation, is also briefly discussed. Finally, some suggestions for future work are given. It is clear that there are large gaps in the data base and that many aspects of such phenomena are poorly understood. Much work remains to be done.

Dolling, D. S.

1993-01-01

372

Instabilities in the boundary layer over a permeable, compliant wall  

NASA Astrophysics Data System (ADS)

Local linear stability of swept and unswept incompressible boundary layers developing over compliant, fluid-saturated, porous plates is considered in the limit of small permeability. The analysis is meant to yield preliminary indications on the possible stabilization induced on the flow's hydrodynamic and hydroelastic modes by poroelastic media, such as those occurring in many natural and technological settings. As far as hydrodynamic modes are concerned, the main stabilizing effect is that of compliance, which however couples weakly to low-frequency crossflow modes. Permeability plays a damping role on hydroelastic modes, which here take the form of travelling wave flutter instabilities. The passive control of instabilities through poroelastic coatings specifically designed to selectively exploit the effect of compliance and/or permeability is a subject worthy of future research efforts.

Pluvinage, Franck; Kourta, Azeddine; Bottaro, Alessandro

2014-08-01

373

Combined core/boundary layer transport simulations in tokamaks  

SciTech Connect

Significant new numerical results are presented from self-consistent core and boundary or scrape-off layer plasma simulations with 3-D neutral transport calculations. For a symmetric belt limiter it is shown that, for plasma conditions considered here, the pump limiter collection efficiency increases from 11% to 18% of the core efflux as a result of local reionization of blade deflected neutrals. This hitherto unobserved effect causes a significant amplification of upstream ion flux entering the pump limiter. Results from coupling of an earlier developed two-zone edge plasma model ODESSA to the PROCTR core plasma simulation code indicates that intense recycling divertor operation may not be possible because of stagnation of upstream flow velocity. This results in a self-consistent reduction of density gradient in an intermediate region between the central plasma and separatrix, and a concomitant reduction of core-efflux. There is also evidence of increased recycling at the first wall.

Prinja, A.K.; Schafer, R.F. Jr.; Conn, R.W.; Howe, H.C.

1986-04-01

374

Small particle transport across turbulent nonisothermal boundary layers  

NASA Technical Reports Server (NTRS)

The interaction between turbulent diffusion, Brownian diffusion, and particle thermophoresis in the limit of vanishing particle inertial effects is quantitatively modeled for applications in gas turbines. The model is initiated with consideration of the particle phase mass conservation equation for a two-dimensional boundary layer, including the thermophoretic flux term directed toward the cold wall. A formalism of a turbulent flow near a flat plate in a heat transfer problem is adopted, and variable property effects are neglected. Attention is given to the limit of very large Schmidt numbers and the particle concentration depletion outside of the Brownian sublayer. It is concluded that, in the parameter range of interest, thermophoresis augments the high Schmidt number mass-transfer coefficient by a factor equal to the product of the outer sink and the thermophoretic suction.

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

1982-01-01

375

Turbulent thermal boundary layer on a permeable flat plate  

SciTech Connect

Scaling laws are established for the profiles of temperature, turbulent heat flux, rms temperature fluctuation, and wall heat transfer in the turbulent boundary layer on a flat plate with transpiration. In the case of blowing, the temperature distribution represented in scaling variables outside the viscous sublayer has a universal form known from experimental data for flows over impermeable flat plates. In the case of suction, the temperature distribution is described by a one-parameter family of curves. A universal law of heat transfer having the form of a generalized Reynolds analogy provides a basis for representation of the heat flux distributions corresponding to different Reynolds numbers and transpiration velocities in terms of a function of one variable. The results are obtained without invoking any special closure hypotheses.

Vigdorovich, I. I. [Moscow State University, Institute of Mechanics (Russian Federation)], E-mail: vigdorovich@imec.msu.ru

2007-06-15

376

Logarithmic boundary layers in strong Taylor-Couette turbulence.  

PubMed

We provide direct measurements of the boundary layer properties in highly turbulent Taylor-Couette flow up to Re=2×106) (Ta=6.2×10(12)) using high-resolution particle image velocimetry and particle tracking velocimetry. We find that the mean azimuthal velocity profile at the inner and outer cylinder can be fitted by the von Kármán log law u+=1/? lny+ +B. The von Kármán constant ? is found to depend on the driving strength Ta and for large Ta asymptotically approaches ??0.40. The variance profiles of the local azimuthal velocity have a universal peak around y+?12 and collapse when rescaled with the driving velocity (and not with the friction velocity), displaying a log dependence of y+ as also found for channel and pipe flows. PMID:23848878

Huisman, Sander G; Scharnowski, Sven; Cierpka, Christian; Kähler, Christian J; Lohse, Detlef; Sun, Chao

2013-06-28

377

Transition and Breakdown to Turbulence in Incompressible Boundary Layers  

NASA Technical Reports Server (NTRS)

We have developed a code where the nonlinear terms are treated implicitly. The equations are discretized using the two-point fourth order compact scheme in the y-direction and the backward Euler method in the x-direction. We investigated the transition process in a Blasius boundary layer due to fundamental type breakdown. With 8 modes in the w and 3 planes, we could compute the evolution of disturbances up to Re(x)=910, which is well into the strongly nonlinear region. The transition onset point is located around Re(x)=850. The comparison with the measurements and with the DNS computations are very good up to Re(x)=880.

Balakumar, Ponnampalam

1998-01-01

378

The large Reynolds number - Asymptotic theory of turbulent boundary layers.  

NASA Technical Reports Server (NTRS)

A self-consistent, asymptotic expansion of the one-point, mean turbulent equations of motion is obtained. Results such as the velocity defect law and the law of the wall evolve in a relatively rigorous manner, and a systematic ordering of the mean velocity boundary layer equations and their interaction with the main stream flow are obtained. The analysis is extended to the turbulent energy equation and to a treatment of the small scale equilibrium range of Kolmogoroff; in velocity correlation space the two-thirds power law is obtained. Thus, the two well-known 'laws' of turbulent flow are imbedded in an analysis which provides a great deal of other information.

Mellor, G. L.

1972-01-01

379

Boundary layer height estimation by sodar and sonic anemometer measurements  

NASA Astrophysics Data System (ADS)

In this paper an analysis of different methods for the calculation of the boundary layer height (BLH) using sodar and ultrasonic anemometer measurements is presented. All the methods used are based on single point surface measurements. In particular the automatic spectral routine developed for Remtech sodar is compared with the results obtained with the parameterization of the vertical velocity variance, with the calculation of a prognostic model and with a parameterization based on horizontal velocity spectra. Results indicate that in unstable conditions the different methods provide similar pattern, with BLH relatively low, even if the parameterization of the vertical velocity variance is affected by a large scatter that limits its efficiency in evaluating the BLH. In stable nocturnal conditions the performances of the Remtech routine are lower with respect to the ones in unstable conditions. The spectral method, applied to sodar or sonic anemometer data, seems to be the most promising in order to develop an efficient routine for BLH determination.

Contini, D.; Cava, D.; Martano, P.; Donateo, A.; Grasso, F. M.

2008-05-01

380

Simulation of Aeroelastic Mesoflaps for Boundary-Layer Interaction  

NASA Astrophysics Data System (ADS)

A novel concept involving an array of mesoflaps that allow for aeroelastic recirculating transpiration has the capability to control shock/boundary-layer interactions. The concept consists of a matrix of small flaps (rigidly fixed at their upstream end and covering an enclosed cavity) which are designed to undergo aeroelastic deflection to achieve proper mass bleed or injection when subjected to gas dynamic shock loads. To investigate the static behavior of the mesoflap system, a loosely coupled aeroelastic finite element scheme was developed. The technique uses an unstructured grid for both the fluid and solid domains to allow for potentially complex geometries. Issues of optimum fluid cycles per aeroelastic iteration, under-relaxation, and adaptive mesh re-gridding versus motion were considered in the context of the flap deflection. The aeroelastic convergence was accelerated and improved by employing such techniques.

Wood, B.; Loth, E.; Geubelle, P.

2002-12-01

381

Control of Unstable Waves in Three Dimensional Boundary Layers  

NASA Technical Reports Server (NTRS)

Stability experiments are conducted in the Arizona State University Unsteady Wind Tunnel on a 45 deg. swept airfoil. The pressure gradient is designed to provide purely crossflow-dominated transition; that is, the boundary layer is subcritical to Tollmien- Schlichting (T-S) disturbances. The airfoil surface is hand polished to a 0.25 micron rms finish. Under these conditions, stationary crossflow disturbances grow to nonuniform amplitude due to submicron surface irregularities near the leading edge. Spectral de-compositions isolate single-mode growth rates for the fundamental and harmonic disturbances. The measurements show early nonlinear growth causing amplitude saturation well before transition. Comparisons with nonlinear PSE calculations show excellent agreement in both the amplitude saturation and the disturbance mode shape.

Saric, William S.

1996-01-01

382

Analysis of the photodiode boundary layer transition indicator  

SciTech Connect

The photodiode transition indicator is a device which has been successfully used to determine the onset of boundary layer transition on numerous hypersonic flight vehicles. The exact source of the electromagnetic radiation detected by the photodiode at transition was not understood. In some cases early saturation of the device occurred, and the device failed to detect transition. Analyses have been performed to determine the source of the radiation producing the photodiode signal. The results of these analyses indicate that the most likely source of the radiation is blackbody emission from the heatshield material bordering the quartz window of the device. Good agreement between flight data and calculations based on this radiation source has been obtained. Analyses also indicate that the most probable source of the radiation causing early saturation is blackbody radiation from carbon particles which break away from the nosetip during the ablation process.

Kuntz, D.W.; Wilken, A.C.; Payne, J.L.

1994-01-01

383

Logarithmic boundary layers in highly turbulent Taylor-Couette flow  

E-print Network

We provide direct measurements of the boundary layer properties in highly turbulent Taylor-Couette flow up to $\\text{Ta}=6.2 \\times 10^{12}$ using high-resolution particle image velocimetry (PIV). We find that the mean azimuthal velocity profile at the inner and outer cylinder can be fitted by the von K\\'arm\\'an log law $u^+ = \\frac 1\\kappa \\ln y^+ +B$. The von K\\'arm\\'an constant $\\kappa$ is found to depend on the driving strength $\\text{Ta}$ and for large $\\text{Ta}$ asymptotically approaches $\\kappa \\approx 0.40$. The variance profiles of the local azimuthal velocity have a universal peak around $y^+ \\approx 12$ and collapse when rescaled with the driving velocity (and not with the friction velocity), displaying a log-dependence of $y^+$ as also found for channel and pipe flows [1,2].

Huisman, Sander G; Cierpka, Christian; Kahler, Christian J; Lohse, Detlef; Sun, Chao

2013-01-01

384

Laminar boundary-layer flow of non-Newtonian fluid  

NASA Technical Reports Server (NTRS)

A solution for the two-dimensional and axisymmetric laminar boundary-layer momentum equation of power-law non-Newtonian fluid is presented. The analysis makes use of the Merk-Chao series solution method originally devised for the flow of Newtonian fluid. The universal functions for the leading term in the series are tabulated for n from 0.2 to 2. Equations governing the universal functions associated with the second and the third terms are provided. The solution together with either Lighthill's formula or Chao's formula constitutes a simple yet general procedure for the calculation of wall shear and surface heat transfer rate. The theory was applied to flows over a circular cylinder and a sphere and the results compared with published data.

Lin, F. N.; Chern, S. Y.

1979-01-01

385

Resonant Alfven wave heating of the plasma sheet boundary layer  

NASA Astrophysics Data System (ADS)

The exchange of energy between the plasma mantle and the plasma sheet boundary layer (PSBL) is examined with a one-dimensional magnetotail model. The energy exchange occurs via Poynting flux generated by the localized mode conversion of a surface wave to an Alfven wave. This Poynting flux propagates through the lobe and into the PSBL where it is absorbed by two processes. The first arises from a gradient in the plasma beta causing a smooth absorption of Poynting flux. The second process results from the localized mode conversion of the decaying surface wave to an Alfven wave, causing a localized absorption of energy. A numerical solution of the linearized ideal MHD equations is obtained by assuming an adiabatic equation of state.

Harrold, B. G.; Goertz, C. K.; Smith, R. A.; Hansen, P. J.

1990-09-01

386

Hypersonic Boundary-Layer Trip Development for Hyper-X  

NASA Technical Reports Server (NTRS)

Boundary layer trip devices for the Hper-X forebody have been experimentally examined in several wind tunnels. Five different trip configurations were compared in three hypersonic facilities, the LaRC 20-Inch Mach 6 Air Tunnel, the LaRC 31 -Inch Mach 10 Air Tunnel, and in the HYPULSE Reflected Shock Tunnel at GASL. Heat transfer distributions, utilizing the phosphor thermography and thin-film techniques, shock system details, and surface streamline patterns were measured on a 0.333-scale model of the Hyper-X forebody. Parametric variations include angles-of-attack of 0-deg, 2-deg, and 4-deg; Reynolds numbers based on model length of 1.2 to 15.4 million: and inlet cowl door simulated in both open and closed positions. Comparisons of transition due to discrete roughness elements have led to the selection of a trip configuration for the Hyper-X Mach 7 flight vehicle.

Berry, Scott A.; Auslender, Aaron H.; Dilley, Authur D.; Calleja, John F.

2000-01-01

387

Auroral ionospheric signatures of the plasma sheet boundary layer in the evening sector  

NASA Technical Reports Server (NTRS)

We report on particles and fields observed during Defense Meteorological Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/auroral oval boundary in the evening magnetic local time (MLT) sector. Season-dependent, latitudinally narrow regions of rapid, eastward plasma flows were encountered by DMSP near the poleward boundary of auroral electron precipitation. Ten DE 2 orbits exhibiting electric field spikes that drive these plasma flows were chosen for detailed analysis. The boundary region is characterized by pairs of oppositely-directed, field-aligned current sheets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electrons either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 generally detected intense fluxes of accelerated electrons and weak fluxes of ions, both with average energies between 5 and 12 keV. In two instances, precipitating ions with energies greater than 5 keV spanned both current sheets. Comparisons with satellite measurements at higher altitudes suggest that the particles and fields originated in the magnetotail inside the distant reconnection region and propagated to Earth through the plasma sheet boundary layer. Auroral electrons are accelerated by parallel electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near-Earth magnetic mirror. Electric field spikes driving rapid plasma flows along the poleward boundaries of intense, keV electron precipitation represent ionospheric responses to the field-aligned currents and conductivity gradients. The generation of field-aligned currents in the boundary layer may be understood qualitatively as resulting from the different rates of earthward drift for electrons and protons in the magnetotail's current sheet.

Burke, W. J.; Machuzak, J. S.; Maynard, N. C.; Basinska, E. M.; Erickson, G. M.; Hoffman, R. A.; Slavin, J. A.; Hanson, W. B.

1994-01-01

388

On the Coupling Between a Supersonic Turbulent Boundary Layer and a Flexible Structure  

NASA Technical Reports Server (NTRS)

A mathematical model and a computer code have been developed to fully couple the vibration of an aircraft fuselage panel to the surrounding flow field, turbulent boundary layer and acoustic fluid. The turbulent boundary layer model is derived using a triple decomposition of the flow variables and applying a conditional averaging to the resulting equations. Linearized panel and acoustic equations are used. Results from this model are in good agreement with existing experimental and numerical data. It is shown that in the supersonic regime, full coupling of the flexible panel leads to lower response and radiation from the panel. This is believed to be due to an increase in acoustic damping on the panel in this regime. Increasing the Mach number increases the acoustic damping, which is in agreement with earlier work.

Frendi, Abdelkader

1996-01-01

389

Boundary layer eddies at the Goodnoe Hills site  

SciTech Connect

Data from nine instrumented meteorological towers at the MOD-2 wind turbine site at Goodnoe Hills in Washington State were analyzed to evaluate high-frequency perturbations, which were observed in the lower boundary-layer flow. Horizontal winds and temperature measurements for a period of 8 min, undisturbed by turbine operation, were available for this study. The data are in 1-s values from June 27, 1985. Throughout the study, departures from the mean for the period and for each sensor were used on area maps and on line-time and tower-time cross sections. Conventional streamline and isotach analyses were employed; they show highly organized flow fields with embedded perturbations traversing the site. Most of the flow fields have a well-developed vortical structure that reaches from the surface through the top level of the highest tower. These structures consist of a system of clockwise and counter-clockwise circulations. The wave length is about 500 to 600 m. Their wave speed is slightly greater than the mean wind speed and their movement is in the general direction of the mean flow. The results of the study show two main reasons why wind conditions and turbine power output in a wind farm may vary in a remarkable and abrupt fashion in space and time under certain circumstances: (1) The boundary-layer flow contains highly organized coherent perturbations with a typical size of 300 {times} 300 M{sup 2}. (2) The transition zones between the perturbations moving through a wind farm are associated with very definitive changes in the wind field that are on the order of meters and seconds. 2 refs., 11 figs.

Aspliden, C.I.; Wendell, L.L.; Clem, K.S.; Gower, G.L.

1991-05-01

390

LASE measurements of convective boundary layer development during SGP97  

NASA Technical Reports Server (NTRS)

The Southern Great Plains 1997 (SGP97) field experiment was conducted in Oklahoma during June-July 1997 to validate the models used for computing remote soil moisture using measurements by microwave radiometers. One of the objectives of SGP97 was to examine the effect of soil moisture on the evolution of the Atmospheric Boundary Layer (ABL) and clouds over the Southern Great Plains (SGP) during the warm season. The LASE (Lidar Atmospheric Sensing Experiment) airborne DIAL (Differential Absorption Lidar) system, which was flown autonomously on the NASA ER-2 aircraft during previous missions, was reconfigured to fly on the NASA P3 research aircraft. During SGP97 LASE was used to study the morning evolution of the ABL, particularly as manifested in the development of the convective boundary layer, and to study the influence of soil moisture variations on the development of ABL. The ABL development is strongly influenced by the surface energy budget, which is in turn influenced by soil moisture, mesoscale meteorology, clouds, and solar insolation. LASE data acquired during this mission are being used to study the ABL water vapor budget, the development of the ABL, spatial and temporal variabilities in the ABL, and the meteorological factors that influence the ABL development. This field experiment also permitted comparisons of LASE water vapor measurements with water vapor profiles acquired by radiosondes launched at the DOE (Department of Energy) Atmospheric Radiation Measurement (ARM) Southern Great Plain (SGP) site and at NASA/Wallops Flight Facility, as well as with measurements from other SGP97 aircraft.

Ismail, Syed; Browell, Edward V.; Ferrare, Richard A.; Senff, Christoph; Davis, Kenneth J.a; Lenschow, Donald H.; Kooi, Susan; Brackett, Vince; Clayton, Marian

1998-01-01

391

Numerical Computations of Hypersonic Boundary-Layer over Surface Irregularities  

NASA Technical Reports Server (NTRS)

Surface irregularities such as protuberances inside a hypersonic boundary layer may lead to premature transition on the vehicle surface. Early transition in turn causes large localized surface heating that could damage the thermal protection system. Experimental measurements as well as numerical computations aimed at building a knowledge base for transition Reynolds numbers with respect to different protuberance sizes and locations have been actively pursued in recent years. This paper computationally investigates the unsteady wake development behind large isolated cylindrical roughness elements and the scaled wind-tunnel model of the trip used in a recent flight measurement during the reentry of space shuttle Discovery. An unstructured mesh, compressible flow solver based on the space-time conservation element, solution element (CESE) method is used to perform time-accurate Navier-Stokes calculations for the flow past a roughness element under several wind-tunnel conditions. For a cylindrical roughness element with a height to the boundary-layer thickness ratio from 0.8 to 2.5, the wake flow is characterized by a mushroom-shaped centerline streak and horse-shoe vortices. While time-accurate solutions converged to a steady-state for a ratio of 0.8, strong flow unsteadiness is present for a ratio of 1.3 and 2.5. Instability waves marked by distinct disturbance frequencies were found in the latter two cases. Both the centerline streak and the horse-shoe vortices become unstable downstream. The oscillatory vortices eventually reach an early breakdown stage for the largest roughness element. Spectral analyses in conjunction with the computed root mean square variations suggest that the source of the unsteadiness and instability waves in the wake region may be traced back to possible absolute instability in the front-side separation region.

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

2010-01-01

392

Spatially Developing Secondary Instabilities in Compressible Swept Airfoil Boundary Layers  

NASA Technical Reports Server (NTRS)

Two-dimensional eigenvalue analysis is used on a massive scale to study spatial instabilities of compressible shear flows with two inhomogeneous directions. The main focus of the study is crossflow dominated swept-wing boundary layers although the methodology can also be applied to study other type of flows, such as the attachment-line flow. Certain unique aspects of formulating a spatial, two-dimensional eigenvalue problem for the secondary instability of finite amplitude crossflow vortices are discussed, namely, fixing the spatial growth direction unambiguously through a non-orthogonal formulation of the linearized disturbance equations. A primary test case used for parameter study corresponds to the low-speed, NLF-0415(b) airfoil configuration as tested in the ASU Unsteady Wind Tunnel, wherein a spanwise periodic array of roughness elements was placed near the leading edge in order to excite stationary crossflow modes with a specified fundamental wavelength. The two classes of flow conditions selected for this analysis include those for which the roughness array spacing corresponds to either the naturally dominant crossflow wavelength, or a subcritical wavelength that serves to reduce the growth of the naturally excited dominant crossflow modes. Numerical predictions are compared with the measured database, both as indirect validation for the spatial instability analysis and to provide a basis for comparison with a higher Reynolds number, supersonic swept-wing configuration. Application of the eigenvalue analysis to the supersonic configuration reveals that a broad spectrum of stationary crossflow modes can sustain sufficiently strong secondary instabilities as to potentially cause transition over this configuration. Implications of this finding for transition control in swept wing boundary layers are examined.

Li, Fei; Choudhari, Meelan M.

2011-01-01

393

Nanoscale Hot-Wire Probes for Boundary-Layer Flows  

NASA Technical Reports Server (NTRS)

Hot-wire probes having dimensions of the order of nanometers have been proposed for measuring temperatures (and possibly velocities) in boundary-layer flows at spatial resolutions much finer and distances from walls much smaller than have been possible heretofore. The achievable resolutions and minimum distances are expected to be of the order of tens of nanometers much less than a typical mean free path of a molecule and much less than the thickness of a typical flow boundary layer in air at standard temperature and pressure. An additional benefit of the small scale of these probes is that they would perturb the measured flows less than do larger probes. The hot-wire components of the probes would likely be made from semiconducting carbon nanotubes or ropes of such nanotubes. According to one design concept, a probe would comprise a single nanotube or rope of nanotubes laid out on the surface of an insulating substrate between two metallic wires. According to another design concept, a nanotube or rope of nanotubes would be electrically connected and held a short distance away from the substrate surface by stringing it between two metal electrodes. According to a third concept, a semiconducting nanotube or rope of nanotubes would be strung between the tips of two protruding electrodes made of fully conducting nanotubes or ropes of nanotubes. The figure depicts an array of such probes that could be used to gather data at several distances from a wall. It will be necessary to develop techniques for fabricating the probes. It will also be necessary to determine whether the probes will be strong enough to withstand the aerodynamic forces and impacts of micron-sized particles entrained in typical flows of interest.

Tedjojuwono, Ken T.; Herring, Gregory C.

2003-01-01

394

Comparison of Methods for Determining Boundary Layer Edge Conditions for Transition Correlations  

NASA Technical Reports Server (NTRS)

Data previously obtained for the X-33 in the NASA Langley Research Center 20-Inch Mach 6 Air Tunnel have been reanalyzed to compare methods for determining boundary layer edge conditions for use in transition correlations. The experimental results were previously obtained utilizing the phosphor thermography technique to monitor the status of the boundary layer downstream of discrete roughness elements via global heat transfer images of the X-33 windward surface. A boundary layer transition correlation was previously developed for this data set using boundary layer edge conditions calculated using an inviscid/integral boundary layer approach. An algorithm was written in the present study to extract boundary layer edge quantities from higher fidelity viscous computational fluid dynamic solutions to develop transition correlations that account for viscous effects on vehicles of arbitrary complexity. The boundary layer transition correlation developed for the X-33 from the viscous solutions are compared to the previous boundary layer transition correlations. It is shown that the boundary layer edge conditions calculated using an inviscid/integral boundary layer approach are significantly different than those extracted from viscous computational fluid dynamic solutions. The present results demonstrate the differences obtained in correlating transition data using different computational methods.

Liechty, Derek S.; Berry, Scott A.; Hollis, Brian R.; Horvath, Thomas J.

2003-01-01

395

Large eddy simultations of the atmospheric boundary layer east of the Colorado Rockies  

SciTech Connect

Large eddy simulation, LES, has often been carried out for the idealized situation of a simple convective boundary layer. Studies of dual Doppler radar and aircraft data from the Phoenix II experiment indicate that the boundary layer of the Colorado High Plains is not a purely convective boundary layer and it is influenced by the mountains to the west. The purpose of this study is to investigate the atmospheric boundary layer on one particular day on the Colorado High Plains. This research applies a LES nested within larger grids, which contain realistic topography and can simulate the larger-scale circulations initiated by the presence of the mountain barrier. How and to what extent the atmospheric boundary layer of the Colorado High Plains is influenced by larger scale circulations and other phenomena associated with the mountain barrier to the west is investigated. The nested grid LES reproduces the characteristics of the atmosphere for the case study day reasonably well. The mountains influence the atmospheric boundary layer over the plains to the east in several ways. The mountains contribute to the vertical shear of the horizontal winds through the thermally-induced mountain-plains circulation. As a consequence of the wind shear, the boundary layer that develops over the mountains is advected eastward over the top of the plains boundary layer, which is developing separately. This layer is marked by a mixture of gravity waves and turbulence and is atypical of a purely convective boundary layer. Just below this layer, the capping inversion of the plains boundary layer is weak and poorly defined compared to the inversions capping purely convective boundary layers. Gravity waves, triggered by the obstacle of the Rocky Mountains and by convection in the mountain boundary layer, also influence the atmosphere above the Colorado High Plains. These influences are found to have significant effects on the turbulence statistics and the energy spectra.

Costigan, K.R.; Cotton, W.R.

1992-10-22

396

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

397

Analytical study of subsonic turbulent boundary layer separation including viscous-inviscid interaction  

NASA Technical Reports Server (NTRS)

The prediction of boundary layer deceleration and separation in regions of adverse pressure gradients are discussed as an unsolved problem of fundamental practical importance in aerodynamics. A three layered theoretical flow model of boundary layer separation including viscous-inviscid interaction was conceived for the case of subsonic two dimensional steady laminar flow.

Inger, G. R.

1974-01-01

398

Friction of a slider on a granular layer: Nonmonotonic thickness dependence and effect of boundary conditions  

E-print Network

Friction of a slider on a granular layer: Nonmonotonic thickness dependence and effect of boundary the effective friction encountered by a mass sliding on a granular layer as a function of bed thickness and boundary roughness conditions. The observed friction has minima for a small number of layers before

Kudrolli, Arshad

399

Alternate Designs of Ultrasonic Absorptive Coatings for Hypersonic Boundary Layer Control  

E-print Network

Alternate Designs of Ultrasonic Absorptive Coatings for Hypersonic Boundary Layer Control Guillaume to parametrically investigate hypersonic boundary layers over ultrasonic absorptive coatings consisting of a uniform ultrasonic absorptive coating (UAC), which consists of a thin perforated layer of reg- ular microstructure

Dabiri, John O.

400

Moisture Transport, Lower-Tropospheric Stability, and Decoupling of Cloud-Topped Boundary Layers  

Microsoft Academic Search

Decoupling during the ''Lagrangian'' evolution of a cloud-topped boundary layer advected equatorward by the trade winds in an idealized eastern subtropical ocean is studied using a mixed-layer model (MLM). The sea surface temperature is gradually warmed while the free tropospheric sounding remains unchanged, causing the boundary layer to deepen, the surface relative humidity to decrease, and surface latent heat fluxes

Christopher S. Bretherton; Matthew C. Wyant

1997-01-01

401

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

402

The effects of forcing on a single stream shear layer and its parent boundary layer  

NASA Technical Reports Server (NTRS)

Forcing and its effect on fluid flows has become an accepted tool in the study and control of flow systems. It has been used both as a diagnostic tool, to explore the development and interaction of coherent structures, and as a method of controlling the behavior of the flow. A number of forcing methods have been used in order to provide a perturbation to the flow; among these are the use of an oscillating trailing edge, acoustically driven slots, external acoustic forcing, and mechanical piston methods. The effect of a planar mechanical piston forcing on a single stream shear layer is presented; it can be noted that this is one of the lesser studied free shear layers. The single stream shear layer can be characterized by its primary flow velocity scale and the thickness of the separating boundary layer. The velocity scale is constant over the length of the flow field; theta (x) can be used as a width scale to characterize the unforced shear layer. In the case of the forced shear layer the velocity field is a function of phase time and definition of a width measure becomes somewhat problematic.

Haw, Richard C.; Foss, John F.

1990-01-01

403

A Source-Term Based Boundary Layer Bleed/Effusion Model for Passive Shock Control  

NASA Technical Reports Server (NTRS)

A modeling framework for boundary layer effusion has been developed based on the use of source (or sink) terms instead of the usual practice of specifying bleed directly as a boundary condition. This framework allows the surface boundary condition (i.e. isothermal wall, adiabatic wall, slip wall, etc.) to remain unaltered in the presence of bleed. This approach also lends itself to easily permit the addition of empirical models for second order effects that are not easily accounted for by simply defining effective transpiration values. Two effusion models formulated for supersonic flows have been implemented into this framework; the Doerffer/Bohning law and the Slater formulation. These models were applied to unit problems that contain key aspects of the flow physics applicable to bleed systems designed for hypersonic air-breathing propulsion systems. The ability of each model to predict bulk bleed properties was assessed, as well as the response of the boundary layer as it passes through and downstream of a porous bleed system. The model assessment was performed with and without the presence of shock waves. Three-dimensional CFD simulations that included the geometric details of the porous plate bleed systems were also carried out to supplement the experimental data, and provide additional insights into the bleed flow physics. Overall, both bleed formulations fared well for the tests performed in this study. However, the sample of test problems considered in this effort was not large enough to permit a comprehensive validation of the models.

Baurle, Robert A.; Norris, Andrew T.

2011-01-01

404

Receptivity of the Flat-Plate Boundary Layer to Free-Stream Turbulence  

Microsoft Academic Search

The linear problem of generation of perturbations of a flat-plate boundary layer by external turbulence is solved. The turbulence is represented in the form of a set of space- and time-periodic vortex modes. It is shown that the boundary layer is most receptive to low-frequency longitudinal vorticity modes. The mean-square velocity fluctuations in the boundary layer and their spectrum are

M. V. Ustinov

2003-01-01

405

Investigations of Suction in a Transitional Flat-Plate Boundary Layer  

Microsoft Academic Search

\\u000a For the maintenance of a laminar boundary layer flow on transonic wings, it is necessary to integrate a boundary layer suction\\u000a unit in the nose region. This concept of the Hybrid Laminar Flow control is realized through a suction area adapted to the\\u000a outer pressure distribution by an array of suction holes. With this, a stabilization of the boundary layer

Stefan Becker; Jovan Jovanovic

2010-01-01

406

Boundary layers in cataclysmic variables: The HEAO-1 X-ray constraints  

NASA Technical Reports Server (NTRS)

The predictions of the boundary layer model for the X-ray emission from novae are summarized. A discrepancy between observations and theory in the X-ray observations is found. Constraints on the nature of the boundary layers in novae, based on the lack of detections of novae in the HEAO-1 soft X-ray survey are provided. Temperature and column densities for optically thick boundary layers in novae are estimated.

Jensen, K. A.

1983-01-01

407

Boundary layers in cataclysmic variables - The HEAO 1 X-ray constraints  

NASA Technical Reports Server (NTRS)

The predictions of the boundary layer model for the X-ray emission from novae are summarized. A discrepancy between observations and theory in the X-ray observations is found. Constraints on the nature of the boundary layers in novae, based on the lack of detections of novae in the HEAO-1 soft X-ray survey are provided. Temperature and column densities for optically thick boundary layers in novae are estimated. Previously announced in STAR as N84-13046

Jensen, K. A.

1984-01-01

408

Characterization of the interaction between a rough boundary layer and multiple cylinders wakes  

NASA Astrophysics Data System (ADS)

Among many ecologically important aspects of fish locomotion, turbulence is thought to create large stability challenges for fishes. Turbulence is a ubiquitous, highly variable feature of aquatic habitats (Denny 1988). Species that are more prevalent in "energetic water" (high flow, high turbulence) have more effective control systems and greater ability to generate propulsive power to maneuver. There are direct engineering applications of such work: the design of fishways, fish ladders, culverts, etc. No work to date has explored the interaction of a rough boundary layer (typical of natural environments), with wake flows in the context of fish responses to turbulent fluctuations. The research performed at IMFT under the umbrella of the NSF IREE grant used complimentary laboratory experimental studies to further apply the results from our previous field observations (Cotel et al. 2005) and current laboratory experiments by determining how a rough turbulent boundary layer interacts with the flow structures created by obstacles (cylinders arrays) in the channel. PIV data were acquired for a variety of flow regimes. The results show a strong interaction between the turbulent boundary layer created by the roughness elements and the wakes behind the cylinder arrays, having strong implications for fish behavior in such environments.

Cotel, Aline; Eiff, Olivier; Pradhan, Pratik

2010-11-01