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Last update: November 12, 2013.

1

Active control of turbulent boundary layer using an array of piezo-ceramic actuators

This paper presents preliminary results from an experimental exploration on drag reduction in a turbulent boundary layer using an array of piezo-ceramic actuators. The actuator array consisting of 16 actuators can generate wall-normal oscillations and, given a phase shift between two adjacent actuators, a spanwise travelling wave. A sinusoidal waveform with four different amplitudes was investigated while oscillating in a

H. L. Bai; Y. Zhou

2009-01-01

2

NASA Astrophysics Data System (ADS)

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

Melnick, Blake; Lang, Amy

2008-11-01

3

NASA Astrophysics Data System (ADS)

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

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

2011-11-01

4

Large Eddy Simulation study of fully developed thermal wind-turbine array boundary layers

NASA Astrophysics Data System (ADS)

It is well known that when wind turbines are deployed in large arrays, their efficiency decreases due to complex interactions among themselves and with the atmospheric boundary layer (ABL). For wind farms whose length exceeds the height of the ABL by over an order of magnitude, a “fully developed” flow regime can be established. In this asymptotic regime, changes in the stream-wise direction can be neglected and the relevant exchanges occur in the vertical direction. Such a fully developed wind-turbine array boundary layer (WTABL) has recently been studied1 using Large Eddy Simulations (LES) under neutral stability conditions. The simulations showed the existence of two log-laws, one above and one below the wind turbine region. This enabled the development of more accurate parameterizations of the effective roughness scale for a wind farm. Now, a suite of Large Eddy Simulations, in which wind turbines are also modeled using the classical “drag disk” concept are performed but for non-neutral conditions. The aim is to study the effects of different thermal ABL stratifications, and thus to study the efficiency and characteristics of large wind farms and the associated land-atmosphere interactions for realistic atmospheric flow regimes. Such studies help to unravel the physics involved in extensive aggregations of wind turbines, allowing us to design better wind farm arrangements. By considering various turbine loading factors, surface roughness values and different atmospheric stratifications, it is possible to analyze the influence of these into the induced surface roughness, and the sensible heat roughness length. These last two can be used to model wind turbine arrays in simulations of atmospheric dynamics at larger (regional and global) scales2, where the coarse meshes used do not allow to account for the specifics of each wind turbine. Results from different sets of simulations under stable and unstable conditions will be presented, for which also the corresponding effective roughness length-scales will be determined. Simulations use imposed heat flux at the bottom or an imposed temperature. The simulation results will be analyzed to determine how stratification affects momentum and scalar transport processes in the wind turbine wakes.

Calaf, M.; Meneveau, C. V.; Parlange, M. B.

2010-12-01

5

Boundary-layer receptivity to three-dimensional roughness arrays on a swept-wing

NASA Astrophysics Data System (ADS)

On-going efforts to reduce aircraft drag through transition delay focus on understanding the process of boundary-layer transition from a physics-based perspective. For swept-wings subject to transition dominated by a stationary crossflow instability, one of the remaining challenges is understanding how freestream disturbances and surface features such as surface roughness create the initial amplitudes for unstable waves. These waves grow, modify the mean flow and create conditions for secondary instabilities to occur, which in turn ultimately lead to transition. Computational methods that model the primary and secondary instability growth can accurately model disturbance evolution as long as appropriate initial conditions are supplied. Additionally, transition delay using discrete roughness arrays that exploit known sensitivities to surface roughness has been demonstrated in flight and wind tunnel testing; however, inconsistencies in performance from the two test platforms indicate further testing is required. This study uses detailed hotwire boundary-layer velocity scans to quantify the relationship between roughness height and initial disturbance amplitude. Naphthalene flow visualization provides insight into how transition changes as a result of roughness height and spacing. Micron-sized, circular roughness elements were applied near the leading edge of the ASU(67)-0315 model installed at an angle of attack of -2.9° in the Klebanoff-Saric Wind Tunnel. Extensive flow quality measurements show turbulence intensities less than 0.02% over the speed range of interest. A survey of multiple roughness heights for the most unstable and control wavelengths and Reynolds numbers of 2.4 x 106, 2.8 x 106 and 3.2 x 106 was completed for chord locations of 10%, 15% and 20%. When care was taken to measure in the region of linear stability, it was found that the disturbance amplitude varies almost linearly with roughness height. Naphthalene flow visualization indicates that moderate changes in already-low freestream turbulence levels can have a significant impact on transition behavior.

Hunt, Lauren Elizabeth

6

Micromachined microphone array on a chip for turbulent boundary layer measurements

NASA Astrophysics Data System (ADS)

A surface micromachined microphone array on a single chip has been successfully designed, fabricated, characterized, and tested for aeroacoustic purposes. The microphone was designed to have venting through the diaphragm, 64 elements (8x8) on the chip, and used a capacitive transduction scheme. The microphone was fabricated using the MEMSCAP PolyMUMPs process (a foundry polysilicon surface micromachining process) along with facilities at Tufts Micro and Nano Fabrication Facility (TMNF) where a Parylene-C passivation layer deposition and release of the microstructures were performed. The devices are packaged with low profile interconnects, presenting a maximum of 100 mum of surface topology. The design of an individual microphone was completed through the use of a lumped element model (LEM) to determine the theoretical performance of the microphone. Off-chip electronics were created to allow the microphone array outputs to be redirected to one of two channels, allowing dynamic reconfiguration of the effective transducer shape in software and provide 80 dB off isolation. The characterization was completed through the use of laser Doppler vibrometry (LDV), acoustic plane wave tube and free-field calibration, and electrical noise floor testing in a Faraday cage. Measured microphone sensitivity is 0.15 mV/Pa for an individual microphone and 8.7 mV/Pa for the entire array, in close agreement with model predictions. The microphones and electronics operate over the 200--40 000 Hz band. The dynamic range extends from 60 dB SPL in a 1 Hz band to greater than 150 dB SPL. Element variability was +/-0.05 mV/Pa in sensitivity with an array yield of 95%. Wind tunnel testing at flow rates of up to 205.8 m/s indicates that the devices continue to operate in flow without damage, and can be successfully reconfigured on the fly. Care has been taken to systematically remove contaminating signals (acoustic, vibration, and noise floor) from the wind tunnel data to determine actual turbulent pressure fluctuations beneath the turbulent boundary layer to an uncertainty level of 1 dB. Analysis of measured boundary layer pressure spectra at six flow rates from 34.3 m/s to 205.8 m/s indicate single point wall spectral measurements in close agreement to the empirical models of Goody, Chase-Howe, and Efimtsov above Mach 0.4. The MEMS data more closely resembles the magnitude of the Efimtsov model at higher frequencies (25% higher above 3 kHz for the Mach 0.6 case); however, the shape of the spectral model is closer to the model of Goody (50% lower for the Mach 0.6 case for all frequencies). The Chase-Howe model does fall directly on the MEMS data starting at 6 kHz, but has a sharper slope and does not resemble the data at below 6 kHz.

Krause, Joshua Steven

7

With increasing fuel costs, research into reducing drag over solid surfaces in high Reynolds number flows is still an area of interest. There have been many studies examining the boundary layer flow over two-dimensional microgeometries (e.g. riblets), but very few studies involving three dimensional microgeometries. The main objective of this study was to examine how embedded vortices, forming in hexagonal

Blake Melnick; Amy Lang

2008-01-01

8

Boundary Layer Simulator Improvement.

National Technical Information Service (NTIS)

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

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

1989-01-01

9

Streamwise development of the wind turbine boundary layer over a model wind turbine array

NASA Astrophysics Data System (ADS)

The streamwise development of turbulence statistics and mean kinetic energy in a model wind farm consisting of 3 × 5 wind turbines is studied experimentally in a wind tunnel. The analysis uses planar Particle Image Velocimetry data obtained at the centerline plane of the wind farm, covering the inflow as well as four planes in between five downstream wind turbines. The data analysis is organized by dividing these measurement planes into three regions: the above-rotor, rotor-swept, and below-rotor regions. For each field, flow development is quantified using a properly defined relative difference norm based on an integration over each of the regions. Using this norm, it is found that the mean streamwise velocity approaches a fully developed state most rapidly, whereas the flow development is more gradual for the second-order statistics. The vertical entrainment flux of the mean kinetic energy by the Reynolds shear stress, __, is observed to develop at a rate similar to that of the Reynolds shear stress rather than the mean streamwise velocity component. Its development is slowest in the layer nearest to the ground. Analysis of various terms in the mean kinetic energy equation shows that the wind turbine boundary layer has not yet reached fully developed conditions by the fifth turbine but that it is approaching such conditions. By comparing the vertical entrainment flux with the horizontal flux due to the mean flow, it is found that the former increases, whereas the latter decreases, as function of downstream distance, but that the former is already an important contributor in the developing region.__

Newman, Jensen; Lebron, Jose; Meneveau, Charles; Castillo, Luciano

2013-08-01

10

Boundary layer transition studies

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

Jonathan H. Watmuff

1995-01-01

11

The atmospheric boundary layer

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

The Atmospheric Boundary Layer

ERIC Educational Resources Information Center

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

Tennekes, Hendrik

1974-01-01

13

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

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

1986-01-01

14

The Atmospheric Boundary Layer

ERIC Educational Resources Information Center

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

Tennekes, Hendrik

1974-01-01

15

NASA Astrophysics Data System (ADS)

A prototype sensor designed to measure atmospheric gas absorption using scattered solar light at SWIR wavelengths has been developed and tested in the lab. This sensor combines a novel use of modern digital detector arrays with gas filter correlation radiometry (GFCR). The combination overcomes tradition difficulties, involving spatially varying albedo, in the spectral analysis of nadir SWIR observations from moving platforms. GFCR provides exceptional spectral selectivity due to typical effective resolving power of 105 and normalized difference signals that are ideally insensitive to spectrally flat attenuators, such as aerosol. By de-focusing the two beams produced by the GFCR and using detector arrays to obtain beam intensity through simple pixel summation, the instrument achieves dual-beam GFCR observations that are exactly matched in time and space. This characteristic nearly eliminates sensitivity to the time and space variation of surface albedo. Plus, measuring beam intensity by summing detector pixel output provides almost unlimited dynamic range, considering the flexibility of integration times provided with modern detector arrays. This poster presents the instrument design and modeling results. Funded under the NASA Planetary Instrument Definition and Design Program, PIDD, the work has focused on evaluating a potential Mars methane measurement near 3.3 microns. Results evaluating gas spectral sensitivity, insensitivity to albedo, non-linearity effects, dynamic range, signal drift, dark current, background emission and other effects are presented. Application to other gases is briefly discussed.

Crowther, B.; Peterson, J.; Gordley, L. L.; Hervig, M. E.; Burton, J.; Fish, C. S.; Diskin, G. S.; Sachse, G. W.

2010-12-01

16

NASA Astrophysics Data System (ADS)

Data from a wind-tunnel experiment on the flow within a 3 x 3 array of lightly loaded wind turbine models operating inside a turbulent boundary layer over a rough surface are analyzed. The data are acquired using X hot-wire anemometry and the focus of the analysis is on the possible differences of the flow structures above and below the canopy of wind turbines. Here this question is addressed using quadrant analysis. Conditional averages of turbulent dissipation (a 1-D surrogate) at various heights at 5 diameters downstream is performed for each of the 4 quadrants as well as different ``hole-sizes.'' The results imply significantly less inter-scale correlations in the low-shear region at the bottom of the wind turbine wake than at other wake locations. Inter-scale correlations above and below the wake are also significantly greater than at that low-shear region. Spectral analysis is performed to determine which scales are mostly responsible for the various levels of Reynolds stresses as functions of position in the wind turbine wake.

Gibson, Max; Kang, Hyung-Suk; Meneveau, Charles; Bayoan Cal, Raul

2009-11-01

17

Boundary layer receptivity - Theory

NASA Astrophysics Data System (ADS)

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

Kerschen, E. J.

18

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

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

1984-01-01

19

Laminar vortex boundary layers

NASA Astrophysics Data System (ADS)

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

Arrese, Juan C.; Lopez, John M.

1996-11-01

20

Receptivity of boundary layers to convected gusts

NASA Astrophysics Data System (ADS)

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

Parekh, D. E.

1993-03-01

21

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

22

Magnetohydrodynamic boundary layer control system

NASA Astrophysics Data System (ADS)

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

Meng, James C.

1993-12-01

23

Minimalist turbulent boundary layer model

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

L. Moriconi

2009-01-01

24

Momentum transfer in boundary layers

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

T. Cebeci; P. Bradshaw

1977-01-01

25

Minimalist turbulent boundary layer model.

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

Moriconi, L

2009-04-06

26

Minimalist turbulent boundary layer model

NASA Astrophysics Data System (ADS)

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

Moriconi, L.

2009-04-01

27

In this paper, flow separation control using an array of three round synthetic jets issued upstream of a separated laminar flow over an inclined flap is investigated using a 3D numerical simulation. The simulation results are validated using experimental data and a qualitative agreement is achieved. The simulations are undertaken at three synthetic jet operating conditions. The vortical structures produced

Jue Zhou; Shan Zhong

28

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

29

Stability of Compressible Boundary Layers.

National Technical Information Service (NTIS)

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

A. H. Nayfeh

1989-01-01

30

Boundary Layer Over Uniform Roughness.

National Technical Information Service (NTIS)

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

J. Wu

1970-01-01

31

Laminar boundary layer over riblets

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

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

1994-01-01

32

Boundary layer control of rotating convection systems.

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

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

2009-01-15

33

Boundary layer sensitivity and receptivity

NASA Astrophysics Data System (ADS)

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

Airiau, Christophe; Walther, Steeve; Bottaro, Alessandro

34

Bottom boundary layer flow profiling system

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

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

1984-04-01

35

Boundary layer receptivity and control

NASA Astrophysics Data System (ADS)

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

Hill, D. C.

1993-12-01

36

Boundary Layer Transport of Small Particles.

National Technical Information Service (NTIS)

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

D. McCready

1984-01-01

37

Nonequilibrium Chemistry Boundary Layer Integral Matrix Procedure.

National Technical Information Service (NTIS)

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

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

1973-01-01

38

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

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

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

39

Synoptic Controls on Boundary-Layer Characteristics

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

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

2010-01-01

40

Boundary condition effects on turbulent boundary layer wall pressure fluctuations

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

Timothy Allen Brungart

1997-01-01

41

Boundary layer receptivity to convected gusts and sound

NASA Astrophysics Data System (ADS)

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

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

42

Microgravity Effects on Plant Boundary Layers

NASA Astrophysics Data System (ADS)

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

Stutte, Gary; Monje, Oscar

2005-08-01

43

Synoptic Controls on Boundary-Layer Characteristics

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

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

2010-01-01

44

Synoptic Controls on Boundary-Layer Characteristics

NASA Astrophysics Data System (ADS)

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

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

2010-03-01

45

Compressible Ekman–Hartmann boundary layers

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

Krzysztof A. Mizerski; David W. Hughes

2010-01-01

46

Turbulent boundary layer on a moving surface

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

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

1976-01-01

47

Boundary layers on a rotating disk

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

Tuncer Cebeci; D. E. Abbott

1975-01-01

48

Cyclone separator having boundary layer turbulence control

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

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

1985-01-01

49

Boundary Layers of Air Adjacent to Cylinders

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

Nobel, Park S.

1974-01-01

50

Problems of matter-antimatter boundary layers

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

B. Lehnert

1977-01-01

51

Boundary layer processes in the Martian magnetosphere

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

52

A comparison of boundary layer receptivity mechanisms

NASA Astrophysics Data System (ADS)

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

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

53

Boundary layers on longitudinally grooved walls (riblets)

NASA Astrophysics Data System (ADS)

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

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

1987-10-01

54

Turbulent boundary layer manipulation by outer-layer devices

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

G. Iuso; M. Onorato

1995-01-01

55

Porous Material Development for Boundary Layer Control.

National Technical Information Service (NTIS)

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

G. May W. B. Giles

1964-01-01

56

Analysis of Nosetip Boundary Layer Transition Data.

National Technical Information Service (NTIS)

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

M. L. Finson

1976-01-01

57

Radiation from Air-Teflon Boundary Layers.

National Technical Information Service (NTIS)

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

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

1970-01-01

58

Sound from Turbulent Boundary Layer Excited Panels.

National Technical Information Service (NTIS)

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

H. G. Davies

1969-01-01

59

Reflection of sound by a boundary layer

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

R. S. Brand; R. T. Nagel

1982-01-01

60

Turbulent Boundary Layers with Assigned Wall Shear.

National Technical Information Service (NTIS)

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

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

1974-01-01

61

Boundary-layer friction in midlatitude cyclones

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

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

2006-01-01

62

BUBBLE – an Urban Boundary Layer Meteorology Project

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

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

2005-01-01

63

Numerical Simulation of Active Control of Boundary Layer Transition,

National Technical Information Service (NTIS)

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

H. F. Fasel L. D. Kral

1988-01-01

64

Klebanoff modes in 3D boundary layers

NASA Astrophysics Data System (ADS)

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

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

2004-11-01

65

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

NASA Astrophysics Data System (ADS)

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

Eastman, Timothy E.

66

Boundary Layer Cloudiness Parameterizations Using ARM Observations

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

67

Hypersonic Boundary Layer\\/Shockwave Interaction Problems

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

Haile Lindsay; Frederick Ferguson

68

Hypersonic Boundary Layer\\/Oblique Shockwave Interaction

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

Haile Lindsay

2005-01-01

69

Boundary layer energy transport in plasma devices

NASA Astrophysics Data System (ADS)

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

Orton, Nigel Paul

2000-11-01

70

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.

71

Aerosol buffering of marine boundary layer cloudiness

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

72

Waves which travel upstream in boundary layers

NASA Astrophysics Data System (ADS)

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

Rogler, H. L.

1985-07-01

73

Field observations of the wave bottom boundary layer

NASA Astrophysics Data System (ADS)

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

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

2000-08-01

74

Electric arc behavior in a boundary layer

NASA Astrophysics Data System (ADS)

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

Rosa, Richard; Farrar, Larry; Trudnowski, Dan

1988-10-01

75

Boundary layer halogens in coastal Antarctica.

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

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

2007-07-20

76

Runup and boundary layers on sloping beaches

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

77

Boundary-layer flows in rotating cavities

NASA Astrophysics Data System (ADS)

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

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

1988-06-01

78

Viscous drag reduction in boundary layers

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

Dennis M. Bushnell; Jerry N. Hefner

1990-01-01

79

Hair receptor sensitivity to changes in laminar boundary layer shape.

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

Dickinson, B T

2010-02-16

80

Boundary layer control device for duct silencers

NASA Astrophysics Data System (ADS)

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

Schmitz, Fredric H.; Soderman, Paul T.

1993-11-01

81

Wall functions for unsteady turbulent boundary layers

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

A. N. Menendez; B. R. Ramaprian

1985-01-01

82

Simulating observed boundary layer clouds on Mars

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

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

2010-01-01

83

Interaction between Trapped Waves and Boundary Layers

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

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

2006-01-01

84

Diagnosis of boundary-layer circulations.

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

Beare, Robert J; Cullen, Michael J P

2013-04-15

85

Reduction of turbulent drag: Boundary layer manipulators

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

E. Coustols

1989-01-01

86

Turbulent boundary layer structure, drag reduction

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

Marten T. Landahl; Joseph H. Haritonidis

1986-01-01

87

Particle-laden boundary layers and singularities

NASA Astrophysics Data System (ADS)

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

Foster, M. R.

2007-11-01

88

Linear Optimal Controllers for Turbulent Boundary Layers

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

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

2001-01-01

89

Boundary layer relaxation from convex curvature

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

Amy Elsa Alving

1988-01-01

90

Turbulent boundary layer characteristics over streamwise grooves

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

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

1984-01-01

91

A Wake Boundary Layer Mixing Experiment.

National Technical Information Service (NTIS)

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

P. J. Pot

1979-01-01

92

INDIVIDUAL TURBULENT CELL INTERACTION: BASIS FOR BOUNDARY LAYER ESTABLISHMENT

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

93

Compressible Ekman layers on curved boundaries

NASA Astrophysics Data System (ADS)

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

Greenspan, H. P.

1984-04-01

94

Nonuniqueness in wakes and boundary layers

NASA Astrophysics Data System (ADS)

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

Smith, F. T.

1983-05-01

95

Boundary layer effects on sound in a circular duct

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

R. T. Nagel; R. S. Brand

1982-01-01

96

Boundary Homogenization for Periodic Arrays of Absorbers

We introduce a homogenization procedure for reaction-diffusion equations in domains whose boundary consists of small alternating regions with prescribed Dirichlet and Neumann data of comparable areas. The homogenized problem is shown to satisfy an effective Dirichlet boundary condition which depends on the geometry of the small-scale boundary structure. This problem is also related to finding the effective trapping rate for

Cyrill B. Muratov; Stanislav Y. Shvartsman

2008-01-01

97

Bursting frequency prediction in turbulent boundary layers

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

LIOU,WILLIAM W.; FANG,YICHUNG

2000-02-01

98

Turbulent Boundary Layers on Rough Surfaces

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

Junghwa Seo; Luciano Castillo

2003-01-01

99

Active Control of Laminar Boundary Layer Disturbances

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

M. Gaster

100

Turbulent boundary layer drag reduction using riblets

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

M. J. Walsh

1982-01-01

101

Reduction of turbulent drag: Boundary layer manipulators

NASA Astrophysics Data System (ADS)

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

Coustols, E.

1989-03-01

102

Linearization of turbulent boundary-layer equations

NASA Astrophysics Data System (ADS)

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

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

1984-12-01

103

Effect of variability on boundary layer separation

NASA Astrophysics Data System (ADS)

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

Gudkov, V. A.

1991-10-01

104

Calculation and Measurement of Separated Turbulent Boundary Layers.

National Technical Information Service (NTIS)

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

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

1982-01-01

105

Linear Stability Theory and Three-Dimensional Boundary Layer Transition.

National Technical Information Service (NTIS)

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

R. E. Spall M. R. Malik

1992-01-01

106

Daytime Evolution of Relative Humidity at the Boundary Layer Top

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

M. Ek; L. Mahrt

1994-01-01

107

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

108

A simple model of the hurricane boundary layer

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

Roger K. Smith

2002-01-01

109

Mixing length in low Reynolds number compressible turbulent boundary layers

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

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

1975-01-01

110

Stability and drag reduction in a boundary layer with microbubbles

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

Joao M. Neves

1988-01-01

111

THE EFFECTS OF PERIODIC WAKE STRUCTURES ON TURBULENT BOUNDARY LAYERS

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

R. M. Holland; R. L. Evans

1996-01-01

112

National Technical Information Service (NTIS)

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

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

2006-01-01

113

The efficient calculation of turbulent boundary layers

NASA Astrophysics Data System (ADS)

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

Rotta, J. C.

1983-12-01

114

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

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

S.-K. Kao

1960-01-01

115

The minisodar and planetary boundary layer studies

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

Coulter, R.L.

1996-06-01

116

Consideration on boundary-layer receptivity

NASA Astrophysics Data System (ADS)

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

Nishioka, Michio

1991-09-01

117

Chaotic motion in an oscillatory boundary layer.

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

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

1996-12-01

118

Turbulent boundary layer pressure fluctuations at large scales

NASA Astrophysics Data System (ADS)

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

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

2002-05-01

119

Near Critical Phenomena in Laminar Boundary Layers

NASA Astrophysics Data System (ADS)

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

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

120

Lidar probing the urban nocturnal boundary layer

NASA Astrophysics Data System (ADS)

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

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

1998-08-01

121

High Energy Benthic Boundary Layer Experiment: HEBBLE

NASA Astrophysics Data System (ADS)

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

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

122

Turbulent boundary layers developing over compliant surfaces

NASA Astrophysics Data System (ADS)

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

Lekoudis, S. G.; Sengupta, T.

1983-05-01

123

Linear Optimal Controllers for Turbulent Boundary Layers

NASA Astrophysics Data System (ADS)

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

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

2001-11-01

124

Boundary-layer instability noise on aerofoils

NASA Astrophysics Data System (ADS)

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

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

1999-03-01

125

Boundary layer receptivity to freestream turbulence

NASA Astrophysics Data System (ADS)

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

Kendall, James M.

1990-06-01

126

Turbulent boundary layer drag reduction using riblets

NASA Astrophysics Data System (ADS)

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

Walsh, M. J.

1982-01-01

127

Halogen chemistry in the marine boundary layer

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

128

Boundary layer (shear-band) in frustrated viscoplastic flows

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

129

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

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

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

2011-01-01

130

A novel boundary-confined method for microlens arrays fabrication

We present a technique to improve microlens arrays (MLAs) uniformity after the thermal reflow process. Traditional photo resist thermal reflow processes cause micro lenses merge together easily due to an inexact reflow time and temperature distribution. This results in poor uniformity and low lens height. A new MLAs fabrication method, called the boundary-confined method, was proposed and demonstrated. By two

Hsin-Ta Hsieh; Vinna Lin; Guo-Dung J. Su

2011-01-01

131

Improved boundary layer depth retrievals from MPLNET

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

132

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

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

R. Maddahian; B. Farhanieh; B. Firoozabadi

2011-01-01

133

Quadrature formulas for functions with a boundary-layer component

NASA Astrophysics Data System (ADS)

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

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

2011-11-01

134

Self-preservation of rough-wall turbulent boundary layers

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

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

2001-01-01

135

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

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

1985-01-01

136

NASA Astrophysics Data System (ADS)

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

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

2008-05-01

137

NASA Astrophysics Data System (ADS)

Analyses of previous boundary-layer transition experiments over axisymmetric bodies indicates a potential for achieving substantial amounts of laminar flow over such shapes. Achievement of natural laminar flow over portions of nonlifting aircraft geometries, such as fuselage to forebodies, tip tanks or engine nacelles, could significantly contribute to the reduction of total aircraft viscous drag. A modern surface-panel method, a streamwise boundary-layer analysis method, and streamwise linear stability theory (E(sup n)-method) are used to correlate several previous transition measurements along axisymmetric geometries to study the transition characteristics of a nonaxisymmetric body geometry, a flight investigation was conducted to measure the transition location and analyze the mode of transition over the nonaxisymmetric forebody of an existing light twin-engine propeller-driven airplane. A summary of the inviscid flow field over the forebody of the aircraft at various body angles is presented, indicating the relatively small magnitude of inviscid crossflow along the forebody at typical cruising attitudes. The transition instrumentation installed in the airplane fuselage is described, together with relative surface-waviness measurements along the forebody. The macroscopic location of the transitional front, obtained from arrayed hot-film sensors, is presented for a matrix of flight conditions with various unit-Reynolds numbers, angles of attack and sideslip, and engine power settings.

Vijgen, Paul M. H. W.

1990-11-01

138

On Internal Gravity Waves Associated with the Stable Boundary Layer

NASA Astrophysics Data System (ADS)

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

Chimonas, George

139

Helical circulations in the typhoon boundary layer

NASA Astrophysics Data System (ADS)

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

Ellis, Ryan; Businger, Steven

2010-03-01

140

Pattern Formation in Cathode Boundary Layer Microdischarges

NASA Astrophysics Data System (ADS)

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

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

2003-10-01

141

The Near-Calm Stable Boundary Layer

NASA Astrophysics Data System (ADS)

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

Mahrt, Larry

2011-09-01

142

DNS of Hypersonic Turbulent Boundary Layers

NASA Astrophysics Data System (ADS)

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

Pino Martin, M.

2004-11-01

143

Excitation of secondary instabilities in boundary layers

NASA Astrophysics Data System (ADS)

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

Crouch, J. D.

1997-04-01

144

The Coriolis effect on coherent structures in planetary boundary layers

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

Igor N Esau

2003-01-01

145

The Boundary Layer on a Flat Plate in Anisotropic Magnetohydrodynamics.

National Technical Information Service (NTIS)

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

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

1965-01-01

146

The mesoscale responses of a locally heated planetary boundary layer

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

Peter John Sousounis

1990-01-01

147

Impact of Injection Schemes on Transition in Hypersonic Boundary Layers.

National Technical Information Service (NTIS)

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

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

2009-01-01

148

Surface Roughness Effects on the Hypersonic Turbulent Boundary Layer.

National Technical Information Service (NTIS)

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

T. Kubota D. E. Berg

1977-01-01

149

Vertical Transport of Water in the Martian Boundary Layer.

National Technical Information Service (NTIS)

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

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

1993-01-01

150

Entrainment process of carbon dioxide in the atmospheric boundary layer

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

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

2004-01-01

151

Bristled shark skin: a microgeometry for boundary layer control?

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

152

Boundary layer phenomena in combustion-driven MHD power generators

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

J. W. Daily

1975-01-01

153

Development of instrumentation for boundary layer transition detection

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

Steven B. Harrison

1991-01-01

154

Modelling the low-latitude boundary layer with reconnection entry

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

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

1994-04-01

155

Beta limitation of matter-antimatter boundary layers

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

B. Lehnert

1987-01-01

156

On fluid dynamic drag reduction in some boundary layer flows

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

K. Vajravelu; R. N. Mohapatra

1990-01-01

157

Effects of coastal forcing on turbulence and boundary- layer structure

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

Linda Maria Viktoria Strom

1999-01-01

158

Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration

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

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

1985-01-01

159

Microbubble Drag Reduction in Liquid Turbulent Boundary Layers

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

Charles L. Merkle; Steven Deutsch

1992-01-01

160

Drag reduction in liquid boundary layers by gas injection

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

Charles L. Merkle; Steven Deutsch

1990-01-01

161

Modelling the low-latitude boundary layer with reconnection entry

NASA Astrophysics Data System (ADS)

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

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

1994-04-01

162

Dusty boundary layer in a surface-burst explosion

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

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

1993-08-01

163

Acoustic receptivity of laminar boundary layers over wavy walls

NASA Astrophysics Data System (ADS)

Acoustic receptivity of laminar boundary layers over surfaces with nonlocalized low-amplitude periodic waviness is experimentally investigated. An array of 2D strips is used to simulate continuous wall waviness. Particular attention to measurement techniques is required to minimize facility-dependent flow and acoustic field anomalies. Balanced arrays of acoustic sources upstream and downstream of the test section are used to generate a traveling acoustic mode, effectively eliminating the standing modes that would distort the results of continuous receptivity measurements. Hot-wire measurements of Tollmien-Schlichting (T-S) mode shapes arc shown to be in excellent agreement with theory. By increasing the number of wall wavelengths, it is demonstrated that an effectively infinite wavy wall can be produced by a small region near lower branch. It is also shown that the ratio between receptivity at distributed waviness and at localized surface inhomogeneities is of O(10). The effect of mismatching the T-S wavelength with the geometric wall wavelength is investigated by changing the free stream velocity at a constant forcing frequency. Strong receptivity occurs only over a narrow band where the wall wavelength and the T-S wavelength are matched. The results of this experiment are shown to match trends as well as absolute amplitudes predicted by receptivity theory.

Wiegel, M.; Wlezien, R. W.

1993-07-01

164

Effects of Normal Swirling Jet on a Boundary Layer Flow

NASA Astrophysics Data System (ADS)

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

Caballero, Irving I.; Subramanian, Chelakara S.

2002-11-01

165

Entrainment and detrainment from a model boundary layer

NASA Astrophysics Data System (ADS)

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

Sun, Meihong; Lichter, Seth

2003-06-01

166

Boundary layer development in axial compressors and turbines. Part 1 of 4: Composite picture

Comprehensive experiments and computational analyses were conducted to understand boundary layer development on airfoil surfaces in multistage, axial-flow compressors and LP turbines. The tests were run over a broad range of Reynolds numbers and loading levels in large, low-speed research facilities which simulate the relevant aerodynamic features of modern engine components.Measurements of boundary layer characteristics were obtained by using arrays of densely packed, hot-film gauges mounted on airfoil surfaces and by making boundary layer surveys with hot wire probes. Computational predictions were made using both steady flow codes and an unsteady flow code. This is the first time that time-resolved boundary layer measurements and detailed comparisons of measured data with predictions of boundary layer codes have been reported for multistage compressor and turbine blading. Part 1 of this paper summarizes all of the experimental findings by using sketches to show how boundary layers develop on compressor and turbine blading. Parts 2 and 3 present the detailed experimental results for the compressor and turbine, respectively. Part 4 presents computational analyses and discusses comparisons with experimental data. Readers not interested in experimental detail can go directly from Part 1 to Part 4.

Halstead, D.E.; Wisler, D.C.; Shin, H.W. [GE Aircraft Engines, Cincinnati, OH (United States); Okiishi, T.H. [Iowa State Univ., Ames, IA (United States); Walker, G.J. [Univ. of Tasmania, Hobart (Australia); Hodson, H.P. [Univ. of Cambridge (United Kingdom)

1997-01-01

167

Theoretical and modeling studies of the marine planetary boundary layer

NASA Astrophysics Data System (ADS)

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

Randall, David A.

1990-12-01

168

Planetary boundary layer feedbacks in climate system

NASA Astrophysics Data System (ADS)

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

Zilitinkevich, S.; Esau, I.

2009-09-01

169

Matter-antimatter boundary layers with a magnetic neutral sheet

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

B. Lehnert

1978-01-01

170

Marine Boundary-layer Height Estimated From The Hirlam Model

NASA Astrophysics Data System (ADS)

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

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

171

Energy dissipating structures in turbulent boundary layers

NASA Astrophysics Data System (ADS)

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

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

2011-11-01

172

Boundary layer aerosols at Trivandrum tropical coast

NASA Astrophysics Data System (ADS)

Most of the atmospheric particulate matter produced from the earth's surface and injected into the Atmospheric Boundary Layer (ABL) through various natural and anthropogenic processes subsequently intrude into the free troposphere and above depending on prevailing turbulent and meteorological features. Properties of ABL aerosols at Trivandrum (8.5°N, 77 °E), a typical coastal station situated near the Arabian Sea uncontaminated by any major industrial sources, are studied using a High Volume Sampler (HVS), a Low Pressure Impactor (LPI), a CW lidar at 0.5?m wavelength (CWL) and a multi-wavelength (0.4-1.02 ?m) solar radiometer (MWR). While HVS provides hourly variation of surface TSP (Total Suspended Particles) concentration, LPI measures the size resolved aerosol mass concentration (in the aerodynamic radius range 0.05 to 14 ?m) in 12 size-bins averaged for a few days in a month, CWL gives altitude profile of aerosol extinction and number density (Na) up to ~2km and MWR gives the columnar aerosol optical depth (AOD). While the diurnal variation of TSP concentration strongly depends on features of mesoscale circulation, the seasonal TSP pattern depends on local meteorology and synoptic circulation. In winter, TSP concentration is low during sea breeze (SB) and high during land breeze (LB). This TSP contrast decreases in summer and reverses during the monsoon period. On an average, TSP concentration varies in the range 30 to 160 ?g/m 3 with two maxima, during winter and monsoon periods (attributed respectively to increased production of continental aerosols and that of sea-spray aerosols by surface winds). Relative concentration of small particles (radius r < 0.15?m) is high in winter and low during the monsoon period, while that for particles in the intermediate size range (0.15

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

173

Turbulent Transfer in the Marine Planetary Boundary Layer.

National Technical Information Service (NTIS)

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

S. Jodha S. Khalsa H. P. Hanson

1994-01-01

174

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

National Technical Information Service (NTIS)

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

N. R. Subramanian M. I. James

1987-01-01

175

Structure of the Laminar Ablating Air-Teflon Boundary Layer.

National Technical Information Service (NTIS)

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

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

1968-01-01

176

Computational Evaluation of Quiet Tunnel Hypersonic Boundary Layer Stability Experiments.

National Technical Information Service (NTIS)

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

M. L. Manning

2000-01-01

177

Stability of Boundary Layers at High Supersonic and Hypersonic Speeds.

National Technical Information Service (NTIS)

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

T. Herbert

1992-01-01

178

Cross Stream Differencing for Integral Boundary Layer Equations.

National Technical Information Service (NTIS)

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

D. Hally

1987-01-01

179

ATMOSPHERIC DISPERSION MODELING BASED UPON BOUNDARY LAYER PARAMETERIZATION

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...

180

Hybrid spectral element/asymptotic method for boundary layers problems

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

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

1997-12-01

181

Differences in the structure of a planetary magnetopause boundary layer

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

182

Investigation of the Stable Atmospheric Boundary Layer at Halley Antarctica

NASA Astrophysics Data System (ADS)

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

Rodrigo, Javier Sanz; Anderson, Philip S.

2013-09-01

183

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

National Technical Information Service (NTIS)

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

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

1996-01-01

184

Some Topics Relating to Modelling of Dispersion in Boundary Layer.

National Technical Information Service (NTIS)

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

F. Pasquill

1975-01-01

185

The Behaviour of Similar Solutions in a Compressible Boundary Layer.

National Technical Information Service (NTIS)

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

J. B. McLeod J. Serrin

1968-01-01

186

Measurements of the fluctuating wall pressure space-time field were made with a linear array of 48 hydrophones beneath a fully developed turbulent boundary layer of water on a flat plate. Autospectra, cross-spectra, and wavenumber-frequency spectra were calculated from digitized hydrophone signals. Boundary layer parameters were estimated from streamwise velocity profiles that were measured with a laser doppler anemometry system and

Bruce Matthew Abraham

2000-01-01

187

A multidisciplinary optimization method for designing boundary layer ingesting inlets

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

188

Benthic boundary layer processes in coastal environments: An introduction

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

M. D. Richardson; W. R. Bryant

1996-01-01

189

The behavior of vertically integrated boundary-layer winds

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

René V. Cormier

1975-01-01

190

Boundary Layer Structure and Processes in Mid - Ocean Storms

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

Nicholas A. Bond

1986-01-01

191

Drag and heat transfer relations for the planetary boundary layer

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

Robert R. Long; Larry J. Guffey

1977-01-01

192

Boundary-Layer Turbulence Over The Nebraska Sandhills

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

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

2001-01-01

193

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

194

An investigation of boundary-layer behavior on wavy walls

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

Suxun Li; Guang Li Chen

1989-01-01

195

High Reynolds number thick axisymmetric turbulent boundary layer measurements

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

K. M. Cipolla; W. L. Keith

2003-01-01

196

Turbulent oceanic western-boundary layers at low latitude

NASA Astrophysics Data System (ADS)

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

Quam Cyrille Akuetevi, Cataria; Wirth, Achim

2013-04-01

197

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

National Technical Information Service (NTIS)

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

L. W. Reidy G. W. Anderson

1988-01-01

198

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

National Technical Information Service (NTIS)

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

B. Kosovic J. K. Lundquist

2004-01-01

199

Multi-point perspectives of reconnection induced boundary layers

NASA Astrophysics Data System (ADS)

Cluster, Double Star and THEMIS close conjunctions at the magnetopause allow exploration of the conjugate response of the dayside magnetopause on the dawn/dusk flanks. In particular, during the April to July 2007 epoch, the array of four Cluster spacecraft, separated at large distances (10,000 km), were traversing the dawn-side magnetopause at high and low latitudes; the five THEMIS spacecraft were often in a 4+1 configuration, traversing the low latitude, dusk-side magnetosphere, and the Double star, TC-1 spacecraft was in an equatorial orbit between the local times of the THEMIS and Cluster orbits. This combination of 10 spacecraft provided simultaneous monitoring across a wide range of local times. The distribution and grouping of spacecraft allow multi-scale analysis of local phenomena operating on both flanks of the magnetopause, such as the occurrence and location of reconnection sites; extent and orientation of the X-line, and associated boundary layer properties. These near simultaneous encounters with reconnection sites are consistent with both a tilted X-line in the LLBL, together with anti-parallel sites extending to flank locations. Smaller scale configurations of Cluster and TC-1 have shown evidence (centering on repeated multi-point sampling of the ion diffusion region and associated null magnetic field) for high-latitude reconnection at a dayside location where the magnetic field orientations inside and outside the magnetopause lie close to anti-parallel, which is closely following a period of low latitude reconnection.

Dunlop, Malcolm

2010-05-01

200

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

Paul M. H. W. Vijgen

1990-01-01

201

ON AERODYNAMIC AND BOUNDARY LAYER RESISTANCES WITHIN DRY DEPOSITION MODELS

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...

202

A novel boundary-confined method for microlens arrays fabrication

NASA Astrophysics Data System (ADS)

We present a technique to improve microlens arrays (MLAs) uniformity after the thermal reflow process. Traditional photo resist thermal reflow processes cause micro lenses merge together easily due to an inexact reflow time and temperature distribution. This results in poor uniformity and low lens height. A new MLAs fabrication method, called the boundary-confined method, was proposed and demonstrated. By two tones of photoresist (PR), positive and negative, only one photo mask and two photolithography steps are needed in the process. After lithography processes, the positive PR is a slightly little smaller than the circular pattern on a photo mask and negative PR is slightly larger than it. Two tones of PR increase tolerance to mask alignment. Fill-factor is high because of high resolution on a thin boundary. All of flowing PR is stopped by the boundary; uniformity is improved without tight thermal dose constrains. Meanwhile, microlenses with a large height are achievable due to "no cling" effect. The method has advantages, not only for large area MLAs but also for a microlens that require precision diameter or positioning. Besides, we replicate MLAs with the optical polymer to verify some optical specifications. Both the fabrication and replication are straightforward and reliable. Our results show that the microlens is approximately a hemispherical profile. The gap between microlenses with 48 ?m diameter in hexagonal arrangement is 2 ?m and the height of microlens is 22 ?m.

Hsieh, Hsin-Ta; Lin, Vinna; Su, Guo-Dung J.

2011-02-01

203

A Volume-Imaging Radar Wind Profiler for Atmospheric Boundary Layer Turbulence Studies

This paper describes the turbulent eddy profiler (TEP), a volume-imaging, UHF radar wind profiler designed for clear-air measurements in the atmospheric boundary layer on scales comparable to grid cell sizes of large eddy simulation models. TEP employs a large array of antennas—each feeding an independent receiver—to simultaneously generate multiple beams within a 288 conical volume illuminated by the transmitter. Range

James B. Mead; Geoffrey Hopcraft; Stephen J. Frasier; Brian D. Pollard; Christopher D. Cherry; Daniel H. Schaubert; Robert E. McIntosh

1998-01-01

204

Aeroelastically deflecting flaps for shock/boundary-layer interaction control

NASA Astrophysics Data System (ADS)

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

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

2003-06-01

205

Numerical studies of boundary-layer receptivity

NASA Astrophysics Data System (ADS)

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

Reed, Helen L.

1995-09-01

206

Acoustic interactions within a sonar array are known to affect the array performance as well as the life expectancy of the transducers themselves. The ability to predict array performance is required for large close-packed arrays of acoustic transducers. In these cases, traditional full finite element or coupled finite element-boundary element methods prove costly. The general approach is to combine a

J. B. Blottman; A. J. Kalinowski

2000-01-01

207

NASA Astrophysics Data System (ADS)

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

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

2002-06-01

208

Surface boundary-layer variability off Northern California, USA, during upwelling

NASA Astrophysics Data System (ADS)

A five-element mooring array is used to study surface boundary-layer transport over the Northern California shelf from May to August 2001. In this region, upwelling favorable winds increase in strength offshore, leading to a strong positive wind stress curl. We examine the cross-shelf variation in surface Ekman transport calculated from the wind stress and the actual surface boundary-layer transport estimated from oceanic observations. The two quantities are highly correlated with a regression slope near one. Both the Ekman transport and surface boundary layer transport imply curl-driven upwelling rates of about 3×10 -4 m s -1 between the 40 and 90 m isobaths (1.5 and 11.0 km from the coast, respectively) and curl-driven upwelling rates about 1.5×10 -4m s -1 between the 90 and 130 m isobaths (11.0 and 28.4 km from the coast, respectively). Thus curl-driven upwelling extends to at least 25 km from the coast. In contrast, upwelling driven by the adjustment to the coastal boundary condition occurs primarily inshore of the 40-m isobath. The upwelling rates implied by the differentiating the 40-m transport observations with the coastal boundary condition are up to 8×10 -4 m s -1. The estimated upwelling rates and the temperature-nitrate relationship imply curl-driven vertical nitrate flux divergences are about half of those driven by coastal boundary upwelling.

Dever, E. P.; Dorman, C. E.; Largier, J. L.

2006-12-01

209

The Asymmetric Boundary layer Flow Under a Translating Hurricane.

NASA Astrophysics Data System (ADS)

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

Shapiro, Lloyd J.

1983-08-01

210

A numerical study of compressible turbulent boundary layers

NASA Astrophysics Data System (ADS)

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

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

2011-01-01

211

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

212

Hypersonic turbulent wall boundary layer computations

NASA Astrophysics Data System (ADS)

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

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

1988-05-01

213

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

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

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

2010-01-01

214

Saltating Particles in a Turbulent Boundary Layer

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

215

Spatial Optimal Disturbances in Three-Dimensional Boundary Layers

NASA Astrophysics Data System (ADS)

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

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

216

Lag model for turbulent boundary layers over rough bleed surfaces

NASA Astrophysics Data System (ADS)

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

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

1994-07-01

217

Nonlocalized receptivity of boundary layers to three-dimensional disturbances

NASA Astrophysics Data System (ADS)

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

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

1992-01-01

218

Measurement of boundary-layer receptivity at suction surfaces

NASA Astrophysics Data System (ADS)

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

Wlezien, R. W.

1989-03-01

219

Kahuku kite wind study. I. Kahuka beach boundary layer

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

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

1982-09-01

220

Boundary-layer dynamics of a developing vortex

NASA Astrophysics Data System (ADS)

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

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

1981-02-01

221

Velocity measurements in a turbulent natural convection boundary layer

NASA Astrophysics Data System (ADS)

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

Cheesewright, R.; Ierokiopitis, E.

222

Extended self-similarity in boundary layer turbulence

NASA Astrophysics Data System (ADS)

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

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

1997-06-01

223

Skin friction in zero-pressure-gradient boundary layers.

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

Yakhot, Victor

2010-10-04

224

Grain-boundary layering transitions in a model bicrystal

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

225

Skin friction in zero-pressure-gradient boundary layers

NASA Astrophysics Data System (ADS)

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

Yakhot, Victor

2010-10-01

226

A Theory of Gravity Wave Absorption by a Boundary Layer

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

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

2006-01-01

227

Surface heating due to turbulent boundary-layer flows

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

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

1981-01-20

228

The collapse of turbulence in the atmospheric boundary layer

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

229

On the origin of the high-latitude boundary layer

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

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

2002-01-01

230

CFD simulation of the atmospheric boundary layer: wall function problems

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

231

Aircraft measurements within the planetary boundary layer over water

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

232

A Boundary-Layer Parametrisation for Martian General Circulation Model

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

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

1996-01-01

233

A Qualitative Description of Boundary Layer Wind Speed Records

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

Radhakrishnan Nagarajan

2006-01-01

234

Iodine monoxide in the Western Pacific marine boundary layer

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

235

Iodine monoxide in the Western Pacific marine boundary layer

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

236

ATMOSPHERIC DISPERSION IN THE ARCTIC: WINTERTIME BOUNDARY-LAYER MEASUREMENTS

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

237

Sources of iodine in the tropical marine boundary layer

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

238

The core-mantle boundary layer and deep Earth dynamics

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

Thorne Lay; Quentin Williams; Edward J. Garnero

1998-01-01

239

Acoustic explorations of the upper ocean boundary layer

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

Svein Vagle

2005-01-01

240

On the Stability of 3D Boundary Layers

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

Th. Herbert

1996-01-01

241

Thickness and concentration profile of the boundary layer in electrodialysis

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

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

1997-01-01

242

Torpedo Drag Reduction Using Magnetohydrodynamic Boundary-Layer Control.

National Technical Information Service (NTIS)

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

Y. K. Wu G. F. Anderson

1972-01-01

243

ANALYTICAL PARAMETERIZATIONS OF DIFFUSION: THE CONVECTIVE BOUNDARY LAYER

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...

244

Stable Boundary-Layer Scaling Regimes: The Sheba Data

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

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

2005-01-01

245

Propagation of propeller tone noise through a fuselage boundary layer

NASA Astrophysics Data System (ADS)

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

Hanson, D. B.; Magliozzi, B.

1984-01-01

246

Basic study on blockage effects in turbulent boundary layer flows

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

M. Noda; H. Utsunomiya; F. Nagao

1995-01-01

247

On boundary-layer transition in transonic flow

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

R. I. Bowles; F. T. Smith

1993-01-01

248

Irregularity excitation associated with charged dust cloud boundary layers

NASA Astrophysics Data System (ADS)

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

Mahmoudian, A.; Scales, W. A.

2012-02-01

249

The role of acoustic feedback in boundary-layer instability

NASA Astrophysics Data System (ADS)

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

Wu, Xuesong

2013-10-01

250

Beta Limitation of Matter-Antimatter Boundary Layers.

National Technical Information Service (NTIS)

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

B. Lehnert

1987-01-01

251

Beta limitation of matter-antimatter boundary layers

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

B. Lehnert

1988-01-01

252

Vertical velocity structure of nonprecipitating continental boundary layer stratocumulus clouds

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

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

2010-01-01

253

Nighttime Chemistry in the Polluted Boundary Layer (Invited)

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

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

2009-01-01

254

Shock Wave / Boundary Layer Interaction Experiment on Control Surface.

National Technical Information Service (NTIS)

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

L. Prochazka S. Schlamp T. Rosgen

2007-01-01

255

COMPRESSIBLE TURBULENT BOUNDARY-LAYER FLOW CONTROL OVER A WEDGE

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

M. Xenos; E. Tzirtzilakis; N. Kafoussias

256

The effects of cylindrical surface modifications on turbulent boundary layers

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

J. B. Johansen; C. R. Smith

1983-01-01

257

Power laws for rough wall turbulent boundary layers

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

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

2003-01-01

258

Theoretical investigation of turbulent boundary layer over a wavy surface

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

L. C. Chang

1975-01-01

259

Atmospheric boundary layer modification in the marginal ice zone

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

Theodore J. Bennett; Kenneth Hunkins

1986-01-01

260

Why Rolls are Prevalent in the Hurricane Boundary Layer

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

Ralph C. Foster

2005-01-01

261

Numerical Solution of the Turbulent-Boundary-Layer Equations.

National Technical Information Service (NTIS)

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

A. M. O. Smith T. Cebeci

1967-01-01

262

Unsteady boundary layers with an intelligent numerical scheme

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

Tuncer Cebeci

1986-01-01

263

A boundary-layer collision in a curved duct

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

KEITH STEWARTSON; TUNCER CEBECI; K. C. Chang

1980-01-01

264

Calculation of incompressible rough-wall boundary-layer flows

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

T. Cebeci; K. C. Chang

1978-01-01

265

Drag reduction by microbubbles in a turbulent boundary layer

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

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

2010-01-01

266

Polymer drag reduction of a zero pressure gradient boundary layer

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

John E. Koskie; William G. Tiederman

1991-01-01

267

Boundary layer effects above a Himalayan valley near Mount Everest

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

268

Turbulent dispersion in the Atmospheric Convective Boundary Layer

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

269

The bottom boundary layer of the deep ocean

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

Laurence Armi; Robert C. Millard

1976-01-01

270

Retinal layer segmentation of macular OCT images using boundary classification

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

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

2013-01-01

271

Simultaneous profiling of the Arctic Atmospheric Boundary Layer

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

S. Mayer; M. Jonassen; J. Reuder

2009-01-01

272

Impulse response of laminar boundary layer and receptivity

NASA Astrophysics Data System (ADS)

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

Sengupta, Tapan K.

273

On the growth of turbulent regions in laminar boundary layers

NASA Astrophysics Data System (ADS)

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

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

1981-09-01

274

TURBULENCE PARAMETERS IMPACTING DISPERSION IN AN URBAN CONVECTIVE BOUNDARY LAYER

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,...

275

Flat plate turbulent boundary-layer control using vertical LEBUs

NASA Astrophysics Data System (ADS)

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

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

276

Navier-Stokes simulation of boundary-layer transition

NASA Astrophysics Data System (ADS)

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

Reed, Helen L.

1990-05-01

277

Velocity Measurements of a Cylindrical Turbulent Boundary Layer in a Submarine Wake

NASA Astrophysics Data System (ADS)

High resolution stereo-PIV measurements were made on a long, small diameter cylinder towed from the control surface of a 1/18^th scale submarine model. The experiments were performed in the tow tank at NSWCCD at 5 kts. 3-D velocity fields over ten body lengths downstream were obtained. The cylinders were approximately neutrally buoyant and towed through a stationary laser sheet oriented perpendicular to the tow direction. The objective of the study was to quantify the effect of the flow behind the submarine control surfaces and propeller on the boundary layer development and dynamics of a model towed array where ?>> the cylinder radius, a. Analysis required stepwise tracking of the array through the dissipating wake field and isolation of the contribution from the cylindrical boundary layer. Approximately 40 instantaneous vector fields were obtained for each location. Mean and fluctuating streamwise and cross-stream velocities will be presented. Initial results indicate that the contribution of the wake to the total flow field and its effect on the boundary layer development varies in the streamwise direction.

Furey, Deborah; Atsavapranee, Paisan; Cipolla, Kimberly; Bretall, Damien

2006-11-01

278

The RAP: a ring array processor for layered network calculations

The authors have designed and implemented a ring array processor, RAP, for fast implementation of layered neural network algorithms. The RAP is a multi-DSP system targeted at continuous speech recognition using connectionist algorithms. Four boards, each with four Texas Instruments, TMS 320C30 DSPs, serve as an array processor for a 68020-based host running a real-time operating system. The overall system

N. Morgan; J. Beck; P. Kohn; J. Bilmes; E. Allman; J. Beer

1990-01-01

279

Using data sets collected during the Lake Atmosphere Turbulent Exchanges (LATEX, convectively unstable conditions) and the Snow Horizontal Array Turbulence Study (SnoHATS, convectively stable conditions) field experimental campaigns, we study the impact of this convective stability on the physics of small scale turbulence in the atmospheric boundary layer flow and the implications for modeling the subgrid scales stresses and fluxes

Elie Bou-Zeid; Nikki Vercauteren; Chad Higgins; Hendrik Huwald; Marc B. Parlange; Charles Meneveau

2009-01-01

280

Numerical investigation of the stable nocturnal boundary layer

NASA Astrophysics Data System (ADS)

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

Shi, Xingzhong

1997-10-01

281

Characteristics of nonlinear evolution of wavepackets in boundary layers

NASA Astrophysics Data System (ADS)

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

Yu, Min; Luo, JiSheng; Li, Jia

2013-02-01

282

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, 800

Wu, Xiaohua; Moin, Parviz

2010-08-01

283

The influence of bulges on boundary-layer instability

NASA Astrophysics Data System (ADS)

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

Elli, S.; Vandam, C. P.

284

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

NASA Astrophysics Data System (ADS)

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

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

2011-06-01

285

On a numerical solution of incompressible turbulent boundary layer flow

NASA Astrophysics Data System (ADS)

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

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

286

Behaviour of Atmospheric Boundary Layer Height at Dome C, Antarctica

NASA Astrophysics Data System (ADS)

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

Pietroni, I.; Argentini, S.

2009-09-01

287

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

288

NASA Astrophysics Data System (ADS)

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

Hujeirat, A.

1995-03-01

289

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

Paul M. H. W. Vijgen

1990-01-01

290

Asymptotic analysis of the k-epsilon turbulent boundary layer

NASA Astrophysics Data System (ADS)

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

Bogucz, E. A.

291

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

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

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

1988-01-01

292

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

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

P. R. Bandyopadhyay; R. D. Watson

1987-01-01

293

NASA Astrophysics Data System (ADS)

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

Wood, Robert; Bretherton, Christopher S.

2004-09-01

294

Boundary Layer Structure and Dynamics in Outer Hurricane Rainbands.

NASA Astrophysics Data System (ADS)

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

Powell, Mark Dillon

295

A numerical study of compressible turbulent boundary layers

NASA Astrophysics Data System (ADS)

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

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

2010-11-01

296

Mixing of Ozone at Boundary Layer Top - A Lidar Study

NASA Astrophysics Data System (ADS)

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

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

2009-09-01

297

Transitionally rough zero pressure gradient turbulent boundary layers

NASA Astrophysics Data System (ADS)

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

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

2008-01-01

298

Studies of the Marine Boundary Layer at Tarapur

NASA Astrophysics Data System (ADS)

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

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

1986-12-01

299

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

NASA Astrophysics Data System (ADS)

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

Kiesow, Robert; Plesniak, Michael

1999-11-01

300

Dynamical Simulation of Cloudy Boundary Layer Flow during Cold Air Outbreaks

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

Chiu-Wai Yuen

1983-01-01

301

Investigating boundary layer turbulence for aeolian sand transport

NASA Astrophysics Data System (ADS)

Recent and ongoing studies into the mechanisms of aeolian sand transport have found that turbulence parameters, rather than mean velocities, are fundamental for understanding the dynamics of moving sand. A more comprehensive understanding of turbulent motions in the atmospheric boundary layer and how this turbulence interacts with sediment has been permitted thanks to technological improvements in sensor technology, to record sand transport and three-dimensional wind velocities at high spatio-temporal resolutions. This paper presents results from novel fieldwork conducted to investigate the role of turbulence in aeolian sand transport by measuring wind velocities in a tight vertical array. Co-located saltation impact sensors (Safires) and sand traps were used to simultaneously monitor the resulting transport. Measuring wind velocities in three dimensions at a series of different heights above the surface allows a unique opportunity to explore atmospheric turbulence, to try and ascertain how and where turbulent motions are generated within the flow and to see how these motions change with respect to height above the surface. Furthermore, the co-located transport sensors allow the sand transport response to these turbulent motions to be realized. Turbulence is quantified through the calculation of the stress tensor, shear velocity, turbulence kinetic energy and turbulent structures from quadrant analysis. These parameters are visualized spatially and temporally using vertical correlations and contour plots. The results show that shear stress is not a consistent parameter throughout the flow, rather it is changeable with height above the surface, and furthermore, there is no consistent pattern with height above the surface. Over relatively short timescales shear stress is shown to evolve in patterns which appear to demonstrate the presence large eddies within the flow. With the experimental design presented here, it is possible to track these eddies down towards the surface and observe the sand transport response at the co-located safires and traps. Sand transport events are found to relate to peaks in resultant shear stress at a number of timescales. From these analyses it is clear that the mechanisms of aeolian sand transport cannot be fully understood from recording wind velocities at 0.5 m above the surface, or higher, it is important to monitor turbulent parameters as close to the surface as possible.

Lee, Z. S.; Baas, A. C. W.

2012-04-01

302

Boundary layer ozone - An airborne survey above the Amazon Basin

NASA Astrophysics Data System (ADS)

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

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

1988-02-01

303

The Boundary Layer Flows of a Rivlin-Ericksen Fluid

NASA Astrophysics Data System (ADS)

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

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

304

Clues and modelling for missing boundary layer in cataclysmic variables

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

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

1996-12-31

305

Aerosol Observations by Lidar in the Nocturnal Boundary Layer

NASA Astrophysics Data System (ADS)

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

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

1999-07-01

306

On multiscale homogenization problems in boundary layer theory

NASA Astrophysics Data System (ADS)

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

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

2012-06-01

307

Boundary layer study. Phase 4: Experimental validation test plan

NASA Astrophysics Data System (ADS)

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

Kehtarnavaz, H.; Coats, D. E.

1990-11-01

308

Receptivity of boundary layers - Asymptotic theory and experiment

NASA Astrophysics Data System (ADS)

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

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

1990-06-01

309

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

310

Calculation of hypersonic laminar boundary layers with chemical kinetics

NASA Astrophysics Data System (ADS)

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

Noel, F.; Boukhobza, P.

1986-11-01

311

Entropy generation in the viscous parts of turbulent boundary layers

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

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

2008-06-01

312

Boundary-layer equations in generalized curvilinear coordinates

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

Argyris G. Panaras

1987-01-01

313

A zonal grid algorithm for DNS of turbulent boundary layers

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

Michael Manhart

2004-01-01

314

Weak Boundary Layers in Styrene-Butadiene Rubber

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

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

1995-01-01

315

The Amazon Boundary Layer Experiment: Wet season 1987

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

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

1990-01-01

316

The Turbulent Structure of Drag Reducing Boundary Layer Flows

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

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

317

Nanodiamonds in the Younger Dryas Boundary Sediment Layer

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

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

2009-01-01

318

Correlation study in shock wave–turbulent boundary layer interaction

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

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

2011-01-01

319

Automated large-eddy simulations of realistic atmospheric boundary layers

NASA Astrophysics Data System (ADS)

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

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

2009-09-01

320

Transient boundary-layer flows in combustion environments

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

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

1980-11-01

321

The effects of cylindrical surface modifications on turbulent boundary layers

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

J. B. Johansen; C. R. Smith

1985-01-01

322

Spatially developing secondary instabilities in compressible swept airfoil boundary layers

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

Fei Li; Meelan M. Choudhari

2011-01-01

323

Some physical aspects of shock wave/boundary layer interactions

NASA Astrophysics Data System (ADS)

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

Délery, Jean; Dussauge, Jean-Paul

2009-12-01

324

Strained layer superlattice focal plane array having a planar structure

An infrared focal plane array (FPA) is disclosed which utilizes a strained-layer superlattice (SLS) formed of alternating layers of InAs and In.sub.xGa.sub.1-xSb with 0.ltoreq.x.ltoreq.0.5 epitaxially grown on a GaSb substrate. The FPA avoids the use of a mesa structure to isolate each photodetector element and instead uses impurity-doped regions formed in or about each photodetector for electrical isolation. This results in a substantially-planar structure in which the SLS is unbroken across the entire width of a 2-D array of the photodetector elements which are capped with an epitaxially-grown passivation layer to reduce or eliminate surface recombination. The FPA has applications for use in the wavelength range of 3-25 .mu.m.

Kim, Jin K; Carroll, Malcolm S; Gin, Aaron; Marsh, Phillip F; Young, Erik W; Cich, Michael J

2012-10-23

325

Perturbed boundary layer diffusion flames. Ph. D. thesis

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

Ang, J.A.

1987-03-01

326

Acoustic explorations of the upper ocean boundary layer

NASA Astrophysics Data System (ADS)

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

Vagle, Svein

2005-04-01

327

A planetary boundary layer observational capability in Kansas

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

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

1997-03-01

328

Linear stability of three-dimensional boundary layers

NASA Astrophysics Data System (ADS)

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

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

2008-03-01

329

Concentration boundary layers in osmotic membrane transport processes

NASA Astrophysics Data System (ADS)

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

Jensen, Kaare; Bohr, Tomas; Bruus, Henrik

2009-11-01

330

An analytical model of capped turbulent oscillatory bottom boundary layers

NASA Astrophysics Data System (ADS)

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

Shimizu, Kenji

2010-03-01

331

Boundary layer structure during sea breeze conditions at Ahtopol, Bulgaria

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

332

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

333

Transient deployment of flat winglets inside a turbulent boundary layer

NASA Astrophysics Data System (ADS)

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

Pierides, Alexis; Andreopoulos, Yiannis

2008-11-01

334

Effect of thermally induced perturbation in supersonic boundary layers

NASA Astrophysics Data System (ADS)

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

Yan, Hong; Gaitonde, Datta

2010-06-01

335

Non-intrusive generation of instability waves in a planar hypersonic boundary layer

NASA Astrophysics Data System (ADS)

The aim of this work is to show the possibility of non-intrusively exciting second-mode instability waves with arbitrary frequency and amplitude in a hypersonic, planar boundary layer, by means of optical methods. Surface heat flux sensors were used to measure natural and artificially excited instability waves on a flat plate at zero angle of attack. The measurements were made using a stream-wise array of flush-mounted high-frequency heat flux sensors. In addition, surface pressure sensors were applied and show the instability waves, as well. The possibility to generate such waves by locally heating the model surface is shown.

Heitmann, D.; Kähler, C.; Radespiel, R.; Rödiger, T.; Knauss, H.; Wagner, S.

2011-02-01

336

Probing high-Reynolds-number effects in numerical boundary layers

NASA Astrophysics Data System (ADS)

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

Pirozzoli, Sergio; Bernardini, Matteo

2013-02-01

337

The Benthic Boundary Layer: Transport Processes and Biogeochemistry

NASA Astrophysics Data System (ADS)

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

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

338

National Technical Information Service (NTIS)

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

R. J. Gloss

1971-01-01

339

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

National Technical Information Service (NTIS)

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

C. Mundt F. Monnoyer

1990-01-01

340

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

National Technical Information Service (NTIS)

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

M. F. Blair

1979-01-01

341

National Technical Information Service (NTIS)

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

M. F. Blair M. J. Werle

1980-01-01

342

Phased array ultrasound imaging through planar tissue layers.

Conventional ultrasound imaging devices are designed based on the assumption of a homogeneous tissue medium of constant acoustic velocity = 1540 m/sec. However, the body consists of tissue layers of varying thicknesses and velocities which range from 1470 m/sec in fat to 3200 m/sec in skull bone. Refraction effects from these layers degrade ultrasound image quality. In this paper, pulse-echo ultrasound imaging is modeled as imaging an organ of interest through an intervening planar tissue layer, such as liver through fat in the abdomen or brain through skull bone in the adult head. Refraction effects from planar tissue layer interfaces are analyzed using Snell's law and measured using phantoms. We also introduce an on-line phased array correction technique based on planar tissue layers to restore ultrasound image quality. We conclude that fat/organ planar interfaces do not degrade image quality significantly. However, refraction effects at a skull/brain planar interface degrades resolution and target acquisition and introduces geometric distortion. Our plane layer phased array correction technique significantly improves image quality in phantoms through lucite aberrators and improves adult cephalic ultrasound image quality when used through the top of the adult skull. The correction technique is robust even in the presence of inaccurate estimates of skull thickness. PMID:3962008

Smith, S W; Trahey, G E; von Ramm, O T

1986-03-01

343

Influence of a Two-scale Surface Roughness on a Neutral Turbulent Boundary Layer

NASA Astrophysics Data System (ADS)

Flow in the urban boundary layer is strongly influenced by the surface roughness, which is composed principally of isolated buildings or groups of buildings. Previous research has shown that the flow regime depends on the characteristic height of these obstacles ( H), and the spacing between them ( W). In reality, the urban boundary layer contains roughness elements with a wide range of length scales; in many practical situations these can be classified into large-scale roughness—buildings, or groups of buildings—and small-scale roughness, such as street furniture and elements on the façades and roofs. It is important to understand how the small-scale roughness might modify mass and momentum transfer in the urban boundary layer, but relatively little information is available concerning the potential interaction between large- and small-scale roughness elements in the different flow regimes. This problem has been studied using wind-tunnel experiments, by measuring vertical velocity profiles over a two-dimensional obstacle array, adding small-scale roughness elements to the top of larger parallel square bars. The experiments were performed for different cavity aspect ratios: the results show that the small-scale roughness increases the turbulence intensities and the momentum transfer when the large-scale obstacles are closely packed ( H/ W > 1) but it has very little effect for more widely-spaced obstacles ( H/ W < 1).

Salizzoni, Pietro; Soulhac, Lionel; Mejean, Patrick; Perkins, Richard J.

2008-04-01

344

Bifurcation of the cusp: Implications for understanding boundary layers

NASA Astrophysics Data System (ADS)

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

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

345

Coupling the dynamics of boundary layers and evolutionary dunes.

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

Ortiz, Pablo; Smolarkiewicz, Piotr K

2009-04-20

346

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

Jiaxiang Yan; Bin Yu

1991-01-01

347

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

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

348

The Adaptation of the Atmospheric Boundary Layer to a Change in Surface Roughness

The adaptation of the atmospheric boundary layer to a change in the underlying surface roughness is an interesting problem and hence much research, theoretical, experimental, and numerical, has been undertaken. Within the atmospheric boundary layer an accurate numerical model for the turbulent properties of the atmospheric boundary layer needs to be implemented if physically realistic results are to be obtained.

S. D. Wright; L. ELLIOTT; D. B. INGHAM; M. J. C. HEWSON

1998-01-01

349

Computation of transonic cascade flows with shock and boundary layer interaction

Viscous phenomena in transonic turbomachine blade flow calculations are represented superimposing the effects of a boundary layer and wake on a inviscid flow. The inviscid flow is determined using a time marching calculation method. The boundary layer calculation is developed to treat the interaction effects of shock and expansion waves. The boundary layer is also modified to account for the

U. K. Singh

1980-01-01

350

The effects of placing a parallel-plate turbulence manipulator in a boundary layer are documented through flow visualization and hot wire measurements. The boundary layer manipulator was designed to manage the large scale structures of turbulence leading to a reduction in surface drag. The differences in the turbulent structure of the boundary layer are summarized to demonstrate differences in various flow

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

1981-01-01

351

Boundary layers as the primary transport regions of the earth's magnetotail

A comprehensive survey of ISEE and IMP LEPEDEA plasma measurements in the earth's magnetotail reveals that the magnetospheric boundary layer and the plasma sheet boundary layer are the primary transport regions there. These plasma measurements also distinguish various components of the plasma sheet, including the central plasma sheet and plasma sheet boundary layer. A significant new result reported here is

T. E. Eastman; L. A. Frank; C. Y. Huang

1985-01-01

352

The effects of external conditions in turbulent boundary layers

NASA Astrophysics Data System (ADS)

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

Brzek, Brian G.

353

Interior versus boundary mixing of a cold intermediate layer

NASA Astrophysics Data System (ADS)

The relative importance of interior versus boundary mixing is examined for the erosion of the cold intermediate layer (CIL) of the Gulf of St. Lawrence. Based on 18 years of historical temperature profiles, the seasonal erosion of the core temperature, thickness and heat content of the CIL are, respectively, ?min = 0.24 ± 0.04°C mo-1, ?min = -11 ± 2 m mo-1 and ? = 0.59 ± 0.09 MJ m-3 mo-1. These erosion rates are remarkably well reproduced with a one-dimensional vertical diffusion model fed with turbulent diffusivities inferred from 892 microstructure casts. This suggests that the CIL is principally eroded by vertical diffusion processes. The CIL erosion is best reproduced by mean turbulent kinetic energy dissipation rate and eddy diffusivity coefficient of ? ? 2 × 10-8 W kg-1 and K ? 4 × 10-5 m2 s-1, respectively. It is also suggested that while boundary mixing may be significant it may not dominate CIL erosion. Interior mixing alone accounts for about 70% of this diffusivity with the remainder being attributed to boundary mixing. The latter result is in accordance with recent studies that suggest that boundary mixing is not the principal mixing agent in coastal seas.

Cyr, F.; Bourgault, D.; Galbraith, P. S.

2011-12-01

354

Coastal boundary layer transition within tropical cyclones at landfall

NASA Astrophysics Data System (ADS)

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

Howard, James Robert

355

Drag reducing outer-layer devices in rough wall turbulent boundary layers

The ability of outer-layer devices to reduce wall shear stress over a substantial streamwise distance in rough-wall turbulent boundary layers has been studied experimentally. The devices examined are a pair of thin flat ribbons placed in tandem as well as those having symmetric airfoil sections. The wall conditions examined are smooth, d- and k-type transverse-groove and sandgrain roughnesses. The wall

P. R. Bandyopadhyay

1986-01-01

356

Photoacoustic emission from Au nanoparticles arrayed on thermal insulation layer.

Efficient photoacoustic emission from Au nanoparticles on a porous SiO(2) layer was investigated experimentally and theoretically. The Au nanoparticle arrays/porous SiO(2)/SiO(2)/Ag mirror sandwiches, namely, local plasmon resonators, were prepared by dynamic oblique deposition (DOD). Photoacoustic measurements were performed on the local plasmon resonators, whose optical absorption was varied from 0.03 (3%) to 0.95 by varying the thickness of the dielectric SiO(2) layer. The sample with high absorption (0.95) emitted a sound that was eight times stronger than that emitted by graphite (0.94) and three times stronger than that emitted by the sample without the porous SiO(2) layer (0.93). The contribution of the porous SiO(2) layer to the efficient photoacoustic emission was analyzed by means of a numerical method based on a one-dimensional heat transfer model. The result suggested that the low thermal conductivity of the underlying porous layer reduces the amount of heat escaping from the substrate and contributes to the efficient photoacoustic emission from Au nanoparticle arrays. Because both the thermal conductivity and the spatial distribution of the heat generation can be controlled by DOD, the local plasmon resonators produced by DOD are suitable for the spatio-temporal modulation of the local temperature. PMID:23571958

Namura, Kyoko; Suzuki, Motofumi; Nakajima, Kaoru; Kimura, Kenji

2013-04-01

357

NASA Astrophysics Data System (ADS)

A zero-pressure gradient turbulent boundary layer is perturbed by a spatially impulsive patch of two-dimensional roughness elements, which are actuated dynamically to alternate between smooth and rough surface conditions, and the downstream response is measured by hot-wire anemometry and particle image velocimetry. The dynamic perturbation is observed to contribute a periodic signature to the downstream flow-field, which manifests itself in critical-layer type behavior. The downstream flow field is reconstructed in a phase-locked sense in order to compare the observed behavior with asymptotic representations of the expected behavior at matched flow conditions. Perturbation using a periodic disturbance is shown to reveal underlying features of the turbulent boundary layer which are intimately connected to the critical layer framework for turbulent pipe flow proposed by McKeon & Sharma (see the DFD-2010 presentation on `Structure from the critical layer framework in turbulent flow' by Sharma & McKeon), while simultaneously providing practical insight on the manipulation of the structure of boundary layers.

Jacobi, Ian; McKeon, Beverley J.

2010-11-01

358

Vertical ozone characteristics in urban boundary layer in Beijing.

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

2012-11-06

359

Non-uniqueness in wakes and boundary layers

NASA Astrophysics Data System (ADS)

In streamlined flow past a flat plate aligned with a uniform stream, it is shown that (a) the Goldstein near-wake and (b) the Blasius boundary layer are nonunique solutions locally for the classical boundary layer equations, whereas (c) the Rott-Hakkinen (1965) very-near-wake appears to be unique. In each of (a) and (b) an alternative solution exists, which has reversed flow and which apparently cannot be discounted on immediate grounds. So, depending mainly on how the alternatives for (a), (b) develop downstream, the symmetric flow at high Reynolds numbers could have two, four or more steady forms. Concerning nonstreamlined flow, for example past a bluff obstacle, new similarity forms are described for the pressure-free viscous symmetric closure of a predominantly slender long wake beyond a large-scale separation. Features arising include nonuniqueness, singularities and algebraic behavior, consistent with nonentraining shear layers with algebraic decay. Nonuniqueness also seems possible in reattachment onto a solid surface and for nonsymmetric or pressure-controlled flows including the wake of a symmetric cascade.

Smith, F. T.

1984-01-01

360

The electrostatic potential of a two-layer three-element symmetric array

NASA Astrophysics Data System (ADS)

The focusing of charged-particle beams by periodic electrostatic fields in such devices as linear accelerators and microwave oscillators can be carried out using a set of electrostatic lenses. An analysis is presented of the spatial distribution of electrostatic potential in this type of system in the form of a two-layer three-element symmetric array. The solution to the boundary value problem for the Laplace equation in the rigorous formulation is obtained by the method of the Riemann-Hilbert problem. The results are presented in the form of equipotential and force lines of the electric field.

Litvinenko, L. N.; Pogarskii, S. A.; Saprykin, I. I.; Sedykh, V. M.

361

The interaction of synthetic jets with turbulent boundary layers

NASA Astrophysics Data System (ADS)

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

Cui, Jing

362

A thermal plume model for the Martian convective boundary layer

NASA Astrophysics Data System (ADS)

The Martian planetary boundary layer (PBL) is a crucial component of the Martian climate system. Global climate models (GCMs) and mesoscale models (MMs) lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the "thermal plume" model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single-column models. The aim of this physically based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in large-eddy simulations (LESs). We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near-surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface-atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near-surface lander measurements. Using a thermal plume model moreover enables a first-order estimation of key turbulent quantities (e.g., PBL height and convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.

Colaïtis, A.; Spiga, A.; Hourdin, F.; Rio, C.; Forget, F.; Millour, E.

2013-07-01

363

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

NASA Astrophysics Data System (ADS)

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

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

2007-11-01

364

Boundary layer polarization and voltage in the 14 MLT region

NASA Astrophysics Data System (ADS)

Viking midlatitude observations of ions and electrons in the postnoon auroral region show that field-aligned acceleration of electrons and ions with energies up to a few kiloelectron volts takes place. The characteristics of the upgoing ion beams and the local transverse electric field observed by Viking indicate that parallel ion acceleration is primarily due to a quasi-electrostatic field-aligned acceleration process below Viking altitudes, i.e., below 10,000-13,500 km. A good correlation is found between the maximum upgoing ion beam energy and the depth of the local potential well determined by the Viking electric field experiment within dayside 'ion inverted Vs.' The total transverse potential throughout the entire region near the ion inverted Vs. is generally much higher than the field-aligned potential and may reach well above 10 kV. However, the detailed mapping of the transverse potential out to the boundary layer, a fundamental issue which remains controversial, was not attempted here. An important finding in this study is the strong correlation between the maximum up going ion beam energy of dayside ion inverted Vs and the solar wind velocity. This suggests a direct coupling of the solar wind plasma dynamo/voltage generator to the region of field-aligned particle acceleration. The fact that the center of dayside ion inverted Vs coincide with convection reversals/flow stagnation and upward Birkeland currents on what appears to be closed field lines (Woch et al., 1993), suggests that field-aligned potential structures connect to the inner part of an MHD dyanmo in the low-latitude boundary layer. Thus the Viking observations substantiate the idea of a solar wind induced boundary layer polarization where negatively charged perturbations in the postnoon sector persistently develops along the magnetic field lines, establishing accelerating potential drops along the geomagnetic field lines in the 0.5-10 kV range.

Lundin, R.; Yamauchi, M.; Woch, J.; Marklund, G.

1995-05-01

365

A multidisciplinary optimization method for designing boundary layer ingesting inlets

NASA Astrophysics Data System (ADS)

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 performance can offset and even overwhelm this potential advantage. For both designs, the tight coupling between the aircraft aerodynamics and the propulsion system poses a difficult design integration problem. This dissertation presents a design method that solves the problem using multidisciplinary optimization. A Navier-Stokes flow solver, an engine analysis method, and a nonlinear optimizer are combined into a design tool that correctly addresses the tight coupling of the problem. The method is first applied to a model 2D problem to expedite development and thoroughly test the scheme. The low computational cost of the 2D method allows for several inlet installations to be optimized and analyzed. The method is then upgraded by using a validated 3D Navier-Stokes solver. The two candidate engine installations are analyzed and optimized using this inlet design method. The method is shown to be quite effective at integrating the propulsion and aerodynamic systems of the Blend-Wing-Body for both engine installations by improving overall performance and satisfying any specified design constraints. By comparing the two optimized designs, the potential advantages of ingesting boundary layer flow for this aircraft are demonstrated.

Rodriguez, David Leonard

2001-07-01

366

Dual Doppler measurement of a sheared, convective boundary layer

NASA Astrophysics Data System (ADS)

The Phoenix 2 experiment was conducted in the spring of 1984 on the high plains of eastern Colorado, using dual Doppler radar as the primary observing tool in a study of the convective planetary boundary layer. Extensive support was provided by in situ sensors (instrumented aircraft, micrometeorological tower and surface network) for verification and extension of radar data. The two experiment days with the deepest boundary layers (which proved to be strongly sheared) were chosen for intensive study, and six 20-minute segments of continuous data from each day (about one each hour during the afternoon) were analyzed. The time resolution of the data is approximately two minutes, and the resolution of the analysis grid (which is 9 km by 9 km by 4 km deep) is 200 m in all directions. Application of available dual Doppler synthesis and integration techniques to define the four-dimensional velocity fields produced an unacceptably high noise level in the vertical velocity and derived statistics, prompting an exploration of alternative analysis methods, and a revival of coplanar integration to reduce the error variance to an acceptable level. The success of the coplanar technique is also demonstrated in a propagation of error analysis. Pressure and buoyancy fluctuations were retrieved from the wind fields using the Gal-Chen/Hane thermodynamic recovery, although over a smaller range of physical scales. The computed momentum and buoyancy fluxes are the basis of a study of the sheared, convective boundary layer over the course of two afternoons, including turbulent kinetic energy budgets and vertical velocity variance budgets.

Schneider, Jeanne Maples

367

Spherical bubble motion in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

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 bubble-bubble interaction were negligible. The experimental diagnostics included ensemble-averaged planar laser intensity profiles for bubble concentration distribution, as well as Cinematic Particle Image Velocimetry with bubble tracking for bubble hydrodynamic forces. A variety of void distributions within the boundary layer were found. For example, there was a tendency for bubbles to collect along the wall for higher Stokes number conditions, while the lower Stokes number conditions produced Gaussian-type profiles throughout the boundary layer. In addition, three types of bubble trajectories were observed-sliding bubbles, bouncing bubbles, and free-dispersion bubbles. Instantaneous liquid forces acting on individual bubbles in the turbulent flow were also obtained to provide the drag and lift coefficients (with notable experimental uncertainty). These results indicate that drag coefficient decreases with increasing Reynolds number as is conventionally expected but variations were observed. In general, the instantaneous drag coefficient (for constant bubble Reynolds number) tended to be reduced as the turbulence intensity increased. The averaged lift coefficient is higher than that given by inviscid theory (and sometimes even that of creeping flow theory) and tends to decrease with increasing bubble Reynolds number.

Felton, Keith; Loth, Eric

2001-09-01

368

The Deposition of the K/T Boundary Layer: Atmospheric Chaos and the Double Layer

NASA Astrophysics Data System (ADS)

The mechanics of impact ejecta deposition are not well understood, especially for planets with atmospheres where complex interactions occur between the ejected particles and the surrounding atmosphere. The K/T boundary ejecta layer is found world-wide and is thought to represent material from the vapor plume produced by the Chicxulub impact. We modeled a simplified Chicxulub scenario using KFIX-LPL, a two-phase fluid flow code which allows us to simulate deposition of the K/T boundary layer through the atmosphere. Air is modeled as a perfect gas and the spherules (condensed from the vapor plume), which are injected into the atmosphere, are modeled as a simple incompressible fluid with the properties of basaltic glass. The particles fall through the thin upper atmosphere, pushing the atmosphere downwards until the particles decelerate due to drag and increasing atmospheric pressure. The particles accumulate at ~50-km altitude and the deceleration heats the atmosphere around the particles (>700 K), causing expansion of the atmosphere and creating a sharp transition between hot dense atmosphere below the deceleration boundary and cool thin atmosphere above. Surface deposition of the global K/T boundary layer occurs on the scale of a few hours. Our models also shed light on the mysterious dual layer observed in several North American localities. Adding an initial injection of terrestrial ejecta (from the ejecta curtain) into our model atmosphere, as would occur at such intermediate distances from the impact, produces two distinct layers due to the alteration of the atmosphere's structure. Deposition of the lower terrestrial layer on the ground begins at ~80 minutes and that of the upper fireball layer begins at ~130 minutes. Our models show dramatic changes to the atmosphere, which have important environmental implications and provide the starting conditions and timeframes for chemical models examining the environmental consequences of Chicxulub. Our models, which include an accurate treatment of thermal radiation, also provide support for the delivery of significant thermal radiation to the Earth's surface.

Goldin, T. J.; Melosh, H. J.

2006-12-01

369

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

370

Magnetohydrodynamic Boundary Layer Flow Past a Flat Plate

NASA Astrophysics Data System (ADS)

The Carrier-Greenspan equations footnote H.P. Greenspan and G.F. Carrier, J. Fluid Mech. 6, 77 (1959) for the magnetohydrodynamic boundary layer flow past a flat plate are considered. Group theoretical arguments are used to deduce the asymptotic properties of the solution. A perturbative procedure due to Bender et. al. footnote C.M. Bender, K.M. Milton, S.S. Pensky and L.M. Simmons Jr., J. Math. Phys. 30, 1447 (1989) is used to find an approximate analytic solution of the Carrier-Greenspan equations which confirms the asymptotic properties given above.

Rollins, David; Shivamoggi, Bhimsen

1997-11-01

371

Boundary Layer Control by Means of Plasma Actuators

The development of controlled transition in a flat-plate boundary layer is investigated using Large Eddy Simulations (LES) with the dynamic Smagorinsky model. The analysis of flow control with the objective to optimize the effects of Tollmien-Schlichting waves on a flat plate by means of plasma actuators was studied. The plasma effect is modeled as a body force in the momentum equations. These equations are solved in a uniform grid using a 2nd-order finite difference scheme in time and space. The response of plasma actuators operating in different time-dependent conditions, produced by transient or periodic inputs at different frequencies, is also analyzed.

Quadros, R. [UFRGS/PPGMAp-TUD-Stroemungslehre und Aerodynamik, Technische Universitaet Darmstadt, Petersenstr. 30, 64287 Darmstadt (Germany); Bortoli, A. L. de [UFRGS/DMPA-Departamento de Matematica Pura e Aplicada, Bento Goncalves 9500, Agronomia-P.O. Box 15080, Porto Alegre-RS (Brazil); Tropea, C. [TUD/SLA-Stroemungslehre und Aerodynamik, Technische Universitaet Darmstadt, Petersenstr. 30, 64287 Darmstadt (Germany)

2007-09-06

372

Evaluation of Local Similarity Functions in the Convective Boundary Layer.

NASA Astrophysics Data System (ADS)

A parameterization method developed by Sorbjan is used to derive expressions for various statistical moments of vertical velocity, potential temperature, and humidity (or passive scalar concentration) in the convective boundary layer. The method is based on decomposing statistical moments into nonpenetrative and residual components, and their local (height-dependent) scaling. The resulting expressions are compared with atmospheric and laboratory data, and also with the results of large-eddy simulation models. An agreement between the similarity functions and the experimental data is obtained.

Sorbjan, Zbigniew

1991-12-01

373

Boundary layer flow of nanofluid over an exponentially stretching surface

The steady boundary layer flow of nanofluid over an exponential stretching surface is investigated analytically. The transport equations include the effects of Brownian motion parameter and thermophoresis parameter. The highly nonlinear coupled partial differential equations are simplified with the help of suitable similarity transformations. The reduced equations are then solved analytically with the help of homotopy analysis method (HAM). The convergence of HAM solutions are obtained by plotting h-curve. The expressions for velocity, temperature and nanoparticle volume fraction are computed for some values of the parameters namely, suction injection parameter ?, Lewis number Le, the Brownian motion parameter Nb and thermophoresis parameter Nt.

2012-01-01

374

Influence of radiation energy transfer on boundary layer temperature drops

NASA Astrophysics Data System (ADS)

The influence of thermal radiation on wall surface temperatures in a typical stationary conjugate heat transfer problem is studied. Free convective heat transfer, accounted for phenomenologically through the introduction of heat transfer coefficients, is supplemented by surface thermal radiation. The calculations clearly indicate that surface radiation can change significantly the surface temperatures which are, in general, reduced with increasing emissivities of the walls. In particular, in the case of small convection heat transfer coefficients, small thermal transmittances of the walls and high values of emissivities, the temperature difference across the temperature boundary layers adjacent to the walls could even be reversed.

Kranjc, T.; Peternelj, J.

2013-09-01

375

Relating statistical moments and entropy in the stable boundary layer

NASA Astrophysics Data System (ADS)

Fourth order moments and their connection with other statistics, including second order moments, skewness and entropy, in stable boundary layers are investigated with a large eddy simulation model (LES), wind tunnel experiment data (WT) and measurements on a meteorological tower in an urban area (MT). The relationship between skewness and kurtosis has been studied through two formulae, whose coefficients are determined for the three data sets. Shannon entropy is analysed as an index of the turbulent flow organization in order to further understand the possible reason for the failure of the QN hypothesis. To quantify this relationship between Shannon entropy and kurtosis, a power function is proposed.

Quan, L.; Ferrero, E.; Hu, F.

2012-01-01

376

Turbulence models for boundary layers on axisymmetric bodies

NASA Astrophysics Data System (ADS)

This report describes two turbulence models for boundary layers on axisymmetric bodies. The turbulence models are the algebraic eddy-viscosity formulation of Cebeci and Smith and the form of the two-equation transport approach suggested by Hanjalic and Launder. In both cases, the governing equations are solved in finite-difference form using Keller's box scheme and the computer program described by Bradshaw, Cebeci, and Whitelaw. The results computed by both models are evaluated for several test cases and discussed in the report. In addition, input and output instructions of a computer program based on the K-epsilon model are given together with a sample calculation.

Li, C.; Cebeci, T.

1985-07-01

377

LDA spectral measurements in a turbulent boundary layer

Fluid velocity measurements in a turbulent boundary layer are performed by means of laser-Doppler anemometry (LDA) in order to analyze, in the time and frequency domain, the trend of turbulent quantities very close to the wall; a large number of data are achieved at a high rate. Three velocity components are simultaneously measured using two independent LDA systems; correlation and spectral density functions together with integral and Taylor time scales at each distance from the wall are calculated. An evaluation of the amplitude of the anisotropic region in the frequency domain is also performed. 10 refs.

Cenedese, A.; Costantini, A.; Romano, G.P. (Karlsruhe, Universitaet, (Germany))

1992-05-01

378

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

379

Drag on intruders in granular beds: A boundary layer approach

NASA Astrophysics Data System (ADS)

We performed a parametric study of the drag on vertical intruders with uniform cross sections of different sizes and shapes, from which we developed a semiempirical model. Baffling techniques were used to isolate the contributions of the intruder's different subsurfaces, and we observed size effects and force focusing on edges. We propose a boundary layer approach, whereby the drag is the surface integral of an effective stress over a monolayer of particles contacting the intruder. The stress has a simple lithostatic dependence and is a function of the orientation relative to the intruder's direction of motion. This approach is experimentally verified and is consistent with the semiempirical model.

Goldsmith, Jonathan; Guo, Hong; Hunt, Shelby N.; Tao, Mingjiang; Koehler, Stephan

2013-09-01

380

Research applications of a boundary-layer wind profiler

A small UHF radar wind profiler was operated over a 40-day period during the summer of 1990 at a site on the windward coast of the island of Hawaii. It provided continuous measurements of winds up to the height of the trade-wind inversion, which varied in altitude from about 2 to 4 km during the course of the experiments. The inversion was readily discernible in the data as an elevated layer of high reflectivity, caused by the sharp gradient of refractive index at that level. With a wavelength of 33 cm, the profiler has about the same sensitivity to light rain as to moderately reflective clear air. The data have provided unexpected information on rain development, wave motions on the inversion, sustained vertical air motions at low levels, and interactions between convection and the inversion echo. This paper gives examples of some of the observations, indicating the wide range of applications of boundary-layer profilers.

Rogers, R.R.; Ethier, S.A. (McGill Univ., Montreal, Quebec (Canada)); Ecklund, W.L.; Carter, D.A.; Gage, K.S. (NOAA, Boulder, CO (United States))

1993-04-01

381

Effect of bulges on the stability of boundary layers

NASA Astrophysics Data System (ADS)

The instability of flows around hump and dip imperfections is investigated. The mean flow is calculated using interacting boundary layers, thereby accounting for viscous/inviscid interaction and separation bubbles. Then, the two-dimensional linear stability of this flow is analyzed, and the amplification factors are computed. Results are obtained for several height/width ratios and locations. The theoretical results have been used to correlate the experimental results of Walker and Greening (1942). The observed transition locations are found to correspond to amplification factors varying between 7.4 and 10.0, consistent with previous results for flat plates. The method accounts for both viscous and shear-layer instabilities. Separation is found to increase significantly the amplification factor.

Nayfeh, Ali H.; Ragab, Saad A.; Al-Maaitah, Ayman A.

1988-04-01

382

A novel boundary-confined method for high numerical aperture microlens array fabrication

NASA Astrophysics Data System (ADS)

In this paper, we present a technique to improve microlens array (MLA) uniformity after the thermal reflow process. It achieved high uniformity and high-numerical aperture (NA) simultaneously without sacrificing the fill factor. In order to improve the fill factor, a residual photoresist (PR) between the photoresist cylinders is used to make photoresist flow outward in standard thermal reflow processes. The PR cylinders, however, merge together easily due to an inexact reflow time and temperature distribution. This results in low uniformity and small lens height or low NA. We propose a boundary-confined method to pattern thin PR holes to prevent PR microlenses from merging together even after a long reflow time. Thick PR cylinders are patterned inside thin PR holes serving as boundaries. PR microlenses are formed after reflowing the thick photoresist cylinders. Both the uniformity and the height of a microlens can be well controlled. Besides, the fill factor is high due to the high resolution at the thin photoresist layer in photolithography. Our results show that the microlens is approximately a hemispherical profile. The gap between microlenses with a 48 µm diameter in hexagonal arrangement is 2 µm and the height of the microlens is 22 µm.

Hsieh, Hsin-Ta; Su, Guo-Dung John

2010-03-01

383

An arrangement of boundary conditions is described and demonstrated that facilitates the large-eddy simulation (LES) of inhomogeneous boundary layers such as internal boundary layers. In addition to the domain where the internal boundary layer develops, the method requires a section of domain over the upwind surface that is of the order of 10 boundary layer thicknesses and thus similar in

Shane D. Mayor; Philippe R. Spalart; Gregory J. Tripoli

2002-01-01

384

Long-Term Observations of the Dynamics of the Continental Planetary Boundary Layer

Time series of mixed layer depth, zi, and stable boundary layer height from March through October of 1998 are derived from a 915-MHz boundary layer profiling radar and CO 2 mixing ratio measured from a 447-m tower in northern Wisconsin. Mixed layer depths from the profiler are in good agreement with radiosonde measurements. Maximum zi occurs in May, coincident with

Chuixiang Yi; Kenneth J. Davis; Bradford W. Berger; Peter S. Bakwin

2001-01-01

385

Further Development and Testing of a Second-Order Bulk Boundary Layer Model.

National Technical Information Service (NTIS)

A one-layer bulk boundary layer model is developed. The model predicts the mixed layer values of the potential temperature, mixing ratio, and u- and v-momentum. The model also predicts the depth of the boundary layer and the vertically integrated turbulen...

R. D. Krasner

1993-01-01

386

NASA Astrophysics Data System (ADS)

We study a driven many particle system comprising two identical lanes of finite lengths. On one lane, particles hop diffusively with a bias in a specific direction. On the other lane, particles hop in a specific direction obeying the mutual exclusion rule. In addition, the two lanes are connected with each other through exchange of particles with certain rules. The system, at its two ends, is in contact with particle reservoirs which maintain specific particle densities at the two ends. In this paper, we study boundary induced phase transitions exhibited by this system and predict the phase diagram using the technique of fixed point based boundary layer analysis. An interesting manifestation of the interplay of two density variables associated with two lanes is found in the shock phase in which the particle density profile across the lane with unidirectional hopping shows a jump discontinuity (shock) from a low- to a high-density region. The density profile on the diffusion-lane never exhibits a shock. However, the shock in the other lane gives rise to a discontinuity in the slope of the diffusion-lane density profile. We show how an approximate solution for the slope can be obtained in the boundary layer analysis framework.

Saha, Bappa; Mukherji, Sutapa

2013-09-01

387

Large Eddy Simulations of boundary layer flow over fractal trees

NASA Astrophysics Data System (ADS)

A large-eddy simulation (LES) of flow over a canopy of fractal trees in the atmospheric boundary layer (ABL) is performed. The fractal trees provide complex boundary- turbulence interactions while maintaining tractable characteristics that can be systematically studied. LES are performed using the immersed boundary method following the implementation of Chester et al. (2007, J. Comp. Phys.). Simulations are performed for each fractal generation and the Reynods stresses and drag forces are computed. The LES results are used to analyze the impact of multiscale geometry on the Reynolds stress distribution and drag forces as a function of the generation number. This effort is also a feasiblity study for Renormalized Numerical Simulations (RNS) which is a methodology that allows for drag forces of unresovled generations to be computed by combining renormalizing techniques with the information from the resolved generations. Results are used to gain insight on scaling relationships between the drag forces and the generation number, and ultimately lead to better renormalization techniques for RNS.

Graham, Jason; Meneveau, Charles

2009-11-01

388

Stereoscopic PIV Measurements in an Urban-type Boundary Layer

NASA Astrophysics Data System (ADS)

An experimental investigation of the flow through urban-type boundary layers (4 rows of 3 cuboid Plexiglas blocks) in a modeled atmospheric boundary layer, will be presented. This study utilizes SPIV, hot-wire and oil-film interferometry measurements. Hot-wire measurements provide input on the incoming flow while direct measurements of the wall shear stress are realized using oil-film interferometry. Flow dispersion in urban areas is highly 3-D; therefore, a 2D traverse system carrying the entire SPIV system was designed which allows us to precisely position the measurement plane. All three velocity components are measured in 2-D planes throughout the model. More than 300 data planes in a 102mm by 50mm by 500mm domain corresponding to the middle street of the urban model are presented. The spacing between adjacent planes is chosen in order to resolve details close to the edges of the blocks. 3D streamlines, vorticity contours, isosurfaces of the second invariant of velocity gradient and Reynolds stresses will be presented and serve as a unique database for the numerical model being developed in parallel at IIT (see talk by Kandala, Rempfer, Wark and Fischer).

Monnier, Bruno; Neiswander, Brian; Wark, Candace; Rempfer, Dietmar

2007-11-01

389

Nearly Free Convection in Thermally Stratified Boundary Layers

NASA Astrophysics Data System (ADS)

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

Narasimha, Roddam

2005-11-01

390

Lidar wind shear measurements in the planetary boundary layer

NASA Astrophysics Data System (ADS)

Lidar measurements of wind velocity profiles by slant sounding in the PBL accompanied by ground-based measurements and a comparison with radio- and kytoon data have been carried out in the country during a period of 32 hours. We also carried out similar investigations studying the influence of various micro- and mesoscale phenomena on the wind velocity stratification. In the present work, parts of the preliminary results from two lidar campaigns (1988, 1990) in the region of Sofia are described. Some results of lidar measurements of wind velocity profiles in case of a stable PBL formation after the sunset are presented. The results are obtained during the BLEX'90 (Boundary Layer Experiment). This campaign aimed to perform an investigation of various phenomena and processes in the PBL over an urban area. Some observations of a wind shear appearance during a cold front invasion over the region of Sofia are also presented. These results are derived in the course of the International Boundary Layer Experiment ZOND '88 (teams from the Institute of Atmospheric Optics/Siberian Branch of the USSR Academy of Sciences and from the Institute of Electronics of the Bulgarian Academy of Sciences).

Kolev, Ivan N.; Parvanov, Orlin; Kaprielov, Boiko

1992-08-01

391

Optimal Disturbances and Receptivity of 3D Boundary Layers

NASA Astrophysics Data System (ADS)

We will present spatial optimal disturbances in a Falkner-Skan-Cooke boundary layer and illuminate how these can be used to determine the receptivity of crossflow vortices to freestream disturbances. Optimal disturbances, which are obtained by solving a parabolized set of equations, initially take the form of vortices tilted against the direction of the mean crossflow shear. Further downstream they evolve into bended streaks and finally into crossflow disturbances. A large potential for initial non-modal growth becomes apparent where both the lift-up effect and the Orr-mechanism are identified as responsible physical mechanisms. We inquire if non-modal growth is related to a receptivity mechanism for modal instabilities in 3D boundary layers. We therefore use continuous modes from the Orr-Sommerfeld/Squire spectrum as a model for freestream turbulence and project them onto initial optimal disturbances in order to obtain receptivity coefficients. A parametric study concerning optimal growth and receptivity will be presented as well as a comparison to existing DNS and experimental data.

Tempelmann, David; Hanifi, Ardeshir; Henningson, Dan

2009-11-01

392

Vortex Roll Breakup in Three-Dimensional Turbulent Boundary Layers

NASA Astrophysics Data System (ADS)

Large helical vortex rolls with axes in the general direction of the mean wind commonly appear in the unstably stratified atmospheric boundary layer. When a rapid shift in the mean wind direction occurs, the vertical transport of momentum and heat flux is sharply reduced compared to the equilibrium value. At long times, this non-equilibrium turbulent flow may develop back into a stable pattern of organized vortex rolls, now aligned with the new wind direction. This transition process is studied via direct numerical simulation of plane channel flow heated from below with impulsively started transverse pressure gradient (Ri = -Ra/PrRe^2 = -0.25, Ra = 10^7, and Pr = 0.71). The timescale for heat flux recovery is approximately the same for turning angles larger than 30 degrees. For higher turning angles, however, the Nusselt number will temporarily drop below one due to a significant reduction in vertical transport. Horizontal velocity and temperature spectra suggest that scale separation between large-scale, organized convective motions and turbulent eddies can prevent heat transfer reduction in transversely accelerated three-dimensional turbulent boundary layers.

Hamman, Curtis; Moin, Parviz

2011-11-01

393

Boundary Layer Height and Structure during the NATO LASIE Campaign

NASA Astrophysics Data System (ADS)

The NATO Ligurian Air-Sea Interaction Experiment (LASIE) took place in 2007, from 16 to 22 June, in the Mediterranean Sea. This filed campaign was organized under the auspices of the NATO Undersea Research Centre (NURC), located in La Spezia, Italy. The main scientific goal was to contribute to the evaluation and development of parameterizations of the oceanic and atmospheric boundary layers and their interactions. Extensive meteorological and oceanographic measurements were collected, on board the research vessels Leonardo, Planet, and Urania, and from the spar buoy ODAS Italia 1. In this study ceilometer (Vaisala CL31) and atmospheric radiosondes (Vaisala DigiCORA) measurements are used to assess the evolution of the marine atmospheric boundary layer (MABL) structure and height during the LASIE cruise. The ceilometer measured continuously the cloud height base, while the radiosondes, launched every 3 hours, recorded vertical profiles of wind speed, wind direction, potential temperature and relative humidity. Several methods available in the literature are used to determine the height of the MABL from observations. The results from these methods are compared with the MABL heights from the limited-area numeric weather prediction models WRF (Weather Research and Forecasting) and MM5 (Fifth-Generation Mesoscale Model).

Tomé, R.; Sempreviva, A. M.; Schiano, E.; Bozzano, R.; Miranda, P. M.; Pensieri, S.; Semedo, A.; Teixeira, J.

2009-09-01

394

Evolution of Reynolds stresses in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

Understanding Reynolds shear-stress events in a turbulent boundary layer is of crucial importance for modelling and controlling turbulent wall-flows. In this study, we examine the evolution in time and space of these shear-stress events by performing time-resolved PIV measurements in a stream-wise wall-normal plane of a turbulent boundary layer at Re? 2500. The conditions are similar to the experiment by Dennis & Nickels (J. Fluid Mech. 2011, vol. 673), who performed measurements at Re?=4700. Four high-speed cameras positioned next to each other, 4-5 m downstream of a glass rod trip, captured a region of flow spanning approximately 2? in stream-wise and 0.5? in wall-normal direction. This zoomed-in field-of-view enables high spatial, l^+ 20, and temporal resolution, ?t^+ 1 which will allow us to describe the evolution of shear-stress events in time and space. In the talk, detailed analyses including instantaneous tracking of Reynolds shear-stress events, quadrant decomposition and spectra of the stream-wise, wall-normal and Reynolds shear-stress fluctuations will be presented.

de Kat, Roeland; Gan, Lian; Dawson, James; Ganapathisubramani, Bharathram

2011-11-01

395

Boundary-layer turbulence characteristics during aeolian saltation

NASA Astrophysics Data System (ADS)

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

Li, Bailiang; McKenna Neuman, Cheryl

2012-06-01

396

Boundary-Layer Stress Instabilities in Neutral, Rotating Turbulent Flows

NASA Astrophysics Data System (ADS)

Boundary-layer instabilities are studied by analyzing the results of laboratory simulations of wall turbulence in a shear-driven rotating flow. The experiments were carried out in the Turin University Laboratory rotating water tank, where a circular flow was generated by either increasing (spin-up) or decreasing (spin-down) the rotation speed of the platform. The flow was measured using a Particle Image Velocimetry technique and the developed turbulence analyzed. Two cases were accounted for, in the former the measurements were performed over a smooth surface (bottom of the tank), while in the latter a rough-to-smooth transition was considered. The turbulent boundary layer developed inside the tank is analyzed by means of vertical profiles of mean and turbulent quantities and on the basis of drag coefficients. Then turbulent structures developed in the different cases are shown and discussed in terms of the vorticity fields. Finally, an analysis based on the concept of swirling strength was carried out to select among the vortex extremes those associated with a coherent structure.

Ferrero, Enrico; Mortarini, Luca; Manfrin, Massimigliano; Longhetto, Arnaldo; Genovese, Rita; Forza, Renato

2009-03-01

397

Identification of Lagrangian Coherent Structures in a Turbulent Boundary Layer

NASA Astrophysics Data System (ADS)

In this study, we identify Lagrangian coherent structures (LCS) in a flat plate turbulent boundary layer at Re? 0f 19:100. To detect the LCS, we compute direct Lyapunov exponents (DLE) (Haller, G., Physica D, vol 149, pp 248-277, 2001). Specifically we use the velocity field obtained from stereo PIV measurements to compute trajectories, x (t,t0,x 0), from initial positions, x0, at time t0. For fixed integration times, | t-t0|, we numerically differentiate the flow map, given by Ft0^t(x0) = x(t, t0, x0), and then compute the deformation gradient tensor field ?^tt0(x0) = [ ?Ft0^t(x0) ]^T [ ?Ft0^t(x0) ]. The DLE field is then found as DLEt0^t(x0) = ( ?max ( ?t0^t(x0) ) )/ (2| t-t0| ). Two dimensional gradient climbing is then used to find points on the locally maximizing, LCS surfaces of the field, DLEt0^t(x0). To determine whether these surfaces truly repel (attract) near by fluid particles, the hyperbolicity criterion is applied (Mathur et al., Phys. Rev. Lett., vol 98, pp 144502, 2007). In particular we compute normal strain rates,

Wilson, Zachary; Tutkun, Murat; Bayoan Cal, Raul

2010-11-01

398

High frequency boundary layer profiling with reusable radiosondes

NASA Astrophysics Data System (ADS)

A new system for high frequency boundary layer profiling based upon radiosondes and free balloons was tested during the field phases of the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST 2011, Lannemezan, France) and of the Hydrological cycle in the Mediterranean Experiment (HyMeX, 2012). The system consists of a conventional Vaisala receiver and a GPS radiosonde (pressure, wind, humidity and temperature) that is tied to a couple of inflated balloons. The principle of the sounding system is to permit the first balloon to detach from the rawinsonde at a predetermined altitude, allowing the rawinsonde to slowly descend with the second balloon to perform a second, new sounding. The instrumentation is then eventually recovered. The expecting landing area is anticipated before the flight by estimating the trajectory of the probe from a forecasted wind profile and by specifying both the balloon release altitude and the mean ascent and descent rates of the system. The real landing point is determined by the last transmission of the radiosonde GPS and the visual landmark provided by the second balloon. About 70 soundings were performed with a recovery rate of more than 80%. Recovered radiosondes were generally reused several times, often immediately after recovery, which definitely demonstrates the high potential of this system.

Legain, D.; Bousquet, O.; Douffet, T.; Tzanos, D.; Moulin, E.; Barrie, J.; Renard, J.-B.

2013-04-01

399

Low Cost Geothermal Separators BLISS Boundary Layer Inline Separator Scrubber

A new compact, low cost, and high performance separator is being developed to help reduce the installed and O and M cost of geothermal power generation. This device has been given the acronym ''BLISS'' that stands for ''Boundary Layer Inline Separator Scrubber''. The device is the first of a series of separators, and in the case of injectates, scrubbers to address the cost-reduction needs of the industry. The BLISS is a multi-positional centrifugal separator primarily designed to be simply installed between pipe supports, in a horizontal position. This lower profile reduces the height safety concern for workers, and significantly reduces the total installation cost. The vessel can demand as little as one-quarter (25%) the amount of steel traditionally required to fabricate many large vertical separators. The compact nature and high separating efficiency of this device are directly attributable to a high centrifugal force coupled with boundary layer control. The pseudo isokinetic flow design imparts a self-cleaning and scale resistant feature. This polishing separator is designed to remove moderate amounts of liquid and entrained solids.

Jung, Douglas; Wai, King

2000-05-26

400

Boundary layer structure over areas of heterogeneous heat fluxes

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

401

Boundary layer structure over areas of heterogeneous heat fluxes

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 within 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. Moveover, 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 discuses 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

402

Boundary layer structure over areas of heterogeneous heat fluxes

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

403

Boundary layer structure over areas of heterogeneous heat fluxes

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 within 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. Moveover, 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 discuses 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.

1993-04-01

404

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

P. E. Roach

1987-01-01

405

Stability of boundary layers within high-speed viscous flows

NASA Astrophysics Data System (ADS)

A numerical study was undertaken to predict the stability of a variety of high-speed boundary-layer flows. Using a finite-volume code, the Navier-Stokes equations were solved for a series of flows around spherically blunted cones. These solutions were used to perform linear-stability analyses for second-mode disturbances. Two investigations were undertaken using an ideal-gas model: the Stetson experiment and a recent experiment conducted at the Institute of Theoretical and Applied Mechanics in Russia. Comparisons were made with both basic-state and disturbance state quantities. For both cases, linear-growth regions have been identified. For the Stetson case, using an experimentally determined wall-temperature distribution for the basic-state appeared to give better agreement with the experimentally measured growth than does the classical adiabatic-wall boundary condition. For the Russian experiment, initial comparisons were made in order to continue a careful collaboration. A third investigation was made which used a chemical non-equilibrium model, considering a Mach 13.5 flow in upper-atmospheric conditions. The goal of this investigation was to evaluate the sensitivity of second-mode growth predictions to changes (within accepted uncertainties) in thermodynamic, reaction-rate; and transport models. The magnitude of change in the stability results correlated strongly with changes in the basic-state thermal boundary-layer profile, consistent with second-mode theory. The largest change in the stability behavior was observed for the case where the transport model was changed. For high-speed flows, the development of computational techniques is in some ways ahead of the experimental community's ability to verify the results. As these techniques are applied to flows in thermochemical non-equilibrium, the fidelity of the constitutive relationships should be considered.

Lyttle, Ian John

2003-10-01

406

An experimental investigation of the three-dimensional flow through an urban-type array (four rows of three cuboid Plexiglas\\u000a blocks) in a laboratory modelled neutrally stratified atmospheric boundary layer is presented. We concentrate on the effect\\u000a of the streamwise spacing between adjacent rows defining two different flow regimes (wake interference and skimming flow)\\u000a as well as the effect of the incident angle

Bruno Monnier; Brian Neiswander; Candace Wark

2010-01-01

407

Directed assembly of one-dimensional magic cluster arrays by domain boundaries

NASA Astrophysics Data System (ADS)

The most commonly occurring boundary separating Ge-5×5 domains grown on Si(111) is the B[2¯2] domain boundary. We demonstrate that this boundary can be used to template one-dimensional arrays of identical In and Ga magic clusters with a lattice constant of 3.3 nm. This is larger than the lattice constant of the two-dimensional magic cluster arrays templated by the Si(111)-7×7 surface reconstruction (2.7 nm), although the magic clusters have the same structure. We also demonstrate that a necessary condition for cluster growth at the domain boundary is the presence of faulted dimer-adatom-stacking-fault-7×7 half unit cells. The relatively unexplored possibility of exploiting the unique structural and electronic properties of domain boundaries in nanostructured materials is discussed.

Mark, Andrew G.; McLean, A. B.

2012-05-01

408

Pathways of soil moisture controls on boundary layer dynamics

NASA Astrophysics Data System (ADS)

Soil moisture controls on precipitation are now receiving significant attention in climate systems because the memory of their variability is much slower than the memory of the fast atmospheric processes. We propose a new model that integrates soil water dynamics, plant hydraulics and stomatal responses to water availability to estimate root water uptake and available energy partitioning, as well as feedbacks to boundary layer dynamics (in terms of water vapor and heat input to the atmospheric system). Using a simplified homogenization technique, the model solves the intrinsically 3-D soil water movement equations by two 1-D coupled Richards' equations. The first resolves the radial water flow from bulk soil to soil-root interface to estimate root uptake (assuming the vertical gradients in moisture persist during the rapid lateral flow), and then it solves vertical water movement through the soil following the radial moisture adjustments. The coupling between these two equations is obtained by area averaging the soil moisture in the radial domain (i.e. homogenization) to calculate the vertical fluxes. For each vertical layer, the domain is discretized in axi-symmetrical grid with constant soil properties. This is deemed to be appropriate given the fact that the root uptake occurs on much shorter time scales closely following diurnal cycles, while the vertical water movement is more relevant to the inter-storm time scale. We show that this approach was able to explicitly simulate known features of root uptake such as diurnal hysteresis of canopy conductance, water redistribution by roots (hydraulic lift) and downward shift of root uptake during drying cycles. The model is then coupled with an atmospheric boundary layer (ABL) growth model thereby permitting us to explore low-dimensional elements of the interaction between soil moisture and ABL states commensurate with the lifting condensation level.

Siqueira, M.; Katul, G.; Porporato, A.

2007-12-01

409

NASA Astrophysics Data System (ADS)

An experimental investigation focused on the study of the physics of unsteady turbulent boundary layer separation under conditions relevant to the dynamic stall process that occurs in helicopter rotors is presented. A flat boundary layer development plate allows for the growth of a turbulent boundary layer of thickness sufficient for high spatial resolution measurements. Downstream of the flat plate, a convex ramp section imposes a streamwise adverse pressure gradient that gives rise to boundary layer separation. In order to impose an unsteady pressure gradient, an airfoil equipped with leading edge plasma flow control is located above the ramp section. Plasma flow control is used to alternately attach and separate the airfoil flow which gives rise to unsteady turbulent boundary layer separation on the convex ramp. Measurements of the resulting unsteady turbulent boundary layer separation via phase-locked two-component PIV, unsteady surface pressure measurements, and high speed digital imaging capture and quantify the dynamics the separation process at the wall and throughout the unsteady boundary layer. Two-component LDA measurements are used to characterize the motions of ejection and sweep events within the unsteady boundary layer using a quadrant splitting technique. Large amplitude quadrant 4 sweep events are the most dynamically significant in the near wall region during the unsteady separation process. The adverse pressure gradient boundary layer profiles throughout the unsteady cycle collapse remarkably well when scaled with embedded shear layer parameters. The implications of the experimental results for the development of flow control strategies for unsteady boundary layer separation are discussed.

Schatzman, David M.

410

A general approach for the prediction of localized instability generation in boundary layer flows

NASA Astrophysics Data System (ADS)

The present approach to the prediction of instability generation that is due to the interaction of freestream disturbances with regions of subscale variations in surface boundary conditions can account for the finite Reynolds number effects, while furnishing a framework for the study of receptivity in compressible flow and in 3D boundary layers. The approach is illustrated for the case of Tollmien-Schlichting wave generation in a Blasius boundary layer, due to the interaction of a freestream acoustic wave with a localized wall inhomogeneity. Results are presented for the generation of viscous and inviscid instabilities in adverse pressure-gradient boundary layers, supersonic boundary layer instabilities, and cross-flow vortex instabilities.

Choudhari, Meelan; Ng, Lian; Streett, Craig L.

411

Influence of an Axial Magnetic Field on the Steady Linear Ekman Boundary Layer

A hydromagnetic version of the Ekman boundary layer is developed in a simple form in order to study how the geophysically important Ekman suction velocity is affected by magnetic fields. The problem treated consists of a viscous, incompressible, conducting fluid in the presence of an infinite, flat, insulating boundary which rotates at speed ?0. Outside the boundary layer, the fluid

Peter A. Gilman; Edward R. Benton

1968-01-01

412

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

National Technical Information Service (NTIS)

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

L. W. Reidy G. W. Anderson

1988-01-01

413

Multi-scale model analysis of boundary layer ozone over East Asia

This study employs the regional Community Multiscale Air Quality (CMAQ) model to examine seasonal and diurnal variations of boundary layer ozone (O3) over East Asia. We evaluate the response of model simulations of boundary layer O3 to the choice of chemical mechanisms, meteorological fields, boundary conditions, and model resolutions. Data obtained from surface stations, aircraft measurements, and satellites are used

M. Lin; T. Holloway; T. Oki; D. G. Streets; A. Richter

2009-01-01

414

A Lagrangian Study of Southeast Pacific Boundary Layer Clouds

NASA Astrophysics Data System (ADS)

Low clouds lie at the heart of climate feedback uncertainties. The representation of clouds in global climate models relies on parameterization of many sub-grid scale processes that are crucial to understanding cloud responses to climate; low clouds in particular exist as a result of tightly coupled microphysical, mesoscale, and synoptic mechanisms. The influence of anthropogenic aerosols on cloud properties could have important ramifications for our understanding of how clouds respond to a changing climate. The VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS REx) sampled the persistent stratocumulus cloud deck located off the coast of Peru and Chile in the southeastern Pacific ocean. Several cloud features found in the stratocumulus deck during VOCALS exhibit signs of interesting aerosol-cloud interactions, including pockets of open cells (POCs). POCs are regions of open-cellular convection surrounded by closed cell stratocumulus, exhibiting not only a marked transition in mesoscale organization and cloud morphology, but also sharp microphysical gradients (especially in droplet concentration) across the boundary between open-cellular and closed cellular convection. In addition, precipitation is often higher at the POC boundaries, hinting at the importance of precipitation in driving their formation. In order to evaluate the microphysical characteristics of POCs prior cloud breakup, we use Lagrangian trajectories coupled with geostationary satellite imagery and cloud retrievals, as well as observational data from VOCALS REx and model data. In three of our case studies, we found regions of anomalously low droplet concentration 18-24 hours prior to POC formation (coupled with liquid water path similar to or higher than surrounding cloud), supporting a precipitation driven mechanism for POC formation. Another group of features with interesting aerosol-cloud interactions observed during VOCALS were mesoscale hook-like features of high droplet concentration which extend far offshore into regions of normally very clean cloud. We use Lagrangian trajectories to investigate the source of the high droplet concentrations of the mesoscale "hooks", and evaluate whether boundary layer transport of coastal pollutants alone can account for their extent. We find that boundary layer trajectories past 85 W do not pass sufficiently close to the coastline to explain high aerosol concentrations offshore.

Painter, Gallia

415

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

416

Numerical investigation of lutocline in oscillatory boundary layer

NASA Astrophysics Data System (ADS)

Formation of lutocline and fluid mud, driven by tidal currents has been an interest of research due to considerable amount of transport beneath the lutocline. Theoretical and experimental studies in idealized condition showed that the formation of the lutocline can be explained through a balance between turbulence-induced particle suspension and the suppression of turbulence due to particle induced density stratification (Huppert, Turner & Hallworth, 1995. J. Fluid Mech., 289, 263-293). Recent field observations suggest the formation of lutocline under waves is also critical to the major transport of fluid mud, especially in conjunction with downslope gravitational effects (Traykovski, Wiberg & Geyer, 2007, Cont. Shelf Res., 27, 375-399). Evidence of wave-induced fluid mud and lutocline wave is also related to energy dissipation of free-surface waves. From modeling perspective, it is hard to accurately model turbulence modulation due to sediment, especially for intense turbulence damping that causes laminar wave boundary layer. A 3D turbulence resolving numerical simulation tool Ozdemir et al. (Ozdemir, Hsu & Balachandar, J. Fluid Mech., in press) vary the available sediment amount in the oscillatory boundary layer and observe four different flow regimes ranging from well-mixed sediment, formation of lutocline and laminarization. With additional simulations for different settling velocities, sediment concentration and Reynolds number, to be presented, it is clear that the dynamics of lutocline is related to the transition of these regimes. Our preliminary analysis shows that with decrease in settling velocity, the lutocline location migrates upward with a milder concentration gradient showing the transition from well-mixed condition to the formation of lutocline is gradual with no clear critical threshold. However, the transition from lutocline to laminarized flow is quite abrupt except intermittent wave-like bursts may occur during transition. Hence, a critical threshold for laminarization (saturation condition) can be proposed for fine sediment transport in wave boundary layer. Concentration iso-contours and ensemble-averaged concentration profiles for three simulations: i) passive ii) 10 g/l volume averaged concentration iii) 50 g/l of volume-averaged concentration. Top panel shows the sample phases. The second panel is the iso-contour plot at C = 13 g/l for the second simulation. Third panel shows the iso-contour of 500 g/l for the third simulation.

Ozdemir, C. E.; Hsu, T.; Balachandar, S.

2010-12-01

417

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

2013-03-01

418

Lagrangian stochastic modeling based on the Langevin equation has been shown to be useful for simulating vertical dispersion of trace material in the convective boundary layer or CBL. This modeling approach can account for the effects of the long velocity correlation time scales, skewed vertical velocity distributions, and vertically inhomogeneous turbulent properties found in the CBL. It has been recognized that Langevin equation models assuming skewed but homogenous velocity statistics can capture the important aspects of diffusion from sources in the CBL, especially elevated sources. We compare three reflection boundary conditions using two different Langevin-equation-based numerical models for vertical dispersion in skewed, homogeneous turbulence. One model, described by Ermak and Nasstrom (1995) is based on a Langevin equation with a skewed random force and a linear deterministic force. The second model, used by Hurley and Physick (1993) is based on a Langevin equation with a Gaussian random force and a non-linear deterministic force. The reflection boundary conditions are all based on the approach described by Thompson and Montgomery (1994).

Nasstrom, J.S.; Ermak, D.L.

1997-04-01

419

Thermal dissociation of anharmonic oscillators in a boundary layer

A study was made to evaluate how the vibration anharmonicity, when taken into account in the expression for the macroscopic dissociation rate, modifies the correction for the motion of the gas. It was assumed that diatomic molecules, Morse anharmonic oscillators, constitute a small impurity in a monoatomic inert gas. It is shown that in the case of harmonic oscillators the motion of the gas in the nonisothermal boundary layer leads to the repopulation of the higher oscillatory levels of the anharmonic oscillators. The diffusion approximation obtained led to a solution which gives an effect of the motion on the reaction rate weaker than that given by the numerical solution. A nitrogen-argon mixture was used to illustrate the results.

Makashev, N.K.; Strakhov, L.B.

1988-03-01

420

On the origin of the high-latitude boundary layer

NASA Astrophysics Data System (ADS)

This paper presents a statistical study of the high-latitude boundary layer (HLBL) performed on 53 Interball-1 magnetopause crossings. In the study we verify if antiparallel merging is the main source of HLBL formation when the IMF is nearly horizontal. To provide such a study we designed a new coordinate system which allowed us to analyze HLBL under varied interplanetary conditions. This coordinate system floats over the dayside magnetopause following the changes in the instant location of the reconnection site. Despite very different interplanetary conditions, the observed HLBL plasma regimes manifest systematic behavior in the "reconnection" frame of reference. We explain the observed pattern in terms of sporadic patchy reconnection in the high magnetic shear region of the magnetopause.

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

421

Structure of three-dimensional turbulent boundary layers

NASA Astrophysics Data System (ADS)

The changes that occur in the Reynolds-stress-producing motion when a cross-stream pressure gradient is applied to an initially two-dimensional turbulent flow are discussed. Two examples are used: (1) a temporal simulation of a channel flow with crossflow applied by a spanwise pressure gradient for t is greater than 0; and (2) a spatial simulation of the boundary layer on an infinite swept wing. Evidence examined to date suggests that the structural changes in the two cases are similar, but the mechanisms may be significantly different, even if effects peculiar to the viscous wall region are ignored. The results from (2) are provisional, based on too short a time series for accurate statistical averages to be obtained. We treat turbulence 'statistics' (solutions of the Navier-Stokes equations in three space dimensions and time) in the same way as experiments: both have limitations of accuracy but both are acceptable representations of real fluid flows.

Bradshaw, P.; Sendstad, O.

1990-12-01

422

Growth Of The Summer Daytime Convective Boundary Layer At Anand

NASA Astrophysics Data System (ADS)

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

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

423

Unsteady turbulent boundary layers in adverse pressure gradients

NASA Astrophysics Data System (ADS)

A number of characteristics of an unsteady turbulent boundary layer have been measured on the after part of the upper surface of an 0012 airfoil (X/c = 0.69 and 0.94). The data is taken at chord Reynolds number of 700,000 and over a range of reduced frequencies (based upon the semichord) of 0.5 to 6.4. Mean and unsteady velocity profiles, as well as Reynolds stress profiles, are presented, as is the nonlinear coupling from the unsteady motion into the steady motion. Data is presented and discussed which shows that for this experiment the periodic unsteady turbulent velocity profile tends toward a universal shape.

Covert, E. E.; Lorber, P. F.

1982-06-01

424

Analysis of the photodiode boundary layer transition indicator

NASA Astrophysics Data System (ADS)

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.

425

A helicopter observation platform for atmospheric boundary layer studies

NASA Astrophysics Data System (ADS)

Spatial variability of the Earth's surface has a considerable impact on the atmosphere at all scales and understanding the mechanisms involved in land-atmosphere interactions is hindered by the scarcity of appropriate observations. A measurement gap exists between traditional point sensors and large aircraft and satellite-based sensors in collecting measurements of atmospheric quantities. Point sensors are capable of making long time series of measurements, but cannot make measurements of spatial variability. Large aircraft and satellites make measurements over large spatial areas, but with poor spatial and temporal resolution. A helicopter-based platform can make measurements on scales relevant for towers, especially close to the Earth's surface, and can extend these measurements to account for spatial variability. Thus, the Duke University Helicopter Observation Platform (HOP) is designed to fill the existing measurement gap. Because measurements must be made in such a way that they are as uncontaminated by the platform itself as much as is possible, it is necessary to quantify the aerodynamic envelope of the HOP. The results of an analytical analysis of the location of the main rotor wake at various airspeeds are shown. Similarly, the results of a numerical analysis using the commercial Computational Fluid Dynamics software Fluent are shown. The optimal flight speed for the sampling of turbulent fluxes is found to be around 30 m/s. At this airspeed, the sensors located in front of the nose of the HOP are in advance of the wake generated by the main rotor. This airspeed is also low enough that the region of high pressure due to the stagnation point on the nose of the HOP does not protrude far enough forward to affect the sensors. Measurements of differential pressures, variables and turbulent fluxes made while flying the HOP at different airspeeds support these results. No systematic effects of the platform are seen at airspeeds above about 10 m/s. Processing of HOP data collected using the current set of sensors is discussed, including the novel use of the Empirical Mode Decomposition (EMD) to detrend and filter the data. The EMD separates the data into a finite number of Intrinsic Mode Functions (IMFs), each of which is unique and orthogonal. The basis is determined by the data itself, so that it need not be known a priori, and it is adaptive. The EMD is shown to be an ideal tool for the filtering and detrending of the HOP data gathered during the Cloud and Land Surface Interaction Campaign (CLASIC). The ability of the HOP to accurately measure atmospheric profiles of atmospheric variables is demonstrated. During experiments conducted in the marine boundary layer (MBL) and the convective boundary layer (CBL), HOP profiles of potential temperature are evaluated using an elastic backscatter lidar. The HOP and the lidar agree on the height of the boundary layer in both cases, and the HOP effectively locates other atmospheric structures. Atmospheric sensible and latent heat fluxes, turbulence kinetic energy (TKE) and horizontal momentum fluxes are also measured, and the resulting information is used to provide context to tower-based data collected concurrently. A brief comparison made over homogeneous ocean conditions yields good results. A more exhaustive evaluation is made using short HOP flights performed above an orchard during the Canopy Horizontal Turbulence Study (CHATS). Randomly selected one-minute sections of tower data are used to calculate fluxes to which the HOP fluxes can be more directly compared, with good results. Profiles of atmospheric fluxes are used to provide context to tower-based measurements. In conclusion, the research conducted here demonstrates unambiguously that the HOP is a unique platform that fills an important gap in observation facilities for the atmospheric boundary layer. It is now available to the scientific community for performing research, which is likely to help bridging existing knowledge gaps in various aspects of Earth surface (continental and maritime) -- atmosphere interactio

Holder, Heidi Eichinger

426

Zero pressure gradient boundary layer at extreme Reynolds numbers

NASA Astrophysics Data System (ADS)

Experiments were conducted in a zero pressure gradient flat plate boundary layer using the Princeton/ONR High Reynolds number Test Facility (HRTF). The HRTF uses highly compressed air, up to 220 atmospheres, to produce Reynolds numbers up to Re?=225,000. This corresponds to a &+circ;=65,000 which is one of the highest Reynolds numbers ever measured in a laboratory. When using pressure to achieve high Reynolds numbers the size of the measurement probes become critical, thus the need for very small sensors is acute. The streamwise component of velocity was investigated using a nanoscale thermal anemometer (NSTAP) as well as a 200?m pitot tube. The NSTAP has a spatial resolution as well as a temporal resolution one order of magnitude better than conventional measurement techniques. The data was compared to recent data from a high Reynolds number turbulent pipe flow and it was shown that the two flows are more similar than previous data suggests.

Hultmark, Marcus; Vallikivi, Margit; Smits, Alexander

2011-11-01

427

An algorithm for the automatic control of boundary layer flow

NASA Astrophysics Data System (ADS)

Interest has recently been renewed in the use of distributed suction for the production of laminar flow over substantial areas of the surface of aircraft wings and engine nacelles. Suction may be most efficiently applied by using a number of independently controllable panels through which fluid is withdrawn. It becomes necessary to determine the distribution of suction flowrates that results in a given streamwise location of boundary layer transition with minimum power consumed in providing suction. This problem is formulated here in terms of nonlinearly constrained optimization. An algorithm is presented which has proved successful in both experimental and numerical studies in determining the optimal suction distribution. Of particular concern in this work is the stability of the basic algorithm and the limits to its rate of convergence to the optimal solution.

Nelson, P. A.; Rioual, J.-L.

1994-05-01

428

Small particle transport across turbulent nonisothermal boundary layers

NASA Astrophysics Data System (ADS)

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.

1982-10-01

429

Transition of MHD boundary layer flow past a stretching sheet

NASA Astrophysics Data System (ADS)

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

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

2010-02-01

430

Boundary layer receptivity to unsteady free-stream pressure gradients

NASA Astrophysics Data System (ADS)

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.

431

Asymptotically optimal unsaturated lattice cubature formulae with bounded boundary layer

NASA Astrophysics Data System (ADS)

This paper describes a new algorithm for constructing lattice cubature formulae with bounded boundary layer \\displaystyle \\int_{\\Omega}f(x)\\, dx\\approx h^n\\sum_{\\substack{k\\in{Z}^n \\\\ \\rho(hk,\\Omega)\\le Ch^{\\gamma}}} c_k(h) f(hk), where \\displaystyle \\gamma<\\frac12, \\qquad c_k(h)=1, if \\displaystyle \\rho(hk, R^n\\setminus\\Omega)\\ge Ch^{\\gamma}. These formulae are unsaturated (in the sense of Babenko) both with respect to the order and in regard to the property of asymptotic optimality on W_2^m-spaces, m\\in(n/2,\\infty). Most of the results obtained apply also to W_2^\\mu({R}^n)-spaces with a hypoelliptic multiplier of smoothness \\mu. Bibliography: 6 titles.

Ramazanov, M. D.

2013-07-01

432

Formation of HD Molecules in the Boundary Layer of TEXTOR

NASA Astrophysics Data System (ADS)

In this paper, we describe the formation of HD molecules in front of a graphite surface in the boundary layer of TEXTOR. The surface release of HD molecules was observed under different experimental conditions: in neutral beam experiments, in surface saturation experiments and in gas injection experiments. Molecular line emission of HD could be measured for the first time in a high temperature boundary plasma by means of Fulcher-?-band spectroscopy. The Q-branches of the first four diagonal bands were identified and analysed with respect to their rotational and vibrational population. No differences between isotopomers could be ascertained in the vibrational ground state population. Calibration spectra, which describe the isotope differences, as well as first experimental results with regard to the isotope exchange are shown. The main result is that HD molecules have to be included into the determination of the particle flux of recycled hydrogen isotopomers. By means of a model constructed on the chemical law of mass action, the release of the isotopomer H2, HD, and D2 can be described in the form of concentration ratios.

Brezinsek, S.; Greenland, P. T.; Mertens, Ph.; Pospieszczyk, A.; Samm, U.; Schweer, B.; Sergienko, G.

433

New particle formation in the marine boundary layer

Aerosol measurements were made in the marine boundary layer along the coast of Washington State during the Pacific Stratus Sulfur Investigation. On April 22 the particle concentration increased to levels much higher than usual for the clean marine boundary layer. The total particulate number concentration greater than 3 nm diameter increased rapidly from about 250 cm[sup [minus]3] to 3,200 cm[sup [minus]3], remained near that level for 7 hours, and then decreased over the next 2 hours to less than 400 cm[sup [minus]3]. The change could not be attributed to either local or distant contamination. Immediately before the increase particulate surface area concentration dropped from 25 [mu]m[sup 2] cm[sup [minus]3] to less than 5 [mu]m[sup 2] cm[sup [minus]3]. The SO[sub 2] concentration increased from about 20 pptv to 40-60 pptv just before the increase in particle concentration. While these measurements cannot distinguish between changes in number concentration caused by particle nucleation versus advection or vertical mixing, clearly there was recent or continuing particle production on a mesoscale in the air mass. Related aircraft measurements and model results support the hypothesis of new particle formation. These data provide evidence that at times high concentrations of new, ultrafine particles are formed at low SO[sub 2] concentrations under mareine conditions. This homogeneous nucleation, as opposed to heterogeneous condensation on existing particles, is strongly and inversely dependent on the concentration of existing particles. 19 refs., 4 figs.

Covert, D.S. (Univ. of Washington, Seattle (United States)); Kapustin, V.N. (Inst. of Atmospheric Physics, Moscow (Russian Federation)); Quinn, P.K.; Bates, T.S. (NOAA Pacific Marine Environmental Lab., Seattle, WA (United States))

1992-12-20

434

Nighttime Chemistry in the Polluted Boundary Layer (Invited)

NASA Astrophysics Data System (ADS)

Chemistry in the urban nocturnal boundary layer (NBL) has received surprisingly little attention in the past. Surface observations often see low ozone and high NO levels, which lead to low nocturnal radical levels and consequently slow chemistry near the ground. Above the surface, however, ozone and radical levels, for example of NO3, are considerably higher, and more efficient chemical pathways for the removal of gaseous pollutants such as nitrogen oxides, ozone, and hydrocarbons, are active. The influence of nocturnal chemistry on aerosol composition is also largest aloft. These processes are poorly understood due to a lack of observations in the altitude range from 20 - 500m. The strong influence of vertical mixing and transport on the composition of the NBL poses an additional challenge, requiring the measurement of vertical concentration profiles and the use of chemical transport models for their interpretation. In addition, heterogeneous processes on the ground and on aerosol surfaces play an important role in the nocturnal atmosphere. In this presentation we will review our current understanding of nocturnal chemistry in the lowest 300m of the polluted atmosphere, with a focus on nitrogen compounds. A number of field experiments in recent years have given insight into the vertical distribution of some of the most important nocturnal trace gases in urban areas, such as ozone, NO2, NO3, N2O5, and HONO. In particular, two 6-week long experiments in Houston, TX, in 2006 and 2009, have shown the strong and persistent impact of vertical mixing on the distribution of all trace gases, as well as the chemistry in the lowest 300m of the atmosphere. These observations were accompanied by detailed meteorological observations and in-situ measurements of chemical species at 70m above the ground. The observations in Houston were interpreted with a 1D chemical transport model that allows quantification of chemistry and transport at night. Our results identify gaps in our understanding of the polluted nocturnal urban boundary layer will be discussed.

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

2009-12-01

435

Drag reduction by microbubbles in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

An experimental characterization of the turbulent boundary layer over a flat plate in the presence of small amounts of microbubbles is performed. The average diameter of the injected bubbles is comparable with the local Kolmogorov lengthscale, and the bulk void fraction C is approximately 0.1%. The velocity field of the liquid phase, as well as the bubble characteristics, is acquired by optical techniques. Even at the small void fraction typical of this investigation, the interaction between microbubbles and turbulence leads to significant modifications of the underlying flow field. In accordance with previous investigations, the main global effect consists in a reduction of the magnitude of the viscous drag. This has been checked with preliminary tests conducted in a towing tank, as well as by inferring the wall stress from the boundary layer velocity profile measured in a laboratory facility at the same conditions. Here, the local wall stress is found to drop by approximately 25%. An analysis of the turbulent statistics shows that the reduction in drag is accompanied by a substantial reduction of the momentum flux throughout most of the inner region and by a decrease of the characteristic dimension of the turbulent scales involved in the production of kinetic energy. The distribution of turbulent kinetic energy among different scales is found to be substantially altered in the two flows. In particular, an enhancement of the energy content at smaller scales is observed in the bubbly case. These effects are discussed in relation with the decrease of the coherence of near-wall flow structures induced by the bubble forcing.

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

2010-11-01

436

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

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

1996-01-01

437

Coherent Structure of a Turbulent Boundary Layer in a Convected Reference Frame.

National Technical Information Service (NTIS)

This is an interim annual report for an ongoing research program studying coherent structure in turbulent boundary layers. The design and development of a water channel facility for use in flow visualization studies of coherent turbulent boundary structur...

C. R. Smith S. L. Huston J. J. Brown

1977-01-01

438

National Technical Information Service (NTIS)

The feasibility of using satellite derived thermal data to generate realistic synoptic scale winds within the planetary boundary layer (PBL) is examined. Diagnostic modified Ekman wind equations from the Air Force Global Weather Central (AFGWC) Boundary L...

C. L. Belt H. E. Fuelberg

1984-01-01

439

Influence of Solid Surface on Relaxation, Crosslinking Processes in Polymer Boundary Layers.

National Technical Information Service (NTIS)

The article briefly examines data on distinctive features of the properties, particularly the crystallization properties, manifested by low-molecular substances in boundary layers. The methods and results of determination of the thicknesses of boundary la...

Y. M. Malinskii

1970-01-01

440

Large-Eddy Simulations of the Convective Atmospheric Boundary Layer over Heterogeneous Land Surfaces

The parameterization of the atmospheric boundary layer is essential for accurate numerical weather predictions. The near-surface values of air temperature or wind speed for instance are highly dependent on the complex land - atmosphere interactions over heterogeneous terrain. Over such surfaces, several open challenges remain regarding the growth of internal boundary layers, the determination of mixing layer heights and the

D. Nadeau; V. Kumar; C. W. Higgins; M. B. Parlange

2009-01-01

441

Mean flow in turbulent boundary layers disturbed to alter skin friction

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

P. R. Bandyopadhyay

1986-01-01

442

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

The Dorodnitsyn finite element method for turbulent boundary layer flow with surface mass transfer is extended to include axisymmetric swirling internal boundary layer flow. Turbulence effects are represented by the two-layer eddy viscosity model of Cebeci and Smith (1974) with extensions to allow for the effect of swirl. The method is applied to duct entry flow and a 10 degree

C. A. J. Fletcher

1985-01-01

443

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

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

1993-09-01

444

Structure of the nocturnal boundary layer over a complex terrain

The complex nature of the nocturnal boundary layer (NBL) has been shown extensively in the literature Project STABLE was conducted in 1988 to study NBL turbulence and diffusion over the complex terrain of the Savannah River Site (SRS) near Augusta, Georgia. The third night of the study was particularly interesting because of the unusual phenomena observed in the structure of the NBL. Further analyses of microscale and mesoscale data from this night are presented using data from SRS network of eight 61 m towers over 900 km{sup 2}, from six launches of an instrumented tethersonde, from permanent SRL meteorological instrumentation at seven levels of the 304 m (1,000 ft) WJBF-TV tower near SRS, and additional data collected at 36 m (CC) by North Carolina State University (NCSU) including a one dimensional sonic anemometer, fine wire thermocouple, and a three dimensional propeller anemometer. Also, data from the nearby Plant Vogtle nuclear power plant observation tower and the National Weather Service at Augusta`s Bush Field (AGS) are presented. The passage of a mesoscale phenomenon, defined as a microfront (with an explanation of the nomenclature used), and a vertical composite schematic of the NBL which shows dual low level wind maxima, dual inversions, and a persistent, elevated turbulent layer over a complex terrain are described.

Parker, M.J. [Westinghouse Savannah River Co., Aiken, SC (United States); Raman, S. [North Carolina State Univ., Raleigh, NC (United States). Dept. of Marine, Earth and Atmospheric Sciences

1992-08-01

445

Structure of the nocturnal boundary layer over a complex terrain

The complex nature of the nocturnal boundary layer (NBL) has been shown extensively in the literature Project STABLE was conducted in 1988 to study NBL turbulence and diffusion over the complex terrain of the Savannah River Site (SRS) near Augusta, Georgia. The third night of the study was particularly interesting because of the unusual phenomena observed in the structure of the NBL. Further analyses of microscale and mesoscale data from this night are presented using data from SRS network of eight 61 m towers over 900 km{sup 2}, from six launches of an instrumented tethersonde, from permanent SRL meteorological instrumentation at seven levels of the 304 m (1,000 ft) WJBF-TV tower near SRS, and additional data collected at 36 m (CC) by North Carolina State University (NCSU) including a one dimensional sonic anemometer, fine wire thermocouple, and a three dimensional propeller anemometer. Also, data from the nearby Plant Vogtle nuclear power plant observation tower and the National Weather Service at Augusta's Bush Field (AGS) are presented. The passage of a mesoscale phenomenon, defined as a microfront (with an explanation of the nomenclature used), and a vertical composite schematic of the NBL which shows dual low level wind maxima, dual inversions, and a persistent, elevated turbulent layer over a complex terrain are described.

Parker, M.J. (Westinghouse Savannah River Co., Aiken, SC (United States)); Raman, S. (North Carolina State Univ., Raleigh, NC (United States). Dept. of Marine, Earth and Atmospheric Sciences)