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Boundary layer - Wikipedia, the free encyclopedia In the atmosphere the boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface. The thickness of the velocity boundary layer is normally defined as the distance from the solid body at which the flow velocity is 99% of the freestream velocity, that is, the velocity that is calculated at the surface of the body in an inviscid flow solution. It is referred to as a bladeless turbine because it uses the boundary layer effect and not a fluid impinging upon the blades as in a conventional turbine. en.wikipedia.org /wiki/Boundary_layer   (1571 words)

Boundary layer transition - Wikipedia, the free encyclopedia The initial stage of the natural transition process is known as the receptivity phase and consists of the transformation of external distrubances in the outer freestream flow over the boundary layer (such as freestream turbulence, surface roughess, acoustic noise etc.) into internal instability oscillations within the boundary layer. Due to the distortion of the boundary layer, inflexional mean profiles develop and a fourth stage is reached, where the boundary layer becomes unstable to three-dimensional high-frequency disturbances. Numerous experiments in recent decades have revealed that the extent of the amplification region, and hence the location of the transition point on the body surface, is strongly dependent not only upon the amplitude and/or the spectrum of external disturbances but also on their physical nature. en.wikipedia.org /wiki/Boundary_layer_transition   (416 words)

Boundary layer The boundary layer is a very thin layer of air flowing over the surface of an aircraft wing, or airfoil, (as well as other surfaces of the aircraft). Each layer of molecules within the boundary layer moves faster than the layer that is closer to the surface of the wing. At its transition point, the boundary layer changes from laminar, where the velocity changes uniformly as one moves away from the object's surface, to turbulent, where the velocity is characterized by unsteady (changing with time) swirling flows inside the boundary layer. www.centennialofflight.gov /essay/Dictionary/boundary_layer/DI134.htm   (452 words)

Thermal - StormWiki   (Site not responding. Last check: 2007-10-11) The boundary layer is the layer of air near the earth's surface that is affected by the surface on scales of an hour or so. Boundary layer physics is a subfield of atmospheric physics or meteorology, but the scales (and therefore the forces) of interest are different. A convective boundary layer is a few hundred meters to 3 km thick, depending on the amount of incoming solar energy, the amount of moisture in the ground, the larger-scale weather (high or low pressure), the wind speed, and other factors. stormwiki.unk.edu /index.php/Thermal   (1074 words)

Consult von Karman Institute for Fluid Dynamics In the past years, the research was concentrated on two aspects: boundary layer transition and shock wave boundary layer interaction with wall-to-recovery temperature ratio effects [MP7]. Boundary layer transition prediction is a very important problem in the aerodynamic design of aeroplanes. The characteristics of the boundary layer around the model in the VKI-H3 Mach 6 wind tunnel are predicted by both the semi-analytical prediction code and Navier-Stokes computations. www.vki.ac.be /research/themes/aeros99/4.html   (1221 words)

GAMBIT MODELING GUIDE: 3. MESHING THE MODEL By attaching a boundary layer to the face that represents the pipe wall, you can increase the mesh density near the wall and decrease the density near the center of the cylinder, thereby obtaining sufficient information to characterize the gradients in both regions while minimizing the total number of mesh nodes in the model. For boundary layers that are attached to uniformly graded edges, the shape of the boundary layer is independent of the algorithm specified for its construction. The transition pattern determines the arrangement of mesh nodes in the region near the outermost row of the boundary layer. www.ent.ohiou.edu /~juwt/HTMLS/fluent/gambit2/html/modeling_guide/mg0301.htm   (2887 words)

DNS Simulation ; High Performance Computing For this purpose, as a first step a parallelized version of a boundary-layer transition code is being developed that will allow for the first time a well-resolved direct numerical simulation of the spatial process of laminar-turbulent transition, comprising the stages of initial development of linear disturbances up to the fully developed turbulent boundary layer. The algorithmic details of the direct numerical simulation of transition to turbulence in a boundary layer based on a formulation in terms of vertical velocity and vertical vorticity are presented. The presence of a $\Lambda$-vortex, significance of streamwise vorticity, spanwise vorticity, shear layer, and speed of propagation of these structures are discussed in detail as well as their role in the transition process and in the process of breakdown to turbulence. www.umaine.edu /MechEng/Kiran/transition.html   (647 words)

KTH mechanics / project information   (Site not responding. Last check: 2007-10-11) Previously transition associated with localized disturbances have been investigated, and at present the transition in boundary layers starting with a pair of oblique waves is investigated. A new direction in the project is to simulate transition in a boundary layer subjected to free-stream turbulence, where the turbulence is initiated close to the leading edge by a random distribution of continuous spectrum modes. Secondary instability of cross-flow vortices in Falkner-Skan-Coke boundary layers. www.mech.kth.se /new/info_projects.asp?ProjectID=36   (636 words)

Unsteady boundary layer transition   (Site not responding. Last check: 2007-10-11) "Effect of unsteady wake-passing frequency on boundary layer transition --- experimental investigation and wavelet analysis" Schobeiri, M.T., Read K., and Lewalle, J. Detailed experimental and theoretical investigations have been carried out to study the effect of unsteady wake passing frequency on the boundary layer transition along a curved plate under zero longitudinal pressure gradients. Periodic unsteady flow with different passing frequencies is generated utilizing an unsteady flow research facility with a rotating cascade of rods positioned upstream of the curved plate. www.ecs.syr.edu /faculty/lewalle/asme-95-gt-437.html   (156 words)

Passive Hypervelocity Boundary Layer Control Using an Acoustically Absorptive Surface   (Site not responding. Last check: 2007-10-11) A series of exploratory boundary layer transition experiments were performed on a 5.06 degree half-angle cone at zero angle-of-attack in the T5 Hypervelocity Shock Tunnel in order to test a novel hypersonic boundary layer control scheme. Recently performed linear stability analyses suggested that transition could be delayed in hypersonic boundary layers by using an ultrasonically absorptive surface that would damp the second mode (Mack mode). Detailed comparisons were performed to insure that previous results obtained in similar boundary layer transition experiments (on a regular smooth surface) were reproduced and the results were extended to examine the effects of the porous surface. www.galcit.caltech.edu /Seminars/Fluids/PastFluids/2000-2001/Rasheed_abs.html   (222 words)

Continuation methods in boundary-layer transition research: A fluid dynamicist's point of view - W. Koch   (Site not responding. Last check: 2007-10-11) The goal of transition research is to identify and understand the physical and corresponding mathematical mechanisms that cause laminar breakdown and use this knowledge to control transition by either delaying or accelerating it. For the pure identification of transition mechanisms it is advantageous to circumvene the difficult topic of receptivity and establish clearly defined conditions. Unfortunately, in boundary layers the equilibrium solutions of the above mentioned classes are practically always unstable and the computed equilibria are never observed physically for higher Reynolds numbers. www.cs.kuleuven.ac.be /~kurt/EUROMECH383/ABSTRACTS/koch.html   (512 words)

Instructions The flow inside a turbulent boundary layer is unsteady and irregular and it is the time-averaged boundary layer equations that must be solved in this case. The WALZ surface properties dialog box therefore asks you to specify whether the leading edge is sharp (initial boundary layer thickness zero) or rounded (initial non-zero boundary layer thickness computed from the inviscid velocity distribution you specify using the stagnation point solution of the Navier Stokes). Even though the boundary layer method becomes invalid when the flow separates like this, you may be able to very crudely model this situation using the applets by treating the laminar separation location as though it were a transition location, and continuing calculations from this point using one of the turbulent boundary layer applets. www.engapplets.vt.edu /fluids/bls2/instructions.html   (2930 words)

KTH mechanics / defense information   (Site not responding. Last check: 2007-10-11) Traditional research on laminar-turbulent transition has focused on scenarios that are caused by the exponential growth of eigensolutions to the linearized disturbance equations, e.g. In the late transition stage, large velocity uctuations are found at ow positions associated with steep spanwise gradients between the streaks. The mechanism continuously interact with the boundary layer and the resulting transition scenario is characterized by the growth of streamwise streaks. www.mech.kth.se /new/info_defense.asp?DefenseID=1   (379 words)

Transition in Hypervelocity Flows   (Site not responding. Last check: 2007-10-11) Boundary layer transition has been observed to take place through formation, growth and merger of turbulent spots in both incompressible and compressible flows (e.g. In studying transition in high enthalpy impulse facilities, the frequency of spot production and the growth rate of spots may influence the interpretation of averaged heat transfer and skin friction data if an insufficient averaging period is used. Excrescences of different heights relative to the boundary layer thickness were found to be capable of generating turbulent wedges in an otherwise laminar boundary layer and a tripping strip was used to cause early transition. www.mech.uq.edu.au /~mee/transition.html   (262 words)

The Effect of Thin Liquid Films on Boundary-Layer Transition   (Site not responding. Last check: 2007-10-11) The physical mechanisms which give rise to this effect are not known, but one possibile cause is the interaction between the liquid (rain) layer and the air boundary layer, and the effect of this interaction on the transition of the air boundary layer. We examine the effect of the liquid film on the boundary-layer transition, using a vibrating ribbon to excite the air boundary layer at discrete frequencies. We show that the liquid layer does, in fact, change the response of the air boundary layer, either destabilizing it or stabilizing it depending on the air and liquid Reynolds numbers and the excitation frequency. flux.aps.org /meetings/BAPSDFD96/abs/S800005.html   (214 words)

FEDSM2001-18036: Discharge Coefficient of Super-Accurate Critical Nozzles Accompanied With the Boundary Layer ... By virtue of the lower pressure at the downstream nozzles in the series connection, the boundary layer transition occurred in the upstream nozzle could be measured by the downstream nozzles accompanied with the laminar boundary layer. After estimating the tendency of the decrease of the discharge coefficient of an upstream nozzle caused by the transition, errors caused by the transition in the downstream nozzles at higher Reynolds number were corrected applying the same tendency. The discharge coefficient in the turbulent boundary layer regime obeys a certain curve, named the universal curve, which is larger than ISO 9300 by 0.2% at the Reynolds number of 1E+06 as shown in the picture. asme.pinetec.com /fedsm2001/abstracts/FEDSM2001-18036.html   (296 words)

Coastal boundary layer transition within tropical cyclones at landfall   (Site not responding. Last check: 2007-10-11) 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. 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. 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. digitalcommons.lib.ttu.edu /dissertations/AAI3124477   (472 words)

Strong-shock interface interactions   (Site not responding. Last check: 2007-10-11) Specifically, the boundary layer over a rotor suction surface is subjected to substantial unsteadiness that stems from impinging wakes of the upstream stator or rotor. The transition from laminar flow on the left, to turbulent flow on the right side of the plate is clearly visible. The study seeks to determine the mechanism that triggers the transitioning of the boundary layer flow from laminar to turbulent. www.psc.edu /research/asci/durbin/durbin.html   (545 words)

The Role of Receptivity in Boundary Layer Transition   (Site not responding. Last check: 2007-10-11) Accurate prediction of boundary-layer transition is crucial to the solution of a wide range of fluid dynamics problems, from the design of low-drag airfoils to the estimation of re-entry heat loads to the swing of a cricket ball. The response of the boundary layer to a three-dimensional disturbance generated by the pulsed displacement of a small wing was also measured. Finally, the response of the boundary layer to turbulent fluctuations in a thin wake are discussed. www.aeromech.usyd.edu.au /wwwdocs/98s2sem8.html   (296 words)

U of M: Department of Mechanical Engineering: Events: Seminars. Key to the prediction of both the fluid mechanics and the heat transfer, and key to pushing the technology further, is an understanding of boundary layer transition. Existing transition models are not always adequate, and it is believed that if a better understanding of the physics of transition were incorporated into the models, they might be improved. The transition is complex, and includes intermittent flow, switching between a badly disturbed non-turbulent state and a turbulent state. www.me.umn.edu /events/seminars/s01/seminar12.shtml   (469 words)

CV Moshe Rosenfeld,   (Site not responding. Last check: 2007-10-11) Seifert, A. and Hodson, H.P., “Periodic Turbulent Strips and Calmed Regions in a Transitional Boundary Layer (TN)”, AIAA J., Vol. Seifert, A., “Non-linear Evolution of Point-Source Disturbances in an Adverse Pressure Gradient Laminar Boundary Layer”, Proceedings of IUTAM symposium on nonlinear instability and transition in three-dimensional boundary layers, Seifert, A. and Tumin, A., “Non-linear Localized Disturbances in an Adverse Pressure Gradient Boundary Layer Transition: Experiment and Linear Stability Theory”, Proceedings of the 8th Beer-Sheva symposium on Turbulence and MHD, 1996, published by AIAA series  Prog. www.eng.tau.ac.il /~seifert/publications.html   (1135 words)

- A I M S   (Site not responding. Last check: 2007-10-11) We study boundary-layer turbulence using the Navier-Stokes-alpha and Leray-alpha models obtaining extensions of the Prandtl equations for the averaged flow in a turbulent boundary layer. In the case of a zero pressure gradient flow along at plate, we derive an extension of the Blasius equation, which we study analytically and prove the existence of a two-parameter family of solutions satisfying physical boundary conditions. Matching these parameters with the skin friction coefficient and the Reynolds number based on momentum thickness, we get an agreement of the solutions with experimental data in the laminar and transitional boundary layers, as well as in the turbulent boundary layer for moderately large Reynolds numbers. aimsciences.org /AIMS-Conference/jsp/display-conference-abstract.jsp?id=2344   (122 words)

Wind-US Utilities - tmptrn   (Site not responding. Last check: 2007-10-11) keyword block is used when a point-by-point surface temperature boundary condition is being used, and/or when point-by-point boundary layer transition information is being specified. Within a zone, boundary layer transition and/or wall temperature data may be specified on a j or k boundary surface as a function of the i index. Boundary layer transition is indicated by a number from 0.0 to 1.0, indicating laminar and fully turbulent flow, respectively. www.grc.nasa.gov /WWW/winddocs/utilities/tmptrn.html   (615 words)

Numerical Study of Wall Influence on Boundary-Layer Transition for Two-Dimensional NACA 16-012 and 4412 Hydrofoil ...   (Site not responding. Last check: 2007-10-11) One of the difficulties in hydrofoil model tests is the relatively low Reynolds number of the test piece and the presence of the test section walls. The corresponding boundary-layer calculations using the CERT calculation scheme are presented to show the influence of the nearby walls on shifting the location of the boundary-layer laminar-turbulent separation as well as turbulent separation. By introducing an effective angle of attack, it is possible to obtain close agreement in the calculated and measured suction side pressure distortion as well as the locations of the boundary-layer separation and transition. www.sname.org /index_of_publications/abstracts/j3413904.htm   (155 words)

Experimental Study of Boundary Layer Transition With Elevated Freestream Turbulence on a Heated Flat Plate A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Conditional sampling performed in the transitional boundary layer indicated the existence of a nearwall drop in intermittency, pronounced at certain low intermittencies, which is consistent with the crosssectional shape of turbulent spots observed by others. Nonturbulent and turbulent profiles in transitional boundary layers cannot be simply treated as Blasius and fully turbulent profiles, respectively. gltrs.grc.nasa.gov /cgi-bin/GLTRS/browse.pl?1991/CR-187068.html   (383 words)

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