Evolution of the Laminar Boundary Layer Over a Flat Plate Under a Free Stream Turbulence Intensity of 1.3%

2008 ◽  
Author(s):  
E. J. Walsh ◽  
F. Brighenti ◽  
D. M. McEligot
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Laminar Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Early Stage ◽  
Turbine Blades ◽  
Leading Edge ◽  
Bypass Transition ◽  
Free Stream Turbulence

The evolution of the laminar boundary layer over a flat plate under a free stream turbulence intensity of 1.3% is analysed. The effect of free stream turbulence on the onset of transition is one of the important sources leading to bypass transition. Such disturbances are of great interest in engineering for the prediction of transition on turbine blades. The study concentrates on the early part of the boundary layer, starting from the leading edge, and is characterised by the presence of streamwise elongated regions of high and low streamwise velocity. It is demonstrated that the so called “Klebanoff modes” are not entirely representative of the flow structures, due to the time-averaged representations used in most studies. For the conditions of this investigation it is found that the urms and the peak disturbances remain constant in the early stages of the transition development. This region, in which the streaks strength is constant, is problematic for many theories as it is not known where on a surface to initiate a growth theory calculation, and hence the prediction of transition onset is difficult. The observation that a constant urms region exists within the boundary layer under these conditions may be the source of great difficulty in predicting transition onset under turbulence levels around 1%. This region suggests that the streaks are either continuously generated and damped, or do not grow during the early stage of transition, and highlights the importance of continuous influence of the free stream turbulence along the boundary layer edge. This work concludes that the first is more likely, and furthermore the measurements are shown to agree with recent direct numerical simulations.

Studies on Wake-Induced Bypass Transition of Flat-Plate Boundary Layers Under Pressure Gradients and Free-Stream Turbulence

2006 ◽  
Author(s):  
Ken-Ichi Funazaki ◽  
Eitaro Koyabu
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Turbine Blades ◽  
Plate Boundary ◽  
Influential Factors ◽  
Bypass Transition ◽  
Pressure Gradients ◽  
Free Stream Turbulence ◽  
Transitional Behavior

This study investigates wake-induced bypass transition of a flat-plate boundary layer that experiences favorable and adverse pressure gradients. The effect of free-stream turbulence is also examined. This paper mainly focuses on how the pressure gradients affect the transitional behavior of the wake-disturbed boundary layer with and without the influence of free-stream turbulence. A wall-contouring of the test duct generates the flow acceleration and deceleration on the flat plate, emulating aft-loading or front-loading pressure distributions of actual turbine blades. A spoked-wheel type wake generator is used to produce wakes moving over the boundary layer. Detailed boundary layer measurements are performed by use of a hot-wire anemometer. In addition to velocity fluctuations, which clearly indicate transition process, intermittency factors are obtained and compared with the results given by a transition model. Noticeable differences in transitional behavior are observed between the cases with and without the enhanced free-stream turbulence. It is also confirmed that the wake width as well as the direction of the bar movement are influential factors to the bypass transition.

Effects of Weak Free Stream Nonuniformity on Boundary Layer Transition

2005 ◽  
Vol 128 (2) ◽  
pp. 247-257 ◽  
Author(s):  
Jonathan H. Watmuff
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Free Stream ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Mean Flow ◽  
Low Speed ◽  
Free Stream Turbulence ◽  
Thickness Variations

Experiments are described in which well-defined weak Free Stream Nonuniformity (FSN) is introduced by placing fine wires upstream of the leading edge of a flat plate. Large amplitude spanwise thickness variations form in the boundary layer as a result of the interaction between the steady laminar wakes from the wires and the leading edge. The centerline of a region of elevated layer thickness is aligned with the centerline of the wake in the freestream and the response is shown to be remarkably sensitive to the spanwise length-scale of the wakes. The region of elevated thickness is equivalent to a long narrow low speed streak in the layer. Elevated Free Stream Turbulence (FST) levels are known to produce randomly forming arrays of long narrow low speed streaks in laminar boundary layers. Therefore the characteristics of the streaks resulting from the FSN are studied in detail in an effort to gain some insight into bypass transition that occurs at elevated FST levels. The shape factors of the profiles in the vicinity of the streak appear to be unaltered from the Blasius value, even though the magnitude of the local thickness variations are as large as 60% of that of the undisturbed layer. Regions of elevated background unsteadiness appear on either side of the streak and it is shown that they are most likely the result of small amplitude spanwise modulation of the layer thickness. The background unsteadiness shares many of the characteristics of Klebanoff modes observed at elevated FST levels. However, the layer remains laminar to the end of the test section (Rx≈1.4×106) and there is no evidence of bursting or other phenomena associated with breakdown to turbulence. A vibrating ribbon apparatus is used to examine interactions between the streak and Tollmien-Schlichting (TS) waves. The deformation of the mean flow introduced by the streak is responsible for substantial phase and amplitude distortion of the waves and the breakdown of the distorted waves is more complex and it occurs at a lower Reynolds number than the breakdown of the K-type secondary instability that is observed when the FSN is not present.

Receptivity to free-stream vorticity of flow past a flat plate with elliptic leading edge

2010 ◽  
Vol 653 ◽  
pp. 245-271 ◽  
Author(s):  
L.-U. SCHRADER ◽  
L. BRANDT ◽  
C. MAVRIPLIS ◽  
D. S. HENNINGSON
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Three Dimensional ◽  
Low Frequency ◽  
Leading Edge ◽  
Sound Waves ◽  
Streamwise Vorticity ◽  
Low Frequencies ◽  
Free Stream Turbulence

Receptivity of the two-dimensional boundary layer on a flat plate with elliptic leading edge is studied by numerical simulation. Vortical perturbations in the oncoming free stream are considered, impinging on two leading edges with different aspect ratio to identify the effect of bluntness. The relevance of the three vorticity components of natural free-stream turbulence is illuminated by considering axial, vertical and spanwise vorticity separately at different angular frequencies. The boundary layer is most receptive to zero-frequency axial vorticity, triggering a streaky pattern of alternating positive and negative streamwise disturbance velocity. This is in line with earlier numerical studies on non-modal growth of elongated structures in the Blasius boundary layer. We find that the effect of leading-edge bluntness is insignificant for axial free-stream vortices alone. On the other hand, vertical free-stream vorticity is also able to excite non-modal instability in particular at zero and low frequencies. This mechanism relies on the generation of streamwise vorticity through stretching and tilting of the vertical vortex columns at the leading edge and is significantly stronger when the leading edge is blunt. It can thus be concluded that the non-modal boundary-layer response to a free-stream turbulence field with three-dimensional vorticity is enhanced in the presence of a blunt leading edge. At high frequencies of the disturbances the boundary layer becomes receptive to spanwise free-stream vorticity, triggering Tollmien–Schlichting (T-S) modes and receptivity increases with leading-edge bluntness. The receptivity coefficients to free-stream vortices are found to be about 15% of those to sound waves reported in the literature. For the boundary layers and free-stream perturbations considered, the amplitude of the T-S waves remains small compared with the low-frequency streak amplitudes.

The generation of Tollmien-Schlichting waves by free-stream turbulence

1996 ◽  
Vol 312 ◽  
pp. 341-371 ◽  
Author(s):  
P. W. Duck ◽  
A. I. Ruban ◽  
C. N. Zhikharev
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Stokes Equations ◽  
Leading Edge ◽  
Wave Generation ◽  
Generation Process ◽  
Viscous Boundary Layer ◽  
Free Stream Turbulence ◽  
Tollmien Schlichting

The phenomenon of Tollmien-Schlichting wave generation in a boundary layer by free-stream turbulence is analysed theoretically by means of asymptotic solution of the Navier-Stokes equations at large Reynolds numbers (Re → ∞). For simplicity the basic flow is taken to be the Blasius boundary layer over a flat plate. Free-stream turbulence is taken to be uniform and thus may be represented by a superposition of vorticity waves. Interaction of these waves with the flat plate is investigated first. It is shown that apart from the conventional viscous boundary layer of thickness O(Re−1/2), a ‘vorticity deformation layer’ of thickness O(Re−1/4) forms along the flat-plate surface. Equations to describe the vorticity deformation process are derived, based on multiscale asymptotic techniques, and solved numerically. As a result it is shown that a strong singularity (in the form of a shock-like distribution in the wall vorticity) forms in the flow at some distance downstream of the leading edge, on the surface of the flat plate. This is likely to provoke abrupt transition in the boundary layer. With decreasing amplitude of free-stream turbulence perturbations, the singular point moves far away from the leading edge of the flat plate, and any roughness on the surface may cause Tollmien-Schlichting wave generation in the boundary layer. The theory describing the generation process is constructed on the basis of the ‘triple-deck’ concept of the boundary-layer interaction with the external inviscid flow. As a result, an explicit formula for the amplitude of Tollmien-Schlichting waves is obtained.

Leading Edge Separation Bubbles on Turbomachine Blades

1995 ◽  
Vol 117 (1) ◽  
pp. 115-125 ◽  
Author(s):  
R. E. Walraevens ◽  
N. A. Cumpsty
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Free Stream ◽  
Turbine Blades ◽  
Leading Edge ◽  
Compressor Blades ◽  
Free Stream Turbulence ◽  
Pressure Distributions ◽  
Small Influence ◽  
Separation Bubbles

Results are presented for separation bubbles of the type that can form near the leading edges of thin compressor or turbine blades. These often occur when the incidence is such that the stagnation point is not on the nose of the aerofoil. Tests were carried out at low speed on a single aerofoil to simulate the range of conditions found on compressor blades. Both circular and elliptic shapes of leading edge were tested. Results are presented for a range of incidence, Reynolds number, and turbulence intensity and scale. The principal quantitative measurements presented are the pressure distributions in the leading edge and bubble region, as well as the boundary layer properties at a fixed distance downstream, where most of the flows had reattached. Reynolds number was found to have a comparatively small influence, but a raised level of free-stream turbulence has a striking effect, shortening or eliminating the bubble and increasing the magnitude of the suction spike. Increased free-stream turbulence also reduces the boundary layer thickness and shape parameter after the bubble. Some explanations of the processes are outlined.

scholarly journals The effect of a low-viscosity near-wall film on bypass transition in boundary layers

2015 ◽  
Vol 772 ◽  
pp. 330-360 ◽  
Author(s):  
Seo Yoon Jung ◽  
Tamer A. Zaki
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Transition To Turbulence ◽  
Bypass Transition ◽  
Fluid Interface ◽  
Free Stream Turbulence ◽  
Wall Film ◽  
The Impact ◽  
Two Fluid

Bypass transition in a two-fluid boundary layer is examined using direct numerical simulations (DNSs). A less-viscous wall film is considered and the impact on transition location is evaluated at two different viscosity ratios and free-stream turbulence intensities. The less-viscous wall film absorbs the mean shear from the outer stream, weakens the lift-up mechanism, and alters the disturbance field inside the boundary layer. These effects all favour a delay in the onset of bypass transition. However, the viscosity and mean-shear discontinuities across the two-fluid interface introduce a new mechanism for the generation of wall-normal vorticity in the boundary layer, and can therefore promote transition to turbulence. Conditionally averaged statistics and streak tracking techniques are adopted in order to examine the impact of the wall film on the bypass transition process. It is shown that the weaker amplification of the streaks in the outer fluid can delay breakdown to turbulence, despite the additional disturbance generation at the two-fluid interface. The efficacy of the wall film in delaying transition is demonstrated at moderate level of free-stream turbulence intensity, but is reduced as the turbulence intensity is increased.

Bypass Transition Modelling: A New Method Which Accounts for Free-Stream Turbulence Intensity and Length Scale

2000 ◽  
Author(s):  
Paul E. Roach ◽  
David H. Brierley
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Test Surface ◽  
Length Scale ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Free Stream Turbulence

The publication of the present authors’ boundary layer transition data in 1992 (now widely known as the ERCOFTAC test case T3) has led to a spate of new experimental and modelling efforts aimed at improving our understanding of this problem. This paper describes a new method of determining boundary layer transition with zero mean pressure gradient. The approach examines the development of a laminar boundary layer to the start of transition, accounting for the influences of free-stream turbulence and test surface geometry. It is presented as a “proof of concept”, requiring a significant amount of work before it can be considered as a practically applicable model for transition prediction. The method is based upon one first put forward by G.I. Taylor in the 1930’s, and accounts for the action of local, instantaneous pressure gradients on the developing laminar boundary layer. These pressure gradients are related to the intensity and length scale of turbulence in the free-stream using Taylor’s simple isotropic model. The findings demonstrate the need to account for the separate influences of free-stream turbulence intensity and length scale when considering the transition process. Although the length scale has less of an effect than the intensity, its influence is, nevertheless, significant and must not be overlooked. This fact goes a long way towards explaining the large scatter to be found in simple correlations which involve only the turbulence intensity. Intriguingly, it is demonstrated that it is the free-stream turbulence at the leading edge of the test surface which is important, not that found locally outside the boundary layer. The additional influence of leading edge geometry is also shown to play a major role in fixing the point at which transition begins. It is suggested that the leading edge geometry will distort the incident turbulent eddies, modifying the effective “free-stream” turbulence properties. Consequently, it is shown that the scale of the eddies relative to the leading edge thickness is a further important parameter, and helps bring together a large number of test cases.

Numerical simulations of boundary-layer bypass transition due to high-amplitude free-stream turbulence

2008 ◽  
Vol 613 ◽  
pp. 135-169 ◽  
Author(s):  
VICTOR OVCHINNIKOV ◽  
MEELAN M. CHOUDHARI ◽  
UGO PIOMELLI
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Free Stream ◽  
Leading Edge ◽  
High Amplitude ◽  
Length Scale ◽  
Normal Velocity ◽  
Bypass Transition ◽  
Turbulent Spots ◽  
Free Stream Turbulence

Direct numerical simulations (DNS) of bypass transition due to high-amplitude free-stream turbulence (FST) are carried out for a flat-plate boundary layer. The computational domain begins upstream of the plate leading edge and extends into the fully turbulent region. Thus, there is noad hoctreatment to account for the initial ingestion of FST into the laminar boundary layer. We study the effects of both the FST length scale and the disturbance behaviour near the plate leading edge on the details of bypass transition farther downstream. In one set of simulations, the FST parameters are chosen to match the ERCOFTAC benchmark case T3B. The inferred FST integral length scaleL11is significantly larger (RL=UL11/ν = 6580) than that employed in previous simulations of bypass transition (RL≃ 1000). An additional set of simulations was performed atRL= 1081 to compare the transition behaviour in the T3B case with that of a smaller value of FST length scale. The FST length scale is found to have a profound impact on the mechanism of transition. While streamwise streaks (Klebanoff modes) are observed at both values of the FST length scale, they appear to have clear dynamical significance only at the smaller value ofRL, where transition is concomitant with streak breakdown. For the T3B case, turbulent spots form upstream of the region where streaks could be detected. Spot precursors are traced to quasi-periodic spanwise structures, first observed as short wavepackets in the wall-normal velocity component inside the boundary layer. These structures are reoriented to become horseshoe vortices, which break down into young turbulent spots. Two of the four spots examined for this case had a downstream-pointing shape, similar to those found in experimental studies of transitional boundary layers. Additionally, our simulations indicate the importance of leading-edge receptivity for the onset of transition. Specifically, higher fluctuations of the vertical velocity at the leading edge of the plate result in higher levels of streamwise Reynolds stress inside the developing boundary layer, facilitating breakdown to turbulence.

High Resolution Measurements of Heat Transfer, Near-Wall Intermittency, and Reynolds-Stresses Along a Flat Plate Boundary Layer Undergoing Bypass Transition

2019 ◽  
Author(s):  
Holger Albiez ◽  
Christoph Gramespacher ◽  
Matthias Stripf ◽  
Hans-Jörg Bauer
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Flat Plate ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Bypass Transition ◽  
Reynolds Stresses ◽  
Length Scales ◽  
Near Wall

Abstract A new experimental dataset focusing on the influence of high free-stream turbulence and large pressure gradients on boundary layer transition is presented. The experiments are conducted in a new wind tunnel equipped with a flat plate test section and a new kind of turbulence generator which allows for a continuous variation of turbulence intensity. The flat plate features an elliptic nose and is mounted midway between contoured top and bottom walls. Two different wall contours can be implemented to create pressure distributions on the flat plate that are typical for the pressure and suction side of high pressure turbine cascades. A large variation of Reynolds number from 3.0 · 105 to 7.5 · 105 and inlet turbulence intensity between 1.1 % and 8 % is realized, resulting in more than 100 test cases. Measurements comprise highly resolved heat transfer, near-wall intermittency and free-stream Reynolds stress distributions. Near-wall intermittency is measured using a traversable hotfilm sensor embedded in a steel-belt that is running around the flat plate while free-stream Reynolds stresses are measured simultaneously at the same position with a revolvable X-wire probe. Additionally, turbulent length scales are analyzed using the X-wire signal along the flat plate. Results show that heat transfer and near wall intermittency distributions are in good agreement and that heat transfer at high turbulence levels increases prior to the formation of first turbulence spots. Transition onset is found to be influenced by the turbulence Reynolds number, i.e. turbulent length scales. At constant inlet turbulence intensity, transition onset moves upstream, when the turbulent Reynolds number is decreased.

Simulations of bypass transition

2001 ◽  
Vol 428 ◽  
pp. 185-212 ◽  
Author(s):  
R. G. JACOBS ◽  
P. A. DURBIN
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Free Stream ◽  
Low Frequency ◽  
Numerical Data ◽  
Bypass Transition ◽  
Mean Velocity ◽  
Free Stream Turbulence ◽  
Turbulent Inflow ◽  
Irregular Motion

Bypass transition in an initially laminar boundary layer beneath free-stream turbulence is simulated numerically. New perspectives on this phenomenon are obtained from the numerical flow fields. Transition precursors consist of long backward jets contained in the fluctuating u-velocity field; they flow backwards relative to the local mean velocity. The jets extend into the upper portion of the boundary layer, where they interact with free-stream eddies. In some locations a free-stream perturbation to the jet shear layer develops into a patch of irregular motion – a sort of turbulent spot. The spot spreads longitudinally and laterally, and ultimately merges into the downstream turbulent boundary layer. Merging spots maintain the upstream edge of the turbulent region. The jets, themselves, are produced by low-frequency components of the free-stream turbulence that penetrate into the laminar boundary layer. Backward jets are a component of laminar region streaks.A method to construct turbulent inflow from Orr–Sommerfeld continuous modes is described. The free-stream turbulent intensity was chosen to correspond with the experiment by Roach & Brierly (1990). Ensemble-averaged numerical data are shown to be in good agreement with laboratory measurements.

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