Comparison of the Structures of a Perturbed and Unperturbed Boundary Layer of the Same Reynolds Number

Volume 1 ◽  
2004 ◽  
Author(s):  
O. Oyewola
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Porous Wall ◽  
Initial Boundary ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Streamwise Location ◽  
Initial Boundary Condition ◽  
The Difference ◽  
Made In

The comparison of the structures of the boundary layer with and without suction of the same momentum thickness Reynolds number Rθ have been made in a turbulent boundary subjected to concentrated suction, applied through a short porous wall strip. The results indicate that, relative to σ = 0, the mean velocity collapses reasonably well but there are some discrepancies in the Reynolds stresses distributions. These discrepancies are also noted in the distributions of the anisotropy invariant tensor, skewness and flatness factors. The result would suggest that the differences are a result of the difference in the initial boundary condition, which influences the flow structures to a significant streamwise location.

Flow and Heat Transfer Behavior in Transitional Boundary Layers With Streamwise Acceleration

1996 ◽  
Vol 118 (2) ◽  
pp. 314-326 ◽  
Author(s):  
F. J. Keller ◽  
T. Wang
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Temperature Fluctuations ◽  
Heat Fluxes ◽  
Temperature Profiles ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Free Stream Turbulence

The effects of streamwise acceleration on a two-dimensional heated boundary layer undergoing natural laminar-turbulent transition were investigated with detailed measurements of momentum and thermal transport phenomena. Tests were conducted over a heated flat wall with zero pressure-gradient and three levels of streamwise acceleration: K ≡ (v/U∞2) (d/U∞/dx) = 0.07, 0.16, and 0.25 × 10−6. Free-stream turbulence intensities were maintained at approximately 0.5 percent for the baseline case and 0.4 percent for the accelerating cases. A miniature three-wire probe was used to measure mean velocity and temperature profiles, Reynolds stresses, and Reynolds heat fluxes. Transition onset and end were inferred from Stanton numbers and skin-friction coefficients. The results indicate that mild acceleration delays transition onset and increases transition length both in terms of distance, x, and Reynolds number based on x. Transition onset and length are relatively insensitive to acceleration in terms of momentum thickness Reynolds number. This is supported by the boundary layer thickness and integral parameters, which indicate that a favorable pressure gradient suppresses boundary layer growth and development in the transition region. Heat transfer rates and temperature profiles in the late-transition and early-turbulent regions lag behind the development of wall shear stress and velocity profiles. This lag increases as K increases, indicating that the evolution of the heat transport is slower than that of the momentum transport. Comparison of the evolution of rms temperature fluctuations to the evolution of Reynolds normal stresses indicates a similar lag in the rms temperature fluctuations.

Reynolds-number scaling of the flat-plate turbulent boundary layer

2000 ◽  
Vol 422 ◽  
pp. 319-346 ◽  
Author(s):  
DAVID B. DE GRAAFF ◽  
JOHN K. EATON
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Extensive Study ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Mean Velocity ◽  
Law Of The Wall ◽  
The Mean

Despite extensive study, there remain significant questions about the Reynolds-number scaling of the zero-pressure-gradient flat-plate turbulent boundary layer. While the mean flow is generally accepted to follow the law of the wall, there is little consensus about the scaling of the Reynolds normal stresses, except that there are Reynolds-number effects even very close to the wall. Using a low-speed, high-Reynolds-number facility and a high-resolution laser-Doppler anemometer, we have measured Reynolds stresses for a flat-plate turbulent boundary layer from Reθ = 1430 to 31 000. Profiles of u′2, v′2, and u′v′ show reasonably good collapse with Reynolds number: u′2 in a new scaling, and v′2 and u′v′ in classic inner scaling. The log law provides a reasonably accurate universal profile for the mean velocity in the inner region.

Experimental Investigation of the Effect of Upstream Conditions on Smooth and Rough Zero Pressure Gradient Turbulent Boundary Layer Flows at Very High Reynolds Numbers

2002 ◽  
Author(s):  
Luciano Castillo ◽  
Junghwa Seo ◽  
T. Gunnar Johansson ◽  
Horia Hangan
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Wind Tunnel ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
The Mean ◽  
High Reynolds ◽  
Very High

A 2D turbulent boundary layer experiment in a zero pressure gradient (ZPG) has been carried out using two cross hot-wire probes. The mean velocity and all non-zero Reynolds stresses were measured in a number of positions, 14–28 m from the inlet of the wind tunnel over a rough and a smooth surface. Wind tunnel speeds of 10 m/s and 20 m/s were set up in order to test the effect of the upstream conditions on the downstream flow. The long test section allowed us to investigate the mean velocity and Reynolds stresses dependence on the local Reynolds number and the initial conditions at very high Reynolds number (i.e. Rθ ∼ 120,000). Furthermore, it will be shown that the mean velocity deficit profiles and some of the Reynolds stresses collapse when the upstream conditions are kept fixed for smooth and rough surface.

Flow and Heat Transfer Behavior in Transitional Boundary Layers With Streamwise Acceleration

1994 ◽  
Author(s):  
F. Jeffrey Keller ◽  
Ting Wang
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Temperature Fluctuations ◽  
Heat Fluxes ◽  
Temperature Profiles ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Free Stream Turbulence

The effects of streamwise acceleration on a two-dimensional heated boundary layer undergoing natural laminar-turbulent transition were investigated with detailed measurements of momentum and thermal transport phenomena. Tests were conducted over a heated flat wall with zero pressure-gradient and three levels of streamwise acceleration: K≡νU¯∞2dU¯∞dx= 0.07, 0.16, and 0.25 × 10−6. Free-stream turbulence intensities were maintained at approximately 0.5% for the baseline case and 0.4% for the accelerating cases. A miniature three-wire probe was used to measure mean velocity and temperature profiles, Reynolds stresses, and Reynolds heat fluxes. Transition onset and end were inferred from Stanton numbers and skin-friction coefficients. The results indicate that mild acceleration delays transition onset and increases transition length both in terms of distance, x1 and Reynolds number based on x. Transition onset and length are relatively insensitive to acceleration in terms of momentum thickness Reynolds number. This is supported by the boundary layer thickness and integral parameters which indicate that a favorable pressure gradient suppresses boundary layer growth and development in the transition region. Heat transfer rates and temperature profiles in the late-transition and early-turbulent regions lag behind the development of wall shear stress and velocity profiles. This lag increases as K increases, indicating that the evolution of the heat transport is slower than that of the momentum transport. Comparison of the evolution of RMS temperature fluctuations to the evolution of Reynolds normal stresses indicates a similar lag in the RMS temperature fluctuations.

Flow Characteristics of Transitional Boundary Layers on an Airfoil in Wakes

2000 ◽  
Vol 122 (3) ◽  
pp. 522-532 ◽  
Author(s):  
H. Lee ◽  
S.-H. Kang
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Skin Friction ◽  
Velocity Fluctuation ◽  
Plate Boundary ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Transitional Region ◽  
Mean Velocity ◽  
Measured Velocity

Transition characteristics of a boundary layer on a NACA0012 airfoil are investigated by measuring unsteady velocity using hot wire anemometry. The airfoil is installed in the incoming wake generated by an airfoil aligned in tandem with zero angle of attack. Reynolds number based on the airfoil chord varies from 2.0×105 to 6.0×105; distance between two airfoils varies from 0.25 to 1.0 of the chord length. To measure skin friction coefficient identifying the transition onset and completion, an extended wall law is devised to accommodate transitional flows with pressure gradient and nonuniform inflows. Variations of the skin friction are quite similar to that of the flat plate boundary layer in the uniform turbulent inflow of high intensity. Measured velocity profiles are coincident with families generated by the modified wall law in the range up to y+=40. Turbulence intensity of the incoming wake shifts the onset location of transition upstream. The transitional region becomes longer as the airfoils approach one another and the Reynolds number increases. The mean velocity profile gradually varies from a laminar to logarithmic one during the transition. The maximum values of rms velocity fluctuations are located near y+=15-20. A strong positive skewness of velocity fluctuation is observed at the onset of transition and the overall rms level of velocity fluctuation reaches 3.0–3.5 in wall units. The database obtained will be useful in developing and evaluating turbulence models and computational schemes for transitional boundary layer. [S0098-2202(00)01603-5]

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Paul Ziadé ◽  
Mark A. Feero ◽  
Philippe Lavoie ◽  
Pierre E. Sullivan
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Base Flow ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Layer Development

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

Turbulent Wake of a Stack and the Influence of Velocity Ratio

2006 ◽  
Author(s):  
M. S. Adaramola ◽  
D. Sumner ◽  
D. J. Bergstrom
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Reynolds Shear Stress ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Turbulent Wake ◽  
Mean Velocity ◽  
Distinct Structure ◽  
Flow Reynolds Number ◽  
Cross Flow Velocity

The effect of the jet-to-cross-flow velocity ratio, R, on the turbulent wake of a cylindrical stack of AR = 9 was investigated with two-component thermal anemometry. The cross-flow Reynolds number was ReD = 2.3×104, the jet Reynolds number ranged from Red = 7×103 to 4.6×104, and R was varied from 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-height ratio of δ/H = 0.5 at the location of the stack. The flow around the stack was broadly classified into three flow regimes depending on the value of R, which were the downwash (R < 0.5), cross-wind dominated (0.5 < R < 1.5), and jet-dominated (R > 1.5) regimes. Each flow regime had a distinct structure to the mean velocity (streamwise and wall-normal directions), turbulence intensity (streamwise and wall-normal directions), and Reynolds shear stress fields.

Direct simulations of low-Reynolds-number turbulent flow in a rotating channel

1993 ◽  
Vol 256 ◽  
pp. 163-197 ◽  
Author(s):  
Reidar Kristoffersen ◽  
Helge I. Andersson
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Stokes Equations ◽  
Navier Stokes ◽  
Complete Suppression ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Computational Mesh ◽  
Developed Pressure ◽  
Rotating Channel

Direct numerical simulations of fully developed pressure-driven turbulent flow in a rotating channel have been performed. The unsteady Navier–Stokes equations were written for flow in a constantly rotating frame of reference and solved numerically by means of a finite-difference technique on a 128 × 128 × 128 computational mesh. The Reynolds number, based on the bulk mean velocity Um and the channel half-width h, was about 2900, while the rotation number Ro = 2|Ω|h/Um varied from 0 to 0.5. Without system rotation, results of the simulation were in good agreement with the accurate reference simulation of Kim, Moin & Moser (1987) and available experimental data. The simulated flow fields subject to rotation revealed fascinating effects exerted by the Coriolis force on channel flow turbulence. With weak rotation (Ro = 0.01) the turbulence statistics across the channel varied only slightly compared with the nonrotating case, and opposite effects were observed near the pressure and suction sides of the channel. With increasing rotation the augmentation and damping of the turbulence along the pressure and suction sides, respectively, became more significant, resulting in highly asymmetric profiles of mean velocity and turbulent Reynolds stresses. In accordance with the experimental observations of Johnston, Halleen & Lezius (1972), the mean velocity profile exhibited an appreciable region with slope 2Ω. At Ro = 0.50 the Reynolds stresses vanished in the vicinity of the stabilized side, and the nearly complete suppression of the turbulent agitation was confirmed by marker particle trackings and two-point velocity correlations. Rotational-induced Taylor-Görtler-like counter-rotating streamwise vortices have been identified, and the simulations suggest that the vortices are shifted slightly towards the pressure side with increasing rotation rates, and the number of vortex pairs therefore tend to increase with Ro.

Separated and Reattached Flow Over Square, Rectangular and Semi-Circular Blocks

2007 ◽  
Author(s):  
M. Agelinchaab ◽  
M. F. Tachie
Keyword(s):  
Boundary Layer ◽  
Cross Sections ◽  
Recirculation Region ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Block Height ◽  
Turbulent Statistics ◽  
Image Velocimetry ◽  
The Mean ◽  
Separated And Reattached Flow

A particle image velocimetry is used to study the characteristics of separated and reattached turbulent flow over two-dimensional transverse blocks of square, rectangular and semi-circular cross-sections fixed to the bottom wall of an open channel. The ratio of upstream boundary layer thickness to block height is considerably higher than in prior studies. The results show that the mean and turbulent statistics in the recirculation region and downstream of reattachment are significantly different from the upstream boundary layer. The variation of the Reynolds stresses along the separating streamlines is discussed within the context of vortex stretching, longitudinal strain rate and wall damping. It appears wall damping is a more dominant mechanism in the vicinity of reattachment. The levels of turbulence diffusion and production by the normal stresses are significantly higher than in classical turbulent boundary layers. The bulk of turbulence production occurs in mid-layer and transported into the inner and outer layers. The results also reveal that the curvature of separating streamline, separating bubble beneath it as well as the mean velocity and turbulent quantities depend strongly on block geometry.

Scaling of Favorable Pressure Gradient Turbulent Boundary Layers With Eventual Relaminarization

2005 ◽  
Author(s):  
Rau´l Bayoa´n Cal ◽  
Xia Wang ◽  
Luciano Castillo
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Reynolds Shear Stress ◽  
Velocity Profiles ◽  
Turbulent Boundary Layers ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Favorable Pressure Gradient ◽  
The Mean

Applying similarity analysis to the RANS equations of motion for a pressure gradient turbulent boundary layer, Castillo and George [1] obtained the scalings for the mean deficit velocity and the Reynolds stresses. Following this analysis, Castillo and George studied favorable pressure gradient (FPG) turbulent boundary layers. They were able to obtain a single curve for FPG flows when scaling the mean deficit velocity profiles. In this study, FPG turbulent boundary layers are analyzed as well as relaminarized boundary layers subjected to an even stronger FPG. It is found that the mean deficit velocity profiles diminish when scaled using the Castillo and George [1] scaling, U∞, and the Zagarola and Smits [2] scaling, U∞δ*/δ. In addition, Reynolds stress data has been analyzed and it is found that the relaminarized boundary layer data decreases drastically in all components of the Reynolds stresses. Furthermore, it will be shown that the shape of the profile for the wall-normal and Reynolds shear stress components change drastically given the relaminarized state. Therefore, the mean velocity deficit profiles as well as Reynolds stresses are found to be necessary in order to understand not only FPG flows, but also relaminarized boundary layers.

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