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.

scholarly journals Wind Turbulence Statistics of the Atmospheric Inertial Sublayer under Near-Neutral Conditions

Atmosphere ◽  
2020 ◽  
Vol 11 (10) ◽  
pp. 1087
Author(s):  
Eslam Reda Lotfy ◽  
Zambri Harun
Keyword(s):  
Boundary Layer ◽  
Atmospheric Stability ◽  
Turbulent Velocity ◽  
Stability Conditions ◽  
Mean Velocity ◽  
Law Of The Wall ◽  
Turbulent Statistics ◽  
The Mean ◽  
The Law ◽  
Velocity Variances

The inertial sublayer comprises a considerable and critical portion of the turbulent atmospheric boundary layer. The mean windward velocity profile is described comprehensively by the Monin–Obukhov similarity theory, which is equivalent to the logarithmic law of the wall in the wind tunnel boundary layer. Similar logarithmic relations have been recently proposed to correlate turbulent velocity variances with height based on Townsend’s attached-eddy theory. The theory is particularly valid for high Reynolds-number flows, for example, atmospheric flow. However, the correlations have not been thoroughly examined, and a well-established model cannot be reached for all turbulent variances similar to the law of the wall of the mean-velocity. Moreover, the effect of atmospheric thermal condition on Townsend’s model has not been determined. In this research, we examined a dataset of free wind flow under a near-neutral range of atmospheric stability conditions. The results of the mean velocity reproduce the law of the wall with a slope of 2.45 and intercept of −13.5. The turbulent velocity variances were fitted by logarithmic profiles consistent with those in the literature. The windward and crosswind velocity variances obtained the average slopes of −1.3 and −1.7, respectively. The slopes and intercepts generally increased away from the neutral state. Meanwhile, the vertical velocity and temperature variances reached the ground-level values of 1.6 and 7.8, respectively, under the neutral condition. The authors expect this article to be a groundwork for a general model on the vertical profiles of turbulent statistics under all atmospheric stability conditions.

scholarly journals Effects of Rear Angle on the Turbulent Wake Flow between Two in-Line Ahmed Bodies

Atmosphere ◽  
2020 ◽  
Vol 11 (4) ◽  
pp. 328
Author(s):  
Ebenezer Essel ◽  
Subhadip Das ◽  
Ram Balachandar
Keyword(s):  
Wake Flow ◽  
Recirculation Region ◽  
Wake Region ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Slant Angle ◽  
The Road ◽  
Rear Angle ◽  
Turbulent Statistics ◽  
The Mean

Understanding the wake characteristics between two in-line vehicles is essential for improving and developing new strategies for reducing in-cabin air pollution. In this study, Ahmed bodies are used to investigate the effects of the rear slant angle of a leading vehicle on the mean flow and turbulent statistics between two vehicles. The experiments were conducted with a particle image velocimetry at a fixed Reynolds number, R e H = 1.7 × 10 4 , and inter-vehicle spacing distance of 0.75 L , where H and L are the height and length of the model. The rear slant angles investigated were a reference square back, high-drag angle ( α = 25 ° ) and low-drag angle ( α = 35 ° ). The mean velocities, Reynolds stresses, production of turbulent kinetic energy and instantaneous swirling strength are used to provide physical insight into the wake dynamics between the two bodies. The results indicate that the recirculation region behind the square back Ahmed body increases while those behind the slant rear-end bodies decreases in the presence of a follower. For the square back models, the dominant motion in the wake region is a strong upwash of jet-like flow away from the road but increasing the rear slant angle induces a stronger downwash flow that suppresses the upwash and dominates the wake region.

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.

Measurement of the Reynolds stresses and the mean-flow field in a three-dimensional pressure-driven boundary layer

1982 ◽  
Vol 119 ◽  
pp. 121-153 ◽  
Author(s):  
Udo R. Müller
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Flow Field ◽  
Three Dimensional ◽  
Turbulent Shear ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Mean Velocity ◽  
Flow Acceleration ◽  
The Mean

An experimental study of a steady, incompressible, three-dimensional turbulent boundary layer approaching separation is reported. The flow field external to the boundary layer was deflected laterally by turning vanes so that streamwise flow deceleration occurred simultaneous with cross-flow acceleration. At 21 stations profiles of the mean-velocity components and of the six Reynolds stresses were measured with single- and X-hot-wire probes, which were rotatable around their longitudinal axes. The calibration of the hot wires with respect to magnitude and direction of the velocity vector as well as the method of evaluating the Reynolds stresses from the measured data are described in a separate paper (Müller 1982, hereinafter referred to as II). At each measuring station the wall shear stress was inferred from a Preston-tube measurement as well as from a Clauser chart. With the measured profiles of the mean velocities and of the Reynolds stresses several assumptions used for turbulence modelling were checked for their validity in this flow. For example, eddy viscosities for both tangential directions and the corresponding mixing lengths as well as the ratio of resultant turbulent shear stress to turbulent kinetic energy were derived from the data.

Characterizing Surface Roughness Effects on Supersonic Turbulent Boundary Layers

2021 ◽  
Author(s):  
Rozie Zangeneh
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Practical Importance ◽  
Rough Wall ◽  
Reynolds Stresses ◽  
Roughness Effects ◽  
Mean Velocity ◽  
Modeling Tools ◽  
Turbulent Statistics ◽  
The Impact

Abstract Flight vehicles traveling at supersonic or hypersonic speeds are vulnerable to the onset of surface roughness, which can result in changes in the state of the boundary layer, ultimately affecting the performance of the vehicle. While the majority of the wetted surface area of a vehicle is relatively smooth, every vehicle will contain roughness on some level. The concept of similarity between smooth- and rough-wall flows is of great practical importance as most computational and analytical modeling tools rely on it either explicitly or implicitly in predicting flows over rough walls. While a number of important questions have yet to be answered, significant progress has been made in the understanding of flows over rough surfaces in recent years. This paper will be conducting numerical research in rough-wall-bounded turbulent flows in supersonic regimes. Wall-modeled Large Eddy Simulation (WMLES) on a flat plate with various roughness ratios will be conducted at M∞ = 2 to evaluate the boundary layer responses. These responses will be characterized in ensemble averaged mean velocity characteristics as well as turbulent intensity responses through the Reynolds Stresses. The second goal is to characterize the streamwise development of mechanical distortions in the domain. In addition, the near-wall coherent structures will be analyzed to determine the impact of roughness effects. The mean and turbulent statistics scaled by the roughness friction velocity will be compared to other results.

PIV Study of Separated and Reattached Open Channel Flow Over Surface Mounted Blocks

2008 ◽  
Vol 130 (6) ◽  
Author(s):  
Martin Agelinchaab ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Channel Flow ◽  
Open Channel ◽  
Open Channel Flow ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Mean Velocity ◽  
High Background ◽  
The Mean

A particle image velocimetry is used to study the mean and turbulent fields of separated and redeveloping flow over square, rectangular, and semicircular blocks fixed to the bottom wall of an open channel. The open channel flow is characterized by high background turbulence level, and the ratio of the upstream boundary layer thickness to block height is considerably higher than in prior experiments. The variation of the Reynolds stresses along the dividing streamlines is discussed within the context of vortex stretching, longitudinal strain rate, and wall damping. It appears that wall damping is a more dominant mechanism in the vicinity of reattachment. In the recirculation and reattachment regions, profiles of the mean velocity, turbulent quantities, and transport terms are used to document the salient features of block geometry on the flow. The flow characteristics in these regions strongly depend on block geometry. Downstream of reattachment, a new shear layer is formed, and the redevelopment of the shear layer toward the upstream open channel boundary layer is studied using the boundary layer parameters and Reynolds stresses. The results show that the mean flow rapidly redeveloped so that the Clauser parameter recovered to its upstream value at 90 step heights downstream of reattachment. However, the rate of development close to reattachment strongly depends on block geometry.

LDA Measurements on Rough Turbulent Boundary Layers Subject to Strong Favorable Pressure Gradients

2006 ◽  
Author(s):  
Rau´l Bayoa´n Cal ◽  
Brian Brzek ◽  
Gunnar Johansson ◽  
Luciano Castillo
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Rough Surface ◽  
Reynolds Stress ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Velocity Deficit ◽  
The Mean ◽  
Stress Profiles

Laser-Doppler anemometry (LDA) measurements of the mean velocity and Reynolds stresses are carried out on a rough surface favorable pressure gradient (FPG) turbulent boundary layer. These data is compared with smooth FPG turbulent boundary layer data possessing with the same strength of pressure gradient and also with rough zero pressure gradient (ZPG) data. The scales for the mean velocity deficit and Reynolds stresses are obtained through means of equilibrium similarity analysis of the RANS equations [1]. The mean velocity deficit profiles collapse, but to different curves when normalized using the free-stream velocity. The effects of the pressure gradient and roughness are clearly distinguished and separated. However, these effects are removed from the outer flow when the profiles are normalized using the Zagarola and Smits [2] scaling. It is also found that there is a clear effect of the roughness and pressure gradient on the Reynolds stresses. The Reynolds stress profiles augment due to the rough surface. Furthermore, the strength of the pressure gradient imposed of the flow changes the shape of the Reynolds stress profiles especially on the < v2 > and < uv > components. The rough surface influence is mostly noticed on the < u2 > component of the Reynolds stress, where the shape of the profiles change entirely. The boundary layer parameter δ*/δ shows the effects of the roughness and a dependence on the Reynolds number for the smooth FPG case. The pressure parameter, A, describes a development of the turbulent boundary layer and no influence of the roughness is linked with the parameter, k+. The boundary layers grow differently and depict the influence of the studied effects in their development. These measurements are the first of their nature due to the extensive number in downstream locations (12) and the combination of the studied external conditions (i.e., the strength of the pressure gradient and the surface roughness).

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.

LDA-Measurements of Transitional Flows Induced by a Square Rib

2001 ◽  
Vol 124 (1) ◽  
pp. 108-117 ◽  
Author(s):  
S. Becker ◽  
C. M. Stoots ◽  
K. G. Condie ◽  
F. Durst ◽  
D. M. McEligot
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Boundary Layers ◽  
Laser Doppler Anemometry ◽  
Plate Boundary ◽  
Index Of Refraction ◽  
Wall Region ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Turbulent Statistics

New fundamental measurements are presented for the transition process in flat plate boundary layers downstream of two-dimensional square ribs. By use of laser Doppler anemometry (LDA) and a large Matched-Index-of-Refraction (MIR) flow system, data for wall-normal fluctuations and Reynolds stresses were obtained in the near wall region to y+<0.1 in addition to the usual mean streamwise velocity component and its fluctuation. By varying velocity and rib height, the experiment investigated the following range of conditions: k+=5.5 to 21, 0.3<k/δ1<1,180<Rek<740,6×104<Rex,k<1.5×105,ReΘ660,−125<x−xk/k<580. Consequently, results covered boundary layers which retained their laminar characteristics through those where a turbulent boundary layer was established shortly after reattachment beyond the forcing rib. For “large” elements, evolution of turbulent statistics of the viscous layer for a turbulent boundary layer y+<∼30 was rapid even in flows where the mean velocity profile still showed laminar behavior.

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.

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