PIV-POD Investigation of the Wake of a Sharp-Edged Flat Bluff Body Immersed in a Shallow Channel Flow

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Arindam Singha ◽  
A.-M. Shinneeb ◽  
Ram Balachandar
Keyword(s):  
Velocity Field ◽  
Channel Flow ◽  
Bluff Body ◽  
Maximum Flow ◽  
Velocity Fields ◽  
Central Plane ◽  
Mean Flow ◽  
Mean Velocity ◽  
Near Surface ◽  
The Mean

This paper reports particle-image velocimetry measurements of instantaneous velocity fields in the wake of a sharp-edged bluff body immersed vertically in a shallow smooth open channel flow. The maximum flow velocity was 0.19 m/s and the Reynolds number based on the water depth was 18,270. The purpose of the present study is to show the vertical variation of the velocity field in the near region of a shallow wake. Measurements of the flow field in the vertical central plane and in the horizontal near-bed, mid-depth, and near-surface planes were taken. Then, the mean flow quantities such as the mean velocity, turbulence intensity, and Reynolds stress fields were investigated. In addition, the proper orthogonal decomposition technique was used to reconstruct the velocity fields to investigate the energetic vortical structures. The results showed that the largest recirculation zone in the mean velocity fields occurred in the mid-depth velocity field, while the smallest one occurred near the bed. Also, the fluid was entrained from the sides toward the wake central plane in the three horizontal velocity fields but with different rates. This behavior was attributed to the existence of quasi-streamwise vortices near the boundaries. In addition, patterns of ejection and sweep events near the free surface similar to the features commonly observed near the wall-bounded flows were observed.

Three-dimensional structure and momentum transfer in a turbulent cylinder wake

1999 ◽  
Vol 394 ◽  
pp. 303-337 ◽  
Author(s):  
A. VERNET ◽  
G. A. KOPP ◽  
J. A. FERRÉ ◽  
FRANCESC GIRALT
Keyword(s):  
Velocity Field ◽  
Saddle Point ◽  
Three Dimensional ◽  
Symmetry Plane ◽  
Half Width ◽  
Small Scale ◽  
Spanwise Direction ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Simultaneous velocity and temperature measurements were made with rakes of sensors that sliced a slightly heated turbulent wake in the spanwise direction, at different lateral positions 150 diameters downstream of the cylinder. A pattern recognition analysis of hotter-to-colder transitions was performed on temperature data measured at the mean velocity half-width. The velocity data from the different ‘slices’ was then conditionally averaged based on the identified temperature events. This procedure yielded the topology of the average three-dimensional large-scale structure which was visualized with iso-surfaces of negative values of the second eigenvector of [S2+Ω2]. The results indicate that the average structure of the velocity fluctuations (using a triple decomposition of the velocity field) is found to be a shear-aligned ring-shaped vortex. This vortex ring has strong outward lateral velocities in its symmetry plane which are like Grant's mixing jets. The mixing jet region extends outside the ring-like vortex and is bounded by two foci separated in the spanwise direction and an upstream saddle point. The two foci correspond to what has been previously identified in the literature as the double rollers.The ring vortex extracts energy from the mean flow by stretching in the mixing jet region just upstream of the ring boundary. The production of the small-scale (incoherent) turbulence by the coherent field and one-component energy dissipation rate occur just downstream of the saddle point within the mixing jet region. Incoherent turbulence energy is extracted from the mean flow just outside the mixing jet region, but within the core of the structure. These processes are highly three-dimensional with a spanwise extent equal to the mean velocity half-width.When a double decomposition is used, the coherent structure is found to be a tube-shaped vortex with a spanwise extent of about 2.5l0. The double roller motions are integral to this vortex in spite of its shape. Spatial averages of the coherent velocity field indicate that the mixing jet region causes a deficit of mean streamwise momentum, while the region outside the foci of the double rollers has a relatively small excess of streamwise momentum.

Numerical study of turbulent magnetohydrodynamic channel flow

2007 ◽  
Vol 572 ◽  
pp. 179-188 ◽  
Author(s):  
THOMAS BOECK ◽  
DMITRY KRASNOV ◽  
EGBERT ZIENICKE
Keyword(s):  
Channel Flow ◽  
Numerical Study ◽  
Layer Structure ◽  
Momentum Balance ◽  
Mean Flow ◽  
Mean Velocity ◽  
Static Approximation ◽  
The Mean ◽  
Magnetohydrodynamic Channel ◽  
Normal Magnetic Field

Mean flow properties of turbulent magnetohydrodynamic channel flow with electrically insulating channel walls are studied using high-resolution direct numerical simulations. The Lorentz force due to the homogeneous wall-normal magnetic field is computed in the quasi-static approximation. For strong magnetic fields, the mean velocity profile shows a clear three-layer structure consisting of a viscous region near each wall and a plateau in the middle connected by logarithmic layers. This structure reflects the significance of viscous, turbulent, and electromagnetic stresses in the streamwise momentum balance dominating the viscous, logarithmic, and plateau regions, respectively. The width of the logarithmic layers changes with the ratio of Reynolds- and Hartmann numbers. Turbulent stresses typically decay more rapidly away from the walls than predicted by mixing-length models.

Near-surface particle image velocimetry measurements in a transitionally rough-wall atmospheric boundary layer

2007 ◽  
Vol 580 ◽  
pp. 319-338 ◽  
Author(s):  
SCOTT C. MORRIS ◽  
SCOTT R. STOLPA ◽  
PAUL E. SLABOCH ◽  
JOSEPH C. KLEWICKI
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Environmental Science ◽  
Particle Image ◽  
Shear Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
Near Surface ◽  
Image Velocimetry ◽  
The Mean

The Reynolds number dependence of the structure and statistics of wall-layer turbulence remains an open topic of research. This issue is considered in the present work using two-component planar particle image velocimetry (PIV) measurements acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility in western Utah. The Reynolds number (δuτ/ν) was of the order 106. The surface was flat with an equivalent sand grain roughness k+ = 18. The domain of the measurements was 500 < yuτ/ν < 3000 in viscous units, 0.00081 < y/δ < 0.005 in outer units, with a streamwise extent of 6000ν/uτ. The mean velocity was fitted by a logarithmic equation with a von Kármán constant of 0.41. The profile of u′v′ indicated that the entire measurement domain was within a region of essentially constant stress, from which the wall shear velocity was estimated. The stochastic measurements discussed include mean and RMS profiles as well as two-point velocity correlations. Examination of the instantaneous vector maps indicated that approximately 60% of the realizations could be characterized as having a nearly uniform velocity. The remaining 40% of the images indicated two regions of nearly uniform momentum separated by a thin region of high shear. This shear layer was typically found to be inclined to the mean flow, with an average positive angle of 14.9°.

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.

scholarly journals Application of entropic approach to estimate the mean flow velocity and Manning roughness coefficient in a high-curvature flume

2016 ◽  
Vol 48 (3) ◽  
pp. 634-645 ◽  
Author(s):  
D. Termini ◽  
T. Moramarco
Keyword(s):  
Flow Velocity ◽  
Maximum Flow ◽  
Roughness Coefficient ◽  
Mean Flow ◽  
Mean Velocity ◽  
Bed Deformation ◽  
Side Walls ◽  
The Mean ◽  
Manning Roughness ◽  
Mean Flow Velocity

The entropy-based approach allows the estimation of the mean flow velocity in open channel flow by using the maximum flow velocity. The linear relationship between the mean velocity, umax, and the mean flow velocity, um, through the dimensionless parameter Φ(M), has been verified both in natural rivers and in laboratory channels. Recently, the authors of this study investigated the reliability of the entropy-based formula in a straight channel and under different bed and side-walls' roughness conditions. The present study aims to further validate the entropy-based approach and to explore the effectiveness of entropy-based formula in high curvature channels. Results show that as the effect of the downstream variation of the channel's curvature the value of the parameter Φ(M) varies along the bend. When the bed deformation is evident, the variation of the parameter Φ(M) is strongly reduced compared to that obtained in absence of bed deformation. Results also show that the Manning's roughness coefficients determined through entropy-based formula are in agreement with those estimated by applying other literature's expressions but, unlike the latter, through the parameter Φ(M) the entropy-based formula could account for the effects due to the advective momentum transport by cross-circulation along the strongly curved reaches of the channel.

Velocity profiles assessment in natural channels during high floods

2011 ◽  
Vol 42 (2-3) ◽  
pp. 162-170 ◽  
Author(s):  
Tommaso Moramarco ◽  
Carla Saltalippi ◽  
Vijay P. Singh
Keyword(s):  
Flow Velocity ◽  
Maximum Flow ◽  
Velocity Profiles ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Flow Area ◽  
The Mean ◽  
Logarithmic Law ◽  
Maximum Flow Velocity

The accuracy of three different approaches for velocity profiles assessment during high floods, when the velocity points sampling is carried out only in the upper portion of the flow area, has been investigated. The first two methods assume the classical logarithmic law with additional terms, to take account of the dip-phenomenon in the velocity profile. The third one is based on the entropy theory and uses the maximum flow velocity occurring in the flow area. A sample of velocity measurements carried out at Pontelagoscuro gauged section (Po River, Italy), has been considered for the analysis. Six flood events have been selected and the accuracy of the investigated methods has been evaluated in terms of mean error in estimating both the mean velocity along each sampled vertical and the mean flow velocity. For high floods, the logarithmic law and the entropic approach were found quite accurate; however, the ability of the latter in reproducing the velocity profiles only by sampling the maximum flow velocity has been shown. Therefore, a procedure for velocity measurements based on the entropic approach has been proposed. The procedure allows one to both to shorten remarkably the time of the velocity sampling and to quickly estimate the discharge.

Passage Flow Structure and Its Influence on Endwall Heat Transfer in a 90 deg Turning Duct: Mean Flow and High Resolution Endwall Heat Transfer Experiments

1997 ◽  
Vol 119 (1) ◽  
pp. 39-50 ◽  
Author(s):  
B. G. Wiedner ◽  
C. Camci
Keyword(s):  
Heat Transfer ◽  
High Resolution ◽  
Velocity Field ◽  
Cross Section ◽  
Three Dimensional ◽  
Mean Flow ◽  
Mean Velocity ◽  
Pressure Fields ◽  
The Mean ◽  
Three Components

Three-dimensional measurements of the mean velocity field have been made in a square-cross-sectional, strongly curved, 90 deg turbulent duct flow. The mean radius to duct width ratio was 2.3. The study was performed as part of an overall investigation of the physics of endwall convective heat transfer. All three components of the velocity vector and the static and total pressure fields were measured using a five-hole probe at four duct cross sections: inlet, 0, 45, and 90 deg. Preliminary turbulence measurements using a single sensor hot wire at the inlet cross section were also obtained to provide streamwise fluctuation levels through the boundary layer. The endwall heat transfer coefficient distribution was determined using a steady-state measurement technique and liquid crystal thermography. A high-resolution heat transfer map of the endwall surface from far upstream of the curve through the 90 deg cross section is presented. The three-dimensional velocity field measurements indicate that a highly symmetric, strong secondary flow develops in the duct with a significant transfer of streamwise momentum to the transverse directions. The cross-stream vorticity components within the measurement plane were estimated using the five-hole probe data and an inviscid from of the incompressible momentum equation. The development of the total and static pressure fields, the three-dimensional mean velocity field, and all three components of the vorticity field are discussed. The endwall heat transfer distribution is interpreted with respect to the measured mean flow quantities.

The structure of a highly decelerated axisymmetric turbulent boundary layer

2021 ◽  
Vol 929 ◽  
Author(s):  
N. Agastya Balantrapu ◽  
Christopher Hickling ◽  
W. Nathan Alexander ◽  
William Devenport
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Large Scale ◽  
Convection Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Experiments were performed over a body of revolution at a length-based Reynolds number of 1.9 million. While the lateral curvature parameters are moderate ( $\delta /r_s < 2, r_s^+>500$ , where $\delta$ is the boundary layer thickness and r s is the radius of curvature), the pressure gradient is increasingly adverse ( $\beta _{C} \in [5 \text {--} 18]$ where $\beta_{C}$ is Clauser’s pressure gradient parameter), representative of vehicle-relevant conditions. The mean flow in the outer regions of this fully attached boundary layer displays some properties of a free-shear layer, with the mean-velocity and turbulence intensity profiles attaining self-similarity with the ‘embedded shear layer’ scaling (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592–642). Spectral analysis of the streamwise turbulence revealed that, as the mean flow decelerates, the large-scale motions energize across the boundary layer, growing proportionally with the boundary layer thickness. When scaled with the shear layer parameters, the distribution of the energy in the low-frequency region is approximately self-similar, emphasizing the role of the embedded shear layer in the large-scale motions. The correlation structure of the boundary layer is discussed at length to supply information towards the development of turbulence and aeroacoustic models. One major finding is that the estimation of integral turbulence length scales from single-point measurements, via Taylor's hypothesis, requires significant corrections to the convection velocity in the inner 50 % of the boundary layer. The apparent convection velocity (estimated from the ratio of integral length scale to the time scale), is approximately 40 % greater than the local mean velocity, suggesting the turbulence is convected much faster than previously thought. Closer to the wall even higher corrections are required.

scholarly journals Global energy fluxes in turbulent channels with flow control

2018 ◽  
Vol 857 ◽  
pp. 345-373 ◽  
Author(s):  
Davide Gatti ◽  
Andrea Cimarelli ◽  
Yosuke Hasegawa ◽  
Bettina Frohnapfel ◽  
Maurizio Quadrio
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Energy Dissipation ◽  
Velocity Field ◽  
Flow Control ◽  
Reynolds Shear Stress ◽  
Power Input ◽  
Energy Fluxes ◽  
Mean Velocity ◽  
The Mean

This paper addresses the integral energy fluxes in natural and controlled turbulent channel flows, where active skin-friction drag reduction techniques allow a more efficient use of the available power. We study whether the increased efficiency shows any general trend in how energy is dissipated by the mean velocity field (mean dissipation) and by the fluctuating velocity field (turbulent dissipation). Direct numerical simulations (DNS) of different control strategies are performed at constant power input (CPI), so that at statistical equilibrium, each flow (either uncontrolled or controlled by different means) has the same power input, hence the same global energy flux and, by definition, the same total energy dissipation rate. The simulations reveal that changes in mean and turbulent energy dissipation rates can be of either sign in a successfully controlled flow. A quantitative description of these changes is made possible by a new decomposition of the total dissipation, stemming from an extended Reynolds decomposition, where the mean velocity is split into a laminar component and a deviation from it. Thanks to the analytical expressions of the laminar quantities, exact relationships are derived that link the achieved flow rate increase and all energy fluxes in the flow system with two wall-normal integrals of the Reynolds shear stress and the Reynolds number. The dependence of the energy fluxes on the Reynolds number is elucidated with a simple model in which the control-dependent changes of the Reynolds shear stress are accounted for via a modification of the mean velocity profile. The physical meaning of the energy fluxes stemming from the new decomposition unveils their inter-relations and connection to flow control, so that a clear target for flow control can be identified.

A wind tunnel simulation of the mean velocity fields behind upright porous fences

2007 ◽  
Vol 146 (1-2) ◽  
pp. 82-93 ◽  
Author(s):  
Zhibao Dong ◽  
Wanyin Luo ◽  
Guangqiang Qian ◽  
Hongtao Wang
Keyword(s):  
Wind Tunnel ◽  
Velocity Fields ◽  
Mean Velocity ◽  
Wind Tunnel Simulation ◽  
The Mean
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