Roughness Effect Downstream of Flow Over a Forward Facing Step

2014 ◽  
Author(s):  
Yaw Y. Afriyie ◽  
Ebenezer E. Essel ◽  
Eric W. Thacher ◽  
Mark F. Tachie
Keyword(s):  
Shear Stress ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Reynolds Shear Stress ◽  
Recirculation Region ◽  
Roughness Effects ◽  
Mean Velocity ◽  
Freestream Velocity ◽  
Image Velocimetry ◽  
Forward Facing Step

This paper presents results of an experimental research conducted to study roughness effects downstream of a forward facing step (FFS). A rough surface and a hydraulically smooth surface were used as a rough-FFS and a smooth-FFS, respectively. The upstream condition was kept smooth. Particle image velocimetry (PIV) technique was used for the velocity measurements. The Reynolds number based on the step height (h) and freestream velocity of the approach flow was kept constant at 8685. The results show that the mean reattachment length for the smooth-FFS (SM-SM) is 1.9h. Roughness reduced the peak values of the streamwise mean velocity, Reynolds shear stress and turbulent kinetic energy by 3%, 45% and 16.7% respectively in the recirculation region. In the early redevelopment region, roughness also reduced the peak values of turbulent kinetic energy and the Reynolds shear stress by 41% and 22% respectively.

PIV Investigation of Separated and Reattached Turbulent Flows Over Ribs of Various Aspect Ratio

2014 ◽  
Author(s):  
Kathryn M. Atamanchuk ◽  
Mark F. Tachie
Keyword(s):  
Kinetic Energy ◽  
Aspect Ratio ◽  
Turbulent Kinetic Energy ◽  
Turbulent Flows ◽  
Reynolds Shear Stress ◽  
Separated Flow ◽  
Particle Image Velocimetry System ◽  
Shear Stresses ◽  
Mean Velocity ◽  
Freestream Velocity

An experimental study is undertaken to investigate the features of separated and reattached flow over surface mounted traverse ribs of varying aspect ratio (1:1, 1:2, and 1:4) in a recirculating open channel turbulent flow. A particle image velocimetry system was used to conduct the velocity measurements. Upstream conditions were kept consistent among all three test cases. The reattachment length of the separated flow was found to decrease as rib aspect ratio increased, primarily as a result of a secondary separation reattachment formation on the ribs of increased aspect ratio. Contour plots of mean velocities, turbulence intensities, turbulent kinetic energy and Reynolds shear stresses, as well as one-dimensional profiles of streamwise mean velocity, turbulent kinetic energy and Reynolds shear stress in the recirculation and reattachment region are presented and discussed. The results show that maximum wall-normal mean velocities are approximately 40% of the approach freestream velocity. The results also indicate that the turbulence levels downstream of the block tend to decrease as the rib aspect ratio increases.

Large-scale modes of turbulent channel flow: transport and structure

2001 ◽  
Vol 448 ◽  
pp. 53-80 ◽  
Author(s):  
Z. LIU ◽  
R. J. ADRIAN ◽  
T. J. HANRATTY
Keyword(s):  
Shear Stress ◽  
Kinetic Energy ◽  
Shear Layer ◽  
Turbulent Kinetic Energy ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Reynolds Numbers ◽  
Upstream Region ◽  
Two Dimensional ◽  
Normal Plane

Turbulent flow in a rectangular channel is investigated to determine the scale and pattern of the eddies that contribute most to the total turbulent kinetic energy and the Reynolds shear stress. Instantaneous, two-dimensional particle image velocimeter measurements in the streamwise-wall-normal plane at Reynolds numbers Reh = 5378 and 29 935 are used to form two-point spatial correlation functions, from which the proper orthogonal modes are determined. Large-scale motions – having length scales of the order of the channel width and represented by a small set of low-order eigenmodes – contain a large fraction of the kinetic energy of the streamwise velocity component and a small fraction of the kinetic energy of the wall-normal velocities. Surprisingly, the set of large-scale modes that contains half of the total turbulent kinetic energy in the channel, also contains two-thirds to three-quarters of the total Reynolds shear stress in the outer region. Thus, it is the large-scale motions, rather than the main turbulent motions, that dominate turbulent transport in all parts of the channel except the buffer layer. Samples of the large-scale structures associated with the dominant eigenfunctions are found by projecting individual realizations onto the dominant modes. In the streamwise wall-normal plane their patterns often consist of an inclined region of second quadrant vectors separated from an upstream region of fourth quadrant vectors by a stagnation point/shear layer. The inclined Q4/shear layer/Q2 region of the largest motions extends beyond the centreline of the channel and lies under a region of fluid that rotates about the spanwise direction. This pattern is very similar to the signature of a hairpin vortex. Reynolds number similarity of the large structures is demonstrated, approximately, by comparing the two-dimensional correlation coefficients and the eigenvalues of the different modes at the two Reynolds numbers.

PIV Investigation of an Open Channel Flow Over a Forward Facing Step

2007 ◽  
Author(s):  
M. K. Shah ◽  
M. F. Tachie
Keyword(s):  
Open Channel ◽  
Particle Image ◽  
Open Channel Flow ◽  
Reynolds Stresses ◽  
Velocity Measurements ◽  
Particle Image Velocimetry Technique ◽  
Mean Velocity ◽  
Freestream Velocity ◽  
Image Velocimetry ◽  
Forward Facing Step

The characteristics of an open channel turbulent flow over a forward facing step (FFS) are investigated in the present study. Two step heights, h = 6 and 9 mm, at Reynolds number, Reh, (based on the approach freestream velocity, U0, and step height, h) of 1900 and 2800 respectively were studied. Particle image velocimetry technique (PIV) was used to obtain detailed velocity measurements upstream of the FFS, in the reattachment region (x/h = 0, 1, 2) and in the redevelopment region (x/h = 4, 10, 15 and 50). The boundary layer integral parameters, mean velocity profiles and Reynolds stresses obtained in the reattachment and redevelopment region are used to document some of the salient features of the flow.

Turbulence Measurements in Channel Flows With Rough and Hydrophobic Walls

2007 ◽  
Author(s):  
N. Ahmad ◽  
R. N. Parthasarathy
Keyword(s):  
Shear Stress ◽  
Rough Surface ◽  
Test Section ◽  
Reynolds Shear Stress ◽  
Turbulent Channel Flow ◽  
Surface Flow ◽  
Wall Region ◽  
Mean Velocity ◽  
Image Velocimetry ◽  
Coated Surface

Particle Image Velocimetry (PIV) measurements were made in a fully-developed turbulent channel flow. The channel test section was 1 ft wide and 1 inch in height and was constructed out of plexiglass. One wall of the test section was made removable. Four walls were used: a plexiglass smooth wall, and three hydrophobic walls: (i) a lotus paint coated plexiglass wall, (ii) a treated aluminum sheet attached to the plexiglass wall and (iii) a treated rough surface attached to the plexiglass wall. The bulk velocity was held constant to yield a Reynolds number (based on the channel half-height) of 5,500. Several images were averaged to obtain mean velocity and Reynolds shear stress and turbulence kinetic energy measurements. It was found that the mean velocities in the near-wall region were higher for the lotus-paint coated surface flow and the treated rough surface flow than the flows with the other two surfaces. The friction velocity estimated from the Reynolds shear stress measurements was significantly lower for these two flows as well. The reduction in the wall shear stress in these flows is attributed to the finite slip that occurs at the hydrophobic surfaces.

scholarly journals Experimental Analysis of Horizontal Turbulence of Flow over Flat and Deformed Beds

2015 ◽  
Vol 62 (3-4) ◽  
pp. 77-99 ◽  
Author(s):  
Donatella Termini
Keyword(s):  
Shear Stress ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Experimental Analysis ◽  
Large Scale ◽  
Laboratory Experiments ◽  
Reynolds Shear Stress ◽  
Turbulent Structures ◽  
Quadrant Method ◽  
Initial Sequence

AbstractLaboratory experiments in a straight flume were carried out to examine the evolution of large-scale horizontal turbulent structures under flat-bed and deformed-bed conditions. In this paper, the horizontal turbulence of flow under these conditions is analyzed and compared. The conditioned quadrant method is applied to verify the occurrence of turbulent events. The distributions of horizontal Reynolds shear stress and turbulent kinetic energy are also presented and discussed. Results show the occurrence of an “initial” sequence of horizontal vortices whose average spatial length scales with the channel width. Under deformed-bed conditions, this spatial length does not change.

PIV Measurements of Turbulent Flow in a Channel With Solid or Perforated Ribs

2010 ◽  
Author(s):  
Lei Wang ◽  
Mirko Salewski ◽  
Bengt Sunde´n
Keyword(s):  
Shear Stress ◽  
Particle Image ◽  
Reynolds Shear Stress ◽  
Area Ratio ◽  
Shear Stresses ◽  
Open Area ◽  
Reattachment Length ◽  
Mean Velocity ◽  
Piv Measurements ◽  
Image Velocimetry

Particle image velocimetry measurements are performed in a channel with periodic ribs on one wall. We investigate the flow around two different rib configurations: solid and perforated ribs with a slit. The ribs obstruct the channel by 20% of its height and are arranged 10 rib heights apart. For the perforated ribs, the slit height is 20% of the rib height, and the open-area ratio is 16%. We discuss the flow in terms of mean velocity, streamlines, vorticity, turbulence intensity, and Reynolds shear stress. We find that the recirculation bubbles after the perforated ribs are significantly smaller than those after the solid ribs. The reattachment length after perforated ribs is smaller by about 45% compared with the solid ribs. In addition, the Reynolds shear stresses around the perforated ribs are significantly smaller than in the solid rib case, leading to a reduction of the pressure loss in the perforated rib case.

Turbulent Flow Over Cavities in an Asymmetric Diverging Channel

2007 ◽  
Author(s):  
M. K. Shah ◽  
M. F. Tachie
Keyword(s):  
Turbulent Flow ◽  
Reynolds Shear Stress ◽  
Adverse Pressure Gradient ◽  
Particle Image Velocimetry Technique ◽  
Mean Velocity ◽  
Freestream Velocity ◽  
Aspect Ratios ◽  
Image Velocimetry ◽  
Diverging Channel ◽  
Single Cavity

An experimental investigation of turbulent flow over a single cavity in an asymmetric diverging channel is presented. Cavities of two different aspect ratios, w/h = 1 and 4, of height h = 6 mm were studied. The Reynolds number based on the approach freestream velocity and cavity height was Reh = 5000. Particle image velocimetry technique (PIV) was used to conduct detailed velocity measurements upstream of the cavity, inside the cavity and as far downstream of the cavity as x/h = 50. Mean velocity, turbulent intensities and Reynolds shear stress were obtained to document the effect of the cavity on the flow in the presence of adverse pressure gradient.

Experimental Study of Reynolds Number Effects on Three-Dimensional Offset Jets

2014 ◽  
Author(s):  
Zacharie M. J. Durand ◽  
Shawn P. Clark ◽  
Mark F. Tachie ◽  
Jarrod Malenchak ◽  
Getnet Muluye
Keyword(s):  
Experimental Study ◽  
Reynolds Number ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Shear Stresses ◽  
Mean Velocity ◽  
Flow Measurements ◽  
Reynolds Number Effects

The effect of Reynolds number on three-dimensional offset jets was investigated in this study. An acoustic Doppler velocimeter simultaneously measured all three components of velocity, U, V and W, and turbulence intensity, urms, vrms, and wrms, and all three Reynolds shear stresses, uv, uw, and vw. Turbulent kinetic energy, k, was calculated with all three values of turbulence intensities. Flow measurements were performed at Reynolds numbers of 34,000, 53,000 and 86,000. Results of this experimental study indicate the wall-normal location of maximum mean velocity and jet spread to be independent of Reynolds number. The effects on maximum mean velocity decay are reduced with increasing Reynolds number. Profiles of mean velocities, U, V and W, turbulence intensities, urms, vrms, and wrms, and turbulent kinetic energy, k, show independence of Reynolds number. Reynolds shear stress uv was independent of Reynolds number while the magnitude of uw was reduced at higher Reynolds number.

Structure of Turbulence Within a Sheared Wake of a Rotor Blade

2006 ◽  
Author(s):  
Francesco Soranna ◽  
Yi-Chih Chow ◽  
Oguz Uzol ◽  
Joseph Katz
Keyword(s):  
Shear Stress ◽  
Kinetic Energy ◽  
Flow Field ◽  
Turbulent Kinetic Energy ◽  
Production Rate ◽  
Rotor Blade ◽  
Energy Production ◽  
Reynolds Shear Stress ◽  
Suction Side ◽  
Velocity Fluctuations

Stereoscopic PIV measurements examine the flow structure and turbulence within a rotor near wake located within a non-uniform field generated by a row of Inlet Guide Vanes (IGVs). The experiments are performed in a refractive index matched facility that provides unobstructed view of the entire flow field. The data are acquired at 10 closely spaced radial planes located near mid-span, enabling measurements of all the components of the phase averaged velocity and strain rate, as well as the Reynolds stress and the triple correlation tensors. The rotor wake is sheared and bent towards the pressure (inner) side by a non-uniform flow field generated by IGV wake segments that propagate along the suction and pressure sides of the rotor passage with different speeds. The axial velocity fluctuations increase along the suction/outer side of the wake, while the other components decay. On the pressure/inner part of the bent wake the circumferential velocity fluctuations are higher. The Reynolds shear stress has a complex distribution, but is higher on the suction side. The turbulent kinetic energy is also consistently higher on the outer (suction) side of the wake. This trend is fundamentally different from those observed in prior studies of curved wakes where turbulence is enhanced on the inner side of the wake due to the destabilizing effect of curvature. To explain the difference, we examine the contributors to turbulent kinetic energy production rate in a curvilinear coordinate system aligned with the wake-centerline. The contribution of streamwise curvature to the production rate of turbulent kinetic energy, although consistent with expected trends, is overwhelmed by effects of wake shearing. The primary contributor to turbulent kinetic energy production rate is the product of Reynolds shear stress with cross-stream gradients of streamwise (in a frame of reference relative to the rotor blade) velocity in the wake. The location of peak in turbulent kinetic energy is almost aligned with that of production rate. The turbulence diffusion term opposes the production rate peaks, but also has high values along the edge of the wake.

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