Experimental Study of Shallow Open Channel Turbulent Flows Over Rough Walls

2014 ◽  
Author(s):  
B. Nyantekyi-Kwakye ◽  
E. E. Essel ◽  
S. Clark ◽  
M. F. Tachie
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Reynolds Number ◽  
Channel Flow ◽  
Open Channel ◽  
Open Channel Flow ◽  
Reynolds Stresses ◽  
Freestream Velocity ◽  
The Mean ◽  
Bed Roughness

An experimental study was undertaken to investigate the effects of bed roughness on the turbulence characteristics of shallow open channel flows. The measurements were performed in a recirculating open channel flow over a reference smooth bed and a three-dimensional rough bed (36-grit sandpaper). The velocity measurements were conducted using a high resolution particle image velocimetry (PIV) system. The Reynolds number based on the depth of flow (h) and freestream velocity (Ue) varied from 21000 to 30000 and the Froude number ranged from 0.46 to 0.65. Two smooth bed experiments were conducted to investigate the effect of Reynolds number on the open channel flow. The mean velocities and Reynolds stresses for the two smooth cases were observed to be weakly dependent on Reynolds number. The effect of bed roughness was observed to penetrate into the outer layer of the boundary layer. The results show that bed roughness significantly increased the skin friction coefficient, wake parameter, boundary layer parameters, as well as the mean velocity, Reynolds stresses and the energy budget terms. A two-point correlation analysis showed that the coherent structures were also significantly modified by bed roughness.

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.

Low Reynolds Number Effect on Open Channel Flow Over a Rib

2014 ◽  
Author(s):  
Ebenezer E. Essel ◽  
Kathryn Atamanchuk ◽  
Samuel d’Auteuil ◽  
Mark F. Tachie
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Open Channel ◽  
Low Reynolds Number ◽  
Open Channel Flow ◽  
Recirculation Region ◽  
Particle Image Velocimetry Technique ◽  
Mean Velocity ◽  
Freestream Velocity ◽  
Low Reynolds

An experimental study was conducted to investigate low Reynolds number effects on open channel flow over a transverse square rib. Particle image velocimetry technique was used to perform detailed velocity measurement in the upstream and recirculation region of a square rib of height, h = 12 mm. The Reynolds number based on the freestream velocity and rib height, Reh = 1510, 2650 and 3950 and the ratio of the boundary layer thickness to step height, δ/h = 2.5 ± 0.2. The results showed that the reattachment length of Reh = 2650 and 3950 increased by 5.7% compared with corresponding value of Reh = 1510. The mean velocities were independent of Reynolds number in the recirculation region but at the reattachment point, Reh = 3650 reduced the streamwise mean velocity and enhanced the wall-normal mean velocity in the region adjacent to the wall. The turbulent kinetic energy beyond the center of the recirculation region increased with increasing Reynolds number.

scholarly journals Turbulence structure of non-uniform rough open channel flow

2018 ◽  
Vol 40 ◽  
pp. 05039
Author(s):  
Priscilla Williams ◽  
Vesselina Roussinova ◽  
Ram Balachandar
Keyword(s):  
Pressure Gradient ◽  
Channel Flow ◽  
Friction Velocity ◽  
Open Channel ◽  
Open Channel Flow ◽  
Shear Stresses ◽  
Turbulence Structure ◽  
Mean Flow ◽  
Rough Bed ◽  
The Mean

This paper focuses on the turbulence structure in a non-uniform, gradually varied, sub-critical open channel flow (OCF) on a rough bed. The flow field is analysed under accelerating, near-uniform and decelerating conditions. Information for the flow and turbulence parameters was obtained at multiple sections and planes using two different techniques: two-component laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Different outer region velocity scaling methods were explored for evaluation of the local friction velocity. Analysis of the mean velocity profiles showed that the overlap layer exists for all flow cases. The outer layer of the decelerated velocity profile was strongly affected by the pressure gradient, where a large wake was noted. Due to the prevailing nature of the experimental setup it was found that the time-averaged flow quantities do not attained equilibrium conditions and the flow is spatially heterogeneous. The roughness generally increases the friction velocity and its effect was stronger than the effect of the pressure gradient. It was found that for the decelerated flow section over a rough bed, the mean flow and turbulence intensities were affected throughout the flow depth. The flow features presented in this study can be used to develop a model for simulating flow over a block ramp. The effect of the non-uniformity and roughness on turbulence intensities and Reynolds shear stresses was further investigated.

Open Channel Flow with Varying Bed Roughness

1979 ◽  
Vol 105 (9) ◽  
pp. 1167-1183 ◽  
Author(s):  
Donald W. Knight ◽  
J. Alasdair Macdonald
Keyword(s):  
Channel Flow ◽  
Open Channel ◽  
Open Channel Flow ◽  
Bed Roughness

Discussion of “Open Channel Flow with Varying Bed Roughness”

1980 ◽  
Vol 106 (9) ◽  
pp. 1554-1554
Author(s):  
Peter R. Wormleaton ◽  
Panos Hadjipanos ◽  
John Allen
Keyword(s):  
Channel Flow ◽  
Open Channel ◽  
Open Channel Flow ◽  
Bed Roughness

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.

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