Effects of Upstream Wall Roughness on Separated Turbulent Flow over a Forward Facing Step in an Open Channel

The effect of upstream surface roughness over a
smooth forward facing step in an open channel was investigated
using a particle image velocimetry technique. Three different
upstream surface topographies consisting of hydraulically smooth
wall, sandpaper 36 grit and sand grains were examined. Besides the
wall roughness conditions, all other upstream flow characteristics
were kept constant. It was also observed that upstream roughness
decreased the approach velocity by 2% and 10% but increased the
turbulence intensity by 14% and 35% at the wall-normal distance
corresponding to the top plane of the step compared to smooth
upstream. The results showed that roughness decreased the
reattachment lengths by 14% and 30% compared to smooth upstream.
Although the magnitudes of maximum positive and negative
Reynolds shear stress in separated and reattached region were 0.02Ue
for all the cases, the physical size of both the maximum and
minimum contour levels were decreased by increasing upstream
roughness.

Authors:

• S. M. Rifat
• André L. Marchildon
• Mark F. Tachie

Keywords:

• Forward facing step
• open channel
• separated and reattached turbulent flows
• wall roughness.

References:

[1] M. Sherry, D. Lo Jacono and J. Sheridan, “An experimental
investigation of the recirculation zone formed downstream of a forward
facing step,” J. Wind Eng. Ind. Aerodyn., Vol. 98(12), pp. 888–894.
[2] H. Ren and Y. Wu, “Turbulent boundary layers over smooth and rough
forward-facing steps,” Phys. Fluids. 23 (2011), 045102.
[3] Y. Wu and H. Ren, “On the impacts of coarse-scale models of realistic
roughness on a forward-facing step turbulent flow,” Int. J. Heat Fluid
Flow, Vol. 40, pp. 15–31.
[4] E.E. Essel, S. Mali, E.W. Thacher and M.F. Tachie, “Upstream
roughness effects on reattached turbulent flow over forward facing step,”
10th International ERCOFTAC Symposium on Engineering Turbulence
Modelling and Measurements, Spain, September 2014.
[5] J. F. Largeau and V. Moriniere, “Wall pressure fluctuations and
topology in separated flows over a forward-facing Step,” Exp. Fluids,
Vol. 42, pp. 21–40.
[6] V. De Brederode and P. Bradshaw, “Three-dimensional flow in
nominally two-dimensional separation bubbles: flow behind a rearwardfacing
step,” Imp. Coll. Aeronaut. Rep., pp. 72–19.
[7] K. A. Flack and M. P. Schultz, “Review of Hydraulic Roughness Scales
in the Fully Rough Regime,” J. Fluids Eng. 132, 041203–10.
[8] D. J. Forliti, P. J. Strykowski, and K. Debatin, “Bias and precision errors
of digital particle image velocimetry,” Exp. Fluids. 28 (2000), pp. 436–
447.
[9] L. Casarsa and P. Giannattasio, “Three-dimensional features of the
turbulent flow through a planar sudden expansion,” Phys. Fluids. 20
(2008), 015103.
[10] H. Hattori, and Y. Nagano, “Investigation of turbulent boundary layer
over forward-facing step via direct numerical simulation,” Int. J. Heat
Fluid Flow, 31(3), pp. 284–294.
[11] E.E. Essel, A. Nematollahi, E.W. Thacher and M.F. Tachie, “Effects of
upstream roughness and Reynolds number on separated and reattached
turbulent flow,” Journal of Turbulence, Vol. 16, Issue 9, 2015.