Effect of Reynolds Number on Wall-normal Turbulence Intensity in a Smooth and Rough Open Channel Using both Outer and Inner Scaling

Authors: Md Abdullah Al Faruque, Ram Balachandar

Abstract:

Sudden change of bed condition is frequent in open channel flow. Change of bed condition affects the turbulence characteristics in both streamwise and wall-normal direction. Understanding the turbulence intensity in open channel flow is of vital importance to the modeling of sediment transport and resuspension, bed formation, entrainment, and the exchange of energy and momentum. A comprehensive study was carried out to understand the extent of the effect of Reynolds number and bed roughness on different turbulence characteristics in an open channel flow. Four different bed conditions (impervious smooth bed, impervious continuous rough bed, pervious rough sand bed, and impervious distributed roughness) and two different Reynolds numbers were adopted for this cause. The effect of bed roughness on different turbulence characteristics is seen to be prevalent for most of the flow depth. Effect of Reynolds number on different turbulence characteristics is also evident for flow over different bed, but the extent varies on bed condition. Although the same sand grain is used to create the different rough bed conditions, the difference in turbulence characteristics is an indication that specific geometry of the roughness has an influence on turbulence characteristics. Roughness increases the contribution of the extreme turbulent events which produces very large instantaneous Reynolds shear stress and can potentially influence the sediment transport, resuspension of pollutant from bed and alter the nutrient composition, which eventually affect the sustainability of benthic organisms.

Keywords: Open channel flow, Reynolds Number, roughness, turbulence.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1127613

References:

[1] Patel V. C. (1998). “Perspective: Flow at high Reynolds number and over rough surfaces – Achilles heel of CFD.” Journal of Fluids Engineering, 120(3), 434-444.
[2] Kirkgöz, M. S., and Ardiçhoğlu, M. (1997). “Velocity profiles of developing and developed open channel flow.” Journal of Hydraulic Engineering, 123(2), 1099-1105.
[3] Dancey, C. L., Balakrishnan, M., Diplas, P., and Papanicolaou, A. N. (2000). “The spatial inhomogeneity of turbulence above a fully rough, packed bed in open channel flow.” Experiments in Fluids, 29(4), 402-410.
[4] Kaftori, D., Hetsroni, G., and Banerjee, S. (1995). “Particle behavior in the turbulent boundary layer. I. Motion, deposition, and entrainment.” Physics of Fluids, 7(5), 1095-1106.
[5] Balachandar, R., and Patel, V. C. (2002). “Rough wall boundary layer on plates in open channels.” Journal of Hydraulic Engineering, 128(10), 947-951.
[6] Tachie, M. F., Bergstrom, D. J., and Balachandar, R. (2004). “Roughness effects on the mixing properties in open channel turbulent boundary layers.” Journal of Fluids Engineering, 126, 1025-1032.
[7] Tachie, M. F., Bergstrom, D. J., and Balachandar, R. (2003). “Roughness effects in low-Re open-channel turbulent boundary layers.” Experiments in Fluids, 35, 338-346.
[8] Nezu, I. (2005). “Open-channel flow turbulence and its research prospect in the 21st century.” Journal of Hydraulic Engineering, 131(4), 229-246.
[9] Balachandar, R., and Bhuiyan, F. (2007). “Higher-order moments of velocity fluctuations in an open channel flow with large bottom roughness.” Journal of Hydraulic Engineering, 133(1), 77-87.
[10] Afzal, B., Faruque, M. A. A., and Balachandar, R. (2009). “Effect of Reynolds number, near-wall perturbation and turbulence on smooth open channel flows.” Journal of Hydraulic Research, 47(1), 66-81.
[11] Faruque, M. A. A. (2009). “Smooth and rough wall open channel flow including effects of seepage and ice cover.” PhD thesis, University of Windsor, Windsor, ON, Canada.
[12] Tachie, M. F. (2001). “Open-channel turbulent boundary layers and wall jets on rough surfaces.” PhD thesis, University of Saskatchewan, Saskatchewan, Canada.