Variation of Streamwise and Vertical Turbulence Intensity in a Smooth and Rough Bed Open Channel Flow

An experimental study with four different types of bed conditions was carried out to understand the effect of roughness in open channel flow at two different Reynolds numbers. The bed conditions include a smooth surface and three different roughness conditions, which were generated using sand grains with a median diameter of 2.46 mm. The three rough conditions include a surface with distributed roughness, a surface with continuously distributed roughness and a sand bed with a permeable interface. A commercial two-component fibre-optic LDA system was used to conduct the velocity measurements. The variables of interest include the mean velocity, turbulence intensity, correlation between the streamwise and the wall normal turbulence, Reynolds shear stress and velocity triple products. Quadrant decomposition was used to extract the magnitude of the Reynolds shear stress of the turbulent bursting events. The effect of roughness was evident throughout the flow depth. The results show that distributed roughness has the greatest roughness effect followed by the sand bed and the continuous roughness. Compared to the smooth bed, the streamwise turbulence intensity reduces but the vertical turbulence intensity increases at a location very close to the bed due to the introduction of roughness. Although the same sand grain is used to create the three different rough bed conditions, the difference in the turbulence intensity is an indication that the specific geometry of the roughness has an influence on turbulence structure.

Authors:

• Md Abdullah Al Faruque
• Ram Balachandar

Keywords:

• Open channel flow
• smooth bed
• rough bed
• Reynolds number
• turbulence.

References:

[1] Kirkgöz, M. S., and Ardiçhoğlu, M. (1997). “Velocity profiles of
developing and developed open channel flow.” Journal of Hydraulic
Engineering, 123(2), 1099-1105.
[2] Nezu, I. (2005). “Open-channel flow turbulence and its research
prospect in the 21st century.” Journal of Hydraulic Engineering, 131(4),
229-246.
[3] 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.
[4] Schlichting, H. (1979). Boundary-Layer theory. McGraw-Hill Classic
Textbook Reissue Series, McGraw-Hill, Inc., United States of America.
[5] Townsend, A. A. (1976). “The structure of turbulent shear flow.”
Cambridge University Press.
[6] Raupach, M. R., Antonia, R. A., Rajagopalan, S. (1991). “Rough wall
turbulent boundary layers.” Applied Mechanics Review, 44 (1), 1.
[7] 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.
[8] Krogstad, P-A., and Antonia, R. (1999). “Surface roughness effects in
turbulent boundary layers.” JExperiments in Fluids, 27, 450-460.
[9] Tachie, M. F. (2001). “Open-channel turbulent boundary layers and wall
jets on rough surfaces.” PhD thesis, University of Saskatchewan,
Saskatchewan, Canada.
[10] Nezu, I. and Nakagawa, H. (1993). Turbulence in open-channel flows.
IAHR Monograph, A. A. Balkema, The Netherlands.