Open channel flow past a train of rib roughness

Abstract

In this paper, the effect of depth on turbulent open channel flow past a train of rib elements is examined. The two-dimensional square ribs spanning the width of the channel are located throughout the length of the flume. The experiments are conducted in the fully rough regime to document the turbulence characteristics of the flow at two different rib spacing (pitch-to-height ratios [ p/k ] of 9 and 18) conforming to the classical definition of k-type roughness. The streamwise mean velocity profiles demonstrate a shift from a smooth wall flow but varied marginally between the two cases of p/k and were also not affected by the change of flow depth. At shallow depths, roughness with p/k = 9 show a substantial increase of turbulence intensities and Reynolds shear stress in the outer layer compared to the reference flow case on a smooth bed. In the case of p/k = 18, the roughness effect is confined to the region less than 3 k from the wall and an outer layer similarity is confirmed with no effect of flow depth. Quadrant analysis shows an increase of ejection and sweep events in the outer layer at shallow depths for p/k = 9. Near the reattachment point, strong sweeps penetrate into the outer layer for both rough wall flows only at shallow depth. Strong ejections, on the other hand, seem to be more sensitive to the roughness and p/k ratio. The present results indicate that at shallow depths with p/k = 9, most of the flow in the outer layer is affected, while with a larger spacing (p/k = 18), the flow characteristics are less sensitive to the change of depth.

Keywords:

• turbulence
• two-dimensional roughness
• open channel flow
• LDV
• quadrant analysis