Higher-order moments of velocity fluctuations in an open-channel flow with large bottom roughness

The present study was undertaken to examine the effect Of Surface roughness on the higher-order velocity moments in a turbulent open-channel flow. The wall roughness is large and the ratio of the roughness size to the depth of flow ranges from 0.1 to 0.15. Flow over two types of roughness conditions (ribs and dunes) are examined and compared with that over a smooth open channel. In the case of the rib roughness, three different spacings are used: p/k=4.5, 9, and 18; here, p=distance between the leading edges of two consecutive ribs and k=height of a rib with square cross section. The shape of the smooth wall dunes was geometrically similar to that commonly used in previous studies. In an effort to understand the influence of local wall roughness, rough-wall duties are also used in the present study. The variables of interest include higher-order moments, conditional statistics based on a quadrant analysis, relationship of higher-order moments to the Reynolds stress and energy production. Although there are some similarities between open-channel flows and turbulent boundary layers, there are important differences as indicated by the third-order moments. The triple products and the second quadrant events are sensitive to the wall condition. Ejection events are found to be prevalent throughout the depth. It is also clear from the study that one needs to explicitly account for the surface condition in any model development to be able to predict transport characteristics.