Abstract
The effects of large roughness group of square columns on the flow structure in a downstream region were investigated experimentally in an open channel flow. Two roughnesses were located in parallel and their interval length was changed, and one roughness was placed backward with 2 kinds of distance on the center line of the front roughnesses. The velocity was measured by using PIV method under two discharge conditions where the roughness was emerged and submerged. In the submerged cases, the effects of lateral and longitudinal distance of each roughness did not so affect the flow structures. In the case of three roughnesses, when the lateral interval length was short, the longitudinal distance between the front and back roughnesses affects significantly to the mean flow and turbulent structures. The long distance setting can keep a low velocity and low turbulence region behind the roughness group. This is caused by the difference of large-scale periodic vortices generated by the roughnesses.
