We present the velocity profile measured within a porous bed below rough turbulent open channel flow. Mean streamwise velocity has a minimum in the top pore, just below the bed surface, and then increases towards a constant value in the deeper pores. We argue that the velocity minimum in the top pore is due to enhanced turbulence generated within the external flow. This produces a flow regime similar to rough turbulent pipe flow, with very efficient momentum extraction. In the deeper pores turbulence intensities are damped so that momentum extraction is less efficient, allowing higher mean velocities to develop. (c) 2007 American Institute of Physics.
- homogeneous fluid