Experimental Investigation of Flow Drag and Turbulence Intensity of a Channel Flow with Rough Walls

Turbulent flow over rough walls is investigated through acoustic doppler velocimeter measurements. Smooth rods with a diameter of 6 mm are used as roughness elements. The rods are arranged at the channel bottom wall in three ways: longitudinally (along the flow direction); transversely (orthogonally to flow direction); and mesh-shaped (in a staggered mesh). The transverse roughness elements produce higher disturbance and flow drag than longitudinal roughness. Both turbulence intensity and flow drag for mesh-shaped roughness are not significantly different from those of transverse roughness, indicating that the transverse roughness elements mainly affect turbulence characteristics. Both turbulence intensity and flow drag are greatest for transverse rough walls at w/k=7; likewise, both increase with decreasing w/k for longitudinal rough walls. Compared with channel flow over a smooth wall, the turbulence intensity increases considerably, while the flow drag only increases slightly when w/k is small for the three arrangements. This is beneficial for enhancing heat transfer and mixing in channel flows with relatively small flow resistance.