Investigation of Surface Roughness Effects in Turbulent Flow around a Cylinder

Pipelines that traverse a river are often buried beneath the river bed. However, the pipeline may be exposed due to scoured riverbed during floods. The exposed pipeline vibrates in a frequency band depending upon the flow velocity, size, and shape of the pipe. These vibrations are detrimental to the pipeline safety and stability due to their cyclic nature. In fact, these vibrations are induced by the turbulence around the cylinder which is a function of bed roughness apart from the diameter of the cylinder and the flow velocity. Therefore, the objective of this paper is to investigate the effect of surface roughness on the turbulence characteristics of flow around a cylindrical object placed on the gravel bed. The effects of surface roughness on distribution of velocity defect and resultant bed shear stress distributions in the flow field around the cylinder were also studies. The Acoustic Doppler Velocimetry (ADV) system was used for measuring the point velocity data in the vicinity of the cylinder. The raw data was properly post processed and analyzed. From the analysis of the velocity measurements, the approach flow was found to be similar to canonical near wall turbulent flow. The approach flow velocity retards in the immediate upstream of the cylinder. The boundary layer separates on the top of the cylinder. Wake region was formed just downstream of the cylinder and flow recirculation occur in this region. Enhanced turbulence intensities were found in the wake region of the cylinder, because of the separated shear layer. At small distance downstream of cylinder, this separated flow reattached to the bottom of the channel. In addition, steep increase in the Reynolds shear stress was observed in the separated flow on the cylinder. The present investigation will boost the understanding of hydraulics of flow around the bed-mounted cylindrical objects in rough bed natural streams.