The Effect of Surface Roughness on Hydrodynamic Drag and Turbulence

Abstract

The ability to accurately predict the drag forces on a ship before it is built would lead to more efficient designs. To do this, the effect of surface roughness on frictional drag must be well understood. The goal of this project is to identify the appropriate roughness scaling parameters for simple three-dimensional roughness with similar length scales to those found on ship hulls. In order to study and ultimately predict the effects of surface roughness on fluid flow and drag, flat plates with smooth and rough surface conditions were tested. Mesh and sandpaper were chosen as the rough surfaces because they are three dimensional, which is characteristic of naturally occurring surfaces. The first phase of testing involved towing the plates in a tow tank to determine the overall frictional drag. These tests were done in the 115 m long tow tank located in the USNA Hydromechanics Laboratory. Detailed velocity measurements were also obtained with the plates in a re-circulating water channel located in the Hydromechanics Laboratory to determine the effect of the roughness on the turbulence near the surface. The velocity measurements were obtained with a laser Doppler velocimeter (LDV). The drag results from both sets of tests showed excellent agreement. It was also observed that beyond a few roughness heights from the wall, the normalized turbulence was independent of the roughness. Proper scaling for the sandpaper was found to be a function of the roughness height, while the mesh surfaces were discovered to be a function of both roughness height and wire spacing. The results from this project will aid in the development of a general model of overall frictional drag from physical measures of the surface alone.