Quantification of Drag Change of Rough Surfaces by Submicron Resolution Long-Distance Micro-PTV

Paper in proceeding, 2019

One of the major questions in fluid mechanics is the prediction of the mean flow and turbulence structure of
boundary layers over surfaces with different roughness types. The current paper shows a method to obtain
the roughness function of different surface roughness by using a small-scale rig with rotating discs and
long-distance microscopic PTV measurements in order to obtain submicron resolution inside the boundary
layer of rotating disc flows. This rig has been designed and constructed for the optical measurements and
consists of an electric motor driving the discs that are rotating inside of a 20-liter water tank. The PTV
measurements are performed on surfaces with different degree of roughness and a smooth case. The rough
cases correspond to two applications of a marine antifouling paint. The measurements are performed for
two rotational speeds (300 and 600 RPM). Field mean velocity profiles are calculated by averaging vertical
bins with 1 px width and every point of the mean velocity profile corresponds to 200 instant velocity values
from 2000 image pairs. The wall shear stress for every rough case is determined from the torque and the
wall shear stress for the smooth case disc with same diameter. The mean velocity profiles are plotted and
the roughness function is directly obtained and compared with results from indirect methods using resolved
CFD simulations and torque measurements in the same facility.