Accurate determination of drag from flow channels with asymmetric roughness of field biofilms

The presence of marine biofilms is known to cause significant financial and operational setbacks as they are ubiquitous to almost every ship and marine vehicle in the ocean. The vast majority of hydrodynamic research on these surfaces uses an artificial analog or monocultures in a lab to represent the biofilm. A field-deployable water tunnel has been designed to investigate the fundamental knowledge relating to the hydrodynamics of natural biofilms (i.e. field biofilms). The channel has smooth top and side walls with the biofilm panel being the closing side on the bottom. The pressure gradient (dp/dx) and flow rate (Q) are measured to determine the wall shear stress and frictional drag coefficient of the biofilm panel. However, the global measurements need to be corrected since they are a function of both the smooth and rough walls. This cannot be directly calculated since the boundary layer displacement from the channel centerline and smooth wall shear stress are both functions of the rough wall. An iterative scheme has been developed to estimate the smooth and rough wall shear stress by assuming a known smooth wall shear stress distribution, logarithmic velocity profile, linear shear stress distribution across the channel, and the boundary layer displacement from the centerline is proportional to the difference in wall shear stress. Panels covered with 220, 320, and 500-grit sandpaper were tested in the channel to establish the accuracy of the developed methodology. The resulting equivalent sandgrain roughness were compared with values in the literature for the same surfaces at similar friction Reynolds number. These results show the efficacy of using an asymmetrically rough channel in determining the drag of field biofilms.