Quantifying Roughness Effects by Ship Viscous Flow Calculations

This paper addresses the effects of hull roughness on viscous flows around ships. These effect are computed replacing the typically non-uniform roughness of the hull surface by a uniform sand roughness. The calculations are performed with the RANS-code PARNASSOS using the SST k − ω model. No wall functions are applied, and the roughness effect is introduced via a change in the ω wall boundary condition.
For a tanker, a container ship and a car carrier, the flow is computed at model and full scale Reynolds numbers for sand-grain roughness heights ranging from 0 (smooth wall) to 300µm. Each case is computed on six nearly geometrically similar grids to allow a fair estimate of the numerical uncertainty.
The results confirm that an increase of the roughness height leads to an increment of the friction and pressure resistance coefficients and the wake fraction. It is clear from the data that there is a significant scale effect, depending not only on the global Reynolds number (based on the ship length) but also on the roughness Reynolds number (based on the roughness height).
The increase of the viscous resistance with the roughness height obtained numerically is compared with existing empirical formulae. The computed results show the best agreement with the empirical correlation proposed by Townsin et al.