Turbulent boundary layers on rough surfaces with and without a pliable overlayer: A simulation of marine fouling

Abstract

A wind tunnel simulation of two boundary layer flows (with and without an adverse pressure gradient) on two surfaces with different roughness was carried out. One of the surfaces was covered throughout with an irregular coarse roughness and the other was identical but additionally overlaid with a large number of deformable nylon on tufts (to simulate the growth of biological fouling on a marine object).

Detailed mean flow and turbulence measurements were carried out to perceive the effect of the overlayer on the boundary layer development with different pressure gradient conditions. Generally, the overlayer affected significantly a majority of the monitored quantities, i.e., the physical growth, displacement, momemtum loss, wall friction, normal and shear turbulent stresses, mixing length, eddy viscosity, etc.

However, with the pressure gradient (of nearly constant second derivative) the effect on the physical growth and the displacement thickness parameters was somewhat reduced. The latter flow resulted in a near equilibrium boundary layer at the rear of the test plate.

Macro- and micro-scales of turbulence were hardly affected by the overlayer, but the dissipation length was reduced by it. A lag-entrainment method (modified here for the particular surface roughnesses) predicted the present experimental data well. Another family of integral methods did worse.