Mean and unsteady loading on square prisms with rounded edges: Hard marine growth, incidence, and Reynolds number effects

A set of wind tunnel experiments was performed to study the average and fluctuating wind loading on an ”infinite” 2D square prism with rounded edges of r/D = 0.16 for different heights of simulated hard marine growth at sub-to transcritical Reynolds numbers. It has already been shown, that a change in surface roughness from ks/D = 4.5×10−6 (smooth) to 1×10−3 (rough) on a similar prism at α = 0° and 45° had large effects on the widths and the Reynolds numbers marking the boundaries of the various flow states. At α = 45° it furthermore led to a large Reynolds-number independency of the lift and drag forces and of the Strouhal number. For the present wind tunnel tests two additional values of ks/D were selected to obtain smaller increments in surface roughness: 4.5×10-4 (slightly rough) and 1.4×10−3 (very rough). Mean and fluctuating lift and drag forces, the main vortex shedding frequency, distributions of the mean surface pressures in the mid-span cross-section of the prism and the mean wake profiles were recorded simultaneously. The same two static angles of incidence, α = 0° and 45°, were investigated for ReD = 60,000–12 million. For the prisms with smooth up to rough surfaces strong effects on the behaviour of all aerodynamic parameters with Reynolds number were found, whereas a further increase in ks/D induced hardly any additional effects. The absolute values of the aerodynamic parameters were found to be independent of a change in ks/D for the sub-to supercritical flow states. For the smooth and slightly rough prisms at α = 0° the supercritical flow state was present up to ReD = 12 million, whereas for the rougher prisms the upper transition and the transcritical flow states appeared at high Reynolds numbers. A change to α = 45° induced little variation with ks/D for all aerodynamic coefficients. The overall behaviour of the force coefficients and the Strouhal number with ks/D and ReD were caused by motions of the transition, separation and reattachment locations.