Single and tandem cylinders and rounded square-section prisms in cross-flow: influence of Reynolds number, incidence angle, edge roundness, surface roughness, and gap spacing​"

Abstract

The worldwide increasing demand for clean energy has led to a major growth of existing and new offshore wind farms. To increase their energy yield to an optimum, modern horizontal-axis wind turbines are constantly being upscaled and the locations of new offshore wind farms are more and more shifted towards the deep sea to capture faster, more consistent winds with less turbulence. Manufacturers of offshore wind turbines are thereby increasingly relying on new designs with ever-larger rotor diameters and higher turbine shafts which requires an increase of their draft to improve the floating platform’s heave motion performance. Besides being a fundamental hydrodynamic bluff-body problem that is highly complex and very challenging, the highly dynamic flow around the upscaled submerged circular foundation elements may introduce large vortex-induced vibrations (VIV). In the cases in which these flow-induced vibrations do occur, this can reflect in large maximum and low minimum tendon forces on the platform’s mooring lines, resulting in a considerable reduction of their fatigue life. One possibility to significantly reduce their susceptibility to VIV is by replacing the circular columns by prismatic ones having squared cross-section and rounded longitudinal edges. Because the latter cross-sectional shape can be seen as a transition geometry between the tow extremes, hence, a circle and a square with sharp edges of 90 degrees, it is to be expected that the behaviour of the flow around rounded square-section prisms is a mixture of both, whereby the amount of edge roundness plays an important role. In the last 10 years more than a dozen experimental studies have been performed in the High-Pressure Wind Tunnel of the DLR in Göttingen to investigate the unsteady flow around stationary circular cylinders and square-section prisms in isolated and tandem configurations up to Reynolds numbers of 10 million. Influencing parameters like the surface roughness height (i.e. the simulated coverage by marine fouling), edge roundness value, incidence angle, and gap spacing were varied, and their decoupled and combined effects on the aerostatics were recorded. Results on both the distributions of the mean and fluctuating surface pressures, 2D and 3D aerodynamic force and pitch moment coefficients, as well as results of the power spectral densities (PSD) of the unsteady lift and drag forces, and the Strouhal numbers - based on the main vortex shedding frequencies in the PSDs - are presented and compared to published data. It is shown that a variation of each of those influencing parameters induces drastic changes in the aerostatics.