Numerical analysis of fluid flow dynamics around a yawed half-submerged cylinder inside an open channel

The drag and side force coefficients of a half-submerged cylinder in a free-surface flow were calculated through numerical simulations, with the aim of supporting the numerical modelling of log transport in rivers. The variability of these coefficients with the yaw angle with respect to the flow direction and with the ratio between the flow depth and the diameter of the cylinder were investigated. Simulations were performed with the three-dimensional code ANSYS/CFX, employing the volume of fluid multiphase technique to reproduce the critical interaction between the free surface and the cylinder. The numerical tests, showing the rise of the drag force coefficient for increasing yaw angles passing from flow-parallel to flow-perpendicular cylinder and the peak of the side force coefficient for flow-oblique cylinder, were validated by comparison with the results of laboratory experiments. The simulations were then extended to conditions with significant blockage in the vertical direction which had not been previously experimented, revealing a strong increase in the force coefficients for decreasing ratios between the flow depth and the cylinder diameter. A detailed description of the reproduced flow features in the proximity of the cylinder for the different cases was furthermore obtained. Such report, in addition to the analysis of the force coefficients, could serve a much wider range than that of log transport, i.e., any case in which a floating cylinder interacts with free-surface flow.