01. Experimental/Computational Fluid Dynamics

Systems in which an object is placed in a uniform flow are received attentions in fluid dynamics, and have been studied for various objects, including cylinders and airfoils. For rotating objects, many analyses have been conducted around a circular cylinder, and the Magnus effect, in which lift is generated by the rotation, is well known. When non-circular objects rotate, it is of interest to consider how the moving object surface affects the flow. In this study, a two-dimensional CFD analysis was conducted around a rotating square cylinder at a constant angular velocity in a uniform flow. The finite difference method was used on an equally spaced orthogonal mesh, and the immersed boundary method was used to embed the rotating prism into the fluid flow simulation.　The flow field was evaluated qualitatively by visualization of vorticity and quantitatively by frequency analysis of the time history of the lift coefficient. When β was small, irregular and complex vortices were lined up in the wake; when β was large, the wake flow field became regular, with alternating vortex trains emanating from the upper and lower sides of the prism and smaller vortices emanating from the lower part of the prism. Frequency analysis of the lift coefficient revealed one frequency component that was independent of the rotational velocity ratio and another frequency component that was four times the rotational frequency of the prism. The latter corresponds to the fact that the prism takes the same posture for every 2π/4 rotation. When the rotational speed ratio was small, the former was dominant, but as the rotational speed ratio increased, the influence of the latter became stronger.