Flexibility and curvature effects on vortex dynamics and fluid-structure interactions

Abstract

Experiments were conducted to illustrate the ways in which flexibility, shape, and boundary conditions affect the vortex dynamics of the flow past a cylinder, particularly in the Reynolds number regime that characterizes biological and environmental flows. First, the flow past a freely-vibrating, flexible cantilevered cylinder is found to exhibit discontinuous regions of vortex-induced vibrations, called ``states,'' as a function of the reduced velocity $U^*$. These states are demarcated by discrete changes in the dominant eigenmodes of the structural response as the cylinder vibrates in progressively higher structural modes with increasing $U^*$. The wake response between different states is also found to have distinct characteristics; of particular note is the occurrence of a P+S wake over one of these regions, which is associated with the constructive interference of contributing eigenmodes.

Second, a rigid, curved cylinder aligned with the flow direction with a concave orientation is found to exhibit regions of non-shedding separated flow as a consequence of the axial flow induced by curvature and also by the free end. The wake characteristics depend on the curvature, aspect ratio, and Reynolds number. Increasing curvature or decreasing aspect ratio mitigates vortex shedding. The vortex shedding in the laminar regime depends only on the local inclination of the cylinder along the span, though this result does not apply in the transitional regime. The wake transition is linked to a concurrent change in the Strouhal number and the shedding angle.

Lastly, the no-slip boundary condition on a straight, normal cylinder is replaced with a liquid-infused or liquid-coated surface intended to produce a partial slip condition. The drag and Strouhal number are unaffected by the surface treatment, contrary to previous findings on superhydrophobic surfaces. A small increase in shear upstream of the separation point is noted in the time-averaged vorticity fields.