Wake-structure interaction of flow over a freely rotating hydrofoil in the wake of a cylinder

Abstract

The two-dimensional flow characteristics of the wake-structure interference between a cylinder and a freely rotating hydrofoil are analysed using the unsteady Reynolds-averaged Navier–Stokes method. The effects of different rotation centres and spacing ratios on the mechanical properties, flow-field structures, surface pressure distributions, turbulent fluctuations, vortex shedding and interference, and the resultant radiated noise and fluid resonant oscillation of the cylinder–hydrofoil system are systematically investigated. The results show that the hydrofoil decreases the shedding frequency of the cylinder wake vortex, whereas the cylinder accelerates the steady flow on the hydrofoil surface through the backward movement of the rotation centre, enhancing the dynamic stall characteristics. The turbulent vortex structures have a major suppression effect in the interaction region. At small spacing ratios, the mechanical properties of the system fluctuate considerably. As the spacing increases, the cylinder Karman vortex street is gradually released and plays a dominant role in generating turbulent fluctuations. The surface pressure increasingly recovers, and the turbulent kinetic energy and vortex intensity show jump peaks at the leading edge of the hydrofoil due to wake impingement. The primary-vortex shedding behind the cylinder impacts the hydrofoil surface, resulting in alternating shedding of the large-scale vortex street with a gradual loss in strength and coherence. This periodic variation in the hydrodynamic characteristics is reflected in the radiated noise, which is confirmed by employing a feedback model, as the system sound pressure level becomes stronger at spacing ratios of L/d=1.0, 1.5, and 3.0 due to the flow resonant oscillation.