The hydrodynamic analysis of multiple hydrofoils translating in tandem in presence of a free surface

Abstract

Hydrofoils find its presence in various engineering applications including high speed marine crafts, blades of hydro-kinetic turbines etc. So, depending on the type of marine application, the foils are arranged in different complex arrangements (i.e. in line, or in tandem) that can alter flow dynamics of the working fluid flowing around them. In order to design efficient marine propulsion systems, it is important to study the effect of different arrangements of hydrofoil systems on its hydrodynamic characteristics (lift, drag, pressure coefficient, velocity contour, etc). In this study, the numerical simulation of fluid flow over a tandem hydrofoils is carried out in presence of a free surface. The present study is limited for a constant Reynolds number of 166,784.8, and at a Froude number of 0.57. The unsteady RANS equations are solved using the volume of fluid (VOF) method embedded in commercial software STAR-CCM+ incorporating $k-ϵ$ turbulence model. The first part of the study focuses on varying spacing ratios (l/c) that range from 0.1 to 4. In the second part of the study, the angle of attack of the downstream hydrofoil was varied over zero degrees to stall keeping the upstream hydrofoil at a zero degree angle of attack. For both studies, pressure and velocity distributions, lift as well as drag coefficients were evaluated. It is concluded that the presence of a free surface in a close proximity to the foils, different spacing between the hydrofoils, and varying angle of attacks of the downstream hydrofoil have significant effects on the overall performance.