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The hydrofoils are essential element in tidal current turbine and in high speed marine crafts. Hence, the study of hydrodynamic characteristics of hydrofoils are important as they play a vital role in improving the performance of these propulsion devices (hydro-kinetic turbine, marine craft). In the present study, the dynamics of unsteady and viscous flow around a two hydrofoil system is investigated for two different configuration: (a) tandem, and in (b) stagger arrangement. The incompressible Navier-Stokes equations were solved using Finite Volume Solver in STAR-CCM+ commercial software package. k-ε turbulence model is incorporated in the present simulation in order to explain the turbulence flow physics while the free surface is captured using Volume of Fluid (VOF) technique. Further, the obtained numerical simulation results were compared with experimental data available in the literature as a validation purpose. The objective of the present study is to investigate the effects of spacing distance on the lift and drag coefficient generated by two foil system in tandem and stagger arrangements which is one of the important design parameter for a tidal turbine blades in presence of free sea surface. It could be observed that the hydrofoils are arranged in tandem configuration, the lift coefficients of the upstream and downstream hydrofoils are higher in comparison to single hydrofoil. Interference effect didn’t die out even at large spacing for 2-D hydrofoil.

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 [Formula: see text] 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.

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