Evaluating Hydrofoil Resistance Components from RANS and Lifting-Line Fluid Simulations

Hydrofoils have become a popular design addition to different kinds of vessels, particularly yachts and race boats. Predicting hydrodynamic forces acting on hydrofoils remains challenging, due to a wide operating regime and complex hydrodynamical phenomena. In this paper we investigate methods of decomposing the total drag force of a hydrofoil from high-fidelity Reynolds-Averaged Navier Stokes (RANS) and low-fidelity lifting line computational fluid dynamics simulations into its physical components. Namely, these are the wave-pattern, induced, and viscous drag components. These components were calculated using wake surveys in RANS, while a separation of downwash coefficients was used in the lifting line formulation. Reynolds numbers between 2.57x10^5 and 2.06x10^6 were simulated, which corresponded to chord-based Froude numbers between 0.5 and 4. We present various Froude number-dependent trends in these components which we observed from RANS. Lifting-line models were modified to account for free surface effects using a mirror image of the hydrofoil and benchmarked against RANS results to gauge their accuracy. Reasonable agreement was seen on the total drag coefficient, however there were discrepancies between drag components due to differences in solver fidelity. Two different versions of the free surface boundary condition were simulated—the wall condition and the biplane condition. Simulations with the biplane condition yielded consistently higher drag predictions than with the wall condition, and better matched the total drag from RANS.