Verification and Validation of URANS Wave Resistance for Air Cushion Vehicles, and Comparison With Linear Theory

Abstract

Verification and validation of URANS wave-resistance predictions for straight-ahead and yawed air-cushion vehicles in calm deep and shallow water are performed. The nonlinear and linear theories are compared to explicate their trends for large cushion pressures, water depth, and cushion dimensions, and the nonlinear theory sinkage and trim trends are discussed. The grid-verification study shows monotonically converged solutions with averaged uncertainty of 4% and 10% for straight-ahead motion in deep and shallow water, respectively. URANS predictions agree with the experimental data to within 6% and 9% for straight-ahead deep and shallow water simulations, respectively. The smooth-edged cushion-pressure simulations predict lower resistance than the sharp-edged case, whereas no significant dependence is obtained for Reynolds number and turbulence modeling. URANS predicts attenuation in the resistance secondary hump as the cushion-pressure level increases. On the other hand, the linear theory does not account for the effect of cushion-pressure level. The linear and nonlinear theories compare within 4.5% for static cushion-pressure-to length ratios less than 0.025 and Froude number greater than 0.5 for both deep and shallow water. The nonlinear theory predicts the effect of water depth better than the linear theory, when compared with the experiments. Both the theories agree well in predicting the decrease in resistance with the decrease in cushion width. The nonlinear theory does not show unrealistically large resistance and side force for sharp-edged cushion pressure for yawed cases, as observed in the linear theory. However, both the theories compare well for the resistance and side-force predictions for the smooth-edged cushion pressure, where the results agree within 10% of the deep-water experimental data. The nonlinear theory predictions for the sinkage and trim are in good agreement with the experimental data, but sinkage is overpredicted and trim is underpredicted. URANS wave-elevation patterns display transverse and diverging waves, which compare well with the Kelvin waves for Froude number less than 0.6 and greater than 1.0, respectively. URANS predicts breaking waves for large cushion pressures for Froude number less than 0.6.