Wake pattern and wave resistance for anisotropic moving objects

Abstract

We present a theoretical study of gravity waves generated by an anisotropic moving disturbance. We model the moving object by an elliptical pressure field of given aspect ratio $\mathcal W$. We study the wake pattern as a function of $\mathcal W$ and the longitudinal hull Froude number $Fr = V/\sqrt{gL}$, where $V$ is the velocity, $g$ the acceleration of gravity and $L$ the size of the disturbance in the direction of motion. For large hull Froude numbers, we analytically show that the rescaled surface profiles for which $\sqrt{\mathcal W}/Fr$ is kept constant coincide. In particular, the angle outside which the surface is essentially flat remains constant and equal to the Kelvin angle, and the angle corresponding to the maximum amplitude of the waves scales as $\sqrt{\mathcal W}/Fr$, thus showing that previous work on the wake's angle for isotropic objects can be extended to anisotropic objects of given aspect ratio. We then focus on the wave resistance and discuss its properties in the case of an elliptical Gaussian pressure field. We derive an analytical expression for the wave resistance in the limit of very elongated objects, and show that the position of the speed corresponding to the maximum wave resistance scales as $\sqrt{gL}/\sqrt{\mathcal W}$.