COMPUTATION OF COBBLESTONE EFFECT WITH UNSTEADY VISCOUS FLOW UNDER A STERN SEAL BAG OF A SES

Abstract

The concept of a surface effect ship (SES) is to lift the hull partly by the air cushion enclosed within two side hulls, a bow skirt and a stern seal. Consequently, it results in lower draft, resistance and motions than equivalent length catamarans in most sea states. In very low sea states, however, there is a significant design problem, which is high vertical accelerations, referred to as the cobblestone effect. The oscillations are based on resonance phenomena and are caused by the change of the cushion volume due to the incident waves. The resonance oscillations have an important damping mechanism which is derived from the air leakage flow under the stern seal bag of a SES. Hence, the accurate prediction of the leakage flow is required for the estimation of the cobblestone effect. In order to solve the unsteady flow field under the stern seal bag, a viscous flow code for numerically simulating two-dimensional incompressible flows has been developed. The governing equations to be solved are the time-dependent Navier–Stokes equations, using the artificial compressibility approach. The spatial discretization is based on a cell-centred finite volume formulation. The inviscid fluxes are evaluated by Roe's scheme with the third-order-accurate MUSCL approach. Time integration is conducted by the second-order accurate backward Euler formula and the linear equation system is solved by an approximate Newton relaxation scheme with the symmetric Gauss–Seidel iteration approach. For the resulting time integration to be conservative on a moving grid system, a geometric conservation law is introduced. A numerical procedure is presented and contributions of the viscous effects to the cobblestone effect problem are discussed.