Boundary conditions and numerical method

The boundary conditions used here for the NLDE are Thompson's characteristic method [2] which allows introducing a disturbance at the incoming boundary by deriving an expression for one of the incoming characteristics with a source term. Various wind conditions can be considered in this way at incoming boundaries. At the outflow boundaries, the boundary conditions are essentially non-reflecting. The ship superstructure and ocean surface are both treated as hard wall boundary conditions.

The NLDE are cast in a generalized coordinate system and solved numerically using a finite difference based scheme. The discretized equations are solved in a time accurate manner by taking advantage of computational aeroacoustics (CAA) methods. The spatial flux derivatives are calculated using seven point stencils of the fourth order optimized Dispersion Relation Preserving (DRP) scheme of Tam and Webb [3]. The time integration is a fourth order accurate Runge-Kutta method.

The NLDE solver is implemented portably on parallel computers, such as, the IBM SP2 and SGI Power Challenge. Efficient computing performance is achieved by using a three dimensional domain decomposition strategy. The whole computational domain is divided into many three dimensional zones. The codes are written in fortran 77 plus Message Passing Interface (MPI) and it is scalable in three dimensions.

In the helicopter/ship interface problem, The most important data for coupling the airwake solution to the dynamics analysis of the main rotor blades of helicopter are the mean flow field, the intensity of flow perturbations, and its dominant frequencies. In this paper, we will present such data.