Novel Eulerian Vorticity Transport Wake Module for Rotorcraft Flow Analysis

Accurate flow prediction is essential to the design and development of rotorcraft. While current numerical analysis tools can, in principle, model the complete flow field surrounding a rotorcraft, in practice the accuracy of these tools is hampered by various inherent numerical deficiencies. For example, Lagrangian methods lose accuracy when the rotor wake undergoes large distortions since the detailed dynamics of vortical structures within the wake must then be modeled empirically. Eulerian primitive variable CFD methods, on the other hand, introduce non-physical dissipation that can quickly smear regions of vorticity near to the rotorcraft leading to poor performance predictions. An approach that combines the first-principles physical modeling capability of Eulerian schemes with the vortex preservation capabilities of Lagrangian vortex techniques has long been needed. Recently, it has been shown that numerical diffusion of the rotor wake can be controlled in an Eulerian solver - albeit one based on a vorticity-velocity rather than primitive variable formulation - by carefully constructing the flux formula and selecting an appropriate flux limiter. The Vorticity Transport Model comprehensive rotorcraft analysis code, which uses an inviscid vorticity-velocity formulation of the Navier-Stokes equations, has demonstrated good conservation of both the strength and structure of the rotor wake, but has yet to be developed to the extent where it can provide good a-priori predictions of blade aerodynamics. This paper describes ongoing work that addresses the issues of accurately modeling the rotor blade aerodynamics and the wake through the development of an Eulerian wake module (VorTran-M) based on the techniques employed by the Vorticity Transport Model. A coupled arrangement has been developed, where a conventional PV-based Navier-Stokes code is used to resolve regions of compressible and viscous flow near the blades and VorTran-M is used for the remaining, vortex-dominated flow domain containing the rotor wake.