Towards the Next Generation of Grid-Based Vorticity-Velocity Solvers for General Rotorcraft Flow Analysis

Accurate flow prediction is essential to rotorcraft development, and while current numerical methods can, in theory, model the entire flow, the reliability of these tools is limited by various inherent numerical deficiencies. Hybrid CFD methods, where a body-fitted CFD solver is coupled to a vortex method, have been developed to address the cost and vorticity preservation issues associated with full-unsteady rotorcraft simulation. Results obtained with several of these hybrid grid-based solvers illustrate the capability of such an approach, when correctly coupled, to resolve vortex-dominated flow fields with lower cell counts than pure RANS/Euler methods; however several fundamental issues preclude the widespread adoption of hybrid solvers. This paper describes recent work directly addressing four of these issues, namely maintaining a solenoidal vorticity field, general multi-resolution adaptive gridding, consistent and efficient implementation of the viscous terms and computation of the unsteady pressure contribution. The associated numerical methods are embodied in the VorTran-M2 code oriented for hybrid operation and assessed by monitoring various integral flow invariants and comparison against theoretical and prior numerical predictions involving viscous and inviscid vortex ring structures.