The effect of grid topology and density on inviscid hovering rotor solutions

Grid dependence is examined for Euler simulations of multibladed rotors in hover. The numerical simulation of rotor flows poses a unique problem for flow solvers. The solution is extremely sensitive to the accuracy of capture of the vortical wake over several turns; this means that a much finer grid density is required away from the blade than for a fixed-wing case, resulting in excessive run-times. An attempt is made to determine the number of grid points required to obtain practical results, by performing grid convergence tests for O-H and O-C grid topologies and determining the optimum ratio of grid densities in each parametric direction, for two- and four-bladed test cases. An upwind Euler solver is used on O-H and O-C structured grids, generated by transfinite interpolation along with a periodic transformation. It is shown that O-H grids produce more accurate solutions than O-C grids with the same number of points, and that 3 x 10(5) grid points produce inviscid solutions of sufficient accuracy for initial design. It is also shown that solutions can be improved by choosing optimum ratios of grid density in each parametric direction.