A robust low speed preconditioning formulation for viscous flow computations

A modification of a low speed preconditioning formulation is proposed to perform robust computations for both internal and external flows. This formulation is based on the isentropic Mach number to derive an efficient scheme for viscous flows. The preconditioning technique is coupled with a point-implicit symmetric Gauss-Seidel relaxation scheme in conjunction with a finite volume discretization for structured mesh topologies. Results of computations of standard test cases with attached and separated boundary layers are presented to validate and assess the method in terms of convergence, accuracy and robustness. The robustness and the effectiveness of the preconditioning formulation is then demonstrated by presenting crosswind air intake computations. This application features a strong acceleration of the flow field around the inlet lips combined with complex three-dimensional separations and therefore it constitutes a challenging test case for preconditioning simulations. It is found that the separation line is better predicted with the preconditioning procedure and good agreement between measured and computed data of flow topology is obtained.