Verification and validation of forces generated by an unstructured flow solver

A primary goal of computational fluid dynamics is the accurate prediction of the forces and moments on ships, aircraft, turbines and similar complicated geometries, especially when viscous effects are important. By using unstructured grids, much of the detail of these complicated geometries can be captured with the grid and hopefully with the solution generated by the flow solver. As unstructured flow solvers mature, the forces predicted by them should become more accurate. For unsteady maneuvering cases, the accuracy of the forces and moments is critical for accurate predictions of the location and orientation of the body in motion, because errors tend to accumulate and grow as the maneuver proceeds. Accurate simulations of maneuvers have been obtained with the structured flow solver UNCLE. However, its unstructured equivalent, U²NCLE, has produced anomalies that are not fully understood. In an effort to isolate and identify potential inaccuracies in the unstructured flow solver, the flow solver has undergone a thorough re-evaluation of each component and the interactions between the components. Particular attention has been paid to the effects of discretization error. The unstructured flow solver uses mixed element types to resolve the boundary, so the discretization error for the non-simplical element types was investigated. Several methods to discretize the viscous terms have been investigated, to determine whether they are linearity preserving. A new inviscid variable extrapolation method (Unstructured MUSCL) has been developed, which has a smaller discretization error than the previous method. Finally, effects of asymmetries in the grid and in the solution algorithm have been investigated.