Numerical Prediction of Interference Drag of Strut-Surface Intersection in Transonic Flow

The numerical evaluation of the interference drag produced by a streamlined strut intersecting a flat wall in transonic flow is presented. The calculations were performed on unstructured grids using the FUN2D and FUN3D codes and the Spalart-Allmaras turbulence model (Spalart, P. R., and Allmaras, S. R., 'A One-Equation Turbulence Model for Aerodynamic Flows,' AIAA Paper 92-0439, Jan. 1992). The grids were generated with AFLR2 and VGRIDns. The impact of the thickness-to-chord ratio of the strut, the Reynolds number, and the effect of the dihedral angle made by the strut with the wall are studied for both inviscid and viscous flows. To better understand the flow in the region where the strut and the wall intersect, the flowfield is analyzed in a detailed fashion for the thicker strut with pressure contours and stream traces. When the thickness-to-chord ratio of the strut is reduced, the flowfield is disturbed only locally at the intersection of the strut with the wall. The interference drag is calculated as the drag increment of the arrangement compared to an equivalent two-dimensional strut of the same cross section. The results show a rapid increase of the interference drag as the angle of the strut deviates from a position perpendicular to the wall. Separation regions appear for low intersection angles, but the viscosity generally provides a positive effect in alleviating the strength of the shock near the junction and, thus, the drag penalty. This study provides an equation to estimate the interference drag of simple intersections in transonic flow.