Analysis of Interference Drag for Strut-Strut Interaction in Transonic Flow

Computational Fluid Dynamics (CFD) simulations were performed to predict the interference drag produced by two streamlined struts intersecting at various angles in transonic flow since no relevant experimental data are available for use in design studies. The one-equation Spalart-Allmaras turbulence model was employed in a RANS formulation assuming fully-turbulent flow. Selected cases were run using the two-equation k-ω shear stress transport (SST) turbulence model for comparison. NACA 64A series airfoils with thickness ratios of 5% and 7.5% were studied at Mach 0.8 and 0.85 for intersection angles between 45 and 90 degrees at an altitude of 12.2 km (chord-based Reynolds numbers of approximately 5 million). The commercial CFD code FLUENT was used for the analysis. In order to better understand the flow behavior, contours of surface pressure and velocity near the interaction region were examined. It is observed that the flow field is disturbed due to shock-induced separation only near the interaction region. There is little effect on viscous drag due to the strut-strut interaction, but changes in the pressure drag result in net interference drag. It is noted that there is an unexpected rise in the interference drag when the struts intersects at 90 degrees due to significant shock-induced flow separation on both sides of the intersecting strut, whereas smaller intersection angles show flow separation only on the side with the acute angle. A response surface for the interference drag coefficient as a function of Mach number, thickness ratio, and intersection angle was generated using the numerical simulations for use in the Multidisciplinary Design Optimization (MDO) studies where efficient means of estimating the interference drag are needed.