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Description
A two-dimensional computational analysis was performed on a tandem wingbox that is the foundation of a supersonic Magnetically Levitated test sled designed and manufactured at General Atomics and the Air Force at Holloman Air Force Base. The effects of Mach number, ground clearance and distance between the wingboxes on the supersonic flow characteristics, and lift and drag loads were investigated. Computational results were compared against experiments on the tandem wingbox flow performed in the SDSU supersonic blow down tunnel. CAD and grid models were generated in Gambit so that grid cells aligned with shocks. Fluent, a commercially available Computational Fluid Dynamics software, was used to simulate the flows. The compressible, turbulent flows over the tandem wingbox were modeled with a Reynolds Averaged Navier-Stokes model closed based on the standard k-_ turbulence model. A grid convergence study was conducted with third-order MUSCL scheme ensuring a minimal effect of numerical errors. The flow between the wingbox and ground behaves similar to the intake flows of high-speed propulsions systems. At a lower Mach number of 2, a combination of back pressure and relatively low frontal clearance area, results in the formation of a detached normal shock upstream of the tandem wingbox configuration. At a higher Mach number of 4, the detached shock is effectively swallowed by the intake, and an oblique shock system forms. The wake behind the front wingbox determines the flow over the rear wingbox. The wake is deflected away from the ground for all cases considered here, since the pressure below the wingboxes is higher than the pressure above the wingbox. As a result a shear layer impinges on the rear wingbox. The pressure force further creates a significant mass flow from below the wingbox to above the wingbox. At Mach 2, a decreased ground clearance resulted in an increased pressure along the bottom of the front wingbox, hence increasing lift and drag. The mass flow is changing due to the change in ground clearance, therefore decreasing the pressure as ground clearance increases. At Mach 4 the front wing box load increased with decreasing ground clearance, whereas the rear wingbox showed a maximum lift and drag at medium range ground clearances. A moving wall resulted in a more accurate representation of ground effect because the boundary layers did not develop along the wall as with a stationary wall, which lead to a separation bubble before the rear wingbox. The computational results were just in reasonable agreement with experimental results. The two rods that hold the two wing boxes together were not modeled in the computation. The interaction of the rods with the wakes and shocks behind the first wingbox are considered the primary reasons for disagreements between computation and experiment.
