Characterization of Unsteady Separation on a Surrogate Cargo Aircraft Model with Edge Rounding

Aft sections of military cargo aircraft fuselages manifest high upsweep angles to accommodate ramp doors, yielding a wake dominated by an unsteady, counter-rotating longitudinal vortex pair that increases the risk of paratrooper injury, cargo drop errors, tail strikes, and drag. Streamwise oriented cylinders truncated at an angle have provided useful insights, but the sharp edge between the cylinder and the base localizes flow separation, potentially eliminating low-frequency unsteady features on aircraft fuselages which are smooth. This work uses Large Eddy Simulations to explore the effect of edge rounding which allows the separation line to adapt continually to local conditions, with accompanying effects on the wake, thus facilitating extraction of key distinctions from the corresponding sharp-edged configuration dynamics. The interpretation of instantaneous streamlines is achieved with meaningful, synchronized instantaneous wall shear stress components and connected to the visualization and characterization of separation locus dynamics. The range of separation line movement is relatively small near the upstream apex (start of the basal upsweep), but increases in the circumferential direction towards the downstream apex on the upper side. The separation locus is well described by Spectral Proper Orthogonal Decomposition modes at low frequencies, specifically St = 0.13, approximately an order of magnitude lower than key unsteady features in the sharp-edge case.