Drag Augmentation of a Sphere in High-Speed Multi-Phase Flow

The impact of a gas/liquid two-phase flow onthe drag of a sphere at supersonic conditions is investigated experimentally. The freestream of a Mach-4 Ludwieg tube is seeded with monodisperse liquid Di-Ethyl-Hexyl-Sebacate (DEHS) droplets with diameters ranging from 7μm to 16μm, to simulate a cloud-like environment. A free-flight methodology is employed, whereby the spheres are released impulsively into the freestream and allowed to respond to the experienced forces unhindered. Sphere trajectories are recorded using a high-speed bi-telecentric visualization system, from which drag accelerations can be accurately derived. Initial theoretical estimates of drag augmentation based on the increased effective density of the particle-laden flow yield increases of the order of 0.1% in comparison to an equivalent clean flow; however, the experimental measurements indicate a drag increment of more than an order of magnitude higher, up to approximately 4%. Particle simulations indicate that the majority of the droplets at the present conditions are impacting the sphere surface supersonically, suggesting that shock impingement and/or the water-hammer effect may be responsible for the unexpectedly high drag increase.