Investigation into the Effects of Aeolian Scaling Parameters on Sediment Mobilization below a Hovering Rotor
Baharani, Ajay Kumar
Leishman, John G
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Flow visualization and particle image velocimetry (PIV) experiments were conducted on a small-scale rotor hovering over a ground plane covered with a mobile sediment bed to help understand the effects of certain selected scaling parameters on the processes of sediment mobilization, entrainment, and uplift as induced by the rotor. Flow visualization using high-speed videography was used to study the rotor flow structures, their evolution in the rotor wake, and their interaction with the ground plane. Time-resolved PIV measurements of the rotor wake flow at the sediment bed quantified the flow velocities where most of the sediment mobilization was observed to occur. Dual-phase PIV experiments were conducted using ten different sediment samples of known characteristics to vary the values of five of the similarity parameters: 1. Particle diameter-to-rotor radius ratio, 2. Particle-to-fluid density ratio, 3. Ratio of characteristic flow (or wind) speed to particle terminal speed, 4. Densimetric Froude number, and 5. Threshold friction velocity ratio. The particle-to-fluid density ratio was shown to have the greatest effect on the resulting two-phase flow, followed by the threshold friction velocity ratio. The flow was also sensitive to changes in the particle diameter-to-rotor radius ratio. Changes in the densimetric Froude number and ratio of the characteristic flow speed to particle terminal speed also showed good correlations to observations of the quantity of uplifted particles. The effects of the passage of the tip vortex near the bed was shown to increase the shear stresses on the bed, which was observed to be closely correlated to an increase in the quantity of entrained sediment particles if the threshold conditions for particle mobility was exceeded. The observations and results were used to make recommendations regarding scaling on dual-phase experiments to better simulate the problem of rotorcraft brownout in the laboratory environment.