Comparisons of Predicted Brownout Dust Clouds with Photogrammetry Measurements

Results from a Lagrangian dust-cloud simulation were compared to photogrammetry measurements of the dust clouds generated by a helicopter during taxi-pass and approach-to-touchdown maneuvers. An inviscid-viscous matching method was used for predicting the flow field arising from a combination of the rotor flow (modeled as an inviscid potential flow using a free-vortex wake method) and the viscous flow region adjacent to the ground (to model sediment mobilization). Although not all of the data required to conduct the simulations were measured (e.g., exact weight of the helicopter, wind conditions, blade control angles, trim state), the comparisons to the measured evolution of the overall shapes and characteristic dimensions of the dust clouds were found to be relatively good. The results also showed the sensitivity of the predicted dust clouds to the weight of the helicopter, the ambient winds, and the exact flight-path that the helicopter followed toward the ground. An analysis of the dust clouds predicted using different particle sizes showed that the clouds contained mostly small particles that were entrained into the flow by unsteady pressure uplift and bombardment ejection mechanisms.