Predictions of Brownout Dust Clouds Compared to Photogrammetry Measurements

Abstract

Results from a Lagrangian-Lagrangian-based brownout simulation were compared to photogrammetric measurements of the evolving dust clouds generated by a helicopter during low altitude taxi-pass and approach-to-touchdown maneuvers. An inviscid-viscous method was used to represent the carrier phase from a combination of the rotor flow field (modeled as an inviscid flow using a free-vortex wake) and a semiempirical boundary layer flow model at the ground. The mobilization and initial uplift of dust particles were represented using semiempirical models for the effects of shear, unsteady pressures, and bombardment mechanisms, the subsequent particle motion being solved for by using a subset of the Basset-Boussinesq-Oseen equations. Although not all the measurements required for validation were available, the results showed good agreement to the measured evolution of the dust clouds. The results showed a significant sensitivity to the weight of the helicopter, as well as the initial flight path that it followed toward the ground. Small ambient winds were also found to affect both the initial development and the subsequent movement of the dust clouds. A particle quantity analysis showed that the majority of the smaller sized particles were uplifted by direct entrainment and bombardment ejection mechanisms, which when acting together rapidly increased the net quantity of suspended dust.

Methodology for Rotorcraft Brownout Mitigation Through Flight Path Optimization

“…Both saltation bombardment and reingestion bombardment processes were modeled in the present simulations. The dust cloud simulation used in the present study has been shown to exhibit good agreement with the measured evolution of actual brownout clouds [36].…”

“…The dynamic motion of these particles is then modeled in synchronicity with the solution for the rotor flow, as well as in the subsequent interactions of particles with the sediment bed. For example, dust particles in saltation or those that are reingested through the rotor disk can impact the sediment bed with greater momentum and eject many more particles in a process called bombardment [2,36,37]. Both saltation bombardment and reingestion bombardment processes were modeled in the present simulations.…”

“…For the present calculations, a semi-empirical, composite model based on the exceedance of a threshold friction velocity [35] with corrections for unsteady pressure effects [36] was used for the initiation of sediment motion on the ground. The dynamic motion of these particles is then modeled in synchronicity with the solution for the rotor flow, as well as in the subsequent interactions of particles with the sediment bed.…”

Applications and challenges for MMW and THz sensors

“…In the graph, the particle sizes are shown near the bottom in frequency equivalent to that of electro-magnetic radiation whose wavelength is equal to the particle diameter. Actual particle sizes [7][8][9][10] are tabulated in Table 3. The obscurants listed above cause multiple scattering with the result that at shorter wavelengths, most of the detected photons have been scattered many times off the obscurant particles resulting in essentially zero contrast signals.…”

Effects of Model Scaling on Sediment Transport in Brownout

“…Computational approaches, such as the computational fluid dynamics (CFD) work performed by Kalra [31] and Thomas [43], attempt to model the brownout problem numerically. Several other CFD studies searched for ways to reduce the problem [44][45][46]. CFD can be useful as it allows for the investigation of many different aspects of brownout physics that can be hard to model in the laboratory, but experiments also necessary.…”