Helicopter brownout blocks out pilots’ sight resulting in difficulty of landing and nap-of-the-earth flight, and induces flight accidents. A sand particle discrete element method (DEM) model based on discrete dynamics and sand particle-flow interaction model are established and coupled with rotor viscous vortex particle model and viscous ground aerodynamic model to account for translation and collision of sand particles under the effect of rotor flow field, and an analysis method of helicopter brownout is then proposed to analyze the characteristics of helicopter brownout. The flight test of EH-60L brownout in the US Army Yuma Proving Ground is used to comparison. It is shown that the predicted shape, position and height of uplift, process of the sand cloud are consist with the flight test. Compared with the Lagrangian dust cloud simulation based on splash entrainment, process of bombardment, particle flux and threshold velocity with assumption, the predicted profile of the sand cloud is more accurate, and compared best with the flight test. Then, the processes of brownout of helicopter in hover and forward are investigated. It is shown that wall jet induced by rotor tip-vortex and ground interaction yields translation and sediment trapping of sand, and it also induces bombardment ejection and saltation. Uplift of sediment induced by the flowfield generates dust cloud, and covers front view of helicopter, and thus forms brownout.
[1] Acquisition and Technology Programs Task Force (ATP TF).Department of Defense Aviation Safety Technologies Report[R]. Washington, DC: Defense Safety Oversight Council, Office of the Under Secretary of Defense for Personnel and Readiness, 2009: 22-23
[2] WHITEHOUSE G R, WACHSPRESS D A, QUACKENBUSH T R, KELLER J D.Exploring aerody-namic methods for mitigating brownout[C]//The American Helicopter Society 65th Annual Forum, Alexandria, AHS, 2009: 349-364.
[3] TRITSCHLER J K, SYAL M, CELI R, LEISHMAN J G.A methodology for rotorcraft brownout mitigation using rotor design optimization[C]// The American Helicopter Society 66th Annual Forum, Alexandria, AHS, 2010: 1674-1690.
[4]Wong O D, TANNER P E.Photogrammetric measure-ments of an EH-60L brownout cloud[J].Journal of the American Helicopter Society, 2016, 61(1):1-10
[5] SYDNEY A, LEISHMAN J G.Measurements of ro-tor/airframe interactions in ground effect over a sediment bed[C]//The American Helicopter Society 69th Annual Fo-rum, Alexandria, AHS, 2013.
[6]MILLUZZO J I, LEISHMAN J G.Vortical sheet behavior in the wake of a rotor in ground effect[J].AIAA Journal, 2017, 55(1):24-35
[7] WACHSPRESS D A, QUACKENBUSH T R, BOSCHITSCH A H.First-principles, free-vortex wake model for helicopters and tiltrotors[C]//The American Helicopter Society 59th Annual Forum, Alexandria, AHS, 2003: 307-330.
[8] WACHSPRESS D A, WHITEHOURSE G R, KELLER J D, YU K.A high fidelity brownout model for real-time flight simulations and trainers[C]//The American Helicopter Society 65th Annual Forum, Alexandria, AHS, 2009: 1281-1304.
[9] ANDREA A D.Numerical analysis of unsteady vortical flows generated by a rotorcraft operating on ground: a first assessment of helicopter brownout[C]//The American Helicopter Society 65th Annual Forum, 2009: 423-446.
[10] KELLER J D, WHITEHOUSE G R, WACHSPRESS D A, TESKE M E, QUACKENBUSH T R.A physics-based model of rotorcraft brownout for flight simulation appli-cations[C]//The American Helicopter Society 62nd Annual Forum, Alexandria, AHS, 2006: 1097-1107.
[11]Syal M, Leishman J G.Predictions of brownout dust clouds compared to photogrammetry measurements[J].Journal of the American Helicopter Society, 2013, 58(2):1-18
[12] GOVINDARAJAN B, LEISHMAN J G.Predictions of rotor and rotor/airframe configurational effects on brown-out dust clouds[C]//The American Helicopter Society 70th Annual Forum, Montreal, Alexandria, AHS, 2014: 407-433.
[13]胡健平, 徐国华, 史勇杰, 吴林波.基于-耦合数值模拟的全尺寸直升机沙盲形成机理[J].航空学报, 2020, 41(3):159-173
[14]HU J P, XU G H, SHI Y J, et al.Formation mechanism of brownout in full-scale helicopter based on CFD-DEM couplings numerical simulation[J].Acta Aeronautica et Astronautica Sinica, 2020, 41(3):123363-173
[15]TAN J F, SUN Y M, BARAKOS, G N.Vortex approach for downwash and outwash of tandem rotor in ground ef-fect[J].Journal of Aircraft, 2018, 55(6):2491-2509
[16]谭剑锋, 周天熠, 王畅, 于领军.旋翼地面效应的气动建模与特性[J].航空学报, 2019, 40(6):122602-12
[17]TAN J F, ZHOU T Y, WANG C, et al.Aerodynamic model and characteristics of rotor in ground effect[J].Acta Aeronautica et Astronautica Sinica, 2019, 40(6):122602-12
[18]TAN J F, CAI J G, BARAKOS G N, Wang C, Huang M Q.Computational study on the aerodynamic interference between tandem rotors and nearby obstacles[J].Journal of Aircraft, 2020, 57(3):456-468
[19]谭剑锋, 王浩文, 吴超, 林长亮.基于非定常面元粘性涡粒子混合法的旋翼平尾非定常气动干扰研究[J].航空学报, 2014, 35(3):643-656
[20]TAN J F, WANG H W, WU C, LIN C L.Ro-torempennage unsteady aerodynamic interaction with unsteady panelviscous vortex particle hybrid method[J].Acta Aeronautica et Astronautica Sinica, 2014, 35(3):643-656
[21] CUNDALL P A, STRACK O D L.A discrete numerical model for granular assemblies[J][J].Geotechnique, 1979, 29(1): 47-65
[22] MINDLIN R D, DERESIEWICZ H.Elastic spheres in contact under varying oblique forces[J]. [J].Transactions of ASME, Series E. Journal of Applied Mechanics,, 1993, , 20(1):327-344
[23] RENZO A D, MAIO F P D.Comparison of contact-force models for the simulation of collisions in DEM-based granular flow codes[J]. [J].Chemical Engineering Science,, 2004, , 59:(1):525-541.
[24] Lee, T.E., Leishman, J. G., and Ramasamy, M. Fluid dynamics of interacting blade tip vortices with a ground plane[J].[J].Journal of the American Helicopter Society,, 2010, 55(2):22005-1, -22005-16.
[25] Lakshminarayan, V.K., Kalra, T. S., Baeder, J. D. De-tailed computational investigation of a hovering microscale rotor in ground effect[J]. [J].AIAA Journal,, 2013; , 51 (4):893-909.
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