Advances on helicopter brownout
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摘要: 直升机近地飞行时，旋翼近地面干扰诱发非定常流动，推动地面沙尘扬起，形成“沙盲”现象，严重威胁直升机飞行安全，针对“沙盲”现象开展气动基础问题研究非常重要。本文从直升机“沙盲”计算方法、试验方法、形成机理、抑制方法等4个角度介绍直升机“沙盲”现象相关研究进展。综合相关研究成果可以发现：耦合涡方法（或CFD方法）和拉格朗日沙粒跟踪方法能够实现沙云轮廓和“沙盲”现象模拟，但尚需进一步体现复杂沙床表面、脉动湍流及沙粒迁移、聚集、起跳和扬起等关键因素；基于高速PIV和相机搭建的“沙盲”试验系统能够获得沙云形态数据，但仍需进一步研究沙云两相流测试技术和沙云空间浓度测试技术，深入探索沙粒驱动机制和“沙盲”演化机理，以及能够有效抑制“沙盲”现象的方法和技术途径。Abstract: The interference of the helicopter rotor and the ground causes the lifting of dust, which leads to the phenomenon of brownout. The phenomenon threatens the flight safety of helicopter seriously, and therefore, it is necessary to carry out relevant researches on the aerodynamic basis for the brownout. In this paper, the research progress of helicopter brownout is deeply analyzed from four aspects: numerical methods, experimental techniques, formation mechanism, and suppression methods. Investigations demonstrate that the coupled vortex method or CFD method and the Lagrange sand tracking method can realize simulations of the sand cloud profile and the phenomenon of brownout, but the key factors such as the complex sand bed surface, fluctuating turbulence, migration of sand particles, aggregation, and lifting of sand particles need to be concentrated on. The brownout experimental system built with high-speed PIV techniques and high-speed camera can obtain the morphology data of the sand cloud, but it is still necessary to study the simulation methods of the complex sand bed and the measurement techniques of dust spatial concentration, to explore the driving mechanism of sand particles and the evolution mechanism of brownout, and to develop the design methods and technical approaches to effectively weaken brownout.
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图 23 旋翼近地面干扰的宏观流动[57]
Figure 23. Macroscopic flow of rotor and ground interference[57]
图 31 沙云中不同粒径沙粒的浓度及质量密度[66]
Figure 31. Concentration and mass density of sand particles of different particle size scales in dust clouds[66]
图 34 “沙盲”现象的流场与沙尘特性示意图[54]
Figure 34. A schematic of characteristics of flow field and sand dust in brownout phenomenon[54]
图 35 基于平均下洗速度与尾流强度衡量直升机沙盲严重性[69]
Figure 35. Levels of brownout for different types of helicopters based on the average downwash speed and wake intensity[69]
图 37 UH–60直升机基准旋翼和优化旋翼在下降飞行状态下形成的沙云特性[70]
Figure 37. Sand cloud characteristics in descending flight for UH–60 benchmark rotor and optimized rotor[70]
图 38 EH–101直升机旋翼与沙云抑制效果[71]
Figure 38. EH–101 helicopter rotor and suppression effect of dust cloud[71]
图 39 英国S–61直升机换装的旋翼与沙盲云[70]
Figure 39. Rebladed British S–61 helicopter and brownout cloud[70]
图 40 直升机在低能见度环境中的降落程序示意[73]
Figure 40. Helicopter landing procedure in degraded visual environ-ments[73]
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