直升机"沙盲(Brownout)"现象阻挡飞行员视线,导致直升机垂直起降、贴地飞行困难,甚至引发飞行事故。为研究直升机的"沙盲"特性,建立基于离散动力学的沙粒DEM(Discrete Element Method)模型和沙粒-流场耦合模型,嵌入旋翼黏性涡粒子和黏性地面气动模型,体现旋翼流场作用下沙粒移动、碰撞等特性,构建直升机"沙盲"现象计算方法。通过与美国陆军Yuma试验场EH-60L"沙盲"飞行试验结果对比,表明本文"沙盲"计算方法得到的"沙云"形状、扬起位置、高度、变化过程与飞行试验测量结果基本一致,且相比于基于沙粒夹带模型、沙粒起跳模型、速度罚值沙粒通量模型的拉格朗日沙粒跟踪方法,本文方法得到的"沙云"轮廓精度更高,与飞行测量结果更吻合。随后研究悬停和前飞状态"沙盲"形成特性,结果表明旋翼桨尖涡与地面干扰形成地面射流,推动沙粒移动堆积,诱发沙粒碰撞起跳,并在旋翼/地面干扰流场作用下扬起形成沙云,覆盖直升机前方视野,构成"沙盲"现象。
Helicopter brownout blocks out pilots’ sight, leads to difficulty in landing and nap-of-the-earth flight, and induces flight accidents. A sand particle Discrete Element Method (DEM) model based on discrete dynamics and a sand particle-flow interaction model are established and coupled with the rotor viscous vortex particle model and viscous ground aerodynamic model to account for translation and collision of sand particles under the effect of the 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 for comparison. It is shown that the predicted shape, position and height of uplift, process of the sand cloud are consistent with those of 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 in better agreement with the flight test result. Then, the processes of helicopter brownout in hover and forward flight are investigated. Results show that the wall jet induced by the rotor tip-vortex and ground interaction yields translation and sediment trapping of sand, producing bombardment ejection and saltation. Uplift of sediment induced by the flowfield generates dust cloud, and covers the front view of the helicopter, thus forming brownout.
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