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Development of a Lagrangian-Lagrangian methodology to predict brownout dust clouds.

机译：拉格朗日-拉格朗日方法的发展，以预测电力不足的尘埃云。

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摘要

A Lagrangian-Lagrangian dust cloud simulation methodology has been developed to help better understand the complicated two-phase nature of the rotorcraft brownout problem. Brownout conditions occur when rotorcraft land or take off from ground surfaces covered with loose sediment such as sand and dust, which decreases the pilot's visibility of the ground and poses a serious safety of flight risk. The present work involved the development of a comprehensive, computationally efficient three-dimensional sediment tracking method for dilute, low Reynolds number Stokes-type flows. The flow field generated by a helicopter rotor in ground effect operations over a mobile sediment bed was modeled by using an inviscid, incompressible, Lagrangian free-vortex method, coupled to a viscous semi-empirical approximation for the boundary layer flow near the ground. A new threshold model for the onset of sediment mobility was developed by including the effects of unsteady pressure forces that are induced in vortically dominated rotor flows, which can significantly alter the threshold conditions for particle motion. Other important aspects of particle mobility and uplift in such vortically driven dust flows were also modeled, including bombardment effects when previously suspended particles impact the bed and eject new particles. Bombardment effects were shown to be a particularly significant contributor to the mobilization and eventual suspension of large quantities of smaller-sized dust particles, which tend to remain suspended. A numerically efficient Lagrangian particle tracking methodology was developed where individual particle or clusters of particles were tracked in the flow. To this end, a multi-step, second-order accurate time-marching scheme was developed to solve the numerically stiff equations that govern the dynamics of particle motion. The stability and accuracy of this scheme was examined and matched to the characteristics of free-vortex method. One-way coupling of the flow and the particle motion was assumed. Particle collisions were not considered. To help reduce numerical costs, the methodology was implemented on graphic processing units, which gave over an order of magnitude reduction in simulation time without any loss in accuracy. Validation of the methodology was performed against available measurements, including flow field measurements that have been made with laboratory-scale and full-scale rotors in ground effect operations. The predicted dust clouds were also compared against measurements of developing dust clouds produced by a helicopter during taxi-pass and approach-to-touchdown flight maneuvers. The results showed that the problem of brownout is mostly driven by the local action of the rotor wake vortices and the grouping or bundling of vortex filaments near the sediment bed. The possibilities of mitigating the intensity of brownout conditions by diffusing the blade tip vortices was also explored. While other means of brownout mitigation may be possible, enhancing the diffusion of the tip vortices was shown to drastically reduce the quantity of mobilized particles and the overall severity of the brownout dust cloud.
机译：拉格朗日-拉格朗日尘云模拟方法已经开发出来，以帮助更好地理解旋翼飞机停电问题的复杂的两相性质。当旋翼飞机降落或从覆盖有松散沉积物（如沙子和灰尘）的地面上起飞时，会出现电源不足的情况，这会降低飞行员的地面可见度，并带来严重的飞行安全风险。目前的工作涉及开发一种全面的，计算效率高的三维泥沙追踪方法，用于稀薄的低雷诺数斯托克斯型流。通过使用不粘，不可压缩的拉格朗日自由涡旋方法，结合地面附近边界层流的粘性半经验近似，对直升机旋翼在移动沉积床上的地面效应操作中产生的流场进行了建模。通过包括在以涡流为主的转子流中引起的不稳定压力的影响，开发了一种新的泥沙运动阈值模型，该模型可以显着改变粒子运动的阈值条件。还模拟了在这种垂直驱动的粉尘流中颗粒迁移和抬升的其他重要方面，包括先前悬浮的颗粒撞击床层并喷射出新颗粒时的轰击效应。轰击效应被证明是动员和最终悬浮大量倾向于保持悬浮状态的大量较小尺寸灰尘颗粒的一个特别重要的贡献。开发了一种数值有效的拉格朗日粒子跟踪方法，该方法在流中跟踪单个粒子或粒子簇。为此，开发了一种多步，二阶精确时间行进方案，以解决控制粒子运动动力学的数值刚性方程。研究了该方案的稳定性和准确性，并与自由涡旋法的特点相匹配。假定流与粒子运动的单向耦合。没有考虑粒子碰撞。为了帮助减少数值成本，该方法在图形处理单元上实施，从而使仿真时间缩短了一个数量级，而没有任何精度损失。该方法论的验证是针对可用的测量结果进行的，包括在地面效应操作中使用实验室规模和满量程转子进行的流场测量。还将预测的尘埃云与直升机在滑行和进近着陆飞行机动过程中产生的尘埃云的测量结果进行了比较。结果表明，节电问题主要是由转子尾流涡流的局部作用以及沉积床附近涡流丝的成束或成束引起的。还探索了通过扩散叶片尖端涡流来减轻掉电状况强度的可能性。尽管可以采用其他方法来减少掉电，但已证明，增强尖端涡流的扩散可以大大减少动员颗粒的数量，并可以降低掉电尘埃云的总体严重性。

著录项

  • 作者

    Syal, Monica.;

  • 作者单位

    University of Maryland, College Park.;

  • 授予单位 University of Maryland, College Park.;
  • 学科 Engineering Aerospace.
  • 学位 Ph.D.
  • 年度 2012
  • 页码 406 p.
  • 总页数 406
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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