Modelling and Simulation of helicopters and tilt-rotors in Vortex Ring State

基于Navier-Stokes方程的计算流体力学的进展使得能够在真实的时间尺度内计算螺旋桨周围的流动。这种模拟最成熟的方法是采用非定常雷诺平均的N-S方法，这种方法在精度和效率之间取得了很好的折衷。这是因为URANS不需要解析所有的流动尺度，这导致CFD网格的大小为数百万个单元，时间步长是粒子弦旅行时间的1/100。结果可用于计算平均压力场和速度场，以及流动中存在的最慢频率。当转换为声学时，该方法只能给出音调噪声，而宽带谱仍然未被分辨。从几何复杂性来看，使用结构或非结构网格是可能的，通过使用重叠或滑动网格，预测可以考虑安装的螺旋桨系统机翼和机舱之间的相互作用。格拉斯哥的直升机多区块方法具备了基于URANS方法计算已安装螺旋桨周围流动的所有要素，因此它是这一领域研究工作的良好起点。另一方面，为了解决螺旋桨流动中存在的越来越多的谐波以及声谱的宽带部分，需要更复杂的技术，这些技术基于湍流的模拟而不是建模。格拉斯哥在计算武器舱和直升机旋翼内的流动方面拥有丰富的分离涡流模拟经验。原则上，DES可用于螺旋桨，并应允许以牺牲更多的计算资源为代价，对大部分流动光谱进行分辨。对于螺旋桨叶片周围的流动，采用这种方法时应采用千万单元数量级的网格，时间步长应为弦线走时的1/10000。当然，这导致所需的CPU时间与URAN相比增加了100倍(由于在HMB解算器中使用了高效的时间积分方案)。以避免与此方法相关的惩罚。最近开发的结构自适应模拟或SAS应该进行螺旋桨流动的测试。这种方法的结果应该接近DES，而成本几乎是URANS方法的两倍。因此，建议采用三重策略，首先评估螺旋桨流动的URANS和DES，然后将SAS的潜在收益与承诺的与DES方法相比减少的CPU时间进行比较。上述任何一种方法的CFD结果都可以与许多工具相结合，以便进一步利用远场气动声学方法来开发压力场。格拉斯哥拥有在直升机旋翼领域流行的FW-H方法的经验。该方法利用CFD产生的非定常压力，基于线性化的声学方程，在远离噪声源的距离处产生螺旋桨的声学特征，厚度、载荷和宽带噪声源可以在近场分辨，并利用FWH方法在CFD域进一步传播。此外，尾缘噪声应该通过细网格DES解决方案来解决。第二种方法可以与CFD结果相结合，从而预测配备螺旋桨的飞机驾驶舱内的噪声水平。这将产生一个集成的仿真环境，在这个环境中可以同时研究螺旋桨的性能和声学，并用于远场和舱室噪声预测。

Progress with computational fluid dynamics based on the Navier-Stokes equations allows for the computation of flows around propellers within realistic time-scales. The most established method for such simulations is to employ the Unsteady Reynolds-Averaged Navier-Stokes method that is a good compromise between accuracy and efficiency. This is because URANS doesn't need to resolve all flow scales which leads to CFD meshes of the size of millions of cells and time steps that are of 1/100 of the chord travel-time of particles. The results can be used for the computation of the mean pressure and velocity fields as well as the slowest frequencies present in the flow. When translated to acoustics, the method can only give tone-noise while the broadband spectrum remains under-resolved.In terms of geometric complexity, the use of structured or unstructured grids is possible, and by employing overset or sliding meshes, the predictions can account for the interaction between the wing and nacelle of an installed propeller system. The Helicopter Multi-Block method of Glasgow has all the ingredients for computing the flow around installed propellers based on the URANS approach and it is therefore a good starting point for research work in this area. On the other hand, the need to resolve more and more harmonics as well as the broad-band part of the acoustic spectrum present in propeller flows, requires more sophisticated techniques that are based on simulation rather than modelling of turbulence. Glasgow has substantial experience with Detached Eddy Simulation for the computation of flows inside weapon bays and helicopter rotors. DES could, in principle, be used for propellers and should allow for the resolution of a large part of the flow spectrum at the expense of more computational resources. For a flow around a propeller blade, grids of the order of 10 million cells should be used with this method and time steps of the order of 1/10000 of the chord travel-time. This of course leads to an increase of the required CPU time by a factor of 100 in comparison to URANS (due to the use of efficient time-integration schemes in the HMB solver). To avoid the penalty associated with this method. The recently-developed Structure-Adaptive-Simulation or SAS should be tested for propeller flows. This method should give results close to the DES at almost twice the cost of the URANS method. It is therefore advisable to adopt a triple strategy that begins with the evaluation of the URANS and DES for propeller flows and then compare the potential gains of SAS with the promised reduction in CPU time in comparison to the DES method. The CFD results of any of the above method can be combined with a number of tools for the further exploitation of the pressure field in conjunction with far-field aeroacoustics methods.Glasgow has experience with the FW-H method that is popular in the field of helicopter rotors. The method uses the CFD-generated unsteady pressure and based on the linearized acoustics equations, produces the acoustic signature of the propeller at distances far apart from the source of noise.Thickness, loading and broadband noise sources could be resolved in the near-field and propagated further of the CFD domain with the FWH method. In addition, trailing edge noise should be resolved by a fine-mesh DES solution.A second method that could be combined with the CFD results could lead to the prediction of the noise-level inside the cabin of an aircraft equipped with propellers. This would lead to an integrated simulation environment where the propeller performance and its acoustics could be studied and used for the far-field and cabin noise predictions at the same time.