Large-eddy simulations of complex turbulent flows

湍流发生在大多数流体流动中。它的复杂性是由于湍流传输主要是由于连贯的、螺旋状的运动（“涡流”），这些运动在性质上是确定的，但在随机的时间和地点出现。例如，在决定飞机的阻力或热交换器的传热时，湍流传输起着主要作用。明确解析涡流的求解方法越来越受欢迎，因为可以对流进行更高的精度和完整性分析；其中包括大涡模拟（LES）。该提案将首先侧重于开发改进的LES模型和方法，以减少其计算需求并加速其向工业过渡；第二，关于LES在复杂流程中的应用。我的团队最近开发了一个新的模型来参数化小涡流[Piomelli et al.， J. Fluid Mech.]。[j]，在最初的测试中，它已经证明自己比现有模型更准确，并且在计算上更便宜。我们建议修改它以更好地响应湍流的局部状态，并将其扩展到更现实的流动。此外，我们将努力开发一个“最优LES”，其中计算网格适应流动，使网格尺寸与湍流特征长度尺度的比例是均匀的。这种改进有可能将模拟的成本降低几个数量级，因为目前的应用程序使用的网格在某些区域过于精细，而在其他区域则处于边缘。第二个主题是LES在技术相关问题上的应用，与其他技术相比，LES提供的对流动物理的更详细的理解可以带来显着的好处。首先，我们建议研究粗糙度对传热的影响，并探索确定能量传输的未知机制，为什么当粗糙度存在时与动量传输的类比不成立，以及如何建模。其次，我们将研究河流沙丘的流动，将我们之前的研究扩展到现实配置，包括沙的运动和沙丘的动态演化。最后，我们将应用LES来研究前缘凸起如何影响机翼的性能，以及哪种形状可以改善机翼的气动性能和声发射。总之，这项建议将采取双管齐下的办法，协调方法改进、基础研究和应用。该模型的开发旨在显著降低计算成本，基础研究和应用将使用LES增加的能力来理解复杂的物理，以前无法接近的问题。这一建议还将导致至少四名HQP的培训。

Turbulence occurs in most fluid flows. Its complexity is due to the fact that turbulent transport is mostly due to coherent, whorl-like motions (“eddies”), which are deterministic in character, but appear at random times and locations. Turbulent transport plays the primary role in determining, for instance, the drag of an aircraft or the heat transfer in a heat exchanger. Solution methods in which the eddies are explicitly resolved are gaining popularity because of the higher accuracy and completeness with which the flow can be analyzed; among them is the large-eddy simulation (LES). This proposal will focus, first, on the development of improved models and methodologies for LES, to decrease its computational demands and accelerate its transition to industry; second, on the application of LES to complex flows. My group has recently developed a new model to parameterize the small eddies [Piomelli et al., J. Fluid Mech., 766:499-527, 2015], which, in initial tests, has shown itself to be accurate and computationally cheaper than existing models. We propose to modify it to respond better to the local state of the turbulence, and extend it to more realistic flows. Furthermore, we will work towards the development of an "optimal LES" in which the computational grid adapts to the flow so that the ratio of grid size to the characteristic length-scale of turbulence is uniform. This improvement has the potential of decreasing the cost of a simulation by orders of magnitude, since present applications use grids that are overly refined in some regions and marginal in others. The second theme is the application of LES to problems of technological relevance, in which the more detailed understanding of the flow physics that LES affords in comparison to other techniques can result in significant benefits. First, we propose to study the effect of roughness on heat transfer, and explore the unknown mechanisms that determine the transport of energy, why the analogy with the transport of momentum does not hold when roughness is present, and how to model it. Second, we will investigate the flow over river dunes, extending our previous research to real-life configurations, including the sand motion and the dynamical evolution of the dune. Finally, we will apply LES to study how leading-edge protuberances can affect the performance of wings, and which shapes can improve the aerodynamic behaviour and sound emission of the wing. In summary, this proposal will take a two-pronged approach that coordinates methodological improvements, fundamental studies and applications. The model development is designed to decrease significantly the cost of the calculations, and the fundamental studies and applications will use the increased capabilities of LES to understand the physics of complex, previously unapproachable, problems. This proposal will also result in the training of at least four HQP.