Flow control and origins of turbulence

该项目的重点是制定利用分布式强迫进行流动操纵/控制/重新安排的战略。术语“分布式作用力”在这里指的是在固体/流体界面产生弱力的各种方法；这与局部施加的集中和/或强作用力相反。产生分布式强迫的方法可以依赖于使用地表粗糙度、微电子机械装置(MEMS)、壁面蒸腾、离子风效应、加热等，并且可以应用于层流、过渡流和湍流。与强强迫相比，弱分布强迫的产生与较轻的重量和较小的压力损失惩罚有关。虽然强强迫总是被认为会在气流中产生巨大的变化，但弱强迫只有在具有正确的时空分布的情况下才可能有效。据估计，弱强迫可能会导致其模式的特征尺度的流动的量级的变化，而不仅仅是其量级的变化。这种策略为了有效，必须利用各种流动不稳定性，这些不稳定性以一种自然的方式在不同模式之间重新分配总流动能量。如果控制/重新安排策略设计得当，只需要少量外部能量输入即可启动重新安排过程。初步结果表明，只要强迫的空间分布相似，无论产生强迫的物理机制是什么，流动系统的响应都表现出一般的相似性；这一事实突显了所提出的方法的稳健性及其广泛的适用性。该项目将探索的应用包括设计燃料电池的微型混合器、微型热交换器、层流翼型、可导致低阻力的表面纹理、在湍流中操纵大型结构的技术、依靠表面振动的推进装置和哮喘诊断程序等。

The project is focused on the development of strategies for flow manipulation/control/re-arrangements using distributed forcing. The term "distributed forcing" denotes here various methods of producing weak forces at the solid/fluid interface; this is opposite to the concentrated and/or strong forcing applied locally. Methods of generating distributed forcing may rely on the use of surface roughness, micro-electro-mechanical devices (MEMS), wall transpiration, ionic wind effect, heating, etc., and can be applied to laminar, transitional and turbulent flows. Weak distributed forcing is of interest because its creation is associated with lower weight and smaller pressure loss penalty as compared with that required by a strong forcing. While strong forcing is always expected to produce large changes in the flow, a weak forcing may be effective only if it has correct spatial/temporal distribution. It is expected that weak forcing may create changes in the flow of the order of the characteristic scale of its pattern rather than just of the order of its magnitude. This strategy, in order to be effective, must take advantage of various flow instabilities, which re-distribute in a natural way the total flow energy among different modes. Only a small external energy input will be required in order to initiate the re-arrangement process if the control/re-arrangement strategy is properly designed. Preliminary results show that responses of flow systems show generic similarities regardless of the physical mechanisms used to generate forcing as long as the spatial distribution of forcing is similar; this fact underlines the robustness of the proposed approach and its wide range of applicability. Applications to be explored in this project include design of micro-mixers for fuel cells, micro-heat exchangers, laminar airfoils, surface textures that could lead to lower drag, techniques for manipulation of large structures in turbulent flows, propulsive devices relying on surface vibrations and asthma diagnosis procedures, among many others.