Flow control and origins of turbulence

该项目的重点是利用分布式强迫开发流量操纵/控制/重新安排策略。“分布力”一词在这里指在固/液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。产生分布强迫的方法可以依赖于使用表面粗糙度、微机电装置（MEMS）、壁面蒸腾作用、离子风效应、加热等，并且可以应用于层流、过渡流和湍流。弱分布强迫之所以令人感兴趣，是因为与强强迫相比，它的产生与较低的重量和较小的压力损失损失有关。虽然强强迫总是预期会产生大的流量变化，但弱强迫只有在具有正确的时空分布时才可能有效。预计弱强迫可能造成其流型特征尺度量级的变化，而不仅仅是其量级量级的变化。该策略必须充分利用各种流动不稳定性，使总流动能量在不同模态之间进行自然的重新分配。如果控制/重排策略设计得当，启动重排过程只需要很小的外部能量输入。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如何，流动系统的响应都具有一般的相似性；这一事实强调了所提出的方法的稳健性及其广泛的适用性。在这个项目中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、降低阻力的表面纹理、在湍流中操纵大型结构的技术、依靠表面振动和哮喘诊断程序的推进装置等等。

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.