Smart skin for control of wall-bounded turbulent flows

边界层是固体表面附近的一层薄薄的流体，其中流体速度从固体表面的速度转变为整体流体速度。流体中的剪应力产生这种速度梯度，并以“表面摩擦”的形式投射在表面上。在大多数工业情况下，例如，在飞机上方或管道内的流动中，边界层中流体元素的运动是混沌的。这样的流体层被称为湍流边界层(TBL)，并且具有更大的速度梯度，从而产生更大的表面摩擦。此外，当增加的压力梯度施加在TBL上时，如在扩散器或飞机机翼上，TBL可能会从表面分离。这就产生了“压力阻力”和强烈的流体-结构相互作用。因此，TBL是飞机、潜水器、船舶和管道中高油耗、高污染和结构振动的主要来源。由于这种综合效应，在控制TBL方面取得的任何进展都将对我们的社会经济和环境保护需求产生深远的影响。这项研究计划的主题是使用一种新型的“智能皮肤”来控制附着和分离的TBL。为了实现这一目标，我们将首先对附着和分离的TBL中大范围运动的表面压力进行实验表征，以开发一个用于湍流控制的预测模型。同时，我们将开发可变形的表面，可以根据需要产生局部表面凹陷或突出。具有与压力足迹匹配的频率和位移的线性执行器将被集成在可变形的皮肤下。我们将把一系列主动可变形面板和表面压力传感器安装到一种“智能皮肤”中。最后，将在用于主动控制附着式和分离式TBL的闭环系统中对“智能皮肤”的性能进行评估。从长远来看，拟议的计划将增加我们对TBL中相干运动的基础知识，并为如何控制TBL提供指导。开发的“智能皮肤”将使操纵各种湍流成为可能，包括TBL。智能皮肤还将成为先进控制算法、人工智能和机器学习应用的试验台。这样的方法最终可以使我们处理动荡的方式现代化。它可以通过将湍流控制引入各种工业部门来实现突破，以提高效率并减少环境足迹。此外，这一研究计划的多学科性质将培养在各种学科中接受培训的高素质人员。

A boundary layer is a thin layer of fluid in the immediate vicinity of a solid surface where fluid velocity transitions from the velocity of the solid surface to the bulk fluid velocity. The shear stress in the fluid generates this velocity gradient, and projects as `skin-friction' on the surface. In most industrial situations, for example, the flow over an aircraft or inside a pipeline, the motions of fluid elements in the boundary layer are chaotic. Such a fluid layer is known as a turbulent boundary layer (TBL), and has a larger velocity gradient that generates greater skin-friction. In addition, when an increasing pressure gradient is imposed on a TBL, like in a diffuser or on an aircraft wing, the TBL may separate from the surface. This generates `pressure drag' and strong fluid-structure interactions. As a result, the TBL is the chief source of high fuel consumption, pollution, and structural vibration in aircrafts, submersibles, ships, and pipelines. Due to this integral effect, any progress in the control of TBLs will have a profound impact on our socio-economic and environmental protection needs. The subject of this research program is the control of attached and separated TBLs using a novel `smart skin'. To achieve this goal, we will first experimentally characterize the surface pressure of large-scale motions in attached and separated TBLs to develop a predictive model for turbulence control. In parallel, we will develop deformable surfaces that can generate on-demand local surface depression or protrusion. Linear actuators, with a frequency and displacement that matches the pressure footprint, will be integrated under a deformable skin. We will implement an array of active deformable panels and surface pressure sensors into a `smart skin'. Finally, the performance of the `smart skin' will be evaluated in a closed-loop system for active control of attached and separated TBLs. In the long-term, the proposed program will increase our fundamental knowledge of coherent motions in TBLs and provide guidelines on how to control TBLs. The developed `smart skin' will make it possible to manipulate a variety of turbulent flows including TBLs. The smart skin will also become a test-bench for the application of advanced control algorithms, artificial intelligence, and machine learning. Such an approach can ultimately modernize how we handle turbulence. It can make a breakthrough by bringing turbulence control to a variety of industrial sectors to increase efficiency and reduce environmental footprints. In addition, the multidisciplinary nature of this research program will train highly qualified personnel who are trained in a variety of disciplines.