Extraction of separation in three-dimensional unsteady and turbulent experimental flows

流体力学中的分离现象既普遍又不可预测。它会造成飞机机翼的气动失速，降低涡轮机械的性能，限制水轮机的效率。虽然人们早就知道它，但到目前为止，我们既不能探测到它，也不能预测它。当它发生时，沿着固体壁面流动的流体会突然分离。为了检测它，常识告诉我们，分离点应该与壁面上流体摩擦力为零的位置重合。这使得路德维希·普朗特尔在1904年定义了一个预测二维稳定流分离的标准。结果表明，在存在三维或非定常现象的情况下，该准则在实例中没有得到验证。尽管许多研究人员已经研究了这个问题，但确定一个严格而实用的预测流动分离的准则仍然具有挑战性。因此，我们不得不等到2004年才看到乔治·哈勒（George Haller）提出的一个精确而完整的理论。这个新理论超出了简单的检测标准。事实上，它预测，对于具有明确的稳定平均值的流动，分离线或表面在流动内部运动时，必须保持固定在固体壁上。这些理论基础没有对所研究的流动的性质作任何假设，因此它适用于任何条件。

The separation phenomenon in fluid dynamics is as common as unpredictable. It causes the aerodynamic stall of aircraft wings, reduces the performance of turbomachines and limits the efficiency of hydraulic turbines. Although it has long been known, to date we can neither detect nor predict it. When it occurs, the fluid flowing along a solid wall suddenly separates. To detect it, common sense suggests us that the separation point should coincide with the location where fluid friction on the wall is zero. This led Ludwig Prandtl to define in 1904 a criterion which predicts the separation of two-dimensional steady flows. It turns out that in the presence of three-dimensional or unsteady phenomena, this criterion is not verified in ex. Although many researchers have examined the problem, the determination of a rigorous and practical criterion to predict flow separation is still challenging. We thus had to wait until 2004 to see an exact and complete theory formulated by George Haller. This new theory is beyond the simple detection criterion. Indeed, it predicts that for flows with a well-defined steady mean, the separation line, or surface, while moving inside the flow, must remain fixed on the solid wall. These theoretical foundations make no assumptions about the nature of the flow studied, so that it applies to any conditions.