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

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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. The objective of this research program is to use the most recent measurement techniques to capture separation phenomena and to analyze and understand the physical mechanisms that generate them. In the medium term, we will check, validate or complement the ideas emanating from theories and in the longer term it is essential to develop a practical tool for separation detection to tackle engineering problems.