Secondary Flows in Boundary Layers and Jets

Abstract

In a gas or fluid flow having lost its stability parallel with quickly changing components of the motion (disturbances) there are also developing relatively long-living large-scale vortex formations — coherent structures that are characterized by shape, dimensions and intensity. The orientation of the vortex axis to a main flow appears to be a characteristic of paramount importance of such formations. Thus, if in some region of the flow the coordinates x, y and z are chosen in such a way that the flow has a shape of a simple gradient flow, then it is natural to call the vortices directed along these axes longitudinal, transverse and lateral. The interaction of such vortices with a main flow is different. A transverse vortex becomes strongly deformed by a main flow and has an evident tendency to a fast transformation into an inclined one close to longitudinal one. It is difficult to observe it and complicated to compute. The longitudinal and lateral vortices become deformed by a main flow relatively slow, they are successfully recorded by the up-to-date means of diagnostics and in the theoretical developments the mechanisms of their formation are studied. It has been elucidated that they may arise and develop due to the nonlinear interaction of different kind fluctuations and wave motions taking place in the unstable flow. The carried out estimates and calculations for a number of characteristic flows (in channels, boundary layers and jet flows) have shown a reasonable nature and quantitative confirmation of the mechanisms suggested.