Streaky Structures in a Wing Boundary Layer at High Free Stream Turbulence and Modelling of the Transition in Such Flow

摘要

Raised interest of researchers to a problem of the laminar-turbulent transition at high free stream turbulence can be observed recently. The given interest is connected to a lot of reasons both of the scientific nature and requirements of the modern engineering with point of view its new opportunities. It is known that laminar-turbulent transition at low free stream turbulence is occurred caused by the process of generation and development of the instability waves so-called Tollmien-Schlichting (TS) waves. This type of transition was in details investigated by many researchers and continues to be studied in nowadays. A more complex way of understanding of the laminar flow breakdown mechanism into the turbulent state for the wall shear layers where the boundary layer flow modulated by longitudinal localized in spanwise direction structures. In this case the classical scenario of transition due to the mechanism of spatial evolution of a TS wave ceases to work and it is necessary to understand what processes result in occurrence of longitudinal structures and what mechanism of their breakdown. By one of such transition mechanisms is the transition mechanism at high free stream turbulence. Experiments in the natural conditions were shown that the flat plate boundary layer at high free stream turbulence was modulated by streaky structures in spanwise direction. It was shown that the spanwise scale of these structures correlated with a boundary layer thickness. On the other hand all researchers observed the turbulent spots origination at last stages of transition. However the more detailed development characteristics of the given structures as well as the mechanism of their generation development and breakdown was not investigated.