给出了转捩边界层中流向涡产生的物理过程。这个过程是A-涡和二次涡环相互作用产生旋涡轴向失稳断裂造成的。

This study is concerned with transition in flat plate boundary layer flow. Sets of results are obtained as follows: (1) Very clear pictures of the formation and the development of the butterfly-like structures rather than?-structures in the K-regime of boundary layer transition are obtained. (2) A chain of ring like vortices, which generate the high-frequency spikes on the time traces of velocity and still present periodical behaviour, at the tip of each?-vortex, which is the part of the butterfly-like structure, are visualized for the frst time. (3) A wave-like structure is observed to occupy the whole boundary layer, extending from the near-wall region to the outer edge of the boundary layer.

An experimental investigation was undertaken to reveal the characteristic flow structure in the regime of nonlinear boundary layer instability and onset of turbulence. A controlled Tollmien-Schlichting wave was introduced into a two-dimensional boundary layer over a flat plate to study the growth and evolution of the controlled disturbances using both hot film measurements and an improved hydrogen bubble visualization technique. The results demonstrate that the actual breakdown of the laminar boundary layer and the breakdown of the L vortices do not imply the immediate onset of turbulence. Rather, the onset occurs later with the breakdown of the long streaks. Therefore, an alternative transitional scenario was developed.

One mechanism by which laminar flow becomes turbulence is through the instability of vortex interaction. The nonlinear vvolution of the interactions between the A-vortex and the secondary closed vortex has been in-vestigated. This secondary closed vortex has been in-vestigated. This nonlinear instability process leads to some fundamental changes in the topology of flows Experi-mental observations are presented herre showing the for-mation of a chain of ring-like vortices,s which generate high-frequency spikes on time traces in a transitional boundary layer.

A model is presented to describe the dynamic physical processes that occur in a transitional boundary layer flow. The CS-solitons, the closed vortex, the secondary closed vortex and the chain of ring-like vortices are postulated to be the basic flow structures of the transitional boundary layer as well as the turbulent boundary layer. It is argued that the central features of transitional and developed turbulent boundary layer flows can be explained in terms of how the series vortices interact with each other, and with the CS-solitons. The physical process that leads to the regeneration of the new closed vortex along the border of the CS-soliton, as well as the processes of evolution of the vortices to high frequency vortices, is described. The model is supported by the important developments in a number of "kernel" experiments, which illustrate both transitional and developed turbulent boundary layer. An important aspect of the model is that it, has been formulated to be consistent with accepted rational mechanics concepts that are known to provide a proper physical description of other flows.