This work investigates, mainly by means of flow visualization, experimental aspects of nonlinear stages, aiming at the K-regime in boundary layer transition. Soliton-like coherent structures (CS solitons) and a new physical mechanism for the formation of vortices along the border of the CS soliton are found.

This chapter presents direct comparisons of related visualizations and hot film measurement studies of the nonlinear, late stages of transition in a boundary layer. The earlier nonlinear stages of the transition process are well-documented in previous studies. The visualizations and measurements were both performed with controlled disturbance conditions excited by an instability wave in a flat-plate boundary layer. The threedimensional wave packets called soliton-like coherent structures (SCS), the Λ-vortex, the secondary closed vortex and the chain of ring vortices are postulated to be the basic flow structures of the transitional boundary layer. New mechanisms for their formation are also given. Despite somewhat different initial disturbance conditions used in these and previous experiments, the flow structures were found to be practically the same. The experimental results show new dynamic processes and new flow structures, i.e. the secondary closed vortex, in a transitional boundary layer.

We experimentally investigate the breakup process of a circular oil film with a hole on water surface using PIV and flow visualization. The hole moves to the bound of the oil slick driven by surface tension, and then the slick breaks into smaller drops. Typical flow maps and velocity fields are given. It is shown that a pair of vortices forms at initial stage, the position of the vortices varies as the hole approaches the bound of the slick. Satellite drops arise when breakup occurs and the size of the satellite drops depend on the shape of the slick.

The experiment was carried out to study the low frequency surface waves due to the horizontal high frequency excitation. The feature of the phenomenon was that the big amplitude axisymmetric surface wave frequency was typically about 1/50 of the excitation frequency. The viscous effect of water was neglected as a first approximation in the earlier papers on this subject. In contrast, we found the viscosity was important to achieve the low frequency water wave with the cooperation of hundreds of "finger" waves. Photographs were taken with stroboscopic lighting and thereafter relevant quantitative results were obtained based on the measurements with Polytec Scanning Vibrometer PSV 400.

A large number of experiments have been devoted to the study of the generation and development of the high-frequency structures, i.e. the chain of ring-like vortices and breakdown of the chain of ring-like vortices, to the investi-gation of their relevance to laminar-turbulent transition in a boundary layerow; however, the origins of turbulence via the process of ow randomization are still not well understood. For this reason a detailed experiment was performed to document the important shape and con guration of the late-stage transitional structures and to investigate some of the still insu

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.

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。

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.

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.