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