Abstract
In the presented paper the results of experiments on transitional boundary layer are presented. The boundary layer was generated on smooth flat wall with zero pressure gradient forming one side of the channel of rectangular cross section. The hairpin vortices, packets of hairpin vortices, turbulent spots and calmed regions were experimentally investigated using time-resolved PIV technique.
Highlights
The transition from laminar to turbulent flow is of great practical interest
Velocity of the air-flow was about 4.6 m s in the channel inlet, the top-hat profile was with intensity of fluctuations less than 0.1% and the deviations of the mean velocity were less than 1% throughout the cross-section
EFM11 on distance from the boundary layer origin Rex 1.4 105. In this position the turbulent spots appeared intermittently, so we could conclude that the last stage of the transition process is running there
Summary
The transition from laminar to turbulent flow is of great practical interest. The final phase of laminar boundary layer transition starts always with the occurrence of first turbulent spots. The turbulent spots followed by calmed regions are defined structures that dominate the last stage of transition. The presence of hairpin shaped vortical structures in boundary layers during transition process to turbulence has been postulated and pursued by a number of investigators over the past half century since the original work of Theodorsen [4]. Hairpin packets arising from upstream fragmented structures are found to be instrumental in the breakdown of the boundary layer bypass transition. A calmed region is attached at the rear of a turbulent spot in a laminar or transitional boundary layer. The PIV method applied to the plains parallel to the wall has been used for study of the transitional boundary layer structure was suggested for the first time in Longmire, Ganapathisubramani, Marusic, Urness & Interrante [9]
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