Experiment Investigation On Bypass Transition Influenced By The Cylinder Wake

Utilizing the water trough sink in Tianjin University fluid dynamics laboratory, the present investigation on the boundary layer bypass transition induced by cylinder wake was carried out by virtue of TR PIV particle image analysis system.First, the basic flow field without disturbance would be measured with TR PIV. The experimental profile of average flow velocity is in accord with Blasuis classical solutions, which confirmed that the basic flow field is laminar boundary layer. Second, the cylinder would be positioned respectively at two different places of the front edge of the flat in the water trough sink. The images of flow fields induced by cylinder in different places were shot by use of TR PIV. After image processing, the sequence signals of instantaneous velocity in two dimensions were obtained.According to the characteristic quantities such as velocity profile and turbulence intensity, the starting point and end point of transition can be determined. The influence on the starting point, end point of transition and the length of transition region, which might be caused by the relative distance between cylinder and the front of flat, was also analyzed.Finally, a multi-scale decomposition in two dimensions of speed signals in the transition zone was accomplished by multi-scale wavelet analysis. According to Q criteria, coherent structures of different scales were extracted. The evolution of the secondary vortex in the transition zone was then identified and tracked.In the bypass transition zone, after phase averaging the fluctuating velocity and the vorticity of the flow, a low-speed hump was found in the boundary layers of the bypass transition zone. The'one-to-one'relationship between secondary transverse vortices and wake vortices is evidenced. With the development of the flow, the shape of the low-speed hump can be visualized more clearly. In the 30cm-38cm region in the end of the transition zone,horseshoe vortex was extracted by phase averaging the vorticity, we found that hairpin heads are affected by three induction forces: the mutual induction, the mean shear effect and the wake-vortex induction.