Comparative Study of Coherent Vortex Structures in a Boundary Layer with Adverse Pressure Gradient by Means of a New Experimental Method

Abstract : There are several kinds of coherent structures found in previous investigations in a transitional Blasius boundary layer. Spikes, A-vortices, 3D high-shear layers, and ring-like vortices are among them. These structures were observed in experimental and numerical (DNS) investigations. Very similar flow pattens were also found in other transitional near-wall shear flows, such as the plane channel flow and the pipe flow, and also in the developed wall turbulence. Based on these observations, it was suggested that these structures are very universal and represent an inherent feature of the process of turbulence production in all near-wall flows in both transitional and turbulent regimes. Moreover, the mechanism of formation and interaction of these structures can be regarded as a universal mechanism of turbulence production. The main goal of the present study is to check this idea for the case of the APG boundary-layer transition, whose late stages have never been studied experimental despite the great practical importance of this case. The only currently available study was performed numerically for a rather large APG (which is difficult to examine in the wind tunnel).