Statistical properties of hairpin vortices in a turbulent boundary layer under stalled-airfoil-type flow conditions

Hairpin structures in the outer region of a turbulent boundary layer (TBL) subjected to a severe adverse pressure gradient (APG), which has also experienced an abrupt transition from strong favourable to strong adverse pressure gradients, have been studied using particle image velocimetry (PIV). The experimental set-up was designed to achieve flow conditions corresponding to trailing-edge stall of an airfoil. Large sets of instantaneous velocity fields are acquired by PIV in streamwise-wall-normal planes at three different streamwise positions in the adverse pressure gradient region. These positions cover a position near the pressure peak up to the separation point in order to study the evolution of turbulence structures in the adverse pressure gradient zone. Investigation of the instantaneous velocity fields shows that the upper region of the boundary layer is densely populated with signatures of hairpin vortices and hairpin packets qualitatively similar to those encountered in zero-pressure-gradient (ZPG) TBL. The hairpin packets average growth angle in the streamwise direction is more important than in the zero-pressure-gradient turbulent boundary layer. The individual hairpin vortices are also slightly more inclined with respect to the wall and more closely spaced, when scaled with the boundary layer thickness, in the APG TBL. These differences of hairpin properties are consistent with the differences in the mean strain rates, in particular rates of streamwise contraction and wall-normal extension, in both flows.

Keywords:

Hairpin Structures , Turbulent Boundary Layer , Adverse Pressure Gradient , Spanwise Vortices , Particle Image Velocimetry