Detection of coherent structures in a turbulent boundary layer with zero, favourable and adverse pressure gradients

Abstract

The paper presents the analysis of turbulent coherent structures found in a turbulent boundary layer subjected to zero, favourable and adverse pressure gradient. The analysis of the shape of conditionally averaged traces of u and v velocity components obtained by VITA (Variable Interval Time Averaging) technique based on signals recorded by X-wire probe allows to detect four types of coherent structures. The paper documents a presence of coherent vortical structures of positive or negative vorticity moving upward and downward. It is shown also that the acceleration and deceleration of the mean streamwise velocity, due to the pressure gradient, modifies the vortex convection velocity magnitude and the angle. This effect is clearly seen especially for the adverse pressure gradient flow, where Q events in first and fourth quadrants are enhanced near the wall.