Analysis of wall shear turbulence in a swirling annular decaying flow induced by a tangential fluid inlet

This work is dedicated to an experimental study of the wall shear stress in a turbulent swirling decaying flow induced by means of a tangential inlet in an annular cell. The flow pattern near both cylinders of the annulus is analyzed by means of measurements of wall velocity gradients using an elect rochemical method involving micro-electrodes working in inert wall. The ana lysis of the axial and circumferential evolutions of the mean wall velocity gradient and of its power spectral density emphasizes the complexity and h ighly three-dimensional nature of the flow pattern of the annular swirling decaying flow induced by means of a single tangential inlet. Near the inner wall of the annulus, a recirculating zone, consisting of several pairs of contra-rotating cells, develops following the main helical motion of the fl uid along the flow path. Near the external wall of the annular cell, high l ocal velocities and energetic turbulent structures are detected in the vici nity of the tangential inlet. These eddies rapidly decay along the flow pat h because of the decrease of the swirl intensity. The setup of a recirculat ion bubble near the inner wall induces a quasi-axisymmetric flow behavior a t the outer cylinder of the annulus. Near both walls, the power spectral de nsities of the velocity gradient reveal a superposition of small turbulent eddies having a low energetic level in the high-frequency domain.