Experiments on the turbulent shear flow in a turn-around duct(II)—The structure of turbulence

Abstract

Detailed measurements including two-component mean velocities (U andV), RMS of turbulent fluctuations (u′ andv′) and turbulent cross-correlation (uv) were made throughout a turn-around duct. Two-component velocity data were obtained using a Laser Doppler Velocimeter. With the aid of a digitized data acquisition system, energy spectra were estimated using a Fast Fourier Transform. Along the outer wall, the flow is affected more than the half across the channel height due to the centrifugal instability. The measured results are consistent with the turbulence production mechanism for stabilizing and destabilizing curved flows. Energy production and dissipation are reduced along the convex wall and amplified along the concave wall. In the quasi-laminar region, turbulent fluctuations and cross-correlations are damped. The mean flow and turbulence structure in this region are influenced mainly by the streamwise pressure gradient rather than curvature. The flow in the downstream part of the turn is dominated by the inertial effect. The turbulent large eddy motions along the concave wall are strongly anisotropic.