Wavelet multi-resolution analysis of initial condition effects on near-wake turbulent structures

Abstract

The effects of initial conditions on turbulence structures of various scales in a near wake have been investigated for two wake generators with the same characteristic dimension, i.e., a circular cylinder and a screen of 50% solidity, based on the wavelet multi-resolution analysis. The experimental investigation used two orthogonal arrays of sixteen X-wires, eight in the (x, y)-plane, and eight in the (x, z)-plane. Measurements were made at x/h (x is the streamwise distance downstream of the cylinder and h is the height of the wake generator) = 20. The wavelet multi-resolution technique was applied to decomposing the velocity data, obtained in the wakes generated by the two generators, into a number of wavelet components based on the central frequencies. The instantaneous sectional streamlines and vorticity field were thus 'visualized' for each wavelet component or central frequency. It was found that the behavior of large- and intermediate-scale structures depend on the initial conditions and the small-scale structures are independent of the initial conditions. The contributions from the wavelet components to the time-averaged Reynolds stresses and vorticity were estimated. Both the large-scale and intermediate longitudinal structures make the most significant contributions to Reynolds stresses in the circular cylinder wake, but the contribution from the large-scale structures appears dominating in the screen wake. The relatively small scale structures of the circular cylinder wake contribute most to the total rms spanwise vorticity.