Application of a wavelet cross-correlation analysis to DNS velocity signals

A wavelet cross-correlation analysis is applied to the velocity signals derived from axisymmetric direct numerical simulations of a coaxial jet configuration. It is shown that this technique allows determination of the time intervals and scales contributing to the Reynolds stresses, and thus an assessment of the level of intermittency of this contribution at the various scales. The availability of the vorticity field from the numerical simulations makes possible association of the roll-up, passage and pairing of vortical structures with average, spectral and instantaneous contributions to the Reynolds stresses. High values of Reynolds stresses are found in the roll-up region, where the axial and radial velocity fluctuations have a small phase lag for significant times intervals. Conversely, further downstream the passage of approximately symmetrical vortices, although giving rise to high velocity fluctuations, does not produce significant Reynolds stresses because the two velocity signals are nearly orthogonal for most of the time. This demonstrates that eduction schemes based on the energy of the velocity fluctuations might not adequately reproduce the flow features characterizing the Reynolds stress production.