PIV measurements of convection velocities in a turbulent mixing layer

Particle image velocimetry (PIV) experiments that resolve a range of scales from 1.5L to 10.4 eta, where L and eta are the integral and Kolmogorov length scales respectively, are performed in the self similar region of a nominally two dimensional planar mixing layer. Data is acquired at 7.25Hz with a field of view that extends to 1.5L thereby permitting spatio-temporal correlations of large and intermediate scale velocity fluctuations to be performed. The overall convection of velocity fluctuations is found to be similar to the mean flow, although fluctuations on the low speed side of the mixing layer on average convect at speeds greater than the mean and fluctuations on the high speed side of the mixing layer are observed to convect at speeds less than the mean. The convection of fluctuations of different length scales in the flow is then observed. it is found that large-scale structures, of the order of the integral scale, convect at speeds that are greater than the mean in the low speed side of the mixing layer and that small scale fluctuations convect at speeds significantly lower than the mean. A crossover wavenumber is also found at which the fluctuations in the low speed side of the mixing layer convect at greater velocities than those on the high speed side of the mixing layer. This wavenumber is found to be Reynolds number dependent.