images or could be obtained using spatial correlation of the images referring next to fig8 the average spatial correlation over 50 instantaneous streamwise images similar to those in fig6 and 7 is shown spatial correlation is a statistical technique which shows quantitatively how well organized or how random the large scale turbulence structures in a given flow is if there is no organization of structures then the peak correlation level of 1 at zero x d separation in fig8 will continuously drop as the separation is increased this is typical of the baseline case with actuators off if structures are well organized then following 1 at zero x d the correlation will decrease with x d but will go through local peaks and valleys as is the case for example for actuators operating at 9 khz the distance between two peaks or valleys is directly related to the forcing frequency these results are consistent with the flow visualization results shown in fig7 a and 7b the distance between the local maxima in fig8 is equivalent to the spatial wavelength or the spacing of the periodic structures this wavelength can be used along with the convective velocity of the structures to determine the shedding frequency of the structures the average convective velocity for this.