Perturbing spanwise modes in turbulent boundary layers

Abstract

The objective of the current study was to manipulate the coherent vortex packets in a turbulent boundary layer at Re_tau=2480 by inserting a small scale cylinder array and to improve the understanding of the downstream flow stability issues. The height of the cylinders was H/delta=0.2 (H+=500) with aspect ratio (AR=cylinder height/base diameter) of 4, and three cases were studied using single array of 0.2 delta, 0.4 delta and 0.6 delta spaced cylinders. Both fixed location data and flying data were acquired at z+=296 using PIV, and the spanwise scales of the packets and the wake-packet interactions downstream of the cylinder array were also discussed. The non-perturbed flow was studied first and the dominant spanwise scale of the vortex packets was found to be approximately 0.6 delta. From the flying data, the organization of vortex packets was found to persist over a streamwise distance of approximately 8 delta. The averaged results of the perturbed cases showed a spanwise variation of the streamwise velocity downstream of the cylinder array, and the spanwise scales of the low speed regions were most stable for the 0.6 delta spacing case. Also, distinct downwash behavior was observed directly behind each cylinder. The flying data showed frequent spanwise interactions of cylinder wakes in the 0.2 delta case and the downstream structures were affected greatly by the incoming flow condition. The 0.4 delta and 0.6 delta cases were discussed based on the relative spanwise location of the upstream vortex packets and the cylinders and it was concluded that the organization of flow structures was most stable when the perturbation scale was the same as the dominant spanwise mode of the non-perturbed flow.