POD analysis of the turbulent boundary layer flow downstream of miniature vortex generators

Abstract

In this study we investigate the influence of an array of miniature vortex generators (MVGs) in a zero-pressure-gradient turbulent boundary layer (ZPG TBL) by means of a large-eddy simulation of rectangular MVGs in a spatially evolving moderate Reynolds number ZPG TBL up to Reτ=1350. The MVG array consists of pairs of rectangular blades arranged in spanwise oriented arrays in the flow. The turbulent intensities and streamwise inclination angles related to the large streamwise extent high-momentum regions (HMRs) and low-momentum regions (LMRs) induced by the MVGs are presented. The instantaneous velocity fluctuation is decomposed into a turbulent velocity component and a spatial velocity component based on the triple velocity decomposition, where the spatial velocity component represents a spatial variation of the time-averaged flow induced by the MVGs. The streamwise turbulent velocity fluctuation associated with the HMRs and the LMRs is further investigated using proper orthogonal decomposition (POD), where we further examine a reduced-order reconstruction of the HMRs and the LMRs, using two-dimensional data. POD has also been used to determine the streamwise inclination angle of wall-attached structures in the HMRs and the LMRs. An examination of the streamwise inclination angle associated with the POD modes has been performed. Results suggest that the streamwise inclination angle of higher energy POD modes may be related to that of the relatively large wall-attached structures reported in the literature. In addition, if the lower energy POD modes are also retained, the value of the streamwise inclination angle tends to approach the value of mean streamwise inclination angle of wall-attached structures. The results suggest that the decreasing trend in streamwise inclination angle may be related to the contributions of the small-scale wall-attached motions.