Abstract
In this study, the effect of ramp-type micro vortex generator (MVG) on multi-scale turbulent motions in a canonical turbulent boundary layer (TBL) is studied. Three-component velocity fields in several cross-stream planes, which cover the near-to-far-wake regime in the downstream of a spanwise array of MVGs, were measured by stereoscopic particle image velocimetry. The method of quasi-bivariate variational mode decomposition was applied to decompose the fluctuating velocity fields into several discrete intrinsic mode functions with different scales, each of which was further processed by proper orthogonal decomposition to extract the corresponding statistical representative structures. Subsequent detailed analysis on pre-multiplied spanwise spectrograms, multi-scale kinematic statistics, energy-bearing motions, turbulent kinetic energy (TKE) budget characteristics, and inter-scale amplitude modulation (AM) effect is performed. It is found that the modulation of the MVG wake on the TBL lies in two aspects. One is the increment of TKE in higher flow layer where large-scale streamwise vortex pairs (SVPs) are present, and the other is the reduction of TKE in the near-wall region. The former quickly dissipates in the downstream along with the dissipation of SVPs, while the latter persists till the far-wake regime, providing a new perspective for the turbulent control by MVG. Such a long-term TKE suppression effect can be explained by the suppression of inter-scale AM effect, which suppresses both the production and inter-scale transfer of TKE.
Funder
National Natural Science Foundation of China