Response of Time-Depended Flowfield Structure Behind an Active Vortex Generators Pair

Abstract

This paper is focused on the time-depended mean flow structure of longitudinal vortices (LV) downstream of a pair of vortex generators controlled angle of pitch with common-flow up configuration. A smart control of turbulent shear flow, separation and drag is the most currently challenging projects. One of the effective control devices is to employ LV for the shear flow manipulation, separation modification and drag reduction. Then, it is important to understand the transitional interacting processes between boundary layer and LV downstream of an active vortex generator (AVG). The flowfield, downstream of the AVG, is expected a time-depended response to the height of AVG periodically changing. The interests are the time scales which govern the periodically behaved flowfield of LV. The important time-scale is the convection time of LV and the other is the driving time of the AVG which participate the formation of LV. Close to the surface, the LV is formed quickly but the convection velocity is slow. Near the center of LV, the LV is formed with time-delay depended on the driving time of AVG but convection velocity is fast. The flowfield structure of the longitudinal vortices downstream of the AVG is expected a complex response to the height of AVG. Therefore, the objective of this paper is to experimentally make clear the time-depended flowfield structure of the LV submerged in a turbulent boundary layer on a flat surface for a smart active control. The experimental results present rapid response of LV to the height of AVG, in case of early-stage of up-phase and down-phase. It makes clear that the high velocity tongue which transports high momentum fluid beneath the LV acts an important roll on the response. Also, the estimation method of the response-time of LV to the height of AVG is proposed in case of early-stage of up-phase. The estimation shows that the convection velocity contributes little on the near surface phenomena of LV. Good agreement with experiments close to the surface is obtained considering that the LV is formed at 70 % height of AVG. The modified response-time supposing that the effects of LV are transmitted as fast as the driving-time of AVG shows good agreement with experiments. NOMENCRATURE