PIV measurement of mildly controlled flow over a straight-wing model

This paper follows our previous work [Tang et al. Exp. Therm. Fluid Sci. 57:1-10 (2014)] to further demonstrate the effectiveness of a newly developed synthetic-jet (SJ) array in flow separation control over a straight-wing model using tomographic particle image velocimetry (Tomo-PIV) measurements. The wing model is tested in a wind tunnel at a fixed angle of attack 19° and chord Reynolds number 1.2 × 105, with the SJ array operating with a momentum coefficient of 4.2 × 10−5 and a reduced frequency of 7.2. Different from all existing SJ-based flow separation control, the present control is so mild that the controlled flow sometimes attaches to and sometimes separates from the wing surface. Both time-averaged and phase-locked flow fields are presented and analyzed. It is found that the control effect reaches its maximum at the nominal phase angle 180°, which is close to the instant when the SJ array achieves its maximum blowing. Significant reduction of the shape factor confirms the effectiveness of the current SJ array in overcoming flow separation. The proper orthogonal decomposition (POD) analysis reveals that, no matter whether the SJ array is actuated or not, the first mode consists of a 2D plane jet-like flow over the wing surface, carrying more than 70% of the total fluctuation energy. Although not strong enough to change this dominant mode, the present mild SJ control significantly increases the occurrence rate of flow attachment. The conditional turbulent kinematic energy (TKE) is also analyzed. It is found that while the introduction of the SJs has little influence on the conditional TKE for the separated flows, it reduces the thickness of the wall-attached layer of high TKE values for the attached flows.