Interaction of in-line twin synthetic jets with a separated flow

An experimental investigation is carried out in a water tunnel to investigate the interaction of in-line twin synthetic jets (SJs) with a separated laminar boundary layer over inclined plates. The flow structures induced by the in-line twin SJs at four phase differences and their resulting flow separation delay are examined using dye visualization and particle imagevelocimetry(PIV)measurements. It is found that, in most of the cases, the heads of hairpin vortices that are produced from the upstream SJ actuator do not change too much, and the vortex legs are highly stretched by the separated shear layer. An exception is the case with 90° phase difference where the combined vortex head appears as a reversed letter “S” and the combined vortex legs are high enough to escape from the influence of the separated flow. Compared to the single SJ, the twin SJs generally exert greater influence on the separated flow regardless of the phase difference. The PIV results in the mid-span plane reveal that the periodic passage of vortex structures influences the separated flow significantly, causing the flapping of the upper edge of the reversed flow region that contributes to the flow separation delay. The delay of separation is also demonstrated by a streak of forward flow protrusion in the wall-parallel PIV results. It is found that the streak varies a lot at different phase differences. Proper orthogonal decomposition analysis is also conducted and two major types of energetic flow structures in the SJ controlledflow are identified: a strip of back-and-forth fluctuation along the upper edge of the reversed flow, and the vortex structures produced by the twin SJs. It is found that the fluctuation strip is most energetic in the single SJ case and the case with 270° phase difference, whereas the convective mode pair is most energetic in the case with 90° phase difference.