Dye Visualization of In-Line Twin Synthetic Jets in Crossflows—A Parametric Study

Abstract

This paper presents a parametric study on the interaction of twin circular synthetic jets (SJs) that are in line with a crossflow over a flat plate. The resulting vortex structures under different actuation, and flow conditions are investigated using two-plane dye visualization in a water tunnel. The influence of four independent nondimensional parameters, i.e., the Reynolds number (ReL), Strouhal number (St), velocity ratio (VR), and phase difference (Δϕ), on the behavior of the twin SJs is studied. It is found that the increase of Reynolds number causes the SJ-induced vortex structures more turbulent, making the twin SJ interaction less organized. The increase of velocity ratio pushes the occurrence of interaction further away from the wall and makes the resulting vortex structures more sustainable. The St has no obvious influence on the interaction. And three types of vortex structures are observed under different phase differences: one combined vortex, two completely separated vortices, and partially interacting vortex structures. Based on this parametric study, a simple model is proposed to predict the resulting vortex pattern for the twin SJ interaction.

Evolution and Near-Wall Effect of the Vortex Structures Induced by In-Line Twin Synthetic Jets in a Crossflow

“…It is also seen that at ∆φ = 3π/2 (Figure 7d), unlike its counterpart, the head of the SJ issued from the upstream actuator is not affected at all by the near-wall particles. All these observations support our previous statement, i.e., in any case, the SJ issued from the up-stream actuator can better maintain its vortical structure than the SJ issued from the downstream actuator [23,24], and hence it dominates the interaction. The reason is obvious: the vortex produced by the upstream actuator has sufficient time to develop itself before the interaction, while the SJ issued from the downstream actuator is disturbed from the very beginning of its emergence.…”

“…At ∆φ = 0 (Figure 7a) and π (Figure 7c), the legs of one hairpin vortex are entrained into the head of the other, forming partially interacting vortex structures. As such, the twin SJs have distinct vortex strengths at various phase differences, as revealed in the authors' previous PIV experiments [23,24]. Furthermore, it is seen that, no matter which type of interaction occurs, the green particles issued from the downstream actuator are mainly scattered in the outer layer of the interacting structures, whereas the red particles is-sued from the upstream actuator are mainly enclosed inside.…”

“…The density of these particles is set to that of water, and the discrete particle model (DPM) is used to track them. Similar to our dye visualization experiment [23,24], the particles issued from the upstream actuator are colored in red, whereas those from the downstream actuator are in green.…”

“…The twin SJ actuators used in the present numerical study are identical to those used in our previous experiment [23,24]. The SJ cavity has an orifice with a diameter of D o = 5 mm.…”

“…The actuators have a diaphragm displacement of ∆ = 0.158 mm with a frequency of f = 1 Hz. To characterize the performance of SJs in a laminar boundary layer, the dimensionless stroke length L and velocity ratio VR are also defined according to Wen and Tang [24]. The stroke length L is defined as…”