Numerical study on the flow field generated by a double-orifice synthetic jet device

In the last few years Synthetic Jet (SJ) actuators have shown their full potential in controlling and manipulate an incoming crossflow. Indeed, these devices have been able to control separated flows over aerodynamic bodies, delay or anticipate transition to turbulence, suppress or enhance turbulence and control liquid jets and sprays. In many applications arrays of SJ actuators or multiple-orifice (or multi-slot) devices are preferred to single-orifice/single-slot actuators. Multi-orifice strategies have been often employed for the control of separated flows, in order to cover the entire spanwise length of the flow to be controlled or to introduce a spanwise modulation of the control. Moreover, such devices are also employed in cooling applications, since multiple-orifice devices exhibit a larger heat dissipation with respect to a single, centred orifice one. Despite this fact, a great part of the studies concerning the design of a SJ actuator have been based on single-slot or single-orifice configurations. As a consequence, the present work is focused on the interaction between the jets generated by a multiple-orifice actuator. In particular, the external flow field generated by a double-orifice SJ actuator is investigated. The analyzed actuator is sealed at one side by an elastic diaphragm, which is composed of a piezoelectric disk and a flexible shim, and connected to the external environment via two circular orifices. The numerical setup matches the flow parameters of the experiments and the preliminary numerical simulations reported in [1]. A series of numerical simulations are carried out, varying the distance between the orifices. The computational domain includes the entire cavity, the orifices, and the external environment. Differently from [1], the investigation focuses on the development of the external flow field, rather than on the vortex motion near the exit plane and within the cavity. The instantaneous flow field is characterized by the presence of two, in-phase, zero-net-mass-flux jets. These jets become turbulent, converge towards each other and merge. The characteristics of the flow are strongly dependent on the distance between the orifice centers and their momentum [2]. It is important to find a scaling law for the merging point streamwise position as a function of these parameters, since jet merging is responsible for circulation cancellation and could be detrimental for flow control applications. Time-averaged flow fields are obtained, and their features are compared with the (time-averaged) characteristics of three-dimensional, continuous twinjets [3]. Moreover, time and phase-averaged velocity fields and fluctuations are compared with those of single-orifice actuators. Finally, spectral analysis of probes (located along the jets trajectories) and dynamic mode decomposition (DMD) are used to investigate the inner shear-layer interactions and recognize shifts in the dominant frequency along the streamwise direction. The latter analyses are useful to detect the vortical motions which are responsible of the jet convergence, of the entrainment of external fluid and of the far-field behaviour of the jet.