Experimental and computational study of a Zero-Net Mass-Flux synthetic jet actuator

An experimental study of a Zero-Net-Mass-Flux (ZNMF) actuator was conducted to evaluate its performance (in particular the peak amplitude of the jet velocity), for an applied voltage in the range (0 volts-6 volts), and frequency in the range (200Hz--300Hz). The ZNMF actuator employed in this study was a three-ounce electrodynamic actuator. For the study, an actuator casing was designed and fabricated which allowed for slot jets of very high aspect ratio to exit from both sides of the casing due to the actuator action. The jet velocity measurements were taken for various slit openings (0.0075in, 0.010in, 0.015in and 0.020in) by a hot-wire anemometer. To validate the experimental data, both 2D and 3D computational studies were conducted by modeling the actuator and the cavity using the commercially available Computational Fluid Dynamics (CFD) solver FLUENT. Numerical simulations included one, two and three jets emanating from single, double and triple slits in the casing of a single actuator. Simulations were performed using both the incompressible and compressible Unsteady Reynolds-Averaged Navier-Stokes (URANS) solvers in conjunction with a two-equation k-&egr; turbulence model. Significant differences were found in the magnitude of the jet velocity between the incompressible and compressible computations for single jet flows for slit openings smaller than 0.010in; the jet velocity in compressible flow solutions was significantly lower than that in incompressible flow solutions and matched the experimental data. The discrepancies between the incompressible and compressible flow solutions diminished with bigger slit width opening (greater than 0.010in) and for larger number of slits. Our calculations have clearly established that compressibility must be taken into account when performing the CFD simulations of ZNMF actuators. Using the experimental data and the numerical results, a dynamic model of the actuator was developed, which includes key parameters such as the electromagnetic force produced by the magnets, coil length, diameter, number of turns, material, resistance, inductance and mass of the diaphragm. These parameters, which influence the movement of the diaphragm for an applied voltage, determine the maximum amplitude of the jet velocity. This model was compared with a fluid dynamic-based model recently published in the literature, which includes pressure terms but does not distinguish between incompressible and compressible behavior of the gas in the actuator cavity. Nevertheless, good agreement between the two models was obtained for the variation in jet velocity with applied voltage. In addition to actuator modeling, simulations were also performed to determine the Active How Control (AFC) characteristics of the synthetic jets by performing the computations of their interaction with a cross-flow boundary layer on a flat plate. Reduction in the skin friction coefficient downstream of the synthetic jets was obtained for all the computed cases.