Numerical Modeling of Synthetic Jets in Quiescent Air with Moving Boundary Conditions

Flow control is a key factor in optimizing the performance of any vehicle moving through fluids. Particularly, in aerodynamics there are many potential benefits for implementing synthetic jets to achieve aircraft designs with less moving parts, uper- maneuverability, and separation control for fuel economy. Piezoelectric synthetic jets are of special interest because of their lightweight and low power consumption. Numerous publications on such jets are available. Actuator properties and boundary conditions relevant to this particular application however are often overlooked. The focus of this project is to numerically model synthetic jets in quiescent air to study the influence of cavity geometry and boundary conditions of the piezoelectric diaphragm on jet velocity. Numerical simulation is performed for two synthetic jet cavities of different height and orifice diameter. The numerical modeling utilizes a turbulent RNG κ ε model and a moving boundary condition with two oscillating deflection profiles, parabolic and logarithmic, applied to the diaphragm. The actuators modeled are typical Bimorph and Thunder piezoelectric actuators. The initial conditions for the actuators are obtained experimentally resulting in 0.396mm and 0.07mm respectively when driven with a sinusoidal wave input at 1524 V/m and 4064 V/m. Although the velocity boundary numerical model gave overall better results than the current moving-boundary numerical model, the moving-boundary model is more accurate since it better approximates the movement of the diaphragm. From an optimizing viewpoint the moving boundary is more suitable to attempt to optimize the design because displacement magnitude of the diaphragm can be measured directly from experiments. For the higher displacement Bimorph actuator, a logarithmic profile matches the experimental results, whereas the parabolic profile provided better results for the relatively small displacement Thunder actuator. It is thus hypothesized that both tested actuators, Bimorph and Thunder, oscillate according to the specified logarithmic and parabolic profile respectively. Cavity height was briefly investigated for the Bimorph actuator. Results show that cavity height did not make a difference in the centerline velocity for the numerical model. The model fails to consider the important effect of the dynamic coupling of the actuator displacement and the pressure that develops inside the cavity. The pressure values obtained are comparable to the theoretical blocking pressure for the Bimorph in the cavity. The results of this study show that jet formation and development has unique characteristics for each actuator and cavity configuration. The smaller orifice cavity configuration produced a faster, longer, thinner jet with larger vortices than the bigger orifice. During max expulsion, t = 0.25T, and max ingestion, t = 0.75T, a low-pressure area localized at the corners of the orifice, inlet and exit respectively, were observed. All cavity configurations passed all three known jet formation criterions that include, Lo/Do\u3e1, Re \u3e 50, and Re/S2 \u3e 0.16