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Abstract
Zero-Net Mass-Flux (ZNMF) actuators or synthetic jet actuators are versatile micro scale devices with numerous applications in the field of fluid mechanics. The primary focus of the current work is to use time-accurate simulations to study the interaction of these jets with cross flows and to optimize their performance for the control of boundary layer separation.
This study consists of four parts. In the first part, a class of phenomenology-based models is proposed to reproduce the flow associated with synthetic jets in grazing flows and simplify the task of ZNMF-based flow control simulations. The proposed models have a non-uniform jet velocity profile with only two spatial degrees of freedom and a uniform slip velocity on the slot-flow boundary. A comparison of key integral quantities associated with the momentum, energy and vorticity fluxes shows that the models with a non-uniform jet velocity during the expulsion phase and uniform jet velocity during the ingestion phase can predict these quantities with good accuracy, whereas a simple plug flow model with a zero slip and uniform jet velocity under-predicts these three quantities during the expulsion phase. Based on our initial analysis, three of the simplest models are selected for further study, including an assessment of their performance for a canonical separated flow at different forcing frequencies. A key finding is that a simple plug-flow type model can predict incorrect trends for separation reduction with the jet frequency. A preliminary attempt is also made to provide empirical closure to these models.
The effect of synthetic jets orientation on its interaction with a zero pressure gradient laminar boundary layer is explored in the second part. A rectangular slot is chosen in this study and streamwise and spanwise orientations of this slot are examined. The orientation of the slot is found to have a significant impact on its interaction with the boundary layer. The dominant feature in the streamwise oriented slot is a pair of strong counter-rotating streamwise vortices and these are missing in the spanwise oriented slot configuration. A variety of flow statistics are computed that indicate that despite the smaller overall blockage represented by the streamwise oriented slot, it is more effective in increasing the momentum of the boundary layer and therefore is expected to be more effective in separation control. For further investigation, the effect of slot orientation at different operational conditions (jet velocity and frequency) on a separated flow over a wall-mounted hump is examined in the third part of this work. Comparison of various instantaneous integral quantities, time average aerodynamic parameters and separation bubble size between two slot configurations shows that the streamwise oriented slot provides better performance in all jet velocities and most forcing frequencies of the range of study.
In the fourth and final part of the study, a canonical separated flow configuration is used to explore the rich flow physics and nonlinear dynamics of separated flows in a systematic manner and to establish a flow-physics based understanding of effective forcing schemes. This configuration consists of a three dimensional 9.5% thick flat plate with an elliptical leading edge and a blunt trailing edge with separation induced through the application of blowing and suction on the top boundary of the computational domain. The key aspect of this approach is that the separation bubble can be located in any position along the suction side of the plate with desired size along with separate control on the Reynolds number which is not possible by variation of angle of attack and/or free-stream velocity for an airfoil. This study shows that even this simple canonical separated flow is dominated by nonlinear interactions between the shear layer, separation bubble, and wake instabilities in a manner that is representative of more complex airfoil separation. The simple boundary condition developed in the first part of this work is used to represent the synthetic jet in a cross flow for separation control. Numerical simulations of sinusoidal forcing show that active control can move the separated flow system to other lock-on states that either reduce or enhance separation. Forcing the flow at an excitation frequency that is close to the separation bubble frequency causes a significant reduction in the size of separation bubble.
