The goal of this Grant Opportunities and Academic Liaison with Industry (GOALI) investigation is to understand how multiple Synthetic Jet Actuators (SJAs) can work together in arrays and to advance the field of SJA-based active flow control toward practical application. The proposed research involves 1) the extension and experimental validation of analytical models of synthetic jet actuators that incorporates the dynamic coupling of multiple actuators and the surrounding flow fields; and 2) use of these models for creating synthetic jet arrays capable of facilitating closed-loop active flow control on real flight platforms. The proposed research includes introducing new modeling techniques that mitigate the need for empirical data and allow for inclusion in closed-loop control algorithms. To achieve this multiple straight vortex tubes will be aligned to simulate the issuing jet arrays. This new modeling technique will allow for the addition of multiple jets in close proximity and due to its low computational expense, is well suited for plant implementation and closed-loop control. The ability to quickly model multiple actuator configurations and include them in a control law will enable the optimization of synthetic jet actuator arrays for a multitude of applications. This fundamental design ability is the link between synthetic jet technology and the realization of active flow control on full-size aerospace vehicles.
Synthetic jet actuator-based active flow control holds the potential for widespread application throughout the aerospace industry. However, the current state of the art of synthetic jet performance is inadequate to realize this potential on full-scale applications. Increased structural efficiency, simplification of control surfaces and increased variability of aerodynamic properties are a few example benefits this technology will substantially contribute to and through which it will impact the ?Green Aircraft? revolution. For example, ?virtual shape change? is the displacement of streamlines without the use of traditional control surfaces such as ailerons and flaps. By using a spatially distributed SJA array, the wing can be structurally optimized to minimize weight and complexity while SJAs displace the streamlines to create a virtual airfoil shape that also optimizes aerodynamic performance. The outcome of this goal will be fundamental insights into the use of multiple actuators for creating synthetic jets with performance output, controllability and versatility heretofore not seen in the industry.
Synthetic Jet Actuators (SJAs) are fluidic devices capable of adding momentum to static or non-static bodies of fluid without adding mass. They are categorized as a zero-net-mass-flux (ZNMF) momentum source. In its simplest compact form a SJA consists of an oscillatory surface connected to a cavity with a single exit orifice through which the fluid enters and exits. SJA technology has been utilized in applications ranging from boundary layer control over aerodynamic surfaces to fluidic mixing in dispersion applications. The ZNMF nature of the technology means it is not subject to constraints experienced by traditional momentum sources that require the addition of mass in order to impart momentum. The momentum that can be added by a single SJA is limited by the energy transfer capabilities of the oscillating surface. In modern SJAs this surface usually is a piezoceramic/metal composite driven by a high voltage AC signal. For applications such as flow control over aerodynamic surfaces, modern SJAs are used in an array configuration and are capable of altering the flow momentum by values ranging from 0.01-10%. While it is possible to build larger actuators to increase this value the benefits associated with the compact size would be lost. It is therefore desirable to tune other parameters associated with SJA arrays to increase this value. The specific motivation for this study comes from the desire to control the momentum addition capacity of a specific SJA array, without having to alter any geometric parameters. In a broader sense this study focuses on understanding the physics of SJA interaction in array configuration through experiments which are then used to guide in the design of modeling technique that predicts SJA array behavior in cross-flows. The first half of the project focused on understanding SJA behavior through modeling. Numerical techniques were initially used to model SJA and SJA arrays in cross-flows. Reduced numerical models were then developed from the full momentum equations. An analytic wave equation based solution to the stream and vorticity formulation of the momentum equations was also implemented to predict SJA behavior. For the experimental component of the project, a finite span high aspect ratio orifice SJA was designed and characterized. Two of these SJA were then placed in close proximity to one another. The relative phase of operation between the two jets was altered and the resulting flow field was measured through Particle Image Velocimetry (PIV). This process was repeated for different sets of array spacing, and SJA to cross-flow velocity ratio. For specific choices of these parameters a 40% increase in momentum addition was observed. The major contributions of this research to the field of SJA fluid dynamics can be summarized as follows: 1) Predicting array behavior through modeling 2 dimensional unsteady CFD was used to demonstrate that phenomenon such as vortex splitting and switching governed the dynamics of the flow. Three parameters were identified as being critical in characterizing the array behavior. Models were used to study the sensitivity of flow field features to these parameters. Limits under which these models were deemed accurate were also established and explained. An analytic solution was also calculated and validated. While limited in accuracy and range of operating conditions, this solution can be used for closed loop control applications. The developed analytical solution also the potential of being applied to more complicated flow fields. 2) Developing and characterizing Synthetic Jet Actuators suitable for array use A low cost, robust finite span actuator that would satisfy the geometric constrains of array implementation as well as the modeling requirements of 2D flow was developed. Constant Temperature Anemometry was initially used to study the jet and characterize it for frequency and voltage response. 3) Model validation and enhancement through array operation While there exist several studies that detail the physics of single SJA interaction with cross-flows, research on experimental analysis of the physics is lacking. PIV experiments were initially used to establish bounds of operating ranges for the modeling techniques. Modeling and PIV results revealed that it was possible to control the amount of momentum that an array adds to a cross-flow. The benefits of array configuration were demonstrated through improved efficiency of actuators when operating under array conditions. Up to 30% increase in performance was measured. The experimental results verified that for an array of synthetic jets tuning the geometric and operating parameters could lead to an improvement in the momentum coefficient. A sensitivity study performed on arrays indicated that altering the relative phase of operation had the most impact on the flow field. 4) Demonstration of array operation for dynamic virtual shape control Experimental results were used to study the effect of actuation on stream line displacement as function of phase angle. The results indicated that the streamline pattern could be altered significantly without having to alter any of the geometric parameters.
