Aerodynamics of aerial and underwater vehicles

Aug 22, 2011    

 

Optimum aerodynamic performance that avoids flow separation on wing surfaces has been traditionally achieved by appropriate aerodynamic design of airfoil sections.  However, when the wing design is driven by non-aerodynamic constraints (stealth, payload, etc.), the forces and moments of the resulting unconventional airfoil shape may be much smaller than on a conventional airfoil.  Therefore, either active or passive flow control can be used to maintain aerodynamic performance throughout the normal flight envelope.  Although passive control devices such as vortex generators have proven, under some conditions, to be quite effective in delaying flow separation, they offer no proportional control and introduce a drag penalty when the flow does not separate (or when they are not needed).

In contrast, active control enables coupling of the control input to flow instabilities that are associated with flow separation and thus may enable substantial control authority at low actuation levels.  Furthermore, active actuation is largely innocuous except when activated and has the potential for delivering variable power.  In previous studies, active control efforts have employed a variety of techniques including external and internal acoustic excitation, vibrating ribbons or flaps, and steady or unsteady blowing.

Over the last decade, the synthetic jet actuator has emerged as a versatile actuator for active flow control.  The formation and evolution of synthetic jets are described in detail in the work of Smith & Glezer, Glezer & Amitay, and Amitay & Cannelle.  The effectiveness of fluidic actuators based on synthetic jets is derived from the interaction of these jets with the flow near the flow boundary that can lead to the formation of a quasi-closed recirculating flow region, resulting in a virtual modification in the shape of the surface.  Past research work has focused on the use of open-loop actuation strategies to generate the required modulated input signals to jet arrays, which is highly dependent on the availability of accurate and comprehensive wind tunnel-validated flow models.  However, the underlying flow mechanisms and interactions of jet arrays are usually very complicated and highly nonlinear.  Moreover, it is extremely difficult to accurately model the changes in the system dynamics due to varied flight conditions, inevitable external disturbances, measurement noise, actuator anomalies, and failures.  Therefore, the development of closed-loop nonlinear adaptive flow control techniques that can automatically compensate for modeling errors and adapt to changes in the system dynamics, are particularly attractive to realize the full potential of synthetic jet technology.

The aerodynamic research in CeFPaC has four main objectives: (1) to understand the flow mechanisms associated with the interaction between the flow and the actuators, (2) to explore, experimentally and numerically, the feasibility of using active flow control for flight control, (3) to develop low order models of the flow, and (4) to develop a closed-loop control schemes.

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