Ashvin VarsaniAerospace Engineering MEng
Numerical Analysis of Orifice Closure and Jet Pitching Mechanisms in Synthetic Jet Actuators
A significant expenditure of research into active flow control devices using synthetic jet actuators (SJAs) has been undertaken over the past decades due to its vast potential for expanding the control of flow regimes on aerofoils. A synthetic jet is produced at the edge of an orifice by the action of a periodically vibrating diaphragm enclosed within a sealed cavity. However, due to its design, the potential for debris to enter the orifice of the actuator and clog the device or erode its internal components has been acknowledged. Once entered, removal of the debris will be a significant problem due to the lack of a high pressure blowing source (e.g. engine bleed air) that can eject the debris. Since SJAs will be incorporated into the aircraft wing structure, routine, physical maintenance of these devices will also be difficult to achieve. It is therefore of great importance to begin looking into closure mechanisms that can be used to protect the orifice from environmental debris, such as dust, ice, insects and water that will be typically experienced by an aircraft in flight. In this work, computational fluid dynamics (CFD) software has been used to model and investigate potential orifice closure mechanisms with an additional purpose to provide jet pitching and velocity output optimisation capabilities. This can be achieved by means of angled orifice walls such that a convergent nozzle arrangement can be made. The closure mechanisms have been analysed in both static and cross flow conditions to determine the best suited closure concept based on the strength of the vortices synthesised at the edge of the orifice. FEA analysis has also been conducted on the closure concepts in order to determine their feasibility for withstanding the appropriate loads imposed on an aircraft during take-off as well as cruise conditions. This investigation shows that a pitched orifice can have a beneficial effect on the nature of the vortex that propagates along the wall of the wing. It was found that an aperture shutter concept with walls shaped in a convergent nozzle arrangement produced the strongest jet out of the orifice exit when operated symmetrically or with a single side activated. Under cross flow conditions of 100ms-1 a peak vorticity of 8x104s-1 was observed at half the period of the SJA with a single wall activated, compared with a vorticity magnitude of 7.03x104s-1 for the standard orifice. This indicates that by pitching the synthetic jet such that a single, stronger vortex is formed, there is potential improve the SJA function by maintaining a stronger vortex across the bottom wall of the flow field. The FEA analysis also determined that all mechanisms investigated can operate effectively in cruise conditions without failing. However during take-off, a particular mechanism based off of utilisation of electro-active polymer technology failed returning a safety factor of 0.58. This indicates that in the current design stage, it would be ineffective for the use on aircraft, although it did show to improve the performance of an SJA together with the aperture concept.