Towards the noise reduction of synthetic jet actuators using lobed orifices

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonWith increasing strain on the civil aviation industry to meet strict targets to reduce the adverse effects aviation has on the environment by 2050, significant advances in aircraft design and research are required. Aerodynamic improvements have been a focus for several decades now, however, current and future civil transport aircraft are based on traditional designs originating from the 1950s. Optimisation of aircraft external geometry for aerodynamic gain is reaching maturity and is becoming increasingly non-cost-effective. New advances in sensor and actuator technology has allowed for the development of active flow control (AFC) devices that have shown promising results in laboratory and even full-scale flight conditions, as seen by the joint NASA-Boeing ecoDemonstrator. One such device is the synthetic jet actuator (SJA), that synthesises periodic jets without the requirement for external air supply, while adding momentum to the surrounding flow. For this reason, SJAs are also referred to as zero-net-mass-flux actuators. There exists extensive work on the use of these devices for flow control applications in a laboratory setting. One of the key issues that remains unresolved, hindering successful aircraft application to-date, is the actuator self-noise generated. The noise level of SJAs can be so severe that they were rejected for application on the ecoDemonstrator in favour of a higher authority, quieter AFC device. SJAs were only considered for use in emergency situations on aircraft. Furthermore, the actuators were also not permitted to operate simultaneously at full power, which may severely limit scope for flow control on aircraft. Other applications that would benefit from SJAs include heat transfer for cooling in electronic devices. Studies in this field identify the same problem with noise levels of up to 73 dB reported. It is clear that work towards the self-noise reduction of SJAs is required to harness the full potential of this actuator technology. In the work presented, passive and active noise control measures in the form of lobed orifices and antiphase operation of two jets, respectively, on the noise reduction of SJAs are ii investigated. Noise sources of synthetic jet actuators include mechanical (diaphragm) and jet induced noise, where the focus of this work is on the latter type. Tests were conducted in quiescent conditions using jet velocity measurements, acoustic measurements, and flow visualisation. Tests were carried out using a single chamber SJA with variable cavity height and both circular and lobed orifices. These tests helped identify a SJA self-noise generation mechanism when using a circular orifice. This mechanism is characterised by a constant frequency behaviour visible in acoustic spectra for a specific jet Reynolds number range of 600< Rej<750 and Strouhal number range of 0.22<St