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<0.50. The geometries of the lobed orifices used in this work differ in lobe count and penetration. It was shown that a broadband noise reduction is possible with such orifices, with a maximum noise reduction of 14 dB at particular frequencies. The results indicate that a high number of lobes and penetration are preferred for noise reduction, however, at the expense of quickly dissipating downstream jet velocity. Flow visualisation reveals that this adverse effect is caused by enhanced mixing of lobed jets with ambient air that leads to earlier and more aggressive breakup of flow structures. A double chamber SJA is also used to demonstrate the noise attenuation through the antiphase operation of two cavities, caused by the interference pattern of the sound field of each source. The maximum reduction measured using this actuator configuration is 14 dB, depending on directivity.EPSRC & RAe

Experimental investigation of the aeroacoustics of synthetic jet actuators in quiescent conditions

In this paper, the aeroacoustic characteristics of a circular orifice, synthetic jet actuator in quiescent conditions is investigated. Electromagnetic actuation, in the form of a shaker-driven actuator with latex diaphragm, proved to be desirable over piezoelectric actuation for this work due to the reduced diaphragm noise contribution to overall actuator self-noise, hence making it easier to identify jet-related noise. Acoustic and velocity data, collected from microphone measurements in an anechoic chamber and hotwire measurements respectively, were compared for correlation. Schlieren visualization was also used to show synthetic jet development near the orifice. Flow-induced sound in the form of an audible whistling was found to occur for a Strouhal number range of 0.2

Towards the noise reduction of piezoelectrical-driven synthetic jet actuators

This work details an experimental investigation aimed at reducing the noise output of piezoelectrical-driven synthetic jet actuators while minimising peak jet velocity reduction. The study considers double-chamber actuator for anti-phase noise suppression and lobed orifice as a method to enhance jet turbulent mixing to suppress jet noise. The study involved the design, manufacture and bench test of interchangeable actuator hardware. Hot-wire anemometry and microphone recordings were employed to acquire velocity and sound pressure level measurements respectively across a range of excitation frequencies for a fixed diaphragm clamping and input voltage. The data analysis indicated a 26% noise reduction (16 dB) from operating a single-chamber, round orifice actuator to a double-chamber, lobed orifice one at the synthetic jet resonant frequency. Results also showed there was a small reduction in peak jet velocity of 7% (∼3 m/s) between these two cases based on orifices of the same discharge area. The electrical-to-fluidic power conversion efficiency of the double-chamber actuator was found to be 15% for both orifice types at the resonant frequency; approximately double the efficiency of a single-chamber actuator

Exploring Supervisors’ Decisions about Procedural Entrustment in Simulation-based and Workplace-based Settings

Entrustment decision-making is a fundamental goal of competency-based medical education (CBME). How supervisors engage their ‘rater cognition’ when judging for entrustment using workplace-based and simulation-based assessments is unclear. We carried out an interview-based, constructivist grounded theory-informed qualitative study that aimed to explore how supervisors make entrustment decisions in both settings using endoscopic polypectomy as a relevant entrustable professional activity (EPA). Gastroenterology supervisors completed EPAs for endoscopic polypectomy after both a single workplace-based and simulation-based assessment and were interviewed after each. Transcribed data were coded iteratively using constant comparison to generate themes. We found participants had difficulty making entrustment decisions and scored EPAs idiosyncratically due to variable personal meanings of entrustment. Each setting was found to have unique strengths in entrustment decision-making when combined in a program of assessment. These data support re-evaluation of the current usage of EPA assessments within CBME systems.M.Sc

An app with remote support achieves better adherence to home exercise programs than paper handouts in people with musculoskeletal conditions: a randomised trial

Question: Do people with musculoskeletal conditions better adhere to their home exercise programs when these are provided to them on an app with remote support compared to paper handouts? Design: Randomised, parallel-group trial with intention-to-treat analysis. Participants: Eighty participants with upper or lower limb musculoskeletal conditions were recruited to the trial. Each participant was prescribed a 4-week home exercise program by a physiotherapist at a tertiary teaching hospital in Australia. Participants were randomly assigned via a computer-generated concealed block randomisation procedure to either intervention (n = 40) or control (n = 40) groups. Intervention: Participants in the intervention group received their home exercise programs on an app linked to the freely available website www.physiotherapyexercises.com. They also received supplementary phone calls and motivational text messages. Participants in the control group received their home exercise programs as a paper handout. Outcome measures: Blinded assessors collected outcome measures at baseline and 4 weeks. The primary outcome was self-reported exercise adherence. There were five secondary outcomes, which captured functional performance, disability, patient satisfaction, perceptions of treatment effectiveness, and different aspects of adherence. Results: Outcomes were available on 77 participants. The mean between-group difference for self-reported exercise adherence at 4 weeks was 1.3/11 points (95% CI 0.2 to 2.3), favouring the intervention group. The mean between-group difference for function was 0.9/11 points (95% CI 0.1 to 1.7) on the Patient-Specific Functional Scale, also favouring the intervention group. There were no significant between-group differences for the remaining outcomes. Conclusion: People with musculoskeletal conditions adhere better to their home exercise programs when the programs are provided on an app with remote support compared to paper handouts; however, the clinical importance of this added adherence is unclear. Trial registration: ACTRN12616000066482. [Lambert TE, Harvey LA, Avdalis C, Chen LW, Jeyalingam S, Pratt CA, Tatum HJ, Bowden JL, Lucas BR (2017) An app with remote support achieves better adherence to home exercise programs than paper handouts in people with musculoskeletal conditions: a randomised trial. Journal of Physiotherapy 63: 161–167