This paper details an experimental investigation aimed at reducing the noise output of piezoelectrical-driven synthetic jet actuators without compromising peak jet velocity. Specifically, the study considers double-chamber ('back-to-back') actuators for anti-phase noise suppression and corrugated-lobed orifices as a method to enhance turbulent mixing of the jets 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 noise measurements respectively for each chamber configuration and orifice plate across a range of excitation frequencies and for a fixed input voltage. The data analysis indicated a 32% noise reduction (20 dBA) from operating a singlechamber, circular orifice SJA to a double-chamber, corrugated-lobed orifice SJA at the Helmholtz 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.

Jet noise of a synthetic jet actuator is isolated from its diaphragm noise to assess aeroacoustic characteristics  The actuator aeroacoustic response, in the form of audible whistling, is found to occur in a similar Strouhal number range as other pipe and orifice flow systems  A threshold in the jet Reynolds number is established for the onset of flow-induced sound  Good agreement between the acoustic spectra, velocity spectra and Schlieren visualisation of the synthetic jet

In this paper the effect of lobed orifice geometry on the self-noise generated by synthetic jet actuators (SJA) is investigated. Two different lobed orifices were designed, one 4-lobed and one 6-lobed. These were then compared to a baseline circular orifice with the aid of CFD simulations, where all orifices have the same effective diameter, de = 2mm. It was shown that the 6-lobed orifice would be the most suitable orifice for the task of noise reduction, as the increased turbulent kinetic energy due to increased mixing occurs further away from the jet core and would enhance mixing without aggressively impacting the jet core velocity. The 6-lobed orifice was then experimentally validated. An analysis of the jet velocity profile at several downstream locations from the orifice exit indicates that though the exit velocities are the same in both orifice cases there is increased mixing causing the jet core to dissipate at a faster rate downstream compared to the baseline circular orifice. Also a noise spectrum analysis shows that there is a broadband noise reduction using the 6-lobed orifice over mid to high frequency range. NomenclatureA = Orifice area, m 2 a = Speed of sound, m/s D = Cavity diameter, mm d = Orifice diameter, mm de = Effective orifice diameter, mm rD = Orifice radius, m = density, kg/m 3 H = Cavity height, mm h = Orifice height, mm f = Actuation frequency, Hz fh = Helmholtz resonance frequency, Hz fd = Diaphragm resonance frequency, Hz E = Young's Modulus, Pa td = Diaphragm thickness, mm = Empirically obtained damping coefficient k 2 = dimensionless frequency parameter = Diaphragm Poisson's ration = Kinematic viscosity of air, m 2 /s 1 PhD Student, College of Engineering, Design and Physical Sciences, UB8 3PH