Characterisation of an independently controlled coaxial synthetic jet

Abstract

This paper presents the hot-wire measurements to characterise the flow field of a novel coaxial synthetic jet (CSJ) actuator. The CSJ actuator is formed by positioning two single-acting synthetic jet (SJ) actuators coaxially (0° orientation angle) along with their own flow actuation mechanism (piezo-electric diaphragms). The CSJ is formed by the periodic suction and expulsion of the fluid into a quiescent medium employing the piezo diaphragms through the inner and annular openings (orifices), which have an equal hydraulic diameter (dh) of 3 mm. This novel configuration facilitates independent control of excitation voltage (V), frequency (f) for inner and annular jets, and additionally, various phase differences (∅s) between the inner and annular jets. The resonance frequency of both the inner and annular cavities are obtained separately as 350 Hz, and 320 Hz, respectively. The inner jet is operated at Reynolds number Re=1100 (V=50 V, and f=350 Hz), which remains fixed for all the test cases. The CSJ flow field is analysed for three mass flux ratios (Mrs) as 0.7, 1.0, and 2.0, which are achieved by operating both the diaphragms independently at various excitation voltages for a fixed f=350 Hz, and at various fs for a fixed excitation voltage of 50 V. Moreover, the effect of ∅s (range: 0° to 180° in the step of 45°) by leading the inner or annular jet is examined while the CSJ is operated at these Mrs. It has been observed that the strength and spreading of the CSJ can be enhanced by providing ∅s between the jets, without investing any additional power for flow excitation. It is important to notice that leading the weaker jet enhances its contribution to the creation of the CSJ, which improves the CSJ's strength and spread. The CSJ flow field is compared with that of the inner SJ, and annular SJ, operated individually, to illustrate the superior performance of the CSJ. The paper conveys that the CSJ’s strength and spreading can be manipulated significantly with the independent control facility, which is feasible with this configuration.