Effect of synthetic jet on circular cylinder radiated noise in laminar flow state

Abstract

This article focuses on the effect of active control via synthetic jets (SJs) on sound generated by a two-dimensional circular cylinder by using the acoustic analogy method. The cylinder is immersed in a uniform upstream flow, where the corresponding Reynolds number is 100 and the Mach number is 0.1. A pair of SJs is placed near the cylinder’s separation point issuing periodically varying forcing, with different combinations of forcing frequency and phase difference. The lattice Boltzmann method (LBM) is coupled with the multi-direct forcing immersed boundary method to solve the near-field flow dynamics. The mechanism of the sound generation lies in the fact that pressure pluses are induced by the periodic vortex shedding from the cylinder’s surface, i.e. dipoles. In the case with active flow control, extra monopoles are generated by the unsteady flow rate resulting from the SJs' periodic blow/suction. The interaction between monopoles and dipoles is confirmed to have a big influence on the acoustic field. The acoustic analogy method is used in various cases with a wide range of control parameters, because it has a considerably lower computational cost than the direct simulation method. Taking into account the effect of the monopole, the acoustic analogy method is developed for solving two-dimensional sound field by substituting the Green’s function. Results indicate that the primary lock-on and the secondary lock-on occur in the case of specified control parameters. The frequency of vortex shedding is related to the SJs’ frequency, deviating from the unforced frequency. Owing to the noise induced by flow, the frequency and phase difference of the SJs also have significant influence on sound field. The far-field noise is enlarged although the SJs reduce drag, due to the induced extra monopole, as well as the strengthened hydrodynamic fluctuation. Further increasing SJs’ frequency or reducing the phase difference will enlarge the far-field noise and make the directivity transformed from dipole to monopole, since the SJs’ self-noise is stronger. Moreover, it is found that the acoustic power increases approximately 4–18 dB compared with the unforced circular cylinder and the drag dipole is strengthened in all combinations of control parameters. This study deepens the understanding of the effect of SJs on sound field, and provides a reference for future studying the control strategies of suppressing noise generated from bluff bodies.