合成射流对层流状态下圆柱辐射噪声的影响

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

主动流动控制是降低流动噪声的新型途径,近年来吸引了研究者的广泛关注.利用二维格林函数,推导得到了忽略四极子影响的声比拟方程的二维时域解.在此基础之上,基于格子Boltzmann方法为核心的流场求解器以及声比拟方程为核心的声场求解器,在低雷诺数(Re=100)下,开展了合成射流频率和相位差对圆柱绕流近壁流场和声辐射远场影响的数值模拟研究.结果表明,在利用合成射流减小圆柱阻力时,出现了两种频率锁定状态,且远场辐射噪声会发生明显变化.提高射流频率或减小相位差会增强射流的自噪声,导致远场辐射声压进一步增大,并使得指向性由偶极子类型转变为单极子类型.在各个控制参数组合下,远场的总辐射噪声增加,与未施加控制时相比声功率增加了4到18 dB,同时,阻力方向的噪声始终增强.本研究可为探索低辐射噪声的合成射流控制策略提供参考数据.

This article focuses on the effect of active control via synthetic jets(SJs)on sound generated by a twodimensional 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.