## 1 Introduction  
Grazing flows over side-wall cavities might generate high-level noise if the acoustic resonance frequency of the cavity matches with the flow oscillations [1; 2; 3]. This acoustic resonance noise, which is narrowband in nature, could not only cause psychological discomfort for humans, but also could strengthen the structural load and hence give rise to fatigue damage. Further, this problem is also an important issue for some weapons or underwater vehicles to reduce their noise radiation and remain invisible from detectors. Consequently, the generation mechanism of this flow-induced cavity resonance as well as its control methods have drawn a great deal of interests for several decades. Both passive and active strategies can be considered to reduce this flow-induced resonance noise. [PERSON] and [PERSON] [4] gave a comprehensive review on the topic of simulation and control of cavity flows, in which over 60 experimental and computational studies before 2011 were summarized and various passive strategies were presented. In recent years, it has also been reported that flow-induced cavity resonance could be alleviated by adding sawtooth spoilers [5] or a span-wise rod [6] at the leading edge, attaching sound-absorbing materials at the cavity bottom [7], or even reshaping the cavity with slanted rear and front walls [8]. Compared with passive strategies, external energy is generally necessary for active methods, where actuators are utilized to achieve better control performance. A comprehensive review on active strategies before 2008 can be found in [9], where different kinds of actuators and sensors as well as different control methods were summarized and compared. Among them, the most common method is active flow control, in which leading-edge actuators are usually used to regulate the flow field on the opening surface of the cavity, so that both the flow oscillations and acoustic resonance could be reduced. It has been reported by recent works that using plasma actuators [10,11] as well as injecting extra flows [12; 13; 14] at the leading edge could effectively suppress the cavity resonance noise. [PERSON] et al. [15; 16; 17] developed the surface perturbation technique, in which a piezo actuator was designed and installed upstream in a duct system so that the resonance noise of the downstream side-wall cavity could be reduced. [PERSON] et al. [18] also presented a closed-loop control method, where a series of linear controllers are iteratively designed to control the nonlinear oscillations of resonating flows. Although different methods of active flow control have been confirmed to be effective to suppress the cavity resonance noise, an obvious drawback of such methods is that the noise level at other frequencies would possibly be enhanced, as could be observed in [10; 11; 12; 13; 14; 15; 16; 17; 18]. Since the final objective is to reduce resonance noise, active noise control (ANC) [19] can naturally be considered as another approach. In an ANC system, a secondary noise is generated by a loudspeaker with the same amplitude but the opposite phase when compared with the primary noise. Thus, when the two noise interfere with each other, the total noise level could be greatly reduced. Some early attempts [20,21] have been performed around 2000. [PERSON] et al. [20] gave simulation results with an ANC system. The filtered-x least mean square (FxLMS) algorithm [19], which is the most basic adaptive algorithm in ANC literatures, was used to control the cavity resonance. It was pointed out that only local noise reduction could be expected around the position of the error microphone. [PERSON] et al. [21] compared the noise reduction performance with a fixed analog controller and a digital adaptive controller. It was claimed that the adaptive controller would lose its efficiency when dealing with multiple acoustic modes of the cavity.