Modal decomposition techniques, flow field and spectral analysis are employed to investigate the wake dynamics and destabilisation mechanisms of a four-bladed marine propeller with or without a nozzle. Numerical simulations are conducted using the Delayed Detached Eddy Simulation (DDES) model for the wake and the Arbitrary Mesh Interface (AMI) method for the blade rotation. The presence of the nozzle significantly reduces the wake's streamwise velocity, delays the wake destabilisation, increases the wake length, and changes the morphologies of wake vortices. In particular, the hub vortex in the ducted propeller wake is broken down into chaotic turbulence by the perturbation of the backflow. From modal analysis, the spatial scale of flow phenomena decreases with the increase of modal frequency. Underlying destabilisation mechanisms in the wake correspond to some characteristic frequencies. The interaction of each sheet vortex with the previously shed tip (leakage) vortices occurs at blade passing frequency (BPF). The pairing of adjacent tip (leakage) vortices occurs at half-BPF. The long-wave instability of the hub vortex and the wake meandering are stochastic processes, each of which occurs at a frequency lower or equal to shaft frequency (SF). These four destabilisation mechanisms can approximately reconstruct the large-scale flow phenomena in the wake. Moreover, each sheet vortex's alternating connection and disconnection with the previously shed tip (leakage) vortices cause the short-wave instability of the tip (leakage) vortices and generate the secondary vortices. The radial expansion motion of large-scale helical vortices in the outer slipstream dominates the wake meandering phenomenon.

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The output common-mode voltage (CMV) will bring many negative effects in a three-phase inverter, which is controlled by the sinusoidal pulse-width modulation (SPWM) method. The carrier phase-shift method and the carrierfrequency modulation method can reduce the peaks of the CMV at the switching frequency and its harmonic frequencies, but they may cause increase of the low-frequency (LF) baseband harmonics. A new reverse injection scheme is proposed to suppress the LF harmonics. In this scheme, the parameters of the interference peaks are obtained through CMV detection and real-time harmonic parameters estimation. The estimated phase errors from the detection-control delay are amended by the method of temporary addition of the standard phase signal. Then, the sinusoids with a reverse phase to the LF harmonics are generated and injected in the reference sinusoid of each phase to produce the SPWM. Thus, the LF peaks of the CMV are counteracted. The feasibility and the validity of this scheme are verified through the experiments under different control strategies.

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