Analyses on flow structures behind submersible with flexible appendage during floating based on continuous wavelet transform and dynamic mode decomposition

This study propounded a new method of installing flexible appendage on the surface of a submersible to modify the wake structure of the submersible as it floats, and explored a method of drag reduction of floating submersible. A comparative analysis of the flow structure of submersibles with varying appendage lengths was done to understand the disturbance characteristics of the wake flow structure of submersible, and the high-speed particle image velocimetry (PIV) measurement experiment was conducted at a Reynolds number of 13448. The analysis of the time-averaged and transient flow fields revealed that flexible appendage was able to break up the large-scale vortices in the wake stream as the submersible floats, but this ability diminished as the length increasing of flexible appendage. Furthermore, in order to investigate the mechanism of this phenomenon, continuous wavelet transform (CWT) and dynamic mode decomposition (DMD) were performed. The results suggested that the flexible appendage inhibited large-scale flow, and the inhibition effect decreased with increasing of flexible appendage length based on CWT. It was demonstrated that the flexible appendage was capable of reducing the energy dominance of the large-scale vortex structure, thereby facilitating the transition from large-scale to small-scale vortices based on DMD. Concurrently, it was found that the intrusion of excessively lengthy appendage into the wake region resulted in the generation of additional disturbances and impeded the process of small-scale vortex shedding.