Mode characteristics of transition flow in the boundary layer of the revolved body under wire spoiler disturbances

The boundary layer transition of underwater vehicles produces strong noise, which seriously affects the communication and detection performance. In order to explore the main flow structures that induce the flow noise, a SUBOFF model is used as the research object, and the experiment and the large eddy simulation method are used to obtain the boundary layer flow field of the underwater revolved body under wire spoiler disturbances. The flow in the boundary layer first forms a separation bubble downstream of the wire spoiler, and a transition occurs in the posterior part of the separation bubble, which induces a large number of multi-scale unsteady vortices. Furthermore, based on the proper orthogonal decomposition method, the mode characteristics of the streamwise and normal velocity pulsation as well as the pressure pulsation in the boundary layer transition zone are studied. It is found that the dominant mode of the streamwise velocity pulsation is the separation bubble and its breakdown along the streamwise direction as well as streamwise vortex downstream. The streamwise direction pulsation in the separated bubble area has a composite frequency of < 3000 Hz, and in the downstream vortex region has a low-frequency structure of < 200 Hz. The mode characteristics of the normal velocity pulsation and pressure pulsation are similar. They are both high-frequency, broad-frequency modal structures with the greatest intensity near the separation bubble. It is inferred that the separation bubble and its breakdown are the main inducements of boundary layer transition, the induced normal velocity pulsation is the main source of high/broadband noise, and the streamwise velocity pulsation is only related to low frequency noise.