Experimental study on tail cavity structure and pressure characteristics of underwater vehicle with tail jet

The tail of an underwater vehicle with a tail jet often creates an attached tail cavity when moving at high speed. The strong coupling between the tail cavity and high-speed jet affects the kinematics characteristics of underwater vehicles. This paper compares the influence of Froude number (Fr) and ventilation coefficient (C_Qv) on the tail cavity at area ratio A/A_e = 92.5 and 236.7 by water tunnel experiment. It was found that at a small Fr, the critical ventilation coefficient of the transition from intact (IC) to partially broken (PBC) and PBC to pulsating foam (PFC) at A/A_e = 92.5 is greater than that of A/A_e = 236.7. The cavitation number (σ_c) at A/A_e = 92.5 is generally smaller than that of A/A_e = 236.7. With increased C_Qv, PBC's dimensionless time-averaged length (L_c‾/D) decreases sharply and then gradually flattens. The L_c‾/D of A/A_e = 236.7 is larger than that of A/A_e = 92.5. The fluctuation amplitude of the PBC length is smaller than that of the PFC, and the fluctuation frequency (f_Lc) of the PFC length is about 120 Hz. The change in the tail cavity will affect the pressure characteristics of the vehicle's bottom. With the increase of C_Qv, the peak frequency of the bottom pressure fluctuation (f_peak) will continue to rise in the PBC until the transition to the PFC, and the f_peak will remain at about 120 Hz. The correlation coefficient is 0.74 between the f_Lc and the f_peak. In addition, the pressure oscillation intensity at the vehicle's bottom (p_brms) is small in the IC region. When the tail cavity changes from PBC to PFC, p_brms increases gradually at first, then tends to be flat.