A vibration isolator system with nonlinear stiffness is designed and tested to solve problems on marine noise measuring equipment which are caused by underwater flow. The low frequency response characteristics of the vibration isolation system are evaluated from static function and dynamic simulation with Harmonic Balance Method. The theoretical analysis results show that the total stiffness value of the isolator system will be kept in 0 point around when parallel springs stay in equilibrium position. In order to assess the practical efficiency of Quasi-Zero-Stiffness (QZS) isolator, a vibration response test is carried out. By comparing the acceleration signals at 5 Hz and 100 Hz between linear and nonlinear system in time domain and frequency domain, the transmissibility curves are obtained. The experimental results demonstrate that the isolation system expands the frequency range of operation effectively by decreasing the natural frequency of the QZS vibration isolator. In addition, the QZS isolation system decreases the transmissibility of vibration by reducing the amplitude of output signals. The QZS isolator has more outstanding performance in contrast to linear spring in ultralow frequency isolation field.

The hydrodynamic performance and near wake of tidal current turbines are investigated via numerical simulation and experimental methods in this study. Based on large eddy simulation, the performance and wake characteristics of a tidal 10 kW turbine are then studied at different TSR (Tip Speed Ratio), and the velocity deficit and wake expansion along the flow direction are compared. The equal proportion hydraulic turbine model is designed by the similarity theory, and the experiment was carried out in a water channel, the test parameters including power and torque. The simulation and experimental results show that due to the boundary effect and blocking effect of the flume, there is a certain deviation between the experimental data and the simulation data, but the trend is the same. With the increase of TSR, the pitch of the helix formed by tip vortex gradually decreases, and the tip vortex and root vortex are destroyed earlier in the process of moving downstream. The axial velocity deficit decreases with the increase of axial distance. Finally, the hydrodynamic performance of a turbine is analyzed through experiment and simulation. The experimental results verify and support the simulation results, but there are quantitative differences, and when the TSR = 5, the turbine efficiency has the maximum value.

A disc-shaped double-layer shell structure reinforced by stiffeners is introduced for underwater gliders. Based on the finite element method integrated with automatic matching layer (FEM/AML) technology and the direct boundary element method (DBEM), the acoustic response of a disc-shaped double-layer shell with six longitudinal ribs within the frequency range of 10–500 Hz is obtained. The resonant frequencies of the sound pressure level (SPL) correlate with the structural–acoustic modes. At resonance frequencies, the acoustic directivity and spatial sound pressure distribution of the double-layer shell exhibit symmetry relative to the mid-cross-section. The influence of longitudinal rib counts on vibro-acoustic behavior is investigated. The analysis results of frequency–spatial spectrum for radiated sound pressure reveal that the resonant frequencies migrate to the mid-high frequency with increases in the longitudinal rib quantity.

Underwater gliders are extensively employed in oceanographic observation and detection. The structural characteristics of thin-wall shells are more susceptible to vibrations from internal mechanical components; this noise emission becomes more complex with the presence of water surfaces. The finite element method (FEM) is introduced to discuss the dynamic performance of cylindrical shells with different lengths. The acoustic-structure coupling, together with the effect of the water surface, is validated by comparisons with experimental or analytical solutions under three cases: half-filled, half-submerged, and partially submerged in fluid. Compared to the verification result, the relative error of the eigenfrequency derived from the numerical result is less than 3%, and then the mesh division and boundary conditions are adjusted to calculate the vibroacoustic response of a disc-type glider. During its water entry process, there are six distinct bright curves in frequency–depth spectra of sound pressure radiated from a partially immersed disc-type glider. The first curve is continuous, while the remaining five curves display discontinuities around a region where the geometric curvature changes gradually. As the submerged depth increases, this causes a shift in the resonance frequencies, evidenced by the curves transitioning from higher to lower frequencies.