In order to study the characteristics of a bidirectional shaft tubular pump with S-type symmetric airfoil blades, a prototype model was designed, manufactured, and tested. The energy characteristics, cavitation characteristics, and runaway characteristics of the pump were obtained under forward and reverse operating conditions for five different blade angles. Based on the basic equations of the pump and the inlet and outlet velocity triangles, combined with model tests and numerical simulations, the hydraulic performance of the pump was extensively analyzed and evaluated. In addition, semi-empirical equations for reverse efficiency and runaway characteristics were proposed. The dynamic pressure-drop coefficients were introduced to compare the cavitation performance under different flow rates in forward and reverse operations. The results reveal that the efficiency of the pump in reverse operation is greater than that of forward operation only under a very small flow rate. While the cavitation performance of the bidirectional pump in the two operating modes is almost the same, the runaway speed and backflow rate in forward operation are considerably greater than those of reverse operation. The results provide an important reference for the safe and stable operation of bidirectional shaft tubular pumps.

This study experimentally investigated the transient flow characteristics of single and twin parallel circular impinging jets using Particle Image Velocimetry (PIV) and Proper Orthogonal Decomposition (POD). High spatiotemporal resolution data were collected to analyze and visualize the distribution and interactions of multi-scale vortex structures. The experimental data were evaluated for PIV uncertainty and statistical convergence. The flow field distribution of the fountain that may occur in the twin impinging jets was also discussed. The results revealed that, for the oblique single jet, high-energy large-scale structures primarily concentrate in downhill regions and gradually decompose into smaller-scale structures. In the combination of twin jets, vortices in the shear layers merge in an alternating pattern. The frequencies of the inner and outer shear layers are sensitive to jet spacing with different characteristic interaction patterns repeatedly observed based on varying spacings. Three distinct cases are defined based on the interaction of the twin jets with the impingement plate. Additionally, three distinct flow field structures of fountain are defined to facilitate, indicating the degree of combination as well. The velocity profiles of the fountain are similar and adhere to a Gaussian distribution. Furthermore, a semi-empirical equation was developed to describe the centerline velocity of the fountain.

The flow past a 2D circular cylinder with control rods is numerically simulated in the present paper. The suppression effects of the control rods on the vortex-induced vibration (VIV) characteristics of the cylinder are investigated using the overlapping grid method and user defined function. By setting the cylinder’s vibration amplitude and drag force coefficient as the expected objective function, a multi-objective optimization is carried out to enhance the suppression performance at different reduced velocities, for which the rod-to-cylinder diameter ratio, gap ratio, incidence angle and reduced velocity are used as design variables. The regression expressions, which are obtained by Box–Behnken design (BBD), are employed in nondominated sorting genetic algorithm II (NSGA-II) to minimize the objectives. It is found that both the optimized vibration amplitude and drag force coefficient exhibit a monotonically decreasing linear trend with the reduced velocity, and as the reduced velocity decreases, the range of the optimized points gradually decreases. Subsequently, the optimized cases are verified by CFD simulations and compared with the original cases. For the samples at the reduced velocity of 4, 5, and 6, the vibration amplitude after optimization decreases by 15.1%, 24.8%, and 21.6%, respectively.

The dynamics of cavitation bubbles critically affect the stability of gas layers and induce intense impact forces on adjacent surfaces. This study numerically investigates the complex interaction mechanisms among a cavitation bubble, a free surface, and three representative biomimetic gas–entrapping microstructured surfaces. A compressible volume of fluid multiphase model with equations of state is developed and validated against experimental observations of bubble morphology, gas layer evolution, and free surface deformation. The effects of bubble–surface (γf = 0.25–1.0) and bubble–wall (γw = 0.8–1.5) stand–off distances are systematically examined, focusing on liquid jet characteristics, shock wave propagation, wall shear stress, and the resulting force and impulse on the microstructured surface. The results reveal that the hierarchical microstructure provides superior gas layer retention, sustaining a higher gas volume fraction (αg = 0.87) and attenuating impact loading (p/p* = 45) compared to micropillar and re-entrant microstructures. The evolution of wall shear stress shows alternating outward and inward patterns driven by bubble-induced radial flows, with peak values of 282 kPa (outward) and 89 kPa (inward) for the hierarchical microstructure. Furthermore, variations in γf and γw significantly influence the jet dynamics, modify the spatial distribution of wall pressure and shear stress, and determine the resulting force and impulse on the microstructured surface. These findings offer theoretical insight into mitigating cavitation-induced erosion and enhancing gas layer stability in marine hydrodynamics and fluid machinery.