This study focuses on the influence and drag force of tandem square cylinders of variable size (with a side length ratio of 1.5) in order to determine the flow past tandem square cylinders in a constrained boundary under different gap ratios at a Reynolds number of 100.

Based on the lattice Boltzmann method with the bounce-back boundary condition, comparisons of the effects of tandem cylinders of equal and variable size are made under different gap ratios.

The results indicate that cylinders of variable size can accelerate the change of the flow field pattern to a certain extent. However, they can also delay the change of the flow pattern at a gap ratio of 6. In addition, they can achieve a resistance reduction of 9.16% at most compared with the flow past a single square cylinder. However, tandem cylinders of equal size can achieve 7.76% at most. On the other hand, compared with tandem cylinders of equal size, those of variable size can achieve a resistance reduction of 67.79%. Thus, tandem cylinders of variable size can obtain a better reduction in drag force, making them a feasible way to achieve flow control and drag reduction.

The results of this study can provide basic guidance for the optimization of drag and flow control for flow past square cylinders in a constrained boundary.

The purpose of this paper is to study the effects of cavitation models on the simulation of full-scale propeller cavitation.

The minimum rotational speed of the propeller required for cavitation is predicted by the Bernoulli equation, and the propeller's cavitation condition is observed by full-scale experiment. After determining the boundary layer mesh thickness and establishing the hydrodynamic model, meshes are generated. In the numerical simulation, the turbulence model is selected from either the standard $k-\epsilon$ model or Realizable $k-\epsilon$ model, and the cavitation model is selected from either the Schnerr−Sauer (S−S) model or the Zwart−Gerber−Belamri (ZGB) model. The maximum gas volume fraction on the surface of the propeller is tracked and recorded. In the meantime, the reliability of the numerical simulation is evaluated by observing the gas distribution on the surface of the propeller. And the simulation accuracy of the related models is compared.

The simulation results show that compared with the standard $k-\epsilon$ turbulence model, the cavitation region obtained with the Realizable $k-\epsilon$ model is significantly more consistent with the experimental results. By monitoring the maximum gas volume fraction on propeller's surface, it is found that the choice of turbulence model has little effect on the cavitation intensity, and the cavitation intensity obtained by S−S model is significantly higher than that obtained by the ZGB model.

This study shows that the turbulence model has a great influence on the area of the cavitation region, and cavitation models have a great influence on cavitation intensity. In terms of the simulation of propeller cavitation, the accuracy of the Realizable $k-\epsilon$ model is higher than that of the standard $k-\epsilon$ model.

This study aims to carry out systematic research on the hydrodynamic performance of a small unmanned catamaran.

Based on STAR-CCM+ numerical simulation software, the hydrodynamic performance of a small unmanned catamaran in a hydrostatic state and self-propelled state under different Froude (Fr) numbers is numerically simulated. The unsteady RANSE model is selected and the Dynamic Fluid Body Interaction (DFBI) model and overset grid function are used to simulate the trim and heave of the catamaran. A body-force propeller is used to replace the propeller effect. The empirical formula of the friction resistance coefficient is used to verify the simulation results of the hydrostatic state. The self-propelled experimental results are then compared to the simulation results of the self-propelled state to verify the accuracy of the simulation results.

When the propeller speed is 3000 r/min, the difference in total resistance between the self-propelled and hydrostatic states of the small unmanned catamaran is 21.099%. Under different propeller rotation speeds, the relative errors of the thrust between the simulation and experimental results of the self-propelled catamaran is less than 10%.

The comparison between the simulation results and experimental results verifies the reliability of the simulation. The hydrodynamic performance of the self-propelled ship hull studied by the volume method is quite different from that under hydrostatic conditions. The numerical method can provide valuable references for further predicting the hydrodynamic performance of small unmanned catamarans.