水动力学研究与进展:英文版最新文献

The sheet/cloud cavitation is of a great practical interest since the highly unsteady feature involves significant fluctuations around the body where the cavitation occurs. Moreover, the cavitating flows are complicated due to the thermal effects. The present paper numerically studies the unsteady cavitating flows around a NACA0015 hydrofoil in the fluoreketone and the liquid nitrogen with particular emphasis on the thermal effects and the dynamic evolution. The numerical results and the experimental measurements are generally in agreement. It is shown that the temperature distributions are closely related to the cavity evolution. Meanwhile, the temperature drop is more evident in the liquid nitrogen for the same cavitation number, and the thermal effect suppresses the occurrence and the development of the cavitating flow, especially at a low temperature in the fluoroketone. Furthermore, the cavitating flows are closely related to the complicated vortex structures. The distributions of the pressure around the hydrofoil is a major factor of triggering the unsteady sheet/cloud cavitation. At last, it is interesting to find that one sees a significant thermal effect on the cavitation transition, a small value of σ/2α is required in the thermo-sensitive fluids to achieve the similar cavitation transition that occurs in the water.

To understand the effect of the compressibility on the cavitating flow, a compressible, multiphase, single component Reynolds averaged Navier-Stokes (RANS) solver is used to study the cavitating flow on a wedge in the present work. A barotropic equation of status is used. A non-linear model for compressibility in the mixture is adopted to capture the effect of the compressibility within the complex cavitation bubbly mixtures. An unsteady cavitation phenomenon is found in the numerical simulation. The numerical results of local compressibility and Mach number in the bubbly mixture are given. The mechanism responsible for the unsteady shedding of the bubbly mixture is discussed based on the numerical results.

The water medium (WM) retarder is an auxiliary braking device that could convert the kinetic energy of the vehicle to the thermal energy of the coolant, and it is used instead of the service brake under non-emergency braking conditions. This paper analyzes the flow distribution based on a mathematical model and analyzes the key factors that could affect the filling ratio and the braking torque of the WM retarder. Computational fluid dynamics (CFD) simulations are conducted to compute the braking torque, and theresults are verified by experiments. It is shown that the filling ratio and the braking torque can be expressed by the mathematical model proposed in this paper. Compared with the Reynolds averaged Navier-Stokes (RANS) turbulent model, the shear stress transport (SST) turbulent model can more accurately simulate the braking torque. Finally, the flow distribution and the flow character in the WM retarders are analyzed.

In this paper, recent measurements of tip vortex flow with and without cavitation carried out in Cavitation Mechanism Tunnel of China Ship Scientific Research Center (CSSRC) are presented. The elliptic hydrofoil with section NACA 66₂-415 was adopted as test model. High-speed video (HSV) camera was used to visualize the trajectory of tip vortex core and the form of tip vortex cavitation (TVC) in different cavitation situations. Laser Doppler velocimetry (LDV) was employed to measure the tip vortex flow field in some typical sections along the vortex trajectory with the case of cavitation free. Stereo particle image velocimetry (SPIV) system was used to measure the velocity and vorticity distributions with and without cavitation. Series measurement results such as velocity and vorticity distributions, the trajectory of tip vortex core, the vortex core radius, cavity size and cavitation inception number were obtained. The results demonstrated that the minimum pressure coefficient in the vortex core obtained by flow field measurement was quite coincident with the tip vortex cavitation inception number obtained under the condition of high incoming velocity and low air content. And TVC would decrease the vortex strength comparing with the case without cavitation.

Stable attached partial cavitation in separated flows can transition to cloud shedding, and the mechanism of transition has been attributed to the presence of a re-entrant liquid jet. Our findings have revealed the presence of propagating bubbly shock waves as an alternative dominant mechanism of shedding when the compressibility of the bubbly mixture is appreciable. In the present paper, we discuss dynamics associated with these bubbly shock waves, interaction of shock waves with obstacles in their path, and means to manipulate their properties to control the shedding process by non-condensable gas injection.

The tip vortex cavitation (TVC) noise of marine propellers is of interest due to the environmental impacts from commercial ships as well as for the survivability of naval ships. Due to complicated flow and noise field around a marine propeller, a theoretical approach to the estimation of TVC noise is practically unrealizable. Thus, estimation of prototype TVC noise level is realized through extrapolation of the model TVC noise level measured in a cavitation tunnel. In this study, for the prediction of prototype TVC noise level from a model test, a novel scaling law reflecting the physical basis of TVC is derived from the Rayleigh-Plesset equation, the Rankine vortex model, the lifting surface theory, and other physical assumptions. Model and prototype noise data were provided by Samsung Heavy Industries (SHI) for verification. In applying the novel scaling law, similitude of the spectra of nuclei is applied to assume the same nuclei distribution in the tip vortex line of the model and the prototype. It was found that the prototype TVC noise level predicted by the novel scaling law has better agreement with the prototype TVC noise measurement than the prototype TVC noise level predicted by the modified ITTC noise estimation rule.

In this study, the effect of the free surface on the cloud cavitating flow around a blunt body is investigated based on the water tank experiment and the CFD method. Numerical results are in good agreement with experimental data, and the mesh independence of the methods is verified. The cavity evolution process includes the cavity growth, the re-entrant jet, the cavity shedding, and the collapse, which can all be observed from the water tank experiment. The effects of the free surface on the cavity length, the thickness, and the cavity evolution period are analyzed by comparing the difference between the cavitating flows on the upper and lower sides of the body. This study also examines the effect of the distance between the free surface and the model through a series of water tank experiments and numerical simulations. The cavity stability and asymmetry, as well as the thickness and the velocity of the re-entrant jet inside the cavity, which varies with the submerged depth, are discussed with consideration of the effect of the free surface. The effect of the free surface on the cavitating flow around the blunt body is enhanced with the decrease of the submerged depth.

A two-dimensional model is built to describe the translation and the rotation of the hovering flapping movement. The equations of motion are derived for insect's flapping movement, and the model is implemented by the computational fluid dynamics (CFD) software FLUENT and it's user defined function (UDF). It is shown that the lift coefficient changes slowly in the intermediate stage, there are two areas in which the lift coefficient changes dramatically, and the drag coefficient behaves quite differently when flapping up and down. The vortex distribution, the pressure distribution, and the velocity vector distribution in the advanced mode at different times follow quite various rules.

The vortex structure and the hydrodynamic performance of a tadpole undulating in the wake of a D-section cylinder are studied by solving the Navier-Stokes equations for the unsteady incompressible viscous flow. A dynamic mesh fitting the tadpole's deforming body surface is used in the simulation. It is found that three main factors can contribute to the thrust of the tadpole behind a D-cylinder: the backward jet in the wake, the local reverse flows on the tadpole surface and the suction force caused by the passing vortices. The tadpole's relative undulating frequency and the distance between the D-cylinder and the tadpole have a great influence on both the vortex structure and the hydrodynamic performance. At some undulating frequency, a tadpole may break or dodge vortices from the D-cylinder. When the vortices are broken, the tadpole can gain a great thrust but will consume much energy to maintain its undulation. When the vortices are dodged, the tadpole is subject to a small thrust or even a drag. However, it is an effective way to save much energy in the undulating swimming, as the Kármán gait does. As the tadpole is located behind the D-cylinder at different distances, three typical kinds of wake are observed. When an incomplete Kármán vortex street forms between the D-cylinder and the tadpole, the tadpole is subject to the highest thrust.