Journal of Hydrodynamics最新文献

In the context of a sudden contraction plug conduit, the near-wall area experiences a significant shearing effect of water flow, however, the extent to which this shearing effect occurs in bubble-water flow and the related variation mechanisms of air bubble size and number remain unclear. This study employs a model test method to investigate the diffusion process of bubble-water flow in a sudden contraction plug conduit. The size and number of bubbles, as well as their distribution along the shearing section under varying initial air volume conditions, are studied in detail using a high-speed image acquisition system. The experimental findings reveal a self-similar relationship between the number and size of bubbles and their cross-sectional distribution over time. The bubble number and size vary in three stages, i.e., quasi-suspension, shearing, and shearing completion stages. The direction perpendicular to the conduit exhibits peak values in bubble number distribution over the three stages, with peak value location varying with the near-wall area. As time progresses, the peak value increases, and a larger initial air volume corresponds to a smaller distance of the peak value location from the wall. The size of air bubbles near the wall is consistent with the minimum diameter of air bubbles in shear flow and is hardly affected by the initial air volume. These results aid in comprehending the change law of two-phase water and air flow under a strong shearing effect in the plug conduit, and provide useful insights for hydraulic design in fluid engineering.

Tip clearance cavitation is one of the most common cavitation phenomena exist on duct propellers, pumps and some hydraulic turbines, which may lead to erosion of the components. Due to the influence of the nearby wall, cavitation inside the tip clearance is more complicated than other cases without interaction. So far, the understanding about the impact mechanism of tip clearance cavitation is still limited. In this paper, to obtain the impact behavior of tip clearance cavitation, a high-speed camera was used to capture the cavitation behavior inside the tip clearance of a hydrofoil, and surface paint coating peeling method was applied to show the impact region. Results indicated that cavitation around the tip of the hydrofoil was composed of a tip separation cavity and a tip leakage vortex cavity, and the one with contribution to impact was the tip separation cavity. Through the comprehensive analysis of the paint peeling region and dynamic behavior of tip separation cavity, the impact was found to be related to the local collapse and rebound of the cloud cavitation shed from the attached part. In addition, the influence of tip clearance size on the behavior of tip clearance cavitation was also investigated. As the tip clearance size increased, the tip separation cavity tended to transfer from sheet cavitation to vortex cavitation. These findings can provide a sound basis for evaluating the erosion risk arising from the tip clearance cavitation.

Wall-modeled large eddy simulation (WMLES) is used to investigate turbulent fluctuations around an axisymmetric body of revolution. This study focuses on evaluating the ability of WMLES to predict the fluctuating flow over the axisymmetric hull and analyzing the evolution of turbulent fluctuations around the body. The geometry is the DARPA SUBOFF bare model and the Reynolds number is 1.2×10⁷, based on the free-stream velocity and the length of the body. Near-wall flow structures and complex turbulent fluctuation fields are successfully captured. Time-averaged flow quantities, such as time-averaged pressure and skin-friction coefficients, and time-averaged velocity profiles on the stern, achieved great agreements between WMLES results and experimental data. Self-similarity of time-averaged velocity defects within a self-similar coordinate up to twelve diameters from the tail. A comprehensive analysis of second-order statistics in the mid-body, stern, and wake regions is condutced. Numerical results agree well with experimental data and previous wall-resolved large eddy simulation (WRLES) results about root mean square (rms) of radial and axial fluctuating velocities at the stern. Turbulent fluctuations including turbulent kinetic energy (TKE) and second-order velocity statistics are identified as dual peak behavior and non-self-similar over the wake length, consistent with previous findings in the literature. This assessment enhances the understanding of WMLES capabilities in capturing complex fluctuating flow around axisymmetric geometries.

The collapse of the cavitation bubble near the rigid wall emits shock waves and creates micro-jet, causing cavitation damage and operation instability of the hydraulic machinery. In this paper, the millimeter-scale bubble near the rigid wall was investigated experimentally and numerically with the help of a laser photogrammetry system with nanosecond-micron space-time resolution and the open source package OpenFOAM-2212. The morphological characteristics of the bubble during its growth phase, collapse phase and rebound phase were observed by experiment and numerical simulation, and characteristics of the accompanying phenomena including the shock wave propagation and micro-jet evolution were well elucidated. The numerical results agree well with the experimental data. The bubble starts from a tiny small size with high internal pressure and expands into a sphere with a radius of 1.07 mm for γ = d / R_max = 1.78. The bubble collapses into a heart shape and moves towards to the rigid wall during its collapse phase, resulting in a higher pressure load for the rigid wall in the second collapse. The maximum pressure of the shock wave of the first bubble collapse phase reaches 5.4 MPa, and the velocity of the micro-jet reaches approximately 100 m/s. This study enriches the existing experimental and numerical results of the dynamics of the near-wall cavitation bubble.

Laser-induced cavitation bubble has been widely used to investigate the mechanisms of hydraulic machinery cavitation erosion and to explore applications in atomization, alloy strengthening, ultrasonic chemistry, biomedicine, surface cleaning and materials processing. This paper consolidates existing research findings on the cavitation bubble dynamics near different boundaries and provides insights for future research work. Firstly, the dynamics of a single cavitation bubble in an infinite field is presented. Subsequently, the focus shifts to the dynamics of cavitation bubble near a rigid wall, angular walls, particles and hydrofoil. Lastly, the paper delves into the dynamics of cavitation bubble within a droplet, revealing the microscopic mechanism of droplet breakup induced by cavitation bubble.

Cavitation occurs widely in nature and engineering and is a complex problem with multiscale features in both time and space due to its associating violent oscillations. To understand the important but complicated phenomena and fluid mechanics behind cavitation, a great deal of effort has been invested in investigating the collapse of a single bubble near different boundaries. This review aims to cover recent developments in the collapse of single bubbles in the vicinity of complex boundaries, including single boundaries and two parallel boundaries, and open questions for future research are discussed. Microjets are the most prominent features of the non-spherical collapse of cavitation bubbles near boundaries and are directed toward rigid walls and away from free surfaces. Such a bubble generally splits, resulting in the formation of two axial jets directed opposite to each other under the constraints of an elastic boundary or two parallel boundaries. The liquid jet penetrates the bubble, impacts the boundary, and exerts a great deal of stress on any nearby boundary. This phenomenon can cause damage, such as the erosion of blades in hydraulic machinery, the rupture of human blood vessels, and underwater explosions, but can also be exploited for applications, such as needle-free injection, drug and gene delivery, surface cleaning, and printing. Many fascinating developments related to these topics are presented and summarized in this review. Finally, three directions are proposed that seem particularly fruitful for future research on the interaction of cavitation bubbles and boundaries.

Providing accurate predictions of extreme water levels through numerical simulation has become essential for disaster prevention and damage mitigation in coastal wetland areas. This study applies the FVCOM model to simulate storm surges caused by several typhoons in the Bohai Sea and the North Huanghai Sea. The vegetation drag force caused by salt marsh plants is inserted into the FVCOM model for model improvement with vegetation effect by integrating RS and GIS technologies. A parametric typhoon model is coupled with background wind fields derived to acquire the spatio-temporal variations of wind and pressure fields in the computational domain. The simulation results reproduce the extreme storm surges induced by typhoon events very well. The modeling results are compared by validating with literature results to examine the effect of vegetation on tidal waves in tidal mud flats. Moreover, the coupled model is also applied to explore storm surge attenuation and land intrusion during Typhoon Winnie in the wetlands of the Liao River Estuary. The simulation results indicate that salt marsh plants can reduce the flow current with little impact on tide flooding/ebbing in vegetated regions. Furthermore, the results show that typhoon presence increases the inundation depth and extendes the flood time in the tidal wetlands of the study region. The FVCOM model incorporating the method with vegetation drag force can provide new insights to understand the comprehensive impact of tidal wetland plants on hydrodynamic characteristics in the Bohai Sea and other waters, hence presents a more accurate quantification of the hydrological process of storm surge in the tidal wetlands.

The relationship between entropy production and vortex evolution affects the efficiency and stability of rotating machinery. This study investigated the energy characteristics of a rocket turbopump and revealed the correlated mechanisms of the entropy production rate using the dissipation effects and characteristic vortex evolution. For the first time, direct and turbulent dissipation and rigid and shear vorticity decomposition methods were utilized to analyze the correlation between flow loss and characteristic vorticities in rotating machinery. With an increase in the flow rate, the hydraulic losses of the dissipation effects and wall decreased by 60% and 38.3%, respectively, and the proportions of the input energy decreased (from 13% to 8%) and remained stable (8%), respectively. The local direct dissipative entropy production (DDEP) in the inducer-impeller is strongly related to shear entropy, and the correlated effect of total enstrophy on DDEP is weaker than that of shear vorticity, indicating that rigid enstrophy suppresses direct dissipation. The correlation between turbulent dissipation and rigid enstrophy was significantly weaker in the static flow passage of the turbopump owing to the weak rigid rotational effect. The correlation between the rigid entropy and local turbulent dissipative entropy production (TDEP) gradually increased with increasing flow rate, reaching a medium correlation (the maximal correlated degree in the turbopump) and exhibiting rigid rotation effects on the hydraulic loss. Moreover, the flow rate significantly affected the correlation (except for the diffuser), and the two characteristic vorticities reached a maximum at the designed flow rate owing to optimal efficiency and minimum hydraulic loss.

Based on high-speed photographic experiments, this study presents a detailed qualitative and quantitative analysis of the dynamics of a single cavitation bubble near the boundary of a rigid wall in asymmetric settings. The main findings are reported as follows: (1) The non-sphericity of the bubble interface decreases with increasing spacing between the bubble and the boundary, and the asymmetry of the bubble becomes more significant with increasing asymmetry angle. (2) The motion mode of the bubble cluster in the second oscillation cycle can be divided into two typical modes depending on the direction of movement. (3) The angle between the oblique jet pointing towards the upper wall surface and the horizontal direction in the second oscillation cycle decreases as the dimensionless spacing decreases.

In order to investigate the resistance performance of an ultra-high-speed aerodynamically alleviated marine vehicle (AAMV), finite volume method (FVM)-based computational fluid dynamics (CFD) software STAR CCM+ is used to simulate the forward motion of this vehicle. The calculated results are validated as they reach good agreement with experimental data. Comparing the motions of models with and without aero-wings, the hybrid aerodynamic and hydrodynamic mechanism of this novel hull is discussed. Study is subsequently performed that how step configuration, spray rail and deadrise angle act on hull behavior and resistance. The results show that models with double steps and spray rail possess better resistance characteristics at high speeds, and planing surface with variable deadrise angle could further improve the overall navigation performance.