Journal of Hydrodynamics最新文献

To research the dynamics of the cavitation bubble under the interaction of particle clusters, the bubble morphological evolutionary characteristics near three equal-sized spherical particles are theoretically explored in the present study based on the Weiss theorem and the velocity potential superposition theory. The three particles are arranged symmetrically, and the fluid velocity field near the three particles and the cavitation bubble is obtained. Moreover, the effects of the bubble-particle distance and the maximum radius of the cavitation bubble on the fluid velocity are investigated, and the contribution mechanisms of the fluid velocity field constituents are compared. The analysis has found that: (1) The fluid velocity between the bubble and the particle is lower than that at the other locations in both the growth and collapse phases, thus the bubble cannot always maintain a standard spherical shape. (2) The bubble-particle distance and the maximum radius of the cavitation bubble are the key parameters affecting the circumferential inhomogeneity of the radial velocity of the fluid around the bubble. The larger the maximum radius or the smaller the bubble-particle distance is, the more visible the non-circularity of the bubble morphology. (3) The image bubbles and the linear sinks contribute oppositely to the fluid velocity field, and the presence of the image bubble reduces the fluid velocity. In the low velocity region, the image bubble is the main mechanism contributing to the effect of the particle on the fluid velocity.

This paper presents a 2-D numerical investigation on interaction between regular waves and a fully submerged horizontal cylinder. A mathematical model of numerical wave tank with two-way fluid-solid interactions were developed and validated. The wave-induced vibrations of a single-degree-of-freedom cylinder were simulated at eleven gap ratios (d / a = 8, 10, 12, 14, 16, 18, 20, 22, 24, 28 and 32). Numerical results indicate that significant nonlinear characteristics are introduced into the originally linear waves with the presence of cylinder. Based on the variation characteristics of cylinder vibration amplitude, the gap ratios can be divided into three ranges, i.e., the uncertain range (8 ≤ d / a ≤ 14), quasi-linear range (14 ≤ d / a ≤ 20) and linear range (20 ≤ d / a ≤ 32). Under the same wave condition, the gap ratio does not affect the frequencies of vortex shedding and cylinder vibration. The presence of the cylinder complicates the flow field and suppress the vortex shedding around the cylinder.

Cavitation is one of the main causes of deteriorating stability of bulb turbines. To enhance their stability, this study examines the effects of runner cavitation on draft tube pressure fluctuation and vibration in bulb turbine through experimental methods. With varying cavitation coefficients, a synchronous test system, including a high-speed camera, vibration acceleration sensors and pressure pulsation sensors, is applied to obtain cavitation images of the runner, vibration and internal fluid pressure pulsation data of the draft tube. The results show that the correlated component of pressure pulsation signals during the cavitation process is the synchronous pressure pulsation of 16f_n With the development of cavitation, the amplitude of synchronous pressure pulsation increases first and then decreases. Cavitation enhances the high-frequency vibration on the wall of runner chamber. The root mean square (rms) of the vertical vibration component IMF3, the horizontal vibration components IMF2, IMF4 are linearly negatively correlated with the cavitation coefficient. The associated component between cavitation-induced vibration and pressure pulsation signal is 16f_n and its harmonics. In the process of cavitation, pressure pulsation plays a leading role in vibration.

Experimental research was conducted on the performance curves and the cavity evolution for different flow and geometric parameters in jet pumps for zero flow ratio (ZFR) conditions. New pressure ratio, P_r, flow ratio, q_r, were used in place of the conventional performance parameters h, q, to characterize the jet pump flow performance. A super cavitation cavity in the jet pump was observed to fill most of the flow channel, which hindered further increases of the flow rate and increased q_r to one, thus, created a critical point on the new P_r - q ²_r curve. Before the critical point, P_r was proportional to q ²_r with a coefficient that was much more sensitive to the area ratio than the relative throat length and the diffusion angle. After the critical point, the flow rate reached its maximum, the limiting flow rate, which only depended on the total inlet pressure and the area ratio. The total inlet pressure was proportional to the square of the limiting flow rate with a flow coefficient that was only a quadratic function of the area ratio.

We have investigated the hydrodynamic and acoustic performance of a hydrofoil with a wave leading edge that is being ingested in a cylindrical wake, to explore the interaction and noise reduction mechanism with the use of near flow field and far field noise decoupled prediction methods of large eddy simulation (LES) and Ffowcs Williams-Hawkings (FW-H). Our results indicate that the wave leading edge has minimal effect on the hydrodynamic performance, however, it has demonstrated the ability to significantly improve the acoustic performance. Through the comparison of sound pressure level (SPL) and acoustic directivity, we have observed that the wave leading edge can significantly reduce the broadband noise in the far field. This is due to its ability to break the large-scale structure of the incoming flow, which weakens the direct impact and therefore reduces the tone noise. Additionally, the interaction between the broken vortex and the boundary layer around the hydrofoil surface is weakened, leading to a reduction in surface pressure pulsation and broadband noise intensity. The wave structure primarily affects the flow structure near the leading edge, resulting in a reduction in flow disturbance and sound source intensity, and an improvement in the acoustic feedback loop between the foil and the fore-cylinder.

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.