Journal of Hydrodynamics最新文献

The spectrogram, based on a short-time Fourier transform, can visualize the time-dependent frequency spectrum of waves and is easy to compute. This time-frequency analysis method provides crucial information about waves generated by moving vessels and has been utilized to analyze Kelvin ship waves and internal waves. To further study the internal waves induced by a submerged body, an experiment is conducted for the towed and self-propelled SUBOFF model in a stratified fluid. The internal wave elevation signals are captured using electronic conductivity probes. Comparing with the calculation results of theoretical model, the high-frequency component of internal waves is identified. The high-frequency component has the exact same characteristics in both the towed and self-propelled model experiment and is consistent with the theoretical results for all Froude numbers. Therefore, this component is composed mainly of lee waves. Through spectral characteristics identification, a low-frequency component is discovered in the spectrogram in addition to the lee wave component. The intensity of the low-frequency component is tightly related to the vortex structure behind the submerged body. The vortex structure depends on the net momentum imparted by the submerged body. Therefore, this component is composed mainly of wake waves induced by the vortex structure.

Using the 3-D sono-elasticity method and the simplified nonstructural mass method, the different dynamic modeling methods of the added water for a single-hull structure are first analyzed in this study. Then, the complete internal flow field method and the simplified nonstructural mass method of the contained water between the double hulls of a double-hull structure are investigated. Finally, based on the calculation and analysis under multiple conditions, a reasonable and simplified dynamic modeling method of added water and contained water is obtained. It is indicated that the mass of added water for a single-hull structure is closely related to the mass of total underwater displacement of the structure. With the increase in the analysis frequency, the mass of added water is characterized by first decreasing rapidly and then decreasing gradually and smoothly. The contained water between the double hulls is distributed to the pressure hull and the light shell based on the ratio of the impedances of the double hulls. The results can basically reflect the acoustic radiation characteristics of the double-hull structure.

Cavitation commonly induces performance deterioration and system vibration in many engineering applications. This paper aims to investigate the effects of air injection on cavitation evolution, pressure pulsation and vibration in a centrifugal pump with inducer. In this paper, the high-speed camera is used to capture the gas flow pattern and cavitation evolution process in the inducer. The impacts of air injection on the inlet pressure pulsation and vibration are also investigated. The results show that the cavitation development in the inducer undergoes four patterns: incipient cavitation, sheet cavitation, cloud cavitation and super cavitation. During the development of cavitation, the main frequency of the pressure pulsation shifts to lower frequencies, and the amplitude of the vibration increases. In addition, air injection promotes the incipient cavitation but delays the cavitation development. A small amount of air can effectively decrease amplitudes of pressure pulsation and vibration. But as the air content increases, the fluctuations and amplitudes of pressure pulsation and vibration increase.

This study presents results from a vegetation-induced flow experimental study which investigates 3-D turbulence structure profiles, including Reynolds stress, turbulence intensity and bursting analysis of open channel flow. Different vegetation densities have been built between the adjacent vegetations, and the flow measurements are taken using acoustic Doppler velocimeter (ADV) at the locations within and downstream of the vegetation panel. Three different tests are conducted, where the first test has compact vegetations, while the second and the third tests have open spaces created by one and two empty vegetation slots within the vegetated field. Observation reveals that over 10% of eddies size is generated within the vegetated zone of compact vegetations as compared with the fewer vegetations. Significant turbulence structures variation is also observed at the points in the non-vegetated row. The findings from burst-cycle analysis show that the sweep and outward interaction events are dominant, where they further increase away from the bed. The effect of vegetation on the turbulent burst cycle is mostly obvious up to approximately two-third of vegetation height where this phenomenon is also observed for most other turbulent structure.

The gas-liquid two-phase homogenous flow has been extensively investigated without the effect of gas release. However, the dissolved gas will release when internal water pressure drops below saturation pressure during hydraulic transients. This results in inaccuracy or even invalidity of the existing model for homogenous flows, especially for the reproduction of two-phase mass transfer processes. To address this problem, this paper couples the gas release model with conservation equations of homogenous flows, which are numerically solved by the second-order Godunov-type scheme (GTS). Specifically, a virtual-cell method is adopted at system boundaries to achieve the same second-order accuracy as interior cells, which is realized by the monotonic upwind scheme for conservation laws (MUSCL-Hancock scheme). Simulated pressure curves by the proposed model are compared with a series of analytical, numerical and experimental results. It indicates that the proposed model with gas release effects reproduces actual pressure responses most accurately, with minimum relative error and root mean squared error compared with experimental data. Moreover, the gas release leads to dynamic synchronous fluctuations of void fraction, wave speed and pressure head, including the opposite trends of void fraction and pressure, and higher void fraction leading to greater wave speed depression. Furthermore, sensitivity analysis is concluded with recommended Courant number, and different gas release effects in different initial void fractions. Present research increases the basic understanding of two-phase mass transfer processes and their implications for hydraulic transients.

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.

Abstract

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 16 f_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 16 f_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 - q²_r curve. Before the critical point, P_r was proportional to q²_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.

Abstract

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.