Journal of Hydrodynamics最新文献

Self-propelled particles are commonly found in a large number of planktonic organisms such as bacteria, fungi, and algae in nature, and researchers have taken a long interest in exploring their swimming mechanisms for more than a century. Especially in the past 20 years, with the development of computational fluid dynamics and flow display technology, as well as the need for the design of synthetic self-propelled particles and micro-swimming devices, self-propelled particles have become the forefront and hotspot of current research in the field of fluid mechanics. This paper first introduces the swimming characteristics of common self-propelled particles, leading to a classic “squirmer” type self-propelled particle model. On this basis, a systematic introduction and summary of the theoretical and numerical simulation research of “squirmer” will be conducted. Finally, the main challenges and opportunities faced by the current research will be summarized.

Riblets are a series of small protrusions formed along the flow direction, which have been extensively studied as a passive turbulent drag reduction technique. Experiments and numerical simulations have shown that well-designed riblets can significantly reduce drag in turbulent flows, making them highly promising and valuable for various applications. In this study, we focus on a scalloped riblet, which is designed by smoothly connecting two third-order polynomials, and thus the sharpness of the tip and the curvature of the valley can be well defined. We conduct direct numerical simulations of turbulent channel with smooth plate, scalloped riblet-mounted and triangular riblet-mounted walls. Width in wall units of W⁺ = 20 and height-width ratio of γ = 0.5 are selected for both riblet cases. Compared with the smooth plate case, the scalloped riblet case achieves an 8.68% drag reduction, while the triangular riblet case achieves a 4.79% drag reduction. The obtained drag reduction rate of the triangular riblet is consistent with previous experiments and simulations, and the results indicate that the scalloped riblet is more effective in reducing drag and deserves further investigation. We compare turbulent statistics of the scalloped riblet case with those of the triangular riblet case. The mean velocity profiles of riblets are similar, but both the Reynolds shear stress and second-order statistics of velocity fluctuations and Liutex are significantly reduced in the scalloped riblets controlled turbulent channel, indicating that the scalloped riblet can more effectively suppress the spanwise and wall-normal turbulent intensity near the wall. We also compare the pre-multiplied spectra of streamwise velocity and streamwise Liutex component for the three cases to investigate the energy distribution and characteristics of Liutex distribution. The Liutex vortex identification method is also utilized to analyze the instantaneous flow field, which provides insights into the flow field and could be beneficial for the further optimization of riblet.

The flow around a circular cylinder for Re = 1000 is characterized by flow separation and Karman vortex street. The typical flow features can be captured to study the correlation between fluid fields and sound fields. In this paper, the three-dimensional circular cylinder is taken as the research object, and the probes of surface fluctuating pressure and far field sound pressure are arranged every 10°. The directional diagram and the coherence of fluctuating pressure and sound pressure are analyzed. The relationship between the flow mode and hydrodynamic noise is studied by using dynamic mode decomposition (DMD). The characteristics of the dipole and quadrupole sound source term of a long span cylinder are studied. The results show that at the angles between 30°–120° and 190°–350°, the fluctuating pressure contributes more to the generation of dipole sounds. The quadrupole sound source shows three-dimensional effects, which is more obvious in a cylinder with large spanwise length.

Unsteady cavitating flow often contains vapor structures with a wide range of different length scales, from micro-bubbles to large cavities, which issues a big challenge to precisely investigate its evolution mechanism by computational fluid dynamics (CFD) method. The present work reviews the development of simulation methods for cavitation, especially the emerging Euler-Lagrange approach. Additionally, the progress of the numerical investigation of hot and vital issues is discussed, including cavitation inception, cloud cavitation inner structure and its formation mechanism, cavitation erosion, and cavitation noise. It is indicated that the Euler-Lagrange method can determine cavitation inception point better. For cloud cavitation, the Euler-Lagrange method can reveal the source of microbubbles and their distribution law inside the shedding cloud. This method also has advantages and great potential in assessing cloud cavitation-induced erosion and noise. With the ever-growing demands of cavitation simulation accuracy in basic research and engineering applications, how to improve the Euler-Lagrange method’s stability and applicability is still an open problem. To further promote the application of this advanced CFD simulation technology in cavitation research, some key issues are to be solved and feasible suggestions are put forward for further work.

This paper presents an efficient time-domain method for simulating nonlinear ship waves. The proposed method, implemented in an earth-fixed coordinate system, integrates a compact boundary element domain within a high-order spectral layer, enabling accurate modeling of both near-field and far-field ship waves. An overset mesh method and an attention mechanism are employed to track the moving ship. The effectiveness of the method is validated through simulations of Wigley and Series 60 ships sailing at various speeds. The numerical results, including the nonlinear wave run-up at the ship bow, surface pressure distribution on the hull, and the ship resistance, agree well with experimental data and published numerical results, confirming that the method is capable of accurately simulating the nonlinear ship waves.

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.