Journal of Hydrodynamics最新文献

The flow field structure within the clearances of turbomachinery is complex and diverse, exhibiting high-dimensional nonlinearity. How to accurately extract the main structures that affect the internal flow within the turbine from the complex clearance flow has always been a key issue. To explore the impact of the dynamic structure of the clearance flow on the mainstream region in a centrifugal pump, this study combines the dynamic mode decomposition (DMD) method to conduct a thorough analysis of the velocity and pressure pulsation frequencies in the multi-physics fields within the clearance. The study has identified the main characteristic structures under different physical conditions in the clearance and has established the relationship between the characteristic structure frequencies in different physical fields and the impeller frequency. The research indicates that the internal flow within the clearance affects the occurrence of vortices in the volute. Under design conditions, the velocity field within the clearance is primarily influenced by high-order harmonic frequencies of the impeller, and the pressure field is mainly affected by low-order harmonic frequencies of the impeller. This reflects the crucial influence of impeller frequency and inlet flow on the coherent structures within the clearance flow. The research results offer new insights and methods for analyzing complex internal flows in large turbomachinery.

Marine turbines have been extensively utilized to harness tidal stream energy from free-flowing tides and currents. However, the assessment of the influences of these marine structures on the surrounding environment is still in its early stage. In this study, a numerical model that couples hydrodynamics and sediment transport is developed to simulate the scour processes around a monopile-supported horizontal axial tidal stream turbine under steady currents. The flow characteristics are calculated by solving the 3-D Navier-Stokes equations with the k -ω shear stress transport (SST) turbulence model for closure. The simulation of sediment bed elevation is achieved by solving the Exner equation. The turbine rotor is parameterized using the actuator line method. The developed model is validated against wake velocity and scour depth measurement obtained from previous literature, showing a good agreement. Subsequently, the effects of tip clearance on the flow characteristics around the turbine model on a rigid flatbed are examined. Finally, the scour processes of the turbine model are presented, along with the vortex system within the scour hole. The numerical model proposed in this study has the potential to contribute to the understanding of the scour mechanism of the tidal stream turbines.

In this work, computational fluid dynamics (CFD)—based simulations and linear diffraction analysis are carried out to investigate the interaction between water waves and metamaterials composed of an array of C-shaped cylinders. The flow visualization by CFD-based simulations reveals that local resonance is a result of constructive interference between the incident wave and the wave radiated from the cavity of the C-shaped cylinder. The wave-induced water motion inside the cavity acts as a source of generating this radiated wave, which has the same angular wave frequency and wavenumber but opposite propagation direction as the incident wave. In addition, it is found from the CFD-based simulations that the energy dissipation increases as the opening of the C-shaped cylinder becomes shorter and sharper, along with an increase in its outer radius, and the variation trend of energy dissipation is only affected by the outer radius. Meanwhile, except for very small opening lengths, variations in opening length, width, and outer radius do not significantly impact the wave attenuation effect of the C-shaped cylinder array. Moreover, the results obtained by CFD and the linear potential flow model are compared. The linear potential flow theory is proven to be a reliable approach for accurately predicting the local resonant frequency and transmission coefficients within the local resonant band across a range of geometric parameters. However, it is noted that this theory may have limitations when applied to cases with extremely small opening lengths, where it struggles to accurately predict the local resonant frequency and the intensity of local resonance.

Metal contaminants from surface water pollution events often enter hyporheic zones, under certain conditions, they may be released back into streams, causing secondary pollution to the water quality. The present study investigated the effects of adsorption, permeability, and anisotropy of sediment beds on the release of zinc ions (Zn²⁺) from the hyporheic zone into overlying turbulent flows using large-eddy simulations (LES). The volume-averaged Navier-Stokes equations and advection-diffusion equation with adsorption term were used to describe the sediment in-flow, adsorption, and convective diffusion of Zn²⁺ within the sediment layer. The effects of sediment permeability on the Zn²⁺ concentration distribution and mass transfer processes were investigated by time-averaged statistics of flow and concentration fields. The results show that adsorption becomes stronger as the pH value increases, leading to a slow increase in Zn²⁺ concentration in the overlying water layer and reaching a lower steady-state concentration. Higher overall permeability of the sediment layer can enhance mass and momentum exchange near the sediment-water interface (SWI), and intensify the release of Zn²⁺ from the sediment layer into the overlying water. As the wall-normal permeability of the sediment layer increases, the normal turbulent intensity strengthens, momentum transport enhances, the wall-normal Zn²⁺ concentration flux increases, the effective diffusion coefficient increases, and the concentration in the overlying water increases.

This study evaluates computational fluid dynamics (CFD) turbulence closures for Reynolds-averaged Navier-Stokes (RANS) equations against experimental data to model complex open channel flows, like those occurring over dune-shaped salmon spawning nests called “redds”. Open channel flow complexity, characterized by near-bed turbulence, adverse pressure, and free surfaces, requires suitable turbulence closure capable of capturing the flow structure between streambed and water surface. We evaluated three RANS models: Standard k - ω, shear-stress transport (SST) k - ω and realizable k - ε, along with four wall treatments for the realizable k - ε: Standard, and scalable wall functions, enhanced wall treatment, and an unconventional closure combining standard wall function with near-wall mesh resolving the viscous sublayer. Despite all models generally capturing the bulk flow characteristics, considerable discrepancies were evident in their ability to predict specific flow features, such as flow detachments. The realizable k - ε model, with standard wall function and mesh resolving viscous sublayer, outperformed other closures in predicting near-wall flow separations, velocity fields, and free surface elevation. This realizable k - ε model with a log-layer resolved mesh predicted the free surface elevation equally well but lacked precision for near-wall flows. The SST k - ω model outperformed in predicting turbulent kinetic energy and provided better predictions of the near-boundary velocity distributions than realizable k - ε closure with any of the conventional wall treatments but overestimated the separation vortex magnitude. The standard k - ω model also overestimated near-wall separation. This study highlights the variability in accuracy among turbulence models, underlining the need for careful model selection based on specific prediction regions.

Since 1970s, several experimental works revealed that the cavitation sheet inception does not occur at the minimum pressure location but further downstream at the location of a laminar/turbulent transition. Most of the cavitation models use the saturation vapour pressure as a threshold to initiate the production of vapour and therefore, are not able to capture such flows. In this paper, three modifications of the Schnerr and Sauer cavitation model are proposed and coupled with an algebraic laminar/turbulent transition model. Application to a NACA 16 012 profile shows the ability of the modifications to move the cavitation inception at the right location compared with the experiment. One of them, based on the multiplication of the evaporation term by the square of the turbulent intensity seems promising.

As one of the most common river patterns in nature, meandering river has very complex flow structures in its curved channel bends, including secondary flow structure and primary flow velocity redistributions. To date, most of the studies have been carried out on the flow structures in channel bends with unavoidable influences from inlet and outlet boundaries, while a streamwise periodic boundary can overcome this shortcoming elegantly. In this paper, large eddy simulations (LES) are employed to investigate the complex flow structures in periodically continuous sharp sine-generated bends. The influence of width-to-depth ratios and dimensionless curvature radiuses are studied. The results highlight two additional vortex structures beyond the commonly known secondary currents: The recirculation zone (RZ) and the inner bank cell (IBC). The width-to-depth ratio shows the determining effect on the recirculation zone. The size of recirculation zone is usually bigger in sine-generated-curve (SGC) channel with large width-to-depth ratios. The biggest recirculation zones appear between the zero-curvature section and the apex section. The inner bank cell only forms in SGC channels with small width-to-depth ratios and low curvature. For SGC channel with large width-to-depth ratios, only one circulation cell is observed near the inner bank. The spatial variations of turbulent features are also revealed by statistical analysis based on the LES sampling data. Results highlight remarkable effect of width-to-depth ratio and dimensionless curvature radius on the turbulent kinetic energy (TKE) and bed shear stress in SGC channels.

This study delved into the acoustic spectrum of bubble clusters, each consisting of 352 vapor bubbles across volume fractions ranging from 0.005% to 40%. The clusters, organized in five distinct layers, were modeled using the volume of fluid (VOF) method to capture the bubble interfaces, and the Ffowcs Williams-Hawkings (FW-H) methodology to compute the far-field acoustic pressure from bubble collapse. Further analysis revealed distinct sound pressure behaviors across different volume fractions: For 25%–40%, time-domain analysis shows that the peak acoustic pressure pulses from the two innermost layers of bubbles are significantly higher than those from the outer layers. In the frequency domain, the octave decay rate of the acoustic pressure levels is relatively low, around −3dB/octave. For 0.5%–25%, four acoustic pressure pulses with similar widths and peak values were observed in the time domain. In the frequency domain, there are three distinct peaks in sound pressure levels (SPL), directly linked to the difference in collapse times of bubbles within the cluster, and the octave decay rate accelerates as the volume fraction decreases, stabilizing at −6dB/octave when the volume fraction is reduced to 17.5%. For 0.005%–0.5%, as the volume fraction decreases from 0.5% to 0.1%, the number of acoustic pressure pulses significantly reduces. Below 0.1% volume fraction, only a single wider pulse is observed. In the frequency domain, the octave decay rate gradually increases with decreasing volume fraction, significantly exceeding −10dB/octave when it drops below 0.1%, reaching up to −11.7dB/octave.

In simulating vegetated flows using the porous approach, the reasonableness of the drag coefficient significantly impacts the calculation results. This study employs large eddy simulation (LES) to quantitatively investigate the effect of drag parameters on key flow characteristics in submerged vegetated flows. The results indicate that changes in the drag coefficient significantly alter the velocity in the middle of the vegetation layer and near the water surface in the free-flow layer. Compared with longitudinal velocity, the drag coefficient has a more pronounced effect on the vertical distribution of Reynolds stress, especially its peak at the top of the vegetation layer. The porous approach can accurately reproduce the vertical distribution of longitudinal velocity and Reynolds stress, consistent with experimental measurements, only when shear-scale flow dominates. Due to the high-intensity secondary flow under moderate vegetation density, fluctuations in the drag coefficient have a more significant impact on the numerical results than in very dense vegetation. Therefore, selecting the drag coefficient value should be done cautiously, especially in the absence of experimental measurements for validation.

Visual and pressurized pipeline systems with single- and multi-undulation layouts were used to study experimentally and analyze theoretically the transient characteristics of water-air two-phase flow during water fillings in undulation pipelines based on the combination action analyses of both the communicating pipe and the gravity of the water-air two-phase flows in the descending pipe. For the single undulation pipeline, the complex two-phase flow-pattern evolutions including full pipe flow and stratified flow for low, medium, high water-filling velocity cases, respectively, lead to a great difference in transient pressure, flow pattern and the water-filling duration. Especially for low and medium water-filling velocity cases, the hydraulic theories related to hydraulic drop and hydraulic jump were employed to investigate the entrapped air pocket evolutions in the descending pipe, and the mechanism of negative pressure at the top of the undulation pipes was analyzed. For the same multi-undulation pipeline, due to the different elevations of the three undulation points along flow direction, namely three different types of pipeline layout, high-medium-low case (high elevation undulation point, medium one, and low one), low-medium-high and high-low-medium ones, their water-filling durations are significantly different, i.e., approximately 80.02 s, 227.34 s and 617.78 s. Meanwhile, there are significant differences in flow patterns in water filling, namely larger entrapped air pockets in three descending pipes for the high-medium-low case, entrapped air pockets in the first two descending pipes and open channel stratified flow in the last one for low-medium-high case, some bubbles in three descending pipes for the high-low-medium case.