Journal of Hydrodynamics最新文献

Bow wave breaking is a common phenomenon during ship navigation, especially at a high speed, involving complex physical mechanism such as interface mixing, air entrainment, and jet splashing. This study uses the delayed detached eddy simulation (DDES) turbulence model on the OpenFOAM platform to simulate flow around a KRISO Container Ship (KCS) model for a Froude number of 0.35, examining trim angles of 0°, 0.5°, 1°. This paper analyzes the statistical and power spectral density (PSD) characteristics of bow wave heights. The analysis shows root mean square (rms) and mean difference between top and bottom views indicate wave breaking. As the trim angle increases, peaks of rms in the bottom view become much higher than that in the top view, reaching 38% at 1°. PSD analysis reveals that resistance and wave height periods differ by no more than 5%, with small-scale structures like jetting and splashing causing non-dominant periodic and high-frequency wave height variations.

This study conducts a comparative analysis between detached eddy simulation (DES) and Unsteady Reynolds-averaged Navier-Stokes (URANS) models for simulating pressure fluctuations in a stilling basin, aiming to assess the URANS mode’s performance in modeling pressure fluctuation. The URANS model predicts accurately a smoother flow field and its time-average pressure, yet it underestimates the root mean square of pressure (RMSP) fluctuation, achieving approximately 70% of the results predicted by DES model on the bottom floor of the stilling basin. Compared with DES model’s results, which are in alignment with the Kolmogorov −5/3 law, the URANS model significantly overestimates low-frequency pulsations, particularly those below 0.1 Hz. We further propose a novel method for estimating the RMSP in the stilling basin using URANS model results, based on the establishment of a quantitative relationship between the RMSP, time-averaged pressure, and turbulent kinetic energy in the boundary layer. The proposed method closely aligns with DES results, showing a mere 15% error level. These findings offer vital insights for selecting appropriate turbulence models in hydraulic engineering and provide a valuable tool for engineers to estimate pressure fluctuation in stilling basins.

Through direct numerical simulations, we investigated the flow structure and heat transfer of the centrally confined 2-D Rayleigh-Bénard (RB) convection over the Rayleigh number range 9 × 10⁵ ≤ Ra ≤ 10⁹ at a fixed Prandtl number Pr = 4.3. It is found that with increasing Ra, the number of convection rolls in the central vertical channel increases from zero to three. When there is no rolls in the vertical channel, the convective flow in central region is significantly influenced by the boundary layer, whereas when the convection rolls is generated in the vertical channel, the convective flows in central regions is free from the boundary layer limitation, and by defining the characteristic length, one obtains the heat transfer scaling law relation in vertical channel, i.e., Nu_vc ∼ Ra ^0.476±0.005_vc , which could be the evidence of “ultimate regime”.

Ship bow wave breaking contains complex flow mechanism, which is very important for ship performance. In this study, a practical numerical simulation scheme for bow wave breaking is proposed and the scheme is applied to the simulation of bow wave breaking of KCS ship model with Fr = 0.26, 0.30, 0.35, 0.40, analyzing the impact of speed on the bow wave breaking. The results indicate that an increase in speed leads to a significant rise in viscous pressure resistance and more pronounced bow wave breaking. Moreover, it is found that the traditional wave height function in OpenFOAM is not suitable for detailed studies of bow wave breaking. This study extracts different free surfaces through top and bottom views to further analyze the free surface overturning, droplet splashing, and cavity entrainment in bow wave breaking. Additionally, the spatial and temporal distribution of cavities at Fr = 0.40 is analyzed, revealing that cavity distribution is closely related to vortex structures and exhibits a periodic pulsation characteristic of approximately 12 s.

Large wood in rivers can lead to accumulations in the river channel, affecting local flow structures, aquatic habitats, and the river’s topography. This plays a crucial role in the ecological restoration of the river. This paper presents flow field measurements downstream of six types of logjams at different flow velocities using acoustic Doppler velocimetry (ADV) for artificially designed engineered logjams. The results indicate that the presence of logjams reduces the flow velocity and increases the turbulent kinetic energy in the wake region, and as the distance downstream increases, the flow velocity and turbulence intensity in the wake region gradually return to the upstream level. The minimum values of normalized flow velocity under different conditions are located in the region of the bottommost logs. The differences in normalized flow velocity at various flow rates are not significant. Jets are less likely to be generated in logjams with larger and more concentrated projection areas, but the strength of the jet is influenced by the physical structure of the logjam (projection area, gap ratio). The flow distribution behind the logjam is primarily influenced by the proportion of the projected area in different regions. Changes in the vertical physical structure of the logjam have minimal effect on the lateral flow distribution. Flow velocity in the gap area (b₀) at the bottom of different logjams is influenced by their physical structure. The larger the overall blockage area of the logjams, the larger the flow velocity in the bottom gap area will be. The flow velocity in the bottom gap area of a densely placed logjam is mainly influenced by the gap ratio. The velocity of flow in the gap area can impact the initiation and deposition of sediment near the logjam. However, the internal structure complexity of the logjam does not significantly affect river energy dissipation and flow attenuation. This study broadens the applicability of certain theoretical models and explores the impact of logjams on river ecology and channel geomorphology. The findings can serve as a theoretical foundation for ecological restoration, timber management, and logjam construction in rivers.

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.