Journal of Hydrodynamics最新文献

A two-phase flow model accelerated by graphical processing unit (GPU) is developed to solve fluid-solid interaction (FSI) using the sharp-interface immersed boundary method (IBM). This model solves the incompressible Navier-Stokes equations using the projection-based fractional step method in a fixed staggered Cartesian grid system. A volume of fluid (VOF) method with second-order accuracy is employed to trace the free surface. To represent the intricate surface geometry, the structure is discretized using the unstructured triangle mesh. Additionally, a ray tracing method is employed to classify fluid and solid points. A high-order stable scheme has been introduced to reconstruct the local velocity at interface points. Three FSI problems, including wave evolution around a breakwater, interaction between a periodic wave train and a moving float, and a 3-D moving object interacting with the free surface, were investigated to validate the accuracy and stability of the proposed model. The numerical results are in good agreement with the experimental data. Additionally, we evaluated the computational performance of the proposed GPU-based model. The GPU-based model achieved a 42.29 times speedup compared with the single-core CPU-based model in the three-dimension test. Additionally, the results regarding the time cost of each code section indicate that achieving more significant acceleration is associated with solving the turbulence, advection, and diffusion terms, while solving the pressure Poisson equation (PPE) saves the most time. Furthermore, the impact of grid number on computational efficiency indicates that as the number of grids increases, the GPU-based model outperforms the multi-core CPU-based model.

Tip leakage flow (TLF) trajectory in a pump with gas entrainment is investigated via visualization experiments and numerical simulations. Starting position of tip leakage vortex (TLV) is determined accurately by numerical simulation. Under high liquid flow rate (Q_l) and high inlet gas volume fraction (IGVF) conditions, TLF flows from suction surface to pressure surface near the leading edge of blade, and the direction of TLF gradually changes along the chord which flows from pressure surface to suction surface near the tailing edge. The angle between TLF and blade mean camberline increases progressively as either Q_l or IGVF decreases, and starting position of TLV moves towards leading edge direction. As Q_l or IGVF decreases, value of vorticity increases and high vorticity region moves towards leading edge. The entropy production rate at blade tip clearance is high, and entropy diffuses from pressure surface to suction surface due to jet flow in blade tip clearance. The greater the amount of accumulated gas there is, the greater the amount of entropy in the area. In addition, when gas is entrained in pump, there are many low frequency fluctuations generated in blade tip clearance.

The local distributions of both the temperature and pressure have a great influence on the rheological characteristics of the drilling fluid, thereby affecting its flow law in a wellbore. Along these lines, in this work, the rheology of water-based drilling fluid samples under high-temperature (30°C–210°C) and high-pressure (34.5 MPa–172.4 MPa) (HTHP) conditions was systematically analyzed. The constitutive model of the variation of the apparent viscosity of the drilling fluid with the temperature and pressure was successfully established. The analysis revealed that, among the Bingham model, the Power law model, the Herschel-Bulkley (H-B) model, and the Casson model, the H-B model can accurately describe the rheology of the drilling fluid under HTHP conditions. Therefore, the H-B model was used to perform numerical simulations of the flow law of the water-based drilling fluid in the wellbore. The simulation results demonstrated that the drilling fluid viscosity decreased as the depth of the wellbore increased, and was mainly influenced by the temperature. The maximum viscosity inside the drill pipe was mainly concentrated in the middle region, and that of the fluid when flowing in the annulus was mainly concentrated on the side near the outer wall of the annulus. This work provides valuable insights for setting the key parameters of the drilling fluid and wellbore cleaning in the drilling operation of a 1×10⁴ m deep well.

Unsteady cloud cavitating flow is detrimental to the efficiency of hydraulic machinery like pumps and propellers due to the resulting side-effects of vibration, noise and erosion damage. Modelling such a unsteady and highly turbulent flow remains a challenging issue. In this paper, cloud cavitating flow in a venturi is calculated using the detached eddy simulation (DES) model combined with the Merkle model. The adaptive mesh refinement (AMR) method is employed to speed up the calculation and investigate the mechanisms for vortex development in the venturi. The results indicate the velocity gradients and the generalized fluid element strongly influence the formation of vortices throughout a cavitation cycle. In addition, the cavitation-turbulence coupling is investigated on the local scale by comparing with high-fidelity experimental data and using profile stations. While the AMR calculation is able to predict well the time-averaged velocities and turbulence-related aspects near the throat, it displays discrepancies further downstream owing to a coarser grid refinement downstream and under-performs compared to a traditional grid simulation. Additionally, the AMR calculation is unable to reproduce the cavity width as observed in the experiments. Therefore, while AMR promises to speed the process significantly by refining the grid only in regions of interest, it is comparatively in line with a traditional calculation for cavitating flows. Thus this study intends to provide a reference to employing the AMR as a tool to speed up calculations and be able to simulate turbulence-cavitation interactions accurately.

We examine the genesis of coherent vortices in submerged vegetated flows by means of a linear stability analysis. The mathematical framework is comprised of the conservation equations of fluid mass and momentum. The problem is tackled by imposing normal mode perturbations over an underlying undisturbed flow. We find that the growth rate of perturbations takes maximum magnitude for a specific wavenumber, termed as the critical wavenumber. The critical wavenumber indicates the most favorable wavenumber of coherent vortices emerging in submerged vegetated flows. The critical wavenumber amplifies as the flow Reynolds number, and vegetation height and density augment. The migration velocity of incipient coherent vortices characterizes minimum magnitude for a selected value of the vegetation height. The unstable zone in the stability diagram embarks beyond a critical Reynolds number. The critical Reynolds number designates the onset of coherent vortex appearance in submerged vegetated flows. The predictions of the present study are congruent with the existing theoretical and experimental works.

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.