Journal of Hydrodynamics最新文献

In this paper, the dynamic characteristics of the cylindrical bubbles under triple-frequency acoustic excitation are investigated theoretically. The analytical solution of the primary-superharmonic-subharmonic (PRI-SUPER-SUB) simultaneous resonance is obtained through the multi-scale method. Based on the analysis of the frequency response, the influencing mechanisms of the primary parameters (e.g., the total amplitude, amplitude ratio, liquid viscosity, polytropic exponent, and bubble equilibrium radius) on the resonance are investigated quantitatively. The main conclusions include: (1) The solution for the simultaneous resonance of the cylindrical bubble exhibits jumping and hysteresis phenomena in the vicinity of the resonance frequency. (2) As the total amplitude, amplitude ratio, and equilibrium radius increase, the response amplitude of the PRI-SUPER-SUB simultaneous resonance increases, while the influence of the viscosity is the opposite. (3) The regions dominated by the instability of the simultaneous resonance is significantly affected by the system parameters.

Wave height forecast (WHF) is of great significance to exploit the marine renewables and improve the safety of ship navigation at sea. With the development of machine learning technology, WHF can be realized in an easy-to-operate and reliable way, which improves its engineering practicability. This paper utilizes a data-driven method, Gaussian process regression (GPR), to model and predict the wave height on the basis of the input and output data. With the help of Bayes inference, the prediction results contain the uncertainty quantification naturally. The comparative studies are carried out to evaluate the performance of GPR based on the simulation data generated by high-order spectral method and the experimental data collected in the deep-water towing tank at the Shanghai Ship and Shipping Research Institute. The results demonstrate that GPR is able to model and predict the wave height with acceptable accuracy, making it a potential choice for engineering application.

Compared with periodic structures, quasi-periodic structures have superior band gap properties and topological interface states. In this paper, a one-dimensional quasi-periodic Fibonacci water wave metamaterial model that can be used to apply quasi-periodic structures to shallow-water wave systems is presented. The fluctuation characteristics of periodic and quasi-periodic structures are examined using finite element numerical calculations based on the shallow-water wave equation. The research results show that the band characteristics of quasi-periodic structures are complex, enabling flexible control of the propagation of shallow-water waves. Furthermore, the mirror-symmetrical design of Fibonacci quasi-periodic water wave metamaterials was created to engineer the topological interface states in shallow-water wave systems, ultimately achieving successful localization of wave energy. This research will greatly enrich our understanding of topology, expand the potential applications of quasi-periodic structures, and provide new insights for manipulating water waves and harvesting energy.

A coastal forest combined with a backward-facing step is an efficient facility to reduce tsunami damage to residential areas behind sea embankments. This study establishes a generalized model, and experimentally explores the water level changes upstream of the vegetation-step mitigation model as well as its energy dissipation effect under different initial Froude numbers, step heights, and vegetation conditions. The results show that the relative backwater rise increases with the growth of vegetation density, patch length and initial Froude number, representing a slowing down of the tsunami inundation. As for energy dissipation, it is mainly caused by the additional resistance of the vegetation and the hydraulic jump. And the vegetation condition not only affects the energy dissipation due to stem-scale turbulence within the patch, but also changes the hydraulic jump process of water falling from the step in cooperation with the step height. As a result, the energy dissipation efficiency always increases with the growth of vegetation density, vegetation patch length and step height. With the criterion that the energy dissipation efficiency and its growth rate can hardly change with vegetation parameters, this study innovatively defines the threshold slope and gives the principle of judging the most cost-effective vegetation conditions at different step heights. These results are expected to provide an important reference for the design of composite tsunami mitigation facilities.

A laboratory study was conducted using particle image velocimetry (PIV) to measure flow velocity distributions over two-dimensional smooth and rough fixed dunes. It comprised 28 tests, each yielding 146 velocity profiles over one complete dune length. Two kinds of double-averaged velocity profiles were computed, one based on all the 146 lines of data (called global average), and the others from only some of them (called partial average). The results show that the global average velocity distribution is generally close to the partial average profile derived from evenly-distributed three or five lines along one dune length. Furthermore, the global average velocity profile can also be reasonably approximated using a single profile, measured at the representative line in this paper. The representative line is found to locate near the reattachment point. This result would be helpful to simplify measurements of general velocity distribution for a flow over dunes. The paper also applies the concept of representative line to the description of distributions of turbulence characteristics.

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.