Journal of Hydrodynamics最新文献

The effect of rotation-curvature correction and inviscid spatial discretization scheme on the aerodynamic performance and flow characteristics of Darrieus H-type vertical axis wind turbine (VAWT) are investigated based on an in-house solver. This solver is developed on an in-house platform HRAPIF based on the finite volume method (FVM) with the elemental velocity vector transformation (EVVT) approach. The present solver adopts the density-based method with a low Mach preconditioning technique. The turbulence models are the Spalart-Allmaras (SA) model and the k-ω shear stress transport (SST) model. The inviscid spatial discretization schemes are the third-order monotone upstream-centered schemes for conservation laws (MUSCL) scheme and the fifth-order modified weighted essentially non-oscillatory (WENO-Z) scheme. The power coefficient, instantaneous torque of blades, blade wake, and turbine wake are compared and analyzed at different tip speed ratios. The extensive analysis reveals that the density-based method can be applied in VAWT numerical simulation; the SST models perform better than the SA models in power coefficient prediction; the rotation-curvature correction is not necessary and the third-order MUSCL is enough for power coefficient prediction, the high-order WENO-Z scheme can capture more flow field details, the rotation-curvature correction and high-order WENO-Z scheme reduce the length of the velocity deficit region in the turbine wake.

With the global demand for more electricity, and for that electricity to be produced using low-carbon generation, a turbine was designed to extract energy from underutilised flows. The mechanism by which the turbine operates makes it highly demanding to represent using mesh-based numerical schemes, resulting in a need to investigate alternative methods. The Smoothed Particle Hydrodynamics (SPH) software, DualSPHysics, utilising the Chrono solid body solver, was used to represent the turbine as a free body in a 2-D environment allowing for evaluation of the free-spin velocity to be assessed. The aim of this being to ascertain the applicability of SPH to the modelling of vertical axis turbines with multiple moving parts, and also develop an understanding of the design itself. The model was found to compare favourably with lab results, showing that a vertical axis turbine may be represented in this fashion. The resilience of the device, a design driver and previously untested mode, was assessed by considering post-damage scenarios. From this, future flume study and parallel numerical modelling can guide this or other vertical axis turbines towards improved performance.

In this paper, a passive control method based on a porous material is applied to the surface of a hemispherical cylinder to control a cavitating flow, and the control effect of this method at different cavitation numbers (σ) is evaluated through the cavity morphology and volume, which is important for the application in engineering. The results indicate that the control effect is improved with a reduction in the cavitation number, for the reduction of vapor volume increases from 22%–50% with σ decreasing from 0.50–0.20. Further investigation indicates that the cavity inception at different cavitation numbers is still induced by the Kelvin-Helmholtz instability, while the spatial distribution of the vapor changes significantly. Moreover, the porous material suppresses the cavitating flow in the front region but enhances it downstream at large cavitation numbers. When σ = 0.20, the cavitating flow is controlled in both the front and rear regions.

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.