Journal of Hydrodynamics最新文献

To enhance understanding of the flow characteristics around a sphere in both stratified and unstratified (UNS) fluids, large eddy simulations (LES) were conducted using a temperature-dependent density model at Re = 3 700. The simulations were performed for flow around a sphere under UNS and stratified conditions (Fr = 3). Horizontal and vertical vorticity, velocity, and streamline distributions were compared, and the evolution of vortex structures in the wake was analyzed. Furthermore, we quantified the velocity deficit, the root mean square (rms) of velocity components in all directions, and the turbulent kinetic energy (TKE) distribution. Additionally, the horizontal and vertical wake lengths were examined. The results demonstrate that the employed numerical simulation method accurately captures the behavior of stratified fluids, with outcomes in close agreement with experimental and numerical findings from previous studies. In the case of homogeneous fluid, a lower density value results in a faster decay of the velocity deficit. In stratified fluids, the vortex structures in the wake evolve through three distinct stages: 3-D, non-equilibrium (NEQ), quasi-two-dimensional (Q2D). For x / D > 2, the rms velocity in the vertical direction exceeds that in the other two directions. In UNS fluid, the TKE distribution forms a vertically elongated spindle shape, while in stratified fluid, it assumes an elliptical shape, being vertically compressed and horizontally expanded. The vertical extent of the density and density gradient distributions surpasses that of the wake.

Navigating efficiently across vortical flow fields presents a significant challenge in various robotic applications. The dynamic and unsteady nature of vortical flows often disturbs the control of underwater robots, complicating their operation in hydrodynamic environments. Conventional control methods, which depend on accurate modeling, fail in these settings due to the complexity of fluid-structure interactions (FSI) caused by unsteady hydrodynamics. This study proposes a deep reinforcement learning (DRL) algorithm, trained in a data-driven manner, to enable efficient navigation of a robotic fish swimming across vortical flows. Our proposed algorithm incorporates the LSTM architecture and uses several recent consecutive observations as the state to address the issue of partial observation, often due to sensor limitations. We present a numerical study of navigation within a Kármán vortex street created by placing a stationary cylinder in a uniform flow, utilizing the immersed boundary-lattice Boltzmann method (IB-LBM). The aim is to train the robotic fish to discover efficient navigation policies, enabling it to reach a designated target point across the Kármán vortex street from various initial positions. After training, the fish demonstrates the ability to rapidly reach the target from different initial positions, showcasing the effectiveness and robustness of our proposed algorithm. Analysis of the results reveals that the robotic fish can leverage velocity gains and pressure differences induced by the vortices to reach the target, underscoring the potential of our proposed algorithm in enhancing navigation in complex hydrodynamic environments.

This paper considers local scour around a pipeline under turbulent flow. The Navier-Stokes equations are solved with a shear stress turbulence model. The original bed deformation equation based on an analytical sediment transport model is used to describe the changes in the bottom surface. The proposed sediment transport equation is based on Coulomb’s friction law for granular flow, Prandtl’s friction law for turbulent flow, and agrees with a large number of phenomenological formulas by other authors. A numerical algorithm for solving the mathematical model of bed surface erosion is implemented in OpenFOAM. Numerical simulations of the problem show that under the influence of turbulent flow generated at the pipeline streamline, a characteristic bottom wave of low steepness appears, the parameters of which asymptotically agree with the experimental data. Based on the analysis of experimental and numerical studies of the considered case, an assumption about the self-similar behavior of the bed surface evolution is made. Based on this assumption, a new method of constructing the self-similar dependence of the bed surface on time and space coordinates is proposed. In the proposed approach, the average values of tangential bottom stresses are determined for a number of self-similar bottom surface shapes, and then the rates of change of bottom wave lengths and amplitudes are calculated using the proposed analytical model. A comparison with experimental data and numerical calculations shows that the solution error does not exceed a few percent and the computational time is reduced by up to 30 times.

The placement of baffles in culvert structures has a significant impact on local hydrodynamics and sediment transport, which are essential for improving fish passage and enhancing aquatic habitats. Large eddy simulations (LESs) are performed to study the influence of triangular baffle designs on flow structure and sediment transport in standard box culverts with varying baffle sizes and spacings. The Reynolds numbers based on the baffle size for these conditions are 37 778 or 75 012. The LES method is validated using experimental data of mean streamwise velocities, and a good agreement is achieved. The simulations demonstrated that the presence of baffles with various designs significantly altered the flow field in their vicinity. Various distinct flow features, such as separation zones, recirculation zones, and resting zones (RZs), are captured. The RZ and low positive velocity zone (LPVZ) show a positive correlation with the baffle size in the culvert, while the negative velocity zone (NVZ) exhibits a negative correlation. The flow field is primarily characterized by a large recirculation zone between the baffles, with the baffle size being the predominant influencing factor. The spacing between the baffles partially constrains the extension of the recirculation zone. The range of turbulence parameters, including the turbulent kinetic energy (TKE) and Reynolds shear stress (RSS), expands as the baffle size increases and the baffle spacing decreases. The presence of baffles suppresses ejections and sweeps interaction turbulence events downstream of the baffles. The near-bed coherent structures are responsible for the erosion zone between the baffles.

This study aims to investigate the influence of various vegetation patches with varying porosities on the hydraulic properties of a vegetated open channel under subcritical flow conditions. This research work investigated three types of vegetation patches: Rigid, flexible, and a combination of the two. In total five vegetation patches with three different porosities for each patch were investigated. Effect of these vegetation patches on various hydraulic parameters such as backwater rise, energy reduction, water surface slope in the vegetation patch, hydraulic jump formation on the downstream side of the vegetation patch, reduction in fluid force index (RFI), moment index (RMI), overflow volume (ΔQ) were studied. The findings revealed that the backwater rise increased in the case of rigid patch as the initial Froude number increased, whereas it decreased in the case of flexible and combined vegetation patches. It was observed that as the porosity increased from low (P_r = 0.90) to high (P_r = 0.99), the backwater rise decreased for all vegetation patches. The relative energy reduction rate increased for the rigid patch and showed a reverse trend for the flexible and combined vegetation patches with increasing initial Froude number. In the combined vegetation arrangement, the energy reduction values were highest for the alternate rigid and flexible (ARF) vegetation patches and lowest for the longitudinal rigid and flexible (LRF) vegetation patches. This study identified the presence of a hydraulic jump downstream of the vegetation patch, as indicated by the Froude number in the range of 1.0–1.7. The study also found that RFI, RMI, ΔQ had the highest values of 19.05%, 19.05%, 80.20%. The results of this study provide insight into the impact of vegetation patches with varying porosities on open-channel flow characteristics and can help develop sustainable vegetation management strategies.

Due to the continuous enhancement of wafer processing precision requirements in the semiconductor industry, ultrasonic cleaning technology has garnered significant attention for its distinctive advantages in wafer cleaning processes. This study aims to investigate the mechanism of ultrasonic cavitation during wafer processing, with a specific focus on the dynamic behavior of transient and steady-state cavitation at varying ultrasonic frequencies, as well as the distribution characteristics of the flow field during acoustic cavitation. In this paper, we comprehensively analyze the mechanism by which microjet impact generates a robust shear force for efficient particle removal from solid surfaces, while quantitatively evaluating the effects of different microjet velocities on wall deformation and potential damage. This research not only enhances our understanding of acoustic cavitation cleaning principles but also provides substantial scientific support for enhancing precision device cleaning efficiency and reducing potential damage.

Flow around a ship that advances at a constant speed V in calm water of uniform finite depth D is considered within the practical, realistic and commonly-used framework of the Green-function and boundary-integral method in conjunction with potential-flow theory. This framework entails accurate and efficient numerical evaluation of flows due to singularities (sources, dipoles) distributed over flat or curved panels of diverse geometries (quadrilaterals, triangles) that are employed to approximate the ship hull surface. This basic core element of the Green-function and boundary-integral method is considered for steady ship waves in the subcritical flow regime gD / V² > 1 and the supercritical flow regime gD / V² < 1, where g is the acceleration of gravity. The special case of deep water is also considered. An analytical representation of flows due to general distributions of singularities over hull-surface panels is given. This flow-representation adopts the Fourier-Kochin method, which prioritizes spatial integration over the panel followed by Fourier integration, in contrast to the conventional method in which the Green function (defined via a Fourier integration) is initially evaluated and subsequently integrated over the panel. The mathematical and numerical complexities associated with the numerical evaluation and subsequent panel integration of the Green function for steady ship waves in finite water depth are then circumvented in the Fourier-Kochin method. A major advantage of this method is that panel integration merely amounts to integration of an exponential-trigonometric function, a straightforward task that can be accurately and efficiently performed. The analytical flow-representation proposed in the study offers a smooth decomposition of free-surface effects into waves, defined by a regular single Fourier integral, and a non-oscillatory local flow, characterized by a double Fourier integral featuring a smooth integrand that primarily dominates within a compact region near the origin of the Fourier plane. Illustrative numerical applications to the flow potentials and velocities associated with a typical distribution of sources over a panel show that the flow-representation given in the study yields a practical method well suited for accurate and efficient numerical evaluation.

We experimentally investigate the 3-D flow characteristics caused by synthetic jet in the turbulent boundary layer (TBL), with the aim of analyzing the differences and similarities of hairpin vortices generated by jet of different hole diameters. For flow fields with hole diameters of 3 mm, 4 mm, 5 mm, the 2D time-resolved particle image velocimetry (TR-PIV) is used for preliminary experiment to determine the generation region of these hairpin vortices, and then the three-dimensional instantaneous snapshots of the region are obtained by tomographic PIV (Tomo-PIV). The statistical average results show that the downstream velocity deficit area is positively correlated with the hole diameter, and the drag reduction effect looks better with small hole diameter. The phase average extracts the three-dimensional morphology of the hairpin vortices produced by synthetic jet, and its distribution tends to be dense with the hole diameter, which is related to the velocity deficit. The two-point cross-correlation coefficient represents the scale of the coherent structure, and the three component scales of these hairpin vortices are smaller with large hole diameter, which is due to insufficient space for development. The flow fields are divided into high-energy and low-energy by proper orthogonal decomposition (POD). It is found that the increase of hole diameter can transfer the generated hairpin vortices from low-energy to high-energy, showing that the strength of high-energy hairpin vortices is positively correlated with the hole diameter.

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.