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This work presents a new approach to modelling of free surface non-Newtonian (viscoplastic or viscoelastic) fluid flows on dynamically adapted octree grids. The numerical model is based on the implicit formulation and the staggered location of governing variables. We verify our model by comparing simulations with experimental and numerical results known from the literature. [ABSTRACT FROM AUTHOR]

Landslide phenomenon in accumulated erodible bed sediments in a reservoir is one of the issues in hydraulic and sedimentation sciences that has received little attention. We intend to model two-dimensional changes of the water surface in a reservoir and of an erodible bed caused by a non-rigid landslide using a particle-based meshless approach. In this study, a fully explicit three-step algorithm is used. In this method, approximate numerical solution to the equations of the fluid dynamics is obtained by replacing the fluid with a set of particles. The governing equations for water flow and sand mass movement are solved for each particle. The movement of each particle, which is in interaction with other particles, is tracked. Experiments of a dam break on a dry bed, and submarine rigid and non-rigid landslides have been used to validate the method. Results indicate that the model was successfully calibrated against the measured data. Moreover, good agreement with the measured data demonstrates high capabilities of this method in simulating free-surface flows and wave-related phenomena. After the model validation, changes of erodible bed in a reservoir due to a non-rigid landslide were modelled. In this study, non-rigid landslide masses and sediment materials were modelled by non-Newtonian Carreau-Yasuda fluid, which is the novelty in the analysis of this type of natural hazard. Two possible scenarios were analyzed—one with the sliding material lighter, and the other with the sliding material heavier than the deposited sediments. The model was run until the landslide completely collapsed and its full impact was applied to the reservoir bed sediments. Additionally, we waited until the water level reached a steady state. These examples demonstrate that the model presented in this paper can be used as a reliable tool for modelling these phenomena. [ABSTRACT FROM AUTHOR]

An iterative time-marching scheme is developed to investigate the hydrodynamic interactions between multiple ships. Such an unsteady interactive effect could be magnified in restricted waterways, e.g., a channel or harbor area. To the author's knowledge, nearly all the research on the ship-to-ship interaction neglecting the free surface effects. The free surface is usually treated as a rigid wall. This assumption is only reasonable when the speed of the ships is very low in deep water condition, due to the hydrodynamic interaction between the ships is mainly induced by near-field disturbances. However, when the moving speeds are moderately higher, especially with a small lateral separation between ships, the far-field effects arising from the ship waves become important. The main objective of the present paper is to develop an iterative time-matching algorithm to solve the hydrodynamic interaction between high-speed ships taking into account the nonlinear free surface boundary condition in time domain. [ABSTRACT FROM AUTHOR]

Urban hydraulic pipelines may be subject to considerable damages while a sudden flow event occurs. However, a transition between free surface and pressurized flow arises; both overpressures and depressions will appear. Controlling this phenomenon becomes a necessity and must be integrated in pipe dimensioning. Most of earlier works were focused on the fictitious piezometric slot. In this work, we aimed to simulate this flow passage as a shock wave and using the Saint Venant mathematical model. Although, in order to take into account the pressurized state; we modified the pressure term. The transition from a type of flow to the other is composed of two discontinues states. Therefore, we solve it as a Riemann problem. To arrive to the most appropriate numerical scheme for the solution, we compare between the results of the Lax Fridricks, lax Wendroff and Godunov schemes. We do this considering the process time, the standard deviation and the Courant Friediricks Levy stability condition. On another hand, we carried out experimental tests, on a transparent and closed circular pipe, to measure pressure change with the flow rate. Hence, we give the physical stationary solution. Finally, we compare numerical results to experimental ones and deduce that the Godunov scheme is the most recommended tool to simulate the flow discontinuity between free surface and pressurized flow. [ABSTRACT FROM AUTHOR]

• A novel technique is developed for measuring the free surface and surface velocity. • Measurement accuracy is verified by wave probes, planes and fringe projection method. • The measurement technique is applied to the flood propagation in a compound channel. River flooding can pose a significant threat to people's lives and properties on the floodplain. Due to the difficulty and potential danger of monitoring field river floods, laboratory experiments have become the main and key method for exploring flood propagation and evaluating potential flood risks. However, the complex three-dimensional flow state of floods in the compound channel makes current measurement techniques inadequate for meeting monitoring requirements. To this end, a new technique is developed in this study that can simultaneously monitor the flood free surface and the surface velocity in the laboratory compound channel. The developed measurement technique combines stereoscopy and the particle tracking velocimetry (PTV) method through the pixel coordinate fitting method, so that the coordinates of the tracer particles can be reconstructed in the three-dimensional state. This method can not only increase the basic data in the free surface construction but can also capture the complex three-dimensional flood flow behavior. The proposed technology was applied to measure the free surface of flood propagation in a laboratory compound channel. Water level, flow velocity, wave velocity, and wave height were simultaneously measured. The measurement accuracy was verified using wave probes, spatial planes, and fringe projection techniques. The results indicate that the water level measurement error is approximately 1.6 mm, and the flow velocity measurement error is around 3 %. The developed measurement system is simpler than previous methods based on light reflection or absorption. Most importantly, it can simultaneously measure both the flood free surface and surface velocity by only sowing tracer particles on the water surface. The current measurement technology offers a new tool for studying flood propagation in a laboratory compound channel. [ABSTRACT FROM AUTHOR]

This study theoretically and numerically investigates the dynamics of an underwater explosion bubble near a free surface. A refined theoretical model is proposed, based on the potential flow theory, in order to improve its accuracy and extend its range of application to near-field scenarios. Furthermore, the two-dimensional axisymmetric Structured Arbitrary Lagrangian-Eulerian (S-ALE) method implemented in LS-DYNA is utilized to capture the bubble-water interface in numerical simulations. A convergence study is performed on the sizes of mesh and computational domain to reduce computation time while maintaining adequate accuracy in the results. The chosen finite element modeling strategy is validated by the Gilmore model, classical empirical formulas, and a reference experiment. After that, the evolution of the bubble shape is simulated for various values of γ f (the dimensionless distance from the initial bubble centroid to the free surface) and δ (the buoyancy parameter). Observations reveal that the free surface results in a slight increase in the maximum bubble radius, which is quantified as a linear fitting equation to allow a precise determination of the initial bubble radius. By combining the present model, simulations, and literature data, it is found that there are differences in the relationships between the dimensionless bubble oscillation period τ and γ f among UNDEX and cavitation bubbles, as well as among UNDEX bubbles with varying maximum radii. In any case, there is a positive correlation between τ and γ f , while the influence of δ on τ varies depending on the exact value of γ f. Meanwhile, the present model's predictions concerning migration history and collapse position generally match with the simulations and reference data. A reversal in the direction of bubble migration is observed when γ f is very small, which is caused by the substantial dependence of the Bjerknes force on the pressure gradient between the bubble interior and the ambient environment. • The refined theory was fully validated by sophisticated numerical simulations. • A method was proposed to more accurately calculate the initial bubble radius. • The refined theory applies even to bubble very close to a free surface (γ f = 1). • The study showed in detail how γ f and δ affect bubble dynamics. • Differences in behaviors among cavitation and UNDEX bubbles were presented. [ABSTRACT FROM AUTHOR]

A series of tow tank experiments are performed to observe the free surface deformation around a surface piercing cylinder with 39 mm diameter, at a range of Froude numbers 0. 36 < F r d < 3. 99. Particular attention is paid to the bow wave at the front of the cylinder and 6 distinct flow regimes are observed to appear as the velocity of the cylinder is increased, from the appearance of a steady hydraulic jump at the leading edge of the bow wave, progressing to a spilling and then plunging breaking wave. Time-resolved measurements of the height and upstream propagation distance of the bow wave are used to quantify the size and shape of the wave, with variations in the mean and fluctuating components of these measurements aligning with the observed transitions in the flow regime of the wave. Retrospective analysis of data from Hay (1947) is used to begin mapping the dependency of these flow regimes on the Froude, Weber, and Reynolds numbers. It is proposed that for such deep water flows, the Froude number could be defined using the largest wavelength of gravity waves produced at the bow (F r λ ), such that the number would by definition represent the ratio of flow velocity to maximum gravity wave celerity, in agreement with the hydraulic analogy. This allows for the flow regime transition point at which the surface waves no longer travel upstream of the cylinder, to occur by definition at F r λ = 1. • Tow tank experiments reveal 6 distinct flow regimes in the bow wave of a surface-piercing cylinder. • Retrospective analysis of literature begins to map occurrence of regimes non-dimensionally. • Quantitative data on bow wave characteristics presented, useful for validating numerical codes. • Proposes a Froude number that is consistent with the hydraulic analogy. [ABSTRACT FROM AUTHOR]

Due to the nonlinearities caused by free surfaces and discontinuities at turning points, analyzing unconfined seepage flow with corner singularities has always been a challenging issue in hydraulic engineering. The numerical manifold method (NMM) provides an almost perfect way to model continuous–discontinuous problems in a unified mathematical form and allows for combining analytical and numerical solutions. Using the NMM, this paper develops a high-precision and grid-independent method for unconfined flow with corner singularities involved in complex hydraulic structures. First, an iteration scheme is proposed to update the free surfaces. Second, a singular cover function for asymptotic solutions at turning points is introduced to the physical covers on singular points. This method can eliminate the necessity for remeshing when iterating seepage analysis and easily reflect strong singularity at the corner. Finally, several numerical experiments are conducted to validate the feasibility and applicability of the proposed method. The results illustrate that the enhanced NMM is efficient and accurate in homogeneous and heterogeneous models for free-surface locating. It also achieves precise results for challenging examples with various singular points, thus laying a solid foundation for further analyzing the coupled effects of seepage and corner singularity on complex structures. [ABSTRACT FROM AUTHOR]

Despite a lot of research on the flow past cylinders in an infinite medium, there is a great deal that is unknown about flow structure affected by the free surface at high Froude numbers (F r). Additionally, at high F r , the study of vortex-induced vibrations (VIV) is very important, which has been underrepresented in previous studies. In the present study, the free-surface flow and VIV are investigated at high F r. Our study has the privilege of performing a comprehensive parametric study on the free-surface parameters. Based on the results, F r has a prominent effect on flow parameters, as a 59 % reduction in the drag coefficient, and an increase of 100 % and 30 % in the lift coefficient and in the Strouhal number are recorded compared to low F r. Additionally, with a 4-fold increase of the free-surface distance to the cylinder, the lift coefficient drops by 27 %, and the Strouhal number increases by 42 %. Also, by approaching the free surface, the vortex shedding is suppressed. Eventually, study of the VIV indicates the substantial impact of the free surface on lock-in phenomenon, i.e., the resonance occurs earlier. Findings of the present study can significantly improve marine design considerations to prevent heavy and irreparable damages. [Display omitted] • The novelty of including free surface effect on flow structure at high Froude numbers. • An inclusive parametric study on free-surface parameters. • The novelty of evaluating vortex-induced vibrations for severe free surface deformation. • A 100 % increases in the lift coefficient and a 30 % enhancement in the Strouhal number compared to low Froude numbers. • Substantial impact of the free surface on lock-in phenomenon and reducing the oscillations. [ABSTRACT FROM AUTHOR]

Submerged bodies are commonly used in many fields such as scientific researches, military and commercial applications. Especially in military applications, submarines have a significant role as a silent and deterrent vehicle. Contrary to popular belief, submerged bodies also operate in shallow depth that free surface effects come into play. This causes the visual identification of submarines while protecting them from sonar detection. This study focuses on the investigation of free surface effects on submarine hydrodynamics moving forward in different depths. The numerical calculations have been conducted at different Reynolds numbers ranging from 1.5x107 to 3.5x107 for both bare and appended forms of DARPA Suboff. A commercial CFD solver has been used to solve URANS equations with k-ε turbulence model. The numerical approach has first been verified and validated with the available experimental data. Later, the numerical results have been discussed in terms of total resistance, resistance components and free surface deformations. It has been concluded that the submergence depth has a significant role on resistance components for depth Froude number larger than 0.7 and the appendages have little effect on free surface deformations in all depths.

Stepped spillway is hydraulic structure designed to dissipate the excess in kinetic energy at the downstream of dams and can reduce the size of stilling basin at the toe of the spillway or chute. The flow on a stepped spillway is characterised by the large aeration that can prevent or reduce the cavitation damage. The air entrainment starts where the boundary layer attains the free surface of flow; this point is called “point of inception”. Within this work the inception point is determined by using software Ansys Fluent where the volume of fluid (VOF) model is used as a tool to track the free surface thereby the turbulence closure is derived in the k − ε turbulence standard model. This research aims to find new formulas for describe the variation of water depth at step edge and the positions of the inception point, at the same time the contour map of velocity, turbulent kinetic energy and strain rate are presented. The found numerical results agree well with experimental results like the values of computed and measured water depth at the inception point and the numerical and experimental inception point locations. Also, the dimensionless water depth profile obtained by numerical method agrees well with that of measurement. This study confirmed that the Ansys Fluent is a robust software for simulating air entrainment and exploring more characteristics of flow over stepped spillways.

Free surface electrospinning is a continuous electrospinning method for low-cost, massive production of nanofibers. The interjet distance λ is a critical parameter in free surface electrospinning, which directly determines the nanofiber production efficiency. In this investigation, we studied the interjet distance during free surface electrospinning based on electrohydrodynamic instability theoretically and experimentally, with special interest focused on the effect of surface tension and electric field intensity on the interjet distance. The experimental results indicated that the critical parameters affecting the interjet distance were the surface tension and applied voltage, which was in good compliance with the theoretical prediction. The relationship between interjet distance λ and surface tension followed an allometric law with positive exponential function, and the relationship between interjet distance λ and applied voltage followed an allometric law with negative exponential function. The present results can be used to understand the basic parameters which determine the interjet distance in free surface electrospinning.

The accuracy of experimental procedure used to calculate the drag coefficient of an Autonomous underwater vehicle (AUV) in a towing tank is investigated using computational fluid dynamics. Effects of struts, used to connect the AUV model to towing carriage, on the hydrodynamics coefficient of the AUV at various relative submergence depths, at AUV speeds of 1.5 and 2.5 m/s are numerically simulated. Various numerical modeling are performed to investigate the effects of free surface with and without presence of struts on the drag coefficient of the AUV. Volume of fluid (VOF) model is used to solve the two phase flow RANS equations. The drag coefficients obtained from two phase flow simulations are compared with those obtained from single phase flow at corresponding velocities. The results obtained from experiments conducted in the towing tank of the Subsea Science and Technology centre, on a full-scale model of the AUV developed in this Centre, agreed well with those obtained by numerical simulations.

This paper presents the results of an experimental investigation on the impact of dam-break-induced surges on a vertical wall. The instantaneous surge height and dynamic pressure on a vertical wall were measured for surges with different reservoir depths of H = 200 mm, 250 mm, and 300 mm. The time-histories of horizontal pressure on the wall were measured using the miniaturized pressure transducers, and the surge heights were recorded with an ultrasonic sensor. The relationships between dynamic pressure and surge height on the vertical wall and during the impact were obtained from recorded raw data. The experimental results highlighted detailed processes on the variation of impact pressure during the surge propagation, impact on the wall, runup, falling, and breakup of the turbulent flow. The time-histories of surge height and dynamic pressure were analyzed, and the results were compared with the hydrostatic pressure on the wall to study wave breaking mechanism of tsunami waves on the wall. Dynamic pressures at the impact instant were found to be approximately three times the corresponding static pressure in the bed, in good agreement with previous research Moreover, the maximum surge runup heights on the wall were between 2.1 and 2.3 times the corresponding initial reservoir depths. The vertical distributions of impact pressure were divided into two hydrodynamic regimes. Based on the impact duration, the first regime occurred less than 0.1 s after the impact with highly non-linear pressure distributions, and the second regime showed a semi-hydrostatic pressure distribution from 0.5 s to 0.7 s. The results presented in this study are suitable for the design of coastal infrastructures and can be used to validate numerical models.

A magnetohydrodynamics solver (“mhdCompressibleInterFoam”) has been developed for a compressible two-phase flow with a free surface by extending “compressibleInterFoam” solver within OpenFOAM suite. The primary goal is to develop a tool to simulate compression of magnetic fields in vacuum and simplified magnetized plasma targets by imploding rotating liquid metal liners in the context of a Magnetized Target Fusion (MTF) concept in pursuit by General Fusion Inc. At present, the solver is limited to axisymmetric problems and the magnetic field evolution is solved in terms of toroidal field component and poloidal flux functions. The solver has been validated and verified using a number of test cases for which analytical or other numerical solutions are provided. Those tests cases include: (i) compression of toroidal and poloidal magnetic fields in vacuum and cylindrical geometry, (ii) axisymmetric annular Hartmann flow, and (iii) compression of magnetized target initialized with a Grad–Shafranov equilibrium state in a cylindrical geometry. A methodology to incorporate conductive solid regions into simulation has also been developed. Capability of the code is demonstrated by simulating a complex case of compressing a magnetized target, which is injected during implosion of a rotating liquid metal liner with an initially soaked poloidal magnetic field. An application of the solver to simulate compression of a magnetized target in a geometry and parameters relevant to the Fusion Demonstration Plant (FDP) being developed by General Fusion Inc. is also demonstrated.

The paper reviews the results of numerical analyses for the micro-and mesoscale deformation-induced surface phenomena in three-dimensional polycrystals with the explicit account for the grain structure. The role of the free surface and grain boundaries in the appearance of the grain-scale stress concentrations and plastic strain nucleation is illustrated on the examples of aluminum polycrystals. Special attention is paid to the discussion of mesoscale deformation-induced surface roughening under uniaxial tension. [ABSTRACT FROM AUTHOR]

In this article, high‐order finite difference solutions of the exact potential‐flow problem for nonlinear water waves are developed which treat both the free surface and the uneven sea bottom boundary conditions using an immersed boundary method (IBM). The convergence, accuracy, and stability of this approach is first established for the linear problem, using various orders of scheme and different grid discretization strategies. The nonlinear wave problem on a flat bottom is then considered, and the solutions are compared with the highly accurate stream function theory solution. The convergence performance of the numerical solution for this nonlinear wave problem is established. Finally, linear wave shoaling and nonlinear wave propagation over a submerged bar are also tested. A preconditioned iterative solution strategy is also developed and shown to provide optimal scaling of the solution effort with increasing number of unknowns. Compared with existing free surface tracking methods, the IBM retains the same level of accuracy and stability when solving the linear problem. For the nonlinear problem, the IBM behaves reasonably in terms of accuracy, but generally with slightly higher computational effort. When it comes to the wave–body interaction problem, it is expected that the IBM will be advantageous compared with the σ‐transform method, since it removes the necessity of constructing an artificial continuous free‐surface inside the body. [ABSTRACT FROM AUTHOR]

Numerical simulation of dam-break water flow over a horizontal dry bed has been performed. We have modified and verified computational techniques, including methods for determining the position of the interface and the continuum model for the surface tension force implemented in the PIFI code, as well as the OpenFOAM software with the interFoam solver and various cases of the two-parameter () model with corrections made taking into account the behavior of flows in regions of low Reynolds numbers. Calculated integral characteristics of dam-break flow were analyzed and compared with measured data. It is shown that taking into account surface tension and using an adequate turbulence model leads to deceleration of water flow and hence to a decrease in the velocity of the wave front, resulting in better agreement between the results of calculations and laboratory experiments. [ABSTRACT FROM AUTHOR]

The present paper focuses on the numerical investigation of the flow around the fully submerged 2D and 3D hydrofoils operating close to a free surface. Iterative boundary element method is implemented to predict the flow field. This study aims to investigate the aspect ratio effect on the free surface interactions and hydrodynamic performance of the hydrofoils under a free surface by using potential flow theory. Three different submergence depths and aspect ratios are studied in the wide range of Froude Numbers. In 3D cases, spanwise width of the numerical wave tank model is selected both equal and wider to the foil span, to observe the sidewall effects. Wave field seems to be two dimensional at low Froude numbers. On the other hand, signs of three dimensionalities are observed on the free surface structure for higher Fn, even the predicted wave elevations are very close to 2D calculations in the midsection. Increment in the Fn give a rise to the amplitude of the generated waves first, however a further increase in Fn has a lowering effect with the beginning of waves spill in the spanwise direction in the form of Kelvin waves. Free surface proximity and resultant wave field are also seeming to be linked with the lift force on the hydrofoil. As aspect ratio of the foil increase, 3D lift values are getting closer to those of 2D calculations. However, it is seen that, 3D BEM predictions of a hydrofoil under free surface effect cannot be considered two-dimensional even the aspect ratio is equal to 8. [ABSTRACT FROM AUTHOR]

Summary: In order to solve incompressible, nonideal magnetohydrodynamic (MHD) free‐surface flows, two weakly compressible smoothed particle hydrodynamics models, with and without the consideration of magnetic induction, are developed. The SPH formulation for magnetic induction magnetohydrodynamics (SPMHD), which is popular in astrophysical studies, is applied for the first time to incompressible free‐surface MHD flows, such as liquid metal flows, with the consideration of nonideal MHD effects and boundaries with arbitrary electric conductivity. An SPMHD implementation using the inductionless approximation is also proposed for both electrically conductive and insulating boundaries, in which a Poisson equation is solved to compute the Lorentz force instead of evolving the magnetic induction equation. Both proposed methods are validated against MHD benchmarks, including free‐surface MHD cases. The proposed inductionless SPMHD implementation has the advantages of stability and relaxed time‐step restrictions, but is only accurate at a low range of Hartmann numbers. For high Hartmann number problems, magnetic induction SPMHD model is more accurate. The computational efficiency and conservation error of the two models are compared and discussed. [ABSTRACT FROM AUTHOR]

The present dataset provides experimental measurements of the 3D liquid/gas interface shape during lateral water sloshing in a partially filled cylindrical container. The measurement technique used to acquire the data is the Reference Image Topography [1] based on a Synthetic Schlieren Free-Surface Reconstruction method [2]. A modified version of the processing algorithm has been used. This one transforms the coordinate system from Cartesian to polar so that the computational domain only includes the area where the fluid is present. Moreover, it uses the conservation of the fluid volume into the investigated area that permits to obtain the absolute height. This allows overcoming the strong limitation of the RIT method regarding the inability to detect changes of the mean surface height, at the condition that the complete and only liquid domain recorded in the images is used in the inversion algorithm. The complete details of the post-processing of the images is reported in the paper associated to this DIB [3]. The dataset includes the external excitation history and maps of the liquid/gas interface acquired during the experiment. These data are considered fundamental for the validation of CFD simulations of sloshing and of simplified theoretical models. A set of 12 test cases are reported in this DIB. A part of these test cases refers to steady state sloshing and a part to sloshing damping. In the last cases, also the 3D map of damping coefficients, calculated using the logarithmic decrement method, is provided.

The flooding of railway ballasts can cause extensive damage. This process has been the subject of several experimental investigations. In the present work, a relatively easy to implement approach to modelling this fluid flow is presented. It is shown that good agreement with the experimental results is obtained. The fluid flow is modelled by Darcy’s law, which we extend to the free fluid flowing above the ballast. The main complexity is in determining the free surface position, which is accomplished using an iterative procedure. The equations are solved using the finite element method. The method is illustrated by careful numerical calculations that are carefully compared with the experimental results reported in the literature. The method is then extended to realistic railway ballast, including the effects of ballast fouling. It is shown that when the flow begins to overtop the ballast, the free surface shifts to greatly increase the chance of ballast scouring.

Tidal current is a promising renewable energy source. Previous studies have investigated the influence of surface waves on tidal turbines in many aspects. However, the turbine wake development in a surface wave environment, which is crucial for power extraction in a turbine array, remains elusive. In this study, we focus on the wake evolution behind a single turbine and its interaction with surface waves. A numerical solver is developed to study the effects of surface waves on an industrial-size turbine. A case without surface wave and two cases with waves and different rotor depths are investigated. We obtain three-dimensional flow field descriptions near the free surface, around the rotor, and in the near- and far-wake. In a comparative analysis, the time-averaged and instantaneous flow fields are examined for various flow characteristics, including momentum restoration, power output, free surface elevation and vorticity dynamics. A model reduction technique is employed to identify the coherent flow structures and investigate the spatial and temporal characteristics of the wave–wake interactions. The results indicate the effect of surface waves in augmenting wake restoration and reveal the interactions between the surface waves and the wake structure, through a series of dynamic processes and the Kelvin–Helmholtz instability. [ABSTRACT FROM AUTHOR]

Over the last two decades, interest in the free-surface behaviour of gravity-driven shallow turbulent flows has increased considerably. It is believed that observation of free-surface behaviour can provide useful information about the hydrodynamic characteristics of the flow and enable remote retrieval of these characteristics to non-invasively and rapidly monitor river flows. At the current state the literature presents scattered knowledge and also exhibits non-uniformity in the terminology used. This paper is a review of the state-of-art of this area of research and was created with two objectives: to gather the information relevant to understand the linkages between the free-surface behaviour and underpinning hydrodynamic processes while using a uniform terminology, and to analyse the gaps in our knowledge of this critical topic. [ABSTRACT FROM AUTHOR]

In the present study, the effect of free surface on the hydrodynamic forces acting on the motion of an autonomous underwater vehicle (AUV) has been investigated. The AUV is powered by solar energy. Using computational fluid dynamics, the Reynolds averaged Navier Stokes (RANS) equations for the flow around the AUV are solved, and the free surface effect is simulated using the volume of fluid (VOF) two-phase flow model. For this purpose, the commercial code ANSYS FLUENT 18 was used [1]. The results of the numerical solution are compared with experimental results of the AUV model in the surface motion in the towing tank of the Persian Gulf National Laboratory with a scale of 1:1. The experiment was performed in a fixed draft and the velocity was ranging from 0.2 m/s to 1.4 m/s (according to Reynolds number 2:4 × 105 to 1.7 × 106). [ABSTRACT FROM AUTHOR]

Experiments were conducted on two splitter plates of different structural rigidities, attached to a rigid cylinder and exposed to cross-flow at different submergence depths in order to study the influence of free surface. It was found that the presence of flexible splitter plate results in drag reduction at all submergence depths when compared with a plain cylinder, with the reduction being dependent on the rigidity of the splitter plate. The lift force was also found to increase abruptly at low depths for the cases involving splitter plates. The results from the analyses of tip kinematics on flexible splitter plate indicated that it performs vertical oscillations symmetrically about its mounting position at high submergence depths but is displaced in one direction up to an order of cylinder diameter and undergoes minimal oscillations when located near the free surface. [ABSTRACT FROM AUTHOR]

In this study, we perform simulations for free surface wave tracking inside a numerical tank. A Navier–Stokes solver, combined with the arbitrary Lagrangian–Eulerian method, is introduced. To generate free surface waves, a mass source internal wave-maker, proposed by Lin and Liu (J Waterw Port Coast Ocean Eng 125(4):207–215, 1999), is employed and implemented in OpenFOAM. A line mass source distribution throughout the water depth is used to mimic a real-world wave-maker. Specifically, two types of line source wave-makers for shallow water, namely the piston and flap types, are established, together with the block mass source model used by Lin and Liu (J Waterw Port Coast Ocean Eng 125(4):207–215, 1999). Numerical results of free surface profiles and horizontal velocity profiles are validated by experimental data (Umeyama in J Waterw Port Coast Ocean Eng 137(2):85–94, 2010). It is concluded that in the case of shallow-water wave propagation, the line source model is better than the block source model at maintaining the wave form. The internal wave-making method has its ceiling (wave height and water depth ratio H / h = 0.1 ) for applications in numerical wave-making. Flow contamination induced by the wave-maker itself must be considered in complex flow situations such as wave-current interactions. [ABSTRACT FROM AUTHOR]

The viscoplastic fluid flow initiated in a circular pipe/plane channel during its filling in the gravity field at the flow rate specified at the inlet section is investigated. A mathematical formulation of the problem is stated based on complete equations of motion, the continuity equation, the natural boundary conditions on a free surface, and no-slip boundary condition on the solid wall. The rheological behavior of the medium is described by the Schwedoff–Bingham law, which presupposes the existence of quasi-solid motion zones (unyielded zones) in regions of low strain rates. The numerical solution of the problem is based on the finite-difference approach including the finite volume method and SIMPLE algorithm for calculating velocity and pressure fields at the internal nodes of a staggered grid. The method of invariants is used to satisfy the boundary conditions on the free surface. To provide a through computation of the flow with unyielded regions, regularization of the rheological equation is implemented. The behavior of a free boundary, flow structure, and flow characteristics as a function of the main parameters is investigated. It is found that in the course of time the initially flat free boundary acquires a stationary convex shape, which remains invariant while moving through the pipe/channel at the rate-average velocity. In the flow near and far away from the free boundary, one can distinguish fountain flow zones and one-dimensional flow regions, respectively. The typical flow structures with different numbers and various locations of unyielded regions in the flow are shown. The topograms of the above-mentioned flow structures as functions of the ratio of viscous and gravity forces and plasticity in the fluid flow are plotted. The stable and unstable behavior of the free boundary shape is shown to be related to the values of the constitutive parameters. [ABSTRACT FROM AUTHOR]

ABSTRACT The traditional approach for the hydrodynamic characterization of the flow down stepped spillways is through physical modeling, which is susceptible to scale effects and has limitations related to experimental apparatus, laboratory space and the spatial discretization of data collection. Computational fluid dynamics (CFD) is an important tool for hydrodynamic analysis because, if used properly, it presents great potential for application in hydraulics. In this work, CFD was used to model the skimming flow down a stepped spillway to investigate the effects of possible pressure measurement errors due to uncertainties in the position of the sensors within the steps. The numerical model was validated through literature velocity profiles and pressure experimental data. The results showed that the best values of water fraction (α) to define free surface are α = 0.30 in the nonaerated region and α = 0.10 in the aerated region. Statistical parameters were calculated using experimental data to estimate extreme pressures. These parameters and the simulation results were used to determine that the extreme maximum and minimum pressures occur, respectively, in the region of 0.81 < x/l < 0.98, in the horizontal faces, and in the region of 0.93 < y/h < 0.98, in the vertical faces.

This article focuses on vortex structure identification methods and their implementation into smoothed particle hydrodynamics (SPH) framework. The most common criteria based on the local velocity gradient tensor analysis like Q-criterion, Δ-criterion, and λ2-criterion are introduced together with their implementation for SPH data. A two-dimensional ‘double dam break’ problem was chosen as a test case because it results in a violent transient free-surface flow with emerging and vanishing eddies of various sizes and intensities. Q-criterion and Δ-criterion results were virtually identical, whereas λ2-criterion appeared to be the most restrictive in vortex identification, so it was the best in suppressing incorrect findings due to the numerical solution imperfection. The analysis of the problem was therefore conducted using the λ2-criterion.

The simulations of submarine sailing near the free surface with long-crested waves have been conducted in this study using an in-house viscous URANS solver with an overset grid approach. First, the verification and validation procedures were performed to evaluate the reliability, with the results showing that the generation of irregular waves is adequately accurate and the results of total resistance are in good agreement with EFD. Next, three different submerged depths ranging from 1.1D to 3.3D were selected and the corresponding conditions of submarine sailing near calm water were simulated, the results of which were then compared with each other to investigate the influence of irregular waves and submerged depths. The simulations of the model near calm water at different submerged depths demonstrated that the free surface will cause increasing resistance, lift, and bow-up moments of the model, and this influence decreases dramatically with greater submerged depths. The results of the irregular wave simulations showed that irregular waves cause considerable fluctuations of hydrodynamic force and moments, and that this influence remains even at a deeper submerged depth, which can complicate the control strategies of the submarine. The response spectrum of hydrodynamic forces and moments showed slight amplitudes in the high-frequency region, and the model showed less sensitivity to high-frequency excitations.

The submarine is usually affected by free surface and the navigation resistance increases when sailing near the surface. In order to study the specific resistance characteristics of submarine sailing near the surface, the SUBOFF with appendages was taken as the research object, and the calculation model was built based on Star CCM+ fluid simulation software, and the resistance coefficients under different submarine depths and speeds were calculated. Through comparative analysis, the influence of the depth and speed of the submarine on the resistance components was obtained, and the cause of the formation was analyzed. The results show that the influence of the depth of submarine on friction resistance coefficient is small in general. With the increase of the depth of the submarine, the pressure resistance coefficient decreases, and the wave amplitude decreases. The shear wave of Kelvin wave system is more obvious and the effect of scattering is weakened, which is of great significance for the study of submarine concealment. With the increase of speed, friction resistance coefficient decreases, the overall change trend of pressure resistance coefficient is first increased and then decreases. The interference effect between free surface and hull increases first and then decreases at each depth. The wave shape changes and resistance results mutually confirm. The free surface mainly generates waves by interacting with the hull, which affects the resistance characteristics of the submarine. The interference effect is greatly affected by the depth and speed of the submarine.

The single-phase free surface lattice Boltzmann model (LBM) was used to investigate the counter-gravity mould filling process of large plate castings. Computational efficiency can be improved by ignoring the variation of the gas lattice. In this paper, a weight coefficient redistribution method was proposed for dealing with the liquid phase discharge problem and distribution in a grid. First, the model was used to calculate the anti-gravity filling process of a large plate cavity with a single gate, with the same parameters, the water filling experiment of high speed camera is taken as a reference, the characteristics of the flow field and the fluid morphology of numerical simulation are in good agreement with the experimental results. Furthermore, the velocity profile of the flow field is used to analyze the characteristics of the flow field, and a criterion called "the altitude difference of free surface" is introduced to evaluate the stability of the fluid. Subsequently, the counter-gravity filling process of a plate with a double gate and the same plate with one gate and a cylindrical obstruction to turbulent flow are investigated. Due to the mutual influence between the two gates, the vortex formed by the double gates is greater than that formed by the single gate. However, the obstacles under the conditions of cylindrical turbulent flow obstruction could reduce the degree of fluid sloshing and improve the stability of the filling.

In order to analyze the characteristics of propeller exciting force, the hybrid grid is adopted and the numerical prediction of KCS ship model is performed for hull–propeller–rudder system by Reynolds-Averaged Navier Stokes (RANS) method and volume of fluid (VOF) model. Firstly, the numerical simulation of hydrodynamics for bare hull at oblique state is carried out. The results show that with the increasing of the drift angle, the coefficients of resistance, side force and yaw moment are constantly increasing, and the bigger the drift angle, the worse the overall uniformity of propeller disk. Then, propeller bearing force for hull–propeller–rudder system in oblique flow is calculated. It is found that the propeller thrust and torque fluctuation coefficient peak in drift angle are greater than that in straight line navigation, and the negative drift angle is greater than the positive. The fluctuation peak variation law of coefficient of side force and bending moment are different due to various causes.

In this study, effect of lateral vibrations on the thermo-solutocapillary convection and surface behavior in a liquid bridge of toluene/n-hexane solution has been studied numerically under microgravity. The Navier-Stokes equations coupled with the concentration diffusion equation are solved on a staggered grid, and the level set approach is used to capture the deformation of free surface. Present results indicate the interface deformation is convex below the midheight of the liquid bridge and relatively complex at the upper height of the liquid bridge under different frequency vibrations. The lateral vibrations restrain radial and axial velocities on the surface under lateral vibrations with frequencies of 5 Hz and 10 Hz. While the radial and axial velocities on the surface increase under the lateral vibration with frequency of 15 Hz, which illustrates that the effect of lateral vibrations on the free surface has uncertainty. [ABSTRACT FROM AUTHOR]

The problem of two-dimensional flow with the free surface of the jet in a region between two semi-infinite straights intersections at point O is calculated analytically for each angle Beta and numerically for each of the various values of the Weber number and angle Beta. By assuming that the flow is potential, irrotational and that the fluid is incompressible and inviscid, and by taking account only the surface tension for a numerical method using the series truncation, and without the effect of gravity and surface tension for the analytic method utilize the hodograph transformation. The obtained results confirmed a good agreement between them when the Weber number tends to infinity, and the comparison of these surface shapes is illustrated. [ABSTRACT FROM AUTHOR]

Summary: We present a robust numerical method for solving incompressible, immiscible two‐phase flows. The method extends both a monolithic phase conservative level set method with embedded redistancing and a semi‐implicit high‐order projection scheme for variable‐density flows. The level set method can be initialized conveniently via a simple phase indicator field instead of a signed distance function (SDF). To process the indicator field into a SDF, we propose a new partial differential equation‐based redistancing method. We also improve the monolithic level set scheme to provide more accuracy and robustness in full two‐phase flow simulations. Specifically, we perform an extra step to ensure convergence to the signed distance level set function and simplify other aspects of the original scheme. Lastly, we introduce consistent artificial viscosity to stabilize the momentum equations in the context of the projection scheme. This stabilization is algebraic, has no tunable parameters and is suitable for unstructured meshes and arbitrary refinement levels. The overall methodology includes few numerical tuning parameters; however, for the wide range of problems that we solve, we identify only one parameter that strongly affects performance of the computational model and provide a value that provides accurate results across all the benchmarks presented. This methodology results in a robust, accurate, and efficient two‐phase flow model, which is mass‐ and volume‐conserving on unstructured meshes and has low user input requirements, making it attractive for real‐world applications. [ABSTRACT FROM AUTHOR]