Overset Mesh Research Articles

A CFD investigation of the effects of passing ship on a small size ice floe

A RANS (Reynolds Averaged Navier-Stokes) CFD STAR-CCM+ model is used to investigate the hydrodynamic interactions between a ship and a circular ice floe. The overset mesh technique is employed to simulate the advancing movement of the passing ship. The ship has a hull of similar configuration to the KCS MOERI container ship. The ice floe has a diameter of 30% of the ship’s length and a thickness of 3 m. The paper primarily focuses on the motions of the ice floe and the hydrodynamic forces induced by the ship progressing in different speeds and calm water conditions. A parametric study on the influence of the ice floe surge and sway motions and associated hydrodynamic loads is presented. It is concluded that ship-generated waves can significantly affect the motions of the ice floe and the influence on sway is greater than that on surge. The ship speed is also found to have a distinct impact on ice floe motions.

Numerical investigations of two vibrating cylinders in uniform flow using overset mesh

Abstract In this research, flow around two vibrating cylinders in a tandem arrangement is simulated at Reynolds number Re = 200 using the dynamic overset mesh technique in finite volume-based commercial software. This investigation aims to study the combined influences of the spacing between the two identical circular cylinders and their excitation frequency in the flow. The cylinders are excited by a transverse forced vibration in a uniform cross-flow by applying a simple harmonic motion. The gap distance between the vibrating cylinders is chosen to be L/D = 1.5 and 4, and the vibration amplitude is kept constant at A/D = 0.25. The study focuses on three frequency ratios of the cylinders’ excitation frequency to Strouhal shedding frequency of the single stationary cylinder f e/f s = 0.8, 1.0, and 1.2. Simulation results showed that the flow characteristics over the two vibrating circular cylinders differ from that of a single vibrating cylinder. Also, it is observed that the lock-in state (resonance) for the two vibrating cylinders and the vortex wake patterns are highly affected by the gap distance between cylinders and the excitation frequency.

Experimental and numerical determination of the lubrication force between a spherical particle and a micro-structured surface

In many particulate processes suspensions need to be handled. Hydrodynamic forces in presence of a liquid as a surrounding continuum medium can significantly affect the particle collision behaviour. When particles approach a wall, lubrication force can become dominant with decreasing distance. This force was described analytically by different authors for a smooth flat wall. Roughness was found to be an important factor in this context, but the mechanisms are still not fully understood. In this work, the effects of topology on the lubrication force were studied using a regular prismatic micro-structured titanium surface produced by micro-milling. A nanoindentation setup was modified for the direct measurement of this force during the particle approach to polished and micro-structured surfaces in liquid. For a more detailed insight on the behaviour of the fluid in the decreasing gap between particle and surface microstructure, resolved computational fluid dynamics (CFD) simulations were performed using an overset mesh method. The comparison of simulation results with nanoindentation tests and analytical solution showed a good agreement. The effects of structure size and particle contact location at various approaching velocities on the lubrication force were investigated.

Influence of a walking mechanism on the hydrodynamic performance of a high-speed wheeled amphibious vehicle

Abstract. In order to reduce the resistance and increase speed for a high-speed wheeled amphibious vehicle, a wheel-retracting mechanism was applied to a walking mechanism and the influence was researched. Firstly, to obtain a reliable numerical method, a realizable shear stress transport (SST) k-ω turbulence model and computational fluid dynamic (CFD) model built by an overset mesh technique was used and compared with the corresponding model tests. Secondly, the effect of the wheels' flip angle on resistance, heave and pitch was investigated. Then, the wheel well was optimized by numerical simulation. Finally, the results showed that the influence of the wheels on resistance was more significant, and the larger the wheels' flip angle was, the more significant the resistance reduction would be. An optimized wheel well was beneficial to resistance reduction. Furthermore, the running attitude became steadier, thereby decreasing the heave and pitch.

Investigation on Numerical Simulation of VIV of Deep-Sea Flexible Risers

The vortex-induced vibration (VIV) of flexible risers is a complex fluid–structure interaction (FSI) phenomenon. In this study, we conducted a numerical simulation method based on the slicing method to study the vortex-induced vibration (VIV) of deep-sea flexible risers with different slenderness ratios and uniform flow velocities. The method combines the finite element model of the riser structure with the two-dimensional flow field slices solved by the Fluent solver. The fluid–structure interaction was realized by a self-compiled UDF program and the overset mesh technique. The numerical results were validated by comparing them with experimental data. The VIV characteristics of the riser, such as the vibration track, vibration mode, vibration frequency and wake vortex shedding mode, were analyzed. The article reveals the nonlinear dynamic features of flexible riser vibration, such as multi-frequency vibration, wide-frequency vibration and multi-modal vibration. The article also provides insights into the fluid–structure interaction mechanism of VIV of deep-sea flexible risers.

IKA-FLOW : A Flexible Body Overset Mesh Implementation for Fish Swimming

Simulation of inertial aquatic swimmers requires fluid structure interactions with temporal body geometry deformation. In practice, his results in a change of the computational domain boundaries that represent the ”swimmer.” These simulations are traditionally done sing body-fitted mesh and mesh morphing methods, but have drawbacks of negative cell volumes and small time-steps to account for the complex swimming motion. In contrast, the overset mesh method, also provided by OpenFOAM®, overcomes most of the drawbacks of the mesh morphing method at the expense of interpolation error. The current OpenFOAM® overset motion library only supports rigid body motion and cannot be used to resolve a body undergoing undulation. A modified motion solver is presented that allows for the complex mesh motion of an overset mesh for four body-caudal fin (BCF) virtual swimmers. The results of this solver are compared with published data of body-fitted meshes. The effect of different simulation parameters (including number of solving iterations, time delay, and temporal resolution) is investigated. Additionally, a novel simulation and comparison of the Ostraciiform locomotion mode with Anguilliform, Carangiform, and Thunniform modes are made investigating the wake, drag and lift. It is concluded that fish undulation has a marked effect on reducing lift generation. Lastly, a comparison of turbulence models (Spalart-Allmaras, k − ω SST, and k − kL − ω) at multiple Reynolds numbers shows that all three models have similar performance at lower Reynolds numbers but diverge at higher numbers.

An Experimental and Numerical Evaluation of the Aerodynamic Performance of a UAV Propeller Considering Pitch Motion

Considering the vibration generated by a propeller-driven UAV or encountering gust, the propeller will perform a very complex follower motion. A pitch and rotating coupled motion is proposed in the present work that can take more complex unsteady performance of follower force than a regular fixed-point rotating motion. In order to evaluate the unsteady follower force and conduct parametric study, an extensive ground test bench was designed for this purpose where the whole test system was driven by a linear servo actuator and the follower force was measured by a 6-component balance. For CFD simulation, coupled motion in particular needs detailed unsteady aerodynamic model; therefore, a high-fidelity CFD-based study integrated with the overset mesh method was complemented to solve the unsteady fluid of varying conditions. The results suggest that a significant influence on unsteady follower force is observed, and the mean value of in-plane force does not equal to zero during the coupled motion process. Compared with the regular fixed-point rotation of propeller, the fluctuation frequency of follower force in present work couples the rotation and pitch motion frequencies. In addition, the oscillation amplitude of out-plane force and torque is positively related with the pitch frequency, pitch amplitude, and relative length from leading edge of wing to the rotation center. For example, the oscillation amplitude of 1-blade’s out-plane force and torque increases by 57.122% and 66.542% for the 5 Hz-5 deg case compared with the 5 Hz-3 deg case, respectively. However, the torque is not sensitive to frequency of pitch motion. The generally excellent agreement evident between the ground test and numerical simulation results is important as guidance for our future investigation on “dynamic” aerodynamic performance of a propeller-driven UAV.

Vertical water entry of a hydrophobic sphere into waves: Numerical computations and experiments

The water entry cavity evolution and its flow structures for a sphere interacting with periodic waves are investigated numerically and experimentally. The large eddy simulation is applied in the simulation to accurately capture the turbulent flow near the surface and within the cavity of the sphere. An overset mesh-based numerical wave tank is developed, integrating an overset mesh with a method for generating regular waves, to ensure high resolution simulation of velocity fields around the water entry cavity in waves. To validate the numerical model, a physical experiment system is developed, featuring a free-falling setup and an asynchronous pulse trigger system. This experimental setup allows for precise control of the vertical water entry of a sphere at a predetermined phase of a periodic wave. The computed cavity shape and the sphere motion are in good agreement with the experimental results. Notably, the hydrodynamic forces exerted on the sphere exhibit two distinct peaks at the moment of impact and the pinch-off of the cavity, respectively. The gas-phase force acting on the dry surface of the sphere, as the cavity forms and evolves, experiences significant fluctuations along the direction of the sphere's descent. These fluctuations are caused by the accelerating gas flow prior to the pinch-off of the cavity. The changes of the hydrodynamic forces on the sphere for the cases of different water entry phase locations of waves and Froude numbers are discussed.

A Study of the Resistance Components of Planing Hull Using Interceptors

The extreme motions created a negative effect on planing hull. Otherwise, interceptors enable the improvement of hydrodynamic characteristics of the highspeed craft. The total drag of the ship can be divided into two components: pressure and shear forces. This project specifically aims to predict the drag component based on CFD simulation with and without an interceptor based on resistance components. The volume of fluid is used in a compound with a RANS solver to determine drag and trim in calm water conditions. SIMPLE algorithm using overset mesh and DFBI was modeled free in heave and pitch to simulate the rigid body motion as a response to shear and pressure drag exerted by the fluid. A grid independence study has been used to predict uncertainties using time-step and grid spacing. Additionally, to guarantee to proceed with the simulation, ITTC recommendations had been followed. The benchmark study was undertaken as an experimental comparison by Park et al. In general, ship performance was controlled by pressure force in the displacement phase and planing phase, the shear and pressure forces are balanced. The interceptor can decrease the pressure force by 5% in the hump region and planing phase.

On the cavity flow of a cylinder exiting water obliquely

The water exit of a high-speed cavitation vehicle is a highly complex problem. Especially when the vehicle exits the water at different angles, the cavity shape and motion stability are affected by the air-water interface, resulting in significant changes in the mechanical environment and dynamics of the vehicle during the water exit. To solve this problem, a CFD method based on VOF multiphase flow interface-capturing technique and overset mesh technique is adopted to numerically calculate the interaction of vapor-liquid-air multiphase flow during the oblique water exit of a high-speed cylinder. Based on the verification of the computational model by mesh convergence and water-entry experiment, the cavitation evolution and flow distribution characteristics of the high-speed cylinder during oblique water exit is analyzed. Moreover, the effects of water-exit angles on the surface pressure distribution and drag characteristics of the cylinder are studied. The results show that the fluid at the end of the cavity impacts the shoulder of the cylinder during oblique water exit, resulting in the pressure difference between the upper and lower surfaces of the cylinder, which may finally leads to motion deviation. With the decreasing water-exit angle, the negative peak drag coefficient decreases while the peak lateral force coefficient increases, and there is a critical range of water-exit angel for lateral force and pitch angle variation.

A numerical study of passing ship effects on a moored ship in confined waterways with new benchmark cases

This study aims at a comprehensive investigation of passing ship effects on a moored ship in confined waterways with high blockage, referring to the new benchmark cases from the latest MASHCON conference. The whole process of a KCS container ship passing a moored Neo-Panamax container ship is simulated by combining the URANS method with the dynamic overset mesh technique. The hydrodynamic forces and moments, attitudes of the moored ship, and ship-generated waves are predicted. Firstly, the case with 10% UKC is simulated for validation. The results indicate that the numerical layout with a larger initial longitudinal distance between the two ships presents more accurate results agreeing well with the test data. Then, based on the validated method, the effects of tank width, water depth, lateral passing distance and passing ship speed are analyzed and are quantified by curve fittings in terms of the amplitudes of hydrodynamic quantities. It is found that these parameters have remarkable impacts on the moored ship, among which the passing speed is the most significant factor. In addition, the ship-generated drawdown dominates in the whole passing process, which causes prominent variations of the hydrodynamic quantities and induces pronounced pressure reductions on the moored ship.

Assessment of turbulence models and mesh types for the simulation of multi-body separation by coupling the CFD and rigid body dynamics

Turbulence models are considered one of the most important sources of numerical errors in the CFD studies on different aeronautical problems. The mesh type can also affect the numerical accuracy and computational efficiency. In this paper, various simulations on the store separation from an aircraft model in the transonic flow have been carried out by coupling the CFD and the rigid body dynamics on the overset mesh. The influence of S-A model, Standard k-epsilon model, SST k-omega model and inviscid model on the simulation results is examined. Different mesh types of both unstructured mesh and structured mesh as component part in overset are also considered. Grid-independence study are first carried out. The effects of different turbulence models and mesh types are then examined by comparing the trajectories, velocities and Eulerian angles. Recommendations are given to get better results for the simulation of multi-body separation problem.

Numerical Analysis of Propeller Wake Evolution under Different Advance Coefficients

Propeller wake fields in an open-water configuration were compared between two loading circumstances using large-eddy simulation (LES) with a computational domain of 48 million grids and an overset mesh technique. To validate the results of the numerical simulation, available experimental data are compared, which indicates that the grid systems are suitable for the present study. The results indicate that the present LES simulations describe the inertial frequency range well for both high and low-loading conditions. Under high-loading conditions, the interlaced spirals and secondary vortices that connect adjacent tip vortices amplify the effects of mutual inductance, ultimately triggering the breakdown of the propeller wake systems. At a great distance from the propeller, the vortex system loses all coherence and turns into a collection of smaller vortices that are equally scattered across the wake. In contrast, under light-loading conditions, the wake vortex system exhibits strong coherence and has a relatively simple topology. The elliptic instability and pairing processes are only observed at a far distance from the propeller. The convection velocity transferring tip vortices downstream is larger under the light-loading condition, which leads to the larger pitch of the helicoidal vortices. The larger pitch weakens the mutual inductance or interaction effects among tip vortices, which delays the instability behaviors of the whole vortex system. The results and implications of this study serve as a guide for the development and improvement of next-generation propellers that function optimally when operating behind aquaculture vessels.

Fluid–structure interaction modelling of a positive-displacement Total Artificial Heart

For those suffering from end-stage biventricular heart failure, and where a heart transplantation is not a viable option, a Total Artificial Heart (TAH) can be used as a bridge to transplant device. The Realheart TAH is a four-chamber artificial heart that uses a positive-displacement pumping technique mimicking the native heart to produce pulsatile flow governed by a pair of bileaflet mechanical heart valves. The aim of this work was to create a method for simulating haemodynamics in positive-displacement blood pumps, using computational fluid dynamics with fluid–structure interaction to eliminate the need for pre-existing in vitro valve motion data, and then use it to investigate the performance of the Realheart TAH across a range of operating conditions. The device was simulated in Ansys Fluent for five cycles at pumping rates of 60, 80, 100 and 120 bpm and at stroke lengths of 19, 21, 23 and 25 mm. The moving components of the device were discretised using an overset meshing approach, a novel blended weak–strong coupling algorithm was used between fluid and structural solvers, and a custom variable time stepping scheme was used to maximise computational efficiency and accuracy. A two-element Windkessel model approximated a physiological pressure response at the outlet. The transient outflow volume flow rate and pressure results were compared against in vitro experiments using a hybrid cardiovascular simulator and showed good agreement, with maximum root mean square errors of 15% and 5% for the flow rates and pressures respectively. Ventricular washout was simulated and showed an increase as cardiac output increased, with a maximum value of 89% after four cycles at 120 bpm 25 mm. Shear stress distribution over time was also measured, showing that no more than 4.5times 10^{-4}% of the total volume exceeded 150 Pa at a cardiac output of 7 L/min. This study showed this model to be both accurate and robust across a wide range of operating points, and will enable fast and effective future studies to be undertaken on current and future generations of the Realheart TAH.

Interceptor Impact on the Step Planing Hull: A Computational Investigation

Studies have shown that step modifications support the planing hull's ability to accelerate quickly; nevertheless, it also produces excessive drag and creates the hump area extremely trim. Engaging integrated with the interceptor will improve the efficiency of the step hull. Planing hull interceptors are placed at the transom stern to regulate the trim angle and reduce movements caused by seas. This study is focused on the effects of step hulls with interceptors on pressure distribution and how they affect the planing hull's drag, heave, and trim. The environment was modeled according to the two-degree of freedom condition to simulate trim and heave. A combination of an overset mesh and the Finite Volume Method (FVM), the Reynolds-Averaged Navier-Stokes equation, was used to evaluate this study. The water and air phases are represented by the turbulent K- and VOF (Volume of Fluid) models. The errors driven by grid spacing and time-step have been estimated using grid convergence studies. The numerical approach was tested experimentally by Park et al. to ensure accuracy. It has been determined through calculations of the drag, trim, and heave results that adding an interceptor to a step vessel is extremely valuable for minimizing drag and controlling trim

Numerical Analysis on Dynamic Behavior Characteristics of an Amphibious Assault Vehicle during Water Entry

In the present study, the dynamic behavior characteristics of an amphibious assault vehicle during water entry were analyzed using STAR-CCM+, a commercial computational fluid dynamics(CFD) code. All computations were performed using an overset mesh system and a RANS based flow-solver coupled with dynamic fluid-body interaction(DFBI) solver for simulating three degrees of freedom motion. For numerical validation of the solver, a water entry simulation of inclined circular cylinder was conducted and it was compared between an existing experiment data and CFD results. The pitch angle variation and the trajectory of the circular cylinder during water entry shows good agreement with previous experimental and numerical studies. For the water entry simulations of the amphibious assault vehicle, the analysis of dynamic behaviors of the amphibious assault vehicle with different slope angles, submerged depths and initial velocities were conducted. It is confirmed that the steep slope angle increases the submerged volume of the amphibious assault vehicle, so the buoyancy acting on the vehicle is increased and the moved distance for the re-flotation is decreased. It is also revealed that the submerged volume is increased, bow-up phenomenon occur earlier.

Research on the Hydrodynamic Performance of a Pentamaran in Calm Water and Regular Waves

As a typical multi-hull ship, the pentamaran has a wider deck, higher damage stability, and low-speed seaworthiness in harsh sea conditions compared with other multi-hull types such as the catamaran and trimaran, having more potential for becoming a new type of freight merchant ship in line with the development trend of the marine economy. In this paper, on the basis of the finite volume method, the numerical simulations and detailed comparative analysis of the monohull and pentamaran in viscous flow were carried out. The resistance prediction in calm water and regular waves and the changes of each resistance component of the pentamaran were explored. The dynamic fluid body interaction (DFBI) module was used for the calculations in calm water, and the overset mesh was used for the calculations in regular waves. The results show that in calm water, the adverse effect of the side hulls on the resistance was mainly reflected in the low-speed range. In the high-speed range, the influence of the side hulls on the resistance can be ignored. At high speeds, the main hull and side hulls of the pentamaran will produce favorable wave-making interference, thereby improving the wave-making performance of the pentamaran. In regular waves, although the added resistance of the pentamaran increased most of the time, its amplitude was smaller than that of the monohull under the same wave condition. This work provides a technical basis for the research on the design and hydrodynamic performance of the pentamaran.

Multiscale model for blood flow after a bileaflet artificial aortic valve implantation

Cardiovascular diseases are the leading cause of mortality in the world, mainly due to atherosclerosis and its consequences. The article presents the numerical model of the blood flow through artificial aortic valve. The overset mesh approach was applied to simulate the valve leaflets motion and to realize the moving mesh, in the aortic arch and the main branches of cardiovascular system. To capture the cardiac system’s response and the effect of vessel compliance on the outlet pressure, the lumped parameter model has been also included within the solution procedure. Three different turbulence modeling approaches were used and compared — the laminar, k-ϵ and k-ω model. The simulation results were also compared with the model excluding the moving valve geometry and the importance of the lumped parameter model for the outlet boundary condition was analyzed. Proposed numerical model and protocol was found as suitable for performing the virtual operations on the real patient vasculature geometry. The time-efficient turbulence model and overall solving procedure allows to support the clinicians in making decisions about the patient treatment and to predict the results of the future surgery.

Investigation of Inclusion Removal at Steel–Slag Interface toward a Small‐Scale Criterion for Particle Separation

Interactions between inclusion particles and the steel–slag interface directly affect the inclusion removal efficiency and thus influence steel cleanliness. Herein, the three‐phase interactions are resolved using the volume of fluid (VOF) method coupled with a dynamic overset mesh. The simulation is able to capture the instantaneous interface deformation and predict the particle motion driven by capillary force. The model validity is first demonstrated by comparison with analytical results. Then, a parameter study is conducted to examine the most influential factors governing the separation process. The results show that the system's wetting condition and the slag viscosity have a decisive effect on particle behavior at the interface (separation or entrapment). From an energy perspective, a better wetting condition generates more energy sources, and the interfacial energy is efficiently transformed into the particle's kinetic energy within a less viscous environment, thus leading to better separation. Besides, a criterion for predicting particle behavior is developed based on a modified Reynolds number (, relevant to fluid properties) and a quantity related to particle dynamics (ζ). The current work brings insights into the interfacial phenomenon during inclusion removal, which can be incorporated into large‐scale simulations to estimate the removal efficiency more accurately.

Numerical Study on the Water Entry of a Freely Falling Unmanned Aerial-Underwater Vehicle

The unmanned aerial–underwater vehicle (UAUV) is a new type of vehicle that can fly in the air and cruise in water and is expected to cross the free water surface several times to perform continuous uninterrupted observation and sampling. To analyze the hydrodynamic and motion characteristics of the vehicle, the whole water-entry process of a multi-degree-of-freedom UAUV with various velocity and pitch angle was investigated through a Reynolds-averaged Navier–Stokes method. The computational domain was meshed by trimmer cells. The relative movement between the vehicle and fluid domain was simulated using moving reference frame overset mesh to delineate the interaction region around vehicle body. To reduce the computational cost, adaptive mesh refinement and adaptive time-stepping strategy were used to capture the slamming pressure accurately with reasonable computational effort. First, convergence study is considered. Simulations of the vehicle with various initial velocities and different pitch angles were performed. The variable physical properties were analyzed, and detailed results through the time-varying force and velocity were shown. Initial velocity and pitch angle are found to significantly influence hydrodynamic behavior, including the time-varying force, while thickness ratio has a great impact on added mass and pressure. The results show that higher entry velocity results in greater peak vertical force. The transverse hydrodynamic characteristics for oblique water entry of the vehicle with varies pith angle are quite different.