Open-source Platform OpenFOAM Research Articles

The effect of the four-tentacled collaboration on the self-propelled performance of squid robot

This study conducts a numerical investigation of the self-propelled performance of a bio-inspired squid robot equipped with four rigid tentacles, exploring three sets of collaborative modes. Leveraging the open-source platform OpenFOAM, we develop a self-propulsion module incorporating the dynamic overset grid technique to manipulate the complex motion of rigid tentacles. The driving system of a single tentacle is simplified into a two-link mechanism, where the phase difference between the links effectively emulates the oscillatory pattern of fish-like locomotion. The interaction of four tentacles gives rise to three distinct driving modes: reverse, homologous, and interlace modes. The results indicate that the homologous mode follows the hydrodynamic characteristics of fish-like waves, the interlace mode can cause the robot to deviate from the initial path, and the reverse mode outperforms the other two modes, exhibiting a higher ultimate cruising speed. Regardless of the propulsion process, the cruising performance of the robot is significantly influenced by the maximum amplitude angle θmax. An increase in θmax also contributes to an elevation in the instantaneous longitudinal force coefficient CFx, with the most pronounced impact observed in the homologous mode. The disparity among the three modes is also evident in the periodic pressure variation and flow field evolution patterns. The vortex distribution during steady-state moments systematically reveals the collaborative effects among the tentacles in different modes on the self-propulsion performance.

Numerical simulation of spray atomization in crossflow using an empirical primary atomization model

The hybrid Eulerian-Lagrangian approach is used to simulate the atomization process of spray jet in subsonic crossflow. A coupled Volume of Fluid (VOF) - Lagrangian Particle Tracking (LPT) solver is implemented in the open-source platform OpenFOAM to solve the governing equations for the continuous and discrete phase respectively. An empirical model for primary atomization is proposed, in which the aerodynamic effect is taken into account, along with the turbulence effect. Thus, it is suitable for the flow at a low liquid-gas density ratio, i.e., less than 500. The performance of the presented model is evaluated and verified by comparisons of the simulated results with the experimental data. The flow characteristics of flow fields and distribution of droplet diameters are analyzed in detail. It is revealed that the vortex structures have a direct impact on the distribution of the droplets. The effects of the empirical constants in the atomization model are also examined. Finally, it is shown that the model performs better for the flow at a higher Weber number, in which the aerodynamic effect is more significant.

OpenFuelCell2: A new computational tool for fuel cells, electrolyzers, and other electrochemical devices and processes

Fuel cells/electrolyzers are efficient and clean electrochemical devices that convert chemical energy directly into electricity and vice versa. They have attracted sustainable attention over the past decade from multiple experimental and numerical studies. However, detailed experimental investigations are typically expensive and challenging for providing a number of operating conditions and designs. Computational analysis offers an alternative approach for these studies. With the steadily increasing high-performance computing resources available, the limitations of numerical simulations have substantially decreased. This contribution details the design choice and code structure of modern electrochemical devices, which have been implemented as a versatile C++ library named openFuelCell2 within the open-source platform OpenFOAM, allowing for large-scale parallel calculations to be performed. The solver considers the major transport phenomena in a typical electrochemical device, including fluid flow, heat and mass transfer, species and charge transfer, and electrochemical reaction. This enables numerical simulations on popular electrochemical devices, such as fuel cells and electrolyzers, to be conducted. The paper also describes the domain decomposition, and parallel performance issues, as well as future applications. Program summaryProgram Title:openFuelCell2CPC Library link to program files:https://doi.org/10.17632/cvmb5xgy7h.1Developer's repository link:https://github.com/openFuelCell2/openFuelCell2Licensing provisions: GPLv3Programming language: C++Journal reference of previous version: S.B. Beale, H.-W. Choi, J.G. Pharoah, H.K. Roth, H. Jasak, D.H. Jeon, Open-source computational model of a solid oxide fuel cell, Computer Physics Communications 200 (2016) 15–26.Does the new version supersede the previous version?: NoReasons for new version: This new version aims to account for additional electrochemical applications.Summary of revisions: The new version employs new code structure designs and is capable of considering two different electric potential fields and two-phase flows.Nature of problem: This software library provides a set of models for simulating electrochemical devices, such as fuel cells and electrolyzers, as well as other similar devices. These systems consist of multiple components with distinct features but are coupled through properties such as temperature, species concentrations, and electric current/potentials. This software utilizes a computational fluid dynamics (CFD) approach to enable the simulation of multiregion and multiphysics problems, accounting for heat and mass transfer, single and two-phase flow, multicomponent diffusion, electron and ion transfer, and electrochemical reactions. This toolbox provides researchers with a powerful tool for studying electrochemical devices in a comprehensive and efficient manner.Solution method: This software utilizes the finite volume method (FVM) to discretize and solve all conserved variables governed by partial differential equations. It employs a variant of the SIMPLE and PISO algorithms, known as PIMPLE, to solve the pressure-velocity coupling. All of the transport equations are solved sequentially, except for the enthalpy equation which is solved in an implicitly coupled manner within the global domain.Additional comments including restrictions and unusual features:•The two-phase flow consists of continuous and dispersed phases, without resolving the phase interfaces.•The two-phase flow is solved with an Eulerian-Eulerian approach.•A local-time-stepping method was typically used in the two-phase solution.•The electrochemical reactions take place in volumetric regions.•Species diffusion is described by Fick's law.

Implementation and Validation of Explicit Immersed Boundary Method and Lattice Boltzmann Flux Solver in OpenFOAM

In this work, the explicit boundary-condition-enforced immersed boundary method (EIBM) and the lattice Boltzmann flux solver (LBFS) are integrated into OpenFOAM to efficiently solve incompressible flows with complex geometries and moving boundaries. The EIBM applies the explicit technique to greatly improve the computational efficiency of the original boundary-condition-enforced immersed boundary method. In addition, the improved EIBM inherits the accurate interpretation of the no-slip boundary condition and the simple implementation from the original one. The LBFS uses the finite volume method to discretize the recovered macroscopic governing equations from the lattice Boltzmann equation. It enjoys the explicit relationship between the pressure and density, which avoids solving the pressure Poisson equation and thus saves much computational cost. Another attractive feature of the LBFS lies in its simultaneous evaluation of the inviscid and viscous fluxes. OpenFOAM, as an open-source CFD platform, has drawn increasing attention from the CFD community and has been proven to be a powerful tool for various problems. Thus, implementing the EIBM and LBFS into such a popular platform can advance the practical application of these two methods and may provide an effective alternative for complicated incompressible flow problems. The performance of the integrated solver in OpenFOAM is comprehensively assessed by comparing it with the widely used numerical solver in OpenFOAM, namely, the Pressure-Implicit with Splitting of Operators (PISO) algorithm with the IBM. A series of representative test cases with stationary and moving boundaries are simulated. Numerical results confirm that the present method does not have any streamline penetration and achieves the second-order accuracy in space. Therefore, the present method implemented in the open-source platform OpenFOAM may have good potential and can serve as a powerful tool for practical engineering problems.

Numerical Two-Phase Simulations of Alkaline Water Electrolyzers

Alkaline water electrolyzers (AWE) have several advantages over other types of electrolyzers, including their high efficiency and especially their relatively low cost due to the usage of non-precious metal catalysts, such as nickel and iron, for the electrodes. Information about local quantities and physical phenomena such as the formation of gas bubbles, current densities, temperatures or local species concentrations within a running cell are important for their improvement. Multiphysical computational fluid dynamics (CFD) simulations of electrochemical components using detailed three-dimensional models can provide valuable insight on local behaviors and characteristics that are difficult or impossible to measure experimentally.This work extends the CFD library openFuelCell2 1, which has been implemented using the open-source platform OpenFOAM®, to simulate AWE cells. The model considers the major transport phenomena, including two-phase fluid flow, heat and mass transfer, electrochemical reactions, species transfer and charge transfer in the various functional regions of the cell. It employs an Eulerian-Eulerian approach to characterize the behavior of each phase comprising interphase mass transport, momentum exchange and heat transfer. Appropriate mapping functions are used to couple the physically distinct regions together. A Butler-Volmer equation characterizes the electrochemical reactions that are assumed to occur in electrodes of finite thickness.This model is used to simulate a single zero-gap AWE cell, depicted in Fig. 1, for different operating conditions such as varying temperatures and volumetric flow rates. The conducted studies provide insight into the local formation of the created gas phase (bubbles), the distribution of species within the gas and electrolyte and their impact towards the performance of the running cell. These numerically obtained results are compared to in-house available and gathered experimental data. Figure 1 demonstrates that the polarization curves obtained at various temperatures are in good agreement with the experimental data. Figure 1

Influence of jet flow on hydrodynamic performance of a ducted propeller

This study introduces a concept that jet technology in the aeronautical field is used for active flow control to improve the hydrodynamic performance of a ducted propeller. Jet flow is added in front of the ducted propeller, and it produces a circumferential velocity that is opposite to the rotation direction of the rotor. An international standard ducted propeller was adopted to demonstrate this concept. The unsteady Reynolds-averaged Navier–Stokes method and the shear stress transport k−ω turbulence model were employed for the simulations. The open-source platform OpenFOAM was utilized. The overall efficiency η0 of the ducted propeller first increases and then decreases with increasing the jet flow velocity Rjf from 1 to 3 and the distance L to the rotation center from 0.2D to 0.4D. When the jet flow is at the optimal condition of Rjf=2 and L=0.3D, the maximum efficiency improvement of 3.1% is achieved for the ducted propeller. The reason is that the jet flow contributes to a pressure increase in the flow through the rotor. This effect is related to tip and hub vortices, which are disrupted by the jet flow and have relatively low vorticity magnitudes compared to the reference case without jet. The findings in this study have the potential to advance the development of active flow control technology for ships.

Numerical investigation of combustion instability in a liquid rocket engine: Interaction effect between hydrodynamics and acoustic mode

In this paper, we present the numerical evidence of multiple bifurcations in thermoacoustic instability as we decrease the equivalence ratio (ϕ) from fuel-rich (ϕ = 1.5) to fuel-lean (ϕ =0.59) within a single-element coaxial liquid rocket engine burning hydrogen peroxide and methane. An axisymmetric compressible Large Eddy Simulation solver in an open-source platform OpenFOAM combined with a global methane chemical kinetical mechanism was used to model the turbulent mixing and combustion process. After the validation of grid independence and simulation results, the synchronization phenomenon between hydrodynamic instability, acoustics and combustion is the main focus of this paper. The results here first revealed that in a direction of decreasing equivalence ratios, the first transition to limit cycle state from combustion noise state was observed via intermittency. A second bifurcation was also observed, leading to quasi-period thermoacoustic instability from the limit cycle state. Intrinsic vorticity modes and natural acoustic modes could be regarded as oscillators. The competition and cooperation between the two modes led to a phenomenon beyond limit cycle states. The vortex dynamics of thermoacoustic systems are dominated by the interaction of intrinsic vorticity modes and natural acoustic modes and exhibit an adjustment of vortex shedding frequencies, resulting in different stability regions. Herein, the flame index was performed in more detail to distinguish between the premixed flame and diffusion flame. Results indicated that the burning mode was an important driver in promoting instability whilst it also served as a medium for competition and cooperation between the natural acoustic modes and intrinsic vorticity modes.

Effects of the spacing between two hydrokinetic turbines on the bedforms by numerical simulations

Accelerating hydrokinetic renewable energy development towards endurance requires investigating interactions between the hydrokinetic turbine and its surrounding physical environment. Interactions between hydrokinetic turbines (HT) and mobile sediment bed are considered as a critical area of assessment, however limited research studies have been published to address this issue. Hill et al. (2015) have shown experimentally that the presence of either single or multiple turbines and the rotation of the blades affect the bed morphology. Musa et al. (2019) have investigated experimentally the local effect of streamwise aligned turbines on the bedload, they found as a result that the geomorphic effects are stronger with increasing shear stress due to the presence of the rotors, inducing an alternating scour-deposition phenomenon. Chen et al. (2017) have investigated the influence of rotor blade tip clearance (distance between blades and seabed) on the scour rate of pile-supported horizontal axis current turbine. The results suggest that the decrease in tip clearance increases the scour depth, hence more sediment transport. Recently, Khaled et al. (2021) have studied the impact of hydrokinetic turbine on erodible sand banks, they showed numerically a significant interaction between the confinement of the turbine and its impact on the near bottom.
 
 In the present issue, we study the impact of two interacted turbines on the near bedform morphology. A modelling framework is derived to predict the significant transport induced by the turbines, such as the Euler-Euler (EE) multiphase model for sediment transport and the Blade Element Method (BEM) to model the forces generated by the turbines, using the open source platform OpenFOAM and the library SedFoam (Chauchat et al. 2017). A phase of validation is presented for the combined model (EE and BEM) using experimental results of Hill et al. (2015). The present study consists in considering one sediment class, sand of diameter of 0.25 mm, and two horizontal axis turbines with an axial flow direction corresponding to the riverine case. The approach is configured with four different axial inter-turbines distances. The wake distribution behind the second turbine is altered by the wake of the upstream turbine in all configurations (fig. 2). This interaction promotes the erosion under the turbines and the deposition along the axis of the turbines in the wake due to relation between the dynamics of ripples generation and the wake effects of turbines (fig. 3).

Effects of the perturbation inlet on the evolution and oscillation characteristics of multiple rotating detonation waves

A numerical simulation using the open-source platform OpenFOAM is conducted to investigate the effect of periodic perturbations at the inlet on the evolution and oscillation characteristics of multiple rotating detonation waves (RDWs). The study uses a two-step chemical reaction mechanism of C2H4/Air. The results reveal that periodic perturbations have a significant effect on the pressure, detonation wave height, heat release rate, thrust, and specific impulse of the rotating detonation engine. The oscillation pattern of these parameters is found to be highly correlated with the form of the inlet perturbations. The study also examines the evolution mechanism of multiple detonation waves and identifies three primary propagation stages under inlet perturbations: chaotic propagation, self-adjustment, and oscillatory propagation. The multiple detonation waves originate from the chaotic propagation and are mainly adjusted in terms of the number and direction of detonation waves through the double-wave collision. The results demonstrate that increasing the amplitude of perturbations poses a significant risk to the survival of the RDW, while high-frequency perturbations may disrupt the three primary propagation stages, leading to predominantly the chaotic propagation. Furthermore, an analysis of the fast Fourier transform of the thrust and specific impulse shows that the oscillation forms of the specific impulse and thrust differ in the chaotic propagation stage.

Numerical investigation of the local impact of hydrokinetic turbine on sediment transport—Comparison between two actuator models

Hydrokinetic turbines interact with the dynamics of the sedimentary bottom at small and large scale. Despite the interest that the study of these interactions deserves, little research has been published in the field. In this paper, we investigate by numerical simulation the interaction between modeled hydrokinetic turbines and bed load under clear-water scour conditions. The mixture of water and sediment is accounted for by an Eulerian multiphase model developed in the open source platform OpenFOAM. The turbine blades are parameterized by two models: the Blade Element Method (BEM) and the Actuator Disc Theory (AD). A good agreement with measurements is obtained in the near wake. The effects of the two different turbine models on the bedload sediment transport and on the wake characteristics are then examined. The local impact of the turbine modeled by BEM is more pronounced on the bed than the one modeled by the AD. So, the BEM modeling is preferable to the AD one for local studies of the impact of a turbine on the bed morphology. However, combining BEM with the two-phase fluid-sediment is time consuming and make difficult the use of this method to study a farm of several stream turbines. In such a case the use of AD is still remains recommended for now. Moreover, more studies must be done at real scale as the turbine rotational speed is relatively low and could induced less impact of the swirl on the bottom shear stress.

A Simulation Framework for Electrochemical Processes with Electrolyte Flow

Software tools for simulating electrochemical processes (e.g. COMSOL Multiphysics, ELSYCA) are mostly of commercial type. Besides, three-dimensional simulations in complex cell geometries are known to become resource-expensive, as typically thin concentration boundary layers need to be resolved. This work presents a simulation framework for electrochemical processes based on the open source platform OpenFOAM. The finite volume method used and combined with domain decomposition is able to efficiently benefit from multi-core computer architectures. Our framework takes into account electrolyte flow, which is well known to affect mass transfer, and allows to consider multi-species electrolytes and forcing of the electrolyte. The stability and fast convergence of the method presented is found to rely on the linearization of the Butler-Volmer condition in the iterative solver. The framework is validated against an analytical solution valid for simplified conditions and an electrodeposition process at a conically shaped electrode in an external magnetic field. The latter exhibits transient departure of the concentration boundary layer from the cathode, and excellent agreement with COMSOL simulation results is found.

Large eddy simulation of combustion instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine: Intermittency route to period-2 thermoacoustic instability

This paper presents the first numerical evidence of an intermittency route to period-2 thermoacoustic instability in a subcritical single-element liquid rocket engine burning hydrogen peroxide/kerosene as we decrease the equivalence ratio (ϕ) from fuel-rich to fuel-lean. To achieve this, three-dimensional compressible large eddy simulation algorithms combined with the Euler–Lagrangian framework are used. A one-equation eddy sub-grid turbulence model with a partially stirred reactor sub-grid combustion model is employed to simulate the spray turbulent combustion process in a high-pressure liquid-fueled combustor based on open-source platform OpenFOAM. This paper focuses on examining the transition process of the dynamical states in the thermoacoustic system and the synchronization between multiple subsystems. The results indicate that, as the equivalence ratio reduces continuously (1.5 ≤ ϕ ≤ 0.5), the system dynamics shift from period-1 oscillations (ϕ = 1.5) to period-2 oscillations (ϕ = 0.5) via intermittency (1.3 ≤ ϕ ≤ 0.9). Under the equivalence ratio of 0.7 (ϕ = 0.7), a transient mode switching between period-1 and period-2 was also observed. The synchronization processes between the pressure and combustion subsystems in terms of phase-locking and frequency-locking are responsible for the emergence of complex dynamical states. The cycle snapshots analysis also provides more details on the synchronization processes between the pressure and the multiple subsystems, such as vortex dynamics, mixture fraction, and combustion heat release. In summary, this paper sheds light on the complex non-linear thermoacoustic oscillations and the underlying physical mechanisms related to the two-phase flow of spray combustion in liquid rocket engines using three-dimensional large eddy simulations, paving the way for developing passive or active control methods.

Three-dimensional thermal-hydraulic characteristics analysis of plate-type fuel reactor core based on OpenFOAM

In this paper, a full scale three-dimensional thermal-hydraulic analysis method for plate-type fuel reactor core is proposed through integrating traditional sub-channel analysis concepts into CFD platform. The coupled heat transfer model between fuel plates and narrow rectangular coolant channels is established. The three-dimensional thermal-hydraulic analysis code CorTAF-PT is developed based on open-source CFD platform OpenFOAM. The CorTAF-PT is validated against with the Qiu-TAKAHASHI experiment and IAEA 10 MW MTR. The code simulation results agree well with the experiment and MTR operation data with relative error within 5%. The full-scale MTR simulation under steady state and LOFA is carried out through channel-level resolution. In partial blockage accident, maximum temperature of coolant, fuel pellet and cladding increase by 9.75 K, 60.65 K and 60.52 K respectively. Furthermore, the double temperature peaks of fuel plates and local coolant reflux with −0.121 m s−1 are revealed under partial blockage accident. This work provides reference value for correlational studies on thermal-hydraulic characteristics of plate-type fuel reactor core.

Immiscible two-phase model for air blasts created during natural avalanches

An immiscible two-phase model based on the incompressible Navier-Stokes (N-S) equations is used to simulate the air blast generated by an avalanche. For simplicity, the avalanche is treated as an assembly of monodisperse spherical grains and described as a continuous media. The constitutive law of local µ(I) rheology is introduced to model the moving granular material. The motion of the avalanche and the induced air blast fits into a unified framework that combines the N-S−type governing equations with a µ(I)-rheology−based kinematic viscosity and a constant viscosity. The avalanche-air interface is treated using the volume-of-fluid method. A numerical program was developed on the open-source platform OpenFOAM specifically for this model to simulate the entire evolutionary process of the avalanche as well as the air blast generated. The model was validated by comparing the results of numerical simulations with those from inclined-plane laboratory experiments. With terrain input from the Shuttle Radar Topography Mission data, the model was further applied to simulate the air blast generated in two natural avalanches, namely, the Baige and Wenjia valley avalanches fo China, which occurred in 2008 and 2018, respectively. The simulation results were found to be consistent with field observations following a statistical analysis of the properties of the air blast including flow speed and area of impact of the above-mentioned natural events.

Numerical study of statistically developed gas-liquid intermittent flows by the lens of three-dimensional simulations

Two-phase intermittent flows are characterized by an alternation between a region almost fully occupied by liquid (a liquid plug) and another one composed by a gas bubble. This type of flow is frequently found in industry, particularly in the transportation of oil and gas through pipelines. Their dynamics is commonly modeled with a one-dimensional approach, since long pipelines are employed, making 2D or 3D simulations unfeasible. The models always require an assessment of wall and interfacial friction factors, which are usually obtained by empirical correlations. In this work, from full 3D velocity fields across statistically developed slug unit cells, flow parameters needed for 1D models were determined. For that, several three-dimensional numerical simulations were performed using the open source platform OpenFOAM®. The two-phase flow was determined within a computational domain corresponding to a single slug unit cell, with periodic boundary conditions under imposed pressure gradient and overall void fraction, employing the Volume of Fluid (VOF) method. Both laminar and turbulent flows were studied. Turbulence was modeled with the k-ω Shear Stress Transport (SST) model. The axial velocity profile along the cross section, as well as slug parameters, were compared with available experimental data, showing a quite good agreement. From the results validated by experimental data, friction factors and momentum flux parameters were determined, allowing the development of corrections to improve the available 1D models.

Influence of Vibration–Dissociation Coupling and Number of Reactions in Hypersonic Nonequilibrium Flows

Abstract During the atmospheric cruising of a hypersonic vehicle, the thermal and chemical nonequilibrium effects characterize the flow field within the shock layer. Therefore an understanding of nonequilibrium flow is essential for the efficient design of a hypersonic vehicle. The present numerical study uses various canonical configurations to study the thermochemical nonequilibrium effects in hypersonic flows. The present study investigates the influence of the vibration–dissociation (V–D) coupling method and the number of reactions on shock standoff distance (SSD), vibrational relaxation process, and surface properties. A finite volume method-based solver using the open-source platform openfoam has been developed to analyze the thermochemical nonequilibrium effects in the hypersonic flow field. The current results show that thermal and chemical nonequilibrium flow assumptions significantly affect SSD, and hence these assumptions are necessary to study the cases with a higher degree of nonequilibrium. The number of reactions influences the vibrational relaxation of diatomic gases in the air. At the same time, the V–D coupling method used to calculate reaction rate constants has a negligible impact on the vibrational relaxation process. Moreover, the V–D coupling method and the number of reactions marginally affect surface pressure. However, in the case of surface heat flux, the 11 reaction model predicts higher peak values than the 17 reaction model.

Coupled neutronics and thermal-hydraulics transient simulation of a gas-cooled reactor in the aircraft nuclear propulsion system

Aircraft nuclear propulsion (ANP) has many historical explorations, but also features with high power density and high operating temperature. These challenges lead to numerical investigations for the transient behaviors of gas-cooled reactors to be used for ANP system. This prismatic-block type reactor is featured by a large number of internal discrete cooling holes with high aspect ratio. When using conjugate heat transfer (CHT) model in CFD analyses for the entire solid core of such type of reactors, the refined mesh resolution near the interface between the fuel matrix and the flow channels leads to high computational costs. Therefore, we proposed a new porous heat transfer (PHT) model with Newton’s law of cooling to predict three-dimensional distributions of temperature in the solid and fluid domain of the reactor core. The verification of the PHT model is conducted in the presented study firstly, and shows that the accuracy of this model is deeply influenced by the prediction of convective heat transfer coefficients. Sequentially, a neutronics (N) and thermal-hydraulics (TH) transient coupling code is developed in the open-source platform OpenFOAM. The point kinetic model is employed to evaluate the dependency of heat power on the time. Doppler broadening effect is considered in the coupling process by delivering fuel average temperatures. Then coupled N/TH simulations are carried out to analyze the gas-cooled reactor during the start-up process and the blockage accident. The obtained results show that the wall temperature is always lower than the design limit even in a rapid start-up process, indicating that the reactor is safe at extreme operations. Moreover, simulation of the blockage accident shows that if several flow channels are totally occupied by debris, the fuel temperature increment is less than 300 K, and the position of the peak fuel temperature transforms from the core center to the zone near the blockage.

Transient characteristics of coupled thermal radiation and natural convection in a three-dimensional cylindrical cavity containing a heated plate

In this study, a numerical study of transient combined natural convection and surface radiation in a cylindrical cavity with a heated plate placed inside is carried out using the open-source platform OpenFOAM. The effects of the Rayleigh number (105 ? Ra ? 107), inclination angle (0? ? ? ? 90?), and surface emissivity (0 ? ? ? 1) on the velocity components, temperature field, and Nusselt number are investigated in detail. Results show that the Rayleigh number has a greater effect on the radiative heat transfer than on natural convection. Radiative heat transfer increases monotonously with the increase of surface emissivity, and its contribution could be over 50% of the total heat transfer rate. The inclination angle of the plate affects the flow structure, but the difference in total Nusselt number is less than 10% for various inclination angles. Results in this study provide insights into the transient characteristics of coupled thermal radiation and natural convection in a three-dimensional cavity and will guide the optimal design of related devices.

Numerical analysis of electrohydrodynamic instability in dielectric-liquid-gas flows subjected to unipolar injection.

In this work, the electrohydrodynamic instability induced by a unipolar charge injection is extended from a single-phase dielectric liquid to a two-phase system that consists of a liquid-air interface. A volume-of-fluid model-based two-phase solver was developed with simplified Maxwell equations implemented in the open-source platform OpenFOAM. The numerically obtained critical value for the linear stability matches well with the theoretical values. To highlight the effect of the slip boundary at interface, the deformation of the interface is ignored. A bifurcation diagram with hysteresis loop linking the linear and finite-amplitude criteria, which is U_{f}=0.059, was obtained in this situation. It is concluded that the lack of viscous effect at interface leads to a significant increase in the flow intensity, which is the reason for the smaller instability threshold in two-phase system. The presence of interface also changes the flow structure and results in a shear distribution of electric force, which may play an important role in the interface deformation.

MODEL- AND FULL-SCALE RANS SIMULATIONS FOR A DRIFT TANKER

The flow around a full-scale (FS) ship can be simulated by means of Reynolds-Averaged Naiver-Stokes (RANS) method, which provides a way to obtain more knowledge about scale effects on ship hydrodynamics. In this work, the viscous flow around a static drift tanker in full scale is simulated by using the RANS solver based on the open source platform OpenFOAM. The k - w SST model is employed to approximate the eddy viscosity. To reduce computational time, wall function approach is applied for the FS simulation. The flow around the ship in model scale is simulated as well, but without using any wall function, i.e., using Low-Reynolds number mode. In order to verify the computations, de- tailed studies on the computational grid including investigation of the sensitivity of computed forces to y+ (dimension- less distance of first grid point to wall) and grid dependency study are carried out. The computed forces are compared with available measured data. The scale effects are analysed and discussed by comparisons.