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•We employed neural networks for the prediction of 2D porous media properties.•We setup a workflow for the creation of a dataset of CFD simulations on OpenFOAM.•The results of CFD ...simulations were the training data for the neural networks.•The permeability and the filtration rate were efficiently predicted by the networks.•The neural networks predicted better the results compared to analytical correlations.
In this work we developed an open-source work-flow for the construction of data-driven models from a wide Computational Fluid Dynamics (CFD) simulations campaign. We focused on the prediction of the permeability of bidimensional porous media models, and their effectiveness in filtration of a transported colloidal species. CFD simulations are performed with OpenFOAM, where the colloid transport is solved by the advection–diffusion equation. A campaign of two thousands simulations was performed on a HPC cluster, the permeability is calculated from the simulations with Darcy’s law and the filtration (i.e. deposition) rate is evaluated by an appropriate upscaled parameter. Finally a dataset connecting the input features of the simulations with their results is constructed for the training of neural networks, executed on the open-source machine learning platform Tensorflow (integrated with Python library Keras). The predictive performance of the data-driven model is then compared with the CFD simulations results and with traditional analytical correlations.
Violent fluid oscillations occur inside narrow gaps between closely side-by-side structures, threating the safety of the operations. In the current study, fluid resonance in the gap between two ...side-by-side different boxes is investigated by a two-dimensional viscous-flow numerical wave tank, which can simultaneously predict the viscous damping and free-surface nonlinearity. The accuracy of the established numerical model is fore validated by the published experimental data. The upstream box keeps fixed and has six varied breadths, the downstream box heaves freely under wave actions and remains a constant breadth. The current work aims to study the effects of the breadth ratio and heave motion of the lee-side box on the hydrodynamics of gap resonance. The wave amplification in the gap, the heave motions of the lee-side box, the reflection, transmission, and energy loss coefficients, and the response time and damping time are concerned. Results show that the gap resonance frequency is insensitive to the variation of the breadth ratio. As the breadth ratio increases from 0.4 to 1.4. The maximum wave height amplification in the gap reduces more than 25%. The maximum response time of the fluid resonance occurs when two boxes have equal breadth.
•OpenFOAM® is employed to study the steady-state gap resonance occurring in-between two non-identical side-by-side boxes.•Coupled effects of the breadth ratio between two boxes and the heave motion of the lee-side box are mainly focused.•The gap resonant frequency is found to be insensitive to the variation of the breadth ratio.•The wave height amplification in the gap and the heave motion of the lee-side box decrease as the breadth ratio increases.•Both the transmission coefficient and the energy loss coefficients decrease as well with the rise of the breadth ratio.
The Oscillating Water Column (OWC) wave energy converter has been shown to have high potential, thus rendering extensive development in recent years. In order to further accelerate its development, ...highly accurate yet computationally efficient tools are necessary particularly when studying the interaction of multiple OWC devices. This paper proposes a new framework for fixed OWC devices with an orifice, that uses the input from a high fidelity non-linear numerical model to improve the accuracy of a low fidelity linear numerical model keeping computational costs low. This is done by accounting for the non-linearities in the pressure-flow of an orifice in the input to the linear numerical model. Experimental data is used to validate the framework, thus providing an accurate and computationally efficient linear numerical model, that can be used for the preliminary analysis of fixed OWC devices.
•A new framework combining low-fidelity and high-fidelity numerical methods for fixed OWC systems.•Accounting for non-linearities in pressure-flow in an orifice in a linear BEM model.•Validation with experimental data.•Use of validated open-source codes.
•Cavitating flow around a 3-D Hydrofoil with Wavy Leading Edges (WLE) is investigated using the LES approach in OpenFOAM.•Comparison of cavity features of wavy leading edges (WLE) hydrofoil with ...straight leading edge (SLE) one.•Force coefficients, re-entrant jet and separation are assessed during the cloud cavity evolution around WLE and SLE hydrofoils.•The mechanisms of the laminar separation bubble (LSB) and low-pressure zone behind the WLE hydrofoil are illustrated.•Role of vorticity stretching and vorticity dilatation with the presence of cavity for the WLE and SLE hydrofoils are investigated.
The present study seeks to conduct numerical investigations of the cavitating flow characteristics around a sinusoidal wavy leading edge (WLE) 3-D hydrofoil underlying a NACA 634–021 profile with an aspect ratio of 4.3. Cavitational and non-cavitational characteristics of hydrofoils are numerically examined at a chord-based Reynolds number of 7.2 × 105. The sinusoidal leading edge geometries include two WLE amplitudes of 5% and 25% and two WLE wavelengths of 25% and 50% of the mean chord length. We examined the cavitating flow around the hydrofoils in different cavitation numbers, namely σ = 0.8 and σ = 1.2. The flow over the protuberances of the WLE hydrofoil is considered at varying chord lengths and a constant angle of attack α = 6°, where significant spanwise variations in all flow properties, in contrast to the straight leading edge (SLE) hydrofoil, were observed. Large eddy simulation (LES) and Kunz mass transfer models are employed to simulate the dynamic and unsteady behavior of the cavitating flow. Besides, the compressive volume of fluid (VOF) method is used to track the cavity interface. Simulation is performed under the two-phase flow solver —interPhaseChangeFoam— of the OpenFOAM package. Compared to the SLE hydrofoil, we provided an exhaustive report of the time-averaged and instantaneous fluid dynamic characteristics of the cavitating flow around the sinusoidal leading edge hydrofoil, i.e., pressure, velocity, and vorticity fields, as well as lift and drag coefficients, and turbulent kinetic energy are reported. Furthermore, detailed analyses of the instantaneous cavity leading edge and flow separation treatment, vortical structure of the flow, vorticity stretching and dilatation, details of the spanwise flow, the formation of a low-pressure zone behind the WLE hydrofoil, streamwise velocity fluctuation, and evolution of the cavity dynamics through a complete cycle are reported. Results show that early development of the laminar separation bubble (LSBs) on the suction side of WLE hydrofoil prevents significant flow separation. Furthermore, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding.
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•Three node CHyQMOM is successfully implemented in OpenFOAM-7.•The algorithm can capture particle trajectory crossing without delta shocks.•For wall bounded problems results are not ...accurate.•Attention should be paid to prevent over-accumulation.
The modeling of dilute gas-particle flows is challenging due to particle-trajectory crossing (PTC). Lagrangian particle tracking can be used, but requires a large number of parcels resulting in high computational costs. A less costly method is the Eulerian number density function (NDF) approach, based on the Boltzmann equation, often solved in terms of lower-order moments of the NDF. In this context the conditional hyperbolic quadrature method of moments (CHyQMOM) was developed and is here implemented for the first time in the OpenFOAM-7, together with high-order advection schemes and a new operator splitting procedure. The resulting solver is used to simulate different test cases: phase segregation problems, collision-less and weakly-collisional PTC flows, asymmetric and symmetric Taylor-Green vortex flow and a dilute gas-particle riser. Results, validated against analytical solutions and predictions obtained with Lagrangian particle tracking, show that the implemented CHyQMOM can be used to handle highly non-equilibrium flows.
In this work, the performance of a buoyancy-modified turbulence model is shown for simulating wave breaking in a numerical wave flume. Reynolds-Averaged Navier-Stokes (RANS) modelling is performed by ...applying both a k-ω and a k-ω SST turbulence model using the Computational Fluid Dynamics (CFD) toolbox OpenFOAM. In previous work of the authors (Devolder et al., 2017), the observed significant decrease in wave height over the length of the numerical wave flume based on RANS turbulence modelling for the case of propagating waves has been avoided by developing a buoyancy-modified k-ω SST model in which (i) the density is explicitly included in the turbulence transport equations and (ii) a buoyancy term is added to the turbulent kinetic energy (TKE) equation. In this paper, two buoyancy-modified turbulence models are applied for the case of wave breaking simulations: k-ω and k-ω SST. Numerical results of wave breaking under regular waves are validated with experimental data measured in a wave flume by Ting and Kirby (1994). The numerical results show a good agreement with the experimental measurements for the surface elevations, undertow profiles of the horizontal velocity and turbulent kinetic energy profiles. Moreover, the underlying motivations for the concept of a buoyancy-modified turbulence model are demonstrated by the numerical results and confirmed by the experimental observations. Firstly, the buoyancy term forces the solution of the flow field near the free water surface to a laminar solution in case of wave propagation. Secondly in the surf zone where waves break, the buoyancy term goes to zero and a fully turbulent solution of the flow field is calculated. Finally and most importantly, the buoyancy-modified turbulence models significantly reduce the common overestimation of TKE in the flow field.
•Simulations of wave breaking in a numerical wave flume using OpenFOAM®.•Performance of a buoyancy-modified k-ω and k-ω SST turbulence model are evaluated.•The buoyancy-modified turbulence models are validated with experimental results.•Accurate predictions of the turbulent flow field at the wave breaking zone.
•Unsteady sheet/cloud cavitating flows are studied using the compressible cavitation solver developed on OpenFOAM.•Cavitation fluid compressibility effects on transient flow structures and dynamics ...are illustrated.•Effects of density variance on velocity divergence are proposed.•The physics for the compressibility effects on cavitation vortex dynamics are investigated based on VTE budget analysis.
The objective of the paper is to investigate the physics involved in the compressible cavitating flows, with emphasis on the compressibility effects. 3-D numerical simulations were conducted on the open source software platform OpenFOAM, using both the native incompressible cavitation solver interPhaseChangeFOAM and implemented compressible cavitation solver, where the cavitation model and turbulence model are kept the same and differences of the two approaches mainly root in the density variances of pure liquid and pure vapor. Results are presented for the transient sheet/cloud cavitating flows around a Clark-Y hydrofoil fixed at attack of angle α=8° at inlet velocity U=10m/s and cavitation number σ=0.8, where both ensemble averaged statistics and transient characteristics are analyzed. Good agreement can be obtained using both the incompressible and compressible approaches when compared with the experiment data. While it is found that compared with the incompressible approach, the compressible approach can predict the unsteady cavitation evolution and cavity shedding frequency better. With the compressibility effects considered, the time averaged void fraction distribution decreases, and the cavity size (i.e. cavitation area) becomes smaller. The re-entrant flow thickness normalized by local cavity thickness predicted by the compressible approach is larger than that by the incompressible approach, indicating that the compressible approach can predict the re-entrant jet dynamics well. The velocity divergence analysis show that compared with that in incompressible approach, where velocity divergence mainly comes from the mass transfer between phases, in compressible approach, the velocity divergence originates from both the cavitation two-phase fluid compressibility and mass transfer, and the fluid density variance dominates in compressible results. Following, the budget analysis of vorticity transport equation (VTE) show that the vortex stretching term dominates the cavitation vortex dynamics. Compressibility effects will significantly increase the dilatation term and decrease baroclinic term by decreasing the misalignment between density gradient and pressure gradient. Finally, the temperature and density variance in different cavitation structures are presented.
This paper numerically investigates the effect of concentration gradient on the re-initiation process of H2/O2 detonation propagating from a vertical channel to a horizontal channel. Two different ...types of concentration gradient distribution are considered in this study: from fuel-rich near the upper wall to fuel-lean near the lower wall (gradient > 0), from fuel-rich near the lower wall to fuel-lean near the upper wall (gradient < 0). The results show that the detonation can be re-initiated successfully for the two different types of concentration gradient, but the formation modes of the transverse waves in the re-initiation process are different. When the gradient > 0, the transverse waves are mainly formed after the 2nd reflection. When the gradient < 0, the transverse waves are mainly formed after the 1st reflection. However, whether the concentration gradient > 0 or < 0, the transverse waves are mainly formed on the fuel-lean side. The earlier transverse waves are mainly originated from the transverse waves of the transverse detonation. The detonation speed under fuel-rich condition is much higher as predicted by 1D simulation in the current study, and a local reactant distribution from fuel-lean to fuel-rich is formed by the leading shock in the horizontal direction. Thus, the transverse waves of the transverse detonation on the fuel-lean side can overtake the Mach detonation. Furthermore, the number of the transverse waves decreases with steeper concentration gradient, leading to the cell structure more irregular and the detonation transition from multi-head mode to single-head mode.
Although OpenFOAM is a widely-used open source computational fluid dynamics (CFD) tool, it is limited to numerical simulations of multi-dimensional reacting/nonreacting flows at relatively-low ...pressures. This is not only because real-fluid models that can evaluate thermophysical properties at high pressures are not available in the thermophysicalModels library of OpenFOAM, but also because the existing mixing model cannot handle various mixing rules of real-fluid models. In the present study, we develop a novel algorithm applicable for a mixture model incorporating various mixing rules in OpenFOAM. Based on the new algorithm, we update the thermophysicalModels library of OpenFOAM 6.0 by implementing a set of real-fluid models such as the Soave-Redlich-Kwong/Peng-Robinson equation of state, Chung's model for dynamic viscosity and thermal conductivity, mixture averaged model for mass diffusivity using Takahashi's correction for binary diffusion coefficients at high pressure. The new library is validated against experimental data and is further assessed for compressible reacting flows by performing two-dimensional numerical simulations of axisymmetric laminar non-premixed counterflow flames and one-dimensional numerical simulations of premixed CH4/air flames at high pressures. The developed library can be used for any reacting flow solvers in OpenFOAM 6.0 that adopt a set of implemented real-fluid models.
Program title: Real-fluid thermophysicalModels
CPC Library link to program files:https://doi.org/10.17632/n8zb2wpjp6.1
Developer's repository link:https://github.com/danhnam11/realFluidThermophysicalModels-6
Licensing provisions: GPLv3
Programming language: C++
Nature of problem: The mixing rules required for evaluating the real-fluid based thermophysical properties of mixtures are relatively complicated and different from each other, while the existing thermophysicalModels library offers only a mixing rule based on the mass fraction weighted average of each species, for which overloading operator functions (“*” and “+=”) are utilized to update parameters for a mixture. However, this approach is not applicable for complicated mixing rules in the real-fluid models.
Solution method: To build a mixture class containing different mixing rules in different models such as the equation of states and thermodynamics/transport properties in OpenFOAM, we adopt void-type functions (parameter updating functions) to update parameters of a mixture instead of the original overloading operator functions in OpenFOAM. The parameters in the mixing rules that depend on pressure and temperature are handled by decomposition such that they can be updated in the mixture class.
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