Publications
420 results found
Adam A, Pavlidis D, Percival JR, et al., 2017, Dynamic Mesh Adaptivity for Immiscible Viscous Fingering, Pages: 788-802
The unstable displacement of one fluid by another in a porous medium occurs frequently in various branches of enhanced oil recovery. It is now well known that when the invading fluid is of lower viscosity than the resident fluid, the displacement front is subject to a Saffman-Taylor instability and is unstable to transverse perturbations. These instabilities can grow, leading to fingering of the invading fluid. Numerical simulation of viscous fingering is challenging. The physics is controlled by a complex interplay of viscous and diffusive forces and it is necessary to ensure physical diffusion dominates numerical diffusion to obtain converged solutions. This typically requires the use of high mesh resolution and high order numerical methods. This is computationally expensive, particularly in 3D. We use IC-FERST, a novel control volume finite element (CVFE) code that uses dynamic mesh adaptivity on unstructured meshes to simulate 2D and 3D viscous fingering with higher accuracy and lower computational cost than conventional methods. We provide evidence that these unstructured mesh simulations in fact yield better results that are less influenced by grid orientation error than their structured counterparts. We also include the effect of capillary pressure and show three examples that are very challenging to simulate using more conventional approaches.
Joulin C, Xiang J, Latham J-P, et al., 2017, A New Finite Discrete Element Approach for Heat Transfer in Complex Shaped Multi Bodied Contact Problems, 7th International Conference on Discrete Element Methods (DEM), Publisher: SPRINGER-VERLAG SINGAPORE PTE LTD, Pages: 311-327, ISSN: 0930-8989
Latham, Obeysekara, Xiang J, et al., 2016, Modelling hydro-geomechanical behaviour of fractured and fracturing rock masses: application to tunnel excavation-induced damage, Conferenze di Meccanica e Ingegneria delle Rocce
Salinas P, Pavlidis D, xie Z, et al., 2016, Improving the convergence behaviour of a fixed-point-iteration solver for multiphase flow in porous media, International Journal for Numerical Methods in Fluids, Vol: 84, Pages: 466-476, ISSN: 1097-0363
A new method to admit large Courant numbers in the numerical simulation of multiphase flow is presented.The governing equations are discretised in time using an adaptive -method. However, the use of implicitdiscretisations does not guarantee convergence of the non-linear solver for large Courant numbers. In thiswork, a double-fixed point iteration method with backtracking is presented that improves both convergenceand convergence rate. Moreover, acceleration techniques are presented to yield a more robust non-linearsolver with increased effective convergence rate. The new method reduces the computational effort bystrengthening the coupling between saturation and velocity, obtaining an efficient backtracking parameter,using a modified version of Anderson’s acceleration and adding vanishing artificial diffusion.
Mostaghimi P, Kamali F, Jackson MD, et al., 2016, Adaptive mesh optimization for simulation of immiscible viscous fingering, SPE Journal, Vol: 21, Pages: 2250-2259, ISSN: 1086-055X
Viscous fingering can be a major concern when waterflooding heavy-oil reservoirs. Most commercial reservoir simulators use low-order finite-volume/-difference methods on structured grids to resolve this phenomenon. However, this approach suffers from a significant numerical-dispersion error because of insufficient mesh resolution, which smears out some important features of the flow. We simulate immiscible incompressible two-phase displacements and propose the use of unstructured control-volume finite-element (CVFE) methods for capturing viscous fingering in porous media. Our approach uses anisotropic mesh adaptation where the mesh resolution is optimized on the basis of the evolving features of flow. The adaptive algorithm uses a metric tensor field dependent on solution-interpolation-error estimates to locally control the size and shape of elements in the metric. The mesh optimization generates an unstructured finer mesh in areas of the domain where flow properties change more quickly and a coarser mesh in other regions where properties do not vary so rapidly. We analyze the computational cost of mesh adaptivity on unstructured mesh and compare its results with those obtained by a commercial reservoir simulator on the basis of the finite-volume methods.
Yang P, Xiang J, Chen M, et al., 2016, The immersed-body gas-solid interaction model for blast analysis in fractured solid media, International Journal of Rock Mechanics and Mining Sciences, Vol: 91, Pages: 119-132, ISSN: 1873-4545
Blast-induced fractures are simulated by a novel gas-solid interaction model, which combines an immersed-body method and a cohesive zone fracture model. The approach employs a finite element fluid model and a combined finite-discrete element solid model. This model is fully coupled and simulates the whole blasting process including gas pressure impulse, shock wave propagation, gas expansion, fragmentation and burden movement phases. In the fluid model, the John-Wilkins-Lee equation of state is introduced to resolve the relationship between pressure and density of the highly compressible gas in blasts and explosions. A Q-scheme is used to stabilise the model when solving extremely high pressure situations. Two benchmark tests, blasting cylinder and projectile fire, are used to validate this coupled model. The results of these tests are in good agreement with experimental data. To demonstrate the potential of the proposed method, a blasting engineering simulation with shock waves, fracture propagation, gas-solid interaction and flying fragments is simulated.
Xiao D, Yang P, Fang F, et al., 2016, A non-intrusive reduced-order model for compressible fluid and fractured solid coupling and its application to blasting, Journal of Computational Physics, Vol: 330, Pages: 221-224, ISSN: 0021-9991
This work presents the first application of a non-intrusive reduced order method to model solid interacting with compressible fluid flows to simulate crack initiation and propagation. In the high fidelity model, the coupling process is achieved by introducing a source term into the momentum equation, which represents the effects of forces of the solid on the fluid. A combined single and smeared crack model with the Mohr–Coulomb failure criterion is used to simulate crack initiation and propagation. The non-intrusive reduced order method is then applied to compressible fluid and fractured solid coupled modelling where the computational cost involved in the full high fidelity simulation is high. The non-intrusive reduced order model (NIROM) developed here is constructed through proper orthogonal decomposition (POD) and a radial basis function (RBF) multi-dimensional interpolation method.The performance of the NIROM for solid interacting with compressible fluid flows, in the presence of fracture models, is illustrated by two complex test cases: an immersed wall in a fluid and a blasting test case. The numerical simulation results show that the NIROM is capable of capturing the details of compressible fluids and fractured solids while the CPU time is reduced by several orders of magnitude. In addition, the issue of whether or not to subtract the mean from the snapshots before applying POD is discussed in this paper. It is shown that solutions of the NIROM, without mean subtracted before constructing the POD basis, captured more details than the NIROM with mean subtracted from snapshots.
Xie Z, Pavlidis D, Salinas P, et al., 2016, A balanced-force control volume finite element method for interfacial flows with surface tension using adaptive anisotropic unstructured meshes, Computers and Fluids, Vol: 138, Pages: 38-50, ISSN: 0045-7930
A balanced-force control volume finite element method is presented for three-dimensional interfacial flows with surface tension on adaptive anisotropic unstructured meshes. A new balanced-force algorithm for the continuum surface tension model on unstructured meshes is proposed within an interface capturing framework based on the volume of fluid method, which ensures that the surface tension force and the resulting pressure gradient are exactly balanced. Two approaches are developed for accurate curvature approximation based on the volume fraction on unstructured meshes. The numerical framework also features an anisotropic adaptive mesh algorithm, which can modify unstructured meshes to better represent the underlying physics of interfacial problems and reduce computational effort without sacrificing accuracy. The numerical framework is validated with several benchmark problems for interface advection, surface tension test for equilibrium droplet, and dynamic fluid flow problems (fluid films, bubbles and droplets) in two and three dimensions.
Salinas P, Pavlidis D, Xie Z, et al., 2016, Dynamic unstructured mesh adaptivity for improved simulation of nearwellbore flow in reservoir scale models, 15th European Conference on the Mathematics of Oil Recovery, Publisher: EAGE
It is well known that the pressure gradient into a production well increases with decreasing distanceto the well and may cause downwards coning of the gaswater interface, or upwards coning ofwateroil interface, into oil production wells; it can also cause downwards coning of the water table,or upwards coning of a saline interface, into water abstraction wells. To properly capture the localpressure drawdown into the well, and its effect on coning, requires high grid or mesh resolution innumerical models; moreover, the location of the well must be captured accurately. In conventionalsimulation models, the user must interact with the model to modify grid resolution around wells ofinterest, and the well location is approximated on a grid defined early in the modelling process.We report a new approach for improved simulation of nearwellbore flow in reservoirscale modelsthrough the use of dynamic unstructured adaptive meshing. The method is novel for two reasons.First, a fully unstructured tetrahedral mesh is used to discretize space, and the spatial location of thewell is specified via a line vector. Mesh nodes are placed along the line vector, so the geometry ofthe mesh conforms to the well trajectory. The well location is therefore accurately captured, and theapproach allows complex well trajectories and wells with many laterals to be modelled. Second,the mesh automatically adapts during a simulation to key solution fields of interest such as pressureand/or saturation, placing higher resolution where required to reduce an error metric based on theHessian of the field. This allows the local pressure drawdown and associated coning to be capturedwithout userdriven modification of the mesh. We demonstrate that the method has wideapplication in reservoirscale models of oil and gas fields, and regional models of groundwaterresources.
Takabatake K, Sun X, Sakai M, et al., 2016, Numerical study on a heat transfer model in a Lagrangian fluid dynamics simulation, International Journal of Heat and Mass Transfer, Vol: 103, Pages: 635-645, ISSN: 0017-9310
Phenomena related to phase change heat transfer are often encountered in engineering. These phenomena are regarded to be complex, since not only phase transition from solid to liquid occurs but also movement of fluid interface has to be taken into consideration. Detailed numerical modeling of these complex systems is essential to better understand them and optimize industrial designs. Lagrangian methods are promising for simulating such complex systems. The Moving Particle Semi-implicit (MPS) method, which is one of the Lagrangian methods, is employed here to simulate the free surface fluid flows involving heat transfer and phase change. On the other hand, the existing MPS method could not apply Neumann boundary condition such as heat flux in the heat transfer simulations. This is because the surface direction could not be readily defined on the surface of the spherical fluid particles in the MPS method. Hence, prescribing the heat fluxes becomes problematic in the existing MPS method. To solve this problem, a new heat flux model is developed, where the divergence operator is applied in the heat transfer simulation. Simple verification tests are performed to demonstrate the heat flux model, where the calculation results are compared against analytically derived solutions. In addition, application of the signed distance function is also investigated in the heat transfer simulation for arbitrary shaped boundary. In simple verification tests, the computation results are shown to agree well with the analytical solutions. Consequently, adequacy of the novel heat transfer model developed here is shown in the Lagrangian fluid dynamics simulation.
Gomes JLMA, Pavlidis D, Salinas P, et al., 2016, A force-balanced control volume finite element method for multi-phase porous media flow modelling, International Journal for Numerical Methods in Fluids, Vol: 83, Pages: 431-445, ISSN: 1097-0363
A novel method for simulating multi-phase flow in porous media is presented. The approach is based on acontrol volume finite element mixed formulation and new force-balanced finite element pairs. The novelty ofthe method lies in: (a) permitting both continuous and discontinuous description of pressure and saturationbetween elements; (b) the use of arbitrarily high-order polynomial representation for pressure and velocityand (c) the use of high-order flux-limited methods in space and to time avoid introducing non-physicaloscillations while achieving high-order accuracy where and when possible. The model is initially validatedfor two-phase flow. Results are in good agreement with analytically obtained solutions and experimentalresults. The potential of this method is demonstrated by simulating flow in a realistic geometry composed ofhighly permeable meandering channels.
Fang F, Pain C, Navon I, et al., 2016, An efficient goal based reduced order model approach for targeted adaptive observations, International Journal for Numerical Methods in Fluids, Vol: 83, Pages: 263-275, ISSN: 0271-2091
An efficient adjoint sensitivity technique for optimally collecting targeted observations is presented. The targeting technique incorporates dynamical information from the numerical model predictions to identify when, where, and what types of observations would provide the greatest improvement to specific model forecasts at a future time. A functional (goal) is defined to measure what is considered important in modelling problems. The adjoint sensitivity technique is used to identify the impact of observations on the predictive accuracy of the functional, then placing the sensors at the locations with high impacts. The adaptive (goal) observation technique developed here has the following features: (1) over existing targeted observation techniques, its novelty lies in that the interpolation error of numerical results is introduced to the functional (goal) which ensures the measurements are a distance apart; (2) the use of proper orthogonal decomposition (POD) and reduced order modeling (ROM) for both the forward and backward simulations, thus reducing the computational cost; and (3) the use of unstructured meshes. The targeted adaptive observation technique, is developed here within an unstructured mesh finite element model (Fluidity). In this work, a POD ROM is used to form the reduced order forward model by projecting the original complex model from a high dimensional space onto a reduced order space. The reduced order adjoint model is then constructed directly from the reduced order forward model. This efficient adaptive observation technique has been validated with two test cases: a model of an ocean Gyre and a model of 2D urban street canyon flows.
Lin Z, Xiao D, Fang F, et al., 2016, Non-intrusive reduced order modelling with least squares fitting on a sparse grid, International Journal for Numerical Methods in Fluids, Vol: 83, Pages: 291-306, ISSN: 0271-2091
This article presents a non-intrusive reduced order model (NIROM) for general, dynamic partial differential equations. Based upon proper orthogonal decomposition (POD) and Smolyak sparse grid collocation, the method first projects the unknowns with full space and time coordinates onto a reduced POD basis. Then we introduce a new least squares fitting procedure to approximate the dynamical transition of the POD coefficients between subsequent time steps taking only a set of full model solution snapshots as the training data during the construction. Thus, neither the physical details nor further numerical simulations of the original PDE model is required by this methodology and the level of non-intrusiveness is improved compared to existing ROMs. Furthermore, we take adaptive measures to address the instability issue arising from reduced order iterations of the POD coefficients.This model can be applied to a wide range of physical and engineering scenarios and we test it on a couple problems in fluid dynamics. It is demonstrated that this reduced order approach captures the dominant features of the high fidelity models with reasonable accuracy while the computation complexity is reduced by several orders of magnitude.
Xiao D, fang F, pain C, et al., 2016, Non-intrusive Reduced Order Modelling of Waterflooding in Geologically Heterogeneous Reservoirs, ECMOR XV - 15th European Conference on the Mathematics of Oil Recovery
Obeysekara A, Lei Q, Salinas P, et al., 2016, A fluid-solid coupled approach for numerical modeling of near-wellbore hydraulic fracturing and flow dynamics with adaptive mesh refinement, 50th US Rock Mechanics/Geomechanics Symposium
Xiao D, Lin Z, Fang F, et al., 2016, Non-intrusive reduced order modeling for multiphase porous media flows using smolyak sparse grids, International Journal for Numerical Methods in Fluids, Vol: 83, Pages: 205-219, ISSN: 0271-2091
In this article, we describe a non-intrusive reduction method for porous media multiphase flows using Smolyak sparse grids. This is the first attempt at applying such an non-intrusive reduced-order modelling (NIROM) based on Smolyak sparse grids to porous media multiphase flows. The advantage of this NIROM for porous media multiphase flows resides in that its non-intrusiveness, which means it does not require modifications to the source code of full model. Another novelty is that it uses Smolyak sparse grids to construct a set of hypersurfaces representing the reduced-porous media multiphase problem. This NIROM is implemented under the framework of an unstructured mesh control volume finite element multiphase model. Numerical examples show that the NIROM accuracy relative to the high-fidelity model is maintained, whilst the computational cost is reduced by several orders of magnitude.
Savre J, Percival J, Herzog M, et al., 2016, Two-Dimensional Evaluation of ATHAM-Fluidity, a Nonhydrostatic Atmospheric Model Using Mixed Continuous/Discontinuous Finite Elements and Anisotropic Grid Optimization, Monthly Weather Review, Vol: 144, Pages: 4349-4372, ISSN: 0027-0644
This paper presents the first attempt to apply the compressible nonhydrostatic ATHAM-Fluidity solver to a series of idealized atmospheric test cases. ATHAM-Fluidity uses a hybrid finite-element discretization where pressure is solved on a continuous 2nd order grid while momentum and scalars are computed on a 1st order discontinuous grid (also known as 1DG–2). ATHAM-Fluidity operates on two- and three-dimensional unstructured meshes, using triangular or tetrahedral elements respectively, with the possibility to employ an anisotropic mesh optimization algorithm for automatic grid refinement and coarsening during run-time. The solver is evaluated using two-dimensional only dry idealized test cases covering a wide range of atmospheric applications. The first three cases, representative of atmospheric convection, reveal the ability of ATHAM-Fluidity to accurately simulate the evolution of large scale flow features in neutral atmospheres at rest. Grid convergence without adaptivity as well as the performances of the Hermite-WENO slope limiter are discussed. These cases are also used to test the grid optimisation algorithm implemented in ATHAM-Fluidity. Adaptivity can result in up to a six-fold decrease in computational time and a five-fold decrease in total element number for the same finest resolution. However, substantial discrepancies are found between the uniform and adapted grid results, thus suggesting the necessity to improve the reliability of the approach. In the last three cases, corresponding to atmospheric gravity waves with and without orography, the model ability to capture the amplitude and propagation of weak stationary waves is demonstrated. This work constitutes the first step towards the development of a new comprehensive limited area atmospheric model.
Adam A, Pavlidis D, Percival J, et al., 2016, Higher-order conservative interpolation between control-volume meshes: Application to advection and multiphase flow problems with dynamic mesh adaptivity, Journal of Computational Physics, Vol: 321, Pages: 512-531, ISSN: 1090-2716
A general, higher-order, conservative and bounded interpolation for the dynamic and adaptive meshing of control-volume fields dual to continuous and discontinuous finite element representations is presented. Existing techniques such as node-wise interpolation are not conservative and do not readily generalise to discontinuous fields, whilst conservative methods such as Grandy interpolation are often too diffusive. The new method uses control-volume Galerkin projection to interpolate between control-volume fields. Bounded solutions are ensured by using a post-interpolation diffusive correction. Example applications of the method to interface capturing during advection and also to the modelling of multiphase porous media flow are presented to demonstrate the generality and robustness of the approach.
du J, Zhu J, Fang F, et al., 2016, Ensemble data assimilation applied to an adaptive mesh ocean model, International Journal for Numerical Methods in Fluids, Vol: 82, Pages: 997-1009, ISSN: 0271-2091
In this study, a first attempt has been made to introduce mesh adaptivity into the ensemble Kalman fiter (EnKF) method. The EnKF data assimilation system was established for an unstructured adaptive mesh ocean model (Fluidity, Imperial College London). The mesh adaptivity involved using high resolution mesh at the regions of large flow gradients and around the observation points in order to reduce the representativeness errors of the observations. The use of adaptive meshes unavoidably introduces difficulties in the implementation of EnKF. The ensembles are defined at different meshes. To overcome the difficulties, a supermesh technique is employed for generating a reference mesh. The ensembles are then interpolated from their own mesh onto the reference mesh. The performance of the new EnKF data assimilation system has been tested in the Munk gyre flow test case. The discussion of this paper will focus on (a) the development of the EnKF data assimilation system within an adaptive mesh model and (b) the advantages of mesh adaptivity in the ocean data assimilation model.
Yang P, Xiang J, Fang F, et al., 2016, Modelling of fluid–structure interaction with multiphase viscous flows using an immersed-body method, Journal of Computational Physics, Vol: 321, Pages: 571-592, ISSN: 1090-2716
An immersed-body method is developed here to model fluid–structure interaction for multiphase viscous flows. It does this by coupling a finite element multiphase fluid model and a combined finite–discrete element solid model. A coupling term containing the fluid stresses is introduced within a thin shell mesh surrounding the solid surface. The thin shell mesh acts as a numerical delta function in order to help apply the solid–fluid boundary conditions. When used with an advanced interface capturing method, the immersed-body method has the capability to solve problems with fluid–solid interfaces in the presence of multiphase fluid–fluid interfaces. Importantly, the solid–fluid coupling terms are treated implicitly to enable larger time steps to be used. This two-way coupling method has been validated by three numerical test cases: a free falling cylinder in a fluid at rest, elastic membrane and a collapsing column of water moving an initially stationary solid square. A fourth simulation example is of a water–air interface with a floating solid square being moved around by complex hydrodynamic flows including wave breaking. The results show that the immersed-body method is an effective approach for two-way solid–fluid coupling in multiphase viscous flows.
Xiao D, Yang P, Fang F, et al., 2016, Non-intrusive reduced order modelling of fluid-structure interactions, Computer Methods in Applied Mechanics and Engineering, Vol: 303, Pages: 35-54, ISSN: 0045-7825
A novel non-intrusive reduced order model (NIROM) for fluid–structure interaction (FSI) has been developed. The model is based on proper orthogonal decomposition (POD) and radial basis function (RBF) interpolation method. The method is independent of the governing equations, therefore, it does not require modifications to the source code. This is the first time that a NIROM was constructed for FSI phenomena using POD and RBF interpolation method. Another novelty of this work is the first implementation of the FSI NIROM under the framework of an unstructured mesh finite element multi-phase model (Fluidity) and a combined finite-discrete element method based solid model (Y2D).The capability of this new NIROM for FSI is numerically illustrated in three coupling simulations: a one-way coupling case (flow past a cylinder), a two-way coupling case (a free-falling cylinder in water) and a vortex-induced vibration of an elastic beam test case. It is shown that the FSI NIROM results in a large CPU time reduction by several orders of magnitude while the dominant details of the high fidelity model are captured.
Buchan AG, Pain CC, 2016, An efficient space-angle subgrid scale discretisation of the neutron transport equation, Annals of Nuclear Energy, Vol: 94, Pages: 440-450, ISSN: 1873-2100
Heaney CE, Buchan AG, Pain CC, et al., 2016, A reduced order model for criticality problems in reactor physics varyingcontrol rod settings, Proceedings of the 24 th UK Conference of the Association for Computational Mechanics in Engineering
Souli M, Kultsep AV, Al-Bahkali E, et al., 2016, Arbitrary Lagrangian Eulerian Formulation for Sloshing Tank Analysis in Nuclear Engineering, Nuclear Science and Engineering, Vol: 183, Pages: 126-134, ISSN: 0029-5639
Fluid-structure interaction plays an important role in nuclear engineering design, where several numerical and experimental tests need to be performed on new tank design before getting into the production process. The design can be performed for fluid storage tanks that require knowledge of sloshing frequencies and hydrodynamic pressure distribution on the structure. These can be very useful for engineers and designers to define appropriate material properties and shell thickness of the structure to be resistant under seismic loading. Data presented in current tank seismic design codes such as Eurocode are based on simplified assumptions for the geometry and material tank properties. Fuel tanks may undergo different types of loading, including seismic loading, where the behavior of storage tanks includes material nonlinearities, which are caused by material yielding. The Arbitrary Lagrangian Eulerian formulation based on finite element analysis presented in the paper takes into account material properties of the structure as well as the complex geometry of the tank. The formulation uses a moving mesh with a mesh velocity defined through the structure motion. In this paper, we use different approaches to solve a fluid-structure coupling problem. The first one uses the full Navier-Stokes equation for the fluid with projection method, and the second approach uses potential flow theory. The problem consists of a sloshing deformable tank submitted to acceleration loading.
Pavlidis D, Gomes JLMA, Xie Z, et al., 2016, Compressive advection and multi-component methods for interface-capturing, International Journal for Numerical Methods in Fluids, Vol: 80, Pages: 256-282, ISSN: 0271-2091
This paper develops methods for interface-capturing in multiphase flows. The main novelties of these methods are as follows: (a) multi-component modelling that embeds interface structures into the continuity equation; (b) a new family of triangle/tetrahedron finite elements, in particular, the P1DG-P2(linear discontinuous between elements velocity and quadratic continuous pressure); (c) an interface-capturing scheme based on compressive control volume advection methods and high-order finite element interpolation methods; (d) a time stepping method that allows use of relatively large time step sizes; and (e) application of anisotropic mesh adaptivity to focus the numerical resolution around the interfaces and other areas of important dynamics. This modelling approach is applied to a series of pure advection problems with interfaces as well as to the simulation of the standard computational fluid dynamics benchmark test cases of a collapsing water column under gravitational forces (in two and three dimensions) and sloshing water in a tank. Two more test cases are undertaken in order to demonstrate the many-material and compressibility modelling capabilities of the approach. Numerical simulations are performed on coarse unstructured meshes to demonstrate the potential of the methods described here to capture complex dynamics in multiphase flows.
Vire A, Spinneken J, Piggott MD, et al., 2016, Application of the immersed-body method to simulatewave–structure interactions, European Journal of Mechanics B: Fluids, Vol: 55, Pages: 330-339, ISSN: 1873-7390
This study aims at demonstrating the capability of the immersed-body method to simulate wave–structure interactions using a non-linear finite-element model. In this approach, the Navier–Stokes equations are solved on an extended mesh covering the whole computational domain (i.e. fluids and structure). The structure is identified on the extended mesh through a nonzero solid-concentration field, which is obtained by conservatively mapping the mesh discretising the structure onto the extended mesh. A penalty term relaxes the fluid and structural velocities to one another in the regions covered by the structure. The paper is novel in that it combines the immersed-body method with wave modelling and mesh adaptivity. The focus of the paper is therefore on demonstrating the capability of this new methodology in reproducing well-established test cases, rather than investigating new physical phenomena in wave–structure interactions. Two cases are considered for a bottom-mounted pile. First, the pile is placed in a numerical wave tank, where propagating waves are modelled through a free-surface boundary condition. For regular and irregular waves, it is shown that the wave dynamics are accurately modelled by the computational fluid dynamics model and only small discrepancies are observed in the close vicinity of the structure. Second, the structure is subjected to a dam-break wave impact obtained by removing a barrier between air and water. In that case, an additional advection equation is solved for a fluid-concentration field that tracks the evolution of the air–water interface. It is shown that the load associated with the wave impact on the structure compares well with existing numerical and experimental data.
Xiao D, Fang F, Pain C, et al., 2016, Non-intrusive reduced order modelling of waterflooding in geologically heterogeneous reservoirs
Production optimisation and history matching are two applications that require the engineer to run numerous flow simulations of flow in the subsurface. Each flow simulation can be very computationally intensive, especially if reservoir is geologically complex. In some cases it may not be feasible to perform the optimisation sufficiently quickly for it to be useful. This has driven the development of reduced order modelling (ROM) techniques. The problem with most ROMs is that they have to be hard-coded into the flow simulator and so cannot be used with the commercial simulators that are used by most oil companies. In addition most applications of ROMs have assumed that the geological description of the reservoir is known. This is generally not the case, indeed the aim of history matching is to adjust the geological model of the reservoir until the flow through the model replicates that which is observed. In this paper a non-intrusive reduced order model (NIROM) is presented that enables the engineer to vary the permeabilities within a heterogeneous reservoir for a fixed well pattern and then estimate the resulting waterflood performance. The NIROM uses a Smolyak sparse grid interpolation method, radial basis functions (RBF) and proper orthogonal decomposition (POD) is presented. 'Non-intrusive' means that the NIROM is implemented independently of the underlying flow model. Here we use it in conjunction with an unstructured mesh, control volume finite element (CVFEM), multiphase flow model. The NIROM is demonstrated using three reservoir models: one with four material layers, one with four baffles and one with eight baffles. The results compare well with those from a high fidelity full model and reduce the CPU time by a factor of a thousand.
Adam AG, Pavlidis D, Percival JR, et al., 2016, Simulation of immiscible viscous fingering using adaptive unstructured meshes and controlvolume galerkin interpolation
Displacement of one fluid by another in porous media occurs in various settings including hydrocarbon recovery, CO2 storage and water purification. When the invading fluid is of lower viscosity than the resident fluid, the displacement front is subject to a Saffman-Taylor instability and is unstable to transverse perturbations. These instabilities can grow, leading to fingering of the invading fluid. Numerical simulation of viscous fingering is challenging. The physics is controlled by a complex interplay of viscous and diffusive forces and it is necessary to ensure physical diffusion dominates numerical diffusion to obtain converged solutions. This typically requires the use of high mesh resolution and high order numerical methods. This is computationally expensive. We demonstrate here the use of a novel control volume - finite element (CVFE) method along with dynamic unstructured mesh adaptivity to simulate viscous fingering with higher accuracy and lower computational cost than conventional methods. Our CVFE method employs a discontinuous representation for both pressure and velocity, allowing the use of smaller control volumes (CVs). This yields higher resolution of the saturation field which is represented CV-wise. Moreover, dynamic mesh adaptivity allows high mesh resolution to be employed where it is required to resolve the fingers and lower resolution elsewhere. We use our results to re-examine the existing criteria that have been proposed to govern the onset of instability. Mesh adaptivity requires the mapping of data from one mesh to another. Conventional methods such as collocation interpolation do not readily generalise to discontinuous fields and are non-conservative. We further contribute a general framework for interpolation of CV fields by Galerkin projection. The method is conservative, higher order and yields improved results, particularly with higher order or discontinuous elements where existing approaches are often excessively diffusive.
Adam AG, Pavlidis D, Percival JR, et al., 2016, Simulation of immiscible viscous fingering using adaptive unstructured meshes and controlvolume galerkin interpolation
Displacement of one fluid by another in porous media occurs in various settings including hydrocarbon recovery, CO2 storage and water purification. When the invading fluid is of lower viscosity than the resident fluid, the displacement front is subject to a Saffman-Taylor instability and is unstable to transverse perturbations. These instabilities can grow, leading to fingering of the invading fluid. Numerical simulation of viscous fingering is challenging. The physics is controlled by a complex interplay of viscous and diffusive forces and it is necessary to ensure physical diffusion dominates numerical diffusion to obtain converged solutions. This typically requires the use of high mesh resolution and high order numerical methods. This is computationally expensive. We demonstrate here the use of a novel control volume - finite element (CVFE) method along with dynamic unstructured mesh adaptivity to simulate viscous fingering with higher accuracy and lower computational cost than conventional methods. Our CVFE method employs a discontinuous representation for both pressure and velocity, allowing the use of smaller control volumes (CVs). This yields higher resolution of the saturation field which is represented CV-wise. Moreover, dynamic mesh adaptivity allows high mesh resolution to be employed where it is required to resolve the fingers and lower resolution elsewhere. We use our results to re-examine the existing criteria that have been proposed to govern the onset of instability. Mesh adaptivity requires the mapping of data from one mesh to another. Conventional methods such as collocation interpolation do not readily generalise to discontinuous fields and are non-conservative. We further contribute a general framework for interpolation of CV fields by Galerkin projection. The method is conservative, higher order and yields improved results, particularly with higher order or discontinuous elements where existing approaches are often excessively diffusive.
Smith P, Lillington J, Pain C, et al., 2016, Directions in Radiation Transport, INTERNATIONAL JOURNAL OF MULTIPHYSICS, Vol: 10, Pages: 355-377, ISSN: 1750-9548
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