22 results found
Gawecka K, Cui W, Taborda D, et al., 2021, Predictive modelling of thermo-active tunnels in London Clay, Geotechnique: international journal of soil mechanics, Vol: 71, Pages: 735-748, ISSN: 0016-8505
Thermo-active structures are underground facilities which enable the exchange of thermal energy between the ground and the overlying buildings, thus providing renewable means of space heating and cooling. Although this technology is becoming increasingly popular, the behaviour of geotechnical structures under additional thermal loading is still not fully understood. This paper focuses on the use of underground tunnels as thermo-active structures and explains their behaviour through a series of finite element analyses based on an existing case study of isothermal tunnels in London Clay. The bespoke finite element codeI CFEP is adopted which is capable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. The complex coupled interactions between the tunnel and the surrounding soil are explored bycomparing results from selected types of coupledand uncoupled simulations. It is demonstratedthat: (1) the thermally-induceddeformation of the tunnel and the ground are more critical design aspects than the thermally-induced forces in the tunnel lining, and (2) the modelling approach in terms of the type of analysis, as well as the assumed permeability of the tunnel lining, have a significant effect on the computed tunnel response and,hence, must be chosen carefully
Gawecka KA, Bascompte J, 2021, Habitat restoration in spatially explicit metacommunity models, JOURNAL OF ANIMAL ECOLOGY, Vol: 90, Pages: 1239-1251, ISSN: 0021-8790
Gawecka K, Taborda D, Potts D, et al., 2020, Finite element modelling of heat transfer in ground source energy systems with heat exchanger pipes, International Journal of Geomechanics, Vol: 20, Pages: 04020041-1-04020041-14, ISSN: 1532-3641
Ground source energy systems (GSES) utilise low enthalpy geothermal energy and have been recognised as an efficient means of providing low carbon space heating and cooling. This study focuses on GSES where the exchange of heat between the ground and the building is achieved by circulating a fluid through heat exchanger pipes. Although numerical analysis is a powerful tool for exploring the performance of such systems, simulating the highly advective flows inside the heat exchanger pipes can be problematic. This paper presents an efficient approach for modelling these systems using the finite element method (FEM). The pipes are discretised with line elements and the conductive-advective heat flux along them is solved using the Petrov-Galerkin FEM instead of the conventional Galerkin FEM. Following extensive numerical studies, a modelling approach for simulating heat exchanger pipes, which employs line elements and a special material with enhanced thermal properties, is developed. The modelling approach is then adopted in three-dimensional simulations of two thermal response tests, with an excellent match between the computed and measured temperatures being obtained.
Cui W, Tsiampousi A, Potts D, et al., 2020, Numerical modelling of time-dependent thermally induced excess pore fluid pressures in a saturated soil, Journal of Geotechnical and Geoenvironmental Engineering - ASCE, Vol: 146, Pages: 04020007-1-04020007-15, ISSN: 0733-9410
A temperature rise in soils is usually accompanied by an increase in excess pore fluid pressure due to the differential thermal expansion coefficients of the pore fluid and the soil particles. To model the transient behaviour of this thermally induced excess pore fluid pressure in geotechnical problems, a coupled THM formulation was employed in this study, which accounts for the non-linear temperature-dependent behaviour of both the soil permeability and the thermal expansion coefficient of the pore fluid. Numerical analyses of validation exercises (where there is an analytical solution), as well as of existing triaxial and centrifuge heating tests on Kaolin clay, were carried out in the current paper. The obtained numerical results exhibited good agreement with the analytical solution and experimental measurements respectively, demonstrating good capabilities of the applied numerical facilities and providing insight into the mechanism behind the observed evolution of the thermally induced pore fluid pressure. The numerical results further highlighted the importance of accounting for the temperature-dependent nature of the soil permeability and the thermal expansion coefficient of the pore fluid, commonly ignored in geotechnical numerical analysis.
Cui W, Potts DM, Zdravković L, et al., 2019, Formulation and application of 3D THM-coupled zero-thickness interface elements, Computers and Geotechnics, Vol: 116, Pages: 1-11, ISSN: 0266-352X
Interface elements are frequently employed in finite element (FE) analyses to represent soil-structure interfaces or rock joints. The modelling of coupled thermo-hydro-mechanical (THM) problems in geotechnical engineering requires equally a coupled and robust THM formulation for interface elements. This paper presents such a formulation which is capable of reproducing the coupled THM behaviour of discontinuities and soil-structure boundaries, and is compatible with other types of finite elements used to discretise the soil and structural domains (e.g. solid and shell elements). The coupled THM three-dimensional (3D) zero-thickness interface element is implemented into the bespoke FE code employed in this research and its features are verified using a number of numerical exercises. To demonstrate their performance, the proposed interface elements are employed in the simulation of the coupled THM behaviour of a fissured triaxial sample subjected to a thermal load and the influence of the presence of fissures on soil behaviour is presented.
Liu R, Taborda D, Gawecka K, et al., 2019, Computational study on the effects of boundary conditions on the modelled thermally induced axial stresses in thermo-active piles, XVII European Conference on Soil Mechanics and Geotechnical Engineering
Cui W, Gawecka KA, Taborda D, et al., 2019, Time-step constraints for finite element analysis of two-dimensional transient heat diffusion, Computers and Geotechnics, Vol: 108, Pages: 1-6, ISSN: 0266-352X
In a FE analysis of transient heat transfer, a lower limit of the time-step size exists below which numerical oscillations of temperatures may occur. Although time-step constraints for simulating 1D heat diffusion have been well established in the literature, the conclusions cannot be directly applied to 2D cases. In this paper, both analytical and computational studies are carried out to obtain the time-step constraints for 2D linear and quadratic elements. It is noted that in the simulation of 2D heat diffusion employing quadratic elements is not always beneficial. Recommendations are provided on selecting the numerical scheme to minimise numerical oscillations.
Gawecka KA, Potts DM, Cui W, et al., 2018, A coupled thermo-hydro-mechanical finite element formulation of one-dimensional beam elements for three-dimensional analysis, Computers and Geotechnics, Vol: 104, Pages: 29-41, ISSN: 0266-352X
Finite element (FE) analysis in geotechnical engineering often involves entities which can be represented as one-dimensional elements in three-dimensions (e.g. structural components, drains, heat exchanger pipes). Although structural components require an FE formulation accounting only for their mechanical behaviour, for the latter two examples, a coupled approach is necessary. This paper presents the first complete coupled thermo-hydro-mechanical FE formulation for one-dimensional beam elements for three-dimensional analysis. The possibility of deactivating each of the systems enables simulation of both coupled and uncoupled behaviour, and hence a range of engineering problems. The performance of these elements is demonstrated through various numerical simulations.
Cui W, Potts DM, Zdravkovic L, et al., 2018, A coupled thermo-hydro-mechanical finite element formulation for curved beams in two-dimensions, Computers and Geotechnics, Vol: 103, Pages: 103-114, ISSN: 0266-352X
To enable the use of beam elements in the modelling of coupled thermo-hydro-mechanical (THM) geotechnical problems, a fully coupled and robust THM formulation is required. This paper presents such a formulation which allows both fluid flow and heat transfer along a 2D curved beam, while ensuring compatibility with coupled THM solid elements commonly used to discretise soils. Verification exercises and application with the proposed coupled beam element are carried out to demonstrate its satisfactory behaviour. The results of these analyses are compared against closed form solutions, solutions obtained using solid elements, and field measurements, showing an excellent agreement.
Cui W, Gawecka KA, Potts DM, et al., 2018, A Petrov-Galerkin finite element method for 2D transient and steady state highly advective flows in porous media, Computers and Geotechnics, Vol: 100, Pages: 158-173, ISSN: 0266-352X
A new Petrov-Galerkin finite element method for two-dimensional (2D) highly advective flows in porous media, which removes numerical oscillations and retains its precision compared to the conventional Galerkin finite element method, is presented. A new continuous weighting function for quadratic elements is proposed. Moreover, a numerical scheme is developed to ensure the weighting factors are accurately determined for 2D non-uniform flows and 2D distorted elements. Finally, a series of numerical examples are performed to demonstrate the capability of the approach. Comparison against existing methods in the simulation of a benchmark problem further verifies the robustness of the proposed method.
Cui W, Tsiampousi A, Potts DM, et al., 2018, Finite element modelling of excess pore fluid pressure around a heat source buried in saturated soils, London, 9th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: Taylor Francis Group, Pages: 741-749
Potts D, Cui W, Gawecka KA, et al., 2018, Numerical modelling of coupled thermo-hydro-mechanical problems: Challenges and pitfalls, 9th European Conference on Numerical Methods in Geotechnical Engineering, Publisher: Taylor Francis Group
Cui W, Potts DM, Zdravkovic L, et al., 2018, An alternative coupled thermo-hydro-mechanical finite element formulation for fully saturated soils, Computers and Geotechnics, Vol: 94, Pages: 22-30, ISSN: 0266-352X
Accounting for interaction of the soil’s constituents due to temperature change in the design of geo-thermal infrastructure requires numerical algorithms capable of reproducing the coupled thermo-hydro-mechanical (THM) behaviour of soils. This paper proposes a fully coupled and robust THM formulation for fully saturated soils, developed and implemented into a bespoke finite element code. The flexibility of the proposed formulation allows the effect of some coupling components, which are often ignored in existing formulations, to be examined. It is further demonstrated that the proposed formulation recovers accurately thermally induced excess pore water pressures observed in undrained heating tests.
Gawecka KA, Taborda DMG, Potts DM, et al., 2017, Numerical modelling of thermo-active piles in London Clay, Proceedings of the Institution of Civil Engineers - Geotechnical Engineering, Vol: 170, Pages: 201-219, ISSN: 1353-2618
Thermo-active foundations utilise heat energy stored in the ground to provide a reliable and effective means ofspace heating and cooling. Previous studies have shown that the effects of temperature changes on their response arehighly dependent on their interaction with the surrounding ground. Consequently, it is necessary to consider thisinteraction and include both the thermal and mechanical behaviour of the ground in design. This paper addresses thisissue by performing state-of-the-art finite-element analyses using the Imperial College Finite Element Program, which iscapable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. First, the LambethCollege pile test is analysed to demonstrate the capability of the adopted modelling approach to capture the observedresponse under thermo-mechanical loading. Subsequently, a detailed study is carried out, demonstrating the impact ofcapturing the fully coupled thermo-hydro-mechanical response of the ground, the use of appropriate boundary conditionsand the uncertainty surrounding thermal ground properties. It is demonstrated that the modelling approach has a largeimpact on the computed results, and therefore potentially on the design of thermo-active piles. Conversely, the effects ofthermal conductivity and permeability of the soil are shown not to influence the pile behaviour significantly.
Gawecka KA, Taborda DMG, Potts DM, et al., 2016, Effects of transient phenomena on the behaviour of thermo-active piles, 1st International Conference on Energy Geotechnics
Cui W, Gawecka KA, Potts DM, et al., 2016, Numerical analysis of coupled thermo-hydraulic problems in geotechnical engineering, Geomechanics for Energy and the Environment, Vol: 6, Pages: 22-34, ISSN: 2352-3808
Ground sources energy systems, such as open-loop systems, have been widely employed in recent years due to their economic and environmental benefits compared to conventional heating and cooling systems. Numerical modelling of such geothermal system requires solving a coupled thermo-hydraulic problem which is characterised by a convection-dominated heat transfer which can be challenging for the Galerkin finite element method (GFEM). This paper first presents the coupled thermo-hydraulic governing formulation as well as the coupled thermo-hydraulic boundary condition, which can be implemented into a finite element software. Subsequently, the stability condition of the adopted time marching scheme for coupled thermo-hydraulic analysis is established analytically. The behaviour of highly convective problems is then investigated via a series of analyses where convective heat transfer along a soil bar is simulated, with recommendations on the choice of an adequate discretisation with different boundary conditions being provided to avoid oscillatory solutions. Finally, the conclusions from the analytical and numerical studies are applied to the simulation of a boundary value problem involving an open-loop system, with the results showing good agreement with an approximate solution. The main objective of this paper is to demonstrate that the GFEM is capable of dealing with highly convective geotechnical problems.
Cui W, Gawecka KA, Taborda DMG, et al., 2015, Time-step constraints in transient coupled finite element analysis, International Journal for Numerical Methods in Engineering, Vol: 106, Pages: 953-971, ISSN: 1097-0207
In transient finite element analysis, reducing the time-step size improves the accuracy of the solution. However, a lower bound to the time-step size exists, below which the solution may exhibit spatial oscillations at the initial stages of the analysis. This numerical ‘shock’ problem may lead to accumulated errors in coupled analyses. To satisfy the non-oscillatory criterion, a novel analytical approach is presented in this paper to obtain the time-step constraints using the θ-method for the transient coupled analysis, including both heat conduction–convection and coupled consolidation analyses. The expressions of the minimum time-step size for heat conduction–convection problems with both linear and quadratic elements reduce to those applicable to heat conduction problems if the effect of heat convection is not taken into account. For coupled consolidation analysis, time-step constraints are obtained for three different types of elements, and the one for composite elements matches that in the literature. Finally, recommendations on how to handle the numerical ‘shock’ issues are suggested.
Cui W, Gawecka KA, Potts DM, et al., 2015, Numerical modelling of open-loop ground source energy systems, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
The environmental and economic benefits of utilising the ground for extracting and storing heat have been known for a long time. However, only recently have government sustainability policies and rising energy prices encouraged the use of this renewable energy resource. In open-loop systems water is abstracted from one well and re-injected into another after exchanging energy with a building’s heating/cooling system using a heat pump. In order to guarantee a good performance of the system, it is fundamental that the possibility of thermal breakthrough occurring is minimised, i.e. that the temperature of the water being abstracted remains unaffected by the injection of warmer/cooler water at the other well. In this paper, the Imperial College FiniteElement Program (ICFEP), which is capable of simulating fully coupled thermo-hydro-mechanical behaviour of porous materials, was used to perform two-dimensional analyses of open-loop ground source heat systems. The parametric studies carried out highlight the relative impact on the occurrence of thermal break-through of the hydraulic ground conditions andthe geometric characteristics of the system, providing an invaluable insight into possible improvements to the current design procedure.
Martinez Calonge D, Gawecka KA, Zdravkovic L, et al., 2015, Development of a new temperature-controlled triaxial apparatus for saturated soils, 16th European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: ICE Publishing
In recent years, the study of the Thermo-Hydro -Mechanical (THM) behaviour of geomaterials has become a growing area in geotechnical engineering due to the increasing interest in energy geostructures and underground nuclear waste disposal. Advanced laboratory testing is essential in gaining an understanding of the THM behaviour of soils and solving these complex geomechanical problems. This paper describes the development of a new triaxial apparatus at the Imperial College Geotechnics Laboratory, capable of testing saturated soils at temperatures up to 85°C and pressures up to 800kPa. In order to aid its design, numerical analysis of the thermal response of the cell was conducted using the Imperial College Finite Element Program (ICFEP) with its newly developed THM capabilities.
Cui W, Gawecka KA, Potts DM, et al., 2015, Investigations on numerical analysis of coupled thermo-hydraulic problems in geotechnical engineering, International Symposium on Energy Geotechnics (1st.: 2015: Barcelona)
Pedraza F, Liu H, Gawecka KA, et al., The role of indirect effects in coevolution as mutualism transitions into antagonism
<jats:title>Abstract</jats:title><jats:p>Species interactions have evolved from antagonistic to mutualistic and back several times throughout life’s history. Yet, it is unclear how changes in the type of interaction between species alter the coevolutionary dynamics of entire communities. This is a pressing matter, as transitions from mutualisms to antagonisms may be becoming more common with human-induced global change. Here, we combine network and evolutionary theory to simulate how shifts in interaction types alter the coevolution of empirical communities. We show that as mutualistic networks shift to antagonistic, selection imposed by direct partners begins to outweigh that imposed by indirect partners. This weakening of indirect effects is associated with communities losing their tight integration of traits and increasing their rate of adaptation. The above changes are more pronounced when specialist consumers are the first species to switch to antagonism. A shift in the outcome of species’ interactions may therefore reverberate across communities and alter the direction and speed of coevolution.</jats:p>
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