CERES :: Browsing by Author "Drikakis, Dimitris"

Browsing by Author "Drikakis, Dimitris"

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Open Access
Addressing the challenges of implementation of high-order finite volume schemes for atmospheric dynamics of unstructured meshes
(European Congress on Computational Methods in Applied Sciences and Engineering, 2016-06-30) Tsoutsanis, Panagiotis; Drikakis, Dimitris
The solution of the non-hydrostatic compressible Euler equations using Weighted Essentially Non-Oscillatory (WENO) schemes in two and three-dimensional unstructured meshes, is presented. Their key characteristics are their simplicity; accuracy; robustness; non-oscillatory properties; versatility in handling any type of grid topology; computational and parallel efficiency. Their defining characteristic is a non-linear combination of a series of high-order reconstruction polynomials arising from a series of reconstruction stencils. In the present study an explicit TVD Runge-Kutta 3rd -order method is employed due to its lower computational resources requirement compared to implicit type time advancement methods. The WENO schemes (up to 5th -order) are applied to the two dimensional and three dimensional test cases: a 2D rising.
Open Access
Advanced computational modelling and simulation of transition to turbulence in separated suddenly-expanded channel flows
(Cranfield University, 2010-09) Vamvakoulas, Christos; Drikakis, Dimitris; Shapiro, Evgeniy
The main scope of this PhD thesis is the analysis of unsteady laminar and transitional suddenly expanded flows. For this reason Implicit Large Eddy Simulation (ILES) approach was used in combination with high order, high resolution numerical methods. The numerical methods examined are a 2nd order Monotonic Upwind Scheme for Scalar Conservation Laws (MUSCL) with Van Leer limiter, a high order (3rd) interpolation and a 5th order Weighted Essentially Non-Oscillatory scheme (WENO). First the numerical data for three low (steady state) Reynolds numbers and for two unsteady ( in the form of primary frequencies) were compared to the experimental data and were found in good agreement. A grid convergence study was undertaken for two Reynolds numbers demonstrating grid convergence and justifying the selection of the grid. The three numerical methods were evaluated for two Reynolds numbers showing good agreement for Reynolds number 412 and discrepancies at Reynolds number 900 between MUSCL and WENO with the MUSCL demonstrating a very dissipative behavior. The physical behavior of the flow in a wide range of Reynolds numbers were examined. For this range the flow behavior changed from steady to unsteady laminar and finally exhibiting localized transition to turbulence. The behavior of the main recirculation areas was described and the vortex shedding that occur there and how this change with the Reynolds number. The flow was observed to change from an unsteady quasi three dimensional flow at Reynolds number 412 to an increased transitional state with three dimensional vortical structures at Reynolds number 550. Kinetic energy spectra were calculated for the aforementioned range of Reynolds numbers. The primary frequencies are increasing with Reynolds number as expected. The slopes that were calculated for the inertial subrange revealed a trend. As the Reynolds number is increasing the slopes are decreasing approaching the value given by Kolmogorov -5/3.
Open Access
Artificial compressibility, characteristics-based schemes for variable density, incompressible, multi-species flows. Part I. Derivation of different formulations and constant density limit
(Elsevier, 2005-12-10) Shapiro, Evgeniy; Drikakis, Dimitris
The paper presents various formulations of characteristics-based schemes in the framework of the artiﬁcialcompressibility method for variable-density incompressible ﬂows. In contrast to constant-density incompressible ﬂows, where the characteristics-based variables reconstruction leads to a single formulation, in the case of variable density ﬂows three diﬀerent schemes can be obtained henceforth labeled as: transport, conservative and hybrid schemes. The conservative scheme results in pseudo-compressibility terms in the (multi-species) density reconstruction. It is shown that in the limit of constant density, the transport scheme becomes the (original) characteristics-based scheme for incompressible ﬂows, but the conservative and hybrid schemes lead to a new characteristics-based variant for constant density ﬂows. The characteristics-based schemes are combined with second and third-order interpolation for increasing the computational accuracy locally at the cell faces of the control volume. Numerical experiments for constant density ﬂows reveal that all the characteristics-based schemes result in the same ﬂow solution, but they exhibit diﬀerent convergence behavior. The multigrid implementation and numerical studies for variable density ﬂows are presented in Part II of this study.
Open Access
Artificial compressibility, characteristics-based schemes for variable-density, incompressible, multispecies flows: Part II. Multigrid implementation and numerical tests
(Elsevier, 2005-12-10) Shapiro, Evgeniy; Drikakis, Dimitris
The paper presents an investigation of the accuracy and efficiency of artificial compressibility, characteristics-based (CB) schemes for variable-density incompressible ﬂows. The CB schemes have been implemented in conjunction with a multigrid method for accelerating numerical convergence and a fourth-order, explicit Runge–Kutta method for the integration of the governing equations in time. The implementation of the CB schemes is obtained in conjunction with first-, second- and third-order interpolation formulas for calculating the variables at the cell faces of the computational volume. The accuracy and efficiency of the schemes are examined against analytical and experimental results for difusion broadening in two- and three-dimensional microﬂuidic channels, a problem that has motivated the development of the present methods. Moreover, unsteady, inviscid simulations have been performed for variable-density mixing layer. The computations revealed that accuracy and eﬃciency depend on the CB scheme design. The best multigrid convergence rates were exhibited by the conservative CB scheme, which is obtained by the fully conservative formulation of the variable-density, incompressible equations.
Open Access
Assessment of high-order finite volume methods on unstructured meshes for RANS solutions of aeronautical configurations.
(Elsevier, 2017-01-03) Antoniadis, Antonis F.; Tsoutsanis, Panagiotis; Drikakis, Dimitris
This paper is concerned with the application of k-exact finite volume methods for compressible Reynolds-Averaged Navier-Stokes computations of flows around aeronautical configurations including the NACA0012, RAE2822, MDA30P30N, ONERA-M6, CRM and DLR-F11. High-order spatial discretisation is obtained with the Weighted Essentially Non-Oscillatory and the Monotone-Upstream Central Scheme for Conservation Laws methods on hybrid unstructured grids in two- and three- dimensions. Schemes of fifth, third and second order comprise the foundation of the analysis, with main findings suggesting that enhanced accuracy can be obtained with at least a third-order scheme. Steady state solutions are achieved with the implicit approximately factored Lower-Upper Symmetric Gauss-Seidel time advancing technique, convergence properties of each scheme are discussed. The Spalart-Allmaras turbulence model is employed where its discretisation with respect to the high-order framework is assessed. A low-Mach number treatment technique is studied, where recovery of accuracy in low speed regions is exemplified. Results are compared with referenced data and discussed in terms of accuracy, grid dependence and computational budget.
Open Access
Assessment of high-resolution methods in hypersonic real-gas flows
(Cranfield University, 2010-07) Tissera, Shiroshana; Drikakis, Dimitris; Titarev, Vladimir
The interest in hypersonic flow phenomena has peaked in recent years where number of experimental and computational work has been carried out. The Computational Fluid Dynamics (CFD) is fast becoming an invaluable tool to investigate compressible hypersonic flow phenomena that are extremely complex in nature. Mathematical models employed to describe complex physical phenomena that take place at hypersonic speeds inherit varying degrees of accuracy and reliability. Therefore, further studies, numerical and experimental, are needed to clarify and improve these models. Numerical computation is one of the tasks that are vital in the overall hypersonic flow research effort. This work investigated the applicability and performance of higher resolution methods to simulate high enthalpy real gas flows. Furthermore, gas-surface interaction and ablation effects are also investigated. In order to achieve the set task, it is imperative that the numerical code (CNS3D) used is equipped with necessary numerical and physical models to tackle flow behaviour typically unique to hypersonic flow. Therefore, the implementation of mathematical models that describe the real gas phenomena, such as vibrational effects, chemical dissociation, diffusion, and high enthalpy effects, has been carried out. The test cases, the HB-2 flare and the double-cone have been considered for the purposes of verification and validation. The experimental data for heat transfer and pressure are compared with numerical predictions to assess the behaviour of modified CNS3D overall and each numerical scheme with regards to reconstruction methods. The overall agreement between the predicted results for both cases and the experimental data is satisfactory. The stagnation point values of pressure and heat flux for HB-2 flare testcase at varying Mach numbers from 5 to 17.8 has been established; these values are expected aid future validation efforts. It was also found that very high-order schemes, such as WENO 5th and 9th -order methods, may provide slightly better results for free stream Mach numbers less than 10; however, there are no obvious benefits over second-order methods for Mach numbers greater than 10. Furthermore, it has been substantiated that increasing order of accuracy compared to increments in the grid resolution is much more effective way of gaining accuracy in the case of real gas flows.
Open Access
Boundary slip dependency on surface stiffness
(American Physical Society, 2010-06-24T00:00:00Z) Asproulis, Nikolaos; Drikakis, Dimitris
The paper investigates the effects of surface stiffness on the slip process aiming to obtain a better insight of the momentum transfer at nanoscale. The surface stiffness is modeled through the stiffness, kappa, of spring potentials, which are employed to construct the thermal walls. It is shown that variations of stiffness, kappa, influence the slip mechanism either toward slip or stick conditions. Increasing the values of kappa alters the oscillation frequency and the mean displacement of the wall particles toward higher and lower values, respectively. Our results suggest that the amount of slip produced as a function of stiffness follows a common pattern that can be modeled through a fifth-order polynomial function.
Open Access
Collision dynamics of nanoscale Lennard-Jones clusters
(American Institute of Physics, 2006-12-11) Kalweit, Marco; Drikakis, Dimitris
An investigation of collision dynamics of nanoparticles for a broad range of impact factors and collision speeds is presented. The investigation is based on molecular dynamics simulations in conjunction with the Lennard-Jones interaction potential thus making the results applicable for a broad range of material properties. Identification criteria are used to classify the collision dynamics into different collision modes and submodes. Detailed analysis of the collision processes reveals the existence of coalescence and stretching separation modes, which are further classified according to their dynamics into sticking; slide-and-locking; droplet; normal stretching separation; stretching separation with satellite droplets; and shearing-off modes. Qualitative and quantitative comparisons with previous molecular dynamic studies and analytical prediction models derived for macroscopic droplet collisions are also discussed. The investigation reveals that the reflexive separation mode, which has been observed in macroscopic droplet collisions, does not occur for nanoparticles consisting of 10 000 (or less) atoms.
Open Access
Computational modelling of acoustic scattering of a sound source in the vicinity of the ground
(International Association of Engineers, 2009-12-31T00:00:00Z) Pantazopoulou, Panagiota; Drikakis, Dimitris
The paper presents a computational model for acoustic scattering of a near-the- ground sound source around a body moving in a uniform flow. Using the method of images and the concept of reflection coefficient, the half space Green's function in a uniform flow is derived in the framework of the boundary element method (BEM). The method is validated against analytical solutions and is further applied in the case of acoustic scattering around an airfoil moving in the vicinity of the ground both for rigid and soft ground conditions. The results show the importance of ground impedance in the attenuation of sound.
Open Access
Computational Modelling of Cavity Arrays with Heat Transfer using Implicit Large Eddy Simulations
(Cranfield University, 2010-01) Malick, Zeshan; Drikakis, Dimitris; Shapiro, Evgeniy
This PhD programme was sponsored by the United Kingdom Atomic Energy Authority (UKAEA). The aim of this study is to conduct advanced computational modelling of a cooling device used in the fusion process which recycles waste energy. The development of efficient, water cooled tiles, that can sustain heat loads of approximately 20 MW (in quasi-steady state conditions) is the motivation of the current work. The information presented here will contribute to thermal-mechanical analysis, to be conducted at the Joint European Torus (JET) in future years. The devices known as “Hypervapotrons” have been used successfully at JET to provide a ion dump that dissipates residual energy from the fusion process. A capability to model the flow structure and heat transfer, across a large number of geometric and material options is provided within. Differences in geometry, result in changes to the flow structure and heat transfer rates. The desire to optimise such designs relies upon the fundamental understanding of the flow field within the main section, where the geometry may be defined as a cavity array. The benchmark case of a lid driven cavity flow was used for the validation of the flow field solution. Solutions using high resolution methods in the formulation provided a good comparison with established experimental data. Therefore, validation of incompressible, Implicit Large Eddy Simulations (ILES) for a wall bounded, three dimensional, turbulent flow is provided within. The sensitivity of the high order reconstruction in conjunction with the characteristics based scheme (Drikakis & Rider, 2005), to resolve turbulent flow structure is provided here. The solution response to grid resolution and a regularised velocity profile at the upper lid surface is also detailed. The investigation provided insight and confidence in the turbulence modelling approach which is relatively recent. It was also demonstrated through the lid driven case (and later in the Hypervapotron cases) that high order reconstruction was a simulation prerequisite, based on grid resolutions used within. Additional validation was also provided against numerical and analytical solutions for the Conjugate Heat Transfer (CHT) and scalar temperature field. Where appropriate both unsteady and steady problems based on a composite, three layer medium are detailed to provide preliminary validation for the implementation of the temperature scalar and conjugate boundary conditions. Unfortunately, it was not feasible to solve the coupled problem with an explicit solver as used in this study. However, it is suggested that the initial stages of thermal boundary layer development may be observed leading to the locations of incipient boiling. Two different Reynolds numbers were considered for the Hypervapotron ”Standard” geometry, Re=12000 and Re=18000. The different flow structures show that the cavity aspect ratio of the Standard design promotes lower flow speeds at the cavity base, since two or three counter rotating vortices coexist inside the cavities depending on Reynolds number. A detailed analysis on the impact of the number of repeating units within the computational domain is also provided. Results are presented of ensemble averaged quantities based on the Reynolds decomposition. The temperature distribution present in the solid, fluid and its interface for the thermally developing case is achieved. In addition the total and decomposed heat fluxes are presented for the Hypervapotron (Standard design) which provides similar comparison with recent Reynolds Averaged Navier-Stokes (RANS) simulations.
Open Access
Computational modelling of environment flows featuring gas dispersion
(Cranfield University, 2010-07) Gerousi, Loukia; Patel, Sanjay; Drikakis, Dimitris
This study particularly aims at understanding flow and pollutant dispersion when flat terrain, single hill and hill with obstacles are present. The emissions of ethylene from a point source are located in eight different positions. For the hill cases, the sources are located downwind from the top of the hill, and the data are collected at various locations. The commercial software packages Gambit 2.4.6, Fluent 6.3.26 and Tecplot 360 are used for the two-dimensional mesh generation, for the flow simulation and for the validation respectively. The numerical results are compared with experimental and numerical data for the single hill case and for the point source using the Spalart- Allmaras model, k-ε Standard, k-ε RNG and k-ε Realizable models. The comparison of the results shows that the k-ᵋ Standard model is in good agreement with the experimental and numerical data. Results also show that the mass fraction of ethylene is highest for the flat terrain case. The next highest mass fraction of ethylene is found for the case with the hill and obstacles, and the single-hill case has the lowest. Moreover, upwind of the first obstacle the average mass fraction is larger than inside the first and the second canyons, and the minimum pollutant is downwind of the last obstacle. The average mass fraction of ethylene is measured at the corners of the canyons, and the results show that generally the bottom left corners have a higher mass fraction than the middle and bottom right side.
Open Access
Computational modelling of instability and transition using high-resolution methods
(Cranfield University, 2007) Patel, Sanjay; Drikakis, Dimitris
This thesis concerns the numerical investigation of suddenly expanded flows featuring separation, instabilities and transition, in the context of Implicit Large Eddy Simulation (ILES). The study of separated flows through suddenly expanded geometries is a classic yet complex area of research. These types of flows feature instabilities which may lead to bifurcation. Non-linear bifurcation is of great importance when considering hydrodynamic stability and the mechanism of laminar to turbulent flow transition. A detailed numerical investigation of various high-resolution methods and their ability to correctly predict the flow through a suddenly expanded and contracted geometry demonstrates that the choice of the particular numerical method employed can lead to an incorrect solution of the flow. The key di erence between the various highresolution methods employed is in the calculation of the nonlinear wave-speed dependent term. It is shown that the nonlinearity of this term provides an asymmetric dissipation to the flow which triggers symmetry-breaking bifurcation in a fully symmetric computational set-up. High-resolution simulations of three-dimensional flow through a plane suddenly expanded channel at low Reynolds numbers show that this type of flow is characterised by a symmetric separation of the fluid which is nominally two-dimensional in the spanwise direction. Increasing the Reynolds number reveals a symmetry-breaking bifurcation of the fluid flow which becomes three-dimensional as Reynolds number is further increased. Simulations confirm that it is this threedimensional disturbance which leads to the onset of time-dependent flow characterised by the periodic shedding of vortices from the upstream recirculation zones. Preconditioning techniques which aim to alleviate sti ness in the calculation of the advective fluxes for low Reynolds number flows are shown to be unsuitable for flows featuring instabilities. The added dissipation to the flow causes the prediction of an incorrect stable solution or to an improper estimation of the size of the separation bubbles. Simulations of a synthetic jet issuing into quiescent air using various slope limiters manage to capture the flow physics relatively well. Limiters are used to avoid a scheme from being oscillatory and provide non-linear dissipation in the region of excessively large gradients. The various limiters di er with regards to the amount of dissipation they provide to the flow, hence the solution obtained is dependent on the limiter used.
Open Access
Computational modelling of the HyperVapotron cooling technique for nuclear fusion applications
(Cranfield University, 2010-10) Milnes, Joseph; Drikakis, Dimitris
Efficient heat transfer technologies are essential for magnetically confined fusion reactors; this applies to both the current generation of experimental reactors as well as future power plants. A number of High Heat Flux devices have therefore been developed specifically for this application. One of the most promising candidates is the HyperVapotron, a water cooled device which relies on internal fins and boiling heat transfer to maximise the heat transfer capability. Over the past 30 years, numerous variations of the HyperVapotron have been built and tested at fusion research centres around the globe resulting in devices that can now sustain heat fluxes in the region of 20 – 30MW/m2 in steady state. Unfortunately, there have been few attempts to model or understand the internal heat transfer mechanisms responsible for this exceptional performance with the result that design improvements are traditionally sought experimentally which is both inefficient and costly. This thesis seeks to develop an engineering model of the HyperVapotron device using commercial Computational Fluid Dynamics software. To establish the most appropriate modelling choices, in-depth studies were performed examining the turbulence models (within the Reynolds Averaged Navier Stokes framework), near wall methods, grid resolution and boiling submodels. Validation of the models is accomplished via comparison with experimental results as well as high order Implicit Large Eddy Simulation methods. It is shown that single phase cavity flows and their related heat transfer characteristics (time-averaged) can be accurately captured if the SST k-omega turbulence model is employed using a fine nearwall grid throughout the cavity (e.g. y+ < 1 throughout). Separately, multiphase solutions with tuned wall boiling models also showed reasonable agreement with experimental data for vertical boiling tubes. As more complex multiphase HyperVapotron models were constructed, it became clear that there is an intrinsic incompatibility between the fine grids required for the single phase heat transfer predictions and the coarser grids plus wall functions required by the boiling model. Ultimately, the full 3D solution was based on the coarser grids as the fall off in accuracy in single phase heat transfer only becomes significant for HyperVapotron designs with deeper cavities. Since it is also shown here that deeper cavities are generally less efficient, these grid induced errors become less relevant if the primary objective is to find optimised performance.Comparing the CFD solutions with HyperVapotron experimental data suggests that a RANS-based, multiphase model is indeed capable of predicting performance over a wide range of geometries and boundary conditions. Whilst a definitive set of design improvements is not defined here, it is expected that the methodologies and tools developed will enable designers of future High Heat Flux devices to perform significant virtual prototyping before embarking on the more costly build and test programmes.
Open Access
Computational nanoscience and molecular modelling of shock wave interactions with biological membranes
(Cranfield University, 2011) Sourmaidou, Damiani; Asproulis, N.; Drikakis, Dimitris
Lateral diffusion of membrane components (lipids and proteins) is an important membrane property to measure since the essential process of absorption of anti-cancer and other drugs -some of which are not soluble in lipids and therefore would not be able to penetrate the cell membrane through passive diffusion- lies on it. In particular, the procedure of diffusion into the cell cytoplasm is reliant on free volumes in the membrane (passive diffusion) as well as carrier proteins (facilitated diffusion). By enhancing the mobility of lipids and/or proteins, the possibility of the carrier protein to "encapsulate" pharmacological components maxim- izes, as a "scanning" of the proteins gets performed due to the fluid phase of a biological membrane. At the same time, the increased mobility of the lipids facilitates the passage of lipid-soluble molecules into the cell. Thus, given that the success of anticancer treatments heavily depends on their absorption by the cell, a significant enhancement of the cell mem- brane permeability (permeabilisation) is rendered vital to the applicability of the technique. For this reason, there is augmented interest in combined methods such as Nanotechnology based drug delivery that is focused on the development of optimally designed therapeutic agents along with the application of shock waves to enhance the membrane permeability to the agents. This study examines the impact of shock waves on a numerical model of a biological membrane. Cont/d.
Open Access
Computational nanoscience and multiscale modeling of DNA molecules
(Cranfield University, 2010-10-26) Lai, Massimo; Drikakis, Dimitris
Molecular Dynamics is a very powerful technique for the investigation of matter at nanoscopic level. However, it’s application in many fields, such as the investigation of many relevant processes of cell biology, is restricted by issues of computational cost. Therefore, in recent years, a growing interest has been generated by the introduction of Coarse-Grained (CG) models, that allow the investigation of bigger systems for longer timescales. In this thesis, Molecular Dynamics was used in order to gain a quantitative understanding of mechanical and di.usive processes of DNA molecules in solution, and in order to parametrise a Coarse Grained model of DNA capable of a qualitative description of the mechanical behaviour of the all-atom model at equilibrium. A software package for the computation of Coarse-Grained interaction force-fields, making use of the recently developed Multiscale Coarse-Grained Method (MSCG) by Izvekov and Voth [1] was implemented. We tested and validated the method by performing a one-point-per-molecule coarse graining of TIP3P water. The resulting model was able to reproduce the fluid structure (its radial distribution function) in a satisfactory and nearly quantitative way. Finally, we applied the MSCG method to a more demanding problem, namely the parametrisation of a 3-point-per-residue coarse-grained model of double-stranded DNA. As a consequence, the agreement of the obtained CG model with the atomistic structure was still not quantitative. In particular, the helical geometry was qualitatively preserved and the Root-Mean-Square Displacement (RMSD) of the coarse-grained model was stable over the trajectory, but higher than its all-atom counterpart. We suggest several possible routes for future improvements. In particular, the explicit modeling of torsional degrees of freedom of the DNA backbone, and the use of recently introduced methods for the refinement of the MSCG estimation of force-field parameters, and a more accurate treatment of Coulombic interactions.
Open Access
Computational nanoscience of flow and mass transport through biological membranes
(Cranfield University, 2008-01-16) Lechuga, Javier; Drikakis, Dimitris
The study presented in this document is the result of three years of research into the complex world of Molecular Dynamics applied to biological cell membranes. The simulation of biological tissues involves not only an excellent knowledge of the numerical calculus and its related tools, but a profound comprehension of the biological and medical literature associated with the phenomenon. By the other hand, the use of high performance facilities is essential for the computation of the Molecular Dynamics models in order to obtain results in acceptable times, so the latest technological advances have played a decisive and important part in this eld of research. The presented obtained results about shock wave interaction with biological membranes, as well as the air ow through the alveolar surface, are part of a new line of research usually known as "virtual experimental". This name comes from the fact that any physical or chemical situation can be re-created into a computer system to calculate its propagation in time. The results of the interaction of shock waves with biological cell membranes have been particularly satisfactory and they have opened a new line of investigation into cancer research. A numerical proportional relation between the shock wave impulse and the value of lateral di usion (from 9.80 to 12.84 10 .7 cm2 s ), as well as the simulation of the transient provoked by the wave into a NPT ensemble are a successful achievement. Other computations of this type of interaction have been simulated into an NVE ensemble as well, however the obtained results for the lateral di usion, in the order of 10 .7 cm2 s , showed no trend regarding the shock wave and the transient e ect could not be simulated. On the other hand, the recreation of the air ow through the alveolar surface is an initial step into the solution of all the controversy surrounding this extremely complex system known as alveolar surface network. An alveolar membrane of around 7 nm has been successfully simulated in agreement with Scarpelli's experiments. This lipid-protein membrane model simulated can serve as a virtual experiment in order to solve the controversy about the alveolar surface. It points to the possibility of air ow through a stable two-layered DPPC phospholipid structure either from a numerical or physical and biological point of view and the existence of an alveolar membrane at the end of the bronchial tubes.
Open Access
Computational science of turbulent mixing and combustion
(Cranfield University, 2010-09) Shimada, Yosuke; Drikakis, Dimitris; Thornber, Ben
Implicit Large Eddy Simulation (ILES) with high-resolution and high-order computational modelling has been applied to flows with turbulent mixing and combustion. Due to the turbulent nature, mixing of fuel and air and the subsequent combustion still remain challenging for computational fluid dynamics. However, recently ILES, an advanced numerical approach in Large Eddy Simulation methods, has shown encouraging results in prediction of turbulent flows. In this thesis the governing equations for single phase compressible flow were solved with an ILES approach using a finite volume Godunov-type method without explicit modelling of the subgrid scales. Up to ninth-order limiters were used to achieve high order spatial accuracy. When simulating non chemical reactive flows, the mean flow of a fuel burner was compared with the experimental results and showed good agreement in regions of strong turbulence and recirculation. The one dimensional kinetic energy spectrum was also examined and an ideal k−5/ 3 decay of energy could be seen in a certain range, which increased with grid resolution and order of the limiter. The cut-off wavenumbers are larger than the estimated maximum wavenumbers on the grid, therefore, the numerical dissipation sufficiently accounted for the energy transportation between large and small eddies. The effect of density differences between fuel and air was investigated for a wide range of Atwood number. The mean flow showed that when fuel momentum fluxes are identical the flow structure and the velocity fields were unchanged by Atwood number except for near fuel jet regions. The results also show that the effects of Atwood number on the flow structure can be described with a mixing parameter. In combustion flows simulation, a non filtered Arrhenius model was applied for the chemical source term, which corresponds to the case of the large chemical time scale compared to the turbulent time scale. A methane and air shear flow simulation was performed and the methane reaction rate showed non zero values against all temperature ranges. Small reaction rates were observed in the low temperature range due to the lack of subgrid scale modelling of the chemical source term. Simulation was also performed with fast chemistry approach representing the case of the large turbulent time scale compared to the chemical time scale. The mean flow of burner flames were compared with experimental data and a fair agreement was observed.
Open Access
Computational simulation of freely falling water droplets on graphics processing units
(Cranfield University, 2013-08) Appleyard, J.; Drikakis, Dimitris
This work describes and demonstrates a novel numerical framework suitable for simulating the behaviour of freely falling liquid droplets. The specific case studied is designed such that the properties of the system are similar to those of raindrops falling through air. The study of raindrops is interesting from both an engineering standpoint and from a standpoint of pure curiosity. As a natural phenomenon, rainfall is something which is experienced by everybody, yet its properties are often misunderstood. The primary engineering application is in improving the ability of radar to determine the characteristics of rainfall for meteorological purposes. The significant original contributions to knowledge within this work come from several areas. The numerical methods used are a unique combination of a high order incompressible implicit large eddy simulation method, a conservative level set method, and a pressure projection method. These methods have all been implemented on a highly parallel GPU architecture, with a resulting performance increase of approximately ten times when a single GPU was compared to a single CPU core. The water droplets were simulated in a regime not previously studied by three dimensional methods. The results of these simulations confirmed the validity of the numerical model by reproducing several important experimental results. New insight was then gained regarding the behaviour of droplet wakes, an area with little previous research. The results of the test simulations show great promise for future use of the numerical framework developed. While the simulations todate have been of air-water interactions, there is little reason the model should be constrained to such a system. In theory almost any low speed isothermal interaction of immiscible Newtonain fluids, with length scales of greater than 1mm, could be modeled accurately by these methods.
Open Access
Design and construction of an acoustic measuring device to investigate automotive tyre noise
(Cranfield University, 2015) Zissimos, I.; Drikakis, Dimitris
Tyre noise measuring procedures are used worldwide and covered by appropriate International Standards, but from the literature search and the author's 30 years' expertise in acoustics, it became clear that there are still problems to overcome. The objective of this project is to design and construct a prototype stationary acoustic device that can produce and measure the noise emitted by the grooves and treads of automotive tyres and develop a new acoustic tyre measuring procedure. Through this new procedure, the noise measurement can be performed in a controlled laboratory environment, no expensive measuring platforms are needed, the cost of the tyre developers will be kept to a minimum, different models or a portion of the tyres can be used (even before tyre production starts), the tyres can be "noise classified" by public authorities, and the procedure will be isolated from the varying surfaces, e.g. roughness and porosity of the road pavement. The constructed apparatus / device is an open platform, and by using this procedure and methodology, research and many investigations can be performed in the future, to suit each investigator's objectives, both in tyre design and in pavement design. In the second stage of the thesis, it was necessary to compare the measured noise from the developed device with an actual rolling tyre, measured according to standard procedures. So a towed one wheel trailer, installed with microphones, has been constructed and tested. Validation of the trailer's noise data has been established conducting a further literature search and has proven satisfactory. In the third stage of the thesis, the main platform of the device was used to investigate further, more tyre related acoustic phenomena. Keywords: Tyre, road, acoustics, noise, measurement, spectral analysis, sound level meter, CPX trailer, pass-by, pavement interaction, porosity, texture, roughness, horn effect, noise regeneration.
Open Access
Eddy viscosity turbulence models for compressible mixing
(Cranfield University, 2011) Mihaiescu, Adrian N.; Drikakis, Dimitris
The K - L and K - ϵ turbulence models are used to simulate the turbulent mixing induced by the Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The models contain additional source terms for the turbulence kinetic energy which depend on the type of the instability. A new criterion based on ratio of the averaged flow and turbulence time scales is introduced for differentiating between the two types of instabilities. The original formulation of the turbulence kinetic energy source present in the K - ϵ model is modified in order to accurately capture the evolution of the Richtmyer-Meshkov instability in both heavy/light and light/heavy configurations. Additional constraints are imposed to the models in order to prevent non-physical solutions when strong gradients are present in the flow. Three test problems are considered and the performance of the turbulence models is assessed by comparing their solutions with the results obtained by high resolution Implicit Large Eddy Simulations (ILES). First, the classical Rayleigh-Taylor and Richtmyer-Meshkov problems are solved. A new approach for initializing the turbulence models in proposed for the Rayleigh-Taylor problem. It is found that both turbulence models describe successfully the self similar growth of the Rayleigh-Taylor and Richtmyer-Meshkov instabilities and can predict accurately the spatial distribution of the fluid concentrations and of the turbulent kinetic energy. The last problem involves the mixing induced at two planar interfaces by multiple shock reflections and refractions. The turbulence models estimate correctly the evolution of the mixing and of the total kinetic energy in the mixing zones. The transport equations of the turbulence models are solved numerically and the influence of the numerical schemes on the results is investigated. It is concluded that the numerical schemes do not have an important influence on the results in the case of the classical Rayleigh-Taylor problem (provided that grid convergence has been achieved and the turbulence models have been initialized using the method proposed here). However, in the presence of shocks (such as in the case of the Richtmyer-Meshkov instability), the HLLC Riemann solver should be used together with a reconstruction scheme of third or higher order of accuracy.