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1
Hällqvist, Thomas. "Large Eddy Simulation of Impinging Jets." Doctoral thesis, KTH, Mekanik, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3858.
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This thesis deals with Large Eddy Simulation (LES) of impinging air jets. The impinging jet configuration features heated circular jets impinging onto a flat plate. The problem addressed here is of generic nature, with applications in many engineering devices, such as cooling of components in gas turbines, in cars and electronic devices. The flow is inherently unsteady and contains relatively slowly varying coherent structures. Therefore, LES is the method of choice when the Reynolds number is large enough to exclude Direct Numerical Simulations (DNS). The present LES model is a basic model without explicit Sub-Grid-Scale (SGS) modeling and without explicit filtering. Instead, the numerical scheme is used to account for the necessary amount of dissipation. By using the computational grid as a filter the cutoff wavenumber depends explicitly on the grid spacing. The underlying computational grid is staggered and constructed in a Cartesian coordinate system. Heat transfer is modeled by the transport equation for a passive scalar. This is possible due to the negligible influence of buoyancy which implies constant density throughout the flow field. The present method provides accurate results for simple geometries in an efficient manner. A great variety of inlet conditions have been considered in order to elucidate how the dynamics of the flow and heat transfer are affected. The considered studies include top-hat and mollified mean velocity profiles subjected to random and sinusoidal perturbations and top-hat profiles superimposed with solid body rotation. It has been found that the shape of the mean inlet velocity profile has a decisive influence on the development of the flow and scalar fields, whereas the characteristics of the imposed artificial disturbances (under consideration) have somewhat weaker effect. In order to obtain results unequivocally comparable to experimental data on turbulent impinging jets both space and time correlations of the inflow data must be considered, so also the spectral content. This is particularly important if the region of interest is close to the velocity inlet, i.e. for small nozzle-to-plate spacings. Within this work mainly small nozzle-toplate spacings are considered (within the range of 0.25 and 4 nozzle diameters), which emphasizes the importance of the inflow conditions. Thus, additional to the basic methods also turbulent inflow conditions, acquired from a precursor pipe simulation, have been examined. Both for swirling and non-swirling flows. This method emulates fully developed turbulent pipe flow conditions and is the best in the sense of being well defined, but it demands a great deal of computing power and is also rather inflexibility. In case of the basic randomly perturbed methods the top-hat approach has been found to produce results in closest agreement with those originating from turbulent inlet conditions. In the present simulations the growth of individual instability modes is clearly detected. The character of the instability is strongly influenced by the imposed boundary conditions. Due to the lack of correlation random superimposed fluctuations have only a weak influence on the developing flow field. The shape of the mean profile, on the other hand, influences both the growth rate and the frequency of the dominant modes. The top-hat profile yields a higher natural frequency than the mollified. Furthermore, for the top-hat profile coalescence of pairs of vortices takes place within the shear-layer of the axial jet, whereas for the mollified profile (for the considered degree of mollification) it takes place within the wall jet. This indicates that the transition process is delayed for smoother profiles. The amount of wall heat transfer is directly influenced by the character of the convective vortical structures. For the mollified cases wall heat transfer originates predominantly from the dynamics of discrete coherent structures. The influence from eddy structures is low and hence Reynolds analogy is applicable, at least in regions of attached flow. The top-hat and the turbulent inflow conditions yield a higher rate of incoherent small scale structures. This strongly affects the character of wall heat transfer. Also the applied level of swirl at the velocity inlet has significant influence on the rate of heat transfer. The turbulence level increases with swirl, which is positive for heat transfer, and so also the spreading of the jet. The latter effect has a negative influence on wall heat transfer, particularly in the center most regions. This however depends also on the details of the inflow data.QC 20100831
2
Cavallo, Marincola Fabrizio. "Large eddy simulation of coal combustion." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/34316.
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In this work an in-house code for large-eddy simulations of coal combustion is developed and tested, with a special focus on the issue of modelling radiative heat transfer effects inside a furnace. An Eulerian-Lagrangian approach is used to describe the continuous gas phase and the discrete particle phase, with a two-way coupling between the two phases (implemented by another group member). The radiative transfer equation is solved using the discrete ordinates method, testing several different angular and spatial discretisation schemes. The spectral properties of the participating media are approximated with different grey gas models of varying complexity and accuracy. The accuracy of the radiative solver is initially assessed on simple idealised static cases in both two- and three-dimensions, and validated against benchmark data found in literature. The code is then integrated, parallelised and optimised with the LES flow and combustion solver, and used to simulate a large 2.4 MW coal combustion furnace. The results of the simulations are compared quantitatively against experimental data in terms of velocity, temperature, species distribution and solid particle analysis, showing a good agreement overall. A parametric study is then also performed on the variables and parameters of the radiation solver, showing great sensitivity on the outcome of the simulations in certain cases, further highlighting the importance of accurate radiation modelling for closed coal combustion furnaces.
3
Worthy, Jude. "Large eddy simulation of buoyant plumes." Thesis, Cranfield University, 2003. http://hdl.handle.net/1826/92.
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A 3d parallel CFD code is written to investigate the characteristics of and differences
between Large Eddy Simulation (LES) models in the context of simulating a thermal
buoyant plume. An efficient multigrid scheme is incorporated to solve the Poisson
equation, resulting from the fractional step, projection method used to solve the Low
Mach Number (LMN) Navier-Stokes equations.
A wide range of LES models are implemented, including a variety of eddy models,
structure models, mixed models and dynamic models, for both the momentum stresses
and the temperature fluxes. Generalised gradient flux models are adapted from their
RANS counterparts, and also tested.
A number of characteristics are observed in the LES models relating to the thermal
plume simulation in particular and turbulence in general. Effects on transition,
dissipation, backscatter, equation balances, intermittency and energy spectra are all
considered, as are the impact of the governing equations, the discretisation scheme,
and the effect of grid coarsening. Also characteristics to particular models are
considered, including the subgrid kinetic energy for the one-equation models, and
constant histories for dynamic models.
The argument that choice of LES model is unimportant is shown to be incorrect as a
general statement, and a recommendation for when the models are best used is given.
4
Ouro, Barba Pablo. "Large eddy simulation of tidal turbines." Thesis, Cardiff University, 2017. http://orca.cf.ac.uk/103301/.
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Understanding of hydrodynamics involved in the flow around tidal turbines is essential to enhance their performance and resilience, as they are designed to operate in harsh marine environments. During their lifespan, they are subjected to high velocities with large levels of turbulence that demand their design to be greatly optimised. Experimental tests have provided valuable information about the performance of tidal stream devices but these are often conducted in constricted flumes featuring turbulent flow conditions different to those found at deployment sites. Additionally, measuring velocities at prospective sites is costly and often difficult. Numerical methods arise as a tool to be used complementary to the experiments in investigations of tidal stream turbines. In this thesis, a high-fidelity large-eddy simulation computational approach is adopted and includes the immersed boundary method for body representation, due to its ability to deal with complex moving geometries. The combination of these numerical methods offers a great balance between computational resources and accuracy. The approach is applied and validated with simulations of vertical and horizontal axis tidal turbines, among other challenging cases such as a pitching airfoil. An extensive validation of predicted hydrodynamics, wake developed downstream of the devices or structural loadings, outlines the accuracy of the proposed computational approach. In the simulations of vertical axis tidal turbines, the blade-vortex interaction is highlighted as the main phenomenon dominating the physics of these devices. The horizontal axis tidal turbine is simulated under different flow and turbulence intensity conditions, in both flat and irregular channel bathymetries. This thesis seeks to assess and enhance the performance, resilience and survivability of marine hydrokinetic devices in their future deployment at sea.
5
Xie, Xuping. "Large Eddy Simulation Reduced Order Models." Diss., Virginia Tech, 2017. http://hdl.handle.net/10919/77626.
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This dissertation uses spatial filtering to develop a large eddy simulation reduced order model (LES-ROM) framework for fluid flows. Proper orthogonal decomposition is utilized to extract the dominant spatial structures of the system. Within the general LES-ROM framework, two approaches are proposed to address the celebrated ROM closure problem. No phenomenological arguments (e.g., of eddy viscosity type) are used to develop these new ROM closure models.
The first novel model is the approximate deconvolution ROM (AD-ROM), which uses methods from image processing and inverse problems to solve the ROM closure problem. The AD-ROM is investigated in the numerical simulation of a 3D flow past a circular cylinder at a Reynolds number $Re=1000$. The AD-ROM generates accurate results without any numerical dissipation mechanism. It also decreases the CPU time of the standard ROM by orders of magnitude.
The second new model is the calibrated-filtered ROM (CF-ROM), which is a data-driven ROM. The available full order model results are used offline in an optimization problem to calibrate the ROM subfilter-scale stress tensor. The resulting CF-ROM is tested numerically in the simulation of the 1D Burgers equation with a small diffusion parameter. The numerical results show that the CF-ROM is more efficient than and as accurate as state-of-the-art ROM closure models.Ph. D.
6
Keays, John F. "Large eddy simulation of premixed combustion." Thesis, Imperial College London, 2007. http://hdl.handle.net/10044/1/11284.
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Shi, Shaoping. "Large-eddy simulation of ship wakes." Morgantown, W. Va. : [West Virginia University Libraries], 2001. http://etd.wvu.edu/templates/showETD.cfm?recnum=2217.
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Thesis (Ph. D.)--West Virginia University, 2001.Title from document title page. Document formatted into pages; contains xv, 211 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 200-211).
8
Gobert, Christian. "Large Eddy Simulation of particle-laden flow." kostenfrei, 2010. https://mediatum2.ub.tum.de/node?id=829484.
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Hawkes, Evatt Robert. "Large eddy simulation of premixed turbulent combustion." Thesis, University of Cambridge, 2001. https://www.repository.cam.ac.uk/handle/1810/251761.
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Ma, Tingguang. "Large-eddy simulation of variable density flows." College Park, Md. : University of Maryland, 2006. http://hdl.handle.net/1903/4185.
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Thesis (Ph. D.) -- University of Maryland, College Park, 2006.Thesis research directed by: Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
11
García-Villalba, Navaridas Manuel. "Large eddy simulation of turbulent swirling jets." Karlsruhe : Univ.-Verl. Karlsruhe, 2006. http://deposit.d-nb.de/cgi-bin/dokserv?idn=979664586.
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O'Mahoney, T. S. D. "Large-eddy simulation of turbine rim seals." Thesis, University of Surrey, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.553681.
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This thesis describes the application of Large-Eddy Simulation (LES) to the internal flows within turbomachines. In particular, the work sought to investigate whether LES could give an improved predictive tool for flows through a turbine rim seal where other methods of modelling turbulence give disappointing results for the simulation of hot annulus gas ingestion into the rim seal cavity. The applicability of LES to flow regimes within a rotor-stator cavity were investigated with reference to two experimental studies, those of Daily & Nece [1] and of Itoh et al. [2]. LES simulations were run using the commercial Computational Fluid Dynamics (CFD) code FLUENT and the Rolls-Royce CFD code HYDRA. Both gave good agreement for the velocity field even when the boundary layer was not fully resolved. HYDRA was then used to simulate a turbine rim seal cavity with external flow through an annulus with NGV and rotor blades, modelled using a small sector with periodic boundary conditions in the circumferential direction. LES was found to predict higher levels of ingestion than Unsteady Reynolds- Averaged Navier-Stokes (URANS) simulations at a number of different values of cavity throughflow. This resulted in better, but not close, agreement with the experiments of Gentilhomme [3]. The sensitivity of the LES to changes in the size of the sector and to the resolution of the CFD grid were investigated. A larger sector simulation, corresponding to 40°, gave almost identical results to those on the smaller sector, 13.3°. Refinement to the CFD grid did lead to different results, particularly in the annulus flow, though critically the prediction of ingestion in the stator boundary layer was largely unaffected.
13
Ferraris, Sergio Adrian. "Large eddy simulation of under-ventilated fires." Thesis, Kingston University, 2007. http://eprints.kingston.ac.uk/20325/.
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The present thesis constitutes an important contribution to the understanding of a partially premixed combustion system associated with the hazardous backdraft phenomena. Backdraft may occur when fresh air is suddenly introduced into a vitiated environment where fire has already died out due to lack of oxygen but there are still unbumt fuel and products of incomplete combustion left. In the context of backdraft or deflagration, a complex flame structure is expected. Both, non-premixed and diffusion combustion might, in principle, be present. The present study focuses on the development of sub-grid scale (SaS) models to facilitate the study of such complex flame structure using the large eddy simulation (LES) technique. Before applying the model to the backdraft simulation, the individual sas models were firstly validated using simple configurations where detailed experimental data is available. A flamelet-like model for premixed combustion was introduced and thereafter coupled with the non-premixed combustion model through a "flame index" parameter. This concept makes use of the gradient signs of oxygen and fuel mass concentrations to distinguish between premixed and diffusion combustion regimes. In the present implementation in LES, an improved version of the flame index concept developed by Domingo et al. [67] was adopted. The model takes into account the fluctuations of both gradients at sub-grid level, which subsequently, might affect the filtered flame index value. In order to track the premixed flame front we used an approach which filters the progress variable balance equation using a filter larger than the actual LES grid. This approach has the advantage that it represents a physical meaningful variable and is stable from the numerical point of view because of the smooth gradients of the progress variable at the flame front and the species concentrations related to it. The flame front tracking technique was tested with an unstrained planar hydrogen flame front. Reasonably good results in burning speed and density ratio were obtained. The non-premixed or diffusion combustion regime was modelled using a flamelet-like model which considers the flame to be located at the stoichiometric value of the mixture fraction and it is related to the strain rate imposed by the counter flow of oxygen and fuel mass concentration feeding streams. This approach has the advantage that it has been tested for different scenarios and it is relatively fast as the variables can be pre-stored in a table. The flame index approach was tested using a laminar triple flame configuration. It was observed that the model could capture the different combustion regimes and predicted the lift-off height with reasonable accuracy. The location of the triple point was well predicted and the three branches further downstream could also be easily discerned from the predictions. Subsequently, a partially premixed turbulent lifted flame was simulated. In this case, it was ne,cessary to introduce the augment of burning velocity induced by the wrinkling of the flame front at sub-grid level. The sas flame front wrinkling factor is defined as the sub-grid scale flame surface divided by its projection in the resolved propagating direction. This can be regarded as the ratio of the sub-grid turbulent flame speed at grid scale (SrLl) and the laminar flame speed (s?). Reasonably good agreement was found on the lift-off prediction, the flame structure, and the mixture fraction profiles. A stabilization mechanism was discussed based on concepts previously exposed where the flame base faces a high velocity flow and a flammable mixture. Thereafter, the flame attempts to find its way upstream through low-speed flammable sections of the flow. It was found that in this process the stabilization point, herein identified as the maximum premixed heat release, plays an important role driving the flame base upstream the flow. Finally, two real scenarios of backdraft in a full scale fire test were simulated. These include the full scale backdraft experiment of Gojkovic [94] and the reduced scale experiment of Weng and Fan [254] . Unfortunately, there exist neither extensive nor accurate measurements før the former one and hence, the comparison against the numerical simulation was largely carried out on qualitative grounds. Five different stages were identified: 1) initial phase, 2) spherical propagation, 3) planar propagation, 4) flame front stretching and 5) fire ball. Qualitatively, the simulation agreed well with the experiment. The ignition delay time (the time from the opening of the hatch until the time when the ignition occurs) was well predicted by the simulation. It was also observed that the flame structure in the backdraft was predominantly premixed. More detailed measurements were available in the tests of Weng and Fan [254]. These included the upper layer temperatures, mass concentrations and pressures at the openings. Different opening geometries were used and the total mass flow rates in and out of the container were also measured. Overall, the predictions were in good agreement with the measurements and the model predicted the correct trend for pressure and mass flow rates in the tests with different openings. Furthermore, the predicted occurrence and non-occurrence of backdraft in different geometrical configurations was in line with the experimental observations in which backdraft did not always happen. During the earlier stages of the study, some effort was devoted to improving the sas turbulence models and to implement a CMC type SGS combustion model into the code. Unfortunately both models were later found to be unsuitable for the backdraft simulation. The first one suffered numerical instabilities caused by the under prediction of the Smagorinsky constant when applied to the backdraft case. The second one was deemed inappropriate due to its requirement of an homogeneous plane of conditional values. Nevertheless, some reasonably good results have been obtained with both models during the validation using simple geometrical configurations, namely a buoyant plume, a backward facing step and the Sandia-D non-premixed turbulent flame. The effort in this direction is therefore still included in the thesis as summarised below. A Lagrangian SGS turbulence model was implemented. Good results were found for classical benchmarking flow configurations such as the buoyant turbulent plume and the backward facing step. It was, however, found that the SGS turbulence viscosity became negative in a larger percentage than originally stated by Maneveour. Because of this, the model is prone to numerical instability. When applied to the backdraft simulation, the dynamically calculated Smagorinsky constant using this model was found to be consistently lower than the conventional range (0.1-0.23). This caused stability problem and made it very difficult to achieve converged solutions. The conditional source estimation (CSE) approach, which is a variation of the Conditional Moment Closure (CMC) approach, was also implemented. This model produced good results for the classical turbulent diffusion flame (SANDIA flame D). Even though the present implementation is not capable of predicting extinction/re-ignition events it was showed that it is very economic from computational point .of view. However, as explained above, this model was also considered as unsuitable for the backdraft simulation.
14
König, Marcel. "Large-Eddy Simulation Modelling for Urban Scale." Doctoral thesis, Universitätsbibliothek Leipzig, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-142714.
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In this work the model ASAM is enriched with new eddy viscosity based dynamic Smagorinsky subgrid-scale models. Therefore the model is more physically based to study atmospheric flow configurations at several atmospheric scales with main focus to urban scale flow with building-resolved resolution.
The implemented dynamic procedures work well and showed good agreement to literature data. In a convective atmospheric boundary layer (ABL) the dynamic Smagorinsky coefficient reaches maximum values of 0.15 and decreases towards the surface or in stable stratified flow regimes. Vertical profiles of the Smagorinsky coefficient in a diurnal cycle of ABL depict typical behaviour of the dynamic Smagorinsky coefficient in near surface flow, free-stream, or stable stratified flow.
Furthermore a modified inflow generation approach is proposed to produce fully turbulent flow fields. To modify a mean flow turbulent fluctuations are generated by superposition of sinusoidal and cosinesoidal modes. Due to the implementation of this inflow method the model ASAM has the ability to reproduce a given wind field with information from its mean wind speed and their fluctuation energy spectrum.
The model configuration developed in this work is able to reproduce flow structure in a complex urban geometry. The Mock Urban Setting Test (MUST) experiment represent an urban roughness geometry by placing 120 shipping containers ordinary arranged in an array. The used building-resolved resolution is able to capture dynamic flow structures like specific wake flow, recirculation regions or eddy detachment. The dynamic fluctuating behaviour of the wind velocity components is reproduced by the model with regard to peak magnitudes and their temporal occurrence. Satisfying agreement is found between tracer gas dispersion field measurements and the model results by capturing the fluctuating concentration magnitude and in some extend the mean values.
15
Semlitsch, Bernhard. "Large Eddy Simulation of Turbulent Compressible Jets." Doctoral thesis, KTH, Mekanik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-156230.
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Acoustic noise pollution is an environmental aggressor in everyday life. Aero- dynamically generated noise annoys and was linked with health issues. It may be caused by high-speed turbulent free flows (e.g. aircraft jet exhausts), by airflow interacting with solid surfaces (e.g. fan noise, wind turbine noise), or it may arise within a confined flow environment (e.g. air ventilation systems, refrigeration systems). Hence, reducing the acoustic noise levels would result in a better life quality, where a systematic approach to decrease the acoustic noise radiation is required to guarantee optimal results. Computational predic- tion methods able to provide all the required flow quantities with the desired temporal and spatial resolutions are perfectly suited in such application areas, when supplementing restricted experimental investigations. This thesis focuses on the use of numerical methodologies in compressible flow applications to understand aerodynamically noise generation mechanisms and to assess technologies used to suppress it. Robust and fast steady-state Reynolds Averaged Navier-Stokes (RANS) based formulations are employed for the optimal design process, while the high fidelity Large Eddy Simulation (LES) approach is utilized to reveal the detailed flow physics and to investigate the acoustic noise production mechanisms. The employment of fast methods on a wide range of cases represents a brute-force strategy used to scrutinize the optimization parameter space and to provide general behavioral trends. This in combination with accurate simulations performed for particular condi- tions of interest becomes a very powerful approach. Advance post-processing techniques (i.e. Proper Orthogonal Decomposition and Dynamic Mode Decomposition) have been employed to analyze the intricate, highly turbulent flows. The impact of using fluidic injection inside a convergent-divergent nozzle for acoustic noise suppression is analyzed, first using steady-state RANS simulations. More than 250 cases are investigated for the optimal injection location and angle, amount of injected flow and operating conditions. Based on a-priori established criteria, a few optimal candidate solutions are detected from which one geometrical configuration is selected for being thoroughly investigated by using detailed LES calculations. This allows analyzing the unsteady shock pattern movement and the flow structures resulting with fluidic injec- tion. When investigating external fluidic injection configurations, some lead to a high amplitude shock associated noise, so-called screech tones. Such unsteady phenomena can be captured and explained only by using unsteady simulations. Another complex flow scenario demonstrated using LES is that of a high ve- locity jet ejected into a confined convergent-divergent ejector (i.e. a jet pump). The standing wave pattern developed in the confined channel and captured by LES, significantly alters the acoustic noise production. Steady-state methods failed to predict such events. The unsteady highly resolved simulations proved to be essential for analyzing flow and acoustics phenomena in complex problems. This becomes a very powerful approach when is used together with steady-state, low time-consuming formulations and when complemented with experimental measurements.QC 20141202
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Wein, Lars [Verfasser]. "Large-Eddy-Simulation von Deckbandlabyrinthdichtungen / Lars Wein." Hannover : Gottfried Wilhelm Leibniz Universität Hannover, 2020. http://d-nb.info/1222160447/34.
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Ranga-Dinesh, K. K. J. "Large eddy simulation of turbulent swirling flames." Thesis, Loughborough University, 2007. https://dspace.lboro.ac.uk/2134/21086.
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Large eddy simulation (LES) is attractive as it provides a reasonable compromise between accuracy and cost, and is rapidly evolving as a practical approach for many engineering applications. This thesis is concerned with the application of large eddy simulation to unconfined swirl in turbulent non-premixed flames and isothermal flows. The LES methodology has been applied for the prediction of turbulent swirling reacting and non-reacting flows based on laboratory scale swirl burner known as the Sydney swirl burner, which has been a target flame of the workshop series of turbulent non-premixed flames (TNF). For that purpose a LES code was developed that can run wide range of applications. An algorithm was developed for LES of variable density reacting flow calculations. Particular attention was given to primitive conservation (mass, momentum and scalar) and kinetic energy of the flow and mixing field. The algorithm uses the primitive variables, which are staggered in both space and time. A steady laminar flamelet model which includes the detailed chemical kinetics and multi component mass diffusion, has been implemented in the LES code. An artificial inlet boundary condition method was implemented to generate instantaneous turbulent velocity fields that are imposed on the inflow boundary of the Cartesian grid. To improve the applicability of the code, various approaches were developed to improve stability and efficiency. LES calculations for isothermal turbulent swirling jets were successful in predicting experimentally measured mean velocities, their rms fluctuations and Reynolds shear stresses. The phenomenon of vortex breakdown (VB) and recirculation flow structures at different swirl and Reynolds numbers were successfully reproduced by the present large eddy simulations indicating that LES is capable of predicting VB phenomena which occurs only at certain conditions. For swirling flames, the LES predictions were able to capture the unsteady flow field, flame dynamics and showed good agreement with experimental measurements. The LES predictions for the mean temperature and major species were also successful.
18
König, Marcel. "Large-Eddy Simulation Modelling for Urban Scale." Doctoral thesis, Leibniz-Institut für Troposphärenforschung, 2013. https://ul.qucosa.de/id/qucosa%3A12447.
Full textAbstract:
In this work the model ASAM is enriched with new eddy viscosity based dynamic Smagorinsky subgrid-scale models. Therefore the model is more physically based to study atmospheric flow configurations at several atmospheric scales with main focus to urban scale flow with building-resolved resolution.
The implemented dynamic procedures work well and showed good agreement to literature data. In a convective atmospheric boundary layer (ABL) the dynamic Smagorinsky coefficient reaches maximum values of 0.15 and decreases towards the surface or in stable stratified flow regimes. Vertical profiles of the Smagorinsky coefficient in a diurnal cycle of ABL depict typical behaviour of the dynamic Smagorinsky coefficient in near surface flow, free-stream, or stable stratified flow.
Furthermore a modified inflow generation approach is proposed to produce fully turbulent flow fields. To modify a mean flow turbulent fluctuations are generated by superposition of sinusoidal and cosinesoidal modes. Due to the implementation of this inflow method the model ASAM has the ability to reproduce a given wind field with information from its mean wind speed and their fluctuation energy spectrum.
The model configuration developed in this work is able to reproduce flow structure in a complex urban geometry. The Mock Urban Setting Test (MUST) experiment represent an urban roughness geometry by placing 120 shipping containers ordinary arranged in an array. The used building-resolved resolution is able to capture dynamic flow structures like specific wake flow, recirculation regions or eddy detachment. The dynamic fluctuating behaviour of the wind velocity components is reproduced by the model with regard to peak magnitudes and their temporal occurrence. Satisfying agreement is found between tracer gas dispersion field measurements and the model results by capturing the fluctuating concentration magnitude and in some extend the mean values.:1 Introduction 1
2 Fundamentals of Large-Eddy Simulation in atmospheric boundary layers 7
2.1 The atmospheric boundary layer . . . . . . . . . . . . . . . . . . . . . 7
2.2 Atmospheric turbulence . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Basic equations of LES . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Subgrid-scale modelling 15
3.1 Eddy viscosity subgrid-scale models . . . . . . . . . . . . . . . . . . . 15
3.1.1 Smagorinsky subgrid-scale model . . . . . . . . . . . . . . . . 16
3.1.2 Dynamic Smagorinsky subgrid-scale model . . . . . . . . . . . 18
3.1.3 Scale-dependent dynamic Smagorinsky subgrid-scale model . . 23
3.2 Implementation in the All Scale Atmospheric Model (ASAM) . . . . . 26
3.2.1 General description of ASAM . . . . . . . . . . . . . . . . . . 26
3.2.2 Subgrid-scale modelling in ASAM . . . . . . . . . . . . . . . . 27
3.3 Applications to meteorological situations . . . . . . . . . . . . . . . . 37
3.3.1 Stable and unstable stratified atmospheric boundary layers . . 37
3.3.2 Flow over periodic sinusoidal hill . . . . . . . . . . . . . . . . 49
4 Generation of turbulent inflow conditions 51
4.1 The necessity of turbulent inflow . . . . . . . . . . . . . . . . . . . . 51
4.2 Synthetic turbulent inflow generation method . . . . . . . . . . . . . 53
4.3 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4 2D simulation results . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5 Mock Urban Setting Test Experiment (MUST) 65
5.1 Micro-scale urban simulation . . . . . . . . . . . . . . . . . . . . . . . 65
5.2 Description of the experiment . . . . . . . . . . . . . . . . . . . . . . 68
5.3 Wind tunnel measurenments of MUST . . . . . . . . . . . . . . . . . 70
5.4 Numerical MUST simulation with ASAM . . . . . . . . . . . . . . . . 72
5.4.1 Choice of initial condition . . . . . . . . . . . . . . . . . . . . 75
5.4.2 Results of simulating case 2682353 . . . . . . . . . . . . . . . 81
5.4.3 Results of simulating case 2681829 . . . . . . . . . . . . . . . 98
5.4.4 Case resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6 Summary and outlook 111
6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7 Bibliography 117
List of Figures 127
List of Tables 135
Acronyms 137
Nomenclature 139
Acknowledgement 143
List of Publications 145
19
Sheen, Shaw-Ching. "Large eddy simulation of subsonic mixing layers." Diss., Virginia Tech, 1993. http://hdl.handle.net/10919/40183.
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Ghelfi, Matteo. "Large Eddy Simulation in internal combustion Engine." Phd thesis, Matteo Ghelfi, 2013. https://tuprints.ulb.tu-darmstadt.de/3600/7/Diss_Matteo.pdf.
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Abstract
Large Eddy Simulation in internal combustion Engine
Dipl.-Ing Matteo Ghel�
The internal combustion (IC) engine simulation is nowadays one of the most difficult proceeding during engine development. Simultaneously this is one of the most important phase because it leads to better acknowledgment of in-cylinder phenomena and suggests new method of emission reduction and control.\\
Large Eddy Simulation (LES) can help this process because of its instationary nature, in perfect agreement with motion, injection and combustion, typical instationary components in this kind of study.\\
This work focuses on a method which includes all the phenomena happening during IC engine cycle in a LES. \\ The main purpose of this work is to include combustion on the simulation obtaining a complete engine cycle. The work concentrates in particular on models development and investigation of the perfect computational domain movement, necessary to reach an optimal resolution during all the engine cycle phases. \\
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Mompó, Laborda Juan Manuel. "Engineering Large Eddy Simulation of Diesel Sprays." Doctoral thesis, Universitat Politècnica de València, 2014. http://hdl.handle.net/10251/37345.
Full textAbstract:
The main objective of this PhD thesis is the study of Diesel sprays under
evaporative conditions by means of Large Eddy Simulations (LES) techniques.
This study has been performed implementing a precise, low-demanding LES
model in the free, full-purpose Computational Fluid Dynamics (CFD) code
OpenFOAM.
The starting point was a careful and exhaustive review of the physical processes
involved in sprays. An emphasis in CFD methodology, particularly for
LES methods, was essential for the thesis, as we were able to find the possible
problems and limitations of our approximation. Moreover, as the most
widely used techniques for the industrial simulation of sprays are based on
the Reynolds-Averaged Navier-Stokes models, we have highlighted the many
advantages of LES modeling. As the latter are, by definition, more computationally
expensive than RANS, we made an optimal configuration that, while
it is able to recover accurately the experimental results, its characteristic time
is in the same order of magnitude that RANS ones. As applicability is a must
in this thesis, we use the surname ¿Engineering¿ LES.
One of the key points of the thesis has been the correct configuration of
the flow turbulent conditions on the inlet. In order to get accurate results,
the turbulent structures coming from this inlet need to be time- and spacecoherent.
An adequate calibration of this conditions is needed to perform any
spray simulation.
Last but not least, all the simulations performed where validated against
experiments, obtaining a very good agreement even close to the nozzleMompó Laborda, JM. (2014). Engineering Large Eddy Simulation of Diesel Sprays [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/37345
TESIS
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Hickel, Stefan. "Implicit turbulence modeling for large-eddy simulation." kostenfrei, 2008. http://mediatum2.ub.tum.de/doc/654921/654921.pdf.
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Renze, Peter Clemens-August. "Large eddy simulation of film cooling flows /." Aachen : Shaker, 2008. http://d-nb.info/989959031/04.
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Hinterberger, Christof. "Dreidimensionale und tiefengemittelte Large-Eddy-Simulation von Flachwasserströmungen." Karlsruhe : Univ.-Verl, 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=973338083.
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Fukagata, Koji. "Large eddy simulation of particulate turbulent channel flows." Doctoral thesis, KTH, Mechanics, 1999. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-2911.
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El-Asrag, Hossam Abd El-Raouf. "Large Eddy Simulation Subgrid Model for Soot Prediction." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/14652.
Full textAbstract:
Soot prediction in realistic systems is one of the
most challenging problems in theoretical and applied combustion. Soot formation as a chemical process is very complicated and not fully understood up to the moment. The major difficulty stems from the chemical complexity of the soot formation processes as well as
its strong coupling with the other thermochemical and fluid processes that occur simultaneously. Soot is a major byproduct of incomplete combustion, having a strong impact on the environment, as well as the combustion efficiency. Therefore, it needs to be predicted in realistic configurations in an accurate and yet computationally efficient way.
In the current study, a new soot formation subgrid model is developed and reported here. The new
model is designed to be used within the context of the Large Eddy Simulation (LES) framework, combined with Linear Eddy Mixing (LEM)
as a subgrid combustion model. The final model can be applied equally to premixed and non-premixed flames over any required geometry and flow conditions in the free, the transition, and the
continuum regimes. The soot dynamics is predicted using a Method of Moments approach with Lagrangian Interpolative Closure (MOMIC) for the fractional moments. Since, no prior knowledge of the
particles distribution is required, the model is generally applicable. The effect of radiation is introduced as an optically thin model. As a validation the model is first applied to a
non-premixed non-sooting flame, then a set of canonically premixed flames. Finally, the model is validated against a non-premixed jet
sooting flame. Good results are predicted with reasonable accuracy.
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Pringuey, Thibault Roland Christophe Maurice. "Large eddy simulation of primary liquid-sheet breakup." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/244655.
Full textAbstract:
This research project aims at providing the aeronautical industry with a modelling capability to simulate the fuel injection in gas turbine combustion chambers. The path to this objective started with the review of state-of-the-art numerical techniques to model the primary breakup of liquid fuel into droplets. Based on this and keeping in mind the requirements of the industry, our modelling strategy led to the generation of a mass-conservative method for efficient atomisation modelling on unstructured meshes. This goal has been reached with the creation of high-order numerical schemes for unstructured grids, the development of an efficient numerical method that transports the liquid-vapour interface accurately while conserving mass and the implementation of an algorithm that outputs the droplet boundary conditions to separate combustion codes. Both high-order linear and WENO schemes have been created for general polyhedral meshes. The notorious complexity of high-order schemes on 3D mixed-element meshes has been handled by the creation of a series of algorithms. These include the tetrahedralisation of the mesh, which allows generality of the approach while remaining efficient and affordable, together with a novel approach to stencil generation and a faster interpolation of the solution. The performance of the scheme has been demonstrated on typical two-dimensional and three-dimensional test cases for both linear and non-linear hyperbolic partial differential equations. The conservative level set method has been extended to unstructured meshes and its performance has been improved in terms of robustness and accuracy. This was achieved by solving the equations for the transport of the liquid volume fraction with our novel WENO scheme for polyhedral meshes and by adding a flux-limiter algorithm. The resulting method, named robust conservative level set, conserves mass to machine accuracy and its ability to capture the physics of the atomisation is demonstrated in this thesis. To be readily applicable to the simulation of atomisation, the novel interfacecapturing technique has been embedded in a framework — within the open source CFD code OpenFOAM — that solves the velocity and pressure fields, outputs droplet characteristics and runs in parallel. In particular, the production of droplet boundary conditions involves a set of routines handling the selection of drops in the level set field, the calculation of relevant droplet characteristics and their storage into data files. An n-halo parallelisation method has been implemented in OpenFOAM to perform the computations at the expected order of accuracy. Finally, the modelling capability has been demonstrated on the simulation of primary liquid-sheet breakup with relevance to fuel injection in aero-engine combustors. The computation has demonstrated the ability of the code to capture the physics accurately and further illustrates the potential of the numerical approach.
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Langella, Ivan. "Large eddy simulation of premixed combustion using flamelets." Thesis, University of Cambridge, 2016. https://www.repository.cam.ac.uk/handle/1810/254303.
Full textAbstract:
Large Eddy Simulation (LES) has potential to address unsteady phenomena in turbulent premixed flames and to capture turbulence scales and their influence on combustion. Thus, this approach is gaining interest in industry to analyse turbulent reacting flows. In LES, the dynamics of large-scale turbulent eddies down to a cut-off scale are solved, with models to mimic the influences of sub-grid scales. Since the flame front is thinner than the smallest scale resolved in a typical LES, the premixed combustion is a sub-grid scale (SGS) phenomenon and involves strong interplay among small-scale turbulence, chemical reactions and molecular diffusion. Sub-grid scale combustion models must accurately represent these processes. When the flame front is thinner than the smallest turbulent scale, the flame is corrugated by the turbulence and can be seen as an ensemble of thin, one-dimensional laminar flames (flamelets). This allows one to decouple turbulence from chemistry, with a significant reduction in computational effort. However, potentials and limitations of flamelets are not fully explored and understood. This work contributes to this understanding. Two models are identified, one based on an algebraic expression for the reaction rate of a progress variable and the assumption of fast chemistry, the other based on a database of unstrained flamelets in which reaction rates are stored and parametrised using a progress variable and its SGS variance, and their potentials are shown for a wide range of premixed combustion conditions of practical interest. The sensitivity to a number of model parameters and boundary conditions is explored to assess the robustness of these models. This work shows that the SGS variance of progress variable plays a crucial role in the SGS reaction rate modelling and cannot be obtained using a simple algebraic closure like that commonly used for a passive scalar. The use of strained flamelets to include the flame stretching effects is not required when the variance is obtained from its transport equation and the resolved turbulence contains predominant part of the turbulent kinetic energy. Thus, it seems that SGS closure using unstrained flamelets model is robust and adequate for wide range of turbulent premixed combustion conditions.
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Branley, Niall Thomas. "Large eddy simulation of non-premixed turbulent flames." Thesis, Imperial College London, 2000. http://hdl.handle.net/10044/1/8584.
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Wille, Matthias Kurt Wilhelm. "Large eddy simulation of jets in cross flows." Thesis, Imperial College London, 1997. http://hdl.handle.net/10044/1/8322.
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Yu, Shaoxi. "Large eddy simulation of deflagration to detonation transition." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/41080.
Full textAbstract:
Deflagration to detonation transition (DDT) is a very important research project for both national defense and energy industry. It is the process where a subsonic deflagration transits into a supersonic detonation, which generates shock waves. In the past years, the simulations of DDT were limited in a small domain, usually sev- eral cubic centimeters. If we want to simulate it in a larger space without improving the numerical method, we need to use the more powerful computer. When the computing resources are limited, we must improve the numerical method to achieve the big-domain simulating. There are two technical paths, one is the adaptive mesh refinement and the other is the large eddy simulation. Both of them are difficult to realize. In this project, we focus on the usage of the LES method for simulating DDT. The main challenge in this work is to develop a reliable model. In this research, a new approach for LES modelling was developed. It is a fully compressible variant of the artificial thickened flame model, which adopts the opt- ing functions on the reference flame thickness. This method ensures that the flame is not over-thickened in deflagration or detonation. To control the options on the flame thickness, a detonation sensor is utilized during the computing.
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Alkishriwi, Nouri. "Large eddy simulation of low mach number flows /." Aachen : Shaker, 2007. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=016487054&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
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Papachristou, Charikleia. "Implicit Large Eddy Simulation of Environmental Urban Flows." Thesis, Cranfield University, 2010. http://dspace.lib.cranfield.ac.uk/handle/1826/4574.
Full textAbstract:
Many environmental flows are turbulent flows. Depending on the physical aspects
of the wind and the urban topology, turbulence might result into unfavourable
or even dangerous conditions for the pedestrians. Turbulence can also play
a very important role in the transport of toxic pollutants from accidental or intentional
releases. Thus, it is vital to understand its complex characteristics so that its features
are accurately predicted when computational methods are used. Real urban environment
involving separation and reattachment regions provides an excellent testcase for
investigating such complex flows.
This thesis is focused on analysing the physics involved in flows around building models
pertinent to environmental flows in urban areas and to evaluate the applicability
of Implicit Large-Eddy Simulation in simulating the specific type of flows. For this
purpose, a number of high resolution schemes in the context of Implicit Large-Eddy
Simulation (each representing di erent degrees of spatial discretisation accuracy) was
assessed.
The evaluation of the schemes involved direct validation against experimental data as
well as comparisons with DNS and LES data regarding flows within roughness element
arrays in staggered arrangements. Initially, the flow within an uniform height
cubical matrix was simulated. Four numerical schemes were tested in three di erent
grid resolutions. The results were found in very good agreement with the Laser
Doppler Anemometry data and they even exhibit DNS-like characteristics in specific
locations of comparisons. Thus, it was concluded that high order spatial discretisation
schemes allow the accurate representation of reality even in relatively coarse computational
meshes.
The second case under investigation involved flows within a more realistic representation
of urban topology. Results obtained within an array of sixteen elements with five
di erent heights reveal that although the roughness of the area is increased, the wind’s
velocity profile above the obstacles shares almost the same slope as in the case of the
array of the four cubical element.
It is believed that this thesis has expanded the range of applications in the context
of Implicit Large Eddy Simulation using high resolution schemes and contributed in
persuading the scientific community for its potentials.
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Mylonas, Antonios Athanassios. "Implicit large eddy simulation of turbulent duct flows." Thesis, Cranfield University, 2010. http://dspace.lib.cranfield.ac.uk/handle/1826/5700.
Full textAbstract:
Ducts can be found in ventilation systems, cooling ducts and blade passages of turbines,
centrifugal pumps and many other engineering installations. The properties of the flow in
ducts can significantly affect the performance and efficiency of these installation areas. The
majority of the flows in ducts and engineering applications are turbulent.
The work presented in this thesis focuses on the analysis of turbulent flows inside square
sectioned ducts and ducts with bends. The accuracy of three different high resolution high
order schemes in the context of Implicit Large Eddy Simulation (ILES) is analysed. The
influence of a low Mach limiting technique, Low Mach Number Treatment (LMNT) is also
studied. The schemes employed are Monotonic Upwind Scheme for Scalar Conservation
Laws (MUSCL) with a 2nd order Monotonized Central (MC) and 5th order limiter, and a
9th order Weighted Essential Non-Oscillatory (WENO) limiter.
The first case studied is a duct of square cross section . In the absence of experimental
data for the duct case, the data from a plain channel flow is used to shed light on the results.
The flow analysis points out the generation of secondary motions created by the existence of
surrounding walls. All schemes employed lead to a similarly developed turbulent flow that
is used to provide the turbulent boundary profile for the following case. LMNT proves to
significantly assist MUSCL 2nd and 5th, that use it, in providing a turbulent profile similar
to that of WENO 9th that did not employ the technique but is inherently less dissipative.
The second case under study is that of a square sectioned duct with a 90o bend. The
simulation output is in good agreement both qualitatively and quantitatively with the experimental
data available in the literature. The generation of secondary flows inside the bend
is observed without flow separation. Although the turbulent flow entering the domain is
almost the same for all cases, differences between the schemes are noticed especially after
the middle of the bend. LMNT leads to an overprediction of turbulence after that area for
both schemes employing it while WENO 9th without LMNT provides the most accurate
results compared to those provided by the experiment.
The results demonstrate applicability of ILES to strongly confined flows with secondary
motions and shed light on cognitive properties of a wide range of state of the art schemes.
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Xiao, Feng. "Large Eddy Simulation of liquid jet primary breakup." Thesis, Loughborough University, 2012. https://dspace.lboro.ac.uk/2134/10148.
Full textAbstract:
Atomisation of liquid fuel jets is an important determinant of combustion performance in gas turbine engines, and thus is the prime research driver here. Since the first stage of the atomisation process primary breakup has not been well understood due to its complexity, the objective of the current project is to develop a robust algorithm for Large Eddy Simulation (LES) to predict primary breakup. In order to provide realistic turbulent inflows for LES of liquid jet primary breakup, a rescaling/recycling method has been developed and validated. Three interface capturing ethods, namely Level Set (LS), Volume of Fluid (VOF), and coupled Level Set and VOF (CLSVOF), have been implemented and evaluated. The CLSVOF technique is adopted as the interface-tracking method in order to combine the advantages of LS and VOF methods. Due to the discontinuity of density and viscosity across the interface, simulations can become unstable due to numerical errors when a conventional discretisation approach is applied. Therefore, the governing equations are discretised here by introducing an extrapolated liquid velocity to minimise the interface momentum error, showing significant improvement in accuracy and robustness for simulations of primary breakup. For several reasons, single drop breakup in a uniform air flow is chosen as a benchmark test case for validation of the developed methodology for modelling atomisation. It is shown that the predicted drop breakup agrees quantitatively well with experiments for different Weber numbers. The solver is then applied to simulate primary breakup of liquid jets, which are more relevant to industrial applications. By simulating single round water jet atomisation in high-speed coaxial air flow, it is found that the predicted liquid core breakup lengths at different air/liquid velocities agree closely with measured data, but only when appropriate turbulent inflow conditions are specified. In simulations of liquid jet breakup in air crossflow, the penetration of the liquid jet is also well reproduced when turbulent inflows are used. In both simulations, it is found that the turbulence convected downstream from the injection nozzles affects significantly the primary breakup process, and the liquid turbulence rather than the gas turbulence plays a dominant role in initial disturbance of the liquid jet surface.
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Lyra, Sgouria. "Large eddy simulation of isothermal and reacting sprays." Thesis, Imperial College London, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526391.
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Liu, Weiyun. "INVESTIGATION OF FILTERING METHODS FOR LARGE-EDDY SIMULATION." UKnowledge, 2014. http://uknowledge.uky.edu/me_etds/46.
Full textAbstract:
This thesis focuses on the phenomenon of aliasing and its mitigation with two explicit filters, i.e., Shuman and Padé filters. The Shuman filter is applied to velocity components of the Navier--Stokes equations. A derivation of this filter is presented as an approximation of a 1-D “pure math” mollifier and extend this to 2D and 3D. Analysis of the truncation error and wavenumber response is conducted with a range of grid spacings, Reynolds numbers and the filter parameter, β. Plots of the relationship between optimal filter parameter β and grid spacing, L2-norm error and Reynolds number to suggest ways to predict β are also presented. In order to guarantee that the optimal β is obtained under various stationary flow conditions, the power spectral density analysis of velocity components to unequivocally identify steady, periodic and quasi-periodic behaviours in a range of Reynolds numbers between 100 and 2000 are constructed. Parameters in Pade filters need not be changed. The two filters are applied to velocities in this paper on perturbed sine waves and a lid-driven cavity. Comparison is based on execution time, error and experimental results.
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Xu, Baopeng. "Large eddy simulation of evaporating two-phase flows." Thesis, Kingston University, 2006. http://eprints.kingston.ac.uk/20324/.
Full textAbstract:
The objective of this study is to develop a CFD tool for performing reliable large eddy simulation (LES) of the compressible evaporating two-phase turbulent flow in a gas turbine combustor. The KIVA-3V code originally developed by Los Alamos National Laboratory is used as a baseline code. The KIVA-3V code has been modified to facilitate LES calculations. Both the temporal and spatial accuracies of the original KIVA-3V code have been improved to second order. A one-equation subgrid scale (SGS) turbulence model is implemented to describe the unresolved turbulent subgrid effect. To ensure that there are sufficient particle numbers to capture the dynamic droplet dispersion process, the ETAB breakup model coupled with a new hybrid droplet-particle algorithm is also implemented into the code. Furthermore, the effect of the subgrid scale (SGS) velocity on the droplet dispersion is included. The SGS velocity is computed from the subgrid turbulent kinetic energy predicted by the one-equation SGS turbulence model. A new collision model based on the concept of "particle cloud" is proposed and implemented in the code. The new model greatly reduces the grid-dependence of the original O'Rourke model in a Cartesian mesh. The gas solver of the new LES version of KIVA-3V code, which will be referred as KIVA-LES hereby) is validated against large eddy simulations of natural and forced plane impinging jets. Predictions were carried out for different inflow conditions, which include a natural plane impinging jet with a random perturbation on the inflow plane and a forced plane impinging jet with a Strouhal number of 0.36, locked both in phase and laterally in space. The first simulation was performed to quantitatively study the mean flow and turbulence statistics. The computed field variables and turbulence intensity of streamwise velocity agreed well with the experimental results. The second simulation was performed to study the vortex structures of a forced plane impinging jet. The predictions captured the typical vortex structures of this kind of flow, such as spanwise rollers, successive ribs, cross ribs and wall ribs were reproduced by the simulation, which were also previously detected by the experiment of Sakakibara et al. (103) with digital particle image velocimetry (DPIV) system, but to our -best knowledge never wholly reproduced by numerical simulations to date. Moreover, the study has also led to some new findings related to the formation and evolution of successive ribs, cross ribs and wall ribs. The new collision model is tested against analytical solutions of simplified realistic collision problems in a box volume. The grid-dependence of the model is also checked against some spray test cases. The new collision scheme is computationally more efficient than the frequently used O'Rourke's (87) scheme since it abandons a sampling procedure to compute the collision number. The new model delivers sufficient accuracy in calculating the collision numbers in cases with uniformly distributed droplets although O'Rourke's model seems to perform better for these scenarios. However, for the prediction of a real spray in Cartesian gird, the new model has delivered much improved results. The predictions of the new model do not show any grid-dependent artefacts. KIVA-LES with the Lagrangian spray models is used to predict non-evaporating and evaporating diesel fuel sprays. The computed results are compared with the experimental data by Hiroyasu and Kadota (55) and Naber and Siebers (81), as well as the predictions of the original KIVA-3V. The predictions are in good agreement with the data. The large scale vortical structures are reproduced by the LES simulations, which cause "branch-like" spray shape and influence the spray penetration depth. The predictions have also captured the differences between the dense and dilute regions of the sprays. The LES analysis of diesel sprays has also demonstrated that SGS velocity has significant influence on the predicted spray angles. Most importantly, grid-convergent results, which were difficult to obtain with the original KIVA-3V, have been obtained in the present study. Finally, the validated code is used to study evaporating two-phase spray flow in a coaxial gas turbine model combustor. The predictions were compared with some published experimental data. This is a first step towards a more comprehensive numerical analysis of practical industrial combustors where multiple inlets and more complex combustor geometry are encountered. Good agreement with the data is achieved. The predictions have captured the "ring-like" vortex just downstream the annulus and "worm-like" streamwise vortical structure further downstream. The axial droplet mass flux and Sauter mean radius (SMR) are well predicted. Overall the present study has demonstrated the capability of KIVA-LES with the newly developed collision model to provide reasonably accurate predictions of evaporating two-phase flows in coaxial gas turbine combustors.
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Chua, Ken. "Large Eddy Simulation of flow around bridge abutments." Thesis, Cardiff University, 2018. http://orca.cf.ac.uk/116655/.
Full textAbstract:
Extreme hydrological events have increased the frequency of ooding scenarios in recent years, resulting in significant bridge inundation and associated damages. Turbulence structures within the ow field are highly energetic and possess high sediment entrainment capacity which will lead to the scour formation around the bridge foundation and consequently causes structural instability or even failure of the structure. This research employs the method of Large Eddy Simulation (LES) to elucidate the complex ow mechanisms around bridge abutments in changing conditions. The level set method (LSM) is adopted in LES code to predict the complex water surface profiles and an extensive validation of the method against complementary experiment is presented. A faithful representation of a natural river which consists of an asymmetrical compound channel with a parabolic main channel and two variable-length abutments with sloped sidewalls and rounded corners, and a bridge deck is presented in this thesis. The LES code is used to analyse the effect of bridge abutment length on the turbulence structure and ow field through the bridge opening. Extensive analysis by means of streamwise velocity contours, 2D and 3D streamlines, isosurfaces of Q-criterion, contours of wall-normal vorticity, probability density functions, quadrant analysis, power density spectra, and water surface elevation contours has been carried out and have shown significant differences between the different abutment lengths. The findings attempt to contribute to the design of resilient hydraulic structures especially on considering the shape and size of an abutment. The investigation of ow mechanisms around bridge abutments under different scour conditions (i.e. pre-scour and equilibrium scour) is presented in the later part of the thesis. Through 3D streamlines and contours of vertical velocity and turbulent kinetic energy, the equilibrium scour case reveals an increase in the three-dimensionality of the ow around the left abutment in the scour region when compared with the at bed case. Focusing on the near bed quantities, i.e. bed shear stress and near bed turbulent kinetic energy, the equilibrium scour case shows a significant relaxation at the vicinity of the left abutment, indicating a drastic reduction in sediment activities.
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Prasad, Vinayaka Nakul. "Large eddy simulation of partially premixed turbulent combustion." Thesis, Imperial College London, 2011. http://hdl.handle.net/10044/1/11871.
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Baba, Ahmadi Mohammad Hassan. "Construction of inlet conditions for large eddy simulation." Thesis, University of Exeter, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.740749.
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Volavý, Jaroslav. "Řešení turbulentního dvoufázového proudění metodou Large Eddy Simulation." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2013. http://www.nusl.cz/ntk/nusl-234161.
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Doctoral thesis deals with the numerical simulations of two-phase flows, especially with prediction of movement of dispersed phase (particles) carried by fluid. The Euler-Lagrange approach was applied for description of the system fluid-particles. It means that the fluid is considered to be continuum and its movement is described using Euler approach. Particles are regarded as mass points and their movement is solved using Lagrangian approach. The Large Eddy Simulation method was adopted for solution of the fluid flow. The series of simulations of the backward-facing step flow laden with particles were performed. The concentration of the particles in the flow was high enough for consideration of the influence of particles on the turbulence of the carrier phase. The developed scheme for generation of turbulence on the inlet is applied. The influence of anisotropic decomposition of subgrid energy on movement of particles was studied in the frame of this work.
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Lettieri, Claudio. "Large eddy simulation of two-phase reacting flows." Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/11285.
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Niall, Branley. "Large eddy simulation of non-premixed turbulent flames." Boston Spa, U.K. : British Library Document Supply Centre, 1999. http://ethos.bl.uk/OrderDetails.do?did=1&uin=uk.bl.ethos.314128.
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ABRUNHOSA, JOSE DINIZ MESQUITA. "TURBULENT COMPLEX FLOW SIMULATION WITH CLASSICAL MODELING AND LARGE EDDY SIMULATION." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2003. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=4346@1.
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CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOUma investigação da capacidade de previsão de modelos de turbulência baseados na modelagem estatística clássica e de grandes escalas é apresentada. A modelagem estatística clássica de turbulência (média de Reynolds) foi analisada, através da solução de escoamentos complexos, como, por exemplo, o escoamento turbulento em degrau (backstep). Especial atenção foi dada aos modelos kapa-epsilon de baixo Reynolds e as variantes renormalizadas (RNG). O comportamento dos vários termos da equação da energia cinética turbulenta na região da parede foram analisados em detalhes, especialmente o termo de difusão de pressão. Avaliou-se a importância da correta modelagem do termo de difusão de pressão sobre as predições dos modelos de baixo número de Reynolds, nas regiões de recirculação. Alguns modelos, propostos na literatura para o termo de difusão de pressão, foram também avaliados teórica e numericamente. A capacidade de previsão da metodologia de simulação de grandes escalas (LES por Large Eddy Simulation) também foi realizada. O desempenho do modelo de Smagorinsky para prever escoamentos limitados por fronteiras sólidas foi avaliado do ponto de vista computacional. Utilizou-se o método de volumes finitos para integrar tanto as equações médias de Reynolds quanto as equações LES. O escoamento turbulento em canal foi resolvido de modo bidimensional e tridimensional. Já o escoamento em degrau (backstep) foi resolvido exclusivamente de modo bidimensional, enquanto o escoamento em um duto de seção quadrada foi simulado de modo tridimensional. Os resultados foram comparados com aqueles obtidos pelos modelos de baixo Reynolds, analisando-se a relação custo-benefício.
An investigation of turbulence models prediction capacity based on classical statistical modeling and large eddy simulation (LES) is presented. The classical statistical modeling (average of Reynolds) was analyzed, by investigating the solution of complex flows, as, for example, the turbulent flow past a backwardfacing- step (backstep). Special attention was given to low Reynolds number k-e models and models derived by renormalization group theory (RNG). The behavior of the different terms in the turbulent kinetic energy equation in the near wall region was examined in details, specially the pressure diffusion term. It was evaluated the importance of the correct modeling of the pressure diffusion term on the predictions of the low Reynolds number models, in recirculating flows. A few models, proposed in the literature for the pressure diffusion term, were also evaluated theoretically and numerically. The prediction capacity of large eddy simulation (LES) technique was also investigated. The ability of Smagorinsky model to predict complex limited wall flows was analyzed from a computational standpoint. The finite-volume method was employed to integrate both the Reynolds average and LES equations. The fully developed turbulent channel flow was solved in two- dimensional and three-dimensional numerical simulations. The turbulent flow over a backward-facing-step was computed exclusively in a twodimensional manner, while the fully developed turbulent flow in a straight square duct was simulated in a three-dimensional manner. The results were compared with those obtained by the low Reynolds models, analyzing the cost-benefit relation.
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Stone, Christopher. "Large-Eddy simulation of combustion dynamics in swirling flows." Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/13430.
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Ziefle, Jörg. "Large-eddy simulation of complex massively-separated turbulent flows /." Zürich : ETH, 2008. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17846.
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Porumbel, Ionuţ. "Large Eddy Simulation of premixed and partially premixed combustion." Available online, Georgia Institute of Technology, 2006, 2006. http://etd.gatech.edu/theses/available/etd-11042006-042840/.
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Thesis (Ph. D.)--Aerospace Engineering, Georgia Institute of Technology, 2007.Yeung, Pui-Kuen, Committee Member ; Lieuwen, Tim, Committee Member ; Menon, Suresh, Committee Chair ; Seitzman, Jerry, Committee Member ; Syed, Saadat, Committee Member.
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Porumbel, Ionut. "Large Eddy Simulation of premixed and partially premixed combustion." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/14050.
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Large Eddy Simulation (LES) of bluff body stabilized premixed and partially premixed combustion close to the flammability limit is carried out in this thesis. The LES algorithm has no ad-hoc adjustable model parameters and is able to respond automatically to variations in the inflow conditions.
Algorithm validation is achieved by comparison with reactive and non-reactive experimental data.
In the reactive flow, two scalar closure models, Eddy Break-Up (EBULES) and Linear Eddy Mixing (LEMLES), are used and compared. Over important regions, the flame lies in the Broken Reaction Zone regime. Here, the EBU model assumptions fail. The flame thickness predicted by LEMLES is smaller and the flame is faster to respond to turbulent fluctuations, resulting in a more significant wrinkling of the flame surface. As a result, LEMLES captures better the subtle effects of the flame-turbulence interaction.
Three premixed (equivalence ratio = 0.6, 0.65, and 0.75) cases are simulated. For the leaner case, the flame temperature is lower, the heat release is reduced and vorticity is stronger. As a result, the flame in this case is found to be unstable. In the rich case, the flame temperature is higher, and the spreading rate of the wake is increased due to the higher amount of heat release
Partially premixed combustion is simulated for cases where the transverse profile of the inflow equivalence ratio is variable. The simulations show that for mixtures leaner in the core the vortical pattern tends towards anti-symmetry and the heat release decreases, resulting also in instability of the flame. For mixtures richer in the core, the flame displays sinusoidal flapping resulting in larger wake spreading.
More accurate predictions of flame stability will require the use of detailed chemistry, raising the computational cost of the simulation. To address this issue, a novel algorithm for training Artificial Neural Networks (ANN) for prediction of the chemical source terms has been implemented and tested. Compared to earlier methods, the main advantages of the ANN method are in CPU time and disk space and memory reduction.
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Kirtaş, Mehmet. "Large Eddy Simulation of a High Aspect Ratio Combustor." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/14134.
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The present research investigates the details of mixture preparation and combustion in a two-stroke, small-scale research engine with a numerical methodology based on large eddy simulation (LES) technique. A major motivation to study such small-scale engines is their potential use in applications requiring portable power sources with high power density. The investigated research engine has a rectangular planform with a thickness very close to quenching limits of typical hydrocarbon fuels. As such, the combustor has a high aspect ratio (defined as the ratio of surface area to volume) that makes it different than the conventional engines which typically have small aspect ratios to avoid intense heat losses from the combustor in the bulk flame propagation period. In most other aspects, this engine involves all the main characteristics of traditional reciprocating engines. A previous experimental work has identified some major design problems and demonstrated the feasibility of cyclic combustion in the high aspect ratio combustor. Because of the difficulty of carrying out experimental studies in such small devices, resolving all flow structures and completely characterizing the flame propagation have been an enormously challenging task. The numerical methodology developed in this work attempts to complement these previous studies by providing a complete evolution of flow variables. Results of the present study demonstrated strengths of the proposed methodology in revealing physical processes occurring in a typical operation of the high aspect ratio combustor. For example, in the scavenging phase, the dominant flow structure is a tumble vortex that forms due to the high velocity reactant jet (premixed) interacting with the walls of the combustor. LES gives the complete evolution of this flow structure, from its beginning to its eventual decay after the scavenging period is over. In addition, LES is able to predict the interaction between the bulk flow at top dead center (TDC) and the turbulent flame propagation. The success of this depends on the ability of the model in predicting turbulent flow structure including its length and velocity scales.