82 results found
Patel M, Navarro-Martinez S, 2023, Heat transfer to proximal cylinders in hypersonic flow, Physics of Fluids, Vol: 35, ISSN: 1070-6631
Uncontrolled atmospheric entry of meteors, satellites, and spacecraft components often leads to their partial or complete demise. In this destructive process, driven by hypersonic ablation, reentry objects fragment, interact, and alter each other's aerothermal environment. The effect of these interactions on the heat transfer to the fragments has not been investigated, despite the heat transfer's importance in hypersonic ablation and reentry demise. This study focuses on the numerical investigation of heat transfer to proximal circular cylinders in a thermochemically frozen flow and in two dimensions. First, binary body configurations at Mach numbers 2, 4, and 8 revealed that the heat load and peak heat transfer can be augmented for either or both proximal bodies by +20% to -90% of an isolated body. Second, different clusters of five proximal bodies showed that the heat load to any given body can range from +40% to -90% of an isolated body. Moreover, the average heat load in a cluster is found to vary between +20% and -60% of an isolated body. Intuitively, clusters which are thin in the direction perpendicular to free-stream velocity and long in the direction parallel to the free-stream velocity have their cluster-averaged heat load reduced. In contrast, thick and thin clusters, in directions perpendicular and parallel to the free-stream velocity, are subject to an increased cluster-averaged heat load.
Zenkner G, Navarro-Martinez S, 2023, A flexible and lightweight deep learning weather forecasting model, Applied Intelligence, ISSN: 0924-669X
Numerical weather prediction is an established weather forecasting technique in which equations describing wind, temperature, pressure and humidity are solved using the current atmospheric state as input. This study examines deep learning to forecast weather given historical data from two London-based locations. Two distinct Bi-LSTM recurrent neural network models were developed in the TensorFlow deep learning framework and trained to make predictions in the next 24 and 72 h, given the past 120 h. The first trained neural network predicted temperature at Kew Gardens with a forecast accuracy of ± 2 ∘ C in 73% of instances in a whole unseen year, and a root mean squared errors of 1.45 ∘ C. The second network predicted 72-h air temperature and relative humidity at Heathrow with root mean squared errors 2.26 ∘ C and 14% respectively and 80% of the temperature predictions were within ± 3 ∘ C while 80% of relative humidity predictions were within ± 20%. Both networks were trained with five years of historical data, with cloud training times of over a minute (24-h network) and three minutes (72-h).
Rathore O, Navarro-Martinez S, 2023, Flame Dynamics Modelling Using Artificially Thickened Models, Flow, Turbulence and Combustion, ISSN: 1386-6184
Thickened flame models are prolific in the literature and offer an effective method of resolving flame dynamics on coarse LES meshes. The current state of the art relies heavily on the use of efficiency functions to compensate for impaired wrinkling of the thickened flame. However in practice these functions can involve parameters that are difficult to determine, perform poorly outside of certain ranges or require a posteriori analysis to evaluate performance. An alternative based on a generalised thickening is evaluated across a range of canonical configurations. The approach is demonstrated to perform well across a large range of thickening factors in capturing phenomena such as localised quenching and pinch off as well as generation of flame surface. Including good performance even in the case of large flame dynamics under acoustic forcing where the model has a clear advantage over DNS in achieving grid independence. Finally the approach is unified into an Large Eddy Simulation/Adaptive Mesh Refinement framework and applied to a turbulent Bunsen flame. The results show that even if the internal flame structure is poorly resolved on the original mesh, the global system behaviour is well predicted and compares favourably with other approaches.
Tretola G, Navarro-Martinez S, Vogiatzaki K, 2022, Primary break-up characterisation and droplet statistics of multi-hole sprays using a probabilistic surface density methodology, INTERNATIONAL JOURNAL OF MULTIPHASE FLOW, Vol: 152, ISSN: 0301-9322
Montgomery K, Bruce PJK, Navarro-Martinez S, 2022, Dynamics of varying thrust coefficients for supersonretro-propulsion during Mars EDL
This paper presents the results from a computational study of a supersonic retro-propulsion geometry with various adaptations to account for differing nozzle configurations. The study considers a central configuration with different diameter linearly diverging nozzles and focusses on the effects of different thrust coefficients on the drag of the craft, as well as the unsteady behaviour that arises from the jet-bow shock interaction. Results show both the long penetration mode and blunt penetration mode, as well as the transition between the two modes. The results presented in this paper are consistent with previous studies, which showed that axial force augmentation occurs for large thrust coefficients, while low values reduce axial force by changing the shape of the interaction. For the large diameter nozzle presented, there is no value of (formula presented) that decreases axial force below that of drag alone.
Tretola G, Vogiatzaki K, Navarro-Martinez S, 2021, Implementation of a probabilistic surface density volume of fluid approach for spray atomisation, COMPUTERS & FLUIDS, Vol: 230, ISSN: 0045-7930
Tretola G, Vogiatzaki K, Navarro-Martinez S, 2021, Effect of the density ratio variation on the dynamics of a liquid jet injected into a gaseous cross-flow, PHYSICS OF FLUIDS, Vol: 33, ISSN: 1070-6631
Mukundan AA, Tretola G, Menard T, et al., 2021, DNS and LES of primary atomization of turbulent liquid jet injection into a gaseous crossflow environment, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 38, Pages: 3233-3241, ISSN: 1540-7489
Almeida Y, Navarro-Martinez S, 2021, Joint-velocity scalar energy probability density function method for large eddy simulations of compressible flow, PHYSICS OF FLUIDS, Vol: 33, ISSN: 1070-6631
Prat A, Sautory T, Navarro-Martinez S, 2020, A Priori Sub-grid Modelling Using Artificial Neural Networks, INTERNATIONAL JOURNAL OF COMPUTATIONAL FLUID DYNAMICS, Vol: 34, Pages: 397-417, ISSN: 1061-8562
Christodoulou L, Karimi N, Cammarano A, et al., 2020, State prediction of an entropy wave advecting through a turbulent channel flow, JOURNAL OF FLUID MECHANICS, Vol: 882, ISSN: 0022-1120
Ahmed A, Tretola G, Navarro-Martinez S, et al., 2020, ATOMIZATION MODELING USING SURFACE DENSITY AND STOCHASTIC FIELDS, ATOMIZATION AND SPRAYS, Vol: 30, Pages: 239-266, ISSN: 1044-5110
Tretola G, Vogiatzaki K, Navarro-Martinez S, 2020, Sub-grid effects in atomisation process using stochastic fields
Liquid atomization involves the interaction of a broad range of length and time scales, which make impractical the use of Detailed Numerical Simulation to resolve all scales. In the present work we use Large Eddy Simulations, where large scales are resolved directly and small scales require closure models. To model sub-grid liquid breakup, the paper use a novel framework, Σ-Y-PDF. The model solves the sub-grid joint probability density function of the liquid volume fraction and surface density. The probability density function allows an instantaneous statistical description of the two variables, and therefore provides sub-grid droplets (or fragments) distribution. The probability density function equation is solved using the Monte Carlo method of Eulerian stochastic fields. The method is used to investigate the influence of sub-grid liquid structures on spray dynamics in two academic configurations: a liquid flow in a cross jet (prototype of a gas-turbine injector) and a liquid turbulent jet (akin to a diesel injector). The same approach is used in both configurations to investigate the influence of small scales in the major spray characteristics: spray liquid penetration, liquid break-up point and droplet distribution. The sensitivity of the suggested framework to the number of stochastic fields and model parameter is also investigated.
Navarro-Martinez S, Tretola G, Yosri MR, et al., 2019, An investigation on the impact of small-scale models in gasoline direct injection sprays (ECN Spray G), INTERNATIONAL JOURNAL OF ENGINE RESEARCH, Vol: 21, Pages: 217-225, ISSN: 1468-0874
Almeida YP, Navarro-Martinez S, 2019, Large Eddy simulation of supersonic combustion using the eulerian stochastic fields method, Flow, Turbulence and Combustion, Vol: 103, Pages: 943-962, ISSN: 1386-6184
The development of supersonic combustion engines (scramjet-type) presents several challenges. Numerical simulations of scramjet engine combustion have become an attractive alternative to experimental investigations. However, supersonic combustion simulations still have several challenges: such as adequate modelling of shock/boundary layer and turbulence/chemistry interactions. The present work presents two large-eddy simulation probability density function (LES-PDF) novel formulations developed for high-speed applications. One is a conservative joint-scalar approach, where the joint-probability for reactive scalars and energy is solved, while the other is a joint velocity-scalar. The LES-PDF transport equations are solved using the Eulerian stochastic fields technique implemented in a density-based compressible solver. The performance of the models is verified through the simulations of two and three-dimensional supersonic reacting mixing layers and compared against DNS data from the literature. The results show that the joint-scalar formulation is accurate and robust, while the joint velocity-scalar closures require further development.
Zeng W, Vogiatzaki K, Navarro-Martinez S, et al., 2019, Modelling of sub-grid scale reaction rate based on a novel series model: Application to a premixed bluff-body stabilised flame, Combustion Science and Technology, Vol: 191, Pages: 1043-1058, ISSN: 0010-2202
In this paper, a new model for closing the sub-grid reaction rate is proposed based on the series expansion of the chemical source term around the filtered value. For validation, large eddy simulations of a bluff-body stabilised premixed flame are performed at three different grid resolutions, and results are compared with experimental data. Simulations neglecting the sub-grid contributions of the source term are also conducted to examine the relative sub-grid contribution. The results show that the series model reproduces correctly key characteristics such as flame anchoring, recirculation zones and shear layers. Statistically, good agreement with experimental data is obtained by the series model, in terms of time-averaged profiles of velocity and its fluctuations, and temperature as well as the size of the recirculation region. With increasing mesh refinement, the “no-model” approach results improve and the predictions are similar (albeit always worse) to those of the series model.
Noh D, Navarro-Martinez S, 2019, Investigation of the jet-flame interaction by large eddy simulation and proper decomposition method, Combustion Science and Technology, Vol: 191, Pages: 956-978, ISSN: 0010-2202
Large eddy simulation (LES) results are presented for a premixed methane/air turbulent flame arising from a confined laboratory-scale single-nozzle burner. The jet issuing from an off-centered nozzle facilitates the development of a large-scale, dominant lateral recirculation zone that stabilizes the flame. A self-sustained jet oscillation is present, which intermittently causes extreme flame fluctuations such as blowout and relight events in the bottom section of the combustion chamber. The combined probability density function transport approach with the Eulerian stochastic fields method is used to numerically investigate the influence of this jet oscillation on combustion stability at the operating condition near lean blowout. The general structure of the flow, including the formation of the recirculation zones depending on the location of the flapping jet, is well-reproduced together with the mean and fluctuating velocity profiles. The behavior of the jet oscillation is investigated using a popular decomposition method known as proper orthogonal decomposition (POD) based on the predicted three-dimensional flow fields. Thanks to POD, the evolution of the simulated flame structure featuring a pronounced flame fluctuation is compared against that experimentally measured according to the phase angles of the low-order modeled jet motion. The absence of the most dominant coherent structure at a single frequency is due to a feedback mechanism between the jet oscillation and combustion process. The simulation shows that a low-frequency jet flapping causes the flame blowout and flashback in the bottom section of the combustor and a stable flame persists as long as the jet flapping rate exceeds a critical value.
Almeida YPD, Navarro-Martinez S, 2019, Large Eddy Simulation of a supersonic lifted flame using the Eulerian stochastic fields method, Proceedings of the Combustion Institute, Vol: 37, Pages: 3693-3701, ISSN: 0082-0784
Scramjet propulsion systems can be the key to deliver the next generation of hypersonic planes. The high costs and complexity of gathering experimental data is a limiting factor in the development of such engine. In this context, numerical simulation has become increasingly popular to investigate supersonic combustion phenomena that otherwise would be prohibitively expensive. Despite recent progress, the simulation of high-speed compressible and reactive flows is still very challenging and presents many associated challenges. The chemical source term is highly non-linear and most combustion models are designed to operate in low-Mach number conditions. The present work investigates the use of Probability Density Function (PDF) in the context of Large Eddy Simulation models under supersonic conditions. Two approaches are considered: an extension of the joint scalar-enthalpy PDF for high-speed flows and a novel joint velocity-scalar-energy PDF model. Both formulations use the Eulerian stochastic fields approach implemented in a fully compressible density-based CFD code. The performance of the models are investigated in a supersonic lifted flame, comparing the stochastic formulations with traditional models that neglect sub-grid fluctuations. The results show that sub-grid contributions are important at coarse meshes and the stochastic fields approach can reproduce the experimental data and the scatter observed. The simulations suggest that the scalar-enthalpy PDF is the most robust formulations and the sub-grid closures of the joint velocity-scalar PDF need further investigation.
Noh D, Karlis E, Navarro-Martinez S, et al., 2019, Azimuthally-driven subharmonic thermoacoustic instabilities in a swirl-stabilised combustor, Proceedings of the Combustion Institute, Vol: 37, Pages: 5333-5341, ISSN: 0082-0784
A joint experimental and computational study of thermoacoustic instabilities in a model swirl-stabilised combustor is presented. This paper aims to deliver a better characterisation of such instabilities through the examination of measurements in conjunction with the numerical results obtained via Large Eddy Simulation (LES). The experimental configuration features a cylindrical combustion chamber where the lean premixed methane/air flame experiences self-sustained thermoacoustic oscillations. The nonlinear behaviour of the acoustically excited flame is experimentally investigated by broadband chemiluminescence and dynamic pressure measurements. In LES, the Eulerian stochastic field method is employed for the unknown turbulence/chemistry interaction of the gas-phase. Comparisons of the predicted frequency spectrum and phase-resolved flame structure with measurements are found to be in good agreement, confirming the predictive capabilities of the LES methodology. The presence of Period 2 thermoacoustic oscillations, as a consequence of period-doubling bifurcation, is also confirmed in LES. Through the application of nonlinear analysis both to the experimental and numerical acoustic fluctuations, it is highlighted that the nonlinear behaviour of combustion instabilities in the burner under investigation follows a pattern typical of Period 2 oscillations. Furthermore, the current work demonstrates a useful approach, through the use of dynamic mode decomposition (DMD), for the investigation of unstable flame modes at a specific frequency of interest. The experimental and numerical DMD reconstructions suggest that hot combustion products are convected azimuthally at a rate dictated by the subharmonic frequency.
Abbas F, Tretola G, Cleary MJ, et al., 2019, On the use of LES-PDF form of the Eulerian-Lagrangian Spray Atomisation (ELSA) model to simulate air blast atomisation
© Asia-Pacific Conference on Combustion, ASPACC 2019.All right reserved. This paper employs the Eulerian-Lagrangian Spray Atomisation (ELSA) model to predict the structure of atomising air-blast spray jets stabilised on the Sydney needle burner. The subgrid joint probability density function (PDF) of liquid volume fraction and surface density is solved in the LES framework to characterise the spray breakup. Two spray jets are used for validation and predictions and measurements of the normalised liquid volume fraction are compared. This paper focuses on reporting the effects of mesh size and subgrid Schmidt number, and it is found that the lower Weber number spray case is more sensitive to these parameters. While significant development is still needed, the initial results are encouraging and point to a good prospect in resolving the structure of the primary atomisation region.
Abbas F, Tretola G, Cleary MJ, et al., 2019, On the use of LES-PDF form of the Eulerian-Lagrangian Spray Atomisation (ELSA) model to simulate air blast atomisation
This paper employs the Eulerian-Lagrangian Spray Atomisation (ELSA) model to predict the structure of atomising air-blast spray jets stabilised on the Sydney needle burner. The subgrid joint probability density function (PDF) of liquid volume fraction and surface density is solved in the LES framework to characterise the spray breakup. Two spray jets are used for validation and predictions and measurements of the normalised liquid volume fraction are compared. This paper focuses on reporting the effects of mesh size and subgrid Schmidt number, and it is found that the lower Weber number spray case is more sensitive to these parameters. While significant development is still needed, the initial results are encouraging and point to a good prospect in resolving the structure of the primary atomisation region.
Picciani MA, Richardson ES, Navarro-Martinez S, 2018, Resolution requirements in stochastic field simulation of turbulent premixed flames, Flow, Turbulence and Combustion, Vol: 101, Pages: 1103-1118, ISSN: 1386-6184
The spatial resolution requirements of the Stochastic Fields probability density function approach are investigated in the context of turbulent premixed combustion simulation. The Stochastic Fields approach is an attractive way to implement a transported Probability Density Function modelling framework into Large Eddy Simulations of turbulent combustion. In premixed combustion LES, the numerical grid should resolve flame-like structures that arise from solution of the Stochastic Fields equation. Through analysis of Stochastic Fields simulations of a freely-propagating planar turbulent premixed flame, it is shown that the flame-like structures in the Stochastic Fields simulations can be orders of magnitude narrower than the LES filter length scale. The under-resolution is worst for low Karlovitz number combustion, where the thickness of the Stochastic Fields flame structures is on the order of the laminar flame thickness. The effect of resolution on LES predictions is then assessed by performing LES of a laboratory Bunsen flame and comparing the effect of refining the grid spacing and filter length scale independently. The usual practice of setting the LES filter length scale equal to grid spacing leads to severe under-resolution and numerical thickening of the flame, and to substantial error in the turbulent flame speed. The numerical resolution required for accurate solution of the Stochastic Fields equations is prohibitive for many practical applications involving high-pressure premixed combustion. This motivates development of a Thickened Stochastic Fields approach (Picciani et al. Flow Turbul. Combust. X, YYY (2018) in order to ensure the numerical accuracy of Stochastic Fields simulations.
Picciani MA, Richardson ES, Navarro-Martinez S, 2018, A thickened stochastic fields approach for turbulent combustion simulation, Flow, Turbulence and Combustion, Vol: 101, Pages: 1119-1136, ISSN: 1386-6184
The Stochastic Fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the Stochastic Fields equations that require grid spacing much finer than the filter scale used for the Large Eddy Simulation. The conventional approach of using grid spacing equal to the filter scale yields substantial numerical error, whereas using grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Thickened Stochastic Fields approach is developed in this study in order to provide physically-accurate and numerically-converged solutions of the Stochastic Fields equations with reduced compute time. The Thickened Stochastic Fields formulation bridges between the conventional Stochastic Fields and conventional Thickened-Flame approaches depending on the numerical grid spacing utilised. One-dimensional Stochastic Fields simulations of freely-propagating turbulent premixed flames are used in order to obtain criteria for the thickening factor required, as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the Stochastic Fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory premixed Bunsen flame. The results demonstrate that the Thickened Stochastic Fields method produces accurate predictions even when using a grid spacing equal to the filter scale. The present development therefore facilitates the accurate application of the Stochastic Fields approach to industrially-relevant combustion systems.
Noh D, Gallot-Lavallée S, Jones WP, et al., 2018, Comparison of droplet evaporation models for a turbulent, non-swirling jet flame with a polydisperse droplet distribution, Combustion and Flame, Vol: 194, Pages: 135-151, ISSN: 0010-2180
The current investigation aims to present the numerical results of a turbulent spray jet flame in the framework of Large Eddy Simulation. The injection of n-heptane liquid fuel is achieved through the use of a pressure-swirl atomiser generating a lifted spray flame. The evolution of the sub-grid probability density function of relevant scalars is accounted for by the Eulerian stochastic field method. A non-reactive spray computation is conducted in a preliminary stage. The simulation allows for a stochastic breakup formulation in combination with a stochastic dispersion model to confirm their predictive capabilities. This is achieved as the liquid properties such as droplet diameter and velocity profiles are found to be in excellent agreement between the simulated results and measurements. The main target of the work is to assess the performance of the most widely accepted evaporation models in a turbulent droplet-laden flame. In this paper, a detailed comparison of the evaporation models is conducted thanks to a recent development in measurement techniques which allow to permit the spray temperature profiles. The time-averaged droplet temperature across the spray flame is therefore investigated in order to validate several droplet vaporisation models. All the models under consideration are found to capture the formation of a double reaction zone flame and the measured lift-off height is reproduced within a satisfactory level of accuracy. This good reproduction of the flame morphology additionally confirms the performance of the pdf approach as a closure for the unknown turbulence–chemistry interaction in this type of spray flames. However, the simulated wet-bulb temperature in the hot burnt gas between the two reaction zones in addition to the profile along the spr ay centreline show a large discrepancy when compared to measurements. This large difference thus requires further investigation both in the modelling of evaporation and in the accuracy of measureme
Almeida YP, Navarro-Martinez S, 2018, Large eddy simulation of supersonic combustion using the Eulerian stochastic fields method, 9th International Symposium on Turbulence, Heat and Mass Transfer (THMT), Publisher: BEGELL HOUSE, INC, Pages: 1037-1048, ISSN: 2377-2816
Navarro-Martinez S, 2018, Conditional Moment Closure Methods for Turbulent Non-premixed Combustion, MODELING AND SIMULATION OF TURBULENT COMBUSTION, Editors: De, Agarwal, Chaudhuri, Sen, Publisher: SPRINGER-VERLAG SINGAPORE PTE LTD, Pages: 291-310, ISBN: 978-981-10-7409-7
Tretola G, Vogiatzaki K, Navarro-Martinez S, 2017, Detailed simulation of air-assisted spray atomization: effect of numerical scheme at intermediate Weber number, 28th European Conference on Liquid Atomization and Spray Systems (ILASS), Publisher: UNIV POLITECNICA VALENCIA, Pages: 249-256
Pesmazoglou I, Kempf A, Navarro-Martinez S, 2017, Large eddy simulation of particle aggregation in turbulent jets, Journal of Aerosol Science, Vol: 111, Pages: 1-17, ISSN: 0021-8502
Aggregation is an inter-particle process that involves a multitude of different physical and chemical mechanisms. Aggregation processes often occur within turbulent flows; for example in spray drying, soot formation, or nanoparticle formation. When the concentration of particles is very large, a direct simulation of individual particles is not possible and alternative approaches are needed. The present work follows the stochastic aggregation modelling based on a Lagrangian framework by Pesmazoglou, Kempf, and Navarro-Martinez (2016) and implements it in the Large Eddy Simulation context. The new coupled model is used to investigate particle aggregation in turbulent jets. Two cases are considered: an existent Direct Numerical Simulation of nanoparticle agglomeration in a planar jet and an experimental configuration of nanoparticles in a round jet. The results show a good agreement in both cases, demonstrating the advantages of the Lagrangian framework to model agglomeration and it capacity to describe the full particle size distribution.
Noh D, Gallot Lavallee S, Jones WP, et al., 2017, Validation of droplet evaporation models for a polydisperse spray in a non-swirling jet flame, European Combustion Meeting 2017, Publisher: Combustion Institute
The present work aims to deliver a numerical investigation of the turbulent spray jet flame in the context of LargeEddy Simulation. Several droplet evaporation models are studied in order to confirm their predictive capabilities interms of the time-averaged droplet temperature in the two-phase reactive flow. All the models under considerationare found to capture the formation of a double reaction zone flame and the measured lift-off height within a goodlevel of accuracy. However, the simulated wet-bulb temperature in the hot burnt gas between the two reaction zonesis considerably higher in comparison to measurements. This large discrepancy thus requires a further investigation inthe evaporation modelling.
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