scholarly journals Numerical Framework for the Computation of Urban Flux Footprints Employing Large-eddy Simulation and Lagrangian Stochastic Modeling

2017 ◽  
Author(s):  
Mikko Auvinen ◽  
Leena Järvi ◽  
Antti Hellsten ◽  
Üllar Rannik ◽  
Timo Vesala
Keyword(s):  
Large Eddy Simulation ◽  
Urban Landscape ◽  
Computational Cost ◽  
Source Area ◽  
Target Volume ◽  
Particle Analysis ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Flux Measurements ◽  
Urban Footprint

Abstract. Conventional footprint models cannot account for the heterogeneity of the urban landscape imposing a pronounced uncertainty on the spatial interpretation of eddy-covariance (EC) flux measurements in urban studies. This work introduces a computational methodology that enables the generation of detailed footprints in arbitrarily complex urban flux measurements sites. The methodology is based on conducting high-resolution large-eddy simulation (LES) and Lagrangian stochastic (LS) particle analysis on a model that features a detailed topographic description of a real urban environment. The approach utilizes an arbitrarily sized target volume set around the sensor in the LES domain, to collect a dataset of LS particles which are seeded from the potential source-area of the measurement and captured at the sensor site. The urban footprint is generated from this dataset through a piecewise post-processing procedure, which divides the footprint evaluation into multiple independent processes that each yield an intermediate result that are ultimately selectively combined to produce the final footprint. The strategy reduces the computational cost of the LES-LS simulation and incorporates techniques to account for the complications that arise when the EC sensor is mounted on a building instead of a conventional flux tower. The presented computational framework also introduces a result assessment strategy which utilizes the obtained urban footprint together with a detailed land cover type dataset to estimate the potential error that may arise if analytically derived footprint models were employed instead. The methodology is demonstrated with a case study that concentrates on generating the footprint for a building-mounted EC measurement station in downtown Helsinki, Finland, under neutrally stratified atmospheric boundary layer.

scholarly journals Numerical framework for the computation of urban flux footprints employing large-eddy simulation and Lagrangian stochastic modeling

2017 ◽  
Vol 10 (11) ◽  
pp. 4187-4205 ◽  
Author(s):  
Mikko Auvinen ◽  
Leena Järvi ◽  
Antti Hellsten ◽  
Üllar Rannik ◽  
Timo Vesala
Keyword(s):  
Large Eddy Simulation ◽  
Urban Landscape ◽  
Computational Cost ◽  
Source Area ◽  
Target Volume ◽  
Particle Analysis ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Flux Measurements ◽  
Urban Footprint

Abstract. Conventional footprint models cannot account for the heterogeneity of the urban landscape imposing a pronounced uncertainty on the spatial interpretation of eddy-covariance (EC) flux measurements in urban studies. This work introduces a computational methodology that enables the generation of detailed footprints in arbitrarily complex urban flux measurements sites. The methodology is based on conducting high-resolution large-eddy simulation (LES) and Lagrangian stochastic (LS) particle analysis on a model that features a detailed topographic description of a real urban environment. The approach utilizes an arbitrarily sized target volume set around the sensor in the LES domain, to collect a dataset of LS particles which are seeded from the potential source area of the measurement and captured at the sensor site. The urban footprint is generated from this dataset through a piecewise postprocessing procedure, which divides the footprint evaluation into multiple independent processes that each yield an intermediate result. These results are ultimately selectively combined to produce the final footprint. The strategy reduces the computational cost of the LES–LS simulation and incorporates techniques to account for the complications that arise when the EC sensor is mounted on a building instead of a conventional flux tower. The presented computational framework also introduces a result assessment strategy which utilizes the obtained urban footprint together with a detailed land cover type dataset to estimate the potential error that may arise if analytically derived footprint models were employed instead. The methodology is demonstrated with a case study that concentrates on generating the footprint for a building-mounted EC measurement station in downtown Helsinki, Finland, under the neutrally stratified atmospheric boundary layer.

Validation of Species-Based Extended Coherent Flamelet Model in a Large Eddy Simulation of a Homogeneous Charge Spark Ignition Engine

2020 ◽  
Author(s):  
Y. See ◽  
M. Wang ◽  
J. Bohbot ◽  
O. Colin
Keyword(s):  
Large Eddy Simulation ◽  
Computational Cost ◽  
Spark Ignition ◽  
Spark Ignition Engine ◽  
Cylinder Pressure ◽  
Flamelet Model ◽  
Progress Variable ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Homogeneous Charge

Abstract The Species-Based Extended Coherent Flamelet Model (SB-ECFM) was developed and previously validated for 3D Reynolds-Averaged Navier-Stokes (RANS) modeling of a spark-ignited gasoline direct injection engine. In this work, we seek to extend the SB-ECFM model to the large eddy simulation (LES) framework and validate the model in a homogeneous charge spark-ignited engine. In the SB-ECFM, which is a recently developed improvement of the ECFM, the progress variable is defined as a function of real species instead of tracer species. This adjustment alleviates discrepancies that may arise when the numerical treatment of real species is different than that of the tracer species. Furthermore, the species-based formulation also allows for the use of second-order numeric, which can be necessary in LES cases. The transparent combustion chamber (TCC) engine is the configuration used here for validating the SB-ECFM. It has been extensively characterized with detailed experimental measurements and the data are widely available for model benchmarking. Moreover, several of the boundary conditions leading to the engine are also measured experimentally. These measurements are used in the corresponding computational setup of LES calculations with SB-ECFM. Since the engine is spark ignited, the Imposed Stretch Spark Ignition Model (ISSIM) is utilized to model this physical process. The mesh for the current study is based on a configuration that has been validated in a previous LES study of the corresponding motored setup of the TCC engine. However, this mesh was constructed without considering the additional cells needed to sufficiently resolve the flame for the fired case. Thus, it is enhanced with value-based Adaptive Mesh Refinement (AMR) on the progress variable to better capture the flame front in the fired case. As one facet of model validation, the ensemble average of the measured cylinder pressure is compared against the LES/SB-ECFM prediction. Secondly, the predicted cycle-to-cycle variation by LES is compared with the variation measured in the experimental setup. To this end, the LES computation is required to span a sufficient number of engine cycles to provide statistical convergence to evaluate the coefficient of variation (COV) in peak cylinder pressure. Due to the higher computational cost of LES, the runtime required to compute a sufficient number of engine cycles sequentially can be intractable. The concurrent perturbation method (CPM) is deployed in this study to obtain the required number of cycles in a reasonable time frame. Lastly, previous numerical and experimental analyses of the TCC engine have shown that the flow dynamics at the time of ignition is correlated with the cycle-to-cycle variability. Hence, similar analysis is performed on the current simulation results to determine if this correlation effect is well-captured by the current modeling approach.

Large Eddy Simulation for Turbines: Methodologies, Cost and Future Outlooks

2013 ◽  
Vol 136 (6) ◽  
Author(s):  
James Tyacke ◽  
Paul Tucker ◽  
Richard Jefferson-Loveday ◽  
Nagabushana Rao Vadlamani ◽  
Robert Watson ◽  
...  
Keyword(s):  
Large Eddy Simulation ◽  
Computational Cost ◽  
Quality Data ◽  
Real Surface ◽  
Internal Cooling ◽  
Design Environment ◽  
Roughness Effects ◽  
Eddy Simulation ◽  
Wide Range ◽  
Large Eddy

Flows throughout different zones of turbines have been investigated using large eddy simulation (LES) and hybrid Reynolds-averaged Navier–Stokes-LES (RANS-LES) methods and contrasted with RANS modeling, which is more typically used in the design environment. The studied cases include low and high-pressure turbine cascades, real surface roughness effects, internal cooling ducts, trailing edge cut-backs, and labyrinth and rim seals. Evidence is presented that shows that LES and hybrid RANS-LES produces higher quality data than RANS/URANS for a wide range of flows. The higher level of physics that is resolved allows for greater flow physics insight, which is valuable for improving designs and refining lower order models. Turbine zones are categorized by flow type to assist in choosing the appropriate eddy resolving method and to estimate the computational cost.

Large-Eddy Simulation of Atomizing Spray With Stochastic Modeling of Secondary Breakup

2003 ◽  
Author(s):  
Sourabh V. Apte ◽  
Mikhael Gorokhovski ◽  
Parviz Moin
Keyword(s):  
Large Eddy Simulation ◽  
Droplet Size ◽  
Particle Tracking ◽  
Hybrid Approach ◽  
Computational Cost ◽  
Fuel Injector ◽  
Combustion Dynamics ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Secondary Breakup

Large-eddy simulation (LES) of reacting multi-phase flows in practical combustor geometries is essential to accurately predict complex physical phenomena of turbulent mixing and combustion dynamics. This necessitates use of Lagrangian particle-tracking methodology for liquid phase in order to correctly capture the droplet evaporation rates in the sparse-liquid regime away from the fuel injector. Our goal in the present work is to develop a spray-atomization methodology which can be used in conjuction with the standard particle-tracking schemes and predict the droplet-size distribution accurately. The intricate process of primary atomization and lack of detailed experimental observations close to the injector requires us to model its global effects and focus on secondary breakup to capture the evolution of droplet sizes. Accordingly, a stochastic model for LES of atomizing spray is developed. Following Kolmogorov’s idea of viewing solid particle-breakup as a discrete random process, atomization of liquid blobs at high relative liquid-to-gas velocity is considered in the framework of uncorrelated breakup events, independent of the initial droplet size. Kolmogorov’s discrete model of breakup is represented by Fokker-Planck equation for the temporal and spatial evolution of droplet radius distribution. The parameters of the model are obtained dynamically by relating them to the local Weber number. A novel hybrid-approach involving tracking of individual droplets and a group of like-droplets known as parcels is developed to reduce the computational cost and maintain the essential features and dynamics of spray evolution. The present approach is shown to capture the complex fragmentary process of liquid atomization in idealized and realistic Diesel and gas-turbine combustors.

Large eddy simulation as a fast and accurate engineering approach for the simulation of rotary blood pumps

2021 ◽  
pp. 039139882110416
Author(s):  
Jia-Dong Huo ◽  
Peng Wu ◽  
Liudi Zhang ◽  
Wei-Tao Wu
Keyword(s):  
Large Eddy Simulation ◽  
Turbulence Modeling ◽  
Computational Cost ◽  
Cost Effective ◽  
Design And Optimization ◽  
Engineering Approach ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Blood Pumps ◽  
Rotary Blood Pumps

An accurate representation of the flow field in blood pumps is important for the design and optimization of blood pumps. The primary turbulence modeling methods applied to blood pumps have been the Reynolds-averaged Navier–Stokes (RANS) or URANS (unsteady RANS) method. Large eddy simulation (LES) method has been introduced to simulate blood pumps. Nonetheless, LES has not been widely used to assist in the design and optimization of blood pumps to date due to its formidable computational cost. The purpose of this study is to explore the potential of the LES technique as a fast and accurate engineering approach for the simulation of rotary blood pumps. The performance of “Light LES” (using the same time and spatial resolutions as the URANS) and LES in two rotary blood pumps was evaluated by comparing the results with the URANS and extensive experimental results. This study showed that the results of both “Light LES” and LES are superior to URANS, in terms of both performance curves and key flow features. URANS could not predict the flow separation and recirculation in diffusers for both pumps. In contrast, LES is superior to URANS in capturing these flows, performing well for both design and off-design conditions. The differences between the “Light LES” and LES results were relatively small. This study shows that with less computational cost than URANS, “Light LES” can be considered as a cost-effective engineering approach to assist in the design and optimization of rotary blood pumps.

scholarly journals Comparison of dealiasing schemes in large-eddy simulation of neutrally stratified atmospheric flows

2018 ◽  
Vol 11 (10) ◽  
pp. 4069-4084 ◽  
Author(s):  
Fabien Margairaz ◽  
Marco G. Giometto ◽  
Marc B. Parlange ◽  
Marc Calaf
Keyword(s):  
Large Eddy Simulation ◽  
Stokes Equations ◽  
Computational Cost ◽  
Navier Stokes ◽  
Partial Sums ◽  
Simulation Code ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Layer Flow ◽  
Neutrally Stratified

Abstract. Aliasing errors arise in the multiplication of partial sums, such as those encountered when numerically solving the Navier–Stokes equations, and can be detrimental to the accuracy of a numerical solution. In this work, a performance and cost analysis is proposed for widely used dealiasing schemes in large-eddy simulation, focusing on a neutrally stratified, pressure-driven atmospheric boundary-layer flow. Specifically, the exact 3∕2 rule, the Fourier truncation method, and a high-order Fourier smoothing method are intercompared. Tests are performed within a newly developed mixed pseudo-spectral finite differences large-eddy simulation code, parallelized using a two-dimensional pencil decomposition. A series of simulations are performed at varying resolution, and key flow statistics are intercompared among the considered runs and dealiasing schemes. The three dealiasing methods compare well in terms of first- and second-order statistics for the considered cases, with modest local departures that decrease as the grid stencil is reduced. Computed velocity spectra using the 3∕2 rule and the FS method are in good agreement, whereas the FT method yields a spurious energy redistribution across wavenumbers that compromises both the energy-containing and inertial sublayer trends. The main advantage of the FS and FT methods when compared to the 3∕2 rule is a notable reduction in computational cost, with larger savings as the resolution is increased (15 % for a resolution of 1283, up to a theoretical 30 % for a resolution of 20483).

Large eddy simulation for aerodynamics: status and perspectives

2009 ◽  
Vol 367 (1899) ◽  
pp. 2849-2860 ◽  
Author(s):  
Pierre Sagaut ◽  
Sébastien Deck
Keyword(s):  
Large Eddy Simulation ◽  
Computational Cost ◽  
Flow Simulation ◽  
Navier Stokes ◽  
Eddy Simulation ◽  
Error Quantification ◽  
Geometrical Complexity ◽  
First Case ◽  
Large Eddy ◽  
First Results

The present paper provides an up-to-date survey of the use of large eddy simulation (LES) and sequels for engineering applications related to aerodynamics. Most recent landmark achievements are presented. Two categories of problem may be distinguished whether the location of separation is triggered by the geometry or not. In the first case, LES can be considered as a mature technique and recent hybrid Reynolds-averaged Navier–Stokes (RANS)–LES methods do not allow for a significant increase in terms of geometrical complexity and/or Reynolds number with respect to classical LES. When attached boundary layers have a significant impact on the global flow dynamics, the use of hybrid RANS–LES remains the principal strategy to reduce computational cost compared to LES. Another striking observation is that the level of validation is most of the time restricted to time-averaged global quantities, a detailed analysis of the flow unsteadiness being missing. Therefore, a clear need for detailed validation in the near future is identified. To this end, new issues, such as uncertainty and error quantification and modelling, will be of major importance. First results dealing with uncertainty modelling in unsteady turbulent flow simulation are presented.

scholarly journals Towards the Large-Eddy Simulation of a full engine: Integration of a 360 azimuthal degrees fan, compressor and combustion chamber. Part I: Methodology and initialisation

2021 ◽  
pp. 1-16
Author(s):  
Carlos Pérez Arroyo ◽  
Jérôme Dombard ◽  
Florent Duchaine ◽  
Laurent Gicquel ◽  
Benjamin Martin ◽  
...  
Keyword(s):  
Large Eddy Simulation ◽  
Combustion Chamber ◽  
Computational Cost ◽  
Thermodynamic Cycle ◽  
Turbine Engine ◽  
Fully Integrated ◽  
Integrated Simulation ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Impact

Optimising the design of aviation propulsion systems using computational fluid dynamics is essential to increase their efficiency and reduce pollutant as well as noise emissions. Nowadays, and within this optimisation and design phase, it is possible to perform meaningful unsteady computations of the various components of a gas-turbine engine. However, these simulations are often carried out independently of each other and only share averaged quantities at the interfaces minimising the impact and interactions between components. In contrast to the current state-of-the-art, this work presents a 360 azimuthal degrees large-eddy simulation with over 2100 million cells of the DGEN-380 demonstrator engine enclosing a fully integrated fan, compressor and annular combustion chamber at take-off conditions as a first step towards a high-fidelity simulation of the full engine. In order to carry such a challenging simulation and reduce the computational cost, the initial solution is interpolated from stand-alone sectoral simulations of each component. In terms of approach, the integrated mesh is generated in several steps to solve potential machine dependent memory limitations. It is then observed that the 360 degrees computation converges to an operating point with less than 0.5% difference in zero-dimensional values compared to the stand-alone simulations yielding an overall performance within 1% of the designed thermodynamic cycle. With the presented methodology, convergence and azimuthally decorrelated results are achieved for the integrated simulation after only 6 fan revolutions.

An Improvement of a Cavitation Model and the Application to LES

2008 ◽  
Author(s):  
Shin-Ichi Tsuda ◽  
Naoki Tani ◽  
Nobuhiro Yamanishi ◽  
Chisachi Kato
Keyword(s):  
Large Eddy Simulation ◽  
Turbulent Flows ◽  
Volume Fraction ◽  
Computational Cost ◽  
High Accuracy ◽  
Cavitation Model ◽  
Trade Off ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Future Potential

In this paper, we have improved a cavitation model implemented in “Front Flow/Blue (FFB)”, which is a solver of turbulent flows using the large-eddy simulation (LES) technique with high accuracy. To improve the cavitation model, we have carried out a survey of conventional cavitation models and performed a trade-off between the models based on some evaluation points such as accuracy, achievement, future potential, and computational cost. In the new cavitation model, the surface area of cavitation bubbles in each cell is also solved in addition to the volume fraction of the bubbles. Although the validation is in progress, the new cavitation model is expected to be useful to reproduce a detailed cavitation structure.

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