scholarly journals Linear-eddy subgrid model for reacting large-eddy simulations - Heat release effects

1997 ◽  
Author(s):  
William Calhoon, Jr. ◽  
Suresh Menon ◽  
William Calhoon, Jr. ◽  
Suresh Menon
Keyword(s):  
Heat Release ◽  
Large Eddy Simulations ◽  
Subgrid Model ◽  
Large Eddy

Stochastic backscatter in large-eddy simulations of boundary layers

1992 ◽  
Vol 242 ◽  
pp. 51-78 ◽  
Author(s):  
P. J. Mason ◽  
D. J. Thomson
Keyword(s):  
Large Scale ◽  
Flow Simulation ◽  
Surface Region ◽  
Large Eddy Simulations ◽  
Flow Profile ◽  
Mean Velocity ◽  
Near Surface ◽  
Eddy Simulation ◽  
Subgrid Model ◽  
Large Eddy

The ability of a large-eddy simulation to represent the large-scale motions in the interior of a turbulent flow is well established. However, concerns remain for the behaviour close to rigid surfaces where, with the exception of low-Reynolds-number flows, the large-eddy description must be matched to some description of the flow in which all except the larger-scale ‘inactive’ motions are averaged. The performance of large-eddy simulations in this near-surface region is investigated and it is pointed out that in previous simulations the mean velocity profile in the matching region has not had a logarithmic form. A number of new simulations are conducted with the Smagorinsky (1963) subgrid model. These also show departures from the logarithmic profile and suggest that it may not be possible to eliminate the error by adjustments of the subgrid lengthscale. An obvious defect of the Smagorinsky model is its failure to represent stochastic subgrid stress variations. It is shown that inclusion of these variations leads to a marked improvement in the near-wall flow simulation. The constant of proportionality between the magnitude of the fluctuations in stress and the Smagorinsky stresses has been empirically determined to give an accurate logarithmic flow profile. This value provides an energy backscatter rate slightly larger than the dissipation rate and equal to idealized theoretical predictions (Chasnov 1991).

Understanding partial fuel stratification for low temperature gasoline combustion using large eddy simulations

2020 ◽  
pp. 146808742092104
Author(s):  
Priya Priyadarshini ◽  
Aimilios Sofianopoulos ◽  
Sotirios Mamalis ◽  
Benjamin Lawler ◽  
Dario Lopez-Pintor ◽  
...  
Keyword(s):  
Low Temperature ◽  
Heat Release ◽  
Direct Injection ◽  
Large Eddy Simulations ◽  
Compression Ignition ◽  
Load Range ◽  
Injection Strategy ◽  
Compression Ignition Engines ◽  
Fuel Stratification ◽  
Large Eddy

The development of gasoline compression ignition engines operating in a low temperature combustion mode depends heavily on robust control of the heat release profile. Partial fuel stratification is an effective method for controlling the heat release by creating a stratified mixture prior to autoignition, which can be beneficial for operation across a wide load range. In this study, three-dimensional large eddy simulations were used to model a double direct injection strategy for which 80% of the fuel was injected during the intake stroke, and 20% of the fuel was injected at varying timing during the compression stroke. The simulations replicated a set of experiments performed at Sandia National Laboratories on a 1-L single-cylinder research engine using E10 gasoline (gasoline fuel containing 10% vol. ethanol). The objective of this study is to analyze the effects of the double direct injection strategy on the compositional and thermal stratification of the mixture, and understand the best use of this operating strategy. The modeling results indicated that by retarding the start of the second injection, the mixture stratification increases, which can be used to control the autoignition timing and the combustion phasing. Ignition and CA50 (crank angle of 50% mass fraction burned) are dictated by the mass concentration of the richest zones in the combustion chamber, as well as their location. The richer zones have the lowest temperatures before ignition primarily due to evaporative cooling from direct fuel injection. Overall, this study enhances the understanding of partial fuel stratification that can be used for controlling the heat release in gasoline compression ignition engines.

A comparative study of gasoline skeletal mechanisms under partial fuel stratification conditions using large eddy simulations

2021 ◽  
pp. 146808742110313
Author(s):  
Gaurav Guleria ◽  
Dario Lopez-Pintor ◽  
John E Dec ◽  
Dimitris Assanis
Keyword(s):  
Low Temperature ◽  
Heat Release ◽  
Chemical Kinetic ◽  
Large Eddy Simulations ◽  
Low Temperature Combustion ◽  
Fuel Stratification ◽  
Large Eddy ◽  
Kinetic Mechanisms ◽  
Temperature Combustion ◽  
Research Grade

Partial fuel stratification (PFS) is a low temperature combustion strategy that can alleviate high heat release rates of traditional low temperature combustion strategies by introducing compositional stratification in the combustion chamber using a split fuel injection strategy. In this study, a three-dimensional computational fluid dynamics (CFD) model with large eddy simulations and reduced detailed chemistry was used to model partial fuel stratification at three different stratified conditions. The double direct injection strategy injects 80% of the total fuel mass at −300 CAD aTDC and the remaining 20% of the fuel mass is injected at three different timings of −160, −50, −35 CAD to create low, medium, and high levels of compositional stratification, respectively. The PFS simulations were validated using experiments performed at Sandia National Laboratories on a single-cylinder research engine that operates on RD5-87, a research-grade E10 gasoline. The objective of this study is to compare the performance of three different reduced chemical kinetic mechanisms, namely SKM1, SKM2, and SKM3, at the three compositional stratification levels and identify the most suitable mechanism to reproduce the experimental data. Zero-dimensional chemical kinetic simulations were also performed to further understand differences in performance of the three reduced chemical kinetic mechanisms to explain variations in CFD derived heat release profiles. The modeling results indicate that SKM3 is the most suitable mechanism for partial fuel stratification modeling of research-grade gasoline. The results also show that the autoignition event progresses from the richer to the leaner compositional regions in the combustion chamber. Notably, the leaner regions that have less mass per unit volume, can contribute disproportionately more toward heat release as there are more cells at leaner equivalence ratio ranges. Overall, this study illuminates the underlying compositional stratification phenomena that control the heat release process in PFS combustion.

Large-eddy simulations of turbulent mixing layers using the stretched-vortex model

2011 ◽  
Vol 671 ◽  
pp. 507-534 ◽  
Author(s):  
T. W. MATTNER
Keyword(s):  
Large Scale ◽  
Large Eddy Simulations ◽  
Mixing Layers ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Subgrid Model ◽  
Large Eddy ◽  
Schmidt Numbers ◽  
Self Similar ◽  
Large Scale Flow

The stretched-vortex subgrid model is used to run large-eddy simulations of temporal mixing layers at various Reynolds and Schmidt numbers, with different initial and boundary conditions. A self-similar flow is obtained, during which the growth rate, mean velocity and Reynolds stresses are in accord with experimental results. However, predictions of the amount of mixed fluid, and of the variation in its composition across the layer, are excessive, especially at high Schmidt number. More favourable comparisons between experiment and simulation are obtained when the large-scale flow is quasi-two-dimensional; however, such states are not self-similar and not sustainable. Present model assumptions lead to predictions of the continued subgrid spectrum with a viscous cutoff that is dependent on grid resolution.

scholarly journals Evaluation of Large-Eddy Simulations via Observations of Nocturnal Marine Stratocumulus

2005 ◽  
Vol 133 (6) ◽  
pp. 1443-1462 ◽  
Author(s):  
Bjorn Stevens ◽  
Chin-Hoh Moeng ◽  
Andrew S. Ackerman ◽  
Christopher S. Bretherton ◽  
Andreas Chlond ◽  
...  
Keyword(s):  
Parameter Space ◽  
Large Eddy Simulations ◽  
Liquid Water Path ◽  
Marine Stratocumulus ◽  
Thermodynamic Structure ◽  
Subgrid Model ◽  
Large Eddy ◽  
Cloud Liquid Water Path ◽  
The One ◽  
Vertical Spacing

Abstract Data from the first research flight (RF01) of the second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study are used to evaluate the fidelity with which large-eddy simulations (LESs) can represent the turbulent structure of stratocumulus-topped boundary layers. The initial data and forcings for this case placed it in an interesting part of parameter space, near the boundary where cloud-top mixing is thought to render the cloud layer unstable on the one hand, or tending toward a decoupled structure on the other hand. The basis of this evaluation consists of sixteen 4-h simulations from 10 modeling centers over grids whose vertical spacing was 5 m at the cloud-top interface and whose horizontal spacing was 35 m. Extensive sensitivity studies of both the configuration of the case and the numerical setup also enhanced the analysis. Overall it was found that (i) if efforts are made to reduce spurious mixing at cloud top, either by refining the vertical grid or limiting the effects of the subgrid model in this region, then the observed turbulent and thermodynamic structure of the layer can be reproduced with some fidelity; (ii) the base, or native configuration of most simulations greatly overestimated mixing at cloud top, tending toward a decoupled layer in which cloud liquid water path and turbulent intensities were grossly underestimated; (iii) the sensitivity of the simulations to the representation of mixing at cloud top is, to a certain extent, amplified by particulars of this case. Overall the results suggest that the use of LESs to map out the behavior of the stratocumulus-topped boundary layer in this interesting region of parameter space requires a more compelling representation of processes at cloud top. In the absence of significant leaps in the understanding of subgrid-scale (SGS) physics, such a representation can only be achieved by a significant refinement in resolution—a refinement that, while conceivable given existing resources, is probably still beyond the reach of most centers.

scholarly journals Incipient thermal choking and stable shock-train formation in the heat-release region of a scramjet combustor. Part II: Large eddy simulations

2015 ◽  
Vol 162 (4) ◽  
pp. 907-920 ◽  
Author(s):  
Johan Larsson ◽  
Stuart Laurence ◽  
Iván Bermejo-Moreno ◽  
Julien Bodart ◽  
Sebastian Karl ◽  
...  
Keyword(s):  
Heat Release ◽  
Large Eddy Simulations ◽  
Shock Train ◽  
Scramjet Combustor ◽  
Large Eddy
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