scholarly journals Scalar Forcing Methodology for Direct Numerical Simulations of Turbulent Stratified Mixture Combustion

2022 ◽  
Author(s):  
Peter Brearley ◽  
Umair Ahmed ◽  
Nilanjan Chakraborty
Keyword(s):  
Numerical Simulations ◽  
Root Mean Square ◽  
Mixture Fraction ◽  
Turbulent Flame ◽  
Passive Scalar ◽  
Direct Numerical Simulations ◽  
Mean Square ◽  
Fraction Distribution ◽  
Stratified Flames ◽  
Scalar Integral

AbstractScalar forcing in the context of turbulent stratified flame simulations aims to maintain the fuel-air inhomogeneity in the unburned gas. With scalar forcing, stratified flame simulations have the potential to reach a statistically stationary state with a prescribed mixture fraction distribution and root-mean-square value in the unburned gas, irrespective of the turbulence intensity. The applicability of scalar forcing for Direct Numerical Simulations of stratified mixture combustion is assessed by considering a recently developed scalar forcing scheme, known as the reaction analogy method, applied to both passive scalar mixing and the imperfectly mixed unburned reactants of statistically planar stratified flames under low Mach number conditions. The newly developed method enables statistically symmetric scalar distributions between bell-shaped and bimodal to be maintained without any significant departure from the specified bounds of the scalar. Moreover, the performance of the newly proposed scalar forcing methodology has been assessed for a range of different velocity forcing schemes (Lundgren forcing and modified bandwidth forcing) and also without any velocity forcing. It has been found that the scalar forcing scheme has no adverse impact on flame-turbulence interaction and it only maintains the prescribed root-mean-square value of the scalar fluctuation, and its distribution. The scalar integral length scale evolution is shown to be unaffected by the scalar forcing scheme studied in this paper. Thus, the scalar forcing scheme has a high potential to provide a valuable computational tool to enable analysis of the effects of unburned mixture stratification on turbulent flame dynamics.

A consequence of the zero fourth cumulant approximation

1962 ◽  
Vol 13 (3) ◽  
pp. 369-382 ◽  
Author(s):  
Edward E. O'Brien ◽  
George C. Francis
Keyword(s):  
Energy Spectrum ◽  
Numerical Computation ◽  
Root Mean Square ◽  
Isotropic Turbulence ◽  
Initial Conditions ◽  
Passive Scalar ◽  
Turbulent Energy ◽  
Length Scale ◽  
Mean Square ◽  
Stationary Turbulence

Recent investigations by Kraichnan (1961) and Ogura (1961) have raised doubts concerning the usefulness of the zero fourth cumulant approximation in turbulence dynamics. It appears extremely tedious to examine, by numerical computation, the consequences of this approximation on the turbulent energy spectrum although the appropriate equations have been established by Proudman & Reid (1954) and Tatsumi (1957). It has proved possible, however, to compute numerically the sequences of an analogous assumption when applied to an isotropic passive scalar in isotropic turbulence.The result of such computation, for specific initial conditions described herein, and for stationary turbulence, is that the scalar spectrum does develop negative values after a time approximately $2 \Lambda | {\overline {(u^2)}} ^{\frac {1}{2}}$, Where Λ is a length scale typical of the energy-containing components of both the turbulent and scalar spectra and $\overline {(u^2)}^{\frac {1}{2}}$ is the root mean square turbulent velocity.

scholarly journals Direct Numerical Simulations of the Impact of High Turbulence Intensities and Volume Viscosity on Premixed Methane Flames

2011 ◽  
Vol 2011 ◽  
pp. 1-12 ◽  
Author(s):  
Gordon Fru ◽  
Gábor Janiga ◽  
Dominique Thévenin
Keyword(s):  
Numerical Simulations ◽  
Heat Release ◽  
Turbulent Flame ◽  
Flame Structure ◽  
Turbulent Flames ◽  
Direct Numerical Simulations ◽  
Turbulent Reynolds Number ◽  
Volume Viscosity ◽  
Flame Structures ◽  
The Impact

Parametric direct numerical simulations (DNS) of turbulent premixed flames burning methane in the thin reaction zone regime have been performed relying on complex physicochemical models and taking into account volume viscosity (κ). The combined effect of increasing turbulence intensities (u′) andκon the resulting flame structure is investigated. The turbulent flame structure is marred with numerous perforations and edge flame structures appearing within the burnt gas mixture at various locations, shapes and sizes. Stepping upu′from 3 to 12 m/s leads to an increase in the scaled integrated heat release rate from 2 to 16. This illustrates the interest of combustion in a highly turbulent medium in order to obtain high volumetric heat release rates in compact burners. Flame thickening is observed to be predominant at high turbulent Reynolds number. Via ensemble averaging, it is shown that both laminar and turbulent flame structures are not modified byκ. These findings are in opposition to previous observations for flames burning hydrogen, where significant modifications induced byκwere found for both the local and global properties of turbulent flames. Therefore, to save computational resources, we suggest that the volume viscosity transport term be ignored for turbulent combustion DNS at low Mach numbers when burning hydrocarbon fuels.

Settling regimes of inertial particles in isotropic turbulence

2014 ◽  
Vol 759 ◽  
Author(s):  
G. H. Good ◽  
P. J. Ireland ◽  
G. P. Bewley ◽  
E. Bodenschatz ◽  
L. R. Collins ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Isotropic Turbulence ◽  
Direct Numerical Simulations ◽  
Inertial Particles ◽  
Mean Square ◽  
Particle Inertia ◽  
Turbulent Eddies ◽  
Air Turbulence ◽  
Large Particles ◽  
Linear Drag

AbstractWe investigate the settling speeds and root mean square (r.m.s.) velocities of inertial particles in isotropic turbulence with gravity using experiments with water droplets in air turbulence from 32 loudspeaker jets and direct numerical simulations (DNS). The dependence on particle inertia, gravity and the scales of both the smallest and largest turbulent eddies is investigated. We isolate the mechanisms of turbulence settling modification and find that the reduced settling speeds of large particles in experiments are due to nonlinear drag effects. We demonstrate using DNS that reduced settling speeds with linear drag (e.g. see Nielsen, J. Sedim. Petrol., vol. 63, 1993, pp. 835–838) only arise in artificial flows that, by design, eliminate preferential sweeping by the eddies. Gravity and inertia both reduce the particle r.m.s. velocities and falling particles are more responsive to vertical than to horizontal fluctuations. The model by Wang & Stock (J. Atmos. Sci., vol. 50, 1993, pp. 1897–1913) captures these trends.

scholarly journals Влияние числа Рейнольдса на распределение пульсационной составляющей вихря скорости по сечению плоского канала

2019 ◽  
Vol 89 (3) ◽  
pp. 347
Author(s):  
Ю.Г. Чесноков
Keyword(s):  
Reynolds Number ◽  
Numerical Simulations ◽  
Cross Section ◽  
Liquid Flow ◽  
Direct Numerical Simulations ◽  
Mean Square ◽  
Flat Channel ◽  
Analysis Of Results ◽  
The Cross ◽  
Vortex Velocity

AbstractBased on the analysis of results from different authors using direct numerical simulations of the liquid flow in a flat channel, the effect of Reynolds number on the distribution of mean-square values of projections of a pulsed component of vortex velocity through the cross-section of a flat channel has been studied.

Direct numerical simulations of turbulent flame expansion in fine sprays

2009 ◽  
Vol 32 (2) ◽  
pp. 2283-2290 ◽  
Author(s):  
Andrew P. Wandel ◽  
Nilanjan Chakraborty ◽  
E. Mastorakos
Keyword(s):  
Numerical Simulations ◽  
Turbulent Flame ◽  
Direct Numerical Simulations

Numerical Investigations of Passive Scalar Transport in Turbulent Taylor-Couette Flows: Large Eddy Simulation Versus Direct Numerical Simulations

2012 ◽  
Vol 134 (4) ◽  
Author(s):  
Yacine Salhi ◽  
El-Khider Si-Ahmed ◽  
Gérard Degrez ◽  
Jack Legrand
Keyword(s):  
Finite Element ◽  
Large Eddy Simulation ◽  
Numerical Simulations ◽  
Passive Scalar ◽  
Direct Numerical Simulations ◽  
Scalar Transport ◽  
Eddy Simulation ◽  
Passive Scalar Transport ◽  
Large Eddy ◽  
Spectral Finite Element

The highly turbulent flow occurring inside (electro)chemical reactors requires accurate simulation of scalar mixing if computational fluid dynamics (CFD) methods are to be used with confidence in design. This has motivated the present paper, which describes the implementation of a passive scalar transport equation into a hybrid spectral/finite-element code. Direct numerical simulations (DNS) and large eddy simulation (LES) were performed to study the effects of gravitational and centrifugal potentials on the stability of incom-pressible Taylor-Couette flow. The flow is confined between two concentric cylinders with an inner rotating cylinder while the outer one is at rest. The Navier-Stokes equations with the uncoupled convection–diffusion–reaction (CDR) equation are solved using a code named spectral/finite element large eddy simulations (SFELES) which is based on spectral development in one direction combined with a finite element discretization in the remaining directions. The performance of the LES code is validated with published DNS data for channel flow. Velocity and scalar statistics showed good agreement between the current LES predictions and DNS data. Special attention was given to the flow field, in the vicinity of Reynolds number of 68.2 with radii ratio of 0.5. The effect of Sc on the concentration peak is pointed out while the magnitude of heat transfer shows a dependence of the Prandtl number with an exponent of 0.375.

scholarly journals An Annual Cycle of Submesoscale Vertical Flow and Restratification in the Upper Ocean

2019 ◽  
Vol 49 (6) ◽  
pp. 1439-1461 ◽  
Author(s):  
Xiaolong Yu ◽  
Alberto C. Naveira Garabato ◽  
Adrian P. Martin ◽  
Christian E. Buckingham ◽  
Liam Brannigan ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Root Mean Square ◽  
Vertical Velocity ◽  
Annual Cycle ◽  
Observational Evidence ◽  
Upper Ocean ◽  
Vertical Flow ◽  
Mean Square ◽  
Ocean Stratification ◽  
Density Equation

AbstractNumerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification and thus restratify the upper ocean via vertical flow. However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical midocean area of the northeast Atlantic. Vertical velocities inferred using the nondiffusive density equation are substantially stronger at submesoscales (horizontal scales of 1–10 km) than at mesoscales (horizontal scales of 10–100 km), with respective root-mean-square values of 38.0 ± 6.9 and 22.5 ± 3.3 m day−1. The largest submesoscale vertical velocities and rates of restratification occur in events of a few days’ duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.

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