Abstract
Scalar 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.
Highlights
Direct Numerical Simulations (DNS) often use numerical velocity forcing schemes (Eswaran and Pope 1988a; Alvelius 1999; Lundgren 2003; Klein et al 2017) in order to maintain the root-mean-square value of the turbulent velocity field and obtain a1 3 Vol.:(0123456789)Flow, Turbulence and Combustion statistically stationary state
The present analysis focuses on the development of scalar forcing methodology to maintain the mixture inhomogeneity in the unburned gas region in Direct Numerical Simulations (DNS) of turbulent stratified flames in canonical configurations without any mechanism for the generation of scalar and velocity fluctuations
The scalar forcing identified in this paper maintains both the mean and rms of the scalar, and retains the initial specified scalar probability density functions (PDFs) along with upper and lower bounds of the scalar in question for both passive scalar mixing and turbulent stratified mixture combustion subjected to a variety of different turbulent velocity forcing schemes
Summary
Direct Numerical Simulations (DNS) often use numerical velocity forcing schemes (Eswaran and Pope 1988a; Alvelius 1999; Lundgren 2003; Klein et al 2017) in order to maintain the root-mean-square (rms) value of the turbulent velocity field and obtain a. In the absence of any inherent turbulence generation mechanism, the forcing of the turbulent velocity field enables turbulent premixed flames to reach a statistically stationary state in a ‘flame in a box’ type canonical configuration. It is desirable to have a scalar forcing mechanism employed in conjunction with turbulent velocity forcing in order to maintain both turbulence intensity and the scalar rms value during the simulation of stratified mixture combustion. Scalar forcing schemes have not been implemented in chemically reacting combustion simulations This study addresses this gap in the existing literature and provides a commentary on the prospects of scalar forcing for the use in DNS analysis of stratified mixture combustion. The summary of the main findings is provided, and the potential usage of the scalar forcing in the future research into turbulent stratified mixture combustion is discussed in the final section The summary of the main findings is provided, and the potential usage of the scalar forcing in the future research into turbulent stratified mixture combustion is discussed in the final section (i.e. Sect. 5) of this paper
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