13:30
Reacting and compressible flows 1
Chair: Stefan Hickel
13:30
15 mins
|
RAYLEIGH-TAYLOR-INDUCED TURBULENT MIXING LAYERS
Nicolas Schneider, Serge Gauthier
Abstract: We study mixing layers generated by Rayleigh-Taylor instability. It develops in various physical situations, such as supernovae explosions and inertial confinement fusion, which aims at obtaining thermonuclear ignition by compressing a pellet filled with deuterium-tritium. The numerical simulations we have carried out start from rest and several physical models are involved: Boussinesq, anelastic, and compressible. This allows for a wide exploration of parameters. Results from DNS spectral simulations with around 900
millions collocation points are presented. Anisotropy, compressibility effects, characteristics of turbulence and mixing, are explored.
|
13:45
15 mins
|
Large-eddy simulation of combustion instability in a back-step flow
Tomoaki Kitano, Ryoichi Kurose, Satoru Komori
Abstract: A large-eddy simulation of combustion instability in a back-step flow is performed, and the effect of equivalent ratio on the combustion instability is investigated. Methane is used as the fuel and a two-step global reaction model is used for the reaction. As the turbulent combustion model, a dynamic thickened flame model is used. The results show that flame is stably formed behind the step by the recirculation flow. Large pressure oscillation and periodical change of flame shape are observed in the case of equivalent ratio of 1.0, and the power spectra of the pressure oscillation has peaks whose frequencies and ntensities well agree with those of the previous experimental study. The intensity of the pressure oscillation becomes much smaller in the cases of equivalent ratio of 0.8 and 1.3, and the change of flame shape also becomes smaller.
|
14:00
15 mins
|
TRANSITION AND TURBULENCE IN A WALL BOUNDED CHANNEL FLOW AT HIGH MACH NUMBER
Sahadev Pradhan and Viswanathan Kumaran Sahadev Pradhan and Viswanathan Kumaran
Abstract: TITLE : TRANSITION AND TURBULENCE IN A WALL BOUNDED CHANNEL FLOW AT HIGH MACH NUMBER
AUTHORS: S. Pradhan and V. Kumaran
AFFILIATION: Department of Chemical Engineering, Indian Institute of Science, Bangalor-560 012, India
ABSTRACT: The flow in a 3D wall bounded channel, simulated using the direct simulation Monte Carlo (DSMC) method, has been used as a test bed for examining different aspects of transition and turbulence at high Mach M = Um / (( kB Tw /m), and Reynolds numbers Re = (ρm Um H)/w. Here, H is the channel half-width, Um is the mean velocity, ρm is the mean density, Tw is the wall temperature, m is the molecular mass, w is the molecular viscosity based on the temperature at the isothermal wall, and kB is the Boltzmann constant.
The laminar-turbulent transition is accompanied by a discontinuous change in the friction factor even at high Mach number. The transition Reynolds number increases faster than linearly with Mach number, and the Knudsen number at transition (also proportional to the ratio of Mach and Reynolds numbers) passes through a maximum as the Mach number is increased. This maximum value is small, less than 0.009, indicating that transition is a continuum phenomenon even at high Mach numbers.
In a high Mach turbulent flow, wall slip in the temperature and the velocities are significant. Slip occurs because the temperature/velocity of the molecules incident on the wall could be very different form that of the wall, even though the temperature/velocity of the reflected molecules is equal to that of the wall. There is slip even in the mean velocity as well as the intensity of the turbulent velocity fluctuations tangential to the wall.
In a compressible turbulent channel flow, we examine the result that the Kolmogorov scale, ~ (H Re-3/4) becomes asymptotically smaller than the mean free path, λ ~ (H M/Re), for M >> Re1/4. The simulation show that the ratio (mean free path to Kolmogorov scale) does decrease as Re-1/4, but it does not increase linearly with Mach number. This is due to the decrease in the local Mach number within the channel, due to the increase in the temperature by viscous heating.
|
14:15
15 mins
|
TURBULENT TRANSPORT OF CHEMICALLY REACTING GASEOUS ADMIXTURES
Tov Elperin, Nathan Kleeorin, Michael Liberman, Igor Rogachevskii
Abstract: We study turbulent diffusion of chemically reacting gaseous admixtures in a developed turbulence. In our previous study [Phys. Rev. Lett. 80, 69 (1998)] using a path-integral approach for a delta-correlated in time random velocity field, we demonstrated a strong modification of turbulent transport in fluid flows with chemical reactions or phase transitions. In the present study we use the spectral tau approximation, that is valid for large Reynolds and Peclet numbers, and show that turbulent diffusion of the reacting species can be strongly depleted by a large factor that is the ratio of turbulent and chemical times (turbulent Damköhler number). We have demonstrated that the derived theoretical dependence of turbulent diffusion coefficient versus the turbulent Damköhler number is in a good agreement with that obtained previously in the numerical modelling of a reactive front propagating in a turbulent flow and described by the Kolmogorov-Petrovskii-Piskunov-Fisher equation. We have found that turbulent cross-effects, e.g., turbulent mutual diffusion of gaseous admixtures and turbulent Dufour-effect of the chemically reacting gaseous admixtures, are less sensitive to the values
of stoichiometric coefficients. The mechanisms of the turbulent cross-effects are different from the molecular cross effects known in irreversible thermodynamics. In a fully developed turbulence and at large Peclet numbers the turbulent cross-effects are much larger than the molecular ones. The obtained results are applicable also to heterogeneous phase transitions.
|
14:30
15 mins
|
COMPRESSIBLE RAYLEIGH-TAYLOR TURBULENT MIXING UNDER DIFFERENT ACCELERATION HISTORIES
You-sheng Zhang, Bao-lin Tian, Xin-liang Li
Abstract: Compressible Rayleigh-Taylor turbulent mixing (CRTM) induced by Rayleigh-Taylor instability occurs when a compressible fluid of heavy density is accelerated or supported against gravity by a compressible fluid of light density, and is of fundamental importance in applications from combustion, to inertial confinement fusion, and to astrophysics. Traditionally, CRTFs are studied under constant acceleration histories. Due to the nature of the processes, however, it is necessary to study CRTF under general acceleration histories g(t). In this aspect, the evolution of Rayleigh-Taylor turbulent mixing under complex acceleration histories, including changes in signs, have been studied numerically[1] and experimentally[2] for incompressible flows, leaving an open question on that of compressible flows. In fact, most engineering problems are compressible. In addition, the available engineering turbulence models cannot capture the variation of mixing width for CRTM with complex acceleration histories, such as the gravity reversal. In order to better understanding the dynamic of CRTM under different variation histories, several DNS cases with different acceleration histories have been conducted and analyzed.
|
14:45
15 mins
|
An LIA+EDQNM strategy to study shocked turbulent mixtures
Jerome Griffond, Benoît-Joseph Gréa, Olivier Soulard
Abstract: Direct numerical simulations (DNS) of the interaction of shock waves with turbulent mixtures are only affordable for very low Reynolds numbers. We propose here to couple the Linear Interaction Analysis (LIA) with an Eddy-Damped Quasi-Normal Markovian (EDQNM) model to get inexpensive valuable information at large Reynolds number.
|
|