S. Ukai

On Richtmyer–Meshkov instability in dilute gas-particle mixtures

Richtmyer–Meshkov instability (RMI) in gas-particle mixtures is investigated both numerically and analytically. The linear amplitude growth rate for a RMI in a two-phase mixture is derived by using a dusty gas formulation for small Stokes number (St⪡1.0), and it is shown that the problem can be characterized by mass loading and St. The model predictions are compared with numerical results under two conditions, i.e., a shock wave hitting (1) a perturbed species interface of air and SF6 surrounded by uniformly distributed particles, and (2) a perturbed shape particle cloud in uniform air. In the first case, the interaction between the instability of the species perturbation and the particles is investigated. The multiphase growth model accurately predicts the growth rates when St⪡1.0, and the amplitude growth normalized by the two-phase RMI velocity shows good agreement with the single-phase RMI growth rate as well. It is also shown that the two-phase model results are in accordance with the growth rates ob...

Large Eddy Simulation of Re-shocked Richtmyer-Meshkov Instability

Richtmyer-Meshkov instability is caused by the impulsive acceleration of two different media, and the instability causes the formation of small scales of turbulence structures. When the initial shock is reflected and passes again through the perturbed field, additional scales of turbulence larger than perturbation are generated. Here, direct numerical simulation of 2D RMI is used first to investigate flow structures similar to previous experimental and numerical studies and, comparisons show good agreement. Also, spectra of kinetic energy followed k 3 , which is commonly accepted to the 2D turbulence. Large-eddy simulation of 3D Richtmyer-Meshkov instability is also conducted using a localized subgrid closure. Even though mixing growth rate before reshock is overestimated, the growth rate after reshock shows good agreement with the experimental results. Vorticity structures are visualized by iso-surface of Q-criteria, and two regimes with strong vorticity are found at the edges of mixing region. The kinetic energy spectra shows an inertial range of k 5/3 at the later stage of reshocked turbulent decay. However, spectra of density showed no or only a small range of inertial range, which is shallower than k 5/3 .

Certain Aspects of Conditional Moment Closure for Spray Flame Modelling

Large-eddy simulations (LES) have been coupled with a conditional moment closure (CMC) method for the improved modelling of small scale turbulence-chemistry interactions in turbulent spray flames. Partial pre-evaporation of the liquid fuel prior to exiting the injection nozzle requires a modified treatment for the boundary conditions in mixture fraction space and mixture fraction subgrid distribution and conditionally averaged subgrid dissipation need to be known. Different modelling approaches for the subgrid distribution of mixture fraction have been assessed, but the modelling of subgrid scalar dissipation that is responsible for the subgrid fuel transport from the droplet surface towards the cell filtered mean has not been forthcoming. Instead, we introduce a new conditioning method based on two sets of conditional moments conditioned on two differently defined mixture fractions: the first mixture fraction is a fully conserved scalar, the second mixture fraction is based on the fluid mass originating from the liquid fuel stream and is strictly not conserved due to the evaporation process. The two-conditional moment approach is validated by comparison with measurements from a turbulent ethanol spray flame and predicted temperature and velocity profiles could significantly be improved when compared to conventional LES-CMC modelling.