5.2. exec/multispec – low-Mach multispecies mixtures

Purpose

Simulates multispecies liquid mixtures in the low-Mach limit, with stochastic mass and momentum fluxes. Optional features include charged species (electrodiffusion with a Poisson solve), multiphase mixtures with surface tension, and gravity. Typical uses are giant fluctuations, diffusion-driven instabilities, spinodal decomposition, droplets and bubbles, and ionic liquids.

Libraries

src_multispec, src_hydro, src_gmres, src_rng, src_analysis, src_common.

Algorithm

algorithm_type selects the time integrator implemented in the application directory:

  • 0 – inertial integrator (AdvanceTimestepInertial.cpp);

  • 6 – Boussinesq integrator (AdvanceTimestepBousq.cpp).

Other values stop the run with “algorithm_type not supported”.

Key parameters

Parameter

Meaning

nspecies, molmass, rhobar, rho0

Number of species, mass per molecule (not molar mass), pure component densities and reference density.

Dbar

Maxwell–Stefan diffusion coefficients. They are given as the lower triangle of a symmetric matrix, row by row: D_12; D_13, D_23; D_14, D_24, D_34; ... (nspecies*(nspecies-1)/2 values).

is_ideal_mixture

1 (default) assumes an ideal mixture (thermodynamic factor \(\Gamma = I\)); 0 uses the non-ideal model.

c_init_1, c_init_2

Initial mass fractions, interpreted by prob_type in Init.cpp.

variance_coef_mass, variance_coef_mom

Amplitude of the stochastic mass and momentum fluxes.

bc_mass_lo/hi

-1 periodic, 1 wall, 2 reservoir.

use_charged_fluid, charge_per_mass, dielectric_const, bc_es_lo/hi

Electrodiffusion for charged species and boundary conditions for the electric potential (1 Dirichlet, 2 Neumann).

use_multiphase, alpha_gex, n_gex, kc_tension

Multiphase free energy and surface tension.
Example inputs

inputs_regression_equil_* (equilibrium fluctuations), inputs_regression_detbubble_* (deterministic bubble), inputs_regression_spinodal_* (charged spinodal decomposition), inputs_regression_cap_2d, and many production cases (inputs_fh_* giant fluctuations, inputs_droplet_*, inputs_rtil_* ionic liquids).

Note

MAX_SPEC in the GNUmakefile must be at least nspecies, and MAX_ELEM must equal MAX_SPEC*(MAX_SPEC-1)/2.