Introduction to Mfix

1
Introduction to Mfix

• M. Syamlal, Fluent, Inc.
• National Energy Technology Lab
• Morgantown, WV 26507-0880

2
Outline

• Mfix overview
• Setting up a run
• Hands on example
• Numerical technique
• Mfix code
• Code modification

3
Mfix Overview

• Equations
• Validation problems
• Application

4
Setting up a Run

• Mfix user interface
• Input data file mfix.dat
• List of keywords
• Array dimensions param.inc file
• Compilation make_mfix
• Running Mfix
• Debugging setup read .Log and .Out files

5
Setting up a Run

• Run time messages
• Obtaining convergence
• Output data files .Res and .Spx
• Viewing output graphically animate_mfix
• Creating data tables post_mfix
• Restarting the run

6
Hands on Example

• A slurry flows into a 20 cm x 20 cm cavity
  through a 2 cm slot at the top left hand side and
  exits through a 2 cm slot on top right hand side.
• Simulate the cavity using a 20x20 grid.
• Find the solids build up in the cavity as a
  function of time for 10 s.

7
Hands on Example
fluid density 1g/cc fluid viscosity 0.01
g/(cms) solids density 2 g/cc particle dia.
0.1 cm e 0.8 f 0.0 e 0.4
Vel 100 cm/s e 0.7
8
Numerical Technique

• Numerical grid
• Scalar transport equation
• Iteration procedure
• Momentum equation discretization
• Pressure correction equation
• Underrelaxation

9
Numerical Technique

• Solids volume fraction correction
• Energy and species balance equations
• Calculating residuals and judging convergence
• Linear equation solvers

10
Mfix Code

• Directory structure
• Types of files .f, .inc, default files, machine
  dependent files
• Examine make_mfix
• Naming conventions coordinates, variables,
  storage location
• List of major Fortran variables

11
Mfix Code

• List of functions function.inc
• List of include files
• List of subroutines
• Examine .inc file structure
• Examine .f file structure

12
Mfix Code walk through

• a solver thread
• mfix time_march iterate solve_velstar
  conv_dif_u_g source_u_g partial_elim_u
  calc_resid_u under_relax_u solve_lin_eq
  leq_sor
• reading input data file and error checking
• initial and boundary condition routines
• writing .Res and .Spx files

13
Code Modification

• An equation for modeling particle attrition
  tracks the mean particle diameter

14
Code Modification

• Create new Fortran symbols and add to common
  blocks
• field variables d_s, d_s0 (fldvar.inc)
• constants a_1, a_2 (constant.inc)
• initial conditions IC_d_s (ic.inc)
• boundary conditions BC_d_s (bc.inc)
• control Attrition_Eq (run.inc)

15
Code Modification

• Add new symbols in namelist
• a_1, a_2, Ic_d_s, Bc_d_s, Attrition_eq in
  namelist.inc
• initialize namelist variables in init_namelist.f
• Check for input errors (a_1³ 0, Ic_d_s ³ 0, and
  Bc_d_s ³ 0)
• Set initial conditions for d_s
• Set boundary conditions for d_s

16
Code Modification

• Modify update_old to include d_s0
• Increase the dimensions of variables defining
  under relaxation factors, number of linear
  equation iterations, residuals, residual display
  variables, and N_spx
• Modify write_out to echo new input data
• Modify write_res and write_spx files to include
  d_s

17
Code Modification

• Copy the files for a scalar equation (e.g., T_s)
  and modify to create a solver for the attrition
  equation
• Call the new solver from iterate
• Include the effect of diamter change in drag_gs
• Modify residual calculation and convergence check

18
Code Modification

• use make_make to create a new make file
• Develop a test case and write mfix.dat and
  param.inc files
• Compile and test the code