# Using DL_MESO_LBE¶

## Summary¶

DL_MESO_LBE uses a number of input files to read in the details of an LBE simulation and produces output files containing the results. The Java GUI and utilities supplied with DL_MESO can be used to create/modify LBE simulation inputs and work with outputs to enable users to visualise and analyse simulations.

## Input and output files¶

The required input files for DL_MESO_LBE are:

• lbin.sys: gives the basic simulation controls, specifying properties such as the lattice scheme, number of timesteps, relaxation times, properties for initial and boundary conditions etc.
• lbin.spa: provides information on which lattice sites include boundary conditions and their types.

The lbin.sys file consists of identifying phrases with numerical values or words to specify particular properties, while the lbin.spa file is given as a list of Cartesian coordinates, each with a boundary condition code. DL_MESO_LBE will work out the initial configuration (velocities, densities etc.) based on values given in the lbin.sys file, but if you wish to modify this at various grid points, you can also supply an lbin.init file (similar to lbin.spa but specifying initial physical properties at each grid point).

The outputs from DL_MESO_LBE can include:

• Screen/standard output: a summary of the LBE simulation with timings and diagnostic information on fluid masses and system momenta.
• lbout.dump: state of simulation recorded at time intervals, continuously overwritten (last one provided) - written in binary, can be used to restart simulation or used with utilities.
• lbout*.vts: trajectory data (i.e. simulation snapshots at user-specified time intervals) - written in structured grid XML-based VTK format, includes macroscopic velocity, fluid densities etc. at each grid point.

If DL_MESO_LBE is run on multiple processor cores, the default behaviour is for each core to write its own file per timestep. To reduce the number of files written, options can be specified in lbin.sys to gather data among cores in different dimensions: if gathering in all dimensions for a system is specified, a single file will be produced per timestep.

Further details of these file formats can be found in Chapter 6 of the DL_MESO User Manual.

## DL_MESO_LBE utilities¶

The most useful ones are:

• init.exe: creates a lbin.init file based on changes made to the initial velocities, densities etc. specified in lbin.sys - this can be used to insert circular/spherical drops of fluid or rectangular/cuboidal regions of solute concentration or temperature;
• vtk.exe: gathers together data in XML-based VTK files from parallel DL_MESO_LBE by creating linking files (lbout*.pvts) to put together *.vts files from individual processor cores when plotting in Paraview;
• dump_to_init.exe: creates a lbin.init file from a previous simulation’s lbout.dump restart file.

These are all run in the same manner as the serial version of DL_MESO_LBE. The utility init.exe will require user input to specify properties, grid points etc. where these deviate from the standard values.

## Analysis and visualisation tools¶

For the exercises supplied, we recommend using Paraview for visualisation and analysis, since this will read the lbout*.vts produced by DL_MESO_LBE and can display, animate and analyse large grid-based datasets. Paraview is available for all operating systems and can either be downloaded directly from its website or installed using package managers.