Global Timestepping Module

This module was developed by ATA Engineering as an add-on to the Loci/CHEM computational fluid dynamics (CFD) solver. The module can be used to enable a non-constant global time step for first order temporal accuracy. The time step is selected based off of a user specified CFL ramp. This time integration scheme is helpful for steady state flow simulations that have initial stability problems that disappear as the solution evolves.

Dependencies

This module depends on both Loci and CHEM being installed. Loci is an open source framework developed at Mississippi State University (MSU) by Dr. Ed Luke. The framework provides a rule-based programming model and can take advantage of massively parallel high performance computing systems. CHEM is a full featured open source CFD code with finite-rate chemistry built on the Loci framework. CHEM is export controlled under the International Traffic In Arms Regulations (ITAR). Both Loci and CHEM can be obtained from the SimSys Software Forum hosted by MSU. This module also requires a compiler with C++11 support.

Installation Instructions

First Loci and CHEM should be installed. The LOCI_BASE environment variable should be set to point to the Loci installation directory. The CHEM_BASE environment variable should be set to point to the CHEM installation directory. The installation process follows the standard make, make install procedure.

make
make install

Usage

First the module must be loaded at the top of the vars file. The global_cfl options list can be used to specify the parameters of the CFL ramp. The time_integration method should be set to euler, the urelax parameter should be set to 1, and the cflmax parameter should not be used.

loadModule: globalTimeStepping

time_integration: euler
urelax: 1.0
global_cfl: <start=0.001, exponent=1.5, coefficient=1e-4, max=100>

The CFL ramp is calculated based on the iteration, $n$, as shown below where $A$ is the specified coefficient, $B$ the specified exponent, $C$ the specifed starting value, and $D$ the specified maximum CFL.

$$ CFL = min \left( D, A n^B + C \right) $$