Chapter 3
Running the solver

The CompressibleFlowSolver can be run to solve the Cylinder Subsonic problem.

Task: 3.1 Run the solver by typing the following command on the command line: $NEK/CompressibleFlowSolver CylinderSubsonic_NS.xml

Tip: To reduce the solution time on computers with multiple processors, MPI can be used to run the simulation in parallel. Note that, for binaries compiled from source, the Cmake option NEKTAR_USE_MPI must have been set ON. To run in parallel, prefix the command in the previous task with mpirun -np X, replacing X by the number of parallel processes to use. For example, to use 32 processes:

mpirun -np 32 $NEK/CompressibleFlowSolver CylinderSubsonic_NS.xml

The simulation has now produced 10 .chk binary files and a final .fld binary file. These binary files contain the result of the simulation every 100 time-steps. This output interval has been chosen through the parameter IO_CheckSteps in PARAMETERS tag. Also, it is possible to note that every 100 time-steps the solver outputs the physical time of the simulation and the CPU time required for doing 100 time-steps. The interval of 100 time-steps is decided through the parameter IO_InfoSteps.

Tip: Stability plays a crucial role in the Compressible Flow solver. To ensure the solution is not polluted leading to numerical instabilities, for long simulations the .chk files can be checked before the simulation ends.