cwipiFoam

Establishes CWIPI coupling of OpenFOAM-v2212 and Nektar++ Acoustic Solver version 5.3.0

The aim of this library is to enable memory-based source coupling of compressible OpenFOAM solvers and Nektar++'s AcousticSolver. The OpenFOAM solver is used to calculate the unsteady hydrodynamic source fields, which are then used as input sources by AcousticSolver to compute the propagation of noise into the far-field. The method of memory-based coupling avoids the I/O bottleneck associated with file-based data transfer since the data is exchanged through system memory via CWIPI.

Installation instructions:

1. Compile Nektar++ with AcousticSolver and CWIPI; install into $HOME/opt

2. Compile OpenFOAM-v2212

3. Create a file named loadNektar.sh and add the following lines:

   export PATH=$HOME/opt/bin:$PATH
   export LIBRARY_PATH=$HOME/opt/lib64:$LIBRARY_PATH
   export LIBRARY_PATH=$HOME/opt/lib64/nektar++:$LIBRARY_PATH
   export LD_LIBRARY_PATH=$HOME/opt/lib64:$LD_LIBRARY_PATH
   export LD_LIBRARY_PATH=$HOME/opt/lib64/nektar++:$LD_LIBRARY_PATH
   

4. Add these directories to your system paths via:

   source loadNektar.sh
   

5. Navigate to the cwipiFoam source code directory and build the coupling library via:

   ./Allwmake
   

Instructions for creating new CWIPI solvers

An edited version of the rhoCentralFoam solver has been provided as an example of how to edit the source code of compressible OpenFOAM solvers to use this CWIPI coupling.

The call

cwipiSwitch cwipi(runTime);

is used to determine whether or not the solver should be run in coupled mode by looking for the cwipiSwitch entry in system/controlDict. One must first run the solver in decoupled mode by setting the entry cwipiSwitch to false in system/controlDict:

cwipiSwitch       false;

The call

cwipiFields couplingFields(mesh, runTime, U, thermo);

establishes the computation of the Lamb vector, entropy and local speed of sound fields which are necessary for the coupling to work. These fields are computed regardless of whether the solver is running in coupled or decoupled mode since it is necessary to time-average these fields before the coupling begins. The solver should be run in decoupled mode until the start-up transient period has passed. The user should then add time-averaging for the base flow fields, namely for U, c, T, rho, s and L to the bottom of the controlDict file:

functions
{
  fieldAverage0
  {
    type            fieldAverage;
    libs            (fieldFunctionObjects);
    fields
    (
      U
      {
        mean          true;
        prime2Mean    false;
        base          time;
        allowRestart  true;
      }
      c
      {
        mean          true;
        prime2Mean    false;
        base          time;
        allowRestart  true;
      }
      T
      {
        mean          true;
        prime2Mean    false;
        base          time;
        allowRestart  true;
      }
      rho
      {
        mean          true;
        prime2Mean    false;
        base          time;
        allowRestart  true;
      }
      s
      {
        mean          true;
        prime2Mean    false;
        base          time;
        allowRestart  true;
      }
      L
      {
        mean          true;
        prime2Mean    false;
        base          time;
        allowRestart  true;
      }
    );
    writeControl      writeTime;
  }
}

The solver should then be run, once again in decoupled mode, until the time-averaged flow fields are statistically stationary. Once this is the case, the solver can then be run in coupled mode with the following switches:

cwipiSwitch       true;
cwipiDim          2;
cwipiStep         1;
cwipiLambVector   true;
cwipiEntropy      true;
cwipiDsDt         true;

In this mode, the solver will create a pointwise interpolation of the flow fields, along with a cwipiPstream object:

volPointInterpolation pInterp(mesh);
cwipiPstream coupling(runTime, mesh, thermo, couplingFIelds, pInterp);

and will send the source fields to AcousticSolver at the correct time step with the following:

if (coupling.sendNow())
{
    coupling.send();
}

The aforementioned combination of parameters enables CWIPI coupling for a Nektar solution grid with 2 spatial dimensions, sending all of the acoustic source fields at each time step. The individual acoustic sources can be turned on/off with their respective named entries (cwipiLambVector, cwipiEntropy and cwipiDsDt).

The cwipiStep parameter governs the frequency with which the source fields are sent to AcousticSolver; this parameter must be defined such that cwipiStep(openfoam) * timeStep(openfoam) = ReceiveSteps(Nektar++) * TimeStep(Nektar++), i.e. that the two solvers step through the same amount of simulated time for every invocation of the send() method. As an example, a value of timeStep for the OpenFOAM solver of 1e-6 and a TimeStep value of 1e-7 in Nektar++ necessitates cwipiStep = 1 and ReceiveStep = 10, since the ratio of time steps is 1/10. AcousticSolver must receive data from OpenFOAM every 10 time steps, while OpenFOAM sends the solution data at every time step.

The ordinary logic of the solver is then executed; for the sake of readability, this has been moved into a header file inserted as:

#include "rhoCentralFoam.H"

The solver then performs its normal I/O and updates the time step of the coupling object, before returning to the beginning of the loop:

runTime.write();
coupling.updateTime();
runTime.printExecutionTime(Info);