So far, our mesh contains only data. This is sufficient for most of the numerical methods that preCICE offers. For nearest-neighbor-gradient mapping, however, preCICE also requires additional gradient data information. In this step, you learn how to add gradient data to the mesh.
Updated 08 Feb 24

When using nearest-neighbor-gradient mapping, we require coupling data and additional gradient data. We have seen in Step 3 how to write data to the mesh. Now, we will learn how to write gradient data to the mesh. For this purpose, we use the following API method:

void writeGradientData(
    precice::string_view          meshName,
    precice::string_view          dataName,
    precice::span<const VertexID> vertices,
    precice::span<const double>   gradients);

Let’s consider an example for writing block vector gradient data corresponding to the vector data v0 = (v0x, v0y) , v1 = (v1x, v1y), ... , vn = (vnx, vny) differentiated in spatial directions x and y. The values are passed as following:

( v0x_dx, v0y_dx, v0x_dy, v0y_dy,
  v1x_dx, v1y_dx, v1x_dy, v1y_dy,
  ... ,
  vnx_dx, vny_dx, vnx_dy, vny_dy  )

Let’s add gradient data to our example code:

precice::Participant precice("FluidSolver", "precice-config.xml", rank, size); // constructor

int meshDim = precice.getMeshDimensions("FluidMesh");
int dataDim = precice.getMeshDimensions("FluidMesh", "Stress");
/* ... */
precice.setMeshVertices("FluidMesh", coords, vertexIDs);

std::vector<double> stress(vertexSize * dataDim);

// create gradient data
std::vector<double> stressGradient(vertexSize * dataDim * meshDim)
/* ... */

while (not simulationDone()){ // time loop
  preciceDt = precice.getMaxTimeStepSize();
  solverDt = beginTimeStep(); // e.g. compute adaptive dt
  dt = min(preciceDt, solverDt);
  precice.readData("FluidMesh", "Displacements", vertexIDs, dt, displacements);

  precice.writeData("FluidMesh", "Stress", vertexIDs, stress);

  // write gradient data
  if (requiresGradientDataFor("FluidMesh")){
    precice.writeGradientData("FluidMesh", "Stress", vertexIDs, stressGradient);

/* ... */

For the example, you can use the following precice-config.xml (note the version specific information above):

<?xml version="1.0"?>

<precice-configuration experimental="true">
    <!-- the gradient flag here is only required vor preCICE version 2.4.0 -->
    <data:vector name="Stress" gradient="on"/>
    <data:vector name="Displacements" />

    <mesh name="FluidMesh" dimensions="3">
      <use-data name="Stress"/>
      <use-data name="Displacements"/>

    <mesh name="StructureMesh" dimensions="3">
      <use-data name="Stress"/>
      <use-data name="Displacements"/>

    <participant name="FluidSolver">
      <provide-mesh name="FluidMesh" />
      <receive-mesh name="StructureMesh" from="SolidSolver"/>
      <write-data name="Stress" mesh="FluidMesh"/>
      <read-data  name="Displacements" mesh="FluidMesh"/>
      <mapping:nearest-neighbor-gradient direction="write" from="FluidMesh"
                                to="StructureMesh" constraint="consistent"/>
      <mapping:nearest-neighbor direction="read" from="StructureMesh"
                                to="FluidMesh" constraint="consistent"/>