wall heat flux calculation

[amat000]

Hello,

I want to calculate and print drag and wall heat flux on a boundary. The ktauChannel example shows how to calculate drag.
How can heat flux be calculated?

I started building a function in udf file which may be called in the UDF_ExecuteStep. I understand that I can compute the temperature gradient with the gradientVolumeKernel. But I am unsure how to find boundary-normal value and integrate it on the boundary.

Function in udf file:

void wallcalc(nrs_t *nrs, dfloat time, int tstep)
{
  mesh_t *mesh = nrs->meshV;
  std::vector<int> bidWall = {1};
  occa::memory o_bidWall = platform->device.malloc(bidWall.size() * sizeof(int), bidWall.data());

  occa::memory &o_gradt= platform->o_mempool.slice0;

  nrs->gradientVolumeKernel(mesh->Nelements, mesh->o_vgeo, mesh->o_D, nrs->fieldOffset, nrs->cds->o_S, o_gradt);
.
.
}

Any suggestions how to calculate boundary-normal gradient and integrate?

[yslan]

Hi,

You can follow the velRecycling plugin that compute the velocity flux:

$$\int_{inlet} \vec{v}\cdot\vec{n}$$

nekRS/src/plugins/velRecycling.cpp

Lines 73 to 94 in 4f87e0e

	// rescale
	getBCFluxKernel(mesh->Nelements,
	bID,
	nrs->fieldOffset,
	o_wrk,
	mesh->o_vmapM,
	mesh->o_EToB,
	mesh->o_sgeo,
	o_area,
	o_flux);
	const int NfpTotal = mesh->Nelements * mesh->Nfaces * mesh->Nfp;
	sumReductionKernel(NfpTotal, o_area, o_flux, o_tmp1, o_tmp2);
	o_tmp1.copyTo(tmp1);
	o_tmp2.copyTo(tmp2);
	dfloat sbuf[2] = {0, 0};
	for (int n = 0; n < Nblock; n++) {
	sbuf[0] += tmp1[n];
	sbuf[1] += tmp2[n];
	}
	MPI_Allreduce(MPI_IN_PLACE, sbuf, 2, MPI_DFLOAT, MPI_SUM, platform->comm.mpiComm);

Or, I have an example of computing heat flux here:
https://github.com/yslan/nekrsExamples/blob/master/nusselt/nekrs_v23/comp_nusselt.hpp