Inconsistent thermal divergence in low-Mach formulation

[bnkeeton]

I've been using the low-Mach formulation in nekRS v23 and found that the evaluation of the thermal divergence seems incorrect for the problem I'm working on. To test this, I've set up a very simple case in a cylindrical domain with a stagnant initial flow superimposed by a spherical hot spot. I run the case for a single small time step, and then output the thermal divergence and compare it to the identical case run with nek5000 by plotting along the line [0,0,0] to [6,0,0]. I have confirmed through separate software that the nek5000 solution is correct. A plot showing the discrepancy is attached, along with both sets of case files. The red curve represents the correct, symmetrical nek5000 thermal divergence, and the blue is nekRS.

One additional item I found interesting is that if you set the thermal conductivity k1 to be constant by changing sigma1=0 in both cases, the two codes then produce identical values for the divergence. I tried inspecting the relevant kernel qtlHex3D but I do not see anything off.

nek5000_jet_1.zip
nekrs_jet_1.zip

Thanks,
Ben Keeton

[stgeke]

Assuming the difference in qtl appears after the first timestep: Does the temperature match?

[bnkeeton]

Hi Stefan, yes, the temperature and thermal conductivity after the first time step are identical, and are shown below in first and second plots, respectively.

Given that $Q_T=\frac{1}{T} \frac{DT}{Dt}= \frac{1}{\rho T} \nabla \cdot (k \nabla T)$ for the single-component mixture considered in this problem, we should expect the thermal divergence to be identical.

Thanks,
Ben

[stgeke]

How about rho and rhoCp?

[bnkeeton]

They are both set to a constant non-dimensional value of 1 in fillProp and uservp.

[bnkeeton]

Hi Stefan, I'm curious if there is any update? I tried using the next branch v24.0.2 (sha:0003f4ae) and also find a solution that differs considerably from nek5000.

The case files originally provided are set up to run the single time step and output the divergence. I'm also happy to run any additional tests that might help resolve the discrepancy.

Thanks,
Ben

[stgeke]

Can you please try the very latest next

[bnkeeton]

Yes, I just tested the latest next branch: v24.0.2 (sha:c6a366da). The divergence is now fixed, and it is identical to that given by nek5000. I will continue to run more complex test cases, and will report back if I notice any strange behavior.

Thanks,
Ben