As presented in the 14th OpenFOAM Conference, when reading gradients we end up with a minor increase in the respective quantity.

We observed this in fluid-fluid coupling (see #60) when coupling {pressure} with {velocity, pressure gradient} and using a coarse mesh. However, this problem also appears in CHT cases (reading heat flux), but we hadn't noticed until now since we are always using very fine meshes there.

In the following figure you can see this error when coupling gradients. When coupling only velocity and pressure, this problem does not appear, although we still get an oscillation around the interface.

We currently suspect a technical problem in the way we read and write gradients, or some other numerical issue.

You can try this in our pipe-pipe tutorial.