Easy: for memeory management,
 

Easy: for memeory management, gradient returns a temporary. Thus:

fvc::grad(p)().internalField()[probeCell]

.internalField() is not strictly needed but makes it easier to understand what you're doing.

Hrv

can I do the same for the temperature gradient at a wall? how can I put out a gradient at a wall over time? can i use the probing utility for this? Thanks a lot!