[Ibrahim_I]

Hello, I am modelling a geothermal system (i.e. pipes underground) and am applying a fixed temperature to the outer wall. This fixed temperature should vary with depth (gradient of 0.1C/m). I'm trying to use a coded boundary condition but unsure how to get the z position.

The T value should be T0 - 0.1*z , where z is negative

Some attempts to get z based on other posts are:
const volVectorField& U = mesh().lookupObject<volVectorField>("U");
const volScalarField z = U.mesh().C() & vector(0,0,1);

const scalarField& z = mesh_.C().component(2);

But I get error: undeclared identifier 'mesh'. I tried this in 0/T file

Thanks