boussinesqapproximation

mkk

Hi,
we tried to a new solver with boussinesq-approximation.
We had some problems with Momentum equation:
We used:
fvVectorMatrix UEqn
(
fvm::ddt(U)
+ fvm::div(phi, U)
- fvm::laplacian(nu, U)
+ g*alpha*(T-TRef)
);
solve( UEqn == -fvc::grad(p) );
We got this error for the line with (t-TRef):
error: no match for \u2018operator+\u2019 in \u2018Foam::operator-(const Foam::tmp<foam::fvmatrix<type> >&, const Foam::tmp<foam::fvmatrix<type> >&) [with Type = Foam::Vector<double>]
Which type is the right for TRef?
Thanks Martin