[ooo]

I want to solve( ddt(U) == F ) in which both U and F are volVectorField.
I get three different results when i use these three approaches!
I expect that all of them should be same

1) forAll(U, cellI) U[cellI] += runTime.deltaTValue()*F[cellI];
2) solve(fvm::ddt(U)==F); //Euler descritization
3) solve(fvm::ddt(U)==F); //CrankNicolson descritization

Do you know why they don't give same result? Because i think all of them do this : ( Unew = Uold + F*dt)