zeroGradient boundary condition simply extrapolates the quantity to the patch from the nearest cell value. The meaning is, the quantity is developed in space and its gradient is equal to zero in direction perpendicular to the patch (perpendicular to the boundary). See figure (). U, k, omega, epsilon, T:

 boundaryField
 {
 ".*_outlet"
    {
        type            zeroGradient;
    }
 }