You can easily couple Pipe Flow Interface with Single-Phase Flow interfaces using multiphysics coupling feature Pipe Connection (). The Pipe Connection node is only available with some COMSOL products (see http://www.comsol.com/products/specifications/).

The Pipe Connection multiphysics coupling couples a 1D pipe segment modeled with the Pipe Flow interface with a 3D single phase flow body. There is an automatic search for possible adjacent geometrical objects. If you want to modify the selections, select the Manual control of selections check box. If you clear it, the selections will be replaced by the automatic ones.

If the automatic selection is not sufficient, select the Manual control of selections check box. Then, select the connected fluid boundaries in the Boundary Selection, Fluid section and the corresponding pipe points in the Point Selection, Pipe section. You can connect only one pipe segment to 2D surface that comprises an orifice in a 3D body.