Use the Velocity node to prescribe a velocity at the pipe ends.

Enter a value or expression for the velocity uin (SI unit: m/s) at the inlet and/or outlet of a pipe. The default is 0 m/s. The velocity uin is defined relative to background flow u0 and thus in the tangential coordinate system. Enable the Show physics symbols from the Graphics and Plot Windows menu on the Preference dialog box in order to visualize the boundary or edge tangent direction. Click the Fluid Properties node to see the tangents as a red arrows.