What I mean by close loop is that the space in the closed loop being blocked from the outer environment. Imagine the simulation to be a liquid cooler with liquid stored in a closed loop tube and driven by a internal motor. The picture below shows a simple sample that I set up for this simulation. I assign the space in the closed loop to be a fluid sub-domain, with water to be the fluid. To make the sample simple enough, I did not assign a heat source nor a rotational region.

Now the problem is that every time that I try to run the simulation, Solidworks asks me to setup boundary conditions. So I setup a rectangular box with 2 faces under default static pressure(behave as open faces) surrounding the closed hollow loop, and run it again. Then there is warning coming up in the info page "Unresolved region-Fluid Subdomain 1". And I can't see any information inside the closed loop such as a preview of temperature or velocity of fluid. So is there anything wrong in the setup?