Help me - Big difference between 2D and 3D computational domain - SolidWorks simulation flow

hello thank you in advance anyone who can help me in this problem that is going bugging me.

I am studying a particular type of valve (Tesla's Valve) has conducted a very simple I want to know the pressure difference between inlet and outlet, before sliding the fluid in one direction and then in the other (I use 2 configurations), the fluid is water and there are no particular boundary conditions.

The problem is that when I study in 2D Computational Domain, the values ​​are similar to those of a scientific comparison but when I use 3D computational domain (that of my greatest interest), the values ​​of pressure decrease dramatically.

I don't understand why this difference of values ​​should be similar, I do not know what to do to find the cause.

If someone would be so kind as to help me I would be very grateful, I could provide more specific details.