Hello,

I'm trying to find the head loss/pressure drop across an open channel for a specified flow rate. The picture below shows how the system looks.

Basically water flows through the left face, and has to go around a bunch of these glass rods, and exits out the other face.

Setup

• Left face is an inlet with specified volumetric flow rate
• Right face is specified to be environmental pressure
• Surface goals are set up to be the average static pressure at inlet, and average static pressure at outlet
• The rods are glass, so I set up a wall condition on them to have a roughness of 5 microinches (found this number online)
• The base and "roof" of the system are steel, so they have a roughness of 200 microinches

Problem

I then run an internal analysis, and find that the pressure difference/head loss between the inlet and outlet is about .82 psi, which is way too high (it should be around .20 psi). This number doesn't change much if I change the roughness of the glass rods. However, if I specify 0 roughness for the glass rods, the head loss is around .20 psi, which is strange.

So, my questions are: Am I doing something wrong in my setup here? How can I diagnose this issue of having too high of a number? Is it possible that the glass rods aren't even recognized, and water's flowing straight through them?

Thanks,

Scott