## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Finding Pressure in Pipe Flow with Fluent?

• Brendan De Leon
Subscriber

Hello! I am a little new to the Fluent program, and as such, I would like a confirmation on some results I have acquired, which I cannot seem to find anywhere else. I am modeling a pipe with an overall length of approximately 5.2 meters and an ID of 0.5 Inches. I have given it an inlet pressure of 30 PSI, an outlet gauge Pressure of 0, and an inlet velocity of.05 m/s, set the fluid to water and turned on energy. I used the laminar flow equation.

Consistently, I get 0 gauge pressure at the outlet. This seems to make sense to me, as there would be a significant head loss- but some other members of my team seem doubtful. Is this a reasonable result? I have posted the pressure contour below. Any information or advice you could offer would be appreciated, thank you!

• Federico Alzamora Previtali
Subscriber

The solution you get is bounded by the boundary conditions that you define. If your outlet BC is set to 0psi gauge pressure, the solver will aim towards that value for its solution. Hence, by definition you should get 0psi for a converged solution.

What is curious to me is that you define both the inlet velocity and pressure at the inlet. The Supersonic/Initial Gauge Pressure for velocity inlets is typically used for supersonic inlets, which is not the case for you. You should leave it at 0psi.

• NickFL
Subscriber

For the boundary conditions, what do you know? The outlet pressure set to zero relative pressure is good. Do you know the inlet mass flow rate? How did you choose the velocity value of 0.05 [m/s]? Keep in mind if you put a uniform inlet velocity in there will be a larger pressure drop as the flow tends to the parabolic shape. If there is a long section upstream, maybe defining this inlet velocity parabola using Named Expressions.

But all that is for later. Before we sit down in front of the computer we need to do hand calculations to get a good estimate so we can accurately model the problem. (Plus we need to show all that time in the Universtiy was not wasted 🙂 ) Use what you learned in your Fluid Dyanmics class to calculate the Reynolds Number (validate the flow is laminar), compute the pressure drop (remember old Moody’s diagram?), plus all the correlations for the bends. Then you have a “good” estimate of what your pressure drop will be. This will give you, in this case, a very good data point that you can use to validate your CFD model.

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