Community
CFD Forum
Welcome to Autodesk’s CFD Forums. Share your knowledge, ask questions, and explore popular CFD topics.
cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 

Static vs. Total Pressure as an Input

3 REPLIES 3
SOLVED
Reply
Message 1 of 4
gschultz
3775 Views, 3 Replies

Static vs. Total Pressure as an Input

Can somebody offer a simple explanation of the difference between static and total pressure when used as an input in SimCFD and advise when the use of each is most appropriate?  For years I have used total pressure as the input in many studies and static as the output (because when starting out with the program I was advised that this was correct) but I have found with a study I am currently running that in some cases using total pressure for the input results in divergence and/or a less accurate result, and changing to a static input seems to correct the problem.

3 REPLIES 3
Message 2 of 4
len.whitehead
in reply to: gschultz

For incompressible analyses, static pressure is the recommended inlet condition when simulating a pressure drop. Total pressure is the sum of the static and dynamic pressure, and is often useful as an inlet for compressible simulations.

Message 3 of 4
gschultz
in reply to: len.whitehead

Then is it possible that using total pressure as an input can result in divergences in an extremely simple model? I am using SimCFD to develop (for reference purposes) a series of curves representing flows through various orifices at pressure differentials between 20 and 100 psig. I'm using a cylindrical volume of ~1 liter, with 10 different orifices ranging from Ø.010 to Ø.375. The orifice length is always 5x the diameter. I initially meshed by selecting the entire model volume and adjusting it to 20%. I was having some trouble at higher pressures with divergence and found that in all cases switching from total to static pressure solved the problem. I also had some results that converged but did not create a smooth curve, with some points being 30-40% off the expected result. Again, static pressure on the input pulled these into line in all but one case. I had noticed that velocity vectors for the questionable results showed some unusual asymmetric patterns. Can this be related to the total pressure input?
Message 4 of 4
waited
in reply to: gschultz

using static pressure boundary conditions will enhance your stability.  When total pressure is used as a boundary condition, then P_total = P_static + 0.5 * density *V^2.  From a computational standpoint, we impose P_static on those boundaries, so P_static = P_total - 0.5 * density * V^2.  Having to subtract off the 0.5 *density * V^2 makes this a more non-linear computation.  As the velocities change on the BC surface, we then update the P_static using these velocities and compute the pressure field based on static pressures.  Using P_static on you BC planes allows for a constant prescribed pressure and eliminates the 0.5 * density * V^2 term being factored in.

 

Dave.

Can't find what you're looking for? Ask the community or share your knowledge.

Post to forums  

Autodesk Design & Make Report