Defined inlet velocity is different with inlet velocity on CFX post

jonpewpew · November 2, 2017, 07:52

Hi there,

I'm new to CFD, and I got problems with CFX post as I don't get my expected results.

My project is simulating flow over a FSAE car to obtain drag and down force.I made the boundary with enclosure on DesignModeller.
I specified inlet velocity as 60 km/h, after the simulation finished I checked the velocity at the inlet using function calculator> sum> velocity, which shows 2150 m/s.

How could this happened as I specified a constant speed at inlet?

Thank you

urosgrivc · November 2, 2017, 08:37

Hi

Why did you use sum? use >area average>velocity
what does velocity contour look like if you make a plane, does it look like 60, or something else at an inlet?

It is almost never the softwares fault, its just a computer it doesent know enithig, the person who uses it gives it all the data and guidance.
When I first started using cfx I was the same, blame it on the cfx, now I know that I just ditnt know ewerithing and I still dont know a lot but I know that if something is wrong it is most probably my fault and not the number crunching machine. This is some life advice that can be implemented in a lot of other cases asveal

I was a FSAE car designer for about 4 years, so I realy hope you make it work and get some good results.

I suggest you use monitor points for drag and downforce and let your convergence criteria be primarely based on these monitor points and not just residuals.

ghorrocks · November 2, 2017, 17:40

The sum function does just that, it sums. So if you have 500 nodes each at 4m/s it will sum to 2000m/s. This is a pretty useless calculation and I suspect it is not what you wanted.

I think you are looking for averaging functions. areaAve or massflowAve look like better choices to me.

November 2, 2017, 07:52	Defined inlet velocity is different with inlet velocity on CFX post	#1
jonpewpew New Member Jonathan Join Date: Nov 2017 Posts: 1 Rep Power: 0	Hi there, I'm new to CFD, and I got problems with CFX post as I don't get my expected results. My project is simulating flow over a FSAE car to obtain drag and down force.I made the boundary with enclosure on DesignModeller. I specified inlet velocity as 60 km/h, after the simulation finished I checked the velocity at the inlet using function calculator> sum> velocity, which shows 2150 m/s. How could this happened as I specified a constant speed at inlet? Thank you

November 2, 2017, 08:37		#2
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 12	Hi Why did you use sum? use >area average>velocity what does velocity contour look like if you make a plane, does it look like 60, or something else at an inlet? It is almost never the softwares fault, its just a computer it doesent know enithig, the person who uses it gives it all the data and guidance. When I first started using cfx I was the same, blame it on the cfx, now I know that I just ditnt know ewerithing and I still dont know a lot but I know that if something is wrong it is most probably my fault and not the number crunching machine. This is some life advice that can be implemented in a lot of other cases asveal I was a FSAE car designer for about 4 years, so I realy hope you make it work and get some good results. I suggest you use monitor points for drag and downforce and let your convergence criteria be primarely based on these monitor points and not just residuals. Last edited by urosgrivc; November 2, 2017 at 09:52.

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November 2, 2017, 17:40		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The sum function does just that, it sums. So if you have 500 nodes each at 4m/s it will sum to 2000m/s. This is a pretty useless calculation and I suspect it is not what you wanted. I think you are looking for averaging functions. areaAve or massflowAve look like better choices to me.