convergence problem in 2D steady supersonic flow

sehs15 · November 4, 2014, 07:10

Hello Guys ..

I'm simulating a flow over a 2D supersonic airfoil with fluent, steady state, density based solver, structured quad cells with yplus around 2 and SA turbulence model, boundaries are velocity inlet and pressure outlet

the problem is the solution doesn't converge such that the residuals are fixed at certain values and the mass flow rate and pressures at certain locations keep oscillating periodically with large amplitude.

i tried to reduce the CFL up to 0.01! oscillations exist with less amplitude.

Can anybody help with this problem im stuck!

cfdkid · November 5, 2014, 06:47

Hello

As you know, there are various reasons why the solution doesn't converge.
I guess the domain of your solution is too small.
How big is the size of domain?
At least the domain behind airfoil must be approximately 10~15 times the chord length.

sehs15 · November 5, 2014, 07:47

yeah the domain behind the airfoil is about 10 times the chord .. i tried pressure inlet boundary and its much much more better than velocity inlet with little oscillations ,, i will try to reduce the CFL with the pressure boundary

cfdkid · November 5, 2014, 18:58

I found same problem with your problem.

Please look at this thread.
http://www.cfd-online.com/Forums/fluent/112445-pressure-far-field-vs-velocity-inlet-pressure-outlet.html

Quote:

Originally Posted by Santos-Dumont

It is pretty straightforward, you can easily find some things about that on the Fluent Theory Guide. Anyway the thing is that VelocityInlet/PressureOutlet are only used for incompressible cases only (velocity under Mach 0.3)

If you are compressible (this is usually set up by setting your material density to Ideal Gas and Viscosity to Sutherland) you shall use Pressure Far Field. You can use your entire boundary (i.e inlet, outlet and sides) and set it up as one single boundary in your meshing software, that saves time while setting up in Fluent.

So velocity inlet for incompressible, and pressure FF for compressible.

sehs15 · November 6, 2014, 07:06

thank you bro ,, but it seems like pressure boundaries also have fluctuations , i will try to extend the domain much more ans see

November 4, 2014, 07:10	convergence problem in 2D steady supersonic flow	#1
sehs15 New Member mohamed mohsen Join Date: Oct 2012 Posts: 29 Rep Power: 13	Hello Guys .. I'm simulating a flow over a 2D supersonic airfoil with fluent, steady state, density based solver, structured quad cells with yplus around 2 and SA turbulence model, boundaries are velocity inlet and pressure outlet the problem is the solution doesn't converge such that the residuals are fixed at certain values and the mass flow rate and pressures at certain locations keep oscillating periodically with large amplitude. i tried to reduce the CFL up to 0.01! oscillations exist with less amplitude. Can anybody help with this problem im stuck!

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November 5, 2014, 06:47		#2
cfdkid New Member AAA Join Date: Nov 2014 Posts: 7 Rep Power: 11	Hello As you know, there are various reasons why the solution doesn't converge. I guess the domain of your solution is too small. How big is the size of domain? At least the domain behind airfoil must be approximately 10~15 times the chord length.

November 5, 2014, 07:47		#3
sehs15 New Member mohamed mohsen Join Date: Oct 2012 Posts: 29 Rep Power: 13	yeah the domain behind the airfoil is about 10 times the chord .. i tried pressure inlet boundary and its much much more better than velocity inlet with little oscillations ,, i will try to reduce the CFL with the pressure boundary

November 6, 2014, 07:06		#5
sehs15 New Member mohamed mohsen Join Date: Oct 2012 Posts: 29 Rep Power: 13	thank you bro ,, but it seems like pressure boundaries also have fluctuations , i will try to extend the domain much more ans see