CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

2D Euler solver using Runge-Kutta, need help/comment

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   August 25, 2011, 03:33
Default 2D Euler solver using Runge-Kutta, need help/comment
  #1
New Member
 
Paul
Join Date: Apr 2011
Posts: 12
Rep Power: 6
syoo is on a distinguished road
Hello,

I am conducting a side project of coding a 2D Euler solver using 4th order Runge-Kutta with 2nd order central difference for spatial derivative. I am using NACA0012 at Mach 0.5 as the test case. I have applied slip-wall boundary condition at the wall (slip condition + no penetration + density extrapolation + pressure extrapolation). I explicitly calculate the energy at wall using the calculated velocity and extrapolated density and pressure at wall. For farfield, I apply freestream boundary condition (distance from airfoil to farfield is 50 chords). I did not include any artificial dissipation.

My solution blows up almost immediately. Any comment/help would be extremely appreciated.

thank you.

-syoo
syoo is offline   Reply With Quote

Old   August 25, 2011, 06:46
Default
  #2
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
Two things:
a) what kind of initial condition are you using? If the computation blows up almost immediately, sounds like your starting vortex screws you...
b) I'm pretty sure that free stream all around your airfoil won't work, since you have no mechanism that allows disturbances in the flow field to vanish/dissipate (since you have a hyperbolic problem). What you should do is use sth like supersonic (extrapolation) outflow on the outflow part of your domain. From what you describe, it really sounds like that would solve your problem!

Cheers!
cfdnewbie is offline   Reply With Quote

Old   August 25, 2011, 10:55
Default
  #3
New Member
 
Paul
Join Date: Apr 2011
Posts: 12
Rep Power: 6
syoo is on a distinguished road
thank you for replying.

a) my initial condition is freestream except at the wall (k=1) and k=2 index.

b) I will try 1st order extrapolation at outflow portion of the grid at farfield. This region is somewhat ambiguous since my grid is an O-grid topology.

thank you.

-syoo
syoo is offline   Reply With Quote

Old   August 25, 2011, 12:07
Default
  #4
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
since your initial conditions are more or less freestream, you will get a shock wave or at least a large pressure disturbance as you start your computation. without a boundary condition suitable for hyperbolic type problems, your disturbances have nowhere to go and end up blowing your simulation to nirvana. so extrapolation BCs with most certainly help, with an O-grid, make sure that your domain boundary is far enough away from the geometry and use good sense for the outflow part of your domain...e.g. consider angle of attack and maybe cd/cl, which will give you an estimate of the angle of deflection of the flow and thus give you a pretty good estimate which part of your BC will be outflow!

cheers!
cfdnewbie is offline   Reply With Quote

Old   September 17, 2011, 23:47
Default
  #5
New Member
 
Paul
Join Date: Apr 2011
Posts: 12
Rep Power: 6
syoo is on a distinguished road
I guess farther help is necessary...... I have implemented non-reflective far field boundary condition. My solution does not blow up immediately anymore. However, after ~10000 iterations or so, my pressure on surface goes negative then solution blows up. My CFL = 0.1 which comes makes it dt = 1e-7 or so. But the amount of time simulated is very short due to small dt, so maybe I can see this as solution still blowing immediately after start?

It seems as though, regardless of the changes I make, the solution seems to remain same / similar.

I am using Runge-Kutta 4th order as described in the book "Computational Fluid Dynmics Volume 2" by Hoffmann.

Is there any common mistake one can make while writing the flow solver for Euler equation for a airfoil?

thank you.
syoo is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
thobois class engineTopoChangerMesh error Peter_600 OpenFOAM 4 August 2, 2014 09:52
2D unstructured Euler solver xz Main CFD Forum 0 August 20, 2004 05:46
Open Source Euler - NS Solver ? MickaŽl PERRIN Main CFD Forum 2 June 24, 2003 12:46
Matlab Unsteady 1D Euler Solver??? Johan Sylwander Main CFD Forum 1 May 24, 2000 15:26
Validation of axisymmetric Euler solver Fernando Velasco Hurtado Main CFD Forum 3 December 19, 1999 19:59


All times are GMT -4. The time now is 21:04.