CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum

Strange oscillating velocity

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

LinkBack Thread Tools Display Modes
Old   April 5, 2006, 12:00
Default Strange oscillating velocity
Posts: n/a

i'm running a simulation past an airfoil like body using my own 2d fvm fractional step cartesian ns code. hence, the body will be represented as steps.

my inlet velocity is either u=1 or slowly increasing from u=0.01 to u=1 in 100 time steps.

when i run at re=100, the u velocity along y=0 (center line) is approx 1 until it nears the body, where it starts to decrease. that's perfectly logical.

however, the strange thing is when run at re=2000, u changes from u~1 (far field upstream) ~0.9 ~0.95 ~0.93 ~0.94 to ~0.8 ~0.84 ~0.76 ~0.8 ~0.6 ~0.4 (just next to body). this answer seems a bit illogical...

my scheme has rhie chow interpolation. moreover, body is about 8 units from upstream so that should be enough right?

Can someone explain to me what's happening? thanks a lot!
  Reply With Quote

Old   April 5, 2006, 12:40
Default Re: Strange oscillating velocity
Posts: n/a
Sounds like a boundary condition effect to me. Depending on the quality of the BC you may need to move the outer boundary much further out. It is not uncommon to see outer boundaries at 50 to 100 chord lengths from the airfoil. I personally would not go less than about 20 (and then only if I had to).
  Reply With Quote

Old   April 6, 2006, 11:38
Default Re: Strange oscillating velocity
Posts: n/a

i've tried lengthening both the upstream as well as the downstream distance. unfortunately, the result is the same.

however, when i increase my grid to 256x128, the solution looks better, ie, the oscillation becomes smaller and the u velocity decreases as it approaches the body. but i wonder if i had really solved the problem.

btw, for a FVM fractional step non-staggered scheme, there is usually a need to define the cell center and face velocity(outward normal).

so for my inlet velocity, i've defined u_center=1, u face east=1 and u face west=-1 for first column of cells. is this reasonable? of course, this will result in a non-solenoidal field. i wonder if this is the cause of the problem. i've no idea how to generate a solenoidal field. it'll be great if someone can direct me some references.

  Reply With Quote


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
Multiple floating objects CKH OpenFOAM 10 September 21, 2011 23:13
Oscillating Airfoil - Dynamic meshing or user-defined velocity profile. DarrenC Main CFD Forum 5 July 19, 2010 22:33
how to compute relative velocity from absolute? spk Main CFD Forum 3 July 9, 2010 08:42
impose oscillating wall velocity IvanCFD FLUENT 0 May 13, 2010 08:53
Velocity in Porous medium : HELP! HELP! HELP! Kali Sanjay Phoenics 0 November 6, 2006 07:10

All times are GMT -4. The time now is 09:33.