|
[Sponsors] |
October 23, 2013, 11:40 |
Drag Coefficient dont convergate.
|
#1 |
New Member
Join Date: Sep 2013
Posts: 12
Rep Power: 12 |
Hi,
i try to simulate a flow around a fishlike body. a want to have the drag coefficient. the stream should be from the front, no walls. the drag coefficient is under zero and the value tramps to zero. so i created a inlet patch with bounery condition for U are fixedValue (uniform (2 0 0) and for p are zeroGradient. the outletpatch boundery condition for U are inletOutlet with a InletValue (0 0 0) and a value uniform (2 0 0) and for p with fixedValue uniform 0. the other bounderieconditions are slip. please look at the txt-files for more. to the solvers: i use the simplefoam (SIMPLE solver) and the k-Omega SST model. the flow is incompressible, steady with k of 1.5*10-6 (water) with turbolences. timesteps are 0.005s long. end is 10s. what i do wrong? please look at the paraview picture. somethink wrong with the stream? i orientatet on the motorbike tuto. |
|
October 24, 2013, 12:29 |
|
#2 |
Senior Member
Artur
Join Date: May 2013
Location: Southampton, UK
Posts: 372
Rep Power: 19 |
Hi,
Your p and U boundary conditions seem OK on the first glance. It seems to me that your domain is far too small with relation to the size of your AUV and so will give you unpredictable results. My advice would be to make it at least 1xlength of your body (L) upstream, 1.5xL up and down of the centreline and 3xL downstream (this will still be a bit too small but it's a start). If your viscous drag will still be wrong I'd recommend checking your nut, k, omega/epsilon boundary conditions. Good luck, A |
|
October 25, 2013, 08:06 |
|
#3 |
New Member
Join Date: Sep 2013
Posts: 12
Rep Power: 12 |
thanks for the answer. i will test your advice.
kind regards |
|
October 28, 2013, 03:25 |
|
#4 |
New Member
Join Date: Sep 2013
Posts: 12
Rep Power: 12 |
ok, the drag coeffient turns from positiv to zero, now. but the stream is the same. after 1-2 cells, it turns from 2 m/s to 0 m/s after some secondes. a dont know, whats wrong there. the hole stream should have a velocity from 2 m/s to calculate the drag coefficient, i think.
|
|
October 28, 2013, 05:55 |
|
#5 |
Senior Member
Artur
Join Date: May 2013
Location: Southampton, UK
Posts: 372
Rep Power: 19 |
Can you please post a picture of the flow features that trouble you and provide a bit more accurate description so it's easier for me to try and help you out?
Thanks, A |
|
October 28, 2013, 06:05 |
|
#6 |
New Member
Join Date: Sep 2013
Posts: 12
Rep Power: 12 |
it looks similar like in the picture of the first post. you see the red strip on the front? thats a velocity of 2m/s. blue ist 0m/s. now i try some other patchconditions for U. i done no picture. but i changed the streamrange or the mesh around the object. the simulation runs crucial faster.
E: now is run with to patches fixedValue uniform (2 0 0) and 2 slip one the sides. inlet or outlet i dont changed. if i change the outlet value to fixedValue uniform (2 0 0) there is a stream on the outlet (red stripe like on the inlet). dont no which order the streams follows. (in or out). if i put all patches fixedValue uniform (2 0 0) except the outlet, i get a stream in a wrong order. so i try now with other conditions :P |
|
October 28, 2013, 06:43 |
|
#7 |
Senior Member
Olivier
Join Date: Jun 2009
Location: France, grenoble
Posts: 272
Rep Power: 17 |
hello,
Your BC are wrong, in the physical sense. You set a 2m/s velocity inlet, strict all uniform, and a zeroGradient for the pressure, just in front of your "fishlike" ! Same at outlet ... NB: if you say "zeroGradient", OF apply zero gradient, not "almost zero Gradient". So increase your box(at least 10x the size of your object in front, 20x in back) if you would stay with this BC, or/and take a look at other more "realistic" BC. (see the airfol tutorial at least). regards, olivier |
|
October 28, 2013, 08:47 |
|
#8 |
New Member
Join Date: Sep 2013
Posts: 12
Rep Power: 12 |
thank you for your comment.
the airfoil tutorial is 2D, isnt it? my object is 3D. so i cannot use conditions like empty oder freestream? may there are analogies for 3D, i dont know. i want a pressure, thats all the same around, because i dont want to have influence to the streaming. what condition should i choose. im a littlebit confused. i think i dont get all the meanings of the boundery condition. E: i added two pictures. one with the same condition as in the first post, but with a bigger mesh. in the secound picture, a changed the condition from low, up, front and backside to wall. its a interesting streaming to the backwall. Last edited by Porisel; October 29, 2013 at 04:06. |
|
October 29, 2013, 11:12 |
|
#9 |
New Member
Join Date: Sep 2013
Posts: 12
Rep Power: 12 |
uff, i've done a huge mistake. for the streamvelocity i choosed the wrong direction. embarrassing. dont tell my mom, please.
|
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Drag Coefficient Convergence Problem | John | FLUENT | 18 | June 24, 2023 10:22 |
problem with saving drag coefficient | colopolo | FLUENT | 5 | April 12, 2013 11:59 |
Incorrect Drag and Drag Coefficient for flow over a cylinder | ozzythewise | Main CFD Forum | 8 | June 13, 2012 07:24 |
Calculation of Drag Coefficient, Help Please | teek22 | CFX | 1 | April 26, 2012 19:41 |
Automotive test case | vinz | OpenFOAM Running, Solving & CFD | 98 | October 27, 2008 09:43 |