CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

3D CFD modelling

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 21, 2000, 12:18
Default 3D CFD modelling
  #1
Elv
Guest
 
Posts: n/a
Hi All, i am running fluent 5.0.4 on NT. I'm solving 3d axial flow turbine problem using RNG + non-equilibrium w.f. I'm basically looking for efficiencyof the turbine at different flow rates (efficeincy is a function of tangential force, axial force, stagnation pressure drop and speed of rotor). I have a few problems ...hope you can help... [1] What pc spec do i need in order to solve a million cells model? [2] For the moment i can only solve 80000 cells models[without any grid independence] and assume that results are valid as all aerofoils are subjected to a same magnitude of numerical error[i.e. i'm making sure to keep same meshing on all blade models]. Is that acceptable? [3] At very low and very high flow rates i'm not getting any convergence. I'm suspecting that for the former, numerical round-off due to relatively low dynamic pressure is preventing conevergence. For the latter, i've noticed flow separation occuring and suspect non-equilibrium effects to be responsible for lack of conevergence. In both cases, i'm monitoring axial force,tangential force and stagnation pressure drop for steadiness and once i'm satisfied that those have stabilised, i'm assuming convergence. Is that o.k?? [4] I dopn't really understand how the turbulent model i'm using is coping with boundary layer transition [re= 0.5 million and 5 million]??????? [5] i haven't touched the realizable k-e at all. Is it worth a try??? Thanks for considering this query.
  Reply With Quote

Old   September 21, 2000, 16:21
Default Re: 3D CFD modelling
  #2
Jonas Larsson
Guest
 
Posts: n/a
A million cells would typically require more than 1 gig memory, at least if you are using the coupled solver - makes a single PC difficult to use. With the segregated solver 1 gig might be enough. Running a one-million case on a single-CPU PC will be very slow though - several days or even weeks. To run this efficiently I'd use a Linux cluster of 10 PCs or so and run it in parallel. Then you can do a run over night.

The convergence problems you are experiencing might be related to unsteady effects - have you tried to run the case unsteady? A simple fix might also be to use the standard k-epsilon model. This will over-predict turbulent energy and this might stabilize your solution, although of course it will also give you a slightly incorrect solution. But sometimes an incorrect solution is better than no solution at all :-/

I definitely think that you should try the Realizable model. It performs quite well in most turbomachinery applications. The RNG model is not as good in my experience. The only time I would consider using the RNG model is when the turbulent Reynolds numbers of the flow is very low - then the Realizable model sometimes behaves badly. This is no problem in axial turbines though.

You asked about transition. With wall-functions you will not predict any transition at all - the boundary layers will be turbulent from the leading edge. This can of course be a source of error if you think that you have a large laminar part in your specific application. There is no transition-model suitable for turbine applications in fluent as far as I know. If you need to predict suction-side transition with Fluent you will have problems. If this is important I'd use a more turbomachinery oriented code where you have ad-hoc transition models (Mayle or Abu-Ghannan&Shaw or something like that). There is not much use trying the more advanced low-Re models - although in theory they might predict by-pass transition in simple cases like flat-plates they are not capable of handling something as complex as a turbine-flow.
  Reply With Quote

Old   September 22, 2000, 08:55
Default Re: 3D CFD modelling
  #3
Elv
Guest
 
Posts: n/a
Thank you Jonas, for your interesting comments. I will investigate the realizable model. In fluent , there's also the Spalart-Allmaras option. I found no comparative literature about it, can you please comment on its possible use for my application? Thank you very much for your kind support. ELv
  Reply With Quote

Old   September 22, 2000, 09:12
Default Re: 3D CFD modelling
  #4
Jonas Larsson
Guest
 
Posts: n/a
The Spalart-Allamaras model can be good for flows over wings etc. You could try to use it in your turbine simulation also - it might help convergence problems. However, if you have separations etc. that you want to model then it is not a good model. I'm also a bit hesitant if it is a suitable model for high-turbulence-level flows, as you have in in turbines. Comments anyone?
  Reply With Quote

Old   September 25, 2000, 16:16
Default Re: 3D CFD modelling
  #5
Oliver
Guest
 
Posts: n/a
Hi Jonas

The Spalart Almaras model is a one-equation model. For 3D flows the flow downstream of the turbine blade, will contain 3D effects, therefore the model to use is a RSM model. However if computational resources are limited, I would recommend the use of low swirl dominated RNG model coupled with the differential viscosity option.

Regards Oliver
  Reply With Quote

Old   September 27, 2000, 03:28
Default Re: 3D CFD modelling
  #6
Jonas Larsson
Guest
 
Posts: n/a
I disagree, RNG models behave badly in many turbomachinery applications and RSM models are difficult to use and not general enough for this type of applications. The only applications where I would consider using RSM is for highly swirling flows in cyclones etc. In turbomachinery flows the main importance of the turbulence model is to predict the boundary layers correctly - this is not the strong side of the RSM models in Fluent!
  Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
CFD modelling of blood flow anne Main CFD Forum 1 October 11, 2004 10:40
CFD JOBS and Expected Salary.... Noel Harrison Main CFD Forum 11 November 22, 2000 08:15
ASME CFD Symposium, Atlanta, 22-26 July 2001 Chris R. Kleijn Main CFD Forum 16 October 2, 2000 10:15
Can we quantify the fruits of CFD? Brady Brown Main CFD Forum 14 December 15, 1999 10:42
Which is better to develop in-house CFD code or to buy a available CFD package. Tareq Al-shaalan Main CFD Forum 10 June 13, 1999 00:27


All times are GMT -4. The time now is 11:37.