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

numerical vs. ideal solutions

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

LinkBack Thread Tools Display Modes
Old   September 20, 1999, 15:10
Default numerical vs. ideal solutions
Gotfred Berntsen
Posts: n/a
Hi !

I have a general question regarding CFD-solvers.

If I do an inviscid calculation on a geometry with a known ideal solution, are there any reasons why the numerical solution would differ from the ideal solution based on stream function theory?

My case is a 2D NACA 0015 hydrofoil, and I'm doing some preliminary calculations to get to know the solver (Fluent 5). The numerically calculated velocity ratio (v/V) tends to be 10% smaller than the ideal solutions based on equations from "theory of wing sections".

Here v is local velocity at the surface, and V is free stream velocity.

I have also done the same inviscid calculation to a 2D cylindrical geometry, and the same tendency appears here. (while maximum value of (v/V) should be 2, I get as low as 1.8)

Gotfred Berntsen
  Reply With Quote

Old   September 20, 1999, 15:34
Default Re: numerical vs. ideal solutions
John C. Chien
Posts: n/a
(1). I was asking question about whether CFD is science or art. (2). In old days, to be qualified as an engineering answer, the error must be less than 5%, because people were using slide rules. (3). I can only say that, try other codes to find out whether they all have the same characteristics. (4). If you start fooling around with the code and answer, then I guess the answer will be on the art side. (5). Last Friday, an engineer came to me and show me that he was getting the same wrong answer from the code. Earlier, I sent him a few pages of notes showing the wrong answer I got from the 3-D commercial code. (It is not too bad for you, it is only 10% off. ) (6). I would strongly suggest that you run a couple of more test cases to check out the code. There is not much one can do with a black box code. In industries, normally, there are many other codes available to double check the results. In most cases, test data are also available.
  Reply With Quote

Old   September 22, 1999, 05:48
Default Re: numerical vs. ideal solutions
Antonio Filippone
Posts: n/a
Your inviscid solution should tend to the analytical ideal solution as you increase the size of your grid.

It is hard to say whether you have wrong input files, wrong settings or wrong theory, but the statement above is a basic concept in CFD. Thin wing theory does not apply to an airfoil that is 15 % thick.

I suggest that you do this:

Take some analytical airfoils (ex. Van de Vooren), compute the analytical solution (your benchmark), change your Fluent input file, run the code with increasing grid size and compare (make sure you have no viscosity).

  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
Numerical viscosity due to the MUSCL and HLL coulpled scheme sonsiest Main CFD Forum 0 May 23, 2011 15:37
Numerical Weather Modelling : Time and space resolution issues chriswatt Main CFD Forum 0 February 15, 2011 22:32
Two-Phase Buoyant Flow Issue Miguel Baritto CFX 4 August 31, 2006 12:02
numerical scheme ado Main CFD Forum 3 October 12, 2000 08:20
New Books and Numerical Software Eleuterio TORO Main CFD Forum 0 December 18, 1998 13:41

All times are GMT -4. The time now is 21:23.