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-   -   Solver Precision in CFX-5. (http://www.cfd-online.com/Forums/cfx/18609-solver-precision-cfx-5-a.html)

cfd guy August 14, 2001 16:55

Solver Precision in CFX-5.
 
Hi people,
I know it sounds a weird question, but how do I know if I'm running in single or double precision in the CFX-5? I've already searched the manual, but I found nothing it could help me. Where I can change it? In the Solver Manager? Build 5?
Regards,
cfd guy

Dan Williams August 14, 2001 20:24

Re: Solver Precision in CFX-5.
 
The problem is that no double recision version of the CFX-5 solver has been released yet. There will be when CFX-5.5 is released this fall.

What sort of problem are you doing that you need double precision? Certainly the coefficient matrix does not benifit much from double precision. Geometric calculations may be another story depending on the grid that you are using.

Dan.

John C. Chien August 14, 2001 20:39

Re: Solver Precision in CFX-5.
 
(1). I had spent a lot of time trying to identify the source of error when using CFX-TASCflow in 3-D turbine applications some times ago. (2). I was convinced that the error was related to the single precision math used in the code. The double-precision version was not available then. My application required fine mesh and low Re turbulence model (two-layer model was available and was used). (3). This limited the Y+ values and the Reynolds number one can compute. (4). My feeling is: in the professional field, the double-precision math is required, for the solution to be possible or even useful. ( I had similar bad experience with an in-house cfd code, which uses single precision math in I/O and each time the calculation do a restart read, it change the accuracy of the computed variables. To be in the CFD field, you must be a professional programmer and mathematician first. It is just a waste of time when you are creating garbages all the time.)

Dan Williams August 15, 2001 00:50

Re: Solver Precision in CFX-5.
 
As you say, double precision math may be necessary, but it really only depends on the grid. With hexahedral meshes used in CFX-TASCflow, when the elements have large aspect ratios then double precision may be necessary. It depends on the application. Double precision math only matters when accuracy to the 5th or 6th significant figure becomes important.

It is not clear that the coefficient matrix benefits from double precision as the coeffients all come from a linearisation of the various non-linear terms anyways. And yes, you are correct, a double precision code should really restart in double precision.

Dan.


cfd guy August 15, 2001 10:15

Re: Solver Precision in CFX-5.
 
Thanks Dan,
Actually, I was just curious about it. Right now, I'm using hexa meshes and I didn't have any convergence problems yet.
Regards,
cfd guy


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