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Why double precision for Y+<1?

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Old   January 2, 2013, 07:46
Question Why double precision for Y+<1?
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Omer
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Hi ,

I am aerodynamic designer and I used CFD for some of my analysis. The Best practise guide http://www.cfd-online.com/Wiki/Best_...omachinery_CFD suggests I use double precision for Y+<1.

I would like to know why we need to do this. I know the aspect ratios when Y+<1 condition is met, can be very high. But I don't understand why we need to use double-precision. What are the underlying mathematics that needs double-precision.

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Omer
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Old   January 2, 2013, 12:09
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re: Double precision and grid quality

all boils down to round off error for very small terms.

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Originally Posted by Omer View Post
Hi ,

I am aerodynamic designer and I used CFD for some of my analysis. The Best practise guide http://www.cfd-online.com/Wiki/Best_...omachinery_CFD suggests I use double precision for Y+<1.

I would like to know why we need to do this. I know the aspect ratios when Y+<1 condition is met, can be very high. But I don't understand why we need to use double-precision. What are the underlying mathematics that needs double-precision.

Regards
Omer
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Old   January 2, 2013, 12:34
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Hi ,

Thanks mjgraf. I would appreciate a few more details to understand this in more dept.

I do understand it is to reduce the round-off errors. Single-precision gets us about 5 to 6 digits after the decimal, whereas double-precision gets us about 15 to 16 digits . But why does this become more pertinent when y+<1? Does this have to do with the very small cell volumes near the wall? And how does this affect our solution?


Thanks in advance.

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Omer
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Old   January 2, 2013, 17:24
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Mohammed
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Please can any body help me in post-cfx
I want to know how can I take values to draw the relationships yplus, k plus, epsilon plus.
can I find the values of friction factors directly from CFX or by analytical equations?
Thanks.
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Old   January 3, 2013, 19:04
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Glenn Horrocks
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Omer: This is all basic numerics. Whether double precision is required is problem dependant, but in general when you are resolving fine gradients in a large field you can run into round off error problems so double precision can help. Just try it with and without and find out the difference for yourself. Even better, write a little test program (doing the heat equation of somethign simple) and see for yourself the effects of numerical precision.

Mohammed: Please start a new thread for a new question.
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Old   January 3, 2013, 23:46
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Omer
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Thanks Glenn.

I will take up your suggestion and comeback with something.

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Omer
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Old   January 21, 2014, 01:22
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maaz ahmad
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hi,
i am working on fluent to obtain pressure coefficient on hip roof low rise building, i want to know how to obtain value of y plus above 30
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Old   January 22, 2014, 08:27
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Quote:
Originally Posted by maaz View Post
hi,
i am working on fluent to obtain pressure coefficient on hip roof low rise building, i want to know how to obtain value of y plus above 30
You should post this as a new thread on the Fluent forum.
Also: How should I ask my question on the CFX forum to get the best possible answer?
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Old   January 25, 2014, 02:54
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pressure coefficient on hip roof low rise building
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