# Why double precision for Y+<1?

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 January 2, 2013, 07:46 Why double precision for Y+<1? #1 New Member   Omer Join Date: Aug 2012 Posts: 4 Rep Power: 7 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

January 2, 2013, 12:09
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http://www.cfd-online.com/Forums/cfx...html#post82819

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

Quote:
 Originally Posted by Omer 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

 January 2, 2013, 12:34 #3 New Member   Omer Join Date: Aug 2012 Posts: 4 Rep Power: 7 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. Regards Omer

 January 2, 2013, 17:24 #4 New Member   Mohammed Join Date: Oct 2012 Posts: 27 Rep Power: 7 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.

 January 3, 2013, 19:04 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,993 Rep Power: 107 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.

 January 3, 2013, 23:46 #6 New Member   Omer Join Date: Aug 2012 Posts: 4 Rep Power: 7 Thanks Glenn. I will take up your suggestion and comeback with something. Regards Omer

 January 21, 2014, 01:22 #7 New Member   maaz ahmad Join Date: Dec 2011 Location: india Posts: 21 Rep Power: 7 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

January 22, 2014, 08:27
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Bruno
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Quote:
 Originally Posted by maaz 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.

 January 25, 2014, 02:54 #9 New Member   maaz ahmad Join Date: Dec 2011 Location: india Posts: 21 Rep Power: 7

 Tags double precision, yplus

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