# Implement Exponential Integral

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 January 24, 2008, 08:28 Hi, I'm wondering anyone kn #1 Member   Erik Arlemark Join Date: Mar 2009 Location: Eindhoven, Netherlands Posts: 47 Rep Power: 9 Hi, I'm wondering anyone knows if the Exponential integral function is implemented in OpenFOAM. If not, maybe someone can give me a hint of how to implement it in my solver myself? Thanks /Erik

 January 24, 2008, 16:04 hi, isnt there are expansio #2 Senior Member   Stephan Gerber Join Date: Mar 2009 Location: Germany Posts: 118 Rep Power: 9 hi, isnt there are expansion of this integral (at least for special arguments like pure complex or real..)?? just wondering stephan

 January 25, 2008, 07:11 Hi Erik, I don't know if it #3 New Member   George Kokotis Join Date: Mar 2009 Posts: 9 Rep Power: 9 Hi Erik, I don't know if it would be of any use to you, but I have used in the past some asymptotic methods to approximate exponential integrals. Relevant methods are described in the following book: Title: Asymptotic Expansions Author: A. Erdelyi Publisher: Dover Publications (June 1, 1956) ISBN-10: 0486603180 ISBN-13: 978-0486603186 At that time (1993), direct calculation was numerically heavy, involving (if I am not mistaken) reformulation of the integrand in terms of Bessel functions and direct numerical integration... On what field are you working?

 January 25, 2008, 07:44 Hi George, Thank you very m #4 Member   Erik Arlemark Join Date: Mar 2009 Location: Eindhoven, Netherlands Posts: 47 Rep Power: 9 Hi George, Thank you very much for the reference! Im working on micro gas flows and I want to incorporate a geometry dependent viscosity (varying with wall-distance) based on viscosities dependence on the mean free path (MFP) (which with physical reasoning also should vary with wall-distance). The thing is: An established expression of this effective mean free path is formulated using the exponential integral (Ei). I have derived a similar and approximate formulation of the geometry dependent MFP which only uses functions known to OpenFOAM. So if the Ei function isn't defined I'm actually happy because it means an numerical advantage for my model (even if approximate). Thank you /Erik

 July 2, 2010, 15:13 #5 New Member   Join Date: Jul 2010 Posts: 3 Rep Power: 8 Hi, I currently have the same problem. I have to use the exponential integral function but I can't find out how to integrate it to my C programme. I've tried "::Ei" but it didn't work. Is there a special library to include at the beginning? How did you manage to solve tour case? Thank you to answer because I'm really stuck. Ingrid

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