# Define characteristic length in matix flow in 3DEC
**URL:** https://forum.itascainternational.com/t/define-characteristic-length-in-matix-flow-in-3dec/1243
**Category:** 3DEC
**Created:** [November 9, 2023, 12:19pm UTC](https://forum.itascainternational.com/t/define-characteristic-length-in-matix-flow-in-3dec/1243 "2023-11-09T12:19:01Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@ShanLee](https://forum.itascainternational.com/u/ShanLee)
#### Post date: [November 9, 2023, 12:19pm UTC](https://forum.itascainternational.com/t/define-characteristic-length-in-matix-flow-in-3dec/1243/1 "2023-11-09T12:19:01Z")
Hi everyone,
In the 3DEC manual, the characteristic length is calculated as the ratio of the volume of the flow domain to the surface area of the flow domain. However, I’m seeking further clarification on the concept of the ‘volume of the flow domain.’ Could someone provide an example? My project focuses on DFN hydraulic-mechanical coupling under varying discharge rates.
Thank you for your assistance.
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### Author:  [@jhazzard](https://forum.itascainternational.com/u/jhazzard)
#### Post date: [November 13, 2023, 2:40am UTC](https://forum.itascainternational.com/t/define-characteristic-length-in-matix-flow-in-3dec/1243/2 "2023-11-13T02:40:28Z")
I don’t actually know what this equation means, but it isn’t used anywhere else in the document so it isn’t really important.
A characteristic length for a fluid diffusion process would be the average length of the flow path through the medium
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### Author:  [@ShanLee](https://forum.itascainternational.com/u/ShanLee)
#### Post date: [November 17, 2023, 2:11am UTC](https://forum.itascainternational.com/t/define-characteristic-length-in-matix-flow-in-3dec/1243/3 "2023-11-17T02:11:41Z")
Dear Jim,
Thank you very much, I will take it into consideration