# Basic question about 'convect'

**URL:** <https://community.freefem.org/t/basic-question-about-convect/4202>\
**Category:** General Discussion\
**Created:** [January 26, 2026, 3:50pm UTC](https://community.freefem.org/t/basic-question-about-convect/4202 "2026-01-26T15:50:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![jna1g18](https://avatars.discourse-cdn.com/v4/letter/j/77aa72/32.png) [@jna1g18](https://community.freefem.org/u/jna1g18)\
**Post date:** [January 26, 2026, 3:50pm UTC](https://community.freefem.org/t/basic-question-about-convect/4202/1 "2026-01-26T15:50:09Z")

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Hi all,

I simply need to know if I’m using the convect function in the correct way. Is the following correct:

 ![Screenshot from 2026-01-26 15-49-02](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/2X/a/aa5eb7f81c2fcd6ebe54136897d4847dd81af712.png)

where ‘n-1’ denotes the result at the previous timestep.

Many thanks.

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**Author:** ![zxss](https://avatars.discourse-cdn.com/v4/letter/z/bc8723/32.png) [@zxss](https://community.freefem.org/u/zxss)\
**Post date:** [January 27, 2026, 9:25am UTC](https://community.freefem.org/t/basic-question-about-convect/4202/2 "2026-01-27T09:25:31Z")

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My understanding is

\frac{Du}{Dt} \approx \frac{1}{\Delta t}[u^{n+1} - convect(u^n,-\Delta t, u^n)]

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<div class="post-metadata">

**Author:** ![jna1g18](https://avatars.discourse-cdn.com/v4/letter/j/77aa72/32.png) [@jna1g18](https://community.freefem.org/u/jna1g18)\
**Post date:** [January 27, 2026, 9:42am UTC](https://community.freefem.org/t/basic-question-about-convect/4202/3 "2026-01-27T09:42:42Z")

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Ah, of course, I forgot the minus sign on dt. Thanks for taking the time to answer!