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Issue on page /chapter2/ns_code2.html #234

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@mcastel1

The natural boundary condition for the Poisson-like equation for \phi cannot be satisfied, it is satisfied only to within $O(\Delta t)$, see https://fenicsproject.discourse.group/t/natural-boundary-condition-is-not-satisfied/16606/8?u=mekong .

This should be clarified in the webpage.

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  1. jorgensd commented on Jan 3, 2025

    @jorgensd
    Owner

    I don't think it needs to be added. This tutorial does not have the goal of analysing the complete splitting scheme.
    This is the job of: https://computationalphysiology.github.io/oasisx/splitting_schemes.html
    where there also are references to several theoretical papers on the proposed method.

    You can also find the improved method where one keep an extra term in the equation:
    image
    image
    (the term that is dropped in this tutorial).
    In the Navier stokes eq this becomes:
    image

    Please also note that this is an open source project. If you have a good way of formulating what you want to have on the web-page, you can simply modify the source code and add a pull request.

  2. mcastel1 commented on Jan 3, 2025

    @mcastel1
    Author

    Do as you wish, but a boundary condition whose residual is huge in the first few steps, and stays large in the following, needs a comment, to the very least. This was confusing not only to me, but also to another colleague of mine.

  3. jorgensd commented on Jan 3, 2025

    @jorgensd
    Owner

    Do as you wish, but a boundary condition whose residual is huge in the first few steps, and stays large in the following, needs a comment, to the very least. This was confusing not only to me, but also to another colleague of mine.

    In general, when starting with a zero flow as initial condition, one sees these kind of «spikes» in the pressure/phi, as p_old is 0. This is not unique to this splitting scheme.

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