This is just archiving the post on the forum: https://forum.taichi-lang.cn/t/a-way-to-store-the-shape-of-a-convec-polygon-and-divide-them-in-fix-grid-in-taichi/1247?u=wyq977

Main questions

Hi guys, I am having a bit of trouble with describing a polygon in a 2d grid, hoping someone could suggest anything. Thx in advance

I couldn’t find a proper way to handle geometry of a cell using immersed boundary method in a fixed grid i.e. 1000 x 1000. My overall goal is to re-implemented the LBIBCell in Taichi. However, it seems pretty difficult to for me to think of a way for the shape of a polygon (cell) in lattice grid in Taichi.

I have the code here: https://github.com/wyq977/taichi_lbibcell.

As shown in the gif on homepage, the simulation went well for Computational fluid dynamics (CFD) using Lattice Boltzmann methods, inspired by LBM_Taichi.

Since I cannot initialize another variable to store upon each iteration, one has to come up with a plan to store a list of points (f32 or f64) and their connection.

Also, because the polygon (cell) are subject to tension/forces between each other, increasing mass (not described here) one has to find a way to re-mesh the polygon and finally divide along the longest axis or a random axis (representing cell division).

To put it simply, one has to come up with the following functionality to describe the reaction-diffusion as well as the fluid dynamics:

1. Find the closest lattice grid point(i, j) to the polygon geometry, (kind of done, see below, i, j are integer
2. Mark the closest as inner or outer
3. Re-mesh the geometry:
   • For example, the 4 points for the cubes are too far away from each other, more points have to be inserted between those 4 points to capture the forces (tension)
4. Divide a polygon along certain axis

In the final version or current LBIBCell, the number of cells are not fixed due to division.

1

        self.boundary = ti.Vector.field(2, dtype=ti.f32, shape=(4,)) # store 4 points of a cube

Currently, I could have one or multiple variables representing multiple polygon (cell) geometry in the beginning. But the problem with that, I limit the resolution of the polygon since I cannot change/re-mesh edges/connection later in simulation.

Ideal situation would be like this:

LBIBCell’s way to finding closest grid points, figure of illustration below

1. for each edge (connection) AB, calculate all the coordinates of points that AB intersects with grid line (x=1, 2,… and y = 1,2,..),basically finds the y=ax +b from AB coordinates
2. Finds the closest grid point p(i, j) to those intersection
3. Find out the ones that are outside of the polygon

Since polygon like ABCDE may have edges horizontally or vertically align with the gird/axis, a little perturbation will be done before to avoid cases like AB is parallel to x = 0 or y = 0.

Thx you guys in advance and any suggestions would be much appreciated

Follow-up

Due to the limits on url in the forum for new users: the github issue might be a bit more informative: https://github.com/wyq977/taichi_lbibcell/issues/1

The idea is inspired by the homework:https://forum.taichi-lang.cn/t/homework0/506/26

Credit to them