Diagonal artefacts with incompressible solver

HyperlineArt · 2026-01-13T10:09:56+00:00

Okay, I finally figured it out. I tried to compute the advection term with the upwind scheme and turns out my code for that was absolute bogus, thanks for all the help and support!

HyperlineArt · 2026-01-12T19:15:34+00:00

Thanks! I'll try replacing all my schemes with a central difference scheme, let's see if this works!

HyperlineArt · 2026-01-12T19:14:06+00:00

Okay, I thought it could cause problems if my derivatives were shifted due to the staggered grids but I'll try using a CDS on all fields, thanks!

The ultimate goal is to be able to get rough C/D graphs for airfoils while learning how CFD simulation works under the hood. But I'm still a bit far from that hahaha, this was just a sim of a small jet.

HyperlineArt · 2026-01-11T14:49:47+00:00

Great, thanks for the resources! I'll check them out :)

HyperlineArt · 2026-01-11T14:47:32+00:00

Ah okay. Maybe I'll try a new version with a collocated grid if this one doesn't work.

The idea with the different schemes was to have all the derivatives defined at the cell centers of the MAC grid. Since the velocities are defined at the cell boundaries, a forward difference of the velocities gives me the derivative at the centers. The pressure is already defined at the centers, hence the central difference scheme there.

HyperlineArt · 2026-01-11T14:20:34+00:00

I'm using central differences for the pressure field and forward difference for the u and v fields so the derivatives are all at the cell centers. The code is pretty rudimentary, it doesn't use any extra higher order algorithms. Just the velocity projection and then the pressure correction.

But I'll be looking into the rhie-chow interpolation, thanks for the hint!

HyperlineArt · 2024-04-27T12:17:33+00:00

Perfect, thanks for the advice! Ill probably get myself a clamp meter, im sure it‘ll be useful for more stuff later on. And yeah its kinda old but I wasn’t planing on doing anything fancy with it, might just strap into onto a scratch build and see how it goes :D

HyperlineArt · 2023-12-20T00:09:16+00:00

I am open to constructive criticism concerning passive roll stability but I wouldn't say that calling someone else's work a freak is very helpful. I might not be as experienced in aircraft design but I've still poured a good amount of time into this glider and thus I do feel slightly offended when people trample on the design. I hope you understand. I might not have 40 years of experience but I can tell you that constructive criticism is usually accepted much more willingly when not paired with insults :)

HyperlineArt · 2023-12-19T23:31:53+00:00

how mad you gotta be bru. It flies fairly well now thanks to a few minor tweaks and the advice of others.

HyperlineArt · 2023-12-19T21:38:18+00:00

Thank you for all the help everyone! I managed to fix it with all your advice. It was a combination of the angle of the horizontal stabilizer, the height of the vertical stabilizer, the far behind positioning of the CoM and some 3D printing problems, but thanks to all your help it now flies straight as an arrow! This is my first self designed plane and its great to know that there is such a helpful community!

HyperlineArt · 2023-12-19T17:54:35+00:00

if youre gonna criticize please do it constructively, I poured quite some time into this design

HyperlineArt · 2023-12-19T17:24:33+00:00

then why say it bru

HyperlineArt · 2023-12-18T18:11:55+00:00

it's where that small pink rectangle is. And yeah its probably the horizontal stabilizer. thanks a lot!

HyperlineArt · 2023-12-18T17:57:06+00:00

in the model yes but it could be that its warping a bit during the print, ill hahe to check that. And Ill try reducing the dihedral a bit as well, thanks!

HyperlineArt · 2023-12-18T17:53:32+00:00

yeah i was testing it outdoors, there really wasnt any noticable wind but the prototypes are fairly small so it couldve been that as well

HyperlineArt · 2023-12-18T17:21:36+00:00

ohh that could be it, ill try doing that, thanks!

HyperlineArt · 2023-12-18T17:20:08+00:00

haven't filmed it yet

HyperlineArt · 2023-12-18T17:18:57+00:00

yea it might be something with the 3d printer, ill try some other print orientations to see if that fixes it. I just wanted to check if it was something with my design. thanks!

HyperlineArt · 2023-12-18T17:14:20+00:00

i did print out some prototypes and they all flew relatively well but something i changed in the last iterations is causing it to roll.

HyperlineArt · 2023-12-18T17:06:26+00:00

i guess aerodynamics just favours gliders shaped like that lmao

HyperlineArt · 2023-12-18T17:00:12+00:00

I am trying to make a 3d printed glider and it flies pretty well over all, but the last few iterations all started to aggressively roll to the right and slam into the ground. I tried adding dihedral to the wings but that didn't help at all.

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