[CFD] Aspect Ratio Warnings in CFD

Рет қаралды 17,604

Күн бұрын

A physical explanation of how cell aspect ratio affects the numerics of steady-state and transient CFD simulations.
Timestamps:
0:00 Introduction
1:13 Definition of Aspect Ratio
4:44 Steady State Example
17:10 Physical explanation of coefficient change
18:39 Effect of advection/convection
22:22 Transient CFD
25:01 Boundary layer cells
26:50 Mesh Example 1
29:30 Mesh Example 2
32:02 Summary
33:53 Outro
References:
C. Greenshields, H. Weller 'Notes on Computational Fluid Dynamics: General Principles', Section 8.2. 2022.
doc.cfd.direct/notes/cfd-gene...
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Disclaimer
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The methods, algorithms, equations, formulae, diagrams and explanations in this talk are for educational and demonstrative purposes only. They should never be used to analyse, design, accredit or validate real scientific / engineering / mathematical structures and flow systems. For such applications, appropriate trained, qualified and accredited (SQEP) engineers / scientists should be consulted along with the appropriate documentation, procedures and engineering standards. Furthermore, the information contained within this talk has not been verified, peer reviewed or checked in any way and is likely to contain several errors. It is therefore not appropriate to use this talk itself (or any of the algorithms, equations, formulae, diagrams and explanations contained within this talk) as an academic or technical reference. The reader should consult the original references and follow the verification and validation processes adopted by your company / institution when carrying out engineering calculations and analyses. Fluid Mechanics 101 and Dr. Aidan Wimshurst are not accountable or liable in any form for the use or misuse of the information contained in this talk beyond the specific educational and demonstrative purposes for which it was intended.

Пікірлер: 61

@mohame.k5399 Жыл бұрын

i fkn hit hike before even watching, ths should say how much we appreciate your admirable efforts, Aiden , you have all my gratitudes and support. i wish you all the best in this life now lets go back watching and taking notes

@francootaola9172 Жыл бұрын

Great to have videos about meshing! 😁

@asifraj321 Жыл бұрын

Your way of teaching is always outstanding. it's easy to understand.

@jessbuildstech Жыл бұрын

What a great video on meshing! Thank you Dr Aidan 🎉

@antares190 Жыл бұрын

Amazing pitch, as always, thanks a lot for this masterclass video!

@ankitsinghbhadouriya5801 6 ай бұрын

Great video. Thanks.

@sergiodavid4590 Жыл бұрын

That's a good video, currently I understand a mesh quality aspects in a transient simulation!! Thank you PhD.

@botonb4eg1 Жыл бұрын

Great video! Thank you very much!

@lorenzosalvatori7156 Жыл бұрын

The examples shown on the video were really good! It helped me a lot :) Thank you Dr Aidan

@danieltarraf7881 Жыл бұрын

What a great video, Thank you !

@sergniko Жыл бұрын

Very usefull video! Example with meshing of 90 degree corner has cleared a lot ! Thanks for your work!

@professionalprocrastinator8103 Жыл бұрын

Masterful course, Aidan! Plenty of teachings and I gained a different insight on meshing (not only does it allow a gain in accuracy but also in stability)

@harri30121993 Жыл бұрын

That was a great explanation on aspect ratios!

@uladzimir8042 Жыл бұрын

Great explanation, thank you! 😊

@pritampriyadarshi4530 10 ай бұрын

Thanks Dr Adrian for the insightful lectures

@nikhilnarale Жыл бұрын

Great video, thank you.

@abhipatil1 Жыл бұрын

Appreciate your efforts for making this video.

@wakil16 Жыл бұрын

Thanks for your videos ! from France

@jaydevdave7369 Жыл бұрын

Amazing insights, thank you very much

@cwong9241 Жыл бұрын

Thanks a lot for your lecture. A step closer to become a CFD expert.🤓

@akshayghorpade8624 11 ай бұрын

Beautiful Explaination. Thank you

@mayuragrawal1060 Жыл бұрын

Thanks a lot. It is good informative video

@anirbandas4201 Жыл бұрын

Good informative video.

@mdshupto183 Жыл бұрын

this is so helpful and interesting . thank you so much for your effort

@oliviermpukuta2726 Жыл бұрын

Great informative session.

@ahmad.s94 5 ай бұрын

Thanks a lot Dr Aidan

@mahmoodmousavi8614 Жыл бұрын

Excellent thanks

@anudeepgupta3799 Жыл бұрын

I am cleared with doubts having on Aspect ratios and what exactly happening in the transient calculations..... Thank you buddy.

@romirder Жыл бұрын

Holy crap. My dissertation is on CFD FSUK aerodynamics, and i wish i found this channel sooner. Been tryint to mesh a front and rear wing on the same geometry but it refused to work. I now understand why. I'll update this if i actually managed to fix it. Thank you for these

@DefaultMando 2 ай бұрын

Any update?

@husseinjusab Жыл бұрын

Please start the podcasts again! They were amazing

@fluidmechanics101 Жыл бұрын

Will do! I've got a few speakers in mind 😎

@dylanharris1601 Ай бұрын

Nice video! If you included examples of what good looks like (desired aspect ratios for varying mesh problems) for the SS and references for that it might enhance the experience.

@Tyrim Жыл бұрын

i was wondering just last weekend why did my transient simulation with structured mesh took so long! problem fixed now, thanks!

@fluidmechanics101 Жыл бұрын

Awesome! That's just what I was hoping for 😃

@yto5338 Жыл бұрын

I sure you are being in my mind! ...or you are watching me somewhere! seriousely thank you for lecture!

@Michallote Жыл бұрын

I suppose that's why Fluent favours polyhedral meshes so much!

@fluidmechanics101 Жыл бұрын

Yes!

@Maxakch Жыл бұрын

👍👍👍👍👍

@engineeringarts4509 Жыл бұрын

This video is so great to explain the implication of aspect ratios in CFD!!! (I usually just care about skewness and non-orthogonality more.) I'm thinking in high aspect ratio mesh, if the long-side of the cell is parallel to the flow in transient flow problem, it can mean Co number is still controllable. Does it also mean to have a good pressure correction to avoid excessive iterations? I'm struggling about the mathematical relationship between aspect ratio and pressure correction. I ask this question it's because I'm thinking in what situation CFD allows to have high aspect ratio. Thanks a lot!!

@fluidmechanics101 Жыл бұрын

You will probably be ok, because the pressure field from the previous time step becomes the initial guess for the next time step, so I don't think aspect ratio will have as much of a detriment as the steady solver where you start from nothing!

@rentionpasolari7230 Жыл бұрын

Thank you for the amazing video! I run the simple case of the flow around a circular cylinder in OpenFOAM using PimpleFOAM, and I realized that when I use structured mesh with higher aspect ratios the number of the outer correctors to converge is much higher than a low aspect ratio case. I can imagine, after watching this video that this is because of the diffusion dominant terms in the pressure equation. My question is, when my Courant number is low(around 0.5), and I use high aspect ratio cells, is it okay to perform just one outer corrector, or should I allow the solver to decide for this (since it needs more to converge)?

@fluidmechanics101 Жыл бұрын

You are absolutely correct. High aspect ratio cells needs more outer corrections, due to the diffusion term in the pressure equation. As for the courant number question, I would check your residuals (and monitors) within each time step. If you can convince yourself you are sufficiently converged within a time step, then 1 outer corrector is fine. An easy way to check this is to run 2 simulations, one with 1 outer corrector and another with 2 outer correctors. The compare your drag and lift coefficients. If there is no difference then you are fine to use 1 outer corrector

@jaydevdave7369 Жыл бұрын

Hello Adrian, do Fluid Mechanics 101 have a discord or reddit channel?

@fluidmechanics101 Жыл бұрын

Yep, discord is here: discord.gg/S9s2Zjbegf

@finfoil8544 Жыл бұрын

What about airfoil boundary layer cells in steady state simulations? I guess the pressure correction in not much a problem close to the wall because it's mostly diffused over the large face? And what about the wake?

@fluidmechanics101 Жыл бұрын

Yes correct. The pressure is mostly diffused normal to the wall, but the streamwise contributions are important, especially on the suction surface, so the high aspect ratios are always going to slow things down.

@finfoil8544 Жыл бұрын

@@fluidmechanics101 And doesn't the transient case also include a pressure correction step? What would be your best approach to calculate stall polars on low Mach number airfoils (M

@fluidmechanics101 Жыл бұрын

I am not sure what you mean by 'stall polars'. If you want to find where the aerofoil is stalling, you can just run a set of steady state simulations, incrementing the angle of attack by 0.5 degrees each time (changing the inlet velocity components). You can each initialize each solution with the solution from the previous angle of attack, so you won't need many iterations (say 500 per angle of attack). You only really need dynamic or moving mesh if you have a transient inflow (like a pitching aerofoil, or a vertical axis wind turbine)

@finfoil8544 Жыл бұрын

@@fluidmechanics101 yes that's what I currently do. I was wondering if the transient case would suffer less from the pressure correction in high aspect cells, in your video I get the feeling the transient case is less sensitive to it, but I don't understand why.

@fluidmechanics101 Жыл бұрын

Unfortunately there are some questions even I cannot answer 😅 the video shows my current understanding. If I find out anymore I will be sure to let you know!

@maximilianmaigler8651 Жыл бұрын

could you please share the LaTeX approach you employ for your Beamer slides? It seems you either have a markdown or .tex template for that

@fluidmechanics101 Жыл бұрын

It's just the beamer package for latex with the boadilla theme 👍

@maximilianmaigler8651 Жыл бұрын

@@fluidmechanics101 thanks for sharing. the schematics you create in tikz I assume

@fluidmechanics101 Жыл бұрын

Nope. I create them all in inkscape. I've never been a fan of tikz

@ravichandrakumarkb5551 Жыл бұрын

Hi there it really gave a clear picture, I am working on an aerofoil post-stall aoa so i am running in transient condition, for K omega sst iIam keeping wall y+ 1 for a RE no. 2.2E6 in that first cell height I am getting around 1.1e -5m near the aaerofoil. For this first cell height I need to keep the time step size less than 1e-5 s to keep courant no. less tha 1 at all the sides of an aerofoil but unfortuanetly i don't have computer resources to deal with this time step size. If i go with larger time steps Cl and flow seperation is completley off but Cd is matching within 10% error. If I use Spalart-Allmaras turbulence model, i went with wall y+ 100 the first cell height is 1e-3m so time step size was maintained at 1e-4s in this Cl and flow seperation is matcing even at post stall but Cd is at error of 70%. Kindly advise me on how to proceed further. Thank you.

@fluidmechanics101 Жыл бұрын

If you are limited by computer resources then you can just do the best you can and make it really clear to the assessors / whoever is reviewing your work that that is the best you can do. If they want a more accurate result, then they need to get you access to a better computer!

@ravichandrakumarkb5551 Жыл бұрын

@@fluidmechanics101 thank you

@adithyaajay8135 10 ай бұрын

How aspect ratio affect steady incompressible flows?

@fluidmechanics101 10 ай бұрын

It mostly affects the rate of convergence of the pressure equation, due to the coefficients in the A matrix (which are calculated from the discretised Laplacian)