[CFD] Pseudo-Transients for Steady State CFD (Part 3)

[CFD] Pseudo-Transients for Steady State CFD (Part 3) - Fluid Timescale

Рет қаралды 5,720

Күн бұрын

An introduction to pseudo-transients and how they can be used to calculate steady-state solutions in CFD (Part 3). Timestamps
0:00 Introduction
0:56 Recap
5:01 Solids vs Fluids
7:00 Diffusion in Fluids
12:20 Diffusion Example
14:15 Convection in Fluids
18:36 Compressible Flow
20:50 Reynolds & Mach Number
23:57 Buoyancy, Rotation etc.
26:31 Summary
28:41 Outro
#pseudoTransient #fluidmechanics101 #cfd
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Some useful references
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1. Ferziger & Peric, 'Computational Methods for Fluid Dynamics', 3rd Edition, Springer-Verlag, Berlin, Heidelberg, New York, 2002.
www.amazon.co.uk/Computationa... [Page 118, 149]
2. ANSYS CFX Solver Theory Guide
Chapter 1: Basic Solver Capability Theory
Section 1.10: Automatic Timescale calculation
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Want to learn more?
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Grab a copy of my CFD Fundamentals Course (for beginners):
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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: 37

@nicotrivium 4 ай бұрын

Dude, amazing explanations... I really love them. I couldn't find a better one so far, related to TSF calculations. Unfortunately, channels like this are so underrated... I am really glad I found it, thanks a lot man!!

@carlosmoreno4682 Жыл бұрын

Right now my PC dying in a 76h simulation VOF (3 phases) in Pseudo-transient regime, just to add to the importance of this nugget of knowledge to me. Thank you Aidan!

@fluidmechanics101 Жыл бұрын

76h ..... That's a long one!

@edgarfernandoreytorra7106 Жыл бұрын

didn't expect that neat piece of information about characteristic lenght and velocity, thank you

@Michallote Жыл бұрын

As always a pleasure to listen to your lectures Dr Aidan

@saravananvenkatesh5717 Жыл бұрын

The lecture was very informative and clear. Than you Dr. Aidan.

@sergniko Жыл бұрын

Very useful talk! I think a better understanding is reached :)

@Sam12347398 Жыл бұрын

Thanks for those presentation, great and clear so far 👍

@MrLote83 Жыл бұрын

Perfect! as always. Thank you Dr. Aidan for such very very informative video.

@jessbuildstech Жыл бұрын

Perfect, part 3 is just what I needed! Thank you Dr Aidan (:

@julioalvespereirafilho6564 10 ай бұрын

J usar

Жыл бұрын

Nice series! Thank you very much.

@anirbandas4201 Жыл бұрын

Great Clarification. Thnks for such informative content.

@guidofranceschini9931 Жыл бұрын

Love you Aidan!

@ganeshyng5403 Жыл бұрын

Thank you so much

@forestshaner8994 Ай бұрын

What a wonderful lecture series. Once again, well done! I do have one ambiguity that I'm trying to work through: if we're considering compressible flow, and our Mach number is greater than 1, shouldn't our characteristic speed be either the maximum velocity in the cells or the maximum velocity at the boundaries, rather than the speed of sound? For example, for a Mach number of 2, shouldn't the local velocity be twice the speed of sound? If we're aiming to be conservative by taking the greatest representative speed, shouldn't we use the local velocity (U_cell) in this situation?

@user-oz6ow5qr1o Жыл бұрын

Thank you very much for your all lectures. They are really helpful. Can you elaborate on how f is chosen (fraction) of time? Another thing, can you make another video on courant number. how does it differ from steady to transient, and from pressure based to density based. Thanks again for your helpful videos

@fluidmechanics101 Жыл бұрын

f is normally set to 1 and is then left as a variable for the user to adjust. If the calculation is unstable then the user can reduce f, to reduce the pseudo time step and get a more stable calculation. Courant number should be the same, regardless of your type of solver or solution algorithm 👍

@saisreechandras9295 8 ай бұрын

Amazing video explanation. Thank you Dr. Aidan. I have a question. You have explained pseudo transients based on solving for temperature profile in the domain. I am currently trying to solve a fluid problem (external flow) to visualize the velocity profiles. Does only the convection and compressibility effects hold for my case in calculating U0 or do I need to use a different approach?

@daronisaac3226 Жыл бұрын

Great content. Thanks for creating this great video series. Point of clarification about characteristic velocity for compressible flows. I'm not understanding your statement at 19:23. The speed of sound depends on temperature, a=√(gamma R T), and not on Mach number. So it seems to to me that the characteristic velocity based on the speed of sound would always dominate when computing characteristic velocity for compressible and other low speed flows. What am I misunderstanding? Thanks

@thirumech6970 Жыл бұрын

Thank you very much for all your crisp and informative series Dr. Aidan!!! Will it be possible for a content covering segregated and coupled solvers? Thank you!!

@fluidmechanics101 Жыл бұрын

Yes! Coupled solvers is going to be a tricky one and is definitely on my list. I want to do all of the big topics in CFD eventually 🙂

@thirumech6970 Жыл бұрын

@@fluidmechanics101 Thank you! 🙂

@angirekulavenu4028 Жыл бұрын

Hi Aidan, thanks for the wonderful lecture. 🙏 Does OpenFOAM support Pseudo transient simulations?

@fluidmechanics101 Жыл бұрын

I think it does in the more recent versions of PimpleFOAM

@ShakeelAhmad-zh6ol Жыл бұрын

Dear Aidan, how we can implement pseudo-transient solution for multiphaseInterFoam solver in OpenFOAM?

@Jialei-dw3li Жыл бұрын

Thank you for the fantastic lectures. I am a little confused: is the coupled solver in Fluent or StarCCM+ use the same algorithm? Looking for your explanation. Thank you very much.

@fluidmechanics101 Жыл бұрын

As these are proprietary codes, we can never really know how their algorithms work exactly. However, they should be using these ideas in their algorithm

@Jialei-dw3li Жыл бұрын

@@fluidmechanics101 I see that in coupled solver the Courant number should be set rather than URF. In this case, what's the relationship between Courant number and the pseudo-time step and why Courant number can be so large (e.g. 100 or 500)? Thank you.

@fluidmechanics101 Жыл бұрын

Ah ok, yes they are using local Courant number instead of pseudo time step as they are related to eachother. As you are aiming for a steady state solution you can go with Co > 1, and should probably use the values that they recommend. If the solver is a bit unstable then try reducing Co a bit until you are happy

@oskarelmgren Жыл бұрын

Pseudo-transient should then on average be able to reach a converged solution faster, and be more stable than true steady state solving with a fixed relaxation factor? Even in the case of a single domain fluid?

@fluidmechanics101 Жыл бұрын

That's quite a big claim! I think the best we can say is that they are different methods for achieving a steady state solution. Different CFD codes may be optimised for different approaches, so I don't think we can really know for sure. My hope with the videos is that you can at least see how they are related ☺️

@minjoong6687 Жыл бұрын

Thank you sir! I just have one question. The density and viscosity are a property of a certain fluid and should be constant for given initial & boundary condition. However, the maximum fluid velocity along the cell should be 'calculated' prior to determining the pseudo timestep. Which needed to be determined 'before' calculation. I wonder how maximum fluid velocity is calculated when we don't even started the calculation. Once again, thanks for your excellent lecture. I appreciate it alot.

@fluidmechanics101 Жыл бұрын

It will be based on the initial condition 👍

@minjoong6687 Жыл бұрын

@@fluidmechanics101 Then, I think the initialization should be done by hybrid of FMG intitialization cuz the standard initialization often assume that velocity or other conditions are all the same for given case. Am I right?

@fluidmechanics101 Жыл бұрын

Yep, you've got the right idea. Normally this is something to experiment with and see what happens

@Maxakch Жыл бұрын

Thanks Aidan! You make great videos, I love it. It helps me look at certain aspects of CFDs more broadly. But you have error in this video at slide kzfaq.info/get/bejne/bqxmgrlyuqnHiH0.html . The error is related to the speed of sound. Formula 16 doesn't work for a flow with Mach number (M) low than 1, it gives as reference speed equal to speed of sound. And that is wrong. Also, by defenition M=U/c, so if M > 1 than c < U.