[CFD] Pseudo Transients for Steady-State CFD (Part 1) - Pseudo vs True Transients
ฝัง
- เผยแพร่เมื่อ 14 มิ.ย. 2024
- An introduction to pseudo transients and how they can be used to calculate steady-state solutions in CFD. Timestamps:
0:00 Introduction
0:58 Recap
3:24 Time derivative
5:50 Diagonal Dominance
6:58 Variable relaxation
8:04 Pseudo Transient
10:28 True Transient
15:59 Multiple Domains
21:57 Pseudo Time Step
26:27 Example
27:58 Domain Timescale
32:20 Pseudo Transient vs Relaxation
34:13 Summary
35:24 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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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. - วิทยาศาสตร์และเทคโนโลยี
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I can't explain how much I benefit your videos as a graduate student. I like to thank you for amazing effort!
Relaxation factor had never be so clear to me ! Thank you !
I found this very easy to digest, I was casually watching having food, and still was able to follow whole video. Kudos
You're a godsend mate, loving the series. Textbooks seem to have a knack for making it tougher than it needs to be.
Finally the wait is over for a fresh new video full of information!!!
Nice and clear! Looking forward for dual time stepping explanation!
Thank you, Aiden!! This definitely helped me understand the difference between true and pseudo time steps
Very clear explanation. Thank you so much!
Wow! I had been waiting for this one! Thank you!😁
These are amazing lectures man. Your videos give an intuitive idea of cfd. I wish I found these lectures when I was in my bachelor's.
I found this talk very usefull! This is the most wanted series Ive ever watched :)
Ultimate!
As usual, amazing talk!
Thank You! Dr.Aiden 😄
Thank you Aidan for this talk that is very clear and understandable. I did have no idea why when I do a true transient simulation, convergence always jumped in any time step. Now, I have a good idea about it thanks to your video, that brings things that you don't find easily in many literature or software manual.
Thanks a lot for your work !
brilliant explanation
A fantastic new video to help us, thank you! 🙌
Love your content!
Wonderful.... Just wonderful quality of content ...🎉
Fantastic. Thank you!
Great lecture again. Thanks a lot 🙂
Great video! Thank you!
Thanks a lot. It was very helpful
Nice lecture
Great talk!
Fantastic Thank You ✌✌👍👍
Thank you very much.
excellent video!.. thank you... could u please make a talk on what is p_rgh in OF and its advantage in numerical method? and what is the equivalent one on other solvers like fluent
great work, thanks a lot. I wish you also cover opensource tools that pseudo-transient approach is applicable.
What I personally do is dividing the volume of domain by the inlet volumetric flow rate and multiplying by 3. That gives a characteristic time for the flow to develope.
I like it! Nice method ☺️
very supportive
Wow, it is always a pleasure to watch your high quality content! I'm working on my thesis right now. (Air cooling in ANSYS Fluent) I'm wondering if I should buy your online course. Do you think on an academic standpoint that your course is "citeable" ? Do you have further literature recommended in your courses?
Thank you very much and greetings from Germany!
I would love to say that it is citable but some professors are quite traditional and like to see books and papers only in references. To be honest I don't think you should risk it. I always try and provide the original sources, so I would cite them instead 👍
Great presentation - very informative! One question - what is the difference between using a pseudo-transient method vs. a "true"-transient method where only one sub-iteration is used for each timestep? Conceptually, it seems the two methods should be the same. However, when I run a test case using a fixed timestep to compare the two approaches in ANSYS FLUENT, the results are different so I must be missing something... Thanks!
Agreed. However, with commercial codes we can never really know what all the differences are, and there may be some other small differences. I have only tried to cover the main conceptual difference here
Hi Aidan, I have a question wrt the transient simulation. As known, it's common to initialize the transient simulation by a steady-state simulation. But what is the cause of the unsteadiness during this switch if the residuals are so small and the steady results converge so great? (Though the flow is inherently unsteady, e.g. vortex shedding)
Such a good Video!
Does it make sense to first run a simulation with relaxation-fectors and than using the results of the simulation as initial values for a pseudo-transient simulation? Or do you only use one of the two methods?
Thank you, Simon
You can use either. As you are doing a steady state calculation it doesn't really matter how you get to your final solution, so you can choose either. Normally I just pick one and go with it (some CFD codes don't have the option and you just have to use one of them). I often go with relaxation first. If that proves tricky to converge, switching to pseudo transient is sometimes better and changing the pseudo time step seems to be easier than changing the relaxation factors arbitrarily (because you can tie it to a physical time step)
@@fluidmechanics101 thank you a lot!
@@fluidmechanics101 Why don't we just go ahead with pseudo-transient from the beginning instead of starting with relaxation and then switching to pseudo-transient if it proves tricky to converge? What's the ups and downs of just sticking to pseudo-transient for every steady state solution? Btw, I'm learning quite a lot from your videos and I'm really enjoying them, thanks for the quality content.
There is no real benefit. You can really go with whatever you prefer. It is steady state, so it doesn't matter how you get there 😊 I suppose the default in fluent is relaxation factors and I am lazy so normally just go ahead with them rather than pseudo transient. Also, I'm glad you are learning a lot from my video! that's great
In ANSYS Fluent under method option we have two option if we want to turn on or off Pseudo Time Method. Now when I turn on this option in Data File Quantities I can choose Cell convective Courant Number but when I turn of Pseudo Time method this option vanishes from data file quantities.
So question is why can't we find courant number if Pseudo Time method is not used ?
Good question, I'm not sure!
Thank you for the amazing and clear lectures man. Can you please shed some light on why pesudo transient method is only supports by coupled solver, why not other solver?
There is a pseudo transient method supported for the SIMPLE/C/PISO Algorithms in Ansys fluent.
Yes, in principle you could apply the pseudo transient method to any pressure velocity coupling algorithm. I think the reason may be historic. SIMPLE with under relaxation was proposed first, so many codes copied the original implementation and used under relaxation with SIMPLE. Coupled with pseudo transient was proposed later. So perhaps it has taken some time for codes to be updated and make them completely flexible (i.e giving you independent choice of pressure velocity coupling and steady state method). We should also remember that these options are often at the heart of CFD codes, so they can be tricky to change if they are buried in thousands of lines of code!
Thank you @Chris B
@@fluidmechanics101 Thank you Dr. Aidan
I don’t quite understand one thing: if we solve equations in an implicit formulation using iterative solvers, won’t the full and pseudo methods coincide?
Is cp of concrete really greater than cp of air? Or am I missing something? Thanks for a response.
Good point. The specific heat capacity is quite similar. However, the density of concrete is much higher. So for a similar sized region in you CFD mesh, the heat capacity (specific heat capacity multipled by mass) is much higher for concrete. Thanks for pointing this out!
so "normally" for a steady state analysis we would set the time derivate to zero and solve through jacobi methods. But in this case we keep the time derivative in and use it to our advantage, keeping in mind its steady state the whole time?
Yes! However, it is worth noting that if you don't include the time derivative or under relaxation, the steady state solution process is likely to be unstable when we solve using a Jacobi style iteration. So we either include relaxation or keep the time derivative and use these to stabilise the matrices, then we can use Jacobi /Gauss-Seidel/Conjugate Gradient to solve
@@fluidmechanics101 very interesting. I think I'm across the topic now. Thanks for your lecture series. Much appreciated.
Hello Aidan, my question that arises from you presentation is, then, why do SS simulations and not always the pseudo transient simulations?
some of SS solvers are faster. For example if you have a single local and global minimum of your response surface. However some problems will have multiple local minima, some kind of valleys etc. In that case pseudo method has additional advantage: it physically guides your solution through this rocky area)
@@makavelilcf Thanks for the clarification 😀
First
Hello Aidan,
I am working on some CFD validation project and have some basic doubts related to surface Cp distribution contour. I am a rookie in CFD and learning new things everyday.
Does surface Cp distribution plot helps you tp identify the flow separation point on the wing? If yes, can you explain how?
I assume that flow accelerates on the upper surface causing Cp to come down upto a limit and again Cp increases downstream on the surface plot of a wing. But will that interpret the flow separation point on line on the wing or it is just a representation of variation of pressure from the freestream????
Please help me to understand what all things can we interpret from a typical Cp distribution plot of 3D wing.
If you are interested in flow separation, you want surface contours of wall shear stress 👍 Look for the contour where the wall shear stress component in the x direction turns negative (assuming your flow is in the x direction). This will show you where you have flow separation
excellent video!.. thank you... could u please make a talk on what is p_rgh in OF and its advantage in numerical method? and what is the equivalent one on other solvers like fluent