
[Sponsors] 
December 24, 2011, 19:23 

#21 
Senior Member
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 713
Rep Power: 19 
We are not going anywhere. It seems that you have made up your mind and no amount of *discussion* is going to change it.
Time is important thing, so I have to limit it to some point. I can not spend more than what i have so far into this discussion. So believe in whatever you believe in. Good luck. 

December 24, 2011, 19:37 

#22 
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 
Too bad, I am sure we could have all benefitted from this, but the discussion speaks for itself, then. Just please do me one favor and answer my question in red: were you talkung about the convergence of your matrix invert all the time? That would explain all the trouble ....
Cheers friend! 

December 24, 2011, 20:58 

#23  
Senior Member
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 713
Rep Power: 19 
Quote:
Nope. I was talking about the fact that chosing a first order BC would not render the solution to 1st order everywhere. Saying it will is plain wrong. So in the end, if you like to believe otherwise, then go ahead and do it. I certainly do not agree with it. 

December 24, 2011, 21:06 

#24  
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 
Quote:
Good luck in your further CFD career, all the best! Cheers! 

December 24, 2011, 21:09 

#25 
Senior Member
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 713
Rep Power: 19 

December 24, 2011, 21:25 

#26 
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 
You just wont let it go, will you? The keyword here is overall, not local. As I have written above, integral. But I can tell that your background is more on the practical than on the theoretical side, so I will not bother you further with mathematical concepts.


December 25, 2011, 04:31 

#27  
Senior Member
duri
Join Date: May 2010
Posts: 160
Rep Power: 9 
Quote:
Quote:
Quote:
To demonstrate the concept of hmethod and pmethod i am attaching two results. Both case ran in fluent, first with 4800 cells and second order Roe scheme (ramp_2nd.jpg) and other case with first order on same mesh but adapted whole region twice so that grid spacing is 4times smaller (ramp_1st.jpg). Check the results, it demonstrates the relation between accuracy, order and mesh size. Beware that even though accuracy of the results are almost same the order is different. Any interesting observations are welcomed. Quote:
Quote:


December 25, 2011, 08:12 

#28  
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 
Hi duri,
thanks for your great reply! Quote:
But in general, it fully depends on the flow physics and the size of your domain whether low order inflow is acceptable or not. It's the engineer's or scientist's job to make sure that your bcs do not (greatly) affect the outcome of your solution. I don't understand why you worry so much about h>0, this concept is from calculus to handle derivatives at a point. (remember point is hypothetical concept). Quote:
Quote:
Quote:
Here are two I can remember of the top of my head: http://www.sciencedirect.com/science...21999182900699 http://www.sciencedirect.com/science...96300305003620 Quote:
Thank you so much, I was almost giving up on this forum Have a nice time! Cheers 

December 25, 2011, 20:36 

#29  
Senior Member
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 713
Rep Power: 19 
That is because that is what I was responding to, when i first commented. There is no doubt in anyone's mind that order of BC could affect the solution.
Quote:
I ran a LES simulation with say 10 million points. So how are you calculating the Taylor series expansion using 10 million points and get order of polynomial overall. Are you doing a curve fitting for a polynomial using 10 million points and then saying that since I used 1st order BC this polynomial is first order because of BC. Exactly. This sums up very well. 

December 26, 2011, 08:08 

#30  
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 
Quote:
We are talking about the order of the scheme, not of the solution here, that's two very different things. I will walk you through how the process is done in code verification, see e.g. the report by the Sandia labs: http://prod.sandia.gov/techlib/acces...000/001444.pdf 1) Generate an analytical solution to the NSequations 2) Create a source term as a right hand side (steps 1 and 2 are just to make sure that you have an exact solution for sth as complex as the NS equations). 3) Add the source term into your code, and doing your calculations 4) Let's say your exact solution is steady, but works with unsteady as well 5) For your steady state, compute the error of your numerical solution against the exact solution, in any suitable norm, let's say L2 for the WHOLE domain. 6) Decrease mesh spacing and repeat 7) compute order from E=Ch**p Let's go back to step 5: Let's say your code is of order 2, so all your errors will go as h**2, so integrating over the whole domain will still give you an "squared" behavior. Let's now assume that a single cell goes with h**1, because of whatever error is there. Then, the whole process with not give you p=2, but will go to p=1, as the linear error dominates. I have attached a zip file with an excel sheet for you to test, I have set up a case with 100 O(2) cells and just a sinle O(1) cell, the overall error is O(1), as you can see. This whole process is of course harder to do if you are doing an LES, but the concept is still valid. Hope this clarifies things! Cheers 

December 26, 2011, 08:12 

#31 
Senior Member
cfdnewbie
Join Date: Mar 2010
Posts: 557
Rep Power: 13 
I do not mean to nitpick here, but it does not necessarily have to be piecewise, think of global methods. Also, curve fit sounds like "interpolation" to me, which is also not the only way to do it, but maybe I'm just misinterpreting your use of the word "curve fit"


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Implementation of boundary conditions for FVM  Tom  Main CFD Forum  7  August 26, 2014 05:58 
2nd order upwind vs 2nd order upwind!!!  Far  Main CFD Forum  7  March 14, 2013 13:29 
Water subcooled boiling  Attesz  CFX  7  January 5, 2013 04:32 
divide a face in order to have different boundary conditions  antonio  ANSYS Meshing & Geometry  3  June 7, 2010 05:37 
Boundary conditions?  Tom  Main CFD Forum  0  November 5, 2002 02:54 