Computational Aerodynamics Questions & Answers

Question by Student 201427564 Professor, when I run paraview, the error occurred that can not open 'post.' data file. I checked Assign4 and Assign7 file, and there were no post.vtk file in Assign7. Is it a cause of this problem or something else?	05.14.17

If what you mean is that there is no post.vtk file in the tgz package, then this is normal. You need to create this post.vtk using warp of course. This is obvious..

Question by Student 201427564 Professor, Should I have to create post.vtk and data.01 files for each case? And I have another error ; 'tec360 : command not found'. Could you give me an advice?

Well yes, you should create those for each case. I don't recall mentioning in class about the use of tec360..? Why are you using this?

Question by Student 201427102 At "Coefficient form of discretization" You used 1st order formular for time deriviative But for x deriviative, you use 2nd order. Why you choose 2nd order??	05.15.17

This is a good question. If we choose 1st order, we'll face another problem other than the spurrious oscillations. I'll explain this again in class tomorrow. 2 points bonus.

Question by Student 201227147 Professor, I have a question about Assignment #7. Last class, you changed mf as 1, 2, and 4 and grid became as $21*21$, $41*41$, and $81*81$ because $is=1$ and $ie=1+round(20*mf)$. But, I think $ie$ should be $round(20*mf)$ so that grid becomes $20*20$, $40*40$, and $80*80$. Why $ie=1+round(20*mf)$?

This is to make sure that there is one node at $x=0,~y=0$. If ie would be set to 20, 40, etc, instead of 21, 41, etc, then there wouldn't be a node precisely at the origin. 1 point bonus.

Question by Student 201227141 Professor, at assign#7 i just changed mf and xiverge then i got this wrong picture. I can't understand what i did wrong.	05.16.17

There's not enough information provided for me to help you. You need to isolate the problem and indicate clearly what change in the control file created this problem.

Question by Student 201427142 Professor, I have problem with Q#2 at Ass. #7. You said $r=\sqrt2$. However, when I set $mf=2\sqrt2$, there is no (0, 0) point instead ($\pm$0.0175439,$\pm$0.0175439). You intend to reduce the scale, but I think it cannot be done. How can I to do about it?	05.17.17

That's correct: I did this on purpose so you need to find a way to approximate $\delta_x P$ at the origin as well as possible when there is no node exactly at the origin. There are several ways this can be done. Explain clearly in your solutions how you do this. 2 points bonus.

Question by Student 201227147 In class, you defined $\epsilon _f^{rel}$ as following: $\epsilon _f^{rel}$ $\equiv$ $\vert\frac{(\delta_{x}\phi)_{c}-(\delta_{x}\phi)_{f}}{(\delta_{x} \phi)_f}\vert$. However, in the table you uploaded, it says that $\epsilon _f^{disc, rel}$ $\equiv$ $\frac{(\delta_{x}\phi)_{f}-(\delta_{x}\phi)_{c}}{(\delta_{x} \phi)_f}$. Which definition is right?

Good observation. It's better to use the definition in the tables. Make modifications to your class notes accordingly. 2 points bonus.

Question by Student 201527110 Professor, I have one question during finding order of accuracy P. In definition, $$P=\frac{1}{ln(R)}*ln(\frac{(\delta_xP)_3-(\delta_xP)_2}{(\delta_xP)_2-(\delta_xP)_1})$$, however what if the value in log function is negative? Is it okay to take absolute value for that sequence?

Well, if the value is within the second log is negative it means you're not within the asymptotic range of convergence.. 2 points bonus.

Question by Student 201527110 Professor, I have one question about the finding of $GCI_c$ what we did at the lecture of today. I understand we can consider $\mid\frac{(\delta_xP)_f}{(\delta_xP)_c}\mid$ as '1' in an asymptotic range, but in case of mesh 4,5 in today's lecture, is it okay to consider $\mid\frac{(\delta_xP)_f}{(\delta_xP)_c}\mid$ as '1' as well? Or should I substitute each values of $\delta_x$ in that form? In other word, if I assume mesh 4 and 5 is also in an asymptotic range, that means I can consider $\mid\frac{(\delta_xP)_f}{(\delta_xP)_c}\mid$ as '1' as well?

05.23.17

The GCI$_c$ parameter is only used to check if the solution is in the asymptotic range. To check if the solution is in the asymptotic range, yes, you need to set the $\delta_x P$s to zero. If you don't do this, then computing GCI$_c$ is useless as it will be a good check of whether we're in the asymptotic range or not. 1 point bonus.

Question by Student 201427102 I wrote at terminal ./warp -r rarefaction.wrp -i out.01 -oi out.01i -q But Terminal said Data file has 20 grid lines along i but the control file specifies 40 grid lines. How can I correct this problem?

Hm, this means that the file out.01 was created with a different mesh size than is now specified in your control file. I recommend not to use the interpolation file flags -oi or -ii if this gives you trouble (this saves on some computing time, but is a bit harder to operate). Simply create a new out.01 with the -o flag without reading in out.01i. That will work just as well but will be a bit slower.

Question by Student 201427564 Professor, I have a question about Question#2. Is it possible that ${P}_{i+1}$ increase while ${P}_{i-1}$ decrease?

I guess you mean an increase/decrease from one mesh level to the other? Yes, it's possible. This means the derivative is becoming larger. 1 point bonus.

Question by Student 201427564 Professor, I have a qeustion about calculating '$p$'. In your table, $r=\frac {{\Delta x}_{c}} {{\Delta x}_{f}} $. By this definition, is it correct that $r=\frac {1} {\sqrt{2}}$ ? Because the fine one is $\sqrt {2}$ times more fine than coarse one.

No, it's the opposite.. This was mentioned in class.

Question by Student 201238707 Professor, When I start "Assignment 7 - Question #2". I just use $(R_\Delta)_\max = 1E-3$ or find convergence value of $(R_\Delta)_\max$	05.24.17

You can keep xiverge set to 1E-3. 1 point bonus.

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