How to understand the calculation result?

zhang-qiang · February 11, 2023, 07:23

I am learning the tutorial: TestCases/incomp_navierstokes/bend, and the printed information is:

Simulation Run using the Single-zone Driver
+----------------------------------------------------------------+
| Inner_Iter| rms[P]| rms[U]| CL| CD|
+----------------------------------------------------------------+
| 0| -2.382554| -16.581105| 0.000000| 0.000000|
| 1| -2.522488| -1.896718| 0.000000| 0.000000|
| 2| -2.655079| -2.244733| 0.000000| 0.000000|
| 3| -2.589785| -2.523275| 0.000000| 0.000000|
| 4| -2.891179| -2.722327| 0.000000| 0.000000|
| 5| -3.306787| -2.959667| 0.000000| 0.000000|
| 6| -3.414430| -3.092187| 0.000000| 0.000000|
| 7| -3.491685| -3.079000| 0.000000| 0.000000|
| 8| -3.506499| -3.109794| 0.000000| 0.000000|
| 9| -3.502077| -3.107762| 0.000000| 0.000000|
| 10| -3.501823| -3.121293| 0.000000| 0.000000|

How to understand the parameters rms[P], rms[U], CL, CD? Does the [P] means pressure? What's the meaning of [U], CL, CD?

Any suggestion is appreciated~~~

bigfootedrockmidget · February 11, 2023, 08:53

The RMS values are the root mean square values of the residuals of the linear system, so they are a measure for how good the current solution approximates the solution to the discrete system of equations. These values should go to 0. Since we take log10(RMS), a value of -10 means that the root mean square of the residual is 10^-10. So the values shown on the screen should go to large negative values. Usually, the best you can get with computers is -16.

Here is a better explanation from the CFD101 channel:

https://www.youtube.com/watch?v=v9OnNeYH4Ok

The values for the lift and drag coefficient Cl and Cd are 0 because no surface is defined for them.

Are you at university? Do you have the opportunity to follow a course on Computational Fluid Dynamics?

zhang-qiang · February 11, 2023, 09:29

Thanks for kindly reply. I am not at university. I am learning SU2 by myself. Could you please recommand a online course?

bigfootedrockmidget · February 11, 2023, 10:10

Best is the cfd101 series on youtube. But if you really want to understand what is going on in CFD, you will need to study many other topics like linear algebra, calculus, differential equations, and numerical analysis. And then there's the specific physics, like fluid dynamics, heat transfer, etc.. What is your current knowledge level? I briefly explained the RMS values, but it is hard to understand if you did not study linear algebra yet.

zhang-qiang · February 11, 2023, 23:33

I am PhD on Biomedical Imaging Engineering, I can understand the meaning of RMS. I am learning CFD by myself, and I have learnt the basic concept of CFD. I think I need to have a deep learning about CFD, and learn more details about SU2. I have found the cfd101 series: https://www.youtube.com/watch?v=yMj-...7lMY1tMvZoYArH. And I will learn it. Thanks for your kindly reply.

February 11, 2023, 07:23	How to understand the calculation result?	#1
zhang-qiang Member zhangqiang Join Date: Nov 2022 Posts: 43 Rep Power: 3	I am learning the tutorial: TestCases/incomp_navierstokes/bend, and the printed information is: Simulation Run using the Single-zone Driver +----------------------------------------------------------------+ \| Inner_Iter\| rms[P]\| rms[U]\| CL\| CD\| +----------------------------------------------------------------+ \| 0\| -2.382554\| -16.581105\| 0.000000\| 0.000000\| \| 1\| -2.522488\| -1.896718\| 0.000000\| 0.000000\| \| 2\| -2.655079\| -2.244733\| 0.000000\| 0.000000\| \| 3\| -2.589785\| -2.523275\| 0.000000\| 0.000000\| \| 4\| -2.891179\| -2.722327\| 0.000000\| 0.000000\| \| 5\| -3.306787\| -2.959667\| 0.000000\| 0.000000\| \| 6\| -3.414430\| -3.092187\| 0.000000\| 0.000000\| \| 7\| -3.491685\| -3.079000\| 0.000000\| 0.000000\| \| 8\| -3.506499\| -3.109794\| 0.000000\| 0.000000\| \| 9\| -3.502077\| -3.107762\| 0.000000\| 0.000000\| \| 10\| -3.501823\| -3.121293\| 0.000000\| 0.000000\| How to understand the parameters rms[P], rms[U], CL, CD? Does the [P] means pressure? What's the meaning of [U], CL, CD? Any suggestion is appreciated~~~

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February 11, 2023, 08:53		#2
bigfootedrockmidget Senior Member bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 505 Rep Power: 17	The RMS values are the root mean square values of the residuals of the linear system, so they are a measure for how good the current solution approximates the solution to the discrete system of equations. These values should go to 0. Since we take log10(RMS), a value of -10 means that the root mean square of the residual is 10^-10. So the values shown on the screen should go to large negative values. Usually, the best you can get with computers is -16. Here is a better explanation from the CFD101 channel: https://www.youtube.com/watch?v=v9OnNeYH4Ok The values for the lift and drag coefficient Cl and Cd are 0 because no surface is defined for them. Are you at university? Do you have the opportunity to follow a course on Computational Fluid Dynamics?

February 11, 2023, 09:29		#3
zhang-qiang Member zhangqiang Join Date: Nov 2022 Posts: 43 Rep Power: 3	Thanks for kindly reply. I am not at university. I am learning SU2 by myself. Could you please recommand a online course?

February 11, 2023, 10:10		#4
bigfootedrockmidget Senior Member bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 505 Rep Power: 17	Best is the cfd101 series on youtube. But if you really want to understand what is going on in CFD, you will need to study many other topics like linear algebra, calculus, differential equations, and numerical analysis. And then there's the specific physics, like fluid dynamics, heat transfer, etc.. What is your current knowledge level? I briefly explained the RMS values, but it is hard to understand if you did not study linear algebra yet.

February 11, 2023, 23:33		#5
zhang-qiang Member zhangqiang Join Date: Nov 2022 Posts: 43 Rep Power: 3	I am PhD on Biomedical Imaging Engineering, I can understand the meaning of RMS. I am learning CFD by myself, and I have learnt the basic concept of CFD. I think I need to have a deep learning about CFD, and learn more details about SU2. I have found the cfd101 series: https://www.youtube.com/watch?v=yMj-...7lMY1tMvZoYArH. And I will learn it. Thanks for your kindly reply.