How to extract Lift and Drag coefficients from Ansys Fluent

Abdul_Rehman · June 12, 2018, 09:28

I'm doing a project on the analysis of this underwater glider. I have to extract lift and drag values at different wing spans. I set the boundary conditions and run the calculations but when results are displayed, I can't understand which value is the lift or drag coefficient. I'm new to CFD so, I lack basic information.
I want to know how can I extract lift and drag coefficients from it and also how can I set angle of attack?

P.S I'm using k-epsilon model, with an inlet water velocity of 0.3 m/s. I have attached some pictures.

RaiderDoctor · June 12, 2018, 11:49

Hey, welcome to the world of CFD. Luckily, you're problem seems like an easy one. Solving for the lift and drag coefficients need to be set prior to calculation. I'm not quite sure of a way to do it afterward...

First, check out this video as it basically answers your question.

https://www.youtube.com/watch?v=WrqwX1GQfeg
It wouldn't be a bad idea to watch parts 1-6, just to get a better understanding of the program.

So, what you need to do is set up a solution monitor for both coefficients that will calculate the values in real-time. This is a very important part of every simulation, as it can let help you in judging convergence (more on that in a second).

In case you are running a new version of Ansys than what was described in the video, the steps involved in setting up a solution monitor are as follows: Double-click Report Definitions. A new menu should pop up, click New, and highlight Force Report. In the drop-down menu that appears, there should be one for both Lift and Drag. Click Drag first (the method for setting up Lift is the exact same). This will bring up a new menu, called Drag Report Definition. Name it "cd-1" or whatever you like, and select the object that you'd like to calculate drag over underneath the Wall Zones section. Underneath the Create section, select Report File, Report Plot, and Print to Console. These are important, as a file will enable you to plot the values for later graphs, a plot will show you the calculated value in real time, and you'll be able to see the exact value in the console. Click OK to close the menu.

In the Report Definitions menu, click Report File Definitions. This will bring up a new menu. Here, we want to specify the frequency that Fluent will record the calculated values. Select the rfile of the report definition you just created (i.e. if you named the definition cd-1, select cd-1-rfile). Click Edit. This will bring up a new menu. Down towards the bottom, specify the frequency you'd like the data to be recorded. I personally like to see the calculated value every iteration. This helps me judge convergence better. If you are doing a steady-state simulation, this will be the default answer. Click OK to close the menu, and Close to close the Report File Definitions menu. Now, click Report Plot Definitions, which will bring up a new menu. Select rplot of the definition you just created (i.e. if you named the definition cd-1, select cd-1-rplot). Click Edit, which will bring up a new menu. Again, I like to get data every iteration, so that I can judge convergence. Change plot title if you like, although I never do. Click OK to exit this menu, and Close to exit the Report Plot Definitions menu.

Repeat the process for the other coefficient. And that's about it. As you calculate, you will see real time solutions of both coefficients in Fluent. As the solution progresses, both quantities should eventually converge to a final answer. This is important, because it helps you monitor how your solution is actually doing. For instance, if your simulation converges via continuity, but has not reached a final value for the lift coefficient, your solution is not really converged. So keep calculating.

Happy simulating, and good luck!

pramodhkumaresh · January 1, 2021, 18:42

Hi, this explanation is quite helpful. But I would also like to know how I can extract data from this plot.

For instance, if I perform a transient simulation is there a possibility to extract data points of lift and drag with respect to the time/ number of iterations as a txt or excel file.

AlexanderZ · January 4, 2021, 02:20

you can write them into file, this setting is available when you are making these monitors

pramodhkumaresh · January 4, 2021, 04:14

Thank you for your reply.

Yes, I have created a report file. Could you please tell me how I can read this file for the required data? becos I think a .out file is created

AlexanderZ · January 4, 2021, 04:31

just read it with any text editor, excel is also possible
you can put any other format instead of .out

eliyaz123321 · February 23, 2021, 00:26

Hi, I am also doing a project for my final year university using ANSYS Fluent/ CFD to analyse aerodynamically Drag and Lift on a vehicle.
i have already done my mesh and solution fully converged.
i am trying to find out if there is anyway to plot the drag around the different parts of the vehicle.
by this i mean the different values of drag coefficient from the front to the rear of the car.
i have shown an image as an example of what i mean.

thanks in advanced

eliyaz123321 · February 23, 2021, 00:29

forgot to post example of image.
its basically the development of Cd over the vehicles length.
how would i go about doing this on my own car?

eliyaz123321 · February 23, 2021, 00:30

https://postimg.cc/r0jNj2zJ

RaiderDoctor · March 22, 2021, 09:37

Hm, what about if you take an isoclip of some width over the length of the car, and then extract the average measurements from it?

June 12, 2018, 11:49		#2
RaiderDoctor Senior Member Join Date: Dec 2016 Posts: 152 Rep Power: 10	Hey, welcome to the world of CFD. Luckily, you're problem seems like an easy one. Solving for the lift and drag coefficients need to be set prior to calculation. I'm not quite sure of a way to do it afterward... First, check out this video as it basically answers your question. https://www.youtube.com/watch?v=WrqwX1GQfeg It wouldn't be a bad idea to watch parts 1-6, just to get a better understanding of the program. So, what you need to do is set up a solution monitor for both coefficients that will calculate the values in real-time. This is a very important part of every simulation, as it can let help you in judging convergence (more on that in a second). In case you are running a new version of Ansys than what was described in the video, the steps involved in setting up a solution monitor are as follows: Double-click Report Definitions. A new menu should pop up, click New, and highlight Force Report. In the drop-down menu that appears, there should be one for both Lift and Drag. Click Drag first (the method for setting up Lift is the exact same). This will bring up a new menu, called Drag Report Definition. Name it "cd-1" or whatever you like, and select the object that you'd like to calculate drag over underneath the Wall Zones section. Underneath the Create section, select Report File, Report Plot, and Print to Console. These are important, as a file will enable you to plot the values for later graphs, a plot will show you the calculated value in real time, and you'll be able to see the exact value in the console. Click OK to close the menu. In the Report Definitions menu, click Report File Definitions. This will bring up a new menu. Here, we want to specify the frequency that Fluent will record the calculated values. Select the rfile of the report definition you just created (i.e. if you named the definition cd-1, select cd-1-rfile). Click Edit. This will bring up a new menu. Down towards the bottom, specify the frequency you'd like the data to be recorded. I personally like to see the calculated value every iteration. This helps me judge convergence better. If you are doing a steady-state simulation, this will be the default answer. Click OK to close the menu, and Close to close the Report File Definitions menu. Now, click Report Plot Definitions, which will bring up a new menu. Select rplot of the definition you just created (i.e. if you named the definition cd-1, select cd-1-rplot). Click Edit, which will bring up a new menu. Again, I like to get data every iteration, so that I can judge convergence. Change plot title if you like, although I never do. Click OK to exit this menu, and Close to exit the Report Plot Definitions menu. Repeat the process for the other coefficient. And that's about it. As you calculate, you will see real time solutions of both coefficients in Fluent. As the solution progresses, both quantities should eventually converge to a final answer. This is important, because it helps you monitor how your solution is actually doing. For instance, if your simulation converges via continuity, but has not reached a final value for the lift coefficient, your solution is not really converged. So keep calculating. Happy simulating, and good luck! Abdul_Rehman and pramodhkumaresh like this.

January 1, 2021, 18:42	extracting data	#3
pramodhkumaresh New Member Pramodh K Join Date: Jul 2020 Posts: 2 Rep Power: 0	Hi, this explanation is quite helpful. But I would also like to know how I can extract data from this plot. For instance, if I perform a transient simulation is there a possibility to extract data points of lift and drag with respect to the time/ number of iterations as a txt or excel file.

January 4, 2021, 02:20		#4
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	you can write them into file, this setting is available when you are making these monitors pramodhkumaresh likes this. __________________ best regards ****************************** press LIKE if this message was helpful

January 4, 2021, 04:31		#6
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	just read it with any text editor, excel is also possible you can put any other format instead of .out pramodhkumaresh likes this. __________________ best regards ****************************** press LIKE if this message was helpful

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January 4, 2021, 04:14		#5
pramodhkumaresh New Member Pramodh K Join Date: Jul 2020 Posts: 2 Rep Power: 0	Thank you for your reply. Yes, I have created a report file. Could you please tell me how I can read this file for the required data? becos I think a .out file is created

February 23, 2021, 00:26		#7
eliyaz123321 New Member Eliyaz Saraj Join Date: Feb 2021 Posts: 3 Rep Power: 5	Hi, I am also doing a project for my final year university using ANSYS Fluent/ CFD to analyse aerodynamically Drag and Lift on a vehicle. i have already done my mesh and solution fully converged. i am trying to find out if there is anyway to plot the drag around the different parts of the vehicle. by this i mean the different values of drag coefficient from the front to the rear of the car. i have shown an image as an example of what i mean. thanks in advanced

February 23, 2021, 00:29		#8
eliyaz123321 New Member Eliyaz Saraj Join Date: Feb 2021 Posts: 3 Rep Power: 5	forgot to post example of image. its basically the development of Cd over the vehicles length. how would i go about doing this on my own car?

February 23, 2021, 00:30		#9
eliyaz123321 New Member Eliyaz Saraj Join Date: Feb 2021 Posts: 3 Rep Power: 5	https://postimg.cc/r0jNj2zJ

March 22, 2021, 09:37		#10
RaiderDoctor Senior Member Join Date: Dec 2016 Posts: 152 Rep Power: 10	Hm, what about if you take an isoclip of some width over the length of the car, and then extract the average measurements from it?