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Restart file solution gives weird outputs

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Old   March 17, 2021, 05:12
Default Restart file solution gives weird outputs
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Ravi
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Hello all,

I am currently trying to obtain the plots of C_{l} and C_{d} vs. convective time units (non-dimensional) for an airfoil in deep stall (having an angle of attack of 60) using the DDES approach, using the SA turbulence model and a 2nd order numerical scheme (dual-time stepping scheme).

I have an issue with plotting the results and comparing them with the ones obtained from experimental data from Swalwell as attached with the comment below. For some reason, when I restart my solution from somewhere in between, there is a mismatch in the residuals that I obtain. This does not seem to make sense from the numerical perspective, as the solution should simply start from where the solver left off.

Furthermore, the fluctuations in C_{l} and C_{d} seem to have diminished for the given time traces, when there is actually supposed to a significant deviation from a certain mean value for both coefficients. Is this on account of high numerical dissipation in the current scheme?

Hoping to hear from you soon.

Ravi
Attached Images
File Type: png Cl_Cd_orig_delt.png (30.1 KB, 8 views)
File Type: png Cl_Cd_Swalwell_experim.PNG (81.5 KB, 7 views)
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Old   March 24, 2021, 04:04
Default Follow-up to weird graphs
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Quote:
Originally Posted by raviramesh10 View Post
Hello all,

I am currently trying to obtain the plots of C_{l} and C_{d} vs. convective time units (non-dimensional) for an airfoil in deep stall (having an angle of attack of 60) using the DDES approach, using the SA turbulence model and a 2nd order numerical scheme (dual-time stepping scheme).

I have an issue with plotting the results and comparing them with the ones obtained from experimental data from Swalwell as attached with the comment below. For some reason, when I restart my solution from somewhere in between, there is a mismatch in the residuals that I obtain. This does not seem to make sense from the numerical perspective, as the solution should simply start from where the solver left off.

Furthermore, the fluctuations in C_{l} and C_{d} seem to have diminished for the given time traces, when there is actually supposed to a significant deviation from a certain mean value for both coefficients. Is this on account of high numerical dissipation in the current scheme?

Hoping to hear from you soon.

Ravi

Hello all, so as a follow-up to the previous post that I had put up last week, I had found out that this error was due to the fact that I had been plotting the time averaged values of C_{l} and C_{d} all this while for this case.

I had also run a URANS k-\omega model simulation to compare my results with them. It seems like the DDES model does not have an option of plotting the C_{l} and C_{d} values for each time step in the history file. I tried looking under the options of AERO_COEFF, but the history file only seems to be writing the time averaged values for DDES simulations and not for URANS ones, and hence, the fluctuations in C_{l} and C_{d} are captured in the latter but not the former. Can someone please help me out with this?

I have attached relevant snapshots of the history as well as the configuration files for the two cases with the post for reference.

Hoping to hear from the others soon.

Ravi
Attached Images
File Type: png config_file_outputs.PNG (7.2 KB, 6 views)
File Type: png tavg_cl_cd.PNG (1.3 KB, 6 views)
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Old   March 25, 2021, 07:08
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From your first plot it looks like you are plotting all the inner iterations.
But without more information it is impossible to help.
Attach your config, the screen output..., the history file..., otherwise the only people in the world that might understand the problem you are describing are the ones who already had it.
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Old   March 25, 2021, 07:45
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Quote:
Originally Posted by pcg View Post
From your first plot it looks like you are plotting all the inner iterations.
But without more information it is impossible to help.
Attach your config, the screen output..., the history file..., otherwise the only people in the world that might understand the problem you are describing are the ones who already had it.
Thank you for your reply, Pedro. Indeed, I did not put up the original files, and I will share it again. Hope it helps better with understanding the problem at hand.

With warm regards,

Ravi
Attached Files
File Type: txt naca0021_ddes.txt (13.7 KB, 3 views)
File Type: txt history_155401_test.txt (7.0 KB, 2 views)
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Old   July 1, 2021, 09:00
Default Solution to problem
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Originally Posted by raviramesh10 View Post
Thank you for your reply, Pedro. Indeed, I did not put up the original files, and I will share it again. Hope it helps better with understanding the problem at hand.

With warm regards,

Ravi
Dear all,

As a late update to this, this issue was caused because of the fact that the time-averaged windowing part was turned on, because of which the instantaneous solution remained more or less constant with the number of time iterations.
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