LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAST

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Arash.HemmatiTopkanloo
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Arash.HemmatiTopkanloo » Wed Jun 15, 2016 3:08 pm

Dear Jason,

Thank you very much for your reply. So you mean that aerodynamic damping is quadratic and it is not correct to use free decay formula for that?

I am trying to extract damping of system to incorporate in my structural model (I guess damping I get comes from aerodynamic+hydrodynamic+structural).

Lets assume I just wish to find damping for the condition without waves. Should I impose initial displacement in steady wind? Or should I impose a pulse wind (for example having a steady wind with 8 m/s till T=50 sec (dying out transient period), then at this time a wind pulse of 14 m/s for 10 sec. Does this give a more realistic aeroelastic damping rather than imposing an initial displacement at the beginning of simulation?

Thank you very much for your help.

Best Regards,

Arash,

Jason.Jonkman
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Jason.Jonkman » Thu Jun 16, 2016 7:28 am

Dear Arash,

You can use a free-decay simulation to quantify linear and/or quadratic damping. I was simply saying that you can't expect to calculate constant linear damping for different cycles of the free-decay response if the damping is dominated by quadratic effects.

It shouldn't matter how you initialize your free-decay simulation (whether by initial conditions or forced motion), as long as you neglect the first couple cycles of the free-decay response before calculating the damping.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Arash.HemmatiTopkanloo
Posts: 47
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Arash.HemmatiTopkanloo » Thu Jun 16, 2016 1:37 pm

Dear Jason,

Thank you for your reply.

Best Regards,

Arash,

Arash.HemmatiTopkanloo
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Arash.HemmatiTopkanloo » Fri Jun 17, 2016 3:26 pm

Dear Jason,

Thank you for you reply.

I calculated the aerodynamic damping based on free decay simulation for different wind speeds and I came up with this graph (https://app.box.com/s/4kq7or4ubbt8ot2y4jg8p1fzfh97m7r8). As you can see in low wind speeds aerodynamic is low (around 1%) and it reaches to the maximum in the rated wind speed and again it falls as wind speed increases. Do you think the trend and values are correct?

Thank you again.

Best Regards,

Arash,

Jason.Jonkman
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Jason.Jonkman » Fri Jun 17, 2016 10:18 pm

Dear Arash,

The graph seems plausible to me. Are these results generated with variable-speed control below rated and the pitch-control system active above rated?

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Arash.HemmatiTopkanloo
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Arash.HemmatiTopkanloo » Sat Jun 18, 2016 5:26 am

Jason.Jonkman wrote:Dear Arash,

The graph seems plausible to me. Are these results generated with variable-speed control below rated and the pitch-control system active above rated?

Best regards,


Dear Jason,

Thank you for your reply. I did not change the settings of Pitch control and Generator control. Both of them are user-defined from Bladed-style DLL as below.

PCMode=5
VSContrl=5

And about active pitch control, I defined start time of override pitch maneuver as longer than time simulation

0 TPCOn - Time to enable active pitch control (s) [unused when PCMode=0]
9999.9 TPitManS(1) - Time to start override pitch maneuver for blade 1 and end standard pitch control (s)

It would appreciate if you could tell me I need to change it?

Thank you very much,

Arash,

Jason.Jonkman
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Jason.Jonkman » Mon Jun 20, 2016 6:11 am

Dear Arash,

That all sounds correct.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Arash.HemmatiTopkanloo
Posts: 47
Joined: Mon Apr 20, 2015 3:35 am
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Arash.HemmatiTopkanloo » Tue Jun 28, 2016 2:51 pm

Dear Jason,

Thank you for your constant support.

I am trying to find aerodynamic damping ratio from free decay simulation. I used to calculate aerodynamic damping from the initial period ( Usually after removing two first cycles ).

Now I tried to do another approach. What I am doing now is to apply uniform wind till time=40s and then for 4 sec I increase wind speed by 50%, then after time =44 sec again wind speed comes back to its constant value and then I calculate damping ratio because of this pulse wind speed.

The thing that is interesting is that for wind speed less than 12m/s, the results are similar to the ones I get from initial period (after removing first two cycles), but for wind speeds more than 12 m/s, in some wind speed, literally there is negligible damping and for some wind speed there is high damping.

To illustrate, I uploaded the time history plot for 16m/s, 18m/s and 20 m/s. You can see for 18 m/s, vibration dies out quickly, but for 20m/s and 16m/s, it almost does not dampen at all. Also, I uploaded the aerodynamic damping obtained from two approaches (Figure A).
Please find the figures here (https://app.box.com/s/rjdn2r9jc24damv2yr3wuqq2e6317zgh)

What do you think is the reason that the results differ this much? Please note that all situations are the same, I just changed wind speed (uniform wind speed).

Thank you very much for your help.

Best Regards,

Arash,

Jason.Jonkman
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby Jason.Jonkman » Wed Jul 06, 2016 12:58 pm

Dear Arash,

I agree that the results look odd, but I'm not sure what the problem might be. One guess is that the controller is not behaving as you'd expect--have you looked at the rotor speed, pitch angle, and generator torque to see if the responses are reasonable during the transient? It is hard for me to identify the problem without knowing anything about your model. Perhaps you should run the same study with the rotor speed spinning at a fixed rate to see if the results are as you expect.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

KumaraRaja.Eedara
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAS

Postby KumaraRaja.Eedara » Sat May 02, 2020 5:09 pm

Jason.Jonkman wrote:Dear Arash,

Aerodynamic damping is not directly specified in the FAST input files,.... but is included intrinsically as part of the aero-elastic solution. That is, the aerodynamic force calculated within FAST is dependent on the structural velocity. In your mathematical model, the aerodynamic force is taken from FAST and is not directly dependent on the structural velocity of your mathematical model. Thus, a difference in phasing of the structural velocity between the models will lead to a different level of aerodynamic damping between them.

Best regards,


Dear Jason,

By the aerodynamic damping, are you referring to the effects because of "structural velocity of blade" and "structural velocity of tower" which are considered in the calculation of "resultant velocity" seen by blade which in turn is used for aerodynamic force calculations? This may have influence on blade oscillations but not so much on tower oscillations. Right?
or , the tower aerodynamic loads (i.e since tower is oscillating in wind, it experiences drag force as a distributed load) ? Also, when we specify damping ratios (zeta) for fore-aft or side-side modes in FAST, how are the damping Coefficients (c) calculated inside FAST? using the formula c= 2*zeta*mass*omega_n?
Thanks.

Regards,
Kumara

Jason.Jonkman
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Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAST

Postby Jason.Jonkman » Mon May 04, 2020 9:21 am

Dear Kumara,

In my response to Arash, I was referring to the aerodynamic damping of the rotor, which results from a combination of blade, tower, and wind velocities. For most wind turbines operating normally, the aerodynamic damping from the rotor dominates over the aerodynamic damping from the tower, at least in the fore-aft direction (along the wind direction). But the aerodynamic damping of the rotor has a strong impact on the tower response.

Yes, your equation for the tower structural damping (c) is correct, except that FAST uses the equivalent expression in terms of stiffness, rather than mass:

c = 2*zeta*stiffness/omega

Regarding this last point, see my post dated Oct 08, 2013 in the following topic for more information: viewtopic.php?f=3&t=789&p=3781.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

KumaraRaja.Eedara
Posts: 45
Joined: Mon Sep 12, 2016 8:34 pm
Organization: IIT Bombay
Location: Mumbai

Re: LARGE DISCREPANCY BETWEEN MY RESULTS AND THOSE FFROM FAST

Postby KumaraRaja.Eedara » Mon May 04, 2020 4:17 pm

Dear Jason,

The link you mentioned is very helpful. Thanks alot.


Regards,
Kumara


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