Rotor effective wind speed

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Rotor effective wind speed

Postby Filip.Spasov » Tue Mar 19, 2019 10:07 am

Dear Jason.Jonkman,
Dear All,

I am trying to implement a Lidar system in feed forward CPC. Therefore, I'm trying to extract the wind speed that the turbine actually 'feels'. Lets call it here rotor effective wind speed (rews). The way I currently do that is based on a simple drive train model (J*OmegaDot=Qa-Qg-Qloss) and leads at the end to the equation: Cp(lambda,beta)/lambda^3 = 2(Q_Lss + J*OmegaDot)/ rho*pi*R^5*Omega^2

To avoid issues with calculating the Cp, I perform a look up table on Cp/lambda^3. (I use a pre calculated Cp surface (attached as .zip), not done by me but seemed reasonable, at least at the beginning :) )
Since the needed turbine data is known (I extract Q_Lss =Qg+Qloss from strain gauges on the LSS), i go with the known pitch angle (from output channel) to the look-up, compare it with the right side of the equation and so, I find the corresponding TSR. From the TSR the rotor effective wind speed is derived: v_eff = Omega*R/TSR

As I tested this with uniform wind and wind steps (12 to 25 m/s with increment of 1m/s, see attached stepInputAndStepResponse.png) i found that there is a bias between the uniform wind speed and the rews. Now I am trying to find out what causes the bias. I've checked everything else twice so one of the possible options left is that I actually have a wrong Cp surface (look up table). Attached a graph with the Cp from FAST, Cp calculated with my lookup table and Cp calculated with my look up table but with the TSR input from FAST (RtTSR). So according to this I have a few questions.

Currently, only the gen DOF is enabled. AFAeroMod =1 (steady aerodynamics) Prandtl tip/hub losses are on. No tower shadow or potential flow.

Would you please tell me the correct configuration of settings to calculate relative precise Cp (TSR,beta) surface. Does the aerodynamics model matter? (read in the forum that Cp above .5 can be computed at steady aerodynamics)

I am also using the AeroDyn output channels (RtAeroCp, RtTSR, RtVAvgxh) as refference but could not find how are these calculated.
How exactly is the averagad x-wind speed calculated?

And at the end :) Do You think that the method I am using to extract the rews is correct at all or You would do it in a different way?

Thank You massively for Your support!!
StepInputAnd StepResponse.png
StepInputAnd StepResponse.png (45.83 KiB) Viewed 351 times
powerCoefficient.png (61.64 KiB) Viewed 351 times
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Re: Rotor effective wind speed

Postby Jason.Jonkman » Tue Mar 19, 2019 5:39 pm

Dear Filip,

One problem I see with your approach is that you calculate the aerodynamic torque as:

Qa = Q_Lss + J*OmegaDot

where you say that Q_Lss is the torque in the low-speed shaft. But, presumably, J in this equation is the total rotational inertia of the drivetrain (J_Rotor + J_Gen*GBRatio^2). Because the low-speed shaft torque only effects the rotor inertia, the aerodynamic torque should be computed as follows:

Qa = Q_Lss + J_Rotor*OmegaDot

as described in my post dated Sep 29, 2011 in the following forum topic: viewtopic.php?f=4&t=525&p=1907.

I agree with your settings for computing a Cp curve. The methods to do this have been discussed several times on this forum.

The way in which AeroDyn v15 calculates RtAeroCp and RtVAvgxh are described in the following forum topic: viewtopic.php?f=2&t=1679&p=7882. RtTSR is calculated by taking omega*R_max/RtVAvgxh, where R_max is the instantaneous radial distance (normal to the shaft) to the most outboard blade aerodynamic analysis node.

I hope that helps.

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

National Renewable Energy Laboratory (NREL)
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