Dear Mohamed,
What input perturbation have you applied to the linear model? Does the output from your linear model match that of the nonlinear model when no input perturbations to the linear model are specified, such that all states, state derivatives, and input perturbations are zero? This should be the case because the output perturbations from the linear model are then zero, and when the periodic OP is added the perturbation, the result should match the nonlinear model.
Best regards,
Dear Jason
Thank you for your explanation.
However, I found less deviation between the linearized model and the nonlinear model, although I set all perturbations to zeros. I add the periodic OPs that are retrieved from the linearization txt file (*.lin). The operating conditions of the nonlinear model are the same as the linearization point. Moreover, I made another simulation with accounting the periodicity of A, B, C, and D matrices, but, it gives me the same results.
The Blade RootMFlp1 VS Simulation time ( kN.m VS seconds).
The yellow line for the nonlinear model. The blue line for the linear model.
The connection diagram is shown below
Regards
M.Fekry
Dear Mohamed,
I still have not fully understood your Simulink model, but if the perturbations zero, then the linear and nonlinear model should agree if the OP from the linear model is derived from the conditions of the nonlinear model. Are you adding the periodic OP to perturbation (which are all zeros for now) when comparing the linear and nonlinear models, including inputs, states, and outputs?
Best regards,
Dear Dr. Jason
The connection diagram of the Simulink is shown below:
Continuing from above
However, The RootMFlp1, RootMFlp2, and RootMFlp3 have some deviation as shown below:
RootMFlp1 kN.m VS Time S
Are there any precautions should I take when dealing with C and D matrices?
Regards
M.Fekry
Dear Mohamed,
Are the perturbed outputs from the linear model (dy = Cdx + Ddu) all zero? From your Simulink model (Linear Model Diagram.jpg), it still looks like you are adding Xop to dx before multiplying it by C.
And where is the periodic OP of the blade flapwise outputs added to the perturbed outputs from the linear model, i.e., y = yop(Azimuth) + dy, after transformation of dy from the nonrotating to the rotating frame? Given that dy in your case should be zero (both rotating and nonrotating), the blade flapwise output from the linear model should just be the OP value, which should match the nonlinear model.
Best regards,
Dr. Jason
Thank you very much for your explanation
it works now.
RootMFlp1 kN.m VS Time S
Dear Jonkman,
First, thank your help on topic “Obtaining sensitivity of aerodynamic power to rotor-collective blade pitch”, I have successfully gotten the data. My version is OPENFAST 2.4.
However, I have some questions.
First, what is the difference between AD RtAeroPwr and ED RotPwr. Maybe the GenEff 94.4 in ServoDyn? I compared them in linearization files. They are not the relation about GenEff.
Second, I looked at the sensitivity of them to rotor-collective blade pith. For ED RotPwr, it is -5.904E+03. For AD RtAeroPwr, it is -5.134E+07. Owing to their units, one is KW, the other is W, so I think the sensitivity of them should differ 10^3, but it is nearly 10^4 now. I am a litter puzzled.
Could you please help me?
Best regards!
Dear Dezheng.Zhu,
A similar question was asked and answered in the following forum topic: FAST_v8.15 OutList parameters - #2 by Jason.Jonkman.
If the generator or drivetrain DOFs are enabled in ElastoDyn, I would expect large differences in the derivatives calculated from a linearization analysis between RotPwr from ElastoDyn and RtAeroPwr from AeroDyn because the former is effected by rotor acceleration/deceleration while the later is not. If the generator and drivetrain DOFs are disabled in ElastoDyn, then I would expect quite good agreement between the two.
Best regards,
