The power will be minimally effected by the structural DOFs, but the loads will be strongly effected by the structural DOFs in most cases.
The nodes will move in all directions. They’ll move in the direction of rotation due to the rotor; they’ll move in the flapwise and edgewise direction due to blade bending, and they’ll move in the radial direction due to the radial shortening effect (associated with blade deflection). The radial shortening results from geometric nonlinearity (not a linear effect).
do you think it is a bad idea to turn on the structural dof and when doing the post-processing assume that they are off??
do you think this will affect my results in calculating the average axial induction factor?i am cheecking mainly on the loads so i cannot turn them off but i do not know how to output my radial positions of the nodes in the blade. what i want to do is to use my intial radial postions that are defined and turn on the structural dof and assume that my nodes are not moving in my postprocessing. do you think this assumption is a bad idea?
yours mostafa
I would not recommend enabling the structural DOFs, but assuming the model is rigid for the purposes of post-processing in general. However, I don’t really know what you need the average induction factor for or how accurate you need it to really comment on if this is an OK assumption for your application.
I wanted to ask in fast where do you measure the Uinf to calculate the axial induction factor?
is it at the beginning of the domain or what exactly? I was thinking about measuring the Uinf in an LES simulation so calculate the axial induction factor in a similar way to the method used in Fast but I was wondering where should my U inf be in fast ? should I choose it at the beginning of the domain or where exactly? Not to mention are you using the same Uinf for all axial induction factors ??
thank you so much
mostafa
In the AeroDyn aerodynamics module of FAST, the undisturbed ambient wind velocity (Uinf) is taken as the local ambient wind speed at the aerodynamic analysis node from the InflowWind wind-inflow module. That is, InflowWind effectively acts as a function that returns the ambient wind velocity dependent on (X,Y,Z) and time. (See the InflowWind User’s Guide for more information: wind.nrel.gov/nwtc/docs/InflowWind_Manual.pdf.)
The aerodynamic analysis nodes are defined within AeroDyn and move with rotor rotation and structural deflection.
I do not understand a few things.
How come are you saying that "In the AeroDyn aerodynamics module of FAST, the undisturbed ambient wind velocity (Uinf) is taken as the local ambient wind speed at the aerodynamic analysis node from the InflowWind wind-inflow module. " is not the aerodynamics analysis node are at the rotor ?should not U inf be a huge distance away of the turbine ? Like should not it be at the beginning of my domain?
yours
mostafa
The aerodynamic analysis nodes in AeroDyn are distributed along the blades and tower (but induction is only calculated at the aerodynamic analysis nodes along the blade).
The InflowWind module processes the ambient wind data.
When thinking of the “CFD equivalent”, InflowWind can be considered a function that identifies ambient wind from a precursor simulation (e.g. a CFD simulation without the effect of the wind turbine present). AeroDyn can be considered a model the accounts for the change in flow as a result of placing the wind turbine in the CFD domain.
If i understand correctly, the axial induction factor is calculated for each aerodynamic node is that right?
It is calculated by using this formula
Uinf -U_Rotor/Uinf
What i understand is that U inf is the velocity in far upwind from the turbine (at the beginning of the domain or very far away from the turbine or you could say it is the wind speed far away of the turbine ) and the U rotor is the velocity at the aerodynamic node.
What I was wondering is whether for each axial induction factor, there is a different U inf or not (this mean all the axial induction factors )or the same U inf is used for all of the axial induction factors for different node?
You said that U inf is calculated at the analysis node which I could not understand as what I know is that U inf should be very far away from the first turbine.
could you please explain more
I agree with your equation for the axial induction factor. In AeroDyn, Uinf is different for each aerodynamic analysis node. But Uinf in AeroDyn is not calculated “far upwind from the turbine”. Instead, Uinf in AeroDyn is calculated as the ambient (without the effect of the aerodynamic load) at the aerodynamic analysis node. This definition of Uinf is possible in AeroDyn because it is not a CFD solution. Instead, the ambient wind is determined by the wind data specified within the InflowWind module (which is unchanged by the aerodynamic loads).
I agree that in CFD it is not possible to define Uinf in this way, because the applied aerodynamic load will impact the wind field. Defining Uinf to be the ambient wind “far upwind from the turbine” would be appropriate in CFD.
I got some problems in simulating the parked turbine. I calculated the aerodynamic forces on the blades (RtAeroFxh, RtAeroFyh and RtAeroFzh) in 2 different parked position (Azimuth) as is shown in Fig.1~ Fig.3. In both of the cases, the 3 blades were pitched to 90 with the “NacYaw” set to different values (0 or 90). In order to exclude the effect of aero-elastic and turbulence, the DOFs of blades and tower were all set to false and the steady wind was used. By these settings, I expect to estimate the mean wind load on 3 blades. However, I found something inconsistent with my common sense.
In the cases when the “NacYaw” were set to 0, I found the “RtAeroFxh”, “RtAeroFyh” and “RtAeroFzh” are much larger when “Azimuth” was set to 270 than those when “Azimuth” was set to 0. This can be seen in Table.1. In the beginning , I thought it may be caused by the “Skewed Wake Correction”. But when the “WakeMod” was set to 0. I want to know if the “Skewed Wake Correction” works when the “WakeMod” was set to 0? If not, what is the cause of this problem?
I estimated the mean wind load on blades with Matlab considering the ShftTilt (5 deg) and PreCone (-2.5 deg) . The drag force and lift force on each blade element per unit length is calculated as FD=0.5ρ(U^2)CD; FL=0.5ρ(U^2)CL. The results are in agreement with those calculated with FAST only when the “NacYaw” was set to 0. I also checked the angle of attack, they are in good agreement with those calculated with FAST. Can you provide me with some suggestions in calculating the mean wind load on blades in parked conditions?
In a parked or idling simulation, you should disable induced velocity (WakeMod=0); this will disable the skewed-wake correction. And for cases with high angle of attack, you should also disable unsteady airfoil aerodynamics (AFAeroMod=1). In this case, the angle of attack is determined geometrically by AeroDyn and the results should be simple to confirm by a hand calculation. You say you’ve pitched the blades to 90 degrees, but Figure 2 shows the airfoil flat into the wind, which implies a pitch of 0 degrees. I would expect angles of attack close to zero degrees in a parked/idling condition without yaw error. I would also verify that the aerodynamic loads computed by FAST/AeroDyn are in fact steady by simulating multiple time steps.
Regarding (1), I would guess the difference in results for different azimuth angles depends on the rotor tilt or wind shear.
Regarding (2), if you are hand-calculating the correct angles of attack, the force coefficients can be predicted by table lookup and the forces can be computed with these coefficients and the dynamic pressure. If you can account for compound angles correctly, I would think your hand calculation would match the result from FAST/AeroDyn. AeroDyn can output more than just the local angle of attack at each cross section, which you can use to double check each step of the calculation.
Dear Jason，
Thanks for your reply.
In the question1, I made a mistake expressing my yaw angle. I set the“NacYaw” to 90, BlPitch(1), BlPitch(2) and BlPitch(3) to 90. Just in this case, when the “Azimuth” were set to 0, I looked the angle of attack of blade 1 from FAST. I found that the angle of attack was about -280 deg, but the “Airfoil” file just covers -180 deg to 180 deg. Would you please tell me how did FAST simulate the drag coefficient in this situation？In the beginning, I thought whether FAST change this angle (-280 deg) to the range from -180deg to 180 deg.That’s to say, -280+360=80 deg, then FAST obtain drag coefficient by linear interpolation. But in “Airfoil” file, this coefficient should be much larger than those from FAST. I felt so confused.
Sincerely
I would expect the angle of attack to be converted (MODULO arithmetic) to between -180 and 180 degrees so that -280 = 80 degrees. I asked Bonnie Jonkman and she said that she recalls the angle of attack being output incorrectly in an older version of AeroDyn v15, which is a bug that has now been fixed. Which version of FAST/OpenFAST are you using? Does switching to the newest release of OpenFAST solve your problem?
The version of FAST I used earlier was FAST (v8.16.00) and AeroDyn (v15.03.00）.When I switch to OpenFAST-v2.0.0,the problem has been solved.
Thank you very much.
" Re: Printing the axial induction factor in FAST
Post by Jason.Jonkman » Tue Aug 14, 2018 8:26 pm
Dear Mostafa,
I agree with your equation for the axial induction factor. In AeroDyn, Uinf is different for each aerodynamic analysis node. But Uinf in AeroDyn is not calculated “far upwind from the turbine”. Instead, Uinf in AeroDyn is calculated as the ambient (without the effect of the aerodynamic load) at the aerodynamic analysis node. This definition of Uinf is possible in AeroDyn because it is not a CFD solution. Instead, the ambient wind is determined by the wind data specified within the InflowWind module (which is unchanged by the aerodynamic loads).
I agree that in CFD it is not possible to define Uinf in this way, because the applied aerodynamic load will impact the wind field. Defining Uinf to be the ambient wind “far upwind from the turbine” would be appropriate in CFD.
Best regards, "
What are then the factors that render the undisturbed velocities different from the wind velocity far upstream?
To give you a concrete example in the context of simulation, I have prescribed options in the Inflow Wind module so that a uniform and steady velocity field of 10.59 m/s would be passed on to OpenFAST. Looking at the variables ‘WindNVelX’ (where N is a user selected wind point), I obtain the value 10.59 m/s for every wind point. However, looking at the variables ‘BbNnVUndx’ (the undisturbed wind velocity, where ‘b’ and ‘n’ are the blade and node numbers respectively), the values are all < 10.59 m/s and they seem to be the same for the same node radial position. Below I’m showing a picture of the undisturbed velocities at all 3 blades’ nodes:
You mentioned that the undisturbed wind velocity is taken as the ambient wind velocity at the blade nodes discarding the effect of the aerodynamic loading. My question is then, what physical phenomena/model causes the decrease in velocity between ‘WindNVelX’ and ‘BbNnVUndx’ since the effect of the aerodynamic loading does not come into play? Is induction at work here, even if it is an undisturbed velocity?
The AeroDyn outputs BbNnVUndx are the wind speeds taken directly from InflowWind, without modification. I expect the values differ from your prescribed value of 10.59 m/s due to a change in coordinate system The AeroDyn outputs BbNnVUndx are expressed in the local blade coordinate system described in Figures 4.3 and 4.4 of the online AeroDyn documentation: openfast.readthedocs.io/en/main … index.html.
