## Printing the axial induction factor in FAST

Discuss the theory and modeling of rotor aerodynamics.

Moderator: Bonnie.Jonkman

Mostafa.Haggag
Posts: 34
Joined: Tue Jun 26, 2018 7:50 am
Organization: self
Location: Egypt

### Re: Printing the axial induction factor in FAST

i have a very small question.
first, I am speechless from your help and I greatly appreciate it.I owe you a favor and i hope to meet you to thank you in person.
secondly, My file has 17 nodes I can output only 9 nodes right? i get to choose any of these 9 nodes right from the 17 node ?
also which nodes should i pick up ? i mean should i choose the first 9 nodes after each other or should I pick one and leave one I mean what combination of nodes should I pick ? or do you think it does not matter? I was thinking that picking a combination of nodes to cover the whole blade to be outputted is the best idea. i mean for example instead of outputting nodes :1 2 3 4 5 6 7 8 9 to be 1 3 5 7 9 11 13 15 17 .I meant to make sure that I have nodes covering the whole blade or what do you think?i am not sure I study mechatronics engineering and i do not have much experience with aerodynamics but i am using my common sense hoping i can get some output .
Also if i turn on the blade structural degrees of freedom, how is the radius changing? I mean are the nodes moving linearly? I mean for example node 0 is at position 0 at time step 0 and node 1 is at position 1.In the next time step, does node 0 get positioned at position 1 and node 1 get positioned at position 2 or how does it work ? do you have any idea how it works?I mean this information will be very very very helpful and i can use it to just post-processing the data instead of editing in the source code.The problem im not an expert in fortran and tried to understand the aerodyn codes but it does not seem to be a simple task.
thank you so so much
mostafa

Jason.Jonkman
Posts: 5737
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear Mostafa,

Correct, outputting nodes 1 3 5 7 9 11 13 15 17 would be a good distribution of the output nodes.

The deflection is not linear, but if you disable the structural degrees of freedom in ElastoDyn, then the blades will be rigid and the aerodynamic analysis node radial positions you set in the AeroDyn input file will be fixed over time.

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

Mostafa.Haggag
Posts: 34
Joined: Tue Jun 26, 2018 7:50 am
Organization: self
Location: Egypt

### Re: Printing the axial induction factor in FAST

a very small question ! i have opened ElastoDyn file and i am looking for the manual of it.My problem is in the structural degrees of freedom i do not find a certain paramter called the structural degrees of freedom .i can only find these parameters to switch on and off but there is not parameter called structural degree of freedom
i am sure that you are talking about one of these parameters but which one exactly:
True FlapDOF1 - First flapwise blade mode DOF (flag)
True FlapDOF2 - Second flapwise blade mode DOF (flag)
True EdgeDOF - First edgewise blade mode DOF (flag)

Jason.Jonkman
Posts: 5737
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear Mostafa,

Yes, I mean the disable all of the blade structural DOFs i.e. FlapDOF1 = FlapDOF2 = EdgeDOF = False.

Of course, if you wish to model a rigid turbine, you may as well use the standalone driver for AeroDyn v15 rather than coupled to FAST. Running the standalone driver is much simpler than running AeroDyn coupled to FAST. Documentation on the standalone driver for AeroDyn v15 is provided along with the AeroDyn documentation.

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

Mostafa.Haggag
Posts: 34
Joined: Tue Jun 26, 2018 7:50 am
Organization: self
Location: Egypt

### Re: Printing the axial induction factor in FAST

Is closing these structural degrees of freedom affect in my loads and power calculations? I mean i will get less realistic results?
Also as you can see in the picture this is a representation of blade with nodes. When we said that the nodes are moving, are they moving in which direction? In the aerodyn file you enter the initial postion of the radial distance to each node starting from 0 to 63.as my rotor radius is 63.when you said that the nodes move in the radial direction you mean the direction in the picture or the direction along the rotor radius (meaning that the intial radial distances of the nodes are changing? )
If my second assumption is correct (concerning that my intial radial distance are changing like the node at postion 62 m will move a bit down to62.1), is it correct to say that the nodes are moving up and down is that right?due to contraction and extraction of the blade? Is it true
If its true, according to what parameter does it extend or contract? What parameters says when the node to move up or down?what parameter say that the blade is extending or not?
Also is their a maximum to the amount of extention of the blade?
Is it possible to assume that my nodes are moving all linearly with a constant value. I mean lets say that all my nodes move down by 0.01 and at another instant all nodes goes up by 0.01?
There is a reason why i donot want to close the structural dof. I have checked on the loads with them on and off and there seemed to be a difference between structural dof on and off.
What i hope to do is to assum that the nodes are moving linearly in the radial direction with a consant value it contract or extend according to a certain parameter i would love to know. With this information i can post process my data assuming its moving linearly.
Attachments
20180706_102821.jpg (1.62 MiB) Viewed 7004 times

Jason.Jonkman
Posts: 5737
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear Mostafa,

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).

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

Mostafa.Haggag
Posts: 34
Joined: Tue Jun 26, 2018 7:50 am
Organization: self
Location: Egypt

### Re: Printing the axial induction factor in FAST

Jason.Jonkman wrote:Dear Mostafa,

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).

Best regards,

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

Jason.Jonkman
Posts: 5737
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear 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.

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

Mostafa.Haggag
Posts: 34
Joined: Tue Jun 26, 2018 7:50 am
Organization: self
Location: Egypt

### Re: Printing the axial induction factor in FAST

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

Jason.Jonkman
Posts: 5737
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear 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: https://wind.nrel.gov/nwtc/docs/InflowWind_Manual.pdf.)
The aerodynamic analysis nodes are defined within AeroDyn and move with rotor rotation and structural deflection.

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

Mostafa.Haggag
Posts: 34
Joined: Tue Jun 26, 2018 7:50 am
Organization: self
Location: Egypt

### Re: Printing the axial induction factor in FAST

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

Jason.Jonkman
Posts: 5737
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

Dear 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.

I hope that helps.

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

Mostafa.Haggag
Posts: 34
Joined: Tue Jun 26, 2018 7:50 am
Organization: self
Location: Egypt

### Re: Printing the axial induction factor in FAST

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.

Jason.Jonkman
Posts: 5737
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Printing the axial induction factor in FAST

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,
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

Bowen.Jiang
Posts: 7
Joined: Sat Aug 17, 2019 4:41 am
Organization: Chongqing University
Location: China

### Re: Printing the axial induction factor in FAST

Dear Jason,

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.
1. 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?
2. 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?
fig.1-fig.3.png (46.49 KiB) Viewed 2518 times
table.1 and table.2.png (28.59 KiB) Viewed 2518 times
table.3.png (46.76 KiB) Viewed 2518 times

Sincerely

Bowen