# Dynamic Inflow

Hello everyone,
I am performing an analysis of a variable speed-pitch controlled wind turbine.
In the pictures attacched you can see the result of an analysis (in terms of RootMyb) carried out using an ETM wind with an average wind speed of 8.5 m/s.
The analysis has been perfomed with ( blue line) and without dynamic inflow ( red line) activated.
As you can see, when the dynamic inflow is activated, the result shows strong oscillations.
I imposed rigid blade and rigid tower motion and, when the dynamic inflow is activated, I see almost the same oscillations .
I also checked the tip sections angles of attack and, when the dynamic inflow is on, they oscillate between -100 and 100 deg.
Do you think that this oscillations could be ascribed to a dynamic inflow model instability and not to a “real” behaviour?
Best Regards
Ferdinando Scherillo

Dear Ferdinando,

Your result from the dynamic inflow model certainly seems to be numerically unstable. It is well known that the generalized dynamic wake (GDW or DYNIN) model currently implemented in AeroDyn is numerically unstable for highly loaded rotors (i.e., for high axial induction factors, say, higher than a = 1/3). An 8-m/s limit is hard coded into the code based on the analysis of several wind turbine models, even though the exact limit is likely turbine dependent. Does dynamic inflow give reasonable results for your model at higher wind speeds?

Best regards,

Dear Jason,
About your question on the other wind speed velocities: Yes I have an instability just for this wind speed (8.5) that is the rated wind speed.
Best regards
Ferdinando

Dear Ferdinando,

Your results confirm my suspicion–that the instability is related to the known limitations with the dynamic inflow model. I suggest that you use the equilibrium (EQUIL, BEM) model below rated wind speed.

Best regards,

Dear Jason,

I am sending this message as we’re facing issues with the dynamic flow options.
We would like to do some tests in order to see the influence of yaw variations on parameters like angles of attack, lift coefficients or power coefficients. We are using a FAST v8.15 with Aerodyn v14.
Firstly, we tested a simple case with the following characteristics:
-constant steady wind of 10 m/s ;

• inviscid conditions;
-without yaw (yaw angle =0);
• compute controller is not activated, ElastoDyn degrees of freedom are all false.

Our blade is a simple one: only NACA0018 airfoils: 25 sections with a cosinus distribution (32 meters).

In this steady conditions, we were expecting to find the same final value using EQUIL or DYNIN inflow models.
Our issue is that we cannot obtain the same results. I am copying the following results of lift coefficient versus time at mid span (Attachment 1):

We also tried to change the parameter ATOLER which had a significant effect on every EQUIL case, but it did not change anything concerning the DYNIN cases (always the same curve occurs).

If we look at this lift coefficient versus span at the last time step, it seems that the EQUIL case is closer to reality than the DYNIN one:

Are we missing anything? Do you think we have to use a DYNIN mode to study the yaw influence? Later, we will also study shear and turbulent wind influence. Could the choice of this inflow model influence the results?

Thank you in advance, I am looking forward to hearing from you.

Panagiotis Panousis

Dear Panagiotis,

By “inviscid”, do you mean that you set the drag to zero or something else?

I wouldn’t expect AeroDyn v14 input parameter AToler to effect the results of DYNIN, as this input is only used in the EQUIL solution. We normally recommend setting AToler to 0.005, which seems consistent with your results as well.

As discussed briefly in the following forum topic, under steady conditions, both EQUIL and DYNIN would ideally predict the same response, but this is not true in practice: http://forums.nrel.gov/t/question-concerning-turbsim-v1-5/227/6. However, your results are showing much bigger differences between EQUIL and DYNIN than I would normally expect. I expect that you doing something wrong in the simulation or in the interpretation of the results, but I don’t know what that might be. The DYNIN model is known to be numerically unstable for heavily loaded rotors (i.e. low wind speeds). Your simulation is close to this at 10 m/s; do you experience the same differences between EQUIL and DYNIN for higher operational wind speeds?

Our typical recommendation is to use EQUIL for simulations involving an operational wind turbine below rated wind speed and DYNIN for simulations involving an operational wind turbine above rated speed.

Best regards,

Dear Jason
Thank you very much for your quick response.

By “inviscid”, I meant inviscid polars as input (generated thanks to xfoil software) and so drag coefficient is set to zero.

Following your advice, I re-started simulation with a higher wind speed of 15 m/s and as expected the gap between EQUIL and DYNIN option is reduced in this case:

And with 12m/s wind speed:

At 8 m/s, as you said, the DYNIN mode doesn’t seem reliable:

In order to work with the generalized dynamic wake model, I will change the wind velocity.