Airfoil analysis software for simulating very high angle of attacks (post stall, high separation) ?

[Victarion]

Hello. I designed a 3 bladed HAWT rotor with airfoil S1210 12% from UIUC database. Now, I must validate the results using a blade element momentum method (Chapter 6, sub chapter 6.1 and 6.8 in the book "Gasch, R., & Twele, J. (2012). Wind Power Plants Fundamentals, Design, Construction and Operation"). One of the required parameters is the lift and drag coefficients Cl and Cd.

From my calculations using the BEM method (implemented in Matlab), some of my wind turbine blade segments (it has twist per segment) will operate at high angle of attacks.

The problem: I have tried to use the built in XFOIL Direct Analysis feature in QBlade, XFLR5, and even the XFOIL software itself... but every software failed to obtain results for very high angle of attacks. XFOIL did the best and managed to get results up to AoA of 28.5 deg for the high range of AoAs. Thus, I do not have the Cl and Cd values for angles over 28.5 deg, which I suppose the Cl will be very low due to stall. I believe this has something to do with XFOIL not being compatible with stall applications.
As I quote from JavaFoil, another airfoil analysis software:
JavaFoil results will be incorrect if larger areas of flow separation are present. Massive separation, as it occurs at stall, is modeled to some extent by empirical corrections, so that maximum lift can be predicted approximately for "conventional" airfoils. If you analyze an airfoil beyond stall, the results will be quite inaccurate. It is questionable, whether any two dimensional analysis method can be used at all in this regime, as the flow field beyond stall is fully three dimensional with spanwise flow and possibly strong three dimensional vortices.

For the BEM calculation using Matlab, I tried setting the Cl and Cd values for AoA higher than 28.5 deg the same as the Cl and Cd values for 28.5 deg, however the power calculated was still too high at 5500-5600 W. The QBlade multi parameter BEM simulation shows a power of approximately 5000 W. I believe this is because the Cl and Cd values at AoA higher than 28.5 deg leads to lower power, as demonstrated by QBlade multi parameter BEM simulation.

The big question is: which airfoil software is capable of simulating very high AoA, which is obviously post stall operation and has massive flow separation?

Thank you very much!

 

[rb1957]

a better forum for your questions !

why are you "so" interested in high/extreme AoA ? twist along the blade "should" take out most of the rotational component ?

another day in paradise, or is paradise one day closer ?

 

[Victarion]

@rb1957
I am puzzled as well. But let me show you my calculations.

Below is the twist and pitch data for my wind turbine rotor. Twist is obtained automatically from QBlade twist optimization tool

I calculate my angle of attack based on this diagram. theta (relative wind angle) = beta + phi(pitch) + alpha

Relative wind angle (here it is phi) is obtained from this diagram from the book by Robert Gasch as mentioned in my original post.

And an example result from Matlab to calculate phi and angle of attack:


And this is an example for the results I obtained:

Here, v_1 is free flow or upstream wind speed in m/s. omega is rotational speed (rad/s) for each v_1. w_1 is the relative wind velocity upstream for every segment of the blade (I divided the blade into 20 segments. coincidentally I simulate the blade for 20 different upstream wind speed). phideg is the relative wind speed angle for every segment of the blade, as seen in the diagram from Robert Gasch's book. Gamma is the pitch angle for every upstream wind speed (e.g. gamma = 6 is for v_1 = 9 m/s). Beta is the twist angle for every segment of the blade. Then alpha is calculated by substracting beta and gamma from phi.

As you can see, for the first 5 segments the AoA is higher than 20. Does this make sense in practice, or at least does my calculations make sense?

Thank you.

 

[Sparweb]

Victarion,
Welcome to Eng-tips.
Those are very challenging questions - I certainly haven't tackled them.
I have the book from Gasch and Twele that you referenced, and it was instrumental to me when I built a BEM analysis of my own wind turbines. I had a distinct lack of interest in blade angles close to stall, however, as I was most interested in an optimization tool for typical performance, not exploring the extremes of operation.

Are you building an analysis to predict the WT's start-up behaviour? Or is it speed controlled with pitch-to-stall? This sounds like the main purpose of high AoA data.

I can't claim to have checked this, but don't all BEM, lifting-line, panels and other methods rely in whole or in part on curves that are linear or at least "well mannered functions"? Even with a high-functioning CFD package this seems like a dicey problem to solve.

http://www.sparweb.ca