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Angle of attack calculate
Hi David
I recently encountered a problem regarding finding the effective angle of attack. I hope you can help me answer it.
I took the blade tip segment based on the airfoil in the qblade and wanted to verify the angle of attack calculation to determine whether my theory is correct. But the angle of attack I calculated was as high as 89 degrees. The qblade calculation shows that the angle of attack of the blade tip is about 5 degrees. I gave a simple small program based on the method given by A. van Garrel by solving the normal vector a3 and the tangential vector a1. The resultant velocity is obtained by using the inflow velocity V0 minus the linear velocity omega*r plus the induced velocity Vind, although here I didn’t give an induced velocity, but I’d like to know, where is my problem that causes the angle of attack to be so large? Did I overlook something?
Thank you
BR,
Roby
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Hi Roby,
even without any induced velocity you should not get such high angles of attack in the tip region, see the velocity triangle below:
Most likely there is an error somewhere in your vector operations or AoA evaluation.
In QBlade the AoA is obtained in the folowing way through a projection of the total velocity into the cross sectional plane of an aerodynamic panel:
m_AoA = atan2((m_V_total.dot(a3)),(m_V_total.dot(a1)))*180/PI;
where a3 is the normal vector in the chordwise direction and a1 is the normal vector in the direction of the airfoils suction side.
BR,
David
Hi david
One problem I’m having is querying the lift coefficients by the angle of attack of a control point, which I do by obtaining the angle of attack of the control point and then taking that angle of attack and bringing it into the airfoil corresponding to the two boundary points adjacent to that control point three times spline querying the coefficients to get the coefficients, and then averaging the two.
This is a good match for changing the lift coefficients more than anything else, but I find that for the drag coefficients, the step is a bit large,and I’d like to ask how you deal with these?
BR,
Roby
Hello Roby,
in QBlade, we use linear interpolation for the coefficients of lift (Cl), drag (Cd), and moment (Cm) between blade stations.
In my experience the linear interpolation is accurate enough, and by changing the overall aerodynamic blade discretization there is still some control regarding the accuracy of the distribution of aerodynamic coefficients over the blade.
BR,
David
Hi David
I placed the control points in the middle of each section of blade, and by using the angle of attack corresponding to the control points went and looked up the lift coefficients of the corresponding airfoils at the boundary points at each end and averaged them.
In my calculations, the combined velocity agrees with the qblade, and the angle of attack shows some problems for the blade element 4, 5, 6, and 7 except for the section corresponding to the rounded airfoil at the front, which I got by subtracting the angle of twist from the angle of income, and I suspect that something has gone wrong here, but I’m not sure what the problem is.
I suspect that there is a problem here, but I am not sure where the problem lies. Perhaps due to a problem with the angle of attack or the selection of the control point location, I have some problems interpolating the drag coefficients later on, and the drag coefficients have a step in the drag coefficients.
Do you have any good suggestions?
BR,
Roby
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Hi Roby,
the region where you see the differences is the transition region between circular (zero lift) airfoils and lift generating airfoils. In this region the circulation gradient is generally largest and this creates strong vortices in the wake and on the blade. Therefore I would expect the differences to stem from the vortex core desingularizon, possibly also from the iteration procedure, which is quite sensitive at large circulation gradients.
BR,
David
Quote from David on 22. May 2024, 10:39Hello Roby,
in QBlade, we use linear interpolation for the coefficients of lift (Cl), drag (Cd), and moment (Cm) between blade stations.
In my experience the linear interpolation is accurate enough, and by changing the overall aerodynamic blade discretization there is still some control regarding the accuracy of the distribution of aerodynamic coefficients over the blade.
BR,
David
Hi David
For your last mention about linear interpolation. I would like to define it again, for example, if the control point 3 is located in the middle of the blade boundary point 3 and boundary point 4, do you interpolate the angle of attack to get the coefficients of 3 and 4 separately, and then average them, or do you add and average 3 and 4 first and then interpolate them?
BR,
Roby
Hi David
I tried the two methods I just mentioned, implemented using cubic spline and linear interpolation respectively, and found that the results did not have much impact.
Then the problem of comparing my lift coefficients with the results obtained from qblade calculations centers on control points 4 to 8.
I have kept the same segmentation of the blade as qblade, 19 boundary points and 18 control points, each control point is located in the middle of the boundary points.
The lift drag coefficients are also from the airfoil files provided by qblade.
Is there any special treatment for the airfoil lift coefficient calculation if these positions are not correct? I’m using rated wind speed and RPM, no pitch angle.
Also, the difference between my angle of attack and yours isn’t that much, but the difference in lift drag is missing a lot.
BR，
Roby
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Hi Roby,
in general, the aerodynamic discretization during a simulation can be independent of the discretization used in the blade definition.
During the simulation, the angles of attack (AoAs) are evaluated at the control points of an aerodynamic panel.
The polar data at each control point is then obtained through linear interpolation based on the polar data of the adjacent blade stations from the blade definition.
The AoA is not interpolated, as it is directly evaluated at the control point.
BR,
David
Hi David
I encountered some problems, that is, I found that the induced velocity of the first row of wake (bound vortex) to the control point was very large, which was the three components of the induced velocity of my blade panel from 1 to 18. I found that after I simulated 60 time steps, the induced velocity was still very large, and the radius of my vortex core was 10% of the chord length, while the circulation was not large. I took some filaments close to the control point separately and found that they induced the control point at a speed of up to 3m/s. These have a very large effect on the Angle of attack. I would like to know how you dealt with these problems. Thanks.
BR,
Roby
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