### BEM method Implementation

Posted:

**Tue Jul 12, 2011 10:53 am**Dear,

I'm writing a Wind Generators FE model in Matlab. I'm implementing the BEM model you suggest in the AaeroDyn Theory Manual. I use the Tip-Loss Model, the Hub-Loss Model and the Buhl Correction for evaluate the axial induction factor (a) when CT (thrust coefficient) >0.96*F. I don't consider the Dynamic Stall Model and the Skewed Wake Correction (I assume the wind flow always perpendicular to the rotor plane).

In the iterative process to evaluate the axial induction factor (a) and the tangential induction factor (a'), i encounter these problems:

1) Often when the local angle of attack (α) is negative, CT results negative and for the relation [27] page 10 of the AeroDyn theory Manual, the axial induction factor (a) results >1 and the local flow angle (ϕ) results negative. So the Tip-Loss Factor and the Hub-Loss Factor results complex number! I have to evaluate the Tip-Loss Factor and the Hub-Loss Factor assuming the absolute value of the local flow angle, i.e. |ϕ| ?

Furthermore, the pressure normal to the rotor plane acting on this blade portion results of opposite direction respect to the wind flow, is physically correct ?

2) Sometimes, the iterative process doesn't converge to calculate the axial induction factor (a) ? So i force the convergence. Have you ever encounter this event ? How can i do ? Which could be the reason ?

3) If the wind flow i considered stochastic, do you assume U∞ (= mean wind speed) + the fluctuating contribution of the wind speed for the calculation of the axial induction factor ? In your relations doesn't appear the fluctuating part of the wind speed, why ?

I really thank you for your support, best regards

Lorenzo Montanari

I'm writing a Wind Generators FE model in Matlab. I'm implementing the BEM model you suggest in the AaeroDyn Theory Manual. I use the Tip-Loss Model, the Hub-Loss Model and the Buhl Correction for evaluate the axial induction factor (a) when CT (thrust coefficient) >0.96*F. I don't consider the Dynamic Stall Model and the Skewed Wake Correction (I assume the wind flow always perpendicular to the rotor plane).

In the iterative process to evaluate the axial induction factor (a) and the tangential induction factor (a'), i encounter these problems:

1) Often when the local angle of attack (α) is negative, CT results negative and for the relation [27] page 10 of the AeroDyn theory Manual, the axial induction factor (a) results >1 and the local flow angle (ϕ) results negative. So the Tip-Loss Factor and the Hub-Loss Factor results complex number! I have to evaluate the Tip-Loss Factor and the Hub-Loss Factor assuming the absolute value of the local flow angle, i.e. |ϕ| ?

Furthermore, the pressure normal to the rotor plane acting on this blade portion results of opposite direction respect to the wind flow, is physically correct ?

2) Sometimes, the iterative process doesn't converge to calculate the axial induction factor (a) ? So i force the convergence. Have you ever encounter this event ? How can i do ? Which could be the reason ?

3) If the wind flow i considered stochastic, do you assume U∞ (= mean wind speed) + the fluctuating contribution of the wind speed for the calculation of the axial induction factor ? In your relations doesn't appear the fluctuating part of the wind speed, why ?

I really thank you for your support, best regards

Lorenzo Montanari