Blade Element Theory for Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 53

Finally, the lift and drag coefficients are required as a function of the Reynolds
number and angle of attack. A very simple function to do this is LandD_0012.m
which uses the data correlations for the NACA 0012 aerofoil based on those of [ 3 ]
as described in Chap. 4 . It will be seen in Chap. 5 that the inputting of measured
lift and drag is more cumbersome than the simple situation shown here.
function [cl, cd] = LandD_0012(aoa, Re)
% Function to calculate the lift, cl, and drag, cd, coefficient of
% a NACA0012 aerofoil using the correlations of McCroskey (1987).
if aoa > 12.0, aoa = 12.0; end % Eq. 4.3 for Cl is only valid…
if aoa < -12.0 aoa = -12.0; end% for aoa < |12 deg|.
cl = aoa*(0.1025 + 0.00485*log10(Re/10^6));
cd0 = 0.0044 + 0.018*Re^(-0.15); % Eq. 4.4 for minimum Cd
delcd = (cl/1.2)^2*0.009; % A data fit to obtain Cd at other angles
cd = cd0 + delcd;
return
The more complex and realistic power_calc.m and its associated programs are
presented and described in Chap. 5 . They have the same basic layout as shown here.
In other words, the blade geometry and the aerofoil lift and drag data are determined
separately, to allow the same main program to handle a wide range of different
configurations and aerofoils. It is common for large wind turbines for the aerofoil
section to change along the blade, but this does not occur often for small turbines.
The reasons for this difference are explored in Chaps. 4 - 6 , but for now it is suffi-
cient to note the simplicity of using the same section in the blade element program.