A fast Blade Element Method algorithm with guaranteed convergence, based off the equations reduced by Ning (2014)
The dcp_calc function outputs the contribution to Cp and forces experienced by an element of the blade. The input parameters are as follows:
- twist - the twist of the blade element
- polars - a cell array containing the polars of the airfoil used at the element - see below for more details
- z - the radial position of the element
- R - the maximum radius of the blade
- tsr - the tip speed ratio of the blade
- B - the total number of blades used in the completed design
- chord - the chord of the aerofoil used
- r0 - the distance from the centre of rotation to the root of the blade
- V - incoming fluid velocity
Polars is a cell array containing four different fits. Elements 1 and 2 are fits of the lift coefficient and drag coefficient respectively, against angle of attack in radians, between -10 and 30 degrees. This can be obtained from XFOIL.
Elements 3 and 4 are fits of Cl and Cd respectively, but between angles of attack betwene -180 degrees and +180 degrees. These have been separated from the other polars as any extrapolation method used typically results in some inaccuracies in the normal angle range. This data is only used while finding the root; if the optimal angle of attack of the aerofoil is between -10 and 30 degrees - which it almost certainly is - then the higher accuracy data will be used for calculating dcp and the forces once converged parameters have been found.