Interpolation of aerodynamic coefficients

To compute the Aerodynamic loads on a blade element, the BEM algorithm requires the lift and drag coefficients. These are provided for different angles of attack and different Reynolds numbers in the Airfoil polar file

1 Angle of attack interpolation

When the BEM algorithm requires aerodynamic coefficients for an angle of attack that has not been provided in the Airfoil polar file, Ashes carries out a linear interpolation between the two closest available angles. For a linear interpolation, it is assumed that the aerodynamic coefficient, for example the lift coefficient 
can be expressed as

 is the angle of attack, and 
 are the slope and the intercept (see
If the lift coefficients 
are given at the angles of attack 
, respectively, then the following equation applies:
$$C_L(\alpha) = C_{L1}+(\alpha-\alpha_1)\frac{C_{L2}-C_{L1}}{\alpha_2-\alpha_1}$$
Equation 1Linear interpolation of the lift coefficient

If the polar is only provided for a number of angles of attack that does not extend from -180 to 180 degrees, the Viterna extrapolation is used to produce coefficients for the whole range of angles.

2 Reynolds number interpolation

It is also possible to provide polars for different Reyniolds numbers. In this case, Ashes will compute the Reynolds number at any given Blade aerodynamical station and the BEM algorithm will use the aerodynamic coefficients based on that Reynolds number.
For example. to find the lift coefficient
for an angle of attack
and a Reynolds number
, the process is as follows:
  1. The BEM algorithms asks for the lift coefficient 
    $$C_L(\alpha, Re)$$
  2. Ashes finds the two polars with Reynolds numbers closest to 
  3. Within each of these two polars, Ashes finds the two angles of attack closest to 
     and performs a linear interpolation (as described in Equation 1). This produces two lift coefficients, 
  4. A new interpolation is perfoemrd between these two lift coefficients. This interpolation can be either Linear or Logarithmic (as defined by the Polar interpolation scheme in the Advanced section of the Aerodynamics tab)

2.1 Linear interpolation

If a linear interpolation is selected, the aerodynamic coefficients are computed as described in Equation 1. For example, the lift coefficient will be
$$C_L(\alpha,Re) = C_{L}(\alpha,Re_1)+(Re-Re_1)\frac{C_L(\alpha,Re_2)-C_L(\alpha,Re_1)}{Re_2-Re_1}$$

2.2 Logarithmic interpolation

With regards to the variation of aerodynamic coefficients with respect the Reynolds number, Thoms (2016) shows that aerodynamic coefficients exhibit a logarithmic dependency. In his work, it is shown that a good fit for the lift and drag coefficients is obtained when assuming the following functions:


This implies that the aerodynamic coefficients can be expressed as 
$$C_L(\alpha,Re) = C_{L}(\alpha,Re_1)+\log_{10}(Re/Re_1)\frac{C_L(\alpha,Re_2)-C_L(\alpha,Re_1)}{\log_{10}(Re_2/Re_1)}$$

$$\log_{10}(C_D(\alpha,Re)) = \log_{10}(C_{D}(\alpha,Re_1))+\log_{10}(Re/Re_1)\frac{\log_{10}(C_D(\alpha,Re_2)/C_D(\alpha,Re_1))}{\log_{10}(Re_2/Re_1)}$$

Note: the moment coefficient is treated assuming the same behaviour as the lift coefficient