DEFAULT and Recommended Polar and Curled Wake Settings in FAST.Farm

[jjonkman]

The FAST.Farm documentation says that the DEFAULT value of k_VortexDecay for the curled wake model is 0.0001, while the r-test input files say 0 and the source code actually uses 0.01. All should be changed to zero.

Also, the modeling guidance in the FAST.Farm documentation for dr is incorrect. The modeling guidance for the polar wake model recommends dr <= c_max, which is too fine. The modeling guidance for the curled wake model recommends dr < D/10, which may be too coarse. The recommendation should be changed to dr <= D^Rotor/15 for both models.

Also, the modeling guidance in the FAST.Farm documentation has typo in the recommended value of DT_Low for the curled wake model; that is, DT_Low is not displayed properly.

Moreover, the modeling guidance in the FAST.Farm documentation for DS_Low is incorrect for the curled wake model. The recommended value of DS_Low <= C_Meander * D^Wake * V_Hub/(150 m/s) is correct for both, but the second expression of DT_Low * V_Hub^2/(15 m/s) is only valid for the polar wake model. For the curled wake model, the second equation should be C_Meander * DT_Low * V_Hub^2/(5 m/s), which follows from other recommendations, i.e., D^Wake = D^Rotor = 15 * dr = 15 * 2 * V_hub * DT_Low = 30 * V_hub * DT_Low.

This bug will not effect any tests because the curled wake model is not current tested.

[ebranlard]

The default value for k_VortexDecay has been updated in #1785
(https://github.com/OpenFAST/openfast/pull/1785/files#diff-7c57d87989b0512a8bafedb7b94b68cf6a7de256981a361b2cd5caaf9c04fa6aR856)