Hi Stefan,
The EQUIL model is a quasi-steady model that assumes that the wake is always in equilibrium with the inflow and structural response. The DYNIN model is a dynamic model such that wake responds dynamically to changes in the inflow or structural response.
Under steady conditions, both models would ideally predict the same response. But this is not true in practice as the Prandtl hub- and tip-losses that are part of the EQUIL solution are not used in the DYNIN soluion. Instead, DYNIN calculates the variation of induction along the rotor disk (radially and azimuthally) using internal flow states and there are not enough states implemented in the current version of AeroDyn to properly capture the hub- and tip-losses (more states would mean slower execution time). We plan to add more DYNIN states as an option in a future version of AeroDyn.
Another limitation of the DYNIN model in the current version of AeroDyn is that it is numerically unstable for heavily-loaded rotors (i.e., low wind speeds). So, only the EQUIL model can be applied at low wind speeds. In fact, when using full-field turbulent winds, the current version of AeroDyn will automatically switch the option from DYNIN to EQUIL when the mean wind speed is less than 8 m/s (the mean wind speed of the entire full-field file is used in this calculation).
Given the limitations of the DYNIN model in the preceeding two paragraphs, I decided to apply the EQUIL model (across all wind speeds) when calculating the steady-state behavior of the NREL 5-MW baseline turbine for Chapter 9 of its specifications report.
Best regards,