FAST.Farm wake plane overtaking/crossing

[luwang00]

In FAST.Farm simulations, it has been observed that the wake planes of a a rotor can occasionally cross each other, i.e., a newer wake plane overtakes its downwind neighbor. This problem is likely caused by errors in the numerical integration of wake-plane center positions. The forward Euler method used coupled with large low-resolution-grid time step can cause a wake plane to overshoot past its downwind neighbor.

To minimize the occurrence of such events, users can decrease f_c and/or DT_low in the FAST.Farm input file. In the absence of a more permanent solution, PR #3071 introduced a check for wake-plane crossing. If detected, the offending wake plane is simply removed to avoid overlapping wake elements. A warning is also given. In the long term, a more permanent and robust solution needs to be developed for this issue.

[jjonkman]

Just to clarify the issue arises from a combination of issues like:

• the simple Euler method in the passive tracer (as mentioned by Lu)
• large time step of the low-resolution domain (as mentioned by Lu)
• floater motion (of an FOWT) changing the location of where wake planes are released from the rotor
• the possible presence of localized gusts in the flow domain,
• the fact that the flow field is not always guaranteed to satisfy continuity (based on how the ambient inflow is generated)

Best regards

[luwang00]

Thanks @jjonkman for the additional clarification. The fact that the flow field is not guaranteed to satisfy the continuity equation could be a major factor here.