I built a floating offshore wind turbine myself, based on OpenFAST version 4.2. The lower floating foundation and mooring system are models I developed myself, while the upper turbine is the IEA 15 MW reference wind turbine. In my research today, I established a case with waves only, without wind or current. To represent the windless condition, I disabled the InflowWind module and the ServoDyn module, set the pitch angle to 90°, and set the rotor speed to zero.
I found that after running the simulation for a while, the Mach number exceeded 1, and the following error messages appeared:
FAST_Solution:FAST_UpdateStates:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates:UpdatePhi(node 39, blade 3):BEMT_UnCoupledSolve:There is no valid value of phi for these operating conditions:
Vx = -1.28864E-03, Vy = -0.46931, rlocal = 94.477, theta = 1.5425, geometric phi = -3.1388. This warning will not be repeated though the condition may persist. (See GeomPhi output channel.)
Time: 110 of 1500 seconds. Estimated final completion at 11:11:38 (in 0.021 days).
FAST_Solution:FAST_UpdateStates:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOutput(node 39, blade 3):UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be repeated though the condition may persist.
Time: 120 of 1500 seconds. Estimated final completion at 11:11:41 (in 0.021 days).
FAST_Solution:FAST_UpdateStates:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOutput(node 20, blade 1):UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be evaluated.
OpenFAST encountered an error at simulation time 123.49 of 1500 seconds.
Simulation error level: FATAL ERROR
Later, I disabled the AeroDyn module in the .fst file, and this issue was resolved—the error messages no longer appeared. However, I am quite confused about this, because when I simulate extreme conditions with an inflow wind speed of 57 m/s, with the rotor stationary and the blades feathered, this problem does not occur. So why does the Mach number become large under windless conditions, especially when the rotor is not spinning?
I built a floating offshore wind turbine myself, based on OpenFAST version 4.2. The lower floating foundation and mooring system are models I developed myself, while the upper turbine is the IEA 15 MW reference wind turbine. In my research today, I established a case with waves only, without wind or current. To represent the windless condition, I disabled the InflowWind module and the ServoDyn module, set the pitch angle to 90°, and set the rotor speed to zero.
FAST_Solution:FAST_UpdateStates:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates:UpdatePhi(node 39, blade 3):BEMT_UnCoupledSolve:There is no valid value of phi for these operating conditions:
Vx = -1.28864E-03, Vy = -0.46931, rlocal = 94.477, theta = 1.5425, geometric phi = -3.1388. This warning will not be repeated though the condition may persist. (See GeomPhi output channel.)
Time: 110 of 1500 seconds. Estimated final completion at 11:11:38 (in 0.021 days).
FAST_Solution:FAST_UpdateStates:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOutput(node 39, blade 3):UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be repeated though the condition may persist.
Time: 120 of 1500 seconds. Estimated final completion at 11:11:41 (in 0.021 days).
FAST_Solution:FAST_UpdateStates:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOutput(node 20, blade 1):UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be evaluated.
OpenFAST encountered an error at simulation time 123.49 of 1500 seconds.
Simulation error level: FATAL ERROR
Later, I disabled the AeroDyn module in the .fst file, and this issue was resolved—the error messages no longer appeared. However, I am quite confused about this, because when I simulate extreme conditions with an inflow wind speed of 57 m/s, with the rotor stationary and the blades feathered, this problem does not occur. So why does the Mach number become large under windless conditions, especially when the rotor is not spinning?