### Simulation error for low turbulent values of wind velocity

Posted:

**Tue Jun 04, 2019 3:10 pm**Dear Jason,

I am performing a batch of aeroelastic simulations under turbulent winds at several hub height average velocities, starting from 5 m/s (5 m/s, 7, 9,11...). With the lower ones, such us 5 m/s I suffer a crash in the simulations when Vx (at hub height) reaches minimum peaks, of about 2 m/s. The given error is this one:

"FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:BEMT_CalcOutput(node 9,

blade 2):Compute_UA_AirfoilCoefs:UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be

evaluated."

Or this one if I set unesteady aerodynamics off:

"FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 9, blade

2):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating

conditions! Vx = -6.63006E-03, Vy = 5.8556, rlocal = 1.438, theta = 0.12822"

This error occurs either if I set the yaw DOF to True or to False.

From the output time series (which are attached in this post) I see that when the failure occurs the rotational speed (around 50 rpm) is too high for the incoming wind velocity (2 m/s more or less). This leads to high TSR and phi angles, and FAST crashes. Initially I have tried a Simple VS control from ServoDyn module of FAST v8 (VSContrl=1), reproducing a fixed pitch passive stall turbine as we discussed in previous topics, line blue in the figure corresponds to this implementation. I have also tried to model the torque control system with a user defined routine (VSContrl=3) (line orange in the figure) setting the generator torque to zero for wind velocities lower than the cut-in velocity (which is 3 m/s, and omega=51 rpm, more or less), and the same curve shape than in the simple VS torque control implementation but the error remains.

I hope that with your experience you could give me advice on this topic.

Thank you very much in advance,

Best regards,

Alvaro Olcoz Alonso

Student at Public University of Navarre

I am performing a batch of aeroelastic simulations under turbulent winds at several hub height average velocities, starting from 5 m/s (5 m/s, 7, 9,11...). With the lower ones, such us 5 m/s I suffer a crash in the simulations when Vx (at hub height) reaches minimum peaks, of about 2 m/s. The given error is this one:

"FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:BEMT_CalcOutput(node 9,

blade 2):Compute_UA_AirfoilCoefs:UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be

evaluated."

Or this one if I set unesteady aerodynamics off:

"FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 9, blade

2):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating

conditions! Vx = -6.63006E-03, Vy = 5.8556, rlocal = 1.438, theta = 0.12822"

This error occurs either if I set the yaw DOF to True or to False.

From the output time series (which are attached in this post) I see that when the failure occurs the rotational speed (around 50 rpm) is too high for the incoming wind velocity (2 m/s more or less). This leads to high TSR and phi angles, and FAST crashes. Initially I have tried a Simple VS control from ServoDyn module of FAST v8 (VSContrl=1), reproducing a fixed pitch passive stall turbine as we discussed in previous topics, line blue in the figure corresponds to this implementation. I have also tried to model the torque control system with a user defined routine (VSContrl=3) (line orange in the figure) setting the generator torque to zero for wind velocities lower than the cut-in velocity (which is 3 m/s, and omega=51 rpm, more or less), and the same curve shape than in the simple VS torque control implementation but the error remains.

I hope that with your experience you could give me advice on this topic.

Thank you very much in advance,

Best regards,

Alvaro Olcoz Alonso

Student at Public University of Navarre