Error calculating the wind speed at a given position with OLAF.

[marcschwa]

HI,

I am currently working with OLAF in OpenFAST 2.4.0 (binaries downloaded directly from the release page) and I am getting the following error at a given point of my simulation (150 s out of 600 s):

FAST_Solution:FAST_AdvanceStates:SolveOption2c_Inp2AD_SrvD:InflowWind_CalcOutput:CalcOutput:IfW_Un iformWind_CalcOutput:GetWindSpeed:Height must not be negative. Error calculating the wind speed at position (1208.1, -20.183, -1.74038E-02) in the wind-file coordinates

Strangely, when I try to run using the same Elastodyn and Inflowfiles but change the Aerodyn to use BEM, the model works just fine. Furthermore, the platform degrees-of-freedom are disabled and the system is fully rigid, so the turbine is completely still. At the point that the simulation crashes, the results between OLAF and BEM are also quite comparable:

Here are some of the parameters that I am using for OLAF:

------------------- WAKE REGULARIZATIONS AND DIFFUSION ----------------------------------------- 0 DiffusionMethod Diffusion method to account for viscous effects {0: None, 1: Core Spreading, "default": 0}
0 RegDeterMethod Method to determine the regularization parameters {0: Constant, 1: Optimized, 2: Chord-scaled, 3: dr-scaled, default: 0 }
3 RegFunction Viscous diffusion function {0: None, 1: Rankine, 2: LambOseen, 3: Vatistas, 4: Denominator, "default": 3} (switch) 3 WakeRegMethod Wake regularization method {1: Constant, 2: Stretching, 3: Age, default: 1} (switch)

------------------- SPEEDUP OPTIONS ----------------------------------------------------------- 2 VelocityMethod Method to determine the velocity {1:Biot-Savart Segment, 2:Particle tree, 3: Segment tree, default: 1} 1.5 TreeBranchFactor Branch radius fraction above which a multipole calculation is used {default: 2.0} [only if VelocityMethod=2] 1 PartPerSegment Number of particles per segment [only if VelocityMethod=2]

I can see a problem related to vortices below the ground in #1237 but it doesn't seem to be the problem I am having, since in my case, there is no warning, but a complete crash on the simulation. Any suggestions or ways to overcome the issue?

[ebranlard]

This issue is likely due to the internal extrapolation of inputs that happens in OpenFAST, leading to points being requested below the ground. Unfortunately, we do not have a solution for this at the moment.

Yet, I would recommend using the latest version of OpenFAST if possible, there have been some modifications to InflowWind when OLAF was being used, which should prevent errors when points are requested outside of the domain.

An alternative is to place the hub height higher, but that's not viable when shear is present.