Wind Veer Parameter

Hi @Jason.Jonkman:
I am getting the below error In TurbSim when I change value of Cohexp to make it greater than 0.35; so for 0.40 I got the error. I would like to use a Cohexp value of 0.85. I believe the error has something to do with my grid size, say. If I use the following model parameters I get no error with a Cohexp of 0.85 .
--------Turbine/Model Specifications-----------------------
31 NumGrid_Z - Vertical grid-point matrix dimension
31 NumGrid_Y - Horizontal grid-point matrix dimension
0.05 TimeStep - Time step [seconds]
3600 AnalysisTime - Length of analysis time series [seconds] (program will add time if necessary: AnalysisTime = MAX(AnalysisTime, UsableTime+GridWidth/MeanHHWS) )
3600 UsableTime - Usable length of output time series [seconds] (program will add GridWidth/MeanHHWS seconds unless UsableTime is β€œALL”)
119.0 HubHt - Hub height [m] (should be > 0.5GridHeight)
210 GridHeight - Grid height [m]
210 GridWidth - Grid width [m] (should be >= 2
(RotorRadius+ShaftLength))
0 VFlowAng - Vertical mean flow (uptilt) angle [degrees]
0 HFlowAng - Horizontal mean flow (skew) angle [degrees]

However, if I use the following model parameters I get the error:
--------Turbine/Model Specifications-----------------------
31 NumGrid_Z - Vertical grid-point matrix dimension
31 NumGrid_Y - Horizontal grid-point matrix dimension
0.05 TimeStep - Time step [seconds]
3600 AnalysisTime - Length of analysis time series [seconds] (program will add time if necessary: AnalysisTime = MAX(AnalysisTime, UsableTime+GridWidth/MeanHHWS) )
3600 UsableTime - Usable length of output time series [seconds] (program will add GridWidth/MeanHHWS seconds unless UsableTime is β€œALL”)
150.0 HubHt - Hub height [m] (should be > 0.5GridHeight)
275 GridHeight - Grid height [m]
750 GridWidth - Grid width [m] (should be >= 2
(RotorRadius+ShaftLength))
0 VFlowAng - Vertical mean flow (uptilt) angle [degrees]
0 HFlowAng - Horizontal mean flow (skew) angle [degrees]

TurbSim Error
Reading the input file β€œ150m_88mps_wind_shearJCS1_test.inp”.
Calculating the spectral and transfer function matrices:
u-component matrices (general coherence model)
CalcFourierCoeffs:CalcFourierCoeffs_General:Error in Cholesky factorization: LAPACK_SPPTRF:
Leading minor order 111 of A is not positive definite, so Cholesky factorization could
not be completed.
The error occurred in the u-component coherence matrix at frequency 1 (2.76817E-04 Hz) Check the input file for invalid physical properties or modify the coherence.

I think the error is due to the size of my grid. However, I need a sufficiently large grid width to prevent blades from deflecting outside of the grid and causing an error in OpenFAST.

Please let me know if you have any ideas on how to solve this problem.

Thank you.

Regards,
AOAW

Hi @Jason.Jonkman:
I solved the problem by using a coarser grid. Instead of 31 x 31, I used 21 x 21. This coarser grid allows me to use Cohexp of 0.85 without getting an error.

Regards,
AOAW

Hi @Andre.White,

Makes sense. A Cholesky factorization error is typically caused by the use of too high coherence for the given grid spacing.

Best regards,

1 Like

Thanks @Jason.Jonkman. Another thing learnt.

Regards,
AOAW