How to understand what pyFAST and equivalentload.py are doing

Wind & Water Computer-Aided Engineering Software Tools
1

A while back i downloaded and installed a python toolbox for the openFAST simulation software.

File not found (github.com)

this python toolbox used a script called EquivalentLoad.py and it calculated the DEL of the supplied openfast file.

For example, it calculated a DEL of 67.25 kN

but i am now trying to figure out what this actually means, and what to do with it?

Any help and understanding of what to do with this value would be great.

thanks

Matt

2

Dear @Matt.Thomas,

This python scripts computes a damage-equivalent load (DEL) based on the provided time series of load, using the Wholer material exponent and a rainflow cycle counting algorithm. For more information on DEL, see the old MLife Theory Manual: https://www.nrel.gov/wind/nwtc/assets/pdfs/mlife-theory.pdf.

FYI: What used to be the python-toolbox is now called the openfast_toolbox in the OpenFAST GitHub repository: GitHub - OpenFAST/openfast_toolbox: Miscellaneous Python tools for OpenFAST,

Best regards,

1 Like
3

ok, reading the mlife theory manual, what does DEL exactly mean? its a little tricky to understand.

Actually, let me tell you what i think it means and please add to it what i am missing

If my DEL is 65kN, to me, that means it is a single static load that is constantly applied to the blade. IE, loading cement bags onto the blade until 65000 netwons of cement is statically pushing on the blade. But im not sure how this translates or relates to cyclic loading?

I am used to watching blades bounce up and down from a fatigue machine for a million or 2 cycles.

thanks

4

Dear @Matt.Thomas,

I don’t agree with your explanation. The damage equivalent load is the range of a cyclic load of fixed amplitude and frequency that results in the same (equivalent) structural damage as a stochastic load of varying amplitudes/frequencies.

Best regards,

1 Like
5

What you wrote is right from the theory manual, and like i said above, it is tricky to understand :slight_smile:

But i think i get it now, after reading more stuff.

If i had the following load history.

50 kN for 1000 cycles
70 kN for 500 cycles
30 kN for 2000 cycles

And DEL was 65 kN, that would mean if we applied 65 kN repeatedly it would end up being the equivalent damage as what the load history above would generate over time.

Now that i have 65 kN, i can use equation 27 ( if i remember correctly) from the theory manual to calculate N, if i know my other values such as L_ult and L_mf