IECWind EWS

[Rebecca.Sykes]

Hi All - though I think this may be in Marshall's domain...

Marshall,
This is partly a question about IEC Wind and partly a question of standards.
The GenEWS routine has a commented out value of alpha which was used in the determination of v(z,t) via ShrMax; now the params value of alpha is used, whose value is dependent on whether 61400-1 or 61400-3 is specified.

Code: Select all

SUBROUTINE GenEWS (FileNum)

! Generate the EWS condition.

USE            Params

IMPLICIT       NONE


! Passed variables
INTEGER        FileNum

! Local variables
!mlb Start of proposed change.
!REAL         ::Alpha = 0.2   !Shear definition constant
!mlb Start of proposed change...
...
!mlb Start of proposed change.
!ShrMax=2.*(2.5+0.2*VG50*(TurbRat)**0.25)/Vhub
ShrMax=2.*(2.5+Alpha*VG50*(TurbRat)**0.25)/Vhub...


Eqn 26 of IEC 61400-1 contains a 0.2, which is not a variable but which has been interpreted in IECWind as the variable alpha and therefore varies with 61400-x. Do you know why this was made a variable? I can't find a justification in 61400-3. Seems especially unusual if it is the vertical shear exponent as it influences the horizontal shear. Can you help me understand?

Rebecca

[Marshall.Buhl]

Dear Rebecca,

In Section 6.3 Wind Conditions of IEC 61400-3 Ed. 1, Equation 3 shows the power law and the comment following it says "the power law exponent, Alpha, is 0.14." It's at the top of Page 22 in my copy.

I think the reasoning is that because the ocean is typically a smoother surface than land is, it should use a smaller shear exponent.


Marshall

[Rebecca.Sykes]

HI Marshall,
Thank you for your reply. I agree alpha is certainly different for 61400-3 to 61400-1. It is 0.14 rather than 0.2, and you are correct, it is outlined at the top of p22 in my copy of 61400-3 also.
However what I am querying is the interpretation of the expression for EWS in 61400-1 p30 eqn 26 and 27. There is a 0.2 which has been interpreted as alpha in IECWind (subroutine GenEWS), but which I don't think it should have been.

So for EWS with positive horizontal shear from eqn27 in 61400-1...

V(y,z,t)=Vhub(z/zhub)^alpha+-(y/D)*(2.5+0.2*beta*sig1*...

and this 0.2 is interpreted as alpha in the GenEWS variable, ShrMax.

I was trying to ascertain if there was a reason for this?

All the best,
Rebecca

[Marshall.Buhl]

Rebecca,

Jason agreed with you that I should not have changed the 0.2 to Alpha, so I repaired the program. You can download the new version here:

http://wind.nrel.gov/designcodes/preprocessors/iecwind/

Thanks!


Marshall

[Rebecca.Sykes]

Thanks for the update Marshall

Its good to feedback into the software, to improve as well as use.