# IECWind EWS

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]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+0.2VG50*(TurbRat)**0.25)/Vhub
ShrMax=2.(2.5+AlphaVG50*(TurbRat)**0.25)/Vhub…
[/code]
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

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

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+0.2betasig1

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

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

Thanks for the update Marshall

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