Strouhal number of jet for noise calcs?

[MattC1234]

I am currently looking into some noise calcs for control valves with the latest edition published by the IEC (60534-8-4) however i have come across something slightly confusing and am wondering if anyone can shed some light on the problem.

The problem is that the calculation for Nstr (Corrected Strouhal Number) in the valve has been specified in equation 12 of the spec.

However the annex A at the back of the specification shows a worked example, for which we have been provided with a slightly modified equation to calculate Nstr. The annex calc (when inputting the example data) does come out with the answer they claim, however i want to know why this equation has changed, i cant find a reason why within the standard or by other soruces.

Any ideas? / Need me to show the equations?

Cheers

Matt

Mechanical Engineer

 

[ione]

Matt,

Could you post the two formulations of the Strouhal number?

 

[MattC1234]

Forgive the formatting, can't do anything about that i'm afraid

Where NSTR =( 0.02 x FL^2 x Cv / 1.17 x XFZP1^1.5 x d x d0 ) x (( 1 / (P1 - Pv ))^0.57)

This is the equation found in equation 12.
FL is the valve pressure recovery factor.
d = valve nominal bore diamater
do = diameter of the seat
XFZP1 = is the characteristic pressure ratio at the inlet pressure P1.

The second equation:

Where NSTR =( 0.036 x FL^2 x Cv x Fd^0.75 / 1.17 x XFZP1^1.5 x d x d0 ) x (( 1 / (P1 - Pv ))^0.57)

Where Fd is the valve style modifier.

Hope this helps.

Cheers

Matt