Twin orifice plate noise

[maxh]

Hi everyone,

Here is a question that has vexed me for a while.

If I have two orifice plates in-line say 10D apart, if each orifice plate has a pressure drop that is higher than the criticle pressure ratio for the inidividual plate, then in calculating the noise level after the second orifice plate do I simply add both the individual noise levels together or is the noise level after the second plate due only to the noise generated at the second plate ?

Or to put the question another way - in simplistic terms as I am a Mechanical Engineer, does noise energy in this sort of case travel and "build" through a system ?

Please note - I am assuming that you get a shock wave at each criticle pressure ratio pressure drop.

Any comments would be appreciated.

 

[hacksaw]

they add more or less on a dB basis but you have to account for the distances to the meaS. point etc.

 

[maxh]

Thanks Hacksaw,

OK, I understand about the distance to source aspect, however, are you saying that the sound energy from the first plate will travel across the second and be radiated by the subsequent pipeline ?

I guess the question here is, do sonic flow conditions modify sound transmission - any thoughts ?

 

[GregLocock]

Generally you get an increase in the flow noise from an obstruction if the preceeding flow is not smooth. To some extent this may be because the vortices in the incoming flow are ready made nucleation sites for vortices to form in the second orifice.

I have not done this with orifice plates, but it is a well known property of fans.

Incidentally you should add the pressures using 10 log(p2+p1), as they will be uncorrelated.





Cheers

Greg Locock