I have read about a method for reducing cleaning jet (speed around 100m/sec) noise by introducing a lower-speed stream, which surrounds the jet and decrease the velocity gradient. Because of the lower gradient, the strength of the vortical structure will be reduced.

I have used Google to search for relative articles and books, but found few. Would anyone tell me where to find more detailed information. I am especially interested in the nozzle design that will have optimum performance on noise reduction.

Not sure if this is the same thing but Jet engines for planes work by passing a large volume of slow relatively cool air round the outside of the inside of the engine, which mixes with the hot air coming from the combustion chamber, although this has other functions, primarily thrust and fuel efficiency, it does reduce the engine noise. It could be looking into this area might throw up some relevant science.

Hope this helps, probably way of topic though!

Cheers

Andy

Trying looking up words like "aeroacoustics of co-axial jets" in the search engine... One of the outcomes of having a large bypass ratio in jet engines is in fact noise reduction. In some fundamental way, it is to reduce the bulk velocity of the jet exhaust, so that noise is reduced at the same thrust level (by making up the necessary thrust with mass flow), according to Lighthill's analogy.

A brief review about noise reduction (with a list of papers for reference) can be found in 'Flow control opportunities in gas turbine engines' by W.K.Lord and others, where noise reduction and many other problems related to gas turbine engines are discussed.

Hope it helps