Perforated Engine Intakes/Exhausts

[polarbearjim]

I have recently noticed that many Jet intakes and exhausts have a perforated surface just forward of the fan and around the diffusing cone. I found this appropriately titled photo of a 757's intake on the Flickr website to illustrate what I mean. The perforated surface can been seen on the left-hand side. The photo is copyright material so I'll just post a link.

Flickr Photo Download: What's that shiny thing do? (http://www.flickr.com/photos/29374594@N00/2066037457/sizes/l/)

I have also noticed in another photo:

http://farm4.static.flickr.com/3218/2717096078_e10f717d66_o.jpg

that the outside surface of the nacelle, corresponding to the same area off the 757's intake appears to be temporarily warped on takeoff to a pitted golf ball like shape possibly due to low pressure inside. Could anybody tell me if this is related to the perforated inner intake panneling and if so, why it was designed like this?

[dixi188]

Hi Jim.
The small holes are part of the accoustic lining that absorbs noise.
We call them pepperpot linings.
As for the golf ball effect, I don't know, but I don't think its to do with the lining.
Hope this helps.

[command]

spot on, just put your ear right up close to it, its quite trippy.

[Mark 1]

The perforated sheets are acoustic liners, as has been stated.
These are tuned devices that are designed specifically to target the radiated fan noise predominantly at the approach condition of the noise certification test requirements. The perforate sheet usually covers a honeycomb of hexagonal cells bonded to the underlying panel and the perforated facing sheets.

In simple terms part of the sound wave gets partly reflected from the perforated surface and part travels into the honeycomb cell, is reflected off the bottom and emerges through the facing sheet a short while later. The phase difference between the two partial reflections creates an interference that attenuates the radiated noise. Similar treatment is also found in the bypass duct and between the LP turbine and hot nozzle, each tuned to attenuate the most problematic turbomachinery noise.

You may also see some intakes covered in a fine woven material that does a similar job which Rohr have patented as DynaRohr absorbent panels.

[Rainboe]

polarbear, I have checked other pictures of 757 taking off, and it confirms there is no sign of cowling oil-caning under pressure, or vacuum. It is observed here because IMO an old, not very good, cowling panel with a bit of bend was used. This area would not be subject to internal cowling vacuum, or external high pressure. It's a matter of the light direction and quality of metalwork!

[polarbearjim]

Thankyou for your intellegent and informed answers folks.

Command, I shall have to give that a try!