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I was at the science museum in London, and was looking at the Rolls Royce RB 211-22B. This particular engine was, I believe, the sixth one ever produced. The surface texture of the fan inlet looks like a half-complete honeycomb, composed of grey and black dots. Does anyone know what the function of these dots are, and, why they seems to be arranged in this semi-random hexagonal structure? My guess is something to do with thickening the boundary layer before it hits the fan tips, but I am especially curious as to why they chose such a specific structure. Rolls Royce RB 211-22B inlet

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    $\begingroup$ Probably an acoustic liner, as explained here. $\endgroup$
    – Ralph J
    Feb 4 at 23:56

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I believe this is an acoustic liner. The holes on the surface are on a regular grid. Underneath, there is a honeycomb layer. That honeycomb is also uniform in size. The alignment / misalignment of the holes and the honeycomb give rise to the semi-random looking pattern.

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  • $\begingroup$ Thank you - you're right! I wonder if the misalignment between the holes and the honeycomb is deliberate (as opposed to each hole being in the centre of a honeycomb). Perhaps the reason for the misalignment is so that multiple acoustic harmonics can be dampened? $\endgroup$ Feb 5 at 19:33
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    $\begingroup$ I know vanishingly little about acoustic liners. I know that the size of the hole and the size of the volume behind the hole are very important parameters. I have no idea about the effect of multiple holes vs. a single hole per chamber. $\endgroup$ Feb 5 at 20:57
  • $\begingroup$ Maybe to avoid any resonation in the flow pattern? $\endgroup$ Feb 11 at 3:31

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