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What are Inlet Disturbances? Help needed please!

Mutleybm1996

I know this is more Physics however I tend to get faster responses on the Chemistry forum and fluid dynamics is a nice cross-over between Physics and Chemsitry.

The paper attached makes the following statement, I would appreciate it if someone could possibly help me by explaining what it means by 'inlet disturbances to the pipe'. I'm researching Reynolds number for a homework in Physics(A-Level) and am having a little difficulty.

Thank you

http://arxiv.org/ftp/arxiv/papers/1007/1007.0810.pdf

'Reynolds himself observed that turbulence was triggered by inlet disturbances to the pipe and the laminar state could be maintained to Re ≈12,000 if he took great care in
minimizing external disturbances to the flow. By careful design of pipe entrances
Ekman (1910) has maintained laminar pipe flow up to a Reynolds number of 40,000
and Pfenniger (1961) up to 100,000 by minimising ambient disturbances.'