Reynolds and inlet disturbances

Mutleybm1996

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

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.'

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(edited 9 years ago)

Reply 1

amongsttheweeds

Original post by Mutleybm1996

Find the user 'Barry Belmont' on YouTube, then look at 'Lecture 11. Turbulence'. It's an old lecture, but will help you to see what is going on. At one point the inlet is changed and you can see the effect.

Re = (D rho v)/mu. Usually, at Re >2100 the flow is no longer laminar. At Re >4000 the flow is turbulent. Effectively the fluid doesn't flow in a predictable fashion, and a phenomenon known as Reynolds stress enters the equation. Eddies form and fluid flows sideways or even backwards, against the prevailing flow.

Adjusting the inlet minimises these effects to preserve laminar flow at high Re.

Edit: one thing I missed was the bit about 'ambient disturbances'. This could be anything which causes vibrations; a busy road, a set of stairs, a drill, etc.

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(edited 9 years ago)

Reply 2

Mutleybm1996

Original post by addylad

I don't suppose you would know how a
Hysteresis-like S loop would be possible from looking at the viscosities would you? My results are -ve exponential.

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Reply 3

amongsttheweeds

Original post by Mutleybm1996

I don't suppose you would know how a
Hysteresis-like S loop would be possible from looking at the viscosities would you? My results are -ve exponential.

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You are welcome to share the info you have and I will have a look. (PM if necessary.)

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Reply 4

Mutleybm1996

Original post by addylad

You are welcome to share the info you have and I will have a look. (PM if necessary.)

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Here

(edited 9 years ago)

Original post by addylad

You are welcome to share the info you have and I will have a look. (PM if necessary.)

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The Reynolds number at the higher temperatures gets really large(about 18000), I'm using Stokes law to calculate the viscoisties and i'm aware that it can only be used for Re<1, but i read somewhere that it can be used for laminar flow 0<Re<4000 as well? Could i get away with saying that it's only valid for Re<1, but can be used as a good approximation for these?