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Reynolds Number

PostPosted: Wed Dec 16, 2009 12:54 am
by Jon
As a question was asked at the weekend as to what Reynolds number is, here is a brief answer.

Reynolds number relates inertial forces to viscous forces in a fluid - i.e. the ratio of the momentum of the air to how 'sticky' the air is. At low Reynolds numbers (less than 500,000 for a flat plate wing section) the air flow is more affected by how sticky it is and so is more stable - laminar flow. At higher numbers the momentum of the air is more dominant, and so the air becomes more unstable - turbulent.

The Reynolds number can be affected by the velocity, density and viscosity of the air of the air, and the length of the path of the air over the object. Increasing velocity, density or length increases the Reynolds number, while increasing the viscosity decreases the number.

Reynolds number is particularly useful in scaling parts for testing as, if a model aircraft in a wind tunnel experiences the same Reynolds number as the real aircraft the air flows will be equivalent, so you don't need to build a tunnel for a full size aircraft and pressurise it to 35,000ft.

A piece of coursework I am currently writing uses Reynolds number to allow the conditions in the lining of a jet engine combustion chamber to be represented by a perspex model at lower temperatures and air speeds - cheaper, easier to take readings and safer than using the 'real' thing.

Feel free to improve this if you want Alex.

Re: Reynolds Number

PostPosted: Sun Dec 12, 2010 5:32 pm
by baldrick
That's one of the better suggestions as to what are Reynolds numbers well done.

So it's the Full cat test for you next week then

Re: Reynolds Number

PostPosted: Fri Jan 21, 2011 7:51 pm
by LAK 12
Baldrick

I thought the reynolds number was the relationship expressesd as a ratio of how many fags and cups of tea he could get through whist doing the CoA on the Bejhave