reynolds numbers

[flybike]

Sorry if this is a dumb question, but could someone please explain reynolds numbers to me? Or a link to a good webpage? I know that it has something to do with the size of the air molecules remaining constant as the airplane changes size, but how can you apply this? What are numbers based on? Thanks, Hans

[davidfee]

Hi Hans,
You're right about the concept of the Reynolds number. For our purposes, basically it's a parameter used to help determine how the airflow will behave around an object. It, as you mentioned, depends on the relative size of the object, but also its speed. Here's a little more information.

The following is from Martin Hepperle's web page:
Reynolds, Osborne (1842-1912)

Worked for 37 years as a professor of engineering at the university of Manchester, Great Britain. In 1883 his experiments lead him to the definition of a dimensionless parameter, which indicated transition from laminar to turbulent flow in pipes. This parameter was later known as Reynolds number.
For the flow inside a tube of diameter l he determined the critical value of this parameter to be 2300.

Re= (Vo*l)/v

l=length
Vo=velocity
v=kinematic viscosity


Reynold's number is a very important parameter in fluid dynamics. Flows at the same Reynolds number are similar, which make it possible to use smaller models (length l) for tests at higher velocities (velocity v) to achieve the same Reynolds number as the full scale airplane.


http://beadec1.ea.bs.dlr.de/Airfoils/index.htm

A more specific derivation from the Team Ariane website:
http://www.delago.de/ariane/EHome.htm
Re= (t*v*p)/n

t=chord
v=velocity
p=air density
n=dynamical viscosity

We use Reynolds numbers to compare different airfoils at the same speed/size, or the same airfoil at different sizes/speeds... etc.

Hope this is useful!
David

[Sparky Paul]

Do a search here for "reynolds".. Lots of discussion..

[e-sailpilot86]

I saw this post thinking about the same question....went to search for reynolds...and got a mix of Reynold's #'s and Reynolds Wrap :(

Which Discussion are they in?

[e-sailpilot86]

I take it back... with a bit of a throbbing brain trying to comprehend it, this stuff is a bit like latin to me..:p I'm starting to understand this stuff

http://www.rcgroups.com/forums/showthread.php?s=&threadid=48955&highlight=Reynolds+numbers

[davidfee]

You're most likely to find discussions of Reynolds number in the Modeling Science and Sailplane discussion sections.

But because airspeeds are not constant, Reynolds numers are usually just approximated for a couple different speeds (say full speed and glide/landing).

Just pick a Re and plug it in... that's how it goes. ;)

Have fun!
-David

[winans]

In a physical sense, the Reynolds number expresses the ratio of inertial forces to viscous forces acting on a body due to its characteristic size, its speed, and the media through which it is passing.

Scott