Reynold´s Number

[Capt Wannabe]

Could somebody please explain in fairly simple terms what a Reynold´s Number is?
Thanks in advance,

CW

[Dan Winterland]

It is a fudge factor to allow models to be used in wind tunnels for aerodynamic research. The idea being by factoring your results by the RN, you get the aerodynamic characteristics of the full sized aircraft.

It shouldn't be of too much interest to pilots, but unfortunately some aerodynamics books have got a bit hung up on them, probably because the author is on a mission to prove how much more they know than the reader IMHO.

[Capt Wannabe]

Yes I gathered it was a bit of a fiddle factor, but we are being taught it and need to know about it for the ATPL ! Unfortunately a few of us can´t quite get a grip on it.

[Tinstaafl]

My imperfect understanding:

As bodies get smaller, the frictional effects as they move through fluids becomes increasingly more significant in affecting the interaction between the bodies & the fluids.

For a example, to a microscopic animal (say a flagellate or similar), moving through water would seem more as if it was trying to force its way through treacle.

A reynold's number is a way of accounting for the proportional differences caused by changes in size & speed. Once the proportional effect is known then it can be factored for when extrapolating the effects seen in one body, compared to a different body eg a model of an aircraft vs the real full size aircraft.

Genghis, where are you when I need you? :)

[G-OOFY]

If you can get a hold of "Engineering Fluid Mechanics" Prof I Clayton (Hodder and Stoughton 93) from a Library there is a small chapter on Renolds number which explains it in simple terms. Although it is only really used by Mechanical, Marine and Aerodynamic Engineers as a fudge factor it is extreamly useful in design.

This is because models do not function in scale with there proportions. ie a one fifth scale model in a wind tunnel should be exposed to an fluid flow less than one fifth of the original vehicle. speed of a/c = 100m/s in the wind tunnel the model ought to be subjected to less than 20m/s. (factor Rennolds number in here ie 20 times renolds number = correct m/s. This is from memory when working at Rolls and Bae during my summer holidays at Uni.

I don't really think it ought to be on a syllabus for Pilots..

Renolds is used because molecoles are relativly bigger to a model than the real thing and therefore they are 'sticker' ie less boundary layer for flow separation to occur. Molecules are effectivly bigger and do not flow as easily around a body.

hope this helps

Incidentaly is there any thermodynamics in the new JAR syllabus?

[This message has been edited by G-OOFY (edited 10 September 2000).]

[Dan Winterland]

Sorry, but you did ask for a simple answer. I qualify for giving simple answers.

But I did say that RN should be of litttle intrest to pilots - it doesn't suprise me in the least that it is being examined under the JAR syllabus!

[Oz_Pilot]

It's the ratio of viscous to inertial forces on an aerodynamic body.

On a small body, the inertia of the air will be more significant than on a large body. Conversely, a large body will be affected more by viscous forces (i.e. skin drag) than a small body.

Calculating Reynolds number gives a means of adjusting scale data to give some sort of meaningful result from scale testing. Another way around it is to use a different gas in a scale test to give the same Reynolds number as the full-scale body.

Have I confused anyone??

[WOK]

This is only explanation I ever understood. ready.....?

YOU CAN'T SCALE THE AIR DOWN

Not scientific I know, but non-dimensional constructs I find hard to visualise and this helped.

If the sensible and educated responses above seemed a bit abstract they may be a bit more digestible with this in mind. Or not.

Personally I put reynolds no.s in the same category as long division. Don't matter how much effort I put in I just can't do it.

[foghorn]

Wow, takes me back to my old Chemical Engineering days.

Buckingham's Pi Theorum suggests that a certain system can be described by 'dimensionless groups' and that to create a correct model of that system the value of these groups must be the same in the model as the modelled system. Reynolds number is one of many dimensionless groups (Mach number is another one).

These dimensionless groups are technically not in themselves fudge factors as suggested (they are not false numbers pulled out of the air to make a correlation agree), however they are often used in engineering correlations because it is then easy to pull out meaningful quantities for any system.

Re = fluid speed x scale x density
-----------------------------
viscosity

where scale is a chosen dimension in m e.g. flow width, pipe diameter etc.

Wow, I've impressed myself that I remember all that junk - I standby for my memory to be proved wrong :)

Oops, looking at the first post, too much detail me thinks... sorry http://www.pprune.org/ubb/NonCGI/redface.gif

[This message has been edited by foghorn (edited 14 September 2000).]