Reynolds number
Reynolds number
Guest
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high speed= more k.e. to boundary layer, and later b.l. seperation, thus a higher Cl max
decreased density (at alt) = less energy = lower Cl max
as for viscosity... I'll learn about it when they fit a viscosity indicator in front of me
Diving for cover already!
decreased density (at alt) = less energy = lower Cl max
as for viscosity... I'll learn about it when they fit a viscosity indicator in front of me
Diving for cover already!
Guest
Posts: n/a
Reynolds number is just a reference number used to compare the behaviour of a fluid in different conditions, for example:
If you test a model in a wind tunnel, you won't receive exactly the same results as on a life-size aircraft, unless the Reynolds numbers of the two situations are rigged to be the same.
The actual number depends on: fluid (air) density, (air)speed, a reference length (explained in a minute) and the viscosity of the fluid (air).
As you will have seen from watching TV programmes whose special effects use models, water always looks 'wrong' - waves the wrong shape, droplets too big etc. So the actual SCALE of the subject fluid makes a difference. To make the water look 'right' you should change the Reynolds number of the 'model' water (by increasing the density, decreasing the viscosity etc.) so that it is the same as the REAL situation - this is where the reference length comes in.
With me so far?
Say you use the chord of a wing on the wind-tunnel model you are testing as your reference 'length', it is the scale between this and the SAME distance on the real wing which tells you how much to change the other factors in the reynolds number by to get results that are equivalent to real life
eg. if your model is 1/10th size, you could increase the density by 10 (cryogenic cooling, or a different fluid such as water), or double the density and increase the speed by 5 etc. - in fact any combination of the factors as long as the final Re nos. are the same between model and Life-size aeroplane.
In general terms, it is a number used for aerodynamic research and experimemts etc. to allow fair comparison of fluid dynamic behaviour.
Clear as mud?
Took me while too, and I don't know if I've explained it v. well! I'll try and simplify answers to any questions...
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So what's my left hand for?
If you test a model in a wind tunnel, you won't receive exactly the same results as on a life-size aircraft, unless the Reynolds numbers of the two situations are rigged to be the same.
The actual number depends on: fluid (air) density, (air)speed, a reference length (explained in a minute) and the viscosity of the fluid (air).
As you will have seen from watching TV programmes whose special effects use models, water always looks 'wrong' - waves the wrong shape, droplets too big etc. So the actual SCALE of the subject fluid makes a difference. To make the water look 'right' you should change the Reynolds number of the 'model' water (by increasing the density, decreasing the viscosity etc.) so that it is the same as the REAL situation - this is where the reference length comes in.
With me so far?
Say you use the chord of a wing on the wind-tunnel model you are testing as your reference 'length', it is the scale between this and the SAME distance on the real wing which tells you how much to change the other factors in the reynolds number by to get results that are equivalent to real life
eg. if your model is 1/10th size, you could increase the density by 10 (cryogenic cooling, or a different fluid such as water), or double the density and increase the speed by 5 etc. - in fact any combination of the factors as long as the final Re nos. are the same between model and Life-size aeroplane.
In general terms, it is a number used for aerodynamic research and experimemts etc. to allow fair comparison of fluid dynamic behaviour.
Clear as mud?
Took me while too, and I don't know if I've explained it v. well! I'll try and simplify answers to any questions...
------------------
So what's my left hand for?
