wingtip vortices
 

i am trying to do a project on the aerodynamics of small airfoils, especially wingtip vortices. i will be testing in a water tank, and at slow speed, so will have reynolds numbers of around 10^3. does anyone know anything about?
please help
thanks

wingtip vortices
 

Many years ago (late 1940's) there was an article in one of the model magazines about small (6 or 8 inch wing span) model gliders that had a slight bouyancy and were released at the bottom of a swimming pool (upside down) to glide slowly to the top of the water. Small pellets of potasium permanganate were attached to each wing tip. The bright purplish dye left spiral tracks of the tip vortices that was easy to see. Potasium permanganate is quite poisonous and should be handled with skin protection.

tank test
 

what an elegant experimental configuration. Nice thinking.

wingtip vortices
 

I've been thinking about it and the models had to be trimmed for center of bouyancy (CB) instead of center of gravity. This usually meant adding weight to the tail because the volume behind the desired location of the CB was greater than the volume ahead of the CB. When the model was trimmed to glide at a high angle of attack (slow glide) the vortices were bigger than when trimmed at a lower angle of attack (fast glide). By putting a third pellet at the center of the wing, the downwash angle could also be directly observed.

If I remember correctly, the density of water is about 700 times greater than the density of air in a standard atmosphere. That would mean that the speed in water times the chord in water would be about 700 times greater than for another chord and speed in air for the same reynolds number to apply. If, for example, the chord in air were ten times the chord in water, then the speed in water would have to be 1/70th of the speed in air for the same reynolds number in the two cases.

wingtip vortices
 

ollie: i think you mixed up something...im pretty sure reynolds numbers decrease when the density increases
oh well

can anybody who knows anything even remotely related to this please say something
any help i can get would be great

thanks
mike

wingtip vortices
 

Reynolds number is equal to density times length times velocity divided by viscosity. In my previous post I erred by not taking into acount the difference in viscosity between air and water and by getting the ratio of densities wrong. Air, in a standard atmosphere, has a viscosity about 50 times smaller than water. Water is about 800 times denser than air. The velocity-length products will be in the ratio of about 16:1 from air to water at the same reynolds number. That is, the product of length times velocity in air will be about 16 times greater than in water for the same reynolds number.

I hope I got it right this time.

wingtip vortices
 

Mike,

The general form of Reynolds Number is -

R = kcv

k = constant that includes fluid density
c = "chord length"
v = velocity

And as a little bit of advice, listen carefully to what Ollie says. Of all the contributors on this board he is one who would thoroughly deserve a 6 star rating (and that is out of 5).

wingtip vortices
 

There is one thing (or more--G!) that I have never understood about all these calculations on Reynolds numbers, etc. They keep comparing gases (air) and liquids (water) as if they behave in the same way. Gases, such as air, are very compressible. Liquids, such as water, are for all practical purposes, totally incompressible. So why are they being treated as behaving in the same way? Please enlighten me.
Puff

wingtip vortices
 

As an object nears the speed of sound the air is constrained by its inability to get out of its own way because the pressure wave can't travel faster than the speed of sound. Well below the speed of sound, the air is not constrained and, the air flows in response to the pressure differences rather than compressing. When air is compressed below the speed of sound the air is constrained from flowing by some kind of a container. The atmosphere is contained in a gravity well created by the gravitational field of the earth. This gravity well container is open on top rather than being a closed container so it does not constrain the subsonic air from flowing rather than being compressed.

All material objects are compressible if the pressure is great enough. Neutron stars are material that is compressed so that the electrons are squeezed into protons, thus creating neutrons as atoms collapse under the extreme pressure. If the pressure is even higher, matter collapes even farther into a black hole disapearing from our ability to see waht is going on because even light can't escape the gravitational field. Fortunately, these extremely high pressures aren't close enough to affect our everyday life much.

wingtip vortices
 

Just to quantify what Ollie said...

Compressibility effects refer to the changes in the density of the fluid as is flows around the airfoil or whatever. In air, these density changes are tiny at low speeds. The equation for the change in the density "rho" is...

delta(rho)/rho = -M^2 * delta(V)/V

i.e. the fractional change in density is equal to
the negated fractional change in velocity times the Mach number squared. This equation is strictly valid only for small fractional velocity changes, but it gets the point across. Say an RC plane is flying at 75 mph, or M = 0.1 . As the air goes over the top of the airfoil, it might increase its speed by 20%, so delta(V)/V = 0.2 . The density change at the same point over the top of the airfoil is

delta(rho)/rho = -0.1^2 * 0.2 = -0.002

or -0.2%. i.e. negligible. So in this case the air can be assumed to be incompressible just like water.



PS. This post incorrectly ended up in the adjacent thread after I got diverted to the login window. Weird.

wingtip vortices
 

Ollie and drela--

Both of your replies DO make sense. Thanks for the excellent clarification!
Puff