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Craig Robbins September 4, 1998 12:41

coanda effect
To whom it may concern,

I'm a 4th year medical student at the University of Miami School of Medicine in Miami, Florida. Yesterday,while performing a spinal tap (placing a long needle through the spine to obtain fluid), my attending physician commented on the fact that the fluid ran down the side of the needle before dropping into the collection container (analagous to water running down the side of a pitcher while pouring into a cup). I said that that was an example of the coanda effect. He stared blankly and said "the what?"

After that preamble, I was hoping you might be able to direct me to a "layman's" site on the internet whereby I can find some information on the coanda effect. By the way, was I correct?

Jonas Larsson September 4, 1998 15:49

Re: coanda effect
The effect you saw is not what is usually called the coanda effect. The coanda effect is the fact that a jet tends to attach and follow a surface. This is a pure fluid dynamics effect related to the low pressure region inside the fast moving jet. Speaking in simple terms this low pressure region "sucks the jet to the wall" (or if the wall is flexible, sucks the wall to the jet - try blowing past a flexible piece of paper and you'll see it clearly).

The effect you saw - that liquid tends to attach to, and in this case follow, a solid surface, is a molecular effect. It has to to with molecular forces between the solid and the liquid molecules. The same forces are also responsible for the capillary effect (is it called that in english? I'm not sure, anyway, it is the phenomena that liquid can be "sucked up" in a thin tube). You can also see these forces if you look closely at the surface of the water in a glass - you'll see that the water surface is slightly higher close to the glass wall as if the glass attracts the fluid.

I hope I'm making sence. Can't give you a web page I'm afraid.

R Sukumar September 4, 1998 16:49

Re: coanda effect

The terminology you were searching for was "Surface Tension".

An interesting real application of the Coanda effect was its use by the Russian Antonov aircraft bureau in designing the AN-32(turboprop) and AN-72(jet) transports. The engines are mounted high over the wing and the nozzle exhaust is directed over the wing surfaces and flaps to drastically improve STOL (lift) performance.

Craig look for any basic fluid dynamics textbook in your library.

Pavel September 7, 1998 04:25

Re: coanda effect
This is not Coanda effect, this is all about surface tension (like capilarity). But if you want to know more about Coanda effect look at these pages: - explanation with picture - really interesting pages about utilization of Coanda effect These pages are about the inventor Henri Coanda.

Jonas Larsson September 8, 1998 13:42

Re: coanda effect (micro gas-turbines)
Sounds like an interesting design... reminds me of a seminar I attended a few years ago. Epstein, Greitzer, Waitz and a couple of other Professors from the MIT Gas Turbine Lab presented a very fascinating project. They were working on designing a micro gas-turbine the size of a shirt button! These small gas-turbines would be micro-machined in silicon I think. Epstein kept assuring it was indeed a very serious project - it is a bit difficult to believe if you haven't heard about it. Apparantly at that time they already had a working combustor (not a very trivial thing to design in that size I guess).

One of the applications they were speculating about was to distribute hundreds of these micro gas-turbines on the wings of an airplane and thereby increase lift and inject energy just where it is needed... fuel supply must be a bit of a problem I guess. They had a lot of other cool examples of how these micro gas-turbines could be used. I think the main application was as fuel-cells replaceing batteries.

I haven't seen any news about these micro gas-turbines since that seminar... I'm still waiting for the revolution. I wonder if they are still working on them at the gas turbine lab. Anyone have any more recent insight into this project?

R.D.Prabhu September 8, 1998 23:21

Re: coanda effect (micro gas-turbines)

At microscales the conventional priciples of rotating turbomachinery may not be valid. (Ref: Chi-Ming Ho's review in Ann.Rev.flu.Mechanics 1998). In fact the very concept of 'friction' is to be redefined on these scales. But there is this interesting idea of viscous-pump being tried now a days on microscales, with a rotating cylinder placed in the 'pump' region of a fluid. Seems a net flow of 10% of the surface speed of the rotating cylinder was observed for small reynolds numbers ( 0.1 -10)!! This is the work of Gad-el-Hak's group at Univ.Notre Dame Indiana , US. I guess we are in for lot more surprises from the micro-world!!



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