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Supersonic Wind Tunnel - Alternative?  
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Posted (8 years 4 months 1 week 6 days 22 hours ago) and read 4701 times:

Most supersonic wind tunnels conist of a chamber that is subject to high pressure and temperature such that when valves are opened, the initial mach is about 3-6 and drops to zero when the pressure dies out -- it doesn't last long. My school's tunnel takes 45 minutes to fill it up and we get 30 seconds of testing. All we can test is coin-sized copper wedges to take photos of the shockwave and use the measured pressures and tempeatures to figure out the Mach number.

It's lame IMO, and does a diservice to supersonic aerodynamics by teaching the students that, 'hey, there is no market for this stuff, this is just to infrom you' I think we need something that lasts longer than a minute, for one, that can allow for force-moment measurement of models.

I am reading from my text; "Mechanics of Thhermodynamics and Propulsion", 2nd ed, by Hill & Peterson 1992; chapter 9 talks about centrifugal (not centripital) compressors. On page 426, it says, and I quote without permission:

Quote:
In recent years designers have made great progess in raising the stage pressure ratio and in improving the efficiency of centrifugal compressors. Single-stage centrifugal compressors have been built with pressure ratios of 5:1 (and adiabatic efficiencies of 85%) and even 10:1 (with adiabatic efficiencies exceeding 80%).
These achievements have required major advances in both aerodynamic and mechanical design. Rotor tip speeds are now as high as 650 m/s, so that the velocity at te rotor exist is definitely supersonic; means have had to be found for decelerating this supersonic flow without excessive shock-induced or other losses.

This got me thinking. With proper design of the compressor and the motor to run it, could we create a supersonic wind tunnel that may last longer than a few minutes? Pressures may be high, but we could alter the temperature throughout the compression process so we don't alter the speeds. 650 m/s is almost Mach 2 at sealevel.


The meaning of life is curiosity; we were put on this planet to explore opportunities.
6 replies: All unread, jump to last
 
User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 1, posted (8 years 4 months 1 week 6 days 18 hours ago) and read 4667 times:

Quoting Lehpron (Thread starter):
With proper design of the compressor and the motor to run it, could we create a supersonic wind tunnel that may last longer than a few minutes?

The big problem is that traditionally you can to recover the kinetic head, so centrifugal compressors have a volute. If you want to keep the flow as fast as possible, you're goin to have to work out a way of channeling it without creating shocks. If you're going down this route, an axial compressor works much better.

And incidently, axial compressors are used, with a convergent-divergent nozzle, for supersonic windtunnels up to about Mach 2.


User currently offlineGrandTheftAero From United States of America, joined Nov 2003, 254 posts, RR: 5
Reply 2, posted (8 years 4 months 1 week 3 days 10 hours ago) and read 4569 times:

Quoting Lehpron (Thread starter):
It's lame IMO, and does a diservice to supersonic aerodynamics by teaching the students that, 'hey, there is no market for this stuff, this is just to infrom you' I think we need something that lasts longer than a minute, for one, that can allow for force-moment measurement of models.

Lame perhaps... but practical. It takes an ENORMOUS amounts of power to run continuous supersonic tunnels. Also, the speed of modern data acquisition systems usually makes it unnecessary to run wind tunnels continuously. It would be easy to take thousands of data points on a force-moment balance, pressure tap, total pressure rake, etc, within a few seconds.


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 3, posted (8 years 3 months 6 days 11 hours ago) and read 4336 times:

Quoting 777236ER (Reply 1):
And incidently, axial compressors are used, with a convergent-divergent nozzle, for supersonic windtunnels up to about Mach 2.



Quoting GrandTheftAero (Reply 2):
It takes an ENORMOUS amounts of power to run continuous supersonic tunnels

Can I figure this prospect alone makes the idea of going supersonic on a fan driven system alone, either impossible or quite difficult? What kind of RPM and/or horsepower requires that?

Quoting GrandTheftAero (Reply 2):
Also, the speed of modern data acquisition systems usually makes it unnecessary to run wind tunnels continuously. It would be easy to take thousands of data points on a force-moment balance, pressure tap, total pressure rake, etc, within a few seconds.

What about vibration from the initial burst of flow? In my mind, from a point where there is no net force exerted on a model then to suddenly open the valves, well I don't know the frequency, but only att he equilibrium can measurements be made. Longer than just a few seconds...?



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineBHMBAGLOCK From United States of America, joined Jul 2005, 2698 posts, RR: 5
Reply 4, posted (8 years 3 months 5 days 20 hours ago) and read 4287 times:

Don't whine! You get a lot less time from a shock tube or tunnel and they're a lot more work to prep for each shot as well. It doesn't change the fact that they are very useful tools.

45 minutes seems a bit long particularly for a small test section as you describe; 10-20 minutes would be more typical in my experience(RPI and Auburn tunnels FYI). Of course, in my experience we would rarely run more than 8-10 seconds; just enough time to snap a Schlieren and take any other data from a balance, pressure sensors, etc. Anything beyond this is gratuitous and more for fun than anything else. Nothing wrong with that as long as nobody gets hurt. Of course there's a lot more opportunities for goofing off and getting hurt in subsonic tunnels but we'll save that for another thread.

Is the limitation due to pump capacity or did the tunnel designer go cheap on air dryers? My experience has been that the dryer can often be the limiting factor after the first few runs of the day, particularly if the humidity is high. Designs with multiple dryers and the ability to "cook" one or more while using another really help a lot.

btw, continuous flow supersonic tunnels do exist. Nothing magic about them and really any type of compressor will do in theory. It takes a lot of power to drive it though and beyond that it will typically require chillers if it's a closed circuit design.



Where are all of my respected members going?
User currently offlineOly720man From United Kingdom, joined May 2004, 6843 posts, RR: 11
Reply 5, posted (8 years 3 months 5 days 4 hours ago) and read 4251 times:

Quoting BHMBAGLOCK (Reply 4):
btw, continuous flow supersonic tunnels do exist. Nothing magic about them and really any type of compressor will do in theory. It takes a lot of power to drive it though and beyond that it will typically require chillers if it's a closed circuit design.

Yep, and they're hellishly expensive to run.

http://www.cr.nps.gov/history/online_books/butowsky4/space4.htm

These are big, but 11ft x 11ft transonic or a 9-by 7-Foot Supersonic Tunnel (Mach 1.55 to 2.50), and an 8-by 7-Foot Supersonic Tunnel (Mach 2.45 to 3.45) have 134MW electic power to drive the air. If you scale on area you'll get some idea of power requirements. 1ft x 1ft continuous may be in the 100's of kW.

http://www.ara.co.uk/supersonic_wind_tunnel.htm

ARA have a smaller SST (0.69m*0.76m) that uses a 13000HP compressor - nearly 10MW.

To be honest there's enough analytical/experimental data out there for people not to need to do supersonic tests unless absolutely necessary. You can blame the 1950s and the cold war for that. At M5 and above where thermochemistry becomes important then tunnels or shock tubes are very useful. Otherwise it's high transonic where experiments need to be done.



wheat and dairy can screw up your brain
User currently offlineBHMBAGLOCK From United States of America, joined Jul 2005, 2698 posts, RR: 5
Reply 6, posted (8 years 3 months 5 days 3 hours ago) and read 4244 times:

Quoting Oly720man (Reply 5):
Yep, and they're hellishly expensive to run.

 checkmark 

Just the infrastructure to supply power at the required levels can easily cost millions.

The US also has a large high speed test complex in TN at AEDC run by a contractor for the USAF and its contractors.

Quoting Oly720man (Reply 5):
To be honest there's enough analytical/experimental data out there for people not to need to do supersonic tests unless absolutely necessary.

Absolutely true although small intermittent tunnels are great, relatively inexpensive teaching tools.

Quoting Oly720man (Reply 5):
At M5 and above where thermochemistry becomes important then tunnels or shock tubes are very useful.

Yes, although I think we are rapidly approaching the point where the errors inherent in tunnel corrections, model scaling, and the like will exceed the errors in the CFD models.

One of the more interesting shock tunnels was built by GE in the 60s to test the Apollo re-entry and was dismantled in the 70s after doing some preliminary space shuttle work. This was a true beast capable of M 100+ using highly compressed hydrogen and oxygen combustion to drive the initial shock. Last I heard, RPI was trying to raise money to put it back together after GE donated it around 1980 to the school.

Unfortunately, I'll bet nothing ever came of it as it would have been horrendously expensive as it would require many unusual(expensive) features in an installation, i.e. special building with one wall designed to fall out in case of a catastrophic failure to relieve pressure relatively harmlessly, remote, very remote, control room for safety, refrigerated gas storage facility, etc.

Beyond this, I'd be shocked if the city would have given them a permit. The only viable location was in the middle of the particle accelerator which was built on top of a hill in what was at the time a rural location. Pretty much surrounded by houses now.



Where are all of my respected members going?
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