Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 76 users online :: 4 registered, 0 hidden and 72 guests


Most users ever online was 155 on Mon Aug 15, 2016 1:40 am

Registered users: Bing [Bot], Google [Bot], MSNbot Media, Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

Wind Tunnel

All non-spudgun related discussion goes here such as projects, theories, serious questions, etc. All "off-topic" posts (aka useless posting, determined by moderators) will be removed.
Sponsored 
  • Author
    Message

Wind Tunnel

Unread postAuthor: boilingleadbath » Fri Feb 22, 2008 6:51 pm

Alright, I've been conscripted to complete my vocational-technology instructor's wind tunnel. Some questions follow.

1) How should I design the collimator screen?

Here's some design data on the tunnel:
Tunnel is about 50" long; that's 8" of expander, 33" to the test object, and 9" to the end of the tunnel. About.
Tunnel is roughly square; 11"x12"
Fan (1/3rd HP squirrel cage) pushes air into the tunnel through an asymmetric expander
Wind speed is about 15 mph in the center of the tunnel.
Instructor is less interested in accuracy than I am (right now our instrumentation reads in .1g increments... for measuring drag and lift on small objects in 15mph winds!)

2) It's not possible to control the speed of a standard (syncronous?) AC motor by installing a dimmer switch on the circuit, is it?

3) Is there a equation for the Reynolds number over a hemisphere-capped cylinder?
  • 0

User avatar
boilingleadbath
Moderator
Moderator
 
Posts: 1642
Joined: Sat Mar 12, 2005 10:35 pm
Location: Pennsylvania, USA
Reputation: 0

Unread postAuthor: TurboSuper » Fri Feb 22, 2008 6:59 pm

How much current does your motor draw?

If the Triac in the dimmer switch can pass that much current, you shouldn't have a problem...

There's also the option of a variable transformer.
  • 0

"If at first you dont succeed, then skydiving is not for you" - Darwin Awards

TurboSuper
Brigadier General
Brigadier General
 
Posts: 986
Joined: Thu Jun 16, 2005 1:44 pm
Reputation: 0

Unread postAuthor: Intel Xeon » Fri Feb 22, 2008 7:26 pm

Well, for #2, No. Ac motors should run at a constant 3600rpm, I believe. (Unless you're not in the USA, where the AC is pulsed at 60hz)
  • 0


Intel Xeon
Sergeant
Sergeant
 
Posts: 84
Joined: Sat Mar 17, 2007 9:43 pm
Location: Philly, PA. (home's in Worcester,MA)
Reputation: 0

Unread postAuthor: boilingleadbath » Fri Feb 22, 2008 8:42 pm

Yeah, that was my understanding (on the motors) too.

Is there any way to change the frequency of the AC?
Run it through a rectifier and than a modified inverter?

I'm sure I'll lose some torque - but this is a 1/3rd HP motor blowing air through a 4' duct; I have some to spare.

********

It's an AC motor - current draw is load dependent. So the dimmer switch should be able to handle it. Not that it's going to matter.
  • 0

User avatar
boilingleadbath
Moderator
Moderator
 
Posts: 1642
Joined: Sat Mar 12, 2005 10:35 pm
Location: Pennsylvania, USA
Reputation: 0

Unread postAuthor: jrrdw » Fri Feb 22, 2008 9:51 pm

boilingleadbath wrote:Yeah, that was my understanding (on the motors) too.

Is there any way to change the frequency of the AC?
Run it through a rectifier and than a modified inverter?

I'm sure I'll lose some torque - but this is a 1/3rd HP motor blowing air through a 4' duct; I have some to spare.

********

It's an AC motor - current draw is load dependent. So the dimmer switch should be able to handle it. Not that it's going to matter.


You would have to start the motor at full power or dam close to it. I beleave all motors need to draw a higher power at start up then can run at less power. My aircompressor needs I think 16 amps to start, then like 10 to run.
  • 0

When life gives you lemons...throw them back they suck!
User avatar
jrrdw
Donating Moderator
Donating Moderator
 
Posts: 6538
Joined: Wed Nov 16, 2005 5:11 pm
Location: Maryland
Country: United States (us)
Reputation: 25

Unread postAuthor: BigGrib » Sat Feb 23, 2008 5:06 am

You said it's a 1/3 HP motor, is it 120V 60hz single phase 3 phase what?? if it's a single phase you should be able to put a variable rheostat or potentiometer inline with the power supply after the main power switch. If it's single phase as well the motor should not need to start at full power, although it is good to usually start a motor at full power and then throttle it down, you can still start it at no power and throttle it up. like I said you should be able to put it in after the main power switch so that you flip the switch and then throttle it up from zero. If you were to use a sliding dimmer switch you can get at lowes or home depot for lighting if it's rated correctly for the right amp load, you should be able to get away with that for your variable speed.
  • 0

Yea, that's definitely going to get you at least a tazer.

<a href="http://www.youtube.com/watch?v=AkMkGOpAF4s">DONT TAZE ME BRO.. DONT TAZE ME... AHHHH</a>
facebook.com/biggrib
User avatar
BigGrib
Colonel
Colonel
 
Posts: 652
Joined: Mon Oct 01, 2007 11:43 pm
Location: TriCities, WA
Reputation: 0

Unread postAuthor: Jeeperforlife » Sat Feb 23, 2008 11:14 am

If the motor has a capacitor on it then it will mess up when you lower the voltage. that is why it pulls so much current right off the bat. If you want a infitly variable speed just go with a brushed style fan, like drills, vacuum sweepers, most grinders ect. look at a celling fan they are 120 volt 60hz and are easy to regulate. They do make variable brushless motors but they have different windings and several different wires going in to put power to.
I built a generator out of a old water pump put a few years ago that worked well enough to power drills and a compressor wile my nephew was building a tree house. It was a brushless style, wile working on it and looking up ways to do it I learned this by accident.
  • 0

User avatar
Jeeperforlife
Staff Sergeant
Staff Sergeant
 
Posts: 108
Joined: Tue Jan 15, 2008 4:51 pm
Country: United States (us)
Reputation: 0

Sponsored

Sponsor
 


Unread postAuthor: Spitfire » Sat Feb 23, 2008 1:22 pm

Here in South Africa, you can put a dimmer onto any fan, our houses power supply is 240V and 60Hz.
If you want higher wind speeds, make the tunnel into a venturi form. So you will get your 15mph wind in the front, then as you move the object more to the center of the venturi the wind speed increases.

Theres one close to us that has that effect. at the intake winds are 100km/h and in the middle 315km/h. The fan that runs it sucks the air into the wind tunnel.
  • 0

User avatar
Spitfire
2nd Lieutenant
2nd Lieutenant
 
Posts: 202
Joined: Fri Nov 16, 2007 5:34 am
Location: South Africa
Reputation: 0

Unread postAuthor: starman » Sat Feb 23, 2008 2:00 pm

Have you thought about damming the air flow with vent louvers before it hits the chamber? The louvers will have to be fairly substantial to bear up to the flow but it shouldn't be that difficult to do with a small squirrel cage. This would allow fully variable flow speed without having to fool with modding the motor voltage or the motor.

Also, a good wind tunnel should be turbulence free meaning the air should be blowing straight through and not twisting and tumbling inside the chamber. You can build a "straightener" (or your collimator) by stacking your 11" x 12" opening FULL of 12" - 18" (longer the better but all identical) sections of 3/4" pvc pipe lengthwise with the proposed flow direction of the air. This may take 120 - 130 PVC sections for your opening so building a box to hold these would be necessary but easy. Just direct the fan's airflow into one side of this "box of holes" and the air will exit the other side reasonably straight. Don't worry about the gaps between the pipes, those spaces will straighten the airflow too.

You'll need to glue the PVC pipes together for stability of the unit as a whole but keep the glue to a minimum...don't glom up the air flow or introduce irregularities in the spaces between the pipes. Keep the ends of the pipe flat...don't taper or flare the openings, particularly at the exit.

I haven't performed any air flow physics on the above recommendation so I can't give you the math behind it. If you'll need all of that, I can't help you.
  • 0

User avatar
starman
Donating Moderator
Donating Moderator
 
Posts: 3041
Joined: Wed Dec 12, 2007 12:45 am
Location: Simpsonville, SC
Reputation: 0

Unread postAuthor: boilingleadbath » Sat Feb 23, 2008 2:51 pm

My understanding is that many wind tunnels combine a fine mesh with the "straitener".

What type of netting fineness would be sufficient for a wind tunnel of this scale, and how much would you expect that to decrease the windspeed?

Sorry I can't give you more info on the motor until I get to see it again on monday...
  • 0

User avatar
boilingleadbath
Moderator
Moderator
 
Posts: 1642
Joined: Sat Mar 12, 2005 10:35 pm
Location: Pennsylvania, USA
Reputation: 0

Unread postAuthor: BigGrib » Sat Feb 23, 2008 3:01 pm

Um you could probably get away with regular window screen
  • 0

Yea, that's definitely going to get you at least a tazer.

<a href="http://www.youtube.com/watch?v=AkMkGOpAF4s">DONT TAZE ME BRO.. DONT TAZE ME... AHHHH</a>
facebook.com/biggrib
User avatar
BigGrib
Colonel
Colonel
 
Posts: 652
Joined: Mon Oct 01, 2007 11:43 pm
Location: TriCities, WA
Reputation: 0

Unread postAuthor: starman » Sat Feb 23, 2008 3:09 pm

boilingleadbath wrote:My understanding is that many wind tunnels combine a fine mesh with the "straitener".

What type of netting fineness would be sufficient for a wind tunnel of this scale, and how much would you expect that to decrease the windspeed?


I can't predict that being ignorant of the mesh and its purpose...possibly as a diffuser of sorts to more even out the flow. You may have to experiment with it some.
  • 0

User avatar
starman
Donating Moderator
Donating Moderator
 
Posts: 3041
Joined: Wed Dec 12, 2007 12:45 am
Location: Simpsonville, SC
Reputation: 0

Return to Non-Spudgun Related Discussion

Who is online

Registered users: Bing [Bot], Google [Bot], MSNbot Media, Yahoo [Bot]

Reputation System ©'