SpudFiles

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?

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.

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)

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.

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.

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.

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.

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.

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.

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...

Um you could probably get away with regular window screen

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.