Quote:
Originally Posted by ChrstphrR
Phil,
I'm curious why you had the test section open, as opposed to having it closed in with say, acrylic or plexiglas?
The mention that it's for flow visualization only... only begs the question, do you have pictures of you smoke testing your models too?
Furthermore, what would have to be added to do force measurements, so that Cd figures could be deduced?
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ChrstphrR,my first tunnel had a Plexiglas sidewall/access panel in to a closed test section.
It was a single-pass tunnel,exactly like the Wright Brothers used.
It had very little utility,as once the section is closed you are limited in how you can interact with the model inside.
With the open test area I'm free to move the tuft wand anywhere I choose,even change models without turning off the power.
Tunnel 'blockage' issues are not as prevalent as the flow is not confined.
A commercial smoke generator,in 1990 dollars are $1,100 ( US) and all the 'air' is wasted with the smoke and cannot be recirculated as it will soon foul the vision as food color will in a water tunnel.
Alan Pope recommends a 20-25 hp 17-foot length tunnel as a 'minimum' for a classroom,with a minimum 100 mph airspeed.I can't afford to operate anything like that.
Model scale verisimilitude for Reynolds number dictates a jet velocity @:
* full-scale 20-mph + air
* 1/2 scale needs 40 mph +
* 1/4 scale ------ 80 mph +
* 1/8 ------------ 160 mph +
* 1/16 ----------- 320 mph +
and so on
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The easiest way to do pure drag measurements is to suspend the model just above the tunnel floor on a wire-hung,pendulum ground board of streamlined construction which has been configured for proper boundary layer effects at the 'ground',and independently tested to determine its own aerodynamic 'tare' drag.
Another wire connects to the pendulum running over a low drag pulley,down to a scale or weight bucket.
When a model is tested on the pendulum,the force is 'balanced' and measured,then the 'tare' is subtracted out,leaving the drag force.
Knowing the frontal area of the model allows the Cd to be determined
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If you want to measure lift and yaw,a triple balance of precision knife edges must be constructed,or of strain gauges.Texas Tech has a "STING" out of the tail of a military jet which has three-axis strain gauges embedded within it.My memory remembers a figure like $66,000 for that piece of equipment.