Of Wide-Angle Diffusers and Kitchen Equipment

The first component that will be designed and built is likely to be the wide-angle diffuser. Now that I am resolve to building a blower tunnel (with the option of attaching a smoke unit). I'm ready to start construction etc. While reading up on wide-angle diffusers, I noticed that we have (as of the publishing of the 1977 paper I'm reading) absolutely no idea how they work -- all our techniques are emperical.

Accepting the fact that I'm going to have to use emperical methods, I've taken a look at some historical data and decided that a value of 25-degrees seems pretty reasonable for 2-theta (Mehta, 69). From there, I'll have to use emperical methods.

On an unrelated note: I'll soon be purchasing a centrifugal blower from lowes or something for testing and development, and an airspeed meter.

REFERENCES:
1. R.D. Mehta, THE AERODYNAMIC DESIGN OF BLOWER TUNNELS WITH WIDE-ANGLE DIFFUSERS; Progressive Aerospace Science, 1977, Vol. 18, pp. 59-120.

Welcome Back

The old is gone and the new is in. Happily settled into the new job, I am now back to working on the wind tunnel.

After much though, I've decided to drop the ambition-level of the project from a closed-circuit wind tunnel to an open-circuit tunnel. Doing so will, eventually, afford me the flexibility to turn it into a smoke tunnel, and, in the mean time, it allows me to make the test-section larger.

Back to the proverbial drawing board.

Compartmentalized

The debate of whether to make an open or closed-circuit wind tunnel has come to a close. The winner is: both? In the interests of convenience, I'm going to make the various components of the tunnel highly separable. I have a collar mechanism in mind, and will try to put the information up on the website, sometime tomorrow. For the sake of making the engineering easy, I'm going to try to have two primary sizes: test-section, pre-constriction (also known as small and large).

If you model the wind tunnel as a test section, followed by a really long diffuser, then a contraction, then the axial fan would, usually, go somewhere in the middle of the diffuser. In my case, diffusers are complicated to build, and so I'm going to try to make the diffuser fit between the test-section and the second corner. Doing so would allow me to hide all of the intermediate sizes in three sections (first diffuser section, first corner, second diffuser section). All other collars would be either small or large.

I'll start designing tomorrow, and start stubbing out a python file to perform the pressure-drop estimates, etc., and we'll see how feasible this plan is.

Design Order

I have, now, had a chance to read a decent amount in the wind-tunnel design book (~50 pages). The equations and methods used in the wind-tunnel book are starting to click with those employed in the introduction-to-flight book, cementing my general aerodynamic understanding. My parents and brother will be arriving in just over three weeks, and I am hoping to have a component designed and ready for construction by the time they all arrive -- thus the design-order question.

The test-section is seeming to be the most logical thing to start with, as, for the initial version, I won't require the ability to take precise measurements (provided I have, at the very least, a good idea of how to amend that shortcoming later). The next component that I would foresee us working on would be the diffuser, immediately proceeding the test-section, as doing so does not require knowledge of the target size.

Partners In Crime

Two of my swing-dancing buddies might be helping out, once construction begins. Autumn has done a great deal of wood-working in the past, and Nick is a fabricator by trade.

Moving On

It does not appear as though construction of a high-reynolds-number wind-tunnel is going to happen. The wind-tunnel testing book states pretty clearly that small tunnels like this one will not reach reynolds numbers that even approach reality. This project is a bit of a hail-mary as-is, so I shouldn't be trying to push the edges of too many envelopes at once.

Brick Walls - Theoretical Limit

So, apparently, most of the data of interest that can be collected from a wind-tunnel is going to be governed by the Reynolds number, not the freestream speed. Consequently, I need to design my wind tunnel to maximize the Reynolds number. Not only does the tunnel need to be designed to maximize the Reynolds number, the model does, as well. The Reynolds number (Re) is proportional to the length along the aerodynamic body. If your aerodynamic body is 1/20th the size of the 'real thing', then you have to up the other components of the Re by a factor of 20 to compensate...That puts the theoretical max for my toy truck's simulated speed at about 15 mi/hr...

Let the Posting Begin

I ran my first 'experiment' last night with my first 'wind-tunnel'. The 'test-section' was the size of the room, and the axial fan used, maybe, 1/8 hp. It was very 'scientific'.