Intro on Lab

” All models are wrong; some models are usefull”

-George Box

Learning about concepts in class can only take you so far. They present the world, often in simplified terms, as a totally understandable place governed by simple, knowable equations. They say things like, ignore viscous drag, assume acceleration due to gravity is equal to 9.81 m/s^2, assume spherical cow. These models are good for analysis, but they don’t represent the world as it truly is. Part of a proper Engineering/science/math education is understanding under what conditions these models and break down. Labs are often where students get to explore the limits of the models by conducting “experiments”, under controlled conditions.

Flow Visualization Lab

The final lab I had to take in my undergraduate aerospace degree was an Aerospace Concepts lab, which I’ll just refer to as Aero Lab. The final “experiment” in my Aero Lab was flow visualization. In this lab we set up a tube, and a NACA 0012 airfoil in a water channel. Picture a wind tunnel but filled instead with water. Once setup, we started up the pumps to establish the flow and let it run for a minute or two to eliminate transient behaviors.

Unfortunately the flowing water, like air, is optically clear. To fix this we released some dye into the flow to see the flow disruption due to the objects. The dyes we used were food dyes mixed with milk and isopropanol alcohol. The milk is added to give the dye some opacity, but it and the dye both have a greater density than the water in the channel. If we just stopped here, then the dye mixture would be more dense then the water and sink to the bottom. We wouldn’t be observing the flow of the channel, but the flow with an added downward component. In order to match the density of the dye to that of water, we added in isopropanol, which has a lower density then water. We matched the density of ideal water as close as we could with our equipment but the water in the channel isn’t ideal. The water in the channel has impurities in it. To mitigate these differences we added an equal volume of water from the channel to the dye mixture. This served to cut the error between the two in half. We were running htis lab shortly before Christmas so we decided to use red and green dye.

Circular Tube

For the circular airfoil in low Reynolds number flow we can see the Karman Vortex street formed behind the blunt body.

NACA – 0012 Airfoil

We can see this street again in the low angle of attack NACA-0012 airfoil. Here, there are some differences form the blunt body above. For example, the pooling of the green dye occurs in front of the dye tap and then extends further back. This helps show the “bubble” that the dye is trapped in on the airfoil. This bubble is a mixture of two phenomena. The first is a wake bubble following the airfoil. The second phenomena is attached flow and it is the portion of the green dye that unsurprisingly looks attached to the wing. Only attached flow on a wing that generates lift. If too much of the flow on a airplane wing separates then the plane stalls and begins to fall. At higher angles of attack it is easier to separate the flow from the wing.

A Comment on Lab reports

“The only difference between Science and screwing around is writing it down”

-Adam Savage

Labs would not be complete without lab reports. These are where you write up your methods, results, analysis, and conclusion. They are invaluable in the real world in case someone wants to know how the experiment went, or to recreate it to check the results themselves. Lab reports are different then normal writing. While both are meant to convey an idea, the lab report is supposed to allow someone who has never met you to to recreate exactly what you have done. It’s supposed to be unambiguous.

This is a hard writing style to pickup, and lab classes often put harsh restrictive requirements on their lab reports to force people to write in this style. Unfortunately, these harsh restrictions meant to help, often force people to write dry, sterile reports that just check off this list of requirements. They don’t think about what should go in the report; why, or how each of the components work together. Students then get in a habit of doing this and carry on writing these dry technical reports over into the future. The real world often is not as clear as a structured lab and the report style they learned is inadequate. Now they have to either learn how to think critically about lab reports and what goes in them, or continue writing bad lab reports. Unfortunately, a review of technical reports will indicate that a large portion of engineers choose the second option.

Can we restructure our teaching methods for how to write a lab report in a way that causes student to think about what they are writing and that is scale-able to a 20-30 person class? I’m not sure, but this post is getting a bit too long so I’ll end this section here. If you have ideas about a better way we can teach technical report writing email me at ari@gereshes.com . If there’s enough interest I’ll put together a post about different styles and ideas.

If you want more Gereshes

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If you can’t wait for next weeks post and want some more Gereshes I suggest

How to land a lunar Lander

Rollout of a rocket motor test stand

How to pump a swing using math

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