Fall 2012 ME 395 - GSI Josh Lacey

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 pointers for Lab 8 reports

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GSI Overlord

Posts : 126
Join date : 2012-08-30

PostSubject: pointers for Lab 8 reports   Thu Dec 06, 2012 12:44 pm

I will basically reiterate and expand a bit on what Professor Thouless covered in lecture on Monday. If you refer to the lecture slide notes that are posted on CTools, you will see that Professor Thouless has done a nice job of laying out the logical flow of the report for everyone.

I think the most challenging aspect of the report is the determination of the final drag coefficient, as it's going to take a bit of finesse in the course of report writing. The process laid on Monday is somewhat iterative (although it should only take 1 iteration). The first thing you will need to do is to plot the Cd vs. Re for what you measured in lab. Based on your observations of this curve, you will need to pick off an initial guess for the coefficient of drag of the full-scale vehicle. Your assumption here is that Santa's vehicle is going to be traveling at high rates of speed (after all, he has a lot of deliveries to make in a short time), so the Cd estimate would need to be at a high Re. What you will hopefully see is that at high Re, your drag coefficient for the model isn't changing much, so you will be able to estimate what the Cd will do at high Re for the full-scale vehicle. You will then use this initial guess to calculate a max velocity and Re for the full-scale vehicle. It is at this point you'll see that the Re for the full-scale vehicle is well outside the range of the Re you tested in lab for the model. This is where that whole business about extrapolation comes into play. You will need to come up a reasonable extrapolation argument to find a more accurate representation of the drag coefficient for the full-scale vehicle (possibly using the Cd vs Re curve for a similar shape from some outside source). Once you have formulated your extrapolation argument, you will have a final Cd for the full-scale vehicle, and you will be able to calculate a final maximum velocity and Re. As a sanity check, the final Re you calculate will hopefully be close (within experimental uncertainty) to what you calculated using the initial estimate of Cd.

I will have all my normal office hours next week, so feel free to drop by if there are additional questions, or post to the forum.
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