Lab4_template

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University of Colorado, Boulder *

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1070

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Mechanical Engineering

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Apr 3, 2024

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docx

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Lab 4 Report Name: ____________________ Partners: ________________ , _______________ Please paste your Excel data and graph below this line. Highlight everything you need in Excel, copy, and then use ‘Paste Special’ to paste as an image (e.g., PNG, PDF, etc.). You can take a screenshot as well and paste it here, but please make sure it is easily viewable by your TA. Please remove the example image below before pasting your own. Question 1 (10 points):

a. Is your experimental value of n greater or less than the expected value of 2? Our experimental value of n was 2.4104 which is greater than the expected value of 2. b. Choose 1 condition (A-D from lab manual) that you think makes the most sense here for your experiment’s physical shortcomings. Which one did you choose? Condition C made the most sense for our experimental shortcomings. c. For the condition you chose, what problems with the setup or experimental procedures could have caused this specific measurement to be higher/lower than expected? The manometer’s readings kept fluctuating making it difficult to pin point an exact reading. Question 2 (10 points): a. What kind of vehicle was your model car? (often written on the bottom of the vehicle) Our car was the C3PO car. b. What was the wind speed (m/sec) you measured when the model blew out of the tunnel? The wind speed was 11.9 m/sec when our car blew out of the tunnel c. Based on your experiment, what wind speed in miles per hour (multiply m/sec by 2.3 to get mph) would move or turn over a full-size vehicle (remember to also use the scaling factor of 4 here)? 11.9 m/sec x 4 x 2.3 = 109.48 mph

d. Does your predicted wind speed (for the full-size vehicle) seem high enough to blow your type of vehicle off a real road? Justify your answer. Yes the wind speed seems high enough because high wind warnings are issued when there is sustained winds of 40 mph or greater and our predicted wind speed is more than double that. e. Give two differences between the model sitting in the wind tunnel and a real car in a real windstorm on a real road (Besides the aforementioned differences of scale). The model car was taking more concentrated wind gusts than a car on an open road. Another difference is that. Another difference is that a real car would have more weight because of what was inside of it and what it’s made of. Question 3 (10 points): a. What was your measured wind speed (m/sec) for this part of the experiment? The measured wind speed was 7.3 m/sec b. Multiply your wind speed in m/sec by 2.3 to obtain the speed in m.p.h. 7.3 m/sec x 2.3 = 16.79 mph c. In order to scale your measurements, multiply the speed in m.p.h. by 10 . What do you get? 16.79 mph x 10 = 167.9 mph d. What was the lowest negative pressure (i.e., which negative value was the most negative) measured on the hut? The lowest negative pressure was -0.0504 kPa

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e. Multiply your answer in Question 3d by 2100 * to arrive at scaled pressure units of pounds per square foot (lbs/ft 2 ). *The 2100 number converts pressure from kPa to lbs/ft 2 and scales the P value by 10 2 since we scaled V by 10 (remember dynamic pressure equation). -0.0504 kPa x 2100 = -105.84 lbs/ft^2 Question 4 (10 points): Consider a 100 square foot section of roof (10 feet by 10 feet, or 100 ft 2 ) a. Using the pressure from Question 3e, what is the magnitude of the upward force [pressure (lbs/ft 2 ) times area (ft 2 )] exerted on this section of roof by the winds? Show work and units! -105.84 lbs/ft^2 x 100 ft^2 = -10,584 lbs b. A 10 by 10 foot section of an average asphalt roof (with underlying wood) puts roughly 800 lbs of downward force on the roof. Divide the magnitude of the upward force due to the wind (your answer to 4a) with this 800 lb downward force due to gravity. This is now a ratio of upward to downward forces. How many times larger or smaller is the magnitude of the upward force when compared with the downward force (your ratio should give you this directly)? Show your work! 10,584 lbs / 800 lbs = 13.23 c. Based on your calculation, what do you think your wind force would do to this average roof? I think our wind force would blow this average roof off

Conclusion Question 1 (10 points): Identify 1 potential physical or experimental source of error for Experiment 1 and explain how it may have affected the final graph of dynamic pressure vs. wind speed. The manometer’s readings were fluctuating to the point that it was difficult to read. This could’ve affected our Y axis because the data might not have been completely accurate. Conclusion Question 2 (10 points): Identify 1 potential physical or experimental source of error for Experiment 3 and explain how it may have affected the final graph of dynamic pressure vs. angular position on the Quonset hut. The dashes on the hut could’ve been spaced out incorrectly making us measure at the wrong degree. This would’ve thrown off our X axis affecting our graph. Conclusion Question 3 (5 points): Sum up all pressures for all angles around the Quonset hut. Does this value come out as negative or positive? Does this mean that the air has an overall lower or higher pressure than ambient pressure? The value came out as negative meaning that the air had an overall lower pressure than ambient pressure.

Conclusion Question 4 (5 points): Some airfoils show some resemblance to the shape of the Quonset hut. The wind across the foil moves from left to right. Assume that the air flowing underneath the foil has a pressure about equal to ambient pressure, while the the air flowing over the top has a pressure like the one you deduced in Conclusion Question 3. Would this vertical pressure gradient (change in pressure with height) create lift or not? Explain your answer. The vertical pressure gradient would create lift because there is a negative pressure going above and it would try to balance it by lifting up.

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