My (Spring 2022) Exam 2

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School

Purdue University *

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Course

308

Subject

Mechanical Engineering

Date

Feb 20, 2024

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pdf

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15

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ME308 Exam 2 Bora Haller TOTAL POINTS 103 / 100 QUESTION 1 1 Problem 1a 8 / 4 - 0 pts Correct + 2 pts BONUS: Re is explained but not how is shapes the delta_BL + 4 pts BONUS - 1 pts Fully laminar sketch of boundary layer not drawn or incorrect (flow over a flat plate will never separate or a fully laminar flow should not have transition to turbulence, no velocity profiles have been drawn) - 1 pts Transitional boundary layer sketch not drawn or incorrect (possible errors: transition to turbulence has been depicted as flow separation, no velocity profiles have been drawn) - 1 pts Lower Reynolds-number sketch not redrawn or incorrect (a lower Reynolds-number boundary layer is thicker or simply shorter in 'x'; the reference boundary layer was already fully laminar, no velocity profiles have been drawn) - 1 pts Reynolds number expression not written. - 4 pts No answer or incorrect + 4 Point adjustment QUESTION 2 2 Problem 1b 4 / 4 - 0 pts Correct - 2 pts The answer doesn't explicitly mention that modern cars reduce the pressure drag or provide a physical explanation - 2 pts Streamlines are qualitatively incorrect or not drawn at all - 4 pts streamlines are not sketched properly or at all, no explanation for reduced drag is provided QUESTION 3 3 Problem 1c 4 / 4 - 0 pts Correct - 2 pts Streamlines are qualitatively incorrect. - 2 pts Bernoulli not invoked to connect changes in velocity with changes in pressure, or written without any explanation, or incorrectly applied. Or it is argued that pressure on top is higher than pressure at the bottom of the airfoil. Or it is argued that the height difference (Delta z) matters. QUESTION 4 4 Problem 2 18 / 18 - 0 pts Correct - 5 pts Correct procedure everywhere, significant calculation error - 7 pts Missing/incorrect answer to (a) - 6 pts Missing/incorrect answer to (b) - 5 pts Missing/incorrect answer to (c) - 18 pts missing/incorrect answer to all
QUESTION 5 5 Problem 3 15 / 15 - 0 pts Correct - 2 pts Missing/incorrect Pi group for $$e$$ - 2 pts Missing/incorrect Pi group for $$L$$ - 3 pts Missing/incorrect Pi group for $$\mu$$ - 3 pts Missing/incorrect Pi group for $$\Delta p$$ - 3 pts Write dependent Pi group as function of indep Pi groups. - 1 pts Missing or incorrect Pi group name Re. - 1 pts Missing or incorrect Pi group name rel. surface roughness QUESTION 6 6 Problem 4 14 / 15 - 0 pts Correct - 3 pts Skin friction varies along sheet of plywood. Need to integrate from leading edge to trailing edge. - 2 pts Reynolds number indicates turbulent flow over most of the plywood. - 3 pts Mistakes in calculating Reynolds number or didn't calculate Reynolds number - 2 pts Incorrect or missing expression for calculating coefficient of drag. - 2 pts Did not correctly convert from $$lb_m ft / s^2$$ to $$lb_f$$. - 2 pts Mistakes in dealing with imperial units or didn't report answer in imperial units - 2 pts Did not choose correct area for computing flat plate drag. - 2 pts computation error - 1 pts Work not shown - 3 pts Coefficient of drag is not drag force. - 2 pts Chose incorrect material properties. - 15 pts Complete miss or not attempted. 1 Where are these calculations? QUESTION 7 7 Problem 5 20 / 20 - 0 pts Correct Did not use EBE at all, leading to wholly incorrect answer. - 5 pts Incorrect (or no) statement of EBE (for example, with respect to losses and flow direction) - 2 pts Incorrect evaluation $$\Delta p$$ in EBE (have to start at tank surfaces otherwise there's not enough information to evaluate it) - 2 pts Incorrect $$\Delta z$$ (sign is important based on flow direction in EBE written) - 2 pts Incorrect major loss, each is of the form $$f\frac{L}{D}\frac{\bar{V}^2}{2g}$$ - 4 pts Incorrectly split major loss; need to split into 3 parts due to 3 different pipe diameters (taking into account that also $$\bar{V}$$ is different in each segment) - 3 pts Incorrect (or major error in) approach to friction factor calculation (for example, didn't find $$Re$$ from $$V$$ from given $$Q$$ or didn't use Moody or Haaland) - 2 pts Further error(s) leading to incorrect value $$\Delta z\approx 20.59$$ m from simplified EBE QUESTION 8 8 Problem 6 20 / 20
- 0 pts Correct Did not use EBE at all/Errors In EBE, leading to a wholly incorrect answer. - 5 pts Incorrect statement of EBE with respect to losses and flow direction. - 2 pts Incorrect evaluation $$\Delta p$$ in EBE (have to start at tank surfaces otherwise there's not enough information to evaluate it) - 2 pts Incorrect $$\Delta z$$ (sign is important based on flow direction in EBE written) - 2 pts Didn't eliminate $$\bar{V}$$ using $$Q = \bar{V}A$$ - 2 pts Incorrect major loss $$f\frac{L}{D}\frac{\bar{V}^2}{2g}=f\frac{L}{D}\fra c{Q^2}{2gA^2}$$ - 2 pts Incorrect system/Pump head as a function of $$Q$$ into EBE - 5 pts Incorrect solution (or didn't solve) for $$Q\approx 0.814$$ cfs between RHS and LHS of simplified EBE - 2 pts Correct solution solution approach for $$Q$$ but algebra/arithmetic mistake (only applies if rubric item 3 was not applied) Page 3
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