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C 7.3: Drag Force of a Sign Mounted on a Moving Vehicle

C 7.3: Drag Force of a Sign Mounted on a Moving Vehicle. Rebecca Currier February 15, 2007 BIEN 301- Individual Project. The problem. Pizza delivery sign mounted on car going 40 mph

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C 7.3: Drag Force of a Sign Mounted on a Moving Vehicle

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  1. C 7.3: Drag Force of a Sign Mounted on a Moving Vehicle Rebecca Currier February 15, 2007 BIEN 301- Individual Project

  2. The problem • Pizza delivery sign mounted on car going 40 mph • If the sign is mounted facing bluntly into the wind, how much will that effect the drag force as opposed to putting the sign parallel to the wind?

  3. Given • Sign is 1.5 ft by 5 ft (7.5 feet2) • Kinematic viscosity of air: 1.61e-4 ft2/s • Density of air: 2.34 e-3 slug/ft3 • CD of car is 0.4 • A of car is 40 ft2 • Rolling resistance is 40 lbf • Car provides 10 hp / gal gas / hr

  4. Required • (a) Drag on sign alone in both orientations • (b) Total drag on car and sign in both orientations • (c) Horsepower required by engine to drive car in both orientations • (d) Fuel efficiency in mi/gal for both orientations

  5. Assumptions • Standard temperature and pressure • Airflow around car is turbulent and viscous • Car has no acceleration • One dimensional airflow • When sign faces parallel to wind, identical boundary layers form; when sign faces bluntly into the wind, boundary layer forms on each side of sign

  6. Setting it up • A// = 7.5 ft2, Af = 15 ft2 (two boundary layers) (7.45 for flat plate in turbulent flow) CD// = 0.004, CDf = 1.19 (Table 7.3 for rectangular flat plate flow, b/h = 5/1.5)

  7. Drag forces • FDfsign = 71.96 lbf • FD//sign = 0.12 lbf (7.62), therefore For drag on sign + car, simply add CDs and Areas together in equation • FDf = 346.52 lbf • FD// = 76.04 lbf

  8. Power • Pf = 378 hp • P// = 114 hp

  9. Fuel efficiency • FEf = 1.1 mi/gal • FE// = 3.5 mi/gal

  10. Conclusion • Mounting sign parallel to wind reduces large drag force to almost nothing • Drag force with sign into wind adds a huge force to the car, almost twice as large as the rolling resistance • Even with minimized drag, 10 hp is not enough to power this car efficiently

  11. Biomedical Application • This problem illustrates the important effects of orientation on drag. • When placing a catheter into blood flow, an engineer must carefully design both the catheter and the insertion device so that the implanted object is properly orientated to minimize drag.

  12. Questions?

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