Force on dog's head out of car sunfoof

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Discussion Overview

The discussion revolves around calculating the force exerted by wind on a dog's head when it is out of a car sunroof while traveling at 50 mph. Participants explore the application of aerodynamic principles, specifically focusing on drag force calculations using different methods and parameters.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents two attempts at calculating the force: one based on wall loading calculations and another using the drag equation, yielding similar results around 19N.
  • Another participant notes that both calculations are incorrect, emphasizing the need for correct units in the drag equation and questioning the drag coefficient (Cd) value of 2 for a dog's head.
  • A further comment suggests that the dog's tongue flapping may contribute to an anomalous drag coefficient value.
  • A participant reiterates the need for correct units and proposes a corrected version of the drag equation calculation using the appropriate speed in meters per second.
  • There is a consensus that the drag coefficient of 2 may be too high, with a suggestion that it could be influenced by an engineering factor of safety.

Areas of Agreement / Disagreement

Participants express disagreement regarding the correctness of the initial calculations and the appropriateness of the drag coefficient used. There is no consensus on the correct value of the drag coefficient or the final force calculation.

Contextual Notes

Limitations include the potential misapplication of units in the drag equation and the uncertainty surrounding the appropriate drag coefficient for a dog's head, which may vary based on additional factors like tongue position.

oneshortplanck
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(i have made this thread as i previously had it in the aero section and it was probably not getting the attention such an important question deserves.)

lets use the sunroof as an example for simplicity.
what force is the wind acting on the dog at 50mph/22.3m/s?
dog head diameter = 0.2m
therefore frontal area of dog head is 0.0314m^2
Drag coefficient = 2
air density = 1.22

here is my attempt (#1) based on wall loading calculations:
0.00256 * 50 * 50 = 6.4 lb/ sq feet = 306.4Pa

0.0314 * 306.4 * 2=19.25N

here is my attempt (2) based on drag equation
2 x ((1.22 x 50^2)/2) * 0.0314 = 19Nany thoughts
 
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Yayyy! They're approximately equal.
 
But both equally wrong unfortunately! You need to use the correct units in the drag equation, mph won't do. Also, 2 for Cd seems high for a dogs head, where did you get that from?
 
The tongue flapping contributes to an anamalous CD value.
 
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billy_joule said:
But both equally wrong unfortunately! You need to use the correct units in the drag equation, mph won't do. Also, 2 for Cd seems high for a dogs head, where did you get that from?

the first is correct units, but the second should read
2 x ((1.22 x 22.352^2)/2) * 0.0314 = 19N

yes, i agree on the 2 cd, perhaps its the engineering factor of safety.
 

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