14.11. As we explained in Chapter 7, the air resistance to motion of a vehicle is something important that engineers investigate. As you may also know, the drag force acting on a car is determined experimentally by placing the car in a wind tunnel. The air speed inside the tunnel is changed, and the drag force acting on the car is measured. For a given car, the experimental data is generally represented by a single coefficient that is called drag coefficient. It is defined by the following relationship: Fa Cả where Ca = drag coefficient (unitless) Fa = measured drag force (N or 1b) p= air density (kg/m³ or slugs/ft') V = air speed inside the wind tunnel (m/s or ft/s) A = frontal area of the car (m? or ft') The frontal area A represents the frontal projection of the car's area and could be approximated simply by multiplying 0.85 times the width and the height of a rectangle that outlines the front of a car. This is the area that you see when you view the car from a direction normal to the front grills. The 0.85 factor is used to adjust for rounded comers, open space below the bumper, and so on. To give you some idea, typical drag coefficient values for sports cars are between 0.27 to 0.38, and for sedans are between 0.34 to 0.5. The power requirement to overcome air resistance is computed by P= F,V where P= power (watts or ft· lb/s) 1 horse power (hp) = 550 ft lb/s and 1 horse power (hp) = 746 W The purpose of this exercise is to see how the power requirement changes with the car speed and the air temperature. Determine the power requirement to overcome air resistance for a car that has a listed drag coefficient of 0.4 and width of 74.4 in. and height of 57.4 in. Vary the air speed in the range of 15 m/s < V < 35 m/s, and change the air density range of 1.11 kg/m³
14.11. As we explained in Chapter 7, the air resistance to motion of a vehicle is something important that engineers investigate. As you may also know, the drag force acting on a car is determined experimentally by placing the car in a wind tunnel. The air speed inside the tunnel is changed, and the drag force acting on the car is measured. For a given car, the experimental data is generally represented by a single coefficient that is called drag coefficient. It is defined by the following relationship: Fa Cả where Ca = drag coefficient (unitless) Fa = measured drag force (N or 1b) p= air density (kg/m³ or slugs/ft') V = air speed inside the wind tunnel (m/s or ft/s) A = frontal area of the car (m? or ft') The frontal area A represents the frontal projection of the car's area and could be approximated simply by multiplying 0.85 times the width and the height of a rectangle that outlines the front of a car. This is the area that you see when you view the car from a direction normal to the front grills. The 0.85 factor is used to adjust for rounded comers, open space below the bumper, and so on. To give you some idea, typical drag coefficient values for sports cars are between 0.27 to 0.38, and for sedans are between 0.34 to 0.5. The power requirement to overcome air resistance is computed by P= F,V where P= power (watts or ft· lb/s) 1 horse power (hp) = 550 ft lb/s and 1 horse power (hp) = 746 W The purpose of this exercise is to see how the power requirement changes with the car speed and the air temperature. Determine the power requirement to overcome air resistance for a car that has a listed drag coefficient of 0.4 and width of 74.4 in. and height of 57.4 in. Vary the air speed in the range of 15 m/s < V < 35 m/s, and change the air density range of 1.11 kg/m³
Chapter2: Loads On Structures
Section: Chapter Questions
Problem 1P
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Transcribed Image Text:14.11. As we explained in Chapter 7, the air resistance to motion of a vehicle is
something important that engineers investigate. As you may also know, the drag
force acting on a car is determined experimentally by placing the car in a wind
tunnel. The air speed inside the tunnel is changed, and the drag force acting on the
car is measured. For a given car, the experimental data is generally represented by
a single coefficient that is called drag coefficient. It is defined by the following
relationship:
Fa
Cả
where
Ca = drag coefficient (unitless)
Fa = measured drag force (N or 1b)
p= air density (kg/m³ or slugs/ft')
V = air speed inside the wind tunnel (m/s or ft/s)
A = frontal area of the car (m? or ft')
The frontal area A represents the frontal projection of the car's area and could be
approximated simply by multiplying 0.85 times the width and the height of a
rectangle that outlines the front of a car. This is the area that you see when you
view the car from a direction normal to the front grills. The 0.85 factor is used to
adjust for rounded comers, open space below the bumper, and so on. To give you
some idea, typical drag coefficient values for sports cars are between 0.27 to 0.38,
and for sedans are between 0.34 to 0.5.
The power requirement to overcome air resistance is computed by
P= F,V
where
P= power (watts or ft· lb/s)
1 horse power (hp) = 550 ft lb/s
and
1 horse power (hp) = 746 W
The purpose of this exercise is to see how the power requirement changes with the
car speed and the air temperature. Determine the power requirement to overcome
air resistance for a car that has a listed drag coefficient of 0.4 and width of 74.4 in.
and height of 57.4 in. Vary the air speed in the range of 15 m/s < V < 35 m/s,
and change the air density range of 1.11 kg/m³ <p<1.29 kg/m?. The given air
density range corresponds to 0°C to 45°C. You may use the ideal gas law to relate
the density of the air to its temperature. Present your findings in both kilowatts
and horsepower as shown in the accompanying spreadsheet. Discuss your
findings in terms of power consumption as a function of speed and air
temperature.
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