A car travels at 70 mph to the right (assumed + n^-_x) on a level road on a calm
ID: 1996939 • Letter: A
Question
A car travels at 70 mph to the right (assumed + n^-_x) on a level road on a calm, 80 deg, F day. You may assume the ambient pressure is 14.7 psia. First, assume the car is moving and the ambient air is stationary. Find the relative air velocity far away from the car. What is the pressure far away from the car? Find the absolute air velocity on the front bumper of the car, which is assumed to be a stagnation point in the flow. Find the relative velocity at this same point on the bumper. Find the total pressure at this same point on the bumper. Find the static pressure at a location on the top of the car where the local relative air velocity is100 mph in the -n^-_x direction. Find the local absolute velocity of the air at this same location on the top of the car.Explanation / Answer
(i) If the air is stagnent, and the car is moving with a velocity 70mph in +nx direction, then the relative velocity of the air with respect to car will be 70mph in -nx direction.
(ii) Far awar from the car, the air will have same static pressure which is equal to 14.7 psia
(iii) The absolute velocity of air on the front side of the bumber will be zero, since the air is stagnent.
(iv) The relative velocity of air on the fron side of the bumper will be opposite to that of car forward velocity, which is equal to 70 mph (-nx).
(v) The total pressure air will be same as that of air which is equal to 14.7 psia
(vi) The air is moving in the direction -nx with the relative velocity 10 mph. On the top of the car, the pressure due to motion of the air is perpendicular to the surface of the top of the car. Hence the pressure component will be zero, on the top of the car. But the static pressure due to weight will be 14.7 psia.
(vii) The velocity of car is 70 mph +nx direction and the relative velocity of the air is 100 mph in -nx direction. Hence the absolute velocity of the air will be 100 - 70 = 30 mph
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