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NATIONAL UNIVERSITY OF SINGAPORE Department of Mechanical Engineering

ME2135 Fluid Mechanics II

Part 2 External Incompressible Viscous Flow  

Tutorial 4

1. A hot-air balloon roughly spherical in shape has a volume of 2000 m3 and a weight of 2.2 kN (including passengers, basket, balloon fabric, etc.). If the outside air temperature is 27° C and the temperature within the balloon is 74° C, estimate the rate at which it will rise under steady-state conditions if the atmospheric pressure is 101 kPa.

2. On a day without any wind, your car fuel economy is 10 L/100km when you drive at a

constant speed 60 km/hr from point A to point B and back to point A. Assume that you repeat the journey, driving at the same speed, on another day when there is a steady wind 30 km/hr blowing from B to A. Find the ratio of energy expended for the two trips and your fuel economy for this windy round-trip?

3. A 134 kW sports car of frontal area 1.72 m2, with a drag coefficient of 0.31, has a drivetrain efficiency of 88%. The rolling resistance is 1.2 percent of the car weight and the car mass is 1250 kg. At what speed does aerodynamic drag first exceed rolling resistance? What is the maximum speed and acceleration? What are the maximum speeds if the drag coefficient was reduced by 10% or the rolling resistance was reduced by 20%?