A 1:10 scale model of a tractor-trailer rig is tested in a wind tunnel. The model frontal area is Am = 0.1 m2. When tested at Vm = 75 m/s in standard air, the measured drag force is FD = 350 N. Evaluate the drag coefficient for the model conditions given. Assuming that the drag coefficient is the same for model and prototype, calculate the drag force on a prototype rig at a highway speed of 90 km/hr. Determine the air speed at which a model should be tested to ensure dynamically similar results if the prototype speed is 90 km/hr. Is this air speed practical? Why or why not?
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