) A wind-tunnel experiment is performed on a small 1:5 scale model of a car, in

Question

) A wind-tunnel experiment is performed on a small 1:5 scale model of a car, in order to assess the drag force F on the design. A dimensionless "drag coefficient" Cd is defined by


where A is the maximum cross-sectional area of the car in the flow. With the model, a force of 2 N was recorded at a flow velocity u of 6 m s–1. Assuming that flow conditions are comparable (i.e., at the same Reynolds number), calculate the expected drag force for the full-sized car when the flow velocity past it is 31 m s–1 (equivalent to 70 miles per hour). [The density of air ? = 1.2 kg m–3.]

Explanation / Answer

so drag force has the equation F=1/2 v2 Cd A so F scales quadratically with v

so F2/F1=v22/v12

so F2/2=312/62

so F2=53.39 N

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