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The
photo was taken in a wind tunnel, looking upstream toward the rear of
the car. A grid with small cotton tufts attached to it is placed in the
wake of the car, and the tufts align themselves with the local flow direction
and indicate the streamline direction. Large counter-rotating vortices
are revealed. Next time you see a car on a cold day, notice how the exhaust
fumes (mostly condensed water) are entrained in the wake of the car to
show the presence of the vortical flows. A car travelling along a dirt
road on a dry day will also reveal these flow patterns.
From Visualized Flow, Japan Society of Mechanical Engineers, Pergamon Press, 1988.
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