Turbulent Wakes

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Airflow past ordinary size objects is only laminar at low speed.  As the airspeed increases, a boundary layer forms. This boundary layer fills the region behind the object with a turbulent wake.

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The figure above shows the airflow past a cylinder as the airspeed and therefore the Reynolds number increase.  Remember

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In pictures 1-3 the Reynolds number is below 2000, in picture 4 it is approximately 10000, and in picture 5 it is above 100000.