A correlation of the hydrodynamic drag force on a cylinder for a periodic motion is demonstrated. The correlation indicates the dependance of the unsteady flow drag coefficient on the wake volume parameter. This parameter is a measure of the volume of flow through the boundary layer and into the wake in a half-cycle. For a laminar boundary layer, this dimensionless parameter is proportional to the Keulegan-Carpenter number and inversely proportional to the square root of the Reynolds number. Using wake volume scaling, drag coefficients were effectively collapsed into a single curve.

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