The boundary layer around a circular cylinder was located from observations of total head taken near the surface with an exceedingly small tube. The diameter of the cylinder and the range of wind speed selected allowed the particular range of V0D/v from 105 to 3.5 × 105, through which a marked change in the character of the flow around a cylinder occurs, to be covered in the experiments. The analysis shows that the distinctive traits of the boundary layer around the cylinder resemble those common to the layer along a plane surface; that is, there is a critical point on the cylinder where a transition from the laminar to a turbulent state of flow in the boundary layer begins and also the transitional region is marked by a rapid opening‐out of the layer. The critical value of Reynolds' number for the cylinder is of the same order of magnitude as that obtained by Van der Hegge Zijnen for a plane surface. The separation of the boundary layer from the surface occurs just beyond the critical point.
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