A shape factor is used in boundary layer flow to determine the nature of the flow.
where H is the shape factor, is the displacement thickness and θ is the momentum thickness. The higher the value of H, the stronger the adverse pressure gradient. A high adverse pressure gradient can greatly reduce the Reynolds number at which transition into turbulence may occur.
Conventionally, H=2.59 (Blasius boundary layer) is typical of laminar flows, while H=1.3-1.4 is typical of turbulent flows.
Read more about this topic: Boundary-layer Thickness
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