Laminar Separation Criteria

Since in Thwaites method we essentially assume a shape for the profile, we can tell when the flow in the boundary layer reverses. This happens when:

The exact value is not very important since λ changes quickly near the area of separation.

Another criterion that does not require numerical integration of the boundary layer equations is one due to Stratford. This criterion asserts that laminar separation occurs when:

Cp' and x' are the canonical pressure coefficient and effective boundary layer length.

Stratford's laminar separation criteria appropriately reflects the deleterious effect of the adverse gradient's severity and length.

Because the laminar boundary layer is prone to transition in an adverse gradient, it is difficult to predict whether the flow will transition or separate first. Sometimes the flow separates, transitions, and then reattaches in what is called a laminar separation bubble. The length of the bubble is a function of the pressure gradient and Reynolds number, growing longer as the Reynolds number is reduced. In any case, laminar separation is to be avoided in airfoil design. This is done in several ways including forcing transition with surface roughness elements (grit) or building in a special transition region in the pressure distribution.

Once the flow is turbulent, we must apply an entirely different set of separation criteria, described in following sections.