Crossflow gradient effects

Although the infinite swept wing has a 3D boundary layer with crossflow, one simple explanation why the crossflow is immaterial to the boundary layer development is that there is no crossflow gradient, in the spanwise z direction in that case. In contrast, in more general 3D boundary layers with nonzero crossflow gradients, such as the one sketched in Figure 4.21, the presence of the crossflow will certainly have an effect.

In general, any lateral gradient in the crossflow will cause lateral convergence/divergence effects as in the axisymmetric case, except here the convergence or divergence occurs only near the wall rather than over the whole boundary layer thickness. However, the overall effect is the same, with the boundary layer growth being increased or decreased relative to the 2D case without crossflow. Figure 4.21 shows a case with crossflow convergence, causing an additional boundary layer thickening. Crossflow divergence would have the opposite effect.