The field source distribution in a typical compressible flow-field over a 2D airfoil is shown in Figure 8.1. The acceleration over the front half of the airfoil produces a positive source area, while the deceleration over the rear half produces a negative source (sink) area. This has two major effects:
1. Increase in the overall velocity over the airfoil, as sketched on the left in Figure 8.2, and also quantitatively plotted in Figure 8.3.
3. Since the airfoil surface is fixed, the streamtubes can only thicken outward, which results in a lateral dilation of the overall velocity and pressure field isolines, which is quantitatively shown in Figure 8.5.
Figure 8.1: Contours of constant a(r) c/V» with increment 0.1, near NACA N66-010 airfoil at a = 0°, MTO = 0.77. Region over most of airfoil’s front half has a > 0 (sources), and region over rear half has a< 0 (sinks).
Figure 8.2: Comparison of velocity vectors and streamlines between incompressible (dashed) and compressible (solid) flows. Source and sink regions in the compressible case increase the velocity adjacent to the airfoil (left), and also cause streamtube thickening (right).