Flow separation and stalling

In Chapter 1 we described how, at high angles of attack, the air flow separates, and fails to follow the contours of an aerofoil, resulting in stalling. To see how this happens, and why it is connected with the boundary layer, we need to look again at how the pressure varies around a wing section.

Figure 3.3(a) shows a typical low speed wing section under normal flight conditions. The pressure reaches its minimum value at a point A, somewhere around the position of maximum thickness on the upper surface. After this, the pressure gradually rises again, until it returns to a value close to the original free-stream pressure, at the trailing edge at B.

This means, that over the rear part of the upper surface, the air has to travel from low to high pressure. The air can do this by slowing down and giving up some of the extra kinetic energy that it possessed at A, according to the Bernoulli relationship p + pV2 is constant. The situation can be likened to that of a cyclist who can free-wheel up a hill, as long as he is going fast enough at the bottom.

Close to the surface, in the boundary layer, however, some of the available energy is dissipated in friction, and the air can no longer return to its original free-stream conditions at B, just as a cyclist would not be able to free-wheel up a hill quite as high as the one that he had just coasted down.

If the increase in pressure is gradual, then the process of turbulent mixing or molecular impacts allows the outer layers to effectively pull the inner ones along. The boundary layer merely thickens, leaving a slow-moving wake at the trailing edge, as in Fig. 3.3(a).

If the rate of increase in pressure is rapid, the mixing process is too slow to keep the lower part of the layer moving, and a dead-water region starts to form. The boundary layer flow stops following the direction of the surface, and separates, as shown in Fig. 3.3(b). Air particles in the dead-water region tend to move towards the lower pressure, in the reverse direction to the main flow. This mechanism is the primary cause of stalling. As the aerofoil angle of attack is increased, the pressure difference between A and B increases, and the separa­tion position moves forward, as in Fig. 3.3(c). (See also, Fig. 1.19.)

(c)

The process of mixing in the turbulent boundary layer is much more rapid than the process of molecular mixing and impacts in the laminar layer, so a tur­bulent boundary layer is less prone to separation than a laminar one of similar thickness. This represents the other important difference between the two types of layer. You will see, therefore, that the type of layer affects the stalling char­acteristics of the aerofoil.