• Adverse pressure gradient (Figure 6.4) - A laminar layer cannot exist when… — Page 119, Lesson 135

Let's pick this up right at the heart of the matter: the adverse pressure gradient. This is the single most important concept for understanding why drag exists on an aerofoil, so I want to build it up carefully. First, the definition. An adverse pressure gradient exists when pressure is increasing in the direction of flow. Think about that word "adverse" — it means working against you. The flow is moving along, and instead of pressure helping it along, the pressure is rising ahead of it, pushing back against the motion. That's the adverse pressure gradient. Now, here's the critical rule I want you to lock in: a laminar layer cannot exist when pressure is rising in the direction of flow. Laminar flow is that smooth, orderly, layered flow right at the surface. The moment you have an adverse pressure gradient, that smooth laminar layer is doomed. It will transition to turbulent flow. On a curved surface like an aerofoil, the transition point — where laminar becomes turbulent — is usually at, or very near to, the point of maximum thickness. Why there? Because that's where the pressure starts rising. And here's the key contrast: because of the adverse pressure gradient on a curved surface, the transition point will be further forward than if the surface were flat. A flat surface doesn't have that pressure rise, so the transition happens later. The curvature pushes it forward. Now, let's follow what happens after transition. This is where form drag is born. Form drag — also called pressure drag — results from the pressure at the leading edge of a body being greater than the pressure at the trailing edge. That pressure difference is the source of the drag. But how do we get there? Let me walk you through the chain of events. Overall, skin friction causes a continual reduction of boundary layer kinetic energy as the flow continues back along the surface. The boundary layer is that thin layer of air right next to the surface, and skin friction is constantly stealing its energy, slowing it down. Then, the adverse pressure gradient behind the transition point causes an additional reduction in kinetic energy of the boundary layer. So now you have two things draining energy: skin friction and the adverse pressure gradient. Here's where it gets critical. If the boundary layer does not have sufficient kinetic energy in the presence of the adverse pressure gradient, the lower levels of the boundary layer — the air right at the surface — stop moving. They stagnate. The upper levels of the boundary layer will overrun at this point, and that point is called the separation point. The boundary layer separates from the surface right there. And here's the fascinating part: surface flow aft of the separation point — that is, behind it, toward the trailing edge — will actually be forward, toward the separation point. That's a flow reversal. The air behind the separation point is moving backward, toward the separation point, not forward toward the trailing edge. Because of this separation, there will be a lower pressure at the trailing edge than at the leading edge. And an aerodynamic force will act in the direction of the lower pressure — that force is form drag. Let me give you the clean summary statement, because this is the takeaway: Separation will occur when the boundary layer does not have sufficient kinetic energy in the presence of a given adverse pressure gradient. It's a balance. If the boundary layer has enough energy, it can push through the pressure rise. If not, it stalls and separates. Now, one practical note about the picture. The vertical scale of the boundary layer in the sketch is greatly exaggerated — it's not drawn to scale. Typically, boundary layer thickness is from about 2 millimetres at the leading edge, increasing to about 20 millimetres at the trailing edge. So it's a very thin layer, but it's doing all the work. So the whole story hangs together: adverse pressure gradient kills the laminar layer, forces transition forward on a curved surface, drains the boundary layer's kinetic energy, and if the energy runs out, the flow separates, pressure drops at the trailing edge, and form drag appears. That's the complete mechanism.