We can see the role played by friction drag (sometimes called viscous drag) and pressure drag (sometimes called form drag or profile drag) by considering an airfoil at different angles of attack. At small angles of attack, the boundary layers on the top and bottom surface experience only mild pressure gradients, and they remain attached along almost the entire chord length. The wake is very small, and the drag is dominated by the viscous friction inside the boundary layers. However, as the angle of attack increases, the pressure gradients on the airfoil increase in magnitude. In particular, the adverse pressure gradient on the top rear portion of the airfoil may become sufficiently strong to produce a separated flow. This separation will increase the size of the wake, and the pressure losses in the wake due to eddy formation Therefore the pressure drag increases. At a higher angle of attack, a large fraction of the flow over the top surface of the airfoil may be separated, and the airfoil is said to be stalled. At this stage, the pressure drag is much greater than the viscous drag .
When the drag is dominated by viscous drag, we say the body is streamlined, and when it is dominated by pressure drag, we say the body is bluff. Whether the flow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape of the body. A streamlined body looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks like a brick, a cylinder, or an airfoil at large angles of attack. For streamlined bodies, frictional drag is the dominant source of air resistance. For a bluff body, the dominant source of drag is pressure drag. For a given frontal area and velocity, a streamlined body will always have a lower resistance than a bluff body. For example, the drag of a cylinder of diameter $D$ can be ten times larger than a streamlined shape with the same thickness (see figure 1).
Figure 1. Drag coefficients of blunt and streamlined bodies. |
Figure 2. Drag coefficient as a function of Reynolds number for smooth circular cylinders and smooth spheres. |
Figure 3. Flow patterns for flow over a cylinder: (A) Reynolds number = 0.2; (B) 12; (C) 120; (D) 30,000; (E) 500,000. Patterns correspond to the points marked on figure 2. |
A laminar boundary layer has less momentum near the wall than a turbulent boundary layer, as shown in figure 4, because turbulence is a very effective mixing process. More importantly, turbulent transport of momentum is very effective at replenishing the near-wall momentum. So when a turbulent boundary layer enters a region of adverse pressure gradient, it can persist for a longer distance without separating (compared to a laminar flow) because the momentum near the wall is higher to begin with, and it is continually (and quickly) being replenished by turbulent mixing .
Figure 4. Boundary layer profiles for laminar and turbulent flow. |
It follows that, if the boundary layer of a sphere can be made turbulent at a lower Reynolds number, then the drag should also go down at that Reynolds number. This is the case, as we can show by using a trip wire. A trip wire is simply a wire located on the front face of the sphere and it introduces a large disturbance into the boundary layer. This disturbance causes an early transition to turbulence, and it effect on the size of the wake, and the total drag is quite dramatic, as shown in figure 5 .
Figure 5. Flow over a sphere: (a) Reynolds number = 15,000; (b) Reynolds number = 30,000, with trip wire. |