Design Tips – boundary layers
The flow of air over a solid surface gives rise to what is known as a boundary layer. This is caused by the air sticking to the surface, actually being slowed to a stop in contact with the surface. As we move away from the surface the air moves faster and faster until it reaches its maximum (freestream) velocity. Boundary layers come in two types. These are known as laminar and turbulent, and they have different characteristics.
Laminar boundary layers consist of layers of air molecules that smoothly flow on top of one another, so that the air at any given point in the boundary layer is moving in the main flow direction.
Turbulent boundary layers are more chaotic, however, and small eddies swirl the molecules up from the surface away to the edge of the boundary layer, and vice-versa. This is shown in the schematic diagram below.
Source: NASA Glenn
Laminar boundary layers tend to be thinner than turbulent boundary layers. Due to their more organised nature, laminar boundary layers produce much less skin friction drag than turbulent boundary layers. Unfortunately, while boundary layers start out laminar, they usually transition into turbulent boundary layers fairly quickly. This is caused by any imperfections in the surface, either manmade or natural. Even dead bugs on a smooth surface can be enough to trigger turbulent transition.
While laminar boundary layers are advantageous for reducing skin friction drag, turbulent boundary layers play an important part in reducing pressure drag, as we will now describe.
Flow Separation
As we showed on the page about drag coefficient, the ability of air flow to follow the surface of an object is crucial in minimising pressure drag. When the air is unable to follow an object's shape it separates. This is shown on the right for an aerofoil, which while being a streamlined shape, will cause flow separation if it is tilted so that it is more oblique to the flow. This is known as aerofoil stall.
In order to prevent flow separation it is best to avoid sharp changes in flow direction, but making a smooth shape with slow changes in curvature.
Hence, the design of what we typically call streamlined vehicles is actually an attempt at eliminating flow separation. This is usually possible, such as with an aerofoil with a sharp trailing edge, but practical consideration do not always allow us the freedom to build the perfect shape. Road cars, for instance, have squared-off backs as it is more important to have plenty of room inside the car and boot than to reduce the drag to an absolute minimum level.
There is another way to prevent flow separation though. This relates to boundary layers. Turbulent boundary layers may cause more skin friction drag, but they tend to be thicker and separate later. So for objects in which the pressure drag is dominant, it can be beneficial to artificially trigger a boundary layer to go turbulent to reduce the flow separation effect. The most well-known example of this is the golf ball.
Laminar boundary layer on a smooth ball means flow separates at around halfway point (12 o'clock and 6 o'clock positions), causing a wide wake.
Turbulent boundary layer due to dimples delays separation past halfway around ball, around 1 o'clock and 5 o'clock positions. This reduces pressure drag significantly.
A similarly shaped object in a Formula One race car is the driver's helmet. If you look closely you will see that some helmets have strips along the top that trigger the boundary layer to become turbulent to prevent the flow from separating over the top of the helmet. This is to both reduce drag and to prevent dirty air being ingested into the engine, via the intake above the driver's head.
So you can see how important the boundary layer is in causing drag. For how this applies to Greenpower race car design go to the Aerodynamic Design page.
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