Aerodynamics Fundamentals: What Causes Drag?
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What Causes Drag?
03.49
| 00:00 | Having discussed what causes downforce, it's time to do the same with drag. |
| 00:04 | As we covered in the previous module, angling a flat plate and applying a pressure differential on it will inherently create a rearwards facing force vector, and this is what is known as pressure drag. |
| 00:16 | This is drag that results from any pressure differential on a surface with a rearwards facing normal vector. |
| 00:22 | This could be drag from suction induced on rearwards facing parts of a wing, or pressure on the forwards facing nose of the car. |
| 00:30 | At the rear in the car's wake there's usually a lower than ambient pressure, and this will cause the rearwards facing portions of the car around the boot to have a substantial amount of pressure drag. |
| 00:42 | On a road going car, the high pressure region at the front and the low pressure region at the back are the two most dominant sources of pressure drag. |
| 00:50 | There is however another component which is known as skin friction drag. |
| 00:55 | Essentially, this drag comes from the formation of the boundary layer and the wall friction that results from it. |
| 01:01 | When the boundary layer is formed, there's a shearing action between the wall and freestream particles, and this shearing generates a force that's applied at the wall, which is along the direction of travel of the flow. |
| 01:12 | The magnitude of the shear force at the wall is proportional to the steepness of the velocity gradient in the boundary layer, and the speed of the fluid squared. |
| 01:21 | At lower Reynolds numbers, the boundary layer is more likely to be laminar, and this results in a fairly gentle velocity gradient. |
| 01:29 | As a result, laminar boundary layers have less skin friction drag. |
| 01:34 | Turbulent boundary layers have more mixing and a steeper velocity gradient, so we'd expect more skin friction drag with these. |
| 01:41 | Generally speaking, this means that to reduce skin friction drag, we want as smooth a surface as possible, as decreasing the surface roughness decreases the production of instabilities within the boundary layer, and so delays the onset of the turbulent boundary. |
| 01:56 | Cars that generate lots of downforce typically have their drag dominated by the pressure drag component. |
| 02:02 | Any suction on any rearwards facing surface will cause drag, and when we increase downforce with things like wings and diffusers, we are inherently generating this rearwards facing suction. |
| 02:12 | In these cases, we don't care so much about skin friction drag, as it's a comparatively small component. |
| 02:19 | Skin friction is far more critical on lower drag cars like solar record cars, due to their very large surface area, and comparatively lower amounts of downforce and lift. |
| 02:29 | These large, flat, horizontal surfaces produce very small pressure differentials, but at all points, the surface is exposed to the airstream, and it will experience skin friction drag. |
| 02:42 | This makes situations like teardrop car shapes tricky. |
| 02:45 | We can teardrop shape a car more with a longer tail that reduces the pressure drag. |
| 02:50 | However, we've added so much surface area that the skin friction drag is notably increased. |
| 02:56 | Ideal balance for these two components of drag lies depends completely on the specifics of the car being designed, and there is no ideal rule to follow here. |
| 03:04 | Everything must be assessed on a case-by-case basis. |
| 03:07 | Let's finish up this module by looking at the main takeaways. |
| 03:11 | Drag consists of two main components, pressure drag and skin friction drag. |
| 03:16 | Pressure drag arises from pressure differentials on rearwards facing surfaces, such as the suction on a rear wing or low pressure regions in a car's wake. |
| 03:26 | Skin friction drag, on the other hand, comes from shearing forces within the boundary layer, with smoother surfaces and laminar boundary layers reducing drag whereas turbulent boundary layers increase it. |
| 03:38 | For high downforce cars, pressure drag dominates, while for low drag designs such as solar cars, skin friction drag plays a larger role. |
