What is Aerodynamic Drag?

Описание: In this video, we will be discussing aerodynamic drag:
Aerodynamic drag is the force that you need to overcome, as you move through the air at a certain velocity. You will feel this for example when you ride a bike, because even at speeds as low as 20 km/h, the force that you need to overcome to push the air away, already accounts for more than half of the push that you need to deliver. In this video, we'll be looking at a speed skier to explain the two components of drag - pressure drag and friction drag.


Pressure drag
When the air hits the front of the skier, the pressure builds up, so that creates a force. At the back of the skier, air is dragged along, lowering the pressure, creating a wake (or draft zone) behind the object as well. If we integrate the pressure over the entire surface of the skier, we obtain the total force acting on the skier. If we want to know the drag, we just filter out the component of the total force that is directed along the wind direction: this is called the pressure drag.

Now if we want to learn more on which parts of the skier are contributing most to drag, we must zoom in and have a look at the local pressure. Clearly, the bigger the pressure, the more it can contribute to drag. But if the surface on which the pressure is acting is actually parallel to the wind direction, it doesn't impact the pressure drag. If the high pressure is, however, working on a surface that is perpendicular to the wind direction, like the front of the helmet or the hands, it does contribute a lot to drag.
To make things easier, we've multiplied this local orientation of the surface with the local pressure to give you an image that shows the direct contributions to drag. You will notice for example that the sides of the arms and the legs only show neutral green color, not contributing to drag. And that's it for the pressure drag.


Friction drag
Next to pushing and pulling on the surface, the air also slides across the surface. This generates friction forces, and although they are typically much smaller than the pressure force, they are relevant as well. In the case of the skier, they only contribute to 4% of the total drag.
If you visualize this, you again get a color map, which is called the friction map. It looks quite different compared to the pressure map. At the front of the helmet for example, where we had a lot of pressure drag, we now have almost zero friction drag, because the air comes to a complete standstill and so there is no relative velocity. On the other hand, where the air needs to curve around the sides of the object, air speeds up and there is a lot of local friction and thus a lot of contribution to the friction drag.

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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).
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