AirShaper

In this video, we will be discussing Drag Coefficients.
What are they? How can you calculate them? And most of all, how you can use them in your design process!


Theory 
Drag coefficients are used to calculate the hydrodynamic (in water) or aerodynamic (in air) force on an object, given the density Rho (ρ), the speed (u) and the frontal area (A) of an object. So if you know the force on an object at a certain speed, for example after a wind tunnel test, you can calculate the drag coefficient yourself using this formula. (0:28)

Once you know the drag coefficient for a certain geometry, you can calculate the force for different object sizes or different velocities
That is very useful for example when you need to size engines, calculate required battery capacity, etc.
But keep in mind that the drag coefficient can vary in function of the Reynolds number. So be careful with large extrapolations to other speeds, sizes or densities.


Practical use
A drag coefficient allows you to analyze the aerodynamic efficiency of an object, irrespective of its size or velocity. That makes it possible to compare a cyclist for example to a building. They are quite different, but still, they have a normalized aerodynamic coefficient.
It is also quite useful within a design process: when you are looking at different concepts for example for your new project or new vehicle, you can rank them according to their drag coefficient. Or you could get inspired by aerodynamic shapes coming from a completely different sector (Mercedes once had a car design inspired by fish!).


Typical Values
A drop shape, which is quite efficient, can have a drag coefficient as low as 0,05, whereas a building typically has one above 1. Lower means more streamlined. 
So if you are working on a drone that needs to fly as far as possible on a single charge or a cyclist that wants a higher top speed, you will want to reduce the drag coefficient as much as possible. 



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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, ...).
We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

What is a Drag Coefficient?

Drag Coefficients Cd are used to quantify the resistance of an object as it moves through a fluid. It is a dimensionless quantity which allows aerodynamicists to analyse the aerodynamic efficiency of an object.

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, ...).
We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

What is Aerodynamic Drag?

Aerodynamic drag is the force an object needs to overcome as it moves through air at a certain velocity. Aerodynamic drag consists of pressure drag and friction drag.

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