Understanding Aerodynamic Drag

Thanks to CuriosityStream for sponsoring this video. Watch thousands of documentaries and get access to Nebula for free when you sign up using the link in the description. When fluid flows past an object, or an object moves through a stationary fluid, the fluid exerts a force on the object. We can split the force into two components - one acting in the same direction as the

fluid flow, which is called drag, and one acting perpendicular to the flow direction, which is called lift. If the fluid is a gas, like air, we call these aerodynamic forces, and if it's water or any other liquid we call them hydrodynamic forces. This video is going to focus on the drag force, and I'll cover lift in a separate video.

Most of the time drag forces are undesirable. They can have a huge effect on the fuel consumption and performance of vehicles, for example. And so engineers go to great lengths to minimise them. We'll explore some interesting ways of reducing drag later on in the video, including how the airline industry could save millions of dollars a year in fuel costs by using a drag-reducing

innovation based on artificial shark skin. But to optimise the design of objects affected by drag forces we first need to understand where these forces come from, so let's start by covering the basics. Drag forces are caused by two different types of stress which act on the surface of an object. First we have the wall shear stresses.

These stresses act tangential to the object's surface, and are caused by frictional forces that arise because of a fluid's viscosity. Then we have the pressure stresses. They act perpendicular to the object's surface, and are caused by how pressure is distributed around the object. The drag force is the resultant of these two stresses in the direction of the flow.