Viscosity is a key property in fluid mechanics that describes how easily a fluid will flow. But there’s more too it than that!

The animated video above covers all of the key things you need to know about viscosity. It explains Newton’s Law of Viscosity, the equation that describes the linear relationship between the shear stress in a flowing fluid fluid and velocity gradient, the constant of proportionality between the two terms being the fluid viscosity, $\mu$.

It also explores how viscosity varies with temperature – the viscosity of liquids reduces with increasing temperature, but for gases temperature has the opposite effect. This is explained by looking at what causes viscosity on the molecular level.

Another interesting aspect of viscosity covered in the video is non-Newtonian fluids, like shear thinning or shear thickening fluids, for which the relationship between the shear stress and the strain rate is non-linear.

Humanity has long been obsessed with heavier-than-air flight, and to this day it remains a topic that is shrouded in a bit of mystery.

The video below takes a detailed look at lift, starting with how it is closely linked to the pressure distribution a body. It covers a few different theories of lift, including the Bernoulli Principle and Newton’s Third Law explanations.

The lift force generated by an airfoil depends not only on the shape of the airfoil, but also on its angle of attack. Stalling is a a dangerous loss of lift that occurs when the angle of attack exceeds a critical value. This is covered in detail in the video.

Circulation is another key aspect of lift that it is important to grasp to develop a more complete understanding so that’s covered too, as is the Kutta condition, the principle that explains how flow leaves the trailing edge of the airfoil.

Drag and lift are the forces that act on a body moving through a fluid, or that act on a stationary object in a flowing fluid. We call these aerodynamic forces if the fluid is a gas like air, or hydrodynamic forces if the fluid is a liquid.

The video below is an animated introduction to aerodynamic drag. To learn about lift check out the page Understanding Aerodynamic Lift.

There are two main causes of drag – the first is the pressure distribution around the object. If flow separation occurs as the object moves through the fluid an area of low pressure is created behind it, which can significantly increase the drag force on the object. The other component of drag is caused by the shear stresses acting on the object. This component is most significant for streamlined bodies, because the surfaces of the object tend to be more aligned with the direction of flow.