# How Mastering fluid dynamics Work

Mastering Fluid Dynamics, the branch of classical physics that deals with the response of matter to shear stress, is critical in many fields including engineering, chemistry and biology. Flow influences transport and mixing of materials, as well as the behavior of living organisms, and it has applications in a wide range of practical disciplines such as mechanical, aerospace and civil engineering, meteorology and astrophysics. This Learning Track provides a strong foundation in the fundamental principles of fluid mechanics, including fluid statics, the study of fluids at rest; and fluid dynamics, which studies the effect of forces on the motion of liquids or gases.

The rheology of fluids is a complex topic, and understanding how a fluid behaves requires an appreciation of the interaction between the fluid particles, a knowledge of their size and shape, and an ability to predict how those properties change over time. This learning track starts with a brief overview of the properties of a fluid, followed by lessons that cover the rotation of a fluid, visualization and measurement techniques for a fluid's state, the different coordinate systems used to mathematically model a fluid's motion, and the various equations that describe the force exerted on a fluid.

A key property of any fluid is its viscosity, which is a measure of the fluid's resistance to shear deformation and reflects how "sticky" or thick it feels. In fluids, a dynamic viscosity is directly proportional to the shear stress (du/dy) divided by the velocity gradient (du/dy/s). This relationship can be described as the dynamic viscosity of a fluid in terms of a formula that has been named after French physiologist Jean-Baptiste Poiseuille (1799-1869), and the SI unit for viscosity is the pascal second (Pa s).

While fluids may exhibit a variety of behaviors, most flows are either laminar or turbulent. A laminar flow has a uniform velocity over the entire domain of the flow, while a turbulent flow has a high level of kinetic energy and is more likely to disperse or deform the fluid particles in a given volume. In most cases, it is desirable for a flow to be laminar because it is more efficient and less energy is lost.

Some flows, such as jet streams and water currents on the surface of lakes or rivers, are almost always turbulent. In most other cases, however, the fluid is a smooth and predictable stream with a low Reynolds number, where the magnitude of inertial effects is relatively small compared to the magnitude of viscous effects. Flows with a low Reynolds number are often referred to as Newtonian, and the fluid in such a flow is said to be ideal.

On a much larger scale, fluid dynamics is responsible for hurricanes, tornadoes, flooding and other natural phenomena that can dramatically alter environments. In addition, foraging birds and insects depend on fluid flows to move them and deliver food supplies. Even the smallest organisms such as caddis fly larvae and black flies use the flow of streams to find and capture food.