Mastering Fluid Dynamics

In the world of physics, there are many fluid concepts that are applicable to liquids that are either at rest or in motion. Physicists have created the subdiscipline of fluid dynamics that studies the flow of these liquids and gases. The fluids that are most often of interest to scientists and engineers are those that are in motion, such as air and water.

The study of such liquids in motion is called Mastering Fluid Dynamics and is a vital aspect of physics. It can have numerous practical applications, such as reducing the drag of automobile bodies and designing more efficient airplanes and wind turbines.

As a result, fluid mechanics is an important part of the physics curriculum at all levels, from elementary school through graduate school. One of the fundamental concepts in fluid mechanics is the principle of momentum. This principle states that a moving parcel of a fluid experiences a net force equal to the sum of inertial forces and the velocity gradient (the difference between the fluid’s velocity and its speed). The fluid will continue in this direction until another force is applied that opposes its momentum. This is why it is essential to understand the force-velocity relationship when learning fluid mechanics.

Mastering Viscosity and Centipoise

The resistance of a fluid to the application of shear stress is described by its dynamic viscosity, which is a function of the tangential force per unit area required to move a plane in one layer past a plane in another layer at a specified distance apart. Viscosity is commonly measured using a viscometer or rheometer and is usually expressed in poises, where one poise is equal to 0.1 Pas. However, it is also sometimes reported in centipoise, where 1 cP is equal to 1/10 of a poise. This is because water, a common reference fluid, has a dynamic viscosity of 1 cP at 20 degC.

A rough broad division can be made between fluids that offer little resistance to shear stress and those that are highly resistant. The former are known as Newtonian fluids, named for the English physicist who observed that their behavior obeyed his law of friction: the shear stress in a fluid at any given moment is proportional to the rate of change in its velocity.

Non-Newtonian fluids, on the other hand, are sensitive to changes in the force exerted on them. They do not always obey the principle of conservation of energy and, as a result, they can sometimes experience an unbalanced asymmetry in the distribution of mass within their control volume, which is also known as a stagnation point.

Flow Visualization

For the purposes of visualizing flow, physicists often use figures called streamlines, streaklines and pathlines. Streamlines are straight lines that represent the tangent at each point in the fluid. A streakline is the location of a fluid element that has been moved from a point in space to a point at some time t, while a pathline is the line of movement for the fluid elements themselves over time.