Fluid Systems

You are driving with your parents when suddenly the car violently bounces up and down. Realizing it's just a pothole, you think nothing of it until you hear the dreaded "flapping noise" indicating a flat tire. Now, you are forced to spend the afternoon at the auto body shop waiting for the tire to be fixed. After some time, the mechanic places your car on the hydraulic lift. You may not know it, but that lift is an example of a fluid system. The lift involves an incompressible fluid and a U-shaped tube with movable pistons. The tube design involves one end with a smaller opening than the other because small forces applied to a smaller area can balance larger forces applied to a larger area. Keeping this example in mind, let this article further introduce the concept of fluid systems through definitions and examples.

Fig. 1- A hydraulic lift is an example of a fluid system.

Fluid Systems Definition

In our educational career, we are taught that there are three states of matter: solids, liquids, and gases. However, it is important to understand that liquids and gases are categorized as fluids. Fluids are systems of particles that easily move and change position.

Force is related to the pressure exerted on a fluid by the equation, \( F=PA \), where \(F\) is force, \( P \), is pressure, and \( A\) is area. Let's try a quick example to reinforce our understanding.

Solids, on the other hand, have a distinct shape and correspond to Newton's second law, \( F=ma\), where \( F\) is force, \( m\) is mass, and \( a \) is acceleration.

Fluid System Types

Fluids are classified into four categories depending on certain properties. These categories include ideal fluids, real fluids, Newtonian fluids, and non-Newtonian fluids. Ideal fluids are incompressible and have zero viscosity. Incompressible refers to fluids whose volume and density do not change due to pressure. Viscosity refers to friction because viscosity causes resistance to motion by causing shear or frictional forces to be present in between particles. No viscosity indicates that no friction is present in an ideal fluid. However, it is important to note that this type of fluid does not exist in reality. Real fluids can be compressible, and viscosity is present, which implies that friction is present. Compressible refers to fluids whose volume and density change due to pressure. Examples of real fluids include castor oil, petrol, and kerosene. Newtonian fluids have constant viscosity where viscosity is not affected by shear stress. Common examples of Newtonian fluids include water and air. Non-Newtonian fluids have variable viscosity meaning viscosity changes with respect to shear stress. Common examples of non-Newtonian fluids include salts, blood, toothpaste, and corn starch.

Shear Stress

When parallel objects slide past one another, this action is known as shearing. This phenomenon occurs in fluids and causes shearing stress.

Shearing stress is one of two types of stress fluids undergo. In physics, stress refers to a force per unit area acting on an infinitesimal surface. Stress is a vector quantity and is divided into normal stress and tangential stress. Normal stress includes pressures that act inward and perpendicular to the surface. Tangential stress includes shear stress. The main reason shear stress is present in fluids is friction due to viscosity. Fluids cannot resist shear stress. This means that when shear stress is applied to a fluid at rest, the fluid moves as it is unable to remain at rest.

Fluid System Components

The components that define fluids, as well as solids, are microscopic molecules. The movement of these molecules determines a substance's state of matter. In solids, the arrangement of particles is fixed as a result of not enough thermal energy present to overcome intermolecular interactions between particles. Hence, solids have a more compact structure because molecules can vibrate but cannot move or switch positions with neighboring molecules. Consequently, solids have definite shape and volume.

Fig.2 - The molecules in solids are arranged in a repeating pattern and cannot move freely.

In contrast, liquids, and gases have a more loosely packed structure. Liquids have partial energy that enables them to overcome intermolecular interactions. This fact allows the particles to move about freely while still being in close proximity to each other. Liquids have definite volume but no shape.

Fig. 3- Molecules in a liquid remain in close proximity while moving.

Gases, however, have enough energy to completely overcome intermolecular interactions. Meaning that the particles completely separate from one another and move about freely. Hence, gases have no definite shape or volume.

Fig.4- Molecules in a gas move about freely.

Importance of Fluid Systems

To understand the importance of fluid systems, let's look at hydraulics. Hydraulics is the practical application of fluids and uses fluid mechanics as its theoretical foundation. Fluid mechanics focus on the forces that arise due to the behavior of fluids. Fluid mechanics is divided into two parts: fluid dynamics and fluid statics. Fluid statics is the study of fluids at rest, while fluid dynamics is the study of fluids in motion. Hydraulic engineers apply fluid mechanics to flowing water within pipes, pumps, or open channels, i.e., lakes or rivers, as well as the containment of water in dams or tanks. Understanding fluid mechanics allows hydraulic engineers to design structures, and hydraulic systems powered by the pressure of a fluid, to withstand intense pressure. Consequently, engineers who specialize in this field deal with the technical issues that accompany the design and implementation of fluid infrastructure. However, in addition to fluid infrastructure, hydraulic engineers also design hydraulic machinery. When designing hydraulic machinery, they must carefully select the appropriate hydraulic fluid, a fluid that acts as an energy transfer, to ensure that a machine operates successfully.

Fluid Systems Examples

Fluid systems use the force of flowing liquids or gases to transport power. An easy way to understand this is to think about the act of breathing. For a fluid to move, a pressure difference is necessary. We create high-pressure and low-pressure areas every time we breathe that enable air to move in and out of our lungs. When we inhale, we do work as we expand our chest cavities to create an area of low pressure inside our lungs. An area of higher pressure exists outside our lungs, which then forces air into our lungs. When we exhale, we work to shrink our lung volume, which in turn increases the air pressure inside our lungs and forces air out. Now recall that power and work are related because power is the rate at which work is done. Thus, the act of breathing is an example of a fluid system.