A fluid is a subset of the phases of matter and includes liquids, gases, plasmas, and, to a lesser degree, plastic solids. A fluid is defined as a material that is continually (deformed) under constant shear stress, regardless of how slight.
The states of matter that we are acquainted with are solid, liquid, gas, and plasma. We are also aware of how these materials respond to stress or force. We now know how molecules change in solids, liquids, and gases.
In this article, we discuss what fluid is, its properties, types, and classifications.
Let’s drive in!
What is Fluid?
Fluids constantly distort, meaning they never stop flowing. A fluid is a material that readily responds to outside pressure and lacks a distinct form. Any liquid, gas, or substance that cannot tolerate a tangential or shearing force when at rest is considered a fluid.
The fluid undergoes a constant form change when the aforementioned forces are applied to it. Substances that have zero shear modulus, or the inability to withstand shear stress, are called fluids.
We would have heard the dehydrated patient’s recommendation to drink additional fluids to make up for the water loss in our daily lives. We must consume more fluids since they aid in the digestion of meals.
Properties of Fluids
Below are the main properties of fluids, included:
Cavitation:
Cavitation occurs when a fluid has a sudden decrease in pressure below the vapor pressure, causing bubbles or voids to develop inside the fluid.
Vapour Pressure:
The equilibrium pressure of a vapor above its liquid is known as the vapour pressure of a liquid. Temperature affects a liquid’s vapour pressure. The vapor pressure rises in tandem with the temperature.
Surface Tension:
Liquid surfaces have a propensity to reduce their surface area. Cohesive forces between liquid molecules produce surface tension. Note: Although insects are denser than water, surface tension is the primary cause of their ability to float on the water.
Specific Gravity:
It is the specific weight of the fluid in question divided by the specific weight of a reference fluid. Relative density is another name for specific gravity. It has no unit and is symbolized by the letter “S.”
Specific Weight:
The weight per unit volume of a fluid is known as its specific weight (γ). N/m3 is the SI unit of specific weight. The relationship between specific weight and density is γ = ρg. Temperature affects a fluid’s specific weight.
Specific Volume:
Its definition is the material’s volume divided by its mass. The reciprocal of the material’s density is its specific volume. Density and specific volume are negatively correlated. m3.kg-1 is the SI unit of specific volume.
Pressure:
The force that a fluid applies per unit area is its pressure. There are two circumstances in which fluid pressure occurs: open and closed. Hydrostatic pressure is the pressure in a static fluid at any given location.
Temperature:
The temperature determines the body’s level of warmth or coolness. Kelvin, Celsius, or Fahrenheit are used to measure the fluid’s temperature. The Kelvin scale is used in accordance with the International System of Units.
Viscosity:
Viscosity controls how much resistance a fluid provides to shear stress. A fluid is said to be inviscid if it provides no resistance to shear force.
Density:
The mass of a fluid per unit volume is its density. A substance’s density changes in proportion to its volume. kg.m-3 is the SI unit of density.
Types Of Fluids
Fluids may be roughly categorized into the following kinds based on the connection between shear and velocity gradient:
- Ideal fluid: An ideal fluid is one that has no viscosity and is incompressible. Since all fluids have some viscosity, the ideal fluid is only a hypothetical one.
- Real fluid: A fluid is considered real if it has viscosity. In real life, every fluid is a genuine fluid.
- Newtonian fluid: A real fluid is referred to as a Newtonian fluid if the shear stress is directly proportional to the rate of shear strain (or velocity gradient).
- Non-Newtonian fluid: A fluid is considered non-Newtonian if its shear stress is not proportional to its shear strain rate (or velocity gradient). Such a fluid has an arbitrary functional connection, either explicit or implicit, between the shear stress and the rate of strain.
- Ideal plastic fluid: The perfect plastic fluid An ideal plastic fluid is a real fluid where the shear stress is proportional to the rate of shear strain (or velocity gradient) and greater than the yield value.
- Incompressible fluid: A fluid is said to be incompressible if its density remains constant while an external force is applied.
- Compressible fluid: A fluid is said to be compressible when its density varies when an external force is applied. As can be seen below, density and viscosity vary depending on the kind of fluid.
Classification Of Fluids
Fluids are classified into four types depending on their flow; they are:
Viscous or non-viscous:
Fluids that are thicker or more viscous are referred to as viscous fluids; they are often quite gloppy. Shampoo and motor oil are two examples.
Non-viscous fluids are those that have a relatively low viscosity or thickness. These are fluids that have very little or no resistance to internal friction. No kinetic energy is lost during the flow of non-viscous fluids.
For example, the superfluid liquid helium
Compressible fluid and incompressible fluids:
Fluids are categorized as either incompressible or compressible according to their Mach Number. The compressible fluid’s Mach Number ranges from 0.3 to 1, whereas the incompressible fluid’s is less than 0.3.
Steady fluid:
A fluid that maintains a consistent density throughout its flow is said to be steady.
Unsteady fluid:
A fluid that varies in velocity between any two places while flowing is said to be unsteady.
Rotational or irrotational
A fluid element is said to be rotating if, while traveling through the flow, the angle between the two intersecting lines of its boundary changes. The fluid is divided into two categories based on its angular motion: rotational fluid and irrotational fluid.
An irrotational flow occurs when the fluid rotates as a whole without any change in the angles between the boundary lines. They are divided into two categories based on the study of fluids:
Fluids statics: The method by which fluids at rest or in motion exert pressure on someone is known as fluid statics.
Fluids dynamics – is the study of how fluids flow while they are moving. Aerodynamics and hydrodynamics are two well-known subfields of fluid dynamics.
FAQs
What is a fluid?
A fluid is a material that readily responds to external pressure and lacks a distinct structure.
The study of the flow of fluids in motion is known as?
Fluid dynamics is the study of fluid flow when in motion.
Define steady fluid?
A fluid is said to be steady if its density doesn’t change as it flows.
State true or false: Fluids do not oppose deformation.
It’s true.
Is air a fluid?
Air is a fluid, yes. A fluid is any material that flows.