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Fluid mechanics

What is a fluid in physics? Types, properties, and examples

What is a fluid in physics? Types, properties, and examples

Fluids are substances that can flow, such as water or oil. In physics, flow is defined as the movement of fluids. These substances do not have a fixed shape, so that they can adapt to the container's shape.

Fluid mechanics is the branch of physics that studies the behavior of fluids. This branch is divided into dynamic (if they are in motion) and static (if they are at rest).

For this, it is necessary to understand the concept of fluid, its main characteristics, the different types they are classified, and the properties that define them.

What is a fluid?

A fluid is a type of continuous medium made up of a set of particles held together by a weak attractive force.

Fluids are one of the four categories of matter. Matter can be divided into solids, liquids, gases, and plasma

Unlike solids, a fluid cannot withstand shear stress for any appreciable time.

Liquids have a definite shape but a variable volume. Liquids can change shape and adapt to the containers that contain them but cannot freely expand or contract. Gases, however, have neither a definite shape nor volume.

Fluid types

Fluids can be classified into several types, depending on their behavior in response to forces:

Newtonian fluids

In Newtonian fluids, the viscosity is considered constant. Newtonian fluids follow Newton's laws for the movement of fluids in which the viscosity is assumed to be constant. Therefore, the curve that describes the relationship between the stress against its strain rate is linear.

Most common liquids and gases fall into this category.

Non-Newtonian fluids

A non-Newtonian fluid is a liquid that does not behave according to Newton's law of viscosity.

When a non-newtonian fluid is at rest, it behaves like a liquid. However, its viscosity changes by exerting a force or changing the temperature. That is, it does not have a defined and constant viscosity value.

In general, viscosity decreases when the temperature rises.

Most non-Newtonian fluids are more viscous when subjected to a slow force and less viscous when subjected to a fast force.

A fluid with increased shear force rates can have a lower viscosity, leading to a lower pressure drop. Low viscosity is the most common type of non-Newtonian fluid.

Shear thinning is the non-newtonian behavior of fluids whose viscosity decreases under shear strain. It is sometimes considered synonymous with pseudo-plastic behavior and is usually defined as excluding time-dependent effects. A thixotropic fluid is a fluid that takes a finite time to attain equilibrium viscosity when introduced to an abrupt change in shear rate.

The Shear Thinning behavior is when a fluid behaves strangely when passed through an opening or at a very low speed, like in the paint on walls.

Dilatant material

A dilatant material is one in which the viscosity increases as the strain rate increases. An example of a non-Newtonian fluid is a shear-thickening fluid. It is not often found in pure materials but can occur when there are particles suspended in the liquid (suspension).

What is an ideal fluid?

The movement of a real fluid is very complex. To simplify its description, we will consider the behavior of an ideal fluid whose characteristics are the following:

  1. Non-viscous fluid: Internal friction between different parts of the fluid is neglected.

  2. Steady flow: The velocity of this type of fluid at a point is constant with time.

  3. Incompressible fluid: The density of the fluid remains constant with time.

  4. Irrotational flow: It does not present eddies; there is no angular momentum of the fluid concerning any point.

Fluids properties in physics

A fluid's properties define how fluid or gas behaves at rest and in motion. There are primary properties and secondary properties of the fluid.

Primary properties 

  • Pressure indicates force per unit of surface area.

  • Density indicates the mass per unit volume. A compressible fluid changes its density when we exert a force on it.

  • The fluid's specific weight is the ratio of the fluid's weight to the fluid's volume.

  • The specific volume of a fluid is defined as the ratio of fluid volume to the fluid's mass.

  • Temperature measures the notion of the heat of a fluid. The temperature is related to the kinetic energy of the particles in the system.

  • Internal energy is the total energy contained in a thermodynamic system.

  • A fluid's enthalpy is the amount of heat that is contained within the fluid. This can be used to measure the temperature of a fluid, as well as its specific heat capacity.

  • Entropy is the thermodynamic property that allows calculating the part of heat energy that cannot be used to produce work.

  • Specific heat is the amount of heat required to raise the temperature to a given value.

  • Viscosity is the difficulty in flowing.

  • Specific gravity is defined as the ratio of the density of a liquid compared to the density of water at 60 degrees F.

Secondary properties 

They characterize the specific behavior of fluids.

  • The viscosity of a fluid represents its resistance to flow.

  • Thermal conductivity indicates the ease with which heat passes through the fluid.

  • Surface tension is a manifestation of intermolecular forces within liquids. Surface tension means that the surface of any liquid behaves as if there were a tension membrane on it. 

  • Compression is the variation experienced by a fluid due to pressure stresses.

Examples of fluids

Some examples of fluids are:

  • Water is the most common fluid on Earth. We can find it statically in a jar or motion flowing in rivers or the sea when it is liquid. However, it behaves as a gas over the ebullition point of water.

  • An oil-hydraulic fluid has the mission to transmit the hydraulic power produced by the pump to one or several receptor organs. At the same time, it must lubricate the moving parts, protect the system from corrosion, and clean and cool or dissipate heat.

  • Air is a fluid composed of molecules in constant motion. When compressed, it becomes denser and exerts pressure. When heated, it expands and becomes less dense. The movement of air molecules makes wind possible.

  • Volcanic magma is a fluid that is found in the earth's mantle. It comprises molten rock, hot gases, and other materials.

  • Paint fluid is a liquid used to apply paint to surfaces. It can be made from various substances, but the most common is water-based paint fluid. This type of paint fluid is typically made from water, pigments, resins, and additives. Paint fluid can also be made from oil-based substances.

  • Noble gases neon (Ne), xenon (Xe), krypton (Kr), helium (He)

  • Blood is a fluid that circulates throughout the body and is composed of plasma, red blood cells, white blood cells, and platelets.

  • Wet mixtures of water with flour or water with cement.

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Publication Date: September 27, 2022
Last Revision: September 29, 2022