Table of Contents
Fluid is defined as substance which has capacity to flow.
It is a substance that deform continuously under the section of shear stress, no matter if it is small
A fluid is a substance that flows and takes the shape of its container. It is a state of matter that can include liquids, gases, and plasmas. Fluids are characterized by their ability to deform under the influence of applied forces, enabling them to flow and exert pressure.
Characteristics and properties of fluids:
Flowability: Fluids have the ability to flow and change shape when subjected to external forces. They lack a definite shape and conform to the shape of their container.
Incompressibility (for liquids): Liquids are generally considered to be incompressible, meaning their density remains nearly constant under ordinary pressures. However, gases and plasmas are compressible, and their density can change significantly with changes in pressure and temperature.
Viscosity: Viscosity is the measure of a fluid’s resistance to flow. It determines the internal friction within the fluid and influences its flow behavior. High-viscosity fluids flow more slowly and have a higher resistance to deformation, while low-viscosity fluids flow more easily.
Pressure: Fluids exert pressure on their surroundings. The pressure exerted by a fluid at a given point is perpendicular to the surface and acts in all directions. Pressure in a fluid increases with depth due to the weight of the fluid above.
Buoyancy: Fluids exert an upward force on objects immersed in them, known as buoyant force. This force is due to the difference in pressure between the top and bottom of the submerged object and is responsible for the phenomenon of objects floating or sinking in fluids.
Fluid dynamics: Fluid dynamics is the study of how fluids behave when they are in motion. It involves the study of fluid flow, turbulence, boundary layers, and various phenomena such as drag, lift, and flow patterns.
Fluids play a fundamental role in numerous areas of science, engineering, and everyday life. They are crucial in disciplines such as physics, chemistry, engineering, and environmental sciences. The understanding of fluid behavior and the principles of fluid mechanics are applied in fields like aerodynamics, hydrodynamics, hydraulics, chemical engineering, and many others.
Fluid mechanics is a branch of physics and engineering that studies the behavior of fluids, both liquids and gases, and the forces acting upon them. It is concerned with understanding and analyzing the motion, deformation, and interactions of fluids under various conditions.
The field of fluid mechanics encompasses a wide range of topics,
Fluid statics: This branch deals with fluids at rest, studying the equilibrium conditions and pressure distribution within a fluid. It includes concepts such as hydrostatic pressure, buoyancy, and the behavior of floating and submerged objects.
Fluid dynamics: Fluid dynamics focuses on the study of fluids in motion. It involves analyzing the forces and motions of fluids, the flow patterns, and the effects of viscosity, turbulence, and boundary layers. Key topics in fluid dynamics include conservation laws, fluid flow equations, flow visualization, and the study of flow phenomena such as drag, lift, and circulation.
Flow measurements: Techniques and instruments are used to measure various properties of fluid flow, including flow rate, pressure, velocity, and turbulence. Common measurement techniques include flowmeters, pressure sensors, velocity probes, and particle image velocimetry (PIV).
Fluid kinematics: This branch deals with the study of fluid motion without considering the forces causing it. It focuses on describing the motion of fluid particles, flow patterns, and the concept of streamlines, pathlines, and streaklines.
Fluid properties: Fluid mechanics also involves understanding the properties of fluids, such as density, viscosity, surface tension, and compressibility. These properties influence the behavior and flow characteristics of fluids.
Computational fluid dynamics (CFD): CFD is a numerical simulation technique used to analyze and predict fluid flow behavior using computational methods. It involves solving fluid flow equations and simulating complex flow phenomena using computer algorithms and models.
Type of fluids
The fluids may be classified into following types
1) Ideal fluid
A fluid, which is incompressible and having no viscosity, is known as ideal fluid.
In fluid mechanics, the term “ideal fluid” refers to a hypothetical fluid that possesses certain idealized properties. While real fluids, such as water or air, have various complexities and interactions, ideal fluids are used in theoretical analyses and mathematical models to simplify calculations and understand fundamental fluid behavior. Here are the key characteristics of an ideal fluid:
Incompressibility: An ideal fluid is assumed to be incompressible, meaning its density remains constant regardless of the applied pressure. This assumption allows for simplified calculations and eliminates the need to account for changes in density.
Zero viscosity: Ideal fluids are considered to have zero viscosity, implying that they flow without any internal friction or resistance. This characteristic implies that ideal fluids do not experience any energy losses due to viscosity, allowing for easier analysis of fluid flow patterns.
Irrotational flow: Ideal fluids are assumed to exhibit irrotational flow, meaning that there are no vortices or rotational motion within the fluid. This simplifying assumption eliminates the consideration of swirling or rotating fluid behavior.
Steady flow: Ideal fluids are often assumed to flow in a steady manner, meaning that the fluid properties, such as velocity and pressure, do not change with time at any point within the fluid. This assumption simplifies the analysis and allows for the determination of properties at a given point without considering temporal changes.
Conservation of energy: Ideal fluids obey the principle of conservation of energy, which means that the total energy within the fluid remains constant as it flows. This principle helps in understanding the behavior of ideal fluids in various situations.
2) Real fluid
A fluid which posses viscosity, is known as real fluid. All fluids in actual practice are real fluid.
Real fluids are fluids that exist in the physical world and exhibit properties that deviate from those of ideal fluids. Unlike ideal fluids, real fluids have viscosity, compressibility, and may experience turbulence. Here are some key characteristics of real fluids:
Viscosity: Real fluids have viscosity, which is the internal friction or resistance to flow within the fluid. Viscosity causes fluid layers to exert forces on each other when they move relative to one another. This property affects the flow behavior and can cause energy losses and fluid deformation.
Compressibility: Unlike ideal fluids, real fluids are compressible to some extent. When subjected to changes in pressure, real fluids experience changes in density. The degree of compressibility varies among different fluids and depends on factors such as pressure, temperature, and molecular properties.
Turbulence: Real fluids can exhibit turbulent flow, which is characterized by chaotic and irregular fluid motion. Turbulence occurs when the flow velocity exceeds a critical value, causing the fluid to form eddies, vortices, and fluctuations. Turbulent flow is more complex and harder to analyze than laminar flow.
Non-ideal behavior: Real fluids may deviate from ideal behavior in various ways. For example, they may exhibit non-Newtonian behavior, where the relationship between shear stress and shear rate is not linear. Real fluids can also experience phase changes, such as boiling or condensation, which affect their properties.
Energy losses: Due to viscosity and other factors, real fluids experience energy losses as they flow. These losses manifest as frictional losses, heat dissipation, and pressure drops along the flow path. Understanding and managing these energy losses are crucial in practical applications of fluid mechanics.
3) Newtonion fluid
A real fluid, in which the shear is directly proportional to the rate of Shear strain is known as newtonion fluid.
A Newtonian fluid is a type of real fluid that exhibits linear, proportional behavior between shear stress and shear rate. It follows Newton’s law of viscosity, which states that the shear stress (τ) within the fluid is directly proportional to the shear rate (du/dy), where du is the change in velocity in the direction perpendicular to the flow and dy is the perpendicular distance. Mathematically, this relationship can be expressed as:
τ = μ (du/dy)
where μ (mu) is the dynamic viscosity of the fluid, which represents its resistance to flow.
Key characteristics of Newtonian fluids include:
Constant viscosity: Newtonian fluids have a constant viscosity that does not change with shear rate. Regardless of the applied shear stress, the fluid’s resistance to flow remains consistent. Common examples of Newtonian fluids include water, air, and many oils.
Linear relationship: The relationship between shear stress and shear rate is linear. When plotted on a graph, the data points form a straight line passing through the origin. The slope of this line represents the dynamic viscosity of the fluid.
Shear-independent behavior: The behavior of Newtonian fluids is not affected by the magnitude or direction of the shear rate. As long as the fluid is within its applicable range of temperatures and pressures, its viscosity remains constant.
It’s important to note that not all fluids behave as Newtonian fluids. Some fluids, called non-Newtonian fluids, exhibit shear-thinning or shear-thickening behavior, meaning their viscosity changes with shear rate. Examples of non-Newtonian fluids include ketchup, toothpaste, and blood. The study of fluid mechanics considers both Newtonian and non-Newtonian fluids to analyze and understand the behavior of real fluids in various applications.
4) Non-newtonion fluid
A real fluid in which the shear stress is not proportional to the rate of shear strain is known as Non-newtonion fluid.
A non-Newtonian fluid is a type of real fluid that does not follow Newton’s law of viscosity. Unlike Newtonian fluids, the relationship between shear stress and shear rate in non-Newtonian fluids is nonlinear and can exhibit different behaviors under varying conditions. Non-Newtonian fluids can have viscosity that varies with shear rate, time, pressure, or other factors.
Types of non-Newtonian fluids
Shear-thinning fluids (pseudoplastic): These fluids exhibit a decrease in viscosity as the shear rate increases. As the shear stress is applied, the fluid becomes thinner and flows more easily. Examples of shear-thinning fluids include certain types of paints, gels, and some polymers.
Shear-thickening fluids (dilatant): These fluids experience an increase in viscosity as the shear rate increases. The fluid becomes thicker and resists flow more as the shear stress is applied. Silly putty and some mixtures of cornstarch and water are examples of shear-thickening fluids.
Bingham plastics: These fluids have a yield stress, meaning they require a certain minimum stress to start flowing. Below the yield stress, they behave like a solid, but once the yield stress is exceeded, they flow like a fluid. Toothpaste and certain drilling muds are examples of Bingham plastics.
Thixotropic fluids: These fluids exhibit a time-dependent decrease in viscosity under constant shear stress. Over time, the fluid becomes less viscous and flows more easily. When the shear stress is removed, the fluid gradually regains its original viscosity. Some types of printing inks and clay suspensions are thixotropic fluids.
Rheopectic fluids: These fluids exhibit a time-dependent increase in viscosity under constant shear stress. Over time, the fluid becomes more viscous and resists flow more. After the shear stress is removed, the fluid gradually returns to its original viscosity. Certain lubricants and some polymer solutions exhibit rheopectic behavior.
5) Ideal plastic fluid
A fluid in which shear stress is more than the yield value and shear stress is proportional to rate of strain is known as ideal plastic fluid.
The term “ideal plastic fluid” is not commonly used in fluid mechanics or rheology. The term “plastic fluid” itself is somewhat contradictory since fluids, by definition, exhibit continuous flow under applied shear stress. Plasticity, on the other hand, refers to the ability of a material to undergo non-reversible deformation without flowing.
However, in certain contexts, “ideal plastic” can be used as an approximation to describe a fluid-like material that exhibits a yield stress and flows plastically once the yield stress is exceeded. This can be seen in materials such as Bingham plastics, which behave as solid-like substances below the yield stress but flow as fluids above it.
In the context of ideal plastic behavior, some simplifying assumptions can be made, similar to those used for ideal fluids.
These assumptions may include
Yield stress: The material has a well-defined yield stress below which it behaves as a solid and does not flow. Above the yield stress, the material starts to deform and flow like a fluid.
No dependency on shear rate: Once the yield stress is exceeded, the material flows with a constant viscosity, independent of the applied shear rate. This assumption simplifies the analysis and allows for the prediction of the material’s behavior under various conditions.
No time-dependent behavior: Ideal plastic fluids do not exhibit time-dependent changes in their viscosity or flow behavior once the yield stress is surpassed. This assumption eliminates the consideration of thixotropy or rheopectic behavior.
Application of Fluid Mechanics
Fluid mechanics has numerous practical applications in various fields. Here are some notable areas where fluid mechanics is applied:
Fluid mechanics plays a vital role in designing and analyzing aircraft, spacecraft, and other aerial vehicles. It is used to understand aerodynamic forces, lift and drag, and the behavior of fluids in propulsion systems, such as jet engines and rockets.
Fluid mechanics is essential for designing and optimizing the performance of vehicles. It is used to study airflow around vehicles, analyze fuel combustion in engines, optimize cooling systems, and improve vehicle aerodynamics for reduced drag and increased fuel efficiency.
Civil Engineering and Hydraulic Engineering
Fluid mechanics is applied in the design of water supply systems, drainage systems, and wastewater treatment plants. It is used to analyze the flow of water in rivers, canals, and pipes, and to design structures such as dams, channels, and bridges to withstand fluid forces.
Fluid mechanics is used to study and model the movement of pollutants and contaminants in air and water. It helps understand dispersion patterns, pollution control measures, and the behavior of fluids in natural environments.
Chemical and Process Engineering
Fluid mechanics is crucial for the design and optimization of chemical processes and industrial systems involving fluid flow. It is used to understand mixing, reaction kinetics, heat transfer, and mass transfer in chemical reactors and industrial equipment.
Fluid mechanics plays a role in understanding blood flow in the circulatory system, air flow in the respiratory system, and fluid transport in cells and tissues. It is applied in medical devices, such as ventilators and artificial hearts, and in the study of drug delivery systems.
Energy and Power Generation
Fluid mechanics is employed in the design and analysis of power plants, including hydraulic turbines, wind turbines, and nuclear reactors. It helps optimize energy extraction from fluid flows and improve efficiency in energy conversion processes.
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