Glossary

1.1 Fluid properties

6 min read•august 20, 2024

Fluid properties are the foundation of aerodynamics. Understanding , pressure, , and is crucial for analyzing how air behaves around aircraft. These properties determine how fluids flow, exert forces, and respond to changes in their environment.

This knowledge forms the basis for more complex aerodynamic concepts. By grasping these fundamental properties, you'll be better equipped to tackle advanced topics like lift, drag, and boundary layer theory in aircraft design and performance analysis.

Density and specific gravity

  • Density is a fundamental property of fluids that describes the mass per unit volume of a substance
  • compares the density of a fluid to a reference fluid (water for liquids, air for gases) at a specified temperature
  • Understanding density and specific gravity is crucial in aerodynamics for determining the behavior of fluids, such as air, in various flow conditions

Pressure in static fluids

Absolute vs gauge pressure

Top images from around the web for Absolute vs gauge pressure

  • Pressure Definition; Absolute & Gauge Pressure – Foundations of Chemical and Biological ... View original

    Is this image relevant?

  • Gauge Pressure, Absolute Pressure, and Pressure Measurement | Physics View original

    Is this image relevant?

  • Pressure Definition; Absolute & Gauge Pressure – Foundations of Chemical and Biological ... View original

    Is this image relevant?

1 of 3

Top images from around the web for Absolute vs gauge pressure

  • Pressure Definition; Absolute & Gauge Pressure – Foundations of Chemical and Biological ... View original

    Is this image relevant?

  • Gauge Pressure, Absolute Pressure, and Pressure Measurement | Physics View original

    Is this image relevant?

  • Pressure Definition; Absolute & Gauge Pressure – Foundations of Chemical and Biological ... View original

    Is this image relevant?

1 of 3
  • measures the total pressure exerted by a fluid, including atmospheric pressure
  • measures the pressure relative to the local atmospheric pressure
  • In aerodynamics, both absolute and gauge pressures are used depending on the specific application and reference frame

Hydrostatic pressure variation

  • is the pressure exerted by a fluid at rest due to its weight
  • Pressure in a static fluid increases linearly with depth, following the equation p=ρghp = \rho gh, where pp is pressure, ρ\rho is fluid density, gg is acceleration due to gravity, and hh is the depth below the surface
  • Hydrostatic pressure variation is important in understanding the forces acting on submerged objects and the stability of floating bodies

Viscosity of fluids

Dynamic vs kinematic viscosity

  • , also known as absolute viscosity, is a measure of a fluid's resistance to shear stress and is expressed in units of Pa⋅sPa \cdot s (pascal-seconds)
  • is the ratio of dynamic viscosity to fluid density and is expressed in units of m2/sm^2/s (square meters per second)
  • In aerodynamics, both dynamic and kinematic viscosities are used to characterize the flow behavior and resistance to deformation in fluids

Newtonian vs non-Newtonian fluids

  • exhibit a linear relationship between shear stress and strain rate, with a constant viscosity independent of the applied shear stress (examples: air, water)
  • have a viscosity that varies with the applied shear stress or strain rate (examples: blood, paint)
  • Most fluids encountered in aerodynamics, such as air, are considered Newtonian fluids under normal conditions

Compressibility of fluids

Bulk modulus of elasticity

  • The , denoted by KK, is a measure of a fluid's resistance to uniform compression
  • It is defined as the ratio of the change in pressure to the fractional change in volume, expressed as K=−VdpdVK = -V \frac{dp}{dV}, where VV is the initial volume, dpdp is the change in pressure, and dVdV is the change in volume
  • The compressibility of fluids is an important consideration in high-speed aerodynamics, where significant pressure changes can occur

Speed of sound

  • The , denoted by cc, is the speed at which pressure waves propagate through a fluid medium
  • It is related to the bulk modulus of elasticity and fluid density by the equation c=Kρc = \sqrt{\frac{K}{\rho}}, where KK is the bulk modulus and ρ\rho is the fluid density
  • The speed of sound is a critical parameter in aerodynamics, as it determines the behavior of fluids in compressible flow regimes (examples: supersonic and hypersonic flows)

Surface tension and capillary effects

  • is a property of fluids that arises from the cohesive forces between molecules at the surface
  • It is responsible for the formation of droplets, bubbles, and menisci, and plays a role in capillary action, where fluids rise or fall in narrow spaces (examples: water in a glass tube, ink in a pen)
  • While surface tension and are less significant in large-scale aerodynamic flows, they can be important in micro-scale fluid systems and in understanding the behavior of liquid fuels

Fluid statics

Buoyancy and Archimedes' principle

  • is the upward force exerted by a fluid on an object immersed in it, due to the pressure difference between the top and bottom of the object
  • states that the buoyant force acting on an object is equal to the weight of the fluid displaced by the object
  • Buoyancy and Archimedes' principle are important in understanding the stability and behavior of objects submerged in fluids, such as aircraft fuel tanks and floating structures

Hydrostatic force on submerged surfaces

  • is the force exerted by a fluid at rest on a submerged surface due to the pressure distribution
  • The magnitude of the hydrostatic force depends on the fluid density, the surface area, and the depth of the centroid of the surface below the fluid surface
  • Calculating hydrostatic forces is crucial in designing aircraft components that interact with fluids, such as control surfaces and fuel tanks

Ideal fluid concept

Inviscid flow assumption

  • An inviscid fluid is a hypothetical fluid with zero viscosity, meaning it offers no resistance to shear stress
  • The simplifies the analysis of fluid flow by neglecting the effects of viscosity, which is a reasonable approximation for high-Reynolds-number flows away from solid boundaries
  • In aerodynamics, the inviscid flow assumption is often used in preliminary design and analysis, such as in potential flow theory and thin airfoil theory

Irrotational flow condition

  • Irrotational flow is a type of fluid flow in which the fluid particles do not rotate about their own axes
  • Mathematically, the is expressed as ∇×V⃗=0\nabla \times \vec{V} = 0, where V⃗\vec{V} is the velocity vector field and ∇×\nabla \times is the curl operator
  • Irrotational flow is a common assumption in potential flow theory and simplifies the analysis of fluid flow in many aerodynamic applications

Fluid kinematics

Streamlines and pathlines

  • are curves that are everywhere tangent to the velocity vector field at a given instant in time
  • are the actual paths followed by individual fluid particles over time
  • In , streamlines and pathlines coincide, while in , they may differ
  • Understanding streamlines and pathlines is essential for visualizing and analyzing fluid flow patterns in aerodynamics

Steady vs unsteady flow

  • Steady flow is a type of fluid flow in which the flow properties (velocity, pressure, density) at any point do not change with time
  • Unsteady flow, also known as non-steady or transient flow, is a type of fluid flow in which the flow properties vary with time
  • Many aerodynamic problems can be approximated as steady flow for simplicity, but unsteady flow analysis is necessary for capturing time-dependent phenomena (examples: turbulence, vortex shedding)

Laminar vs turbulent flow

  • is a type of fluid flow characterized by smooth, parallel layers of fluid with no mixing between the layers
  • is a type of fluid flow characterized by chaotic and irregular motion, with mixing between fluid layers
  • The transition from laminar to turbulent flow depends on the Reynolds number, which is a dimensionless quantity that relates the inertial forces to the viscous forces in a fluid
  • Understanding the differences between laminar and turbulent flow is crucial in aerodynamics, as it affects drag, heat transfer, and mixing processes

Fluid dynamics

Conservation of mass

  • The principle states that mass cannot be created or destroyed in a closed system
  • For fluid flow, this principle is expressed by the , which relates the time rate of change of fluid density to the divergence of the mass flux
  • In aerodynamics, the conservation of mass is a fundamental principle used in deriving the governing equations of fluid motion

Conservation of momentum

  • The principle states that the total momentum of a closed system remains constant in the absence of external forces
  • For fluid flow, this principle is expressed by the , which relate the time rate of change of fluid momentum to the forces acting on the fluid (pressure, viscous, and body forces)
  • The conservation of momentum is a key principle in aerodynamics, used in analyzing the forces acting on aircraft and predicting their motion

Conservation of energy

  • The principle states that energy cannot be created or destroyed, only converted from one form to another
  • For fluid flow, this principle is expressed by the energy equation, which relates the time rate of change of fluid energy to the work done by the fluid and the heat transfer
  • In aerodynamics, the conservation of energy is important in analyzing the performance of aircraft engines, heat exchangers, and other thermal systems

Key Terms to Review (31)

Absolute Pressure: Absolute pressure is the total pressure exerted on a fluid, measured relative to a perfect vacuum. This concept is crucial in understanding fluid behavior, as it combines both the atmospheric pressure and the gauge pressure of a fluid. It is essential for various applications in fluid mechanics and aerodynamics, influencing calculations involving buoyancy, flow dynamics, and other fluid properties.
Archimedes' Principle: Archimedes' Principle states that any object submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. This fundamental concept is crucial in understanding how fluids behave and interact with objects, providing insights into fluid properties and fluid statics, including pressure variations and buoyancy effects.
Bulk Modulus of Elasticity: The bulk modulus of elasticity is a measure of a material's resistance to uniform compression. It quantifies how much pressure is required to decrease the volume of a substance by a certain amount, indicating how compressible or incompressible the fluid is under pressure. A high bulk modulus suggests that a fluid is less compressible, while a low bulk modulus indicates greater compressibility, which is critical for understanding fluid behavior in various applications such as aerodynamics and hydraulics.
Buoyancy: Buoyancy is the upward force experienced by an object submerged in a fluid, which opposes the weight of the object and is determined by the fluid's density and the volume of the displaced fluid. This phenomenon is essential for understanding how objects float or sink in different fluids, linking it to the properties of fluids and their static behaviors.
Capillary Effects: Capillary effects refer to the phenomenon that occurs when liquid rises or falls in a narrow space due to the balance of adhesive and cohesive forces. This behavior is especially noticeable in thin tubes or porous materials, where the interactions between the liquid and the solid surfaces play a crucial role. Understanding capillary effects is important as it influences various fluid behaviors in engineering and natural systems.
Compressibility: Compressibility is a measure of how much a fluid's density changes when pressure is applied. This property is crucial for understanding fluid behavior under various conditions, particularly at high velocities where the effects of pressure changes become significant. Compressibility influences how fluids expand and contract, impacting phenomena such as shock waves, flow separation, and heat transfer in aerodynamics.
Conservation of Energy: Conservation of energy is a fundamental principle stating that energy cannot be created or destroyed, only transformed from one form to another. This principle is crucial in understanding fluid dynamics as it relates to how energy is conserved in systems involving fluid flow, allowing us to analyze various phenomena including pressure changes, velocity variations, and temperature effects in moving fluids.
Conservation of Mass: Conservation of mass is a fundamental principle stating that mass cannot be created or destroyed in an isolated system. This principle is critical in fluid dynamics, as it dictates how mass flows through a system and forms the foundation for analyzing fluid behavior, ensuring that the mass entering a control volume equals the mass leaving it. Understanding this concept allows for insights into the interactions of fluid properties, the formulation of conservation laws, and the behavior of isentropic flow under varying conditions.
Conservation of Momentum: Conservation of momentum is a fundamental principle stating that the total momentum of a closed system remains constant over time, provided that no external forces are acting on it. This concept is crucial in understanding fluid dynamics, where changes in fluid velocity and density are governed by how momentum is transferred and conserved in interactions, making it essential for analyzing flow patterns and forces in fluids.
Continuity equation: The continuity equation is a fundamental principle in fluid dynamics that expresses the conservation of mass within a fluid flow. It states that for an incompressible fluid, the mass flow rate must remain constant from one cross-section of a flow to another, linking the velocity and area of flow at different points. This relationship is crucial in understanding how fluids behave in various conditions, from static scenarios to dynamic flow through nozzles and turbulent environments.
Density: Density is a measure of mass per unit volume of a substance, usually expressed in kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³). It plays a critical role in understanding fluid behavior, as it influences buoyancy, pressure distributions, and flow characteristics. Additionally, density affects how sound waves propagate through fluids and helps in the visualization of fluid flow patterns, as well as the interactions of fluids with surfaces, impacting heat transfer and skin friction.
Dynamic Viscosity: Dynamic viscosity is a measure of a fluid's resistance to flow when subjected to shear stress. It is a fundamental property that describes how 'thick' or 'sticky' a fluid is, and plays a crucial role in understanding the behavior of fluids in motion. This property is essential in the study of fluid mechanics, influencing factors such as skin friction and heat transfer in various applications.
Gauge pressure: Gauge pressure refers to the pressure of a fluid relative to atmospheric pressure. It measures how much pressure is above or below the surrounding atmospheric pressure, which means that gauge pressure can be positive or negative depending on whether the fluid pressure exceeds or is less than atmospheric pressure. Understanding gauge pressure is essential for analyzing fluid behavior and is commonly used in various applications, such as in hydraulics and aerodynamics.
Hydrostatic Force: Hydrostatic force is the pressure exerted by a fluid at rest due to the weight of the fluid above it. This force acts perpendicular to any surface in contact with the fluid and is crucial for understanding how fluids behave under static conditions, especially in terms of pressure variation with depth.
Hydrostatic Pressure: Hydrostatic pressure is the pressure exerted by a fluid at rest due to the weight of the fluid above it. This pressure increases with depth, reflecting the influence of gravity on the fluid's weight, which is crucial in understanding fluid behavior and characteristics. Hydrostatic pressure plays a significant role in fluid properties, influencing how fluids interact with surrounding structures and systems, and is essential in analyzing scenarios involving fluids at rest.
Inviscid Flow Assumption: The inviscid flow assumption is a simplification in fluid dynamics where the effects of viscosity are neglected, allowing for the analysis of fluid behavior without accounting for internal friction. This assumption is crucial when analyzing idealized flows, as it simplifies the governing equations and provides insights into the flow characteristics of fluids at high speeds or in low-viscosity scenarios.
Irrotational Flow Condition: Irrotational flow condition refers to a type of fluid motion where the flow has no vorticity, meaning that there is no local rotation of fluid particles about their center of mass. This condition implies that the flow can be described using potential functions, simplifying many analyses in fluid dynamics. In the context of fluid properties, irrotational flow is important because it allows for the application of potential flow theory and helps in understanding the behavior of inviscid fluids.
Kinematic Viscosity: Kinematic viscosity is a measure of a fluid's resistance to flow under the influence of gravity, defined as the ratio of dynamic viscosity to fluid density. It is an essential property that describes how a fluid behaves in motion, particularly in the context of how it flows in different situations like pipes or around objects. Understanding kinematic viscosity helps predict flow patterns, energy losses, and overall fluid behavior in various applications.
Laminar flow: Laminar flow is a type of fluid motion where the fluid flows in parallel layers with minimal disruption between them, resulting in smooth and orderly movement. This flow regime is characterized by low velocities and high viscosity, allowing for predictable behavior that can be analyzed using simplified mathematical models.
Navier-Stokes Equations: The Navier-Stokes equations are a set of nonlinear partial differential equations that describe the motion of viscous fluid substances. They form the foundation for fluid dynamics and are crucial for understanding various phenomena, including turbulence and boundary layer behavior, making them essential in aerodynamics and related fields.
Newtonian Fluids: Newtonian fluids are a class of fluids that have a constant viscosity regardless of the shear rate applied to them. This means that they exhibit a linear relationship between shear stress and shear rate, making their behavior predictable and easy to model in fluid dynamics. Common examples include water, air, and most gases, which are essential for understanding fluid properties in various engineering applications.
Non-Newtonian Fluids: Non-Newtonian fluids are substances whose viscosity changes with the applied stress or shear rate, unlike Newtonian fluids which have a constant viscosity regardless of these conditions. This unique property means that non-Newtonian fluids can behave in complex ways, such as becoming thicker when stirred or thinner when left to sit. Understanding these behaviors is crucial when examining fluid properties, as it affects how fluids flow and interact under various conditions.
Pathlines: Pathlines are the trajectories that individual fluid particles follow over time as they move through a flow field. They provide insight into the motion of fluid elements and can help visualize how fluids behave, connecting fluid properties, visualization techniques, and post-processing methods to understand flow patterns more comprehensively.
Specific Gravity: Specific gravity is a dimensionless quantity that compares the density of a fluid to the density of a reference substance, typically water at 4°C. It provides insight into how a fluid will behave under various conditions, particularly in determining buoyancy and stability within fluids. Understanding specific gravity is essential for predicting how fluids interact with forces such as pressure and gravity, especially when analyzing fluid properties and statics.
Speed of sound: The speed of sound is the rate at which pressure waves travel through a medium, often defined in meters per second (m/s). This speed varies based on the properties of the medium, such as its temperature, density, and elasticity, and is crucial for understanding how sound behaves in different fluid environments.
Steady Flow: Steady flow refers to a fluid flow condition where the velocity of the fluid at any given point does not change with time. In this state, properties such as pressure, density, and velocity remain constant at any specific location within the flow field, allowing for simplified analysis of fluid behavior. This concept is crucial for understanding various fluid dynamics principles, as it establishes a baseline for examining how fluids interact with forces and structures over time.
Streamlines: Streamlines are imaginary lines that represent the flow of fluid in a streamline flow, where the fluid moves in a steady and continuous manner. These lines are tangential to the velocity vector of the flow at any point, illustrating how fluid elements move through space and providing insights into fluid behavior around objects and surfaces.
Surface Tension: Surface tension is the property of a liquid that causes its surface to behave like a stretched elastic membrane. This phenomenon arises from the cohesive forces between liquid molecules, which create a tendency for the surface to minimize its area. Surface tension plays a crucial role in various fluid behaviors, including droplet formation, capillarity, and the stability of floating objects.
Turbulent flow: Turbulent flow is a type of fluid motion characterized by chaotic and irregular fluctuations in velocity and pressure, resulting from the interactions between layers of fluid. This complex flow pattern leads to mixing and energy dissipation, making it critical for understanding various aerodynamic phenomena such as lift, drag, and heat transfer.
Unsteady Flow: Unsteady flow refers to a fluid motion where the velocity at a point changes over time, meaning that the flow conditions are not constant. This type of flow can lead to varying pressures and forces acting on objects in the fluid, making it crucial to understand in scenarios such as oscillating airflow or rapidly changing environmental conditions. The effects of unsteady flow can significantly impact the behavior and performance of fluids in different applications, linking closely with fluid properties and conservation principles.
Viscosity: Viscosity is a measure of a fluid's resistance to deformation and flow, often described as the 'thickness' or 'stickiness' of a fluid. It plays a crucial role in determining how fluids behave under various conditions, affecting everything from fluid properties to the movement of air over surfaces. Understanding viscosity is essential for analyzing forces acting on fluids, predicting flow patterns, and studying the performance of aerodynamic bodies.
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Back
Practice QuizGlossary
Practice QuizGlossary
Next guide