Fluid: Difference between revisions
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{{Continuum mechanics}} | {{Continuum mechanics}} | ||
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==Overview== | |||
A '''fluid''' is defined as a substance that continually deforms (flows) under an applied [[shear stress]] regardless of how small the applied stress. All liquids and all gases are fluids. Fluids are a subset of the [[Phase (matter)|phases of matter]] and include [[liquid]]s, [[gas|gases]], [[Plasma physics|plasmas]] and, to some extent, [[plasticity (physics)|plastic solids]]. | A '''fluid''' is defined as a substance that continually deforms (flows) under an applied [[shear stress]] regardless of how small the applied stress. All liquids and all gases are fluids. Fluids are a subset of the [[Phase (matter)|phases of matter]] and include [[liquid]]s, [[gas|gases]], [[Plasma physics|plasmas]] and, to some extent, [[plasticity (physics)|plastic solids]]. | ||
Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between [[plasticity (physics)|solids]] and fluids is not entirely obvious. The distinction is made by evaluating the [[viscosity]] of the substance. | Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between [[plasticity (physics)|solids]] and fluids is not entirely obvious. The distinction is made by evaluating the [[viscosity]] of the substance. Silly Putty can be considered either a solid or a fluid, depending on the time period over which it is observed. | ||
Fluids display such properties as:<br /> | Fluids display such properties as:<br /> | ||
* not resisting deformation, or resisting it only lightly ([[viscosity]]), and<br /> | * not resisting deformation, or resisting it only lightly ([[viscosity]]), and<br /> | ||
* the ability to flow (also described as the ability to take on the shape of the container).<br /> | * the ability to flow (also described as the ability to take on the shape of the container).<br /> | ||
These properties are typically a function of their inability to support a [[shear stress]] in static | These properties are typically a function of their inability to support a [[shear stress]] in static equilibrium. | ||
Solids can be subjected to shear stresses, and to normal stresses - both | Solids can be subjected to shear stresses, and to normal stresses - both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called [[pressure]]. Real fluids display [[viscosity]] and so are capable of being subjected to low levels of [[shear stress]]. | ||
In a solid, shear stress is a function of [[Strain (materials science)|strain]], but in a fluid, shear stress is a function of rate of strain. A consequence of this behavior is [[Pascal's law]] which describes the role of [[pressure]] in characterizing a fluid's state. | In a solid, shear stress is a function of [[Strain (materials science)|strain]], but in a fluid, shear stress is a function of rate of strain. A consequence of this behavior is [[Pascal's law]] which describes the role of [[pressure]] in characterizing a fluid's state. | ||
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*[[Non-Newtonian fluid]]s : where stress is proportional to rate of strain, its higher powers and derivatives. | *[[Non-Newtonian fluid]]s : where stress is proportional to rate of strain, its higher powers and derivatives. | ||
The behavior of fluids can be described by the [[Navier-Stokes equations]] - a set of | The behavior of fluids can be described by the [[Navier-Stokes equations]] - a set of partial differential equations which are based on:<br /> | ||
* continuity ([[continuity equation#fluid dynamics|conservation of mass]]), | * continuity ([[continuity equation#fluid dynamics|conservation of mass]]), | ||
* conservation of [[linear momentum]] | * conservation of [[linear momentum]] | ||
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==See also== | ==See also== | ||
*[[Entropy]] | *[[Entropy]] | ||
*[[Rheology]] | *[[Rheology]] | ||
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*[[Superfluid]] | *[[Superfluid]] | ||
*[[Perfect fluid]] | *[[Perfect fluid]] | ||
[[Category:Fluid mechanics]] | [[Category:Fluid mechanics]] | ||
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[[Category:Phases of matter]] | [[Category:Phases of matter]] | ||
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{{WikiDoc Sources}} | {{WikiDoc Sources}} | ||
Revision as of 17:40, 29 May 2009
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Overview
A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of how small the applied stress. All liquids and all gases are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.
Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between solids and fluids is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered either a solid or a fluid, depending on the time period over which it is observed.
Fluids display such properties as:
- not resisting deformation, or resisting it only lightly (viscosity), and
- the ability to flow (also described as the ability to take on the shape of the container).
These properties are typically a function of their inability to support a shear stress in static equilibrium.
Solids can be subjected to shear stresses, and to normal stresses - both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called pressure. Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.
In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of rate of strain. A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a fluid's state.
Depending on the relationship between shear stress, and the rate of strain and its derivatives, fluids can be characterized as:
- Newtonian fluids : where stress is directly proportional to rate of strain, and
- Non-Newtonian fluids : where stress is proportional to rate of strain, its higher powers and derivatives.
The behavior of fluids can be described by the Navier-Stokes equations - a set of partial differential equations which are based on:
- continuity (conservation of mass),
- conservation of linear momentum
- conservation of angular momentum
- conservation of energy.
The study of fluids is fluid mechanics, which is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion.