|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| __NOTOC__
| | Jerrie is what you would call me but Which i don't like when somebody use my full nick name. The job I've been occupying for years is a real people manager. Guam is even I've always been income. What I truly doing is fish are [http://Thesaurus.com/browse/suffering suffering] from acne and I'll be building something else along to barefoot jogging. Go to some of my website to find apart more: http://prometeu.net<br><br>my web blog :: clash of clans hack tool; [http://prometeu.net simply click the following webpage], |
| A '''Power-law fluid''', or the '''[[Wilhelm Ostwald|Ostwald]]–[[Armand de Waele|de Waele]] relationship''', is a type of [[generalized Newtonian fluid]] for which the [[shear stress]], ''τ'', is given by
| |
| | |
| :<math>\tau = K \left( \frac {\partial u} {\partial y} \right)^n </math>
| |
| | |
| where:
| |
| *''K'' is the ''flow consistency index'' ([[SI]] units Pa•s<sup>''n''</sup>),
| |
| *∂''u''/∂''y'' is the [[shear rate]] or the [[velocity]] [[gradient]] perpendicular to the plane of shear (SI unit s<sup>−1</sup>), and
| |
| *''n'' is the ''flow behavior index'' (dimensionless).
| |
| | |
| The quantity | |
| | |
| :<math>\mu_{\operatorname{eff}} = K \left( \frac {\partial u} {\partial y} \right)^{n-1} </math>
| |
| | |
| represents an ''apparent'' or ''effective [[viscosity]]'' as a function of the shear rate (SI unit Pa•s).
| |
| | |
| Also known as the [[Wilhelm Ostwald|Ostwald]]–[[Armand de Waele|de Waele]] power law<ref name=OdW>e.g. [[G. W. Scott Blair]] ''et al.'', ''J. Phys. Chem''., (1939) 43 (7) 853–864. Also the ''de Waele-Ostwald'' law, e.g [[Markus Reiner]] ''et al.'', ''Kolloid Zeitschrift'' (1933) 65 (1) 44-62</ref><ref name=Ostwald1929>Ostwald called it the de Waele-Ostwald equation: ''Kolloid Zeitschrift'' (1929) 47 (2) 176-187</ref> this [[mathematical]] relationship is useful because of its simplicity, but only approximately describes the behaviour of a real [[non-Newtonian fluid]]. For example, if ''n'' were less than one, the power law predicts that the effective viscosity would decrease with increasing shear rate indefinitely, requiring a fluid with infinite viscosity at rest and zero viscosity as the shear rate approaches infinity, but a real fluid has both a minimum and a maximum effective viscosity that depend on the [[physical chemistry]] at the [[molecular]] level. Therefore, the power law is only a good description of fluid behaviour across the range of shear rates to which the coefficients were fitted. There are a number of other models that better describe the entire flow behaviour of shear-dependent fluids, but they do so at the expense of simplicity, so the power law is still used to describe fluid behaviour, permit mathematical predictions, and correlate experimental data.
| |
| | |
| Power-law fluids can be subdivided into three different types of fluids based on the value of their flow behaviour index:
| |
| :{|align="left" border="1" cellspacing="0"
| |
| |''n''
| |
| |Type of fluid
| |
| |-
| |
| |<1
| |
| |[[Pseudoplastic]]
| |
| |-
| |
| |1
| |
| |[[Newtonian fluid]]
| |
| |-
| |
| |>1
| |
| |[[Dilatant]] (less common)
| |
| |}
| |
| | |
| <br style="clear: left"/>
| |
| | |
| == Pseudoplastic fluids ==
| |
| [[Pseudoplastic]], or ''shear-thinning'' fluids have a lower apparent viscosity at higher shear rates, and are usually [[solution]]s of large, [[polymer]]ic molecules in a solvent with smaller molecules. It is generally supposed that the large molecular chains tumble at random and affect large volumes of fluid under low shear, but that they gradually align themselves in the direction of increasing shear and produce less resistance.
| |
| | |
| A common household example of a strongly shear-thinning fluid is styling gel, which primarily composed of water and a fixative such as a vinyl acetate/vinylpyrrolidone copolymer (PVP/PA). If one were to hold a sample of hair gel in one hand and a sample of [[corn syrup]] or [[glycerine]] in the other, they would find that the hair gel is much harder to pour off the fingers (a low shear application), but that it produces much less resistance when rubbed between the fingers (a high shear application).
| |
| | |
| == Newtonian fluids ==
| |
| A [[Newtonian fluid]] is a power-law fluid with a behaviour index of 1, where the shear stress is directly proportional to the shear rate:
| |
| | |
| :<math>\tau = \mu \frac {\partial u} {\partial y} </math> | |
| | |
| These fluids have a constant viscosity, ''μ'', across all shear rates and include many of the most common fluids, such as [[water]], most aqueous [[solution]]s, [[oil]]s, [[corn syrup]], [[glycerine]], [[air]] and other [[gas]]es.
| |
| | |
| While this holds true for relatively low shear rates, at high rates most oils in reality also behave in a non-Newtonian fashion and thin. Typical examples include oil films in automotive engine shell bearings and to a lesser extent in geartooth contacts.
| |
| | |
| == Dilatant fluids ==
| |
| [[Dilatant]], or ''shear-thickening'' fluids increase in apparent viscosity at higher shear rates. They are rarely encountered, but one common example is an uncooked paste of [[cornstarch]] and [[water]]. Under high shear the water is squeezed out from between the [[starch]] [[molecule]]s, which are able to interact more strongly.
| |
| | |
| While not strictly a dilatant fluid, [[Silly Putty]] is an example of a material that shares these viscosity characteristics. Another use is in a [[viscous coupling]] in which if both ends of the coupling are spinning at the same (rotational) speed, the fluid viscosity is minimal, but if the ends of the coupling differ greatly in speed, the coupling fluid becomes very viscous. Such couplings have applications as a lightweight, passive mechanism for a passenger automobile to automatically switch from two-wheel drive to four-wheel drive such as when the vehicle is stuck in snow and the primary driven axle starts to spin due to loss of traction under one or both tires.
| |
| | |
| == Velocity profile in a circular pipe ==
| |
| | |
| Just like a [[Newtonian fluid]] in a circular pipe gives a quadratic velocity profile (see [[Hagen–Poiseuille equation]]), a power-law fluid will result in a power-law velocity profile,
| |
| | |
| :<math>u(r) = \frac{n}{n+1}\left(\frac{dp}{dz}\frac{1}{2K}\right)^\frac{1}{n}\left[R^\frac{n+1}{n}-r^\frac{n+1}{n}\right] </math> | |
| | |
| where <math>u(r)</math> is the (radially) local axial velocity, <math>dp/dz</math> is the pressure gradient along the pipe, and <math>R</math> is the pipe radius.
| |
| | |
| ==See also==
| |
| *[[Power law]]
| |
| *[[Rheology]]
| |
| *[[Navier-Stokes equations]]
| |
| *[[Fluid]]
| |
| *[[First-Order Fluid]]
| |
| *[[Cross fluid]]
| |
| *[[Carreau fluid]]
| |
| *[[Generalized Newtonian fluid]]
| |
| *[[Herschel-Bulkley fluid]]
| |
| | |
| ==References==
| |
| {{reflist}}
| |
| | |
| [[Category:Non-Newtonian fluids]]
| |
Jerrie is what you would call me but Which i don't like when somebody use my full nick name. The job I've been occupying for years is a real people manager. Guam is even I've always been income. What I truly doing is fish are suffering from acne and I'll be building something else along to barefoot jogging. Go to some of my website to find apart more: http://prometeu.net
my web blog :: clash of clans hack tool; simply click the following webpage,