Redigerer Navier-Stokes-ligningene

{{Kildeløs|Helt uten kilder.|dato=10. okt. 2015}}
'''Navier–Stokes-ligningene''', oppkalt etter [[Claude-Louis Navier]] og [[George Gabriel Stokes]], er en [[ligning (matematikk)|ligning]] som beskriver bevegelse av [[Viskositet|viskøse]] [[væske]]r og [[gass]]er. Ligningen er en [[Ikke-lineært system|ikke-lineær]], partiell [[differensialligning]].

Vektorligningen er

<math>\rho \left(\frac{\partial \mathbf{v}}{\partial t} + \mathbf{v} \cdot \nabla \mathbf{v}\right) = -\nabla p + \nabla \cdot\mathbb{T} + \mathbf{f}.</math>

For et newtonsk [[fluid]] kan leddet <math> \nabla \cdot\mathbb{T} </math> erstattes med <math>  \mu \nabla^2 \mathbf{v} </math>, der <math> \mu </math> er den dynamiske viskositetskonstanten for fluidet.

==Kartesiske koordinater==

Ved å skrive ut komponentene i vektorligningen over får vi følgende ligninger for impulsen i 3-D,

:<math> \rho \left(\frac{\partial u}{\partial t} + u \frac{\partial u}{\partial x} + v \frac{\partial u}{\partial y}+ w \frac{\partial u}{\partial z}\right) =  -\frac{\partial p}{\partial x} + \mu \left(\frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} + \frac{\partial^2 u}{\partial z^2}\right) + \rho g_x</math>

:<math> \rho \left(\frac{\partial v}{\partial t} + u \frac{\partial v}{\partial x} + v \frac{\partial v}{\partial y}+ w \frac{\partial v}{\partial z}\right) = -\frac{\partial p}{\partial y} + \mu \left(\frac{\partial^2 v}{\partial x^2} + \frac{\partial^2 v}{\partial y^2} + \frac{\partial^2 v}{\partial z^2}\right) + \rho g_y</math>

:<math> \rho \left(\frac{\partial w}{\partial t} + u \frac{\partial w}{\partial x} + v \frac{\partial w}{\partial y}+ w \frac{\partial w}{\partial z}\right) = -\frac{\partial p}{\partial z} + \mu \left(\frac{\partial^2 w}{\partial x^2} + \frac{\partial^2 w}{\partial y^2} + \frac{\partial^2 w}{\partial z^2}\right) + \rho g_z</math>

For en ikke-kompressibel væske gir kontinuitetsligningen:

:<math>{\partial u \over \partial x} + {\partial v \over \partial y} + {\partial w \over \partial z} = 0</math>

===Sylinderkoordinater===

Et variabelskifte på ligningssettet i kartesiske koordinater gir impulsligningene for r, θ, og ''z'':

:<math>
\rho \left(\frac{\partial u_r}{\partial t} + u_r \frac{\partial u_r}{\partial r} + \frac{u_{\theta}}{r} \frac{\partial u_r}{\partial \theta} + u_z \frac{\partial u_r}{\partial z} - \frac{u_{\theta}^2}{r}\right) = 
-\frac{\partial p}{\partial r} +
\mu \left[\frac{1}{r}\frac{\partial}{\partial r}\left(r \frac{\partial u_r}{\partial r}\right) + \frac{1}{r^2}\frac{\partial^2 u_r}{\partial \theta^2} + \frac{\partial^2 u_r}{\partial z^2} - \frac{u_r}{r^2} - \frac{2}{r^2}\frac{\partial u_{\theta}}{\partial \theta}\right] + \rho g_r</math>

:<math>
\rho \left(\frac{\partial u_{\theta}}{\partial t} + u_r \frac{\partial u_{\theta}}{\partial r} + \frac{u_{\theta}}{r} \frac{\partial u_{\theta}}{\partial \theta} + u_z \frac{\partial u_{\theta}}{\partial z} + \frac{u_r u_{\theta}}{r}\right) = 
-\frac{1}{r}\frac{\partial p}{\partial \theta} +
\mu \left[\frac{1}{r}\frac{\partial}{\partial r}\left(r \frac{\partial u_{\theta}}{\partial r}\right) + \frac{1}{r^2}\frac{\partial^2 u_{\theta}}{\partial \theta^2} + \frac{\partial^2 u_{\theta}}{\partial z^2} + \frac{2}{r^2}\frac{\partial u_r}{\partial \theta} - \frac{u_{\theta}}{r^2}\right] + \rho g_{\theta}</math>

:<math>
\rho \left(\frac{\partial u_z}{\partial t} + u_r \frac{\partial u_z}{\partial r} + \frac{u_{\theta}}{r} \frac{\partial u_z}{\partial \theta} + u_z \frac{\partial u_z}{\partial z}\right) = 
-\frac{\partial p}{\partial z} + \mu \left[\frac{1}{r}\frac{\partial}{\partial r}\left(r \frac{\partial u_z}{\partial r}\right) + \frac{1}{r^2}\frac{\partial^2 u_z}{\partial \theta^2} + \frac{\partial^2 u_z}{\partial z^2}\right] + \rho g_z</math>

Kontinuitetsligningen gir:

:<math>
\frac{1}{r}\frac{\partial}{\partial r}\left(r u_r\right) + 
\frac{1}{r}\frac{\partial u_\theta}{\partial \theta} + 
\frac{\partial u_z}{\partial z} = 0.</math>

===Kulekoordinater===

:<math>
\rho \left(\frac{\partial u_r}{\partial t} + u_r \frac{\partial u_r}{\partial r} + \frac{u_{\phi}}{r \sin(\theta)} \frac{\partial u_r}{\partial \phi} + \frac{u_{\theta}}{r} \frac{\partial u_r}{\partial \theta} - \frac{u_{\phi}^2 + u_{\theta}^2}{r}\right) = -\frac{\partial p}{\partial r} + \rho g_r</math>
::<math>
\mu \left[
\frac{1}{r^2} \frac{\partial}{\partial r}\left(r^2 \frac{\partial u_r}{\partial r}\right) + 
\frac{1}{r^2 \sin(\theta)^2} \frac{\partial^2 u_r}{\partial \phi^2} + 
\frac{1}{r^2 \sin(\theta)} \frac{\partial}{\partial \theta}\left(\sin(\theta) \frac{\partial u_r}{\partial \theta}\right) - 
2 \frac{u_r + \frac{\partial u_{\theta}}{\partial \theta} + u_{\theta} \cot(\theta)}{r^2} + 
\frac{2}{r^2 \sin(\theta)} \frac{\partial u_{\phi}}{\partial \phi}
\right]
</math>

:<math>
\rho \left(\frac{\partial u_{\theta}}{\partial t} + u_r \frac{\partial u_{\theta}}{\partial r} + \frac{u_{\phi}}{r \sin(\theta)} \frac{\partial u_{\theta}}{\partial \phi} + \frac{u_{\theta}}{r} \frac{\partial u_{\theta}}{\partial \theta} + \frac{u_r u_{\theta} - u_{\phi}^2 \cot(\theta)}{r}\right) = -\frac{1}{r} \frac{\partial p}{\partial \theta} + \rho g_{\theta}</math>
::<math>
\mu \left[
\frac{1}{r^2} \frac{\partial}{\partial r}\left(r^2 \frac{\partial u_{\theta}}{\partial r}\right) + 
\frac{1}{r^2 \sin(\theta)^2} \frac{\partial^2 u_{\theta}}{\partial \phi^2} + 
\frac{1}{r^2 \sin(\theta)} \frac{\partial}{\partial \theta}\left(\sin(\theta) \frac{\partial u_{\theta}}{\partial \theta}\right) + 
\frac{2}{r^2} \frac{\partial u_r}{\partial \theta} - 
\frac{u_{\theta} + 2 \cos(\theta) \frac{\partial u_{\phi}}{\partial \phi}}{r^2 \sin(\theta)^2}
\right]
</math>

:<math>
\rho \left(\frac{\partial u_{\phi}}{\partial t} + u_r \frac{\partial u_{\phi}}{\partial r} + \frac{u_{\phi}}{r \sin(\theta)} \frac{\partial u_{\phi}}{\partial \phi} + \frac{u_{\theta}}{r} \frac{\partial u_{\phi}}{\partial \theta} + \frac{u_r u_{\phi} + u_{\phi} u_{\theta} \cot(\theta)}{r}\right) = -\frac{1}{r \sin(\theta)} \frac{\partial p}{\partial \phi} + \rho g_{\phi}</math>
::<math>
\mu \left[
\frac{1}{r^2} \frac{\partial}{\partial r}\left(r^2 \frac{\partial u_{\phi}}{\partial r}\right) + 
\frac{1}{r^2 \sin(\theta)^2} \frac{\partial^2 u_{\phi}}{\partial \phi^2} + 
\frac{1}{r^2 \sin(\theta)} \frac{\partial}{\partial \theta}\left(\sin(\theta) \frac{\partial u_{\phi}}{\partial \theta}\right) + 
\frac{2 \frac{\partial u_r}{\partial \phi} + 2 \cos(\theta) \frac{\partial u_{\theta}}{\partial \phi} - u_{\phi}}{r^2 \sin(\theta)^2}
\right]
</math>

Kontinuitetsligningen gir:

:<math>
\frac{1}{r^2}\frac{\partial}{\partial r}\left(r^2 u_r\right) + 
\frac{1}{r \sin(\theta)}\frac{\partial u_\phi}{\partial \phi} + 
\frac{1}{r \sin(\theta)}\frac{\partial}{\partial \theta}\left(\sin(\theta) u_\theta\right) = 0</math>
{{Autoritetsdata}}

[[Kategori:Fluiddynamikk]]
[[Kategori:Partielle differensialligninger]]