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Navier-Stokes-ligningene

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Navier-Stokes-ligningene
Kartesiske koordinaterSylinderkoordinaterKulekoordinater

Navier–Stokes-ligningene, oppkalt etter Claude-Louis Navier og George Gabriel Stokes, er en ligning som beskriver bevegelse av viskøse væsker og gasser. Ligningen er en ikke-lineær, partiell differensialligning.

Kildeløs: Denne artikkelen mangler kildehenvisninger, og opplysningene i den kan dermed være vanskelige å verifisere. Kildeløst materiale kan bli fjernet. Helt uten kilder. (10. okt. 2015)

Vektorligningen er

ρ ( ∂ v ∂ t + v ⋅ ∇ v ) = − ∇ p + ∇ ⋅ T + f . {\displaystyle \rho \left({\frac {\partial \mathbf {v} }{\partial t}}+\mathbf {v} \cdot \nabla \mathbf {v} \right)=-\nabla p+\nabla \cdot \mathbb {T} +\mathbf {f} .} {\displaystyle \rho \left({\frac {\partial \mathbf {v} }{\partial t}}+\mathbf {v} \cdot \nabla \mathbf {v} \right)=-\nabla p+\nabla \cdot \mathbb {T} +\mathbf {f} .}

For et newtonsk fluid kan leddet ∇ ⋅ T {\displaystyle \nabla \cdot \mathbb {T} } {\displaystyle \nabla \cdot \mathbb {T} } erstattes med μ ∇ 2 v {\displaystyle \mu \nabla ^{2}\mathbf {v} } {\displaystyle \mu \nabla ^{2}\mathbf {v} }, der μ {\displaystyle \mu } {\displaystyle \mu } 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,

ρ ( ∂ u ∂ t + u ∂ u ∂ x + v ∂ u ∂ y + w ∂ u ∂ z ) = − ∂ p ∂ x + μ ( ∂ 2 u ∂ x 2 + ∂ 2 u ∂ y 2 + ∂ 2 u ∂ z 2 ) + ρ g x {\displaystyle \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}} {\displaystyle \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}}
ρ ( ∂ v ∂ t + u ∂ v ∂ x + v ∂ v ∂ y + w ∂ v ∂ z ) = − ∂ p ∂ y + μ ( ∂ 2 v ∂ x 2 + ∂ 2 v ∂ y 2 + ∂ 2 v ∂ z 2 ) + ρ g y {\displaystyle \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}} {\displaystyle \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}}
ρ ( ∂ w ∂ t + u ∂ w ∂ x + v ∂ w ∂ y + w ∂ w ∂ z ) = − ∂ p ∂ z + μ ( ∂ 2 w ∂ x 2 + ∂ 2 w ∂ y 2 + ∂ 2 w ∂ z 2 ) + ρ g z {\displaystyle \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}} {\displaystyle \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}}

For en ikke-kompressibel væske gir kontinuitetsligningen:

∂ u ∂ x + ∂ v ∂ y + ∂ w ∂ z = 0 {\displaystyle {\partial u \over \partial x}+{\partial v \over \partial y}+{\partial w \over \partial z}=0} {\displaystyle {\partial u \over \partial x}+{\partial v \over \partial y}+{\partial w \over \partial z}=0}

Sylinderkoordinater

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

ρ ( ∂ u r ∂ t + u r ∂ u r ∂ r + u θ r ∂ u r ∂ θ + u z ∂ u r ∂ z − u θ 2 r ) = − ∂ p ∂ r + μ [ 1 r ∂ ∂ r ( r ∂ u r ∂ r ) + 1 r 2 ∂ 2 u r ∂ θ 2 + ∂ 2 u r ∂ z 2 − u r r 2 − 2 r 2 ∂ u θ ∂ θ ] + ρ g r {\displaystyle \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}} {\displaystyle \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}}
ρ ( ∂ u θ ∂ t + u r ∂ u θ ∂ r + u θ r ∂ u θ ∂ θ + u z ∂ u θ ∂ z + u r u θ r ) = − 1 r ∂ p ∂ θ + μ [ 1 r ∂ ∂ r ( r ∂ u θ ∂ r ) + 1 r 2 ∂ 2 u θ ∂ θ 2 + ∂ 2 u θ ∂ z 2 + 2 r 2 ∂ u r ∂ θ − u θ r 2 ] + ρ g θ {\displaystyle \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 }} {\displaystyle \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 }}
ρ ( ∂ u z ∂ t + u r ∂ u z ∂ r + u θ r ∂ u z ∂ θ + u z ∂ u z ∂ z ) = − ∂ p ∂ z + μ [ 1 r ∂ ∂ r ( r ∂ u z ∂ r ) + 1 r 2 ∂ 2 u z ∂ θ 2 + ∂ 2 u z ∂ z 2 ] + ρ g z {\displaystyle \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}} {\displaystyle \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}}

Kontinuitetsligningen gir:

1 r ∂ ∂ r ( r u r ) + 1 r ∂ u θ ∂ θ + ∂ u z ∂ z = 0. {\displaystyle {\frac {1}{r}}{\frac {\partial }{\partial r}}\left(ru_{r}\right)+{\frac {1}{r}}{\frac {\partial u_{\theta }}{\partial \theta }}+{\frac {\partial u_{z}}{\partial z}}=0.} {\displaystyle {\frac {1}{r}}{\frac {\partial }{\partial r}}\left(ru_{r}\right)+{\frac {1}{r}}{\frac {\partial u_{\theta }}{\partial \theta }}+{\frac {\partial u_{z}}{\partial z}}=0.}

Kulekoordinater

ρ ( ∂ u r ∂ t + u r ∂ u r ∂ r + u ϕ r sin ⁡ ( θ ) ∂ u r ∂ ϕ + u θ r ∂ u r ∂ θ − u ϕ 2 + u θ 2 r ) = − ∂ p ∂ r + ρ g r {\displaystyle \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}} {\displaystyle \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}}
μ [ 1 r 2 ∂ ∂ r ( r 2 ∂ u r ∂ r ) + 1 r 2 sin ⁡ ( θ ) 2 ∂ 2 u r ∂ ϕ 2 + 1 r 2 sin ⁡ ( θ ) ∂ ∂ θ ( sin ⁡ ( θ ) ∂ u r ∂ θ ) − 2 u r + ∂ u θ ∂ θ + u θ cot ⁡ ( θ ) r 2 + 2 r 2 sin ⁡ ( θ ) ∂ u ϕ ∂ ϕ ] {\displaystyle \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]} {\displaystyle \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]}
ρ ( ∂ u θ ∂ t + u r ∂ u θ ∂ r + u ϕ r sin ⁡ ( θ ) ∂ u θ ∂ ϕ + u θ r ∂ u θ ∂ θ + u r u θ − u ϕ 2 cot ⁡ ( θ ) r ) = − 1 r ∂ p ∂ θ + ρ g θ {\displaystyle \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 }} {\displaystyle \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 }}
μ [ 1 r 2 ∂ ∂ r ( r 2 ∂ u θ ∂ r ) + 1 r 2 sin ⁡ ( θ ) 2 ∂ 2 u θ ∂ ϕ 2 + 1 r 2 sin ⁡ ( θ ) ∂ ∂ θ ( sin ⁡ ( θ ) ∂ u θ ∂ θ ) + 2 r 2 ∂ u r ∂ θ − u θ + 2 cos ⁡ ( θ ) ∂ u ϕ ∂ ϕ r 2 sin ⁡ ( θ ) 2 ] {\displaystyle \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]} {\displaystyle \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]}
ρ ( ∂ u ϕ ∂ t + u r ∂ u ϕ ∂ r + u ϕ r sin ⁡ ( θ ) ∂ u ϕ ∂ ϕ + u θ r ∂ u ϕ ∂ θ + u r u ϕ + u ϕ u θ cot ⁡ ( θ ) r ) = − 1 r sin ⁡ ( θ ) ∂ p ∂ ϕ + ρ g ϕ {\displaystyle \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 }} {\displaystyle \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 }}
μ [ 1 r 2 ∂ ∂ r ( r 2 ∂ u ϕ ∂ r ) + 1 r 2 sin ⁡ ( θ ) 2 ∂ 2 u ϕ ∂ ϕ 2 + 1 r 2 sin ⁡ ( θ ) ∂ ∂ θ ( sin ⁡ ( θ ) ∂ u ϕ ∂ θ ) + 2 ∂ u r ∂ ϕ + 2 cos ⁡ ( θ ) ∂ u θ ∂ ϕ − u ϕ r 2 sin ⁡ ( θ ) 2 ] {\displaystyle \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]} {\displaystyle \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]}

Kontinuitetsligningen gir:

1 r 2 ∂ ∂ r ( r 2 u r ) + 1 r sin ⁡ ( θ ) ∂ u ϕ ∂ ϕ + 1 r sin ⁡ ( θ ) ∂ ∂ θ ( sin ⁡ ( θ ) u θ ) = 0 {\displaystyle {\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} {\displaystyle {\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}
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