雷諾平均納維-斯托克斯方程

雷諾平均納維－斯托克斯方程（英語：Reynolds-averaged Navier–Stokes equations，簡稱RANS）是流體力學中一種用來描述湍流的時均納維－斯托克斯方程。其思想是將湍流運動看作時間平均與瞬時脈動兩種流動的疊加，即任一物理量 $\phi$ 滿足：

\phi ={\bar {\phi }}+\phi '.

其中， ${\bar {\phi }}$ 為時均值， $\phi '$ 為脈動值。時均值可定義為：

{\bar {\phi }}={\frac {1}{\Delta t}}\int _{t}^{t+\Delta t}\phi (t)dt.

如果不考慮密度脈動的影響，對納維－斯托克斯方程中的物理量按上述方法取時間平均，可得到可壓縮流體平均流動的控制方程（即雷諾平均方程）：^{[注 1]}

{\frac {\partial \rho }{\partial t}}+\operatorname {div} (\rho \mathbf {u} )=0

{\frac {\partial (\rho u)}{\partial t}}+\operatorname {div} (\rho u\mathbf {u} )=\operatorname {div} (\mu \ \operatorname {grad} u)-{\frac {\partial p}{\partial x}}+\left[-{\frac {\partial (\rho {\overline {u'^{2}}})}{\partial x}}-{\frac {\partial (\rho {\overline {u'v'}})}{\partial y}}-{\frac {\partial (\rho {\overline {u'w'}})}{\partial z}}\right]+S_{u}

{\frac {\partial (\rho v)}{\partial t}}+\operatorname {div} (\rho v\mathbf {u} )=\operatorname {div} (\mu \ \operatorname {grad} v)-{\frac {\partial p}{\partial y}}+\left[-{\frac {\partial (\rho {\overline {u'v'}})}{\partial x}}-{\frac {\partial (\rho {\overline {v'^{2}}})}{\partial y}}-{\frac {\partial (\rho {\overline {v'w'}})}{\partial z}}\right]+S_{v}

{\frac {\partial (\rho w)}{\partial t}}+\operatorname {div} (\rho w\mathbf {u} )=\operatorname {div} (\mu \ \operatorname {grad} w)-{\frac {\partial p}{\partial z}}+\left[-{\frac {\partial (\rho {\overline {u'w'}})}{\partial x}}-{\frac {\partial (\rho {\overline {v'w'}})}{\partial y}}-{\frac {\partial (\rho {\overline {w'^{2}}})}{\partial z}}\right]+S_{w}

如果使用張量中的指標符號，則又可表示為：

{\frac {\partial \rho }{\partial t}}+{\frac {\partial }{\partial x_{i}}}(\rho u_{i})=0

{\frac {\partial }{\partial t}}(\rho u_{i})+{\frac {\partial }{\partial x_{j}}}(\rho u_{i}u_{j})=-{\frac {\partial p}{\partial x_{i}}}+{\frac {\partial }{\partial x_{j}}}(\mu {\frac {\partial u_{i}}{\partial x_{j}}}-\rho {\overline {u_{i}'u_{j}'}})+S_{i}

上式中的 $-{\overline {u_{i}'u_{j}'}}$ 被稱作雷諾應力，即：

\tau _{ij}=-{\overline {u_{i}'u_{j}'}}

注釋