Nikiya Anton Bettey 2021: 09-22-2021-0824

Wednesday, September 22, 2021

09-22-2021-0824 - vorticity equation

The vorticity equation of fluid dynamics describes evolution of the vorticity $ω$ of a particle of a fluid as it moves with its flow, that is, the local rotation of the fluid (in terms of vector calculus this is the curl of the flow velocity). The equation is:

{\begin{aligned}{\frac {D{\boldsymbol {\omega }}}{Dt}}&={\frac {\partial {\boldsymbol {\omega }}}{\partial t}}+(\mathbf {u} \cdot \nabla ){\boldsymbol {\omega }}\\&=({\boldsymbol {\omega }}\cdot \nabla )\mathbf {u} -{\boldsymbol {\omega }}(\nabla \cdot \mathbf {u} )+{\frac {1}{\rho ^{2}}}\nabla \rho \times \nabla p+\nabla \times \left({\frac {\nabla \cdot \tau }{\rho }}\right)+\nabla \times \left({\frac {\mathbf {B} }{\rho }}\right)\end{aligned}}

where $D / Dt$ is the material derivative operator, $u$ is the flow velocity, $ρ$ is the local fluid density, $p$ is the local pressure, $τ$ is the viscous stress tensor and $B$ represents the sum of the external body forces. The first source term on the right hand side represents vortex stretching.

The equation is valid in the absence of any concentrated torques and line forces, for a compressible Newtonian fluid.

In the case of incompressible (i.e. low Mach number) and isotropic fluids, with conservative body forces, the equation simplifies to the vorticity transport equation

{\frac {D{\boldsymbol {\omega }}}{Dt}}=\left({\boldsymbol {\omega }}\cdot \nabla \right)\mathbf {u} +\nu \nabla ^{2}{\boldsymbol {\omega }}

where $ν$ is the kinematic viscosity and $\nabla 2$ is the Laplace operator.

https://en.wikipedia.org/wiki/Vorticity_equation

Nikiya Anton Bettey 2021

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Wednesday, September 22, 2021

09-22-2021-0824 - vorticity equation

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