Misplaced Pages

Enstrophy

Article snapshot taken from Wikipedia with creative commons attribution-sharealike license. Give it a read and then ask your questions in the chat. We can research this topic together.

In fluid dynamics, the enstrophy E {\displaystyle {\mathcal {E}}} can be interpreted as another type of potential density; or, more concretely, the quantity directly related to the kinetic energy in the flow model that corresponds to dissipation effects in the fluid. It is particularly useful in the study of turbulent flows, and is often identified in the study of thrusters as well as in combustion theory and meteorology.

Given a domain Ω ⊆ R n {\displaystyle \Omega \subseteq \mathbb {R} ^{n}} and a once-weakly differentiable vector field u ∈ H 1 ( R n ) n {\displaystyle u\in H^{1}(\mathbb {R} ^{n})^{n}} which represents a fluid flow, such as a solution to the Navier-Stokes equations, its enstrophy is given by:

E ( u ) := ∫ Ω | ∇ u | 2 d x {\displaystyle {\mathcal {E}}({\bf {u}}):=\int _{\Omega }|\nabla \mathbf {u} |^{2}\,d\mathbf {x} }

where | ∇ u | 2 = ∑ i , j = 1 n | ∂ i u j | 2 {\displaystyle |\nabla \mathbf {u} |^{2}=\sum _{i,j=1}^{n}\left|\partial _{i}u^{j}\right|^{2}} . This quantity is the same as the squared seminorm | u | H 1 ( Ω ) n 2 {\displaystyle |\mathbf {u} |_{H^{1}(\Omega )^{n}}^{2}} of the solution in the Sobolev space H 1 ( Ω ) n {\displaystyle H^{1}(\Omega )^{n}} .

Incompressible flow

In the case that the flow is incompressible, or equivalently that ∇ ⋅ u = 0 {\displaystyle \nabla \cdot \mathbf {u} =0} , the enstrophy can be described as the integral of the square of the vorticity ω {\displaystyle \mathbf {\omega } } :

E ( ω ) ≡ ∫ Ω | ω | 2 d x {\displaystyle {\mathcal {E}}({\boldsymbol {\omega }})\equiv \int _{\Omega }|{\boldsymbol {\omega }}|^{2}\,d\mathbf {x} }

or, in terms of the flow velocity:

E ( u ) ≡ ∫ Ω | ∇ × u | 2 d x {\displaystyle {\mathcal {E}}(\mathbf {u} )\equiv \int _{\Omega }|\nabla \times \mathbf {u} |^{2}\,d\mathbf {x} }

In the context of the incompressible Navier-Stokes equations, enstrophy appears in the following useful result:

d d t ( 1 2 ∫ Ω | u | 2 ) = − ν E ( u ) {\displaystyle {\frac {d}{dt}}\left({\frac {1}{2}}\int _{\Omega }|\mathbf {u} |^{2}\right)=-\nu {\mathcal {E}}(\mathbf {u} )}

The quantity in parentheses on the left is the kinetic energy in the flow, so the result says that energy declines proportional to the kinematic viscosity ν {\displaystyle \nu } times the enstrophy.

See also

References

  1. ^ Navier-Stokes equations and turbulence. Ciprian Foiaş. Cambridge: Cambridge University Press. 2001. pp. 28–29. ISBN 0-511-03936-0. OCLC 56416088.{{cite book}}: CS1 maint: others (link)
  2. Doering, C. R. and Gibbon, J. D. (1995). Applied Analysis of the Navier-Stokes Equations, p. 11, Cambridge University Press, Cambridge. ISBN 052144568-X.

Further reading

Categories: