Lamb–Oseen Vortex
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In
fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...
, the Lamb–Oseen vortex models a line vortex that decays due to viscosity. This vortex is named after Horace Lamb and
Carl Wilhelm Oseen Carl Wilhelm Oseen (17 April 1879 in Lund – 7 November 1944 in Uppsala) was a theoretical physicist in Uppsala and Director of the Nobel Institute for Theoretical Physics in Stockholm. Life Oseen was born in Lund, and took a Fil. Kand. degre ...
.


Mathematical description

Oseen looked for a solution for the Navier–Stokes equations in cylindrical coordinates (r,\theta,z) with velocity components (v_r,v_\theta,v_z) of the form :v_r=0, \quad v_\theta=\fracg(r,t), \quad v_z=0. where \Gamma is the
circulation Circulation may refer to: Science and technology * Atmospheric circulation, the large-scale movement of air * Circulation (physics), the path integral of the fluid velocity around a closed curve in a fluid flow field * Circulatory system, a bio ...
of the vortex core. Navier-Stokes equations lead to :\frac = \nu\left(\frac - \frac \frac\right) which, subject to the conditions that it is regular at r=0 and becomes unity as r\rightarrow\infty, leads to :g(r,t) = 1-\mathrm^, where \nu is the kinematic viscosity of the fluid. At t=0, we have a potential vortex with concentrated vorticity at the z axis; and this vorticity diffuses away as time passes. The only non-zero vorticity component is in the z direction, given by :\omega_z(r,t) = \frac \mathrm^. The pressure field simply ensures the vortex rotates in the circumferential direction, providing the centripetal force : = \rho , where ''ρ'' is the constant density


Generalized Oseen vortex

The generalized Oseen vortex may obtained by looking for solutions of the form :v_r=-\gamma(t) r, \quad v_\theta= \fracg(r,t), \quad v_z = 2\gamma(t) z that leads to the equation :\frac -\gamma r\frac = \nu \left(\frac - \frac \frac\right).
Self-similar solution In the study of partial differential equations, particularly in fluid dynamics, a self-similar solution is a form of solution which is similar to itself if the independent and dependent variables are appropriately scaled. Self-similar solutions ap ...
exists for the coordinate \eta=r/\varphi(t), provided \varphi\varphi' +\gamma \varphi^2=a, where a is a constant, in which case g=1-\mathrm^. The solution for \varphi(t) may be written according to Rott (1958)Rott, N. (1958). On the viscous core of a line vortex. Zeitschrift für angewandte Mathematik und Physik ZAMP, 9(5-6), 543–553. as :\varphi^2= 2a\exp\left(-2\int_0^t\gamma(s)\,\mathrm s\right)\int_c^t\exp\left(2\int_0^u \gamma(s)\,\mathrm s\right)\,\mathrmu, where c is an arbitrary constant. For \gamma=0, the classical Lamb–Oseen vortex is recovered. The case \gamma=k corresponds to the axisymmetric stagnation point flow, where k is a constant. When c=-\infty, \varphi^2=a/k, a Burgers vortex is a obtained. For arbitrary c, the solution becomes \varphi^2=a(1+\beta \mathrm^)/k, where \beta is an arbitrary constant. As t\rightarrow\infty, Burgers vortex is recovered.


See also

* The
Rankine vortex The Rankine vortex is a simple mathematical model of a vortex in a Viscosity, viscous fluid. It is named after its discoverer, William John Macquorn Rankine. The vortices observed in nature are usually modelled with an Potential flow#Examples of ...
and Kaufmann (Scully) vortex are common simplified approximations for a viscous vortex.


References

{{DEFAULTSORT:Lamb-Oseen vortex Vortices Equations of fluid dynamics