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Navier-Stokes equations (Rev #5, changes)

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Content

Idea

from Wikipedia

In physics the Navier–Stokes equations, describe the motion of fluid substances. These equations arise from applying Newton’s second law to fluid motion, together with the assumption that the fluid stress is the sum of a diffusing viscous term (proportional to the gradient of velocity), plus a pressure term.

The equations are useful because they describe the physics of many things of academic and economic interest. They may be used to model the weather, ocean currents, water flow in a pipe, air flow around a wing, and motion of stars inside a galaxy. The Navier–Stokes equations in their full and simplified forms help with the design of aircraft and cars, the study of blood flow, the design of power stations, the analysis of pollution, and many other things. Coupled with Maxwell’s equations they can be used to model and study magnetohydrodynamics.

Details

The general form of Navier-Stokes (1) is

ρ(vt+v.v)=p+T+f\rho \left( \frac {\partial v}{\partial t} + v.\nabla v\right) = -\nabla p + \nabla T +f

and v is the flow velocity vector, ρ\rho is the fluids density, p is pressure and T is a stress tensor and f are the body forces.

and conservation of mass:

vt+(ρv)=0\frac {\partial v}{\partial t} + \nabla \left(\rho v\right) = 0

It is possible to state stochastic differential equations such that the expectation value of the solution process is a solution to the Navier-Stokes equations. This seems to be a fairly recent result of Peter Constantin and Gautam Iyer:

  • Peter Constantin, Gautam Iyer: A stochastic-Lagrangian particle system for the Navier-Stokes equations (arXiv)

Turbulence

One of the most mysterious and striking features of solutions of the Navier-Stokes equations is turbulence.

References

category: methodology