The Leray projection is a mathematical tool used to describe the motion of fluids like air or water. It takes a vector fieldâÂÂessentially a description of how something moves at each point in spaceâÂÂand extracts the part that represents incompressible (divergence-free) flow. This is especially useful in studying fluid dynamics, such as in the NavierâÂÂStokes equations that describe how fluids move.
It is named after Jean Leray.
The basic idea of the Leray projection is that any vector-field in three-dimensions admits a decomposition into a curl-free part, and a divergence-free part. This is known as the Helmholtz decomposition. (More generally, the Hodge decomposition applies in higher dimensions: see for instance the Euler-Arnold equations.)
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One can show that a given vector field on can be decomposed as
Different than the usual Helmholtz decomposition, the HelmholtzâÂÂLeray decomposition of is unique (up to an additive constant for ). Then we can define as
The Leray projector is defined similarly on function spaces other than the Schwartz space, and on different domains with different boundary conditions. The four properties listed below will continue to hold in those cases.
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For vector fields (in any dimension ), the Leray projection is defined by
This definition must be understood in the sense of pseudo-differential operators: its matrix valued Fourier multiplier is given by
Here, is the Kronecker delta. Formally, it means that for all , one has
where is the Schwartz space. We use here the Einstein notation for the summation.
The Leray projection has the following properties:
The incompressible NavierâÂÂStokes equations are the partial differential equations given by
where is the velocity of the fluid, the pressure, the viscosity and the external volumetric force.
By applying the Leray projection to the first equation, we may rewrite the Navier-Stokes equations as an abstract differential equation on an infinite dimensional phase space, such as , the space of continuous functions from to where and is the space of square-integrable functions on the physical domain :
where we have defined the Stokes operator and the bilinear form by
The pressure and the divergence free condition are "projected away". In general, we assume for simplicity that is divergence free, so that ; this can always be done, by adding the term to the pressure.