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LaTeX Math Block
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\frac{D \rho}{ Dt} = \frac{\partial \rho}{\partial t} + \rho \cdot \nabla {\bf u} = 0

which is equivalent to

LaTeX Math Block
anchordivergence
alignmentleft
 \nabla {\bf u} = 0


with With account of Continuity equation:

LaTeX Math Block
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\frac{\partial \rho}{\partial t} + \nabla (\rho \, {\bf u}) = 0

the Incompressible flow criteria simplifies to:

LaTeX Math Block
anchordivergence
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 \nabla {\bf u} = 0


which means that flow velocity is It means that velocity of Incompressible flow is solenoidal.


The term Incompressible flow is a misnomer as it does not necessarily means that the fluid itself is incompressible

In many practical applications condition 

LaTeX Math Block Reference
anchordivergence
 is met for compressible fluids (usually when fluid compressibility is relatively small) and the fluid flow satisfies
LaTeX Math Block Reference
anchordivergence
 and is called
incompressible flow.

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