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## Main Question or Discussion Point

In a paper, the authors write:

[tex]\rho \textbf{u} \otimes \textbf{u} = \rho (\textbf{u} \cdot \nabla)\textbf{u}[/tex],

but otherwise, I have no idea what the [tex]\otimes[/tex] symbol means.

From what I know of the standard incompressible equations, it seems thatAt the continuum level, the dynamics of incompressible flow has to obey conservation laws of mass and momentum:

[tex]\rho \partial_t \textbf{u} = \nabla \cdot \tau[/tex]

and

[tex]\nabla \cdot \textbf{u} = 0[/tex]

where the momentum flux [tex]-\tau = \rho \textbf{u} \otimes \textbf{u} - \tau_d[/tex]. Here [tex]\rho[/tex] is the density of the fluid which is assumed to be constant, [tex]\textbf{u} = (u,v)[/tex] is the velocity field, and [tex]\tau_d[/tex] is the stress tensor.

[tex]\rho \textbf{u} \otimes \textbf{u} = \rho (\textbf{u} \cdot \nabla)\textbf{u}[/tex],

but otherwise, I have no idea what the [tex]\otimes[/tex] symbol means.