What is the exact definition of an incompressible fluid flow ? Is it a flow with constant density OR Is it a flow with divergence free velocity field OR Is it a flow with Mach number less than 0.3?
As per continuity eqn,
$$\frac{\partial \rho}{\partial t}+ \nabla \cdot \rho \overrightarrow{V}=0$$
If I assume density as constant it will lead to a divergence free vector field
$\nabla \cdot \overrightarrow{V}=0$
In certain places it is defined as $\hspace{1cm}$ $Ma<0.3$ with an assumption of $\frac{\partial \rho}{\partial p} \approx 0\hspace{0.5cm}$ or $\hspace{0.5cm}\frac{\partial p}{\partial \rho} \approx \infty$