We extend the dynamic van der Waals model introduced by A. Onuki [Phys. Rev.<br />
Lett. 94, 054501 (2005)] to the description of cohesive granular flows under a<br />
plane shear to study their hydrodynamic instabilities. Numerically solving the<br />
dynamic van der Waals model, we observe various heterogeneous structures of the<br />
density in steady states, where the viscous heating is balanced with the energy<br />
dissipation caused by inelastic collisions. Based on the linear stability<br />
analysis, we find that the spatial structures are determined by the mean volume<br />
fraction, the applied shear rate, and the inelasticity, where the instability<br />
is triggered if the system is thermodynamically unstable, i.e. the pressure,<br />
$p$, and the volume fraction, $\phi$, satisfy $\partial p/\partial\phi&lt;0$.