Comment on Kinetic theory models for granular mixtures with unequal granular temperature: Hydrodynamic velocity [Phys. Fluids 33, 043321 (2021)]

Comment on the paper J. Solsvik and E. Manger, Kinetic theory models for granular mixtures with unequal granular temperature: Hydrodynamic velocity, Phys. Fluids textbf{33}, 043321 (2021).