Inertial force, as the name implies is the force due to the momentum of the fluid. This is usually expressed in the momentum equation by the term (ρv)v(ρv)v. So, the denser a fluid is, and the higher its velocity, the more momentum (inertia) it has. As in classical mechanics, a force that can counteract or counterbalance this inertial force is the force of friction (shear stress). In the case of fluid flow, this is represented by Newtons law, . This is only dependent on the viscosity and gradient of velocity. For more details please refer NCERT Book.