# Why is viscosity relevant to such physical situations as spherical radial flow?

I have some difficulties trying to understand the viscosity term in the Rayleigh-Plesset equation:

$$R\frac{d^2R}{dt^2} +\frac{3}{2}(\frac{dR}{dt})^2 + \frac{4\nu_L}{R}\frac{dR}{dt} +\frac{2\gamma}{\rho_LR} + \frac{\Delta P(t)}{R} = 0$$

This equation describes the change in the radius of a spherical bubble. The main thing i don't understand is the inclusion of a viscous term - a far as i understand, viscosity is relevant to physical situations where there is shear flow, or equivalently when the gradient of pressure has component orthogonal to the gradient of flow velocity (for example, a laminar flow between two stationary planar plates). In the case of expanding bubble, the flow has spherical symmetry, and that means that both pressure and flow speed are only radius-dependent, hence viscous effects are irelevant here. So what am i misunderstanding here? obviously i have some mistakes in my conception of the radial fluid flow.