Figure 3: Dependency of the maximum shear stress at the wall on the viscosity in the case of a purely viscous fluid. The two curves correspond to two different values of the frequency of eye rotations (dashed line 20 rad/s; solid line 10 rad/s; A = 20 deg = rad). W: water; S.O.: silicon oils ( Kg/m^{3}, Pa·s, and Pa·s). In the figure we also report with symbols the values of the maximum wall shear stress obtained in the case of a viscoelastic fluid and adopt the rheological properties measured in [18, 19]. Solid square: complex viscosity Pa·s, rad/s [18]; empty square: Pa·s, rad/s [18]; solid circle: Pa·s, rad/s [19]; and empty circle: Pa·s, rad/s [19].