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/m3,  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].