Defocusing PTV applied to an open wet clutch: from macro to micro

Defocusing particle tracking velocimetry (defocusing PTV) is applied in various levels of magnification to investigate the flow in an open wet clutch with radial grooving. The magnifications range from M=0.8 up to M=15 , whereas the corresponding defocusing sensitivities s^Z , denoting the rate of the particle image diameter change along the optical axis, range from 67.1 to 9.0 µm/pixel. For flows with sub-millimeter structures, as it is the case of this clutch flow application, a high spatial resolution is necessary to fully understand the flow phenomena in these confined geometries. It is demonstrated in this study that all optical configurations can resolve the miniature flow structures that are present in an open wet clutch. Moreover, the M=0.8 set-up, featuring the largest field of view, captures the entire region between the inner and the outer radius of the clutch, providing a comprehensive overview of the three-dimensional clutch flow field, ranging from the smallest structures to the largest. It is found that the M=0.8 set-up also yields the lowest uncertainties in the determination of the particle locations in x , y direction as well as in z direction along the optical axis. The particle image diameter uncertainty is 2σ _d_i=0.24 pixel, which translates (by multiplying the defocusing sensitivity of s^Z=67.1 µm/pixel) into a particle z location determination uncertainty of 2σ _z=16.1 µm. These results give evidence to the fact that not necessarily the best defocusing sensitivity determines the accuracy of the particle z location estimation; to a great extent the signal-to-noise ratio and the width of the outer ring of the defocused particle image influence the accuracy. Graphical Abstract