Effects of Fluid Properties on Fluid Entry of Free-Falling Disk

The characteristics of fluid entry of free-falling disks in stagnant ambient have many applications in environment protection and engineering design. To further study the solid–fluid interaction, six fluid mixtures with different fluid properties were tested to explore the impacts of fluid properties on the dynamics of free-falling disks and crown formation. Polyacrylamide (PAM) powder was added to water to form fluid mixtures with different fluid densities, viscosities, and yield stresses. The concentration of the additive was varied to change the fluid properties while maintaining the transparency of the fluid for flow visualization. Image analysis techniques were implemented in combination with an in-house MATLAB algorithm to extract information for disk movement, deceleration, and crown geometries, and to study the pinch-off phenomenon. Experimental data indicated that the fluid properties significantly altered the crown formation characteristics and pinch-off properties. Based on the results, the variation of pinch-off time and pinch-off depth versus the fluid entry followed similar trends for all ranges of fluid properties except in relatively low PAM concentration of 0.01 wt.%, where the values of the pinch-off time and pinch-off depth first decreased after increasing the polymer concentration up to 0.01 wt.% and then increased and followed an asymptote. The height of the crown increased at the beginning of the entry, followed by a jump, and it sharply reduced afterward.