Air cavity deformation by single jointed diver model entry bodies, An experimental apparatus for generating homogeneous isotropic turbulence
Authors: Gregorio Elizabeth , Johnson Blair 
Published: January 2024

Air cavity deformation by single jointed diver model entry bodies

Competitive divers are able to attain much higher scores when they perform a splash-less entry. To achieve this goal, they use the “rip” entry maneuver where they roll their body forward immediately after impact. This dynamic shape change after impact separates them from previously studied entry bodies. An experimental study of a geometrically simplified hinged diver model is presented. The results for five different hinged models are reported. Geometric and hinge stiffness changes are used to identify the most important aspects of the maneuver. The trajectory of these models after impact and the estimated size of the entrained air cavity are reported. The models that deform the fastest also have the largest final estimated air cavity. A non-dimensional time based on the time to complete deformation is found to collapse the estimated size of the air cavity for all models that deform before the trailing air cavity collapses. Fixed models are introduced to compare the hinged models to non-deforming entry bodies. The hinged models are found to have between 42% and 154% larger estimated air cavities during the final measurement than their fixed counterpart. At the moment of deep seal, two distinct air cavities are formed: one that connects to the atmosphere above the pool that has smooth walls and a lower cavity largely composed of small bubbles. This composition is analogous to observations of competitive divers. Entry bodies that deform are found to significantly change the shape and formation of the air cavity.

An experimental apparatus for generating homogeneous isotropic turbulence

An experimental apparatus using synthetic jets to generate zero mean flow homogeneous isotropic turbulence (HIT) in the center of a cubic water tank is presented. Pumps drive jets at the corners and midpoints of the tank edges, producing center- facing flow to generate turbulence. The array of synthetic jets is controlled by a random forcing algorithm (Variano et al. in Exp Fluids 37:613–615, 2004) to optimize generation of turbulence while minimizing mean and secondary flows. We explore different combinations of mean percentage of jets on, 𝛷on, mean on-time, Ton, and pump outlet velocity, VP, to determine their respective roles in turbulence generation. Particle image velocimetry (PIV) and acoustic Doppler velocimetry (ADV) are used to measure the velocity in the central isotropic region of the tank to determine turbulence statistics including mean velocities, mean flow strength, turbulent kinetic energy, spectra, integral scales, dissipation rates, Kolmogorov scales, Taylor microscales, and the Taylor-scale Reynolds number. We identified a range of input parameters to vary energy and length scales of the flow while maintaining homogeneity and isotropy in the central core of the facility. Magnitudes of the Taylor- scale Reynolds number ranged from 68 to 176 in the apparatus. Negligible mean recirculations were found, with mean flow strength values ranging from 1.26 to 2.99%, while ratios of root mean square turbulent velocities remained between 0.93 and 0.98, indicating a high degree of isotropy.