Efficient Levitation in Vertical Wind Tunnels: A Novel Design for Stable Liquid Droplet Studies

The efficient study of liquid droplets, ranging from micrometers to a few centimeters, through levitation, is often hindered by limitations in conventional designs. This is primarily due to continuous droplet deformation in the test section, which is further aggravated by the reaching critical Weber velocity for breakup before reaching the terminal velocity, particularly for larger droplets (diameter > capillary number). Additionally, as the droplet size increases, deviations from the spherical (0,0) mode become more prominent due to reduced surface tension effects. This results in continuous transitions to other energy-minimizing modes. Here, we propose a novel test section configuration for stabilizing large droplets in a vertical wind tunnel to overcome these challenges. The unconventional design introduces a diverging cross-section to introduce a decreasing vertical velocity with height and a symmetric airfoil rotated about its chord (spheroid) is proposed to make a velocity bucket keep the droplets levitating in the center of the test section. This offers flexibility for experimental researchers to customize the levitation (tunnel) profile for studying droplets of various sizes, ultimately enhancing the efficiency of droplet investigations. Numerical tools are employed to analyze longitudinal and lateral velocity variations in the test section, comparing the concepts for minimal turbulence intensity.