Evolution of Gas Volume Fraction in the Wake of 2-D and 3-D Ventilated Supercavities*

Vehicles submerged in water experience significant friction drag, degrading performance.

Partial- and super- cavities can significantly reduce the skin friction drag by replacing the near-

surface flow with gas. The flow of gas into and out of the cavity greatly influences the amount of

drag reduction, cavity shape, and stability. Understanding the flow physics in the gas phase is

critical for optimizing supercavitating objects that can make effective use of the drag reduction.

Invasive measurements of the gas fraction alter the flow around the cavity, thus making it

challenging to generalize the results. Radiation based measurement techniques, such as X-ray

densitometry and tomography, allow for quantification of the gas fraction without disturbing the

underlying flow field. We measure the gas fraction in the wake of a nominally 2-D,

supercavitating, ventilated bluff body using X-ray densitometry in the Michigan 8-inch water

tunnel. A similarly scaled, axisymmetric cavitator for a 3-inch round test section is developed

and preliminary results from the Michigan Scanning Electron Beam X-ray Tomography system

are presented.