The following photos show experiments run on on a cavitating, oscillating hydrofoil in the Low Turbulence Water Tunnel on the California Institute of Technology campus.
The foil is oscillated about a spanwise axis, thus creating a periodic angle of attack. The resulting periodicity in suction surface pressure results in some interesting cavitation structures. Most notable are cloud like regions of high bubble concentration, termed cloud cavitation, and a plume of water that penetrates underneath the cavitation attached to the suction surface of the foil, termed a re-entrant jet.
In an attempt to minimize the noise and damage caused by cavitation, experiments have been run investigating the effects of air injection on these cavitation structures. The two photos below show an oscillating hydrofoil under identical flow conditions, with the exception that air is injected into the flow in the second photo.
The flow is from left to right in both photos, with the suction
surface of the foil facing the camera. The flow
conditions were as follows:
Cavitation number: 1.2
Oscillation frequency: 13.4 Hz
Instantaneous angle of attack: 12.3 degrees
Photo by Beth McKenney
In the first photo, note the presence of a very well defined re-entrant jet (the white spear shaped region). As this sheet of water penetrates underneath the attached region of cavitation vapor on the suction surface of the foil, it creates the well defined frothy region shown. The progress of this jet towards the leading edge of the foil is thought be be closely related to the formation of the cloud cavitation structures previously mentioned.
Photo by Beth McKenney
The second photo shows the effects of air injection. The re-entrant jet is not as clearly defined in this case. Since the re-entrant jet is related to the detachment of coherent cloud cavitation, it is a structure of significant interest in cavitation research.
Photos by Garrett Reisman and Mark Duttweiler
The sequence above shows selected sequential (but not necesarily consecutive) frames from a high speed movie of the cavitation structure in the presence of air injection. The flow in these pictures is from right to left. Note the lack of a coherent cloud cavitation structure in (d). The collapse of these cloud cavitation structures (often present in similar experiments without air injection) is responsible for very intense levels of radiated noise. The lack of these structures in experiments with air injection offers a possible explanation for the reduction in radiated noise observed when air injection is present.