The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Twenty-First Symposium on NAVAL HYDRODYNAMICS
260 mm long. Between 10 and 20 seconds of steady flow can be produced with velocities in the range of 5 to 25 m/s (in the empty test section). The test section velocity is determined by measuring the pressure difference between the test section entrance and the pressure near the entrance of the contraction. This pressure difference was related to the test section velocity using LDV measurements of the average flow velocity in the center of the empty test section. The free and dissolved air content of the BDWT can be qualitatively controlled through deaeration and by allowing free gas bubbles to reach the free surfaces in the two tanks.
In the study of the detachment region of an attached cavity, results are presented here for the cavity flow over a 25.4 mm diameter brass sphere which was mounted on a sting within the BDWT test section. The surface of the sphere was either highly polished or roughened. The sting was plumbed to permit injection of air into the cavity, and a separate pressure tap was used to measure the difference between the cavity pressure and the static pressure of the flow upstream of the model. Five 203 µm diameter holes were placed in the sphere at an angular location of 22.5 degrees measured from the stagnation point, and fluorescein dye was injected
Figure 1: The 25.4 mm diameter sphere mounted on a sting in the BDWT test section. A ventilated cavity flow behind the sphere was created to study the region of cavity separation.
through these holes to permit visualization of the flow near the surface of the sphere. Flash photography and high speed video imaging were used to capture images of the dye streak as it flowed over the sphere. This setup is shown in Figure 1.
The flow streamlines near the surface of the sphere were recorded using particle tracers. The flow was illuminated with a continuous light sheet produced with an Argon-Ion laser, and silvered hollow glass spheres of nominally 10 µm diameter were added to the flow as tracers. Photographic images of the streamlines near the cavity were produced by time exposing the image of the flowing particles.
A wedge was used to produce a partial cavity in the study of the closure of partial cavitation. The wedge had an angle of 26.5 degrees and a step height of 19 mm, and was mounted to the wall of the test section. Partial cavities formed on the vertex of this wedge. An image of this setup is shown in Figure 2. Double pulsed Particle Imaging Velocimetry (PIV) was used to investigate the flow field in the closure region of the cavity. The details of the PIV experimental setup is provided in Tassin et al. (1995) and Yu (1995). Two frequency-doubled Nd-YAG lasers were used to produce a pulsed light sheet, and the flow was seeded with fluorescent latex particles with an average
Figure 2: A wedge mounted on the wall of the BDWT test section. A natural cavity separates from the vertex of the wedge.