Trans RINA, Vol 152, Part A4, Intl J Maritime Eng, Oct-Dec 2010
source of harmonic vibration excitation. However, when the vibration response on the ship is not of this simple form real-time analysers can be used to advantage in identifying more complicated signals producing results such as those described later.
Whilst the ship scaled rms pressure levels at bpf were often higher than desirable, those at twice, three times and even four times bpf were unusual as described in some detail later. It was clear, however, that at first sight these results could not be used to explain the source of the vibration on the ship.
4. FLOW OBSERVATIONS
The observations of propeller cavitation under stroboscopic lighting showed wide arcs where blade back cavitation was present in the form of leading-edge vortex cavitation and some surface cavitation, whilst in the arcs void of back cavitation surface cavitation on the blade face was usually present. This was to be expected considering the propeller loadings tested, the nominal inflow wakes and cavitation numbers at representative blade radius in the tdc position.
the
When looking downstream however, both near and far from the propellers, heavily cavitating propeller tip vortices were seen in the outboard arcs of the slipstreams extending from near tdc, 0°, to bdc, 180°, for both propellers. It appeared as though imaginary longitudinal vertical planes near the propeller shaft axes and parallel with them inhibited the presence of tip vortex cavitation on their inboard sides. This was unusual in the author’s experience and noteworthy as in most cases where vortex breakdown has occurred with strongly cavitating tip vortices the vortices continued to cavitate through 360 degrees after the singularity.
Unfortunately it was not possible to obtain pictures of the downstream flow just described because of the awkward viewing situation. It could be seen by eye however, when articulating one’s head and viewing at shallow angles through the tunnel’s bottom windows.
Observations of the cavitation on the propeller blades and in the aperture space immediately ahead of the rudder were also captured using a video camera. At first sight these recordings simply appeared to confirm the observations made by eye under stroboscopic lighting, until a frame was found showing details not seen earlier, see Fig. 1. This shows a propeller blade on the Port side approaching tdc and the observer, with its accompanying back cavitation, and two cavitating helical tip vortices shed from preceding blades. The first of these shed vortices had reached the outboard side of the port rudder whilst the second was at the aperture just upstream of the rudder’s leading-edge, where it had transformed into the large whitish-grey region which is believed to include the site of a cavitating vortex breakdown or collapse. The fact that this region is whitish-grey in appearance instead
of translucent indicates it is a region of micro-bubbles reflecting the incident stroboscopic light back to the observer. This description is enhanced by the merger of the whiter interior region into the fuzzy light-grey region at the periphery where the density of the micro-bubbles has decreased presumably.
Figure 1. Cavitating Vortex Collapse
The vortex breakdown site itself is believed to lie within the overall whitish-grey region near the top where it is brighter than the remainder, which appears to include a trail of micro-bubbles following the helical path that the tip vortex and inboard helical vortex sheet would have taken had the tip vortex not collapsed.
This observation marks a notable difference between previous observations of model propeller tip cavitating vortex breakdowns where, after the disappearance of the cavitating vortex core for a short distance, the cavitating core re-establishes itself - as if the breakdown had not occurred. A similar observation was made by the authors of Reference [1] for a hydrofoil tested in the CalTech water tunnel. The disappearance of the cavitating cores of the tip vortices on their inboard sides in their respective slipstreams, referred to earlier, suggests the circulation of the downstream vortices had weakened to an extent that cavitation was not supported.
5. OTHER SOURCES OF CAVITATING VORTEX BREAKDOWNS
(Excluding propeller-hull vortex collapse) 5.1
OSCILLATING HYDROFOILS
Cavitating vortex breakdowns in the tip vortices shed by oscillating hydrofoils, as described by C Brennan of CalTech and his collaborators, help to throw light on what is believed to be taking place in the case of the cavitating propeller tip vortices in this example. For instance, the position of tip vortex cavitation where it becomes a cavitation cloud, consisting of a region of densely packed micro-bubbles, is responsible for the very high pressures experienced in the fluid on its collapse. The cloud of micro-bubbles is unstable and collapses
A-176 ©2010: The Royal Institution of Naval Architects
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