Propeller RPM: 1700
Yaw: 0 degrees
Advance Ratio: 0.68
Cavitation Number: 0.19
The images below summarize some of my initial research investigating the cavitating behavior of Navy Model Propeller 5236. The experiments were run in the Low Turbulence Water Tunnel on the California Institute of Technology campus.
Typically, experiments performed on cavitating propellers are quantified in terms of two convenient dimensionless parameters -- the cavitation number and the advance ratio. The cavitation number is defined as the ratio of the pressure margin above vapor pressure and the dynamic pressure of the flow (based on propeller tip speed). The advance ratio is given by the free stream velocity divided by the product of the propeller rotation rate (in cycles per second) and the propeller diameter. The advance ratio thus represents a ratio of the incoming flow speed and the propeller tip speed. Two other parameters varied in the current experiments were the propeller yaw (the angle between the propeller axis and the incoming flow) and the propeller rotation rate.
The first picture gives a general overview of what the propeller looks like when cavitating.
Propeller RPM: 1700
Yaw: 0 degrees
Advance Ratio: 0.68
Cavitation Number: 0.19
Many of the common features of propeller cavitation are present. An attached sheet cavity and bubbly caviation are both visible on the propeller blades. Also, very distinct tip vortices can be seen as they are swept downstream of the propeller by the incoming flow.
The next picture shows a close up view of the a propeller blade nearing the dead bottom in its rotation around the hub.
Propeller RPM: 1850
Yaw: 0 degrees
Advance Ratio: 0.67
Cavitation Number: 0.15
In this picture, the propeller (painted red for better constrast) is exhibiting a phenomenon known as supercavitation, where the attached sheet cavity (the clear, striated region extending rearward from the leading edge) extends beyond the trailing edge of the propeller blade.
As the first two pictures show, the experimental setup is such that the propeller can be operated either upstream (first picture) or downstream (second picture) of the gearcase housing. Much of my early work with the propeller focused on quantifying the behavior of the cavitation at varying advance ratios and cavitation numbers, when operated upstream of the gearcase housing. The graph below summarizes the extent of cavitation, defined by the fraction (t/c) of the blade chord covered by either clear or frothy cavitation.
Propeller RPM: 1700
Yaw: 0 degrees
Advance Ratio: varying
Cavitation Number: varying
When the propeller is operated downstream of the gearcase housing, an very interesting periodic instability is observed on the propeller. This instability is characterized by a fluctuation in extent and type of cavitation seen on the blades. The instability generally occurs at a frequency of about 9.5 Hz, is quite variable in amplitude, and occurs uniformly across all rotational locations.
The following sequence of images (taken from high speed video footage obtained with a CCD camera at 500 frames/second) shows several of the important phases of this instability cycle.
Propeller RPM: 1850
Yaw: 0 degrees
Advance Ratio: 0.67
Cavitation Nuber: 0.15
In these pictures, the number in the lower left corresponds to the fraction of the instability cycle that has elapsed. As the instability cycle begins (0.00), the cavitation extent is at a minimum, and frothy cavitation is dominant. As the cycle progresses, a region of attached cavitation begins to grow back from the leading edge (0.25). As this occurs, a re-entrant jet structure (indicated by the white arrow) is swept backwards from the leading edge. Finally, the attached cavitation reaches a maximum extent (0.70). However, the re-entrant jet then returns, penetrating forward towrads the leading edge (0.92). It eventually progresses forward nearly to the leading edge, returning the cavitation to the beginning of the cycle (0.00).