The never ending battle with propeller cavitation

Even though propeller cavitation and its downsides, e.g. noise, vibration and erosion, could be suppressed in many cases for a conventional propeller, the absence of cavitation indicates that the design could be pushed further to gain higher efficiency and performance. This means that where high efficiency and performance are required, cavitation is unavoidable, which calls for silky skills at the design stage to manage the downsides of cavitation. Despite the considerable progress made during the last ten years in Computational Fluid Dynamics (CFD) on simulation of cavitation dynamics, experimental model testing is still the most reliable and costefficient way of making the final assessment of the cavitation behaviour for commercial ship projects. However, CFD simulations can be a very valuable tool during the design process to make several design iterations giving a better design for the model tests.

As a par t of a PhD project at Chalmers University of Technology, extensive cavitation experiments were carried out in order to investigate the mechanisms behind the formation of potentially erosive cavitation. Many of the experiments were designed to generate different par ticular types of cavitation and to study their behaviour, whereas other experiments were used to identify the existence and development of different types of cavitation. The focus has been on examining different behaviours that might appear, and try to understand why and how they can be erosive.

Complex micro-scale processes

Cavitation damage is ultimately caused by complex physical micro-scale processes resulting in high pressure pulses, velocities and temperatures in the fluid. This will cause highly localised transient surface stress in the body material if the cavity collapses close enough to the body surface. The micro-scale processes are usually extremely rapid and very difficult to observe. The focus in the PhD project has been on processes of observable scale controlling the initial conditions for the micro-scale events.

Coupling large scale to micro scale hydrodynamics

Having early large-scale hydrodynamics related to subsequent erosive processes enables the identification of the importance of global parameters for the cavitation development, which can be useful knowledge at the design stage,during the experimental assessment of cavitation and during the analysis of CFD simulations of cavitation. The approach to and concept of relating large-scale hydrodynamics to the micro-scale, combined with a consideration of energy focusing during the cavity collapse, was developed as a tool for the analysis of high-speed video recordings of cavitation observations in the Erocav project. The concept, presented in the Erocav handbook, has been used and developed fur ther in later EC projects (Virtue and HTA).

Selecting tools for experimental assessment of cavitation

Experimental cavitation assessment normally involves judging the risk of erosion as regards the corresponding full-scale device as well as a statement of possible underlying physical causes, which can be a difficult task. One of the main problems when analysing cavitation is the wide span of the spatial and time resolution that needs to be covered to reveal important proper ties to understand why and how a potentially erosive cavitation collapse develops. Standard video recordings using stroboscopic lighting and noise measurement can be very effective tools to cover long time statistics and the largest scales of cavitation.

The need for extreme frame rates and optics…

However, when it comes to understanding the continuous development of a par ticular cavitation behaviour during its collapse, one may need extreme frame rates and optics to resolve the motion of the most violent collapses. As an example, it can be mentioned that a frame rate of 90 000 fps (frames per second) can be insufficient to resolve the late stage of a violent cavitation collapse, the collapse motion is simply too fast and also too small.

From noise measurements, it was seen that, for the most violent cavitation collapses, a noise measurement system having a rise time less than 0.5 μs and a sampling rate of 5 MHz is not fast enough to measure the real pressure pulse generated at the cavitation collapse. Even though the real shape of such a pressure pulse cannot be resolved, noise measurements can be very useful in revealing the focusing in time of the collapse, which is an important parameter when judging the risk of erosion.

… and standard video

Even though a high-speed video recording with a frame rate of 90 000 fps may be unable to resolve important dynamics of a collapse, a standard video recording (30 fps) can be very useful for analysing the statistical properties. This may seem contradictory but the fact is that the information obtained by means of both these techniques does not overlap; they are complementary to each other, providing different pieces of information for an analysis according to the approach in the Erocav handbook. 

Complementing visual observations with, for example, noise measurements and erosion tests by SSPA’s standard paint coating method increases the certainty of the cavitation assessment and increases the knowledge of the particular cavitation process. Since the available tools gives different pieces of information, it is important to make a selection of tools based on the requirements and circumstances for each particular case.

Photos and illustrations

One of the set-ups used in the cavitation tunnel at SSPA Sweden AB to investigate mechanisms behind the formation of potentially erosive cavitation. The upstream foil, to the right, oscillates and generates gusts on the stationary downstream foil to generate transient cavitation. Transducers for acoustic measurements are installed in the support seen in front on the foils.

Images from a high-speed video showing the collapse of a very erosive mixed glassy and cloudy cavity at the blade root of a propeller blade. There is erosion even in the bronze on the propeller model. The energy focused at the collapse of the glassy sheet (frame 1700–1717) is cascaded to the cloudy part, which is forced into a very violent collapse. An extensive rebound, as is seen here, is an important sign to look for when performing cavitation erosion assessments.