Cavitation effects on ship propellers lead to increased erosion and a loss of efficiency. In the cavitation process vapour cavities occur and dissolve by implosion. Short-term but considerable side effects in this course are extremely high accelerations, temperatures and pressures. They result in a damage on the surface of solid objects, like e. g. ship propellers, where in the course of time larger particles break away from the surface. Nevertheless, a detailed scientific explanation for this effect does not yet exist.
KonKav project - optical measuring of cavitation effects
The KonKav project aims at better understanding this physically very complex effect, at making it mathematically comprehensible and predictable in order to finally take it into account in construction and operation. Manufacturers of ship propellers but also those of steering systems can thus further consolidate their position on world market. Fraunhofer IGD supports the consortium consisting of ship model basins, Hamburg University of Technology and Rostock University with its competence in the processing of underwater images.
System Structure and Algorithms
Cavitation tests with a ship propeller are recorded in the flow channel using a multiple high-speed camera system. Fraunhofer IGD analyses the sequences of images by means of image processing and reconstructs the non-spherical bubble entities as 3D models in order to be able to examine their extension in space. The greatest challenges are in the transition from air to water as well as in the relatively poor accessibility of the channel making the positioning of cameras and lights rather difficult, and in the great number of disturbing particles and small bubbles within the water flow which make the evaluation of images difficult.
The approach chosen makes use of the reflections at the border between bubble entity and surrounding water as well as of coherence features between the single images. For this purpose, the arrangement of two and three high-speed cameras respectively and a stroboscope as light source has been optimised. For the segmentation, realised in a first step, different procedures were evaluated. On this basis the NormalizedCut algorithm was chosen and adapted according to the task.
The following spatial registration of the images forms the basis of the 3D reconstruction. It makes use of a procedure referring to the visual hull approach and is able to work even with a small number of views due to the use of symmetry features of the bubble entities. The resulting 3D model is used by the project partners to validate and optimise the developed mathematical models. The algorithms and software modules have been completed by works in camera calibration, image registry and design of a user surface as well as by the development of an interactive visualisation module.
Die entwickelten Algorithmen und Softwaremodule wurden durch Arbeiten zur Kamerakalibrierung, Bildregistrierung und Erstellung einer Benutzungsoberfläche sowie der Entwicklung eines interaktiven Visualisierungsmoduls komplettiert.
In a follow-up project the flow channel results are to be transferred onto full-scale. For this purpose, cameras are placed in the stern which observe the propeller and the wake under realistic conditions. The automatic evaluation of these images will be a new challenge for Fraunhofer IGD scientists.