Recently van Rijsbergen et al. (2012), by simultaneous observation of cavitation and acoustic emission measurements, and Petkovsek and Dular (2013), by simultaneous observation of both cavitation structures and cavitation damage, have pointed to the fact that the small scale structures and the topology of the cavitation clouds play a significant role in cavitation erosive potential. Despite the two, before mentioned, studies opened some new insights to the physics of cavitation damage, many new questions appeared. In the present study we attached a thin aluminum foil to the surface of a transparent Venturi section using two sided transparent adhesive tape. The surface was very soft - prone to be severely damaged by cavitation in a very short period of time. Using high speed cameras, which captured the images at 30,000 frames per second, we simultaneously recorded cavitation structures (from several perspectives) and the surface of the foil. Analysis of the images revealed that five distinctive damage mechanisms exist - spherical cavitation cloud collapse, horseshoe cavitation cloud collapse, the twister cavitation cloud collapse and in addition it was found that pits also appear at the moment of cavitation cloud separation and near the stagnation point at the closure of the attached cavity.
COBISS.SI-ID: 14036763
We are comparing results of numerical simulations against high speed simultaneous observations of cavitation and cavitation erosion. We performed fully compressible, cavitating flow simulations to resolve the formation of the shock waves at cloud collapse % these are believed to be directly related to the formation of the damage. Good agreements were noticed between calculations and tests. Two high pressure peaks were found during one cavitation cycle. One relates to the cavitation collapse and the other one corresponds to the cavitation shed off, both contributing to a distinctive stepwise erosion damage growth pattern. Additional, more precise, simulations with much shorter time step were performed to investigate the processes of cavitation collapse and shedding off in more detail. There the importance of small cavitation structures which collapse independently of the main cloud was found. The present work shows a great potential for future development of techniques for accurate predictions of cavitation erosion by numerical means only.
COBISS.SI-ID: 13879067
A new method based on ultra-fast X-ray imaging was developed in this work for the investigation of the dynamics and the structures of complex two-phase flows. In this paper, cavitation was created inside a millimetric 2D Venturi-type test section, while seeding particles were injected into the flow. Thanks to the phase-contrast enhancement technique provided by the APS (Advanced Photon Source) synchrotron beam, high definition X-ray images of the complex cavitating flows were obtained. These images contain valuable information about both the liquid and the gaseous phases. By means of image processing, the two phases were separated, and velocity fields of each phase were, therefore, calculated using image cross-correlations. The local vapour volume fractions were also obtained, thanks to the local intensity levels within the recorded images. These simultaneous measurements, provided by this new technique, afford more insight into the structure and the dynamic of two-phase flows as well as the interactions between them, and hence enable to improve our understanding of their behaviour. In the case of cavitating flows inside a Venturi-type test section, the X-ray measurements demonstrate, for the first time, the presence of significant slip velocities between the phases within sheet cavities for both steady and unsteady flow configurations.
COBISS.SI-ID: 15738139
When dealing with liquid flows, where operating temperature gets close to the liquid critical temperature, cavitation cannot be assumed as an isothermal phenomenon. Due to the relatively high density of vapor, the thermodynamic effect (decrease of temperature in the bulk liquid due to latent heat flow) becomes considerable and should not be neglected. For applications like pumping cryogenic fuel and oxidizer in liquid propulsion space launchers, consideration of the thermodynamic effect is essential - consequently the physical understanding of the phenomenon and its direct experimental observation has a great value. This study presents temperature measurements in a cavitating flow on a simple convergent-divergent constriction by infrared (IR) thermography. Developed cavitating flow of hot water (-100 °C) was evaluated by high-speed IR thermography and compared with conventional high-speed visualization, at different operating conditions with the velocity range at the nozzle throat between 9.6 and 20.6 m/s and inlet pressure range between 143 and 263 kPa. Temperature depression near the nozzle throat - near the leading edge of cavitation was measured in a range up to [vartriangle]T = 0.5 K. This confirms the presence of the thermodynamic effects by cavitation phenomenon and it is in agreement with its theory. In the study, average temperature fields, fields of temperature standard deviation and time-resolved temperatures, are presented and discussed. In addition, statistical analysis between temperature drop and cavitation flow characteristics is shown.
COBISS.SI-ID: 15578139
This paper presents a novel non-contact method for simultaneous analysis of pressure and velocity conditions in cavitating flows. The method (implemented in our software ADMflow) is based on high-speed camera flow visualization and was evaluated in an experiment with ultrasonically induced acoustic cavitation of different intensities. Attached cavitation with clearly visible cavitation structures occurred on the tip of an ultrasonic probe immersed in distilled water. Using the high-speed imaging data, pressure fluctuations were calculated by a computer-aided algorithm based on the Brennen’s theory of cavitation cloud kinematics and a modified version of the Rayleigh-Plesset equation. Reference measurements of pressure pulsations were conducted by a hydrophone installed at the bottom of the liquid container. The analysis of cavitation structure dynamics was complemented by calculation of velocity fields from the imaging data, the algorithm for which is based on the advection-diffusion equation. Calculated pressure fluctuations were analyzed in the spatial, temporal and spectral domain. Presented results indicate a strong correlation between the fields of velocity and pressure fluctuations during the growth and collapse of cavitation structures. A comparison of time series and power spectra demonstrates that our cavitation analysis method is in a reasonably good agreement with results of the reference measurement methods and can therefore be used for non-contact analysis of pressure and velocity conditions in cavitating flows.
COBISS.SI-ID: 20392982