Non-Invasive Detection of Vortex Street Cavitation (original) (raw)
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Cavitating vortex characterization based on acoustic signal detection
IOP conference series, 2016
In hydraulic turbines operating at part loads, a cavitating vortex structure appears at runner outlet. This helical vortex, called vortex rope, can be cavitating in its core if the local pressure is lower that the vaporization pressure. An actual concern is the detection of the cavitation apparition and the characterization of its level. This paper presents a potentially innovative method for the detection of the cavitating vortex presence based on acoustic methods. The method is tested on a reduced scale facility using two acoustic transceivers positioned in "V" configuration. The received signals were continuously recorded and their frequency content was chosen to fit the flow and the cavitating vortex. Experimental results showed that due to the increasing flow rate, the signal-vortex interaction is observed as modifications on the received signal's high order statistics and bandwidth. Also, the signal processing results were correlated with the data measured with a pressure sensor mounted in the cavitating vortex section. Finally it is shown that this non-intrusive acoustic approach can indicate the apparition, development and the damping of the cavitating vortex. For real scale facilities, applying this method is a work in progress.
In present paper application of computer vision techniques to propeller cavitation experiments is presented. These techniques are widely adopted in many different environments and therefore they are well documented. They are also attractive from an economic point of view, due to relative low cost of the hardware involved. Nevertheless their application to study propeller behavior in cavitation tunnel is not straightforward, because of the nonstandard environment. However the adoption of these techniques may open a wide field of investigation and can result in a deepening of knowledge in propeller cavitation phenomena. In particular, obtained data can be linked to connected topics, such as propeller radiated noise or pressure signature, providing a better understanding on the sources of these effects, and invaluable information for validation of computer simulations. Present paper traces a possible path to develop an experimental technique, covering theoretical points as well as data analysis strategies and other practical aspects. All techniques are presented through practical application, thus making clearer their points of strength and their shortcomings. Besides achieved results, possible improvements and future developments are outlined.
Acoustic Detection of Cavitation Inception
Journal of Applied Fluid Mechanics, 2017
Cavitation phenomenon can cause deterioration of the hydraulic performance, damage by pitting, material erosion, structure vibration and noise in fluid machinery, turbo-machinery, ship propellers and in many other applications. Therefore, it is important to detect inception of cavitation phenomenon. An experimental study has been carried out in order to investigate the noise radiated by various cavitating sources to determine the validity of noise measurements for detecting the onset of cavitation. Measurements have been made measuring the noise radiated by a number of configurations in a water tunnel at various operating condition to determine the onset of cavitation. The measurements have been conducted over a frequency range of 31.5 Hz to 31.5 kHz in one-third octave bands. The onset of cavitation was measured visually through a Perspex side of the working section of the water tunnel. Moreover, a theoretical estimate of the pressure radiated from the cavitation nuclei at their critical radii and their frequency was presented. Tests indicated that, generally, at the point of visual inception there was a marked rise of the sound pressure level in the high-frequency noise, whilst the low-frequency noise increased as the cavitation developed. This finding was supported by the theoretical estimate of the pulsating frequency of cavitation nuclei. The results illustrated that the visual observations of inception confirm the noise measurements.
Dynamic behaviour of cavitation clouds: visualization and statistical analysis
Journal of the Brazilian Society of Mechanical Sciences and Engineering
High-speed visualization and subsequent statistical analysis of a high-speed cavitating water jet were performed with a custom image acquisition and processing system. In a cavitating water jet, cavity clouds form and collapse with an unsteady, periodic tendency where the frequency depends on the flow conditions. The aim of the presented investigations was to examine and analyse the dynamic behaviour of these cavity clouds to understand the effect of the influencing experimental working conditions, such as injection pressure, nozzle geometry and shape (convergent or divergent) on the size, integrity and life cycle of the clouds. The results show oscillation patterns in the geometry of the clouds (thickness, length, area, etc.), through shedding, growing, shrinking, which are related to upstream pressure fluctuations caused by the plunger pump and the interaction between the jet and the surroundings. The corresponding characteristic oscillation frequencies of the cavitating jets were also determined through cloud shape analysis.
Non-contact Method for Analysis of Cavitating Flows
Ultrasonics, 2017
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 advectiondiffusion 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.
Journal of Hydrodynamics, Ser. B, 2016
A method is presented to determine significant frequencies of oscillations of cavitation structures from high-speed camera recordings of a flow around a 2-D hydrofoil. The top view of the suction side of an NACA 2412 hydrofoil is studied in a transparent test section of a cavitation tunnel for selected cloud cavitation regimes with strong oscillations induced by the leading-edge cavity shedding. The ability of the method to accurately determine the dominant oscillation frequencies is confirmed by pressure measurements. The method can resolve subtle flow characteristics that are not visible to the naked eye. The method can be used for noninvasive experimental studies of oscillations in cavitating flows with adequate visual access when pressure measurements are not available or when such measurements would disturb the flow.
FME Transactions, 2007
In order to study and understand the jet structure and the behavior of cloud cavitation within time and space, visualization of highly submerged cavitating water jet has been done using 4-Quik-05 camera. This included obligatory synchronization technique and different types of lens-objectives. The influencing parameters, such as: injection pressure, downstream pressure and cavitation number were experimentally proven to be very significant. The recordings of sono-luminescence phenomenon proved the bubble collapse everywhere along the jet. In addition, the effect of temperature on sono-luminescence was a point of investigation .
Modeling of bubble generated noise in tip vortex cavitation inception
2007
Observation of bubble nuclei in tip vortex flows indicate bubble elongation followed by a splitting. Non-spherical simulations capture this dynamics and show reentrant jet formation and emission of very high noise. However, applying the non-spherical method to a field of nuclei is prohibitively expensive. In this paper, we attempt to overcome this difficulty by performing simulations with an improved spherical model accounting for the pressure gradient through a Surface Averaged Pressure (SAP) method and also accounting for bubble splitting. Nonspherical numerical simulations are used to develop splitting criteria and to characterize the offspring split sub-bubbles and the resulting noise. These criteria are implemented in the SAP spherical model. Numerical results with single bubble and with a realistic field are presented.
Assessment of cavitation regime in divergent vortex flows
IOP conference series, 2019
The analysis of a cavitating swirling flow in a configuration similar to the flow at the entrance of a hydraulic turbine draft tube, except the rotational component, is proposed. This flow behaviour is essential for the hydraulic turbine operation in extended range and its understanding is essential for the turbine design improvement. Advanced 2D measurements of the velocity field using PIV techniques and unsteady pressure measurements are used to determine the σ number and velocity field for various stages of cavitation development.