Browsing by Author "Vokurka, Karel"
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- ItemExperimental study of the thermal behavior of spark generated bubbles in water(Elsevier Science Inc, 2013) Vokurka, Karel; Plocek, JaroslavIn the paper thermal behavior of spark generated bubbles is studied experimentally using an optic sensor (a photodiode) and an acoustic sensor (a hydrophone). It is shown that there is plasma in the bubble interior during the whole first bubble oscillation. A simple method is used, based on several assumptions, which allows determining an estimate of the radiated optic energy and from it the surface temperature of the bubble when the thermal radiation reaches its maximum. This temperature can be then exploited when computing a number of other quantities. An example of the energy partition in the first bubble growth phase and in the first compression and the following expansion phases is given. The estimate of the surface plasma temperature when the bubble is compressed to its first minimum volume is also given and it has been found to be approximately 5900 K in a selected experiment. It is also shown that spark generated bubbles do not behave adiabatically and that the plasma in the bubble interior represents a highly non-homogeneous medium far from an equilibrium state, for which the equation of the state of an ideal gas cannot be used. (C) 2013 Elsevier Inc. All rights reserved.
- ItemExploitation of vibration and noise signals cyclostationarity in condition based maintenance(Inst of Noise Control Engineering, Poughkeepsie, NY, United States, 1997) Vokurka, KarelA traditional approach to the analysis of vibration and noise in condition based maintenance of machinery is to assume stationarity of the measured signals. However, in the case of machinery working cyclically (e.g., gearboxes, engines, turbines) the generated vibration and noise are basically nonstationary (cyclostationary). This feature, in the traditional approach partially suppressed by continuous time averaging, can be exploited to give more detailed information on the condition of the monitored machinery. To achieve this, periodic time averaging is used. In this paper, the second‐order statistical characteristics of the cyclostationary signals are considered. These are double correlation functions, double autospectral densities, and instantaneous autospectra. The disadvantage of these statistical characteristics is their greater complexity. However, they yield a more detailed description of measured signals. Another advantage is a greater immunity to interfering signals. Algorithms for computation of the characteristics will be presented. Theoretical conclusions will be demonstrated on a concrete example of vibration data measured on a passenger car gearbox. [Work supported by the Grant Agency of the Czech Republic.] [See NOISE‐CON Proceedings for full paper.]
- ItemTime-frequency statistical characteristics of cyclostationary signals(IEEE, Piscataway, NJ, United States, 1998) Vokurka, KarelTime-frequency statistical characteristics of cyclostationary signals are studied. The characteristics considered are real and complex instantaneous autospectra. Unfortunately, as both these characteristics can take on negative values, they are not spectral densities. Moreover, the second characteristics is a complex function of time and frequency. The first characteristics, on the other hand, can be deteriorated by artifacts occurrence. The origin of these artifacts is shown to be the difference between the support area, on which the autospectrum is unambiguously defined, and the area, which represents the basic period of the autospectrum. However, both characteristics can be computed with very good time and frequency resolution.