Research On Fractal Characteristics Of Choking Cavication Noise Signal

Choking cavitation is a new technology where the water cavitation phenomenon is combined with choking phenomenon of gas-liquid two phase flow. It has potential broad applications in the waste water treatment and industrial cleaning.It introduced the basic knowledge of cavitation theory and fractal theory. The choking cavitation noise signal test platform had built. The choking cavitation noise signal had collected with the hydrophone under the conditions of different flow and back pressure. The power spectral density estimation, the single fractal features and the multifractal properties of the choking cavitation noise signals had searched via Welch, fractal box dimension and multifractal spectrum.The results of power spectrum analysis show that the energy amplitude of choking cavitation noise signal is mainly concentrated in the high frequency band, and an optimum range of back pressure exists.The results of single fractal analysis show that for the choking cavitation noise signal in time domain the cavitation intensity is the highest as the fractal box dimension values D≤1.71, the cavitation intensity is higher as 1.71<D<1.74 and the cavitation intensity is lower as D≥1.74. The range of optimal back pressure is 120-190 k Pa for the high cavitation intensity.The results of multifractal properties analysis show that for the choking cavitation noise signal in time domain the cavitation intensity is the highest as the multifractal spectrum width values Δα ≤ 0.1, the cavitation intensity is higher as 0.1 < Δα < 0.15 and the cavitation intensity is lower as Δα≥0.15. the choking cavitation noise signal in frequency domain the cavitation intensity is the highest as the multifractal spectrum width values Δα≤0.2, the cavitation intensity is higher as 0.2<Δα<0.4 and the cavitation intensity is lower asΔα≥0.4. The range of optimal back pressure are respectively 130-170 k Pa and 110-170 k Pa for the high cavitation intensity.Based on the fractal theory, the single fractal and the multifractal features for choking cavitation noise signal were researched. The fractal box dimension and multifractal spectrum width are taken as the feature indices estimating status of choking cavitation. When the cavitation intensity is high, the cavitation signal is dominant in the collecting signals which have regularity, correlation and stationarity. So the magnitude of the fractal box dimension and multifractal spectrum width are small. On the contrary, When the cavitation intensity is low, the randomness signal is dominant in the collecting signals. So the magnitude of the fractal box dimension and multifractal spectrum width are large. The fractal box dimension and multifractal spectrum width can describe quantitatively the cavitation status, which provides a new method for estimating cavitation status.