A measuring method of the echo reduction of passive materials by using the time reversal（TR） technique is presented. To measure the echo reduction of a sample with this approach, the received signals are firstly foc...A measuring method of the echo reduction of passive materials by using the time reversal（TR） technique is presented. To measure the echo reduction of a sample with this approach, the received signals are firstly focused according to the TR theory. Then, the sample is removed and the TR processing is again employed to realize the focus of the received signal.Finally, the echo reduction of the sample is evaluated with these focusing signals. Besides, to calibrate the measured echo reduction via the TR technique, a standard sample is employed to measure a constant coefficient that only depends on the measurement environment. An aluminum plate sample and a steel plate sample with the same size of 1.1 mxl.O m x0.005 m axe tested in a wave guide tank. The experimental results show that the calibrated values are well consistent with theoretical results under the free field at the measured frequency range of0.5-20 kHz. The relative errors of all the measured values are less than 10% and the values of the expanded uncertainty are less than 1.5 dB. The TR processing focuses the energy in spatial domain and temporal domain, so it can be used to measure the echo reduction of passive materials in the environments with reflections induced by boundaries and low frequency sources.展开更多
Noise reduction of different airfoils is important because these sections are used in wind turbines,propellers,and aircraft wings.Several methods are used for passive noise reduction of sections.One of these methods i...Noise reduction of different airfoils is important because these sections are used in wind turbines,propellers,and aircraft wings.Several methods are used for passive noise reduction of sections.One of these methods is the use of surface treatment.In this research,the effect of the typical surface treatment element(finlets)on the vortex structure at different frequencies in the turbulent flow created on the NACA2412 section is investigated.For this purpose,one-dimensional hot wire probe is used.The used surface treatment in this research has a special geometry and the distance between two consecutive finlets is 6 mm(S=6 mm).This study shows that this surface treatment element can be used for noise reduction in high frequencies.Another result of this research is the suggestion of the most suitable position to install this special surface treatment element to reduce vortex energy in all frequency ranges.This installation location is determined based on a dimensionless parameter(X_(aft)/h).展开更多
文摘A measuring method of the echo reduction of passive materials by using the time reversal（TR） technique is presented. To measure the echo reduction of a sample with this approach, the received signals are firstly focused according to the TR theory. Then, the sample is removed and the TR processing is again employed to realize the focus of the received signal.Finally, the echo reduction of the sample is evaluated with these focusing signals. Besides, to calibrate the measured echo reduction via the TR technique, a standard sample is employed to measure a constant coefficient that only depends on the measurement environment. An aluminum plate sample and a steel plate sample with the same size of 1.1 mxl.O m x0.005 m axe tested in a wave guide tank. The experimental results show that the calibrated values are well consistent with theoretical results under the free field at the measured frequency range of0.5-20 kHz. The relative errors of all the measured values are less than 10% and the values of the expanded uncertainty are less than 1.5 dB. The TR processing focuses the energy in spatial domain and temporal domain, so it can be used to measure the echo reduction of passive materials in the environments with reflections induced by boundaries and low frequency sources.
文摘Noise reduction of different airfoils is important because these sections are used in wind turbines,propellers,and aircraft wings.Several methods are used for passive noise reduction of sections.One of these methods is the use of surface treatment.In this research,the effect of the typical surface treatment element(finlets)on the vortex structure at different frequencies in the turbulent flow created on the NACA2412 section is investigated.For this purpose,one-dimensional hot wire probe is used.The used surface treatment in this research has a special geometry and the distance between two consecutive finlets is 6 mm(S=6 mm).This study shows that this surface treatment element can be used for noise reduction in high frequencies.Another result of this research is the suggestion of the most suitable position to install this special surface treatment element to reduce vortex energy in all frequency ranges.This installation location is determined based on a dimensionless parameter(X_(aft)/h).
