Nonaxisymmetric wave propagation in an inviscid fluid with a pipeline shear flow is investigated. Mathematical equation is deduced from the conservations of mass and momentum, leading to a second-order differential equation in terms of the acoustic pressure. Meanwhile a general boundary condition is formulated to cover different types of wall configurations. A semianalytical method based on the Fourier-Bessel theory is provided to transform the differential equation to algebraic equations. Numerical analysis of phase velocity and wave attenuation in water is addressed in the laminar and turbulent flow. Meanwhile comparison among different kinds of boundary condition is given. In the end, the measurement performance of an ultrasonic flow meter is demonstrated.
A robust discrete Fourier transform (DFT)-based channel estimation for burst orthogonal frequency division multiplexing (OFDM) systems is proposed in this paper. The conventional DFT-based channel estimation methods improve the performance by neglecting nonsignificant channel taps. However, the error floor occurs at high signal-to-noise when useful channel information is discarded, especially in non-sample-spaced channel. To solve this problem, we propose a modified DFTbased channel estimation method. Based on the noise power estimated in the frequency domain, which does not suffer from the energy leakage of channel path, we design the threshold for detecting the significant channel taps. Moreover, a wider region with fixed width is designed to decrease the energy loss of channel. Simulation results show that the proposed method is robust to the distribution of channel paths, and the error floor is eliminated.
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