Range Of Propagation Distances Research Articles

Split-step simulations to assess the effects of atmospheric boundary layer turbulence on the dose variability of N-waves and shaped booms

The effects of atmospheric boundary layer turbulence on the loudness variability of a sonic boom N-wave and shaped boom are examined with split-step simulations. The shaped boom is representative of a design iteration of the NASA X-59 aircraft. Inhomogeneous atmospheric boundary layer turbulence is generated in the computational domain by a Fourier synthesis method. The N-wave and shaped boom are propagated through turbulent fields representing eight different convection levels measured at the NASA Kennedy Space Center and the NASA Armstrong Flight Research Center. Probability density functions of the formation of caustic regions along the propagation direction are computed from the N-wave results, and a parameter to collapse the caustic PDFs that accounts for both fluctuation intensities and length scales is proposed. Statistical results concerning loudness metric variability are presented, and the standard deviations of several metrics are shown to collapse across different convection levels of turbulence for small nondimensional propagation distances. The loudness metric distributions are observed to be well approximated by a normal distribution for a given range of propagation distances, and become increasingly skewed as distance increases. A model function for the dose variability is proposed, and the function parameters are found to be related to the convection level of the turbulence. The model for the dose variability distribution is compared to simulation data that were not used to find the regression parameters of the model. At several nondimensional propagation distances, agreement is observed between the model and the simulation data. These results indicate that the model may be suitable for providing quick estimates of noise dose variability in the primary carpet region across a wide range of atmospheric boundary layer conditions.

Air-coupled detection of nonlinear Rayleigh surface waves to assess material nonlinearity

This research presents a new technique for nonlinear Rayleigh surface wave measurements that uses a non-contact, air-coupled ultrasonic transducer; this receiver is less dependent on surface conditions than laser-based detection, and is much more accurate and efficient than detection with a contact wedge transducer. A viable experimental setup is presented that enables the robust, non-contact measurement of nonlinear Rayleigh surface waves over a range of propagation distances. The relative nonlinearity parameter is obtained as the slope of the normalized second harmonic amplitudes plotted versus propagation distance. This experimental setup is then used to assess the relative nonlinearity parameters of two aluminum alloy specimens (Al 2024-T351 and Al 7075-T651). These results demonstrate the effectiveness of the proposed technique – the average standard deviation of the normalized second harmonic amplitudes, measured at locations along the propagation path, is below 2%. Experimental validation is provided by a comparison of the ratio of the measured nonlinearity parameters of these specimens with ratios from the absolute nonlinearity parameters for the same materials measured by capacitive detection of nonlinear longitudinal waves.

Wind turbine noise propagation: Comparison of measurements and predictions by a method based on geometrical ray theory

The objective of the present work has been to investigate the accuracy of the Nord2000 propagation model when used to predict noise from high sources like wind turbines. The model is representative to a new generation of �engineering� models developed on basis of analytical and heuristic solutions. The accuracy is investigated by comparison with measurements. Three experiments have been carried out for this purpose: 1) downwind and upwind propagation over flat grass-covered ground from an elevated loudspeaker, 2) downwind and upwind propagation over non-flat ground from a loudspeaker placed on top of a wind turbine nacelle, and 3) downwind propagation over non-flat ground from a single wind turbine. Experiment 1) showed for downwind propagation a very fine agreement between measured and predicted A-weighted noise levels in the considered range of propagation distances (1500 m). The bias error was 0.1 dB with a standard uncertainty of 0.7 dB. The agreement for upwind propagation was not as good. In experiment 2) the accuracy was slightly reduced compared to the downwind results of experiment 1) with a bias error of 0.5 dB and a standard uncertainty of 1.9 dB. In experiment 3) where a wind turbine was used as source the agreement was less than seen in the loudspeaker experiments.

Laser-ultrasonic measurements of thin-film properties using surface acoustic waves

High-frequency, laser-ultrasonic methods to evaluate the mechanical properties of thin films have been developed. The approach is based on the optical generation and detection of surface acoustic waves (SAWs) to determine the frequency dependence of the phase velocity (dispersion relation). Broadband or variable-frequency, quasi-plane wave SAWs are generated by a line-focused, 200-ps pulsed laser. The out-of-plane SAW displacement amplitudes are measured using a path-stablized Michelson interferometer with line-focus detection. The apparatus incorporates differential photodiode detection with a −3-dB bandwidth of 40 kHz–800 MHz. With this system, several displacement waveforms over a range of propagation distances are acquired and used to determine the SAW dispersion. Dispersion relations up to 200 MHz are presented for a variety of specimens, including a series of Si wafers with TiN films 0.2–1.3 micrometers thick and containing residual stress. The results illustrate the applicability of this technique to various conditions such as stiffening and loading films and isotropic and anisotropic substrates. The experimental dispersion curves obtained with this method are compared to theoretical predictions to evaluate the film elastic properties.

Parametric characterization of Gaussian beams propagating through active media

On the basis of the intensity moment formalism, within the thin-sheet approach, the beam width, the beam quality and the kurtosis parameter of a Gaussian beam propagating through active media are investigated. The analysis considers both the homogeneously and inhomogeneously broadened cases. The dependence of the above three parameters on the small-signal gain, saturation intensity and input beam size is studied. The validity of the thin-sheet approximation for the range of propagation distances and physical parameters of the system is also discussed.

Quasiparticle description of pulse propagation in a lossy dispersive medium

It has been known from extensive saddle point analysis that pulses propagating in a lossy dispersive Lorentzian medium exhibit a high frequency forerunner and frequency chirping. The quasiparticle approximation permits an accurate and numerically efficient calculation of the forerunner frequency chirping with a simple physical explanation. For the case of a Gaussian incident pulse, the error in the second order quasiparticle approximation becomes smaller as the pulse propagates; hence one may apply the quasiparticle method for a wide range of propagation distances with error estimates that can be readily calculated.