No abstract

Measurements of the wind noise measured at the ground surface outdoors are analyzed using the mirror flow model of anisotropic turbulence by Kraichnan [J. Acoust. Soc. Am. 28(3), 378-390 (1956)]. Predictions of the resulting behavior of the turbulence spectrum with height are developed, as well as predictions of the turbulence-shear interaction pressure at the surface for different wind velocity profiles and microphone mounting geometries are developed. The theoretical results of the behavior of the velocity spectra with height are compared to measurements to demonstrate the applicability of the mirror flow model to outdoor turbulence. The use of a logarithmic wind velocity profile for analysis is tested using meteorological models for wind velocity profiles under different stability conditions. Next, calculations of the turbulence-shear interaction pressure are compared to flush microphone measurements at the surface and microphone measurements with a foam covering flush with the surface. The measurements underneath the thin layers of foam agree closely with the predictions, indicating that the turbulence-shear interaction pressure is the dominant source of wind noise at the surface. The flush microphones measurements are intermittently larger than the predictions which may indicate other contributions not accounted for by the turbulence-shear interaction pressure.

A large porous wind fence enclosure has been built and tested to optimize wind noise reduction at infrasonic frequencies between 0.01 and 10 Hz to develop a technology that is simple and cost effective and improves upon the limitations of spatial filter arrays for detecting nuclear explosions, wind turbine infrasound, and other sources of infrasound. Wind noise is reduced by minimizing the sum of the wind noise generated by the turbulence and velocity gradients inside the fence and by the area-averaging the decorrelated pressure fluctuations generated at the surface of the fence. The effects of varying the enclosure porosity, top condition, bottom gap, height, and diameter and adding a secondary windscreen were investigated. The wind fence enclosure achieved best reductions when the surface porosity was between 40% and 55% and was supplemented by a secondary windscreen. The most effective wind fence enclosure tested in this study achieved wind noise reductions of 20-27 dB over the 2-4 Hz frequency band, a minimum of 5 dB noise reduction for frequencies from 0.1 to 20 Hz, constant 3-6 dB noise reduction for frequencies with turbulence wavelengths larger than the fence, and sufficient wind noise reduction at high wind speeds (3-6 m/s) to detect microbaroms.

Biological efficiency of pesticide droplets is affected by leaf surface fine structures; however, few reliable methods exist to physically measure and quantify surface roughness. A 3D optical surface profiler was evaluated for its effectiveness as a novel and reliable method to measure and quantify leaf surface roughness in terms of areal roughness parameters. Evaluations included its accuracy for measuring 3D roughness parameters relating mean roughness length, S a , skewness, S sk , and kurtosis, S ku . Their values were compared with the wettability of seven leaf types ranging from easy-to-wet to very difficult-to-wet. Measurement accuracy was validated by a qualitative visual analysis comparing 3D surface renderings of measured leaf surfaces generated by the profiler and micrographs taken with a scanning electron microscope (SEM). The accuracy was also validated by measuring and comparing the micrometer and sub-micrometer scaled roughness on leaf types with hierarchical (multi-scale) structuring and smooth surfaces. Both the renderings and the SEM showed visual agreement in surface variations from waxes and trichomes. The measured roughness lengths for the multi-scale and smooth surfaces were on the same order of magnitude for micrometer scale roughness, approximately 1×10 0 μm, but different orders of magnitude for sub-micrometer scale roughness, approximately 1×10 −1 μm and 1×10 −3 μm, respectively. Comparisons for S sk and S ku to wettability were inconclusive, however, comparisons between S a and wettability showed a positive linear fit, suggesting that S a could be a viable metric for relating leaf surface roughness to wettability. The results from the micrometer and sub-micrometer scale surface roughness quantification could be used to improve pesticide spray deposition quality, leading to reductions in pesticide use and negative environmental impact.

In previous research [Raspet et al., J. Acoust. Soc. Am. 123(3), 1260-1269 (2008)], predictions of the low frequency turbulence-turbulence and turbulence-mean shear interaction pressure spectra measured by a large wind screen were developed and compared to the spectra measured using large spherical wind screens in the flow. The predictions and measurements agreed well except at very low frequencies where the turbulence-mean shear contribution dominated the turbulence-turbulence interaction pressure. In this region the predicted turbulence-mean shear interaction pressure did not show consistent agreement with microphone measurements. The predicted levels were often much larger than the measured results. This paper applies methods developed to predict the turbulence-shear interaction pressure measured at the ground [Yu et al., J. Acoust. Soc. Am. 129(2), 622-632 (2011)] to improve the prediction of the turbulence-shear interaction pressure above the ground surface by incorporating a realistic wind velocity profile and realistic turbulence anisotropy. The revised prediction of the turbulence-shear interaction pressure spectra compares favorably with wind-screen microphone measurements in large wind screens at low frequency.