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  • A Tutorial on the Rapid Distortion Theory Model for Unidirectional, Plane Shearing of Homogeneous Turbulence

    Abstract: The theory of near-surface atmospheric wind noise is largely predicated on assuming turbulence is homogeneous and isotropic. For high turbulent wavenumbers, this is a fairly reasonable approximation, though it can introduce non-negligible errors in shear flows. Recent near-surface measurements of atmospheric turbulence suggest that anisotropic turbulence can be adequately modeled by rapid-distortion theory (RDT), which can serve as a natural extension of wind noise theory. Here, a solution for the RDT equations of unidirectional plane shearing of homogeneous turbulence is reproduced. It is assumed that the time-varying velocity spectral tensor can be made stationary by substituting an eddy-lifetime parameter in place of time. General and particular RDT evolution equations for stochastic increments are derived in detail. Analytical solutions for the RDT evolution equation, with and without an effective eddy viscosity, are given. An alternative expression for the eddy-lifetime parameter is shown. The turbulence kinetic energy budget is examined for RDT. Predictions by RDT are shown for velocity (co)variances, one-dimensional streamwise spectra, length scales, and the second invariant of the anisotropy tensor of the moments of velocity. The RDT prediction of the second invariant for the velocity anisotropy tensor is shown to agree better with direct numerical simulations than previously reported.
  • Vibration Survey of Room 47 with a Laser Doppler Vibrometer: Main Laboratory Basement, U.S. Army ERDC-CRREL

    ABSTRACT:  Plans are underway to create an acousto-optic laboratory on the campus of the Cold Regions Research and Engineering Laboratory. For this purpose, existing space in the basement of the Main Laboratory will be renovated. Demanding measurement techniques, such as interferometry, require a sufficiently quiet vibration environment (i.e., low vibration levels). As such, characterization of existing vibration conditions is necessary to determine vibration isolation requirements so that highly sensitive measurement activities are feasible. To this end, existing vibro-acoustic conditions were briefly surveyed in Room 47, a part of the future laboratory. The survey measured ambient noise and ambient vertical floor vibrations. The ambient vibration environment was characterized according to generic velocity criteria (VC), which are one-third octave band vibration limits. At the time of the survey, the ambient vibration environment fell under a VC-A designation, where the tolerance limit is 2000 µin/s across all one-third octave bands. Under this condition, highly sensitive measurement activities are feasible on a vibration-isolated working surface. The conclusion of this report provides isolation efficiency requirements that satisfy VC-E limits (125 µin/s), which are necessary for interferometric measurements.
  • PUBLICATION NOTICE: Preliminary Measurements on the Geography of Urban VHF Radio-Frequency Noise

    Abstract: Radio-frequency (RF) noise has typically been measured at a handful of fixed, representative locations within the urban environment (Achatz, Lo, Papazian, Dalke, & Hufford, 1998; Achatz & Dalke, 2001; Dalke, Achatz, Lo, Papazian, & Hufford, 1997; Wepman & Sanders, 2011; Wagstaff & Merricks, 2005; Spaulding & Disney, 1974). In this work, we discuss the development of a mobile RF noise measurement system and the necessary geospatial and statistical post-processing techniques required to characterize the variations in noise on the street-scale in the VHF sections (60 – 300 MHz) of the spectrum. We discuss the design of our mobile noise measurement system, with special focus on the choice and calibration of preselection filters and preamplifiers necessary to reliably measure low RF noise levels while avoiding intermodulation distortion problems that arise in an environment with many strong emitters. Additionally, we describe post-processing techniques developed to reliably merge and interpolate the RF data with geolocation data which are collected on two very different (microsecond and multisecond, respectively) timescales. We use a preliminary urban dataset from Boston, MA to show that the geo-statistical properties of RF noise power can vary appreciably over street-scale distances, and that these spatial variations are repeatable over tactically relevant times.
  • PUBLICATION NOTICE: Isarithmic mapping of radio-frequency noise in the urban environment

    Abstract: Radio-frequency (RF) background noise is a spatially-varying and critical parameter for predicting radio communication system and electromagnetic sensor performance in urban environments. Previous studies have measured urban RF noise at fixed, representative locations. The Cold Regions Research and Engineering Laboratory (CRREL) has developed a tunable system for conducting mobile RF noise measurements in the VHF and UHF and shown that urban RF noise characteristics vary significantly and repeatably at a scale of tens of meters (Haedrich & Breton, 2019). CRREL also found high-powered regions in Boston, MA that are persistent over time. However, since previous studies conducted stationary measurements or measurements along linear transects, little is known about the 2-dimensional topography of urban noise and the spatial distribution and characteristics of these high-powered regions. In this paper, we present the results of a dense, block-grid survey of downtown Boston, MA at 142 and 246.5 MHz with measurements taken every meter along each street. We present isarithmic maps of median noise power and describe the spatial distribution, shape and other characteristics of the high-powered regions. We compare the rate of noise power decay around high-powered regions to losses predicted by a power law model of path loss.