The Structure of Atmospheric Turbulence and its Application to the Design of Pipe Arrays

Abstract

Summary Turbulent boundary layers at the surface of the Earth limit the detection of infrasonic waves with periods greater than 1 s. Pipe arrays designed to improve the signal-to-noise ratios of infrasonic waves usually assume that the background noise due to this turbulent boundary layer is incoherent between the array inlets. The power at various points on a surface was measured; coherences between these points were determined and they were found to be significant in the period range 1-100 s. Such coherent noise must be considered when pipe arrays are designed. Infrasonic waves of periods greater than 1 s travel long distances due to low absorption and are therefore important in studies of the structure of the atmosphere. The detection of such waves is limited, however, by the presence of a turbulent boundary layer at the surface of the Earth. The effects of such turbulent noise can be reduced by using multiple-sampling pipe arrays as inlets to the pressure measuring transducers (Daniels 1959). A lack of statistical data on the nature of this turbulent boundary layer has hampered the design of pipe arrays. There have been investiga- tions into the variations of turbulence with altitude (e.g. Davenport 1961) but few data are available for determinations of the horizontal structure of turbulence in the period range common to the periods of infrasonic waves. The objective of the research presented here was to determine the power due to turbulence on a flat surface and then to find the coherence between recording points as a function of distance of separation, confining the observations and results to the period range 1-100s. This information was then used to design pipe arrays with optimum signal-to-noise ratios. In our studies we used as the source of turbulence the steady prevailing wind of high summer in North Texas. Velocities were determined from cross-spectra; these clearly indicated that the turbulence travelled at the velocity of the wind as determined by an anemometer. The coherence fell rapidly with increasing distance and we concluded that the power levels were associated with the wind-derived turbu- lence and not with propagating signals. Experimental conditions Priestley (1966) has performed one of the few investigations of the structure of atmospheric turbulence in the period range 1-100 s. The present investigation was initially intended to repeat and extend that of Priestley, but, as will be shown later, different criteria were used in the selection of the data.