The present study utilizes the ultrasonic travel time technique to diagnose grid generated turbulence. Ultrasonic flow metering technology relies on the measurement and computation of small perturbations in the travel time of acoustic ultrasonic waves through the dynamic medium. The statistics of the travel time variations of ultrasonic waves that are caused by turbulence probably affect the performance of ultrasonic flow meters. Motivation for the study stems from the large travel time variations observed in typical ultrasonic flow and circulation meters. Turbulent flow data was collected downstream of a grid introduced in a uniform flow in the wind tunnel using ultrasonic techniques. Grid turbulence is well defined in literature, and is nearly homogeneous and isotropic. The experimental investigation was performed under well-controlled laboratory conditions. The grid mesh sizes varied from 0.25-0.5in, and flow velocities from 0-20m/s. The ultrasonic transducers were of 100 kHz working frequency; and all of the data was collected with them oriented perpendicular to the mean flow. Path lengths were increased from 2-10in; and the data acquisition and control system featured a very high speed data acquisition card with an analog to digital converter that enabled excellent resolution of ultrasonic signals. Experimental data was validated by comparison to other studies. The work aims to investigate the influence of the grid-generated turbulent flow on acoustic wave propagation, in terms of the variance of the travel time. The effect of turbulence on acoustic wave propagation was observed. The experimental data was used to compute average travel times, acoustic travel time variances, and standard deviation amplitude fluctuations. The data was collected in the region estimated to be homogeneous and isotropic. Average travel time data support the assumption that only the large (as compared to the wavelength ) turbulent inhomogeneities influence acoustic wave propagation. Variance data confirm the presence of a non-linear trend in the acoustic travel times with increasing path length. Amplitude fluctuations data confirm a correlation between areas of caustic formation and large amplitude fluctuations.
Worcester Polytechnic Institute
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Meleschi, Shangari B., "Ultrasonic Technique In Determination Of Grid-Generated Turbulent Flow Characteristics And Caustic Formation" (2004). Masters Theses (All Theses, All Years). 496.
caustics, turbulence, wave propagation, ultrasonic flow meter, Turbulence, Numerical grid generation (Numerical analysis), Turbulent flow