Recording and analysis of fluctuating signals using a microcomputer

Abstract

This paper describes the application of both the Acorn Atom microcomputer and the Apple II microcomputer to the recording and analysis of the fluctuating signals associated with turbulent flow. Sampling rates up to 20 kHz are achieved using fast analogue-to-digital converters and machine code programs, while sample periods approaching one second can be employed by using all the available memory of the 48K Apple II computer. Subsequent digital analysis of the data provides accurate and rapid evaluation of the mean and rms values, the autocorrelation function and, for transitional flow, the intermittency factor. In order to check the accuracy of these systems and the DISA Amplitude Probability Analysis System for the measurement of intermittency, an Intermittency Simulator was developed. This unit included an analogue gate controlled by two decade counters, so that precise 1% variations could be obtained in the intermittency of transmission of the adjustable sinusoidal waveform from a signal generator. The calibrations indicated that the computer systems were well within 1% accurate in evaluating the intermittency factor, while the Amplitude Probability Analysis System could be in error by 4% at the higher values of intermittency factor. Fast analogue-to-digital converters used with microcomputers can therefore provide a rapid, accurate and convenient method for the capture and analysis of fluctuating signals associated with turbulent flows. The Acorn system is also being used in waterwave experiments at very much lower frequencies.