Instability and turbulence in confined swirling flow investigated by laser Doppler anemometry

Loader, Alan James (1981) Instability and turbulence in confined swirling flow investigated by laser Doppler anemometry. University of Southampton, Doctoral Thesis.

Abstract

Point velocities in confined swirling water flow have been measured using the technique of laser Doppler anemometry (LDA). The technique has been extended to recording real time turbulence data for subsequent statistical analysis by a high speed computer.The cylindrical Perspex tube [diameter, D = 190mm, length,L = 190-1900mm] had twin tangential inlets [diameter 27mm] and an axial downstream outlet [diameter, d = 25, 50 or 75mm]. Flow rates with Reynolds numbers in the range 150-23,000 (based on mean axial velocity and tube diameter) were used, and reversed flow and precession of the vortex core were present in most cases. The effect of Reynolds number and tube geometry on core stability and turbulence was investigated. Tangential and axial velocities were measured with a frequency offset, forward-scatter LDA, developed previously by Blackmore, and a frequency-offset, back-scatter LDA was devised and developed by the author to measure radial velocities and also tangential velocities when an air core was present in the vortex. Successful operation of the latter, probing up to 350mm into the flow, was dependant on a high laser power (300mw), addition of titanium dioxide seeding particles and use of a plate glass end wall. A frequency tracking filter, devised previously by Wilmshurst, processed the LDA signal to give mean and turbulent (r.m.s.) velocities. Real time fluctuating velocity data was analysed in the computing facility of the Institute of Sound and Vibration Research to provide velocity statistics, including core precession frequency, power spectra, autocorrelations and turbulence scales. An investigation of the accuracy of LDA's was carried out, and the error in turbulent velocity measurement was shown to be generally less than 1% for both the optical systems used, due mainly to the excellent design of the frequency tracker. However, core precession introduced a large error (up to 100%) into the measurement of turbulent velocity near the tube centre and a technique was successfully developed for correcting the measurements, by statistical analysis of the real time velocities.

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