An improved laser-Rayleigh scattering photodetection system

Abstract

An improved photodetection system for high resolution laser-Rayleigh scattering measurements has been developed that utilizes a solid state detector coupled to a custom-designed, lownoise, transimpedance amplifier. The resulting system, based on a PIN photodiode is less expensive, inherently safer, less delicate and, depending on the detected light level, may exhibit higher signal-tonoise ratios than photodetection systems based on photomultiplier tubes. The frequency response of the system was designed to be uniform (3% peak variation) from DC to nearly 100 kHz. Concentration fluctuation spectra of a high scattering cross-section label [jet fluid) gas discharging into a density-matched, low scattering crosssection quiescent reservoir gas were measured using this system. Spectral signal-to-noise ratios as high as 7 decades were achieved under some conditions in parts of the spectrum.

Similarity of the concentration field of gas-phase turbulent jets

“…They are the same ones employed by Dowling & Dimotakis (1988), and are described in detail in Dowling (1988), and Dowling, Lang & Dimotakis (1989). The experimental findings, with comparisons and discussion, are presented in $ 3.…”

“…The small bumps that occur in the noise floor are the scaled values of 60 Hz, 120 Hz, and 360 Hz. The diagnostic system that allowed these spectra to be measured with a dynamic range of nearly seven decades is described in Dowling et al (1989). The spectra collapse well in spite of the modest Reynolds number of the flow.…”

“…Laser-Rayleigh scattering was used to determine the instantaneous concentration of the binary mixture of jet and reservoir gases in a small focal volume within the turbulent cone of the jet. Laser-Rayleigh scattering has been successfully used and documented in many previous studies (see discussion and citations in Dowling et al 1989), and is only described briefly here. The technique makes use of the fact that gas molecules elastically scatter photons, and that different molecules have different Rayleigh-scattering cross-sections.…”

“…The absolute accuracy of the calibration was about f 1 parts per lo3, which resulted from mild drift in the laser power and electronics. The techniques used for data processing and data reduction are described in Dowling (1988), and Dowling et al (1989).…”

Measurements of scalar power spectra in high Schmidt number turbulent jets

“…The noise spectrum was assumed to be white, as was found to be the case in separate measurements of this quantity (see also Dowling, Lang & Dimotakis 1989, for examples). Nevertheless, one can appreciate that the result is not sensitive to the assumed shape of the noise spectrum at low frequencies, where S4(w) dominates.…”