Techniques for high-speed digital imaging of gas concentrations in turbulent flows

Techniques have been developed to measure the concentration of nozzle gas quantitatively in transitional and turbulent flow regimes at high framing rates in two dimensions. Elastically scattered light from a seeded jet, illuminated by a sheet of laser light, was recorded in three ways. The first involved the use of a rotating mirror to sequentially place images onto different portions of a relatively slow vidicon-based imaging system to achieve a high effective framing rate (2–10 kHz) for up to four frames. The second used a portion of a monolithic 128×128 photodiode array connected directly to a high-speed analog to digital converter to achieve a framing rate of 1.136 kHz. The third used a fast video camera and recorder (6 kHz) to store images on magnetic tape in analog form, which were later digitized upon playback. A comparison of these techniques is presented along with a discussion of factors important in the use of high-speed digital imaging systems. It is shown that the temporal development of large-scale structures can be digitally recorded in two dimensions, with the advantage of being both quantitative and visually interpretable.