Instantaneous Two-Dimensional Gas Concentration Measurements by Light Scattering

A light-scatteri ng technique which makes it possible to map out the instantaneous concentration field at 104 points in a thin sheet intersecting a nonreacting turbulent flow is described. The concentration is inferred from the intensity of the Lorenz/Mie scattered radiation from a seeded flow. The signal is detected by a computer- controlled, low-light-level TV camera and the resulting data have the advantage of being both quantitative and visually interpretable. The technique can be modified to obtain temporal as well as spatial information. In addition, the use of other scattering mechanisms for similar measurements is discussed. ITH the realization of the importance of coherent structures in turbulent flows, there has been an increased interest in experimental means of obtaining in- formation on the nature of these structures. The use of multipoint probes, conditional sampling techniques, and flow visualization has provided much new data. The picture is still incomplete, however, since most simultaneous multipoint measurements have been limited to a relatively small number of points, usually along a single line. Also, flow visualization, which provides a large amount of information, is generally only qualitative. In order to provide a large number of simultaneous, quantitative measurements, a nonintrusive light-scattering technique has been developed which makes it possible to measure gas concentration at 10,000 points within a plane intersecting a nonreacting turbulent flow. The technique used is an extension of one first developed by Rosensweig1 and later employed by several other researchers. A thorough review of the method has been given by Becker.2 The technique is based on the fact that, if a gas is seeded with light-scattering particles having dimensions so small that there is negligible slip between trie particles and the surrounding gas, each unit mass of the seeded gas will be marked by ap- proximately the same number of aerosols in the course of its motion. (The size of the particles is sufficiently large so that there is negligible Brownian diffusion of particles due to molecular agitation. A precise estimate of how well the particles follow the fluid motion is given later.) If the seeded gas mixes with a second unseeded gas, the number of scat- tering particles in each unit volume of the mixture will be reduced and the scattered radiation will be proportionall y less. Early experiments of this type used a bright lamp to illuminate a smoke-laden flow. Later, Shaughnessy and Morton3 found that using a laser as the illumination source improved the signal/noise ratio. By a system of lenses and slits, the scattering from a small volume of seeded gas was collected and focused onto a photomultiplie r tube (PMT). The PMT output was then shown to be proportional to the seeded gas concentration, and it was possible to measure average concentration, concentration fluctuations, and power spectra at a single point. Our experiments invoke the same principle but, rather than measuring the time evolution of the