Measurements in a subsonic turbulent jet using a quantitative schlieren technique

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Abstract
The development of optical methods for the quantitative study of the fluctuating properties of turbulent flows can provide a supplement to conventional hot-body anemometry techniques. In particular, the study of high-speed flows by hot-wire or hot-film anemometry is often difficult owing to the presence of temperature and velocity fluctuations in the flow, thereby complicating the correct interpretation of measured signals. In addition, restrictions are placed on the application of such anemometers by their physical strength, frequency-response characteristics and the introduction of disturbances by the measuring probes into the flow.The operation of an optical detection system depends primarily on the mechanism by which the detected radiation intensity is modulated by the flow. Methods which have been used successfully include scattering or absorption of incident light by tracer constituents or particles, the absorption or emission of infra-red radiation by the flow and quantitative adaptations of the schlieren and interferometer systems which are sensitive to the flow density structure. All these systems detect a summation of signals from different parts of the flow and in consequence it is necessary to consider in detail the relation of the integrated signal to the local properties of the flow. This paper deals in particular with the application of the schlieren principle to an axisymmetric turbulent jet.