Claude M. Veret
Proceedings Volume 16th Intl Congress on High Speed Photography and Photonics, (1985) https://doi.org/10.1117/12.967955
During the tests in wind tunnel, flow visualization techniques offer to the aerodynamicists general views of the whole or part of flows they are studying. Even when the image observation only provides qualitative informations, the high density of these informations leads to a synthetic interpretation of the very complex physical phenomena which take place in the course of the interaction between a flow and a solid in relative motion. The variety of these techniques arises from the problem diversity according to the fluid relative velocity and to the solid wall proximity. On the wall, informations are obtained by examination of the flows of more or less viscous, colored or not, liquid spread on the surface, or still thin tufts motion. Out of the wall, the techniques to be used are different according that the external flow velocity is more or less fast as that of the sound in this medium. In subsonic flow, the air is seeded with tracers in the shape of small solid or liquid particles. Observed individualy or in groups, the particle motion informs on this of the surrounding air. Images are obtained thanks to the particle scattering properties regarding the illumination light. Changes in the illuminating conditions (total illumination or by a light sheet) allow to show particular features such as vortices, separation and recirculating zones, ..In the supersonic domain, new physical phenomena appear, providing rough or progressive changes of the gas density (particularly, shock wave apparition). These density variations involve propagation light perturbations (phase changes in a light wave or light ray de-viations in a beam). It is the domain where techniques such as interferometry, schlieren and shadowgraphy are developed. They provide not only qualitative visualizations, but, in certain cases, such as two-dimensional or axisymetrical flows, they allow density distribution determinations. Some phenomena are very brief (detonation wave propagation) or evolve fast (turbulent structures). In order to allow a detailed analysis, it is then necessary to fix temporarily some aspects of these phenomena. Separate pictures are obtained thanks to fast shutters, pulse light illuminations (spark lamps or lasers), or image converter cameras. Sequences with high repetition rate (from 25 frames per second to beyond a million of frame per second) necessitate the use of special materials: high speed cameras (ordinary or with compensating prism), rotating drum or mirror cameras, image converters,..To all these recording means on photographic film, is now added the video, recording on magnetic band. Although it does not yet reach the same number of pixels in an image, the video presents some advantages for its easy use, fast restitution and long play recording. in the field of high speed video, frame rates of 200 and 2000 frames per second, in full image, are obtained. More, the video development now allows to take advantage of the image observation informative richness to facilitate the interpretation of large data sets acquired either by measuring means or at the computer output. The aerodynamicists are particularly interested with it for their result presentation and comparison between experiment and calculation.
