Shadow, Schlieren and Color Interferometry

This article describes the three main methods for visualization of a gas in motion based on local refractive index variations: shadowgraph, schlieren and interferometric techniques. These non-invasive methods have been used by Onera for visualizing and measuring the gas density fields of two-dimensional and axisymmetric flows, varied examples of which are presented here. The originality of this paper lies, on the one hand, in recent developments in polychromatic holographic interferometry using transmission and reflection panchromatic plates and, on the other hand, the results obtained in digital color holographic interferometry. Although the CCD resolution and size are not as good as that of holographic plates, digital three-wavelength holographic interferometry appears, for the near future, to be the best candidate to characterize future complex flows.