Observation of flow turbulence using aero-optical techniques: from schlieren to holographic interferometry

This keynote discusses the way fluid dynamicists at ONERA take advantage of aero-optical effects to visualize and quantatively study turbulent flows in transparent media.

Since the early developments of wind tunnels, various techniques have been devised to provide visualizations and field measurements of turbulent flows. For flows displaying density gradients, which is the case for aero-thermal and supersonic flows for example, it was soon recognized that aero-optical effects could be efficiently leveraged to yield informative flow visualizations. Indeed, one can exploit the deflection of light induced by refractive index gradients, themselves proportional to flow density gradients, to obtain integrated projection images of relevant flow features. Particularly, the seminal works of E. Mach, H. Schardin and more recently G.E. Settles, have raised both shadowgraphy and Schlieren visualization to the status of classical and essential experimental techniques for wind tunnel testing.
More recently, Background Oriented Schlieren (BOS) has become a very practical alternative to these techniques by providing access to larger fields of view and outdoor applications with simple optical arrangements. Moreover, the BOS technique directly provides quantitative data in the form of integrated light ray angular deviations that can be used for tomographic reconstruction of the associated density fields.
For applications requiring high spatial resolution measurements, other strategies based on optical wavefront analysis may be considered, at the price of a more complex and restrictive optical setup. Among them, Digital Holographic Interferometry (DHI), providing phase variations of the optical wavefront passing through a field of non-homogeneous refractive index, is of particular interest for the study of certain classes of turbulent flows.

These techniques will be reviewed in this keynote and recent application cases studied at ONERA will be more particularly discussed.