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

Keynote addresses will be delivered to set the underlying tone and summarize the core message. These speeches will be done by specialists from laboratories and international companies recognized for their expertise in the field.

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

David Donjat & Olivier Léon
ONERA, The French Aerospace Lab, France

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.


David Donjat is a research scientist working at ONERA (The French Aerospace Lab) - Department of Aerodynamics and Energetics Modeling. Olivier Léon currently works at the Multi-Physics Department (DMPE) of ONERA. He does research in Fluid Mechanics. His current projects focus on Near-Wall, Wake and Jet Turbulence, Heat Transfer and Optical Measurement Techniques.