Three color digital holographic imaging and challenges in acoustics and fluid mechanics – IMHOTEC

In various domains such as acoustics, vibrations or fluid mechanics, the validation of models and/or numerical simulations of studied phenomena needs more and more to develop non invasive metrological tools allowing full field measurements with very high spatial and temporal resolutions. Furthermore fine experimental data are necessary to initiate numerical computing and they are in unavoidable for validating numerical simulations. At this moment, very few optical methods answer simultaneously to such a requirement and thus strongly limit the validation of analytical and numerical modelling of involved phenomena. The goal of the IMHOTEC project is to propose new optical full field imaging methods with a high spatial and temporal resolution allowing the characterisation and the fine analysis of vibrating cinematic fields, of complex acoustic fields for a frequency range varying from 0 to 50kHz (audible acoustics) and of complex vortex flows or unsteady flows for subsonic and transonic Mach number.
Proposed project IMHOTEC aims to overcome limitations reached in previous works and to synthesize the competences acquired in the past years by the partners in order to develop adaptable and new optical imaging methods having for properties, full field imaging, high spatial resolutions, high temporal resolutions, giving absolute data and allowing dynamic three dimensional measurements.
To do this, we propose to study opportunities given by the numerical wave front reconstruction from the recording of digital color holograms. In such an approach, one can take advantage of using simultaneously three reference waves and three measuring waves with three different wavelengths (red, green, blue for example). The 3 color digital holograms may then be reconstructed in delayed time via a numerical simulation of diffraction for each wavelength. Compared to classical holography in which the reconstruction process is pure physical laser diffraction, the main advantage of digital holography is that there is no need for chemical treatment of holograms and no consumables such as in photographic plate processes. Furthermore, the object phase is directly computed from the digital hologram. This direct computation of the optical phase is the key for contact less metrology since it encodes refractive index variations, displacements, deformations, and also polarising properties of the object under test. In addition, a three wavelength illuminating of the object can lead to multi-sensitivity and thus to simultaneous three dimensional metrology. Butit gives also absolute optical phase determination which is compulsory for fluid metrology. Indeed, the last one often needs to solve the discontinuities in the refractive index through shock waves or to accede to unsteady phenomena through the real-time character of the technique.
Such new apparatus giving fine experimental data will offer new opportunities in dynamic photo-mechanics for studying some current open problems. Indeed, challenges aim to answer questions related to fluid mechanics, thermo-acoustic and granular material properties. In vibrations, challenge concerns measurement of 3D vibration motions of a medium for getting insight in the fundamental dynamics of disordered granular materials; in acoustics, challenge concerns analysis of heat transfer in acoustically driven flow and in fluid mechanics, challenge concerns aerodynamic flow in a schematic model of a generic launcher (Ariane).