Digital holographic interferometry at F4-ONERA hypersonic wind tunnel

The feasibility of digital holographic interferometry using a CW laser @ 532 nm and a Photron APX camera is demonstrated in the F4-ONERA hypersonic wind tunnel. This optical technique is used to point out the possible combustion of a small hydrogen jet in the shock layer developed on the model. The optical technique generates in the field under analysis, interference micro-fringes which are used as carrier spatial frequency. The field of optical thickness is obtained with a FFT2D post processing. Then, if the flow is assumed to be two-dimensional, the map of the refractive index can be estimated. The adjustment of the optical bench is easy to implement and the results obtained with this technique can be used for several interesting comparisons. From the phase difference maps, the holographic images and schlieren images in the x and y directions can be recalculated, which is not possible with a conventional schlieren optical setup. In addition, if the characteristics of the ionized gas are well known, the value of the gas density upstream and in the shock layer can be estimated.