Validation of CFD Density Field Validation in Supersonic Axisymmetric Flows Using BOS and Differential Interferometry

Abstract

Density measurements have been performed using Background Oriented Schlieren (BOS) and differential interferometry, in the case of supersonic flows around a cone-cylinder body, a hemisphere profile and two spike-tipped models for Mach numbers 2 and 3, and no angle of attack. Experiments have been driven in the blow-down wind tunnel of the FrenchGerman Research Institute of Saint-Louis (ISL). For BOS technique a specific Abel transform algorithm has been developed to rebuild density field from ray deflections. For differential interferometry an original and simple calibration and filtering process allows to obtain quantitative results in spite of experimental noise during the blow-down. Performances of these two very different techniques – accuracy, spatial resolution, limitations – have been evaluated and compared in the basic cases of cone and hemisphere. BOS measurements have been 2% accurate and spatial resolution has been 1 millimeter in a 100*100mm2 flowfield. In the same flowfield, differential interferometry has given 2% accurate measurements and spatial resolution has been 0.25 mm. Pretty good agreement has been found with RANS numerical simulations outside from boundary layers and bow shock regions. The latter is the main limitation to both of methods. In a bow shock region, BOS experimental set up is not “stigmatic”, while a “parasite” shadowgraph appears in differential interferometry. The lack of information leads to under-estimation of density in such a region. Having knowledge about performances and limitations of these two techniques, Background Oriented Schlieren and Differential Interferometry have been used to measure density in the case of a supersonic flow around a spike-tipped body: a disk-spike and a biconical-spike for Mach numbers 2 and 3 have been under study. Outside bow shock regions, a pretty good agreement has been found between measurements by BOS and by Differential Interferometry, which allow them to be used later for CFD validation.