Visual-hull based 3D reconstruction of shocks in under-expanded supersonic bevelled jets.

• Three-dimensional shock structures produced by Mach 1.45 supersonic bevelled jets were digitally reconstructedbased on schlieren photography and a voxel-based visual hull technique. By taking advantage of the strong edgefeatures commonly found in schlieren images of shock waves, the proposed technique demonstrates the possibilityof performing shock wave reconstruction in supersonic jet applications without prior knowledge of theglobal density or velocity field. Semi-synthetic camera parameters were introduced as a method to circumventcamera calibration issues faced in the reconstruction procedure. This is key to achieving accurate and highresolutionreconstructed shock waves for both axisymmetric and asymmetric test cases with an average of 2.5%error when validated against raw schlieren images. When applied to bevelled jets with non-uniform nozzle exitgeometries, an additional assumption was made to address the problem of schlieren line-of-sight blockage by thenon-conventional nozzle, and reconstruction errors were found to be larger near regions of poorer shock wavecontrast. Current results indicate that the technique is robust and fast during image calibration and processing,with accuracy of reconstructed shock waves in both conventional and non-conventional nozzles strongly dependenton shock wave contrast. Compared to existing techniques that can be used to reconstruct 3D shockstructures, the proposed technique has the advantage of being totally non-intrusive as compared to point orparticle-based measurements, requires significantly less computation than tomographic methods, offers highresolution reconstruction even with limited camera resolution and projected schlieren views, and is easy and costeffective to implement.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Strömningsmekanik och akustik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Fluid Mechanics and Acoustics (hsv//eng)

Nyckelord

Schlieren Three-dimensional image processing Flow diagnostics Camera calibration Visual hull Supersonic jets

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