Application of PIV to over-expanded supersonic flows: Possibilities and limits
Two-dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using a digital particle image velocimetry (PIV). Mean velocities, vorticity field and volume dilatation field were obtained from PIV images using 0.33 mum titanium dioxide (TiO2) particle. The seeding particle of larger size, 1.4 mum TiO2, was also used for the experimental comparison of velocity lag downstream of shock waves. The results have been compared and analyzed with schlieren photographs for the locations of shock waves and over-expanded, shock structure to inspect possibilities and limits of a PIV technique to over-expanded supersonic flows. It is found that although the quantitative velocity measurement using PIV on over-expanded supersonic flows with large velocity and pressure gradients is limited, the locations of normal shock and oblique shock waves can be resolved by the axial/radial velocity fields, and over-expanded shock structure can be predicted by vorticity field and volume dilatation field which are acquired from the spatial differential of the velocity field.
piv over-expanded supersonic flows mean velocity vorticity volume dilatation