  A Tomographic Background-Oriented Schlieren (TBOS) technique is developed to
aid in the visualization of compressible flows. An experimental setup was
devised around a sub-scale rocket nozzle, in which four cameras were set up in
a circular configuration with 30{\deg} angular spacing in azimuth. Measurements
were taken of the overexpanded supersonic jet plume at various nozzle pressure
ratios (NPR), corresponding to different flow regimes during the start-up and
shut-down of rocket nozzles. Measurements were also performed for different
camera parameters using different exposure times and f-stops in order to study
the effect of measurement accuracy. Density gradients and subsequently
two-dimensional line-of-sight integrated density fields for each of the camera
projections are recovered from the index of refraction field by solving a
Poisson equation. The results of this stage are then used to reconstruct
two-dimensional slices of the (time-averaged) density field using a tomographic
reconstruction algorithm employing the filtered back-projection and the
simultaneous algebraic reconstruction technique. By stacking these
two-dimensional slices, the (quasi-) three-dimensional density field is
obtained. The accuracy of the implemented method with a relatively low number
of sparse cameras is briefly assessed and basic flow features are extracted
such as the shock spacing in the overexpanded jet plume.
