Mattias Rahm

Evaluation of optical arrangements for ballistic imaging in sprays

This work investigates the imaging performance, in terms of contrast and resolution, of two different time-gated ballistic imaging setups commonly used in spray research. It is shown that the two setups generate similar spatial resolution in the presence of scattering media. The simpler (2f) setup, however, is less sensitive to component misalignments and time-gate induced aberrations than the commonly used (4f) system. Measurements comparing both arrangements indicated slightly higher contrast for the 2f system under the densest conditions for small scatterers. Subsequent computational modeling confirmed the observed tolerance of the 2f system to misalignment and gate effects. The best performing setup was also compared experimentally to its non-time-gated shadow-imaging equivalent, to establish when the time-gate enhances imaging performance. It is shown that the time-gated setup generates higher contrast under almost all of the scattering conditions tested, while the non-time-gated setup generates higher spatial resolution only in the lower scatterer size range at the lowest scatterer concentrations.

Optical sectioning for measurements in transient sprays

We describe a practical arrangement for optical sectioning by means of time-gated backscatter imaging using ultrafast illumination and a CS2-based optical Kerr effect shutter. This arrangement can reveal additional information when probing transient turbid media such as fuel injection sprays or complex multiphase flows which require single-shot imaging with sufficient time resolution to freeze the dynamics of the flow.

Visualization of acceleration in multiphase fluid interactions

Probing the dynamics of structures in turbid media is important for understanding the internal forces that drive the time evolution of many fluid systems; the breakup of fuel injection sprays is a prime example. We demonstrate a three-pulse configuration for time-gated ballistic imaging, applied to a turbulent, steady spray allowing the acquisition of time-correlated image data. Coupled with targeted region-matching analysis, the detected image triplets are used to generate time-resolved velocity and acceleration vectors representing motion and forces involved in spray development.

Optical Arrangements for Time-Gated Ballistic Imaging

We report on a comparison of two optical setups used in time-gated ballistic imaging simulating monodisperse scattering environments with polystyrene spheres in different sizes and concentrations suspended in water.