Mixing analysis of a swirling recirculating flow using DNS and experimental data

Fluid flow and mixing of a recirculating, swirling flow at moderate Reynolds number was investigated by direct numerical simulation (DNS) as well as experimentally using laser Doppler velocimetry (LDV) and spontaneous Raman scattering. Using these techniques together allows for a cross validation of the data. Furthermore, each technique can compensate for the restrictions of the other making the full data set more complete. Detailed one and two point statistics for velocity and scalar concentration are presented and discussed. The high swirl momentum leads to a fluid mechanical instability, a so-called precessing vortex core (PVC), which will be characterized especially with respect to its interaction with the scalar field. The results show that due to the reverse flow zone the PVC strongly modifies the mixing process leading to a precessing scalar concentration field. However, the level of the mean square deviation for the scalar field is considerably smaller than the corresponding value for the velocity field due to the different nature of the radial mean velocity and scalar concentration profiles in the near nozzle region.