• CFD
• Diffuser shape optimization
• Passive flow control
• Turbulent flow regime

The object of the present work is testing the capabilities of a passive method
for improving the efficiency of a plane symmetric diffuser. The passive control
method consists in introducing small localized flow recirculations. These
localized flow separations are obtained modifying the geometry of the diffuser
diverging walls using couples of appropriately contoured cavities. The turbulent
flow regime is considered. The Reynolds number, based on the diffuser
width at the inlet section and the inlet velocity on the axis in a certain section
is Re = 20000. The diffuser area ratio in 4.7 and the divergence angle is 5°.
The inlet flow is fully-developed turbulent. A preliminary mesh sensitivity
analysis for two turbulence models (SST k-ω and RSM) is performed for the
flow inside the reference diffuser configuration (without cavities). Based on
this analysis the SST k-ω model is chosen for an optimization procedure in
order to identify the cavity geometries that maximize the pressure recovery
in the diffuser and minimize the main boundary separation extent. Three
different configuration have been considered: one, two and three couples
of subsequent couples of general contoured cavities. In the optimized diffuser
configuration with one couple of contoured cavities there are not small
localized flow recirculations, while in the other two optimized diffuser configurations
with two and three couples of contoured cavities small localized flow
recirculations are present. The passive control increases the performance of
the diffuser in all three configurations (the maximum pressure recovery is
reached with the optimized diffuser configuration with three couples of cavities),
leading to an increase of the mean pressure recovery coefficient by
reducing the extent of the main flow separation region. In particular, the
success of the passive control is due both to a virtual geometry modification
of the flow inside the diffuser and to a favourable effect of the cavities in
reducing the momentum losses near the wall. A classical shape optimization
with Bézier curve is also carried out in order to mazimixe the pressure recovery.
The resulting optimized diffuser shape with Bézier curve does not show
small local flow recirculations but produces a diverging wall shape similar
to the one optimized with contoured cavities. Since the optimized diffuser
with Bézier curve provides a pressure recovery very similar but not equal to
the one of the optimized diffuser with three couples of cavities it seems that
the "optimum" configuration can be given by the union of a high degrees of
freedom Bézier curve and the presence of small localized flow recirculation placed
at precise locations.