• BARC benchmark
• diffuser
• numerical simulations
• optimization
• rectangular cylinder
• separation control
• unstructured grids
• VMS-LES

The present work gives a contribution to the investigation of separated flows
and to the set-up of strategies for their control by means of numerical simulations. The manuscript is divided in two parts. The first part concerns
the appraisal of a passive control method aimed at reducing and, possibly,
eliminating boundary-layer separation. The control strategy consists in the
introduction in solid walls of appropriately-shaped cavities. As a paradigmatic example of internal flow of engineering interest, to which the passive
control can be applied, we consider herein a plane diffuser. The flow Reynolds
number is kept very low (Re = 500, based on the diffuser height and on the
inlet velocity on the axis), so that turbulence and three-dimensional effects
can be neglected. A configuration characterized by an expansion rate of 2 is
studied, while the diverging angle is chosen such that, in the considered conditions, without the introduction of the control, the flow inside the diffuser
is characterized by a large zone of boundary-layer separation. The numerical
simulations are validated and the different simulation parameters are set by
comparing the results obtained by three different codes. From a qualitative
viewpoint, in all the simulations, the flow inside the diffuser is steady and
is characterized by a zone of asymmetrical separated flow. Moreover, all the
simulations give very similar quantitative predictions of the flow main quantities. In order to reduce the separated zone and to increase the efficiency
of the diffuser, a couple of symmetric cavities is introduced in the diffuser
walls. An optimization procedure is developed to identify the best cavity
geometry, which can maximize the pressure recovery in the diffuser and minimize the boundary layer separation extent. The most important geometrical
parameters are identified. The introduction of the optimal cavities leads to
an increase in pressure recovery of more than 13% and to a strong reduction
of the separation extent. The robustness of the control to small changes in
the geometrical parameters of the cavities is also investigated. It is found
that the control is effective as far as the flow is able to reattach immediately
downstream the cavity.
The second part of the present work is a computational contribution to
the Benchmark on the Aerodynamics of a Rectangular 5 : 1 Cylinder, BARC.
Variational multiscale large-eddy simulation (VMS-LES)is used in order to
numerically simulate the high Reynolds, low-turbulence incoming flow around
a stationary, sharp-edged rectangular cylinder of infinite spanwise length and
of breadth-to-depth ratio equal to 5. Two different eddy-viscosity subgrid
scale (SGS) models are used to close the VMS-LES equations, viz. the
Smagorinsky model and the WALE models. A proprietary research code
is used, which is based on a mixed finite-volume/finite-element method apliplicable to unstructured grids for space discretization and on linearized implicit time advancing. The influence of SGS modeling, grid refinement and
Reynolds number on the results is investigated. Two different unstructured
grids are considered; similarly two Reynolds numbers values are investigated
(Re = 20000 and 40000 based on the freestream velocity and on the cylinder
depth). The assessment of quality and reliability of VMS-LES results is addressed: the results obtained are compared together and with other available
numerical results and experimental data. The VMS-LES approach is shown
to be capable of giving results of comparable accuracy to those obtained in
classical LES simulations on noticeably coarser grids. The near wake flow
and the mean drag coefficient prediction are found to be almost the same in
all the simulations, while the flow on the cylinder lateral sides, as well as the
time fluctuations of lift, are highly sensitive to all the considered parameters.
The vorticity dynamics on the cylinder lateral sides is finally investigated and
typical vortex configurations are identified.