## Tesi etd-05132012-110657 |

Tipo di tesi

Tesi di dottorato di ricerca

Autore

GROZESCU, ANNABELLA NICOLETA

URN

etd-05132012-110657

Titolo

Numerical simulation and control of separated flows

Settore scientifico disciplinare

ING-IND/06

Corso di studi

INGEGNERIA AEROSPAZIALE

Commissione

**tutor**Salvetti, Maria Vittoria

Parole chiave

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

Data inizio appello

01/06/2012;

Consultabilità

completa

Riassunto analitico

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.

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.

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