The present work gives a contribution to the investigation on separated flows and to the set-up of strategies for their control by means of numerical simulations. In particular, an appraisal is carried out of a passive control method aimed at reducing and, possibly, eliminating boundarylayer separation. The control strategy consists in the introduction in solid walls of appropriately-shaped cavities. As a paradigmatic example of an 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 area ratio of 2 is studied, while a divergence angle of 7 degrees is chosen, so that, 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 point of view, 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 long as the flow is able to reattach immediately downstream of the cavity.
A passive method for flow separation control: application to a 2D diffuser conguration.
MARIOTTI, ALESSANDRO;BURESTI, GUIDO;SALVETTI, MARIA VITTORIA
2012-01-01
Abstract
The present work gives a contribution to the investigation on separated flows and to the set-up of strategies for their control by means of numerical simulations. In particular, an appraisal is carried out of a passive control method aimed at reducing and, possibly, eliminating boundarylayer separation. The control strategy consists in the introduction in solid walls of appropriately-shaped cavities. As a paradigmatic example of an 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 area ratio of 2 is studied, while a divergence angle of 7 degrees is chosen, so that, 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 point of view, 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 long as the flow is able to reattach immediately downstream of the cavity.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
