Active Control of the Turbulent Flow Downstream of a Backward Facing Step with Dielectric Barrier Discharge Plasma Actuators

Active Control of the Turbulent Flow Downstream of a Backward Facing Step with Dielectric Barrier Discharge Plasma Actuators
Author: Patricia Sujar Garrido
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Total Pages: 0
Release: 2014
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This thesis is part of an international project (MARS) to improve air transport efficiency by active flow control strategy. In this context, the presented experimental works are focused on a surface Dielectric Barrier Discharge (DBD) as a solution to control the turbulent flow separation downstream a backward-facing step (BFS) at Reh = 30000. Two different plasma discharges are investigated: an ac-DBD resulting in a electrohydrodynamic force and a ns- DBD producing a pressure wave. Thanks to the versatility of plasma discharges and in order to optimize its effects on the flow, different locations of the DBD actuator have been investigated. Furthermore, an extended parametric study regarding the input variables of the discharge has been carried out by stereoscopic PIV. Among the obtained results, the mean reattachment length has been reduced up to 20%. In addition, other averaged quantities such as Reynolds stress components, the kinetic energy and the vorticity thickness of the separated shear layer have been analyzed to provide more extended information about the effects of the DBD actuator. The last part includes a dynamical analysis of the modifications produced by an optimal actuation. For that aim, the dominant structures are investigated by their signature in the frequency domain and by proper orthogonal decomposition (POD). All the results lead to the definition of an optimal actuation for which the mean reattachment position is reduced and the vortex shedding street can be reinforced by a lock-on control mechanism.

Direct Numerical Simulation of Turbulent Flow Over a Backward-facing Step

Direct Numerical Simulation of Turbulent Flow Over a Backward-facing Step
Author: Michal Andrzej Kopera
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Total Pages:
Release: 2011
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A three-dimensional, turbulent flow in a channel with a sudden expansion was studied by direct numerical simulation of the incompressible Navier-Stokes equations. The objective of this study was to provide statistical data of backwardfacing step flow for turbulence modelling. Additionally, analysis of the statistical and dynamical properties of the flow is performed. The Reynolds number of the main simulation was Reh = 9000, based on the step height and mean inlet velocity, with the expansion ratio ER = 2:0. The discretisation is performed using the spectral/hp element method with stiffly-stable velocity correction scheme for time integration. The inlet boundary condition is a fully turbulent velocity and pressure field regenerated from a plane downstream of the inlet. A constant flowrate was ensured by applying Stokes flow correction in the inlet regeneration area. Time and spanwise averaged results revealed, apart from the primary recirculation bubble, secondary and tertiary corner eddies. Streamlines show an additional small eddy at the downstream tip of the secondary corner eddy, with the same circulation direction as the secondary vortex. The analysis of the 3D, timeonly average shows the wavy spanwise structure of both primary and secondary recirculation bubble, that results in spanwise variations of the mean reattachment location. The visualisation of spanwise averaged pressure uctuations and streamwise velocity showed that the interaction of vortices with the recirculation bubble is responsible for the apping of the reattachment position. The characteristic frequency St = 0:078 was found. The analysis of small-scale energy transfer was performed to reveal large backscatter regions in strong Reynolds stress areas in the mixing layer. High correlation of small-scale transfer with non-linear interaction of large-scale velocity and small-scale vorticity was found. The data of the flow fields was archived. It contains the averages for velocities, pressure and Reynolds stress tensor, as well as 3D instantaneous pressure and velocity history.