ENERGY-EFFICIENT FLOW CONTROL OF BACKWARD-FACING STEP FLOWS USING PASSIVE AND ACTIVE AERODYNAMIC TECHNIQUES

Date

2026-6-26

Author

Hasanli, Matin

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Energy losses associated with flow separation are encountered in a wide range of engineering systems, including diffusers, duct flows, HVAC installations, turbomachinery, and aerodynamic applications. These losses reduce pressure recovery, increase energy demand, and often lead to undesirable flow unsteadiness. The backward-facing step (BFS) is one of the most widely studied benchmark configurations for investigating separated-flow behaviour and evaluating flow-control strategies. This thesis examines the effectiveness of both passive and active flow-control methods for manipulating the separated flow downstream of a backward-facing step and improving its aerodynamic performance. Passive flow control is implemented using rigid and elastic cantilevered plates positioned within the recirculation region, while active flow control is achieved through synthetic jet actuation. Numerical simulations are performed to investigate the influence of key design parameters, including plate length, structural stiffness, synthetic-jet forcing frequency, and momentum input. The numerical methodology is first validated against experimental benchmark data, demonstrating good agreement with measured flow characteristics. The results show that both passive and active flow-control approaches effectively modify the separated flow structure, reduce the reattachment length, and improve pressure recovery downstream of the step. Among the passive configurations, rigid plates provide the greatest aerodynamic benefit, with the optimum configuration reducing the reattachment length by approximately 12% and improving the base-pressure characteristics by about 15% relative to the uncontrolled case. Elastic plates also improve the flow behaviour; however, their effectiveness is strongly influenced by structural stiffness, with excessive deformation reducing the control authority. Among all configurations investigated, synthetic jet actuation provides the strongest flow-control authority, achieving reductions in reattachment length of up to around 65% together with an increase in base pressure of approximately 159%. Crucially, system-level efficiency evaluations reveal that while this aggressive active control operates at an energetic deficit, optimized low-momentum actuation yields a net energy saving of up to 89%. Overall, both passive and active flow-control methods are shown to improve the flow characteristics downstream of the backward-facing step by reducing the extent of the recirculation region and enhancing pressure recovery. While synthetic jet actuation achieves the greatest reductions in reattachment length and the largest increase in base pressure, rigid plates provide substantial improvements without requiring external energy input. The results highlight the trade-off between control effectiveness and energy expenditure and demonstrate the potential of both approaches for the management of separated flows. Finally, the study evaluates these control strategies in the context of practical engineering applications, assessing their viability based on ease of implementation, maintenance requirements, and overall energy sustainability.

Subject Keywords

Active flow control, backward-facing step, flow separation, passive flow control, synthetic jets.

URI

https://hdl.handle.net/11511/120089

Collections

Northern Cyprus Campus, Thesis