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Effects of boundary layer suction control on flow through an axisymmetric diverging channel

ABSTRACT The conical diffuser is one of the oldest and most versatile hydraulic devices, however its performance is often less than optimal. This is because boundary layer separation and instabilities in the flow lead to energy losses through internal friction. Boundary layer suction offers a method for controlling and improving this flow, and some experimental studies have shown this to be effective – though they leave questions as to its net efficiency. This study takes a more comprehensive approach to suction control of the conical diffuser through the use of numerical studies coupled with optimisation. The flow through a conical diffuser with semi-divergence angle was simulated by solving the full steady-state incompressible Navier-Stokes equations in the fully laminar Reynolds Number range . Uniform suction control was then applied in the divergent region, and optimisation methods used to determine the most effective suction strength for two objectives: (a) maximising the diffuser efficiency, and (b) minimising the separation length. It was found that suction control could increase the performance of the diffuser and was efficient even when considering the power to run the control. The results suggest that fluidic control of the diffuser should be reconsidered as it shows significant promise, particularly for applications where the constraints on the geometry of the diffuser are tight, for example in aircraft turbine engines.