Passive and active flow augmentation: From diffusers to multi-rotor machines

Flow augmentation consists in modifying mass flow across the actuation plane of a rotor to enhance energy extraction or propulsive efficiency. The talk sketches the distinction between passive and active rotor augmentation strategies. Power coefficient trends are compared analytically while numerical results illustrate differences in flow topology. Rotors are stylized as actuator disks that exert homogeneous normal forces on the steady flow of inviscid fluids to highlight the distinctive features of each augmentation principle. Passive augmentation principles have been well documented because they guide the design of ducted, shrouded and diffuser-augmented wind turbines1-6. These axisymmetric bodies decrease average static pressures on the rotor plane to increase mass flux and power coefficient. Rotor-body interactions are dominated by conservative forces5,7: the bodies don’t exchange energy with the fluid but act as augmenting devices and affect global energy balance by changing rotor state. Virtual work arguments show that bodies exert streamwise forces4,6 that can be related with the power coefficient through the law of de Vries1,6. Active flow augmentation is a rather recent theoretical concept8. Its simplest energy extraction embodiment consists of an upstream actuator that accelerates flow onto a downstream actuator. This augmentation strategy is coined as active because the upstream actuator injects (spends) energy into the flow for the downstream actuator to extract (produce) energy from a greater mass flux than if it were alone. The interaction mechanism depends on the action of non-conservative forces and actuators interact exclusively through changes in total flow enthalpy when they are sufficiently far apart. No pressure interactions occur in this asymptotic case and a closed solution exists together with an analytical power coefficient law. Parallels can be drawn with wake ingestion propeller setups9 but no practical energy extraction realizations have been attempted yet. Passive and active flow augmentation concepts are different but we hope that parallels between them shed further light on the physics of energy extraction from ideal fluid flows. The communication concludes with a few reflections meant to trigger an open discussion about the implications and applicability of the discussed theories.