Turbulent flow control of three types of flow is numerically studied using hybrid RANS/LES approaches. Flow control effect and control mechanisms are studied through both instantaneous and statistically-averaged flow properties. Behaviours of hybrid RANS/LES methods in these three types of flow are also investigated.
Flow over a backward facing step is investigated, with piezoelectric actuators implemented on the step to control flow separation and recirculation. The simulation results agree well with the experiments. For the controlled flow, a slightly reduced primary recirculation between two adjacent actuators was observed. In the exploring study of actuators with a control velocity similar to the free stream flow, counter-rotating vortex pairs are generated, interacting with the separated shear layer. The primary recirculation becomes much smaller, and the recovered flow has a smaller skin friction.
Pitched and skewed jet vortex generators are applied to the NACA0015 aerofoil to study their control effect on the trailing edge separation induced by a gradual adverse pressure gradient. Both the simulation and experiments show that after a certain time, the originally separated flow is forced to be attached by the blowing jets. The lift coefficient is enhanced and the drag coefficient is reduced. When the jets are switched off, the fully attached flow recovers to the originally separated flow. The jet-removal process has about 70% longer transient time than the jet-deployment process.
Flow through fractal orifices in pipes is studied to investigate the passive control effect of the orifice geometries in flow mixing and decay. The simulation results are in good agreement with the available experimental data. With a higher fractal level, the vena contracta velocity decreases and the unrecoverable pressure loss becomes smaller. The higher level fractal orifices generate more organized vortices, maintaining a high turbulent kinetic energy for a longer distance and a slower decay. Axis-switching is observed for all these fractal orifices.
