Plasma actuation for boundary layer separation control in engine ducts

Turbulent boundary layer separation is an important problem for a variety of applications, including S-shaped aircraft engine intakes and inter-turbine ducts (ITDs). Turbulent boundary layer separation in the engine intakes causes inlet flow distortion, resulting in deteriorated engine performance and reduced engine component life. The design of ITDs is likely to become more aggressive in response to requirements for lighter, more efficient and environment-friendly aircraft engines. Such aggressive ITDs would likely suffer large pressure losses due to turbulent boundary layer separation leading to reduced engine performance, and therefore require the application of flow control techniques. Various flow control techniques have been studied to control turbulent boundary layer separation, such as vortex generators, vortex generator jets and synthetic jets. The recent advent of dielectric barrier discharge (DBD) plasma actuators can potentially provide a more effective and robust alternative. Plasma actuators convert electricity directly into flow momentum near the surface through partial ionization of air. Compared to other flow control techniques, these electrical solid-state actuators have fast response time and are simple, potentially robust and easy to integrate and non-intrusive (low to zero protrusion), which make them ideal for aerodynamic applications.;The present work studies the effectiveness of DBD plasma actuators in the suppression of turbulent boundary layer separation in S-shaped engine ducts. First, numerical and experimental studies on the control of turbulent boundary layer separation by plasma actuation are carried out in 2-D generic diffusers, as simplified models of 3-D S-shaped engine ducts. The findings from both CFD simulations and experiments are similar. Results show that plasma actuation can reduce and even suppress turbulent boundary layer separation in both continuous and pulsed modes. The effectiveness of the actuation (in terms of pressure recovery) increases with the actuator strength. The optimal actuator location is generally close to and upstream of the boundary layer separation point. In the pulsed mode, the optimal pulsing frequency corresponds to a dimensionless frequency (based on freestream velocity and separation length) on the order of 1 and is equal to (within measurement resolution) the dominant frequency in the non-actuated turbulent boundary layer. This implies that the pulsed actuation works through resonance of dominant turbulence flow structures and improves flow mixing and momentum transfer from outer high-momentum flow to near-surface low-momentum flow. The actuation effectiveness increases with duty cycle. However, for optimal efficiency in terms of effectiveness versus power consumption in pulsed actuation, the duty cycle should be chosen between 10% and 50%. In addition, pulsed actuation at the optimal frequency is more effective than continuous actuation for similar power consumption.;With the results from 2-D diffusers as a general guide, numerical and experimental studies are conducted on an annular inter-turbine duct (ITD) to demonstrate the effectiveness of plasma actuation in controlling turbulent boundary layer separation in 3-D S-ducts in continuous actuation mode. The trends in terms of the influence of actuator location and strength on effectiveness (reduction of total pressure loss from boundary layer separation) are consistent with those from the 2-D diffusers studies. Moreover, empirical design rules for continuous plasma actuation, obtained from data collapse of computational simulations in 2-D generic diffusers, successfully provide a conservative prediction of the required actuator strength for reduction of turbulent boundary layer separation in the ITD.