Active flow control of ramp flow by fluidic oscillators

International audienceThe study of actuators for active flow control has been in rapid expansion in the last several decades, pursuing different goals such as reducing drag on bluff bodies 1 , increasing lift of airfoils 2, 3 or enhancing mixing in combustion chambers 4, 5. Compared to traditional passive control methods or steady blowing method, the active flow control based on periodic fluidic excitations is much more efficient, with a gain of two orders of magnitude in terms of added momentum coefficient, as demonstrated by numerous researches (e.g. Greenblatt and Wygnanski 6). These periodic fluidic disturbances can be provided by various kinds of actuators such as ZNMF (Zero Net Mass Flow) actuators, plasma actuators or MEMS (Micro-Electro-Mechanical-Systems) 7. Among them, fluidic oscillators can emit oscillating jets in a large operating frequency and velocity range when supplied with a pressurized fluid without requiring any moving part, since their oscillations are totally self-induced and self-sustained and only depend on the internal flow dynamics, which is a great advantage in terms of reliability and robustness 8-10. A typical fluidic oscillator is basically composed of an inlet nozzle N, two feedback loops F1 and F2 and two outlets O1 and O2, as shown in Figure 1a. Its behavior is based on the Coanda effect: the jet issuing from nozzle N attaches one of the two walls W1 or W2. The attachment either to wall W1 or wall W2 depends on the initial conditions or is the result of specific actions on the jet. If there was no feedback loop and if the outlet sections were large, the attachment to wall W1 or wall W2 would be stable and the flow would exit through the corresponding outlet, O1 or O2, respectively. With feedback loops, when the jet is attached to wall W1, part of the flow fills in the feedback loop F1 and a pressure increase in the left side of the device is observed, due to the hydraulic restriction at outlet O1. This pressure increase forces the jet to switch toward the right side. Following the jet switching, the same phenomenon develops in the right side of the oscillator and results in a self-sustained oscillating behavior, with a pulsed flow alternatively exiting outlets O1 and O2. In the current design, the feedback loops are plastic tubes as shown in Figure 1b, connected perpendicularly to the base plate. The channels of the oscillator's central part are milled in the base plate in a depth of 370 µm while the outlet slot is milled in the cover plate with an area of 0.5 mm 2. The throat section of inlet nozzle has a width of about 200 µm. The outlet jet has the same direction as the inlet air. After examining by hot wire the frequency and velocity response of an isolated oscillator to inlet pressure, an array of 12 identical fluidic oscillators is integrated in a ramp with a 25° slant angle by assembling the cover plate, the base plate and the ramp together as shown in Figure 1c