On dynamics of fluidic jet switching and shear layer instabilities

Résumé 0

Active flow control systems pave the way for increased cycle efficiency and reduced operational costs of next-generation jet engines. An active fluidic switching device is here proposed as the technological building block required for such a system. A novel switching mechanism based on acoustic excitation of natural flow instabilities is developed for that purpose and, what's more, investigated experimentally, numerically and analytically. The novel actuation mechanism outperforms the current state of the art and thus offers great potential to be used in future aerospace applications. Unlike conventional actuators, acoustic excitation is shown to achieve the balancing act between high bandwidth, sufficient control authority and cost effectiveness. A proof of concept study using low-frequency excitation and low subsonic flow velocities reveals that the excitation and amplification of the natural shear layer instability results in detachment, switching and reattachment of the flow in a Coanda based fluidic device. The switching mechanism is explained by an enhanced roll-up of vortical structures on the unattached side of the jet causing an asymmetry in entrainment that ultimately leads to a pressure drop sufficient to counteract the attachment force. The detailed analysis of the switching mechanism then allows the development of a second device that operates up to sonic conditions using ultrasound. However, the switching mechanism of the second device is found to rely on the excitation of a different flow instability; namely, the jet preferred mode as well as the first subharmonic of the natural shear layer instability. This can be attributed to a change in the shear layer characteristics as the Reynolds number increases. In addition, ambiguous data obtained from both Large-Eddy Simulation and Particle Image Velocimetry suggest a fundamental different relationship between the natural shear layer instability and the jet preferred mode with respect to the currently accepted point of view. This is investigated separately by means of the downstream development of the shear layer characteristics of an excited round jet subject to different initial conditions.