Rotating Flow and Electroactive Actuators – ETAE

The ETAE project suggests significant advancements about the emergence and control of hydrodynamic instabilities in closed recirculating flows with a free surface. This generic flow configuration is present in numerous industrial contexts. The present aim is, from well-designed excitations by electro-active actuators, to manipulate the flow, and thus to identify the mechanisms promoting large-scale vortical instabilities arising in the presence of external mechanical noise. Bringing together the experimental/numerical skills on rotating flows at LIMSI, and the experience of GEEPS about modelling and conception of active actuators, will address important issues about the effect of parasitic noise on closed fluid systems. The exploratory side about actuators opens wide perspectives on the application of new measurement and control techniques in a fluid set-up, in close interaction with the development of new active materials expected to contribute in the future to fluid control strategies.

The study of instabilities in closed rotating flows, triggered by rotating disks, has been one of the key topics for which LIMSI is internationally recognised. Such flows have now become classical topics due to their genericity and their importance in geophysical or industrial contexts. Using an experimental device consisting of a rotating vessel partially filled with liquid and a free surface, the team at LIMSI has shown evidence for instability modes due to the free surface. The flow before the instability is axisymmetric, and this axisymmetry is broken by instability modes above a given threshold (for the angular velocity of the disk). Two cases can be identified: weak deformations of the free surface where the instability manifests itself as a array of large-scale vortices, versus strong deformations where the free surface itself has broken its axisymmetry. From an experimental point of view, measurements of the free surface height in real time demands novel techniques. Besides, the strong deformation case remains even today a true challenge for numerical simulation. However, even in the weal deformation case, threshold measurements have revealed significant departures between experimental results and numerical predictions. Sensibility methods, developed only recently in the context of open flows, appear as relevant tools to understand the effect of generic external unsteadiness (of weak amplitude and mechanical origin) on the fluid system. Moreover, adding a perfectly controlled vibration to a given flow should explain, and more importantly reduce the mismatch between observed thresholds. Devising such actuators, the associated measurement methods, and integrating them into an efficient feedback loop represent as for today important technological challenges. This project is at the junction between active control of rotating flows at LIMSI and modelling of active-material-based actuators at GEEPS. Bringing together such skills is expected to lead to both fundamental and practical progress about the sensibility of confined flows to unavoidable parasitic vibrations. The exploratory side about actuators in the large deformation regime opens new important perspectives on the development of fluid-structure simulation codes as well as on the characterisation of electro-active materials.