Professor Gilead Tadmor Electrical & Computer Engineering, Northeastern University Professor Gilead Tadmor Electrical & Computer Engineering, Northeastern University "Enablers for control oriented Galerkin Models in Fluid Dynamic Systems" Abstract. The combined nonlinearity and high dimension of computational fluid dynamics models (O(10^4) at the very low end) form a serious if not insurmountable impediment to their use in real time feedback design: This applies to the use of practical analytical nonlinear design methods, to the ability to reliably estimate a large number of flow states from few and typically noisy sensor readings, and to real time computations necessary for feedback implementation. Seeking low order models, the issue of an ample dynamic envelope becomes critical in the context of feedback control, where free and actuated transient deviations from designated target states or orbits, are in the essence. We shall review some enablers to the development of very low order, design oriented Galerkin models (GMs). These enablers include physics based identification of modes that play important dynamic roles, estimation of turbulence and pressure effects, modes from multiple operating points and tunable models and actuation models. In particular, work curried now by NU students include studies of dynamic mean field corrections and the mechanisms by which nearly singular oscillatory actuation at the boundary, communicates with flow instability. The dynamic manifold that defines the model's validity envelope must be respected - but can also be exploited - in observer and control design, such as by a restriction to slow drift in the system's periodic behavior, enabling the use of simplifying dynamic phasor models. Finally, the talk will highlight some intrinsic performance limitations in GM based feedback flow control.