Identifying dynamic modes of separated flow subject to ZNMF-based control from surface pressure measurements

Fluid systems are most efficient for fully attached flows, and designers therefore seek to avoid flow separation. Active flow control can help achieve this goal, and closed-loop control offers improved performance at off-design conditions. However, this requires feedback of accurate state estimates to the controller in real time. This motivates a physics-based, state-estimation technique that economically extracts key dynamical features of the flow. This work aims to extract dynamical characteristics of a laminar separation bubble on a flat plate at a chord Reynolds number of 10⁵ using a linear array of unsteady surface pressure measurements. First, DynamicMode Decomposition (DMD) is employed on high-dimensional time-resolved PIV velocity and corresponding estimated pressure fields to identify the dynamically relevant spatial structure and temporal characteristics of the separated flow. Then, results are presented of various open-loop control cases using pulse-modulation of a zero-net mass-flux actuator slot located just upstream of separation. Real-time estimates of the dynamical characteristics are provided by performing on- line DMD onmeasurements froma linear array of 13 unsteady surface pressure transducers. The results show that this method provides reliable estimates of the modal characteristics of the sep- arated flow subject to forcing at a rate much faster than the characteristic time scales of the flow. Therefore, online DMD applied to the surface pressure measurements provides a time-varying linear estimate of the evolution of the controlled flow, thereby enabling closed-loop control.