POD analysis of momentumless trailing edge wake using synthetic jet actuation

In this work, periodic addition of momentum using a Trailing Edge Synthetic Jet Actuation (TESJA) was used as an open-loop forcing mean to control the wake of an elongated plate. Two synthetic jet actuators were positioned at the trailing edges, spanning the width of the plate, to provide either in-phase or out-of-phase simultaneous actuation. The actuation frequency was fixed to 75% of the natural vortex shedding frequency, while a high momentum addition coefficient of Cμ=24% was applied to balance momentum deficit in the wake. Two component velocity measurements had been recorded using Particle Image Velocimetry (PIV) at mid span around the plate. These measurements allowed the flow dynamic investigation of both natural and actuated wakes at Reynolds number, based on the model height h, Re=U∞ h/ν of 7200.The measurements revealed that the application of TESJA changes the wake flow topology, reduces the extent of the recirculation bubble and redistributes energy among the Reynolds stresses. Coherent structures and their dynamics in the wake were evaluated for both simultaneous actuation strategy (in-phase and out-of-phase). Axial velocity fluctuations spectra showed that these modifications are caused by suppressing the natural vortex shedding and generating new structures which are locked-in on the actuation frequency.The Proper Orthogonal Decomposition (POD) was used to identify the most energetic modal contributions and extract the effect of the actuation on the spatial structure of the corresponding modes. POD modes were found to be associated in pairs corresponding to the downstream convection of flow structures. Actuation affected especially the first mode pairs; i.e. the initial symmetric alternate-signed structures present in the natural case were converted into two anti-symmetric parallel rows of alternate-signed structures in the in-phase actuation, while for out-of-phase actuation modes have the same alignment of structures as the natural case.The most energetic POD modes and the phase-averaged approach are used to reconstruct low-dimensional evolution. The low-order reconstruction highlights the actuation mechanism and exemplifies the key roles of actuation structures. This is evidenced by the fact that the redistribution of the injected momentum is mainly controlled by these generated structures. © 2012 Elsevier Inc.