We study an array of streamwise-oriented Dielectric Barrier Discharge (DBD) plasma actuators as an active control technique in turbulent flows. The analysis aims at elucidating the mechanism of interaction between the structures induced by the DBD-plasma actuators and the convective heat transfer process in a fully developed turbulent boundary layer. The employed flush-mounted DBD-plasma actuator array generates pairs of counter-rotating, stationary, streamwise vortices. The full three-dimensional, velocity field is measured with stereoscopic PIV and convective heat transfer at the wall is assessed by infrared thermography. The plasma actuator forcing diverts the main flow, yielding a low-momentum region that grows in the streamwise direction. The suction effect promoted on top of the exposed electrodes confines the vortices in the spanwise direction. Eventually, the pair of streamwise vortices locally reduces the convective heat transfer with a persistence of several outer lengthscales downstream of the actuation.

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This study focuses on an active strategy for unsteady-load control by means of a trailing-edge flap. Experiments on a 2D pitching and heaving NACA 0018 airfoil with an actuated trailing-edge flap are performed at a reduced frequency κ = 0.1 in a free-stream flow with Re = 1.3 × 10⁵. The model is equipped with 24 differential pressure transducers to provide with the time-resolved distribution of the pressure difference between pressure and suction sides. Results show that an actuated flap significantly decreases the magnitude of unsteady loads. A reduced order model of the flap effect on the loads is proposed. The loads are estimated by adding the contribution of the clean wing with the flap one. This model can be applied to unsteady loads for both attached flow and dynamic stall conditions, constituting an effective control strategy for dynamic loads, if the airfoil transition location is properly controlled.

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The analysis of the unsteady flow field of an axisymmetric subsonic jet at Reynolds numbers between 5,000 and 20,000 and computation of its unsteady pressure field is carried out from single snapshots of tomographic PIV measurements (acquisition rate 0.8 Hz). This is achieved by a recently developed pressure-evaluation technique based upon the vortex-in-cell (VIC) methodology. The technique allows for a finite time-marching of the instantaneous 3D velocity fields obtained from low-repetition PIV systems. A time series of velocity fields in 3D is obtained, which is integrated into unsteady pressure by use of the Navier-Stokes momentum equation. Despite the limitations in the finite-time marching of the measured structures due to the size of the acquired tomographic volume, spectra of pressure fluctuations can be extracted in a frequency range between 800Hz and 20kHz. Fair agreement was found between the experimentally computed pressure spectra and the respective values found in literature. Further work is dedicated to exploit the full potential of the technique by attempting a sound pressure level integration of the noise sources from the unsteady fields.@en

The unsteady three-dimensional flow organization of jets issued from a duct with swirl vanes at Reynolds number equal to 1000 and swirl number ranging between 0 and 0.8 is investigated. Time-resolved tomographic particle image velocimetry returns the instantaneous flow structure and its evolution by visualization of velocity and vortical features. The most relevant coherent motions are identified and characterized with the aid of dynamic mode decomposition. The time-averaged flow topology indicates that the vanes used to impart the swirling motion have a significant impact on the azimuthal modulation of momentum, with the jet exhibiting four sectors separated by a thin cross-like wake resulting from the boundary layer developed along the vane walls. The flow field is thus characterized by inner and outer shear regions. An increase in swirl, up to moderate levels, causes larger jet spreading angles. Further increase of the swirl number is accompanied by the appearance of a central recirculation zone due to vortex breakdown at which increases in size and is triggered upstream for increasing. Although no shear layer instability development is observed at, already at the swirling motion promotes the growth of helical vortices appearing as Kelvin-Helmholtz waves that deform the outer axial shear layer. The downstream evolution features successive pairing, which is observed for all the considered swirl numbers. The initial development of the instability is independent for each vane, whereas a mutual interaction between the vanes occurs after the vortex pairing. The reconnection from the four sectors vortices induces a significant increase of azimuthal vorticity, which affects the dynamical behaviour of the precessing vortex core. The latter is visualized by a low-order spatio-temporal reconstruction based on few dynamical modes. At a higher swirl number, the axial vorticity component dominates the flow field; it interacts with the azimuthal vorticity, which penetrates inward through the meanders of the vane wakes and forces the vortex core precession and breakdown.

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Dynamic stall limits the operative life of rotors, due to a periodic occurrence of force and moment peaks that can lead to blade structural-failure. The aim of this study is to investigate the possibility of controlling cyclic forces on wind turbine blades by means of Trailing Edge Flaps. The periodic motion of a rotor blade is reproduced by a 2D wing pitching and plunging in a free-stream flow at 푹풆=ퟏ.ퟑ× ퟏퟎퟓ. The wing is equipped with 50 pressure taps able to provide with a time-resolved distribution of the pressure on the wing. A trailing-edge flap is installed in the wing and actuated with respect to the wing motion. Flap actuations in phase opposition to the rotor motion are found to maximize the lift integral over the period while minimizing the load fluctuations over time. @en