Juan D'Adamo

Flow Control with Electrohydrodynamic Actuators

Theabilityofanelectrohydrodynamicactuatortomodifythecharacteristicsofa e owovera e atplateisanalyzed. Thedeviceconsidered usese ush-mounted electrodes and a dcpowersupply to createa plasma sheeton thesurface of the plate. We analyze the mechanism of formation of this plasma sheet, which has some similarities with the phenomenon of streamer formation. Experimental results are presented concerning e ow visualizations obtained at low e ow velocities (o 1 m/s) and velocity e elds obtained with a particle image velocimetry technique for higher e ow velocities (range 11.0-17.5 m/s). These results show that the discharge can induce an important acceleration of the e ow close to the surface. ORONAS are self-sustaining discharges characterized by a strong inhomogeneity of the electric e eld cone guration and electrodes having a low curvature radius. This cone guration cone nes the ionization process to regions close to the high-e eld electrodes. Thus, in this phenomenon, there are active electrodes, surrounded by ionization regions where free charges are created, a low-e eld drift region where charged particles drift and react, and low-e eld passive electrodes. Coronas can be unipolar or bipolar if oneorbothelectrodesareactiveelectrodes.Bipolarcoronascanlead to the formation of streamers, weakly conducting plasma e laments extending from one electrode and carrying their own ionization re- gion ahead of themselves. Positive streamers are cathode directed and negative streamers anode directed. The physics of corona discharge occurring in a gas close to an insulating surfacehasnotbeen as widely studied as coronaswithout anyextraneousbodiesinthevicinityofthedischarge. 1;2 Becausethe discharge involves the movement of ions as well as a large amount of neutral particles, this situation becomes of special interest in aerodynamics for e ow and instabilities control. The induced e uid motion is usually called electroconvection or sometimeselectricwind.Coulombianelectroconvectiontakesplace ifthecoulombianforcesactingonthee uidparticlesarepredominant in relation to the polarization ones. This is usually the case when the e uid medium is air. The way the electric forces act on e uid particles may be explained by considering that ions in their drift motion from one electrode to the other will exchange momentum with the neutral e uid particles and induce their movement. Because currents involved in the process are so lowthat magnetic effects can be disregarded the phenomenon is described by the set of equations used in electrohydrodynamics (EHD) problems.

Variational assimilation of POD low-order dynamical systems

With this work, we propose improvements to the construction of low-order dynamical systems (LODS) for incompressible turbulent external flows. The model is constructed by means of a proper orthogonal decomposition (POD) basis extracted from experimental data. The POD modes are used to formulate an ordinary differential equation (ODE) system or a dynamical system which contains the main features of the flow. This is achieved by applying a Galerkin projection to the Navier–Stokes equations. Usually, the obtained LODS presents stability problems due to modes truncation and numerical uncertainties, specially when working on experimental data. We perform the model closure with a variational method, data assimilation, which refines the state variables within an iterative scheme. The technique allows as to correct the dynamic system coefficients and to identify and ameliorate the issued experimental data.

Circular cylinder drag reduction by three-electrode plasma actuators

The drag reduction in a circular cylinder was explored by means of a novel three electrode plasma actuator (DBDE). The DBDE actuator can reduce the drag coefficient up to a ~25% respect to the base flow drag coefficient. It has been demonstrated that, within the present experimental conditions, the DBDE actuator, for a fixed value of the power coefficient, adds a higher momentum to the flow and, consequently, produces a higher drag reduction than the DBD actuator with the same power coefficient. The actuator efficiency was analysed in terms of the momentum added to the flow revealing similar behaviour for both kind of actuators. However to produce similar levels of actuation both kind of actuators require different values of VAC voltages that resulted always lower for D BDE. The reduction in this high voltage value is highly beneficial as is directly related to: a lower HV AC source power requirement, a reduction in the dielectric breakdown probability of the device and a reduction in leakage currents.

The scenario of two-dimensional instabilities of the cylinder wake under electrohydrodynamic forcing: a linear stability analysis

We propose to study the stability properties of an air flow wake forced by a dielectric barrier discharge (DBD) actuator, which is a type of electrohydrodynamic (EHD) actuator. These actuators add momentum to the flow around a cylinder in regions close to the wall and, in our case, are symmetrically disposed near the boundary layer separation point.Since the forcing frequencies, typical of DBD, are much higher than the natural shedding frequency of the flow, we will be considering the forcing actuation as stationary.In the first part, the flow around a circular cylinder modified by EHD actuators will be experimentally studied by means of particle image velocimetry (PIV). In the second part, the EHD actuators have been numerically implemented as a boundary condition on the cylinder surface. Using this boundary condition, the computationally obtained base flow is then compared with the experimental one in order to relate the control parameters from both methodologies.After validating the obtained agreement, we study the Hopf bifurcation that appears once the flow starts the vortex shedding through experimental and computational approaches. For the base flow derived from experimentally obtained snapshots, we monitor the evolution of the velocity amplitude oscillations. As to the computationally obtained base flow, its stability is analyzed by solving a global eigenvalue problem obtained from the linearized Navier–Stokes equations. Finally, the critical parameters obtained from both approaches are compared.

Spatiotemporal spectral analysis of a forced cylinder wake.

The wake of a circular cylinder performing rotary oscillations is studied using hydrodynamic tunnel experiments at Re=100. Two-dimensional particle image velocimetry on the midplane perpendicular to the axis of a cylinder is used to characterize the spatial development of the flow and its stability properties. The lock-in phenomenon that determines the boundaries between regions of the forcing parameter space where the wake is globally unstable or convectively unstable [see Thiria and Wesfreid, J. Fluids Struct. 25, 654 (2009) for a review] is scrutinized using the experimental data. A method based on the analysis of power density spectra of the flow allows us to give a detailed description of the forced wake, shedding light on the energy distribution in the different frequency components and in particular on a cascade-like mechanism evidenced for a high amplitude of the forcing oscillation. In addition, a calculation of the drag from the velocity field is performed, allowing us to relate the resulting force on the body to the wake properties.

Reduced order models for wake control with a spinning cylinder

We study the formulation of reduced order models for a circular spinning cylinder in laminar vortex shedding regime. Proper orthogonal decomposition techniques based on the snapshots generated from velocity fields and a Galerkin projection are used to generate a low dimensional representation of the infinite dimensional system described by the Navier Stokes equation. This kind of problem has generally been studied numerically, without taking into consideration the constraints associated to physical experimental conditions or under the assumption that actuations do not alter significantly the time averaged velocity field. We propose a reduction technique that enables to obtain a control function based on the mean velocity field deviations from the unperturbed reference flow. This function is educed with a Gram Schmidt construction and gives rise to an extended base in which the velocity fields of the perturbed modes are expanded. The analysis allows for easy determination of a suitable model to describe a large range of the control parameter space, without needing to recalculate the coefficients that define the system for new operating conditions.

Centrifugal instability of Stokes layers in crossflow: the case of a forced cylinder wake

The wake flow around a circular cylinder at Re≈100 performing rotatory oscillations has been thoroughly discussed in the literature, mostly focusing on the modifications to the natural Bénard–von Kármán vortex street that result from the forced shedding modes locked to the rotatory oscillation frequency. The usual experimental and theoretical frameworks at these Reynolds numbers are quasi-two-dimensional, because the secondary instabilities bringing a three-dimensional structure to the cylinder wake flow occur only at higher Reynolds numbers. In this paper, we show that a three-dimensional structure can appear below the usual three-dimensionalization threshold, when forcing with frequencies lower than the natural vortex shedding frequency, at high amplitudes, as a result of a previously unreported mechanism: a pulsed centrifugal instability of the oscillating Stokes layer at the wall of the cylinder. The present numerical investigation lets us in this way propose a physical explanation for the turbulence-like features reported in the recent experimental study by the present authors.

Wake flow stabilization with DBD plasma actuators for low Re numbers

We propose to study the stability properties of a wake air flow forced by electrohydrodynamic (EHD) actuators. EHD actuators can add momentum to the flow around an object in regions close to the wall. Hence, we adopt the hypothesis of the forcing actuation modeled as a slip-wall condition. We study the flow around a cylinder modified by EHD actuators, dielectric barrier discharge (DBD) type, by means of particle image velocimetry (PIV) measures. For the sake of simplicity we observed flows at low Reynolds(Re) numbers (~ 200) where the flow is mainly two-dimensional. As the forcing frequencies, characteristic of DBD, are much higher than the natural shedding frequency of the flow (kHz vs. Hz), we consider in this work only the forcing actuation as stationary. The actuators are disposed symmetrically near the boundary layer separation point. Thus, flow symmetry is conserved, as it remains weakly non parallel. This aspect allows us to perform stability analysis of the flow and to determine its characteristics which are governed by a strong mean flow correction. We also present results on the global mode evolution under forcing. The main motivation of this work is to optimize the forcing on the flow, leading to a more effective energy consumption of EHD actuators.

Reduced order models for EHD controlled wake flow

We present in this work a way to construct reduced order model (ROM) for wake flows at low Reynolds numbers, from experimental data of particle image velocimetry (PIV). The proper orthogonal decomposition method (POD) allows the extraction of a small number of functions or modes that describes the flow velocity field. Galerkin projection of Navier Stokes equations onto these modes leads to obtain a system of ordinary equations, the reduced order model. We take interest particularly in the study of the modifications introduced by an electrohydrodynamic (EHD) actuator on the flow. EHD action is obtained from a gaseous discherge of dielectric barrier type (DBD), produced by electrodes flush mounted on a cylinder surface. The reduced order model presents as an effective tool in order to analyse the coupled electrical and hydrodynamic phenomena. We focus on the flow changes considering the voltage between the electrodes as a control parameter.

The scenario of two-dimensional instabilities of the cylinder wake under EHD forcing: A linear stability analysis

We propose to study the stability properties of an air flow wake forced by a dielectric barrier discharge (DBD) actuator, which is a type of electrohydrodynamic (EHD) actuator. These actuators add momentum to the flow around a cylinder in regions close to the wall and, in our case, are symmetrically disposed near the boundary layer separation point. Since the forcing frequencies, typical of DBD, are much higher than the natural shedding frequency of the flow, we will be considering the forcing actuation as stationary. In the first part, the flow around a circular cylinder modified by EHD actuators will be experimentally studied by means of particle image velocimetry (PIV). In the second part, the EHD actuators have been numerically implemented as a boundary condition on the cylinder surface. Using this boundary condition, the computationally obtained base flow is then compared with the experimental one in order to relate the control parameters from both methodologies. After validating the obtained agreement, we study the Hopf bifurcation that appears once the flow starts the vortex shedding through experimental and computational approaches. For the base flow derived from experimentally obtained snapshots, we monitor the evolution of the velocity amplitude oscillations. As to the computationally obtained base flow, its stability is analyzed by solving a global eigenvalue problem obtained from the linearized Navier–Stokes equations. Finally, the critical parameters obtained from both approaches are compared.