Guillermo Artana

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.

Influence of a DC corona discharge on the airflow along an inclined flat plate

Abstract Visualizations and measurements by Particle Imaging Velocimetry are conducted in a wind tunnel in order to determine the influence of a DC corona discharge established between a wire and a plate collecting electrode on the properties of an airflow around a flat plate. Results show that the kinetic energy induced by the ionic wind inside the boundary layer allows a drag reduction for low Reynolds numbers (ReL

Electric wind produced by surface plasma actuators: a new dielectric barrier discharge based on a three-electrode geometry

Active flow control is a rapidly developing topic because the associated industrial applications are of immense importance, particularly for aeronautics. Among all the flow control methods, such as the use of mechanical flaps or wall jets, plasma-based devices are very promising devices. The main advantages of such systems are their robustness, their simplicity, their low-power consumption and that they allow a real-time control at high frequency. This paper deals with an experimental study about the electric wind produced by a surface discharge based on a three-electrode geometry. This new device is composed of a typical two-electrode surface barrier discharge excited by an AC high voltage, plus a third electrode at which a DC high voltage is applied in order to extend the discharge region and to accelerate the ion drift velocity. In the first part the electrical current of these different surface discharges is presented and discussed. This shows that the current behaviour depends on the DC component polarity. The second part is dedicated to analysing the electric wind characteristics through Schlieren visualizations and to measuring its time-averaged velocity with a Pitot tube sensor. The results show that an excitation of the electrodes with an AC voltage plus a positive DC component can significantly modify the topology of the electric wind produced by a single DBD. In practice, this DC component allows us to increase the value of the maximum induced velocity (up to +150% at a few centimetres downstream of the discharge) and the plasma extension, to enhance the depression occurring above the discharge region and to increase the discharge-induced mass flow rate (up to +100%), without increasing the electrical power consumption.

Effect of a dc discharge on the supersonic rarefied air flow over a flat plate

Recent studies have shown atmospheric plasma discharges to be an effective means of air flow control. If in subsonic conditions the plasmas effect is explained by a transfer of momentum from the charged particles to the neutral ones, in supersonic conditions it seems that the effects are mainly of thermal origin but some authors think that this effect is not the only one to act. This paper presents experimental results of stagnation pressure, spectroscopic emission and drag performed in a rarefied Mach 2 flow over a flat plate model with a half-wedge leading edge. Changes caused by a negative dc discharge located on the upper surface of the flat plate are investigated in two cases. In the first case the negative potential is applied on the upstream electrode and in the second case it is applied downstream. The second electrode is grounded. The measurements carried out indicate two opposite effects depending on the localization of the negative potential.

Stall control at high angle of attack with periodically excited EHD actuators

We analyze the modifications of the flow around a NACA 0015 airfoil when the flow is periodically perturbed with an electrohydrodynamic (EHD) actuator. The device used consists of two bare electrodes flush mounted on the surface of the profile operated in a discharge regime characterized by the formation of a plasma sheet contouring the body. In this study, we analyze the influence of the frequency of a periodic actuation on the aerodynamic performance of the airfoil. The analysis is undertaken with measurements of the surface pressure distribution and of the flow fields with Particle Image Velocimetry technique. The experiments indicate that at moderate Reynolds numbers (150000 15°).

THEORETICAL ANALYSIS OF LINEAR STABILITY OF ELECTRIFIED JETS FLOWING AT HIGH VELOCITY INSIDE A COAXIAL ELECTRODE

Abstract In this article we develop a temporal linear stability analysis of a circular electrified jet flowing inside a cylindrical coaxial electrode. The problem of a high-velocity jet going in a gaseous atmosphere is examined and we analyze the results in order to bring out the influence of the electrification, the surface tension, the velocity and other parameters on the stability of the jet. From this theory we finally state the changes in the breakup phenomena that are expected to be observed when electrifying these jets.

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.

Contribution to the analysis of the flow electrification process of powders in pneumatic conveyers

Abstract In this paper we examine the process of charge development in powders for two different configurations: one is a fluidized bed between two porous electrodes, the other is a flow in a pneumatic conveyer. It is proposed a theoretical approach for both phenomena that is in good agreement with experimental results.

Electrical and plasma characteristics of a quasi-steady sliding discharge

A quasi-steady sliding discharge at atmospheric pressure is generated by combining a surface dielectric barrier together with a DC corona discharge in a three-electrode geometry. The discharge extends along the whole side-length of the electrodes (150 mm) and covers the full inter-electrode gap (30 mm). It is found that this discharge is composed of repetitive streamers that are uniformly distributed along the whole electrode length and that propagate along the inter-electrode gap with an average velocity of ~2 × 107 cm s−1, and with an average electric field of ~120 kV cm−1 and a total particle number of ~5 × 108 at the streamer head. Assuming that the electron distribution function reaches an equilibrium value with the electric field, an electron temperature of 9 eV at the streamer head is obtained. The streamer frequency is around 5 × 104 Hz for a well-developed sliding discharge regime, and the time-averaged electron density amounts to 1.5 × 107 cm−3.

A fluid motion estimator for schlieren image velocimetry

In this paper, we address the problem of estimating the motion of fluid flows that are visualized through a Schlieren system. Such a system is well known in fluid mechanics as it enables the visualization of unseeded flows. As the resulting images exhibit very low photometric contrasts, classical motion estimation methods based on the brightness consistency assumption (correlation-based approaches, optical flow methods) are completely inefficient. This work aims at proposing a sound energy based estimator dedicated to these particular images. The energy function to be minimized is composed of (a) a novel data term describing the fact that the observed luminance is linked to the gradient of the fluid density and (b) a specific div curl regularization term. The relevance of our estimator is demonstrated on real-world sequences.