Steady control of laminar separation over airfoils with plasma sheet actuators Sosa Roberto Artana Guillermo Laboratorio de Fluidodinámica, Universidad de Buenos Aires, CONICET, Buenos Aires, Argentina

Introduction In last years there has been a growing interest in the use of ionization to add localized momentum to the flow by the collisions process of migrating charged particles with the neutral species of the air These EHD technologies have been considered as good candidates to enhance the aerodynamic perfomance of airfoils These EHD technologies have been considered as good candidates to enhance the aerodynamic perfomance of airfoils

Objectives In this work we study the improvement of the aerodynamic performance of an airfoil at very low Re (Re<50000) by means of an EHD actuator Our study focuses on the relative distance of the actuator location and the position of separation line.

Flow configurations at low Re aerodynamics Position and size of the laminar bubble separation (LBS) changes with airflow velocity at a fixed angle and with the angle at a fixed airflow Short LBS Large LBS

Layout 1. Interest. 2. EHD actuators. 3. Electromechanical coupling. 4. Experimental setup. 5. Results & discussion.  Laminar boundary layer :  Partially attached  With separation Laminar boundary layer partially separated Laminar boundary layer partially separated Laminar boundary layer fully separated Laminar boundary layer fully separated  Flow with a laminar separation bubble (LSB) Fully reattached turbulent boundary layer Fully reattached turbulent boundary layer With downstream separation of the reattached turbulent boundary layer With downstream separation of the reattached turbulent boundary layer 6. Conclusions.

Some Area of Interest Power Control of Wind Turbines Wind turbines are usually designed to produce electrical energy with a maximum output at wind speeds around 15 metres per second. In case of stronger winds it is necessary to waste part of the excess energy of the wind in order to avoid damaging the wind turbine. Around two thirds of the wind turbines currently being installed in the world are stall controlled machines. The geometry of the fixed angle blades profiles is designed to ensure that the moment the wind speed becomes too high, it stalls. This stall prevents the lifting force of the rotor blade from acting on the rotor In practical application, stall control is not very accurate and many stall-controlled turbines do not meet their specifications. Deviations of the design-power in the order of tens of percent are regular

Corona discharge. Dielectric barrier discharge. Sliding discharge. Kind of ehd actuators

Previous research on ehd excited airfoils Actuator offActuator on Near Post stall Regime Time averaged flow fields with associated streamlines. Angle of attack 15.8º, Re= , U0=10 m/s

Previous reserach on ehd excited airfoils Near Post stall Regime Time averaged flow fields with associated streamlines. Angle of attack 19.8º, Re= , U 0 =25 m/s Actuator off Actuator on

Electric forces: Through collisional process the force on the fundamental carriers becomes the force on the medium n, m, and V represent the density number, the mass, the frequency of collisions and the relative velocity of the charged carriers to the medium macroscopic velocity (identifying positive ones with subindex + and negative ones with subindex -) n, m, and V represent the density number, the mass, the frequency of collisions and the relative velocity of the charged carriers to the medium macroscopic velocity (identifying positive ones with subindex + and negative ones with subindex -) Neglecting both the magnetic effects and the interactions between charged particles, and considering that the macroscopic force is an average of the forces acting only on the heavy charge carriers (ions) of charge q, the force transmitted to the medium may be reduced to the coulombian force density expression: Neglecting both the magnetic effects and the interactions between charged particles, and considering that the macroscopic force is an average of the forces acting only on the heavy charge carriers (ions) of charge q, the force transmitted to the medium may be reduced to the coulombian force density expression: Alteration of physical properties of the gas (density, viscosity,..). Electromechanical coupling

Experimental set up Airfoil model: NACA 0015 PMMA Actuation 0.55<x/c<0.78 Power of Actuation <20W Power/unit surface actuation<1000W/m 2

Experimental set up: Wind tunnel Flow Open wind tunnel of low Turbulence level Test section: 450*450mm Air stream: 0-7 m/s Surface pressure measurements (micromanometer) Flow visualization (smoke injection and laser sheet)

Layout 1. Low Reynolds Aerodynamic control. 2. EHD actuators. 3. Electromechanical coupling. 4. Experimental setup. 5. Results & discussion.  Laminar boundary layer :  Partially attached  With separation Laminar boundary layer partially separated Laminar boundary layer partially separated Laminar boundary layer fully separated Laminar boundary layer fully separated  Flow with a laminar separation bubble (LSB) Fully reattached turbulent boundary layer Fully reattached turbulent boundary layer With downstream separation of the reattached turbulent boundary layer With downstream separation of the reattached turbulent boundary layer 6. Conclusions.

Partially attached Laminar boundary layer The plateu in the pressure is associated with flow separation Separation occurs in the interelectrode space (x/c  0.65 ) Wake mass deficit compensation Actuation Separation elimination

Laminar boundary layer partially separated Flow Flow separation upstream the actuator location (x/c  0.35 ) Actuation Separation elimination Actuation Inviscid behaviour

Laminar boundary layer fully separated Flow separation upstream the actuator location (x/c  0.1 ) Actuation Flow acceleration Intermitent reattachment

Flow with a laminar separation bubble (LSB) Fully reattached turbulent boundary layer Laminar separation bubble extended approximately from x/c = 0.1 to x/c = 0.5 Actuation Slight Flow acceleration of the attached flow Actuation on an turbulent attached flow (Large LBS)

Flow with laminar separation bubble (LSB) With downstream separation of the reattached turbulent boundary layer Actuation Very Slight Flow acceleration of the separated flow Laminar separation bubble approximately from x/c = 0 to x/c = 0.05 (analogue to a turbulator) Separation of turbulent boundary layer at x/c  0.15 Actuation on turbulent separated flow (Short LBS)

Difference on coefficients of aerodynamic performance as a consequence of EHD actuation.  (º) Re* Flow On actuator CwActuation EffectC l (off) ClCl C l /C Dp (off)  (C l /C Dp ) Separated LBL (x/c≈0.65) 8.7Reattacment Separated LBL (x/c≈0.35) 101.2Reattachment Separated LBL (x/c≈0.10) 101.2Intermitent reattachment TBL (upstream Large LBS ) (x/c≈1) 8.7Sligth acceleration of the attached flow Separated TBL (upstream Short LBS ) (x/c≈0.15) 8.7Sligth acceleration of the separated flow Actuator location 0.55<x/c<0.78 Non dimensional power coeffcient Angle of attack Lift coefficient Lift and drag presssure ratio

Conclusions The relative distance between actuator and separation line reveals a crucial parameter for low aerodynamics flow control. Reattachment of the flow requires lesser power when separation occurs at close vicinity of the actuator, whatever the free airstream velocity. When separation occurs far upstream the actuation may be even not capable to reattach the flow, whatever the free airstream velocity. The actuator activation on flows that did not experience separation close to the actuator does not produce a significant improvement on the aerodynamics airfoil performance.

Merci beaucoup !!!