Steady control of laminar separation over airfoils with plasma sheet actuators

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

2. 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

3. 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.

4. Large LBS Short LBS 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

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

6. 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

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

8. Previous research on ehd excited airfoils Near Post stall Regime Time averaged flow fields with associated streamlines. Angle of attack 15.8º, Re= 133333, U0=10 m/s Actuator off Actuator on

9. Previous reserach on ehd excited airfoils Actuator off Actuator on

10. Electromechanical coupling • 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 -) • 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,..).

11. Experimental set up Airfoil model: NACA 0015 PMMA Actuation 0.55<x/c<0.78 Power of Actuation <20W Power/unit surface actuation<1000W/m2

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

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

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

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

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

17. Slight Flow acceleration of the attached flow Actuation 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 on an turbulent attached flow (Large LBS)

18. Very Slight Flow acceleration of the separated flow Actuation Flow with laminar separation bubble (LSB)With downstream separation of the reattached turbulent boundary layer 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)

19. Difference on coefficients of aerodynamic performance as a consequence of EHD actuation Lift and drag presssure ratio Actuator location 0.55<x/c<0.78 Non dimensional power coeffcient Angle of attack Lift coefficient .

20. 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.

21. Merci beaucoup !!!