Plasma control of flow in an airfoil

Flow separation

Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer relative to the object falls almost to zero . The fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices

It leads to drastic increase in pressure drag

Present measures taken to reduce flow separation

VORTEX GENERATOR- A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface

Basic principle: They generate an array of streamwise vortices that promote increased mixing of the high speed air in the main stream and outer boundary layer, thereby the boundary layer is reenergized.

Disadvantages of VGs

They cannot be used for active stall control

They also generate considerable amount of parasitic drag during cruise.

Introduction to plasma

Plasma is defined as the fourth state of matter.

Plasma is created when sufficient energy is added to a gas so that it becomes ionized. The result is a quasi-neutral particle system

containing free electrons, ions and, depending on the degree of ionization , neutral particles.

Types of Plasma

Hot plasma -A plasma is referred to as being "hot" if it is nearly fully ionized.

Cold plasma-A plasma is called as "cold" if only a small fraction (for example 1%) of the gas molecules are ionized

Plasma control of flow over an airfoil

Plasma actuator is the new alternative without the drag penalty to control the separation it is also called as plasma vortex generator

Plasma actuator works due to the momentum added by the discharge-induced electric wind, or to the laminar to turbulent transition induced by the disturbances generated by the plasma actuator.

Types of Plasma actuator

There are two main forms of discharge used for the separation controlalternating current (AC)-DBD and nanosecond pulse (NS)-DBD discharge

When AC high voltage is applied to the electrodes ,it ionizes the air and creates an ionic wind of few m/s ( 1.5 m/s 8 m/s)

DBD is driven by repetitive nanosecond pulses ,it can offer highly efficient non-thermal plasma at atmospheric pressure and is widely used for plasma applications

Factors affecting ionic velocity

Electrode gap (5 < d < 15 mm)

Grounded electrode width (1 < L < 50 mm)

Frequency (100 Hz < f < 2 kHz)

Voltage amplitude (8 < V < 30 kV)

Material permittivity (3 < er < 10)

Dielectric layer thickness (1 < a < 3 mm)

References

Optimization of a dielectric barrier discharge actuator by stationary and non-stationary measurements of the induced flow velocity: application to airflow control - M. Cazalens et al. Springer

Lift and drag performances of an axisymmetric airfoil controlled by plasma actuator - N. Benard et al. Elsevier

Flow separation control on swept wing with nanosecond pulse driven DBD plasma actuators - Wu Yun et al. Chinese Journal of Astronautics.

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