Flow control in dielectric liquid with ElectroHydroDynamic actuator

The control of gas flow with plasma actuator is well known for 15 years with different applications in the fields of aerodynamic including separated flows, boundary layer flows, turbulent mixing, noise reduction, etc. In the framework of the LABEX INTERCATIFS project, we develop an experimental demonstrator of flow control in dielectric liquid. In order to produce an effective manipulation of a liquid flow, we use ElectroHydroDynamic (EHD) actuator such as the Dielectric Barrier Injection (DBI) one. The main advantages of these EHD actuators are: a direct conversion of electric energy into inertial energy (without movable part), a low cost, and an easy implementation on model with small scale. This type of actuator generates a flow motion in dielectric liquid. The charge injection phenomenon is used to produce electro-convective flows until 1m/s. These electro-convective jet flows can be used in many applications: mixing, flow control, cooling systems etc. For instance, EHD phenomena is now commonly used in pumps dedicated to microfluidic applications.

For LABEX INTERACTIFS, we have built a silicon oil tunnel of around 80 Liters where the dielectric liquid is driven by a gear pump [0; 40cm/s]. The tunnel is composed of a test section with a square section of 10x10cm and length of 70cm. A NACA0015 hydrofoil profile is placed in the test section and is equipped with a DBI actuator. This actuator is composed of a high voltage electrode (HV) placed on the leading edge of the NACA and a ground electrode embedded inside the NACA profile under the upper surface. Then we connect the HV electrode to a DC power supply [0;-30kV] that induces a charge injection inside the liquid. The electric field induced by the electrodes, generates ions at the tip of the HV electrode which follow the upper side of the NACA profile and it creates an EHD force. This DBI actuator produces a wall jet whose velocity reaches a few centimeters per second along the upper side of the airfoil and directed from the leading edge to the trailing edge.

The experiments are conducted with an inlet flow of a few centimeters per second [5cm/s;40cm/s] with different angle of attack from 0° to 16°. The Reynolds number is low with an order of magnitude of 1000. Measurements using Particle Image velocimetry (PIV) technique have been performed in order to obtain the velocity fields around the NACA profile, this for a large number of angle of attack and inlet flow velocities. This study shows for the first time that the actuator can influence a naturally separated flow due to adverse pressure gradient. More precisely for a whole detached flow (angle of attack of 11°), we succeed in reducing the area of the recirculation bubble by 47% on the suction side of the hydrofoil by using a DC voltage UHV=-20kV (see Fig. a and b). By simply increasing the amplitude of the voltage UHV, a fully reattached flow can be observed on the upper side of the NACA0015 (see figure c). From the results of this experiment, it is concluded that EHD can be useful to manipulate a separated liquid flow. The full flow reattachment at large incidence suggests that the lift and drag coefficients can be adjusted accordingly to the applied voltage amplitude.

To our knowledge, this experiment of flow control in dielectric liquid is the first one showing the influence of an electrical discharge on a separated flow and our recent results open a new field regarding the manipulation of internal liquid flow with potential applications in cooling system or mass transfer with low energy consumption control system.