The electric wind

Reconstitution of the first experiment (Courtesy of Bernard Thomas)

The phenomenon of the electric wind was first observed in the Electrostatic Experiments in the years 1600 long before the nature of electricity was understood and among a lot of new and impressive phenomena (Attraction, Repulsion, Sparks, Luminescence...). Four century later, the Electric Wind remains a sort of laboratory curiosity, which has lost a great part of its mystery but for complex reasons (see intro and common objections) is still unexplored.

The Electric Wind is in fact a movement of the neutral fluid induced in the bulk of the fluid by electrical forces. To obtain such an effect it is necessary to obtain at list one area were there is a dissymmetry between positive and negative forces i.e. a region outside from electric neutrality. This is not the case in hot plasmas or even in the cold plasma of a glow discharge

Figure1 (neutralisation in an usual glow)

In the case of homogenous low pressure glows or high pressure arcs, both charges are created in every place in such a way that the electric charge is globally null in every point of the discharge. In such a case the electric force is also null and no neutral wind is observed.

The charges of the same polarity than the point are expulsed from the plasma.....

In the case of the corona discharge, the ionisation region where both charges are created is limited to a small region near the strained electrode where the field is high enough to create ionisation by an avalanche process. As a consequence the charges of the opposite polarity than the point one are attracted and fall toward the point surface where they are collected. The charge of the same polarity than the point are on their side expelled from the luminescent region toward an external region where only one charge polarity is present. This region is called the monopolar region.

.....and create a large monopolar area

Moreover, the highest value of he electrical force is obtained very close to the strained electrode where both the charge density and the electric field take their maximum values. The localised action of a force in a fluid creates what is called in fluid mechanics a jet.

The fluid movement is very different from the electric field lines. The fluid lines have a tendency to concentrate before to reach the active area where the electric force is maximum and to continue in nearly strait lines. Then in the same manner than in the case of a fan, you can feel a strong blowing in the front side and nearly nothing at the same distance in the back side. This strong dissymmetry is not easy to understand and is related to the fact that the fluid problem is strongly non linear.

The narrow jet type profile

Note that the axially symmetrical profile is obtained in the case of a punctual source of force (a corona point) whereas the two-dimensional profile is obtained in a case of a force exerted along a line (wire or blade coronas).

Full jet profile

One very important consequence is that the electric field covers an area a lot larger then the one covered by the flow. In case off the presence a near lateral wall, the electrical charges will interfere with it whereas the neural flow will stay away from it. This is a real problem if we want to make the flow interfere with walls (see drag reduction page).

In practise the neutral speeds in the flow reaches values up to 8 m/s, these speeds are equivalent to a pressure rise of 100 Pa.

The electric wind may have a wide range of   applications such as loudspeakers pumps or propulsion but also some specific applications such as boundary layer blowing (see drag reduction page), active control of stream (flight control, active absorption) and even shock wave reduction (see applications), but many of the applications need strong and homogenous interaction of he flow with walls and we really don't know yet to what extent this will be possible.