Introduction

A clearly defined field.....

This site deals with: "mechanical effects created by the action of an electrical force in the bulk of a fluid". This situation arises when electrons or ions are injected, in the presence of a high electric field, in a fluid mainly composed of neutral particles.

In order to obtain a net electric force, a monopolar conduction or at least a very dissymmetric conduction should be achieved. In such a situation, the electrical charge modifies to a great extend the electric field that will exist without the injected charges. Then in the general case exists a strong coupling between, electrical field, charged particles flows, and neutral flow.

Note that this area of physics is sometime referred as Electro-Hydro-Dynamics (E.H.D.) or sometimes Electro-Fluid-Dynamics (E.F.D.). However because a global charge is needed to create a resulting force that do not exist in the case of a perfect conductive fluid, the electric field behaviour is closer to electrostatic rules than it is to usual electrodynamics rules. The term electrified seems then more adapted than the root "electro", to describe a situation were a global electrical charge is present. Note that the other solution is to use the world "electrostatic" in all situations where a global charge is present, this is often done in practise even in situations that are not "static" at all (Electrostatic machines for instance), but the use of the radical "static" in dynamic situations even if it can be partially justified, is really embarrassing.  In the same spirit, as far as we will mainly consider mechanical effects, avoiding to a great extend the thermodynamic aspect, it is more proper to use mechanics instead of dynamics in such a situation. Then the correct domain's name we are going to study should rather be called Electrified-Fluid-Mechanics or E.F.M. Unfortunately the most common term is E.H.D., then, although the domain involved is more restricted than the full E.H.D. domain and  in order to keep a common  nomenclature, we will use and abuse of E.H.D. in the following.

More generally E.H.D. is, with its counterpart M.H.D. (Magneto-Hydro-Dynamics), a part of Plasmas Dynamics.

With very old roots.....

Several century's ago, among many other observations, two spectacular electrostatic experiments were involving such mechanical effects: the blowing of a candle and the rotation of the corona wheel (see Electrostatic experiments). Although clearly related to our main topic, these experiments were done at a time where even basic electricity rules were unknown, the discrete nature of electricity charges was ignored (electrons and ions were discovered nearly two centuries later) and only a very general term was used to describe such observations: effluvia.

 

The usual experiments are made up of two distinct aspects :

- The Corona glow which is a glow discharge at atmospheric pressure, characterised by a very localised blue light. The Corona discharge is in fact a very practical source of ions used in many practical applications such as copiers, dust cleaners....

- The Electric wind which is the resulting neutral flow induced by the collisions of the ions with the neutrals. The neutral flow usually propagate a lot farther than the original discharge area.

Some quasi-magic and sulphuric aspects.....

As we will see later on, although known since centuries, many aspects of high voltage discharges in air, involving at the same time light, sounds, smelling (ozone), mechanical effect, fire, are really complex and have kept up to now a great part of mystery. Then high voltages discharges are a natural pole of attraction of many curious but also of dreamers and believers of all kinds. Then if you are looking for a new "little green man" explanation or a new "anti-electro-gravito-free-energy" theory, you miss your step, here we deal with usual standard science and our work is based on giants such as Newton, Ampere, Biot, Maxwell, Navier, Stokes, Boltzman, Mach, and many others....

However there is a real sulphuric aspect with E.H.D. in the sense that this specific field has remained up to now and for complex reasons on the fringes of academic science. A great part of the work described here refers to quite old works and publications and is widely ignored by what we may call the "standard scientific community". Many researchers try to justify their poor knowledge using common objections against E.H.D., but these objections are usually everything but rational. The main reason why E.H.D. has not been developed is probably because it lies somewhere between Plasma physics and Hydrodynamics. The physicists specialized in fluid dynamics are generally ignorant on Electricity and plasma behaviour and reciprocally.

There is then a sort of "dead angle" between Electricity and Fluid-mechanics (For specialists between Maxwell Equations and Navier-Stokes Equations). Of course the general case is very complicated (Non-equilibrium plasma submitted to electro-magnetic forces) but  they are simplifications such as Extended Euler Model  or Extended Navier-Stokes Model (for specialists) which give some very general results in good agreement with experiments. The reader with a general scientific background should start by reading the Demystification page and then to get a little more of general knowledge and properties he could try to follow the Basic E.H.D. course.

But in fact, a complex, rich in applications, and nearly virgin territory....

The mixing of these two very different fields presents some unexpected difficulties. The first one is related to the fact that both electricity and hydrodynamics are mature sciences and then the existence of a deep dead angle cannot be even seriously evoked in modern times where nearly everybody is convinced that only refinements and numeric simulations in very specialized areas remain to be done. Then a researcher that will try to fill the gap between the two domains will have to dig several century in the past to derive extended versions of  electrostatics and Bernoulli energy conservation principle adapted to the case. Then such a scientist will be more considered like a science archaeologist then like the builder of a new promising field. The second more pernicious difficulty is strongly related to the first one and has something to do with irreversibility of science evolution. In a certain manner we may consider that science is a kind of dissipative jet of matter introduced in a resting fluid. At the origin the flow is very strong, coherent and stationary, but, if you follow the flow with time it became larger because of the action of viscosity forces stating into movements particles originally at rest. At the same time the intensity of the flow reduces. This is the same for science evolution, the science widens but the local intensity decrease. Back to our dissipative flow, as the flow widens it became instable and complexity arise rapidly, it become soon impossible to describe the flow in all its completeness, some information is lost and only partial description is allowed. Science evolve in the same manner, in a chaotic way, the more specific the study, the more evanescent the related knowledge. But there is a last aspect that may have a tremendous importance, as the flow evolved it looses its predictability for the future and its memory of the past. The same concern arise in scientific education, in order to obtain performing engineers and researchers, a great importance is set to the use of recent tools such as simulations and recent optimized models and less and less time is devoted to the basic laws and tools used in the past to established the current science. Then present researchers are very specialised persons able to use present models and tools but very few of them are able to link the tools to the origins and to understand the underlying principles that were used to build them. Then education in order to preserve the goal of showing performing results was forced to drop very important steps in the progression of the young student. This one is nowadays more able to use a set of given equations, to use order of magnitude tools such as averages and proportionality than he is able to understand potential theory, entropy concerns and the related differential geometry and operators. Present researchers are then in their great majority totally unable to derive new models strongly linked to solid roots.

The present situation is puzzling. Although the field is rich in potential applications, the realisation of industrial products or even of prototypes seems to encounter special difficulties. The most obvious example is what may be called the "ionic pump failure". In the years 1950 two researchers (see Robinson and Stuetzer in references), where working on the possibility to build pumps using the ion drag effect on neutrals. Although they were able to measure a pressure rise on the axis on the 100 Pa range, they where not able to obtain any practical realisation able to deliver more than a few Pa pressure. The problem, that is still not overcome yet, was probably due to an oversimplification of the flow problem (see ion drag pumps for more details). The researchers have assumed that the flow will be homogenous in the whole discharge section whereas we have discovered later on (see Ballerau work) that the neutral flow has in fact a jet structure which is not at all homogenous and presents strong dissipative effects. The most puzzling aspect is that at the same time, in the fluid mechanics scientific community the general behaviour for an accelerated neutral flow to form a narrow jet was known since centuries. This illustrates again the absence of communication between the two domains: Electricity and fluid dynamics.

The other common mistake is to consider that the field will be easy to manage with the powerful existing tools and will lead to simple models. Although there is no mystery in the way the electric field, the ions, and the neutrals behave, each of the three different problems presents particular difficulties. For instance the electric field in the presence of a non neutral charge, is strongly influenced by the charges, themselves strongly influenced by the electric field. This situation, leads to a non linear behaviour and is responsible of the shielding effect that is observed in plasmas and conductors. But also this situation (when neutral coupling is taken into account) leads to a very special form of instability called the E.H.D. instability. The charges flows are also very complex because of convection, diffusion, mobility processes through neutrals and chemical reactions such as ionisation, attachment, charge transfer, recombination.... But another very complex aspect is the charge interaction with non conductive solid walls where the charges have a strong tendency to deposit and accumulate, leading to local noisy breakdowns.  The neutral flow on its side presents also a non linear behaviour which leads to very complex aspects such as boundary layer flows, instabilities, turbulences.

In E.H.D. these three complex problems are melted. We can guess, if we remind the century of experimental work that was done in fluid mechanics before a correct modelling of usual flows was obtained,  that this will lead to centuries of experimental and theoretical work before an overall understanding of the E.H.D. maximum performances and limitations comes to be complete. Moreover we can doubt on the ability of present structures to do the job, on one side because of the lack of motivations, strongly related to market rules more oriented to short time profit than to long time prospective, and in an other side, because of the lack of human solid competence in simultaneously all the fields involved. Of course, with a lot of money, solid teams can be assembled but this will be done only if some attractive results are obtained at least in one of the potential applications of E.H.D. field. So E.H.D.'s future is more relying now on private investigations than in long term state projects.

Accessible to nearly everybody !!!

A fascinating consequence is that anybody with a basic general knowledge in electronics can do innovative experiments in the field (see for instance the lifters work of J.L.Naudin and the thrusters work of Saviour) and can built at home with a small budget a lab totally in the "state of the art" (see How to build my own lab) . There is so many improvements in sustentation, propulsion, pumps, acoustic generation, advanced flow control that are in the catch of the hand that we can be sure that there will be several breakthrough in the field shortly.