...MHD is a very large field of investigation.
The inventor of the basic concepts is the famous British scientist Michael Faraday.
MHD has two aspects.
-
On one hand, one can accelerate fluids, through Lorentz force J x
B which refers to MHD accelerators. -
On the other one, one can convert the kinetic energy of a fluid, its enthalpy,
into electric energy. This refers to MHD generators.
In such plants, when a gas flow crosses the magnetic lines of a transverse magnetic
field B at velocity V, an induced electric field V x
B acts on charged particles, free electrons or ions.
...A very good book exists (now, only in scientific
libraries) :
Sutton & Sherman : "Engineering magnetohydrodynamics",
Mac Graw Hill Books Cie, 1967
for physicists and post graduate students.
...I have a personal experience in
MHD, since 1964. Between 1964 and 1972 I worked at the Institut de Mécanique
des Fluides de Marseille. Following, a shortened (devoted to more recent works)
bibliography :
(1) J.P.Petit : "Is supersonic flight possible ?" Eighth Inter.
Conf. on MHD Electr. Power Generation. Moscow 1983.
(2) J.P.Petit & B.Lebrun : "Shock wave cancellation in a gas by Lorentz force
action". Ninth Inter. Conf. On MHD Electr. Power Generation. Tsukuba, Japan,
1986
(3) B.Lebrun & J.P.Petit : "Shock wave annihilation by MHD action in supersonic
flows. Quasi-one dimensional steady analysis and thermal blockage". European
Journal of Mechanics; B/Fluids, 8 , n°2, pp.163-178, 1989
(4) B.Lebrun & J.P.Petit : "Shock wave annihilation by MHD action in supersonic
flows. Two-dimensional steady non-isentropic analysis. Anti-shock criterion,
and shock tube simulations for isentropic flows". European Journal of Mechanics,
B/Fluids, 8 , pp.307-326, 1989
(5) B.Lebrun : "Theoretical approach to the suppression of shock waves forming
around a slender obstacle placed in an ionized argon flow. Thesis in Energetics
n° 233. University of Poitiers, France, 1990.
(6) B.Lebrun & J.P.Petit : "Theoretical analysis of shock wave annihilation by
Lorentz force field". International MHD symposium, Peking 1990.
...Then I shifted to astrophysics
and theoretical cosmology. During the sixties we built short-duration MHD generators,
based on shock tube driven hot wind tunnels. This device could produce short
duration flows, high temperature and high velocity, with quite high pressures.
This is not a low pressure wind tunnel. The typical parameters are :
Gas : argon
Velocity : 2,700 m/s
Temperature : 10,000°K
Pressure : one bar
Electrical conductivity : 4000 mkhos/m
...We are presently building a new
lab, with private fundings. I think it will be operational within the end of
2001. The activity will cover different fields :
-
Hot gas experiments
-
Cold gas experiments (supersonic flows of air at atmospheric
pressure) -
Low pressure gas experiments (simulations).
-
Numerical experiments.
-
Hydraulic experiments (high velocity submarine models).
...At first, why do we restart such
activity after a 13 year stop ? Because we have new ideas. In 1975 I imagined
that supersonic flight could be possible, in dense air, without creating sonic
boom, nor turbulence. I published papers in scientific journals about that.
At this time it seemed to be a foolish idea. In 1975, with my colleague Maurice
Viton we built a hydraulic experiment with a one tesla magnet. Such B-field
was necessary to modify a water flow around a tiny model, in a free surface
flow (8cm/s). The model was a cylinder (7mm diameter). The experience was a
success and the front wave (specialists in fluid mechanics know that the waves
created by a ship are very similar to shock waves) was completely cancelled.
Then I thought that the idea could not be so fancy as it looked at first glance.
...During the next ten years everything
was very difficult, not from a scientific point of view but, let us say, from
a "political point of view". Obviously, this new concept was connected
to the UFO case. In gas experiments, such "MHD aerodyne" would be
surrounded by a luminous plasma, reddish at low regime, almost white at larger
ones. Electrodes, when machines would have some, would look like "windows".
In addition, disk-shaped machine (as named in a first in a "Compte rendu
de l'Académie des Sciences de Paris", in 1975) was optimum, on scientific
grounds (and MHD grounds, which can be somewhat different from classical fluid
mechanics grounds), so that the scientific community was not so enthusiastic
about such project, even if the scientific grounds were perfectly clear and
clean.
...I tried to install some research,
first in the French CNES (Centre National d'Etudes Spatiales) in 1979-1982,
then in a laboratory of the CNRS (Centre National de la Recherche Scientifique)
in 1984-1986. This second laboratory was located in Rouen, and its director
was Professor Valentin, today retired. Through the CNES, some experiments
were installed in Toulouse, in the French CERT (Centre d'Etude et de Recherche
Technique), a lab in close connexion with the army. Some money was given but,
in these two trials, the army stupidly obliged the labs to kick me out of the
scientific job. Unfortunately, the scientific background of the involved persons,
in both cases, was not adequate and the research failed in the two. A lot of
money was stupidly wasted. So that in 1987 I decided to give up definitively.
But, very recently, new ideas incited me to make a come back in the field.
By experience, I knew that if any collaboration with French institutions was
initiated, the military would immediately rot the situation, as they did many
times before. So that we decided to restart with our own forces and resources.
It can seem to be foolish. But, as far as I see all this research can be operated
with obsolete materials. In addition, the price of electronics and computers
has been considerably lowered since 20 years. Many very good researchers, now
retired, joined us. So that we have decided to set up a lab, in the south of
France. Presently we collect obsolete systems : capacitor banks, electric
supplies with various voltages, low and high, ignitrons, klystrons, optical devices
and so on, and we store it. When we will have all the necessary, we will go
into action, soon, we hope.
...Now, let us have a look at some
of the new ideas.
...If you click there,
you will read the Note aux Comptes rendus de l'Académie des sciences,
published in 1975. If you don't read French, some brief explanations. Following,
the three first illustrations. On figure 1, a disk-shaped machine, with an equatorial
solenoid, whose alternating electric current creates an alternating magnetic
field. This one (thanks to Mr Maxwell) creates an induced electric field E'
which tends to create circular induced electric currents. Combining these currents
J' with the instantaneous value of the magnetic field we get radial forces (Hall
effect is considered as negligible) we get a system of time-dependent radial
forces J ' x
B, which are directed towards the outside and towards the center, alternately.
So that the idea was the following : Suppose we can create a non-steady ionization
at the vicinity of the disk, controlled in time, we could act on the fluid,
using radial centrifugal forces when they are present, at the top of the disk
and radial centripetal forces, when this force is reversed, at the bottom of
the disk-shaped machine.
...On the next figure, the expected
induced gas flow; around our disk-shaped machine :
...The computations said that the
suction effect could be very strong, strong enough to cancel any shock wave
formation at the stagnation point of the machine (it moves along its axis).
The technical problem was to modulate the ionization at the vicinity of the
wall, in time. We imagined first a device looking like a "wolf trap"
:
...Imagine a "small size"
conical hole, in the wall and, along its axis, a "pin". At the junction
cone-plane, a circular electrode (anode). The central electrode (pin like) is
charged negatively. Then an electric discharge forms in the surrounding air,
as shown above. The subsequent magnetic field, associated to this electric discharge,
tends to push the free electrons away and to give them energy. We thought that
it could produce short lifetime negative ions in air, making possible MHD interaction
during the lifetime of such ions. In a well equipped lab such research could have
been carried out, but we had none. Since 1973 I was in an astronomical observatory,
which is not an optimum place to perform plasma physics experiments.
...Anyway, at this end of the seventies,
some interesting aspect of the machine was discovered. Good plasma specialists
know that magnetic pressure tends to blow away electric discharges. We experienced
that in low pressure air experiments. The solution was rapidly found. Instead
of creating a B field whose maximum value was in the symmetry plane (created by
a single equatorial coil), we decided to use three, a big one and two smaller,
as shown on the next figure :
...Left : the axis of the machine.
Up and right, the schematic disposition of the three coils, showing the way
the electric currents flow in. On the figure, the magnetic lines. We see that
the maximum B surface is close to a portion of a cone (containing the circles
of two coils). Gray area : the confinement volume, in which the plasma tends
to lie. Immediate success, experimentally. Then the wall of the machine, to
optimize the MHD interaction, must be orthogonal to the magnetic lines. Finally,
this is the typical aspect of our electrodeless MHD aerodyne, using induction
phenomena and pulsed ionization, optimized on MHD grounds :
...You see why we got some problems
with the scientific community, army, politicians, and so on.
...Short-duration ionization was
difficult to manage.
Cold gas MHD experiments.
...But, recently, a new idea came,
which is summarized on the following figure :
...The walls of the model are teflon
made. In the two teflon shells, coils are imbedded (the equatorial one and the
two "confinement coils"). Two klystrons will be used, connected to
two separate wave guides. These two are shown on the figure. They are coaxial
brass cylinders. When one is in operation, the second one is out, and so on.
Mingled thin brass lamellas (red) act as a diffuser and spray microwaves. The
brass plate located in the plane of symmetry prevents the microwaves used to
ionize the air on the opposite side of the machine. On the next figure we show the
model when the microwaves flow along the central channel, are diffused by the
thin mingled brass lamellas, at the top part, cross the teflon then and create
a thin layer of ionized air. 3 GHz microwaves are optimum to ionize atmospheric
pressure air, and ionized gas absorbs microwaves. That's why the ionization
will be confined in a thin layer.
..It seems
relatively simple, with such device, to create time-dependent ionization state
at the vicinity of a disk-shaped model. It is simple, too, to synchronize the
alternating electric currents in the three coils. So that such model could suck
the air just in front very strongly. Depends on the intensity of the magnetic
field, of course (the MHD interaction parameter must be strong enough). It is
not necessary to cancel the shock waves during a long time. Only short duration
tests can be operated, in a short duration wind tunnel. We will have to build
it. Schematically, this wind tunnel is based on a big vacuum tank, whose content
is removed, at each test, by a powerful vacuum pump (we already have it). Left
: the supersonic channel. Between the channel and the tank : a mylar membrane.
When the pumps makes the pressure low enough in the tank, the membrane breaks.
Typical duration of the flow : several tenth of second.
...This simple idea was exciting
enough to decide to restart MHD research. Next image : another view of the disk-shaped
model :
...Next : expected gas flow, around
a "passive" model. Shock waves system.
Next : the flow, after shock wave cancellation by Lorentz forces
action :
...We have planned to make experiments
on "MHD hydrodynes", i.e. large velocity submarines.
Hot gas MHD experiments.
...In the lab a shock driven wind
tunnel will be built (so-called shock tube). On the next figure, a schematic
view of the MHD plant.
..Next figure : the MHD part of the
plant.
...The next image shows the two coils
and the MHD channel :
...In such plants, in 1967, plasma
accelerations had been successfully operated at the Institut de Mécanique
des Fluides de Marseille, evidencing the efficiency of the Lorentz forces to
control a supersonic flow. Inlet velocity (pure argon, one bar, 10,000°K)
: 2750 m/s. Outlet velocity : 8000 m/s !!
Next, the position of the flat wing model in the supersonic wind
tunnel, in the experiment planned in 1987 :
...The PhD thesis of Bertrand Lebrun
(1987), publications in the European Jr of Mechanics, plus two presentations
in international MHD meetings (Tsukuba, 1987, Pekin, 1990) focussed on front
shock wave cancellation in hot argon flow (10,000°K) provided by a shock
driven wind tunnel. Today we intend to set up this key-experiment. Schematically
the existence of an attached shock, at the front part of some sort of "wing"
( 2d flow) can be evidenced by laser interferometry (developed in the Institute
of Fluids Mechanics of Marseille in 1965). On the right, the picture we expect
to get, is the front shock is cancelled in this hot argon flow.
...As we have now the (theoretical)
possibility to operate in cold gas, this experiment in hot argon can seem superfluous.
But we prefer to deal with these two goals, as "parallel researches".