Diagnostic techniques for EHD and MHD
interaction
Creators: name: Neretti, Gabriele <1980>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2009
Subjects: ING-IND/31 Elettrotecnica

description: The impact of plasma technologies is growing both in the academic and in the industrial fields. Nowadays, a great interest is focused in plasma applications
in aeronautics and astronautics domains.
Plasma actuators based on the Magneto-Hydro-Dynamic (MHD) and Electro-
Hydro-Dynamic (EHD) interactions are potentially able to suitably modify
the fluid-dynamics characteristics around a flying body without utilizing
moving parts. This could lead to the control of an aircraft with negligible
response time, more reliability and improvements of the performance. In
order to study the aforementioned interactions, a series of experiments and
a wide number of diagnostic techniques have been utilized.
The EHD interaction, realized by means of a Dielectric Barrier Discharge
(DBD) actuator, and its impact on the boundary layer have been evaluated
by means of two different experiments.
In the first one a three phase multi-electrode flat panel actuator is used. Different
external flow velocities (from 1 to 20m/s) and different values of the
supplied voltage and frequency have been considered. Moreover a change of
the phase sequence has been done to verify the influence of the electric field
existing between successive phases.
Measurements of the induced speed had shown the effect of the supply voltage
and the frequency, and the phase order in the momentum transfer phenomenon.
Gains in velocity, inside the boundary layer, of about 5m/s have
been obtained. Spectroscopic measurements allowed to determine the rotational
and the vibrational temperature of the plasma which lie in the range
of 320 ÷ 440°K and of 3000 ÷ 3900°K respectively. A deviation from thermodynamic
equilibrium had been found.
The second EHD experiment is realized on a single electrode pair DBD actuator
driven by nano-pulses superimposed to a DC or an AC bias. This
new supply system separates the plasma formation mechanism from the acceleration
action on the fluid, leading to an higher degree of the control of
the process. Both the voltage and the frequency of the nano-pulses and the
amplitude and the waveform of the bias have been varied during the experiment.
Plasma jets and vortex behavior had been observed by means of fast Schlieren
imaging. This allowed a deeper understanding of the EHD interaction process.
A velocity increase in the boundary layer of about 2m/s had been
measured.
Thrust measurements have been performed by means of a scales and compared
with experimental data reported in the literature. For similar voltage
amplitudes thrust larger than those of the literature, had been observed.
Surface charge measurements led to realize a modified DBD actuator able to obtain similar performances when compared with that of other experiments.
However in this case a DC bias replacing the AC bias had been used.
MHD interaction experiments had been carried out in a hypersonic wind
tunnel in argon with a flow of Mach 6. Before the MHD experiments a
thermal, fluid-dynamic and plasma characterization of the hypersonic argon
plasma flow have been done.
The electron temperature and the electron number density had been determined
by means of emission spectroscopy and microwave absorption measurements.
A deviation from thermodynamic equilibrium had been observed.
The electron number density showed to be frozen at the stagnation region
condition in the expansion through the nozzle.
MHD experiments have been performed using two axial symmetric test bodies.
Similar magnetic configurations were used. Permanent magnets inserted
into the test body allowed to generate inside the plasma azimuthal currents
around the conical shape of the body. These Faraday currents are responsible
of the MHD body force which acts against the flow.
The MHD interaction process has been observed by means of fast imaging,
pressure and electrical measurements. Images showed bright rings due to the
Faraday currents heating and exciting the plasma particles.
Pressure measurements showed increases of the pressure in the regions where
the MHD interaction is large. The pressure is 10 to 15% larger than when
the MHD interaction process is silent.
Finally by means of electrostatic probes mounted flush on the test body lateral
surface Hall fields of about 500V/m had been measured. These results
have been used for the validation of a numerical MHD code.