A simultaneous three-directional laser absorption technique for
the
study of a shock-induced Richtmyer–Meshkov instability mixing zone
is reported. It is an improvement
of a CO2 laser absorption technique, using three detectors
during the same run,
through three different directions of the test section, for the
simultaneous thickness
measurement of the mixing zone near the corner, near the wall and at the
centre of
a square-cross-section shock tube. The three-dimensional mean front and
rear shapes
of the mixing zone, its thickness and volume are deduced from the experimental
measurements. The cases when the shock wave passes from a heavy gas to
a light
one, from one gas to another of similar densities and from a light gas
to a heavy
one, are investigated before and after the mixing zone compression by the
reflected
shock, for different incident shock wave Mach numbers. It is shown that
the mixing
zone is strongly deformed by the wall boundary layer when it becomes turbulent.
Consequently, the thickness of the mixing zone is not constant along the
shock tube
cross-section, and the measurement of the mean volume of the mixing zone
appears
to be more appropriate than its mean thickness at the centre of the shock
tube.
The influence of the incident shock wave Mach number is also studied. When
the
Atwood number tends to zero, we observe a limit-like regime and the thickness,
or
the volume, of the mixing zone no longer varies with the incident shock
wave Mach
number. Furthermore, a series of experiments undertaken with an Atwood
number
close to zero enabled us to define a membrane-induced minimum mixing thickness,
L0, depending on the initial configuration of the experiments.
From the experimental
data, a hypothesis about the mixing zone thickness evolution law with
time is deduced
on the basis of L0. The results are found to follow
two very
different laws depending
on whether they are considered before or after the establishment of the
plenary
turbulent regime. However, no general trend can be determined to describe
the entire
phenomenon, i.e. from the initial conditions until the turbulent stage.
Flow Structure behind a Shock Wave Discharged from a Square Cross Section Shock Tube.
