New instabilites of unsteady transonic flows
with non-equilibrium phase transition
are presented including unsymmetric flow patterns with moving oblique shock
systems in supersonic nozzles with perfectly symmetric shapes. The phenomena
were
first detected when performing experiments in our supersonic wind tunnel
with
atmospheric supply and could be perfectly reproduced by numerical simulations
based
on the Euler equations, i.e. neglecting the viscosity of the fluid. The
formation of
the liquid phase is modelled using the classical nucleation theory for
the steady state together with the Hertz–Knudsen droplet growth law
and
yields qualitatively and
quantitatively excellent agreement with experiments in the unsteady flow
regime with
high-frequency oscillations including the unstable transient change of
the structure
from symmetric to unsymmetric flow.
For engineering applications the sudden increase or decrease of the
frequency by
a factor 2 or more and of the pressure amplitude at the bifurcation limits
is of
immediate practical interest, e.g. for flutter excitation of turbomachinery
blading.