30 research outputs found
A Laboratory Investigation of Supersonic Clumpy Flows: Experimental Design and Theoretical Analysis
We present a design for high energy density laboratory experiments studying
the interaction of hypersonic shocks with a large number of inhomogeneities.
These ``clumpy'' flows are relevant to a wide variety of astrophysical
environments including the evolution of molecular clouds, outflows from young
stars, Planetary Nebulae and Active Galactic Nuclei. The experiment consists of
a strong shock (driven by a pulsed power machine or a high intensity laser)
impinging on a region of randomly placed plastic rods. We discuss the goals of
the specific design and how they are met by specific choices of target
components. An adaptive mesh refinement hydrodynamic code is used to analyze
the design and establish a predictive baseline for the experiments. The
simulations confirm the effectiveness of the design in terms of articulating
the differences between shocks propagating through smooth and clumpy
environments. In particular, we find significant differences between the shock
propagation speeds in a clumpy medium compared to a smooth one with the same
average density. The simulation results are of general interest for foams in
both inertial confinement fusion and laboratory astrophysics studies. Our
results highlight the danger of using average properties of inhomogeneous
astrophysical environments when comparing timescales for critical processes
such as shock crossing and gravitational collapse times.Comment: 7 pages, 6 figures. Submitted to the Astrophysical Journal. For
additional information, including simulation animations and the pdf and ps
files of the paper with embedded high-quality images, see
http://pas.rochester.edu/~wm
Spontaneous Transition of Turbulent Flames to Detonations in Unconfined Media
Deflagration-to-detonation transition (DDT) can occur in environments ranging
from experimental and industrial systems to astrophysical thermonuclear (type
Ia) supernovae explosions. Substantial progress has been made in explaining the
nature of DDT in confined systems with walls, internal obstacles, or
pre-existing shocks. It remains unclear, however, whether DDT can occur in
unconfined media. Here we use direct numerical simulations (DNS) to show that
for high enough turbulent intensities unconfined, subsonic, premixed, turbulent
flames are inherently unstable to DDT. The associated mechanism, based on the
nonsteady evolution of flames faster than the Chapman-Jouguet deflagrations, is
qualitatively different from the traditionally suggested spontaneous reaction
wave model, and thus does not require the formation of distributed flames.
Critical turbulent flame speeds, predicted by this mechanism for the onset of
DDT, are in agreement with DNS results.Comment: 4 pages, 3 figures; accepted to Physical Review Letter
Turbulence generation by a shock wave interacting with a random density inhomogeneity field
When a planar shock wave interacts with a random pattern of pre-shock density
non-uniformities, it generates an anisotropic turbulent velocity/vorticity
field. This turbulence plays an important role at the early stages of the
mixing process in the compressed fluid. This situation emerges naturally in
shock interaction with weakly inhomogeneous deuterium-wicked foam targets in
Inertial Confinement Fusion (ICF) and with density clumps/clouds in
astrophysics. We present an exact small-amplitude linear theory describing such
interaction. It is based on the exact theory of time and space evolution of the
perturbed quantities behind a corrugated shock front for a single-mode
pre-shock non-uniformity. Appropriate mode averaging in 2D results in closed
analytical expressions for the turbulent kinetic energy, degree of anisotropy
of velocity and vorticity fields in the shocked fluid, shock amplification of
the density non-uniformity, and sonic energy flux radiated downstream. These
explicit formulas are further simplified in the important asymptotic limits of
weak/strong shocks and highly compressible fluids. A comparison with the
related problem of a shock interacting with a pre-shock isotropic vorticity
field is also presented.Comment: This article corresponds to a presentation given at the Second
International Conference and Advanced School "Turbulent Mixing and Beyond,"
held on 27 July - 07 August 2009 at the Abdus Salam International Centre for
Theoretical Physics, Trieste, Italy. That Conference Proceeding will be
published as a Topical Issue of the Physica Scripta IOP scienc