21,359 research outputs found
Tuning Process for the Modified Magnussen Combustion Model
In the application of CFD to turbulent reacting flows, one of the main limitations to predictive accuracy is the chemistry model. Using a full or skeletal kinetics model may provide good predictive ability, however, at considerable computational cost. Adding the ability to account for the interaction between turbulence and chemistry improves the overall fidelity of a simulation but adds to this cost. An alternative is the use of simple models, such as the Magnussen model, which has negligible computational overhead, but lacks general predictive ability except for cases that can be tuned to the flow being solved. In this paper, a technique will be described that allows the tuning of the Magnussen model for an arbitrary fuel and flow geometry without the need to have experimental data for a particular case. The tuning is based on comparing the results of the Magnussen model and full finite-rate chemistry when applied to perfectly and partially stirred reactor sim- ulations. In addition, a modification to the Magnussen model is proposed that allows the upper kinetic limit for the reaction rate to be set, giving better physical agreement with full kinetic mechanisms. In order to improve the agreement with flame temperatures, the thermal properties of the product species is adjusted to better match the mixture proper- ties of the full mechanism. The combustion model is then applied to the simulation of a representative scramjet flowpath, and the results compared to experimental data and other kinetic models. This procedure allows a simple reacting model to be used in a predictive manner, and affords significant savings in computational costs for CFD simulations
Nonlinear shear wave interaction at a frictional interface: Energy dissipation and generation of harmonics
Analytical and numerical modelling of the nonlinear interaction of shear wave
with a frictional interface is presented. The system studied is composed of two
homogeneous and isotropic elastic solids, brought into frictional contact by
remote normal compression. A shear wave, either time harmonic or a narrow band
pulse, is incident normal to the interface and propagates through the contact.
Two friction laws are considered and their influence on interface behavior is
investigated : Coulomb's law with a constant friction coefficient and a
slip-weakening friction law which involves static and dynamic friction
coefficients. The relationship between the nonlinear harmonics and the
dissipated energy, and their dependence on the contact dynamics (friction law,
sliding and tangential stress) and on the normal contact stress are examined in
detail. The analytical and numerical results indicate universal type laws for
the amplitude of the higher harmonics and for the dissipated energy, properly
non-dimensionalized in terms of the pre-stress, the friction coefficient and
the incident amplitude. The results suggest that measurements of higher
harmonics can be used to quantify friction and dissipation effects of a sliding
interface.Comment: 17 pages, 10 figure
The Ubiquity and Dual Nature of Ultra Compact Dwarfs
We present the discovery of several Ultra Compact Dwarfs (UCDs) located in
field/group environments. Examination of these objects, plus literature
objects, confirms the existence of two distinct formation channels for UCDs. We
find that the UCDs we have discovered around the group elliptical NGC3923 (and
UCDs generally) have properties consistent with their being the most luminous
members of the host galaxy's globular cluster (GC) system. We describe UCDs of
this type as giant GCs (GGCs). In contrast, the UCD we have found associated
with the isolated S0 NGC4546 is clearly the result of the stripping of a
nucleated companion galaxy. The young age (~3.4 Gyr) of the UCD, the lack of a
correspondingly young GC population, the apparently short dynamical friction
decay timescale (~0.5 Gyr) of the UCD, and the presence of a counterrotating
gas disc in the host galaxy (co-rotating with the UCD) together suggest that
this UCD is the liberated nucleus remaining after the recent stripping of a
companion by NGC4546. We suggest a general scheme that unifies the formation of
GCs, UCDs, and galaxy nuclei. In this picture "normal" GCs are a composite
population, composed of GCs formed in situ, GCs acquired from accreted
galaxies, and a population of lower mass stripped dwarf nuclei masquerading as
GCs. Above a "scaling onset mass" of 2x10^6 Msun (Mv ~ -10), UCDs emerge
together with a mass-size relation and a likely mass-metallicity relation (the
"blue tilt"). In the mass range up to 7x10^7 Msun (Mv ~ -13) UCDs comprise a
composite population of GGCs and stripped nuclei. Above 7x10^7 Msun, UCDs must
be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist
to populate such an extreme of the GCLF.Comment: 23 pages, 16 figures, accepted for publication in MNRA
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