3,776 research outputs found
High-fidelity simulation of an ultrasonic standing-wave thermoacoustic engine with bulk viscosity effects
We have carried out boundary-layer-resolved, unstructured fully-compressible
Navier--Stokes simulations of an ultrasonic standing-wave thermoacoustic engine
(TAE) model. The model is constructed as a quarter-wavelength engine,
approximately 4 mm by 4 mm in size and operating at 25 kHz, and comprises a
thermoacoustic stack and a coin-shaped cavity, a design inspired by Flitcroft
and Symko (2013). Thermal and viscous boundary layers (order of 10
m) are resolved. Vibrational and rotational molecular relaxation
are modeled with an effective bulk viscosity coefficient modifying the viscous
stress tensor. The effective bulk viscosity coefficient is estimated from the
difference between theoretical and semi-empirical attenuation curves.
Contributions to the effective bulk viscosity coefficient can be identified as
from vibrational and rotational molecular relaxation. The inclusion of the
coefficient captures acoustic absorption from infrasonic (10 Hz) to
ultrasonic (100 kHz) frequencies. The value of bulk viscosity depends on
pressure, temperature, and frequency, as well as the relative humidity of the
working fluid. Simulations of the TAE are carried out to the limit cycle, with
growth rates and limit-cycle amplitudes varying non-monotonically with the
magnitude of bulk viscosity, reaching a maximum for a relative humidity level
of 5%. A corresponding linear model with minor losses was developed; the linear
model overpredicts transient growth rate but gives an accurate estimate of
limit cycle behavior. An improved understanding of thermoacoustic energy
conversion in the ultrasonic regime based on a high-fidelity computational
framework will help to further improve the power density advantages of
small-scale thermoacoustic engines.Comment: 55th AIAA Aerospace Sciences Meeting, AIAA SciTech, 201
Electroweak phase transition in the MSSM with four generations
By assuming the existence of the sequential fourth generation to the minimal
supersymmetric standard model (MSSM), we study the possibility of a strongly
first-order electroweak phase transition. We find that there is a parameter
region of the MSSM where the electroweak phase transition is strongly first
order. In that parameter region, the mass of the lighter scalar Higgs boson is
calculated to be above the experimental lower bound, and the scalar quarks of
the third and the fourth generations are heavier than the corresponding quarks.Comment: 12 pages, 2 tables, 2 figure
Phase transition in a supersymmetric axion model
In a supersymmetric axion model where the scale for both supersymmetry
breaking and Peccei-Quinn symmetry breaking is around GeV, we find
that there is a reasonable parameter space for a strongly first order phase
transition at the scale.Comment: 5 pages, 1 figur
Electroweak phase transition in a nonminimal supersymmetric model
The Higgs potential of the minimal nonminimal supersymmetric standard model
(MNMSSM) is investigated within the context of electroweak phase transition. We
investigate the allowed parameter space yielding correct electroweak phase
transitoin employing a high temperature approximation. We devote to
phenomenological consequences for the Higgs sector of the MNMSSM for
electron-positron colliders. It is observed that a future linear
collider with GeV will be able to test the model with regard
to electroweak baryogenesis.Comment: 28 pages, 5 tables, 12 figure
Understanding the effect resonant magnetic perturbations have on ELMs
All current estimations of the energy released by type I ELMs indicate that,
in order to ensure an adequate lifetime of the divertor targets on ITER, a
mechanism is required to decrease the amount of energy released by an ELM, or
to eliminate ELMs altogether. One such amelioration mechanism relies on
perturbing the magnetic field in the edge plasma region, either leading to more
frequent, smaller ELMs (ELM mitigation) or ELM suppression. This technique of
Resonant Magnetic Perturbations (RMPs) has been employed to suppress type I
ELMs at high collisionality/density on DIII-D, ASDEX Upgrade, KSTAR and JET and
at low collisionality on DIII-D. At ITER-like collisionality the RMPs enhance
the transport of particles or energy and keep the edge pressure gradient below
the 2D linear ideal MHD critical value that would trigger an ELM, whereas at
high collisionality/density the type I ELMs are replaced by small type II ELMs.
Although ELM suppression only occurs within limitied operational ranges, ELM
mitigation is much more easily achieved. The exact parameters that determine
the onset of ELM suppression are unknown but in all cases the magnetic
perturbations produce 3D distortions to the plasma and enhanced particle
transport. The incorporation of these 3D effects in codes will be essential in
order to make quantitative predictions for future devices.Comment: 32 pages, 9 figure
The CP properties of the lightest Higgs boson with sbottom effects
In the framework of the recently proposed gluino-axion model, using the
effective potential method and taking into account the top-stop as well as the
bottom-sbottom effects, we discuss the CP--properties of the lightest Higgs
boson, in particular its CP--odd composition, which can offer new opportunities
at collider searches. It is found that although the CP-odd composition of the
lightest Higgs increases slightly with the inclusion of the sbottom effects, it
never exceeds %0.17 for all values of the renormalization scale Q ranging from
top mass to TeV scaleComment: 24 pp, 12 eps fig
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