28,345 research outputs found
Fine Scale Features of Turbulent Shear Flows
This thesis presents an investigation into kinematic features of fine scale turbulence in free
shear flows. In particular it seeks to examine the interaction between the different length scales
present in shear flow turbulence as well as the interaction between the strain-rate tensor and
the rotation tensor, which are the symmetric and skew-symmetric components of the velocity
gradient tensor respectively.
A new multi-scale particle image velocimetry (PIV) technique is developed that is capable
of resolving the flow at two different dynamic ranges, centred on inertial range scales and on
dissipative range scales, simultaneously. This data is used to examine the interaction between
large-scale fluctuations, of the order of the integral scale, and inertial and dissipative range
fluctuations. The large-scale fluctuations are observed to have an amplitude and frequency
modulation effect on the small scales, and the small scales are shown to have a slight effect on
the large scales, illustrating the two way nature of the energy cascade. A mechanism whereby
integral scale rollers leave behind a wake of intense small-scale fluctuations is proposed.
The interaction between strain and rotation is examined with regards to the rate of enstrophy
amplification (ωiSijωj). It is found that the mechanism that is responsible for the
nature of enstrophy amplification is the alignment tendency between the extensive strain-rate
eigenvector and the vorticity vector. This mechanism is also observed to be scale dependent for
ωiSijωj > 0, but independent for ωiSijωj < 0. This is subsequently confirmed with new dual-plane
stereoscopic PIV experiments performed as part of this study. Finally, computational
data is used to examine the effect of experimental noise and variation of spatial resolution on
the observation and understanding of this strain - rotation interaction
Metabolic regulation by p53 family members
The function of p53 is best understood in response to genotoxic stress, but increasing evidence suggests that p53 also plays a key role in the regulation of metabolic homeostasis. p53 and its family members directly influence various metabolic pathways, enabling cells to respond to metabolic stress. These functions are likely to be important for restraining the development of cancer but could also have a profound effect on the development of metabolic diseases, including diabetes. A better understanding of the metabolic functions of p53 family members may aid in the identification of therapeutic targets and reveal novel uses for p53-modulating drugs
Stress-Induced Cocaine Seeking Requires a Beta-2 Adrenergic Receptor-Regulated Pathway from the Ventral Bed Nucleus of the Stria Terminalis That Regulates CRF Actions in the Ventral Tegmental Area
The ventral bed nucleus of the stria terminalis (vBNST) has been implicated in stress-induced cocaine use. Here we demonstrate that, in the vBNST, corticotropin releasing factor (CRF) is expressed in neurons that innervate the ventral tegmental area (VTA), a site where the CRF receptor antagonist antalarmin prevents the reinstatement of cocaine seeking by a stressor, intermittent footshock, following intravenous self-administration in rats. The vBNST receives dense noradrenergic innervation and expresses β adrenergic receptors (ARs). Footshock-induced reinstatement was prevented by bilateral intra-vBNST injection of the β-2 AR antagonist, ICI-118,551, but not the β-1 AR antagonist, betaxolol. Moreover, bilateral intra-vBNST injection of the β-2 AR agonist, clenbuterol, but not the β-1 agonist, dobutamine, reinstated cocaine seeking, suggesting that activation of vBNST β-2 AR is both necessary for stress-induced reinstatement and sufficient to induce cocaine seeking. The contribution of a β-2 AR-regulated vBNST-to-VTA pathway that releases CRF was investigated using a disconnection approach. Injection of ICI-118,551 into the vBNST in one hemisphere and antalarmin into the VTA of the contralateral hemisphere prevented footshock-induced reinstatement, whereas ipsilateral manipulations failed to attenuate stress-induced cocaine seeking, suggesting that β-2 AR regulate vBNST efferents that release CRF into the VTA, activating CRF receptors, and promoting cocaine use. Last, reinstatement by clenbuterol delivered bilaterally into the vBNST was prevented by bilateral vBNST pretreatment with antalarmin, indicating that β-2 AR-mediated actions in the vBNST also require local CRF receptor activation. Understanding the processes through which stress induces cocaine seeking should guide the development of new treatments for addiction
Lie point symmetries and the geodesic approximation for the Schr\"odinger-Newton equations
We consider two problems arising in the study of the Schr\"odinger-Newton
equations. The first is to find their Lie point symmetries. The second, as an
application of the first, is to investigate an approximate solution
corresponding to widely separated lumps of probability. The lumps are found to
move like point particles under a mutual inverse-square law of attraction
Oscillatory Modes of a Prominence-PCTR-Corona Slab Model
Oscillations of magnetic structures in the solar corona have often been
interpreted in terms of magnetohydrodynamic waves. We study the adiabatic
magnetoacoustic modes of a prominence plasma slab with a uniform longitudinal
magnetic field, surrounded by a prominence-corona transition region (PCTR) and
a coronal medium. Considering linear small-amplitude oscillations, the
dispersion relation for the magnetoacoustic slow and fast modes is deduced
assuming evanescent-like perturbations in the coronal medium. In the system
without PCTR, a classification of the oscillatory modes according to the
polarisation of their eigenfunctions is made in order to distinguish modes with
fast-like or slow-like properties. Internal and external slow modes are
governed by the prominence and coronal properties respectively, and fast modes
are mostly dominated by prominence conditions for the observed wavelengths. In
addition, the inclusion of an isothermal PCTR does not substantially influence
the mode frequencies, but new solutions (PCTR slow modes) are present.Comment: Accepted for publication in Solar Physic
Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations
We have carried out an extensive survey of magnetic field strengths toward
dark cloud cores in order to test models of star formation: ambipolar-diffusion
driven or turbulence driven. The survey involved hours of observing
with the Arecibo telescope in order to make sensitive OH Zeeman observations
toward 34 dark cloud cores. Nine new probable detections were achieved at the
2.5-sigma level; the certainty of the detections varies from solid to marginal,
so we discuss each probable detection separately. However, our analysis
includes all the measurements and does not depend on whether each position has
a detection or just a sensitive measurement. Rather, the analysis establishes
mean (or median) values over the set of observed cores for relevant
astrophysical quantities. The results are that the mass-to-flux ratio is
supercritical by , and that the ratio of turbulent to magnetic energies
is also . These results are compatible with both models of star
formation. However, these OH Zeeman observations do establish for the first
time on a statistically sound basis the energetic importance of magnetic fields
in dark cloud cores at densities of order cm, and they lay
the foundation for further observations that could provide a more definitive
test.Comment: 22 pages, 2 figures, 2 table
Thermodynamic formalism for the Lorentz gas with open boundaries in dimensions
A Lorentz gas may be defined as a system of fixed dispersing scatterers, with
a single light particle moving among these and making specular collisions on
encounters with the scatterers. For a dilute Lorentz gas with open boundaries
in dimensions we relate the thermodynamic formalism to a random flight
problem. Using this representation we analytically calculate the central
quantity within this formalism, the topological pressure, as a function of
system size and a temperature-like parameter \ba. The topological pressure is
given as the sum of the topological pressure for the closed system and a
diffusion term with a \ba-dependent diffusion coefficient. From the
topological pressure we obtain the Kolmogorov-Sinai entropy on the repeller,
the topological entropy, and the partial information dimension.Comment: 7 pages, 5 figure
Spatial evolution of the turbulent/turbulent interface geometry in a cylinder wake
This study aims to examine the spatial evolution of the geometrical features
of the turbulent/turbulent interface (TTI) in a cylinder wake. The wake is
exposed to various turbulent backgrounds in which the turbulence intensity and
the integral length scale are independently varied and comparisons to a
turbulent/non-turbulent interface (TNTI) are drawn. The turbulent wake was
marked with a high-Schmidt-number () scalar and a planar laser induced
fluorescence (PLIF) experiment was carried out to capture the interface between
the wake and the ambient flow from = 5 to 40 where is the streamwise
coordinate from the centre of the cylinder and is the cylinder's diameter.
It is found that the TTI generally spreads faster toward the ambient flow than
the TNTI. A transition region of the interfaces' spreading is found at , after which the interfaces propagate at a slower rate than
previously (upstream) and the mean interface positions of both TNTI and TTI
scale with the local wake half-width. The location of both the TNTI and TTI
have non-Gaussian probability density functions (PDFs) in the near wake because
of the influence of the large-scale coherent motions present within the flow.
Further downstream, after the large-scale coherent motions have dissipated, the
TNTI position PDF does become Gaussian. For the first time we explore the
spatial variation of the ``roughness'' of the TTI, quantified via the fractal
dimension, from near field to far field. The length scale in the background
flow has a profound effect on the TTI fractal dimension in the near wake,
whilst the turbulence intensity only becomes important for the fractal
dimension farther downstream
Nucleation and Growth of GaN/AlN Quantum Dots
We study the nucleation of GaN islands grown by plasma-assisted
molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In
particular, we assess the variation of their height and density as a function
of GaN coverage. We show that the GaN growth passes four stages: initially, the
growth is layer-by-layer; subsequently, two-dimensional precursor islands form,
which transform into genuine three-dimensional islands. During the latter
stage, island height and density increase with GaN coverage until the density
saturates. During further GaN growth, the density remains constant and a
bimodal height distribution appears. The variation of island height and density
as a function of substrate temperature is discussed in the framework of an
equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure
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