844 research outputs found
Finite-Difference Solution for Laminar or Turbulent Boundary Layer Flow over Axisymmetric Bodies with Ideal Gas, CF4, or Equilibrium Air Chemistry
A computer code was developed that uses an implicit finite-difference technique to solve nonsimilar, axisymmetric boundary layer equations for both laminar and turbulent flow. The code can treat ideal gases, air in chemical equilibrium, and carbon tetrafluoride (CF4), which is a useful gas for hypersonic blunt-body simulations. This is the only known boundary layer code that can treat CF4. Comparisons with experimental data have demonstrated that accurate solutions are obtained. The method should prove useful as an analysis tool for comparing calculations with wind tunnel experiments and for making calculations about flight vehicles where equilibrium air chemistry assumptions are valid
Differential function and maturation of human stem cell-derived islets after transplantation
Insulin-producing stem cell-derived islets (SC-islets) provide a virtually unlimited cell source for diabetes cell replacement therapy. While SC-islets are less functional when first differentiated in vitro compared to isolated cadaveric islets, transplantation into mice has been shown to increase their maturation. To understand the effects of transplantation on maturation and function of SC-islets, we examined the effects of cell dose, transplantation strategy, and diabetic state in immunocompromised mice. Transplantation of 2 and 5, but not 0.75 million SC-islet cells underneath the kidney capsule successfully reversed diabetes in mice with pre-existing diabetes. SQ and intramuscular injections failed to reverse diabetes at all doses and had undetectable expression of maturation markers, such as MAFA and FAM159B. Furthermore, SC-islets had similar function and maturation marker expression regardless of diabetic state. Our results illustrate that transplantation parameters are linked to SC-islet function and maturation, providing ideal mouse models for preclinical diabetes SC therapy research
Vesicle shape, molecular tilt, and the suppression of necks
Can the presence of molecular-tilt order significantly affect the shapes of
lipid bilayer membranes, particularly membrane shapes with narrow necks?
Motivated by the propensity for tilt order and the common occurrence of narrow
necks in the intermediate stages of biological processes such as endocytosis
and vesicle trafficking, we examine how tilt order inhibits the formation of
necks in the equilibrium shapes of vesicles. For vesicles with a spherical
topology, point defects in the molecular order with a total strength of
are required. We study axisymmetric shapes and suppose that there is a
unit-strength defect at each pole of the vesicle. The model is further
simplified by the assumption of tilt isotropy: invariance of the energy with
respect to rotations of the molecules about the local membrane normal. This
isotropy condition leads to a minimal coupling of tilt order and curvature,
giving a high energetic cost to regions with Gaussian curvature and tilt order.
Minimizing the elastic free energy with constraints of fixed area and fixed
enclosed volume determines the allowed shapes. Using numerical calculations, we
find several branches of solutions and identify them with the branches
previously known for fluid membranes. We find that tilt order changes the
relative energy of the branches, suppressing thin necks by making them costly,
leading to elongated prolate vesicles as a generic family of tilt-ordered
membrane shapes.Comment: 10 pages, 7 figures, submitted to Phy. Rew.
Towards a sensitive search for variation of the fine structure constant using radio-frequency E1 transitions in atomic dysprosium
It has been proposed that the radio-frequency electric-dipole (E1) transition
between two nearly degenerate opposite-parity states in atomic dysprosium
should be highly sensitive to possible temporal variation of the fine structure
constant () [V. A. Dzuba, V. V. Flambaum, and J. K. Webb, Phys. Rev. A
{\bf 59}, 230 (1999)]. We analyze here an experimental realization of the
proposed search in progress in our laboratory, which involves monitoring the E1
transition frequency over a period of time using direct frequency counting
techniques. We estimate that a statistical sensitivity of |\adota| \sim
10^{-18}/yr may be achieved and discuss possible systematic effects that may
limit such a measurement.Comment: 8 pages, 7 figure
The compensation of Gaussian curvature in developable cones is local
In this paper we use the angular deficit scheme [V. Borrelli, F. Cazals, and
J.-M. Morvan, {\sl Computer Aided Geometric Design} {\bf 20}, 319 (2003)] to
determine the distribution of Gaussian curvature in developable cones (d-cones)
[E. Cerda, S. Chaieb, F. Melo, and L. Mahadevan, {\sl Nature} {\bf 401}, 46
(1999)] numerically. These d-cones are formed by pushing a thin elastic sheet
into a circular container. Negative Gaussian curvatures are identified at the
rim where the sheet touches the container. Around the rim there are two narrow
bands with positive Gaussian curvatures. The integral of the (negative)
Gaussian curvature near the rim is almost completely compensated by that of the
two adjacent bands. This suggests that the Gauss-Bonnet theorem which
constrains the integral of Gaussian curvature globally does not explain the
spontaneous curvature cancellation phenomenon [T. Liang and T. A. Witten, {\sl
Phys. Rev. E} {\bf 73}, 046604 (2006)]. The locality of the compensation seems
to increase for decreasing d-cone thickness. The angular deficit scheme also
provides a new way to confirm the curvature cancellation phenomenon.Comment: 12 pages; 5 figure
Dynamics of filaments and membranes in a viscous fluid
Motivated by the motion of biopolymers and membranes in solution, this
article presents a formulation of the equations of motion for curves and
surfaces in a viscous fluid. We focus on geometrical aspects and simple
variational methods for calculating internal stresses and forces, and we derive
the full nonlinear equations of motion. In the case of membranes, we pay
particular attention to the formulation of the equations of hydrodynamics on a
curved, deforming surface. The formalism is illustrated by two simple case
studies: (1) the twirling instability of straight elastic rod rotating in a
viscous fluid, and (2) the pearling and buckling instabilities of a tubular
liposome or polymersome.Comment: 26 pages, 12 figures, to be published in Reviews of Modern Physic
Angular dependence of the radiation power of a Josephson STAR-emitter
We calculate the angular dependence of the power of stimulated terahertz
amplified radiation (STAR) emitted from a voltage applied across a stack
of intrinsic Josephson junctions. During coherent emission, we assume a
spatially uniform Josephson current density in the stack acts as a surface
electric current density antenna source, and the cavity features of the stack
are contained in a magnetic surface current density source. A superconducting
substrate acts as a perfect magnetic conductor with on its
surface. The combined results agree very well with recent experimental
observations. Existing BiSrCaCuO crystals atop perfect
electric conductors could have Josephson STAR-emitter power in excess of 5 mW,
acceptable for many device applications.Comment: 3 pages 3 figure
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