2,340 research outputs found
Comparison of acoustic and strain gauge techniques for crack closure measurements
A quantitative study on the systems performances of the COD gauge and the acoustic transmission techniques to elastic deformation of part-through crack and compact tension specimens has been conducted. It is shown that the two instruments measure two completely different quantities: The COD gauge yields information on the length change of the specimen whereas the acoustic technique is sensitive directly to the amount of contract area between two surfaces, interfering with the acoustic signal. In another series of experiments, compression tests on parts with specifically prepared surfaces were performed so that the surface contact area could be correlated with the transmitted acoustic signal, as well as the acoustic with the COD gauge signal. A linear relation between contact area and COD gauge signal was obtained until full contact had been established
Theoretical investigation of finite size effects at DNA melting
We investigated how the finiteness of the length of the sequence affects the
phase transition that takes place at DNA melting temperature. For this purpose,
we modified the Transfer Integral method to adapt it to the calculation of both
extensive (partition function, entropy, specific heat, etc) and non-extensive
(order parameter and correlation length) thermodynamic quantities of finite
sequences with open boundary conditions, and applied the modified procedure to
two different dynamical models. We showed that rounding of the transition
clearly takes place when the length of the sequence is decreased. We also
performed a finite-size scaling analysis of the two models and showed that the
singular part of the free energy can indeed be expressed in terms of an
homogeneous function. However, both the correlation length and the average
separation between paired bases diverge at the melting transition, so that it
is no longer clear to which of these two quantities the length of the system
should be compared. Moreover, Josephson's identity is satisfied for none of the
investigated models, so that the derivation of the characteristic exponents
which appear, for example, in the expression of the specific heat, requires
some care
Riccati parameter modes from Newtonian free damping motion by supersymmetry
We determine the class of damped modes \tilde{y} which are related to the
common free damping modes y by supersymmetry. They are obtained by employing
the factorization of Newton's differential equation of motion for the free
damped oscillator by means of the general solution of the corresponding Riccati
equation together with Witten's method of constructing the supersymmetric
partner operator. This procedure leads to one-parameter families of (transient)
modes for each of the three types of free damping, corresponding to a
particular type of %time-dependent angular frequency. %time-dependent,
antirestoring acceleration (adding up to the usual Hooke restoring
acceleration) of the form a(t)=\frac{2\gamma ^2}{(\gamma t+1)^{2}}\tilde{y},
where \gamma is the family parameter that has been chosen as the inverse of the
Riccati integration constant. In supersymmetric terms, they represent all those
one Riccati parameter damping modes having the same Newtonian free damping
partner modeComment: 6 pages, twocolumn, 6 figures, only first 3 publishe
Thermal denaturation of fluctuating finite DNA chains: the role of bending rigidity in bubble nucleation
Statistical DNA models available in the literature are often effective models
where the base-pair state only (unbroken or broken) is considered. Because of a
decrease by a factor of 30 of the effective bending rigidity of a sequence of
broken bonds, or bubble, compared to the double stranded state, the inclusion
of the molecular conformational degrees of freedom in a more general mesoscopic
model is needed. In this paper we do so by presenting a 1D Ising model, which
describes the internal base pair states, coupled to a discrete worm like chain
model describing the chain configurations [J. Palmeri, M. Manghi, and N.
Destainville, Phys. Rev. Lett. 99, 088103 (2007)]. This coupled model is
exactly solved using a transfer matrix technique that presents an analogy with
the path integral treatment of a quantum two-state diatomic molecule. When the
chain fluctuations are integrated out, the denaturation transition temperature
and width emerge naturally as an explicit function of the model parameters of a
well defined Hamiltonian, revealing that the transition is driven by the
difference in bending (entropy dominated) free energy between bubble and
double-stranded segments. The calculated melting curve (fraction of open base
pairs) is in good agreement with the experimental melting profile of
polydA-polydT. The predicted variation of the mean-square-radius as a function
of temperature leads to a coherent novel explanation for the experimentally
observed thermal viscosity transition. Finally, the influence of the DNA strand
length is studied in detail, underlining the importance of finite size effects,
even for DNA made of several thousand base pairs.Comment: Latex, 28 pages pdf, 9 figure
Investigating microstructural evolution during the electroreduction of UO2 to U in LiCl-KCl eutectic using focused ion beam tomography
Reprocessing of spent nuclear fuels using molten salt media is an attractive alternative to liquid-liquid extraction techniques. Pyroelectrochemical processing utilizes direct, selective, electrochemical reduction of uranium dioxide, followed by selective electroplating of a uranium metal. Thermodynamic prediction of the electrochemical reduction of UO2 to U in LiCl-KCl eutectic has shown to be a function of the oxide ion activity. The pO2− of the salt may be affected by the microstructure of the UO2 electrode. A uranium dioxide filled “micro-bucket” electrode has been partially electroreduced to uranium metal in molten lithium chloride-potassium chloride eutectic. This partial electroreduction resulted in two distinct microstructures: a dense UO2 and a porous U metal structure were characterised by energy dispersive X-ray spectroscopy. Focused ion beam tomography was performed on five regions of this electrode which revealed an overall porosity ranging from 17.36% at the outer edge to 3.91% towards the centre, commensurate with the expected extent of reaction in each location. The pore connectivity was also seen to reduce from 88.32% to 17.86% in the same regions and the tortuosity through the sample was modelled along the axis of propagation of the electroreduction, which was seen to increase from a value of 4.42 to a value of infinity (disconnected pores). These microstructural characteristics could impede the transport of O2− ions resulting in a change in the local pO2− which could result in the inability to perform the electroreduction
Predominance diagrams of uranium and plutonum species in both lithium chloride-potassium chloride eutectic and calcium chloride
Electro-reduction of spent nuclear fuel has the
potential to significantly reduce the amount of high level
waste from nuclear reactors. Typically, spent uranium and
plutonium are recovered via the PUREX process leading to
a weapons-grade recovery; however, electro-reduction
would allow spent nuclear fuel to be recovered effectively
whilst maintaining proliferation resistance. Here, we pres-
ent predominance diagrams (also known as Littlewood
diagrams) for both uranium and plutonium species in
molten lithium chloride–potassium chloride eutectic (LKE)
at 500
C and in calcium chloride at 800
C. All diagrams
presented depict regions of stability of various phases at
unit activity in equilibrium with their respective dissociated
ions. The diagrams thermodynamically define the electro-
chemical system leading to predictions of reaction condi-
tions necessary to electrochemically separate species. The
diagrams have been constructed using a pure thermody-
namic route; identifying stable species within the molten
salt with an assumption of unit activity for each of the
phases. These thermodynamically predicted diagrams have
been compared to the limited available experimental data;
demonstrating good correlation. The diagrams can also be
used to predict regions of stability at activities less than
unity and is also demonstrated
Introduction to Research Dialogues
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141903/1/jcpy82.pd
Visualization of Flowfield Modification by RCS Jets on a Capsule Entry Vehicle
Nitric oxide planar laser-induced fluorescence (NO PLIF) has been used to visualize the flow on the aft-body of an entry capsule having an activated RCS jet in NASA Langley Research Center's 31-Inch Mach 10 wind tunnel facility. A capsule shape representative of the Apollo command module was tested. These tests were performed to demonstrate the ability of the PLIF method to visualize RCS jet flow while providing some preliminary input to NASA's Orion Vehicle design team. Two different RCS nozzle designs - conical and contoured - were tested. The conical and contoured nozzles had area ratios of 13.4 and 22.5 respectively. The conical nozzle had a half-angle of 10 . Low- and high-Reynolds number cases were investigated by changing the tunnel stagnation pressure from 350 psi to 1300 psi, resulting in freestream Reynolds numbers of 0.56 and 1.8 million per foot respectively. For both of these cases, three different jet plenum pressures were tested (nominally 56, 250 and 500 psi). A single angle-of-attack was investigated (24 degrees). NO PLIF uses an ultraviolet laser sheet to interrogate a slice in the flow containing seeded NO; this UV light excites fluorescence from the NO molecules which is detected by a high-speed digital camera. The system has spatial resolution of about 200 microns (2 pixel blurring) and has flow-stopping time resolution (approximately 1 microsecond). NO was seeded into the flow two different ways. First, the RCS jet fluid was seeded with approximately 1-5% NO, with the balance N2. This allowed observation of the shape, structure and trajectory of the RCS jets. Visualizations of both laminar and turbulent flow jet features were obtained. Visualizations were obtained with the tunnel operating at Mach 10 and also with the test section held at a constant pressure similar to the aftbody static pressure (0.04 psi) obtained during tunnel runs. These two conditions are called "tunnel on" and "tunnel off" respectively. Second, the forebody flow was seeded with a very low flowrate (<100 standard cubic centimeters per minute) of pure NO. This trace gas was entrained into and allowed visualization of the shear layer forming between the expansion fan on the shoulder of the model and the recirculating separated flow in the wake of the model. This shear layer was observed to be laminar in the absence of the RCS jet operation and turbulent above a certain RCS jet flowrate. Furthermore, the operation of the RCS jet is seen to push the shear layer out away from the model, with a higher jet pressures resulting in larger deflections. Figures show some data from this test, partially processed. In the final paper, these images will be processed and rendered on a three dimensional visualization of the test hardware for clearer visualization and interpretation of the flowfields
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