12,026 research outputs found
Influence of shear reinforcement corrosion on the performance of under-reinforced concrete beams
The in-service performance of reinforced concrete beams can be severely affected through cor-rosion of the steel reinforcement when it becomes subjected to harsh corrosive environments containing chlo-rides and carbon dioxide. In such instances, corrosion is likely to occur in the steel reinforcement, with the expansive nature of the corrosion products likely to induce cracking and spalling of the concrete. A loss of structural integrity (stiffness) will occur and this can severely influence the serviceability of the member. The purpose of this paper is to investigate the relationship between degree of corrosion and loss of stiffness in corrosion damaged under-reinforced concrete beams. Beams (100mm x 150mm cross section) were subjected to accelerated corrosion in the laboratory and subsequently tested in flexure to failure. The paper reports on the results of these tests and relates the degree of corrosion in the main steel to the percentage loss in stiffness in the concrete beams
Robustness from flexibility in the fungal circadian clock
Background
Robustness is a central property of living systems, enabling function to be maintained against environmental perturbations. A key challenge is to identify the structures in biological circuits that confer system-level properties such as robustness. Circadian clocks allow organisms to adapt to the predictable changes of the 24-hour day/night cycle by generating endogenous rhythms that can be entrained to the external cycle. In all organisms, the clock circuits typically comprise multiple interlocked feedback loops controlling the rhythmic expression of key genes. Previously, we showed that such architectures increase the flexibility of the clock's rhythmic behaviour. We now test the relationship between flexibility and robustness, using a mathematical model of the circuit controlling conidiation in the fungus Neurospora crassa.
Results
The circuit modelled in this work consists of a central negative feedback loop, in which the frequency (frq) gene inhibits its transcriptional activator white collar-1 (wc-1), interlocked with a positive feedback loop in which FRQ protein upregulates WC-1 production. Importantly, our model reproduces the observed entrainment of this circuit under light/dark cycles with varying photoperiod and cycle duration. Our simulations show that whilst the level of frq mRNA is driven directly by the light input, the falling phase of FRQ protein, a molecular correlate of conidiation, maintains a constant phase that is uncoupled from the times of dawn and dusk. The model predicts the behaviour of mutants that uncouple WC-1 production from FRQ's positive feedback, and shows that the positive loop enhances the buffering of conidiation phase against seasonal photoperiod changes. This property is quantified using Kitano's measure for the overall robustness of a regulated system output. Further analysis demonstrates that this functional robustness is a consequence of the greater evolutionary flexibility conferred on the circuit by the interlocking loop structure.
Conclusions
Our model shows that the behaviour of the fungal clock in light-dark cycles can be accounted for by a transcription-translation feedback model of the central FRQ-WC oscillator. More generally, we provide an example of a biological circuit in which greater flexibility yields improved robustness, while also introducing novel sensitivity analysis techniques applicable to a broader range of cellular oscillators
Large Structure Dynamics and Entrainment in the Mixing Layer at High Reynolds Number
Observations were made on a turbulent mixing layer in a water channel at Reynolds numbers up to 3 x 10^6 . Flow visualization with dyes revealed (once more) large coherent structures and showed their role in the entrainment process; observations of the reaction of a base
and an acid indicator injected on the two sides of the layer, respectively, gave some indication of where molecular mixing occurs. Autocorrelations of streamwise velocity fluctuations, using an LDV, revealed a fundamental
periodicity associated with the large structures. The surprisingly long correlation times suggest time scales much longer than had been supposed; it is argued that the mixing layer dynamics at any point is coupled to the large structure further downstream, and some possible
consequences about the effects of initial conditions and,. of the influence of apparatus geometry are discussed
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Identification of free amino acids in several crude extracts of two legumes using thin-layer chromatography
Multi-scale chemistry modelling for spacecraft atmospheric re-entry
We aim to develop a model capable of simulating the surface chemistry and material erosion involved when a re-entry vehicle descends through the atmosphere. Our starting point is to simulate the erosion of a fcc crystal slab due to cluster bombardment, using the model Lennard-Jones potential. From this, we plan to scale up towards Direct Monte Carlo Simulation approaches for the gas dynamics above the surface
Tunneling conductance in Superconductor/Ferromagnet junctions: a self consistent approach
We evaluate the tunneling conductance of clean Ferromomagnet/Superconductor
junctions via a fully self-consistent numerical solution of the microscopic
Bogoliubov-DeGennes equations. We present results for a relevant range of
values of the Fermi wavevector mismatch (FWM), the spin polarization, and the
interfacial scattering strength. For nonzero spin polarization, the conductance
curves vary nonmonotonically with FWM. The FWM dependence of the
self-consistent results is stronger than that previously found in
non-self-consistent calculations, since, in the self-consistent case, the
effective scattering potential near the interface depends on the FWM. The
dependence on interfacial scattering is monotonic. These results confirm that
it is impossible to characterize both the the FWM and the interfacial
scattering by a single effective parameter and that analysis of experimental
data via the use of such one-parameter models is unreliable.Comment: 12 pages, including 8 figure
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