Constant Gravity Research Articles

Observables in loop quantum gravity with a cosmological constant

In many quantum gravity approaches, the cosmological constant is introduced by deforming the gauge group into a quantum group. In three dimensions, the quantization of the Chern-Simons formulation of gravity provided the first example of such a deformation. The Turaev-Viro model, which is an example of a spin-foam model, is also defined in terms of a quantum group. By extension, it is believed that in four dimensions, a quantum group structure could encode the presence of $\mathrm{\ensuremath{\Lambda}}\ensuremath{\ne}0$. In this article, we introduce by hand the quantum group ${\mathcal{U}}_{q}(\mathfrak{s}u(2))$ into the loop quantum gravity (LQG) framework; that is, we deal with ${\mathcal{U}}_{q}(\mathfrak{s}u(2))$-spin networks. We explore some of the consequences, focusing in particular on the structure of the observables. Our fundamental tools are tensor operators for ${\mathcal{U}}_{q}(\mathfrak{s}u(2))$. We review their properties and give an explicit realization of the spinorial and vectorial ones. We construct the generalization of the $\mathrm{U}(N)$ formalism in this deformed case, which is given by the quantum group ${\mathcal{U}}_{q}(\mathfrak{u}(N))$. We are then able to build geometrical observables, such as the length, area or angle operators, etc. We show that these operators characterize a quantum discrete hyperbolic geometry in the three-dimensional LQG case. Our results confirm that a quantum group structure in LQG can be a tool to introduce a nonzero cosmological constant into the theory. Our construction is both relevant for three-dimensional Euclidian quantum gravity with a negative cosmological constant and four-dimensional Lorentzian quantum gravity with a positive cosmological constant.

Light and gravity signals synergize in modulating plant development.

Tropisms are growth-mediated plant movements that help plants to respond to changes in environmental stimuli. The availability of water and light, as well as the presence of a constant gravity vector, are all environmental stimuli that plants sense and respond to via directed growth movements (tropisms). The plant response to gravity (gravitropism) and the response to unidirectional light (phototropism) have long been shown to be interconnected growth phenomena. Here, we discuss the similarities in these two processes, as well as the known molecular mechanisms behind the tropistic responses. We also highlight research done in a microgravity environment in order to decouple two tropisms through experiments carried out in the absence of a significant unilateral gravity vector. In addition, alteration of gravity, especially the microgravity environment, and light irradiation produce important effects on meristematic cells, the undifferentiated, highly proliferating, totipotent cells which sustain plant development. Microgravity produces the disruption of meristematic competence, i.e., the decoupling of cell proliferation and cell growth, affecting the regulation of the cell cycle and ribosome biogenesis. Light irradiation, especially red light, mediated by phytochromes, has an activating effect on these processes. Phytohormones, particularly auxin, also are key mediators in these alterations. Upcoming experiments on the International Space Station will clarify some of the mechanisms and molecular players of the plant responses to these environmental signals involved in tropisms and the cell cycle.

Experimental Study on Cracking Load and Ultimate Load of Masonry Walls with Window Openings Strengthened with Sprayed GFRP

A contrast investigation of the seismic behavior of unreinforced masonry walls with window openings strengthened with Carbon Fiber Reinforced Polymer (CFRP) sheets and sprayed Glass Fiber Reinforced Polymer (GFRP) is presented. Three wall specimens in the scale of 1/2 were tested by the horizontal cyclic loading combined with constant gravity loads. The seismic strengthening effects by two different FRP retrofitting schemes are compared in aspect of the failure modes, shear capacity, hysteresis response, deterioration of rigidity and ability of energy dissipation. The experimental results indicate that the increasing degree of shear capacity and the improvement of seismic behavior of the SGFRP-strengthened wall are significantly superior to that of the wall strengthened with epoxy-based CFRP; and the overcoat of sprayed GFRP can bond tightly and work well together with the masonry to play a critical role in earthquake resistance.

Nonlinear Finite Element Analysis on Shear Bearing Capacity of Masonry Wall Strengthened by SGFRP

4 finite element models of ANSYS were constructed according to the masonry wall specimens of the experimental study. A material nonlinear finite element analysis by the finite element calculation software ANSYS on masonry walls strengthened by SGFRP under the horizontal cyclic loads combined with constant gravity loads was performed. The load-displacement curves of the models obtained from ANSYS simulation were described. The ultimate loads of the 3 models strengthened by SGFRP increased significantly, and the fiber length affect the bearing capacity and the deformation capacity of the wall. The results of the nonlinear finite element analysis of the models agree well to the experimental results. Some effective and reasonable reinforcement suggestions are proposed.

Abundance analysis for long-period variables

Asymptotic giant branch (AGB) stars play a key role in the enrichment of galaxies with heavy elements. Due to their large amplitude variability, the measurement of elemental abundances is a highly challenging task that has not been solved in a satisfactory way yet. Following our previous work we use hydrostatic and dynamical model atmospheres to simulate observed high-resolution near-infrared spectra of 12 variable and non-variable red giants in the globular cluster 47 Tuc. The 47 Tuc red giants are independently well-characterized in important parameters (mass, metallicity, luminosity). The principal aim was to compare synthetic spectra based on the dynamical models with observational spectra of 47 Tuc variables. Assuming that the abundances are unchanged on the upper giant branch in these low-mass stars, our goal is to estimate the impact of atmospheric dynamics on the abundance determination. We present new measurements of the C/O and 12C/13C ratio for 5 non-variable red giants in 47Tuc. The equivalent widths measured for our 7 variable stars strongly differ from the non-variable stars and cannot be reproduced by either hydrostatic or dynamical model atmospheres. Nevertheless, the dynamical models fit the observed spectra of long-period variables much better than any hydrostatic model. For some spectral features, the variations in the line intensities predicted by dynamical models over a pulsation cycle give similar values as a sequence of hydrostatic models with varying temperature and constant surface gravity.

Turbulent entrainment into fluid mud gravity currents

The entrainment of ambient water into non-Newtonian fluid mud gravity currents was investigated in this study. Constant volume release gravity currents were generated in a lock-exchange tank for a wide range of experimental conditions. A technique similar to the so-called light attenuation technique was used to find the boundary of the current, allowing for the calculation of both temporal and bulk entrainment parameters (in terms of the temporal and bulk entrainment velocities, respectively). It was found that the temporal entrainment velocity is dependent on different parameters in the different propagation phases. The slumping phase begins with an adjustment zone (henceforth, non-established zone) in which the temporal entrainment velocity is not a function of the current front velocity, followed by the established zone in which the temporal entrainment velocity is a function of the current front velocity. This dependence of the temporal entrainment velocity on the current front velocity carries through to the inertia-buoyancy phase. As expected, temporal entrainment velocity in the viscous-buoyancy phase was negligible in comparison to average entrainment velocity in the other phases. It is observed that the temporal entrainment characteristics in the non-established zone is governed by the competition between the entrainment-inhibiting density stratification effects and the entrainment-favouring effects of the Kelvin–Helmholtz billows that are quantified by the Richardson number and the Reynolds number of the gravity current, respectively. In the established zone, Reynolds number effects were observed to dominate over Richardson number effects in dictating temporal entrainment characteristics. A parameterization for the temporal entrainment velocity for non-Newtonian fluid mud gravity currents is developed based upon the experimental observations. This study also found that the bulk entrainment characteristics for the non-Newtonian fluid mud gravity currents can be parameterized by the Newtonian bulk entrainment parameterizations that rely solely on a bulk Richardson number. Interestingly, it was found that the non-Newtonian characteristics of the gravity current have little to no effect on the entrainment of the Newtonian ambient fluid.

Missing Gains from Trade?

In a class of trade models which satisfy a constant elasticity gravity equation, the welfare gains from trade can be computed using the open economy domestic trade share and a constant trade elasticity. The measured welfare gains from trade from this quantitative approach are typically relatively modest. In this paper, we suggest a channel for welfare gains that this quantitative approach typically abstracts from: trade-induced changes in domestic productivity. Using a model of sequential production, in which trade induces a reorganization of production that raises domestic productivity, we show that the welfare gains from trade can become arbitrarily large.

Experimental Research on Seismic Behavior of Seismically Damaged RC Frame Column Strengthened with Sprayed Hybrid BF/CFRP

This experimental program was designed for investigating the seismic behaviors and strengthening effect of pre-damaged RC frame columns retrofitted with sprayed BFRP and hybrid BF/CFRP. Four RC frame column specimens, among which one was unstrengthened and three was pre-damaged and strengthened with sprayed FRP, were tested under an incremental loading procedure of the pseudo-static, cyclic shear loads combined with constant gravity loads. The test results including the failure mode, ultimate bearing load capacity, load-displacement hysteresis curves and ductility of specimens were obtained and analyzed. It indicates that spraying hybrid BF/CFRP strengthening scheme can effectively improve the ductility and energy dissipation ability of pre-damaged concrete frame columns. Although the improvement of the peak loads and ultimate lateral deformation of damaged frame columns were not obvious compared with those of the reference column, but it should be pointed out that the strengthened columns were pre-damaged seriously with yielded steel bars and the recover of load bearing ability resulted from spraying FRP retrofit can not be neglected. It also shows that increasing the thickness of spraying overcoat can effectively improve the energy dissipation ability of damaged frame columns.

Effect of initial excess density and discharge on constant flux gravity currents propagating on a slope

The effect of the upstream conditions on propagation of gravity current over a slope is investigated using three-dimensional numerical simulations. The current produced by constant buoyancy flux, is simulated using a large eddy simulation solver. The dense saline solution used at the inlet is the driving force of the flow. Higher replenishment of the current is possible either by a high inflow discharge or high initial fractional density excess. In the simulations, it is observed that these two parameters affect the flow in different ways. Results show that the front speed of the descending current is proportional to the cube root of buoyancy flux, \((g_o^{\prime } Q)^{1/3}\), which agrees with the previous experimental and numerical observations. The height of the tail of the current grows linearly in the streamwise direction. Formation of a strong shear layer at the boundary of mixed upper layer and dense lower layer is observed within the body and the tail of the current. Over the tail of the current far enough from the inlet, the vertical velocity and density profiles are compared to the ones from an experimental study. Distance from the bed to the point of maximum velocity increases with an increase in inflow discharge, while it remains practically unchanged with increasing initial fractional excess density in the simulations. Even though the velocity profiles are in good agreement, some discrepancies are observed in fractional excess density profiles among experimental and numerical results. Possible reasons for these discrepancies are discussed. Generally, gravity current type of flows could be expressed in layer-integrated formulation of governing equations. However, layer integration introduces several constants, commonly known as shape factors, to the equations of motion. The values of these shape factors are calculated based on simulation results and compared to the values from experiments and to the favorably used ‘top hat’ assumption.

Seismic retrofit of non-ductile reinforced concrete frames with chevron braces

The use of chevron braces for seismic strengthening of existing non-ductile reinforced concrete frames was investigated experimentally and numerically. Five strengthened and two as-built reference portal frame specimens were tested under a constant gravity load and increasing cyclic lateral displacement excursions. The test results indicated that steel brace retrofitting increased the lateral stiffness, strength and energy dissipation capacity significantly. Specimens strengthened with square hollow sections experienced significant strength degradation upon brace buckling, but a lateral strength of about 3·5 times the strength of bare frames was maintained up to a drift ratio of about 2% in all strengthened specimens. Non-linear cyclic analysis was conducted to simulate the behaviour of the test specimens. A satisfactory agreement with estimated and observed load–deformation responses was observed. Using test observations and simulation results, the deformation capacity limits of chevron brace strengthened deficient frames were discussed. A parametric study was conducted to investigate the effectiveness of the chevron brace retrofitting for different brace sizes and column axial loads. Finally, a non-linear time history analysis was performed for a building frame of a deficient existing four-storey three-bay reinforced concrete structure by using two chevron brace sections in this study.

Effect of Gravity on Movement and Muscle Characteristics for Daily Occupational Performance in Sitting and Side-Lying Positions in Adults

The purpose of this study was to compare differences in the activation of biceps and triceps, if any, in patients with stroke in a simple reaching and grasping task under two body orientations, one performed under constant gravity and the other against gravity. Twenty healthy adults from the midwestern United States were randomly assigned to either an against gravity (sitting) or a constant gravity (side-lying) body orientation in a within-subject repeated measure design. Each participant performed the task of moving a filled soda can from a fixed start and end position. The results showed that the reaching velocity was significantly lower ( p < .0001) and muscle activations of biceps and triceps as measured by surface electromyography were significantly less pronounced ( p < .00001) in the side-lying position compared to the sitting position. The data suggest that the muscle activation patterns differ under different biomechanical constraints created by different gravity situations. Thus, the same functional task done in different body orientations may not necessarily fully recruit the same muscle groups in all orientations.

Design and Development on a Kind of Fiber Automatic Slicing Instrument

Abstract: Y172 fiber slicer (also says Hartz slicer) is often used in slicing fiber in fabric inspection. However, there are some problems for this kind of slicer, such as low success，low efficiency，poor safety and so on. A new fiber automatic slicing instrument is proposed in this paper. Some creative designs are done, in aspects of slicing thickness adjustment, automatic fiber cutting, blade constant clamping and gravity pressure. This fiber slicing instrument changed traditional slicing method, and effectively resolved the problems of Y172 fiber slicer. A reliable and efficient instrument is provided for fast and accurate slicing fiber.

Analytic non-integrability of the Suslov problem

In this work, we consider the Suslov problem, which consists of a rotation motion of a rigid body, whose center of mass is located at one axis of inertia, around a fixed point O in a constant gravity field restricted to a nonholonomic constraint. The integrability and non-integrability has been established by a number of authors for the nongeneric values of b = (b1, b2, b3) which is the unit vector along the line connecting the point O with the center of mass of the body. Here, we prove the analytic non-integrability for the remaining (generic) values of b.

Fidelity for kicked atoms with gravity near a quantum resonance

Kicked atoms under a constant Stark or gravity field are investigated for experimental setups with cold and ultracold atoms. The parametric stability of the quantum dynamics is studied using the fidelity. In the case of a quantum resonance, it is shown that the behavior of the fidelity depends on arithmetic properties of the gravity parameter. Close to a quantum resonance, the long-time asymptotics of the fidelity is studied by means of a pseudoclassical approximation introduced by Fishman et al. [J. Stat. Phys. 110, 911 (2003)]. The long-time decay of fidelity arises from the tunneling out of pseudoclassical stable islands, and a simple ansatz is proposed which satisfactorily reproduces the main features observed in numerical simulations.

Effect of chemical composition on the ζ-potential of chromite

Chromite samples from Africa, Brazil, Philippines and Russia were investigated through electrophoretic mobility and method of Mular and Roberts measurements. Chemical composition, lattice constant and specific gravity were determined. According to the chemical composition of the samples, empirical equations to predict the point of zero charge (PZC) of oxides and hydroxides were applied (Parks’ and Yoon’s treatments). The isoelectric point (IEP) of chromite is independent of the electrolyte concentration and confirmed that KNO3 is an indifferent electrolyte for this mineral. The PZC of the chromites investigated is around pH 6.5. The maximum difference found between measured and calculated values was 1.0 pH unit. The results obtained by the method of Mular and Roberts are close to the predicted PZC for three of the four chromite samples. The results obtained by microelectrophoresis point to a difference between measured and calculated values of 0.6 pH unit for samples from Russia and Africa, and 1.0 pH unit for the sample from the Philippines. The four investigated samples display a good atomic ordering in the spinel structure. The presence of lattice vacancies, as well as the existence of sites proner to oxidation and/or hydration may account for the difference between the calculated and measured values.

De Sitter symmetry and quantum theory

de Sitter symmetry on quantum level implies that operators describing a given system satisfy commutation relations of the de Sitter algebra. This approach gives a new perspective on fundamental notions of quantum theory. We discuss applications of the approach to the cosmological constant problem, gravity, and particle theory.

Experimental response of double-wythe masonry panels strengthened with glass fiber reinforced polymers subjected to diagonal compression tests

Many existing unreinforced masonry buildings are seismically vulnerable and require retrofitting. The present experimental study, performed on half scale double-wythe brick masonry panels strengthened by glass fiber reinforced polymers (GFRPs), aimed to investigate the efficiency of a shear reinforcement technique using externally bonded FRP composites. The test specimens were built in a manner to simulate the traditional masonry walls built in Iran. A series of two unreinforced brick panels and five GFRP strengthened panels were subjected to diagonal tension tests. The two unreinforced diagonal tension specimens presented a brittle failure due to splitting along the compressed diagonal. The key parameter was the strengthening configuration. Experimental results pointed out that GFRP reinforcement provided a less brittle failure and a noticeable load bearing capacity increase. Performances of the different reinforcement configurations were compared in terms of strength, deformation capacity, ductility, energy absorption, stiffness and mechanism of failure. Also, near the end of the paper, experimental results of three perforated masonry shear walls are reported. The walls were tested under constant gravity load and incrementally increasing in-plane loading cycles. Two of these walls and two of the diagonal tension specimens had the same geometrical reinforcement ratio in the diagonal direction.

Monte Carlo simulation of defects in hard-sphere crystal grown on a square pattern

Monte Carlo simulations of hard-sphere (HS) crystal grown on a square patterned wall under gravity have been performed. While previous simulations were performed with step-wise controlled gravity, in the present simulations constant gravity has been applied from the first. In the case in which a flat wall is used as the bottom wall, if a large gravity is suddenly applied, the system does polycrystallize. On the other hand, in the present simulations, despite the sudden application of gravity, the system has not polycrystallize. Crystalline nucleation on the square pattern and successive crystal growth upward are suggested to overcome the homogeneous nucleation inside and result in. Defect disappearance, which has been essentially the same as that for the case with step-wise controlled gravity, has also observed for the present case. The characteristic of the square patterned bottom wall simulation with a large horizontal system size has been existence of triangular defects suggesting stacking tetrahedra.

Cyclic behavior of perforated masonry walls strengthened with glass fiber reinforced polymers

Many of the existing unreinforced masonry buildings are seismically vulnerable and require retrofitting. In this experimental study, the cyclic behavior of six, one-half scale, perforated unreinforced brick walls, before and after retrofitting, using Glass Fiber Reinforced Polymers (GFRPs), is investigated. The walls were built using one-half scale solid clay bricks and cement mortar to simulate the traditional walls built in Iran during the last 40 years of the 20th century. These walls had a window opening at their center. One brick wall was unreinforced and considered a reference specimen. Three walls were directly upgraded after construction using GFRPs. The fifth wall was first strengthened and tested. Then, the seismically damaged specimen was retrofitted, using GFRPs, and tested again. Each specimen was retrofitted on the surface of two sides. All specimens were tested under constant gravity load and incrementally increasing in-plane loading cycles. During the test, each wall was allowed to displace in its own plane. The key parameter was the strengthening configuration including the cross layout, grid layout, and combined layout. Strengthening by means of GFRPs significantly improved the strength, deformation capacity, and energy absorption of the brick wall. The increase in performance parameters was dependent upon GFRP layout.

Long-term lateral displacement of geosynthetic-reinforced soil segmental retaining walls

The service limit-state design of Geosynthetic-Reinforced Soil (GRS) retaining walls requires accurate estimation of the lateral facing displacement at the end of construction as well as after years of creep. However, before a simplistic but rational methodology for this purpose can be developed, mechanisms governing the short-term and long-term lateral facing displacements must be clarified. In this study, extensive Finite Element analyses were carried out using a calibrated Finite Element procedure to investigate and attempt to better understand the lateral facing displacements of segmental GRS walls at the end of construction and after 10 years of creep under constant gravity loading. The study found that among the two main components of lateral facing displacement, the deformation of reinforced soil zone was largely governed by reinforcement spacing and reinforcement stiffness, while the influence of reinforcement length was negligible. Soil stiffness also played an important role in the lateral deformation if large reinforcement stiffness and/or small reinforcement spacing were used. In contrast, reinforcement length to a very large extent determined the lateral displacement at the back of reinforced soil zone. With constant reinforcement length, the reinforced soil zone could be treated as a deep beam. The displacement at the back of reinforced soil zone was then determined by the earth pressure, beam depth, and beam stiffness, the last of which is a function of soil stiffness, reinforcement spacing, reinforcement stiffness, and facing stiffness. The study found that isochrone stiffness can be used to interpret the lateral deformation of GRS walls under working stress condition.