ON THE SPECTRAL INSTABILITY AND BIFURCATION OF 2D-QUASI-GEOSTROPHIC POTENTIAL VORTICITY EQUATION

The analysis on hydrodynamic stability of shear flows is an active research direction in fluid dynamics. In this article, the spectral instability and bifurcation of forced shear flows governed by the 2D quasi-geostrophic equation with a generalized Kolmogorov forcing are investigated. We prove that the corresponding eigenvalue problem can be transferred into a family of algebraic equations with infinity number of variables, and the nontrivial solutions to the algebraic equations are expressed in form of continuous fractions. After obtaining the asymptotic estimate for the ratio of the imaginary parts of eigenvalues to a control parameter R as it approaches to infinity, we show that there exists a critical value Rc above which, the forced shear flows become unstable, where the control parameter R is the product of Reynolds number Re and the intensity of the curl of the forcing. To shed light on the bifurcation involved in the losing stability of the forced shear flows, a natural method used to reduce the quasi-geostrophic equation to ODEs is introduced. Based on numerical experiments on the coefficients in the ODEs, we show that both supercritical and subcritical Hopf bifurcations occur in the forced shear flows, which only depend on the type of generalized Kolmogorov forcing

Activation Mode Effects on the Shear Bond Strength of Dual-cured Resin Cements

Objectives: This study evaluated the immediate (10-minute) and delayed (24-hour) bond strength of dual-cured resin cements that are light-activated either immediately or delayed (after five minutes) or chemically-activated only. Materials and Methods: Three dual-cured resin cements were evaluated: RelyX ARC, Panavia F and Enforce. Cylinders of resin cement were built up over resin composite blocks following the manufacturers’ instructions for each luting agent. The cements were mixed, inserted into the molds and light-activated either immediately or after five minutes (delayed light activation). When no light activation was performed, the materials were protected from light exposure until testing. Half of the samples were tested at 10 minutes; the remaining samples were stored at 100% relative humidity for 4 hours when they were then test ed (n=10). Data were submitted to three-way ANOVA and post-hoc Tukey’s tests (α=0.05). The failure mode was evaluated under SEM. Results: RelyX ARC presented the highest values of shear bond strength, followed by Enforce. Panavia F showed the lowest values. Both immediate and delayed light activation caused the cements to present the highest means of shear bond strength. There was an improvement in bond strength after 24 hours of storage. Conclusions: RelyX ARC produced the highest bond strength, which was improved by light activation and storage for 24 hours

Satellite testing of a gravitationally induced quantum decoherence model

Quantum mechanics and the general theory of relativity are two pillars of modern physics. However, a coherent unified framework of the two theories still remains an open problem. Attempts to quantize general relativity have led to many rival models of quantum gravity, which, however, generally lack experimental foundations. We report a quantum optical experimental test of event formalism of quantum fields, a theory which attempts to present a coherent description of quantum fields in exotic spacetimes containing closed timelike curves and ordinary spacetime. We experimentally test a prediction of the theory with the quantum satellite Micius that a pair of time-energy entangled particles probabilistically decorrelate passing through different regions of the gravitational potential of Earth. Our measurement results are consistent with the standard quantum theory and hence do not support the prediction of event formalism

Gas Path Health Monitoring for a Turbofan Engine Based on a Nonlinear Filtering Approach

Different approaches for gas path performance estimation of dynamic systems are commonly used, the most common being the variants of the Kalman filter. The extended Kalman filter (EKF) method is a popular approach for nonlinear systems which combines the traditional Kalman filtering and linearization techniques to effectively deal with weakly nonlinear and non-Gaussian problems. Its mathematical formulation is based on the assumption that the probability density function (PDF) of the state vector can be approximated to be Gaussian. Recent investigations have focused on the particle filter (PF) based on Monte Carlo sampling algorithms for tackling strong nonlinear and non-Gaussian models. Considering the aircraft engine is a complicated machine, operating under a harsh environment, and polluted by complex noises, the PF might be an available way to monitor gas path health for aircraft engines. Up to this point in time a number of Kalman filtering approaches have been used for aircraft turbofan engine gas path health estimation, but the particle filters have not been used for this purpose and a systematic comparison has not been published. This paper presents gas path health monitoring based on the PF and the constrained extend Kalman particle filter (cEKPF), and then compares the estimation accuracy and computational effort of these filters to the EKF for aircraft engine performance estimation under rapid faults and general deterioration. Finally, the effects of the constraint mechanism and particle number on the cEKPF are discussed. We show in this paper that the cEKPF outperforms the EKF, PF and EKPF, and conclude that the cEKPF is the best choice for turbofan engine health monitoring

Wall-to-floor connection behaviour in a low-damage concrete wall building

Following the 2010/2011 Canterbury earthquakes, approximately 60% of multi-story buildings with reinforced concrete walls required demolition. Both practitioners and researchers have increasingly realized that low-damage structural systems could be an alternative to improve the seismic behaviour of concrete buildings and to reduce the economic and social impact of structural damage in future earthquakes. To verify the seismic response of a low-damage concrete wall building representing state-of-art design practice, a shake table test on a two-story concrete building was recently conducted as part of an ILEE-QuakeCoRE collaborative research program. The building utilized flexible wall-to-floor connections in the long span direction and isolating wall-to-floor devices in the short span direction to provide a comparison of their respective behaviour. Additionally, the wall-to-floor interaction such as effects of wall uplift on the link slab, and force transfer mechanism from floor to the wall will be discussed in this paper

Seismic performance of mortarless reinforced masonry walls

Reinforced masonry walls are commonly used as a primary lateral load resisting system in buildings in regions of moderate or high seismicity. Mortarless reinforced masonry (MLRM) walls are constructed using concrete masonry units placed without mortar and then interconnected by concrete grout. The absence of mortar can overcome several issues during the construction of conventional reinforced masonry walls. Previous research mainly focused on mortarless systems using three-dimensional interlocking blocks without vertical reinforcement. In this study, a total of six MLRM walls were constructed and tested under cyclic loadings to investigate their seismic behaviour. Experimental parameters included axial load ratio, shear span ratio, and vertical reinforcement ratio of the walls. Test results showed that the MLRM walls exhibited high stiffness, good deformation capacity, and reasonable energy dissipation capacity. The response of all six walls under cyclic loadings was controlled by 3–4 cracks formed at bed joints between the concrete masonry units, and their failure was dominated by flexure with the occurrence of slight shear sliding. The load-displacement envelope, strength, and energy dissipation of all tested MLRM walls are discussed

Impulse Noise Removal by L1 Weighted Nuclear Norm Minimization

In recent years, the nuclear norm minimization (NNM) as a convex relaxation of the rank minimization has attracted great research interest. By assigning different weights to singular values, the weighted nuclear norm minimization (WNNM) has been utilized in many applications. However, most of the work on WNNM is combined with the l2data-fidelity term, which is under additive Gaussian noise assumption. In this paper, we introduce the L1-WNNM model, which incorporates the l1-data-fidelity term and the regularization from WNNM. We apply the alternating direction method of multipliers (ADMM) to solve the non-convex minimization problem in this model. We exploit the low rank prior on the patch matrices extracted based on the image non-local self-similarity and apply the L1-WNNM model on patch matrices to restore the image corrupted by impulse noise. Numerical results show that our method can effectively remove impulse noise

Cyclic loading test of three-bay RC space frame strengthened with X-shape RC braces

© 2015, RILEM. Previous studies showed improved seismic performance of reinforced concrete (RC) plane frames by adding RC braces. However, in literature, the studies on the efficiency of RC braces for the improvement of seismic performance of RC space frame (RCSF) are very limited. This paper presents the experimental results on two 1/4-scale, two-storey and three-bay RCSFs under cyclic loading, i.e. a control RCSF and another RC X-braces strengthened RCSF (termed as CBRCSF). The test results show that compared with the control frame, the seismic performance of the CBRCSF was improved significantly in terms of lateral strength, stiffness degradation and energy dissipation capacity due to the addition of RC X-braces. Moreover, the CBRCSF could continue to bear loads after the rupture of the RC braces, thereby revealing redundancy of the CBRCSF because of the use of RC X-braces