Slowly rotating charged black holes in anti-de Sitter third order Lovelock gravity

In this paper, we study slowly rotating black hole solutions in Lovelock gravity (n=3). These exact slowly rotating black hole solutions are obtained in uncharged and charged cases, respectively. Up to the linear order of the rotating parameter a, the mass, Hawking temperature and entropy of the uncharged black holes get no corrections from rotation. In charged case, we compute magnetic dipole moment and gyromagnetic ratio of the black holes. It is shown that the gyromagnetic ratio keeps invariant after introducing the Gauss-Bonnet and third order Lovelock interactions.Comment: 14 pages, no figur

Spatially homogeneous Lifshitz black holes in five dimensional higher derivative gravity

We consider spatially homogeneous Lifshitz black hole solutions in five dimensional higher derivative gravity theories, which can be possible near horizon geometries of some systems that are interesting in the framework of gauge/gravity duality. We show the solutions belonging to the nine Bianchi classes in the pure R^2 gravity. We find that these black holes have zero entropy at non-zero temperatures and this property is the same as the case of BTZ black holes in new massive gravity at the critical point. In the most general quadratic curvature gravity theories, we find new solutions in Bianchi Type I and Type IX cases.Comment: 15 pages, no figure; v2, refs added, version to appear in JHE

Reissner-Nordstrom Black Holes and Thick Domain Walls

We solve numerically equations of motion for real self-interacting scalar fields in the background of Reissner-Nordstrom black hole and obtained a sequence of static axisymmetric solutions representing thick domain walls charged black hole systems. In the case of extremal Reissner-Nordstrom black hole solution we find that there is a parameter depending on the black hole mass and the width of the domain wall which constitutes the upper limit for the expulsion to occur.Comment: 18 pages, 10 figures, accepted for Phys. Rev.

Rotating Black Branes in the presence of nonlinear electromagnetic field

In this paper, we consider a class of gravity whose action represents itself as a sum of the usual Einstein-Hilbert action with cosmological constant and an $U(1)$ gauge field for which the action is given by a power of the Maxwell invariant. We present a class of the rotating black branes with Ricci flat horizon and show that the presented solutions may be interpreted as black brane solutions with two event horizons, extreme black hole and naked singularity provided the parameters of the solutions are chosen suitably. We investigate the properties of the solutions and find that for the special values of the nonlinear parameter, the solutions are not asymptotically anti-deSitter. At last, we obtain the conserved quantities of the rotating black branes and find that the nonlinear source effects on the electric field, the behavior of spacetime, type of singularity and other quantities.Comment: 7 pages, 5 figures, to appear in EPJ

Electromyography-based Adaptive Cooperative Control for a Wrist Orthosis

This paper proposes an adaptive cooperative control method for a wrist orthosis, consisting of a trajectory tracking controller, an admittance controller integrated with an electromyography (EMG)-driven musculoskeletal model-based approach. The admittance controller adaptively alters the reference trajectory based on the estimated joint torque by the EMG-driven musculoskeletal model. The admittance parameters are regulated by accessing the wrist joint condition in real-time. Three experiments are conducted including, trajectory tracking control (TTC), fixed cooperative control (FCC), adaptive cooperative control (ACC) with two cooperative ratios of 0.3 and 0.6 respectively. Preliminary results demonstrate that the cooperative control strategies have smaller root-mean-square-errors compared with the TTC when the subject’s intention is detected. The proposed method can modify the wrist orthosis’s compliance in real-time in response to the wrist joint stiffness changes, which shows its potential to improve the efficiency and safety in rehabilitation

Holographic Lovelock Gravities and Black Holes

We study holographic implications of Lovelock gravities in AdS spacetimes. For a generic Lovelock gravity in arbitrary spacetime dimensions we formulate the existence condition for asymptotically AdS black holes. We consider small fluctuations around these black holes and determine the constraint on Lovelock parameters by demanding causality of the boundary theory. For the case of cubic Lovelock gravity in seven spacetime dimensions we compute the holographic Weyl anomaly and determine the three point functions of the stress energy tensor in the boundary CFT. Remarkably, these correlators happen to satisfy the same relation as the one imposed by supersymmetry. We then compute the energy flux; requiring it to be positive is shown to be completely equivalent to requiring causality of the finite temperature CFT dual to the black hole. These constraints are not stringent enough to place any positive lower bound on the value of viscosity. Finally, we conjecture an expression for the energy flux valid for any Lovelock theory in arbitrary dimensions.Comment: 31 pages, 1 figure, harvmac, references added, calculation of viscosity/entropy ratio include

Transient heat conduction through a substrate of brine-spongy ice

An analytical model for heat conduction through brine-spongy ice is developed. This model fills a gap in knowledge related to transient heat conduction to a two-phase substrate which is crucial for modeling transient icing and deicing of cold surfaces in contact with salt water. The core of the model is based on the phase change of pure ice and brine pockets trapped in the structure of spongy ice. Freezing of brine pockets causes the release of the latent heat of fusion that is considered as the source of heat generation distributed throughout the brine-spongy ice. A nonlinear partial differential equation and a number of equations of state for ice, brine, and brine-spongy ice create governing equations of heat transfer through brine-spongy ice. A standard numerical scheme solves the set of equations in various initial conditions. The variation of temperature, volume fraction of brine and salinity of brine pockets are calculated numerically. Experimental samples of brine-spongy ice are examined under transient conditions and their surface temperatures are captured using an infrared thermal camera. The numerical results, which are for various overall salinities, are closely aligned with the measured surface temperatures

Droplet trajectories of wave-impact sea spray on a marine vessel

Marine icing phenomena are strongly dependent on the rate of water impact to marine vessels. The most important source of this incoming water is wave-impact sea spray. There is limited understanding of droplet size and velocity distributions of wave-impact sea spray. Initial distributions of the size and velocity of droplets are crucial for the calculation of the droplet path and consequently for determining the water impact to every individual place on marine vessels. This paper develops a new model of wave-impact sea spray using a distribution of the size and velocity of droplets at the edge of the vessel. The concepts of water-sheet breakup and droplet breakup lead to an inverse dependence between the size and velocity of droplets after the breakup process. Droplets take different paths and form a spray cloud in front of the vessel. The liquid water content in front of the vessel can be calculated by considering the arrangement, sizes, and velocities of a set of droplets in the spray cloud. The response of the droplet trajectory model to various initial conditions with different sets of droplet sizes and velocities is examined. The numerical results are compared to real data from field observations. Droplet sizes are inversely proportional to droplet velocities, as verified by liquid water content data obtained by the field observations. This paper proposes the use of this inverse relationship based on physics of the breakup process as the initial data for calculating the wave-impact sea spray trajectory in front of a vessel

Droplet size and velocity distributions of wave-impact sea spray over a marine vessel

The spatial distribution of droplets in a spray cloud created by wave-impact sea spray and the distribution of their sizes and velocities over a vessel deck are investigated. Existing mono-size and mono-velocity models of sea spray are not accurate enough for modelling marine icing phenomena. Wave-impact sea spray creates numerous droplets in front of and around a vessel. Droplets are the result of sheet and droplet breakup of sea water. The velocity-size dependence of the resultant droplets causes the creation of various sizes and velocities of droplets. A droplet trajectory method employs the velocity-size dependence of the droplets to find their spatial distributions in the cloud of spray over the vessel deck. Drag and body forces overcome the initial velocities of the droplets, and consequently, they follow the wind and gravitational directions. The motion of the droplets affects the shape and extent of the spray cloud over the vessel. A numerical scheme is used to find the distribution of sizes and velocities of the droplets over a vessel. Results show that neither the smallest nor the largest droplets reach the maximum height. The medium-sized droplets can reach the maximum height of the spray cloud. As the spray cloud travels over the deck, the droplet velocities become almost the same. Comparing the numerical results with field observations shows that the predicted results are consistent and have reasonable agreement with the field measurements

Extended Functionalities of Photovoltaic Systems with Flexible Power Point Tracking:Recent Advances

The power system is experiencing an ever-increasing integration of photovoltaic power plants (PVPPs), which leads demand on the power system operators to force new requirements to sustain with quality and reliability of the grid. Subsequently, a significant quantity of flexible power point tracking (FPPT) algorithms have been proposed in the literature to enhance functionalities PVPPs. The intention of FPPT algorithms is to regulate the PV power to a specific value imposed by the grid codes and operational conditions. This will inevitably interfere the maximum power point tracking (MPPT) operation of PV systems. Nevertheless, the FPPT control makes PVPPs much more grid-friendly. The main contribution of this paper is to comprehensively compare available FPPT algorithms in the literature from different aspects and provide a benchmark for researchers and engineers to select suitable FPPT algorithms for specific applications. A classification and short description of them are provided. The dynamic performances of the investigated algorithms are compared with experimental tests on a scaled-down prototype. Directions for future studies in this area are also presented.MOE (Min. of Education, S’pore)Accepted versio