This paper investigates model predictive control (MPC) of a single sea wave energy converter (WEC). By using control schemes which constrain certain quantities, such as the maximum size of the ...feedback force, the energy storage for actuators and relative heave motion, it is possible for control to not only improve performance but to directly impact strongly on design and cost. Motivated by this fact, a novel objective function is adopted in the MPC design, which brings obvious benefits: First, the quadratic program (QP) derived from this objective function can be easily convexified, which facilitates the employment of existing efficient optimization algorithms. Second, this novel design can trade off the energy extraction, the energy consumed by the actuator and safe operation. Moreover, an alternative QP is also formulated with the input slew rate as optimization variable, so that the slew rate limit of an actuator can be explicitly incorporated into optimization. All these benefits promote the real-time application of MPC on a WEC and reduced cost of hardware.
•A novel predictive control scheme for a sea wave energy converter is proposed.•It systematically optimizes energy output and hardware constraints.•The resulting optimization is convex which facilitates the implementation.•Other optimal control schemes are also discussed and compared.
In these proceedings we discuss PHENIX results on small systems at RHIC, specifically those relating to collectivity and flow. We discuss the small systems geometry scan, which comprises collisions ...of p+Au, d+Au, and 3He+Au at SNN=200GeV and is designed to exploit differences in intrinsic geometry. We also discuss the small systems beam energy scan, which comprises d+Au collisions at SNN=200GeV, 62.4 GeV, 39 GeV, and 19.6 GeV and is designed to explore differences in system size and lifetime while keeping the geometry fixed. We find the data in nearly all cases are well-described by hydrodynamical calculations involving a QGP phase, suggestion formation of QGP droplets in a wide variety of collisions.
Abstract
The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous active galactic nuclei. The continuum ...has a high-energy turnover, typically in the range of one to several 100 keV and is suggestive of Comptonization by thermal electrons. We are studying hard X-ray spectra of AGN obtained with NuSTAR after correction for X-ray reflection and under the assumption that coronae are compact, being only a few gravitational radii in size as indicated by reflection and reverberation modelling. Compact coronae raise the possibility that the temperature is limited and indeed controlled by electron–positron pair production, as explored earlier (Paper I). Here, we examine hybrid plasmas in which a mixture of thermal and non-thermal particles is present. Pair production from the non-thermal component reduces the temperature leading to a wider temperature range more consistent with observations.
Theoretical calculations have shown the possibility of P-violating bubbles in the QCD vacuum, which in combination with the strong magnetic field created in off-central heavy-ion collisions lead to ...novel effects such as the Chiral Magnetic Effect (CME) and the Chiral Separation Effect (CSE). A coupling between the CME and the CSE produces a wave-like excitation called the Chiral Magnetic Wave (CMW). The CMW produces a quadrupole moment that always has the same sign and is therefore present in an average over events. We present a series of charge-dependent anisotropic flow measurements in Pb–Pb collisions at sNN=2.76TeV by ALICE, using two- and three-particle correlators with unidentified hadrons. The relation of these measurements to the search for the CMW is discussed.
We demonstrate that deterministic sea wave prediction (DSWP) combined with constrained optimal control can dramatically improve the efficiency of sea wave energy converters (WECs), while maintaining ...their safe operation. We focus on a point absorber WEC employing a hydraulic/electric power take-off system. Maximizing energy take-off while minimizing the risk of damage is formulated as an optimal control problem with a disturbance input (the sea elevation) and with both state and input constraints. This optimal control problem is non-convex, which prevents us from using quadratic programming algorithms for the optimal solution. We demonstrate that the optimum can be achieved by bang–bang control. This paves the way to adopt a dynamic programming (DP) algorithm to resolve the on-line optimization problem efficiently. Simulation results show that this approach is very effective, yielding at least a two-fold increase in energy output as compared with control schemes which do not exploit DSWP. This level of improvement is possible even using relatively low precision DSWP over short time horizons. A key finding is that only about 1 second of prediction horizon is required, however, the technical difficulties involved in obtaining good estimates necessitate a DSWP system capable of prediction over tens of seconds.
► We investigate constrained optimal control of sea wave energy converters. ► The objective is to maximize energy take-off while minimizing the risk of damage. ► The optimal control problem is analyzed using Pontryagin's minimum principle. ► Dynamic programming is employed to resolve the on-line optimization problem. ► Simulation result shows at least a two-fold increase in energy output.
Context. In past years, several observations of AGN and X-ray binaries suggested the existence of a warm (T ~ 0.5 − 1 keV) and optically thick (τcor ~ 10 − 20) corona covering the inner parts of the ...accretion disk. These properties are directly derived from spectral fitting in UV to soft-X-rays using Comptonization models. However, whether such a medium can be both in radiative and hydrostatic equilibrium with an accretion disk is still uncertain. Aims. We investigate the properties of such warm, optically thick coronae and put constraints on their existence. Methods. We solve the radiative transfer equation for grey atmosphere analytically in a pure scattering medium, including local dissipation as an additional heating term in the warm corona. The temperature profile of the warm corona is calculated assuming that it is cooled by Compton scattering, with the underlying dissipative disk providing photons to the corona. Results. Our analytic calculations show that a dissipative thick corona (τcor in the range 10–12) on top of a standard accretion disk can reach temperatures of the order of 0.5–1 keV in its upper layers provided that the disk is passive. However, in the absence of strong magnetic fields, the requirement of a Compton cooled corona in hydrostatic equilibrium in the vertical direction sets an upper limit on the Thomson optical depth τcor ≲ 5. We show that this value cannot be exceeded independently of the accretion disk parameters. However, magnetic pressure can extend this result to larger optical depths. Namely, a dissipative corona might have an optical depth up to ~20 when the magnetic pressure is 100 times higher than the gas pressure. Conclusions. The observation of warm coronae with Thomson depth larger than ≃5 puts tight constraints on the physics of the accretion disk/corona systems and requires either strong magnetic fields or vertical outflows to stabilize the system.
A soft X-ray excess above the 2–10 keV power-law extrapolation is generally observed in the X-ray spectra of active galactic nuclei. The origin of this excess is still not well understood. Presently ...there are two competitive models: blurred ionized reflection and warm Comptonization. In the case of warm Comptonization, observations suggest a corona temperature in the range 0.1–2 keV and a corona optical depth of about 10–20. Moreover, radiative constraints from spectral fits with Comptonization models suggest that most of the accretion power should be released in the warm corona and the disk below is basically non-dissipative, radiating only the reprocessed emission from the corona. However, the true radiative properties of such a warm and optically thick plasma are not well known. For instance, the importance of the Comptonization process, the potential presence of strong absorption and/or emission features, and the spectral shape of the output spectrum have been studied only very recently. Here, we present simulations of warm and optically thick coronae using the
TITAN
radiative transfer code coupled with the
NOAR
Monte-Carlo code, the latter fully accounting for Compton scattering of continuum and lines. Illumination from above by hard X-ray emission and from below by an optically thick accretion disk are taken into account, as well as (uniform) internal heating. Our simulations show that for a large part of the parameter space, the warm corona with sufficient internal mechanical heating is dominated by Compton cooling and neither strong absorption nor emission lines are present in the outgoing spectra. In a smaller part of the parameter space, the calculated emission agrees with the spectral shape of the observed soft X-ray excess. Remarkably, this also corresponds to the conditions of radiative equilibrium of an extended warm corona covering a non-dissipative accretion disk almost entirely. These results confirm that warm Comptonization is a valuable model that can explain the origin of the soft X-ray excess.
Context. Compton scattering is involved in many astrophysical situations. It is well known and has been studied in detail for the past fifty years. Exact formulae for the different cross sections are ...often complex, and essentially asymptotic expressions have been used in the past. Numerical capabilities have now developed to a point where they enable the direct use of exact formulae in sophisticated codes that deal with all kinds of interactions in plasmas. Although the numerical computation of the Compton cross section is simple in principle, its practical evaluation is often prone to accuracy issues. These can be severe in some astrophysical situations but are often not addressed properly. Aims. In this paper we investigate numerical issues related to the computation of the Compton scattering contribution to the time evolution of interacting photon and particle populations. Methods. An exact form of the isotropic Compton cross section free of numerical cancellations is derived. Its accuracy is investigated and compared to other formulae. Then, several methods to solve the kinetic equations using this cross section are studied. Results. The regimes where existing cross sections can be evaluated numerically are given. We find that the cross section derived here allows for accurate and fast numerical evaluation for any photon and electron energy. The most efficient way to solve the kinetic equations is a method combining a direct integration of the cross section over the photon and particle distributions and a Fokker-Planck approximation. Expressions describing this combination are given.
Context. High resolution X-ray spectra of black hole X-ray binaries (BHBs) show blueshifted absorption lines suggesting the presence of outflowing winds. Furthermore, observations show that the disk ...winds are equatorial and they occur in the Softer (disk dominated) states of the outburst and are less prominent or absent in the Harder (power-law dominated) states. Aims. We want to test whether the self-similar magneto-hydrodynamic (MHD) accretion-ejection models can explain the observational results for accretion disk winds in BHBs. In our models, the density at the base of the outflow from the accretion disk is not a free parameter. This mass loading is determined by solving the full set of dynamical MHD equations without neglecting any physical term. Thus, the physical properties of the outflow depend on and are controlled by the global structure of the disk. Methods. We studied different MHD solutions characterized by different values of the disk aspect ratio (ε) and the ejection efficiency (p). We also generate two kinds of MHD solutions depending on the absence (cold solution) or presence (warm solution) of heating at the disk surface. Such heating could be either from dissipation of energy due to MHD turbulence in the disk or from illumination of the disk surface. Warm solutions can have large (>0.1) values of p, which would imply larger wind mass loading at the base of the outflow. We use each of these MHD solutions to predict the physical parameters (distance, density, velocity, magnetic field, etc.) of an outflow. Motivated by observational results, we have put limits on the ionization parameter (ξ), column density, and timescales. Further constraints were derived for the allowed values of ξ from thermodynamic instability considerations, particularly for the Hard SED. These physical constraints were imposed on each of these outflows to select regions within it, which are consistent with the observed winds. Results. The cold MHD solutions are found to be inadequate and cannot account for winds because of their low ejection efficiency. On the contrary, warm solutions can have sufficiently high values of p( ≳ 0.1), which are required to explain the observed physical quantities in the wind. From our thermodynamic equilibrium curve analysis for the outflowing gas, we find that in the Hard state a range of ξ is unstable. This constraint makes it impossible to have any wind at all in the Hard state. Conclusions. Using the MHD outflow models we are able to explain the observed trends, i.e. that the winds are equatorial and that they are observable in the Soft states (and not expected in the Hard state) of the BHB outbursts.
Context.
The X-ray spectra of X-ray binaries are dominated by emission of either soft or hard X-rays which defines their soft and hard spectral states. While the generic picture is relatively well ...understood, little is known about the interplay of the various media at work, or about the reasons why some sources do not follow common behavior. Cygnus X-3 is amongst the list of X-ray binaries that show quite complex behavior, with various distinct spectral states not seen in other sources. These states have been characterized in many studies. Because of its softness and intrinsic low flux above typically 50 keV, very little is known about the hard X/soft gamma-ray (100–1000 keV) emission in Cygnus X-3.
Aims.
Using the whole INTEGRAL data base, we aim to explore the 3–1000 keV spectra of Cygnus X-3. This allows to probe this region with the highest sensitivity ever, and search for the potential signature of a high-energy non-thermal component as sometimes seen in other sources.
Methods.
Our work is based on state classification carried out in previous studies with data from the
Rossi
X-Ray Timing Explorer. We extend this classification to the whole INTEGRAL data set in order to perform a long-term state-resolved spectral analysis. Six stacked spectra were obtained using 16 years of data from JEM-X (3–25 keV), ISGRI (25–300 keV), and SPI (20–400 keV).
Results.
We extract stacked images in three different energy bands, and detect the source up to 200 keV. In the hardest states, our purely phenomenological approach clearly reveals the presence of an additonnal component > 50 keV in addition to the component usually interpreted as thermal Comptonization. We apply a more physical model of hybrid thermal/nonthermal corona (
EQPAIR
) to characterize this nonthermal component and compare our results with those of previous studies and analyses. Our modeling indicates a more efficient acceleration of electrons in states where major ejections are observed. We also evaluate and find a dependence of the photon index of the power law as a function of the strong orbital modulation of the source in the Flaring InterMediate state. This dependence could be due to a higher absorption when Cygnus X-3 is behind its companion. However, the uncertainties on the density column prevent us from drawing any firm conclusions.
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