Entropy Perturbations in Low-Density Universe Models

We study adiabatic ~curvature! and entropy ~isocurvature! perturbations produced during a period of cosmological inflation that is driven by multiple scalar fields with an arbitrary interaction potential. A local rotation in field space is performed to separate out the adiabatic and entropy modes. The resulting field equations show explicitly how on large scales entropy perturbations can source adiabatic perturbations if the background solution follows a curved trajectory in field space, and how adiabatic perturbations cannot source entropy perturbations in the long-wavelength limit. It is the effective mass of the entropy field that determines the amplitude of entropy perturbations during inflation. We present two applications of the equations. First, we show why one in general expects the adiabatic and entropy perturbations to be correlated at the end of inflation, and calculate the cross correlation in the context of a double inflation model with two non-interacting fields. Second, we consider two-field preheating after inflation, examining conditions under which entropy perturbations can alter the large-scale curvature perturbation and showing how our new formalism has advantages in numerical stability when the background solution follows a non-trivial trajectory in field space.

The role of cosmic-ray-modified contact discontinuities and pressure balance structures in two-fluid cosmic-ray hydrodynamics in one Cartesian space dimension are investigated by means of analytic and numerical solution examples, as well as by weakly nonlinear asymptotics. The fundamental wave modes of the two-fluid cosmic-ray hydrodynamic equations in the long-wavelength limit consist of the backward and forward propagating cosmic-ray-modified sound waves, with sound speed dependent on both the cosmic-ray and thermal gas pressures; the contact discontinuity; and a pressure balance mode in which the sum ofthe cosmic ray and thermal gas pressure perturbations is zero. The pressure balance mode, like the contact discontinuity is advected with the background flow. The interaction of the pressure balance mode with the contact discontinuity is investigated by means of the method of multiple scales. The thermal gas and cosmic-ray pressure perturbations satisfy a linear diffusion equation, and entropy perturbations arising from nonisentropic initial conditions for the thermal gas are frozen into the fluid. The contact discontinuity and pressure balance eigenmodes both admit nonzero perturbations in the thermal gas, whereas the cosmic-ray-modified sound waves are isentropic. The total entropy perturbation is shared between the contact discontinuity and pressure balance eigenmodes, and examples are given in which there is a transfer of entropy between the two modes. In particular, N-wave type density disturbances are obtained which arise as a result of the entropy transfer between the two modes. A weakly nonlinear geometric optics perturbation expansion is used to study the long timescale evolution of the short-wavelength entropy wave and the thermal gas sound waves in a slowly varying, large-scale background flow. The weakly nonlinear geometric optics expansion is also used to generalize previous studies of squeezing instability for short-wavelength sound waves in the two fluid model, by including a weakly nonlinear wave steepening term that leads to shock formation, as well as the effect of long time and space dependence of the background flow. Implications of cosmic-ray-modified pressure balance structures and contact discontinuities in models of the interaction of traveling interplanetary shocks and compression and rarefraction waves with the solar wind termination shock are briefly discussed.

When can preheating affect the CMB?

In order to study the heat exchanger simulation and recovery device design of waste heat boiler of gas turbine generator set on Ocean platform, the theory of thermal cycle of gas turbine and the principle of steam turbine are introduced. The waste heat of exhaust gas of a certain type of gas turbine generator set on Ocean platform is taken as the object, to design the waste heat recovery device of this type of gas turbine generator set. According to the characteristics of the selected waste heat boiler, the mathematical model is reasonably simplified, and the steady-state simulation model of the waste heat boiler is established by using the computer simulation software, thus forming the whole model of the waste heat boiler. It is attempted to recover the waste heat from the exhaust gas of gas turbine generator set in Ocean platform by using gas-steam combined cycle power generation, which has certain reference significance for the design, application and improvement of a common gas turbine waste heat recovery device on Ocean platform. The established steady-state model simulation model and thermal efficiency calculation model have good accuracy, and have a certain auxiliary role in design calculation and performance research.

Abstract. The response of clouds to sudden decreases in the flux of galactic cosmic rays (GCR) – Forbush decrease events – has been investigated using cloud products from the space-borne MODIS instrument, which has been in operation since 2000. By focusing on pristine Southern Hemisphere ocean regions we examine areas where we believe that a cosmic ray signal should be easier to detect than elsewhere. While previous studies have mainly considered cloud cover, the high spatial and spectral resolution of MODIS allows for a more thorough study of microphysical parameters such as cloud droplet size, cloud water content and cloud optical depth, in addition to cloud cover. Averaging the results from the 22 Forbush decrease events that were considered, no statistically significant correlations were found between any of the four cloud parameters and GCR, when autocorrelations were taken into account. Splitting the area of study into six domains, all of them have a negative correlation between GCR and cloud droplet size, in agreement with a cosmic ray – cloud coupling, but in only one of the domains (eastern Atlantic Ocean) was the correlation statistically significant. Conversely, cloud optical depth is mostly negatively correlated with GCR, and in the eastern Atlantic Ocean domain that correlation is statistically significant. For cloud cover and liquid water path, the correlations with GCR are weaker, with large variations between the different domains. When only the six Forbush decrease events with the largest amplitude (more than 10% decrease) were studied, the correlations fit the hypothesis slightly better, with 16 out of 24 correlations having the expected sign, although many of the correlations are quite weak. Introducing a time lag of a few days for clouds to respond to the cosmic ray signal the correlations tend to become weaker and even to change sign.

The Met Office Unified Model is run for a case observed during Intensive Observation Period 18 (IOP18) of the Convective Storms Initiation Project (CSIP). The aims are to identify the physical processes that lead to perturbation growth at the convective scale in response to model‐state perturbations and to determine their sensitivity to the character of the perturbations. The case is strongly upper‐level forced but with detailed mesoscale/convective‐scale evolution that is dependent on smaller‐scale processes. Potential temperature is perturbed within the boundary layer. The effects on perturbation growth of both the amplitude and typical scale length of the perturbations are investigated, and perturbations are applied either sequentially (every 30 minutes throughout the simulation) or at specific times.The direct effects (within one time step) of the perturbations are to generate propagating Lamb and acoustic waves and produce generally small changes in cloud parameters and convective instability. In exceptional cases a perturbation at a specific grid point leads to switching of the diagnosed boundary‐layer type or discontinuous changes in convective instability, through the generation or removal of a lid. The indirect effects (during the entire simulation) are changes in the intensity and location of precipitation and in the cloud‐size distribution. Qualitatively different behaviour is found for strong (1 K amplitude) and weak (0.01 K amplitude) perturbations, with faster growth after sunrise found only for the weaker perturbations. However, the overall perturbation growth (as measured by the root‐mean‐square error of accumulated precipitation) reaches similar values at saturation, regardless of the perturbation characterization. Copyright © 2010 Royal Meteorological Society and Crown Copyright.

We study axisymmetric, free oscillations driven by gravity and surface tension at the interface of two viscous, immiscible, radially unbounded fluids, analytically and numerically. The interface is perturbed as a zeroth-order Bessel function (in space) and its evolution is obtained as a function of time. In the linearised approximation, we solve the initial value problem (IVP) to obtain an analytic expression for the time evolution of wave amplitude. It is shown that a linearised Bessel mode temporally evolves in exactly the same manner as a Fourier mode in planar geometry. We obtain novel analytical expressions for the time varying vorticity and pressure fields in both fluids. For small initial amplitudes, our analytical results show excellent agreement with those obtained from solving the axisymmetric Navier–Stokes equations numerically. We also compare our results with the normal mode approximation and find the latter to be an accurate representation at very early and late times. The deviation between the normal mode approximation and the IVP solution is found to increase as a function of viscosity ratio. The vorticity field has a jump discontinuity at the interface and we find that this jump depends on the viscosity and the density ratio of the two fluids. Upon increasing the initial perturbation amplitude in the simulations, nonlinearity produces qualitatively new features not present in the analytical IVP solution. Notably, a jet is found to emerge at the axis of symmetry rising to a height greater than the initial perturbation amplitude. Increasing the perturbation amplitude further causes the jet to undergo end pinch off, giving birth to a daughter droplet. This can happen either for an advancing or a receding jet, depending on the viscosity ratio. A relation is found between the maximum jet height and the perturbation amplitude. Hankel transform of the interface demonstrates that at large perturbation amplitudes higher wavenumbers emerge, sharing some of the energy of the lowest mode. When these additional higher modes are present, the interface has pointed crests and rounded troughs.

We experimentally and numerically study the dynamics of a liquid jet issued from a rotating orifice, whose breakup is regulated by a vibrating piezo element. The helical trajectory of the spiralling jet yields fictitious forces varying along the jet whose longitudinal projections stretch and thin the jet, affecting the growth of perturbations. We show that by quantifying these fictitious forces, one can estimate the jet intact length and size distribution of drops formed at jet breakup. The presence of the locally varying fictitious forces may render high-frequency perturbations, that would otherwise be stable in the abscence of stretching, unstable, as observed similarly in the case of straight jets stretching under gravity. The perturbation amplitude then dictates how strong the perturbation is coupled to the jet compared with random noise that is inherently present in any experimental set-up. In the present study we exploit the slenderness of the jet to separate the calculation of the base flow and the growth of perturbations. The fictitious forces calculated from the base flow trajectory are then used in a nonlinear slender-jet model, which treats the spiralling jet as a quasi-straight jet with locally varying body forces. We show both experimentally and numerically that jet breakup characteristics (e.g. intact length and drop size distribution) can be controlled by finite-amplitude perturbations created by mechanically induced pressure modulations. Finally, we revisit the integrated net gain approach developed for straight jets under gravity and we provide simple analogous relations for spiralling jets.

In this study, we present the evolution of cosmological perturbations in a universe consisting of standard matter and interacting vacuum. We use the 1 + 3 covariant formalism in perturbation framework and consider two different models for the interacting vacuum; namely, a linear interacting model and interaction with creation pressure model. For both models, we derive the evolution equations governing the growth of linear perturbations for both radiation- and dust-dominated universe. We find numerical solutions in appropriate limits, namely long and short wavelengths. For both models, the perturbations grow with time (decay with redshift), showing that structure formation is possible in an accelerated cosmic background. The perturbation amplitudes — and their relative scalings with those of [Formula: see text]CDM — depend on the values of the interaction parameters considered, and in a way that can be used to constrain the models using existing and future large-scale structure data. In the vanishing limits of the coupling parameters of the interaction, we show that standard [Formula: see text]CDM cosmology, both background and perturbed, is recovered.

We extensively study the evolution and distinct signatures of cosmological models, in which dark energy interacts directly with dark matter. We first focus on the imprints of these coupled models on the cosmic microwave background temperature power spectrum, in which we discuss the multipole peak separation together with the integrated Sachs-Wolfe effect. We also address the growth of matter perturbations, and disentangle the interacting dark energy models using the expansion history together with the growth history. We find that a disformal coupling between dark matter and dark energy induces intermediate-scales and time-dependent damped oscillatory features in the matter growth rate function, a unique characteristic of this coupling. Apart from the disformal coupling, we also consider conformally coupled models, together with models which simultaneously make use of both couplings.

The cosmic stories : beginning, evolution, and present days of the universe

We study the growth of energy-density perturbations in inflationary-universe models, applying an extension of Bardeen's gauge-invariant framework derived in a previous paper. The complete analysis is exemplified in the case of the ''new inflationary universe'' of Linde and Albrecht and Steinhardt. For this model we obtain the following result: the amplitude of energy-density fluctuations at horizon crossing is of order 50, far too large to match with the usual pictures of galaxy formation. Our result agrees with other published analyses. We conclude that the amplification of energy-density perturbations is determined by the change in the equation of state between initial and final Hubble radius crossing. It is, however, independent of the phase structure between the two crossings. In particular, it is independent of the reheating mechanism. We also derive a simple formula which describes the complete evolution of perturbations outside the horizon whenever entropy and anisotropic stress perturbations are unimportant. Finally, we comment on previous published methods and discuss their limitations.

Efficient and cost-effective harvesting of exhaust waste heat represents significant opportunities for improving the overall energy efficiency of combined heat and power cycles, transportation vehicles, and industrial processes. Pyroelectric thermal energy harvesting is interesting for its potentially high efficiency. We report a design concept and related thermal models for harvesting of exhaust waste heat based on the pyroelectric effect. The design concept utilizes switchable thermal interfaces formed on duct walls of exhaust gas and liquid coolants to achieve temporal thermal cycling of pyroelectric materials. Our thermal model explicitly accounts for spatial variations in the exhaust gas and coolant temperatures for practical waste heat harvesting systems, for example, those that can be incorporated into micro-CHP (combined heat and power) units. Model prediction results for example cases are presented to illustrate the impact of design parameters, including the heat capacity rate and the convective heat transfer coefficients.

A key issue for inertial confinement fusion (ICF) is the hydrodynamic stability of the imploding capsule. Imperfections on the capsule surface can grow into large perturbations that degrade capsule performance. Understanding this process is crucial if the authors are to successfully predict requirements for future high-gain ICF capsules. Experiments on the Nova laser at Lawrence Livermore National Laboratory have directly measured perturbation growth on planar foils, and three experimental groups have investigated backlit perturbation growth using imploding spheres. In addition to these efforts, which concentrate on indirectly driven implosions, is work investigating the hydrodynamic stability of directly driven ICF capsules. In these direct-drive experiments the laser light shines directly on the capsules, causing the implosion and providing the seed for perturbation growth. This article reports measurement, via emission from spectroscopic tracers, of the full process of perturbation growth leading to pusher-fuel mix in spherical implosions, and shows perturbation growth dependence on initial perturbation amplitude and wavelength. In contrast to the cited direct-drive work, the authors have in this experiment separated the drive from the perturbation seed.

The observations and research on the neutrinos provide a kind of indirect way of revealing the properties of dark matter particles. For the detection of muon neutrinos, the main issue is the large atmospheric background, which is caused by the interactions between the cosmic rays and atoms within the atmosphere. Compared with muon neutrinos, tau neutrinos have a smaller atmospheric background especially for the downward-going direction. Except for the classical neutrino sources, dark matter particles can also annihilate into the neutrinos and are the potential high energy astrophysical sources. The annihilation rate of dark matter particles is proportional to the square of number density; therefore, the annihilation rate is large near the center of dark matter halos especially for the new kind of dark matter structures named ultracompact dark matter minihalos (UCMHs). In previous works, we have investigated the potential muon neutrino flux from UCMHs due to dark matter annihilation. Moreover, since the formation of UCMHs is related to the primordial density perturbations of small scales, we get the constraints on the amplitude of the primordial curvature perturbations of small scales, $1 \lesssim k \lesssim 10^{7} ~\rm Mpc^{-1}$. In this work, we focus on the downward-going tau neutrinos from UCMHs due to dark matter annihilation. Compared with the background of tau neutrino flux we get the constraints on the mass fraction of UCMHs. Then using the limits on the mass fraction of UCMHs we got the constraints on the amplitude of the primordial curvature perturbations which are extended to the scale $k \sim 10^{8} ~ \rm Mpc^{-1}$ compared with previous results.