Cosmology and stellar equilibrium using Newtonian hydrodynamics with general relativistic pressure

We revisit the analysis made by Hwang and Noh [JCAP 1310 (2013)] aiming the construction of a Newtonian set of equations incorporating pressure effects typical of the General Relativity theory. We explicitly derive the Hwang-Noh equations, comparing them with similar computations found in the literature. Then, we investigate $i)$ the cosmological expansion, $ii)$ linear cosmological perturbations theory and $iii)$ stellar equilibrium by using the new set of equations and comparing the results with those coming from the usual Newtonian theory, from the Neo-Newtonian theory and from the General Relativity theory. We show that the predictions for the background evolution of the Universe are deeply changed with respect to the General Relativity theory: the acceleration of the Universe is achieved with positive pressure. On the other hand, the behaviour of small cosmological perturbations reproduces the one found in the relativistic context, even if only at small scales. We argue that this last result may open new possibilities for numerical simulations for structure formation in the Universe. Finally, the properties of neutron stars are qualitatively reproduced by Hwang-Noh equations, but the upper mass limit is at least one order of magnitude higher than the one obtained in General Relativity.Comment: 15 pages, 4 figures. Section 2 greatly extended with a post-Newtonian analysis. Final results strengthe

Perturbative analysis of generalized Einstein's theories

The hypothesis that the energy-momentum tensor of ordinary matter is not conserved separately, leads to a non-adiabatic expansion and, in many cases, to an Universe older than usual. This may provide a solution for the entropy and age problems of the Standard Cosmological Model. We consider two different theories of this type, and we perform a perturbative analysis, leading to analytical expressions for the evolution of gravitational waves, rotational modes and density perturbations. One of these theories exhibits satisfactory properties at this level, while the other one should be discarded.Comment: 14 pages, Latex fil

Multiple-scale turbulence modeling of boundary layer flows for scramjet applications

As part of an investigation into the application of turbulence models to the computation of flows in advanced scramjet combustors, the multiple-scale turbulence model was applied to a variety of flowfield predictions. The model appears to have a potential for improved predictions in a variety of areas relevant to combustor problems. This potential exists because of the partition of the turbulence energy spectrum that is the major feature of the model and which allows the turbulence energy dissipation rate to be out of phase with turbulent energy production. The computations were made using a consistent method of generating experimentally unavailable initial conditions. An appreciable overall improvement in the generality of the predictions is observed, as compared to those of the basic two-equation turbulence model. A Mach number-related correction is found to be necessary to satisfactorily predict the spreading rate of the supersonic jet and mixing layer

Density perturbations in an Universe dominated by the Chaplygin gas

We study the fate of density perturbations in an Universe dominate by the Chaplygin gas, which exhibit negative pressure. We show that it is possible to obtain the value for the density contrast observed in large scale structure of the Universe by fixing a free parameter in the equation of state of this gas. The negative character of pressure must be significant only very recently.Comment: Latex file, 5 page

Regularity and stability of electrostatic solutions in Kaluza-Klein theory

We investigate the family of electrostatic spherically symmetric solutions of the five-dimensional Kaluza-Klein theory. Besides black holes and wormholes, a new class of geodesically complete solutions is identified. A monopole perturbation is carried out, enabling us to prove analytically the stability of a large class of solutions, including all black holes and neutral solutions.Comment: 2 pages, "mprocl.sty" with LATEX 2.09, contribution to the 9th Marcel Grossmann meeting (MG9), Rome, July 200

Electrostatic solutions in Kaluza-Klein theory: geometry and stability

We investigate the family of electrostatic spherically symmetric solutions of the five-dimensional Kaluza-Klein theory. Both charged and neutral cases are considered. The analysis of the solutions, through their geometrical properties, reveals the existence of black holes, wormholes and naked singularities. A new class of regular solutions is identified. A monopole perturbation study of all these solutions is carried out, enabling us to prove analytically the stability of large classes of solutions. In particular, the black hole solutions are stable, while for the regular solutions the stability analysis leads to an eigenvalue problem.Comment: Latex file, 21 page

Thermodynamic Losses in a Gas Spring: Comparison of Experimental and Numerical Results

Reciprocating-piston devices can be used as high-efficiency compressors and/or expanders. With an optimal valve design and by carefully adjusting valve timing, pressure losses during intake and exhaust can be largely reduced. The main loss mechanism in reciprocating devices is then the thermal irreversibility due to the unsteady heat transfer between the compressed/expanded gas and the surrounding cylinder walls. In this paper, pressure, volume and temperature measurements in a piston-cylinder crankshaft driven gas spring are compared to numerical results. The experimental apparatus experiences mass leakage while the CFD code predicts heat transfer in an ideal closed gas spring. Comparison of experimental and numerical results allows one to better understand the loss mechanisms in play. Heat and mass losses in the experiment are decoupled and the system losses are calculated over a range of frequencies. As expected, compression and expansion approach adiabatic processes for higher frequencies, resulting in higher efficiency. The objective of this study is to observe and explain the discrepancies obtained between the computational and experimental results and to propose further steps to improve the analysis of the loss mechanisms