History of cosmic evolution with modified Gauss-Bonnet-dilatonic coupled term

Gauss-Bonnet-dilatonic coupling in four dimension plays an important role to explain late time cosmic evolution. However, this term is an outcome of low energy string effective action and thus ought to be important in the early universe too. Unfortunately, phase-space formulation of such a theory does not exist in the literature due to branching. We therefore consider a modified theory of gravity, which contains a nonminimally coupled scalar-tensor sector in addition to higher order scalar curvature invariant term with Gauss-Bonnet-dilatonic coupling. Such an action unifies early inflation with late-time cosmic acceleration. Quantum version of the theory is also well-behaved.Comment: 13 pages, 5 figures, To appear in EPJC (2017

Viability of Noether symmetry of F(R) theory of gravity

Canonization of F(R) theory of gravity to explore Noether symmetry is performed treating R - 6(\frac{\ddot a}{a} + \frac{\dot a^2}{a^2} + \frac{k}{a^2}) = 0 as a constraint of the theory in Robertson-Walker space-time, which implies that R is taken as an auxiliary variable. Although it yields correct field equations, Noether symmetry does not allow linear term in the action, and as such does not produce a viable cosmological model. Here, we show that this technique of exploring Noether symmetry does not allow even a non-linear form of F(R), if the configuration space is enlarged by including a scalar field in addition, or taking anisotropic models into account. Surprisingly enough, it does not reproduce the symmetry that already exists in the literature (A. K. Sanyal, B. Modak, C. Rubano and E. Piedipalumbo, Gen.Relativ.Grav.37, 407 (2005), arXiv:astro-ph/0310610) for scalar tensor theory of gravity in the presence of R^2 term. Thus, R can not be treated as an auxiliary variable and hence Noether symmetry of arbitrary form of F(R) theory of gravity remains obscure. However, there exists in general, a conserved current for F(R) theory of gravity in the presence of a non-minimally coupled scalar-tensor theory (A. K. Sanyal, Phys.Lett.B624, 81 (2005), arXiv:hep-th/0504021 and Mod.Phys.Lett.A25, 2667 (2010), arXiv:0910.2385 [astro-ph.CO]). Here, we briefly expatiate the non-Noether conserved current and cite an example to reveal its importance in finding cosmological solution for such an action, taking F(R) \propto R^{3/2}.Comment: 16 pages, 1 figure. appears in Int J Theoretical Phys (2012

Numerical investigation of a porous media combustor in a small-scale diesel engine

The application of porous media in compression ignition engines has significant effects on its combustion behavior. In this work, a Computational Fluid Dynamics (CFD) analysis of combustion in diesel engine is performed for 100% load, and the effects of porous media addition in the combustion chamber are quantified. With a porosity of 66.7%, silicon carbide is applied as porous media of cylindrical shape in the modified piston bowl in the conventional engine. The combustion analysis outputs include average cylinder-pressure, temperature; Nitrogen Oxides (NOx), mean mixture fraction, turbulent kinetic energy, total energy and modified Peclet number. The results of the CFD study for the cases of non-porous media are validated against the performed baseline experimental analysis, whereas porous media predictions are compared to the state-of-the-art studies available in the literature. In presence of porous media, the average peak pressure and temperature are found to drop by similar to 26 bar and similar to 550 K, respectively, as compared to that of non-porous media. Furthermore, NOx emissions are significantly reduced up to 97%. The generation of turbulent kinetic energy is enhanced by 86% for PM leading to an increment of similar to 36% in the thermal energy conversion than without a porous media. (C) 2019 Elsevier Ltd. All rights reserved