arXiv subjects

A. Paglietti

Publications and source records attributed to A. Paglietti.

6 recordsLinked to original sources

arXiv2023

On the impossibility of isothermal heat transfer and its implications for thermal physics

The physical impossibility of heat transfer under isothermal conditions implies that the classical expression for the entropy of the ideal gas may not be compatible with the internal energy of the gas itself. A corrected expression of the ideal gas entropy is derived here. It is independent on volume. This result is shown to be at a variance with the statistical interpretations of entropy as a quantity that is related to the number of microstates compatible with the macroscopic state of the system. It also offers a better understanding of the thermodynamic notion of entropy. The present analysis also establishes a general equation that links the entropy of a system to its internal energy.

cond-mat.stat-mech↗

arXiv2023

Conservation of angular momentum and the existence of absolute time and space

The law of balance of angular momentum is shown to imply the existence of absolute time, a fundamental physical quantity that is independent of the motion or position of the observer. Absolute time implies the notion of absolute simultaneity, which in turn leads to the notion of absolute distance between two points. The existence of absolute space follows as a consequence. These concepts apply to every field of physics to which the angular momentum balance law applies, and in particular to the theory of special relativity. The paper also shows that in a vacuum, the independence of the speed of light from the motion of its source makes it possible to determine the absolute positions of all points in space. The same independence also allows us to determine the state of absolute rest or motion of a reference frame from within the frame itself.

physics.gen-ph↗

arXiv2020

Angular momentum balance and vortex production in wall-bounded flows

To produce a vortex, a torque must be applied to the fluid. In viscous fluids, the torques that produce turbulent vortices result from the loss of symmetry of the stress tensor, once the viscous friction exceeds the shear stress resistance of the fluid. In wall-bounded flows, in particular, the turbulent vortices form in a thin layer of fluid adjacent to the wall, practically coinciding with the so-called viscous sublayer, where the viscous friction reaches the largest values. The present paper determines a vortex structure for this sublayer, consistent with the well-known linearity of the diagram of the mean streamwise velocity of this region. The analysis enables us to calculate the diameter, angular velocity, and interaxis of the vortices in the viscous sublayer in steady-state conditions. The lifting force that makes the vortices migrate from the wall towards the mainstream flow is determined, and the crucial role played by gyroscopic precession in the reorientation of the vortex axis is discussed.

physics.gen-ph↗

arXiv2017

The laminar-to-turbulent transition in viscous fluid flow

The onset of turbulence in laminar flow of viscous fluids is shown to be a consequence of the limited capacity of the fluid to withstand shear stress. This fact is exploited to predict the flow velocity at which laminar flow becomes turbulent and to calculate, on a theoretical basis, the corresponding critical value of the Reynolds number. A constitutive property essential to the present analysis is the ultimate shear stress of the fluid. The paper shows how this stress can be determined experimentally from a test in plane Couette flow. For water at 20 °C, the value of the ultimate shear stress is calculated from the experiments reported in the literature. This value is then is employed to predict the Reynolds number corresponding to the onset of turbulence in Taylor-Couette flow and in pipe flow of circular cross section. The results are realistic and their significance is assessed critically. The procedure can be applied to predict the onset of turbulence in any non-turbulent flow, provided that the velocity field of the flow is known.

physics.gen-ph↗

arXiv2009

Interaction with thermal radiation in the free expansion and mixing of ideal gases and Gibbs' paradox in classical thermodynamics

The standard theory of ideal gases ignores the interaction of the gas particles with the thermal radiation (photon gas) that fills the otherwise vacuum space between them. This is an unphysical feature of the theory since every material in this universe, and hence also the particles of a gas, absorbs and radiates thermal energy. The interaction with the thermal radiation that is contained within the volume of the body may be important in gases since the latter, unlike solids and liquids, are capable of undergoing conspicuous volume changes. Taking this interaction into account makes the behaviour of the ideal gases more realistic and removes Gibbs' paradox.

physics.class-ph↗

arXiv2009

Thermal Radiation Effect in the Free Expansion of an Ideal Gas and Gibbs' Paradox in Classical Thermodynamics

The standard theory of ideal gases ignores the interaction of the gas particles with the thermal radiation (photon gas) that fills the otherwise vacuum space between them. This is an unphysical feature since every material absorbs and radiates thermal energy. This interaction may be important in gases since the latter, unlike solids and liquids are capable of undergoing conspicuous volume changes. Taking it into account makes the behaviour of the ideal gases more realistic and removes Gibbs' paradox.

physics.class-ph↗