Narrow Results

Lattice Boltzmann (LB) models for ideal gases retrieve the Navier-Stokes equation in the incompressible limit. Nevertheless, the high values of the isothermal compressibility introduce serious drawbacks. In the simulation of liquid flows through porous media, the high pressure gradients through throats and tortuous paths produce significant errors when conventional LB models are used, especially when the interest is the transient flow. In this work a significant reduction of the isothermal compressibility is reached by adjusting the equilibrium distribution moments to those obtained imposing a van der Waals pressure-density dependence and adopting a lattice with a high number of speeds. Simulation results for the shock tube problem, the velocity step problem and the transient pressure response in a tortuous channel are presented and compared with available analytical results.

In this paper we study in detail the sound speed — , which is directly related to the classical perturbations — of dynamical dark energy (DE). We consider especially the case with an equation of state crossing the cosmological constant boundary, and show its implications for cosmic microwave background (CMB) anisotropy. With the present observational data on the CMB, the Type Ia supernova (SNIa) and galaxy clustering, we perform a global analysis to constrain the sound speed of DE, using the Markov chain Monte Carlo method. We find that the sound speed of DE is weakly constrained by current observations, and thus futuristic precision measurements of the CMB on a very large angular scale (low multipoles) are necessary.

The problem of the cosmic acceleration is here revisited by using the fact that the adiabatic speed of sound can be assumed to be negligible small. Within the context of general relativity, the total energy budget is recovered under the hypothesis of a vanishing speed of sound by assuming the existence of one fluid only. We find a cosmological model which reproduces the main results of the ΛCDM paradigm at late-times, showing an emergent cosmological constant, which is not at all related with the vacuum energy term. As a consequence, the model presented here behaves as a unified dark energy (DE) model.

The acoustic properties of the supraspinatus tendon insertions of three adult Japanese white rabbits (male, 3.2–3.5 kg) were measured by scanning acoustic microscopy (SAM). After cutting parallel to the supraspinatus tendon fibers, specimens were fixed with 10% neutralized formalin, embedded in paraffin and sectioned at 5 μm thickness. The sound speed and the attenuation constant along the articular surface of the tendon from the insertion site to the musculotendinous junction were measured by SAM. The two-dimensional distribution of the acoustic properties was displayed with color-coded scales and was compared to the histologic structure. Toluidine-blue staining and immunohistochemical staining against type II collagen were performed to determine the presence of fibrocartilage besides a standard Elastica–Masson staining to assess the overall histologic structure.

The distribution of the sound speed and the attenuation constant showed an almost identical pattern. Both parameters were lower in the non-mineralized fibrocartilage than those in the tendon proper. The area indicating low sound speed corresponded well to the area of metachromasia and type II collagen. The non-mineralized fibrocartilage indicated softer material properties than the tendon proper, which could be interpreted as an adaptation for the various biomechanical stresses at the insertion site.

We report on progress in observing the hydrogen-bonded structure of water and sea water by deep-ocean Raman spectroscopy. In the normal ocean, the abundance of single H₂O species is less than 20%. The principal form is the tetrahedral pentamer, and the (H₂O)${}_{\text{n}}\leftrightarrow$ nH₂O equilibrium plays a dominant role in controlling physicochemical processes and the speed of sound. The enthalpy of the H-bond in water is 2.624 kcal/mol, and in sea water 2.258 kcal/mol due to the quantity of non-HB water in the solvation shell of sea salt ions. The activation energy of viscous flow is 4.28 kcal/mol at one atmosphere pressure, consistent with the need to break hydrogen bonds and overcome the van der Waals intermolecular forces. The activation energy decreases exponentially with depth/pressure and is 10% less at 4,000 m depth. We suggest this is due to the pressure-induced shift in the water equilibrium to favor the less compressible quasi-planar pentamer, tetramer, and so on forms. We address the long-standing conclusion that microbial swimming by means of rotating flagella is only 1%–2% efficient. This neglects the need for microbes to overcome the activation energy of viscous flow; $\sim$95% of the mechanical energy produced by the flagella must be used to break H-bonds and overcome intermolecular forces.

A two-fluids linearized model was expressed for incompressible and compressible assumptions. The amplitude comparison showed significant incompressible first order waves and compressible second and third order waves with propagation speeds closely related to both the mixture flow velocity and the sound speed in the disperse medium.