Diffusion in dense supercritical methane from quasi-elastic neutron scattering measurements

Methane, the principal component of natural gas, is an important energy source and raw material for chemical reactions. It also plays a significant role in planetary physics, being one of the major constituents of giant planets. Here, we report measurements of the molecular self-diffusion coefficient of dense supercritical CH4 reaching the freezing pressure. We find that the high-pressure behaviour of the self-diffusion coefficient measured by quasi-elastic neutron scattering at 300 K departs from that expected for a dense fluid of hard spheres and suggests a density-dependent molecular diameter. Breakdown of the Stokes–Einstein–Sutherland relation is observed and the experimental results suggest the existence of another scaling between self-diffusion coefficient D and shear viscosity η, in such a way that Dη/ρ=constant at constant temperature, with ρ the density. These findings underpin the lack of a simple model for dense fluids including the pressure dependence of their transport properties.

Vortex shedding by a circular cylinder in lock-exchange density current

<p>Flow around bluff bodies have been attracting the interest of the research community for more than a century. The physical mechanisms associated with the vortex shedding in the wake of bluff bodies is still of fundamental research interest. However, flow-structure interaction in density currents has not received enough attention. The transient nature of the interaction between the density driven flow and a stationary object constitutes the motivation for the present laboratory study aiming at investigating the vortex generation and fate on the wake of a circular cylinder in a density current.</p><p>The experiments were conducted in a horizontal and rectangular cross-section channel with 3.0 m long, 0.175 m wide and 0.4 m deep. The gravity current was generated using the classic lock-exchange configuration. A sliding stainless-steel gate with 1 mm thickness, sealed by PVC board glued in the sidewall, was positioned at 0.3 m from the left hand side of the channel. The experiment starts when the gate is suddenly removed, leaving the dense fluid to flow along the bottom of the channel, while the ambient fluid moves above in the opposite direction. The dense fluid consists in a mixture of fresh water and salt while the ambient fluid is a solution fresh water and ethanol (96%). The amount of salt and alcohol added in each mixture was determined in order to obtain a given density difference and to ensure the same refractive index in both fluids. Two different currents were tested with reduced gravity equal to 0.06 ms<sup>-2</sup> and 0.24 ms<sup>-2</sup>. For each test ten repetitions were carried out. Instantaneous velocity maps were acquired with a Particle Image Velocimetry system at 15 Hz. Polyamide seeding particles of density equal to 1.03 were added in both dense and ambient fluids.</p><p> The Reynolds number varied between 1500 and 4000. The results show that vortex shedding varies as the current reaches and overtakes the cylinder. Boundary layer detachment and shear instability is initiated shortly before the snout reaches the cylinder. A pattern of well-defined symmetrical vortexes is formed as a result of the initial shear instability. As the head of the current engulfs the cylinder, stronger turbulence diffusion contributes to reduce vortex coherence. Vortexes are smaller and detach sooner, while is not clear if shedding is alternate or simply random. The formation length is smaller than that of a steady flow with the same Re. When the back of the current passes, the formation length is increased and vortex shedding becomes periodical again. A striking feature is that the Von Kármán street is frequently symmetrical rather than exhibiting a pattern of alternate vortices.</p><p>This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).</p>

Gravity induced vertical motion of dense fluids into saturated granular beds

<p>Gravity currents propagating over and within porous layers occurs in natural environments and in industrial processes. The particular modes by which the dense fluid flows into the porous layer is a subject that is not sufficiently understood. To overcome this research gap, we conducted laboratory experiments aimed at describing experimentally the dynamics of the drainage flow.</p><p>The experiments were conducted in a horizontal channel with a rectangular cross-section. The channel is 3.0 m long, 0.05 m wide. The porous bottom was composed of 5 cm and 10 cm layers of 3 mm borosilicate spheres – unimodal bed – and of a mixture of 3 mm (50% in weight) and 5 mm spheres (50%) – bi-modal bed. The porosity of the unimodal bed ranged between 0.60 and 0.64 (compatible with loose packing). The porosity of the bi-modal bed ranged between 0.61 and 0.65. All gravity currents were generated by releasing suddenly denser fluid locked by a thin vertical barrier placed at 0.2 m from the channel end. The dense fluid consists in a mixture of freshwater and salt (coloured with Rhodamine) while the ambient fluid is a solution of freshwater and ethanol. The density difference between the ambient fluid and the current, and the need to maintain the same refractive index, determine the amount of salt and alcohol added in each mixture. Here we report the findings of currents with a reduced gravity of 0.06 ms<sup>-2</sup>.</p><p>Each experiment was recorded by an high-speed camera with a frame-rate of 386 Hz and a resolution of 2320 x 1726 pxxpx. Measurements were based on light absorption techniques: a LED light panel 0.3 m high and 0.61 m long was used as back illumination. All images were calibrated to ascribe, pixel by pixel, a concentration value from a 8 bit gray level. Different calibrations were performed for the porous layer and for the surface current.</p><p>Results show that, in the slumping phase, the gravity current flows with velocities compatible with those over rough beds. As the current progresses further attenuation of momentum is noticed owing to mass loss to the porous bed.</p><p>The flow in the porous bed reveals plume instability akin to a Saffman-Taylor instability. The growth of the plumes seems independent from the initial fluid height in both types of porous beds. The wavelength and the growth rate of the plumes depends on the bed material. Plumes grow faster in the case of the bi-modal bed and the wavelength of the bi-modal bed is about 1.5 as that of the unimodal bed. It is hypothesised that the gravity-induced porous flow is best parameterized by a Péclet number defined as a ratio of dispersive (mechanical diffusion) and advective modes of transport. Smaller wavelengths and slower growths are attained for stronger dispersion, characterisitic of the unimodal bed. For bimodal beds, permeability is larger, and thus also advection. This causes the flow to concentrate in faster growing but farther apart plumes.</p><p> </p><p>This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).</p>

Confinement-induced stabilization of the Rayleigh-Taylor instability and transition to the unconfined limit

The prevention of hydrodynamic instabilities can lead to important insights for understanding the instabilities’ underlying dynamics. The Rayleigh-Taylor instability that arises when a dense fluid sinks into and displaces a lighter one is particularly difficult to arrest. By preparing a density inversion between two miscible fluids inside the thin gap separating two flat plates, we create a clean initial stationary interface. Under these conditions, we find that the instability is suppressed below a critical plate spacing. With increasing spacing, the system transitions from the limit of stability where mass diffusion dominates over buoyant forces, through a regime where the gap sets the wavelength of the instability, to the unconfined regime governed by the competition between buoyancy and momentum diffusion. Our study, including experiment, simulation, and linear stability analysis, characterizes all three regimes of confinement and opens new routes for controlling mixing processes.

Cystic lymphangioma of the jejunal mesentery in a young adult: a rare case report

Lymphangioma is an uncommon congenital malformation of the lymphatic system that manifest as a benign tumor. They are most commonly seen in children and rarely in adults. Lymphangiomas in the peritoneal cavity are extremely rare in adults, comprising of less than 1% of all lymphangiomas. We report a case of 18-year-old healthy female who was diagnosed with cystic mesenteric lymphangioma of the small intestine on CT scan as a part of her routine health check-up. CT scan reported a large well-defined cystic mass in mesentery of the small intestine filled with a hyper dense fluid. The lesion had thin capsule and internal septa. Radiological diagnosis was confirmed intra-operatively on laparoscopy. The patient underwent laparotomy with resection of the cyst and the involved jejunum segment followed by end to end anastomosis. The specimen was sent for histo-pathological examination and the findings were consistent with the diagnosis of cystic lymphangioma of the jejunal mesentery. Cystic mesenteric lymphangiomas are rare entities in a clinical setting that can be easily missed on clinical examination or may present as asymptomatic disease. CT scan is the gold standard investigation for diagnosis of mesenteric lymphangiomas. The preferred surgical approach depends on the location of the lesion, the clinical features and the spread of the lesion to surrounding structures.