RANS-VOF Simulations of Fully Developed Density-Stratified Air-Water Flow in a 3D Rectangular Duct

We perform RANS-VOF simulation of turbulent, fully developed, density-stratified air-water flow in a 3D rectangular duct cross section of height twice the width. Flow through an open or partially-filled duct is characterized by the presence of an air-water interface interacting with a solid wall, forming a mixed-boundary corner. A novel feature of the mixed-boundary corner for turbulent flow is the interaction of wall turbulence with the air-water interface. In the current study, the RANS-VOF equations (for fully developed flow) are solved in a rectangular duct, using periodic inlet/outlet boundary conditions. The flow is completely specified by the (common) driving pressure gradient down the duct and by the fill factor (relative height of the heavier phase to the total height of the duct). Varying the pressure gradient and fill factor results in different flow combinations, namely, laminar air/laminar water, turbulent air/laminar water, turbulent air/turbulent water, laminar air/turbulent water. Since RANS-VOF simulations are computationally less expensive compared to LES and DNS, we systematically investigated a range of flow combinations. The Reynolds stresses are tracked near the mixed-boundary corner for the different flow combinations. The structure of the secondary vortices near the mixed-boundary corner differs from that in the corner formed by the solid vertical and horizontal duct walls.

COUPLING OF A UNIFORM FLOW WITH A RADIAL FLOW

The authors consider the problem of conjugation of a uniform turbulent water flow and a radial flow. The solution uses a simple wave, which allowed us to obtain an analytical solution at all points of the flow. The bottom of the channel, into which the flow flows from a rectangular pipe, is assumed to be horizontal and smooth. The article provides a step-by-step calculation algorithm. The method is intended for use by designers of hydraulic structures.

Continuous in situ measurements of anchor ice formation, growth, and release

In northern rivers, turbulent water becomes supercooled (i.e. cooled to slightly below the freezing point) when exposed to freezing air temperatures. In supercooled turbulent water, frazil (small ice disks) crystals are generated in the water column, and anchor ice starts to form on the bed. Two anchor ice formation mechanisms have been reported in the literature: either by the accumulation of suspended frazil particles, which are adhesive (sticky) in nature, on the riverbed or by in situ growth of ice crystals on the bed material. Once anchor ice has formed on the bed, the accumulation typically continues to grow (due to either further frazil accumulation and/or crystal growth) until release occurs due to mechanical (shear force by the flow or buoyancy of the accumulation) or thermal (warming of the water column which weakens the ice-substrate bond) forcing or a combination of the two. There have been a number of detailed laboratory studies of anchor ice reported in the literature, but very few field measurements of anchor ice processes have been reported. These measurements have relied on either sampling anchor ice accumulations from the riverbed or qualitatively describing the observed formation and release. In this study, a custom-built imaging system (camera and lighting) was developed to capture high-resolution digital images of anchor ice formation and release on the riverbed. A total of six anchor ice events were successfully captured in the time-lapse images, and for the first time, the different initiation, growth, and release mechanisms were measured in the field. Four stages of the anchor ice cycle were identified: Stage 1: initiation by in situ crystal growth; Stage 2: transitional phase; Stage 3: linear growth; and Stage 4: release phase. Anchor ice initiation due to in situ growth was observed in three events, and in the remainder, the accumulation appeared to be initiated by frazil deposition. The Stage 1 growth rates ranged from 1.3 to 2.0 cm/h, and the Stage 2 and 3 growth rates varied from 0.3 to 0.9 cm/h. Anchor ice was observed releasing from the bed in three modes: lifting of the entire accumulation, shearing of layers of the accumulation, and rapid release of the entire accumulation.

Peltae subacquee e specchiature marmoree La forma dell’acqua tra storia dell’arte e filosofia

This paper focuses on two canonical representations of water in 12th-century Venetian churches: (i) the so-called ‘peltae pattern’, usually defined as ‘geometric decoration’ and recognized as the symbol of water; and (ii) the ‘marble slab’, usually included among non-iconic decorations and recognized as il mare. Why did the medieval masters represent the same natural element in the same type of location in these two different ways? Our hypothesis is that (i) represented the turbulent water of terrestrial life, while (ii) represented heavenly water. We argue that support for both claims can be found by retracing the sources of the two decorative models and looking at them from an art historical point of view, and by analyzing them from a philosophical and perceptological standpoint in order to retrieve universal perceptual patterns that can sustain the iconological reading.