1,721,285 research outputs found
Natural grid stretching for DNS of wall-bounded flows
Full text linkWe propose a natural stretching function for DNS of wall-bounded flows, which blends uniform near-wall spacing with uniform resolution in terms of Kolmogorov units in the outer wall layer. Numerical simulations of pipe flow are used to educe optimal value of the blending parameter and of the wall grid spacing which guarantee accuracy and computational efficiency as a result of maximization of the allowed time step. Conclusions are supported by DNS carried out at sufficiently high Reynolds number that a near logarithmic layer in the mean velocity profile is present. Given a target Reynolds number, we provide a definite prescription for the number of grid points and grid clustering needed to achieve accurate results with optimal exploitation of resources
Tancaite-(Ce), a new molybdate from Italy
No full textTancaite-(Ce), ideally FeCe(MoO4)3•3H2O, is a new mineral occurring within cavities in the quartz veins which cut the granite at Su Senargiu, Sarroch (CA), Sardinia, Italy. It is a secondary mineral formed in the oxidation zone of a sulphide ore vein. Associated minerals are quartz, muscovite, molybdenite, pyrite and mendozavilite. Tancaite-(Ce) is red or pale brown in colour, with a vitreous to adamantine lustre. Electron microprobe analyses give (wt.): SiO2 0.34, CaO 0.09, Fe2O3 11.29, SrO 0.02, La2O3 5.04, Ce2O3 10.35, Pr2O3 1.07, Nd2O3 3.66, Sm2O3 0.19, ThO2 2.58, UO2 0.17, MoO3 58.62, H2O* 7.43, sum 100.85. It gives the formula: Fe3+1.03(Ce0.46La0.23Nd0.16Pr0.05Sm0.01U0.01Th0.07)Σ0.99(Mo2.96 Si0.04)Σ3.00O12•3H2.00O, which can be simplified as Fe3+(REE)(MoO4)3•3H2O. The calculated density is 3.834 g/cm3. The presence of H2O was confirmed by micro-Raman spectrometry (stretching and bending vibrations of O-H). The X-ray diffraction pattern of tancaite is characterised by a set of strong reflections, which point to a cubic unit cell, with a 6.80 Å and space group Pm(-3)m, plus a set of weaker reflections. By measuring only the strong reflections with a 4-circle diffractometer, it was possible to solve and refine the “average structure” of tancaite (R 0.032 for 149 unique reflections). The crystal structure consists of FeO6 octahedra centred at the origin of the cubic cell and linked together through MoO4 tetrahedra by corner sharing. The molybdate tetrahedra are statistically distributed in four symmetry-related positions, with 1⁄4 occupancy. In the centre of the unit cell the REE cations exhibit 6+3 coordination, bonding six oxygen atoms and three H2O molecules. The refined structure is clearly an average structure, characterised by statistical disorder which involves the Mo cations, the H2O molecules and the ligands of the REE cations at the centre of the unit cell. Two intensity data collections were performed in order to measure the weaker reflections and to solve the real crystal structure of tancaite. The former was performed at the synchrotron facility Elettra, at the XRD1 beamline; the latter was carried out by means of a CCD-equipped Oxford diffractometer, operating with MoKα radiation. Both sets of data indicated a larger super-cell with respect to the previous cubic cell with a 6.80 Å. One of the possible super-cells showed rhombohedral symmetry, with a 19.2, c 47.4 Å in the hexagonal setting. The new mineral has been approved by the IMA CNMNC (# 2009-097). The name is after Giuseppe Tanca, mineral collector
Transitional and turbulent flows in rectangular ducts: budgets and projection in principal mean strain axes
No full textWe carry out Direct Numerical Simulation (DNS) of flows in closed rectangular ducts with several aspect ratios. The Navier–Stokes equations are discretised through a second-order finite difference scheme, with non-uniform grids in two directions. The duct cross-sectional area is maintained constant as well as the flow rate, which allows to investigate which is the appropriate length scale in the Reynolds number for a good scaling in the laminar and in the fully turbulent regimes. We find that the Reynolds number based on the half length of the short side leads to a critical Reynolds number which is independent on the aspect ratio ((Formula presented.)), for ducts with (Formula presented.). The mean and rms wall-normal velocity profiles are found to scale with the local value of the friction velocity. At high friction Reynolds numbers, the Reynolds number dependence is similar to that in turbulent plane channels, hence flows in rectangular ducts allow to investigate the Reynolds number dependency through a reduced number of simulations. At low Re, the profiles of the statistics differ from those in the two-dimensional channel due to the interaction of flow structures of different size. The projection of the velocity vector and of the Reynolds stress tensor along the eigenvectors of the strain-rate tensor yields reduced Reynolds stress anisotropy and simple turbulence kinetic energy budgets. We further show that the isotropic rate of dissipation is more difficult to model than the full dissipation rate, whose distribution does not largely differ from that of turbulence kinetic energy production. We expect that this information may be exploited for the development of advanced RANS models for complex flows
Marinellite, a new feldspathoid of the cancrinite-sodalite group
No full textMarinellite, [(Na,K to the cancrinite-sodalite group. The crystal structure of a twinned crystal was preliminary refined in space group P31c, but space group P6̄2c could also be possible. It was found near Sacrofano, Latium, Italy, associated with giuseppettite, sanidine, nepheline, haüyne, biotite, and kalsilite. It is anhedral, transparent, colourless with vitreous lustre, white streak and Mohs' hardness of 5.5. The mineral does not fluoresce, is brittle, has conchoidal fracture, and presents poor cleavage on {001}. Dmeas is 2.405(5) g/cm3, Dcalc is 2.40 g/cm3. Optically, marinellite is uniaxial positive, non-pleochroic, ω = 1.495(1), ε = 1.497(1). The strongest five reflections in the X-ray powder diffraction pattern are [d in Å (I) (hkl)]: 3.725 (100) (214), 3.513 (80) (215), 4.20 (42) (210), 3.089 (40) (217), 2.150 (40) (330). The electron microprobe analysis gives K2O 7.94, Na2O 14.95, CaO 5.14, Al2O3 27.80, SiO2 32.73, SO3 9.84, Cl 0.87, (H2O 0.93), sum 100.20 wt %, less O = Cl 0.20, (total 100.00 wt %); H2O calculated by difference. The corresponding empirical formula, based on 72 (Si + Al), is (Na31.86K11.13Ca6.06)Σ=49.05 (Si35.98Al36.02)Σ=72O144.60 (SO4)8.12Cl1.62·3.41H2 O. The crystal structure of marinellite may be described as formed by the stacking along c of 12 layers containing six-membered rings of tetrahedra: the stacking sequence is ABCBCBACBCBC..., where A, B, and C represent the positions of the rings within the layers. Its structure consists of two liottite cages superimposed along [0, 0, z] and of columns of cancrinite and sodalite cages along [1/3, 2/3, z] and [2/3, 1/3, z]. Sulphate groups, surrounded by sodium, calcium and potassium cations, occupy the liottite cages. Chlorine anions and sulphate groups occupy the sodalite cages, whereas H2O molecules are located within the cancrinite cages, bonded to Na cations. This structural model was refined in the space group P31c, conventional R = 0.098 for 2155 reflections. The structural relationships between marinellite and tounkite are discussed
RAMSBECKITE, (CU,ZN)15(OH)22(SO4)4.6H2O, A FIRST OCCURRENCE FOR ITALY FROM LA-VENEZIANA MINE, VALLE-DEI-MERCANTI, VICENZA
No full textRamsbeckite is described from "La Veneziana' mine, Valle dei Mercanti, Vicenza (first occurrence in Italy). It occurs in a limonitic gangue as euhedral emerald-green crystals up to 0.5 mm in size. X-ray diffraction data yield: a = 16.110(8), b = 15.602(4). c = 7.112(2) Å. β = 90.27(3)°, space group P21/a. Dobs + 3.41(1) g/cm3; biaxial negative, 2Vobs = 38(2)° with X.~//c, Y = b, Z//a and nα = 1.669(2), nβ = 1.703(2), nγcalc = 1.707(2). Microprobe analysis (H2O content by difference) results in the chemical formula (Cu10.97Sn4.18)15.15(SO4)4(OH)22.27.4H2
Marinellite, a member of the cancrinite-sodalite supergroup. Forty years of synergy between academic research and field work by amateur groups to deal with this complex group of minerals.
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Cymrite and benstonite from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy): first Italian occurrence
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