CFD simulations of hydrocyclones with an air core - Comparison between large eddy simulations and a second moment closure

CFD simulations of the 75 mm, hydrocyclone of Hsieh (1988) have been conducted using Fluent TM. The simulations used 3-dimensional body fitted grids. The simulations were two phase simulations where the air core was resolved using the mixture (Manninen et al., 1996) and VOF (Hirt and Nichols, 1981) models. Velocity predictions from large eddy simulations (LES), using the Smagorinsky-Lilly sub grid scale model (Smagorinsky, 1963; Lilly, 1966) and RANS simulations using the differential Reynolds stress turbulence model (Launder et al., 1975) were compared with Hsieh's experimental velocity data. The LES simulations gave very good agreement with Hsieh's data but required very fine grids to predict the velocities correctly in the bottom of the apex. The DRSM/RANS simulations under predicted tangential velocities, and there was little difference between the velocity predictions using the linear (Launder, 1989) and quadratic (Speziale et al., 1991) pressure strain models. Velocity predictions using the DRSM turbulence model and the linear pressure strain model could be improved by adjusting the pressure strain model constants.

Estimation of concentration-dependent diffusion coefficient in drying process from the space-averaged concentration versus time with experimental data

The estimation of a concentration-dependent diffusion coefficient in a drying process is known as an inverse coefficient problem. The solution is sought wherein the space-average concentration is known as function of time (mass loss monitoring). The problem is stated as the minimization of a functional and gradient-based algorithms are used to solve it. Many numerical and experimental examples that demonstrate the effectiveness of the proposed approach are presented. Thin slab drying was carried out in an isothermal drying chamber built in our laboratory. The diffusion coefficients of fructose obtained with the present method are compared with existing literature results.

An integral equation method for a mixed initial boundary value problem for unsteady Stokes system in a doubly-connected domain

We present a novel numerical method for a mixed initial boundary value problem for the unsteady Stokes system in a planar doubly-connected domain. Using a Laguerre transformation the unsteady problem is reduced to a system of boundary value problems for the Stokes resolvent equations. Employing a modied potential approach we obtain a system of boundary integral equations with various singularities and we use a trigonometric quadrature method for their numerical solution. Numerical examples are presented showing that accurate approximations can be obtained with low computational cost.

Transition of planar Couette flow at infinite Reynolds numbers

An outline of the state space of planar Couette flow at high Reynolds numbers (Re<105) is investigated via a variety of efficient numerical techniques. It is verified from nonlinear analysis that the lower branch of the hairpin vortex state (HVS) asymptotically approaches the primary (laminar) state with increasing Re. It is also predicted that the lower branch of the HVS at high Re belongs to the stability boundary that initiates a transition to turbulence, and that one of the unstable manifolds of the lower branch of HVS lies on the boundary. These facts suggest HVS may provide a criterion to estimate a minimum perturbation arising transition to turbulent states at the infinite Re limit. © 2013 American Physical Society.

CFD and bubble column reactors:simulation and experiment

The simulation of two-phase flow in bubble columns using commercially available software fromFluent Incorporated is presented here. Data from a bubble column with a ratio of height to thecolumn diameter of 5 : 1 are compared with simulations and experimental results for time-averaged velocity and Reynolds stress proles are used to validate transient, two-dimensional simulations.The models are based on multiphase biological reactors with applications in the food industry. An example case of the mass transfer of oxygen through the liquid phase is also presented.

An alternating method for the stationary Stokes system

An alternating procedure for solving a Cauchy problem for the stationary Stokes system is presented. A convergence proof of this procedure and numerical results are included.

Stokes waves revisited:exact solutions in the asymptotic limit

The Stokes perturbative solution of the nonlinear (boundary value dependent) surface gravity wave problem is known to provide results of reasonable accuracy to engineers in estimating the phase speed and amplitudes of such nonlinear waves. The weakling in this structure though is the presence of aperiodic “secular variation” in the solution that does not agree with the known periodic propagation of surface waves. This has historically necessitated increasingly higher-ordered (perturbative) approximations in the representation of the velocity profile. The present article ameliorates this long-standing theoretical insufficiency by invoking a compact exact n-ordered solution in the asymptotic infinite depth limit, primarily based on a representation structured around the third-ordered perturbative solution, that leads to a seamless extension to higher-order (e.g., fifth-order) forms existing in the literature. The result from this study is expected to improve phenomenological engineering estimates, now that any desired higher-ordered expansion may be compacted within the same representation, but without any aperiodicity in the spectral pattern of the wave guides.

Processed 2 minutes-averaged continuous VM-ADCP (vessel-mounted Acoustic Doppler Current Profiler) profiles during POLARSTERN cruise ANT-XXVIII/3

This data set was obtained during the R. V. POLARSTERN cruise ANT-XXVIII/3. Current velocities were measured nearly continuously when outside territorial waters along the ship's track with a vessel-mounted TRD Instruments' 153.6-kHz Ocean Surveyor ADCP. The transducers were located 11 m below the water line and were protected against ice floes by an acoustically transparent plastic window. The current measurements were made using a pulse of 2s and vertical bin length of 4 m. The ship's velocity was calculated from position fixes obtained by the Global Positioning System (GPS). Heading, roll and pitch data from the ship's gyro platforms and the navigation data were used to convert the ADCP velocities into earth coordinates. Accuracy of the ADCP velocities mainly depends on the quality of the position fixes and the ship's heading data. Further errors stem from a misalignment of the transducer with the ship's centerline. The ADCP data were processed using the Ocean Surveyor Sputum Interpreter (OSSI) software developed by GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel. The averaging interval was set to 120 seconds. The reference layer was set to bins 5 to 16 avoiding near surface effects and biases near bin 1. Sampling interval setting: 2s; Number of bins: 80; Bin length: 4m; Pulse length: 4m; Blank beyond transmit length: 4m. Data processing setting: Top reference bin: 5; Bottom reference bin: 16; Average: 120s; Misalignment amplitude: 1.0276 +/- 0.1611, phase: 0.8100 +/- 0.7190. The precision for single ping and 4m cell size reported by TRDI is 0.30m/s. Resulting from the single ping precision and the number of pings (most of the time 36) during 120seconds the velocity accuracy is nearly 0.05m/s. (Velocity accuracy = single ping precision divided by square root of the number of pings).

Averaged sedimentation rates in the Northwest Pacific and Bering Sea during the first and the second oxygen isotope stages

Isotope chronostratigraphy of Upper Quaternary sediments from the Northwest Pacific and the Bering Sea was established by oxygen isotope records in planktonic and benthic foraminifera. The main regularities of temporal variations of calcium carbonate, organic carbon and opal contents, as well as of magnetic susceptibility in sediments of the study region with regard to climatic variations and productivity were established by means of isotopic-geochemical and lithophysical analyses of bottom sediments. Correlation of volcanogenic interbeds in the sediments was carried out, and their stratigraphy and age were preliminarily ascertained. Correlation was accomplished of A.P. Jouse diatom horizons determined by an analysis of the main ecological variations in diatom assemblages in Upper Quaternary sediments of the Northwest Pacific, Bering and Okhotsk Seas, and their comparison with similar variations in sediment cores with standard oxygen isotope stages. Also variations in lithology and contents of biogenic components in sediments of the region and in the cores were taken into account. A ratio of abundance of "neritic" species to the sum of "neritic" and oceanic species abundance (coefficient Id) can be a criterion of ecological changes of diatom assemblages in the studied region. It is determined by climate variability and mostly by sea ice influence. Schemes of average sedimentation rates in the Northwest Pacific and Bering Sea for periods of the first and the second oxygen isotope stages (12.5-1 and 24-12.5 ka, respectively) were plotted on the basis of obtained results and correlation of diatom horizons and lithological units in early studied cores with the oxygen isotope stages. Closure of the Bering Strait and aeration of the north-eastern shelf of the Bering Sea during the second stage induced increase of sedimentation rates in the Bering Sea, as compared with the first stage, and suspended material transport from the Bering Sea through the Kamchatka Strait into the Northwest Pacific and its accumulation in the southeast direction.

Dimensional dependence of the Stokes-Einstein relation and its violation

We generalize to higher spatial dimensions the Stokes-Einstein relation (SER) as well as the leading correction to diffusivity in finite systems with periodic boundary conditions, and validate these results with numerical simulations. We then investigate the evolution of the high-density SER violation with dimension in simple hard sphere glass formers. The analysis suggests that this SER violation disappears around dimension du = 8, above which it is not observed. The critical exponent associated with the violation appears to evolve linearly in 8 - d, below d = 8, as predicted by Biroli and Bouchaud [J. Phys.: Condens. Matter 19, 205101 (2007)], but the linear coefficient is not consistent with the prediction. The SER violation with d establishes a new benchmark for theory, and its complete description remains an open problem. © 2013 AIP Publishing LLC.