Multiscaling in Models of Magnetohydrodynamic Turbulence

From a direct numerical simulation of the MHD equations we show, for the first time, that velocity and magnetic-field structure functions exhibit multiscaling, extended self similarity (ESS), and generalized extended self similarity (GESS). We also propose a new shell model for homogeneous and isotropic MHD turbulence, which preserves all the invariants of ideal MHD, reduces to a well-known shell model for fluid turbulence for zero magnetic field, has no adjustable parameters apart from Reynolds numbers, and exhibits the same multiscaling, ESS, and GESS as the MHD equations. We also study dissipation-range asymptotics and the inertial- to dissipation-range crossover.Comment: 5 pages, REVTEX, 4 figures (eps

A 3-D Transient CFD Model of a Reciprocating Piston Compressor with Dynamic Port Flip Valves

In this paper a full transient 3-D CFD model of a complete single piston compressor system along with dynamic inlet and outlet port flip valves is presented. The model includes the motion of the cam driven piston, and also predicts the motion of the dynamic flip valves. All activities related to model creation, simulation and post-processing were performed within the simulation software Simerics–PD (also known as PumpLinx).  The mesh creation and mesh motion for the piston chamber and the flip valve volumes were performed using meshing algorithms implemented within the simulation software. In addition, a novel mesh motion algorithm was implemented to enable a fully coupled simulation of the inlet port flip valve and the piston surface: which have intersecting swept volumes and may interfere during the operation of the compressor. The entire simulation was performed in a fully coupled manner in which the motion of the valves is calculated within Simerics–PD during the transient CFD simulation. This is achieved by a Fluid Structure Interaction (FSI) methodology where the pressure distribution acting on the valve is used to calculate the valve motion using a robust ordinary differential equation (ODE) solver. Important input parameters of the rotational dynamics (such as torsion constant) of the flip valve were calculated from the beam bending theory. In addition a Full Closure model for flip valves implemented within Simerics–PD ensures that there is no leakage through the valves when they are in the fully closed position, which are essential for accurate compressor simulation.  In the present work, detailed descriptions are provided of the model creation and simulation methodology for an industrial single piston compressor system. The model creation was performed on the real world geometry without simplifications and real-gas fluid properties were used for the simulation. The fully coupled 3-D transient CFD simulation of the piston compressor and the flip valves is shown to have excellent simulation speed and several relevant results are presented. The model set-up and performance of the simulation are demonstrated to be robust and user friendly and the algorithms developed can be readily applied to single piston compressor systems with dynamic port valves

Morphology parameters for intracranial aneurysm rupture risk assessment

OBJECTIVE—The aim of this study is to identify image-based morphological parameters that correlate with human intracranial aneurysm (IA) rupture. METHODS—For 45 patients with terminal or sidewall saccular IAs (25 unruptured, 20 ruptured), three-dimensional geometries were evaluated for a range of morphological parameters. In addition to five previously studied parameters (aspect ratio, aneurysm size, ellipticity index, nonsphericity index, and undulation index), we defined three novel parameters incorporating the parent vessel geometry (vessel angle, aneurysm [inclination] angle, and [aneurysm-to-vessel] size ratio) and explored their correlation with aneurysm rupture. Parameters were analyzed with a two-tailed independent Student's t test for significance; significant parameters (P < 0.05) were further examined by multivariate logistic regression analysis. Additionally, receiver operating characteristic analyses were performed on each parameter. RESULTS—Statistically significant differences were found between mean values in ruptured and unruptured groups for size ratio, undulation index, nonsphericity index, ellipticity index, aneurysm angle, and aspect ratio. Logistic regression analysis further revealed that size ratio (odds ratio, 1.41; 95% confidence interval, 1.03−1.92) and undulation index (odds ratio, 1.51; 95% confidence interval, 1.08−2.11) had the strongest independent correlation with ruptured IA. From the receiver operating characteristic analysis, size ratio and aneurysm angle had the highest area under the curve values of 0.83 and 0.85, respectively. CONCLUSION—Size ratio and aneurysm angle are promising new morphological metrics for IA rupture risk assessment. Because these parameters account for vessel geometry, they may bridge the gap between morphological studies and more qualitative location-based studies

Inertial- and Dissipation-Range Asymptotics in Fluid Turbulence

We propose and verify a wave-vector-space version of generalized extended self similarity and broaden its applicability to uncover intriguing, universal scaling in the far dissipation range by computing high-order (\leq 20\/) structure functions numerically for: (1) the three-dimensional, incompressible Navier Stokes equation (with and without hyperviscosity); and (2) the GOY shell model for turbulence. Also, in case (2), with Taylor-microscale Reynolds numbers 4 \times 10^{4} \leq Re_{\lambda} \leq 3 \times 10^{6}\/, we find that the inertial-range exponents (\zeta_{p}\/) of the order - p\/ structure functions do not approach their Kolmogorov value p/3\/ as Re_{\lambda}\/ increases.Comment: RevTeX file, with six postscript figures. epsf.tex macro is used for figure insertion. Packaged using the 'uufiles' utilit

A study of fluid structure and thermal interactions in the lubricating interface between gears and lateral bushes in external gear machines

A novel Fluid Structure Interaction (FSI) model of lateral lubricating gaps in External Gear Machines (EGM) is presented in this study. Modeling effort is directed toward improvement of the performance of the lubricating interface in EGMs in terms of power losses associated with this internal lubricating gap flow during the operation of this particular kind of positive displacement machine. This is achieved by modeling the phenomena involved with this lubricating gap flow, with particular focus on accurate prediction of lubricating film gap heights, pressures and viscous and volumetric losses arising from the lubricating interface. The modeling approach towards achieving the aim of predicting the performance of the lubricating interface is discussed, with details such as generation of finite volume meshes, coupling with other related models, development of fluid, structural and thermo-elastic finite volume solvers and development of a fully coupled FSI-Thermal algorithm. Simulation results of the proposed FSI model for a reference case representative of the most common EGM design solution have been presented. In addition, two experimental validations for the FSI model are presented: using experimental lateral gap leakage measurements and capacitive film thickness measurements. The FSI model has also been used to create and simulate novel design solutions of the lubricating interfaces in EGMs. Simulation results from several such solutions are discussed with certain designs showing markedly improved performance. The generality of the proposed FSI model is also demonstrated and it has been used to simulate and improve upon EGMs with non-standard configurations. Modeling approaches and experimental results for these designs are described. The first such model of EGMs, the FSI model has large potential to drive the design of future EGMs with improved efficiency and reliability, with low losses as well as low chances of wear

Anomalously rough sandpiles in one dimension: exact decimation results

We categorize the possible large-scale behaviors of one-dimensional nearest-neighbor hopping "sandpile" slope models on a circle of N sites with nonconserving noise, via decimation. Defining the height at site i to be hi, we find a continuous infinity of fixed points of the decimation, parametrized by an index n (1&#8804;n&#8804;2) with (h0-hN/2)2~N2/n. The fixed point n=2 corresponds to our ordinary notion of a rough interface, while the others are new and far rougher

[Marco Aur??lio Nunes Ferreira de Queiroz, t??cnico da FJP]

Marco Aur??lio Nunes Ferreira de Queiroz, t??cnico da Funda????o Jo??o Pinheiro (FJP)