A phenomenological model for particle dispersion and clustering

The objective of this thesis is to propose a new model for particle dispersion and clustering for use within an (unsteady)-Reynolds Averaged Navier-Stokes ((u)RANS) computational framework. The need for an improved model stems from industrial requirements to address certain limitations of the currently used models. Namely, low predicted particle entrance into recirculation zones for particles with large Stokes numbers and unrealistically spatial and temporal uniform predicted particle concentrations. Abstract The literature review presented within this thesis examines the various computational tools available for modeling the Lagrangian phase and identifies Kinematic Simulations (KS) as potentially capable of reproducing accurate Lagrangian statistics and particle clustering across a range of physical scales while at the same time requiring a modest increase of computational resources relative to the presently used methods. Abstract The thesis proposes a combination of (u)RANS and KS in a coupled Eulerian-Lagrangian framework.The (u)RANS calculations will be responsible for modeling the large coherent motions while the KS will be employed to model the effects of all the other scales, that are represented statistically in the (u)RANS context, on particle motion. In other words, the representation of the velocity field within the `eddies' will be simulated by tracking a particle through an isotropic turbulent field constructed with the aid of KS. The extent of scales and the energy content of the isotropic field modeled by KS at every instance is determined by the local properties of the under-resolved `eddy' as determined by the Eulerian framework. Abstract The proposed model is evaluated on an axisymmetric sudden expansion test case through comparisons with experimental results, LES calculations as well as RANS simulations employing the current industry standard dispersion model. Improved overall performance was observed with significant differences between the particle trajectories computed with the proposed model and those with a model widely used in industry. This last point is of particular significance as one of the limitations of the currently used models was the high degree of spatial uniformity in the predicted particle distribution.Open Acces

Modelling Risers With Partial Strake Coverage

This paper compares model test data for risers with partial strake coverage to predictions made with the Vortex-Induced Vibration (VIV) prediction program SHEAR7 Versions 4.5 and 4.6. It is shown that new features in Version 4.6 substantially enhance the capability for predicting the VIV response of risers with partial coverage. Experimental data is taken from two large L/D tests: the NDP 38 m long riser tests and the Deepstar-Miami, 500 foot long riser. New methods are described for modeling risers partially covered with helical strakes. Key SHEAR7 parameters are recommended, based on parametric investigations to calibrate the model against the available experimental data. Recommended modeling procedures are described so as to facilitate implementation by SHEAR7 users in their VIV modeling tasks.SHEAR7 JI

Response Variability in Flexible Cylinder VIV Model Test Data

This paper presents some results from the 2011 SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented 38m long flexible cylinders at Reynolds numbers up to 220,000. The main objective it to present the experimental results in a manner that describes the response variability that exists in the measured response data. Despite the fact that VIV is known to be a stochastic process, this is rarely addressed in the literature and currently there is no framework or 'best-practice' in the VIV community that can address statistically non-stationary data. In this paper, the experimental measurements are treated like non-stationary time-series and all statistical quantities which are typically of interest are computed with the use of short duration moving windows (or time-gates). A novel way of plotting and presenting VIV response data for flexible cylinders is introduced that is capable of revealing the inherent variability that exists in the cylinder's response.SHEAR7 Joint Industry Projec

The Identification of Power-In Region in Vortex-Induced Vibration of Flexible Cylinders

This paper proposes a novel method to identify the power-in regions of long flexible cylinders subjected to vortex-induced vibration (VIV). It also attempts to address a practical problem: “Will a secondary power-in region appear after the primary power-in region is covered with suppression devices?” The source of data is a recent model test on a 38 meter long flexible cylinder, densely instrumented with fiber optic strain gauges and accelerometers. For pipes with partial coverage of suppression devices in uniform flow, the bare region would be expected to be a power-in region and the section with suppression devices is expected to be a power-out region. Experimental data from these types of tests are used to benchmark the proposed power-in zone identification method. The method is then used to identify the power-in zones on a bare cylinder in a sheared flow. This paper also explores the occurrence of secondary power-in regions that may exist, when suppression devices are placed in the primary power-in zone. Secondary power-in regions were observed. Lessons learned from the power-in region identification in sheared flows will be a useful tool for designer/engineers choosing where to place suppression devices.DeepStar (Consortium)SHEAR7 JI

Effects of Strake Coverage and Marine Growth on Flexible Cylinder Vortex-Induced Vibrations

This paper presents some results from the recent SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 220,000. The main objective is to present the experimental results describing the effectiveness of different amounts of strake coverage and to explore the influence of simulated marine growth. The data are presented in terms of cross-flow (CF) response amplitudes and rainflow-counted damage rates due to the combined CF and in-line (IL) bending stresses. All the results are compared with the bare cylinder cases which will be used as a reference to determine how effective the strakes are in suppressing vortex-induced vibrations (VIV) and how this effectiveness can be affected by marine growth. The results show that even small bare sections (missing strakes) can lead to significant VIV response. Finally, the tests revealed that even moderate amounts of marine growth can quickly negate any suppression coming from the strakes.SHEAR7 JI

Reynolds Number Effects on the Vortex-Induced Vibration of Flexible Marine Risers

This paper explores the Reynolds number dependence of the Vortex-Induced Vibration (VIV) of flexible marine risers. Emphasis is placed on revealing the trends that exist between the Strouhal number and the Reynolds number and between the dimensionless amplitude (A/D) and Reynolds number. Data is drawn from recent towing tank experiments which used flexible cylinders of three different diameters. The 38m long pipes were exposed to uniform and sheared currents. The Reynolds number range extended from approximately 5,000 to 220,000 — well into the critical regime — with the larger diameter pipes responding in up to the 13th mode and the smaller diameter pipe responding well above the 20th mode. The results and trends from this set of experiments are compared to previous results from laboratory and field experiments.SHEAR7 JI

Shadowing of Ultrahigh Energy Neutrinos

The rise with energy of the neutrino--nucleon cross section implies that at energies above few TeV the Earth is becoming opaque to cosmic neutrinos. The neutrinos interact with the nucleons through the weak charged current, resulting into absorption, and the weak neutral current, which provides a redistribution of the neutrino energy. We Mellin transform the neutrino transport equation and find its exact solution in the moment space. A simple analytical formula is provided, which describes accurately the neutrino spectrum, after the neutrinos have traversed the Earth. The effect of the weak neutral current is most prominent for an initial flat neutrino spectrum and we find that at low energies (around 1 TeV) the neutrino intensity is even enhanced.Comment: gziped, tar file of LaTeX paper plus 2 postscript figures, 13 page

Consolidation of Empirics for Calculation of VIV Response

The present paper consolidates available experimental results for both sub-critical and critical Reynolds numbers and varying surface roughness and formulates a coefficient excitation model that aims at unbiased response estimates when using semi-empirical VIV prediction programs. A simplified procedure is suggested to account for higher order effects when relevant. The paper discusses the use of a modified coefficient excitation model with the objective of capturing or correctly reflecting certain specific features that have been observed in sub-critical and supercritical VIV experiments. The first part of this paper shows how the available low Reynolds number hydrodynamic data that currently forms the basis for most semi-empirical prediction software needs to be modified to correctly reflect the available experimental observations at sub-critical Reynolds numbers. The latter part of this paper looks at the available high Reynolds experimental data and suggests ways whereby the previously identified force coefficient database might be modified to reflect what is currently known about the VIV response of smooth and rough surfaced cylinders in the critical and super-critical Reynolds regimes.Norway Deepwater Progra

Rapid Stochastic Acceleration of Protons to Energies Above 100~TeV in the Accretion Column Of Hercules X-1

An investigation into the acceleration of protons by scattering off relativistic Alfv\'{e}n waves in the accretion column of Hercules X-1 is presented. The mechanism is shown to achieve mean particle energies of 30~TeV under very reasonable assumptions about the environment, and 250~TeV is available under some circumstances. The highest individual energy attained is almost 1~PeV. The protons emerge in the form of a narrow beam directed at the inner edge of the accretion disk, which is favourable because of the reduced power requirement and presence of target material for gamma-ray production.Comment: 14 pages of uuencoded compressed postscript including 10 figures, accepted by Astroparticle Physics, ADP-AT-94-

Manifestations of Extra Dimensions in a Neutrino Telescope

Theories with large extra dimensions provide the possibility that a flavor neutrino, localized in a 3+1 brane, can mix with a singlet neutrino living in the bulk. This mixing leads to unconventional patterns of neutrino matter oscillations and we examine in details how these oscillations depend upon two parameters: the brane-bulk coupling $\xi$ and the effective mass $\mu$ of the flavor neutrino inside matter. We find that high energy $(E \ge 50$ GeV) $\nu_\mu$ neutrinos, to be detected by neutrino telescopes, can give signals of extra dimensions. With a 1 k$m^{3}$ neutrino telescope extra dimensions with radius down to $1\mu m$ can be tested directly, while for smaller radius an indirect evidence can be established.Comment: 14 pages, 5 figures, added conclusion