197 research outputs found
A large eddy simulation model for two-way coupled particle-laden turbulent flows
In this paper we propose a new modeling framework for large eddy simulations
(LES) of particle-laden turbulent flows that captures the interaction between
the particle and fluid phase on both the resolved and subgrid-scales. Unlike
the vast majority of existing subgrid-scale models, the proposed framework does
not only account for the influence of the sugrid-scale velocity on the particle
acceleration but also considers the effect of the particles on the turbulent
fluid flow. This includes the turbulence modulation of the subgrid-scales by
the particles, which is taken into account by the modeled subgrid-scale stress
tensor, and the effect of the unresolved particle motion on the resolved flow
scales. Our new modeling framework combines a recently proposed model for
enriching the resolved fluid velocity with a subgrid-scale component, with the
solution of a transport equation for the subgrid-scale kinetic energy. We
observe very good agreement of the particle pair separation and particle
clustering compared to the corresponding direct numerical simulation (DNS).
Furthermore, we show that the change of subgrid-scale kinetic energy induced by
the particles can be captured by the proposed modeling framework
A hybrid immersed boundary method for dense particle-laden flows
A novel smooth immersed boundary method (IBM) based on a direct-forcing
formulation is proposed to simulate incompressible dense particle-laden flows.
This IBM relies on a regularization of the transfer function between the
Eulerian grid points (to discretise the fluid governing equations) and
Lagrangian markers (to represent the particle surface) to fulfill the no-slip
condition at the surfaces of the particles, allowing both symmetrical and
non-symmetrical interpolation and spreading supports to be used. This enables
that local source term contributions to the Eulerian grid, accounting for the
boundary condition enforced at a Lagrangian marker on the surface of a
particle, can be present on the inside of the particle only when this is
beneficial, for instance when the Lagrangian marker is near another particle
surface or near a domain boundary. However, when the Lagrangian marker is not
near another particle surface or a domain boundary, the interpolation and
spreading operators are locally symmetrical, meaning a ``classic'' IBM scheme
is adopted. This approach, named hybrid IBM (HyBM), is validated with a number
of test-cases from the literature. These results show that the HyBM achieves
more accurate results compared to a classical IBM framework, especially at
coarser mesh resolutions, when there are Lagrangian markers close to a particle
surface or a domain wall
Conservative finite-volume framework and pressure-based algorithm for flows of incompressible, ideal-gas and real-gas fluids at all speeds
A conservative finite-volume framework, based on a collocated variable
arrangement, for the simulation of flows at all speeds, applicable to
incompressible, ideal-gas and real-gas fluids is proposed in conjunction with a
fully-coupled pressure-based algorithm. The applied conservative discretisation
and implementation of the governing conservation laws as well as the definition
of the fluxes using a momentum-weighted interpolation are identical for
incompressible and compressible fluids, and are suitable for complex geometries
represented by unstructured meshes. Incompressible fluids are described by
predefined constant fluid properties, while the properties of compressible
fluids are described by the Noble-Abel-stiffened-gas model, with the
definitions of density and specific static enthalpy of both incompressible and
compressible fluids combined in a unified thermodynamic closure model. The
discretised governing conservation laws are solved in a single linear system of
equations for pressure, velocity and temperature. Together, the conservative
finite-volume discretisation, the unified thermodynamic closure model and the
pressure-based algorithm yield a conceptually simple, but versatile, numerical
framework. The proposed numerical framework is validated thoroughly using a
broad variety of test-cases, with Mach numbers ranging from 0 to 239, including
viscous flows of incompressible fluids as well as the propagation of acoustic
waves and transiently evolving supersonic flows with shock waves in ideal-gas
and real-gas fluids. These results demonstrate the accuracy, robustness and the
convergence, as well as the conservation of mass and energy, of the numerical
framework for flows of incompressible and compressible fluids at all speeds, on
structured and unstructured meshes
Estimation of curvature from volume fractions using parabolic reconstruction on two-dimensional unstructured meshes (Supporting data)
This data accompanies the paper "Estimation of curvature from volume fractions using parabolic reconstruction on two-dimensional unstructured meshes", published in Journal of Computational Physics (2017). The document "supportingdata.pdf" gives a description of the data provided in the txt-files
On the kinetics of thermal oxidation of the thermographic phosphor BaMgAL10O17:Eu
Decreased photoluminescence of the phosphor BaMgAL10O17:Eu due to oxidation of the europium dopant at high temperatures has been a subject of study for many years in relation to its use in lighting applications. However, understanding of the underlying effects that cause this reduction in photoluminescence remains incomplete and some of the mechanisms proposed in the literature are contradictory. Recent use of this phosphor as a thermal history sensor has extended the range of exposure conditions normally investigated in lighting applications to higher temperatures and multiple exposure times. The kinetics of the process were investigated by means of spectroscopy and material characterisation techniques. It was found that changes in the luminescence are the result of two simultaneous processes: the oxidation of Eu2+ ions (through a process of diffusion) and a phase transition. The level of degradation of the phosphor is suggested to follow the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model above 900 °C and thus can be predicted with knowledge of the exposure time and temperature. This is useful in applications of the phosphor as a temperature sensor
Phase proper orthogonal decomposition of non-stationary turbulent flow
A phase proper orthogonal decomposition (Phase POD) method is demonstrated,
utilizing phase averaging for the decomposition of spatio-temporal behaviour of
statistically non-stationary turbulent flows in an optimized manner. The
proposed Phase POD method is herein applied to a periodically forced
statistically non-stationary lid-driven cavity flow, implemented using the
snapshot proper orthogonal decomposition algorithm. Space-phase modes are
extracted to describe the dynamics of the chaotic flow, in which four central
flow patterns are identified for describing the evolution of the energetic
structures as a function of phase. The modal building blocks of the energy
transport equation are demonstrated as a function of the phase. The triadic
interaction term can here be interpreted as the convective transport of
bi-modal interactions. Non-local energy transfer is observed as a result of the
non-stationarity of the dynamical processes inducing triadic interactions
spanning across a wide range of mode numbers
EXPERIMENTS AND MODELLING OF MICRO-JET ASSISTED FLUIDIZATION OF NANOPARTICLES
The fluidization of nanoparticle agglomerates can be largely improved by using downward pointing micronozzles, creating a high-velocity jet, as experimentally shown. By discrete particle simulations – treating the agglomerates as single particles – we show that the main reason is probably the reduction of the agglomerate size by agglomerate-agglomerate collisions
The Effect of the Presence of Very Cohesive Geldart C Ultra-Fine Particles on the Fluidization of Geldart A Fine Particle Beds
The effect of the presence of ultra-fines (d < 10 μm) on the fluidization of a bed containing fine particles (d < 100 μm), is the subject of this paper. Practically, it can happen due to breakage or surface abrasion of the fine particles in some processes which totally changes the size distribution and also fluidization behaviour. The materials used in this study are both ground calcium carbonate (GCC); fine is CALCIT MVT 100 (Geldart’s group A) and ultra-fine is CALCIT MX 10 (group C). The experimental results for different binary mixtures of these materials (ultra-fines have 30%, 50%, or 68% of the total mixture weight) show that the physical properties of the mixtures are close to those of pure ultra-fine powders. Using mean values of the bed pressure drop calculated from several independent repetitions, the fluidization behaviour of different mixtures are compared and discussed. The fluidization behaviour of the mixtures is non-reproducible and includes cracking, channelling and agglomeration (like for pure ultra-fine powders). Increasing the portion of ultra-fine materials in the mixture causes a delay in starting partial fluidization, an increase in the bed pressure drop as well as a delay in reaching the peak point
A detailed characterization of BaMgAl10O17 : Eu phosphor as a thermal history sensor for harsh environments
Knowledge of component temperatures in gas turbines is essential for the design of thermal management systems and to maintain the lifetime of highly loaded parts as the firing temperature increases in pursuit of improved thermal efficiency. When on-line methods such as pyrometers and thermocouples are not suitable, a thermal history sensor can be used to record the maximum temperatures and read them out after operation. Currently, temperature sensitive paints are applied to obtain temperature profiles in gas turbine components but they present some limitations. A new method based on irreversible changes in the optical properties of thermographic phosphors can potentially overcome some of these difficulties. In particular, a sensor based on the oxidation of europium based phosphors has shown great potential. In this work the temperature sensing capabilities of the phosphor BaMgAl10O17:Eu are investigated in the temperature range from 700 °C to 1200 °C, and suitable measurands defined. The influence of practical factors comprising excitation fluence, exposure time, dopant concentration, cooling down time and atmosphere composition, on measurement accuracy and sensitivity are also reported
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