100 research outputs found
Continuum simulation of the discharge of the granular silo: a validation test for the mu(I)-visco-plastic flow law
Using both a continuum Navier-Stokes solver, with the mu(I)-flow-law
implemented to model the viscous behavior, and the discrete Contact Dynamics
algorithm, the discharge of granular silos is simulated in two dimensions from
the early stages of the discharge until complete release of the material. In
both cases, the Beverloo scaling is recovered. We first do not attempt
quantitative comparison, but focus on the qualitative behavior of velocity and
pressure at different locations in the flow. A good agreement is obtained in
the regions of rapid flows, while areas of slow creep are not entirely captured
by the continuum model. The pressure field shows a general good agreement. The
evolution of the free surface implies differences, however, the bulk
deformation is essentially identical in both approaches. The influence of the
parameters of the mu(I)-flow-law is systematically investigated, showing the
importance of the dependence on the inertial number I to achieve quantitative
agreement between continuum and discrete discharge. The general ability of the
continuum model to reproduce qualitatively the granular behavior is found to be
very encouraging.Comment: 12 pages, 15 figure
An adaptive solver for viscoelastic incompressible two-phase problems applied to the study of the splashing of slightly viscoelastic droplets
We propose an adaptive numerical solver for the study of viscoelastic 2D
two-phase flows using the volume-of-fluid method. The scheme uses the robust
log conformation tensor technique of Fattal & Kupferman (2004,2005} combined
with the time-split scheme proposed by Hao & Pan (2007}. The use of this
time-split scheme has been proven to increase the stability of the numerical
computation of two-phase flows. We show that the adaptive computational
technique can be used to simulate viscoelastic flows efficiently. The solver is
coded using the open-source libraries provided by the \basilisk \cite{Basilisk}
platform. In particular, the method is implemented for Oldroyd-B type
viscoelastic fluids and related models (FENE-P and FENE-CR). The numerical
scheme is then used to study the splashing of weakly viscoelastic drops. The
solvers and tests of this work are freely available on the Basilisk web sit
Breakup of finite-size liquid filaments: Transition from no-breakup to breakup including substrate effects
This work studies the breakup of finite-size liquid filaments, when also
including substrate effects, using direct numerical simulations. The study
focuses on the effects of three parameters: Ohnesorge number, the ratio of the
viscous forces to inertial and surface tension forces, the liquid filament
aspect ratio, and where there is a substrate, a measure of the fluid slip on
the substrate, i.e. slip length. Through these parameters, it is determined
whether a liquid filament breaks up during the evolution toward its final
equilibrium state. Three scenarios are identified: a collapse into a single
droplet, the breakup into one or multiple droplets, and recoalescence into a
single droplet after the breakup (or even possibly another breakup after
recoalescence). The results are compared with the ones available in the
literature for free-standing liquid filaments. The findings show that the
presence of the substrate promotes breakup of the filament. The effect of the
degree of slip on the breakup is also discussed. The parameter domain regions
are comprehensively explored when including the slip effects. An experimental
case is also carried out to illustrate the collapse and breakup of a
finite-size silicon oil filament supported on a substrate, showcasing a
critical length of the breakup in a physical configuration. Finally, direct
numerical simulations reveal striking new details into the breakup pattern for
low Ohnesorge numbers, where the dynamics are fast and the experimental imaging
is not available; our results therefore significantly extend the range of
Ohnesorge number over which filament breakup has been considered
Adaptive Cartesian meshes for atmospheric single-column models: a study using Basilisk 18-02-16
It is well known that the representation of certain atmospheric conditions in
climate and weather models can still suffer from the limited grid resolution
that is facilitated by modern-day computer systems. Herein we study a simple
one-dimensional analogy to those models by using a single-column model
description of the atmosphere. The model employs an adaptive Cartesian mesh
that applies a high-resolution mesh only when and where it is required. The
so-called adaptive-grid model is described, and we report our findings
obtained for tests to evaluate the representation of the atmospheric boundary
layer, based on the first two GEWEX ABL Study (GABLS) inter-comparison cases.
The analysis shows that the adaptive-grid algorithm is indeed able to
dynamically coarsen and refine the numerical grid whilst maintaining an
accurate solution. This is an interesting result as in reality, transitional
dynamics (e.g. due to the diurnal cycle or due to changing synoptic
conditions) are the rule rather than the exception.</p
A projection method for multiphase flows
An Eulerian projection approach for incompressible variable-density two-phase flows is presented. The Navier-Stokes equations governing these flows are reformulated to take the form of the corresponding equations for the lighter phase with a constant density, which can be efficiently solved using standard numerical methods. The effect of the additional mass in the heavier phase is accounted for by a forcing term, which is determined from the solution of an artificial velocity field. This artificial field is subjected solely to inertial and gravity forces as well as the force coupling the flow field and the artificial field. The phase interface in this purely Eulerian approach is described using the level-set method. Results for two-dimensional simulations of the Rayleigh-Taylor instability are presented to validate the new method
Fluctuations of elastic interfaces in fluids: Theory and simulation
We study the dynamics of elastic interfaces-membranes-immersed in thermally
excited fluids. The work contains three components: the development of a
numerical method, a purely theoretical approach, and numerical simulation. In
developing a numerical method, we first discuss the dynamical coupling between
the interface and the surrounding fluids. An argument is then presented that
generalizes the single-relaxation time lattice-Boltzmann method for the
simulation of hydrodynamic interfaces to include the elastic properties of the
boundary. The implementation of the new method is outlined and it is tested by
simulating the static behavior of spherical bubbles and the dynamics of bending
waves. By means of the fluctuation-dissipation theorem we recover analytically
the equilibrium frequency power spectrum of thermally fluctuating membranes and
the correlation function of the excitations. Also, the non-equilibrium scaling
properties of the membrane roughening are deduced, leading us to formulate a
scaling law describing the interface growth, W^2(L,T)=L^3 g[t/L^(5/2)], where
W, L and T are the width of the interface, the linear size of the system and
the temperature respectively, and g is a scaling function. Finally, the
phenomenology of thermally fluctuating membranes is simulated and the frequency
power spectrum is recovered, confirming the decay of the correlation function
of the fluctuations. As a further numerical study of fluctuating elastic
interfaces, the non-equilibrium regime is reproduced by initializing the system
as an interface immersed in thermally pre-excited fluids.Comment: 15 pages, 11 figure
In Search of Durable Sandphobic Thermal/Environmental Barrier Coatings for Rotorcraft Gas Turbine Engines
AIAA Scitech 2021 ForumThe article of record as published may be found at https://doi.org/10.2514/6.2021-0874The objective of this research is to develop new thermal/environmental barrier coatings (T/EBCs) that exhibit greater durability and CMAS (Calcia-Magnesia-Alumino-Silicates) resistance than any of the current state-of-the-art rotorcraft turbine engine coatings. Commercial/Military aircraft engines, especially helicopter engines undergo severe damage to critical components when they need to operate over sandy terrains or volcanic zones. Typical high pressure turbine vanes/blades with current coatings undergo damages that include blade coating wear, sand glazing, Calcia-Magnesia-Alumina-Silicates (CMAS) attack, oxidation, plugged cooling holes, all of which can cause rapid engine performance loss and in severe cases ending up in loss of aircraft. Design of novel T/EBCs for high temperature operation is presented in this paper based on ongoing work in understanding the fundamental governing parameters affecting CMAS adhesion, build-up, and chemical attack. The paper intends to report specific objectives and findings obtained thus far from an ambitious T/EBC research program funded by OSD’s Strategic Environmental Research and Development Program (SERDP). Systematic sand-phobic development research efforts and methodologies from modeling to engine relevant high-temperature environmental test evaluations are described in this paper to innovate improved T/EBCs for both Ni-superalloy based substrates and emerging SiC-SiC Ceramic Matrix Composite (CMC) based substrates.This research was performed while the author held an NRC Research Associateship award at Field Robotics Laboratory, Naval Postgraduate SchooThe authors acknowledge the support from received from DoD’s Strategic Environmental Research and Development Program (SERDP) for this research. The research reported in this document is being performed in connection with contract/instrument W911QX-16-D-0014 with the U.S. Army Research Laboratory. The authors are thankful to Dr. Robin Nissan (SERDP Program Manager) and Mr. Braxton Lewis (Principal Scientist, SERDP) for their support to this SERDP project# WP20-1281
The SOLAS air-sea gas exchange experiment (SAGE) 2004
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 58 (2011): 753-763, doi:10.1016/j.dsr2.2010.10.015.The SOLAS air-sea gas exchange experiment (SAGE) was a multiple-objective study investigating
gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in
high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the
expansive Subpolar Zone of the southern oceans. This paper provides a general introduction and
summary of the main experimental findings. The release site was selected from a pre-voyage desktop
study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the
south-east of New Zealand and the experiment conducted between mid-March and mid-April 2004. In
common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a
Lagrangian study quantifying key biological and physical drivers influencing the air-sea gas exchange
processes of CO2, DMS and other biogenic gases associated with an iron-induced phytoplankton
bloom. A dual tracer SF6/3He release enabled quantification of both the lateral evolution of a labelled
volume (patch) of ocean and the air-sea tracer exchange at the 10’s of km’s scale, in conjunction with
the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the
gas exchange coefficient on windspeed that is widely applicable and describes air-sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties, and wind speed were quantified to further assist the development of gas exchange models for high-wind environments.
There was a significant increase in the photosynthetic competence (Fv/Fm) of resident phytoplankton
within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary
production and column-integrated chlorophyll a concentrations had only doubled relative to the
unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions
totalling 1.1 tonne Fe2+, this was a very modest response compared to the other mesoscale iron
enrichment experiments. An investigation of the factors limiting bloom development considered co-
limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst
incident light levels and the initial Si:N ratio were the lowest recorded in all FeAX’s to date, there was
only a small seed-stock of diatoms (less than 1% of biomass) and the main response to iron addition
was by the picophytoplankton. A high rate of dilution of the fertilised patch relative to phytoplankton
growth rate, the greater than expected depth of the surface mixed layer and microzooplankton grazing
were all considered as factors that prevented significant biomass accumulation. In line with the limited
response, the enhanced biological draw-down of pCO2 was small and masked by a general increase in pCO2 due to mixing with higher pCO2 waters. The DMS precursor DMSP was kept in check through grazing activity and in contrast to most FeAX’s dissolved dimethylsulfide (DMS) concentration declined through the experiment. SAGE is an important low-end member in the range of responses to iron addition in FeAX’s. In the context of iron fertilisation as a geoengineering tool for atmospheric CO2 removal, SAGE has clearly demonstrated that a significant proportion of the low iron ocean may not produce a phytoplankton bloom in response to iron addition.SAGE was jointly funded through
the New Zealand Foundation for Research, Science and Technology (FRST) programs
(C01X0204) "Drivers and Mitigation of Global Change" and (C01X0223) "Ocean
Ecosystems: Their Contribution to NZ Marine Productivity." Funding was also provided for
specific collaborations by the US National Science Foundation from grants OCE-0326814
(Ward), OCE-0327779 (Ho), and OCE 0327188 OCE-0326814 (Minnett) and the UK Natural
Environment Research Council NER/B/S/2003/00282 (Archer). The New Zealand
International Science and Technology (ISAT) linkages fund provided additional funding
(Archer and Ziolkowski), and the many collaborator institutions also provided valuable
support
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