140 research outputs found
A methodology for the integration of stiff chemical kinetics on GPUs
Numerical schemes for reacting flows typically invoke the method of fractional steps in order to isolate the chemical kinetics model from diffusion/convection phenomena. Here, the reaction fractional step requires the solution of a collection of independent ODE systems which may be severely stiff. Recently, researchers have begun to explore the highly parallel structure of graphics processing units (GPUs) in order to accelerate integration schemes for these ODE systems. However, much of the existing work concentrates on explicit integration algorithms which may fall short in the presence of stiffness. In this light, we have carefully reimplemented in OpenCL C the Fortran 77 program of the 3-stage/5th order implicit Runge–Kutta method Radau5 by Hairer and Wanner (1991) and tested it extensively in the context of a transient equilibrium scheme for the flamelet model. Our implementation can easily be integrated with any existing reactive flow software in order to solve the reaction fractional step on an OpenCL-enabled GPU. Moreover, it is suited for any Chemkin-format reaction mechanism with ≲200≲200 species without incurring a loss in occupancy and it reaches its limit speedup (which is largely independent of the mechanism size) at a small problem size (≈500 ODE systems). In view of memory constraints, we include an optimized scheme for splitting the ODE systems across several kernel invocations and overlapping the kernel execution with data transfers. An in-depth evaluation is based upon runtime measurements of the CPU and the GPU implementation on a user level and a high-end CPU/GPU for an increasing number of ODE systems, reduced and detailed reaction mechanisms and a range of time step sizes
The role of actors in the policy design process: introducing design coalitions to explain policy output
Despite a renaissance of policy design thinking in public policy literature and a renewed interest in agency in the policy process literature, agency in the policy design process has, so far, not received systematic attention. Understanding the agency at play when designing policy, however, is crucial for better comprehension of policy design choices and varia- tion in policy design across cases. Here, we build on the hierarchical structure of design elements that constitute each policy and analyse how actors position themselves during a policy design process in relation to individual design elements. Our aim is to establish dif- ferent actors’ roles in shaping the policy output in an inductive, single-case study using the empirical case of the Swiss renewable energy feed-in tariff. Notably, we find agency in the form of coalitions which emerge around particular design elements. Based on our repre- sentative analysis, we derive the generalisable concept of design coalitions that we define as relational structures of actors who gather around and advocate for specific policy design elements during the policy design process. Policy design coalitions are dynamic through- out the design process and strategic and constitute the determinants in translating policy problems into final policy designs during policy designing. Our approach allows us to shed light on the role of agency in the policy design process in general
Monitoring of the accelerator beam distributions for internal target facilities
We describe a direct method for monitoring the geometrical dimensions of a
synchrotron beam at the target position for internal target installations. The
method allows for the observation of the proton beam size as well as the
position of the beam relative to the target. As a first demonstration of the
technique, we present results obtained by means of the COSY-11 detection system
installed at the cooler synchrotron COSY. The influence of the stochastic
cooling on the COSY proton beam dimensions is also investigated.Comment: 7 pages, 8 figures, Submitted to Nucl. Inst. & Meth.
Population balance modelling and laser diagnostic validation of soot particle evolution in laminar ethylene diffusion flames
Laminar diffusion flames present an elementary configuration for investigating soot formation and validating kinetic models before these are transferred to turbulent combustors. In the present article, we present a joint experimental and modelling investigation of soot formation in a laminar co-flow burner. The diffusion flames are analysed with the aid of laser diagnostic techniques, including elastic light scattering (ELS), planar laser-induced fluorescence of OH (OH-PLIF) and line-of-sight attenuation (LOSA), to measure the spatial distribution of soot, gas phase species and the line-of-sight integrated soot volume fraction (ISVF), respectively. The experimental dataset is supplemented by location-specific TEM images of thermophoretically sampled soot particles. The simulation of the sooting flames is carried out with a recently developed discretisation method for the population balance equation (Liu and Rigopoulos, 2019, Combust. Flame 205, 506-521) that accomplishes an accurate prediction of the particle size distribution, coupled with an in-house CFD code. By minimising numerical errors, we ensure that the discrepancies on the modelling side are mainly due to kinetics and are able to carry out an investigation of alternative models. We include a complete set of soot kinetics for PAH-based nucleation and condensation, HACA-based surface growth and oxidation as well as size-dependent aggregation, and consider three different gas phase reaction mechanisms (ABF, BBP and KM2). Based on predictions of the gas phase composition and particle size distribution of soot, modelled counterparts of the laser diagnostic signals are computed and compared with the experimental measurements. The approach of directly predicting signals circumvents the difficulties of explicitly representing the OH concentration in terms of the measured OH-PLIF data and avoids using ‘hybrid’ modelled and measured values to approximate the OH concentration. Moreover, the LOSA signal is directly converted to the line-of-sight ISVF instead of a measure of local soot volume fraction to avoid tomographic inversion errors. Lastly, the predicted ELS signal is computed in terms of the particle size distribution resolved by the population balance model, thus circumventing the approximation of an integral soot property using a presumed size distribution. While we cannot obtain quantitative agreement between experiments and simulations, the accuracy of the numerical approach and the direct prediction of experimental signals allow us to conduct sensitivity analyses of key empirical parameters and investigate the importance of the PAH chemistry and its influence on the competition between nucleation, condensation and surface growth
Near Threshold K+K- Meson-Pair Production in Proton-Proton Collisions
The near threshold total cross section and angular distributions of K+K- pair
production via the reaction pp --> ppK+K- have been studied at an excess energy
of Q = 17 MeV using the COSY-11 facility at the cooler synchrotron COSY. The
obtained cross section as well as an upper limit at an excess energy of Q = 3
MeV represent the first measurements on the K+K- production in the region of
small excess energies where production via the channel pp --> pp Phi --> ppK+K-
is energetically forbidden. The possible influence of a resonant production via
intermediate scalar states f0(980) and a0(980) is discussed.Comment: 8 pages, 6 figures, replaced with revised version, accepted for
publication in Phys. Lett.
Collision damping in the pi 3He -> d'N reaction near the threshold
We present a simple quantum mechanical model exploiting the optical potential
approach for the description of collision damping in the reaction pi 3He -> d'N
near the threshold, which recently has been measured at TRIUMF. The influence
of the open d'N -> NNN channel is taken into account. It leads to a suppression
factor of about ten in the d' survival probability. Applications of the method
to other reactions are outlined.Comment: RevTeX4, 14 pages, 3 Postscript figures, uses epsfig.sty, to appear
in Phys.Rev.
Low-Energy \Lambda-\p Scattering Parameters from the Reaction
Constraints on the spin-averaged scattering length and effective
range have been obtained from measurements of the reaction
close to the production threshold by comparing model phase-space Dalitz plot
occupations with experimental ones. The data fix well the position of the
virtual bound state in the system. Combining this with information
from elastic scattering measurements at slightly higher energies,
together with the fact that the hyperdeuteron is not bound, leads to a new
determination of the low energy scattering parameters.Comment: 18 pages, 7 figure
Heavy Meson Production at COSY - 11
The COSY-11 collaboration has measured the total cross section for the pp -->
pp eta-prime and pp --> pp eta reactions in the excess energy range from
Q = 1.5 MeV to Q = 23.6 MeV and from Q = 0.5 MeV to Q = 5.4 MeV,
respectively.
Measurements have been performed with the total luminosity of 73 nb^(-1) for
the pp --> pp eta reaction and 1360 nb^(-1) for the pp --> pp eta-prime one.
Recent results are presented and discussed.Comment: Invited talk at 4th International Conference on Physics at Storage
Rings (STORI 99), Bloomington, Indiana, USA, September 12-16, 199
Near-Threshold eta Meson Production in Proton-Proton Collisions
The production of eta mesons has been measured in the proton-proton
interaction close to the reaction threshold using the COSY-11 internal facility
at the cooler synchrotron COSY. Total cross sections were determined for eight
different excess energies in the range from 0.5 MeV to 5.4 MeV. The energy
dependence of the total cross section is well described by the available
phase-space volume weighted by FSI factors for the proton-proton and proton-eta
pairs.Comment: 9 pages, 1 table, 5 figure
Total and Differential Cross Sections for the pp-->pp eta-prime Reaction Near Threshold
The eta-prime meson production in the reaction pp-->pp eta-prime has been
studied at excess energies of Q = 26.5, 32.5 and 46.6 MeV using the internal
beam facility COSY-11 at the cooler synchrotron COSY. The total cross sections
as well as one angular distribution for the highest Q-value are presented. The
excitation function of the near threshold data can be described by a pure
s-wave phase space distribution with the inclusion of the proton-proton final
state interaction and Coulomb effects. The obtained angular distribution of the
eta-prime mesons is also consistent with pure s-wave production.Comment: 6 pages, 5 figures, submitted to Eur. Phys. J.
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