2,456 research outputs found
An efficient flamelet progress-variable method for modeling non-premixed flames in weak electric fields
Combustion stabilization and enhancement of the flammability limits are
mandatory objectives to improve nowadays combustion chambers. At this purpose,
the use of an electric field in the flame region provides a solution which is,
at the same time, easy to implement and effective to modify the flame
structure. The present work describes an efficient flamelet progress-variable
approach developed to model the fluid dynamics of flames immersed in an
electric field. The main feature of this model is that it can use complex
ionization mechanisms without increasing the computational cost of the
simulation. The model is based on the assumption that the combustion process is
not directly influenced by the electric field and has been tested using two
chemi-ionization mechanisms of different complexity in order to examine its
behavior with and without the presence of heavy anions in the mixture. Using a
one- and two-dimensional numerical test cases, the present approach has been
able to reproduce all the major aspects encountered when a flame is subject to
an imposed electric field and the main effects of the different chemical
mechanisms. Moreover, the proposed model is shown to produce a large reduction
in the computational cost, being able to shorten the time needed to perform a
simulation up to 40 times.Comment: 26 pages, 13 figures, paper accepted for publication on Computers and
Fluid
Multipass automata and group word problems
We introduce the notion of multipass automata as a generalization of pushdown
automata and study the classes of languages accepted by such machines. The
class of languages accepted by deterministic multipass automata is exactly the
Boolean closure of the class of deterministic context-free languages while the
class of languages accepted by nondeterministic multipass automata is exactly
the class of poly-context-free languages, that is, languages which are the
intersection of finitely many context-free languages. We illustrate the use of
these automata by studying groups whose word problems are in the above classes
Retirement expectations, pension reforms, and their effect on private wealth accumulation
We estimate the effect of pension reforms on households’ expectations of retirement outcomes and private wealth accumulation decisions exploiting a decade of intense Italian pension reforms as a source of exogenous variation in expected pension wealth. The Survey of Household Income and Wealth, a large random sample of the Italian population, elicits expectations of the age at which workers expect to retire and of the ratio of pension benefits to pre-retirement income between 1989 and 2002. We find that workers have revised expectations in the direction suggested by the reform and that there is substantial offset between private wealth and perceived pension wealth, particularly by workers that are better informed about their pension wealth
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