3,106 research outputs found
An optimal path to transition in a duct
This paper is concerned with the transition of the laminar flow in a duct of
square cross-section. Like in the similar case of the pipe flow, the motion is
linearly stable for all Reynolds numbers, rendering this flow a suitable
candidate for a study of the 'bypass' path to turbulence. It has already been
shown \citep{Biau_JFM_2008} that the classical linear optimal perturbation
problem, yielding optimal disturbances in the form of longitudinal vortices,
fails to provide an 'optimal' path to turbulence, i.e. optimal perturbations do
not elicit a significant nonlinear response from the flow. Previous simulations
have also indicated that a pair of travelling waves generates immediately, by
nonlinear quadratic interactions, an unstable mean flow distortion, responsible
for rapid breakdown. By the use of functions quantifying the sensitivity of the
motion to deviations in the base flow, the 'optimal' travelling wave associated
to its specific defect is found by a variational approach. This optimal
solution is then integrated in time and shown to display a qualitative
similarity to the so-called 'minimal defect', for the same parameters. Finally,
numerical simulations of a 'edge state' are conducted, to identify an unstable
solution which mediates laminar-turbulent transition and relate it to results
of the optimisation procedure
Transition to turbulence in duct flow
The transition of the flow in a duct of square cross-section is studied. Like
in the similar case of the pipe flow, the motion is linearly stable for all
Reynolds numbers; this flow is thus a good candidate to investigate the
'bypass' path to turbulence. Initially the so-called 'linear optimal
perturbation problem' is formulated and solved, yielding optimal disturbances
in the form of longitudinal vortices. Such optimals, however, fail to elicit a
significant response from the system in the nonlinear regime. Thus,
streamwise-inhomogeneous, sub-optimal disturbances are focussed upon; nonlinear
quadratic interactions are immediately evoked by such initial perturbations and
an unstable streamwise-homogeneous large amplitude mode rapidly emerges. The
subsequent evolution of the flow, at a value of the Reynolds number at the edge
between fully developed turbulence and relaminarization, shows the alternance
of patterns with two pairs of large scale vortices near opposing parallel
walls. Such edge states bear a resemblance to optimal disturbance
The purpose and role of product standards in the commercialization of new energy technologies : a preliminary analysis
This paper discusses the role of product standards in a
commercialization program. The usefulness of standards is explained as
arising from the existence of transaction costs and other market failures
in the operation of markets, and the effects of standards upon market
operation are broadly discussed. The role of standards in a
commercialization program is then explored and is seen as justified by
the existence of market failures and the lack of suitable options other
than standards available for remedying certain particulars of the
situation. A description of the voluntary standards system follows to
show how its use in a commercialization program may modify the
governmental role
Comments on proposed rulemaking concerning electric rates for solar users
Energy Laboratory Utility Systems Program
The changing landscape of the Liesbeek River valley : an investigation of the use of an environmental history approach in historical research and in classroom practice
Includes bibliography.This dissertation has two components, one History and one Education, and the central unifying theme is Environmental History. The History component examines the historiography of this sub-discipline, and then applies an environmental analysis as an example of its use in historical research. The second component explores the use of Environmental History in the teaching of school history, and presents a curriculum model which uses this approach. Both components use the Liesbeek River valley in the Cape Peninsula as a case-study
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