4,013 research outputs found
Statistics of turbulent fluctuations in counter-rotating Taylor-Couette flows
The statistics of velocity fluctuations of turbulent Taylor-Couette flow are
examined. The rotation rate of the inner and outer cylinder are varied while
keeping the Taylor number fixed to
(). The azimuthal velocity component of the flow
is measured using laser Doppler anemometry (LDA). For each experiment
datapoints are acquired and carefully analysed. Using extended
self-similarity (ESS) \cite{ben93b} the longitudinal structure function
exponents are extracted, and are found to weakly depend on the ratio of the
rotation rates. For the case where only the inner cylinder rotates the results
are in good agreement with results measured by Lewis and Swinney \cite{lew99}
using hot-film anemometry. The power spectra shows clear -5/3 scaling for the
intermediate angular velocity ratios , roughly -5/3 scaling for , and no clear scaling law can be found for
(inner cylinder rotation only); the local scaling exponent of the spectra has a
strong frequency dependence. We relate these observations to the shape of the
probability density function of the azimuthal velocity and the presence of a
neutral line
Railway timetabling from an operations research
In this paper we describe Operations Research (OR) models andtechniques that can be used for determining (cyclic) railwaytimetables. We discuss the two aspects of railway timetabling: ()the determination of arrival and departure times of the trains atthe stations and other relevant locations such as junctions andbridges, and () the assignment of each train to an appropriateplatform and corresponding inbound and outbound routes in everystation. Moreover, we discuss robustness aspects of bothsubproblems.
Turbulence strength in ultimate Taylor-Couette turbulence
We provide experimental measurements for the effective scaling of the
Taylor-Reynolds number within the bulk , based
on local flow quantities as a function of the driving strength (expressed as
the Taylor number Ta), in the ultimate regime of Taylor-Couette flow. The data
are obtained through flow velocity field measurements using Particle Image
Velocimetry (PIV). We estimate the value of the local dissipation rate
using the scaling of the second order velocity structure
functions in the longitudinal and transverse direction within the inertial
range---without invoking Taylor's hypothesis. We find an effective scaling of
, (corresponding
to for the dimensionless
local angular velocity transfer), which is nearly the same as for the global
energy dissipation rate obtained from both torque measurements
() and Direct Numerical Simulations
(). The resulting Kolmogorov length
scale is then found to scale as
and the turbulence intensity as . With both the local dissipation rate and the local
fluctuations available we finally find that the Taylor-Reynolds number
effectively scales as Re in the
present parameter regime of .Comment: 15 pages, 8 figures, J. Fluid Mech. (In press
Comparison of heuristic approaches for the multiple depot vehicle scheduling problem
Given a set of timetabled tasks, the multi-depot vehicle scheduling problemis a well-known problem that consists of determining least-cost schedulesfor vehicles assigned to several depots such that each task is accomplishedexactly once by a vehicle. In this paper, we propose to compare theperformance of five different heuristic approaches for this problem,namely, a heuristic \\mip solver, a Lagrangian heuristic, a columngeneration heuristic, a large neighborhood search heuristic using columngeneration for neighborhood evaluation, and a tabu search heuristic. Thefirst three methods are adaptations of existing methods, while the last twoare novel approaches for this problem. Computational results on randomlygenerated instances show that the column generation heuristic performs thebest when enough computational time is available and stability is required,while the large neighborhood search method is the best alternative whenlooking for a compromise between computational time and solution quality.tabu search;column generation;vehicle scheduling;heuristics;Lagrangian heuristic;large neighborhood search;multiple depot
Strategic Investment Under Uncertainty: Merging Real Options with Game Theory
As becomes apparent from the standard text books in industrial organization (cf.Tirole, 1988, The Theory of Industrial Organization), the analysis of the e.ects of uncertainty within this field is yet underdeveloped.This paper shows that the new theory of strategic real options can be used to fill this empty hole .Based on the work by Smets (1991) standard models are identified, and they are analyzed by applying a method involving symmetric mixed strategies.As an illustration, extensions regarding asymmetry, technology adoption and decreasing uncertainty over time are reviewed.Among others, it is found that the value of a high cost firm can increase in its own cost.Furthermore, it is established to what extent investments are delayed when technologial progress is anticipated, and it is found that competition can be bad for welfare.productivity;innovation;technology;corporate finance;valuation;capital budgeting;investment;game theory;uncertainty
Re-scheduling in railways: the rolling stock balancing problem
This paper addresses the Rolling Stock Balancing Problem (RSBP). This problem arises at a passenger railway operator when the rolling stock has to be re-scheduled due to changing circumstances. These problems arise both in the planning process and during operations. The RSBP has as input a timetable and a rolling stock schedule where the allocation of the rolling stock among the stations does not fit to the allocation before and after the planning period. The problem is then to correct these off-balances, leading to a modified schedule that can be implemented in practice.For practical usage of solution approaches for the RSBP, it is important to solve the problem quickly. Therefore, the focus is on heuristic approaches. In this paper, we describe two heuristics and compare them with each other on some (variants of) real-life instances of NS, the main Dutch passenger railway operator. Finally, to get some insight in the quality of the proposed heuristics, we also compare their outcomes with optimal solutions obtained by solving existing rolling stock circulation models.heuristics;railway planning;integer linear programming;rolling stock re-scheduling
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