3,691 research outputs found
Caravan Awnings: a Geometrical Problem
Two questions regardingthe design of caravan awnings were posed by a company.The company wishes to produce awnings with a pretty appearance. When an awning is attached to a caravan, some wrinkles could appear. We developed some methods to avoid the wrinkles. The problem is restricted to awnings which are made from one piece of cloth
Transition to turbulence in particle laden flows
Suspended particles can alter the properties of fluids and in particular also
affect the transition from laminar to turbulent flow. In the present
experimental study, we investigate the impact of neutrally buoyant, spherical
inertial particles on transition to turbulence in a pipe flow. At low particle
concentrations, like in single phase Newtonian fluids, turbulence only sets in
when triggered by sufficiently large perturbations and, as characteristic for
this transition localized turbulent regions (puffs) co-exist with laminar flow.
In agreement with earlier studies this transition point initially moves to
lower Reynolds number (Re) as the particle concentration increases. At higher
concentrations however the nature of the transition qualitatively changes:
Laminar flow gives way to a globally fluctuating state following a continuous,
non-hysteretic transition. A further increase in Re results in a secondary
instability where localized puff-like structures arise on top of the uniformly
fluctuating background flow. At even higher concentration only the uniformly
fluctuating flow is found and signatures of Newtonian type turbulence are no
longer observed
Conspicious Consumption, Economic Growth, and Taxation: A Generalization
This paper studies the infuence of consumption externalities in the Ramsey model. In contrast to the recent literature, a quite general specification of preferences is used and the concept of the effective intertemporal elasticity of substitution is introduced. We give conditions for the observational equivalence between economies with consumption externalities and externality-free economies. An additional key result is that there exist several types of instantaneous utility functions in which the decentralized solution coincides with the socially planned one in spite of the presence of consumption externalities. The conditions for optimal taxation are also derived.Social status, Conspicuous consumption, Economic growth
Status Seeking in the Small Open Economy
In our modified version of the small open economy Ramsey model, we assume that agents have preferences over consumption and status which, in turn, is determined by relative wealth. This extension potentially eliminates the standard model's counterfactual result that an impatient country over time mortgages all of its capital and labor income. We show that the steady-state values of net assets and consumption, the speed of convergence and, in particular, the direction of adjustment during the transition depend crucially upon the degree of status consciousness. The latter also influences the economy's response to macro-economic shocks.Status seeking, Relative wealth, Open economy dynamics
The Quest for Status and Endogenous Labor Supply. The Relative Wealth Framework
This paper introduces the quest for status into the Ramsey model with endogenous labor supply. We focus our attention on relative wealth preferences. In contrast to relative consumption preferences, they allow for the possibility that agents work too little in the long run, while under both specifications the steady-state levels of consumption and the stock of physical capital exceed their socially optimal counterparts. The initial phase of transitional dynamics is unambiguously characterized by under-consumption and excessive work effort. The social optimum can be replicated by taxing capital income, where the optimal tax rate increases as physical capital accumulates.Status, Relative consumption, Relative wealth, Endogenous labor supply
Experimental investigation of transitional flow in a toroidal pipe
The flow instability and further transition to turbulence in a toroidal pipe
(torus) with curvature (tube-to-coiling diameter) 0.049 is investigated
experimentally. The flow inside the toroidal pipe is driven by a steel sphere
fitted to the inner pipe diameter. The sphere is moved with constant azimuthal
velocity from outside the torus by a moving magnet. The experiment is designed
to investigate curved pipe flow by optical measurement techniques. Using
stereoscopic particle image velocimetry, laser Doppler velocimetry and pressure
drop measurements, the flow is measured for Reynolds numbers ranging from 1000
to 15000. Time- and space-resolved velocity fields are obtained and analysed.
The steady axisymmetric basic flow is strongly influenced by centrifugal
effects. On an increase of the Reynolds number we find a sequence of
bifurcations. For Re=4075 a supercritical bifurcation to an oscillatory flow is
found in which waves travel in the streamwise direction with a phase velocity
slightly faster than the mean flow. The oscillatory flow is superseded by a
presumably quasi-periodic flow at a further increase of the Reynolds number
before turbulence sets in. The results are found to be compatible, in general,
with earlier experimental and numerical investigations on transition to
turbulence in helical and curved pipes. However, important aspects of the
bifurcation scenario differ considerably
Relative Consumption and Endogenous Labour Supply in the Ramsey Model: Do Status-Conscious People Work Too Much?
This paper introduces consumption externalities into a Ramsey-type model with endogenous labour supply and homogeneous agents. The instantaneous utility of any consumer is assumed to depend on work effort, own consumption and relative consumption, where the latter determines the individual's status in the society. Appropriate normality conditions with respect to consumption and leisure ensure that at least in the long run status-conscious individuals consume and work too much, compared to the social optimum, and that the capital stock is too high. Public policy can, however, induce the private sector to attain the social optimum by designing an optimal consumption tax policy.Status, Relative consumption, Work effort
Exceeding the asymptotic limit of polymer drag reduction
The drag of turbulent flows can be drastically decreased by addition of small
amounts of high molecular weight polymers. While drag reduction initially
increases with polymer concentration, it eventually saturates to what is known
as the maximum drag reduction (MDR) asymptote; this asymptote is generally
attributed to the dynamics being reduced to a marginal yet persistent state of
subdued turbulent motion. Contrary to this accepted view we will show in the
following that for an appropriate choice of parameters polymers can reduce the
drag beyond the suggested asymptotic limit, eliminating turbulence and giving
way to laminar flow. However at higher polymer concentrations the laminar state
becomes unstable, resulting in a fluctuating flow with the characteristic drag
of the MDR asymptote. The asymptotic state is hence dynamically disconnected
from ordinary turbulence.Comment: 6 pages, 6 figure
Dynamics of viscoelastic pipe flow in the maximum drag reduction limit
Polymer additives can substantially reduce the drag of turbulent flows and
the upper limit, the so called "maximum drag reduction" (MDR) asymptote is
universal, i.e. independent of the type of polymer and solvent used. Until
recently, the consensus was that, in this limit, flows are in a marginal state
where only a minimal level of turbulence activity persists. Observations in
direct numerical simulations using minimal sized channels appeared to support
this view and reported long "hibernation" periods where turbulence is
marginalized. In simulations of pipe flow we find that, indeed, with increasing
Weissenberg number (Wi), turbulence expresses long periods of hibernation if
the domain size is small. However, with increasing pipe length, the temporal
hibernation continuously alters to spatio-temporal intermittency and here the
flow consists of turbulent puffs surrounded by laminar flow. Moreover, upon an
increase in Wi, the flow fully relaminarises, in agreement with recent
experiments. At even larger Wi, a different instability is encountered causing
a drag increase towards MDR. Our findings hence link earlier minimal flow unit
simulations with recent experiments and confirm that the addition of polymers
initially suppresses Newtonian turbulence and leads to a reverse transition.
The MDR state on the other hand results from a separate instability and the
underlying dynamics corresponds to the recently proposed state of
elasto-inertial-turbulence (EIT).Comment: 18 pages, 5 figure
The Quest for Status and Endogenous Labor Supply: The Relative Wealth Framework
This paper introduces the quest for status into the Ramsey model with endogenous labor supply. We focus our attention on relative wealth preferences. In contrast to relative consumption preferences, they allow for the possibility that agents work too little in the long run, while under both specifications the steady-state levels of consumption and the stock of physical capital exceed their socially optimal counterparts. The initial phase of transitional dynamics is unambiguously characterized by under-consumption and excessive work effort. The social optimum can be replicated by taxing capital income, where the optimal tax rate increases as physical capital accumulates.Status, Relative Consumption, Relative Wealth, Endogenous Labor Supply
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