1,637 research outputs found
Amplification of Cosmological Inhomogeneities by the QCD Transition
The cosmological QCD transition affects primordial density perturbations. If
the QCD transition is first order, the sound speed vanishes during the
transition and density perturbations fall freely. For scales below the Hubble
radius at the transition the primordial Harrison-Zel'dovich spectrum of density
fluctuations develops large peaks and dips. These peaks grow with wave number
for both the hadron-photon-lepton fluid and for cold dark matter. At the
horizon scale the enhancement is small. This by itself does not lead to the
formation of black holes at the QCD transition. The peaks in the
hadron-photon-lepton fluid are wiped out during neutrino decoupling. For cold
dark matter that is kinetically decoupled at the QCD transition (e.g., axions
or primordial black holes) these peaks lead to the formation of CDM clumps of
masses .Comment: 39 pages, 10 figures, RevTeX; (1) ETH Zuerich, (2) Univ. Frankfurt;
improved presentation of 'Introduction' and 'Collisional Damping at Neutrino
Decoupling', results unchanged; accepted for publication in Phys. Rev.
The Role of Social Capital in the Industrialization of the Food System
Selfishness of preferences alone will not support the coordination necessary for the industrialization of the food system. Social capital relationships of mutual sympathy (caring) yield socio-emotional goods that are important in the more personal business world of evolving incomplete contracts and alliances involving input suppliers, processors, and labor. Relationships are also critical when consumers are buying image as well as physical products. Management and policy alternatives constitute investment in social capital that can affect opportunism, risk, loyalty, and trust.Agricultural Finance,
A physics-based approach to flow control using system identification
Control of amplifier flows poses a great challenge, since the influence of environmental noise sources and measurement contamination is a crucial component in the design of models and the subsequent performance of the controller. A modelbased approach that makes a priori assumptions on the noise characteristics often yields unsatisfactory results when the true noise environment is different from the assumed one. An alternative approach is proposed that consists of a data-based systemidentification technique for modelling the flow; it avoids the model-based shortcomings by directly incorporating noise influences into an auto-regressive (ARMAX) design. This technique is applied to flow over a backward-facing step, a typical example of a noise-amplifier flow. Physical insight into the specifics of the flow is used to interpret and tailor the various terms of the auto-regressive model. The designed compensator shows an impressive performance as well as a remarkable robustness to increased noise levels and to off-design operating conditions. Owing to its reliance on only timesequences of observable data, the proposed technique should be attractive in the design of control strategies directly from experimental data and should result in effective compensators that maintain performance in a realistic disturbance environment
Modal and nonmodal stability analysis of electrohydrodynamic flow with and without cross-flow
We report the results of a complete modal and nonmodal linear stability
analysis of the electrohydrodynamic flow (EHD) for the problem of
electroconvection in the strong injection region. Convective cells are formed
by Coulomb force in an insulating liquid residing between two plane electrodes
subject to unipolar injection. Besides pure electroconvection, we also consider
the case where a cross-flow is present, generated by a streamwise pressure
gradient, in the form of a laminar Poiseuille flow. The effect of charge
diffusion, often neglected in previous linear stability analyses, is included
in the present study and a transient growth analysis, rarely considered in EHD,
is carried out. In the case without cross-flow, a non-zero charge diffusion
leads to a lower linear stability threshold and thus to a more unstable low.
The transient growth, though enhanced by increasing charge diffusion, remains
small and hence cannot fully account for the discrepancy of the linear
stability threshold between theoretical and experimental results. When a
cross-flow is present, increasing the strength of the electric field in the
high- Poiseuille flow yields a more unstable flow in both modal and
nonmodal stability analyses. Even though the energy analysis and the
input-output analysis both indicate that the energy growth directly related to
the electric field is small, the electric effect enhances the lift-up
mechanism. The symmetry of channel flow with respect to the centerline is
broken due to the additional electric field acting in the wall-normal
direction. As a result, the centers of the streamwise rolls are shifted towards
the injector electrode, and the optimal spanwise wavenumber achieving maximum
transient energy growth increases with the strength of the electric field
Algebraically diverging modes upstream of a swept bluff body
Classical stability theory for swept leading-edge boundary layers predicts eigenmodes in the free stream with algebraic decay far from the leading edge. In this article, we extend the classical base flow solution by Hiemenz to a uniformly valid solution for the flow upstream of a bluff body, which includes a three-dimensional boundary layer, an inviscid stagnation-point flow and an outer parallel flow. This extended, uniformly valid base flow additionally supports modes which diverge algebraically outside the boundary layer. The theory of wave packet pseudomodes is employed to derive analytical results for the growth rates and for the eigenvalue spectra of this type of mode. The complete spectral analysis of the flow, including the algebraically diverging modes, will give a more appropriate basis for receptivity studies and will more accurately describe the interaction of perturbations in the free stream with disturbances in the boundary laye
Spontaneous spiking in an autaptic Hodgkin-Huxley set up
The effect of intrinsic channel noise is investigated for the dynamic
response of a neuronal cell with a delayed feedback loop. The loop is based on
the so-called autapse phenomenon in which dendrites establish not only
connections to neighboring cells but as well to its own axon. The biophysical
modeling is achieved in terms of a stochastic Hodgkin-Huxley model containing
such a built in delayed feedback. The fluctuations stem from intrinsic channel
noise, being caused by the stochastic nature of the gating dynamics of ion
channels. The influence of the delayed stimulus is systematically analyzed with
respect to the coupling parameter and the delay time in terms of the interspike
interval histograms and the average interspike interval. The delayed feedback
manifests itself in the occurrence of bursting and a rich multimodal interspike
interval distribution, exhibiting a delay-induced reduction of the spontaneous
spiking activity at characteristic frequencies. Moreover, a specific
frequency-locking mechanism is detected for the mean interspike interval.Comment: 8 pages, 10 figure
Algebraically decaying modes and wave packet pseudo-modes in swept Hiemenz flow
The modal structure of the swept Hiemenz flow, a model for the flow near the attachment line of a swept wing, consists of eigenfunctions which exhibit (super-)exponential or algebraic decay as the wall-normal coordinate tends to infinity. The subset of algebraically decaying modes corresponds to parts of the spectrum which are characterized by a significant sensitivity to numerical discretization. Numerical evidence further suggests that a continuous spectrum covering a two-dimensional range of the complex plane exists. We investigate the family of uniform swept Hiemenz modes using eigenvalue computations, numerical simulations and the concept of wave packet pseudo-modes. Three distinct branches of the family of algebraically decaying eigenmodes are identified. They can be superimposed to produce wavefronts propagating towards or away from the boundary layer and standing or travelling wave packets in the free stream. Their role in the exchange of information between the free stream and the attachment-line boundary layer for the swept Hiemenz flow is discussed. The concept of wave packet pseudo-modes has been critical in the analysis of this problem and is expected to lead to further insights into other shear flows in semi- or bi-infinite domain
Experimental control of natural perturbations in channel flow
International audienceA combined approach using system identification and feed-forward control design has been applied to experimental laminar channel flow in an effort to reduce the naturally occurring disturbance level. A simple blowing/suction strategy was capable of reducing the standard deviation of the measured sensor signal by 45 %, which markedly exceeds previously obtained results under comparable conditions. A comparable reduction could be verified over a significant streamwise extent, implying an improvement over previous, more localized disturbance control. The technique is effective, flexible, and robust, and the obtained results encourage further explorations of experimental control of convection-dominated flows
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