Damped Fluctuations Research Articles

Bioreactor scale‐down studies of suspended plant cell cultures

The effect of dissolved oxygen fluctuations on suspended plant cells was investigated using a scale‐down stirred bioreactor system to simulate the heterogeneous conditions found in large‐scale vessels. Transgenic Nicotiana tabacum cells expressing IgG1 antibody and Thalictrum minus cells producing berberine were subjected to gas switching cycles based on a stochastic model of log‐normal circulation time distributions. Because pumping severely damaged the cells, the scale‐down approach was restricted to a single‐vessel system. Dissolved oxygen oscillations of amplitude about ±5% air saturation were achieved in the N. tabacum cultures; however, most simulations produced highly damped fluctuations due to oxygen transfer limitations and no significant effect on the cultures was observed. In contrast, berberine production by T. minus cells was reduced when fluctuations of ±10–15% air saturation were superimposed on mean dissolved oxygen tensions of 25 and 35% air saturation. This work demonstrates that dissolved oxygen gradients such as those found in large‐scale mixing vessels can be detrimental to plant cell cultures even when sufficient average dissolved oxygen levels are provided. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4281–4288, 2018

Nonlinear growth of structure in cosmologies with damped matter fluctuations

We investigate the nonlinear evolution of structure in variants of the standard cosmological model which display damped density fluctuations relative to cold dark matter (e.g. in which cold dark matter is replaced by warm or interacting DM). Using N-body simulations, we address the question of how much information is retained from different scales in the initial linear power spectrum following the nonlinear growth of structure. We run a suite of N-body simulations with different initial linear matter power spectra to show that, once the system undergoes nonlinear evolution, the shape of the linear power spectrum at high wavenumbers does not affect the non-linear power spectrum, while it still matters for the halo mass function. Indeed, we find that linear power spectra which differ from one another only at wavenumbers larger than their half-mode wavenumber give rise to (almost) identical nonlinear power spectra at late times, regardless of the fact that they originate from different models with damped fluctuations. On the other hand, the halo mass function is more sensitive to the form of the linear power spectrum. Exploiting this result, we propose a two parameter model of the transfer function in generic damped scenarios, and show that this parametrisation works as well as the standard three parameter models for the scales on which the linear spectrum is relevant.

Версия элементарной неомарксистской модели экономических колебаний

The world economic and financial crisis has revealed the weakness of modern economics mainstream theories and the related models in explaining the capitalist economy dynamics. These are first of all real business cycles and New Keynesian dynamic stochastic general equilibrium models. They treat economic fluctuations as a result of unexpected shocks. The Marx's reproduction theory can become an alternative as it analyses cyclicality as an immanent feature of the capitalist mode of production arising from fundamental contradictions between labour and capital. Some of its concepts have been formalized in the predator-prey Class-Struggle Model by R. Goodwin. The model was a theoretical tool to investigate interactions between changes in employment rate (a labour market indicator) and share of labour in national income (representing a distribution of the value produced between workers and capitalists). Nevertheless, nor it neither subsequent modifications were empirically valid. Researchers who tried to verify the models by bringing them to data have pointed to a misspecification as a possible reason. To our mind, the point is that the models by R. Goodwin and his disciples reflect a distributive conflict between labour and capital which is subordinate to the main contradiction regarding production relations. The article states a prey-predator type model version based on the reproduction theory by K. Marx. The variables are productive capital and surplus value which change in time along damped oscillations trajectories. Within the present framework the economic dynamics is seen as a subsequence of damped fluctuations each of which is characterised by own specific parameters (amplitude, duration), which in turn depend on fostered innovations and their diffusion in the economy.

Stochastic resonance of a linear harmonic oscillator with non-linear damping fluctuation

Although non-linear noise exists far more widely in actual systems than linear noise, the study on non-linear noise is far from meeting the needs of practical situations as yet. The phenomenon of stochastic resonance (SR) is a non-linear cooperative effect which is jointly produced by signal, noise, and system, obviously, it is closely related to the nature of the noise. As a result, the non-linear nature of the non-linear noise has an inevitable impact on the dynamic behavior of a system, so it is of great significance to study the non-linear noise's influence on the dynamic behavior of the system. The linear harmonic oscillator is the most basic model to describe different phenomena in nature, and the quadratic noise is the most basic non-linear noise. In this paper, we consider a linear harmonic oscillator driven by an external periodic force and a quadratic damping fluctuation. For the proposed model, we focus on the effect of non-linear nature of quadratic fluctuation on the system's resonant behavior. Firstly, by the use of the Shapiro-Loginov formula and the Laplace transform technique, the analytical expressions of the first moment and the steady response amplitude of the output signal are obtained. Secondly, by studying the impacts of noise parameters and system intrinsic frequency, the non-monotonic behaviors of the steady response amplitude are found. Finally, numerical simulations are presented to verify the effectiveness of the analytical result. According to the research, we have the following conclusions: (1) The steady response amplitude is a non-monotonic function of coefficients of the quadratic damping fluctuation. Furthermore, the non-linear damping fluctuation is easier to contribute the system's enhancing response to the external periodic signal than the linear fluctuation. (2) The evolution of the steady response amplitude versus noise intensity presents more resonant behaviors. One-peak SR phenomenon and double-peak SR phenomenon are observed at different values of coefficients of the quadratic noise, particularly, the SR phenomenon disappears at the positive quadratic coefficient of the quadratic noise. (3) The evolution of the steady response amplitude versus the system intrinsic frequency presents true resonance, i. e. the phenomenon of resonance appears when the external signal frequency is equal to the system intrinsic frequency. True resonance is not observed in the linear harmonic oscillator driven by a linear damping fluctuation as yet. In conclusion, all the researches show that the non-linear nature of non-linear noise plays a key role in system's resonant behavior, in addition, the non-linear damping fluctuation is conductive to the detection and frequency estimation of weak periodic signal.

Communication: Memory effects and active Brownian diffusion.

A self-propelled artificial microswimmer is often modeled as a ballistic Brownian particle moving with constant speed aligned along one of its axis, but changing direction due to random collisions with the environment. Similarly to thermal noise, its angular randomization is described as a memoryless stochastic process. Here, we speculate that finite-time correlations in the orientational dynamics can affect the swimmer's diffusivity. To this purpose, we propose and solve two alternative models. In the first one, we simply assume that the environmental fluctuations governing the swimmer's propulsion are exponentially correlated in time, whereas in the second one, we account for possible damped fluctuations of the propulsion velocity around the swimmer's axis. The corresponding swimmer's diffusion constants are predicted to get, respectively, enhanced or suppressed upon increasing the model memory time. Possible consequences of this effect on the interpretation of the experimental data are discussed.

Nocturnal oscillations in plasma renin activity during sleep in hypertensive patients: the influence of perindopril

In previous studies, we established a strong concordance between nocturnal oscillations in plasma renin activity (PRA) and REM-NREM sleep cycles. To determine whether this relation persists in the case of moderate essential hypertension and if it is influenced by antihypertensive therapies affecting renin release, six normal subjects and six hypertensive patients were studied. The normal subjects underwent one control night. The hypertensive patients were studied during a first night when a placebo was given. Four of them underwent a second night following a single dose of an angiotensin-converting enzyme (ACE) inhibitor, perindopril; and a third night, 45 days later, with the antihypertensive treatment. In addition, two of the patients underwent two night-studies, after a single and repeated doses of a beta-blocker, atenolol, to see whether preventing renin release modified the sleep structure. The relationship between the nocturnal PRA oscillations and the sleep stage patterns persisted in hypertensive patients receiving placebo. In patients who had low PRA levels, the increases associated with NREM sleep were small. However, the mean relative amplitude of the oscillations, expressed as a percentage of the nocturnal mean, was about 60%, which was similar to that in normotensive subjects. Active renin and PRA oscillations were closely coupled. ACE activity profiles displayed damped fluctuations and no systematic relationship with sleep stages.(ABSTRACT TRUNCATED AT 250 WORDS)

Stochastic response of nonlinear structures with parameter random fluctuations

The random response of a nonlinear structural system is examined when its parameters are experiencing random fluctuations with time. The treatment is based on the recent developments in the mathematical theory of stochastic differential equations. These include the Ito stochastic calculus and the Fokker-Planck equation approach to derive a general differential equation that describes the evolution of the statistical moments of the response coordinates. The differential equation is found to constitute an infinite coupled set of differential equations that are closed via two different closure schemes. The system response is determined in the neighborhood of internal resonance condition and for various random intensities of the system parameters. It is found that the random modal interaction is governed mainly by the internal resonance ratio and the stiffness fluctuation intensity. The effect of the random damping fluctuation on the system response is found to be very small compared to the stiffness fluctuation effect.