Perturbation Timescale Research Articles

Experimental study on curvature effects and preferential diffusion for perturbed laminar premixed ammonia-air flames

Ammonia-air flames are known for low reactivity and have been posing as a huge hindrance in employing the chemical as a sustainable fuel of tomorrow. Curvature is a parameter that could influence the flame structure and so the position of the maximum heat release rate. Flame-acoustic interactions on a Bunsen burner are performed to study the local flame response to highly perturbed flows. NH2* chemiluminescence is used to study the reactivity of these flames. Non-perturbed flames are used as a reference to understand the inherent behaviour of Bunsen ammonia flames. A case study has been chosen for an equivalence ratio ranging between 1.0 and 1.4 at atmospheric conditions to study perturbed flames. The objective is to study the effect of curvature induced by the perturbations on the reactivity of the flame. It was seen that this given case study was quite complex as the flame response was to multiple factors like the effect of Lewis number, convective-diffusion velocities, decomposition of ammonia into hydrogen, thereby, promoting preferential diffusion of hydrogen in both large-scale and locally for certain cases apart from the generated acoustic perturbation which itself dictates the flow regime of the fresh gases, etc. Since the Damköhler number was around 1, the perturbation time scales and the reactivity time scales were comparable and so none of the effects could be ignored. It was concluded that for richer flames where Le>1, the negative curvature promoted the production of hydrogen leading to local enhancement in reactivity. A change in the local thickness due to the induced curvature was seen for all conditions.

Low perturbation limit decoherence analyzed by scaling the Double Quantum Hamiltonian

By varying the magnitude of the effective interaction between spins in relation to the perturbations, we study the decoherence behavior in a connected proton system. Making use of the Magnus expansion, we introduce a NMR pulse sequence that generates an average Hamiltonian with Double Quantum terms multiplied by a scaling factor, δ, with the possibility to take positive and negative values. The performance of the pulse sequence for different values of the scaling factors was validated in polycrystalline adamantane, by observing the evolution of the polarization. A time reversal procedure, accessible through the change of sign in the controlled Hamiltonian, was necessary to observe multiple quantum coherences. The spin counting develops a characteristic growth in two species of clusters for the scaled time. The influence of the scaling factor on the reversibility was observed through the behavior of the Loschmidt echoes, which decayed faster as the scaling factor increases. From the analysis of dynamics and its reversibility, we extracted characteristic times for the spin diffusion, T2δ and the intrinsic decoherence decay, T3δ for each scaling factor δ, and perturbation time scale, TΣ. Observing the dependence of reversibility vs. perturbation rates, both normalized with the spin diffusion rate, we find that in the limit of low perturbations, T2δ/T3δ deviates from the linear dependence on T2δ/TΣ that corresponds to strong perturbation. The asymptotic value T2/T3≈0.15 as T2δ/TΣ vanishes, gives evidence that the main source of irreversibility is the intrinsic decoherence associated to the chaotic many-body dynamics of the system.

Examining AGN UV/Optical Variability beyond the Simple Damped Random Walk

We present damped harmonic oscillator (DHO) light-curve modeling for a sample of 12,714 spectroscopically confirmed quasars in the Sloan Digital Sky Survey Stripe 82 region. DHO is a second-order continuous-time autoregressive moving-average process, which can be fully described using four independent parameters: a natural oscillation frequency (ω 0), a damping ratio (ξ), a characteristic perturbation timescale (τ perturb), and an amplitude for the perturbing white noise (σ ϵ ). The asymptotic variability amplitude of a DHO process is quantified by σ DHO—a function of ω 0, ξ, τ perturb, and σ ϵ . We find that both τ perturb and σ ϵ follow different dependencies with rest-frame wavelength (λ RF) on either side of 2500 Å, whereas σ DHO follows a single power-law relation with λ RF. After correcting for wavelength dependence, σ DHO exhibits anticorrelations with both the Eddington ratio and the black hole mass, while τ perturb—with a typical value of days in the rest frame—shows an anticorrelation with the bolometric luminosity. Modeling active galactic nuclei (AGN) variability as a DHO offers more insight into the workings of accretion disks close to the supermassive black holes at the center of AGN. The newly discovered short-term variability (characterized by τ perturb and σ ϵ ) and its correlation with bolometric luminosity pave the way for new algorithms that will derive fundamental properties (e.g., Eddington ratio) of AGN using photometric data alone.

Resonance capture and long-term evolution of planets in binary star systems

Aims. The growing population of planets discovered in orbit around one stellar component of a binary star raises the question of the influence of the binary companion on the formation process of planetary systems. The aim of this work is to study the impact of a binary companion on the evolution of two-planet systems during both the Type-II migration phase and their long-term evolution after the dissipation of the protoplanetary disk. Methods. We used the symplectic integrator SyMBA, modified to include a wide binary companion. We also included the Type-II migration of giant planets during the protoplanetary disk phase with suitable eccentricity and inclination damping as well as the gravitational potential acting on the planets due to the disk and the nodal precession of the disk induced by the binary companion. We considered various inclinations, eccentricities, and separations of the binary companion. Results. Disk migration allows for the formation of planet pairs in mean-motion resonances despite the presence of the binary companion. When the binary separation is wide (1000 au), the timescale of the perturbations that it raises on the planets is longer than the disk’s lifetime and resonant pairs are routinely formed in the 2:1, 5:2, and 3:1 commensurabilities. Provided the planet-planet interaction timescale is smaller than the timescale of binary perturbations, these systems can remain in resonance long after the disk has dissipated. When the binary separation is smaller (250 au), only planets in the 2:1 resonance tend to remain in a resonant state and more chaotic evolutions are observed, as well as more ejections. After those ejections, the remaining planet can become eccentric due to the perturbations from the binary companion in addition, for strongly inclined binary companions, captures in the von Ziepel-Lidov-Kozai resonance can occur. Whereas in systems with two planets, this mechanism is quenched by planet-planet interactions. Our simulations reveal that the interplay between planet-disk, planet-planet, and planet-binary interactions can lead to the formation of resonant pairs of planets which remain stable over timescales much longer than the disk’s lifetime.

A Comprehensive Perturbative Formalism for Phase Mixing in Perturbed Disks. I. Phase Spirals in an Infinite, Isothermal Slab

Galactic disks are highly responsive systems that often undergo external perturbations and subsequent collisionless equilibration, predominantly via phase mixing. We use linear perturbation theory to study the response of infinite isothermal slab analogs of disks to perturbations with diverse spatiotemporal characteristics. Without self-gravity of the response, the dominant Fourier modes that get excited in a disk are the bending and breathing modes, which, due to vertical phase mixing, trigger local phase-space spirals that are one- and two-armed, respectively. We demonstrate how the lateral streaming motion of slab stars causes phase spirals to damp out over time. The ratio of the perturbation timescale (τ P) to the local, vertical oscillation time (τ z ) ultimately decides which of the two modes is excited. Faster, more impulsive (τ P < τ z ) and slower, more adiabatic (τ P > τ z ) perturbations excite stronger breathing and bending modes, respectively, although the response to very slow perturbations is exponentially suppressed. For encounters with satellite galaxies, this translates to more distant and more perpendicular encounters triggering stronger bending modes. We compute the direct response of the Milky Way disk to several of its satellite galaxies and find that recent encounters with all of them excite bending modes in the solar neighborhood. The encounter with Sagittarius triggers a response that is at least 1–2 orders of magnitude larger than that due to any other satellite, including the Large Magellanic Cloud. We briefly discuss how ignoring the presence of a dark matter halo and the self-gravity of the response might impact our conclusions.

Emergent decoherence induced by quantum chaos in a many-body system: A Loschmidt echo observation through NMR

In the long quest to identify and compensate the sources of decoherence in many-body systems far from the ground state, the varied family of Loschmidt echoes (LEs) became an invaluable tool in several experimental techniques. A LE involves a time-reversal procedure to assess the effect of perturbations in a quantum excitation dynamics. However, when addressing macroscopic systems one is repeatedly confronted with limitations that seem insurmountable. This led to the formulation of the central hypothesis of irreversibility stating that the timescale of decoherence, ${T}_{3}$, is proportional to the timescale of the many-body interactions we reversed, ${T}_{2}$. We test this by implementing two experimental schemes based on Floquet Hamiltonians where the effective strength of the dipolar spin-spin coupling, i.e., $1/{T}_{2}$, is reduced by a variable scale factor $k$. This extends the perturbation timescale, ${T}_{\mathrm{\ensuremath{\Sigma}}}$, in relation to ${T}_{2}$. Strikingly, we observe the superposition of the normalized Loschmidt echoes for the bigger values of $k$. This manifests the dominance of the intrinsic dynamics over the perturbation factors, even when the Loschmidt echo is devised to reverse that intrinsic dynamics. Thus, in the limit where the reversible interactions dominate over perturbations, the LE decays within a timescale, ${T}_{3}\ensuremath{\approx}{T}_{2}/R$ with $R=(0.15\ifmmode\pm\else\textpm\fi{}0.01)$, confirming the emergence of a perturbation independent regime. These results support the central hypothesis of irreversibility.

Variation cascades: resource pulses and top-down effects across time scales.

Top-down and bottom-up theories of trophic control have been fundamental to our understanding of community dynamics and structure. However, most ecological theories have focused on equilibrium dynamics and do not provide predictions for communities' responses in temporally fluctuating environments. By deriving the frequency response of populations in different trophic communities, we extend the top-down and bottom-up theories of ecology to include how temporal fluctuations in potential primary productivity percolate up the food chain and are re-expressed as population variability. Moreover, by switching from a time-based representation into the frequency domain, we provide a unified method to compare how the time scale of perturbations determines communities' responses. At low frequencies, primary producers and secondary consumers have the highest temporal variability, while the primary consumers are relatively stable. Similar to the Exploitation Ecosystem Hypothesis, top-down effects drive this alternating pattern of variability. We define the top-down effect of consumers on the variability of lower trophic levels as a variation cascade. However, at intermediate frequencies, variation cascades can amplify temporal variation up the food chain. At high frequencies, variation cascades weaken, and fluctuations are attenuated up the food chain. In summary, we provide a novel theory for how communities will respond to fluctuations in productivity, and we show that indirect species interactions play a crucial role in determining community dynamics across the frequency spectrum.

Evaluation of Stochastic Perturbed Parameterization Tendencies on Convective-Permitting Ensemble Forecasts of Heavy Rainfall Events in New York and Taiwan

Abstract The representation of model error in ensemble prediction systems (EPSs) can be limited by the assumptions within parameterization schemes. Stochastic perturbed parameterization tendencies (SPPT) is one representation of model error that randomly perturbs parameterized physical tendencies using a spatially and temporally correlated red-noise field. This research investigates the sensitivity of ensemble rainfall forecasts produced by the Weather Research and Forecasting (WRF) Model to the configuration of SPPT and independent SPPT (iSPPT) for three meso–synoptic-scale heavy rainfall events over the United States and Taiwan, primarily focusing on the ensemble mean and standard deviation as well as forecast skill. Thirty-two 20-member ensembles, which represent a combination of eight configurations of the stochastic perturbation time scale, length scale, and amplitude scale, and four perturbed parameterization schemes, as well as an unperturbed control simulation, are examined for each event. In each case, rainfall standard deviation is most sensitive to the perturbation time scale and amplitude scale. Moreover, microphysics tendency perturbations are associated with the largest standard deviation in two of the three events, followed by perturbations to the total (nonmicrophysics), turbulent mixing, and radiation parameterized tendencies. Additionally, microphysics tendency perturbations are associated with an increase in the areal coverage of heavy rainfall compared to the control forecast, regardless of whether the control forecast over or underrepresents the observed rainfall distribution.

Investigation into the thermo-hydrodynamics of ferrofluid flow under the influence of constant and alternating magnetic field by InfraRed Thermography

Convective flow of single-phase ferrofluids under the influence of constant and alternating magnetic field has attracted attention as an effective strategy for enhanced heat transfer in mini/micro thermal systems. In the present study, an attempt has been made to gain deep insight of the heat transfer characteristics of single-phase ferrofluid flow in a heated stainless steel tube under the influence of constant and time-varying magnetic field. The governing parameters are mainly the magnetic flux density (B) and perturbation frequency (f) of the applied magnetic field. Three magnetic flux density value of B = 0 G, 700 G and 1080 G have been used for constant magnetic field. Constant value of B = 1080 G was used for alternating magnetic field while, frequencies of applied magnetic field has been varied from 0.1 Hz to 5 Hz. Flow Reynolds number was kept constant to Re = 66. Some 2-D numerical simulations have also been performed to qualitatively support the experimental data. The study is focused to delineate the mechanism of augmentation of heat transfer through the interaction of available force fields, i.e., interplay of magnetic force and inertia of the flow, and also the effect of various time scales on the flow and thermal behavior. Major inferences of the study are (a) on the application of external magnetic (constant and alternating), heat transfer augments (b) existence of a threshold frequency of external magnetic field for maximum augmentation as outcome of advective time-scale and magnetic perturbation time-scale. InfraRed Thermography (IRT) has been used to measure the wall temperature, while, some bright field visualizations have also been done to qualitatively support the explanations of experimental data.

Plasma flow reactor studies of H2/O2/Ar kinetics

In the present study, a plasma flow reactor (PFR) facility designed to perform both ex situ and in situ experiments of stable (H2 and O2) and intermediate (OH radicals) species detection was used to examine the plasma-assisted characteristics of hydrogen oxidation at 1 atm pressure for temperatures ranging from 420 K to 1100 K. Experiments were performed at nearly isothermal conditions, by heavily diluting reactive mixtures in argon, in an attempt to mitigate temperature changes from exothermic chemical reactions. This technique allows experimental results to be interpreted from a perspective that plasma and thermal (neutral) heat release effects are decoupled, essentially isolating the effects of the plasma-chemistry and its influence on the neutral-chemistry. Results showed no thermal reaction until 860 K, at which point hydrogen was rapidly consumed within the flow residence time associated with the reactor. With the plasma discharge, the onset of oxidation was extended to lower temperatures (T < 860 K), while exhibiting a steady increase in the rate of oxidation starting from 470 K, and eventually consuming all the initial hydrogen by 800 K. Absolute measurements of OH mole fraction reveal that at conditions well below the dominance of the thermal chain-branched chemistry (at T = 668 K), the plasma induced a chain-propagating effect on OH formation, which was entirely confined to the boundaries of the plasma discharge section of the reactor. Furthermore, temporal measurements were also performed, showing that the extent of OH formation and subsequently its global effect on the fuel consuming chemistry can be manipulated based on the plasma perturbation timescale (i.e., the time scale at which the high-voltage pulses are administered to the reactive flow to generate the plasma discharge). Experimental results are compared to modeling calculations and show relatively good agreement, with the model predicting similar kinetic trends as a function of temperature. These results demonstrate new insight into the kinetics governing plasma-assisted combustion, and provide new experimental data to facilitate the development and validation of PAC-specific kinetic mechanisms.

Noninvasive, real-time measurements of plasma parameters via optical emission spectroscopy

Plasma process control applications require acquisition of diagnostic data at a rate faster than the characteristic timescale of perturbations to the plasma. Diagnostics based on optical emission spectroscopy of intense emission lines permit rapid noninvasive measurements with low-resolution (∼1 nm), fiber-coupled spectrographs, which are included on many plasma process tools for semiconductor processing. Here the authors report on rapid analysis of Ar emissions with such a system to obtain electron temperatures, electron densities, and metastable densities in argon and argon/mixed-gas (Ar/N2, Ar/O2, Ar/H2) inductively coupled plasmas. Accuracy of the results (compared to measurements made by Langmuir probe and white-light absorption spectroscopy) are typically better than ±15% with a time resolution of 0.1 s, which is more than sufficient to capture the transient behavior of many processes, limited only by the time response of the spectrograph used.

Abstract 5092: Metabolomic profiling of Oct-1 and c-myc deficient cells to obtain clinical and biological insights into tumorigenicity

Abstract Oct-1 and C-myc have recently been shown to be pro-tumorigenic and their expression is increased in various cancers. Despite recent surge in understanding the role of pluripotency related transcription factors of stem cells in tumorigenicity, little is understood of the role of pluripotency factors oct-1, c-myc, and oct-1 in cancer metabolism. We have used metabolic profiling of oct-1 and c-myc deficient and overexpressed cells to characterize intermediary metabolism and identify the critical nutrients in cancer metabolism. Cancer cell metabolism was investigated using systems biology tools such as metabolic flux analysis (MFA) to reveal intracellular ES cell metabolic flux distribution. Metabolic flux analysis (MFA) refers to a methodology whereby intracellular fluxes (i.e. conversion rates of metabolites through individual reactions) are calculated using a stoichiometric model for the major intracellular reactions and applying mass balances around intracellular metabolites. MFA is a powerful approach for understanding and comparing different metabolic states, and offer insights into the functional capabilities of a metabolic network. The power of this approach is that it takes into account a large set of measurements as well as the complex inter-dependence among the various pathways due to the sharing of a common pool of co-factors such as NADH, NADPH, thus providing a more complete and integrated picture of the metabolic state than isolated measurements relevant to a few pathways. Given that the turnover rate of intracellular metabolites is very fast compared to the time scale of perturbations imparted on the cells, we can assume a “pseudo-steady-state” whereby intracellular levels of metabolites do not change significantly. Under these conditions, the intracellular fluxes are linearly related to measured rates of uptake and release of extracellular metabolites. Since, the metabolites produced by oct-1 and c-myc deficient cells present a unique signature which is different from the metabolic profile of oct-1 and c-myc over expressed cells, we hypothesized that metabolic profiling based approach could lead to understanding of the role of these factors in cancer metabolism. The developed strategy first revealed pathways that were down regulated in the c-myc and oct-1 deficient cells. Second, the obtained optimal flux solutions were used to guide the up-regulation of various pathways and hence, determining the appropriate nutrient and hormonal supplements necessary for loss of oncogenic transformations. Combined biological and clinical studies using metabolic profiling of oct-1 and c-myc deficient and overexpressed cells is likely to have a major impact on the development of therapeutics targeting metabolism in cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5092.

On the Origins of Eccentric Close-In Planets

Strong tidal interaction with the central star can circularize the orbits of close-in planets. With the standard tidal quality factor Q of our solar system, estimated circularization times for close-in extrasolar planets are typically shorter than the ages of the host stars. While most extrasolar planets with orbital radii -->a 0.1 AU indeed have circular orbits, some close-in planets with substantial orbital eccentricities have recently been discovered. This new class of eccentric close-in planets implies that either their tidal Q factor is considerably higher, or circularization is prevented by an external perturbation. Here we constrain the tidal Q factor for transiting extrasolar planets by comparing their circularization times with accurately determined stellar ages. Using estimated secular perturbation timescales, we also provide constraints on the properties of hypothetical second planets exterior to the known ones.

On the Origins of Eccentric Close-in Planets

AbstractStrong tidal interaction with the central star can circularize the orbits of close-in planets. With the standard tidal quality factorQof our solar system, estimated circularization timescales for close-in extrasolar planets are typically shorter than the age of the host stars. While most extrasolar planets with orbital radiia≲ 0.1 AU indeed have circular orbits, some close-in planets with substantial orbital eccentricities have recently been discovered. This new class of eccentric close-in planets implies that either their tidalQfactor is considerably higher, or circularization is prevented by an external perturbation. Here we constrain the tidalQfactor for transiting extrasolar planets by comparing their circularization times with accurately determined stellar ages. Using estimated secular perturbation timescales, we also provide constraints on the properties of hypothetical second planets exterior to the known eccentric close-in planets.

Shell sources as a probe of relativistic effects in neutron star models

A perturbing shell is introduced as a device for studying the excitation of fluid motions in relativistic stellar models. We show that this approach allows a reasonably clean separation of radiation from the shell and from fluid motions in the star, and provides broad flexibility in the location and timescale of perturbations driving the fluid motions. With this model we compare the relativistic and Newtonian results for the generation of even parity gravitational waves from constant density models. Our results suggest that relativistic effects will not be important in computations of the gravitational emission except possibly in the case of excitation of the neutron star on very short time scales.

Measurements and image analysis of the turbulent field in an axisymmetric jet subject to a sudden velocity decrease

The unsteady turbulent far field of a transient axisymmetric turbulent jet is studied experimentally. The initial flow perturbation consists of a sudden and large decrease in the ejection velocity. Flow visualization provides a qualitative understanding of the response of the jet structure, and the temporal evolution of the mean aand fluctuating velocity field is measured by using X-hot-wire probes. The longitudinal adaptation of the perturbation time scale to the local jet time scale can be described by showing a local self-similarity property of the flow. Transient mean radial velocities are deduced, and a large radial inflow of external fluid is found to occur during the phase of velocity decrease. The anisotropic reaction of the turbulent second-order moments reflects the particular energy-transfer mechanisms encountered in free shear flows. A local negative energy production from the turbulent field to the mean field appears on the jet axis during the transient evolution. Moreover, we can expect that the kinetic energy budget measured in steady jets is perturbed.

Phase averaged velocity field in an axisymmetric jet subject to a sudden velocity decrease

An unsteady transient axisymmetric turbulent jet was studied experimentally. The initial flow perturbation consisted of a sudden and large decrease in the ejection velocity. The temporal evolution of the mean and fluctuating unsteady velocity field was measured by using X hot-wire probes. In the jet far field, adaptation of the externally imposed unsteadiness to the local jet time scale is confirmed quantitatively. The main features of the phase averaged velocity field are presented and comments are made about the instantaneous state of the turbulence energetics. Transient mean radial velocities are deduced and an important increase of the instantaneous rate of entraining external fluid into the jet is found. Finally, we show that the pressure effect due to radial impusle terms plays an important role in the propagation of the mean perturbation. The longitudinal adaptation of the perturbation time scale driven by the local jet time scale provides a turbulent flow that is intermediate to quasi-static flows and rapidly distorted flows.

Effect of particle inertia on the instability of a particle-laden flow

The instability of a particle-laden flow subjected to velocity shear and differential particle loading was studied using the perturbation method. The momentum equations for particles and fluid are coupled at the order of the perturbation variable. The fluid-induced particle fluctuation is included explicitly. Therefore, it is not necessary to assume that particles are immobile at the perturbation time-scale (i.e. large Stokes number). A modified Rayleigh equation was derived. For a piece-wise linear velocity distribution and a step-function particle concentration distribution, the dispersion equation is an eighth-order polynomial equation, which was solved using the numerical and symbolic computation software MATHEMATICA. It was found that particle inertia not only modifies the shear instability but also gives rise to a second instability that depends critically on differential particle loading. The effects of particle mass loading, the Stokes number (the ratio of the particle response time and the flow time-scale), and the wave number of an infinitesimal disturbance, were examined.