Additive noise induces front propagation

The effect of additive noise on a static front that connects a stable homogeneous state with an also stable but spatially periodic state is studied. Numerical simulations show that noise induces front propagation. The conversion of random fluctuations into direct motion of the front's core is responsible of the propagation; noise prefers to create or remove a bump, because the necessary perturbations to nucleate or destroy a bump are different. From a prototype model with noise, we deduce an adequate equation for the front's core. An analytical expression for the front velocity is deduced, which is in good agreement with numerical simulations.     

热粘性对发动机排气管道中噪声传播的影响

声波在气体中传播时,气体的热粘性效应会使声波产生一定程度的衰减,且气体的声吸收系数随温度的升高而增大。由于发动机的排气温度较高,热粘性效应引起的排气管道中的噪声衰减应加以考虑。基于准平面波理论,首次计算了考虑热粘性效应时不同温度、流速和管道尺寸下排气管道中的传递损失,分析了各参数对管道中噪声衰减的影响。结果表明,随着温度和频率的升高热粘性声衰减增强,而气流流速和管道直径的增加会降低直管中的热粘性声衰减。对于简单膨胀腔,传递损失的预测结果表明,热粘性效应使通过频率处的声衰减有所改善。     

Lag-driven motion in front propagation

Front propagation is a ubiquitous phenomenon. It arises in physical, biological and cross-disciplinary systems as diverse as flame propagation, superconductors, virus infections, cancer spread or transitions in human prehistory. Here we derive a single, approximate front speed from three rather different time-delayed reaction–diffusion models, suggesting a general law. According to our approximate speed, fronts are crucially driven by the lag times (periods during which individuals or particles do not move). Rather surprisingly, the approximate speed is able to explain the observed spread rates of completely different biophysical systems such as virus infections, the Neolithic transition in Europe, and postglacial tree recolonizations.     

Noise-induced dynamic bistability of front propagation

We study noise-induced front propagation in a bistable system of the activator–inhibitor type. By varying the intensity of the multiplicative noise, the velocity of the front exhibits a transition to a bistable regime, where the actual velocity and direction of front motion depends mainly on the (random) initial conditions.     

Effect produced on sound propagation by an internal temperature front moving over the shelf

Results of observing the changes that occur in the vertical distribution of water temperature under the effect of an intense atmospheric cyclone and the influence of these changes on sound propagation in the shelf region of the Sea of Japan are presented. The measurement results refer to the autumn conditions. The measuring equipment includes a vertical acoustic-hydrophysical measuring system, a broadband transmitter (both of them being connected with the shore station by cable lines), and a self-contained resonance (320 Hz) transmitter of the electromagnetic type. The sound (tone signals) propagation is studied on a 510-m-long constant-depth (38 m) track (TON-310 Hz) and a 10.6-km-long track (TON-320 Hz), which is set up by placing the self-contained transmitter at the bottom (at a depth of 65 m). Results of field experiments are presented along with those of numerical simulation of the effect produced by an internal temperature front moving toward the coast and formed by the seasonal thermocline on the propagation of 320-Hz sound signals through it. It is shown that refraction and scattering of sound waves propagating through the temperature front moving along the acoustic track may cause intensity variations of acoustic field at the reception point, which occur synchronously at different depths and have amplitudes of up to 14 dB and a period of about 40 min.     

Motorway noise modelling based on perpendicular propagation analysis of traffic noise

A model for motorway traffic noise has been obtained from measurements along the the Bangkok-Chonburi motorway. The model’s parameters include traffic volume and combination, the average spot speed of each type of vehicle and the physical conditions of the motorway in terms of right-of-way width, number of lanes, lane width, shoulder width, and median width for both of the main carriageways and frontage roads. The noise level that is generated by each type of vehicle has been analyzed according to the propagation in the direction perpendicular to the center line of motorway’s carriageway. The total traffic noise is then analyzed from traffic volume of all vehicle types on both sides of carriageways and frontage roads. The basic noise levels used in the motorway traffic noise model are modified according to the effective ground effect along the propagation path. The final result of this study is that a motorway traffic noise model based on the perpendicular propagation analysis technique performs well in a statistical goodness-of-fit test against the field data, and therefore, can be used effectively in traffic noise prediction for related or similar motorway projects.     

Flow-frequency effect upon Huygens front propagation

A premixed flame within a turbulent flow exhibits a decreasing enhancement of fuel consumption rate with increasing turbulence intensity, an effect known as the bending effect. Denet has shown that flow time correlations may be one cause of the bending effect. Using a Damköhler-Huygens front propagation model, we illustrate that the removal of flow components with reduced frequencies greater than unity (ω >kS _L) causes a small reduction in front area but a large reduction in the flow intensity, which is the bending effect (ω is the frequency and k is the wavenumber). To be effective in producing front area, a flow mode must have a phase velocity, ω/k, smaller than the laminar burning velocity, S _L.     

Fluctuation-regularized front propagation dynamics in reaction-diffusion systems

We introduce and study a new class of fronts in finite particle-number reaction-diffusion systems, corresponding to propagating up a reaction-rate gradient. We show that these systems have no traditional mean-field limit, as the nature of the long-time front solution in the stochastic process differs essentially from that obtained by solving the mean-field deterministic reaction-diffusion equations. Instead, one can incorporate some aspects of the fluctuations via introducing a density cutoff. Using this method, we derive analytic expressions for the front velocity dependence on bulk particle density and show self-consistently why this cutoff approach can get the correct leading-order physics.     

Noise-induced front propagation in a bistable system

A bistable system satisfying a generalized Maxwell condition exhibits an interface solution deterministically at rest. It is shown that state-dependent (multiplicative) noise induces an average motion of this interface into the region of higher noise intensity.     

Plastic wave front propagation in cylindrical bars

Making use of the dynamic, elastic-plastic finite element program, the elastic-plastic wave propagation in a cylindrical bar with a circular cross section is analysed for linear strain hardeningt Special attention is given to the influence of strain rate on. the axial stress distribution for variou. impact loading conditions. Comparison is made between numerical two-dimensional and analytical one-dimensional results for structural low alloy steel (C-Mo-Cr) having strain rate dependens dynamic mechanical properties. The implications of practical importance for experimental dynamic plasticity studies by means of Hopkinson Split Pressure Bar (HSPB) technique are discussed in detail.     

Exact solutions in front propagation problems with superdiffusion

Front propagation in a number of reaction–superdiffusion problems is studied. Specifically, traveling wave propagation, domain wall pinning, and systems of waves governed by a bistable single equation as well as FitzHugh–Nagumo equations are considered. The reaction terms in the equations are taken in the form of piecewise linear functions, which allow for exact solutions to be obtained. The effect of superdiffusion on front propagation is discussed.     

Role of weighting in the dynamics of front propagation

Non-equilibrium front propagation in a two-dimensional network modelling wildfire propagation was studied. The model includes deterministic long-range interactions due to radiation and a time weighting procedure. Three weight-dependent propagation regimes were found: dynamical, static, and non-propagative. The dynamical regime shows saturation for small weight values and a percolation transition area depending on the weight and size of the interaction domain. From the scaling interface exponents, the model seems to belong to the dynamical percolation universality class. In the limit of static regime it belongs to the random deposition class.     

Kink propagation induced by multiplicative noise

The influence of spatio-temporal external multiplicative fluctuations on a single kink in a bistable distributed system is studied. For this purpose we derive a stochastic dynamic equation for the position of the shifted kink. An analytical estimate for spatio-temporally uncorrelated fluctuations is represented and discussed. We draw the conclusion that multiplicative noise induces a propagation of the most probable kink into the region of larger noise. This effect is demonstrated in numerical simulations.     

Correlation noise in the acoustic wave front inversion

Noise that accompanies the wave front inversion of sound signals is considered. It is shown that this noise is a consequence of the interference phenomena occurring in complex media with multiple signal reflections, which lead to a noiselike nature of the spectrum modulus. A method of the wave front inversion that allows one to reduce the interference noise is proposed. The aforementioned phenomena are numerically analyzed.     

A two-particle representation of front propagation in diffusion-reaction systems

A two-particle model is formulated which approximates the motion of the forwardmost particle in a lattice gas, which has recently been analyzed and numerically simulated. The lattice gas, which evolves on a linear chain, consists of particles which jump to each vacant nearest neighbor site with rate /2 and also create new particles at these sites with rate 1/2. This model is known to exhibit statistically steady propagation of the forwardmost particle, with mean propagation velocity converging to (2)^1/2 for large . Here, a two-particle representation is used to estimate the propagation velocity for finite . The results are in good agreement with numerical simulations of the lattice gas. Implications concerning front propagation in diffusion-reaction systems are discussed.