Front Speed Research Articles

Tracking an Eruptive Prominence Using Multiwavelength and Multiview Observations on 2023 March 7

In this paper, we carry out multiwavelength and multiview observations of the prominence eruption, which generated a C2.3 class flare and a coronal mass ejection (CME) on 2023 March 7. For the first time, we apply the revised cone model to three-dimensional reconstruction and tracking of the eruptive prominence for ∼4 hr. The prominence propagates nonradially and makes a detour around the large-scale coronal loops in active region NOAA 13243. The northward deflection angle increases from ∼36° to ∼47° before returning to ∼36° and keeping up. There is no longitudinal deflection throughout the propagation. The angular width of the cone increases from ∼30° and reaches a plateau at ∼37°. The heliocentric distance of the prominence rises from ∼1.1 to ∼10.0 R ☉, and the prominence experiences continuous acceleration (∼51 m s−2) over 2 hr, which is probably related to the magnetic reconnection during the C-class flare. The true speed of the CME front is estimated to be ∼829 km s−1, which is ∼1.2 times larger than that of the CME core (prominence). We conclude that both acceleration and deflection of eruptive prominences in their early lives could be reproduced with the revised cone model.

Dynamics for a Diffusive Epidemic Model With a Free Boundary: Spreading Speed

ABSTRACTWe study the spreading speed of a diffusive epidemic model proposed by Li et al. [Dynamics for a diffusive epidemic model with a free boundary: spreading‐vanishing dichotomy, Zeitschrift für Angewandte Mathematik und Physik 75 (2024): Article No. 202], where the Stefan boundary condition is imposed at the right boundary, and the left boundary is subject to the homogeneous Dirichlet and Neumann condition, respectively. A spreading‐vanishing dichotomy and some sharp criteria were obtained in Li et al. In this paper, when spreading happens, we not only obtain the exact spreading speed of the spreading front described by the right boundary, but derive some sharp estimates on the asymptotical behavior of solution component . Our arguments depend crucially on some detailed understandings for a corresponding semi‐wave problem and a steady‐state problem.

Optimum design of contra-rotating wind turbines with adjacent rotors

A downstream installed contrarotating rotor can efficiently extract the residual kinetic energy of a wind turbine wake, thus increasing the overall power coefficient. A significant interaction between the two rotors exists so that, to completely exploit the device potential, it is not advisable to adopt a classical design procedure conceived for a single-rotor configuration, as commonly seen in the existing literature. In particular, current approaches also oversimplify the design of the back rotor by merely mirroring the front rotor geometry, rather than optimising it for its power extraction duties. To fill this knowledge gap, the paper presents an original design strategy expressly conceived for contra-rotating wind turbines. This new approach optimises the geometry of both rotors, fully accounting for the interaction between them and the wake rotation at the outlet of the front wheel. The procedure adopts a calculus of variations approach aimed at maximising the overall power coefficient. It relies on a classical actuator disk model and is it valid for two adjacent rotors with the same diameter. As a result, the optimal chord and pitch distributions of the two rotors are evaluated for any combination of the front and back tip speed ratios. The procedure can be applied to marine turbines, too.

Structural dynamics of contractile injection systems

The dynamics of many macromolecular machines is characterized by chemically mediated structural changes that achieve large-scale functional deployment through local rearrangements of constitutive protein subunits. Motivated by recent high-resolution structural microscopy of a particular class of such machines, contractile injection systems (CISs), we construct a coarse-grained semianalytical model that recapitulates the geometry and bistable mechanics of CISs in terms of a minimal set of measurable physical parameters. We use this model to predict the size, shape, and speed of a dynamical actuation front that underlies contraction. Scaling laws for the velocity and physical extension of the contraction front are consistent with our numerical simulations and may be generally applicable to related systems.

Speed and Shape of Population Fronts with Density-Dependent Diffusion

There is growing empirical evidence that animal movement patterns depend on population density. We investigate travelling wave solutions in reaction-diffusion models of animal range expansion in the case that population diffusion is density-dependent. We find that the speed of the selected wave depends critically on the strength of diffusion at low density. For sufficiently large low-density diffusion, the wave propagates at a speed predicted by a simple linear analysis. For small or zero low-density diffusion, the linear analysis is not sufficient, but a variational approach yields exact or approximate expressions for the speed and shape of population fronts.

LES evaluation of hydrogen explosion disaster in interconnected vessels

Using the LES method, this study is aimed at evaluating the hydrogen explosion disaster in interconnected vessels. The flame evolution, flame front speed, explosion overpressure, vorticity and velocity vectors are obtained by changing volume ratio, pipe length, pipe diameter and ignition position. The result indicated that as the volume ratio increases, the flame vorticity decreases, resulting in a lower maximum flame front speed. However, due to stronger pre-compression, the maximum explosion overpressure increases. An increase in pipe diameter significantly reduces flame vorticity, leading to a reduction in both maximum flame front speed and maximum explosion overpressure. With increasing pipe length, the flame vorticity increases, and the maximum flame front speed exhibits a monotonic increase, while the maximum explosion overpressure follows a trend of “initially slightly decreasing, then remaining nearly constant.” The ignition in the smaller vessel resulted in the highest vorticity and flame front speed, followed by ignition in the big vessel, with the lowest flame front speed observed when the ignition occurred at the center of the pipe. The ignition in the big vessel produced the highest pre-compression and explosion overpressure, followed by ignition in the smaller vessel, with the lowest explosion overpressure observed when the ignition occurred at the center of the pipe.

Automatic determination of directional stability parameters of an electric vehicle with a two-motor scheme as part of the torque distribution controller

In case if the front and rear axle of the electric vehicle are not mechanically linked, the presence of automatic torque distribution to the axles or wheels becomes not only an addition to increase the level of consumer properties, but also a necessity. A method for determining the directional stability condition of an electric four-wheel drive vehicle is developed as part of the research by comparing the target and measured yaw rate, and is based on the formulated conditions of drift, skid, and counter steer registration. A method for detecting slipping of the driving wheels of an electric vehicle with a dual-motor scheme is developed, which is based on comparing the target and measured velocity differences of the front and rear axle and includes an auto oscillation detection function. A method for determining the target front and rear axle speed difference in a corner is described. Based on the tests performed, it is concluded that the proposed methods are applicable to the active torque distribution function between the axles of an all-wheel drive electric vehicle being developed by the authors of the paper, which provides improved directional stability and controllability, and counteracts slipping of the drive wheels.

Decay of a hydrogen-air flame front to cup-like cells in a narrow horizontal gap

Infrared visualization was employed to observe the decay of a hydrogen-air flame front propagating in a narrow gap similar to a Hele-Shaw cell. Based on the images, the flame front speeds and sizes for mixtures with hydrogen concentrations of 7 % and 10 % by volume in gaps with widths of 3–5 mm were calculated. The shapes of the individual cells formed were determined, the mechanism of flame front decay due to the critical influence of diffusive-thermal instability was described, and a criterion for flame front decay was proposed. The obtained results may be used in the design of fuel cells and MEMS hydrogen devices to ensure the safety of their operation.

Compressive failure and fragmentation of fused silica glass under quasi-static loading

This paper investigates the failure and post-failure fragmentation processes of fused silica glass cylinders under quasi-static compression, addressing the effects of surface finish on apparent strength. The compressive strengths of cylinders with side-polished sides under dry friction conditions (PSG-D) or lubricated conditions (PSG-L) were 2.52 GPa and 2.32 GPa, respectively. In situ observations showed that these specimens fractured from either the top or bottom contact surfaces, and that friction between the specimen and the loading anvil delayed the development of surface cracks. The unpolished rough side specimens (RSG-D) under dry friction conditions failed at a much lower level of 1.54 GPa, initiated by defects in the side surfaces. Specimens "explosively" fragmented upon failure. The propagation speed of the failure front ranged from 1800 to 2800 m/s. The average sizes and distributions of the fragments were determined through statistical analysis, with results showing reasonably close agreement with theoretical estimations.

Weak shock compaction on granular salt

This study conducted integrated experiments and computational modeling to investigate the speeds of a developing shock within granular salt and analyzed the effect of various impact velocities up to 245 m/s. Experiments were conducted on table salt utilizing a novel setup with a considerable bore length for the sample, enabling visualization of a moving shock wave. Experimental analysis using particle image velocimetry enabled the characterization of shock velocity and particle velocity histories. Mesoscale simulations further enabled advanced analysis of the shock wave’s substructure. In simulations, the shock front’s precursor was shown to have a heterogeneous nature, which is usually modeled as uniform in continuum analyses. The presence of force chains results in a spread out of the shock precursor over a greater ramp distance. With increasing impact velocity, the shock front thickness reduces, and the precursor of the shock front becomes less heterogeneous. Furthermore, mesoscale modeling suggests the formation of force chains behind the shock front, even under the conditions of weak shock. This study presents novel mesoscale simulation results on salt corroborated with data from experiments, thereby characterizing the compaction front speeds in the weak shock regime.

Simulation Analysis of the Freezing Process For Achieving Bidirectional Freezing Coating Via the Freeze-Casting Technique

To delve deeper into the shapes and positions of the solidification front, as well as the detailed temperature distributions under various solidification conditions, this study employed the commercial software ANSYS FLUENT to conduct a meticulous numerical simulation of the two-dimensional solidification process in freeze casting. As part of the research, the enthalpy-porous media solidification/melting model was chosen as the core theoretical framework, and the accuracy of the numerical calculations was verified by comparing them with locally measured temperature changes from experiments. Subsequently, we further examined the influence mechanisms of factors such as cold source temperature, solid content, mold depth, and mold thermal conductivity on the speed of the solidification front. This is crucial because, in the freeze casting process, the speed of the solidification front directly determines the formation and evolution of macropore structures. The results of the simulation analysis indicate that an increase in mold thermal conductivity, a decrease in cold source temperature, a shallower mold depth, and a higher solid content all lead to a corresponding increase in the movement speed of the solidification front. These simulation outcomes not only provide strong theoretical support for optimizing the process parameters of freeze casting but also aid in achieving more precise control over the microstructure of materials, thus enabling a more efficient and accurate material preparation process.

Investigation on the explosion of ammonia/hydrogen/air in a closed duct by experiments and numerical simulations

In this paper, the evolution of an explosion of hydrogen/ammonia/air flames is studied by experiments and numerical simulations. A two-dimensional (2D) model is utilized, employing the detached eddy simulation (DES) method along with the thickened flame model. Stoichiometric hydrogen/ammonia/air with ∂ = 25% and 50% (ammonia blending ratio in fuel) is considered. The results indicate that the numerical simulations accurately replicate the experimental findings. Both the experimental observations and numerical simulations reveal the formation process of the “tulip” flame. With increasing ∂, both the flame front speed and overpressure decrease. A change in ∂ can alter the contribution of intermediate reactions, thereby influencing the explosion behavior.

The effect of pressure evolution on flame propagation dynamics of H2/CH4 mixtures explosion in a horizontal closed duct

To investigate the effect of explosion pressure on flame propagation of H2/CH4 mixtures, the pressure dynamics of H2/CH4 mixtures explosion with different volume fractions of hydrogen (0, 0.2, 0.4, 0.6, 0.8, 1 (×100/%)) and equivalence ratios (0.8, 1, 1.2, 1.4) is measured using 2 pressure sensors in a horizontal duct with ignition electrode position not at the end. The flame propagation characteristic is studied using images recorded by a high-speed camera. Through the analysis and discussion of the experimental results, the interaction among explosion pressure, flame front, and flow field are discussed. The results show that when the volume fraction of hydrogen is greater than 0.6, the rate of explosion pressure rises, the flame front position, and the flame front speed are greatly improved. At higher and lower equivalence ratios, it is easier to form distorted “tulip” flames. As the volume fraction of hydrogen increases, the inclination angle of the flame front decreases, and becomes a typical “tulip” flame gradually. The convection of the flow field is an important cause of pressure oscillation, which makes the pressure distribution uneven, and its influence on Pmax, (dP/dt)max, and tc is weak. As the volume fraction of hydrogen increases, the duration of pressure oscillation decreases, and the intensity of pressure oscillation increases. The pressure wave has a substantial impact on the flame front propagation of distorted “tulip” flames and the pressure history of typical “tulip” flames.

Nonlinear wave propagation in a bistable optical chain with nonreciprocal coupling

The propagation of nonlinear waves, such as fires, weather fronts, and disease spread, has drawn attention since the dawn of time. A well-known example of nonlinear wave–fronts–in our daily lives is the domino waves, which propagate equally toward the left or right flank due to their reciprocal coupling. However, there are other situations where front propagation is not fully understood, such as bistable fronts with nonreciprocal coupling. These couplings are characterised by the fact that the energy emitter and receiver are not interchangeable. Here, we study the propagation of nonlinear waves in a bistable optical chain forced by nonreciprocal optical feedback. The spatiotemporal evolution and the front speeds are characterised as a function of the nonreciprocal coupling. We derive an equation to describe the interacting optical elements in a liquid crystal light valve with nonreciprocal optical feedback and compare the experimental results with numerical simulations of the coupled bistable systems.

Flame propagation dynamic characteristics and mechanisms of methane–syngas–air mixtures in a semi-enclosed duct

The composition of syngas is complex, often leading to the generation of methane, which impacts the explosive characteristics of syngas. The analysis focused on the kinetic characteristics and mechanisms of flame propagation in methane–syngas–air mixtures in a semi-enclosed duct under various equivalence ratios (φ = 0.8–1.4) and methane volume ratios (R = 0 %–70 %) under ambient temperature and pressure. The results indicated that the flame speed and pressure propagation of methane–syngas–air mixtures could be divided into three stages: a stable stage, an oscillatory increase stage, and an oscillatory decrease stage. In the stable stage, as the methane proportion in the premixed gas increased, the oscillations in the flame and pressure decreased. Turbulent flow of the mixed gas was observed due to restrictions and obstructions within the duct. This led to an increase in the peak pressure and flame speed of the mixed gas, thereby resulting in an oscillatory increase stage. Distorted tulip-shaped flames were also formed in the semi-enclosed duct. The flame of the pure syngas/air mixture exhibited a hemispherical shape and a finger-shaped and slope-shaped propagation structure. When 30 %, 50 %, or 70 % of the methane in the methane–syngas–air mixtures was added, the flame exhibited a propagation structure characterized by a hemispherical shape, finger shape, slope shape, flat shape, tulip shape, or distorted tulip shape. The flame front evolved with time from a linear growth stage to a nonlinear growth stage. The flame front speed of the methane–syngas–air mixtures was closely related to the pressure and flame structure. With the increase in the methane proportion in the methane–syngas–air mixtures, the concentration of key radicals and the rate of OH* generation and consumption gradually decreased. Sensitivity analysis revealed that with increasing methane proportion in the premixed gas, the most important reaction dominating OH* consumption changed from R15 (H + O2(+M) <=> HO2(+M)) to R138 (CH4 + OH <=> CH3 + H2O) and then to R112 (2CH3(+M) <=> C2H6(+M)); moreover, the most important reaction dominating OH* generation remained R1 (H + O2 <=> O + OH). The addition of methane interrupted the hydrogen–oxygen chain reaction of the methane–syngas–air explosion process.

Combustion characteristics of refuse-derived fuel pellets having varying plastic compositions.

The plastic fraction of refuse-derived fuel (RDF) pellets influences fuel's physico-chemical, and mechanical properties, which in turn might affect the combustion behavior of RDF. In the present study, the combustion behavior of different plastic fraction-simulated RDF pellets is reported. The simulated pellets were prepared based on the Indian commercial RDF composition. Initially, the plastic content varied from the existing fraction in Indian commercial RDF, i.e., 35% (RDF-1), to a lower plastic content of 5% (RDF-2). Physico-chemical characterization showed that a higher plastic fraction in RDF-1 reduces its pellet density by 25% as compared to RDF-2. The changes in RDF physico-chemical properties due to plastic variation are reflected in the RDF conversion process. Single-particle and packed-bed studies concluded that the lower density for higher plastic RDF-1 leads to lower conversion times (36%), and higher flame front speed (11%), which are desirable conditions for faster conversion. However, packed-bed studies also showed limitations regarding the utilization of high plastic RDF as RDF-1 has a narrow range of operating air mass flux due to the early advent of convective cooling of the bed. Finally, considering the constraints associated with the utilization of high plastic fraction (~ 35%) RDF and to maximize the effective utilization of plastic in RDF, a workable plastic fraction of 15% in RDF was proposed and tested for its combustion properties. RDF with 15% plastic showed faster conversion, higher flame front speed, and a wider range of operating air mass flux before the advent of convective cooling of the bed.

Direct and Indirect Effects of Mountain Heights on Heavy Rainfall in the Hokitika Region of New Zealand

In the Hokitika region, on the west coast of the South Island of New Zealand on 18 June 2015, very heavy stratiform precipitation (&gt;200 mm/per day) occurred under north-westerlies with small CAPE (&lt;25 J/kg). Analyses of model simulations and observations showed that this heavy rainfall was due to cold-front lifting enhanced by orographic lifting over the Southern Alps. At 1.5 km grid-length, the model terrain underestimated the average height of the 103 tallest mountains over the South Island by ~800 m. This led to weaker orographic lifting and mountain blocking, and a faster-moving and stronger cold front in the Hokitika region. As a result, large errors in the heavy rainfall prediction occurred. By increasing either the resolved or the sub-grid mountain heights, the simulated rainfall errors were largely reduced through stronger orographic lifting and mountain blocking, and simulation of the cold front movement and strength was improved. All the experiments have the same “flow-over” regime with mountain waves and/or wave breaking (Fm ranges 0.61–1.21). However, the rainfall amount and distribution on the windward side of the mountains varied significantly. Our new findings were that the Southern Alps can have significant indirect effects on heavy rainfall by altering the speed and strength of the cold front, in addition to the well-known direct dynamical effects (i.e., orographic lifting and mountain blocking). A combination of these direct and indirect effects makes the heavy rainfall simulation sensitive to mountain heights even under the same “flow-over” regime.

Travelling waves for a fast reaction limit of a discrete coagulation-fragmentation model with diffusion and proliferation.

We study traveling wave solutions for a reaction-diffusion model, introduced in the article Calvez et al. (Regime switching on the propagation speed of travelling waves of some size-structured myxobacteriapopulation models, 2023), describing the spread of the social bacterium Myxococcus xanthus. This model describes the spatial dynamics of two different cluster sizes: isolated bacteria and paired bacteria. Two isolated bacteria can coagulate to form a cluster of two bacteria and conversely, a pair of bacteria can fragment into two isolated bacteria. Coagulation and fragmentation are assumed to occur at a certain rate denoted by k. In this article we study theoretically the limit of fast coagulation fragmentation corresponding mathematically to the limit when the value of the parameter k tends to . For this regime, we demonstrate the existence and uniqueness of a transition between pulled and pushed fronts for a certain critical ratio between the diffusion coefficient of isolated bacteria and the diffusion coefficient of paired bacteria. When the ratio is below , the critical front speed is constant and corresponds to the linear speed. Conversely, when the ratio is above the critical threshold, the critical spreading speed becomes strictly greater than the linear speed.

Large eddy simulations of sheet-to-cloud cavitation transitions with special emphasis on the simultaneous existence of the re-entrant jet and shock waves

Sheet-to-cloud cavitation transitions are very complex owing to the simultaneous appearance of the re-entrant jet and shock waves. The objective of this paper is to investigate the physics of compressible cavitating flows with emphasis on the simultaneous existence of the re-entrant jet and shock waves. A compressible cavitating solver, which considers the compressibility effects of both the liquid and the vapour, was used to account for the different shedding characteristics induced by the re-entrant jet mechanism (RJM), the shock wave mechanism (SWM), and both the re-entrant jet and the shock waves (RJM-SMW). The solver used Large Eddy Simulations (LES) and the Schnerr-Sauer cavitation model. The numerical results are first compared with available experimental data [1] which showed satisfactory agreement for various aspects of the flow, including the pressure distribution, cavity shapes, condensation front speed, and shedding frequency. The results first show three different cavity shedding processes induced by the re-entrant jet and the shock waves. Frequency analyses show that the shock waves produce a wider frequency distribution which indicates that shock waves induce more shedding than the re-entrant jet. The quasi-periodic characteristics of the transitions between the various cavity shedding characteristics are mainly induced by the variations of the pressure, residual cavity volume and medium compressibility. Further analyses show that the different pressure, residual cavity volume and medium compressibility amplitudes determine the cavity shedding characteristics in the subsequent shedding cycle. When the conditions favour both the re-entrant jets and the shock wave effects, both the re-entrant jet and the shock waves impact the cavity shedding.

The advancing wave front on a sloping channel covered by a rod canopy following an instantaneous dam break

The drag coefficient Cd for a rigid and uniformly distributed rod canopy covering a sloping channel following the instantaneous collapse of a dam was examined using flume experiments. The measurements included space x and time t high resolution images of the water surface h(x, t) for multiple channel bed slopes So and water depths behind the dam Ho along with drag estimates provided by sequential load cells. Using these data, an analysis of the Saint-Venant equation (SVE) for the front speed was conducted using the diffusive wave approximation. An inferred Cd=0.4 from the h(x, t) data near the advancing front region, also confirmed by load cell measurements, is much reduced relative to its independently measured steady-uniform flow case. This finding suggests that drag reduction mechanisms associated with transients and flow disturbances are more likely to play a dominant role when compared to conventional sheltering or blocking effects on Cd examined in uniform flow. The increased air volume entrained into the advancing wave front region as determined from an inflow–outflow volume balance partly explains the Cd reduction from unity.