Linear Velocity Gradient Research Articles

Computational Studies of Insect-Sized Flapping Wings In Inclined Stroke Plane Under the Influence Of Temporally Varying Shear Inflow

2D computational studies on the effects of temporally varying shear inflow condition on forces generated by an insect-sized flapping wing in the inclined stroke plane have been carried out for Re = 150. Temporally varying shear inflow condition was represented by a combination of a mean wind speed term, a temporally fluctuating velocity term and a spatial velocity gradient term. The mathematical form followed the expression UG Uw = U∞ Uw +    Ug Uw    sin    2π fg fw t    ±    Vgrad Uw    y. Here, UG is the gusty inflow velocity in m/s, U∞ is mean free stream velocity in m/s, Ug is the amplitude of the temporally sinusoidal velocity fluctuation in m/s, Vgrad is the linear velocity gradient along the Y-axis in m/s per m, Uw is the root mean square average of the flapping cycle velocity of the wing in m/s, fg is the frequency of the temporally sinusoidal velocity fluctuation in Hz, fw is the wing’s flapping frequency in Hz, y is dimension along Y-axis in m and t is time in second. For the present studies Ug/Uw = 1 and fg/fw = 0.1 were chosen. Five cases with linear velocity gradient ratio along the Y-axis, Vgrad/Uw = ± 5, ± 2.5 and 0 were considered. Findings were analyzed by plots of instantaneous and cycle-averaged force coefficients, phase space plots, global recurrence plots and windowed recurrence quantification analysis. Numerical investigations revealed that negative Vgrad/Uw induced a considerable increase in vertical force and marginal decrease in the horizontal force. Positive Vgrad/Uw induced a marginal increment in horizontal force but caused a substantial decrement in vertical force.

Enhancing the Spun Yarn Properties by Controlling Fiber Stress Distribution in the Spinning Triangle with Rotary Heterogeneous Contact Surfaces

Control of tension distribution in the spinning triangle region that can facilitate fiber motion and transfer is highly desirable for high quality yarn production. Here, the key mechanisms and a mechanical model of gradient regulation of fiber tension and motion with rotary heterogeneous contact surfaces were theoretically analyzed. The linear velocity gradient, effected on a fiber strand using rotary heterogeneous contact surfaces, could balance and stabilize the structure and stress distribution of spinning triangle area, which could capture exposed fiber to reduce hairiness formation and enhance the internal and external fiber transfer to strengthen the fiber utilization rate. Then, varied yarns spun without and with the rotary grooved and rotary heterogeneous contact surfaces were tested to compare the property improvement for verifying above-mentioned theory. The hairiness, irregularity, and tensity of the yarns spun with rotary heterogeneous contact surfaces spun yarns were significantly improved compared to other spun yarns, which effectively corresponded well to the theoretical analysis. Based on this spinning method, this effective, low energy-consuming, easy spinning apparatus can be used with varied fiber materials for high-quality yarn production.

Possible Explosive Dispersal Outflow in IRAS 16076-5134 Revealed with ALMA

We present 0.9 mm continuum and CO(3–2) line emission observations retrieved from the Atacama Large Millimeter/submillimeter Array archive toward the high-mass star formation region IRAS 16076-5134. We identify 14 dense cores with masses between 0.3 and 22 M ☉. We find an ensemble of filament-like CO(3–2) ejections from −62 to +83 km s−1 that appear to arise radially from a common central position, close to the dense core MM8. The ensemble of filaments has a quasi-isotropic distribution in the plane of the sky. The radial velocities of several filaments follow a linear velocity gradient, increasing from a common origin. Considering the whole ensemble of filaments, we estimate the total mass to be 138 and 216 M ☉, from its CO emission, for 70 K and 140 K, respectively. Also, assuming a constant velocity expansion for the filaments (of 83 km s−1), we estimate the dynamical age of the outflowing material (3500 yr), its momentum (∼104 M ☉ km s−1), and its kinetic energy (∼1048–49 erg). The morphology and kinematics presented by the filaments suggest the presence of a dispersal outflow with explosive characteristics in IRAS 16076-5134. In addition, we make a raw estimate of the lower limit of the frequency rate of the explosive dispersal outflows in the galaxy (one every 110 yr), considering a constant star formation rate and efficiency, with respect to the galactocentric radius of the galaxy. This may imply a comparable rate between dispersal outflows and supernovae (approximately one every 50 yr), which may be important for the energy budget of the and the link between dispersal outflows and high-mass star formation.

Simultaneous Evidence of Edge Collapse and Hub-filament Configurations: A Rare Case Study of a Giant Molecular Filament, G45.3+0.1

We study multiwavelength and multiscale data to investigate the kinematics of molecular gas associated with the star-forming complexes G045.49+00.04 (G45E) and G045.14+00.14 (G45W) in the Aquila constellation. An analysis of the FUGIN 13CO(1–0) line data unveils the presence of a giant molecular filament (GMF G45.3+0.1; length ∼75 pc, mass ∼1.1 × 106 M ⊙) having a coherent velocity structure at [53, 63] km s−1. The GMF G45.3+0.1 hosts G45E and G45W complexes at its opposite ends. We find large-scale velocity oscillations along GMF G45.3+0.1, which also reveals the linear velocity gradients of −0.064 and +0.032 km s−1 pc−1 at its edges. The photometric analysis of point-like sources shows the clustering of young stellar object (YSO) candidate sources at the filament’s edges where the presence of dense gas and H ii regions are also spatially observed. The Herschel continuum maps along with the CHIMPS 13CO(3–2) line data unravel the presence of parsec scale hub-filament systems (HFSs) in both sites, G45E and G45W. Our study suggests that the global collapse of GMF G45.3+0.1 is end dominated, with the addition to the signature of global nonisotropic collapse at the edges. Overall, GMF G45.3+0.1 is the first observational sample of filament where the edge-collapse and the hub-filament configurations are simultaneously investigated. These observations open the new possibility of massive star formation, including the formation of HFSs.

Flame whirling characteristics of fire in a room with vent

SummaryVelocity distribution and motion of a whirling flame in a room fire with vent were studied analytically in this article. Using Rankine vortex model, whirling flame models with and without decay of the rotation velocity were established. The whirling flame model and the cross‐wind velocity boundary conditions were used to study the velocity distribution of the rotating flame under the action of side wind. When there is a linear velocity gradient in the lateral wind, the whirling flame still revolves around the same axis at different heights in the model without attenuation of rotation velocity, but the axis becomes tilted. In the rotation velocity attenuation model, the axis of the whirling flame changes from a straight line to a curve, and the change of the shape of the whirling flame is related to the flame height, the rotation angular velocity, and the maximum wind velocity. Under the assumption of Rankine vortex model with the same rotation angular velocity, the uniform lateral wind and lateral wind with linear velocity gradient can only affect the position and shape of the whirling flame axis but not change the rotation angular velocity.

Kinematic modelling of clusters with Gaia: the death throes of the Hyades

ABSTRACT The precision of the Gaia data offers a unique opportunity to study the internal velocity field of star clusters. We develop and validate a forward-modelling method for the internal motions of stars in a cluster. The model allows an anisotropic velocity dispersion matrix and linear velocity gradient describing rotation and shear, combines radial velocities available for a subset of stars, and accounts for contamination from background sources via a mixture model. We apply the method to Gaia DR2 data of the Hyades cluster and its tidal tails, dividing and comparing the kinematics of stars within and beyond 10 pc, which is roughly the tidal radius of the cluster. While the velocity dispersion for the cluster is nearly isotropic, the velocity ellipsoid for the tails is clearly elongated with the major axis pointing towards the Galactic centre. We find positive and negative expansions at ≈2σ significance in Galactic azimuthal and vertical directions for the cluster but no rotation. The tidal tails are stretching in a direction tilted from the Galactic centre while equally contracting as the cluster in Galactic vertical direction. The tails have a shear (A) of 16.90 ± 0.92 $\mathrm{m}\, \mathrm{s}^{-1}\, \mathrm{pc}^{-1}$ and a vorticity (B) of −6.48 ± 1.15 $\mathrm{m}\, \mathrm{s}^{-1}\, \mathrm{pc}^{-1}$, values distinct from the local Oort constants. By solving the Jeans equations for flattened models of the Hyades, we show that the observed velocity dispersions are a factor of ≈2 greater than required for virial equilibrium due to tidal heating and disruption. From simple models of the mass loss, we estimate that the Hyades is close to final dissolution with only a further ≲30 Myr left.

Flow-accelerated platelet biogenesis is due to an elasto-hydrodynamic instability

Blood platelets are formed by fragmentation of long membrane extensions from bone marrow megakaryocytes in the blood flow. Using lattice-Boltzmann/immersed boundary simulations we propose a biological Rayleigh-Plateau instability as the biophysical mechanism behind this fragmentation process. This instability is akin to the surface tension-induced breakup of a liquid jet but is driven by active cortical processes including actomyosin contractility and microtubule sliding. Our fully three-dimensional simulations highlight the crucial role of actomyosin contractility, which is required to trigger the instability, and illustrate how the wavelength of the instability determines the size of the final platelets. The elasto-hydrodynamic origin of the fragmentation explains the strong acceleration of platelet biogenesis in the presence of an external flow, which we observe in agreement with experiments. Our simulations then allow us to disentangle the influence of specific flow conditions: While a homogeneous flow with uniform velocity leads to the strongest acceleration, a shear flow with a linear velocity gradient can cause fusion events of two developing platelet-sized swellings during fragmentation. A fusion event may lead to the release of larger structures which are observable as preplatelets in experiments. Together, our findings strongly indicate a mainly physical origin of fragmentation and regulation of platelet size in flow-accelerated platelet biogenesis.

South Atlantic Transect: Variations in Oceanic Crustal Structure at 31°S

AbstractWe present an analysis of geophysical data acquired along a transect of 0–62 Ma crust located on the western flank of the Mid‐Atlantic Ridge at 31°S; all crust was formed at the same ridge segment. Crustal thickness, constrained by five wide‐angle profiles, has mean values of 5.6 km at 6.6 and 15.2 Ma, 7.0 km at 30.6 Ma, 5.5 km at 49.2 Ma, and 3.6 km at 61.2 Ma. Crustal thickness is uniform along each ridge‐parallel profile (standard deviations 0.1–0.3 km), indicating uniform along‐axis magmatic accretion over lateral distances of 40–60 km. The crustal structure of 61.2 Ma crust is not only anomalously thin compared to the other profiles but also contains regions with a linear velocity gradient from seafloor to Moho, which suggests that intense fracturing may extend to the base of the thin crust. Abyssal hill root‐mean‐square heights in the study region are 57–142 m and have an inverse correlation with spreading rate. These values are lower than the average root‐mean‐square height of 196 m elsewhere on the southern Mid‐Atlantic Ridge and indicate relatively high mantle temperatures in our study area. Unsedimented or lightly sedimented basement highs are prevalent at all ages; we argue that bottom currents scour the high topography, transporting sediment into adjacent basement lows. All drillsites planned for International Ocean Discovery Program Expeditions 390 and 393 are within 1–10 km of unsedimented or lightly sedimented basement highs, which should facilitate fluid flow and continued geochemical exchange between crust and seafloor.

Numerical Study on Flow Field Characteristics of Wind Turbine with Double Fork Tip Structure

Based on the double fork tip structure of wing of Boeing 737MAX, herein, we designed a new type of wind turbine with double fork tip structure blade. By the means of computational fluid dynamics method, fork we simulated wind turbine with original tip structure and two type of double fork tip structures; wind velocity was set at 10 m/s, tip velocity ratio was 4.5, and the effect of wind turbine fluid field from different tip structure were investigated. The results show that double fork tip structure was able to increase the pressure difference between pressure surface and suction surface by 4.3% and 7.7%, comparing with the original tip structure; as a result, the rotor power coefficient was increased. The double fork tip structure increased the linear velocity and velocity gradient of the blade tip, the linear velocity was increased by 3.5% and 4.3% comparing with original tip blade. The wake and tip vortex of double fork tip structure wind turbine tend to move inward, the tendency increases with a smaller angle.

Droplets. II. Internal Velocity Structures and Potential Rotational Motions in Pressure-dominated Coherent Structures

Abstract We present an analysis of the internal velocity structures of the newly identified sub-0.1 pc coherent structures, droplets, in L1688 and B18. By fitting 2D linear velocity fields to the observed maps of velocity centroids, we determine the magnitudes of linear velocity gradients and examine the potential rotational motions that could lead to the observed velocity gradients. The results show that the droplets follow the same power-law relation between the velocity gradient and size found for larger-scale dense cores. Assuming that rotational motion giving rise to the observed velocity gradient in each core is a solid-body rotation of a rotating body with a uniform density, we derive the “net rotational motions” of the droplets. We find a ratio between rotational and gravitational energies, β, of ∼0.046 for the droplets, and when including both droplets and larger-scale dense cores, we find β ∼ 0.039. We then examine the alignment between the velocity gradient and the major axis of each droplet, using methods adapted from the histogram of relative orientations introduced by Soler et al. We find no definitive correlation between the directions of velocity gradients and the elongations of the cores. Lastly, we discuss physical processes other than rotation that may give rise to the observed velocity field.

Investigating the complex velocity structures within dense molecular cloud cores with GBT-Argus

ABSTRACT We present the first results of high-spectral resolution (0.023 km s−1) N2H+ observations of dense gas dynamics at core scales (∼0.01 pc) using the recently commissioned Argus instrument on the Green Bank Telescope (GBT). While the fitted linear velocity gradients across the cores measured in our targets nicely agree with the well-known power-law correlation between the specific angular momentum and core size, it is unclear if the observed gradients represent core-scale rotation. In addition, our Argus data reveal detailed and intriguing gas structures in position–velocity (PV) space for all five targets studied in this project, which could suggest that the velocity gradients previously observed in many dense cores actually originate from large-scale turbulence or convergent flow compression instead of rigid-body rotation. We also note that there are targets in this study with their star-forming discs nearly perpendicular to the local velocity gradients, which, assuming the velocity gradient represents the direction of rotation, is opposite to what is described by the classical theory of star formation. This provides important insight on the transport of angular momentum within star-forming cores, which is a critical topic on studying protostellar disc formation.

Boundary conditions with adjustable slip length for the lattice Boltzmann simulation of liquid flow

The phenomenon of liquid slip has been studied by many researchers using the lattice Boltzmann method. However, boundary conditions for the lattice Boltzmann simulation of liquid flow are far from perfect and how to specify the boundary conditions for liquid flow with slip accurately is still a challenge. In this work, we introduce four widely used slip boundary conditions in gaseous flow into the simulation of liquid flow, two half-way schemes and two modified schemes. Theoretical analysis shows that all half-way schemes are equivalent in principle, so are the modified schemes. According to the equivalence of these schemes, these slip boundary conditions are improved by expanding the range of the combination parameters from [0,1] to [0,2] to surmount the barrier of limited simulated slip length. And the relations between the combination parameters and the slip length are deduced strictly in theory. The specified combination parameter is decided by the given slip length and the relaxation time. The discrete effects of these slip boundary conditions are analyzed. If the grid is fine enough, the discrete effects can be ignorable and the local flow at the wall can be approximated as flow with linear velocity gradient. The accuracy and reliability of our method have been verified by the simulations of the Couette flow, the Poiseuille flow and the unsteady Womersley flow. The cylindrical Couette flow is also implemented to explore the possibility of simulating liquid flows with curved boundary.

Temperature structure and kinematics of the IRDC G035.39–00.33

Aims. Infrared dark clouds represent the earliest stages of high-mass star formation. Detailed observations of their physical conditions on all physical scales are required to improve our understanding of their role in fueling star formation. Methods. We investigate the large-scale structure of the IRDC G035.39-00.33, probing the dense gas with the classical ammonia thermometer. This allows us to put reliable constraints on the temperature of the extended, pc-scale dense gas reservoir and to probe the magnitude of its non-thermal motions. Available far-infrared observations can be used in tandem with the observed ammonia emission to estimate the total gas mass contained in G035.39-00.33. Results. We identify a main velocity component as a prominent filament, manifested as an ammonia emission intensity ridge spanning more than 6 pc, consistent with the previous studies on the Northern part of the cloud. A number of additional line-of-sight components are found, and a large scale, linear velocity gradient of ~0.2 km s$^{-1}$ pc$^{-1}$ is found along the ridge of the IRDC. In contrast to the dust temperature map, an ammonia-derived kinetic temperature map, presented for the entirety of the cloud, reveals local temperature enhancements towards the massive protostellar cores. We show that without properly accounting for the line of sight contamination, the dust temperature is 2-3 K larger than the gas temperature measured with NH$_3$. Conclusions. While both the large scale kinematics and temperature structure are consistent with that of starless dark filaments, the kinetic gas temperature profile on smaller scales is suggestive of tracing the heating mechanism coincident with the locations of massive protostellar cores.

A CENSUS OF LARGE-SCALE (≥10 PC), VELOCITY-COHERENT, DENSE FILAMENTS IN THE NORTHERN GALACTIC PLANE: AUTOMATED IDENTIFICATION USING MINIMUM SPANNING TREE

ABSTRACT Large-scale gaseous filaments with lengths up to the order of 100 pc are on the upper end of the filamentary hierarchy of the Galactic interstellar medium (ISM). Their association with respect to the Galactic structure and their role in Galactic star formation are of great interest from both an observational and theoretical point of view. Previous “by-eye” searches, combined together, have started to uncover the Galactic distribution of large filaments, yet inherent bias and small sample size limit conclusive statistical results from being drawn. Here, we present (1) a new, automated method for identifying large-scale velocity-coherent dense filaments, and (2) the first statistics and the Galactic distribution of these filaments. We use a customized minimum spanning tree algorithm to identify filaments by connecting voxels in the position–position–velocity space, using the Bolocam Galactic Plane Survey spectroscopic catalog. In the range of , we have identified 54 large-scale filaments and derived mass ( ), length (10–276 pc), linear mass density (54–8625 pc−1), aspect ratio, linearity, velocity gradient, temperature, fragmentation, Galactic location, and orientation angle. The filaments concentrate along major spiral arms. They are widely distributed across the Galactic disk, with 50% located within ±20 pc from the Galactic mid-plane and 27% run in the center of spiral arms. An order of 1% of the molecular ISM is confined in large filaments. Massive star formation is more favorable in large filaments compared to elsewhere. This is the first comprehensive catalog of large filaments that can be useful for a quantitative comparison with spiral structures and numerical simulations.

The method of calculating the velocity field and shear stresses in incomressible fluid

The velocity field calculation method is based on the use of two special cases of the Newtonian fluid motion equations, not including the Navier-Stokes equations. Two shear stress calculation methods are considered. The first method is the differentiation of the velocity field equation, and the second one requires the solution of the first-order differential equation. The second method provides the distribution of shear stresses for any continuous medium, including the Newtonian fluid.Calculation equations for a laminar flow in a round pipe are found. It is shown that a parabolic velocity distribution along the radius is a special case of a more general equation.The factors affecting the shear stresses for the three flow models are found. Stresses are determined by the linear velocity gradients in the laminar flow. In the 3D vortex, they can be found by various equations, which include vorticity. Total stresses for the averaged turbulent flow are calculated by summing the previously found stresses.The equations of the method are incomplete and may be used for the accurate solution of simple problems.

Structural signature of a sheared granular flow

Various types of mesoscopic structures form in jammed granular materials due to the self-organization of their constituent particles. Internal structural degrees of freedom are introduced in addition to the translational degree of freedom, and both impact the intrinsic properties of granular materials from its constituent ordinary objects. In this study, we perform numerical simulations of plane shear granular flows confined with a constant pressure. Using the radical tessellation method, we investigate the temporal and spatial evolutions of granular structures. The simulation results show that the degree of local fivefold symmetry (LFFS) is a unique structural indicator due to its significant spatial heterogeneity and dramatic variance with shearing. In the steady state, regions with small LFFS possess large linear velocity gradients, large angular velocities, and consequently large fluctuations in kinetic energy and elastic energy. Thus, LFFS may link the internal structures and dynamic properties of granular materials. Inspired by this spatial distribution scenario consisting of LFFS and energy, we propose a structural unit composed of a strong force network (SFN) and a weak force network (WFN). The SFN has a high shear resistance that acts as the elastic backbone that supports the entire matrix of a granular assembly, whereas the WFN with high energy dissipation consists of embedded “inclusions” among the SFN. This structural analysis aids the understanding of the complex phenomena of granular materials and provides insight into the mechanisms.

Palomar 5 and its tidal tails: a search for new members in the tidal stream

In this paper, we present the results of a search for members of the globular cluster Palomar 5 and its associated tidal tails. The analysis has been performed using intermediate and low-resolution spectroscopy with the AAOmega spectrograph on the Anglo-Australian Telescope. Based on kinematics, line strength and photometric information, we identify 39 new red giant branch stars along ∼20° of the tails, a larger angular extent than has been previously studied. We also recover eight previously known tidal tail members. Within the cluster, we find seven new red giant and one blue horizontal branch members and confirm a further 12 known red giant members. In total, we provide velocity data for 67 stars in the cluster and the tidal tails. Using a maximum likelihood technique, we derive a radial velocity for Pal 5 of −57.4 ± 0.3 km s−1 and a velocity dispersion of 1.2 ± 0.3 km s−1. We confirm and extend the linear velocity gradient along the tails of 1.0 ± 0.1 km s−1 deg−1, with an associated intrinsic velocity dispersion of 2.1 ± 0.4 km s−1. Neither the velocity gradient nor the dispersion change in any significant way with angular distance from the cluster, although there is some indication that the gradient may be smaller at greater angular distances in the trailing tail. Our results verify the tails as kinematically cold structures and will allow further constraints to be placed on the orbit of Pal 5, ultimately permitting a greater understanding of the shape and extent of the Galaxy's dark matter halo.

ALMA OBSERVATIONS OF INFALLING FLOWS TOWARD THE KEPLERIAN DISK AROUND THE CLASS I PROTOSTAR L1489 IRS

We have conducted ALMA observations in the 1.3 mm continuum and 12CO (2–1), C18O (2–1), and SO (56–45) lines toward L1489 IRS, a Class I protostar surrounded by a Keplerian disk and an infalling envelope. The Keplerian disk is clearly identified in the 12CO and C18O emission, and its outer radius (∼700 AU) and mass (∼0.005 M☉) are comparable to those of disks around T Tauri stars. The protostellar mass is estimated to be 1.6 M☉ with the inclination angle of 66°. In addition to the Keplerian disk, there are blueshifted and redshifted off-axis protrusions seen in the C18O emission pointing toward the north and the south, respectively, adjunct to the middle part of the Keplerian disk. The shape and kinematics of these protrusions can be interpreted as streams of infalling flows with a conserved angular momentum following parabolic trajectories toward the Keplerian disk, and the mass infalling rate is estimated to be ∼5 × 10−7 M☉ yr−1. The specific angular momentum of the infalling flows (∼2.5 × 10−3 km s−1 pc) is comparable to that at the outer radius of the Keplerian disk (∼4.8 × 10−3 km s−1 pc). The SO emission is elongated along the disk major axis and exhibits a linear velocity gradient along the axis, which is interpreted to mean that the SO emission primarily traces a ring region in the flared Keplerian disk at radii of ∼250–390 AU. The local enhancement of the SO abundance in the ring region can be due to the accretion shocks at the centrifugal radius where the infalling flows fall onto the disk. Our ALMA observations unveiled both the Keplerian disk and the infalling gas onto the disk, and the disk can further grow by accreting material and angular momenta from the infalling gas.

Fast acoustic velocity tomography of focusing operators

A 2D velocity model was estimated by tomographic imaging of overlapping focusing operators that contain one-way traveltimes, from common-focus points to receivers in an aperture along the earth’s surface. The stability and efficiency of convergence and the quality of the resulting models were improved by a sequence of ideas. We used a hybrid parameterization that has an underlying grid, upon which is superimposed a flexible, pseudolayer model. We first solved for the low-wavenumber parts of the model (approximating it as constant-velocity pseudo layers), then we allowed intermediate wavenumbers (allowing the layers to have linear velocity gradients), and finally did unconstrained iterations to add the highest wavenumber details. Layer boundaries were implicitly defined by focus points that align along virtual marker (reflector) horizons. Each focus point sampled an area bounded by the first and last rays in the data aperture at the surface; this reduced the amount of computation and the size of the effective null space of the solution. Model updates were performed simultaneously for the velocities and the local focus point positions in two steps; local estimates were performed independently by amplitude semblance for each focusing operator within its area of dependence, followed by a tomographic weighting of the local estimates into a global solution for each grid point, subject to the constraints of the parameterization used at that iteration. The system of tomographic equations was solved by simultaneous iterative reconstruction, which is equivalent to a least-squares solution, but it does not involve a matrix inversion. The algorithm was successfully applied to synthetic data for a salt dome model using a constant-velocity starting model; after a total of 25 iterations, the velocity error was [Formula: see text] and the final mean focal point position error was [Formula: see text] wavelength.

Investigations on the Formation of Planar Flames in Mesoscale Divergent Channels and Prediction of Burning Velocity at High Temperatures

Flame stabilization studies of preheated mesoscale channels of various divergent angles and aspect ratios are reported in this article. Flame propagation modes such as planar flame and negatively and positively stretched flames were observed for a range of mixture flow rates and equivalence ratios. The present investigation is focused on the formation of the planar flames in these channels. The effect, of various parameters such as channel aspect ratio, divergence angle, and heating rate on the formation of planar flames and thereby on the prediction of laminar burning velocity of various fuel–air mixtures at high temperatures are discussed. Detailed investigations show that stretch-free planar flames can be stabilized in high-aspect-ratio channels with linear velocity and temperature gradient in the axial direction. Detailed numerical simulations confirm a negligible effect of heat loss on the burning velocity due to external preheating of the channel walls. Apparatus independence of the burning velocity is confirmed through experiments in channels of various aspect ratios and divergence angles for various fuel–air mixtures. The proposed method of burning velocity measurement at high temperature is quite promising.