Tip Velocity Research Articles

Numerical investigation of premixed methane-air flame in two-dimensional half open tube in the early stages

Premixed methane-air flame propagation in a two-dimensional half-open tube is studied using a computational fluid dynamic method. The flame is simulated using a laminar combustion model with simplified reaction mechanism. The first purpose of this work is to determine whether a 2D planar flame model can be used to predict flame propagation in a square-section tube. Another purpose is to investigate the evolution of the flame tip velocity after the flame skirt touches the wall, using the relationship between the flame surface area and the burnt gas volume growth. The simulation results show that the 2D planar flame model is not suitable for predicting the flame propagation in a square-section tube, and that the simulated position of the flame tip is reliable only when the flame skirt touches the wall. There is an approximate proportional relationship between the volume growth of burnt gas and the flame tip velocity even after the lateral flame touches the wall. The widely used equation for the relationship between burnt gas volume growth and flame surface area fails after the flame touches the wall, and then holds at several later times. The balance is broken by the continuous extinction of the flame skirt. Finally, we report the dimensionless time interval between when the flame skirt touches the wall and flame front inversion, as well as the dimensionless velocity of the flame skirt. There are some differences between the values of the dimensionless time interval in 2D planar tubes and in 3D cylindrical tubes.

On the Theory of Stable Mode of Dendritic Growth in the Presence of Convective Heat and Mass Transfer Boundary Conditions

The dendritic form is one of the most common forms of crystals growing from supercooled melts and supersaturated solutions. In recent decades, an analytical theory has been developed that describes a stable dendrite growth mode under the conditions of a conductive heat and mass transfer process. However, in experiments, the growth of dendritic crystals is often observed under the conditions of convective fluid flow. In the present work, the theory of the growth of dendritic crystals is developed taking into account the convective mechanism of heat and mass transfer at the crystal-melt interface. A stable mode of dendritic growth in the case of intense convective flows near the steady-state growing dendritic tip is analyzed. The selection theory determining a stable growth mode in the vicinity of parabolic solutions as well as the undercooling balance condition are used to find the dendrite tip velocity and its tip diameter as functions of the melt undercooling. It is shown that the theoretical predictions in the case of convective boundary conditions are in agreement with experimental data for small undercoolings. In addition, the convective and conductive heat and mass transfer mechanisms near the growing dendritic surfaces are compared. Our calculations show that the convective boundary conditions essentially influence the stable mode of dendritic growth.

Morphological transformation of the process zone at the tip of a propagating crack. II. Geometrical parameters of the process zone.

The morphological transformation of the process zone at the tip of a propagating crack occurs with the increase of the crack velocity. The zone configuration changes its shape from concave to convex, dropletlike form. The latter exhibits a metastable wake. We prove that the transformation takes place as soon as the crack velocity exceeds Gordon's speed V_{G}. The latter is the velocity of motion of the interface between the stable and overheated metastable phases. We further analyze the dependence of geometrical parameters of the zone and wake on the crack tip velocity. We show that at a constant velocity, the size of the process zone grows with the approach to the binodal. However, it decreases by over three orders of magnitude as the crack's velocity increases. In contrast, the interval length where the zone or the wake comes in direct contact with the crack surface increases at 0≤V<V_{G}, achieves its maximum at V=V_{G}, and then decreases with the further velocity increase. The zone vanishes as soon as the crack's velocity exceeds a critical speed V_{cr}.

Effect of inner liner material on penetration behavior of reactive material double-layered liner shaped charge

To improve the insufficient penetration depth of the traditional single reactive liner shaped charge, the penetration enhancement behaviors of a reactive material double-layered liner (RM-DLL) shaped charge are investigated. This RM-DLL consists of an inner liner with metal materials and an outer liner with (polytetrafluoroethylene) PTFE/Al reactive materials. Based on the platform of AUTODYN-2D code, the influence of inner liner material on the jet formation of RM-DLL shaped charge and its penetration performance of multi-layered space plates were conducted. The numerical results indicated that, during the jet formation stage, the inner metal liner mainly formed a high-velocity precursor jet and the outer reactive liner became a major part of the slug. With increasing the material density of inner liner, the jet tip velocity and tip diamater decreased, and the effective mass of precursor jet also dropped off. For a given penetration time, with the increase in the material density of inner liner, the penetration capability of the RM-DLL shaped charge increased, whereas the mass of reactive materials entering the penetrated steel target decreased significantly. This RM-DLL shaped charge, incorporating the penetration capability of a precursor metal jet and the deflagration effects of the follow-thru reactive materials, will produce extremely damage to the desired target, typically such as the armored fighting vehicles.

Simulation study on the jet formation and penetration capability of hypervelocity double-layer liner shaped charges

The jet formation and penetration capability of hypervelocity double-layer liner shaped charges (HDLLSCs) against rolled homogeneous armor (RHA) targets are investigated by numerical simulation. The HDLLSCs with different cone angle and relative position of disc are simulated to investigate the influence of these parameters on penetration capability and compare with a traditional conical shaped charge (CSC). The simulation results show that, the tip velocity of the jet formed by HDLLSCs with a tantalum disc is lager by 6.8% compared with that formed by the CSCs. The three stages of jet formation for HDLLSC including converge, formation, and secondary impact are revealed and discussed. The penetration capability of HDLLSCs is influenced by the coupled effect of cone angle and relative position of disc, a larger relative position of disc is more suitable for a large cone angle. In addition, the standoff also has a significant effect on the penetration depth of HDLLSCs, the penetration depth increases from 3.31 charge diameter (CD) to 6.52 CD with the standoff increasing from 1.5 CD to 4.0 CD. Moreover, the penetration depth of the jet formed by HDLLSCs is larger by 18.5% compared with that formed by the CSCs at the standoff of 4.0 CD.

Numerical simulation for dendrite growth in directional solidification using LBM-CA (cellular automata) coupled method

To predict the dendrite morphology and microstructure evolution in the solidification of molten metal, numerically, lattice Boltzmann method (LBM) - cellular automata (CA) model has been developed by integrating the LBM to solve the mass transport by diffusion and convection during solidification and the CA to determine the phase transformation with respect to the solid fraction based on the local equilibrium theory. It is successfully validated with analytic solutions such as Lipton-Glicksman-Kurz (LGK) model in static melt, and Oseen-Ivantsov solution under the fluid flow conditions in terms of tip radius and velocity of the dendrite growth. The proposed LBM-CA model does not only describe different types of dendrite formations with respect to various solidification conditions such as temperature gradient and growth rate, but also predict the primary dendrite arm spacing (PDAS) and the secondary dendrite arm spacing (SDAS), quantitatively, in directional solidification (DS) experiment with Ni-based superalloy.

Nanopumping of water via rotation of graphene nanoribbons

In this paper, we perform molecular dynamics simulations to propose a novel bio-inspired nanopumping mechanism that is achieved through the rotation of graphene nanoribbons. Due to the rotation and interaction with water, the graphene nanoribbons undergo morphological transformation. It is shown that with appropriate geometrical and spatial parameters, the resulting morphology is twisted ribbon, which is efficient in pumping of water through a channel. This mimics the propulsive behavior of bacterial flagella through continual rotation at the base and causing morphology of the geometry into twisted ribbons, thus driving flow. It was observed that the maximum flux rate decreases upon reaching the optimal configuration even with increasing rotational speed and graphene width. This is due to the development of cavitation near the region of the nanoribbon with tip velocities approaching the speed of sound in water. The simulation shows promising results where the flux rate of the driven flow outperforms various nanopump configurations that have been reported in recent literature by more than one order.

The role of intense convective flow on dendrites evolving with n-fold symmetry

Abstract Convection plays an essential role in the growth of dendrites. It may influence the transport of heat and substances as well as mechanical deformation of dendritic crystals. In this paper, the effect of convective heat and mass transport that substantially changes the dendrite tip diameter and its tip velocity is demonstrated. Using currently developed models for dendrite growth with flow and the selection theory in the case of n-fold crystal symmetry, we present the results of commutations and quantitatively compare them with experimental data. Namely, we give the critical analysis of experimental data for solidifying T i 45 A l 55 samples processed in electromagnetic levitators on the Ground and in microgravity conditions. The theory is tested against experimental data for both tip velocity and tip diameter. It is shown that convection plays an essential role in formation of habitus of crystals, morphological stability and growth kinetics of crystal interfaces.

Influence of displacement constraints to the surface reconstruction of stressed bicrystal thin films

In this work, surface morphology evolution of bicrystal thin films under the combined action of grain boundary and surface diffusion is investigated by considering different mechanical constraints. 2D surface topographies of thin films, that are (a) freestanding, (b) strongly bonded to its substrate and (c) strongly bonded to its substrate and one of sidewalls, are simulated using a numerical implementation of an irreversible thermo-kinetics model. Relationships which give the groove depth as a function of time are obtained. Results show that mechanical loading conditions are effective in determining the morphology and kinetics of grooving. For the three scenarios that had been investigated, it was found that the groove depth evolves linearly with different tip velocities under the same level of uniaxial tension. In freestanding films groove tip evolves faster; i.e. as the film gets constrained from its substrate and/or one of its sidewalls, the tip velocity slows down. It was also observed that high triple junction mobilities at low levels of applied stress hinder the effects of displacement constraints to groove shape, even in the case of asymmetric stress distributions inside the film. On the other hand, low triple junction mobilities at moderate applied stresses allow formation of asymmetric grain boundary grooves due to the induced asymmetry in the driving force for surface diffusion with respect to the grain boundary.

Theoretical modeling of crystalline symmetry order with dendritic morphology

Stable growth of a crystal with dendritic morphology with n-fold symmetry is modeled. Using the linear stability analysis and solvability theory, a selection criterion for thermally and solutally controlled growth of the dendrite is derived. A complete set of non-linear equations consisting of the selection criterion and an undercooling balance (which determines the implicit dependencies of the dendrite tip velocity and tip diameter on the total undercooling) is formulated. The growth kinetics of crystals having different lattice symmetry is analyzed. The model predictions are compared with phase field modeling data on ice dendrites grown from pure undercooled water.

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.

Investigation on millimeter-scale W1/O/W2 compound droplets generation in a co-flowing device with one-step structure

Millimeter-scale W1/O/W2 compound droplets generation in a co-flowing microfluidic device with one-step structure was symmetrically observed through experiments, and the formation process is distinguished according to the variation law of thread tip velocity. Specially, under a wide range of operation conditions, W1/O/W2 compound droplets form accompanying with the generation of oil phase droplet. The formation ratio of compound droplets to oil phase droplet always depends on the outer phase Capillary number, Caw2, and the flow rate ratio, R, of middle to inner phases, and thus the flow pattern diagram is obtained. Further the influences of all phases flow rates, outer phase viscosity as well as the interfacial tension on the formation period and geometric sizes are extensively studied. Finally, a prediction correlation for dimensionless inner and outer diameters with several dimensionless numbers is also obtained. The results could provide an experimental basis for controllably preparing monodisperse compound droplets with millimeter-scale.

Radar Signatures of Drones Equipped With Heavy Payloads and Dynamic Payloads Generating Inertial Forces

Due to the availability of cheap commercial and customizable drones, the potential for using them to carry threat payloads has increased significantly. In this study, radar signatures of drones carrying simulated threat payloads have been investigated experimentally. Two different scenarios were considered: 1) drones carrying heavy payloads and 2) the dynamic response of a drone subject to inertial recoil forces which mimic the effect of a firearm attached to the drone. Experimental data for the two scenarios was collected with 24 and 94 GHz Doppler radar systems. Micro-Doppler analysis has revealed that (i) the degree of fluctuation in helicopter rotor modulation (HERM) lines in long integration spectrograms does not correlate with the presence or absence of a heavy payload and (ii) the blade flashes in fully sampled, short integration spectrograms confirm that the tip velocity and rotation rate increase with payload weight as extra thrust is required. However, in both cases, these effects are difficult to attribute exclusively to the presence of the heavy payloads as they can also be attributed to other factors affecting flight dynamics such as wind or platform maneuvers. Finally, we present what we believe to be the first measurements of a simulated recoil scenario in which distinct signatures in the bulk Doppler of the fuselage are clearly attributable to the applied recoil. Analysis shows that these signatures are consistent with the inertial forces which would be imparted by a 9 mm parabellum round fired from a Glock 22 pistol if it was attached to the drone.

Study of the diesel engine working process during its operation with a fuel injection pressure of 300 MPa

The paper presents the simulation result of the influence of the ratio of the diameter of the combustion chamber Dкс to its depth hкс and boost pressure рк on the characteristics of a 1ChN 12/13 single-cylinder engine with an injection pressure of 300 MPa at a crankshaft speed of 1400 min-1. The simulation was performed with Dкс/hкс from 3,4 to 10,0, and рк from 0,15 to 0,45 MPa. The results show that the engine achieves the best performance, nitrogen oxides NOx in the exhaust gases decreases at Dкс/hкс = (7,8-10), and the pressure рк from 0,25 to 0,35 MPa. At рк = 0,35 MPa, Dкс/hкс = 10, the indicated power increases by 7,1 %. NOx reduces by 68 % but soot, CO, HC increase 4,5, 9,5, and 2,2 times, respectively. The results also show the impact of the boost pressure on spray characteristics. The boost pressure increases, the penetration, and the tip velocity decrease, but the spray angle changes a little. While the combustion chamber diameter changes, the penetration, and the spray angle change a little, and the tip velocity varies much. Changing the boost pressure is a means of redistributing the amount of fuel burned in the jet and near the wall of the combustion chamber. With an increase in the boost pressure, the proportion of fuel that burns at the beginning of the combustion process under conditions of volumetric mixing increases, while at the end of the combustion process, a large concentration of fuel is located near the combustion chamber wall.

A study of bubble growth in the compressible Rayleigh–Taylor and Richtmyer–Meshkov instabilities

Within the framework of modified Layzer-type potential flow theory [V. N. Goncharov, “Analytical model of nonlinear, single-mode, classical Rayleigh-Taylor instability at arbitrary Atwood numbers,” Phys. Rev. Lett. 88, 134502 (2002)], we study bubble growth in compressible Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities. It is known from adiabatic equations that the density ρ and adiabatic index γ are compressibility-related factors for a given static pressure p. Here, we introduce a dynamically varying stagnation point pressure P̃=p±12ρ̃η̇02, which relates time-varying quantities, such as fluid density ρ̃, pressure P̃, and bubble tip velocity η̇0, and then, we analytically derive the governing equations for time evolution of bubbles in the RT and RM instabilities of compressible fluids. For the RT instability, the upper fluid adiabatic index γu and density ρu increase the bubble amplitude and velocity, but they decrease the bubble curvature radius at the early stage, while the lower fluid adiabatic index γl and density ρl have opposite effects on those of γu and ρu, which is consistent with recent results. For the RM instability, γu and ρu decrease the bubble amplitude and velocity, but they increase the bubble curvature radius at the early stage; however, γl and ρl have opposite effects on those of γu and ρu. Moreover, we find a good agreement between our three-dimensional results of the RM bubble amplitude and recent numerical simulations.

3D Motion Trajectories Recognition using Optimal Set of Geometric Primitives, Angular and Statistical Features

This paper presents the 3D motion trajectories (lower case 3D alphabetic characters) recognition using optimal set of geometric primitives, angular and statistical features. It has been observed that the different combinations of these features have not been used in the literature for recognition of 3D characters. The standard dataset named “CHAR3D” has been used for analysis purpose. The dataset consists of 2858 character samples and each character sample is 3 dimensional pen tip velocity trajectory. In this dataset only single pen down segmented characters have been considered. The recognition has been performed using Random Forest (RF) and multiclass support vector machine (SVM) classifier on the optimal subset of extracted features. The best obtained recognition accuracy of 83.4% has been recorded using 3D points, angular and statistical features at 10 fold cross validation using SVM classifier. Moreover, the highest recognition accuracy of 96.88% has been recorded using an optimal subset of 32 dimensional features namely, geometric primitives, angular and statistical features at 10 fold cross validation by RF classifier.

Experimental study on pressure dynamics and self-ignition of pressurized hydrogen flowing into the L-shaped tubes

To investigate the effects of varying right-angle corner locations inside the L-shaped tube on self-ignition induced by high-pressure hydrogen release, a series of experiments were carried out on L-shaped tubes with different right-angle corner locations and a straight tube was adopted for comparison. It is found that compared with the straight tube, the propagation of shock wave in the L-shaped tubes becomes more complicated due to the existence of reflected shock wave. The pressure profiles detected by pressure transducers before the right-angle corner will undergo secondary rapid rise. The varying right-angle corner locations inside the L-shaped tube have a significant influence on self-ignition of hydrogen. The closer the right-angle corner is to the burst disk, the lower the critical pressure that causes hydrogen self-ignition is. Three possible mechanisms of self-ignition inside the L-shaped tubes are discussed. Nevertheless, different right-angle corner locations have no obvious effects on development process of out-tube jet flame, only the velocity of flame tip and the length and width of jet flame have slight differences.

Evaluation of drag force of a thrip wing by using a microcantilever

Tiny flight-capable insects such as thrips utilize a drag-based mechanism to generate a net vertical force to support their weight, owing to the low associated Reynolds number. Evaluating the drag generated by such small wings is of considerable significance to understand the flight of tiny insects. In this study, a self-sensing microcantilever was used to measure the drag force generated by an actual wing of a thrip. The wing of a thrip was attached to the tip of the microcantilever, and the microcantilever along with the wing was affixed perpendicular to a constant airflow at the middle of a bench-top wind tunnel. The drag generated by the wing under airflow velocities in the range of 0–4.8 m/s was obtained. In addition, the drag generated by the wing was verified by performing a three-dimensional computational fluid dynamics analysis. At a biological average wing tip velocity of 0.7 m/s, the difference between the measured drag force (290 nN) and calculated drag force (300 nN) was merely 3.3%. This new approach of evaluating the drag force generated by tiny insects could contribute to enhancing the understanding of microscale flight.

Theoretical Study of the Jet Formation of a Shaped Charge Liner Driven by Strong Electromagnetic Energy

On the basis of the RCL circuit model and Pugh–Eichelberger–Rostoker (PER) theory of jet formation and combining with Maxwell equations, the calculation method for the electromagnetic force and the collapse velocity of the liner is obtained through the infinitesimal method. The theoretical model of jet formation of a liner driven by a strong magnetic force is established. The model can calculate the collapse process and jet formation of the liner, jet and tail velocity, jet length, diameter, and other important parameters at any time. Simultaneously, the jet and tail velocity parameters and the shape of the jet under different voltages are calculated using the theoretical model established in this research. To verify the correctness of the theoretical model, a liner has been calculated and compared with the experimental results. The relative error of the jet tip velocity is found to be 3.7%. The results show that the theoretical model established in this research can accurately calculate the jet forming process and the related parameters driven by strong electromagnetic energy.

Design and Analysis of Vegetable Transplanter Based on Five-bar Mechanism

As one of the important components of automatic transplanter, duckbilled planting device, is a significant factor affecting the efficiency of the transplanter. Aiming at the problems of low transplanting rate, poor upright angle and poor stability of the transplanter, a five-pole transplanting mechanism is designed. Firstly, the parameters of the five-pole transplanting mechanism are analyzed and confirmed by MATALAB software. After virtual prototype model is established by SolidWorks software, simplified model is imported into ADAMS software for kinematics simulation, and then the trajectory, velocity and acceleration curve of the duckbilled tip are obtained. The simulation results verify the rationality of the structure and parameters and show that the trajectory of duckbilled planter meets the agronomic requirements of vegetable cultivation.