Velocity Spread Research Articles

Validation of two-layer model for underexpanded hydrogen jets

Previous studies have shown that the two-layer model more accurately predicts hydrogen dispersion than the conventional notional nozzle models without significantly increasing the computational expense. However, the model was only validated for predicting the concentration distribution and has not been adequately validated for predicting the velocity distributions. In the present study, particle imaging velocimetry (PIV) was used to measure the velocity field of an underexpanded hydrogen jet released at 10 bar from a 1.5 mm diameter orifice. The two-layer model was the used to calculate the inlet conditions for a two-dimensional axisymmetric CFD model to simulate the hydrogen jet downstream of the Mach disk. The predicted velocity spreading and centerline decay rates agreed well with the PIV measurements. The predicted concentration distribution was consistent with data from previous planar Rayleigh scattering measurements used to verify the concentration distribution predictions in an earlier study. The jet spreading was also simulated using several widely used notional nozzle models combined with the integral plume model for comparison. These results show that the velocity and concentration distributions are both better predicted by the two-layer model than the notional nozzle models to complement previous studies verifying only the predicted concentration profiles. Thus, this study shows that the two-layer model can accurately predict the jet velocity distributions as well as the concentration distributions as verified earlier. Though more validation studies are needed to improve confidence in the model and increase the range of validity, the present work indicates that the two-layer model is a promising tool for fast, accurate predictions of the flow fields of underexpanded hydrogen jets.

Design and Efficiency Enhancement of a Ka-Band Industrial Gyrotron

The key subsystems of a high efficiency material processing gyrotron are designed at an operating frequency of 27.885 GHz. A triode-type electron gun is designed to launch hollow spiraling electron beam. The outer diameter of the gun is kept at 50 mm to lower inside diameter of the magnet. Start oscillation current for TE02 operating as well as competing modes is plotted. The output power and efficiency are predicted to be 18.4 kW and 46%, respectively, based on single mode time-dependent analytical equation. Beam-wave interaction is studied by employing particle-in-cell (PIC) simulation. Electronic efficiency and output power are increased by operating the tube in hard excitation region. An axially slotted cavity is adopted to abridge mode competitions. The output power and electronic efficiency are obtained to be 20.08 kW and 50.2%, respectively, without considering any velocity spread of the electron beam. However, the efficiency is reduced to 45%, when a moderate transverse velocity spread of 5% is considered. The power efficiency is enhanced to 67.07% by introducing a 3-staged depressed collector.

On the Influence of the Electron Velocity Spread on the Passband Properties of Gyroklystrons

We present the results of studying the passband properties of gyroklystrons theoretically and experimentally and analyze the main causes of limitations in the bandwidth of the frequencies that they amplify. It is shown that in the range of low beam currents, where the electron velocity spread is rather low, the amplification bandwidth of a two-cavity gyroklystron is close to the bandwidth of the output cavity loaded with an electron beam. In the range of high currents, the amplification bandwidth depends only weakly on the parameters of the output cavity and is determined mainly by the spread in the times of electron passing through the drift space, which is caused by the difference in their translational velocities. Good qualitative agreement of the calculated and experimental dependences indicates that the proposed method can be applied to the calculation and interpretation of frequency characteristics of gyroklystrons. The possibility to expand the passband of operating frequencies in the high-current region by decreasing the influence of the electron velocity spread on beam bunching in the three-cavity gyroklystron operating in the maximum acceleration regime is demonstrated experimentally. Theoretically, the developed method for calculation of frequency characteristics can be expanded to the case of a three-cavity gyroamplifier with slight modifications.

Universal Subterahertz Large-Orbit Gyrotron: Operation at the Second and Third Cyclotron Harmonics

Coherent subterahertz radiation in the continuous-wave regime was obtained in a large-orbit gyrotron (LOG) operated at a high harmonic of the cyclotron frequency. An electron–optical system containing a gun with a cusp followed by a section of adiabatic magnetic compression with a factor of 1000 ensures the formation of a 30 keV/0.7 A helical axis-encircling electron beam with acceptable velocity spread and pitch factor in a magnetic field of 5 T. Stable singlemode operation was obtained at frequencies of 0.394 and 0.267 THz with radiation powers of 0.37 and 0.9 kW at the third and second cyclotron harmonics, respectively. Tuning of the generation frequency in the gyrotron during its operation at the second cyclotron harmonic and complex regimes of competition of two waves excited at the second and third cyclotron harmonics are also studied.

Mechanisms of flame spread and burnout in large enclosure fires

Knowledge of the first principles defining fire behaviour in large enclosures remains limited despite their common use in modern tall buildings. The evolution of a fire in large enclosures can be defined by the relationship between the flame front and burnout velocities (VS/VBO). This relationship can be classified into three distinct fire spread modes being Mode 1 (VS/VBO → ∞), Mode 2 (VS/VBO > 1), and Mode 3 (VS/VBO ≈ 1). The mechanisms governing flame spread and burnout are investigated using four full-scale enclosure fire experiments with high porosity wood cribs with similar enclosure geometries. Flame and burnout front positions and velocities are estimated using video data. Velocities are affected by the heat feedback from the enclosure and smoke layer to the fuel. The spread velocity shows two regimes, a minimum external heat flux above which there is surface spread (q˙s,min″) and a heat flux that defines the onset of very rapid flame spread (q˙rs,crit″). A phenomenological model is developed to help identify the underlying mechanisms controlling the transition between the different spread modes. Both the model and data show that for wood cribs, the dependence of the burnout front velocity to the external radiation is weak, whereas the dependence of the flame spread velocity to the external and flame heat flux is strong. A transition from Mode 3 (VS/VBO ≈ 1) to Mode 2 (VS/VBO > 1) occurs with increasing external heat fluxes above q˙s,min″. The transition to Mode 1 (VS/VBO → ∞) is further defined once q˙rs,crit″ is attained due to a sudden increase in the flame heat flux by changing the ventilation condition, or by significant increases in the external heat flux from the enclosure.

Effects of dust dispersibility on the suppressant enhanced explosion parameter (SEEP) in flame propagation of Al dust clouds

This work presents an overview about the suppressant enhanced explosion parameter (SEEP) phenomenon in aluminum dust explosion moderation. The SEEP phenomenon can be attributed to either the flammable gas produced by decomposition of insufficient chemical suppressant so as to form an explosible hybrid mixture, or to the improvement in dust dispersibility caused by small amounts of thermal inhibitor so as to form better dispersed dust clouds. Aluminum (Al) and four particle sizes of alumina (Al2O3) were used to confirm a physically caused SEEP phenomenon by performing flame propagation experiments. Higher flame spread velocities (FSVs) in Al dust clouds were found in the presence of 5 or 10% <150 and <45-µm Al2O3 powder. Adding micro-sized Al2O3 disrupted inter-particle contact in combustible dusts, decreased inter-particle forces, and formed dust clouds with better dispersibility, thereby decreasing the effective particle size distribution (PSD) of Al dust. A strong thermal effect brought about by 2.5 µm Al2O3 overcame the negative effect of improved dispersion, preventing SEEP from occurring. The addition of 50 nm Al2O3 increased cohesion of powder mixtures, and decreased dust dispersibility. With benefits from both dispersion suppression and the thermal effect, Al flame propagation was well quenched.

Ultra-High Velocity Ratio in Magnetron Injection Guns for Low-Voltage Compact Gyrotrons

Low-voltage compact gyrotron is under development at the University of Electronic Science and Technology of China (UESTC) for industrial applications. Due to the low operating voltage, the relativistic factor is weak, and interaction efficiency could not be high. Therefore, a magnetron-injection gun (MIG) with an extremely high-velocity ratio α (around 2.5) is selected to improve the interaction efficiency. As beam voltage drops, space charge effects become more and more obvious, thus a more detailed analysis of velocity-ratio α is significant to perform low-voltage gyrotrons, including beam voltage, beam current, modulating voltage, depression voltage, cathode magnetic field, and magnetic depression ratio. Theoretical analysis and simulation optimization are adopted to demonstrate the feasibility of an ultra-high velocity ratio, which considers the space charge effects. Based on theoretical analysis, an electron gun with a transverse to longitudinal velocity ratio 2.55 and velocity spread 9.3% is designed through simulation optimization. The working voltage and current are 10 kV and 0.46 A with cathode emission density 1 A/cm2 for a 75 GHz hundreds of watts’ output power gyrotron.

The Future and Evolution of COVID-19 Worldwide

This research exposes a new view about the future of COVID-19 evolution and transformation(s) from a holistic approach. We renamed to COVID-19 in the year 2020 as the COVID-20. We are interested in evaluating the transition from COVID-20 to COVID-21 (from January 2021). This research wants to prove that COVID-21 can transform its composition and mutation(s) more rapidly anytime and anywhere than COVID-20. This research assumes that COVID-21 is changing rapidly and more deadly. According to our current simulator results, the COVID-21 will be more potent than COVID-20 with new symptoms and different transmission ways and speed of spreading worldwide. The new simulator is called “The COVID-X Simulator.” The COVID-X Simulator presents many equations, functions, and graphical models to evaluate different possible future scenarios and prevention measures to reduce its impact worldwide. The main objective of the COVID-X Simulator is to assess and monitoring COVID in coming years at different places under different characteristics of transmission, the velocity of spread, alternative control methods, treatments, and its final possible eradication. We can confirm that COVID will stay longer with or without vaccination or medication for COVID worldwide.

PIV measurement inside single and double droplet interaction on a solid surface

In this study, the spreading velocities within the droplet impact on a sapphire glass surface is investigated with the aid of particle image velocimetry (PIV) method. Experiments are performed for unheated and heated surfaces and droplets with impact velocities ranging from 1.12 to 2.40 m/s which correspond to Weber numbers in the range 40–190. It was observed that the radial velocity is linear throughout a relatively large range of spreading radius. However, the velocity profiles show a non-linear shape outside radial positions owing to the capillary and viscous forces over time. For high-Weber numbers, the linearity of radial velocity profile is more evident due to the viscosity effects in the lamella which are insignificantly relative to the inertia forces. Also, the spreading velocities within the droplet pair are investigated at room temperature using the same methods. Another stagnation point formation was observed in the interaction area. In the last part, radial velocity measurements within the liquid lamellas were compared with analytical and computational models for the temperature of unheated surface. For high-Weber case, the analytical model quite agrees with the linear parts of the radial velocity profiles in the interior radial positions. For moderate Weber case, the predicted radial velocity profile only agrees well with the linear parts of experimental data during early spreading process, but the inconsistency between the analytical model and PIV results rises in the later spreading and receding phases. Comparing the results with the computational simulation show that there is a good agreement for both linear and non-linear parts in radial velocity profiles.

Flame spread and combustion characteristics of two adjacent jatropha oil droplets

The present study investigated the flame spread and combustion characteristics of two jatropha oil droplets, droplet A and B. Specifically, the pyrolysis characteristics of jatropha oil were studied using thermogravimetric and differential scanning calorimetry (DSC) analysis. Liquid chromatography-mass spectrometry (LC-MS) was used to analyze the components in jatropha oil. Then, the flame spread from droplet A to droplet B and combustion dynamics of two droplets were studied in a constant volume chamber under atmospheric pressure and room temperature (300 K). The effects of normalized droplets spacing S/d0 (2.7–4.9) on flame spread were also studied. The results showed that the combustion of jatropha oil droplet containing three stages: initial expansion stage, quasi-steady combustion stage and micro-explosive combustion stage. Periodic puffing and micro-explosion occurred during micro-explosive combustion stage. This is due to the superheating of low boiling-point components produced by pyrolysis of long-chain fatty acids in jatropha oil. The duration of initial expansion stage increases significantly with the increase of droplet spacing. When the normalized droplet spacing reaches 4.9, the second droplet cannot ignite and reached flame spread limit. Interestingly, the flame spread velocity increase at first and then decrease with increase droplet spacing. This is because the diffusion flame is cooled by unburned droplet when the spacing is relatively small. However, when the spacing is relatively large, the flame spread velocity mainly depends on the duration of second droplet in first stage.

Reaction-diffusion modeling of the spread of spruce budworm in boreal ecosystem

Boreal forest in Canada has two main recurrent disturbances: one is fire and the other one is spruce budworm. The defoliation by spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae) was first started in British Columbia of Canada in 1957 near Liard River. Budworm outbreaks were observed in the 1960’s to 1970’s and from the mid 1980’s to the present. Spruce budworm disturbance is one of the major issues of the forest management because of the potential losses of timber and non-timber resources in the boreal forest. To develop a sustainable management of the boreal ecosystem, mathematical modeling approaches for the budworm outbreak have been progressed. For this understanding, we propose a single species reaction-diffusion model appended with Holling type II functional response. In this paper, our main goal is to understand the spreading phenomenon of spruce budworm and subsequently identify its spreading velocity. In order to determine the spreading velocity, we focus on the one-dimensional traveling wave solutions of the model. Our results suggest that the minimal speed of traveling wave solutions can exhibit the spreading velocity of the spruce budworm in boreal forests.

Systematic Time-Dependent, Multimode Simulation Study of a Ka-Band, Three-Cavity Second-Harmonic Gyroklystron Amplifier

The multimode beam–wave interaction behavior in the RF interaction structure of a second-harmonic, 32.3 GHz gyroklystron amplifier operating in the TE02 mode has been studied by simulating the device using a commercial 3-D particle-in-cell code, “CST Studio Suite.” The effect of the variation of numerous parameters, such as driver frequency, RF power, magnetic-field strength, and beam current, has been investigated to realize the sensitivity of the device performance to these parameters. The effect of varying the position of the buncher cavity with respect to input and output cavity on the 3 dB gain and efficiency has also been investigated. The simulation results show that the designed gyroklystron amplifier produces a constant RF output power of ~315 kW at 32.3 GHz center frequency for a 70 kV, 20 A electron beam. The 3 dB gain of the device is obtained as ~26 dB with ~22.5% electronic efficiency and ~11 MHz bandwidth. The velocity spread is assumed as 0% for the present work. For validation, the simulated results have been compared with the previously reported experimental results and the previously reported analytical results, which are found to be in good agreement (i.e., less than 5% variation).

Analytical model for predicting maximum spread of droplet impinging on solid surfaces

In the present study, we develop a theoretical approach to predict the maximum spread of a liquid droplet on a dry solid surface. By using the dynamics of the gas layer entrapped underneath the droplet during initial stages of spreading, we determine the initial spread velocity of the droplet. The predicted spread velocity is used to model viscous dissipation and spread time of the droplet, post-impact. We also reformulate the surface energy of the droplet at the maximum spread to account for the presence of a rim formed at the periphery of the droplet. Incorporating the renewed terms into an energy conservation equation, the maximum spread of the droplet is predicted. The constructed model is validated with both the in-house experiments and the literature performed for various liquids and surfaces. The study also examines the existing scaling laws available to predict the maximum spread in inertial and viscous regimes and compares them with the model. Results reveal that the proposed model effectively predicts maximum spread values even at a low Weber number, despite variations in wettability values. The scaling laws were found to be inefficient in predicting the maximum spread for water at a low Weber number as they do not account for the effect of the surface wettability.

Giant Metrewave Radio Telescope Detections of Two High-opacity Hi 21 cm Absorbers at z ≈ 1.2

Abstract We report the discovery of two remarkable high-opacity Hi 21 cm absorbers against low-luminosity active galactic nuclei (AGNs), at z = 1.2166 toward J0229+0044 and at z = 1.1630 toward J0229+0053. The absorbers were detected in an unbiased Giant Metrewave Radio Telescope survey for Hi 21 cm absorption against radio sources in the DEEP2 survey fields, covering z ≈ 0.73–1.53, and including sources without known redshifts. The velocity-integrated Hi 21 cm optical depths are (74.2 ± 7.8) km s−1 (J0229+0044) and (78.41 ± 0.81) km s−1 (J0229+0053), higher than that of any known redshifted Hi 21 cm absorber at z &gt; 0.12, and implying high Hi column densities, &gt;1022 cm−2. The emission redshift of J0229+0044 is consistent with the Hi 21 cm absorption redshift, while the strength and velocity spread of the absorption against J0229+0053 suggest that it too arises from gas in the AGN environment: both absorbers are thus likely to be “associated” systems. The two AGNs have low rest-frame 1.4 GHz radio and 1215 Å ultraviolet luminosities (≲1026.1 W Hz−1 and ≲1021.7 W Hz−1, respectively), both significantly lower than the typical luminosities of AGNs against which Hi 21 cm searches have hitherto been carried out at z ≳ 1. The paucity of Hi 21 cm absorbers at z ≳ 1 may be due to a luminosity bias in high-z AGN samples that have been searched for Hi 21 cm absorption, where the high AGN ultraviolet luminosity affects physical conditions in its environment, ionizing the neutral hydrogen.

Image analysis of flame behavior for polyolefins and polystyrene in vertical flame test

AbstractAlthough the UL‐94 vertical flame test is often used to evaluate the flammability of polymers based on ignition time and combustion duration parameters, a significant amount of information regarding the time course of combustion is difficult to analyze in detail. Herein, image analysis of the time course of the upper and lower end heights, total area, and color division of flame was performed for polyolefins and polystyrene with different molecular weights in the vertical flame test. The combustion process was classified into two stages: before and after the first drip. In the first stage, the upward spread velocity, oscillation, and color of flame were analyzed. It was assumed that the difference in the fuel production rates or thermal decomposition products depends on the molecular structure. In the second stage, the melt flowability, flame position, and combustion continuity differed vastly depending on the molecular structure or molecular weight.

Morphological Evolution and Interfacial Effects Analysis of Drop Motion in Transverse Vibration of Inclined Plate

Based on experimental and simulation research, analysis of the morphological evolution and interfacial effects of drop motion in the transverse vibration of inclined micro-textured plate are studied. Experimental results show the morphological evolution of drop involves an oscillation stage, spreading and migration stage, and infiltration stage. The spread diameter increases from the initial 3.02 to 5.12 mm. Meanwhile, based on the real experimental morphology of the drop dynamic wettability, a two-phase flow theoretical model of motion evolution of forced vibration drop was established to simulate the drop spreading process. The analysis result shows the calculated results are close to the experimental results, and the on micro-textured surface is faster spreading coefficient is S-shaped and increases with the increase of time. The spreading velocity than the smooth one, and there is low-speed rotating airflow in the micro-textured pit. The vortex cushion effect and vortex wheel effect are the main reasons for the acceleration of drop motion. Two interfacial effects reduce the friction resistance and impel fluid movement.

Velocity fluctuations of stochastic reaction fronts propagating into an unstable state: Strongly pushed fronts.

The empirical velocity of a reaction-diffusion front, propagating into an unstable state, fluctuates because of the shot noises of the reactions and diffusion. Under certain conditions these fluctuations can be described as a diffusion process in the reference frame moving with the average velocity of the front. Here we address pushed fronts, where the front velocity in the deterministic limit is affected by higher-order reactions and is therefore larger than the linear spread velocity. For a subclass of these fronts-strongly pushed fronts-the effective diffusion constant D_{f}∼1/N of the front can be calculated, in the leading order, via a perturbation theory in 1/N≪1, where N≫1 is the typical number of particles in the transition region. This perturbation theory, however, overestimates the contribution of a few fast particles in the leading edge of the front. We suggest a more consistent calculation by introducing a spatial integration cutoff at a distance beyond which the average number of particles is of order 1. This leads to a nonperturbative correction to D_{f} which even becomes dominant close to the transition point between the strongly and weakly pushed fronts. At the transition point we obtain a logarithmic correction to the 1/N scaling of D_{f}. We also uncover another, and quite surprising, effect of the fast particles in the leading edge of the front. Because of these particles, the position fluctuations of the front can be described as a diffusion process only on very long time intervals with a duration Δt≫τ_{N}, where τ_{N} scales as N. At intermediate times the position fluctuations of the front are anomalously large and nondiffusive. Our extensive Monte Carlo simulations of a particular reacting lattice gas model support these conclusions.

Development of Third-Harmonic 1.2-THz Gyrotron With Intentionally Increased Velocity Spread of Electrons

We develop the concept of a continuous wave (CW) 1.185 THz (wavelength about 250- $\mu \text{m}$ ) gyrotron with an output power of several watts, intended for dynamic nuclear polarization (DNP)/nuclear magnetic resonance (NMR) spectroscopy applications. For a 15-T magnetic field provided by a commercially available cryomagnet, the required frequency can be achieved by operating at the third cyclotron harmonic. Under conditions of extreme density of the mode spectrum, we propose using interaction with an electron beam intentionally formed with a high (up to 40%) velocity spread as a selection mechanism. This ensures suppression of the most dangerous parasitic traveling modes at the first and the second cyclotron harmonics, which are very sensitive to the spread. The main parameters of the gyrotron are determined both using the start currents analysis and within the framework of a nonstationary self-consistent model. Experimental feasibility of the single-mode third-harmonic generation under conditions of mode competition is verified based on 3-D particle-in-cell (PIC) simulations.

Integrated breakup modeling solutions from debrisat analysis

On-orbit hypervelocity breakup models are of increasing importance as the space environment becomes more congested and issues of debris lethality and lifetime become more relevant to space operators. One such fragmentation model is The Aerospace Corporation's IMPACT, which simulates debris from explosions and hypervelocity collisions. IMPACT is a mass-based, semi-empirical model combining empirical distributions for parameters such as number, spreading velocity, fragment dimensions, and area-to-mass ratio (AMR), with physical conservation laws and boundary conditions. The original empirical relationships were based upon limited ground-test data during the early 1990s and were then updated during the late 2000′s following an extensive evaluation of over 11,000 pieces of debris from more than three-dozen historical on-orbit fragmentation events. The availability of extensive fragmentation data from the DebriSat project, including data for smaller debris fragments, now enables a fresh look at the overall model.This paper addresses observations from evaluation of specific components of the model, such as mass and density distributions, fragment shapes as a function of material, and AMR distributions, and discusses the integration of these components into a cohesive model. The combination of detailed ground test results with key parameters available from on-orbit observations provides a fuller understanding of the implications of any on-orbit fragmentation event.

Optical cycling, radiative deflection and laser cooling of barium monohydride (138Ba1H)

We present the first experimental demonstration of radiation pressure force deflection and direct laser cooling for barium monohydride (BaH) molecules resulting from multiple photon scattering. Despite a small recoil velocity (2.7 mm s−1) and a long excited state lifetime (137 ns), we use 1060 nm laser light exciting the X → A electronic transition of BaH to deflect a cryogenic buffer-gas beam and reduce its transverse velocity spread. Multiple experimental methods are employed to characterize the optical cycling dynamics and benchmark theoretical estimates based on rate equation models as well as solutions of the Lindblad master equation for the complete multilevel system. Broader implications for laser cooling and magneto-optical trapping of heavy-metal-containing molecules with narrow transition linewidths are presented. Our results pave the way for producing a new class of ultracold molecules—alkaline earth monohydrides—via direct laser cooling and trapping, opening the door to realizing a new method for delivering ultracold hydrogen atoms (Lane 2015 Phys. Rev. A 92, 022511).