Lift-off Velocity Research Articles

Lift-off velocity on the surface of a binary asteroid system

This paper investigates the characteristics of lift-off velocity with analytically derived vector expression. Compared with a single asteroid system, there is an external perturbed body that has influence on the dynamic environment of the target body surface for a binary asteroid system. Binary asteroid systems modeled by ellipsoid-sphere model are classified as two cases: synchronous system and non-synchronous system. The effects of different factors on the lift-off velocity related with the dynamic characteristics of the systems are analyzed. In the synchronous system, the lift-off velocity curves are similar with those in the single asteroid system. The maximum relative difference in lift-off velocity caused by perturbed bodies of different masses is less than 8%, according to maximum and minimum lift-off velocity distributions. The density/spin parameter and ellipticity parameter have other different influences on lift-off velocity. As for a non-synchronous system, the distributions of the maximum and minimum lift-off velocity with different poses are studied and their trends for one period are summarized. The minimum lift-off velocity is found to be close to zero in the low latitude at some certain time points that are affected by the spin ratio.

Propagation of tribrachial flames with ozone in a laminar non-premixed jet

Effects of ozone on the propagation characteristics of the tribrachial flame in a laminar co-flow jet are experimentally investigated. Ozone is generated by a coaxial plasma reactor which can produce a maximum O3 concentration of 2850 ppm with a power consumption of 76.5 W. A high-speed camera is employed to record the stationary and propagating flames both with and without ozone, by which the flame displacement and propagation speeds can be determined. It is shown that the flame stability characteristics are altered with the addition of O3, in which a flame with decreased lift-off height and enhanced lift-off, reattachment and blowout velocities is observed. The enhanced flame stability with O3 addition is due to an increase in flame displacement and propagation speeds. A scaling argument shows that the enhanced flame propagation speed is strongly related to the O3 kinetic enhancement on laminar flame speed. Due to the distinct flame structure, the accompanied hydrodynamic enhancement by O3 addition facilitates the tribrachial flame propagation, implying an overprediction of the O3 kinetic enhancement. The hydrodynamic enhancement with O3 presence originates from a stronger flow redirection, due to the higher sensitivity of off-stoichiometric flames to O3 enhancement on laminar flame speed, and a more slanted flame front, due to the increased velocity gradient.

An Investigation on the Application of Plasma-Activated Gas to Laminar Lifted Nonpremixed Jet Flames

This paper is to experimentally investigate the effect of plasma-activated gas on the characteristics of a laminar lifted nonpremixed jet flame in a coflow stream. A coaxial dielectric-barrier discharge is employed to activate fuel (C3H8) and oxidizer, respectively. It is shown that an enhancement of 12% and 50% in the liftoff velocity is observed for oxidizer and fuel activation, respectively. With the addition of O3 (1650 ppm) by oxidizer activation, the flame liftoff height is continuously decreased and the blowout velocity is extended. In contrast, a larger liftoff height, accompanied with a smaller blowout velocity, exhibits for flames with fuel activation. The inconsistent effect of fuel activation on the flame liftoff is presumably a contribution of the gas expansion, the conversion rate, and the corresponding product selectivity. The gas expansion is due to the fuel activation that produces lighter hydrocarbons than propane. The conversion rate and the H2 and CH4 selectivities are found to be associated with the plasma energy density. The flame propagation can be greatly enhanced to produce a nozzle-attached flame when the plasma energy density is high enough to effectively decompose C3H8 into H2. Nevertheless, a decrease in plasma energy is more sufficient for the cleavage of C–C bonds, resulting in a significant decrease in H2 selectivity, but a nearly unaltered CH4 selectivity. The reduced conversion due to the lower plasma energy, along with the gas expansion, produces a lifted flame with a larger liftoff height than the nonactivated flame.

On the Liftoff of Diffusion Flame: An Experimental and Analytical Study

Abstract The effect of lip thickness of fuel nozzle on the liftoff of an attached diffusion methane flame with and without a coflow was experimentally and analytically investigated. Three fuel nozzles made of a long pipe with the same internal diameter but different nozzle lip thickness were tested. The co-airflow was also varied to assess its impact on the liftoff of jet diffusion flame. The results showed that the effect of fuel nozzle lip thickness on the liftoff of a free jet flame (i.e., no coflow) is marginal. However, the presence of a weak to moderate coflow (0.05 m/s < Uco < 0.3 m/s) reduced the liftoff velocity of an attached flame. The reduction rate of the liftoff velocity with co-airflow was found to be more significant for the fuel nozzle with the smaller lip thickness. At a relatively higher coflow (Uco > ∼0.3 m/s), a more pronounced drop in the liftoff velocity with coflow was observed. Flow field characteristics obtained using PIV measurements showed that the coflow stream experienced a transition to the turbulent regime for Uco > ∼0.3 m/s range which is believed to cause the inception of the flame liftoff to occur. A model for predicting the liftoff velocity of an attached flame in the presence of a coflow was developed based on the stoichiometric mixture velocity equation. This model showed good agreement with the present experimental data. However, although it can predict successfully the observed trends in the literature, further experiments are required to generalize it.

Analytical prediction of electrowetting-induced jumping motion for droplets on hydrophobic substrates

Electric voltage applied in electrowetting can induce spreading of a liquid droplet on solid substrates and yield significant contact angle reduction, which has been widely used for manipulating individual droplets in microfluidics and lab-on-a-chip devices, and even for creating jumping motion of droplets. Here, we present a theoretical closed-form expression of lift-off velocity to predict electrowetting-induced jumping motion of a droplet on hydrophobic substrates. In particular, we consider a liquid droplet wetting on a hydrophobic surface with a voltage applied between the droplet and the substrate. By turning off the applied voltage, the energy stored in the droplet deformation by electrowetting releases and may be sufficient to overcome the energy barrier for detachment. Based on the energy conservation of the droplet-substrate system, we derive a closed-form formula to predict the droplet jumping velocity in terms of the Young contact angle, the Lippmann-Young contact angle, and the Ohnesorge number. The validity of the theoretical prediction is confirmed by comparing the predicted jumping velocities with both experimental observations and numerical simulations. The predictive formula indicates that the jumping motion can be enhanced by increasing the Young contact angle and decreasing the Lippmann-Young contact angle or the Ohnesorge number. Also, a phase diagram of droplet jumping motion is constructed based on this model, which provides insights on accurate control of the electrowetting-induced jumping motion of droplets on hydrophobic surfaces.

Effect of pressure on the characteristics of lifted flames

The effects of pressure on the characteristics of lifted flames in a coflow with propane fuel were investigated experimentally in a pressure chamber. Changing the pressure influenced the density, reaction kinetics, and flame propagation speed. The pressure range tested was P = 0.5–5.5 atm. As the fuel jet velocity increased, a nozzle-attached flame transitioned to a lifted flame before blowout occurred. Depending on pressure, the onset conditions of liftoff and blowout occurred in the laminar, transition, or turbulent regimes. When P < 1.6 atm, the flame was lifted and had a tribrachial edge structure in the laminar regime, and the liftoff height (HL) increased with increasing pressure. Both the liftoff and blowout velocities decreased with decreasing pressure, and they merged at 0.5 atm. A correlation was derived in terms of the Schmidt number (Sc) and the Reynolds number (Re): Pn/2(HLPn/2)(Sc−1)/(2Sc−1)∝Re. The reattachment velocity in the laminar and transition regimes linearly decreased with pressure. The liftoff height had two local minimum points at given Reynolds number in the transition regime, and then increased linearly in the turbulent regime.

Observation and modelling of bubble dynamics in isolated bubble regime in subcooled flow boiling

Abstract Experiments of subcooled flow boiling in isolated bubble regime were conducted to understand the dynamics of individual bubbles. The test fluid was water, the flow direction was vertical upward, and the pressure was slightly higher than the atmospheric pressure. In the present experiments, the bubbles produced at nucleation sites were commonly lifted off the vertical heated surface. They were then propelled into the subcooled bulk liquid to disappear due to heat transfer with subcooled liquid. It was frequently observed that the bubbles were suddenly accelerated in the lateral direction before the disappearance. Using the present observation results, new correlations were developed for the bubble lift-off velocity and the bubble acceleration phenomenon after the lift-off. It was shown that the bubble trajectories and the time-variation of bubble size calculated using the present correlations are in fairly good agreement with the experimental results.

Plasma-Assisted Stabilization of Lifted Non-premixed Jet Flames

The present study experimentally investigates the effects of plasma discharges on the stabilization of lifted non-premixed jet flames in a stream of co-flow air. The plasma discharge is produced on the sharp edge of the fuel nozzle exit, facilitating its impact on flame stabilization. It is observed that the application of plasma discharges has an impact on the enhancement of flame lift-off velocity, lift-off height, and hysteresis phenomenon and leads to plasma-attached flames, plasma-enhanced lifted flames, and plasma-ineffective lifted flames, depending on flame lift-off conditions. A maximum enhancement of approximately 84% is observed for the flame lift-off velocity when the co-flow velocity is sufficiently low. As the co-flow velocity or the jet Reynolds number is low, the flame is anchored at the nozzle by the discharge. As the co-flow velocity or the jet Reynolds number is increased, the flame detaches but with a decrease in lift-off height compared to the flame without the discharge. If the co-fl...

Analysis of variations in the saltating sand grain transport velocity

A technique is developed for retrieving the vertical profile of the mass saltation flux from measurements of the saltating sand mass concentration and wind velocity in the surface air layer. The technique is based on the solution of direct and inverse problems of the saltation sand dynamics and aimed at determining the sliding coefficient of sand grains. The calculation results for the sliding coefficient of sand grains are compared with data of measurements in wind channels. The conditions under which saltating sand trajectories fall in the height ranges from 5 to 10 mm and from 10 to 20 mm are revealed. The effect of variations in the diameter, lift-off velocity and angle, and friction velocity on the sliding coefficient of sand grains is analyzed. The sliding coefficient for 100-μm diameter sand particles is estimated for the first time.

Experimental investigation on burner lip thickness effect on the liftoff and blowout velocities of jet diffusion flame

This experimental study addresses issue on the effect of burner lip thickness on the liftoff and blowout velocities of jet diffusion flame discharging into quiescent air. Burner tubes of two kinds of inner diameter (2 and 3 mm) with a wide range of lip thicknesses (0.25–16.5 mm) are implemented with methane or propane jet diffusion flame, respectively. The results show that the burner lip thickness has a profound effect on flame liftoff velocities, especially the blowout velocities. With the increase of the lip thickness, the blowout velocities firstly increase, then decrease and lastly remain unchanged. Specifically, the blowout velocities of 2 and 3 mm inner diameters tubes reach the maximum values when the corresponding burner lip thicknesses are 2 and 1.5 mm, respectively. In addition, compared with free (unconfined) jet diffusion flame, the jet confinement results in slight reductions of flame liftoff and blowout velocities. The existence range of lifted flame issuing from larger diameter burner tube is wider than that of smaller diameter burner tube. The existence range of propane lifted flame is wider than that of methane.

Sand particle lift-off velocity measurements and numerical simulation of mass flux distributions in a wind tunnel

Lift-off velocity of saltating sand particles in wind-blown sand located at 1.0 mm above the sand bed surface was measured using a phase Doppler particle analyzer in a wind tunnel. The results show that the probability distribution of lift-off velocity can be expressed as a lognormal function, while that of lift-off angle follows an exponential function. The probability distribution of lift-off angle conditioned for each lift-off velocity also follows an exponential function, with a slope that becomes steeper with increasing lift-off velocity. This implies that the probability distribution of lift-off velocity is strongly dependent on the lift-off angle. However, these lift-off parameters are generally treated as an independent joint probability distribution in the literature. Numerical simulations were carried out to investigate the effects of conditional versus independent joint probability distributions on the vertical sand mass flux distribution. The simulation results derived from the conditional joint probability distribution agree much better with experimental data than those from the independent ones. Thus, it is better to describe the lift-off velocity of saltating sand particles using the conditional joint probability distribution. These results improve our understanding of saltation processes in wind-blown sand.

Lift-Off Velocity on Solar-System Small Bodies

The lift-off velocity is defined as the velocity at which a particle, moving tangentially on the surface of some arbitrary body, will lift off from that surface and enter orbit. A vectorial expression for this velocity was previously derived for dimensionless particles. This theory is extended to be applicable to surface exploration craft with some finite size that are envisioned for missions to the small bodies of our solar system. For realistic mission scenarios, the lift-off velocity is shown to be independent of the craft size, and to depend strongly on the radius of curvature of the target body; this velocity is useful in the operational design of surface craft. A similar expression is derived for the case of motion on a ridge or asperity, in which the lift-off velocity becomes independent of the surface curvature. This second case has implications to the friction force experienced by a surface vehicle or rock. A methodology is developed to compute the curvature of a spherical-harmonic shape model; an established method that yields the curvature of a vertex-facet model is reviewed. The two methods are then applied to compute the lift-off-velocity distributions on the surfaces of asteroids Alpha, Bennu, and Eros.

Insights into the formation and dynamics of coignimbrite plumes from one‐dimensional models

AbstractCoignimbrite plumes provide a common and effective mechanism by which large volumes of fine‐grained ash are injected into the atmosphere. Nevertheless, controls on formation of these plumes as a function of eruptive conditions are still poorly constrained. Herein, two 1‐D axysymmetric steady state models were coupled, the first describing the parent pyroclastic density current and the second describing plume rise. Global sensitivity analysis is applied to investigate controls on coignimbrite plume formation and describe coignimbrite source and the maximum plume height attained. For a range of initial mass flow rates between 108 and 1010 kg/s, modeled liftoff distance (the distance at which neutral buoyancy is attained), assuming radial supercritical flow, is controlled by the initial flow radius, gas mass fraction, flow thickness, and temperature. The predicted decrease in median grain size between flow initiation and plume liftoff is negligible. Calculated initial plume vertical velocities, assuming uniform liftoff velocity over the pyroclastic density current invasion area, are much greater (several tens of m/s) than those previously used in modeling coignimbrite plumes (1 m/s). Such velocities are inconsistent with the fine grain size of particles lofted into coignimbrite plumes, highlighting an unavailability of large clasts, possibly due to particle segregation within the flow, prior to plume formation. Source radius and initial vertical velocity have the largest effect on maximum plume height, closely followed by initial temperature. Modeled plume heights are between 25 and 47 km, comparable with Plinian eruption columns, highlighting the potential of such events for distributing fine‐grained ash over significant areas.

Hysteresis in stabilization of methane diffusion flames with plasmas

The stabilization characteristics of liftoff and reattachment in methane diffusion flames have been investigated experimentally with addition of plasmas. The flame liftoff and reattachment velocities have been measured by varying the applied voltages on plasmas. The results show that the liftoff and reattachment velocities are strongly affected by the plasmas. The difference between these two velocities as well as the flame hysteresis region is reduced and eventually disappeared with the addition of plasmas. A topological approach is put forward to explore the mathematic laws of hysteresis phenomena, and the existent hysteresis and catastrophe laws of flame structure transition are interpreted under the typical operation routes. It is concluded that a change of flame stabilization will occur when the fuel velocity or the applied voltage passes through stabilization boundaries in a special direction.

The lift-off velocity on the surface of an arbitrary body

An expression is developed for the velocity at which a particle, moving tangentially on the surface of a body with an arbitrary shape, rotation, and gravitational field, will lift off from that surface and enter orbit. The osculating departure plane in which this lift-off motion occurs is defined by the net particle acceleration and the desired lift-off direction. The body surface is approximated within this plane, at the departure point, with some radius and center of curvature, allowing for a universal, frame-independent lift-off velocity expression. Applying the geometry of plane-ellipsoid intersections, we perform a validation of the full lift-off velocity expression on a number of rotating ellipsoids. Finally, we derive a limit expression for the case of lift-off from a rotating plane, and compare this with the results on curved bodies. This theory has numerous applications to lander/rover surface mobility operations on asteroids, comets, and small moons, as well as to geophysical processes encountered on these bodies.

強制対流サブクール沸騰の数値解析における気泡の伝熱面離脱条件の影響

Numerical simulation of subcooled flow boiling in a vertical rectangular duct was carried out using a one-way bubble tracking method; this method treats each bubble as an individual particle. Although a bubble behavior after lifting off the heated surface depends on the bubble size and velocity at lift-off, these lift-off conditions are not taken into consideration in existing subcooled flow boiling models. In the present simulation, the distributions of lift-off velocity and bubble size were considered. It was demonstrated that the calculated void fraction in subcooled flow boiling is influenced significantly by the lift-off conditions.

Near-Field Characteristics of a Rectangular Jet and Its Effect on the Liftoff of Turbulent Methane Flame

Understanding the stability of turbulent flames is a key for the design of efficient combustion systems. The present paper reports an experimental study on the effect of the internal geometry of a rectangular orifice on the characteristics/stability of a turbulent methane flame. Three rectangular nozzles with different orifice lengths having an identical exit aspect ratio (AR) of 2 were used. The co-airflow strength was also varied to evaluate its effect on the jet flow emerging from the rectangular nozzle. The experimental data revealed that the jet initial conditions affect both the flow characteristics and the liftoff of turbulent diffusion methane flame. That is, increasing the orifice length of the rectangular nozzle resulted in delaying the occurrence of the axis-switching phenomenon, reducing the length of the jet potential core, and accelerating the liftoff transition of the attached flame. The co-airflow was found to reduce the velocity strain rate in the shear layer, displace the occurrence of axis-switching farther downstream of the jet, and delay flame detachment. The results revealed also that there is a clear interplay between the flame liftoff and the jet near-field molecular mixing and flow characteristics. That is, a rectangular jet which spreads faster and generates higher near-field velocity strain and turbulence intensity causes flame detachment at a lower fuel jet velocity. Based on this, a correlation was found between the flame liftoff velocity, the fuel molecular thermal diffusivity, the stoichiometric laminar flame speed, and the fuel jet strain rate at the nozzle exit. This relationship was shown to successfully predict the liftoff velocity of methane flame as well as other common gaseous hydrocarbons and hydrogen flames.

Measurement of the movement parameters of saltating sand over a flat sand bed using a high-speed digital camera

The movement parameters of saltating sand are basic physical characteristics in aeolian physics, but remain poorly understood because of limitations in the available measurement technology. High-speed photography provides an efficient method to track the trajectories of saltating particles. From a digital recording of the trajectory, movement parameters can be derived. However, accurate and quick reconstruction of particle trajectories from video images is not yet possible, as this requires the use of prohibitively laborious manual methods. In addition, light reflection from the bed decreases contrast between near-surface particles and the bed. This makes saltation information near the surface difficult to obtain, even though this data is vitally important for studying particle entrainment processes and the mechanisms by which particles leave the surface. In this paper, we describe a novel method for tracking the trajectories of saltating particles. Using motion-detection algorithms based on the subtraction of consecutive images and dynamic thresholds, we developed a new series of high-contrast images from the original images. We then tracked particles by combining a manual operation with a computerized operation, after which a polynomial fitting method could be used to obtain various movement parameters of the saltating particles, such as the lift-off velocity and angle, impact velocity and angle, characteristic length and height, and instantaneous velocity vector at each point in the trajectory. The new method can quickly and accurately calculate the movement parameters of saltating sand particles in air and near the bed surface.

Effects of Diluents on Lifted Turbulent Methane and Ethylene Jet Flames

The effects of diluents on the liftoff of turbulent, partially premixed methane and ethylene jet flames for potential impact in industrial burner operation for multifuel operation have been investigated. Both fuel jets were diluted with nitrogen and argon in separate experiments, and the flame liftoff heights were compared for a variety of flow conditions. Methane flames have been shown to liftoff at lower jet velocities and reach blowout conditions much more rapidly than ethylene flames. Diluting ethylene and methane jets with nitrogen and argon, independently, resulted in varying trends for each fuel. At low dilution levels (∼5% by mole fraction), methane flames were lifted to similar heights, regardless of the diluent type; however, at higher dilution levels (∼10% by mole fraction) the argon diluent produced a flame which stabilized farther downstream. Ethylene jet flames proved to vary less in liftoff heights with respect to diluent type. Significant soot reduction with dilution is witnessed for both ethylene and methane flames, in that flame luminosity alteration occurs at the flame base at increasing levels of argon and nitrogen dilution. The increasing dilution levels also decreased the liftoff velocity of the fuel. Analysis showed little variance among liftoff heights in ethylene flames for the various inert diluents, while methane flames proved to be more sensitive to diluent type. This sensitivity is attributed to the more narrow limits of flammability of methane in comparison to ethylene, as well as the much higher flame speed of ethylene flames.

Mode and hysteresis of laminar methane/oxygen inverse diffusion flame

Flame modes and liftoff hysteresis of the methane/oxygen inverse diffusion flame (IDF) are experimentally studied in still air. The effects of gas velocity on flame mode and liftoff hysteresis are investigated by changing the gas flow rate, and the influences of gas velocity on OH* distribution in different modes of flame are investigated using an ultraviolet camera. The results show that methane velocity, oxygen velocity and history of the flame mode are the key factors in determining the flame mode. Flame mode regimes are identified according to the three factors. The OH* profile along the axis of the nozzle in the IDF indicates that the reaction zone is narrow in fuel rich condition and broad in fuel lean condition. The hysteresis characteristics of the IDF are significantly influenced by the coaxial methane velocity. With the increase of coaxial methane velocity, the liftoff velocity and attachment velocity of the IDF decrease linearly, while the transition velocity increases linearly from partly premixed flame to IDF.