10.1063/5.0040336.2

Control of smoke flow in tunnel fires using longitudinal ventilation systems – a study of the critical velocity

In order to investigate the interaction between double spherical tungsten fragments and Composition B (Comp B) covered by tantalum coverplates, the numerical simulation was carried out by using AUTODYN-3D software. The influence on the shock initiation characteristics of double fragments at various distance separation b1, time separation h1, impact angle α with the same or different velocities were analyzed, and the critical initiation velocity of covered Comp B was obtained. The obtained simulation results show that, under certain circumstances, the superposition effect of shock waves caused by double fragments impact on covered Comp B is obvious. The critical initiation velocity increases with the increase of b1 when fragments impacting on covered charge at the same time with same or different velocities, with α=0, and approaches critical velocity of a single fragment. The critical initiation velocity decreases initially and then increases with the increase of h1 when fragments impacting asynchronously on covered charge with different velocities and α=0. When fragments impacting synchronously on covered Comp B with different velocities, the critical velocity decreases initially and then increases with the increase of α. When fragments impacting asynchronously on covered Comp B with different velocities, the critical velocity increases with the increase of h1. Therefore, a shock initiation of covered Comp B rather be achieved by multiple fragments at lower velocity than by a single fragment at higher.

The transport of solid particles is a concern for several industries and has been the source of extensive research for many years. One topic that has been studied is the critical velocity required to transport particles, but the transport behavior used to determine the critical velocity can be different. For example, one commonly defined critical velocity is the minimum velocity necessary to keep the particles fully suspended. However, this study examined the sand behavior with two different definitions for critical velocity. The first definition simply requires the sand to move enough to prevent a permanent bed from forming. The sand behavior when this velocity requirement is met can be described as intermittent scouring and stopping. All particles do not need to move continuously. The second definition of critical liquid velocity requires the sand particles to continuously move along the bottom of the pipe. The sand behavior when this velocity requirement is met is described as continuous scouring and not fully suspended. Numerous past experiments have amassed a significant amount of slurry data, but the majority of the data is for particle volume concentrations greater than 1%. However for many cases such as in oil and gas production, particle concentrations much below 1% are present. So to examine transport for lower concentrations, experiments were conducted for volume concentrations of 0.01%, 0.1%, and 1%. Three different sand diameters and two different liquid viscosities were also used to observe their effects on the critical liquid velocities. All tests were conducted in transparent, horizontal pipe to observe particle behavior. Results have shown that as particle size, concentration, and liquid viscosity increase, the required critical liquid velocity also increases. Experimental results are compared to existing data and models to examine the effect of concentration, focusing on volume concentrations below 1%.

Experimental study and visualization of impacting spherical hydrophobic particles on an air – Liquid interface: Newtonian and Boger liquid analysis

This study delves into the dynamics of generating microdroplets by impacting a droplet onto a micropore on superhydrophobic copper substrates. It identifies the necessary impact velocities for single microdroplet formation for each micropore and characterizes microdroplet size in relation to micropore diameter. The results underscore the significant role of viscosity, especially as the diameter of the micropore decreases. For micropores measuring 400 μm, an increase in viscosity up to 8 cP does not alter the critical impact velocities, while smaller diameters of 50 and 100 μm see a notable change in critical velocities with even minor increases in viscosity. Remarkably, the diameter of the microdroplet remains consistent regardless of changes in the liquid viscosity or impact velocity. This research showcases two practical uses of single microdroplets: printing on paper and fabricating microbeads. The insights gained from these findings pave the way for advancements in printing technology and microfabrication techniques.

The onset of water-droplet detachment from the cathode GDL (gas diffusion layer)/channel interface in PEM (proton exchange membrane or polymer electrolyte membrane) fuel cells was studied numerically and analytically in the inertia-dominating flow regime. This is the flow regime of interest in real-world PEM fuel-cell applications. Our three-dimensional (3-D) numerical model employs the VOF (volume of fluid) method to track the dynamic liquid/gas interface as water droplet is being deformed and eventually detached by flowing-air drag. Our simplified, analytical model is based on the force balance between pressure drag tending to detach the droplet and surface tension tending to hold the droplet in place. Using our numerical model and for a given initial droplet size, we compute the 3-D droplet shape as a function of air-flow velocity — this enables us to determine the critical velocity above which the droplet is detached. Analytically, we develop an explicit expression that relates the critical air-flow velocity to droplet diameter, channel height, and static contact angle. We compare computed critical velocities at various droplet diameters with experimental data available from the literature and reasonably good agreements are obtained.

One of the most important advantages of twin-fluid atomizers over other atomizers is the opportunity of atomizing liquids in a wide range of liquid properties efficiently. Thus, the fundamentals of spray dynamics, such as spray pattern, liquid breakup length, and spatial droplet size evolution using different liquid properties need to be investigated in some detail. The purpose of this study is therefore to examine and describe the influence of the liquid viscosity and surface tension on the spray performance of prefilming air-blast atomizers. A high-speed background shadowgraphy technique associated with particle tracking and Phase Doppler Anemometry (PDA) as a non-intrusive high-resolution local measurement technique were utilized to study the atomization process regarding the influence of the liquid properties on the spray quality. Generally, the break up mechanism is considerably influenced by the liquid viscosity. The breakup length ( ) is dependent on liquid viscosity with for Pa = 0.25 and 0.6 bar air pressure, respectively. Droplet size distribution profiles of different liquids in axial and radial directions are studied. No significant influences of the surface tension and viscosity were observed on the mean droplet velocity downstream of the spray. A minor increase of the droplet mean diameter was observed with liquid viscosity variations from 22 to 111 mPas, however, the liquid surface tension was verified to play an important role in the atomization process at prefilming airblast atomizers as well. According to the available experimental data, it has subsequently been possible to develop an original and descriptive model based on a dimensional approach at different air pressure ranges.

Droplet motion induced by superposition of shear flow and horizontal surface vibration

Based on example investigation, this research categorised the failure modes of the column into three typical types, which are one plastic hinge, two plastic hinges and three plastic hinges models, respectively. The first two modes would appear when the sliding body impacted the RC column, while the last mode would occur when the structures are subjected to huge rockfall. Subsequently, the relationship between the critical velocity of sliding body and the properties of the RC column were obtained. To investigate the interaction between a sliding body and a structure, a typical type of RC column was selected for the analysis. The dependence of the critical velocity and the impact angle of the sliding body on the density of the sliding body and the height of the column are determined. It also took the width of the cross section, the ultimate bending moment of the column and the horizontal displacement at the top of the column into consideration. In order to validate the applicability of present model, a case study was enumerated to research the critical velocity and the mass of the rockfall. Meanwhile, sensitivity of parameters, which are the distance between the impact location and the bottom end of the column, and the ultimate bending moment of the column are conducted. In addition, the relationship between the torsional moment of the column and the impact velocity in different impact angles for sliding body and rockfall was also discussed.

In the standard scenario of planet formation, terrestrial planets, ice giants, and cores of gas giants are formed by the accumulation of planetesimals. However, there are few N-body simulation studies of planetesimal accretion that correctly take into account the merging condition of planetesimals. In order to investigate a realistic accretion process of planetesimals, it is necessary to clarify the merging criteria of planetesimals at collision. We perform numerical collision experiments using smoothed particle hydrodynamics and obtain the merging criteria as a function of planetesimal mass and impact parameters for undifferentiated rocky and icy planetesimals and differentiated icy planetesimals. We vary the total mass of colliding planetesimals, their mass ratios, and the impact angle and obtain the critical impact velocity as the merging criteria distinguishing merging from hit-and-run collision. We find that the critical impact velocity normalized by the two-body surface escape velocity decreases with increasing impact angle. The critical impact velocity does not depend on the total mass, while it has a weak positive dependence on the mass ratio. These results barely depend on the composition and internal structure of the planetesimals.

This paper reports important parameters that affect water ricochet studies for artillery projectiles. The factors affecting ricochet are critical angle and critical velocity of impact. This study has developed mathematical models for ricochet studies and derived expressions for critical angle and critical velocity. Simulations have been carried out for standard bullets, the data for which are available in open literature. The effects of mass, diameter, and length of the projectile on critical velocity and angle are studied through two non-dimensional parameters, µ and θ. The results suggest how ricochet conditions can be avoided by defining initial data at launch.

Sticking/rebound criterion for collisions of small adhesive particles: Effects of impact parameter and particle size

The corrugated plate dryer is a very important equipment for the steam-water separation in the steam generator. Its separation efficiency determines the economic indicators of nuclear power plants. The study of the complicated steam-water separation mechanism of the corrugated plate dryer is very helpful to improve its separation efficiency. Flow and breakdown characteristics under horizontal shear of water film falling down vertical corrugated plate dryer is investigated. Air and water are used as the two phases in the research. This paper also derives a model calculating the critical airflow velocity of water film breakdown and completes the comparison of the experimental result and the result calculated by the model. The relationship between the water film thickness and critical airflow velocity of water film breakdown is investigated through the experiment. In addition, the water film thickness is measured according to CCD high-speed camera acquisition system and the Planar Laser Induced Fluorescence (PLIF) method is also used for the measurement. The experimental result reveals that the critical airflow velocity of water film breakdown is related to the corrugated plate structural parameters, the properties of the two phases and the water film thickness. The critical airflow velocity of water film breakdown and the water film thickness are negatively correlated. The result calculated by the model is in good agreement with the experimental result as the water film thickness is large in a certain range.

Density, viscosity, and surface tension of coal liquids have been experimentally determined at temperatues up to 850/degree/F and pressures up to 3200 psia. Measurements were made on liquids produced with the Exxon Donor Solvent process from Illinois and Wyoming coals. Several measurements were also made to determine the effect of dissolved hydrogen on the physical properties of coal liquids. These data were used to investigate the applicability of the existing physical property correlations to coal-derived liquids. Results indicate that the existing correlations are generally satisfactory for the temperature and pressure dependence of density but are unsatisfactory for the viscosity and surface tension of coal-derived liquids. 27 refs.

Direct measurements have been made of the torques required to produce angular acceleration and retardation of liquid helium II contained in a can filled with closely spaced disks at a temperature of 1.27°K. The acceleration and retardation processes are not equivalent, and it is suggested that this asymmetry is due to the presence of vortex lines in the uniformly rotating superfluid. It is found that the forces producing superfluid acceleration are dependent on the past history of the helium and are greatly enhanced if the disk surfaces are roughened; furthermore, the superfluid flow shows a strong tendency to be macroscopically irrotational during retardation. These properties suggest that the forces are associated with turbulence in the superfluid, and it is found that a consistent description of the results can be given in terms of a mutual friction due to such turbulence. Values of the mutual friction appropriate to a steady flow are deduced and found to be in good agreement with values deduced from the results of other isothermal flow experiments; they can be represented, above a critical velocity, by a modification of the law introduced by Gorter & Mellink (1949), namely Fsn = A p sp n with A=23+/-2 posie -1 . V 0 =-0.6 +/- 0.2 c, s -1 Below the critical velocity a persistent current with a lifetime of at least 25 min has been observed. In the last section quantitative results on the growth and decay of turbulence in a non-equilibrium situation are presented, and laws describing the growth and decay processes are formulated.