Critical Height Research Articles

The influence of the liquid layer height on the velocity field and evaporation during local heating

ABSTRACT The Particle Image Velocity method was used to investigate the velocity fields when the liquid layer height changed from 0.6 to 5.1 mm. The loss of stability of the velocity field during local heating is realized at the Bond number (Bo) 0.01–0.02, which is 10–20 times lower than the critical value of Bo(cr) compared to heat exchange at a uniform wall temperature. The Marangoni flow depends both on the height and the layer diameter. The critical height of the layer 2–2.5 mm leads to a sharp change in the flow pattern.

Load Transfer in Geosynthetic-Reinforced Piled Embankments with a Triangular Arrangement of Piles

A geosynthetic-reinforced piled embankment situated along a newly constructed high-speed rail line was fully instrumented and monitored to explore soil arching in an equilateral-triangular arrangement of piles with circular cap configuration. During construction, the settlement and vertical stress on the pile caps and subsurface at ground level and the tensile strain in the geogrid were recorded. A modified analytical arching model is derived from the established concentric arches (CA) model to elucidate the load transfer mechanism, specifically for a triangular array of piles. The general framework assumes that the embankment load, not resting on the subsurface beneath the arches, is transferred outward along the arches and distributed uniformly over the improved ground. Compared to the conventional CA model, the proposed model gives a lower proportion of load carried by piles, more in line with field measurements. Given the friction and adhesion between soil and geosynthetics, a novel quartic equation is applied to characterize the tensioned membrane effect of geosynthetic reinforcement based on Pham’s circular and parabolic models. The stiffness variation of the biaxial geogrid was critically assessed for the triangular pile arrangement, and its implications for the design were revealed. Finally, the pile efficacy and critical arch height were determined from different analytical methods and checked against field observations, demonstrating the potential of the proposed model.

DNS Study on Turbulent Transition Induced by an Interaction between Freestream Turbulence and Cylindrical Roughness in Swept Flat-Plate Boundary Layer

Laminar-to-turbulent transition in a swept flat-plate boundary layer is caused by the breakdown of the crossflow vortex via high-frequency secondary instability and is promoted by the wall-surface roughness and the freestream turbulence (FST). Although the FST is characterized by its intensity and wavelength, it is not clear how the wavelength affects turbulent transitions and interacts with the roughness-induced transition. The wavelength of the FST depends on the wind tunnel or in-flight conditions, and its arbitrary control is practically difficult in experiments. By means of direct numerical simulation, we performed a parametric study on the interaction between the roughness-induced disturbance and FST in the Falkner–Skan–Cooke boundary layer. One of our aims is to determine the critical roughness height and its dependence on the turbulent intensity and peak wavelength of FST. We found a suppression and promotion in the transition process as a result of the interaction. In particular, the immediate transition behind the roughness was delayed by the long-wavelength FST, where the presence of FST suppressed the high-frequency disturbance emanating from the roughness edge. Even below the criticality, the short-wavelength FST promoted a secondary instability in the form of the hairpin vortex and triggered an early transition before the crossflow-vortex breakdown with the finger vortex. Thresholds for the FST wavelengths that promote or suppress the early transition were also discussed to provide a practically important indicator in the prediction and control of turbulent transitions due to FST and/or roughness on the swept wing.

A Hydrosuction Siphon System to Remove Particles Using Fan Blades

Sedimentation in dam reservoirs can cause problems that lead to loss of storage capacity and decrease in the flood control volume. Hydrosuction sediment removal is one of the methods used to remove sediments from within a reservoir using the suction energy provided by the effective head. In this study, a new tool has been developed by attaching the reservoir to a suction pipe intake point and using a simple fan blade mechanism for the hydrosuction sediment removal system. This mechanism is used to create a vortex flow to suspend the settled particles. This paper investigated the effects of the fan blade angles, effective head, and inlet height from the surface of layer particles on the performance and efficiency of fan blades hydrosuction sediment removal (FBHSSR) and hydrosuction sediment removal (HSSR) systems based on the geometric scour hole parameters. Results from the experimental tests indicated the effectiveness of the FBHSSR system, with the fan blade angles of 30°, 45°, and 60° leading to approximately 800%, 200%, and 117%, respectively, removed particles greater than those of the HSSR system. Furthermore, the maximum depth and diameter of the scour hole were increased by 206%, 200%, and 137% and 135, 112%, and 117%, respectively, for each angle. The effective head or experiment time also enhanced system performance by increasing the suction discharge, but no change was observed in terms of efficiency. The critical inlet heights for the FBHSSR and HSSR systems are 1 time and 2.54 times, respectively, more than the diameter of the suction pipe. Thus, it can be concluded that using fan blades in HSSR systems is a good approach to improve the properties of the scour hole.

Physical modeling of slag carryover in the last stage of ladle teeming during continuous casting with dynamic change of slide gate opening

To investigate slag carryover in the last stage of ladle teeming during continuous casting, a water model system was established. The nozzle flow rate of model ladle was controlled by slide gate, which was automatically governed by a control module. The prototype teeming process can be accurately modelled through the system. However, under the condition that the slide gate is fully opened, the teeming process in water model has a great deviation with the actual. The vortex critical height in the mode of dynamic changing of slide gate is obviously less than that under the condition of 100% opening. Furthermore, under the dynamic changing mode, effects of operating parameters on vortex formation and slag carryover have been investigated. The vortex critical height decreases as the residence time before teeming increases. The effect of initial water level on critical height of vortex is little. As the flow rate increases, the critical height of vortex increases significantly. The variation of vortex diameter experiences developing stage and matured stage. When oil is added into model ladle, an important change is that the critical height of vortex decreases. By contrast, the vortex diameter obviously increases. The vortex critical height decreases while oil is added into ladle, but it increases as the oil layer thickness increases.

Suppressing condensation frosting using micropatterned ice walls

Condensation frosting is encountered ubiquitously in our daily life and numerous industrial applications, rooting many detrimental consequences in heat transfer and mechanical stabilities. In latest decades, both active de-frosting and passive anti-frosting methods have been developed. In this work, we adopt the concept of three-dimensional overlapping dry zones by patterning microscopic ice walls on a smooth substrate, engendering a sustainable frost coverage of less than 13 %. Patterned ice walls prevent vapor deposition on the substrate base by absorbing all incoming moisture on upper sidewalls, leaving the substrate base free of contaminates from both frost and condensate. To ensure vapor preferentially depositing on upper sidewalls, a critical ice wall height exists, below which fine ice grains initiate in the adjacent of the substrate base and grow substantially faster, compromising the anti-frosting performance. The proposed surface shows reasonable durability when exposed to varying substrate supercooling, and frost coverage may increase due to covering and fracturing of the out-of-plane growth of feather-like ice clusters at large substrate supercooling. Our findings provide an effective anti-frosting surface design that doesn’t require any chemical or mechanical modifications on the targeted surfaces, showing great advantages compared to conventional icephobic surface designs.

Mechanical instability of heavy column with rotational spring

Abstract In previous research on the mechanical instability of trees based on mechanical theory, wild tree has been modeled as a cantilever which was perfectly attached to the ground. However, experimental research has identified two failure modes, including root turnover and self-buckling of the trunk. This suggests that the imperfect fixation caused by root-soil interaction must be considered when discussing tree stability. The purpose of this study is to clarify the self-buckling characteristics of wild trees considering soil instability. To account for the resistance moment caused by the interaction between the root and soil, trees as cantilevers fixed to the ground by a rotational spring were modeled. In this model, the self-buckling problem was formulated considering the rotational rigidity of the spring, and the formula derived for the critical height and buckling mode. As a result, the formula for critical height considering rotational rigidity was obtained, and it was found that the buckling modes can be classified into the rigid-body mode and beam mode based on the rotational rigidity. By comparing this result with the statistical law based on the measurement of real trees reported in previous research, it was determined that real trees were designed based on beam mode. This suggests that the wild tree skillfully balances the moment of resistance caused by the interaction between the root and soil to prevent “uprooting,” which is extremely fatal for trees. Moreover, it was also found that the safety factor of trees for self-buckling is ensured enough to prevent the beam mode.

Evaporation in thin film region of water at ambient temperature under reduced pressure conditions: Flow and interface characteristics

In this paper, the flow and interface characteristics of the thin liquid film region near the contact line of water under reduced pressure conditions are experimentally observed by using a multilayer nanoparticle image velocimetry technique. The results demonstrate that the flow in this region is promoted by reducing the pressure. The average velocities in the top layer of this region are 57.99, 61.52, 65.20, 66.00, and 68.25 μm/s at the degree of reduced pressure of 10–50 kPa with an interval of 10 kPa. Reducing the pressure shortens the evaporation time and advances the depinning time in the constant contact radius mode. Entering the constant contact angle mode, the initial height of the film increases, the critical height is elevated, and the decrease rate of film thickness increases, which leads to the shortening of the overall evaporation time. This work provides insight into the microscopic transport mechanisms of coolant under reduced pressure conditions.

Numerical Analysis of Grouting of Water-Enriched Karst Highway Tunnel Based on Critical Water-Enriched Height

The occurrence state of groundwater in a karst area has a significant impact on the tunnel lining structure. The impact on the groundwater is more significant during construction. This paper used numerical simulation to study the grouting reinforcement range of the Liaoshan Tunnel. This was based on the critical water rich height. Under Grade IV and Grade V surrounding rock, the critical points of grouting reinforcement were 230 m and 240 m deep, respectively. The vertical and horizontal stress and seepage flow of the tunnel arch crown and invert decreased as the circumferential grouting reinforcement range increased. In the case of no grouting reinforcement, 1/4 circumferential grouting reinforcement, 1/2 circumferential grouting reinforcement, and all circumferential grouting reinforcement, the seepage flow was 7 m3/dm, 4 m3/dm, 3 m3/dm, and 0 m3/dm, respectively. With Grade V surrounding rock, when the groundwater level was 260–270 m away from the tunnel arch crown, two schemes could be adopted: 1 m radial grouting reinforcement and 1/2 circumferential grouting reinforcement. The results have guiding significance for developing a construction scheme of a karst water rich tunnel.

Two-dimensional spreading of frictionless adhesive oil droplets.

When sand flows out of a funnel onto a surface, a three dimensional pile that is stabilized by friction grows taller as it spreads. Here we investigate an idealized two dimensional analogue: spreading of a pile of monodisperse oil droplets at a boundary. In our system the droplets are buoyant, adhesive, and in contrast to sand, friction is negligible. The buoyant droplets are added to the pile one-at-a-time. As the aggregate grows, it reaches a critical height and the 2D pile spreads out across the barrier. We find that, while granularity is important, the growth process is reminiscent of a continuum liquid. A "granular capillary length", analogous to the capillary length in liquids, sets the critical height of the aggregate through a balance of buoyancy and adhesion. At a coarse-grained level, the granular capillary length is capable of describing both steady-state characteristics and dynamic properties of the system, while at a granular level repeated collapsing events play a critical role in the formation of the pile.

Effect of initial generating eddy height on formation and flame geometry of fire whirl

Background Fire whirl is an extreme fire behaviour in wildland fires, and an essential factor for its formation is the surrounding generating eddy. No systematic experimental study has been conducted on natural fire whirls with varying heights of the initial generating eddy. Aims The aim of this research was to provide a comprehensive experimental study on the effect of initial generating eddy height on fire whirl formation and flame characteristics. Methods The experiments were conducted in a fixed-frame facility with varying channel wall height (representing the initial generating eddy height). A 20-cm-diameter propane burner (10.0–100.0 kW in heat release rate) was used. Key results The critical channel wall height for fire whirl formation decreases with the heat release rate. The mean flame height grows remarkably with initial generating eddy height for large heat release rates, but it varies only slightly at relatively small heat release rates. Conclusions The formation of fire whirl depends on the initial generating eddy height, rotational strength, and heat release rate. A flame height correlation of the fire whirl is obtained by considering the initial generating eddy height. Implications This work provides a basis for improving the prediction accuracy of natural fire whirls in wildland fires.

Deep learning method for rapidly estimating pig body size

Context During pig breeding, a change in a pig’s body size is an important indicator that reflects its health. However, it is difficult to extract the necessary features from images to estimate pig body size without contact. Aims It is crucial to develop a fast and accurate body size estimation algorithm to meet the practical needs of farms, i.e., numerous body size detections. Methods This report presents a rapid pig body size estimation technique based on deep learning. The YOLOv5 model is enhanced by integrating MobilenetV3, and a lightweight object detection network is introduced as the feature extraction network. An attention mechanism is also added to this system. Following these improvements, the proposed YOLOv5_Mobilenet_SE model is more suitable for the small-target detection of key parts of live pigs. A depth camera was used at a fixed height to capture the pig’s back information, which enables calculations of the critical height, i.e., the body height, of live pigs. Other key measuring points on the pig are generated according to the detection frame of the key parts located by the model. A gradient boosting regression algorithm is used to establish the body size prediction model based on the Euclidean distance between the key measuring points and the actual body size data. Key results The upgraded YOLOv5_Mobilenet_SE model achieves a mean average precision of 3.9%, which is higher than that obtained using the original YOLOv5 model. The model size is reduced from 91.2 to 10.2 M, and the average detection time for each image is 4.4 ms. The mean absolute percent errors in terms of body size, body width, and body height are 2.02%, 1.95%, and 1.84%, respectively, relative to manual measurements. Conclusions This method greatly reduces the model size and detection time while ensuring accuracy, and therefore, this method can cut costs for farms performing pig body size measurements. Implications The results of this study can provide technical support for automated and digital monitoring in the pig breeding industry.

Ionospheric Variability over the Brazilian Equatorial Region during the Minima Solar Cycles 1996 and 2009: Comparison between Observational Data and the IRI Model

The behavior of the Brazilian equatorial ionosphere during the solar minimum periods, 1996 and 2009, which cover the solar cycles 22/23 and 23/24, respectively, is investigated. For this, the F2 layer critical frequency (foF2) and peak height (hmF2) registered by a Digisonde operated at São Luis (2.33° S; 44° W) are carefully analyzed. The results show that the seasonal mean values of the foF2 and the hmF2 in the equinoxes and winter during 2009 were lower than in 1996. In the summer, an anomalous response to solar variability was observed. In this case, the hmF2 in 2009 is higher than in 1996 during a specific daytime interval. Besides that, it was verified that the prereversal enhancement of the zonal electric field (PRE) during the equinoxes in 2009 occurred a few minutes earlier than in 1996. Additionally, a Fast Fourier Transform (FFT) analysis was used to investigate the impacts of solar atmospheric tides (amplitude, diurnal, semidiurnal, and terdiurnal modes) on foF2 and hmF2 parameters with respect to its seasonality. Significant differences were observed between their values during the two minima, mainly in the amplitude of hmF2, which was higher in 1996 than in 2009 for all days analyzed. Moreover, the seasonality in the diurnal and semidiurnal modes for both periods presented an annual variability, while the terdiurnal mode exhibited annual and semiannual components. The results are compared with the International Reference Ionosphere (IRI) model, and the main differences between the observation and the model results are discussed in this work.

Assessment of reasonability of production wells transfer to well workover stage

The article discusses predicting the critical parameters of production wells to assess the need to transfer them to well workover stage and put on the program of geological and technical measures (GTM). A methodology for substantiating and assessing the influence of critical values of reservoir parameters on the operation of production wells is presented. Three stages of transferring wells to workover are considered: geological, technological and analytical, and the procedure for assessing their effectiveness. The procedure for calculating the shut-in time of wells and gas losses during the transition of production wells to workover, determining the parameters of the technological regime after geological and technical measures are considered. A method has been developed for predicting the time of well shut-in due to flooding (self-killing) and assessing the timing of its operation with production flow rates at the final stage of field development. The results of the study of dependence of the critical height of the liquid column on the bottomhole pressure and the average daily flow rate of the Cenomanian well, the scheme for revealing the dependence of gas water contact level on the reservoir pressure and the volume of the sampled gas are presented. The procedure for calculating the technical and geological-production state of wells is considered according to the diagnostic rating assessment. Keywords: field; gas well; prediction; critical parameter; cenomanian; flooding; flow rate; pressure.

Characterization of viscous Newtonian and non-Newtonian liquid rope coiling onto static and moving surface

In this paper, the coiling characteristics of silicon oil and epoxy resin are compared on static as well as moving base plates. Jetting, coiling and dripping regimes are observed in the case of silicon oil. Coiling and dripping are observed for epoxy resin. All flow regimes of Epoxy Resin are found to be viscous dominated than silicon are inertia-gravity dominated. A critical height of the stack of the coil is noticed before it collapses, specifically for epoxy resin. As the base plate starts a movement, Translated Coil, side-kick and meandering are noticed depending on the relative forces of shear and inertia. Translated coils are observed for inertia dominated regime, Side-kick observed when inertia and viscous are comparable, and meandering are observed for the viscous dominated regime. Epoxy Resin is better controlled in terms of frequency and wavelength of the pattern formation on the moving base surface.

Three-dimensional physical modeling of load transfer in basal reinforced embankments under differential settlement

This study developed a large-scale laboratory apparatus to evaluate the load transfer behavior of basal reinforced embankment fill because of soil arching and geogrid tensile force. A 3D trapdoor-like test system performed five scaled model tests using analogical soil. The instrumentation system involved multiple earth pressure cells, dial gauges, multipoint settlement gauges, and geogrid strainmeters. Comprehensive measurements were performed to investigate the evolution of soil stress and displacement at specific fill elevations with variations in the area replacement ratio and geogrid stiffness. The critical height of the soil arching was determined to be ∼1.1–1.94 times the clear pile spacing based on the soil stress and displacement profiles. The distribution of the geogrid tensile strain between and above the adjacent caps demonstrated that the maximum geogrid strains occur on top of the caps, and the tensioned geogrid effect on the load transfer efficiency was evaluated. The cap size and center-to-center pile spacing affect the pile efficacy more significantly than the stiffness of the geogrid. The measured critical heights of arching, stress concentration ratios, and geogrid strain matched those calculated by several well-recognized analytical methods. This experimental program facilitates the development of arching models that account for differential settlement impact.

Characterizing model uncertainty of upper-bound limit analysis on slopes using 3D rotational failure mechanism

The slope stability assessment is a classical problem in geotechnical engineering. This topic have attracted many researcher’s attention and various theoretical models for predicting critical slope heights or safety factors in the light of the limit equilibrium (LE) method and the kinematical approach of limit analysis (LA) method. Meanwhile, a large number of experimental studies have been conducted to check the slope stability. Using centrifuge testing results, this paper aims to employ Bayesian method to characterize the model uncertainties of the classical three-dimensional rotational failure mechanism proposed by Michalowski and Drescher (2009) to predict critical slope heights in frictional soils, by incorporating the test uncertainties and parameter uncertainties. The obtained results show that the LA three-dimensional rotational failure mechanism overestimates the critical slope height compared with the LE method, and the experimental observational uncertainty has negligible influences on the posterior statistics of model uncertainty.

Critical diffraction of irregular structure detonations and their predictability from experimentally obtained [formula omitted] data

The present work reports new experiments of detonation diffraction in a 2D channel configuration in stoichiometric mixtures of ethylene, ethane, and methane with oxygen as oxidizer. The flow field details are obtained using high-speed schlieren near the critical conditions of diffraction. The critical initial pressure for successful diffraction is reported for the ethylene, ethane and methane mixtures. The flow field details revealed that the lateral portion of the wave results in a zone of quenched ignition. The dynamics of the laterally diffracting shock front are found in good agreement with the recent model developed by Radulescu et al. (Physics of Fluids 2021). The model provides noticeable improvement over the local models using Whitham’s characteristic rule and Wescott, Bdzil and Stewart’s model for weakly curved reactive shocks. These models provide a link between the critical channel height and the critical wave curvature. The critical channel heights and global curvatures are found in very good agreement with the critical curvatures measured independently by Xiao and Radulescu (Combust. Flame 2020) in quasi-steady experiments in exponential horns for three mixtures tested. Furthermore, critical curvature data obtained by others in the literature was found to provide a good prediction of critical diffraction in 2D. These findings suggest that the critical diffraction of unstable detonations may be well predicted by a model based on the maximum curvature of the detonation front, where the latter is to be measured experimentally and account for the role of the cellular structure in the burning mechanism. This finding provides support to the view that models for unstable detonations at a meso-scale larger than the cell size, i.e., hydrodynamic average models, are meaningful.

Drop impact on a sessile soap bubble

The dynamics of a water droplet impacting onto a soap bubble sitting on a solid substrate was experimentally investigated using a high-speed camera. At low impact velocities, the droplet bounced on the soap bubble. Varying the size of the soap bubble, we discovered two regimes at high impact velocities. For a soap bubble larger than a critical dimensionless height of 3.18, the water droplet passed through the soap film, while the soap bubble burst as the bubble height was less than the critical value. Theoretical models were proposed for the critical impact velocities needed to reach these two regimes, with their intersection correctly capturing the critical bubble height separating them. Two mechanisms for the bursting of the soap bubble were identified depending on the location where the bursting started, either from the center below the impacting droplet or from the periphery of the soap film. Moreover, the bursting of the soap bubble could be suppressed by replacing the solid substrate below the bubble with a tube.

Transient simulation of slag entrapment in a tundish

A transient multiphase flow model is established by solving the heterogeneous dynamic equation to simulate the process of the slag entrapment in the tundish, which can provide data for the trial production to reduce the tundish residue on the premise of ensuring no slag entrapment. A numerical simulation of critical slag entrapment height of 60 t tundish under different steel throughputs is analyzed, and the following conclusions are obtained: when the steel throughput is 3.5 t/min, 4.5 t/min, 5.5 t/min, and 6.4 t/min, the critical time of the risk of slag entrapment are 1042 s, 691 s, 561 s, and 476 s after casting end, respectively. The heights of slag entrapment are 186.9 mm, 202.9 mm, 220.5 mm, and 222.5 mm, and the heights without slag entrapment are 202.4 mm, 214.6 mm, 236.1 mm, and 238.0 mm with the throughput of 3.5 t/min, 4.5 t/min, 5.5 t/min, and 6.4 t/min.