Interface Height Research Articles

A numerical study of a liquid compound drop solidifying on a horizontal surface

In this study, we present the three-phase numerical simulation results of an axisymmetric compound drop solidifying on a cold plate by the front-tracking method. The compound drop initially has a hemispherical shape with a concentric inner gas core (i.e. gas bubble). The temperature of the plate is lower than the fusion temperature of the shell liquid and thus causes the shell liquid to solidify from the plate surface. It is found that the solid-liquid interface propagates parallelly to the plate during the initial stage of solidification. Then, the region near the outer interface moves faster than that near the inner gas core does. The solidification process completes around the inner core first, and the drop top is the last region to be solidified. Accordingly, a break point is available on the temporal variation of the average height of the solid-liquid interface. Besides, in the presence of volume expansion, the solidified compound drop also has an apex at the top. The numerical results show that the presence of the inner gas bubble prolongs the solidification time. We also find that varying the Stefan number St (in the range of 0.032 – 1.0) or the Bond number Bo (in the range of 0.18 – 3.2) has a minor effect on the inner gas core after complete solidification even though the solidification time decreases with an increase in St or Bo. The effects of the Prandtl number and volume change on the final product of the solidification process are also considered.

Large Eddy Simulation of Wake Formation around a Free Surface–Piercing Circular Cylinder

Guo, H.; Bahrian, F.; Zhang, W.; Sun, C., and Hu, J., 2021. Large eddy simulation of wake formation around a free surface–piercing circular cylinder. Journal of Coastal Research, 37(1), 132–142. Coconut Creek (Florida), ISSN 0749-0208.Numerical simulation of flow past a free surface–piercing circular cylinder using large eddy simulation was performed at Reynolds number Re = 2.7 × 104 and Froude number Fr = 0.8. Commercially available code in ANSYS Fluent was used to investigate the free surface and underlying vortical structures. The dynamic Smagorinsky subgrid-scale model and volume of fluid method for capturing the free surface were used in the simulation. The mean velocities and interface height from the simulation are in good agreement with the available experimental data. The organized Karman vortex generation in deep flow was found to attenuate near the free surface. The overall force developed on the cylinder also decreased due to the presence of the free surface. The simulation shows that the vortical structures present in the flow correlate with the mean parameters of the free surface. Vorticity magnitude and Reynolds shear stress were found to show significantly different behavior close to the free surface as compared to that in deep flow.

Numerical study of ADS windowless spallation target based on diffuse interface method

The study of spallation targets is an important part of ADS system research. Liquid windowless spallation target is the current research hotspot. The working fluid forms a turbulent flow containing gas and liquid in the windowless target, and an accurate prediction of the working conditions is an important part of the study of spallation targets. First, briefly introduce the numerical method of the diffuse interface method, and use water as the working fluid, based on thise method, the four types turbulence model of k-ε model, k-ω model, SST model and Spalart-Allmaras model are used to perform the hydraulic numerical calculation of the ADS liquid windowless spallation target. Through experimental comparison and calculation cost comparison, it is found that the k-ε model is most suitable for the coupled diffuse interface method to predict the working conditions of windowless spallation targets. At the same time, turbulence analyses are performed on the calculation results of the four turbulence models. Then, continue to use water as the working medium, and use the diffuse interface method and k-ε model to perform detailed numerical calculation and analysis of the liquid windowless spallation target. The effects of outlet pressure and inlet velocity on the length of the free interface and the recirculation zone are studied. Finally, using liquid Lead-Bismuth Eutectic as the working fluid, using the diffuse interface method and k-ε model, the effects of outlet back pressure and inlet velocity on the height of the free interface and the pressure along the way were studied.

Thermo-Fluid Dynamics Analysis of Fire Smoke Dispersion and Control Strategy in Buildings

Smoke is the main threat of death in fires. For this reason, it becomes extremely important to understand the dispersion of this pollutant and to verify the influence of different control systems on its spread through buildings, in order to avoid or minimize its effects on living beings. Thus, this work aims to perform thermo-fluid dynamic study of smoke dispersion in a closed environment. All numerical analysis was performed using the Fire Dynamics Simulator (FDS) software. Different simulations were carried out to evaluate the influence of the exhaust system (natural or mechanical), the heat release rate (HRR), ventilation and the smoke curtain in the pollutant dispersion. Results of the smoke layer interface height, temperature profile, average exhaust volumetric flow rate, pressure and velocity distribution are presented and discussed. The results indicate that an increase in the natural exhaust area increases the smoke layer interface height, only for the well-ventilated compartment (open windows); an increase in the HRR accelerates the downward vertical displacement of the smoke layer and that the 3 m smoke curtain is efficient in exhausting smoke, only in the case of poorly ventilated compartments (i.e., with closed windows).

Experimental investigations of upward-inclined stratified oil-water flows using simultaneous two-line planar laser-induced fluorescence and particle velocimetry

Experiments are performed in low-inclination (≤ 5°) upward stratified oil (Exxsol D140) and water flows. The flows are investigated using a novel two-line laser-based diagnostic measurement technique that combines planar laser-induced fluorescence and particle image/tracking velocimetry to obtain two-dimensional (2-D) space- and time-resolved phase and velocity information. The technique enables direct measurements in the non-refractive-index-matched fluids of interest, as opposed to substitute fluids which are matched optically but whose properties may be less representative of those in real field applications. Flow conditions span in situ Reynolds numbers in the range 1300-3630 in the oil phase and 1810-11540 in the water phase, and water cuts of 10% and 20%. Instantaneous velocity vector-fields reveal the presence of complex flow structures in the water phase at low mixture velocities, which become less coherent with increasing pipe inclinations. These structures contribute to the generation of interfacial waves, increase the unsteadiness of the flow and the rate of momentum transfer to the oil phase. Statistical information on the interface heights, mean axial and wall-normal velocity profiles and fluctuations, Reynolds stresses, and mixing lengths is obtained from the analysis of the spatiotemporally resolved phase and velocity data. The normalised mean and rms velocity characteristics (velocity fluctuations and Reynolds stress) are shown to be weakly-dependent on the pipe inclination as the mixture velocity increases. Finally, predictions from a linear mixing-length model agree reasonably well with measurements for the water layer and near-interface regions.

Conductance-Based Interface Detection for Multi-Phase Pipe Flow.

Sediment and flow depth monitoring in sewers is important for informing flow models and for predicting and mitigating against sewer blockage formation and surcharge. In this study, a novel sensor based on conductance measurement has been developed and tested under a laboratory environment and validated by a finite-element model. The relative conductance is measured between pairs of adjacent electrodes to provide a conductance profile along the sensor length. A piecewise linear relationship between conductance and electrode length was derived and the interface positions between sediment, water, and air can be determined from the profile. The results demonstrated that the root mean square error of the model and the measured interface level are within 1.4% and 2.6% of sensor’s measurement range. An error distribution of interface height shows that all anticipated errors are within the resolution of the electrode length increments. Furthermore, it was found that the conductivity of the measured medium is proportional to the gradient of the linear relationship of conductance and electrode length. It could therefore prove a valuable new tool for the accurate quantification of sediment and flow levels in sewer conduits, coastal environments, drainage systems for transport networks, and other industrial or academic applications.

An Easy Method for Pressure Measurement in Microchannels Using Trapped Air Compression in a One-End-Sealed Capillary.

Pressure is one basic parameter involved in microfluidic systems. In this study, we developed an easy capillary-based method for measuring fluid pressure at one or multiple locations in a microchannel. The principal component is a commonly used capillary (inner diameter of 400 μm and 95 mm in length), with one end sealed and calibrated scales on it. By reading the height (h) of an air-liquid interface, the pressure can be measured directly from a table, which is calculated using the ideal gas law. Many factors that affect the relationship between the trapped air volume and applied pressure (papplied) have been investigated in detail, including the surface tension, liquid gravity, air solubility in water, temperature variation, and capillary diameters. Based on the evaluation of the experimental and simulation results of the pressure, combined with theoretical analysis, a resolution of about 1 kPa within a full-scale range of 101.6–178 kPa was obtained. A pressure drop (Δp) as low as 0.25 kPa was obtained in an operating range from 0.5 kPa to 12 kPa. Compared with other novel, microstructure-based methods, this method does not require microfabrication and additional equipment. Finally, we use this method to reasonably analyze the nonlinearity of the flow-pressure drop relationship caused by channel deformation. In the future, this one-end-sealed capillary could be used for pressure measurement as easily as a clinical thermometer in various microfluidic applications.

Experimental and computational study of smoke dynamics from multiple fire sources inside a large-volume building

Fire in large-volume buildings poses separate and distinct challenges from compartmented spaces. In particular, the risk it poses on life safety is mostly due to the smoke it produces, its effects on the evacuation process, and how it can affect the firefighters when they approach the building to fight the fire and rescue the occupants. This is why smoke control in large-volume enclosures is of utter importance for life safety, property protection and business continuity. This research presents an experimental and numerical study of the smoke dynamics from four fire sources (combined heat release rate of 5.2 MW) with different ignition times and under transient ventilation conditions, inside a 20 m high cubic atrium. Temperature measurements in the plume and close to the walls have been recorded using 59 thermocouples. The data was used in the determination of the smoke layer interface height, with the least-square and the N-Percent methods. Results show that significantly worse conditions are induced by multiple sources than those in a single fire of equal power. The results have been obtained using the computational fluid dynamics code FDS (Fire Dynamics Simulator, v6.7.1). The comparison shows that in the far field the temperature predictions are accurate inside the smoke layer (10 m and 15 m) with discrepancies lower than 10%, whereas higher significant discrepancies were observed at the smoke layer interface, i.e. 5 m high, with discrepancies up to 20%. Furthermore, a grid analysis has been performed showing that grid resolutions, D*/δ, based on the global HRR of the fire might be sufficient to predict the fire induced conditions far from the fire. The analysis confirmed the applicability of FDS to the assessment of the smoke dynamics under complex fire conditions away from flames. Ultimately, the present work shows the importance of considering fires with multiple sources due to the faster smoke production compared with single fires.

Deformation of a Liquid Near an AFM Tip: Molecular Dynamics Approach.

The interaction between an atomic force microscopy (AFM) probe and a thin film of water deposited over a flat substrate is studied using molecular dynamics (MD). The effects of the film thickness and the probe radius on both the deformation height of the liquid interface and the distance of the jump to contact at which the liquid comes in direct contact with the probe are investigated. The dynamics of the surface deformation and the role of interface fluctuations are studied in detail. The systems considered belong to the thin-film regime described in a semianalytical model previously established by Ledesma-Alonso et al. (Langmuir 2013, 29, 7749-7757). MD simulations predict that for shallow films, both the distance at which the jump to contact occurs and the surface maximal deformation height increase steadily with the layer thickness regardless of the probe radius, which is in agreement with the previously proposed theoretical model. The deformation of the surface was shown to be unstable because of the strong effect of thermal fluctuations. For each of the considered systems, the film thickness was such that interface fluctuations induced the jump to contact. The comparison of the deformation obtained in MD with the profiles predicted by the continuous model points out the complementarity between the two approaches. The results of the molecular approach not only are consistent with those of the continuous model but also provide more information on the description of nanoscale phenomena. In particular, MD results point out the importance of fluctuations when it comes to the description of the particular dynamics of nanosystems involving soft interfaces. This shows the need to improve continuous models by complementing them with a molecular approach for a better accuracy.

Partial phase flow rate measurements for stratified oil-water flow in horizontal wells

To accurately measure and evaluate the oil-water production profile of horizontal wells, a dynamic measurement experiment of oil-water two-phase flow in horizontal wells and numerical simulation were combined to establish a method for measuring the partial phase flow rate of oil-water two-phase stratified flow in horizontal wells. An experimental work was performed in horizontal oil-water two-phase flow simulation well using combination production logging tool including mini-capacitance sensor and mini-spinner. The combination tool provides a recording of holdup and velocity profiles at five different heights of the borehole cross-section. The effect of total flow rate and water-cut on the response of spinner and capacitive sensor at five measured positions were investigated. The capacitance water holdup interpolation imaging algorithm was used to determine the local fluid property and oil-water interface height, and the measured local fluid speed was combined with the numerical simulation result to establish an optimal calculation model for obtaining the partial phase flow rate of the oil-water two-phase stratified flow in the horizontal well. The calculated flow rates of five measured points are basically consistent with the experimental data, the total flow rate and water holdup from calculation are in agreement with the set values in the experiment too, suggesting that the method has high accuracy.

Bathymetry constrains ocean heat supply to Greenland’s largest glacier tongue

Mass loss from the Greenland ice sheet has increased over the past two decades, currently accounting for 25% of global sea-level rise. This is due to increased surface melt driven by atmospheric warming and the retreat and acceleration of marine-terminating glaciers forced by oceanic heat transport. We use ship-based profiles, bathymetric data and moored time series from 2016 to 2017 of temperature, salinity and water velocity collected in front of the floating tongue of the 79 North Glacier in Northeast Greenland. These observations indicate that a year-round bottom-intensified inflow of warm Atlantic Water through a narrow channel is constrained by a sill. The associated heat transport leads to a mean melt rate of 10.4 ± 3.1 m yr–1 on the bottom of the floating glacier tongue. The interface height between warm Atlantic Water and colder overlying water above the sill controls the ocean heat transport’s temporal variability. Historical hydrographic data show that the interface height has risen over the past two decades, implying an increase in the basal melt rate. Additional temperature profiles at the neighbouring Zachariae Isstrom suggest that ocean heat transport here is similarly controlled by a near-glacier sill. We conclude that near-glacier, sill-controlled ocean heat transport plays a crucial role for glacier stability. Ocean heat transport underneath the floating tongue of 79 North Glacier, Greenland, is controlled by a sill in the inflow channel, according to ship-based and mooring data as well as bathymetric data.

Analytical and numerical predictions of hydrogen gas flow induced by wall and corner leakages in confined space

This contribution addresses a newly developed semi-analytical model coupling the zone model, virtual point source buoyancy plume theory and mirror theory to predict the gas flow behaviors of leaked hydrogen restricted by a wall or a corner in confined space with an opening. The effects of leaked hydrogen mass flux, opening geometry and the leakage location on interface height, outflow velocity and hydrogen molar fraction in upper layer, were thoroughly investigated at steady stage. A computational fluid dynamics tool, FLACS, was employed to simulate the dispersion process in different leakage scenarios and validate the capability of the derived analytical model. The results show that in all center, wall and corner leakage circumstances, the interface height declines with larger leakage mass flux, whereas the outflow velocity and hydrogen molar fraction change inversely. The interface height, outflow velocity and hydrogen molar fraction are positively, negatively and negatively correlated with the opening dimension, respectively. The opening height plays a more important role in determining the interface height and hydrogen molar fraction but hardly affects the outflow velocity. The interface height keeps unchanged with varying leakage locations when other parameters remain constants. However, according to the mirror theory the outflow velocities in corner and wall leakage conditions are 0.63 and 0.4 times of those in center leakage case. Meanwhile, the hydrogen molar fractions of corner and wall leakages are 1.59 and 2.52 times of the ones in center leakage. All these ratios are validated by the corresponding analytical and numerical predictions. The credibility of the analytical model is verified by the good agreement with the numerical estimations.

New approaches to determine the interface height of fire smoke based on a three‐layer zone model and its verification in large confined space

SummaryLarge confined space has high incidence of fires, which seriously threatens the safety of people working there. Understanding the distribution of smoke in such large space is critical to fire development prediction and smoke control. Three improved methods for the stratification interface prediction of fire smoke are developed, including of improved intra‐variance, integral ratio and N‐percentage methods. In these methods, the interface height is determined by the vertical temperature distribution based on a three‐layer smoke zone model, which is an improvement of a two‐layer zone model. Thereafter, the three improved methods are applied to several typical fire cases simulated CFD to predict the smoke interface, and their applicability and reliability are verified by comparison of the smoke stratification results with the filed simulation results. Results show that the three improved methods can effectively determine the location of the three‐layer zone model's interface, and they have the ability to predict smoke interface for fires with different fire source types and ventilation conditions.

Hydrodynamic limit of a $(2+1)$-dimensional crystal growth model in the anisotropic KPZ class

We study a model, introduced initially by Gates and Westcott [11] to describe crystal growth evolution, which belongs to the Anisotropic KPZ universality class [19]. It can be thought of as a $(2+1)$-dimensional generalisation of the well known ($1+1$)-dimensional Polynuclear Growth Model (PNG). We show the full hydrodynamic limit of this process i.e the convergence of the random interface height profile after ballistic space-time scaling to the viscosity solution of a Hamilton-Jacobi PDE: $\partial _{t}u = v(\nabla u)$ with $v$ an explicit non-convex speed function. The convergence holds in the strong almost sure sense.

Critical surface coning due to a line sink in a vertical drain containing a porous medium

The withdrawal of water with a free surface through a line sink from a two-dimensional, vertical sand column is considered using the hodograph method and a novel spectral method. Hodograph solutions are presented for slow flow and for critical, limiting steady flows, and these are compared with spectral solutions to the steady problem. The spectral method is then extended to obtain unsteady solutions and hence the evolution of the phreatic surface to the steady solutions when they exist. It is found that for each height of the interface there is a unique critical coning value of flow rate, but also that the value obtained is dependent on the flow history.
 
 doi:10.1017/S1446181119000099

An investigation of the effect of make-up air velocity on smoke layer height with asymmetric openings and rotational air flow in atrium fires

The results of full-scale tests of smoke interface height in atrium fires with different make-up air velocities and opening arrangements, including asymmetric openings and openings that resulted in rotational air flows, are presented in this paper. The non-symmetric opening arrangements included openings on 3 sides of the atrium, 2 adjacent sides and 1 side. Make-up air velocities of 1, 1.5 and 2 m/s and fire sizes of 1, 2 and 5-MW were considered. Temperature was recorded along the height of the atrium by setting up thermocouple trees, and the temperatures measurements were used to determine the experimental smoke interface height using the N-percentage method. Predicted smoke layer height from the atrium plume correlation is compared with the full-scale test data. The results show that even a 1 m/s make-up air velocity can cause a significant decrease in the smoke interface height in the case of an asymmetric opening arrangement and rotational air flows in the atrium. The full-scale test data are used to propose a correlation to calculate the reduction of smoke interface height for the design of smoke exhaust systems in atria.

Experimental investigation of interfacial wave in stratified low-viscosity-oil and water flow in horizontal and upward pipes

ABSTRACT The effects of oil–water two-phase-stratified flow rates, inlet water cut and pipe inclinations on the characteristics of the interfacial wave were investigated experimentally. The experiment was conducted in a transparent acrylic tube with 20 mm ID, from horizontal to inclined (0°, +3°, +5°, and +10°), using water and No.15 industrial white oil (11.984 mPa·s at 40°C) as test fluids. A high-speed digital camera has been equipped for visualization and a pair of conductivity probes for oil–water interface height measurement. The stratified flow pattern at different pipe inclinations and wave celerity were obtained using a camera. In addition, from the time series of the conductivity probes, the wave amplitude and wavelength were calculated. Results showed that the interfacial wave characteristics are strongly affected by flow rates, inlet water cut and pipe inclinations, and the analysis of these effects are based on the stabilizing force and destabilizing force of the oil–water flow.

Compartment temperature estimation of a multiple-layer cable tray fire with different cable arrangements in a closed compartment

Fire hazard analysis of multiple-layer cable tray is an important part of nuclear safety analysis. Large-scale cable fire experiments with a three-layer horizontal cable tray were conducted in a closed compartment. The vertical temperature profile in the middle of the room was acquired. Distinctive stratification phenomena were found in the vertical temperature distribution. The interface between the upper and lower layer was located at approximately the height of the top cable layer. Heat transfer between the smoke and compartment walls occurred mainly above the smoke interface. A modified non-steady energy balance model in a closed compartment which included the effect of smoke interface height was used to estimate the compartment temperatures. Compared with the experimental results, the modified model for the multiple-layer cable tray fire in a closed compartment provides better estimation than the original model.

Wall-bounded flow over a realistically rough superhydrophobic surface

Direct numerical simulation (DNS) is performed for two wall-bounded flow configurations: laminar Couette flow at $Re=740$ and turbulent channel flow at $Re_{\unicode[STIX]{x1D70F}}=180$, where $\unicode[STIX]{x1D70F}$ is the shear stress at the wall. The top wall is smooth and the bottom wall is a realistically rough superhydrophobic surface (SHS), generated from a three-dimensional surface profile measurement. The air–water interface, which is assumed to be flat, is simulated using the volume-of-fluid (VOF) approach. The two flow cases are studied with varying interface heights $h$ to understand its effect on slip and drag reduction ($DR$). For the laminar Couette flow case, the presence of the surface roughness is felt up to $40\,\%$ of the channel height in the wall-normal direction. Nonlinear dependence of $DR$ on $h$ is observed with three distinct regions. A nonlinear curve fit is obtained for gas fraction $\unicode[STIX]{x1D719}_{g}$ as a function of $h$, where $\unicode[STIX]{x1D719}_{g}$ determines the amount of slip area exposed to the flow. A power law fit is obtained from the data for the effective slip length as a function of $\unicode[STIX]{x1D719}_{g}$ and is compared to those derived for structured geometry. For the turbulent channel flow, statistics of the flow field are compared to that of a smooth wall to understand the effects of roughness and $h$. Four cases are simulated ranging from fully wetted to fully covered and two intermediate regions in between. Scaling laws for slip length, slip velocity, roughness function and $DR$ are obtained for different penetration depths and are compared to past work for structured geometry. $DR$ is shown to depend on a competing effect between slip velocity and turbulent losses due to the Reynolds shear stress contribution. Presence of trapped air in the cavities significantly alters near-wall flow physics where we examine near-wall structures and propose a physical mechanism for their behaviour. The fully wetted roughness increases the peak value of turbulent intensities, whereas the presence of the interface suppresses them. The pressure fluctuations have competing contributions between turbulent pressure fluctuations and stagnation due to asperities, the near-wall structure is altered and breaks down with increasing slip. Overall, there exists a competing effect between the interface and the asperities, the interface suppresses turbulence whereas the asperities enhance them. The present work demonstrates DNS over a realistic multiphase SHS for the first time, to the best of our knowledge.

Displacement ventilation with radiant panel for hospital wards: Measurement and prediction of the temperature and contaminant concentration profiles

Displacement ventilation (DV) is known to provide high air quality and ventilation efficiency. With DV, a contaminant interface is formed in a room, and the air quality in the occupant zone below the contaminant interface height can be kept clean. This paper proposes a DV system to solve the serious odor problem in hospital wards. A vertical radiant panel that can be controlled individually is suggested as a complementary heating system. In order to study the influence of the panel on the displacement ventilated room, the temperature and contaminant concentration profiles were examined under different panel conditions: the distance between the panel and bed, height of the panel, surface temperature of the panel, and supply airflow rate. When the radiant panel was heated, it created a stronger plume than the human body, which produced contaminated air. When there was space between the radiant panel and bed, the contaminated air was locked up before reaching the ceiling. Personal exposure of a standing person was also investigated because the contaminated interface is generally lower than the breathing zone of a standing person with DV. The zonal model and improved zonal model were validated by a comparison of their results with the measured contaminant concentrations.