Plume Centerline Research Articles

Effects of selected cathode materials on a magnetically enhanced vacuum arc thruster

We discuss the effects of the ion distribution of a magnetically-enhanced Vacuum Arc Thruster using different cathode materials (Fe, Al, Cu) and magnetic field strengths. We develop a formalism, which is based on physical and geometric ideas, that describes the data for the inclusion of a magnetic field better than previous models (R2 > 0.89 for worst case). Our tests were performed on a Vacuum Arc Thruster with 270 A discharge current and an average pulse length of 3.5 ms. The magnetic field generator, driven by a separate capacitive discharge circuit, produced field strengths ranging from 0 to 250 mT along the thruster's centerline. We obtained the thrust correction factor and theoretical thrust from our measurements. It was found that application of magnetic fields increased ion density and axial momentum along the plasma plume's centerline. At higher magnetic field strengths (∼250 mT) the thrust factor decreased across all materials. For our electrodes, the inclusion of a protruding magnetic nozzle was seen to cancel the benefits arising from beam collimation due to the magnetic field. The iron electrode performed best, which may be due to its ferromagnetic nature.

Enhancing in situ biodegradation in groundwater using pump and treat remediation: a proof of concept and modelling analysis of controlling variables.

A remediation approach which uses pump and treatment (PAT) to enhance the biodegradation of organic contaminants by increasing dispersive mixing between plumes and groundwater was evaluated for a phenol-contaminated aquifer, using a reactive transport model which simulates kinetic reactions between an electron donor (ED) in the plume and electron acceptor (EA) in the groundwater. The influence of system design and operation was examined in six modelling scenarios. Injection or extraction of groundwater increases biodegradation above no action and the location, pumping rate, and distance between well(s) are important variables which influence biodegradation. An increase in pumping rate, distance of the wells from the plume centreline, and changing the flow direction increase dispersive mixing between the plume and groundwater. This increases plume spreading and the plume fringe interface, providing a greater flux of dissolved EAs for biodegradation. In general, injection of groundwater containing natural EAs enhances biodegradation more than extraction. The enhancement of biodegradation is sensitive to the relative fluxes of ED and EA, as controlled by the arrangement of the wells. In the best performing scenario, biodegradation was enhanced by 128%, compared with no action.

Hydro‐Sedimentary Processes of a Plunging Hyperpycnal River Plume Revealed by Synchronized Remote Imagery and Gridded Current Measurements

AbstractThe present knowledge of plunging hyperpycnal river plumes is mainly based on two‐dimensional (confined) laboratory experiments. Several hypotheses on three‐dimensional (unconfined) flow processes have been made, but not tested in situ. In this field study, the dominant three‐dimensional hydro‐sedimentary processes related to unconfined plunging were elucidated by synchronizing autonomous time‐lapse camera images with boat‐towed acoustic Doppler current profiler measurements. It was found that the flow field complies with two‐dimensional conceptualizations along the plume centerline. Perpendicular to the centerline, the plume slumped laterally due to its density excess and simultaneously converged laterally due to its vertical divergence. This combination led to a narrowing of the plume near the surface, resulting in a sediment‐rich triangle‐shaped pattern on the surface near the inflow, and a rather stable width near the bed. The formation of secondary flow cells transporting riverine water and suspended particulate matter (SPM) away from the plume near the bed, up toward the surface and back toward the plume near the surface, was revealed. An increase of the average SPM concentration and the SPM flux in the main flow direction indicates net sediment erosion under the investigated conditions of high discharge and sediment load. This suggests transient storage of sediment during conditions of lower discharge and sediment load, and high morphological activity in the plunging area. These findings allowed extending a classical conceptual plunging model to laterally unconfined sediment‐laden plunging inflows for conditions of well‐mixed ambient water in the plunging region and inflow densimetric Froude numbers exceeding 1, common in nature.

Experimental and numerical investigation into burning rate and fire plume characteristic of pool fire with plate obstacle

Pool fire is a common type of fire accident in the petrochemical industry, which often causes significant casualties and property damage. In most real fire scenarios involving the interaction between pool fire and obstacles, the surrounding obstacles would impact the development of the pool fire significantly, altering the results of fire risk assessment. To address these knowledge gaps, a comprehensive experimental and numerical analysis was conducted to explore the pool fire behavior with plate obstacle. The results show that as the obstacle height decreases or diameter increases gradually, the average burning rate increases obviously. The temperature and flow fields were obtained through numerical simulation, and indentified as three distinct patterns. It was found that the vortices formed near the obstacles facilitate the mixing of air and fuel vapor, thereby changing the fire plume behavior. In addition, the classical model for the plume centerline temperature was modified for pool fire with plate obstacle. This work enhances understanding of pool fire behavior with plate obstacles, which directly contribute to mitigating fire damage and improving energy application safety.

Effect of Compartment Height with Fire Source on Horizontal Vent Flow and Temperature Distribution

The effect of compartment height with a fire source on the horizontal vent flow and temperature distribution was numerically investigated. Two compartment heights were examined, namely 1.1 m (H1.1) and 5.5 m (H5.5). Horizontal vent areas were set at 1% (HV01) and 10% (HV10) of the floor area, and fire sources were positioned at the center (FSC) and side (FSS). H1.1 exhibited a larger mass flow rate and higher velocity of vent flow than H5.5 for FSC cases with HV01 and HV10. In the case of HV01 with FSS, the mass flow rates between H1.1 and H5.5 were comparable. However, in the case o f HV10 w ith F SS, H5.5 d emonstrated a larger m ass flow r ate than H 1.1. V ent flow v elocities a ppeared similar between H1.1 and H5.5 for FSS cases with both HV01 and HV10. Meanwhile, H1.1 exhibited a higher temperature distribution than H5.5 for HV01 cases with FSC and FSS. For HV10 with FSC, H5.5 showed a higher temperature distribution than H1.1. Conversely, for HV10 with FSS, the opposite trend was observed. The previous correlation overestimated the results of the present numerical simulation regarding the fire plume centerline temperature rise. It was speculated that mixing within the compartment affected the trend of fire plume centerline temperature rise.

Infrared radiation characteristics of dagger-type hypersonic missile

Hypersonic vehicles emit strong infrared radiation from their high-temperature exhaust plume and body, which is critical for infrared early warning, tracking, and guidance. In this work, a comprehensive analysis is conducted on the factors involved in air dissociation reaction within the shock layer of hypersonic missile heads, as well as the multi-component afterburning effect of the exhaust plume. A novel Reverse Monte Carlo Method (RMCM) is proposed for infrared radiation calculation, which utilizes two-dimensional Low-Discrepancy Sequences (LDS) to improve computational accuracy. The numerical calculations for a dagger-type missile show that afterburning reactions increase the temperature on the centerline of the outlet exhaust plume by about 1000 K. The total infrared radiation intensity of the missile is the highest in the 1–3 μm band, with the high-temperature wall of the nozzle being the primary source of solid radiation, and gas radiation primarily coming from H2O. The radiation intensity of the missile exhaust plume in the 3–5 μm band is the highest, with radiation sources primarily coming from CO2, CO, and HCl. Afterburning reactions of the exhaust plume increase the total infrared radiation intensity of the missile by about 0.7 times. These results can provide reference for the detection and guidance of hypersonic missiles.

Rock core VOC profiles diagnostic of aquitard occurrence and integrity in a multi-layered sedimentary rock aquifer flow system

Aquitard assessment poses a challenge for practitioners and researchers managing dense non-aqueous phase liquid (DNAPL) contaminants in sedimentary rock aquifer systems. Within these systems, DNAPL migration and bulk hydraulic properties are controlled by fracture networks which are difficult to characterize at relevant scales with conventional methods. A previous study at a DNAPL contaminated site used contrasts in high-resolution vertical gradients to delineate four aquitards and two surfaces with poor vertical connectivity within a thick (>200 m) sedimentary rock aquifer system primarily composed of layered sandstones. In this study, we corroborate that method by using the existing DNAPL contamination as a tracer to identify and characterize the aquitards in the sequence and assess aquitard integrity. Closely spaced samples of rock from four continuous cores were collected and analyzed for volatile organic compounds (VOCs). The cores were collected along a transect perpendicular to groundwater flow and 75 m downgradient of the DNAPL source zone. Dissolved phase contaminant concentrations were collected using high-resolution multilevel systems placed along the plume centerline, downgradient from the source. The rock core and groundwater VOC concentration profiles suggest a primary aquitard limits vertical migration of both DNAPL and the associated dissolved phase contaminants, despite relatively large, downward gradients across its lower boundary. A calibrated, three-dimensional numerical groundwater flow model indicates an anisotropy factor of 1,000 for this aquitard. The Kv values for all four aquitards are only slightly lower than the values for the aquifers. However, the aquitard Kv values are 1–3 orders of magnitude less than their Kh values, and the aquitard Kh values are often similar to or even greater than the Kh values for the aquifers. Therefore, large anisotropy, rather than exceptionally low Kv values, distinguish the aquitards from the aquifers in this system. Fracture network characterization of nearby outcrops showed the anisotropy of the aquitards can be attributed to laterally extensive, bedding parallel partings coupled with short, frequently terminating, and poorly connected vertical fractures. In addition, the rock VOC profiles combined with fracture network data show that fracture termination surfaces can be powerful inhibitors of downward DNAPL migration providing additional evidence for the importance of these features in sedimentary rock flow systems. Ultimately, this study reveals the important influence of anisotropic aquitards on DNAPL migration, accumulation, and persistence, as well as associated plume migration. Furthermore, it demonstrates that subtle contrasts critical to aquitard integrity may be easily missed with long screened wells and other conventional, low resolution site characterization methods.

General solutions of the plume equations: towards synthetic plumes and fountains

Previous mathematical models of quasi-steady turbulent plumes and fountains have described the flow that results from a prescribed input of buoyancy. We offer a new perspective by asking, what input of buoyancy would give rise to a plume, or fountain, with given properties? Addressing this question by means of an analytical framework, we take a first step toward enabling a plume with specific characteristics, i.e. a synthetic plume, to be designed. We develop analytical solutions to the conservation equations that describe four kinds of turbulent flow: axisymmetric plume, starting fountain, line plume and wall plume. Crucially, our solutions do not require the buoyancy distribution to be specified, whether this be the source or off-source distribution. Key to our approach, we specify a function for the volume flux, $Q=f(z)$ , and take advantage of the weak coupling between the conservation equations to uniquely express general solutions in terms of $f$ . We show that any analytic function $f$ can form the basis for a set of solutions for the fluxes, local variables and local Richardson number, though $f/({\rm d}f/{\rm d}z)>0$ is a necessary condition for physically realistic solutions. As an example of plume synthesis, we show that an axisymmetric plume can have an invariant radius if there is an exponentially increasing input of buoyancy to the plume centreline. We also consider how plume synthesis could be achieved practically.

Experimental study on the morphology of turbulent diffusion flames with different aspect ratios under slope conditions

In this paper, the morphology of turbulent diffusion flames with varying aspect ratios under slope conditions was investigated. A simulated fire source using propane gas with an area of 80 cm2 was utilized. The influences of slope angle and aspect ratio of the fire source on the flame tilt angle (between the plume centerline and the slope), flame length, and flame height were systematically studied. The aspect ratio of the fire source (L/W) was changed from 1 to 8, and the slope angle was changed from 0° to 40°. Results indicate that the presence of a slope causes asymmetrical air entrainment on both sides of the fire plume, causing a pressure difference that leads to a decrease in the flame tilt angle as the slope angle increases. Additionally, the flame length increases monotonically with the slope angle due to the enhanced component of thermal buoyancy along the slope. Under the same slope condition, the flame tilt angle and flame height (perpendicular to the slope) decrease as the aspect ratio increases. This phenomenon is attributed to the increased entrainment restriction effect of the slope on the fire plume. New models for calculating flame tilt angle, flame length, and flame height, which consider the slope angle and aspect ratio of the fire source were established by using the equivalent perimeter of plume entrainment. To further verify the reliability of the newly established models, reference data with wide heat release rate (6.15 kW–231 kW), different slope inclination angle (0°–80°), pool size (equivalent diameter: 9 cm–44.1 cm), and different fuels (propane, ethylene) were used to against with the predicted values of our correlations. It is found that the maximum error of all correlations is within 15–20%, their universality and accuracy are further verified.

Experimental investigation and integral simulation of a thermal annular plume

Thermal plumes issued with mean exit velocities of 0.527 and 0.702 m/s from internally closed annular nozzles are investigated experimentally, theoretically, and numerically. The experimental setup and the measurement and analysis methodologies for flow and mixing are described in detail. The experimental setup consists of an air heating box with an annular slot of 0.12-m mean diameter and 0.01-m width, accompanied by equipment for setup operation and measurement of mean velocities and temperatures. The trajectory of maximum velocities and the isovelocity contours indicate reattachment due to plume self-merging at z_{textrm{P0}}= 0.161 m and around 0.20 m in correspondence to exit velocities. It was found that z_{textrm{P0}} depends mainly upon initial Froude number and annulus equivalent diameter. Experiments detected a vortex ring right after the exit followed by a stagnation point at z_{textrm{P1}}cong 0.055 m on the plume centreline. The annular plume core obtains almost uniform temperature and on average 15 times higher dilutions than bulk dilutions of the equivalnt round plume. To predict the self-merging trajectory, a novel integral model was developed named self-merging approach (SMA). SMA is based on the solution of the ordinary differential equations of momentum and buoyancy applied to an infinitesimal sector of the annular plume, which is equivalent to two identical slot plumes. These plumes are reattached due to outer entrainment, inner entrainment starvation, and a series of sinks along the annular plume centreline with strengths calculated by an original relationship. SMA predictions agree very well with the observations. The present findings contribute to a better understanding of the phenomenon and offer new ideas for designing buoyancy-operating disposal systems with low momentum, using annular- or rosette-type diffusers mainly in shallow water bodies.

Quantifying the Effect of Wind on Volcanic Plumes: Implications for Plume Modeling

AbstractThe considerable effects that wind can have on estimates of mass eruption rates (MERs) in explosive eruptions based on volcanic plume height are well known but difficult to quantify rigorously. Many explicitly wind‐affected plume models have the additional difficulty that they require the use of centerline heights of bent‐over plumes, a parameter not easily obtained directly from observational data. We tested two such models by using the time series of varying plume heights and windspeeds of the 2010 eruption. The mapped fallout and photos taken during this eruption allow us to estimate the plume geometry and to empirically constrain input parameters for the two models tested. Two strategies are presented to correct the difference in maximum plume height and centerline height: (a) based on plume radius, and (b) by using the plume type parameter Π, which quantifies the relative influence of buoyancy and cross‐wind on the plume dynamics, to discriminate weak, intermediate and strong plumes. The results indicate that it may be more appropriate to classify plumes as either wind‐dominated, intermediate or buoyancy‐dominated, where the relative effects of both wind and MER define the type. The analysis of the Eyjafjallajökull data shows that the MER estimates from both models are considerably improved when a plume‐type dependent centerline‐correction is applied. For one model, we varied the wind entrainment coefficient β. For this particular eruption, we find that the best value for β lies between 0.28 and 0.36, unlike previous suggestions that set this parameter to 0.50.

Berechnungsverfahren für Temperaturen im Brandnahbereich

Abstract Für die ingenieurgemäße Untersuchung brandschutztechnischer Fragestellungen, die im Rahmen von individuellen Brandschutzkonzepten vorgenommen werden, ist häufig die Kenntnis von Umgebungstemperaturen im Umfeld des Brandereignisses wesentlich. Dabei stehen Fragen nach dem Auslösezeitpunkt von Sprinklern, die Temperaturbeaufschlagung von Bauteilen und das Erreichen kritischer Temperaturen Für Menschen oder Baustoffe im Vordergrund. Vielfach wird diese Berechnung mit Zonenmodellen oder überschlägig mit Plume-Ansätzen vorgenommen. Bei dieser Vorgehensweise werden häufig lokale Effekte nicht beachtet oder in Fällen, bei denen sich während des Brandverlaufes eine heiße Rauchgasschicht bildet, zu geringe Plume-Temperaturen berechnet. Mit Hilfe von modifizierten Ansätzen, wie sie in diesem Bericht vorgestellt worden sind, besteht die Möglichkeit, realistischere Werte zu berechnen, da sie insbesondere eine Vermischung der im Plume aufsteigenden Verbrennungsgasen mit den heißen Rauchgasen einer ausgebildeten Rauchgasschicht mit endlicher Temperatur in die Betrachtung einbeziehen.

Improved Domenico solution for three-dimensional contaminant transport

The Domenico solution is a widely used analytical solution in screening tools such as BioScreen, FATE5, BioChlor, ART3D, ArcNLET, Quick-Domenico, etc., to simulate the three-dimensional transport of reactive contaminants in groundwater. While being approximate, this solution presents a convenient closed-form expression for computing plume contaminant concentrations without the need for numerical integration. Previous studies have mathematically shown that the Domenico solution provides correct estimates of contaminant plume concentrations when the value of longitudinal dispersivity is zero. However, under non-zero longitudinal dispersivity values, the errors in the Domenico solution increase, especially at plume locations beyond the advective front. The nature of these errors places severe limitations on the applicability of the Domenico solution in simulating realistic contaminant transport problems. Here, we present a new closed-form solution for three-dimensional (3D) reactive contaminant transport that overcomes this limitation. We introduce the concept of characteristic residence time, which represents the average time taken by a contaminant particle to reach a given (x, y, z) location. We show that along the plume centerline, the characteristic residence time exhibits three features, (i) initial linear response with a slope equal to the inverse of advection velocity, (ii) final saturation response with a maximum value of simulation time (iii) transition between the initial and final responses at the region surrounding the advective front, with sharper transitions at lower longitudinal dispersivities and smoother transitions at higher longitudinal dispersivities. We calibrate the Ramberg-Osgood function to fit the behavior of the characteristic residence time along the plume centerline and use this to develop a new and improved closed-form approximation for the Domenico solution. Without any additional computational effort, our new and improved solution better approximates the solute concentrations of a three-dimensional contaminant plume under realistic values of longitudinal dispersivity. Our improved solution closely matches the exact analytical solution, especially at plume locations beyond the advective front, where the Domenico solution performs poorly. Existing screening tools can be readily be retrofitted with the new and improved solution to provide more accurate estimates for plume concentrations.

Flow fields and outputs of the hydrothermal plume at the Wocan-1 field based on an <italic>in-situ</italic> video

<p indent="0mm">Deep-sea hydrothermal plumes are an essential medium for exchanging the heat and elements between the lithosphere and hydrosphere and contribute to elemental cycling and marine chemistry evolution. In this paper, based on field video footage of a black smoker recorded at the Wocan-1 hydrothermal field on the slow-spreading Carlsberg Ridge in the northwestern Indian Ocean, the flow fields of the “Huoyanshan” hydrothermal plume were calculated using a cross-correlation algorithm. In addition, the heat and chemical outputs of the hydrothermal vent are investigated using the estimated source volume flux and fluid properties obtained from <italic>in-situ</italic> measurements. The results reveal that as the plume rises, the vertical velocity along the plume centerline increases, reaching a maximum value of approximately <sc>15.5 cm/s.</sc> This tendency is attributed to the combined effect of initial momentum flux and buoyancy flux. The maximum horizontal velocity along the plume centerline is approximately <sc>10.3 cm/s,</sc> indicating a strong deep ocean current on the Carlsberg Ridge. Due to plume entrainment, the volume flux of the hydrothermal vent linearly increases with increasing plume rise. Furthermore, the heat and iron outputs from the “Huoyanshan” hydrothermal vent were estimated as <sc>1.35±0.11 MW</sc> and <sc>3661±323 kg/a,</sc> respectively. These results provide valuable insights into the plume dynamics of submarine hydrothermal vents and the deep-sea thermal and elements balance.

Study on the merged flame height and radiative heat flux distribution from square propane fire arrays

SummaryThe remote ignition and accelerated flame spread in multiple fires accidents are mainly due to the thermal radiation. The merged flame height and external radiative heat flux distribution of the square fire arrays were investigated numerically. Different burner spacing, total number of burners and heat release rate (HRR) of an individual burner were considered. Results show that the fire plume centerline temperature can be predicted accurately by achieving grid independence on a fine enough grid. A correlation for predicting the merged flame height of the multiple fires is developed involving the number of burners, HRR of each burner and burner spacing. The comparison of calculated and simulated data in this work validated the accuracy of the proposed correlation. The radiative heat flux distributions surrounding the fire array can be divided into two regions, that is, the near‐ and far‐field regions. The total heat release rate of the fire array is shown to be the dominant effect on the radiation distribution throughout the whole region, whilst the burner spacing is shown to only have an effect in the near‐field region. To distinguish the two regions, the critical distance of 0.7 ( is the characteristic fire array length) was obtained.

Experimental Study on Axial Temperature Profile of Jet Fire of Oil-Filled Equipment in Substation

With the widespread use of substations around the world, oil jet fire accidents from transformer oil-filled equipment in substations caused by faults have occurred from time to time. In this paper, a series of transformer oil jet fire experiments are carried out by changing the external heat source (30 cm and 40 cm) and the inner diameter of the container (5 cm, 8 cm and 10 cm) to study the axial centerline temperature distribution of the transformer oil jet fire plume of the transformer oil-filled equipment in the substation. The experiment uses K-type thermocouple, electronic balance and CCD to measure and assess the temperature distribution of the axial centerline of the fire plume of the transformer oil jet. The result demonstrates that the axial centerline temperature of the fire plume increases with the external heat release rate and the inner diameter of the container. In addition, a novel axial temperature distribution prediction model of the transformer oil jet fire plume is established. This model can effectively predict the oil jet fire plume temperature of transformer oil- filling equipment in substations, and provide help for substation fire control.

Simulations of dispersion through an irregular urban building array

Following the release of a harmful substance within an urban environment, buildings and street canyons create complex flow regimes that affect dispersion and localized effluent concentrations. While some fast-response dispersion models can capture the effects caused by individual buildings, further research is required to refine urban characterizations such as plume channeling and spreading, and initial dispersion, especially within the presence of a nonhomogeneous array of structures. Field, laboratory, and modeling experiments that simulate urban or industrial releases are critical in advancing current dispersion models. This project leverages the configuration of buildings used in a full-scale, mock urban field study to examine the concentrations of a neutrally buoyant tracer in a series of wind tunnel and Embedded Large Eddy Simulation (ELES) experiments. The behavior, propagation, and magnitude of the plumes were examined and compared to identify microscale effects.After demonstrating excellent quantitative and qualitative comparisons between the wind tunnel and ELES via lateral and vertical concentration profiles, we show that a nonlinear least squares fit of the Gaussian plume equation well represents these profiles, even within the array of buildings and network of street canyons. The initial plume dispersion depended strongly on the structures immediately adjacent to the release, and consequently, the near-surface plume spread very rapidly in the first few street canyons downwind of the source. The ELES modeling showed that under slightly oblique incoming wind directions of 5° and 15°, an additional 5° and 14° off-axis channeling of the plume occurred at ground level, respectively. This indicates how building structures can cause considerable plume drift from the otherwise expected centerline axis, especially with greater wind obliquity. Additionally, AERMOD was used to represent the class of fast-running, Gaussian dispersion models to inform where these types of models may be usefully applied within urban areas or groups of buildings. Using an urban wind speed profile and other parameters that may be locally available after a release, AERMOD was shown to qualitatively represent the ground-level plume while somewhat underestimating peak concentrations. It also overestimated the lateral plume spread and was challenged in the very near-field to the source. Adding a turbulence profile from the ELES data into AERMOD's meteorological input improved model estimates of lateral plume spread and centerline concentrations, although peak concentration values were still underestimated in the far field. Finally, we offer some observations and suggestions for Gaussian dispersion modeling based on this mock urban modeling exercise.

Scaling patch analysis of turbulent planar plume

Proper scaling in turbulent planar plumes is investigated here using a scaling patch approach. Based on the scaled boundary conditions, a proper velocity scale for the mean axial flow is the plume centerline velocity Uref=Uctr, and a proper temperature scale for the temperature excess is Θref=Tctr−T∞, where Tctr is the plume centerline temperature and T∞ is the ambient fluid temperature. By seeking an admissible scaling, a key concept in the scaling patch approach, for the mean continuity, mean momentum, and mean energy equations, respectively, the following is found: (1) a proper scale for the mean transverse flow is Vref=(dδ/dx)Uctr, where dδ/dx is the growth rate of the plume width. (2) A proper scale for the Reynolds shear stress is Rvu,ref=UctrVref=(dδ/dx)Uctr2, a mix of the scales for the mean axial and transverse flows. (3) A proper scale for the turbulent heat flux is Rvθ,ref=VrefΘctr, a mix of the scales for the mean transverse flow and mean temperature excess. The mean transverse flow thus plays a critical role in the scaling of turbulent planar plumes. Approximate functions are developed for the scaled mean transverse flow, Reynolds shear stress, and turbulent temperature flux, and are found to agree favorably with experimental and numerical simulation data. The integral analysis of the mean momentum equation yields a Richardson number Ri, which remains invariant in the axial direction. The Richardson number is defined as Ri=defgβΘctrδt/(UctrVref)≈1/2, where g is the gravitational acceleration, β is the thermal expansion coefficient, and δt is the plume half-width based on the mean temperature profile. This Richardson number arises directly from the scaling patch analysis of the mean momentum equation, including both the streamwise and transverse velocity scales.

Variability of physical meteorology in urban areas at different scales: implications for air quality.

Air quality in cities is influenced not only by emissions and chemical transformations but also by the physical state of the atmosphere which varies both temporally and spatially. Increasingly, tall buildings (TB) are common features of the urban landscape, yet their impact on urban air flow and dispersion is not well understood, and their effects are not appropriately captured in parameterisation schemes. Here, hardware models of areas within two global mega-cities (London and Beijing) are used to analyse the impact of TB on flow and transport in isolated and cluster settings. Results show that TB generate strong updrafts and downdrafts that affect street-level flow fields. Velocity differences do not decay monotonically with distance from the TB, especially in the near-wake region where the flow is characterised by recirculating winds and jets. Lateral distance from an isolated TB centreline is crucial, and flow is still strongly impacted at longitudinal distances of several TB heights. Evaluation of a wake-flow scheme (ADMS-Build) in the isolated TB case indicates important characteristics are not captured. There is better agreement for a slender, shorter TB than a taller non-cuboidal TB. Better prediction of flow occurs horizontally further away and vertically further from the surface. TB clusters modify the shape of pollutant plumes. Strong updrafts generated by the overlapping wakes of TB clusters lift pollutants out of the canopy, causing a much deeper tracer plume in the lee of the cluster, and an elevated plume centreline with maximum concentrations around the TB mean height. Enhanced vertical spread of the pollutants in the near-wake of the cluster results in overall lower maximum concentrations, but higher concentrations above the mean TB height. These results have important implications for interpreting observations in areas with TB. Using real world ceilometer observations in two mega-cities (Beijing and Paris), we assess the diurnal seasonal variability of the urban boundary layer and evaluate a mixed layer height (MLH) empirical model with parameters derived from a third mega-city (London). The MLH model works well in central Beijing but less well in suburban Paris. The variability of the physical meteorology across different vertical scales discussed in this paper provides additional context for interpreting air quality observations.

Experimental study of the effects of ullage height on fire plume centerline temperature with a new virtual origin model

Investigating the evolution of the fire plume centerline temperature has both fundamental and practical importance to process safety, risk analysis and disaster control. In practice, ullage height (distance between the fuel surface and the container upper rim) is a common boundary condition that affects the burning intensity and the flaming characteristics. In this study, a series of experiments were conducted to reveal the effects of ullage height on the evolution of the heptane fire plume centerline temperature. The tested ullage height was systematically changed from zero to the value at which the flame self-extinguished. Results showed that, as ullage height increased, the fire plume centerline temperature first decreased and then slightly increased. By comparing the model predictions of previous classic models with the tested results, this study proved that previous classic models are not capable in predicting temperature profiles under high ullage height conditions. Based on physical and dimensionless analysis, a new virtual origin model was established to quantify the effects of ullage height on the virtual origin. A more universal plume centerline temperature model, which is capable to predict the centerline temperature profiles with any ullage height conditions, was proposed and validated by literature experimental data.