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Air curtains are produced by thin vertical planar air jets and are used to prevent exchange flows between two fluids with a horizontal density gradient. They have been shown to work effectively provided the deflection modulus (DM) is greater than a critical value (DM = 0.14). Herein, results are presented from an experimental and modeling study of the initial transient development of the buoyancy distribution within an enclosure containing a round buoyant turbulent plume that is segmented by a co-flowing planar turbulent jet. A theoretical model based on filling box theory is developed to predict the leakage of buoyant fluid across the planar jet and the time at which the planar jet is no longer able to contain the buoyant plume outflow. Results from a series of analog salt-bath experiments are also presented that show that the model accurately predicts the rate of transport of buoyant fluid across the planar jet up until DM falls below the critical value of 0.14. The experiments show that, following the breakdown of the model there is a transition period after which the horizontal distribution of buoyant fluid throughout the enclosure is the same as it would be in the absence of the planar jet.

The ability of green roof systems to impact stormwater runoff from buildings has been covered in many research studies. However, a lot of these studies looked at long-term retention, with much less work focused on how to model a green roof's response to larger design storms. Work that has examined flow routing for individual rainfall events has focused on empirical routing models that are tuned to the specific roof being modelled. This paper presents a new physics-based model for flow routing based on the green roof module geometry and soil properties, which requires only a single discharge coefficient to be measured. The results of this model were compared with the results of a series of experiments to quantify a modular green roof system's hydraulic response to drawdown and steady rainfall.

Satellite measurements of lake surface temperature can benefit several environmental applications such as estimation of lake evaporation, predictions of lake overturning, and meteorological forecasts. Using a one-dimensional lake simulation that incorporates satellite measurements of lake surface temperature, the average diurnal variation in lake surface temperature was obtained. The satellite measurements were obtained from the MODIS instrument aboard the Aqua and Terra satellites. Herein the functional form for the diurnal variation in surface temperature is presented for each of the five major lakes in the Savannah River Basin, which are located in South Carolina and Georgia: Lakes Jocassee, Keowee, Hartwell, Russell, and Thurmond. Differences in the diurnal variation in surface temperature between each of these lakes are identified and potential explanations for these differences are presented.

This paper examines the relationship between the flow rate and outflow depth for flow in a rectangular pipe with the outflow running partially full of a free nappe through a series of compu...

Quantifying evaporative loss from reservoirs plays a critical role in sound water-availability management plans and in reservoir management. Various methods are used to quantify reservoir evaporation; however, each method carries a degree of uncertainty that propagates to model predictions of available water within a reservoir or a reservoir network. Herein, we explore the impact of uncertainty in reservoir evaporation on model outputs of historical and future water availability throughout the five major reservoirs in the Savannah River Basin in South Carolina, USA, using four different evaporation methods. Variability in the total available water is evaluated using the United States Army Corps of Engineers (USACE) 2006 Drought Contingency Plan hydrologic model of the Savannah River Basin, which incorporates recent water-management plans and reservoir controls. Results indicate that, during droughts, reservoir evaporation plays a large role in water-availability predictions, and uncertainty in evaporative losses produces significant uncertainty in modeled water availability for extreme events. For example, the return period for an event in which the availability of water in Lake Hartwell was reduced to 50% of full pool capacity varied from 38.2 years to 53.4 years, depending on the choice of evaporation parameterization. This is a variation of 40% in the return period, depending on the choice of evaporation method.

We assessed the effectiveness of an integrated engineering public-speaking class relative to a traditionally taught public-speaking class. The integrated class was designed to meet the grow...

Air curtain performance is a function of the source angle ( θ ), deflection modulus ( D M ) and the neutral height ( h n ). For given values of h n and θ there is a critical D M . C above which the air curtain will reach the floor. However, the air curtain alters the location of the neutral height. Herein the simultaneous solution of the integral forms of the momentum and angular momentum equations leads to a prediction of the change in the neutral height due to the presence of the air curtain. For a vertical two-dimensional air curtain the predicted value is h n / H = 1 / 3 and leads to D M . C = 0 . 167 . Experimental and computational studies show that above this D M . C the curtain effectiveness increases to a peak at D M . m a x and then decreases. The peak effectiveness being above the critical D M is typically attributed to installation issues. However, it is shown that the application of an entrainment model for modeling the air curtain volume flux leads to a prediction of the peak effectiveness occurring at D M . m a x = 1 . 26 D M . C even for an idealized two-dimensional flow. This is almost identical to the minimum air curtain safety factor of 1.3 found in the literature. The model shows that the reduction in effectiveness as D M decreases from D M . m a x to D M . C is not due to three-dimensional effects but is due to the increased path length of the air curtain as D M approaches D M . C .

The flight of compact debris can be highly stochastic in nature. However, standard flight equations are deterministic after assuming a spherical shape for gravel. This study proposes a stochastic model to resolve some key aspects of these uncertainties that originate due to changes in orientation of the debris particle during flight. The model numerically solves the differential equations of motion for a large number of gravel pieces taken from five different size gradations. The amount by which the drag and lift coefficients ( δ C D and δ C L ), the lift force direction ( δ θ ) and the projected area ( δ α ) are varied at each time-step during the flight simulation of a single debris are the four parameters used to fit the model to the results obtained from gravel drop experiments. An optimization criterion ( e ) was introduced and the model was optimized for each gravel gradation tested. It was found that only small perturbations in the lift coefficient are needed to model the flight. However, larger perturbations in the direction of the lift force are required to correctly predict the statistics of the landing locations.

Results are presented from of experiments to determine the susceptibility of rooftops to embers staying in contact during wildfires. Wind tunnel experiments were run in which the rooftops of model houses were covered with model embers and exposed to the wind. Tests covered a range of wind speeds, wind directions, and roof geometries. Experiments were filmed and image analysis showed regions on the rooftops where embers remained after prolonged exposure to the wind. Rooftop ember removal is controlled by the flow separation over the roof and around the building as well as the building geometry. Counterintuitively, the flow separation means that, for certain wind directions, increasing the roof slope actually results in larger regions of the rooftop where embers are retained. Internal corners, such as around dormers, can be regions of high ember stability. However, these regions of high stability are also dependent on the wind direction. For one building tested, the fraction of the roof that remained covered varied by 46% for the same wind speed but different wind angles. The results show that ember retention on rooftops is highly dependent on the flow over and around the whole building and, therefore, tests should model the entire building.

Aerodynamics of the firebrands' flight is fundamental to fire spotting; yet, this phenomenon is relatively poorly understood. Variants of Tachikawa's transport models have been developed for different shapes of debris but, only simplified versions are implemented in firebrand transport models. Failure to include lift and rotational forces reduces the problem to a two-dimensional one, in contrast to the 3D trajectories observed. As such, many studies may have under-/overestimated the flight distance and failed to capture the spotting distribution, accurately. Also, there is virtually no detailed quantitative experimental validation available for rod-like debris transport models. Hence, a set of free-fall experiments with non-combusting model firebrands were run in the absence of cross-wind. The results are employed for verification and performance evaluation of a 3D deterministic 6-Degrees-Of-Freedom (DOF) rod-like debris transport model. It is shown that a transport model must include the complete 6-DOF aerodynamics of the debris for estimating the flight characteristics, such that results are not statistically significantly different from the experimental data. The findings have applications beyond firebrand transport, as they represent the most comprehensive experimental data set and analysis of rod-like debris transport, currently in the literature.

Firebrand shower and its subsequent spot fires are responsible for more than half of the ignitions reported during wildfires, in particular at wildland urban interface (WUI) areas. The firebrand transport is a highly stochastic and nonlinear problem which directly influences the spotting distribution. Hence, a coupled stochastic model of firebrand showers, that is thoroughly and systematically validated against large scale wind tunnel experiments of lofting and downwind transport of model firebrands, is presented. It is shown that the developed model predicts the first and second order statistics of the flight accurately in relation to the experimental data. The sensitivity of the model to the initial conditions of the flight as well as the velocity field characteristics are examined.

To date, due to difficulties in making measurements during wildfires, much of what is known about firebrand showers and the subsequent fire spotting comes from mathematical modeling of the lofting and downwind transport of firebrands. However, these models lack experimental validation. Hence, the coupled lofting and downwind transport of non-combusting rod-like firebrands is experimentally modeled by releasing them through the velocity field of a large scale boundary layer wind tunnel. Complete trajectories of model firebrands are resolved using image processing algorithms. The results show a strong positive correlation between the maximum rise height (zmax) and the landing location (xl) of model firebrands. In addition, it is shown that, given the velocity field, the empirical probability density functions (PDF) of x l / z max are similar regardless of the firebrands' aspect ratio. This implies that the lofting and downwind transport processes cannot be decoupled in transport models. Analysis of the data reveals that, the larger the aspect ratio of firebrands, the more sensitive their landing locations are to the variability in the velocity field through which they are released. The data set presented herein serves as the most comprehensive experimental evidence for not only firebrand transport studies but also for validating mathematical models for the flight of rod-like debris/brands within the velocity field of other extreme events such as hurricanes.

Satellite measurements of water surface temperature can benefit several environmental applications such as predictions of lake evaporation, meteorological forecasts, and predictions of lake overturning events, among others. Limitations on the temporal resolution of satellite measurements can restrict these improvements. A model of the diurnal variation in lake surface temperature could potentially increase the effective temporal resolution of satellite measurements of surface temperature, thereby enhancing the utility of these measurements in the above applications. As a step in this direction, herein a one-dimensional thermal model of a lake is used in combination with surface temperature measurements from the moderate resolution imaging spectroradiometer instrument aboard the Aqua and Terra satellites, along with ambient atmospheric conditions from local weather stations, to calculate the diurnal surface temperature variation for Lake Hartwell in South Carolina. The calculated solutions are used to obtain a functional form for the diurnal surface temperature variation of this lake, a result which has not been obtained heretofore. This functional form was obtained by averaging over several years worth of data and, therefore, represents the diurnal variation of surface temperature of the average day. Accordingly, attempts to use this averaged function to predict surface temperature in between satellite overpasses on any given day did not perform well due to day-to-day variations in cloud cover, wind speed, and other factors. It is possible that use of this averaged function combined with daily meteorological data may enable better performance.

Knowledge of the evaporative loss from lakes and reservoirs is critical to water resources managers as well as to the overall understanding of the water balance in a given basin, geographical region, or continent. Existing methods for ascertaining evaporation from lakes and reservoirs include point measurements, water balance and mass transfer calculations, and proxy measurements using a pan. Point measurements using the eddy flux covariance method can be accurate, but are resource intensive and unsuited for determining spatial variation over a lake, or for obtaining measurements over many lakes. Mass balance methods cannot provide spatial variability and their accuracy depends on other portions of the water balance that can be challenging to obtain, such as leakage. Similarly, relatively recently deployed scintillation methods provide only an average for a strip across a lake and are also resource intensive and not suited for multi-lake studies. Evaporation pan measurements can also be used, though their accuracy is poor. Herein, we use a combination of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite measurements of water surface temperature, measurements of wind speed, air temperature, and relative humidity from local NWS stations, and a mass transfer method, to demonstrate multi-lake evaporation measurements. Specifically, the seasonal variation in evaporation is obtained for the five major lakes in the Savannah River Basin (in South Carolina, USA): Lakes Jocassee, Keowee, Hartwell, Russell, and Thurmond. Since this approach requires only an existing satellite resource with global coverage and existing NWS stations, this method can potentially be ported to any lake where there is a nearby meteorology station. Hence, this method could be used by both water resource managers and limnologists alike. The possibility is discussed of extending this approach beyond a single basin to encompass an entire geographical region or continent.

Highly buoyant plumes, such as wildfire plumes, in low to moderate wind speeds have initial trajectories that are steeper than many industrial waste plumes. They will rise further into the atmosphere before bending significantly. In such cases the plume's trajectory will be influenced by the vertical variation in horizontal velocity of the atmospheric boundary layer. This paper examined the behavior of a plume in an unstratified environment with a power-law ambient velocity profile. Examination of previously published experimental measurements of plume trajectory show that inclusion of the boundary layer velocity profile in the plume model often provides better predictions of the plume trajectory compared to algebraic expressions developed for uniform flow plumes. However, there are many cases in which uniform velocity profile algebraic expressions are as good as boundary layer models. It is shown that it is only important to model the role of the atmospheric boundary layer velocity profile in cases where either the momentum length (square root of source momentum flux divided by the reference wind speed) or buoyancy length (buoyancy flux divided by the reference wind speed cubed) is significantly greater than the plume release height within the boundary layer. This criteria is rarely met with industrial waste plumes, but it is important in modeling wildfire plumes.

Experimental results are presented for the rate of flushing of a dense fluid from an idealized two-dimensional square urban canyon as a function of the Richardson number of the flow, rangin...

A warming climate leads to a moister atmosphere and more rapid hydrologic cycle. As such, many parts of the country are predicted to experience more total rainfall per year and more frequent extreme rainfall events. Most regions of the country have stormwater systems designed to a standard that matches outflow rates to pre-development values for specified return period storms. Increases in these return period storm depths, as predicted by many global climate models, will stress existing stormwater infrastructure. This paper examines how rainfall patterns will change over the remainder of the century across the state of South Carolina. Rainfall simulations from 134 realizations of 21 global climate models were analyzed across the state of South Carolina through 2099. Results show that there will be increases in both annual total rainfall (ATR) and 24-hour design storm depth for a range of return period storms. Across South Carolina, ATR is predicted to increase by approximately 2.3-4.0 inches over the forecast period while the 100 year design storm depth is predicted to increase by 0.5-1.2 inches depending on location. However there are significant regional variations with the Savannah River Basin experiencing smaller increases in ATR compared to the rest of the state.

Results are presented for a computational study of free overfall from a smooth, horizontal, circular pipe that is flowing full upstream. The limiting discharge value that delineates the par...

Results of a simplified analysis of the hydrologic performance of undrained porous pavements, pavements without underdrains but that allow infiltration, are reported. The characterization of porous pavement behavior presented is appropriate for initial pavement design purposes. During any given storm, there are four distinct phases of porous pavement behavior: infiltration, accumulation, runoff, and drawdown. Analytical and graphical methods are presented for describing each of these phases from which a nonconvoluted approximate runoff hydrograph can be derived. This approach approximates peak runoff and time of that peak for a given storm depth, rainfall distribution, and pavement design. Second, a generalized method for calculating a porous pavement’s effective curve number (ECN) based on the National Resources Conservation Service (NRCS) [formerly Soil Conservation Service (SCS)] runoff curve number is presented. The analysis shows that for undrained porous pavements, the ECN is a function not only of the pavement and soil properties, but also of the storm depths and hyetographs used in the analysis. A standard calculation and presentation method is suggested that will allow local regions or municipalities to generate a local ECN design tool appropriate for their intensity-duration-frequency (IDF) data, which are only a function of the effective storage depth and the daily effective soil infiltration depth. A plot of the maximum ECN for all pavement depths and soil types is presented for each NRCS storm type as a reference for generating local ECN figures.

Though the presence of a vertical porosity distribution in porous pavements has been recognized, the impact of this distribution on the permeability of the pavement has not been considered. This research investigates the impact of a vertical porosity distribution on the overall permeability of pervious concrete. First, an unimpaired porosity–permeability relationship was generated using pervious concrete specimens which had uniform porosity throughout. This relationship was then used in conjunction with the vertical porosity distribution of surface compacted pervious concrete specimens to generate a vertical permeability distribution. An effective permeability value was used to summarize the overall behavior of the permeability distribution and was shown to have a much better correlation with the actual permeability of the specimens than the permeability predicted using the average porosity of the specimen. Additionally, an analysis of the relationship between the permeability distribution and the effective permeability showed that the minimum permeability (and therefore minimum porosity) tends to have the largest impact on the effective permeability.

This technical note outlines an initial assessment of the feasibility of using water sprays to intercept and extinguish airborne embers, as a means to protect buildings during wildfires. An analytical model was developed to calculate the probability of inter-particle collisions within two intersecting streams of particles, and was then applied to a range of test cases involving embers and water droplets. Results from this simplified analysis indicated that water sprays could effectively protect buildings from ‘ember attack’ in this manner, but only when either: i) large water flow rates were used (in the order of 1 L s−1 per metre of building perimeter to be protected), or ii) the sprays were comprised of very small (~0.1 mm) droplets at moderate water flow rates (~0.1 L s−1 m−1). It is likely that the quantity of water required to satisfy (i) would not be available in many circumstances, and further investigation is required to determine whether sprays of ~0.1 mm droplets could operate effectively in the conditions of a wildfire. The analysis presented herein would be a suitable basis for further investigation into these spray systems, and for quantitative comparison with other types of wildfire sprinkler systems.

A method for determining the distribution of porosity across a porous pavement sample using image analysis was developed and evaluated. Its measured average porosity compares favorably to other porosity methods including two volumetric based methods and ASTM D7063 which uses a vacuum sealing device. The impact of the representative elemental area (REA), which is the minimum pavement cross sectional area that must be analyzed to have a statistically significant measure of the pavement porosity, was investigated. The REA was experimentally determined to be 83.9 cm 2 (13 in. 2 ) for the different gradations of porous pavements evaluated, which were ASTM standard gradations No. 89, No. 78, No. 7, and No. 67. Though this method can be used to measure a one dimensional porosity distribution in different directions (vertically, horizontally, or radially in cylinders), this work focused on its application to vertical porosity distributions. Using the results of the image analysis and the REA, a smoothed profile of the porosity distribution was produced which showed that, for a porous pavement sample compacted in a single lift, the porosity is high at the surface, due to the surface texture, decreases drastically to a minimum at approximately 2–3 cm (∼1 in.) in depth and then linearly increases until approximately two centimeters from the bottom of the sample where the porosity increases drastically due to the wall effects. The porosity distribution produced agrees with previously proposed distributions.

The present paper explores the influence of room geometry on the overturning of smoke owing to a centrally located floor fire, and examines the implications on smoke filling times. The focus is on presenting practical design guidelines based on the theoretical predictions of the model of Kaye and Hunt. An engineering platform is developed for the prediction of smoke filling times, and a rational basis is provided by way of which smoke behaviour can be specified for simple room designs. The time taken for smoke to fill a room to a given height is critically affected by the room aspect ratio and the characteristic size of the buoyancy source. At large times, taller (small aspect ratio) rooms are shown to fill with smoke at a faster rate than wide (large aspect ratio) rooms owing to large-scale overturning and engulfing of ambient air during the initial transients. Larger area sources of buoyancy also decrease significantly the smoke filling times, with important implications for fire and smoke safety design. Simplified design curves incorporating the main findings have been developed for use as a tool by practising fire-safety engineers.

The purpose of this study was to determine how a real-world, sustainability-focused engineering capstone course affected students' critical thinking skills, student knowledge of sustainability, and student and faculty workloads. The research also investigated the effectiveness of a classwide jigsaw team approach. A combination of qualitative and quantitative assessment tools, including surveys, journals, interviews, and timecards, was employed to investigate the research questions. Results revealed that a real-world project with a focus on sustainability positively impacted students' critical thinking skills and led to increased knowledge of sustainability, but it also correlated with a high workload for students and faculty. Additionally, the jigsaw organization structure proved successful and yielded a positive team- building experience for the students. These results suggest open-ended problems with real project constraints can yield a uniquely beneficial learning experience without sacrificing the quality of student design or project deliverables. DOI: 10.1061/(ASCE)EI.1943-5541.0000141. © 2013 American Society of Civil Engineers. CE Database subject headings: Sustainable development; Students; Engineering education; Undergraduate study. Author keywords: Capstone; Sustainability; Real-world project; Critical thinking; Integrative thinking; Student workload; Faculty workload; Jigsaw team; Community-based project; Undergraduate engineering education.

The results of a simple computational model for differential settling are presented illustrating the significant role that particle size distribution plays in collision frequency and sedimentation rate of particles in a quiescent environment. The model tracks a large number of particles (order 105) with log-normally distributed diameters, as they settle at their Stokes settling velocities. Particle collisions are detected and result in larger particles that fall more rapidly. A number of simplifying assumptions are made in the model in order to avoid empirical correlations for phenomena such as collision efficiency and particle shape. These simplifying assumptions were needed to isolate and quantify the role of the particle size distribution. Simulated concentration profiles indicate that, even in the absence of collisions, the standard deviation (σD) of the particle size strongly influences the bulk mass settling rate as, for larger σD, more mass is concentrated in larger, faster falling particles. The collision frequency is also a strong function of σD. For a given mass concentration the collision frequency first increases linearly with increasing σD as greater variation in particle size leads to greater variation in particle velocity, and shorter times for particles to catch each other. For larger σD more mass is concentrated in larger particles, so, for a given mass concentration, there are fewer particles per unit volume, increasing the mean distance between the particles and reducing the collision frequency. The implications of these results for sedimentation measurement using optical attenuation techniques are discussed.

A new experimental technique has been developed to measure the rate of removal of aggregate from built-up roofs in severe storms. The technique was used to quantify the rate of removal of aggregate from a two-dimensional building as a function of the building parapet height and the wind speed. The experimental results indicate that the mass flux varies over time. Initially the mass flux is quite high, and there is significant movement of aggregate within the roof cavity formed by the parapet. After this initial adjustment, the mass flux reduces by a factor of four and then remains steady. The adjustment time was found to be independent of the parapet height but strongly dependent on the wind speed. This suggests that the primary role of the parapet is to prevent removal of aggregate from the roof cavity, but that parapets are less effective at preventing aggregate motion within the roof cavity. Increasing the parapet height significantly reduces the mass flux. A parapet height equal to 15% of the building height reduces the blow-off rate by a factor of 100 compared to a roof without a parapet. Further, the addition of a parapet always reduced the mass flux. This stands in contrast to studies of the wind speeds required to initiate motion in which small parapets have been observed to reduce the critical wind speed required to initiate blow-off. Although the experiments were run at relatively small scale, the results were not significantly influenced by viscosity, indicating that small scale wind tunnel studies of mass blow-off rate may, with care, be scaled up to full-size buildings. Again this is in contrast to blow-off initiation studies, in which the wind speed required to start mass removal is strongly dependent on the Reynolds number, and only full scale experimental results should be used in design.

A detailed computational parametric study of a perforated pipe underdrain surrounded by loose aggregate has been conducted. Several controlling geometric parameters (i.e., trench width, head, aggregate depth over the pipe, pipe wall perforation area per unit length of pipe, and the area of individual perforations) were considered and the variation of pipe discharge characteristics as a function of these parameters has been quantified. Results indicate that, for each combination of these parameters, there is a finite pipe length after which discharge does not increase with increasing pipe length. This pipe length depends on the pipe wall perforation area per unit length of pipe, pipe cross-sectional area, and losses associated with them. It was found that the effective pipe system discharge coefficient is independent of the channel width, aggregate depth, and head over the pipe for the range of parameters tested. The computational fluid dynamics (CFD) results were used to develop an equation for th...

This paper presents the results of a computational fluid dynamics (CFD) model study of the hydraulics of groundwater flow and porous pipe underdrains. The study was conducted using a three-dimensional CFD model built in ANSYS Fluent. The model was validated by replicating previous experimental results of saturated subsurface flow (water surface level above the aggregate) for a 10.2-cm perforated pipe shrouded in loose laid aggregate. The CFD model consistently overpredicted the flow rate for a given head and aggregate depth by an average of 11%. After considering the effect of pipe perforation blockage due to aggregate, the average overprediction decreased to only 6%. The discharge coefficient for the perforated pipe computed using the CFD model was 0.54 compared with 0.49 from experiments. It was also found that the discharge was quite small at the upstream end of the pipe, with the bulk of the water entering the pipe in the vicinity of the outlet. Finally, the computational results showed that, ...

This study investigated methods for predicting and managing surface water spread and depth on porous pavement bike lanes adjacent to impervious traffic lanes. Results are presented from a t...

The hydrologic behavior of an underdrained porous pavement system was characterized using an effective curve number (ECN), which is synonymous to the common runoff curve number (RCN). A model previously developed for porous pavements without an underdrain, which generates the rainfall-runoff data for the system, was modified to include an underdrain. However, the addition of an underdrain adds three new parameters (drain size, drain placement height, and drain spacing) that both influence the ECN and fundamentally change the design procedure. A sensitivity analysis was completed to quantify the impacts of different rainfall depths and distributions on this modified model. The RCN equations were then fit to the rainfall-runoff data to find the ECN value. ECN values for a wide range of pavement and underdrain configurations were summarized in figures that can be used as preliminary design aids for designers and regulators. These figures can be used to estimate the ECN value for a pavement design usi...

Experimental results are presented for the shear-driven flushing of a dense fluid from a square street canyon. The flow is both qualitatively and quantitatively influenced by the density of the fluid in the canyon, which is parameterized in terms of the flow Richardson number. A total of 26 experiments were conducted for Richardson numbers ranging from 0.08 to 4.5. For low Richardson numbers the canyon remains well mixed during the flushing process and the density difference decays exponentially (as observed in previous studies flushing a neutrally buoyant tracer). As the Richardson number increases the canyon becomes more stably stratified and the decay rate reduces. For very high Richardson numbers, a two-layer stratification is observed. Over time, for the two-layer case, the lower dense layer gets thinner, indicating that dense fluid is being skimmed from the top of the layer, and less dense, indicating that ambient fluid is being mixed down into the dense layer. Previous studies only observed flushing by skimming. The layer buoyancy decreases linearly over time until the shear layer reaches the bottom of the canyon at which point the rate of layer buoyancy reduction dramatically increases. Results also indicate that the density interface is thinner than the shear layer that is driving the mixing, and that the difference in thicknesses increases with an increasing Richardson number. This result is consistent with previous studies of mixing in stratified shear flows. The initial exponential decay rate was calculated for each experiment and is given by k = 1 / ( 18 + 84 R i ) 1.21 . Following the initial period of exponential decay, the decay rate increases as the stratification weakens.

The mechanics of shear-driven flushing of a dense fluid from a canyon is considered through a series of laboratory experiments. Experiments are presented for a range of canyon aspect ratios and flow Richardson numbers. The dynamics of the mixing is discussed qualitatively and four separate flow regimes are identified. The flow regime observed is shown to be dependent on both the canyon aspect ratio and the flow Richardson number. For taller, narrower canyons and higher Richardson numbers a two-layer stratification is observed throughout the flushing process. For wider canyons and intermediate Richardson numbers the canyon remains stably stratified but no distinct layers are observed. For low Richardson numbers and wide canyons, the canyon is relatively well mixed during the whole flushing process. A regime diagram showing the flow observed in Richardson number–aspect ratio space is presented.

A new experimental technique is presented for calculating the critical conditions under which loose gravel on a roof becomes airborne. The critical condition for gravel blow-off from the top of a roof depends on the building geometry, particle properties, and the wind conditions. A series of two-dimensional wind tunnel tests were run to measure the critical condition for particle removal. The experimental results demonstrate that the critical condition for blow-off, parameterized in terms of a particle densimetric Froude number, is a function of the dimensionless particle size (d⁎) and the building geometry. Results for buildings without a parapet show that the critical particle densimetric Froude number has a power-law relationship with the dimensionless particle size as F r d 2 = 8.1 ( d ⁎ ) − 0.44 for the range of parameters tested. For buildings with a parapet, the densimetric Froude number for the critical condition depends on both Reynolds number and parapet height to building height ratio. The experimental results indicate that buildings without a parapet are not always the most prone to blow-off, and that under certain conditions, a small parapet height can increase the risk of gravel removal. As the critical Froude number is dependent on the Reynolds number, raw data from small scale experiments cannot simply be scaled up by using a Froude number. Further, it is demonstrated that the current approach of using the Shields Diagram (or equivalent data) to scale results from small to large scale are also flawed as the motion initiation mechanism is different. Therefore, existing design guides should be re-visited and full-scale experiments should be conducted in order to fully analyze the risk of blow-off.

Results of an experimental study of the merging of unequal parallel round turbulent jets are presented. Experiments were conducted for a jet axial separation to nozzle diameter ratio of 3.0 and the Reynolds numbers ranging from 8 000 to 15 000. The distance to the point where the jets are merged was measured for a range of jet source momentum flux ratios. Three different merger criteria were used based on the mean velocity profile, mean passive tracer concentration profile, and Reynolds stress profile. The results show that the concentration profile merges closest to the jet sources followed by the velocity profile with the Reynolds stress profile merging furthest from the nozzles. For all three profiles the merge distance is relatively insensitive to the momentum flux ratio, consistent with previous findings for slot jets and buoyant round jets. The measured merge distances are consistent with previously published results for equal round jets, though the poor spatial resolution of data in the literature means that limited comparison is possible. There are no studies of unequal jet merger currently in the literature that could be used for comparison.

The properties of the atmospheric boundary layer (ABL) influence a range of physical phenomena in urban areas such as wind loading on buildings, wind-driven ventilation flows, pollution dispersion, and the lift off and transport of loose debris. In order to accurately model the ABL velocity profile at laboratory scale, it is important to establish the shear velocity u∗, surface roughness z0, and zero plane displacement d in either a wind tunnel or water flume. Current techniques for establishing these parameters are based on either an analysis of the boundary layer surface geometry or iterative curve fitting techniques that use mean velocity and/or turbulent kinetic energy profiles, occasionally combined with empirical correlations for z0. A new curve fitting method for calculating these logarithmic velocity profile parameters is presented. This new method calculates u∗, z0, and d directly from time-averaged velocity profile data in just two steps without any iteration. A comparison is presented between the results of the new method and other available methods applied to a range of velocity profile measurements in air and water. The comparison shows that, although the new method requires less data and fewer steps, it calculates u∗ and z0 with greater accuracy than existing techniques, and d with equivalent accuracy to existing techniques.

The stochastic nature of debris flight is investigated through a series of Monte Carlo simulations based on the debris flight equations for compact debris presented by Holmes (2004) . Any given debris flight situation presents a number of uncertainties such as the size of the piece of debris and the time-varying turbulent wind flow. Current debris flight models are largely deterministic and do not account for such uncertainty in input parameters. The simulations presented model the flight of a single spherical particle whose diameter is given by a probability distribution function, driven by a turbulent wind with velocity fluctuations appropriate to the atmospheric boundary layer. The model predicts the mean and standard deviation of the particle flight distance and impact kinetic energy. Results show that introducing uncertainty in particle diameter, horizontal turbulence intensity, or vertical turbulence intensity leads to larger mean values for flight distance and impact kinetic energy, compared to the condition where there is no variability in input parameters. Introducing input parameter variability also leads to variability in flight distance and impact kinetic energy that is quantified in this study. While the simulations presented do not realistically characterize the complex flow within an urban canopy, the results provide significant physical insight into the influence of particle size variability and turbulence on the mean and standard deviation of the flight distance and impact kinetic energy.

The classical plume model due to Morton, Taylor & Turner (Proc. R. Soc. Lond. A, vol. 234, 1956, pp. 1–23) is re-cast in terms of the non-dimensional plume radius, the plume ‘laziness’ defined as the squared ratio of the source radius and the jet length, and the buoyancy flux. It is shown that many of the key results of this classical model can then be read straight from the equations without recourse to solving them. Based on this observation, derivative models that consider plumes propagating through stratified environments or undergoing chemical reactions are similarly re-cast. We show again that key results can be read straight from the governing equations and results that have previously only been demonstrated numerically can be found analytically. In particular, we unify two previously distinct models that consider plumes propagating through stable and unstable stratified environments whose stratification has a power-law dependence on height. We present analytical solutions for the range of stratification power-law decay rates for which straight-sided plumes are possible. This result unifies the sets of solutions by Batchelor (Q. J. R. Meteorol. Soc., vol. 80, 1954, pp. 339–358) and Caulfield & Woods (J. Fluid Mech., vol. 360, 1998, pp. 229–248). We are able to explain the unstable behaviour previously found when the power lies in the range (−4, −8/3). Finally we show that this method also has limited advantages when applied to plumes with unsteady source conditions.

A series of two-dimensional computational fluid dynamics (CFD) simulations of merging turbulent plane jets were conducted to establish the point at which the jets can be regarded as having merged to form a single jet. Simulations were conducted to quantify the influence of the nozzle width, jet axis separation, Reynolds number, and the relative jet strength on the distance from the source to the merger point. The simulation results were then compared with experimental results published in literature. As the jet fluid exited the two nozzles, the shear layer between the jet and the ambient fluid drove the ambient fluid entrainment into the jets. However, a finite volume of fluid trapped was between the two jets, and a recirculation zone with two counterrotating vortices was established. This vortex pair reduced the pressure between the jets, drawing them together more rapidly than theoretical models predicted. The simulation results indicated that equal-strength jets merged at a distance downstream of 1.4 t...

In this study the discharge coefficient for circular orifices of different size in two different sizes of riser pipe is investigated experimentally. This type of outlet structure is common in detention ponds to achieve runoff volume control from developed areas in order to meet outflow discharge and water quality requirements. The discharge coefficient is determined by recording the drop in pond water level with time as water flows out of the orifice. The discharge coefficient is found to be a function of head over the orifice, location of the orifice above the floor of the tank, and the ratio of the orifice diameter to riser pipe diameter. The discharge coefficient increases as the head over the orifice decreases and height of the orifice above the floor of the tank decreases. The discharge coefficient reduces as the ratio of the orifice diameter to pipe diameter increases and eventually reaches an asymptotic value. Also, with all other variables being the same, the discharge coefficient is lower for larger size riser pipe. An equation that is a function of head over the orifice and height of the orifice above the floor of the tank is fitted for each ratio of orifice diameter to piper diameter. The coefficients of the equation are found to be a function of the ratio of orifice diameter to piper diameter. A minimum coefficient of determination of 0.78 for the fitted equation suggests that the fit can be used to determine the discharge coefficient for orifices in circular riser pipes.

We examine the role of diffusivity, whether molecular or turbulent, on the steady-state stratification in a ventilated filling box. The buoyancy-driven displacement ventilation model of Linden et al. (J. Fluid Mech., vol. 212, 1990, p. 309) predicts the formation of a two-layer stratification when a single plume is introduced into an enclosure with vents at the top and bottom. The model assumes that diffusion plays no role in the development of the ambient buoyancy stratification: diffusion is a slow process and the entrainment of ambient fluid into the plume from the diffuse interface will act to thin the interface resulting in a near discontinuity of density between the upper and lower layers. This prediction has been corroborated by small-scale salt bath experiments; however, full-scale measurements in ventilated rooms and complementary numerical simulations suggest an interface that is not sharp but rather smeared out over a finite thickness. For a given plume buoyancy flux, as the cross-sectional area of the enclosure increases the volume of fluid that must be entrained by the plume to maintain a sharp interface also increases. Therefore the balance between the diffusive thickening of the interface and plume-driven thinning favours a thicker interface. Conversely, the interface thickness decreases with increasing source buoyancy flux, although the dependence is relatively weak. Our analysis presents two models for predicting the interface thickness as a function of the enclosure height, base area, composite vent area, plume buoyancy flux and buoyancy diffusivity. Model results are compared with interface thickness measurements based on previously reported data. Positive qualitative and quantitative agreement is observed.

We examine the role of heat source geometry in determining rates of airflow and thermal stratification in natural displacement ventilation flows. We modify existing models to account for heat sources of finite (non-zero) area, such as formed by a sun patch warming the floor of a room. Our model allows for predictions of the steady stratification and ventilation flow rates that develop in a room due to a circular heat source at floor level. We compare our theoretical predictions with predictions for the limiting cases of a point source of heat (yielding a stratified interior), and a uniformly heated floor (yielding a mixed interior). Our theory shows a smooth transition between these two limits, which themselves result in extremes of ventilation, as the ratio of the heat source radius to the room height increases. Our model for the transition from displacement to mixing ventilation is compared to previous work and demonstrates that the transition can occur for smaller sources than previously thought, particularly for rooms with large floor area compared to ceiling height.