R.F. Griffiths

An improved method for the estimation of surface roughness of obstacle arrays

Abstract The Lettau (1969, Journal of Applied Meteorology 8 , 828–832) relationship has been used for almost three decades by meteorologists and wind-tunnel engineers to estimate surface roughness from the geometry of regular arrays of roughness elements. However, Lettau’s relationship is limited to low roughness element densities. It does not account for the nonlinear decrease of the roughness length at high roughness densities, nor does it account for drag differences caused by different obstacle shapes or layouts. In this paper an improvement upon the Lettau relationship is derived starting from fundamental principles, based on certain assumptions about the velocity profile. The improved model includes the obstacle drag coefficient explicitly, and accounts for the peak in the z 0 versus area density curve.

Field experiments of dispersion through regular arrays of cubic structures

To investigate the effect of plan area density on the near-field dispersion of pollutant plumes in built-up areas, scaled field measurements have been made of the dispersion of a plume released upwind of regular arrays of cubes of varying plan area density.It was found that the lateral concentration profiles were Gaussian in all cases. Close to the source, the lateral dispersion parameter σy was increased, relative to that of a plume in open terrain and was highest for the most dense array. Despite the increased plume dimensions in the array, the reduction in advection velocity resulted in ground-level concentrations that were in general not too different from those of a plume in open terrain. This behaviour can be modelled by a Gaussian-plume-type expression for time-averaged concentration.

Field and wind tunnel investigations of plume dispersion around single surface obstacles

The near-wake entrainment behaviour of a limited range of isolated obstacles, representative of those found in urban areas, was investigated in the field at a nominal scale between 1/10 and 1/20. Tracer gas was released from a continuous point source located upwind of the obstacles. It is shown that concentrations in the lee of different obstacles depend on the proportion of the plume entrained and on the dimensions of the region of recirculating flow. The presence of taller obstacles results in a reduction of ground level concentrations. The effect of longitudinal, lateral and vertical source displacement was also examined for the different obstacles in the field. The field experimental programme was supported by physical modelling in the wind tunnel at a nominal scale of approximately 1/100. Dispersion around a single cube was mainly investigated in the wind tunnel and the results confirm in general the findings of the field trials. However, comparisons between wind tunnel and field results clearly show that the plume is more dispersed in the field, attributable to the effect of additional wind meander occurring in the atmosphere. Accordingly, centreline concentrations are higher in the wind tunnel and the effect of lateral source displacement is more pronounced in the wind tunnel than in the field.

The effect of uncertainties in human toxic response on hazard range estimation for ammonia and chlorine

Abstract Major hazards risk assessment has become a topic of widespread importance throughout society and its institutions. Sources of uncertainty are outlined for the release and dispersion phases of toxic and/or combustible materials. Attention is then focused on the wide variation in the statements available in the literature on human acute inhalation toxic response to ammonia and chlorine. Using values of probit functions or equivalent statements of toxicity that have been used in a number of published estimation studies, it is shown that these yield a wide spread of results for the hazard ranges and areas calculated using a particular dense gas dispersion model, DENZ, assuming notional rapid releases of ammonia and chlorine from pressurized containers. The importance of concentration intermittency is discussed and it is concluded that its neglect leads to an underestimate of possible detriments. It is apparent that the uncertainties associated with this aspect of major hazards risk assessment are larger than has been made explicit in some studies.

Concentration measurements around an isolated building: A comparison between wind tunnel and field data

Field experiments have been conducted in flat terrain using a model building, which represents real structures at a scale of between 13 and 110. The building is rotatable, and thus may be positioned in any orientation relative to the wind direction. A tracer gas is released upwind of the building and detectors are used to measure concentration values at a frequency of 10 Hz. Similar experiments were undertaken at 350 of the field scale using a wind tunnel. The measured concentration values are non-dimensionalised to enable the two sets of results to be compared directly. The highest mean concentrations found around the building tend to be over-estimated by the wind tunnel, whilst the minimum concentrations tend to be underestimated. This suggests that there is more dispersion of the plumes in the atmosphere than in the wind tunnel. A likely explanation for this is the differences in turbulence scales relative to the size of the model building. Data sets from the field experiments are also divided into sections each of 5 min duration. This indicates that there is a great deal of variation at a single location throughout each experiment. This is caused by the continual changes in the wind direction (both mean and standard deviation) leading to continual changes in the direction of travel of the plume. Intermittency (the proportion of time for which no gas is observed) and conditional means (which exclude zero sections of data) are calculated. Both parameters exhibit considerable variation between each 5 min period at a single location.

Local characteristics of atmospheric dispersion within building arrays

Abstract Dispersion of pollutants within building arrays was examined in the field, using model obstacles that represented real structures at a nominal scale between 1 10 and 1 20 . The main purpose of the experiments was to examine the local characteristics of flow and dispersion in the vicinity of individual obstacles embedded in an array of cuboid buildings. Two building array configurations were used, namely in-line and staggered. The spacing of the buildings in the array was S/H=1.5 (where S is the space between two consecutive array elements). This spacing was chosen after a wind tunnel flow visualisation study of the effect of obstacle spacing on dispersion. Dispersion around isolated model buildings of similar shape and orientation to the mean wind direction has already been investigated in the field (Mavroidis et al., 2000, Atmospheric Environment, submitted for publication). A comparison is presented between dispersion around an isolated building and around the same building embedded in an array of cubes, which represented an urban area. The results suggest that enhanced mixing and dispersion occur within the array. Furthermore, differences in concentrations measured in the wake of obstacles of different shape and orientation with respect to the mean wind direction are reduced within the array, compared with the isolated obstacle case.

Errors in the use of the Briggs parameterization for atmospheric dispersion coefficients

Abstract Attention is drawn to errors promulgated in the literature on the Briggs parametrization which is widely used in dispersion calculations. The significance of these errors is discussed and reference is made to the form in which the parameterization was originally expressed, and the limitations as to its use which are sometimes ignored by users. Additional errors that are sometimes generated by users are also discussed. Users of the parameterization and authors of computer codes incorporating it are urged to check that these errors have not been incorporated into software versions of dispersion models.

Flow and dispersion around an isolated building

Abstract Experimental investigations have been carried out on flow and dispersion around an isolated building, paying particular attention to the fluctuating components of the dispersion. Results are presented of experiments conducted in neutral or slightly unstable weather conditions in flat terrain using a model building, which represents real structures at a scale of between 1 3 and 1 10 . A particular feature of the building is that it is rotatable, enabling it to be positioned at any desired orientation relative to the mean wind direction. A tracer gas was released upwind of the building and concentrations measured using a detector system with a response time of approximately 1 s. Experiments were undertaken for the purpose of comparing the concentration distributions measured at various locations in the field with those measured in earlier experiments conducted in a wind tunnel. Comparisons were made of a range of statistical parameters of the concentration distribution about the building including mean, standard deviation, intermittency and concentration fluctuation intensity. Cumulative distribution functions of concentration were also constructed. It was found that, in general, concentration fluctuation intensities occurring in the field were larger than those measured in the wind tunnel, except in the near-wake region.

Full-scale experiments on dispersion around an isolated building using an ionized air tracer technique with very short averaging time

Abstract Experiments are reported in which negatively ionized air is used as a tracer on the flow and dispersion in the vicinity of an isolated building. The technique permits very rapid response concentration measurements to be made, so that the characteristics of concentration fluctuations can be determined. Experiments have been carried out using both continuous and pulsed ion sources, and with several detectors deployed to reveal aspects of their sequential activation as a puff of ions is carried on a trajectory in the wake region. Statistical aspects of the multi-detector experiments are presented, and suggestions based on these results are put forward concerning further use of the method in examining this type of flow and dispersion behaviour.

REACTPOOL: a code implementing a new multi-compound pool model that accounts for chemical reactions and changing composition for spills of water reactive chemicals

All chemicals that react violently with water or in contact with water liberate toxic gas are included in the list of substances covered by the majority of the international legislation on major hazards. This category includes a large number of chemicals that are used widely in the process industries. A survey of accidents that occurred in the last 10 years in the USA shows numerous major incidents that involved spillages of these substances. Even so, there are almost no experimental data on the behaviour of these chemicals on release. Furthermore, there are very few published studies on modelling the behaviour of such spillages, except in the case of hydrogen fluoride. In previous work we reported a new theoretical model [J. Haz. Mat. 62 (1998) 101-129, J. Haz. Mat. 62 (1998) 131-142, J. Haz. Mat. A67 (1999) 9-40], that describes accidental spills of SO(3) and oleum, which are substances with very complex behaviour that belong to this category. It describes both the pool [J. Haz. Mat. 62 (1998) 101-129, J. Haz. Mat. 62 (1998) 131-142] and the cloud behaviour [J. Haz. Mat. A67 (1999) 9-40]. In the work reported here the pool model was modified in a generic form in order to include other water reactive chemicals. REACTPOOL is a new code that can be used for both instantaneous and continuous liquid releases under a wide range of input parameters (steady or varying). It can be used for all liquids irrespective of their volatility and reactivity, and it describes pools consisting of more than one liquid that can have changing composition and properties. The purpose of this paper is to present the general procedure followed in REACTPOOL and to show how the new model has been modified and implemented for substances other than SO(3) and oleum. The modelling procedure has been implemented in a computer code written in Visual Basic, and results of the model have been generated using this code. It should be noted that this model requires validation data, but that the availability of such data awaits the performance of suitable experimental investigations.