G. Brusasca

Particle model simulation of diffusion in low wind speed stable conditions

Abstract A Lagrangian particle model (LAMDA), previously developed and applied to the simulation of atmospheric dispersion in neutral and convective windy conditions, was modified to deal with stable low wind speed conditions. These last are among the most difficult to be treated. In fact, on the one hand, nearly calm situations, associated to strong stability and air stagnation, make the lower layers of the atmosphere poorly diffusive, and, on the other hand, the large fluctuations in the wind direction (meandering), spread the airborne pollutants over wide angular sectors. An ad hoc algorithm to simulate the effect of meadering on the dispersion is proposed. The model is validated by comparing its simulation results to three tracer experiments held in stable low wind speed conditions by the Idaho National Engineering Laboratory (U.S.A.) in 1974. These experiments present plume spread of different width (48, 138 and 360°, respectively, at an arc located 200 m downwind from the source) and are comprehensive of a wide set of conditions, ranging from strong to weak stability and from low wind speed to calm. The results of the comparison are discussed. The ability of the model to simulate the g.l.c. distributions with a good degree of confidence is illustrated.

Comparison between the results of a Monte Carlo atmospheric diffusion model and tracer experiments

Abstract A Lagrangian statistical (Monte Carlo) model for airborne pollutant dispersion is presented. Its ability to simulate the atmospheric dispersion both in homogeneous and inhomogeneous turbulence by comparison with an analytical solution and with the Willis and Deardorff water tank experiments, respectively, has been stated in previous papers. In the present paper the model is used to simulate dispersion in the real atmospheric PBL. The numerical results obtained are verified against experimental data from the Karlsruhe Nuclear Research Center tracer experiments. The model is applied to the problem of predicting the ground level concentration of two different tracers simultaneously released from two heights (160 and 195 m) at the Karlsruhe meteorological tower. Convectively unstable and neutral conditions were prevailing during the two tracer experiments which have been simulated. Model performance was evaluated through two statistical indexes: relative mean bias and normalized mean square error. The cumulative frequency distribution of the point-by-point ratio between observed and predicted ground level concentrations (glcs) was also computed. The simulated concentrations agree very well with observations. The tracer data were also compared to the simulations of 10 Gaussian models. They differed one another for the choice of dispersion sigma curves and for the way to insert the wind speed and direction. None of them proved to perform better than our particle model in all the exercises.

A simple way of computing buoyant plume rise in Lagrangian stochastic dispersion models

Abstract A simple and easy to use method to include Eulerian plume rise in Lagrangian particle models is presented. This approach takes into account the vertical variation of wind and stability. Its ability to realistically simulate plume rise and spread, both through numerical experiments under typical atmospheric conditions and by comparison with actual plumes detected by a Differential Lidar in a case study, is shown. Despite its simplicity, our method proved to describe the main plume rise characteristics in a satisfactory way and to yield a fair agreement among observed and predicted plume centreline heights and standard deviations.

BOUNDARY-LAYER FLOW OVER ANALYTICAL TWO-DIMENSIONAL HILLS: A SYSTEMATIC COMPARISON OF DIFFERENT MODELS WITH WIND TUNNEL DATA

Two mass consistent models (MATHEW and MINERVE) and two dynamic linearized models (MS3DJH/3R and FLOWSTAR) are used to simulate the mean flow over two-dimensional hills of analytical shape and of varying slope. The results are compared with detailed wind tunnel data (RUSHIL experiment at US EPA). Different numerical experiments have been performed, varying input data and control parameters, to test the data-processing methodology and to evaluate the minimum input data (for mass consistent models only) necessary to obtain a reliable flow field. The models behave differently according to the physical assumptions made and numerical procedure used: an assessment is then made in order to identify the proper solution for the different conditions of topography and wind data.

A study of the assessment of air temperature, and sensible-and latent-heat fluxes from sonic-anemometer observations

SummaryIn the past ten years, use of sonic anemometers to provide fast-response wind sensing in the atmospheric surface layer became more and more intensive, due to their increasing availability, decreasing cost and increasing number of publications regarding their data treatment. In this paper, we present an algorithm we have developed in order to analyse the data of one of the most diffused types of sonic anemometer,i.e. the Gill sonic anemometer. Our software allows to evaluate the air temperature and the sensible- and latent-heat fluxes if also low-response measurements of air humidity at the same level of sonic anemometer, and of net radiation and ground surface flux are available. We let our software the option of evaluating the fluxes along the mean local streamline (thus the turbulent fluxes are associated with transport across mean streamline surfaces). By using the data gathered in two field experiments, we discuss the applications of the proposed formulations and check the accuracy of our method by comparing the evaluated fluxes with those measured by fluxmeter.

Simulation of atmospheric diffusion in low windspeed meandering conditions by a Monte Carlo dispersion model

SummaryConditions of low windspeed associated to strong stability and air stagnation correspond to highly nonstationary and inhomogeneous diffusion situations. In such «poor» diffusion cases, integrated models (like Gaussian models) are no longer valid. New numerical methodologies, like Monte Carlo methods, are needed. Recently a Monte Carlo particle model for airborne pollutant dispersion has been developed by our team. Its ability to predict ground-level concentrations in convective and neutral conditions with a high degree of accuracy was verified against tracer experiments. In the present paper our model is used to simulate the dispersion under low windspeed conditions. Particular emphasis is given to the problem of wind meandering. We have demonstrated the need of recording the wind statistics over averaging periods of the order of a few minutes and anad hoc algorythm to treat low windspeed situation when only hourly average data are available is suggested. The model results are compared to tracer data obtained by the Idaho National Engineering Laboratory (U.S.A.) in 1974.

An assessment of mixing-length closure schemes for models of turbulent boundary layers over complex terrain

The effectiveness of closure assumptions implemented in turbulent boundary-layer models is rather uncertain over complex terrain. Different closure schemes for Reynolds shear stress based on the mixing-length concept are compared with data from wind tunnel experiments over complex terrain and the results are analysed on the basis of second-order moment equations. A good estimation of the vertical momentum flux velocity scale turns out to be given by the standard deviation of the vertical velocity while the turbulent kinetic energy scaling gives less satisfactory results in regions where turbulence anisotropy is large. Fairly good results are given by closure models implementing a shear-limited mixing-length already proposed for non-logarithmic wind profiles, while large errors characterize traditional mixing-length formulations.

Calibration with experimental data of a 2D sea breeze model by heating parametrization

SummaryA new scheme of vertical distribution of sensible heat flux has been introduced in a bidimensional numerical sea breeze model which uses a prescribed flux at the ground. The scheme has been suggested by inspection of thermal and dynamical fields observed during a case study along the coast of northern Adriatic Sea. Agreement with data is shown to be considerably increased.RiassuntoUn nuovo schema per la distribuzione verticale del flusso di calore sensibile è stato introdotto in un modello bidimensionale di brezza di mare in cui l’input termico è dato dal valore al suolo del flusso stesso. Lo schema è stato suggerito dall’analisi dei campi termici e dinamici osservati durante un «case study» nel comprensorio Alto Adriatico, ed ha contribuito a migliorare sensibilmente i risultati della simulazione.

Hydraulic simulation of the interaction between stack emissions and nearby buildings

SummaryA physical simulation in hydraulic channel has been performed, concerning the behaviour of buoyant and neutral plumes in the presence of a building of schematic shape placed near the stack. The effect of the aerodynamic wake, due to the stack itself and to the building, on the plume geometrical behaviour has been studied. This was done for various stack-building relative positions and letting both the ratios between exit and wind velocity and between stack and building height vary. The aim of the experiment was to confirm or better define the simple criteria normally applied for excluding in practical cases the occurrence of dangerous plume trapping phenomena. The results confirm these criteria and even suggest the possibility of relaxing them slightly. The simulation also allowed an indirect check of the validity of Briggs and Bryant plume rise models, as well as a study of the plume radius trendvs. distance from the source andvs. rise above the emission height. Particularly interesting in this context is the fact that the data put into evidence an enlarged plume spread in the cases of partial trapping with respect to the imperturbed cases and that the linear entrainment relationship between radius and rise is reassessed, with an entrainment parameter β=0.6 as in the atmosphere in the case of buoyant plumes.