This is the review of CFD (Computational Fluid Dynamics) guidelines for dispersion modeling in the USA, Japan and Germany. Most parts of this review are based on the short report of the special meeting on CFD Guidelines held at the International Symposium on Computational Wind Engineering (CWE2014), University of Hamburg, June 2014. The objective of this meeting was to introduce and discuss the action program to make worldwide guidelines of CFD gas-dispersion modeling. The following six gas-dispersion guidelines including Verification and Validation (V&V) schemes are introduced by each author; (1) US CFD guidelines; (2) COST/ES1006; (3) German VDI (Verein Deutscher Ingenieure) guidelines; (4) Atomic Energy Society of Japan; (5) Japan Society of Atmospheric Environment; (6) Architectural Institute of Japan. All guidelines were summarized in the same format table shown in the main chapters in order to compare them with each other. In addition to the summary of guidelines, the overview of V&V schemes and many guidelines of CFD modeling in the USA are explained.

This article deals with the state-of-the-art of experimental and numerical studies carried out regarding air pollutant dispersion in urban environments. Since the simulation of the dispersion field around buildings depends strongly on the correct simulation of the wind-flow structure, the studies performed during the past years on the wind-flow field around buildings are reviewed. This work also identifies errors that can produce poor results when numerically modelling wind flow and dispersion fields around buildings in urban environments. Finally, particular attention is paid to the practical guidelines developed by researchers to establish a common methodology for verification and validation of numerical simulations and/or to assist and support the users for a better implementation of the computational fluid dynamics (CFD) approach.

Computational Fluid Dynamics (CFD) was used to simulate the grit and sand separation effectiveness of a typical hydrodynamic vortex separator (HDVS) system. The analysis examined the influences on the separator efficiency of: flow rate, fluid viscosities, total suspended solids (TSS), and particle size and distribution. It was found that separator efficiency for a wide range of these independent variables could be consolidated into a few curves based on the particle fall velocity to separator inflow velocity ratio, Ws/Vin. Based on CFD analysis it was also determined that systems of different sizes with length scale ratios ranging from 1 to 10 performed similarly when Ws/Vin and TSS were held constant. The CFD results have also been compared to a limited range of experimental data.

This article considers numerical simulations of the ventilation environment within a large 300m indoor military firing range. The range tunnel is 5.5m wide ×4.3m tall ×300m long. The tunnel is preceded by a large armament room that permits tracked and wheeled vehicles to fire weapons downrange. Combusted gases and particles within the range have been observed to stagnate and obscure the range even after firing short automatic bursts. Computations were performed for existing configurations of supply and exhaust vents, barriers and doors to compare with actual flow measurements made within the range. The numerical model was found to represent the existing flows quite well, so the model was then used to investigate alternate flow configurations.

For over fifty years the common presumption has been that computational fluid dynamics (CFD) and experimental fluid dynamics (EFD) were mutually exclusive and competitive. Often the question was posed: When can we get rid of our physical modeling facilities? This question does not recognize the tremendous synergistic leverage of combining the best qualities of both CFD and EFD as a research and design methodology. Coordinating the application of both CFD and EFD in a hybrid management approach can expedite results, improve understanding of flow phenomena, and often reduce research costs and time. This paper considers some of the common questions that arise as one considers hybrid research or design methods as it is applied to wind engineering and the built environment.

Computational fluid dynamics (CFD) was used to simulate the mixing characteristics of a small circular anaerobic digester tank (diameter 6 m) equipped sequentially with 13 different plunger type vertical linear motion mixers and two different type internal draft-tube mixers. Rates of mixing of step injection of tracers were calculated from which active volume (AV) and hydraulic retention time (HRT) could be calculated. Washout characteristics were compared to analytic formulae to estimate any presence of partial mixing, dead volume, short-circuiting, or piston flow. Active volumes were also estimated based on tank regions that exceeded minimum velocity criteria. The mixers were ranked based on an ad hoc criteria related to the ratio of AV to unit power (UP) or AV/UP. The best plunger mixers were found to behave about the same as the conventional draft-tube mixers of similar UP.

Partially-filled pipe flow as occurs in subsurface drains and sewers is computed by Manning's resistance equation or using the cross-sectional velocity distribution. Yet, Manning's equation is valid only for turbulent flow and no theoretical solutions and experiments are available for laminar, partially-filled pipe flow, although fully-filled pipe flow is well understood. This research solves the Navier–Stokes equations, using bipolar coordinates and the Fourier transform, for partially-filled pipe flow under steady uniform conditions, resulting in theoretical solutions for the cross-sectional velocity distribution, discharge, boundary shear stress and friction coefficient. Although the solutions are not tested with laminar flow data (a research need), they satisfy all boundary conditions and special cases. Particularly, their graphical interpretations agree qualitatively with related turbulent flow data, providing a benchmark for formulating analytical or empirical solutions for turbulent flow in the future. The proposed stage–discharge relationship is also useful for discharge measurements in drainage and sewerage systems.

Numerical calculations were performed to reproduce the transport and dispersion of the instantaneous release of finite volumes of dense gases over homogeneous flat, sloped, and complex terrain surfaces for calm and windy situations. The 1981 Porton Trial Test 8 field study of the release of 40m3 of a Freon-air mixture was used as a validation case to evaluate the behavior of volume integrated, cross-section integrated , depth integrated and full 3-d CFD models. In addition wind-tunnel measurements by Meroney & Lohmeyer (1983, 1984) about an instantaneous cloud of dense gas released in a wind were reproduced by similar calculations. All results were inter-compared, and time-dependent cloud dimensions, arrival times, and concentrations were considered. Very similar behavior was found for all models which tend to validate all methods as useful predictors.

Numerical calculations are performed to reproduce the transport and dispersion of the continuous release of dense gases over flat homogeneous surfaces with and without the mitigating influence of a downwind water curtain. Frequently such plumes are released as a result of a chemical manufacturing, storage or gas transportation accident resulting in a ground-level hazard due to gas flammability or toxicity. A field situation in which cold carbon dioxide was released upwind of water curtains (Moodie et al., 1981) was simulated using the open-source software FDS (Fire Dynamic Simulator) a full 3-d CFD model. Only water-spray enhancement of dispersion was considered; hence, no chemical removal or reactions were present or simulated. Wind-tunnel measurements for a 1:28.9 scale replication of the Moodie experiments are also compared with the 3-d CFD results. Concentration distributions, percent dilution and forced diffusion parameters were compared in scatter diagrams. Concentration field contours with and without active spray curtains are also presented. (C) 2012 Elsevier Ltd. All rights reserved.

A series of unsteady atria fire calculations are performed using a finite-volume CFD program on two and three dimensional generic buildings immersed in simulated atmospheric boundary layers. The model results reveal that external winds can modify the infiltration and exfiltration of air through external doors and windows, distort thermal and smoke columns rising above test fires in the atria, cause the plumes to impact directly against atria walls, and modify the resultant filling of elevated atria spaces. In some cases aggressive fire “whirls” form, which can enhance fire strength, enclosure mixing, and exposure. Results are compared qualitatively with similar physical model experiments.

Stepped spillways are becoming increasingly popular due to the low-cost in construction and high efficiency. Numerical modeling of stepped spillways is very complicated and challenging because of the high roughness and velocity recirculation regions. Two types of multiphase flow models are used: a mixture multiphase flow model (MMF) and a volume of fluid multiphase flow model (VOF). The differences between both models are the phases interpenetrating and the phase velocities. In both models, the realizable k-ε model is chosen to simulate turbulence. The computational results are compared with large-scale experimental data from Colorado State University. The spillway was 1.22m wide and consisted of 25 horizontal steps each 0.61m high and 1.22m long. The discharge was varied from 0.57 to 3.28 m 3 /s. The data series obtained for model comparison include; velocity profiles, air concentration, and characteristics of flow. Both models can satisfactorily simulate the flow pattern and the recirculation regions. The velocity profiles are more accurately simulated using the VOF model. The MMF model gave satisfactory results for air concentration.

Computational Fluid Dynamics (CFD) was used to simulate the mixing characteristics of four different circular anaerobic digester tanks (diameters of 13.7, 21.3, 30.5, and 33.5m) equipped with single and multiple draft impeller tube mixers. Rates of mixing of step and slug injection of tracers were calculated from which digester volume turnover time (DVTT), mixture diffusion time (MDT), and hydraulic retention time (HRT) could be calculated. Washout characteristics were compared to analytic formulae to estimate any presence of partial mixing, dead volume, short-circuiting, or piston flow. CFD satisfactorily predicted performance of both model and full-scale circular tank configurations.

Numerical methods are used to examine the development of fires within porous urban canopies. Idealized generic porous models of 2-D and 3-D city structures are used to- gether with inserted heat sources to predict combined flow circulations. The presence of struc- tures of different densities, height and distribution produce unique flowfields associated with the porous canopy structure of cities, but the buoyancy produced by combustion products modifies, and for large fires, dominates the urban canopy flow.

Uncontrolled fires and their associated smoke have been part of mankind’s hazard environment since prehistoric times. Fires caused by lightening or volcanic activity moved across the earliest vegetative landscape whether grassland or forest scourging away all life before its path. Later, as man collected into groups and tribes, villages, towns and cities were routinely wiped away as natural, accidental, war or arson sources provided ignition. Most cities were not burned to the ground once, but multiple times. Even today massive wild fires in forests occur every year all over the world, and the threat of mass fires in cities haunt the minds of those concerned by large petrochemical accidents, wars or terrorism.

A computational fluid dynamics (CFD) code including Lagrangian prediction of the gravity driven but stochastic trajectory descent of droplets is considered to predict plume rise and surface drift deposition from mechanical draft cooling towers. CFD drift deposition calculations are performed for a specific urban cooling-tower situation with and without the urban buildings surrounding the cooling-tower complex present to produce a set of multiplicative factors that could be used to correct seasonal or annual predictions for the presence of large urban structures.

Drift of small water droplets from mechanical and natural draft cooling tower installations can contain water treatment chemicals such that contact with plants, building surfaces and human activity can be hazardous. Prediction of drift deposition is generally provided by analytic models such as the US Environmental Protection Agency approved Industrial Source Complex Short Term Version 3 (ISCST3) or Seasonal-Annual Cooling Tower Impact (SACTI) codes; however, these codes are less suitable when cooling towers are located midst taller structures and buildings. A computational fluid dynamics (CFD) code including Lagrangian prediction of the gravity driven but stochastic trajectory descent of droplets is considered and compared to data from the 1977 Chalk Point Dye Tracer Experiment. The CFD program predicts plume rise, surface concentrations, plume centerline concentrations and surface drift deposition within the bounds of field experimental accuracy.

Continuing advances in computer speed and the availability of user friendly software has made computational fluid dynamics a cost effective compliment and at times alternative to physical modeling in the field of civil engineering hydraulics. Validation of the CFD models for open channel flow conditions remains limited due to the significant cost of obtaining data. Furthermore, once the flume study has been conducted, a numerical study is typically no longer required. This paper explores the ability of CFD to reproduce free surface flow in a trapezoidal channel in a bend. Laboratory data was collected at MIT during the late 1950's and early 1960's to obtain an understanding of shear stress distribution and flow patterns in bends. A portion of those studies was reproduced using CFD and a comparison made between observed and predicted values. The objective of the paper is to demonstrate that CFD can offer a cost effective alternative and compliment to physical modeling of smooth, rigid boundary conditions. The CFD software Fluent (1998) was used to model flow in a trapezoidal channel using two turbulence models, K-Epsilon and Reynolds Stress. Model results show that observed and predicted water surface elevations typically differ by less than 2.5 percent, and predicted shear stresses differ by less than 10 percent. Model results are also used to illustrate the limitations of the K-Epsilon model as well as conditions under which it produces very similar results to the significantly more expense Reynolds Stress model. Results of the study clearly demonstrate the strengths and weaknesses of using CFD as an alternative or compliment to physical modeling.

HE DRIFT OF SMALL WATER DROPLETS frommechanical and natural draft cooling tower installa-tions can contain water treatment chemicals thatcan be hazardous if they make contact with plants,building surfaces, or human activity. Prediction ofdrift accretion is generally provided by analyticmodels as found in the US EPA-approved ISCST3 orSACTI codes. However, these codes are not suitablewhen cooling towers are located in the midst oftaller structures and buildings. A CFD calculationincluding a Lagrangian prediction of the stochastic,gravity-driven trajectory descent of droplets is a better approach in this kind of environment. Onesuch calculation has been performed and comparedto data from the 1977 Chalk Point Dye TracerExperiment in preparation for using such methodsin more complex building configurations. Thenumerical analysis predicts plume rise, surface con-centrations, plume centerline concentrations, andsurface drift accretion within the bounds of fieldexperimental accuracy. Estimation of the impact of cooling tower drift onthe downwind deposition of droplet-born toxins isdifficult. A few field studies performed between1965 and 1984 examined cooling tower plume rise, visibility, and downwind concentrations.Unfortunately, only a couple of these actually meas-ured deposition rates downwind. Despite limitedfield data, concern about drift and deposition led tothe development of more than a dozen separateanalytic models to predict downwind ground-levelconcentrations and accretion rates. Chen [1] com-pared ten drift deposition models using a set ofstandard input conditions for a natural-draft coolingtower, and found that most of the models agreedwithin a factor of three. However, when all ten mod-els were compared, the predicted maximum driftParticle-laden exhaust flows in a typical urban setting where cooling towers emit 300 micron particles in an 8.5m/sec exhaust stream, using reference wind speeds of 5m/s at an angle of 240° from true North; pathlines are shown at left, and particle tracks are shown at rightChalk Point Coal Fired Power Station (2640 MW), Maryland