Data and Models

• Operational Street Pollution Model (OSPM)
• The Microscale Model CHENSI
• German Guideline Dispersion Models - PROKAS_V and MISKAM (free demo-version !)
• The Model PROKAS_B for built-up streets
• The mesoscale model MEMO
• The microscale model MIMO
• The microscale model TASCflow

Operational Street Pollution Model (OSPM)

Concentrations of exhaust gases are calculated using a combination of a plume model for the direct contribution and a box model for the recirculating part of the pollutants in the street.

The direct contribution is calculated using a simple plume model. It is assumed that both the traffic and emissions are uniformly distributed across the canyon. The emission field is treated as a number of infinitesimal line sources aligned perpendicular to the wind direction at the street level. The cross wind diffusion is disregarded. The wind direction at the street level is assumed to be mirror reflected with respect to the roof level wind. The plume expression for a line source is integrated along the path defined by the street level wind. The length of the integration path depends on the extension of the recirculation zone.

The contribution from the recirculation part is calculated using a simple box model. It is assumed that the canyon vortex has the shape of a trapeze, with the maximum length of the upper edge being half of the vortex length. The ventilation of the recirculation zone takes place through the edges of the trapeze but the ventilation can be limited by the presence of a downwind building if the building intercepts one of the edges. The concentration in the recirculation zone is calculated assuming that the inflow rate of the pollutants into the recirculation zone is equal to the outflow rate and that the pollutants are well mixed inside the zone.

The turbulence within the canyon is calculated taking into account the traffic created turbulence.

The model can be used for streets with irregular buildings or even buildings on one side only but it is best suited for regular street-canyon configurations. The model should not be used for crossings or for locations far away from the traffic lanes.

The NO₂ concentrations are calculated taking into account NO-NO₂-O₃ chemistry and the residence time of pollutants in the street.

The model is designed to work with input and output in the form of one-hour averages.

More information on OSPM and a free evaluation version of WinOSPM (OSPM with a Windows user interface) is available now.

The Microscale Model CHENSI

Unsteady 3D wind fields are explicitly calculated using two-equation turbulence model (turbulent kinetic energy and its dissipation rate) and concentrations of pollutant species are determined by on-line or off-line resolution of scalar transport partial differential equations.

A procedure of finite difference or volume approximation is used on staggered non-uniform Cartesian grids. The numerical scheme is explicit in time and the continuity condition is directly satisfied by the iterative artificial compressibility method. Various boundary conditions can be selected: Dirichlet, Neumann, Laplace, symmetry, periodicity, law of the wall.

The model is currently used for in-streets wind flows and traffic pollutant dispersion at local scales. Input conditions are inflow wind field, turbulence, potential temperature and background pollutant concentrations, traffic speed and intensity, number of traffic lanes, car pollutant emissions (as line/volume sources) for unlimited number of species. Turbulence induced by traffic is simulated through an extra generation term in the turbulent kinetic energy equation. Complex recirculating flow structures are calculated and residence times of chemical species are deduced from the resulting concentration field time variations.

CHENSI is intended to be coupled with an atmospheric fast-chemistry model for the NO_x-O₃ system.

The model is best suited for urban canopies with a distribution of a few streets and regular buildings, canyon streets with crossroads

CHENSI runs currently on the Group's 4-CPU SiliconGraphics Origin 200 computer of the Group and on Cray C-90 computers located at the National Computer Institute (IDRIS) of the National Centre for Scientific Research (CNRS). TECPLOT software is used for plotting and visualising calculated wind and pollutant fields.

For more information: Dr. Jean-Francois Sini (e-mail: Jean-Francois.Sini@ec-nantes.fr)

German Guideline Dispersion Models - PROKAS_V and MISKAM

PROKAS_V

An interface exists to MOBILEV (program for determination of traffic induced emissions), to PROKAS_B and to WinMISKAM (programs for determination of concentrations in built-up streets).

Input requirements for PROKAS_V are

• files containing the co-ordinates of the streets, the annual mean of the emissions of up to 3 air pollutants, for example benzene, soot, and NO_x in these streets, and for each street additional terms to the dispersion parameter counting for near field disturbances of the flow (traffic induced turbulence etc.)
• the Gaussian dispersion parameters for smooth and rough terrain
• the co-ordinates of the receptor points and the background concentrations at these points
• the distribution of the emissions for the single hours of the week
• the meteorological statistics (3-dimensional statistics of wind speed, wind direction and atmospheric stratification)

The model determines the concentrations (annual mean values and 98-percentils) of the air pollutants under consideration (incl. NO_x and NO₂) at arbitrary receptor points.

PROKAS_V is not applicable

• if the wind field is inhomogenious in the area under consideration or in areas with valley drainage flows
• in cases when the influences of buildings have to be considered in detail
• in distances less than 10 m from the line source.

MISKAM

MISKAM consists of a 3-dimensional non hydrostatic flow model and an Eulerian dispersion model. The physical basis are the complete 3-dimensional equations of motion of the flow field and the advection-diffusion equation to determine the concentrations of substances with neutral density. The calculated result is the stationary flow and pressure field, diffusion coefficients and the concentration field in an area of typically 300 m x 300 m (60 x 60 cells or more, non equidistant grid). The main application is in built-up areas where neutral atmospheric stratifications are dominant.

Input parameters are:

• Information about the grid, the height and the base area of the buildings,
• the aerodynamic roughness of the areas between the buildings and on the building surfaces as well as
• position and strength of the sources
• initial information about the wind direction, the wind speed and the reference height of the wind speed.

WinMISKAM has an interface to MOBILEV (program to determine traffic induced emissions) and to PROKAS_V (program to determine the concentrations by surrounding streets). The superposition of the concentrations determined by PROKAS_V and the concentrations determined by WinMISKAM is time correlated in order to assure a correct determination of the 98-percentile.

The calculated results are the concentrations (annual mean values and 98-percentils) of the air pollutants under consideration (incl. NO_x and NO₂).

WinMISKAM is not applicable

• in strongly structured terrain,
• if the area, modelled in WinMISKAM, is too small to take into account features influencing the flow from outside the modelled area
• when the assumption of neutral atmospheric stratification is not appropriate (incl. areas with valley drainage flows)
• when the geometry of the buildings differs significantly from a rectangular grid

MISKAM is applicable on common PCs under Windows 3.xx, Win95 or WindowsNT. It is not a public domain program, but a full version can be provided by IBAL for use during a limited time. Additionally a free version of WinMISKAM restricted to a horizontal grid size of 20x20 grids and a fixed statistic of wind speed and wind direction is available for downloading. Further information can be obtained from Universitaet Mainz, Institut fuer Physik der Atmosphaere, (Dr. Eichhorn), Mainz, Germany or Ingenieur-Büro Dr. Lohmeyer (Achim.Lohmeyer@Lohmeyer.de) or see the German guideline VDI 3782/8E (March 1998), published by Verein Deutscher Ingenieure Düsseldorf, Germany. A detailed description is available on the Model Documentation System (MDS)

The Model PROKAS_B for built-up streets

PROKAS_B is an additional module to PROKAS_V. It calculates the concentrations in built-up streets caused by the emissions in these streets while PROKAS_V provides the contributions of the surrounding streets. PROKAS_B is based on precalculated dimensionless results with the microscale flow and diffusion model WinMISKAM for 21 building patterns. These patterns depend on the ratio of height of buildings to width of street, presence of buildings on one or both sides of the street, and the percentage of gaps along the length of the street. Modelling of up to 5000 streets with the surrounding buildings within a network of streets is possible. The influence of traffic induced turbulence is included. An interface exists to MOBILEV (program to determine traffic induced emissions).

The calculated results are the concentrations (annual mean values and 98-percentils) of the air pollutants under consideration (incl. NO_x and NO₂) in 1 m distance from the walls of the surrounding buildings and at 1.5 m height above the ground for the side of the street, where the meteorological conditions result in the highest concentrations.

PROKAS_B is not applicable

• if the building patterns vary strongly within 100 m along the street
• if the assumption of neutral atmospheric stratification in the street is not appropriate or in areas with valley drainage flows
• in cases when the influences of buildings have to be considered in detail.

PROKAS_B is applicable on common PCs under Windows 3.xx, Win95 or WindowsNT. It is not public domain program, but a full version can be provided by IBAL for use during a limited time. Further information can be obtained from Ingenieur-Büro Lohmeyer (Achim.Lohmeyer@Lohmeyer.de). A detailed description is available on the Model Documentation System (MDS)

The mesoscale model MEMO

MEMO is a prognostic mesoscale model which allows describing the air motion over complex terrains. Within MEMO, the conservation equations for mass, momentum, and scalar quantities as potential temperature, turbulent kinetic energy and specific humidity are solved in terrain-influenced co-ordinates. Non-equidistant grid spacing is allowed in all directions. The numerical solution is based on second-order discretisation applied on a staggered grid. Conservative properties are fully preserved within the discrete model equations. The discrete pressure equations are solved with a fast elliptic solver in conjunction with a generalised conjugate gradient method. Advective terms are treated with the TVD scheme.

Turbulent diffusion can be described with either a zero-, one- or two-equation turbulence model. At roughness height similarity theory is applied. The radiative heating/cooling rate in the atmosphere is calculated with an implicit multilayer method for shortwave radiation. The surface layer over land is computed from the surface heat budget equation. The soil temperature is calculated by solving an one dimensional heat conduction equation for the soil. MEMO allows performing "telescopic" simulations, via nesting MEMO-in-MEMO. In this sense, meteorological information may be input to MEMO at rather large distances from the area of interest.

As one of the core models of the European Zooming Model, MEMO has been successfully applied for various European airsheds including the Upper Rhine Valley and the areas of Heilbronn, Basel, Graz, Barcelona, Lisbon, Madrid, Milano, London, Cologne, Lyon, the Hague and Athens.

The microscale model MIMO

MIMO has been developed on the basis of MEMO. The conservation equations for mass, momentum, and scalar quantities as potential temperature, turbulent kinetic energy and specific humidity are solved in terrain-influenced co-ordinates. Non-equidistant grid spacing is allowed in all directions. The numerical solution is based on second-order discretisation applied on a staggered grid. Conservative properties are fully preserved within the discrete model equations. The discrete pressure equations are solved with a fast elliptic solver in conjunction with a generalised conjugate gradient method. Advective terms are treated with the TVD scheme. Turbulent diffusion can be described with the two-equation k-e turbulence model.

The model is currently used for the simulation of the flow fields and the dispersion and transformation of pollutants around building aggregates and within street canyons. The concentrations of NO and NO₂ are calculated taking into account NO-NO₂-O₃ chemistry. MIMO can also be coupled with the mesoscale flow model MEMO (one way coupling) to simulate close-to-real cases where mesoscale flow processes, such as katabatic/anabatic wind systems or channeling of the flow within a valley, are considered.

The microscale model TASCflow

TASCflow is a general purpose CFD analysis system using a flexible multi-block grid system, a modern graphical user interface and many sophisticated modelling tools. TASCflow comes with advanced grid generation and pre- and post-processing capabilities. Within TASCflow, the conservation equations for mass, momentum, and scalar quantities as temperature, turbulent kinetic energy and any number of species are solved in generalised curvilinear co-ordinates. Non-equidistant grid spacing is allowed in all directions. The numerical solution is based on first-order in time and second-order in space discretisation, applied on a co-located grid arrangement. Conservative properties are fully preserved within the model equations. The discrete momentum and continuity equations are solved with a coupled elliptic solver. An efficient algebraic multi-grid solution technique is adopted, giving a practically constant rate of convergence, regardless of the level of the grid refinement. Turbulent diffusion can be described with the two-equation k-e turbulence model.

Additional capabilities of TASCflow comprise Lagrangian particle tracking, reacting/combusting species, rotating frame of reference, conjugate heat transfer, surface to surface radiation, compressible flows, local grid refinement and generalised periodic connections. TASCflow is routinely used in automotive design, turbomachinery, combustion, process industries and building ventilation design.