Dispersion of Air Pollution & Penetration into the Local Environment

Computational Fluid Dynamics

The FLUIDITY Code

FLUIDITY is general-purpose CFD code developed at Imperial College by Dr. C.C. Pain. The code is capable of modelling a wide range of fluid problems involving single and multiphase (solid/liquid/gas) flows by numerically solving the Navier-Stokes equations and the accompanying field equations. The number of field variables solved for include temperature, chemical species, variable density and viscosity. A fuller description of the code and its computational structure and techniques used can be found here (hyperlinked to http://julia.et.ic.ac.uk/). In brief, FLUIDITY incorporates:

Simulated Airflows within the DAPPLE-designated area using FLUIDITY

In order to carry out the CFD simulations using FLUIDITY, the intersection of interest (Marylebone Road with Gloucester Road) has been represented by a computational domain that consists of two intersecting street canyons with four buildings of different dimensions at each corner of the intersection. The domain is currently oriented in such a way so that it coincides with the wind direction as measured during the tracer release experiment in May 2003. At the inlet, a constant velocity boundary condition is specified, whilst the turbulence is generated by three rows of roughness elements placed normal to the flow and close to the inlet. At the free surfaces of the domain, impermeable, no shear conditions were applied (i.e. flow normal to the surface of interest is zero), whilst the no-slip boundary condition (i.e. all three velocity components are zero) were applied at the solid wall boundaries.

The release of a pollutant has also been considered in the simulations, by specifying a constant pollution source for a certain time duration at a fixed point in the domain. The pollutant concentration predictions will ultimately be used in comparison with the real personal exposure data and assist in their analysis and interpretation.

An important feature currently being incorporated within the FLUIDITY code is the ability to represent ÒmovingÓ sources, thus effectively simulating the effect of a moving vehicle on the surrounding flow field and subsequently on the pollution concentrations.

In summary, the CFD models presently consider:

  1. Representation of the Marylebone/Gloucester Road intersection with the four main buildings at the corners of the intersection.
  2. Wind direction so that the computational domain is oriented in the same direction as the wind.
  3. Roughness elements for the generation of turbulence Ð in a similar way as in the wind tunnel studies.
  4. Constant velocity inlet condition; no-slip boundary conditions at the building walls and the bottom of the domain.
  5. Stationary pollutant sources.
  6. Moving vehicles sources and their effect on the surrounding flow field and concentration profiles.
  7. Many vehicles modelled as well as trees.

Examples of our results can be seen in the following attached PowerPoint presentations. They correspond to a simulation with a Southwesterly wind, as observed during the May 2003 field campaign.

Presentation 1 (6.1MB) Flow Variations within Marylebone Road: From East to West.

The slides show how the flow patterns generated within a vertical cross-section (perpendicular to Marylebone) vary as one moves from the East end of Marylebone to the West. Interesting flow patterns are formed both in the East as well as the West of the intersection. On the far East of the intersection, a single flow circulation pattern is observed with flow moving vertically from the south side of the road to the north. As one approaches the intersection from the East, a more complicated pattern emerges with the appearance of two circulation patterns of different widths. As we pass the intersection, two distinct circulation patterns are observed.

Presentation 2 (2.6MB) Flow Variations within Gloucester Place: From South to North.

These slides show the flow variation within vertical cross-sections along Gloucester Place. The patterns are not as interesting as along Gloucester Place, until we reach the intersection. Looking at a vertical cross-sections parallel to and within Marylebone, we can see the complex vertical flow patterns generated.

Presentation 3 (1MB) Horizontal Flow Patterns

These slides show the flow patterns in horizontal planes at different heights, within the whole domain.

Presentation 4 (4.5MB) Concentration Profiles

Concentration profiles are shown within the whole domain, after two-fixed pollution sources were considered: (a) south of Gloucester Place, and (b) West of Marylebone Road. For the first source, it is clear that with a southwesterly wind the pollution moves along the South part of Gloucester Place, and then follows the flow pattern and moves to the East of Marylebone road. It spreads within the Eastern part of Marylebone, and goes as high as the two buildings (Bickenhall Mansions and Dorset house) but it does not seem to go above the buildings.

For the second source location, the pollution spread with in the Western part of Marylebone, and moves above Marathon House, and spreads into the Northern part of Gloucester Place but at heights above the building heights. It also spreads into the Eastern part of Marylebone road, moving initially close to Dorset House (i.e. the northern part of the road).

Future Work

A essential part of the simulation work is validation of the results. As a first step, we will evaluate the FLUIDITY results of the DAPPLE study area with the corresponding wind-tunnel studies.

Further development work is also planned to investigate the treatment of boundary conditions at wall boundaries; alternatives to the current no-slip solid wall boundary condition will be tested. In addition, further development on the turbulence modelling work is anticipated.