Air Quality in Urban Areas: Traffic Induced Pollutants Concentration and Dispersion

A contribution to subproject SATURN

Miroslav Jicha, Jaroslav Katolicky and Jiri Pospisil

Technical University of Brno, Faculty of Mechanical Engineering,

Power Engineering Institute, Technicka 2, 61669 Brno, Czech republic

  1. Summary


An Eulerian-Lagrangean method was developed to predict traffic induced flow rate and turbulence as a result of moving vehicles. The method is based on CFD calculations using Eulerian approach to the continuous phase and Lagrangean approach to the „discrete phase“ of moving objects - cars. First, a road tunnel is taken into consideration and different traffic rates and speed of cars is taken into account to obtain the flow rate entrained throughout the tunnel and correct velocity and concentration field at the tunnel portal. Second the same procedure is applied to an open street canyon with a specified wind speed at roof level and different traffic conditions. Three different formulas for extra sources of kinetic energy of turbulence are tested for the traffic induced turbulence showing a non-negligible effect on the total flow rate.
 

  1. Aim of the research
The principal aim is to establish the influence of traffic in specific urban situations like road tunnel openings and street canyons and to predict the influence of different traffic conditions on the dispersion of pollutants in microscale. Also the impact of different meteorological conditions like thermal stratification and solar radiation inside the street canyon will be studied and their influence established.
 
  1. Activities during the year
The aformentioned Eulerian-Lagrangean method to model moving vehicles was developed and tested. As a basis for the tests, a road tunnel was chosen for which many numerical tests have been done to establish influence of tunnel length, traffic rate (cars/hour) and vehicle speed. Three different formulas for additional generation of kinetic energy of turbulence were tested (Jicha and Katolicky, 1998). The same method has been applied to a street canyon. Different aspect ratioo of the canyon has been modelled as well as different traffic situation comprising traffic rate, speed and one-or multi-lane traffic.
 
 
  1. Principal results
Results of the predictions related to a vehicle tunnel are presented in Fig. 1 and 2. In Fig. 3 there is a comparison of the numerical modelling with a one-dimensional analysis of the flow inside a tunnel for the case without and with additional source of kinetic energy of turbulence. The importance of additional generation of turbulence is evident. First, the flow field in the cross section tends to be more uniform and the flow rate is reduced. In the Fig. 4 and 5 there are results of the velocity and concentration fields in a street canyon when vehicles move along the street with the specified velocity and density, thus inducing a strong flow in their direction.
 
 

Fig. 1. Traffic induced flow rate as a function of tunnel length for different speed of cars
 
 

Fig. 2 Traffic induced flow rate as a function of traffic rate in 100m tunnel, speed 57.6 km/hour

Fig. 3 Influence of additional generation of turbulence
 
 
 
 

  1. Main conclusion
An Eulerian-Lagrangean method has been developed for the prediction of the flow rate of the air induced by moving vehicles. The method has been tested and compared with other method and experiments. Flow entrained by moving vehicles reduces the concentration inside the road tunnel and street canyon as well. The importance of additional generation of turbulence has been showed. Additional generation of turbulent kinetic energy increases turbulent diffusion and makes the concentration field more uniform.

Higher traffic rate inside a street canyon causes higher velocity to spread over larger cross section of the canyon and more intense longitudinal flow throughout the canyon. As a result we can observe lower concentrations inside the canyon. Two-way traffic shows highest concentrations inside the canyon compared to one-way traffic.
 
 

Traffic rate: 360 cars/hour/lane
 
A. B. C.


Cmax          0.20E-01                              &nbs p; 0.25E-01                                    0.35E-01
 
 

Traffic rate: 720 cars/hour/lane

Cmax:         0.30E-01                             0.45E-01                                 0.70E-01
 

Fig. 4 Concentration field in the street canyon (passive scalar) - steady situation
 
 

Velocity at roof level: 3 m/s Width/Height of the canyon: 1.27

A. One-way traffic (2 parallel lanes)

B. One-way traffic (4 parallel lanes)

C. Two-way traffic (2 and 2 opposite lanes)
 
 
 
 

Traffic rate: 360 cars/hour/lane
 
A. B. C.

 

Traffic rate: 720 cars/hour/lane

Maximum velocity Wmax = +13 m/s, -11 m/s

Fig. 5 Velocity field in the street direction - steady situation
 
 

  1. Aim for the coming year
In the coming year the already developed and tested Eulerian-Lagrangean method for moving objects and traffic induced flow field will be applied and tested for an infinite as well as 3D street canyon. Different conditions will be imposed starting with traffic speed and rate and roof level speed and direction of the wind. Energy balance and heat fluxes in the street canyon will be analysed with the aim to establish the role of buoyancy force resulting from heating the walls and road by solar irradiation.

A street canyon structure composed of individual canyons and intersections will be studied with the aim to predict the flow and concentration field resulting from unsteady traffic, like increasing traffic rate during peek hours or jamed traffic. The main goal be will to predict dispersion of pollutants in the vicinity of traffic in the scale of several streets.

  1. Acknowledgement

    2.  
The authors gratefully acknowledge the partial financial support from the Czech Eureka program of the Ministry of Education of the Czech republic.
     
  1. References
Jicha, M., Katolicky J., Eulerian-Lagrangian computational model for traffic induced flow field and turbulence inside a vehicle tunnel, Proc. 5th Int. Conference on Harm. Atm. Disp. Mod. Reg. Purp., Rhodos (1998), 549-556

Crowe, G.T., Sharma, M.P., Stock, D.E., The Particle-Source-In-Cell Model for Gas-Droplet Flows, J.Fluid Eng., vol.99 (1977), 325-332

Sini J. F., Mestayer P. G., Traffic-induced urban pollution: A numerical simulation of street dispersion and net production, 22nd NATO/CCMS International Technical Meeting on Air Pollution Modelling and its Application, Clermont-Ferrand (1997)

Jicha M., Katolicky J., Almbauer R., Modelling of Pollutants Dispersion from a City Road Tunnel, 22nd NATO/CCMS International Conference, Clermont-Ferrand (1997) 485-6