Dispersion of Air Pollution & Penetration into the Local Environment

The EnFlo Wind Tunnel, University of Surrey

The EnFlo wind tunnel was built to provide a facility capable of simulating both stable and unstable atmospheric boundary layer conditions. The working section was therefore designed to accommodate a wide range of configurations (i.e. to model sea to land or land to sea interfaces, etc.).

The key features of the tunnel are shown in the isometric view below. They are:

  1. an open circuit 'suck-down' design,
  2. a simple inlet followed by a 15 layer inlet flow heater,
  3. a working section comprising interchangeable 1x1.5m panels, separate roof or floor heating and cooling panels, 3-zone heated wall panels (to minimise differences between wall and flow temperatures), and
  4. twin fans, preceded by a heat exchanger to return the flow to ambient temperatures.
Isometric drawing of tunnel

The tunnel is fitted with a three-dimensional traverse system which covers the final 12m of the working section. Access to the working section is through two doors, one on either side at the downstream end of the section. The technical specification of the tunnel is:

Working section dimensions
Overall length
Air speed range
20 x 3.5 x 1.5 m
27.2 m
0.3 to 4.0 ms-1
Co-ordinate origin tunnel floor centre
at working section entrance
Traverse gear travel
longitudinal
lateral
vertical

8 to 20 m
-1.5 to 1.5 m
0 to 1.5 m
Inlet heater
Number of layers
Maximum heating rate
Maximum temperature gradient

15 @ 0.1 m
400 kW
80Cm-1
Panel Heating and Cooling
Individual panel size
Maximum heating rate
Maximum surface temperature
Maximum cooling rate
Minimum surface temperature

1 x 1.5 m
5 kWm-2
120C
1 kWm-2
~ 10C (depending on dewpoint)
Systems drawing of tunnel

This figure shows a typical experimental set-up in which the tunnel's heating and cooling facilities are used. The associated gas emission, sampling and analysis systems are also shown, along with the tunnel control and condition monitoring equipment. Reference flow conditions are measured by two ultrasonic anemometers, one held at a fixed reference location and the other positioned as required, and two propeller anemometers mounted on either side of the traverse carriage; the motor shaft speed is also measured. Temperature conditions are monitored by thermocouple rakes in the flow and individual thermocouples in each tunnel panel. The pressure drop across the inlet is also monitored, primarily to indicate the state of the inlet screens.

The DAPPLE model in the EnFlo tunnel

Upstream view of model

The 1:200 scale model installed in the wind tunnel. The picture is taken into the wind and shows the boundary layer generation devices and surface roughness upwind of the model. This is the simplest model, where all buildings have been reduced to simple blocks with flat roofs. More details, such as courtyards and roof shapes will be added later as we investigate the level of importance of such detail in local flow and dispersion processes.

This is a view looking SW across the model; Westminster Council House's tower is visible near the centre of the picture. The red structure above the model is the traverse gear which moves the probe vertically; the silver object at its apex is the Fast FID, used for measuring concentration fluctuations. Measurements are being taken at the tip of the probe, which is in York St. The curved pipe in York St is supplying release gas to a solenoid valve, used to generate "puff" releases.

Upstream view of model