Experimentally Modelling Urban Air Pollution Individual Tall Building Case
The aim of this study is to measure the effect that an individual tall building has on scalar dispersion within a modelled urban boundary layer.
Intended Measurement techniques are planar PIV-PLIF along the centerline of the flume, and stereo PIV-PLIF in streamwise slices
This was done in the University of Southampton’s Recirculating Water Tunnel
Left: Experimental setup on water tunnel floor for stereo PIV-PLIF measurements.
Right: Experimental setup on water tunnel floor for streamwise planar PIV-PLIF measurements
Left: Image of the inside of the recirculating water tunnel and false floors used to generate an atmospheric boundary layer. This photo was taken from the area the 3D print is intended to be located in the tunnel.
The roughness elements are a collection of spires and cubes designed to thicken the log-law region of the boundary layer downstream.
Objectives:
How can we Model Urban Air Pollution Experimentally?
In order to Model Urban Air Pollution, it is necessary to be able to track the location and concentration of a scalar species introduced to the flow.
This is done using the Planar Laser Induced Fluorescence (PLIF) method.
This method uses a fluorescent dye as the species to be introduced and a planar laser sheet to take a cross section of dye concentration.
This technique is primarily conducted in liquids.
Right: Laser sheet being tested for freestream measurements in the recirculating water tunnel
Tomos Rich, Dr Christina Vanderwel
The tall building acts to significantly change the path of the impinging species, immediately re-directing it out of the centreplane, creating a low-concentration zone in the immediate wake of the building.
In the far wake, spreading increases due to the increased turbulence from the building, creating a broad region of medium-concentration.
Results:
Diagram of the University of Southampton’s Recirculating Water Tunnel
The graphs and diagrams shown to the right show both the streamwise and cross-stream slices of scalar dispersion around the tall building.
Top right shows a streamwise slice of normalized dye concentration and variance against downstream distance.
Bottom right shows cross-stream slices of normalized dye concentration downstream of the building. Downstream distances measured in building heights from back of tall building: a = 2H, b = 4H, c = 6H, d = 8H, e = 10H, f = 12H. These graphs show the dual peak dye shape explained in the diagram at top right.
Experimental Setup
Vertical turbulent and advective flux maps around and in the wake of the tall building. Advective flux is equal to (CW/CsU∞), turbulent flux is equal to (c′w′/CsU∞). Displayed on a log scale, the chequered box is an erroneous section of data.
Streamwise velocity, normalized by freestream velocity
Normalised Vertical Advective Flux
Normalised Vertical Turbulent Flux
Vertical cumulative total against downstream and cross-stream distance from the dye source (∫ C dz).
Normalised Species Concentration Variance
Normalised Species Concentration
Normalised Species Concentration
The diagram to the right shows the predicted diagram of highest intensity concentration areas.