Flow variability in the vicinity of a busy intersection in Central London A. Balogun, A.S. Tomlin, C. R. Wood, J. Barlow, S. Belcher, R. Smalley, J. Lingard, S. Arnold, A. Dobre, A. Robins, D. Martin and D. Shallcross Universities of Leeds, Reading, Bristol, Surrey.
Introduction The aim of the paper: To assess the influence of the roof top background flow on the flow characteristics at the 2 DAPPLE intersection sites. DAPPLE (Dispersion of Air Pollution and Penetration into the Local Environment) 2007 field campaign. An overview of the DAPPLE project and comprehensive description of the measurement site and set-up have been presented in Arnold et al. (2004) for the 2003 campaign and Wood et al. (2009) for the 2007 field measurements. Further information is also available at www.dapple.org.uk. .
Site coordinates & instrumentation layout Measurement Period: May 22-July 4, 2007 R-hand Cartesian coordinate system used (inset). U & V velocity components aligned along Marylebone Rd & Gloucester Pl. 3D ultrasonic sonic anemometers 1 on roof-top reference R 4 @ intersection (~4m and 8m) 1 @ street canyon (~4m )
Complexity of site geometry • Marylebone Rd. is widest canyon so expect strong in street channelling component along MRd. • Weaker channelling along Gloucester Pl. 3 • Buildings different heights so expect asymmetries. 2 1 • Open arc may add to complexity. • Site 2 further into canyon than site 1.
Heights of measurement Z ~ 2-3H H= Z = H 21 m Z roof = Flow influenced by 18.4m Z ~ 0.3H buildings Thanks to Janet Barlow
Influence of 15 minute mean roof-top flow direction θ ref on in street flow θ ij θ ij 11 12 32 Where I = 1- 3 for site and J = 1- 2 for top & bottom sonics 21 22 U ref : ♦ < 1.2 ms -1 , ♦ 1.2 - 3 ms -1 , ♦ > 3 ms -1
Influence of 15 minute mean roof-top flow direction θ ref on in street flow θ ij Site 3 shows classic in canyon helical flow pattern. Site 2 - closer to canyon flows than site 1. • Strong channelling along MRd. • U ref : ♦ < 1.2 ms -1 , ♦ 1.2 - 3 ms -1 , ♦ > 3 ms -1
Influence of 15 minute mean roof-top flow direction θ ref on in street flow θ ij Site 3 shows classic in canyon Site 2 - closer to canyon flows than site 1. helical flow pattern. • Strong channelling along MRd. • In street recirculation for θ ref with significant cross street component • Weak channelling along GPl (Geometry). U ref : ♦ < 1.2 ms -1 , ♦ 1.2 - 3 ms -1 , ♦ > 3 ms -1
Site 1 bottom sonic • Combination of complex flow types leading to high degree of scatter in mean in street flow angle. • In street channelling for only narrow range of θ ref . • Combination of flow channelling, recirculation, corner vortices and bifurcation all possible for θ ref oblique. Uref : ♦ < 1.2 ms -1 , ♦ 1.2 - 3 ms -1 , ♦ > 3 ms -1 .
Site 1 bottom sonic • Further analysis of complex regions required to help explain scatter in mean flow angles! 46° ≤ θ ref ≤ 75° • Wind vector roses & Pdfs were employed. -134° ≤ θ ref ≤ -75°
15 minute mean wind roses for oblique reference flows: 46° ≤ θ ref ≤ 75° Roof Site 1 shows possible flow bifurcation leading to mean in street angles of anywhere between 0° ≤ θ 12 ≤ 120 ° → i.e. large scatter in mean flow angle. Site 2 shows evidence of channelling along MRd, cross canyon recirculation and turning of in street angle – corner vortex?
Hourly probability density functions (pdf’s) for oblique reference flows. GPl → MRd • Bifurcated flow leads to bi-modal in street pdf’s at both intersection sites. → MRd Vortex? • Slight change in θ ref pdf causes strength of Recirculation channelling from different streets to shift at site 1 → scattered mean flow angle. • Site 2 more consistent – further in canyon.
Hourly probability density functions (pdf’s) for oblique reference flows. GPl → MRd • Bifurcated flow leads to bi-modal in street pdf’s at both intersection sites. → MRd Vortex? • Changes in θ ref causes strength of Recirculation channelling from different streets to shift at site 1 as the flow is bifurcated → scattered mean flow angle. • Site 2 more consistent – further in canyon.
Flow visualization and wind tunnel modelling ( θ ref =51°) Top left Source MRd (south), horizontal light sheet shows intermittent vortex at the south-west corner of the intersection. 2 1 Bottom right Source at GPl (centre), horizontal light sheet shows vortex at the south-east corner of the intersection. Carpentieri et al, 2009
15 minute mean wind roses for oblique reference flows: -134° ≤ θ ref ≤ -105° Site 1 shows complex Roof combination of flow patterns leading to huge scatter in mean flow angle: • bifurcation of channelled flows along MRd and down GPl • influence of in canyon recirculation leading to weak channelling up GPl. Site 2: • stronger channelling up GPl than down • influence of in canyon recirculation • backing to the left may suggest presence of spiralling.
Example hourly pdf’s for -134° ≤ θ ref ≤ -105° Recirculation → MRd ↓ GPl Site 1: Still evidence of channelled modes but far more complex combination of flow patterns than Recirculation for opposing oblique flows: Bifurcation, recirculation, channelling & influence of site geometry. → MRd Site 2: ↓ GPl Recirculation dominant.
Fluctuating background flow leads to changes in relative importance of different flow features → complexity! Cross canyon Fluctuating helical vortex roof-top wind Corner vortex Flow bifurcation
Conclusions • Intersection flows appear to be a complex combination of various flow patterns (bifurcation, flow recirculation and corner vortices). Results suggests intersection flow pattern is 3D for oblique rooftop flows. • Site 2 behaviour is closer to typical canyon flows than site 1 even though it is not deep into the canyon. • Small changes in background wind direction have pronounced influences on the behaviour of intersection flow patterns. • Consequently, short time-scale variability in background flow direction can lead to highly scattered in-street mean flow angles. • Averaged flow angles hide the true multi-modal features of the flow! • Geometric features at the intersection corners also significantly influence the variability of flow behaviour at both sites. Modelling effort will need to incorporate this for improved accuracy. •
Acknowledgements We acknowledge EPSRC and the UK Home Office for DAPPLE funding. We also thank DAPPLE colleagues, staff at WCCH, Transport for London and the School of Earth and Environment at the University of Leeds.
Fluctuating background flow leads to changes in relative importance of different flow features → complexity! Cross canyon Fluctuating helical vortex roof-top wind Corner vortex Flow bifurcation Thank you for your attention
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