Turbulence statistics from DNS and LES - implications for urban canopy models. Omduth Coceal Dept. of Meteorology, Univ. of Reading, UK. Email: [email protected] and Dobre, S.E. Belcher (Reading) T.G. Thomas, Z. Xie, I.P. Castro (Southampton). Interpretation of field experiments.
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Turbulence statistics from DNS and LES - implications for urban canopy models
Omduth Coceal
Dept. of Meteorology, Univ. of Reading, UK.
Email: [email protected]
and
Dobre, S.E. Belcher (Reading)
T.G. Thomas, Z. Xie, I.P. Castro (Southampton)
Can we hope to understand this …
… using this ?
y
h
x
E.g. Urban canopy models (e.g. Martilli et al., 2002; Coceal & Belcher, 2004 )
Triple decomposition of velocity field
Spatial average of Reynolds-averaged momentum equation
Coceal et al., BLM (2006)
Coceal et al., IJC, to appear (2007)
Mean flow is out of screen, 643 gridpoints per cube
Compared with data from Cheng and Castro (2003)
and Castro et al. (2006)
stresses
spectra
velocity
pressure
Different building layouts, same density
Coceal et al., BLM (2006)
0.5hm
Compare results with LES performed by Zhengtong Xie (Southampton)
Velocities are smaller over the random array. The random array exerts more drag.
Spatially-averaged velocities are very similar within arrays.
Inflection is much weaker in random array.
Qualitatively similar behaviour in the two arrays
Spatial fluctuations are very significant within the canopy
In the random array, the peaks in tke are less strong, but are still quite pronounced.
They occur at the height of the tallest building, not at the mean or modal building height.
Profiles of uw component of dispersive stress are very similar below z=h_m.
Dispersive stresses are NOT small within the canopy (also Kanda, 2004).
TKE from shear layers shed from vertical edges of tallest building dominates above half the mean building height.
Wind speed-up around the tall building in relation to the background flow, especially at lower levels.
Tallest building (1.72 times the mean building height) exerts 22% of the total drag!
The 5 tallest buildings (out of 16) are together responsible for 65% of the drag.
The shapes of the drag profiles are in general similar for many of the tallest buildings (17.2m, 13.6m, 10.0m) except when they are in the vicinity of a taller building.
The profile shapes of the shortest buildings (6.4m and 2.8m) are very different - but these buildings do not exert much drag.
w’
u’ < 0
w’ > 0
u’ > 0
w’ > 0
u’
u’ < 0
w’ < 0
u’ > 0
w’ < 0
Decompose contributions to shear stress <u’w’> according to signs of u’, w’
Ejections (Q2)
Sweeps (Q4)
Ejections and sweeps contribute most to the Reynolds stress <u’w’>
They are associated with organized motions
Kanda et al. (2004), Kanda (2006)
Exuberance
From DNS
Real field data (Christen, 2005)
Exuberance is a measure of how disorganized the turbulence is
Magnitude of Exuberance is smallest near canopy top in DNS (uniform building heights)
Increases slowly above building canopy, rapidly within canopy
DNS
Indicates character of the organized motions
Ejection dominance well above the canopy
Sweep dominance close to/within the canopy. Cross-over point is at z = 1.25 h
Real field data (Christen, 2005)
Effects of building layout
Effects of random building heights:
Effects of tall buildings:
Ejections and sweeps are associated with eddy structures
Fluctuating windvectors
Unsteady coupling of flow within and above canopy