Omduth Coceal Dept. of Meteorology, Univ. of Reading, UK. Email: [email protected]and A. Dobre, S.E. Belcher (Reading) T.G. Thomas, Z. Xie, I.P. Castro (Southampton) Urban turbulence - flow statistics, dynamics and modelling A numerical study using direct numerical simulations (DNS) over groups of idealized buildings Seminar given at UK Met Office, Exeter, 1 May 2007
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Omduth Coceal Dept. of Meteorology, Univ. of Reading, UK. Email: [email protected] and A.Dobre, S.E. Belcher (Reading) T.G. Thomas, Z. Xie, I.P. Castro.
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Omduth Coceal Dept. of Meteorology, Univ. of Reading, UK.
- Compute these spatially-averaged quantities from the DNS data
- Don’t resolve horizontal heterogeneity at the building/street scale- Take horizontal averages: resolve vertical flow structure
h
y
x
€
D u
Dt+
1
ρ
∂ p
∂x=
∂
∂zu'w' +
∂
∂z˜ u ˜ w − D
€
u = u + ˜ u + u'
Spatial average of Reynolds-averaged momentum equation
Triple decomposition of velocity field
Spatially averaged statistics - uniform arrays
Different building layouts, same density(Detailed explanation of this plot in Coceal et al., 2006)
Arrays with random building heights (same density)
0.5hm
Compare results with LES performed by Zhengtong Xie (Southampton)
Same building density and staggered layout as in uniform array
Spatial averages - mean velocity
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.
Spatial averages - stresses
In the random array, the peaks are less strong, but still quite pronounced. They occur at the height of the tallest building, not at the mean or modal building height.
Spatial averages - dispersive stresses
Profiles of uw component of dispersive stress are very similar below z=h_m.
Spatial fluctuations
Qualitatively similar behaviour in the two arrays
Energy partitioning
(i) mke dominates above the canopy, but rapidly becomes a negligible fraction of the total k.e. within the canopy, while the fraction of dke and tke both increase. (ii) the fractions of mke, dke and tke for the two arrays are very similar below z=h_m; energy is partitioned roughly in the same proportion.(iii) above the canopy, the tke fraction over the random array is roughly twice as large as that over the regular array.
Buildings of variable heights - TKE
TKE from shear layers shed from vertical edges of tallest building dominates above half the mean building height.
Buildings of variable heights - Umag
The effect of the tallest building is more pronounced w.r.t. the total velocity magnitude.
Buildings of variable heights - Drag profiles (I)
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.
Buildings of variable heights - Drag profiles (II)
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.
Summary (I): Effect of building geometry on statistics
Effects of building layout
Mean flow structure and turbulence statistics vary substantially with layout
Effect of packing density still needs to be properly documented
Effects of random building heights:
Less strong shear layer on average
Inflection in spatially-averaged mean wind profile much less pronounced
Larger drag/roughness length
Below the mean building height, spatial averages are very similar to regular array
Effects of tall buildings:
Strong shear layers associated with tall buildings - high TKE
They exert a large proportion of the drag
They cause significant wind speed-up lower down the canopy
(II) Unsteady dynamics
Quadrant analysisDecompose contributions to shear stress <u’w’> according to signs of u’, w’
u’
w’
u’ > 0
w’ > 0
u’ < 0
w’ > 0
u’ < 0
w’ < 0
u’ > 0
w’ < 0
Which quadrants contribute most to the Reynolds stress <u’w’> ?
Ejections (Q2)
Sweeps (Q4)
Quadrant analysis
Profiles of fractional frequency and fractional contribution of each quadrant
Ejections and sweeps dominateThey are associated with turbulent organized motions
Quadrant analysis - ExuberanceExuberance
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Ex =S1 + S3
S2 + S4
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
Real field data (Christen, 2005)From DNS
Quadrant analysis - Q2 vs Q4 (I)
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
€
ΔS0 = S4 − S2
Real field data (Christen, 2005)
DNS
Fluctuating velocity vectors in x-z plane
Ejections and sweeps are associated with eddy structures
Mean flow from left to right. Local mean subtracted from velocity vectors.
Spatial distribution of ejections and sweeps
Fluctuating windvectors
Unsteady coupling of flow within and above canopy
Red = sweep eventsBlue = ejection events
Give information on lengthscale and spatial structure of organized motionsCorrelation lengthscale increases with height of reference point Small at z = h and within canopyStructures above canopy are inclined; inclination angle is a function of height
Two-point correlations Ruu
Instantaneous structures above buildings in 3d
Lower Reynolds number of 1200 (Re = 125, still fully rough flow)
Clearly reveals vortex structures (red) and low momentum regions (blue)
Vortex cores identified using isosurfaces of negative 2
3d structure of the conditional vortex
Hairpin-like conditional vortex obtained by conditional averaging of a large number of instantaneous realisations
Role of canopy-top shear layer
y
Intermittent impinging of shear layer on downstream buildings drives a recirculation.
cf Louka et al. (2000).
Space-time correlation Ruu with negative time delay of -0.4T; ref is at (8, 0.75).
T is an eddy turnover time of the largest eddies shed by the cubes.
Effect of shear layer on flow within canopy
z at z = 0.5 h x at y = 0.5 h
Interacting vertical shear layers Vortex tilting and stretching
Shear layers within the canopy
Small-scale circulations within canopy
Instantaneous windvectors in y-z plane within a cavity (flow is out of screen )
Summary (II): A conceptual model of the unsteady dynamics
Three flow regimes:
Flow well above canopy is a classical rough wall flow and its structure resembles that over a smooth wall boundary layer, although there are quantitative differences.
Flow near the canopy top is dominated by shear layer shed off top of cubes and by larger boundary layer eddies.
Flow within canopy is complicated by interaction of above with shear layers shed off vertical faces of the buildings, vortex stretching and tilting and distortion by roughness.