Janet Barlow and Omduth Coceal Based on Technical Report 527 - - PowerPoint PPT Presentation

A review of urban roughness sub-layer turbulence

Janet Barlow and Omduth Coceal

Based on Technical Report 527 prepared for the UK Met Office

Oke, 1988 H/W = 0.6 H/W = 1.0

Isolated roughness H/W < 0.3 Skimming flow H/W > 0.65 Wake interference 0.3 < H/W < 0.65

Field studies Numerical modelling Physical modelling stability traffic realistic sources and sinks repeatable stationary whole domain high Re too complex?! too simple?! no resolution issues idealised layouts 2D: street canyons/bar roughness/cavity flow 3D regular: cubes/cuboids 3D complex: “real” geometry

Field Phys. mod. Num. mod. total 2D 13 6 19 38 3D regular 3 10 23 36 3D complex 12 3 1 16 total 28 19 43 90

Papers reviewed on urban RSL flow

• Papers with significant study of flow within RSL
• Mainly flow and momentum exchange rather than

scalars

Talk Structure

Are common features emerging?

• Rough-wall Boundary Layer flow
• Canopy flow
• Scaling
• Urban Morphology
• Current modelling approaches
• Conclusions and open questions

Rough-wall boundary layer flow

Flow over smooth/rough surfaces (Raupach et al 1991)

inner layer

• uter layer

Boundary layer depth δ Viscous lengthscale Inertial sublayer: friction velocity u* For rough surface, ADD Roughness element lengthscales h, LX, LY, spacing Inertial sublayer: friction velocity u*

• Jimenez (2004) flow over rough walls: h/δ < 0.025
• Castro et al. (2006): urban areas = flow over “very rough walls”?
• Review: mean h/δ = 0.11, range 0.03 to 0.24

Atmospheric Boundary Layer

boundary layer mixed layer ~2-5h ~0.1zi z Roughness sublayer zi~1km windspeed potential temperature inertial sublayer surface layer

RSL depth

• Raupach et al 1991:

depth 2-5h

• Can be 10-15h in

unstable conditions (Roth 2000)

• Rotach 1995: RSL can

“squeeze out” ISL

• Cheng and Castro 2002:

is there sufficient fetch to grow an ISL?

• Cheng et al. 2007: no

ISL detected for λf=0.0625 (aligned cubes)

Feddersen, 2005, PhD thesis Wind tunnel model of Basel Christen, 2005, PhD thesis Basel-Sperrstraβe tower

The BUBBLE project (Basel, Switzerland)

• Criterion: height of min.

scatter in stress profiles; stress near constant with height above this

• Depth = 3.3h ± 0.6h
• cf. fullscale: Stress near

constant for z > 1.5h, max. height of measurement 2.2h

Canopy flow

Spatial variability within urban canopy

staggered aligned Velocity Turbulent kinetic energy Shear stress Dispersive stress Coceal et al. (2007b), z = 0.25h

Spatial mean cf. spatial standard deviation?

Significant dispersive stress within canopy

• Velocity moments vary strongly with height in vegetation canopies (“family

portrait”, Raupach et al. 1996)

• Christen 2005, “The Basel Family”
• Integral lengthscales minimum near urban canopy top
• Coceal et al. 2007b: mixing length at minimum near canopy top

Mixing layer hypothesis (Raupach et al 1996)

• Inflection point in mean wind profile unstable, leads to growth of coherent

structures

• Responsible for mixing throughout vegetation canopy depth
• Turbulence highly efficient (e.g. Ruw > surface layer values)

• Mixing layer

analogy yields universal result for vegetation canopies

• Turbulence scales
• n vorticity

thickness δω

• Test for urban

canopies?

Coherent structures: field study evidence

• Oikawa and Meng 1995, Salmond et al. 2004, Christen et al. 2007
• Quadrant analysis and skewness profiles:

sweeps dominate within canopy, ejections above

• Feigenwinter et al. (2005) in

Basel

• Ensemble averaged coherent

structure

• Ejection-sweep cycle
• Temperature microfront

Coherent structures: modelling

• Kanda 2006a, Coceal et al. 2007c, Castro et al. 2006, Inagaki and

Kanda 2008

• Consensus not yet reached about coherent structures over urban

surfaces – form, generation Structures detected in the field at a single point using wavelet analysis – comparable to EOF analysis of 3D datasets? Most modelling results obtained over uniform sized roughness elements

• Coceal et al. 2007c: “cartoon”

Scaling velocity and height

Feigenwinter 2005: mean in “constant stress layer” KK and Rotach 2004: peak stress Moriwaki and Kanda 2006d: a) scaled with top measurement b) stress extrapolated to z=h

• Cheng and Castro 2002: Better

scaling of log law profile using surface stress derived from a) form drag b) average of ISL and RSL stress profile

• Schultz (2007) – left, idealised cube type surface
• Kastner-Klein and Rotach (2004) – right, model of Nantes in wind

tunnel Peak stress not at z = h, nearer maximum height of buildings Peak stress present in individual profiles – insufficient spatial sampling?

• Xie et al. 2008 (based on Cheng and Castro 2002, non-uniform

layout): tallest buildings contribute significantly to drag Peak stress parameterisation too sensitive to local tall building influence?

Taller buildings’ contribution to drag

Urban Morphology

(or, why did we ever expect buildings to be like trees in the first place…?)

Layout

Kanda 2006a: LES, staggered layout higher drag MacDonald 1998: parameterisation includes factor to “correct” drag of individual roughness elements Can we formulate a morphological parameter to quantify element layout? e.g. “gap ratio”? (pic – thanks Anil Padhra)

Wind direction

• Kim and Baik 2004 – RANS

modelling shows change in mean flow patterns Studies only now emerging showing systematically effect of wind direction changes on flow, drag, dispersion

Urban areas contain other things too…

• DAPPLE site

(London, UK)

• Trees as roughness

elements Gromke and Rock 2007: wind tunnel simulation of trees and dispersion Pardyjak 2009: adding canopy to QUIC- URB

• Traffic

Kastner-Klein et al 2003: parameterisation of TKE due to traffic Pic courtesy of Microsoft Virtual Earth (thanks to Ahmed Balogun)

Current modelling approaches

1) Urban canopy models

MacDonald et al. 2000a:  Simple  Assumes constant mixing length in canopy Bentham and Britter 2003:  Simpler!!  Assumes no height variation in canopy windspeed Martilli et al. 2002:  Can implement in e.g. NWP models  What is Cd? Belcher et al. 2003; Coceal and Belcher 2003/4:  Allows for adjusting flows  What is cd(z)?

Current modelling approaches

2) Empirical parameterisations

MacDonald et al. 1998 (z0/h, d/h)  Simple  Based on cubes Rotach 2001:  Based on shear stress max, clear feature  …assuming there is one! Kastner-Klein and Rotach 2004:  Based on shear stress displacement ht  Doesn’t include BL depth

Current modelling approaches

3) Models based on mean flow structure

Caton et al. 2003:  Simple, based on street canyon vortex  No unsteadiness represented Dobre et al. 2005:  Street canyon vortex, allows for wind direction change  Only applicable to “street canyon” type flows Brown; Pardyjak; Addepalli et al. 2007 QUIC-URB:  Based on mean flow patterns, apply mass consistency  Relies on robust mean flow features, simplistic turbulent exchange

Conclusions: (very) rough surface?

• Fetch may not be long enough for an ISL to
• form. Is h/δ relevant?
• RSL depth observations fall into 2-5H.

Definition of depth can be local (single profile)

• r neighbourhood (spatial average of profiles).

Relationship with morphology not clear, e.g. height variability increases RSL depth

• Nature of coherent structures not yet

established for a range of morphologies – form, production mechanism?

Conclusions: canopy?

• Spatial averaging impossible to achieve with

fullscale data – substitute averaging over wind directions

• Large spatial variability, large bluff elements,

large wake size. Need to explore greater range

• f morphology parameters
• Flows are similar in some respects (“The Basel

Family Portrait” from BUBBLE). Scaling parameters??

• BUT mixing layer analogy not yet tested fully

(behaviour of lengthscales different near canopy top). Model of turbulent production?

Barlow, J.F. and Coceal, O. (2009) A review of urban roughness sub-layer turbulence, Technical Report 527, UK Met Office http://www.metoffice.gov.uk/publications/ NWP papers and reports (registration needed)