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Scale effects in wind tunnel modeling of an urban atmospheric boundary layer


Precise urban atmospheric boundary layer (ABL) wind tunnel simulations are essential for a wide variety of atmospheric studies in built-up environments including wind loading of structures and air pollutant dispersion. One of key issues in addressing these problems is a proper choice of simulation length scale. In this study, an urban ABL was reproduced in a boundary layer wind tunnel at different scales to study possible scale effects. Two full-depth simulations and one part-depth simulation were carried out using castellated barrier wall, vortex generators, and a fetch of roughness elements. Redesigned “Counihan” vortex generators were employed in the part-depth ABL simulation. A hot-wire anemometry system was used to measure mean velocity and velocity fluctuations. Experimental results are presented as mean velocity, turbulence intensity, Reynolds stress, integral length scale of turbulence, and power spectral density of velocity fluctuations. Results suggest that variations in length-scale factor do not influence the generated ABL models when using similarity criteria applied in this study. Part-depth ABL simulation compares well with two full-depth ABL simulations indicating the truncated vortex generators developed for this study can be successfully employed in urban ABL part-depth simulations.

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b G :

vortex generator width

d :

displacement height

f :


f M :

peak frequency

h :

object height

h G :

vortex generator height

l :

large eddy mean size

l G :

vortex generator length

t :


u :

absolute velocity in the x-direction

\( \bar u \) :

mean velocity component in the x-direction

\( {\bar u_z} \) :

mean velocity component in the x-direction at height z

u τ :

friction velocity

\( {\bar u_{ref}} \) :

reference velocity

\( {\bar u_\delta } \) :

freestream velocity

u′,v′,w′ :

fluctuating velocity component in the x-, y-, z-direction

x :

distance in the direction of the flow

y :

spanwise distance from test section centerplane

z :

vertical distance from wind tunnel floor

z ref :

reference height

z 0 :

aerodynamic surface roughness length

I u :

turbulence intensity in the x-direction

Je :

Jensen number

L u,x :

longitudinal integral length scale of turbulence

Re R :

roughness Reynolds number

R u,x :

longitudinal correlation coefficient

S :

length scale factor

S u (f):

power spectrum of longitudinal velocity fluctuations

T :

total record length

α :

power law exponent

δ :

boundary layer thickness

κ :

von Karman constant

λ :

similarity coefficient

ρ :

air density

σ u :

standard deviation of u

τ :

Reynolds stress

v :

air viscosity


model (wind tunnel)


prototype (full-scale)


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This work was conducted in the Institute of Aerodynamics, Technische Universität München. The support of the Croatian Ministry of Science and Technology, the German Academic Exchange Service (DAAD), and the Croatian Academy of Sciences and Arts (HAZU) is gratefully acknowledged. The author acknowledges many helpful discussions with Prof. Boris Laschka, Dr. Albert Pernpeintner, and Dr. Joseph Fischer. Special thanks needs to be expressed to the TUM departmental technical staff for the manufacturing of the simulation hardware. The Fulbright Foundation supported a research stay at the University of Notre Dame.

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Correspondence to Hrvoje Kozmar.

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Kozmar, H. Scale effects in wind tunnel modeling of an urban atmospheric boundary layer. Theor Appl Climatol 100, 153–162 (2010).

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  • Wind Tunnel
  • Turbulence Intensity
  • Reynolds Stress
  • Atmospheric Boundary Layer
  • Vortex Generator