Journal of Heat Island Institute International Vol. 9-2 (2014)

Academic Article

First Results of High Resolution Large-Eddy Simulations of the
Atmospheric Boundary Layer
M. Keck*1

S. Raasch*1

M. O. Letzel*2

E. Ng*3*1 Institute of Meteorology and Clymatology, Leibniz Universität Hannover, Germany*2 Lohmeyer Consulting Engineers, Karlsruhe, Germany
3* School of Architecture, Chinese University of Hong Kong, Hong Kong
Corresponding author: M. KECK, keck@muk.uni-hannover.deABSTRACT
As one of the most densely populated cities in the world Macau has to face particular difficulties in terms ofcreating a comfortable living environment. Narrow streets and high-density districts significantly reduce the naturalventilation. Especially in Macau's humid subtropical climate the natural ventilation should be used to decrease
thermal stress. As an aggravating factor, Macau's population is still growing. In order to create new habitable space,five reclamation areas are currently being built around Macau's present coasts. This study investigates thepedestrian level ventilation in Macau using the parallelized large-eddy simulation model PALM. The aims of thestudy are a) to evaluate the impact of the planned reclamation areas on the ventilation within the present citydistricts, b) to investigate how urban morphological quantities, e.g. the surface cover fraction, affect the ventilationin cities and c) to show the advantages of urban LES over RANS simulations. The results reveal that the planned
reclamation areas might only have a small impact on the average ventilation within the present city districts ofMacau.
Key Words: Urban Boundary Layer, Large-Eddy Simulation, PALM, Urban Climate, Thermal Comfort

1. Introduction

repeated including the planned reclamation areas. Comparisons

Macau is one of the most densely populated cities in the world.

detailed information about their effect on the ventilation. WithThus, it is a particular challenge to create a comfortable living

this information suggestions for a better alignment of theenvironment. Especially in Macau's humid subtropical climate

buildings in the planned areas could be made. Furthermore, thepeople are likely to suffer from thermal stress. Thermal comfort

gustiness in some selected public places and parks ismostly depends on wind velocity, air temperature, radiation, andinvestigated. For this purpose, virtual measuring stations areof the simulations with and without the reclamation areas give(1)

humidity . Even in densely populated cities the velocity ratio,placed in those selected locations.

defined as the ratio of wind velocity at a height of 2m above theAlthough using Reynolds-averaged (RANS) models is the mostground to the undisturbed velocity of the free flow above the city,common approach to simulate flows around obstacles,

can be significantly improved by an appropriate alignment oflarge-eddy simulation (LES) is used in this study. Using an LESbuildings(3).

model is far more expensive than using a RANS model, but it

In order to extend the urban area, the government of Macau is

has two advantages: In contrast to RANS, LES does not only

about to create five new reclamation areas around Macau's

supply the averaged mean flow field but also resolves thepresent coasts (Fig.1). The goal is to align the buildings in the

instantaneous turbulent fields. In the urban canopy layer these

new areas in such a way that the air ventilation within the cityturbulent fields are of particular importance, because the comfort

does not change for the worse.

of pedestrians does not only depend on the mean air ventilation,

In the present study, the turbulent wind field in Macau isbut also on the occurrence of gusts. For example, an area withsimulated and analyzed to get a general impression about the air

low mean wind velocities still might be considered asventilation within the city. Afterwards, the simulations arecomfortable, if the wind is gusty and exceeds a certain threshold- 39 -

only very few changes.
3. SetupThis study is restricted to neutral stratification. For all
simulations a geostrophic wind of 1.5m/s is prescribed. Cyclicboundary conditions are used in spanwise direction. In
streamwise direction non-cyclic boundary conditions are applied,so that the wind leaving at the outflow does not re-enter at theinflow. Thereby, the buildings and hills within the model domain
do not affect the flow approaching the city. In order to guarantee
a turbulent inflow, a turbulence recycling method is used(7). Byusing this setup the model domain can be kept quite small.
Sensitivity studies showed that a topography-free zone of about
1.5km width is required upstream of the city, in order toguarantee a good performance of the turbulence recycling
method. This topography-free zone is still shorter than it would
have been if cyclic boundary conditions were being used,because they require a large topography-free buffer zone
upstream of the built-up areas, in order to prevent that theFig. 1 Map of the Peninsula de Macau and the northern part ofbuilding induced turbulence re-enters at the inflow and affects

Macau's southern island. The planned reclamation areas are

the turbulence structures approaching the city.

highlighted in orange.

All simulations cover a time span of 6h and all time-averaged
values are averaged over the last 5 hours of a simulation. That isvalue from time to time. This kind of data cannot be provided bybecause the turbulence needs approx. 1h to properly develop andRANS at all and has to be estimated with empirical laws.

5h proved to be the shortest statistically representative averaging

Another advantage is that even the mean flow around obstacles

time.

(4)

is calculated more accurately with LES compared to RANS .

Sensitivity studies concerning the grid size have been performed.
Figure 2 a) shows the Frequency distributions of the velocity
ratio for a simulation with 4m grid spacing and a simulation with

2. Model description

2m grid spacing. It can be seen that very low and very high
The LES code used for this project is the Parallelized

velocity ratios occur more often if a 4m grid spacing is used

LES-Model (PALM), which has been developed at the Institutebecause the flows in narrow streets are not resolved sufficiently.

of Meteorology and Climatology of the Leibniz Universität

Figure 2 b) shows that the differences between 2m and 1m grid

(8)

Hannover since 1997 . The basic model is based on thespacing are quite small. These sensitivity studies demonstrated

non-hydrostatic, incompressible Boussinesq equations, the firstthat using the initially desired grid size of 1m is not reasonable,

law of thermodynamics and an equation for subgrid-scale

since simulations with 2m grid spacing provide sufficiently

turbulent kinetic energy. The aforementioned equations areresolved results and are about 16 times cheaper. Nonetheless, afiltered implicitly using the volume-balance approach ofsimulation with a resolution of 2m still requires an immense

Schumann(9). The 1.5-order Deardorff scheme is used foramount of computational power.

(2)

turbulence closure . At each surface, both horizontal andThe model domain for such a simulation contains more thanvertical, a Prandtl layer is assumed. For the time-integration a

2.5*109 grid points and each run takes about 45h on 4096

3rd-order Runge-Kutta scheme is used. The advection scheme

processor cores of an SGI Altix ICE 8200 Plus to simulate a timeused is a fifth-order scheme which was developed by Wicker andspan of 6h.

(10)

Skamarock.

In this study the velocity ratios vr for the four most occurring

PALM provides high performance on state-of-the-art massively

wind directions in Macau are investigated (S, SSE, SE and N). vr

parallel computers which is mandatory to simulate the turbulent= vp / v∞ is used as a ventilation indicator and is defined as thewind field for whole cities. It is validated for flows around solid

ratio of the wind velocity vp at pedestrian level, 2 m above

obstacles(5) and has already been used to study the wind comfort

ground, and v∞ near the top of the model domain (z = 2.5km)

in city districts of Hong Kong(6). Therefore, the code required

where the flow is no longer affected by the city. The model- 40 -domain for the simulations with southeast wind covers about
8x6km² and only contains the Peninsula de Macau (see Fig. 3).
In these simulations a grid spacing of 2m is used. The model
domains for all the other simulations cover about 16x8km² andalso include Macau's southern island. Due to the very large
model domain the grid spacing had to be coarsened to 4m for thesimulations including the southern island.
4. Results
The results of all simulations show that the time averaged windvelocities at pedestrian level are spatially highly variable. In
many cases vr demonstrates high values for streets aligned witha)

the mean wind and low values for those streets perpendicular to
the mean wind direction. However, the results also reveal that
there are numerous exceptions, which are proof for thecomplexity of an urban canopy flow.
Figure 4 shows the effect of the planned reclamation areas on vrfor wind from southeast. For this wind direction the reclamation
areas showed the greatest impact on the flow field of the Macau
Peninsula. The model domain is rotated so that the wind enters
the domain from the left. The strongest effect can be seen in thewake of the reclamation area A at the bottom of the graphic,
where the ventilation is strongly reduced. However, the areas in
the already existing parts of Macau that will be affected by theb)reclamation areas are quite small. In some of these parts theFig. 2 Frequency distributions of the velocity ratio; a)ventilation will even increase, due to channelling andcomparison between 4m and 2m resolution, b) comparison

overflowing. The other simulations confirm that the planned

between 2m and 1m resolution.

reclamation areas only have a small effect on the ventilation
within the already existing districts, regardless of the winddirection.
Figure 5 shows that there is a linear correlation between vr anda) the surface cover fraction (SCF) and b) the frontal area
density (FAD). SCF is defined as the area covered withbuildings divided by the total surface area, and FAD is the areaof all building walls facing the mean wind direction divided by
the total surface area. For low vr values there seems to be asecond linear correlation, which can especially be seen in Figure
5 b). All of those data points with very low vr values belong toan area around a small but steep hill. This leads to theassumption that the linear correlations between vr and FAD (andalso vr and SCF) depend on the complexity of the orography, i.e.
that the linear correlations in Fig. 5 (green lines) are only validfor flat terrain. The impact of complex terrain on the correlations
between vr and SCF, and vr and FAD is currently being
investigated. All values were averaged over an area of 1km2. InFig. 3 Model domain of the simulation with southeast wind. Theorder to get more data points, the averaging areas overlap. This

topography is rotated so that the wind enters the model domain

way it is also guaranteed that results are not influenced byfrom the left side.

random positioning of the averaging areas. Yoshie et al. found asimilar linear correlation between the SCF and vr using wind- 41 -

tunnel experiments and field measurements(11). Furthermore, theresults showed that the building volume per surface area is alsolinearly correlated to vr (not shown in the figures).
In order to investigate the additional benefits of LES over RANS
simulations 54 virtual measuring stations were place in themodel domain. These stations saved time series of the local
velocity ratios. Figure 6 shows as an example the averagevelocity ratios in a small park as well as a wind rose createdfrom a time series that was virtually measured at a point in thepark (black dot). The average velocity ratio at the measuring
point is 0.11 and the average wind direction is SSE. However,
the wind rose shows that the wind blows mostly fromsoutheasterly directions but approximately 20% of the time also
from westerly directions. Moreover, it can be seen that velocity
ratios greater than 0.22 (double of the average value) aredetected more than 11% of the time.

Fig. 4 Relative deviations of between the results of a simulation
without and a simulation including the planned reclamation
areas for wind from the southeast; red: lower velocity ratios withreclamation areas, blue: higher velocity ratios with reclamation
areas; the wind enters the model domain from the left side.

a)

a)

b)
Fig. 6 a) Average vr in a small park for wind from the southeast.
The black dot shows the position of the virtual measuring station.

b)
Fig. 5: Correlation between vr and a) the surface cover fractionb) Wind rose of vr values detected at the virtual measuringand b) the frontal area density.

station. The wind enters the model domain from the left side.
- 42 -(10) L. J. Wicker and W. C. Skamarock, Time-splitting methods

5. Conclusions

for elastic models using forward time schemes. Mon. Wea. Rev.,
The study revealed that the effect of the planned reclamation

130, 2088-2097 (2002)

areas on the average wind velocities in the present Macau city(11) R. Yoshie, H. Tanaka, T. Shirasawa and T. Kobayashi,districts is rather small. Only in very few open places and wide

Experimental study on air ventilation in a built-up area withroads vr was significantly decreased. However, in some areas theclosely-packed high-rise buildings. J. Environ. Eng., 73,

wind velocities even increase due to channelling and661-667 (2008)

overflowing effects. Most parts of the city won't be affected by
the creation of the planned reclamation areas. Analysis of the(Received Nov. 30, 2014, Accepted Dec. 27, 2014)

output data showed that there are linear correlations between vrand SCF, and vr and FAD, respectively. For city planners these
correlations can be used as an orientation for the expected
average ventilation within city districts. The conducted virtual
measurements demonstrated that LES could be an important tool
for future urban studies, because it not only gives the averagewind velocities and directions but also information aboutvariances and peak values. The ongoing study is currently
extended for the effects of thermal stratification, in order tobetter capture the real meteorological conditions in Macau.
References(1) V. Cheng and E. Ng, Thermal comfort in urban open spaces
for Hong Kong, Architectural Science Review, 49, 236-242(2006)(2) J. W. Deardorff, Stratocumulus-capped mixed layers derived
from a three-dimensional model, Bound-Lay. Meteorol., 18,495-527 (1980)(3) J. Hang, Y. Li, M. Sandberg, R. Buccolieri and S. Di
Sabatino, The influence of building height variability on
pollutant dispersion and pedestrian ventilation in idealized
high-rise urban areas. Building and Environment, 56, 346-360(2012)(4) M. O. Letzel, M. Kanda and S. Raasch, A new dimension ofurban climate modelling with parallel large-eddy simulation. 6thInternational Conference on Urban Climate, 296-299 (2006)(5) M. O. Letzel, M. Krane and S. Raasch, High resolution urban
large-eddy

simulation

studies

fromstreet

canyontoneighborhood scale. Atm. Env., 42, 8770-8784 (2008)(6) M. O. Letzel, C. Helmke, E. Ng, X. An and S. Raasch, LES
case study on pedestrian level ventilation in two neighbourhoodsin Hong Kong. Meteorol. Z., 21, 575-589 (2012)(7) H. Kataoka and M. Mizuno, Numerical flow computation
around aeroelastic 3D square cylinder using inflow turbulence.
Wind and Structures, 5, 379-392 (2002)(8) S. Raasch and M. Schröter, PALM – A large-eddy simulation
model performing on massively parallel computers. Meteorol. Z.,
10, 363-372 (2001)(9) U. Schumann, Subgrid scale model for finite difference
simulations of turbulent flows in plane channels. J. Comp. Phys.,
18, 376-404 (1975)- 43 -