1 Corresponding author: Tel. + 31 527 248662; Fax. + 31 527 248582; E-mail address: eddy.willemsen@dnw.aero

Design for wind comfort in The Netherlands:

procedures, criteria and open research issues

Eddy Willemsena,1

Jacob A. Wisseb

aGerman-Dutch Wind Tunnels (DNW), P.O. Box 175, Emmeloord, the Netherlands

bEindhoven University of Technology, P.O. Box 513, Eindhoven, the Netherlands

Abstract

A code for the assessment of wind comfort and wind danger in the built environment in The Netherlands is presented.

The code is not another legal building requirement, but a helping hand to incorporate wind comfort in a building

programme. It regulates technical procedures and some quality control. A decision scheme indicates which building

plans need professional assessment by wind engineers.

Local wind speed data are predicted from wind tunnel or CFD simulations. The code puts requirements to the

providing institutes to ensure a good quality of their data.

The statistics of a reference wind speed at the building location is needed to calculate the frequency of occurrence that

a threshold wind speed is exceeded. The code prescribes a method to calculate the statistics of the reference wind speed

at a height of 60m. This method is built in a software package and incorporates the most up-to-date data from a

network of meteorological stations in The Netherlands.

Five grades of wind comfort A-E are distinguished and defined by the frequency of occurrence that the local wind

speed exceeds a threshold value of 5 m/s for the hourly mean value. For three different activity areas these grades of

wind comfort are assessed in terms of a poor, moderate or good local wind climate.

The code is unique in its kind and eliminates possible differences in the approach of wind comfort assessment by

prescribing the underlying procedures.

Keywords: wind comfort; comfort criteria; comfort evaluation; wind tunnel comparison; town planning

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1. Introduction

Growing awareness by the general public and local authorities on wind comfort in

public space, a considerable increase in high rise building and a confusing diversity of

applied wind comfort criteria called for an initiative. Local authorities, architects, city

planners, project developers and scientists were invited by The Netherlands

Normalisation Institute NEN to a workshop. This workshop established a need for a

code on wind comfort assessment. Consequently, NEN conducted a project in close

cooperation with eight Dutch cities, three wind tunnel institutes and many other parties

concerned.

The present paper presents the final results of this project, which resulted in the

publication of NEN 8100, March 2006. This code is not a legal building requirement

but offers a strict method to incorporate wind comfort in a building programme.

It is in line with the present day practice in The Netherlands, where since about 30 years

three wind tunnel laboratories have been testing building plans for (mechanical) wind

comfort. They have developed a rapport with their clients and received feed-back from

them. The validations of building plans were based on wind comfort criteria by Lawson

& Penwarden (1975) and introduced to the Dutch market by Visser (1980). However, in

later years several cities adapted different criteria for reasons of clearness and

comprehensibility to users and the general public, as well as adaptations for building

near the coastline.

The present code is based on the non-documented experience of these laboratories and

of course on the state of the art acquired by the international wind engineering

community. Apart from published research, much has been learned from work within

and outside the framework of the EC COST-C14 project, entitled The Impact of Wind

and Storm on City Life and Built Environment (see www.costc14.bham.ac.uk). A

summary of wind comfort criteria used by eight participating laboratories is given in

Koss (2004) and in Sanz-Andres and Cuerva (2004).

2. Wind comfort analysis, yes or no

Architecture and urban planning with respect to the wind climate is in its infancy. Urban

wind climate design virtually does not exist. Designers, their clients and local

authorities have very little notion of the relation between urban geometry and pedestrian

comfort. This state of affairs is realistic because of three issues.

Firstly, the perception of urban space by the public is badly understood. Sociological,

cultural and psychological influences are of importance as well as exposure to rain,

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sunshine and wind. Urban designers do not know professionally to what extent

pedestrian comfort is important.

Secondly, the correlation between urban geometry and local climate is poorly

documented, even in wind engineering literature. Available literature is

incomprehensible to designers. Including urban climate into their design is unrealistic if

the subject is a closed domain of technical people.

Thirdly, these technical people may advise expensive alterations or additions, which

may even be ugly or unpractical.

The new Dutch code NEN 8100 does not bridge the gap between designers, their clients

and authorities. It aims to bring some order in the field by giving decision rules to

discriminate between building plans which do or do not require action with regard to

wind comfort. It offers assistance in making a building programme by offering a format

for requirements with regard to urban wind climate. Parties may agree on a level of

wind comfort to strive for. The code also includes requirements to obtain a good quality

wind tunnel or CFD simulation.

The decision scheme that indicates which building plans need professional assessment

by wind engineers is based on experience and on research from Beranek & Van Koten

(1980) and Bottema (1993). The decision scheme discriminates between building height

smaller than 15m and 30m as well as sheltered or unsheltered buildings. For sheltered

buildings up to a height of 15m no further evaluation regarding the wind climate is

necessary. For sheltered buildings with a height between 15m and 30m and for

unsheltered buildings up to a height of 30m the assessment from an expert is necessary

to decide whether or not a CFD or wind tunnel investigation is necessary. For buildings

higher than 30m such an investigation is always necessary.

A building and its direct environment are sheltered when for all wind sectors the

following conditions are satisfied:

the local surface area is covered for 20% or more with obstacles like trees (tops) and

buildings up to a radius of 300m,

the building is lower than 150% of the average height h of the obstacles within a

radius of 300m,

the distance from the building to the surrounding obstacles does not exceed ten

times the average height h.

Stathopoulos (2006) mentions some comparable criteria for Montréal, where an

exclusion clause for low buildings is dealt with by specifying a minimum height (like

the 15m mentioned above) and adding the condition of exposed height (building height

higher than the height of the surrounding buildings) to be more than a third. In addition

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there is an intermediate category of buildings that requires attention but not necessarily

a boundary layer wind tunnel study. In such cases a report from a wind engineering

expert is generally sufficient.

A test is a full assessment which includes wind tunnel or CFD simulation of the

building plan, evaluation of alternative geometries or additional measures. Generally,

remedial measures are decided in dialogue between wind engineer and client.

3 Methodology

The authors discussed the methodology to assess wind comfort in Willemsen and Wisse

(2002); see also Blocken et al. (2004). Wind speed ratios CV between pedestrian level

wind and a relevant reference wind are determined as function of the natural wind

direction ß in a wind tunnel experiment (or through CFD calculations).

)ß(V

)ß(V)ß(C

WT,REF

WT,PED

WT,V and )ß(V

)ß(V)ß(C

IS,REF

IS,PED

IS,V (1)

VPED is the wind speed at pedestrian level and VREF the wind speed at a reference

height. The subscript “wt” denotes the wind tunnel and “is” the in-situ condition.

In The Netherlands time-mean wind speeds are used. There is a pending issue about the

optimal wind parameter: mean wind speed_

V , gust duration and gust speed or an

effective wind speed VEFF. An effective wind speed may be defined as follows:

σ.k_VVEFF (2)

σ is the standard deviation and V/σ the turbulence intensity. In literature k has a value

between 0.0 and 3.5; see e.g. ASCE (1999). Heated thermistors are used in The

Netherlands as measuring devices. These sensors provide some kind of a time-mean

effective wind speed with yet unknown response to the dynamic wind speed component,

i.e. unknown k-value. However, the existing experience with wind comfort assessment

is based on this technique (Visser, 1990), making further refinements redundant.

In a perfect simulation the calculated CV (ß) values are the same as in-situ. In-situ

pedestrian level wind speeds can then be calculated:

)ß(V)ß(C)ß(V IS,REFWT,VIS,PED (3)

Quality criteria for wind comfort consist of the combination of a threshold wind speed

VTHR and a probability P that a local wind speed VPED,IS exceeds the threshold value

VTHR. At a certain level of P the public will experience the location as not comfortable.

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The frequency distribution of the pedestrian level wind can be calculated from the wind

tunnel or CFD results with Equation 3, if the frequency distribution of the in-situ

reference wind speed is known. Consequently, the probability that a threshold wind

speed VTHR is exceeded can be calculated, i.e. P(VPED,IS >VTHR).

There exists a fairly good correlation between P(VPED,IS >VTHR) and VTHR values, which

has been discussed by Koss (2004) and Sanz-Andres and Cuerva (2004). The present

authors analyzed a rather common wind comfort investigation in a wind tunnel

(Willemsen and Wisse, 2003). Figure 1 shows the sensitivity of the process. When at

5 m/s the VTHR value is changed with only 0.5 m/s (i.e. 10%), the P(VPED,IS >VTHR)

value is changed with 25% (for example: a P-value of 16% might change towards 12%

or 20%, depending on an increase or decrease of the VTHR-value).

Figure 1 demonstrates that a threshold wind speed is a relative, arbitrary criterion. No

information is available on either the acceptable degree of discomfort or the acceptable

maximum percentage of the people in public space that experience discomfort.

The authors derived figure 2 from data presented by Jackson (1978). We may conclude

that a mean wind speed of 5 m/s indicates an above-average degree of wind discomfort,

say two of every three passers-by. We do not know what level of discomfort is

acceptable to the public. Neither in terms of a score, nor in terms of mean wind speed.

Inherently, a maximum probability cannot be defined in an absolute sense. VTHR and

PTHR(VPED,IS>VTHR) are values on a relative scale.

4 Criteria

The current practice by some European laboratories has been discussed by a European

working group of the late Cost-C14 project. Results are reviewed and analyzed by Koss

& Sahlmen (2002) and Delpech et al. (2005). It appears that the range of criteria used

can be partly explained by the dependence between threshold wind velocity and

maximum probability of occurrence; see fig.1.

To set criteria for mechanical wind comfort is to choose a value for k in equation 2 as

well as values of VTHR and the maximum value for the probability P(VPED,IS >VTHR).

These choices have to depend on the foreseen function of the public space or

alternatively of a certain grade of wind comfort to be achieved. The choices made in the

code NEN 8100 are given in table 1. The choices are discussed in Willemsen & Wisse

(2003). To summarize: because literature is inconclusive and codification has to be a

consolidation of current practice, k is chosen to be zero. This means that the code is not

based on criteria of for example Hunt (1976), Gandemer (1978), Melbourne (1978) and

others. There methods require information on the standard deviation, which is not

measured in a standard wind climate investigation in a wind tunnel in The Netherlands.

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A prognosis of the standard deviation in an urban canopy is difficult; see for example

Rotach (1995). Isyumov (private communication, 2003) stated that in general the mean

wind speed is a good indicator, but in his experience discomfort in some cases could

only be explained by gustiness. A 3-second gust is used by Murakami (1986). In the

code NEN 8100 the threshold in the mean wind speed is 5 m/s for all grades of wind

comfort and 15 m/s for danger by wind forces on pedestrians. Longstanding practice in

The Netherlands is based on this mean wind speed, which had been chosen after

Lawson & Penwarden (1975). Too little is known about the relation between the

probability P(VIS>VTHR) and the perception of comfort and danger by the public. No

documentation is found on its dependence of the local weather climate, apart from some

intuitive statements.

The present paper is relevant for mechanical pedestrian comfort only.

The choice of the probability P(VIS > 5 m/s) in NEN 8100 is based on feed-back by

clients, especially by developers of shopping-centres. The code defines five grades of

wind comfort A-E. The corresponding P-ranges are in agreement with the attainable

overall accuracy in wind tunnel simulations. The maximum shift in comfort grade due

to possible errors is one grade. The acceptable P(VPED, IS >VTHR) values are dependent

of the foreseen function of the public. The code specifies grades of comfort, not

functions of the urban place. Firstly, urban places such as building entrances or squares

are meant for different use by all kinds of public. Secondly, it is preferable to leave the

decision on the demanded grade of comfort to the client, the local authorities and the

urban planners.

Quality criteria may be mathematically precise and physically correct but will always

also be used for ill-defined applications. Civil officers need clarity and precision.

Therefore, an explaining consultant who can assess the situation at hand is necessary.

5 Quality of wind tunnel and CFD simulations

Wind tunnel experiments have a long standing history of feedback from building

practice. CFD is a newcomer and still in the stage that it has to prove itself. It is

considered unpractical to exclude the newcomer. For both techniques specific

knowledge and expertise is required. Three quality demands are defined for the

involved institutes, offices and alike. The first demand is sufficient capabilities and

education of the involved personnel, regarding among others aerodynamic theory, wind

climate and modelling techniques. The second demand is the production of a quality

document in English, to be updated every five years. The quality document should

address issues like description of the wind tunnel or CFD-code, modelling of the

atmospheric boundary layer, quality of the applied instruments, procedure for defining

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the model characteristics, and alike. The third demand is the completion of a technical

information sheet for every measurement or calculation, which should be attached to the

final report to the client. It shall include prescribed items and contain the most relevant

information regarding the model, test set-up or grid definition, data management and

deliverables. The purpose of this form is to allow a review by the client.

6 Climatological input

To calculate P(VPED, IS >VTHR) from values of CV, WT (ß) generated by a wind tunnel or

CFD simulation, a climatology of VREF, IS is needed; see equation 3. Therefore, data of at

least one meteorological station has to be transformed into probabilistic wind data at the

building site. This transformation is discussed by Willemsen & Wisse (2002) and by De

Wit et al. (2003). Before the new code was accepted in The Netherlands, a rather wide

range of methods was in use in choosing the appropriate meteorological station and to

transform its data to values for the location of interest. To eliminate the corresponding

differences in the final assessment of the wind climate at pedestrian level height, a

computer application was custom made by the Royal Netherlands Meteorological

Institute KNMI to form one of the pillars of NEN 8100. The code prescribes this

improved and standardized method. The output of the computer application is statistical

wind data at the project location at a reference height of 60 meter.

The KNMI application is a spin-off of a project called HYDRA, whereby an update of

the wind climate statistics all over the country, including the water-land (and vice versa)

transition zones was obtained. The project was funded by the National Institute for

Coastal and Marine Management (RIKZ) and the Institute for Inland Water

Management and Waste Water Treatment (RIZA) in The Netherlands, who are legally

obliged to redo their risk assessment of the Dutch dike systems regularly. On the

IJsselmeer, the Waddenzee and parts of the Zeeland waters wind is the main generator

of waves. Available wind speed records were re-analysed and updated, resulting in a set

wind climate data of high standard. The initial focus of the HYDRA project was on

extreme wind speeds, but could easily be translated to yearly mean wind speeds.

For the transformation of the wind statistics from one place to another a two-layer

model, with a neutral surface layer wind profile up to 60m and a geostrophic drag

relation from Deacon (1973) was used. Internal boundary layer development is

modelled following Panofsky and Dutton (1984). A description of underlying principles

is given by Verkaik & Smits (2001), as well as by Wieringa (1976, 1986). The wind

speed at an arbitrary location is estimated from wind velocity statistics at upstream

meteorological stations, implying that the choice of the meteorological station varies

with wind direction. Mesoscale roughness lengths are determined from high resolution

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land use maps, obtained from satellite images with a resolution of 25 meter. The land

use map distinguishes 46 different zo-values, between 0.0002m for the muddy flat

condition and 1.6m for a built-up urban area. Figure 3 shows a land use map for a

building site near Schiphol Airport, illustrating the high level of input data.

7 Open research issues

Drafters of the code are aware of the relative importance of consulting with respect to

wind comfort in urban planning. In fact, urban planners have to satisfy very complex

programmes in an environment of local politics and economy. To include urban physics

in their design is difficult. Research and demonstration projects are needed. Application

of wind engineering to problems in urban planning is born out of a definite need to

analyse and improve too windy places. Research has been mainly conducted by wind

engineers in order to find a more firm basis for their consults. What is needed to

improve our contribution to urban planning is cooperation with urban planners and

architects in a collective research in the perception of urban space. The effect of very

different parameters has to be quantified. Perhaps, at the end, we might get some

information on the maximum acceptable probability P(VIS > VTHR) if sociological,

psychological, cultural and architectural features as well as patterns of the use of urban

space are compared to the impact of weather parameters in a scientific way. Moreover,

the experience of wind engineers in consulting in urban planning and the feedback of

their clients is not objectively documented and analyzed. If so, we could learn the value

of this experience. A European working group of the late Cost-C14 programme did set

up a project to compare the criteria used by different European laboratories. Partly, this

was discussed by Koss & Sahlmen (2002) and Delpech et al. (2005). Research on

thermal wind comfort has been started; see e.g. Sasaki et al. (2000) and Stathopoulos et

al. (2004). This may widen the perspective of the contribution of physics to the

appreciation of an urban place to urban planning. However, in the opinion of the present

authors it is not likely that new design rules will emerge. If temperature and humidity

can be designed at all, it is by planning of green areas and water surfaces at a much

larger scale than urban design; in contrast to local wind and radiation which are

dependent of local urban geometry.

Even more specific for wind engineers is research into the aerodynamics of urban flow.

What is a roughness length for cities in dependence of their geometry? Is it possible to

define roughness lengths for cities at all?

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8. Acknowledgements

The initiatives and management of the Netherlands Normalisation Institute NEN was

essential for the success of the codification project. Especially the endeavours of

Ir. R. Nouwen should be mentioned. Also the contributions of experts and their

institutes of TNO, NLR/DNW, Peutz and TU/e were indispensable. The project has

been funded by the cities of Amersfoort, Amsterdam, Lelystad, Rotterdam, The Hague,

Tilburg, Utrecht and Maastricht

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Bottema, M., 1993. Wind climate and urban geometry. PhD thesis Technical University Eindhoven,

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Figure captions

Figure 1 Correlation between frequency of occurrence and threshold wind speed

Figure 2 Discomfort score versus mean wind speed

Figure 3 Land use map near Schiphol Airport

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Table 1 Criteria for wind comfort and danger

wind comfort

P(VIS>5m/s)

in % hours per year

activity area

grade business

walking

pedestrian

walking sitting & standing

< 2.5 A good good good

2.5–5.0 B good good moderate

5.0–10 C good moderate poor

10–20 D moderate poor poor

> 20 E poor poor poor

wind danger

P(VIS>15 m/s) limited risk 0.05-0.3 % hours per year

dangerous > 0.3 % hours per year

13

Figure 1

Figure 2

0

20

40

60

80

100

0 1 2 3 4 5 6 7

mean wind speed [m/s]

dis

co

mfo

rt s

co

re

14

Figure 3

Figure 3 (in colour; for web site use only)