Actiflow performs around 100 studies per year to advise governments, architects and developers about wind in the urban environment. An example of a wind study as performed by Actiflow, is the wind study for the Amstel III area in Amsterdam, performed according to the Dutch NEN8100 norm, using CFD computer imulations.

In a CFD study, a three dimensional computer model is built of a part of a city, and OpenFOAM is used to simulate the wind behavior in the entire city area for varying wind directions. The simulation results are then combined with the local wind statistics, in order to produce maps with local wind hindrance classes and local wind danger classes. The wind hindrance classes are defined by the probability of exceeding a local wind speed of 5 m/s. From a functional point of view, the wind classes describe whether a certain location is suitable for “staying for a long period in time”, “strolling” or “walking through” (see Table 1). To determine the risk for wind danger, the NEN-8100 norm looks at the probability of exceeding a local wind speed of 15 m/s. (See Table 2)

Table 1. Definition of wind hindrance classes

Table 2. Definition of wind danger classes

Amstel III is an area of approximately 1 km2 at the south-east of Amsterdam. Currently the area is mainly occupied by offices. The average height of the current buildings is roughly 25m, and the buildings typically have a square footprint.

The municipality of Amsterdam has the desire to transform this area into a lively urban district with mixed residential, office and recreational use. Between now and 2027, roughly 5.000 new houses will be realized, together with shops and new office spaces. Obviously, the municipality also envisions a comfortable outdoor space in this area. To realize the goals of the municipality, a significant amount of building volumes will be added to the area, leading to the introduction of high-rise buildings and a denser urban fabric, as shown in figure 1. In the early planning phase, the municipality wanted to know if this increase in building volume would lead to problems concerning wind.

Figure 1. Overview of the Amstel III region near Amsterdam

In Figure 1, the wind hindrance map is shown for the simulated area. The results are shown in a cross section which is 1.75m above ground level. In the map it can be seen that there are several significant regions of wind class D and E, which have to be avoided in public areas.

Figure 2. Wind hindrance classes in the Amstel III region

An important reason for these problematic areas is the orientation of the existing street plan. A large part of the streets is straight and SW-NE oriented. As the dominant wind direction in the Netherlands is SW, the wind from this direction can accelerate along the streets and cause problems at the corners of several buildings. A second reason is the addition of high- rise buildings. The wind that flows over the older (lower) buildings hits the façade of the high-rise buildings, and then causes wind hindrance on pedestrian level through the downwash effect. On the other hand, there are also enough regions with wind class A and B, which are perfectly suited for outdoor sitting and outdoor activities.

Next to the wind comfort, also the risk for wind danger is plotted according to the NEN8100 (Table 2). Wind danger needs to be avoided at all times. In Figure 3, it can be seen that there 2 zones with significant wind danger, which needs to be mitigated. Apart from that, there are also zones with limited risk for wind danger. In consultation with the municipality of Amsterdam, it was decided to also try to mitigate these zones. As the zones with (limited) wind danger are the same as the zones with wind comfort class D and E, local mitigations will improve the wind comfort and reduce the risk of wind danger.

Figure 3. Wind danger classes in the Amstel III region

Based on the results of the wind study, the following recommendations are formulated:

• As the dominant wind direction is SW, it is advisable to have rather low buildings in the SW part of Amstel III, and to slowly increase the maximum height of the buildings towards the NE.
• Critical buildings should be placed on a plinth or should have canopies that reduce the effect of the downwash on pedestrian level.
• Preferably, buildings should not be placed in a square grid, but rather in an irregular pattern, to avoid accumulation of wind from the SW.
• Design the public space as much as possible in accordance with the governing wind classes.
• Make use of natural elements like trees, hedges and other vegetation to break down the wind as much as possible.

Based on this study, the city of Amsterdam formulated guidelines and requirements for real estate developers in the Amstel III area. Each developer now has the obligation to prove in an early design phase that the design of their building(s) meets the requirements in terms of wind comfort. This is where CFD or Actiflow helps managing pedestrian level comfort in the built environment.