Geoscience Reference
In-Depth Information
N wind is systematically channeled. If we look back to Figure 5.13 (e.g. point x),
where PET is expressed, we see that these are the areas that remain relatively cool in
warm nights and also cool on windy summer nights due to higher wind speed. For
PET calculation, wind was modeled using another software (
ENVI-met
) and when
wind is channeled its speed increases; so the wind tunnel experiment has also helped
to validate
ENVI-met
simulations. G-shaped buildings create particularly sheltered
areas. Inside the courtyards (such as y on Figure 5.16) airflow is trapped and there is
no air renewal; there will only be eddies and recirculation of the same air, that may
move pollutants and particulate matter. As some of these courtyards are used for car
parking, one may guess that the air quality inside them will frequently be poor (no
measurements to prove it as yet).
Numerous other experiments have been conducted by Lopes [LOP 03] for the
three prevailing wind directions: N, NW, and W. Figure 5.16 synthesizes the main
airflows and indicates clearly the areas where circulation is more disturbed. In most
cases, airflow within the city district is similar when gradient wind blows from the N
and NW, so the same arrows refer to both gradient wind directions. Western
gradient wind originates other flow directions in the city district. It is noticed that
the worst ventilated areas (inside G-shaped buildings) correspond to the areas where
sand accumulation persisted during the experiments and where whirls as well as
(bad quality) air recirculation occurred. Advice should be given to transform the
parking lots into green spaces, in order to improve air quality.
The I and L typologies led to wind channeling following the same paths,
although parts of the courtyards remain in the “wind shadow” and generate sheltered
areas. In the main W-E street, N and NW gradient winds blow from the W, after
being channeled between the windward L- and I-shaped buildings. As referred
above, the tower-like buildings allow airflow, although causing deviation as the
wind diverges upwind of each tower and converges downwind. The experiments
with NW wind have shown that the wind speed will decrease not only due to urban
roughness, but also to the fact that the wind forms a 45º angle with the prevailing N-
S direction of the buildings. When the wind blows from the W direction the
existence of the N-S highway (
Eixo Norte-Sul
), which is higher than the surrounding
areas, causes eddies near the slopes of the embankment on which the highway is
built. The tower-like buildings hinder W wind circulation more effectively than N
and NW winds. Wind measurements inside the scale model with Pitot-Prandtl tubes
have helped to validate several of the previous experiments. It was shown that the
increase of z
0
from 0.02 to 1.5 m in the windward side of Telheiras city district
originates a severe drop of near ground wind speed. Wind profiles performed in the
scale model showed that when z
0
changes
from 0.02 to 1.5 m there is a reduction of
46% in wind speed near the ground, 30% at 50 m high and only 10% at 200 m.
The different experiments show clearly that if the city continues to grow, the N
wind circulation will be progressively hindered inside this city district (and others in
similar geographical position), with severe negative impacts on inhabitants comfort
and health.
