Urban Heat Island (UHI) CFD Simulation on a Real Zone

Description

Introduction

The study of Urban Heat Island (UHI) is one of the important issues in the world and is popular among researchers. We investigate these issues using CFD analysis.

Auckland is a large metropolitan city on the North Island of New Zealand. The most populous urban area in the country and the fifth largest city in Oceania.

Auckland lies between the Hauraki Gulf to the east, the Hanau Ranges to the southeast, Manuka Harbor to the southwest, and the Waitakere Ranges. The surrounding hills are covered in rainforest, and the landscape is dotted with 53 volcanic Fields.

Auckland has an oceanic climate. Meanwhile, according to the National Institute of Water and Atmospheric Research (NIWA), its climate is considered subtropical. This city has hot and humid summers and mild and humid winters.

It is the warmest main center of New Zealand. The average daily maximum temperature is 23.7 °C (74.7 °F) in February and 14.7 °C (58.5 °F) in July. The maximum recorded temperature was 34.4 °C (93.9 °F) on 12 February 2009. But, the minimum temperature was −3.9 °C (25.0 °F).

Auckland occasionally suffers from air pollution due to fine particle emissions. There are also occasional breaches of guideline levels of carbon monoxide. While maritime winds normally disperse the pollution relatively quickly, they can sometimes become visible as smog, especially on calm winter days.

The mean high-pressure belt in the New Zealand sector of the Southern Hemisphere is centered near 30° S so that westerly winds predominate over the country.

On a day-to-day basis, however, there is great variability in the pressure distribution. Sometimes intense anticyclones occur in the country’s south with depressions to the north. They cause an easterly flow with the reversal of the usual weather pattern. These blocking situations may be rather persistent. They interrupt the more common westerlies associated with the eastward progression of weather systems.

Urban Heat Island Description

The purpose of this simulation is to observe the effects of ocean winds on urban roads in Auckland city. Thus, we select the urban plan of Auckland University of Technology (UHI) for this issue to investigate.

This study aims to analyze wind comfort conditions in the area and find places where wind speed is higher than 3.8 m/s. For thermal comfort analysis, the campus area should have no place with a temperature above 295 K.

All the buildings and structures in the space under consideration were extracted and named with Google earth pro (Fig. 1). With the map of the desired area in hand, the geometry and volumes corresponding to each building were plotted within the ANSYS Design Modeler software.

Figure 1: The exact position of the main domain surrounded by the subdomain

Figure 2: Dimensions of all domains

Wind Comfort

Step 1: 

The airflow over Auckland is predominantly from the southwest. This is particularly so in winter and spring, but in summer, the proportion of winds from the northeast increases. This arises from the changing location of the high-pressure belt, which is further south in summer and early autumn than in winter and spring.

In addition, sea breezes add to the proportion of easterlies in eastern areas in summer and early autumn. Figure 3 shows surface wind’s mean annual wind frequencies based on hourly observations from selected stations.

Mean wind speed data (average wind speeds taken over the 10 minutes preceding each hour) are available for several sites in Auckland, illustrating the region’s different wind regimes. Coastal areas (e.g., Auckland Airport) tend to be windier than sheltered inland areas throughout the year.

Figure 3: average wind speeds

Figure 4: average wind speeds

As urbanization grows, more population exists in jam city centers. Wind trouble, or unstable wind conditions, are more significant as an agent for location and structure designs, as these new tall constructions can lead to higher wind velocities and more powerful jet streams at the pedestrian level.

To appraise the pedestrian wind comfort in a specific place for existing or future urban areas, three origins of Historical meteorological data, Local wind conditions, and Specific comfort criteria need to be specified and then incorporated.

There are many basic scales available to assist in recognizing the anticipated wind comfort in the planning step by providing factors of what should be obtained to remain within desirable situations. Any construction, tall towers, short buildings, bridge, or tunnels, will affect its circumambient.

By determining pedestrian wind comfort, municipal employer designers can forecast the wind flow behavior around buildings while they are in the planning step. Considering wind comfort issues is imperative for urban renovations and designs. Figure 5 shows the table of wind comfort for different pedestrian circumstances.

Figure 5: Example of mechanical wind comfort criteria

Figure 6: wind speed 1m from the grand surface (m/s)

Table 1:

The contour of the velocity distribution of wind from southeast to northwest shows that the wind speed around the buildings that are directly exposed to the airflow even reaches 30 km/h at some point (black arrows), and it is not suitable at all with the criteria of measuring the comfort of the flow speed in the passages.

With the passage of air inside the passages, the speed also decreases and reaches about 10 km/h, which is a good amount. The speed felt by the wind in the last row of buildings is much lower (red arrow), and the wind is damped inside the passages.

Temperature Comfort

Step 1: Solar Radiation

The solar radiation is modeled using DO Model and based on geographical coordinates in the middle of February (13 pm).

Figure 7: Solar radiation in the domain

The results show that the taller building has absorbed more radiation and less radiation in the passages between them due to the shadow created by the buildings. The minimum solar incident radiation value is 1560 W/m2 (black arrow), and the maximum is 1700 W/m2 (red arrow).

However, there was no significant difference in people’s thermal sensations between different solar radiation levels in less shaded sites. The hypothesis was that visual comfort brought by shading could affect how people perceive thermal comfort in outdoor environments.

Step 2: Building’s Heat Flux

Most of the Auckland region experiences mean annual temperatures between 14 °C and 16 °C, with eastern areas generally warmer than western areas. They experience lower mean annual temperatures over higher elevations (e.g., Hanau Ranges; 12°C) due to the temperature decrease with altitude.

There is a deal of variability in this figure, with the high ground being relatively colder under windy conditions, while on cold nights’ hill tops may be warmer than the low ground because of cold air drainage. We show the areal variation of annual median average temperature in Figure 6.

Figure 8: Annual median average temperature

To see the temperature changes in the passages and the area, we show the temperature contour at 1.5 meters from the ground. The temperature of the surface of the earth is 286.15 K, and the temperature of the free stream is considered to be 288.15 K based on meteorological data. We use the DO model to apply the effect of solar radiation.

The amount of heat flux that is emitted from buildings, according to comprehensive investigations of building science, for an office building in normal conditions is equal to 0.6 w/m2, which is also applied to the walls of these buildings.

To analyze the temperature results, the temperature contour shows that the buildings heat their surroundings by applying heat flux and form the urban heat island phenomenon, in which in the face of the wind of 288.15 K.

Figure 9: Wind Temperature contour

table 2:

The buildings at the beginning of the route reach a suitable temperature. But the buildings in the back get a little less cold. The meeting hall is a building with a low height but with a larger area (specified with a black arrow).

Compared to other buildings, it cools down later and still experiences higher temperatures in a part of its roof, and the maximum temperature also occurs at this point.

Figure 10: Temperature in the domain. Left up 0.5m. Right up 1m down left 1.5m and downright 2m from the ground surface

table 3:

At a distance of 0.5 meters from the earth’s surface, it is, on average, 287.46 Kelvin. The warm environment between the buildings results from heat rejection from a roofed environment. The highest temperature is 295.77 K.

The average temperature increases slightly at one meter above the ground. It reaches 287.96 K. In this contour, heat transfer from the previous hot platform can be seen. This caused the maximum temperature to reach 292.58 K.

The changes are insignificant at a distance of 1.5 meters from the earth’s surface compared to before. The maximum temperature has not changed.

At a distance of two meters from the ground, as well as at a distance of half a meter, a hot platform can be seen that is transferring heat to the environment and has caused an increase in the temperature in the surrounding environment. The maximum value in this contour has increased and reaches 294.87 Kelvin.

Urban Heat Island Conclusion

In this project, we investigate the urban condition of Auckland from two different aspects. In the first step, related to the wind comfort analysis, the results prove that the wind speed exceeds the standard distance suitable for pedestrian conditions in several areas based on Figures 5 and 6, especially in the suburbs.

Thus, the municipality can consider installing windcatchers or vegetation reproduction in the suburban area to overcome the problem or also make more distance between the close buildings leading to the narrow street canyons.

On the other hand, the thermal comfort investigated in the second step has different conditions. By considering the solar radiation applied to the buildings, there isn’t any specific problem for residents. In the worst-case scenario, considering a heat flux for the buildings, still, the thermal criteria (295K) are satisfied, and there wouldn’t be any hardship for the citizens.