Introduction
Urban areas have their own unique microclimate and differ significantly compared to their rural surroundings. This phenomenon is called the urban heat island. Urban buildings shape airflow in the urban sublayer and the resulting aerodynamic conditions strongly affect pedestrian comfort, ventilation potential of urban spaces and building energy demand [1]. This project aims to analyze convective heat transfer at ground surfaces for different building configurations.
Building Configurations
The configurations chosen are based on a reference case which consists of 16 cubic buildings, whose side (L) and the distance (D) between buildings is 32mm. In order to investigate how flow changes in different configurations, the array has been modified on three different parameters: distance, orientation and height. The distance between buildings was both halved and doubled. For the orientation, the array is rotated by 30° and 45°. For the height parameter, all the buildings are doubled in height.
Wind Tunnel Measurements
Flows through the buildings are analyzed according to two different criteria. The first is wind speed, measured with a Cobra Probe only performed on the reference configuration and the second is temperature, taken with an infrared camera. All the measurements were performed in a wind-tunnel at a wind speed of 7m/s. Figure 1 shows the results of the Cobra Probe measurements. The lines in between the buildings parallel to the wind flow have a similar behavior. Wind acquires speed as it enters the cube array. Right before exiting the array, the flow accelerates and decelerates after exiting. It is also visible from the graphs that the velocity is unsteady near the ground and along the height of the buildings, while it becomes constant as the height increases when the flow is not influenced by the buildings. Overall, the wind speed is higher outside the configuration (blue line). The se configurations have also been compared with the reference configuration for the thermal imaging.
Figure 1: Comparison of wind speeds
In the windward direction, the flow between the buildings varies in Configuration 1 and 2. In the latter, there is a so-called interaction flow, which indicates that two corner streams originating and separating at the passage-entrance corners interact and merge together in a single wide passage jet causing higher wind speed – hence lower temperature – than those in a single corner stream [2]. On the other hand, Configuration 3 has an isolated flow, which means that there is no interaction between the corner streams in the passage [2]. Standing vortices and low wind speed are present downwind and upwind of every building.
The stagnation area of Configuration 4 both upwind and downwind is significantly bigger than the one in the base case. The most critical areas are in the corner stream at the side of the buildings, since increasing the height of buildings causes higher speeds.
In the 30° rotated configuration, the wind speed in the area downwind greatly increases. There are many critical areas where the wind speed is high, both between the buildings and around the configuration.
In the 45° rotated configuration, the wind flow has the most influence on the heat transfer. There are no standing vortices upwind because the wind hit the edges of the building, hence the corner streams do not reach high speeds. Nevertheless, the corner streams join downstream of the building, increasing the overall wind speed.
Figure 2: Thermal imaging during wind tunnel experiments
Conclusions
The change of parameters in building configurations has a big impact on the thermal convective heat transfer at the ground level. Every configuration creates different patterns and each of them has advantages and drawbacks.
Wind speed and thermal images can be used effectively at an early stage of the design phase, when the building’s cross section or the arrangement of buildings are examined from the viewpoint of the pedestrian-level wind environment. However, the application of this system is limited to simple arrangements.
Bibliography
[1] Blocken, B. (2012). Fundamentals and applications in urban physics and wind engineering. Eindhoven: Technische Universiteit Eindhoven
[2] Blocken, B., Carmeliet, T., & Stathopoulos, T. (2007). CFD evaluation of wind speed conditions in passages between parallel buildings – effect of wall-function roughness modifications for the atmospheric boundary layer flow. Wind engineering and industrial aerodynamics, 941 – 962.
