The evaluation of snowdrifting simulation by snow and wind tunnel for urban design in winter cities

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The evaluation of snowdrifting simulation by snow and wind tunnel for urban design in winter cities

Kondo En

Tsuyoshi Setoguchi

HU, Sapporo, Japan

Abstract

In winter cities, Controlling the impact of snow and wind is important in ur-ban design. Snow and wind simulations have been performed to evaluate the urban designs for suppressing the impact mainly by wind tunnels. This study regards CFD of Snowdrift (herein-after called snow CFD) as one of the techniques to simulate snowdrift for urban design. For es-tablishing the snow and wind simulation process using snow CFD, this study evaluated the usa-bility of snow CFD by comparing with wind tunnel and measurements of snow depth. It was revealed that snow CFD is able to indicate the approximate locations of snowdrift but is difficult to grasp the size or height of snowdrifts. Based on the results, we propose a snow and wind simulation process linking with an urban design process. In this process, it is desired to properly use snow CFD and wind tunnel in response to an urban design process.

Keywords: winter city, wind tunnel, CFD, urban block design, snow simulation

Кондо Эн, Тсуёши Сетогучи

Университет Хоккайдо, Саппоро, Япония

Исследование возникновения сугробов и ветряных потоков в условиях имитации зимних городов

Абстракт

simulate snow wind

В городах с холодным зимним климатом при городском проектировании важно учитывать факторы снежных осадков и ветровой нагрузки. Имитация снега и ветра выполняется в основном с помощью аэродинамических труб, которые позволяют определить качество городских планов для снижения воздействия погодных условий. Это исследование рассматривает вычислительную гидродинамику снежных заносов (здесь и далее ВГД снега) как один из способов смоделировать снежные заносы в условиях городской среды. В исследовании проведена оценка пригодности ВГД посредством сравнения ветряных потоков и измерений высоты снежного покрова. Установлено, что ВГД способна определять приблизительные локации снежных заносов, однако, при этом сложно понять размер или высоту сугробов. Основываясь на результатах исследования, авторы предлагают методику симуляции снега и ветра, которая найдет применение при городском планировании.

Ключевые слова: зимний город, аэродинамическая труба, ВГД (вычислительная гидродинамика), городской дизайн, симуляция снега.

1. Background and Purpose of This Study

In the field of the environmental engineering of wind, CFD have already been practiced to simulate the wind situation in urban scale. As technologies of numerical analysis development, CFD have been tried to simulate the snowdrift around the buildings.

This study regards CFD as one of the techniques to simulate snowdrift for urban design. For establishing the snow and wind simulation process using snow CFD, this study evaluated the usability of snow CFD by comparing with wind tunnel and measurements of snow depth. In addition, we propose the snow and wind simulation process linking with the urban design process after revealing the properties of snow CFD, wind tunnel and measurements of snow depth.

2. Method

The target area was selected. A city block in downtown Sapporo, Japan was used as a case study of urban blocks in high-rise and high-density districts (Fig. 1).

Simulations of snowdrift were performed by snow CFD and wind tunnel.

Snow depth in Kita 3-jo square (in Fig. 1) was measured.

The results of snow CFD, wind tunnel, and measurement of snow depth were compared.

The usability of snow CFD was evaluated based on 4).

Considering 5), we propose a snow and wind simulation process linking with an urban design process.

Fig. 1. Target area

3. Outline of Snowdrifting simulations

Snow simulation by snow CFD was performed using the standard k-є model based on Reynolds-averaged equations. Wind of 10 m/s (Japan Meteorological Agency defines snowstorms as snowfalls with wind speeds of over 10 m/s) and the dominant wind direction in downtown Sapporo (2012-2017) were applied to the weather situation of the simulation: wind speed is 10 m/s at 10 m (surface roughness is 0.0001), and wind direction is northwest.

Snow simulation by wind tunnel was performed using the boundary-layer wind tunnel. The building models were manufactured at a 1:500 scale. The average wind speed and the dominant wind direction in downtown Sapporo (2012-2017) were applied to the weather situation of the simulation: wind speed is 3.26 m/s at 59.5 m (surface roughness is 0.3), and wind direction is northwest.

The wind speed of snow CFD was set higher than that of wind tunnel so that snow CFD tend to output snowdrifts insensitively.

4. Comparison of results of snow CFD and wind tunnel

Snowdrift is generated by winds blowing on the ground. In urban areas, these winds are often generated by tall buildings. Four main areas [1, c.30], in which strong winds blow were sampled; separated flow area, counter flow area, turbulent area, wake area (Fig. 2).

Separated flow and counter flow were set as factors of snow blow, turbulent and wake were set as factors of snowdrift.

Fig. 2. The relation between buildings and winds

Distribution maps (Fig. 3) were made by coloring in twenty grades from 0 mm to the maximum value. In these maps, snowdrift areas were grouped into separated flow area or counter flow area, snow blow areas were grouped into turbulent area or wake area. Snowdrift areas were compared in location, size, shape and height. Snow blow areas were compared in location, size and shape.

As a result of the comparison of two maps: five common points were confirmed, snowdrift in the turbulent area (A and D), snow blow in the separated flow area (a and e), snow blow in the counter flow area (b and g), snow blow in the separated flow area (c and h) and snow blow in the counter flow area (d and j). Snowdrift B and C in the map of wind tunnel didn't correspond to those in the map of snow CFD. Snowdrift a and P in the map of snow CFD didn't correspond to those in the map of wind tunnel.

Fig. 3. The results of simulation

Also, these results of simulation were compared to the result of measurement of snow depth (Fig. 4). In the east side of the square, the tendency for snowdrift to accumulate from south to north was confirmed in all results. On the other hand, in the west side of the square, there was no tendency to correspond each other.

Fig. 4. The results of simulation and measurement

Conclusion

The following 3 points were clarified as evaluation of usefulness of snow CFD.

The locations of snowdrifts and snow blows in snow CFD roughly correspond to those in wind tunnel. Snow CFD is useful in simulating the locations of snowdrifts and snow blows.

The sizes, shapes and heights of snowdrifts and snow blows in snow CFD didn't correspond to those in wind tunnel. It is necessary to consider the use of wind tunnel in addition to snow CFD to simulate the detailed snow distribution tendency of the snow depth and snowdrift areas.

Higher education methodology topical issues

As the accuracy of the snow CFD decreases as the analysis time is shortened, it is necessary to consider the analysis time in consideration of the analysis purpose.

Toward the development of environmental evaluation method of snow and wind, linked with urban design in winter cities, we propose snow and wind simulation process using snow CFD, wind tunnel, and measurement of snow depth.

The fundamental characteristics of the three methods are summarized and characteristics of each method which become important in the process were extracted (Snow CFD: a point that can output the snow particle concentration in the air and the wind speed distribution in an arbitrary cross section, a point that can easily change weather conditions, building shapes, etc. Wind tunnel: a high accuracy of data that can be output. Measurement of snow depth: a point that we can grasp the snow distribution cover in real space.).

Snow CFD which is the bottoming evaluation axis in the process is placed on the base shaft. Wind tunnel is used as means for grasping detailed snow distribution. Measurement of snow depth is used as a means for grasping the snow distribution in real space. From the above arrangement, we propose snow and wind simulation process based on snow CFD (Fig. 5).

Fig. 5 Snow and Wind Simulation Process

At the stage of Setting assignments in a planning, snow and wind simulation process is required to build a simulation system and set multidimensional viewpoints of evaluation. First, Snow CFD analysis is performed. The criteria for determining the measurement area, the accuracy of the model, the modeling area etc. in wind tunnel are obtained from the result of snow CFD. Second, Wind Tunnel experiment is performed. Compared with the result of snow CFD, when a large difference appears in the position of the snow distribution tendency, the calculation condition is improved again and snow CFD is performed to fix the basic analysis condition. Based on the results of wind tunnel and snow CFD after the improvement, the items of environmental evaluation of snow and wind are set up.

At the stage of designing volumes and forms, snow and wind simulation process is required to output quickly and the flexibility. At this stage, snow CFD is mainly used. Snow CFD has many types of data that can be output (snow particle concentration in the air, wind speed distribution in an arbitrary cross section, etc.) and, its modeling time and cost are low. Snow CFD can easily change analysis conditions (wind speed, wind direction, precipitation amount). Simulation of volume design and form design are performed stepwise in conjunction with design process. For best plan, snow CFD is performed for multiple cases in which weather conditions are changed. At the final stage of design process, snow and wind simulation process requires the accuracy of the output data. Wind tunnel is mainly used. Snow removal energy and walk environment are quantitatively evaluated by using the result of wind tunnel with high accuracy of output data.

References

1. Wind Engineering Institute. Basic knowledge of Strong Wind Around Tall Buildings, Kajima Institute Publishing Co., Ltd., 2005.

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