Documentation

Pedestrian Wind Comfort using Lattice Boltzmann Method – SimScale

Overview

With the increase in number of High-rise buildings all around the World, proper planning of the vicinity for comfort and safety becomes important. Computational Fluid Dynamics seems to be an apt solution in assessing these comfort and safety levels even before the buildings are erected and also helps in faster design iterations. Pedestrian-level (micro-climate) condition is one of the first microclimativ issues to be considered in modern city planning and building design [1].

Wind analysis results using CFD simulation are now seen as reliable sources of quantitative and qualitative data, and they are frequently used to make important design decisions. However, to have full confidence in those decisions, extensive verification and validation of the CFD results are necessary.

For that reason, the team of application engineers at SimScale make sure to validate all the major features that are rolled out over time, comparing the simulation results to analytical or experimental data. Recently, we used experiments from the Architectural Institute of Japan (AIJ) to validate the results gained from the SimScale platform.

Flow Around Buildings
CFD simulation of wind around buildings carried out in a web browser with SimScale.

Architectural Institute of Japan (AIJ) Pedestrian Wind Comfort Experiments

The Architectural Institute of Japan (AIJ) is a Japanese professional organization for architects, building designers, and engineers. It was founded in 1886 and has gathered over 38,000 members since. It publishes several journals, technical standards for architectural design and construction, and research committee studies.

The wind analysis test case for this validation was taken from the “Guidebook for Practical Applications of CFD to Pedestrian Wind Environment around Buildings”, published by AIJ in 2008, which sets the standards for cross-comparison between the results of CFD predictions, wind tunnel tests, and field measurements, and helps validate the accuracy of CFD codes for pedestrian wind comfort assessments.


Case E: WIND ANALYSIS IN AN URBAN AREA

The case being validated is Case E, which is a simplified geometry of a complex of buildings. The urban area model treated here was an actual city block in Niigata city, Japan, with low-rise houses jammed closely together and one target high-rise building at 60m high. Wind tunnel experiments at 1/250 scale were performed on this model in a turbulent boundary layer with a power law exponent of 0.25.

Out of the many scenarios presented in Case E, the impact of the winds from the north, east, south, and west were used to validate the Lattice Boltzmann code of SimScale.

Results from SimScale LBM solver shows great correlation with the experimental results.

Geometry and Methodology

CAD Model
AIJ Case E CAD Model

 

The above picture shows the AIJ Case-E geometry with the main buildings highlighted. The original CAD model was downloaded as the .DXF file from the AIJ website and converted it to an STL file. This STL file was then imported to SimScale directly and used for simulation.

Lattice Boltzmann Methodology
Lattice Boltzmann Methodology

 

Simulation Setup

Inlet Velocity and Turbulent Kinetic Intensity profiles were applied using the table-input in SimScale.

Velocity and Turbulent Intensity Profiles
Velocity and Turbulent Intensity Profiles

In comparison to the traditional CFD solvers, which uses K-Epsilon turbulence model, the SimScale-pacefish solver uses a Delayed Detached Eddy Simulaion model (DDES). The DDES model has an advantage of switching between the Large Eddy Simulation (LES) at regions where the Mesh Lattice is fine enough and the RANS model where the solid boundaries are present to well resolve the boundary layers.

CFD Analysis Results

To compare the CFD wind analysis results to the experimental data, the velocities were normalized with the reference inlet velocity at a height of 15.9m from the ground. The measurements were taken at points listed in the AIJ guidebook. The CFD results obtained from the SimScale platform were plotted against the experimental results to see the correlation.

Correlated results are posted here for four wind directions (North, South, West and East).

The correlation between the AIJ experiments and SimScale results was highly linear, with a majority of the points being within 0.2 in relative velocity.

North – Wind

Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Northerly Wind
Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Northerly Wind

South – Wind

Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Southerly Wind
Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Southerly Wind

East – Wind

Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Easterly Wind
Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Easterly Wind

West – Wind

Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Westerly Wind
Relative Velocity Comparison Between Experimental and SimScale CFD Results for a Westerly Wind

The results shown here are statistically averaged from the transient output from SimScale. SimScale provides the feasibility of stastistical analysis of data points and plotting peak values. These results show that, as the correlation plot suggested, the CFD simulation with SimScale and the wind tunnel experiments produce very similar data. The main advantage of using this new Lattica Boltzmann solver – pacefish with SimScale mainly stands out in imporvment of accurary over the normal OpenFOAM models. General artifacts like overprediction of velocity values at higher velocity regimes are tackled by the use of LBM solver with DDES model. SimScale-pacefish solver cuts down the runtime drastically from over 2 days of simulation (for one wind direction) to about 10 hours. This is a major avantage over the comupational time reduction which turns out to be a time and cost saver especially in Architechtural Aerodynamic simulations.

Conclusion

Buildings in urban areas have complicated shapes and are distributed in an irregular manner, making physical testing difficult and expensive. With the accuracy of CFD codes steadily increasing, simulation has become a viable substitute, and it has been adopted by architecture and construction companies all over the world for assessing pedestrian wind comfort, wind loads on buildings, skyscraper aerodynamics and more. A strict verification and validation of simulation results, however, remains critical for engineers to be able to use the obtained data with confidence and base important design decisions on it.

References

[1] (104-106, 397-407) (2012) “Designing for pedestrian comfort in response to local climate.” Journal of Wind Engineering and Industrial Aerodynamics – Wu and Kriksic
[2] (2006) “AIJ Guideline for Practical Applications of CFD to Wind Environment around Buildings” The Fourth International Symposium on Computational Wind Engineering – Akashi Mochida, Yoshihide Tominaga and Ryuichiro Yoshie.
[3] Guidebook for Practical Applications of CFD to Pedestrian Wind Environment around Buildings – Architectural Institute of Japan.
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