City of London: Mesh and Geometry Guidelines

While simulating a Pedestrian Wind Comfort (PWC) analysis, mesh resolution is of prime importance. There are certain requirements that need to be taken care of while performing CFD simulations to get high-quality and reliable results that resolve the flow features accurately. These requirements ensure that the modeling approaches do not vary drastically, and stay within defined parameters.

Similarly, when it comes to simulating a building or a region within London to assess the wind comfort, there are certain meshing guidelines that should be followed. These guidelines are standard and are referred to as the City of London Guidelines.

The City of London Guidelines are discussed below:

• The 3D CAD model should be resolved for building features larger than 0.5 \(m\) near pedestrian areas to account for the localized wind conditions.
• Critical locations like entrances, corners, or regions of specific interest must have cell size no greater than 0.3 \(m\).
• A minimum of 10 cells should be included between buildings and across street canyons. However, cell sizes beyond the region of interest can be larger to avoid high-density meshes.
• Features that are part of landscapes should be ignored if they don’t exceed 8 \(m\) in height. Large trees or hills might be included under careful consideration of their impacts since limited published guidance exists for the modeling of such landscape features.

Figures 1 and 2 show the cartesian mesh generated, around ‘The Shard, London’ in SimScale complying with the City of London guidelines. CAD geometry credits to Accucities \(^3\).

The Ideal SimScale Setup

In order to have the most efficient setup complying with the guidelines, it is advised to use the following settings:

• Radius of the region of interest: 385 \(m\)
• Mesh Fineness: “very fine”
• These settings will lead to a minimum mesh size of 0.5 \(m\).
• In the critical areas an additional region refinement with a manual sizing at a target size of 0.3 \(m\) can be placed (leading to a local cell size of 0.25 \(m\))

More details on how to access the mesh and control its settings in PWC analysis can be found here.

Wind Comfort and Safety Criteria

Wind Comfort Criteria

City Lawson Criteria ( called “London LDDC Comfort” in SimScale), which is a modified version of the Lawson LDDC criteria, is accepted as a comfort criteria. The criteria is summarized in the following table:

Category	Mean and GEM wind speed (5% exceedance) \(m/s\)	Description
Frequent Sitting	2.5	Acceptable for frequent outdoor sitting use, e.g. restaurant, café.
Occasional Sitting	4	Acceptable for occasional outdoor seating, e.g. general public outdoor spaces, balconies and terraces intended for occasional use, etc.
Standing	6	Acceptable for entrances, bus stops, covered walkways or passageways beneath buildings.
Walking	8	Acceptable for external pavements, walkways.
Uncomfortable	>8	Not comfortable for regular pedestrian access.

The table above can be explained with the following example: If the mean and GEM wind speed over 8 \(m/s\) achieves 5% exceedance, then it is categorized as Uncomfortable. Here, GEM stands for Gust Equivalent Mean. This value is obtained by dividing the peak gust wind speed by 1.85.

Wind Conditions in PWC

Wind Safety Criteria

The safety criteria has a single rule: If the Mean and GEM wind speed from any wind direction is greater than or equal to 15 \(m/s\) and it achieves 0.022% exceedance, then consider it as Dangerous.

Last updated: March 28th, 2023