Mesh Settings for PWC

The mesh for a Pedestrian Wind Comfort analysis (PWC) is quite different than for the Finite-Volume-based fluid dynamics analysis types on SimScale, as it is based on the lattice-Boltzmann method (LBM). Here a cartesian background mesh is generated, which is composed only of cube elements that are not necessarily aligned with the geometry of the buildings or the terrain. 

To take into account the exact geometry, there exists a sub-grid model that accounts for the interfaces between the geometry and the fluid domains.

The big advantage of the lattice-Boltzmann method is that the grid size is not bound by the small details of the geometry and that it is a lot more robust with respect to inaccuracies in the CAD model such as small holes, overlaps, or intersections.

The sizing for the background mesh for the PWC analysis is highly automated and targeted towards the specific use case, thus it works best for model sizes of 1 – 10 \(km^2\) and a region of interest radius of 200 \(m\) – 500 \(m\).

Global Mesh Settings

The main logic of the mesh algorithm is as follows:

• The finest cells are located near the surfaces of the geometry. In the region of interest, there is a two-level finer minimum cell size used than outside of it.
• Close to the surface there will be a layer of 4-6 cells of the smallest cell size.
• Away from the surfaces, roughly every 4 cells, there is an increase in cell size by a factor of 2 where again 4 cells of constant size are added.
• Additional refinements are added in the leeward side of the buildings (there we have roughly 16 cells of the same size) in order to accurately resolve the wake region. This refinement is adapted for each wind direction individually in accordance with the actual wind direction.
• The value of the maximum and minimum cell size used depends on the fineness level and the radius of the region of interest. In general, the smaller the region of interest, the smaller the minimum cell size. Details can be deduced from the table below (Table 1):

Fineness	Number of Refinements Levels	Number of Cells per Disc Diameter
Very Coarse	4	16
Coarse	5	16
Moderate	5	24
Fine	6	16
Very Fine	6	24

$$ Maximum\ Cell\ Size = \frac{ Disc\ Diameter}{Number\ of\ Cells\ Per\ Disc\ Diameter} $$

$$ Minimum\ Cell\ Size = \frac{ Maximum\ Cell\ Size}{2^{Number\ of\ Refinement\ Levels}} $$

The following example (Table 2) shows maximum and minimum cell sizes per disc radius, using automatic meshing process.

Fineness	Very Coarse	Coarse	Moderate	Fine	Very Fine
Relative minimum size	1	1/2	1/3	1/4	1/6
Minimum size per 100 \(m\) radius	0.78	0.39	0.26	0.20	0.13
Relative cell count approximation	1	2	4	8	16

• If explicitly a minimum cell size target is needed, this can also be defined using the Target size option. This makes sure that the minimum cell size in the region of interest is at most as large as the target cell size (it can be lower). The exact value is reported in the run info. This option is specifically useful if one wants to make sure to meet the cell size requirements that certain wind standards (such as London Wind Microclimate Guidelines or the Dutch NEN8100 Wind Standard) require.

• The actual cell count depends on multiple factors, such as the diameter of the (ROI) region of interest \((D)\), the height of the buildings \((h)\), the size of the wind tunnel and the complexity of the CAD model. A rough estimation for cell count for different diameter to height ratios can be seen below (Table 4), assuming a Large wind tunnel is selected from the Region of interest > Advanced settings and a relatively simple CAD model:

Ratio \(D/h\)	Very Coarse	Coarse	Moderate	Fine	Very Fine
\(D/h\) = 2	10e6	12e6	25e6	45e6	100e6
\(D/h\) = 4	4e6	5e6	12e6	18e6	45e6
\(D/h\) = 10	1.5e6	3e6	7e6	10e6	20e6

Reynolds Scaling

The Reynolds scaling factor can be accessed under Mesh settings. The Reynolds number is defined as \(Re= \frac{UL}{\nu} \) where \(L\) is the reference length, \(U\) is the velocity, and \(\nu \) is the kinematic viscosity of the fluid. When a scaling factor is applied, instead of sizing the geometry down, the viscosity is increased to ensure that the Reynolds number is reduced to the correct scaling.

In most urban scale flows, an Automatic Reynolds scaling is recommended to ensure accurate results. Like wind tunnel models, we use Reynolds scaling assuming that bluff bodies (block-like buildings) and high Reynolds numbers are present. This enables the concept of similarity of large Reynolds numbers. When both these conditions are met, automatic scaling is valid. Automatic Reynolds scaling helps you get accurate results, cheaper without extensive expertise.

Advanced users operating outside the typical assumptions of urban scales are welcome to use manual scaling, however, they should consider the impacts on wall modeling and rely on their expertise. A scale of 1 represents full scale, and 1/250 or 1/400 would represent a typical wind tunnel scale. Automatic values in SimScale are typically in the order of 1/10.

Progressive Refinement

Progressive refinement is a feature that allows users to speed up their pedestrian wind comfort studies by utilizing coarser meshes in towards the beginning of the simulation and transitioning into finer meshes as the simulation goes beyond a certain percentage of progress. This feature enables to drastically reduce the run times (typically between 35% and 45%) without impacting the results significantly.

To understand the benefit of progressive refinements, it is important to recognize that PWC studies consist of transient simulations that oftentimes involve large flow domains and meshes.

The initial condition for velocity of a PWC simulation is zero velocity globally. As such, the main objective towards the beginning of a PWC run is to develop the flow field. By using a coarser mesh in this stage, you can develop the flow while spending less computational resources.

The setup of a progressive refinement requires two parameters:

• Base refinement fraction: this field represents up to what percentage of progress the simulation will be run using a coarse mesh. The mesh in this stage will be 2 levels of refinement coarser than the final mesh will be. The default setting of 0.4 indicates that the first 40% of the simulation will be run on the coarsest configuration
• Full refinement fraction: this field represents after what percentage of progress the simulation will start using the final mesh with the refinements prescribed by the user. The default setting of 0.3 indicates that the last 30% of the simulation will be run on the final mesh. Between the base and full refinement fractions, the mesh is progressively refined. This documentation page contains an example of a fully refined mesh and meshes that are 1 and 2 levels coarser.

Local Mesh Refinements

Sometimes it is required to put additional focus on specific regions like entrance areas, narrow street canyons between high rise buildings, complex terrain or vegetation, and local mesh refinements in these regions are desired.

In such cases, either a Surface refinement or a Region refinement can be added.

Surface Refinement

A surface refinement is best suited for cases where a specific building or a set of its surfaces or solids should be refined. 

The fineness can be defined analogously to the global mesh sizing from Very coarse to Very fine. Also, in the same way, as it is done for the global mesh sizing inside the region of interest, the surface refinement will result in a layer of 4-6 cells of the smallest cell size and gradually increasing with a larger distance from the surface.

Region Refinement

In case a region between two buildings or a specific open area that is of interest would need to be refined, a region refinement would be best suited. In order to locate the region where the mesh should be refined, a geometry primitive of type sphere or box can be created using the ‘+’ button and assigned by activating the slider in front of it.

The sizing for the region of interest is the same as for the global sizing and the surface refinement, but in contrast to them, the smallest cell size will be kept throughout the whole assigned domain.

Last updated: July 9th, 2024

What's Next

Starting a PWC Run and Accessing Run Information

part of: Pedestrian Wind Comfort Analysis