Swept Away: Tall Buildings Whip Up the Wind
Roopinder Tara posted on October 16, 2019 |
A first look at SimScale’s simulation to predict wind speeds around tall buildings.

When walking to work in any city, you know that around a certain corner you will be hit with a wind blast. You put your head down and soldier through it. In less than a block, it’s gone. You straighten out your hair. 

You have experienced a corner vortex, says Haitham Aboshosha, assistant professor of structural engineering at Ryerson University in Toronto. It happens as wind from two faces of a building meet in a rush and swirl. In other places, you may have effects like channeling—where wind attenuates going down a street—or downwashing—where the winds near the top floors cascade down its walls. 

For architects who are creating public spaces, and even developers of waterfront projects or even restaurants who are wondering if plates and napkins will stay on the table, wind effects like these can be a disaster.

Buildings can magnify winds, causing pedestrian hazards. Toronto had to put climbing ropes along Front Street to keep citizens from being swept away from wind attenuated by tall buildings. (Picture from video by Natalie Nanowski, CBC)

A number of cities, including Toronto, Boston and London, are asking builders and planners to certify that new buildings will not put their citizens at risk or discomfort. Planners will need to model the wind around their planned buildings. Toronto is demanding all buildings of 12 stories undergo a wind assessment. 

wind speeds over Boston Seaport Village

Winds from the Atlantic hit a portion of the Boston Seaport District in this simulation. Boston’s builders were so concerned with the wind that they made the floors of the 121 Seaport building (PTC headquarters, not shown) elliptical and oriented the building to minimize wind effects. (Picture courtesy of SimScale.)

Professor Aboshosha offers one solution: a subsonic wind tunnel at Ryerson University, where wind tunnel, smoke trails and a scale model can determine how winds will play out in the complex geometry that many downtown areas have become. 

Enter SimScale

A number of simulation vendors offer another method: CFD, or computational fluid dynamics. Normally the simulation tool for the aerospace and automotive industries, SimScale has come up with an application for AEC and BIM.

But if you are an architect or city planner, you may not know how to even spell CFD. 

No problem, says Richard Szöke-Schuller who showed us SimScale’s new LBM (lattice Boltzman method) solver and their latest addition to it, the Pedestrian Wind Comfort Analysis application, or PWCA, their solution to nasty urban wind effects. 

Our software is meant for those who work with buildings and urban landscapes, says Szöke-Schuller, not planes and cars. You don’t have to be a simulation specialist, in other words. 

SimScale allows input of up to 36 different wind directions and speeds. Note the circle which defines the area of interest over which resulting wind speeds will be calculated.

PWCA codes resulting speeds by user’s choice of pedestrian comfort criteria, either the Dutch wind nuisance standard NEN 8100, or the Davenport or Lawson (shown) criteria.

Indeed, the software solves for the effects of prevailing winds around a city model with almost zero time and attention devoted to meshing. In fact, the volumetric mesh common to CFD simulation, is never displayed. The program coordinates with Google Maps to extract location information. You import the blocky (low level of detail) building from a CAD source, such as Rhino or SketchUp. You input the winds (multiple wind directions at once) and press “solve.” 

The simulation is then sent off to SimScale’s GPU-laden servers (near super computers) to be solved. You can use your computer for whatever else in the meantime. In less than two hours, the resulting wind patterns are unfurled across your screen, the highest winds shown in concentrations of red lines. You can also elect to have the results animated so you can “see” the wind blow. The color corresponds to different levels of comfort, such as established by T.V, Lawson with six levels of pedestrian comfort that go from comfortable while sitting for a long time (wind speeds of less than 5 m/s) to dangerous, over 15 m/s (or 34 mph). You will know right away where not to plan an outdoor café. The wind can be studied at a set height above the ground, say 6 feet, to see if you that will stay on. It can be set to 500 feet to see if your penthouse cabanas will stay put. 

Will it handle hills? asks the San Francisco-based editor.

It can handle San Francisco, says Richard.

The software cannot handle wind moving anything but laterally, such as updrafts and downdrafts. It will also not handle thermal effects, which may be created by urban heat islands like parking lots or heat produced by the buildings themselves. 

It can handle incompressible and isothermal flow only, says Szöke-Schuller.

To Solve a Wind Tunnel, You Have to Make One

The city model shown consisted of about 50 buildings of varying size and height. The software has you create a “wind tunnel” over an area of concern. Care is taken to make the wind tunnel, rectangular in section, wide and tall enough so that the effects of wind near the surface of the tunnel are far enough away that they don’t affect the area of concern. A wind tunnel about three times as wide as the diameter of the area of concern and maybe 4 times the height of the tallest building (I’m estimating here) seems to work. 

Multiple wind speeds and directions can be applied to the model, and the PWCA will run them all at once. The wind loads can come from a third party wind database, which some cities may already have, and include annual data. 

Although you never see them, we find out afterwards that the model involved 6.6 million cells. Invisibly, they were clustered around critical areas like around building edges and sparse as you rose up from the buildings. 

The application runs on SimScale GPU solvers, each one costing $50,000—quite beyond the budget of the average small- or medium-sized architectural firm. But the cost of one simulation the SimScale way? Less than $100.

To use PWCA, you have to have a SimScale professional account.

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