While accurate wind pressure coefficients are critical to evaluating building design, most engineering software for energy and thermal analysis oversimplifies treatment of wind pressure, which can adversely impact cooling, ventilation, overheating, and fresh air rates assessments. This presentation shows how SimScale provides accurate calculations of wind pressures, quickly and cost-efficiently, to deliver a more comprehensive evaluation of building performance. Watch the webinar recording here: https://www.youtube.com/watch?v=VU4-PN9PYDM

1. CFD webinar
Pressure Coefficients on
Building Facades for
Building Simulation

2. In today’s presentation we will explore the
importance of accurate wind pressure
coefficients, and why these are critical in
building design.
This is especially important if you use any
building simulation tools for thermal
modelling, ventilation design, and
compliance.
Wind pressure
coefficients

3. Why are wind pressure coefficients (Cp) so important?
1. The wind pressure at an opening (Pw) and hence ΔP - difference between
internal and external pressure determines how building simulation tools
calculate air flow (natural ventilation, fresh air etc.) through windows/openings.
1. This air exchange will have a ΔT - difference between internal and external
temperature
1. Therefore the amount of air coming in/out of openings has a direct impact on
building:
a. Fresh air and air exchanges - are you calculating the ventilation and air
quality values correctly?
a. Heating and cooling loads - because the amount of air coming in/out of
your building at a certain temperature will either add or remove
thermal energy from your spaces
a. Energy - Gas and electricity due to heating and cooling costs
a. Thermal comfort - air speed, distribution, temperature and quantity of
flow rate.

4. 1. Pw is dependent on Cp
2. Pw is used to calculate ΔP
3. ΔP is used to calculate mass flow rate exchange between
inside and outside (There is also a ΔT)
4. Mass flow rate is an input into the calculation of:
a. Heating load
b. Cooling load
c. Overall energy (KWh) and Cost ($)
d. Thermal comfort
e. Overall zone and building energy balance
f. Impacts on air velocities in rooms for comfort
purposes
Imagine a 50% error in your Cp and how that error propagates
through all of the above
Why are wind pressure coefficients (Cp) so important?

5. What factors should you watch out for?
Most thermal modelling and building simulation tools use simple airflow
network models to calculate bulk air movement. They DO NOT capture the
following effects:
● Wind turbulence
● External wind conditions
● Local building context
● Terrain is an oversimplified constant
● Exposure types are poorly understood and seldom applied. They are
also based on very simplified wind tunnel testing conducted a long
time ago.
Have you ever thought of how your wind pressure coefficients can be more
accurately calculated?

6. Setup and run a
simulation
Pressure
coefficient
results
Import into thermal
modelling solution
1
3-Part Workflow
1. Run a PWC simulation on SimScale.
2. Download results and calculate the pressure
coefficients in ParaView from the pressure results.
3. Query the results in ParaView and use the results
to obtain thermal model’s with higher accuracy.
How do we assess pressure coefficients
on building facades?
2
3

7. The Workflow for Pressure Coefficients
Pressure Coefficients on Building Facades for Building
Simulation

8. Step 1
● Traditionally the process for running a CFD
simulation has been very difficult.
● SimScale has simplified this into a 3 step process
and considered all the best practices and
documentation.
● A simulation can be setup and run within 10
minutes in most cases.
Setup

9. Step 2
Pressure Coefficient
Results
● To obtain pressure coefficients, the results need to
be post processed
● The pressure results are normalised using the
wind speed at the same height in the wind profile.
● Results can be queried by simply selecting points
on the building.

10. These pressure coefficients can be used for many
different things including increasing the accuracy of
thermal models.
Step 3
Import Into Thermal
Modelling Solution

11. Setting up a simulation
Pressure Coefficients on Building Facades for Building
Simulation

12. Define region
Define wind rose
Define pedestrian
zones
1
3
2-Minute Setup
It’s that simple.
1. Define region
2. Define wind rose
3. Define pedestrian zones
How simple can it be?
2

13. Step 1
● Position and size the cylindrical region of interest
● Define which direction is North
Define City Region

14. Step 2
Define Wind Rose
Location was searched by address and the wind data
automatically imported from the connected database
(Wind speed and direction).

15. Step 3
● Pedestrian region was set to be 1.5m from the
floor.
● All terrain at pedestrian level was select to be
offset.
● In more complex scenarios, terraces, balconies or
planes at set heights can be used to record results.
Define pedestrian region

16. Doing this yourself
● This can of course all be done by you!
● The Example project setup is here
● The ParaView state is here
This will allow you to create pressure coefficients to
the eurocode wind profile, but can be modified to
suit your needs.
Useful Links

17. ● All these wind directions are computed in parallel,
a transient (real time wind) simulation takes less
than 2 hours.
● Results are exported on the building surfaces.
8 Directions, At Once!
Results

18. ● Pressure coefficients on windward side are higher
than 1 and on the leeward side, less than 1 as
expected.
● A pattern is observed that on the windward face of
the building the pressure coefficients are fairly
uniform.
● On the next slide we will look at how this alters
due to shielding.
Wind Pressure
Coefficients
Results
South facade h/H = 0.2 S Cp 0.18

19. ● When the wind is coming from the south the
pressure is mostly unaffected by the closely
surrounding buildings.
● This is different when there is an easterly wind,
the shielding from the nearby building on the
windward side causes the pressure coefficient to
drop at building height.
Wind Pressure
Coefficients
Results

20. Results at 0° for SW
Direction
Nearby
Building

21. When comparing to pressure coefficients obtained in building
simulation models we see the following:
● Pressure higher up the building is much higher in
comparison to standard methods used, where flow is
much faster than expected for this wind profile due
to upstream obstruction.
● Lower down, it is much less as surrounding buildings
are actively obstructing the flow.
● Up to 100% difference in Cp values across the length
and width of the building.
● Imagine the error which propagates through to
calculating air flow rates, loads, comfort and more!
Summary
How will this impact on your:
○ Overheating assessments?
○ Fresh air rates?
○ Energy/Loads?
○ Compliance?