[Bldg-sim] CFD wind modelling using Lawson Criteria

[Tariq Abbas]

Dear Claire,

*         How many wind directions are required? Lawson and the BRE
DG520 suggest 12 but I have heard of 8 directions being used. There's a
big difference in processor/ post-processing time between the two.

 

Depends if it goes to public enquiry, I had to revise my 8 directions to
16. Remember wind tunnel guys will probably do at least 16, possibly
more so you are contending with them.

 

*         How are the results from the CFD usually post-processed to
produce the acceptability criteria throughout the domain? Is it a case
of writing some kind of script for the postprocessor using weighting
functions based on the local wind data? Are there any examples or
guidance on this? 

 

Statistical analysis based on ESDU criteria (this is something you need
to purchase) and the book by lawson. You will need to insert notional
monitoring points in all the key locations where you expect issues to
occur and post process based on those points. Typically around 25 to 30
locations minimum. In a wind tunnel they will pick these locations and
insert Irwin probes or similar to monitor the conditions at those
points. 

 

The ESDU criteria also tells you how to set up the upstream wind profile
based on a desktop analysis of what is even further upstream. 

 

*         Any guidance on suitable turbulence models for CFX?

 

At least 2 equation model k-E, the real issue is that you are probably
doing steady state and therefore not accounting for gusts. The wind
tunnel guys will pick on this aspect here.

 

*         Finally what wind speed is used for the actual modelling, I
assume the 10 year average speed then post processing takes account of
variation in local wind speed, peak gust etc?

 

>From memory i think i used 5m/s at 10m height, corrected by ESDU
profile. It really doesn't matter too much as the results you get for
5m/s need to be correlated to your weather file (8760 entries) for the
nearest location. Also, there are several Lawson criteria that are in
play, and i think one set was called CDDC or DDC or something similar
which related to projects in canary wharf area only. Don't use this
criteria. I would do a google search on wind studies for various
council's / local gov agencies and see what criteria they used. 

 

The wind tunnel players in UK are BRE, RWDI and BMT. If you find their
submissions online you will see what they have used. PS - they are not
in agreement with each other!!

 

Best of luck...

 

Tariq.

 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Claire Das
Bhaumik
Sent: 14 March 2013 20:53
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] CFD wind modelling using Lawson Criteria

 

I need to carry out a Pedestrian wind comfort (Lawson) Assessment using
ANSYS CFX, I'm reading 'Wind Effects on Buildings' and the Franke et al
paper etc. but it's still not entirely clear to me. If you have any
guidance that would be great, particularly:

 

*         How many wind directions are required? Lawson and the BRE
DG520 suggest 12 but I have heard of 8 directions being used. There's a
big difference in processor/ post-processing time between the two.

 

*         How are the results from the CFD usually post-processed to
produce the acceptability criteria throughout the domain? Is it a case
of writing some kind of script for the postprocessor using weighting
functions based on the local wind data? Are there any examples or
guidance on this? 

 

*         Any guidance on suitable turbulence models for CFX?

 

*         Finally what wind speed is used for the actual modelling, I
assume the 10 year average speed then post processing takes account of
variation in local wind speed, peak gust etc?

 

Thanks,

 

Dr Claire Das Bhaumik CEng MCIBSE

Senior Environmental Modeller

 

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