Arup uses Harpoon and EnSight
Sheffield City Hall airflow
by Erin Hatfield and Paul Gilfrin
How do you test the effectiveness of
something you cannot see?
That is the task that faced engineers at
international consultancy firm providing engineering design services for the
refurbishment of Sheffield City Hall. The City
Hall, one of Sheffield’s key multi use venues,
is nearing completion of a £12.5 million refurbishment project to bring better patron access to seating and facilities, improved performer
areas, new balconies and seating, and advanced conferencing and IT equipment.
Part of the internal overhaul introduces a new
full fresh air displacement ventilation system to the 2300-seat Oval Hall. Using computational fluid dynamics (CFD) and
high-end visualization tools, Arup was able to see the new ventilation system’s
output. This allowed Arup to understand predicted comfort conditions and
provide the opportunity to review possible improvements.
Creating an automatic CFD mesh
Arup’s design included for the reuse and
adaptation of many existing builders work ventilation ducts and risers with a
major part of the work involving the construction of a new timber framed floor
plenum in which individual seat diffusers would be located.
To help understand the airflow systems,
Arup created a CAD model of the Oval Hall in based on 2D AutoCAD data and other
information. The initial CAD model took
three days to construct and was particularly complicated due to the curved
seating and roof, and the high level of detail required for the pedestal, floor
and step-mounted diffusers for the CFD mesh.
Since the hall is symmetrical, the model
was divided in half and imported as an STL file into Harpoon automatic meshing
software from Sharc (www.sharc.co.uk).
Harpoon generated a hex-dominant mesh, and prism layers were applied in
the areas where necessary to maintain a good wall function in the CFD
analysis. The final model contained four
million fluid cells.
“During the CFD analysis, we refined the
CAD model and meshes several times,” says Darren
Woolf, an Associate with Arup. “Harpoon’s mesh generation speed allowed us
to deal with these iterations with minimal time delay.”
One of Harpoon’s strengths is that it can
handle massive changes in scale which is ideal for the types of geometries
faced in large HVAC studies. Features of the software such as boundary layer
creation and cell stretching allow the user to reduce the number of cells in a
mesh while retaining the necessary precision.
The surfaces of the auditorium were
initially modeled as resistive surfaces with fixed adjacent temperatures, since
all surfaces were internal and not exposed to radiant heating or cooling from
outside weather. Arup later adjusted
this model to allow for radiant heat transfer between the surfaces and internal
objects, in particular the ceiling and audience.
Seeing is believing
The fully meshed model was imported into a
leading solver for CFD analysis. Woolf
and his team used existing and estimated new heat-load data to create the
heating and cooling scenarios within the Hall.
During the analysis, cases were explored using a full auditorium with
lights and other heating elements. Cool
air was supplied at seat level and extracted above, a direct inverse of the
CFD results were imported into CEI’s
EnSight visualization software. There,
temperature distribution was visualized and air movement vector fields were
identified for the recommended cooling system modifications.
Based on the CFD results and EnSight
visualizations, Arup discovered that the system produced good mixing throughout
the main auditorium, but there was a heat buildup toward the back of the
balconies, in particular the lower balcony.
Despite this, Arup’s solution was workable, since the higher occupied
zone temperatures were acceptable as overall comfort conditions showed a
significant improvement over the existing arrangement.
“We used EnSight visualizations to confirm
our initial design ideas and highlight a few compromise areas,” says David
Clixby with Arup. “Having these images
at our fingertips helped our credibility in client and project sponsor presentations.
The Hall is now open playing host to nearly
400 events a year – from conferences and concerts to bar nights and dances.