This study used computational fluid dynamics (CFD) to analyze the impact of helicopter downwash on pedestrian comfort levels in an urban streetscape. The CFD modeling was validated by comparing simulated air velocities to experimental data. The study found that pedestrian comfort significantly decreased in the immediate area of the helicopter, while minor effects were observed further downstream. Taller buildings tended to improve street-level comfort by disrupting airflow, while narrow streets surrounded by tall buildings funneled downwash toward pedestrians, reducing their comfort. The main conclusion is that while downwash effects are smaller than wind, local mitigation is needed to address its impact on pedestrians.

1. Investigation of the helicopter downwash
effect on pedestrian comfort using CFD
Paul Bernardo BA
Project Engineer, Department of Computational Fluid Dynamics, Arup,
Dublin, Ireland
Réamonn Mac Réamoinn BA, ME
Senior Engineer, Department of Computational Fluid Dynamics, Arup,
Dublin, Ireland
Patrick Young BSc, ME
MSc student, School of Mechanical Engineering, University College Dublin,
Dublin, Ireland
Diarmuid Brennan BSc, ME
MSc student, School of Civil Engineering, University College Dublin, Dublin,
Ireland
Philip Cardiff BE, PhD
Assistant Professor, School of Mechanical Engineering, University College
Dublin, Dublin, Ireland (Orcid:0000-0002-4824-427X)
Jennifer Keenahan BE, PhD, CEng MIEI
Assistant Professor, School of Civil Engineering, University College Dublin,
Dublin, Ireland (corresponding author: jennifer.keenahan@ucd.ie)
(Orcid:0000-0002-1258-2728)
This study employed computational fluid dynamics (CFD) to investigate the impact of helicopter downwash on
pedestrian comfort in a representative low-rise streetscape. A time-averaged approach was adopted, where
propulsion from the helicopter blades was included using the so-called rotor disc method, as implemented in the
open-source software OpenFoam. The modelling approach was validated by comparing downstream air velocities
with experimental measurements. The effect of helicopter downwash on pedestrian comfort in a low-rise built
environment, representative of an Irish city streetscape, was then analysed. It was found that pedestrian comfort
significantly decreased in the immediate vicinity of the helicopter, while minor propagating effects were felt further
downstream. The effects of building height, street width and prevailing winds were then examined. In general, it
was found that taller buildings tended to improve street-level pedestrian comfort, while narrow streets surrounded
by tall buildings tended to funnel the downwash towards the street level, decreasing pedestrian comfort. The main
conclusion is that although the effect of helicopter downwash is smaller in magnitude compared with that of
prevailing winds, a local mitigation must be established to deal with it.
Introduction
Helicopter activities within the built environment have become
more commonplace. Helicopters are used for purposes such as life
safety, transport and broadcasting (Wahono, 2013). Due to their
relatively small infrastructure demands compared with airplanes,
helipad facilities are commonly found on the top of buildings,
including hotels, stadiums and hospitals. As a helicopter generates
thrust to propel it from the ground, it induces a downwash effect
(Wahono, 2013). Due to the relative proximity of a helipad to the
ground, it can adversely affect pedestrians in the vicinity.
Therefore, when making assessments of the wind microclimate in
urban environments, it is important to account for the interaction
between the helicopter and the built environment.
When helicopters are used in confined areas and near pedestrians,
structures, equipment, ground vehicles and other aircraft, there is
a chance of rotor-wash-related incidents (Ferguson, 1994). Most
incidents occur when helicopters are hovering close to the ground
level, at take-off or at landing (Ferguson, 1994). When helicopters
are near the ground, the flow field of the rotor can be described as
a radial wall jet. The flow exits almost perpendicular to the plane
of rotation of the rotor, which then turns as it approaches the
ground so that it moves outwards from the helicopter (Wahono,
2013). Several incidents of severe injury to personnel around
helicopter landing areas have occurred, as well as significant
damage to vehicles in the vicinity, such as broken windscreens,
and damage to vehicle bodies due to impacts (Ferguson, 1994).
There are also reports of indirect effects due to fine particles, such
as sand, snow, gravels and soils, which are accelerated by the
downwash of helicopters; this can be damaging to people’s eyes,
skin and respiratory systems (Wang et al., 2015). The primary
risk associated with helicopter downwash for pedestrians is its
overturning ability; this risk is directly affected by a person’s
height, weight, training and awareness (Soligo et al., 1998). The
direct damage to structures tends to be more complex than simply
overturning and can include doors being ripped off hinges,
damage and cracking to building facades and windowpanes due to
deflection and fatigue damage to structural elements, such as
fixings and connections (Ferguson, 1994).
Urban authorities and councils are beginning to recognise the
importance of pedestrian wind comfort and wind safety. The
Dutch wind nuisance standard (NEN, 2006), to the best
knowledge of the authors, is the first standard to account for
pedestrian wind comfort in the built environment. For the design
of helicopter landing sites, there are a number of guidelines,
including those by the Australian Queensland health authority
(Queensland Health, 2017) and the International Civil Aviation
Organization (ICAO, 2016).
The initial stage of this research involves the development of a
helicopter downwash model that can capture the primary flow
characteristics to a ‘reasonable’ level. This is done by emulating
the study performed by Leese (1972) using a computational fluid
1
Cite this article
Bernardo P, Réamoinn RM, Young P et al.
Investigation of the helicopter downwash effect on pedestrian comfort using CFD.
Infrastructure Asset Management,
https://doi.org/10.1680/jinam.19.00060
Research Article
Paper 1900060
Received 28/06/2019; Accepted 29/11/2019
ICE Publishing: All rights reserved
Keywords: health & safety/
mathematical modelling/town & city
planning
Infrastructure Asset Management