Downburst dynamics and the implications for engineering structures

1. Background

Over the last few years there has been renewed interest in the effects of extreme wind events, since in a number of cases these events are the most important with respect to wind loading. For example, Sterling et al. (2006) examined the structure of extreme gusts corresponding to a variety of synoptic and non-synoptic winds. From an engineering perspective, the physics associated with wind relating to synoptic events has been well researched – velocity and turbulence intensity profiles are reasonably well understood and can be reproduced with appropriate levels of accuracy in boundary layer wind tunnels.  ISO (2009) has attempted to provide guidance to enable appropriate conversions to be undertaken for other non-synoptic wind events, e.g., thunderstorm downbursts.  However, ISO (2009) acknowledges that the conversion presented is based on limited data and that “No guidance can be given with respect to turbulence intensities in thunderstorms at this stage.” During a downburst a column of air moves vertically downwards and impinges on the ground. This causes the resultant air to be displaced radially outwards from the point of impingement, with a ring vortex travelling away from the stagnation point. The effect of this on the mean streamwise velocity profile is shown in Figure 1, while Figure 2 illustrates the 3-D nature of a downburst with respect to time. The translational velocity of the parent storm (i.e., the environmental flow ‘driving’ the downburst) can also influence the potential wind field and hence the wind loading experienced by a building.

 fig1  fig2

Figure 1. A comparison between flow characteristics of a downburst and a conventional boundary layer. (Lin and Savory, 2006).

Figure 2. 3-D structure of a downburst at 10m above the ground. (Orwig and Schroeder, 2007).

The above work has revealed a number of underlying structures embedded within the wind during a downburst. However, the scarcity in data restricts the ability of this work to be incorporated into the structural design process.  Thus, it is argued that there is a need to undertake a comprehensive examination of the structure of thunderstorm downbursts and to investigate the corresponding wind induced forces which can arise. The scarcity of full-scale data and the difficulty of predicting such events ensure that at present, modelling is a sensible strategy. Furthermore, the uncertainties associated with both physical and numerical modelling (e.g., Reynolds number limitations, lack of calibration data etc.) strongly suggest that a combined physical/numerical modelling programme supplemented by (limited) full-scale data is the best way forward. Without such an examination of the wind field associated with thunderstorm downbursts, the suitability of existing design methods remains an open question.  This is of importance since in many parts of the world wind speeds of this origin constitute the design wind speeds (Letchford et al., 2002).  Even in areas where these events are not dominant, the continued investment and development of society and its related infrastructure ensures that society as a whole is more vulnerable to the effects of such events irrespective of how frequently they occur.

2. Research aim, objectives and milestones

The project has two aims: firstly, to examine the structure of the flow field associated with thunderstorm downbursts through physical and numerical simulation; secondly, to examine the corresponding wind induced forces acting on a number of physical models of low-rise structures subjected to a simulated downburst, in order to understand and hence ensure the resilience of such structures to wind-induced damage.  Related to these aims the following objectives (O have been identified:

  1. O1.   to undertake a series of physical model experiments on two isolated low-rise buildings (for which significant full-scale and wind tunnel data exists corresponding to boundary layer flow) subject to a thunderstorm downburst at different distances from the centre of the impingement;
  2. O2.   to repeat the measurements in (O1) for a variety of surface roughness types in order to assess the effect of variations in ground roughness;
  3. O3.   to repeat the measurements of (O1) and investigate the effect of the motion of the parent storm on the wind forces experienced by the two structures;
  4. O4.   to fully assess the suitably of the physical simulations using full-scale data;
  5. O5.   to use the data obtained from (O1) – (O3) to develop a novel, numerical approach capable of yielding a time series of velocity and wind induced pressures acting on the buildings which be interrogated in order to provide an insight into the downburst dynamics and their effects on low-rise structures.

For more details regarding the project please contact Dr Ian Taylor (ian.taylor@strath.ac.uk), Lecturer at the  Department of Mechanical and Aerospace Engineering at the  University of Strathclyde.
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