Physical Simulation

Due to the impracticality of measuring, for example, the wind loading on a building in a full-scale downburst (as the time and location of a downburst is near impossible to predict) simulations are required in order to gain further understanding of the effect of these events. With the advent of Computational Fluid Dynamics, numerical simulations have become an option, though they are reliant of physical simulations to provide data for validation of the model.

In order to simulate downbursts in the laboratory two main approaches have been used, slot and impinging jets. Slot jets are showing promise for larger scale simulations than possible for the latter. However, slot jets are only able to simulate the downburst outflow, not the downdraft itself, and only give a 2-D simulation of a 3-D event (though it is debatable how much of an issue this presents, due to the large scale of a downburst relative to a structure). The development of slot jet simulators has been due, in a large part, to the work of Lin and his collaborators - see [1] and [2] for more information.

Impinging jets are vertical, fan-driven jets which impinge on a horizontal plate, forming a horizontal, radial outflow. It has been shown that, even for a steady impinging jet in which the fans are permanently on and the outflow is stationary, the vertical velocity profile matches that of a downburst [3]. More advanced impinging jet simulators use fan control to pulse the jet, or a flap/aperture mechanism to block/release the jet, or a combination of both (e.g. [4], [5]). This pulsing of the jet simulates the unsteady nature of a downburst and, unlike the steady jet, the ring vortex associated with a full-scale downburst event is generated, leading to the high velocities seen when the vortex reinforces the outflow.

The University of Birmingham Downburst Simulator uses a 1m diameter impinging jet with computer controlled fans and aperture control flaps, the largest of its kind in the world. Research performed using the simulator has so far examined interference effects and pressure distributions on cubes and portal buildings.

References

[1] LIN, W. E. & SAVORY, E. 2006. Large-scale quasi steady modelling of a downburst outflow using a slot jet. Wind and Structures, 9 (6), 419-440.

[2] LIN, W. E., ORF, L. G., SAVORY, E. & NOVACCO, C. 2007. Proposed large-scale modelling of the transient features of a downburst outflow. Wind and Structures, 10 (4), 315-346.

[3] WOOD, G. A., KWOK, K. C. S., MOTTERAM, N. A. & FLETCHER, D. A. 2001. Physical and numerical modelling of thunderstorm downbursts. Journal of Wind Engineering and Industrial Aerodynamics, 89 (6), 535-335.

[4] MASON, M. S., LETCHFORD, C. W. & JAMES, D. L. 2005. Pulsed wall jet simulation of a stationary thunderstorm downburst, Part A: Physical structure and flow field charaterisation. Journal of Wind Engineering and Industrial Aerodynamics, 93, 557-580.

[5] MCCONVILLE, A. C., STERLING, M. & BAKER, C. J. 2009. The physical simulation of thunderstorm downbursts using an impinging jet. Wind and Structures, 12 (2), 133-149