Dynamics of experimentally simulated microbursts

Numerical simulations of impinging jets with application to downbursts

“…The six snapshots correspond to moments when: (a) the ring-vortex approaches the surface; (b) and (c) the vortex touches down and triggers the unsteady separation-reattachment of the boundary layer; (d) the main ring-vortex and the counter-rotating vortex formed inside the reattachment bubble travel radially; (e) and (f) a second ring vortex (the results of the initial quasi-periodic Kelvin-Helmholtz instability) touches down and the two ring vortices travel radially and dissipate. While the formation of the separation-reattachment secondary vortex might slow down the radial advection of the ring vortex (Alahyari and Longmire, 1995), the two counter-rotating vortices produce a region of intense stretching in between them. This stretching translates into an important local flow acceleration in the immediate vicinity of the wall with maximum time averaged velocities manifested around the radial position r/D ¼ 1.1.…”

“…Given the complexity of the full-scale phenomena, downburst laboratory simulations have been directed so far towards generic experiments of density currents impinging on a wall (Alahyari andLongmire, 1995, Lundgren et al, 1992) or impinging jets (Bakke, 1957, Cooper et al, 1993, Didden and Ho, 1985, Donaldson and Snedeker, 1971, Landreth and Adrian, 1990, Poreh et al, 1967. Flow visualizations or single point measurements were employed in most cases and the experiments were limited by the laboratory scale.…”

“…The main full-scale downburst studies have been originally initiated in relation to air carrier accidents during take off and landing. Programs such as the Northern Illinois Meteorological Research on Downbursts (NIMROD), the Joint Airport Weather Studies (JAWS), the Classify and Avoid Wind Shear (CLAWS), the Microburst and Severe Thunderstorm (MIST) are essentially meteorological studies oriented towards understanding the formation of microbursts and downbursts, see Alahyari and Longmire (1995) for a complete reference list. It is known that a mass of cold and moist air descends suddenly from the thunderstorm cloud base impinging on the ground surface being afterwards convected radially (Fujita, 1985).…”

Large-eddy simulation of starting buoyant jets

“…Generally, the jet is found to penetrate linearly with time in the PFD [7,8], and with the square root of time in the downstream self-similar phase [9][10][11][12] of the PDF. The other extreme of a starting lazy plume has been studied by Lundgren et al [13], Alahyari and Longmire [14], and Pottebaum and Gharib [15]. The first two studies focused on the scaling comparison between laboratory results and the phenomenon of microburst, and showed that an appropriate scaling can significantly simplify the analysis.…”

Physical simulation of dry microburst using impinging jet model with swirl

“…For the complexity of the full scale phenomenon, the physical simulation of the downburst is confined to the generic experiments of density currents impinging on a wall. Alahyari and Longmire (1995), Lundgren et al (1992), Cooper et al (1993), Didden and Ho (1985), Knowles and Myszko(1998) have studied experimental simulation of the downburst. Letchford and Chay(2002), Chay and Letchford(2002) and Sengupta and Sarkar (2007) performed physical modelling to study the flow field characteristics and pressure distribution the stationary and translational downburst.…”