Peak overpressure and impulse due to diffraction over a cylinder and/or multi-reflection of a shock wave in structural design- Part I

Abstract

<jats:p> The design and planning of structural components, such as columns, to resist blast loads is a complex task. Often standard free-field blast loads, specified in design codes or other literature, are used for the analysis of the object. These loads are then further increased or decreased depending on the topology of the surrounding geometry (Mach-Stem), the shape of the impacted object itself, and blast wave reflection. However, most of these research works focus purely on the assessment of the structural components itself, ignoring a complex fluid mechanics phenomenon such as diffraction, which is of particular interest when circular columns are standing next to each other or placed in front of a façade. The article addresses three main topics. First, the article answers the question whether the area behind the column is protected, meaning constituting a shadowed area. We present findings that the close area behind a column is subjected to higher pressure and impulse values as there would be without the column. Hence, the incident pressure sees significant pressure buildup due to diffraction. This pressure buildup is quantified using pressure increase factors and presented together with the accompanying impulse. Second, this pressure buildup is of relevance for realistic design of a façade behind the column, which is not covered in current design codes at all. We discuss relevant parameters in the design process. Third, directly coupled with the assessment of the pressure buildup behind the column due to diffraction is the assessment of pressure and impulse in the area behind the column due to multi-wave reflection at a façade, leading to a significant pressure and impulse scale-up, which might be relevant for design of a column and/or a façade. This article identifies gaps in understanding diffraction and subsequent multi-reflection of blast wave within a structural design framework and provides insights on how to establish safe design accounting for these effects. </jats:p>

Mitigation of blast loading through blast–obstacle interaction

“…Table 3 displays the input parameters that were considered, sub-divided into 5 groups, A through E. These combinations have been carefully selected to cover a wide spread of blast wave properties such as overpressure, specific impulse, positive phase duration, and importantly, the shock wave velocity, which has a direct effect on the formation of vortices in the disturbed wake region (Hahn et al, 2021; Niollet et al, 2015; Ritzel et al, 2018). Group E has been included for comparing the effect of each of the cylinder sizes under consideration (in groups A–D) for a given blast wave intensity and obstruction ratio.…”

“…Therefore, mitigating a blast wave by means of wave interactions described previously may also be evaluated reliably using numerical simulations. Hahn et al (2021) numerically characterized the pressure and impulse changes downstream of a blast wave-circular pole interaction. The explosive charge masses were varied to generate a range of scaled stand-off distances.…”

“…Hahn et al (2021) numerically characterized the pressure and impulse changes downstream of a blast wave-circular pole interaction. The explosive charge masses were varied to generate a range of scaled stand-off distances.…”

Experimental and simulation study on blast loads of cylindrical Shells

“…Therefore, when the distance between the outflow boundary and the charge is greater than 100 mm, the outflow boundary on one side of the charge does not significantly affect pressure distribution on the other side. Previous study has shown that the influence of outflow boundaries was found to only extend 10-20 elements (Hahn et al, 2020). Subsequent numerical models far exceed these requirements.…”

“…For bomb attacks in urban environments, blast loads on structures depend on the geometric shape of structures and blast waves. Previous studies on blast loads mostly focus on spherical charges and flat targets, some design manuals give blast loads on the surface of cuboids (Baker, 1973; Defense, 2008; Henrych and Abrahamson, 1980; Hyde, 1991; Kingery and Bulmash, 1984), a probabilistic blast load model for domes has been proposed based on extensive tests (Qi et al, 2020; Zhi et al, 2019), detailed experimental data on blast loads of cylindrical structures for direct verification of numerical models are not available (Hahn et al, 2020; Kristoffersen et al, 2021; Qasrawi et al, 2015).…”