Reflected overpressure

In Figure 3.8a, a plane shock wave is moving toward a rigid structure. As the incident wave encounters the front wall, the portion striking the wall is reflected and builds up a local, reflected overpressure. For weak waves, the reflected overpressure is slightly greater than twice the incident (side-on) overpressure. As the incident (side-on) overpressure increases, the reflected pressure multiplier increases. See Appendix C, Eq. (C-1.4). 

In summary, an object s blast loading has two components. The first is a transient pressure distribution induced by the overpressure of the blast wave. This component of blast loading is determined primarily by reflection and lateral rarefaction of the reflected overpressure. The height and duration of reflected overpressure are determined by the peak side-on overpressure of the blast wave and the lateral dimensions of the object, respectively. The Blast loading of objects with substantial... 

Blast wave (Overpressure and negative phase pressure relative to atmospheric condition) Diffraction loading—forces on a structure resulting from the direct and reflected overpressure... 

The reflected pressure wave amplitude and impulse for shock waves associated with detonations are well documented, as shown in Figure A. 3 (Ref. 7, Volume II). Less information is available on reflected overpressure and impulse resulting from deflagration pressure waves. Reference 67 documents approaches for evaluating reflected overpressure from weaker blast pressure waves. Forbes (Ref. 71) suggests the following approximate relation to model the more complex relations in Reference 64 ... 

All flow behind the wave is stopped, and pressures are considerably greater than side-on. The pressure in normally reflected waves is usually designated pr(t), and the peak reflected overpressure, Pr. The integral of overpressure over the positive phase, defined in Equation (13), is the reflected specific impulse ir. Durations of the positive phase of normally reflected waves are almost the same as for side-on waves, thigh explosive blast sources than have most blast parameters. 

For shock waves weak enough that air behaves as a perfect gas, there is a fixed and well-known relation between peak reflected overpressure and peak side-on overpressure (References 5 and 11). 

At low incident overpressures (Ps- -0), the reflected overpressure approaches the acoustic limit of twice the incident overpressure. If one were to assume a constant Y = 1.4 for air for strong shocks, the upper limit would appear to be Pr = 8PS. But, air ionizes and dissociates as shock strengths increase, andY is not constant. In fact, the real upper limit ratio is not exactly known, but is predicted by Doering and Burkhardt (Reference 11) to be as high as 20. Brode (Reference 12) has also calculated this ratio for normal reflection of shocks in sea level air, assuming air dissociation and ionization. 

As the wave front moves forward, the reflected overpressure on the face of the structure drops rapidly to the side-on overpressure, plus an added drag force due to the wind (dynamic) pressure. At the same time, the air pressure wave bends or "diffracts" around the structure, so that the structure is eventually engulfed by the blast, and approximately the same pressure is exerted on the sides and the roof. The front face, however, is still subjected to wind pressure, although the back face is shielded from it. 

As indicated in Equation 3.8 and Section 3.3.2, the duration of the reflected overpressure effect, tc, should not exceed that of the free field positive overpressure,... 

Some reduction of reflected overpressure results within a horizontal distance of about twice the barrier wall height. Beyond this distance, the effects of a barrier wail is virtually nil. Quantification of the pressure reduction is difficult and often times requires sophisticated computer modeling. Normally, it is more cost effective to upgrade the strength of the structure to be protected than it is to construct a barrier wall. This is especially true when the structure of interest does not have sufficient blast capacity in the roof to resist the blast load. 

Reflected Overpressure - The rise in pressure produced by a shock wave or pressure wave as felt by a flat surface oriented perpendicular to the direction of wave propagation. 

It was fortunate that, for the BURRO-9 test, RPTs were anticipated and a pressure transducer was located in the air about 30 m from the LNG injection point. The reflected overpressures at various times are shown in Table Vin. Also given on the same table are the TNT equivalents assuming a free-air, point-source explosion. Some equivalents were significantly higher than noted in the smaller Shell Pipeline tests. 

When a shock is reflected from an inelastic surface after normal (head-on) incidence, the pressure rise (reflected overpressure) and the... 

Fig. 38. Peak reflected overpressure (Ap ) following normal reflection of a blast wave from a flat surface dynamic pressure q).

Table IXA. Tentative Estimates for 50% Lethality of Reflected Overpressure Pulses on 70-kg Adult Humans ...

When such reflection occurs, an object precisely at the surface will experience a pressure increase, since the reflected wave is formed instantaneously the value of the overpressure thus experienced at the surface is generally considered to be entirely a reflected pressure. In the region near ground zero, this total reflected overpressure will be more than twice the value of the peak overpressure of incident blast wave. 

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