Blast wind dynamic

Tertiary blast injury - injuries due to the blast wind The blast wind (dynamic overpressure) results from the motion of the combustion products of the explosion. Resultant injuries vary from total disruption to amputation and devastating injuries from impact of the displaced body on the envirorunent. 

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. 

Some indication of the corresponding values of peak overpressure, peak dynamic pressure and maximum blast wind velocity for an ideal shock front in air at sea level are given in Table 9.1. 

When the pressures on different areas of a structure are quickly equalised, because of its small size, the characteristics of the structure or the rapid formation of numerous openings by the action of blast, the diffraction forces operate for a very short time. The response of the structure is then mainly due to the dynamic pressures (or drag forces) of the blast wind, e.g. for telephone poles, radio and television transmitter towers, and tall chimneys. 

In the evaluation of blast damage to structures, a distinction should be made between local and global response of structures. Local response would be associated with response of wall elements relative to their supporting members (girt, purlin, beam and column). For local structural elements the blast and dynamic wind loads are typically associated with only their load on the local structure. 

Table 6.10 presents some damage effects. It may give the impression that damage is related only to a blast wave s peak overpressure, but this is not the case. For certain types of structures, impulse and dynamic pressure (wind force), rather than overpressure, determine the extent of damage. Table 6.10 was prepared for blast waves of nuclear explosions, and generally provides conservative predictions for other types of explosions. More information on the damage caused by blast waves can be found in Appendix B. 

Other properties of tlie blast wave are tlie shock velocity, wliich is tlie rate or speed of tlie blast wave as it travels tluough tlie air, tlie particle velocity (or peak wind velocity), tlie peak dynamic pressure, and tlie peak rejected overpressure. 

Because of the importance of the dynamic pressure q in drag or wind effects and target tumbling, it is often reported as a blast wave property. In some instances drag specific impulse i, defined as... 

For a building with a flat roof (pitch less than 10°) it is normally assumed that reflection does not occur when the blast wave travels horizontally. Consequently, the roof will experience the side-on overpressure combined with the dynamic wind pressure, the same as the side walls. The dynamic wind force on the roof acts in the opposite direction to the overpressure (upward). Also, consideration should be given to variation of the blast wave with distance and time as it travels across a roof element. The resulting roof loading, as shown in Figure 3.8, depends on the ratio of blast wave length to the span of the roof element and on its orientation relative to the direction of the blast wave. The effective peak overpressure for the roof elements are calculated using Equation 3.11 similar to the side wall. 

This blast effect is due to air movement as the blast wave propagates through the atmosphere. The velocity of the air particles, and hence the wind pressure, depends on the peak overpressure of the blast wave. Baker 1983 and Hvf 5-/300 provide data to compute this blast effect for shock waves. In the low overpressure range with normal atmospheric conditions, the peak dynamic pressure can be calculated using the following empirical formula from Afewmark I956 ... 

It may be predicted that, in the absence of physical stress of wind and snow blast, as in most tropical mountains below the snow line, the treeline should be highly dynamic within a long term temporal scale because trees are slow to establish, grow and mature. Thus, it is likely that other limitations to tree seed setting and growth are at play. One of these constraints is in fact seed germination in sites exposed to UV-B radiation (see section 5.3)... 

Nowadays the requirements towards the technical features of electrolysis systems have changed tremendously. Dynamical behavior is required instead of operating closely to the optimum. Wind occurs in strong blasts, sun is hidden by cloud fields. This all happens in seconds. An electrolysis system has to master steepest energy gradients within seconds up and down. It has to be able to be switched off totally and suddenly deal with overload simations accordingly. 

The main purpose of this approach is the identification of major dynamic parameters which can be used for estimating and predicting tank behavior when subjected to severe loading conditions such as earthquake, blast or drastic wind pressure. 

When the shock front reaches a given point, both the overpressure and the dynamic pressure increase almost immediately from zero to their maximum values and then decrease. The dynamic pressure (and wind velocity) will fall to zero some what later than the overpressure beeause of the momentum of the air in motion behind the shock front, but for the purposes of estimating damage the difference is not significant. During the negative (suetion) phase of the blast wave the dynamic pressure is very small and aets in the opposite direetion. 

The forces on a structure associated with a blast wave resulting from an external detonation are dependent upon the peak values and the pressure-time variation of the incident and dynamic wind pressure action, including characteristics of the reflected blast wave caused by interaction with the structure.28... 

The effective loads on structures due to blast and associated dynamic wind loads are a function not only of the dynamic characteristics of the load but also the dynamic response characteristics of the structure, which should be... 

II-5. When the blast wave impulse encounters an obstruction it results in a reflected wave typically two to four times the magnitude of the side-on peak pressure, but of shorter duration, impinging on obstructions perpendicular to the free field or side-on blast wave s direction of travel. As the positive blast wave traverses a building structure, in addition to the reflected pressure on the windward side, it exerts a positive pressure on all walls and the roof of the structure as it passes. Dynamic winds following the blast wave exert a positive pressure (inward) on the windward wall and negative pressures on the side and leeward walls and roof. 

FIG. II-4. Additional side-on blast parameters for TNT U shock front velocity (m/s) u particle velocity behind the shock wave (m/s) Q dynamic wind pressure (Pa) b decay constant. 

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