Particle velocity, blast waves

In the surrounding atmosphere, a blast wave is experienced as a transient change in gas-dynamic-state parameters pressure, density, and particle velocity. Generally, these parameters increase rapidly, then decrease less rapidly to sub-ambient values (i.e., develop a negative phase). Subsequently, parameters slowly return to atmospheric values (Figure 3.7). The shape of a blast wave is highly dependent on the nature of the explosion process. 

Air particles in a blast wave have a certain velocity which, in general, flow in the same direction as the propagation of the blast wave. This explosion wind can sweep... 

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. 

Blast wind (Air mass movement) Drag loading—forces on a structure resulting from the high velocity of the air particles in the blast wave flowing around the structure... 

As a blast wave passes through the air or interacts with and loads a structure or target, rapid variations in pressure, density, temperature and particle velocity occur. The properties of blast waves which are usually defined are related both to the properties which can be easily measured or observed and to properties which can be correlated with blast damage patterns. It is relatively easy to measure shock front arrival times and velocities and entire time histories of overpressures. Measurement of density variations and time histories of particle velocity are more difficult, and few reliable measurements of temperature variations exist. 

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 ... 

Study of the blast contours of HE s provides V (S) data for the lateral initial shock wave during its lifetime, i.e. in the interval between its creation and obliteration by the oncoming pressure wave. It is known that the actual particle-velocity vector of the initial lateral shock wave is not quite perpendicular to the charge axis, but somewhat in the forward position (Ref 53, p 323)... 

Blasters can use this equation to estimate the peak particle velocity of a seismic wave or they can use the graph shown in Fig 2. For example Determine the typical peak particle velocity from a normally confined blast with a maximum charge-weight-per-delay-period of 400 lbs at a distance of 1000 ft from the receiving site. The scaled distance, R/W 4 = 1000/40044 = 50 corresponds to a peak particle velocity of 0.31 ips on the graph in Fig 2... 

It should be emphasized that the expression given in the above equation relating the peak particle velocity, charge-weight-per-delay-period, and distance provides typical values only for planning blasting projects in the absence of seismic data. For further detailed information on blasting situations where the above equation is not applicable, the measurement and interpretation of seismic waves, and techniques necessary to reduce blast vibration, the reader is referred to Refs 1,3 4 Effects of Seismic Waves on Structures. 

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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