Shock front velocity, blast waves

In the free field, the blast wave from an explosion travels at or above the acoustic speed for the propagating medium. TM 5-1300 provides plots of shock front velocity vs. scaled distance for high energy TNT explosives. There are no similar plots available for pressure wave propagation. However, for design purposes it can be conservatively assumed that a pressure wave travels at the same velocity as a shock wave. In the low pressure range, and for normal atmospheric conditions, the... 

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. 

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. 

Blast Effects in Air (Air Blast Effects). When an expl charge is detonated in air, the gaseous products expand rapidly and compress the surrounding air so that it moves outward with high velocity, thus initiating a shock wave. This layer of compressed air is bounded by an extremely sharp front, called the shock front, in which the pressure rises abruptly. The shock front moves outward with an initial velocity much greater than that of sound but, after a short distance, the velocity decreases rapidly. 

For both fluences, only one front was visible until about 100 ns, after which two fronts became distinguishable. As can be seen in Figs. 41a and b, the velocity of this initial shock wave rapidly decays well below the velocity at which the blast wave next appears. This leads to the conclusion that this initial shock is separate from the blast wave or product front. Therefore, the first velocities calculated for both the blast wave and product front in both the 50 and 250 mjcm 2 fluence laser can be questioned, since they are based on the final position of the initial shock. Because of this doubt, these points have been left open in contrast to the filled points used for the remainder of the blast wave and product front points. The apparently separate initial shock is of particular interest, because it appears in both fluence cases and persists for a similar duration even though the propagation distance is quite different. This indicates that a relatively strong material ejection occurs near the peak of the laser pulse and then abruptly stops. In both fluence... 

Fig. 41 Initial shock and blast wave velocities and product front velocities. REPRINTED WITH PERMISSION OF [Ref. 167], COPYRIGHT (1996) Springer Verlag...

For a short interval after the detonation, point 1, there will be no increase in pressure since it takes the blast wave some time to travel the distance from the point of explosion to the given location. Point 2 indicates the time of arrival of the shock front a strong wind commences to blow away from the explosion. This is often referred to as a transient wind, as its velocity decreases fairly rapidly with time. 

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. 

There is a finite time interval required for the blast wave to move out from the explosion centre to any particular location. This time interval is dependent upon the energy yield of the explosion and the distance involved. At 1.6 km from a 1 megaton burst, the arrival time would be about 4 s. Initially, the velocity of the shock front is quite high, many times the speed of sound, but as the blast wave progresses outwards, so it slows down as the pressure at the front weakens. Finally, at long ranges, the blast wave becomes essentially a sound wave and its velocity approaches ambient sound velocity. 

At a combustion velocity of 150-250 m/s, the pressure profile changes and is as shown in Fig. 10.9b. A shock wave precursor followed by a continuing pressure growth at the pressure perturbation front. The pressure wave transforms into a blast... 

Blast wave Fast air pressure change propagating in the form of a pressure perturbation away from the blast epicenter. The leading front pressure jump is known as a shock wave, it is called a compression wave when the pressure rise grows with a moderate velocity. A low intensity pressure wave moves with the sound speed in the surroundings. Finite intensity shock/blast waves propagate with supersonic speed. 

Charges in Air Travel Time, Velocity of Front, and Duration of Shock Waves , BRL-X-127, Ballistic Res Lab, Aberdeen Prov Grnd (1950) 4) J.C. Talley et al, Damage to Aircraft by Blast , NPG-1058, Naval Prov Grnd, Dahlgren (1952) 5) G.A. Young, Base Surge Analysis—... 

The mechanical explosion damage is caused by the blast or shock wave. The explosion starts acoustic waves in the air which travel with the acoustic velocity, c, superposed on the velocity u of the mass motion with which material is convected out from the centre. Since c ylr where Tis the absolute temperature and since both u and c are greater farther back in the wave disturbance it follows that the back of the wave overtakes the front and thus builds up a sharp front. This is essentially discontinuous in both pressure and density. It has been shown that in such a wave front the density just behind the front rises abruptly to six times its value just ahead of the front. In back of the front the density falls down essentially to zero. 

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