Threshold impact velocity

In Section 3, we showed that large-scale molecular-dynamics (MD) simulations could be used to study the effect of impacts upon perfect crystals of high-explosive diatomic molecules whose interactions are modeled by the reactive empirical bond-order (REBO) potential. We showed that perfect crystal shock simulations lead to detonation above a threshold impact velocity, with characteristics that satisfy the ZND theory of detonations. To see if the threshold for initiation of chemical reaction can be lowered, we also introduced a variety of defects into our samples. 

ABSTRACT RARDE work on the ignition or initiation of cased explosives and rocket propellants is reviewed. Attention is focussed on criteria to predict the threshold impact velocity for detonation. Critical energy and power concepts are considered. Experiments show that, for thin cases, a critical energy criterion is adequate but clear evidence is provided for a non-shock initiated detonation regime with thicker cases. Existing criteria cannot describe this phenomenon. The need for a quantitative description of hot spot formation and growth is highlighted. 

The shock pressure P can be calculated from Hugoniot data for explosive, barrier and projectile. Details of these data are included in reference (5). Figures 5-7 show both experimental and calculated plots of threshold impact velocity against barrier thickness. 

Yarin and Weiss[357] also determined the number and size of secondary droplets, as well as the total ejected mass during splashing. Their experimental observations by means of a computer-aided charge-coupled-device camera and video printer showed that the dependence of the critical impact velocity, at which splashing initiates, on the physical properties (density, viscosity, and surface tension) and the frequency of the droplet train is universal, and the threshold velocity may be estimated by ... 

Impact to Detonation Threshold Velocity TATB d, Detonation Threshold Flyer Velocity, mm/y sec 42,46... 

Computation of Impact and Fluid Thresholds—The velocity necessary to sustain a particle in a fluid must be the same as that required to keep it in suspension, or, in other words, the terminal velocity vm. Thus whether the forces acting on a stationary particle causing it to move are vertical or horizontal, they are encompassed in the equations developed in Chapter 2, Eqs (2-9) to (2-11). Since we are generally interested in conditions corresponding to turbulent motion we may write... 

We count the N2 + O2—>N + N+ 0 + 0 channel as a reactive one because all the trajectories we examined yielded a four atom final state only when it was preceded by the formation of NO, however briefly. In addition, this channei has a dynamical energy threshold significantly higher than the endoergicity. The total yield is shown vs. the impact velocity for N2 + O2 molecules embedded in a cluster of 125 rare gas atoms, One can replot this figure in terms of a reduced variable, see Fig. 13. 

Both dissociations and four-center reactions have an energy threshold. Since the diatom molecule inside the cluster (before the collision) is cold, the nominal energy threshold for the bond dissociation will be the binding energy. For the four-center reaction, the threshold will be the minimum energy required to surmount the reaction barrier. These energetic requirements can be easily reached by controlled selections of the impact velocity. 

Ti is the position vector of particle i and rcm is the position of the cluster center of mass. Figure 28 shows the value of p at a large distance from the smface for a reboimding cluster of 125 Ar atoms as a function of the velocity of impact. The onset of shattering when the impact velocity exceeds a threshold, is clear from this plot. 

In this chapter, we focus on the first stage of clogging and investigate the impaction of non-Newtonian power-law fluids on thin fibers. We aim to obtain the threshold radius of impacting droplets in different impact velocities. Effect of shear-thinning and shear-thickening behavior of droplets is evaluated and compared with corresponding Newtonian fluids. For this purpose, volume of fluid method is used and open source OpenFOAM software is applied for simulations. 

In this section, we present the results obtained for water droplets impacting a fiber of radius 350 pm. Physical properties of water are given in Table 1. Threshold radiuses of the droplets in different impact velocities are obtained and compared with those exhibited by Lorenceau et al. [1], All obtained results are shown in Figure 1, which demonstrates the threshold radiuses in different impact velocities. 

Figure 1. Threshold radiuses of the water droplets in different impact velocities...

Variation of the dimensionless threshold radius versus dimensionless impact velocity is plotted in Figure 2, which shows a good agreement with the experimental and theoretical data presented by Lorenceau et al. [1],... 

There are some general observations that are common to Newtonian and non-Newtonian fluids. For all kinds of fluids, the threshold radius of the droplet has a reverse relation with the impact velocity of the droplet. In all cases, at a fixed impact velocity, droplets with a radius greater than the threshold radius have passed the fiber without breakup, and drops with a radius lower than the threshold radius clung to the fiber entirely. 

Impaction of non-Newtonian power-law droplet to the horizontal fiber of circular cross section is investigated in this study. Volume of fluid technique is employed, significantly reducing the computational cost. Outcomes are divided into three parts First, it has been observed that the threshold radius of the droplets decreased with the increase of impact velocity for New-... 

Figure 6. Threshold radiuses of droplets versus impact velocity for Newtonian, shear-thinning and shear-thickening fluids...

Davies GAO, Hitchings D, Wang J. Prediction of threshold impact energy in quasi-isotropic carbon/epoxy composite laminates under low-velocity impact. Compos. Sci Technol 2000 60(l) l-7. 

Fig. 8.5 Photographs of a liquid drop hitting a smooth dry substrate. A 3.4 mm diameter alcohol drop hits a smooth glass substrate at impact velocity 3.74 m/s in the presence of different background pressures of air. Each row shows the drop at four times. The first frame shows the drop just as it is about to hit the substrate. The next three frames in each row show the evolution of the drop at 0.276, at 0.552, and at 2.484 ms after impact. In the top row, with the air at 100 kPa (atmospheric pressure), the drop splashes. In the second row, with the air Just slightly above the threshold pressure, 38 4 kPa, the drop emits only a few droplets. In the third row, at a pressure of 30.0 kPa, no droplets are emitted and no splashing occurs. However, there is an undulation in the thickness of the rim. In the fourth row, taken at 17.2 kPa, there is no splashing and no apparent undulations in the rim of the drop [18]...

Figure 6. Zoomed-in chronophotographs of the impact region, when a hydrophobic sphere (static contact angle 6>q 115°) is falling on an air-water interface at different impact velocities compared with the air entrainment threshold f/ (a) U = 2.4 m/s < f/ and (b) U = 5.0 m/s > f/. The thin liquid film that develops and rises along the sphere in both cases either gathers at the pole to encapsulate the sphere (low velocity), or is ejected from the sphere thus creating an air cavity behind it (high velocity).

FIGURE 16.5 Variation in threshold friction velocity with particle diameter. The plot distinguishes between fluid and impact threshold. 

---