Impact velocities

Stressing by Impact. Size reduction is achieved by the impact of a particle against a soHd surface (Fig. 3c) or another particle (Fig. 3d). The particle can be accelerated to impact against the surface, or the surface can be accelerated to impact the particle, as in an impact mill. The momentum transferred is limited by the mass of the particle and the achievable impact velocity. 

TABLE 19-2 Approximate Impact Velocities of Some Rotating Internal Devices in Mixers ... 

Impact. The m or factor is the peripheral speed of the rotating internal device. Table 19-2 gives impact-velocity data for various mixers. 

Decrease impact velocity to reduce fragmentation Lower-formulation density. Decrease hed-agitation intensity (e.g., mixer impeller speed, fluid-hed excess gas velocity, drum rotation speed). Also strongly influenced hy distributor-plate design in fluid-heds, or impeller and chopper design in mixers. 

Although it is entirely possible for erosion-corrosion to occur in the absence of entrained particulate, it is common to find erosion-corrosion accelerated by a dilute dispersion of fine particulate matter (sand, silt, gas bubbles) entrained in the fluid. The character of the particulate, and even the fluid itself, substantially influences the effect. Eight major characteristics are influential particle shape, particle size, particle density, particle hardness, particle size distribution, angle of impact, impact velocity, and fluid viscosity. 

The conecting term in the pressure reflects the diminution in tire impact velocity of atoms at the containing walls of tire gas due to the attraction of the internal mass of gas, and the volume term reflects the finite volume of the molecules. Data for these two constants are shown in Table 3.4. 

These guns were developed to attain much higher impact velocities than those available with powder guns (Crozier and Hume, 1957 Curtis, 1962). Peak velocities in the 7-8 km/s range can be routinely attained, with peak shock pressures approaching 1 TPa in high impedance materials. 

Weight. The projectile weight must be compatible with the desired muzzle velocity. The greatest challenge is usually to make the projectile as light as possible to achieve a maximum impact velocity, yet meet other criteria such as tilt and integrity. 

Meir and Clifton [12] study shocked <100) LiF (high purity) with peak longitudinal stress amplitudes 0.5 GPa. A series of experiments is reported in which surface damage is gradually eliminated. They find that, while at low-impact velocities the dislocations in subgrain boundaries are immobile and do not affect the dislocation concentration in their vicinity, at high-impact velocities ( 0.1 km/s) dislocations emitted from subgrain boundaries appear to account for most of the mobile dislocations. 

Test data are available for two experiments at different impact velocities in this configuration. In one of the tests the projectile impact velocity was 1.54 km/s, while in the second the impact velocity was 2.10 km/s. This test was simulated with the WONDY [60] one-dimensional Lagrangian wave code, and Fig. 9.21 compares calculated and measured particle velocity histories at the sample/window interface for the two tests [61]. Other test parameters are listed at the top of each plot in the figure. 

The precise impact experiment offers both versatility and the highest of precision. In typical experiments, the projectile is faced with an impactor that is either the same material as the sample to be impacted or a standard material whose properties are known and reproducible. In a symmetric impact configuration the impactor and sample are the same upon impact, precisely one-half of the impact velocity is imparted to the sample. As the impact velocity can be readily measured to an accuracy of 0.1%, this condition provides the most accurately known and best characterized condition achieved in shock-compression science. 

Impact velocities from about 2 to 6 kms which can produce shock pressures in the 110 GPa range, are routinely achieved with two-stage, light-gas guns. These systems are typically limited to diameters of from 12 to 25 mm. To achieve these velocities, propellant is used to accelerate a large piston which serves to compress a reservoir of helium or hydrogen gas to high pressure. The gas then accelerates the projectile over an acceleration distance of perhaps 25 m. 

Fig. 4.2. The technique used to study the piezoelectric behavior of the crystals quartz and lithium niobate used controlled, precise impact loading. The impact velocity can be measured to an accuracy of 0.1%, leading to the most precisely known condition in shock-compression science (after Davison and Graham [79D01]).

As a vessel ruptures, its fragments accelerate rapidly to a maximum velocity. This value is the initial fragment velocity Vj. It is used to calculate either the range of fragment travel or, if collision with an obstacle occurs before maximum range is attained, impact velocity. 

Figure C-4. Impact velocity and injury criteria as a function of side-on overpressure and impulse (Bowen et al. 1968).

Based on the pressure and impulse of the incident blast wave, the maximum velocity can be calculated of a human body during transportation by the explosion wind. Figure C-4 shows the impact velocity for the lethality criterion for whole body impact as a function of side-on overpressure and impulse... 

We now describe the current techniques of deposition. A coating process involves several parameters. There is the nature of the substrate a crystal or an amorphous material, the quality of its polishing and its temperature. There are also the characteristics of the source, as temperature and emission law, and those of the medium in between, as its pressure and composition. In evaporation process the energy of particles is 0.1 eV, or 1100 K their impact velocity is in the range of m.s . With sputtering techniques, the energy lies in between 10-50 eV and the impact velocity is in the range of km.s . ... 

The particles must hit the accumulating bed with a high impact velocity. 

Decrease impact velocity for breakage Decrease excess gas velocity, also distributor plate design may be modified. 

The spherical coordinates of the ejected electron momentum k are k, 0, and < ), where 0 = cos-1 (k v). Because the impact velocity lies along the Z axis, then v v Z. The three Sommerfeld parameters are defined by... 

Figure 20. Electron emissions at 0 = 0° for 40-keV H+ ion impact in H2. The double differential cross section (DDCS = ifia/dfldE ) is plotted against k/v, where v is the impact velocity, k is the ejected-electron momentum, and dU — 2k sin 0 dd. The filled circles represent the experimental data [38], and the CDW-EIS results are given by the solid line [38].

A limited number of polyanion-polycation systems were tested using a droplet/falling annulus method (Fig. 4). This technique, which has been described elsewhere [64] reduces the net impact velocity between the droplet with the oppositely charged counterion fluid. A stream of droplets was directed into a collapsing annular liquid sheet. By matching the velocities of the droplet and sheets, the impact conditions can be moderated. It has been shown to produce monodisperse spherical capsules, though it requires several days of calibration for each new system and is obviously not practical for a massive screening such as was carried out herein. 

To validate the model developed in the present study, the simulations are first conducted and compared with the experimental results of Wachters and Westerling (1966). In their experiments, water droplets impact in the normal direction onto a hot polished gold surface with an initial temperature of 400 °C. Different impact velocities were applied in the experiment to test the effect of the We number on the hydrodynamics of the impact. The simulation of this study is conducted for cases with different Weber numbers, which represent distinct dynamic regimes. 

The simulation shown in Fig. 10 is an impact of a saturated water droplet of 2.3 mm in diameter onto a surface of 400°C with an impact velocity of 65 cm/s, corresponding to a Weber number of 15. This simulation and all others presented in this study are conducted on uniform meshes (Ax — Ay — Az = A). The mesh resolution of the simulation shown in Fig. 10 was 0.08 mm in grid size, although different resolutions are also tested and the results are compared in Figs. 11 and 12. The average time-step in this case is around 5 ps. It takes 4000 iterations to simulate a real time of 20 ms of the impact process. The simulation... 

The simulations were also performed under same conditions as the case of Fig. 10 but for higher impact velocities. The simulated-droplet dynamics and heat-transfer rate at the solid surface at different impact velocities are given in Ge and Fan (2005). 

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