Pressure wave, described

As described in Section 6.2.1., British Gas performed full-scale tests with LPG BLEVEs similar to those conducted by BASF. The experimenters measured very low overpressures firom the evaporating liquid, followed by a shock that was probably the so-called second shock, and by the pressure wave from the vapor cloud explosion (see Figure 6.6). The pressure wave firom the vapor cloud explosion probably resulted from experimental procedures involving ignition of the release. The liquid was below the superheat limit temperature at time of burst. 

If the system under consideration is chemically inert, the laser excitation only induces heat, accompanied by density and pressure waves. The excitation can be in the visible spectral region, but infrared pumping is also possible. In the latter case, the times governing the delivery of heat to the liquid are those of vibrational population relaxation. They are very short, on the order of 1 ps this sort of excitation is thus impulsive. Contrary to a first impression, the physical reality is in fact quite subtle. The acoustic horizon, described in Section VC is at the center of the discussion [18, 19]. As laser-induced perturbations cannot propagate faster than sound, thermal expansion is delayed at short times. The physicochemical consequences of this delay are still entirely unknown. The liquids submitted to investigation are water and methanol. 

Note that attenuation of pressure wave follows p=p0e ax whereas attenuation of intensity is described by I=I0e 2ax. Attenuation is often expressed as a = 3v. For air, water and soft biological tissues, a= 10, 2.10-3, and 0.3 to 1.5 dB cm-1 MHz-1, respectively. 

Size distribution plays a major role in the microbubble stability, behavior in vivo, and the microbubble acoustic response. The Rayleigh-Plesset equation which describes the microbubble response to pressure waves suggests that ultrasound scattering is proportional to the sixth power of the microbubble diameter [46]. It is not possible, however, to inject large bubbles (e.g., 0.1 or 1 mm in diameter) in the bloodstream, because they would be immediately lodged in the vasculature as emboli, severely limiting the blood flow. Fortunately, microbubbles with the size of several micrometers are still quite echogenic in the ultrasound... 

The theories of transient processes leading to steady detonation waves have been concerned on the one hand with the prediction of the shape of pressure waves which will initiate, described in Section VI, A of Ref 66, and on the other hand with the pressure leading to the formation of such.an initiating pulse, described in Section VI, B. In Section V it was shown that the time-independent side boundary conditions are important in determining the characteristics of steady, three-dimensional waves. It now becomes necessary to take into consideration time-dependent rear boundary conditions. For one-dimensional waves, the side boundary conditions are not involved... 

Pi, pi) and is exactly equivalent to the shock wave described in Chapter 1. When heat q is produced in the combustion wave, the Hugoniot curve shifts in the direction indicated by the arrow in Fig. 3.2. It is evident that two different types of combustion are possible on the Hugoniot curve (1) a detonation, in which pressure and density increase, and (2) a deflagration, in which pressure and density decrease. 

The invidual components of the piping system like valves, branches, atomizing nozzles etc. are described by resistance coefficients. Furthermore the modeling of gas-charged dampeners and resonators is possible. The software simulates pressure losses in pipeline as well as radially expansion of pipes due to transition of pressure waves. 

Detonation, as described earlier, is a process where upon a matrix of uniform particles of gas and solid forms a pressure wave. The pressure wave is what causes the bulk of destruction. However, it should be noted that detonation is a completely different process then deflagration and combustion. As previously stated, compounds that detonate must poses certain functional groups. These functional groups are initiated by molecular shocks generated by blasting caps, detonators, and/or boosters. Of coarse not all explosives need to be initiated by blasting caps, detonators, and/or boosters for example, primary explosives (which you will learn much about shortly) can be detonated under relatively easy means by sparks, heat, friction, percussion, fire, and shock. 

The frequency is measured in hertz (Hz) (cycles per second) and can be described as the rate of vibration. The faster the movement, the higher the frequency of the sound pressure waves created. The human ear does not hear all frequencies. Our normal hearing ranges from 20 to 20,000 Hz, or, roughly, from the lowest note on a great pipe organ to the highest note on a violin. 

The first term is connected with isobaric entropy fluctuation, which gives a diffusive component, and the second term is connected with an adiabatic pressure fluctuation, which gives rise to a high-frequency acoustic wave. The pressure wave is an acoustic standing wave oscillating with a period of Tac = A/v. This component decays by a mechanical acoustic damping or run out effect of the wave if the number of the fringes is limited. After the complete decay of this wave, the isobaric wave appears. This wave just stays where it is and decays by the thermal diffusion process as described in Section I1.B.2. This equation may be further expanded as... 

Three observations described in Section 2 are not immediately consistent with the above model of pressure-diffusion at A u. In each case, we will show that these can be explained by the presence of heterogeneous subsurface permeability structures. The first of these observations (iv - Section 2) is that the average speed of propagation of the pressure wave differs between clusters. Analysis of the data in Figure 2 and other vertical projections clearly show that the clusters are on different fault planes. Therefore, the differing speeds of propagation must be caused by different mean transmissivity values on the individual fault planes. 

If we assume the object represented by the mesh can be deformed (otherwise it wouldn t vibrate, and thus wouldn t make any sound), then the physical equation describing the pressure waves in the object is... 

Two types of simulations, namely shock and pressure wave, are reported. In the shock wave simulations, Zi and Zj are fixed at the tenth and tenth to last rings of the nanotube and represent locations of minimum image boundaries described previously. A 10 A plug of fluid is given an initial z velocity. Subsequent fluid motion is tracked very carefully through visualization and through extensive analysis. 

The purpose ofthe pressure wave simulations (the previously described moving membrane simulations) was to determine if molecular dynamics would allow the coherent transfer of mechanical energy across a fluid. In these simulations, if energy transmission is to occur, the membrane vibrational periods must be commensurate with the length ofthe nanotube and speed of transmission. 

The term ultrasound describes sound waves with a frequency range from 16kHz up to several megahertz. Vibrational motions are transmitted by oscillating devices into a fluid and cause pressure waves. This varying sound pressure is superimposed on the static pressure of the liquid. Fluids are generally capable... 

In process plants apparatuses are connected by pipes. Therefore pressure waves generated by an explosion in an apparams can cause pressure loads in other apparatuses. In order to avoid this, isolation of apparatuses can be achieved by flame arresters (or other devices). They are described below following [14—16]. These impede the propagation of an explosion and hence reduce its consequences (level 4 of Table 4.1). They can be applied to mixtures of flammable gases or vapours with oxidants (usually the oxygen in air) or mixtures of dusts and oxidants. 

It should be noted that the term shock waves refers to a pressure wave of large amplitude that arises from sharp and vioient disturbances when the velocity of wave propagation exceeds the veiocity of sound propagation. Characteristicaiiy, an abrupt change of the medium properties (e.g., pressure, stress, density, particie velocity, temperature, etc.) takes piace in a limited space across the shock wave (Schetz and Fuhs, 1996 Shapiro, 1953 Anderson, 1982 Saad, 1992). In the case described in this chapter, the physicai phenomenon of shock wave is restricted to one-dimensional plane wave propagation, in which properties of air in the resonant tube of the wave generator... 

Previously in our theoretical and experimental works [l, it has been shown that the behavioiir of pressure waves of small i Po/Po intensity, their structure, attenuation in a boiling bubble medixam are well described by a model equation... 

The sequence of events in inertial confinement can briefly be described as follows. A small pellet of radius less than 5 mm and containing the fuel mixture (DT, in this case) is symmetrically struck from several directions by intense pulses of energy from either a laser or an ion beam. Absorption of this energy below the pellet s surface leads to local ionization and plasma formation. The consequence of these processes is an outward-directed mass transfer by ablation and - by a rocket type reaction - an inward-directed pressure wave leading to the compression and heating of the target. With the temperature and fuel density sufficiently... 

The medium subjected to cavitation may be a macroscopically homogeneous liquid (before cavitation occurs),or a heterogeneous medium made of immiscible liquids or a solid and a liquid. Any liquid can be subjected to cavitation, which a priori can be triggered by any kind of pressure wave, from infrasonic frequencies up to several MHz. For example, bromine atoms released in brominated water cavitating at infrasonic frequency (<16 Hz) convert maleic to fumaric acid via a mechanism described in Ch. 2 (p. 65). Sonoluminescence, usually observed between 20 kHz and 2 MHz (Fig. 1), was also studied by... 

The dynamic process of pressure propagation can be described by the laws of the conservation of mass, momentum, and energy. From these laws, a theoretical representation may be derived for simple cases, as described below for pressure waves with small pressure amplitudes. Such a pressure wave can be described approximately by the following equation ... 

Sound is transmitted through the air by sound waves which are produced by vibrating objects. The vibrations cause a pressure wave which can be detected by a receiver, such as a microphone or the human ear. The ear may detect vibrations which vary from 20 to 20 000 (typically 50-16 000) cycles each second (or hertz (Hz)). Sound travels through air at a finite speed (342 m/s at 20°C and sea level). The existence of this speed is shown by the time lag between lightning and thunder during a thunderstorm. Noise normally describes loud, sudden, harsh or irritating sounds. 

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