Compaction Wave Research Articles

Investigation of the Process of Deflagration-to-Detonation Transition (DDT) in Granular Secondary Explosives with High-Speed Photography.

The deflagration-to-detonation transition (DDT) in confined charges of granular secondary explosives has been investigated with high-speed photography. Specially designed steel confinements, which incorporate slit windows to allow streak photography, were constructed to house pressed pentaerythritol tetranitrate (PETN) charges of known density. The results indicate that convective burning is only important during the initial stages of the DDTprocess in causing rapid pressure build-up, and does not govern the deflagration process up to the point of DDT. Following convection, the propagation of the combustion front was found to be governed by the compaction wave generation of hot-spot ignition sites, The transition to detonation is finally attained when the front of a plug of highly compacted material, essentially a second compaction waveformed at the combustion front, overtakes the first compaction wave and causes shock initiation of the uncompacted, and therefore. more highly shock sensitive, original-density PETN. The detonation then propagates to the end of the charge at constant velocity and a rearward shock propagates upstream through the highly compacted plug material and intersects the combustion front. The proposed mechanism is supported by the observed deformation of the internal surface of the steel confinement.

Simplified modeling of transition to detonation in porous energetic materials

A simplified model that can predict the transitions from compaction to detonation and shock to detonation is given with the aim of describing experiments in beds of porous HMX. In the case of compaction to detonation, the energy of early impact generates a slowly moving, convective-reactive deflagration that expands near the piston face and evolves in a manner that is characteristic of confined deflagration to detonation transition. A single-phase state variable theory is adopted in contrast to a two-phase axiomatic mixture theory. The ability of the porous material to compact is treated as an endothermic process. Reaction is treated as an exothermic process. The algebraic (Rankine–Hugoniot) steady wave analysis is given for inert compaction waves and steady detonation waves in a piston supported configuration, typical of the experiments carried out in porous HMX. A structure analysis of the steady compaction wave is given. Numerical simulations of deflagration to detonation are carried out for parameters that describe an HMX-like material and compared with the experiments. The simple model predicts the high density plug that is observed in the experiments and suggests that the leading front of the plug is a secondary compaction wave. A shock to detonation transition is also numerically simulated.

Head-on interaction of planar shock waves with polyurethane foams — a semi-empirical model

A semi-empirical model for calculating the velocity of the compaction wave which is transmitted into a polyurethane foam struck head-on by a planar shock wave is developed on the basis of some experimental observations. The compaction wave induced velocity and the induced density are determined. Predictions of the model are compared to experimental results and found to be satisfactory.

Jump conditions across strong compaction waves in gas saturated rigid porous media

Based on experimental results and some additional simplifying assumptions, the general macroscopic two-phase equations governing the flow field which is developed in a gas-saturated rigid porous medium domain were simplified to a form which enabled us to develop two analytical models for calculating the jump conditions across a strong compaction wave. Predictions obtained by these two simplified analytical models are compared to the experimental results of Sandusky and Liddiard (1985) and to predictions of another more complicated model which was proposed by Powers et al. (1989). Fairly good to excellent agreements are evident.

The impact of a shock wave on porous compressible foams

A phenomenological study of the processes occurring when a shock wave interacts with porous polyester and polyether foams has been undertaken. Plane shock waves generated in a shock tube were reflected off a slab of foam mounted against the back wall of the tube. Tests were conducted with an initial shock wave Mach number of 1.4 and a 70 mm thick slab of foam. The reduction in reflected shock wave strength and substantial increase in the back wall pressure over that for rigid wall reflection, found by other workers, were confirmed. Piezoelectric pressure transducers were used to record the pressure before, alongside and behind the foam specimen. Schlieren photographs of the flow were made and showed some features not previously reported. In particular it is shown that there is a flow of gas across the face of the foam at some point of the process. Previous investigations of this interaction process have assumed that the face of the foam is a contact surface. Short duration photographs of the distortion of the foam were taken, enabling the wave propagation in the foam material itself to be studied. It is established that the front of this compaction wave in the foam material moves at considerably lower velocity (- 90 m/s) than the gas wave as detected by the pressure transducers (- 200 m/s). This result contrasts with the assumption made in previous work that the two-phase medium behaves essentially as a homogeneous substance. A simple physical model based on a zone of compacted material in the foam acting as a high-resistance flow barrier, is proposed to explain the observed phenomena.

Analysis of Steady Compaction Waves in Porous Materials

A two-phase continuum mixture model is used to analyze steady compaction waves in porous materials. It is shown that such a model admits both subsonic and supersonic steady compaction waves in response to a piston-driven boundary condition when a Tait equation is used to describe a solid matrix material and a generic static compaction relation is used to describe collapse of the matrix. Parameters for the Tait equation are chosen to match shock and compaction wave data. The model is able to predict compaction wave speed, final pressure, and final volume fraction in porous HMX. The structure of the compaction wave is also studied. A shock preceding the compaction wave structure is predicted for compaction waves travelling faster than the ambient sound speed of the solid. For subsonic compaction waves no leading shock is predicted. The compaction zone length is studied as a function of initial volume fraction, piston velocity, and compaction viscosity.

Numerical Studies of Dynamic Compaction of Inert and Energetic Granular Materials

In this paper a numerical study of dynamic compaction of granular materials is presented. A multiphase mixture model treating two-phase nonequilibrium flow is applied to describe dynamic compaction experiments. Compaction is modeled using a description of grain distortion driven by pressure differences. Various energetic and inert granular materials including HMX, Fluid A (Nitrocellulose), Melamine, and Teflon are studied for various piston-impact conditions and compared to existing data. Numerical analysis of a ramp-wave piston-impact experiment is also examined. The multiphase mixture model predicts reasonably well the observed steady compaction wave characteristics. Dynamic compaction experiments with detailed resolution are needed to better address the transient behavior of compaction waves in granular materials.

The effect of the membrane compression wave on the light scattering spectrum from a protein monomer-dimer equilibrium

The membrane compression wave on the Rayleigh-Brillouin spectrum of a solution of protein in a monomer-timer equilibrium has been calculated. When the frequency of the longitudinal wave is slow compared to the monomer-dimer exchange frequency the compaction wave produces an asymmetry in the spectrum. The asymmetry is small and may be undetectable in the experimental spectra. If the frequency of the compression wave is high compared to the exchange rate then new bands will appear in the experimental spectra. The amplitude and shift of these peaks are related to the amplitude and frequency of the longitudinal wave, respectively. The bands due to the compression wave in this limit should be detectable in the experimental spectrum. Fitting of the experimental spectrum to the form calculated below will yield valuable information about the membrane compression wave.

A proposal on the function of unsaturated fatty acids and ionic lipids: the role of potential compaction in biological membranes

Unsaturated fatty acids are constituents of nearly all biological membranes. They are always present in membranes which possess transmembrane potentials. Two completely different biosynthetic routes have evolved (aerobic and anaerobic) for placing cis double bonds in the 9 position on the fatty acids of membrane lipids. Bacterial membranes contain primarily monounsaturated fatty acids, whereas eukaryote membranes contain a significant fraction of polyunsaturated fatty acids. The polyunsaturated fatty acids are concentrated in organelles, such as chloroplasts and mitochondria that are known to manipulate transmembrane potentials. I propose that the function of the unsaturated fatty acids is to facilitate the transmission of a local compaction of the membrane (in response to a transmembrane potential) laterally through the membrane. The role of the cis double bond at position 9 is twofold: first to create a kink in the chains of a large fraction of membrane fatty acids enhancing the separation of two regions in the membrane and second to enhance the rigidity of the membrane in the region between the head group and the 9 double bond. This ordered region contains those carbons proximal to the 9 carbon and which are in a regular array of trans conformations. The presence of a reasonable proportion of cis double bonds at position 9 will tend to maintain these trans conformations utilizing pi-pi (van der Waals) interactions between adjacent hydrocarbon chains at position 9. The disordered region contains the carbons distal to the 9 carbon. These have greater degrees of freedom and considerable gauche conformations. The role of the double bonds in the polyunsaturated fatty acids distal to carbon 9 is to facilitate trans bilayer pi-pi (van der Waals) interactions enhancing compaction of the bilayer during the electrostriction. I further propose that it is the function of the ionic headgroups to form an interlocking polyionic network which constitutes an elastic sheet. These ionic interactions would serve as the restoring force converting the compaction into a wave. The facilitation of the compaction of the bilayer together with the polyionic restoring force permits the membrane to transmit conformational changes from one transmembrane protein to another. Since transmembrane potentials are created and responded to by proteins each in a single location, it is thus proposed that a potential compaction wave emanates from the first protein in all directions in the plane of the membrane. The proposed wave would have both physical and electrical components. The electrohydrodynamic wave would require that the compaction oscillations be coupled to an oscillating electrical field. These proposals are applied to mitochondrial oxidative phosphorylation, and to transport across biological membranes.