We extend the work of Theofanous and Li [“On the physics of aerobreakup,” Phys. Fluids 20, 052103 (2008)] on aerobreakup physics of water-like, low viscosity liquid drops, to Newtonian liquids of any viscosity. The scope includes the full range of aerodynamics from near incompressible to high Mach number flows. The key physics of Rayleigh–Taylor piercing (RTP, first criticality) and of shear-induced entrainment (SIE, second and terminal criticality) are verified and quantified by new viscosity- and capillarity-based scalings for fluids of any viscosity. The relevance and predictive power of linear stability analysis of the Rayleigh–Taylor and Kelvin–Helmholtz problems (both including viscosity) is demonstrated for the RTP and the SIE regimes, respectively. The advanced stages of breakup and of the resulting particle-clouds are observed and clear definition and quantification of breakup times are offered.

We extend the work of Theofanous and Li [Phys. Fluids 20, 052103 (2008)] on aerobreakup physics of water-like, low viscosity liquid drops, and of Theofanous et al. [Phys. Fluids 24, 022104 (2012)] for Newtonian liquids of any viscosity, to polymerthickened liquids over wide ranges of viscoelasticity. The scope includes the full range of aerodynamics from near incompressible to supersonic flows and visualizations are recorded with μs/μm resolutions. The key physics of Rayleigh-Taylor piercing (RTP, first criticality) and of Shear-Induced Entrainment (SIE, second criticality) are verified and quantified on the same scaling approach as in our previous work, but with modifications due to the shear-thinning and elastic nature of these liquids. The same holds for the onset of surface waves by Kelvin-Helmholtz instability, which is a key attribute of the second criticality. However, in the present case, even at conditions well-past the first criticality, there is no breakup (particulation) to be found; instead the apparently unstable (extensively stretched into sheets) drops rebound elastically to reconstitute an integral mass. Such a resistance to breakup is found also past the second criticality, now with extensive filament formation that maintain a significant degree of cohesiveness, until the gas-dynamic pressure is high enough to cause filament ruptures. Thereby we define the onset of a third criticality peculiar to viscoelastic liquids-SIER, for SIE with ruptures. Past this criticality the extent of particulation increases and the characteristic dimension of fragments generated decreases in a more or less continuous fashion with increasing dynamic pressure. We outline a rheology-based scaling approach for these elasticity-modulated phenomena and suggest a path to similitude (with polymer and solvent variations) in terms of a critical rupture stress that can be measured independently. The advanced stages of breakup and resulting particle clouds are observed and a clear definition and quantification of breakup time is offered. C 2013 American Institute of Physics. Theofanous, Mitkin, and Ng Phys. Fluids 25, 032101 (2013) informs measures and concepts of operation under such an attack, which is a principal motivator of the present work. Notable early work is due to Wilcox et al. 7 and Matta et al. 8 As the solvents utilized in these efforts suggest, 6 in both cases interest derived from the subject of targeted atmospheric dissemination (high-speed delivery of Newtonian liquids yields fine mist which tends to remain airborne, evaporates significantly, and likely fails to impact the target). Wilcox dissolved poly-isobutyl methacrylate (PIBMA), polyvinyl acetate (PVA), or nitrocellulose (NC) in bis (2-ethyl hexyl) hydrogen phosphide (BIS) or di-butyl phthalate (DBP). Matta worked with poly-methyl methacrylate (PMMA) in diethyl malonate (DEM) as a solvent. Working with a shock tube Wilcox documented the essential role of viscoelasticity: "retardation" or "inhibition" of breakup at "concentrations as low as 0.1%. . . ...

We have established previously, that the spreading of liquids in granular porous media at low levels of saturation, typically less than 10% of the available void space, has very distinctive features in comparison to that at higher saturation levels. In particular, we showed that the spreading is controlled by a special type of diusional process, that its physics can be captured by an equation of the super-fast diusion class, and these ndings were supported by rst-of-a-kind experiments. In this paper, we take these ndings to the next level including deeper examination and exposition of the theory, an expanded set of experiments to address scaling properties, and systematic evaluations of the predictive performance against these experimental data, keeping in mind also potential practical applications.

We quantify experimentally the dispersal characteristics of dense particle clouds in high-speed interactions with an atmosphere. Focused on the fundamentals, the experiments, conducted in a large-scale shock tube, involve a well-characterized 'curtain' of (falling) particles that fully occupies the cross-sectional area of the expansion section. The particle material (glass) and size (∼1 mm) are fixed, as is the curtain thickness (∼30 mm) and the particle volume fractions in it, varying from ∼58 % at the top of the curtain to ∼24 % near the bottom. Thus, the principal experimental variable is the impacting shock strength, with Mach numbers varying from 1.2 to 2.6, and flow speeds that cover from subsonic (M IS ∼ 0.3) to transonic and supersonic (M IS ∼ 1.2). The peak shock pressure ratio, 7.6, yields a flow speed of ∼630 m s −1 , and a curtain expansion rate at ∼20 000 g. We record visually (high-speed, particle-resolving shadowgraphic method) the reflected/transmitted pressure waves and the transmitted contact wave, as well as the curtain displacements, and we measure the reflected/transmitted pressure transients. Data analysis yields simple rules for the amplitudes of the reflected pressure waves and the rapid cloud expansions observed, and we discover a time scaling that hints at a universal regime for cloud expansion. The data and these data-analysis results can provide the validation basis for numerical simulations meant to enable a deeper understanding of the key physics that drive this rather complex dispersal process.

The flow pattern around a thin strip horizontally towed at constant velocity in a continuously stratified liquid is visualized by conventional "Vertical slit-Foucault's knife", "Maksoutov's slit-thread" and "horizontal slit-regular grating" methods. Using these sensitive high-resolution methods enables to reveal new kind of the streaky structure including a sequence of thin sloping interfaces both on the strip surface and inside its wake. When velocity or distance from the strip increases, the streaks may be turned into the sloping or nearly horizontal interfaces. Reconnections of outer edges of the streaks result in appearance of a set of symmetrical "butterfly-like" vortices, which are perturbed by a shear flow inside the downstream wake. Lift forces caused by a slope of the strip produce an asymmetry of the wake and lead to fast degeneration of the streaky structures.

We present experiments and numerical simulations for an elementary paradigm of disperse multiphase flow: highly dilute, homogeneous, finite-dimension clouds of particles (curtains) hit by shock/blast waves in one dimension. In the experiments (particle volume fraction ${<}1\,\%$) the blasts that follow the shocks vary from low subsonic to supersonic, and we report data on curtain expansions and volume fraction distributions. The particle-resolving numerical simulations, run for the supersonic case, yield excellent agreement with all of these experimental data. We find that the essential feature for these good predictions is a flow choking phenomenon that entails a (particle) dispersive character of the flow down a volume fraction gradient (as at the downstream portions of the curtain). A most basic effective-field model is made to capture this gas dynamics by emulating the wake behind each particle, as seen in the particle-resolving direct Euler simulation (DES). On this basis, standard drag laws yield excellent agreement with the dispersive behaviour found in the experiment/DES, thus revealing a physics-based path to eventual well posedness of the mathematical model.

Blast traumatic brain injury (bTBI) is a leading contributor to combat-related injuries and death. Although substantial emphasis has been placed on blast-induced neuronal and axonal injuries, co-existing dysfunctions in the cerebral vasculature, particularly the microvasculature, remain poorly understood. Here, we studied blast-induced cerebrovascular dysfunctions in a rat model of bTBI (blast overpressure: 187.8 -18.3 kPa). Using photoacoustic microscopy (PAM), we quantified changes in cerebral hemodynamics and metabolism-including blood perfusion, oxygenation, flow, oxygen extraction fraction, and the metabolic rate of oxygen-4 h post-injury. Moreover, we assessed the effect of blast exposure on cerebrovascular reactivity (CVR) to vasodilatory stimulation. With vessel segmentation, we extracted these changes at the single-vessel level, revealing their dependence on vessel type (i.e., artery vs. vein) and diameter. We found that bTBI at this pressure level did not induce pronounced baseline changes in cerebrovascular diameter, blood perfusion, oxygenation, flow, oxygen extraction, and metabolism, except for a slight sO 2 increase in small veins (<45 lm) and blood flow increase in large veins ( ‡45 lm). In contrast, this blast exposure almost abolished CVR, including arterial dilation, flow upregulation, and venous sO 2 increase. This study is the most comprehensive assessment of cerebrovascular structure and physiology in response to blast exposure to date. The observed impairment in CVR can potentially cause cognitive decline due to the mismatch between cognitive metabolic demands and vessel's ability to dynamically respond to meet the demands. Also, the impaired CVR can lead to increased vulnerability of the brain to metabolic insults, including hypoxia and ischemia.

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