Shear-induced Mixing Research Articles

Mechanical and thermal properties as a function of matrix composition of all-oxide ceramic matrix composites fabricated by a sequential infiltration process

The microstructural design of matrices for all-oxide ceramic matrix composites (Ox/Ox) with damage tolerant fracture behavior is challenging. Therefore, the potential use of different matrix materials might be limited even though they appear to offer advantageous functional properties, such as thermal insulation or corrosion resistance. In this study, we investigated the hypothesis of simultaneously adjusting mechanical and functional properties by separate matrix phases within and between the fiber bundles in Ox/Ox. A sequential infiltration process was used to manufacture Ox/Ox with an alumina-zirconia matrix phase (high damage tolerance) and a mullite-alumina matrix phase (thermal insulation). The effect on the mechanical and thermal properties was governed by the infiltration sequences. A property combination was achieved for either the mechanical or the thermal behavior. This was due to a shear-induced mixing of the matrix phases during the lamination process, which renders it difficult to achieve distinctly separated matrix phases within the composite.

Experimental and numerical investigation on heat transfer analysis of square slotted pin-fin heat sinks

This paper introduces two novel designs of pin-fin heat sink (PHFS), namely the inline slotted pin-fin (ISP) and the V-shaped slotted pin-fin (VSP), to address rapid heat dissipation for one central/graphics processing unit (CPU/GPU) in a single-phase liquid cooling system. Experimental and numerical investigations are conducted to observe the effect of heat transfer for non-slotted and slotted pin-fin heat sinks under different Reynolds numbers (225-720) and heating powers (90-120 W). Parameters such as average wall temperature, Nusselt number, thermal resistance, pressure drop, and hydrothermal performance factor are evaluated. Comparative analyses show that the slotted pin-fin heat sinks exhibit higher Nusselt numbers (Nu) and lower thermal resistance (Rth) levels than that in the non-slotted pin-fin heat sinks, particularly at lower Reynolds numbers. At the Reynolds number 225 and power of 120 W, the inline and V-shaped slotted pin-fin heat sinks experience a 13.98% and 19.69% increase in Nu and a 12.65% and 15.41% reduction in Rth compared to the non-slotted one, respectively. The V-shaped slotted pin-fin heat sink achieves more than 5 ∘C reduction in the maximum wall temperature than the non-slotted one. The slotted structure reduces pressure drop losses by minimizing dead flow areas on one hand. On the other hand, the inclined slots generate vortices behind the fins, increase shear-induced mixing and thus improve the force convection cooling performance.

Bacteria bioluminesce in response to fluid shear.

Many bacteria exhibit bioluminescence, through which they convert chemical energy into light and when in abundance, can even light up the sea. While it is known that mechanical stress can trigger bioluminescence in eukaryotes such as dinoflagellates, it has often been suggested that this is not true for bacteria. In this work, we show, by joining imaging with rheology experiments, that bacteria indeed emit light in response to shear within seconds of stimulation. The light intensity increases sigmoidally with shear rate and the response to ramping of shear exhibits species-dependent hysteresis. The sigmoidal response is consistent with the response of shear-activated ion channels in eukaryotic cells leading to calcium signaling, and hysteresis is a common feature of ion channels in other systems. We capture the full range of experimental observations using a simple two-state kinetic model with adaptation. The two-state dynamics provides the time scales for the opening and closing of the ion channels and are found to be of the order of seconds. The time scale for adaptation of the total number of channels to shear is larger and introduces the unsteadiness in the system that leads to hysteresis. An additional fourth-time scale is found to be present that results in the decay of light under long steady shear. The role of shear-induced mixing of oxygen in the suspensions will also be discussed.

Whither warming in the Galápagos?

The Galápagos Islands play host to an iconic ecosystem—a UNESCO World Heritage Site and the second largest marine reserve in the world, home to several endangered species. The waters off the west coast of the Galápagos are also one of the few places in the world ocean that are presently cooling, with potentially significant ecological consequences of this local reprieve from global warming. Here I show, using a recently developed high-resolution ocean state estimate, that the observed cooling in the Galápagos is the result of a strengthening of the wind-driven equatorial ocean circulation. An acceleration and shift of the Equatorial Undercurrent, which can be attributed to a strengthening of the cross-equatorial component of the trade wind in response to the interhemispheric gradient in surface warming, leads to a 54% increase in upwelling velocity along the western Galápagos Islands as well as increased shear-induced mixing. Analogous to other so-called “cold blobs,” such as the one south of Greenland in the North Atlantic, this is an early and important sentinel of a broader change in the tropical ocean circulation. Thus far, and for perhaps the very near future, the western shores of the Galápagos appear to offer refuge from some of the deleterious impacts of anthropogenic climate change including suppressed upwelling and surface warming.

Complicated Structure Change during Capillary Extrusion of Binary Blends of Polycarbonate and Poly(methyl methacrylate).

The effects of pressure and shear rate on the miscibility of binary blends comprising bisphenol-A polycarbonate (PC) and low molecular weight poly(methyl methacrylate) (PMMA) were investigated using a capillary rheometer. Both pressure and shear rate affected the miscibility. The examination of an extruded strand of the blend provided information about the cause of the phase change. Under high pressure, pressure-induced demixing occurred at temperatures below the lower critical solution temperature (LCST) of the blend. Consequently, the extruded strand became opaque throughout. During shear-induced mixing/demixing, a part of the strand became opaque because of the distribution of the shear rate in the strand. For example, during shear-induced demixing, only the exterior of the strand, i.e., the high shear rate region, became opaque. Above the LCST, shear-induced mixing occurred, and only the center region of the strand became opaque.

Shear-induced mixing of granular materials featuring broad granule size distributions.

Granular flows during a shear-induced mixing process are studied using discrete element methods. The aim is to understand the underlying elementary mechanisms of transition from unmixed to mixed phases for a granular material featuring a broad distribution of particles, which we investigate systematically by varying the strain rate and system size. Here the strain rate varies over four orders of magnitude and the system size varies from ten thousand to more than a million granules. A strain rate-dependent transition from quasistatic to purely inertial flow is observed. At the macroscopic scale, the contact stresses drop due to the formation of shear-induced instabilities that serves as an onset of granular flows and initiates mixing between the granules. The stress-drop displays a profound system size dependence. At the granular scale, mixing dynamics are correlated with the formation of shear bands, which result in significantly different timescales of mixing, especially for those regions that are close to the system walls and the bulk. Overall, our results reveal that although the transient dynamics display a generic behavior, these have a significant finite-size effect. In contrast, macroscopic behaviors at steady states have negligible system size dependence.

The influence of far field stratification on shear-induced turbulent mixing

We compare the properties of the turbulence induced by the breakdown of Kelvin–Helmholtz instability (KHI) at high Reynolds number in two classes of stratified shear flows where the background density profile is given by either a linear function or a hyperbolic tangent function, at different values of the minimum initial gradient Richardson number${{Ri}}_0$. Considering global and local measures of mixing defined in terms of either the irreversible mixing rate$\mathscr {M}$associated with the time evolution of the background potential energy, or an appropriately defined density variance dissipation rate$\chi$, we find that the proliferation of secondary instabilities strongly affects the efficiency of mixing early in the flow evolution, and also that these secondary instabilities are highly sensitive to flow perturbations that are added at the point of maximal (two-dimensional) billow amplitude. Nevertheless, mixing efficiency does not appear to depend strongly on the far field density structure, a feature supported by the evolution of local horizontally averaged values of the buoyancy Reynolds number${Re}_b$and gradient Richardson number${Ri}_g$. We investigate the applicability of various proposed scaling laws for flux coefficients$\varGamma$in terms of characteristic length scales, in particular discussing the relevance of the overturning ‘Thorpe scale’ in stratified turbulent flows. Finally, we compare a variety of empirical model parameterizations used to compute diapycnal diffusivity in an oceanographic context, arguing that for transient flows such as KHI-induced turbulence, simple models that relate the ‘age’ of a turbulent event to its mixing efficiency can produce reasonably robust mixing estimates.

Observational case study of a persistent cold pool and gap flow in the Columbia River Basin

AbstractPersistent cold pools form as layers of cold stagnant air within topographical depressions mainly during wintertime when the near-surface air cools and/or the air aloft warms and daytime surface heating is insufficient to mix out the stable layer. An area often affected by persistent cold pools is the Columbia River Basin in the Pacific Northwest, when a high-pressure system east of the Cascade Range promotes radiative cooling and easterly flow. The only major outflow for the easterly flow is through the narrow Columbia River Gorge which cuts through the north-south oriented Cascade Range and often experiences very strong gap flows. Observations collected during the Second Wind Forecast Improvement Project (WFIP2) are used to study a persistent cold pool in the Columbia River Basin between 10-19 Jan 2017 which was associated with a strong gap flow. We used data from various remote sensing and in situ instruments and a optimal estimation physical retrieval to obtain thermodynamic profiles to address the temporal and spatial characteristics of the cold pool and gap flow and to investigate the physical processes involved during formation, maintenance and decay. While large-scale temperature advection occurred during all phases, we found that the cold pool vertical structure was modulated by the existence of low-level clouds and that turbulent shear-induced mixing and downslope wind storms likely played a role during its decay.

Microstructural progression of shear-induced mixing in a CuNi alloy

Shear deformation has been highlighted in multiple research efforts for its ability to impart novel microstructures that demonstrate improvements in mechanical properties. When used to process and densify powdered material, these shear-based consolidation techniques are commonly referred to as friction consolidation (FC). In this paper, the microstructural evolution from compacted Cu and Ni powders to a consolidated Cu0.5Ni0.5 alloy is examined. Various stages of porosity reduction and deformation are shown. Deformation was observed to accumulate preferentially in the more ductile material early in the process, leading to the formation of a tortuous microstructural zone. Porosity reduction was extensive, decreasing from ~65% in the pre-compacted state to ~1% in the fully consolidated alloy. The final consolidated alloy showed a ~2× hardness improvement over the unalloyed, compacted material. Unique aspects of this work include demonstration of FC processing to produce an equiaxed, sub-micrometer grain size in samples within a 0.5 to 2 min processing time. The results point to future opportunities to implement shear deformation during powder densification to expand the range of property outcomes in bulk materials.

Instabilities of a Time-Dependent Shear Flow

AbstractThis study offers a systematic stability analysis of unsteady shear flows representing large-scale, low-frequency internal waves in the ocean. The analysis is based on the unbounded time-dependent Couette model. This setup makes it possible to isolate the instabilities caused by uniform shear from those that can be attributed to resonant triad interactions or to the presence of inflection points in vertical velocity profiles. Linear analysis suggests that time-dependent spatially uniform shears are unstable regardless of the Richardson number (Ri). However, the growth rate of instability monotonically decreases with increasing Ri and increases with increasing frequency of oscillations. Therefore, models assuming a steady basic state—which are commonly used to conceptualize shear-induced instability and mixing—can be viewed as singular limits of the corresponding time-dependent systems. The present investigation is focused on the supercritical range of Richardson numbers (Ri > 1/4) where steady parallel flows are stable. An explicit relation is proposed for the growth rate of shear instability as a function of background parameters. For moderately supercritical Richardson numbers (Ri ~ 1), we find that the growth rates obtained are less than, but comparable to, those expected for Kelvin–Helmholtz instabilities of steady shears at Ri < 1/4. Hence, we conclude that the instability of time-dependent flows could represent a viable mixing mechanism in the ocean, particular in regions characterized by relatively weak wave activity and predominantly supercritical large-scale shears.

The Lagrangian Coherent Structure and the Sediment Particle Behavior in the Lock Exchange Stratified Flows

Ku, H. and Hwang, J.H., 2018. The Lagrangian Coherent Structure and the sediment particle behavior in the lock exchange stratified flows. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 976–980. Coconut Creek (Florida), ISSN 0749-0208.Shear-induced turbulent mixing in the stratified flow is widely known as an import driving force of the circulations of coastal water and transports of small particles such as suspended sediments and aquatic micro-organisms. Therefore, this work mainly studies the influence of the vertical mixing and stratification on transports of gravitationally-falling sediment particles. In particular, Lagrangian Coherent Structure (LCS) of unsteady flow, which can be determined as finite-time Lyapunov exponents (FTLE) field of fluid particles is compared with the conventional Lagrangian particle trajectories which is obtained by solving particle motion equations. The shear-induced turbulent mixing of water mass and its effect on the transport of particles are investigated by a series of the lock-exchange flow experiments using the large eddy simulation (LES). When two water bodies have sufficiently high density difference, the Kelvin-Helmholtz billows are well developed along the lateral boundary and they trap the sediment particles which were initially located close to a transient layer at the half-depth of the channel. The simulation results also show that the local stretching rate of fluid particles dominates the transport of the gravitationally falling particles. In particular, the sediment particles are trapped within the relatively low FTLE field. The effect of sediment mass dropped the particles downward but not allows particles pass through the lower FTLE fields.

Internal wave turbulence at a biologically rich Mid-Atlantic seamount.

The turbulence regime near the crest of a biologically rich seamount of the Mid-Atlantic Ridge southwest of the Azores was registered in high spatial and temporal resolution. Internal tides and their higher harmonics dominate the internal wave motions, producing considerable shear-induced turbulent mixing in layers of 10–50 m thickness. This interior mixing of about 100 times open-ocean interior values is observed both at a high-resolution temperature sensor mooring-site at the crest, 770 m water depth being nearly 400 m below the top of the seamount, and a CTD-yoyo site at the slope off the crest 400 m horizontally away, 880 m water depth. Only at the mooring site, additionally two times higher turbulence is observed near the bottom, associated with highly non-linear wave breaking. The highest abundance of epifauna, notably sponges, are observed just below the crest and 100 m down the eastern slope (700–800 m) in a cross-ridge video-camera transect. This sponge belt is located in a water layer of depressed oxygen levels (saturation 63±2%) with a local minimum centered around 700 m. Turbulent mixing supplies oxygen to this region from above and below and is expected to mix nutrients away from this biodegraded layer towards the depth of highest abundance of macrofauna.

Shear-driven transport and mixing in the interiors of massive stars

AbstractTurbulent transport and mixing generated by hydrodynamic instabilities triggered by rotation gradients are key mechanisms in the evolution of massive stars. We present here a summary of the progresses on shear-induced mixing obtained with numerical simulations, along with a new prescription for horizontal turbulence.

Internal deformation of a muddy gravity flow and its interaction with the seafloor (site C0018 of IODP Expedition 333, Nankai Trough, SE Japan)

The processes of flow deformation of marine mass-transport sediments, including their ability to affect the underlying substrate and add mass during sediment flow events, are addressed based on sedimentological analyses of strata from the distal part of a ∼61-m-thick mass-transport deposit (MTD 6) drilled during Integrated Ocean Drilling Program (IODP) Expedition 333. Our analyses, supported by 3D seismic data, show a cohesive density flow deformed by folding, faulting and shear, except for its lowermost part (∼7 m), where no deformation and sediment entrainment was identified. While the lowermost part moved as rigid sediment, the underlying sand layer acted as the basal shear zone for this part of the distal MTD 6. This shear zone was restricted to the sand, not involving the overlying sediments. From this, the studied part of MTD 6 was found to represent a case where the flow behaviour at least partly depended on the location and properties of the underlying sand layer, a situation that so far has received little attention in studies of marine flows. Our results also show that shear-induced mixing, located by the initial layering, is an important process in the flow transformation from cohesive slumps to mud flows and that this may occur over short distances (<4 km) without involving disintegration into blocks, probably due to only moderate prefailure consolidation of the sediments involved. In conclusion, we find that the bulk part of the flow was self-contained from a mass balance point of view and that that the overall amount of entrainment was limited.

Stratified coastal ocean interactions with tropical cyclones.

Hurricane-intensity forecast improvements currently lag the progress achieved for hurricane tracks. Integrated ocean observations and simulations during hurricane Irene (2011) reveal that the wind-forced two-layer circulation of the stratified coastal ocean, and resultant shear-induced mixing, led to significant and rapid ahead-of-eye-centre cooling (at least 6 °C and up to 11 °C) over a wide swath of the continental shelf. Atmospheric simulations establish this cooling as the missing contribution required to reproduce Irene's accelerated intensity reduction. Historical buoys from 1985 to 2015 show that ahead-of-eye-centre cooling occurred beneath all 11 tropical cyclones that traversed the Mid-Atlantic Bight continental shelf during stratified summer conditions. A Yellow Sea buoy similarly revealed significant and rapid ahead-of-eye-centre cooling during Typhoon Muifa (2011). These findings establish that including realistic coastal baroclinic processes in forecasts of storm intensity and impacts will be increasingly critical to mid-latitude population centres as sea levels rise and tropical cyclone maximum intensities migrate poleward.

Crystallization of equimolar poly(l-lactide)/poly(d-lactide) blend below the melting point of α crystals under shear

The isothermal crystallization behavior of a 1:1 poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) mixture under pulsed shear was investigated by in-situ polarized light optical microscopy (POM) and wide angle X-ray scattering (WAXS). The effects of shear rate and temperature were studied. The results showed that application of shear flow increased the nucleation density, crystallization rate and degree of orientation for both stereocomplex crystals (SC) and α crystals. Interestingly, when the crystallization temperature was below the melting temperature of α crystals, the phase content of SC increased with shear rate, while the α content displayed a bell-shaped curve with regard to shear rate. The structure formation in this system was closely related to the shear-induced nucleation, phase mixing and chain mobility.

Atypical anticlockwise internal tidal motions in the deep ocean

In the ocean, horizontal motions associated with freely propagating semidiurnal tidal inertia-gravity waves mainly describe an ellipse that is traversed in a clockwise direction in the Northern Hemisphere. In this article, rare observations of anticlockwise polarised semidiurnal motions are presented from deep North-Atlantic mooring sites. Anticlockwise motions are found to occur 1–20% of the time, irrespective of latitude and (weak) vertical density stratification. In a deep narrow channel like Kane Gap however, anticlockwise motions dominate over clockwise. The statistics of turbulence match the occurrence of polarisation change. It is unclear whether these observations represent theoretical predictions of reflected baroclinic Poincaré waves, or those involving the effects of the horizontal Coriolis parameter on internal wave propagation in weak stratification. The observed switching between clockwise and anticlockwise polarisation with time at a fixed position cannot be explained by the internal wave theory using the traditional approximation. Instead, the polarisation switching points at baroclinic effects involving varying background conditions like stratification and shear-induced mixing. It is suggested the polarisation statistics may be used as a diagnostic tool for such varying conditions.

Self-organization of Cu–Ag during controlled severe plastic deformation at high temperatures

Abstract

Microstructure turbulence and diffusivity parameterization in the tropical and subtropical Atlantic, Pacific and Indian Oceans during the Malaspina 2010 expedition

Measurements of microstructure turbulence were carried out, in the upper 300m, in the tropical and subtropical Atlantic and Pacific oceans during the Malaspina 2010 expedition, by using a microstructure turbulence (MSS) profiler. Diapycnal diffusivity (Kρ) was estimated from dissipation rates of turbulent kinetic energy (ε) measured by the MSS profiler, and also from hydrographic and meteorological data by using the K-profile parameterization (KPP). In the mixing layer, averaged Kρ (169×10−4m2s−1) and ε(16.8×10−8Wkg−1) were three and one orders of magnitude higher, respectively, compared to the ocean interior (0.59×10−4m2s−1 and 1.0×10−8Wkg−1). In general, the KPP showed a good agreement with diffusivity estimates derived from microstructure observations, both in the mixing layer and in the ocean interior. The KPP also reproduced the main regional patterns observed in the ocean interior. The analysis of turbulence generation mechanisms below the mixing layer showed that shear-induced mixing was more important in those regions influenced by the equatorial undercurrent, where averaged diffusivity was 2.27–3.62×10−4m2s−1. Favorable conditions for salt fingers formation were more frequently observed in the Atlantic, where, as a consequence of this process, diffusivity could increase up to 20%. This result could have important implications for the transport of heat and dissolved substances in these regions.

Publisher’s Note: Shear-Induced Mixing Governs Codeformation of Crystalline-Amorphous Nanolaminates [Phys. Rev. Lett.113, 035501 (2014)

Publisher’s Note: Shear-Induced Mixing Governs Codeformation of Crystalline-Amorphous Nanolaminates [Phys. Rev. Lett.<b>113</b>, 035501 (2014)