Unidirectional Regimes Research Articles

Colloidal Multi-Dot Nanorods.

Colloidal nanorod heterostructures consisting of multiple quantum dots within a nanorod (n-DNRs, where n is the number of quantum dots within a nanorod) are synthesized with alternating segments of CdSe "dot" and CdS "rod" via solution heteroepitaxy. The reaction temperature, time dependent ripening, and asymmetry of the wurtzite lattice and the resulting anisotropy of surface ligand steric hindrance are exploited to vary the morphology of the growing quantum dot segments. The alternating CdSe and CdS growth steps can be easily repeated to increment the dot number in unidirectional or bidirectional growth regimes. As an initial exploration of electron occupation effects on their optical properties, asymmetric 2-DNRs consisting of two dots of different lengths and diameters are synthesized and are shown to exhibit a change in color and an unusual photoluminescence quantum yield increase upon photochemical doping.

Self-oscillating buckling and postbuckling of a liquid crystal elastomer disk under steady illumination

Self-oscillating systems can absorb energy from steady environment to maintain continuous movements without the aid of extra controller. Different from the existing abundant self-oscillating systems, self-buckling has the advantages of sudden energy explosion and large displacement, and has potential application in the fields of jumping robots, rescue, military industries, mechano-logics and so on. However, it is a challenging task in avoiding the tendency of reaching thermodynamic equilibrium for a self-oscillating system. In this paper, we creatively develop a self-oscillating liquid crystal elastomer (LCE) disk, which is capable of buckling continuously and periodically under steady illumination. Based on nonlinear plate theory and dynamic LCE model, a nonlinear dynamic model of the self-buckling LCE disk is formulated. By series expansions and Runge-Kutta method, the dynamic buckling and postbuckling of the LCE disk under steady illumination is numerically calculated. The LCE disk under steady illumination can develop into two motion regimes: the static regime and the self-buckling regime, including alternating and unidirectional self-buckling regimes. The self-buckling of the disk can be triggered by controlling several key system parameters. In addition, the frequency and amplitude of the self-buckling can also be modulated by these parameters. The self-buckling LCE disk broadens the design ideas in the fields of new robots, energy conversion, and biomimetic.

Seagrass coastal protection services reduced by invasive species expansion and megaherbivore grazing

Abstract Seagrasses provide an important ecosystem service by creating a stable erosion‐resistant seabed that contributes to effective coastal protection. Variable morphologies and life‐history strategies, however, are likely to impact the sediment stabilization capacity of different seagrass species. We question how opportunistic invasive species and increasing grazing by megaherbivores may alter sediment stabilization services provided by established seagrass meadows, using the Caribbean as a case study. Utilizing two portable field‐flumes that simulate unidirectional and oscillatory flow regimes, we compared the sediment stabilization capacity of natural seagrass meadows in situ under current‐ and wave‐dominated regimes. Monospecific patches of a native (Thalassia testudinum) and an invasive (Halophila stipulacea) seagrass species were compared, along with the effect of three levels of megaherbivore grazing on T. testudinum: ungrazed, lightly grazed and intensively grazed. For both hydrodynamic regimes, the long‐leaved, dense meadows of the climax species, T. testudinum provided the highest stabilization. However, the loss of above‐ground biomass by intensive grazing reduced the capacity of the native seagrass to stabilize the surface sediment. Caribbean seagrass meadows are presently threatened by the rapid spread of the invasive opportunistic seagrass, H. stipulacea. The dense meadows of H. stipulacea were found to accumulate fine sediment, and thereby, appear to be effective in reducing bottom shear stress during calm periods. This fine sediment within the invasive meadows, however, is easily resuspended by hydrodynamic forces, and the low below‐ground biomass of H. stipulacea make it susceptible to uprooting during storm events, potentially leaving large regions vulnerable to erosion. Overall, this present study highlights that intensive megaherbivore grazing and opportunistic invasive species threaten the coastal protection services provided by mildly grazed native species. Synthesis. Seagrass meadows of dense, long‐leaved species stabilize the sediment surface and maintain the seabed integrity, thereby contributing to coastal protection. These services are threatened by intensive megaherbivore grazing, which reduces the stability of the surface sediment, and opportunistic invasive species, which are susceptible to uprooting in storms and thereby can leave the seabed vulnerable to erosion.

Sediment resuspension due to near-bed turbulent coherent structures in the nearshore

Laboratory and field experiments have suggested that near-bed sediment resuspension is an intermittent process influenced by turbulent coherent structures. This is in contrast to many sediment transport theories that relate sediment transport rates to mean near bed flow parameters. This study presents observations of turbulent bursting events obtained from two contrasting environments dominated by unidirectional and oscillatory flow regimes with mean currents higher than the critical value for sand transport. In agreement with previous laboratory and field experiments the data investigated in this study (both in unidirectional current and oscillatory flow environments) indicated that sediment resuspension was an event based system with sediment flux controlled by ejections and sweeps. Both the magnitude of the Reynolds stress and duration of the events contributed to sediment resuspension. Wavelet analysis confirmed that over time turbulence occurred in slowly evolving clusters that were closely followed by periods of strong resuspension near the bed, evolving from the primary leading scales at low frequencies, and decaying in time after the termination of the turbulent agitation. The results highlight the necessity of considering the instantaneous Reynolds shear stress effects in the existing sediment transport equations which are developed based on a time-averaged flow parameters and mean a critical velocity. Field observations presented in this study highlight the importance of considering turbulence events along with the concept of impulse as an essential parameter to be considered into future modelling of sediment movement under both unidirectional and oscillatory flows.

Emergent structures in reaction-advection-diffusion systems on a sphere.

We demonstrate unusual effects due to the addition of advection into a two-species reaction-diffusion system on the sphere. We find that advection introduces emergent behavior due to an interplay of the traditional Turing patterning mechanisms with the compact geometry of the sphere. Unidirectional advection within the Turing space of the reaction-diffusion system causes patterns to be generated at one point of the sphere, and transported to the antipodal point where they are destroyed. We illustrate these effects numerically and deduce conditions for Turing instabilities on local projections to understand the mechanisms behind these behaviors. We compare this behavior to planar advection which is shown to only transport patterns across the domain. Analogous transport results seem to hold for the sphere under azimuthal transport or away from the antipodal points in unidirectional flow regimes.

Isolator-free switchable uni- and bidirectional hybrid mode-locked erbium-doped fiber laser.

An Erbium-doped fibre ring laser hybrid mode-locked with single-wall carbon nanotubes (SWNT) and nonlinear polarisation evolution (NPE) without an optical isolator has been investigated for various cavity conditions. Precise control of the state of polarisation (SOP) in the cavity ensures different losses for counter-propagating optical fields. As the result, the laser operates in quasi-unidirectional regime in both clockwise (CW) and counter-clockwise (CCW) directions with the emission strengths difference of the directions of 22 dB. Furthermore, by adjusting the net birefringence in the cavity, the laser can operate in a bidirectional generation. In this case, a laser pumped with 75 mW power at 980 nm generates almost identical 790 and 570 fs soliton pulses with an average power of 1.17 and 1.11 mW. The operation stability and pulse quality of the soliton pulses in both unidirectional regimes are highly competitive with those generated in conventional ring fibre lasers with isolator in the cavity. Demonstrated bidirectional laser operation can find vital applications in gyroscopes or precision rotation sensing technologies.

Asymmetric transmission in prisms using structures and materials with isotropic-type dispersion.

It is demonstrated that strong asymmetry in transmission can be obtained at the Gaussian beam illumination for a single prism based on a photonic crystal (PhC) with isotropic-type dispersion, as well as for its analog made of a homogeneous material. Asymmetric transmission can be realized with the aid of refraction at a proper orientation of the interfaces and wedges of the prism, whereas neither contribution of higher diffraction orders nor anisotropic-type dispersion is required. Furthermore, incidence toward a prism wedge can be used for one of two opposite directions in order to obtain asymmetry. Thus, asymmetric transmission is a general property of the prism configurations, which can be obtained by using simple geometries and quite conventional materials. The obtained results show that strong asymmetry can be achieved in PhC prisms with (nearly) circular shape of equifrequency dispersion contours, in both cases associated with the index of refraction 0<n<1 and n>1. For the comparison purposes, results are also presented for solid uniform non-magnetic prisms made of a material with the same value of n. It is shown in zero-loss approximation that the PhC prism and the ultralow-index material prism (0<n<1) can replace each other in some cases without affecting the scenario of asymmetric transmission. Moreover, the PhC prism and the solid dielectric prism can show the same scenario at n>1. Possible contributions of scattering on the individual rods and diffraction on the wedge to the resulting mechanism are discussed. Analogs of unidirectional splitting and unidirectional deflection regimes, which are known from the studies of PhC gratings, are obtained in PhC prisms and solid uniform prisms, i.e. without higher diffraction orders.

Predictability of dune activity in real dune fields under unidirectional wind regimes

AbstractWe present an analysis of 10 dune fields to test a model‐derived hypothesis of dune field activity. The hypothesis suggests that a quantifiable threshold exists for stabilization in unidirectional wind regimes: active dunes have slipface deposition rates that exceed the vegetation deposition tolerance, and stabilizing dunes have the opposite. We quantified aeolian sand flux, slipface geometry, and vegetation deposition tolerance to directly test the hypothesis at four dune fields (Bigstick, White Sands Stable, White Sands Active, and Cape Cod). We indirectly tested the hypothesis at six additional dune fields with limited vegetation data (Hanford, Año Nuevo, Skagen Odde, Salton Sea, Oceano Stable, and Oceano Active, “inverse calculation sites”). We used digital topographic data and estimates of aeolian sand flux to approximate the slipface deposition rates prior to stabilization. Results revealed a distinct, quantifiable, and consistent pattern despite diverse environmental conditions: the modal peak of prestabilization slipface deposition rates was 80% of the vegetation deposition tolerance at stabilized or stabilizing dune fields. Results from inverse calculation sites indicate deposition rates at stabilized sites were near a hypothesized maximum vegetation deposition tolerance (1 m a−1), and active sites had slipface deposition rates much higher. Overall, these results confirm the hypothesis and provide evidence of a globally applicable, simple, and previously unidentified predictor for the dynamics of vegetation cover in dune fields under unidirectional wind regimes.

Motor Coordination in Long-Distance Transport in Axons

Retrograde transport of nerve growth factor signaling endosomes by microtubular motors, from the axon terminals to cell bodies, is vital for the survival of neurons. The robustness of this fast long-distance axonal transport and biased directionality could be attributed to the cooperative mechanics of multiple motors and/or intracellular regulation mechanisms. Here, we present a comprehensive motion analysis of retrograde nerve growth factor (NGF)-endosome trajectories in axons to show that cooperative motor mechanics and intracellular motor regulation are both important factors determining the endosome directionality. We used quantum dot (QD) to fluorescently label NGF and acquired trajectories of retrograde QD-NGF-endosomes with < 20 nm accuracy at 32 Hz, using pseudo-total internal reflection fluorescence imaging. Using a combination of transient motion analysis and Bayesian parsing, we segregated the trajectories into sustained periods of retrograde (dynein-driven) motion, constrained pauses and brief anterograde reversals. Mean square displacement analysis and the temperature dependence of transient reversals confirm that motors of opposite polarities (dyneins and kinesins) are both active on the endosomes during retrograde transport. Stochastic multi-motor model simulations show that the biased directionality as well as several statistical metrics of NGF-endosome transport can only be simulated reasonably by assuming that the microtubule-binding affinity of kinesin is down-regulated. Specifically, the simulations suggest that the NGF-endosomes are driven on average by 4-7 active dyneins and 1-3 down-regulated kinesins. These observations are corroborated by the dynamics of endosomes detaching under load in axons; showcasing the cooperativity of multiple dyneins and the subdued activity of kinesins. We discuss the ramifications of our results for intracellular transport regulation, in conjunction with recent studies on cellular cargo in a wide range of motility (bidirectional to unidirectional) regimes.

Parameters of solar wind electron heat‐flux pitch‐angle distributions and IMF topologies

Pitch‐angle distributions (PADs) of solar wind heat‐flux (HF) electrons are used as a proxy for interplanetary magnetic field (IMF) topology. Unidirectional PADs yield IMF solar polarities, and bidirectional electron (BDE) PADs are interpreted as signatures of closed fields. A general perception exists that the directionalities are easily distinguished, clearly defining open and closed IMFs. We quantify PADs with the ratios of the HF parallel and anti‐parallel to the IMF to that perpendicular to the IMF plotting these parameters against each other in a directionality distribution for six years of electron data from the 3DP experiment on the Wind satellite. This bimodal plot clearly shows the unidirectional populations, but shows no evidence for a separate bidirectional HF population. A similar plot of magnetic clouds is double‐banded with no evidence of a bifurcation between bidirectional and unidirectional regimes. In conclusion, this basic parameterization shows no distinction between open and closed field topologies.

The effect of flow and morphology on boundary layers in the scleractinians Dichocoenia stokesii (Milne-Edwards and Haime) and Stephanocoenia michilini (Milne-Edwards and Haime).

Mass transfer characteristics of scleractinian corals are affected by their skeletal morphology and the concentration gradients that develop as a consequence of the interactions of their morphology and biomass with the overlying seawater. These interactions can have a profound effect on coral metabolism. In this study, boundary layer characteristics were compared between different size colonies of the corals Dichocoenia stokesii and Stephanocoenia michilini to determine the relative roles of colony size and corallite structures (i.e. surface roughness) in mass transfer. Colonies of both species were rounded in shape, but differed in small-scale roughness as measured by the elevation of corallites. Additionally, D. stokesii had a greater aspect ratio than S. michilini, and their colonies were slightly taller for a given diameter. Boundary layers were characterized by placing dead coral skeletons in a flow tank and estimating shear velocities (u(*)) at different flow speeds. The effects of flow speed, size, and roughness on shear velocities were estimated for two juvenile size classes (10-20 and 30-40 mm diameter) of each species that were exposed to unidirectional flow regimes (4 and 17 cm s(-1)). Shear velocities were significantly greater in high, compared to low flow, and there was a significant interaction between colony size and surface roughness; the interaction was caused by a difference in magnitude, rather than direction, of the effect of roughness and size on u(*). Thus, there was a greater degree of turbulence at high flow compared to low flow, regardless of roughness or size, and the greatest turbulence occurred over large colonies of D. stokesii at high flow. Together, these results suggest that boundary layers around small corals are heavily influenced by upstream roughness elements, and more strongly affected by flow regimes than skeletal features. The relationship between colony morphology (i.e. aspect ratio and, possibly, surface roughness) and boundary layer characteristics may be non-linear in small corals.

Effects of unidirectional and oscillatory water flow on nitrogen fixation (acetylene reduction) in coral reef algal turfs, Kaneohe Bay, Hawaii

Rates of acetylene reduction (nitrogenase activity) by algal turf communities from Kaneohe Bay, Oahu, Hawaii, were measured as a function of increasing water flow speeds under unidirectional and oscillatory flow regimes in an enclosed incubation chamber. Water flow speeds, shear stress, and turbulence intensities were measured with thermistor probes in the chamber and over the turfs in the field. The thickness of the boundary layer varied significantly and linearly with bulk water flow speeds in the field. Although the boundary layer in the chamber also decreased with increasing flow speeds, turbulence intensity and flow speeds in the chamber were mostly lower than those typically measured in the field. Rates of acetylene reduction were positively related to water flow speed. Oscillatory water flow, which increased turbulence intensity five times, resulted in a significant increase in acetylene reduction compared to unidirectional flow. Even at the lowest mean flow speeds measured in the field (<0.1 m s −1), mean rates of acetylene reduction (27 nmol ethylene cm −2 h −1±11 SD) were high under oscillatory flow. Equivalent high rates under unidirectional flow were not achieved until flow speed was more than doubled. The slope of log–log linear regressions of acetylene reduction versus flow speed was 0.5 for both oscillatory and unidirectional regimes. This result suggests that acetylene reduction rates in the chamber were controlled by mass transfer of a rate-controlling solute, such as acetylene or oxygen (inhibitory to nitrogenase), through a laminar diffusion boundary layer. Because coral reefs exist in areas of very low nitrogen availability, nitrogen fixation is fundamentally important for coral reef primary production and biogeochemistry. Yet, current understanding of nitrogen fixation on coral reefs has been derived primarily from measurements made under unnatural conditions of no or low water flow. This study lends support to the importance of water flow as a major control of the metabolism of organisms occupying coral reefs.

Superradiance phenomenon in semiconductor magnetooptics

We have established the feasibility of collective spontaneous recombination and superradiance of electron-hole plasma in strong magnetic fields B ∼ (10 5−10 6)G. The limiting parameters of ultrashort pulses are found. Linear and nonlinear stages of superradiance in unidirectional (without resonator) and single-mode (in resonator) regimes are investigated. The estimates for GaAs show that coherent pulses of recombination superradiance of duration ∼(0.1-1)ps and intensity ⪆100 MW cm -2 may be obtained.

Unusual sedimentary structures in the Oregon coastal dunes

The Oregon coastal dunes have evolved in a temperate, rainy climate with a bimodal onshore wind regime. The south-west winds are the strongest of the two major wind groups, and coincide with the winter rainy season. These winds cause wet sand accumulations on north-facing slipfaces, that eventually fail in the form of huge slump plates. The plates have distinctive features that should make it possible to identify them in ancient rocks. The rainy climate maintains a high water table in the dunes. This water table emerges in places to form freshwater interdunal ponds and sabkhas that also leave distinctive deposits. The alternation of the strong south-west winds during the rainy winter season with the persistent but weaker north-west winds of summer causes episodic advance (winter) and scour and retreat (summer) of the large ‘oblique’ or transverse ridge dunes for which the area is famous. An upwind to downwind exposure of a transverse ridge dune carved by Tenmile Creek revealed a complex internal structure consisting of north-dipping sets of cross strata formed by south-west winds separated by gently sloping bounding surfaces formed by wind reversals. This internal structure is similar to that hypothesised for compound bedform or ‘draa’ deposition in unidirectional wind regimes, because the appearance is that of multiple bedforms migrating down the lee slope of a larger bedform. This pattern of bedform development may be relatively common, as bimodal and complex wind regimes are common throughout the world. The internal structure of the Tenmile Creek dune provides a modern example of bounding surfaces in a draasized dune that are formed by wind reversals. As has already been pointed out bounding surface models cannot always be predicated on continuous forward movement of bedforms. The draa model of compound bedforms may possibly be expanded to include single bedforms with multiple slipfaces in time due to seasonal changes in wind regime.

Flow-reversal flow-injection analysis Enhancement of flow-injection titrations

Most FIA applications involve simple unidirectional continuous-flow regimes. This paper continues the development of the flow-reversal (frFIA) approach and its applications. Flow-reversal flow programming achieves flexibility in operation without requiring manifolds to be refitted. The sensitivity of FIA titrations can be increased by use of frFIA.

The Influence of Feedback on Competition Between SRS and SBS

Abstract The influence of feedback on the competition between the processes of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) has been studied experimentally. The unidirectional regimes of SRS and SBS generation under conditions of feedback creation at the frequency of SRS radiation according to the ring scheme and, in the case of SBS also to the ‘half-resonator’ scheme, have been investigated. A study of the energy characteristics has revealed a significant enhancement of the scattering process which the selective feedback was intended to promote and simultaneous suppression of the other competing process. Analysis of the statistical and temporal characteristics of converted light pulses confirms a higher degree of stimulated scattering development in the presence of feedback. The absence of the influence of selective ring feedback on the SBS-conversion has been observed.

Monitoring surface change on a Namib linear dune

AbstractIn tackling the apparently intractable problem of linear dune initiation and maintenance there has been a move away from large‐scale deductive models to smaller‐scale field studies of individual dunes. This paper reports a study of surface change on a large, complex linear dune in the Namib Desert, southern Africa.The dune surface responds to a markedly seasonal wind regime. In summer westerly winds erode sand from the west flank of the dune and deposit it on the easterly lee side of the dune crest. In winter this pattern is reversed. Easterly winds erode sand from the east slope and deposit it on the west slope. The crest therefore moves back and forth some 15 m each year returning at the end of a year's cycle to its position at the beginning. The position of the base of the dune appears to remain fixed, even though sand is moving throughout the dune system. The dune does extend northward along some resultant of the westerly and easterly winds.Despite relatively high levels of activity, especially at the dune crest, there is no evidence of the breakdown of the linear dune form. The conclusion must therefore be that linear dunes can be maintained in bimodal wind regimes and are not necessarily related to unidirectional parallel regimes as others have suggested.

A combined mixing cell/analytical model to describe two‐dimensional reactive solute transport for unidirectional groundwater flow

A combined two‐dimensional mixing cell/analytical solution is presented which describes multiple reactive solute transport in unidirectional groundwater flow regimes. The model uses a two‐step solving routine which separates chemical equilibria computations (ion exchange and mineral precipitation/dissolution) from calculations of advection/dispersion. Before running the model for a particular grid configuration, an isochemical flow situation is run first to calibrate the model. The calibration technique adjusts for mass losses or gains to and from cells, which occurs when using mixing cell models. Computational time for calculating chemical equilibria in each mixing cell is markedly reduced by the use of a single‐equation solution to solve for multi‐ion exchange equilibria and a rapid iterative technique to solve for calcite equilibria. The application of the model to simulate a solution injection experiment into an unconsolidated sand deposit yielded results in reasonable agreement with field observations.

Flow regime and bedforms in a ridge and runnel system, S.E. Spain

During landward migration, ridge and runnel systems are subjected to asymmetric oscillatory and/or unidirectional flow regimes, depending on the stage of development reached by these systems. In the early stages of evolution, when the ridge is situated in the upper shoreface, the whole system is subjected to asymmetric oscillatory flow. The runnel is under lower flow regime conditions and the ridge may be under upper or lower flow regime according to water depth and wave energy. Later, when the ridge has migrated to a position on the foreshore, the runnel is largely under a unidirectional lower flow regime while the ridge itself is under oscillatory upper flow regime. When the ridge welds to berm, it is largely emergent and exposed to high-tide swash action under upper flow regime conditions. The runnel is eventually filled with sand and transformed into a low-lying area. All these types grade laterally into each other. One or more ridge and runnel systems can occur at the same time. Wave energy, tide level and position of the ridge control the variations in the characteristics of the ridge and also the position of the zones of bedforms found at the upper shoreface.

DISPERSIVE TRANSPORT IN RIVER AND TIDAL FLOWS

Analytical results are presented which describe the mechanisms of longitudinal dispersive mass transport in rectangular channels of finite and infinite widths for both unidirectional (river) and oscillatory (tidal) flow regimes. Emphasis is placed upon the discussion of results and the characteristics of longitudinal dispersive mass transport revealed by the analytical treatment. Expressions presented for the dispersion coefficient were obtained from solutions to four sets of boundary value problems for the velocity and concentration variation components u" and c". Examination of these expressions reveals that in oscillatory flow the dispersive mass transport is described by a type of resonant interaction between the period of oscillation and the time scales of vertical and lateral mixing. The analysis also shows that for oscillatory flow regimes the effect of lateral shear becomes negligible for very wide channels and the three dimensional solution collapses to the two dimensional case in which vertical shear and mixing effects dominate. It is shown analytically that this is not the case in unidirectional flows. For this case the lateral shear and mixing effects dominate the corresponding vertical effects and dispersive mass transport increases without bound with increasing channel widths.