Temporal Instability Research Articles

Using unstable data from mobile phone applications to examine recent trajectories of retail centre recovery

The COVID-19 pandemic has changed the ways in which we shop, with significant impacts on retail and consumption spaces. Yet, empirical evidence of these impacts, specifically at the national level, or focusing on latter periods of the pandemic remain notably absent. Using a large spatio-temporal mobility dataset, which exhibits significant temporal instability, we explore the recovery of retail centres from summer 2021 to 2022, considering in particular how these responses are determined by the functional and structural characteristics of retail centres and their regional geography. Our findings provide important empirical evidence of the multidimensionality of retail centre recovery, highlighting in particular the importance of composition, e-resilience and catchment deprivation in determining such trajectories, and identifying key retail centre functions and regions that appear to be recovering faster than others. In addition, we present a use case for mobility data that exhibits temporal stability, highlighting the benefits of viewing mobility data as a series of snapshots rather than a complete time series. It is our view that such data, when controlling for temporal stability, can provide a useful way to monitor the economic performance of retail centres over time, providing evidence that can inform policy decisions, and support interventions to both acute and longer-term issues in the retail sector.

Spatial and Temporal Instabilities of Optical Discharges

Spatial and Temporal Instabilities of Optical Discharges

Temporal Instability, Wildernesses, and the Otherworld in Early Modern Drama

This article shows how temporal disorder diffuses into the wildernesses within early modern English drama. Those areas beyond the walls of cities and castles in—among other plays—The Two Gentlemen of Verona, Titus Andronicus, Romeo and Juliet, and Macbeth thus flit free from the temporal rules that construct a play’s quotidian world, and the conspicuous partitions that enclose an otherworld in medieval iconography no longer seem clear within them. I argue that these spaces enact an unfamiliar and chaotic ‘otherworld’ within quotidian space, and characters’ ventures into these outer regions at certain points resemble movements into an ‘afterlife’. Journeys into a wilderness, then, parallel a shift from one temporal sphere to another, and characters encounter a post-death state of being within the play’s present.

Series connection of quantum Hall resistance array and programmable Josephson voltage standard for current generation at one microampere

An accurate current can be realized by a series connection of an invariant voltage and resistance with zero interconnection resistance in the framework of Ohm’s law. In practice, however, accurate current realization is limited by the temporal drift and instability of employed resistors, solid-state voltage sources and interconnection wires against environmental fluctuations, such as temperature or atmospheric pressure. Here, we generated a current of 1 μA, determined by the Josephson voltage and quantized Hall resistance array (QHRA). A programmable Josephson voltage standard (PJVS), 1 MΩ QHRA and ultrastable low noise current amplifier (ULCA) are connected in series to generate and measure a current of 1 μA. The expected current is calculated as the ratio of the Josephson voltage to the QHRA plus the output resistance of the PJVS. The relative difference between the current measured by the ULCA and the calculated current is as small as 40 nA A−1 with a combined measurement uncertainty (k = 1) of 0.13 μA A−1.

Optimizing the Spatial Nonuniformity of Irradiance in a Large-Area LED Solar Simulator

The solar simulator has allowed all photovoltaic devices to be developed and tested under laboratory conditions. Filtered xenon arc lamps were the gold-standard source for solar simulation of small-area silicon photovoltaic devices; however, scaling these devices to illuminate large areas is neither efficient nor practical. Large-area solar simulation to meet appropriate spectral content and spatial nonuniformity of irradiance (SNI) standards has traditionally been difficult and expensive to achieve, partly due to the light sources employed. LED-based solar simulation allows a better electrical efficiency and uniformity of irradiance while meeting spectral intensity requirements with better form factors. This work details the design based on optical modeling of a scalable, large-area, LED-based, solar simulator meeting Class AAA performance standards formed for inline testing of printed solar cells. The modular design approach employed enables the illuminated area to be expanded in quanta of ~260 cm2 to any preferred illumination area. A 640 cm2 area illuminated by two adjacent PCB units has a measured total emission of 100 mW/cm2, with a SNI of 1.7% and an excellent approximation to the AM1.5G spectrum over the wavelength range of 350–1100 nm. The measured long-term temporal instability of irradiance (TIE) is <0.5% for a 550-min continuous run. This work identifies the design steps and details the development and measurement of a scalable large-area LED-based solar simulator of interest to the PV testing community, and others using solar simulators.

Roughness Effect on the Flow Past Axisymmetric Bodies at High Incidence

The flow at low Mach numbers and high angles of attack over axisymmetric configurations is not symmetric. The mechanism that triggers the asymmetry is a combination of a global (temporal) instability and a convective (spatial) instability. This latter instability is caused by roughness and other geometrical imperfections, which lead to roll angle dependent forces. The flow at these conditions has a complex vortex sheet structure, with two or three different flow regions. An accurate simulation by means of Computational Flow Dynamics (CFD) is thus very challenging, and many researchers have therefore employed Large Eddy Simulation (LES) codes. This study demonstrates that Unsteady Reynolds Averaged Navier-Stokes (URANS) methods are a suitable alternative, if Scale Adaptive Simulation (SAS) is used. This method is capable of capturing the main flow features, provided that fine meshes, which achieve geometrical similarity between the meshed geometry and the real object, and small-time steps are used. It is also demonstrated that, by using URANS methods in combination with SAS, strong differences in the global and local forces depending on the surface roughness of the model are obtained, a result which coincides with several wind tunnel tests.

Injury severity analysis of time-of-day fluctuations and temporal volatility in reverse sideswipe collisions: A random parameter model with heterogeneous means and heteroscedastic variances

Problem: Sideswipe collisions in the opposite direction often result in more severe injuries than the typical same-direction crashes, especially when light trucks are involved. This study investigates the time-of-day fluctuations and temporal volatility of potential factors that affect the injury severity of reverse sideswipe collisions. Methods: A series of random parameters logit models with heterogeneous means and heteroscedastic variances are developed and utilized to explore unobserved heterogeneity inherent in variables and preclude biased parameter estimation. The segmentation of estimated results is also examined through temporal instability tests. Results: Based on crash data in North Carolina, a number of contributing factors are identified that have profound associations with obvious and moderate injuries. Meanwhile, significant temporal volatility is observed in the marginal effects of several factors such as driver restraint, alcohol or drugs impact, Sport Utility Vehicle (SUV) at fault, and adverse road surface across three different periods. Fluctuations in the time of day indicate that restraint with belts is more effective in mitigating the obvious injury in the nighttime, and high-class roadway sustains a higher probability of resulting in more serious injury compared to the daytime. Practical Applications: The findings of this study could help further guide the implementation of safety countermeasures related to atypical sideswipe collisions.

Temporal instability of partial least squares regressions for estimating leaf photosynthetic traits from hyperspectral information

Partial least squares regression (PLSR) is applied increasingly often to predict plant photosynthesis from reflectance spectra. While its applicability across different areas has been examined in previous studies, its stability across time has yet to be evaluated. In this study, we assessed a series of PLSR models built upon three different band selection approaches (iterative stepwise, genetic algorithm, and uninformative variable elimination), in combination with different spectral transforms (original and first-order derivative spectra), for their stabilities in predicting the maximum carboxylation rate (Vcmax) and maximum electron transport rate (Jmax) from hyperspectral reflectance spectra at different temporal scales (seasonal and interannual). The results showed that both photosynthetic parameters can be estimated from leaf hyperspectral reflectance with moderate to good accuracy across different growing stages (R2 = 0.45–0.84) and years (R2 = 0.37–0.97). We further found that the iterative stepwise selection of informative bands when building PLSR models could greatly improve its predictive capacity compared with that of other PLSR models, especially those based on first-order derivative spectra. However, the selected bands of the models for both photosynthetic parameters were, unfortunately not consistent. Furthermore, we could not have identified any model with fixed spectra performed consistently across different seasonal stages and across different years. However, the blue spectral regions were popularly selected throughout the growing stages and in different years. The results demonstrate that leaf spectra-trait estimation using PLSR models varies with time and thus cast doubt over the use of a specific PLSR model to infer leaf traits across different temporal-spatial contexts. The development of a general applicable PLSR model is still in the works.

Beyond Needle and Thread

Beyond Needle and Thread

ROC approach to forecasting recessions using daily yield spreads.

Even though many studies have established the existence of structural breaks and declining predictability in the relationship between GDP growth and yield spreads, business analysts continue to watch for the inversion of the spread as one of the leading indicators for recessions. We use the Receiving Operating Characteristics (ROC) approach, to reevaluate the enduring power of spread to forecast recessions, notwithstanding the temporal instabilities. We identify the value of the spread that produces the highest discriminatory power as measured by different functionals of the ROC curve e.g., the hit rate, false alarm rate, and the Youden's index. Based on data startingfrom January 2, 1962, we find that the optimal threshold has drifted upwards from zero since the early 1980s, and the deteriorating power of yield spread can largely be restored once the optimal cut-off values are used to issue recession forecasts.

Temporal instability of confined three-dimensional liquid jet with heat and mass transfer under longitudinal acoustic oscillations

The temporal instability of a confined viscous liquid jet surrounded by high-speed co-flowing viscous gas phase is studied in this work. The effect of the longitudinal acoustic oscillations, which is regarded as gas axial velocity oscillations, is also considered. The heat and mass transfer is characterized by the ratio between conduction heat flux and the evaporation heat flux; then, an explicit dispersion relation equation is obtained. The results suggest that more than one unstable region appears because of the gas velocity oscillations, including Kelvin–Helmholtz (K–H) instability and parametric instability regions. Increasing the forcing frequency enhances the K–H instability, while it has a stabilizing effect on the parametric instability. In addition, the liquid jet tends to be more unstable in non-axisymmetric modes when the gas rotating strength is strong. Although the gas viscosity has a destabilizing effect on the gas–liquid interface, the destabilizing effect is weak due to the low viscosity of the gas phase. According to the linear instability theory, the dominant wavenumber will locate in the most unstable region. Moreover, the parametric instability in non-axisymmetric modes may be observable when the Weber number is large.

Numerical Study of the Influence of the Geometrical Irregularities on Bodies of Revolution at High Angles of Attack

The flow at high angles of attack over axisymmetric configurations is not symmetric. The mechanism that triggers the asymmetry may be a combination of a global or hydrodynamic instability (temporal instability) combined with a convective instability (spatial instability) due to microscopic irregularities of the configuration. Poor repeatability of experiments and large differences in the global forces have been obtained with very small changes of the nose tip. In order to study theoretically this phenomenon, numerical simulations have been conducted for an ogive-cylinder configuration at subsonic flow and high angle of attack. For the numerical prediction of the flow about a missile type configuration, an assessment of the effect of structured and unstructured meshes is very important. How the body surface is modelled is very relevant; especially the tip zone of the body. Either configuration resembles a smooth or a rough model. The effect of the turbulence models is also decisive. The analysis has led to the conclusion that only Reynolds stress turbulence models (RSM) combined with Scale Adaptive Simulation (SAS), are the appropriate theoretical tools for the characterization of this flow. The geometrical similarity is very important. There is a roll or orientation angle effect for the unstructured grid, while the structured grid presents a bi-stable solution, one mirror of each other.

Multi-year mapping of flood autumn irrigation extent and timing in harvested croplands of arid irrigation district

ABSTRACT Flood irrigation after crop harvest, e.g. autumn irrigation (AI), is a common irrigation practice in arid and semi-arid regions like Hetao Irrigation District (HID) in Northwest China to increase soil moisture and leach soil salt. Detailed information about the extent, timing, and amount of AI is imperative for modeling agro-hydrological processes and irrigation management. However, little attention is given to the identification of the above AI factors. There are basically three major difficulties in estimating the annual changes in AI, including a suitable index to identify AI, temporal instability of thresholds, and an effective validation method for irrigation timing. Therefore, this study proposes a simple and effective threshold-based method to extract the extent and timing of AI in the HID using MODIS water indices at a daily timescale. The Multi-Band Water Index (MBWI) time series is first reconstructed using an adaptive weighted Savitzky-Golay filter and then used to identify the AI extent and time. The proposed model has a stronger generalization capability both in time and space due to robust thresholds selected from the Z-score normalized feature variable. The model is validated both at pixels generated by the segmentation of Sentinel-derived MBWI using a threshold-based model and at sampling points from the field survey. Results show that the model performed well with an overall accuracy of more than 90.0% for the irrigation area. The overall accuracies of irrigation timing are 76.4% and 91.7% based on the middle-to-late and whole irrigation periods, respectively. We found a decreasing trend in the AI area and a gradual delay in the starting time of AI in the HID, mainly due to changes in cropping patterns, climate, and irrigation fees. Overall, the model is promising in identifying flood irrigation extent and timing in large irrigation districts and is helpful for irrigation scheduling.

Instability of Viscoelastic Liquid Sheets in a Transverse Electric Field

The temporal linear instability of a viscoelastic liquid sheet moving around an inviscid gas in a transverse electrical field is analyzed. The fluid is described by the leaky dielectric model, which is more complex than existing models and enables a characterization of the liquid electrical properties. In addition, the liquid is assumed to be viscoelastic, and the dimensionless dispersion relation of the sinuous and varicose modes between the wavenumber and the temporal growth rate can be derived as a 3 × 3 matrix. According to this relationship, the effects of the liquid properties on the sheet instability are determined. The results suggest that, as the electrical Euler number and the elasticity number increase and the time constant ratio decreases, the sheet becomes more unstable. Finally, an energy budget approach is adopted to investigate the instability mechanism for the sinuous mode.

Evaluating gender differences in injury severities of non-helmet wearing motorcyclists: Accommodating temporal shifts and unobserved heterogeneity

With rapid growth in motorcycle use and relatively low helmet-wearing usage rates, injuries and fatalities resulting from motorcycle crashes in Pakistan are a critical concern. To investigate possible temporal instability and differences in the factors that determine resulting injury severities between male and female non-helmet wearing motorcyclists, this study estimated male and female injury severity models using a random parameter logit approach with heterogeneity in means and variances. Motorcycle crash data between 2017 and 2019 from Rawalpindi, Pakistan, were used for the model estimation. With four possible crash injury severity outcomes (injury, minor injury, severe injury, and fatal injury), a wide variety of explanatory variables were considered, including the characteristics of riders, vehicles, roadways, environments, crashes, and temporal considerations. Temporal shifts in the effects of explanatory variables were confirmed using a series of likelihood ratio tests. While the effects of several explanatory variables are relatively temporally stable, those of most variables vary considerably across the years. In addition, out-of-sample simulations underscore the temporal shifts from year to year and the differences between male and female motorcyclist-injury severity. The findings suggest that factors such as effective enforcement countermeasures and relevant educational campaigns can be implemented to reduce injury severity. The statistically significant differences between male and female non-helmeted injury severity models underscore the importance of policies that separately target male and female motorcycle rider safety.

Climatic filtering and temporal instability shape the phylogenetic diversity of European alpine floras

Alpine ecosystems are hotspots of biodiversity despite their cold climates. Here we investigate spatial patterns in the phylogenetic diversity (i.e. the degree of species relatedness) of European alpine floras and quantify the influence of climatic conditions since the late Pleistocene and historical climatic instability in shaping these patterns. We collected species‐pool data for 22 alpine regions in central and southern Europe and calculated phylogenetic diversity within and between regions using two metrics sensitive to terminal branching in the phylogeny. We regressed phylogenetic diversity against macroclimatic variables representing seasonal extremes between the Last Glacial Maximum (LGM; 21 000 years BP) and the present at 1000‐year intervals. We found the lowest phylogenetic diversity in the Carpathians and the central Alps, and the highest in the north‐eastern and southern Iberian Peninsula. Phylogenetic diversity decreased with temperature seasonality and low winter temperature. While the effect of temperature seasonality was relatively constant over time, the influence of winter temperature decreased after the LGM. We also found that phylogenetic diversity decreased with historical climatic instability. Between regions, phylogenetic diversity was mainly explained by current climatic distance rather than geographic distance, suggesting that alpine floras were primarily driven by species sorting along climatic gradients. Our results confirm the role of environmental filtering in shaping the current phylogenetic diversity of alpine floras, resulting in more closely related lineages in regions with relatively cold and unstable climates. We also highlight the importance of explicitly incorporating climatic variation through time to better understand the processes structuring the current biodiversity of alpine floras.

Convective and absolute instabilities in inclined buoyancy layers

The linear instability of the buoyancy-driven flow adjacent to an inclined heated wall immersed in a thermally stratified medium is studied theoretically and numerically. For the temporally unstable system, spatiotemporal stability analysis is carried out to delineate the parameter space (Grashof number, Prandtl number, and tile angle) for convective/absolute instability. We provide an example of an absolute instability of the buoyancy layer on an inclined buoyancy layer. It is shown that the tile angle and Prandtl number have a dramatic influence on the spatial-temporal properties of the flow. For fixed Pr = 6.7, increasing tile angle decreases the domain of absolute instability, and when tile angle is greater than 20°, the absolute instability disappears. The flow will change from convectively unstable to absolutely unstable with the increase of Pr. Results from the direct numerical simulation are in agreement with the predictions of the linear temporal and spatial-temporal instabilities. These encouraging results should be helpful for understanding such a buoyancy-driven flow system.

Effect of confinement on weakly nonlinear temporal instability analysis of a viscous planar liquid sheet sandwiched between two co-flowing gas streams at equal velocities

Extensive number of applications like rocket jet engine and gas turbines carry out atomization within a confined environment as it provides faster atomization and good mixing efficiency. Literature suggests that the effect of confinement on breakup of droplets, liquid threads, and annular jets is not elementary, but it varies with liquid viscosity, surrounding medium viscosity, and the confinement strength. Through the present weakly nonlinear temporal analysis, we unveil if a similar dynamic effect of confinement persists on the breakup of planar two-dimensional liquid sheets of varying viscosities. Moreover, to mimic air assisted atomization, the surrounding gases are considered to be flowing with non-zero velocities. Our study reveals that presence of confinement at higher gas-to-liquid velocity ratios of 2.5 and 3 produces no apparent improvement in atomization at all liquid Reynolds numbers. However, at relatively low gas-to-liquid velocity ratios of 2 and 2.25, confinement produces faster breakup as compared to an unconfined one, more so when Reynolds number is relatively low (Re < 100). The overall influence of confinement on non-dimensional breakup time is, however, weak as compared to how it affects the linear growth rate. A counteracting effect of confinement on the linear maximum growth rate and normalized second order disturbance amplitude is identified as the primary reason. Our study also identifies the minimum confinement height CH-1 which is just enough for the liquid sheet to achieve breakup without touching the solid walls. Interestingly, the maximum linear growth rate and minimum breakup time for a particular flow condition are always registered when confinement height equals to CH-1. The values of CH-1 display a strong dependence upon Reynolds number and gas-to-liquid velocity ratios.

Image-derived Metrics Quantifying Hemodynamic Instability Predicted Growth of Unruptured Intracranial Aneurysms.

While image-derived predictors of intracranial aneurysm (IA) rupture have been well-explored, current understanding of IA growth is limited. Pulsatility index (PI) and wall shear stress pulsatility index (WSSPI) are important metrics measuring temporal hemodynamic instability. However, they have not been investigated in IA growth research. The present study seeks to verify reliable predictors of IA growth with comparative analyses of several important morphological and hemodynamic metrics between stable and growing cases among a group of unruptured IAs. Using 3D images, vascular models of 16 stable and 20 growing cases were constructed and verified using Geodesic techniques. With an overall mean follow-up period of 25 months, cases exhibiting a 10% or higher increase in diameter were considered growing. Patient-specific, pulsatile simulations were performed, and hemodynamic calculations were computed at 5 important regions of each aneurysm (inflow artery, aneurysm neck, body, dome, and outflow artery). Index values were compared between growing and stable IAs using ANCOVA controlling for aneurysm diameter. Stepwise multiple logistic regression and ROC analyses were conducted to investigate predictive models of IA growth. Compared to stable IAs, growing IAs exhibited significantly higher intrasaccular PI, intrasaccular WSSPI, intrasaccular spatial flow rate deviation, and intrasaccular spatial wall shear stress (WSS) deviation. Stepwise logistic regression analysis revealed a significant predictive model involving PI at aneurysm body, WSSPI at inflow artery, and WSSPI at aneurysm body. Our results showed that high degree of hemodynamic variations within IAs is linked to growth, even after controlling for morphological parameters. Further, evaluation of PI in conjunction with WSSPI yielded a highly accurate predictive model of IA growth. Upon validation in future cohorts, these metrics may aid in early identification of IA growth and current understanding of IA remodeling mechanism.

Intensity instability and correlation in amplified multimode wave mixing

The dynamics of optical nonlinearity in the presence of gain and feedback can be complex leading to chaos in certain regimes. Temporal, spectral, spatial, or polarization instability of optical fields can emerge from chaotic response of an optical chi ^{(2)} or chi ^{(3)} nonlinear medium placed between two cavity mirrors or before a single feedback mirror. The complex mode dynamics, high-order correlations, and transition to instability in these systems are not well known. We consider a chi ^{(3)} medium with amplified four-wave mixing process and study noise and correlation between multiple optical modes. Although individual modes show intensity instability, we observe relative intensity noise reduction close to the standard quantum noise, limited by the camera speed. We observe a relative noise reduction of more than 20 dB and fourth-order intensity correlation between four spatial modes. More than 100 distinct correlated quadruple modes can be generated using this process.