Space-time Measurements Research Articles

Dynamic three-dimensional measurement and analysis of rotating polygons

Rotating polygons are geometric patterns formed spontaneously by the free surface of a rotating fluid driven by a rotating disk at the bottom of a partially filled cylindrical container. Most previous studies mainly predict the number of polygon apexes as a function of disk rotation speed and initial fluid depth. We propose to experimentally characterize this three-dimensional instability by means of highly accurate measurements of the free surface height using Fourier transform profilometry. For a given initial height of 26 mm in a cylindrical cavity of radius 140 mm, these measurements enabled us to not only revisit quantitatively the previous analysis in the fully developed regime but also extend the analysis to the spin-up regime (starting from rest). This latter regime has not been explored. In addition, we describe the mixed-mode regime at the boundary between two clearly established polygons. Thanks to precise space-time resolved surface height measurements, we propose a decomposition of each azimuthal mode that provides valuable information on the growth and dynamics of rotating polygons and can, therefore, serve as a basis for future numerical simulations and theoretical models.

Characteristics of jet noise: A synthesis

It is a distinct privilege to produce this article to honor Professor Tam, the foremost authority on jet aeroacoustics. The fundamental characteristics of jet noise have been studied for 70 years, since the pioneering work of Lighthill in the 1950s. The acoustic analogy, with many variants, has served as the leading theory for nearly 50 years. Many leading researchers in the 1970s formulated theories to interpret the measured trends from subsonic and supersonic jets, using acoustic analogy and flow features as the framework. Quadrupoles, dipoles and monopoles were believed to constitute the sources of noise. The discovery of large-scale organized structures in free shear layers and jets sparked a different avenue of thinking about their importance for noise generation. Tam was the first to clearly demonstrate that these structures are efficient generators of noise and constitute the dominant noise sources, especially in the downstream direction. Now, two schools of thought emerged on the sources of jet noise. Experimental measurements showed that the mean flow as well as the turbulence statistics exhibit a self-similarity in the mixing layer and another similarity in the fully developed jet. Based on these observations, Tam proposed that since noise is generated by the turbulence of the jet, the noise spectra generated by fine-scale and large-scale turbulence should also exhibit self-similarity. By examining a large set of supersonic jet noise data acquired at NASA Langley, Tam offered evidence that the turbulent mixing noise of high-speed jets does consist of two independent self-similar components. In this paper, experimental evidence is compiled from an extensive database that quantifies the effect of several parameters that affect jet spectra. A new scaling method is developed and extended to noise predictions for realistic dual-stream nozzle geometries. The objectives of this paper are to serve as a synthesis of noise characteristics and to focus on application to real-world problems. Results from five different experimental measurements are examined: (1) farfield spectral characteristics; (2) azimuthal and polar correlations in the farfield; (3) correlations of jet turbulence fluctuations and farfield sound; (4) measurement of source distributions with an elliptic mirror; and (5) space-time correlation measurements in the nearfield with a cage array, and nearfield-farfield correlations. Two distinctly different trends are observed in the angular ranges of 50° – ∼120° and ∼120° – 165° for all the parameters investigated with the above five approaches. The salient observations are mutually supporting, and the cumulative weight lends credence to the proposition that there are two distinct sources of turbulent mixing noise.

The flattening of spacetime hierarchy of the N,N-dimethyltryptamine brain state is characterized by harmonic decomposition of spacetime (HADES) framework.

The human brain is a complex system, whose activity exhibits flexible and continuous reorganization across space and time. The decomposition of whole-brain recordings into harmonic modes has revealed a repertoire of gradient-like activity patterns associated with distinct brain functions. However, the way these activity patterns are expressed over time with their changes in various brain states remains unclear. Here, we investigate healthy participants taking the serotonergic psychedelic N,N-dimethyltryptamine (DMT) with the Harmonic Decomposition of Spacetime (HADES) framework that can characterize how different harmonic modes defined in space are expressed over time. HADES demonstrates significant decreases in contributions across most low-frequency harmonic modes in the DMT-induced brain state. When normalizing the contributions by condition (DMT and non-DMT), we detect a decrease specifically in the second functional harmonic, which represents the uni- to transmodal functional hierarchy of the brain, supporting the leading hypothesis that functional hierarchy is changed in psychedelics. Moreover, HADES' dynamic spacetime measures of fractional occupancy, life time and latent space provide a precise description of the significant changes of the spacetime hierarchical organization of brain activity in the psychedelic state.

Examining the relationship between active transport and exposure to streetscape diversity during travel: A study using GPS data and street view imagery

Active transport (AT)—physical activity (PA) during travel—can promote human health. Among built environment factors related to travel research, design refers to the street network features encouraging AT. The advent of street view images (SVIs) presents the potential to measure design during travel by capturing the eye-level built environments. Benefited by SVIs, this study innovatively introduces streetscape diversity—the interconnection of street view-derived built environment factors—during travel as the proxy to measure design from the street view perspective. Exposures to higher streetscape diversity could provide increased access to potential destinations and therapeutic landscapes, thereby promoting AT. Through integrating SVIs and young adults’ Global Positioning System (GPS) trajectories, this study utilized negative binomial regression models to assess the relationship between streetscape diversity and time spent in AT. Results indicate that the inclusion of streetscape diversity improves the model performance, and there is a positive relationship between streetscape diversity and AT. This finding indicates that increased access to travel routes with diverse streetscapes could increase the probability of participating in AT. Furthermore, integrating individual GPS data and SVIs allows more precise space-time measurement of individual environmental exposures.

Measuring the impacts of disruptions on public transit accessibility and reliability

Public transit systems are facing higher risk of system degradation from external disruptions, affecting their ability to deliver reliable accessibility to transit users. Therefore, resilience, the ability to maintain functions during a disruption, becomes a crucial assessment of public transit systems. In this paper, we calculate two space-time prism-based measures with General Transit Feed Specification real-time (GTFS-RT) data: realizable real-time accessibility, a conservative real-time accessibility measure that can be achieved by users subject to delays, and scheduled accessibility, accessibility based on schedule. We also define accessibility unreliability, the deviation between realizable accessibility and scheduled accessibility, to measure the reliability of delivered accessibility. We use the two measures to gauge the resilience of public transit systems and conduct two case studies of short- and long-term disruptions, namely Ohio State football games and the COVID-19 pandemic, on the Central Ohio Transit Authority (COTA) bus system in Columbus, Ohio. We find there are two peaks of high unreliability before and after each football games, with the stadium as the geographic center of the disruption. The after-game peaks are shorter and more intense than the before-game. We also find COVID-19 had persistent negative impacts on accessibility and reliability: Realizable accessibility universally declined during the pandemic, but only part of cities experienced unreliability increase, primarily in urban perimeters and suburbs. Improved traffic conditions during the pandemic may help to reduce unreliability, but the later service cuts increased unreliability. The two case studies prove the effectiveness of the method to detect system disturbances and provide important guidance for public transit system operation and planning.

Demonstrating Photon Ring Existence with Single-baseline Polarimetry

Images of supermassive black hole accretion flows contain features of both curved spacetime and plasma structure. Inferring properties of the spacetime from images requires modeling the plasma properties, and vice versa. The Event Horizon Telescope Collaboration has imaged near-horizon millimeter emission from both Messier 87* (M87*) and Sagittarius A* (Sgr A*) with very long baseline interferometry (VLBI) and has found a preference for magnetically arrested disk (MAD) accretion in each case. MAD accretion enables spacetime measurements through future observations of the photon ring, the image feature composed of near-orbiting photons. The ordered fields and relatively weak Faraday rotation of MADs yield rotationally symmetric polarization when viewed at modest inclination. In this letter, we utilize this symmetry along with parallel transport symmetries to construct a gain-robust interferometric quantity that detects the transition between the weakly lensed accretion flow image and the strongly lensed photon ring. We predict a shift in polarimetric phases on long baselines and demonstrate that the photon rings in M87* and Sgr A* can be unambiguously detected with sensitive, long-baseline measurements. For M87*, we find that photon ring detection in snapshot observations requires ∼1 mJy sensitivity on >15 Gλ baselines at 230 GHz and above, which could be achieved with space-VLBI or higher-frequency ground-based VLBI. For Sgr A*, we find that interstellar scattering inhibits photon ring detectability at 230 GHz, but ∼10 mJy sensitivity on >12 Gλ baselines at 345 GHz is sufficient and is accessible from the ground. For both sources, these sensitivity requirements may be relaxed by repeated observations and averaging.

A projection-based approach to extend digital volume correlation for 4D spacetime measurements

A projection-based approach to extend digital volume correlation for 4D spacetime measurements

Three-dimensional numerical modelling of floods in river corridor with complex vegetation quantified using airborne LiDAR imagery

Excessive flood flow over the historic diversion weir in the vegetated Asahi River in Okayama Prefecture, Japan, was recently recorded for the first time after its renovation work. Fluvial researchers analysed the diversion discharge for flood mitigation measures through laboratory studies and conventional two-dimensional (2-D) depth-averaged simulations. The existing model was insufficient for simulation of certain phenomena such as flow resistance caused by vegetation branches and leaves and vertical flow distribution around the river corridor. Therefore, we developed a three-dimensional (3-D) vegetation resistance porous model by estimating topography, land cover, and vegetation distribution from airborne light detection and ranging (LiDAR) topo-bathymetry (ALB) data. Results show that the water level and flow regime were more reproducible than by referenced 2-D calculations when compared to space-time image velocimetry (STIV) data and field measurements. The diversion discharge designed using the proposed model is feasible with the current riverbed and vegetation conditions.

4D-tracking in the 10-ps range: A technological perspective

The present paper focuses on recent and ready-to-come advancements concerning high-resolution 4D-tracking with a perspective approach. Four-dimensional-tracking techniques (particle tracking with timing information for each detection point) have revealed a necessity for the next and next-to-next generations of high-energy physics experiments to cope with the increasing luminosity and consequent event pile-up in the beam collision region. Such a decisive challenge concerns both detection and processing technologies at an unprecedented level of difficulty. In addition to the high performance required in space–time measurement precision (some tens of picoseconds resolution in timing and about 10 µm resolution in space), an extremely high radiation hardness is demanded for such technologies together with an extremely high read-out and processing capability. Emerging experimental solutions for sensors and electronics against such challenges are presented here.

Research on the Optimization Model of Railway Emergency Rescue Network Considering Space-Time Accessibility

The capability of railway emergency rescue can be enhanced by maintaining the railway emergency rescue network and upgrading its technology. Nowadays, influenced by the factors, such as resource type, personnel distribution, line level, etc., space-time differences may be unavoidable. In the meantime, the general description method of the transportation network may lack the consideration of the rescue transportation particularity, so the strategies of resource allocation, maintenance, and upgrading could be illogical. Hence, in this paper, the gravity model is utilized to improve the classical travel time budget model and to construct the space-time accessibility model, firstly. Then, further exploring the space-time accessibility of nodes and edges of railway emergency rescue network and considering the randomness of travel time, a space-time accessibility measurement method for an emergency network is proposed. Moreover, a global optimization model with accessibility characteristics is then constructed for the maintenance allocation of the emergency rescue transportation network. The results show that the proposed method can solve the maintenance allocation problem of the large-scale rescue network effectively, reduce the risk of maintenance allocation strategy failure caused by unreasonable node index parameters, and provide an effective basis and theoretical support for the rational formulation of railway rescue transportation network maintenance allocation strategy.

Experimental observation of periodic Korteweg-de Vries solitons along a torus of fluid

We report on the experimental observation of solitons propagating along a torus of fluid. We show that such a periodic system leads to significant differences compared to the classical plane geometry. In particular, we highlight the observation of subsonic elevation solitons, and a nonlinear dependence of the soliton velocity on its amplitude. The soliton profile, velocity, collision, and dissipation are characterized using high-resolution space-time measurements. By imposing periodic boundary conditions onto the Korteweg-de Vries (KdV) equation, we recover these observations. A nonlinear spectral analysis of solitons (periodic inverse scattering transform) is also implemented and experimentally validated in this periodic geometry. Our work thus reveals the importance of periodicity for studying solitons and could be applied to other fields involving periodic systems governed by a KdV equation.

Regional income convergence in Colombia: population, space, and long-run dynamics

We examine the trajectory of regional income dynamics in Colombia. Using data on all 33 Colombian departments from 2000 to 2016, we employ extensions of (spatial) Markov chains, space-time mobility measures, along with a fully weighted version of the distribution analysis approach. By considering these extensions, our analysis enables us to answer questions such as whether the role of spatial context influences the distributional dynamics of Colombian departments, or the magnitude of the moderating effect of department’s population. The inclusion of additional measures such as the asymptotic half-life of convergence provides additional results, informing on how long it would take to reach the hypothetical long-run distribution of per capita income. Results, which are reported for both pre- and post-2008 trends, complement previous literature on regional economic convergence in a relevant South American context, showing stronger convergence patterns when controlling for the population living in each department. The patterns do not particularly intensify when controlling for spatial spillovers, since the role of spatial context was already playing a relevant role from the beginning of the period analyzed. Therefore, although the ergodic distributions show a conditional-convergence pattern, addressing the problems of spatial exclusion fully, persistent polarization among geographical departments and populations, along with the relevant core-periphery gaps, still requires the design and implementation of specific policies.

Space-time-resolved measurements of the effect of pinned contact line on the dispersion relation of water waves

Surface wave dynamics in small scale configurations can be substantially modified by edge constraints, in particular a pinned contact line, when the force it exerts is non-negligible with respect to gravity and surface tension. This work develops a hybrid approach, which combines a theoretical model with a complete space-time resolved measurement of the surface deformation and static menisci, allowing an accurate estimation of the contribution of a pinned contact line to shift the dispersion relation towards faster phase velocities.

Stolt migration algorithm for impulse through-the-wall radar imaging based on compressive sensing

Impulse radar technology is commonly used for through-wall radar imaging (TWRI) owing to its simple hardware structure and range resolution enhancement. Compressive sensing technique has been applied to impulse TWRI to reduce the amount of measurement data and provide the enhanced imaging quality, which adopts a ray-tracing model and requires prior information of the transmitted pulse. In this paper, however, we propose a CS-based Stolt migration imaging algorithm for impulse TWRI system. The proposed imaging algorithm deduces the sparse transform between the raw measurement space data and the image space data and does not need a prior knowledge of the transmitted pulse. The numerical simulation results show that compared to the traditional Stolt migration imaging method, the proposed imaging algorithm can provide the improved imaging reconstruction quality and reduce the amount of space-time measurements at the cost of increased computation time.

Quantum measurement of space-time events

The phase space of a relativistic system can be identified with the future tube of complexified Minkowski space. As well as a complex structure and a symplectic structure, the future tube, seen as an eight-dimensional real manifold, is endowed with a natural positive-definite Riemannian metric that accommodates the underlying geometry of the indefinite Minkowski space metric, together with its symmetry group. A unitary representation of the 15-parameter group of conformal transformations can then be constructed that acts upon the Hilbert space of square-integrable holomorphic functions on the future tube. These structures are enough to allow one to put forward a quantum theory of phase-space events. In particular, a theory of quantum measurement can be formulated in a relativistic setting, based on the use of positive operator valued measures, for the detection of phase-space events, hence allowing one to assign probabilities to the outcomes of joint space-time and four-momentum measurements in a manifestly covariant framework. This leads to a localization theorem for phase-space events in relativistic quantum theory, determined by the associated Compton wavelength.

Logsum-type space-time accessibility measures that consider two-stage decision-making

Logsum-type space-time accessibility measures that consider two-stage decision-making

Hacia una sociedad longeva sostenible: instrumentalizando la intergeneracionalidad y el diseño centrado en la persona

Objective: This article aims to decode the opportunities of longevity societies. Methodology: It uses a qualitative analytical methodology that is nourished by the learning of 6 years of research and projects carried out in mYmO1 (entity dedicated to intergenerational innovation). It also relies on technical knowledge and analysis original sources based on the Intergenerational approach and the Human-Centered Design. The proposal presents a clear goal that is to achieve Sustainable Cities and Communities, considering the unit of space-time measurement key to start mapping projects for long-lived societies. The Sustainable Development Goals (SDGs) drawn up by the United Nations are mentioned as a reference for the scope of actions in the text, the target being SDG-11. In addition, all proposed actions are based on a premise that is taken as a basis and that lies in the importance of all actors assuming their responsibility in solving problems. In order to decode the opportunities that longevity societies offer and to achieve sustainable cities and communities, community ownership of change is considered essential. Results: Based on these considerations, a strategic matrix is designed that uses the Theory of Change to establish the relationships between actions and objectives. Desired changes are identified and what should happen to ensure that these changes lead to long-term results is analysed too. In addition, as a consequence of its practical implication, the proposed matrix is useful for small and large interventions, in any development and innovation project. Within this framework, intergenerational approach and human-centered design offer the methodology and tools to put into practice the development of concrete processes and projects. Limitations: It is an essay that shows some initiatives in concrete realities whose results are not universally extrapolated. Practical implications: As a result, new models of participation are made visible in which organizations, public authorities and individuals collaborate to solve current challenges.

Towards a sustainable longevity society: Instrumentalizing intergenerationality and human-centered design

Objective: This article aims to decode the opportunities of longevity societies. Methodology: It uses a qualitative analytical methodology that is nourished by the learning of 6 years of research and projects carried out in mYmO1 (entity dedicated to intergenerational innovation). It also relies on technical knowledge and analysis original sources based on the Intergenerational approach and the Human-Centered Design. The proposal presents a clear goal that is to achieve Sustainable Cities and Communities, considering the unit of space-time measurement key to start mapping projects for long-lived societies. The Sustainable Development Goals (SDGs) drawn up by the United Nations are mentioned as a reference for the scope of actions in the text, the target being SDG-11. In addition, all proposed actions are based on a premise that is taken as a basis and that lies in the importance of all actors assuming their responsibility in solving problems. In order to decode the opportunities that longevity societies offer and to achieve sustainable cities and communities, community ownership of change is considered essential. Results: Based on these considerations, a strategic matrix is designed that uses the Theory of Change to establish the relationships between actions and objectives. Desired changes are identified and what should happen to ensure that these changes lead to long-term results is analysed too. In addition, as a consequence of its practical implication, the proposed matrix is useful for small and large interventions, in any development and innovation project. Within this framework, intergenerational approach and human-centered design offer the methodology and tools to put into practice the development of concrete processes and projects. Limitations: It is an essay that shows some initiatives in concrete realities whose results are not universally extrapolated. Practical implications: As a result, new models of participation are made visible in which organizations, public authorities and individuals collaborate to solve current challenges.

Short-time deterministic prediction of individual waves based on space-time X-band Marine radar measurements

We demonstrate and verify, by the use of both synthetic and real wave data, a newly developed capability of short-time phase-resolved wave prediction based on incoherent X-band marine radar measurements. An inversion algorithm is developed to convert X-band radar sea surface measurements into the phase-resolved wave field and the associated wave spectrum based on the linear gravity wave theory. The wave components obtained from the reconstruction are then used to initialize the wave propagation model that is used to provide a short-time deterministic forecast of wave field evolution downstream. Both wave spectrum and spatial-temporal wave elevation evolution obtained based on the X-band measurements are compared with the independent point wave measurements by Miros RangeFinder. The agreements between them are reasonably well, which has a significant implication on practical applications of short-time deterministic wave prediction in optimal marine operations.

Gait Phase Recognition Using Fuzzy Logic Regulation with Multisensor Data Fusion

The health challenges brought by aging population and chronic noncommunicable diseases are increasingly severe. Scientific physical exercise is of great significance to prevent the occurrence of chronic diseases and subhealth intervention and promote health. However, improper or excessive exercise can cause injury. Research shows that the sports injury rate of people who often exercise is as high as 85%. Aiming at the problem of low accuracy of single sensor gait analysis, a real‐time gait detection algorithm based on piezoelectric film and motion sensor is proposed. On this basis, a gait phase recognition method based on fuzzy logic is proposed, which enhances the ability of gait space‐time measurement. Experimental results show that the proposed gait modeling method based on ground reaction force (GRF) signal can effectively recognize and quantify various gait patterns. At the same time, the introduction of heterogeneous sensor data fusion technology can effectively make up for the accuracy defects of single sensor measurement and improve the estimation accuracy of gait space‐time measurement.