A spatial and spectral Analysis of the Sentinel-2 nighttime Image

Nighttime optical remote sensing provides valuable insights into natural and, in particular, human activities. This study evaluates the nighttime imaging capabilities of the Sentinel-2 mission using the only available nighttime acquisition not limited to ocean observations for dark signal calibration, covering the United Arab Emirates with Dubai in 2015. We checked the detection limit using granules over the Persian Gulf, extracted radiance spectra for different regions of interest, and analysed lighting types and temperatures. Results suggest a conservative nighttime detection limit of approx. 0.37 W/m²/µm/sr for visible/near infrared bands, and 0.08 W/m²/µm/sr for short-wave infrared bands. Sentinel-2’s high spatial resolution and multispectral bands, although designed for daytime observations, were capable of detecting and classifying bright visible/near and short-wave infrared emitters. Comparisons with hyperspectral EnMAP imagery acquired in 2025 validated the classifications and revealed changes in urban lighting over a decade. While limitations apply, this study highlights S2’s potential for nighttime remote sensing and supports considerations of nighttime capabilities for future satellite missions

DendroTime: Progressive Hierarchical Clustering for Variable-Length Time Series

Many effective dissimilarity measures for variable-length time series, such as DTW, MSM, or TWED, are expensive to compute because their runtimes increase quadratically with the time series' lengths. When used in hierarchical agglomerative clustering algorithms that need to compute all pairwise time series dissimilarities, they cause slow runtimes and do not scale to large time series collections. However, there are use cases, where fast, interactive hierarchical clustering is necessary. For these use cases, progressive hierarchical clustering algorithms can improve runtimes and interactivity. Progressive algorithms are incremental algorithms that produce and continuously improve an approximate solution, which eventually converges to the exact solution. In this paper, we present DendroTime, the first (parallel) progressive clustering system for variable-length time series colections. The system incrementally computes the pairwise dissimilarities between the input time series and supports different ordering strategies to achieve progressivity. Our evaluation demonstrates that DendroTime's progressive strategies are very effective for clustering scenarios with expensive time series dissimilarity computations

Hot streak development in hypersonic boundary layer transition on a blunt cone with cooled walls: shock tunnel experiments and numerical simulations

The uncertainty of the boundary layer transition location on a hypersonic vehicle and the corresponding uncertainty in the surface heat flux and skin friction present major challenges for sustained hypersonic flight. One source of uncertainty is the mechanism governing breakdown of instabilities in boundary layers on geometries with highly-cooled walls (low $T_w / T_e$) relevant to flight conditions. An experimental investigation was carried out using a 7$^{\circ}$ half-angle, straight cone with a $2.5~mm$ nose radius and a Reynolds number based on nose radius of $Re_n \approx 5100$ at $0^{\circ}$ angle of attack. The tests were conducted at Mach 7.4 in the High Enthalpy Shock Tunnel Göttingen (HEG) DLR (2018) of the German Aerospace Center (DLR). Dominant primary instabilities, identified as second-mode waves in previous experiments Laurence (2016), were observed as peaks in the power spectral density plot of figure 1 for surface-mounted fast pressure transducers. Broadening of these peaks at the downstream locations on the cone was attributed to nonlinear interactions of instabilities within the boundary layer. Streaks were observed in the nonlinear interaction region, by means of temperature sensitive paint, as shown in figure 2. This suggests nonlinear interaction of the second-mode waves with secondary instabilities and is a unique result from a shock tunnel testing environment. Accompanying numerical investigations of the linear and nonlinear transition regime were carried out for the wind tunnel conditions and the circular cone geometry of the HEG experiments, in order to understand the role of the secondary instability in the transition process. The primary instability investigations using Linear Stability Theory (LST) and low-amplitude wave packet calculations confirmed that axisymmetric second-mode waves are indeed the dominant primary instability. The dominant frequency range obtained from experiments (\cref{PSD_map}) and numerical calculations are in good agreement. The strongly amplified axisymmetric second-mode waves suggest the possibility of a so-called fundamental resonance, where a large amplitude axisymmetric (primary) disturbance wave interacts (resonates) nonlinearly with a pair of lower amplitude (secondary) oblique disturbance waves of the same frequency. Therefore, fundamental resonance calculations were carried out for a wide range of azimuthal wavenumbers ($k_c$). The N-factors of the secondary disturbances reveal for which azimuthal wavenumbers the fundamental resonance is "strongest" (see \cref{fig:nfac_secondary}). A strong fundamental resonance gives rise to the nonlinear generation of steady streamwise modes, which were found to be responsible for the generation of "hot" streaks on the surface of a flared cone in the "cold-flow" experiments carried out at the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University (\cite{Chynoweth2019}). Comparable streaks of high skin-friction have been observed in numerical simulations for the HEG conditions (\cref{fig:streaks_dns}). With the "controlled" simulations a total of 250 streaks around the circumference was obtained, which is in good agreement with the number of streaks observed in the HEG experiments ($220-230$). For the final version of this paper a detailed comparison between experimental measurements and numerical investigations will be provided for the various transition stages (linear and nonlinear)

A Serious Game as a Tool for User-centered Design of Mobility Solutions

Before the implementation of new mobility solutions, it is often difficult to encourage and enable potential users to participate in the design process, to predict acceptance, and to determine the influence of individual design pa-rameters on use intentions. A serious game involving repeated mobility choices and, at the same time, fun to play might be a useful tool to allow for user studies and participation in this situation. We developed a concept of such a game in the context of intermodal transport. A first prototype was im-plemented and subjected to an early test with respect to user experience and usability through an evaluation with seven experts. At the current state of development, flow experience, measured by the flow short scale (FKS), was medium. Usability, measured by the system usability scale (SUS), was low, and the heuristic evaluation yielded many hints for improvement. Based on the insights gained, the development of the game will be continued to pre-pare it for a user test assessing the potential of the method for studying user preferences in the transport system

Monitoring urban green space for climate-resilient development in the face of rapid urbanization: A tale of two Vietnamese cities

Urban green space (UGS) contributes to sustainable and climate-resilient urban development by providing ecosystem services and enhancing public health. In rapidly urbanizing cities, UGS is compromised by expanding built infrastructure, leading to loss and fragmentation of green areas. This study employs a resource-efficient remote sensing approach for monitoring UGS dynamics in two examples of rapid urbanization, Hanoi and Ho Chi Minh City (HCMC) in Vietnam. The approach identifies UGS by applying a ground-truthed threshold to Normalized Difference Vegetation Index quartile maps (NDVI–P75) from nine years of open-access Sentinel-2 imagery before blending it with national census data. The results indicate a pronounced spatial heterogeneity in UGS distributions, with low densities in urban cores and greater availability in the peripheral districts of both metropolises. The temporal analysis shows diverging trends: while UGS areas in Hanoi are relatively stable overall but declining per capita due to ongoing urbanization, HCMC experiences a general decline in both UGS indicators. The findings emphasize the urgent need for implementing integrated UGS strategies that account for the diverse socio-economic drivers of UGS loss. By offering a robust and reproducible methodology for monitoring UGS, this research highlights the potential of remote sensing tools to inform urban planning and policy development. This approach is highly transferable to other urban contexts globally, demonstrating an effective and transparent pathway to foster climate-justice and “sustainable cities and communities” in line with the United Nations’ Sustainable Development Goal No. 11

Dynamics of interstitial molecular-type double donor complexes in silicon

Complementary time-resolved spectroscopies have been applied to study dynamics of molecular-type magnesium-related donors. Large interstate energy gaps of these donors prevent nonradiative decays through a first-order, one-phonon-assisted scattering – the main relaxation mechanism in shallow substitutional donors in low-doped silicon. Analysis reveals very short decay times of the deepest excited states of molecular donors: dephasing within less than 10 ps and relaxation rates above 30/ns. These decays are several times shorter than those observed in single-electron hydrogen-like substitutional donors, but longer than those in helium-like interstitial atomic magnesium centers in silicon. Spectral correlations of temporal dependences of particular transients to the lattice phonon overtones suggest that phonon-assisted electronic scattering contributes also to decoherence of states in these double donors. Such efficient second-order phonon-assisted processes were underestimated for dynamics of deep impurities in semiconductors

An unstructured high-order finite-volume scheme for the simulation of reactive multi-species flows

In this work, a high-order finite-volume method is combined with an iterative projection approach to solve transport equations for reactive fluids in the low-Mach number regime. The proposed solution algorithm is fully collocated in both space and time and employs a vertex-centered -exact discretization to achieve truly third-order spatial accuracy, even on fully unstructured median-dual grids. To enhance both accuracy and robustness, viscous and convective fluxes are treated consistently within the high-order framework. Convective fluxes are discretized using a central face-value approximation augmented with adaptive numerical dissipation control, governed by a novel gradient-limiting strategy that selectively reduces the order of accuracy near strong gradients while minimizing artificial dissipation elsewhere. The performance of the method is assessed against a conventional finite-volume scheme for unstructured grids, with a focus on reducing the number of computational elements required for accurate simulations. Benchmark test cases include the isochoric advection of a hydrogen-oxygen mixture, convection of a pseudo-isentropic vortex, and flame kernel–vortex interaction. As a key extension, a large-eddy simulation of a turbulent hydrogen-nitrogen-air diffusion flame on a fully unstructured three-dimensional grid is presented, demonstrating the method’s capability to handle complex variable-density reactive flows in practical combustion scenarios. Results show that the k-exact scheme achieves accurate predictions even on relatively coarse grids, substantially reducing computational cost while maintaining physical fidelity - underscoring its potential for reactive flow simulations in both industrial and research applications