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Time-series analysis of oxygen as an important environmental parameter for monitoring diversity hotspot ecosystems : an example of a river sinking into the karst underground
Saptashwa Bhattacharyya, Janez Mulec, Andreea Oarga-Mulec, 2023, original scientific article

Abstract: Predicting variations in dissolved oxygen concentration (DO) is important for management and environmental monitoring of aquatic ecosystems. Regression analyses and univariate and multivariate time-series analyses based on autoregressive methods were performed to investigate oxygen conditions in the Pivka River, Slovenia. The monitoring site was established upstream where the river sinks into the karst cave Postojnska jama, which hosts one of the richest subterranean faunas yet studied worldwide. It was found that abnormal variations of DO started to be noticeable at values of DO < 3 mg/L and became more pronounced until the ecosystem reached fully anoxic conditions. The abnormal fluctuations during the critical summer period were due to environmental conditions, organic load and resident biota. Predictions for future detection of anomalies in DO values were made from stable residuals of the measured data, and it was demonstrated that the model could be used to obtain a reliable estimate for a short period, such as one day. The example presented an analysis pipeline based on specific and established threshold DO values, and it is particularly important for ecosystems with diversity hotspots where prolonged low DO values can pose a threat to their biota.
Keywords: karst (geology), aquatic ecosystems, dissolved oxygen, modelling, prediction
Published in RUNG: 05.09.2023; Views: 968; Downloads: 5
.pdf Full text (15,17 MB)
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RELATIVISTIC TIDAL DISRUPTIONS OF REALISTIC STARS BY SUPERMASSIVE BLACK HOLES
Taj Jankovič, 2023, doctoral dissertation

Abstract: Stellar tidal disruption events (TDEs), where a star gets disrupted by strong tidal forces of a supermassive black hole (SMBH), offer a unique opportunity for studies of SMBHs and stellar dynamics in galactic nuclei and provide insights into accretion physics. Currently, there are ≈ 100 observed TDEs, however, this number is expected to increase significantly with the start of new wide-field optical surveys, e.g. with the Vera Rubin Observatory. We focus on hydrodynamic simulations of TDEs with the smoothed particle hydrodynamics code Phantom. To begin with, we simulate TDEs in a general relativistic and Newtonian description of an SMBH’s gravity. Stars, which are placed on parabolic orbits with different parameters β (to be defined here), are constructed with the stellar evolution code MESA and therefore have realistic stellar density profiles. We study the mass fallback rate of the debris Ṁ, a quantity often assumed to determine the TDE light curves, and its dependence on the β, stellar mass and age as well as the black hole’s spin and the choice of the gravity’s description. We find that relativistic disruptions at the same pericenter distance are stronger than disruptions in a Newtonian description of the SMBH’s gravity. We also determine the differences between Ṁ of realistic stars with various ages and masses. In addition, we characterize the effect of SMBH’s rotation on the Ṁ and find that it depends on the orientation of SMBH’s spin vector relative to the stellar orbital angular momentum. Encounters on prograde orbits result in narrower Ṁ curves with higher peak values, while the opposite occurs for retrograde orbits. Stellar disruption results in an elongated stream of gas that partly falls back to the pericenter. Due to apsidal precession, the returning stream may collide with itself, leading to a self-crossing shock that launches an outflow. If the black hole spins, this collision may additionally be affected by Lense-Thirring precession which can cause an offset between the two stream components. We study the impact of this effect on the outflow properties by carrying out local simulations of collisions between offset streams. As the offset increases, we find that the geometry of the outflow becomes less spherical and more collimated along the directions of the incoming streams, with less gas getting unbound by the interaction. However, even the most grazing collisions we consider significantly affect the trajectories of the colliding gas, likely promoting subsequent strong interactions near the black hole and rapid disc formation. We analytically compute the offset to stream width ratio, finding that even slowly spinning black holes can cause both strong and grazing collisions. We propose that the deviation from outflow sphericity may enhance the self-crossing shock luminosity due to a reduction of adiabatic losses, and cause significant variations of the efficiency at which X-ray radiation from the disc is reprocessed to the optical band depending on the viewing angle. These potentially observable features hold the promise of constraining the black hole spin from tidal disruption events.
Keywords: Computer modelling and simulation, hydrodynamics, black holes, infall
Published in RUNG: 29.08.2023; Views: 931; Downloads: 12
.pdf Full text (16,29 MB)

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Follow-up studies on exoplanet and eclipsing binary candidates using GoChile
Anže Pirc, 2022, other monographs and other completed works

Keywords: GoChile telescope, astronomy, software setup, aperture photometry, light curve modelling, candidate exoplanet
Published in RUNG: 10.03.2023; Views: 1423; Downloads: 0
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Converting apprehensive customers to willing customers : building trust in online shopping arena
Saibal K. Pal, 2017, original scientific article

Abstract: This study focuses on knowing about the factors that can affect the trust levels of the consumers involved in online shopping. Trust has been identified as a prominent factor that ultimately leads to the transformation of the apprehensive customers into willing customers. The role of website design features and portal affiliation is tested regarding their impact in affecting the trusting beliefs of the customers. Structural Equation Modeling (SEM) was put to use to do the statistical testing on the data collected through students of tier III city of India. The results showed that the website design features have an important role to play in affecting the trusting beliefs of the people whereas portal affiliation didn’t have much of a role. The study implies that the website managers must focus on their design features if they want to gain the trust of their customers. The portal affiliation wouldn’t be fruitful in the case of a student at least. Better shoppers amongst students must be attracted towards portals through innovative designs and features.
Keywords: online shopping, building trust, website design features, portal affiliation, structural equation modelling, theory of reasoned action
Published in RUNG: 01.04.2021; Views: 1954; Downloads: 0
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8.
A Bottom-Up Building Stock Model for Tracking Regional Energy Targets—A Case Study of Kočevje
Marjana Šijanec-Zavrl, Gašper Stegnar, Andraž Rakušček, Henrik Gjerkeš, 2016, original scientific article

Abstract: The paper addresses the development of a bottom-up building stock energy model (BuilS) for identification of the building stock renovation potential by considering energy performance of individual buildings through cross-linked data from various public available databases. The model enables integration of various EE and RES measures on the building stock to demonstrate long-term economic and environmental effects of different building stock refurbishment strategies. In the presented case study, the BuilS model was applied in the Kočevje city area and validated using the measured energy consumption of the buildings connected to the city district heating system. Three strategies for improving the building stock in Kočevje towards a more sustainable one are presented with their impact on energy use and CO2 emission projections up to 2030. It is demonstrated that the BuilS bottom-up model enables the setting of a correct baseline regarding energy use of the existing building stock and that such a model is a powerful tool for design and validation of the building stock renovation strategies. It is also shown that the accuracy of the model depends on available information on local resources and local needs, therefore acceleration of the building stock monitoring on the level of each building and continually upgrading of databases with building renovation information is of the utmost importance.
Keywords: bottom-up modelling, renovation scenarios, building stock, regional energy policy
Published in RUNG: 25.10.2016; Views: 4483; Downloads: 234
.pdf Full text (5,27 MB)

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Bottom-up modelling of continuous renovation and energy balance of existing building stock: case study Kočevje
Marjana Šijanec Zavrl, Gašper Stegnar, Andraž Rakušček, Henrik Gjerkeš, 2016, published scientific conference contribution

Abstract: A dynamic bottom-up model of the building stock is developed and implemented in a case study of Kočevje urban region. In the model, national register of real estate is cross-linked to data from other registers, e.g. the energy performance certificates (EPC) and the subsidized energy renovation measures. Regular updates of the data in registers enable continual improvement of the model. Therenovation potential is determined with respect to the age of building components after the last renovation, while the energy performance of the building stock is based either on the EPC for a particular building if available or on the energy indicators of corresponding building type from IEE EPISCOPE building typology and IEE RePublic_ZEB. Thus, the bottom-up model of the building stock (BuilS) enable a profound overview of the total heat demand, final energy use and CO2 emissions of the entire stock. In the case study Kočevje, various strategies for improving the buildings towards more sustainable ones are presented with projections to 2030. The strategies, reference and intensive renovation scenario, are compared with more ambitious strategy that the municipality is looking towards in the frame of Covenant of Mayors commitment. The bottom-up model was validated against the metered energy use of buildings connected to district heating. In the case study the model shows how the implementation of various strategies lead to different impacts and how the ambitious municipal plans are going to produce independence from fossil fuels by fostering the use of wood biomass as a locally available sustainable energy supply. The Kočevje case study analysis demonstrates, at the local level how a concept of increasing renewable energy sources utilisation and building energy efficiency stimulated by progressive measures can respond to low carbon society and sustainable energy selfsupply challenges.
Keywords: modelling, renovation scenarios, building stock, region policies
Published in RUNG: 12.04.2016; Views: 5159; Downloads: 0
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