Showing papers by "National Technical University of Athens published in 2022"

Development of a Tourism Carrying Capacity Index (TCCI) for sustainable management of coastal areas in Mediterranean islands – Case study Naxos, Greece

Abstract: Mediterranean Region is nowadays deemed to be a ‘hot spot’ in various respects, including climate change, environment and biodiversity, urbanization and mass tourism, to name a few. This holds true in the case of Mediterranean islands in particular, i.e. places that are globally acknowledged as appealing mass tourism destinations; while at the same time are confronted with severe threats, more intensely presented in islands' overcrowded coastal zones. In fact, intensified Climate Change incidents and related threats, coupled with the escalating – beyond carrying capacity limits – coastal urbanization and mass tourism development, represent currently key policy concerns in Mediterranean islands. These also raise, in a strict and urgent way, the necessity to sketch more sustainable and resilient future pathways, especially in their coastal counterparts. Towards this end, this paper attempts to outline the Tourism Carrying Capacity (TCC) approach as a means for assessing and steadily monitoring tourism development trajectory in coastal areas of the Mediterranean islands. More particularly, by using the Pressure-State-Response (PSR) framework as a foundation, it develops and implements a Tourism Carrying Capacity Index (TCCI) that focuses primarily on environmental and manmade dimensions; incorporates mostly tourism-specific variables; and is capable of providing guidance towards planning more sustainable tourism future pathways in coastal areas. TCCI application, demonstrated for the Island of Naxos, Greece, unveils its power as an easy to handle decision-making tool for serving coastal tourism sustainability assessment and monitoring; but also its replicability potential and barriers to implementation.

ENERDGE: Distributed Energy-Aware Resource Allocation at the Edge

Abstract: Mobile applications are progressively becoming more sophisticated and complex, increasing their computational requirements. Traditional offloading approaches that use exclusively the Cloud infrastructure are now deemed unsuitable due to the inherent associated delay. Edge Computing can address most of the Cloud limitations at the cost of limited available resources. This bottleneck necessitates an efficient allocation of offloaded tasks from the mobile devices to the Edge. In this paper, we consider a task offloading setting with applications of different characteristics and requirements, and propose an optimal resource allocation framework leveraging the amalgamation of the edge resources. To balance the trade-off between retaining low total energy consumption, respecting end-to-end delay requirements and load balancing at the Edge, we additionally introduce a Markov Random Field based mechanism for the distribution of the excess workload. The proposed approach investigates a realistic scenario, including different categories of mobile applications, edge devices with different computational capabilities, and dynamic wireless conditions modeled by the dynamic behavior and mobility of the users. The framework is complemented with a prediction mechanism that facilitates the orchestration of the physical resources. The efficiency of the proposed scheme is evaluated via modeling and simulation and is shown to outperform a well-known task offloading solution, as well as a more recent one.

Social innovation and austerity governance in Athens and Madrid: Rethinking the changing contours of policy and practice:

Abstract: The notion of social innovation (SI) has received significant attention in academic debates and policy, denoting the potential for bottom-up and ‘bottom-linking’ sociospatial transformations and so...

Reheating in Runaway Inflation Models via the Evaporation of Mini Primordial Black Holes

Abstract: We investigate the cosmology of mini Primordial Black Holes (PBHs) produced by large density perturbations that collapse during a stiff fluid domination phase. Such a phase can be realized by a runaway-inflaton model that crosses an inflection point or a sharp feature at the last stage of inflation. Mini PBHs evaporate promptly and reheat the early universe. In addition, we examine two notable implications of this scenario: the possible presence of PBH evaporation remnants in galaxies and a non-zero residual potential energy density for the runaway inflaton that might play the role of the dark energy. We specify the parameter space that this scenario can be realized and we find that a transit PBH domination phase is necessary due to gravitational wave (GW) constraints. A distinct prediction of the scenario is a compound GW signal that might be probed by current and future experiments. We also demonstrate our results employing an explicit inflation model.

Stress-Testing Framework for Urban Water Systems: A Source to Tap Approach for Stochastic Resilience Assessment

Abstract: Optimizing the design and operation of an Urban Water System (UWS) faces significant challenges over its lifespan to account for the uncertainties of important stressors that arise from population growth rates, climate change factors, or shifting demand patterns. The analysis of a UWS’s performance across interdependent subsystems benefits from a multi-model approach where different designs are tested against a variety of metrics and in different times scales for each subsystem. In this work, we present a stress-testing framework for UWSs that assesses the system’s resilience, i.e., the degree to which a UWS continues to perform under progressively increasing disturbance (deviation from normal operating conditions). The framework is underpinned by a modeling chain that covers the entire water cycle, in a source-to-tap manner, coupling a water resources management model, a hydraulic water distribution model, and a water demand generation model. An additional stochastic simulation module enables the representation and modeling of uncertainty throughout the water cycle. We demonstrate the framework by “stress-testing” a synthetic UWS case study with an ensemble of scenarios whose parameters are stochastically changing within the UWS simulation timeframe and quantify the uncertainty in the estimation of the system’s resilience.

Investigation of the gridded flash flood Guidance in a Peri-Urban basin in greater Athens area, Greece

Abstract: In this research work, an implementation of the gridded Flash Flood Guidance (FFG) method is conducted for the prediction and evaluation of flash floods in Greece. The FFG system is a well-established Early Warning System (EWS) for the estimation of flash flood threats, especially in small ungauged basins, where flash floods are the most dominant form of flooding. First, an overview of the gridded FFG system is conducted, where all computations are performed at grid level. The methodology applied consists of the derivation of the threshold rainfall, referred to as the FFG. The catastrophic flash flood event that occurred on the 15th of November 2017 and caused the loss of 24 human lives and extensive economic damage within the Mandra settlement, in the Attica region in Greece, is used to assess the lead time provided by the system. Finally, a sensitivity analysis is performed regarding the two main aspects of the system, i.e. the threshold runoff and the estimation of the soil moisture conditions, and their impact on the generated lead times. Results show that the most crucial aspect of the system is the soil moisture conditions, since in wet conditions a 10% deviation on a scale from zero to one, can result in a complete time-step loss of the possibilities provided lead time, while in dry conditions the deviation is much larger. Finally, concerning the studied event, results show that if the system was operational, and the forecasted rainfall matched the values of the actual rainfall, at least a five-hour lead time warning could have been issued based on the results of the system.

O6-plane backreaction on scale-separated Type IIA AdS3 vacua

Abstract: A bstract We evaluate the backreaction of O6-planes in scale-separated AdS 3 flux vacua of massive Type IIA. Using the appropriate flux scaling we show that the corrections to the various background fields and moduli are controlled and subleading when going from smeared to localized sources. Similarly, the backreaction corrections to the scalar potential are parametrically small in the scale-separation limit, assuming always that the near-O6-plane singularities will find a resolution within string theory, even in the presence of a Romans mass. Our analysis is based on the equations of motion and therefore applies also to the non-supersymmetric vacua.

Comprehensive Fuzzy Logic Coefficient of Performance of Absorption Cooling System

Abstract: The Absorption Cooling System (ACS) coefficient of performance (COP) is a key parameter that describes the degree of efficiency of this type of cooling system. In all previous analyses, this coefficient was determined in a deterministic way, which in the case of the operation of a thermodynamic system, such as ACS, does not ensure its optimal operation. Therefore, in this paper, Comprehensive Fuzzy Logic COP (CFLCOP) is presented, which enables a more flexible operation of ACS and more precise action on those components of ACS that do not contribute to its optimal operation. The CFLCOP includes two fuzzy logic-based COPs: one based on energy efficiency (ENFLCOP) and the other on exergy efficiency (EXFLCOP). The analysis of the use of CFLCOP shows a higher number of hours of ACS operation in the optimal mode, compared to the deterministic COP (DCOP) and a higher degree of continuity of ACS operation at the desired cooling temperature.

Mixing Thermodynamics and Flory–Huggins Interaction Parameter of Polyethylene Oxide/Polyethylene Oligomeric Blends from Kirkwood–Buff Theory and Molecular Simulations

Abstract: In this study, we conduct a full thermodynamic analysis of polyethylene oxide/polyethylene oligomeric blends, building on the methodology introduced by Petris et al. [J. Phys. Chem. B, 2019, 123, 247–57], using which we contribute to the interpretation of large-scale molecular dynamics (MD) oligomeric blend simulations in the light of Kirkwood–Buff (KB) theory, featuring a composition-dependent estimation of the Flory–Huggins interaction parameter. The KB integrals are calculated from NpT MD trajectories using the particle fluctuation method. The component activity coefficients, the excess Gibbs energy of mixing, the volume, enthalpy, and entropy of mixing are extracted as functions of the mole fraction. The Flory–Huggins interaction parameter χ is estimated by interpreting the Gibbs energy of mixing in the framework of Flory–Huggins theory, and its dependence on composition is explored. A structural analysis of the studied oligomeric blends is performed to obtain the mean squared radius of gyration, the molecular pair distribution functions, and the dihedral angle distributions of the two components and is used to interpret the predicted thermodynamic properties. All the results are compared against experimental measurements and previous simulations, where available, and the agreement is found to be very good, validating our proposed methodology.

Biotic and Abiotic Biostimulation for the Reduction of Hexavalent Chromium in Contaminated Aquifers

Abstract: Hexavalent chromium is a carcinogenic heavy metal that needs to be removed effectively from polluted aquifers in order to protect public health and the environment. This work aims to evaluate the reduction of Cr(VI) to Cr(III) in a contaminated aquifer through the stimulation of indigenous microbial communities with the addition of reductive agents. Soil-column experiments were conducted in the absence of oxygen and at hexavalent chromium (Cr(VI)) groundwater concentrations in the 1000–2000 μg/L range. Two carbon sources (molasses and EVO) and one iron electron donor (FeSO4·7H2O) were used as ways to stimulate the metabolism and proliferation of Cr(VI) reducing bacteria in-situ. The obtained results indicate that microbial anaerobic respiration and electron transfer can be fundamental to alleviate polluted groundwater from hazardous Cr(VI). The addition of organic electron donors increased significantly Cr(VI) reduction rates in comparison to natural soil attenuation rates. Furthermore, a combination of organic carbon and iron electron donors led to a longer life span of the remediation process and thus increased total Cr(VI) removal. This is the first study to investigate biotic and abiotic Cr(VI) removal by conducting experiments with natural soil and by applying biostimulation to modify the natural existing microbial communities.

Reversing visibility analysis: Towards an accelerated a priori assessment of landscape impacts of renewable energy projects

Abstract: Impacts to landscapes have been identified as major drivers of social opposition against renewable energy projects. We investigate how the process of mitigating landscape impacts can be improved and accelerated, through a re-conceptualization of visibility analysis. In their conventional format, visibility analyses cannot be implemented in early planning phases as they require the finalized locations of projects as input. Thus, visual impacts to landscapes cannot be assessed until late in development, when licensing procedures have already begun and projects' locations have already been finalized. In order to overcome this issue and facilitate the earlier identification of impactful projects we investigate the reversal of visibility analyses. By shifting the focus of the analyses from the infrastructure that generates visual impacts to the areas that have to be protected from these impacts, visibility analyses no longer require projects' locations as input. This methodological shift is initially investigated theoretically and then practically, in the region of Thessaly, Greece, computing Reverse - Zones of Theoretical Visibility (R-ZTVs) for important landscape elements of the region, in order to then project visual impacts to them by planned wind energy projects. It was demonstrated that reversing visibility analyses (a) enables the creation of R-ZTV-type maps that facilitate the anticipation of landscape impacts of projects from earlier planning stages and (b) discards the requirement for individual visibility analyses for each new project, thus accelerating project development. Furthermore, R-ZTV maps can be utilized in participatory planning processes or be used independently by projects' investors and by stakeholders in landscape protection.