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Dionissios P. Margaris
Fluid Mechanics Laboratory, Department of Mechanical Engineering and Aeronautics, University of Patras, 26054 Patras, Greece

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Journal article
Published: 13 June 2020 in Sustainability
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Since recreational diving activities have increased in recent decades, resulting in additional environmental pressure on the coastal zone, the deployment of artificial reefs as a conservation strategy to divert mass ecotourism from fragile natural reefs has been proposed and realized in many areas of the world. Twelve units of a patented naturoid artificial reef technology developed by the Hellenic Centre for Marine Research (HCMR) were deployed in 2015 in the Underwater Biotechnological Park of Crete (UBPC) in order to create an experimental diving oasis and investigate the potential of achieving this aim for the over-exploited coastal ecosystems of this part of the Eastern Mediterranean. Assessment of the degree of establishment of artificial reefs and their ability to mimic natural ecosystems is often monitored through biological surveys and sampling. The measurement of the chemical, physical, and hydrodynamic characteristics of the water mass surrounding artificial reefs is also essential to fully understand their comparison to natural reefs. In particular, the flow field around reefs has been shown to be one of the most important physical factors in determining suitable conditions for the establishment of a number of key species on reef habitats. However, the combination of biological establishment monitoring and realistic flow-field simulation using computational fluid dynamics as a tool to aid in the design improvement of already existing reef installations has not been fully investigated in previous work. They are often reported separately as either ecological or engineering studies. Therefore, this study examined a full-scale numerical simulation of the field flow around individual already installed naturoid reef shapes, and part of their present arrangement on the sea bottom of the UPBC combined with the field-testing of the functionality of the installed artificial reefs concerning fish species aggregation. The results show that the simulated flow characteristics around the HCMR diving oasis artificial reefs were in good general agreement with the results of former studies, both for flows around a single deployed unit and for flows around a cluster of more than one unit. The results also gave good indications of the performance of individual reef units concerning key desirable characteristics such as downstream shadowing and sediment/nutrient upwelling and resuspension. In particular, they confirmed extended low flow levels (less than 0.3 m/s) and in some cases double vortexes on the downstream side of reef units where observed colonization and habitation of some key fish species had taken place. They also showed how the present distribution of units could be optimized to perform better as an integrated reef cluster. The use of computational fluid dynamics, with field survey data, is therefore suggested as a useful design improvement tool for installed reef structures and their deployment arrangement for recreational diving oases that can aid the sustainable development of the coastal zone.

ACS Style

Dimitrios Androulakis; Costas Dounas; Andrew Banks; Antonios Magoulas; Dionissios Margaris. An Assessment of Computational Fluid Dynamics as a Tool to Aid the Design of the HCMR-Artificial-ReefsTM Diving Oasis in the Underwater Biotechnological Park of Crete. Sustainability 2020, 12, 4847 .

AMA Style

Dimitrios Androulakis, Costas Dounas, Andrew Banks, Antonios Magoulas, Dionissios Margaris. An Assessment of Computational Fluid Dynamics as a Tool to Aid the Design of the HCMR-Artificial-ReefsTM Diving Oasis in the Underwater Biotechnological Park of Crete. Sustainability. 2020; 12 (12):4847.

Chicago/Turabian Style

Dimitrios Androulakis; Costas Dounas; Andrew Banks; Antonios Magoulas; Dionissios Margaris. 2020. "An Assessment of Computational Fluid Dynamics as a Tool to Aid the Design of the HCMR-Artificial-ReefsTM Diving Oasis in the Underwater Biotechnological Park of Crete." Sustainability 12, no. 12: 4847.

Journal article
Published: 28 April 2020 in Computation
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Our study presents the computational implementation of an air lubrication system on a commercial ship with 154,800 m3 Liquified Natural Gas capacity. The air lubrication reduces the skin friction between the ship’s wetted area and sea water. We analyze the real operating conditions as well as the assumptions, that will approach the problem as accurately as possible. The computational analysis is performed with the ANSYS FLUENT software. Two separate geometries (two different models) are drawn for a ship’s hull: with and without an air lubrication system. Our aim is to extract two different skin friction coefficients, which affect the fuel consumption and the CO2 emissions of the ship. A ship’s hull has never been designed before in real scale with air lubrication injectors adjusted in a computational environment, in order to simulate the function of air lubrication system. The system’s impact on the minimization of LNG transfer cost and on the reduction in fuel consumption and CO2 emissions is also examined. The study demonstrates the way to install the entire system in a new building. Fuel consumption can be reduced by up to 8%, and daily savings could reach up to EUR 8000 per travelling day.

ACS Style

Andreas G. Fotopoulos; Dionissios P. Margaris. Computational Analysis of Air Lubrication System for Commercial Shipping and Impacts on Fuel Consumption. Computation 2020, 8, 38 .

AMA Style

Andreas G. Fotopoulos, Dionissios P. Margaris. Computational Analysis of Air Lubrication System for Commercial Shipping and Impacts on Fuel Consumption. Computation. 2020; 8 (2):38.

Chicago/Turabian Style

Andreas G. Fotopoulos; Dionissios P. Margaris. 2020. "Computational Analysis of Air Lubrication System for Commercial Shipping and Impacts on Fuel Consumption." Computation 8, no. 2: 38.

Journal article
Published: 03 April 2020 in Remote Sensing
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The coastal ocean is one of the most important environments on our planet, home to some of the most bio-diverse and productive ecosystems and providing key input to the livelihood of the majority of human society. It is also a highly dynamic and sensitive environment, particularly susceptible to damage from anthropogenic influences such as pollution and over-exploitation as well as the effects of climate change. These have the added potential to exacerbate other anthropogenic effects and the recent change in sea temperature can be considered as the most pervasive and severe cause of impact in coastal ecosystems worldwide. In addition to open ocean measurements, satellite observations of sea surface temperature (SST) have the potential to provide accurate synoptic coverage of this essential climate variable for the near-shore coastal ocean. However, this potential has not been fully realized, mainly because of a lack of reliable in situ validation data, and the contamination of near-shore measurements by the land. The underwater biotechnological park of Crete (UBPC) has been taking near surface temperature readings autonomously since 2014. Therefore, this study investigated the potential for this infrastructure to be used to validate SST measurements of the near-shore coastal ocean. A comparison between in situ data and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra SST data is presented for a four year (2014–2018) in situ time series recorded from the UBPC. For matchups between in situ and satellite SST data, only nighttime in situ extrapolated to the sea surface (SSTskin) data within ±1 h from the satellite’s overpass are selected and averaged. A close correlation between the in situ data and the MODIS SST was found (squared Pearson correlation coefficient-r2 > 0.9689, mean absolute error-Δ < 0.51 both for Aqua and Terra products). Moreover, close correlation was found between the satellite data and their adjacent satellite pixel’s data further from the shore (r2 > 0.9945, Δ < 0.23 for both Aqua and Terra products, daytime and nighttime satellite SST). However, there was also a consistent positive systematic difference in the satellite against satellite mean biases indicating a thermal adjacency effect from the land (e.g., mean bias between daytime Aqua satellite SST from the UBPC cell minus the respective adjacent cell’s data is δ = 0.02). Nevertheless, if improvements are made in the in situ sensors and their calibration and uncertainty evaluation, these initial results indicate that near-shore autonomous coastal underwater temperature arrays, such as the one at UBPC, could in the future provide valuable in situ data for the validation of satellite coastal SST measurements.

ACS Style

Dimitrios N. Androulakis; Andrew Clive Banks; Costas Dounas; Dionissios P. Margaris. An Evaluation of Autonomous In Situ Temperature Loggers in a Coastal Region of the Eastern Mediterranean Sea for Use in the Validation of Near-Shore Satellite Sea Surface Temperature Measurements. Remote Sensing 2020, 12, 1140 .

AMA Style

Dimitrios N. Androulakis, Andrew Clive Banks, Costas Dounas, Dionissios P. Margaris. An Evaluation of Autonomous In Situ Temperature Loggers in a Coastal Region of the Eastern Mediterranean Sea for Use in the Validation of Near-Shore Satellite Sea Surface Temperature Measurements. Remote Sensing. 2020; 12 (7):1140.

Chicago/Turabian Style

Dimitrios N. Androulakis; Andrew Clive Banks; Costas Dounas; Dionissios P. Margaris. 2020. "An Evaluation of Autonomous In Situ Temperature Loggers in a Coastal Region of the Eastern Mediterranean Sea for Use in the Validation of Near-Shore Satellite Sea Surface Temperature Measurements." Remote Sensing 12, no. 7: 1140.

Journal article
Published: 01 September 2018 in Chinese Journal of Chemical Engineering
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The hydrodynamic behavior of multiple bubbles rising upward is a field of ongoing research since various aspects of their interaction require further analysis. Shape deformation, rise velocity, and drag coefficient are some of the uncertainties to be determined in a bubble upward flow. For this study the predictions of the three-dimensional numerical simulations of the volume of fluid (VOF) CFD model were first compared with experimental results available in the literature, serving as benchmark cases. Next, 28 cases of pairs of equal and unequal-sized in-line pairs of bubbles moving upwards were simulated. The bubble size varied between 2.0–10 mm. Breakthrough of the present study is the small initial distance of 2.5 R between the center of the bubbles. To provide a more practical nature in this study material properties were selected to match methane gas and seawater properties at deepsea conditions of 15 MPa and 4 °C, thus yielding a fluid-to-bubble density ratio λ = 7.45 and viscosity ratio n = 100.46. This is one of the few studies to report results of the coalescence procedure in this context. The hydrodynamic behavior of the leading and trailing bubbles was thoroughly studied. Simulations results of the evolution of the rise velocity and the shape deformation with time indicate that the assumption that the leading bubble is rising as a free rising single one is not valid for bubbles between 2.0–7.0 mm. Finally, results of the volume of the daughter bubble exhibited an oscillating nature.

ACS Style

Nikolaos A. Avgerinos; Dionissios P. Margaris. A three-dimensional CFD study of the hydrodynamic behavior of equal and unequal-sized in-line methane bubbles at high pressure. Chinese Journal of Chemical Engineering 2018, 26, 1792 -1802.

AMA Style

Nikolaos A. Avgerinos, Dionissios P. Margaris. A three-dimensional CFD study of the hydrodynamic behavior of equal and unequal-sized in-line methane bubbles at high pressure. Chinese Journal of Chemical Engineering. 2018; 26 (9):1792-1802.

Chicago/Turabian Style

Nikolaos A. Avgerinos; Dionissios P. Margaris. 2018. "A three-dimensional CFD study of the hydrodynamic behavior of equal and unequal-sized in-line methane bubbles at high pressure." Chinese Journal of Chemical Engineering 26, no. 9: 1792-1802.