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Hydrodynamic circulation in the marine environment, characterized by the effects of the intense tide conditions, the atmospheric loading, and the bathymetry, constitutes a complex phenomenon. The present paper deals with the numerical simulation of the hydrodynamic circulation of the Gulf of Evoikos, which is characterized by the existence of a strong tide. The Estuarine and Lake CΟmputer Model three-dimensional hydrodynamic model has been used, which, besides the tide, takes into consideration the atmospheric forces acting on the surface of the study area, such as the solar radiation, the atmospheric pressure, the relative humidity, the rainfall height, the air temperature and the wind speed and direction. The physical and hydrodynamic characteristics of the water circulation under tide conditions are examined such as the salinity, the water temperature, the density stratification, the flow velocities, the tracer concentration, etc. These characteristics are absolutely critical for the diffusion and dispersion of passive pollutants, the water flow renewal, and the general environmental situation.

The mixing characteristics in the marine environment of the Euboean Gulf are studied. The Estuarine and Lake CΟmputer Model three-dimensional hydrodynamic model has been used, to simulate numerically the effects of the strong tide conditions, the atmospheric forcing, and the oceanographic conditions. Water age was calculated in all computational cells and its renewal was examined with the “pure” water of the open sea both on the surface layers, where the effect of tide and wind was pronounced, as well as on the deeper layers and bottom. It was investigated if in surface layers the tide and the wind restore the water of the study area, thus preventing its renewal. In the remote area, the mixing and dilution of the pollutants contained in the treated municipal waste of five installed diffusers in this complex hydrodynamic field, generated by the aforementioned loads, is simulated. It was found that even a slight stratification density has the effect of restricting the vertical mixing and entrapment of dirt in the deeper layers. The combined impact of municipal waste effluents on selected areas of high economic, tourist and environmental value in the Euboean Gulf was assessed. It was found out, that the Diffuser 2 has a great effect in all the five selected areas, because it has greater waste water discharge and because of the tide, who produce a velocity field and transfer the pollutants in longer distances. It was also investigated indicatively in an important area, whether the presence of tide contributes to better mixing and dispersing of pollutants and to better water quality or, on the contrary, as it restores remote pollutants that were released earlier.

We consider the problem of the vertically upwards disposal of heavy brine sewage from a two-dimensional diffuser in a lighter, homogeneous, motionless and shallow ambient sea. The rejected high salinity water of seawater desalination plants for urban and agricultural uses is such a case of a two dimensional fountain. The disposal of brine sewage produces a negative buoyant jet due to its initial momentum, which impinges on the free surface, spreads laterally on it and then sinks downwards, because of the negative buoyancy. Laboratory experiments and dimensional considerations are used in this paper in order to investigate the spreading behavior (width) of the vertical fountain which impinges on the free surface of the shallow ambient fluid. The experimental results have been used to derive an equation relating the width at the free surface with the initial parameters of the flow. In addition, the experimentally measured dilution of the heavier brine sewage on the recipient’s surface is compared with the dilution which was calculated by a numerical simulation of a well-known commercial software package, CORJET (a CORMIX sub model).

A steel plate is one of the critical components of a scroll expander system that can experience cavitation micro-pitting while in service. The content of the present paper consists of two distinct but interrelated parts. The first part aims to highlight that the use of Computational Fluid Dynamics (CFD) simulations in conjunction with experimental measurements can constitute a quite promising tool for the prediction of cavitation erosion areas in scroll expander systems. For this purpose a three-dimensional CFD, steady state numerical simulation of the refrigerant working fluid is employed. Numerical results revealed the critical areas where cavitation bubbles are formed. These numerical critical areas are in direct qualitative agreement with the actual eroded regions by cavitation, which were found by microscopic observations across the steel plate on an after use, scroll expander system. The second part of the paper, aims to further investigate the behaviour and the durability of the steel plate of the studied scroll expander system subjected to cavitation erosion by using an ultrasonic experimental test rig. Scanning Electron Microscopy (SEM) and optical interferometer micrographs of the damaged surfaces were observed, showing the nature of the cavitation erosion mechanism and the morphological alterations of the steel plate samples. Experimental results are explained in terms of the cavitation erosion rates, roughness profile, accumulated strain energy, and hardness of the matrix. The experimental study can serve as a valuable input for future development of a CFD numerical model that predicts both cavitation bubbles formation as well as cavitation damage induced by the bubbles that implode on the steels plates.

Eight vertical flow small-scale constructed wetlands were introduced in a pre-existing wastewater treatment pilot plant consisting of three interconnected waste stabilization ponds—a facultative pond followed by two maturation/aerobic ones. The wetlands were identical in couples but differed in substrate and plant presence (common reed) and were fed from the outflow of the first maturation pond. The system was monitored for two years in order to study its operation and efficiency for the treatment of septage in an open air experiment. Samples were collected twice a month and TSS, BOD5, pH, EC, TP, N- and N- were monitored. BOD5 removal was greater in the summer months. The unplanted wetlands were more efficient in TSS and BOD5 concentrations removal. Still, when estimating the BOD5 mass in the constructed wetlands effluents using Thornthwaite model to adjust the results, it was found that the BOD5 mass of unplanted wetlands effluents is greater than that found in planted ones. The system did not ...

This study investigates the long-term capacity of the North Aegean coastal systems to transport and store conservative pollutants that originate from the Black Sea. Emphasis is placed on modeling the dispersion and accumulation of a passive tracer that represents a Black Sea pollutant (BSP) substance that continuously discharges from the Dardanelles exit into the North Aegean, for a long period of time (16 years). The effects of the Black Sea water (BSW) inflows, meteorological forcing, and seasonal stratification are assessed with a 3D hydrodynamic model (Estuary, Lake and Coastal Ocean Model), after validation with available field data. The salinity, water temperature, and discharge from the Dardanelles Strait are taken to be seasonally varied. According to the authors' best knowledge, the present paper constitutes the first numerical modeling attempt in the literature that apart from the long-term hydrodynamic characteristics that have also been studied in previous works, a suitable tracer is introduced in order to predict the long-term fate, distribution, and accumulation of pollutants that originate from the Black Sea into the North Aegean coastal regions. The overall results of the present investigation indicate that the BSP concentration is very high at the coastal waters of Thassos, Samothraki, and Limnos islands, as well as along the mainland coastal waters between Alexandroupolis and Strymonikos Gulf, during summer and autumn when strong water column stratification occurs. In general, the BSP concentration in the North Aegean surface waters reaches considerable high values (47–58 % of the initial pollutant concentration at Dardanelles inflow) within 16 years. Even for depths more than 500 m the BSP concentration is still remarkable, slightly increasing with time. The increase of the BSP concentration with respect to time at various depths (from free surface up to 750 m) is also investigated.

A numerical simulation of the surface buoyant plume that is formed from the Black Sea brackish water discharge into the North Aegean Sea, through the Dardanelles Straits, has been performed using the ELCOM hydrodynamic model after validation with available field and remote sensing data. Important climatological factors, such as air temperature, relative humidity, wind speed, wind direction, solar radiation, atmospheric pressure and rainfall that affect the water circulation in North Aegean as well as the Coriolis force effect, are taken into account. The seasonal characteristics of the water circulation in the North Aegean are examined using a horizontal grid resolution of 4 km by 4 km. The salinity, the water temperature and discharge from the Dardanelles straits are taken to be seasonally varied. The simulation was conducted for a total flow time of 6 years. According to the authors best knowledge the present paper constitutes the first numerical modeling attempt in the literature that apart from the long-term hydrodynamic characteristics that have also been studied in previous works, suitable tracers are introduced in order to predict the long term fate and distribution of pollutants that are transported from the Black sea into the North Aegean. The overall results of the present investigation indicate that a substantial percentage of pollutants originating from the Black Sea, accumulate in the North part of the Aegean Sea. The Black Sea Pollutant (BSP) concentration in the North Aegean surface waters reaches relatively high values (20–34%) of its initial assumed value (100%) at the Dardanelles exit to the North Aegean, in a relatively short period of 6 years. Even at 500 m depth the BSP accumulation is more than 5% of its initial value.

During floods, the density of river water usually increases due to the increase in the concentration of the suspended sediment that the river carries, causing the river to plunge underneath the free surface of a receiving water basin and form a turbidity current that continues to flow along the bottom. The study and understanding of such complex and rare phenomena is of great importance, as they constitute one of the major mechanisms for suspended sediment transport from rivers into the ocean, lakes or reservoirs. In the present paper a previously tested and verified numerical model [1] is applied in laboratory scale numerical experiments of continuous, high density turbidity currents. The turbidity currents are produced by the steady discharge of fresh water – suspended sediment mixtures, into an inclined channel which is connected at its downstream end to a wide horizontal tank. Both, channel and tank are initially filled with fresh water. This configuration serves as a simplified experimental analog of natural, hyperpycnal turbidity currents that are formed at river outflows in the sea, lakes or reservoirs and usually travel within subaqueous canyon-fan complexes. The main aim is to investigate the exact qualitative and quantitative effect of fundamental, flow controlling parameters in the hydrodynamic and depositional characteristics of continuous, high density turbidity currents. According to the authors’ best knowledge, the present paper constitutes the first attempt in the literature, where the isolated effects of each individual controlling parameter as well as their relative importance on the hydrodynamic characteristics of continuous, high-density turbidity currents are quantitatively evaluated in detail. The numerical model used, is based on a multiphase modification of the Reynolds Averaged Navier–Stokes equations (RANS). For turbulence closure the Renormalization-group (RNG) k–e model is applied, which is an enhanced version of the widely used standard k–e model.

Virtual Water is a recently introduced and widely recognized concept that is considered important for attaining regional and global water security. It refers to the water needed for the production of an agricultural or industrial product and it is contained in the product not in real but in virtual sense. The practical importance of the virtual water concept is mainly twofold, as virtual water trade can act as a tool to achieve water security and efficient water use, while water footprints can act as links between consumption patterns and the impacts on water. This paper applies the virtual water concept at regional level using the Prefectures of Xanthi and Rhodope as case studies. The ensuing results are juxtaposed and commented.

During floods, the density of river water usually increases due to a subsequent increase in the concentration of the suspended sediment that the river carries, causing the river to plunge underneath the free surface of a receiving water basin and form a turbidity current that continues to flow along the bottom. The study and understanding of such complex phenomena is of great importance, as they constitute one of the major mechanisms for suspended sediment transport from rivers into oceans, lakes or reservoirs. Unlike most of the previous numerical investigations on turbidity currents, in this paper, a 3D numerical model that simulates the dynamics and flow structure of turbidity currents, through a multiphase flow approach is proposed, using the commercial CFD code FLUENT. A series of numerical simulations that reproduce particular published laboratory flows are presented. The detailed qualitative and quantitative comparison of numerical with laboratory results indicates that apart from the global flow structure, the proposed numerical approach efficiently predicts various important aspects of turbidity current flows, such as the effect of suspended sediment mixture composition in the temporal and spatial evolution of the simulated currents, the interaction of turbidity currents with loose sediment bottom layers and the formation of internal hydraulic jumps. Furthermore, various extreme cases among the numerical runs considered are further analyzed, in order to identify the importance of various controlling flow parameters.

In a pre-existing wastewater treatment pilot plant consisting of three interconnected waste stabilization ponds –a facultative pond followed by two maturation/aerobic ones – and located in Sindos near Thessalonica, Northern Greece, we introduced eight vertical flow constructed wetlands, identical in couples, that differ in substrate and plant presence and are fed from the outflow of the first maturation pond. This resulted in the creation of a combined waste stabilization ponds–constructed wetland system, which was being monitored systematically for 2 years in order to study its operation. The unit was fed with septage. The monitored parameters were BOD5, pH, conductivity and TSS. This article presents the results obtained from the aforementioned unit, which are juxtaposed to results recorded in the literature for systems with similar characteristics, before focusing on the differences recorded in the results due to the different characteristics of the wetlands (different substrates, presence or absence of plants). To evaluate the system efficiency, results obtained from the constructed wetlands are compared to respective results recorded from the second maturation pond located in the unit, the inflow of which is the same as that of the wetlands. The constructed wetlands that were left unplanted were the ones which exhibited the highest BOD5 and TSS removal and the lowest conductivities.

Greece and Bulgaria, are crossed by three very important transboundary rivers, namely Strouma, Nestos/Mesta, and Maritsa. However, until recently no effective plan concerning their Integrated Water Management has been made. This is rather justified because an Integrated Water Management must be based on accurate and reliable data coming from a large number of observations and measurements and covering large time periods, in order to have an appropriate statistical treatment of the data and extraction of much trustworthy results. In this paper, histograms of mean discharges at the Greek part of the hydrological catchment and correlation coefficients of values relating Greek and Bulgarian stations at 21 river sites in Nestos/Mesta were analyzed in a monthly basis. The main aim of this study is a first-time presentation of useful meteorological data from both countries and to contribute towards an integrated study of the runoff for the whole hydrological basin, in agreement with directive E.U. 60/2000. This was accomplished within the framework of Interreg II transboundary collaboration between Greece and Bulgaria. The hydrological budget equations for the earth’s surface were used. The water balance of Nestos/Mesta and Dospatska rivers to the Greek frontiers, as a total, for the period 1935–95, with observed precipitations, produced an annual average runoff coefficient of 0.482. With corrected precipitations, the coefficient acquires a value of 0.438 revealing the good correlation between the Bulgarian and Greek meteorological stations.

Nestos is one of the most important transboundary rivers flowing through Bulgaria and Greece. In the Greek part of the river, two reservoirs, the Thisavros Reservoir and the Platanovrysi Reservoir, have already been constructed and started operating in 1997 and 1999, respectively. In the first part of the chapter, the reservoir sedimentation effect on the coastal erosion is investigated, for the case of the Nestos River delta and the adjacent shorelines, through a combination of mathematical modeling, modern remote sensing techniques, and field surveying, while in the second part, the mechanical removal as well as the flushing of sediment from the reservoir of Platanovrysi and its disposal in the subbasin downstream of the Platanovrysi Dam up to the Nestos River delta are investigated as potential treatment methods of reducing coastal erosion, using a modification of the same mathematical model that is utilized in the first part of the chapter. The overall findings and conclusions arising from the work presented and discussed in the present chapter contribute to the overall need to thoroughly understand the direct effect of dam construction on coastal erosion, as well as to examine the effectiveness of potential sediment management treatments.

An in situ synchrotron radiographic study of a molten Al-10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode. This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density. ExoMet Project which is co-funded by the European Commission in the 7th Framework Programme (contract FP7-NMP3-LA-2012-280421), by the European Space Agency and by the individual partner organisations, the UK Engineering and Physical Sciences Research Council (EPSRC) (EP/K005804/1 and EP/I02249X/1) and the Research Complex at Harwell.

A steel plate is one of the critical components of a scroll expander system that usually experiences cavitation in service. An experimental study is conducted to study the behaviour of the scroll’s steel plate subjected to cavitation erosion. For this purpose an ultrasonic transducer is utilised to produce cavitation bubbles. Micrographs of damaged surfaces were observed, showing the nature of the cavitation mechanism and the morphology alterations across the steel sample. Experimental results are explained in terms of the cavitation erosion rates, roughness profile, accumulated strain energy, and hardness of the matrix.

Gravity or density currents constitute a large class of natural flows that are generated and driven by the density difference between two or even more fluids. The density difference between two fluids usually arises due to differences in temperature or salinity, but it can also arise due to the presence of suspended solid particles. These particulate currents, in the case of sediment laden water that enters a water basin, are classified according to the density difference with the ambient fluid into three major categories: a) hypopycnal currents, when the density of the sediment laden water is lower than that of the receiving water basin, b) homopycnal currents, when the density of the sediment laden water is almost equal to that of the receiving water basin, and c) hyperpycnal currents when their density is much greater than that of the receiving water body (Mulder & Alexander, 2001). In the case of floods, the suspended sediment concentration of river water rises to a great extent. Hence, the river plunges to the bottom of the receiving basin and forms a hyperpycnal plume which is also known as turbidity current. Such flows are usually formed at river mouths in oceans, lakes or reservoirs, and can travel remarkable distances transferring, eroding and depositing large amounts of suspended sediments (Mulder & Alexander, 2001).

In this paper, hypopycnal and hyperpycnal particulate currents that are formed at river outflows in the sea are studied. The main aim is the use of CFD methods, in order to investigate the plunging mechanism in the case of hyperpycnal currents. Two series of numerical experiments are conducted. The first series, aims to validate the numerical model by checking its ability in capturing the critical suspended sediment concentration for plunging. The second series, aims to investigate the dependence of the plunge depth from the initial flow conditions, in the case of hyperpycnal currents. From the analysis of the results, an empirical equation is derived which relates the plunge depth with the initial conditions. It is also found that secondary hyperpycnal currents, can be generated indirectly, from hypopycnal currents, due to descending sediment fingers. The model predictions, in the present paper are in agreement with previous investigations.

Observations of cavitation damage within a lubricated expander system are studied experimentally. Typical experimental analysis is used to observe cavitation erosion features such as SEM, light-microscopy and lightinterferometer. An experimental test-rig is used to study bubble characteristics within fluids to compare the theoretical analysis and practical in-service results from the expander system. Using an ultra-sonic methodology and high-speed camera techniques the bubbles are observed within the working fluids. A 2D numerical simulation of the scroll was performed to explore the mechanism which generates scroll cavitation. It is found that the pressure is high enough to liquefy instantaneously part of the refrigerant close to the bottom boundary, creating conditions for the generation of cavitation bubbles within the liquefied refrigerant. This finding resolves the puzzle how the refrigerant which enters the scroll in gas phase produces cavitation.

The coastal zone is a transitory zone between land and sea. Due to its importance to man, not only for its high food production but also for recreation, sea transportation and industrial activities, coastal zone receives high environmental pressure from him. This paper deals with degradation phenomena of the coastal zone in the west section of the River Nestos Delta, North Aegean Sea, with special stress on the geomorphological changes in the coastline. The length of the coastline in this part of river Nestos Delta (the Kavala- Chrisoupoli part), from Nea Karvali village to the west, up to the river mouth to the east, is around 35 km long. This section constitutes the biggest and more extended sector of the Nestos Delta; it is the section where the main course and the various branches of the river were located, in the past. Along the coastal zone of this section of the delta many lagoons, sand bars, spits, barrier islands, washover fans, etc. were developed in its geologic past. Some of these geoforms still exist, but the majority of them have been destroyed by physical and/or anthropogenic interventions. Two of the last interventions are the diversion and entrenchment of the river to the east, in early 50’s and the construction of two high dams in the river course inland, in 2000. These human interventions deprived this land of flooding waters and sediments resulting in: (a) drying of most of the river channels and courses crossing this area of the river’s delta, (b) erosion of the coastal landforms and retreat of the shoreline in the majority of the delta coasts. There are, of course, a few places along the coastline where deposition and accretion are still taking place. In more detail, along the coastline taken into consideration in the present paper, one can meet: • stretches with high erosion rates, like the Akroneri Cape (spit), the inner coastline of Keramoti bay (Kokala -Piges coast), the Monastiraki coastline, etc, • stretches with high accretion rates like the Keramoti peninsula/spit, and • stretches at equilibrium or low rate of change like the barrier (spit) west of Akroneri Cape up to Nea Karvali coast and a short stretch of the coastline south-east of Keramoti peninsula. Comparing the Delta coastline of 1945 (from available aerial photographs) and the coastline of 2002 (from high resolution satellite images), before the construction of the Thisavros and Platanovrisi high dams (period 1945-2002), it has been estimated that: 88% of the delta and the adjacent coastlines has been accreted while only 12% has been eroded. In other words, there was a surplus of accretion by 76% and the delta was procreated. Comparing the Delta coastline of 2002 (from high resolution satellite images) and the coastline of 2007 (from high resolution D-GPS field measurements), after the construction of the dams (period 2002-2007), it has been estimated that: only 39% of the delta and the adjacent coastlines has been accreted while 61% has been eroded. In other words, there was a surplus of erosion by 22% and the delta began to retreat. This was due to Δελτίο της Ελληνικής Γεωλογικής Εταιρίας, 2010 Bulletin of the Geological Society of Greece, 2010 Πρακτικά 12ου Διεθνούς Συνεδρίου Proceedings of the 12th International Congress Πάτρα, Μάιος 2010 Patras, May, 2010 the great reduction (by almost 80%) of the river’s sediment load reaching to the sea. Thus, up to 2002, or so, the balance accretion – erosion in the whole delta coastline was positive, whereas after 2002 the erosion and retreat predominates in the delta’s coastline. The prevention of sediments and fresh water flooding in the delta area, has also affected the crops production in the fields in the vicinity of the delta as well as the fish output in the lakes and lagoons of the coastal zone.

The centre dilution, the trajectory and the turbulence structure of a plane buoyant jet issued horizontally are studied. It is found that the trajectory of a thermal water jet can be predicted reasonably well, provided that the conservation of heat flux, instead of buoyancy flux is used. The normalised centerline, turbulence intensity increases with decreasing radius of curvature. A “-3” spectral slope has been observed at the low frequencies of the spectrum of the temperature spectra, next to a “-5/3” inertial subrange.

Although the jet momentum flux has been traditionally accepted as constant, this is not in general true because a weak pressure field is induced in the ambient fluid with positive gradient and because the induced flow field carries momentum flux to the jet. The angle ϕ, at which the induced flow streamlines enter the jet, is the basic parameter which determines whether the jet momentum flux increases, remains constant or decreases. A theoretical solution is presented for the variation of the jet momentum flux in turbulent submerged jets in stationary ambient fluid. The solution presented in this paper generalizes previous theoretical solutions and is in good agreement with existing experimental results. The contribution of the induced pressure field relative to the induced velocity field in varying the jet momentum flux is investigated. The induced flow streamlines are calculated using non-constant jet momentum flux and are compared with Taylor's solution (where constant jet momentum flux was assumed).

The equation usually used to describe the conservation of volume flux in free turbulence (such as mixing layers, jets, etc.) is discussed.