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ÖgeInvestigation of new type wave energy converter systems(Graduate School, 2024-02-09)In consideration of the evolving global economy and increasing population, energy is increasingly occupying a more important place in our lives. As commonly understood, energy sources can be categorized into two groups: non-renewable and renewable. A renewable energy source is defined as an energy source that can exist the next day within the natural order of the Earth. The most significant features of renewable energy sources include reducing carbon dioxide emissions to help protect the environment, decreasing dependence on foreign energy sources due to being local resources, contributing to increased employment, and receiving widespread and strong support from the public. In other words, renewable energy sources possess characteristics of accessibility, availability, and acceptability. Among renewable energy sources, wave energy, despite being approximately 10 times more intense than solar energy per square meter and 5 to 10 times more intense than wind energy, has a very low utilization rate compared to others. Efforts to harness wave energy are rapidly increasing worldwide. It is a predictable fact that our country, surrounded by seas on three sides, has significant potential for wave energy, and the use of this energy type will reduce external dependence. Wave energy converters are used to convert wave energy in the seas into electrical energy. Among these converters, oscillating water column (OWC) type converters are the most efficient. The oscillating water column is a system consisting of two phases: water and air flow. Moving according to the periodic structure of the wave, the water converter compresses the air inside, allowing it to exit at a certain speed from the outlet. Therefore, the parameters affecting this system are the periodic movement of the wave, the dimensions of the converter, and the dimensions of the outlet. Until now, studies have been conducted on OWC energy converters, and prototypes have been developed by testing various models. However, due to the complexity of the energy conversion mechanism, commercial production has not achieved the desired efficiency from these studies. This thesis is composed of three separate articles, each written by designing new oscillating water column-type wave energy converters, calculating their efficiencies, and compiling the results into articles. In the first article, a new tank design is proposed for an oscillating water column (OWC) with an inclined wall to improve performance. Physical experiments were conducted for different orifice damping levels and sea conditions to calculate hydrodynamic performance. The results were compared with the results of a classic OWC design. According to the comparison, it was observed that the new chamber design had a significant impact on OWC efficiency. The proposed chamber design geometry increased OWC performance by up to 31% for wave parameters where the OWC operates effectively. Additionally, the results show that the optimal orifice damping level for the highest efficiency increase is dependent on the chamber design. The aerodynamic geometry of the chamber increased OWC efficiency by 54% maximum and 44% average even when there is internal slosihng in the tank. The second article focuses on optimizing the diameter of the cylindrical front wall entry of the OWC. Specifically, 240 physical experiments were conducted for five different diameter values, various orifice dampings, dimensionless wave frequencies (Kh), and wave heights. It was found that the cylindrical front wall entry geometry consistently improved OWC efficiency under all conditions. Maximum and average efficiency improvements were calculated as 45% and 25%, respectively. The simplicity of the modification at the front entrance makes this improvement even more interesting. A negative correlation was observed between the size of the diameter and Kh. Optimal sizes for the diameter were identified as 12 cm, 7 cm, and 5 cm for low, medium, and large Kh values, respectively. The diameter maximizing efficiency in a specific frequency band was determined based on wave height and orifice ratio. To validate the effectiveness of cylindrical geometry, experiments in free decay were conducted, demonstrating that modifying the front entrance mitigated or reduced flow separation caused by the sharp lower cut. Consequently, the utilization of a cylindrical front wall underlip geometry proved beneficial in reducing shear stresses on both sides of the front wall and enhancing the structural integrity of the OWC model. In the third article, hydrodynamic features evaluating the wave energy collection potential of a double-chambered OWC structure were examined through a series of experiments. The study used efficiency as a measure to quantitatively assess the performance of the OWC system by changing front wall designs, power orifice dampings, and wave conditions. The findings show that the double-chambered configuration consistently exhibited improved performance for the studied orifice ratios, opening heights, and incoming wave frequencies combinations. Increasing the opening height of the second chamber improved the performance of the OWC, as expected. Interestingly, the effect of applied orifice damping on efficiency was found to be relatively insensitive. The most significant improvements in efficiency values were observed for larger wave frequency values. For example, for dimensionless wave frequency (Kh) values of 1.43 and 1.68, efficiency increased from 0.47 to 0.72 and from 0.26 to 0.52, respectively, with the orifice ratio combination of τ1, 0.015 and τ2, 0.018. While the single-chambered OWC effectively operated in the resonance frequency range of 0.94-1.23, the double-chambered design expanded this frequency range, allowing effective wave energy collection over a wider spectrum of wave conditions. The double-chambered configuration offers significant advantages by increasing the effective frequency bandwidth for efficient energy conversion in wave energy conversion applications. Research findings have shown that this innovative design has the potential to significantly increase the efficiency and adaptability of wave energy converters. The results indicate that improvements and innovations in wave energy technologies will make it possible to use this energy source more efficiently and reliably. However, overcoming challenges at the application scale and economic barriers will require further work and investment. This thesis seeks to establish a comprehensive framework for both research and industrial applications within the wave energy field, highlighting the considerable potential of wave energy. In summary, the goal of this investigation is to enhance our understanding, make progress in wave energy technology, and play a role in shaping a sustainable energy future.
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ÖgeDevelopment of drought intensity-duration-frequency curves based on standardized climatic indices using physical variables, deficit in precipitation and deficit in streamflow(Graduate School, 2023-11-13)Drought is a natural disaster that causes water scarcity and affects the population seriously in many ways such as economic, social and environmental impacts. These effects have been further aggravated by rising demands for agricultural, municipal and industrial water, etc. due to the growing population coupled with rising life standards and increasing industrialization in the world. Compared to other natural hazards, drought is among the most damaging disasters as it develops slowly over a considerable period of time that may linger for several years even after the drought has terminated. How important the drought is in the sustainability of water resources requires an extra effort to understand and explain. Domestic use, agriculture, industry, energy, tourism and many more sectors are exposed to drought, which has a direct impact on ecology, economy and society. As one of the least understood hydrological phenomena, drought stands as both a technical and social problem. This is a great motivation to explore drought concepts and methodologies in this thesis by extending them from their quantitative technical context to qualitative social context. Case studies were performed in two river basins with different climatic characteristics. One is Seyhan River Basin in the south of Turkey with a Mediterranean climate, and another is the Kocher catchment in the southwestern corner of Germany with a humid subtropical climate. In the literature, the drought is commonly analyzed by the use of available hydroclimatic or hydrologic data with little in-depth consideration of specific major dry periods experienced over a region. Also, it is not a common practice to assess the probability of drought categories with a rolling time series and hence the changing knowledge for operational drought monitoring. A combination of such quantitative analysis with a comprehensive qualitative assessment of drought as a human-water relation aimed to fill this gap by performing a case study in the Seyhan River Basin, Turkey. Therefore, the first part of this thesis focuses on non-stationary rolling time series analysis together with the stationary full-record time series analysis and the individual major dry period analysis. Six major dry periods were identified from the precipitation time series of 19 meteorological stations. Dry periods extended over 1970-1974, 2002-2008, and 2013-2015, known also as droughts of 1974, 2008, and 2014, respectively, should particularly be noted among all as they affected several sectors in the river basin with the long-lasting deficit in water. Major dry periods were analyzed by rolling time series and full time series, and they were also analyzed individually. The results show that significantly higher probabilities were calculated for extreme droughts with the use of individual major dry periods. An important outcome of the study is that drought is underestimated in practice with the sole use of the whole data record. Also, a major dry period could be important in terms of its duration while another in terms of its severity or intensity. Each major dry period has its own impact on human-water relations that can be influential on drought mitigation, management and governance. The second part of the thesis investigates the statistical properties of drought characteristics for a better understanding of drought to mitigate its negative impacts and improve drought management strategies. Differently from the common approach based on the run theory, the hypothesis is that drought is different from the dry period and dry periods identify droughts of different durations. Up to now, no attention has been paid in this sense to the statistical properties of dry periods versus those of more severe and impactful droughts as separate phenomena. The study shows how different the statistical properties of dry periods and droughts are by quantifying them with the Standardized Precipitation Index (SPI) together with the total probability theorem-coupled frequency analysis. The study tests four concepts in this context: (1) Dry period, (2) Critical drought for a dry period, (3) Critical drought for a year. The results uncover the high number of droughts, which are not addressed in the run theory, and reveal more intense droughts that are masked within the dry periods. Specifically, the drought impact is more often related to drought intensity than severity and duration. In addition, the dry period is less intense than other concepts. Among the droughts newly proposed, critical droughts are the most intense concepts. This demonstrates that drought characteristics calculated for the drought concepts are different from the common approach of the dry period. The difference among the concepts is large enough to carry potential implications for local or regional scale drought management planning. In the third study, drought intensity-duration-frequency (IDF) curves were developed by using the critical drought identified in the second study. The drought IDF curves are based on precipitation and streamflow deficits because drought estimates in terms of physically measurable variables are key knowledge for effective water management. However, how these deficits vary with the drought event severity indicated by commonly used standardized indices is often unclear. Drought characteristics assigned the same value in the index are not necessarily the same in different regions, and in different months of the same region. The study investigates drought to remove this disadvantage of the index-based drought IDF curves and develop intensity-duration-frequency (IDF) curves in terms of the associated deficit. In order to study the variation of deficits, the link between precipitation and streamflow was used, and the associated indices, standardized precipitation index (SPI) and standardized streamflow index (SSI). More specifically, the analysis relies on frequency analysis combined with the total probability theorem applied to the critical drought severity. The critical drought has varying durations and it is extracted from dry periods. IDF curves in terms of precipitation and streamflow deficits for the most severe drought of each drought duration in each year are then subject to the comparison of statistical characteristics of droughts for different return periods. Precipitation and streamflow data from two catchments, the Seyhan River (Turkey) and the Kocher River (Germany) provide examples of two climatically and hydrologically different cases. A comparison of the two cases allows to test a similar method in different hydrological conditions. The results show that precipitation and streamflow deficits vary systematically reflecting seasonality and the magnitude of precipitation and streamflow characteristics of the catchments. Deficits change from one month to another at a given station. Higher precipitation deficits were observed in winter months compared to summer months. Additionally, major droughts experienced in both catchments on the IDF curves show that the major droughts have return periods at the order of years at short durations. This coincides with the observation in the catchments and shows the applicability of the IDF curves. The IDF curves can be considered a tool for the use in a range of specific activities of agriculture, ecology, industry, energy, water supply, etc. This is particularly important to end-users and decision-makers to act against the drought quickly and precisely in a more physically understandable manner. Other than quantitative methodologies, drought was also analyzed by a qualitative approach. Using the causal loop diagram, the drought event of 2008 observed in the Seyhan River Basin was taken as an example to focus on the identification of bidirectional feedback between humans and water under drought conditions. This particular drought is short but severe as it coincided with the least amount of precipitation ever recorded in the river basin. It is an important example because it gave an impetus to the governmental water authorities to initiate basin-scale drought management plans. The quantitative and qualitative data collected for this drought were used in the causal loop analysis. The causal loop diagram shows that short-term measures under economic, structural and conservation frameworks can only provide temporary solutions which may aggravate drought impact. A sustainable water availability under drought conditions can only be achieved by long-term reactions such as drought management plans. In summary, the thesis contributes to drought analysis research by proposing new methodologies and drought concepts. The outcomes of the study underline the need for such drought analyses. The analysis of the conceptualization reveals strong differences among the statistical properties of the drought characteristics. Moreover, the thesis emphasizes the region-specific characteristics of drought that need to be considered for drought management strategies.
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ÖgeFizik tabanlı tam yayılı hidrolojik modellerin kalibrasyonunda uydu tabanlı verilerin kullanılması(Lisansüstü Eğitim Enstitüsü, 2023-01-31)Hidrolojik model parametreleri geleneksel yaklaşımda havza çıkışında gözlenen günlük nehir akım verileriyle tahmin edilmeye çalışılır. Modern yaklaşımda ise akım verileri yanında herkese açık uydu tabanlı uzaktan algılama verilerinden de azami istifade edilmeye çalışılır. Uzaktan algılama verilerinin kullanıldığı yöntem ile ulaşılan alansal model çıktıları, sadece akım verisiyle elde edilen noktasal iyileştirme sonuçlarına göre daha tutarlı ve güvenilirdir. Bu nedenle fizik tabanlı sonlu elemanlar yöntemiyle çalışan matematiksel havza modellerinin uydu verileriyle kalibrasyonu dünyada giderek yaygınlaşmaktadır. Bu çalışmamızın amacı uzaktan algılama yöntemleriyle elde edilmiş buharlaşma ve terleme verilerinin hidrolojik model kalibrasyonuna etkilerini araştırmaktır. Bunun için veri kalitesi yüksek Fransa'nın Vienne havzasında fizik tabanlı tam yayılı mHM modeli kurulmuş ve 11 senaryolu kalibrasyon deneyleri yapılmıştır. 2002-2014 kalibrasyon dönemindeki modelin akım benzeşim performansı modelin günlük akım çıktıları ile gözlenen akım değerleri arasında hesaplanan KGE, modelin alana yayılı fiziksel performansı ise mHM'in uzun dönem (2002-2014) aylık buharlaşma ve terleme (AET) raster çıktı haritaları ile referans MODIS-AET raster haritaları arasında üçer aylık üç dönemde (1. dönem: Mart, Nisan, Mayıs, 2. dönem: Haziran, Temmuz, Ağustos 3. dönem: Eylül, Ekim, Kasım için) hesaplanan SPAEF değerleri ile ortaya konmuştur. 1998-2001 arası 4 yıl ön koşturma (spin-up) dönemi olduğundan performans hesaplarına dahil edilmemiştir. Model parametreleri Ostrich yazılımı içerisinde bulunan 750 iterasyonlu paralel pareto-DDS (PARA-PADDS) yöntemi kullanarak kalibre edilmiştir. Sonuçlara göre, sadece havza çıkışında konumlandırılan bir tek akım gözlem istasyonundan elde edilen akım verilerinin kullanıldığı kalibrasyonda (senaryo 1) modelin akım performansı beklendiği gibi çok yüksek (KGE 0.91, maksimum değeri 1); modelin AET performansı ise üç dönemde de çok düşüktür (SPAEF -0.16, -0.21, -0.26). Havza çıkışındaki AGİ ve nehrin farklı alt kollarında konuşlu 3 AGİ'den alınan akım verileriyle 4 AGİ'li kalibrasyonda (senaryo 2) ortalama KGE 0.91'den 0.37'ye düşerken ve üç dönemin SPAEF değerleri -0.72, 0.76, 0.55 olmuştur. Tek AGİ ve uydu verili kalibrasyonda (senaryo 3) akım performansında 0.91'den düşme çok sınırlı olmuş (KGE 0.90), bunun yanında AET performansı üç dönemde de önemli oranda iyileşmiştir (SPAEF 0.64, 0.77, 0.74). Akım ölçümü hiç olmayan sadece uydu verili kalibrasyonda (4. senaryo) su dengesi sağlanamamış (KGE -0.24), üç dönemde SPAEF 0.39, 0.69, 0.67 değerlerini almıştır. Veri eksikliği yaşanan havzalar için iki senaryo (5 ve 6) kurgulanmıştır. Bir yılda ve sadece her ayın ortasında tek ölçüm, toplamda 12 adet akım verisi ve MODIS-AET uydu verisiyle kalibrasyonda (senaryo 5) su dengesi ve AET performansının iyileştiği gözlemlenmiştir (KGE 0.67 ve üç dönemde SPAEF 0.65, 0.82, 0.76). MODIS-AET yanında sadece bir yıllık günlük akım verileri kullanılan 6. senaryoda KGE 0.72 ve üç dönemde SPAEF 0.64, 0.80, 0,79 değerlerini almıştır. Kurgulanan diğer 5 farklı senaryoda karasal gözlemler yerine global akış verilerinden elde edilen zaman serileri kullanılmıştır. Bu diğerlerinden farklı 5 senaryoda bilhassa hiçbir ölçüm verisinin olmadığı havzalarda çok amaçlı çoklu kalibrasyon yaklaşımının karasal gözlemlerden tamamen bağımsız olarak yürütülüp yürütülemeyeceği sorusuna cevap aranmıştır. ERA5_LAND ve GLDAS akış verilerinin kullanıldığı senaryolarda uydu verisi MODIS-AET kullanılmıştır. Bu senaryolar içinde S11`de GLDAS ve MODIS-AET aylık ortalamalar ile yapılan çok amaçlı çoklu kalibrasyonda su dengesi önemli ölçüde sağlanmıştır. Bu senaryo kurgusunda diğerlerinde olduğu gibi karasal gözlemlerin bulunmadığı kabulünden yola çıkarak akış verisi ve AET verisi ücretsiz olarak uydu ve uydu bazlı yeniden analiz ürünü olarak temin edilmiştir. GLDAS global yeniden analiz ürünü yayılı akış verileri uzun yıllar ortalması 12 aylık zaman serilerine dönüştürülerek MODIS`den alınan yayılı AET verileri ile birlikte kalibrasyonda kullanılmıştır. KGE değerinin 0,37`ye çıkmasına rağmen SPAEF değerlerinin sırası ile 0,63; 0,78 ve 0,75 değerlerini alması diğerleri ile kıyaslandığında su dengesini önemli ölçüde sağlamıştır. S7 senaryosunda 2002-2014 yılları arası ERA5_LAND akış verisinin zaman serileri kullanılırken MODIS AET verileri patern yönünden hedeflenmiştir. Bu kurguda KGE değerleri sıfıra yakın çok küçük değerler alırken SPAEF okumaları HTA ve EEK dönemlerinde yüksek okumalar vermiştir. Fakat su dengesi sağlanamamıştır. S8 kurgusunda ERA5_LAND için 2002-2014 arası uzun dönem aylık veriler kalibrasyonda kullanılmıştır. MODIS AET veriside çok amaçlı çoklu kalibrasyonun diğer amaç fonksiyonuna konu edilen alansal girdisi olmuştur. Bu kurguda SPAEF değerlerinin yüksek değerler alması (0,64; 0,78; 0,73) yanında KGE okumaları sınırlı bir iyileşme göstermiştir (0,18). Her ne kadar havzanın fiziki temsili iyileşmiş olsada su dengesi sınırlı kalmıştır. 9 numaralı senaryoda akış verisi uzun yılların ortalaması alınarak aylık toplamlar şeklinde akış serilerine sokulurken kalibrasyon için bir diğer girdi yine MODIS AET olmuştur. Bu kurguda SPAEF değerleri önemli ölçüde yükselme gösterirken (0,63; 0,77; 0,80) KGE değeri eksi değerler alarak (-0,18) su dengesinden uzaklaşılmıştır. S10 için yapılan kurguda hatanın amaçlandığı akış serileri ve patern yönünden amaçlanan AET verileri ERA5_LAND ürünü olarak temin edilmiş ve kalibrasyonda başka bir veri kullanılmamıştır. ERA5_LAND akış ve AET verileri uzun yıllar aylık ortalamalar şeklinde kalibrasyonda kullanılmıştır. Bu kalibrasyon neticesinde elde edilen debi ve AET simülasyonlarına ait benzeşim başarısı KGE için -0,23 değerini alırken SPAEF değerleri sırası ile 0,02; 0,29 ve 0,37 değerlerini almıştır. Su dengesinin en fazla bozulduğu ve performansı en düşük olan senaryo kurgusu burada ortaya çıkmıştır. Global ürünlerle yapılan kalibrasyonda en iyi performansı S11 senaryosu sağlarken en kötüsünü S10 sağlamıştır. Sonuçlarımız akım ölçümleri eksik ve yetersiz havzalar için ümit vericidir. Bu çalışma, fizik tabanlı modellerin uydu verileri ve uygun amaç fonksiyonları ile kalibre edildiklerinde havzanın fiziğiyle uyumlu, su dengesini de bozmayan optimum parametre setine ulaşılabildiğini göstermiştir. Bir diğer önemli çıktı ise karasal gözlemlerin hiç olmadığı bir havzada dahi ücretsiz uydu verileri ve global ürünler kullanılarak kalibrasyon başarısı sağlanabilmektedir.
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ÖgeAssessing the performance of gridded precipitation products – a comparative analysis of the Black Sea and East Africa regions(Graduate School, 2022-11-04)Hydrological and climatological studies require a good quality of precipitation data. It is difficult to assess the spatial and temporal variability of precipitation for a basin without sufficient gauging stations. Recently, several high-resolution public-domain reanalysis and gridded precipitation products (GPPs) have been produced. The performance of four GPPs in estimating the spatio-temporal distribution of precipitation was examined in this study. The chosen regions are the Rize province (Black Sea region) and the Blue Nile basin (East Africa region). Both basins have similar topograpy and high precipitation. In this study, the observed precipitation data were compared with the GPP datasets using statistical metrics and time-series plots. Then, the performance of these products was evaluated by assessing their capacity in estimating river flow with a calibrated physically based hydrological model- SWAT. On top of that, we have assessed the performance of the multi gauge calibration (MGC) technique to improve the SWAT model efficiency. We assessed four calibration methods to improve the model simulation outcome. The temporal and seasonal data performance of the GPPs was, then, assessed before testing the quality of prediction with various statistical analysis metrics. Finally, the climate change impact on the hydroclimatology of both basins was studied using GCM scenario for future climate predictions. Estimating the parameters that represent the various hydrological processes is one of the main issue hydrologists need to solve since it is impossible to measure all the hydrological parameters. We tested four methods where calibration was done using flow data: only from upstream area (US), only from downstream area (DC), using both upstream and downstream areas of the basin (MGC), and using first the upstream then downstream flow data (UCDC). The results showed that model calibration using the MGC and UCDC techniques proved to have improved the model performance, unlike the single gauge calibrations. The reason is the model obtains much wider information on the basin characteristics to adjust its parameters when more than one gauging station is used simultaneously for calibration. Our study has contributed to the validation of gridded precipitation products for the mountainous regions of the Black Sea and East Africa regions which have scarcity in weather gauging station. In addition to statistical and visual map assessment, we adopted a hydrological model (SWAT) parameterized for these basins to make original assessments on the hydrological responses for each GPP dataset precipitation inputs. The resulting graph showed that the annual cyclic behavior of all data is quite consistent with each other, like having a peak on May. The result for the Ikizdere basin indicated that the peak water yield magnitude estimattion in May is significantly greater than the observed water yield for CFSR and MSWEP datasets. However, the water yield value was lower for APHRODITE. Moreover, water yields were highly overestimated during the periods May-December by the MSWEP simulation, which is due to overestimated precipitation in the first four months (January-April) when most of the precipitation falls in the form of snow. The most comparable simulated flow regime with the observation was that of the ECMWF simulation flow. When we come to the Blue Nile basin, it is considered the most important river basin for the Nile River as it generated much of the annual flow. Hence, having a good accuracy of summer precipitation (rainfall) estimate is essential for Blue Nile basin since much of the annual rainfall falls in the highland region during the summer season (Jun – Sep). The MSWEP precipitation dataset has overestimated the peak flows which is obviously caused by the overestimation of rainfall in the summer season (Jun-Sep). The CFSR dataset performance was poor with negative deviations specifically for the wet season of the year (Jun-Sep). It resulted in underestimation of flow, especially for the peak flows which could be explained by the underestimation of precipitation over the study area. Among the GPPs, ECMWF captures the annual cycle of the measured flow cycle with little deviations. Like the Ikizdere basin, it has proved to perform better for the Blue Nile basin, where both the total water yield and surface flows for the wet has been estimated with good accuracy. The results allign with the previous studies outcomes which concluded that the ECMWF weather data could not only be successfully used in place of surface weather observation records, but also improve hydrological modelling performance. For both study areas, the ECMWF gridded precipitation estimates have been proven to be the most comparable with the observed precipitation suggesting that the dataset could be implimented for mountaneous areas of the world with very scarce ground weather observation stations. Finally, the seasonal average simulated flow of Ikizdere basin was forced with the GCM climate change scenarios and compared with the observed flow at basin outlet. There is consist trend in the forecasted flow for the various GCM projections. There is a dipole of climate projection underperformance in the sense of underestimation for winter & autumn seasons, whereas, overestimating for the spring season. For winter (Dec – Feb) and autumn (Sep-Nov), all the climate models forecast decrease in flow for the river, whereas for the spring season (Mar-May) all the GCMs forecasted an increase in flow. Hence, towards the end of the century, the climate projections indicate a decreasing trend for the winter season and an increasing trend for spring season. This finding agrees with the previous study results where the long-term historical precipitation data analysis demonstrated a decreasing trend in winter season for the Black Sea region. The case is difference for summer season where the GCM scenarios didn't agree on the precipitation projection trend. Generally, the impact of climate change on the seasonal precipitation of the Rize province project a decreasing trend for the winter season and increasing trend for spring season towards the end of the century. When we come to the Blue Nile basin, the GCM scenarios did not agree on the future climate projections. An increase in precipitation as well as river flow from Rosieres is projected by some of the emission scenarios (MIMR and INCM3) throughout the year except in March and April, which increases outflow in the river. Contrary to that, the other scenarios (BCM2 and CSMK3) estimate a decrease in precipitation and outflow. When we have a closer look into the emission scenarios, the A1B scenarios generally predict increased river flow for the Blue Nile river, while B1 predicts decrease in flow. A low flow, especially in summer (Jul-Sep) mean significant reduction in the heads of reservoirs in Ethiopia, Sudan and Egypt which will have a severe impact on the livelihood of people.
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ÖgeHysterical effects in flow characteristics in the wake region of group of cylinders during the passage of gradually varying unsteady flow(Graduate School, 2022-09-06)In recent studies, it has been witnessed that a group of cylinders offer some distinct advantages compared to mono-pile when applied as a support structure in marine and riverine environments. It is well established that an array of cylinders presents a viable option to a single solid cylinder with lower cost, lower scour and contraction effects. Donghai Bridge offshore wind farm in China is a practical real-world example where this type of structures are of significant importance as an offshore wind turbine support structure. In the pertinent literature, an overwhelming majority of flow-body interaction investigations are studied under steady flow conditions. Furthermore, researchers mainly focus on oscillatory flow when dealing with unsteady flow and using steady state approximation for gradually varying unsteady flows. Yet, several flow conditions observed in nature such as tsunamis, meteorology driven flows, tidal currents and river hydrographs fall into the category of gradually varying unsteady flow. In this thesis, the influence of gradually varying unsteady flow around a group of cylinders (Hexagonal arrays of Circular Cylinders, HACCs) was investigated by flume experiments. The experiments are carried out in a rectangular flume with dimension 30 m in length, 1 m in width and 1.25 in depth. The bed was smooth concrete and the sides were plexiglass walls. A honey-comb pattern was used at the inlet to ensure smooth inlet conditions. The water was recirculated through a monobloc pump and the water depth was kept constant at 30 cm through entire experiments. The flow discharge is controlled via an inverter system. By entering the desired input through the pump's control unit, unsteady and steady flow conditions are repeatedly and reliably produced. Two unsteady flow cases generated with same minimum and maximum velocities, but different durations in order to investigate the effect of unsteadiness degree. The flow velocity started at 0.04 m/s and increased to a peak of 0.23 m/s. One steady case had the duration of 90 seconds, the other had 120 seconds. Same experiments also carried out under steady flow conditions for benchmarking purposes. Velocity and water depth measurements were taken with a frequency of 100 Hz. Two resistance type level meters are used, one located 2.5 meters upstream the other downstream with the same distance. Instantaneous recording of water depth and water surface are carried out. Simultaneously, velocity measurements are collected with the help of an Acoustic Doppler Velocimeter. Velocity measurements are taken at both horizontal and vertical planes. In total, 290 measurement points were utilized, 140 points at horizontal plan and 150 points at vertical plan. The resolution of the measurement grid is increased near the obstacle in order to capture turbulence characteristics. The obstacle, HACC, consisted of 7 circular cylinders with a diameter of 3.4 cm. The circumambient diameter of the HACC was 16 cm and the solid volume fraction was 0.32. The HACC placed 12 m from the inlet to ensure fully developed flow conditions. Two arrangements of HACC, namely staggered and regular arrangement, in order to investigate the effect of obstacle orientation. In brief, three flow cases (steady, unsteady 90 seconds, unsteady 120 cases) and two HACC configurations are used. In total, 6 experimental runs are performed. Each experimental run consisted of 290 velocity measurement points. The analysis of collected data is started with the despiking of the raw data. The spikes in the velocity measurements are removed with a method widely used in the literature. Then the synchronization procedure is carried out. Velocity and water depth measurements are synchronized through their computer time recording. 290 velocity measurements that are recorded at different times are synchronized by the help of water surface slope measurements. Since water depth measurements are kept constant through all experiments, they are used as a benchmarking point. Finally, turbulence decomposition is done, turbulent and mean flow recordings are obtained to carry out post-processing. Experimental findings resulted in several significant conclusions. It is found that velocity deficit between wake and contraction region exhibits a hysterical character between rising and falling stages. The negative water surface slope is observed during the falling stage of the unsteady flow. This adverse pressure gradient causes earlier boundary layer separation and broader wake region. This leads to a stark shear layer rear the obstacle and higher lateral momentum transfer. During the passage of unsteady flow, counter-clockwise hysteresis is observed between depth-averaged velocity and turbulence kinetic energy. This means that for a given velocity, the falling stage produces higher turbulence compared to the rising stage. This effect was observable irrespective of the HACC arrangement. Furthermore, it was also observed that this hysterical relationship becomes more evident further downstream. The arrangement of HACC is also observed to be of importance in terms of hysteresis. It is observed that hysterical loops are less dramatic for the regular HACC. The hysteresis is mitigated by the strong bleed jets that occur in the wake region of regular HACC. It is known that due to the particular geometry of regular HACC, a significant amount of flow can go through the obstacle and create bleed flow which highly influences wake characteristics in terms of stabilizing the wake and controlling mixing process. It was concluded that the amplified bleed jets are the primary reason behind the suppressed hysterical behaviour. Another critical point that is outlined is the effect of unsteadiness on pile behaviour in terms of flow-body interaction. The pile is observed to render the pile to behave in a more streamlined manner. For a given velocity, the unsteady cases produced smaller recirculation compared to the steady case. The recirculation zone is also observed to be larger during the falling stage compared to the rising stage. Yet, this effect also seems to be less pronounced for the regular arrangement of HACC. The strong bleed jets are presumed to weaken this behaviour.