Kesit Daralmasının Soliter Dalgalara Olan Etkisinin Ve Sınır Tabakasının Deneysel Olarak İncelenmesi

Abstract

Doğada çok sık gözlenemeyen ve dolayısıyla da araştırmacıların üzerinde pek fazla inceleme yapma şansı bulamadıkları ve bir çeşit soliter dalga olan N-Dalga'sı bu tez kapsamında deneysel olarak labarotuvarda üretilmiş ve incelenmiştir. Özellikle deniz tabanında gerçekleşen heyelanlar ve depremler nedeniyle oluşan tsunamiler bu çeşit tekil dalgalar oluşturmaktadır. Bu nedenle tekil dalgaların incelenmesi büyük felaketler yaratan tsunami gibi afetleri daha iyi anlamamız açısından önem arz etmektedir. Bilim insanları tsunami gibi tekil dalgaları incelemeye başladıkları ilk yıllarda, bu tip dalgaların tek bir dalga tepesinden oluşan soliter dalga tipinde olduğuna kanaat getirmişlerdir. Bu minvalde soliter dalga üzerine çalışmalara ağırlık verilmiş çeşitli teorik yaklaşımlar irdelenerek sayısal ve fiziksel modeller gerçekleştirilmiştir. Mamafih tsunamilerin önceden öngörülememesi ve pek yaygın olmayan görsel kayıt ekipmanlarından dolayı gerçek bir tsunami dalgasının benzeştirilmesi ile ilgili olarak sağlam temelli ve net çalışmalar ortaya konamamıştır. Dijital teknolojideki hızlı gelişmeler sayesinde uzaktan algılama çalışmaları yaygınlaşmış, fotoğraf ve kamera teknolojileri gelişerek bir çok bireyin sahip olabileceği duruma gelmiştir. Böylelikle ilk modern bilimin başlangıcı sayılan gözlem yeteneğimiz tsunamiler konusunda da gelişmemize fırsatlar sunmuştur. 1990'lı yılların sonlarına doğru tsunami dalgasının sadece bir tepeye sahip olmadığı ve ayrıca bir de çukurun eşlik ettiği bir dalga olduğu anlaşıldı. Şeklinin N harfine benzerliğinden ötürü de N-Dalgası olarak isimlendirildi. Ayrıca N-dalgası için dalga tepesinin dalga çukurundan önce geldiği durum için "Leading Elevation N-wave (LEN)" ve dalga çukurunun dalga tepesinden önce geldiği durum için "Leading Depression N-wave (LDN)" tanımları yapılmıştır. Bu ilk gözlemleri takip eden yıllarda araştırmacılar, çeşitli teoriler ve sayısal modeller geliştirmişler. Fakat gerek gerçek bir tsunamiye ait kayıt olamaması gerekse de labaratuvarlarda bu tip dalgarı üretmeye müsait dalga üreticilerinin bulunmamasından dolayı teorilerilerini ve sayısal modellerinin doğruluğunu onaylamakta sıkıntılar yaşamışlardır. 2004 yılına geldiğimizde 26 Aralık günü Hint Okyanusu'nda meydana gelen ve Sumatra Depremi olarak da adlandırılan deniz altı depremi sırasında büyük bir tsunami meydana gelmiştir. Toplam 14 ülkeden 230000 insanın hayatını kaybettiği bu afet sırasında tesadüfen o bölgede bulunan "Mercator" isimli Belçika bandıralı bir yat bu tsunamiye ait bir su seviyesi değişimi kaydı almayı başardı. Bu tarihten sonrada N-Dalgasına ait teoriler gözden geçirildi, yenilendi ve daha sağlam temellere oturtuldu. Güncel çalışmalara göre artık N-Dalgası biri pozitif diğeri negatif genliğe sahip ve aralarında faz farkı bulunan iki adet soliter dalganın süperpozisyon hali olarak tanımlanmaktadır. Halen emekleme aşamasında bulunan araştırmalarda N-Dalgası fiziksel olarak laboratuvarlarda yeni yeni oluşturulmaya başlanmış olup bu konu ile ilgili çok kısıtlı sayıda çalışma mevcuttur. Sadece İstanbul Teknik Üniversitesi Hidrolik Laboratuvar'ı bünyesinde bulunan ve burada geliştirilen özel bir piston mekanizması sayesinde 23.5m x 1.0m x 0.5m boyutlarındaki dalga kanalında N-dalgası üretilebilmektedir. Bu çalışma kapsamında ise N-dalgasının fiziksel özelliklerini ortaya koymak amacıyla bir çalışma yürütülmüştür. Böylelikle bu konudaki literatürde bulunan boşluk doldurulmaya çalışılmıştır. Bu minvalde iki ana başlık altında çalışmalar yürütülmüştür. İlk olarak N-dalgasına ait ortalama akım hızı, türbülans ve taban kayma gerilmesi gibi akım özelliklerini incelemek maksadıyla taban sınır tabakasında ölçümler yapılmıştır. Daha sonrasında ise haliç, boğaz, koy gibi morfolojik kıyı şekillerinin etkilerini daha iyi anlayabilmek açısından basitleştirilmiş bir kanal daraltma işlemi gerçekleştirilmiştir. Böylelikle N-dalgasının fiziksel özelliklerinin kanal daralması ile nasıl değiştiği ortaya konulmuştur. N-dalgasının, doğası gereği permanan ve periyodik olmamasından ötürü fiziksel karakteristiklerinin ortaya konması amacıyla grup ortalaması (ensemble average) yöntemi benimsenmiştir. Bu yöntem permanan olmayan akımların incelenmesinde kullanılan bir yöntemdir (Sumer v.d., 2010). Bu minvalde N-dalgası 40 defa üretilmiş ve eş zamanlı olarak taban yakınında 35 noktada hız profili ve su yüzü değişimi ölçümleri yürütülmüştür. Bunlara ek olarak yine aynı dalgaya ait serbest yüzey akım hızının belirlenmesi amacıyla farklı derinliklerde yine hız profili ölçümleri gerçekleştirilmiştir. Açık denizde oluşan tsunamiler kıyılara yaklaştıkça kıyı morfolojisinden etkilenmektedirler. Özellikle boğaz, nehir gibi morfolojik yapılarda ilerleyişinin araştırılması bakımından ikincil bir deney düzeneği oluşturulmuştur. Bu düzenekle de kanal sırasıyla 5°, 10° ve 15° tedrici olarak daraltılmıştır. Yine bu durumlarda da 2 noktada hız profili ve su yüzü değişimleri kaydedilmiştir. Toplam 70 adet gerçekleştirilen deneyler daha sonra Matlab programı ile oluşturulan kodlarla analiz edilmiştir. Bu analizler sonucunda hız profilleri, kayma gerilmesi, türbülans özellikleri gibi akım özellikleri ortaya konulmuştur. Bu minvalde grafiklerin yanı sıra akımın zamanla değişimini gösteren videolar da Ekler kısmında CD-ROM olarak sunulmuştur. Ayrıca daralmanın etkisi ve su yüzü değişimleri de bu çalışma kapsamında irdelenmiştir.

N-Waves, which are a type of solitary waves, do not occur in the nature frequently so that the researchers can not examine them easily. Solitary waves, particularly N-Waves, are of particular importance in geosciences due especially to tsunamis generated by sea bottom landslides and earthquakes. Investigation of solitary waves is crucial in understanding natural phenomenon such as non-linear waves in nearshore regions or catastrophic disasters like tsunamis which has devastating effects. In the early years of solitary wave investigations, scientists decided that tsunami waves can be represented by a solitary wave (i.e. with only one crest). This decision led to deeper research on solitary waves, substantial amount of theoretical approaches followed by numerical and physical modelling studies. However due to unpredictability of tsunamis and lack of widespread visual recording devices, clear and well founded studies were not achieved in simulation of a real tsunami wave. Due to the recent dramatic advancement in digital technology, remote sensing techniques became widespread and also video and photo capturing technologies improved so advanced that each individual can have a camera of their own. Therefore, observation, our most basic scientific tool, gave us the ability to improve our knowledge on tsunamis. At the end of 90s, a trough accompanied by a crest was discovered in tsunami waves. The geometrical similarity of this wave and Latin letter "N" ended up with the name N-Wave. Additionally, two new definitions were made for N-Waves; if the crest is followed by a trough, 'Leading Elevation N-wave (LEN)' and if the trough is followed by a crest, 'Leading Depression N-wave (LDN)'. In the following years researchers developed different theories and numerical models, but due to lack of real world tsunami records and lack of tsunami type wave generators in laboratories, they had problems in verification of their theories and models in an accurate extent. In 26th of December 2004, a devastating tsunami formed during Sumatra Earthquake in Indian Ocean. During this disaster more than 230000 people from 14 countries lost their lives and incidentally a Belgium boat called 'Mercator' was able to take a record of water level variation during this tsunami. After this recording N-Wave theories were reconsidered, renewed and better established. According to the up-to-date studies, N-Waves have been considered as the independent superposition of two solitary waves in different phases; one has positive and the other has negative amplitude. This new studies are still in the beginning level with rare publications and N-Waves are being generated in laboratories just recently. In this context, an experimental setup was designed the laboratory flume of Istanbul Technical University Hydraulics laboratory such that a typical N-Wave was generated and investigated experimentally in the frame of this PhD thesis. Hydraulic Laboratory of Istanbul Technical University has a unique N-Wave generator with a special vertically-oriented piston mechanism in a wave flume with dimensions 23.5m x 1.0m x 0.5m. Previously a number of researchers investigated run-up characteristics of this kind of waves in this flume. Also, effects of coastal forests on tsunami run-up and coastal erosion, behaviour of rubble-mound and caisson type breakwaters under tsunami waves were investigated in this flume. In this experimental study, geometrical and kinematical properties of N-Waves were investigated to fill the gap in the literature. Accordingly, studies were conducted under two main chapters. Firstly, boundary layer measurements on the flume bottom were performed for better understanding of N-wave generated near-bed flow properties such as time-averaged velocity, turbulence and bed shear stresses. Afterwards as the second chapter, a simplified flume narrowing section was manufactured for better understanding the effects of coastal morphology (such as estuaries, straits, bays etc.) on the progression of N-waves. Therefore, changes in physical properties of investigated N-wave due to narrowing were presented. Furthermore, water elevation time series and velocity measurements were obtained. Resistant type probes were used for water elevation measurements. For this system two resistant type wave probes, one 8 channel HR Wallingford wave monitor and a four channel National Instruments NI-9215 A/D converter were used. Also one channel of the A/D converter was assigned for synchronization between water level measurements and velocity measurements. In other words, synchronization output of the velocity profiler was connected to one channel of the A/D converter in order to ensure simultaneous data recording. Velocity measurements were performed by using the 3D acoustic velocity profiler (Vectorino) manufactured by Nortek AS. This new generation profiler can measure simultaneously three component of the instantaneous velocity in 35 points over a vertical range of 3.5cm, with intervals of 1mm. Moreover, this profiler can perform with a sampling rate up to the 100Hz. During the measurements, this device sends short sound impulses to the flow and listens with receivers placed on its four arm. Consecutively, it compares transmitted and received frequencies of sound and calculates velocity of the flow by using the Doppler effect theory. This real time and simultaneous velocity data was recorded by using a program provided by the manufacturer of the instrument. This type of velocitymeters are widely used in coastal and hydraulic applications and proved their reliability through the years. For generating controlled conditions, a metal sheet with dimensions of 2m (length) x 1m (width) x 0.004m (thickness) was deployed at bottom of the flume in the measurement section. Also this metal sheet was painted to assure a smooth surface. All velocity measurements were performed over the same point on this metal sheet during this study. Thus, surface roughness (corresponding to hydraulically smooth conditions) was kept constant during the experiments. Furthermore, two measurement points were used at section narrowed experiments. To ensure identical bottom roughness properties in consecutive narrowing configurations, side panels used for section narrowing were slid instead of sliding measurement probes. Since N-Waves are unsteady and non-periodic by their nature, time-averaging techniques cannot be used to calculate the mean flow parameters. Therefore, ensemble averaging method was used during the investigation of its physical properties. In this context the identical N-Wave was generated for 40 times in successive experiments and velocity profiles close to the bottom and water elevation time series were measured simultaneously. Additionally, velocity profiles at different vertical ranges from the flume bottom were measured in order to determine free stream velocity for the same N-Wave. These measurements were analysed numerically by the Matlab software tool and results were presented in this thesis. In the light of the findings, the results of water surface profile were matched with N-wave equation with a high positive correlation. Thus, it can be said that produced wave in the context of this study is a typical N-wave. Afterwards free stream velocity, time-average velocity profiles, turbulent fluctuations and wave Reynolds number values were calculated. In the light of the findings, it was concluded that flow was in transitional state from laminar to turbulent regime at the phase of maximum free stream velocity. Also, flow was dominated by the horizontal velocity component (u) and the vertical velocity component (w) was very small. The results further show that the turbulent intensity and turbulent kinetic energy were also dominated by the horizontal component. Besides, turbulence was produced within the boundary layer close to bed and dissipated towards to upper levels. Moreover, it can be said that there is a net longitudinal mass transport in the flow field beneath N-waves. Also, a phase shift between water elevation time series and mean horizontal velocity was not observed, as expected in the case of solitary wave. Additionally, the relationship between free stream velocity and water elevation time series was investigated. It is found that velocities at rising curve of the N-wave were lower than the falling curve. This result is in contradiction with flood wave (gradually varied flow case), another type of unsteady flow. Furthermore, bottom shear stresses were obtained during the experiments. Total bottom shear stress consists of two parts which are viscous shear stress and turbulent shear stress. It was revealed that turbulent stress component was more dominant than the viscous stress counterpart, when the free stream velocity was at its maximum. Furthermore, fluctuations at the shear stress are important because of formation of vortices and turbulent coherent structures. Hence, fluctuation of the shear stress was also presented in this study. Tsunamis, first generated at offshore, are extensively affected by the coastal morphologies as they approach the shoreline. To investigate N-Waves reaching estuaries and river mouths, the second chapter of the experimental set-up was established. In this set-up, the width of the flume was reduced gradually with 5, 10 and 15 degree angles by means of a 2cm thick water-resistant plywood panels. Velocity profiles and water level variations were also measured for these 3 configrations. These measurements were made at two sections which were at the entrance of narrowed section and middle of the that section. In these measurement a significant phase shift was observed at the tail section of the N-wave. Also wave height and period differences were obtained and presented herein. Additionally, velocity differences were studied between narrowed section and un-narrowed section experiments. In this case, free stream velocities at narrowed section experiments were approximately 50% less than the un-narrowed velocities. This is an expected result for a general wave. Experimental measurements, for 70 experiments in total, were analysed in Matlab environment with codes written especially for this purpose. Flow properties like velocity profiles, shear stresses, turbulence properties were produces as results of these analyses. Graphs and videos of the results were given in Appendices. Since the process is highly unsteady, animation videos presenting the time-varying velocity and turbulence measurements are very indicative in terms of the characteristics of N-Wave boundary layers. Additionally, the effect of narrowing and water level variations were also included in the aforementioned visual material.