Ahmet Ozan Celik

The role of impulse on the initiation of particle movement under turbulent flow conditions.

Fundamental to our understanding of erosional and transport phenomena in earth-surface dynamics and engineering is knowledge of the conditions under which sediment motion will begin when subjected to turbulent flow. The onset criterion currently in use emphasizes the time-averaged boundary shear stress and therefore is incapable of accounting for the fluctuating forces encountered in turbulent flows. We have validated through laboratory experiments and analytical formulation of the problem a criterion based upon the impulse imparted to a sediment grain. We demonstrate that in addition to the magnitude of the instantaneous turbulent forces applied on a sediment grain, the duration of these turbulent forces is also important in determining the sediment grains threshold of motion, and that their product, or impulse, is better suited for specifying such conditions.

Role of instantaneous force magnitude and duration on particle entrainment

[1] A new criterion for the onset of entrainment of coarse sediment grains is presented here. It is hypothesized that not only the magnitude, but also the duration of energetic near bed turbulent events is relevant in predicting grain removal from the bed surface. It is therefore proposed that the product of force and its duration, or impulse, is a more appropriate and universal criterion for identifying conditions for particle dislodgement. This conjecture is investigated utilizing two theoretical models, representative of two modes of entrainment: saltation and rolling. In these models, instantaneous, highly fluctuating turbulent forces are simulated as short‐lived pulses of characteristic magnitude and duration, which transfer adequate fluid momentum to the particle, to trigger its entrainment. The analytical solution of the respective equations of motion is employed in deriving representations of threshold conditions in terms of the impulse characteristics. It is shown that hydrodynamic forces of sufficiently high magnitude are capable of entraining a particle only when they last long enough so that their impulse exceeds a critical value. To illustrate further the validity of the critical impulse concept, as well as extend and generalize its application to different entrainment levels of an individual grain, a novel experimental setup is utilized. This setup facilitates observations of angular displacement of a steel mobile particle in air due to electromagnetic pulses of different magnitude and duration. The experimentally obtained conditions for partial or complete entrainment support the concept of a critical impulse.

Impulse and particle dislodgement under turbulent flow conditions

In this study, we investigated the role of turbulence fluctuations on the entrainment of a fully exposed grain near threshold flow conditions. Experiments were carried out to measure synchronously the near bed flow velocity and the particle movement for a range of flow conditions and resulting particle entrainment frequencies. We used a simplified bed geometry consisted of spherical particles to reduce the complexities associated with the variations in the bed and flow details in an effort to identify the underlying dominant physical mechanism. An analysis was performed based on common force approximations using near bed flow velocity. Turbulence fluctuations were treated as impulses, which are products of magnitude and duration of applied force. It is demonstrated that besides the magnitude of the instantaneous forces applied on a sediment grain, their duration is important as well in determining whether a particle will be entrained by a turbulent flow event. Frequency of particle entrainment varied rema...

Nonintrusive Method for Detecting Particle Movement Characteristics near Threshold Flow Conditions

Bed-load measurements comprise an important component in the development of reliable formulas, in an effort to obtain the necessary constitutive relations between the amount of transported material and flow parameters. The uncertainty of such measurements is rather well known, being much more pronounced at lower transport rates. This uncertainty stems from the multitude of factors affecting bed-load transport and the lack of available trustworthy measuring technologies. Predictions of the limiting case of nearly zero bed-load transport, typically reported in literature as threshold of motion or critical condition, are even more challenging. The purpose of this contribution is twofold. First, to examine the sensitivity of bed-load transport measurements at conditions moderately higher than critical, to the presence of a rather unobtrusive trap, designed through several iterations. Even under relatively simple laboratory flume channel and flow conditions, it proved difficult to measure the bed-load transport rate in a completely unbiased way. Second, to develop a methodology, together with the appropriate instrumentation, for determining the condition of incipient motion. The nonintrusive approach described here proved to be reliable in detecting even the slightest movements of a particle. At the same time, it demonstrates the complexity of the problem due to the highly fluctuating nature of turbulent flow.

Distributions of Compressive Strength Obtained from Various Diameter Cores

Test results from cores with diameters smaller than the standard sizes and with length-to-diameter (l/d) ratios less than 1 are considered to be unreliable due to higher variability in concrete strength. There is, however, still a lack of evidence in the literature regarding the unreliability of the compressive strength data for these lower-volume cores. Significantly important is the missing verification of such statements concerning the random variations of concrete strength, supported with statistical approaches. Statistically significant data sets under such conditions are very rare. In this study, the issue was addressed by exploring the features of the compressive strength obtained from different diameter cores using probabilistic concepts—in particular, when the l/d is less than 1— and shed light on the uncertainty regarding the reliability of the data taken under these conditions. The major outcome of this study is that concrete cores with diameters less than the standard-size cores and larger than microcores could be effectively used in practice to represent the variations in concrete strength, provided that the number of core samples and l/d are sufficiently high.

Reassessment of the flood damage at a river diversion hydropower plant site: lessons learned from a case study

Despite the accumulated vast theoretical and practical knowledge on the flood-related topics, the lack of precautions taken against the natural disasters keeps causing fatality and property damage. In this study, a recent flood event in a run-of-river-type hydroelectric power plant in Tavsanli District of Kutahya was analyzed and the main reasons for the occurred damage were investigated. First, a stochastic approach was followed to obtain the essential parameters used in the design procedures. In support of the whole investigation, numerical simulation of the river flow was performed to analyze the river flow behavior using computational fluid dynamics method. In addition, geographical information system was implemented to understand the site-specific topography and its influence. Different types of analyses and fieldwork were combined and used to help reveal the actual scenario and showed that a non-catastrophic natural event resulted in a costly damage due to a chain of wrong actions (misinterpretation of the functions of certain safety structures and lack of taking precautions at the hill slopes) in the local implementation of the project. The damage initially was believed to be due to the rise in the water level in the nearby river. It is believed the picture presented here will be useful in amending arrangements to avoid such damages in the future for a large number of comparable power plants in Turkey.

Investigation of wind loads on the Osmangazi Bridge with numerical modeling

Rüzgâr yükleri, çeşitli yapıların tasarımında önemli dinamik yüklerdir. Özellikle, ulaştırma, yapı ve akışkanlar mekaniği gibi farklı disiplinlerin bir arada değerlendirilmesini gerektirdiği için özel bir mühendislik problemi olarak gösterilebilir. Gelişen teknikler ile birlikte, artan ihtiyaçlara cevap vermek adına köprü ve köprüyol tasarımları daha geniş ve daha yüksek açıklıkları aşmak için sınırları zorlamaktadır. Bu özelliklerdeki yapıların aerodinamik açıdan değerlendirilebilmesi için hazırlanmış yönetmelikler ve rüzgâr tüneli testleri kullanılmaktadır. Ancak, söz konusu yöntemlerin içerdiği kısıtlamalar ve dezavantajlar, sayısal model çalışmalarını daha önemli bir hale getirmiştir. Geliştirilen sonlu elemanlar uygulamaları, başarılı sınır koşulu ve türbülans modelleri ile birlikte yüksek performanslı bilgisayarlar kullanılarak her türlü geometrik yapı için yüksek hassasiyette hesaplamalı akışkanlar dinamiği analizleri yapabilmek mümkün hale gelmiştir. Bu çalışmada, Osmangazi Köprüsü tabliyesinin, farklı rüzgar hızları ve farklı türbülans modelleri için aerodinamik analizi sayısal olarak gerçekleştirilmiştir. Köprüye etki eden rüzgâr yükleri, aerodinamik kuvvetler olarak değerlendirilmiştir. Sonuç olarak, köprü tabliyesinin geometrik olarak akıma uyumlu bir yapıda tasarlandığı ve bu nedenle oluşan rüzgâr direncinin oldukça düşük olduğu tespit edilmiştir. Wind loads are crucial dynamic load for various structures. It is a unique engineering problem particularly because it requires the combined expertise of transportation, structure and fluid mechanics engineers. With the advancing techniques and technologies, also to answer the increasing demand, the design of bridges and bridge-ways tends to move towards wider structures with longer spans. Typically, standards and widn tunnel tests are used to evaluate the design of structures with such characteristics aerodynamically. However, the limitations and disadvantages of these methods make numerical modeling efforts more important. The developed finite element methods, applications of successful boundary conditions and tubulence models together with high performance computing power makes is possible to analyze structures with various geometries with computational fluid dynamics method. In this study, anerodynamic analysis of The Osmangazi Bridge deck slab under a number of wind speeds and turbulence models were performed numerically. The wind loads acting on the bridge were evaluated as the aerodynamic loads. The results show that the deck slab cross section is well streamlined and the wind resistance of the structure is quite low.

Kendiliğinden Yerleşen Beton Davranışının Hesaplamalı Akışkanlar Dinamiği İle Benzetimi

Temel olarak Newtonyen olmayan akiskan akisi Bingham tipi model ile aciklanmaktadir. Bu yontemle Deeb vd., lifsiz KYB akisi sirasinda Lagrange yaklasimi temeline dayanan, sayisal ag gerektirmeyen bir numerik yontem olan yumusatilmis parcacik hidrodinamigi (Smoothed Particle Hydrodynamics) metodunu kullanarak uc boyutlu benzetim agregalarin (8 mm ve daha buyuk) dagilimi uzerine odaklanmislardir [10]

Dere ağzı hidrodinamiğinin 3 boyutlu modellenmesi: Karışım ve tabakalı akım durumları

Dere islahi calismalarinin tasarim sureclerinde, dere agzi yakinlarinda ve haliclerde geleneksel metotlarin karakterize edemedigi ve etkin sonuc vermedigi durumlar yasanabilir. Bunun sebebi olarak tek boyutlu ampirik denklemlerin tabakali akimda veya karisim bolgesinde olayin fizigini yansitamamasi gosterilebilir. Bir dere agzi cikisi yakinlarinda tabakali bir akim durumu oldugu varsayildiginda, akim ust bolgesinde tabakalanan tatli su kati cidarla temas halinde olmayacak ve boylelikle taban puruzlulugu, kanal boyutlandirilmasi gibi hesaplamalarda etkinligini yitirecektir. Bu durumda boyutlandirma icin (kullanilan ama gecerliligi olmayan “n” yardimiyla) Manning denkleminden elde edilecek olan hidrolik yaricap gercekci olmayacaktir. Etkin sonuclar alinamadigi durumlarda ise gereginden daha buyuk boyutlandirmalar ya da yetersiz kalabilecek kesitler ortaya cikabilmektedir. Bu durum taskinlara ve dolayisiyla geri donusu olmayan mal ve can kayiplara yol acabilmektedir. Riva Deresi’nde meydana gelen taskinlar bu duruma verilebilecek bir ornektir. Bu calismada soz konusu dere agzindaki tatli-tuzlu su karisim bolgeleri sayisal olarak modellenmis ve bu modelde gercege yakin kesit ve debiler kullanilmistir. 3 farkli senaryoda karsilastirmali degerlendirme yapilarak kesit tasariminda problem yaratan durumlar belirlenmistir. Uygulanan Hesaplamali Akiskanlar Dinamigi (HAD) modelinin sonuclari ortaya konularak modelin yeterlilikleri ve gelistirilmesine yonelik yorumlar da degerlendirilmistir.

Examination of changes in the Turkish hydropower development, regulation, and planning

Hydropower has a critical role in supplying the energy demand and sustaining the economic and industrial growth in Turkey. Over the past decade, several policy changes have been made to improve the contribution of hydropower in overall national energy production. Turkey is utilizing less than half of the hydropower potential and planning to employ almost full of its potential within the next 10 years. For this plan to be realized there is a need to analyze the strengths and weaknesses of the past and recent regulations. Implications from such an analysis are believed to be necessary for a better national energy strategy. This work fills this gap and presents analysis of data representing utilized and unutilized hydropower potential obtained from the official sources. The analysis coupled with the recent policy changes was discussed within the context of sustainable hydropower structure. The unutilized hydropower potential plays a crucial role in national energy security. It is also evident that hydropower alone will not be meeting the rapidly increasing demand. So far, however, researchers have shown an increased interest in unutilized hydropower potential; there has been no detailed analysis about identifying characteristics of this potential and developing strategies in terms of required technological investments. The efforts toward realizing the long-term national energy security goals need to consider establishing research development and manufacturing capabilities of domestic hydropower equipment which will fit the need to reduce foreign dependence on hydropower energy technology.