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Katı yakıtlı roket motorlarında daimi olmayan akışların ikili zaman adımlaması yöntemi ile sayısal benzetimi
Get PDFA computer code, implementing a novel Dual Time Stepping (DTS) algorithm, is developed in order to simulate two-dimensional unsteady cold flow in Solid Rocket Motors (SRM) involving variable internal geometry. Simulation of unsteady flow at all-speeds is essential, since flow speed of combustion gases through an SRM ranges from incompressible limit to supersonic speeds and internal geometry of the SRM varies in time due to regression of combusting propellant surface. In this respect, present code solves compressible form of time dependent conservation laws, written in Arbitrary Lagrangian-Eulerian (ALE) form, on deforming grids at low Mach numbers (M 1) as well as at supersonic speeds. The present code exploits low Mach number preconditioning technique to modify compressible form of Navier-Stokes equations for simulation of all-speed flows. Time dependent Navier-Stokes equations are written in ALE form in order to solve flow problems involving moving boundaries. Time dependent preconditioned Navier-Stokes equations in ALE form is solved by means of a novel implicit DTS algorithm. The use of novel DTS algorithm is advocated due to following issues: First; the multiplication of preconditioning matrix by spatial derivatives only is sufficient to equalize the convective eigenvalues. Second; because the source terms in the unsteady residual do not affect the range of wave speeds, pseudo time steps are not changed and they are calculated using the matrix, which is obtained from the multiplication of preconditioning matrix by convective flux Jacobian. Present DTS algorithm has a simple form, since it has only two matrix multiplications and no matrix inversion at each R-K stage. The use of dual time stepping lets the physical time step be selected based on the numerical accuracy criterion only. Geometric Conservation Law (GCL) compliant ALE time-stepping scheme, which retains on deforming grids the nonlinear stability and the second order time-accuracy achieved on stationary grids, is used. GCL is solved concurrently with the time dependent equations. A perturbation method based on algebraic Transfinite Interpolation (TFI) is used for rapid generation of deforming grids at each time step. Cell-centered finite volume discretization technique is adopted. Convective terms are evaluated using central differencing scheme. The flux vectors at the midpoint of a cell face are computed by arithmetic averaging of flow variables at two neighboring cells. Scalar artificial dissipation model is adopted in order to prevent odd-even decoupling of the solution. Artificial dissipation terms are modified to let them scale correctly at low Mach numbers. The variables, which are required for the computation of viscous terms, are also averaged at a cell face. Gradients at the midpoints of a cell face are computed by means of Green's theorem. Local time stepping, implicit residual smoothing and multigrid are implemented for convergence acceleration. Injection-induced inviscid steady flow in an SRM is simulated first. Numerical results indicate that pressure slightly decreases in the outflow direction in the propellant section and remains almost constant in the solid wall section of SRM. The flow is supersonic at the downstream of the nozzle throat. Computed results agree with numerical results available in literature. Efficiency of the novel DTS algorithm is assessed for the simulation of unsteady laminar flows inside SRM with and without propellant surface moving due to combustion. Steady state Euler solution computed previously is used as an initial condition to unsteady simulations. First, unsteady laminar flow inside SRM is computed on stationary grid neglecting the propellant regression. Computed head-end pressure oscillations and vortex shedding formation agree with other numerical solutions available in literature. Next, laminar flow inside the SRM involving propellant regression is simulated on deforming grids. Computed results are not validated for propellant regression case, since there are no experimental data or numerical results available in the open literature. However, all the physics (pressure oscillations and vortex formation) related to SRM simulation are computed well and propellant regression is modeled accurately. Numerical results indicate that present code, exploiting the latest and efficient methods like ALE, GCL, preconditioning, multigrid and a novel DTS algorithm, successfully simulates cold flow in SRM like configurations on stationary as well as deforming grids. Novel DTS algorithm has no detrimental effect on the solution accuracy. Keywords: Solid rocket motor, dual time stepping, arbitrary Lagrangian-Eulerian method, preconditioning, deforming grid, multigrid.Değişken iç geometriye sahip katı yakıtlı roket motorlarında iki boyutlu daimi olmayan akışların benzetimi yeni bir kapalı ikili zaman adımlama algoritması kullanılarak gerçekleştirildi. Zamana bağlı denklemler, keyfi Lagrange-Euler sistemi için yazılarak hücre merkezli sonlu hacimler yöntemi ile ayrıklaştırıldı ve yapılı ağlar üzerinde hareketli sınırlar ile çözüldü. Sıkıştırılamaz limitte yakınsama hızını etkileyen akustik ve taşınım hızları arasındaki fark önşartlandırma tekniği ile ortadan kaldırıldı. Yapay sönümleme terimleri düşük Mach sayılarında doğru olarak hesaplanacak şekilde ayarlandı. Önerilen kapalı ikili zaman adımlama algoritmasını kullanan keyfi Lagrange-Euler zaman adımlaması, geometrik korunum yasasını sağlamakta ve hareketsiz ağlarda elde edilen zaman doğruluğunu korumaktadır. Kullanılan keyfi Lagrange-Euler zaman adımlaması, daha az matris çarpımı içermesi ve matris tersinin hesaplanmasını gerektirmediğinden literatürde mevcut olanlara göre daha basittir. Her bir zaman adımında yeni ağ, cebirsel karşısonlu interpolasyon yöntemine dayanan rahatsızlık yöntemi kullanılarak üretildi. Lokal zaman adımlaması, artık düzeltici ve çokluağ yöntemleri kullanılarak yakınsama hızlandırıldı. Katı yakıtlı roket motoru benzeri konfigürasyonda daimi ve daimi olmayan akışların benzetimi hareketsiz ve hareketli ağlar kullanılarak başarı ile yapıldı. Elde edilen hız profilleri, basınç dalgalanmaları ve girdap kopmaları literatürdeki deneysel, analitik ve sayısal sonuçlar ile uyumludur. Hesaplanan sonuçlar, önerilen kapalı zaman algoritması ile geliştirilen akış çözücünün değişken iç geometriye sahip olan katı yakıtlı roket motorlarında iki boyutlu daimi olmayan akışların çözümü ve analizinde kullanılabileceğini göstermektedir. Anahtar Kelimeler: Katı yakıtlı roket motoru, ikili zaman adımlaması, keyfi Lagrange-Euler yöntemi, önşartlandırma, şekil değiştiren ağ, çokluağ.  
Makaralı bükme işleminde ince sacların kenar dalgalanma kusurunun deneysel incelenmesi
Get PDFCold roll forming (CRF) is the process of forming the sheet metal by passing it through a series of rotating rolls arranged in tandem that bend and shape the sheet metal. CRF is gaining more significance between the other metal forming in today's manufacturing environment. The producers request tighter tolerances, improved surface quality and increased variety of cross sections that require the parameters affecting the process to be revisited in order to understand the effects and interactions among them. The design of the process is still based on empirically gained data and practical experience of employees. Systematic experimental researches have been done with limited forms because of high costs. The CAD tools in this field require to be improved in order to simulate and analyze the process in a better way. In this regard the latest design software packages are being sold with finite element analysis interfaces, as the design software packages are limited with the empirical methods & analogy with the previous similar shapes. On the producers' side it is very important to deliver the CRF machine as early as possible once an order is received. However, only after production, the CRF machines can be tuned to prevent any undesired shape defects that require additional time to fix some of which may need a rework on the machine. On the customer side the time loss is the biggest risk as it is a mass production machine and the tuning and design change costs which are always accompanied with time and profit loss. Due to complex deformations unlike bending or deep drawing, CRF requires careful modeling of the process. The most frequent defect observed during CRF process of symmetrical u-shape profiles is the edge waviness. The web-buckling, twist, bend in the horizontal and vertical plane are the other defects, which can be observed during CRF. The introduced empirical equations i.e. forming angle method, forming rate method, Bhattacharyya method, Ona & Jimma method are the ones used by CRF machine & flower pattern designers. However these equations generally offer roll set numbers or flower patterns within a large safety margin. Therefore, for complex shapes the designer?s experience is still the most reliable tool for deciding the design parameters. In this study Taguchi method as used in order to investigate edge waviness defect during CRF process of thin sheet metal applying full factorial L8 design method. During the experimental study the effects of the parameters of roll setup (the angle difference at the roll sets 40 and 55 degrees), flange length (12 mm or 17 mm) and material mechanical properties (mild steel or aluminum) on edge waviness and interrelations between them were investigated. The study shows that the flange length has the biggest effect on the edge waviness formation. The roll setup and material type has equal effects and are observed to have equal magnitudes after flange length. In addition to these effects, the interactions between the flange length, material type and roll setup are observed and detailed in this study. The interaction between roll setup and edge length shows that as the angle increment between two roll sets decrease, the edge waviness decreases for both of the edge length values. In this regard if there is a need to work with changing edge length, the angle increments between the roll sets should be reduced. This interaction result is especially important for the state of the art CRF machine design to manufacture parts with variable cross section. The second interaction between edge length and material type shows that as the edge length decreases the surface waviness decreases for both of the material types of aluminum and mild steel. The importance of this interaction is important in the classical CRF technology to control the material non-homogeneity. In other words using a larger edge length the CRF machine becomes sensitive to material mechanical properties changes. Another defect observed during the experimental study was the (longitudinal) bow. Due to the large angle difference between the last two roll stands a weak correlation is observed between the edge waviness and the bow formation in this study. The experimental results, within the limits of this study, can be summarized as follows: 1) The edge length is the most important parameter to control the edge waviness. 2) The mechanical properties of the materials are equally important with the roll angle increment. 3) There is an interaction between roll angle increment and edge length. 4) There is another interaction between edge length and material properties. Shorter edge length controls the material mechanical properties non-homogeneity. Keywords: Cold roll forming, ondulation, edge waviness, Taguchi method. Makaralı bükme işlemi arka arkaya yerleştirilmiş makara setlerinin hareket ettirilmesi ile sac malzemenin makaraların arasından geçerken kenarının bükülerek istenilen profilin elde edilmesi olarak tarif edilebilir. Çok çeşitli kesitler bu yöntemle imal edilmekte ve sanayi üretiminde makaralı bükme işleminin payı hızla artmaktadır. Yaklaşık yüzyıllık bir işlem olmasına rağmen, tezgah ve makara tasarımı, günümüz teknolojisinde halen deneysel olarak elde edilen bilgilere ve çalışanların pratik deneyimlerine bağlıdır. Yüksek maliyet ve harcamalardan ötürü sistematik olarak deneysel çalışmalar çok az yapılabilmektedir. Makaralı bükme tezgahı tasarımı istenilen profilin en az makara seti kullanılarak elde edilmesi hedefi ile yapılmaktadır. Makaralı bükme işleminde görülen kusurlar içinde kenar yüzeyde dalgalanma en sık görüleni olmakla birlikte, sac yüzeyinde/taban alanında cep oluşumu, dikey ve yatay yönde eğilme, burkulma ve burulma sayılabilir. Bu çalışmada ince sacların makaralı bükme işleminde kenar dalgalanma kusuru deneysel olarak incelenmiştir. Kenar dalgalanmasının davranışının daha iyi anlaşılması ve bu kusurun kontrol altında tutulması makara setleri ve tezgah tasarımının daha düşük maliyetle ve daha hızlı yapılabilmesi açısından önem taşımaktadır. Deneysel çalışmada Taguçi yaklaşımı ile üç değişkenli ve iki düzeyli L8 tam eşleştirmeli deney yöntemi benimsenmiş ve elde edilen sonuçlar aynı yöntemle incelenmiştir. Sac malzemenin kenar yüksekliği değişimi en büyük etki olarak gözlenmiş, ayrıca yüzey kalitesine etkisi olan kenar yüksekliği ile malzeme cinsi ve makara dizilişi ile malzeme cinsi arasında etkileşimler tespit edilmiştir. Anahtar Kelimeler: Makaralı bükme işlemi, makaralı bükme (roll form), ondülasyon, kenar yüzeylerde dalgalanma, Taguçi yöntemi. 
Measuring long-term and constant rate ground motion by satellite radar interferometry: Avcılar case
Get PDFRadar interferometrisi (InSAR), birbirine çok yakın görüntüleme geometrileriyle elde edilmiş iki farklı radar görüntüsü arasında oluşan faz farkını hesaplayan bir tekniktir. Faz farkı, uydu platformunda bulunan radar ile yeryüzünde görüntülenen nokta arasındaki uzaklığa karşılık gelmektedir. Günümüzde InSAR, yüzey deformasyonunun ölçülmesinde etkili bir teknik olarak kabul edilmektedir. Diğer yandan, tekniğin uygulanabilirliği radar sinyalinin korelasyonu ve atmosferik etkilerle sınırlıdır. Bu çalışmada, uzun zamanda oluşan yüzey deformasyonunu ölçmek için, ERS-1 ve ERS-2 uydularından elde edilen radar görüntüleri kullanılarak bir strateji geliştirilmiştir. Yöntem, ham SAR verilerinin seçimi, SAR veri-işleme ve post-veri-işleme adımlarını içermektedir. Çalışmada, Avrupa Uzay Ajansı’nın (ESA) 1992-2002 yılları arasında ERS uydularıyla elde ettiği radar görüntüleri kullanılmıştır. Radar görüntü çiftleri, çeşitli veri-seçimi kriterleriyle belirlenmiştir. Radar görüntülerindeki faz ölçmelerinin yoğunluğu, “permanent saçıcılar” (PS) tekniği kullanılarak azaltılmıştır. 1992 ve 1999 yılları arasındaki maksimum yerdeğiştirme hızı, Avcılar’ın batısında bir noktada -7 mm/yıl olarak belirlenmiştir. Birinci yazarın diğer çalışmalarında detaylı olarak belirtildiği üzere; Avcılar’da uzun zamandır sabit hızda oluşan yer hareketleri, bölgede var olduğu bilinen toprak kaymalarıyla ilişkilidir. Yerdeğiştime hızlarının dağılımı, yağışlı mevsimde süreksiz olarak oluşan toprak kaymalarının, kayma ve oturma şeklinde bütün sene boyunca sürdüğünü göstermektedir. Anahtar Kelimeler: Uydu radar interferometrisi, yüzey deformasyonu, jeodezi, uzaktan algılama. Radar interferometry (InSAR) is a technique that calculates the phase difference between two radar images acquired by slightly different viewing geometries (Massonnet and Feigl, 1998; Hanssen, 2001; Madsen and Zebker, 1998; Burgmann et al., 2000; Rosen et al., 2000; Gens and van Genderen, 1996). The resultant interference pattern is called an “interferogram”. If certain conditions are met, the phase differences form spatially coherent fringes. Each fringe represents a difference of one cycle of the two-way travel time between the radar aboard the satellite and the target on the ground. Interpreted in units of distance, each fringe represents half the radar wavelength in change of distance, or 28 mm for C-band radars aboard ERS and ENVISAT satellites. While conventional InSAR has been proven to be a very effective technique to measure ground motion, its applicability is limited mainly by radar signal correlation and atmospheric effects. Therefore, to measure slow ground motion occurring over long times requires a challenging effort if conventional InSAR techniques are to be used. If one of the scatterers contributing to the pixel is much more stable than the others, then its contribution to the phase and amplitude measurements will dominate those from the other scatterers. If we can identify these pixels represented by a single strong scattarer, then we can simplify the process of interpreting the interferograms. These persistent scatterers (hence called “PS pixels”) are more reliable because they are less susceptible to all processes causing decorrelation. Accordingly, they have less noise then other pixels. Once the data set has been reduced in size by several orders of magnitude by selecting only the reliable PS pixels in the interferograms, the subsequent interpretation and analysis involving unwrapping and modeling are considerably simpler. Several approaches have been proposed for identifying PS pixels since 2000 (e.g. Hanssen and Usai, 1997; Ferretti et al., 2000, 2001; Colesanti et al., 2003a, 2003b; Lyons and Sandwell, 2003; Hooper et al., 2004; Hooper, 2006). In this study, we establish a radar data selection and processing flow for measuring slow ground motion occuring over long times. We combine different methodologies in raw radar data processing and post-processing. All the steps presented in the study are repeatable and applicable to ERS SAR data for measuring slow motion deformation. We use the entire radar data set of ERS-1 and ERS-2 satellites for the data frame titled as Track 336 and Frame 2783 (T336/F2783). This frame covers all Istanbul metropolitan area and contains 48 epochs between 1992 and 2000. For the deformation analysis, we choose a data frame of 4x5 km2 that covers the Avcılar vicinity of Istanbul. We process the raw SAR data using DIAPASON software (Differential Interferometric Automated Process Applied to Survey of Nature) (version 4.1) developed at the French Space Agency (Massonnet et al., 1994; CNES, 1998; CNES, 2003a). The PS pixels persistent to phase decorrelation are selected through their “amplitude dispersion index” DA, as defined by Ferretti et al. (2000, 2001) and multi-coherence γ, as described by Colesanti (2003). We use “General Inversion for Phase Technique” (GIPhT) developed by Feigl and Thurber (2009) for modeling interferograms. GIPhT defines a misfit cost C between the model and the phase observations. By minimizing the cost C, this approach can solve simultaneously for both linear and non-linear parameters. Analysis of 14 image epochs between 1992 and 1999 has revealed downward ground motion around the Avcılar district of Istanbul. We have analyzed a set of 24 interferometric pairs with PS coverage of 7 PS/km2. The PS coverage corresponds to 1% of the image pixels. Three subsidence sources with a constant subsiding rate have been modeled by using a simple 4-parameter Mogi source (easting, northing, volume, depth). Also, a tropospheric parameter (vertical gradient) for each epoch has been included to the parameter set. The final model fits the phase data significantly better than the prior model based on F-test with 95 % confidence. We find a maximum displacement rate of -7 mm/yr at a point located at latitude 40.98ºN and longitude 28.71ºE. Akarvardar et al. (2007) and Akarvardar (2007) suggest that most of the downward ground motion occurs in the landslide areas. It seems likely that the soils once displaced after landslides have continued to slide and settle down with a constant rate between 1992 and 1999. Keywords: Satellite radar interferometry, ground motion, geodesy, remote sensing
The accuracy assessment of InSAR measurements and reducing data points
Get PDFYapay Açıklıklı Radar İnterferometrisi (InSAR), yeryüzü topografyasının ve deformasyonun belirlenmesi için yeni bir jeodezik tekniktir ve yersel ölçümlere ihtiyaç duymaz. InSAR ölçü sadece uydu bakış doğrultusunda bilgi verir. Bu ölçüler yörünge, topografya veya atmosfer kaynaklı hataları içerisinde barındırabilir. Bu ölçülerin doğruluğu İzmit Depremi örnek verisi üzerinde test edilmiştir. İzmit depremiyle ilgili olarak bu bölgedeki GPS noktalarındaki ölçümlerle aynı noktalardaki InSAR ölçüleri karşılaştırılmış, aradaki fark yaklaşık ±4 cm bulunmuştur. InSAR ölçülerinin çözülmesi sırasında başlangıç noktası kaynaklı hatalardan kurtulabilmek amacıyla aynı noktalar arasındaki göreli farklar incelenmiştir. Bu durumda farkların ortalaması ±2 cm’dir. Aradaki farkın nedeninin hem GPS hem de InSAR’ın kendi içerisinde barındırdığı hata kaynakları hem de GPS ölçülerinin deprem sonrası hareketleri içermemesine rağmen, InSAR ölçülerinin elde edildiği deprem sonrasına ait görüntünün depremden 1 ay sonrasına ait olması nedeniyle deprem sonrası hızlı hareketleri içermesi olduğu değerlendirilmiştir. InSAR ölçüleri çok yoğun olması nedeniyle, hesaplama yükü çok ağırdır. Ayrıca bu ölçülerin GPS ve nivelman gibi başka jeodezik yöntemlere ait ölçü gruplarıyla kullanılmaları durumunda çok baskın olmaktadırlar. Bu nedenle seyrekleştirilmelerine ihtiyaç vardır. InSAR ölçülerinin seyrekleştirilmesi üzerine yeni bir algoritma geliştirilmiş, bu algoritmanın sık kullanılan quadtree algoritması ile karşılaştırması yapılmıştır. Yeni algoritmanın quadtree algoritmasına göre InSAR veri kümesini 2/3 oranında daha az noktayla daha doğru bir şekilde temsil ettiği hesaplanmıştır. Bu seyrekleştirme algoritmasının yapılacak çalışmalarda kullanılabileceği değerlendirilmiştir. Anahtar Kelimeler: GPS, InSAR, doğruluk, nokta seyrekleştirme. Interferometric Synthetic Aperture Radar (InSAR) is a new geodetic technique for determining earth topography and deformation and it does not need any land survey. InSAR uses Synthetic Aperture Radar (SAR ) images which are in the region of microwave of electromagnetic spectrum It is a method in which phase component of SAR images are differenced and effects other than the deformation are removed to determine deformation. These phase difference images are called interferograms. In interferograms every 2π cycle of phase corresponds to one wavelength deformation in line of sight (LOS). To determine absolute deformation these cycles have to be unwrap namely added over each other. InSAR measurements can include many types of errors. These errors can be caused by orbit, digital elevation model or atmosphere. Not many studies has been interested on the accuracy of these measurements. In this study the accuracy of InSAR measurements are tested for İzmit Earthquake by GPS measurements Firstly, The interferogram was constructed by ERS-1 SAR images. Because of the inadequacy of the phase unwrapping algorithm, every cycle of the interferogram was digitized. The measurements at GPS points were compared with the InSAR measurements at the same points for İzmit Earthquake. 11 GPS points lay over the interferogram. The comparison was made for north and south of North Anatolian Fault (NAF) separately, 8 of the GPS points stay north of the NAF, 3 of them south. For the aim of comparison, the deformation of GPS in 3 dimensions was projected on LOS of InSAR measurements. For the North of the NAF, the average of the differences is 38 mm and all the differences are in the same direction. The reason for this difference is thought to be the postseismic deformation which was included by InSAR measurements but not by GPS measurements. For South of the NAF the average of the difference is 94 mm and al the differences are in the same direction. The main reason for this differences is the beginning point that is chosen to unwrap phase. It can be seen that the GPS point which is near the beginning point but out of the InSAR measurements coverage area has a 91mm displacement in LOS. This means that the beginning point has a 91 mm shift. In this case the difference for the South of the NAF is only 3 mm. Another test was held to check the accuracy. It was taking in to account the double differences between GPS points. By this namely, differencing displacement between two GPS points and checking InSAR measurements at those point, we can get rid of unwrapping beginning point error. The average of differences is 20 mm and Standard deviation is ±7mm. InSAR measurements are highly dense compared with other geodetic observations. For calculation convenience, this huge number of data points is reduced by different approaches. One of the most used is quadtree algorithm. Quadtree algorithm, divides the data in quadrants and checks the rms value for each quadrant. If the rms value of a quadrant bigger than a threshold value determined at the beginning, algorithm goes on dividing that quadrant to another quadrants. If the rms value is smaller than the threshold value, the algorithm stops for that quadrant. This algorithm generally gives good results but it does not take in to account the natural patterns that exist in data. In this study a new algorithm is investigated which takes into account the patterns in a data set and can represent the data by smaller number than quadtree. In this new developed algorithm contour lines are created for the data with the selected interval taking into account the slope of the data. Then, these contour lines are simplified by the commonly used Douglas-Poiker algorithm. And all the vertices of the simplified contour lines are converted to data points. And for other purposes such as weighting all of these data points are associated with the points in the original dataset that each of them represents. To test the results this new algoritm was compared with quadtree algorithm for İzmit earthquake interferogram. The data set was sampled by quadtree and newly developed algorithm. And by these sampled points, the original data set was tried to be constructed again. Differences are taken with the reconstructed dataset and original dataset for two algorithms. It was observed that the new algorithm represents the original data more accurately with 2/3 less points if compared with the quadtree. Keywords: GPS, InSAR, accuracy, data reduction
Exact solution of circular vertical curves
Get PDFGeçki düşey geometrisinde, iki doğru parçasını birleştirmek için daire yayı veya 2. derece parabolü kullanılmaktadır. Uygulamada, dairesel düşey kurblara ilişkin kırmızı kot ve kilometre hesaplarında kolaylık amacı ile bazı kabuller yapılarak yaklaşık çözümler uygulanmaktadır. Bir ulaştırma yapısının uygulama projesi, yapının tüm niteliklerini kapsar. Bu niteliklerin en önemlilerinden biri olan “geçki düşey geometrisi”, ulaştırma yapısının gerçek (hatasız) düşey geometrisini temsil eder ve sayısal olarak kilometreler, kırmızı kotlar ve kırmızı çizgi eğimleri ile ifade edilir. Söz konusu sayısal büyüklükleri hata dereceleri bakımından üç ana gruba ayırmak mümkündür: Birinci gruptaki büyüklükler, hesap kolaylığı bakımından yuvarlak sayı seçilirler. İkinci gruptakilerin sayısal inceliği boy kesit çiziminin ölçeğine bağlıdır. İkinci gruptaki büyüklüklerin hataları küçüktür. Üçüncü gruptaki büyüklükler ise, bir hesap işlemi sonunda üretildiklerinden hataları diğer gruptakilerden daha büyüktür. Günümüzde demiryolları ve yüksek standartlı kara yolları için, hesapla bulunan kilometrelerin ve proje kotlarının varsayımlardan kaynaklanan hatalardan arındırılmış mm inceliğinde değerler kabul edilebilir hata sınırları içinde hesaplanmalıdır. Günümüzde bilgisayar teknolojisinin gelişmesi ile birlikte bu yaklaşık çözümlerin önemi kalmamıştır. Ayrıca demiryolları ve yüksek standartlı kara yolları için, hesapla bulunan kilometre ve kırmızı kotların mm inceliğinde değerler olması gerekmektedir. Bu nedenle dairesel düşey kurbların çözümünde kesin çözümün kullanılması daha uygun olacaktır. Bu yazıda düşey kurbların (daire ve 2. derece parabol) kesin çözümlerine ilişkin formüller türetilmiş ve çözümü anlatılmıştır. Örnek olarak alınan bir geçkide düşey geometriye ait ana ve ara noktaların kilometrelerinin ve kırmızı kotlarının, yaklaşık ve kesin hesapları yapılarak aradaki farklar gösterilmiştir. Anahtar Kelimeler: Düşey kurb, geçki, daire, parabol, düşey geçki tasarımı. In the vertical geometry of the routes, circular curves or 2nd degree parabola are used for joining two straight lines. In practice, the approximate solutions are used with the aim of simplifying the calculation of heights and kilometers of circular vertical curves. A transportation practice project covers all properties of the structure. One of the most important properties named "vertical geometry of the route" represents real geometry (with no errors) and is expressed by kilometers, elevations and gradients digitally. Those digital values can be divided into the tree parts by means of error levels: Group 1- The values chosen by creator (designer/drafter) with defined criteria (usually, radiuses of vertical circular curves or the lengths of parabolas in horizon plate, entry and exit gradients of back and forward tangents, kilometers of essential points of vertical curve (EPVC) which consist of beginning of vertical curve (BVC), end of vertical curve (EVC) and point of vertical intersection (PVI). Group 2- The values measured by means of graphically. Group 3- The values determined by calculation. The values in the second group are chosen as rounded numbers in order to simplify the computations. The numbers of the digits of the values in the second group are determined based on the scale of the longitudinal section. In most cases, the scale of the longitudinal section is 1:1000 thus the chainage values of the tangent points are obtained within the precision of 0.25 to 0.5 meters. The values in the group one are free of error while the second group values consist small errors. In third group, since the values in the third group are the products of a computation they have more errors according to the other two groups. In most textbooks the computation of vertical geometry is being thought without taking the errors introduced by omitting and assumptions into account. These errors are mainly introduced by not carrying out the computations using enough degrees of a formula which is in a series form or assuming that slope distances can be used as plane distances (Umar and Yayla 1994; Müller, 1984). The main reason of these assumptions is due to the limitations of the computations before 70s. Nowadays these limitations are over come by means of new computation techniques and instruments. And the computations can even be executed easily by a hand calculator. The results of exact computations of circular and parabolic vertical curves are described. The basic concepts of the subject should be summarized prior to calculations. 1) The vertical geometry is designed by using the longitudinal section of horizontal geometry (original surface). All vertical geometry related calculations are to be carried out on a vertical plane defined by K, H perpendicular coordinate system.2) K axis shows the chainages. The points located on the same vertical line naturally will have the same chainages. All distances used in the calculations must be in horizontal plane.3) H shows the point heights. All distances used in the calculations must be in vertical plane. 4) All computations are carried out in stages. The number and the quality of the input values obtained as 1st, 2nd and 3rd group values should be good enough to achieve an unique solution for the stage calculation of a vertical geometry. If the number and the quality of the values are not suitable, the calculations can not be done. In case of having the number of the initial data more than required number, the obtained results are different depending on the calculation method used. The contradictions adverting to the real value concept as a result of these computations must be prevented. The both methods, approximate solution and the exact solution, were applied to the profile data of sag and crest vertical curves to point out differences between two methods and the initial data. Two points are taken as an initial data on every curve and straight line for each. Nowadays, approximate solutions are not necessary due to the recent developments in computer technology. In addition, for the railroads and high standard roads the level of calculation precision should be in millimeter. For these reasons, the exact solutions are more suitable instead of approximate solutions. In this paper, equations of exact solution of vertical curves are evaluated and the solutions are explained. Differences of project heights and kilometers obtained from approximate and exact solutions are having been shown on a sample route. Keywords: Circular curves, route, circle, parabola, circular route design
On wing life optimization of commercial turbofan aircraft engines
Get PDFBir havayolu işletmesinde, turbofan uçak motorlarından sorumlu olan mühendisliğin birincil önceliği, motorların teknik olarak uçuşa elverişliliğini sağlayacak şekilde yönetilmesi ve bununla birlikte uçak motorlarının kullanım maliyetinin, güvenilirliğinin, emniyetinin ve operasyonel esneklik açısından motor performanslarının yeterli seviyelerde olmasının temin edilmesidir. Motorların uçak üzerinden sökülmesi ve motora gereken iyileştirme işlemlerinin yapılması gereken optimum bir zaman aralığı olduğu bilinmektedir. Optimum zaman aralığından önce gerçekleştirilen sökümlerde henüz kullanılabilir durumdaki parçaların vaktinden önce tamir edilmesi nedeniyle, kullanılabilecek parça ve motor ömrü ek bir maliyet olarak ortaya çıkacaktır. Optimum zaman aralığından sonra gerçekleştirilen sökümlerde, tamir edilmesi gereken parçaların sayılarının artması, tamirlerin zor ve maliyetli olması ek bir maliyet olarak ortaya çıkmaktadır. Söz konusu optimum uçuş ömrünün belirlenebilmesi amacıyla bir operatörün motorlarına ait veriler hem analitik hem de istatiksel olarak analiz edilmiştir. Gerçek problemin özelliklerini bozmayacak yaklaşımlarla, problem matematiksel olarak modellenmiştir. Bakım maliyeti, performans, güvenilirlik ve emniyet hedefleri bir arada düşünülmesi gerektiğinden, problem çoklu amaç fonksiyonlu bir optimizasyon problemi şeklinde çözülmüştür. Söz konusu çoklu amaç fonksiyonlu problem, bir havayolu mühendisliğinin öncelikleri göz önüne alınarak dayanıklılık fonksiyonu şekline dönüştürülmüş ve optimizasyon genetik algoritma yöntemi kullanılarak gerçekleştirilmiştir. Optimizasyon sonunda elde edilen sonuçlara göre, ele alınan motor tipi için bir optimum uçuş ömrü, bir başka değişle, optimum söküm aralığı bulunmuşur. Anahtar Kelimeler: Uçak motor bakımı, uçuş ömrü, optimizasyon.The primary objective of an airline powerplant engineer responsible from turbofan aircraft engines is to technically manage the engines to be available for revenue flight, while achieving the desired goals of cost of use, reliability and safety, with adequate engine performance level for operational flexibility. For an airline, flight safety is a must. Thus, aviation authorities throughout the world strictly bound the range in which an airline could move to meet the above goals. Since, approximately 40-45% of total aircraft maintenance cost is directly arising from engine maintenance costs, meeting these objectives becomes a very important issue as far as the competitiveness of the airline business is concerned. Under these circumstances, airlines always seek to find and explore methods in order keep the powerplants running with an optimized cost versus on wing life. The engine on wing maintenance concept is known as "on-condition maintenance". In this concept, engines are continuously monitored during their on wing operation in order to prevent failures and to meet goals on reliability and safety. In other words, engines are kept on wing as long as reliability, safety and performance levels are acceptable. All of the above conditions end up with raising the following important question: What is the time on wing of an aircraft engine that will fulfill all of these goals at the required levels This question forms the basis of analyzes performed in this study. It is known that there is an optimum time at which engine should have essential restoration done, in order to meet an optimized cost for removal. In other words, at times prior to the optimum one; the opportunity costs which are arising due to repairing an engine before all its useful life is consumed result in an increase in maintenance cost. At times greater than the optimum; the number of parts which need repair, the difficulty and cost of repair and the number of parts which must be scrapped increase. As a result, the maintenance cost increases. In order to search for this optimum, available data is gathered from one airline and analyzed both analytically and statistically. The problem is mathematically modeled by making assumptions, without effecting the accuracy of the real problem. In order to achieve a better understanding, the objectives (maintenance cost, performance, reliability and safety) are studied as sub problems. The data relating these objectives to the engine on wing time are searched and analyzed. For each of these objectives, mathematical expressions are derived. As far as the integrity of the goals of maintenance cost, performance, reliability and safety is concerned, a multi objective optimization problem is proposed. The model is converted into a fitness function form by an airline engineering perspective and multi objective problem of on wing life optimization is solved by using a Genetic Algorithm based solver. Genetic Algorithm method has been selected as being a robust and reliable technique for multi objective engineering optimization problems. Since performance deterioration characteristics end up with three different types of deterioration curves, three sub problems have been solved. This ensured the diversity in terms of actual engine performance deterioration characteristics, which is a result of operational, configurational and manufacturing diversities encountered in the real operation. Performance related diversity is believed to cover the above mentioned facts of the actual problem and enables a better forecast on the optimum on wing life of commercial turbofan aircraft engines. All the results are gathered considering airline engineering priorities and the optimum on wing life of a CFM56-3C1 engine is calculated. A removal planned within this optimum removal interval will achieve all the priorities in terms of minimum maintenance cost, adequate performance, while not sacrificing from reliability and safety. The result of the optimization is compared with the actual removals and found to be in line with the actual removal time frame. This proves the proficiency of proposed method and the mathematical model. Additionally, the operational factors affecting the aircraft engine on wing life is discussed in order to derive a generalized mathematical model for adaptation to other airline engine fleets and to other engine types. Thus, a generaized model is formulated in order to find the optimum time on wing for any type of commercial turbofan engine operated in any fleet. Keywords: Aircraft engine maintenance, on wing life, optimization
Service value creation in supplier-buyer interactions
Get PDFTedarik Zinciri Yönetimi (TZY), imalat, yöneylem, müşteri yönetimi ve dağıtım gibi değişik disiplinlerin ilgisini çekmiştir. Küresel tedarik, zaman ve kalite temelli rekabet, teknolojik gelişmeler ve çevresel belirsizlik unsurları sonucunda, TZY akademik araştırmalar için güncel bir başlık haline gelmiştir. Artan küresel rekabet, tedarik zinciri halkalarının birbirleri ile daha yakın koordinasyon sağlamasını ve kendilerini diğer halkaların konumlarını dikkate alarak daha iyi hizalamalarını gerektirmektedir. Bu sebeple, tedarikçi-alıcı çiftlerinin yakından incelenmesi hayati önem taşımaktadır. Fiyat bir ürünün en önemli özelliği haline dönüştükçe karlılık düşmekte, rekabet avantajı azalmaktadır, buna ek olarak bilişim teknolojilerinin tedarikçi-alıcı çiftlerinin daha yakın koordinasyonuna imkan tanımayan araçları geliştikçe, tedarikçiler ürünlerini sundukları hizmetler ile desteklemektedirler. Öyle ki, hizmet kalitesi, ürün kalitesinden daha önemli bir sipariş kazandıran olarak anılmaktadır. Ancak, hizmetler yolu ile farklılaştırma yakalanması zor bir kavramdır. Ürünlere eşlik ederek melez ürün oluşumuna katkıda bulunan hizmetler, tedarikçilerin rekabetçiliklerini daha da geliştirebilecekleri unsurlar arasında yer almaktadır. Ancak, alıcıların hizmetlere dair öznel beklentilerinin gelişimleri ve değişkenlikleri sebebi ile tedarikçiler sunumunda bulunacakları hizmet seviyesi gereklerini öngörmekte zorlanmaktadırlar. Bu çalışmanın hedeflediği amaçlar çeşitlidir. Hizmetlerin Tedarik Zinciri Yönetimi (TZY) içerisindeki önemini ortaya koyarak, tedarikçi-alıcı etkileşimlerinde katma değer yaratabilme kabiliyetlerini ortaya koymak bu amaçlardan ilkidir. Ayrıca, tedarikçilerin alıcılarının hizmet seviyesi gereklerini belirleyebilmeleri için Snell Kanunu’ndan yararlanılarak, tedarikçilere ışık tutacak bir model geliştirmek de amaçlanmaktadır. Anahtar Kelimeler: Tedarik Zinciri Yönetimi, Tedarikçi-Alıcı etkileşimi, hizmetler, değer yaratımı.Supply chain management has attracted attention from various multi-disciplinary fields, which include manufacturing, operations management, customer management and transportation. Global sourcing, time and quality based competition, improvements in technology, and environmental uncertainties have placed supply chain management as a timely topic for academic research. Defect-free, fast and reliable delivery has almost become the necessities to be in the market, rather than competitive advantage. Ever stiff competition in the global arena calls for closer coordination and better alignment between supply chain participants. Therefore, a closer examination of supplier and buyer interactions is vital. As price becomes the most important feature of a product and reduces profitability and competitive advantage, and as information technology tools that help in a closer coordination with buyer and supplier emerge, suppliers get engaged in providing services around their products. Moreover, service quality is referred as a more important order winner than the product quality. Yet differentiation via services dimension is not an easy concept to grasp. Firms often perform services according to the norms they have established over years of experience. Though, the breadth of experience commonly varies from one firm to another. Therefore, norms adopted for services vary, too. A norm considered for a specific service in one company might not exist at another company, or even if exist could be underestimated. When placed in the context of complex supply chains, this presents a supplier with often buyers that have different services' norms. A certain aspect of a supplier's service might be deemed superior at one buyer, while normal or even inferior at another buyer. Therefore, provision of services in supply chain management presents a promising research area. There is a need to understand if supplier's provision of services is capable of generating superior or inferior perceived buyer value. For a firm to gain and retain value additive beneficial relationships with its buyers, its offer should often involve some social values (something that will make buyer's life a better one) either by clearly identifying the needs of its buyers or by outperforming what other suppliers already do. Suppliers demand more information on sophisticated norms and specifications of their buyers to formulate their services. This research attempts to shed the light on the issues discussed, and also uncover the possible causes of superior perceived buyer value creation upon supplier's provision of services in supply chain. An exploratory research model has been developed by the aid of an analogy from Snell's Law of the physics as services creation of value has an instantaneous nature like that of the passage of a beam of light from one medium to another one. Two propositions had stemmed from the exploratory research. Firstly, lower operational medium on supply side is likely to cause inferiority in perceived buyer value for an encounter. Secondly, higher operational medium on supply side is likely to cause superiority in perceived buyer value for an encounter. A panel of experts had been devised to confirm usefulness and meaningfulness of the developed model. The research process for the panel of experts was a qualitative technique based on an interpretive orientation. Participants included 9 provincial individuals. In all, there were 19 interviews, involving 9 individuals. In four instances more than one person participated in an interview. Thirteen of the interviews were face-to-face, and three were conducted by telephone. Interviewees confirmed the model by the practices they are involved within the industry, as well as with the exemplifying issues they have presented. No changes were requested for the developed exploratory model. The main contribution of this research endeavor to academia is the developed exploratory model, which explains creation of superior perceived buyer value via provision of services in a supplier-buyer interaction. Therefore, reference disciplines, including supply chain management and services have received contribution. High levels of services are often costly for suppliers to adopt for the entire buyer base due to two major reasons: the unnecessary high costs incurred for buyers who were not necessarily expecting such high services for value creation, and the dramatically raised expectations which will set the base for all forthcoming services of the supplier that'll force the supplier to perform an ever higher levels of services. Developed model facilitates detection of possible opportunities to improve service levels through which a supplier can create superior perceived buyer value. Keywords: Supply chain management, supplier-buyer interaction, services, value creation
Developing a competitive advantage model for organizations
Get PDFGünümüz işletmelerinin karşı karşıya oldukları en temel güçlüklerden biri, sahip olunan kısıtlı kaynakların dağıtımında temel alınacak kriterler konusundaki belirsizliktir. Yöneticiler çoğunlukla, firmaların performanslarını artırmak için, birbirine benzer görünen alternatifler arasından seçim yaparak, kaynakların geliştirilmesi, korunması, kullanılması, yaygınlaştırılması, vb. konularda karar vermek zorunda kalırlar. Bu yönetsel karar verme sürecinde kritik olan nokta, sonuçta firmaya rekabet üstünlüğü sağlayabilecek alternatiflerin değerlendirilmesinde kullanılabilecek kriterlerin varlığıdır. Ayrıca, çevrede yaşanan hızlı değişimler de, organizasyonların çevresindeki belirsizlikleri artırarak yönetilmelerini daha da karmaşık hale getirmekte ve eski stratejik yaklaşımları da geçersiz kılmaktadır. Bu durumda, değişen çevresel şartlara uyum sağlayacak ve yöneticilerin stratejik karar alternatiflerini önceliklendirebilmelerine ve karar vermelerine yardımcı olacak yeni yaklaşımlara ihtiyaç duyulmaktadır. Bu ihtiyacın karşılanmasına yönelik olarak bu çalışmada, rekabet üstünlüğüne iki farklı bakış açısıyla yaklaşan, Porter’ın rekabet üstünlüğü modeli ile kaynak esaslı modeli bütünleştirecek ve rekabet üstünlüğü kaynaklarının belirlenmesine imkân verecek bir rekabet üstünlüğü modeli kurulmuştur. Kurulan rekabet üstünlüğü modeli, çevrenin ve firma kaynaklarının (kaynaklar ve yetenekler) rekabet üstünlüğü ile ilişkisinin analiz edilmesine ve rekabet üstünlüğü kaynaklarının (içsel ve dışsal) belirlenmesine olanak sağlamaktadır. Modelde çevre, firmalar tarafından ihtiyaç duyulan kaynaklar bakımından ele alınmış; “çevrenin mevcut ve yeni organizasyonları desteklemek, bu organizasyonların büyümelerini, başarılı ve kalıcı olmalarını sağlamak üzere yeterli kaynakları sağlama ölçüsü” olarak tanımlanan “çevresel cömertlik” boyutu üzerine odaklanılmıştır. Firma kaynakları ise, kaynak hiyerarşisindeki, kaynaklar (varlık ve beceriler) ve yetenekler boyutları ile ele alınmıştır. Anahtar Kelimeler: Rekabet üstünlüğü, firma kaynakları, çevresel cömertlik. In a world characterized by durable products, stable customer needs, well-defined national and regional markets, and clearly identified competitors, competition was a "war of position" in which companies occupied competitive space like squares on a chessboard, building and defining market share in clearly defined product or market segments. The key to competitive advantage was where a company chose to compete. How it chose to compete was also important but secondary, a matter of execution. As markets fragment and proliferate, "owning" any particular market segment becomes simultaneously more difficult and less valuable. As product lifecycles accelerate, dominating existing product segments becomes less important than being able to crate new products and exploit them quickly. Meanwhile, as globalization breaks down barriers between national and regional markets, competitors are multiplying and reducing the value of national market share. In this more dynamic business environment, strategy has to become correspondingly more dynamic. Competition is now a "war of movement" in which success depends on anticipation of market trends and quick response to changing customer needs. Successful competitors move quickly in and out of products, markets, and sometimes even entire business. In such an environment, the essence of strategy is not the structure of a company's products and markets but the dynamics of its behaviour. And the goal is to identify and develop the hard-to-imitate firm resources that distinguish a company from its competitors in eyes of the customer. In order to overcome these challenges and to be the winner of this war of movement, organizations must attain competitive advantage and also understand the business milieu the economic and social changes affecting a business. Understanding how these conditions increase the pace of change illuminates why organizations must become more flexible, open, and responsive to milieu conditions. An increased pace of change increases the amount and intensity of competition as new competitors enter markets. Under these circumstances organizations must learn how to attain competitive advantage to respond to the intense competition; in other words, how to implement strategies that exploit their internal strengths, through responding to environmental opportunities, while neutralizing threats and avoiding internal weaknesses. But a firm?s competition does not include only all of its current competitors, but also potential competitors poised to enter its industry at some future date. So the strategy being implemented by that firm must be a value creating strategy, not simultaneously being implemented by any current or potential competitors. From this point of view, in this research, a competitive advantage model for the analysis of the sources of competitive advantage, integrating the two basic streams of competitive advantage theory (i.e., Porter's competitive advantage model and the resource-based view) has been developed. This competitive advantage model makes it possible to analyze the relationship between the environment, firm resources (resources and capabilities) and competitive advantage, and to determine the internal and/or external sources of competitive advantage. The model defines the environment in terms of the resources required by firms and focuses on environmental munificence dimension which is defined as "the extent to which the environment provides enough resources to support established organizations and new entrants and to enable them to grow and prosper". And for firm resources, resource and capability dimensions of the resource hierarchy were taken into consideration. A resource includes all tangible and intangible assets and skills controlled by a firm that enable the firm to conceive of and implement strategies that improve its efficiency and effectiveness and hold the potential of competitive advantage. A capability is the capacity of a firm to perform the basic functional activities better than competitors in order to affect a desired end, by means of resource teams. This study addresses three major questions: (i) Whether industry-specific or firm-specific factors account for a more significant portion of the position of advantage variations "(ii) In order to attain competitive advantage, which firm resources should the firms possess" (iii) In order to attain competitive advantage, in which types of environments should the firms compete. Keywords: Competitive advantage, firm resources, environmental munificence
6 Serbestlik dereceli 6-3, özel yapı 6-3 ve 6-4 paralel mekanizmaların genişletilmiş çalışma uzayı analizi
Get PDFParallel mechanisms, which are closed loop mechanisms, consist of a base platform, a moving platform, and at least two links actuated in parallel. The Stewart Platform Mechanism (SPM), which is originally proposed by D. Stewart as a flight simulator in 1965, is the most renowned parallel manipulator. In 1949, the first working parallel mechanism was designed by Gough. For this reason, such a parallel mechanism is sometimes referred to as the Gough-Stewart Platform. Hunt suggested using parallel manipulators in robotic applications due to their advantages. Recent advances in high-precision technology necessitated the replacement of serial mechanisms by parallel working mechanisms in many industrial applications. The advantages of parallel manipulators are high rigidity, precision, accuracy, load carrying capacity, stiffness, ability to be utilized in high speed applications and ease of control are given in literature by many authors. However, the workspace of parallel mechanism is smaller than serial mechanism. In addition to this, a number of studies on parallel mechanisms, often on the 3 degrees of freedom (DOF) and 6 DOF types exist in open literature. 6-3 SPM, studied in this thesis, consists of a fixed base platform and a mobile platform. Six linear actuators are connected to the base at six points via universal joints and to the top platform at three points via spherical joints. Consecutive linear actuators are attached to each other in groups of two so that there are three points of attachment to the top platform. The particular 6-3 SPM?s moving platform is connected to spherical joints via small rods. 6-4 SPM also consists of a base platform and a mobile platform. Six linear actuators are connected to the base at six points via universal joints and to the top platform at four points via spherical joints. These structures are called 6-UPS (Universal-Prismatic-Spherical) mechanisms due to the types of joints in the architecture. Both platforms have 6 DOF, ability to move positional and orientation in 3 directions. To give a model for the workspace analysis of 6-3 SPM, the particular 6-3 SPM and 6-4 Stewart platform mechanisms kinematics are carried out. Workspace analysis of parallel mechanisms is not generalized so far in literature. However, many problems, like small workspace, poor dexterity and difficulty in design are still open for an efficient exploitation of the concept. Most of the studies in the literature examined two dimensional orientation workspace. Some other studies cover only the boundary scanned 3D orientation workspace. In literature, workspace analysis methods are classified into 3 main groups as the discretization method, geometrical method and the Jacobian matrix technique. After carrying out kinematic analysis, discretization method, which is based on Euler angles, is used to represent the orientation workspace of 6-3 SPM, the particular 6-3 SPM and 6-4 SPM. In order to simplify interpretation, the orientation workspace is illustrated in a cylindrical coordinate system. The orientation workspaces of the 6-3 SPM, the particular 6-3 SPM, and the 6-4 SPM are compared. In this study, the method used is fully scanned orientation workspace, which is extended, in the mostly workable direction for 6-3 SPM, the particular 6-3 SPM and 6-4 SPM. Possible gaps, which are omitted in representation of boundary orientation workspace, can be realized in fully scanned orientation workspace. For these types of working mechanisms, i.e. a mechanical tool used material processing the determination of the points, which makes the workspace maximum be outlined. The workspace of the 6-4 mechanism with trapezoidal upper platform is larger than that of the 6-3 mechanisms with triangular upper platform. However, in the case of the 6-4 mechanism, there are greater gaps within the workspace. Even though the workspace of the 6-4 mechanism is greater than that of the 6-3 mechanism, the trapezoid platform cannot reach every point in the vicinity of C, gravity centre, and at the boundaries of the workspace. A 6-3 SPM should be preferred if it is desired to work close to C and the boundaries of the workspace as in flight simulator case. In addition, a triangular moving platform is structurally more stable than a trapezoidal moving platform. When greater or distributed loads need to be carried by the mechanism, such as in the case of heavy loads lifting, a trapezoidal moving platform should be preferred. Keywords: Parallel mechanism, workspace analysis, Stewart Platform. Paralel mekanizmalar kapalı çevrim yapılardır. Paralel mekanizmalarda hareketli platform, sabit platforma en az iki noktada birbirinden bağımsız kinematik bağlantı elemanlarıyla bağlıdır. Paralel mekanizmaların avantajları yüksek katılık, hassasiyet, doğruluk, yük taşıma kapasitesi ve yüksek hız uygulamalarında çalışabilme olarak sıralanabilir. Fakat çalışma uzayları seri mekanizmalara göre daha küçüktür. Stewart Platform Mekanizması(SPM), ilk olarak 1965 yılında D. Stewart tarafından uçuş simülatörü olarak önerilen en meşhur paralel manipülatördür. Bu çalışmada iki farklı tip olan 6-3, özel yapı 6-3 SPM ve 6-4 SPM kullanılmıştır. 6-3 SPM’sı sabit ve hareketli platformdan oluşmaktadır. 6 adet lineer eyleyici sabit platforma 6 noktadan üniversal mafsallarla ve üst platforma 3 noktadan küresel mafsallarla bağlanmıştır. Ardışık lineer eyleyiciler ikili grup halinde birbirine bağlanarak üst platforma üç noktadan bağlanmışlardır. Aynı zamanda 6-4 SPM’de sabit ve hareketli platformdan oluşmaktadır. 6 adet lineer eyleyici sabit platforma 6 noktadan üniversal mafsallarla ve üst platforma 4 noktadan küresel mafsallarla bağlanmıştır. Çalışma uzayı analizine örnek olarak, 6-3 SPM, özel yapı 6-3 SPM ve 6-4 SPM’lerin ters kinematik analizleri yapılmıştır. Bu denklemler Matematica ve Matlab programları kullanılarak çözülmüştür. 6-3 SPM, özel yapı 6-3 SPM ve 6-4 SPM’lerin yönelme çalışma uzayı analizleri Euler açıları temeline dayanan ayrıklaştırma metodu kullanılarak yapılmıştır. Elde edilen sonuçlar, yorumlama açısından kolay olması için, silindirik koordinatlara dönüştürülerek yönelme çalışma uzayı grafikleri çizdirilmiştir. 6-3 SPM, özel yapı 6-3 SPM ve 6-4 SPM’lerin yönelme çalışma uzayları karşılaştırılmıştır. Aynı zamanda hareketli platformun alanı değiştirilerek 6-3 ve 6-4 SPM’lerin çalışma uzayları karşılaştırılmıştır.Anahtar Kelimeler: Paralel mekanizmalar, çalışma uzayı, Stewart Platformu
Examination of welding distortion via applying thermal tensioning technique on austenitic stainless steels
Get PDFTürk Deniz Kuvvetlerinin modernizasyonu kapsamında İstanbul Tersanesi Komutanlığı/Pendik’te, Alman Abeking&Rasmussen ve Lürssen firmalarının konsorsiyumu ile Aydın Sınıfı mayın avlama gemileri inşa edilmektedir. Aydın Sınıfı mayın avlama gemilerinin, genel olarak dünya bahriyesindeki mayın avlama gemilerinden farkı, bu sınıf gemi inşasında kullanılan ostenitik paslanmaz çeliklerdir. Hali hazırda bu çeliği, Alman, İtalyan ve Kanada donanmaları kullanmaktadırlar. Anılan çeliğin, mayın avlama gemilerindeki kullanım amacı, manyetik olmamaları, deniz suyu korozyonuna dayanımlı olmaları ve yüksek mukavemete sahip olmalarıdır. Ancak manyetik olmayan ostenitik paslanmaz çeliklerin, ısı iletimlerinin kötü, buna mukabil ısıl genleşmelerinin yüksek olması, bu çeliklerin kaynak sonrası distorsiyonlarının, diğer çeliklere oranla, daha fazla olmasına neden olmaktadır. Bu durum, kaynaklı parçaların montajları esnasında, birleştirme problemlerine yol açtığından, her kaynak işleminden sonra kaynaklı parçalar, alevle düzeltme işlemine tabi tutulmaktadır. Bu çalışmanın amacı, İstanbul Tersanesi Komutanlığı/Pendik’te imali devam eden Aydın Sınıfı mayın avlama gemilerinin kaynak işlemi sonrasında meydana gelen distorsiyonlarını en aza indirmek amaçlı, kaynak esnasında uygulanan ilave ısı girdisinin bir yöntem olarak uygulanabilirliğinin ispat edilmesidir. İskoçya Strathclyde Üniversitesi’nde yapılan deneyler sonucunda, kaynak esnasında parçaya ilave ısı girdisi verilerek, Isıl Gerilme Tekniği uygulanmasının, T kaynaklı parçalarda, ondüle şeklindeki (buckling) distorsiyonlarını önlediği ve açısal çarpılmaları azalttığı ispatlanmıştır. Anahtar Kelimeler: Ostenitik paslanmaz çelik, ondüle distorsiyon, ısıl gerilme tekniği.Under the construction of German Mine Hunter Consortium (Abeking&Rasmussen and Lürssen), Istanbul Shipyard/Pendik is building Aydın Class Mine Countermeasures (MCM) Vessels for Turkish Navy. Aydın Class Mine Countermeasures (MCM) Vessels distinct from the other world navies because of their hull made of austenitic stainless steel. Nowadays, German, Italian and Canadian navies use this type of steel for ship hulls. Austenitic stainless steel has many advantages, like resistant to the corrosion of sea water, high yield point and non-magnetic structure. But these metals are having also disadvantages, like poor heat conductivity and high thermal expansivity. These disadvantages create more distortions than normal steels in welding conditions. Fittings of welded structure are very important section in shipbuilding, so geometry of welded structure need to be accurate. For straightening of distortions, flame straightening method is widely using in shipbuilding industry. Nowadays for quality products it is necessary to reduce price. Straightening of distortions via flame straightening method is more time, manpower and material consuming. So improving of straightening of distortions will be beneficial for budget of government. The aim of this thesis is to show benefits of usage of Thermal Tensioning Technique as a method of straightening of distortions for Aydın Class Mine Countermeasures (MCM) Vessels which are built at the Istanbul Shipyard/Pendik. Straightening of distortions via Thermal Tensioning Technique is examined for ship building industry as a preventing method with many academic researches. The stiffened panel made of austenitic stainless steel is used in the experiment of the Strathclyde University/Scotland. In specific condition under gas protection weld (MIG), thermal camera is used for distribution of heat, thermo couples are used for temperature measurement and laser appliance is used for distortion measurements. This paper focuses on the transient temperature fields created by the large heat inputs from arc welding which is the driving force for distortion in the plates. The use of a thermal imaging camera allows the thermal fields to be viewed on a whole-field basis. The use of a welding rig in this kind of investigation is essential as it allows parameters to be kept constant throughout. The moveable track is controlled by stepper motors such that the travel speed of the weld can be adjusted to a tenth of a millimeter per second. The torch is held static while the plate is pulled along at a constant speed by a series of pulleys and small stepper motors. The welding machine was connected to the computer and so all parameters could be entered via the computer so that all the variables could be controlled. The camera was held in place with scaffolding and the tripod for the camera fixed to a bracket that allowed movement in all directions. The CCD thermal imaging camera used here was the ThermaCam SC500 from FLIR systems. The camera is able to provide thermo graphic images in the infrared spectrum from 3 to 12μm. Following initial tests with austenitic stainless steel plates the amount of background radiation reflected from the plate to the camera was seen to be high. This led to the painting of the plates with a high emissivity, two-part zinc silicate primer made up in two parts. To verify thermal images thermocouples were also present at distances of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100mm. The laser appliance is used for distortion measurements. All of austenitic stainless steel plates are scanned before welding and again after welding. The data was used to compare the distortion effects under different restraint conditions. The distortion analysis was carried out using an automated laser scanner which was programmed to cover the whole plate recording data at intervals throughout the scan. The surface deformation data obtained from Labview program was evaluated on the graphics. Austenitic stainless steel plates are tested under four conditions. These are free welding, restrained welding, restrained and additional heat input welding, free and additional heat input welding. As a result, the experiments in Strathclyde University proved that Thermal Tensioning Technique prevent buckling distortions and improves angular distortions on stiffened welded plates. Usage of Thermal Tensioning Technique for Aydın Class Mine Countermeasures (MCM) Vessels and also other type of ship building industry is additional benefit and assistance for economy. Keywords: Austenitic stainless steels, buckling distortions, thermal tensioning technique. 
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