1,721,173 research outputs found
Doğrusal olmayan yapıların doğrudan rezonans tepki tahmini: kanatçıklı diskler üzerine bir örnek incelemesi
No full textThis thesis proposes a novel method that utilizes the Response Dependent Nonlinear Mode (RDNM) concept to directly determine the resonance response of nonlinear
systems without solving the system completely. The RDNM concept is introduced to decrease the nonlinear algebraic equation size in the modal superposition method
by creating a new eigenvalue problem for each response level. As the response level and pattern change, the stiffness matrix of the system is adjusted accordingly. A fur ther reduction is ensured with the Dual Modal Space Method. The proposed method is evaluated on both lumped parameter and large-scale finite element models, incor porating different nonlinearities like one-dimensional dry friction model, cubic stiff ness, and one-dimensional dry friction model with normal load variation. Results
demonstrate the capability of the method to predict resonance peaks accurately, with excellent agreement to frequency response solutions. Conspicuously, the proposed
method reduces computational time significantly compared to the classical modal su perposition method.
This thesis introduces another novel technique for utilizing complex numbered RD NMs in cyclically constrained structures. The method is demonstrated through a finite model of a large-scale cyclically symmetric shrouded blade. The proposed approach involves using complex numbered RDNMs to obtain the steady-state periodic non linear response of the
shrouded blade assembly. By perturbing normal load, contact stiffness, and excitation levels, the resonance peaks are traced using complex num bered RDNMs. The results of the study demonstrate the efficacy of the proposed tech nique in capturing the nonlinear behavior of cyclically constrained structures while ensuring computational advantage.Bu tez, sistemi tamamen çözmeden doğrusal olmayan sistemlerin rezonans tepkisini doğrudan belirlemek için Yanıta Bağlı Doğrusal Olmayan Mod (RDNM) kavramını kullanan yeni bir yöntem önermektedir. RDNM kavramı, her yanıt seviyesi için yeni bir özdeğer problemi yaratarak, modal süperpozisyon yönteminde doğrusal olmayan cebirsel denklem boyutunu azaltmak için tanıtılmıştır. Tepki seviyesi ve patern değiştikçe, sistemin rijitlik matrisi buna göre ayarlanır. İkili Modal Uzayı Metodu ile daha fazla azalma sağlanır. Önerilen yöntem, tek boyutlu kuru sürtünme modeli, kübik rijitlik ve normal yük değişimine sahip tek boyutlu kuru sürtünme modeli gibi farklı doğrusal olmama durumlarını içeren hem toplu parametre hem de büyük ölçekli sonlu eleman modelleri üzerinde değerlendirilmiştir. Sonuçlar, yöntemin rezonans tepe noktalarını doğru bir şekilde tahmin etme yeteneğini, frekans yanıtı çözümleriyle mükemmel bir uyum içinde olduğunu göstermektedir. Dikkat çekici bir şekilde, önerilen yöntem klasik modal süperpozisyon yöntemine kıyasla hesaplama süresini önemli ölçüde azaltır.\par
Bu tez, döngüsel olarak kısıtlanmış yapılarda karmaşık numaralı RDNM'leri kullanmak için başka bir yeni teknik sunmaktadır. Teknik, büyük ölçekli döngüsel olarak simetrik örtülü bir bıçağın sonlu bir modeli aracılığıyla gösterilmiştir. Önerilen teknik, örtülü kanat düzeneğinin sabit durum periyodik doğrusal olmayan tepkisini elde etmek için karmaşık numaralı RDNM'lerin kullanılmasını içerir. Normal yükü, temas sertliğini ve uyarma seviyelerini bozarak, rezonans tepe noktaları, karmaşık numaralı RDNM'ler kullanılarak izlenir. Çalışmanın sonuçları, önerilen tekniğin hesaplama avantajı sağlarken döngüsel olarak kısıtlanmış yapıların doğrusal olmayan davranışını yakalamadaki etkinliğini göstermektedir.M.S. - Master of Scienc
Değişken Temas Yüzeyleri İçin Dinamik Kuru Sürtünme Modeli Geliştirilmesi
No full textKuru sürtünme birbirine temas eden her yüzeyde bulunduğu için mekanik sistemlerin gerçekçi olarak modellenmesinde oldukça önemlidir. Bazı sistemlerde istenmese de kasıtlı olarak kuru sürtünmenin dahil edildiği yapılarda bulunmaktadır. Bu sistemlerdeki amaç istenmeyen titreşimleri kuru sürtünme vasıtasıyla sönümlenmesidir. Bu tip kuru sürtünme sönümleyicileri gaz türbinli motorların türbin kanatçıklarının titreşim genliklerinin düşürülmesinde sıkça kullanılmaktadır ve aynı zaman gaz türbinli motor üreticilerinin en önemli tasarım problemlerinden birini oluşturmaktadır. Bunların dışsında kuru sürtünmeli sönümleyiciler tren vagonlarının ve araç titreşim seviyelerinin düşürülmesi, binaların depreme karşı korunması ve istenmeyen titreşimlerin giderilmesi gibi çeşitli durumlarda da kullanılmaktadır. Bu ve benzer sistemlerin dinamik olarak modellenmesinde karşılaşılan en büyük sorun kuru sürtünmenin doğrusal olmayan yapısı sebebiyle analizleri oldukça karmaşıklaştırmasıdır. Bu projede amaçlanan bu ve benzer sistemlerin dinamik analizi için yeni kuru sürtünme modelleri geliştirilmesidir. Şu anda tahrikli titreşim analizlerinde kullanılmakta olan modeller temas yüzeyinin etki eden kuvvete bağlı olarak değişmesini tam olarak modellememektedirler. Şu anda kullanılmakta olan kuru sürtünme modellerinde temas parametreleri sabit olarak alınmıştır. Halbuki temas parametreleri temas yüzeyinin değişmesiyle birlikte değişebilmektedir. Özellikle eğimli yüzeylerin temasında ortaya çıkan bu durumu modellemek için temas parametrelerinin değişimini de içeren bir kuru sürtünme modeli geliştirilecektir. Dolayısıyla önerilen proje hali hazırda kullanılmakta olan sürtünme modellerindeki önemli eksikliklerin giderilmesini sağlayacaktır. Böylece içinde sürtünme barındıran dinamik sistemlerin sayısal benzetimleri deneysel çalışmalarla daha fazla örtüşecek ve yapılacak deney sayısını en aza indirerek büyük ekonomik kazanca sebep olacaktır
KURU SÜRTÜNMELİ SÖNÜMLEYİCİLERDEKİ AŞINMANIN ÖNGÖRÜLMESİ VE BUNUN TİTREŞİM AZALTIMINA ETKİLERİ
No full textAlthough friction is mostly considered an undesirable phenomenon due to its energy dissipation characteristic, specially designed dry friction dampers utilize this feature to reduce the dynamic response of the system at the resonant frequencies. However, fretting wear due to dry friction during the operational life of the structure may drastically shift the performance from the initial design point and eventually cause drastic failures. Hence, it is important to establish a sophisticated algorithm that couples nonlinear dynamic response solver of frictionally constraint problems with a fretting wear model. In this thesis, the proposed methodology is utilized to find the evolution of dynamic response and surface topography of two widely used case studies which are shrouded blade and grounded blade platforms under ongoing vibration cycles.Sürtünme, sistemin enerjisini sönümleme özelliğinden dolayı çoğunlukla istenmeyen bir fenomen olarak görülse de, özel olarak tasarlanmış kuru sürtünmeli sönümleyiciler, sistemin rezonans frekanslarında dinamik tepkisini azaltmak için bu özelliği kullanır. Bununla birlikte, yapının operasyonel ömrü boyunca kuru sürtünmeden kaynaklanan titreşimsel aşınması, performansı ilk tasarım noktasından büyük ölçüde kaydırabilir ve sonunda ciddi sonuçlara neden olabilir. Bu nedenle, kuru sürtünmeli problemlerinin doğrusal olmayan dinamik tepki çözücüsünü bir sürtünme aşınma modeliyle birleştiren gelişmiş bir algoritma kurmak önemlidir. Bu tezde, tasvir edilen metodoloji kullanılarak, üzerinde sıklıkla çalışılan “shrouded blade” ve “grounded blade platform” örnekleri üzerinden, devamlı titreşim döngüsü altında, dinamik tepki ve yüzey topografisinin değişimi incelenmiştir.M.S. - Master of Scienc
Ayarlanabilir kütleli ve kaldıraç tipli sönümleyiciler kullanılarak sürüş konforunun artırımı.
No full textIn this study, the efficiency of linear and rotational tuned mass dampers (TMD) and lever type vibration isolators (LVI) in improving ride comfort is investigated based on a vehicle quarter-car model. TMDs reduce vibration levels by absorbing the energy of the system, especially at their natural frequencies. Both types of TMDs are investigated in the first part of this study. Although linear TMDs can be implemented more easily on suspension systems, rotational TMDs show better performance in reducing vibration levels; since, the inertia effect of rotational TMDs is higher than the linear TMDs. In order to obtain better results with TMDs, configurations with chain of linear TMDs are obtained in the second part of the study without changing the original suspension stiffness and damping coefficient. In addition to these, the effect of increasing the number of TMDs used in the chain configuration is investigated. Results show that performance deterioration at lower frequencies than wheel hop is reduced by using chain of TMDs. In the third part of this study, various configurations of LVIs with different masses are considered and significant attenuation of vibration amplitudes at both body bounce and wheel hop frequencies is achieved. Results show that TMDs improve ride comfort around wheel hop frequency while LVIs are quite efficient around body bounce frequency. Finally, parameter uncertainty due to aging of components and manufacturing defects are investigated.M.S. - Master of Scienc
Design and production stages of a vibration test fixture
Full text linkRandom vibration tests are performed by attaching a product on an electro dynamic shaker as a part of standardized environmental vibration tests which require specified random vibration profiles applied to products within certain tolerance bands. An ideal test fixture, which will be used to attach a product to a shaker, should be as rigid as possible with no local resonances in the frequency range of the random vibration test profile. However, such a fixture design is rarely possible due to concerns such as fabrication costs, testing time, etc. A rigid fixture usually leads to a design comprised of massive components which may not be economical to fabricate or may not be feasible because of the available payload carrying capacity of the shaker planned to be used for the test. In cases where multiple test items are sought to be tested simultaneously to reduce overall testing time, increased total payload necessitates the fixture designers to reduce the fixture weight. All these practical issues oftentimes force test engineers to design relatively low-weight test fixtures with local resonant behavior in the frequency range of interest. The existence of these local resonances of the fixture would cause resonant vibration behavior which sometimes may prevent the shaker control system from producing the desired random vibration profile. In this study, a random vibration testing scenario is investigated to identify the described issues with producing a desired random vibration profile. The problem is studied both experimentally and through finite element simulations. A remedy, which involves increasing damping in the problematic local modes of the test fixture by adding tuned vibration absorbers, is proposed and shown to be effective through finite element based simulations and experiments.M.S. - Master of Scienc
Tek ve çift duvarlı kavisli karbon nanotüplerin doğrusal olmayan titreşimleri
No full textIn this thesis, effects of Geometric, initial curvature, and van der Waals (vdW) interlayer force nonlinearities on the variation of nonlinear natural frequency of Carbon Nanotubes (CNTs) are investigated in detail throughout several case studies. Galerkin method with a single trial function, which is the eigenfunction of the linear system, is widely used in literature in studying nonlinear vibrations of CNTs. However, eigenfunctions of the nonlinear systems can be significantly different than the eigenfunctions of the linear system. Therefore, depending on the nonlinearity, it may not be possible to capture the nonlinear characteristics by using a single trial function. Consequently, for the first time in this thesis, multiple trial functions are used to investigate the nonlinear free vibrations of CNTs. Moreover, a new solution approach– describing function method– is proposed which has the advantage of expressing the nonlinear force as a nonlinear complex stiffness matrix multiplied by displacement vector, where the off-diagonal terms of the nonlinear stiffness matrix can provide a comprehensive knowledge about the coupling between the trial functions. Depending on the boundary conditions considered, it is hard to find suitable trial functions that satisfies all the boundary conditions; hence, in order to overcome this difficulty, iterative path following method (IPFM) based on differential quadrature method (DQM) is developed which does not require trial functions. It is concluded that DQM based nonlinear solution method is very promising in solving nonlinear continuous systems, since it requires less number of gird points which results in less number of nonlinear equations compared to finite element methods.Ph.D. - Doctoral Progra
Kanatçıklı diskler için doğrusal olmayan titreşim deney test düzeneklerinin geliştirilmesi.
No full textHigh-cycle fatigue is one of the most frequent reason of failure for turbomachines and in early design process, so it is crucial to predict the vibration levels. Various finite element modelling techniques for bladed-disk systems appear in the literature, including both reducing large size FEM and describing the frictional contact interface. However, not only having a large size FEM but also including the nonlinear friction to models makes the task very struggling and time consuming. In order to enhance the working life, resonances should be avoided but, it is not easy to avoid all critical resonances on turbomachinery due to the broad spectrum of aerodynamic excitation. Thus, decreasing the vibration amplitudes become necessary. The vibration amplitudes of the system can be reduced by the initial gap and friction between the shroud contact interface which is a nonlinear contact phenomenon. Therefore, calculating the dynamic properties of system is a major problem. This study is conducted in order to understand the nonlinear frictional contact behavior and gap nonlinearity which affects the damping characteristics of the shrouded blades. To do so, two experimental test setups are developed. First one consists of a shrouded blade to measure dynamic responses of stationary shrouded blade with gap nonlinearity. The effects of different shroud contact angle, shroud positions along radial direction, initial gap and different excitation forces are investigated on the first bending mode of the blade. In the second test, an under platform damper setup is prepared. The effect of friction and normal preload at the contact surface is investigated. During both tests, a modal shaker for excitation and a data acquisition system with accelerometers are used for measurement.Thesis (M.S.) -- Graduate School of Natural and Applied Sciences. Mechanical Engineering
Kuru sürtünmeli motor takozu tasarımı.
No full textAutomotive engine mounts are used to support engine weight, protect engine from road inputs and isolate transmission of vibrations created by the engine, which has a drastic effect on the noise generated inside the passenger cabin. Most common types of engine mounts are elastomeric and hydraulic mounts, the former having better vibration isolation characteristics whereas the latter displays better shock isolation. Elastomeric mounts are widely used for their low initial cost, while hydraulic mounts with inertia track and decoupler are chosen for their good vibration isolation and shock excitation characteristics. However, hydraulic mounts with inertia track and decoupler are not appropriate for small segment and commercial vehicles due their high initial cost. In this thesis, the effect of the addition of a dry friction damper on the performance of elastomeric automobile engine mounts is investigated. Friction dampers are used to attenuate vibration amplitudes in many applications such as gas turbine engines, railway vehicles, space structures and civil buildings. In this study, a friction element is added to the engine mount at its axial direction and its effect is studied. Results show that, the addition of dry friction damping to the original system increases vibration isolation performance significantly at low frequencies; whereas, due to the increased stiffness of the system, at high frequencies dry friction damper has a mitigating effect on performance. In order to overcome this problem, original system parameters are modified. In the modified system a softer mount that increase vibration isolation performance at high frequencies; but, which might cause excessive static deflection due to reduced stiffness of the system is used. On the other hand, addition of dry friction damping prevents excessive static deflections due to the increased stiffness effect and also increases the performance at high frequencies due to the soft mount. Final results showed that vibration isolation performance at low frequencies increases considerably while vibration isolation performance at high frequency is similar and even slightly better than the original system with addition of dry friction damping.M.S. - Master of Scienc
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