| 12391. |
- Khodabandeloo, Babak
(författare)
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Acoustical experiments on the non-linear and non-equilibrium behavior of rocks
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The study of dynamic behavior of non-perfect materials, such as rocks, is a rich topic for developing sensitive material evaluation and damage detection techniques. It has extensive applications in a variety of fields such as Physics, Geology, and Aerospace, Mechanical, Civil, Petroleum, and Material Engineering. It is experimentally known that as the level of damage in any initially 'un-damaged' materials increases, their nonlinear elasticity increases considerably and their dynamic behavior becomes more similar to heterogeneous and micro-inhomogeneous materials such as rocks. Despite the diversity of inherently non-perfect materials, they share some common features, among them are fast nonlinear dynamics, slow dynamics and conditioning.In this thesis, the fast and slow dynamics, recovery and relaxation of geomaterials subjected to external influences such as ultrasound, heat and light are investigated experimentally. The effect of light on rock is monitored using two different material state indicator tests: the measurement of deflection of a granite beam; and, the changes in sound speed. Young's modulus variations for small temperature changes in granite, steel, brass, and graphite are measured, providing us with a quantitative base for separation of softening due to the temperature and to the ultrasonic field respectively.The resonance frequency dependence on amplitude of acoustic wave at temperatures from 15-60 Celsius was investigated. The effects of each of them (the temperature and the acoustic intensity) have been known separately, but here were conducted experiments to see their combined effects. The sound speed was monitored through the resonance frequency by a series of ultrasonic frequency sweeps with successively increasing amplitudes. There was in each temperature set surprisingly one - and only one - temperature at which the sound velocity increased with the ultrasound level. At all other temperatures the sound velocity decreased.In another test, the effect of ultrasonic activation of marble in a constrained boundary condition is studied experimentally for the first time. The observed effects indicate simultaneous expansion and softening of the rock when subjected to acoustic waves
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| 12392. |
- Khodadad, Davood, 1985-
(författare)
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Creating a supportive and effective learning environment for engineering students : Pedagogical strategies, engagement, and enhanced outcomes
- 2023
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Ingår i: International Journal of Engineering Pedagogy. - : International Association of Online Engineering. - 2192-4880. ; 13:8, s. 33-50
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Tidskriftsartikel (refereegranskat)abstract
- Engineering education requires a strong emphasis on problem-solving, critical thinking, and practical application of knowledge. To achieve the highest quality of teaching, educators must create a trusting environment that allows students to feel comfortable asking questions and performing to the best of their abilities. This paper outlines the teaching philosophy and prac-tices of an engineering lecturer who has adapted his pedagogical approach across several universities in Sweden and abroad. The author emphasizes the importance of being flexible and responsive to student needs, offering early and constructive feedback, and providing students a safe and supportive learning environment with opportunities to develop program-ming skills. The paper also includes comments from students that reflect the author’s effec-tiveness as an educator in creating a supportive and challenging learning environment for engineering students.
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| 12393. |
- Khodadad, Davood, 1985-
(författare)
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Digital holography for temperature investigations in space and transparent media
- 2023
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE.
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Konferensbidrag (refereegranskat)abstract
- The paper presents a method for investigating temperature fields in space and transparent media using off-axis digital holographic interferometry. This interferometric technique is particularly well-suited for measuring dynamically changing temperature fields due to its capability to automatically evaluate temperature from a single interferogram. The experimental validation of this technique involved measuring temperature variations within a burning candle's flame. To establish a reference point, an initial hologram was captured without a burning candle, followed by subsequent holograms during the burning process. This allowed the observation of changing refractive index gradient states in the surrounding air. The reconstruction of the complex amplitude facilitated the calculation of the distribution of phase changes. By establishing a relationship between phase changes, thermal coefficients of the air's refractive index, and temperature fluctuations, temperature measurements were achieved at these distinct states. The accuracy of the measurement was estimated to be less than 1 °C in our experimental setup, showing the high precision achievable with this technique. In summary, the paper offers analysis of digital holographic interferometry as a tool for temperature measurements in transparent media. Its potential applications extend to combustion studies, space exploration, atmospheric research, and various other scientific disciplines.
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| 12394. |
- Khodadad, Davood, 1985-, et al.
(författare)
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Temperature Sensing in Space and Transparent Media : Advancements in Off-Axis Digital Holography and the Temperature Coefficient of Refractive Index
- 2023
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Ingår i: Applied Sciences. - : MDPI. - 2076-3417. ; 13:14
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Tidskriftsartikel (refereegranskat)abstract
- An off-axis digital holographic interferometry technique integrated with a Mach–Zehnder interferometer based setup is demonstrated for measuring the temperature and temperature profile of a transparent medium. This technique offers several advantages: it does not require precise optomechanical adjustments or accurate definition of the frequency carrier mask, making it simple and cost-effective. Additionally, high-quality optics are not necessary. The methodology relies on measuring the phase difference between two digitally reconstructed complex wave fields and utilizing the temperature coefficient of the refractive index. In this way, we presented an equation of the temperature as a function of phase changes and the temperature coefficient of refractive index. This approach simplifies the calculation process and avoids the burden of complicated mathematical inversions, such as the inverse Abel transformation. It also eliminates the need for additional work with the Lorentz–Lorentz equation and Gladstone–Dale relation and can be extend for 3D measurements.
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| 12395. |
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| 12396. |
- Khojasteh, D., et al.
(författare)
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Numerical analysis of shipping water impacting a step structure
- 2020
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Ingår i: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 209
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Tidskriftsartikel (refereegranskat)abstract
- Shipping water, the flow washing over and impacting the upper decks of ships and offshore structures, occurs frequently during their service life and often causes structural problems. For engineers to design safe floating structures subjected to shipping water it is essential to gain an in-depth understanding of its depth and flow field, and the resulting impact forces. In this work, Computational Fluid Dynamics (CFD) is applied to understand the physics of shipping water washing over a stepped platform. We find that the most accurate solutions are obtained with the k−ε turbulence closure. The hydrodynamic load generated by the shipping water is found to strongly depends on the kinematic energy of the water hitting the step. It is shown that with smaller values of the freeboard a more dynamic flow ensues, with a stronger vortex and larger velocity gradient resulting in deeper shipping water and a larger impact force.
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| 12397. |
- Khorsand Vakilzadeh, Majid, 1984, et al.
(författare)
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An improved modal approach for model reduction based on input-output relation
- 2012
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Ingår i: International Conference on Noise and Vibration Engineering (ISMA) / International Conference on Uncertainty in Structural Dynamics (USD). KU Leuven, Dept Mech Engn, Leuven, BELGIUM. SEP 17-19, 2012. - 9789073802896 ; 5, s. 3451-3459
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Konferensbidrag (refereegranskat)abstract
- An approximated model of a large scale dynamical system captures its main features by a proper selection of its dominant modes. In this paper, a new metric is presented for modal dominancy analysis. This metric is an explicit formulation for evaluating the participation of each mode, independent from the other modes, into the output. Only eigensolutions with a large contribution to this metric, the dominant eigensolutions, will be retained in the approximating lower order model. The modal truncation technique which includes the proposed dominancy analysis not only has a metric for error estimation, but is able to automatically detect and eliminate unobservable and uncontrollable modes. An example from finite element analysis of an aluminum plate illustrates the approach and shows that the proposed method is very efficient in respect to computational time.
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| 12398. |
- Khorsand Vakilzadeh, Majid, 1984, et al.
(författare)
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Experiment design for improved frequency domain subspace system identification of continuous-time systems
- 2015
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Ingår i: IFAC-PapersOnLine. - : Elsevier BV. - 2405-8963. ; 48:28, s. 886-891
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Konferensbidrag (refereegranskat)abstract
- A widely used approach for identification of linear, time-invariant, MIMO (multi-input/multi output) systems from continuous-time frequency response data is to solve it in discrete-time domain using subspace based identification algorithm incorporated with a bilinear transformation. However, the bilinear transformation maps the distribution of the frequency lines from continuous-time domain to discrete-time domain in a non-linear fashion which may make identification algorithm to be ill-conditioned. In this paper we propose a solution to get around this problem by designing a dedicated frequency sampling strategy. Promising results are obtained when the algorithm is applied to synthetic data from a 6DOF mass-spring model.
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| 12399. |
- Khorsand Vakilzadeh, Majid, 1984, et al.
(författare)
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Manifold Metropolis adjusted Langevin algorithm for high-dimensional Bayesian FE
- 2014
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Ingår i: Proceedings of the International Conference on Structural Dynamic , EURODYN. - 2311-9020. - 9789727521654 ; 2014-January, s. 3029-3036
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Konferensbidrag (refereegranskat)abstract
- Bayesian finite element model updating provides a rigorous framework to take various sources of uncertainty, such as uncertainties in observation, modeling and prior knowledge, into account when characterizing uncertainty in model parameters, through updates of their joint probability density function. The Markov chain Monte Carlo methods are currently the most popular simulation techniques for producing samples from the posterior probability density functions of the uncertain parameters. However, the effectiveness of these approaches is adversely affected by the dimension and correlation structure of the model parameter space. This paper presents the application of a manifold based Metropolis adjusted Langevin algorithm forsolving high-dimensional model updating problems in structural dynamics. It exploits the Riemannian geometry of the model parameters to help construct proposal densities which closely approximate the target density. Thus, the Markov chain explores the target density faster. Practical issues for applicability of the proposed algorithm for the Bayesian FE model updating problem are illustrated using simulated data for a six degrees of freedom mass-spring model with up to 16 parameters to be calibrated.
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| 12400. |
- Khorsand Vakilzadeh, Majid, 1984, et al.
(författare)
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Sequential gauss-newton MCMC algorithm for high-dimensional bayesian model updating
- 2017
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Ingår i: Conference Proceedings of the Society for Experimental Mechanics Series. - Cham : Springer International Publishing. - 2191-5644 .- 2191-5652. ; 3 Part F2, s. 303-314
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Konferensbidrag (refereegranskat)abstract
- Bayesian model updating provides a rigorous framework to account for uncertainty induced by lack of knowledge about engineering systems in their respective mathematical models through updates of the joint probability density function (PDF), the so-called posterior PDF, of the unknown model parameters. The Markov chain Monte Carlo (MCMC) methods are currently the most popular approaches for generating samples from the posterior PDF. However, these methods often found wanting when sampling from difficult distributions (e.g., high-dimensional PDFs, PDFs with flat manifolds, multimodal PDFs, and very peaked PDFs). This paper introduces a new multi-level sampling approach for Bayesian model updating, called Sequential Gauss-Newton algorithm, which is inspired by the Transitional Markov chain Monte Carlo (TMCMC) algorithm. The Sequential Gauss-Newton algorithm improves two aspects of TMCMC to make an efficient and effective MCMC algorithm for drawing samples from difficult posterior PDFs. First, the statistical efficiency of the algorithm is enhanced by use of the systematic resampling scheme. Second, a new MCMC algorithm, called Gauss-Newton MCMC algorithm, is proposed which is essentially an M-H algorithm with a Gaussian proposal PDF tailored to the posterior PDF using the gradient and Hessian information of the negative log posterior. The effectiveness of the proposed algorithm for solving the Bayesian model updating problem is illustrated using three examples with irregularly shaped posterior PDFs.
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