20181207 17:59 
Fast estimation of flutter parameters of steam turbine blades using boundary element methods
Prasad, Chandra Shekhar ; Pešek, Luděk
In the present paper the classical flutter parameters in turbomachinery cascade such as aerodynamic damping estimation using fast numerical method is described. The unsteady aerodynamic loading and the aerodynamic damping (AD) coefficient of the vibrating cascades is estimated by 2D boundary elementmethod such as source and doublet panel method (PM). To estimate the flutter parameters aerodynamic influence coefficient traveling wave mode (TWM)method is adopted. The experimental aerodynamic damping is compared against simulated results. The result demonstrate that potential flow based boundary elements method can estimate the AD with desired accuracy and less computational time.
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20181207 17:59 
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20181207 17:59 
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20181207 17:59 
Experimental investigation of fault tolerant converter connected to unbalanced grid
Valouch, Viktor ; Šimek, Petr ; Škramlík, Jiří
A model of the threephase voltage converter for simulation of its modes of operation in standard as wellas fault states is presented. The presented model can be used to simulate steadystates and also transients at dc and ac side of the gridconnected converter. Based on simulation, an algorithm was designed to identify and localize a fault in some phase of the threephase converter. Rules how to modify hardware, and control and modulation strategies to hold the converter in operation after the fault are presented and tested. the algorithm was experimentally verified.
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20181207 17:59 
Computational time reversal method based on finite element method: influence of temperature
Mračko, Michal ; Kolman, Radek ; Kober, Jan ; Převorovský, Zdeněk ; Plešek, Jiří
Time reversal method is used to focus elastic waves to the location of the original source and reconstruct its source time function. The procedure consists of two steps: Frontal task and Reversal task. In the Frontal task, the medium is excited by an arbitrary source, elastic waves propagate through a body of interest and the dynamic response at few points on boundary is recorded. In the second step (say the Reversal task) the response signal is reversed in time and transmitted back into the medium resulting in focusing in the original source location. It is of practical importance to investigate a case when the medium changes its properties between the frontal and reversal wave propagation steps. An example is a problem of transferring experimentally recorded data to a computational model, where discrepancies in geometry, elastic properties and boundary conditions are expected. Our motivation is to develop a methodology for computation of time reversal problems in commercial finite element software. The results prove that this method is extremely sensitive to the change of temperature and one have to pay special attention to tuning of elastic parameters relevant to the\nexperiment.
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20181207 17:59 
Finite element modeling of the signal propagation in a thin tube and comparison with experimental data
Kruisová, Alena ; Kolman, Radek ; Trnka, Jan ; Mračko, Michal
In finite element modeling of wave propagation problems, both the spatial and temporal discretization lead to dispersion errors. It means that the phase velocity of propagated wave is related to its frequency. In framework of temporalspatial dispersion analysis, the time step size for implicit time integration method based on the Newmark method is proposed for linear and quadratic serendipity plane finite elements. In this paper, we verify the theoretical dispersion analysis by elastic wave propagation in thin tube, where experimental results are known. Such time step size was used in finite element modeling of the stress wave propagating in this thin steel tube, the results of simulations were compared with experimental results.
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20181002 14:38 
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20181002 14:38 
Development, assessment and verification of finite element procedures for contact problems
Gabriel, Dušan ; Kopačka, Ján ; Masák, Jan ; Plešek, Jiří
A frictionless threedimensional contact algorithm based on the prediscretization penalty formulation was proposed. The algorithm proved to be robust, accurate and symmetry preserving—no master/slave surfaces have been introduced. It was implemented in the finite element code PMD (Package for Machine Design) for the solution of complex engineering problems. The capability of the algorithm was demonstrated in creep analysis of Tpiece of the steam distributor of the CHEMOPETROL heating plant T200. Material properties were described by the probabilistic exponential model with damage. The purpose of the analysis was to compare the original version and the proposed design modification applied to one of Tpieces of the distributor when the proposed contact algorithm was employed to simulate the interaction between the collar and the pipe.
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20181002 14:38 
Vibration stability of a vertical flywheel lifted by permanent magnetic rings
Zapoměl, Jaroslav ; Ferfecki, P. ; Sudhakar, A. ; Kozánek, Jan
Lifting the vertical flywheels by permanent magnets is used to reduce energy losses in the support elements. Unstable equilibrium position of pairs of magnetic rings, material damping of the shaft, and nonlinear properties of the support elements can arrive at the flywheel unstable oscillations. Therefore, evaluation of the vibration stability is the essential part of the design process of flywheel devices. In the developed computational procedure the flywheel is represented by a beamlike body, to which an absolutely rigid disc (the actual flywheel) is attached. The determination of the force action between the magnetic rings is based on mutual interaction between two magnetic dipoles. The steady state response of the flywheel on the unbalance excitation is calculated by means of the trigonometric collocation method and its stability is evaluated by application of the Floquet theorem. The developed computational procedures and learning more on behaviour of flywheels lifted by magnetic rings are the main contributions of the carried out research.\n
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20181002 14:38 
Selfexcited oscillation properties of the profile NACA0015 in subsonic flow
Zolotarev, Igor ; Vlček, Václav ; Kozánek, Jan
Selfexcited oscillations of the profile NACA0015 in subsonic flow were realized in the wind tunnel. The plunge support had the same elasticity for all experiments, the pitch support had seven different elasticity parameters. During our experiments different types of selfexcited vibration (flutter, stall flutter) were observed. Range of Mach numbers was M = 0.08–0.45, corresponding range of Reynolds numbers is Re = (1–5)·105, when the chord length is used as the characteristic dimension of the profile.\n
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