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The stability investigation of vibrations of flexibly supported rigid rotors damped by hybrid magnetorheological damping elements
Zapoměl, Jaroslav ; Kozánek, Jan ; Ferfecki, Petr
A usual technological solution how to reduce the time varying forces transmitted between the rotor and its casing consists in application of a flexible suspension and in adding the damping devices to the constraint elements. To achieve their optimum performance their damping effect must be controllable. For this purpose a concept of a hybrid damping device working on the principle of squeezing the layers of normal and magnetorheological oils have been developed. Here in this paper, there is investigated influence of the proposed damping element on stability of the rotor vibrations induced by its imbalance during the steady state operating regimes. The stability is evaluated by two approaches: by utilization of a transition matrix and the Floquet theorem and by the evolutive method. The former determines the stability by means of magnitude of the largest eigenvalue of the transition matrix set up over the span of time of one period, the later by time evaluation of eigenvalues of the system linearized in a small neighbourhood of the steady state phase trajectory of the rotor during one period and calculation of the corresponding damping ratio or logarithmic decrement.
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Coarse-grained particles conveying along a pipe bed
Vlasák, Pavel ; Chára, Zdeněk ; Konfršt, Jiří ; Kysela, Bohuš
The paper describes the results of a flow visualisation of model coarse-grained fully stratified particle-water mixtures. Investigation was focussed on study of the particle behaviour and carrier liquid and conveyed particles local velocities. Glass balls and washed graded pebble gravel of mean diameter of 6 mm were conveyed by water in a horizontal smooth pipe loop with a transparent pipe viewing section of inner diameter 40 mm. Particle movement along the pipe invert and particulate stationary bed, created by two layers of spherical particles of the same size as the conveyed particles, was studied. The effect of stationary bed on local velocity values of the carrier liquid and conveyed particles were determined. It was conducted that the maximum liquid velocity is significantly shifted from pipe centre to a higher position in the flow pattern with stationary bed.
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Engineering Mechanics 2013
Zolotarev, Igor
The objective of the 19th conference is to provide a forum for researchers to discuss the state-of-the-art in Mechanics of Solids, Fluids and Thermomechanics, this particularly in connection with problems under investigation in the Czech Republic. Problems of Engineering Mechanics related to the contemplated topics can be discussed, and their solutions presented, during the sessions of the Conference, held at the Hotel of the ŽĎAS, a.s., situated in the beautiful countryside of the Žďár Highlands.
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Engineering Mechanics 2013 - book of extented abstracts
Zolotarev, Igor
The objective of the 19th conference is to provide a forum for researchers to discuss the state-of-the-art in Mechanics of Solids, Fluids and Thermomechanics, this particularly in connection with problems under investigation in the Czech Republic. Problems of Engineering Mechanics related to the contemplated topics can be discussed, and their solutions presented, during the sessions of the Conference, held at the Hotel of the ŽĎAS, a.s., situated in the beautiful countryside of the Žďár Highlands.
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Parallel numerical simulation of airflow past an oscillating NACA0015 airfoil
Řidký, Václav ; Šidlof, Petr ; Vlček, Václav
This paper focuses on 3D and 2D parallel computation of pressure and velocity fields around an elastically supported airfoil self-oscillating due to interaction with the airflow The results of numerical simulations are compared with data measured in a wind tunnel, where physical model of a NACA0015 airfoil was mounted and tuned to exhibit the flutter instability. The experimental results were obtained previously in the Institute of Thermomechanics by interferometric measurements in a subsonic wind tunnel in Nový Knín. For the numerical solution is implemented in OpenFOAM, an open-source software package based on finite volume method. In the numerical solution is prescribed displacement of the airfoil, which corresponds to the experiment.
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Unsteady aerodynamic forces measured on a fluttering profil
Vlček, Václav ; Zolotarev, Igor ; Kozánek, Jan
The evaluations of optical measurements of the flow field near the fluttering profile NACA0015 with two-degrees of freedom are presented. Mach number of the airflow was M = 0.21 and M = 0.45, the boundaries of the flutter occurrence. Aerodynamic forces acting on the profile were evaluated in the drag and lift components which enabled to obtain independently the forces corresponding to the upper and lower surfaces of the profile. Using the mentioned decomposition, the new information about mechanism of flutter properties was obtained. The force effects on the upper and lower surfaces are in opposite phases and they are partially eliminated as a result of the circulation around the profile. The cycle changes of this forces cause the permanent energy contribution from the airflow to the vibrating system.
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Model of turbine blades bundles
Půst, Ladislav ; Pešek, Luděk
A detail analysis of dynamical properties of five-blades-bundle mathematical model was carried out with the aim to investigate how the damping elements made of special rubber inserted into slots between blades heads influence the response curves at different distribution of exciting harmonic forces. Viscous-elastic linear Voigt–Kelvin model was used for modeling the rheological properties of damping elements. Constant values of stiffness and damping parameters were supposed at analysis, but the frequency dependent parameters are mentioned as well. The effect of complicated form of higher modes of vibration on blades bundle amplitude resonance level is shown. The importance of orthogonality of excitation forces distribution to the other eigenmodes of blades bundle is discovered at isolating selected resonance.
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Finite element modelling of sound pressure around the human head during phonation
Švancara, P. ; Tomeček, V. ; Horáček, Jaromír ; Švec, J. G.
The study presents finite element (FE) model of sound propagation through the vocal tract and around the human head during speech production. Similar experimental studies are not easily realisable. The FE model of the acoustic spaces corresponding to the human vocal tract for Czech vowel [a:] and acoustic space around the human head was created from computer tomography (CT) images. Modal and transient analyses (excitation by a short pulse) are used for analysis of resonant characteristics of the FE model. The production of vowel is then simulated using transient analysis of the FE models excited by Liljencrants-Fant’s (LF) glottal signal model. Formant frequencies detected from computed spectra are in good agreement with results of modal analysis and with literature. The results of numerical simulation enable evaluating of the transfer functions between a reference point and any point in the space around the head.
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In vivo measurements of air pressure, vocal folds vibration and acoustic characteristics of phonation into a straw and a resonance tube used in vocal exercising.
Radolf, Vojtěch ; Laukkanen, A. M. ; Horáček, Jaromír ; Veselý, Jan ; Liu, D.
The study investigates the differences between three most widely used methods in voice training and therapy: Phonation into a glass resonance tube (1) the outer end in the air, (2) the outer end submerged 2-10 cm below water surface in a bowl (‘water resistance therapy’ with bubbling effect), and (3) phonation into a very thin straw. One female speech trainer served as subject. Acoustic samples, electroglottographic signals and both mean and dynamic airpressures in the mouth cavity were registered for repetitions of [pu:pu], and for phonation into the tubes, while the outer end was randomly shuttered, in order to get an estimate of subglottic pressure. Both phonation threshold and ordinary, most comfortable phonation were recorded.
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