Ladislav Půst

VIBRATION OF CIRCULAR BLADED DISK WITH IMPERFECTIONS

The steady state response of a model of circular bladed disk with imperfection is investigated. Disk imperfection results from additional two groups of damping heads fixed on opposite ends of one diameter. These damping heads are introduced into the computing model as additional point mass, damping and stiffness. Such type of imperfection causes the bifurcation of double eigenfrequencies into pairs of close eigenfrequencies. The effect of imperfection is examined both numerically on three-dimensional nonrotating FE-model and analytically on a simplified split 2DOF model of rotating disk excited by single point harmonic force. Nonlinear friction connection is analyzed and equivalent linear damping coefficient is derived and used in the calculation procedure. It is shown that nonproportional distribution of damping strongly influences the high of resonance peaks. Some examples of response curves illustrate the dynamic properties of stationary and rotating disks with mass-damping-stiffness imperfection.

Influence of Dry Friction Damping on Bladed Disk Vibration

The paper deals with influence of dry friction damping on bladed disk vibration. Firstly the 3D FE solution and analysis of the imperfect blade disk dynamics is presented. Then the dry friction effect is studied analytically on linearized spring-dry-friction model consisting of equivalent linear stiffness and damping coefficients. An influence of nonlinearities and unproportional damping distribution on response curves is discussed as well.

Formation of the low-field peak in magnetization loops of high-Tc superconductors

Abstract The positions of the central (low-field) peak in the magnetization hysteresis loops (MHLs) are analyzed in various high- T c superconducting samples comprising several RBa 2 Cu 3 O 7− δ (RBCO; R=rare earths) single crystals of different thicknesses, a laser-ablated YBa 2 Cu 3 O 7− δ (YBCO) thin film, Ag-sheathed (Pb,Bi) 2 Sr 2 Ba 2 Cu 3 O 10+ δ (Bi-2223) mono- and multifilamentary tapes, and a model sample designed to reproduce a layer of grains [M.R. Koblischka et al., Appl. Phys. Lett. 70 (1997) 514]. The single crystals and the thin film show the peak at zero-field or at negative applied fields on the descending field branch according to the critical state models, the Bi-2223 tapes are found to exhibit the peak anomalously in positive applied fields. In order to better understand the magnetization processes leading to the formation of the central peak in the MHLs, the local field distributions in applied fields close to zero were studied using magneto-optic (MO) flux visualization on the same samples. These flux patterns show how the vortices are rearranged when sweeping through zero-field. A large demagnetizing effect (“perpendicular geometry”) facilitates the penetration of vortices of opposite polarity, especially along structural defects, thus, forcing the central peak towards zero or even to very small positive fields. To explain the anomalous behaviour found in Bi-2223 tapes, effects of granularity have to be considered additionally. Further, we discuss the interaction of the central peak with other “peak effects” observed in MHLs.

Stiffening Effect and Dry-Friction Damping of Bladed Wheel Model with “Tie-Boss” Couplings - Numerical and Experimental Investigation

Bladed wheel model with tie-boss couplings for numerical and experimental investigation of stiffening and friction damping between tie-bosses is introduced. The modal behavior of FE numerical models of the wheel for two contact limit states, i.e. open and bonded contacts, was ascertained. The experimental modal analysis of the wheel both for open and pre-stressed contacts were performed, too. For detail stiffening and damping effect investigation the physical model of three-blade-bundle was elaborated. The experiments were performed for different excitation forces, excitation frequencies and contact pre-stresses. The dynamics of the bundle with respect to different contact states was evaluated from vibration attenuation after short resonant excitation. It was observed that if the macroslips arise in contacts that eigen-frequencies of the bundle are very close to the eigen-frequencies of open contact model bundle and high damping effect is achieved. If the microslips arise in contacts the eigen-frequencies are close to eigen-frequencies of the bond contact model and low damping is achieved. Hence the stiffening effect is high only in the case of microslips. The slip transition is conditioned by the level of adhesion that must be exceeded by excitation force. The FE model of the wheel and blade triple model with dynamic frictional contacts in the tie-boss couplings were developed and calculated results are compared with experiment.

Dynamics of Simple Gearing Model with Flexible Planetary Pins

The influence of flexible pins of planet gears on the dynamic properties of planetary gearing box is investigated by means of a general analysis with some alternative values of parameters. For discovering of basic dynamic behaviour of planetary gear system the simplest planetary gear set with neglected compliance of teeth contacts, fixed axes of central sun wheel and fixed planetary carrier, etc is used One mode of free vibration with eigenfrequency Ω proportional to and zero frequency mode Ω→0 corresponding to the rotation with constant angular velocity were analysed. Forced vibrations, analysed by means of response curves, were oriented on ascertaining of influence of driving aggregate characteristic on frequency spectrum of the whole set.

Formation of the central peak in magnetization loops of type-II superconductors

Based on magneto-optical observations of flux distributions on high-Tc single crystals and thin films aroundB=0 T at various temperatures, possible origins for the formation of the central peak commonly found in magnetization loops of hard type-II superconductors are discussed. The appearence of the central peak is closely related to the the full-penetration field,H*. It is shown that a reduction of the sample thickness influences the value ofH*, thus leading to a narrower field range for the rearrangement of the vortices when passingB=0 T. The role of pinning in the formation of the central peak and the role of the central peak in the formation of the fishtail effect is also discussed.