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Non-linear Vibration of Planar Case of Three-blade Bundle with Dry Friction Contacts
Šnábl, Pavel ; Pešek, Luděk ; Půst, Ladislav
Using dry friction couplings (so called tie-bosses) between the blades in turbine rotors can be used to suppress dangerous resonant vibrations of turbine blades and it has been studied on our institute on various models. Experimental setup of three-blade bundle was built along with bladed wheel. The first set-up of the bundle was with blades turned 45 degrees to the disc plane. In that case the deformation of the blades causes turning of the ends of tie-bosses and edge contact together with multi-directional slip occurs. The experimental set-up was simplified herein so that the blades are parallel to the disc plane and in-line slip occurs. Although single electromagnet was used to excite the system, the first mode of vibration with no slip in tie-bosses just occurred. In case of bladed wheel, all blades have coupling through tie-bosses with neighbouring blades which stiffen them and raise their eigenfrequency. In our case, a mistuning by reducing mass of side blades caused similar behaviour. Steel to steel contact pair was used for the first tests.
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Numerical and Experimental Investigation of the Flow Field in Five Blade Linear Cascade in Subsonic Flow
Šnábl, Pavel ; Chindada, Sony ; Bublík, O. ; Procházka, Pavel P. ; Prasad, Chandra Shekhar
In large steam turbines, last stage blades are very long and must be designed very thin and with no shroud to minimize the centrifugal force which leads to low eigenfrequencies and low structural damping. In this case, aero-elastic damping plays important role on last stage bladed disc’s dynamics. Three major aero-elastic issues found in turbomachinery are forced response, non-synchronous vibrations, and flutter. Flutter is an unstable, self-excited vibration resulting from coupling between the structural vibrations and unsteady aerodynamic forces. It is clear that this unstable behaviour must be avoided and predictions of flutter behaviour need to be performed during design stage of the turbine.\n
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Modelling of pulse-triggered running waves on a blade cascade
Šnábl, Pavel ; Pešek, Luděk
An experimental cascade of five NACA 0010 blades with pitch degrees of freedom is excited by a moment pulse to one of the blades. Depending on the flow conditions, such as wind speed and the angle of attack of the wind on the blades, the vibration of the excited blade is transmitted by the wind to the other blades. This can result in a running wave that diminishes over time or in a flutter running wave. A simplified spring-mass-damper model with inter-blade connections and non-linear van der Pol dampers is tuned to match experimentally obtained stability curves and tested to see if it can qualitatively model the response of the cascade to the pulse excitation.\n
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Modální analýza pryží odpruženého kola Stadler se dvěma variantami listových tlumičů
Šnábl, Pavel ; Pešek, Luděk ; Bula, Vítězslav
Cílem těchto modálních analýz bylo identifikovat vlastní frekvence a tlumení pryžo-odpruženého kola a porovnat jak obě varianty tlumičů mezi sebou sebou, tak s dřívějším měřením gumo-odpruženého kola typu Stadler bez tlumičů. Experimentální modální analýza probíhala v laboratoři ÚT s použitím jednobodového buzení (SIMO) na pryžo-odpruženém kole. Z této dynamické analýzy byly získány vlastní frekvence a tvary kmitání kola v pásmu do 6,4 kHz. Následně byly provedeny i detailnější analýzy v úzkém frekvenčním pásmu zaměřené na chování tlumičů se smykovým elementem v důležitých rezonancích pro vznik valivého a kvílivého hluku.
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Dynamic tensile experiments of hard rubber on instron test machine under finite deformations
Šulc, Petr ; Pešek, Luděk ; Bula, Vítězslav ; Šnábl, Pavel
This paper deals with the first experimental tests of hard rubbers under tensile harmonic loading on the axial-torsional servohydraulic Instron 8852 test machine. The objective of these experiments on cylindrical rubber specimens on this machine is to obtain stress-strain curves of rubber specimens, first considering uniaxial finite longitudinal strains for a single load cycle. Next, the hysteresis loops and their skeleton curves are compared for two selected strain measures as a function of their conjugate stress. It is shown how to obtain the dependence between elongation in the axial direction and constriction in the radial direction, which is used to determine the Cauchy stress in this configuration.
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A new blade cascade for flutter studies
Šnábl, Pavel ; Pešek, Luděk ; Procházka, Pavel P. ; Skála, V.
In this paper, a new blade cascade, developed in the Institute of Thermomechanics, is presented. The geometry of the cascade was modified so now the stall flutter can occur. It has some major construction improvements over the original cascade which now allow more precise and reliable measurements. The stability of the new blade cascade with five NACA 0010 profiles with rotational degree of freedom is assessed by Travelling Wave Mode approach and Aerodynamic Influence Coefficient approach.
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Problematics of aerodynamic damping calculation from measured data of 5-blade cascade
Šnábl, Pavel ; Pešek, Luděk ; Prasad, Chandra Shekhar ; Chindada, Sony
Aerodynamic damping as a function of inter-blade phase angle (IBPA), so called S-curve, is crucial for assessment of aeroelastic stability of blade cascades, e.g. turbines, compressors, etc.\nFor constructing the S-curve, the motion-induced controlled flutter is introduced to the bladesof the cascade. As decribed in [1], two testing methods exist: aerodynamic influence coefficient\n(AIC) approach and travelling wave mode (TWM) approach. In TWM approach, all blades in a row oscillate with the same frequency and amplitude with various IBPAs. The response is\nmeasured only on the reference blade. With this approach, several measurements with different IPBAs are needed to construct the S-curve. On the other hand, AIC uses single oscillating\nblade and principle of linear superimposition of aerodynamic influence responses measured on all blades in a cascade. The result of one single measurement can be used for estimation of\naerodynamic damping for any IBPA. In the past year a new 5-blade cascade with rotating symmetrical NACA 0010 profiles was designed and built. The blades of the cascade were placed further apart and thus we are now able to reach stall flutter. Also, the suspension of the blades and sensors were significantly improved. Now, our goal is to evaluate S-curves using AIC approach for different flow conditions and oscillation frequencies.
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Numerical simulations of aeroelastic instabilities in turbine blade cascade by modified Van der Pol model at running excitation
Pešek, Luděk ; Šnábl, Pavel ; Prasad, Chandra Shekhar ; Delanney, Y.
Apart from rotary test rig for evaluation of structural dynamics of the bladed wheels, the control flutter experiments has been performed on the linear cascade model in the subsonic wind tunnel in the Institute of Thermomechanics, of CAS, in Prague. These experiments are aimed at stability evaluation of the cascade at running waves or at stability limit testing by flow speed changes or by force impulses of blades. The onset of flutter and its spreading in the cascade are observed, too. The linear cascade model consists of five NACA010 blades. All the blades can be separately excited with electromagnetic torque excitation mechanism and all of them are instrumented to measure the aerodynamic moments which can be used to calculate the aerodynamic work. A more details about the linear blade cascade experimental set up can be found in [1-2]. To predict a dynamic behaviour in the blade cascade, we have been dealing with simplified theoretical modelling of the aeroelastic instability in turbine blade cascade [3-5]. Due to the application of the reduced cascade model consisting of simple elements – springs, rigid bodies, linear dampers – and aeroelastic forces introduced by analytical Van der Pol model, it facilitates to study the dangerous states of vibration of such complicated turbine parts [6-9]. This study is aimed at examination of aeroelastic instabilities of 10-blade cascade at running excitation that arises due to the wakes flowing from stator the blades to the rotating blades. They cause forced excitation in the narrow frequency range.
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