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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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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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Aeroacoustic Noise Source Investigation in the Case of the Blade Cascade
Procházka, Pavel P. ; Antoš, Pavel ; Šnábl, Pavel
The blade cascade consisted of five NACA 0010 profiles is examined in the framework of project dealing with nonlinear dynamic phenomenon related to stall flutter of flexible profiles with pitch degree of freedom in subsonic flow. Various angles of attack (AoA) of the cascade generates different (broadband) aeroacoustic noise. The goal of this article is to describe the existence of possible noise sources using two experimental methods – Particle Image velocimetry (PIV) and hot-wire (HW) anemometry.
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Experimental and numerical evaluation of blade cascade instability using travelling wave mode approach
Chindada, Sony ; Šnábl, Pavel ; Pešek, Luděk
The stability of experimental blade cascade with five NACA 0010 profiles was measured and evaluated with travelling wave mode approach with three inner blades oscilating. The measured moment of the middle blade consist of not only the aerodynamic moment from the flow but also structural moment from the blade dynamics. Two days of obtaining the aerodynamic moment from the measured data are herein presented and the results compared. In addition, numercial simulations were done and the results from 2D CFD analysis in ANSYS are compared with the experimental data.
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Turbine blade linkages in reduced modal synthesis model
Pešek, Luděk ; Šnábl, Pavel
The paper is the first stage of our modelling turbine blade linkages in reduced modal synthesis model. The modal synthesis method is used for the dynamic behaviour studies of turbine bladed wheels with frictional inter-blade linkages and in case of the microslips the stiffening effect of the linkages play an important role in wheel dynamics. The paper presents the first results of modal analysis of the wheel when springs were used to substitute the linkages in the modal synthesis model.
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Effect of oscillating blade on tonal noise of blade cascade with five NACA 0010 profiles
Šnábl, Pavel ; Pešek, Luděk ; Radolf, Vojtěch ; Antoš, Pavel ; Procházka, Pavel P. ; Prasad, Chandra Shekhar
The airfoil tonal noise is a well known phenomenon which appears on single airfoils in moderate Reynolds numbers at low angles of attack. It can appear on small aircrafts, fans, wind turbines etc. In thispaper, it was observed on a blade cascade consisting of five NACA 0010 profiles placed in a closed rectangular channel with hard walls when for given conditions, i.e. flow velocity and angle of attack, the cascade tunes to one single frequency. During the middle blade oscillation, the single tone splits up to several tones with lower intensity modulated by the excitation frequency. Thus, the oscillation of one blade suppresses the tonal noise generation in the cascade.
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Torque and Flow Field Optical Measurements of The Single Blade Under Stall Flutter
Procházka, Pavel P. ; Uruba, Václav ; Šnábl, Pavel
The single blade NACA0010 was exposed to the incoming flow inside a channel with rectangular cross-section. The blade was investigated for three different working regimes. The first configuration was for fixed blade. The second regime was for fluttering blade due to flow instabilities and the last one was performed for blade under force excitation. The flow field was measured using Particle Image Velocimetry. A force transducer and a small rotary encoder were used to evaluate the force ratios.
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Study of aeroacoustics noise of high-speed aircraft propellers and mitigation of the cause
Prasad, Chandra Shekhar
The project deals with the study of aeroacoustics noise generated by high-speed turboprop propellers and methods to minimize its causes. A reduced order flow solver with aeroacaustic noise capturing characteristics is developed for fast numerical analysis purpose. The acoustical performance will be also studied using flow visualization using article image velocimetry under rotation and acoustics noise will be measured by microphones for both conventional as well as unconventional propeller designs at diferent rotational speeds.
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