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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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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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Flutter methodology using reduced order aeroelastic model
Prasad, Chandra Shekhar ; Pešek, Luděk ; Šnábl, Pavel
The present research project is focused on development of fast and efficient numerical method based on reduced order aeroelastic method (ROAM) for modeling and analysis of \nclassical flutter in the low pressure (LP) stage steam turbine blade. Stability diagram such as aerodynamic damping (AD) of aeroelasticity of cascade of 3D turbine blades used as a prime \ncharacterization factor. In the calculation of the S-curve/AD, the problem of classical flutter formation associated with running waves is considered here. Running waves is simulated by \nthe inter-blade phase shift of the blades in the cascade. Panel method based boundary element flow solver is employed for calculation of unsteady aerodynamic forces. This method is good \ncompromise of speed and accuracy for the estimation of the stability of the blades on a classical flutter. One way loose coupling technique between PM based flow solver and the structural \nmodel. For the structural part modal model synthesis (MMS) method is adopted in the ROAM solver. MMS is adopted to further reduced the computational cost. The ROAM simulated AD \nand pressure distribution over blade is compared high fidelity CFD data on real blade geometry provided by Doosan Skoda Power s.r.o. Furthermore, the ROAM results are also compared \nwith experimentally obtained results on two different linear cascade. The ROAM model shows good agreement with linear cascade results, however, there is noticeable discrepancy with real blade CFD results.
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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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A hybrid boundary element based aeroelastic model for flexible wing.
Prasad, Chandra Shekhar ; Pešek, Luděk
The paper describes development of medium delity aeroelastic numerical model for fast aeroelastic analysis of the flexible aeronautical structures e.g. wings, rotor blades etc. The numerical model is developed particularly for the incompressible low subsonic flow regime application. For the flow field modeling boundary element based hybrid panel method flow solver with viscous-inviscid coupling strategy have been successfully developed and implemented here.The unsteady flow field is modeled using hybrid/modi ed panel method where, integral boundary layer theory (vis-cous part), surface panel method (potential flow inviscid part) and vortex particle method (separated shear layer modeling) are coupled together. The proposed model can simulate both attached and separated flow fields. The estimated aerodynamic lift coefficients and the pressure coefficient are compared with experimental results for static and dynamic stall flow conditions. Furthermore, the results from new aeroelastic model will be compared with classical CFD-CSD based aeroelastic models for efficiency and accuracy check. The proposed methodology for the aeroelastic analysis of long exible aeronautical structure will provide researchers and engineers a fast, cost effective and efficient tool for aeroelastic analysis for different design at preliminary design stage where large numbers of design iteration are required within short time frame.
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Classical flutter analysis of low pressure steam turbine blade cascade using 3D boundary element method
Prasad, Chandra Shekhar ; Pešek, Luděk
In this paper study of aeroelastic stability in steam turbine rotor is carried out using boundary element method. A mesh free fluid\nsolver is developed for fast estimation of unsteady aerodynamic loading and to estimate the aerodynamic damping in 3D blade cascade. The aerodynamic damping is estimated in traveling wave mode. The unsteady incompressible flow field is modeled using 3D surface Panel method. The proposed methodology successfully estimates aerodynamic damping with acceptable accuracy the for the aeroelastic (classical \n flutter) analysis of 3D blade cascade. The simulated results are compared with experimental data. The simulated aerodynamic damping shows good agreement with\nexperimental results. The present methodology shows significant reduction in computational time over computational fluid dynamic solvers.
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