Influence of Intrarow Interaction on the Aerodynamic Damping of an Axial Turbine Stage

Müller, Tobias; Vogt, Damian M.; Vogel, Klemens; Phillipsen, Bent

doi:10.1115/gt2018-76777

Cited by 5 publications

(7 citation statements)

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“…Firstly, it is very common to assume the aerodynamic forces as linear in the structural displacement amplitude û. In the amplitude range relevant for high-cycle fatigue, this assumption has been supported by experimental [11,12] and numerical [13][14][15] evidence. Secondly, it is often assumed that 𝜔 and 𝝍 correspond to a certain normal vibration mode of the structure in vacuum, obtained under linearized contact boundary conditions in joints (i.e.…”

Section: Introductionmentioning

confidence: 70%

Development of a Fully-Coupled Harmonic Balance Method and a Refined Energy Method for the Computation of Flutter-Induced Limit Cycle Oscillations of Bladed Disks with Nonlinear Friction Contacts

Berthold¹,

Gross²,

Frey³

et al. 2021

Preprint

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Flutter stability is a dominant design constraint of modern gas and steam turbines. To further increase the feasible design space, flutter-tolerant designs are currently explored, which may undergo Limit Cycle Oscillations (LCOs) of acceptable, yet not vanishing, level. Bounded self-excited oscillations are a priori a nonlinear phenomenon, and can thus only be explained by nonlinear interactions such as dry stick-slip friction in mechanical joints. The currently available simulation methods for blade flutter account for nonlinear interactions, at most, in only one domain, the structure or the fluid, and assume the behavior in the other domain as linear. In this work, we develop a fully-coupled nonlinear frequency domain method which is capable of resolving nonlinear flow and structural effects. We demonstrate the computational performance of this method for a state-of-the-art aeroelastic model of a shrouded turbine blade row. Besides simulating limit cycles, we predict, for the first time, the phenomenon of nonlinear instability, i.e. , a situation where the equilibrium point is locally stable, but for sufficiently strong perturbation (caused e.g. by an impact), the dry frictional dissipation cannot bound the flutter vibrations. This implies that linearized theory does not necessary lead to a conservative design of turbine blades. We show that this phenomenon is due to the nonlinear contact interactions at the tip shrouds, which cause a change of the vibrational deflection shape and frequency, which in turn leads to a loss of aeroelastic stability. Finally, we extend the well-known energy method to capture these effects, and conclude that it provides a good approximation and is useful for initializing the fully-coupled solver.

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Section: Introductionmentioning

confidence: 70%

Development of a Fully-Coupled Harmonic Balance Method and a Refined Energy Method for the Computation of Flutter-Induced Limit Cycle Oscillations of Bladed Disks with Nonlinear Friction Contacts

Berthold¹,

Gross²,

Frey³

et al. 2021

Preprint

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“…According to equation ( 1), R is relating the reflected to the impinging perturbation amplitude at the spatial placement of the respective boundary condition, while Raaa is relating the corresponding mean perturbation amplitudes in the entire far-field. Since a linear superposition of different transient flow phenomena occurring at the same frequency was proven to be of sufficient validity in Mueller et al, 16 an extraction of the reflected ('R') harmonic pressure distribution is achieved by subtracting the vibrationinduced emitted ('E') harmonic pressure distribution in the frequency domain from the resultant superimposed ('S') harmonic pressure distribution, as shown in equation (1).…”

Section: Evaluation Methodologymentioning

confidence: 99%

“…In all cases, the reflected harmonic pressure distribution is characterized by a pronounced acoustic interference caused by subsequent physical reflections of the induced spurious perturbations when impinging on the rotor blade row. 16 For this reason, the local reflection coefficient R derived at the spatial placement of the outlet boundary condition might be falsified considerably depending on the actual state of local interference. The assessment of the investigated outlet boundary conditions is therefore mainly based on the mean reflection coefficient Raaa, taking into account the perturbation amplitude level throughout the entire outlet far-field.…”

Section: Investigation Of Boundary Conditionsmentioning

confidence: 99%

“…A transonic axial turbine stage of a medium-size turbocharger featuring 33 rotor blades at a tip diameter of approximately 400 mm is applied as test case in this study, 15,16 as shown in Figure 2. The investigated operating point is characterized by subsonic flow conditions such that both the initial flow disturbances, as well as the ones induced by spurious reflections at the far-field boundary conditions, feature the ability to propagate in upstream and downstream direction.…”

Section: Test Casementioning

confidence: 99%

“…A quasi three-dimensional (Q3D) isolated rotor model with prescribed blade vibration according to the first blade bending mode is applied for assessing the reflective behavior of different far-field boundary conditions, as shown in Figure 2. The corresponding slice model represents the mean flow path of a fluid particle at a spanwise position of 85% span for the given operational conditions and is derived by averaging the velocity distribution obtained from a previous 3D steady-state stage calculation in circumferential direction, as outlined in Mueller et al 16 This modeling strategy ensures the following relevant prerequisites:…”

Section: Numerical Modelmentioning

confidence: 99%

See 2 more Smart Citations

On the far-field boundary condition treatment in the framework of aeromechanical computations using ANSYS CFX

Mueller

Vogt

Fischer

et al. 2020

Proceedings of the Institution of Mechanical Engineers, Part A:

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This numerical study aims at predicting the reflective behavior of different conventional inlet and outlet far-field boundary conditions as well as available non-reflecting boundary conditions (NRBC) implemented in the commercial CFD solver ANSYS CFX. An isolated rotor model of an axial turbine stage with prescribed blade displacement is applied as test case to consider a representative application case, while at the same time provoke an unsteady flow field featuring pronounced flow perturbations in the far-field. Since the reflective behavior of the implemented boundary conditions was found inadequate in the given application case, a zonal treatment of the inlet and outlet far-field, based on a modification of the governing Navier-Stokes equations, is investigated. The applied approach has proven its capability to suppress spurious reflections reliably, while at the same time ensures a preservation of the reference flow conditions within the required domain extensions. The results of a case study considering calculation domains of different spatial extent and different treatments of their respective far-fields suggest variations in the steady flow aerodynamics to be of moderate influence on the predicted aerodynamic damping, while spurious reflections were found to falsify the unsteady aerodynamics considerably.

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Analysis of Friction-Saturated Flutter Vibrations With a Fully-Coupled Frequency Domain Method

Berthold

Groß

Frey

et al. 2020

Volume 10A: Structures and Dynamics

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Flutter stability is a dominant design constraint of modern gas and steam turbines. Thus, flutter-tolerant designs are currently explored, where the resulting vibrations remain within acceptable bounds. In particular, friction damping has the potential to yield Limit Cycle Oscillations (LCOs) in the presence of a flutter instability. To predict such LCOs, it is the current practice to model the aerodynamic forces in terms of aerodynamic influence coefficients, derived for some normal modes of the linearized structural model and fixed oscillation frequency. However, this approach neglects that both the nonlinear contact interactions and the aerodynamic stiffness cause a change in the deflection shape and the frequency of the LCO. This, in turn, may have a substantial effect on the aerodynamic damping. The goal of this paper is to assess the technical importance of these neglected interactions. To this end, a state-of-the-art aero-elastic model of a low pressure turbine blade row is considered, undergoing nonlinear frictional contact interactions in the tip shroud interfaces. The LCOs are computed with a fully-coupled harmonic balance method, which iteratively computes the Fourier coefficients of structural deformation and conservative flow variables, as well as the a priori unknown frequency. The coupled algorithm was tested for various combinations of harmonics in both domains and found to provide excellent computational robustness and efficiency. Moreover, a refinement of the conventional energy method is developed and assessed, which accounts for both the nonlinear contact boundary conditions and the linearized aerodynamic influence. It is found that the conventional energy method may not predict a limit cycle oscillation at all while the novel approach presented here can. Furthermore the refined energy method provides deep understanding of the nonlinear aero-elastic interactions.

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Influence of Intrarow Interaction on the Aerodynamic Damping of an Axial Turbine Stage

Cited by 5 publications

References 0 publications

Development of a Fully-Coupled Harmonic Balance Method and a Refined Energy Method for the Computation of Flutter-Induced Limit Cycle Oscillations of Bladed Disks with Nonlinear Friction Contacts

Development of a Fully-Coupled Harmonic Balance Method and a Refined Energy Method for the Computation of Flutter-Induced Limit Cycle Oscillations of Bladed Disks with Nonlinear Friction Contacts

On the far-field boundary condition treatment in the framework of aeromechanical computations using ANSYS CFX

Analysis of Friction-Saturated Flutter Vibrations With a Fully-Coupled Frequency Domain Method

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