C. Bréard

An Integrated Time-Domain Aeroelasticity Model for the Prediction of Fan Forced Response due to Inlet Distortion

The forced response of a low aspect-ratio transonic fan due to different inlet distortions was predicted using an integrated time-domain aeroelasticity model. A time-accurate, nonlinear viscous, unsteady flow representation was coupled to a linear modal model obtained from a standard finite element formulation. The predictions were checked against the results obtained from a previous experimental program known as Augmented Damping of Low-aspect-ratio Fans (ADLARF). Unsteady blade surface pressures, due to inlet distortions created by screens mounted in the intake inlet duct, were measured along a streamline at 85 percent blade span. Three resonant conditions, namely 1F/3EO, IT & 2F/8EO and 2S/8EO, were considered. Both the amplitude and the phase of the unsteady pressure fluctuations were predicted with and without the blade flexibility. The actual blade displacements and the amount of aerodynamic damping were also computed for the former case. A whole-assembly mesh with about 2,000,000 points was used in some of the computations. Although there were some uncertainties about the aerodynamic boundary conditions, the overall agreement between the experimental and predicted results was found to be reasonably good. The inclusion of the blade motion was shown to have an effect on the unsteady pressure distribution, especially for the 2F/1T case. It was concluded that a full representation of the blade forced response phenomenon should include this feature.

Aeroelasticity analysis of air-riding seals for aero-engine applications

This paper presents the results of a feasibility study on air-riding seal aeroelasticity for large-diameter aero-engines. A literature survey of previous seal studies revealed a significant amount of experimental work but numerical modeling using CFD techniques was relatively scarce. Indeed, most existing theoretical studies either deal with the structural behavior, or use simplified flow modeling. The aeroelasticity stability of a simplified air-riding seal geometry, devised for this particular feasibility study, was analyzed in three dimensions for typical engine operating conditions. Both the unsteady flow and structural vibration aspects were considered in the investigation. The boundary conditions and the seal gap were varied to explore the capabilities and limitations of a state-of-the-art unsteady flow and aeroelasticity code. The methodology was based on integrating the fluid and structural domains in a time-accurate fashion by exchanging boundary condition information at each time step. The predicted characteristics, namely lift and flow leakage as a function of pressure and seal gap, were found to be in agreement with the expected behavior. Operating seal gaps were determined from the actual time histories of the seal motion under the effect of the aerodynamic and the restoring spring forces. Both stable and unstable cases were considered. It was concluded that, in principle, the existing numerical tools could be used for the flow and aeroelasticity analyses of hydrostatic seals. However, due to large Mach number variations, the solution convergence rate was relatively slow and it was recognized that a preconditioner was needed to handle seal flows. For small gaps of about 10 microns, typical of spiral groved seals, the flow has a high Knudsen number, indicating that the Navier-Stokes formulations may no longer be valid. Such cases require a totally different treatment for the modeling of steady and unsteady aerodynamics, either by modifying the transport parameters of the Navier-Stokes equations or by considering rarefied gas dynamics.

Computational Study of Intake Duct Effects on Fan Flutter Stability

A detailed e utter analysis is presented of a civil aeroengine fan assembly using an integrated three-dimensional aeroelasticity model. Two different intake ducts are considered. The e rst one is a straight duct, the geometry of whichisrepresentativeoftestrigintakes.Thesecondductisanaxisymmetricversionofthee ightintake.Duringthe e rst phase of the study, a series of e utter analyses was conducted for the 60 ‐80% speed range. Each computation was performed at a single point along the speed characteristic by following an elevated working line that was near the expected e utter boundary. As routinely observed in rig tests, the e utter stability was predicted to drop sharply forsome very narrow speedranges, but thebehaviorwasfound to bemarkedly differentforeach individual intake. To gain further understanding of the intake effects, a large number of calculations were undertaken for thesecond intake alone. An assumed pressure perturbation, due a rotating fan assembly vibrating in a given nodal diameter mode, was imposed at the duct exit. The propagation of this perturbation was monitored at a number of stations along the duct. The cases studied were chosen to cover the combinations of speeds and nodal diameters for which theearlierfanassembly plusintakecalculationshad predictede utter.Itwasshownthat, foragiven nodaldiameter assembly mode, instabilities occurred when the perturbation frequency was sufe ciently close to the duct’ s cuton frequency in theregion closeto thefan.Such a e nding suggeststhat fan e utterand intakeductacoustics arerelated in an integral fashion. Flutter will occur when the pressure perturbation due to fan rotation and blade vibration match, both in frequency and shape, an acoustic mode of the intake.

A fully distributed unstructured Navier-Stokes solver for large-scale aeroelasticity computations

We present the development and validation of a parallel unsteady flow and aeroelasticity code for large-scale numerical models used in turbomachinery applications. The work is based on an existing unstructured Navier-Stokes solver developed over the past ten years by the Aeroelasticity Research Group at Imperial College Vibration University Technology Centre. The single-process multiple-data paradigm was adopted for the parallelisation of the solver and several validation cases were considered. The computational mesh was divided into several sub-sections using a domain decomposition technique. The performance and numerical accuracy of the parallel solver was validated across several computer platforms for various problem sizes. In cases where the solution could be obtained on a single CPU, the serial and parallel versions of the code were found to produce identical results. Studies on up to 32 CPUs showed varying levels of parallelisation efficiency, an almost linear speed-up being obtained in some cases. Finally, an industrial configuration, a 17 blade row turbine with a 47 million point mesh, was discussed to illustrate the potential of the proposed large-scale modelling methodology.

A CFD-BASED NON-LINEAR MODEL FOR THE PREDICTION OF TONE NOISE IN LINED DUCTS

This paper describes the development and application of a time-domain acoustic liner model which is designed for the simulation of sound propagation and attenuation in conjunction with time-accurate unsteady flow computations using large-scale numerical models. The main duct domain is represented by the 3D Euler or Navier-Stokes equations while the resistive part of the liner model consists of a time-independent part and a non-linear time-dependent part. Its reactive part is obtained by solving the ID Euler equations within the liner cavity. A-3D-benchmark test geometry, including a lined intake, was modelled using an advanced aeroelasticity code. First, the liner model was validated for steadystate intake duct flows using a number of numerical benchmarks, with particular emphasis on the stability of the duct/liner boundary condition for a range of cases. The pressure perturbation due to the fan was investigated next via a full unsteady flow calculation. Both continuous and discontinuous liner models were considered. It was found that liner scattering had nonlinear effects on noise attenuation. It was concluded that the liner model could be used for both steady state and unsteady flows.

A non-linear integrated aeroelasticity method for the prediction of turbine forced response with friction dampers

An integrated aeroelasticity model was described for turbine blade forced response predictions. An iterative procedure was developed to determine the resonance shift under the effects of both unste ...

An Integrated Time-Domain Aeroelasticity Model for the Prediction of Fan Forced Response Due to Inlet Distortion

The forced response of a low aspect-ratio transonic fan due to different inlet distortions was predicted using an integrated time-domain aeroelasticity model. A time-accurate, non-linear viscous, unsteady flow representation was coupled to a linear modal model obtained from a standard finite element formulation. The predictions were checked against the results obtained from a previous experimental programme known as “Augmented Damping of Low-aspect-ratio Fans” (ADLARF). Unsteady blade surface pressures, due to inlet distortions created by screens mounted in the intake inlet duct, were measured along a streamline at 85% blade span. Three resonant conditions, namely 1F/3EO, 1T&2F /8EO and 2S/8EO, were considered. Both the amplitude and the phase of the unsteady pressure fluctuations were predicted with and without the blade flexibility. The actual blade displacements and the amount of aerodynamic damping were also computed for the former case. A whole-assembly mesh with about 2,000,000 points was used in some of the computations. Although there were some uncertainties about the aerodynamic boundary conditions, the overall agreement between the experimental and predicted results was found to be reasonably good. The inclusion of the blade motion was shown to have an effect on the unsteady pressure distribution, especially for the 2F/1T case. It was concluded that a full representation of the blade forced response phenomenon should include this feature.Copyright

A Resonance Tracking Algorithm for the Prediction of Turbine Forced Response With Friction Dampers

This paper presents an iterative method for determining the resonant speed shift when non-linear friction dampers are included in turbine blade roots. Such a need arises when conducting response calculations for turbine blades where the unsteady aerodynamic excitation must be computed at the exact resonant speed of interest. The inclusion of friction dampers is known to raise the resonant frequencies by up to 20% from the standard assembly frequencies. The iterative procedure uses a viscous, time-accurate flow representation for determining the aerodynamic forcing, a look-up table for evaluating the aerodynamic boundary conditions at any speed, and a time-domain friction damping module for resonance tracking. The methodology was applied to an HP turbine rotor test case where the resonances of interest were due to the 1T and 2F blade modes under 40 engine-order excitation. The forced response computations were conducted using a multi-stage approach in order to avoid errors associated with “linking” single stage computations since the spacing between the two bladerows was relatively small. Three friction damper elements were used for each rotor blade. To improve the computational efficiency, the number of rotor blades was decreased by 2 to 90 in order to obtain a stator/rotor blade ratio of 4/9. However, the blade geometry was skewed in order to match the capacity (mass flow rate) of the components and the condition being analysed. Frequency shifts of 3.2% and 20.0% were predicted for the 1T/40EO and 2F/40EO resonances in about 3 iterations. The predicted frequency shifts and the dynamic behaviour of the friction dampers were found to be within the expected range. Furthermore, the measured and predicted blade vibration amplitudes showed a good agreement, indicating that the methodology can be applied to industrial problems.© 2000 ASME

Forced Response Assessment Using Modal Force Based Indicator Functions

This paper will focus on core-compressor forced response with the aim to develop two design criteria, the so-called chordwise cumulative modal force and heightwise cumulative force, to assess the potential severity of the vibration levels from the correlation between the unsteady pressure distribution on the blade’s surface and the structural modeshape. It is also possible to rank various blade designs since the proposed criterion is sensitive to changes in both unsteady aerodynamic loads and the vibration modeshapes. The proposed methodology was applied to a typical core-compressor forced response case for which measured data were available. The Reynolds-averaged Navier-Stokes equations were used to represent the flow in a non-linear time-accurate fashion on unstructured meshes of mixed elements. The structural model was based on a standard finite element representation from which the vibration modes were extracted. The blade flexibility was included in the model by coupling the finite element model to the unsteady flow model in a time-accurate fashion. A series of numerical experiments were conducted by altering the stator wake and using the proposed indicator functions to minimize the rotor response levels. It was shown that a fourfold response reduction was possible for a certain mode with only a minor modification of the blade.© 2008 ASME

Flutter stability analysis of a complete fan assembly

A non-linear integrated aeroelasticity method was applied to the utter analysis of a complete civil aeroengine fan assembly, including an intake duct. A viscous unsteady ow representation was used together with nodal diameter assembly modes in order to predict the modal time histories of three di erent con gurations: no intake, symmetric intake and non-symmetric ight intake. The blades dynamic behaviour was found to be di erent for each con guration considered, indicating the in uence of intake ducts on utter stability.