A. I. Sayma

Modeling of Three-Dimensional Viscous Compressible Turbomachinery Flows Using Unstructured Hybrid Grids

An advanced numerical model for the simulation of steady and unsteady viscous compressible e ows for turbomachinery applications is described. The compressible Favre-averaged Navier ‐Stokes equations are used together with a one-equation turbulence model. The e ow domain is discretized using unstructured hybrid grids that can contain a mixture of hexahedral, pentahedral, tetrahedral, and triangular prismatic cells. The e ow equations are discretized using a node-centered e nite volume scheme that relies on representing the mesh using an edge-based data structure. A dual time stepping technique is applied to a point implicit formulation so that time accuracy can be maintained with large Courant ‐Friedrichs‐Lewy numbers. Nonree ecting boundary conditions are applied at the ine ow and oute ow boundaries to prevent any spurious ree ections of the outgoing waves. The model was validated against measured data for two cases. Radial proe les of pressure and temperature rise were determined from the steady e ow analysis of a rig fan blade, and these were found to be in very good agreement with the measured quantities. A rotor/stator interaction was studied next. Detailed comparisons were carried out against measured steady and unsteady e ow data and good agreement was obtained in all cases.

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.

On the Use of Atmospheric Boundary Conditions for Axial-Flow Compressor Stall Simulations

This paper describes a novel way of prescribing computational fluid dynamics (CFD) boundary conditions for axial-flow compressors. The approach is based on extending the standard single passage computational domain by adding an intake upstream and a variable nozzle downstream. Such a route allows us to consider any point on a given speed characteristic by simply modifying the nozzle area, the actual boundary conditions being set to atmospheric ones in all cases. Using a fan blade, it is shown that the method not only allows going past the stall point but also captures the typical hysteresis loop behavior of compressors.

Semi‐structured meshes for axial turbomachinery blades

This paper describes the development and application of a novel mesh generator for the flow analysis of turbomachinery blades. The proposed method uses a combination of structured and unstructured meshes, the former in the radial direction and the latter in the axial and tangential directions, in order to exploit the fact that blade-like structures are not strongly three-dimensional since the radial variation is usually small. The proposed semi-structured mesh formulation was found to have a number of advantages over its structured counterparts. There is a significant improvement in the smoothness of the grid spacing and also in capturing particular aspects of the blade passage geometry. It was also found that the leading- and trailing-edge regions could be discretized without generating superfluous points in the far field, and that further refinements of the mesh to capture wake and shock effects were relatively easy to implement. The capability of the method is demonstrated in the case of a transonic fan blade for which the steady state flow is predicted using both structured and semi-structured meshes. A totally unstructured mesh is also generated for the same geometry to illustrate the disadvantages of using such an approach for turbomachinery blades. Copyright

Computational aeroelastic modelling of airframes and turbomachinery: progress and challenges.

Computational analyses such as computational fluid dynamics and computational structural dynamics have made major advances towards maturity as engineering tools. Computational aeroelasticity (CAE) is the integration of these disciplines. As CAE matures, it also finds an increasing role in the design and analysis of aerospace vehicles. This paper presents a survey of the current state of CAE with a discussion of recent research, success and continuing challenges in its progressive integration into multidisciplinary aerospace design. It approaches CAE from the perspective of the two main areas of application: airframe and turbomachinery design. An overview will be presented of the different prediction methods used for each field of application. Differing levels of nonlinear modelling will be discussed with insight into accuracy versus complexity and computational requirements. Subjects will include current advanced methods (linear and nonlinear), nonlinear flow models, use of order reduction techniques and future trends in incorporating structural nonlinearity. Examples in which CAE is currently being integrated into the design of airframes and turbomachinery will be presented.

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.

Whole-Assembly Flutter Analysis of a Low Pressure Turbine Blade

This paper reports the findings of a flutter investigation on a low pressure turbine blade using a 3D non-linear integrated aeroelasticity method. The approach has two important features: (i) the calculations are performed in a time-accurate and integrated fashion whereby the structural and fluid domains are linked via an exchange of boundary conditions at each time step, and (ii) the analysis is performed on the entire bladed-disk assembly, thus removing the need to assume a critical interblade phase angle. Although such calculations are both CPU and in-core memory intensive, they do not require pre-knowledge of the flutter mode and hence they allow a better understanding of the aeroelasticity phenomena involved.

Multibladerow Forced Response Modeling in Axial-Flow Core Compressors

This paper describes the formulation and application of an advanced numerical model for the simulation of blade-passing and low-engine order forced response in turbomachinery core compressors. The Reynolds averaged Navier-Stokes equations are used to represent the flow in a nonlinear time-accurate fashion on unstructured meshes of mixed elements. The structural model is based on a standard finite-element representation. The fluid mesh is moved at each time step according to the structural motion so that changes in blade aerodynamic damping and flow unsteadiness can be accommodated automatically. A whole-annulus 5-bladerow forced response calculation, where three upstream and one downstream bladerows were considered in addition to the rotor bladerow of interest, was undertaken using over 20 million grid points. The results showed not only some potential shortcomings of equivalent 2-bladerow computations for the determination of the main blade-passing forced response, but also revealed the potential importance of low engine-order harmonics. Such harmonics, due to stator blade number differences, or arising from common symmetric sectors, can only be taken into account by including all stator bladerows of interest. The low engine-order excitation that could arise from a blocked passage was investigated next. It was shown that high vibration response could arise in such cases.