### Publications

Boudet J, Autef VND, Chew JW, Hills NJ, Gentilhomme O (2005) Numerical simulation of rim seal flows in axial turbines, AERONAUTICAL JOURNAL 109 (1098) pp. 373-383 ROYAL AERONAUTICAL SOC

Turner AB, Long CA, Childs PRN, Hills NJ, Millward JA (1997) Review of some current problems in gas turbine secondary systems, American Society of Mechanical Engineers (Paper)

This paper reviews the current position of five major problem areas in gas turbine secondary air system design. Although the problems are of primary interest to the designer of the coolant flow paths, since they directly affect the temperature, the stresses and thus the life of the major rotating components, three of the problems interact with the main gas path and are thus also the concern of the mainstream aerodynamicist. The five problems reviewed are: prediction of the flow distribution and heat transfer in the high pressure compressor drive cone cavity from the turbine to the rim of the HP compressor running underneath the combustion chamber; the flow penetration and heat transfer in the multiple rotating cavities formed by the multiple discs of the high pressure compressor with a rotating shaft running through the bores; the control of ingestion of hot turbine mainstream gas into the rotor-stator wheelspaces through the rim-seals; the problem of compressor and turbine stator-well heating, particularly compressor stator-wells in which excessive temperatures have been occasionally measured and finally, the pre-swirl coolant system which has to take the blade cooling air across from the stationary casing to the rotating turbine disc in the most advantageous manner.

Javiya U, Chew J, Hills N, Scanlon T (2011) A comparative study of cascade vanes and drilled nozzle designs for pre-swirl, Proceedings of the ASME Turbo Expo 5 (PARTS A AND B) pp. 913-920 ASME

Design of pre-swirl systems is important for the secondary air cooling system of gas turbine engines. In this paper, three pre-swirl nozzles, a cascade vane and two drilled nozzles are analysed and their performances are compared. The two drilled nozzles considered are a straight drilled nozzle and an aerodynamically designed nozzle. CFD analyses are presented for stand-alone and pre-swirl system 3D sector models at engine operating conditions near to engine maximum power condition rotational Reynolds number (Re ?) up to 4.6 ! 10 . Nozzle performance is characterised by the nozzle discharge coefficient (C ), nozzle velocity coefficient (?·) and cooling air delivery temperature. Two commonly used eddy viscosity models are employed for the study, the standard º-µ and Spalart-Allmaras models with wall functions. Both models give very similar results for C and · and are in reasonable agreement with available experimental data. Effects of nozzle or vane number and sealing flow have been analysed. The cascade vanes perform slightly better than the aerodynamically designed drilled nozzles but the final design choice will depend on other component and manufacturing costs. An elementary model is presented to separate temperature losses due to the nozzle, stator drag and sealing flow. Copyright © 2011 by Rolls-Royce plc.

Amirante D, Hills NJ, Barnes CJ (2012) Thermo-Mechanical Finite Element Analysis/Computational Fluid Dynamics Coupling of an Interstage Seal Cavity Using Torsional Spring Analogy, Journal of Turbomachinery 134 (5)

The optimization of heat transfer between fluid and metal plays a crucial role in gas turbine design. An accurate prediction of temperature for each metal component can help to minimize the coolant flow requirement, with a direct reduction of the corresponding loss in the thermodynamic cycle. Traditionally, in industry fluid and solid simulations are conducted separately. The prediction of metal stresses and temperatures, generally based on finite element analysis, requires the definition of a thermal model whose reliability is largely dependent on the validity of the boundary conditions prescribed on the solid surface. These boundary conditions are obtained from empirical correlations expressing local conditions as a function of working parameters of the entire system, with validation being supplied by engine testing. However, recent studies have demonstrated the benefits of employing coupling techniques, whereby computational fluid dynamics (CFD) is used to predict the heat flux from the air to the metal, and this is coupled to the thermal analysis predicting metal temperatures. This paper describes an extension of this coupling process, accounting for the thermo-mechanical distortion of the metal through the engine cycle. Two distinct codes, a finite element analysis (FEA) solver for thermo-mechanical analysis and a finite volume solver for CFD, are iteratively coupled to produce temperatures and deformations of the solid part through an engine cycle. At each time step, the CFD mesh is automatically adapted to the FEA prediction of the metal position using efficient spring analogy methods, ensuring the continuity of the coupled process. As an example of this methodology, the cavity flow in a turbine stator well is investigated. In this test case, there is a strong link between the thermo-mechanical distortion, governing the labyrinth seal clearance, and the amount of flow through the stator well, which determines the resulting heat transfer in the stator well. This feedback loop can only be resolved by including the thermo-mechanical distortion within the coupling process. © 2012 American Society of Mechanical Engineers.

Javiya U, Chew J, Hills N, Dullenkopf K, Scanlon T (2012) Evaluation of CFD and coupled fluid-solid modelling for a direct transfer pre-swirl system, Proceedings of the ASME Turbo Expo 4 (PARTS A AND B) pp. 2179-2190

The prediction of the pre-swirl cooling air delivery and disc metal temperature are important for the cooling system performance and the rotor disc thermal stresses and life assessment. In this paper, standalone 3D steady and unsteady CFD, and coupled FE-CFD calculations are presented for prediction of these temperatures. CFD results are compared with previous measurements from a direct transfer pre-swirl test rig. The predicted cooling air temperatures agree well with the measurement, but the nozzle discharge coefficients are under predicted. Results from the coupled FE-CFD analyses are compared directly with thermocouple temperature measurements and with heat transfer coefficients on the rotor disc previously obtained from a rotor disc heat conduction solution. Considering the modelling limitations, the coupled approach predicted the solid metal temperatures well. Heat transfer coefficients on the rotor disc from CFD show some effect of the temperature variations on the heat transfer coefficients. Reasonable agreement is obtained with values deduced from the previous heat conduction solution. Copyright © 2012 by ASME.

Hills NJ, Chew JW, Turner AB (2002) Computational and mathematical modeling of turbine rim seal ingestion, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME 124 (2) pp. 306-315 ASME-AMER SOC MECHANICAL ENG

Lott PT, Hills NJ, Chew JW, Scanlon T, Shahpar S (2009) HIGH PRESSURE TURBINE STAGE ENDWALL PROFILE OPTIMISATION FOR PERFORMANCE AND RIM SEAL EFFECTIVENESS, PROCEEDINGS OF ASME TURBO EXPO 2009, VOL 7, PTS A AND B pp. 1075-1087 AMER SOC MECHANICAL ENGINEERS

Boudet J, Hills NJ, Chew JW (2006) Numerical simulation of the flow interaction between turbine main annulus and disc cavities, Proceedings of the ASME Turbo Expo 6 PART A pp. 553-562

This paper presents numerical simulations of the unsteady flow interactions between the main annulus and the disc cavity for an axial turbine. The simulations show the influence of the main annulus asymmetries (vane wakes, blade potential effect), and the appearance of rim seal flow instabilities. The generation of secondary frequencies due to non-linear interactions is observed, and the possibility of further low frequency effects and resonance is noted. The computations are compared to experimental results, looking at tracer gas concentration and mass-flows. Results are further analysed to investigate the influence of the rim seal flow on the blading aerodynamics. The flow that is ejected through the rim seal influences the unsteady flow impinging the blades. The influence of this rim-seal flow is even observed downstream of the blades, where it distorts the radial profile of stagnation temperature. Copyright © 2006 by ASME.

Sun Z, Chew JW, Hills NJ, Barnes CJ, Valencia AG (2012) 3D coupled fluid-solid thermal simulation of a turbine disc through a transient cycle, Proceedings of the ASME Turbo Expo 4 (PARTS A AND B) pp. 1959-1969

Thermal analysis of a turbine disc through a transient test cycle is demonstrated using 3D computational fluid dynamics (CFD) modeling for the cooling flow and 3D finite element analysis (FEA) for the disc. The test case is a 3D angular sector of the high pressure (HP) turbine assembly of a civil jet engine and includes details of the coolant flow around the blade roots. Proprietary FEA and CFD solvers are used to simulate the metal and fluid domains, respectively. Coupling is achieved through an iterative loop with smooth exchange of information between the FEA and CFD simulations at each time step, ensuring consistency of temperature and heat flux on the coupled interfaces between the metal and fluid domains. The coupled simulation can be completed within a few weeks using a PC cluster with multiple parallel CFD executions. The FEA/CFD coupled result agrees well with corresponding rig test data and the baseline 3D and 2D FEA solutions, which have been calibrated using test data. Provision of upstream boundary conditions and modeling of rapid transients are identified as areas of uncertainty. Averaging of CFD solutions and relaxation is used to overcome difficulties caused by CFD oscillations associated with flow unsteadiness. The present work supports the continued use and development of the FEA/CFD coupling method for industrial applications. Copyright © 2012 by ASME.

Chew JW, Ciampoli F, Hills NJ, Scanlon T (2005) Pre-swirled cooling air delivery system performance, Proceedings of the ASME Turbo Expo 2005, Vol 3 Pts A and B pp. 1129-1137 AMER SOC MECHANICAL ENGINEERS

in a rotor/stator cavity without heat transfer and buoyant flow in a rotor/rotor cavity. The numerical tool used employs a spectral element discretisation in two dimensions and a Fourier expansion in the remaining direction, which is periodic and corresponds to the azimuthal coordinate in cylindrical coordinates. The spectral element approximation uses a Galerkin method to discretise the governing equations, but employs high-order polynomials within each element to obtain spectral accuracy. A second-order, semi-implicit, stiffly stable algorithm is used for the time discretisation. Numerical results obtained for the rotor/ stator cavity compare favourably with experimental results for Reynolds numbers up to Re1 = 106 in terms of velocities and Reynolds stresses. The buoyancy-driven flow is simulated using the Boussinesq approximation. Predictions are compared with previous computational and experimental results. Analysis of the present results shows close correspondence to natural convection in a gravitational field and consistency with experimentally observed flow structures in a water-filled rotating annulus. Predicted mean heat transfer levels are higher than the available measurements for an air-filled rotating annulus, but in agreement with correlations for natural convection under gravity.

Ganine V, Hills NJ, Lapworth BL (2012) Nonlinear acceleration of coupled fluid-structure transient thermalproblems by Anderson mixing, International Journal for Numerical Methods in Fluids

Conjugate heat-transfer problems are typically solved using partitioned methods where fluid and solid subdomains are evaluated separately by dedicated solvers coupled through a common boundary. Strongly coupled schemes for transient analysis require fluid and solid problems to be solved many times each time step until convergence to a steady state. In many practical situations, a fairly simple and frequently employed fixed-point iteration process is rather ineffective; it leads to a large number of iterations per time step and consequently to long simulation times. In this article, Anderson mixing is proposed as a fixed-point convergence acceleration technique to reduce computational cost of thermal coupled fluid-solid problems. A number of other recently published methods with applications to similar fluid-structure interaction problems are also reviewed and analyzed. Numerical experiments are presented to illustrate relative performance of these methods on a test problem of rotating pre-swirl cavity air flow interacting with a turbine disk. It is observed that performance of Anderson mixing method is superior to that of other algorithms in terms of total iteration counts. Additional computational savings are demonstrated by reusing information from previously solved time steps. © All rights reserved 2012 Rolls-Royce plc.

Illingworth JB, Hills NJ, Barnes CJ (2005) 3D fluid - Solid heat transfer coupling of an aero engine pre-swirl system, Proceedings of the ASME Turbo Expo 3 PART A pp. 801-811

This paper examines the application of a newly developed code, which couples a commercial computational fluid dynamics (CFD) code (FLUENT) directly to an in-house finite element solver used by Rolls-Royce plc. The coupling is achieved by passing metal temperatures from the finite element solver to define CFD boundary conditions, and then passing heat fluxes from the resulting CFD solutions back to the finite element model. This coupling method was applied to a real engine test case, modelling the pre-swirl system of the Rolls-Royce Trent 500 aero engine. The initial phase of the analysis couples an axisymmetric finite element whole engine model to two axisymmetric CFD models. The CFD models define the pre-swirl fluid domain during idle and maximum take off engine running conditions. This demonstrates the code's ability to accommodate temperature transients through the use of multiple CFD models. A further analysis is then performed coupling the same finite element engine model to a 3D CFD model replicating stabilised maximum take off conditions. This is compared with the axisymmetric to axisymmetric analysis to identify the approximations inherent in using an axisymmetric model to investigate a three dimensional flow structure. Copyright © 2005 by ASME.

Ganine V, Amirante D, Hills N (2015) Enhancing performance and scalability of data transfer across sliding grid interfaces for time-accurate unsteady simulations of multistage turbomachinery flows, Computers and Fluids 115 pp. 140-153

© 2015 .High fidelity simulations of the flow phenomena around complex geometries for turbomachinery applications require fluid solvers to run on ever increasing processor counts. For fully unsteady predictions in rotor-stator systems most of CFD codes employ the sliding interface technique. However, the scalability and efficiency of current sliding grid parallel implementations are significantly constrained by the computation and communication imbalances. They are associated with data transfer across discrete non-matching interfaces. To prepare for the challenges at extreme scales in this paper we attempt to redesign the algorithm in such a way that it maintains the scalability of the original CFD code on static grids. In the proposed parallel implementation the cell containment search and interpolation workloads are balanced by employing a deterministic geometric decomposition on an intermediate "rendezvous" set of processes. Rapidly changing dynamic communication patterns induced by the grids relative motion are handled with a sparse communication protocol. The scaling behavior and performance of the developed technique are analyzed using realistic test cases on two different computing systems.

Hills N (2012) Foreword to JPAS special issue on applied computational aerodynamics and high performance computing in the UK, Progress in Aerospace Sciences 52

Amirante D, Hills NJ, Barnes CJ (2010) THERMO-MECHANICAL FEA/CFD COUPLING OF AN INTERSTAGE SEAL CAVITY USING TORSIONAL SPRING ANALOGY, PROCEEDINGS OF THE ASME TURBO EXPO 2010, VOL 4, PTS A AND B pp. 1037-1049 AMER SOC MECHANICAL ENGINEERS

A direct numerical simulation of a Batchelor vortex has been carried out in the presence of freely-decaying turbulence, using both periodic and symmetric boundary conditions; the latter most closely approximates typical experimental conditions, while the former is often used in computational simulations for the purposes of numerical convenience. The higher-order velocity statistics were shown to be strongly dependent upon the boundary conditions, but the dependence could be mostly eliminated by correcting for the random, Gaussian modulation of the vortex trajectory commonly referred to as 'wandering' using a technique often employed in the analysis of experimental data. Once corrected for this wandering, the strong peaks in the Reynolds stresses normally observed at the vortex centre were replaced by smaller local extrema located within the core region but away from the centre. The distributions of the corrected Reynolds stresses suggested that the formation and organization of secondary structures within the core is the main mechanism in turbulent production during the linear growth phase of vortex development.

Koli B, Chew JW, Hills NJ, Scanlon T (2014) CFD INVESTIGATION OF A FLUIDIC DEVICE FOR MODULATION OF AERO-ENGINE COOLING AIR, PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2014, VOL 5C AMER SOC MECHANICAL ENGINEERS

Amirante D, Hills NJ, Barnes CJ (2012) A moving mesh algorithm for aero-thermo-mechanical modelling in turbomachinery, International Journal for Numerical Methods in Fluids 70 (9) pp. 1118-1138

This paper describes the development of a mesh deformation method used for aero-thermo-mechanical coupling of turbo-engine components. The method is based on the nonlinear solution of an elastic medium analogy, solved using finite element discretisation and modified to let the boundary nodes be free to slide over the deflected surfaces. This sliding technique relies on a B-spline reconstruction of the moving boundary and increases the robustness of the method in situations where the boundary deflection field presents significant gradients or large relative motion between two distinct boundaries. The performance of the method is illustrated with the application to an interstage cavity of a turbine assembly, subjected to the deformations computed by a coupled thermo-mechanical analysis of the engine component. © 2012 John Wiley & Sons, Ltd.

Amirante D, Hills NJ (2015) Large-Eddy Simulations of Wall Bounded Turbulent Flows Using Unstructured Linear Reconstruction Techniques, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME 137 (5) ARTN 051006 ASME

Javiya U, Chew JW, Hills NJ, Zhou L, Wilson M, Lock GD (2011) CFD analysis of flow and heat transfer in a direct transfer preswirl system, Journal of Turbomachinery 134 (3)

Experimental measurements from a new single stage turbine are presented. The turbine has 26 vanes and 59 rotating blades with a design point stage expansion ratio of 2.5 and vane exit Mach number of 0.96. A variable sealing flow is supplied to the disc cavity upstream of the rotor and then enters the annulus through a simple axial clearance seal situated on the hub between the stator and rotor. Measurements at the annulus hub wall just downstream of the vanes show the degree of circumferential pressure variation. Further pressure measurements in the disc cavity indicate the strength of the swirling flow in the cavity, and show the effects of mainstream gas ingestion at low sealing flows. Ingestion is further quantified through seeding of the sealing air with nitrous oxide or carbon dioxide and measurement of gas concentrations in the cavity. Interpretation of the measurements is aided by steady and unsteady computational fluid dynamics solutions, and comparison with an elementary model of ingestion.

Vinod Kumar BG, Chew JW, Hills NJ (2013) Rotating flow and heat transfer in cylindrical cavities with radial inflow, Journal of Engineering for Gas Turbines and Power 135 (3)

The design and optimization of an efficient internal air system of a gas turbine requires a thorough understanding of the flow and heat transfer in rotating disc cavities. The present study is devoted to the numerical modeling of flow and heat transfer in a cylindrical cavity with radial inflow and a comparison with the available experimental data. The simulations are carried out with axisymmetric and 3-D sector models for various inlet swirl and rotational Reynolds numbers up to 1.2 × 106. The pressure coefficients and Nusselt numbers are compared with the available experimental data and integral method solutions. Two popular eddy viscosity models, the Spalart-Allmaras and the k-É, and a Reynolds stress model have been used. For cases with particularly strong vortex behavior the eddy viscosity models show some shortcomings, with the Spalart-Allmaras model giving slightly better results than the k-É model. Use of the Reynolds stress model improved the agreement with measurements for such cases. The integral method results are also found to agree well with the measurements. © 2013 by ASME.

Design and optimization of an efficient internal air system of a gas turbine requires thorough understanding of the flow and heat transfer in rotating disc cavities. The present study is devoted to numerical modelling of flow and heat transfer in a cylindrical cavity with radial inflow and comparison with the available experimental data. The simulations are carried out with axi-symmetric and 3-D sector models for various inlet swirl and rotational Reynolds numbers upto 2.1×106. The pressure coefficients and Nusselt numbers are compared with the available experimental data and integral method solutions. Two popular eddy viscosity models, the Spalart-Allmaras and the k-e , and a Reynolds stress model have been used . For cases with particularly strong vortex behaviour the eddy viscosity models show some shortcomings with the Spalart-Allmaras model giving slightly better results than the k-e model. Use of the Reynolds stress model improved the agreement with measurements for such cases. The integral method results are also found to agree well with the measurements. Copyright © 2012 by ASME.

Amirante D, Hills NJ (2014) LES OF WALL BOUNDED TURBULENT FLOWS USING UNSTRUCTURED LINEAR RECONSTRUCTION TECHNIQUES, PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2014, VOL 2B AMER SOC MECHANICAL ENGINEERS

The effects of surface roughness on air-riding seals are investigated here using the Rayleigh-pad as an example. Both incompressible and compressible flows are considered using both CFD analysis and analytical/numerical solutions of the Reynolds equation for various 2D or 3D roughness patterns on the stationary wall. A 'unit-based' approach for incompressible flows has also been employed and is shown to be computationally much less expensive than the full-geometry solution. Results are presented showing the effect of surface roughness on the net lift force. The effects of varying the Reynolds number are demonstrated, as well as comparative results for static stiffness.

Ganine V, Javiya U, Hills N, Chew J (2012) Coupled fluid-structure transient thermal analysis of a gas turbine internal air system with multiple cavities, Proceedings of the ASME Turbo Expo 4 (PARTS A AND B) pp. 2167-2177

This paper presents the transient aero-thermal analysis of a gas turbine internal air system through an engine flight cycle featuring multiple fluid cavities that surround a HP turbine disk and the adjacent structures. Strongly coupled fluid-structure thermal interaction problems require significant computational effort to resolve nonlinearities on the interface for each time step. Simulation times may grow impractical if multiple fluid domains are included in the analysis. A new strategy is employed to decrease the cost of coupled aero-thermal analysis. Significantly lower fluid domain solver invocation counts are demonstrated as opposed to the traditional coupling approach formulated on the estimates of heat transfer coefficient. Numerical results are presented using 2D FE conduction model combined with 2D flow calculation in five separate cavities interconnected through the inlet and outlet boundaries. The coupled solutions are discussed and validated against a nominal stand-alone model. Relative performance of both coupling techniques is evaluated. Copyright © 2012 by ASME.

Amirante D, Hills NJ, Barnes CJ (2012) Use of dynamic meshes for transient metal temperature prediction, Proceedings of the ASME Turbo Expo 4 (PARTS A AND B) pp. 2073-2084

This paper describes the thermal analysis conducted on a three-dimensional model of a stator well contained within a turbine assembly. A methodology has been developed for coupled fluid-solid modelling accounting for the boundary deflections predicted by the structural analysis. The coupling is obtained through an iterative process between a finite element code (FEA) performing structural and thermal analysis for the solid part, and a finite volume solver for the CFD. As the engine runs transiently through a specified flight cycle, the FEA predictions of metal deformations and temperatures are passed to the CFD code, which in turns computes the heat fluxes over the metal surfaces. A robust moving mesh technique is used to automatically modify an initial mesh, based on the cold geometry, to the time dependent boundary deflections. Thus, the methodology guarantees that the CFD is always carried out on the hot-running geometry. A thorough investigation into the flow physics involved in the stator well is conducted. It is shown that an accurate thermal modelling for transient regimes necessitates the correct prediction of the time dependent clearances present in the system. Even small changes in the clearances may cause a transition between different dynamic behaviours, egress or ingestion, ultimately leading to drastically different thermal responses. Copyright © 2012 by ASME.

This paper presents transient aero-thermal analysis for a gas turbine disk and the surrounding air flows through a transient slam acceleration/deceleration ?square cycle? engine test, and compares predictions with engine measurements. The transient solid-fluid interaction calculations were performed with an innovative coupled finite element (FE) and computational fluid dynamics (CFD) approach. The computer model includes an aero-engine high pressure turbine (HPT) disk, adjacent structure, and the surrounding internal air system cavities. The model was validated through comparison with the engine temperature measurements and is also compared with industry standard standalone FE modelling. Numerical calculations using a 2D FE model with axisymmetric and 3D CFD solutions are presented and compared. Strong coupling between CFD solutions for different air system cavities and the FE solid model led to some numerical difficulties. These were addressed through improvement to the coupling algorithm. Overall performance of the coupled approach is very encouraging giving temperature predictions as good as a traditional model that had been calibrated against engine measurements.

Chew JW, Hills NJ, Hornsby C, Young C (2003) Recent developments in application of CFD to turbomachinery internal air systems, Proc. 5th European Turbomachinery Conference

Advances in the development and application of computational fluid dynamics (CFD) to gas turbine internal air systems are discussed in this paper. It is shown that the combination of parallel computation, using powerful PC clusters, with relatively robust and flexible CFD solvers is now having considerable impact in industry. At the same time research studies demonstrate that capability continues to expand. Examples are given of 3D, steady and unsteady industrial applications, with discussions of computing times on PC clusters using both ethernet and myrinet network connections. Integration of CFD into the design process is also considered. Research studies, centring on model validation and methods improvement are also illustrated and discussed, with inclusion of very recent work using a 32 processor cluster with fast myrinet networking.

Understanding and modelling of main annulus gas ingestion through turbine rim seals is considered and advanced in this paper. Unsteady 3-dimensional computational fluid dynamics (CFD) calculations and results from a more elementary model are presented and compared with experimental data previously published by Hills et al (1997). The most complete CFD model presented includes both stator and rotor in the main annulus and the inter-disc cavity. The k-µ model of turbulence with standard wall function approximations is assumed in the model which was constructed in a commercial CFD code employing a pressure correction solution algorithm. It is shown that considerable care is needed to ensure convergence of the CFD model to a periodic solution. Compared to previous models, results from the CFD model show encouraging agreement with pressure and gas concentration measurements. The annulus gas ingestion is shown to result from a combination of the stationary and rotating circumferential pressure asymmetries in the annulus. Inertial effects associated with the circumferential velocity component of the flow have an important effect on the degree of ingestion. The elementary model used is an extension of earlier models based on orifice theory applied locally around the rim seal circumference. The new model includes a term accounting for inertial effects. Some good qualitative and fair quantitative agreement with data is shown. Copyright © 2001 by ASME.

Hills N (2007) Achieving high parallel performance for an unstructured unsteady turbomachinery CFD code, AERONAUTICAL JOURNAL 111 (1117) pp. 185-193 ROYAL AERONAUTICAL SOC

Ganine V, Javiya U, Hills N, Chew J (2012) Coupled fluid-structure transient thermal analysis of a gas turbine internal air system with multiple cavities, Journal of Engineering for Gas Turbines and Power 134 (10)

This paper presents the transient aerothermal analysis of a gas turbine internal air system through an engine flight cycle featuring multiple fluid cavities that surround a HP turbine disk and the adjacent structures. Strongly coupled fluid-structure thermal interaction problems require significant computational effort to resolve nonlinearities on the interface for each time step. Simulation times may grow impractical if multiple fluid domains are included in the analysis. A new strategy is employed to decrease the cost of coupled aerothermal analysis. Significantly lower fluid domain solver invocation counts are demonstrated as opposed to the traditional coupling approach formulated on the estimates of heat transfer coefficient. Numerical results are presented using 2D finite element conduction model combined with 2D flow calculation in five separate cavities interconnected through the inlet and outlet boundaries. The coupled solutions are discussed and validated against a nominal stand-alone model. Relative performance of both coupling techniques is evaluated. © 2012 American Society of Mechanical Engineers.

Chew JW, Doherty JW, Gillan M, Hills NJ (2006) practical applications of automated design and optimisation techniques using CFD,

Application of design optimisation techniques using CFD to problems for turbomachinery internal air systems and motorsport will be described and discussed. Developments in these areas build on earlier work on turbomachinery blade and aircraft applications, and present new challenges. Specific examples include turbine cooling air pre-swirl nozzles, turbine rim sealing, and full track optimisation of a Champ Car. Focussing on these specific examples, issues such as choice of optimisation method, automation of mesh generation, geometry parameterisation, solution convergence, model accuracy and method robustness will be considered.

Hills NJ (2007) Whole turbine CFD modelling, Proceedings of the ASME Turbo Expo 2007, Vol 6, Pts A and B pp. 817-824 AMER SOC MECHANICAL ENGINEERS

Chew JW, Hills NJ, Khalatov S, Scanlon T, Turner AB (2003) Measurement and analysis of flow in a pre-swirled cooling air delivery system, American Society of Mechanical Engineers, International Gas Turbine Institute, Turbo Expo (Publication) IGTI 5 B pp. 913-920

Measurements and analysis for a pre-swirl cooling air delivery system are reported here. The experimental rig used is representative of aero-engine conditions, having 18 pre-swirl nozzles, 72 receiver holes, capable of speeds up to 11 000 rpm, and giving differences between total temperature upstream of the pre-swirl nozzles and relative total temperature measured in the receiver holes of up to 26K. Pressure and temperature measurements are reported. An elementary model is developed for calculation of the cooling air delivery temperature. This accounts for the pre-swirl nozzle velocity coefficient, moments on the stationary and rotating surfaces in the pre-swirl chamber, and flows through the inner and outer seals to the chamber. The model is shown to correlate the measurements well for a range of disc speeds and pre-swirl velocity to disc speed ratios.

O'Mahoney T, Hills N, Chew J (2012) Sensitivity of les results from turbine rim seals to changes in grid resolution and sector size, Progress in Aerospace Sciences 52 pp. 48-55

Large-Eddy Simulations (LES) were carried out for a turbine rim seal and the sensitivity of the results to changes in grid resolution and the size of the computational domain are investigated. Ingestion of hot annulus gas into the rotor-stator cavity is compared between LES results and against experiments and Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations. The LES calculations show greater ingestion than the URANS calculation and show better agreement with experiments. Increased grid resolution shows a small improvement in ingestion predictions whereas increasing the sector model size has little effect on the results. The contrast between the different CFD models is most stark in the inner cavity, where the URANS shows almost no ingestion. Particular attention is also paid to the presence of low frequency oscillations in the disc cavity. URANS calculations show such low frequency oscillations at different frequencies than the LES. The oscillations also take a very long time to develop in the LES. The results show that the difficult problem of estimating ingestion through rim seals could be overcome by using LES but that the computational requirements were still restrictive. © 2011 Elsevier Ltd. All rights reserved.

Noor Mohamed S, Chew J, Hills NJ (2016) EFFECT OF BOLTS ON FLOW AND HEAT TRANSFER IN A ROTOR-STATOR DISC, Proceedings of ASME Turbo Expo 2016

Previous studies have indicated some differences between steady CFD predictions of flow in a rotor-stator disc cavity with rotating bolts compared to measurements. Recently time-dependent CFD simulations have revealed the unsteadiness present in the flow and have given improved agreement with measurements. In this paper unsteady Reynolds averaged Navier-Stokes (URANS) 3600 model CFD calculations of a rotor-stator cavity with rotor bolts were performed in order to better understand the flow and heat transfer within a disc cavity previously studied experimentally by other workers. It is shown that the rotating bolts generate unsteadiness due to wake shedding which creates time-dependent flow patterns within the cavity. At low throughflow conditions, the unsteady flow significantly increases the average disc temperature. A systematic parametric study is presented giving insight into the influence of number of bolts, mass flow rate, cavity gap ratio and the bolts-to-shroud gap ratio on the time depended flow within the cavity.

This paper describes the development of a mesh deformation method used for aero-thermo-mechanical coupling of turbo-engine components. The method is based on the nonlinear solution of an elastic medium analogy, solved using finite element discretisation and modified to let the boundary nodes be free to slide over the deflected surfaces. This sliding technique relies on a B-spline reconstruction of the moving boundary and increases the robustness of the method in situations where the boundary deflection field presents significant gradients or large relative motion between two distinct boundaries. The performance of the method is illustrated with the application to an interstage cavity of a turbine assembly, subjected to the deformations computed by a coupled thermo-mechanical analysis of the engine component.

The cooling air in a rotating machine is subject to windage as it passes over the rotor surface, particularly for cases where non-axisymmetric features such as boltheads are encountered. The ability to accurately predict windage can help reduce the quantity of cooling air required, resulting in increased efficiency. Previous work has shown that steady CFD solutions can give reasonable predictions for the effects of bolts on disc moment for a rotor-stator cavity with throughflow but flow velocities and disc temperature are not well predicted. Large fluctuations in velocities have been observed experimentally in some cases. Time-dependent CFD simulations reported here bring to light the unsteady nature of the flow. Unsteady Reynolds averaged Navier-Stokes (URANS) calculations for 120 degree and 360 degree models of the rotor-stator cavity with 9 and 18 bolts were performed in order to better understand the flow physics. Although the rotor-stator cavity with bolts is geometrically steady in the rotating frame of reference, it was found that the bolts generate unsteadiness which creates time-dependent rotating flow features within the cavity. At low throughflow conditions, the unsteady flow significantly increases the average disc temperature.

Verdicchio JA, Chew JW, Hills NJ (2001) Coupled fluid/solid heat transfer computation for turbine discs, Proceedings of the ASME Turbo Expo 3

This paper considers the coupling of a finite element thermal conduction solver with a steady, finite volume fluid flow solver. Two methods were considered for passing boundary conditions between the two codes - transfer of metal temperatures and either convective heat fluxes or heat transfer coefficients and air temperatures. These methods have been tested on two simple rotating cavity test cases and also on a more complex real engine example. Convergence rates of the two coupling methods were compared. Passing heat transfer coefficients and air temperatures was found to give the quickest convergence. The coupled method gave agreement with the analytic solution and a conjugate solution of the simple free disc problem. The predicted heat transfer results for the real engine example showed some encouraging agreement, although some modelling issues are identified. Copyright © 2001 by ASME.

Sun Z, Chew JW, Hills NJ, Lewis L, Mabilat C (2010) COUPLED AERO-THERMO-MECHANICAL SIMULATION FOR A TURBINE DISC THROUGH A FULL TRANSIENT CYCLE, PROCEEDINGS OF THE ASME TURBO EXPO 2010, VOL 4, PTS A AND B pp. 1025-1036 AMER SOC MECHANICAL ENGINEERS

Javiya U, Chew J, Hills N, Zhou L, Wilson M, Lock G (2010) CFD ANALYSIS OF FLOW AND HEAT TRANSFER IN A DIRECT TRANSFER PRE-SWIRL SYSTEM, PROCEEDINGS OF THE ASME TURBO EXPO 2010, VOL 4, PTS A AND B pp. 1167-1178 AMER SOC MECHANICAL ENGINEERS

Alexiou A, Hills NJ, Long CA, Turner AB, Wong LS, Millward JA (2000) Discharge coefficients for flow through holes normal to a rotating shaft, International Journal of Heat and Fluid Flow 21 (6) pp. 701-709

A possible design for a more compact gas turbine engine uses contra-rotating high pressure (HP) and intermediate pressure (IP) turbine discs. Cooling air for the IP turbine stages is taken from the compressor and transferred to the turbine stage via holes in the drive shaft. The aim of this work was to investigate the discharge coefficient characteristics of the holes in this rotating shaft, and, in particular, to ascertain whether the sense of rotation of the shaft with respect to the discs affected these significantly. This paper reports mostly on experimental measurements of the discharge coefficients. Some CFD modelling of this flow was carried out and this has helped to explain the experimental work. The experimental results show the effects on the discharge coefficient of rotational speed, flow rate, and co- and contra-rotations of the shaft relative to the discs. The measured values of the discharge coefficient are compared with established experimental data for non-rotating holes in the presence of a cross-flow. For stationary shaft and discs, co-rotation of the shaft and discs and differential rotation with the disc speed less than the shaft (in the same rotational direction), the discharge coefficients are in reasonable agreement with these data. For differential rotation (including contra-rotation) with the disc speed greater than the shaft, there is a significant decrease in discharge coefficient.

Ganine V, Hills NJ, Lapworth BL (2013) Nonlinear acceleration of coupled fluid-structure transient thermalproblems by Anderson mixing, International Journal for Numerical Methods in Fluids 71 (8) pp. 939-959

Conjugate heat-transfer problems are typically solved using partitioned methods where fluid and solid subdomains are evaluated separately by dedicated solvers coupled through a common boundary. Strongly coupled schemes for transient analysis require fluid and solid problems to be solved many times each time step until convergence to a steady state. In many practical situations, a fairly simple and frequently employed fixed-point iteration process is rather ineffective; it leads to a large number of iterations per time step and consequently to long simulation times. In this article, Anderson mixing is proposed as a fixed-point convergence acceleration technique to reduce computational cost of thermal coupled fluid-solid problems. A number of other recently published methods with applications to similar fluid-structure interaction problems are also reviewed and analyzed. Numerical experiments are presented to illustrate relative performance of these methods on a test problem of rotating pre-swirl cavity air flow interacting with a turbine disk. It is observed that performance of Anderson mixing method is superior to that of other algorithms in terms of total iteration counts. Additional computational savings are demonstrated by reusing information from previously solved time steps. © All rights reserved 2012 Rolls-Royce plc.

O'Mahoney T, Hills N, Chew J (2012) Sensitivity of LES results from turbine rim seals to changes in grid resolution and sector size, Progress in Aerospace Sciences

Javiya U, Chew J, Hills N, Dullenkopf K, Scanlon T (2013) Evaluation of computational fluid dynamics and coupled fluid-solid modeling for a direct transfer preswirl system, Journal of Engineering for Gas Turbines and Power 135 (5)

The prediction of the preswirl cooling air delivery and disk metal temperature are important for the cooling system performance and the rotor disk thermal stresses and life assessment. In this paper, standalone 3D steady and unsteady computation fluid dynamics (CFD), and coupled FE-CFD calculations are presented for prediction of these temperatures. CFD results are compared with previous measurements from a direct transfer preswirl test rig. The predicted cooling air temperatures agree well with the measurement, but the nozzle discharge coefficients are under predicted. Results from the coupled FE-CFD analyses are compared directly with thermocouple temperature measurements and with heat transfer coefficients on the rotor disk previously obtained from a rotor disk heat conduction solution. Considering the modeling limitations, the coupled approach predicted the solid metal temperatures well. Heat transfer coefficients on the rotor disk from CFD show some effect of the temperature variations on the heat transfer coefficients. Reasonable agreement is obtained with values deduced from the previous heat conduction solution. © 2013 by ASME.

Amirante D, Sun Z, Chew JW, Hills NJ, Atkins NR (2016) MODELING OF COMPRESSOR DRUM CAVITIES WITH RADIAL INFLOW, Proceedings of ASME Turbo Expo 2016

Reynolds-Averaged Navier-Stokes (RANS) computations have been conducted to investigate the ?ow and heat trans-fer between two co-rotating discs with an axial through?ow of cooling air and a radial bleed introduced from the shroud. The computational ?uid dynamics (CFD) models have been cou-pled with a thermal model of the test rig, and the predicted metal temperature compared with the thermocouple data. CFD solutions are shown to vary from a buoyancy driven regime to a forced convection regime, depending on the radial in?ow rate prescribed at the shroud. At a high radial in?ow rate, the computations show an excellent agreement with the measured temperatures through a transient rig condition. At a low radial in?ow rate, the cavity ?ow is destabilized by the thermal strati?cation. Good qualitative agreement with the measurements is shown, although a signi?cant over-prediction of disc temperatures is observed. This is associated with under prediction of the penetration of the axial through?ow into the cavity. The mismatch could be the result of strong sensitivity to the prescribed inlet conditions, in addition to possible shortcomings in the turbulence modeling.

Ciampoli F, Hills NJ, Chew JW, Scanlon T (2008) UNSTEADY NUMERICAL SIMULATION OF THE FLOW IN A DIRECT TRANSFER PRE-SWIRL SYSTEM, PROCEEDINGS OF THE ASME TURBO EXPO 2008, VOL 4, PTS A AND B pp. 1647-1655 AMER SOC MECHANICAL ENGINEERS

Previous studies have indicated some differences between steady CFD predictions of flow in a rotor-stator disc cavity with rotating bolts compared to measurements. Recently time-dependent CFD simulations have revealed the unsteadiness present in the flow and have given improved agreement with measurements. In this paper unsteady Reynolds averaged Navier-Stokes (URANS) 3600 model CFD calculations of a rotorstator cavity with rotor bolts were performed in order to better understand the flow and heat transfer within a disc cavity previously studied experimentally by other workers. It is shown that the rotating bolts generate unsteadiness due to wake shedding which creates time-dependent flow patterns within the cavity. At low throughflow conditions, the unsteady flow significantly increases the average disc temperature. A systematic parametric study is presented giving insight into the influence of number of bolts, mass flow rate, cavity gap ratio and the bolts-to-shroud gap ratio on the time depended flow within the cavity.

Unsteady flow dynamics in turbine rim seals are known to be complex and attempts accurately to predict the interaction of the mainstream flow with the secondary air system cooling flows using computational fluid dynamics (CFD) with Reynolds-averaged Navier?Stokes (RANS) turbulence models have proved difficult. In particular, published results from RANS models have over-predicted the sealing effectiveness of the rim seal, although their use in this context continues to be common. Previous studies have ascribed this discrepancy to the failure to model flow structures with a scale greater than the one which can be captured in the small-sector models typically used. This article presents results from a series of Large-Eddy Simulations (LES) of a turbine stage including a rim seal and rim cavity for, it is believed by the authors, the first time. The simulations were run at a rotational Reynolds number Re ¼ 2.2 106 and a main annulus axial Reynolds number Rex ¼ 1.3 106 and with varying levels of coolant mass flow. Comparison is made with previously published experimental data and with unsteady RANS simulations. The LES models are shown to be in closer agreement with the experimental sealing effectiveness than the unsteady RANS simulations. The result indicates that the previous failure to predict rim seal effectiveness was due to turbulence model limitations in the turbine rim seal flow. Consideration is given to the flow structure in this region. K

Experimental measurements from a new single stage turbine are presented. The turbine has 26 vanes and 59 rotating blades with a design point stage expansion ratio of 2.5 and vane exit Mach number of 0.96. A variable sealing flow is supplied to the disc cavity upstream of the rotor and then enters the annulus through a simple axial clearance seal situated on the hub between the stator and rotor. Measurements at the annulus hub wall just downstream of the vanes show the degree of circumferential pressure variation. Further pressure measurements in the disc cavity indicate the strength of the swirling flow in the cavity, and show the effects of mainstream gas ingestion at low sealing flows. Ingestion is further quantified through seeding of the sealing air with nitrous oxide or carbon dioxide and measurement of gas concentrations in the cavity. Interpretation of the measurements is aided by steady and unsteady computational fluid dynamics solutions, and comparison with an elementary model of ingestion.

Large and flexible wind turbine blades may be susceptible to severe blade deformations coupled with dynamic stall. To advance prediction capability for this problem a general deforming mesh computational fluid dynamics (CFD) method has been developed for calculating flows with moving or deforming boundaries using an elastic spring analogy. The method has been evaluated against experimental data for flow around a pitching NACA0012 airfoil in the deep dynamic stall regime where flow is highly separated, and compared with other authors0 CFD simulations for pitching airfoil. The effects of varying the reduced frequency are also investigated. During the upstroke the present results are in generally good agreement with experiment and other CFD studies. During the downstroke some differences with experiment and other CFD models are apparent. This may be due to the sensitivity of the separated flow to modelling assumptions and experimental conditions. Overall, the degree of agreement between CFD and experiment is considered encouraging.

One of the key research topics in the computational fluid dynamics community is to improve the computational efficiency of steady-state finite volume codes. Real-world use cases require the solution to the Navier-Stokes equations for a wide range of Mach numbers, Reynolds numbers and mesh cell aspect ratios. This introduces stiffness in the discretised equations and therefore a slowdown in convergence. The community has pursued in particular two avenues to speed up the convergence of the corresponding error modes: Optimisation of Runge-Kutta coefficients for explicit Runge-Kutta schemes; and the introduction of implicit preconditioners, with a limited investigation of Runge-Kutta coefficients suitable to those implicit preconditioners. After proposing improvements to the implicit preconditioner, the present work proposes an optimisation procedure allowing the optimisation of the Runge-Kutta coefficients specifically for the implicit preconditioner. Employed on a realistic use case, the Runge-Kutta coefficients extracted with this method show a 20%?38% reduction of the number of iterations needed for convergence compared to Runge-Kutta coefficients recommended in the literature for comparable schemes and with the same computational cost per iteration.

Unsteady flow phenomena unrelated to the main gas-path blading have been identified in a number of turbine rim seal investigations. This unsteadiness has significant influence on the sealing effectiveness predicted by the conventional steady RANS (Reynolds-averaged Navier?Stokes) method, thus it is important for turbine stage design and optimisation. This paper presents CFD (computational fluid dynamics) modelling of a chute type rim seal that has been previously experimentally investigated. The study focuses on inherent large-scale unsteadiness rather than that imposed by vanes and blades or external flow. A large-eddy simulation (LES) solver is validated for a pipe flow test case and then applied to the chute rim seal rotor/stator cavity. LES, RANS and unsteady RANS (URANS) models all showed reasonable agreement with steady measurements within the disc cavity, but only the LES shows unsteadiness at a similar distinct peak frequency to that found in the experiment, at 23 times the rotational frequency. The boundary layer profile within the chute rim seal clearance has been scrutinised, which may explain the improvement of LES over RANS predictions for the pressure drop across the seal. LES results show a clockwise mean flow vortex. A more detailed sketch of the rim sealing flow unsteady flow structures is established with the help of the LES results. However, there are some significant differences between unsteadiness predicted and the measurements, and possible causes of these are discussed.

In this research a spectral element method is used to perform direct numerical simulation (DNS) and implicit large-eddy simulation (LES) of flows induced by centrifugal buoyancy in rotating cavities. These flows occur, for instance, in the compressor cavities of gas turbines internal air systems, in which cooling air is used to extract heat from compressor disks. Buoyancy-induced flows are inherently challenging to study using computational fluid dynamics (CFD), since turbulence models based on the Reynolds-averaged Navier-Stokes (RANS) equations are not able to provide an accurate description of the phenomena induced by the interplay between buoyancy and rotation. For this reason, model-free approaches are desirable, since they can provide an accurate description of the flow physics. First, the method is applied to a rotor/stator configuration, in which regions of laminar, transitional and fully turbulent flow coexist, and the results are compared with experimental data from the literature. Subsequently, flow induced by centrifugal buoyancy in a sealed rotating annulus is investigated using linear stability analysis, DNS and LES. It is shown that the onset of convection for a rotating cavity is similar to that for the problem of Rayleigh-Bénard convection. Analysing flow statistics for different values of the Rayleigh number, it is shown that the disk boundary layer behaves as a laminar Ekman layer, both in terms of its thickness and of its velocity profiles. This is observed even when instantaneous profiles are considered, despite the unsteadiness of the solution. The results also show that the shroud thermal boundary layer scaling is consistent with that of natural convection under gravity. Introducing an axial throughflow of cooling air, some features observed in the sealed cavity are maintained, however a strong reduction in the core temperature and a corresponding increase in the shroud heat transfer occur. The axial throughflow also promotes a significant increase in the range of frequencies observed inside the cavity.
In this thesis, improved and faster CFD based aero-thermo-mechanical methods that can be used to optimize engine configurations early in the design process are described. Axisymmetric models of 3D non-axisymmetric features such as protrusions, holes and honeycomb liners are developed for use in this context, and 3D unsteady CFD is used to investigate the flow physics. Initially, the research focussed on modelling of a rotor-stator disc cavity. Steady CFD validations for a plane disc and for a disc with protrusion were carried out and a simplified body force model was developed for including the 3D effects of rotating and stationary bolts into the axisymmetric CFD models. The simplified rotor bolt model was verified and validated by comparing the results with Sussex Windage rig test data and 3D CFD data. The simplified stator bolt model was verified using 3D CFD results. The simplified rotor bolt model was found to predict the drag and windage heat transfer with reasonable accuracy compared to 3D sector CFD results. However, 3D sector CFD under-predicts the high core flow swirl and the adiabatic disc surface temperature inboard of the bolt, compared to experimental data. In the second part of the study, unsteady Reynolds averaged Navier-Stokes (URANS) calculations of the rotating bolts cases were performed in order to better understand the flow physics. Although the rotor-stator cavity with bolts is geometrically steady in the rotating frame of reference, it was found that the rotor bolts generate unsteadiness which creates time-dependent rotating flow features within the cavity. A systematic parametric study is presented giving insight into the influence of the bolt number and the cavity geometric parameters on the time dependent flow within the cavity. The URANS calculations were extended to a high pressure turbine (HPT) rear cavity to show possible unsteady effects due to rotating bolts in an engine case. Following this, the body force model was adapted to model the rotating hole velocity changes and flow through honeycomb liners. The honeycomb and hole models were verified by comparing the results with available experimental data and 3D CFD calculations. In the final part of the study, coupled FE-CFD calculations for a preliminary design whole engine thermo-mechanical (WETM) model for a transient square cycle was performed including the effects of non-axisymmetric features. Six cavities around the HPT disc were modelled using CFD. The coupled approach provides more realistic physical convective heat transfer boundary conditions than the traditional approach. The unvalidated baseline thermo-mechanical model results were verified using the high fidelity coupled FE-CFD solution. It was demonstrated that the FE-CFD coupled calculations with axisymmetric modelling of 3D features can be achieved in a few days time scale suitable for preliminary engine design. The simplified CFD based methods described in this thesis could reduce the computational time of transient coupled FE-CFD calculations several orders of magnitude and may provide results as accurate as 2DFE-3DCFD coupled calculations.