Figure: thermic loading of a high-pressure turbine stage caused byhot gas streaks.
Contact :
DLR - GermanAerospace Center
Institute of Propulsion TechnologyLinder Höhe51147 Cologne
Phone: 0 22 03/ 6 01-45 75Fax: 0 22 03/ 6 43 95
E-Mail: [email protected]@dlr.de
http://www.trace-portal.de
TRACE-Modern Simulation Technologies for Turbomachinery Design
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n turbomachinery design there is a clear trend towards shortened develop-ment cycles. Following the developmentof powerful computing resources, numer-ical simulation technologies now repre-sent an indispensable tool within thedevelopment cycle. At the Institute forPropulsion Technology of the GermanAerospace Center (DLR) a simulationsystem has been developed specificallyfor the calculation and investigation ofturbomachinery flows. After a decadeof development and validation the
simulation system has reached maturityunder the name TRACE (Turbo machineryResearch Aerodynamic ComputationalEnvironment) and become establishedas the standard method for the compu-tation of internal flows within DLR. Theprogram has been successfully used atresearch and university institutes for thescientific analysis of complex, unsteadyturbomachinery flows. Furthermore, thesystem has been applied in industry forseveral years in the development andoptimization of engine components. As
a result of the close cooperation withMTU-Aeroengines, industrial require-ments concerning data handling andefficiency have helped shape thedevelopment of the system.
Figure: turbulent loss areas of a lowpressure turbine within a real aero-engine.
Focus of Future Development
The already sophisticated simulationsystem TRACE continues to be developedfurther by more than ten scientists withinthe framework of internal, national andEuropean research projects. The closecooperation with the industrial partnersensures the orientation of developmentremains relevant to the design drivendemands of industry. An important focusof research is therefore turbulence andtransition modelling, with considerationof unsteady three-dimensional effectsas well as efficiency and roughness forindustrial applications. For the calculationof complex geometries TRACE has beenextended to an unstructured solver basedon the validated numerical techniquesof the structured solver. The couplingof structured and unstructured gridssimplifies grid generation and reducescosts for complex geometries, e.g. casecontouring or cavities. Furthermore,interdisciplinary problems of aeroelas-ticity, aeroacoustics and thermodynamics
are of increasing importance. For thermo-dynamic problems, coupled fluidstructurecalculations enable predictions of theheat transfer on blade surfaces. Anexample of a current numerical study isthe analysis of the thermal loading ofa high-pressure turbine stage withconsideration of hot gas streaks. Along-side the further development of physicalmodels, the development of improvednumerical methods, such as new temporalintegration methods to reduce compu-tation overheads, is important. Withinthe development framework of TRACEsoftware engineering plays an importantrole: the complete simulation processchain is integrated into a version manage-ment system, including the pre- andpost-processing tools, with a standardinterface (CGNS). In summary, theTRACE program has become a highlyaccurate and capable CFD platform formodelling turbo-machinery related flowphenomena.
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Development Status
From the beginning TRACE has beendeveloped specifically to investigate thehighly complex time-dependent turbo-machinery flows. Over time, a team ofscientists has developed a sophisticatedsimulation system with high qualitynumerical models. TRACE is a structuredmulti-block flow solver based on thefully compressible Navier-Stokes equations.An upwind spatial discretization schemecombined with an implicit time integra-tion method guarantees, alongsideefficiency and robustness, a highlyaccurate solution. Non-reflecting boundaryconditions and fully conservative inter-polation techniques enable a realisticreproduction of physical boundary con-ditions as well as a flexible coupling ofrotating and non-rotating grid blocks.
Figure upper left: density gradient ina stator-rotor configuration of a low-pressure compressor.
Figure upper right: pressure distributionon the blade surface of a turbinecascade: experiment-simulation.
Figure lower right: performance mapof a transsonic compressor stage:experiment-simulation
To account for the effects of turbulence,both the Wilcox k-omega model, andan extended version of the well knownSpalart and Allmaras model are available.Furthermore, transition criteria coupleddirectly with the turbulence modelsserve to detect the transient laminar/turbulent junctions common in manyturbomachinery flows. The system iswritten in the C programming languageand is embedded in a parallel environ-ment. The code is hardware independentand may therefore be run on any processorarchitecture. A very efficient parallelscalability is reached on machinesranging from simple PC clusters tomassively parallel super-computers withseveral hundred processors.
To establish confidence in a CFD programrequires extensive validation with existingcomputational and experimental data.Throughout TRACE's development thecode has been continuously validatedagainst numerous test cases. Whencom-paring experimental and compu-tational data a commonly used measurein turbo-machinery applications is thedistribution of static pressure over a bladesurface. The excellent agreement in thedata, and in particular the accurateprediction of the separation bubble atlow Reynolds number, and laminar-turbulent transition by bypass-transitionat higher Reynolds number, verify the
high quality of the simulation systemTRACE. The operating behaviour ofmulti-stage turbomachines can beestimated through integral parameterssuch as pressure rise and flow rate.Good agreement between the measuredand computed characteristic curves ofa given turbomachine indicates that thethree-dimensional flow-phenom-enaare captured to sufficient accuracy inthe numerical simulation
Flow Solutions
On account of component reductionand greater thrust requirements, theaerodynamic and mechanical require-ments of modern engines have risendramatically. As a consequence of thetrend towards compact designs, e.g.through the reduction of the axial-gapbetween adjacent blade rows, thesignificance of the interaction betweenblade rows has grown; and therefore adetailed knowledge of these phenomenais now of great importance in theoptimization of blade design. Theunderstanding of these phenomena isa task ideally suited to the TRACEsystem. As an example of the level ofaccuracy achievable, the results of atime-accurate compressor simulationare shown in terms of instantaneousdensity gradients (numerical Schlieren).
The rotor shock interacts with the up-stream stator inducing strong aero-dynamic and structural forces. Anaccurate prediction of theefficiency andlosses can only be achieved by accuratelyresolving these non-linear effects.
Coupled Applications
As methods are progressively refined,interdisciplinary questions become ofever greater importance. To determinethe properties of materials for instance,know-ledge of the thermal bladeloading is necessary. This can only beobtained through a coupled calculationof the outer flow and the heat conduc-tion within the blade. In a pilot projectthe TRACE code was successfully coupledwith a heat conduction process througha universal interface. Another interdis-ciplinary area of importance is aeroelasti-city. Through the interaction betweenthe flow and the blades, the danger(particularly in heavily loaded bladerows) of exciting blade vibrations exists.Operating conditions with large ampli-tude oscillations need to be excludedin the design phase as they may leadto blade failure in the extreme. For anaccurate prediction of the aeroelasticinteractions the simulation system
TRACE has a module for the inclusionof blade deformation. It has beendeveloped in cooperation with the DLRInstitute of Aeroelasticity in Göttingen.The unsteady aerodynamics and thestructural dynamics of the blades arecalculated simultaneously in a coupledsimulation. Different aeroelastic problemscan be considered: aerodynamically self-excited blade vibration (flutter) as wellas blade vibration excited by an externalaerodynamic perturbation (forced res-ponse). Current studies include forcedresponse calculations of a counter rotat-ing propfan. The excitation of the frontrotor, which is caused by the pressurefield of the rear rotor, can be determinedby analyzing the time-accurate simulationdata.
Acoustic Solutions
Engine noise is a topical research problemof considerable importance. Today, theradiation of discrete frequency sound,tonal noise, can already be calculatedwith the appropriate numerical methods.To accurately capture the sound waves,the amplitudes of which are three tofive orders of magnitude below atmo-spheric pressure, the TRACE code isbeing developed further to incorporate
high-order accurate spatial and temporalnumerical methods. With considerationof the necessary resolution, the firstcalculations have been successfullyperformed. The noise spectrum includesboth discrete frequency tonal andstochastic components, the latter beingknown as broadband noise. A promisingmethod for the calculation of broadbandnoise is the Large Eddy Simulationmethod. In this approach the largerturbulent scales are directly resolved.Simplifying assumptions about turbulenceare avoided. The finer turbulencestructures not resolved are taken intoaccount through so called sub-grid scalemodels. The results of such LES simula-tions are important in understandingthe formation mechanisms of the largeswirl structures that often occur in therear blade area of turbines.
Figure upper left: “Forced-Response”-simulation of a counter rotating propfan.Aerolastic excitation of front rotor bypressure field of rear rotor.
Figure upper right: vortex structures nearthe suction surface of a low pressureturbine as a result of a LES-simulation.
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