Hans-Peter Kersken

Toward Excellence in Turbomachinery Computational Fluid Dynamics: A Hybrid Structured-Unstructured Reynolds-Averaged Navier-Stokes Solver

A three-dimensional hybrid structured-unstructured Reynolds-averaged Navier-Stokes (RANS) solver has been developed to simulate flows in complex turbomachinery geometries. It is built by coupling an existing structured computational fluid dynamics (CFD) solver with a newly developed unstructured-grid module via a conservative hybrid-grid interfacing algorithm, so that it can get benefits from the both structured and unstructured grids. The unstructured-grid module has been developed with consistent numerical algorithms, data structure, user interface and parallelization to those of the structured one. The numerical features of the hybrid RANS solver are its second-order accurate upwind scheme in space, its SGS implicit formulation of time integration, and its accurate modeling of steady/unsteady boundary conditions for multistage turbomachinery flows. The hybrid-grid interfacing algorithm is essentially an extension of the conservative zonal approach that has been previously applied on the mismatched zonal interface of the structured grids, and it is fully conservative and also second-order accurate. Due to the mismatched grids allowed at the block interface, users would have great flexibility to build the hybrid grids even with different structured and unstructured grid generators. The performance of the hybrid RANS solver is assessed with a variety of validation and application examples, through which the hybrid RANS solver has been demonstrated to be able to cope with the flows in complex turbomachinery geometries and to be promising for the future industrial applications.

Time-Linearized and Time-Accurate 3D RANS Methods for Aeroelastic Analysis in Turbomachinery

Graham Ashcroft Institute of Propulsion Technology, German Aerospace Center (DLR), Linder HA¶he, 51147 Cologne, Germany A computational method for performing aeroelastic analysis using either a time-linearized or an unsteady time-accurate solver for the compressible Reynolds averaged Navier--Stokes (RANS) equations is described. The time-linearized solver employs the assumption of small time-harmonic perturbations and is implemented via finite differences of the nonlinear flux routines of the time-accurate solver. The resulting linear system is solved using a parallelized generalized minimal residual (GMRES) method with block-local preconditioning. The time accurate solver uses a dual time stepping algorithm for the solution of the unsteady RANS equations on a periodically moving computational grid. For either solver, and both flutter and forced response problems, a mapping algorithm has been developed to map structural eigenmodes, obtained from finite element structural analysis, from the surface mesh of the finite element structural solver to the surface mesh of the finite volume flow solver. Using the surface displacement data an elliptic mesh deformation algorithm, based on linear elasticity theory, is then used to compute the grid deformation vector field. The developed methods are validated first using standard configuration 10. Finally, for an ultra-high bypass ratio fan, the results of the time-linearized and the unsteady module are compared. The gain in prediction time using the linearized methods is highlighted.

The Distributed Engineering Framework TENT

The paper describes TENT, a component-based framework for the integration of technical applications. TENT allows the engineer to design, automate, control, and steer technical workflows interactively. The applications are therefore encapsulated in order to build components which conform to the TENT component architecture. The engineer can combine the components to workflows in a graphical user interface. The framework manages and controls a distributed workflow on arbitrary computing resources within the network. Due to the utilization of CORBA, TENT supports all state-of-the-art programming languages, operating systems, and hardware architectures. It is designed to deal with parallel and sequential programming paradigms, as well as with massive data exchange. TENT is used for workflow integration in several projects, for CFD workflows in turbine engine and aircraft design, in the modeling of combustion chambers, and for virtual automobile prototyping.

A problem solving environment for multidisciplinary coupled simulations in computational grids

This paper describes a software environment for doing multidisciplinary coupled simulations in computational grids using a combination of various existing software tools.The environment consists of the software integration system TENT for setting up, steering and monitoring the simulation, the code coupling library MpCCI for the numerical coupling of simulation codes, and the MPI implementation MPICH-G2 and the Globus Toolkit for running the coupled codes in distributed environments.

The Discrete Adjoint of a Turbomachinery RANS Solver

Since adjoint flow solvers allow for the computation of sensitivities of global flow parameters under geometric variations in an amount of time which is nearly independent of the number of geometric parameters, automatic shape optimization can be accelerated considerably by the use of an adjoint solver. In this article, a systematic approach for the development of an exact discrete adjoint of a turbomachinery flow solver is described. By using finite differences to differentiate the numerical fluxes, the problems associated with automatic and hand differentiation are circumvented. Moreover, a general treatment of the adjoint numerical boundary conditions is presented. As a result, an exact adjoint boundary condition for the conservative mixing planes is obtained. In combination with nonreflecting boundary conditions the latter are crucial for accurate flow simulations in turbomachinery. The adjoint is validated on the basis of a transonic compressor stage.Copyright

A Harmonic Balance Technique for Multistage Turbomachinery Applications

This article describes a nonlinear frequency domain method for the simulation of unsteady blade row interaction problems across several blade rows in turbomachinery. The capability to efficiently simulate such interactions is crucial for the improvement of the prediction of blade vibrations, tonal noise, and the impact of unsteadiness on aerodynamic performance.The simulation technique presented here is based on the harmonic balance approach and has been integrated into an existing flow solver. A nontrivial issue in the application of harmonic balance methods to turbomachinery flows is the fact that various fundamental frequencies may occur simultaneously in one relative system, each one being due to the interaction of two blade rows. It is shown that, considering the disturbances corresponding to different fundamental frequencies as mutually uncoupled, one can develop an unsteady simulation method which from a practial view point turns out to be highly attractive. On the one hand, it is possible to take into account arbitrarily many nonlinear interaction terms. On the other, the computational efficiency can be increased considerably once it is known that the nonlinear coupling between certain subsets of the harmonics plays only a minor role.To validate the method and demonstrate its accuracy and efficiency a multistage compressor configuration is simulated using both the method described in this article and a conventional time-domain solver.Copyright

Advanced Numerical Methods for the Prediction of Tonal Noise in Turbomachinery—Part II: Time-Linearized Methods

This is the second part of a series of two papers on unsteady computational fluid dynamics (CFD) methods for the numerical simulation of aerodynamic noise generation and propagation. It focuses on the application of linearized RANS methods to turbomachinery noise problems. The convective and viscous fluxes of an existing URANS solver are linearized and the resulting unsteady linear equations are transferred into the frequency domain, thereby simplifying the solution problem from unsteady time-integration to a complex linear system. The linear system is solved using a parallel, preconditioned general minimized residual (GMRES) method with restarts. In order to prescribe disturbances due to rotor stator interaction, a so-called gust boundary condition is implemented. Using this inhomogeneous boundary condition, one can compute the generation of the acoustic modes and their near field propagation. The application of the time-linearized methods to a modern high-bypass ratio fan is investigated. The tonal fan noise predicted by the time-linearized solver is compared to numerical results presented in the first part and to measurements.

Nonreflecting Boundary Conditions for Aeroelastic Analysis in Time and Frequency Domain 3D RANS Solvers

This paper describes the implementation of a set of nonreflecting boundary conditions of increasing approximation quality for time-accurate and time-linearized 3D RANS solvers in the time and frequency domain. The implementations are based on the computation of eigenfunctions, either analytically or numerically, of the linearized Euler or Navier-Stokes equations for increasingly complex background flows. This results in a hierarchy of nonreflecting boundary conditions based on 1D characteristics, 2D circumferential mode decomposition, and 3D circumferential and radial mode decomposition, including viscous effects in the latter, for the frequency domain solver. By applying a Fourier transform in time at the boundaries the frequency domain implementations can be employed in the time domain solver as well. The limitations of each approximation are discussed and it is shown that increasing the precision of the boundary treatment the nonreflecting property of the boundary conditions is preserved for more complex flows without incurring an excessive increase in computing time.Results of a flutter analysis of a low pressure turbine blade obtained by time and frequency domain simulations are validated against each other and against reference results obtained with a 3D Euler frequency domain solver. The comparison of the results for different boundary conditions reveals the importance of using high quality boundary conditions.Copyright

HICFD: Highly Efficient Implementation of CFD Codes for HPC Many-Core Architectures

The objective of the German BMBF research project Highly Efficient Implementation of CFD Codes for HPC Many-Core Architectures (HICFD) is to develop new methods and tools for the analysis and optimization of the performance of parallel computational fluid dynamics (CFD) codes on high performance computer systems with many-core processors. In the work packages of the project it is investigated how the performance of parallel CFD codes written in C can be increased by the optimal use of all parallelism levels. On the highest level Message Passing Interface (MPI) is utilized. Furthermore, on the level of the many-core architecture, highly scaling, hybrid OpenMP/MPI methods are implemented. On the level of the processor cores the parallel Single Instruction Multiple Data (SIMD) units provided by modern CPUs are exploited.

Simulations of Unsteady Blade Row Interactions Using Linear and Non-Linear Frequency Domain Methods

This paper compares various approaches to simulate unsteady blade row interactions in turbomachinery. Unsteady simulations of turbomachinery flows have gained importance over the last years since increasing computing power allows the user to consider 3D unsteady flows for industrially relevant configurations. Furthermore, for turbomachinery flows, the last two decades have seen considerable efforts in developing adequate CFD methods which exploit the rotational symmetries of blade rows and are therefore up to several orders of magnitude more efficient than the standard unsteady approach for full wheel configurations.This paper focusses on the harmonic balance method which has been developed recently by the authors. The system of equations as well as the iterative solver are formulated in the frequency domain.The aim of this paper is to compare the harmonic balance method with the time-linearized as well as the non-linear unsteady approach. For the latter the unsteady flow fields in a fan stage are compared to reference results obtained with a highly resolved unsteady simulation. Moreover the amplitudes of the acoustic modes which are due to the rotor stator interaction are compared to measurement data available for this fan stage.The harmonic balance results for different sets of harmonics in the blade rows are used to explain the minor discrepancies between the time-linearized and unsteady results published by the authors in previous publications. The results show that the differences are primarily due to the neglection of the two-way coupling in the time-linearized simulations.Copyright