Ernesto Casartelli

Assessment of Various Turbulence Models in a High Pressure Ratio Centrifugal Compressor With an Object Oriented CFD Code

The flow field in a high pressure ratio centrifugal compressor with a vaneless diffuser has been investigated numerically. The main goal is to assess the influence of various turbulence models suitable for internal flows with an adverse pressure gradient. The numerical analysis is performed with a 3D RANS in-house modified solver based on an object-oriented open-source library. According to previous studies from varying authors, the turbulence model is believed to be the key parameter for the discrepancy between experimental and numerical results, especially at high pressure ratios and high mass-flow. Particular care has been taken at the wall, where a detailed integration of the boundary layer has been applied. The results present different comparisons between the models and experimental data, showing the influence of using advanced turbulence models. This is done in order to capture the boundary layer behavior, especially in large adverse pressure gradient single stage machinery.

Implementation of Explicit Density-Based Unstructured CFD Solver for Turbomachinery Applications on Graphical Processing Units

For the aerodynamic design of multistage compressors and turbines Computational Fluid Dynamics (CFD) plays a fundamental role. In fact it allows the characterization of the complex behaviour of turbomachinery components with high fidelity.Together with the availability of more and more powerful computing resources, current trends pursue the adoption of such high-fidelity tools and state-of-the-art technology even in the preliminary design phases. Within such a framework Graphical Processing Units (GPUs) yield further growth potential, allowing a significant reduction of CFD process turn-around times at relatively low costs.The target of the present work is to illustrate the design and implementation of an explicit density-based RANS coupled solver for the efficient and accurate numerical simulation of multi-dimensional time-dependent compressible fluid flows on polyhedral unstructured meshes. The solver has been developed within the object-oriented OpenFOAM framework, using OpenCL bindings to interface CPU and GPU and using MPI to interface multiple GPUs.The overall structure of the code, the numerical strategies adopted and the algorithms implemented are specifically designed in order to best exploit the huge computational peak power offered by modern GPUs, by minimizing memory transfers between CPUs and GPUs and potential branch divergence occurrences. This has a significant impact in terms of the speedup factor and is especially challenging within a polyhedral unstructured mesh framework. Specific tools for turbomachinery applications, such as Arbitrary Mesh Interface (AMI) and mixing-plane (MP), are implemented within the GPU context.The credibility of the proposed CFD solver is assessed by tackling a number of benchmark test problems, including Rotor 67 axial compressor, C3X stator blade with conjugate heat transfer and Aachen multi-stage turbine. An average GPU speedup factor of approximately S ≃ 50 with respect to CPU is achieved (single precision, both GPU and CPU in 100 USD price range). Preliminary parallel scalability test run on multiple GPUs show a parallel efficiency factor of approximately E ≃ 75%.Copyright

Assessment of transition modeling and compressibility effects in a linear cascade of turbine nozzle guide vanes

The flow field in a linear cascade of highly loaded turbine nozzle guide vanes has been numerically investigated at low and high subsonic regime, i.e. exit isentropic Mach number of M2is= 0.2 and 0.6, respectively. Extensive experimental data are available for an accurate assessment of the numerical procedure. Aerodynamic measurements include not only vane loading and pressure drop in the wake but also local flow features such as boundary-layer behavior along both pressure and suction sides of the vane, as well as secondary flow structures downstream of the trailing edge. Simulations were performed by using two CFD codes, a commercial one and an open-source based in-house code. Besides computations with the well-established SST k turbulence model assuming fully turbulent flow, transition models were taken into account in the present study. The original version of the -Remodel of Menter was employed. Suluksna - Juntasaro correlations for transition length (Flenght) and transition onset (Fonset) were also tested. The main goal was to establish essential ingredients for reasonable computational predictions of the cascade aerodynamic behavior, under both incompressible and compressible regime. This study showed that transition modelling should be coupled with accurate profiles of inlet velocity and turbulence intensity to get a chance to properly quantify aerodynamic losses via CFD method. However, additional weaknesses of the transition modeling have been put forward when increasing the outlet Mach number.

Development of High Order LES Solver for Heat Transfer Applications Based on the Open Source OpenFOAM Framework

The computation of heat transfer phenomena in gas turbines plays a key role in the continuous quest to increase performance and life of both component and machine.In order to assess different cooling approaches computational fluid dynamics (CFD) is a fundamental tool. Until now the task has often been carried out with RANS simulations, mainly due to the relatively short computational time. The clear drawback of this approach is in terms of accuracy, especially in those situations where averaged turbulence-structures are not able to capture the flow physics, thus under or overestimating the local heat transfer.The present work shows the development of a new explicit high-order incompressible solver for time-dependent flows based on the open source C++ Toolbox OpenFOAM framework. As such, the solver is enabled to compute the spatially filtered Navier-Stokes equations applied in large eddy simulations for incompressible flows.An overview of the development methods is provided, presenting numerical and algorithmic details.The solver is verified using the method of manufactured solutions, and a series of numerical experiments is performed to show third-order accuracy in time and low temporal error levels. Typical cooling devices in turbomachinery applications are then investigated, such as the flow over a turbulator geometry involving heated walls and a film cooling application. The performance of various sub-grid-scale models are tested, such as static Smagorinsky, dynamic Lagrangian, dynamic one-equation turbulence models, dynamic Smagorinsky, WALE and sigma-model. Good results were obtained in all cases with variations among the individual models.Copyright

Object-Oriented Open-Source CFD for Turbomachinery Applications: A Review and Recent Advances

CFD is nowadays an indispensable tool for the design and analysis of high performance turbomachinery. In the community, the commercial codes play a major role, offering specific features, especially in pre- and postprocessing, but also in the solver technology, suited to ease the approach to CFD for the development engineers. Nevertheless, in the last ten years many institutions, both in academia and in industry, have seriously started to look at alternatives in order to have more flexibility in the implementation of specific models and more freedom in the large-scale usage of CFD, like for example in automated optimization-processes and L-DES computations. Among the different possibilities, the open-source toolbox OpenFOAM® has been used in the turbomachinery community over a broad spectrum of applications, showing how well-suited it is for customization and how reliable the results can become, when the implemented solver-technology is upgraded to state-of-the-art models.In the present publication a review of the community and some of the author’s development-activities are presented. About the latter, selected applications are presented, from transonic internal flow to turbine cooling and conjugate heat transfer, showing how specific customizations can allow to achieve industrially relevant results. Details on recent advances like the implementation of a fully-implicit mixing-plane interface and of a transition model show how competitive this environment can be. Moreover an outlook of ongoing future developments of the solver technology, such as a pressure-based coupled solver, is also given.Copyright

AN OBJECT-ORIENTED CFD CODE FOR OPTIMIZATION OF HIGH PRESSURE RATIO COMPRESSORS

The flow fields and performances of different transonic radial compressors of varying geometries and conceptual desi gns have been studied numerically. All the simulations were per formed with a modified in-house 3D RANS solver based on an object-oriented open-source library. The solver uses an Al lMach algorithm with a special treatment for the pressure cor rector equation to deal with highly compressible flows. The 3 D flow field structures, the characteristics and integral quantities have been compared to the results of established, state-of- the-art commercial solvers as well as to measurements whenever poss ible. This paper demonstrates for various configurations tha t the main flow features and the flow characteristics have been captured by the new solver. Furthermore, the new solver is also c apable of computing the delta variations of similar designs.This is an essential step for the broad application of the new solv er for optimization design cycles.

Flow Investigation in a Disk Micropump

The flow field in a disk micropump is investigated using CFD. The results are validated with experimental data, showing good agreement. Flow field details are presented for different pump parameters. In order to better understand the influence of the pump geometry and fluid properties on the pump performance, an analytical model for the pump head has been derived. Since the pumping mechanism is mainly due to viscous forces, a typical turbomachinery approach, which is based on the effect of centrifugal forces, is not appropriate. The proposed analytical model, which is general and can also be used for scale up purposes, shows that the non-dimensional results of CFD and measurements matches very well.Copyright

Assessment of an implicit mixing plane approach for pump-turbine applications

In the design process of pump-turbines, both in pump and turbine mode, the assessment of the components matching is very important. In order to be fast in this task, the usual procedure is based on steady-state methods, like the frozen-rotor or the mixing-plane method. The frozen-rotor approach is straight forward and relatively easy to implement, but can produce unphysical behavior, mainly depending on the relative position of the components. On the other hand, the mixing-plane has a more physical background, delivering to the downstream component the mixed-out state of the upstream flow. On how the mixed-out state is computed and imposed to the downstream component there are different methodologies. In the present paper a novel, fully-implicit mixing-plane method is presented and applied to pump-turbine applications, both in pump and turbine mode. The major advantage of this approach is its robustness, including the ability to handle back flow at the interface, and accuracy. Compared to currently available methods, both in proprietary and commercial codes, the implicit approach leads to a consistent treatment of the interface, enforcing natively the idea of the mixing-plane, i.e. constant spanwise-distribution of the quantities. This allows to obtain excellent results also at part- and over-load.

Implementation of a CFD-Based Aeroelastic Analysis Toolbox for Turbomachinery Applications

The understanding of aeroelastic phenomena is fundamental for the structural integrity of many applications in aerospace and mechanical engineering and even in some other disciplines (e.g. civil engineering) where flexible structures possibly undergo unsteady fluid-dynamic loads. Therefore the availability of accurate analysis tools for the study of the aeroelastic interaction between aerodynamic and elastic forces is an important asset for the design of modern, high performance turbomachinery.Together with the more and more powerful computing resources, current trends pursue the adoption of high-fidelity tools and state-of-the-art technology within the research fields of Computational Structural Dynamics (CSD) and Computational Fluid Dynamics (CFD). This choice is somehow obliged when dealing with highly non-linear aeroservoelastic phenomena.The approach typically used for turbomachinery aeroelastic analysis features the so-called “one-way coupling”, i.e. the loads predicted by the aerodynamic model are transferred to the structural model to evaluate relevant stresses and displacements.The objective of the present work is to illustrate the design and implementation of a platform for solving multidisciplinary non-linear Fluid-Structure Interaction (FSI) problems with a “two-way coupling” or fully coupled approach, that is linking together high-fidelity state-of-the-art CSD and CFD tools by means of a robust, flexible aeroelastic interface scheme.The credibility of the proposed aeroservoelastic analysis toolbox is assessed by tackling a set of aeronautical and turbomachinery-oriented benchmark test problems such as: the evaluation of the fully coupled non-linear aeroelastic trim of HIRENASD (HIgh REynolds Number AeroStructural Dynamics) wing and the identification of the aerodynamic damping coefficient of Standard Configuration 10, high subsonic/transonic, 2D/3D compressor cascade. The results are compared with reference experimental and numerical data available in literature.© 2014 ASME

Reviewing the Implicit Mixing Plane Approach: Theoretical and Applied Cases

The CFD assisted design of modern single- and multi-stage turbomachines is usually performed with the mixing plane approach in order to assess the components matching. While the mixing-plane state-of-the-art is based on a boundary-condition based approach, hereafter called explicit, the authors presented last year a novel, fully implicit method, which shows considerable advantages compared to the explicit one. In the present paper the quality and advantages of the novel approach compared to the state-of-the-art will be shown through a variety of detailed examples. The main issues discussed are the built-in ability to reduce incoming disturbances and to manage backflow at the interface due to the implicit formulation. With selected cases it is shown that no special care has to be taken to avoid reflections at the interface also for inviscid transonic or fully-supersonic cases. Moreover, detailed results for a high-pressure centrifugal compressor are presented, showing that the proposed approach is able to capture both, the global behavior as well as local flow-features over the complete speed-line, while the explicit approach partially fails on the same test case.Copyright