Luca Mangani

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.

Development of a Novel Fully Coupled Solver in OpenFOAM: Steady-State Incompressible Turbulent Flows

In this work a block coupled algorithm for the solution of three-dimensional incompressible turbulent flows is presented. A cell-centered finite-volume method for unstructured grids is employed. The interequation coupling of the incompressible Navier-Stokes equations is obtained using a SIMPLE-type algorithm with a Rhie-Chow interpolation technique. Due to the simultaneous solution of momentum and continuity equations, implicit block coupling of pressure and velocity variables leads to faster convergence compared to classical, loosely coupled, segregated algorithms of the SIMPLE family of algorithms. This gain in convergence speed is accompanied by an improvement in numerical robustness. Additionally, a two-equation eddy viscosity turbulence model is solved in a segregated fashion. The substnatially improved performance of the block coupled approach compared to the segregated approach is demonstrated in a set of test cases. It is shown that the scalability of the coupled solution algorithm with increasing numbers of cells is nearly linear. To achieve this scalability, an algebraic multigrid solver for block coupled systems of equations has been implemented and used as linear solver for the system of block equations. The presented algorithm has been entirely embedded into the leading open-source computational fluid dynamics (CFD) library OpenFOAM.

Development of a Novel Fully Coupled Solver in OpenFOAM: Steady-State Incompressible Turbulent Flows in Rotational Reference Frames

In this article, the fully coupled block algorithm for the solution of three-dimensional incompressible turbulent flows presented in a companion article [1] is extended for use with multiple reference frames and multiple mesh blocks. The implicit block coupling is applied to the extra rotational terms, and to the multiblock interfaces. Furthermore, implementation details on the linearization of cyclic and other boundary conditions are detailed. These modifications allow the coupled solver to retain its improved performance and robustness in addition to mesh size scalability while solving turbomachinery-type applications. The performance and mesh size scalability of the coupled solver is compared to that of a segregated pressure based solver [2] using three industrial-size test cases.

General fully implicit discretization of the diffusion term for the finite volume method

ABSTRACT In this paper, a fully implicit method for the discretization of the diffusion term is presented in the context of the cell-centered finite volume method. The newly developed fully implicit method is denoted by the modified implicit nonlinear diffusion (MIND) scheme. The method is used to solve several isotropic and anisotropic diffusion problems in two-dimensional domains covered with structured (quadrilateral elements) and unstructured (triangular elements) grid systems. The comparison of generated results with similar ones obtained using the semi-implicit scheme demonstrates the superior robustness and accuracy of the MIND scheme and its good convergence characteristics for all types of meshes.

An OpenFOAM pressure-based coupled CFD solver for turbulent and compressible flows in turbomachinery applications

ABSTRACT In this article, a recently developed pressure-based, fully coupled solver capable of predicting fluid flow at all speeds is extended to deal with turbulent flows in a rotating frame of reference and emphasizing turbomachinery applications. The pressure–velocity coupling at the heart of the Navier-Stokes equations is resolved by deriving a pressure equation in a similar fashion to a segregated SIMPLE algorithm but with implicit treatment of the velocity and pressure fields. The resulting system of coupled equations is solved using an algebraic multigrid solver. The above numerical procedures have been implemented within OpenFOAM®, which is an open-source code framework capable of dealing with industrial-scale flow problems. The OpenFOAM-based coupled solver is validated using experimental and numerical data available from reference literature test cases as well as with a segregated solver based on the SIMPLE algorithm. This is done in addition to evaluating its performance by solving an industrial problem. In comparison with the segregated solver, the coupled solver results indicate substantial reduction in computational cost with increased robustness.

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

3D casing-distributor analysis with a novel block coupled OpenFOAM solver for hydraulic design application

Nowadays, computational fluid dynamics is commonly used by design engineers to evaluate and compare losses in hydraulic components as it is less expensive and less time consuming than model tests. For that purpose, an automatic tool for casing and distributor analysis will be presented in this paper. An in-house mesh generator and a Reynolds Averaged Navier-Stokes equation solver using the standard k-ω SST turbulence model will be used to perform all computations. Two solvers based on the C++ OpenFOAM library will be used and compared to a commercial solver. The performance of the new fully coupled block solver developed by the University of Lucerne and Andritz will be compared to the standard 1.6ext segregated simpleFoam solver and to a commercial solver. In this study, relative comparisons of different geometries of casing and distributor will be performed. The present study is thus aimed at validating the block solver and the tool chain and providing design engineers with a faster and more reliable analysis tool that can be integrated into their design process.

A fully coupled OpenFOAM® solver for transient incompressible turbulent flows in ALE formulation

ABSTRACT In this article, the previously developed single block fully coupled algorithm [1,2] for solving three-dimensional incompressible turbulent flows is extended to resolve transient flows in multiple rotating reference frames using the arbitrary Lagrange–Euler (ALE) formulation. Details on the discretization of ALE terms along with a recently developed extension to the conservative and fully implicit treatment of multi-block interfaces into three-dimensional space are presented. To account for turbulence, the kω − SST turbulence model in ALE formulation is solved using Navier–Stokes equations. This multi-block transient coupled algorithm is embedded within the OpenFOAM® Computational Fluid Dynamics (CFD) library, and its performance evaluated in a real case involving a turbulent flow field in a swirl generator by comparing numerical predictions with experimental measurements.

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.

Implementation of boundary conditions in the finite-volume pressure-based method—Part I: Segregated solvers

ABSTRACT The paper deals with the formulation of a variety of boundary conditions for incompressible and compressible flows in the context of the segregated pressure-based unstructured finite volume method. The focus is on the derivation and the implementation of these boundary conditions and their relation to the various physical boundaries and geometric constraints. While a variety of boundary conditions apply at any of the physical boundaries (inlets, outlets, and walls), geometric constraints define the type of boundary condition to be used. The emphasis is on relating the mathematical derivation of the boundary conditions to the algebraic equations defined at each centroid of the boundary elements and their coefficients. All derived boundary conditions are validated through a set of test cases with comparison of computed results to available numerical and/or experimental data.