Investigation of mixed element hybrid grid-based CFD methods for rotorcraft flow analysis

Accurate first-principles flow prediction is essential to the design and development of rotorcraft, and while current numerical analysis tools can, in theory, model the complete flow field, in practice the accuracy of these tools is limited by various inherent numerical deficiencies. An approach that combines the first-principles physical modeling capability of CFD schemes with the vortex preservation capabilities of Lagrangian vortex methods has been developed recently that controls the numerical diffusion of the rotor wake in a grid-based solver by employing a vorticity-velocity, rather than primitive variable, formulation. Coupling strategies, including variable exchange protocols are evaluated using several unstructured, structured, and Cartesian-grid Reynolds Averaged Navier-Stokes (RANS)/Euler CFD solvers. Results obtained with the hybrid grid-based solvers illustrate the capability of this hybrid method to resolve vortex-dominated flow fields with lower cell counts than pure RANS/Euler methods

Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

A Generalized Compressible Cavitation Model

A new multi-phase model for low speed gas/liquid mixtures is presented; it does not require ad-hoc closure models for the variation of mixture density with pressure and yields thermodynamically correct acoustic propagation for multi-phase mixtures. The solution procedure has an interface-capturing scheme that incorporates an additional scalar transport equation for the gas void fraction. Cavitation is modeled via a finite rate source term that initiates phase change when liquid pressure drops below its saturation value. The numerical procedure has been implemented within a multi-element unstructured framework CRUNCH that permits the grid to be locally refined in the interface region. The solution technique incorporates a parallel, domain decomposition strategy for efficient 3D computations. Detailed results are presented for sheet cavitation over a cylindrical headform and a NACA 66 hydrofoil

A coupled CFD approach for combustor-turbine interaction

The current approach in the industry to numerically investigate the flow in a gas turbine considers each component, such as combustor and turbine, as a stand-alone part, involving no or very minor interactions with other parts, mainly applied through static boundary conditions. Efficient and very specialised CFD codes have been developed in the past to address the different flow characteristic occurring in the different regions of the engine. In order to meet the future requirements in terms of fuel consumption and pollutants emissions, an integrated approach capable of capturing all the possible interactions between different components is necessary. An efficient and accurate way to achieve integrated simulations is to couple already existing specialised codes in a zonal type of coupling. In this Thesis work a methodology to couple an incompressible/low-Mach number pressure-based combustion code with a compressible density-based turbomachinery code for industrial application has been developed. In particular two different couplings have been implemented: the first, based on the exchange of existing boundary conditions through files, comes as a completely separated tools from the original codes, of which no modifications are required, and it is applied to steady state simulations; the second instead, based on the exchange of boundary conditions and body forces through message passing, requires some modifications of the source codes and it is applied to both steady and unsteady cases. A simple analysis shows that not all the primitive variables can be made continuous at the coupling interface between the two codes and a compromise was found that allows minor discontinuity in some of the variables while achieving mass flow conservation and continuity of the temperature profiles. The coupling methodology has been applied to a simplified but realistic industrial case, consisting of a RQL (Rich Burn - Quick quench - Lean burn) combustor coupled with the first stage of the HP turbine. The analysis of the steady case has shown that the combustor field is affected as far as 150% axial chord lengths upstream of the blades leading edge, affecting RTDF and OTDF at the interfaces. In the turbine stage significant differences in both efficiency and degree of reaction were found in the coupled cases with respect to standard standalone simulations using radial inlet profiles. The analysis of the unsteady simulation has instead shown the hot streaks behaviour across the turbine, that are only partially mitigated by the stator blades and, due to segregation effect of hot and cold gases, migrate towards the pressure side of the rotor blades

OpenFOAM Simulations of Atmospheric-Entry Capsules in the Subsonic Regime

The open-source Computational Fluid Dynamics software OpenFOAM is gaining wider acceptance in industry and academia for incompressible flow simulations. To date, there has been relatively little utilization of OpenFOAM for compressible external aerodynamic applications. The numerous turbulence models available in OpenFOAM makes it an attractive option for evaluating alternate Reynolds-Averaged Navier-Stokes (RANS) turbulent models to assess separated flow on atmospheric entry vehicles in the subsonic regime, where traditional turbulent models show reduced accuracy. This paper presents simulations of an axisymmetric capsule geometry at subsonic conditions using an OpenFOAM compressible flow solver. These results are compared with results from the NASA CFD code OVERFLOW and experimental data. These OpenFOAM simulations serve as a basis to explore OpenFOAMs extended turbulence models on compressible separated flows such as found on entry capsules

Cfd Analysis Of Helicopter Rotor-fuselage Flow Interaction In Hovering And Forward Flight Conditions

Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2016Thesis (Ph.D.) -- İstanbul Technical University, Institute of Science and Technology, 2016Askı ve ileri uçuş durumunda zorlu rotor-gövde akış etkileşim problemini incelemek için zamana bağlı sıkıştırılabilir akış analizleri gerçekleştirilmiştir. Sistemi oluşturan herbir bileşenin akış yapısı üzerindeki etkilerini irdelemek için izole gövde ve izole rotor konfigürasyonları ele alınmıştır. Daha sonra, bileşenlerin birbirlerine olan etkilerini incelemek amacıyla sistemin tamamı analize tabi tutulmuştur. İzole gövde analizleri RANS tabanlı daimi hesaplamalara dayanmaktadır. Rotor palalarını içeren durumlar için ise URANS çözümleri gerçekleştirilmiştir. Akışın türbülanslı doğasını modellemek için daha güvenilir sonuç ürettiği analizler ile tespit edilmiş olan Realizable k-ε türbülans modeli kullanılmıştır. Zamana bağlı rotor analizleri üç farklı ilerleme oranı için gerçekleştirilmiştir. Hava yükleri nedeniyle palada gözlemlenen dinamik hareketler azimut açısı ile periyodik bir şekilde değişim gösterirken, aynı zamanda ilerleme oranına bağlı olarak da değişim göstermektedir. Palanın tanımlı hareketleri, mevcut kod yetenekleri ile temsil edilememektedir. Fakat, bu dinamik hareketler ticari HAD yazılımı içerisine kullanıcı tarafından yazılan bir kod vasıtasıyla simülasyon modeline dahil edilebilmektedir. Bilhassa ileri uçuş şartlarında daha belirgin olan çırpma ve yunuslama hareketlerini modellemek için birinci mertebe Fourier serilerinden yararlanılarak bir UDF kodu yazılmıştır. Hesaplama hacmi düzensiz yapıda olup karma elemanlardan oluşmaktadır. Dinamik çözüm ağı yaklaşımlarında sıklıkla görülen problemler çözüm ağı deformasyonu ve çözüm ağı oluşturma yöntemlerinin kullanıldığı dinamik ağlar ile aşılmıştır. Mevcut sayısal çalışmanın doğruluğu deneyler ve diğer sayısal çalışmaların sonuçları ile karşılaştırılarak ortaya konmuştur. Benzer başarılı sonuçlar, daha az sayıda çözüm ağı kullanılarak elde edilmiştir. Bu nedenle, mevcut yöntem hesaplama süresinde azalma sağlamakta ve makul hesaplama kaynağı kullanımını mümkün kılmaktadır.Unsteady compressible flow analyses are carried out to investigate the challenging helicopter rotor–fuselage interaction problem in hover and forward flight conditions. First, the isolated fuselage and the isolated rotor configurations are analyzed to examine the individual effects of each component on the flow field. Then, the rotor-fuselage interaction problem is considered. The isolated fuselage analyses are based on the steady RANS computations. URANS simulations are carried out for the cases with rotor blades. The Realizable k-ε turbulence model is found to perform best for the predictions. The time-dependent rotor analyses are simulated at three different advance ratios. The blade dynamic motions excited by the air loads, which vary periodically in the azimuth direction and also differ based on the advance ratio, have been prescribed by a UDF code embedded into the solver, since these motions cannot be directly represented with the existing commercial code capabilities. Azimuthal variations of the flap and pitch motions of the blades are prescribed a priori as a first order Fourier series through User Defined Function feature of the code. The computational domain was modeled by unstructured hybrid mesh elements. Commonly seen dynamic mesh problems are alleviated by appropriately formed dynamic grids using the spring based smoothing and cell re-meshing methods. The accuracy of the present numerical predictions has been demonstrated by the comparison of obtained results with the experiments and other numerical results available in the open literature. The present single grid methodology has given similar successful results with much lower number of grid elements, thus resulting in much shorter computing times, using modest computational power.DoktoraPh.D

Forty Years of Computational Fluid Dynamics Research in India-Achievements and Issues

A review of the emergence and maturing of computational fluid dynamics (CFD) research in India over the last four decades is presented. The status of in-house developed CFD codes in various aerospace laboratories and academic institutions in the country is described along with their strengths and weaknesses. Although, some level of maturity is achieved in CFD to address the external flow problems of an aerospace vehicle, the slow growth of indigenous reacting CFD codes forced Indian aerospace industry to depend solely on the commercial software for addressing the internal flow problems related to propulsion and combustion. A brief account of various technical and managerial issues in CFD development is presented. A roadmap is proposed for the graduation of CFD codes, from analysis tool to design tool.Defence Science Journal, 2010, 60(6), pp.567-576, DOI:http://dx.doi.org/10.14429/dsj.60.60

Numerical study of fluidic oscillators with compressible flow

Se estudiará el fllujo en el interior de osciladores fluídicos mediante el uso de un código abierto de Mecánica de Fluidos Computacional, prestando especial atención al comportamiento con flujo compresible.1. Documentación y estudio del estado del arte. 2. Aprendizaje de los conceptos básicos de la Mecánica de Fluidos Computacional. 3. Aprendizaje del software OpenFOAM. 4. Mallado del oscilador fluídico de referencia. 5. Lanzamiento de las simulaciones. 7. Extracción y análisis de resultados, comparándolos con los resultados obtenidos en simulaciones con flujo incompresible. 8. Conclusiones

A Continuous/Discontinuous FE Method for the 3D Incompressible Flow Equations

A projection scheme for the numerical solution of the incompressible Navier-Strokes equation is presented. Finite element discontinuous Galerkin (dG) discretization for the velocity in the momentum equations is employed. The incompressibility constraint is enforced by numerically solving the Poisson equation for pressure using a continuous Galerkin (cG) discretization. The main advantage of the method is that is does not require the velocity and pressure approximation spaces to satisfy the usual inf-sup condition, thus equal order finite element approximations for both velocity and pressure can be used. Furthermore, by using cG discretization for the Poisson equation, no auxiliary equations are needed as it is required for dG approximations of second order derivatives. In order to enable large time steps for time marching to steady-state and time evolving problems, implicit scheme is used in connection with high order implicit RK methods. Numerical tests demonstrate that the overall scheme is accurate and computationally efficient

A numerical simulation of flow over a NACA0025 airfoil using Large Eddy Simulation turbulence models

The ability of computational fluid dynamic (CFD) to predict critical flow characteristics has always been questionable. Flow separation over lifting surfaces such as airfoils are one of the critical features which can significantly deteriorate their aerodynamic performances. The purpose of this research is to evaluate the accuracy of two CFD methods in predicting the flow separation over a NACA0025 airfoil at low Reynolds numbers. The first code is an in-house code which is based on a 3D compressible Navier-Stokes solver with preconditioning and self-adaptive upwinding methods. The second code is the commercial FLUENT software. In order to accurately simulate the laminar boundary layer separation, the Large Eddy Simulation (LES) method is used for turbulence modeling of both codes. Results comparison shows that Fluent is not able to capture this feature. In addition the results are also compared with another similar numerical simulation and validated with available experimental data