Block-structured grids for complex aerodynamic configurations: Current status

The status of CFD methods based on the use of block-structured grids for analyzing viscous flows over complex configurations is examined. The objective of the present study is to make a realistic assessment of the usability of such grids for routine computations typically encountered in the aerospace industry. It is recognized at the very outset that the total turnaround time, from the moment the configuration is identified until the computational results have been obtained and postprocessed, is more important than just the computational time. Pertinent examples will be cited to demonstrate the feasibility of solving flow over practical configurations of current interest on block-structured grids

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

An implicit hybrid method for the computation of rotorcraft flows

There is a wide variety of CFD grid types including Cartesian, structured, unstructured and hybrids, as well as, numerous methodologies of combining these to reduce the time required to generate high-quality grids around complex configurations. If the grid methodologies were implemented in different codes, they should be written in such a way as to obtain the maximum performance from the available computer resources. A common interface should also be required to allow for ease of use. However, it is very time consuming to develop, maintain and add extra functionally to different codes. This paper examines the possibility of taking an existing CFD solver, the Helicopter Multi-Block (HMB) CFD method, and implementing a new grid type while reusing as much as possible the original code base. The paper presents some of the challenges encountered in extending the code which was written for a single mesh type, to a more flexible solver that is still computationally efficient but can cope with a variety of grid types

Advances in Time-Domain Electromagnetic Simulation Capabilities Through the Use of Overset Grids and Massively Parallel Computing

A new methodology is presented for conducting numerical simulations of electromagnetic scattering and wave propagation phenomena. Technologies from several scientific disciplines, including computational fluid dynamics, computational electromagnetics, and parallel computing, are uniquely combined to form a simulation capability that is both versatile and practical. In the process of creating this capability, work is accomplished to conduct the first study designed to quantify the effects of domain decomposition on the performance of a class of explicit hyperbolic partial differential equations solvers; to develop a new method of partitioning computational domains comprised of overset grids; and to provide the first detailed assessment of the applicability of overset grids to the field of computational electromagnetics. Furthermore, the first Finite Volume Time Domain (FVTD) algorithm capable of utilizing overset grids on massively parallel computing platforms is developed and implemented. Results are presented for a number of scattering and wave propagation simulations conducted using this algorithm, including two spheres in close proximity and a finned missile

A Coupled CFD/CSD Investigation of the Effects of Leading Edge Slat on Rotor Performance

A coupled Computational Fluid Dynamic (CFD) and Computational Structural Dynamics (CSD) methodology is extended to analyze the effectiveness of a leading edge slat (LE-Slat) for mitigating the adverse effects of dynamic stall on rotor blade aerodynamic and dynamic response. This involved the following improvements over the existing CFD methodology to handle a multi-element airfoil rotor: incorporating the so-called Implicit Hole Cutting method for inter-mesh connectivity, implementing a generalized force transfer routine for transferring LE-Slat loads onto the main blade, and achieving increased parallelization of the code. Initially, the structured overset mesh CFD solver is extensively validated against available 2-D experimental wind tunnel test cases in steady and unsteady flight conditions. The solver predicts the measurements with sufficient accuracy for test cases with both the baseline airfoil and that with two slat configurations, S-1 and S-6. As expected, the addition of the slat is found to be highly effective in delaying stall until larger angles for the case of a static airfoil and ameliorating the effects of dynamic stall for a 2-D pitching airfoil. The 3-D coupled CFD/CSD model is extensively validated against flight test data of a UH-60A rotor in a high-altitude, high-thrust flight condition, namely C9017, characterized by distinct dynamic stall events in the retreating side of the rotor disk. The validated rotor analysis tool is then used to successfully demonstrate the effectiveness of a LE-Slat in mitigating (or eliminating) dynamic stall on the rotor retreating side. The calculations are performed with a modified UH-60A blade with a 40%-span slatted airfoil section. The addition of the slat is effective in the mitigation (and/or elimination) of lift and moment stall at outboard stations, which in turn is accompanied by a reduction of torsional structural loads (upto 73%) and pitch link loads (upto 62%) as compared to the baseline C9017 values. The effect of a dynamically moving slat, actuating between slat positions S-1 and S-6, is thoroughly investigated, firstly on 2-D airfoil dynamic stall, and then on the UH-60A rotor. Three slat actuation strategies with upto [1, 3, 5]/rev harmonics, respectively, are considered. However, it is noted that the dynamic slat does not necessarily result in better rotor performance as compared to a static slat configuration. The coupled CFD/CSD platform is further used to successfully demonstrate the capability of the slat (S-6) to achieve upto 10% higher thrust than C9017, which is beyond the conventional thrust limit imposed by McHugh's stall boundary. Stall mitigation due to the slat results in a reduction of torsional load upto 54% and reduction of pitch link load upto 32% as compared to the baseline C9017 flight test values, even for an increase in thrust of 10%

HPCCP/CAS Workshop Proceedings 1998

This publication is a collection of extended abstracts of presentations given at the HPCCP/CAS (High Performance Computing and Communications Program/Computational Aerosciences Project) Workshop held on August 24-26, 1998, at NASA Ames Research Center, Moffett Field, California. The objective of the Workshop was to bring together the aerospace high performance computing community, consisting of airframe and propulsion companies, independent software vendors, university researchers, and government scientists and engineers. The Workshop was sponsored by the HPCCP Office at NASA Ames Research Center. The Workshop consisted of over 40 presentations, including an overview of NASA's High Performance Computing and Communications Program and the Computational Aerosciences Project; ten sessions of papers representative of the high performance computing research conducted within the Program by the aerospace industry, academia, NASA, and other government laboratories; two panel sessions; and a special presentation by Mr. James Bailey

Deformable Overset Grid for Multibody Unsteady Flow Simulation

A deformable overset grid method is proposed to simulate the unsteady aerodynamic problems with multiple flexible moving bodies. This method uses an unstructured overset grid coupled with local mesh deformation to achieve both robustness and efficiency. The overset grid hierarchically organizes the subgrids into clusters and layers, allowing for overlapping/embedding of different type meshes, in which the mesh quality and resolution can be independently controlled. At each time step, mesh deformation is locally applied to the subgrids associated with deforming bodies by an improved Delaunay graph mapping method that uses a very coarse Delaunay mesh as the background graph. The graph is moved and deformed by the spring analogy method according to the specified motion, and then the computational meshes are relocated by a simple one-to-one mapping. An efficient implicit hole-cutting and intergrid boundary definition procedure is implemented fully automatically for both cell-centered and cell-vertex schemes based on the wall distance and an alternative digital tree data search algorithm. This method is successfully applied to several complex multibody unsteady aerodynamic simulations, and the results demonstrate the robustness and efficiency of the proposed method for complex unsteady flow problems, particularly for those involving simultaneous large relative motion and self-deformation

NAS technical summaries. Numerical aerodynamic simulation program, March 1992 - February 1993

NASA created the Numerical Aerodynamic Simulation (NAS) Program in 1987 to focus resources on solving critical problems in aeroscience and related disciplines by utilizing the power of the most advanced supercomputers available. The NAS Program provides scientists with the necessary computing power to solve today's most demanding computational fluid dynamics problems and serves as a pathfinder in integrating leading-edge supercomputing technologies, thus benefitting other supercomputer centers in government and industry. The 1992-93 operational year concluded with 399 high-speed processor projects and 91 parallel projects representing NASA, the Department of Defense, other government agencies, private industry, and universities. This document provides a glimpse at some of the significant scientific results for the year

Investigation on the aerodynamic performance of cycloidal rotors with active leading-edge morphing

A cycloidal rotor is a novel form of propulsion system which has a geometrical design differing completely from the conventional screw propeller. The blades of a cycloidal rotor rotate about the horizontal axis of rotation. A key advantage of this rotor system is the instantaneous control of the net thrust vector, meaning that the thrust can be adjusted to any desired direction, perpendicular to the rotor’s horizontal axis of rotation. Few investigations have been conducted to assess the negative impact dynamic stall has on the cycloidal rotor’s performance characteristics. Dynamic stall is a complex phenomenon associated with unsteady aerofoil pitching motion that generates large hysteresis effects on the blade’s aerodynamic characteristics during the pitch cycle. In this study, an investigation is conducted to assess the effect of active leading-edge morphing on alleviating the negative impact dynamic stall has on the aerofoil aerodynamic characteristics as well as the cycloidal rotor performance characteristics. Computational studies are performed for a large-scale cycloidal rotor and for a single pitch-oscillating symmetric aerofoil operating at a large Reynolds number, Re greater than 1,000,000. Dynamic stall wind tunnel testing of a single NACA0015 aerofoil with an active leading-edge flap is also conducted to validate the effects of leading-edge morphing from the single pitch-oscillating aerofoil’s CFD model. The main findings from this study showed that applying active leading-edge morphing resulted in significant improvements of both the single aerofoil’s aerodynamic characteristics and the cycloidal rotor’s performance characteristics. The results from the CFD for the single pitch-oscillating aerofoil showed that the negative effects of dynamic stall were alleviated when applying active leading-edge morphing. The results from the cycloidal rotor CFD simulations showed that the effects of dynamic stall were alleviated which led to a reduction in the level of blade-wake interference. This led to a significant improvement in the cycloidal rotor performance characteristics, such as a 4-blade cycloidal rotor with active leading-edge morphing applied producing less power dissipation in comparison to a rigid 2-blade cycloidal rotor. The main findings from the experimental analysis showed that active leading-edge morphing reduced negative effects of dynamic stall such as the level of aerodynamic hysteresis, as well as improving the aerodynamic efficiency

Heat Transfer Mechanism In Particle-Laden Turbulent Shearless Flows

Particle-laden turbulent flows are one of the complex flow regimes involved in a wide range of environmental, industrial, biomedical and aeronautical applications. Recently the interest has included also the interaction between scalars and particles, and the complex scenario which arises from the interaction of particle finite inertia, temperature transport, and momentum and heat feedback of particles on the flow leads to a multi-scale and multi-physics phenomenon which is not yet fully understood. The present work aims to investigate the fluid-particle thermal interaction in turbulent mixing under one-way and two-way coupling regimes. A recent novel numerical framework has been used to investigate the impact of suspended sub-Kolmogorov inertial particles on heat transfer within the mixing layer which develops at the interface of two regions with different temperature in an isotropic turbulent flow. Temperature has been considered a passive scalar, advected by the solenoidal velocity field, and subject to the particle thermal feedback in the two-way regime. A self-similar stage always develops where all single-point statistics of the carrier fluid and the suspended particles collapse when properly re-scaled. We quantify the effect of particle inertial, parametrized through the Stokes and thermal Stokes numbers, on the heat transfer through the Nusselt number, defined as the ratio of the heat transfer to the thermal diffusion. A scale analysis will be presented. We show how the modulation of fluid temperature gradients due to the statistical alignments of the particle velocity and the local carrier flow temperature gradient field, impacts the overall heat transfer in the two-way coupling regime