An interactive boundary layer modelling methodology for aerodynamic flows

Computational fluid dynamics (CFD) simulation is a computational tool for exploring flow applications in science and technology. Of central importance in many flow scenarios is the accurate modelling of the boundary layer phenomenon. This is particularly true in the aerospace industry, where it is central to the prediction of drag. Modern CFD codes as applied to modelling aerodynamic flows have to be fast and efficient in order to model complex realistic geometries. When considering viscous flows, the boundary layer typically requires the largest part of computational resources. To simulate boundary layer flow with most current CFD codes, requires extremely fine mesh spacing normal to the wall and is consequently computationally very expensive. Boundary layer modelling approaches offer considerable computational cost savings. One boundary layer method which proved to be very accurate is the two-integral method of Drela (1985). Coupling the boundary layer solution to inviscid external flow, however, is a challenge due to the Goldstein singularity, which occurs as separation is approached. This research proposed to develop a new method to couple Drela‟s two-integral equations to a generic outer flow solver in an iterative fashion. The study introduced an auxiliary equation, which was solved along with the displacement thickness to overcome the Goldstein singularity without the need to solve the entire flow domain simultaneously. In this work, the incompressible Navier-Stokes equations were used for the outer flow. In the majority of previous studies, the boundary layer thickness was simulated using a wall transpiration boundary condition at the interface between viscous and inviscid flows. This boundary condition was inherently non-physical since it added extra mass into the system to simulate the effects of the boundary layer. Here, this drawback was circumvented by the use of a mesh movement algorithm to shift the surface of the body outward without regridding the entire mesh. This replaced the transpiration boundary condition. The results obtained show that accurate modelling is possible for laminar incompressible flow. The predicted solutions obtained compare well with similarity solutions in the case of flat and inclined plates, and with the results of a NACA0012 airfoil produced by the validated XFOIL code (Drela and Youngren, 2001). CopyrightDissertation (MEng)--University of Pretoria, 2012.Mechanical and Aeronautical Engineeringunrestricte

On the possibility (or lack thereof) of agreement between experiment and computation of flows over wings at moderate Reynolds number

The flight of many birds and bats, and their robotic counterparts, occurs over a range of chord-based Reynolds numbers from 1 x 104 to 1.5 x 105. It is precisely over this range where the aerodynamics of simple, rigid, fixed wings becomes extraordinarily sensitive to small changes in geometry and the environment, with two sets of consequences. The first is that practical lifting devices at this scale will likely not be simple, rigid, fixed wings. The second is that it becomes non-trivial to make baseline comparisons for experiment and computation, when either one can be wrong. Here we examine one ostensibly simple case of the NACA 0012 airfoil and make careful comparison between the technical literature, and new experiments and computations. The agreement (or lack thereof) will establish one or more baseline results and some sensitivities around them. The idea is that the diagnostic procedures will help to guide comparisons and predictions in subsequent more complex cases.We are grateful to the Air Force Office of Scientific Research for equipment funding and partial funding of J.T. during this work under grants FA9550-15-1-0255 and FA9550-16-1-0392, both under the management of Doug Smith. L.S.’s work is supported by South African National Aerospace Center, and by a Research Completion Grant from the University of Pretoria, South Africa.http://rsfs.royalsocietypublishing.org2018-02-28hb2017Mechanical and Aeronautical Engineerin

Development of a semi-empirical wake formation and dissipation prediction model for HAHT placed in channel flow

DATA AVAILABILITY : Data will be made available on request.Hydrokinetic (HK) energy production has been primarily developed for use in tidal energy applications. However, where inland water infrastructure systems with sufficient velocities and spatial requirements exist, HK energy may hold great potential. A first order estimate of the wake length and dissipation rate behind a device is necessary for installation design and analysis. Some analytical approximations have been developed to estimate the wake field, although the majority of these approximations do not consider operational conditions in confined flow settings. This paper focuses on the development of a new semi-empirical model for the prediction of the wake formation, dissipation, and flow recovery. Various HK turbines are modelled, and benchmark validated using commercially available computational fluid dynamics software. The developed semi-empirical wake model adequately predicts wake behaviour over a range of performance conditions (linked to the specific turbine thrust), ambient turbulence conditions as well as blockage ratios, which are all important parameters in inland flow applications. The model enables an approximation of the wake behaviour with an accuracy of within 10% over the tested range of turbines. This approximation is valuable for facilitating the planning of turbine placement and determining the spatial requirements for inland hydrokinetic (HK) schemes.The University of Pretoria.https://www.elsevier.com/locate/oceanenghj2023Civil EngineeringMechanical and Aeronautical Engineerin

Potential for energy recovery from boundary-layer ingesting actuator disk propulsion

The theoretical benefits of highly integrated propulsion systems are highlighted herein by assessing the potential for energy recovery utilization using actuator disk propulsion. Decomposing aerodynamic forces into thrust and drag for closely integrated bodies, particularly those employing boundary-layer ingestion, becomes challenging. In this work, a mechanical energy-based approach was taken using the power balance method. This allowed the performance to be analyzed through the mechanical flow power in the fluid domain, disregarding the need for any explicit definition of thrust and drag. Through this, the benefit of boundary-layer ingestion was observed from a wake energy perspective as a decrease in the downstream mechanical energy deposition and associated viscous dissipation. From a propulsion perspective, the reduction in power demand necessary to produce propulsive force indicated the possibility of power savings by utilizing the energy contained within the ingested boundary-layer flow.The University of Pretoria Department of Research and Innovation as well as the Cranfield School of Aerospace, Transport, and Manufacturing.https://arc.aiaa.org/toc/ja/0/0hj2024Mechanical and Aeronautical EngineeringNon

An interactive boundary layer modelling methodology for aerodynamic flows

PURPOSE – The purpose of this paper is to introduce a unique technique to couple the two-integral boundary layer solutions to a generic inviscid solver in an iterative fashion. DESIGN/METHODOLOGY/APPROACH – The boundary layer solution is obtained using the two-integral method to solve displacement thickness point by point with a local Newton method, at a fraction of the cost of a conventional mesh-based, full viscous solution. The boundary layer solution is coupled with an existing inviscid solver. Coupling occurs by moving the wall to a streamline at the computed boundary layer thickness and treating it as a slip boundary, then solving the flow again and iterating. The Goldstein singularity present when solving boundary layer equations is overcome by solving an auxiliary velocity equation along with the displacement thickness. FINDINGS – The proposed method obtained favourable results when compared with the analytical solutions for flat and inclined plates. Further, it was applied to modelling the flow around a NACA0012 airfoil and yielded results similar to those of the widely used XFOIL code. ORIGINALITY/VALUE – A unique method is proposed for coupling of the boundary layer solution to the inviscid flow. Rather than the traditional transpiration boundary condition, mesh movement is employed to simulate the boundary layer thickness in a more physically meaningful way. Further, a new auxiliary velocity equation is presented to circumvent the Goldstein singularity.The Council for Scientific and Industrial Research (CSIR) on Thematic Type A Grant No. TA-2009-013http://www.emeraldinsight.com/journals.htm?issn=0961-5539hj201

Potential for energy recovery of unpowered configurations using power balance method computations

New aircraft developments are made to improve aircraft performance and efficiency. One such method is integrating propulsion into the airframe. This allows for boundary-layer ingestion, which shows promise of significant power benefits. However, these benefits are difficult to quantify as the propulsion system and aircraft body become meticulously integrated. The thrust and drag are coupled and cannot be defined separately, making conventional performance analysis methods inapplicable. The power balance method (PBM) addresses this by quantifying aircraft performance in terms of mechanical flow power and change in kinetic-energy rate. The primary focus of this work was to perform computational studies implementing the PBM on unpowered aerodynamic bodies to evaluate their respective drag contributions. A secondary study was also conducted to quantify the energy recovery potential of various bodies using a potential for energy recovery factor. The computational fluid dynamics case studies showed that drag obtained using the PBM agreed to within 2% of conventional momentum-based approaches. Maximal energy recovery potential was consistently observed at the trailing ends of the geometries, with values ranging between 9 and 12%

Potential for energy recovery from boundary-layer ingesting actuator disk propulsion

The theoretical benefits of highly integrated propulsion systems are highlighted herein by assessing the potential for energy recovery utilization using actuator disk propulsion. Decomposing aerodynamic forces into thrust and drag for closely integrated bodies, particularly those employing boundary-layer ingestion, becomes challenging. In this work, a mechanical energy-based approach was taken using the power balance method. This allowed the performance to be analyzed through the mechanical flow power in the fluid domain, disregarding the need for any explicit definition of thrust and drag. Through this, the benefit of boundary-layer ingestion was observed from a wake energy perspective as a decrease in the downstream mechanical energy deposition and associated viscous dissipation. From a propulsion perspective, the reduction in power demand necessary to produce propulsive force indicated the possibility of power savings by utilizing the energy contained within the ingested boundary-layer flow

Potential for energy recovery of unpowered configurations using power balance method computations

New aircraft developments are made to improve aircraft performance and efficiency. One such method is integrating propulsion into the airframe. This allows for boundary-layer ingestion, which shows promise of significant power benefits. However, these benefits are difficult to quantify as the propulsion system and aircraft body become meticulously integrated. The thrust and drag are coupled and cannot be defined separately, making conventional performance analysis methods inapplicable. The power balance method (PBM) addresses this by quantifying aircraft performance in terms of mechanical flow power and change in kinetic-energy rate. The primary focus of this work was to perform computational studies implementing the PBM on unpowered aerodynamic bodies to evaluate their respective drag contributions. A secondary study was also conducted to quantify the energy recovery potential of various bodies using a potential for energy recovery factor. The computational fluid dynamics case studies showed that drag obtained using the PBM agreed to within 2% of conventional momentum-based approaches. Maximal energy recovery potential was consistently observed at the trailing ends of the geometries, with values ranging between 9 and 12%.The University of Pretoria Department of Research and Innovation as well as Cranfield School of Aerospace, Transport and Manufacturing.https://arc.aiaa.org/toc/ja/58/6hj2022Mechanical and Aeronautical Engineerin

Computational investigation of the aerodynamic performance of an optimised alternative fuselage shape

PURPOSE: The purpose of this study is to re-evaluation fuselage design when the main wing’s has the ability to fulfill stability requirements without the need for a tailplane. The aerodynamic requirements of the fuselage usually involve a trade-off between reducing drag and providing enough length for positioning the empennage to ensure stability. However, if the main wing can fulfill the stability requirements without the need for a tailplane, then the fuselage design requirements can be re-evaluated. The optimisation of the fuselage can then include reducing drag and also providing a component of lift amongst other potential new requirements. DESIGN/METHODOLOGY/APPROACH: A careful investigation of parameterisation and trade-off optimisation methods to create such fuselage shapes was performed. The A320 Neo aircraft is optimised using a parameterised 3D fuselage model constructed with a modified PARSEC method and the SHERPA optimisation strategy, which was validated through three case studies. The geometry adjustments in relation to the specific flow phenomena are considered for the three optimal designs to investigate the influencing factors that should be considered for further optimisation. FINDINGS: The top three aerodynamic designs show a distinctive characteristic in the low aspect ratio thick wing-like aftbody that has pressure drag penalties, and the aftbody camber increased surface area notably improved the fuselage’s lift characteristics. ORIGINALITY/VALUE: This work contributes to the development of a novel set of design requirements for a fuselage, free from the constraints imposed by stability requirements. By gaining insights into the flow phenomena that influence geometric designs when a lift requirement is introduced to the fuselage, we can understand how the fuselage configuration was optimised. This research lays the groundwork for identifying innovative design criteria that could extend into the integration of propulsion of the aftbody.The South African National Research Foundation.https://www.emeraldgrouppublishing.com/journal/aeatMechanical and Aeronautical EngineeringSDG-09: Industry, innovation and infrastructureSDG-12:Responsible consumption and productio

Computational investigation of the aerodynamic performance of an optimised alternative fuselage shape

Purpose – The purpose of this study is to re-evaluation fuselage design when the main wing’s has the ability to fulfill stability requirements without the need for a tailplane. The aerodynamic requirements of the fuselage usually involve a trade-off between reducing drag and providing enough length for positioning the empennage to ensure stability. However, if the main wing can fulfill the stability requirements without the need for a tailplane, then the fuselage design requirements can be re-evaluated. The optimisation of the fuselage can then include reducing drag and also providing a component of lift amongst other potential new requirements. Design/methodology/approach – A careful investigation of parameterisation and trade-off optimisation methods to create such fuselage shapes was performed. The A320 Neo aircraft is optimised using a parameterised 3D fuselage model constructed with a modified PARSEC method and the SHERPA optimisation strategy, which was validated through three case studies. The geometry adjustments in relation to the specific flow phenomena are considered for the three optimal designs to investigate the influencing factors that should be considered for further optimisation. Findings – The top three aerodynamic designs show a distinctive characteristic in the low aspect ratio thick wing-like aftbody that has pressure drag penalties, and the aftbody camber increased surface area notably improved the fuselage’s lift characteristics. Originality/value – This work contributes to the development of a novel set of design requirements for a fuselage, free from the constraints imposed by stability requirements. By gaining insights into the flow phenomena that influence geometric designs when a lift requirement is introduced to the fuselage, we can understand how the fuselage configuration was optimised. This research lays the groundwork for identifying innovative design criteria that could extend into the integration of propulsion of the aftbody