Analysis of the aerodynamic effects of a propeller flowfield on bodies downstream of the propeller is a complex task. These interaction effects can have serious repercussions for many aspects of the vehicle, including drag changes resulting in larger power requirements, stability changes resulting in adjustments to stabilizer sizing, and lift changes requiring wing planform adjustments. Historically it has been difficult to accurately account for these effects at any stage during the design process. More recently methods using Euler solvers have been developed that capture interference effects well, although they don't provide an ideal tool for early stages of aircraft design, due to computational cost and the time and expense of setting up complex volume grids. This research proposes a method to fill the void of an interference model useful to the aircraft conceptual and preliminary designer. The proposed method combines a flexible and adaptable tool already familiar to the conceptual designer in the aerodynamic panel code, with a pseudo-steady slipstream model wherein rotational effects are discretized onto vortex particle point elements. The method maintains a freedom from volume grids that are so often necessary in the existing interference models. In addition to the lack of a volume grid, the relative computational simplicity allows the aircraft designer the freedom to rapidly test radically different configurations, including more unconventional designs like the channel wing, thereby providing a much broader design space than otherwise possible. Throughout the course of the research, verification and validation studies were conducted to ensure the most accurate model possible was being applied. Once the vortex particle scheme had been verified, and the ability to model an actuator disk with vortex particles had been validated, the overall product was compared against propeller-wing wind tunnel results conducted specifically as benchmarks for numerical methods. The method discussed in this work provides a glimpse into the possibility of pseudo-steady interference modeling using vortex particles. A great groundwork has been laid that already provides reasonable results, and many areas of interest have been discovered where future work could improve the method further. The current state of the method is demonstrated through simulations of several configurations including a wing and nacelle and a channel wing.

The vortex particle method (VPM) is a mesh-free approach to computational fluid dynamics (CFD) solving the Navier-Stokes equations in their velocity-vorticity form. The VPM uses a Lagrangian scheme, which not only avoids the hurdles of mesh generation, but it also conserves vortical structures over long distances with minimal numerical dissipation while being orders of magnitude faster than conventional mesh-based CFD. However, VPM is known to be numerically unstable when vortical structures break down close to the turbulent regime. In this study, we reformulate the VPM as a large eddy simulation (LES) in a scheme that is numerically stable, without increasing its computational cost. A new set of VPM governing equations are derived from the LES-filtered Navier-Stokes equations. The new equations reinforce conservation of mass and angular momentum by reshaping the vortex elements subject to vortex stretching. In addition to the VPM reformulation, a new anisotropic dynamic model of subfilter-scale (SFS) vortex stretching is developed. This SFS model is well suited for turbulent flows with coherent vortical structures where the predominant cascade mechanism is vortex stretching. Extensive validation is presented, asserting the scheme comprised of the reformulated VPM and SFS model as a meshless LES that accurately resolves large-scale features of turbulent flow. Advection, viscous diffusion, and vortex stretching are validated through simulation of isolated and leapfrogging vortex rings. Mean and fluctuating components of turbulent flow are validated through simulation of a turbulent round jet, in which Reynolds stresses are resolved directly and compared to experimental measurements. Finally, the computational efficiency of the scheme is showcased in the simulation of an aircraft rotor in hover, showing our meshless LES to be 100x faster than a mesh-based LES with similar fidelity. The ability to accurately and rapidly assess unsteady interactional aerodynamics is a shortcoming and bottleneck in the design of various next-generation aerospace systems: from electric vertical takeoff and landing (eVTOL) aircraft to airborne wind energy and wind farms. For instance, current models used in preliminary design fail to predict and assess configurations that may lead to the wake of a rotor impinging on another rotor or a wing during an eVTOL transition maneuver. In the second part of this dissertation, we address this shortcoming as we present a variable-fidelity CFD framework based on the reformulated VPM for simulating complex interactional aerodynamics. We further develop our meshless LES scheme to include rotors and wings in the computational domain through actuator models. A novel, vorticity-based, actuator surface model (ASM) is developed for wings, which is suitable for rotor-wing interactions when a wake impinges on the surface of a wing. This ASM imposes the no-flow-through condition at the airfoil centerline by calculating the circulation that meets this condition and by immersing the associated vorticity following a pressure-like distribution. Extensive validation of rotor-rotor and rotor-wing interactions predicted with our LES is presented, simulating two side-by-side rotors in hover, a tailplane with tip-mounted propellers, and a wing with propellers mounted mid-span. To conclude, the capabilities of the framework are showcased through the simulation of a multirotor tiltwing vehicle. The vehicle is simulated mid maneuver as it transitions from powered lift to wing-borne flight, featuring rotors with variable RPM and variable pitch, tilting of wings and rotors, and significant rotor-rotor and rotor-wing interactions from hover to cruise. Thus, the reformulated VPM provides aircraft designers with a high-fidelity LES tool that is orders of magnitude faster than mesh-based CFD, while also featuring variable-fidelity capabilities.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Zhongquan Zheng / In this study, a vortex particle method is used to simulate incompressible vortical flows, specifically aircraft wake vortices. This is particularly suitable for a wake vortex system that is slowly varying in the axial direction and has a high Reynolds number and low Mach number. The flow field, in the form of vorticity, is employed as the source in the far-field acoustic calculation using a vortex sound formula that enables computation of acoustic signals radiated from an approximated incompressible flow field. In a two-dimensional vortex system, the stretching effect in the axial direction is neglected. The purpose of this study is to focus on vortex core behaviors. A numerical simulation is performed in a more realistic wake consisting of a counter-rotating vortex pair with inviscid ground effects and shear flows. A Kirchhoff spinning-core vortex model is thus used as a starting point. In a vortex system with multiple vortices, such as a complicated aircraft vortex wake vortices, the sound emission frequency of the unsteady vortex core is subjected to change because of interactions between multiple vortices. The behaviors of the influence, indicated by the ratio between the core size and the distance of the vortices, are investigated as well as the underlining vortex core dynamic mechanisms. Cases of co-rotating vortices and a multiple-vortex system composed of two counter-rotating vortex pairs are studied for applications to aircraft wake vortex sound. In three-dimensional vortices, sinusoidal instabilities, which occur in the axial direction at various length scales, result in significant flow structure changes in these vortices, and thus influence their radiated acoustic signals. Cases of vortex rings and a pair of counter-rotating vortices are studied when they are undergoing both long-wave and short-wave instabilities. Both inviscid and viscous interactions are considered and the effects of turbulence are simulated using sub-grid-scale models. A higher peak frequency than the Kirchhoff frequency appears due to the straining field caused by mutual perturbation, under both long-wave and short-wave instabilities. Vortices with the initial core vorticity of the Gaussian distribution and the elliptic distribution are also studied.

No description available.

Vortex breakdown is an important phenomenon observed in swirling flows involving the development of a stagnation point on the axis of the vortex followed by a region of recirculation when the swirl increases beyond a particular level. It has been studied extensively over past 50 years and various theories have been proposed to explain its various aspects. However, a single model explaining all the aspects together is yet to emerge. Numerical simulations of breakdown have been performed using a variety of grid-based as well as vortex methods. Vortex methods are a Lagrangian alternative to grid-based methods wherein the motion of the vorticity is determined by the local fluid velocity convection, with models for viscous effects when considered. The fluid velocity is obtained from the vorticity field. Only the rotational regions of the flow need to be considered leading to significant economy of computational effort for simulations of vorticity dominated flows, such as vortex breakdown. The inviscid vortex filament method has been used to simulate several aspects of the vortex breakdown phenomenon. The vortex filament method however, cannot easily simulate viscous effects. To simulate the viscous effects the viscous vortex particle method needs to be used. This work was intended to be a first step towards this end by initially evaluating the effectiveness of the inviscid version of the vortex particle method in simulating the breakdown phenomenon. The inviscid vortex particle method was found to satisfactorily simulate most qualitative aspects involved in the formation of vortex breakdown such as the retardation of axial velocity along centerline, radial swelling of the vortex core, formation of stagnation points, creation of azimuthal vorticity gradient from axial vorticity gradient and the turning of vortex lines along with the formation of a bubble-like structure with recirculating flow within. The effect of a wall placed adjacent to the vortex core was simulated by using image vortices. The wall was not found to influence the location of breakdown. However, the initiation of the spiral mode was found to occur earlier when a wall was present. For a quantitative assessment, a simulation of the experimental results of Faler and Leibovich (1978) was attempted. The simulation managed to predict the location of the breakdown and the extent of the bubble. The shape and height of the bubble obtained however were not in accord with the experimental observations. A single vortical cell was obtained in the interior of the bubble.

O presente trabalho tem como objetivo realizar um estudo comparativo entre os três métodos para obtenção das características aerodinâmicas de tabuleiros de pontes: analítico, experimental e numérico. O tema é motivado pelo crescimento das necessidades de conhecimento científico e tecnológico na área de desempenho aerodinâmico de seções típicas de pontes nas etapas mais iniciais de projeto. A precisão de uma ferramenta de previsão certamente acelera a convergência ao modelo final; porém, qualquer metodologia numérica deve ser extensivamente testada antes de ser utilizada nos escritórios de cálculo. Inicialmente mostra-se a lógica da análise de instabilidades provocadas pelo vento, resume-se a modelagem de tabuleiros de pontes para ensaios em túnel de vento e descreve-se o método numérico implementado no programa comercial de análise de pontes RM. Comparam-se dados disponíveis de testes de modelos reduzidos no túnel de vento com as estimativas de aproximação do Método da Vorticidade em Partículas (MVP). Confrontam-se resultados para casos clássicos, um estudo paramétrico, dados dos relatórios de três pontes Brasileiras já ensaiadas no Túnel de Vento Joaquim Blessmann do LAC – PPGEC/UFRGS: Paulicéia, Guamá e Roberto Marinho. Finalmente, considera-se o método experimental como preciso e julga-se o método numérico comparativamente, analisando a confiabilidade dos resultados e procurando definir a sua faixa de aplicabilidade. Examinando a variabilidade dos resultados, bastante baixa, definiu-se sua faixa de aplicação considerando os resultados obtidos como satisfatórios para projetos básicos e executivos de estruturas; necessitando, porém, para os casos especiais, de comprovação experimental em túnel de vento antes da sua execução. Exemplifica-se então o uso do método para alguns problemas de análises de seções. Os resultados e comparações corroboram verificações anteriores do método dos vórtices discretos para verdadeiras seções de pontes e mostram porque o seu uso vem se tornando cada vez mais aceitável. / The work has the objective to realize a comparative study between three methods to obtain bridge aerodynamic coefficients: analytic, experimental and numerical. The theme is motivated by the increasing need of knowledge about the aerodynamic behaviour of bridge decks in the early stages of the design process. The accuracy of a predictive tool will certainly accelerate the convergence to the final design. However, any numerical methodology must be extensively tested ascertain and hence validated before it becomes of current use by designers. Collaboration is made comparing wind tunnel data of reduced models and numerical estimations by a simplified approach. After a brief overview of wind loading and wind tunnel modeling, the bridge wind resistance design in the commercially available bridge design software package RM is shown. The Discrete Vortex Method and the implementation are shortly described. Initially, classical fluid problems are indicated together with the comparison of parametric cross sectional bridge shapes. After, three Brazilian large cablestayed bridge projects are presented as instance of analysis: Paulicéia, Guamá and Roberto Marinho. Bridges aerodynamic coefficients were experimentally obtained at the Boundary Layer Wind Tunnel Joaquim Blessmann at Universidade Federal do Rio Grande do Sul (UFRGS). Finally, experimental results are considered precise and those results evaluated with CFD are compared analyzing the liability and applicability. According to the results variability, quite low, we considered them appropriated for basic and final structural designs; needing, although, for special cases, of experimental probation in wind tunnel tests before the construction phase. Some CFD analysis problems of cross sections are then presented as application examples. The results and similitude corroborate earlier verifications of the method and this implementation, demonstrating why its use is becoming increasingly acceptable.

O presente trabalho tem como objetivo realizar um estudo comparativo entre os três métodos para obtenção das características aerodinâmicas de tabuleiros de pontes: analítico, experimental e numérico. O tema é motivado pelo crescimento das necessidades de conhecimento científico e tecnológico na área de desempenho aerodinâmico de seções típicas de pontes nas etapas mais iniciais de projeto. A precisão de uma ferramenta de previsão certamente acelera a convergência ao modelo final; porém, qualquer metodologia numérica deve ser extensivamente testada antes de ser utilizada nos escritórios de cálculo. Inicialmente mostra-se a lógica da análise de instabilidades provocadas pelo vento, resume-se a modelagem de tabuleiros de pontes para ensaios em túnel de vento e descreve-se o método numérico implementado no programa comercial de análise de pontes RM. Comparam-se dados disponíveis de testes de modelos reduzidos no túnel de vento com as estimativas de aproximação do Método da Vorticidade em Partículas (MVP). Confrontam-se resultados para casos clássicos, um estudo paramétrico, dados dos relatórios de três pontes Brasileiras já ensaiadas no Túnel de Vento Joaquim Blessmann do LAC – PPGEC/UFRGS: Paulicéia, Guamá e Roberto Marinho. Finalmente, considera-se o método experimental como preciso e julga-se o método numérico comparativamente, analisando a confiabilidade dos resultados e procurando definir a sua faixa de aplicabilidade. Examinando a variabilidade dos resultados, bastante baixa, definiu-se sua faixa de aplicação considerando os resultados obtidos como satisfatórios para projetos básicos e executivos de estruturas; necessitando, porém, para os casos especiais, de comprovação experimental em túnel de vento antes da sua execução. Exemplifica-se então o uso do método para alguns problemas de análises de seções. Os resultados e comparações corroboram verificações anteriores do método dos vórtices discretos para verdadeiras seções de pontes e mostram porque o seu uso vem se tornando cada vez mais aceitável. / The work has the objective to realize a comparative study between three methods to obtain bridge aerodynamic coefficients: analytic, experimental and numerical. The theme is motivated by the increasing need of knowledge about the aerodynamic behaviour of bridge decks in the early stages of the design process. The accuracy of a predictive tool will certainly accelerate the convergence to the final design. However, any numerical methodology must be extensively tested ascertain and hence validated before it becomes of current use by designers. Collaboration is made comparing wind tunnel data of reduced models and numerical estimations by a simplified approach. After a brief overview of wind loading and wind tunnel modeling, the bridge wind resistance design in the commercially available bridge design software package RM is shown. The Discrete Vortex Method and the implementation are shortly described. Initially, classical fluid problems are indicated together with the comparison of parametric cross sectional bridge shapes. After, three Brazilian large cablestayed bridge projects are presented as instance of analysis: Paulicéia, Guamá and Roberto Marinho. Bridges aerodynamic coefficients were experimentally obtained at the Boundary Layer Wind Tunnel Joaquim Blessmann at Universidade Federal do Rio Grande do Sul (UFRGS). Finally, experimental results are considered precise and those results evaluated with CFD are compared analyzing the liability and applicability. According to the results variability, quite low, we considered them appropriated for basic and final structural designs; needing, although, for special cases, of experimental probation in wind tunnel tests before the construction phase. Some CFD analysis problems of cross sections are then presented as application examples. The results and similitude corroborate earlier verifications of the method and this implementation, demonstrating why its use is becoming increasingly acceptable.

O presente trabalho tem como objetivo realizar um estudo comparativo entre os três métodos para obtenção das características aerodinâmicas de tabuleiros de pontes: analítico, experimental e numérico. O tema é motivado pelo crescimento das necessidades de conhecimento científico e tecnológico na área de desempenho aerodinâmico de seções típicas de pontes nas etapas mais iniciais de projeto. A precisão de uma ferramenta de previsão certamente acelera a convergência ao modelo final; porém, qualquer metodologia numérica deve ser extensivamente testada antes de ser utilizada nos escritórios de cálculo. Inicialmente mostra-se a lógica da análise de instabilidades provocadas pelo vento, resume-se a modelagem de tabuleiros de pontes para ensaios em túnel de vento e descreve-se o método numérico implementado no programa comercial de análise de pontes RM. Comparam-se dados disponíveis de testes de modelos reduzidos no túnel de vento com as estimativas de aproximação do Método da Vorticidade em Partículas (MVP). Confrontam-se resultados para casos clássicos, um estudo paramétrico, dados dos relatórios de três pontes Brasileiras já ensaiadas no Túnel de Vento Joaquim Blessmann do LAC – PPGEC/UFRGS: Paulicéia, Guamá e Roberto Marinho. Finalmente, considera-se o método experimental como preciso e julga-se o método numérico comparativamente, analisando a confiabilidade dos resultados e procurando definir a sua faixa de aplicabilidade. Examinando a variabilidade dos resultados, bastante baixa, definiu-se sua faixa de aplicação considerando os resultados obtidos como satisfatórios para projetos básicos e executivos de estruturas; necessitando, porém, para os casos especiais, de comprovação experimental em túnel de vento antes da sua execução. Exemplifica-se então o uso do método para alguns problemas de análises de seções. Os resultados e comparações corroboram verificações anteriores do método dos vórtices discretos para verdadeiras seções de pontes e mostram porque o seu uso vem se tornando cada vez mais aceitável. / The work has the objective to realize a comparative study between three methods to obtain bridge aerodynamic coefficients: analytic, experimental and numerical. The theme is motivated by the increasing need of knowledge about the aerodynamic behaviour of bridge decks in the early stages of the design process. The accuracy of a predictive tool will certainly accelerate the convergence to the final design. However, any numerical methodology must be extensively tested ascertain and hence validated before it becomes of current use by designers. Collaboration is made comparing wind tunnel data of reduced models and numerical estimations by a simplified approach. After a brief overview of wind loading and wind tunnel modeling, the bridge wind resistance design in the commercially available bridge design software package RM is shown. The Discrete Vortex Method and the implementation are shortly described. Initially, classical fluid problems are indicated together with the comparison of parametric cross sectional bridge shapes. After, three Brazilian large cablestayed bridge projects are presented as instance of analysis: Paulicéia, Guamá and Roberto Marinho. Bridges aerodynamic coefficients were experimentally obtained at the Boundary Layer Wind Tunnel Joaquim Blessmann at Universidade Federal do Rio Grande do Sul (UFRGS). Finally, experimental results are considered precise and those results evaluated with CFD are compared analyzing the liability and applicability. According to the results variability, quite low, we considered them appropriated for basic and final structural designs; needing, although, for special cases, of experimental probation in wind tunnel tests before the construction phase. Some CFD analysis problems of cross sections are then presented as application examples. The results and similitude corroborate earlier verifications of the method and this implementation, demonstrating why its use is becoming increasingly acceptable.

The flow field and forces on an isolated oscillating NACA 0012 airfoil in a uniform flow is studied using viscous vortex particle method. The simulations are carried out at very low chord (c) based Reynolds number (Re=1000), motivated by the current interest in development of Micro Air Vehicles (MAV). The airfoil is forced to oscillate in both heave and pitch at different normalized oscillation frequencies (f), which is represented by the non-dimensional reduced frequency fc/U).( From the unsteady loading on the airfoil, the net energy transfer to the airfoil is calculated to determine the propensity for the airfoil to undergo self-induced oscillations or flutter at these very low Reynolds numbers. The simulations are carried out using a viscous vortex particle method that utilizes discrete vortex elements to represent the vorticity in the flow field. After validation of the code against test cases in the literature, simulations are first carried out for the stationary airfoil at different angles of attack, which shows the stall characteristics of the airfoil at this very low Reynolds numbers. For the airfoil oscillating in heave, the airfoil is forced to oscillate at different reduced frequencies at a large angle of attack in the stall regime. The unsteady loading on the blade is obtained at different reduced frequencies. This is used to calculate the net energy transfer to the airfoil from the flow, which is found to be negative in all cases studied. This implies that stall flutter or self-induced oscillations are not possible under the given heave conditions. The wake vorticity dynamics is presented for the different reduced frequencies, which show that the leading edge vortex dynamics is progressively more complex as the reduced frequency is increased from small values. For the airfoil oscillating in pitch, the airfoil is forced to oscillate about a large mean angle of attack corresponding to the stall regime. The unsteady moment on the blade is obtained at different reduced frequencies, and this is used to calculate the net energy transfer to the airfoil from the flow, which is found to be positive in all cases studied. This implies that stall flutter or self-induced oscillations are possible in the pitch mode, unlike in the heave case. The wake vorticity dynamics for this case is found to be relatively simple compared to that in heave. The results of the present simulations are broadly in agreement with earlier stall flutter studies at higher Reynolds numbers that show that stall flutter does not occur in the heave mode, but can occur in the pitch mode. The main difference in the present very low Reynolds number case appears to be the broader extent of the excitation region in the pitch mode compared to large Re cases studied earlier. region in the pitch mode compared to large Re cases studied earlier.

No description available.