No description available.

The initial rotation of flat, rectangular plates in quiescent flow were studied experimentally using two-dimensional and stereoscopic particle image velocimetry. The study examined the vortex dynamics of spanwise vorticity created on the upper, leeward surface of each plate of aspect ratio 2 and 4, which consists primarily of a leading-edge vortex. Reynolds numbers of 4,000, 8,000, and 16,000 based on the tip velocity and angles of attack of 25°, 35°, and 45° were investigated at five different azimuthal locations (90°, 180°, 235°, 270°, and 320°). The 25% and 50% spanwise positions for the aspect ratio 4 plate and 50% spanwise position for the aspect ratio 2 plate were studied. For the 25% and 50% spanwise location for the aspect ratio 4 and 2 plate, respectively, the leading-edge vortex structure's shape and coherence appear to be evolving temporally as the plate begins its initial motion. Leading-edge vortex circulation measurements confirm there is a non-monotonic trend showing increasing values until an azimuthal position of approximately 220° where there is a dip in the circulation values, but the circulation then rises towards the end of the range of azimuthal positions investigated. A strong region of counter-rotating vorticity was observed on the surface of the plate beneath the leading-edge vortex from the interaction of the leading-edge vortex with the plate. It was hypothesized that the interactions between the leading-edge vortex and counter-rotating vorticity are an important factor in governing the dynamics and strength of the leading-edge vortex which may ultimately determine whether the leading-edge vortex remains attached. To validate this claim, a transport analysis of the vorticity in the leading-edge vortex was developed to determine the contributions of spanwise flux, tilting of in-plane vorticity components, the shear layer, and annihilation has on the rate of change of circulation of the leading-edge vortex in the spanwise direction. Results of this analysis indicate that annihilation of the leading-edge vortex from entrainment of the counter-rotating vorticity is an important factor in governing the dynamics of the leading-edge vortex.

The aerodynamic performance of low aspect ratio wings at low Reynolds numbers applicable to micro air vehicle design was studied in this thesis. There is an overall lack of data for this low Reynolds number range, particularly concerning details of local flow behavior along the span. Experiments were conducted to measure the local pressure distributions on a wing at various spanwise locations in a Reynolds number range 30000 < Re < 90000. The model wing consisted of numerous wing sections and had a rectangular planform with NACA0012 airfoil shape with aspect ratio of one. One wing section, with pressure ports at various chordwise locations, was placed at different spanwise locations on a wing to effectively obtain the local pressure information. Integration of the pressure distributions yielded the local lift coefficients. Comparison of the local lift distributions to optimal elliptic lift distribution was conducted. This comparison showed a sharply peaked lift distribution near the wing tip resulting in a drastic deviation from the equivalent elliptic lift distributions predicted by the finite wing theory. The local lift distributions were further analyzed to determine the total lift coefficients vs angle of attack curves, span efficiency factors and the induced drag coefficients. Measured span efficiency factors, which were lower than predictions of the elliptic wing theory, can be understood by studying deviations of measured lift from the elliptic lift distribution. We conclude that elliptic wing theory is not sufficient to predict these aerodynamic performance parameters. Overall, these local measurements provided a better understanding of the low Reynolds number aerodynamics of the low aspect ratio wings.

Losses associated with turbulent flows dissipate a significant amount of generated energy. Such losses originate from the drag force, which is often described as the sum of the pressure drag and the friction drag. This thesis sets out to explore the hypothesis that passive wall motions driven by fluid mechanical forces are able to reduce the friction drag in fully developed turbulent boundary layers. Firstly, the streamwise and spanwise opposition controls proposed by Choi et al. (1994, Journal of Fluid Mechanics) are revisited to identify beneficial wall motions. Near-wall streamwise or spanwise velocity fluctuations are measured along a detection plane parallel to the wall (sensing). For streamwise control, the wall velocities are set to be equivalent to the measured streamwise velocity fluctuations, whereas for spanwise control, the wall velocities are set to have the same magnitude but opposite direction as the measured spanwise velocity fluctuations (actuation). Direct numerical simulations of canonical turbulent channel flows are carried out at low (Reτ ≈ 180) and intermediate (Reτ ≈ 1000) Reynolds numbers to quantify the effect of the distance between the wall and the detection plane. The investigation reveals the primary differences between the mechanisms underlying the two active in-plane controls. The modified flow features and turbulence statistics show that the streamwise control amplifies the most energetic streamwise velocity fluctuations and damps the near-wall vorticity fluctuations. In comparison, the spanwise control induces near-wall vorticity in order to counteract the quasi-streamwise vortices of the near-wall cycle and suppress turbulence production. Although, the working principles of the active controls are fundamentally different, both achieve drag reduction by mitigating momentum transfer between the velocity components. Secondly, two theoretical passive compliant wall models are proposed, the aim being to sustain beneficial wall motions identified by active flow control simulations. In the proposed models, streamwise or spanwise in-plane wall motions are governed by an array of independent one-degree-of-freedom damped harmonic oscillators. Unidirectional wall motions are driven by local streamwise or spanwise wall shear stresses. A weak coupling scheme is implemented to investigate the interaction between the compliant surface models and the turbulent flow in the channel by means of direct numerical simulations. A linear analytical solution of the coupled differential equation system is derived for laminar pulsatile channel flows allowing verification and validation of the numerical model. The obtained analytical solution is utilised to map the parameter space of the passive controls and estimate the effect of the wall motions. It is shown that depending on the control parameters, the proposed compliant walls decrease or increase the vorticity fluctuations at the wall similarly to the active controls. This is confirmed by direct numerical simulations. On the one hand, when the control parameters are chosen appropriately, the passive streamwise control damps the near-wall vorticity fluctuations and sustains the same drag reduction mechanism as the active streamwise control. This leads to modest, 3.7% and 2.3% drag reductions at low and intermediate Reynolds numbers. On the other hand, the spanwise passive control is not capable of increasing the near-wall vorticity fluctuations as dictated by the active spanwise control. For this reason, passive spanwise wall motions can increase the friction drag by more than 50%. The results emphasise the necessity of anisotropy for a practical compliant wall design. The present work demonstrates for the first time that passive wall motions can decrease friction drag in fully turbulent wall-bounded flows. The thesis sheds light on the working principle of an active streamwise control, and proposes a passive streamwise control exploiting the same drag reduction mechanism. An analytical model is developed to give a ready prediction of the statistical behaviour of passive in-plane wall motions. Whereas streamwise passive wall motions are found beneficial when the control parameters are chosen appropriately, solely spanwise passive wall motions lead to a drag penalty.

No description available.

No description available.

A robotically-controlled actuation system has been developed and built to perform active open-loop flow control experiments on transitional and turbulent backward-facing step flows in water. Control of the reattaching shear layer used hydraulic suction-and-blowing actuation emanating from 128 individual ports along the separation edge of the step. Each ports perturbation was periodic in time, but individually controlled to produce either spanwise-invariant (2D) or spanwise-varying (3D) spatial actuation profiles. An image processing system and special aqueous tuft were developed to measure the length of the recirculation bubble. Multiple images of a tuft array were time-averaged to do so. In general, 3D forcing was no more effective in reducing bubble length than 2D forcing. However, greater local spanwise reductions in reattachment length were observed for some cases of spanwise-varying forcing. Backlit dye was used to track the evolution of vorticity in the flow in video and still images.

No description available.

A presente tese se refere a três principais objetivos. Um objetivo foi desenvolver um código de simulação numérica direta para simulação de ondas de instabilidade em um escoamento de Poiseuille plano. O outro objetivo foi analisá-Io através do Método das Soluções Manufaturadas (MMS), e por fim, um terceiro objetivo foi estudar o regime não-linear da evolução de trens de ondas modulados em um escoamento de Poiseuille plano. O código resolve numericamente, com diferenças finitas de ordem de precisão alta e métodos pseudo espectrais, as equações de Navier-Stokes tri-dimensionais e incompressíveis numa formulação vorticidade-velocidade. O MMS é um método de verificação de código mais completo que os normalmente usados, por exemplo, comparação com teoria de estabilidade linear. O código usa diferenças finitas de ordem alta de precisão, mas com diferentes ordens em diferentes regiões do domínio. O MMS é pouco utilizado neste tipo de código. Concluiu-se que estes códigos em geral não operam na chamada faixa assintótica de erro. Na faixa de trabalho, a ordem do erro varia no domínio computacional de forma consistente com os métodos numéricos empregados. Isto permite, entre outras coisas, a otimização do esquema numérico. Após os testes de verifição, simulações numéricas dos trens de ondas foram realizadas. A análise dos resultados foi feita através das teorias de instabilidade primária e secundária e teoria fracamente não-linear. Foi estudado um trem de ondas modulado numa região do diagrama de instabilidade onde vários estudos para ondas mono cromáticas foram realizados. Os resultados sugeriram que nesta região o regime não-linear de transição dos trens de ondas modulados é governado pela instabilidade tipo-K. Com a redução da amplitude inicial de perturbação um cenário mais complexo, que pode estar envolvendo outros mecanismos, foi observado. Casos mais próximos ao primeiro ramo do diagrama também foram estudados. Nesta região a teoria linear prevê ondas tri-dimensionais sendo as mais instáveis, fato que poderia favorecer a ocorrência da chamada transição oblíqua. Os resultados indicaram que o trem de ondas modulado se divide em duas regiões que tendem-se afastar uma da outra. Apesar de estar associado com a instabilidade linear, este comportamento não havia sido antecipado na literatura. Neste cenário, para trens de ondas modulados isolados, o regime não-linear não pode ser correlacionado claramente com nenhum dos cenários clássicos de transição. Neste contexto transição oblíqua pode estar restrito a situações que envolvem a interação entre trens de ondas modulados. / The current thesis had three objectives. The first objective was to develop a code of direct numerical simulation (DNS) to simulation of waves of instability in a plane Poiseuille flow. The other main objective was to analyze it through the method of manufactured solutions (MMS). Finally, a third objective was to study the nonlinear regime of spanwise modulated wavetrains in a plane Poiseuille flow. Using high-order finite differences and pseudo-spectral methods, the DNS code solved the incompressible three-dimensional Navier-Stokes equations in a vorticity-velocity formulation. The MMS is a verification method more complete than the others more often used, for example, comparison with linear stability theory. The code used different high-order finite differences in different regions of the domain. The MMS has been little used for this type of code. It was concluded that these codes generally do not operate in the so-called errar asymptotic range. In the working range, the observed arder changes in the computational domain in a manner consistent with the numerical methods employed. This allows, among other things, optimization of the numerical scheme. After testing, numerical simulations of the wavetrains were performed. The analysis of the results was made based on the primary and secondary instability theories and weakly non-linear theory. A modulated wavetrain was studied in a region of the stability diagram where several studies for monochromatic waves were performed. The results suggested that in this region the non-linear regime of transition of the modulation wavetrain is governed by the K-type instability. With the reduction of the magnitude of the initial disturbance, a more complex scenario, which may involve other mechanisms, was observed. Cases near the first branch of the instability diagram were also studied. In this region, the linear theory predicts tri-dimensional waves are the most unstable, a fact that could lead to the so-called oblique transition. The results indicated that the modulated wavetrain divided into two regions, which tend move further from each other. Despite being associated with the linear instability, this behavior was not anticipated in the literature. In this scenario, for isolated modulated wavetrains, the non-linear system could not be clearly correlated with any of the classic scenarios of transition. In this context, oblique transition may perhaps to situations involving the interaction between modulated wavetrains.

A presente tese se refere a três principais objetivos. Um objetivo foi desenvolver um código de simulação numérica direta para simulação de ondas de instabilidade em um escoamento de Poiseuille plano. O outro objetivo foi analisá-Io através do Método das Soluções Manufaturadas (MMS), e por fim, um terceiro objetivo foi estudar o regime não-linear da evolução de trens de ondas modulados em um escoamento de Poiseuille plano. O código resolve numericamente, com diferenças finitas de ordem de precisão alta e métodos pseudo espectrais, as equações de Navier-Stokes tri-dimensionais e incompressíveis numa formulação vorticidade-velocidade. O MMS é um método de verificação de código mais completo que os normalmente usados, por exemplo, comparação com teoria de estabilidade linear. O código usa diferenças finitas de ordem alta de precisão, mas com diferentes ordens em diferentes regiões do domínio. O MMS é pouco utilizado neste tipo de código. Concluiu-se que estes códigos em geral não operam na chamada faixa assintótica de erro. Na faixa de trabalho, a ordem do erro varia no domínio computacional de forma consistente com os métodos numéricos empregados. Isto permite, entre outras coisas, a otimização do esquema numérico. Após os testes de verifição, simulações numéricas dos trens de ondas foram realizadas. A análise dos resultados foi feita através das teorias de instabilidade primária e secundária e teoria fracamente não-linear. Foi estudado um trem de ondas modulado numa região do diagrama de instabilidade onde vários estudos para ondas mono cromáticas foram realizados. Os resultados sugeriram que nesta região o regime não-linear de transição dos trens de ondas modulados é governado pela instabilidade tipo-K. Com a redução da amplitude inicial de perturbação um cenário mais complexo, que pode estar envolvendo outros mecanismos, foi observado. Casos mais próximos ao primeiro ramo do diagrama também foram estudados. Nesta região a teoria linear prevê ondas tri-dimensionais sendo as mais instáveis, fato que poderia favorecer a ocorrência da chamada transição oblíqua. Os resultados indicaram que o trem de ondas modulado se divide em duas regiões que tendem-se afastar uma da outra. Apesar de estar associado com a instabilidade linear, este comportamento não havia sido antecipado na literatura. Neste cenário, para trens de ondas modulados isolados, o regime não-linear não pode ser correlacionado claramente com nenhum dos cenários clássicos de transição. Neste contexto transição oblíqua pode estar restrito a situações que envolvem a interação entre trens de ondas modulados. / The current thesis had three objectives. The first objective was to develop a code of direct numerical simulation (DNS) to simulation of waves of instability in a plane Poiseuille flow. The other main objective was to analyze it through the method of manufactured solutions (MMS). Finally, a third objective was to study the nonlinear regime of spanwise modulated wavetrains in a plane Poiseuille flow. Using high-order finite differences and pseudo-spectral methods, the DNS code solved the incompressible three-dimensional Navier-Stokes equations in a vorticity-velocity formulation. The MMS is a verification method more complete than the others more often used, for example, comparison with linear stability theory. The code used different high-order finite differences in different regions of the domain. The MMS has been little used for this type of code. It was concluded that these codes generally do not operate in the so-called errar asymptotic range. In the working range, the observed arder changes in the computational domain in a manner consistent with the numerical methods employed. This allows, among other things, optimization of the numerical scheme. After testing, numerical simulations of the wavetrains were performed. The analysis of the results was made based on the primary and secondary instability theories and weakly non-linear theory. A modulated wavetrain was studied in a region of the stability diagram where several studies for monochromatic waves were performed. The results suggested that in this region the non-linear regime of transition of the modulation wavetrain is governed by the K-type instability. With the reduction of the magnitude of the initial disturbance, a more complex scenario, which may involve other mechanisms, was observed. Cases near the first branch of the instability diagram were also studied. In this region, the linear theory predicts tri-dimensional waves are the most unstable, a fact that could lead to the so-called oblique transition. The results indicated that the modulated wavetrain divided into two regions, which tend move further from each other. Despite being associated with the linear instability, this behavior was not anticipated in the literature. In this scenario, for isolated modulated wavetrains, the non-linear system could not be clearly correlated with any of the classic scenarios of transition. In this context, oblique transition may perhaps to situations involving the interaction between modulated wavetrains.