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Escoamentos sobre superfícies côncavas, como os que ocorrem no intradorso de uma pá de turbina, estão sujeitos à instabilidade centrífuga. A esse tipo de configuração atribui-se possibilidade de transição à turbulência devido a formação dos vórtices de Görtler. Estudos são propostos no sentido de identificar possível influência do gradiente de pressão nos mecanismos de desenvolvimento desses vórtices e sua interação com outras perturbações na transição. O processo de investigação dá-se numericamente por meio do desenvolvimento e uso de um código numérico paralelizado e de alta ordem de precisão. Resultados obtidos caracterizam o gradiente de pressão adverso como mais instável se comparado ao caso neutro ou favorável. Variações no gradiente de pressão não se mostram eficientes no processo de controle da instabilidade. Ao gradiente adverso atribui-se antecipação da região de saturação dos vórtices. Ressalta-se ainda a natureza desestabilizadora do gradiente adverso quanto aos mecanismos de amplificação dos modos varicoso e sinuoso associados à instabilidade secundária. / Flows over concave surfaces are subjected to centrifugal instability and may transition to turbulence. Studies are conducted to identify the role of the external pressure gradient on the development of the Görtler vortices and their interaction with other flow disturbances. Numerical simulations are carried out by the development and use of an in-house parallel code with highorder of accuracy. Adverse pressure gradient configurations are observed to be more unstable than the neutral and favourable ones. Pressure gradient variations do not prove to be an efficient way to control the centrifugal instability. The destabilizing behaviour that is observed by the adverse pressure gradient justifies its influence on the anticipation of the saturation of the primary vortices and growth of the sinuous and varicose secondary modes.

Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak. Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed. <b>Keywords:</b>Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.

Wave Number Selection and Defect Dynamics in Patterns with Hexagonal Symmetry Denis B. Semwogerere 108 Pages Directed by Dr. Michael F. Schatz We report quantitative measurements of wave number selection, secondary instability and defect dynamics in hexagonal patterns. A novel optical technique ("thermal laser writing") is used to imprint initial patterns with selected characteristics in a B뮡rd-Marangoni convection experiment. Initial patterns of ideal hexagons are imposed to determine the band of stable-pattern wave numbers. For small values of control parameter epsilon the measured stable band is found to agree quantitatively with theoretical predictions at the low-wave-number side of the band, and qualitatively at the high-wave-number side. Long-wavelength perturbations of ideal hexagonal patterns suggested by theory are imposed for epsilon=0.46 and their growth rates are measured to investigate the mechanisms of secondary instability. Our results suggest a transverse-phase instability limits stable hexagons at low wave number while a longitudinal-phase instability limits high-wave-number hexagons. Initial patterns containing an isolated penta-hepta defect are imprinted to study defect propagation directions and velocities. The experimental results agree well with theoretical predictions. The experimental investigations are discussed in the context of patterns with hexagonal symmetry formed under nonequilibrium external driving conditions.

<p>Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak.</p><p>Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed.</p><p><b>Keywords:</b>Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.</p>

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19091号 / エネ博第315号 / 新制||エネ||64 / 32042 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 岸本 泰明, 教授 前川 孝, 教授 中村 祐司 / 学位規則第4条第1項該当

The rotating-disk boundary layer is not only a simpler model for the study of cross-flow instability than swept-wing boundary layers but also a useful simplification of many industrial-flow applications where rotating configurations are present. For the rotating disk, it has been suggested that a local absolute instability, leading to a global instability, is responsible for the small variation in the observed laminar-turbulent transition Reynolds number however the exact nature of the transition is still not fully understood. This thesis aims to clarify certain aspects of the transition process. Furthermore, the thesis considers the turbulent rotating-disk boundary layer, as an example of a class of three-dimensional turbulent boundary-layer flows. The rotating-disk boundary layer has been investigated in an experimental apparatus designed for low vibration levels and with a polished glass disk that gave a smooth surface. The apparatus provided a low-disturbance environment and velocity measurements of the azimuthal component were made with a single hot-wire probe. A new way to present data in the form of a probability density function (PDF) map of the azimuthal fluctuation velocity, which gives clear insights into the laminar-turbulent transition region, has been proposed. Measurements performed with various disk-edge conditions and edge Reynolds numbers showed that neither of these conditions a↵ect the transition process significantly, and the Reynolds number for the onset of transition was observed to be highly reproducible. Laminar-turbulent transition for a ‘clean’ disk was compared with that for a disk with roughness elements located upstream of the critical Reynolds number for absolute instability. This showed that, even with minute surface roughness elements, strong convectively unstable stationary disturbances were excited. In this case, breakdown of the flow occurred before reaching the absolutely unstable region, i.e. through a convectively unstable route. For the rough disk, the breakdown location was shown to depend on the amplitude of individual stationary vortices. In contrast, for the smooth (clean-disk) condition, the amplitude of the stationary vortices did not fix the breakdown location, which instead was fixed by a well-defined Reynolds number. Furthermore, for the clean-disk case, travelling disturbances have been observed at the onset of nonlinearity, and the associated disturbance profile is in good agreement with the eigenfunction of the critical absolute instability. Finally, the turbulent boundary layer on the rotating disk has been investigated. The azimuthal friction velocity was directly measured from the azimuthal velocity profile in the viscous sublayer and the velocity statistics, normalized by the inner scale, are presented. The characteristics of this three-dimensional turbulent boundary-layer flow have been compared with those for the two-dimensional flow over a flat plate and close to the wall they are found to be quite similar but with rather large differences in the outer region. / <p>QC 20150119</p>

Escoamentos sobre superfícies côncavas, como os que ocorrem no intradorso de uma pá de turbina, estão sujeitos à instabilidade centrífuga. A esse tipo de configuração atribui-se possibilidade de transição à turbulência devido a formação dos vórtices de Görtler. Estudos são propostos no sentido de identificar possível influência do gradiente de pressão nos mecanismos de desenvolvimento desses vórtices e sua interação com outras perturbações na transição. O processo de investigação dá-se numericamente por meio do desenvolvimento e uso de um código numérico paralelizado e de alta ordem de precisão. Resultados obtidos caracterizam o gradiente de pressão adverso como mais instável se comparado ao caso neutro ou favorável. Variações no gradiente de pressão não se mostram eficientes no processo de controle da instabilidade. Ao gradiente adverso atribui-se antecipação da região de saturação dos vórtices. Ressalta-se ainda a natureza desestabilizadora do gradiente adverso quanto aos mecanismos de amplificação dos modos varicoso e sinuoso associados à instabilidade secundária. / Flows over concave surfaces are subjected to centrifugal instability and may transition to turbulence. Studies are conducted to identify the role of the external pressure gradient on the development of the Görtler vortices and their interaction with other flow disturbances. Numerical simulations are carried out by the development and use of an in-house parallel code with highorder of accuracy. Adverse pressure gradient configurations are observed to be more unstable than the neutral and favourable ones. Pressure gradient variations do not prove to be an efficient way to control the centrifugal instability. The destabilizing behaviour that is observed by the adverse pressure gradient justifies its influence on the anticipation of the saturation of the primary vortices and growth of the sinuous and varicose secondary modes.

No description available.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19091号 / エネ博第315号 / 新制||エネ||64(附属図書館) / 32042 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 岸本 泰明, 教授 前川 孝, 教授 中村 祐司 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM