Two rotor configurations with different non-uniform inflows were studied: a rotor ingesting the wake of an upstream cylinder and a rotor ingesting a thick axially symmetric boundary layer from an upstream centerbody. In both cases, the undisturbed inflow was measured without the rotor present in order to characterize the inflow, in particular to calculate the unsteady upwash velocity distribution at the location of the rotor. In addition, detailed acoustic measurements were completed using a 251-channel large-area microphone array. In all, over 400 conditions covering different advance ratios, angles of yaw, and inflow conditions were measured. Measurements of the sound show that the source has a complex directivity, different from that of a streamwise aligned dipole, due to the inhomogeneous unsteady upwash distribution. In addition, observers at different far field locations will perceive sources from different locations on the rotor disk. The directivity is a function of both the rotor geometry and turbulent inflow. A simplified model of the sound source was developed using these inputs and accurately predicts trends observed in the far field noise. For the cylinder wake ingestion case, on-blade measurements of the flow field show that the wake is drawn to the center of the rotor disk with increasing thrust. This is particularly noticeable if the wake does not strike the center of the rotor disk. The effects of this flow distortion on the far field directivity are well predicted by the model. The effects of yaw to rotate the produced sound field can be inferred from this model as well. A novel beamforming procedure was used to isolate sources across the face of the rotor for the cylinder wake ingestion case for an upstream observer position. This method may be used to isolate different sound sources on a rotor if multiple sources are present or if different regions of the rotor disk need to be isolated. The directivity of a rotor ingesting an axially symmetric boundary layer is far less complex than the ingestion of a two-dimensional cylinder wake, but measurements still show the perceived source location shift with observer location. Overall, the proposed noise modeling technique is an efficient method to predict the directivity of turbulence ingestion noise for inhomogeneous inflows. This can enable quick absolute noise predictions at all far field locations using only a single point measurement or far field noise prediction to establish absolute levels. / Doctor of Philosophy / In many engineering applications, rotors interact with turbulence. Aircraft and ships with rear mounted propellers can have upstream appendages or discontinuities that generate turbulence that travels downstream and is drawn into the propeller. Wind turbines interact with turbulence in the atmosphere and with turbulent wakes from other turbines. Interaction of a rotor with turbulence results in unsteady loading on the rotor blades that can radiate as sound, causing unwanted community noise or vehicle detection. As such, prediction and reduction of noise due to turbulence ingestion is highly desirable and remains an active area of research. Turbulence ingestion noise is well understood from first principles and can be successfully predicted provided an accurate description of the turbulent inflow and unsteady aerodynamic response of the rotor blades. Much work has focused on homogenous, isotropic turbulence ingestion noise, however, in practical applications, the rotor inflow is often non-uniform, anisotropic, and can change dramatically with the thrusting condition of the rotor. Research efforts to develop noise predictions considering these more complex, but practical inflows have focused on the inflow modeling and measurement and have relied on a small subset of sound measurements for validation. The present study seeks to provide new physical insight into inhomogeneous, anisotropic turbulence ingestion noise through wind tunnel experiments. In particular, two rotor configurations with different practical non-uniform inflows are studied: a rotor ingesting the wake of an upstream cylinder and a rotor ingesting a thick axially symmetric boundary layer from an upstream center body. In both cases, the undisturbed inflow was measured without the rotor present in order to characterize the inflow, and detailed acoustic measurements were completed using a 251-channel large-area microphone array. In all, over 400 rotor operating conditions were measured. The acoustic directivity in each case is examined in detail as a function of rotor operating condition. A simplified directivity model is developed and validated with measurements. Ultimately, the directivity model can provide a good engineering approximation of the full directivity with reduced computational time or can be used to extrapolate measured results to positions in the far field where placement of sensors is not possible. The results can also be used to guide the analysis and interpretation of single point or microphone array measurements in the acoustic far field of a rotor.

Both experimental and theoretical studies have beenconsidered on flat plate boundary layers as well as on wakesbehind porous cylinders. The main thread in this work iscontrol, which is applied passively and actively on boundarylayers in order to inhibit or postpone transition toturbulence; and actively through the cylinder surface in orderto effect the wakecharacteristics. An experimental set-up for the generation of the asymptoticsuction boundary layer (ASBL) has been constructed. This studyis the first, ever, that report a boundary layer flow ofconstant boundary layer thickness over a distance of 2 metres.Experimental measurements in the evolution region, from theBlasius boundary layer (BBL) to the ASBL, as well as in theASBL are in excellent agreement with boundary layer analysis.The stability of the ASBL has experimentally been tested, bothto Tollmien-Schlichting waves as well as to free streamturbulence (FST), for relatively low Reynolds numbers (Re). For the former disturbances good agreement is foundfor the streamwise amplitude profiles and the phase velocitywhen compared with linear spatial stability theory. However,the energy decay factor predicted by theory is slightlyoverestimated compared to the experimental findings. The latterdisturbances are known to engender streamwise elongated regionsof high and low speeds of fluid, denoted streaks, in a BBL.This type of spanwise structures have been shown to appear inthe ASBL as well, with the same spanwise wavelength as in theBBL, despite the fact that the boundary layer thickness issubstantially reduced in the ASBL case. The spanwise wavenumberof the optimal perturbation in the ASBL has been calculated andis β = 0.53, when normalized with the displacementthickness. The spanwise scale of the streaks decreases withincreasing turbulence intensity (Tu) and approaches the scale given by optimalperturbation theory. This has been shown for the BBL case aswell. The initial energy growth of FST induced disturbances hasexperimentally been found to grow linearly as Tu2Rexin the BBL, the transitional Reynolds numberto vary as Tu-2, and the intermittency function to have a relativelywell-defined distribution, valid for all Tu. The wake behind a porous cylinder subject to continuoussuction or blowing has been studied, where amongst other thingsthe Strouhal number (St) has been shown to increase strongly with suction,namely, up to 50% for a suction rate of 2.5% of the free streamvelocity. In contrast, blowing shows a decrease ofStof around 25% for a blowing rate of 5% of the freestream velocity in the considered Reynolds number range. Keywords:Laminar-turbulent transition, asymptoticsuction boundary layer, free stream turbulence,Tollmien-Schlichting wave, stability, flow control, cylinderwake. / QC 20100607

In many situations, fluid flows can exhibit a wide range of temporal and spatial phenomena. It has become common to extract physically important features, called modes, as a first step in the analysis of flows with high complexity. One of the most prominent modal analysis techniques in the context of fluid dynamics is Proper Orthogonal Decomposition (POD), which enables extraction of energetically coherent structures present in the flow field. This method does, however, suffer from the lack of connection with the mathematical theory of dynamical systems and its utility in the analysis of arbitrarily complex flows might therefore be limited. In the present work, we instead consider application of the Koopman Mode Decomposition (KMD), which is an approach based on spectral decomposition of the Koopman operator. This technique is employed for modal analysis of the incompressible, two-dimensional ow past two side-by-side cylinders at Re = 60 and with a non-dimensional cylinder gap spacing g* = 1. This particular configuration yields a wake ow which exhibits in-phase vortex shedding during finite time, while later transforming into the so-called flip-flopping phenomena, which is characterised by a slow, periodic switching of the gap ow direction during O(10) vortex shedding cycles. The KMD approach yields modal structures which, in contrary to POD, are associated with specific oscillation frequencies. Specifically, these structures are here vorticity modes. By studying these modes, we are able to extract the ow components which are responsible for the flip-flop phenomenon. In particular, it is found that the flip-flop instability is mainly driven by three different modal structures, oscillating with Strouhal frequencies St1 = 0:023, St2 = 0:121 and St3 = 0:144, where it is noted that St3 = St1 + St2. In addition, we study the in-phase vortex shedding regime, as well as the transient regime connecting the two states of the flow. The study of the in-phase vortex shedding reveals| - not surprisingly - the presence of a single fundamental frequency, while the study of the transient reveals a Koopman spectrum which might indicate the existence of a bifurcation in the phase space of the flow field; this idea has been proposed before in Carini et al. (2015b). We conclude that the KMD offers a powerful framework for analysis of this ow case, and its range of applications might soon include even more complex flows.

In order to study three-dimensional unsteady turbulent flow fields such as the wakes of bluff bodies, a Digital Particle Image Velocimetry (DPIV) system was developed. This system allows non-intrusive two-dimensional and time varying velocity measurements. Software and hardware modifications necessary to enhance the capabilities of the system were preformed, resulting in increased frequency resolution. However, due to hardware limitations and limitations inherited from the implementation of the method, space resolution is reduced. Subsequently, digital image processing tools to improve the space resolutions were developed. The advantages and limitations of the method for the study of turbulent flows are presented in detail. The developed system is employed in the documentation of time-varying turbulent flow fields. Initially we study the spanwise variation of the near wake of a low-aspect ratio, surface-mounted, circular cylinder piercing a free surface. The asymmetry of the end conditions combined with the natural unsteadiness of the vortex shedding generates a very complex flow filed which is difficult to study with conventional methods. By employing the aforementioned system we are able to reveal a departure of the two-dimensional character of the flow in the form of oblique vortex shedding. The effect of free surface on the vortex formation length and on the vortex reconnection process is documented. Near the free surface the alternate mode of vortex shedding is suppressed, leading to simultaneous shedding of vortices in the wake. Indications of vortex dislocations and change of the vortex axis in order to reconnect to the free surface are observed. Finally, a novel approach of reconstructing the three-dimensional, time -varying volume of the flow field by obtaining simultaneous measurements of Laser Doppler Velocimetry and Particle Image Velocimetry planes is presented. The same field is investigated with focus on the streamwise structures. Three-dimensional streamwise vortical structures are known to exist due to instabilities of plane shear layers. Similar streamwise vortices, also known as braid vortices have been observed in the past in the wake of circular cylinders with symmetric boundary conditions. The present spatio-temporal analysis demonstrated coexistence of two types of streamwise vortices in the wake, bilge and braid type of vortices. These may be due to the three dimensionality introduced by the free surface. In addition, the sufficient time resolution allowed the detection of the primary Von-Karman vortex through a plane of interrogation normal to the free stream, thus revealing the spanwise variation of the vortex shedding and its evolution at different downstream stations. The combination of the effect of the asymmetric boundary conditions with a free surface is investigated by adding one more source of three-dimensionality in terms of inclination of the cylinder axis. Hydrogen-bubble and particle-flow visualizations are preformed in combination with Laser-Doppler Velocimetry measurements. From both qualitative and quantitative results the effects of inclination and Froude number are documented. It is proved that the vortex shedding is suppressed for high values of the Froude number, however the inclination counteracts the vortex suppression and favors the vortex shedding mechanism. In addition, in the region of the no-slip boundary condition the flow is dominated by the effect of the horseshoe vortex. The case of a three-dimensional separated flow over a surface-mounted prism is investigated using a modified version of the system. The character of the separated from the leading edge corner shear layer and the formed separation bubble are documented in space and time along the mid-plane of symmetry of the body. Three different flows corresponding to different Reynolds numbers are studied. The unsteadiness of the flow is presented indicating a pseudo-periodic character. Large-scale, low-frequency oscillations of the shear layer that have been observed in the past using point measurement methods are now confirmed by means of a whole field velocity measurement, technique allowing a holistic view of the flow. In addition, the unsteadiness of the point of reattachment is associated with the flapping of the shear layer and the shedding of vorticity in the wake. Finally, it is demonstrated that the apparent vortex shedding mechanism of such flows is dependent on the interaction of the primary vortex of the separation bubble with a secondary vortex formed by the separation of the reverse flow boundary layer. By performing measurements with such time and space resolution the inadequacy of time averaged or point measurement methods for the treatment of such complex and unsteady flow fields becomes evident. In final case we employ Particle-Image Velocimetry to show the effect of unsteady excitation on two-dimensional separated flow over a sharp edged airfoil. It is proved that such an approach can be used to effectively control and organize the character of the flow, potentially leading to lift increase and drug reduction of bluff bodies / Ph. D.

No description available.

"Fluid-structure interaction arises in a wide array of technological applications including naval and marine hydrodynamics, civil and wind engineering and flight vehicle aerodynamics. When a fluid flows over a bluff body such as a circular cylinder, the periodic vortex shedding in the wake causes fluctuating lift and drag forces on the body. This phenomenon can lead to fatigue damage of the structure due to large amplitude vibration. It is widely believed that the wake structures behind the structure determine the hydrodynamic forces acting on the structure and control of wake structures can lead to vibration control of the structure. Modeling this complex non-linear interaction requires coupling of the dynamics of the fluid and the structure. In this thesis, however, the vibration of the flexible cylinder is prescribed, and the focus is on modeling the fluid dynamics in its wake. Low-dimensional iterative circle maps have been found to predict the universal dynamics of a two-oscillator system such as the rigid cylinder wake. Coupled map lattice (CML)models that combine a series of low-dimensional circle maps with a diffusion model have previously predicted qualitative features of wake patterns behind freely vibrating cables at low Reynolds number. However, the simple nature of the CML models implies that there will always be unmodelled wake dynamics if a detailed, quantitative comparison is made with laboratory or simulated wake flows. Motivated by a desire to develop an improved CML model, we incorporate self-learning features into a new CML that is trained to precisely estimate wake patterns from target numerical simulations and experimental wake flows. The eventual goal is to have the CML learn from a laboratory flow in real time. A real-time self-learning CML capable of estimating experimental wake patterns could serve as a wake model in a future anticipated feedback control system designed to produce desired wake patterns. A new convective-diffusive map that includes additional wake dynamics is developed. Two different self-learning CML models, each capable of precisely estimating complex wake patterns, have been developed by considering additional dynamics from the convective-diffusive map. The new self-learning CML models use adaptive estimation schemes which seek to precisely estimate target wake patterns from numerical simulations and experiments. In the first self-learning CML, the estimator scheme uses a multi-variable least-squares algorithm to adaptively vary the spanwise velocity distribution in order to minimize the state error (difference between modeled and target wake patterns). The second self-learning model uses radial basis function neural networks as online approximators of the unmodelled dynamics. Additional unmodelled dynamics not present in the first self-learning CML model are considered here. The estimator model uses a combination of a multi-variable normalized least squares scheme and a projection algorithm to adaptively vary the neural network weights. Studies of this approach are conducted using wake patterns from spectral element based NEKTAR simulations of freely vibrating cable wakes at low Reynolds numbers on the order of 100. It is shown that the self-learning models accurately and efficiently estimate the simulated wake patterns within several shedding cycles. Next, experimental wake patterns behind different configurations of rigid cylinders were obtained. The self-learning CML models were then used for off-line estimation of the stored wake patterns. With the eventual goal of incorporating low-order CML models into a wake pattern control system in mind, in a related study control terms were added to the simple CML model in order to drive the wake to the desired target pattern of shedding. Proportional, adaptive proportional and non-linear control techniques were developed and their control efficiencies compared."

Dans cette thèse, l’étude des interactions entre des particules ultrafines émises par les pots d’échappement et l’écoulement de sillage créé par le véhicule émetteur a été réalisée principalement selon une approche numérique. Une campagne expérimentale a été conduite à des fins de validation. L’objet de la thèse vise à comprendre l’impact des particules issues des pots d’échappement sur l’environnement proche tant du côté piéton que du côté des passagers des véhicules suiveurs. Pour cela, l’écoulement du fluide a été traité avec une approche eulérienne type URANS (Unsteady Reynolds Average Navier-Stokes) combinée à un suivi lagrangien pour les nanoparticules. En effet, cette thèse est conduite en parallèle d’un projet collaboratif financé par l’ADEME (CAPTIHV) dont le but est d’évaluer la qualité de l’air des habitacles des véhicules automobiles, et en particulier de l’infiltration des particules ultrafines issues du trafic environnant. L’étude de la dispersion des particules fines en écoulements turbulents nécessite une analyse fine des structures turbulentes qui s’y développent. Notre étude numérique a donc consisté, en premier lieu, à analyser cette dispersion dans le cas d’un écoulement de sillage classique à l’aval d’un cylindre. Cela nous a permis de caractériser la dynamique d’interactions de nanoparticules solides de carbone avec les structures tourbillonnaires en considérant l’impact de la turbulence et de la diffusion brownienne. Cela a permis d’évaluer l’influence des principaux mécanismes influençant la dispersion. Les résultats de ces simulations nous ont permis de sélectionner les mécanismes/forces importants pouvant influencer la dispersion de telles particules dans le sillage d’un véhicule automobile ; Cela nous a facilité la mise en place et l’analyse des simulations relativement plus complexes de l’aérodynamique du corps d’Ahmed à culot droit en présence des nanoparticules simulant les suies des gaz d’échappement. Les interactions des particules ultrafines avec les structures tourbillonnaires se créant dans le sillage des véhicules ont été évaluées à partir de profils de concentrations et les coefficients de dispersions transversales. La dernière étape a consisté en une campagne d’essais en soufflerie qui nous a permis de caractériser les champs de vitesses moyens et turbulents ainsi que les champs de concentrations particulaires à l’aval du véhicule pour valider les résultats numériques / In this thesis, the study of the interactions between ultrafine particles emitted by the exhaust pipes and the wake flow generated by the emitting vehicle was carried out mainly using a numerical approach. An experimental campaign was conducted for validation purpose. The goal of the thesis is to understand the impact of exhaust particles on the surrounding environment on both the pedestrian and the passengers of the following vehicles. For this purpose, the fluid flow was resolved with an Eulerian type URANS model (Unsteady Reynolds Average Navier-Stokes) combined to the Lagrangian approach for the nanoparticles trajectories calculation. This thesis is conducted simultaneously with a collaborative project funded by ADEME (CAPTIHV) whose purpose is to assess the air quality of automotive car cabins, and particulate infiltration from the surrounding traffic in particular of ultrafine particles. The study of the dispersion of fine particles in turbulent flows requires a fine analysis of the turbulent structures that develop in such flows. Our numerical study therefore consisted, first, in analyzing this dispersion in the case of a classic wake flow downstream of a cylinder. This enabled us to characterize the interaction of solid carbon nanoparticles with vortical structures evaluating at the same time the impact of turbulence and Brownian diffusion. This allowed determining the influence of the main mechanisms influencing nanoparticles dispersion. In a second step, we replaced the cylinder configuration by a simplified geometry of a motor vehicle, Ahmed body configuration. Therefore, simulations with and without of particles presence have been conducted and have allowed to highlight the swirls structures and to characterize the particles dispersion through particle concentration profiles and the particles dispersion coefficients. The results of these simulations allowed us determining the important mechanisms / forces that can influence the dispersion of such particles in the wake of a ground vehicle; this facilitated the implementation and analysis of relatively more complex simulations of the aerodynamics of the square back Ahmed body in the presence of nanoparticles simulating soot from the exhaust gases. The interactions of ultrafine particles with vortical structures appearing in the wake of vehicles were evaluated from concentration profiles and transverse dispersion coefficients. The final step was a wind tunnel experimental campaign that allowed us to characterize the average and turbulent velocity fields as well as the particle concentration fields downstream of the vehicle to validate the numerical results