A new view of nonlinear water waves: the Hilbert spectrum

We survey the newly developed Hilbert spectral analysis method and its applications to Stokes waves, nonlinear wave evolution processes, the spectral form of the random wave field, and turbulence. Our emphasis is on the inadequacy of presently available methods in nonlinear and nonstationary data analysis. Hilbert spectral analysis is here proposed as an alternative. This new method provides not only a more precise definition of particular events in time-frequency space than wavelet analysis, but also more physically meaningful interpretations of the underlying dynamic processes

Intermittency and Self-Organisation in Turbulence and Statistical Mechanics

There is overwhelming evidence, from laboratory experiments, observations, and computational studies, that coherent structures can cause intermittent transport, dramatically enhancing transport. A proper description of this intermittent phenomenon, however, is extremely difficult, requiring a new non-perturbative theory, such as statistical description. Furthermore, multi-scale interactions are responsible for inevitably complex dynamics in strongly non-equilibrium systems, a proper understanding of which remains a main challenge in classical physics. As a remarkable consequence of multi-scale interaction, a quasi-equilibrium state (the so-called self-organisation) can however be maintained. This special issue aims to present different theories of statistical mechanics to understand this challenging multiscale problem in turbulence. The 14 contributions to this Special issue focus on the various aspects of intermittency, coherent structures, self-organisation, bifurcation and nonlocality. Given the ubiquity of turbulence, the contributions cover a broad range of systems covering laboratory fluids (channel flow, the Von Kármán flow), plasmas (magnetic fusion), laser cavity, wind turbine, air flow around a high-speed train, solar wind and industrial application

Crosswind stability of vehicles under nonstationary wind excitation

This work has studied the crosswind stability of vehicles under nonstationary wind excitation in various scenarios. Railway vehicles running on curved and straight track with varying vehicle speed are studied. Road vehicles are classified into different categories. For each vehicle class, a corresponding worst-case vehicle model has been built. As the wind excitation on the vehicle is a stochastic process, a risk analysis has to be carried out and failure probabilities are computed and analyzed

Crosswind stability of vehicles under nonstationary wind excitation

This work has studied the crosswind stability of vehicles under nonstationary wind excitation in various scenarios. Railway vehicles running on curved and straight track with varying vehicle speed are studied. Road vehicles are classified into different categories. For each vehicle class, a corresponding worst-case vehicle model has been built. As the wind excitation on the vehicle is a stochastic process, a risk analysis has to be carried out and failure probabilities are computed and analyzed

Active flow control methods for aerodynamic applications

The cylinder in cross flow has been the subject of many numerical and experimental studies since it provides a deep insight of the physical phenomena occurring in a wide range of flow regimes. Despite a number of investigations at Reynolds number (Re = 3900), there has been a constant debate on the important aspects of the flow such as spanwise resolutions, lateral domain extent, convergence of turbulent statistics in the near wake, the so called U-V streamwise velocity profiles at x = 1D, where D is the cylinder diameter, and the critical Re for the onset of shear layer instability together with its characterization. In this thesis, an attempt has been made to address some of these issues and report new results through Direct numerical simulations (DNS) by employing spanwise domain extents i.e. Lz = 1.5D; 2D; 2.5D; pD at the moderate flow regime i.e. Re = 2000, where boundary layer is still laminar while the near wake has gone fully turbulent. Intermittent bursts of shear layer instability have been spotted at this Re indicating the signs of the incipient laminar to turbulent transition in the separating shear layer. It is further confirmed that the secondary instability develops in the regions between the opposite sign large scale spanwise vortices and features a phase lag of 135 degree. Pseudo-Floquet analysis gives a good prediction of fastest growing mode consistent with the reported numerical and experimental measurements. In the second part of the thesis, active flow control (AFC) past circular cylinder has been thoroughly investigated with the aid of parametric analysis at the same Re. We applied spanwise-dependent fluidic actuation, both steady and time-dependent, on the flow past a circular cylinder at Re = 2000. The actuation takes place in two configurations: in-phase blowing and suction from the slits located at ±90 degree (top and bottom) with respect to the upstream stagnation point for both steady and time periodic actuation, and blowing and suction from the top and bottom slits traveling oppositely with respect to each other in the spanwise direction. Optimal forcing amplitude and wavelength are obtained by sweeping across the parametric space. Spanwise-dependent time-independent forcing with wavelength ¿z = 2D has been found the optimal one in terms of drag reduction and attenuation in lift fluctuations. The time-dependent forcing with sinusoids travelling oppositely with respect to each other along the span produced significant reduction in drag force and lift fluctuations, however, the in-phase time periodic actuation with forcing frequency four times the natural vortex shedding frequency resulted in significant increased drag and lift fluctuation, signalling to a potential candidate for the energy harvesting applications. Finally, in the last part of the thesis, time-dependence of flow inside novel laminar-fluidic-oscillator has been analyzed using DNS. Again, pseudoFloquet stability analysis has been utilized to predict the fastest growing Fourier modes along the homogeneous direction. Supplementary three-dimensional numerical study has also been conducted for the suitable cases at various Re. It has been found that steady flow inside fluidic oscillator’s cavity bifurcates from steady state to time-periodic state through supercritical Hopf bifurcation. The secondary transition inside fluidic oscillator’s cavity occurs through the breaking of flow symmetry about the cavity axis by pitchfork supercritical bifurcation.El cilindro en flujo cruzado ha sido objeto de muchos estudios numéricos y experimentales, ya que proporciona una visión profunda de los fenómenos físicos que ocurren en una amplia gama de regímenes. A pesar de una serie de investigaciones en el número de Reynolds (Re = 3900), ha habido un debate constante sobre los aspectos importantes del flujo, como las resoluciones en el span, la extensión del dominio lateral, la convergencia de estadísticas turbulentas en la estela cercana, el tipo de perfil (U o V) en la estela a x = 1D, donde D es el diámetro del cilindro, y el Re crítico para el inicio de la inestabilidad de la capa de cizalla y su caracterización. En esta tesis, se han intentado abordar algunos de estos problemas e informar nuevos resultados a través de simulaciones numéricas directas (DNS) mediante el uso de extensiones de dominio spanwise de Lz = 1.5D; 2D; 2.5D; pD en un régimen de flujo transicional a Re = 2000, donde la capa límite todavía es laminar mientras que la estela cercana se ha vuelto completamente turbulenta. A este Re h sido detectada inestabilidad intermitente, lo que indicando una transición incipiente laminar-turbulenta de la capa de cizalla. Se confirma además que la inestabilidad secundaria se desarrolla en las regiones entre los vórtices a gran escala del signo opuesto y presenta un desfase de 135 grados. El análisis de pseudo-Floquet da una buena predicción del modo de crecimiento más rápido consistente con las mediciones numéricas y experimentales reportadas. En la segunda parte de la tesis, el control de flujo activo (AFC) sobre el cilindro circular se ha investigado a fondo con la ayuda de análisis paramétrico al mismo Re. Aplicamos una actuación fluídica dependiente de la envergadura, tanto constante como dependiente del tiempo, en el flujo alrededor de un cilindro circular a Re = 2000. La actuación se realiza en dos configuraciones: soplado y succión en fase desde las ranuras ubicadas a ± 90 grados (arriba y abajo) con respecto al punto de estancamiento aguas arriba (tanto para la actuación periódica constante como dependiente del tiempo), y para el soplado y la succión con dependencia temporal tal que viajan en sentido opuesto a lo largo de las ranuras superior e inferior. La amplitud y la longitud de onda de forzado óptimas se obtienen barriendo el espacio paramétrico. Se ha encontrado que el forzado independiente del tiempo pero de amplitud variable en la envergadura con longitud de onda ¿z = 2D es el óptimo en términos de reducción de la resistencia y atenuación en las fluctuaciones de sustentación. El forzado dependiente del tiempo con sinusoides que viajan en sentido opuesto entre sí a lo largo del tramo produce una reducción significativa en la fuerza de resistencia aerodinámica y la fluctuación de la sustentación, sin embargo, la actuación periódica en el tiempo en fase con una frecuencia de forzado cuatro veces mayor que la frecuencia natura de desprendimiento de vórtices resultó en un aumento significativo de la resistencia y fluctuaciones de sustentación, lo cual lo coloca como potencial candidato para aplicaciones de recolección de energía. Finalmente, en la última parte de la tesis, la dependencia temporal del flujo dentro de un nuevo oscilador fluídico laminar se ha analizado utilizando DNS. Nuevamente, el análisis de estabilidad pseudoFloquet se ha utilizado para predecir los modos de Fourier de más rápido crecimiento en la dirección homogénea. También se ha realizado un estudio numérico tridimensional suplementario para varios de los Re considerados. Se ha encontrado que el flujo constante dentro de la cavidad del oscilador fluídico bifurca del estado estacionario al estado periódico en el tiempo mediante una bifurcación supercrítica de Hopf. La transición secundaria dentro de la cavidad del oscilador fluídico ocurre a través de la ruptura de la simetría del flujo en relación al eje de simetría de la cavidad por bifurcación supercrítica pitchfork

New Advances in Fluid Structure Interaction

Fluid–structure interactions (FSIs) play a crucial role in the design, construction, service and maintenance of many engineering applications, e.g., aircraft, towers, pipes, offshore platforms and long-span bridges. The old Tacoma Narrows Bridge (1940) is probably one of the most infamous examples of serious accidents due to the action of FSIs. Aircraft wings and wind-turbine blades can be broken because of FSI-induced oscillations. To alleviate or eliminate these unfavorable effects, FSIs must be dealt with in ocean, coastal, offshore and marine engineering to design safe and sustainable engineering structures. In addition, the wind effects on plants and the resultant wind-induced motions are examples of FSIs in nature. To meet the objectives of progress and innovation in FSIs in various scenarios of engineering applications and control schemes, this book includes 15 research studies and collects the most recent and cutting-edge developments on these relevant issues. The topics cover different areas associated with FSIs, including wind loads, flow control, energy harvesting, buffeting and flutter, complex flow characteristics, train–bridge interactions and the application of neural networks in related fields. In summary, these complementary contributions in this publication provide a volume of recent knowledge in the growing field of FSIs

30th International Conference on Condition Monitoring and Diagnostic Engineering Management (COMADEM 2017)

Proceedings of COMADEM 201

Sensors Fault Diagnosis Trends and Applications

Fault diagnosis has always been a concern for industry. In general, diagnosis in complex systems requires the acquisition of information from sensors and the processing and extracting of required features for the classification or identification of faults. Therefore, fault diagnosis of sensors is clearly important as faulty information from a sensor may lead to misleading conclusions about the whole system. As engineering systems grow in size and complexity, it becomes more and more important to diagnose faulty behavior before it can lead to total failure. In the light of above issues, this book is dedicated to trends and applications in modern-sensor fault diagnosis