A review of experiments on stationary bluff-body wakes

Experimental studies dealing with the wake of isolated stationary bluff-bodies are reviewed. After briefly recalling the pioneering works in this domain, the paper focuses on recent research conducted with the latest experimental methods and techniques. The review encompasses a range of topics, including, the effects of bluff-body geometry (non-circular cross sections and nonuniformity in spanwise direction), steady and unsteady (periodic and non-periodic) inflow conditions; surface proximity (rigid wall, confinement and water free surface) and non-Newtonian fluids. Focus is brought to the flow physics of the wakes, including especially the complex threedimensional and oscillatory behaviours induced by the periodic vortex shedding phenomenon. The paper aims to offer a critical and systematic review of new knowledge and findings on the subject area, as well as emerging? and the most frequently adopted experimental techniques. The review also helps identifying knowledge gaps in the literature that need to be addressed in future investigations

Multi-mode interactions in vortex-induced vibrations of flexible curved/straight structures with geometric nonlinearities

A general low-order fluid-structure interaction model capable of evaluating the multi-mode interactions in vortex-induced vibrations of flexible curved/straight structures is presented. Cross-flow motions due to unsteady lift forces of inclined sagged cables and tensioned beams in uniform currents are investigated. In contrast to a linear equation governing the transverse motion of straight beams or cables typically considered in the literature, coupled horizontal/vertical (axial/transverse) displacements and geometric nonlinearities of curved cable (straight beam) are accounted for. A distributed nonlinear wake oscillator is considered in the approximation of space-time varying hydrodynamics. This semi-empirical fluid force model in general depends on the mass-damping parameter and has further been modified to capture both the effects of varying initial curvatures of the inclined cylinder and the Reynolds number. Numerical simulations are performed in the case of varying flow velocities and parametric results highlight several meaningful aspects of vortex-induced vibrations of long flexible cylinders. These comprise multi-mode lock-in, sharing, switching and interaction features in the space and time domains, the estimated maximum modal and total amplitudes, the resonant nonlinear modes of flexible cylinders and their space-time modifications, and the influence of fluid/structure parameters. A shortcoming of single-mode or linear structural model is underlined. Some quantitative and qualitative comparisons of numerical/experimental results are discussed to demonstrate the validity and required improvement of the proposed modelling and analysis predictions

Benchmark on the Aerodynamics of a Rectangular 5:1 Cylinder: An overview after the first four years of activity

In July 2008, a benchmark study on the aerodynamics of a stationary rectangular cylinder with chord-to- depth ratio equal to 5 (BARC) was launched. This paper gives an outline of the state of the art on the aerodynamics of 5:1 rectangular cylinders prior to the starting of BARC, and summarizes the results obtained by the contributors during the first four years of activity. The results of about 70 realizations of the BARC flow configuration obtained under a nominally common set-up in both wind tunnel experiments and numerical simulations are compared among themselves and with the data available in the literature prior to BARC, in terms of bulk parameters, flow and aerodynamic load statistics, pressure and force spanwise correlations. It is shown that the near wake flow, the base pressure and, hence, the drag coefficient obtained in the different flow realizations are in very good agreement. Conversely, the flow features along the cylinder lateral surfaces and, hence, the lift, are strongly sensitive to set-up and modelling, leading to a significant dispersion of both wind tunnel measurements and numerical predictions. Finally, a possible asymmetry of the time averaged flow has been recognized both in wind tunnel tests and in numerical simulation

Wake Dynamics and Passive Flow Control of a Blunt Trailing Edge Profiled Body

Wake flows behind two-dimensional bodies are dominated mainly by two types of coherent structures, namely, the Karman Benard vortices and the streamwise vortices, also referred to as rolls and ribs respectively. The three-dimensional wake instabilities lead to distinct instability modes (mode-A, mode-B and mode-C or mode S) depending on the flow Reynolds number and geometric shape. The present investigation explores the mechanism by which the flow transitions take place to three-dimensionality in the near wake of a profiled leading edge and blunt trailing edge body. Experiments consisting of a combination of Planar Laser Induced Fluorescence visualizations and Particle Image Velocimetry measurements are conducted for Reynolds numbers ranging from 250 to 46000. The results indicate that three instability modes, denoted by mode-A, mode-B and mode-C, appear in the wake transition to three-dimensionality, but their order of appearance does not occur through the traditional route as observed in circular cylinder flows. It is found that mode-C instability with a spanwise spacing varying between 1.2 to 2.8D (D being the trailing edge thickness) dominates the near wake development. This result is explored further with the aim to devise a simple passive control method to mitigate vortex shedding for blunt trailing edge bodies. The effect of a trailing edge spanwise sinusoidal perturbation (SSP) is investigated for a range of Reynolds numbers (ReD) spanning the transition range from ReD = 550 up to 46000. PIV measurements at different vertical and horizontal locations are performed to study changes in the streamwise and spanwise vortices. The base drag and strength of vortex shedding decrease with wavy trailing edge compared to the straight trailing edge. Proper Orthogonal Decomposition (POD) of the obtained PIV data indicates that the spanwise sinusoidal perturbation redistributes the relative energy, enhancing the streamwise vortices and, as a result, suppressing the Karman Benard rolls

Forces on cylinders in relative fluid motion

This thesis reviews current knowledge concerning the forces between circular cylinders and relatively flowing fluids. The kinematic conditions considered are these of steady, unidirectional unsteady, oscillatory, and that of monochromatic surf act- water waves incident upon a vertical surface piercing cylinder. The complicating effects of cylinder surface roughness and freestream turbulence are also considered. Detailed investigation of an oscillating cylinder experiment carried out by the author, at high Stokes number (Reynolds divided by Keulegan Carpenter number), but covering a low range of Keulegan Carpenter number is presented. Following this, a discussion of wave tank and generator design, culminates in the design of such a facility for the testing of cylinders in progressive waves. These experimental examples are used in conjunction with model theory to explain the deficiencies and similarities between the various kinematic conditions above. The experimental utilization of simpler fluid kinematic conditions to model those that are more complex is shown to be possible within certain limitations. Guidance is given to facilitate selection of the most suitable experimental technique for the investigation of specific fluid-cylinder dynamic problems. An original picture of vortex behaviour throughout a cycle of relative planar oscillatory motion is postulated based upon recorded circumferential pressure distribution history, and resultant in-line and lift forces. The Morison equation, considered as a mathematical model which describes the cylinder-fluid dynamics, is shown to be reasonable for Keulegan Carpenter numbers less than 5 (in the inertia dominant regime), or greater than 25 (in the drag dominant regime). The equation is misleading in the intermediate region (5 to 25) where the drag and inertia force components are each of comparable importance. The neglect of the important transverse (with respect to relative flow direction) force component and the effects of flow history, contained in residual vorticity, are also shown to be important deficiencies. The implications of this work for the understanding of the fluid mechanics of vertical circular cylinders in the sea are also considered

Dynamics of gap flow interference in a vibrating side-by-side arrangement of two circular cylinders at moderate Reynolds number

In this work, the coupled dynamics of the gap flow and the vortex-induced vibration (VIV) on a side-by-side (SBS) arrangement of two circular cylinders is numerically investigated at moderate Reynolds number 100 < Re < 800. Of particular interest is to establish a relationship between the VIV, the gap flow and the near-wake instability behind bluff bodies. We find that the kinematics of the VIV regulates the streamwise vorticity concentration, which accompanies with a recovery of two-dimensional hydrodynamic responses at the peak lock-in stage. On the other hand, the near-wake instability may develop around an in-determinant two-dimensional streamline saddle point along the interfaces of a pair of imbalanced counter-signed vorticity clusters. The vorticity concentration difference of adjacent vorticity clusters and the fluid momentum are closely interlinked with the prominence of streamwise vortical structures. In both SSBS and VSBS arrangements, the flip-flopping frequency is significantly low for the three-dimensional flow, except at the VIV lock-in stage for the VSBS arrangement. A quasi-stable deflected gap flow regime with negligible spanwise hydrodynamic (i.e., two-dimensional) response is found at the peak lock-in stage of VSBS arrangements. Owing to the gap-flow proximity interference, a high streamwise vorticity concentration is observed in its narrow near-wake region. The increase of the gap-flow proximity interference tends to stabilize the VIV lock-in, which eventually amplifies the spanwise correlation length and weakens the streamwise vortical structures. We employ the dynamic mode decomposition procedure to characterize the space-time evolution of the primary vortex wake

Analysis of stationary and non-stationary phenomena in turbulent subcritical flow behind two parallel cylinders

This study presents the analysis of the bistable phenomenon for turbulent flows around two cylinders side-by-side using two methods for data analysis and chaos theory for dynamic analysis. The experimental data were acquired for various Reynolds numbers and pitch-todiameter ratio p/D of 1.16, 1.26, and 1.60, cylinders diameter was 25.1 mm. The experimental technique consists of measuring the velocity fluctuations in an aerodynamic channel using hot-wire anemometry. The study presents the application of the Hilbert-Huang transform (HHT) as a tool of analysis for non-stationary and non-linear signals. The method was first validated using single cylinders and then extended for two cylinders side-by-side. Results show that the HHT method may provide information about particular events in timefrequency space and about the physics of flow scales. The statistical analysis of the experimental data is performed to identify statistical patterns that can be used to characterize the bistable flow. The signals are scanned by a moving window for the statistical analysis, creating blocks of probability density functions (PDFs). The four first statistical moments of each PDF are calculated, and a tendency of behavior based on their variations is established. The dynamics of the bistable flow system are studied applying chaos theory tools, like the largest Lyapunov exponent. The strange attractors of the velocity-time series are reconstructed, and their topology is useful to understand the physics of the bistable system. Each flow wake mode is analyzed separately. A general model of the bistable flow is reconstructed using probability functions. The application of a set of tools in the analysis of the turbulent wake behind cylinders is useful for the comprehension of turbulent phenomena, producing meaningful results and allowing the identification of turbulent structures and flow scales, and a better understanding of the system dynamics.Este estudo apresenta a análise do fenômeno da biestabilidade no escoamento em torno de dois cilindros lado a lado usando dois métodos para análise de sinais, e teoria do caos para a análise da dinâmica. Os dados experimentais foram adquiridos para vários números de Reynolds e várias razões de aspecto p/D de 1,16, 1,26 e 1,60, o diâmetro dos cilindros é de 25,1 mm. A técnica experimental utilizada consiste em medir as flutuações de velocidade em um canal aerodinâmico utilizando anemometria de fio quente. O estudo apresenta a aplicação da transformada de Hilbert-Huang (HHT) como ferramenta de análise para sinais não estacionários e não lineares. O método é primeiramente validado utilizando sinais experimentais para um cilindro sobre escoamento turbulento e após aplicado ao escoamento sobre dois cilindros lado a lado. Resultados mostram que o método de HHT fornece não só uma definição mais precisa de eventos específicos no espaço tempo-frequência, mas também permite uma interpretação física mais significativa dos processos dinâmicos das escalas do escoamento. A análise estatística dos dados experimentais é feita com o objetivo de identificar padrões estatísticos que possam ser utilizados para caracterização do escoamento biestável. Para a análise estatística os dados são varridos por uma janela móvel, criando blocos de funções densidade de probabilidade (PDFs). Os quatro primeiros momentos estatísticos são calculados e é possível estabelecer uma tendência de comportamento baseada em suas variações. A dinâmica do sistema biestável é estudada aplicando ferramentas da teoria do caos, como o maior expoente de Lyapunov. O atrator estranho da série temporal da velocidade é reconstruído e sua topologia é utilizada para melhor compreensão do comportamento físico do fenômeno da biestabilidade. Cada esteira do escoamento biestável é analisada separadamente. Um modelo geral do escoamento biestável é reconstruído utilizando funções de probabilidade. A aplicação de um conjunto de ferramentas para a análise da turbulência das esteiras dos cilindros é útil para a melhor compreensão de fenômenos turbulentos, produzindo resultados significativos e permitindo a identificação de estruturas turbulentas e escalas do escoamento e um entendimento sobre a dinâmica do sistema

Vortex-Induced Vibration of Structures with Broken Symmetry

A bluff body, i.e., an object with a blunt cross-section immersed in cross-flow forms an unstable wake, resulting in the formation of large-scale vortical structures, which induce unsteady forces on the body. If the body is flexible or flexibly mounted, vortex-induced vibration (VIV) results, which can have significant implications for a number of physical systems, from aeolian harps to power transmission lines, towing cables, undersea pipelines, drilling risers and mooring lines used to stabilize offshore floating platforms. VIV has been a major subject of research in recent years. The majority of these studies have focused on symmetric systems in which the structure, its geometry, its boundary conditions and the incoming flow are symmetric. However, in many real-world applications, VIV occurs in asymmetric systems. A flexibly-mounted vertical uniform circular cylinder which is placed in a uniform current is an example of a system with symmetric geometry, boundary conditions and wake. This geometry is usually considered as a canonical configuration in modeling structures undergoing VIV. However, many other configurations can exist in which the geometrical symmetry of the system is broken, such as an inclined cylinder in which the cylinder is placed with an angle with respect to the incoming flow direction, or a tapered cylinder in which the diameter of the circular cylinder varies along its length, or when the geometry of the cross-section changes from circular to other shapes such as square or triangle. Also, in some other cases, such as a circular cylinder which is forced to rotate about its long axis, the symmetry of the surrounding wake is broken by the rotation imposed on the cylinder. These cases among many examples are systems with broken symmetry, and are widely used in the offshore industry. The main objective of this thesis is to study VIV of systems with broken symmetry. For this purpose, experiments are conducted and the VIV response of the systems are studied in terms of amplitude and frequency of oscillations as well as the wake of the cylinder. It is shown that higher harmonics contributions of flow forces play an important role in VIV of structures. Also, various taper ratios in VIV of tapered cylinders are found to be the major parameters influencing the VIV response of tapered cylinders. Different rotation rates in VIV of a rotating cylinder, various angles of attack in VIV of a triangular cylinder and different angles of inclination in VIV of flexible inclined cylinder are found to be the influencing parameters in studying the VIV of structures with broken symmetry