Search

Isothermal turbulent flow around circular cylinders arranged side-by-side was numerically simulated on a commercial finite-volumes platform, ANSYS® CFX, version 2020 R2. The turbulence was modeled by using k-ω shear stress transport (k-ω SST). Three different Reynolds numbers were computed, Red = 200, 1000, and 3000, which were based on the cylinder diameter, d, the free stream velocity, U∞, and the kinematic viscosity of the fluid, ν. Sided cylinders were spaced apart from each other, forming a p/d ratio equal to 2, which was kept constant throughout the computations regardless of changes in the Reynolds number. The drag coefficient, Cd, as well as its time traces, was evaluated along with the different wake topologies experienced by the cylinders (wide wake WW and narrow wake NW). The simulations were able to predict the bistable flow over the cylinders and the Cd changes associated with the wakes. Whenever a new wake topology was identified, the shape drag changed in accordance with the instantaneous pressure distribution. A laminar simulation was carried out for the lowest Reynolds number case, showing that the adopted turbulence model did not affect the dynamic response of the flow. The Red = 3000 case was compared to Afgan’s outcomes, whose simulations were carried out in a 3-D mesh using LES (Large Eddy Simulation), showing great agreement with their results.

We address the task of modelling numerically turbulent flows within random arrays of circular cylinders, relevant for several industrial and environmental applications. Numerical simulations are employed to model infinite domains of randomly placed emergent and rigid cylinders validated by a laboratory database acquired by a PIV system. The main goals are: (i) to discuss the effect of the numerical domain size and the grid resolution on the first and second order moments and (ii) to characterise the spatial distribution of mean flow and turbulence variables in the drag-wake controlled stratum. Five domains of different sizes (9–44 cylinders) and four grid resolutions were independently tested. The results show that the time-averaged velocity field and the Reynolds stress tensor were not significantly affected by the size of the tested numerical domains. The analysis of the grid resolution influence shows how the results improve with mesh refinement, while none of the tested meshes produces un-physical results. The present work provides guidance on the acceptable compromises, in terms of mesh resolution and domain size, when predicting, with eddy resolving computational fluid mechanics tools, first and second-order moments of turbulent flows within infinite arrays or randomly placed cylinders. © 2018 Elsevier Ltd 80 245 261 245-261

Two-dimensional flow over bluff bodies is studied in the unsteady laminar flow regime using numerical simulations. In previous investigations, lift and drag forces have been studied over different cross-sectional shapes like circles, squares, and ellipses. We aim to extend the previous research by studying the variation of hydrodynamic forces as the shape of the body changes from a circular cylinder to a more streamlined or a bluffer body. The different body shapes are created by modifying the downstream circular arc of a circular cylinder into an ellipse, hence elongating or compressing the rear part of the body. The precise geometry of the body is quantified by defining a shape factor. Two distinct ranges of shape factors with fundamentally different behavior of lift and drag are identified. The geometry constituting the limit is where the rear part ellipse has a semi-minor axis of half the radius of the original circle, independent of the Reynolds number. On the other hand, the vortex shedding frequency decreases linearly over the whole range of shape factors. Furthermore, the variation of the forces and frequency with Reynolds number, and how the relations vary with the shape factor are reported.

The flow around static and freely vibrating cylinders are of interest in understanding different phenomena encountered in many practical applications. This work has been partially financially supported by the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (Ref. TRA2017-88508-R). We also acknowledge Red Española de Surpercomputación (RES) for awarding us access to the MareNostrum IV machine based in Barcelona, Spain (Ref. FI-2017-2-0016) and PRACE for awarding us access to SuperMUC (Project ViValdi Ref. 2017174222). D. Pastrana acknowledges support of the CONACyT-SENER graduate fellowship program to study abroad 278102/439162.

In this paper, an introduction to electromagnetic scattering is presented. We introduce the basic concepts needed to face a scattering problem, including the scattering, absorption, and extinction cross sections. We define the vector harmonics and we present some of their properties. Finally, we tackle the two canonical problems of the scattering by an infinitely long circular cylinder, and by a sphere, showing that the introduction of the vector wave function makes the imposition and solution of the boundary conditions particularly simple.

This study reports an efficient reduction of the drag exerted by a flow on a cylinder when the former is forced with a plasma actuator. A three-electrode plasma device (TED) disposed on the surface of the body is considered, and the effect of the actuation frequency and amplitude is studied. Particle image velocimetry (PIV) measurements provided a detailed information that was processed to obtain the time-averaged drag force and to compare the performances of TED actuator and the canonical dielectric discharge barrier actuator. For the Reynolds number considered (Re = 5500), excitations with the TED actuator were more efficient, achieving drag reductions that attained values close to 40% with high net energy savings. The reduction of coherent structures using the instantaneous vorticity fields and a clustering technique allowed us to gain insight into the physical mechanisms involved in these phenomena. This highlights that the symmetrical forcing of the wake flow at its resonant frequency with the TED promotes symmetrical vorticity patterns which favor drag reductions. Fil: D'adamo, Juan Gastón Leonel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina Fil: Leonardo, Leandro. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina Fil: Castro Hebrero, Federico Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina Fil: Sosa, Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina Fil: Duriez, Thomas Pierre Cornil. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina Fil: Artana, Guillermo Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina

The onset of two-dimensional instabilities in the flow past two side-by-side circular cylinders is numerically investigated in the ranges $0.1\leq g\leq 3$ and $\mathit{Re}, with $g$ being the non-dimensional gap spacing between the surfaces of the two cylinders and $\mathit{Re}$ the Reynolds number. A comprehensive, global stability analysis of the symmetric base flow is carried out, indicating that three harmonic modes and one steady antisymmetric mode become unstable at different values of $g$ and $\mathit{Re}$. These modes are known to promote distinct flow regimes at increasing values of $g$: single bluff-body, asymmetric, in-phase and antiphase synchronized vortex shedding. For each mode, the inherent structural sensitivity is examined in order to identify the core region of the related instability mechanism. In addition, by exploiting the structural sensitivity analysis to base flow modifications, a passive control strategy is proposed for the simultaneous suppression of the two synchronized shedding modes using two small secondary cylinders. Its effectiveness is then validated a posteriori by means of direct numerical simulations.

Two-dimensional flow over a circular cylinder placed in-line behind a control rod of the same diameter is studied with and without the influence of a plane wall. Thermal patterns observed during various vortex shedding modes are discussed in detail. POD analysis of temperature data reveals that, with the decrease in wall height, modal structures are moved closer to the cylinder surface, which increases the heat transfer fluctuation. The plane wall increases the heat transfer from the bottom surface of the cylinder and decreases the heat transfer from the top surface, while the rotation of control rod contributes in an opposite manner. As the cylinder is moved closer to the wall, the time-averaged Nusselt number increases for a stationary control rod. In the case of a rotating control rod, the rotation of the control rod acts in the opposite sense to nullify this wall effect. � 2014 Taylor & Francis Group, LLC.

Boundary layers are subject to favorable and adverse pressure gradients because of both the temporal and spatial components of the pressure gradient. The adverse pressure gradient may cause the flow to separate. In a closed loop unsteady tunnel we have studied the initiation of separation in unsteady flow past a constriction (bluff body) in a channel. We have proposed two important scalings for the time when boundary layer separates. One is based on the local pressure gradient and the other is a convective time scale based on boundary layer parameters. The flow visualization using a dye injection technique shows the flow structure past the body. Nondimensional shedding frequency (Strouhal number) is calculated based on boundary layer and momentum thicknesses. Strouhal number based on the momentum thickness shows a close agreement with that for flat plate and circular cylinder. � 2013 Elsevier Ltd.

International audience; In the domain of flow simulation, avoiding the manual conception and numerisation of the domain can lead to the saving of a certain amount of time. Some processes, using heavy devices like LASER metrology, allow the numerical reconstruction of a real object. The aim of this paper is to propose a more simple tool requiring a commercial digital camera (like a smartphone), to transform a digital picture into a ready to use mesh. Besides simplicity, the tool has to be precise enough to bring accurate simulation results. Then, image processing object detection and reconstruction are used to generate a 2D mesh that can be integrated in a finite volume transient CFD simulation. Cars and airfoils are chosen as objects and the DNS fluid flow Gerris solver performs the simulations. After a validation on a circular shape object, simulations, conducted at different Reynolds number, provide accurate results plotting the Von Karman alley regime.

Slender marine structures are subjected to ocean currents, which can cause vortex-induced vibrations (VIV). Accumulated damage due to VIV can shorten the fatigue life of marine structures, so it needs to be considered in the design and operation phase. VIV prediction tools are based on hydrodynamic coefficients, which are obtained from forced motion experiments on a circular cylinder. Most of the forced motion experiments apply harmonic motions in either in-line (IL) or cross-flow (CF) direction. Combined IL and CF forced motion experiments are also reported. However, measured motions from flexible pipe VIV tests contain higher order harmonic components, which have not yet been extensively studied. This paper presents results from conventional forced motion VIV experiments, but using measured motions taken from a flexible pipe undergoing VIV. The IL excitation coefficients were used by semi-empirical VIV prediction software VIVANA to perform combined IL and CF VIV calculation. The key IL results are compared with NDP flexible pipe model test results. By using present IL excitation coefficients, the prediction of IL responses for combined IL and CF VIV responses is improved.

The large eddy simulation (LES) for flow around circular cylinder at Re=4.1×104 is performed in this paper. The mean and root mean square (RMS) of fluctuating pressure on circular cylinder are reasonably presented, with the mean drag coefficient, fluctuating lift coefficient and particularly the vortex-shedding Strouhal number being in good agreement with available reports. It is found that the vortex shedding is not in phases along the span wise direction, while the vortex-shedding energy is concentrated to a limited band of frequency. Statistics show that the fluctuating pressure time series at θ = 90o and θ = 270o on the opposing side of the circular share the same features, and the pressure fluctuating energy is also concentrated on the vortex-shedding frequency. The flow mechanism involved in drag and lift on cylinder is discussed according to the coherence function of the fluctuating pressure between the upper and lower cylinder surface.

The vortex formation and shedding behind bluff structures is influenced by fluid flow parameters such as, Reynolds number, surface roughness, turbulence level, etc. and structural parameters such as, mass ratio, frequency ratio, damping ratio, etc. When a structure is flexibly mounted, the K�rm�n vortex street formed behind the structure gives rise to vortex induced oscillations. The control of these flow induced vibrations is of paramount practical importance for a wide range of designs. An analysis of flow patterns behind these structures would enable better understanding of wake properties and their control. In the present study, flow past a smooth circular cylinder is numerically simulated by coupling the mass, momentum conservation equations along with a dynamical evolution equation for the structure. An active flow control strategy based on zero net mass injection is designed and implemented to assess its efficacy. A three actuator system in the form of suction and blowing slots are positioned on the cylinder surface. A single blowing slot is located on the leeward side of the cylinder, while two suction slots are positioned at an angle ?=100�. This system is found to effectively annihilate the vortex induced oscillations, when the quantum of actuations is about three times the free stream velocity. The dynamic adaptability of the proposed control strategy and its ability to suppress vortex induced oscillations is verified. The exact quantum of actuation involved in wake control is achieved by integrating a control equation to decide the actuator response in the form of a closed loop feed back system. Simulations are extended to high Reynolds number flows by employing eddy viscosity based turbulence models. The three actuator system is found to effectively suppress vortex induced oscillations. � 2012 Elsevier Inc.

The vortex shedding characteristics and the drag force acting on a circular cylinder, attached with an oscillating splitter plate, are investigated by solving the two-dimensional Navier-Stokes equations. The splitter plate is forced to exhibit harmonic oscillation about its attachment point, and the Reynolds number (Re) of the flow is 100. In order to facilitate easy handling of the plate oscillations inside the computational domain, the equations are solved in a Cartesian grid, and the concept of immersed boundary method is used to impose the boundary conditions on the body surface. The characteristic feature of this problem is the complex interaction between the vortices shed from the splitter plate and the cylinder. Three different patterns of vortex shedding are observed in the wake of the circular cylinder depending upon the frequency and amplitude of plate oscillation: normal shedding, chain of vortices and shedding from splitter plate. It is found that the inverse relationship between the vortex formation length and Strouhal number is not applicable when the splitter plate is subjected to oscillation. Additional related interesting fluid dynamics features are also presented. � 2011 Elsevier Ltd.

This paper presents the Hall and Ion-slip effects on electrically conducting couple stress fluid flow between two circular cylinders in the presence of a temperature dependent heat source. The governing non-linear partial differential equations are transformed into a system of ordinary differential equations using similarity transformations and then solved using homotopy analysis method (HAM). The effects of the magnetic parameter, Hall parameter, Ion-slip parameter and couple stress fluid parameter on velocity and temperature are discussed and shown graphically.

Flow past a circular cylinder mounted on a flat plate normal to the flow direction is numerically simulated. Three-dimensional mass and momentum conservation equations are solved to obtain the detailed spatio-temporal dynamics of wake vortices revealed through the velocity and pressure data. Particular attention is focused on the junction between the flat plate and the cylindrical cross-section, which is well known to be a rich source of three-dimensional vortical structures. The wake-boundary layer interaction region generates necklace vortices, which are popularly known as horseshoe vortices. The effect of spanwise height (H/D) of the cylinder on these vortices and their unsteady wake characteristics are investigated. The forces acting on the cylinder and the Strouhal periodicities are validated for Re=200, apart from presenting qualitative features, such as streamlines and vorticity contours. The aspect ratio of the cylinder is found to have predominant influence on the primary and secondary vortex structures. Streakline visualization is developed to study the spatio-temporal dynamics of interaction between the wake and the boundary-layer. An assessment of the two types of boundary conditions, viz., free-slip and no-slip (the latter due to bed friction) are investigated. Vortex shedding is found to be completely suppressed for smaller aspect ratios (H/D)?1.0, with a stabilized flow pattern in the aft of the cylinder, while it persists for H/D>1.0. Copyright � Taylor & Francis Group, LLC.

The flow characteristic around two circular cylinders in tandem arrangement located near a plane wall were investigated experimentally in a uniform flow at a Reynolds Number of 5.3 x 104. The center to center spacing between the two cylinders relative to the cylinder diameter was constantly maintained at P/D = 1.5. The pressure distributions along the surface of the cylinder and the plane wall were measured by varying the gap-to-diameter of cylinder ratio (G/D) in the range of 0 < G/D < 0.467. Surface oil-film techniques were used to investigate the flow patterns on the cylinder. The result showed that for upstream cylinder, in the gap-to-diameter ratio G/D < /D, there is no stagnation point at front side of the upstream cylinder; it is gradually raised as the gap increase. For the downstream cylinder, a peak on the lower side of the front side of the cylinder is apparent in each of the pressure distributions. This peak represents the reattachment of shear layer that separates from lower side of the upstream cylinder. The reattachment point tends to move forward close to the angular position of  = 0o as the gap ratio increase. The shear layer bifurcates into two shear layers. One shear layer continues in the downstream direction, and the other shear layer flows in the upstream direction.

Reynolds averaged Navier–Stokes equations (RANS) are solved to simulate the flow past a circular cylinder. Momentum injection through M oving S urface B oundary-layer C ontrol (MSBC) with zero net mass injection is implemented to achieve wake control. Popular eddy viscosity based closure models are assessed for their predictive capability of turbulent wake characteristics. For the present simulation, sub-critical Reynolds number ( Re ) of 3900 is chosen, where extensive validations are available. A stabilization approach is proposed to model, predict and control vortex shedding behind a circular cylinder. Along with the mass, momentum conservation, turbulent kinetic energy (TKE) and its rate of dissipation equations are solved with the objective of achieving the annihilation of wake structures. To enable momentum injection, two simple rotary type control cylinders are located at 120°, behind the main cylinder. The ratio of the main cylinder, control cylinder and gap between them are fixed at D: 0.1D: 0.01D, respectively. These control cylinders, which are like externally controllable actuators, are assessed for their ability to influence momentum injection and hence wake patterns. The popular finite volume based S emi I mplicit P ressure L inked E quations (SIMPLE) scheme is employed for the numerical calculations. Detailed assessment of different eddy viscosity based turbulence models viz., standard k – e , Renormalization Group (RNG) k – e , realizable k – e and k – e version of Kato–Launder (KL) is carried out. As a precursor, validation of turbulence statistics such as, mean streamwise velocity along the wake centerline, mean pressure coefficient on the cylinder surface and time averaged Reynolds stresses etc. is carried out against known experimental and numerical computations. The role of externally controllable actuators on the fluid flow patterns past a circular configuration is assessed with the help of streaklines, streamlines, vorticity, Reynolds stress contours etc. Complete suppression of vortex shedding is observed for an injection parameter (defined as the ratio of control cylinder velocity ( U c ) to that of the freestream ( U ∞ )) ξ = 6.0 . The results clearly demonstrate the effectiveness of a rather simple momentum injection strategy in suppressing turbulent vortex structures.

The effects of blockage on the hydrodynamic, thermal and mass transfer characteristics of a circular cylinder (CC) and their association with each other are investigated numerically, by considering the influence of blockage (beta=0.333-0.800) on the flow and heat transfer mechanisms in conjunction with moisture diffusivity (D=1 x 10(-8)-1 x 10(-5) m(2)/s) to show how much mass transfer behavior and phenomena are affected. As some comprehensive ANSYS-CFX runs are performed in the hydrodynamic and thermal fields around the CC, the moisture distributions within the CC are evaluated by Alternating Direction implicit method. It is determined that blockage causes thinner hydrodynamic and thermal boundary layers, rises the frictional and thermal activities, and shifts the separation locations (theta(s)) downstream to theta(s)=50.20 degrees, 41.98 degrees and 37.30 degrees for beta=0.333, 0.571 and 0.800. In the complete blockage scenario set, stagnation point heat transfer values are evaluated to be above those of the back-face, signifying the superior heat transfer enhancing capability of the stagnation point momentum activity when compared with the impact of downstream vortex system. The influence of moisture diffusivity on the overall drying times is determined to advance with stronger blockage. As the back face mass transfer coefficients (h(m-bf)) rise with a high beta, the contrary is valid for front face values (h(m-ff)), with the interpreting ratios of (h) over bar (m-bf)/(h) over bar (m)=0.51 and 0.57 and (h) over bar (m-ff)/(h) over bar (m)=1.49 and 1.43 for beta=0.333 and 0.800.

Form deviations of cylindrical features present in the manufactured parts are measured using coordinate measuring machine (CMM) and expressed as circularity or cylindricity errors evaluated using appropriate reference features. In the present work, computational geometric techniques are used to establish a circle as reference feature, and a heuristic algorithm is proposed to get a unique convex inner hull. Using the concept of equi-distant lines and diagrams, minimum circumscribed (MC), maximum inscribed (MI), and minimum zone (MZ) circles are established. For the first time, algorithms purely based on computational geometric concepts have been developed in the present work to arrive at MC, MI, and MZ circular cylinders. As the algorithms and the implementation details are explained with simple data sets, the practitioners can easily understand these methods and implement them in CMMs for the evaluation of circularity and cylindricity errors. The algorithms are also tested on larger datasets, and in all cases, accurate results are obtained in less than a second. � 2010 Springer-Verlag London Limited.

Numerical investigations are carried out to study the behavior of near wake flow for flow past a transversally vibrating square cylinder using developed computational code. Two-dimensional computational study is conducted over a range of frequencies of cylinder vibration. The nature of vortex shedding is found to be contingent of the frequency of excitation of the cylinder and observed to vary characteristically. The temporal evolution of the unsteady wake is presented. The unsteady forces on the vibrating square cylinder are computed and are compared with those on a stationary square cylinder. Further results include dependence of vorticity fields as well as of wake Strouhal number on frequency of excitation of the vibrating cylinder. In addition, a comparison between drag forces for flow past transversely vibrating square and circular cylinders for similar amplitude and frequency of cylinder vibration has been presented as a validation of the tested computational code. � 2010 Begell House, Inc.

Numerical simulation of flow past a circular cylinder at sub-critical Reynolds number Re = 3,900 is performed using three-dimensional, unsteady, Reynolds-Averaged Navier-Stokes (URANS) equations. A nonlinear turbulence model based on the k-? formulation is used to achieve the turbulent closure. The results obtained by the simulations are compared with experimental and previously reported numerical results. The grid used for the present simulation is reasonable, and the accuracy obtained is good considering the computational cost involved in carrying out large-eddy simulations (LES) for the same test case. The test flow is also simulated using standard k-? model, and the results obtained by the nonlinear k-? model are found to be better.

An experimental investigations was carried out on a perforated circular cylinder (0.50m diameter) encircling an impermeable cylinder (0.05m diameter) at a constant water depth of 0.7m for both regular and random waves in the wave flume at the Department of Ocean Engineering, Indian Institute of Technology, Madras, India. The porosity of the outer cylinder was varied from 4.54 to 19.15% to study the influence of porosity on wave forces on and water surface fluctuations in and around the twin cylinder system. A numerical method is developed based on the boundary integral equation method along with a porous body boundary condition, where the porosity is modeled using the resistance coefficient f and added mass coefficient Ca for regular waves. The resistance coefficient increases with the increase in porosity and wave heights except for a porosity of 4.54%, whereas the added mass coefficient is almost zero. Porosity in the range of 10-15% can be recommended for the perforated cylinder based on the experimental results of wave forces and the water surface fluctuations inside the perforated cylinder. � 2008 ASCE.

The paper deals with numerical analyses of interference galloping of two elasticcaly supported circular cylinders of equal diameters. The basis of the analyses is a quasi-steady model of this phenomenon. The model assumes that both cylinders participate in the process of interference galloping and they have two degrees of freedom. The movement of the cylinders is described as a set of four nonlinear differential equations. On the basis of numerical solutions of these equations the author evaluate the correctness of this quasi-steady model. Then they estimate the dependence of a critical reduced velocity on the Scruton number, turbulence intensity and arrangements of the cylinders.

The purpose of this project is the study of the steady streaming phenomenon induced by diff erent distributions of circular cylinders oscillating within a fluid. To understand whether this characteristic flow would be suitable for trapping micro-particles, the infuence of the main parameters governing this phenomenon was analysed, varying them within two di fferent geometrical distributions. In the fi rst one, placing two cylinders symmetrically, to extend and contrast the previous results for this case. And in the second set of experiments, a distribution of four cylinders forming a square was used instead, to analyse its outcome as it was unexplored so far. In order to achieve these objectives, several experiments were performed during this research. To recreate the conditions needed, the laboratory set-up was formed by a tank full of water and glass particles that reflect the light, a laser beam to create a visual plane, and the structure containing the cylinders. Meanwhile, a high-speed camera is in charge of capturing the position of the particles at every given period of time. After gathering all the raw data, the images were processed by several Matlab codes which are able to extract the important information about the movement of the particles and display the tendency they are following. Then, the outcome was compared and contrasted with previous experiments and numerical studies to confi rm its validity and understand its meaning. Through this analysis, it is conclude that the experiments achieved during this project can confi rm the previous hypothesis and ratify former studies. Moreover, it has been possible to provide a wider overview of the importance of the parameters that govern the steady streaming and extend the knowledge about potential uses for new distributions of circular cylinders. Ingeniería en Tecnologías Industriales

Three-dimensional direct numerical simulations of a Bingham fluid flowing past a confined circular cylinder have been used in order to investigate viscoplastic effects in the wake-transition regime. The case of a cylinder confined in a plane channel with a fixed blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2 has been studied, for values of the Bingham number and Reynolds numbers (based on the cylinder diameter and bulk flow velocity) in the range 0 Bn ≤ 5 and 150 ⩽ Re ⩽ 600 , respectively. The critical Reynolds number for the onset of three-dimensional flow regime has been shown to increase linearly with Bingham number, at least in the range of parameters considered. Two distinct modes of three-dimensional instability (modes A and B) have been identified in the flow, and the influence of viscoplastic effects on the evolution of these instabilities have been described. Significant deviations in the structure of the far wake from the Newtonian case have been observed and associated with a disruption of underlying processes that are known to operate in the Newtonian wake. Despite these structural differences, it has been shown that the evolution of the Strouhal number with Reynolds number in the viscoplastic fluid exhibits two discontinuous changes that are associated with the onset of the different instabilities in the flow, a behavior that mirrors the behavior in Newtonian fluids.

This article presents the numerical studies on predicting onset of flow separation and vortex shedding in flow past unconfined two-dimensional elliptical cylinders for various Axis Ratios (AR) and a wide range of Angles of Attack (AOA). An efficient Cartesian grid technique based immersed boundary method is used for numerical simulations. The laminar separation Reynolds number (Res) that marks separation of flow from surface and the critical Reynolds number (Recr) which represents transition from steady to unsteady flow are determined using diverse methods. A stability analysis which uses Stuart-Landau equation is also performed for calculating Recr. The shedding frequency (Stcr) that corresponds to Recr is calculated using Landau constants. The simulated results for circular cylinder are found to be in good agreement with the literature. The effects of AR and AOA on Res, Recr, and Stcr are studied. It is observed that the Res, Recr, and Stcr exhibit a direct/inverse relationship with AR depending upon the given AOA. Correlations of Res, Recr, and Stcr with respect to AR and AOA are proposed with good accuracy. � 2014 AIP Publishing LLC.

The bluff body simulations over canonical forms like circular and square cylinders are very well studied and the correlations for bulk parameters like mean drag coefficient and Strouhal numbers for the same are reported widely. In the case of elliptic cylinder, the literature is very sparse, especially for moderate Reynolds number (Re). Hence, in this work, a detailed study about fluid flow characteristics over an elliptic cylinder placed in a free stream is performed. Simulations are carried out for different Re ranging from 50 to 500 with axis ratio (AR) varied between 0.1 to 1.0 in steps of 0.1. Immersed boundary method is used for the solid boundary condition implementation which avoids the grid generation for each AR and a single Cartesian grid is used for all the simulations. The effect of AR for various Reynolds numbers is also focused on using the in-house code. The influence of AR is phenomenal for all the Re and the values of wake length, drag coefficient, and Strouhal number decrease with decreasing AR for a particular Re. The critical ARs, for vortex shedding and wake formation, are identified for various Re. Detailed correlations for wake length, critical ARs for vortex shedding and wake formation, mean drag coefficient and Strouhal number, in terms of AR, are reported in this work. [DOI: 10.1115/1.4024862]

The dissertation addresses the design of ultra-high towers under the wind action and has a special application for Solar Updraft Power Plants. They are a highly sustainable natural resource for electric power generation, based on a combined sun-wind energy solution. The object of the investigation is a 1-km tall solar tower, made of reinforced concrete and stiffened along the height by stiffening rings. They are usually introduced in the design of solar towers in order to reduce the structural vulnerability to the wind action by enhancing a beam-like behaviour. However, the wind tunnel tests performed within this research showed that the presence of rings along the height of the tower modifies the aerodynamics of the flow around the circular cylinder and creates a bi-stable and asymmetric load condition, even at moderately high Reynolds numbers. This phenomenon is new and unknown. Similar effects were observed in the critical range of Re number and around two side-by-side cylinders, but the conditions of occurrence and the physical reasons are profoundly different. The discovery of the existence of such a bistable and asymmetric load condition induced by rings along the height of a finite length circular cylinder, its interpretation, as well as the cross-checked experimental evidence in different wind-tunnels confirmed also by numerical simulations, are the original contributions of this work. Then, the effect is quantified on the structural response. The bistable asymmetric load on the structure did not result to be a prohibitive load condition for solar towers and the magnitude of the effect depends on the number and on the size of the rings. Mitigation strategies are then proposed in the work. Furthermore, the dissertation evaluates the shell response to the stochastic wind loading process and provides to the designer a general unified simple tool to define design wind loads for quasi-static calculations of ultra-high towers in any atmospheric boundary layer flow., Die hiermit vorgelegte Doktorarbeit befasst sich mit der Einwirkung des natürlichen Windes auf ultra-hohe Turmbauwerke. Die Türme von Aufwindkraftwerken stehen im Vordergrund. Derartige Kraftwerke ermöglichen eine schadstofffreie Erzeugung elektrischer Energie. Als Untersuchungsgegenstand der Arbeit dient beispielhaft ein 1 km hoher Aufwindturm. Das Tragwerk ist als Stahlbetonschale, die durch Aussteifungsringe verstärkt ist, konzipiert. Erstmalig wurden im Rahmen dieser Forschungen Windkanalversuche an Aufwindtürmen durchgeführt. Sie zeigten, dass die außenliegenden Versteifungsringe die aerodynamischen Eigenschaften des Turmes im Vergleich zu einem endlichen Kreiszylinder ohne Ringe erheblich verändern: Zwischen den Ringen entsteht abschnittsweise ein unsymmetrischer, bi-stabiler Strömungs- und Belastungszustand, der bis zu den höchsten untersuchten Reynoldszahlen zu beobachten ist. Originäre Beiträge der Arbeit sind die Entdeckung der abschnittsweise unsymmetrischen, bi-stabilen Windbelastung, die Bestätigung des experimentellen Befundes durch Versuche in einem zweiten Windkanallaboratorium und durch numerische Simulationen, sowie schließlich die Deutung des Phänomens. Weiterhin wird die Auswirkung der zusätzlichen Windbelastung auf die Strukturbeanspruchungen untersucht. Es wird gezeigt, dass Anzahl und Breite der Ringbalken die Größe des Lasteffekts bestimmen; Maßnahmen zu seiner Minderung werden entwickelt. Insgesamt bleiben die Zusatzbeanspruchungen beherrschbar. Abschließend befasst sich die Arbeit mit den Tragwerksbeanspruchungen infolge des stochastischen Windlastprozesses, insbesondere mit den Störungen des Membranzustands in der Umgebung der Ringsteifen. Für die Tragwerksplanung werden verallgemeinerte Wind-Ersatzlasten entwickelt, die geeignet sind, als Grundlage zur Berechnung der statischen und quasi-statischen Beanspruchungen ultra-hoher Türme in beliebiger atmosphärischer Grenzschichtströmung zu dienen.

This paper presents three-dimensional direct numerical simulations of liquid metal flow around a circular cylinder placed symmetrically in a rectangular duct, under a wide range of magnetic field intensities. Results are presented for values of the Hartmann number (based on the duct width) in the range of 0 ≤ Ha ≤ 1120, and the Reynolds number (based on the cylinder diameter and centerline velocity) in the range 0 ≤ Rec ≤ 5000. The generated flow regimes and the associated critical values of parameters are investigated in detail through full three-dimensional simulations. The effect of the magnetic field on the wake structure is discussed in relation to the possible mechanisms for the generation or suppression of vortices, and to previous attempts to model magnetohydrodynamic flows using simplified two-dimensional models. Present results reveal a non-monotonic dependance of the critical Reynolds number for the onset of vortex shedding, with respect to the Hartmann number. For certain combinations of Ha and Re values, this work confirms the onset of a new flow regime, the existence of which has been recently suggested based on quasi-two-dimensional simulations. Unexpectedly, the spanwise distribution of the force coefficients along the cylinder is found to become more three-dimensional with increasing Ha. Furthermore, the three-dimensional nature of the present simulations reveals additional counter-intuitive features of the new regime that could not possibly had been captured by quasi-two-dimensional models. One such feature, shown here for the first time, is an increase in the flow unsteadiness with increasing intensity of the magnetic field. © 2013 Author(s). 25

In the present study, a lumped parameter model for vortex-induced vibrations is analysed. In this work, the vortex-induced vibrations of an elastically mounted rigid cylinder are able to move in-line and transverse to the flow with equal mass ratio and natural frequencies. A simplified lumped mass model is proposed to study the two degree of freedom (dof) structural oscillator. A classical van der Pol equation along with acceleration coupling, models the near wake dynamics describing the fluctuating nature of vortex shedding. The model dynamics is investigated analytically and the results are compared for moderate mass ratios. The results predicted using this model show a good agreement with the experimental data. The dependence of stream-wise displacement on mass and damping is explored. The cause of cross-flow displacement magnification due to freedom to move in stream-wise direction is also explored using the proposed model. Apart from these two degrees of freedom, the cylinder can also undergo rotation about its centre of mass. The effect of freedom to move in this rotational degree of freedom is exploited to our advantage by applying it to the VIVACE (Vortex induced vibration aquatic clean energy) design which was originally proposed by Bernitsas et al. (2008). The original design was not reported to be the optimal one and the set-up was shown to work only for a given flow velocity. But, the flow environment keeps changing and hence there is a need to bring in robustness and optimize the proposed design. The values of optimized spring stiffness have been found using the lumped mass model. The design is made robust by exploiting the rotational mode. This mode is triggered by varying the overhang lengths in accordance with the varying flow velocity in order to strike resonance for a certain flow regime. � 2013 Elsevier Ltd.

The flow past a circular cylinder in the presence of two counter-rotating control cylinders is analytically investigated. It is shown that vortex formation behind a circular cylinder can be suppressed by introducing two small counter-rotating circular cylinders. However, choosing an appropriate position and optimal circulation strength for the control cylinders is central to achieving optimal control cylinder design. Solving the complete Navier-Stokes (NS) equations can be time consuming if we approach parameterizing the position and circulation strength of the control cylinders. Instead, reduced-order models can be employed to save the computational expenditure associated with solving the complete NS models. To this end, a F�ppl model is employed to investigate the influence of position of the control cylinders on the flow separation from the main circular cylinder. Conformal mapping techniques are used to investigate the flow features, from which the optimum value of circulation was found based on pressure distribution and drag forces. Analytical solutions are complemented through finite volume-based viscous flow calculations. � 2013 The Japan Society of Fluid Mechanics and IOP Publishing Ltd.

The influence of rounded corners on the aerodynamic forces and flow interference has been studied in detail for a uniform flow past two side-by-side arranged square cylinders. The Reynolds number (Re) based on the cylinder diameter (D) and free stream velocity (U∞) is 100. Numerical simulations are carried out for seven different transverse gap ratios (T/D), each with a minimum and maximum corner radius. An inbuilt finite difference code with staggered arrangement of flow variables is used to discretize the governing equations. The concept of immersed boundary method (IBM) is employed to simulate flow around rounded corners using the regular Cartesian grids. The computational code was validated for flow past an isolated circular cylinder, square cylinder, and two equal sized circular cylinders and the results were found to be in very good agreement with available literatures. In the present study, results in terms of the mean and rms values of lift and drag coefficients, Strouhal number, phase diagrams, and contours of streamlines and vorticity are presented. As the corner radius is increased, a reduction in the drag force is observed. There exists a significant effect of gap ratio and corner radius on the phase angle of lift and drag coefficients. Three different flow patterns, namely the single bluff body flow, biased gapside flow, and two independent bluff body flows, were observed from this study.

Numerical simulations were performed to analyse the possibility of damping abrupt incoming free stream disturbances upon a porous aerofoil using an unsteady Reynolds-averaged Navier-Stokes (URANS) model. To mimic the turbulence disturbance levels that are typically encountered in the atmosphere, two flow configurations were considered. In the first configuration, the unsteadiness of the flow was created with vortices shed from a circular cylinder installed ahead of a WTEA-TE1 aerofoil. The continuous von Kármán shedding vortices contained within the cylinder wake were convected downstream and projected upon the aerofoil. In the second configuration, an instantaneous pair of discrete vortices was created by a rotational snapping of a flat plate, installed upstream of the aerofoil. Solid and porous aerofoil configurations, with porosity settings of 11 and 22%, were applied on 50% of the chord of the aerofoil starting from the leading edge. Both steady and unsteady flow simulations were performed to assess the performance of the porosity under steady and unsteady effects. The steady state flow simulations revealed a noticeable reduction in the aerofoil lift coefficient for the porous aerofoil. For unsteady solutions with a continuous or distinct series of vortices interacting with the aerofoil, the porosity showed insignificant damping of the lift coefficient amplitude. The porosity values investigated in the current exercise had indiscernible effect upon the unsteady lift-load alleviations caused by free stream disturbances.

This letter presents the design of a reduced acoustic cloak that uses a temperature gradient in order to obtain sound speeds larger than in air. The cloak consists of a circular acoustic crystal made of ten concentric layers of rigid cylinders whose surfaces are heated or cooled in order to get the temperature gradient needed for cloaking behavior. The total pressure field produced by the scattering of sound waves impinging this complex structure is computed and it is shown how acoustic waves are bent in a way similar to that predicted for perfect cloaking devices. © 2012 American Institute of Physics., Work partially supported by the Spanish MICINN under Grant Nos. TEC2010-19751 and CSD2008-66 (CONSOLIDER program) and by the U.S. Office of Naval Research. J.S.-D. acknowledges useful discussions with L.-W. Cai.

International audience; Efficient, robust and accurate algorithms are proposed for solving the multiple scattering problem by M circular obstacles for the whole spectrum of frequency. The representation of the solution is based on an integral equation formulation next solved by using Fourier basis. Numerical examples are provided to show that the approaches are efficient.

The flow field past multiple arbitrarily moving cylinders is calculated using a generalisation of the conventional images method. The flow is presented by a flow from an infinite set of dipoles. For calculation purposes the infinite series must be, however, truncated. The kinetic energy of fluid is presented as a sum over all modelling dipoles. A rather good convergence of the method is observed. However, if the distances between any cylinders tend to zero, the method fails in zones close to the contact point. Nevertheless, even for the case of touching cylinders the series for the kinetic energy remain convergent. In case of just two cylinders the method projects on the classical method of images. Method is validated by several comparisons with solutions known from the literature.

This paper presents a computational fluid dynamics (CFD) approach to the calculation of translational added mass coefficient of axisymmetric underwater bodies in both longitudinal and transverse directions. The proposed method involves CFD simulations of accelerated as well as uniform flows past axisymmetric bodies based on Reynolds averaged Navier-Stokes (RANS) equations and makes use of the results of these simulations to obtain the inertia force acting on a body as a function of time. The translational added mass can be obtained from this inertia force history. Validation of the methodology is presented for the benchmark case of accelerated flow past sphere and infinitely long circular cylinder in cross-flow for which analytical solutions of the added mass problem are well known.

We formulate and study inflation, extension and twisting of prestressed cylindrical shells that are isotropic in the stress free configuration. We establish that if the prestresses vary only radially in the annular cylinder then a deformation field of the form r = r ^ ( R ) , θ = Θ + Ω Z , z = λ Z is possible in annular cylinders made of any incompressible material and find sufficient conditions for the deformation to be possible when made of compressible materials. When the material is capable of undergoing large elastic deformations and has a non-linear constitutive relation, for the cases studied here, there is up to 26 percent variation in the boundary loads required to engender a given boundary displacement between the prestressed and stress free annular cylinders. On the other hand, the difference in the realized deformation field is only marginal (less than 2 percent). These are unlike the case wherein the material obeys Hooke's law and undergoes small deformations. This study has some relevance to the deformation of blood vessels.

The acoustical performances of regular arrays of cylindrical elements, with their axes aligned and parallel to a ground plane, have been investigated through predictions and laboratory experiments. Semi-analytical predictions based on multiple scattering theory and numerical simulations based on a boundary element formulation have been made. Measurements have been made in an anechoic chamber using arrays of (a) cylindrical acoustically-rigid scatterers (PVC pipes) and (b) thin elastic shells. Insertion loss (IL) spectra due to the arrays have been measured without and with ground planes for several receiver heights. Data and predictions have been compared. The minima in the excess attenuation spectrum i.e., attenuation maxima due to the ground alone resulting from destructive interference between direct and ground-reflected sound waves, tend to have an adverse influence on the band gaps (BG) related to a periodic array in the free field when these two effects coincide. On the other hand, the presence of rigid ground may result in an IL for an array near the ground similar to or, in the case of the first BG, greater than that resulting from a double array, equivalent to the original array plus its ground plane mirror image, in the free field. © 2011 Acoustical Society of America., This work was supported by the UK Engineering and Physical Sciences Research Council (Grant Nos. EP/E063136/1 and EP/E062806/1) and by MEC (Spanish Government) and FEDER funds, under Grant No. MAT2009-09438. Authors are also grateful to reviewers for their valuable comments.

This paper presents a comprehensive computational work on the hydrodynamic, thermal, and mass transfer characteristics of a circular cylinder, subjected to confined flow at the cylinder Reynolds number of Re(d) = 40. As the two-dimensional, steady and incompressible momentum and energy equations are solved using ANSYS-CFX (version II.0), the moisture distributions are computed by a new alternating direction implicit method based software. The significant results, highlighting the influence of blockage (beta = 0.200-0.800) on the flow and heat transfer mechanism and clarifying the combined roles of beta and moisture diffusivity (D = 1 X 10(-8)-1 X 10(-5) m(2)/s) on the mass transfer behavior, are obtained for practical applications. It is shown that the blockage augments the friction coefficients (C(f)) and Nusselt numbers (Nu) on the complete cylinder surface, where the average Nu are evaluated as Nu(ave) = 3.66, 4.05, 4.97, and 6.51 for beta = 0.200, 0.333, 0.571, and 0.800. Moreover, the blockage shifts separation (theta(s)) and maximum C(f) locations (theta(Cf-max)) downstream to the positions of theta(s) = 54.10, 50.20, 41.98, and 37.30 deg and theta(Cf-max) = 51.5, 53.4, 74.9, and 85.4 deg. The highest blockage of beta = 0.800 encourages the downstream backward velocity values, which as a consequence disturbs the boundary layer and weakens the fluid-solid contact. The center and average moisture contents differ significantly at the beginning Of drying process, but in the last 5% of the drying period they vary only by 1.6%. Additionally, higher blockage augments mass transfer coefficients (h(m)) on the overall cylinder surface; however, the growing rate of back face mass transfer coefficients (h(m-bf)) is dominant to that of the front face values (h(m-ff)), with the interpreting ratios of (h) over bar (m-bf)/(h) over bar (m)= 0.50 and 0.57 and h (h) over bar (m-ff)/(h) over bar (m) = 1.50 and 1.43 for beta = 0.200 and 0.800.

We study the lasing eigenvalue problems for a periodic open optical resonator made of an infinite grating of circular dielectric cylinders standing in free space, in the E-and H-polarization modes. If possessing a "negative-absorption" refractive index, such cylinders model a chain of quantum wires made of the gain material under pumping. The initial-guess values for the lasing frequencies are provided by the plane-wave scattering problems. We demonstrate a new effect: the existence of specific grating eigenmodes that have a low threshold of lasing even if the wires are optically very thin. © 2010 Optical Society of America.

Классический метод мультиполей для произвольной двоякопериодической системы сплошных цилиндров обобщен для полых цилиндров. Задача сведена к бесконечной системе алгебраических уравнений относительно моментов мультиполей. Получены расчетные выражения для решения системы уравнений методом редукции. Приведены примеры численной реализации полевой задачи и предложенного аналитического метода. A classical multifield method for an arbitrary doubly periodic system of solid cylinders is generalized for hollow cylinders. The problem is reduced to an infinite system of algebraic equations in multifield moments. Computational expressions for solving the system of equations by a reduction method are derived. Examples of numerical implementation of the field problem and the introduced analytical method are given.

An algorithm is proposed to model, predict and control vortex shedding behind a circular cylindrical configuration. The main ingredients of the algorithm include multiple-feedback sensors, actuators (with zero net mass injection) and a control strategy. Along with the mass and momentum conservation equations, a control equation is implemented to enable the desired flow control goals. A number of sensors are chosen in the downstream of the body to report the state of the flow. The role of externally controllable actuators on the fluid flow patterns past a circular configuration is assessed. To enable, zero net mass injection, two simple rotary type mechanical actuators are located at 120�, right behind the main cylinder. The popular finite volume based SIMPLE scheme is employed for the numerical calculations. As a precursor, the scheme simulates flow past an isolated cylinder, which is validated over a moderate range of Reynolds numbers. The design parameters of interest such as Strouhal number, drag and lift coefficients etc are used for the purpose of validation. The simulated flow fields are compared against the flow visualization study, which clearly demonstrates the efficacy of the actuators at discrete levels of rotation. The basic character of the flow is completely modified at Uc/U?�=�2.0 and Re = 100, where a complete suppression of vortex shedding is observed. This is tantamount to complete control of all the global instability modes. Fictitious tracer particles are released to visualize the vortex structures in the form of streaklines. The results clearly demonstrate the effectiveness of a rather simple active control algorithm in suppressing the vortex structures. All the relevant fluid flow features of the bluff-body fluid mechanics under the influence of actuators are studied in the sub-critical Reynolds number range of Re = 100-300. � 2009 Elsevier Masson SAS. All rights reserved.

The immersed boundary method (IB hereafter) is an efficient numerical methodology for treating purely hydrodynamic flows in geometrically complicated flow-domains. Recently Grigoriadis et als. [1] proposed an extension of the IB method that accounts for electromagnetic effects near non-conducting boundaries in magnetohydrodynamic (MHD) flows. The proposed extension (hereafter called MIB method) integrates naturally within the original IB concept and is suitable for magnetohydrodynamic (MHD) simulations of liquid metal flows. It is based on the proper definition of an externally applied current density field in order to satisfy the Maxwell equations in the presence of arbitrarily-shaped, non-conducting immersed boundaries. The efficiency of the proposed method is achieved by fast direct solutions of the two poisson equations for the hydrodynamic pressure and the electrostatic potential. The purpose of the present study is to establish the performance of the new MIB method in challenging configurations for which sufficient details are available in the literature. For this purpose, we have considered the classical MHD problem of a conducting fluid that is exposed to an external magnetic field while flowing across a circular cylinder with electrically insulated boundaries. Two- and three-dimensional, steady and unsteady, flow regimes were examined for Reynolds numbers Red ranging up to 200 based on the cylinder's diameter. The intensity of the external magnetic field, as characterized by the magnetic interaction parameter N, varied from N = 0 for the purely hydrodynamic cases up to N = 5 for the MHD cases. For each simulation, a sufficiently fine Cartesian computational mesh was selected to ensure adequate resolution of the thin boundary layers developing due to the magnetic field, the so called Hartmann and sidewall layers. Results for a wide range of flow and magnetic field strength parameters show that the MIB method is capable of accurately reproducing integral parameters, such as the lift and drag coefficients, as well as the geometrical details of the recirculation zones. The results of the present study suggest that the proposed MIB methodology provides a powerful numerical tool for accurate MHD simulations, and that it can extend the applicability of existing Cartesian flow solvers as well as the range of computable MHD flows. Moreover, the new MIB method has been used to carrry out a series of accurate simulations allowing the determination of asymptotic laws for the lift and drag coefficients and the extent of the recirculation length as a function of the amplitude of the magnetic field. These results are reported herein. © 2009 Elsevier Ltd. All rights reserved. 39 345 358 345-358

The main aim of this work is to investigate experimentally the influence of ultrasonic waves, on the heat transfer enhancement, from a stainless steel circular cylinder to distilled water, in subcooled boiling conditions. This study has carried on for a few years at the Department of Energetics “L.Poggi”. The effect was observed since ’60s: different authors had investigated the cooling effect due to the ultrasonic waves at different heat transfer regimes, especially from a thin platinum wire to water. They had found out that the highest heat transfer coefficient enhancement was been in subcooled boiling conditions. So this paper has the purpose to clarify the physical phenomenon and optimize a large range of variables involved in the mechanism. It reports the experimental results obtained with ultrasound at the frequency of 38 kHz, at two different subcooling degrees, ΔTSUB = 25 and 35°C. The heat fluxes applied on the cylinder, the ultrasonic generator power, Pgen and also the placement of the heater inside the ultrasonic generator tank, are been varied. The ultrasonic waves seem to be very useful for a practical application in the last generation electronic components cooling: they need dissipating huge heat fluxes and avoiding high temperatures (≈150°C), after that they could damage themselves.

In this article, the collocated parameter models are used to estimate the effective thermal conductivity of the two-phase materials. The algebraic equations are derived using a unit-cell-based isotherm approach for a two- and three-dimensional spatially periodic medium. The geometry of the medium is considered as matrix of touching and nontouching in-line square and circular cylinders as well as touching and nontouching in-line solid and hollow cubes. The models are used to predict the thermal conductivity of numerous two-phase materials (maximum conductivity ratio of 1000 and concentration ranging between 0 and 1). The estimated thermal conductivity data is in good agreement with experimental data within ± 16.67% deviation for various two-phase systems. The results are compared with the standard models and experimental data for different types of two-phase systems; it is shown that the touching in-line solid cube model predicts better as compared to other geometrical configurations.

We present results on parametrically forced capillary waves in a circular cylinder, obtained in the limit of large fluid depth, using two low-viscosity liquids whose surface tensions differ by an order of magnitude. The evolution of the wave patterns from the instability to the wave-breaking threshold is investigated in a forcing frequency range (f= ω/2π = 25–100 Hz) that is around the crossover frequency (ωot) from gravity to capillary waves (ωot/2≤ω/2≤4ωot). As expected, near the instability threshold the wave pattern depends on the container geometry, but as the forcing amplitude is increased the wave pattern becomes random, and the wall effects are insignificant. Near breaking, the distribution of random wavelengths can be fitted by a Gaussian. A new gravity–capillary scaling is introduced that is more appropriate, than the usual viscous scaling, for low-viscosity fluids and forcing frequencies 3Hz. In terms of these scales, a criterion is derived to predict the crossover from capillary- to gravity-dominated breaking. A wave-breaking model is developed that gives the relation between the container and the wave accelerations in agreement with experiments. The measured drop size distribution of the ejected drops above the breaking threshold is well approximated by a gamma distribution. The mean drop diameter is proportional to the wavelength determined from the dispersion relation; this wavelength is also close to the most likely wavelength of the random waves at drop ejection. The dimensionless drop ejection rate is shown to have a cubic power law dependence on the dimensionless excess acceleration ε′dan inertial–gravitational ligament formation model is consistent with such a power law.

In this paper we propose a new method to solve the optimal control problem in which the feedback matrix K is computed in an efficient way for complex flows, with large number of degrees of freedom, using an approach similar to adjoint-based control optimization. The idea is to consider the direct-adjoint system as an input-output problem where the input is given by the current state and the output is the control. Since the control has much smaller dimension than the state, the feedback matrix K can be efficiently obtained from the solution of the adjoint of the direct-adjoint system. It can further be shown using the symplectic product that the direct-adjoint system is self adjoint. As a consequence the new adjoint system is equivalent to the direct-adjoint system with suitable initial and terminal conditions. With this method the optimal control problem can be solved efficiently for any value of the control penalty l 2. Results are presented of this novel technique as applied to suppressing the vortex shedding behind a circular cylinder, and compared to the minimal-energy feedback control presented in [4].

In this paper we study the structural sensitivity of the nonlinear periodic oscillation arising in the wake of a circular cylinder for Re47. The sensibility of the periodic state to a spatially localised feedback from velocity to force is analysed by performing a structural stability analysis of the problem. The sensitivity of the vortex shedding frequency is analysed by evaluating the adjoint eigenvectors of the Floquet transition operator. The product of the resulting neutral mode with the nonlinear periodic state is then used to localise the instability core. The results obtained with this new approach are then compared with those derived by Giannetti & Luchini [8]. An excellent agreement is found comparing the present results with the experimental data of Strykowski & Sreenivasan [7].