Wake Research Papers - Academia.edu (original) (raw)

... School of Engineering Springfield, Massachussetts Drag Reduction and Wake Minimization on Marine Vehicles ,,P. ,3 1991 By: , Craig A. Hunter Pasquale Delore Walter M. Presz, Jr. Final Report Office of Naval Research Grant... more

- by
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- Control, Fluid Dynamics, Mixing, Models

Optimisation of propeller/hull configurations based on the Reynolds-Averaged Navier-Stokes (RANS) equations has been very rare so far due to the large computational effort required. Virtually no such optimisation has been carried out at full scale, where only a few RANS methods are at all applicable due to stability problems. The present paper introduces a newly developed RANS solver especially designed for stability and this solver is shown to work well at full scale. Through a link to a commonly used propeller analysis code, predictions of the viscous flow around the full scale ship with an operating propeller may be made.This is utilised in the work reported here, where the flow codes are introduced into a system for automatic optimisation. It is shown that even well designed propellers may be further improved, both in a fixed wake and in the wake behind a fixed hull.

- by Björn Regnström
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- Engineering, Design, Optimization, Conception

Experiments were performed to investigate the effects of amplitude and depth on the drag reduction of a NACA 0012 airfoil plunging near a free surface for a range of frequencies. Beyond the effect of the free surface, at low Strouhal numbers based on amplitude, Sr A , the drag reduction follows a parabolic trend with greater effect for greater amplitude, similar to the Garrick predictions. At Sr A & 0.08, larger amplitudes break from this trend due to leading-edge vortex formation. As a result, smaller amplitudes become preferable for Sr A [ 0.12. In addition, for the first time, vortex lock-in is documented experimentally. The effect of depth is twofold; firstly with decreasing depth, there is a general departure from the Garrick trends. Secondly, a reduction in thrust is observed around a constant unsteady parameter of s = U ? 2pf/g & 0.25; around this value significant free-surface waves form that detract from thrust creation. For depths greater than two chord lengths, there is negligible free-surface effect.

- by David Cleaver
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- Mechanical Engineering, Aerospace Engineering, Generation, Frequency

We compared good sleepers with minimally and highly distressed poor sleepers on three measures of daytime functioning: self-reported fatigue, sleepiness, and cognitive inefficiency. In two samples (194 older adults, 136 college students), we tested the hypotheses that (1) poor sleepers experience more problems with daytime functioning than good sleepers, (2) highly distressed poor sleepers report greater impairment in functioning during the day than either good sleepers or minimally distressed poor sleepers, (3) daytime symptoms are more closely related to psychological adjustment and to psychologically laden sleep variables than to quantitative sleep parameters, and (4) daytime symptoms are more closely related to longer nocturnal wake times than to shorter sleep times. Results in both samples indicated that poor sleepers reported more daytime difficulties than good sleepers. While low-and high-distress poor sleepers did not differ on sleep parameters, highly distressed poor sleepers reported consistently more difficulty in functioning during the day and experienced greater tension and depression than minimally distressed poor sleepers. Severity of all three daytime problems was generally significantly and positively related to poor psychological adjustment, psychologically laden sleep variables, and, with the exception of sleepiness, to quantitative sleep parameters. Results are used to discuss discrepancies between experiential and quantitative measures of daytime functioning. D

- by Catherine Fichten
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- Depression, Cognition, Attention, Fatigue

For moving ahead ship needs thrust, which is generated by rotating propeller behind its hull. A controllable pitch propeller can adjust incidence of incoming flow and can offer better control over the thrust generated by it. Traditionally thrust and torque of a propeller, is predicted by model test. This takes more time, manpower, space and cost. In present time CFD is getting popular for hydrodynamic designs as it give relative saving in time, manpower, space and cost.

- by Aditya Kolakoti
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- Mechanical Engineering, Propeller, Resistance, Wake

A total of 49 extended abstracts are included in this volume of papers presented at the 15th International Workshop on Water Waves and Floating Bodies, held 17 February - 1 March, 2000, Caesarea, Israel. Also see http://www.eng.tau.ac.il/-miloh/iwwwfb. The objective of the Workshop is to provide a forum for informal discussions of fundamental research, of mutual interest to both engineers and scientists, in the broad area of wave interactions with fixed, floating or submerged bodies.

- by TOUVIA MILOH
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- Fluid Mechanics, Fluid Dynamics, Israel, Finite Element Analysis

Rotor blades can be found in many engineering applications, mainly associated with converting energy from fluids to work (or electricity). Rotor blade geometry is a key factor in the mechanical efficiency of the energy conversion process. For example, wind turbines' performance directly depends on the blade geometry and the wake flow formed behind them. We suggest to use a bioinspired blade based on the common swift wing. Common swift ( Apus apus ) is known to be a long-distance flyer, able to stay aloft for long periods of time by maintaining high lift and low drag. We study the near-wake flow characteristics of a freely rotating rotor with swept blades and its aerodynamic loads. These are compared with a straight-bladed rotor. The experiments were conducted in a water flume using particle image velocimetry (PIV) technique. Both blades were studied for four different flow speeds with freestream Reynolds numbers ranging from 23 000 to 41 000. Our results show that the near wake ...

- by Roi Gurka
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- Geology, Medicine, Wake

Furling is the dominant mechanism for over speed and power control of small wind turbines. In this paper we present a consistent model of the dynamics of gravity-controlled furling systems based on a Lagrangian formalism. The aerodynamic forces acting on tail vane and rotor have been modeled using Xfoil and blade element momentum (BEM) theory, respectively. Due to the proximity of tail vane and rotor a model of the near-wake generated by the rotor was incorporated into the model, assuming a parabolic wake shape. The different design parameters, such as lever lengths and axis tilt angles, have been studied in a systematic manner and their impact on the wind speed values for entering and leaving the furling regime have been assessed. In the first part of the study the free-stream in-flow wind speed was fixed at a given value and the system was allowed to reach stable conditions. The steady-state values of the yaw and furling angle were recorded as a function of wind speed both for increasing and decreasing wind speed and the consequences for design choices have been discussed. In the second part, a slow variation of input wind speed was superimposed on the constant wind speed signal and the dynamic response of the system was analyzed. The results of the study are thought to provide an initial roadmap for the design of furling systems.

- by Oliver Probst
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- Engineering, Economics, Wind turbine, Power Control

1] Dunes are present in nearly all fluvial channels and are vital in predicting flow resistance, sediment transport, and deposition within many rivers. Progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field, laboratory, and numerical investigations. Progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune. Additionally, the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood, with the feedback of dune three-dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital. Such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features. This review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes. Five areas are then highlighted and discussed as a possible focus for future research: (1) the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field, (2) the influence of three-dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress, (3) flow field modification resulting from bed form superimposition and amalgamation, (4) the scale and topology of dune-related turbulence and its interactions with sediment transport and the flow surface, and (5) the influence of suspended sediment on the dune flow field and dune morphology. Citation: Best, J. (2005), The fluid dynamics of river dunes: A review and some future research directions,

- by Jim Best
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- Turbulence, Complexity, Morphology, Topology

A three dimensional transient model is developed to simulate the local hydrodynamics of a gas–liquid–solid three-phase fluidised bed reactor using the computational fluid dynamics (CFD) method. The CFD simulation predictions are compared with the experimental data of Kiared et al. [1999. Mean and turbulent particle velocity in the fully developed region of a three-phase fluidized bed. Chemical Engineering & Technology 22, 683–689] for solid phase hydrodynamics in terms of mean and turbulent velocities and with the results of Yu and Kim [1988. Bubble characteristics in the racial direction of three-phase fludised beds. A.I.Ch.E. Journal 34, 2069–2072; 2001. Bubble-wake model for radial velocity profiles of liquid and solid phases in three-phase fluidised beds. Industrial and Engineering Chemistry Research 40, 4463–4469] for the gas and liquid phase hydrodynamics in terms of phase velocities and holdup. The flow field predicted by CFD simulation shows a good agreement with the experimental data. From the validated CFD model, the computation of the solid mass balance and various energy flows in fluidised bed reactors are carried out. The influence of different interphase drag models for gas–liquid interaction on gas holdup are studied in this work.

En el presente trabajo se hace un inventario y una interpretación de los principales símbolos utilizados en el rito de velorio en la ciudad de Maracaibo, Venezuela. Para colectar la información se utilizó el método etnográfico de observación-participación, durante la cual se aplicaron también técnicas como entrevistas, grabaciones y fotografías. Para el análisis interpretativo se utilizó la Semiótica, tal como la proponen Lotman y Greimas. El análisis permite confirmar el papel de los símbolos en la constitución y funcionamiento de la cultura funeraria, en particular su capacidad de condensación, su rol en la memoria colectiva y su singular estabilidad y resistencia al cambio a través del tiempo.

- by Jose Enrique Finol
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- Semiosis, Symbol, Wake, Opción

In the wake of economic globalisation trends, for many seaports, privatisation is under serious consideration. Transfer of property rights from the public to the private sector has a number of consequences right from the way in which key functions in ports are performed. This paper, after presenting the state-of-the-art theory on port management and port privatisation, claims that current classifications are too general.

- by Martin de Jong
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- Civil Engineering, Economics, Property Rights, Public sector

The effects of co-current flows on a rising Taylor bubble are systematically investigated by a front tracking method coupled with a finite difference scheme based on a projection approach. Both the upward (the co-current flows the same direction as the buoyancy force) and the downward (the co-current moves in the opposite direction of the buoyancy force) co-currents are examined. It is found that the upward cocurrent tends to elongate the bubble, while the downward co-current makes the bubble fatter and shorter. For large N f (the inverse viscosity number), the upward co-current also elongates the skirted tail and makes the tail oscillate, while the downward co-current shortens the tail and even changes a dimpled bottom to a round shape. The upward co-current promotes the separation at the tail, while the downward co-current suppresses the separation. The terminal velocity of the Taylor bubble rising in a moving flow is a linear combination of the mean velocity (U C ) of the co-current and the terminal velocity (U 0 ) of the bubble rising in the stagnant liquid, and the constant is around 2 which agrees with the literature. The wake length is linearly proportional to the velocity ratio (U C /U 0 ). The co-currents affect the distribution of the wall shear stresses near the bubble, but not the maximum.

- by Shaoping Quan
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- Engineering, Multiphase Flow, Thin Film, Vortex rings

This article deals with the interaction of co-rotating vortices, in configurations similar to those found in the extended nearwake of typical transport aircraft. The fundamental process of vortex merging is analyzed and modeled in detail in a twodimensional context, giving insight into the conditions for merging and its physical origin, and yielding predictions for the resulting flow. Three-dimensional effects, in the form an elliptic short-wave instability arising in the initial co-rotating vortex flow, are described and analyzed theoretically. They are found to cause significant changes in the merging process, such as earlier merging and larger final vortex cores. Illustrations from recent experimental, numerical and theoretical studies are given, and the relevance of the results for applications to real aircraft wakes is discussed. To cite this article: P. Meunier et al., C. R. Physique 6 (2005).  2005 Académie des sciences. Published by Elsevier SAS. All rights reserved. Résumé Aspects physiques de la fusion de tourbillons. Cet article traite de l'interaction entre tourbillons co-rotatifs, dans des configurations semblables à celles présentes dans le sillage proche et moyen des avions de transport. Le processus fondamental de fusion des tourbillons est analysé et modélisé en détail dans une description bidimensionnelle, donnant accès à des conditions pour la fusion et son origine physique, ainsi qu'aux propriétés de l'écoulement résultant. Les effets tridimensionnels, sous forme d'une instabilité elliptique à courte longueur d'onde des vortex co-rotatifs initiaux, sont décrits et analysés théoriquement. Ils sont à l'origine de modifications importantes de la fusion, comme un démarrage plus rapide du processus et un vortex final plus gros. Des illustrations d'études expérimentales, numériques et théoriques récentes sont données, et la pertinence des résultats pour des applications aux sillages réalistes des avions est discutée. Pour citer cet article : P. Meunier et al., C. R. Physique 6 (2005).

- by Patrice Meunier
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- Interaction, Mathematical Sciences, Physical sciences, Vortex
- by Alan Wadcock and +1
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- Aerodynamics, Interactions, Measurement, Wind Tunnel

Ahmed body is a simplified vehicle model which retains important features of real ground vehicles. Present study aims to investigate effects of rear slant angles (a = 25°, 30°and 35°) on time-averaged and instantaneous flow characteristics downstream of the Ahmed body because angle of the rear slanted surface considerably affects the flow characteristics. The particle image velocimetry (PIV) technique is employed to measure the flow field in the symmetry plane z = 0 downstream of the Ahmed body. Demonstration and detailed discussions of the flow features are provided using time-averaged velocity vectors, streamlines, vorticity and TKE contours. Three critical flow points are revealed in the wake downstream of the Ahmed body. Namely, they are focus point 1 (F 1), focus point 2 (F 2) and saddle point (S). Variations in slant angle a cause shift in the locations of these critical flow points. Effects of change of slant angle a on each critical flow point vary according to their locations in the wake. Instantaneous vorticity contours and spectral analysis of the velocity field are also provided. As a result of the spectral analysis, a single dominant frequency location is only detected for a = 25°, on the other hand two dominant frequency locations are detected for a = 30°and 35°in the wake. Mean stream-wise and vertical velocity components are compared with available data in the literature. Although there are some discrepancies between the present and previous results at certain locations in the flow field, general agreement between these results is reasonably good when taking the difference between Reynolds numbers of the two studies into account.

- by Tural Tunay
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- Engineering, Aerodynamics, Wake

P. (2009) Reduced-order modelling of vortex-induced vibration of catenary riser. Ocean Engineering, 36 (Abstract A new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical-numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid-riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvature. The equations of riser 3-D motion are based on a pinned-pinned, tensioned-beam or flexural cable, modelling which accounts for overall effects of riser bending, extensibility, sag, inclination and structural nonlinearities. The unsteady hydrodynamic forces associated with cross-flow and in-line vibrations are modelled as distributed van der Pol wake oscillators. This hydrodynamic model has been modified in order to capture the effect of varying initial curvature of the inclined flexible cylinder and to describe the space-time fluctuation of lift and drag forces. Depending on the vortex-excited in-plane/out-ofplane modes and system fluid-structure parameters, the parametric studies are carried out to determine the maximum response amplitudes of catenary risers, along with the occurrence of unimodal lock-in phenomenon. The obtained results highlight the effect of initial curvatures on the nonlinear dynamics of riser undergoing vortex-induced vibration.

- by Narakorn Srinil
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- Civil Engineering, Ocean Engineering, Nonlinear dynamics, Modeling

An experimental study is performed on a three-bladed rotor model in two water tanks. The blade pitch, rotational velocity, descent angle, and descent speed are all varied in order to simulate a wide range of rotorcraft operating states, focusing on descent cases where the rotor is operating in or near vortex ring state -an area in which there is currently very little available data. Flow visualization is done by injecting air bubbles and fluorescent dye tangentially from the blade tips to mark the vortex core, showing the development of both short-wave ("sinuous") and long-wave ("leapfrogging") instabilities on the helical vortices in the wake. Strain gages are used to record transient loads, allowing a correlation between the rotor thrust performance and the development of the vortex wake. Reynolds numbers are of order 10 5 and test runs are performed for extended periods -up to 500 rotor revolutions -demonstrating the repeatability of the patterns of thrust variation. The data indicate that as the instabilities develop, adjacent vortices merge and form thick vortex rings, particularly during descent. Periodic shedding of these rings from the wake associated with vortex ring state is observed, resulting in peak-to-peak thrust fluctuations of up to 95% of the mean and occurring at regular intervals of 20-50 rotor revolutions, depending on flow parameters.

- by nasa sava
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- Mechanical Engineering, Aerospace Engineering, Patterns, Air

A detailed experimental study is performed on the separated flow structures around a low aspect-ratio circular cylinder (pin-fin) in a practical configuration of liquid cooling channel. Distinctive features of the present arrangement are the confinement of the cylinder at both ends, water flow at low Reynolds numbers (Re = 800, 1800, 2800), very high core flow turbulence and undeveloped boundary layers at the position of the obstacle. The horseshoe vortex system at the junctions between the cylinder and the confining walls and the near wake region behind the obstacle are deeply investigated by means of Particle Image Velocimetry (PIV). Upstream of the cylinder, the horseshoe vortex system turns out to be perturbed by vorticity bursts from the incoming boundary layers, leading to aperiodical vortex oscillations at Re = 800 or to break-away and secondary vorticity eruptions at the higher Reynolds numbers. The flow structures in the near wake show a complex three-dimensional behaviour associated with a peculiar mechanism of spanwise mass transport. High levels of free-stream turbulence trigger an early instabilization of the shear layers and strong Bloor-Gerrard vortices are observed even at Re = 800. Coalescence of these vortices and intense spanwise flow inhibit the alternate primary vortex shedding for time periods whose length and frequency increase as the Reynolds number is reduced. The inhibition of alternate vortex shedding for long time periods is finally related to the very large wake characteristic lengths and to the low velocity fluctuations observed especially at the lowest Reynolds number.

- by Alessandro Armellini
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- Engineering, Water, Heat Transfer, Particle Image Velocimetry

ABSTRACT This paper presents a three-dimensional numerical model for horizontal axis wind turbines, capable of simulating both the dynamic response of the turbine to changing flow conditions, and the full wake generated by the turbine. The turbine model is coupled to computational fluid dynamics software using Large Eddy Simulation to solve for unsteady flow conditions, with the Smagorinsky method handling sub-grid turbulence. Wind tunnel simulations are compared with experimental data, and then a simulation of a real turbine in complex terrain is compared with LIDAR data from the same site, in both cases agreeing favourably. Copyright © 2011 John Wiley & Sons, Ltd.

- by Angus C . W . Creech and +1
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- Mechanical Engineering, Wind Energy, Fluid, Dynamics

With the steady increase in air traffic, civil aviation authorities are under continuous pressure to increase aircraft handling capacity. One potential approach is to reduce the separation distance between aircraft at takeoff and landing witliout compromising safety. ATC-Wake aims to develop and build an integrated system for ATC (Air Traffic Control) that would allow variable aircraft separation distances, as opposed to the fixed distances presently applied at airports. The ATC-Wake project has developed and built an integrated ATC Wake Vortex Safety and Capacity platform. A variety of subsystems is integrated and used within a test bed role: - To evaluate the interoperability of ATC-Wake with existing ATC systems currently used at various European airports; - To assess the safety and capacity improvements that can be obtained by local installation of the ATC-Wake system at various European airports; - To evaluate operational usability and acceptability of the ATC-Wake system; The ...

- by Frederic Barbaresco
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- Wake

This paper presents numerical simulation results for vortex-induced vibration of a circular cylinder in laminar flow. A vortex method is implemented to solve the two-dimensional Navier-Stokes equations in terms of vorticity. In order to validate the numerical code, the flow past a fixed cylinder is first investigated for which enough experimental and numerical results are available. Basic characteristics of the dynamic response and vortex shedding for an elastically mounted circular cylinder are then investigated for 70 < Re < 170. The lock-in phenomenon is captured at certain reduced velocities where the lift coefficient takes a considerable value associated with a high amplitude response. The wake structure exhibits the 2S or C (2S) modes of vortex shedding in this range of Reynolds numbers, as opposed to the 2P mode which is observed in the turbulent flow regime. The numerical results are in acceptable agreement with available experimental and numerical data.

- by Taneda Singh
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- Mechanical Engineering, Applied Mathematics, Modeling, Fluid Dynamics
- by Mustafa Z . Yousif
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- Physics, Mechanics, Turbulence, Large Eddy Simulation

Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder a t 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Karmin-vortex-street frequency is observed. Beyond this region of decay, largerscale (lower wavenumber) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the centre of the band shifting to lower frequencies as downstream distance is increased.

- by John Cimbala
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- Engineering, Visualization, Fluid Mechanics, Low Frequency

Large-scale vortices, that is, eddies whose characteristic length scale is larger than the local Rossby radius of deformation R d , are ubiquitous in the oceans, with anticyclonic vortices more prevalent than cyclonic ones. Stability or robustness properties of already formed shallow-water vortices have been investigated to explain this cyclone-anticyclone asymmetry. Here the focus is on possible asymmetries during the generation of vortices through barotropic instability of a parallel flow. The initial stage and the nonlinear stage of the instability are studied by means of linear stability analysis and direct numerical simulations of the one-layer rotating shallow-water equations, respectively. A wide variety of parallel flows are studied: isolated shears, the Bickley jet, and a family of wakes obtained by combining two shears of opposite signs.

- by Alexandre Stegner
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- Oceanography, Physical Oceanography, Instability, Shear Flow
- by Bengt Andersson
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- Chemical Engineering, Turbulence, Modeling, Stirred Tank Reactor

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.

- by Narakorn Srinil
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- Engineering, Modeling, Fluid structure interaction, Vibration

The wing-tip vortex of a rectangular wing half-model with a model engine is studied experimentally and numerically. The airfoil has a supercritical BAC 3-11/RES/30/21 geometry with a chord length of c = 150 mm. The investigations include three different spanwise positions of the engine and two jet velocities with a constant free stream velocity of u ∞ = 27 m/s yielding Re c = 2.8 × 10 5 . The numerical simulation is divided into two parts. First the flow around the airfoil is calculated. In a second step the data is taken as an input to calculate the wake downstream of the trailing edge. Experiments include 2C-and 3C-PIV-measurements for the velocity distribution of the jet and the wing-tip vortices. The experimental and numerical results show the significance of the streamwise velocity component for the analysis of the jet/wing-tip vortex interaction.  2003 Elsevier SAS. All rights reserved.

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- by parviz moin
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- Computational Fluid Dynamics, Turbulence, Convergence, Finite Element Analysis

Self-twisting composite marine propellers, when subject to hydrodynamic loading, will not only automatically bend but also twist due to passive bend-twist (BT) coupling characteristics of anisotropic composites. To exploit the BT coupling effects of self-twisting propellers, a two-level (material and geometry) design methodology is proposed, formulated, and implemented. The material design is formulated as a constrained, discrete, binary optimization problem, which is tackled using an enhanced genetic algorithm equipped with numerical and analytical tools as function evaluators. The geometry design is formulated as an inverse problem to determine the unloaded geometry, which is solved using an over-relaxed, nonlinear, iterative procedure. A sample design is provided to illustrate the design methodology, and the predicted performance is compared to that of a rigid propeller. The results show that the selftwisting propeller produced the same performance as the rigid propeller at the design flow condition, and it produced better performance than the rigid propeller at off-design flow conditions, including behind a spatially varying wake.

- by Y. Young
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- Engineering, Modeling, Fluid structure interaction, Vibration

Six turbulence models, including standard k–ε, k–ε RNG, k–ω (88), revised k–ω (98), Reynolds stress transport model (RSTM), and two-fluid model (TFM), were applied to the simulation of a closed conduit polychromatic UV reactor. Predicted flow field and turbulent kinetic energy were compared with the experimental data from a digital particle image velocimetry (DPIV). All of the predicted flow fields were combined with a multiple segment source summation (MSSS) fluence rate model and three different microbial response kinetic models to simulate the disinfection process at two UV lamp power conditions. Microbial transport was simulated using the Lagrangian particle tracking method. The results show that the fluence distributions and the effluent inactivation levels were sensitive to the turbulence model selection. The level of sensitivity was a function of the operating conditions and the UV response kinetics of the microorganisms. Simulations with operating conditions that produced higher log inactivation or utilized microorganisms with higher UV sensitivity showed greater sensitivity to the turbulence model selection. In addition, a broader fluence distribution was found with turbulence models that predicted a larger wake region behind the lamps.

- by Joel Ducoste
- •
- Applied Mathematics, Kinetics, Modeling, Water Treatment

The present work performs a numerical analysis of the plasticity induced crack closure under variable amplitude loading, namely under single tensile overloads and High-Low block loading sequences. The optimisation of the numerical procedure was previously performed under constant amplitude loading for the same material. The physical model was analysed using MARC, a commercial finite element package. The numerical analysis was performed considering constant DK baseline level and stress ratio conditions. The crack opening values were based on the last contact in the first node behind the crack tip. The numerical results are compared with experimental measurements. Crack closure numerical results are consistent with experimental observations under peak overloads and High-Low load blocks, indicating that crack growth retardation under variable amplitude loading sequences is closely linked to the plasticity induced crack closure phenomenon. Moreover, the obtained crack profiles predict the formation of an enlarged hump of residual stretched material ahead of the load variation location. The numerical analysis produced reasonable closure levels when compared to experimental results when the partial closure phenomenon is taken into account in the closure measurements.

- by D.M. Rodrigues
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- System Identification, Plasticity, Modeling, Finite element method
- by Sven Schmitz
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- Mechanical Engineering, Applied Mathematics, Computational Fluid Dynamics, Modeling

UU 1 9b. TELEPHONE NUMBER (Include area code) 206-543-1365 Standard Form 298 (Rev. 8/98)

- by Ren-Chieh Lien
- •
- Turbulence, Vorticity, Kinematics, Laboratory experiment

Synergistic application of experiments and numerical simulations is crucial to understanding the underlying physics of airframe noise sources. The current effort is aimed at characterizing the details of the flow interaction between two cylinders in a tandem configuration. This setup is viewed to be representative of several component-level flow interactions that occur when air flows over the main landing gear of large civil transports. Interactions of this type are likely to have a significant impact on the noise radiation associated with the aircraft undercarriage. The paper is focused on two-dimensional, time-accurate flow simulations for the tandem cylinder configuration. Results of the unsteady Reynolds Averaged Navier-Stokes (URANS) computations with a two-equation turbulence model, at a Reynolds number of 0.166 million and a Mach number of 0.166, are presented. The experimental measurements of the same flow field are discussed in a separate paper by Jenkins, Khorrami, Choudhari, and McGinley (2005). Two distinct flow regimes of interest, associated with short and intermediate separation distances between the two cylinders, are considered. Emphasis is placed on understanding both time averaged and unsteady flow features between the two cylinders and in the wake of the rear cylinder. Predicted mean flow quantities and vortex shedding frequencies show reasonable agreement with the measured data for both cylinder spacings. Computations for short separation distance indicate decay of flow unsteadiness with time, which is not unphysical; however, the predicted sensitivity of mean lift coefficient to small angles of attack explains the asymmetric flowfield observed during the experiments.

- by Meelan Choudhari
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- Mechanical Engineering, Aerospace Engineering, Modeling, Aeroacoustics
- by Francis Gohin
- •
- Remote Sensing, LiDAR, Multidisciplinary, Surface Roughness

The dynamics of a 3D bimodal turbulent wake downstream a square-back Ahmed body are experimentally studied in a wind-tunnel through high-frequency wall pressure probes mapping the rear of the model and a horizontal 2D velocity field. The barycenters of the pressure distribution over the rear part of the model and the intensity recirculation are found highly correlated. Both described the most energetic large-scale structures dynamics, confirming the relation between the large-scale recirculation bubble and its wall pressure footprint. Focusing on the pressure, its barycenter trajectory has a stochastic behavior but its low frequencies dynamics exhibit the same characteristics as a weak strange chaotic attractor system, with two well defined attractors. The low frequencies dynamics associated to the large-scale structures are then analyzed. The largest Lyapunov exponent is first estimated, leading to a low positive value characteristic of strange attractors and weak chaotic systems. Afterwards, analyzing the autocorrelation function of the time-series, we compute the correlation dimension, larger than two. The signal is finally transformed and analyzed as a telegraph signal showing that its dynamics correspond to a quasi-random telegraph signal. This is the first demonstration that the low frequencies dynamics of a turbulent 3D wake are not a purely stochastic process but rather a weak chaotic process exhibiting strange attractors. From the flowcontrol point of view, it also opens the path to more simple closed-loop flow control strategies aiming at the stabilization of the wake and the control of the dynamics of the wake barycenter.

- by Jean-luc Aider
- •
- Physics, Turbulence, Wake, Chaotic

The knifefish species propels itself by generating a reverse Kármán street from an anal fin and without significantly moving its body. This unique feature makes this species' propulsion method highly efficient (Blake, 1983). It has been suggested that there is an optimal swimming range for fish based on the amplitude and frequency of the reverse Kármán street. Experiments have been performed to measure the ratio between the amplitude and wavelength of vortices in the wake of a knifefish. It is suggested that by optimizing the thrust created by the reverse Kármán street the wave efficiency can be estimated for a given spacing ratio, and present observations have an average value of 0.89. The relationship established between spacing ratio and wave efficiency, in addition to the measured parameters, will be invaluable for bio-inspired designs based on the knifefish.

- by Thomas Reesbeck
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- Mechanical Engineering, Robotics, Aerospace Engineering, Physics

The aeroacoustic response of two side-by-side circular cylinders in cross-flow is investigated experimentally. In order to investigate the effect of the gap between the cylinders on the acoustic resonance mechanism, six spacing ratios between the cylinders, in the range of T/D=1.25-3, have been investigated, where D is the diameter of the cylinders and T the centre-to-centre distance between them. Special attention is given to the intermediate spacing ratio range, which exhibits bistable flow regimes in the absence of resonance. During the tests, the acoustic cross-modes of the duct housing the cylinders are self-excited. For the intermediate spacing ratios, T/D=1.25, 1.35, 1.46 and 1.75, two distinct vortex-shedding frequencies at the off-resonance conditions are observed. These are associated with the wide and narrow wakes of the cylinders, as described in the literature. In this case, acoustic resonances occur at a Strouhal number, which is between those observed before the onset of resonance. The acoustic resonance synchronizes vortex shedding in the two wakes and thereby eliminates the bistable flow phenomenon. For large spacing ratios, T/D=2.5 and 3, vortex shedding occurs at a single Strouhal number at which the acoustic resonance is excited.

- by Atef Mohany
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- Engineering, Acoustics, Aeroacoustics, Resonance

The dynamic behavior of a collisionless plasma flowing around an obstacle is investigated by numerical methods. In the present studies, the obstacle is formed by an absorbing cylinder, and a 2-D electrostatic particle-in-cell simulation is used to study the flow characteristics, with extensions to a fully 3-D generalization of the problem demonstrated as well. The formation of irregular filamented density depletions, oblique to the flow, is observed. The structures form behind the obstacle, in a region with a strong velocity shear, but also other instability mechanisms can be identified. The dynamics of these structures is highly dependent on the physical parameters of the plasma, and they can either be quasi-stationary or undergo a dynamic evolution. The structures are found to be associated with phase-space vortices, observed especially in the phase space spanned by the velocity direction perpendicular to the flow and the spatial coordinate in the same direction. The bias of the obstacle with respect to the plasma potential is found to be an important parameter for the dynamics of the structures, but seemingly not for their formation as such. The results can be of interest in the interpretation of structures in space plasmas as observed by instrumented spacecrafts.

- by Patrick Guio
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- Geophysics, Plasma Physics, Bias, Multidisciplinary

A phenomenological discrete bubble model is proposed to help in the design and dynamic diagnosis of bubbling fluidized beds. An activation region mechanism is presented for bubble formation, making it possible to model large beds in a timely manner. The bubbles are modeled as spherical-cap discrete elements that rise through the emulsion phase that is considered as a continuum. The model accounts for the simultaneous interaction of neighboring bubbles by including the trailing effects due to the wake acceleration force. The coalescence process is not irreversible and therefore, the coalescing bubble pair is free to interact with other rising bubbles originating the splitting phenomena. To validate the model, the simulated dynamics are compared with both experimental and literature data. Time, frequency, and state space analysis are complementarily used with a multiresolution approach based on the empirical method of decomposition to explore the different dynamic scales appearing in both the simulated time series and those obtained from experimental runs. It is concluded that the bubble dynamics interactions play the main role as the driver of the resulting bed dynamics, matching the main features of measured bubble dynamics. Exploding bubble phenomena have been identified by establishing a direct relation between the bubble generation, interaction and eruption, and the measured signals. © 2010 American Institute of Chemical Engineers AIChE J, 2011

- by Domingo Santana
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- Chemical Engineering, Design, Time Series, Modeling

Island wakes are areas of a strong eddy activity influencing the availability and transport of organic matter in the ocean which, in turn impact biological productivity. Despite this, eddy formation in the lee of North Atlantic tropical islands is scarcely documented, except for the Canary Islands. Moreover, the occurrence of anticyclones leeward of Madeira has seldom been detected. During the summer of 2011, a multiplatform approach, combining satellite data with in situ measurements, was used to study an anticyclonic eddy generated in the lee of the Madeira Island. The main objective was to confirm recent numerical evidence suggesting that orographically perturbed winds can induce anticyclonic eddies leeward of Madeira, particularly during summer months. The high resolution sampling of the eddy's interior shows a strong downwelling of %100 m of the isopycnal layer below the mixed layer, typical of intrathermocline eddies. The 25 km radius of this anticyclonic structure exceeds the local deformation radius by a factor of 2. The vortex Rossby number remained moderate (Ro 5 0.26) even if the relative core vorticity reached a finite value (f/f 5 20.7). The occurrence of strong trade winds (10-15 m s 21 ) prior to the detection of the first surface eddy signatures (July 2011) concurrent with opposite flowing geostrophic currents, shows that the orographic wind forcing is the main mechanism for generating this mesoscale long-lived eddy. After leaving the shelter of the island, the eddy traveled northwesterly following a perpendicular net Ekman transport pathway at a speed of 5 km/d, for at least 2 months. An interaction with a cyclonic partner generated in the area helped precipitate the northward trajectory. This study presents the first clear evidence of a windinduced mesoscale anticyclone in the lee of Madeira.

- by Alexandre Stegner
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- Multidisciplinary, Boundary Layer, Wake, Mixed layer

The hysteresis effect on the vortex induced vibration (VIV) on a circular cylinder is investigated by the numerical solution of the two-dimensional Reynolds averaged Navier-Stokes equations. An upwind and total variation diminishing (TVD) conservative scheme is used to solve the governing equations written in curvilinear coordinates and the k-ɛ turbulence model is used to simulate the turbulent flow in the wake of the body. The cylinder is supported by a spring and a damper and free to vibrate in the transverse direction. In previous work, numerical results for the amplitude of oscillation and vortex shedding frequency were compared to experimental data obtained from the literature to validate the code for VIV simulations. In the present work, results of practical interest are presented for the power absorbed by the system, phase angle, amplitude, frequency, and lift coefficient. The numerical results indicate that the hysteresis effect is observed only when the frequency of vortex ...

- by Carlos Levi
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- Mechanical Engineering, Civil Engineering, Mathematics, Turbulence

The effects of shape and relative submergence (the ratio of flow depth to obstacle height, d/H) were investigated on the wakes around four different low-aspect-ratio wall-mounted obstacles semi-ellipsoids with the major axes of the base ellipses aligned in the streamwise and transverse directions, two cylinders with aspect ratios matching the ellipsoids. The wake structure of a fully submerged, spherical obstacle was also investigated in the same flow conditions to provide insight into the flow-obstacle interaction with implication to sediment transport. A low-aspect-Abstract Approved:

- by Seyed Hajimirzaie
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- Wake

Bouncing and coalescence of a pair of slightly deformed bubbles rising side by side in a quiescent liquid are experimentally studied. The trajectories and shapes of the bubbles are investigated in detail by using a high-speed video camera. The wakes of bubbles are visualized by using a photochromic dye that is colored with UV light irradiation. We observe that the patterns of the trajectories of rising bubbles are strongly dependent on the Reynolds number. When the Reynolds number is over the critical region, two bubbles approach each other and then collide. After the collision, two types of motions are observed-coalescence and bouncing. We investigate the critical Reynolds number and Weber number over which the bubbles bounce. In the definitions of these numbers, we use vertical velocity, instead of horizontal one, as the characteristic velocity. We clarify that the critical Weber number is around 2 regardless of the Morton number. The critical Reynolds number decreases with an increase in the Morton number. Moreover, the visualization of the wake of bubbles enables us to observe the vortex separation from the rear surface of the bubbles on collision. We find that the vortex separation from the rear of bouncing bubbles causes a decrease in the rising velocity and an increase in the horizontal speed after their collision. We also observe that the behavior of repeatedly bouncing bubbles is significantly influenced by the wake instability of a single bubble rather than by the bubble-bubble interaction.

- by Masao Watanabe
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- Mechanical Engineering, Chemical Engineering, Modeling, Morphology

- by Eva Libman
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- Depression, Cognition, Attention, Fatigue

Numerical results of mixed convective heat transfer from two identical cylinders in a uniform upward flow has been demonstrated at Re = 100. The effect of aiding and opposing buoyancy is brought about by varying Richardson numbers. A stabilized SUPG based finite element technique has been used. Results are discussed for 20 cases by varying the locations of second cylinder with respect to the fixed location of the first cylinder. The effect of buoyancy on force coefficients, Strouhal number and Nusselt number is investigated. Under the same buoyancy induced field the cylinders are found to oscillate at the same frequency. For a particular cylinder spacing, vortex shedding is observed up to Ri = 0.25. Maximum heat transfer is found at the front face of second cylinder. It is found that hydrodynamic instabilities grow and flow shows chaotic phenomena when the system is severely influenced by thermal buoyancy. It is for the first time that such behavior for the Richardson number at this tandem configuration is being reported.

- by Amaresh Dalal
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- Engineering, Modeling, Heat Transfer, Mesh generation

A pilot experiment was conducted in the period from April to June 2008 in the Straits of Florida near Port Everglades, Florida, in order to study the dynamics of far wakes of ships. In this experiment, a small boat with downward-looking sonar made "snakelike" sections through wakes of ships of opportunity during the TerraSAR-X overpasses. The ship and its parameters (length, speed, course, etc.) were identified utilizing an automated identification system. The sonar responded to the clouds of microbubbles generated in the ship wake by the propulsion system and ship-hull turbulence. The ship wakes were traced in the sonar signal typically from 10 to 30 min after the ship's passage. A preliminary analysis of the measurements suggests that the visibility of the centerline ship wake in synthetic aperture radar (SAR) images is correlated with the presence of microbubbles in the wake. This supports the hypothesis that natural surfactants scavenged and brought to the surface by rising bubbles play an important role in the wake visibility in SAR. The influence of the wind-wave field on the ship wake, as well as the effect of screening of the wind-wave field by the ship's hull, adds another level of complexity to wake patterns observed in SAR images.

- by Susanne Lehner
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- Geophysics, Remote Sensing, Turbulence, Synthetic Aperture Radar

The paper presents research to develop a model complex that takes into account the interaction between the wind farm and the atmosphere, and between closely spaced wind farms. Six models have been reviewed and developed/adapted for use in wind farm modelling, covering scales from several hundred kilometres down to the size of the individual wind turbine. Flow within wind farms is difficult to predict. The analytical and modified WAsP/park models show promise; however, these require further development/evaluation. For the flow downwind of the wind farm, several intermediate-scale models fit the available data rather well, and may be candidates for the other half of the two-model complex which we aim at building.

- by Kurt Hansen and +1
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- Mechanical Engineering, Wind Energy, Wind, Wind Farm

The purpose of thtk Letters section is to provide rapid dissemination of important new results in theJields regularly covered by Physics of Fluids A. Results of extended research should not be presented as a series of letters in place of comprehensive articles. Letters cannot exceed three printed pages in length, including space allowed for title, figures, tables, references and an abstract limited to about 100 words. There is a three-month time timit, from date of receipt to acceptance, for processing Letter manuscripts. Authors must also submit a brief statement justifying rapid publication in the Letters section.

- by Michael Triantafyllou
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- Oscillations, Hydrodynamics, Von Karman Vortex Street, Strouhal Number