Download or read book Wake Similarity and Vortex Formation for Two dimensional Bluff Bodies written by Jose V. Nebres and published by . This book was released on 1992 with total page 362 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Twenty First Symposium on Naval Hydrodynamics written by National Research Council and published by National Academies Press. This book was released on 1997-09-11 with total page 1100 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Control and Suppression of Laminar Vortex Shedding Off Two dimensional Bluff Bodies written by Andre Sirilutporn Chan and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The focus of this research is on the control and suppression of vortex shedding of flow past bluff bodies. The motivation of this research stems from the aerodynamic problems encountered in the design and development of hard disk drives (HDD's). Two different computational fluid dynamic methods have been used in this research-- the Semi-Implicit Method for Pressure Linked Equation (SIMPLE), that is widely employed in today's commercial incompressible flow solvers, and the high-order spectral difference (SD) method, recently developed for compressible flow solution. In addition to numerical simulation and verification, complementary experimental measurements have been performed to further validate the results. This research leads to two very different suppression techniques: 1) a passive control using a thin splitter plate positioned downstream of the bluff body; 2) an active control by way of counter rotating a cylinder pair. The passive suppression technique places a thin splitter plate downstream of the bluff body in order to interfere with the vortex wakes and thereby suppress the vortexinduced forces on the bluff body itself. The present investigation examines the suppression of wake instabilities in the laminar shedding regime. Both bounded and unbounded flow conditions are examined. It is found that in the bounded flow condition, the channel walls have an additional stabilizing effect on the shedding control. With proper positioning of the splitter plate, vortex shedding is completely suppressed in a bounded flow with moderate blockage factor. Wind tunnel empirical experiments have also confirmed the effectiveness of a splitter plate in a bounded flow. Active flow control by counter-rotating a pair of cylinders has been numerically investigated. It has also been investigated experimentally in partnership with the Gas Dynamics Laboratory at Princeton University. It is demonstrated that it is possible to suppress unsteady vortex shedding for gap sizes from one to five cylinder diameters, at Reynolds numbers from 100 to 200. The degree of unsteady wake suppression is proportional to the speed and the direction of rotation, and there is a critical rotation rate where a complete suppression of flow unsteadiness can be achieved. In the doublet-like configuration at higher rotational speeds, a virtual elliptic body that resembles a potential doublet is formed, and the drag is reduced to zero. The shape of the elliptic body primarily depends on the gap between the two cylinders and the speed of rotation. Prior to the formation of the elliptic body, a second instability region is observed, similar to that seen in studies of rotating single cylinders. It is also shown that the unsteady wake suppression can be achieved by rotating each cylinder in the opposite direction, that is, in a reverse doublet-like configuration. This tends to minimize the wake interaction of the cylinder pair and the second instability did not make an appearance over the range of speeds investigated here.
Download or read book Control of Two and Three Dimensional Wake Instabilities from Bluff Bodies written by and published by . This book was released on 1995 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: The detailed, instantaneous flow structure of the near-wake of a cylinder subjected to various classes of active (open-loop) and passive control has been characterized using both qualitative flow visualization and high-image-density particle image velocimetry (piv) developed in our laboratories. The first cinema PIV system allows acquisition of images that are highly resolved in both space and time. Concepts of timing of vortex formation, period-doubled patterns of vortices, and modulated and phase-locked systems of vortices are addressed for controlled oscillations of the cylinder. Three-dimensional patterns of instantaneous vorticity allow classification of the modes of response of the near-wake, providing a basis for formulating concepts for attenuation of vortex formation.
Download or read book Research on Bluff Body Vortex Wakes written by and published by . This book was released on 1995 with total page 8 pages. Available in PDF, EPUB and Kindle. Book excerpt: Comparison of laboratory experiments and computational results for two dimensional flows showed that, in impulsively started flows, laboratory flows are two dimensional in the early stages and that three dimensional effects develop after acceleration is complete. In fully developed flow the mean and fluctuating forces are considerably lower (up to 50%) in the laboratory flows (ie with three dimensionality) than in the two dimensional numerical simulations. The experiments in tow tank and water tunnel also revealed the existence of long-time modulations of vortex shedding forces. These have the form of bursts with duration of order 10 vortex shedding periods. A novel method of introducing controlled spanwise components of velocity into otherwise two dimensional flow, at low Reynolds number was to give the cylinder an axial (spanwise) motion, either a steady translation or a periodic oscillation. For unsteady, periodic axial oscillation of the cylinder, the experiments showed how the ratio of the spanwise period to the vortex shedding period determines the patterns of vortex dislocation in the wake, how regimes of chaos are formed and how shedding frequency and wake spectra are affected. (AN).
Download or read book Particle Image Velocimetry written by Ronald J. Adrian and published by Cambridge University Press. This book was released on 2011 with total page 585 pages. Available in PDF, EPUB and Kindle. Book excerpt: Particle image velocimetry, or PIV, refers to a class of methods used in experimental fluid mechanics to determine instantaneous fields of the vector velocity by measuring the displacements of numerous fine particles that accurately follow the motion of the fluid. Although the concept of measuring particle displacements is simple in essence, the factors that need to be addressed to design and implement PIV systems that achieve reliable, accurate, and fast measurements and to interpret the results are surprisingly numerous. The aim of this book is to analyze and explain them comprehensively.
Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1995 with total page 456 pages. Available in PDF, EPUB and Kindle. Book excerpt: Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
Download or read book An Experimental Study of the Origin of Oblique Vortex Shedding from Two dimensional Bluff Bodies written by Mustapha Hammache and published by . This book was released on 1990 with total page 202 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book An Investigation of Flow Around Two Bluff Bodies in Tandem and Staggered Arrangements by the Discrete Vortex Method and Experiment written by Hacı İbrahim Keser and published by . This book was released on 1995 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Using an experimental-numerical integrated approach, flow around and mean pressure fields on a blunt-based flat plate and a downstream circular cylinder in tandem and staggered arrangements have been investigated. The investigation focuses on the effect of a wide range of horizontal and vertical spacings between the plate and the cylinder, on the flow and the associated mean pressure distributions. With different cross-stream length scales, and relatively thicker boundary layers of the upstTeam plate, the plate-cylinder arrangement offers a unique configuration that has not been disclosed before. The experimental part of the study reveals that, similar to that observed for equal diameter cylinders, which has received the most research attention in the related literature, a critical spacing exists. For horizontal spacings smaller than the critical, formation of vortices in front of the downstream cylinder is inhibited. However, flow configuration before the critical spacing is found to be drastically different than those for the circular cylinders. It has been demonstrated here that, the critical spacing of the tandem arrangement is valid for the staggered arrangements as well. That spacing marks the boundary of a region on either side of which different flow interferences take place for both tandem and the staggered arrangements. Experimental study also reveals the variation with respect to spacing of the mean pressure distributions on surfaces of the trailing-end of the plate. The experimental part provides not only a unique set of data, but also a basis for comparison for the numerical part of the study. In the numerical part, two different types of the discrete vortex method has been utilized. Firstly, an inviscid, Lagrangian approach allowed representation of boundary layers separated from the upstream plate by practically zero thickness rows of discrete vortices. The approach is original in combining the conformal-mapping image and the vortex-singularity-distribution techniques for representation of plate and the cylinder surfaces respectively. Secondly, a Lagrangian-Eulerian viscous discrete vortex method is used. Utilization of panel representation of body surfaces along with overlapping rectangular grids, allowed development of a computer code for laminar, two dimensional flow calculation around bodies of arbitrary geometry and number. Both parts of the numerical study could predict number of flow features, implied by the xnean pressure distributions in their experimental counterpart. With a realistic representation of the thick boundary layers of the plate, the viscous method I correctly predicted the experimentally found critical spacing, and, with its clear images of flow evolution, contributed significantly to the understanding of wake interference as function of the distance between the bodies. An extensive comparison of the measured and calculated mean distributions as function of the spacing was made. It has been shown that the drag force acting on the cylinder is primarily dependent on the pressure difference between both ends of the cylinder.
Download or read book Unsteady Combustor Physics written by Tim C. Lieuwen and published by Cambridge University Press. This book was released on 2012-08-27 with total page 427 pages. Available in PDF, EPUB and Kindle. Book excerpt: Developing clean, sustainable energy systems is a pre-eminent issue of our time. Most projections indicate that combustion-based energy conversion systems will continue to be the predominant approach for the majority of our energy usage. Unsteady combustor issues present the key challenge associated with the development of clean, high-efficiency combustion systems such as those used for power generation, heating or propulsion applications. This comprehensive study is unique, treating the subject in a systematic manner. Although this book focuses on unsteady combusting flows, it places particular emphasis on the system dynamics that occur at the intersection of the combustion, fluid mechanics and acoustic disciplines. Individuals with a background in fluid mechanics and combustion will find this book to be an incomparable study that synthesises these fields into a coherent understanding of the intrinsically unsteady processes in combustors.
Download or read book On Coupling Two dimensional Bluff Body Potential Flow to the Periodic Vortex Wake written by R. L. Wardlaw and published by . This book was released on 1966 with total page 64 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Fluid Structure Sound Interactions and Control written by Yu Zhou and published by Springer. This book was released on 2018-05-15 with total page 382 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book presents the proceedings of the Symposium on Fluid-Structure-Sound Interactions and Control (FSSIC), (held in Tokyo on Aug. 21-24, 2017), which largely focused on advances in the theory, experiments on, and numerical simulation of turbulence in the contexts of flow-induced vibration, noise and their control. This includes several practical areas of application, such as the aerodynamics of road and space vehicles, marine and civil engineering, nuclear reactors and biomedical science, etc. Uniquely, these proceedings integrate acoustics with the study of flow-induced vibration, which is not a common practice but can be extremely beneficial to understanding, simulating and controlling vibration. The symposium provides a vital forum where academics, scientists and engineers working in all related branches can exchange and share their latest findings, ideas and innovations – bringing together researchers from both east and west to chart the frontiers of FSSIC.
Download or read book Three Dimensional Aspects of Nominally 2 D and 3 D Bluff Body Wakes written by and published by . This book was released on 1995 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: Over the last decade, there has been much work carried out to understand three-dimensional effects at low Reynolds numbers (Re) in the wake of bluff bodies, yielding the reaction that end effects can influence the vortex shedding across large spanwise lengths. Wake patterns such as parallel shedding, oblique shedding, cellular shedding, transient patterns, such as 'phase shocks' and 'phase expansions', as well as the existence of large-scale 'vortex dislocations', have been investigated, aspects of this research being spearheaded by me studies at Cornell under the above Contract. In particular, we have discovered a new mechanism for oblique wave resonance in the far wake. We have found a means to control the near wake by using suction at the spanwise ends of a body, which has enabled much more precise phenomena to be investigated including the critical conditions for turbulence inception, and a careful study of 3-D wake transition. We have combined our research at Cornell with studies by Dr. Peter Monkewitz at Ecole Polytechnique, Lausanne, who has effectively laid much of the foundation of the description of these near wake patterns in terms of a Guinzburg-Landau equation. For example, the now transient phenomenon known as a 'phase expansion' has been found to be directly analogous to a Prandtl-Meyer expansion found in gas dynamics. Clearly, the work under the support of the ONR has led to a surprisingly rich new understanding of three-dimensional effects in nominally two-dimensional wake flows.
Download or read book Bluff Body Wakes Dynamics and Instabilities written by Helmut Eckelmann and published by Springer Science & Business Media. This book was released on 2013-11-11 with total page 390 pages. Available in PDF, EPUB and Kindle. Book excerpt: Bluff-body wakes play an important role in many fluid dynamics problems and engineering applications. This book gives and up-to-date account of recent results obtained in the study of bluff-body wakes. Experimental, theoretical and numerical approaches are all comprehensively covered and compared. Topics of particular interest include hydrodynamic instability analyses, three-dimensional pattern formation problems, flow control methods, bifurcation analyses, numerical simulations and turbulence modelling. The main originality of thisvolume is that recent conceptual advances made to describe nonlinear phenomena in general are put to the test on a classical problem in fundamental fluid mechanics, namely the wake structure generated behind a bluff object.
Download or read book Introduction to Hydrodynamic Stability written by P. G. Drazin and published by Cambridge University Press. This book was released on 2002-09-09 with total page 278 pages. Available in PDF, EPUB and Kindle. Book excerpt: Instability of flows and their transition to turbulence are widespread phenomena in engineering and the natural environment, and are important in applied mathematics, astrophysics, biology, geophysics, meteorology, oceanography and physics as well as engineering. This is a textbook to introduce these phenomena at a level suitable for a graduate course, by modelling them mathematically, and describing numerical simulations and laboratory experiments. The visualization of instabilities is emphasized, with many figures, and in references to more still and moving pictures. The relation of chaos to transition is discussed at length. Many worked examples and exercises for students illustrate the ideas of the text. Readers are assumed to be fluent in linear algebra, advanced calculus, elementary theory of ordinary differential equations, complex variables and the elements of fluid mechanics. The book is aimed at graduate students but will also be very useful for specialists in other fields.
Download or read book Vortex Methods written by Georges-Henri Cottet and published by Cambridge University Press. This book was released on 2008-04-24 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Vortex methods have matured in recent years, offering an interesting alternative to finite difference and spectral methods for high resolution numerical solutions of the Navier Stokes equations. In the past three decades, research into the numerical analysis aspects of vortex methods has provided a solid mathematical background for understanding the accuracy and stability of the method. At the same time vortex methods retain their appealing physical character, which was the motivation for their introduction. This book presents and analyzes vortex methods as a tool for the direct numerical simulation of impressible viscous flows. It will interest graduate students and researchers in numerical analysis and fluid mechanics and also serve as an ideal textbook for courses in fluid dynamics.
Download or read book Flow Dynamics of Bluff Bodies written by Anirudh Narayan Rao and published by . This book was released on 2012 with total page 139 pages. Available in PDF, EPUB and Kindle. Book excerpt: A computational study investigating the flow past bluff bodies in the low Reynoldsnumbers range (Re ≤ 750) is presented. Two- and three-dimensional investigations are performed to investigate various flow transitions that occur when canonical bluff bodies such as circular cylinders and spheres are placed near a planar boundary, rotated or a combination of the two effects.Control parameters such as [alpha], the non-dimensionalised rotation rate, defined as the ratio of the tangential velocity on the body surface to the oncoming fluid velocity, and gap height G/D, the distance between the body and the wall (G) non-dimensionalised by the diameter D, are extensively used together with the Reynolds number. For these investigations, [alpha] is varied between ±3, where positive values correspond to prograde rotation and negative values correspond to retrograde rotation. The gap height is varied from G/D = ∞ for bodies in freestream to G/D ~ 0 for bodies near a wall.A spectral element based solver is used to solve the Navier-Stokes equations in twoandthree-dimensions. Computational domains are constructed so that the evaluatedflow parameters, such as the force coefficients and the shedding frequency, are accurateto an error of less than 1%. Spatial resolution studies are performed to obtain a tradeoffbetween accuracy and computational time. For all investigations, the results varyby less than 0.5% with respect to the domain with the highest resolution.The first of these studies investigates the onset of various three-dimensional modes inthe wake of a rotating cylinder in freestream as the rotation rate is varied for [alpha] ≤ 2.5 and Re ≤ 400. Two transitions are considered in this study; the first being the transition to periodic flow where vortex shedding occurs. As the rotation rate was increased, the onset of periodic flow was delayed and altogether suppressed for [alpha] ≥ 2.1. The second transition considered is the transition to three-dimensionality using a technique known as linear stability analysis. For rotation rates [alpha] ≤ 1, the onset of the three-dimensional modes occurs in the unsteady regime, and is identical to that observed for a non-rotating cylinder, although the rotation rate delays the onset of transition to higher Reynolds numbers. For higher rotation rates, the three-dimensional scenario becomes increasingly complex, where three new modes bifurcate from the unsteady base flow and two new modes bifurcate from the steady base flow. The spatio-temporal characteristics and the physical mechanism leading to the instability of these modes are discussed.A second study investigates the flow dynamics for a circular cylinder translatingalong a wall at different gap heights. From the two-dimensional computations, the forcecoefficients and the shedding frequencies were quantified. At large spacings, G/D ≥ 0.28, the transition to three-dimensionality was observed on the unsteady base flow, while below this gap height, the three-dimensional transition occurred in the steady regime at Reynolds numbers lower than the transition to periodic flow. Simulations were further carried out to determine the variation of the transitional Reynolds numbers for cylinders rolling along a wall. For forward rolling cases, the transition to unsteady flow occurred at increasingly low Reynolds numbers, while reverse rolling delayed the onset of periodic flow to higher Reynolds numbers and periodic flow was suppressed for [alpha] ≤ -1.5. Linear stability analysis indicated that the onset of three-dimensional flow was lowered as the rotation rate was increased to higher positive values of [alpha], while three-dimensionality was suppressed for negative rotation rates of [alpha] ≤ -2. For the cylinder sliding along a wall ([alpha] = 0), stability analysis at higher Reynolds numbers in the unsteady state shows multiple modes unstable to spanwise perturbations. The three-dimensional simulations indicate that the flow eventually becomes chaotic, possibly due to the interaction between the various modes.The second study was further extended to investigate the flow past multiple bodiesnear a wall. The additional control parameter for this study was the separation distanceS/D, where S is the distance between the cylinders and Reynolds number, while therotation rate was fixed at [alpha] = 0. For cylinders at very small and very large separations, the flow features were identical to that of the singular cylinder. As Reynolds number was increased, unsteady flow was detected at close spacings, which led to an increase in the drag coefficient on the downstream cylinder. Stability analysis showed similar trends for the limiting cases, while for intermediate spacings, the flow first became unstable, and then restabilised at slightly higher Reynolds number. This flow further became unstable at higher Reynolds number. Three-dimensional simulations over a range of separations show the flow transitioning to a chaotic state akin to the singular cylinder.The final study investigated the wake of a forward rolling sphere for Re ≤ 500. At Re ~ 140, vortex shedding occurred by the formation of hairpin vortices which moved away from the wall and convected downstream. A secondary transition involving the loss of planar symmetry occurred at Re ~ 192, where the hairpin vortices were displaced laterally along the wake centreline, giving a sinuous structure to the wake when viewed from above. Beyond this transition, the lateral oscillations exhibited a 7 : 3 resonance with the hairpin vortex shedding. As Reynolds number was increased, the flow progressively became more disorganised and chaotic. At the highest tested Reynolds number of 500, the wake was spatio-temporally chaotic, while retaining its sinuous structure.
