The early stage of development of the wake behind an impulsively started cylinder for 40 Re 4
TL;DR: The time development of the symmetrical standing zone of recirculation, which is formed in the early stage of the flow due to a circular cylinder impulsively set in motion perpendicular to its generators, has been studied using a flow visualization technique.
Abstract: The time development of the symmetrical standing zone of recirculation, which is formed in the early stage of the flow due to a circular cylinder impulsively set in motion perpendicular to its generators, has been studied using a flow visualization technique. The Reynolds numbers (based upon the diameter) range from 40 to 104. Some new phenomena indicated in the flow patterns are revealed, and several different regimes are differentiated by a detailed analysis of the evolution of the main flow characteristics. A correlation with some theoretical results is established.
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01 Jan 1989
TL;DR: In this paper, the authors use hot-wire (HW) or laser velocimetry (LV) to estimate the velocity, vorticity, and pressure fields of wake flows.
Abstract: One of the most challenging and time-consuming problems in experimental fluid mechanics is the measurement of the overall flow field properties, such as the velocity, vorticity, and pressure fields. Local measurements of the velocity field (i.e., at individual points) are now done routinely in many experiments using hot-wire (HW) or laser velocimetry (LV). However, many of the flow fields of current interest, such as coherent structures in shear flows or wake flows, are highly unsteady. HW or LV data of such flows are difficult to interpret, as both spatial and temporal information of the entire flow field are required and these methods are commonly limited to simultaneous measurements at only a few spatial locations.
1,798 citations
TL;DR: In this paper, the authors investigated the dynamic characteristics of the pressure and velocity fields of the unsteady incompressible laminar wake behind a circular cylinder, and the initiation mechanism for vortex shedding and evaluation of the body forces are presented for Reynolds-number values of 100, 200 and 1000.
Abstract: The dynamic characteristics of the pressure and velocity fields of the unsteady incompressible laminar wake behind a circular cylinder are investigated numerically and analysed physically. The governing equations, written in a velocity—pressure formulation and in conservative form, are solved by a predictor—corrector pressure method, a finite-volume second-order-accurate scheme and an alternating-direction-implicit (ADI) procedure. The initiation mechanism for vortex shedding and the evaluation of the unsteady body forces are presented for Reynolds-number values of 100, 200 and 1000.The vortex shedding is generated by a physical perturbation imposed numerically for a short time. The flow transition becomes periodic after a transient time interval. The frequency of the drag and lift oscillations agree well with the experimental data.The study of the interactions of the unsteady pressure and velocity fields shows the phase relations between the pressure and velocity, and the influence of different factors: the strongly rotational viscous region, the convection of the eddies and the almost inviscid flow.The interactions among the different scales of structures in the near wake are also studied, and in particular the time-dependent evolution of the secondary eddies in relation to the fully developed primary ones is analysed.
779 citations
Book•
09 Jan 2006TL;DR: In this paper, the authors describe a planing vessel with wave resistance and propulsion, and planing vessels with planing, whipping, and springing, as well as a semi-displacement vessel.
Abstract: 1. Introduction 2. Resistance and propulsion 3. Waves 4. Wave resistance and wash 5. Surface effect ships 6. Hydrofoil vessels and foil theory 7. Semi-displacement vessels 8. Slamming, whipping and springing 9. Planing vessels 10. Manoeuvring Appendix References Index.
457 citations
TL;DR: In this paper, hot-wire measurements were conducted in the very near wake (x/d⩽10) of a circular cylinder at a Reynolds number based on cylinder diameter.
Abstract: Hot-wire measurements were conducted in the very near wake (x/d⩽10) of a circular cylinder at a Reynolds number based on cylinder diameter, Re
d of 3900. Measurements of the streamwise velocity component with the use of single sensor hot-wire probes were found to be inaccurate for such flowfields where high flow angles are present. An X-array probe provided detailed streamwise and lateral velocity component statistics. Frequency spectra of these two velocity components are also presented. Measurements with a 4-sensor hot-wire probe confirmed that the very near wake region is dominantly two-dimensional, thus validating the accuracy of the present X-array data.
437 citations
TL;DR: In this article, the authors investigated the time scales associated with the shedding of the trailing and leading edge vortices, as well as the corresponding time-dependent forces, and presented a generic mechanism of the frequency selection as a result of unsteady aerodynamics.
Abstract: Motivated by our interest in unsteady aerodynamics of insect flight, we devise a computational tool to solve the Navier–Stokes equation around a two-dimensional moving wing, which mimics biological locomotion. The focus of the present work is frequency selection in forward flapping flight. We investigate the time scales associated with the shedding of the trailing- and leading-edge vortices, as well as the corresponding time-dependent forces. We present a generic mechanism of the frequency selection as a result of unsteady aerodynamics.
435 citations
Cites methods or result from "The early stage of development of t..."
...The computational tool is tested by comparing results on impulsively started flows with analytical and experimental results (Bouard & Coutanceau 1980; Dickinson & Götz 1993)....
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...Secondly, we compare the computed unsteady velocity field with well-documented experiments by Bouard & Coutanceau (1980)....
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...To test our code, we compute flow past a cylinder, a limiting case of an ellipse, and compare the results with experiments by Bouard & Coutanceau (1980)....
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...To test the dynamics in the vortex wake, we followed the set up of Bouard & Coutanceau (1980) and computed the velocity field in the wake along the symmetry axis....
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...In figure 10 we copied the experimental points from figure 18 in Bouard & Coutanceau (1980) and overlaid our numerics on top of that....
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TL;DR: In this article, a visualization method is used to obtain the main features of the hydrodynamic field for flow past a circular cylinder moving at a uniform speed in a direction perpendicular to its generating lines in a tank filled with a viscous liquid.
Abstract: A visualization method is used to obtain the main features of the hydrodynamic field for flow past a circular cylinder moving at a uniform speed in a direction perpendicular to its generating lines in a tank filled with a viscous liquid. Photographs are presented to show the particular fineness of the experimental technique. More especially, the closed wake and the velocity distribution behind the obstacle are investigated; the changes in the geometrical parameters describing the eddies with Reynolds number (5 < Re < 40) and with the ratio λ between the diameters of the cylinder and tank are given. A comparison with existing numerical and experimental results is presented and some remarks are made about the calculation techniques proposed up to the present. The limits of the Reynolds-number range for which the twin vortices exist and adhere stably to the cylinder are determined.
622 citations
TL;DR: Experiments on the near wake of a cylinder are described in this paper in an attempt to present a coherent picture of the events encountered as the Reynolds number increases from small values up to values of a few thousand.
Abstract: Experiments on the near wake of a cylinder will be discribed in an attempt to present a coherent picture of the events encountered as the Reynolds number increases from small values up to values of a few thousand. Much work on this subject has already been done, but there are gaps in our description of these flows as well as more fundamental deficiencies in our understanding of them. The subject has been reviewed several times and most recently by Berger & Wille (1972) whose paper covers much of the ground that will be discussed again here. The present work may be regarded as built upon this latest review. I remember with gratitude many helpful discussions with the late Rudolph Wille who contributed so much to this subject. The investigation has concentrated on circular cylinders, but the wakes of bluff cylinders of different cross sectional shapes have also been observed. Bluff cylinders in general are considered in §§4 and 5, together with the effect of splitter plates on circular cylinders in §9. The experiments concern, almost exclusively, flow visualization of the wakes by means of dye washed from the bodies. The patterns of dye observed are, therefore, filament line representations of the flow leaving the separation lines on the body. It must be stressed that the dye does not make visible the vorticity bearing fluid because at low Reynolds number, vorticity diffuses considerably more rapidly than does dye. The ratio of the molecular diffusivity of momentum to that of mass of dye is of the order of 100.
370 citations
TL;DR: In this article, the Navier-Stokes equations were integrated numerically for the time-dependent flow past an impulsively started circular cylinder, based on the diameter of the cylinder, from 5 to ∞.
Abstract: An accurate method is described for integrating the Navier-Stokes equations numerically for the time-dependent flow past an impulsively started circular cylinder. Results of integrations over the range of Reynolds numbers, based on the diameter of the cylinder, from 5 to ∞ are presented and compared with previous numerical, theoretical and experimental results. In particular, the growth of the length of the separated wake behind the cylinder has been calculated for R = 40, 100 and 200 and is found to be in very good agreement with the results of recent experimental measurements. The calculated pressure distribution over the surface of the cylinder for R = 500 is also found to be in reasonable agreement with experimental measurements for the case R = 560.For Reynolds numbers up to 100 the equations were integrated until most of the features of the flow showed a close approximation to steady-state conditions. The results obtained are in good agreement with previous calculations of the steady flow past a circular cylinder. For R > 100 the integrations were continued until the implicit method of integration broke down by reason of its failure to converge. A secondary vortex appeared on the surface of the cylinder in the case R = 500, but for higher Reynolds numbers, including the case R = ∞, the procedure broke down before the appearance of a secondary vortex. In all cases the flow was assumed to remain symmetrical.
252 citations
TL;DR: In this paper, a finite difference solution for the time dependent equations of motion has been carried out in order to extend the range of available data on steady flow around a cylinder to larger Reynolds numbers.
Abstract: Finite difference solutions for the time dependent equations of motion have been carried out in order to extend the range of available data on steady flow around a cylinder to larger Reynolds numbers. At the termination of the calculations for R = 40 and 200, the separation angle, the drag coefficient and the pressure and vorticity distributions around the surface of the cylinder were very close to their steady-state values. For R = 500 the separation angle and drag coefficient were very close to their steady-state values but the pressure distribution and vorticity distribution at the rear of the cylinder were still changing slightly. The results at R = 500 were found to be quite different from those at R = 200 so it is not clear how closely we approximated the steady solution for R → ∞. The forces on the cylinder due to viscous drag and due to pressure drag are found to be smaller for steady flow than for laboratory experiments where the wake is unsteady.
167 citations