Showing papers on "Vortex shedding published in 1994"
TL;DR: In this article, structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry.
Abstract: Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events’ in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.
1,186 citations
TL;DR: In this article, the Strouhal number and the mean base suction coefficient were measured at the mid-span position Reynolds numbers from about 50 to 4 × 104 were investigated.
Abstract: The investigation is concentrated on two important quantities – the Strouhal number and the mean base suction coefficient, both measured at the mid-span position Reynolds numbers from about 50 to 4 × 104 were investigated Different aspect ratios, at low blockage ratios, were achieved by varying the distance between circular end plates (end plate diameter ratios between 10 and 30) It was not possible, by using these end plates in uniform flow and at very large aspect ratios, to produce parallel shedding all over the laminar shedding regime However, parallel shedding at around mid-span was observed throughout this regime in cases when there was a slight but symmetrical increase in the free-stream velocity towards both ends of the cylinder At higher Re, the results at different aspect ratios were compared with those of a ‘quasi-infinite cylinder’ and the required aspect ratio to reach conditions independent of this parameter, within the experimental uncertainties, are given For instance, aspect ratios as large as L/D = 60–70 were needed in the range Re ≈ 4 × 103–104 With the smallest relative end plate diameter and for aspect ratios smaller than 7, a bi-stable flow switching between regular vortex shedding and ‘irregular flow’ was found at intermediate Reynolds number ranges in the subcritical regime (Re ≈ 2 × 103)
620 citations
TL;DR: In this article, the authors discuss the role of vortices in the flight of modern helicopters and aircraft, and discuss the geometrical boundary geometries that act to promote vortex formation.
Abstract: Situations where an effectively irrotational freest ream contains regions of concentrated vorticity are common in external aerodynamics, where vortices are known to play an important role in the flight of modern helicopters (Carr 1988) and aircraft (Cunningham 1989, Mabey 1989). Vortices may arise as a consequence of shedding from some upstream surface or near certain three-dimensional boundary geometries that act to promote vortex formation. Examples of the former type of vortex gen eration include: 1. vortices that trail from the tips of airfoils (Harvey & Perry 1971) and control surfaces on submarines (Lugt 1983), 2. transverse vortices shed from maneuvering airfoil surfaces and helicopter blades in a process known as dynamic stall (McCroskey 1982, Francis & Keesee 1985, Carr 1988), and 3. shedding from stationary obstacles. Geometry-induced creation can occur in any situation where a flow along a wall approaches a surface-mounted obstacle; examples include: 1. airframe features such as wing/body junctions, 2. conning towers on submarines, and 3 . computer chips mounted on electrical circuit boards. Similar geometries are en-
384 citations
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01 Jan 1994
TL;DR: In this article, the performance of the dynamic subgrid-scale eddy-viscosity model and the suitability of high-order accurate, upwind-biased numerical methods for large eddy simulations of complex flows are investigated in the case of the turbulent wake behind a circular cylinder at Reynolds number 3,900, based on freestream velocity and cylinder diameter.
Abstract: : The performance of the dynamic subgrid-scale eddy-viscosity model and the suitability of high-order accurate, upwind-biased numerical methods for large eddy simulations of complex flows are investigated in the case of the turbulent wake behind a circular cylinder at Reynolds number 3,900, based on freestream velocity and cylinder diameter The numerical method consists of high-order upwind-biased finite difference techniques applied to the compressible Navier-Stokes equations written in generalized coordinates Integration in time is done using a fully implicit, second-order accurate iterative technique The results of three fifth-order accurate simulations performed on identical grids with the least-squares version of the dynamic model, the fixed-coefficient Smagorinsky model, and with no subgrid-scale model are compared in the first 10 diameters of the wake The impact of three-dimensionality is also examined via two and three-dimensional calculations without a subgrid-scale model The effect of numerical dissipation is investigated by comparing two simulations using upwind-biased schemes, the first being fifth-order, and the second seventh-order accurate It is found that the near-wake is highly three-dimensional at this Reynolds number It contains pairs of counter-rotating streamwise vortices, the effect of which cannot be reproduced in two-dimensional calculations Three-dimensional computations are essential for predicting flow statistics of engineering interest (AN)
334 citations
TL;DR: In this paper, it is shown that free shear flows can be substantially altered through direct control of the large coherent vortices present in the flow by placing a foil in the wake of a D-section cylinder, sufficiently far behind the cylinder so that it does not interfere with the vortex formation process.
Abstract: It is shown experimentally that free shear flows can be substantially altered through direct control of the large coherent vortices present in the flow.First, flow-visualization experiments are conducted in Kalliroscope fluid at Reynolds number 550. A foil is placed in the wake of a D-section cylinder, sufficiently far behind the cylinder so that it does not interfere with the vortex formation process. The foil performs a heaving and pitching oscillation at a frequency close to the Strouhal frequency of the cylinder, while cylinder and foil also move forward at constant speed. By varying the phase of the foil oscillation, three basic interaction modes are identified. (i) Formation of a street of pairs of counter-rotating vortices, each pair consisting of one vortex from the initial street of the cylinder and one vortex shed by the foil. The width of the wake is then substantially increased. (ii) Formation of a street of vortices with reduced or even reverse circulation compared to that of oncoming cylinder vortices, through repositioning of cylinder vortices by the foil and interaction with vorticity of the opposite sign shed from the trailing edge of the foil. (iii) Formation of a street of vortices with circulation increased through merging of cylinder vortices with vortices of the same sign shed by the foil. In modes (ii) and (iii) considerable repositioning of the cylinder vortices takes place immediately behind the foil, resulting in a regular or reverse Karman street. The formation of these three interaction patterns is achieved only for specific parametric values; for different values of the parameters no dominant stable pattern emerges.Subsequently, the experiments are repeated in a different facility at larger scale, resulting in Reynolds number 20000, in order to obtain force and torque measurements. The purpose of the second set of experiments is to assess the impact of flow control on the efficiency of the oscillating foil, and hence investigate the possibility of energy extraction. It is found that the efficiency of the foil depends strongly on the phase difference between the oscillation of the foil and the arrival of cylinder vortices. Peaks in foil efficiency are associated with the formation of a street of weakened vortices and energy extraction by the foil from the vortices of the vortex street.
277 citations
TL;DR: In this article, the phase-averaged velocities and turbulence intensities of the turbulent shear layer and the associated recirculation region on the sidewall formed in flow separation from the forward corner of a square cylinder have been studied with one-component laser-Doppler velocimetry.
Abstract: The turbulent shear layer and the associated recirculation region on the sidewall formed in flow separation from the forward corner of a square cylinder have been studied with one-component laser-Doppler velocimetry. Because of vortex shedding, the flow is approximately periodic, and is treated as a separated flow undergoing largeamplitude forcing at the shedding frequency. Phase (ensemble)-averaged velocities and turbulence intensities were obtained, and a close relationship in phase and amplitude between phase-averaged turbulence intensities and gradients of phase-averaged velocity is found in much of the flow region. The similarity behaviour of the phase-averaged profiles in the shear layer as well as the streamwise growth of the shear layer are investigated. While phase-averaged velocity profiles collapse well in similarity coordinates, normalized turbulence intensities exhibit systematic deviations from similarity. Shear-layer growth also departs markedly from the linear growth law of unforced plane mixing layers. The effect of the recirculation is suggested as a possible explanation for some of these deviations. Similarities to and differences from steady and forced mixing layers, steady separated flows with recirculation, and unsteady boundary layers are discussed.
259 citations
TL;DR: In this article, the onset of Karman-vortex shedding is studied experimentally in the wake of different two-dimensional bluff bodies, namely an oblong cylinder, circular cylinders and plates of rectangular cross-section.
Abstract: The onset of Karman-vortex shedding is studied experimentally in the wake of different two-dimensional bluff bodies, namely an oblong cylinder, circular cylinders and plates of rectangular cross-section. Different control measures, such as wake heating, transverse body oscillations and base bleed are investigated. As the steady-periodic Karman shedding has previously been identified as a limit-cycle, i.e. as self-excited oscillations, the experiments are interpreted in the framework of the Stuart–Landau model. The coefficients of the Stuart–Landau equation for the characteristic vortex shedding amplitude, i.e. the linear temporal growth rate, linear frequency and the Landau constant, are fully determined for the two cylinders and in part for the plate. For this purpose transients are generated by suddenly switching transverse body oscillations or base bleed on or off. The analysis of these transients by a refined method based on complex demodulation provides reliable estimates of the model coefficients and yields an experimental validation of the concept that a global instability mode grows or decays as a whole. Also, it is demonstrated that the coefficients of the Stuart–Landau equation are independent of the experimental technique used to produce the transients.
239 citations
TL;DR: In this paper, a global, three-dimensional stability analysis of the steady and the periodic cylinder wake is carried out employing a low-dimensional Galerkin method, and the steady flow is found to be asymptotically stable with respect to all perturbations for Re less than 54.
Abstract: A global, three-dimensional stability analysis of the steady and the periodic cylinder wake is carried out employing a low-dimensional Galerkin method. The steady flow is found to be asymptotically stable with respect to all perturbations for Re less than 54. The onset of periodicity is confirmed to be a supercritical Hopf bifurcation which can be modeled by the Landau equations. The periodic solution is observed to be only neutrally stable for 54 less than Re less than 170. While two-dimensional perturbations of the vortex street rapidly decay, three-dimensional perturbations with long spanwise wavelengths neither grow nor decay. The periodic solution becomes unstable at Re = 170 by a perturbation with the spanwise wavelength of 1.8 diameters. This instability is shown to be a supercritical Hopf bifurcation in the spanwise coordinate and leads to a three-dimensional periodic flow. Finally the transition scenario for higher Reynolds numbers is discussed.
226 citations
TL;DR: In this paper, a computational study of the feedback control of von Karman vortex shedding behind a circular cylinder at low Reynolds numbers is reported, where a single feedback sensor is used and the actuators are 180° out of phase with each other.
Abstract: A computational study of the feedback control of von Karman vortex shedding behind a circular cylinder at low Reynolds numbers is reported. The two‐dimensional Navier–Stokes equations with feedback are solved numerically. The control actuators are a pair of blowing/suction slots located at ±110° from the leading stagnation point. A single feedback sensor is used and the actuators are 180° out of phase with each other. Complete suppression of vortex shedding is achieved for the simulation at Reynolds number Re=60. The suppression window in the feedback sensor location xs is narrow. With the feedback sensor location fixed at the optimum location, vortex shedding becomes suppressed with increasing feedback gain α. However, further increase of the feedback gain destabilizes the flow again. At Reynolds number Re=80, and above, the feedback control stabilizes the primary vortex shedding mode, but a secondary mode which may be lower or higher in frequency than the primary depending upon the phase of the feedback...
158 citations
TL;DR: In this article, a spatial decomposition technique was developed to split the total flow into a primary flow component and a secondary flow component, where the primary flow is comprised of the mean flow and Strouhal vortices, while the secondary flow is the result of the three-dimensional streamwise vortice that are the essence of transition to turbulence.
Abstract: The fine scale three-dimensional structures usually associated with streamwise vortices in the near wake of a circular cylinder have been studied at Reynolds numbers ranging from 170 to 2200. Spatially continuous velocity measurements along lines parallel to the cylinder axis were obtained with a scanning laser anemometer. To detect the streamwise vortices in the amplitude modulated velocity field, it was necessary to develop a spatial decomposition technique to split the total flow into a primary flow component and a secondary flow component. The primary flow is comprised of the mean flow and Strouhal vortices, while the secondary flow is the result of the three-dimensional streamwise vortices that are the essence of transition to turbulence. The three-dimensional flow amplitude increases in the primary vortex formation region, then saturates shortly after the maximum amplitude in the primary flow is reached. In the near-wake region the wavelength decreases approximately like Re(exp -0.5), but increases with downstream distance. A discontinuous increase in wavelength occurs below Re = 300 suggesting a fundamental change in the character of the three-dimensional flow. At downstream distances (x/D = 10-20), the spanwise wavelength decreases from 1.42D to 1.03D as the Reynolds number increases from 300 to 1200.
144 citations
TL;DR: In this article, the authors show that coherent pressure fluctuations observed on delta wings are due to the helical mode instability of the vortex breakdown flow field, which implies increasing wavelength in the streamwise direction.
Abstract: Experiments show that coherent pressure fluctuations observed on delta wings are due to the helical mode instability of the vortex breakdown flowfield. No dominant frequency in the spectra of pressure fluctuations on the wing surface was observed after the breakdown reached the apex of the wing, although the vortex shedding could be detected in the wake. Measurements of pressure fluctuations at different streamwise locations on the wing suggest that the dimensionless frequency fx/U^ is nearly constant for a given geometry, which implies increasing wavelength in the streamwise direction. For different wings, this nondimensional frequency is shown to be a function of nondimensional circulation T/U^x only. Both the wavelength of the disturbances and the core radius increase with the nondimensional circulation at a fixed streamwise location. The wavelength normalized by the core radius is around 3-4, which is much smaller than the predictions for the Q vortex.
TL;DR: In this paper, the formation and growth of a hairpin vortex in a flat-plate boundary layer and its later development into a young turbulent spot were studied. But they did not consider the formation of hairpin heads.
Abstract: Direct numerical simulation was used to study the formation and growth of a hairpin vortex in a flat-plate boundary layer and its later development into a young turbulent spot. Fluid injection through a slit in the wall triggered the initial vortex. The legs of the vortex were stretched into a hairpin shape as it traveled downstream. Multiple hairpin vortex heads developed between the stretched legs. New vortices formed beneath the streamwise-elongated vortex legs. The continued development of additional vortices resulted in the formation of a traveling region of highly disturbed flow with an arrowhead shape similar to that of a turbulent spot.
01 Jan 1994
TL;DR: In this paper, critical point theory has been applied for the description and identification of eddying motions in turbulence and in vortex shedding, including both large scale and fine scale motions.
Abstract: Some applications of critical point theory are shown for the description and identification of eddying motions in turbulence and in vortex shedding. This includes both large scale and fine scale motions. Difficulties in the interpretations of flow topology are outlined with some examples.
TL;DR: In this paper, the lift hysteresis loops are estimated from the shed vorticity flux and the analytical foundation of the method and the various approximations are discussed.
Abstract: The flow field of an airfoil oscillated periodically over a reduced frequency range, 0 ≤ k ≤ 1.6, is studied experimentally at chord Reynolds numbers of Rc = 22000 and 44000. For most of the data, the NACA0012 airfoil is pitched sinusoidally about one quarter chord between angles of attack α of 5° and 25°. The cyclic variation of the near wake flow field is documented through flow visualization and phase-averaged vorticity measurements. In addition to the familiar dynamic stall vortex (DSV), an intense vortex of opposite sign is observed to originate from the trailing edge just when the DSV is shed. The two together take the shape of the cross-section of a large ‘mushroom’ while being convected away from the airfoil. The phase delay in the shedding of the DSV with increasing k, as observed by previous researchers, is documented for the full range of k. It is observed that the sum of the absolute values of all vorticity convected into the wake over a cycle is nearly constant and is independent of the reduced frequency and amplitude of oscillation but dependent on the mean α. The time varying component of the lift is estimated in a novel way from the shed vorticity flux. The analytical foundation of the method and the various approximations are discussed. The estimated lift hysteresis loops are found to be in reasonable agreement with available data from the literature as well as with limited force balance measurements. Comparison of the lift hysteresis loops with the corresponding vorticity fields clearly shows that major features of the lift variation are directly linked to the evolution of the large-scale vortical structures and the phase delay phenomenon.
TL;DR: In this article, the structure of the shear layer which emanates from the leading edge of a 76-deg sweep delta wing and forms the primary vortex is investigated numerically using a Beam-Warming-based algorithm.
Abstract: The structure of the shear layer which emanates from the leading edge of a 76-deg sweep delta wing and forms the primary vortex is investigated numerically. The flow conditions are Mv_ = 0.2, Re = 50,000 and angle of attack of 20.5 deg. Computational results are obtained using a Beam-Warming-t ype algorithm. The existence of a Kelvin-Helmholtz-type instability of the shear layer which emanates from the leading edge of the delta wing is demonstrated. A description is provided of the three-dimensional, unsteady behavior of the smallscale vortices associated with this instability. The numerical results are compared qualitatively with experimental flow visualizations exhibiting a similar behavior.
TL;DR: In this article, an analytical theory for the self-induced translation of an intense vortex relative to a uniform background flow on the β plane is presented, and the equivalent barotropic approximation is used to formulate the initial value problem within a polar coordinate frame translating with the vortex center.
Abstract: An analytical theory is presented for the self-induced translation of an intense vortex relative to a uniform background flow on the β plane. The equivalent barotropic approximation is used to formulate the initial value problem within a polar coordinate frame translating with the vortex center. A contour dynamical model of the vortex is melded with the regular beta-plane model of the residual flow. Evolution of vortex asymmetries for azimuthal mode number one, the so-called beta gyres, which are responsible for the relative vortex motion, is considered for a period of time while the Rossby wave radiation is not important. It is shown for an initially axisymmetric vortex that the beta gyres and corresponding vortex translational velocity consist of two parts. The first one is generated by advection of the background potential vorticity gradient and rotates differentially because of the symmetric vortex circulation. The second part arises due to distortion in the vortex shape represented by displa...
TL;DR: In this article, critical point theory has been applied for the description and identification of eddying motions in turbulence and in vortex shedding, including both large scale and fine scale motions.
Abstract: Some applications of critical point theory are shown for the description and identification of eddying motions in turbulence and in vortex shedding. This includes both large scale and fine scale motions. Difficulties in the interpretations of flow topology are outlined with some examples.
TL;DR: In this article, a numerical investigation of three-dimensional round jets subjected to streamwise and azimuthal perturbations is reported, and a comprehensive discussion of vortex induction mechanisms leading to the reorientation of the initial vorticity both in the ring and braid regions and to the deformation of the rings is presented.
Abstract: In this paper, a numerical investigation of three‐dimensional round jets subjected to streamwise and azimuthal perturbations is reported. The main objective of the study is to give a consistent scenario for the breaking of rotational symmetry in such flows which may ultimately lead to the production of intense side jets. In particular it is shown that the development of the Widnall instability on the primary vortex rings and the evolution of the Bernal and Roshko [J. Fluid Mech. 170, 499 (1986)] streamwise vortices generated by the instability of the braid could be deeply intertwined. A comprehensive discussion of the vortex induction mechanisms leading to the reorientation of the initial vorticity both in the ring and braid regions and to the deformation of the rings is presented. The recent analysis by Monkewitz and Pfizenmaier [Phys. Fluids A 3, 1356 (1991)] is confirmed in the sense that strong radial ejection of fluid is not directly linked to the deformation of the vortex rings but rather to the occurrence of coherent streamwise vortex pairs. However, the final relative position of the streamwise vortex pairs with respect to the deformations of the vortex rings differs slightly from Monkewitz and Pfizenmaier’s proposition.
TL;DR: In this article, the suppression of the fluid forces acting on a circular cylinder (hereafter called the main cylinder) by controlling the flow around it has been investigated by introducing a fine circular cylinder near the main cylinders.
Abstract: The objective of this paper is to investigate the suppression of the fluid forces acting on a circular cylinder (hereafter called the main cylinder) by controlling the flow around it. Flow control was established by introducing a fine circular cylinder (hereafter called the control cylinder) near the main cylinder. Measurements were carried out with variation of the position of the control cylinder in the ranges of G/d = 0.004 [approximately] 0.20 (G is the gap between main cylinder and control cylinder, d is diameter of main cylinder) and [alpha] = 0 [approximately] 180 deg ([alpha] is the angle along circumference from the front stagnation point of main cylinder) at a Reynolds number of 6.5 [times] 10[sup 4]. Subsequently, the steady and unsteady fluid forces, vortex shedding frequency and flow pattern were systematically examined. Furthermore, such matters as the mechanism of the flow control, the nature of the controlled wake, the relationship between the characteristics of the controlled fluid forces, and the behavior of the flow were discussed in detail on the basis of the obtained results regarding fluid forces, vortex shedding frequency and flow pattern.
TL;DR: In this article, the phase drift angle between two points at different spanwise positions was analyzed, showing that at the transition separation (Δz/d≊1) the probability distribution changed from being narrow banded to become more broad banded and closely Gaussian for an intermediate spacing of 2 <Δd/d<4, and almost uniform for Δz/D=6.
Abstract: The three‐dimensionality of the vortex shedding from a circular cylinder at high subcritical Reynolds number (4.3×104) has been studied, focusing on the characteristics of the vortex shedding phase drift and correlation along the cylinder span. Short time integration of the correlation coefficient, based on eight shedding cycles, showed that there were strong oscillations in the degree of correlation of the vortex shedding, and that these oscillations showed strong regularity having periods around 10–20 times the Strouhal period. These oscillations started when exceeding separations of Δz/d≊1 between the measurement points, and remained up to the largest case studied (Δz/d=6). The phase drift angle between two points at different spanwise positions was analyzed, showing that at the transition separation (Δz/d≊1) the probability distribution changed from being narrow banded to become more broad banded and closely Gaussian for an intermediate spacing of 2<Δz/d<4, and almost uniform for Δz/d=6. For all different separations studied (0.25<Δz/d<6), the probability distribution of the phase drift angle had a zero mean, indicating that the vortex shedding was, on the average, parallel to the cylinder.
TL;DR: In this paper, a method to control the spanwise end conditions and wake patterns using end suction was proposed. But the wake, at a given Reynolds number, is receptive to a continuous range of oblique shedding angles (θ), rather than to discrete angles, and there is excellent agreement with the cos θ formula for oblique-shedding frequencies.
Abstract: Recently there has been a surge of new interest in three-dimensional wake patterns In the present work, we have devised a method to control the spanwise end conditions and wake patterns using “end suction”, which is both continuously-variable and admits transient control Classical steady-state patterns, such as parallel or oblique shedding or the “chevron” patterns are simply induced The wake, at a given Reynolds number, is receptive to a continuous range of oblique shedding angles (θ), rather than to discrete angles, and there is excellent agreement with the “cos θ” formula for oblique-shedding frequencies We show that the laminar shedding regime exists up to Reynolds numbers (Re) of 205, and that the immense disparity among reported critical Re for wake transition (Re = 140–190) can be explained in terms of spanwise end contamination Our transient experiments have resulted in the discovery of new phenomena such as “phase shocks” and “phase expansions”, which can be explained in terms of a simple model assuming constant normal wavelength of the wake pattern Peter Monkewitz (Lausanne) also predicts such transient phenomena from a Guinzburg-Landau model for the wake
TL;DR: In this paper, the mean and fluctuating flow fields of a coaxial jet configuration, with ratios between inner and outer diameters and velocities, were studied by means of LDA and hot-wire measurements, using an experimental facility designed for the purpose, and profiles of the mean axial velocity, of the axial and radial turbulence intensities, and of the shear stress were described for the initial and intermediate zones of the near field.
TL;DR: In this article, a straight wing having an NACA 0015 cross section and rectangular planform was attached to a circular splitter plate, and the resulting vortex-dominated flow was characterized in detail using surface pressure measurements and flow visualization.
Abstract: : A straight wing having an NACA 0015 cross section and rectangular planform was attached to a circular splitter plate. This configuration was pitched at a constant rate to angles exceeding the static stall angel. The unsteady, vortex-dominated flow that developed over the wing and splitter plate was characterized in detail using surface pressure measurements and flow visualization. Both types of data showed that the leading-edge vortex underwent profound three-dimensional alterations to cross section and convection over the entire wing span. These changes in leading-edge vortex structure and kinematics were correlated with prominent spanwise variations in force coefficients. When appropriately dissected, visualization results and pressure data suggested physical mechanisms to account for these three-dimensional variations in unsteady forces and surface pressures.
TL;DR: In this article, the authors compute the forces that turn an ellipse settling in a vertical channel of viscous fluid in a regime in which the ellipso oscillates under the action of vortex shedding.
Abstract: We do a direct two-dimensional finite-elment simulation of the Navier–Stokes equations and compute the forces which turn an ellipse settling in a vertical channel of viscous fluid in a regime in which the ellipse oscillates under the action of vortex shedding. Turning this way and that is induced by large and unequal values of negative pressure at the rear separation points which are here identified with the two points on the back face where the shear stress vanishes. The main restoring mechanism which turns the broadside of the ellipse perpendicular to the fall is the high pressure at the ‘stagnation point’ on the front face, as in potential flow, which is here identified with the one point on the front face where the shear stress vanishes.
TL;DR: In this article, the formation of the streamwise vortices in the wake is discussed and it is emphasized that the stretching effect in the saddle region formed by the spanwise Vortices plays a major role in the development of the streams.
TL;DR: In this article, the Navier-Stokes equation for flow around a fixed-cylinder is numerically solved for Reynolds numbers up to 250, and the static rotational stability of the cylinder is analyzed by considering moments around the fixed cylinder at different angles to the flow.
TL;DR: In this article, a numerical analysis of the three-dimensional flow field generated by pitched and skewed air jets issuing into an otherwise undisturbed turbulent boundary layer is presented, and it is demonstrated that each such jet produces a single strong longitudinal vortex.
Abstract: Numerical calculations of the three-dimensional flowfield generated by pitched and skewed air jets issuing into an otherwise undisturbed turbulent boundary layer are presented. It is demonstrated that each such jet produces a single strong longitudinal vortex. The strength of the vortex, as inferred from its effect on the development of skin friction, is shown to be influenced by pitch and skew angles, exit velocity, and downstream distance in ways which accord with published experimental results. The calculated beneficial effect that the longitudinal vortices have on the development of skin friction in an adverse pressure gradient demonstrates the mechanism by which vortex generators delay boundary-layer separation. It follows that the numerical model could be used to optimize arrays of air-jet vortex generators. Furthermore, the facility to quantify the interaction between the vortex and the boundary layer should also be valuable in the application of vane vortex generators, and possible even more generally. 18 refs.
TL;DR: In this paper, a numerical solution of vortex-excited oscillations of a circular cylinder is described, at Reynolds numbers below 130, where the wake behind the cylinder is fully laminar.
TL;DR: In this article, the authors investigated the effect of asymmetric vortex shedding on the wake of two flat plates placed side by side normal to the flow in water at Re=1500 based on an individual plate width, where the spacing between the plates varied from one plate width to two plate widths.
Abstract: Unsteady and asymmetric vortex shedding was investigated experimentally in the wake of two flat plates placed side by side normal to the flow. The experiment was conducted in water at Re=1500 based on an individual plate width. The spacing between the plates was varied from one plate width to two plate widths. For a small spacing between the plates, the deflection of the jet passing through the gap resulted in a larger wake with mostly symmetric vortex shedding on one side and a smaller wake with asymmetric shedding on the other. The intermediate wake for the small plate spacing resembled that of a single bluff body, whereas a more intermittent structure was observed at the wider spacing. The velocity time traces were obtained by laser Doppler velocimetry (LDV) at several locations in the wake. The Fourier spectra of the velocity data at certain positions indicated the presence of more than one dominating frequency. Wavelet analysis mapped the intermittent near‐periodic structures, and quantified their mean energy spectrum in relation to the ensemble mean. These results are interpreted by comparison with flow visualization data.