Showing papers on "Vortex shedding published in 1996"

The structure and development of a wing-tip vortex

Abstract: Experiments have been performed on the tip vortex trailing from a rectangular NACA 0012 half-wing. Preliminary studies showed the vortex to be insensitive to the introduction of a probe and subject only to small wandering motions. Meaningful velocity measurements could therefore be made using hot-wire probes.Detailed analysis of the effects of wandering was performed to properly reveal the flow structure in the core region and to give confidence in measurements made outside the core. A theory has been developed to correct mean-velocity profiles for the effects of wandering and to provide complete quantitative estimates of its amplitude and contributions to Reynolds stress fields. Spectral decomposition was found to be the most effective method of separating these contributions from velocity fluctuations due to turbulence.Outside the core the flow structure is dominated by the remainder of the wing wake which winds into an ever-increasing spiral. There is no large region of axisymmetric turbulence surrounding the core and little sign of turbulence generated by the rotational motion of the vortex. Turbulence stress levels vary along the wake spiral in response to the varying rates of strain imposed by the vortex. Despite this complexity, the shape of the wake spiral and its turbulent structure reach an approximately self-similar form.On moving from the spiral wake to the core the overall level of velocity fluctuations greatly increases, but none of this increase is directly produced by turbulence. Velocity spectra measured at the vortex centre scale in a manner that implies that the core is laminar and that velocity fluctuations here are a consequence of inactive motion produced as the core is buffeted by turbulence in the surrounding spiral wake. Mean-velocity profiles through the core show evidence of a two-layered structure that dies away with distance downstream.

Control of laminar vortex shedding behind a circular cylinder using splitter plates

Abstract: Laminar vortex shedding behind a circular cylinder and its control using splitter plates attached to the cylinder are simulated. The vortex shedding behind a circular cylinder completely disappears when the length of the splitter plate is larger than a critical length, and this critical length is found to be proportional to the Reynolds number. The Strouhal number of the vortex shedding is rapidly decreasing with the increased plate length until the plate length (l) is nearly the same as the cylinder diameter (d). On the other hand, at 1

Low-Reynolds-number separation on an airfoil

Abstract: Unsteady boundary-layer separation from an Eppler 387 airfoil at low Reynolds number is studied numerically. Through a series of computations, the effects of Reynolds number and angle of attack are investigated. For all cases, vortex shedding is observed from the separated shear layer. From linear stability analysis, a KelvinHelmholtz instability is identified as causing shear layer unsteadiness. The low-turbulence wind-tunnel tests of the Eppler 387 airfoil are used to compare with the time-averaged results of the present unsteady computations. The favorable comparison between computational and experimental results strongly suggests that the unsteady largescale structure controls the low-Reynolds-number separation bubble reattachment with small-scale turbulence playing a secondary role. Nomenclature C = chord length CD - drag coefficient CL = lift coefficient Cp = pressure coefficient / = shedding frequency Re = chord Reynolds number R P - reattachment point S P = separation point Sr = Strouhal number U = velocity 9 = momentum thickness Subscripts sep = conditions at separation oo = freestream conditions

Tropical Cyclone Motion and Evolution in Vertical Shear

Abstract: The motion and the evolution of tropical cyclone-like vortices in an environmental flow with vertical shear are investigated using a baroclinic primitive equation model. The study focuses on the fundamental dynamics of a baroclinic vortex in vertical shear, the influence of vortex structure, and the role of diabatic heating. The results show that the initial response of the vortex to the vertical shear is to tilt downshear. As soon as the tilt develops, the upper-level anticyclonic and lower-level cyclonic circulations begin to interact with each other. As a result of these interactions, the tilted axis of the vortex reaches a stable state after an initial adjustment, which varies with the structure of the vortex, its environmental flow shear, and the cumulus convective heating. The motion of an adiabatic vortex in vertical shear is controlled by both the steering of the environmental flow and vertical coupling mechanisms. Most of the vortices move with the environmental flow at about 650 hPa or ...

Computation of quadrupole noise using acoustic analogy

Abstract: Acoustic analogy computations of vortex shedding noise were carried out in the context of a two-dimensional, low-Mach-number laminar flow past a NACA 0012 airfoil at chord Reynolds number of 10 4 . The incompressible Navier-Stokes equations were solved numerically to give an approximate description of the near-field flow dynamics and the acoustic source functions. The radiated far-field noise was computed based on Curle's extension to the Lighthill analogy. This study emphasizes an accurate evaluation of the Reynolds stress quadrupoles in the presence of an extensive wake. An effective method for separating the physical noise source from spurious boundary contributions caused by eddies crossing a permeable computational boundary is presented. The effect of retarded-time variations across the source region is also examined. Computational solutions confirm that the quadrupole noise is weak compared with the noise due to lift and drag dipoles when the freestream Mach number is small. The techniques developed in this study are equally applicable to flows in which the volume quadrupoles act as a prominent noise source.

Experimental and numerical investigations of dynamic stall on a pitching airfoil

Abstract: The dynamic stall process on a pitching NACA 0012 airfoil was investigated by two experimental techniques-particle image velocimetry (PIV) and laser-sheet visualizations-and a numerical code based on the Navier-Stokes equations. The freestream velocity was 28 m/s, leading to a Reynolds number (based on airfoil chord) of 3.73 X 10 5 . The airfoil motion was a sinusoidal function between 5 and 25 deg of incidence, with a frequency of 6.67 Hz corresponding to a reduced frequency (based on airfoil half-chord) of 0.15. The out-of-plane component of the vorticity could be derived from the PIV velocity fields. The comparison between experimental and numerical results was conducted for the four main phases of the dynamic stall process, i.e., attached flow, development of the dynamic stall vortex, poststall vortex shedding, and reattachment. In general, the computational results agreed very well with the experimental results. However, some discrepancies were observed and discussed. The cycle-to-cycle nonreproducibility of the flowfield during the phase of massive separation is also mentioned.

Simulations of the viscous flow normal to an impulsively started and uniformly accelerated flat plate

Abstract: The development of a two-dimensional viscous incompressible flow generated from an infinitesimally thin flat plate, impulsively started or uniformly accelerated normal to the free stream is studied computationally. An adaptive numerical scheme, based on vortex methods, is used to integrate the vorticity–velocity formulation of the Navier–Stokes equations. The results of the computations complement relevant experimental works while providing us with quantities such as the vorticity field and the unsteady forces experienced by the body. For the uniformly accelerated plate the present simulations capture the development of a number of centers of vorticity along the primary separating shear layer. This phenomenon has been observed in experimental works but has not been predicted by inviscid models. The present simulations suggest that this Kelvin–Helmholtz-type instability is driven by the interaction of primary and secondary vorticity near the tips of the plate and depends on the acceleration of the plate.

Recirculation zones of unconfined and confined annular swirling jets

Abstract: An experimental study is presented for flowfields behind an axially mounted cylindrical bluff body of annular swirling jet flows. Both the confined and the unconfined cases are examined. The controlled parameters included the Reynolds number Re and the swirl number S. Smoke streaks were used to observe the dynamic flow structures of the recirculation zone behind the bluff body. Laser-Doppler anemometry was used for velocity measurements at the jet exit and downstream of the annular jet flows. The results indicate that the recirculation zones for both the unconfined and the confined cases can be classified into seven typical flow patterns based on the Reynolds number and swirl number : stable flow, vortex shedding, transition, prepenetration, penetration, vortex breakdown, and attachment. The flow patterns and their domains in (Re, S) space for unconfined and confined cases were basically the same. In the attachment regime, the central recirculation zone for the confined case was larger than that for the unconfined case. The boundaries of the recirculation zone for various flow conditions were investigated. Scaling analysis was used to correlate the lengths of the recirculation zone with the swirl number.

Three-dimensional vortex structures in a cylinder wake

Abstract: The three-dimensionality of the velocity field in the wake of a circular cylinder has excited considerable interest and debate over the past decade. Presented here are experimental results that characterize the underlying vorticity field of such wakes. Using particle image velocimetry (PIV), instantaneous velocity fields were measured and from these the vorticity of the longitudinal vortices lying in the region between Karman vortices was found. Near the saddle point, induced by the stretching of the Karman vortices, the vorticity of the longitudinal vortices was found to be greater than the Karman vortices themselves. Their circulation was of the order of 10% of the Karman vortices. The high levels of vorticity result from the stretching of the longitudinal vortices, as evident in the topology of the vortices. It is shown that the longitudinal vortices are locked in phase to the Karman vortices, effectively riding on their backs in the braid region. While only one mode of longitudinal vortex formation was observed, evidence was found of a step change in the vorticity levels at a Reynolds number of approximately 200. This is consistent with the transition point between the two modes of vortex shedding shown to exist by Williamson (1988). It had previously been proposed that the observed vortex patterns were consistent with the evolution of the longitudinal vortices from perturbations of vortex lines in the separating shear layer which experience self-induction and stretching from the Karman vortices. Evidence is presented that supports this model.

The effect of free-stream turbulence on sectional lift forces on a circular cylinder

Abstract: Wind-tunnel experiments were conducted to examine the effect of grid-generated turbulence on lift forces at sections of a circular cylinder. Turbulence of longitudinal intensity between 0.6% and 18% was employed, with cylinder Reynolds numbers in the range 1 × 105 to 5 × 105. Addition of low-intensity turbulence had the primary effect of inducing the critical transition at Reynolds numbers below that for smooth flow; above transition there was little difference between the forces experienced by the cylinder in smooth or turbulent flow, with no sign of organized vortex shedding.At higher turbulence intensities effects consistent with a return to organized vortex shedding were observed, particularly for the highest intensity and at the upper end of the Reynolds number range; lift coefficients were greater than in smooth supercritical flow, with a broad spectral peak centred near a Strouhal number of 0.23 accompanied by an increase in spanwise correlation lengths of lift force.

Vortex-induced motions of a tethered sphere

Abstract: Despite the practical significance of studying the vibrations of a tethered sphere in a steady fluid flow, there are almost no laboratory investigations of such a problem, and it was previously unknown whether such a system would tend to oscillate or not The present work demonstrates that a tethered sphere will oscillate vigorously, at a peak-to-peak amplitude of about two diameters, over a large range of velocities The RMS amplitude of these oscillations is found to be independent of the tether length when plotted against the reduced velocity VR = U/fnD However, the saturation value of the RMS is found to vary with the mass ratio Also interesting is the observed oscillation frequency, at low mass ratios, which does not correspond to either the natural frequency nor the sphere vortex shedding frequency (in the absence of vibrations) The observed large amplitude oscillations also induce an increase in drag and tether angle of the order of 100% The above results suggest that oscillations are highly significant to predictions of sphere response in a steady flow, and should not be neglected

The instability and breakdown of tall columnar vortices in a quasi-geostrophic fluid

Abstract: We examine the linear stability of elliptical columns of uniform potential vorticity subject to two-dimensional (horizontal) straining within a rapidly rotating, stratified (quasi-geostrophic) fluid. We find that horizontal straining can promote the exponential growth of three-dimensional disturbances when the vortex height-to-width aspect ratio exceeds, qualitatively, three times the ratio of the Coriolis parameter to the buoyancy frequency. This instability is not related to the usual baroclinic instability which operates on shallow vortex columns whose potential vorticity changes sign with height. The nonlinear development of these instabilities is investigated numerically using a high-resolution contour surgery algorithm. Simulations are conducted for both a Boussinesq (ocean-like) fluid and a compressible (atmospheric-like) fluid having exponentially decreasing density with height. The simulations reveal a generic nonlinear development that results in a semi-ellipsoidal baroclinic vortex dome at the lower surface and, in the case of a Boussinesq fluid, another such dome at the upper surface.The related problem of two interacting vortex columns is also examined. A generic three-dimensional instability and nonlinear development occurs no matter how great the distance between the vortex columns, provided that they are sufficiently tall.Our results may bear upon the observed structure of many atmospheric and oceanic vortices, whose height-to-width aspect ratios are consistent with our findings. Remarkably, even strongly ageostrophic vortices, such as tropical cyclones, fit the pattern. Our results furthermore re-open questions about the long-time nature of freely decaying quasi-geostrophic turbulence, for which recent simulations indicate a progressive two-dimensionalization by vortex alignment, while earlier simulations have indicated long-lived baroclinic vortices, not unlike what we find here.

Parietal vortex shedding as a cause of instability for long solid propellant motors - Numerial simulations and comparisons with firing tests

Abstract: This paper describes a new mechanism leading to vortex shedding instabilities in long (large L/D) solid propellant motors. This mechanism is termed "parietal vortex shedding" and has been discovered thanks to numerical simulations of the unsteady, 2D, compressible, Navier-Stokes equations. It seems to involve hydrodynamic instabilities of the mean flow velocity profiles, corresponding to injection induced internal flow (so called Culick or Taylor profiles), that couple with the acoustic frequencies of the chamber. Although this mechanism is found to be very powerful, it seems to need some background noise to feed it. The presented simulations can explain observed instabilities in configurations, without segmentation or without protruding inhibitor rings, of a simplified subscale setup of the Ariane 5 MPS P230 solid propellant motor. Detailed comparisons are proposed and the influence of the propellant combustion response and of the turbulence of the flow field are analyzed by means of recently developed models. INTRODUCTION-OBJECTIVES not precisely known if inhibitor rings are or not destroyed or completely slack; on the other hand non segmented motors have also given rise to a same type of instability as vortex shedding : this is the case of one of the configuration (LP3D) of the LP3 set up presented in a preceding paper. As it is known ' ' , classical linear acoustic balance computations do not give reliable stability predictions in complex internal geometries (such as in the P230 motor), and then an effort was carried out to perform full numerical simulations of the unsteady, compressible internal flow fields. On the other hand, motor internal flows are mostly non-observable, and without numerical simulations it is not possible to describe the path of the aerodynamic instability development. The object of the present paper is to show hydrodynamic instabilities (which drive pressure oscillations) in configurations without shear flows induced by inhibitor rings, and to explain how these instabilities occur by means of numerical simulations. LP3D AND LP3E TEST CASES This work is part of the overall research effort, supported by CNES, accompanying the development of the Ariane 5 P230 MPS solid propellant motor (program ASSM for Aerodynamics of Segmented Solid Motors) and makes use of experimental results obtained during the combustion stability assessment program carried out for BPD and CNES, by delegation of ESA. In this scope, it is the continuation of earlier works about numerical simulations in solid propellant rocket motors ". The Ariane 5 motor, as other large segmented motors (U.S. Space Shuttle and Titan SRM) has been reported to exhibit pressure and thrust oscillations. Until recently, it was believed that such instability was exclusively due to the segmented design : inhibitor rings induce shear layers and vortex shedding driven oscillations. Nevertheless it is * Research scientist, Energetics Dept. 1 Project manager, Energetics Dept., Member AIAA Copyright © 1996 by the American Institute of Aeronautics and Astronautics, Inc., All rights reserved. The test cases are based on two configurations of the LP3 motor. The LP3 motor, which has been presented in reference , is a simplified 1/15 subscale set-up of the Ariane 5 P230 motor used to test several inter-segment arrangements. The configurations of interest here, called LP3D and LP3E, have no prominent obstacles at the mid chamber point, see figure 1. Both configurations have a cylindrical chamber of length 1632 mm and inner diameter 203 mm. At ignition, the main propellant burning surface is cylindrical, inner diameter 90 mm, with a chamfered surface near the aft end. The supersonic outlet nozzle is submerged, with a throat diameter of 56.5 mm, then the total length of the motor is 1650 mm. LP3D and LP3E have a small forward segment of 225 mm length, cylindrical for LP3D (169.5

Algebraic turbulence model simulations of supersonic open-cavity flow physics

Abstract: A time-accurate double thin-layer Navier-Stokes computation is performed for an unsteady supersonic open cavity with a length-to-depth ratio of 2 The results are used to determine the flow-physics mechanisms responsible for the cavity oscillation cycle A new cycle is described and compared to previous descriptions It is found that a shed vortex impinges on the cavity aft lip and forms a pressure pulse that augments or forces, at the vortex shedding frequency, an internal upstream moving wave that has been reflected from the aft corner This upstream moving wave eventually reflects off the cavity forward wall and forces the shedding of a new vortex It was found, however, that the reflected wave dissipates before it reaches the aft wall Instead, a second wave forms beneath the shed vortex and eventually reflects from the aft corner and is forced at the shedding frequency by the shed vortex wave, completing the cycle

Trailing-edge jet control of leading-edge vortices of a delta wing

Abstract: The effect of using a trailing-edge jet to control the leading-edge vortices of a delta wing is investigated experimentally in a water towing tank facility. The Reynolds number, based on the freestream velocity and the root chord, is 9.8 X 103. Both static and dynamic (pitching-up) conditions are tested. For the dynamic cases, the wing is pitched from 10- to 45-deg angle of attack with pitch rates varied from 0.043 to 0.26. From the dye flow visualization, it is shown that a downward vectored trailing-edge jet can significantly delay the vortex breakdown on a delta wing. Strong asymmetric breakdown of the leading-edge vortices can be induced by arranging the vectored jet in an asymmetric configuration. Transient pitching motion delays the onset of the vortex breakdown. The initial delay is independent of the pitch rate. Also, the use of jet control is found to be effective for the dynamic cases. During the initial pitching-up period, the use of jet control has a dominant influence on the propagation of the vortex breakdown. In general, with jet control, the propagation of the vortex breakdown slows down. From instantaneous particle image velocimetry measurements, a quasiperiodic variation of the leading-edge vorticity field is detected before the vortex breakdown. This variation appears to relate to the strong interaction between the separating shear layer, the secondary vortex, and the primary vortex. Along the vortex axis, the velocity distribution changes from a jet-type profile to a wake-type profile, signifying the onset of vortex breakdown.

Feedback control of vortex shedding from a circular cylinder

Abstract: This paper presents an experimental study on the suppressing of vortex shedding from a circular cylinder by feedback sound. Experiments were performed in a wind tunnel, and the feedback sound was generated inside the cylinder and locally introduced into the flow through a thin slit on the cylinder surface. In this way, the shear flow nearest to the slit was directly manipulated during the control. The results show that the suppression of vortex shedding can be achieved at Reynolds numbers ranging from 4×103 to 1.3× 104, according to signals from a hot-wire checking throughout the wake and signals from a remote microphone. This local and one-sided feedback, being different from other control techniques, allows a better understanding of the control mechanism, which in this case probably causes a destructive interaction between two shear flows separated from both sides of the cylinder. The technique has been useful to deepen our understanding to the wake instabilities behind the cylinder.

Numerical study of the blockage effects on viscous flow past a circular cylinder

Abstract: In various numerical solutions of flow around bluff bodies the unbounded physical domain is replaced by a restricted computational one whose extent depends on the size of the computational grid network. The truncation of the solution domain in the cross-flow direction reduces the computer time required for the solution, but introduces numerical blockage effects which influence considerably the values of the various flow parameters. In the present paper the finite element solution of steady and unsteady flow around a circular cylinder at Re = 106 is presented for blockage ratios of 0.05, 0.15 and 0.25. A boundary condition was tested for which the streamfunction values at the outer boundaries were those of the irrotational solution around a circular cylinder. The size of the standing vortices decreases with the blockage ratio when the flow is steady, while the spacing of the vortices decreases in both directions with increasing blockage ratio when the wake becomes unsteady. The hydrodynamic forces on the cylinder and the Strouhal number are magnified as the blockage ratio increases. The application of the streamfunction values derived from the irrotational solution at the outer boundaries reduced blockage effects only at high blockage ratio.

The instability of the separated shear layer from a bluff body

Abstract: It has generally been assumed in previous studies that the normalized shear‐layer frequency (fSL/fK) versus Reynolds number (Re) varies as Re0.5. There has also been a great deal of scatter in quoted values of the critical Reynolds number (Recrit) for the onset of the shear‐layer instability, with values ranging from 350 to 3000. In the present work, we find that Recrit is strongly sensitive to the mode of primary vortex shedding and may vary between 1200–2600 depending on the angle of oblique shedding. In the case of the shear‐layer frequency, we have conducted a careful reevaluation of all previous data in the literature, and find the relationship fSL/fK=0.0235×Re0.67. Surprisingly, not one of the previous studies yields a relationship that is close to Re0.5. We suggest an explanation for the variation Re0.67 based on simple physical grounds.

Systematic study of the correlation between geometrical disturbances and flow asymmetries

Abstract: Recently, large efforts were directed to numerical simulations of high-incidence flows around slender bodies of revolution to gain more insight into the problem of flow asymmetry. One of the significant advantages in using numerical simulation to investigate these flows is the ability to obtain a disturbance-free base solution. This base solution can then be perturbed for parametric studies of controlled disturbances. The experimentally observed asymmetric flow is numerically simulated by the introduction of a simulated disturbance (bump) placed near the body apex. A series of asymmetric flows over an ogive cylinder at a high angle of attack is computed by systematically varying the bump size. The results reproduce the experimentally observed effect of nose imperfections on flow asymmetry and are used to demonstrate the role of the sectional side force variation along the body. Analysis of the numerical results and the correlation between the disturbances and the asymmetric flow point to a convective instability mechanism as the origin of flow asymmetry.

Vortex shedding in steady oscillation of a flue organ pipe

Abstract: A simplified model for the prediction of the amplitude of the oscillation in a flue organ pipe is developed. The model is based on a flow visualisation study of the interaction of the jet with the labium in a small flue organ pipe with a recorder-like geometry. Experiments and theoretical analysis demonstrate that flow separation and subsequent vortex shedding at the edge of the labium is essential for the determination of the sound amplitude and spectral distribution. Further research should now investigate the validity of those conclusions for other flue instruments.