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Showing papers on "Starting vortex published in 1998"


Journal ArticleDOI
TL;DR: In this article, the authors used a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, and showed that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing-suction near the leading edge with low-level power input.
Abstract: By using a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, we show that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing–suction near the leading edge with low-level power input. This unsteady forcing can modulate the evolution of the separated shear layer to promote the formation of concentrated lifting vortices, which in turn interact with trailing-edge vortices in a favourable manner and thereby alter the global deep-stall flow field. In a certain range of post-stall angles of attack and forcing frequencies, the unforced random separated flow can become periodic or quasi-periodic, associated with a significant lift enhancement. This opens a promising possibility for flight beyond the static stall to a much higher angle of attack. The same local control also leads, in some situations, to a reduction of drag. On a part of the airfoil the pressure fluctuation is suppressed as well, which would be beneficial for high-α buffet control. The computations are in qualitative agreement with several recent post-stall flow control experiments. The physical mechanisms responsible for post-stall flow control, as observed from the numerical data, are explored in terms of nonlinear mode competition and resonance, as well as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing edge are two basic constituents of unsteady post-stall flow and its control. Since the former has a rich spectrum of response to various disturbances, in a quite wide range the natural frequency of both constituents can shift and lock-in to the forcing frequency or its harmonics. Thus, most of the separated flow becomes resonant, associated with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced, causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the unfavourable trailing-edge vortex is pushed downstream. The wake pattern also has a corresponding change: the shed vortices of alternate signs tend to be aligned, forming a train of close vortex couples with stronger downwash, rather than a Karman street.

327 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigate the three-dimensional instability of a counter-rotating vortex pair to short waves, which are of the order of the vortex core size, and less than the inter-vortex spacing.
Abstract: In this paper, we investigate the three-dimensional instability of a counter-rotating vortex pair to short waves, which are of the order of the vortex core size, and less than the inter-vortex spacing. Our experiments involve detailed visualizations and velocimetry to reveal the spatial structure of the instability for a vortex pair, which is generated underwater by two rotating plates. We discover, in this work, a symmetry-breaking phase relationship between the two vortices, which we show to be consistent with a kinematic matching condition for the disturbances evolving on each vortex. In this sense, the instabilities in each vortex evolve in a coupled, or ‘cooperative’, manner. Further results demonstrate that this instability is a manifestation of an elliptic instability of the vortex cores, which is here identified clearly for the first time in a real open flow. We establish a relationship between elliptic instability and other theoretical instability studies involving Kelvin modes. In particular, we note that the perturbation shape near the vortex centres is unaffected by the finite size of the cores. We find that the long-term evolution of the flow involves the inception of secondary transverse vortex pairs, which develop near the leading stagnation point of the pair. The interaction of these short-wavelength structures with the long-wavelength Crow instability is studied, and we observe significant modifications in the longevity of large vortical structures.

299 citations


Journal Article
TL;DR: In this paper, the authors focus on the mechanisms for the dynamical generation and evolution of vortical structures in the flow field, in particular the counter-rotating vortex pair associated with the jet cross-section.
Abstract: Among the important physical phenomena associated with the jet in crossflow is the formation and evolution of vortical structures in the flow field, in particular the counter-rotating vortex pair (CVP) associated with the jet cross-section. The present computational study focuses on the mechanisms for the dynamical generation and evolution of these vortical structures. Transient numerical simulations of the flow field are performed using three-dimensional vortex elements. Vortex ring rollup, interactions, tilting, and folding are observed in the near field, consistent with the ideas described in the experimental work of Kelso, Lim & Perry (1996), for example. The time-averaged effect of these jet shear layer vortices, even over a single period of their evolution, is seen to result in initiation of the CVP. Further insight into the topology of the flow field, the formation of wake vortices, the entrainment of crossflow, and the effect of upstream boundary layer thickness is also provided in this study.

241 citations


Proceedings ArticleDOI
12 Jan 1998
TL;DR: In this paper, the control of separated flow on an unconventional airfoil using synthetic jet actuators was investigated experimentally, and the effect of control location and amplitude was investigated for different angles of attack.
Abstract: The control of separated flow on an unconventional airfoil using synthetic jet actuators was investigated experimentally. A symmetric airfoil based on the aft portion of a NACA four-digit series airfoil with a cylindrical leading edge was used in the experiment. The tests were conducted at Rec=3(10)5. For a>5°, the flow separated from the airfoil surface. Applying synthetic jet control near the leading edge, upstream of the separation point, reattached the separated flow fixangle of attack up to 18°. The effect of control location and amplitude was investigated for different angles of attack. Hot wire measurements in the nearwake of the airfoil revealed a transient passing of vortices associated with the transition from separated to reattached flow on the airfoil.

238 citations


Journal ArticleDOI
TL;DR: The formation time scale of axisymmetric vortex rings is studied numerically for relatively long discharge times in this paper, and it is shown that the scaled circulation of the pinched off vortex is relatively insensitive to the details of the formation process, such as the velocity programme, velocity profile, vortex generator geometry and the Reynolds number.
Abstract: The formation time scale of axisymmetric vortex rings is studied numerically for relatively long discharge times. Experimental findings on the existence and universality of a formation time scale, referred to as the `formation number', are confirmed. The formation number is indicative of the time at which a vortex ring acquires its maximal circulation. For vortex rings generated by impulsive motion of a piston, the formation number was found to be approximately four, in very good agreement with experimental results. Numerical extensions of the experimental study to other cases, including cases with thick shear layers, show that the scaled circulation of the pinched-off vortex is relatively insensitive to the details of the formation process, such as the velocity programme, velocity profile, vortex generator geometry and the Reynolds number. This finding might also indicate that the properly scaled circulation of steady vortex rings varies very little. The formation number does depend on the velocity profile. Non-impulsive velocity programmes slightly increase the formation number, while non-uniform velocity profiles may decrease it significantly. In the case of a parabolic velocity profile of the discharged flow, for example, the formation number decreases by a factor as large as four. These findings indicate that a major source of the experimentally found small variations in the formation number is the different evolution of the velocity profile of the discharged flow.

188 citations


10 Apr 1998
TL;DR: The formation of vortex rings generated through impulsively started jets is studied experimentally in this article, where the velocity and vorticity field of the leading vortex ring formed is disconnected from that of the trailing jet.
Abstract: The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using digital particle image velocimetry (DPIV) for a wide range of piston stroke to diameter (L/D) ratios. The results indicate that the flow field generated by large L/D consists of a leading vortex ring followed by a trailing jet. The vorticity field of the leading vortex ring formed is disconnected from that of the trailing jet. On the other hand, flow fields generated by small stroke ratios show only a single vortex ring. The transition between these two distinct states is observed to occur at a stroke ratio of approximately 4, which, in this paper, is referred to as the ‘formation number’. In all cases, the maximum circulation that a vortex ring can attain during its formation is reached at this non-dimensional time or formation number. The universality of this number was tested by generating vortex rings with different jet exit diameters and boundaries, as well as with various non-impulsive piston velocities. It is shown that the ‘formation number’ lies in the range of 3.6–4.5 for a broad range of flow conditions. An explanation is provided for the existence of the formation number based on the Kelvin–Benjamin variational principle for steady axis-touching vortex rings. It is shown that based on the measured impulse, circulation and energy of the observed vortex rings, the Kelvin–Benjamin principle correctly predicts the range of observed formation numbers.

157 citations


Journal ArticleDOI
TL;DR: In this paper, the axial and radial velocities in a meridional plane for non-wavy and wavy Taylor-Couette flow in the annulus between a rotating inner cylinder and a fixed outer cylinder with fixed end conditions were measured.
Abstract: The stability of supercritical Couette flow has been studied extensively, but few measurements of the velocity field of flow have been made. Particle image velocimetry (PIV) was used to measure the axial and radial velocities in a meridional plane for non-wavy and wavy Taylor–Couette flow in the annulus between a rotating inner cylinder and a fixed outer cylinder with fixed end conditions. The experimental results for the Taylor vortex flow indicate that as the inner cylinder Reynolds number increases, the vortices become stronger and the outflow between pairs of vortices becomes increasingly jet-like. Wavy vortex flow is characterized by azimuthally wavy deformation of the vortices both axially and radially. The axial motion of the vortex centres decreases monotonically with increasing Reynolds number, but the radial motion of the vortex centres has a maximum at a moderate Reynolds number above that required for transition. Significant transfer of fluid between neighbouring vortices occurs in a cyclic fashion at certain points along an azimuthal wave, so that while one vortex grows in size, the two adjacent vortices become smaller, and vice versa. At other points in the azimuthal wave, there is an azimuthally local net axial flow in which fluid winds around the vortices with a sense corresponding to the axial deformation of the wavy vortex tube. These measurements also confirm that the shift-and-reflect symmetry used in computational studies of wavy vortex flow is a valid approach.

152 citations


Proceedings ArticleDOI
12 Jan 1998
TL;DR: In this article, numerical simulations of active flow control applied to an airfoil using the Reynolds-averaged Navier-Stokes equations are presented, and two flow-control techniques for a NACA0012 airfoin at a chord Reynolds number of 8.5 x 10 6 are investigated.
Abstract: Results of numerical simulations of active flow control applied to an airfoil using the Reynoldsaveraged Navier-Stokes equations are presented. The simulations are first compared with the poststall separation control experiments of Seifert et al.1'12 on a NACA0015 at 1.2 x 106 chord Reynolds number. The jet is introduced tangential to the surface at the leading edge of the airfoil. The calculated lift increments are in good agreement with the experimental data. Two flow-control techniques for a NACA0012 airfoil at a chord Reynolds number of 8.5 x 10 6 are investigated. The first technique utilizes a small, 0.5% chord, steady jet, and the second method employs a synthetic jet of a similar scale. Performance benefits are obtained by placing the actuators very near the airfoil leading edge on the suction surface. A significant increase in lift (29%) is obtained using the synthetic jet actuator in the post-stall regime. At lower lift, the steady jet actuator significantly reduces drag by rotating the lift vector upstream.

146 citations


Journal ArticleDOI
TL;DR: In this article, the authors describe the vorticity dynamics that drive axial flow to smaller-scale, increasingly isotropic motions at later times, following the initial formation of discrete and intertwined vortex loops and the mutual interactions of multiple vortex tubes in close proximity.
Abstract: A companion paper (Part 1) employed a three-dimensional numerical simulation to examine the vorticity dynamics of the initial instabilities of a breaking internal gravity wave in a stratified, sheared, compressible fluid. The present paper describes the vorticity dynamics that drive this flow to smaller-scale, increasingly isotropic motions at later times. Following the initial formation of discrete and intertwined vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of multiple vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets gives the vortex tubes axial variations with both axisymmetric and azimuthal-wavenumber-2 components. The axisymmetric variations excite axisymmetric twist waves or Kelvin vortex waves which propagate along the tubes, drive axial flows, deplete the tubes' cores, and fragment the tubes. The azimuthal-wavenumber-2 variations excite m = 2 twist waves on the vortex tubes, which undergo strong amplification and unravel single vortex tubes into pairs of intertwined helical tubes; the vortex tubes then burst or fragment. Reconnection often occurs among the remnants of such vortex fragmentation. A common mutual interaction is that of orthogonal vortex tubes, which causes mutual stretching and leads to long-lived structures. Such an interaction also sometimes creates an m = 1 twist wave having an approximately steady helical form as well as a preferred sense of helicity. Interactions among parallel vortex tubes are less common, but include vortex pairing. Finally, the intensification and roll-up of weaker vortex sheets into new tubes occurs throughout the evolution. All of these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

92 citations


Patent
05 Jun 1998
TL;DR: An active control method and apparatus for encouraging the early destruction of trailing vortices formed by a lifting body having at least two control surfaces on each side and creating multiple vortex pairs is provided in this article.
Abstract: An active control method and apparatus for encouraging the early destruction of trailing vortices formed by a lifting body having at least two control surfaces (68, 70) on each side and creating multiple vortex pairs is provided. The method includes actuating the control surfaces in a manner leading to the direct excitation of one or more of the Transient Growth Mechanism, the Short Wavelength Instability, the Long Wavelength Instability, and the Crow Instability to result in the accelerated breakup of the trailing vortices. A method of selecting control parameters in an active control system includes identifying the near-field vorticity distribution of the aircraft, selecting at least one amplification mechanism for distorting the trailing vortices, determining amplitudes and wavelengths for imposed vortex perturbations, simulating trailing vortex evolutions, determining effectiveness of perturbations and amplification mechanisms, determining vortex positions as a function of control surface deflections, and determining control parameters to govern the movement of the wing control surfaces.

56 citations


Journal ArticleDOI
TL;DR: In this paper, a new simple explanation of the Ranque effect of energy separation in the vortex tube is proposed, which takes place due to radial motion of turbulent micro-volumes with differing tangential velocities in the strong centrifugal field.
Abstract: The efficiency of thermal insulation of a microwave-generated plasma using reverse vortex flow was investigated experimentally and by numerical simulations. Comparison with the conventional vortex method of plasma insulation was made. Changing the location of the vortex inlet to the exit end of the plasma torch leads to a significant decrease of the heat loss to the wall; from 30% to 5%. This result contradicts the traditional explanation of the Ranque effect. A new simple explanation of the Ranque effect of energy separation in the vortex tube is proposed. Energy separation takes place due to radial motion of turbulent micro-volumes with differing tangential velocities in the strong centrifugal field. The new model of energy separation explains such apparently mysterious phenomena as the counter-rotation of the central vortex flow layers observed in some experiments and in numerical simulations. A new approach for consideration of the confined vortex flows is described.

Journal ArticleDOI
TL;DR: In this article, the experimental and numerical study of the vortex formation during the passage of a solitary wave over a rectangular structure submerged in water is described, and the transient patterns of the separated flow (R= 82,000) behind the block are visualized by tracing the motion of injected dye in a laboratory flume.
Abstract: This paper describes the experimental and numerical study of the vortex formation during the passage of a solitary wave over a rectangular structure submerged in water. The transient patterns of the separated flow (R= 82,000) behind the block are visualized by tracing the motion of injected dye in a laboratory flume. The related numerical solution is calculated by using the stream function-vorticity formulation in a transient boundary-conformed grid system to satisfy the full nonlinear free-surface conditions. Both experimental and numerical results illustrate that a large vortex is first formed at the rear of the block, accompanied by a secondary eddy below it, as the solitary wave hits this block. Then the vortex is bulgy but decays with time, and the secondary eddy continues to grow in both size and strength as the primary vortex diffuses. The time development of these calculated flow patterns, including vortex shedding and vorticity transportation, agrees quite well with experimental observations.

Journal ArticleDOI
TL;DR: In this article, the effect of the angle of attack, the Reynolds number, and the hydrofoil planform on the tip vortex was investigated, and a good agreement in pressure distribution and oil flow pattern was achieved between the numerical solution and available experimental data.
Abstract: The flow over a finite-span hydrofoil creating a tip vortex was numerically studied by computing the full Navier-Stokes equations. A good agreement in pressure distribution and oil flow pattern was achieved between the numerical solution and available experimental data. The steady-state roll-up process of the tip vortex was described in detail from the numerical results. The effect of the angle of attack, the Reynolds number, and the hydrofoil planform on the tip vortex was investigated. The axial and tangential velocities within the tip-vortex core in the near-field wake region were greatly influenced by the angle of attack. A jet-like profile in the axial velocity was found within the tip-vortex core at high angle of attack, while a wake-like profile in the axial velocity was found at low angle of attack. Increasing the Reynolds number was found to increase the maximum axial velocity, but only had a slight impact on the tangential velocity. Finally, a swept hydrofoil planform was found to attenuate the strength of the tip vortex due to the low-momentum boundary layer traveling into the tip vortex on the suction side

Journal ArticleDOI
TL;DR: In this article, a typical experimental vortex generator was perturbed by inclining the exit orifice, leading to an initial vortex roll-up with maximum and minimum circulation at the shortest and longest cylinder locations, respectively.
Abstract: A typical experimental vortex generator was perturbed by inclining the exit orifice. Instantaneous velocity fields were measured with particle image velocimetry at a Reynolds number, Γ0/ν, of 2800, which falls in the laminar regime for the axisymmetric case. Despite the nearly uniform velocity of the axisymmetric piston, the velocity exiting the cylinder is spatially and temporally non-uniform. Specifically, the exit velocity and the entrainment are larger on the short cylinder side. This fluid motion leads to an initial vortex roll-up with maximum and minimum circulation at the shortest and longest cylinder locations, respectively. A highly complex vortex structure forms, consisting of a primary vortex ring with varying circulation and branched vortex tubes that initially extend from the primary ring upstream toward the cylinder. The variation of the circulation in the primary ring and the strength of the branched vortex tubes increase with incline angle. The branched vortex tubes induce a strong cross-stream sweep of fluid toward the long cylinder side. The branched tubes convect across the cylinder exit with the sweep, break free of the cylinder, and pass through the primary ring. Beyond this time, the vortex structure consists of two closed-loop branches connected on the short cylinder side. As the flow progresses, the center of momentum moves toward the short cylinder side. As the cylinder incline angle is increased, the migration away from the centerline increases, and the flow becomes increasingly disorganized. The propagation speed and penetration distance are reduced because of the loss of coherent circulation. Qualitatively similar velocity fields and flow visualization photographs are presented for a larger (nominally turbulent) Reynolds number of 23000.

Journal ArticleDOI
TL;DR: In this paper, a study of three-dimensional interaction between a blade and an initially columnar vortex was performed using flow visualization experiments and computer simulations for problems where the blade is translated normal to and impacts upon the vortex core.
Abstract: A study of three-dimensional interaction between a blade and an initially columnar vortex was performed using flow visualization experiments and computer simulations for problems where the blade is translated normal to and impacts upon the vortex core. The reported research focuses on cases where the vortex is sufficiently strong that the blade boundary layer separates before the blade penetrates into the vortex core. The study examines the topology and evolution of the separated secondary vorticity from the blade and its interaction with the primary vortex. In particular, it is observed that the separated vorticity from the blade has the form of a series of vortex loops that wrap around the outer surface of the primary vortex core and spread both upstream and downstream along the vortex axis. Both the initial formation of these vortex loops and their eventual interaction with the primary vortex are investigated. Computations based on a series of idealized models of the secondary vorticity are performed in order to examine the primary vortex response to wrapped vortex loops.

Patent
05 Jun 1998
TL;DR: In this article, a method for inhibiting dynamic stall of an airfoil by causing a fluid to flow out of at least one location on the air-foil was proposed.
Abstract: A method for inhibiting dynamic stall of an airfoil by causing a fluid to flow out of at least one location on the airfoil. This location may be anywhere on the airfoil; but if the location is within one-quarter of the airfoil chord from the leading edge and the fluid flow has non-zero net mass flux, then the fluid flow is modulated at a frequency described by a Strouhal ratio greater than one.

Journal ArticleDOI
TL;DR: In this paper, a dye visualization study of a triangular vortex in a rotating fluid is presented, where the emergence and subsequent break-up of the vortex structure are described, and a simple point-vortex model is used to simulate this breakup.

Journal ArticleDOI
TL;DR: In this paper, a laser-Doppler velocimetry was employed to experimentally investigate two-dimensional airfoil-vortex interaction, and the most violent interaction occurs when the disturbing vortex passes very near over the loaded blade.

Journal ArticleDOI
TL;DR: In this paper, a low-speed wind tunnel was used to obtain information on the distribution of phase-locked velocity and vorticity components across the vortex, as well as the phaselocked circulation associated with the evolving vortex, and the data indicated that the average trajectory of the oscillating vortex in the near e eld is very nearly the same as for a stationary wing at the mean incidence.
Abstract: were performed in a low-speed wind tunnel at a Reynolds number of 1 :8 £ 10 5 . The wing had a semiaspect ratio of 2. It was oscillated at a frequency of 1 Hz and an amplitude of 5 deg around a mean incidence of 10 deg. The instantaneous LDV data were used to obtain information on the distribution of phase-locked velocity and vorticity components across the vortex, as well as the phase-locked circulation associated with the evolving vortex. The data indicate that the average trajectory of the oscillating vortex in the near e eld is very nearly the same as for a stationary wing at the mean incidence. The length and circulation scales, as well as the maximum circulation carried by the vortex e ow under the conditions studied, are modulated in a signie cantly nonquasisteady manner. Nevertheless, beyond about 0.7 chordlength, the normalized circulation distribution across most of the inner part of the vortex exhibits, at all times, the same universal behavior as the vortex behind a stationary wing.

Dissertation
01 Oct 1998
TL;DR: In this article, an experimental parametric sturdy of vane and air-jet vortex generators in a turbulent boundary layer has been carried out, where cross-stream data were measured at a number of downstream locations using a miniature five-hole pressure probe, such that local cross stream velocity vectors could be derived.
Abstract: An experimental parametric sturdy of vane and air-jet vortex generators in a turbulent boundary layer has been carried out. Experiments were carried out in two facilities, one with a free-stream velocity of 20 m/s and a boundary layer thickness (6) of 41.5 mm, and one in a high speed facility at free-stream Mach numbers of between 0.45 and 0.75 and a boundary layer thickness of 20 mm. Cross-stream data were measured at a number of downstream locations using a miniature five-hole pressure probe, such that local cross-stream velocity vectors could be derived. Streamwise vorticity was calculated using the velocity vector data. In the low speed study, vortex generator parameters were as follows: " Vane vortex generators: thin rectangular vanes with a vane aspect ratio of unity (2h/c = 1), free-stream velocity 20 m/s, incidence (cc = 10', 15', 18', 20'), height-to-boundary- layer- thickness-ratio (h/8 0.554,0.916,1.27,1.639), and strearnwise distance from the vortex generator (x/6 = 3.855,12.048,19.277,26.506). " Air-jet vortex generators: circular jet nozzles, free-stream velocity = 20 m/s, jet nozzle pitch and skew angles (cc, P= 30', 45', 60'), hole diameter-to-boundary-layer-thickness-ratio (D/5 = 0.098,0.193,0.289), jet-to-free-stream-velocity ratio (VR = 0.7,1.0,1.3,1.6,2.0), and strearnwise distance from the vortex generator (x/8 = 3.855,12.048,19.277,26.506). In the high-speed study, the vortex generator parameters were as follows: Vane vortex generators: thin rectangular vanes with an aspect ratio of unity, incidence ((X 1505 20'), he i ght-to- boundary- I ayer-th i ckne s s-rati o (h/8 = 0.75), strearnwise distance from the vortex generator (x/6 = 8.755 16.25,23.75), and free-stream Mach numbers of 0.45,0.6 and 0.75. Air-jet vortex generators: jet pitch ((x = 30', 45'), jet skew angle (P = 30', 45', 60'), hole diameter-to-boundary-layer-thickness-ratio (D/8 = 0.15,0.3), j et-to- free- strearn-ve loc ity ratio (VR = 1.6), and strearnwise distance from the vortex generator (x/6 = 8.75,16.25,23.75, 31.25), and free-stream Mach numbers of 0.50,0.6 and 0.75. Streamwise vorticity data from the experiment was used to generate prediction techniques that would allow the vorticity profiles, downstream of vane or air-jet vortex generators, to be predicted. Both techniques are based on the approximation of the experimental cross-stream vorticity data to Gaussian distributions of vorticity through the vortex centre. The techniques, which are empirically derived, are simple equations that give the peak vorticity and vortex radius based on the vortex generator parameters. Use of these descriptors allows the assembly of the Gaussian vorticity equation. Both techniques are compared with the experimental data set and were seen to produce peak vorticity results to within 12% and 20% (for the vanes and air-jets respectively), 15% for the radius of the vortex, and 15% and 20% in vortex circulation (for the vanes and air-jets respectively). The two simple prediction techniques allow good prediction…

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a self-noise mechanism from a fan modeled as a linear cascade of semi-inevegetable at plates, and compared it to a single airfoil without adjacent blade scattering.
Abstract: Self-noise from fans and rotors is generated by blade boundary-layer turbulence interacting with the trailing edges of the blades. Previous theories describing this mechanism have considered only an isolated airfoil and have shown that acoustic scattering from the sharp trailing edge is responsible for the sound that propagates to the far e eld. In aeroengine applications, fans or stators have relatively high solidity, and there will be acoustic scattering from adjacent blades as well as the blade locally excited by the e ow. The theory is given for self-noise from a fan modeled as a linear cascade of semi-ine nite e at plates. The magnitude of the self-noise mechanism is compared with the e eld that would be generated by a single airfoil without adjacent blade scattering.

Journal ArticleDOI
TL;DR: In this article, sound propagation in the bulk of a homogeneous flow made of a single intense vortex in air is studied using an adaptive-coherent average technique, and the mean amplitude and phase of the distorted wave fronts are reconstructed.
Abstract: We report an experimental study of sound propagation in the bulk of a homogeneous flow made of a single intense vortex in air. By using an adaptive-coherent average technique, the mean amplitude and phase of the distorted wave fronts are reconstructed. A large phase shift is observed on the wave fronts on either side of the ``shadow'' region behind the vortex. The shape of the averaged amplitude, as a function of space coordinates, shows that caustics are also formed in the acoustic field. In a nonstationary flow, these effects make it possible to monitor the vortex strength and position.

Journal ArticleDOI
TL;DR: In this paper, a supersonic streamwise vortex is brought about when moderate and strong streamwise vortices encounter the bow shock in front of a circular cylinder at Mach 2.49 and the main features of the vortex/cylinder interaction are the formation of a blunt-nosed conical shock with apex far upstream of the undisturbed shock stand-off distance, and a vortex core which responds to passage through the apex of the conical cascade by expanding into a turbulent conical flow structure.
Abstract: The head-one interaction of a supersonic streamwise vortex with a circular cylinder reveals a vortex breakdown similar in many ways to that of incompressible vortex breakdown. In particular, the dramatic flow reorganization observed during the interaction resembles the conical vortex breakdown reported by Sarpkaya (1995) at high Reynolds number. In the present study, vortex breakdown is brought about when moderate and strong streamwise vortices encounter the bow shock in front of a circular cylinder at Mach 2.49. The main features of the vortex/cylinder interaction are the formation of a blunt-nosed conical shock with apex far upstream of the undisturbed shock stand-off distance, and a vortex core which responds to passage through the apex of the conical shock by expanding into a turbulent conical flow structure. The geometry of the expanding vortex core as well as the location of the conical shock apex are seen to be strong functions of the incoming vortex strength and the cylinder diameter. A salient feature of the supersonic vortex breakdown is the formation of an entropy-shear layer, which separates an interior subsonic zone containing the burst vortex from the surrounding supersonic flow. In keeping with the well-established characteristics of the low-speed vortex breakdown, a region of reversed flow is observed inside the turbulent subsonic zone. The steady vortex/cylinder interaction flow fields generated in the current study exhibit many characteristics of the unsteady vortex distortion patterns previously observed during normal shock wave/vortex interactions. This similarity of the instantaneous flow structure indicates that the phenomenon previously called vortex distortion by Kalkhoran et al. (1996) is a form of supersonic vortex breakdown.

Journal ArticleDOI
TL;DR: In this article, the dynamic behavior of shedding vortices in turbulent wake behind a tapered circular cylinder is experimentally studied by hot-wire measurements in a low-speed wind tunnel.

Journal ArticleDOI
TL;DR: The initial value problem for the motion of an intense, quasi-geostrophic, equivalent-barotropic, singular vortex near an infinitely long escarpment is studied in this paper.
Abstract: The initial value problem for the motion of an intense, quasi-geostrophic, equivalent-barotropic, singular vortex near an infinitely long escarpment is studied in three parts. First, for times small compared to the topographic wave timescale the motion of the vortex is analysed by deriving an expression for the secondary circulation caused by the advection of fluid columns across the escarpment. The secondary circulation, in turn, advects the primary vortex and integral expressions are found for its velocity components. Analytical expressions in terms of integrals are found for the vortex drift velocity components. It is found that, initially, cyclones propagate away from the deep water region and anticyclones propagate away from the shallow water region. Asymptotic evaluation of the integrals shows that both cyclones and anticyclones eventually propagate parallel to the escarpment with shallow water on their right at a steady speed which decays exponentially with distance from the escarpment. Secondly, it is shown that for times comparable to, and larger than, the wave timescale, the vortex always resonates with the topographic wave field. The flux of energy in the topographic waves leads to a loss of energy in the vortex and global energy and momentum arguments are used to derive an equation for the distance (or, equivalently, the vortex velocity) of the vortex from the escarpment. It is shown that cyclones, provided they are initially within an O(1) distance (here a unit of distance is dimensionally equivalent to one Rossby radius of deformation) from the escarpment, drift further away from the deep water (i.e. toward higher ambient potential vorticity), possibly crossing the escarpment and accumulate at a distance of approximate to 1.2 on the shallow side of the escarpment. For distances larger than 1.2 there is essentially no drift of the vortex perpendicular to the escarpment. Anticyclones display similar behaviour except they drift in the opposite direction, i.e. away from the shallow water or toward lower ambient potential vorticity. Third, the method of contour dynamics is used to describe the evolution of the vortex and the interface representing the initial potential vorticity jump between the shallow and deep water regions. The contour dynamic results are in good quantitative agreement with the analytical results.

Proceedings ArticleDOI
12 Jan 1998
TL;DR: In this paper, a delta wing with a 60° sweep angle at a Reynolds number of 260,000 and angles of attack varying from 12 to 20° was used to demonstrate the effect of the jet blowing on the vortex core.
Abstract: Injection of jets from nozzles embedded in a delta wing were used to add momentum into the core of the leading edge vortices and thus delay their breakdown. The tests were performed on a delta wing with a 60° sweep angle at a Reynolds number of 260,000 and angles of attack varying from 12 to 20°. At a typical angle of attack of 15°, the vortex breakdown location was moved from its natural location at 55% of the wing chord to 90% near the trailing edge, using a blowing coefficient of Cu = 0.007. Though optimized for a = 15°, the jet blowing was effective in the entire range of angles of attack studied. Flow visualizations, which were performed by illuminating smoke with a laser sheet, showed the effect of the jet blowing on the vortex core and were used to determine the vortex trajectory and breakdown location. The mean axial velocity and turbulence intensity were mapped across the vortex at several axial locations using Laser Doppler Velocimetry. Vortex breakdown resulted in strong deceleration of the mean velocity in the core region, the appearance of negative flow and increased turbulence intensity. Fluctuations in the vortex breakdown location at a Strouhal number of 0.13 were recorded. The control jet injection accelerated the core velocity and stabilized the vortex as evidenced by a reduction in the turbulence intensity and a substantial shift of the vortex breakdown location towards the trailing edge. Nomenclature

Patent
Wendell G. Huotari1
13 Mar 1998
TL;DR: In this paper, a multiple flow metering system includes first and second vortex flowmeters for measuring a flow of a fluid, which includes a first vortex flowmeter including a first bluff body having a leading edge and a shape for inducing a first stream of vortices in the fluid as a function of the flow at a wavelength λ.
Abstract: A multiple flow metering system includes first and second vortex flowmeters for measuring a flow of a fluid. The first vortex flowmeter includes a first bluff body having a leading edge and a shape for inducing a first stream of vortices in the fluid as a function of the flow at a wavelength λ. The first vortex flowmeter also includes a first vortex sensor which generates a first output based on a frequency of the first stream of vortices. The second vortex flowmeter includes a second bluff body having a leading edge and a shape for inducing a second stream of vortices in the fluid as a function of the flow at the wavelength λ. The second vortex flowmeter also includes a second vortex sensor which generates a second output based on the frequency of the second stream of vortices. The leading edge of the second body is spaced from the leading edge of the first body by 1.15λ to 2.0λ.

Patent
20 Feb 1998
TL;DR: In this article, a system and method for reducing the primary vortex wake structure generated by a lifting body mounted on an object moving through a fluid is described, which is achieved by altering the generated initial vortex wake to make it vulnerable to rapid breakup and, second, producing disturbances to this wake with secondary vortices from auxiliary lifting surfaces, called vortex leveraging tabs, to instigate this breakup.
Abstract: This invention relates to a system and method for reducing the primary vortex wake structure generated by a lifting body mounted on an object moving through a fluid. This is achieved by first, altering the generated initial vortex wake to make it vulnerable to rapid breakup; and, second, producing disturbances to this wake with secondary vortices from auxiliary lifting surfaces, called vortex leveraging tabs, to instigate this breakup. This invention relates to various fields of uses to include vortices generated by any type of lifting body moving through a fluid to include aircraft and watercraft, such as surface vessels and submarines.

Journal ArticleDOI
TL;DR: In this article, an analytical method is developed to predict the structure of a fully developed trailing vortex with a viscous core, where vortex structure is calculated from the load distribution on the generating wing and fundamental conservation laws are satisfied.
Abstract: An analytical method has been developed to predict the structure of a fully developed trailing vortex with a viscous core. Vortex structure is calculated from the load distribution on the generating wing and fundamental conservation laws are satisfied. The present rollup model implicitly addresses viscous effects in the vortex core region by assuming a turbulent mixing process in the core during formation. Mixing theory suggests the appropriate functional form of the solution velocity profiles within this region, with constants that are determined uniquely by the method for arbitrary wing loading distributions. Important structural properties such as vortex strength, core size, and peak swirl velocity are calculated directly from these solution constants. The viscous core model was validated against two recent experimental studies, which provided new insight into vortex growth

Journal ArticleDOI
TL;DR: In this paper, an experimental study of the flowfield generated by the interaction of a streamwise vortex having a strong wake-type axial Mach number profile and a two-dimensional oblique shock wave was conducted in a Mach 2.49 flow.
Abstract: An experimental study of the flowfield generated by the interaction of a streamwise vortex having a strong wake-type axial Mach number profile and a two-dimensional oblique shock wave was conducted in a Mach 2.49 flow. The experiments were aimed at investigating the dynamics of supersonic vortex distortion and to study downstream behavior of a streamwise vortex during a strong shock wave/vortex encounter. The experiments involved positioning an oblique shock generator in the form of a two-dimensional wedge downstream of a semi-span, vortex generator wing section so that the wing-tip vortex interacted with the otherwise planar oblique shock wave. Planar laser sheet visualizations of the flowfield indicated an expansion of the vortex core in crossing a spherically blunt-nose shock front. The maximum vortex core diameter occurred at a distance of 12.7 mm downstream of the wedge leading edge where the vortex had a core diameter of more than double its undisturbed value. At distances further downstream the vortex core diameter remained nearly constant, while it appeared to become more diffused at distances far from the wedge leading edge. Measurements of vortex trajectory revealed that the vortex convected in the freestream direction immediately downstream of the bulged-forward shock structure, while it traveled parallel to the wedge surface at distances further downstream. The turbulent distorted vortex structure which formed as a result of the interaction, was found to be sensitive to downstream disturbances in a manner consistent with incompressible vortex breakdown. Physical arguments are presented to relate behavior of streamwise vortices during oblique and normal shock wave interactions.