An experimental study of the effect of external turbulence on the decay of a single vortex and a vortex pair

2011 ◽  
Vol 670 ◽  
pp. 214-239 ◽  
Author(s):  
J. P. J. van JAARSVELD ◽  
A. P. C. HOLTEN ◽  
A. ELSENAAR ◽  
R. R. TRIELING ◽  
G. J. F. van HEIJST
Keyword(s):  
Vortex Breakdown ◽  
Symmetry Plane ◽  
Vortex Pair ◽  
Azimuthal Velocity ◽  
Single Vortex ◽  
Image Velocimetry ◽  
Vortex Centre ◽  
Observed Behaviour ◽  
External Turbulence ◽  
Oseen Vortex

This study is concerned with the effect of external turbulence on the decay of vortices. Single vortices and vortex pairs were generated with wing(s) mounted in the sidewalls of a wind tunnel. The distance between the two vortices could be adjusted such that they just touched each other or overlapped. The intensity of the turbulence could be controlled with a turbulence grid. The development of the vortex was measured at a number of downstream stations with particle image velocimetry for a range of wing settings. The results indicate that the single vortex can be described by the ‘two length scales’ model of Jacquin, Fabre & Geffroy (AIAA, vol. 1038, 2001, p. 1). A vortex core, which decays like a Lamb–Oseen vortex, is embedded in a region with an almost constant radius and a much lower azimuthal velocity that obeys a ~r−β power law, with r being the radius measured from the vortex centre. For the turbulence levels and the test section length used in this study, the single-vortex behaviour is independent of the external turbulence and in contrast with the decay of the vortex pair that strongly depends on the external turbulence. In the initial stages of the vortex pair evolution, the vortices decay due to cancellation of vorticity at the symmetry plane. At a later stage, Crow oscillations are observed, followed by a breakdown of the vortices. This vortex breakdown might be due to direct turbulent action. The observed behaviour is in agreement with the theoretical model of Crow & Bate (J. Aircraft, vol. 13, 1976, p. 476).

Development of Swirling Flow in a Rod Bundle Subchannel

2002 ◽  
Vol 124 (3) ◽  
pp. 747-755 ◽  
Author(s):  
Heather L. McClusky ◽  
Mary V. Holloway ◽  
Donald E. Beasley ◽  
Michael E. Conner
Keyword(s):  
Angular Momentum ◽  
Swirling Flow ◽  
Axial Direction ◽  
Azimuthal Velocity ◽  
Lateral Velocity ◽  
Full Field ◽  
Rod Bundle ◽  
Image Velocimetry ◽  
Oseen Vortex ◽  
Support Grid

Experimental measurements of the axial development of swirling flow in a rod bundle subchannel are presented. Swirling flow was introduced in the subchannel from a split vane pair located on the downstream edge of the support grid. Particle image velocimetry using an optical borescope yielded full-field lateral velocity data. Lateral flow fields and axial vorticity fields at axial locations ranging from 4.2 to 25.5 hydraulic diameters downstream of the support grid were examined for a Reynolds number of 2.8×104. The lateral velocity fields show that the swirling flow was initially centered in the subchannel. As the flow developed in the axial direction, the swirling flow migrated away from the center of the subchannel. Radial distributions of azimuthal velocity and circulation are presented relative to the centroid of vorticity, and are compared to that of a Lamb-Oseen vortex. The angular momentum decreased as the flow developed in the axial direction. The spatial decay rate of the angular momentum is compared to that of decaying, swirling flow in a pipe.

Transient energy growth of a swirling jet with vortex breakdown

2018 ◽  
Vol 856 ◽  
pp. 288-322 ◽  
Author(s):  
Gopalsamy Muthiah ◽  
Arnab Samanta
Keyword(s):  
High Speed ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Azimuthal Velocity ◽  
Transient Growth ◽  
Transient Energy ◽  
Orr Mechanism ◽  
Streamwise Streaks ◽  
Short Time ◽  
Oseen Vortex

We investigate the existence of short-time, local transient growth in the helical modes of a rapidly swirling, high-speed jet that has transitioned into an axisymmetric bubble breakdown state. The time-averaged flow consisting of the bubble and its wake downstream constitute the base state, which we show to exhibit strong transient amplification owing to the non-modal behaviour of the continuous eigenspectrum. A pseudospectrum analysis mathematically identifies the so-called potential modes within this continuous spectrum and the resultant non-orthogonality between these modes and the existing discrete stable modes is shown to be the main contributor to such growth. As the swirling flow develops post the collapsed bubble, the potential spectrum moves further toward the unstable half-plane, which along with the concurrent weakening of exponential growth from the discrete unstable modes, increases the dynamic importance of transient growth inside the wake region. The transient amplifications calculated at several locations inside the bubble and wake confirm this, where strong growths inside the wake far outstrip the corresponding modal growths (if available) at shorter times, but especially at the higher helical orders and smaller streamwise wavenumbers. The corresponding optimal perturbations at initial times consist of streamwise streaks of azimuthal velocity, which if concentrated inside the core vortical region, unfold via the classical Orr mechanism to yield structures resembling core (or viscous) Kelvin waves of the corresponding Lamb–Oseen vortex. However, in contrast to that in Lamb–Oseen vortex flow, where critical-layer waves are associated with higher transient gains, here, such core Kelvin modes with the more compact spiral structure at the vortex core are seen to yield the maximum transient amplifications.

Thermal and Fluid Dynamic Structures of a Laboratory-Scale Fixed-Frame Fire-Whirl

2001 ◽  
Author(s):  
Mohamed I. Hassan ◽  
A. Helali ◽  
Kozo Saito
Keyword(s):  
Heat Flux ◽  
Fluid Dynamic ◽  
Azimuthal Velocity ◽  
Video Camera ◽  
Liquid Pool ◽  
Flame Height ◽  
Buoyancy Flow ◽  
Image Velocimetry ◽  
Fire Whirl ◽  
Dynamic Structures

Abstract Fire whirl is one of the most destructive phenomena in mass fires. To study thermal and fluid dynamic structures of a fire whirl in a laboratory, a fire whirl generator consisting of two vertically oriented split-cylinders were placed in an asymmetric position to form a compartment leaving two open slits in each end. A 5-cm diameter liquid pool fire was placed at the center of the compartment floor, the fire generated buoyancy flow moved upwardly, and fresh air entered to the compartment creating swirl motion. The visible flame height of the generated fire whirl was measured by a video camera, 2-D azimuthal velocity profiles at several different heights by particle image velocimetry (PIV), and the average heat flux input to the fuel surface by a Gardon gauge type heat flux meter.

HOW THE BODY CONTRIBUTES TO THE WAKE IN UNDULATORY FISH SWIMMING

2001 ◽  
Vol 204 (16) ◽  
pp. 2751-2762 ◽  
Author(s):  
ULRIKE K. MÜLLER ◽  
JORIS SMIT ◽  
EIZE J. STAMHUIS ◽  
JOHN J. VIDELER
Keyword(s):  
Body Wave ◽  
Vortex Pair ◽  
Maximum Flow ◽  
The Body ◽  
Double Row ◽  
Image Velocimetry ◽  
Swimming Efficiency ◽  
Swimming Mode ◽  
Thrust Production ◽  
Tail Beat

SUMMARY Undulatory swimmers generate thrust by passing a transverse wave down their body. Thrust is generated not just at the tail, but also to a varying degree by the body, depending on the fish's morphology and swimming movements. To examine the mechanisms by which the body in particular contributes to thrust production, we chose eels, which have no pronounced tail fin and hence are thought to generate all their thrust with their body. We investigated the interaction between body movements and the flow around swimming eels using two-dimensional particle image velocimetry. Maximum flow velocities adjacent to the eel's body increase almost linearly from head to tail, suggesting that eels generate thrust continuously along their body. The wake behind eels swimming at 1.5Ls-1, where L is body length,consisted of a double row of double vortices with little backward momentum. The eel sheds two vortices per half tail-beat, which can be identified by their shedding dynamics as a start—stop vortex of the tail and a vortex shed when the body-generated flows reach the `trailing edge' and cause separation. Two consecutively shed ipsilateral body and tail vortices combine to form a vortex pair that moves away from the mean path of motion. This wake shape resembles flow patterns described previously for a propulsive mode in which neither swimming efficiency nor thrust is maximised but sideways forces are high. This swimming mode is suited to high manoeuvrability. Earlier recordings show that eels also generate a wake reflective of maximum swimming efficiency. The combined findings suggest that eels can modify their body wave to generate wakes that reflect their propulsive mode.

Performance of a Low Speed Axial Fan Under Distortion: An Experimental Investigation

2020 ◽  
Author(s):  
Sumit Tambe ◽  
Ugaitz Bartolomé Oseguera ◽  
Arvind Gangoli Rao
Keyword(s):  
Flow Field ◽  
Symmetry Plane ◽  
Vortex Pair ◽  
Axial Fan ◽  
Low Speed ◽  
Fan Performance ◽  
Fuel Burn ◽  
Flow Phenomena ◽  
Order Of Magnitude ◽  
Distortion Index

Abstract In the pursuit of reducing the fuel burn, future aircraft configurations will feature several types of improved propulsion systems, e.g. embedded engines with boundary layer ingestion, high-bypass ratio engines with short intakes, etc. Depending on the design and phase of flight, the engine fan will encounter inflow distortion of varying strength, and fan performance will be adversely affected. Therefore, investigation of the flow phenomena causing performance losses in fan and distortion interaction is important. This experimental study shows the effect of varying distortion index on four aspects of fan performance: distortion topology, upstream redistribution, performance curve, and flow unsteadiness. A low speed fan is tested under 60° circumferential distortion of varying strength, generated using distortion screens. The flow field in the upstream redistribution region is measured using PIV (planar and stereo). The fan performance is obtained using total pressure measurements. The noise spectra measured by a microphone are used to quantify the unsteadiness in the flow field. The distortion index (DC60) varies linearly with the grid porosity at constant wall thickness and aspect ratio of the grid cells. However, the distortion topology is significantly different as a stream-wise vortex pair appears in distorted flow at higher DC60. The vortices are stronger at higher DC60, but their order of magnitude is much lower than the circulation corresponding to fan itself. The spinner, distortion index and topology significantly affect the upstream redistribution mechanism. The vortex pair redistributes the flow which results in lower asymmetry in the symmetry plane. With increasing distortion, the performance is reduced and the unsteadiness is increased.

Bubble and conical forms of vortex breakdown in swirling jets

2019 ◽  
Vol 873 ◽  
pp. 322-357 ◽  
Author(s):  
Pradeep Moise ◽  
Joseph Mathew
Keyword(s):  
Vortex Breakdown ◽  
Initial Conditions ◽  
Azimuthal Velocity ◽  
Linear Instability ◽  
Subcritical State ◽  
Experimental Investigations ◽  
Global Mode ◽  
Swirling Jets ◽  
Conjugate State ◽  
Centreline Velocity

Experimental investigations of laminar swirling jets had revealed a new form of vortex breakdown, named conical vortex breakdown, in addition to the commonly observed bubble form. The present study explores these breakdown states that develop for the Maxworthy profile (a model of swirling jets) at inflow, from streamwise-invariant initial conditions, with direct numerical simulations. For a constant Reynolds number based on jet radius and a centreline velocity of 200, various flow states were observed as the inflow profile’s swirl parameter $S$ (scaled centreline radial derivative of azimuthal velocity) was varied up to 2. At low swirl ($S=1$) a helical mode of azimuthal wavenumber $m=-2$ (co-winding, counter-rotating mode) was observed. A ‘swelling’ appeared at $S=1.38$, and a steady bubble breakdown at $S=1.4$. On further increase to $S=1.5$, a helical, self-excited global mode ($m=+1$, counter-winding and co-rotating) was observed, originating in the bubble’s wake but with little effect on the bubble itself – a bubble vortex breakdown with a spiral tail. Local and global stability analyses revealed this to arise from a linear instability mechanism, distinct from that for the spiral breakdown which has been studied using Grabowski profile (a model of wing-tip vortices). At still higher swirl ($S=1.55$), a pulsating type of bubble breakdown occurred, followed by conical breakdown at 1.6. The latter consists of a large toroidal vortex confined by a radially expanding conical sheet, and a weaker vortex core downstream. For the highest swirls, the sheet was no longer conical, but curved away from the axis as a wide-open breakdown. The applicability of two classical inviscid theories for vortex breakdown – transition to a conjugate state, and the dominance of negative azimuthal vorticity – was assessed for the conical form. As required by the former, the flow transitioned from a supercritical to subcritical state in the vicinity of the stagnation point. The deviations from the predictions of the latter model were considerable.

Particle Image Velocimetry Measurements of Turbulent Jets Issuing From Twin Elliptic Nozzles With Various Orientations

2020 ◽  
Vol 143 (2) ◽  
Author(s):  
Ella Marie Morris ◽  
Neelakash Biswas ◽  
Seyed Sobhan Aleyasin ◽  
Mark Francis Tachie
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Symmetry Plane ◽  
Velocity Profiles ◽  
Equivalent Diameter ◽  
Linear Contraction ◽  
Turbulent Characteristics ◽  
Image Velocimetry ◽  
Nozzle Orientation ◽  
Minor Plane

Abstract The effects of nozzle orientation on the mixing and turbulent characteristics of elliptical free twin jets were studied experimentally. The experiments were conducted using modified contoured nozzles with a sharp linear contraction. The centers of the nozzle pair had a separation ratio of 5.5. Two nozzle configurations were tested, twin nozzles oriented along the minor plane (Twin_Minor) and twin nozzles oriented along the major plane (Twin_Major) and the results were compared with a single jet. In each case, the Reynolds number based on the maximum jet velocity and the equivalent diameter was 10,000. A planar particle image velocimetry (PIV) system was used to measure the velocity field in the jet symmetry plane. It was observed that the velocity decay rate is not sensitive to nozzle orientation. However, close to the jet exit, the spread rate was highest in the minor plane. In addition, contour plots of swirling strength, Reynolds shear stress and turbulent intensities revealed significant differences between the minor and major planes. Velocity profiles showed little variation close to the jet exit, while further downstream the variations between the velocity profiles were more pronounced between the major and minor planes.

scholarly journals The naturally oscillating flow emerging from a fluidic precessing jet nozzle

2008 ◽  
Vol 606 ◽  
pp. 153-188 ◽  
Author(s):  
CHONG Y. WONG ◽  
GRAHAM J. NATHAN ◽  
RICHARD M. KELSO
Keyword(s):  
Near Field ◽  
Vortex Pair ◽  
Downstream Face ◽  
Convergence Region ◽  
Azimuthal Component ◽  
Image Velocimetry ◽  
Flow Convergence ◽  
Jet Nozzle ◽  
First Time ◽  
Direction Opposite

Phase-averaged and directionally triggered digital particle image velocimetry measurements were taken in longitudinal and transverse planes in the near field of the flow emerging from a fluidic precessing jet nozzle. Measurements were performed at nozzle inlet Reynolds and Strouhal numbers of 59000 and 0.0017, respectively. Results indicate that the jet emerging from the nozzle departs with an azimuthal component in a direction opposite to that of the jet precession. In addition, the structure of the ‘flow convergence’ region, reported in an earlier study, is better resolved here. At least three unique vortex-pair regions containing smaller vortical ‘blobs’ are identified for the first time. These include a vortex-pair region originating from the foci on the downstream face of the nozzle centrebody, a vortex-pair region shed from the edge of the centrebody and a vortex-pair region originating from the downstream surface of the nozzle exit lip.

Characteristics of 3D Turbulent Wall Jets and Offset Jets With Small Offset Ratios

2009 ◽  
Author(s):  
M. Agelinchaab ◽  
M. F. Tachie
Keyword(s):  
Three Dimensional ◽  
Symmetry Plane ◽  
Shear Stresses ◽  
Wall Jets ◽  
Particle Image Velocimetry Technique ◽  
Image Velocimetry ◽  
Turbulent Wall Jets ◽  
The Mean ◽  
Turbulent Wall ◽  
Proper Orthogonal

This paper reports an experimental study of turbulent three-dimensional generic wall jets and offset jets. The jets were created from a long circular pipe. A particle image velocimetry technique was used to conduct velocity measurements in the symmetry plane of the jet. From these measurements, the salient features of the flows are reported in terms of the mean velocities, turbulence intensities and Reynolds shear stresses. The energy spectra and profiles of reconstructed turbulence intensities and Reynolds shear stresses from low order proper orthogonal decomposition modes are also reported.

Flow structure behind two staggered circular cylinders. Part 1. Downstream evolution and classification

2008 ◽  
Vol 607 ◽  
pp. 51-80 ◽  
Author(s):  
J. C. HU ◽  
Y. ZHOU
Keyword(s):  
Initial Conditions ◽  
Shear Layers ◽  
Circular Cylinders ◽  
Flow Structures ◽  
Incident Flow ◽  
Flow Topology ◽  
Single Vortex ◽  
Single Body ◽  
Image Velocimetry ◽  
Different Types

Flow structures, Strouhal numbers and their downstream evolutions in the wake of two-staggered circular cylinders are investigated at Re=7000 using hot-wire, flow-visualization and particle-image velocimetry techniques. The cylinder centre-to-centre pitch, P, ranges from 1.2d to 4.0d (d is the cylinder diameter) and the angle (α) between the incident flow and the line through the cylinder centres is 0° ~ 90°. Four distinct flow structures are identified at x/d ≥ 10 (x is the downstream distance from the mid-point between the cylinders), i.e. two single-street modes (S-I and S-II) and two twin-street modes (T-I and T-II), based on Strouhal numbers, flow topology and their downstream evolution. Mode S-I is further divided into two different types, i.e. S-Ia and S-Ib, in view of their distinct vortex strengths. Mode S-Ia occurs at P/d ≤ 1.2. The pair of cylinders behaves like one single body, and shear layers separated from the free-stream sides of the cylinders roll up, forming one street of alternately arranged vortices. The street is comparable to that behind an isolated cylinder in terms of the topology and strength of vortices. Mode S-Ib occurs at α ≤ 10° and P/d > 1.5. Shear layers separated from the upstream cylinder reattach on or roll up to form vortices before reaching the downstream cylinder, resulting in postponed flow separation from the downstream cylinder. A single vortex street thus formed is characterized by significantly weakened vortices, compared with Mode S-Ia. Mode S-II is identified at P/d=1.2~2.5 and α>20° or 1.5≤P/d≤4.0 and 10° < α≤20°, where both cylinders generate vortices, with vortex shedding from the upstream cylinder at a much higher frequency than from the downstream, producing two streets of different widths and vortex strengths at x/d≤5.0. The two streets interact vigorously, resulting in a single street of the lower-frequency vortices at x/d≥10. The vortices generated by the downstream cylinder are significantly stronger than those, originating from the upstream cylinder, in the other row. Mode T-I occurs at P/d≥2.5 and α=20°~88°; the two cylinders produce two streets of different vortex strengths and frequencies, both persisting beyond x/d=10. At P/d≥2.5 and α≥88°, the two cylinders generate two coupled streets, mostly anti-phased, of the same vortex strength and frequency (St≈0.21), which is referred to as Mode T-II. The connection of the four modes with their distinct initial conditions, i.e. interactions between shear layers around the two cylinders, is discussed.

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