Time Resolved PIV Measurements of a Slot Lobed Jet Issuing Into a Crossflow

2021 ◽  
Author(s):  
Michael Lewandowski ◽  
Paul Kristo ◽  
Abdullah Weiss ◽  
Mark Kimber
Keyword(s):  
Velocity Ratio ◽  
Leading Edge ◽  
Residual Effects ◽  
Low Velocity ◽  
Mean Velocity ◽  
Time Resolved ◽  
Freestream Velocity ◽  
Round Jet ◽  
Jet In Crossflow ◽  
Turbulent Statistics

Abstract The near field mixing phenomenon created by a round jet with three slot lobes exhausting into a crossflow are investigated at a velocity ratio of 0.5. Time-resolved particle image velocimetry measurements provide instantaneous velocity fields of the slotted jet in crossflow, allowing for evaluation of the first and second order turbulent statistics in two perpendicular planes of interest. The independently controlled jet exit and crossflow inlet are first characterized extensively to confirm the velocity ratio and anticipated momentum exchanges. Spanwise and transverse mean velocity profiles reveal that the interaction of the three slot lobes and the center round jet primarily occur in the immediate jet exit region, though residual effects are also found in the wake. Evaluation of the Reynold stresses aims to quantify the near region mixing between the jets collated geometric features and their interaction with the crossflow. Frequency analysis reveals that low-frequency harmonics in the wake region provide greater energy contributions than that of the higher-frequency harmonics found along the leading edge shear layer. This behavior is attributed to the low velocity ratio, where the freestream velocity is twice as large as the jet exit velocity. The experimental data and observations herein serve analogous computational modeling efforts for the slotted jet in crossflow at low velocity ratios, with ample information to inform necessary boundary conditions, fluid properties, and flow fields for validation.

scholarly journals Time-resolved particle image velocimetry measurements of a tandem jet array in a cross flow at low velocity ratios

2020 ◽  
Author(s):  
Paul Kristo ◽  
Mark L. Kimber
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Velocity Fields ◽  
Instantaneous Velocity ◽  
Low Velocity ◽  
Time Resolved ◽  
Turbulent Statistics ◽  
Image Velocimetry

Investigation of the near field dynamics of a single and tandem array of three jets are provided by 2-D time-resolved particle image velocimetry (TR-PIV) measurements. Instantaneous velocity fields are examined in the transverse and spanwise planes with jet to cross flow velocity ratios in the range from 0.9 to 1.7. Previous studies have shown that for high ratios (≥2), the leading jet provides sufficient shielding to ensure that all jets downstream exhibit nearly identical flow characteristics. The current transverse plane measurements exhibit more unique and localized features as a result of the competing effects of pressure gradients and vortex mechanisms assessed via the jet exit profiles, first and second order turbulent statistics, streamline trajectories, recirculation areas, and penetrations depths. Proper orthogonal decomposition (POD) is applied to the spanwise plane instantaneous velocity fields to determine the statistically dominant features of the single and tandem jet configurations at equivalent velocity ratios. The velocity fields are then reconstructed using the truncated POD modes to provide further insight into the shear layer and wake vortices that drive these configurations. Vortex identification algorithms are applied to the reconstructed velocity fields to determine the statistical characteristics of the vortices, including their centroids, populations, areas, and strengths, each of which exhibit largely different dependencies on jet configuration and velocity ratio. Several of the investigated metrics are found to exhibit different behaviors below and above a velocity ratio of unity, and also as a function of increasing velocity ratio between 1 and 2, implying that several transitions mechanisms are present in the low velocity ratio regime investigated herein.

Turbulent Mixing Process of a Round Jet With Slot Lobes

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
Paul J. Kristo ◽  
Coleman D. Hoff ◽  
Ian G. R. Craig ◽  
Mark L. Kimber
Keyword(s):  
Turbulent Mixing ◽  
Near Field ◽  
Streamwise Velocity ◽  
Velocity Profiles ◽  
Separate Flow ◽  
Geometric Feature ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Round Jet ◽  
Turbulent Statistics

Abstract Turbulent mixing in the near region of a round jet with three slot lobes is examined via mean velocity and turbulent statistics and structures at a Reynolds number of 15,000. The design utilizes separate flow motivations upstream of each geometric feature, deviating from conventional nozzles or orifice plates. Immediate outlet velocity profiles are heavily influenced by opposing pressure gradients between the neighboring round and slot streams. Spanwise mean velocity profiles reveal the majority of the convective exchange between a given slot and the round center occurs in the immediate near field, but has lasting effects on the axial centerline profiles downstream. This is also reflected by the velocity half-widths, exhibiting asymmetry across the entirety of available measurements. Centerline turbulence intensities exhibit strong and short-lived isotropy. The increasingly anisotropic intensities found downstream are lower than similar geometries from the literature, implying that mixing development is inhibited. Reynolds stresses at the round-slot interface are significantly smaller than the round-stagnant exchange, but achieve a symmetric condition at x/D ≅ 4. Two-point spatial correlations of the fluctuating streamwise velocity exhibit stronger dependence toward the axial centerline at the round-slot interface in comparison to the nominal round radius. In contrast, spanwise velocity fluctuations exhibit nearly identical, localized behaviors on each side of the jet. Corresponding differences in streamwise integral length scale peak in the range 1.0 ≤ x/D ≤ 1.5, and so too do the turbulent structures in this area, as a result of the collated jet geometry.

scholarly journals Transition scenario of the round jet in crossflow topology at low velocity ratios

2014 ◽  
Vol 26 (8) ◽  
pp. 084101 ◽  
Author(s):  
Tristan Cambonie ◽  
Jean-Luc Aider
Keyword(s):  
Low Velocity ◽  
Round Jet ◽  
Jet In Crossflow ◽  
Transition Scenario

Organized motions in a jet in crossflow

2001 ◽  
Vol 444 ◽  
pp. 117-149 ◽  
Author(s):  
A. RIVERO ◽  
J. A. FERRÉ ◽  
FRANCESC GIRALT
Keyword(s):  
Velocity Field ◽  
Velocity Ratio ◽  
Mean Field ◽  
Vortex Pair ◽  
Leeward Side ◽  
Mean Velocity ◽  
Jet In Crossflow ◽  
Planar Laser ◽  
The Mean ◽  
Counter Rotating Vortex Pair

An experimental study to identify the structures present in a jet in crossflow has been carried out at a jet-to-crossflow velocity ratio U/Ucf = 3.8 and Reynolds number Re = UcfD/v = 6600. The hot-wire velocity data measured with a rake of eight X-wires at x/D = 5 and 15 and flow visualizations using planar laser-induced fluorescence (PLIF) confirm that the well-established pair of counter-rotating vortices is a feature of the mean field and that the upright, tornado-like or Fric's vortices that are shed to the leeward side of the jet are connected to the jet flow at the core. The counter-rotating vortex pair is strongly modulated by a coherent velocity field that, in fact, is as important as the mean velocity field. Three different structures – folded vortex rings, horseshoe vortices and handle-type structures – contribute to this coherent field. The new handle-like structures identified in the current study link the boundary layer vorticity with the counter-rotating vortex pair through the upright tornado-like vortices. They are responsible for the modulation and meandering of the counter-rotating vortex pair observed both in video recordings of visualizations and in the instantaneous velocity field. These results corroborate that the genesis of the dominant counter-rotating vortex pair strongly depends on the high pressure gradients that develop in the region near the jet exit, both inside and outside the nozzle.

Effect of Upstream Shear on Flow and Heat (Mass) Transfer Over a Flat Plate

2008 ◽  
Author(s):  
Kalyanjit Ghosh ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Velocity Ratio ◽  
Leading Edge ◽  
Surface Velocity ◽  
Freestream Velocity ◽  
Inner Region

A parametric study is conducted to investigate the effect of wall shear on a two-dimensional turbulent boundary layer. The shear is imparted by a moving belt, flush with the wall, translating in the flow direction. Velocity and mass transfer experiments have been performed for four surface-to-freestream velocity ratios (0, 0.38, 0.52, 0.65) with a Reynolds number based on the momentum thickness between 770 and 1776. The velocity data indicate that the location of the ‘virtual origin’ of the turbulent boundary layer ‘moves’ downstream towards the trailing edge of the belt with increasing surface velocity. The highest velocity ratio represents a case which is responsible for the removal of the inner region of the boundary layer. Mass transfer measurements downstream of the belt show the presence of a local minimum in the variation of the Stanton vs. Reynolds number for the highest velocity ratio. Downstream of this minimum, approximately 1 cm from the leading edge of the mass transfer plate, the characteristics of the turbulent boundary layer are restored and the data fall back on the empirical variation of the Stanton number with Reynolds number.

Flow Characteristics of Round Jet Issuing Into Counter-Flowing Uniform Stream

2003 ◽  
Author(s):  
Masafumi Miyata ◽  
Tatsushi Yagi
Keyword(s):  
Stagnation Point ◽  
Turbulence Intensity ◽  
Velocity Ratio ◽  
Water Channel ◽  
Recirculation Region ◽  
Mean Velocity ◽  
Penetration Length ◽  
Round Jet ◽  
Jet Axis ◽  
Uniform Stream

Two-color four beam LDV measurements were conducted for the round jet issuing into a counter-flowing uniform stream under well controlled boundary conditions. A comprehensive set of the velocity data was documented for velocity ratios from 0.8 to 7. The linear dependence of jet penetration length on velocity ratio was confirmed for all conditions investigated, including those with velocity ratios less than 1. The proportional coefficient estimated is substantially larger than those reported for similar jets in a water channel. The streamwise variations of mean velocity and turbulence intensity on the jet axis also depend only on velocity ratio. It is very interesting to know that the streamwise turbulence intensity on the jet axis, non-dimensionalized by uniform velocity, is at a maximum near at the stagnation point and that the maximum has a constant value near 1 for velocity-ratios greater than 2. The static pressure was found to become near to zero at the stagnation point and is negative in the core of the recirculation region.

Dynamics and mixing of vortex rings in crossflow

2008 ◽  
Vol 604 ◽  
pp. 389-409 ◽  
Author(s):  
RAJES SAU ◽  
KRISHNAN MAHESH
Keyword(s):  
Boundary Layer ◽  
Vortex Ring ◽  
Nozzle Exit ◽  
Velocity Ratio ◽  
Leading Edge ◽  
Vortex Rings ◽  
Low Velocity ◽  
Hairpin Vortices ◽  
Stroke Length ◽  
Asymmetric Vortex

Direct numerical simulation is used to study the effect of crossflow on the dynamics, entrainment and mixing characteristics of vortex rings issuing from a circular nozzle. Three distinct regimes exist, depending on the velocity ratio (ratio of the average nozzle exit velocity to free-stream crossflow velocity) and stroke ratio (ratio of stroke length to nozzle exit diameter). Coherent vortex rings are not obtained at velocity ratios below approximately 2. At these low velocity ratios, the vorticity in the crossflow boundary layer inhibits roll-up of the nozzle boundary layer at the leading edge. As a result, a hairpin vortex forms instead of a vortex ring. For large stroke ratios and velocity ratio below 2, a series of hairpin vortices is shed downstream. The shedding is quite periodic for very low Reynolds numbers. For velocity ratios above 2, two regimes are obtained depending upon the stroke ratio. Lower stroke ratios yield a coherent asymmetric vortex ring, while higher stroke ratios yield an asymmetric vortex ring accompanied by a trailing column of vorticity. These two regimes are separated by a transition stroke ratio whose value decreases with decreasing velocity ratio. For very high values of the velocity ratio, the transition stroke ratio approaches the ‘formation number’. In the absence of trailing vorticity, the vortex ring tilts towards the upstream direction, while the presence of a trailing column causes it to tilt downstream. This behaviour is explained. In the absence of crossflow, the trailing column is not very effective at entrainment, and is best avoided for optimal mixing and entrainment. However, in the presence of crossflow, the trailing column is found to contribute significantly to the overall mixing and entrainment. The trailing column interacts with the crossflow to generate a region of high pressure downstream of the nozzle that drives crossflow fluid towards the vortex ring. There is an optimal length of the trailing column for maximum downstream entrainment. A classification map which categorizes the different regimes is developed.

Cavitation about a jet in crossflow

2015 ◽  
Vol 768 ◽  
pp. 141-174 ◽  
Author(s):  
P. A. Brandner ◽  
B. W. Pearce ◽  
K. L. de Graaf
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
High Speed ◽  
Velocity Ratio ◽  
Cavitation Number ◽  
Variable Pressure ◽  
Lower Velocity ◽  
Freestream Velocity ◽  
Jet In Crossflow ◽  
Cavity Closure

Cavitation occurrence about a jet in crossflow is investigated experimentally in a variable-pressure water tunnel using still and high-speed photography. The 0.012 m diameter jet is injected on the centreplane of a 0.6 m square test section at jet to freestream velocity ratios ranging from 0.2 to 1.6, corresponding to jet-velocity-based Reynolds numbers of $25\times 10^{3}$ to $160\times 10^{3}$ respectively. Measurements were made at a fixed freestream-based Reynolds number, for which the ratio of the undisturbed boundary layer thickness to jet diameter is 1.18. The cavitation number was varied from inception (up to about 10) down to 0.1. Inception is investigated acoustically for bounding cases of high and low susceptibility to phase change. The influence of velocity ratio and cavitation number on cavity topology and geometry are quantified from the photography. High-speed photographic recordings made at 6 kHz provide insight into cavity dynamics, and derived time series of spatially averaged pixel intensities enable frequency analysis of coherent phenomena. Cavitation inception was found to occur in the high-shear regions either side of the exiting jet and to be of an intermittent nature, increasing in occurrence and duration from 0 to 100 % probability with decreasing cavitation number or increasing jet to freestream velocity ratio. The frequency and duration of individual events strongly depends on the cavitation nuclei supply within the approaching boundary layer. Macroscopic cavitation develops downstream of the jet with reduction of the cavitation number beyond inception, the length of which has a power-law dependence on the cavitation number and a linear dependence on the jet to freestream velocity ratio. The cavity closure develops a re-entrant jet with increase in length forming a standing wave within the cavity. For sufficiently low cavitation numbers the projection of the re-entrant jet fluid no longer reaches the cavity leading edge, analogous to supercavitation forming about solid cavitators. Hairpin-shaped vortices are coherently shed from the cavity closure via mechanisms of shear-layer roll-up similar to those shed from protuberances and jets in crossflow in single-phase flows. These vortices are shed at an apparently constant frequency, independent of the jet to freestream velocity ratio but decreasing in frequency with reducing cavitation number and cavity volume growth. Highly coherent cavitating vortices form along the leading part of the cavity due to instability of the jet upstream shear layer for jet to freestream velocity ratios greater than about 0.8. These vortices are cancelled and condense as they approach the trailing edge in the shear layer of opposing vorticity associated with the cavity closure and the hairpin vortex formation. For lower velocity ratios, where there is decreased jet penetration, the jet upstream shear velocity gradient reverses and vortices of the opposite sense form, randomly modulated by boundary layer turbulence.

Dynamic Characteristics of Flow Separation From a Low Reynolds Number Airfoil

2007 ◽  
Author(s):  
Daniel R. Morse ◽  
James A. Liburdy
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Large Scale ◽  
Region Of Interest ◽  
Leading Edge ◽  
Chord Length ◽  
Vortical Flow ◽  
Local Circulation ◽  
Time Resolved ◽  
Freestream Velocity

This study examines the generation of large scale vortices caused by flow separation from a flat wing at various angles of attack. Time-resolved particle image velocimetry is used to determine the evolution and convective characteristics of the large scale structures. A rectangular airfoil with aspect ratio of 0.5 is used and data are collected at a Reynolds number of 23,500, for angles of attack from 0° to 20°. Data consists of two dimensional velocity fields obtained at 500 Hz located at the airfoil centerline. The region of interest is near the separation point but fields of view extend over approximately one half of the chord length from the leading edge to document the downstream progression of the large scale vortical flow elements. The velocity data were processed to identify the vorticity field dynamics in terms of the Kelvin-Helmholtz instability occurring near the leading edge. The vortical structures are identified using vortex detection based on local circulation. The convective nature of the vortex elements are shown to consist of merging, stalling and convecting, with convective velocities on the order of 20% of the freestream velocity with an associated Stouhal number based on chord length and freestream velocity of approximately 1.0.

Flow Characteristics of a Three Dimensional Bluff Body With a Single Rotating Cylinder

2012 ◽  
Author(s):  
James R. Bell ◽  
David Burton ◽  
Damien McArthur ◽  
John Sheridan
Keyword(s):  
Bluff Body ◽  
Aerodynamic Drag ◽  
Velocity Ratio ◽  
Three Dimensional ◽  
Rotating Cylinder ◽  
The Body ◽  
Surface Velocity ◽  
Cylinder Surface ◽  
Low Velocity ◽  
Freestream Velocity

This work investigated the application of a rotating cylinder to the upper leeward edge of a three dimensional bluff body in ground proximity. Aerodynamic drag measurements, base pressure contours and wake velocity profiles were obtained in a closed jet wind tunnel for Reynolds Numbers in the range of approximately 220,000 to 660,000. The cylinder of diameter 0.1H was mounted on the upper edge of the leeward face of the body. The ratio of cylinder surface velocity to freestream velocity was varied from 0 to 2.0. A computational model of the geometry was developed and results are presented for various velocity ratios and cylinder diameters. The results of this work demonstrated that, even at low velocity ratios, the cylinder rotation has a large effect on the flow structures in the body wake region. A large downwash is observed that creates two large counter-rotating vortices and a resultant significant increase in drag. The aerodynamic drag changes are presented as a function of velocity ratio and are shown to be Reynolds Number insensitive over the range tested. Aerodynamic drag was shown to increase with increasing velocity ratio over the velocity ratio range 0.25 to 2.0.

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