Analysis of Entrainment at the Turbulent/Non-Turbulent Interface of a Square Jet

2013 ◽  
Author(s):  
Abbas Ghasemi ◽  
Vesselina Roussinova ◽  
Ronald Barron ◽  
Ram Balachandar
Keyword(s):  
Velocity Vector ◽  
Particle Image ◽  
Central Plane ◽  
Axial Distance ◽  
Velocity Vector Field ◽  
Ambient Fluid ◽  
Round Jet ◽  
Image Velocimetry ◽  
Swirling Strength ◽  
Cover Up

Particle image velocimetry measurements are carried out to study the entrainment at the interface between the non-turbulent and turbulent regions in a square jet. Jet Reynolds number based on the hydraulic diameter of the jet is 50,000. Measurements cover up to 25 diameters downstream of the nozzle exit using five horizontal field-of-views in the central plane of the jet. The turbulent/non-turbulent interface is identified using a velocity criterion and a suitable thresholding method. Using vorticity and swirling strength it is shown that the turbulent/non-turbulent interface separates the rotational and irrotational regions of the flow. Instantaneous velocity vector field superimposed with the turbulent/non-turbulent interface are presented. The relation between the vortex cores in the vicinity of the turbulent/non-turbulent interface and the contractions and expansions noticed in the jet velocity field are explained. Entrainment into the jet is evaluated at each axial distance by identifying the points falling inside the turbulent region of the jet. Compared to a round jet, the square jet entrains more ambient fluid. In addition, normal volume fluxes going through the turbulent/non-turbulent interface of the square jet are found to be larger compared to that of a round jet.

The flow around a pipeline with a spoiler

2009 ◽  
Vol 224 (1) ◽  
pp. 109-121 ◽  
Author(s):  
A A Oner
Keyword(s):  
Shear Stress ◽  
Velocity Vector ◽  
Particle Image ◽  
Reynolds Numbers ◽  
Two Dimensional ◽  
Velocity Vector Field ◽  
Smooth Wall ◽  
Particle Image Velocimetry Technique ◽  
Offshore Pipelines ◽  
Image Velocimetry

Offshore pipelines are buried in the seabed to be protected from the damage caused by hydrodynamic forces or by human activities. However, because of soil erosion and interaction of currents with the pipeline on the moveable seabed, the processes of local scouring and, sometimes, self-burial of pipelines take place. To increase the rate and extent of scouring, the technique of attaching a spoiler to the pipeline has been developed. In this study, two-dimensional, steady, turbulent flow around a horizontal pipeline with a spoiler near a smooth wall is investigated experimentally by using the particle image velocimetry technique. The effect of the spoiler was examined for the Reynolds numbers of ReD=840, 1500, 4150, and 9500 based on the pipe diameter. The effect of the spoiler on the process of scouring is investigated through the parameters of the measured instantaneous and time-averaged patterns of the velocity vector field and the streamline topology. The results indicated that the attachment of the spoiler to the pipeline increases the length of the upstream and downstream separation regions and it is also observed that the spoiler does not significantly increase the rate of the flow that passes through the gap and the shear stress acting on the seabed.

Particle Image Velocimetry Measurements Near the Onset of Cavitation in a Converging-Diverging Glass Nozzle

2016 ◽  
Author(s):  
Aaron Schmidt ◽  
B. Terry Beck ◽  
Mohammad H. Hosni
Keyword(s):  
Velocity Vector ◽  
Particle Image ◽  
Time Interval ◽  
Seed Density ◽  
Velocity Vector Field ◽  
Nozzle Throat ◽  
Piv Measurements ◽  
Seed Particles ◽  
Image Velocimetry ◽  
Onset Of Cavitation

Water flow through a converging-diverging glass nozzle experiences a pressure drop and its velocity increases as it flows through the converging section. For an inviscid fluid, the pressure minimum occurs at the nozzle throat, where the cross-sectional area is minimum. If the minimum pressure is below the water vapor pressure, cavitation may occur. The actual minimum pressure through a converging-diverging nozzle depends on many factors and may not occur at the nozzle throat. Additionally, fluid through the nozzle may be driven into the metastable region and subsequently cavitate at a lower pressure than the vapor pressure. All of these factors combine to create a complex and unsteady flow pattern. The precise conditions leading to the onset of cavitation in water flowing in a converging-diverging nozzle are not well understood. Utilization of a clear glass converging-diverging nozzle enabled Particle Image Velocimetry (PIV) measurements of the velocity vector field inside the nozzle without significantly promoting premature cavitation formation. Glass spheres of 10 μm diameter were selected as seed particles for use in the PIV measurements. These seed particles did not significantly affect the formation (or onset) of cavitation in the nozzle; however, larger seed particles (120 μm diameter) provided nucleation sights and promoted cavitation prematurely. The seed particles were injected into the flow significantly upstream from the nozzle to prevent disrupting the flow entering the nozzle. High seed density was needed to supply enough seed particles to interrogate small regions near the nozzle wall; however, high seed density could also cause speckling and reduce the ability to produce meaningful PIV measurements. A Nd:YAG laser provided illumination of the seed particles in the nozzle. Laser reflections off of the nozzle exterior had to be minimized to avoid saturating the PIV camera. A polarizing filter was installed on the camera to reduce reflections. An enclosure that surrounded the nozzle was also designed and utilized. The enclosure was filled with water to reduce laser reflections off of the nozzle exterior wall. The time elapsed between frames had to be adjusted for each section of the nozzle interrogated with PIV. For accurate velocity measurements, particles needed to travel at least two particles diameters but less than 25% of each interrogation cell. The large variation in velocities present in the nozzle prevented one time interval from satisfying the seed particles displacement requirements. The time interval between frames had to be tailored to each section of the nozzle, depending upon the range of velocities seen in that section. Detailed measurement of the velocity profile near the nozzle throat required precise control over all timing parameters and pushed the available hardware to its smallest possible time interval. Detailed PIV measurements near the wall in regions of recirculation and at the cavitation front required the use of a long-distance microscope. This limited the field of view and necessitated a high seed particle density, which presented problems due to the lack of control over the flow of the seed particles in the near wall region. PIV allowed for the measurement of the velocity vector field inside a converging-diverging nozzle without disrupting the flow. These measurements provided detailed velocity and flow pattern information throughout the nozzle, particularly in the regions near the cavitation front where boundary layer separation was observed along with regions of recirculating flow. These detailed velocity profiles were compiled to present a complete PIV analysis of the converging-diverging glass nozzle. Measurements of the velocity field near cavitation onset allowed for a better understanding of the conditions triggering cavitation and the degree to which the water flow was able to be driven into the metastable region.

Comparative Evaluation of Single/Twin Round and Elliptic Jets Using Particle Image Velocimetry

2018 ◽  
Author(s):  
Seyed Sobhan Aleyasin ◽  
Mark Francis Tachie
Keyword(s):  
Particle Image Velocimetry ◽  
High Speed ◽  
Particle Image ◽  
Joint Probability ◽  
Strength Analysis ◽  
Round Jets ◽  
Image Velocimetry ◽  
Velocity Decay ◽  
Swirling Strength ◽  
Single Jet

Twin round and elliptic jets with nozzle spacing of S/d = 2.8 are investigated and the results are compared with those obtained from single jets. The measurements were performed at Re = 10000 using particle image velocimetry. The results show that the twin elliptic jets merge and combine faster than the round jets. However, the twin elliptic jets have lower spreading than their corresponding single jet but in the round jets it is opposite. The vortical structures obtained using swirling strength analysis are more intense in the elliptic jets compared with the round jets; consistent with their higher spreading. In the shear layers, the velocity skewness is considerably positive due to the diffusion of high-speed jet fluid towards the ambient. On the other hand, the streamwise skewness on the centerline is negative because of the entrainment of low-speed ambient fluid; resulting in centerline velocity decay. In addition, the joint and weighted joint probability density functions are used to understand the dominant events which contribute into the mixing of the jets with their surrounding fluid.

Application of Long-Distance Forward-Scattering Particle Image Velocimetry in Micro-Scale Turbulent Jet Flow Tests

2006 ◽  
Author(s):  
Lichuan Gui ◽  
Bernard J. Jansen ◽  
John M. Seiner
Keyword(s):  
Particle Image Velocimetry ◽  
High Speed ◽  
Particle Image ◽  
Forward Scattering ◽  
Particle Image Velocimetry System ◽  
Central Plane ◽  
Long Distance ◽  
Micro Scale ◽  
Air Jet ◽  
Image Velocimetry

A new particle image velocimetry system is applied to measure turbulent air jet flows from a micro-scale nozzle. The applied MPIV system includes a long-distance microscope that enables not only a long working distance, but also a forward-scattering optical setup. By using a high repeating rate Nd:YAG laser and an advanced digital camera, particle image recordings can be captured at 60 fps, i.e. 30 PIV recording pairs per second, with an interframing time of 180 ns, so that a high-speed flow measurement is enabled in micro scale. Measurements were conducted in the central plane of an air jet from a nozzle of 500 μm in diameter at flow velocity up to 110 m/s. Mean velocity and Reynolds stress distributions were determined with statistical analyses of thousands of instantaneous velocity maps.

Measurement and Characterization the Flocculation Processes by PIV

2013 ◽  
Vol 456 ◽  
pp. 644-647
Author(s):  
Jun Feng Gao
Keyword(s):  
Particle Image Velocimetry ◽  
Velocity Field ◽  
Velocity Vector ◽  
Field Data ◽  
Particle Image ◽  
Morphological Changes ◽  
In Situ Observation ◽  
Image Velocimetry ◽  
Flocculation Process

Particle image velocimetry (PIV) was applied to characterize the morphological changes of flocs and to acquired velocity field data in the flocculation process in Taylor-Couette reactor. By use of PIV the morphological of the flocs with ferric trichloride (FeCl3) could be characterized and described with good performance, the velocity vector also could be measurement. It was shown that the flocculation efficiencies reached the maximum values and the size of the generated flocs was the biggest when the roating speed was in the range between 20~60 rpm. It was demonstrated that PIV can be exploited as a useful tool in the in-situ observation the flocculation processes. Keywords: flocs morphology; flocculation efficiencies; velocity vector; PIV

scholarly journals Three-Dimensional Reconstruction of Evaporation-Induced Instabilities Using Volumetric Scanning Particle Image Velocimetry

Optics ◽  
2020 ◽  
Vol 1 (1) ◽  
pp. 52-70
Author(s):  
Mohammad Amin Kazemi ◽  
Janet A. W. Elliott ◽  
David S. Nobes
Keyword(s):  
Particle Image Velocimetry ◽  
Velocity Vector ◽  
Vector Fields ◽  
Particle Image ◽  
Laser Sheet ◽  
Three Dimensional ◽  
Maximum Velocity ◽  
Liquid Volume ◽  
Image Velocimetry ◽  
Out Of Plane

The three-dimensional (3D) flow below the interface of an evaporating liquid at a low pressure is visualized and quantified using scanning particle image velocimetry. The technique presented highlights the use of a single camera and a relatively fast moving laser sheet to image the flow for an application where using more than one camera is difficult. The technique allows collection of the full three-dimensional velocity vector map over the whole liquid volume. The out-of-plane component of the velocity has been determined using two different processing approaches: (i) deriving the full vector from a 3D cross-correlation of the particle volumes and (ii) applying the continuity equation to determine out-of-plane velocities from the calculated in-plane velocity vector fields. The results obtained from both methods showed good agreement with each other. The 3D velocity field reveals the existence of a torus shaped vortex below the evaporating meniscus that was induced by the exposure of the cold liquid to the warmer solid walls. The velocity data also shows that the maximum velocity occurs below the interface, not at the interface which highlights that the observed vortex is not driven by thermocapillary forces that usually govern the flow during evaporation at smaller scales.

scholarly journals Leading-edge vortices over swept-back wings with varying sweep geometries

2019 ◽  
Vol 6 (7) ◽  
pp. 190514 ◽  
Author(s):  
William B. Lambert ◽  
Mathew J. Stanek ◽  
Roi Gurka ◽  
Erin E. Hackett
Keyword(s):  
Particle Image ◽  
Delta Wing ◽  
Leading Edge ◽  
Micro Air Vehicles ◽  
Linear Sweep ◽  
Image Velocimetry ◽  
Gliding Flight ◽  
Swirling Strength ◽  
The Common ◽  
Leading Edge Vortices

Micro air vehicles are used in a myriad of applications, such as transportation and surveying. Their performance can be improved through the study of wing designs and lift generation techniques including leading-edge vortices (LEVs). Observation of natural fliers, e.g. birds and bats, has shown that LEVs are a major contributor to lift during flapping flight, and the common swift ( Apus apus ) has been observed to generate LEVs during gliding flight. We hypothesize that nonlinear swept-back wings generate a vortex in the leading-edge region, which can augment the lift in a similar manner to linear swept-back wings (i.e. delta wing) during gliding flight. Particle image velocimetry experiments were performed in a water flume to compare flow over two wing geometries: one with a nonlinear sweep (swift-like wing) and one with a linear sweep (delta wing). Experiments were performed at three spanwise planes and three angles of attack at a chord-based Reynolds number of 26 000. Streamlines, vorticity, swirling strength, and Q -criterion were used to identify LEVs. The results show similar LEV characteristics for delta and swift-like wing geometries. These similarities suggest that sweep geometries other than a linear sweep (i.e. delta wing) are capable of creating LEVs during gliding flight.

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