Comparison of Near Wake-Flow Structure Behind a Solid Cap With an Attached Bubble and a Solid Counterpart

2003 ◽  
Author(s):  
Hidekazu No ◽  
Michel Call ◽  
Akira T. Tokuhiro
Keyword(s):  
Flow Structure ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Equivalent Diameter ◽  
Velocity Measurements ◽  
Near Wake ◽  
Downward Flow ◽  
Air Bubble ◽  
Square Channel ◽  
Image Velocimetry

An experimental study was conducted on the flow structure in the near-wake of a hollow cap with an air bubble attached underneath and a solid object possessing a bubble-like shape. The objective of the study was to elucidate distinguishing wake flow characteristics of the capped bubble relative to the solid. The experiment was performed in a square channel, 80×80mm2 in cross section. The bubble and solid were separately suspended in downward flow of purified water. Both the capped bubble and the solid were ellipsoidal in shape (the cap was shaped to represent the front of an ellipsoidal bubble) and had an approximate volume of 0.8ml. The Reynolds number for the flow, based on the objects’ equivalent diameter and average downward flow velocity (U = 25cm/s), was Re ≅ 2800. Velocity measurements were taken using Particle Image Velocimetry. The obtained velocity data were analyzed to deduce vorticity, turbulent kinetic energy, production, and Reynolds stress. Graphic and numerical comparisons between the two cases were made. The results to date are discussed.

Measurement in the Wake Region of Two Bubbles in Close Proximity by Combined Shadow-Image and PIV Techniques

1999 ◽  
Vol 121 (1) ◽  
pp. 191-197 ◽  
Author(s):  
A. Tokuhiro ◽  
A. Fujiwara ◽  
K. Hishida ◽  
M. Maeda
Keyword(s):  
Ccd Camera ◽  
Shadow Image ◽  
Wake Flow ◽  
Air Bubbles ◽  
Velocity Measurements ◽  
Square Channel ◽  
Image Velocimetry ◽  
Close Proximity ◽  
Spatio Temporal ◽  
Two Bubbles

An experimental study on flow around two similarly-sized, adjacent air bubbles confined in a 1000 mm vertical, square channel (100 × 100 mm2) with downward flow of water was conducted. The bubbles were D = 11.7 mm in major diameter, ellipsoidal in shape (0.4 ml volume) and 12 mm apart. The Reynolds and Eo¨tvo¨s numbers were 1950 < ReD < 2250, 11 < Eo < 11.5 such that the bubbles oscillated. Velocity measurements were taken using Digital Particle Image Velocimetry, Complemented by Laser Induced Fluorescence. Simultaneously, a second CCD camera recorded the shadow image of the bubble pair’s motions. Visualization revealed that the bubbles move out of phase and do not collide nor coalesce. The velocity data revealed the dynamic interaction of two wake-flow velocity fields with a jet-like flow in-between. From the DPIV data, estimates of the vorticity, Reynolds-stress and turbulent kinetic energy (TKE) distributions confirmed the spatio-temporal nature of the flow. Details will be presented.

scholarly journals Near-Wake Flow Structure of a Suspended Cylindrical Canopy Patch

Water ◽  
2019 ◽  
Vol 12 (1) ◽  
pp. 84 ◽  
Author(s):  
Ayşe Yüksel Ozan ◽  
Didem Yılmazer
Keyword(s):  
Flow Field ◽  
Flow Structure ◽  
Green Infrastructure ◽  
Nutrient Management ◽  
Wake Flow ◽  
Wake Region ◽  
Near Wake ◽  
Vegetation Patch ◽  
Comprehensive Picture ◽  
Flowing Waters

Urban stormwater is an important environmental problem, especially for metropolitans worldwide. The most important issue behind this problem is the need to find green infrastructure solutions, which provide water treatment and retention. Floating treatment wetlands, which are porous patches that continue down from the free-surface with a gap between the patch and bed, are innovative instruments for nutrient management in lakes, ponds, and slow-flowing waters. Suspended cylindrical vegetation patches in open channels affect the flow dramatically, which causes a deviation from the logarithmic law. This study considered the velocity measurements along the flow depth, at the axis of the patch, and at the near-wake region of the canopy, for different submerged ratios with different patch porosities. The results of this experimental study provide a comprehensive picture of the effects of different submergence ratios and different porosities on the flow field at the near-wake region of the suspended vegetation patch. The flow field was described with velocity and turbulence distributions along the axis of the patch, both upstream and downstream of the vegetation patch. Mainly, it was found that suspended porous canopy patches with a certain range of densities (SVF20 and SVF36 corresponded to a high density of patches in this study) have considerable impacts on the flow structure, and to a lesser extent, individual patch elements also have a crucial role.

scholarly journals The Effect of Boattail Angles on the Near-Wake Structure of Axisymmetric Afterbody Models at Low-Speed Condition

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
The Hung Tran
Keyword(s):  
Strouhal Number ◽  
Reynolds Shear Stress ◽  
Flow Behavior ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Turbulent Intensity ◽  
Near Wake ◽  
Wake Structure ◽  
Low Speed ◽  
Speed Condition

The effect of a boattail angle on the structure of the wake of an axisymmetric model was investigated at low-speed condition. Four conical boattail models with angles of 0° (blunt-based body), 10°, 16°, and 22° were selected for this study. The Reynolds number based on the diameter of the model was around 1.97×104. Particle image velocimetry (PIV) was used to measure the velocity of the wake flow. The time-averaged flow characteristics including the length of recirculation of the afterbody, turbulent intensity, and Reynolds shear stress were analyzed and compared among those boattail models. The experimental results showed that the length of recirculation decreases with increasing boattail angle to 16°. At a boattail angle above 16°, the flow was fully separated near the shoulder and near-wake structure was highly changed. The turbulent intensity at a boattail angle of 22° showed a similar level to that in the case of the blunt-based body. Flow behavior on boattail surface should be accounted as an important parameter affecting the wake width and drag of the model. Power spectral density and proper orthogonal decomposition (POD) analyses showed that a Strouhal number of StD=0.2 dominated for the boattail model up to 16°. The fully separated flow was dominated by a Strouhal number of StD=0.03−0.06, which was firstly presented in this study.

Free end effects on the near wake flow structure behind a finite circular cylinder

2000 ◽  
Vol 88 (2-3) ◽  
pp. 231-246 ◽  
Author(s):  
Cheol-Woo Park ◽  
Sang-Joon Lee
Keyword(s):  
Circular Cylinder ◽  
Flow Structure ◽  
Wake Flow ◽  
Near Wake ◽  
End Effects

Experimental Study on the Near Wake Behind Two Staggered Cylinders of Unequal Diameters

2012 ◽  
Author(s):  
Yangyang Gao ◽  
Xikun Wang ◽  
Soon Keat Tan
Keyword(s):  
Reynolds Stress ◽  
Incident Angle ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Circular Cylinders ◽  
Mean Flow ◽  
Near Wake ◽  
Particle Image Velocimetry Technique ◽  
Image Velocimetry ◽  
Spacing Ratio

The wake structure behind two staggered circular cylinders with unequal diameters was investigated experimentally using the particle image velocimetry technique (PIV). This investigation was focused on the variations of flow patterns in terms of incident angle at Reynolds number Re = 1200. Comparisons of the time-averaged flow field of two staggered cylinders with unequal diameters at different angles were made to elucidate the mean flow characteristics. The characteristics of Reynolds shear stress contours at different incident angles and spacing ratios were also investigated. The results showed that with increasing of incident angle, the scale of Reynolds stress contours behind the upstream cylinder becomes larger, as well as the effect of spacing ratio on Reynolds stress contours.

Experimental Analysis of the Near Wake from a Ducted Thruster at True and Near Bollard Pull Conditions Using Stereo Particle Image Velocimetry (SPIV)

2005 ◽  
Vol 127 (3) ◽  
pp. 191-196 ◽  
Author(s):  
S. El Lababidy ◽  
N. Bose ◽  
P. Liu ◽  
D. Walker ◽  
F. Di Felice
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Practical Interest ◽  
Stereoscopic Particle Image Velocimetry ◽  
Wake Flow ◽  
Hydrodynamic Characteristics ◽  
Near Wake ◽  
Image Velocimetry ◽  
Wide Range ◽  
Marine Applications

Thrusters working at low advance coefficients are employed in a wide range of offshore and marine applications on Floating, Production, Storage, and Offloading (FPSO) systems; shuttle tankers; tug boats; and mobile offshore units. Therefore, an understanding of the flow around the thrusters is of great practical interest. Despite this interest, there is lack of knowledge in the description of the hydrodynamic characteristics of a ducted thruster’s wake at bollard pull and low advance coefficient values. This work was aimed at providing detailed data about the hydrodynamic characteristics of a Dynamic Positioning (DP) thruster near wake flow at different low advance coefficient values. Wake measurements were made during cavitation tunnel tests carried out on a ducted propeller model at the Italian Ship Model Basin (INSEAN), Rome, Italy. Through these experiments, the DP thruster near wake velocity components at different downstream axial planes, up to 1.5 diameters downstream, were obtained using a Stereoscopic Particle Image Velocimetry (SPIV) system. These experiments were carried out at different advance coefficient (J) values [bollard pull (J=0), J=0.4 and J=0.45].

Near wake flow analysis of a vertical axis wind turbine by stereoscopic particle image velocimetry

2014 ◽  
Vol 70 ◽  
pp. 47-61 ◽  
Author(s):  
G. Tescione ◽  
D. Ragni ◽  
C. He ◽  
C.J. Simão Ferreira ◽  
G.J.W. van Bussel
Keyword(s):  
Particle Image Velocimetry ◽  
Wind Turbine ◽  
Particle Image ◽  
Flow Analysis ◽  
Vertical Axis ◽  
Stereoscopic Particle Image Velocimetry ◽  
Wake Flow ◽  
Vertical Axis Wind Turbine ◽  
Near Wake ◽  
Image Velocimetry

Near-Wake Flow of a V-Gutter With Slit Bleed (Data Bank Contribution)

1993 ◽  
Vol 115 (1) ◽  
pp. 13-20 ◽  
Author(s):  
Jing-Tang Yang ◽  
Go-Long Tsai
Keyword(s):  
Reverse Flow ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Data Bank ◽  
Wake Flow ◽  
Near Wake ◽  
Cold Flow ◽  
Flame Holder ◽  
Flow Through

The cold-flow characteristics of a v-shape flame holder with flow bleed from a slit located at the leading edge have been investigated. According to experimental evidence, a nonsymmetric wake structure is developed behind the symmetric slit v-gutter. The flow through the slit induces greater reverse flow and greater back pressure in the near wake. It also provokes more extensive transport across the shear layers and reduces both the turbulent intensity and the Reynolds shear stress of the wake flow. These results indicate that the slit v-gutter can have a better flame holding ability and less pressure loss compared with the traditional v-gutter. In view of fluid dynamics features, the slit v-gutter is indeed a potentially useful design of flame holder.

Experimental Investigation of the Wake of a Discontinuous Cylinder

2010 ◽  
Author(s):  
V. S. R. Mandava ◽  
Gregory A. Kopp ◽  
Joan Herrero ◽  
Francesc Giralt
Keyword(s):  
Structural Characteristics ◽  
Dominant Role ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Near Wake ◽  
Vortical Structures ◽  
Image Velocimetry ◽  
Entrainment Process ◽  
Thin Steel Plate

The effects of a discontinuous cylinder geometry on the near wake structures was investigated experimentally. This ‘discontinuous’ circular cylinder has gaps so that solid segments 5D long are followed by gaps 2.5D long, in a repeating pattern, where D is the diameter of the cylinder. A thin steel plate was used to hold all of the cylinder pieces together. Thus, a three-dimensional (3D) wake was created at the origin with the intent to force the near wake flow to have similar structural characteristics as the far wake behind an ‘infinite/continuous’ cylinder, i.e., a near wake flow with horseshoes or double rollers formed by rapid kinking of Ka´rma´n-like vortices. Since the kinetic energy associated with the fluctuations of these near-wake 3D vortical structures is high, the flow system is considered suitable to clarify the role of these velocity patterns in the entrainment process of wake flows, which is still the subject of controversy. Particle Image Velocimetry (PIV) and Hot-Wire Anemometry (HWA) techniques were used to analyze the flow at two Reynolds numbers, Re = 10000 and 4000, in the wake of the discontinuous cylinder up to x/D = 190 downstream. The development of double rollers resulting from the interaction between the high momentum flow through the gaps and the Ka´rma´n-like vortices formed behind the solid cylindrical segments was confirmed. The Strouhal number of the double rollers in the wake is 0.14. These vortices have a dominant role in the initial wake growth. Thus, the overall flow dynamics are similar to the momentum transfer that takes place at the scale of the intermittent turbulent bulges that protrude from the wake in the far region and that were reported to be associated with double rollers.

scholarly journals Turbulent Wake-Flow Characteristics in the Near Wake of Freely Flying Raptors: A Comparative Analysis Between an Owl and a Hawk

2020 ◽  
Vol 60 (5) ◽  
pp. 1109-1122 ◽  
Author(s):  
Krishnamoorthy Krishnan ◽  
Hadar Ben-Gida ◽  
Gareth Morgan ◽  
Gregory A Kopp ◽  
Christopher G Guglielmo ◽  
...  
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Flight Behavior ◽  
Aerodynamic Noise ◽  
Particle Image Velocimetry System ◽  
Near Wake ◽  
Time Resolved ◽  
Turbulent Characteristics

Synopsis Owl flight has been studied over multiple decades associated with bio-inspiration for silent flight. However, their aerodynamics has been less researched. The aerodynamic noise generated during flight depends on the turbulent state of the flow. In order to document the turbulent characteristics of the owl during flapping flight, we measured the wake flow behind a freely flying great horned owl (Bubo virginianus). For comparison purposes, we chose to fly a similar-sized raptor a Harris’s hawk (Parabuteo unicinctus): one is nocturnal and the other is a diurnal bird of prey. Here, we focus on the wake turbulent aspects and their impact on the birds’ flight performances. The birds were trained to fly inside a large-scale wind tunnel in a perch-to-perch flight mode. The near wake of the freely flying birds was characterized using a long duration time-resolved particle image velocimetry system. The velocity fields in the near wake were acquired simultaneously with the birds’ motion during flight which was sampled using multiple high-speed cameras. The turbulent momentum fluxes, turbulent kinetic energy production, and dissipation profiles are examined in the wake and compared. The near wake of the owl exhibited significantly higher turbulent activity than the hawk in all cases, though both birds are similar in size and followed similar flight behavior. It is suggested that owls modulate the turbulence activity of the near wake in the vicinity of the wing, resulting in rapid decay before radiating into the far-field; thus, suppressing the aerodynamic noise at the far wake.

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