The Flow Structure and Statistics of a Passive Mixing Tab

1993 ◽  
Vol 115 (2) ◽  
pp. 255-263 ◽  
Author(s):  
W. J. Gretta ◽  
C. R. Smith
Keyword(s):  
Boundary Layer ◽  
Flow Visualization ◽  
Flow Structure ◽  
Water Channel ◽  
Symmetry Plane ◽  
Streamwise Vortices ◽  
Hairpin Vortices ◽  
Mean Velocity ◽  
Velocity Statistics ◽  
Passive Mixing

Water channel flow visualization and anemometry studies were conducted to examine the flow structure and velocity statistics in the wake of a passive mixing tab designed for enhancement of cross-stream mixing by generation of flow structures characteristic of turbulent boundary layers. Flow visualization reveals that the mixing tab generates a wake comprising a combination of counter rotating, streamwise vortices enveloped by distinct hairpin vortex structures. The counter rotating streamwise vortices are observed to stimulate a strong ejection of fluid along the symmetry plane, which results in very rapid cross-stream mixing. The hairpin vortices are found to undergo successive amalgamation and coalescence downstream of the device, which aids in the streamwise mixing and outward penetration of ejected fluid. After an initially intense mixing process, the mixing tab wake rapidly develops mean velocity, turbulence intensity, and boundary layer integral properties characteristic of a significantly thickened turbulent boundary layer.

scholarly journals Control of Synthetic Hairpin Vortices in Laminar Boundary Layer for Skin-Friction Reduction

2020 ◽  
Vol 8 (1) ◽  
pp. 45
Author(s):  
Bonguk Koo ◽  
Yong-Duck Kang
Keyword(s):  
Boundary Layer ◽  
Flow Visualization ◽  
Skin Friction ◽  
Laminar Boundary Layer ◽  
Water Channel ◽  
Friction Reduction ◽  
Control Technique ◽  
Hairpin Vortices ◽  
Local Skin ◽  
Hot Film

The results of flow visualization and hot-film measurement in a water channel are presented in this paper, in which the effectiveness of controlling synthetic hairpin vortices in the laminar boundary layer is examined to reduce skin friction. In this study, hairpin vortices were generated by periodically injecting vortex rings into a cross flow through a hole on a flat plate. To control the hairpin vortices, jets were issued from a nozzle directly onto the head of the hairpins. The results of the flow visualization demonstrated that the jets destroyed the hairpins by disconnecting the heads from their legs, after which the weakened hairpin vortices could not develop. Therefore, the circulation around the legs was reduced, which suggests that the direct intervention on the hairpin heads resulted in the reduction of streamwise stretching. Data obtained by a hot-film sensor showed that the high-speed regions outside the hairpin legs were reduced in speed by this control technique, leading to a decrease in the associated local skin friction.

Interaction between a spatially growing turbulent boundary layer and embedded streamwise vortices

1996 ◽  
Vol 326 ◽  
pp. 151-179 ◽  
Author(s):  
Junhui Liu ◽  
Ugo Piomelli ◽  
Philippe R. Spalart
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Turbulent Boundary Layer ◽  
Turbulent Kinetic Energy ◽  
Vortex Core ◽  
Eddy Viscosity ◽  
Shear Stresses ◽  
Streamwise Vortices ◽  
Mean Velocity ◽  
Production Term

The interaction between a zero-pressure-gradient turbulent boundary layer and a pair of strong, common-flow-down, streamwise vortices with a sizeable velocity deficit is studied by large-eddy simulation. The subgrid-scale stresses are modelled by a localized dynamic eddy-viscosity model. The results agree well with experimental data. The vortices drastically distort the boundary layer, and produce large spanwise variations of the skin friction. The Reynolds stresses are highly three-dimensional. High levels of kinetic energy are found both in the upwash region and in the vortex core. The two secondary shear stresses are significant in the vortex region, with magnitudes comparable to the primary one. Turbulent transport from the immediate upwash region is partly responsible for the high levels of turbulent kinetic energy in the vortex core; its effect on the primary stress 〈u′v′〉 is less significant. The mean velocity gradients play an important role in the generation of 〈u′v′〉 in all regions, while they are negligible in the generation of turbulent kinetic energy in the vortex core. The pressure-strain correlations are generally of opposite sign to the production terms except in the vortex core, where they have the same sign as the production term in the budget of 〈u′v′〉. The results highlight the limitations of the eddy-viscosity assumption (in a Reynolds-averaged context) for flows of this type, as well as the excessive diffusion predicted by typical turbulence models.

Tip and Junction Vortices Generated by the Sail of a Yawed Submarine Model at Low Reynolds Numbers

2011 ◽  
Vol 133 (3) ◽  
Author(s):  
Juan M. Jiménez ◽  
Alexander J. Smits
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Water Channel ◽  
Reynolds Numbers ◽  
Tip Vortex ◽  
Streamwise Vortices ◽  
Low Reynolds Numbers ◽  
Good Accordance ◽  
Finite Wing ◽  
Yaw Angle

Results are presented on the behavior of the tip and junction vortices generated by the sail of a SUBOFF submarine model at yaw angles from 6 deg to 17 deg for a Reynolds number of 94×103 based on model length. The measurements were conducted in a water channel on a spanwise plane 1.3 chord lengths downstream from the trailing edge of the sail. In the vicinity of the sail hull junction, the presence of streamwise vortices in the form of horseshoe or necklace vortices locally dominates the flow. As the yaw angle is increased from 6 deg to 9 deg, the circulation of the sail tip vortex increases, and is in good accordance with predictions from finite wing theory. However, as the yaw angle is further increased, the sail boundary layer separates with an overall drop in circulation. In contrast, the circulation value for the junction vortex increases with yaw angle, and only drops slightly at the highest yaw angle.

Experimental observation of hairpin auto-generation events in a turbulent boundary layer

2016 ◽  
Vol 795 ◽  
pp. 611-633 ◽  
Author(s):  
Y. Jodai ◽  
G. E. Elsinga
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Shear Layer ◽  
Transition Process ◽  
Wall Region ◽  
Generation Model ◽  
Streamwise Vortices ◽  
Hairpin Vortices ◽  
Time Resolved ◽  
Near Wall

Time-resolved tomographic particle image velocimetry experiments show that new hairpin vortices are generated within a fully developed and unperturbed turbulent boundary layer. The measurements are taken at a Reynolds number based on the momentum thickness of 2038, and cover the near-wall region below $y^{+}=140$, where $y^{+}$ is the wall-normal distance in wall units. Instantaneous visualizations of the flow reveal near-wall low-speed streaks with associated quasi-streamwise vortices, retrograde inverted arch vortices, hairpin vortices and hairpin packets. The hairpin heads are observed as close to the wall as $y^{+}=30$. Examples of hairpin packet evolution reveal the development of new hairpin vortices, which are created upstream and close to the wall in a manner consistent with the auto-generation model (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353–396). The development of the new hairpin appears to be initiated by an approaching sweep event, which perturbs the shear layer associated with the initial packet. The shear layer rolls up, thereby forming the new hairpin head. The head subsequently connects to existing streamwise vortices and develops into a hairpin. The time scale associated with the hairpin auto-generation is 20–30 wall units of time. This demonstrates that hairpins can be created over short distances within a developed turbulent boundary layer, implying that they are not simply remnants of the laminar-to-turbulent transition process far upstream.

Measurements Within Go¨rtler Vortices

1979 ◽  
Vol 101 (4) ◽  
pp. 517-520 ◽  
Author(s):  
S. H. Winoto ◽  
D. F. G. Dura˜o ◽  
R. I. Crane
Keyword(s):  
Boundary Layer ◽  
Flow Visualization ◽  
Laminar Boundary Layer ◽  
Velocity Component ◽  
Water Channel ◽  
Concave Surface ◽  
Vortex Pattern ◽  
Naturally Occurring ◽  
Local Measurements ◽  
Made In

Local measurements of stream wise velocity component have been made in the laminar boundary layer on the concave surface of a water channel, supported by flow visualization. Details of the naturally-occurring Go¨rtler vortex pattern are presented.

Transition in Pressure-Surface Boundary Layers

1987 ◽  
Vol 109 (2) ◽  
pp. 296-302 ◽  
Author(s):  
R. I. Crane ◽  
G. Leoutsakos ◽  
J. Sabzvari
Keyword(s):  
Boundary Layer ◽  
Water Channel ◽  
Outer Wall ◽  
Reynolds Numbers ◽  
Surface Boundary ◽  
Mean Velocity ◽  
Pressure Surface ◽  
Wall Boundary Layer ◽  
Transition Criteria ◽  
Dimensional Boundary

Laminar-to-turbulent transition in the presence of Go¨rtler vortices has been investigated experimentally, in the outer wall boundary layer of a curved water channel. Ratios of boundary layer thickness at the start of curvature to wall radius were around 0.05 and core flow turbulence intensities were between 1 and 3 percent. Measurements of intermittency factor were made by hot film probe and of mean and rms velocity by laser anemometer. At Reynolds numbers low enough to allow considerable nonlinear vortex amplification in the laminar region, transition was found to begin sooner and progress faster at a vortex upwash position than at a spanwise-adjacent downwash position. Measured Go¨rtler numbers at transition onset bore little relationship to those often used as transition criteria in two-dimensional boundary layer prediction codes. Little spanwise variation in intermittency occurred at higher Reynolds numbers, where mean velocity profiles at upwash were much less inflected. Toward the end of curvature, favorable pressure gradients estimated to exceed the Launder relaminarization value corresponded with cases of incomplete transition.

Angled Injection of Jets Into a Turbulent Boundary Layer

1980 ◽  
Vol 102 (2) ◽  
pp. 211-218 ◽  
Author(s):  
L. H. Y. Lee ◽  
J. A. Clark
Keyword(s):  
Boundary Layer ◽  
Friction Coefficient ◽  
Turbulent Boundary Layer ◽  
Physical Model ◽  
Visual Information ◽  
Water Channel ◽  
Skin Friction Coefficient ◽  
Velocity Data ◽  
Mean Velocity ◽  
Plane Jets

Inclined laminar submerged plane jets were injected from a 1 cm slot into a turbulent boundary layer developed on a sidewall of a water channel. Profiles of mean velocities and longitudinal fluctuations were measured to 64 slot widths downstream of the jet exit. Length and velocity similarity scales were obtained from mean velocity data, and local values of skin friction coefficient were determined. Two maxima in the longitudinal fluctuation profiles were established and found to follow precisely loci of vortex formations. The maxima grew exponentially in the downstream direction and peaked at a location where these transverse vortices were at their full strength before coalescence. Effects of different injection angles and velocity ratios were found. Further extension of a physical model to describe the flow is validated based on correlation of mean and fluctuating velocity data with visual information.

The Mean Flow Structure Around and Within a Turbulent Junction or Horseshoe Vortex—Part I: The Upstream and Surrounding Three-Dimensional Boundary Layer

1988 ◽  
Vol 110 (4) ◽  
pp. 406-414 ◽  
Author(s):  
J. D. Menna ◽  
F. J. Pierce
Keyword(s):  
Boundary Layer ◽  
Flow Structure ◽  
Vortex Flow ◽  
Three Dimensional ◽  
Horseshoe Vortex ◽  
Mean Flow ◽  
Complex Flow ◽  
Mean Velocity ◽  
Standard Case ◽  
The Mean

The mean flow structure upstream, around, and in a turbulent junction or horseshoe vortex is reported for an incompressible, subsonic flow. This fully documented, unified, comprehensive, and self-consistent data base is offered as a benchmark or standard case for assessing the predictive capabilities of computational codes developed to predict this kind of complex flow. Part I of these papers defines the total flow being documented. The upstream and surrounding three-dimensional turbulent boundary layer-like flow away from separation has been documented with mean velocity field and turbulent kinetic energy field measurements made with hot film anemometry, and local wall shear stress measurements. Data are provided for an initial condition plane well upstream of the junction vortex flow to initiate a boundary layer calculation, and freestream or edge velocity, as well as floor static pressure, are reported to proceed with the solution. Part II of these papers covers the flow through separation and within the junction vortex flow.

INVESTIGATION ON FLOW STRUCTURE BEHIND A RING BY IMPLEMENTING OPTICAL PARTICLE CHARACTERIZATIONS

2006 ◽  
Vol 20 (25n27) ◽  
pp. 4511-4516
Author(s):  
CHEOL WOO PARK
Keyword(s):  
Flow Structure ◽  
Bluff Body ◽  
Water Channel ◽  
Cross Flow ◽  
Circular Ring ◽  
Outer Side ◽  
Mean Velocity ◽  
Circulating Water ◽  
Image Velocimetry ◽  
Bluff Body Wake

The flow structure behind circular and elliptical type rings embedded in a cross-flow was investigated experimentally using a particle image velocimetry (PIV), implementing optical particle characterizations. The experiments were performed in a circulating water channel with a test section of 0.2m height × 0.3m width × 1.2m length. The Reynolds number based on the ring hoop cord length is about Re =1200. The velocity fields near the ring hoop were measured using the two-frame cross-correlation PIV method. As a result, the flow near the sharp-edged end of ring hoop ascends fast and showed a conventional vortical structure appeared in a bluff body wake. In the mean velocity field behind a circular ring, there were two large vortices rotating in different directions from each other in the near wake regime caused by the interaction between the central jet flow and the entrained ambient fluids from outer side of ring hoop.

Flow Visualization and PIV Measurements of Flow Around a Sport Utility Vehicle Model

2010 ◽  
Author(s):  
Alban Lamy ◽  
Amandine Hamel ◽  
Christina Vanderwel ◽  
Stavros Tavoularis ◽  
Azeddine Kourta
Keyword(s):  
Boundary Layer ◽  
Flow Visualization ◽  
Flow Structure ◽  
Recirculation Zone ◽  
Aerodynamic Drag ◽  
Vertical Plane ◽  
The Body ◽  
Dimensionless Frequency ◽  
Sport Utility Vehicle ◽  
Utility Vehicle

The instantaneous flow structure around a 1:18 scaled model of a square-back sport utility vehicle (Hummer H2) was documented in a low-speed water tunnel. The study comprises both flows with the model fixed on a flat plate and flows with the model’s wheels rolling on an endless belt that moved with the speed of the free stream, thus simulating ground effects. The flow structure was investigated using flow visualization by dye injection as well as particle image velocimetry (PIV) for several Reynolds numbers in the range of 7000 to 27700. The flow along the roof, the sidewalls, and the underbody was observed to separate at the rear edges of the body, creating a recirculation zone at the rear of the vehicle, which is associated with pressure loss and a major contribution to aerodynamic drag. In the vertical plane of symmetry, this recirculation zone appears as two counter-rotating vortices. With a fixed ground, the lower vortex was less energetic than the upper vortex because the boundary layer that developed along the ground upstream of the model reduced the momentum of the flow below the vehicle. This boundary layer was also observed to separate from the ground behind the vehicle, creating a third vortex located further downstream along the ground. This boundary layer separation forced the bottom vortex to remain attached to the base of the vehicle, whereas the upper vortex was advected in the wake. The dimensionless frequency (Strouhal number) of the vortex shedding process from the roof was found to be in the range of 0.1 to 0.9. With a moving ground, the upper vortex behaved similarly to that in the fixed ground configuration; however, in the absence of the boundary layer along the ground, the lower vortex was typically stronger and its location showed some variability. In both configurations, the Reynolds number had little influence on the wake topology, mostly increasing the turbulence intensity without modifying the main flow pattern.

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