On the formation of streamwise vortices by plasma vortex generators

AbstractThe streamwise vortices generated by dielectric-barrier-discharge plasma actuators in the laminar boundary layer were investigated using particle image velocimetry to understand the vortex-formation mechanisms. The plasma vortex generator was oriented along the primary flow direction to produce a body force in the spanwise direction. This created a spanwise-directed wall jet which interacted with the oncoming boundary layer to form a coherent streamwise vortex. It was found that the streamwise vortices were formed by the twisting and folding of the spanwise vorticity in the oncoming boundary layer into the outer shear layer of the spanwise wall jet, which added its own vorticity to increase the circulation along the actuator length. This is similar to the delta-shaped, vane-type vortex generator, except that the circulation was enhanced by the addition of the vorticity in the plasma jet. It was also observed that the plasma vortex was formed close to the wall with an enhanced wall-ward entrainment, which created strong downwash above the actuator.

Download Full-text

Development of a Steady Vortex Generator Jet in a Turbulent Boundary Layer

Journal of Fluids Engineering ◽

10.1115/1.1627833 ◽

2003 ◽

Vol 125 (6) ◽

pp. 1006-1015 ◽

Cited By ~ 15

Author(s):

Gregory S. Rixon ◽

Hamid Johari

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Flow Direction ◽

Vortex Generator ◽

Streamwise Vortex ◽

Freestream Velocity ◽

Local Boundary ◽

Image Velocimetry ◽

Particle Image Velocimetry Method ◽

Vortex Generator Jet

The development of a vortex generator jet within a turbulent boundary layer was studied by the particle image velocimetry method. Jet velocities ranging from one to three times greater than the freestream velocity were examined. The jet was pitched 45 deg and skewed 90 deg with respect to the surface and flow direction, respectively. The velocity field in planes normal to the freestream was measured at four stations downstream of the jet exit. The jet created a pair of streamwise vortices, one of which was stronger and dominated the flow field. The circulation, peak vorticity, and wall-normal position of the primary vortex increased linearly with the jet velocity. The circulation and peak vorticity decreased exponentially with the distance from the jet source for the jet-to-freestream velocity ratios of 2 and 3. The wandering of the streamwise vortex can be as much as ±30% of the local boundary layer thickness at the farthest measurement station.

Download Full-text

Formation of surface trailing counter-rotating vortex pairs downstream of a sonic jet in a supersonic cross-flow

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.455 ◽

2018 ◽

Vol 850 ◽

pp. 551-583 ◽

Cited By ~ 16

Author(s):

Mingbo Sun ◽

Zhiwei Hu

Keyword(s):

Separation Bubble ◽

Three Dimensional ◽

Cross Flow ◽

Vortex Generator ◽

Vortex Pair ◽

Formation Mechanisms ◽

Wall Jet ◽

Recirculating Flow ◽

Sonic Jet ◽

Counter Rotating Vortex Pair

Direct numerical simulations were conducted to uncover physical aspects of a transverse sonic jet injected into a supersonic cross-flow at a Mach number of 2.7. Simulations were carried out for two different jet-to-cross-flow momentum flux ratios ($J$) of 2.3 and 5.5. It is identified that collision shock waves behind the jet induce a herringbone separation bubble in the near-wall jet wake and a reattachment valley is formed and embayed by the herringbone recirculation zone. The recirculating flow in the jet leeward separation bubble forms a primary trailing counter-rotating vortex pair (TCVP) close to the wall surface. Analysis on streamlines passing the separation region shows that the wing of the herringbone separation bubble serves as a micro-ramp vortex generator and streamlines acquire angular momentum downstream to form a secondary surface TCVP in the reattachment valley. Herringbone separation wings disappear in the far field due to the cross-interaction of lateral supersonic flow and the expansion flow in the reattachment valley, which also leads to the vanishing of the secondary TCVP. A three-dimensional schematic of surface trailing wakes is presented and explains the formation mechanisms of the surface TCVPs.

Download Full-text

Vortex Formation near the Intakes to Turbomachinery and Duct Systems

Proceedings of the Institution of Mechanical Engineers ◽

10.1243/pime_proc_1974_188_071_02 ◽

1974 ◽

Vol 188 (1) ◽

pp. 597-605 ◽

Cited By ~ 2

Author(s):

M. J. C. Swainston

Keyword(s):

Boundary Layer ◽

Gas Turbine ◽

Potential Flow ◽

Vortex Formation ◽

Experimental Results ◽

Formation Mechanisms ◽

Layer Potential ◽

Vortex Strength ◽

Duct Systems ◽

Experimental Findings

Experimental results are presented which show vortex formations occurring within a model marine gas-turbine downtake. The basic phenomenon of vortex formation is discussed and the boundary-layer-potential-flow interaction identified as the cause. Further experiments on a model gas-turbine test stand are reported in which the vortex strength was assessed. On this basis, a simplified theory for the occurrence and strength of vortex formations is presented which agrees qualitatively with the principal experimental findings. The formation of air-entraining vortices in hydraulic installations is briefly examined in the light of the explanations previously advanced. Although a number of possible vortex formation mechanisms are identified, it is concluded that further research is required in this area.

Download Full-text

Supersonic boundary-layer interactions with various micro-vortex generator geometries

The Aeronautical Journal ◽

10.1017/s0001924000003353 ◽

2009 ◽

Vol 113 (1149) ◽

pp. 683-697 ◽

Cited By ~ 13

Author(s):

S. Lee ◽

E. Loth

Keyword(s):

Boundary Layer ◽

Flow Separation ◽

Vortex Generator ◽

Wave Drag ◽

Supersonic Boundary Layer ◽

Vortex Generators ◽

Streamwise Vortices ◽

Streamwise Vorticity ◽

Displacement Thickness ◽

Mach 3

Abstract Various types of micro-vortex generators (μVGs) are investigated for control of a supersonic turbulent boundary layer subject to an oblique shock impingement, which causes flow separation. The micro-vortex generators are embedded in the boundary layer to avoid excessive wave drag while still creating strong streamwise vortices to energise the boundary layer. Several different types of µVGs were considered including micro-ramps and micro-vanes. These were investigated computationally in a supersonic boundary layer at Mach 3 using monotone integrated large eddy simulations (MILES). The results showed that vortices generated from μVGs can partially eliminate shock induced flow separation and can continue to entrain high momentum flux for boundary-layer recovery downstream. The micro-ramps resulted in thinner downstream displacement thickness in comparison to the micro-vanes. However, the strength of the streamwise vorticity for the micro-ramps decayed faster due to dissipation especially after the shock interaction. In addition, the close spanwise distance between each vortex for the ramp geometry causes the vortex cores to move upwards from the wall due to induced upwash effects. Micro-vanes, on the other hand, yielded an increased spanwise spacing of the streamwise vortices at the point of formation. This resulted in streamwise vortices staying closer to the floor with less circulation decay, and the reduction in overall flow separation is attributed to these effects. Two hybrid concepts, named ‘thick-vane’ and ‘split-ramp’, were also studied where the former is a vane with side supports and the latter has a uniform spacing along the centreline of the baseline ramp. These geometries behaved similar to the micro-vanes in terms of the streamwise vorticity and the ability to reduce flow separation, but are more physically robust than the thin vanes.

Download Full-text

Nonlinear instability of developing streamwise vortices with applications to boundary layer heat transfer intensification through an extended Reynolds analogy

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2008.0057 ◽

2008 ◽

Vol 366 (1876) ◽

pp. 2699-2716 ◽

Cited By ~ 10

Author(s):

J.T.C Liu

Keyword(s):

Heat Transfer ◽

Boundary Layer ◽

Heat Flux ◽

Reynolds Shear Stress ◽

Streamwise Vortex ◽

Secondary Instability ◽

Streamwise Vortices ◽

Present Contribution ◽

Reynolds Analogy ◽

Transport Effects

The intent of the present contribution is to explain theoretically the experimentally measured surface heat transfer rates on a slightly concave surface with a thin boundary layer in an otherwise laminar flow. As the flow develops downstream, the measured heat transfer rate deviates from the local laminar value and eventually exceeds the local turbulent value in a non-trivial manner even in the absence of turbulence. While the theory for steady strong nonlinear development of streamwise vortices can bridge the heat transfer from laminar to the local turbulent value, further intensification is attributable to the transport effects of instability of the basic steady streamwise vortex system. The problem of heat transport by steady and fluctuating nonlinear secondary instability is formulated. An extended Reynolds analogy for Prandtl number unity, Pr =1, is developed, showing the similarity between streamwise velocity and the temperature. The role played by the fluctuation-induced heat flux is similar to momentum flux by the Reynolds shear stress. Inferences from the momentum problem indicate that the intensified heat flux developing well beyond the local turbulent value is attributed to the transport effects of the nonlinear secondary instability, which leads to the formation of ‘coherent structures’ of the flow. The basic underlying pinions of the non-linear hydrodynamic stability problem are the analyses of J. T. Stuart, which uncovered physical mechanisms of nonlinearities that are crucial to the present developing boundary layers supporting streamwise vortices and their efficient scalar transporting mechanisms.

Download Full-text

Reynolds Stress Field of a Stronger Wall Jet Managed by a Streamwise Vortex: Effect of Periodic Perturbation

Volume 1: Fora, Parts A, B, C, and D ◽

10.1115/fedsm2003-45228 ◽

2003 ◽

Author(s):

Shinsuke Mochizuki ◽

Seiji Yamada ◽

Hideo Osaka

Keyword(s):

Stress Field ◽

Reynolds Stress ◽

Large Scale ◽

Streamwise Vortex ◽

Plane Wall ◽

Periodic Perturbation ◽

Spanwise Direction ◽

Wall Jet ◽

Streamwise Vorticity ◽

Entrainment Process

Six Reynolds stress components were studied experimentally to understand evolution of streamwise vortex and a plane wall jet. It is seen that periodic perturbation are able to modify non-isotropic Reynolds stress field involved in the transport equation for streamwise vorticity. Modified Reynolds stress field accelerates development of vortex radius in spanwise direction. Interaction between streamwise vortex and spanwise eddies in the outer layer of the plane wall jet strengthen both velocity and length scales of large-scale eddies and increase streamwise momentum flux in enhancement of entrainment process.

Download Full-text

Three-Dimensional Structure of a Nominally Planar Turbulent Boundary Layer

Journal of Fluids Engineering ◽

10.1115/1.3448970 ◽

1979 ◽

Vol 101 (3) ◽

pp. 326-330 ◽

Cited By ~ 3

Author(s):

Y. Furuya ◽

I. Nakamura ◽

H. Osaka

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Three Dimensional ◽

Leading Edge ◽

Turbulent Boundary Layers ◽

Main Flow ◽

The Other ◽

Dimensional Structure ◽

Spanwise Direction ◽

Streamwise Vortices

This research is concerned with detailed experiments on spanwise nonuniformity of nominally planar turbulent boundary layers. Two procedures for eliminating spanwise nonuniformity are studied. One method is to remove the original, natural vortices by introducing additional ones arising from protuberances attached to the leading edge of a flat plate, and the other technique is by making the main flow entirely uniform. Effects of artificially controlled streamwise vortices on spanwise nonuniformity are examined. From these experiments, the process by which induced vortices cause nonuniformity of turbulent boundary layer characteristics in the spanwise direction is discussed.

Download Full-text

Wall turbulence manipulation by large-scale streamwise vortices

Journal of Fluid Mechanics ◽

10.1017/s0022112002002574 ◽

2002 ◽

Vol 473 ◽

pp. 23-58 ◽

Cited By ~ 22

Author(s):

GAETANO IUSO ◽

MICHELE ONORATO ◽

PIER GIORGIO SPAZZINI ◽

GAETANO MARIA DI CICCA

Keyword(s):

Large Scale ◽

Time Derivative ◽

Vortex Generator ◽

Turbulent Channel Flow ◽

Wall Turbulence ◽

Spanwise Direction ◽

Streamwise Vortices ◽

Turbulent Structures ◽

Reduced Frequency ◽

Flow Manipulation

This paper describes an experimental study of the manipulation of a fully developed turbulent channel flow through large-scale streamwise vortices originated by vortex generator jets distributed along the wall in the spanwise direction. Apart from the interest in flow management itself, an important aim of the research is to observe the response of the flow to external perturbations as a technique for investigating the structure of turbulence. Considerable mean and fluctuating skin friction reductions, locally as high as 30% and 50% respectively, were measured for an optimal forcing flow intensity. Mean and fluctuating velocity profiles are also greatly modified by the manipulating large-scale vortices; in particular, attenuation of the turbulence intensity was measured. Moreover the flow manipulation caused an increase in longitudinal coherence of the wall organized motions, accompanied by a reduced frequency of burst events, demonstrated by a reduction of the velocity time derivative PDFs and by an higher intermittency. A strong transversal periodic organization of the flow field was observed, including some typical behaviours in each of the periodic boxes originated by the interaction of the vortex pairs. Results are interpreted and discussed in terms of management of the near-wall turbulent structures and with reference to the wall turbulence regeneration mechanisms suggested in the literature.

Download Full-text

Stability of Concave Boundary Layers: Overview of Stability Mechanism and Recent Findings

Volume 1: Symposia, Parts A and B ◽

10.1115/fedsm2008-55092 ◽

2008 ◽

Author(s):

Ladan Momayez ◽

Pascal Dupont ◽

Guillaume Delacourt ◽

Hassan Peerhossaini

Keyword(s):

Heat Transfer ◽

Boundary Layer ◽

Turbulent Boundary Layer ◽

Streamwise Vortex ◽

Turbulent Heat Transfer ◽

Turbulent Heat ◽

Streamwise Vortices ◽

Concave Wall ◽

Centrifugal Instability ◽

Concave Boundary

A series of experimental measurements of flow and heat transfer under streamwise Go¨rtler vortices shows conclusively that the local surface heat transfer rates can exceed that of the turbulent boundary layer even in the absence of turbulence. We have observed unexpected behavior of heat transfer in a laminar boundary layer on a concave wall at low nominal velocity, a configuration ignored in the literature. In this situation, precise measurements of the wall heat flux show that the heat transfer enhancement is extremely elevated, above that corresponding to the case of a turbulent boundary layer on a flat plate. The nonlinearly developing steady streamwise vortex (primary instability) heat transfer can already bridge the local laminar to turbulent heat transfer values in the absence of turbulence. The analysis shows that for a range of velocities less than a certain critical velocity, the transitional boundary layer is dominated by centrifugal instability. However, the steady streamwise vortices, like steady Taylor vortices between coaxial rotating cylinders, are susceptible to secondary instabilities of the varicose and sinuous modes. In experiments both modes appear to coexist and cause waviness of the primary streamwise vortices. Other results confirm this discussion based on analysis of the influence of a forcing upstream disturbance.

Download Full-text

Localized edge states in the asymptotic suction boundary layer

Journal of Fluid Mechanics ◽

10.1017/jfm.2013.20 ◽

2013 ◽

Vol 717 ◽

Cited By ~ 40

Author(s):

T. Khapko ◽

T. Kreilos ◽

P. Schlatter ◽

Y. Duguet ◽

B. Eckhardt ◽

...

Keyword(s):

Boundary Layer ◽

Coherent Structures ◽

Spanwise Direction ◽

Edge States ◽

Streamwise Vortices ◽

Boundary Layer Flows ◽

Edge Tracking ◽

Subcritical Regime ◽

Asymptotic Dynamics ◽

Asymptotic Suction

AbstractThe dynamics on the laminar–turbulent separatrix is investigated numerically for boundary-layer flows in the subcritical regime. Constant homogeneous suction is applied at the wall, resulting in a parallel asymptotic suction boundary layer (ASBL). When the numerical domain is sufficiently extended in the spanwise direction, the coherent structures found by edge tracking are invariably localized and their dynamics shows bursts that drive a remarkable regular or irregular spanwise dynamics. Depending on the parameters, the asymptotic dynamics on the edge can be either periodic in time or chaotic. A clear mechanism for the regeneration of streaks and streamwise vortices emerges in all cases and is investigated in detail.

Download Full-text