Measurements of the Effects of Streamwise Riblets on Boundary Layer Turbulence

2017 ◽  
Vol 139 (11) ◽  
Author(s):  
Derek B. Ancrum ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Coherent Structures ◽  
Wave Packets ◽  
Effective Control ◽  
Turbulent Spot ◽  
Hairpin Vortices ◽  
Cross Sectional ◽  
Vortical Structures ◽  
Turbulent Fluctuations ◽  
Boundary Layer Turbulence

This study presents experimental results on the effects of riblets on the coherent structures of turbulence within a turbulent spot. The riblet spacings of the study correspond to 0.5 and 1.5 times the natural spacing of the low-speed streak. The cross-sectional dimensions of the riblets were chosen to control the spatial distribution of wave packets consisting of streamwise-aligned hairpin vortices. Both riblet spacings demonstrated effective control on the spanwise positioning of the wave packets. The wider-spaced riblets reduced spanwise mutual interaction between wave packets. The closer-spaced riblets promoted this interaction via spanwise-oriented vortical structures which produced stronger turbulent fluctuations.

The behaviour of circular synthetic jets in a laminar boundary layer

2005 ◽  
Vol 109 (1100) ◽  
pp. 461-470 ◽  
Author(s):  
S. Zhong ◽  
F. Millet ◽  
N. J. Wood
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Synthetic Jets ◽  
Operating Conditions ◽  
Vortex Rings ◽  
Hairpin Vortices ◽  
Vortical Structures ◽  
Flow Separation Control ◽  
Wall Flow ◽  
Near Wall

Abstract Dye flow visualisation of circular synthetic jets was carried out in laminar boundary layers developing over a flat plate at a range of actuator operating conditions and freestream velocities of 0·05 and 0·1ms–1. The purpose of this work was to study the interaction of synthetic jets with the boundary layer and the nature of vortical structures produced as a result of this interaction. The effects of Reynolds number (Re), velocity ratio (VR ) and Strouhal number (St) on the behaviour of synthetic jets were studied. At low Re and VR , the vortical structures produced by synthetic jets appear as highly stretched hairpin vortices attached to the wall. At intermediate Re and VR , these structures roll up into vortex rings which experience a considerable amount of tilting and stretching as they enter the boundary layer. These vortex rings will eventually propagate outside the boundary layer hence the influence of the synthetic jets on the near wall flow will be confined in the near field of the jet exit. At high Re and VR , the vortex rings appear to experience a certain amount of tilting but no obvious stretching. They penetrate the edge of the boundary layer quickly, producing very limited impact on the near wall flow. Hence it is believed that the hairpin vortices produced at low Re and VR are likely to be the desirable structures for effective flow separation control. In this paper, a vortex model was also described to explain the mechanism of vortex tilting.

Characterization of coherent vortical structures in a supersonic turbulent boundary layer

2008 ◽  
Vol 613 ◽  
pp. 205-231 ◽  
Author(s):  
SERGIO PIROZZOLI ◽  
MATTEO BERNARDINI ◽  
FRANCESCO GRASSO
Keyword(s):  
Boundary Layer ◽  
Outer Layer ◽  
Stokes Equations ◽  
Supersonic Boundary Layer ◽  
Navier Stokes ◽  
Hairpin Vortices ◽  
Length Scales ◽  
Navier Stokes Equations ◽  
Vortical Structures

A spatially developing supersonic boundary layer at Mach 2 is analysed by means of direct numerical simulation of the compressible Navier--Stokes equations, with the objective of quantitatively characterizing the coherent vortical structures. The study shows structural similarities with the incompressible case. In particular, the inner layer is mainly populated by quasi-streamwise vortices, while in the outer layer we observe a large variety of structures, including hairpin vortices and hairpin packets. The characteristic properties of the educed structures are found to be nearly uniform throughout the outer layer, and to be weakly affected by the local vortex orientation. In the outer layer, typical core radii vary in the range of 5–6 dissipative length scales, and the associated circulation is approximately constant, and of the order of 180 wall units. The statistical properties of the vortical structures in the outer layer are similar to those of an ensemble of non-interacting closed-loop vortices with a nearly planar head inclined at an angle of approximately 20° with respect to the wall, and with an overall size of approximately 30 dissipative length scales.

Formation and decay of coherent structures in pipe flow

2010 ◽  
Vol 655 ◽  
pp. 258-279 ◽  
Author(s):  
JIMMY PHILIP ◽  
JACOB COHEN
Keyword(s):  
Coherent Structures ◽  
Pipe Flow ◽  
Three Dimensional ◽  
Vortex Pair ◽  
Hairpin Vortices ◽  
Cross Sectional ◽  
Instability Waves ◽  
Sectional Plane ◽  
The Cross ◽  
Three Dimensional Coordinate

Experimental investigation of the generation and decay of coherent structures, namely, streaks (accompanied by a counter-rotating vortex pair) and hairpin vortices in pipe flow, is carried out by artificial injection of continuous disturbances. Flow visualization and velocity measurements show that for small amplitudes of disturbances (v0) streaks are produced, and increasing v0 produces instability waves on the streaks, which further break down into an array of hairpin vortices. However, the streaks and hairpins decay along the downstream direction (X). In fact, the critical value of v0 required for the initiation of hairpins at a given Re (Reynolds number) varies with the streamwise distance (in contrast to the previously found scaling of v0 ~ Re−1, valid only close to the location of injection, i.e. smaller X). This is a consequence of the decay of the coherent structures in the pipe. Moreover, the hairpins have been found to decay more slowly with increasing Re. Measurements of energy in the cross-sectional plane of the pipe, and maps of disturbance velocity at various X-locations show the transient growth and decay of energy for relatively low v0. For higher v0 and Re the energy has been seen to increase continuously along the length of the pipe under observation. Owing to the increase in the cross-sectional area occupied by the disturbance along the X-direction, it is observed that energy can transiently increase even when the total disturbance magnitude is decreasing. Observing the similarity of the present work and other investigations wherein decay of turbulence in pipe flow is found, a schematic illustration of the transition surface for pipe flow on a v0−Re−X, three-dimensional coordinate system is presented.

scholarly journals Counter-hairpin vortices in the turbulent wake of a sharp trailing edge

2011 ◽  
Vol 689 ◽  
pp. 317-356 ◽  
Author(s):  
Sina Ghaemi ◽  
Fulvio Scarano
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
High Speed ◽  
High Speed Flow ◽  
Trailing Edge ◽  
Hairpin Vortices ◽  
Low Speed ◽  
Speed Flow ◽  
Low Speed Streaks

AbstractThe unsteady organization and evolution of coherent structures within the turbulent boundary layer and subsequent wake of the sharp symmetric trailing edge of a NACA0012 aerofoil are investigated. The experiments are conducted in an open test-section wind tunnel at ${\mathit{Re}}_{c} = \text{386\hspace{0.167em}000} $ based on the aerofoil chord and ${\mathit{Re}}_{\theta } = 1300$ based on the boundary layer momentum thickness. An initial characterization of the flow field using two-component particle image velocimetry (PIV) is followed by the investigation of the unsteady organization and evolution of coherent structures by time-resolved three-dimensional PIV based on a tomographic approach (Tomo-PIV). The inspection of the turbulent boundary layer prior to the trailing edge in the region between 0.15 and $0. 8\hspace{0.167em} {\delta }_{99} $ demonstrated streaks of low- and high-speed flow, while the low-speed streaks are observed to be more coherent along with strong interaction with hairpin-type vortical structures similar to a turbulent boundary layer at zero pressure gradient. The wake region demonstrated gradual deterioration of both the low- and the high-speed streaks with downstream progress. However, the low-speed streaks are observed to lose their coherence at a faster rate relative to the high-speed streaks as the turbulent flow develops towards the far wake. The weakening of the low-speed streaks is due to the disappearance of the viscous sublayer after the trailing edge and gradual mixing through the transport of the remaining low-speed flow towards the free stream. This transport of low-speed flow is performed by the ejection events induced by the hairpin vortices as they also persist into the developing wake. The higher persistence of the high-speed streaks is associated with counter-hairpin vortical activities as they oppose the deterioration of the high-speed streaks by frequently sweeping the high-speed flow towards the wake centreline. These vortical structures are regarded as counter-hairpin vortices as they exhibit opposite characteristics relative to the hairpin vortices of a turbulent boundary layer. They are topologically similar to the hairpins as they appear to be U-shaped but with inverted orientation, as the spanwise portion is in the vicinity of the wake centreline and the legs are inclined at an approximately $6{0}^{\ensuremath{\circ} } $ to the wake axis in the downstream direction demonstrating a strain-dominated topology. The counter-hairpin vortices are partially wrapped around the high-speed streaks and contribute to the wake development by transporting high-speed flow towards the wake centreline. Similar to the hairpin vortices of a turbulent boundary layer, the occurrence of a complete counter-hairpin vortex is occasional while its derivatives (portions of spanwise or quasi-streamwise vortices) are more frequently observed. Therefore, a pattern recognition algorithm is applied to establish characterization based on an ensemble-averaged counter-hairpin vortex. The formation of the counter-hairpin vortices is due to an additional degree of interaction between the low- and high-speed streaks after the trailing edge across the wake centreline. The shear layer produced along the wake centreline by neighbouring low- and high-speed streaks promotes the formation of spanwise vortices that form the counter-hairpin vortices by connection to quasi-streamwise vortices. Finally, a conceptual model is proposed to depict the three-dimensional unsteady organization and evolution of coherent structures in the wake region based on the hairpin and counter-hairpin vortex signatures.

scholarly journals Optimal wave packets in a boundary layer and initial phases of a turbulent spot

2010 ◽  
Vol 656 ◽  
pp. 231-259 ◽  
Author(s):  
S. CHERUBINI ◽  
J.-C. ROBINET ◽  
A. BOTTARO ◽  
P. DE PALMA
Keyword(s):  
Boundary Layer ◽  
Wave Packet ◽  
Scaling Law ◽  
Initial Perturbation ◽  
Three Dimensional ◽  
Plate Boundary ◽  
Optimal Time ◽  
Spanwise Direction ◽  
Turbulent Spot ◽  
Hairpin Vortices

The three-dimensional global optimal dynamics of a flat-plate boundary layer is studied by means of an adjoint-based optimization in a spatial domain of long – but finite – streamwise dimension. The localized optimal initial perturbation is characterized by a pair of streamwise-modulated counter-rotating vortices, tilted upstream, yielding at the optimal time elongated streaks of alternating sign in the streamwise direction. This indicates that perturbations with non-zero streamwise wavenumber have a role in the transient dynamics of a boundary layer. A scaling law is provided, describing the variation of the streamwise modulation of the optimal initial perturbation with respect to the streamwise domain length and to the Reynolds number. For spanwise-extended domains, a near-optimal three-dimensional perturbation is extracted during the optimization process; it is localized also in the spanwise direction, resulting in a wave packet of elongated disturbances modulated in the spanwise and streamwise directions. The nonlinear evolution of the optimal and near-optimal perturbations is investigated by means of direct numerical simulations. Both perturbations are found to induce transition at lower levels of the initial energy than local optimal and suboptimal perturbations. Moreover, it is observed that transition occurs in a well-defined region of the convected wave packet, close to its centre, via a mechanism including at the same time oscillations of the streaks of both quasi-sinuous and quasi-varicose nature. Hairpin vortices are observed before transition; they have an active role in the breakdown of the streaks and result in a turbulent spot which spreads out in the boundary layer.

Substructures in a turbulent spot

1988 ◽  
Vol 197 ◽  
pp. 389-414 ◽  
Author(s):  
R. Sankaran ◽  
M. Sokolov ◽  
R. A. Antonia
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Leading Edge ◽  
Simple Relation ◽  
Sufficient Evidence ◽  
Streamwise Direction ◽  
Geometrical Construction ◽  
Turbulent Spot ◽  
Hairpin Vortices ◽  
Streamwise Distance

Substructures within a turbulent spot which develops in a slightly heated laminar boundary layer have been identified using arrays of cold wires aligned in either a streamwise direction or in a direction normal to the wall. At any given streamwise distance from the spot origin, histograms of the number of detected substructures exhibit a peak, defining the most probable spot or the spot with the most likely number of substructures. The number of substructures in the most probable spot increases with streamwise distance but all substructures are convected at approximately the same velocity for any given distance from the wall. This velocity is approximately equal to that of the leading edge of the spot and increases slightly with distance from the wall. The increase in the number of substructures accounts for the streamwise growth of the spot. A simple relation is derived for determining the number of substructures at a particular streamwise station and a geometrical construction is proposed for identifying the origin of a new substructure. There is sufficient evidence for suggesting that the new substructures are formed near the trailing edge of the spot. The convection velocity, inclination and lengthscales of the substructures compare favourably with the corresponding characteristics of hairpin vortices.

Direct numerical simulations of riblets to constrain the growth of turbulent spots

2011 ◽  
Vol 668 ◽  
pp. 267-292 ◽  
Author(s):  
JAMES S. STRAND ◽  
DAVID B. GOLDSTEIN
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Immersed Boundary ◽  
Ensemble Averaging ◽  
Hairpin Vortices ◽  
Turbulent Spots ◽  
Vortical Structures ◽  
Flat Wall ◽  
Self Similar ◽  
Good Agreement

A spectral direct numerical simulation (DNS) code was used to study the growth and spreading of turbulent spots in a nominally laminar, zero-pressure-gradient boundary layer. In addition to the flat-plate case, the interaction of these spots with riblets was investigated. The flat plate, riblets and initial spot perturbation were simulated via an immersed boundary method, and a ‘suction wall’ allowed the available channel code to model a boundary layer. In both flat-wall and riblet cases, self-similar arrowhead-shaped spots formed. The λ2 variable of Jeong & Hussain (1995) was used to visualize the vortical structures within a spot, and a spot was seen to consist primarily of a multitude of entwined hairpin vortices. The range of scales of the hairpin vortices was found to increase as the spot matures. Ensemble averaging was used to obtain more accurate results for the spot spreading angle, both for the flat-wall case and the riblet case. The spreading angle for the flat-wall spot was 6.3°, in reasonably good agreement with prior DNS work. The spreading angle for the spot over riblets was 5.4°, a decrease of 14% compared with the flat-wall.

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