Passive control of transition in three-dimensional boundary layers, with emphasis on discrete roughness elements

Saric, William S.; Carpenter, Andrew L.; Reed, Helen L.

doi:10.1098/rsta.2010.0368

Cited by 92 publications

(71 citation statements)

References 36 publications

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“…We consider cases with no laminar flow, and with laminar flow up to Re x = 5 × 10 6 and 10 7 . These are typical of current projections [2]. Typical wing chord Reynolds numbers can reach Re x = 5 × 10 7 , and body Reynolds numbers Re x = 5 × 10 8 .…”

Section: (I) Local and Global Effects On Viscous Dragmentioning

confidence: 95%

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Drag reduction: enticing turbulence, and then an industry

Spalart

Mclean

2011

Phil. Trans. R. Soc. A.

124

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We examine drag-reduction proposals, as presented in this volume and in general, first with concrete examples of how to bridge the distance from pure science through engineering to what makes inventions go into service; namely, the value to the public. We point out that the true drag reduction can be markedly different from an estimate based simply on the difference between turbulent and laminar skin friction over the laminarized region, or between the respective skin frictions of the baseline and the riblettreated flow. In some situations, this difference is favourable, and is due to secondary differences in pressure drag. We reiterate that the benefit of riblets, if it is expressed as a percentage in skin-friction reduction, is unfortunately lower at full-size Reynolds numbers than in a small-scale experiment or simulation. The Reynolds number-independent measure of such benefits is a shift of the logarithmic law, or 'DU + '. Anticipating the design of a flight test and then a product, we note the relative ease in representing riblets or laminarization in computational fluid dynamics, in contrast with the huge numerical and turbulence-modelling challenge of resolving active flow control systems in a calculation of the full flow field. We discuss in general terms the practical factors that have limited applications of concepts that would appear more than ready after all these years, particularly riblets and laminar-flow control.

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Section: (I) Local and Global Effects On Viscous Dragmentioning

confidence: 95%

“…The LFC choices are wide, and it may be a surprise in such a mature field that both natural and hybrid LFC are in contention, as is the 'distributed roughness element' idea [2]. The uncertainty level remains high.…”

Section: Airline Industry Considerations (A) Laminar-flow Controlmentioning

confidence: 99%

Drag reduction: enticing turbulence, and then an industry

Spalart

Mclean

2011

Phil. Trans. R. Soc. A.

124

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“…As such, regions of high shear cool more rapidly than respective low shear regions. Differences in wall temperature captured by the thermal camera can therefore be used for visualisation between laminar and turbulent regions (see also Saric et al (2011)). In addition, using highly sensitive, actively cooled thermal imagers, details in the shear distribution of laminar boundary layers, such as the characteristic streaky footprint of CFVs can be retrieved.…”

Section: Infrared Thermographymentioning

confidence: 99%

Conditioning of cross-flow instability modes using dielectric barrier discharge plasma actuators

2017

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In the current study, selective forcing of cross-flow instability modes evolving on a 45• swept wing at Re = 2.17 · 10 6 is achieved by means of spanwise-modulated plasma actuators, positioned near the leading edge. In the perspective of laminar flow control, the followed methodology holds on the discrete roughness elements/upstream flow deformation (DRE/UFD) approach, thoroughly investigated by e.g. Saric et al. (1998);Malik et al. (1999) and Wassermann & Kloker (2002). The possibility of using active devices for UFD provides several advantages over passive means, allowing for a wider range of operating Re numbers and pressure distributions. In the present work, customised alternating current dielectric barrier discharge plasma actuators have been designed, manufactured and characterised. The authority of the actuators in forcing monochromatic stationary cross-flow modes at different spanwise wavelengths is assessed by means of infrared thermography. Moreover, quantitative spatio-temporal measurements of the boundary layer velocity field are performed using time-resolved particle image velocimetry. The results reveal distinct steady and unsteady forcing contributions of the plasma actuator on the boundary layer. It is shown that the actuators introduce unsteady fluctuations in the boundary layer, amplifying at frequencies significantly lower than the actuation frequency. In line with the DRE/UFD strategy, forcing a sub-critical stationary mode, with a shorter wavelength compared to the naturally selected mode, results in less amplified primary vortices and related fluctuations, compared to the critical forcing case. The effect of the forcing on the flow stability is further inspected by combining the measured actuators body-force with the numerical solution of the laminar boundary layer and linear stability theory. The simplified methodology yields fast and computationally cheap estimates on the effect of steady forcing (magnitude and direction) on the boundary layer stability.

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“…These campaigns made use of very quiet wind tunnels, exhibiting a free stream turbulence level typically lower than 0.1% of the free stream velocity. Surface flow visualisation was performed by means of infrared thermography (Saric et al 2011), hydrogen bubbles tracing (Bippes Three-dimensional organisation of primary and secondary crossflow instability 3 1999), naphthalene sublimation techniques (Dagenhart et al 1989;Saric et al 1998;White & Saric 2000) and fluorescent mineral oil (Serpieri & Kotsonis 2015a,b). Hotwire anemometry (HWA) boundary layer measurements were performed using precision sensor traversing systems (e.g.…”

Section: Background and Present Workmentioning

confidence: 99%

“…Dagenhart et al (1989); Bippes (1999); Saric et al (2003Saric et al ( , 2011) have shown a distinctive transition scenario for this type of boundary layers, characterised by sequences of streaks aligned with, within a few degrees, the flow direction. These streaks are the footprint of stationary boundary layer vortices causing a velocity modulation along the wing span and therefore of the wall skin friction and of the convective heat flux (quantities usually visualised in these experiments).…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional organisation of primary and secondary crossflow instability

2016

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An experimental investigation of primary and secondary crossflow instability developing in the boundary layer of a 45 • swept wing, at a chord Reynolds number of 2.17·10 6 is presented. Linear stability theory is applied for preliminary estimation of the flow stability while surface flow visualisation using fluorescent oil is employed to inspect the topological features of the transition region. Hot-wire anemometry is extensively used for the investigation of the developing boundary layer and identification of the statistical and spectral characteristics of the instability modes. Primary stationary as well as unsteady type-I (z-mode), type-II (y-mode) and type-III modes are detected and quantified. Finally, three-component, three-dimensional measurements of the transitional boundary layer are performed using tomographic particle image velocimetry. This research presents the first application of an optical experimental technique for this type of flow. Among the optical techniques tomographic velocimetry represents, to date, the most advanced approach allowing the investigation of spatially correlated flow structures in threedimensional fields. Proper orthogonal decomposition (POD) analysis of the captured flow fields is applied to this goal. The first POD mode features a newly reported structure related to low-frequency oscillatory motion of the stationary vortices along the spanwise direction. The cause of this phenomenon is only conjectured. Its effect on transition is considered negligible but, given the related high energy level, it needs to be accounted for in experimental investigations. Secondary instability mechanisms are captured as well. The type-III mode corresponds to low frequency primary travelling crossflow waves interacting with the stationary ones. It appears in the inner upwelling region of the stationary crossflow vortices and is characterised by elongated structures approximately aligned with the axis of the stationary waves. The type-I secondary instability consists instead of significantly inclined structures located at the outer upwelling region of the stationary vortices. The much narrower wavelength and higher advection velocity of these structures correlate with the higher-frequency content of this mode. The results of the investigation of both primary and secondary instability from the exploited techniques agree with and complement each other and are in line with existing literature. Finally, they present the first experimental observation of the secondary instability structures under natural flow conditions.

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Passive control of transition in three-dimensional boundary layers, with emphasis on discrete roughness elements

Cited by 92 publications

References 36 publications

Drag reduction: enticing turbulence, and then an industry

Drag reduction: enticing turbulence, and then an industry

Conditioning of cross-flow instability modes using dielectric barrier discharge plasma actuators

Three-dimensional organisation of primary and secondary crossflow instability

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