Receptivity of a swept-wing boundary layer to micron-sized discrete roughness elements

Kurz, H.; Kloker, Markus J.

doi:10.1017/jfm.2014.425

Cited by 51 publications

(43 citation statements)

References 23 publications

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“…They commented on the discrepancies with the experimental results of Reibert et al (1996) pointing at possible small imperfections in the application of roughness elements on the model surface (which is very likely in laboratory conditions given the micrometric size of these elements). The problem of receptivity to surface roughness was further investigated by Kurz & Kloker (2014) in a following DNS study. They found that the amplitude of the fundamental stationary mode scales linearly with the roughness height only when the roughness array features null spanwise-averaged shape and flow blockage.…”

Section: Primary Instabilitymentioning

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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Section: Primary Instabilitymentioning

confidence: 99%

Three-dimensional organisation of primary and secondary crossflow instability

2016

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“…Radeztsky et al (1999); Bippes (1999); Saric et al (2003); Kurz & Kloker (2014)). As such, notwithstanding the chosen flow control strategy, the used actuators should feature extremely low levels of roughness when installed on the model surface.…”

Section: Plasma Actuators For Cross-flow Instability Controlmentioning

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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Section: Introductionmentioning

confidence: 99%

“…Note, however, that the strength of the disturbance flow field may depend nonlinearly on roughness parameters such as as the height, as shown by Luchini (2013) for a generic boundary layer and Choudhari & Duck (1996) and Kurz & Kloker (2014b) for swept-wing flow. In the presence of cross-flow instability, such shallow roughness elements induce unstable steady cross-flow modes; a review on related receptivity studies is given in Kurz & Kloker (2014b). A practical application in the scope of LFC is the stabilisation of a three-dimensional boundary layer by the induction of a control mode, as suggested by Saric, Carrillo & Reibert (1998) and the recent associated DNS by Hosseini et al (2013); see also the detailed simulations by Wassermann & Kloker (2002).…”

Section: Introductionmentioning

confidence: 99%

Mechanisms of flow tripping by discrete roughness elements in a swept-wing boundary layer

Kurz

Kloker

2016

J. Fluid Mech.

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The effects of a spanwise row of finite-size cylindrical roughness elements in a laminar, compressible, three-dimensional boundary layer on a wing profile are investigated by direct numerical simulations (DNS). Large elements are capable of immediately tripping turbulent flow by either a strong, purely convective or an absolute/global instability in the near wake. First we focus on an understanding of the steady near-field past a finite-size roughness element in the swept-wing flow, comparing it to a respective case in unswept flow. Then, the mechanisms leading to immediate turbulence tripping are elaborated by gradually increasing the roughness height and varying the disturbance background level. The quasi-critical roughness Reynolds number above which turbulence sets in rapidly is found to be Re kk,qcrit ≈ 560 and global instability is found only for values well above 600 using nonlinear DNS; therefore the values do not differ significantly from two-dimensional boundary layers if the full velocity vector at the roughness height is taken to build Re kk . A detailed simulation study of elements in the critical range indicates a changeover from a purely convective to a global instability near the critical height. Finally, we perform a three-dimensional global stability analysis of the flow field to gain insight into the early stages of the temporal disturbance growth in the quasi-critical and over-critical cases, starting from a steady state enforced by damping of unsteady disturbances.

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Receptivity of a swept-wing boundary layer to micron-sized discrete roughness elements

Cited by 51 publications

References 23 publications

Three-dimensional organisation of primary and secondary crossflow instability

Three-dimensional organisation of primary and secondary crossflow instability

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

Mechanisms of flow tripping by discrete roughness elements in a swept-wing boundary layer

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