Response of a laminar separation bubble to impulsive forcing

Michelis, Theodoros; Yarusevych, Serhiy; Kotsonis, Marios

doi:10.1017/jfm.2017.217

Cited by 38 publications

(48 citation statements)

References 65 publications

(130 reference statements)

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“…For all cases, the locus of peak RMS in each profile seems to collapse well on the dividing streamline. This is consistent with the development of coherent flow motions due to shear layer instabilities, in a fashion similar to Laminar Separation Bubbles (Michelis et al 2017). Compared to the baseline case, significant increase of velocity fluctuations is observed just downstream of the plasma actuation at F * = 0.25 .…”

Section: Leading Edge Separation Control At = 155 • (Actuation Upstrsupporting

confidence: 81%

Airfoil flow separation control with plasma synthetic jets at moderate Reynolds number

2018

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An array of 26 plasma synthetic jet actuators (PSJA) is flush-mounted on a NACA-0015 airfoil model to control the leadingedge flow separation at moderate Reynolds number (Re c = 1.7 × 10 5). The stall angle of this airfoil is postponed from 15.5 • to approximately 22 • , and the peak lift coefficient is increased by 21%. PSJAs exhibit distinctive separation control mechanisms depending on the relative location between actuation and separation and reduced frequency of actuation (F *). At an angle of attack of = 15.5 • , the non-actuated flow separates approximately 4% chord length downstream of the jet orifices. Plasma synthetic jets (PSJs) applied at F * ≥ 0.5 can displace the separation point downstream to mid-chord position, as a result of the energizing of the incoming boundary layer through mixing enhancement. As a comparison, with actuation frequency of F * ≤ 0.25 , the separation point at = 15.5 • remains near the leading edge and the zero-velocity line is periodically swept towards the suction surface by the convecting spanwise vortices generated from PSJ actuation, leading to a reduction of timeaveraged backflow area. For the case of separation control at = 22 • , the separation point resides always upstream of the actuation position, regardless of actuation frequency. The peak lift coefficient is attained at F * = 1 , and the decreasing lift at high actuation frequency (F * = 2) is ascribed to the severe interaction between adjacent spanwise vortices at short spacing.

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Section: Leading Edge Separation Control At = 155 • (Actuation Upstrsupporting

confidence: 81%

Airfoil flow separation control with plasma synthetic jets at moderate Reynolds number

2018

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“…The forcing signal is constructed by modulating a sinusoidal signal (f c = 5 kHz) with a square signal and the duty cycle is selected such that a 1 ms pulse is produced. Under the experimental conditions described here, the frequencies associated with the LSB vortex shedding and flapping are of the order of 130 and 10 Hz respectively (Michelis et al 2017). Since the carrier frequency (f c = 5 kHz) exceeds the relevant hydrodynamic frequencies by approximately two orders of magnitude, a pulse is equivalent to a single impulsive perturbation.…”

Section: Set-up Overviewmentioning

confidence: 94%

“…The experimental set-up used for this study (figure 1) is identical to the one described in Michelis et al (2017). The flat plate is 1000 mm long, 500 mm wide and 20 mm thick, while its leading edge is a modified super ellipse (Lin, Reed & Saric 1992).…”

Section: Set-up Overviewmentioning

confidence: 99%

“…Given the limited spanwise extent of the field of view relative to the wavelength of the spanwise deformations, determination of the streamwise (λ x ) and spanwise (λ z ) wavelengths is performed through spatial wavelet analysis (Daubechies 1992;Michelis et al 2017). At each recorded time instant, the total Q-criterion value is computed along the dashed streamwise and spanwise monitor lines indicated in figure 8.…”

Section: Spatio-temporal Characteristicsmentioning

confidence: 99%

“…To excite the primary instability mode, the flow is impulsively forced. In line with Michelis, Kotsonis & Yarusevych (2017), forcing is applied in a two-dimensional fashion by means of a spanwise-uniform dielectric-barrier-discharge (DBD) plasma actuator (Corke, Enloe & Wilkinson 2010;Kotsonis 2015), located upstream of the separation point, thus circumventing the shortcomings of localised point forcing discussed earlier. In addition, an essential feature of impulsive forcing is that, for fixed free-stream velocity and turbulent intensity, the bubble experiences elevated disturbance levels only during the forcing impulse, otherwise being subjected to natural level of disturbances.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

On the origin of spanwise vortex deformations in laminar separation bubbles

2018

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This work investigates the three-dimensional, spatio-temporal flow development in the aft portion of a laminar separation bubble. The bubble is forming on a flat plate geometry, subjected to an adverse pressure gradient, featuring maximum reverse flow of approximately 2 % of the local free-stream velocity. Time-resolved velocity measurements are performed by means of planar and tomographic particle image velocimetry, in the vicinity of the reattachment region. The measurements are complemented with a numerical solution of the boundary layer equations in the upstream field. The combined numerical and measured boundary layer is used as a baseline flow for linear stability theory analysis. The results provide insight into the dynamics of dominant coherent structures that form in the separated shear layer and deform along the span. Stability analysis shows that the flow becomes unstable upstream of separation, where both normal and oblique modes undergo amplification. While the shear layer roll up is linked to the amplification of the fundamental normal mode, the oblique modes at angles lower than approximately $30^{\circ }$ are also amplified substantially at the fundamental frequency. A model based on the stability analysis and experimental measurements is employed to demonstrate that the spanwise deformations of rollers are produced due to a superposition of normal and oblique instability modes initiating upstream of separation. The degree of the initial spanwise deformations is shown to depend on the relative amplitude of the dominant normal and oblique waves. This is confirmed by forcing the normal mode through a controlled impulsive perturbation introduced by a spanwise invariant dielectric-barrier-discharge plasma actuator, resulting in the formation of spanwise coherent vortices. The findings elucidate the link between important features in the bubble shedding dynamics and stability characteristics and provide further clarification on the differences in the development of coherent structures seen in recent experiments. Moreover, the results present a handle on the development of effective control strategies that can be used to either promote or suppress shedding in separation bubbles, which is of interest for system performance improvement and control of aeroacoustic emissions in relevant applications.

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