Dynamic pitching effect on a laminar separation bubble

Nati, A.; Kat, Roeland de; Scarano, Fulvio; Oudheusden, B.W. van

doi:10.1007/s00348-015-2031-6

Cited by 36 publications

(26 citation statements)

References 33 publications

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“…the regions of low velocity fluid that appear over the span of the vortex filaments at x/c 0.55 throughout figure 18. The formation of spanwise uniform shear layer vortices is consistent with the observations of Jones et al (2008), Marxen et al (2013) and Nati et al (2015), which according to Michelis et al (2018) is an indication of the relative dominance of normal over oblique modes. Furthermore, the development of spanwise deformations leading to localized regions of vortex breakup is consistent with the vortex breakup mechanism for an LSB proposed by Kurelek et al (2016).…”

Section: Coherent Structuressupporting

confidence: 87%

“…The formed structures dominate the flow development in the aft portion of the bubble (Lengani et al 2014(Lengani et al , 2017 and have been argued to be responsible for inducing mean reattachment (e.g. Marxen have been found to be largely spanwise uniform, but quickly undergo significant three-dimensional deformations prior to the breakdown to turbulence (Jones, Sandberg & Sandham 2008;Marxen, Lang & Rist 2013;Nati et al 2015;Kurelek, Lambert & Yarusevych 2016;Kirk & Yarusevych 2017). In contrast, other investigators have reported highly deformed and spanwise non-uniform vortical structures at formation (Burgmann & Schröder 2008;Hain, Kähler & Radespiel 2009;Wolf et al 2011).…”

Section: Introductionmentioning

confidence: 98%

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Transition in a separation bubble under tonal and broadband acoustic excitation

2018

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Transition and flow development in a separation bubble formed on an airfoil are studied experimentally. The effects of tonal and broadband acoustic excitation are considered since such acoustic emissions commonly result from airfoil self-noise and can influence flow development via a feedback loop. This interdependence is decoupled, and the problem is studied in a controlled manner through the use of an external acoustic source. The flow field is assessed using time-resolved, two-component particle image velocimetry, the results of which show that, for equivalent energy input levels, tonal and broadband excitation can produce equivalent changes in the mean separation bubble topology. These changes in topology result from the influence of excitation on transition and the subsequent development of coherent structures in the bubble. Both tonal and broadband excitation lead to earlier shear layer roll-up and thus reduce the bubble size and advance mean reattachment upstream, while the shed vortices tend to persist farther downstream of mean reattachment in the case of tonal excitation. Through a cross-examination of linear stability theory (LST) predictions and measured disturbance characteristics, nonlinear disturbance interactions are shown to play a crucial role in the transition process, leading to significantly different disturbance development for the tonal and broadband excited flows. Specifically, tonal excitation results in transition being dominated by the excited mode, which grows in strong accordance with linear theory and damps the growth of all other disturbances. On the other hand, disturbance amplitudes are more moderate for the natural and broadband excited flows, and so all unstable disturbances initially grow in accordance with LST. For all cases, a rapid redistribution of perturbation energy to a broad range of frequencies follows, with the phenomenon occurring earliest for the broadband excitation case. By taking nonlinear effects into consideration, important ramifications are made clear in regards to comparing LST predictions and experimental or numerical results, thus explaining the trends reported in recent investigations. These findings offer new insights into the influence of tonal and broadband noise emissions, resulting from airfoil self-noise or otherwise, on transition and flow development within a separation bubble.

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Section: Coherent Structuressupporting

confidence: 87%

Section: Introductionmentioning

confidence: 98%

Transition in a separation bubble under tonal and broadband acoustic excitation

2018

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“…In contrast, more conflicting reports have been made in experimental studies. For example, similar to the DNS results, 9, 10 the investigations of Nati et al 20 and Kurelek et al 21 found the structures to be strongly coherent across the span at formation, followed by the development of spanwise unsteadiness leading to the breakup of the roll-up vortices to smaller scales. On the other hand, some experimental investigations, while observing consistent roll-up of the separated shear layer in streamwise assessments of the flow field, have described the spanwise uniformity of the structures as sporadic, 11,12 or even not present entirely.…”

mentioning

confidence: 51%

“…5. This roll-up process has been observed experimentally, 5,[11][12][13][18][19][20][21] as well as numerically. 6, 8-10, 14, 22 The formed structures tend to dominate the flow development in the aft portion of the bubble, 10,21 and have been argued to be responsible for inducing mean flow reattachment.…”

mentioning

confidence: 79%

The effect of acoustic excitation on the later stages of transition in a laminar separation bubble

Kurelek

Yarusevych

2016

46th AIAA Fluid Dynamics Conference

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The effect of external acoustic excitation on flow development within a separation bubble formed over a NACA 0018 airfoil at a chord Reynolds number of 125 000 and an angle of attack of 4°is investigated experimentally. Two-component, time-resolved particle image velocimetry is used to study both streamwise and spanwise aspects of the flow development. Acoustic excitation is applied at the frequency of the most amplified disturbance in the naturally developing separated shear layer and several excitation amplitudes are considered. The results show that periodic shear layer vortices form in the separation bubble with strong spanwise coherence at roll-up; however, the vortex filaments develop a spanwise unsteadiness which leads to rapid deformation and breakdown to smaller scales. Applying acoustic excitation causes the separation bubble to decrease in size, with the effect becoming more pronounced with increasing excitation amplitude. The changes in the mean bubble topology are directly linked to alterations in the salient characteristics of the shear layer vortices. Specifically, excitation is shown to cause earlier shear layer roll-up while enhancing the spanwise coherence of the dominant structures, with the former resulting in the aft portion of the bubble shifting upstream, and the latter reducing the streamwise distance between the mean roll-up and reattachment locations. Nomenclature c = airfoil chord length C P = surface pressure coefficient; (P − P 0 ) / 1 2 ρU 0 2 f = frequency l = airfoil leading to trailing edge arc length z = spanwise coherence length P = mean local surface pressure P 0 = free-stream static pressure p = fluctuating pressure Re = chord-based Reynolds number; U 0 c/ν St = chord-based Strouhal number; f c/U 0 t, t * = time and non-dimensional time; t * = tU 0 /c U, U = instantaneous and time-averaged x-velocity component u = fluctuating component of U U 0 = free-stream velocity U c = convective velocity of shear layer vortices U e = separation bubble edge velocity v = fluctuating y-velocity component v = root-mean-square of v x, y, z= wall-tangent, wall-normal, and spanwise coordinates x S , x H , x R = mean streamwise locations of separation, maximum bubble height, and reattachment α = angle of attack λ x , λ z = streamwise and spanwise disturbance wavelengths ν, ρ = kinematic viscosity and density of air φ pp = frequency auto-spectral density of p φ uu = wavelength auto-spectral density of u φ vv = frequency auto-spectral density of v ω = spanwise vorticity * Graduate Student

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“…Some studies focusing on the time dependent boundary layer in small pitch amplitudes include the work done by Pascazio et al (1996) which shows a time delay in laminar-turbulent transition during pitching. Nati et al (2015) analyzed the effect of small amplitude pitching on a laminar separation bubble at low Reynolds numbers. Mai and Hebler (2011) and Hebler et al (2013) (Lokatt and Eller, 2017;Lokatt, 2017) qualitatively represent small changes in operating conditions, such as the changes due to structural deformations or small trailing-edge flap deflections.…”

Section: Introductionmentioning

confidence: 99%

Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil

Negi

Vinuesa

Hanifi

et al. 2018

International Journal of Heat and Fluid Flow

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High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Re c = 100, 000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain "off-design" conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at Re c = 400, 000.

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Dynamic pitching effect on a laminar separation bubble

Cited by 36 publications

References 33 publications

Transition in a separation bubble under tonal and broadband acoustic excitation

Transition in a separation bubble under tonal and broadband acoustic excitation

The effect of acoustic excitation on the later stages of transition in a laminar separation bubble

Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil

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