First-mode-induced nonlinear breakdown in a hypersonic boundary layer

Nair, Unnikrishnan Sasidharan; Gaitonde, Datta V.

doi:10.1016/j.compfluid.2019.104249

Cited by 17 publications

(7 citation statements)

References 53 publications

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“…Around of the points are clustered within the boundary layer height of the coldest wall, . A relatively finer wall spacing of is utilized to ensure sufficient resolution of the cooler boundary layers, based on prior considerations for near-adiabatic wall temperatures (Unnikrishnan & Gaitonde 2019 a ). The boundary conditions are similar to those used in the 2-D simulations at the inflow, outflow, wall and far field boundaries, and the spanwise direction is assumed to be periodic.…”

Section: Three-dimensional Transition Mechanisms In Cooled Hblsmentioning

confidence: 99%

Instabilities and transition in cooled wall hypersonic boundary layers

Nair

Gaitonde

2021

J. Fluid Mech.

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Section: Three-dimensional Transition Mechanisms In Cooled Hblsmentioning

confidence: 99%

Instabilities and transition in cooled wall hypersonic boundary layers

Nair

Gaitonde

2021

J. Fluid Mech.

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“…This could explain why skin friction overshoot was not observed in the time-and span-averaged c f , plotted in figure 14(a). A previous study 41 of this HBL where transition was induced through forcing by a monochromatic first-mode oblique wave, clearly identified a skin friction overshoot in the transitional regime.…”

Section: Turbulent Hbl and Near-wall Effectsmentioning

confidence: 63%

Linear, nonlinear and transitional regimes of second mode instability

Unnikrishnan,

Gaitonde

2020

Preprint

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The 2D second-mode is a potent instability in hypersonic boundary layers (HBLs). We study its linear and nonlinear evolution, followed by its role in transition and eventual breakdown of the HBL into a fully turbulent state. Linear stability theory (LST) is utilized to identify the second-mode wave through FS-synchronization, which is then recreated in linearly and nonlinearly forced 2D direct numerical simulations (DNSs). The nonlinear DNS shows saturation of the fundamental frequency, and the resulting superharmonics induce tightly braided "rope-like" patterns near the generalized inflection point (GIP). The instability exhibits a second region of growth constituted by the fundamental frequency, downstream of the primary envelope, which is absent in the linear scenario. Subsequent 3D DNS identifies this region to be crucial in amplifying oblique instabilities riding on the 2D second-mode "rollers". This results in lambda vortices below the GIP, which are detached from the "rollers" in the inner boundary layer. Streamwise vortex-stretching results in a localized peak in length-scales inside the HBL, eventually forming haripin vortices. Spectral analyses track the transformation of harmonic peaks into a turbulent spectrum, and appearance of oblique modes at the fundamental frequency, which suggests that fundamental resonance is the most dominant mechanism of transition. Bispectrum reveals coupled nonlinear interactions between the fundamental and its superharmonics leading to spectral broadening, and traces of subharmonic resonance as well. Global forms of the fundamental and subharmonic modes are extracted which shows that the former disintegrates at the location of spanwise breakdown, beyond which the latter amplifies. Fundamental resonance results in complete breakdown of streamwise streaks in the lower log-layer, producing a fully turbulent HBL. Statistical analyses of near-wall flowfield indicate an increase in large-scale "splatting" motions immediately following transition, resulting in extreme skin friction events, which equilibrate as turbulence sets in.

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“…This could explain why the skin-friction overshoot was not observed in the time-and span-averaged c f , plotted in figure 16(a). A previous study (Unnikrishnan & Gaitonde 2019) of this HBL where transition was induced through forcing by a monochromatic first-mode oblique wave, clearly identified a skin-friction overshoot in the transitional regime. Near-wall features in the boundary layer provide significant insights into the drag penalty on the surface.…”

Section: Turbulent Hbl and Near-wall Effectsmentioning

confidence: 63%

“…At the free stream conditions considered, the unstable growth rates of mode after synchronization can display magnitudes five to ten times (see e.g. Özgen & Kırcalı 2008; Unnikrishnan & Gaitonde 2019) larger than those prior to it. Mode remains damped over this adiabatic wall, since .…”

Section: Linear Analysismentioning

confidence: 96%