Instabilities and transition in cooled wall hypersonic boundary layers

Nair, Unnikrishnan Sasidharan; Gaitonde, Datta V.

doi:10.1017/jfm.2021.84

Cited by 18 publications

(8 citation statements)

References 57 publications

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“…Laurence et al (2016) experimentally demonstrated a similar kink in the development of a wave packet in a high-enthalpy boundary layer and attributed it to nonlinear effects. Unnikrishnan & Gaitonde (2021) showed that wave packets in highly cooled boundary layers become increasingly nonlinear as they elongate, unlike warmer-wall cases where packets split into a clear nonlinear head and a separate, linear trailing region. Thus, the observations made for this burst of content could be attributed to acoustic radiation, nonlinear interactions or a combination of the two.…”

Section: Bandpass Filteringmentioning

confidence: 99%

“…For example, Chuvakhov & Fedorov (2016) suggested that the mechanism may delay the transition to turbulence. Unnikrishnan & Gaitonde (2020, 2021) addressed this postulate, maintaining that the destabilization of perturbations within the boundary layer would have a stronger effect promoting transition than the radiated energy would have in delaying it. Finally, nose bluntness has also been shown to have a significant effect on the supersonic mode, with the LST and DNS of Mortensen (2018) demonstrating a strong coupling between increased nose radius (up to a certain point) and instability growth rate; effects of oxygen dissociation were also observed to have a significant effect.…”

Section: Introductionmentioning

confidence: 99%

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Time-resolved wave packet development in highly cooled hypersonic boundary layers

Paquin,

Hameed,

Parziale

et al. 2024

J. Fluid Mech.

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Boundary-layer disturbances are analysed on a $5^{\circ }$ half-angle blunted cone in Mach 5, high-enthalpy flow ( $h_0 = 9\ {\rm MJ}\ {\rm kg}^{-1}$ ) with a low wall-to-edge temperature ratio, $T_w/T_e = 0.18$ . Schlieren and focused laser differential interferometry (FLDI) are utilized to assess the structures and frequency content associated with disturbances. Wave packets are identified from bursts of modal content on time-resolved spectrograms. Bandpass filtering, proper orthogonal decomposition (POD) and space–time POD are then applied to the schlieren data. Bandpass filtering suggests the presence of wave packet dispersion and elongation indicative of slow-acoustic-wave synchronization. Modal reconstruction techniques indicate the radiation of content outside the boundary layer and distinct orientation changes within disturbances, potentially the first experimental evidence of the supersonic-mode instability in such a flow field. Cross-bicoherence computations are carried out for discrete time segments of data from both schlieren and FLDI data. They demonstrate that the most dominant nonlinear interactions are the fundamental–first-harmonic and the fundamental–low-frequency interactions.

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Section: Bandpass Filteringmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Time-resolved wave packet development in highly cooled hypersonic boundary layers

Paquin,

Hameed,

Parziale

et al. 2024

J. Fluid Mech.

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“…We note that the TWPs and the Mack modes in hypersonic turbulent boundary layer transition share many similar features. For example, (i) they share the same morphology, behaving as the travelling waves with a certain wavenumber, (ii) they are both located close to the wall, (iii) the TWPs are intensified by the rising Mach number and the cooling wall (Yu et al 2019;Unnikrishnan & Gaitonde 2021;Yu & Xu 2021), and the second mode is destabilized by these factors (Mack 1984;Stetson & Kimmel 1992), and (iv) they can both enhance the wall pressure fluctuations (Casper, Beresh & Schneider 2014;Yu et al 2020;Unnikrishnan & Gaitonde 2021). It is probably the different base flows that lead to the disparity in the growth of these perturbations.…”

Section: Transient Growth On a Steady Streakmentioning

confidence: 99%

“…2019; Unnikrishnan & Gaitonde 2021; Yu & Xu 2021), and the second mode is destabilized by these factors (Mack 1984; Stetson & Kimmel 1992), and (iv) they can both enhance the wall pressure fluctuations (Casper, Beresh & Schneider 2014; Yu et al. 2020; Unnikrishnan & Gaitonde 2021). It is probably the different base flows that lead to the disparity in the growth of these perturbations.…”

Section: Numerical Experiments and Flow Dynamicsmentioning

confidence: 99%

On the generation of near-wall dilatational motions in hypersonic turbulent boundary layers

Yu,

Zhou,

Dong

et al. 2024

J. Fluid Mech.

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Dilatational motions in the shape of travelling wave packets have been identified recently to be dynamically significant in hypersonic turbulent boundary layers. The present study investigates the mechanisms of their generation and their association with the solenoidal motions, especially the well-recognized near-wall self-sustaining process of the regeneration cycle between the velocity streaks and quasi-streamwise vortices. By exploiting the direct numerical simulation databases and orchestrating numerical experiments, we explore systematically the near-wall flow dynamics in the processes of the formation and transient growth of low-speed streaks. We conclude via theoretical ansatz that the nonlinearity related to the parallel density and pressure gradients close to the wall due to the restriction of the isothermal boundary condition is the primary cause of the generation of the dilatational structures at small scales. In fully developed turbulence, the formation and the existence of healthy dilatational travelling wave packets require the participation of the turbulence at scales larger than those of the near-wall regeneration cycles, especially the occurrence of the bursting events that generate vortex clusters. This is proven by the less intensified dilatational motions in the numerical experiments in which the Orr mechanism is alleviated and the vortical structures and turbulent bursts are weakened.

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“…Over the insulated wall, the developed boundary layer is thicker, approximately δ h0 ≃ 14 mm, in contrast to δ h0 ≃ 8 mm over the cold wall. Unnikrishnan et al [43] noted that second-mode instabilities tend to exhibit maximum amplitudes near highgradient regions. Consequently, amplification dynamics are intensified closer to the wall under colder surface conditions.…”

Section: Steady Statementioning

confidence: 99%

Coherent structure tracking of the second Mack mode in transitional hypersonic boundary layers

Hammachi,

Piot,

Deniau

et al. 2024

J. Phys.: Conf. Ser.

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The natural transition of hypersonic boundary layers (HBLs) is often expressed in terms of discrete modes and their linear stability. A frequent interpretation revolves around fast and slow acoustic modes interacting in the vicinity of the vortical/entropic branches of the continuous spectrum found from stability analyses. Yet several transition scenarios are contingent upon factors such as the spectral content of the free-stream disturbances, or the interactions between the discrete modes within the boundary layer and the free-stream disturbances near the leading edge which can be decomposed into vortical, acoustic and entropic nature based on the fluid-thermodynamic (FT) components. Yet the interpretations of linear stability applied to discrete modes can lead to semantic conflicts with the terminology of FT components. To clarify the current description of the processes involved, this study chooses an approach aimed at characterizing the dynamics of the second Mack mode in transitional HBLs through coherent structure tracking. The method involves decomposing the flow perturbations into acoustic, vortical and entropic content, and following their associated coherent structures over time. For this purpose, direct numerical simulations are carried out to investigate the dynamics of the second Mack mode instability in two-dimensional HBLs, considering a flow at Mach 6 over a cooled and an insulated wall. It is found that vortical structures coexist at different heights along the wall surface, forming alternating sign doublets around the critical layer and above the relative sonic line. These structures are found to merge in the region of maximum second Mack mode instability.

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Instabilities and transition in cooled wall hypersonic boundary layers

Abstract: Abstract

Cited by 18 publications

References 57 publications

Time-resolved wave packet development in highly cooled hypersonic boundary layers

Time-resolved wave packet development in highly cooled hypersonic boundary layers

On the generation of near-wall dilatational motions in hypersonic turbulent boundary layers

Coherent structure tracking of the second Mack mode in transitional hypersonic boundary layers

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