Linear, nonlinear and transitional regimes of second-mode instability

Nair, Unnikrishnan Sasidharan; Gaitonde, Datta V.

doi:10.1017/jfm.2020.781

Cited by 28 publications

(9 citation statements)

References 73 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…2018; Craig et al. 2019) and theoretically/computationally (for example, Fedorov & Tumin 2011; Sivasubramanian & Fasel 2014; Unnikrishnan & Gaitonde 2020), with studies tending to focus on smoothly varying surface geometries. The outer mould line of true flight vehicles does not always vary smoothly in the streamwise direction however, and may exhibit a sudden increase in angle, e.g.…”

Section: Introductionmentioning

confidence: 99%

“…For axisymmetric or near-axisymmetric geometries, the dominant instability mechanism above Mach numbers of approximately four is the Mack or second mode. Following its discovery (Mack 1975), the second mode has received substantial attention, both experimentally (for example, Demetriades 1974;Stetson & Kimmel 1992;Laurence et al 2012;Laurence, Wagner & Hannemann 2014Casper et al 2016;Kennedy et al 2018;Craig et al 2019) and theoretically/computationally (for example, Fedorov & Tumin 2011;Sivasubramanian & Fasel 2014;Unnikrishnan & Gaitonde 2020), with studies tending to focus on smoothly varying surface geometries. The outer mould line of true flight vehicles does not always vary smoothly in the streamwise direction however, and may exhibit a sudden increase in angle, e.g.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Transitional hypersonic flow over slender cone/flare geometries

Butler

Laurence

2022

J. Fluid Mech.

View full text Add to dashboard Cite

Experiments are performed in a Mach-6 shock tunnel to examine the laminar-to-turbulent transition process associated with a sudden increase in surface angle on a slender body. A cone/flare geometry with a 5 $^\circ$ frustum and compression angles ranging from 5 $^\circ$ to 15 $^\circ$ allow a range of mean flow configurations, spanning an attached shock-wave/boundary-layer interaction to a fully separated one; the unit Reynolds number of the flow is also varied to modify the state of incoming second-mode boundary-layer disturbances. Ultra-high-speed schlieren visualizations provide a global picture of the flow development, supplemented by high-frequency surface pressure measurements. For the 5 $^\circ$ compression, the unsteady flow field is dominated by the second-mode waves, whose breakdown to turbulence is generally accelerated (compared with the straight-cone configuration) by encountering the angle change. As the compression angle is increased to induce separation, lower-frequency disturbances appear along the separated shear layer that exhibit much larger amplification rates than the incoming second-mode waves; the latter effectively freeze in amplitude downstream of the separation point before rapidly breaking down upon reattachment. The shear-layer disturbances become dominant at the largest compression angle tested. Radiation of disturbance energy to the external flow is consistently observed: this generally occurs along mean flow features (flare, separation or reattachment shocks) for the second-mode disturbances and spontaneously for the shear-layer waves. The combined application of spectral proper orthogonal decomposition and a global bispectral analysis allows the identification of important unsteady flow structures and the association of these with prominent nonlinear interactions in the various configurations.

show abstract

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Transitional hypersonic flow over slender cone/flare geometries

Butler

Laurence

2022

J. Fluid Mech.

View full text Add to dashboard Cite

show abstract

“…An explicit approach (Shu & Osher 1988) was also utilized to ensure that the mean and unsteady features of the flow fields are not sensitive to the choice of the time-integration scheme. The DNS solver has been validated and applied to boundary layer and inlet-related problems in prior publications (Unnikrishnan & Gaitonde 2020; Khobragade, Unnikrishnan & Kumar 2021).…”

Section: Methodsmentioning

confidence: 99%

Flow instabilities and impact of ramp–isolator junction on shock–boundary-layer interactions in a supersonic intake

2022

Self Cite

View full text Add to dashboard Cite

Boundary layer instabilities and shock–boundary-layer interactions (SBLIs) critically affect the performance and safe operation of mixed-compression air intakes. We present a computational study anchored in companion experiments, to evaluate the multimodal mechanisms driving the dynamics of the external compression ramp flow and the cowl SBLI in a Mach $3$ intake. Boundary layer transition over the external ramp is first analysed through a global linear analysis, and the linear estimates are further validated through direct numerical simulations. The separation bubble over the ramp harbours three-dimensional stationary instabilities that induce transition, under the influence of secondary instabilities driven by the shear layer modes of the bubble. The interaction of the resulting turbulized boundary layer with the cowl shock at the ramp–isolator junction and its control through geometrical modification constitutes the second part of the study. We tested a faceted (baseline) and a notched (modified) junction design to evaluate its impact on the low-, mid- and high-frequency scales generated at the cowl SBLI region. In relation to the baseline case, the notched geometry effectively locks the separation point of the bubble, thus attenuating the upstream low-frequency breathing motion. The notch also energizes the midfrequency content through vortex shedding in a well-developed shear layer, which persists into the isolator, thus assisting in an efficient compression process through cross-stream mixing of near-wall flow. The isolator boundary layer in the notched design exhibits relatively lower static pressures and higher velocity fluctuations, which are conducive to improving the flow stability, unstart margin and efficiency of high-speed propulsion systems.

show abstract

“…One-dimensional linear stability calculations are conducted to characterize the unstable modes of the boundary-and entropy-layers. These have proven useful in informing the linear stages of laminar-turbulent breakdown in hypersonic configurations with sharp leading edges [35,46,47]. As noted earlier, LST has also been employed to account for the destabilization of modal instabilities in the entropy-layer [19,20].…”

Section: Local Stability Analysismentioning

confidence: 99%

“…While both DMD and SPOD are applicable to linear and nonlinear flow data, they do not explicitly account for the nonlinear interactions. Higher-order statistical tools, such as auto-and cross-bispectrum, have been employed in the literature to quantify some of these interactions in experimental [10] and numerical [46] studies.…”

Section: E Data-driven Flow Decomposition Techniquesmentioning

confidence: 99%

Role of entropic-instabilities in laminar-turbulent transition on a blunted flat plate

Goparaju¹,

Gaitonde²

2021

Preprint

Self Cite

View full text Add to dashboard Cite

The effects of entropic-instabilities on the laminar-turbulent transition dynamics of a blunted flat plate at Mach 4 are numerically investigated through linear and nonlinear approaches. Linear wavepacket analysis reveals amplifying oblique first-modes as well as planar and oblique entropylayer disturbances. The receptivity of entropic-instabilities is found to be largest for actuation seeded in the entropy-layer; the corresponding linear evolution is characterized by an intensification in the wall-normal plane, coupled with streamwise tilting. Disturbances in the entropy-layer interact non-linearly with each other through the oblique breakdown mechanism, inducing streamwise streaks in the boundary-layer. These undergo further destabilization downstream leading to turbulence onset. Different mechanisms of interest are extracted with modal decomposition techniques best suited for each. Dynamic mode decomposition (DMD) reveals sinuous subharmonic oscillations induced by the entropic-disturbances as the dominant streak instability mechanism. On the other hand, spectral proper orthogonal decomposition (SPOD) elicits the slanted hook -shaped structures in the temperature perturbations, which are attributable to the Orr like-mechanism in the entropy-layer. Furthermore, cross-bispectral mode decomposition (CBMD) shows that the temperature perturbations amplify on the crests of the low-speed streaks in the entropy-layer and generates disturbances through triadic interactions. These newer disturbances further interact, resulting in the transfer of perturbation energy into the boundary-layer. This eventually leads to spanwise homogenization and near-wall streak generation, reflecting late nonlinear stages of transition. Towards the end of breakdown region, spectral broadening accompanied by the appearance of an inertial sub-range in the boundary-layer indicates the approach of the flow towards turbulence.

show abstract

Linear, nonlinear and transitional regimes of second-mode instability

Abstract: Abstract

Cited by 28 publications

References 73 publications

Transitional hypersonic flow over slender cone/flare geometries

Transitional hypersonic flow over slender cone/flare geometries

Flow instabilities and impact of ramp–isolator junction on shock–boundary-layer interactions in a supersonic intake

Role of entropic-instabilities in laminar-turbulent transition on a blunted flat plate

Contact Info

Product

Resources

About