Low-frequency unsteadiness mechanisms in shock wave/turbulent boundary layer interactions over a backward-facing step

Hu, Weibo; Hickel, Stefan; Oudheusden, B.W. van

doi:10.1017/jfm.2021.95

Cited by 34 publications

(49 citation statements)

References 71 publications

(183 reference statements)

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“…The spanwise extent of the computational domain is equivalent to 14 incoming boundary layer thickness δ 0 on the suction side. Therefore, the present grid resolution is similar or higher than those employed by other high-fidelity simulations of SBLIs [19,28,47,62]. The same holds valid for the adequacy of the span length of the domain.…”

Section: Flow and Mesh Configurationssupporting

confidence: 63%

“…Modal decomposition techniques have emerged as powerful tools for analyzing the unsteady flows [43] facilitating the assessment of organized structures which may play an important role in the flow dynamics. In recent years, these techniques have been applied for the investigation of low-frequency unsteadiness in SBLIs [28,[44][45][46][47][48][49][50][51][52].…”

Section: Proper Orthogonal Decompositionmentioning

confidence: 99%

“…Several authors have studied the low-frequency events taking place in SBLIs using experimental [11][12][13][14][15][16] and numerical [17][18][19][20][21][22][23][24][25][26][27][28][29] techniques. Based on previous investigations, one can categorize the main flow phenomena in three different frequency bands: low frequency oscillations from the reflected shock motion and large-scale bubble breathing (0.02 < St < 0.05), low to mid-frequency motions of the separation bubble and flapping of the shear layer (St ≈ 0.1), and the K-H instabilities (0.3 < St < 0.5).…”

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confidence: 99%

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Shock-Boundary Layer Interactions in Supersonic Turbine Cascades

Lui,

Ricciardi,

Wolf

et al. 2022

Preprint

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The physics of shock-boundary layer interactions in a supersonic turbine cascade is investigated through a wall-resolved large eddy simulation. Special attention is given to the characterization of the low-frequency dynamics of the separation bubbles using flow visualization, spectral analysis, space-time cross correlations, and flow modal decomposition. The mean flowfield shows different shock structures formed on both sides of the airfoil. On the suction side, an oblique shock impinges on the turbulent boundary layer, whereas a Mach reflection interacts with the pressure side boundary layer. Instantaneous flow visualizations illustrate elongated streamwise structures on the incoming boundary layers and their interactions with the shocks and separation bubbles. The passage of high-speed (low-speed) streaks through the recirculation bubbles leads to the downstream (upstream) motion of the separation point on both suction and pressure sides, resulting in spanwise modulation of the bubbles. Space-time cross-correlations reveal that the near-wall streaks drive the suction side separation bubble motion, which in turn promotes the oscillations of the reattachment shock and shear layer flapping. Space-time correlations also indicate the existence of a π phase jump in the pressure fluctuations along the separation bubble on the suction side.After this phase jump, a downstream propagating pressure disturbance is observed, while prior to this point, the pressure disturbances dominantly propagate in the upstream direction. Finally, the organized motions in the shock-boundary layer interactions and their corresponding characteristic frequencies are identified using proper orthogonal decomposition. I. INTRODUCTIONSupersonic fluid machinery offers size and cost reduction in high-speed propulsion and power generation systems [1, 2] and for compact hydrocarbon cracking [3]. The detailed analysis of supersonic turbines is challenging predominantly due to the shock-boundary layer interactions (SBLIs) which arise when the detached oblique shock waves forming at the stator/rotor leading edges impinge on the boundary layers of the neighboring airfoils. The shocks impose intense adverse pressure gradients on the boundary layers that may cause flow separation. This, in turn, leads to the formation of separation and reattachment shocks that *

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Section: Flow and Mesh Configurationssupporting

confidence: 63%

Section: Proper Orthogonal Decompositionmentioning

confidence: 99%

mentioning

confidence: 99%

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Shock-Boundary Layer Interactions in Supersonic Turbine Cascades

Lui,

Ricciardi,

Wolf

et al. 2022

Preprint

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“…According to Portal-Porras et al [30], the LES-based technique provided better resolution per cell than conventional ones did. The LES is a mathematical turbulence model used in computational fluid dynamics [31,32]. The main idea is to reduce computational costs relative to direct numerical simulation (DNS) via low-pass filtering of the Navier-Stokes equations to model the smallest-length scales, the most computationally expensive to resolve [33].…”

Section: Simulation Physicsmentioning

confidence: 99%

Estimating the Reattachment Length by Realizing a Comparison between URANS k-Omega SST and LES WALE Models on a Symmetric Geometry

et al. 2021

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In this study, a water reattachment length was calculated by adopting two different models. The first was based on Unsteady Reynolds-Averaged Navier–Stokes (URANS) k-omega with Shear Stress Transport (SST); the second was a Large Eddy Simulation (LES) with Wall-Adapting Local Eddy-Viscosity (WALE). Both models used the same mesh and were checked with Taylor length-scale analysis. After the analysis, the mesh had 11,040,000 hexahedral cells. The geometry was a symmetrical expansion–contraction tube with a 4.28 expansion ratio that created mechanical energy losses, which were taken into account. Moreover, the reattachment length was estimated by analyzing the speed values; the change of speed value from negative to positive was used as the criterion to recognize the reattachment point.

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“…In contrast, for the compression ramp and FFS cases, the separation in these two configurations is produced by the flow compression due to the contraction of the geometry. In the published literature, including our previous work (Grilli, Hickel & Adams 2013; Pasquariello, Hickel & Adams 2017; Hu, Hickel & van Oudheusden 2021), the first three SWBLI geometries have been well investigated, whereas seldom has attention been paid to the FFS. In the FFS configuration, the boundary layer separates far upstream of the step and reattaches on the step wall or downstream of the step.…”

Section: Introductionmentioning

confidence: 99%

Unsteady mechanisms in shock wave and boundary layer interactions over a forward-facing step

Hickel

Oudheusden

2022

J. Fluid Mech.

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The flow over a forward-facing step (FFS) at $Ma_\infty =1.7$ and $Re_{\delta _0}=1.3718\times 10^{4}$ is investigated by well-resolved large-eddy simulation. To investigate effects of upstream flow structures and turbulence on the low-frequency dynamics of the shock wave/boundary layer interaction (SWBLI), two cases are considered: one with a laminar inflow and one with a turbulent inflow. The laminar inflow case shows signs of a rapid transition to turbulence upstream of the step, as inferred from the streamwise variation of $\langle C_f \rangle$ and the evolution of the coherent vortical structures. Nevertheless, the separation length is more than twice as large for the laminar inflow case, and the coalescence of compression waves into a separation shock is observed only for the fully turbulent inflow case. The dynamics at low and medium frequencies is characterized by a spectral analysis, where the lower frequency range is related to the unsteady separation region, and the intermediate one is associated with the shedding of shear layer vortices. For the turbulent inflow case, we furthermore use a three-dimensional dynamic mode decomposition to analyse the individual contributions of selected modes to the unsteadiness of the SWBLI. The separation shock and Görtler-like vortices, which are induced by the centrifugal forces in the separation region, are strongly correlated with the low-frequency unsteadiness in the current FFS case. Similarly as observed previously for the backward-facing steps, we observe a slightly higher non-dimensional frequency (based on the separation length) of the low-frequency mode than for SWBLI in flat plate and ramp configurations.

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Low-frequency unsteadiness mechanisms in shock wave/turbulent boundary layer interactions over a backward-facing step

Abstract: Abstract

Cited by 34 publications

References 71 publications

Shock-Boundary Layer Interactions in Supersonic Turbine Cascades

Shock-Boundary Layer Interactions in Supersonic Turbine Cascades

Estimating the Reattachment Length by Realizing a Comparison between URANS k-Omega SST and LES WALE Models on a Symmetric Geometry

Unsteady mechanisms in shock wave and boundary layer interactions over a forward-facing step

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