arXiv subjects

Clément Caillaud

Publications and source records attributed to Clément Caillaud.

4 recordsLinked to original sources

arXiv2025

Reynolds effects on transition to turbulence for hypersonic expansion and compression corner flows

This experimental and numerical study examines transition to turbulence for a Cone-Cylinder-Flare geometry at Mach 7 and across a broad Reynolds number range. The focus is set on both attached boundary layers and separated shock-boundary layer interactions. The campaign is conducted in the R2Ch facility. Unsteady wall pressure fluctuations and high-speed schlieren images are analysed using data-driven techniques and compared with base flow computations and global linear stability analysis. The results distinguish two transition regimes. At high Reynolds numbers, transition is dominated by the second Mack mode and its non-linear interactions on the cone. High-frequency wall pressure measurements and schlieren imaging permit the capture of both fundamental waves and their non-linear harmonics. Non-linear interaction regions are resolved with unprecedented detail, clarifying the boundary-layer state before rapid breakdown at reattachment. At lower Reynolds numbers, the transition scenario is more intricate, marked by the coexistence of low- and high-frequency modes. A complex coupling between separated flow and convective instabilities is revealed, with trapped acoustic waves inside the recirculation region measured experimentally for the first time. Their linear origin is demonstrated through global stability analysis, and a simple acoustic duct model is provided to predict their frequencies. These waves offer a new interpretation of low-frequency pressure signatures and suggest a mechanism for energy transfer from high to low frequencies, ultimately driving transition on the flare. The findings advance understanding of hypersonic boundary-layer transition and its dependence on Reynolds number and flow separation.

physics.flu-dyn↗

arXiv2025

Transition mechanisms in hypersonic wind-tunnel nozzles: a methodological approach using global linear stability analysis

Base-flow computations and stability analyses are performed for a hypersonic wind tunnel nozzle at a Mach number of 6. Isothermal and adiabatic wall boundary conditions are investigated, and moderate stagnation conditions are used to provide representative scenarios to study the transition in quiet hypersonic wind tunnel facilities. Under these conditions, the studied nozzle shows a small flow separation at the convergent inlet. Global stability analysis reveals that this recirculation bubble may trigger a classical three-dimensional stationary unstable global mode. Resolvent analysis reveals Görtler, first and second Mack modes affecting the divergent part of the nozzle, along with a Kelvin-Helmholtz instability induced by the bubble. The present study also highlights the key impact of perturbations located in the convergent inlet on the development of instabilities further downstream in the divergent outlet, helping understand the need and efficacy of a suction lip upstream of the nozzle throat to mitigate instabilities in the divergent nozzle. Detailed knowledge of all these mechanisms is essential for understanding flows in quiet hypersonic wind tunnel nozzles and, consequently, represents a key step toward the optimisation of such nozzles.

physics.flu-dyn↗

arXiv2024

Effect of streaks on hypersonic boundary layer instability

Hypersonic boundary layers exhibit diverse transition pathways, influenced by various flow conditions and environments. Non-modal mechanisms, such as the lift-up effect, are recognized as pivotal contributors to transition, particularly over complex geometries like rough walls or blunted forebodies. In this study, we investigate the impact of steady streaks on the transition to turbulence in hypersonic boundary layers. Streaky baseflows are generated using optimal disturbances at the inlet, forming the basis for our analysis. We conduct linearized direct numerical simulations on these baseflows at Mach number $M_\infty=6.0$, using white noise forcing to trigger instabilities. An efficient extension of the SPOD method is applied to identify and track the instabilities originating from the broadband forcing on the 3D non-homogeneous baseflows. The results highlight the dominant influence of low-frequency first-mode instabilities and streak-associated instabilities in the linear regime. These findings emphasize the critical role of streaks in hypersonic boundary layer transition and provide valuable insights into this complex phenomenon.

physics.flu-dyn↗

arXiv2023

Global stability analysis of a hypersonic cone-cylinder-flare geometry

Characterizing the boundary layer transition to turbulence around realistic hypersonic vehicles is a challenging task due to the numerous parameters that affect the process. To address this challenge, the cone-cylinder-flare (CCF) geometry has been designed to provide a flow topology that captures various transition mechanisms observed on reentry objects, such as absolute and convective instabilities, which are dependent on the free stream conditions. In this study, a global linear stability analysis is performed on the CCF model at Mach=6.0 to investigate and map the dominant instabilities at wind tunnel flow conditions. We examine optimal responses and forcings computed using resolvent analysis, as well as global modes originating from the recirculation bubble at the cylinder flare junction. The effects of bluntness are assessed through analyses of both blunt and sharp configurations. Our results shed light on the linear flow mechanisms that promote the transition to turbulence around such hypersonic objects.

physics.flu-dyn↗