DC6: Boundary Layer Transition Induced by Periodic Surface Roughness
SkillsThis PhD position is part of the FairCFD Doctoral Network, funded by the European “Marie Skłodowska-Curie Actions” (MSCA) programme. The network aims to define and promote numerical sustainability in the field of Computational Fluid Dynamics (CFD). The successful candidate will join a cohort of 15 doctoral researchers distributed across 9 European countries, benefiting from access to cutting-edge training events, advanced scientific and technical courses, and secondments in both academic and industrial environments. The doctoral researcher will be primarily hosted at the Department of Aerodynamics, Aeroelasticity and Acoustics of ONERA – The French Aerospace Lab, located in Meudon, France. The position offers an attractive salary in accordance with MSCA regulations for Doctoral Researchers, including a living allowance, a mobility allowance, and, where applicable, a family allowance. The PhD funding is guaranteed for 36 months. In line with the international mobility rule of the MSCA-DN programme, applicants must not have resided or carried out their main activity in France for more than 12 months within the 36 months preceding the start of the PhD.
|
|
ProjectUnderstanding roughness-induced transition is crucial, as it leads to increased skin-friction drag and can significantly impact the aerodynamic performance of vehicles. Over the past decade, the transition of subsonic boundary-layer flows induced by isolated roughness elements of various shapes has attracted considerable attention. Direct Numerical Simulations (DNS) of the unsteady wake generated by an isolated cylindrical roughness element (Figure 1, left) were performed by Loiseau et al. (2014). The emergence of such unsteady wakes was subsequently examined through global stability analyses of the corresponding three-dimensional steady base flows. The authors showed that sinuous and varicose eigenmodes—associated with distinct physical mechanisms—become unstable once the roughness height exceeds a critical threshold. A similar methodology was employed by Citro et al. (2015) to study the flow past a semi-hemispherical roughness element, confirming the existence of analogous instability mechanisms. More recently, Bucci et al. (2018) investigated subcritical transition around a cylindrical roughness element by combining experimental measurements with numerical simulations. To elucidate the amplification mechanisms underlying the experimentally observed transition, the authors conducted a resolvent analysis, providing new insights into the subcritical amplification processes. The objective of this PhD thesis is to investigate subsonic boundary-layer transition induced by periodically distributed surface roughness. Ma and Mahseh (2023) recently conducted a related study combining Direct Numerical Simulations (DNS) with global stability analysis, providing valuable insight into the instability mechanisms at play. However, such high-fidelity numerical approaches are computationally prohibitive, which limits their applicability to a restricted set of parameters and configurations. In this project, we aim to develop novel analytical and reduced-order numerical methods to investigate the flow dynamics induced by periodic roughness while drastically reducing the computational cost compared to full DNS or global stability analyses. These approaches will make it possible to explore broader parametric spaces—including variations in roughness geometry, spacing, and Reynolds number—and to address more realistic configurations relevant to aeronautical and industrial applications. The first approach considered in this project will leverage the spatial periodicity of the surface roughness. Bloch theory provides a rigorous framework for analyzing wave propagation in spatially periodic, non-dissipative media, such as electromagnetic, elastic, or acoustic waves in metamaterials. In the present context, it will be applied to investigate the non-modal amplification of flow perturbations decomposed into Bloch waves. This formulation enables the singular value problem arising in resolvent analysis to be discretized within a single unit cell. The Bloch wavenumber remains a continuous control parameter, allowing one to explore the amplification of disturbances with arbitrary wavelengths. Figure 2 show the most amplified spatial structures computed in a boundary-layer flow over two-dimensional periodic roughness, for low (left) and high (right) excitation frequencies. In both cases, the entire analysis is performed on the unit cell highlighted in blue. For low frequencies and long wavelengths (left) the analysis recovers the classical Tollmien–Schlichting waves characteristic of smooth-wall boundary layers. At higher frequencies and shorter wavelengths (right), shear-layer instabilities emerge, revealing the strong influence of surface periodicity on the dynamics of short-wavelength perturbations. The second approach explored in this project will rely on homogenization theory. Asymptotic homogenization provides a rigorous framework for describing the macroscale behavior of media containing fine-scale heterogeneities. It does so by replacing the rapidly varying microscopic properties of the medium with equivalent, effective macroscopic parameters. This approach can be used to derive effective boundary conditions defined on a smooth virtual surface, which acts as the boundary of the macroscale problem (Zampogna et al., 2019). In this way, the computationally demanding resolution of the flow inside each individual roughness element is avoided. Within this framework, we will develop and implement methods to compute steady boundary-layer flows over rough surfaces and subsequently to analyze the amplification of unsteady perturbations using the same homogenized formulation. A key question that naturally arises is whether this approach is capable of capturing the high-frequency shear-layer modes (Figure 2, right), which are strongly influenced by the fine-scale geometry of the surface roughness. During the first year, the PhD candidate will become familiar with the underlying mathematical frameworks (Bloch theory and homogenization) and the existing numerical tools, by investigating the transition of boundary-layer flows over two-dimensional surface roughness. The influence of roughness size, shape, and spacing on both the steady base flow and the amplification of perturbations will be systematically analyzed using both approaches. To perform these parametric studies efficiently, the candidate will develop algorithms capable of tracking resolvent modes as parameters vary, thus significantly reducing the overall computational cost. In the second and third years, the study will be extended to three-dimensional roughness configurations, requiring the development of dedicated numerical tools optimized for high-performance computing environments. Results will be systematically compared with those obtained for isolated roughness elements, providing new physical insight into the collective effects of periodic roughness on boundary-layer transition. Beyond the individual research program described above, the candidate will also participate, together with the other FairCFD doctoral researchers, in a network-wide multidisciplinary initiative addressing the environmental and societal impacts of numerical simulation, in line with the objectives of the Marie Skłodowska-Curie Actions. References :J.-C. Loiseau, J.-C. Robinet, S. Cherubini, and E. Leriche. Investigation of the roughness-induced transition: global stability analyses and direct numerical simulations. Journal of Fluid Mechanics, 760:175–211, 2014. V. Citro, F. Giannetti, P. Luchini, and F. Auteri. Global stability and sensitivity analysis of boundary-layer flows past a hemispherical roughness element. Physics of Fluids, 27(8), 2015 M. A. Bucci, D. K. Puckert, C. Andriano, J.-C. Loiseau, S. Cherubini, J.-C. Robinet, and U. Rist. Roughness-induced transition by quasi-resonance of a varicose global mode. Journal of Fluid Mechanics, 836:167–191, 2018. R. Ma and K. Mahesh. Boundary layer transition due to distributed roughness: Effect of roughness spacing. Journal of Fluid Mechanics, 977:A27, 2023. Zampogna, G. A., Magnaudet, J., & Bottaro, A. (2019). Generalized slip condition over rough surfaces. Journal of Fluid Mechanics, 858, 407-436. |
|
CollaborationsAt least two secondments, i.e. research visits of a minimum duration of two months, will be planned in partner universities of the project (IMFT, University of Salerno, University of Cambridge, TU Berlin). |
|
Laboratoire d’accueil à l’ONERADepartment : Aérodynamique, Aéroélasticité, Acoustique Location (centre ONERA) : Meudon Contact : Olivier Marquet, Alessandro Bongarzone email : alessandro.bongarzone@onera.fr |
PhD director Name : Olivier Marquet Laboratory : DAAA Phone. : 0146235197 email : olivier.marquet@onera.fr |