MSCA-DN FairCFD - DC6 - Boundary Layer Transition Induced by Periodic Surface Roughness

ONERA; France; On-site; EUR 34,000 - 38,000; Full time

Job summary

ONERA in Meudon, France, invites applications for a PhD position in fluid mechanics focusing on boundary-layer transition induced by surface roughness, under the FairCFD network. The project combines Bloch theory and homogenization to develop efficient methods for predicting perturbation amplification in periodic roughness.

The candidate will work within the Departement of Aerodynamics, Aeroelasticity and Acoustic and contribute to a Marie Skłodowska-Curie Actions Network.

Qualifications

• PhD applicant in fluid mechanics or related field.
• Experience with boundary-layer theory and stability analysis is advantageous.
• Strong programming and high-performance computing skills.

Responsibilities

• Develop analytical and reduced-order models for boundary-layer transition over rough surfaces.
• Implement Bloch theory and homogenization approaches for periodic roughness.
• Track resolvent modes and compare with high-fidelity simulations.

Job description

Organisation/Company ONERA Department DAAA Research Field Engineering » Aerospace engineering Engineering » Mechanical engineering Engineering » Simulation engineering Researcher Profile First Stage Researcher (R1) Positions PhD Positions Application Deadline 30 Oct 2026 - 00:00 (Europe/Paris) Country France Type of Contract Temporary Job Status Full-time Offer Starting Date 1 Jan 2027 Is the job funded through the EU Research Framework Programme? Horizon Europe - MSCA Marie Curie Grant Agreement Number 101226482 Is the Job related to staff position within a Research Infrastructure? No

Offer Description

Scientific background

Understanding 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 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 measurementswith 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.

Objectives

The objective of this PhD thesis is to investigate 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 problemarising 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.

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.

Your research programm

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 overallcomputational 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.

Where you will work

The PhD candidate will be enrolled during three years in a doctorate school at the University of Paris Saclay. She/he will perform his research activities in the Departement of Aerodynamics, Aeroelasticity and Acoustic from ONERA, located in Meudon, France, where she/he will be based for most of the project and integrated into local research activities on hydrodynamic stability and high-fidelity simulations.

Integration within the FairCFD Network

Within the FairCFD network, you will contribute mainly to WP1, Efficient physics-based numerical methods. You will regularly exchange with other DCs in the network who apply similar approaches to different problems and/or different numerical methods to similar problems.

Interdisciplinary task: co-designing numerical frugality

Beyond your individual research program described above, you will contribute along with all other FairCFD doctoral candidates to anetwork-wide multidisciplinary effort(WP5) addressing the environmental and societal dimensions of numerical simulation.
Each DC will participate in thedefinition of practical metricsfor numerical frugality (computational cost, energy use, resource impact) and contribute data from their simulations to acollective meta-analysis. This initiative will be supported by interdisciplinary experts and accompanied by a dedicated DC in social sciences, who will lead a qualitative study on evolving practices in simulation across the network. Together, we aim to buildconcrete, informed recommendationsfor sustainable scientific computing.

Network Training Program — More Than Just a PhD

As a Doctoral Network funded by Marie Sklodowska-Curie Actions (MSCA-DN), FairCFD will offer to you a rich and engaging training experience, including

• Four one-week training events; (i) an induction week devoted to team-building, open-science practices and sustainability issues, (ii) an Essential Skills Accelerator event combining aiming to to equip DCs with essential technical and transferable skills, (iii) a Hackathon eventwhere DCs will collaborate in teams to solve complex physics problem and compare various simulation strategies in terms of precision and sobriety, and (iv) aCareer and Leadership Development Forum Aiming to equip DCs with transferable skills essential for their future careers.
• Five On-line coursescombining technical training to state-of-the art simulation methods ranging from physics-based approaches to data-driven ones, exposition to industrial applications, along with Social, ethical and environmental aspects of decision-making in modelling practices.
• Involvement in the organisation of scientific events including a mini-symposium as part of a large-audience scientific conference,a scientific symposium allowing to share the output in terms of new methods, innovation, and applications to industrial processes, and a Societal colloquium to deliver the outputs of the multidisciplinary tasks of the network.

This programme is designed to support your growth as a researcher, innovator, and engaged citizen, fully equipped to lead the next generation of responsible simulation science. See our website for more details ( https://www.imft.fr/faircfd/project-presentation/

Where to