AERO2025 - UNSTEADY FLOWS: RECENT DEVELOPMENTS AND APPLICATIONS

The 59th 3AF International Conference on Applied Aerodynamics, held in Strasbourg from March 24 to 26, focused on Unsteady Flows: recent developments and applications. Supported by the Région Grand Est, the ICube Laboratory at the University of Strasbourg and ONERA, this year's event brought together 110 participants from 16 countries to explore technological advances and their applications in a wide range of sectors.technological advances and their applications in key sectors such as aerospace (civil and military), transport and energy.

The five pilot conferences and fifty-nine presentations in twelve sessions highlighted the importance of unsteady flows, which influence the stability, noise and performance of aircraft and helicopters, as well as the efficiency of propulsion systems, where dynamic interactions impact fuel consumption, mechanical strength and durability.and helicopters, as well as the efficiency of propulsion systems, where dynamic interactions impact fuel consumption, mechanical strength and durability. These phenomena also affect strategic applications, from missiles and space launchers to land vehicles subjected to extreme conditions.

Speakers underlined the dual challenge of these flows: a challenge for design and modeling, but also an opportunity for optimization, thanks to advances in high-fidelity numerical simulation and experimental methods. The growing use of High-Performance Computing (HPC), hybrid algorithms (RANS/LES) and techniques such as Particle Image Velocimetry (PIV) or Laser Doppler Velocimetry (LDV) has made it possible to optimize the design and modeling of these flows.locimetry (LDV) now make it possible to study these phenomena with unprecedented precision.

Tribute to Laurent Jacquin

The conference opened with a tribute by Denis Sipp (ONERA) to Laurent Jacquin [1959-2024], a major figure in aerodynamics. His work, spanning more than three decades, had a profound impact on the discipline. His major contributions include:

- Jet/wake vortex interaction

(1996-2007 ): LDV measurement campaigns to characterize jet mixing and destabilization mechanisms.

• Cavity flows (1999-2013): detailed analyses of instabilities and cavity resonance phenomena.
• Tremblement transsonic wing (2000-2016) : international reference database , including pressure-conditioned LDV measurements.
• Turbulence in subsonic jets (1986-2020): exploration of dynamics and transition.

The tribute recalled that his approach combined experimental rigor and theoretical vision.

Conférence Pilote n°1 - A. Le Pape (ONERA) presented the challenges of unsteady aerodynamics for rotary wings, including vortex-blade interactions, dynamic stall and transonic flows. Electric vertical take-off and landing aircraft (ADAVe / eVTOL) and rotor mechanics require accurate fluid-structure coupling (CSD/CFD). Despite the efficiency of CFD simulations, their limitations persist in turbulence, transition and meshing. New methods are emerging for acoustics, but model validation still depends on reliable experimental data.

Session 1A - Rotating airfoils - Among the presentations in this first session, a study by Safran / École Centrale de Lyon showed that the addition of passive control holes located on both the hub and the casing of a radial diffuser, thus linking the channels between the blades to each other, makes it possible to delay stall and improve the stability of a TURBB compressor. on both the hub and the casing of a radial diffuser, thus linking the channels between the blades, helps to delay stall and improve the stability of a Safran Helicopters TURBOCEL compressor.

TURBOCEL compressor - Pressure ratio and isentropic efficiency as functions of mass flow rate.

Credit: SAFRAN / École Centrale Lyon

Session 1B : Measurement techniques - TU-Berlin 's experimental study presented a two-dimensional calorimetric parietal friction sensor, capable of measuring both the amplitude and direction of friction fluctuations. Tested behind a descending step at Re = 15,000, this sensor delivered consistent results and opens up promising prospects for the analysis of unsteady low-speed flows.

Instantaneous friction downstream of a descending step - Credit: TU-Berlin

Session 1C - Adaptive canopies An adaptive canopy prototype, inspired by the A320 model, has been studied by IMFT and the LAPLACE Laboratory. The research combined simulations of coherent Organised-Eddyo- Similation (OES) structures with experiments using composite-fibre piezoelectric actuators. These actuators generate progressive waves applied to the wing surface, forming an active electro-wall capable of controlling the interaction between the shock wave and the boundary layer (SBLI), while reducing the buffeting phenomenon.

The results show a reduction in drag of up to 1.64%, as well as an improvement in the lift/drag ratio of around

+2%, especially when waves are applied

in an optimized zone (for x/c Є [0.514;0.686]).

Simulation of coherent structures (OES) and experimental set-up - Credit: IMFT / LAPLACE

Pilot lecture #2 - The lecture presented by J. Crouch (Boeing) focused on the use of Global Stability Analysis (GSA), a method based on the linearization of the Navier-Stokes equations around stationary RANS solutions. This approach enables early identification of unstable modes (growth, frequency and spatial shape) at a computational cost comparable to that of conventional RANS simulations.

Its application to transonic wobble illustrates the effectiveness of the method: on infinite wings, the wobble results from a global oscillatory instability, while on swept wings, an additional broadband unstable mode appears. This makes GSA a powerful tool for predicting the onset of instabilities and guiding aerodynamic design.

Session 2A - propulsivesystem - Imperial College has studied the unsteady flow in a square duct, representing the cooling circuit of a hydrogen fuel cell aircraft placed in the wake of a propeller.the cooling circuit of a hydrogen fuel cell aircraft placed in the wake of a propeller.

The instabilities were characterized by time-resolved PIV measurements. The study concludes that the amplification of these instabilities could, on the one hand, improve heat transfer for more efficient cooling, but on the other hand, cause local supercooling that could damage the fuel cell.

Session 2B - Laminar debonding and transition

- ISAE-SUPAERO has analyzed the dynamics of laminar separation bubbles on low Reynolds number NACA0012 profiles in the presence of gusts. The results show that longitudinal gusts play a critical role, yet are poorly anticipated by conventional analytical models.

These observations underline the importance of incorporating unsteady, wake and longitudinal pressure gradient effects into these analytical models when modelling lift.

Session 2C - Advanced flow field analysis - ONERA presents an innovative method for quantifying shock wave energy loss in aerodynamic flows. The main objective is to distinguish shock wave losses from other irreversible losses, without requiring a clear boundary between these phenomena.

The study provides a complete derivation of the energy balance equation presented as more robust for handling critical interactions in complex aerodynamic systems such as turbomachinery.

For example, shockwaves forming at the level of the rectifier's fixed vanes (OGV) under blocked flow conditions (cf. figure on the right) are generally excluded from integration by the standard method of calculating shock wave anergy, as they lie in the wake of the fan blades. In contrast, the proposed method takes them into account.