DAEP seminars 2024

Nouvelle traduction

Theoretical and Experimental Study of Mixing in Low-Density Jets

• Monday, December 16, 2024 - 2:00 pm - room 38.137 - by Léo Walter

The aim of this PhD thesis is to study the physical mechanisms which govern the evolution and the mixing of side-jets in low-density binary mixing round jets, using a complementary numerical and experimental approach. The physical mechanisms which are responsible for the generation of side-jets, closely related to the three-dimensionalization of the jet through the development of secondary instabilities, are as of yet not fully understood. As such, a better understanding of the mechanisms at play is a prerequisite for the design of an efficient control strategy to promote the mixing between the jet and ambient fluid.

The objective of the numerical study is to identify the transient mechanisms which influence the growth of three-dimensional disturbances in the low-density round jet, specifically under the physical conditions in which side-jets appear. To that aim, a linear non-modal stability analysis was conducted over the non-linear evolution of a two-dimensional axisymmetric Kelvin—Helmholtz vortex ring which develops in low-density round jets due to the Kelvin—Helmholtz primary instability. The stability analysis was implemented through further numerical development of the existing dalsa academic code. Through the use of a direct-adjoint optimisation method, we identify the spatial structure and temporal evolution of three-dimensional disturbances which yield the highest growth of generalised energy, as well as the underlying physical mechanisms and their relation to side-jets generation in low-density round jets at low Atwood numbers. In particular, we seek to bring a new perspective in order to settle between the two current hypotheses concerning the physical mechanisms at the origin of side-jets. The first hypothesis suggested by Monkewitz & Pfizenmaier (1991) relies on a velocity induction mechanism induced by the longitudinal counter-rotating vortex dipoles developing in the constant-density case. The second one is based on the three-dimensionalisation mechanism associated with longitudinal velocity streaks of opposite sign developing on either side of the hyperbolic stagnation point in the braid identified by Lopez-Zazueta et al. (2016) in the case of variable-density plane mixing layers.

The numerical analysis is conducted in close relation to an experimental investigation of the structure of side-jets in a helium-air binary mixture round jet. The parameters used in the numerical analysis, such as the Reynolds number, the Atwood number and jet aspect ratio, are based on the operating conditions used in the experiment, allowing the theoretical predictions to be compared with the empirical evolution of the helium-air jet. To that aim, we conduct hot-wire anemometry measurements of the jet radial profile and frequency of the primary instability under several operating conditions to characterise the evolution of the governing parameters and relate the experimental conditions to the existing scientific literature.

The objective of the experimental investigation is to study the structure of side-jets and their effect on the mixing of the jet and ambient fluids. To do so, we have designed and assembled a tomographic Background Oriented Schlieren (3DBOS) experimental bench. This bench is designed to observe the deviations of light-rays of the order of 0.5 mrad induced by the change in refractive index in the helium-air jet. The 3DBOS technique employed in this study provides novel reconstructions of three-dimensional density maps of the side-jets which develop over the helium-air jet. Through these novel density maps, we can provide new insight into the structure of side-jets and their induced mixing, and relate them to the predictions of the stability analysis.

Turboprop engines and body-force modeling of installation effects

• Friday, December 6, 2024 - 11:00 am - room 38.051 - by Thibault Kiffer

Recently, in order to reduce the carbon footprint of commercial aviation, new aircraft architectures have been proposed by Airbus. Propeller-driven propulsion configurations are being studied in particular, as they are expected to deliver significant reductions in fuel consumption. The absence of a nacelle makes it possible to increase the diameter of the rotor compared with a turbojet engine, thus boosting propulsive efficiency. However, even though propellers have been in use since the dawn of aviation, there are still a number of integration issues among the architectures studied by aircraft and engine manufacturers. One of these challenges is the interaction of the propeller wake with the wing, which has an impact on aerodynamic performance. It is also a major source of noise, which cannot be addressed by conventional acoustic coatings due to the absence of the nacelle. Consequently, effective simulation methods are needed to assess the aerodynamic and aeroacoustic performance of installed propeller configurations. Body-force modeling (BFM) helps reduce the computational cost of integrated propulsion systems. In this model, the blades are replaced by a volume source term reproducing the work and losses generated by the blades. The aim of this thesis work is to model the aerodynamic and aeroacoustic installation effects of a turboprop engine using a BFM approach.

High-fidelity simulations, theory and machine learning for trailing edge noise

• Friday, November 22, 2024 - 3:00 pm - room 38.137 - by Andrea Arroyo Ramo(presentation)

Understanding the trailing-edge noise mechanisms is a challenging task, in particular in the presence of complex mean flows and geometry installation effects. On the other hand, the design of turbomachinery applications requires tools that are able to perform many accurate evaluations with a low computational cost. Analytical models or hybrid numerical approaches have traditionally been employed for that purpose. However, such methods are typically constrained by simplifying hypotheses, or they are restricted to particular flow conditions and/or applications. In this work, both ends are addressed with a twofold objective: gain a better understanding on the noise generation mechanisms of the trailing-edge noise on the Controlled Diffusion (CD) airfoil by means of Direct Numerical Simulations (DNS) and to develop a fast and reliable surrogate model to predict wall-pressure spectrum by means of Artificial Neural Networks.

Designing a distributed hybrid propulsion light aircraft with enriched powertrain assumptions

• Friday, November 8, 2024 - 2:00 pm - room 38.137 - by Baptiste Legrand(presentation)

Prior to the 2000s, the rise of Computed Assisted Design provided aircraft architects tools to sped up the development of new aircraft. However, the early design stage procedures developed, cannot be applied to distributed, electric or hybrid propulsion aircraft. Therefore, researchers are studying these concepts from different angles. Research projects usually aim at development of an early design procedure which then enriched using specific high-fidelity models. This seminar will present a similar procedure based on higher fidelity powertrain assumptions.

An acoustic liner inside a rotor blade? Really?

• Friday, October 18, 2024 - 2:00 PM - room 38.137 - by Santiago Montoya Ospina

With the rapid rise of drone technology, concerns about increased noise exposure have become more pressing. We investigate a novel approach to reduce drone noise by integrating acoustic liners directly inside rotor blades-addressing the noise source itself. This method offers a fresh alternative to traditional uses of acoustic liners, such as those found in turbofan nacelles. Additionally, we examine the impact of surface roughness introduced by the liner on both the aerodynamic and acoustic performance of the rotor. Finally, we assess the effects of placing the liner on a strut that resembles the drone’s chassis.

Airfoil tonal noise reduction by roughness elements

• Friday, October 4, 2024 - 2:00 pm - room 38.250 - by Tiago de Araújo (ITA, Brazil)

Characterization of lightly loaded shrouded rotors in the presence of flow distortions

• Friday, September 27, 2024 - 11:00 am - room 38.137 - by Vincent Maillet(presentation)

In the context of deploying new means of air mobility, there is a need to develop innovative propulsive effectors. The lightly loaded ducted rotor is one of the most promising answers to this need, and its characterization in the case of a homogeneous inlet flow has been previously studied. However, “breakaway” aircraft architectures, either through installation effects or atypical flight points, give rise to situations where the flow arriving at the thruster is no longer homogeneous (crosswinds, tilting rotors, boundary layer or wake ingestion, etc.). The aim of the work presented at this seminar will be to understand how the rotor-hull assembly will behave aerodynamically in these unusual conditions.

Multidisciplinary modeling of the impact of atmospheric conditions on an aircraft take-off performance in a context of global warming

• Friday, June 28, 2024 - 11:00 am - room 38.137 - by Suzanne Salles

Climate change has an impact on aircraft operations both in terms of performance and safety. The recurrence of extreme weather outside of the aircraft flight envelope at best leads to a loss in cost-effectiveness and at worst represents a threat to safety calling for a reorganization of air operations. More intense and frequent heat waves could for example impact operations with longer take-off distances and weight restrictions being necessary due to a decrease in performance.

This study aims at qualitatively and quantitatively investigating the impact of changes in atmospheric conditions due to global warming on aviation. More precisely, on the take-off phase which is most critical in terms of safety and engine performance and may be severely impacted by meteorological conditions. Future atmospheric variables predicted up to the end of the century according to different socio-economic scenarios dictating greenhouse gas emissions are evaluated in terms of their influence on performance of a commercial aircraft. Due to the stochastic nature of these predictions, it is necessary to consider the associated uncertainties, and to quantify the occurrence probability of critical conditions according to localizations and scenarios.

On the role of deep geometric learning in aerodynamic shape optimization

• Friday, June 21, 2024 - 11:00 am - room 38.137 - by Zhen Wei

In this talk, I will introduce my research on the integration of deep geometric learning in aerodynamic shape optimization (ASO). The focus of the presentation will be on the potential of deep geometric learning to enhance the automation and effectiveness of solving ASO problems. Specifically, I will discuss the development and implementation of two mesh parameterization models and their applications in various scenarios. The results from multiple case studies demonstrate that deep geometric learning techniques can significantly reduce the complexity and cost of modeling and manipulating complex geometries. These advancements pave the way for a more accessible ASO solution and have the potential to impact other computer-aided engineering domains.

Identification of aeroelastic aircraft using graph neural networks

• Friday, April 26, 2024 - 11:00 am - room 38.137 - by Michele Colombo(presentation)

In this seminar, I will present the method of graph neural networks united with the method of neural ordinary differential equations applied to the identification of the discrete burst dynamics of an aeroelastic aircraft model.

Using LES for electromagnetic propagation in realistic turbulent atmospheres

• Friday, April 12, 2024 - 11:00 am - room 38.137 - by Victor Darchy(presentation)

The atmospheric marine boundary layer (ABL) is a complex environment in which various phenomena affect electromagnetic (EM) systems. In particular, tropospheric turbulence is characterized by rapid fluctuations in atmospheric refractive index, which can introduce disturbances in both the amplitude and phase of EM signals, resulting in additional losses. Thus, comprehensive turbulence modeling is essential to quantitatively assess the influence of the atmosphere on EM wave propagation. Conventional methods are based on a purely statistical generation of the phenomenon from Kolmogorov-type spectra. In this presentation, I will propose several more realistic methods for taking turbulence into account in ABL, based on LES simulations of meteorological scenarios.

Two Compressors, Twice the pleasure

• Friday, March 15, 2024 - 11:00 am - room 38.137 - by Xavier Flete

This thesis is part of a CIFRE agreement between Liebherr-Aerospace and ISAE-SUPAERO. This work identifies the coupling phenomena that can influence the stability of a centrifugal bi-compressor fitted with smooth diffusers operating in the vicinity of the pumping zone. An understanding of the aerodynamic instabilities specific to each stage has revealed unfavorable coupling between these two components. The downstream stage plays a dominant role in destabilizing the complete compressor. In order to generalize this result, we need to study a modified configuration where the upstream stage becomes limiting. Following this analysis, local and global stability indicators are deduced. In addition, new design rules are proposed to extend the operating range of this type of compressor. Finally, methods applicable in the pre-design phase are put in place to predict the occurrence of the phenomena of interest, and hence the pumping line.

Simulation of violent transients and pulsating flows in turbines turbines

• Friday, February 2, 2024 - 11:00 am - room 38.137 - by Pierre Bertojo(presentation)

One promising way of improving turbomachinery performance is to use engine cycles with constant-volume (isochore) combustion instead of traditional isobaric combustion. In the aeronautics industry, there are plans to move towards PDE (Pulsating Detonation Engine) or RDE (Rotating Detonation Engine) engines, in order to boost overall performance. These particular propulsion system architectures require very different power supply conditions for the chamber and surrounding components. In particular, a high degree of instationnarity must be taken into account. Thus, the theoretical benefits of isochoric combustion systems can only be envisaged if we can control the performance of the turbines, which are subjected to highly unsteady feed conditions akin to pulsating flows.

This project investigates the influence of turbine blade thickness effects on the design recommendations for a pulse-fed turbine with infinitely thin blades. To this end, an experimental design was carried out, limiting the study to dihedral blade geometries. This led to an optimum geometric configuration for the stator. To obtain a complete stage, it was necessary to add the rotating part with the rotor. The geometry chosen for the rotor is the stator mirror. Simulations using the sliding mesh method were carried out, showing similarities and differences to the isolated stator case. A further analysis was carried out to quantify the entropy generation of each geometry, and the influence of strength on overstress.

Finally, and this is the final objective of the study, it was necessary to complete these configurations by adding pulsed conditions. In order to respect Humphrey’s thermodynamic cycle, combustion must take place at constant volume. The combustion chamber must therefore be isolated, and the direct consequence of this is alternating shock wave propagation and expansion within the channel. The aim of the latest work in this thesis is to characterize the influence of the various cycle parameters and to maximize the over-effort associated with the passage of the shock wave on the geometry and minimize the adverse effects of expansion.

Aerospace and society: insights from science fiction

• Friday, January 26, 2024 - 11:00 am - room 38.137 - by Aleksandar Joksimović(presentation)

The science fiction genre has arguably been the most prominent repository of modern cautionary tales about human innate curiosity, creativity and urge to reach towards the unknown. Comparative reading of extensive body of SciFi work across different media reveals insights into how aerospace might be perceived by humanity in today’s just as well as in arbitrarily far-future contexts. In a pair of conference papers [1, 2] co-written with Richard Pearson from LACS, I explore two such themes:

1. Collective perception of aerospace technologies as enablers of utopian future of the humankind continues a long history of tales of human expansion, but it does not come without its controversies. Some SciFi stories tend to take for granted that aerospace brings people together, yet they simultaneously stay oblivious to its potentially nefarious effects by resorting to (false?) promises of deus ex machina solutions found in the darkness of the unknown. [1]
2. Physical scales made available to us by ever-improving aerospace technologies range from local Earth-based intervals to arbitrarily large cosmic scales of space-time. Taken literally, such extrapolation can invert the historical trend of aerospace being a force for unification of humanity to it being a force for its disintegration. Yet, science-fiction seems to avoid this problem by persistently synergising the hypothetical space-faring humanity with various forms of Artificial Intelligence... [2]

[1] Joksimović, A. and Pearson R., Holding Aerospace in High Esteem: Insights from Science Fiction, presented at AIAA SciTech Forum 2024, Orlando, FL/USA, 08-12 Jan 2024.
[2] Joksimović, A. and Pearson R., AI as Custodian of Space-Faring Humanity: Insights from Science Fiction, presented at AIAA SciTech Forum 2024, Orlando, FL/USA, 08-12 Jan 2024.

Instability and spinning stall in a smooth centrifugal machine diffuser

• Thursday, January 11, 2024 - 2:00 pm - room 38.137 - by Antoine Dazin (LMFL/ENSAM Lille)

The smooth diffusers of centrifugal pumps and compressors can be subject to unstable aerodynamic/hydrodynamic phenomena that limit their operating ranges. Although these phenomena have been highlighted for over 50 years, several hypotheses on their physical origin are discussed in the literature. The work presented here focuses on identifying the various unstable modes that can occur in a smooth diffuser, through experimental analysis and CFD calculations of the complete machine. Several reduced-order models incorporating increasingly complex physics are then proposed to discuss the physical origin of the unstable modes identified. An analysis of the effects of diffuser geometry (radius ratio) or boundary conditions at the input to this component is also discussed.