DAEP Seminars 2023

Motorsport performance: design and simulation

• Friday, December 15, 2023 - 11:00 am - room 38.137 - by Michele Colombo

I’ll be presenting my experience of 4 years in the aerodynamic performance department of a Formula 1 racing team and 8 years of consulting in the motorsport branch of a sports car brand. After a basic introduction to vehicle dynamics, we’ll delve into the simulation methods used and their interaction (performance calculation, telemetry, CFD, wind tunnel measurements) and a few anecdotes.

Direct numerical simulation of nucleate boiling and cavitation in micro-gravity conditions

• Friday, December 8, 2023 - 11:00 am - room 38.137 - by Annafederica Urbano

Behavior of five-hole probes in confined environments

• Friday, June 30, 2023 - 11:00 am - room 38.137 - by Adrien Vasseur

Five-hole probes are a tool for indirect measurement of the velocity vector of a flow. Their robustness makes them a good tool for studying the hot parts of a turbomachine. In the engine, their relative large size and the complexity of the flows modify the relations between the pressures measured at the five holes and the velocity vector. This thesis aims at improving the understanding of these problems in order to prepare the development of corrections of errors related to intrusiveness.

Stability analysis of variable density trailing vortices

• Friday, June 23, 2023 - 11:00 am - room 38.137 - by Julien Sablon

The stability of trailing vortices has been studied for years and remains an essential research subject nowadays. These vortices generated by the wings (or any finite foil) are an unavoidable by-product of the lift generation. They can be hazardous when aircraft are following each other especially during take-off and landing phases. This leads to substantial security distances between each aircraft in the airports and consequently to higher operating costs. Nowadays, the ecological footprint of the aircraft is a major issue for the civil aviation industry. The reduction of the vortex persistence is likely to reduce the occurrence of contrails in the sky by an increase of the mixing of the different gases. As the contrails have a significant contribution to the aviation-induced radiative forcing, their mitigation is expected to reduce the climate impact of the sector. The study of the instabilities which are present on isolated vortices or vortex systems is a necessary first step to better understand the destabilization mechanisms and stands as a key point for control strategies that could be implemented on aircraft.

More specifically, this stability analysis is conducted on an isolated three-dimensional vortex described by an analytical base flow model corresponding to the q-vortex, a parallel version of the Batchelor vortex. A Gaussian density profile is superimposed to the q-vortex to get a plausible variable-density profile. The presentation will highlight the different results obtained in the framework of the modal and non modal stability analyses conducted on this baseflow.

Experimental and analytical-digital aeroelastic analysis of flexible rotor blades

• Friday, June 16, 2023 - 11:00 am - room 38.051 - by Anaïs Chambon

In the study of rotating flexible blades, the structural properties of the blade determine the main bending and twisting modes. At certain operating points (i.e. at certain rotation speeds), these modes can be excited, leading to various instability phenomena such as aeroelastic flutter. This is an unsteady, self-excited vibration in which the structure exchanges energy with the flow. One of the causes of this phenomenon is the coalescence of two structural modes: bending and torsion, which reach the same vibration frequency. In this case, the blade vibrates uncontrollably and aerodynamic performance is severely degraded. This is a general problem with rotors. However, it becomes even more acute with flexible blades (the blade deforms under the centrifugal effect of rotation). One of the consequences of these instabilities is instantaneous structural damage or fatigue damage to the system. It is therefore essential to understand and control this phenomenon.

To this end, an experimental test rig dedicated to the study of rotors with flexible blades at the fixed point was developed, in order to carry out a vibratory and aerodynamic analysis. Blade sets of varying flexibility and thickness were designed using different composite materials, with the aim of studying the influence of flexibility on the occurrence of aeroelastic phenomena. Aerodynamic performance measurements were carried out, as were measurements of the dynamic deformations of the rotating blades.
In addition, an aeroelastic model has been developed, based on the coupling of :

• a finite element approach to model the dynamic behavior of the blades, assumed as a uniform, slender, isotropic and homogeneous beam with out-of-plane bending and torsional deformations, subject to inertial and centrifugal forces due to rotational effects.
• An approximation of unsteady aerodynamic terms.

Finally, experimental and numerical Campbell diagrams are plotted to study the evolution and behavior of eigenmodes as a function of rotor speed. In this way, the presence of coupling between bending and torsion frequencies can be detected, within the operational speed range of the test bench. These crossovers between bending and torsion modes in Campbell’s diagrams can be seen as one of the indicators of the onset of flutter due to mode coalescence, and can thus be used to identify blade flutter speeds.

Artificial intelligence mitigation of unsteady aerodynamic disturbances

• Friday, June 9, 2023 - 11:00 am - room 38.137 - by Brice Martin(Presentation)

Urban air mobility offers great prospects for a cleaner, safer and more efficient mode of transport. Among the challenges to be met, such as electrification, it is essential to develop effective techniques for mitigating disturbances, given that the urban environment is an ideal place for them to occur. Since models describing the impact of disturbances on vehicle aerodynamic performance are unsteady and non-linear, it is difficult to establish effective mitigation strategies. In this work, we seek to develop such strategies through reinforcement learning. Our work is structured around three main axes. First, using a simplified problem, we seek to explain why reinforcement learning is preferable to standard control techniques for the disturbance mitigation problem. Next, we propose a methodology for both detecting and mitigating these disturbances. Finally, we focus on controlling the interaction between disturbances and vehicle boundary layers.

Numerical study of the impact of skidding on the shock wave/boundary layer interaction

• Friday, June 2, 2023 - 11:00 am - room 38.137 - by Thomas Bergier

In order to move away from the canonical configuration and to approach a real case of shock wave/boundary layer interaction, a skid angle is imposed between the shock and the upstream flow direction. The impact of this three-dimensional phenomenon is studied on the mean flow and on the unsteady dynamics of the interaction zone

Exergy analysis for unsteady aerodynamics

• Friday, May 26, 2023 - 11:00 a.m. - room 38.137 - by Juan Pablo Ruscio

The exergy analysis of systems has become a powerful tool into many disciplines of engineering in order to achieve a better understanding of the efficiency of systems in the past decades. The literature defines that the exergy analysis “consists of using the first and second law of thermodynamics together, for the purpose of analyzing the performance in the reversible limit of a system, and for estimating the departure from this limit”. In other words, exergy is the part of the energy with potential to be recovered, and anergy is the part which has no potential of being recovered (linked to entropy). In this way, the first exergy analysis methodology for aerodynamics was developed in 2015 under the hypothesis of steady flows and it proved to be a convenient tool for analyzing the new more integrated propulsion-aiframe configurations.
The objective of the presented work is to develop the exergy analysis methodology for unsteady flows, and validate it through its application on different test cases on which different physical phenomena are present (laminar and turbulent regimes, lifting body, moving shock-waves, non-inertial body movement, heat transfer). Therefore, opening the possibility to know which part of the energy contained on the unsteady wake of an object has potential of being recovered (exergy) and which should be avoided at all to be generated (anergy).

Evaluation of the RANS approach for the study of swirling hot spot flows in cooled turbines

• Friday, April 14, 2023 - 11:00 a.m. - room 38.137 - by Christopher Wingel

High pressure turbine inlet flow is characterized by high levels of turbulence and swirling hot spots that impact the aerodynamics and aerothermics of the turbine stage. The reliable prediction of this swirling hot spot flow must therefore be done upstream with numerical simulations.

In this thesis, the RANS approach is evaluated on the FACTOR project configuration, where LES and experimental data are available. A first state of the art on this high pressure turbine geometry with stationary and unsteady simulations, and with or without cooling, confirms a complex flow to predict, especially from the turbulence point of view. A simplified configuration is therefore developed for the analysis of a turbulent swirling hot spot using LES and for the calibration of RANS simulations. This study reveals that diffusion plays an important role in the transport of a hot spot in a curved channel, the diffusion being driven by the turbulence dissipation scale, which is evaluated through a turbulence integral scale. A turbulent kinetic energy balance is used to describe the main mechanisms governing the flow. Further studies based on Lumley analysis show a strong anisotropy of the turbulence which cannot be properly captured by the RANS turbulence models following the Boussinesq assumption. Thus, the EARSM and RSM models show promise in recovering the sensitivity to curvature and rotation.

With the experience gained from this academic study, the FACTOR configuration is again addressed. Anisotropic turbulence models are evaluated, but the results lack predictability when compared to the tests. Two problems are highlighted in the boundary condition: measurement errors at the inlet plane of the high-pressure turbine (P40) due to high levels of gyration, and an unsteady nature of P40 due to the presence of hydrodynamic instability. The analyses show that the first point is of paramount importance, while the second improves the results to a lesser extent.

Source term modeling of turbomachinery: review of methods and application examples

• Friday, March 31, 2023 - 11:00 am - room 38.137 - by Guillaume Dufour(presentation)

The body force method (BFM) consists in replacing the turbomachinery blades by volume source terms in the area covered by the blades. This reduces the computational cost by reducing the size of the mesh and allowing in particular to model by a stationary approach rotors in the presence of distortion, which require unsteady computations with classical methods. Beyond the diversity of existing BFM models, a priori different approaches, such as the “3D throughflow” or the “coupling between Blade Element Theory and a CFD calculation”, can also be seen in a similar way. Thus, the first objective of this presentation is to give a global view of all these methods, in the more general context of source term modeling methods, insisting on the philosophy of derivation of source terms and the different formulations of forces, in order to arrive at more generic classifications than those generally proposed in the literature. The second objective is to present an overview of the different applications treated by this approach at ISAE, mainly in collaboration with AIRBUS, SAFRAN and ONERA. In particular, we will deal with the modeling of short air inlets at incidence, boundary layer ingestion, and the case of fast propellers. Unsteady modeling to deal with rotating stall in compressors will also be discussed. Finally, approaches to integrate the method into full engine level predictions will be discussed.

Aeroacoustic optimization of MAV rotors

• Friday, March 3, 2023 - 11:00 a.m. - room 38.137 - by Pietro Li Volsi(presentation)

With the increase in the use of quadcopter micro air vehicles (MAVs), the issue of noise pollution is becoming crucial in both military and civilian fields. For defense and security applications, the acoustic stealthiness of MAVs is a major issue, particularly for surveillance and intelligence missions. In the civilian domain, the increasing use of MAVs in urban environments requires public acceptability, which relies heavily on their acoustic discretion. The hovering condition, which has the practical advantage of allowing MAVs to work in close proximity to the target, is studied in this thesis. To improve the acoustic stealthiness of MAVs, it is imperative to reduce the acoustic signature of their rotors. The question of rotor noise is not a new problem, but the efforts of the community have so far focused on larger-sized rotors, such as helicopter rotors. The small size of MAV rotors requires a specific analysis of the aerodynamic phenomena causing the radiated noise. The low Mach and chord-based Reynolds numbers associated with small rotors (typically less than 30 cm in diameter) induce particular aeroacoustic noise effects composed of two main sources: tonal and broadband noise. Here, a multi-objective optimization procedure aiming at reducing the tonal noise and maximizing the flight endurance of an isolated MAV rotor is presented. The optimization is then validated against aerodynamic and aeroacoustic tests performed on optimized rotors printed by means of stereolithographic 3D printers. Particular attention is paid to the aerodynamic and acoustic modeling used for optimization, with distinct analyses of tonal and broadband noise contributions. The role of broadband noise contribution, classically modeled through semi-empirical approaches used for automotive fans, wind turbines, and helicopter rotors, is discussed.