DAEP Seminars 2018

Numerical and experimental study for the passive control of aeroelastic flutter of a wing

• Friday, December 7, 2018 - 11:00 a.m. - Room 38.137 - by Claudia Fernandez-Escudero and Miguel Gagnon(Presentation)

Operation of lightly loaded shrouded rotors: status and similarities

• Friday, November 30, 2018 - 11:00 a.m. - room 38.137 - by Massyl Lagha

Feedback and perspectives of the European H2020 project ULTIMATE (Ultra Low emission Technology Innovations for Mid-century Aircraft Turbine Engines)

• Friday, November 9, 2018 - 11:00 a.m. - room 38.137 - by Nicolás García Rosa

Stability and transition of boundary layers and other activities in flow control

• Friday, July 6, 2018 - 11:00 a.m. - Room 38.137 - by Erwin Gowree

Combustion instability sensing and control in gas turbines

• Friday, June 22, 2018 - 11:00 a.m. - room 38.137 - by Prof. Satyanarayanan Chakravarthy (ITT Madras)

Land based gas turbines used for power generation have to meet stringent low NOx emission standards, for which lean premixed prevaporised combustors are adopted. However, such premixed flames are prone to severe thermo-acoustic instabilities due to feedback of the acoustics generated by the flame fluctuations on the flame’s response. In recent times, data driven and image processing approaches are being adopted for sensing precursors to instabilities so that corrective action can be taken to control them. In the data driven approach, a symbolic time series analysis of the pressure and flame chemiluminescence fluctuations are combined in a mutual-information theoretic framework to develop a robust precursor detection algorithm. A flame sensor has been developed for this purpose by a combination of a fibre-optic probe and quartz windows to be mounted on the combustor wall. Efforts are also focused on combining this with a fibre-Bragg grating for acoustic pressure sensing along with the flame chemiluminescence fluctuations for the mutual information algorithm. High speed imaging of the flame in its approach to instability has been analysed for a quantity termed as the coherent image phase (CIP) based on the Hilbert transform of the images. The CIP provides an excellent basis to evaluate the Rayleigh criterion in comparison with the original image. Next, the scale invariant feature transform (SIFT) algorithm used in computer vision is adopted on the flame images to deduce locations of rotational symmetry in the combustion zone that represent regions of vortex structures involved in causing spatio-temporal fluctuations in heat release rate, leading to instability. Finally, an artificial neural network is adopted to learn from the flame images obtained under thermo-acoustically stable and unstable conditions on the formation of coherent structures followed by the flame so as to detect precursors to instability. A cooled camera probe is being developed to be mounted on the combustor for obtaining real time images to which the above image processing techniques can be employed for precursor detection and corrective action to bring the combustor back to stable conditions.

Progress and challenges in simulating combustion with realistic chemistry

• Friday, June 15, 2018 - 11:00 a.m. - room 38.137 - by Perrine Pepiot (Cornell University)

The vital role of simulations and computational insights in reducing pollutant emissions, designing better engines and better fuels, and assessing the technical and economic viability of radically different combustion technologies, is now clearly established. A key enabling step is the development of computational approaches that allow our increasingly detailed knowledge of the chemical kinetics of realistic fuel oxidation to be applied to the modeling and simulation of combustion reactors. In this talk, I will briefly review the challenges associated with the integration of detailed chemical kinetics in reactive flow simulations. I will then discuss the progress we have made in the analysis and reduction of complex kinetic networks, with a focus on graph-based techniques and the characteristics of the stand-alone reduced models they typically generate. Using Large-Eddy Simulations of turbulent flames as case study, I will show how these techniques are enhanced through careful integration and coupling with CFD tools, wherein the flow characteristics adaptively inform the reduced chemical model to be used. I will conclude on the remaining challenges still to overcome, and potential avenues to do so.

Rotary wing decelerators - research itinerary

• Thursday, June 7, 2018 - 11:00 a.m. - room 38.137 - by Joaquín Piechocki (National University of La Plata)

The presentation will be about the activities of UIDET GTA-GIAI from Universidad Nationale de La Plata, with special focus on the research progression on rotary wing decelerators made during the last years.

Advances and perspectives in aerostructural studies of moderate Reynolds rotors in free and confined environments

• Friday, May 18, 2018 - 11:00 am - room 38.137 - by Sébastien Prothin

Study of the gas mixture produced by Richtmyer-Meshkov instability in the initial periodic weakly diffuse regime

• Friday, April 27, 2018 - 11:00 a.m. - room 38.137 - by Pierre Graumer

The Richtmyer-Meshkov instability (RMI), theorized by Richtmyer and first experimentally implemented by Meshkov, occurs when a perturbed interface between two fluids of different densities is subjected to an impulse acceleration leading to mixing of the two species. This instability plays an important role in many physical phenomena such as supersonic combustion, supernova collapse or inertial confinement fusion (ICF). ICF involves producing nuclear fusion reactions from a mixture of two hydrogen isotopes: deuterium and tritium. To this end, the isotopes are packaged in a cryogenic spherical capsule a few millimetres in diameter and compressed using very high-powered laser radiation. This protocol allows to obtain the pressure and temperature conditions ( 2.107K) necessary to produce a plasma. This plasma expands towards the outside of the target and generates in reaction a shock wave in the opposite direction which should finally allow to reach in the center of the target the conditions of temperature ( 1.108K) and density ( 100g/cm3) essential to the fusion of the isotopes. The realization of such a process requires that the illumination of the target is perfectly homogeneous and that the target is perfectly spherical. In practice, the non-uniform illumination and the geometric defects of the target cause the development of hydrodynamic instabilities (Rayleigh-Taylor and Richtmyer-Meshkov instabilities) at the density interfaces. The resulting mixing decreases the energy yield of the reaction, and may prevent the thermodynamic conditions necessary for the fusion reaction from being reached. The objective of this thesis is a continuation of a previous thesis conducted at ISAE/DAEP (Bouzgarrou) on the experimental analysis of the gas mixture resulting from the Richtmyer-Meshkov instability. The experiments carried out in this previous study were performed in a vertical shock tube, and the analysis of the mixture was conducted using parietal pressure sensors and time-resolved optical measurements (strioscopy, Doppler velocimetry (LDV), particle image velocimetry (PIV)). In order to create a density interface between the two gases inside the shock tube (Air/SF6 interface in the case considered here), a thin 0.5μm thick nitrocellulose membrane was sandwiched between two square mesh metal grids providing both mechanical support for the membrane and the creation of a periodic three-dimensional perturbation pattern at the density interface. This work has highlighted the need to get rid of the membrane in order to properly implement laser measurement techniques such as LDV and PIV. Indeed, when the incident shock wave impacts the membrane, it breaks and produces a large number of micro-debris which generate parasitic light reflections from the laser sheet, preventing the measurement. The objective of our work is therefore to change the experimental protocol by designing and implementing a new density interface generation system that does not use a membrane, in order to be able to carry out a precise study of the turbulent mixing resulting from the instability thanks to optical time-resolved measurements.

Direct numerical simulations of hypersonic transition delay over complex wall impedance

• Thursday, March 29, 2018 - 2:00 p.m. - Room 38.137 - by Carlo Scalo (Purdue University)

The talk will discuss recent results from direct numerical simulations of hypersonic transition delay over porous coatings (Sousa et al., JSR, 2018). Transitional modes in high-speed boundary layers over canonical geometries, such as slender cones, exhibit acoustic wave propagation properties, where the boundary layer (due to its thermally stratified structure) acts as an acoustic waveguide sustaining resonance and growth of the instability waves. The latter are acoustically interacting with the underlying porous cavities, responsible for acoustic absorption and hence mode amplitude attenuation. The dynamics of waves propagating in porous cavities will be analyzed via direct and inverse eigenvalue approaches allowing to derive a broadband complex value of the surface impedance, which is then applied as a boundary condition to the overlying flow via a time-domain technique. Flow conditions and properties of the porous coatings match the ones from experiments carried out by Alex Wagner at the German Space Center (DLR) in Göttinghen, who has successfully demonstrated the capabilities of porous walls composed of carbon-fiber reinforced carbon ceramics to delay transition by attenuating acoustic modes in the boundary layer.

Computational setup of a transitional boundary layer on a slender cone over porous walls.

Advances in multiphysics aircraft wing aerodynamics

• Friday, March 23, 2018 - 11:00 a.m. - room 38.137 - by Éric Laureandeau (École Polytechnique de Montréal)

Feedback on a research trip on resonant electromechanical de-icing systems

• Friday, March 16, 2018 - 11:00 a.m. - Room 38.137 - by Valerie Budinger

This seminar aims to report on a research trip conducted in Germany as part of a test campaign on resonant electromechanical de-icing systems. It will first address the scientific aspects: the issues and challenges of resonant electromechanical de-icing systems. Then the presentation will focus on the human aspects of such an experiment: how to set up a research stay abroad? What are the difficulties to overcome and the benefits of such a stay?

Influence of inter-disc cavity flows on turbine aerodynamics

• Friday, February 16, 2018 - 11:00 a.m. - room 38.137 - by Maxime Fiore

In a turbomachinery turbine, the necessary clearances between the stationary parts connected to the stators and the rotating parts connected to the rotors lead to spaces under the hub called cavities. Hot main stream air can enter the cavities and damage the discs holding the rotor blades located in the cavities. In order to prevent this, compressor air is drawn into these cavities to lower the temperature of the off-vein flow while preventing hot vein air from entering. However, some of this cavity air tends to escape into the main duct leading to additional losses for the turbine. Through different numerical approaches (RANS, LES, LES-LBM), the flow in the cavity and the flow interaction phenomena between the vein and the off-vein are analyzed.

Regenerative dynamic soaring trajectory augmentation over flat terrains

• Friday, February 9, 2018 - 11:00 a.m. - room 38.137 - by Nathan Long

A gliding technique, known as dynamic soaring (DS), replicates the flight pattern of the wandering albatross bird to enable energy neutral, repeatable flight trajectories. This study investigated the potential for DS to act as a basis for UAV battery power regeneration by means of a windmilling propeller placed on the nose of the aircraft. In order to give an indication of the type of atmospheric and environmental conditions necessary to perform regenerative dynamic soaring (RDS), RDS trajectories were optimised for the DT-18 UAV. The optimal flight paths for varying amounts of energy regeneration, surface conditions and periodic constraints were obtained and compared to a base, energy neutral DS case. The findings suggest that by slightly altering the DS flight pattern, significant battery recharge levels can be reached for small drones under certain conditions.

Performance improvement of small unmanned aerial vehicles through gust energy harvesting

• Friday, January 26, 2018 - 11:00 am - room 38.051 - by Nikola Gavrilovic

Recent works driven by the unfailing experience from nature have shown a significant amount of energy available in the atmosphere. This energy is coming through in the form of vertical air motions described as spatial gradients, such as thermals, shear layers, orographic lift and short-period temporal gradients as (for example) gusts. There are strong indications that birds use their feathers for sensing flow perturbations. Being fluffy and subjected to fluttering provoked by small disturbances, birds have natural sensory system, which enables them to feel and exploit flow disorders. However, for a variety of reasons, it is understood that identical copies from nature to manmade technologies are not feasible. Instead, a creative inspiration and conversion into technology are often based on various steps of abstraction.

The performance of mini UAVs being constrained by onboard energy due to their limited size can be significantly enhanced by specific flight strategies according to expected atmospheric formations or even continuous disturbances. Most of the energy harvesting methods rely on an active control system that detects and exploits the energy of atmospheric turbulence through intentional maneuvering of the aircraft.

The presented results are a work of a PhD student of third year. The first part includes theoretical modelling of atmospheric turbulence for both 2D profiles and 3D flying environment and flight simulations with appropriate aerodynamic models with active control for gust energy extraction. The second part is related on bio-inspired wind field estimation with measurements through wing surface pressure distribution. Flight test measurements have been performed in association with ENAC. The third part shows results from international collaboration with RMIT University from Melbourne Australia. It consists from wind tunnel campaign and flight tests for gust energy extraction. Finally some future work will also be presented considering the time left till the end of the thesis.