DAEP Seminars 2016

From the attenuation of a shock wave to the noise generated by a pressurization valve

• Wednesday, October 26, 2016 - 11:00 a.m. - room 38.137 - by Alice Chauvin

Influence of distortion on the tonal noise of a jet engine

• Friday, October 21, 2016 - 11:00 a.m. - room 38.137 - by Majd Daroukh(Presentation)

The evolution of turbojet engines towards very high bypass ratio (UHBR) architectures results in a shortening of the nacelle and the distance between the fan and the rectifiers. A strong azimuthal heterogeneity of the flow, called distortion, is then created at the fan. The influence of this distortion on the tonal noise is studied using unsteady numerical simulations based on the averaged Navier-Stokes equations (URANS). These simulations are coupled with direct acoustic prediction methods, based on the Cantrell & Hart formulation, and hybrid methods, based on the Goldstein formalism.

Analysis, characteristics and control of turbulent boundary layer detachments

• Friday, September 30, 2016 - 11:00 am - room 38.137 - by Antoine Debien

Taking into account environmental constraints is today a major issue for the development of the aeronautical sector, which is moving towards “more economical, safer, cleaner and quieter” transport. In this context, the use of innovative concepts such as flow control seems to be one of the necessary solutions that will ensure this technological breakthrough. This paper will focus on debonding, an undesirable phenomenon in many fluid applications, and its control through two examples. The first example, resulting from the European project PLASMAERO which aimed at demonstrating the efficiency of plasma actuators for the control of aircraft wing flows, corresponds to the design of DBD plasma actuators and their use on a mid-chord disbond (Rec = 1.33×106). The second example, carried out within the framework of the white ANR SePaCode which aimed at the realization of robust control laws based on the physics of the disbond, deals with the control of a turbulent boundary layer disbond (Reθ = 3500) on a sharp-edged ramp configuration performed using pulsed vortex generators. We will thus focus on understanding the physics of the control by comparing results obtained in open-loop control - relying on characteristic frequencies of the loop flow - with the use of closed-loop control laws - synthesized by a genetic algorithm.

Resonant mode control in high-speed boundary layers via wall-impedance

• Friday, 19 August 2016 - 10:00 a.m. - room 38.137 - by Carlo Scalo

The talk will discuss fundamental computational and physical aspects associated with transitional and turbulent boundary layer control via acoustic resonance and absorption in porous walls. The dynamics of waves propagating in porous cavities will be analyzed via direct and inverse eigenvalue approaches with focus on the characterization of the surface impedance. The latter is defined as the ratio between the Fourier transforms of pressure and transpiration velocities at the interface. High-fidelity numerical simulations retaining full aeroacoustic coupling between the overlying flow and porous walls relying on the time-domain technique by Scalo, Bodart and Lele, Phys. Fluids (2015) will be discussed. Finally, plans to perform direct numerical simulations of the experiments by Wagner et al, AIAA 2012-5865 in hypersonic flow control over ultrasonically absorptive porous coatings will be presented in the context of the recently awarded Air Force Grant FA9550-16-1-0209.

High-order schemes for high-fidelity DNS and LES of compressible flows over complex geometries

• Friday, 19 August 2016 - 11:00 a.m. - room 38.137 - by Guido Lodato

Static symmetry breaking modes and unsteady aerodynamics of thick three-dimensional bodies

• Wednesday 20 July 2016 - 14h00 - room 38.137 - by Olivier Cadot

For simple geometries of unshaped 3D bodies, stationary modes of symmetry breaking appear at the first bifurcation in the laminar regimes of wakes. These modes persist at high Reynolds numbers and are present on an industrial scale as well as in ground transportation aerodynamics. They produce a global stochastic dynamics dominating the unsteady aerodynamic forces. Some techniques for stabilizing these modes will be presented.

Analytical modeling for acoustic tonal and broadband noise

• Friday, June 24, 2016 - 11:00 a.m. - room 38.137 - by Michaël Bauerheim

To complement experiment, high-fidelity simulation has already attracted broad interest for acoustic noise predictions. In contrast, analytical models are usually thought valid only for simple cases, and applied for validation purposes only. This presentation describes two recent theoretical analysis, bringing up to date the interest of such analytical tools in an industrial context. First, a non-linear analytical framework for self-sustained vortex-driven tonal noise is proposed. It is based on numerical simulations coupled with an analytical non-linear dispersion relation. It is applied to a T-junction with a grazing flow, as a prototype of classical situations encountered in industrial configurations. Second, the scattering of acoustic or vorticity waves by a blade cascade is investigated, typical of stator-rotor configurations as well as swirlers in combustion chambers. The problem is treated by a non-compact mode-matching technique coupled with the analytical Ovenden’s solution of a slowly-varying duct. It allows the analytical description of the blade cascade taking into account curvature and staggered angle effects. Extension of this framework to 3D cases and trailing edge noise will be discussed.

Performance control of a variable geometry centripetal radial turbine

• Friday, June 3, 2016 - 11:00 am - room 38.137 - by Pierre-Thomas Lauriau(Presentation)

The technological transformation of aeronautical transport, undertaken at the European level, is leading to an evolution towards more sober and less polluting aircraft. This has a strong impact on air conditioning systems (or packs), whose architecture makes greater use of electrical energy compared to conventional systems. For these “electric” packs, the required range of operability is wide. The radial turbine of the air conditioning pack must then be fitted with a variable geometry system to support a wide range of flow rates. However, the integration of the device that ensures the variation of the injection section is responsible for the drop in performance of the turbine stage over the entire operating range. In order to make the use of a radial turbine with variable injection section viable in the context of “electric” air conditioning for aviation, it is necessary to identify the mechanisms that create entropy, in order to be able to control the performance of the turbine stage.

Predicting Turbofan Fan-Stage Noise Using Low-Order Methods

• Friday, May 27, 2016 - 2:00 p.m. - room 38.051 - by Sheryl Grace

On approach when a commercial aircraft’s engines are throttled down, the fan stage becomes the main engine noise source. The noise exists mainly due to the interaction of the fan rotor wake with the fan exit guide vanes (FEGVs). Both tonal and broadband noise is produced. We have developed a computational hybrid method that can be used during the design phase to predict the broadband interaction noise. A low-order cascade response solution forms the backbone of the RSI (rotor-stator interaction) method that will be discussed. Input to RSI consists of rotor wake properties currently taken from either experimental data or a Reynolds Averaged Navier Stokes (RANS) flow simulation. The basis for and outcomes of modeling choices made within the RSI framework will be presented. Comparison between measured and predicted noise levels indicates the method can provide the trend prediction necessary for design.

What would I know that you don’t know?

• Friday, April 15, 2016 - 11:00 a.m. - room 38.137 - by Allan Bonnet(Presentation)

If all goes well, in 263 days, I will have to retire. Correlatively, it is not useless that there is this type of seminar with a somewhat mysterious title. In fact, discussing aerodynamics left and right, it turned out that the perception of some results of our very complex field, is not the same according to the interlocutors. Each one had distinct teachers and teachings, and the propagation of knowledge often reproduces what one has perceived oneself and which undoubtedly convinced us.

The aim of this seminar is then to come back to our aerodynamic models, Navier-Stokes (but not too much...), Euler (and the discontinuities and their facets), or potential flow. This will be an opportunity to see that, with a little imagination (but supported), one can question results that seem to be “concrete”. We will also try to correct results read in the literature that turn out to be inaccurate.

Parallelization in time and its application to CFD: principle and application examples

• Friday, March 18, 2016 - 11:00 am - room 38.137 - by Thibaut Lunet(Presentation)

Since its emergence about fifty years ago, the idea of solving partial differential equations in parallel in time has been gradually attracting the scientific community, in particular in order to be able to make the best use of the next computing resources that will arrive in the next decade (supercomputers that can perform up to 10^18 operations per second). A few years ago, a solution based on the Parareal algorithm (Maday et al. , 2001) seduced by its simplicity of application and its first results. This algorithm was even the starting point for new ideas and algorithms, and has since been joined by many other solutions(PFASST - Emmett and Minion, 2012; PARAEXP - Gander and Güttel, 2013; ...).

However, few have yet tried to apply a time-parallelization method to the study of a turbulent flow solved by direct simulation. However, current CFD codes, massively parallel in space, will reach their scalability limits with tomorrow’s supercomputers, and would therefore benefit particularly from a new dimension of parallelization.

The aim of this seminar is to present the history and the state of the art of time parallelization, and then to focus on the Parareal algorithm, in order to present its functioning in more detail and to illustrate it with several examples. Finally, a particular emphasis will be put on the application of such an algorithm to a turbulent fluid mechanics problem (Isotropic Turbulence), in order to highlight the different issues that need to be mastered in order to achieve a really efficient time parallelization.

Source term modeling of turbomachinery for simulation of aircraft-engine interactions

• Thursday, February 18, 2016 - 11:00 am - room 38.137 - by William Thollet

As the bypass ratio of modern turbojet engines increases, the diameter of civil aircraft engines becomes larger and larger, which increases the weight and drag of the powerplant. One solution considered to counteract this effect is to reduce the length of the air inlet, whose function is to supply the fan with a healthy flow. However, this solution causes the emergence of new aerodynamic interactions between these two components, which must be taken into account during their respective design. Although it is possible to accurately predict these interactions using unsteady simulations of the fan in the inlet, this approach is currently too costly to be used in daily design loops. This thesis work therefore focuses on a particular type of turbomachinery modeling, which aims at taking into account the fan accurately and at low cost. To this end, source terms that reproduce its effect on the flow are injected into the RANS equations, the problem then being to determine how to construct these source terms to represent as faithfully as possible the behaviour of the fan.

Study and control of the stall of a multi-element rigid wing

• Friday, January 29, 2016 - 11:00 a.m. - room 38.137 - by Alessandro Fiumara

The rigid wingsail is the means of propulsion for the America’s Cup catamarans. This new rig has improved the performance of the boats but the crews have shown some difficulty in tuning the wing in all sailing conditions. Indeed, the overall performance envelope of a rigid wing is not yet fully understood and the aerodynamic phenomena fully characterized. An experimental wind tunnel campaign was therefore carried out on a scale model of a bi-element wing to characterize the flow. Numerical RANS simulations were also carried out on the wing under wind tunnel conditions. The wind tunnel domain had to be modeled to reproduce the effects of interaction with the wing which modified the flow characteristics. The numerical results showed a good correspondence with the experimental data in terms of prediction of the coefficients and in terms of reproduction of the flow structure. Differences were noted in the modelling of the slot jet due to the effects of deformation of the model during the wind tunnel tests. Indeed, this jet is very sensitive to the geometry of the slot but it also has a very important influence on the high-lift possibilities of the wing and on its stall behaviour.