Alain Merlen

Numerical simulation of the compressible mixing layer past an axisymmetric trailing edge

approximately equal to 2.9×10 6 . The free-stream Mach number at separation is equal to 2.46, which corresponds to experiments and leads to high levels of compressibility. The present work focuses on the evolution of the turbulence field through extra strain rates and on the unsteady features of the annular shear layer. Both timeaveraged and instantaneous data are used to obtain further insight into the dynamics of the flow. An investigation of the time-averaged flow field reveals an important shear-layer growth rate in its initial stage and a strong anisotropy of the turbulent field. The convection velocity of the vortices is found to be somewhat higher than the estimated isentropic value. This corroborates findings on the domination of the supersonic mode in planar supersonic/subsonic mixing layers. The development of the shear layer leads to a rapid decrease of the anisotropy until the onset of streamline realignment with the axis. Due to the increase of the axisymmetric constraints, an adverse pressure gradient originates from the change in streamline curvature. This recompression is found to slow down the eddy convection. The foot shock pattern features several convected shocks emanating from the upper side of the vortices, which merge into a recompression shock in the free stream. Then, the flow accelerates and the compressibility levels quickly drop in the turbulent developing wake. Some evidence of the existence of large-scale structures in the near wake is found through the domination of the azimuthal mode m = 1 for a Strouhal number based on trailingedge diameter equal to 0.26.

Zonal-detached-eddy simulation of projectiles in the subsonic and transonic regimes

This paper presents nonspinning-projectile computations with advanced turbulence modeling to demonstrate the relevance of using a hybrid method in projectile simulations. A zonal-detached-eddy simulation methodology is used to improve the base-flow prediction without deteriorating the incoming attached flow upstream of the base in the subsonic and transonic regimes. The numerical results are found to compare fairly well with the available experimental wall-pressure data. Both time-averaged and unsteady features are then discussed. Particular attention was paid to the near-wake flowfield and its dependency upon the freestream Mach number. In these calculations, some classical features of massively separated flows, such as the pressure evolution along the base or the wake centerline characteristics, are poorly predicted with the Spalart-Allmaras model. The use of a hybrid method leads to promising results and allows for a physical analysis of the separated flowfield. The flow appears to exhibit self-similar properties, even in the recirculation area, independently of the Mach number value in the range of freestream conditions investigated. Moreover, the instability process leading to the shear-layer growth is found to be similar in the range of parameters investigated and is in accordance with previous results concerning the compressibility effects in free shear flows. Finally, base-pressure spectra are reported and compared with axisymmetric base-flow data.

Exergy-Based Formulation for Aircraft Aeropropulsive Performance Assessment: Theoretical Development

Aircraft have evolved into extremely complex systems that require adapted methodologies and tools for an efficient design process. A theoretical formulation based on exergy management is proposed for assessing the aeropropulsive performance of future aircraft configurations. It consists of the combination of a momentum balance and a fluid flow analysis involving the first and second laws of thermodynamics. The exergy supplied by the propulsion system and its partial destruction within the control volume is associated with the aircraft mechanical equilibrium. Characterization of the recoverable mechanical and thermal outflows is made along with the identification of the irreversible phenomena that destroy their work potential. Restriction of the formulation to unpowered configurations yields connections to some well-known far-field drag expressions and shows that their underlying theory can be related to exergy considerations. Because the exergy balance does not rely on the distinction of thrust and drag, ...

Synthetic jets based on micro magneto mechanical systems for aerodynamic flow control

A magneto-mechanical micro-actuator providing an axisymmetric synthetic microjet for active flow control was designed, fabricated and characterized. The micro-actuator consists of an enclosed cavity with a small orifice in one face and a high flexible elastomeric (PDMS) membrane in the opposite one. The membrane vibration is achieved using a magnetic actuation chosen for its capacity for providing large out of plane displacements and forces necessary for the performances aimed for. The paper presents first numerical simulations of the flow performed during the design process in order to identify a general jet formation criterion and optimize the devices performances. The fabrication process of this micro-magneto-mechanical system (MMMS) is then briefly described. The full size of the device, including packaging and actuation, does not exceed 1 cm3. The evaluation of the performances of the synthetic jet with 600 µm orifice was performed. The results show that the optimum working point is in the frequency range 400–700 Hz which is in accordance with the frequency response of the magnet-membrane mechanical resonator. In this frequency range, the microjet reaches maximum speeds ranging from 25 m s−1 to 55 m s−1 for an electromagnetic power consumption of 500 mW. Finally the axial velocity transient and stream-wise behaviours in the near and far fields are reported and discussed.

Exergy-based Aircraft Aeropropulsive Performance Assessment: CFD Application to Boundary Layer Ingestion

Aircraft have evolved into extremely complex machines that require adapted tools to allow efficient design process. A performance formulation based on an exergy balance is under development at ONERA for assessing future aircraft configurations. A control volume analysis is performed to relate the exergy supplied by the propulsion system, its partial destruction within the control volume and the aircraft mechanical equilibrium. The formulation does not rely on the expression of thrust and drag and is therefore especially suitable for the performance evaluation of blended-wing bodies with boundary layer ingestion. A first step towards such applications is the investigation of a more academic configuration consisting in the ingestion of the complete wake of a simplified fuselage. Investigation is made via 3D RANS computations and it is shown that the benefit is due to lower levels of exergy destruction in the wake/jet of the BLI configuration.

Impact of Drops on Non-wetting Biomimetic Surfaces

We have carried out an experimental study of liquid drop impact on superhydrophobic substrates covered by a carpet of chemically coated nano-wires. The micro-structure of the surface is similar to some biological ones (Lotus leaf for example). In this situation the contact angle can then be considered as equal to 180 degrees, with no hysteresis. Due to its initial inertia, the drop experiences a flattening phase after it hits the surface, taking the shape of a pancake. Once it reaches its maximal lateral extension, the drop begins to retract and bounces back. We have extracted the lateral extension of the drop, and we propose a model that explains the trend. We find a limit initial velocity beyond which the drop protrudes into the nano-wire carpet. We discuss the relevance of practical issues in terms of self-cleaning surfaces or spray-cooling.

To grate a liquid into tiny droplets by its impact on a hydrophobic microgrid

We report on experiments of drop impacting a hydrophobic microgrid of typical spacing a few tens of micrometers. Above a threshold in impact speed, liquid emerges to the other side, forming microdroplets of size about that of the grid holes. We propose a method to produce either a monodisperse spray or a single tiny droplet of volume as small as a few picolitres corresponding to a volume division of the liquid drop by a factor of up to 105. We also discuss the discrepancy of the measured thresholds with that predicted by a balance between inertia and capillarity.

Supercritical parametric phase conjugation of ultrasound. Numerical simulation of nonlinear and nonstationary mode

This paper investigates the saturation mechanism of the nonstationary supercritical mode of parametric wave phase conjugation in a magnetostrictive medium. The numerical simulation considers the two most probable nonlinear mechanisms of interaction between elastic deformation and electromagnetic excitation. For the qualitative study of the dynamics of the system, a one-dimensional numerical simulation is sufficient if applied to an infinite medium with a finite active zone. The temporal form of the conjugate wave is obtained for both hypotheses. Comparison with experiments shows that only one mechanism corresponds to the experimental behavior.

Magnetically Actuated Microvalves for Active Flow Control

This paper describes the design, fabrication and characterization of silicon based, high flow rate microvalves for the active control of separated air flows. The fabricated system provides pulsed microjets with an outlet speed reaching 150 m/s at an actuation frequency ranging from static actuation to 2.2 kHz, using either electromagnetic actuation or a self oscillating mode. After a brief introduction, the microvalve dimensioning and fabrication process are presented. The actuation techniques used are then described and discussed.

MEMS for Flow Control: Technological Facilities and MMMS Alternatives

An introduction of MEMS in general is made. Then the paper provides an overview of essential MEMS devices already elaborated for different problems of flow control in aeronautics. In the last part, attention is focused on solutions based on Micro-Magneto-Mechanical Systems (MMMS).