Les micro-systèmes magnéto-mécaniques

            présentent des possibilités intéressantes en matière de contrôle d'écoulements. Ils

            permettent de remplir les cahiers des charges des constructeurs aéronautiques ou

            automobiles. Ils sont également plus facilement intégrables sur les prototypes que les

            MEMS issus de microtechnologies entièrement intégrées. On présentera quelques tests en

            soufflerie. Les effets sur les écoulements seront discutés et les perspectives

            tracées

GIMENO, Leticia

MERLEN, Alain

PERNOD, Philippe

TALBI, Abdelkrim

VIARD, Romain

English

I-Revues

19ème Congrès Français de Mécanique                                                                           Marseille, 24-28 août 2009 
  1 
Micro-systèmes et contrôle d'écoulements 
A. MERLEN, P. PERNOD, A. TALBI, R. VIARD, L. GIMENO   
a. Institut d’Electronique, Microélectronique et Nanotechnologies,  Cité Scientifique,      Avenue Poincaré 
BP 60069, 59652 Villeneuve d'Ascq 
 
 
Résumé :  
Les micro-systèmes magnéto-mécaniques présentent des possibilités intéressantes en matière de contrôle 
d'écoulements. Ils permettent de remplir les cahiers des charges des constructeurs aéronautiques ou 
automobiles. Ils sont également plus facilement intégrables sur les prototypes que les MEMS issus de 
microtechnologies entièrement intégrées. On présentera quelques tests en soufflerie. Les effets sur le 
écoulements seront discutés et les perspectives tracées. 
Abstract : 
Micro-magnetical mechanical Systems are potential solutions for flow control actuation. They are 
compatible with the requirements of aeronautic and utomobile industries. Their implementation is easir 
than for the MEMS fabricated in totally integrated micro-technologies. Some wind tunnel tests are 
commented. The effects on flowsand some perspectives  are evocated.  
Mots clefs : contrôle d’écoulements. Micro-technologies 
1 Introduction 
In a context of environmental constraints and fuel consumption reduction, flow control is becoming an 
important topic for aeronautic and automobile industrie . Increase of performances or drag reduction are the 
main issue where flow control can bring a substantial contribution. Recently, two flow properties focussed 
the research efforts: separation and wall friction. Controlling the first one means increase in lift or pressure 
drag reduction while controlling the second one reduc s the viscous drag. In these kind of applications, the 
control needed is localized and must be reactive according to the definition of Gad-el-Hak. [1]. Obviously, 
the control must need less energy than it saves. For this reason and because the uncertainty on the 
localization leads to distributed control devices, MEMS technology quickly appeared as a potential solution. 
It has been shown for example that a control can be realized by blowing oscillatory air jets in a boundary 
layer through submillimetric holes situated on the wall surface just up-stream of a separation area [2,3,4,5]. 
For friction control one of the most elaborated works by Kasagi et al [6] demonstrated the possibility of 
controlling a turbulent boundary layer in a channel. Pulsed jets are considered as a good candidate for most 
of the situations not only because they reduce the flow rate relatively to equivalent continuous jets but also 
because they are selective: they can be used to find a specific sensitive frequency instead of acting blindly on 
the whole flow. This is particularly important for aeroacoustic problems or for turbulence control. This is 
obviously the consequence of the non linearity for Navier-Stokes equations where small scale perturbations 
at high frequency may have global consequences at low frequency as suggested for example  by Wiltse-
Glezer [7] or Stanek [8]. As Navier-Stokes equations are the expression of the conservation of momentum 
for fluids,  it is natural to think that the control must modulate sources of momentum. This idea dominates all 
the numerical simulation on flow control. Nevertheless going from the concept to real actuation is not a 
small challenge. It supposes at least an open loop c ntrol with a distributed sensors and actuators arays. 
Wall deformations or fluidic actuators can be considered for this task and pulsed jets stand out becaus  of 
their potential high momentum. 
On the technological point of view the production of a localized source of high momentum with small energy 
19ème Congrès Français de Mécanique                                                                           Marseille, 24-28 août 2009 
  2 
expenses needs a mode of actuation that enables quick and large displacements of a small quantity of fluid. 
Two types of air blowing techniques exist: pulsed jts, providing only air injection and synthetic jets, 
providing alternating air injection and suction capabilities. The wide-spread method to achieve such jets 
relies on bulk mechanical systems that do not provide an independent control of each microjets. Moreover, 
well known actuation are massive and are not suited for the available space on airfoils for aeronautical 
applications. Much work has been achieved recently i  the development of microscale actuators for easier 
integration and independent jet control [9]. Existing solutions, based on electrostatic [10] or piezoelectric [11] 
actuation techniques, currently provide acceptable pu sation frequency or outlet speed. We present in this 
paper a novel architecture of silicon based, micromachined microvalves specifically dedicated for active air 
flow control. The microvalves actuation will be made on electromagnetic method which has attracted a high 
level of interest, since it provides an extended working range, a short response time, a high field energy 
density, and a low actuating voltage. A specific fabrication process involving high cross section silicon 
microchannels is described, as well as the setup of the electromagnetic actuation technique which allows to 
satisfy requirements in terms of performances (e.g.bandwidth, time response, closed-loop controllability, 
output force and displacement amplitudes).  The microvalves can provide high speed (150 m/s) microjets for 
actuation frequencies between DC and 400 Hz. 
2 Valve architecture 
The MEMS architecture consists of a silicon microchannel covered with a flexible PolyDimethylSiloxane 
(PDMS) membrane (60 µm thickness). The architecture of the microvalve is shown in fig.1. The silicon 
block processed over the PDMS membrane allows to increase the robustness, to set the resonance frequency 
higher to obtain a large actuation bandwidth, it’s also allows to enhance the closed mode, all the while 
preserving the elasticity of the membrane. The silicon pad, is free to move upwards along the y axis and/or 
rotate around the z axis under the effect of the pressure distribution introduced by a series of silicon walls 
processed in the microchannel under the pad.  
 
FIG. 1. Microvalve architecture. A silicon pad, processed over the membrane, pinches the microchannel 
when pushed towards the walls 
Pressurization of the inlet induces an increase of the static pressure in the microchannel and the expansion of 
the flexible membrane. The inlet gas is then free to pass through the microsystem, and a static equilibri m is 
reached between the resulting pressure forces and the tensile stress induced in the membrane (200 µm 
vertical displacement, 0.6 rad rotation around the z axis due to the pressure drop under the membrane in open 
mode, nominal inlet pressure of 1.5 Bars). Actuation is obtained by applying a force on the rigid pad normal 
to the channel plane, overcoming the inner pressure forc s and resulting in the mechanical pinching of the 
silicon microchannel. Such a system is represented i  Fig.2. Electromagnetic actuation was chosen for the 
high strain and displacements achievable and the eas  of integration of coil-magnet systems.  A permanent 
NdFeB magnet (cylindrical, 3mm diameter, 2mm height), bonded to the pad and coupled with a 100 
windings, 200 µm diameter wire coil are used to actu te the valve. When fed by the actuation current, the 
coil produces a magnetic field gradient on its proximity, generating a force on the permanent magnet dir cted 
normally to the membrane plane. Electromagnetic actuation means provide the compulsory high force and 
19ème Congrès Français de Mécanique                                                                           Marseille, 24-28 août 2009 
  3 
displacement needed for the aimed high flow rate, low frequency applications. 
 
FIG. 2. Electromagnetic actuation principle: the magnetic field gradient induces the closure force on the 
permanent magnet bonded to the mobile silicon pad. 
Critical dimensions for the microvalve arise from the tradeoff between the targeted high flow rate and the 
nominal inlet pressure Pin. As high deformation of the polymer membrane induces risks of tearing and 
fatigue, the nominal inlet pressure is fixed to 1.5 Bar, corresponding to a stready-state displacement of the 
rigid pad of 200 µm in the direction normal to the channel plane. The outlet flow rate is then maximized by 
minimizing the total pressure drop in the microsystem. For this reason, the microchannel cross section is set 
to 380 µm height and 3 mm width in order to reach the 1mm hydraulic diameter of the outlet hole. Silicon 
pad dimensions are set to 3mm*3mm and the inlet nomi al pressure is set to 1.5 Bars, inducing 200µm 
vertical displacement of the pad and an acceptable 30% elongation of the PDMS membrane. 
3 Characterization 
 
The outlet speed is measured thanks to a 1mm long 55P11 DANTEC hot wire anemometer. The measure is 
made 500µm away form the outlet hole, in a direction parallel to the exit microjet. Independently, a 
shadowgraph system permits the monitoring of the microjet shape, and leakage diagnosis on the measured 
microvalves. Figure 3 shows cartographies of the measured outlet speed obtained using a 30° inclined 
vectoring plate. The maximum outlet speed reaches 150 m/s in the case of a perpendicular vectoring plate. 
 
Fig. 3. Cartographies of the measured outlet speed, 30° vectoring plate, 1mm diameter outlet hole. Figures 
(a),(b),(c), and (d) represent perpendicular cuts of he microjet at a distance respectively equal to 500µm, 
5mm, 10mm and 20 mm of the outlet hole. 
As the measured value is an integration of the speed along the wire length, the presented values are slightly 
underestimated. Fig. 4 shows very weak leakage in closed mode during dynamical actuation (400 Hz, 0.5 A)  
19ème Congrès Français de Mécanique                                                                           Marseille, 24-28 août 2009 
  4 
 
Fig. 4. Exhaust speed vs. time (4 wall valve, 550Hz actuation frequency, 0,5A coil current, 75 m/s max. 
outlet speed, b). Stroboscopic shadowgraph of the ex aust jet (a). The high actuation current can be reduced 
by increasing the coil winding number. 
 
Figure 5 shows measured maximum exit velocities verus the inlet pressure. It can be seen that the valv  
performances  are relatively similar. 
 
Fig.5 Maximum exit velocity versus inlet pressure for 8 valves 
4 Basic flow control experiment 
The general idea is to generate a pressure gradient on a boundary layer on flat wall by a modification of the 
opposite wall curvature (diffuser) in a low velocity Eiffel-type wind tunnel. The diffuser has an expansion 
ratio of about 2 (0.29/0.14). It is equipped with perforated suction holes to prevent separation. Consequently, 
the flow separates on the flat plate. RANS computations have shown that the plate being flat there is nothing 
promoting the reattachment and the separation length could have been as long as 80 cm. A NACA 23012 
airfoil was then positioned at about 7 cm from the flat plate side to cause the reattachment. It was found that 
a chord of 18 cm, an angle of attack of 7 degrees and a distance of the trailing edge to the flat plate equal to 
58 mm were adequate to fix the separation length to a reasonable value. The idea was to avoid obtaining a 
separation length larger that the wind tunnel span (0.3 m) in order to limit 3D effects. The sketch of the 
experimental set-up is represented in figure 6 which in ludes all the useful dimensions. 
19ème Congrès Français de Mécanique                                                                           Marseille, 24-28 août 2009 
  5 
 
Fig 6. Sketch of the Wind tunnel experiment at ONERA Lille. 
The inflow velocity was 31 m/s. The boundary layer was fully turbulent at the actuator location and its height 
was δ0=1.5 cm. The Reynolds number based on the boundary layer displacement thickness was about 4000 
at the actuator location. The stagnation pressure and temperature corresponded to atmospheric conditios. 
The suction velocity through the diffuser porous wall deduced from the measured flow rate was about 0.6 
m/s. It represents about 2% of the main flow rate. The microjets have been disposed at about 14 δ0 upstream 
of the separation point. Figure 7 shows the array of microvalves. 
 
Fig 7. Array of microvalves monted on the backside of the flat plate (without the coils) The spacing between 
each actuator is 15 mm (1 δ0). The squared hole edge dimension is 1 mm and the je  direction is oriented 
with the pitch and skew angles both equal to 45 degrees. 
5 Results and conclusion 
The first results shows that the pulsed jets are abl  to reattached the flow (Figure 8 and 9) for a maxi um 
exit velocity of 50 to 60m/s.  This result is similar for continuous jets and doesn’t seem to depend on the 
frequency (35Hz, 70Hz, 140 Hz have been tested, 70hz corresponding to F+=1 ). Nevertheless, since the 
injected momentum is lower with pulsed jets than with continuous jets (the inflow pressure being the same), 
one can conclude to a better overall efficiency of the pulsed jets which gives the same results than te 
continuous ones at a lower energetic cost (the electric cost being negligible). It is clear that controlling 
turbulent structure will require a more sophisticated control (like a close loop) and in that perspectiv , pulsed 
or synthetic jets are far more promising that continuous jets or VG. In this work we just demonstrated that 
such fluid device without any optimization and close loop control is as efficient as VG or continuous jet . 
But another important results is the demonstration of the technical possibility to implement microsystems in 
a model and to achieve successfully wind tunnel tests without damaging the microvalves. 
19ème Congrès Français de Mécanique                                                                           Marseille, 24-28 août 2009 
  6 
 
Fig 8. Longitudinal velocity field of the uncontrolled flow in the symmetry plane (2D PIV) 
 
Fig 9. Longitudinal velocity with micro-jets (P=1.2 bars, F=70 Hz) 
References  
[1] Gad-el-Hak  “Flow control: passive, active and reactive flow management.” Cambridge University Press, 
London, UK, (2000) 
[2] I. Wygnanski, “Boundary layer and flow control by periodic addition of momentum”, AIAA Paper 97-
2117 (1997). 
[3] J. Park, H. Choi, “Effect of uniform blowing or suction from a spanwise slot on a turbulent boundary 
layer flow”, Phys. Fluids 11 (1999) pp. 3095-3105. 
[4] R. A. Antonia, Y. Zhu, M. Sokolov, “Effect of concentrated wall suction on a turbulent boundary layer”, 
Phys. Fluids 7 (1995) p.2465. 
[5] Y. Chang, S. Scott Collins, S. Ramakrisnan, “Viscous effects in control of near-wall turbulence”, Phys. 
Fluids 14 (2002) pp. 4069-4080. 
[6] Endo T., Kasagi N. and Suzuki Y., 2000, Feedback control of wall turbulence with wall deformation. I t. 
J. Heat Fluid Flow 21, pp. 568-575. [7] J. M. Wiltse and A. Glezer, “Direct high-frequency excitation of 
turbulence in free shear flows,” AIAA Paper 96-0309 (1996).,  
[7] J. M. Wiltse and A. Glezer, “Direct high-frequency excitation of turbulence in free shear flows,” AIAA 
Paper 96-0309 (1996).,  
[ 8] M. J. Stanek, N. Shina, J. M. Seiner, B. Pierce, M.I. Jones.  « High Frequency Flow Control –
Suppression of Aero-Optics In Tactical Directed Energy Beam Propagation  & The Birth of a New Model” 
AIAA 2002-2272  
[9] P. Pernod, V. Preobrazhensky, A. Merlen, O. Duclo x, A. Talbi, L Gimeno, N. Tiercelin, IUTAM 
symposium on flow control and MEMS, LNCS Series, Springer Verlag (2006). 
[10] J.R Frutos, D Vernier,.; F. Bastien,; M. de Labachelerie,; Y. Bailly, IEEE Sensors, 2005 , 7, 30 (2 05). 
[11] C. Warsop, “Aeromems ii : A european research effort to develop mems based on flow control 
technologies,” Proc. AIAA 2nd Flow Control Conferenc , 2004. 
 


Micro-systèmes et contrôle d'écoulements

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