Wall Shear Stress and Flow Direction Thermal MEMS ...

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ScienceDirect Procedia Engineering 168 (2016) 774 – 777

30th Eurosensors Conference, EUROSENSORS 2016

Wall shear stress and flow direction thermal MEMS sensor for separation detection and flow control applications Cecile GHOUILA-HOURI*a,b, Jean-Claude GERBEDOENa, Julien CLAUDELa, Quentin GALLAS b, Eric GARNIER b, Alain MERLEN a,b, Romain VIARD c, Abdelkrim TALBIa, Philippe PERNOD a a

Univ. Lille, CNRS, Centrale Lille, UMR 8520 - IEMN, LIA LICS/LEMAC, F-59000 Lille, France b ONERA, Chemin de la Hunière 91123 Palaiseau, France c Fluiditech, Thurmelec, 68840 Pulversheim, France

Abstract We report a study on a wall shear stress and flow direction hot-wire based sensor operating in anemometric mode. The sensor is designed to achieve fast response time and high sensitivity. The sensor structure consists in free hot wires over insulator bridges and exhibits sensitivity to wall shear stress variations and to flow direction. The sensor can be wall-mounted for applications such as flow control or turbulence analysis. Finite elements simulations have been made to demonstrate a fast thermal response time to fluid velocity of about 300 µs in constant current operating mode and a sensitivity to wall shear-stress fluctuations. Electrical characterizations and wind tunnel experiments were performed to analyze the sensor response to respectively heat power and flow velocity. © 2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: MEMS hot-wire sensors; Wall shear-stress sensor; Separation detection; Flow control

1. Introduction In aeronautics, flow control has several objectives such as increasing lift, decreasing drag, reducing noise and preventing or suppressing flow separation. Reactive flow control using sensors provides the best control performances but requires accurate measurement for real efficiency. Therefore sensors with high spatial and temporal resolutions are needed and Micro-Electro-Mechanical Systems (MEMS) technology allows the development of miniaturized devices that can fulfill such requirements in terms of performances. Sensors for active flow control concern fluid

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Corresponding author. Tel.: +33 3 20 49 56 45 E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi:10.1016/j.proeng.2016.11.278

Cecile Ghouila-Houri et al. / Procedia Engineering 168 (2016) 774 – 777

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parameters such as pressure and wall-shear-stress. Our study is mainly focused on the second parameter. Wall-shearstress can be measured through two methods: a direct method using a floating element or a relative method using heat transfer [1]. Floating element sensors contain a mechanical part which is displaced by the fluid and thus these sensors are more prone to wear than thermal ones. Thermal sensors are indeed based on heat transfer between an electrically heated metallic wire and the surrounding cooler fluid. The latest are preferred for aerodynamic flow control applications or flow analysis. Hot-film and hot-wire sensors have thus been developed and commercialized for a few decades at macro-scale. Nowadays, micro-machining techniques allow to develop miniaturized sensors for more accurate measurements. Hot-film and hot-wire sensors share the same physical principle. Their only difference lies in the designed structure: in hot-film sensors the wire is deposited on a membrane made of various materials such as silicon nitride [2] or glass [3] or polyamide [4], whereas in hot-wire sensors the wire is free from the substrate [5,6].Thermal sensors specially dedicated for flow control and separation detection have been recently developed, as for instance the polyamide based wall hot wires in the AeroMEMS project [7] or the flexible hot films developed by Miau’s team [8]. Both were characterized in a wind tunnel. In this paper we report a thermal sensor designed for flow control applications, operating in both constant current mode and constant temperature mode and sensitive to wall-shear stress variations and flow direction for separation detection. Heater and measurement elements consist of long metallic wires suspended by periodic silicon oxide microbridges as developed in the first part. This design, patented by the IEMN LIA LICS/LEMAC [9,10] allows efficient thermal insulation, fast response time and mechanical robustness. Then, finite elements simulations using COMSOL Multiphysics are presented. This numerical study has been performed to present theoretical sensitivities to wall shear stress and flow direction. Finally, the first electrical characterizations and experimental results obtained in a turbulent boundary layer wind tunnel are presented and discussed in the last part. 2. The thermal sensor prototype The geometry sensor consists in four 1mm-long and 3 µm-wide suspended wires on silicon oxide bridges. One wire is the heater and the three others are sensitive elements. The heater and one of the sensitive elements form a multilayer placed at the center of the sensitive part. A layer of silicon oxide ensures electrical insulation between the heater and the measurement wire. Measure and heating have been uncoupled to improve the signal to noise ratio. The two other sensitive wires are arranged on both sides of the central element. Each wire is separated from the substrate to avoid heat losses by solid conduction in the silicon bulk and to increase gaseous conduction. The wire suspension by 7 µm-wide and 600 nm-high silicon oxide bridges provides mechanical toughness despite the high aspect ratio of the wires. The mechanical stress in this design is engineered using multilayer materials enabling a self-compensating stress. Gold has been chosen for the heater wire and Ni/Pt multi-layer for measurement wires. Figure 1 (a) and (b) are Scanning Electron Microscopy pictures of the fabricated sensor prototype. Figure 1 (b) enables to distinguish the heater and the measurement wire separated by a silicon oxide layer.

Fig.1. SEM picture of the realized hot-wire sensor: general view (a) zoom on the central wire (b)

3. Finite element method simulation The finite element method (FEM) simulation of the design has been performed using COMSOL Multiphysics software in order to evaluate and predict the sensor characteristics. The model takes into account all the geometrical parameters of the sensor. The heater is heated with constant power leading to heat transfer towards the other wires by

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conduction. Figure 2 (a) shows the heat distribution in the fluid flow for 11 mW heating power when the flow is at rest. With low power consumption (11 mW), an increase of about 90 K is performed. Numerical unsteady simulations provide the sensor theoretical response time to the wall shear-stress which is about 300 µs (not shown here). With an incoming simulated turbulent boundary layer, stationary simulations provide the sensor sensitivity response for wall shear stress sensing (Figure 2.(b)) and for flow direction sensing (Figure 2.(c)), both for 11 mW heat power.

Fig.2. COMSOL Multiphysics simulations: (a) heating response (b) shear stress sensitivity (c) flow direction response for 40 Pa wall shear stress

4. Experimental results 4.1. Electrical characterization The first experiments were devoted to electrical and thermal characterization without fluid flow. The first set of experiments consists in using hot plate to heat the whole sensor structure allowing the determining of the temperature coefficient of resistance (TCR) of the Ni/Pt multilayer. The TCR defined by ܶ‫ ܴܥ‬ൌ οܴȀሺοܶǤ ܴ଴ ሻwhere R is the resistance, R0 is resistance of reference and T the temperature, is about 2300 ppm/K. This coefficient is directly linked to the sensor sensitivity. The coefficient of temperature rise was measured using a Keithley 2400 source-meter and expresses the temperature increase according to the heating power: this coefficient reaches about 8 K/mW for the central wire (Figure 3 (a)) and 5 K/mW for lateral wires (Figure 3 (b)). Simulations and electrical measurements performed on different prototypes - only three of them are presented on Figure 3 - are in good agreement. Note that numerical results present a better temperature raise with power since the simulation considers infinite wires. The small differences at high power seen between the sensors tested are due to fabrication process.

Fig.3. Sensors response to heating power for the central wire (a) and one lateral wire (b) of three example sensors

4.2. Wind tunnel results To calibrate the sensor response to fluid flow, experiments in a turbulent boundary layer wind tunnel have been performed in the ONERA Lille center. This wind tunnel has a 30 cm x 30 cm test section. The sensor, operating in constant current mode, has been placed flush to the wind tunnel wall. A Dantec hot-wire was placed in the center of the wind tunnel test section to provide the flow velocity at 10 cm distance away from the wall. As seen from Figure 4, the sensor provides a sensitivity of about 1.5 Ω resistance variation at the wall for 40 m/s flow velocity variation at the wind tunnel center. The wall shear stress sensor is able to accurately capture the flow variations.


Fig.4. Experimental results: resistance variations at the wind tunnel wall and flow velocity variations in the wind tunnel center

5. Conclusion We report here a hot-wire based wall shear-stress MEMS sensor. The device structure is a sort of compromise between hot-films (robust but power consumer and relatively slow), and hot wires (fast and sensitive but fragile) as it consists in hot wires suspended by silicon oxide bridges. First prototypes have been realized using micromachining techniques. Electrical characterizations demonstrate a TCR of 2300 ppm/K and the coefficient of temperature rise of 8K/mW. Then wind tunnel experiments have been done to evaluate the sensor response over the flow: 1.5 Ω resistance variation at the wall for 40 m/s variations in the tunnel center have been achieved. Acknowledgments This work has been financially supported by the French National Research Agency (ANR) in the frame of the ANR ASTRID “CAMELOTT” project. The authors also thank RENATECH the French national nanofabrication network. References [1] L. Lfdahl and M. Gad-el-Hak, MEMS-based pressure and shear stress sensors for turbulent flows, Meas. Sci. Technol (1999)., vol. 10, no. 8, p. 665. [2] E. Vereshchagina, R. M. Tiggelaar, R. G. P. Sanders, R. A. M. Wolters, and J. G. E. Gardeniers, Low power micro-calorimetric sensors for analysis of gaseous samples, Sens. Actuators B Chem 206 (2015) [3] S. Liu, S. Pan, F. Xue, L. Nay, J. Miao, and L. K. Norford, Optimization of Hot-Wire Airflow Sensors on an Out-of-Plane Glass Bubble for 2D Detection, J. Microelectromechanical Syst 24 (2015) [4] J. J. Miau, T. S. Leu, J. M. Yu, J. K. Tu, C. T.Wang, V. Lebiga, D. Mironov, A. Pak, V. Zinovyev, and K. M. Chung, Mems thermal film sensors for unsteady flow measurement, Sens. Actuators Phys. 235 (2015) 113. [5] A. Talbi, L. Gimeno, J-C. Gerbedoen, R. Viard, A. Soltani, V. Mortet, V. Preobrazhensky, A. Merlen, and P. Pernod, A micro-scale hot wire anemometer based on low stress (Ni/W) multi-layers deposited on nano-crystalline diamond for air flow sensing, J. Micromechanics Microengineering 25 (2015). [6] Pernod Philippe, Gimeno-Monge Leticia, Talbi Abdelkrim, Merlen Alain, Viard Romain, Mortet Vincent, Soltani Ali, Preobrazhensky Vladimir, “Hot-wire sensor of submillimeter size and associated method of production,” FR2958754 (A1) 2011-10-14 WO2011128828 (A1) 2011-10-20 FR2958754 (B1) 2012-10-26 EP2561369 (A1) 2013-02-27 US2013125644 (A1) 2013-05-23 JP2013527436 (A) 2013-06-27 US8978462 (B2) 2015-03-17 EP2561369 (B1) 2015-04-01 DK2561369 (T3) 2015-07-06 JP5770828 (B2) 2015-08-26, 2011 [7] U. Buder, R. Petz, M. Kittel, W. Nitsche, and E. Obermeier, AeroMEMS polyimide based wall double hot-wire sensors for flow separation detection, Sens. Actuators Phys. 142 (2008). [8] T. S. Leu, J. M. Yu, J. J. Miau, and S. J. Chen, MEMS flexible thermal flow sensors for measurement of unsteady flow above a pitching wind turbine blade, Exp. Therm. Fluid Sci. 77 (2016) [9] Viard Romain, Talbi Abdelkrim, Pernod Philippe, Merlen Alain, Preobrazhenski Vladimir, Miniaturised Sensor Comprising A Heating Element, And Associated Production Method, FR2977886 (A1) 2013-01-18 WO2013008203 (A2) 2013-01-17 WO2013008203 (A3) 2013-03-07 CN103717526 (A) 2014-04-09 EP2731908 (A2) 2014-05-21 US2014157887 (A1) 2014-06-12 EP2731908 (B1) 2015-09-09 DK2731908 (T3) 2015-12-21, 2013. [10] R. Viard, A. Talbi, A. Merlen, P. Pernod, C. Frankiewicz, J.-C. Gerbedoen, and V. Preobrazhensky, “A robust thermal microstructure for mass flow rate measurement in steady and unsteady flows,” J. Micromechanics Microengineering, 23 (2013)

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