15.6 Control of Separation over Low-Reynolds-Number Wings Recently, researchers from the University of Florida have proposed a MEMS system for controlling sepa- ration at low Reynolds numbers. The primary motivation of the proposed system was to enhance the lift- to-drag ratio in the flight of micro-air-vehicles (MAVs). Because of their small size (a few centimeters characteristic size) and low speed, MAVs experience low Reynolds number flow phenomena during flight. One of these is an unsteady laminar separation that occurs near the leading edge of the wing and affects the aerodynamic efficiency of the wing adversely. Figure 15.33 displays a schematic of the proposed control system components and test model geome- try. The main idea is based on the deployment of integrated MEMS sensors and actuators near the lead- ing edge of an airfoil, or wing section. Additional sensor arrays are to be used near the trailing edge of the wing. The leading edge sensors are intended for detection of the separation location in order to activate those actuators closest to that location for efficient control, as discussed previously. On the other hand, the trailing edge sensors are to be utilized to sense the location of flow reattachment. In this manner, it would be possible to adapt the magnitude and location of actuation in response to changes in the flow and thus, for instance, maintain the flow attached at a particular location on the wing. The ultimate benefit of such a control system is the manipulation of the aerodynamic forces on the wing for increased efficiency as well as maneuverability without the use of cumbersome mechanical systems. In actual implementation, the University of Florida group adopted a hybrid approach whereby conven- tional-scale piezoelectric devices were used for actuation and MEMS sensors were used for measurements. Additionally, it appears that because of the difficulty in detecting the instantaneous separation location, as discussed in the delta wing control problem, a small step in the surface of the wing was introduced near the leading edge at the actuation location. Thus, the location of separation was fixed and there was no need to use leading edge sensors for initial testing of the controllability of the flow. The flow control test model is shown in Figure 15.34. 15.6.1 Sensing To measure the unsteady wall shear stress, platinum-surface hot-wire sensors were microfabricated. The devices consisted of a 0.15 µm thick ϫ 4 µm wide ϫ 200µm long platinum wire deposited on top of a 0.15 silicon nitride membrane. Beneath the membrane is a 10 µm deep vacuum cavity with a diameter of 200µm. Similar to the UCLA/Caltech sensor the evacuated cavity was incorporated in the sensor design to maximize the thermal insulation to cooling effects other than that due to the flow. As a result the sensor Towards MEMS Autonomous Control of Free-shear Flows 15-29 MEMS sensor and actuator array MEMS sensors Dynamic separation Reattachment Flow FIGURE 15.33 Control system components for University of Florida low Reynolds number wing control project. © 2006 by Taylor & Francis Group, LLC exhibited a static sensitivity as high as 11 mV/Pa when operating at an overheat ratio (operating resistance/ cold resistance) of up to 2.0. The sensor details can be seen in the SEM image in Figure 15.35. For detailed characterization of the static and dynamic response of the sensor, refer to Chandrasekaran et al. (2000) and Cain et al. (2000). 15.6.2 Flow Control Static surface pressure measurements and PIV images were used by Fuentes et al. (2000) to characterize the response of the reattaching flow to forcing with the piezoelectric actuators. The 51 mm wide ϫ 16 m long flap-type actuators (see Figure 15.34) were operated at their resonance frequency of 200 Hz. The resulting static pressure (plotted as a coefficient of pressure, CP) distribution downstream of the 1.4 mm high step is given in Figure 15.36.Similar results without forcing are also provided in the figure for comparison.As seen from the figure, the minimum negative peak of CP, corresponding to the location of reattachment, shifts upstream with excitation. The extent of the shift is fairly significant, amounting to about 30% or so of the uncontrolled reattachment length. The reduction in the reattachment length with forcing also can be depicted from the streamline plots obtained from PIV measurements (see Figure 15.37). However, the real benefit of the PIV data was to reveal the nature of the flow structure associated with actuation by capturing images that were phase-locked to different points of the forcing cycle. Those results are provided in Figure 15.38 for an approximately full cycle of the forcing. A convecting vortex structure is clearly seen in the sequence of streamline plots in Figures 15.38a through 15.38d. The observed vortex structures were periodic when an actuation amplitude of about 22µm was used. For substantially smaller forcing amplitude, the generated vortices were found to be aperiodic. 15-30 MEMS: Applications Region of interest for PIV Pressure taps, 0.25 in. apart Piezoactuator (flaps) FIGURE 15.34 Test model for separation control experiments of University of Florida. Vacuum cavity Platinum sensing element Gold contacts FIGURE 15.35 SEM view of University of Florida MEMS wall-shear sensor. © 2006 by Taylor & Francis Group, LLC Similar to the UCLA/Caltech and IIT/UM efforts, the University of Florida work has demonstrated the ability to alter the flow significantly through low-level forcing. Additionally, high-sensitivity MEMS sensors were developed and tested. However, for all three efforts their remains to be a demonstration of a fully autonomous system in operation. Towards MEMS Autonomous Control of Free-shear Flows 15-31 CP 0 10 20 30 40 50 60 –0.65 –0.6 –0.55 –0.5 –0.45 –0.4 –0.35 –0.3 –0.25 –0.2 –0.15 –0.1 –0.05 0 0 Hz 200 Hz X, mm FIGURE 15.36 Static pressure distribution with and without control. X mm 0 5 10 15 20 25 0 5 10 15 20 25 X mm (a) (b) FIGURE 15.37 Streamlines of the flow over the step without (a) and with (b) actuation. © 2006 by Taylor & Francis Group, LLC 15.7 Reflections on the Future When considering the potential use of MEMS for flow control, it is not difficult to find contradictory views within the fluid dynamics community. This is not surprising given the number of challenges facing the implementation and use of the fairly young technology. Challenges aside, however, there are certain capabilities that can be achieved only with MEMS technology. Examples include tens of kHz distributed mechanical actuators; sensor arrays that are capable of resolving the spatio-temporal character of the flow structure in high-Reynolds-number flows; integration of actuators, sensors, and electronics; and more. These are the kind of capabilities that seem to be needed if we are to have any hope of controlling such a 15-32 MEMS: Applications X mm 0 5 10 15 20 (a) X mm 0 5 10 15 20 (b) X mm 0 5 10 15 20 (c) X mm 0 5 10 15 20 (d) FIGURE 15.38 Phase-averaged stream line plots at different phases of the forcing cycle. © 2006 by Taylor & Francis Group, LLC difficult system as that governed by the Navier Stokes equations. Therefore, it is much more constructive to identify the challenges facing the use of MEMS and search for their solutions than to simply dismiss the technology along with its potential benefits. In this section, some of the leading challenges facing the attainment of autonomous MEMS control systems for shear layer control are highlighted. These are accompanied by the author’s perspective on the hope of overcoming these challenges. One of the main concerns regarding the implementation of MEMS devices is regarding their robustness, particularly if they have to be operated in harsh, high-temperature environments. For the most part, this con- cern stems from the micron size of the MEMS devices, which renders them vulnerable to large external forces. However, it is important to remember that as one shrinks a structure, the flow forces acting on it decrease along with its ability to sustain such forces. That is, to a certain extent the microscale devices may be as strong as, if not stronger than, their larger scale equivalents (at least if they are designed well). That is probably why the actuators from Naguib et al. (1997) operated properly while immersed in a Mach-0.8 shear layer, and the actuators and sensors of Huang et al. (2000) successfully completed a test flight while attached to the outside of an F-15 fighter jet. Furthermore, as new microfabrication techniques are devised for more resilient, chemically inert, harder materials than silicon, it will be possible to construct microdevices for harsh, high-temperature, chemically reacting environments. Some of the current notable efforts in this area are those concerned with micromachining of silicon carbide and diamond. The robustness question is probably more critical from a practical point of view. That is, whereas a MEMS array of surface stress sensors deployed over an airplane wing may survive during flight, it may easily be crushed by a person during routine maintenance. However, such issues should, and could, be addressed at the design stage where, for instance, the sensor array might be designed to be normally hidden away and deploy only during flight. Additionally, the inherent array-fabricating ability of MEMS could be used to increase system robustness through redundancy. If a few sensors broke, other on-chip sensors could be used instead. If the number of malfunctioning sensors became unacceptable, the entire chip could be replaced with a new one. The economics of replacing MEMS system modules will likely be justified, as it seems natural that MEMS will eventually follow in the path of the IC chip with its low-cost bulk-fabrication technologies. Beyond robustness, there will be a need to develop innovative approaches to enhance the signal-to- noise ratio of MEMS sensors. As discussed earlier, when shrinking sensors, their sensitivity often, but not always, decreases proportionally. Because for the most part traditional transduction approaches have been used with the smaller sensors, the overall signal-to-noise ratio cannot be maintained at desired lev- els. Hence, there is a need to identify ultrasensitive transduction methods. An example of such methods is the intragrain poly-diamond piezoresistive technology developed recently by Salhi and Aslam (1998). This technology promises the ability to integrate inexpensive poly-diamond piezoresitive gauges with a gauge factor of up to 4000 (20 times more sensitive than the best silicon sensors) into microsensors. Finally, when it comes to actuation, one of the most challenging issues that need to be addressed is the sufficiency of MEMS actuation amplitudes. Notwithstanding the successful demonstrations of the IIT/UM and UCLA/Caltech groups discussed earlier in this chapter, boundary layers in practice tend to be significantly thicker and turbulent at separation than encountered in those experiments. Therefore, it is most likely that the use of MEMS actuators will be confined to controlled experiments in the labora- tory (where they may be used, for example, for proof of concept experiments) and flows in microdevices. For large-scale flows, successful autonomous control systems will most probably be hybrids consolidat- ing macroactuators with MEMS sensor arrays as in the University of Florida work. This will require devel- oping clever techniques for integrating the fabrication processes of MEMS to those of large-scale devices in order to capitalize on the full advantage of MEMS. Acknowledgment The author greatly appreciates the help of Prof. Chih-Ming Ho at UCLA and Prof. Carol Bruce at the University of Florida for providing images and electronic copies of their publications for composition of this chapter. Towards MEMS Autonomous Control of Free-shear Flows 15-33 © 2006 by Taylor & Francis Group, LLC References Alnajjar, E., Naguib, A.M., Nagib, H.M., and Christophorou, C. (1997) “Receptivity of High-Speed Jets to Excitation Using an Array of MEMS-Based Mechanical Actuators,” Proceedings of ASME Fluids Engineering Division Summer Meeting, paper FEDSM97-3224, 22–26 June, Vancouver, BC, Canada. Alnajjar, E., Naguib, A., and Nagib, H. (2000) “Receptivity of an Axi-Symmetric Jet to Mechanical Excitation Using a Piezoelectric Actuator,” AIAA Fluids 2000, AIAA paper number 2000-2557, 19–22 June, Denver, Colorado. Cain, A., Chandrasekaran, V., Nishida, T., and Sheplak, M. (2000) “Development of a Wafer-Bonded, Silicon-Nitride, Membrane Thermal Shear-Stress Sensor with Platinum Sensing Element,” Solid- State Sensor and Actuator Workshop, 4–8 June, Hilton Head Island, South Carolina. Chandrasekaran, V., Cain, A., Nishida, T., and Sheplak, M. (2000) “Dynamic Calibration Technique for Thermal Shear Stress Sensors with Variable Mean Flow,” 38th Aerospace Sciences Meeting & Exhibit, AIAA paper number 2000-0508, 10–13 January, Reno, NV. Corke, T.C., and Kusek, S.M. (1993) “Resonance in Axisymmetric Jets with Controlled Helical-Mode Input,” J. Fluid Mech., 249, pp. 307–36. Corke, T.C., and Cavalieri, D. (1996) “Mode Excitation in a Jet at Mach 0.85,” Bul. Am. Phys. Soc. 41, p. 1700, 49th American Physical Society Meeting, DFD, 24–26 Nov., Syracuse, NY. Drubka, R.E., Reisenthel, P., and Nagib, H.M. (1989) “The Dynamics of Low Initial Disturbance Turbulent Jets,” Phys. Fluids A, 1, pp. 1723–1735. Epstein, A.H., Senturia, S.D., Al-Midani, O., Anathasuresh, G., Ayon, A., Breuer, K., Chen, K-S, Ehrich, F.E., Esteve, E., Frechette, L., Gauba, G., Ghodssi, R., Groshenry, C., Jacobson, S., Kerrebrok, J.L., Lang, J.H., Lin, C-C, London, A., Lopata, J., Mehra, A., Mur Miranda, J.O., Nagle, S., Orr, D.J., Piekos, E., Schmidt, M.A., Shirley, G., Spearing, S.M., Tan, C.S., Tzeng, Y-S., and Waitz, I.A. (1997) “Micro- Heat Engines, Gas Turbines, and Rocket Engines: The MIT Microengine Project,” AIAA Fluid Mechanics Summer Meeting, AIAA Paper 97-1773, 29 June–2 July, Snowmass, CO. Fiedler, H.E., and Fernholz, H H. (1990) “On Management and Control of Turbulent Shear Flows,”Prog. Aerosp. Sci., 27, pp. 305–87. Fuentes, C., He, X., Carroll, B., Lian, Y., and Shyy, W. (2000) “Low Reynolds Number Flows Around an Airfoil with a Movable Flap: Part 1. Experiments,” AIAA Fluids 200 & Exhibit, AIAA paper number 2000-2239, 19–22 June, Denver, CO. Gharib, M., Modarress, D., Fourguette, D., and Taugwalder, F. (1999) “Design, Fabrication and Integration of Mini-LDA and Mini-Surface Stress Sensors for High Reynolds Number Boundary Layer Studies,” ONR Turbulence and Wakes Program Review, 9–10 September, Palo Alto, CA. Grosjean, C., Lee, G., Hong, W., Tai, Y.C., and Ho, C.M. (1998) “Micro Balloon Actuators for Aerodynamic Control,” Eleventh Annual International Workshop on Micro Electro Mechanical Systems (MEMS ’98), pp. 166–71, 25–29 January, Heidelberg, Germany. Ho, C.M., and Huerre, P. (1984) “Perturbed Free Shear Layers,” Annu. Rev. Fluid Mech., 16, pp. 365–424. Ho, C.M., Huang, P.H., Lew, J., Mai, J., Lee, G.B., and Tai, Y.C. (1998) “MEMS: An Intelligent System Capable of Sensing-Computing-Actuating,” Proceedings of 4th International Conference on Intelligent Materials, Society of Non-Traditional Technology, Tokyo, Japan, pp. 300–3. Huang, A., Ho, C.M., Jiang, F., and Tai, Y.C. (2000) “MEMS Transducers for Aerodynamics: A Paradigm Shift,” AIAA 38th Aerospace Sciences Meeting & Exhibit, AIAA paper number 00-0249, 10–13 January, Reno, NV. Huang, C.C., Najafi, K., Alnajjar, E., Christophorou, C., Naguib, A., and Nagib, H.M. (1998) “Operation and Testing of Electrostatic Microactuators and Micromachined Sound Detectors for Active Control of High Speed Flows,” Eleventh Annual International Workshop on Micro Electro Mechanical Systems (MEMS ’98), pp. 81–86, 25–29 January, Heidelberg, Germany. Huang, C.C., Papp, J., Najafi, K., and Nagib, H.M. (1996) “A Microactuator System for the Study and Control of Screech in High-Speed Jets,” Nineth Annual International Workshop on Micro Electro Mechanical Systems (MEMS ’96), pp. 19–24, 11–15 February, IEEE, San Diego, California. 15-34 MEMS: Applications © 2006 by Taylor & Francis Group, LLC Huang, C.C., Naguib, A., Soupos, E., and Najafi, K. “A Silicon Micromachined Microphone for Fluid Mechanics Research,” (2002a) J. Micromech. Microeng., 12, pp. 1–8. Huang, C., Christophorou, C., Najafi, K., Naguib, A., and Nagib, H.M. (2002b) “An Electrostatic Micro- actuator System for Application in High-Speed Jets,” J. Microelectromech. Syst., 11, pp. 222–35. Huerre, P., and Monkewitz, P.A. (1990) “Local and Global Instabilities in Spatially Developing Flows,” Annu. Rev. Fluid Mech., 22, pp. 473–537. Hussain, A.K.M.F., and Zaman, K.B.M.Q. (1978) “The Free Shear Layer Tone Phenomenon and Probe Interference,” J. Fluid Mech., 87, pp. 349–84. Jiang, F., Lee, G.B., Tai, Y.C., and HO, C.M. (2000) “A Flexible Micromaching-Based Shear-Stress Sensor Array and its Application to Separation-Point Detection,” Sensors Actuators, 79, pp. 194–203. Lee, G.B., Ho, C.M., Jiang, F., Liu, C., Tsao, T., Tai, Y.C., and Scheuer, F. (1996) “Control of Roll Moment by MEMS,” in ASME MEMS, Ng, W., ed. Lepicovsky, J., Ahuja, K.K., and Burrin, R.H. (1985) “Tone Excited Jets: Part 3. Flow Measurements,” J. Sound Vib., 102, pp. 71–91. Liu, C., Tai, Y.C., Huang, J.B., and Ho, C.M. (1994) “Surface-Micromachined Thermal Shear Stress Sensor,” Appl. Microfab. Fluid Mech. FED-Vol. 197, ASME, pp. 9–16. Liu, C., Tsao, T., Tai, Y.C., Leu, J., Ho, C.M., Tang, W.L., and Miu, D. (1995) “Out-of-Plane Permanent Magnetic Actuators for Delta Wing Control,” Eighth Annual International Workshop on Micro Electro Mechanical Systems (MEMS ’95), pp. 7–12, 29 January–2 February, IEEE Amsterdam, Netherlands. Miller, R., Burr, G., Tai,Y.C., Psaltis, D., Ho, C.M., and Katti, R. (1996) “Electromagnetic MEMS Scanning Mirrors for Holographic Data Storage,” Technical Digest, Solid-State Sensor and Actuator Workshop, pp. 183–86. Moin, P., and Bewley, T. (1994) “Feedback Control of Turbulence,” Appl. Mech. Rev. 47, part 2, pp. S3–S13. Moore, C.J. (1977) “The Role of Shear-Layer Instability Waves in Jet Exhaust Noise,” J. Fluid Mech., 80, pp. 321–67. Naguib, A., Christophorou, C., Alnajjar, E., and Nagib, H. (1997) “Arrays of MEMS-Based Actuators for Control of Supersonic Jet Screech,” AIAA Summer Fluid Mechanics Meeting, AIAA paper number 97-1963, 29 June–2 July, Snowmass, CO. Naguib, A., Soupos, E., Nagib, H., Huang, C., and Najafi, K. (1999a) “A Piezoresistive MEMS Sensor for Acoustic Noise Measurements,” 5th AIAA/CEAS Aeroacoustics Conference, AIAA paper number 99- 1992, 10–12 May, Bellevue, WA. Naguib, A., Benson, D., Nagib, H., Huang, C., and Najafi, K. (1999b) “Assessment of New MEMS-Based Hot Wires,” Proceedings of the 3rd ASME/JSME Joint Fluids Engineering Conference, 18–22 July, San Francisco. Padmanabhan, A., Goldberg, H.D., Breuer, K.S., and Schmidt, M.A. (1996) “A Wafer-Bonded Floating- Element Shear-Stress Microsensor with Optical Position Sensing by Photodiodes,” J. Microelectromech. Syst., 5, pp. 307–15. Powel, A. (1953) “On the Mechanism of Choked Jet Noise,” Proc. Phys. Soc. (London), 66, no. 408B, pp. 1039–56. Reshotko, E., Pan, T., Hyman, D., and Mehregany, M. (1996) “Characterization of Microfabricated Shear Stress Sensors,” Eighth Beer-Sheva International Seminar on MHD Flows and Turbulence, 25–29 February, Jerusalem, Israel. Salhi, S., and Aslam, D.M. (1998) “Ultra-High Sensitivity Intra-Grain Poly-Diamond Piezoresistors,” Sensors Actuators A, 71, pp. 193–97. Tam, C.K.W. (1986) “Excitation of Instability Waves by Sound: A Physical Interpretation,” J. Sound Vib., 105, pp. 169–72. Tsao, F., Jiang, R., Miller, A., Tai, Y.C., Gupta, B., Goodman, R., Tung, S., and Ho, C.M. (1997) “An Integrated MEMS System for Turbulent Boundary Layer Control,”Technical Digest, Transducers ’97 1, pp. 315–18. Tsao, T., Liu, C., Tai, Y.C., and Ho, C.M. (1994) “Micromachined Magnetic Actuator for Active Fluid Control,” Appl. Microfab. Fluid Mech. FED-Vol. 197, ASME, pp. 31–38. Yang, X., Tai, Y.C., and Ho, C.M. (1997) “Micro Bellow Actuators,” Technical Digest, International Conference on Solid-State Sensors and Actuators, Transducers ’97 1, pp. 45–48, 16–19 June, Chicago. Towards MEMS Autonomous Control of Free-shear Flows 15-35 © 2006 by Taylor & Francis Group, LLC . on Micro Electro Mechanical Systems (MEMS 95 ), pp. 7– 12, 29 January 2 February, IEEE Amsterdam, Netherlands. Miller, R., Burr, G., Tai,Y.C., Psaltis, D., Ho, C .M. , and Katti, R. ( 199 6) “Electromagnetic. robustness, particularly if they have to be operated in harsh, high-temperature environments. For the most part, this con- cern stems from the micron size of the MEMS devices, which renders them vulnerable. result the sensor Towards MEMS Autonomous Control of Free-shear Flows 1 5 -2 9 MEMS sensor and actuator array MEMS sensors Dynamic separation Reattachment Flow FIGURE 15.33 Control system components