The MEMS Handbook MEMS Applications (2nd Ed) - M. Gad el Hak Episode 2 Part 9 pdf

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
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Towards MEMS Autonomous Control of Free-shear Flows 15-35
© 2006 by Taylor & Francis Group, LLC