The shear layer separating from the edge of the delta wing is thin order of 1 mm for the UCLA/Caltechwork and very sensitive to minute changes in the geometry.. Furthermore, when the edg
Trang 1The shear layer separating from the edge of the delta wing is thin (order of 1 mm for the UCLA/Caltech
work) and very sensitive to minute changes in the geometry Therefore, as discussed earlier in this chapter,
the use of microactuators to alter the shear-layer, and ultimately the vortical-structure, characteristics
has good potential for success Furthermore, when the edge of the wing is rounded, rather than sharp,
the specific separation point location will vary with the distance from the wing apex, the flow velocity
and the position of the wing relative to the flow Therefore, a distributed sensor/actuator array is needed
to cover the area around the edge of the delta wing for detection of the separation line and actuation in
the immediate vicinity of it
35.4.1 Sensing
To detect the location of the separation line around the edge of the delta wing, the UCLA/Caltech group
utilized an array of MEMS hot-wire shear sensors The sensors, which are described in detail by Liu et al
(1994), consisted of 2-µm wide × 80-µm long polysilicon resistors that were micromachined on top of
an evacuated cavity (an SEM view of one of the sensors is provided in Figure 35.3) The vacuum cavity
provided thermal isolation against heat conduction to the substrate in order to maximize sensor cooling
by the flow The resulting sensitivity was about 15 mV/Pa, and the frequency response of the sensors was
10 kHz For more comprehensive coverage of this and other MEMS hot-wire sensors, the reader is referred
to Chapter 26
Because of directional ambiguity of hot-wire measurements and the three-dimensionality of the
sep-aration line, it was not possible to identify the location of sepsep-aration from the instantaneous shear-stress
values measured by the MEMS sensors Instead, Lee et al (1996) defined the location of the separation
line as that separating the pressure- and suction-side flows in the vicinity of the edge of the wing The
distinction between the pressure and suction sides was based on the rms level of the wall-shear signal
This was possible, as the unsteady separating flow on the suction side produced a highly fluctuating
wall-shear signature in comparison to the more steady attached flow on the pressure side
A typical variation in the rms value of the wall-shear sensor is shown as a function of the position
around the leading edge of the wing in Figure 35.4 Note that the position around the edge is expressed
in terms of the angle from the bottom side of the edge, as demonstrated by the insert in Figure 35.4 It
should also be pointed out that, because the rms is a time-integrated quantity, the detection criterion
was primarily useful in identifying the average location of separation In a more dynamic situation, where,
FIGURE 35.3 SEM image of the UCLA/Caltech shear stress sensor.
Trang 2The shear layer separating from the edge of the delta wing is thin (order of 1 mm for the UCLA/Caltech
work) and very sensitive to minute changes in the geometry Therefore, as discussed earlier in this chapter,
the use of microactuators to alter the shear-layer, and ultimately the vortical-structure, characteristics
has good potential for success Furthermore, when the edge of the wing is rounded, rather than sharp,
the specific separation point location will vary with the distance from the wing apex, the flow velocity
and the position of the wing relative to the flow Therefore, a distributed sensor/actuator array is needed
to cover the area around the edge of the delta wing for detection of the separation line and actuation in
the immediate vicinity of it
35.4.1 Sensing
To detect the location of the separation line around the edge of the delta wing, the UCLA/Caltech group
utilized an array of MEMS hot-wire shear sensors The sensors, which are described in detail by Liu et al
(1994), consisted of 2-µm wide × 80-µm long polysilicon resistors that were micromachined on top of
an evacuated cavity (an SEM view of one of the sensors is provided in Figure 35.3) The vacuum cavity
provided thermal isolation against heat conduction to the substrate in order to maximize sensor cooling
by the flow The resulting sensitivity was about 15 mV/Pa, and the frequency response of the sensors was
10 kHz For more comprehensive coverage of this and other MEMS hot-wire sensors, the reader is referred
to Chapter 26
Because of directional ambiguity of hot-wire measurements and the three-dimensionality of the
sep-aration line, it was not possible to identify the location of sepsep-aration from the instantaneous shear-stress
values measured by the MEMS sensors Instead, Lee et al (1996) defined the location of the separation
line as that separating the pressure- and suction-side flows in the vicinity of the edge of the wing The
distinction between the pressure and suction sides was based on the rms level of the wall-shear signal
This was possible, as the unsteady separating flow on the suction side produced a highly fluctuating
wall-shear signature in comparison to the more steady attached flow on the pressure side
A typical variation in the rms value of the wall-shear sensor is shown as a function of the position
around the leading edge of the wing in Figure 35.4 Note that the position around the edge is expressed
in terms of the angle from the bottom side of the edge, as demonstrated by the insert in Figure 35.4 It
should also be pointed out that, because the rms is a time-integrated quantity, the detection criterion
was primarily useful in identifying the average location of separation In a more dynamic situation, where,
FIGURE 35.3 SEM image of the UCLA/Caltech shear stress sensor.
Trang 336 Fabrication Technologies for Nanoelectromechanical
Systems
36.1 Introduction
36.2 NEMS-Compatible Processing Techniques
Electron Beam Lithography • X-Ray Lithography • Other Parallel Nanoprinting Techniques • Achieving Atomic Resolution
36.3 Fabrication of Nanomachines: The Interface with Biology
Inspiration from Biology • Practical Fabrication
of Biological Nanotechnology
36.4 Summary Acknowledgments
36.1 Introduction
As discussed in previous chapters of this volume, microelectromechanical systems (MEMS) are typically constructed on the micrometer scale, with some thin layers being perhaps in the nanometer range As has already been demonstrated by microelectronic circuits, the lateral dimensions of MEMS are being pushed into the nanometer range as well This advance has been dubbed “nanoelectromechanical sys-tems,” or NEMS The ultimate utility of nanomachining (that is to say, the application of capable robots
on the molecular scale to solving a range of problems) is limitless Such a regime will likely be attainable only by the “bottom-up” approach in which atoms are individually manipulated to construct macromol-ecules or molecular machines Properties of pure molmacromol-ecules, such as heat conduction, electrical conduc-tion (low power dissipaconduc-tion), speed of performance and strength, without the limits of boundaries to other molecules and resulting materials defects, vastly exceed those of bulk materials
Drexler wrote about molecular machinery that could be modeled after the ultimate existing nanoelec-tromechanical system, the biological cell [Drexler, 1981] He discussed analogs within the cell for such mechanical devices as cables, solenoids, drive shafts, bearings, etc Proteins exhibit a remarkable range
of functionality, and compared with current MEMS technology, are extremely small Reasoning that proteins are ideal models upon which to design nanomachines, Drexler envisioned the development of machinery that would allow us to artificially produce such nanoscale mechanical components as those listed above Imagining complex machinery operating at the molecular scale gives rise to images of
Gary H Bernstein
University of Notre Dame
Holly V Goodson
University of Notre Dame
Gregory L Snider
University of Notre Dame