Deposited on a flexible skin, self-made MEMS (Micro Electronically-Mechanical Systems) thermal film sensors were applied to a contoured wall surface for sensing unsteady flow behaviors. The sensors, each featuring a platinum sensing element 0.1μm in thickness on a polyimide substrate 20μm thick, were about 150–200ohm at room temperature. The frequency response of the sensors could be up to 30kHz when operating in constant temperature mode. In studying the unsteady flow behaviors, the flow information of interest was mainly the frequency contents of the real-time signals measured. In this paper, the three presented cases illustrate how the signals of the MEMS sensors could be used to explore the instantaneous behaviors of the unsteady flows.

When scientific experiments require transmission of powerful laser or radio beams through the atmosphere, the Federal Aviation Administration (FAA) requires that precautions be taken to avoid inadvertent illumination of aircraft. At present, the FAA requires that laser operators use human spotters to protect against accidental illumination. Here, we describe a simple, inexpensive, and highly reliable electronic system for detecting aircraft entering the vicinity of a laser beam that makes use of the air traffic control (ATC) radio transponders required on most aircraft. The radio system uses two antennas, both aligned with the laser beam. One antenna has a broad beam and the other has a narrow beam. The ratio of the transponder power received in the narrow beam to that received in the broad beam gives a measure of the angular distance of the aircraft from the axis that is independent of the range or the transmitter power. This ratio is easily measured and can be used to shutter the laser when the aircraft is too close to the beam. Comparisons of prototype systems operating at both the Apache Point and W. M. Keck Observatory with an FAA database indicate successful identification of commercial airplanes passing near the telescope boresight.

Experiments were conducted for 2D circular cylinders at Reynolds numbers in the range of 1.73 × 105–5.86 × 105. In the experiment, two circular cylinder models made of acrylic and stainless steel, respectively, were employed, which have similar dimensions but different surface roughness. Particular attention was paid to the unsteady flow behaviors inferred by the signals obtained from the pressure taps on the cylinder models and by a hot-wire probe in the near-wake region. At Reynolds numbers pertaining to the initial transition from the subcritical to the critical regimes, pronounced pressure fluctuations were measured on the surfaces of both cylinder models, which were attributed to the excursion of unsteady flow separation over a large circumferential region. At the Reynolds numbers almost reaching the one-bubble state, it was noted that the development of separation bubble might switch from one side to the other with time. Wavelet analysis of the pressure signals measured simultaneously at θ = ±90° further revealed that when no separation bubble was developed, the instantaneous vortex-shedding frequencies could be clearly resolved, about 0.2, in terms of the Strouhal number. The results of oil-film flow visualization on the stainless steel cylinder of the one-bubble and two-bubble states showed that the flow reattachment region downstream of a separation bubble appeared not uniform along the span of the model. Thus, the three dimensionality was quite evident.

We describe here a dual passive aircraft avoidance system involving both infrared and radio detection of aircraft in the vicinity of a transmitted laser beam. The infrared imager detects image motion of a thermally emissive source against the cold backdrop of space, while the radio receiver detects transmissions at 1090 MHz from the aircraft transponder. Together, these systems form a semi-redundant but complementary scheme for aircraft avoidance.

Unsteady, three-dimensional characteristics of vortex shedding were studied with self-made MEMS sensors situated spanwisely on a circular cylinder subjected to uniform incoming flow. Firstly, verification on the reliability of the MEMS sensor signals was made by comparing with the hot-wire signals obtained in the flow simultaneously. Subsequently, the MEMS sensor signals were analyzed with Wavelet and Hilbert transformations and 2D-FFT. By Wavelet and Hilbert transformations, the results obtained indicate that the strongly three-dimensional vortex shedding events are featured with pronounced spanwise variations in the instantaneous phases of vortex shedding, which are further identified as the occurrences of vortex dislocation. Furthermore, the results of 2D-FFT analysis reveal that the spanwise wave numbers of vortex shedding largely falls in a range between -2 to 2, for the MEMS sensors spanned over a spanwise region of 3 D in length.

Experiments were made with 14 MEMS sensors situated along the span of a circular cylinder whose aspect ratio was 5. The signals of the MEMS sensors were sampled simultaneously as flow over the cylinder at Reynolds numbers of 10(4). The results of Wavelet analysis of the signals indicate that the percentage of time during which strong three-dimensionality of vortex shedding was detected is about 10%. As noted, strong three-dimensionality took place when the fluctuating amplitude of the signals was severely modulated and the vortex shedding frequency reduced appeared abnormally high or low. Further noted was that the addition of a splitter plate of 0.5 or one diameter in length behind the circular cylinder was not able to suppress the three-dimensionality of the flow.

Experiments were made with 14 MEMS sensors situated along the span of a circular cylinder of aspect ratio 5. The signals of the MEMS sensors were sampled simultaneously as flow over the cylinder at Reynolds numbers of 10. The results of Wavelet analysis of the signals indicate that the percentage of time during which strong three-dimensionality of vortex shedding was detected is about 10%. As noted, strong three-dimensionality took place when the fluctuating amplitude of the signals was severely modulated and the vortex shedding frequency reduced was abnormally high or low. Further noted was that the addition of a splitter plate of 0.5 or one diameter in length behind the circular cylinder was not able to suppress the three-dimensionality of the flow.

Unsteady characteristics of How separation from a circular cylinder, at Reynolds numbers of 1.65-1.71 X 10(4), were studied using Wavelet analysis of the signals obtained by a micro-electrical-mechanical-system film sensors array flushed with the cylinder surface. At theta = 85 deg near where flow separation took place, it was found that the unsteady flow behaviors were featured with two time scales, one of which was associated with vortex shedding, and the other was at least 1 order of magnitude longer due to excursion of flow separation in a circumferential region of the extent about 5 deg. The Wavelet analysis results enabled one to reduce the percentage of time during which the vortex shedding frequency was detectable at the location measured. In the region upstream of flow separation the percentage values obtained were very close to 100%, followed by a pronounced decrease from theta = 85 to 100 deg. At theta = 100 deg, the percentage values reduced were about 30%. Moreover, low-frequency modulations were noticed in the signals measured by the thermal-film sensors situated upstream of flow separation and the hot-wire in the freestream. The Wavelet analysis with these signals revealed a trend that the lower the vortex shedding frequency, the larger the amplitude of the vortex shedding frequency component.