'A Comparative Study of Image Compression Techniques for Digital Particle Image Velocimetry', W.M. Humphreys, Jr. and A. Naguib. To appear in AIAA Journal in 2002.

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'Characterization of the Pressure Fluctuions Under a Canonical Turbulent Boundary Layer', S. Gravante, A. Naguib, C. Wark and H. Nagib. AIAA Journal 36, No.10, October 1998, pp.1808-1816.

Abstract

'Towards MEMS Autonomous Control of Free-Shear Flows', A. Naguib, The MEMS Handbook , Chapter 33, Mohammed Gad-el-Hak (Ed.), CRC, September 2001.

'On the Order and Accuracy of the Stochastic Estimation of the Flow Structure Associated with Surface Pressure Events in a Turbulent Boundary Layer', A. Naguib and C. Wark, Advances in Turbulence VIII, Proceedings of the Eighth Turbulence Conference, C. Dopazo et al. (Eds.), CIMNE, Barcelona, Spain, June 2000.

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Resolution Enhancement of Doppler Frequency Measurements Using a Centroid Technique

A centroid-based technique for locating peaks in the frequency spectrum is demonstrated to have the potential for substantially enhancing the resolution of Fourier-based Laser Doppler Velocimetery (LDV) measurements. The effect of data windowing and signal to noise ratio on the potential resolution improvement is also analyzed.

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A Comparative Study of Image Compression Techniques for Digital Particle Image Velocimetry

A comparative, quantitative study of image compression techniques for use with digital particle image velocimetry has been performed. Several candidate compression algorithms were selected for the study including a lossless technique and two mathematical transform-based methods. Each of the compression algorithms was implemented using commercial off-the-shelf software packages. Three image sequences were selected to exercise the various compression methods. These sequences included a set of industry standard images and two sets of images obtained from experimental work conducted at NASA Langley Research Center. Evaluation of the various methods was accomplished using quantitative perceptual and meteorological performance measures. The results of the study indicate that several of the tested methods of compression are suitable for digital particle image velocimetry. A lossless LZ77 technique coupled with pixel thresholding of image gray levels before compression yielded excellent performance in terms of compression level and negligible introduction of spatial errors to the images. A lossy JPEG algorithm was shown to provide acceptable performance; however, significant spatial errors and increased numbers of false vectors derived from processing of the compressed images were observed at high compression levels. Finally, a lossy wavelet algorithm was shown to provide excellent performance in terms of minimal introduction of spatial errors and a reduction in the false vector rate over a wide range of compression levels.

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Stochastic Estimation and Flow Sources Associated with Surface Pressure Events in a Turbulent Boundary Layer

The ability of stochastic estimation to capture the conditional streamwise velocity field associated with wall-pressure events was investigated using simultaneous hot-wire and microphone measurements over the Reynolds number range: 1437 < Re < 5670. The velocity measurements were conducted at the same streamwise and spanwise location as the wall-mounted microphone but at various heights in the boundary layer, spanning the buffer and logarithmic regions. The results show that utilization of the wall-pressure as the event for the stochastic estimation requires the inclusion of both the linear and quadratic terms in order to provide an accurate representation of the conditional velocity field. The need for the inclusion of the quadratic term, which is unlike published results based on velocity and Reynolds stress events in wall-bounded flows, is attributed to the influences of the turbulent-turbulent pressure source term. Additionally, examination of the energy content of the quadratic component of the stochastic estimation and its Reynolds number scaling suggests that the quadratic term is associated with motions that scale with the boundary layer thickness.

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Characterization of the Pressure Fluctuations Under a Canonical Turbulent Boundary Layer

to be done The ability of stochastic estimation to capture the conditional streamwise velocity field associated with wall-pressure events was investigated using simultaneous hot-wire and microphone measurements over the Reynolds number range: 1437 £ Req £ 5670. The velocity measurements were conducted at the same streamwise and spanwise location as the wall-mounted microphone but at various heights in the boundary layer, spanning the buffer and logarithmic regions. The results show that utilization of the wall-pressure as the event for the stochastic estimation requires the inclusion of both the linear and quadratic terms in order to provide an accurate representation of the conditional velocity field. The need for the inclusion of the quadratic term, which is unlike published results based on velocity and Reynolds stress events in wall-bounded flows, is attributed to the influences of the turbulent-turbulent pressure source term. Additionally, examination of the energy content of the quadratic component of the stochastic estimation and its Reynolds number scaling suggests that the quadratic term is associated with motions that scale with the boundary layer thickness.

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Receptivity of an Axi-Symmetric Jet to Mechanical Excitation Using a Piezoelectric Actuator

The receptivity of the shear layer surrounding an axi-symmetric jet to mechanical excitation by a cantilever-type piezoelectric actuator was investigated. The excitation was limited to low levels that did not exceed the linear amplification threshold. Hence, it was possible to develop and use a simple linear model for decoupling the actuator's mechanical and acoustic effects. Application of this model demonstrated that the acoustic effects were insignificant compared to the mechanical influences. The dependence of the latter on the actuator radial and streamwise positions, forcing amplitude and jet Reynolds number were examined. The results suggest that the receptivity mechanisms are nonlinear. Moreover, a significant dependence on the actuator streamwise location is observed. This dependence appears to be linked to the development of the natural shear layer instability.

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A Doppler Sensor for High-Resolution Measurements of the Turbulent Wall Pressure at High-Reynolds Numbers

A new unsteady-surface-pressure measurement technique is demonstrated. The technique utilizes the Doppler frequency shift as the basic sensing mechanism. The frequency shift is experienced by a focused laser beam reflected off the aluminized top of a flexible polymer diaphragm subjected to the unsteady pressure. A prototypical sensor based on this concept, with 2-mm sensing size, is developed and characterized. The results reveal a sensor diaphragm sensitivity of about 0.00004 mm/Pa and useful frequency range of 100 < f < 4000 Hz. Furthermore, wall-pressure spectra obtained using the new sensor in a low-Reynolds-number turbulent boundary layer agree well with those obtained from a commercial electret microphone. The new approach has the potential to realize sensor arrays with less than 0.5 mm diameter for use in high Reynolds number diagnostics.

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Assessment of New MEMS-Based Hot Wires

The characteristics of new MEMS hot-wire sensors are examined in detail. Tests conducted include characterization of the new sensors' electrical and thermal properties, operating and long-term stability, static calibration and temperature sensitivity. The results indicate that the MEMS sensors are one to two orders of magnitude more sensitive than hot-wires made from the conventional materials. The static calibration of several wires followed King's law to within a maximum error of 1% over a velocity range of 5-30 m/s. Furthermore, the new sensors have demonstrated excellent stability characteristics and temperature sensitivity that is less than tungsten for the same overheat ratio.

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A Piezoresistive MEMS Sensor for Acoustic Noise Measurements

A MEMS piezoresistive acoustic sensor has been developed for measurement of jet screech noise. The new sensor was calibrated in the sound field of an air siren. Two sensors with a size of 0.51 and 0.71 mm were tested and compared to commercial sensors. The results show that the MEMS sensors are five to eight times more sensitive than the smallest commercially available piezoresistive sensors. Furthermore, the sensors are stable as demonstrated by the agreement between calibrations conducted over two months period. However, the bandwidth of this first generation sensors is somewhat limited by the damping of the diaphragm motion. This effect will be remedied in the next generation sensors through careful minimization of damping effects and increase of the sensors natural frequency.

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