Mohamed Daoud

A Doppler sensor for high spatial resolution measurements of unsteady surface pressure

The Doppler frequency shift is used as the basic sensing mechanism for a new technique for measurement of unsteady surface pressure. 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. Prototype sensors based on this concept, with different sizes and diaphragm materials and thicknesses, are constructed as well as evaluated. The results provide an understanding of the limits of the sensors sensitivity, bandwidth, resolution and noise level. Moreover, analysis of typical wall-pressure spectra beneath boundary layers of high and low Reynolds number in the light of these limits underlines the potential advantage of the new sensor in resolving the signature of small-scale turbulent structures at high Reynolds numbers.

Microphone-Array Measurements of the Floor Pressure in a Low-Speed Cavity Flow

The objective of this paper is to highlight the significance of spatiotemporal data and associated analysis tools in the identification of oscillation modes in cavity flows. To this end, the unsteady wall-pressure field on the floor of a shallow, rectangular cavity is experimentally investigated at a low Mach number of 0.086. The pressure measurements are conducted by using an array of 16 microphones arranged in a line along the centerline of the cavity floor, which spans its full length. Data were acquired for two cavity configurations with a length-to-depth, L/D ratio of 5 and 8. Results show that for L/D = 5, the pressure oscillations are driven by the wake mode; whereas for L/D = 8, Rossiter-type modes are observed. The ability to contrast the wake and Rossiter-type oscillations was made possible through the unique, wall-pressure, wave-number-frequency signature of each of the modes.

Stochastic Estimation of the Flow Structure Downstream of a Separating/Reattaching Flow Using Wall-Pressure Array Measurements

This investigation examines the surface-pressure fluctuations and associated flow structures spatiotemporally in the developing flow downstream of the reattachment point of a fence-with-splitter-plate flow. Simultaneous measurements of the wall-pressure and velocity field were undertaken using a 16-microphone array, extending over the streamwise range 1.67 < x/Xr < 3.33 (where Xr is the mean reattachment length), and X-hotwire sensor at two Reynolds numbers of 8000 and 16000, based on the fence height above the splitter plate. The array data were used to obtain the wavenumber-frequency spectrum of the wall-pressure fluctuation. The results illustrate that Taylor hypothesis of frozen eddies reasonably describes the flow in the investigated zone. This allowed utilization of the time-dependent LSE of the velocity field to estimate the spatial structure of the flow above the microphone array. The results confirm the association of the most-energetic pressure fluctuation with the passage of quasi-periodic vortices.Copyright

Microphone-Array Measurements of Acoustic and Hydrodynamic Wall-Pressure Fluctuations in a Low-Speed Cavity Flow

The unsteady wall-pressure field on the floor of a rectangular shallow (long) cavity has been experimentally investigated at a low Mach number of 0.086. The pressure measurements were conducted using an array of 16 microphones arranged in line along the centerline of the cavity and spanning its full length. Data were acquired for two cavity configurations with length-to-depth, L/D, ratio of 5 and 8. Results show that for L/D = 5, the pressure oscillations are driven by the wake mode, while for L/D = 8, Rossiter-type modes are observed. The latter is different from the traditional Rossiter-type resonance in short cavities, which is associated with the Kelvin-Helmholtz shear-layer modes. This new long-cavity Rossiter-type resonance is associated with the large-scale structure near the end of the reattachment zone on the cavity floor, and hence is referred to as back-step resonance. The ability to contrast the wake and Rossiter type oscillations here was made possible through the unique wall-pressure wavenumber-frequency signature of each of the modes.