Projects

Project 2

Project	A Doppler Sensor for High-Resolution Measurements of the Turbulent wall Pressure at High-Reynolds Numbers
Sponsor	Office of Naval Research
Students	Mohamed Daoud
Objective	The goal of this work is to demonstrate and develop a new surface-pressure sensor concept that is based on the Doppler frequency shift. Because the technique utilizes a laser beam that can be focused to a spot size of a few to tens of microns, sensors of this new type can potentially be constructed with diameters less than 0.5 mm and implemented in an array with sub-mm inter-sensor spacing.
Description	The primary sensing element of the Laser Doppler Hydrophone (LDH)/ Microphone (LDM) consists of a thin polymer diaphragm with a reflective upper surface. The diaphragm is attached to plug substrate on top of a hole, as seen in Figure 1.
	Figure 1 shows a schematic diagram of LDM sensor plug.
	Under the action of the unsteady flow pressure, the diaphragm experiences time-dependent deflection on top of the hole. The instantaneous wall-normal velocity of the diaphragm is measured using the Doppler frequency shift of a laser beam incident on the reflective side of the pressure-sensing foil. The corresponding diaphragm deflection is obtained from time integration of the velocity. Since the wall pressure acting on the diaphragm is directly proportional to the deflection, within the elastic limit of the diaphragm and its bandwidth, one may obtain the pressure from the deflection information. To do so, the deflection sensitivity of the diaphragm needs to be determined independently from sensor calibration.
		Figure 2 displays a photograph of the optical system put together to conduct the Doppler measurements.
	The light source for the Doppler system is a Uniphase 20 mW polarized He-Ne laser with a wavelength of 633 nm. The laser beam passes through a quarter-wavelength plate (QP), which in combination with the laser-head polarizer stops any reflected laser light from entering the laser tube. After QP, the laser beam is split into 'measurement' and 'reference' beams using a 50/50 cube beam splitter (BS1). Each of these two beams is passed through an Interaction Corp. acousto-optic modulator, model AOM-40, for independent frequency biasing to eliminate the sign ambiguity of the Doppler frequency shift. The acousto-optic modulators are driven using an Interaction Corp. dual-channel frequency synthesizer, model DFE. The use of two independent AOM units enables the attainment of a frequency bias between the measurement and reference beams in steps of ±10 kHz. Furthermore, the single-crystal driver of both units ensures temperature-independent frequency shift. The frequency-shifted measurement beam is directed through a second beam splitter (BS2), off mirrors M1 and M2, and through a 500-mm focal-length achromatic lens (AL) towards the pressure-sensor diaphragm. The achromatic lens is used to decrease the spot size of the beam to approximately 0.4 mm.
		The full measurement system and the wind tunnel used to calibrate the sensors are shown in Figure 3.
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Project	Simultaneous PIV and Wall-Pressure-Array Measurements in a Separating/ Reattaching Flow Geometry
Sponsor	NASA Langley Research Center - Advanced Measurement and Diagnostics Branch
Students	Laura M. Hudy
Coworkers	William Humphreys, Jr. and Scott Bartram, NASA Langley Research Center, Hampton, VA
Objective	Compilation of a database for use to understand various aspects of separating/reattaching flow physics. These include: 1. Development of low-order models for estimation of the flow field from surface-pressure measurements in flow control applications. 2. Understanding and modeling of the flow sources responsible for the generation of the wall-pressure in separating/reattaching flows. 3. Using whole-field measurements to explore the flow physics behind the unsteady behavior of the separating shear layer.
Description	The experiment was completed in the Subsonic Basic Research Tunnel at NASA Langley Research Center in Hampton, Virginia in August 2000. The tunnel is an open-circuit wind tunnel with a 0.57 m-wide by 0.84 m-high by 1.85 m-long test section downstream of a 6:1 contraction. The model was placed in the center of the wind tunnel, splitting the test section in half vertically. The flow speed used for the results presented here was 15 m/s, resulting in a model Reynolds number of 8000 based on the total fence height of the model. The geometry investigated was a splitter plate with a fence attached upstream perpendicular to the splitter plate and the flow as seen in Figure 1.
	Figure 1 displays a schematic of the test model.
	The splitter plate was instrumented with 80 flush-mounted Panasonic electret microphones located behind the fence. The microphones were configured in an array that consisted of one streamwise row of 28 microphones along the centerline of the splitter plate. On either side of the centerline were two parallel rows, each with 13 microphones. The microphones were used to measure the dynamic pressure along the surface. Static pressure taps were also used to measure the static pressure along the surface of the model.
		The wall-microphone and static-tap configuration is depicted in the photograph in Figure 2.
	Finally, the Particle Image Velocimetery (PIV) Measurements were conducted over a plane parallel to the streamwise (x) and normal (y) directions and centered on top of the 28 wall microphones along the centerline of the model.
	Figure 3 displays a photograph of a PIV system and the model inside the wind tunnel test section.
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