A critical aspect in the study of a wide variety of fluid flows ranging from aquatic locomotion to turbulence is high resolution pressure measurement near walls and internal to the flow. The present investigation seeks to develop novel micro-optical based pressure sensors for fluid dynamics applications. Currently available full-field flow measurement techniques such as digital particle image velocimetry (DPIV) allow for high accuracy, high resolution measurement of the unsteady fluid velocity in general fluid flows. While highly useful, DPIV measurements and velocity measurements from similar methods do not provide direct measurements of the fluid pressure. In studies of biological propulsion, for example, local pressure measurements on the animal surface would lend great insight into biological methods for controlling the flow and for providing low drag and high efficiency locomotion. Wall pressure measurements in this setting, however, require sensors with high sensitivity, small size (for local measurements with good spatial resolution and minimum disturbance to the animal), and portability so that they can easily move with the deforming surface of the animal. Moreover, direct local measurements of pressure internal to fluid flows (i.e., away from walls and surfaces) would amplify experimental investigations of complex flows such as turbulent jets since this information is difficult to obtain from velocimetry measurements without amplifying measurement noise. The proposed micro-optical pressure sensor has several characteristics that will allow it to address the challenges of measuring pressure on dynamic, flexible surfaces and internal to the flow field. The sensor concept is based the morphology-dependent shifts in resonant frequencies of transparent micro-spheres. This phenomenon is commonly known as the whispering gallery mode (WGM). A change in the morphology of the micro-sphere (size, shape, or optical constants) causes a shift in the resonant frequency (or the WGM). This shift in the resonance can be related to the pressure applied to the micro-sphere. Using specially tailored nanocrystals (quantum dots) coupled to the micro spheres, an external illumination source can be used to excite the WGM and shifts can be monitored by externally observing the emission spectrum of the spheres. This approach allows remote optical measurement of the pressure, with the micro spheres acting as local pressure sensors that can be embedded in a surface or move with the fluid itself. We propose the development of an array of wall pressure sensors and also the development of sensors that will be used to seed the flow allowing pressure measurement at points internal to the flow. The developed and calibrated sensors will be applied to canonical unsteady flows involving vortex rings in order to validate their performance as well as investigate important features of turbulent vortex rings impinging on a wall. The outcome of the proposed work will be a sensor system that can be used for high spatial resolution (micron scale), high pressure resolution, dynamic measurements of pressure at arbitrary points on a surface and internal to a flow. This sensor concept offers a new opportunity to improve the current State of the Art on flow diagnostics and provide an unprecedented capability to experimental fluid dynamicists. The improved flow diagnostic capability could greatly assist the understanding of biological propulsion (important both for animal ecology and engineered propulsion systems that mimic the efficacy of natural systems) and engineering flows ranging from fluid mixing to flow separation in turbulent boundary layers associated with increased vehicle drag. As part of the outreach activities of the project, the research will involve the recruitment and training of undergraduate and high school students to increase the involvement of students, particularly from underrepresented groups, in science and engineering and encourage current engineering students to pursue graduate degrees.
