A photonic wall pressure sensor for fluid mechanics applications.

In this paper, we demonstrate a micro-optical wall pressure sensor concept based on the optical modes of dielectric resonators. The sensing element is a spherical micro-resonator with a diameter of a few hundred micrometers. A latex membrane that is flush mounted on the wall transmits the normal pressure to the sensing element. Changes in the wall pressure perturb the sphere's morphology, leading to a shift in the optical modes. The wall pressure is measured by monitoring the shifts in the optical modes. Prototype sensors with polydimethylsiloxane micro-spheres are tested in a steady two-dimensional channel flow and in a plane wave acoustic tube. Results indicate sensor resolutions of ∼20 mPa and bandwidth of up to 2 kHz.

Demonstration of a laser vorticity probe in turbulent boundary layers.

A laser-based probe for the nonintrusive measurement of velocity gradient and vorticity was demonstrated in turbulent boundary layers. Unlike most other optical methods, the current technique provides an estimate of the velocity gradient, without having to first measure velocity at multiple points. The measurement principle is based on the heterodyne of coherent light scattered from two adjacent particles. The beat frequency of the heterodyne is directly proportional to the velocity gradient. The probe is assembled from commercially available, inexpensive optical components. A laser Doppler velocimeter (LDV) processor is used to analyze the heterodyne signal. A component of vorticity is obtained by using two appropriately aligned velocity gradient probes. The optical probes developed were used in turbulent boundary layers to measure local, time-frozen velocity gradients partial differential u / partial differential y, partial differential v / partial differential x, and partial differential v / partial differential y, as well as the spanwise vorticity. The measurements were compared to those inferred from LDV measurements in the same facility and to data available in the literature.

Laser heterodyne method for high-resolution gas-density measurements.

A new method for noncontact, high-resolution measurement of gas density is described. The method uses a two-frequency Zeeman-split He-Ne laser and cumulative phase-measuring electronics. The measurement is resolved in two dimensions and provides density that is averaged only along the length of the laser beam that passes through the test section. The technique is based on highly accurate measurement of the optical path-length change of the laser beam as it passes through a test cell (in principle, to within 0.001lambda, where lambda is the wavelength of the laser). The technique also provides a very large dynamic range (again, in principle, up to 10(10)), which makes the method additionally attractive. Although the optical path length through the test section is directly related to the index of refraction, and hence to the density of the gas, the method can also be used to measure temperature (if the gas pressure is known) or pressure (if the temperature is known).