Miniature Laser Doppler Velocimeter for Measuring Wall Shear
- Created: Tuesday, 01 February 2005
Interference fringes are configured for sensitivity to a velocity gradient.
A miniature optoelectronic instrument has been invented as a nonintrusive means of measuring a velocity gradient proportional to a shear stress in a flow near a wall. The instrument, which can be mounted flush with the wall, is a variant of a basic laser Doppler velocimeter. The laser Doppler probe volume can be located close enough to the wall (as little as 100 µm from the surface) to lie within the viscosity-dominated sublayer of a turbulent boundary layer.
Like other laser Doppler velocimeters, this instrument includes optics that split a laser beam into two parts that impinge on the probe volume from two different directions to form interference fringes in the probe volume. Also like other laser Doppler velocimeters, this instrument measures the frequency of variation of light reflected by particles entrained in the flow as they pass through the fringes (the velocity component that one seeks to measure is simply the product of this frequency and the fringe spacing). What distinguishes this instrument from other laser Doppler velocimeters is its highly miniaturized design and its unique fringe geometry.
The instrument (see figure) includes a diode laser, the output of which is shaped by a diffractive optical element (DOE) into two beams that have elliptical cross sections with very high aspect ratios. The DOE focuses these beams through two slits a few microns apart on a surface that, in use, is mounted flush with the wall that bounds the flow. Light reflected from flow particles that pass through the fringes is collected through a window (essentially, a third, wider slit). Another DOE acts as focusing lens that couples the collected light into an optical fiber that, in turn, couples the light to an avalanche photodiode. The output of the photodiode is processed to measure the frequency of variation in the intensity of the reflected light
The interference between the laser beams forms fringes that diverge by an amount proportional to the distance from the wall: the fringes appear as radial spokes in the plane that contains a parallel-to-the-wall velocity component to be measured. Because the magnitude of this velocity component also increases linearly with distance from the wall in the viscosity-dominated flow regime and because the corresponding component of shear stress is proportional to the perpendicular-to-the-wall gradient of this velocity component, it follows that the frequency of variation of light reflected by particles entrained in the flow is proportional to the shear stress component that one seeks to measure.
The critical optical components for manipulating the laser light are fabricated on a 0.5-mm-thick quartz substrate in a sequence of microfabrication steps. The front surface (the top surface in the figure) is coated with a thin film of chromium, then further coated with poly(methyl methacrylate) [PMMA]. The slits and window are formed in the chromium film by electron-beam lithography followed by wet etching. The back surface is coated with PMMA, in which the DOEs are formed by electron-beam lithography. The unitary assembly of optical components thus formed is mounted in a compact housing that also holds the diode laser and the fiber-optic coupled photodiode.
Refer to NPO-20851, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Miniature Laser Doppler Velocimeter for Measuring Wall Shear (reference NPO-20851) is currently available for download from the TSP library.
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