Direct Optical Measurements of Vorticity in a Nearly Turbulent Boundary Layer.
A new approach to the historically unsolvable problem of turbulence in fluid flow has been developing in recent years, a result of visual observations of coherent and persistent vortex-like structures in these flows. One particularly intriguing type of structure, the turbulent boundary layer burst, has motivated development of the novel vorticity measurement technique described here. Literature describing the burst is reviewed, emphasizing its putative association with counterrotating vortex pairs oriented in the (streamwise) direction parallel to the mean flow, and located within or near the outer edge of the viscous sublayer region of the turbulent boundary layer. Past attempts at measuring the vorticity field surrounding these structures have failed due to a lack of sufficiently small probes with the capability of accurate measurements near a wall. An optical technique for the direct measurement of vorticity in liquid flows, capable of 50 (mu)m spatial resolution in the slow moving flow near the wall, has been developed and is described here. Small spherical particles suspended in the flow rotate with angular velocity accurately equal to half the local vorticity; measurements of the rotation rates of such particles indicate the vorticity. Transparent spherical particles of less than 50 (mu)m diameter, each containing embedded planar crystal mirrors, have been developed for this purpose and are suspended in a refractive-index -matched liquid. Measurements of the times required for laser reflections from the mirrors to rotate through the small angle defined by a pair of parallel slits yields one component of the angular velocity, and thus, of the vorticity. Production and physical properties of the probe particles are reported. Theoretical capabilities and limitations of the method including accuracy, spatial and temporal resolution, data rate, and background noise are calculated and found to be coupled to the optical geometry, flow field, and orientation of the vorticity vector. Analysis yields procedures for selective optimization of each parameter as dictated by the particular application. Measurements of steady-state, laminar, two-dimensional Poiseulle flows demonstrate the accuracy of the technique and confirm theoretical predictions. The technique was used to study the temporal distribution of spanwise (perpendicular to the mean flow and parallel to the wall) vorticity in the wall region of a nearly turbulent boundary layer at modest Reynolds number. Vorticity fluctuations were discovered having rms values of about 50% of the mean vorticity, peak values as large as seven standard deviations from the mean, and typical frequencies 0.1-0.2 times the mean vorticity at the wall. Details of the flow tunnel construction are reported with evidence that the boundary layer development is normal, and that these newly observed fluctuations are not spurious. Comparison with published measurements of related quantities in similar boundary layers shows that some of these quantities exhibit temporal behaviors similar to those of the new vorticity fluctuations. Limitations of the current optical geometry on measurements of streamwise vorticity are discussed. Improvements in the detection system, designed to enable measurement of the elusive streamwise vorticity component simultaneously with the spanwise component, are currently being developed and are described in limited detail.
- Pub Date:
- June 1981
- Physics: Fluid and Plasma