Channel Constructed to Provide Well Defined Boundary Layers for Vorticity Optical Probe Measurements

摘要

In the period of fall 1983/spring 1984 the authors were predominantly concerned with the design, construction and testing of a new large scale flow system. It appears from preliminary examination that this flow system generates a turbulent boundary layer suitable for wide ranging study of vorticity dynamics. Verification that the flow exhibits normal boundary layer characteristics with both flow visualization and hot-film anemometry is under way. The Vorticity Optical Probe provides a measure of local vorticity fluctuations. It is based upon the tendency of small spherical particles to rotate with angular velocity, omega, as omega = W/2 determined by the local fluid vorticity W. To measure the particle rotation, plane mirrors (15 micron lead carbonate platelets) imbedded in about 25 micron Lucite (PMMA) spheres are the vorticity problem particles. Laser light reflected from the rotating particles allows the local vorticity to be deduced. Clearly, the refractive index of the working fluid must be matched to that of the polymethyl methacrolate particles (n=1.49) to eliminate refraction at their spherical surfaces. The choice of working fluid to match the particle refractive index has a direct effect on the flow system construction design and materials selection: the authors selected a 60 wt.% aqueous solution of ZnI2 (zinc iodide) with n=1.49, density rhonu=0.01 sq cm/sec. This fluid is moderately corrosive, perhaps comparable with sea water, but it does appear practicable for use in moderate scale flow systems and thus extends the usefulness of the vorticity probe. These along with other physical considerations are discussed in part II.