平面射流中时间分辨PIV检测方案(CCD相机)

Impinging jets are commonly employed in engineering applications, such as cooling anddrying processes, requiring relatively high local transport properties. Flow development of aplanar impinging jet and the effect of varying jet parameters are investigated experimentallyin a jet facility that is specifically designed, fabricated, and characterized as part of thecurrent study. The velocity field is measured using time-resolved, planar, two-componentParticle Image Velocimetry. The investigation focuses on two jet parameters: Reynoldsnumber and nozzle-to-plate spacing. Four test cases are investigated, including two Reynoldsnumbers 3000 and 6000, and two nozzle-to-plate spacings, 2B and 4B, where B is the jetwidth.Primary vortices form in the shear layer just downstream of the nozzle exit due toamplification of disturbances through the Kelvin-Helmholtz instability. Vortex sheddingexhibits higher periodicity with increasing Reynolds number but is not significantly affectedby changes to the nozzle-to-plate spacing. Further, vortex shedding location shifts upstreamwith increasing Reynolds number. The Strouhal number based on vortex shedding frequencyvaries between 0.4  StB  0.5 and the vortices are convected downstream at an averageconvective velocity of 57% of the jet centerline velocity for all cases examined. ForReB = 6000, the characteristic wavelength of the primary vortices is 1.1B for both nozzleto-plate spacings. At the lower Reynolds number, the wavelengths increase to approximately1.3B and 1.5B, for nozzle-to-plate spacings of 2B and 4B, respectively. Local decelerationof primary vortices, due to the impinging surface, causes consecutively shed vortices tomerge, with vortex merging observed in all cases investigated.As the primary vortices pass in the outer shear layer of the wall jet region, secondaryvortices of opposingly signed vorticity form due to roll-up of the wall bound vorticity in theinner shear layer. The secondary vortex shedding is marked by amplification of the surfacenormal velocity fluctuations in the inner shear layer when x/B > 3. The secondary vortexshedding shows a lower periodicity compared to the primary vortices but in general haveconvective velocities that are larger than that of the primary vortices. As the Reynoldsnumber is increased, the secondary vortex formation is suppressed, and, as a result, the walljet develops more parallel to the impinging surface. With increasing Reynolds number, theconvective velocities of the secondary vortices also decrease. In all cases, pairing betweena primary and secondary vortex is observed and the paired vortices are convected in thestreamwise direction away from the surface. At the higher Reynolds number, the pairing isimmediately followed by vortex breakdown.