The flow structure in the near field of jets and its effect on cavitation inception, and, Implementation of ferroelectric liquid crystal and birefringent crystal for image shifting in particle image velocimetry.

摘要

Cavitation experiments performed in the near field of a 50-mm diameter (D) jet at ReD = 5 x 105, showed inception in the form of inclined "cylindrical" bubbles at axial distances (x/D) less than 0.55, with indices of 2.5. On tripping the boundary layer, cavitation inception occurred at x/D ≈ 2, as distorted "spherical" bubbles with inception indices of 1.7. To investigate these substantial differences, the near field of the jet was measured using Particle Image Velocimetry (PIV). Data on the primary flow, the strength distribution of the "streamwise" vortices and the velocity profiles within the initial boundary layers were obtained. The untripped case showed a direct transition to three-dimensional flow in the near field (x/D 0.7) even before rolling up to distinct vortex rings. Strong "streamwise" vortices with strengths up to 25% of the jet velocity times the characteristic wavelength were seen. Cavitation inception occurred in the core of these vortices. In contrast, in the tripped jet the vortex sheet rolled up to the familiar Kelvin-Helmholtz vortex rings with weak secondary vortices. Using the measured nuclei distribution, strengths and straining of the "streamwise" structures, the rates of cavitation events were estimated. The estimated results match very well with the measured cavitation rates. Also, the Reynolds stresses in the near field of the jet show similar trends and magnitudes to those of Browand & Latigo (1979) and Bell & Mehta (1990) for a plane shear layer.; In the second part of this essay we discuss the implementation of electro-optical image shifting to resolve directional ambiguity in PIV measurements. The technique uses a ferroelectric liquid crystal (FLC) as an electro-optic half wave plate and a birefringent crystal (calcite) as the shifter. The system can be used with non-polarized light sources and fluorescent particles. The minimum shifting time is approximately 100mus. This compact electrooptical device usually is positioned in front of the camera lens, though it has also been mounted inside the lens body. This device extensively was used to acquire data in the near field of the jet, which is discussed in Chapter 2. Sample vector maps from a turbulent multidirectional flow are also included.